Selective inhibitory effects of niflumic acid on 5-HT-induced contraction of the rat isolated stomach fundus

The effects of niflumic acid (NFA), an inhibitor of calcium-activated chloride currents I(Cl(Ca)), were compared with the actions of the voltage-dependent calcium channel (VDCC) blocker nifedipine on 5-hydroxtryptamine (5-HT)- and acetylcholine (ACh)-induced contractions of the rat isolated fundus. NFA (1 - 30 microM) elicited a concentration-dependent inhibition of contractions induced by 5-HT (10 microM) with a reduction to 15. 5+/-6.0% of the control value at 30 microM. 1 microM nifedipine reduced 5-HT-induced contraction to 15.2+/-4.9% of the control, an effect not greater in the additional presence of 30 microM NFA. In contrast, the contractile response to ACh (10 microM) was not inhibited by NFA in concentrations </=100 microM, although this response was partly inhibited by nifedipine (1 microM) to 67.6+/-11. 8% of the control value. NFA (1 - 30 microM) did not affect contraction induced by either 20 mM or 60 mM KCl, suggesting that this drug was not acting via blockade of VDCCs or activation of potassium channels. In contrast, 3, 5-dichlorophenylamine-2-carboxylic acid and 4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid were less selective in their inhibitory effects, inducing reductions of 60 mM KCl-induced contraction at concentrations >/=10 microM. Our results show that NFA can exert selective inhibitory effects on the chloride-dependent 5-HT-induced contractions of the rat fundus. The data support the hypothesis that activation of Cl((Ca)) channels leading to calcium entry via VDCCs is a mechanism utilized by 5-HT, but not by ACh, to elicit contraction of the rat fundus.

Conditions permitting suppression of stretch-induced and vasoconstrictor tone by basal nitric oxide activity in porcine cerebral artery

This study examined the ability of basal nitric oxide activity to suppress intrinsic and vasoconstrictor tone in isolated rings of porcine cerebral artery. Following stretch of approximately 1 g, N(G)-nitro-L-arginine methyl ester (L-NAME, 100 microM) produced a rise in tone in endothelium-containing but not endothelium-denuded rings. Thus, intrinsic tone was present and was powerfully suppressed by basal nitric oxide activity. Nevertheless, when concentration-response curves were constructed to U46619 and 5-hydroxytryptamine (5-HT), no endothelium-dependent depression of vasoconstriction was observed. It therefore appeared that basal nitric oxide activity was able to suppress intrinsic but not vasoconstrictor tone in these vessels. Stretch-tension curves generated following the application of stretch over the range 0 - 5. 5 g on endothelium-denuded rings showed that tension was stretch-induced. Experiments conducted in the presence of L-NAME (100 microM) revealed that the level of tone present in endothelium-containing rings was substantially higher than in endothelium-denuded rings across the entire range of stretch. When endothelium-containing and endothelium-denuded rings were set at similar levels of stretch-induced tone, rather than similar levels of stretch, the presence of the endothelium now depressed significantly vasoconstrictor responses to U46619 and 5-HT. Thus, when endothelium-containing and endothelium-denuded rings of porcine cerebral artery are set at similar points along their respective stretch-tension curves, rather than at similar levels of stretch, basal nitric oxide activity can be seen to inhibit both stretch-induced and vasoconstrictor tone.

Modulation of 5-hydroxytryptamine efflux from rat cortical synaptosomes by opioids and nociceptin

The modulation of [(3)H]-5-hydroxytryptamine ([(3)H]-5-HT) efflux from superfused rat cortical synaptosomes by delta, kappa, mu and ORL(1) opioid receptor agonists and antagonists was studied. Spontaneous [(3)H]-5-HT efflux was reduced (20% inhibition) by either 0.5 microM tetrodotoxin or Ca(2+)-omission. Ten mM K(+)-evoked [(3)H]-5-HT overflow was largely Ca(2+)-dependent (90%) and tetrodotoxin-sensitive (50%). The delta receptor agonist, deltorphin-I, failed to modulate the K(+)-evoked neurotransmitter efflux up to 0.3 microM. The kappa and the mu receptor agonists, U-50,488 and endomorphin-1, inhibited K(+)-evoked [(3)H]-5-HT overflow (EC(50)=112 and 7 nM, respectively; E(max)=28 and 29% inhibition, respectively) in a norBinaltorphimine- (0.3 microM) and naloxone- (1 microM) sensitive manner, respectively. None of these agonists significantly affected spontaneous [(3)H]-5-HT efflux. The ORL(1) receptor agonist nociceptin inhibited both spontaneous (EC(50)=67 nM) and K(+)-evoked (EC(50)=13 nM; E(max)=52% inhibition) [(3)H]-5-HT efflux. The effect of NC was insensitive to naloxone (up to 10 microM), but was antagonized by [Nphe(1)]nociceptin(1-13)NH(2) (a novel selective ORL(1) receptor antagonist; pA(2)=6.7) and by naloxone benzoylhydrazone (pA(2)=6.3). The ORL(1) ligand [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) also inhibited K(+) stimulated [(3)H]-5-HT overflow (EC(50)=64 nM; E(max)=31% inhibition), but its effect was partially antagonized by 10 microM naloxone. It is concluded that the ORL(1) receptor is the most important presynaptic modulator of neocortical 5-HT release within the opioid receptor family. This suggests that the ORL(1)/nociceptin system may have a powerful role in the control of cerebral 5-HT-mediated biological functions.

Role of uptake inhibition and autoreceptor activation in the control of 5-HT release in the frontal cortex and dorsal hippocampus of the rat

1. Using brain microdialysis, we compared the relative role of 5-hydroxytryptamine (5-HT; serotonin) blockade and somatodendritic 5-HT(1A) and/or terminal 5-HT(1B) autoreceptor activation in the control of 5-HT output. 2. Fluoxetine (10 mg kg(-1) i.p.) doubled the 5-HT output in frontal cortex and dorsal hippocampus. The 5-HT(1A) receptor antagonist WAY 100635, (0.3 mg kg(-1) s.c.) potentiated the effect of fluoxetine only in frontal cortex (to approximately 500 % of baseline). 3. Methiothepin (10 mg kg(-1) s.c.) further enhanced the 5-HT rise induced by fluoxetine+WAY 100635, to 835+/-179% in frontal cortex and 456+/-24% in dorsal hippocampus. Locally applied, methiothepin potentiated the fluoxetine-induced 5-HT rise more in the former area. 4. The selective 5-HT(1B) receptor antagonist SB-224289 (4 mg kg(-1) i.p.) enhanced the effect of fluoxetine (10 mg kg(-1) i.p.) in both areas. As with methiothepin, SB-224289 (4 mg kg(-1) i.p.) further enhanced the 5-HT increase produced by fluoxetine+WAY 100635 more in frontal cortex (613+/-134%) than in dorsal hippocampus (353+/-59%). 5. Locally applied, fluoxetine (10 - 300 microM; EC(50)=28 - 29 microM) and citalopram (1 - 30 microM; EC(50)=1.0 - 1.4 microM) increased the 5-HT output two to three times more in frontal cortex than in dorsal hippocampus. These data suggest that the comparable 5-HT increase produced by systemic fluoxetine in frontal cortex and dorsal hippocampus results from a greater effect of reuptake blockade in frontal cortex that is offset by a greater autoreceptor-mediated inhibition of 5-HT release. As a result, 5-HT autoreceptor antagonists preferentially potentiate the effect of fluoxetine in frontal cortex.

Activation of intrinsic afferent pathways in submucosal ganglia of the guinea pig small intestine

The enteric nervous system contains intrinsic primary afferent neurons that allow mucosal stimulation to initiate reflexes without CNS input. We tested the hypothesis that submucosal primary afferent neurons are activated by 5-hydroxytryptamine (5-HT) released from the stimulated mucosa. Fast and/or slow EPSPs were recorded in submucosal neurons after the delivery of exogenous 5-HT, WAY100325 (a 5-HT(1P) agonist), mechanical, or electrical stimuli to the mucosa of myenteric plexus-free preparations (+/- extrinsic denervation). These events were responses of second-order cells to transmitters released by excited primary afferent neurons. After all stimuli, fast and slow EPSPs were abolished by a 5-HT(1P) antagonist, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide, and by 1.0 microM tropisetron, but not by 5-HT(4)-selective antagonists (SB204070 and GR113808A) or 5-HT(3)-selective antagonists (ondansetron and 0.3 microM tropisetron). Fast EPSPs in second-order neurons were blocked by hexamethonium, and most slow EPSPs were blocked by an antagonist of human calcitonin gene-related peptide (hCGRP(8-37)). hCGRP(8-37) also inhibited the spread of excitation in the submucosal plexus, assessed by measuring the uptake of FM2-10 and induction of c-fos. In summary, data are consistent with the hypothesis that 5-HT from enterochromaffin cells in response to mucosal stimuli initiates reflexes by stimulating 5-HT(1P) receptors on submucosal primary afferent neurons. Second-order neurons respond to these cholinergic/CGRP-containing cells with nicotinic fast EPSPs and/or CGRP-mediated slow EPSPs. Slow EPSPs are necessary for excitation to spread within the submucosal plexus. Because some second-order neurons contain also CGRP, primary afferent neurons may be multifunctional and also serve as interneurons.

An improved in vitro bioassay for the study of 5-HT(4) receptors in the human isolated large intestinal circular muscle

Recently, it was demonstrated that 5-HT induces relaxation of human colon circular muscle through activation of 5-HT(4) receptors and 5-HT(7) receptors. The aim of the current study was to develop a new in vitro bioassay of human colon that would facilitate the pharmacological analysis of 5-HT responses mediated solely by 5-HT(4) receptors. Contracting circular muscle strips with KCl (80 mM) yielded a stable contractile tension and, in contrast to muscarinic cholinoceptor agonists and histamine, a profound reduction of spontaneous contractility. This allowed the establishment of reproducible, fully-defined, agonist concentration-response curves by cumulative dosing. Under these conditions, 5-HT induced a concentration-dependent relaxation (pEC(50) 7.31, Hill slope 0.91). Neither methysergide (10 microM) nor granisetron (1 microM) affected the 5-HT-induced relaxation, suggesting that 5-HT(1), 5-HT(2), 5-HT(3), 5-ht(5), 5-HT(6) or 5-HT(7) receptors are not involved. The lack of effect of tetrodotoxin (0.3 microM) indicated a direct effect of 5-HT on the smooth muscle. The selective 5-HT(4) receptor antagonists GR 113808, GR 125487 and RS 39604 competitively antagonized the 5-HT-induced relaxation (pK(B) 9.43, 10.12 and 8.53, respectively). SB 204070 (1 nM) produced a rightward shift (pA(2) 10.34) and depression of the 5-HT curve. These affinity estimates are similar to those previously reported for 5-HT(4) receptors. The selective 5-HT(4) receptor agonists, prucalopride and R076186, induced relaxations (pEC(50) 7.50 and 7.57, respectively), that were blocked by GR 113808 (3 nM), yielding pA(2) estimates of 9.31 and 9.21, respectively. To summarise, in KCl (80 mM)-contracted muscle strips, 5-HT induces relaxation through activation of a homogeneous smooth muscle 5-HT(4) receptor population. This new bioassay allows the focused, pharmacological characterization of human colonic 5-HT(4) receptors in vitro.

Changes in systolic time intervals-a non-invasive marker for the haemodynamic effects of sumatriptan

AIMS

This study assessed the use of systolic time intervals (STI) as a potential non-invasive marker of the haemodynamic effects of sumatriptan, a 5HT1 receptor agonist.

METHODS

Twenty-six patients undergoing diagnostic cardiac catheterization participated. STIs were derived from haemodynamic pressure tracings at baseline, following placebo injection and following either subcutaneous (n=18) or intravenous injection (n=8) of sumatriptan.

RESULTS

Sumatriptan (i.v. or s.c.) was associated with significant increases in mean arterial pressure (95% C.I. 9,14mmHg, P=0.0001), total electromechanical systole (95% C.I.8,36ms, P<0.0001), pre-ejection period (95%C.I. 8,21ms, P=0.0001) and left ventricular ejection time (95% C.I. 2,12ms, P=0.004). Conclusion STI responses were consistent with sumatriptan-induced changes in afterload. In summary, the measurement of STIs is a potential non-invasive method of investigating the influence of serotonergic compounds on the cardiovascular system.

The recombinant 5-HT1A receptor: G protein coupling and signalling pathways

The 5-hydroxytryptamine 5-HT1A receptor was one of the first G protein coupled receptors whose cDNA and gene were isolated by molecular cloning methods. Transfection of the cDNA of this receptor into cells previously bearing no 5-HT receptors has resulted in the acquisition of large amounts of information regarding potential signal transduction pathways linked to the receptor, correlations of receptor structure to its various functions, and pharmacological properties of the receptor. Transfection studies with the 5-HT1A receptor have generated critical new information that might otherwise have been elusive. This information notably includes the discovery of unsuspected novel signalling linkages, the elucidation of the mechanisms of receptor desensitization, the refinement of models of the receptor pharmacophore, and the development of silent receptor antagonists, among others. The current review summarizes the most important studies of the recombinant 5-HT1A receptor in the decade since the identification of its cDNA.

Characterization of the 5-hydroxytryptamine receptors mediating contraction in the intestine of Suncus murinus

1. The effects of 5-HT and 5-HT agonists to induce contraction and the 5-HT receptors mediating these effects were investigated in the proximal, central and terminal intestinal segments of Suncus murinus. 2. The contraction curves to 5-HT (3 nM - 30 microM) were shifted to the right by methysergide (1 microM) and ritanserin (0.1 microM), without affecting the maximum response. 3. In the central and terminal segments (but not the proximal segments) ondansetron (1 microM) and atropine (1 microM) significantly attenuated the contractions to higher concentrations of 5-HT. The selective 5-HT4 receptor antagonist SB204070 (1 nM), failed to modify 5-HT induced contractions in any segment examined. 4. 5-carboxamidotryptamine, alpha-methyl-5-HT and 5-methoxytryptamine (0.003 - 3.0 microM) induced contractions but unlike 5-HT, higher concentrations of these three agents failed to increase the response or were associated with a decrease in response. 2-methyl-5-HT (0.03 - 1.0 microM) was ten times less potent than 5-HT to induce contraction but achieved the same maximum response. 5. The contractions induced by the lower concentrations of 2-methyl-5-HT (0.03 - 1.0 microM) in all segments were markedly reduced or abolished by methysergide (1.0 microM); the response to the higher concentrations of 2-methyl-5-HT (3 - 30.0 microM) were markedly reduced by atropine (1.0 microM) and ondansetron (1.0 microM). 6. In all segments examined, tetrodotoxin (1 microM) significantly reduced the 5-HT-induced contraction. 7. It is concluded that the 5-HT-induced contraction was mediated via 5-HT2 (ritanserin sensitive) receptors in all regions of the intestine, with 5-HT3 (ondansetron sensitive) receptors mediating an additional major component in the central and terminal regions.

Pharmacological characterization of 5-HT4 receptors mediating relaxation of canine isolated rectum circular smooth muscle

This study aimed to characterize for the first time in vitro 5-HT4 receptors in the canine gastrointestinal tract. For this purpose, we used circular muscle strips of the canine isolated rectum. In the presence of methysergide (60 microM), 5-HT induced relaxation of methacholine (1 microM)-precontracted muscle strips, yielding a monophasic sigmoidal concentration-relaxation curve (pEC50 7.2+/-0.07). Tetrodotoxin (0.3 microM) did not affect the curve to 5-HT, suggesting the inhibitory 5-HT receptor is located on the smooth muscle. Granisetron (0.3 microM) did also not affect the curve to 5-HT, which excludes the 5-HT3 receptor mediating the relaxation to 5-HT. The presence of methysergide rules out the involvement of 5-HT1, 5-HT2 or 5-HT7 receptors. 5-HT, the selective 5-HT4 receptor agonists R076186, prucalopride (R093877) and SDZ HTF-919 and the 5-HT4 receptor agonists cisapride and 5-MeOT relaxed the muscle strips with a rank order of potency R076186 = 5-HT > cisapride > prucalopride > or = SDZ HTF-919 > 5-MeOT. The selective 5-HT4 receptor antagonists GR 125487, RS 39604 and GR 113808 competitively antagonized the relaxations to 5-HT, yielding pK(B) estimates of 9.7, 7.9 and 9.1, respectively. The selective 5-HT4 receptor antagonist SB 204070 shifted the curve to 5-HT rightward and depressed the maximal response (apparent pA2 10.6). GR 113808 (10 nM) produced a parallel rightward shift of the curve to the selective 5-HT4 receptor agonists R076186 (pA2 8.8). It is concluded that 5-HT induces relaxation of the canine rectum circular muscle through stimulation of a single population of smooth muscle 5-HT4 receptors. For the first time, a nonhuman species was shown to exhibit relaxant 5-HT4 receptors in the large intestine.

Studies on the role of serotonin receptor subtypes in the effect of sibutramine in various feeding paradigms in rats

The effect of the 5-hydroxytryptamine (5-HT) and noradrenaline (NA) reuptake inhibitor sibutramine was studied in food deprived, neuropeptide Y (NPY)- or muscimol-injected rats. Sibutramine dose-dependently reduced feeding caused by food-deprivation (ED50 = 5.1+/-0.8 mg kg(-1)) or by NPY injection into the paraventricular nucleus of the hypothalamus (ED50 = 6.0+/-0.5 mg kg(-1)). The increase in food intake caused by muscimol injected into the dorsal raphe was not modified by sibutramine (1-10 mg kg(-1)). The hypophagic effect of 5.1 mg kg(-1) sibutramine in food-deprived rats was studied in rats pretreated with different serotonin receptor antagonists. Metergoline (non-selective, 0.3 and 1.0 mg kg(-1)), ritanserin (5-HT2A/2C, 0.5 and 1.0 mg kg(-1)) and GR127935 (5-HT1B/1D), 0.5 and 1.0 mg kg(-1)) did not modify the hypophagic effect of sibutramine, while SB206553 (5-HT2B/2C, 5 and 10 mg kg(-1)) slightly but significantly reduced it (Fint(2.53) = 3.4; P<0.05). The reduction in food intake caused by 6.0 mg kg(-1) sibutramine in NPY-injected rats was not modified by GR127935 (1.0 mg kg(-1)). The results suggest that, with the possible exception of a partial involvement of 5-HT2B/2C receptors in sibutramine's hypophagia in food-deprived rats, 5-HT1 and 5-HT2 receptor subtypes do not play an important role in the hypophagic effect of sibutramine, at least in the first 2 h after injection.

Pharmacological evidence for the 5-HT7 receptor mediating smooth muscle relaxation in canine cerebral arteries

1. We investigated in the present study whether 5-HT is able to exert direct relaxant responses in canine basilar and middle cerebral arteries via the 5-HT7 receptor. 2. In arterial rings deprived of endothelium and pre-contracted with prostaglandin F2 alpha (2 microM), 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine, sumatriptan or alpha-methyl-5-HT produced further increase in tone and/or slight relaxation. Blockade of 5-HT1B 1D and 5-HT2A receptors with GR127935 (1 microM) and ketanserin (0.1 microM), respectively, antagonized the vasoconstrictor component of the response and unmasked a concentration-dependent relaxation to 5-HT, 5-CT and 5-methoxytryptamine; sumatriptan and alpha-methyl-5-HT remained inactive as relaxant agonists. The rank order of agonist potency in both arteries was 5-CT > 5-HT > 5-methoxytryptamine >> sumatriptan > or = alpha-methyl-5-HT. 3. In dog basilar artery, pre-incubated with GR127935 (1 microM) and ketanserin (0.1 microM) and precontracted with prostaglandin F2 alpha (2 microM), the 5-HT7 ligands, clozapine (1 microM), mesulergine (0.3 microM), methiothepin (3 nM), risperidone (3 nM), spiperone (1 microM) and LY215840 (10-100 nM), produced significant rightward shifts of the concentration-response curves for 5-HT and 5-CT. Only methiothepin and risperidone reduced significantly the maximum relaxant response (Emax), whilst the other drugs behaved as competitive antagonists with affinity values (pKB) that significantly correlated with their binding affinity (pKi) at recombinant 5-HT7 receptors. 4. These data disclosing the involvement of the 5-HT7 receptor in cerebrovascular relaxation may be strongly relevant in the light of: (1) the involvement of 5-HT in migraine; (2) the putative linkage between cephalovascular vasodilatation and migraine headache; and (3) the relatively high 5-HT7 receptor affinity of migraine prophylactic 5-HT antagonists.

Serotonin reuptake inhibitor, fluoxetine, dilates isolated skeletal muscle arterioles. Possible role of altered Ca2+ sensitivity

1. Inhibitors of serotonin reuptake in the central nervous system, such as fluoxetine, may also affect the function of vascular tissues. Thus, we investigated the effect of fluoxetine on the vasomotor responses of isolated, pressurized arterioles of rat gracilis muscle (98 +/- 4 microns in diameter at 80 mmHg perfusion pressure). 2. We have found that increasing concentrations of fluoxetine dilated arterioles up to 155 +/- 5 microns with an EC50 of 2.5 +/- 0.5 x 10(-6) M. 3. Removal of the endothelium, application of 4-aminopyridine (4-AP, an inhibitor of aminopyridine sensitive K+ channels), or use of glibenclamide (an inhibitor of ATP-sensitive K+ channels) did not affect the vasodilator response to fluoxetine. 4. In the presence of 10(-6), 2 x 10(-6) or 10(-5) M fluoxetine noradrenaline (NA, 10(-9)-10(-5) M) and 5-hydroxytryptamine (5-HT, 10(-9)-10(-5)M)-induced constrictions were significantly attenuated resulting in concentration-dependent parallel rightward shifts of their dose-response curves (pA2 = 6.1 +/- 0.1 and 6.9 +/- 0.1, respectively). 5. Increasing concentrations of Ca2+ (10(-4) 3 x 10(-2) M) elicited arteriolar constrictions (up to approximately 30%), which were markedly reduced by 2 x 10(-6)M fluoxetine, whereas 10(-5)M fluoxetine practically abolished these responses. 6. In conclusion, fluoxetine, elicits substantial dilations of isolated skeletal muscle arterioles, a response which is not mediated by 4-AP- and ATP-sensitive K+ channels or endothelium-derived dilator factors. The findings that fluoxetine had a greater inhibitory effect on Ca2+ elicited constrictions than on responses to NA and 5-HT suggest that fluoxetine may inhibit Ca2+ channel(s) or interfere with the signal transduction by Ca2+ in the vascular smooth muscle cells.

Increased dopaminergic and 5-hydroxytryptaminergic activities in male rat brain following long-term treatment with anabolic androgenic steroids

1. The effects of treating groups of rats with four different anabolic androgenic steroids (AAS) (testosterone, nandrolone, methandrostenolone, and oxymetholone) on 5-hydroxytryptamine (5-HT) and dopamine (DA) neurones in different brain regions were examined. The AAS was injected six times with 1 week's interval and the rats were sacrificed 2 days after the final injection. 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA), DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured. The effect on DA and 5-HT synthesis rate was analysed as the accumulation of 3,4-dihydroxyphenyl-alanine (DOPA) and 5-hydroxytryptophan (5-HTP), respectively, after inhibition of the amino acid decarboxylase with NSD-1015 (3-hydroxy-benzylhydrazine dihydrochloride). Additionally, the monoamine oxidase (MAO) activity was analysed in the hypothalamus. 2. The DOPAC + HVA/DA ratio was increased in the striatum in all treatment groups. However, the synthesis rate of DA was significantly increased only in the methandrostenolone treated group. 3. The 5-HIAA/5-HT ratio was increased in all treatment groups in the hippocampus, in the frontal cortex in the methandrostenolone-treated animals and in the hypothalamus in the testosterone- and oxymetholone-treated rats, while the 5-HT synthesis rate was not affected by the AAS-treatments. 4. The MAO-A activity was increased in the oxymetholone-treated rats while the other treatment groups were unaffected. The MAO-B activity was not changed. 5. The results indicate that relatively high doses of AAS increase dopaminergic and 5-hydroxytryptaminergic metabolism in male rat brain, probably due to enhanced turnover in these monaminergic systems.

The inhibition of nicotine-evoked relaxation of the guinea-pig isolated basilar artery by some analgesic drugs and progesterone

1. The purpose of this study was to investigate the mechanism of nicotine-evoked relaxation of the guinea-pig isolated basilar artery and to study the effects of drugs associated with the aetiology or treatment of migraine on the nicotine response. 2. The guinea-pig isolated basilar artery, pre-contracted with prostaglandin F2alpha (PGF2alpha), in the presence of atropine (3 microM) and guanethidine (3 microM), relaxed on addition of nicotine (0.1 mM) in approximately 50% of preparations. The responses to nicotine were of short duration and blocked in preparations pre-treated for 10 min with capsaicin (1 microM) and are therefore probably a consequence of the stimulation of trigeminal C fibre terminals. 3. Responses to nicotine were reduced in the presence of 5-carboxamidotryptamine, 5-hydroxytryptamine and sumatriptan in that order of potency. This is consistent with a 5-HT1 receptor mechanism. These agonists evoked small additional contractions in vessels pre-contracted with PGF2alpha. 4. Indomethacin (0.3-10 microM), aspirin (10-30 microM), and nitro-L-arginine methyl ester (L-NAME, 0.1 mM) reduced nicotine-evoked relaxation of the basilar artery, suggesting the involvement of both nitric oxide and cyclo-oxygenase products in this response. 5. Progesterone (1 microM) markedly reduced the response to nicotine, a possible reflection of the ion channel blocking activity of high concentrations of this compound. 6. The guinea-pig basilar artery is a preparation in which the effects of drugs on responses to stimulation of trigeminal nerve terminals can be studied in vitro and may thus be of interest in assessing the actions of drugs used in treatment of headache.

Studies on the role of dopamine in the degeneration of 5-HT nerve endings in the brain of Dark Agouti rats following 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') administration

1. We investigated whether dopamine plays a role in the neurodegeneration of 5-hydroxytryptamine (5-HT) nerve endings occurring in Dark Agouti rat brain after 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') administration. 2. Haloperidol (2 mg kg(-1) i.p.) injected 5 min prior and 55 min post MDMA (15 mg kg(-1) i.p.) abolished the acute MDMA-induced hyperthermia and attenuated the neurotoxic loss of 5-HT 7 days later. When the rectal temperature of MDMA + haloperidol treated rats was kept elevated, this protective effect was marginal. 3. MDMA (15 mg kg(-1)) increased the dopamine concentration in the dialysate from a striatal microdialysis probe by 800%. L-DOPA (25 mg kg(-1) i.p., plus benserazide, 6.25 mg kg(-1) i.p.) injected 2 h after MDMA (15 mg kg(-1)) enhanced the increase in dopamine in the dialysate, but subsequent neurodegeneration was unaltered. L-DOPA (25 mg kg(-1)) injected before a sub-toxic dose of MDMA (5 mg kg(-1)) failed to induce neurodegeneration. 4. The MDMA-induced increase in free radical formation in the hippocampus (indicated by increased 2,3- and 2,5-dihydroxybenzoic acid in a microdialysis probe perfused with salicylic acid) was unaltered by L-DOPA. 5. The neuroprotective drug clomethiazole (50 mg kg(-1) i.p.) did not influence the MDMA-induced increase in extracellular dopamine. 6. These data suggest that previous observations on the protective effect of haloperidol and potentiating effect of L-DOPA on MDMA-induced neurodegeneration may have resulted from effects on MDMA-induced hyperthermia. 7. The increased extracellular dopamine concentration following MDMA may result from effects of MDMA on dopamine re-uptake, monoamine oxidase and 5-HT release rather than an 'amphetamine-like' action on dopamine release, thus explaining why the drug does not induce degeneration of dopamine nerve endings.

5-hydroxytryptamine receptors mediating vasoconstriction and vasodilation in perinatal and adult rabbit small pulmonary arteries

1. Vasoconstrictor responses to 5-HT, 5-carboxamidotryptamine (5-CT, 5-HT1 receptor agonist), alpha-methyl-5-HT (5-HT2 receptor agonist) and sumatriptan (5-HT1D/1B receptor agonist) were studied in fetal, 0-24 h, 4 day, 7 day and adult rabbit pulmonary resistance arteries (PRAs), alone and in the presence of the NO synthase inhibitor Nomega-nitro-L-arginine methylester (L-NAME). The effect of the selective 5-HT receptor antagonists ketanserin (5-HT2A receptor) and GR55562 (5-HT1B/1D receptor) on vasoconstrictor responses to 5-HT were studied in the presence of L-NAME. Vasodilator responses to 5-CT were also studied in pre-contracted PRAs. 3. 5-HT and alpha-methyl-5-HT were equipotent in causing contraction in the PRAs at each age (e.g. pEC50s for 5-HT and alpha-methyl-5-HT were 6.74+/-0.13 and 6.63+/-0.22 respectively in adult vessels). In the perinatal PRAs, sumatriptan and 5-CT produced negligible contractions, but in adult PRAs, 5-CT and sumatriptan were potent agonists with pEC50s of 6.05+/-0.3 and 5.70+/-0.20 respectively. 4. L-NAME markedly increased the maximum response to 5-HT in the 0-24 h, 4 day and 7 day vessels and increased 5-HT potency in the 4-, 7-day-old and adult rabbit vessels. 5. In perinatal vessels, responses to 5-HT, with L-NAME present, were antagonized by ketanserin (30 nM and 0.1 microM) but not GR55562 (1 microM). A small ketanserin-resistant, GR55562-sensitive component was observed at 0-24 h. In adult vessels, both ketanserin and GR55562 inhibited 5-HT-induced responses. 7. Vasodilator responses to 5-CT were observed in pre-contracted PRAs from 4- and 7-day-old rabbits but not in the fetus, 0-24 h old or adult rabbit vessels. At 4 days the vasodilator response was inhibited both by L-NAME and GR55562. At 7 days the response was only partly blocked by L-NAME and resistant to GR55562. The L-NAME resistant component was antagonized by the 5-HT7 receptor antagonist spiperone (1 microM). 8. The results suggest that 5-HT2A-receptors mediate vasoconstriction in perinatal vessels whilst the 5-HT1D or 5-HT1B receptor contributes in adult rabbit vessels. The 5-HT1D or 5-HT1B receptor mediates NO-dependent vasodilation in vessels from rabbits at 4 days of age whilst 5-HT7 receptors mediate NO-independent vasodilation by 7 days.

5-HT1B receptor-mediated contractions in human temporal artery: evidence from selective antagonists and 5-HT receptor mRNA expression

1. In the human temporal artery both 5-HT1-like and 5-HT2A receptors mediate the contractile effects of 5-hydroxytryptamine (5-HT) and we have suggested that the 5-HT1-like receptors resemble more closely recombinant 5-HT1B than 5-HT1D receptors. To investigate further which subtype is involved, we investigated the blockade of 5-HT-induced contractions by the 5-HT1B-selective antagonist SB-224289 (2,3,6,7-tetrahydro-1'-methyl-5-[2-methyl-4'[(5-methyl-1,2,4-oxadiazole- 3-yl) biphenyl-4-yl] carbonyl] furo[2,3-f]indole-3-spiro-4'-piperidine oxalate) and the 5-HT1D-selective antagonist BRL-15572 (1-phenyl-3[4-3-chlorophenyl piperazin-1-yl] phenylpropan-2-ol). We also used RT-PCR to search for the mRNA of 5-HT1B, 5-HT1D and other 5-HT receptors. 2. The contractile effects of 5-HT in temporal artery rings were partially antagonized by SB-224289 (20, 200 nM) (apparent KB = 1 nM) and ketanserin (1 microM) but not by BRL-15572 (500 nM). 3. Sumatriptan evoked contractions (EC50, 170 nM) that were resistant to blockade by BRL-15572 (500 nM) but antagonized by SB-224289 (20, 200 nM). 4. The potency of 5-HT (EC50) was estimated to be 94 nM for the ketanserin-sensitive receptor and 34 nM for the SB-224289-sensitive receptor. The fraction of maximal 5-HT response mediated through SB-224289-sensitive receptors was 0.20-0.67, the remainder being mediated through ketanserin-sensitive receptors. 5. We detected arterial receptor mRNA for the following receptors (incidence): 5-HT1B (8/8), 5-HT1D (2/8), 5-HT1F (0/4), 5-HT2A (0/8) 5-HT2B (0/8), 5-HT2C (0/8), 5-HT4 (4/8) and 5-HT7 (4/8). 6. We conclude that the ketanserin-resistant fraction of the 5-HT effects and the effects of sumatriptan are mediated by 5-HT1B receptors. The lack of antagonism by BRL-15572 rules out 5-HT1D receptors as mediators of the contractile effects of 5-HT and sumatriptan.

Citalopram-induced hypophagia is enhanced by blockade of 5-HT(1A) receptors: role of 5-HT(2C) receptors

The selective 5-hydroxytryptamine reuptake inhibitor citalopram (10 and 20 mg kg(-1), i.p.) significantly reduced food intake in male rats (CD-COBS) habituated to eat their daily food during a 4-h period. The 5-HT1A receptor antagonist WAY100635 (0.3 mg kg(-1)) administered systemically did not modify feeding but significantly potentiated the reduction in food intake caused by 10 mg kg(-1) i.p. citalopram. The dose of 5 mg kg(-1) i.p. citalopram was not active in animals pretreated with vehicle but significantly reduced feeding in animals pretreated with WAY100635. WAY100635 (0.1 microg 0.5 microl(-1)) injected into the dorsal raphe significantly potentiated the hypophagic effect of 10 mg kg(-1) citalopram. WAY100635 (1.0 microg 0.5 microl(-1)) injected into the median raphe did not modify feeding or the hypophagic effect of 10 mg kg(-1) citalopram. The 5-HT2B/2C receptor antagonist SB206553 (10 mg kg(-1), p.o.) slightly reduced feeding by itself but partially antagonized the effect of WAY100635 administered systemically (0.3 mg kg(-1), s.c.) or into the dorsal raphe (0.1 microg 0.5 microl(-1)) in combination with 10 mg kg(-1) i.p. citalopram. The hypophagic effect of 10 mg kg(-1) i.p. citalopram alone was not significantly modified by SB206553. Brain concentrations of citalopram and its metabolite desmethylcitalopram in rats pretreated with SB206553, WAY100635 and their combination were comparable to those of vehicle-pretreated rats, 90 min after citalopram injection. The hypophagic effect of citalopram was potentiated by blocking 5-HT1A receptors. Only the effect of the WAY100635/citalopram combination seemed to be partially mediated by central 5-HT2C receptors.

Modulation of airway hyperresponsiveness and eosinophilia by selective histamine and 5-HT receptor antagonists in a mouse model of allergic asthma

1. Since both histamine and 5-hydroxytryptamine (5-HT) can be released by murine mast cells, we investigated the possible role of these autacoids on airway hyperresponsiveness (AHR), eosinophil infiltration and serum-IgE levels in a murine model of allergic asthma. 2. Ovalbumin-sensitized mice were exposed to either ovalbumin (2 mg ml(-1)) or saline aerosols on 8 consecutive days. Starting one day before the challenge, animals were injected i.p. twice a day with a 5-HT-type 1 (5-HT1) or type 2 (5-HT2) receptor antagonist (methiotepine, 1.25 or 2.0 mg kg(-1) and ketanserin, 12 mg kg(-1), respectively) or a histamine-type 1 (H1) or type 2 (H2) receptor antagonist (mepyramine, 12 or 20 mg kg(-1) and cimetidine, 10 or 25 mg kg(-1), respectively). Furthermore, animals were injected with a combination of cimetidine and ketanserin or with an alpha-adrenoceptor antagonist (phentolamine, 5 mg kg(-1)). 3. In vehicle-treated ovalbumin-challenged animals airway responsiveness to intravenous injections of methacholine in vivo was significantly (9 fold increase, P<0.01) increased when compared to vehicle-treated saline-challenged animals. Furthermore, ovalbumin challenge of vehicle-treated animals induced a significant increase in both eosinophil numbers in bronchoalveolar lavage (BAL) fluid (0+/-0, vehicle/saline and 15.0+/-5.9 x 10(4) cells vehicle/ovalbumin, P<0.05) and ovalbumin-specific IgE levels in serum (157+/-69 and 617+/-171 units ml(-1), respectively, P<0.05) compared to saline-challenged mice. Virtually no eosinophils could be detected in saline-challenged animals after all different treatments. 4. Treatment with ketanserin or cimetidine resulted in a partial but significant decrease of the ovalbumin-induced AHR compared to ovalbumin-challenged controls (P<0.05) and reduced eosinophil infiltration after ovalbumin challenge by 60% and 58%, respectively. The combination of cimetidine and ketanserin almost completely abolished AHR whereas eosinophilia was decreased by 49%. No effects of these antagonists were observed on IL-16 levels in BAL fluid or on serum antigen-specific IgE levels. Treatment with either the H1-receptor, the 5-HT1-receptor or the alpha-adrenoceptor antagonist, did not decrease the observed ovalbumin-induced airway responsiveness or eosinophilia in vehicle-treated animals. Higher doses of either methiotepine (2.0 mg kg(-1)) or mepyramine (20 mg kg(-1)) did decrease ovalbumin-induced eosinophil infiltration (by 67%, P<0.05 and 73%, respectively), whereas no effects of these antagonists were observed on ovalbumin-specific IgE levels in serum. 5. From these data it can be concluded that both histamine and 5-HT play a role in antigen-induced AHR and eosinophilia in the mouse.

The 5-HT1-like receptors mediating inhibition of sympathetic vasopressor outflow in the pithed rat: operational correlation with the 5-HT1A, 5-HT1B and 5-HT1D subtypes

1. It has been suggested that the inhibition of sympathetically-induced vasopressor responses produced by 5-hydroxytryptamine (5-HT) in pithed rats is mediated by 5-HT1-like receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors. 2. Intravenous (i.v.) continuous infusions of 5-HT and the 5-HT1 receptor agonists, 8-OH-DPAT (5-HT1A), indorenate (5-HT1A), CP 93,129 (5-HT1B) and sumatriptan (5-HT(1B/1D)), resulted in a dose-dependent inhibition of sympathetically-induced vasopressor responses. 3. The sympatho-inhibitory responses induced by 5-HT, 8-OH-DPAT, indorenate, CP 93,129 or sumatriptan were analysed before and after i.v. treatment with blocking doses of the putative 5-HT receptor antagonists, WAY 100635 (5-HT1A), cyanopindolol (5-HT(1A/1B)) or GR 127935 (5-HT(1B/1D)). Thus, after WAY 100635, the responses to 5-HT and indorenate, but not to 8-OH-DPAT, CP 93,129 and sumatriptan, were blocked. After cyanopindolol, the responses to 5-HT, indorenate and CP 93,129 were abolished, whilst those to 8-OH-DPAT and sumatriptan (except at the lowest frequency of stimulation) remained unaltered. In contrast, after GR 127935, the responses to 5-HT, CP 93,129 and sumatriptan, but not to 8-OH-DPAT and indorenate, were abolished. 4. In additional experiments, the inhibition induced by 5-HT was not modified after 5-HT7 receptor blocking doses of mesulergine. 5. The above results suggest that the 5-HT1-like receptors, which inhibit the sympathetic vasopressor outflow in pithed rats, display the pharmacological profile of the 5-HT1A, 5-HT1B and 5-HT1D, but not that of 5-HT7, receptors.

Quantitative evaluation of 5-hydroxytryptamine (serotonin) neuronal release and uptake: an investigation of extrasynaptic transmission

Whether neurotransmitters are restricted to the synaptic cleft (participating only in hard-wired neurotransmission) or diffuse to remote receptor sites (participating in what has been termed volume or paracrine transmission) depends on a number of factors. These include (1) the location of release sites with respect to the receptors, (2) the number of molecules released, (3) the diffusional rate away from the release site, determined by both the geometry near the release site as well as binding interactions, and (4) the removal of transmitter by the relevant transporter. Fast-scan cyclic voltammetry allows for the detection of extrasynaptic concentrations of many biogenic amines, permitting direct access to many of these parameters. In this study the hypothesis that 5-hydroxytryptamine (5-HT) transmission is primarily extrasynaptic in the substantia nigra reticulata, a terminal region with identified synaptic contacts, and the dorsal raphe nucleus, a somatodendritic region with rare synaptic incidence, was tested in brain slices prepared from the rat. Using carbon fiber microelectrodes, we found the concentration of 5-HT released per stimulus pulse in both regions to be identical when elicited by single pulse stimulations or trains at high frequency. 5-HT efflux elicited by a single stimulus pulse was unaffected by uptake inhibition or receptor antagonism. Thus, synaptic efflux is not restricted by binding to intrasynaptic receptors or transporters. The number of 5-HT molecules released per terminal was estimated in the substantia nigra reticulata and was considerably less than the number of 5-HT transporter and receptor sites, reinforcing the hypothesis that these sites are extrasynaptic. Furthermore, the detected extrasynaptic concentrations closely match the affinity for the predominant 5-HT receptor in each region. Although they do not disprove the existence of classical synaptic transmission, our results support the existence of paracrine neurotransmission in both serotonergic regions.

Role of hyperthermia in the protective action of clomethiazole against MDMA ('ecstasy')-induced neurodegeneration, comparison with the novel NMDA channel blocker AR-R15896AR

1. The immediate effect of administration of 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') on rectal temperature and the effect of putative neuroprotective agents on this change has been examined in rats. The influence of the temperature changes on the long term MDMA-induced neurodegeneration of cerebral 5-hydroxytryptamine (5-HT) nerve terminals was also examined. 2. The novel low affinity N-methyl-D-aspartate (NMDA) receptor channel blocker AR-R15896AR (20 mg kg(-1), i.p.) given 5 min before and 55 min after MDMA (15 mg kg(-1), i.p.) did not prevent the MDMA-induced hyperthermia and did not alter either the MDMA-induced neurodegenerative loss of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in cortex, striatum and hippocampus or loss of [3H]-paroxetine binding in cortex 7 days later. 3. The neuroprotective agent clomethiazole (50 mg kg(-1), i.p.) given 5 min before and 55 min after MDMA (15 mg kg(-1)) abolished the MDMA-induced hyperthermic response and markedly attenuated the loss of 5-HT, 5-HIAA and [3H]-paroxetine binding in the brain regions examined 7 days later. 4. When rats treated with MDMA plus clomethiazole were kept at high ambient temperature for 5 h post-MDMA, thereby keeping their body temperature elevated to near that seen in rats given MDMA alone, the MDMA-induced loss of 5-HT, 5-HIAA and [3H]-paroxetine was still attenuated. However, the protection (39%) afforded by the clomethiazole administration was less than seen in rats kept at normal ambient temperature (75%). 5. These data support the proposals of others that NMDA receptor antagonists are neuroprotective against MDMA-induced degeneration only if they induce hypothermia and further suggest that increased glutamate activity may not be involved in the neurotoxic action of MDMA. 6. These data further demonstrate that a proportion of the neuroprotective action of clomethiazole is due to an effect on body temperature but that, in addition, the compound protects against MDMA-induced damage by an unrelated mechanism.

Effect of RX 821002 at 5-HT1A-receptors in rabbit spinal cord in vivo

1. The activity of RX 821002 (2-methoxy idazoxan) at 5-HT1A-receptors in the spinal cord has been investigated in decerebrated, spinalized rabbits. Reflexes evoked in medial gastrocnemius motoneurones by electrical stimulation of the sural nerve were unaffected by intrathecal (i.th.) administration of RX 821002 (111 and 664 nmol cumulative, n = 7), although the highest dose of this drug did produce a significant increase in heart rate of 28 +/- 7 beats min(-1). Subsequent administration of the 5-HT1A-receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) at 300 nmol, i.th., facilitated reflexes to a median of 144% of pre-drug controls, an effect that was partially reversed (to a median value of 120% of pre-drug values) by subsequent administration of the 5-HT1A-receptor antagonist WAY-100635, at 185 nmol i.th. 2. In a separate set of experiments, 8-OH-DPAT was given at 30 nmol i.th. and potentiated reflexes to a median of 170% of pre-drug levels (n = 8). Subsequent administration of RX 821002 (at a cumulative dose of 1.11 micromol, i.th., n = 5) significantly reduced gastrocnemius responses to a median of 154% of control values. 3. After a 3 h recovery period, 8-OH-DPAT was re-administered at 30 nmol, i.th., and increased reflexes to a median value of 151% of pre-drug levels, an effect not significantly different from when it was given alone. WAY-100635 dose-dependently antagonized this effect, causing significant reductions in reflexes at a cumulative dose of 0.55 nmol, i.th., and complete reversal of the effects of 8-OH-DPAT at a cumulative dose of 5.5 nmol. 4. These data show that, at intrathecal doses up to 664 nmol, RX 821002 is devoid of agonist activity at 5-HT1A-receptors. It appears to be a very weak antagonist at these sites in vivo, being some 2000 times less potent than WAY-100635. The inability of WAY-100635 to block completely the effects of high doses of 8-OH-DPAT has been noted previously and can be explained by non-selective actions of the agonist. However, it would appear that a 30 nmol i.th. dose of 8-OH-DPAT is selective for 5-HT1A receptors in this preparation.

Characterization of prejunctional 5-HT1 receptors that mediate the inhibition of pressor effects elicited by sympathetic stimulation in the pithed rat

1. A study was made of the effects of 5-carboxamidotryptamine (5-CT) on pressor responses induced in vivo by electrical stimulation of the sympathetic outflow from the spinal cord of pithed rats. All animals had been pretreated with atropine. Sympathetic stimulation (0.1, 0.5, 1 and 5 Hz) resulted in frequency-dependent increases in blood pressure. Intravenous infusion of 5-CT at doses of 0.01, 0.1 and 1 mg kg(-1) min(-1) reduced the pressor effects obtained by electrical stimulation. The inhibitory effect of 5-CT was significantly more pronounced at lower frequencies of stimulation. In the present study we characterized the pharmacological profile of the receptors mediating the above inhibitory effect of 5-CT. 2. The inhibition induced by 0.01 microg kg(-1) min(-1) of 5-CT on sympathetically-induced pressor responses was partially blocked after i.v. treatment with methiothepin (10 microg kg(-1)), WAY-100,635 (100 microg kg(-1)) or GR127935T (250 microg kg(-1)), but was not affected by cyanopindolol (100 microg kg(-1)). 3. The selective 5-HT1A receptor agonist 8-OH-DPAT and the selective 5-HT(1B/1D) receptor agonists sumatriptan and L-694,247 inhibited the pressor response, whereas the 5-HT1B receptor agonists CGS-12066B and CP-93,129 and the 5-HT2C receptor agonist m-CPP did not modify the pressor sympathetic responses. 4. The selective 5-HT1A receptor antagonist WAY-100,635 (100 microg kg(-1)) blocked the inhibition induced by 8-OH-DPAT and the selective 5-HT(1B/1D) receptor antagonist GR127935T (250 microg kg(-1)) abolished the inhibition induced either by L-694,247 or sumatriptan. 5. None of the 5-HT receptor agonists used in our experiments modified the pressor responses induced by exogenous noradrenaline (NA). 6. These results suggest that the presynaptic inhibitory action of 5-CT on the electrically-induced pressor response is mediated by both r-5-HT1D and 5-HT1A receptors.

Antagonist properties of (-)-pindolol and WAY 100635 at somatodendritic and postsynaptic 5-HT1A receptors in the rat brain

1. The aim of the present work was to characterize the 5-hydroxytryptamine1A (5-HT1A) antagonistic actions of (-)-pindolol and WAY 100635 (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide). Studies were performed on 5-HT1A receptors located on 5-hydroxytryptaminergic neurones in the dorsal raphe nucleus (DRN) and on pyramidal cells in the CA1 and CA3 regions of the hippocampus in rat brain slices. 2. Intracellular electrophysiological recording of CA1 pyramidal cells and 5-hydroxytryptaminergic DRN neurones showed that the 5-HT1A receptor agonist 5-carboxamidotryptamine (5-CT) evoked in both cell types a concentration-dependent cell membrane hyperpolarization and a decrease in cell input resistance. On its own, (-)-pindolol did not modify the cell membrane potential and resistance at concentrations up to 10 microM, but it antagonized the 5-CT effects in a concentration-dependent manner. Similar antagonism of 5-CT effects was observed in the CA3 hippocampal region. (-)-Pindolol also prevented the 5-HT1A receptor-mediated hyperpolarization of CA1 pyramidal cells due to 5-HT (15 microM). In contrast, the 5-HT-induced depolarization mediated by presumed 5-HT4 receptors persisted in the presence of 3 microM (-)-pindolol. 3. In the hippocampus, (-)-pindolol completely prevented the hyperpolarization of CA1 pyramidal cells by 100 nM 5-CT (IC50=92 nM; apparent KB=20.1 nM), and of CA3 neurones by 300 nM 5-CT (IC50=522 nM; apparent KB= 115.1 nM). The block by (-)-pindolol was surmounted by increasing the concentration of 5-CT, indicating a reversible and competitive antagonistic action. 4. Extracellular recording of the firing rate of 5-hydroxytryptaminergic neurones in the DRN showed that (-)-pindolol blocked, in a concentration-dependent manner, the decrease in firing elicited by 100 nM 5-CT (IC50=598 nM; apparent KB= 131.7 nM) or 100 nM ipsapirone (IC50= 132.5 nM; apparent KB= 124.9 nM). The effect of (-)-pindolol was surmountable by increasing the concentration of the agonist. Intracellular recording experiments showed that 10 microM (-)-pindolol were required to antagonize completely the hyperpolarizing effect of 100 nM 5-CT. 5. In vivo labelling of brain 5-HT1A receptors by i.v. administration of [3H]-WAY 100635 ([O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1 -piperazinyl)ethyl-N-(2-pyridyl)cyclo-hexane-carboxamide) was used to assess their occupancy following in vivo treatment with (-)-pindolol. (-)-Pindolol (15 mg kg[-1]) injected i.p. either subchronically (2 day-treatment before i.v. injection of [3H]-WAY 100635) or acutely (20 min before i.v. injection of [3H]-WAY 100635) markedly reduced [3H]-WAY 100635 accumulation in all 5-HT1A receptor-containing brain areas. In particular, no differences were observed in the capacity of (-)-pindolol to prevent [3H]-WAY 100635 accumulation in the DRN and the CAI and CA3 hippocampal areas. 6. Intracellular electrophysiological recording of 5-hydroxytryptaminergic DRN neurones showed that WAY 100635 prevented the hyperpolarizing effect of 100 nM 5-CT in a concentration-dependent manner (IC50=4.9 nM, apparent KB=0.25 nM). In CA1 pyramidal cells, hyperpolarization induced by 50 nM 5-CT was also antagonized by WAY 100635 (IC50 = 0.80 nM, apparent KB= 0.28 nM).

Characteristics of histamine-induced leukocyte rolling in the undisturbed microcirculation of the rat mesentery

1. The main objective of this study was to analyse the role and mode of action of the mast cell mediator histamine in leukocyte-endothelium interactions in small venules in vivo. For this purpose, we used a histological approach (combined with intravital microscopy) that allows studies of rapid mediator-induced venular leukocyte accumulation, reflecting leukocyte rolling, in the undisturbed microcirculation of the rat mesentery where rolling is normally absent. 2. We first examined the relative importance of histamine and 5-hydroxytryptamine (5-HT) in acute mast cell-dependent leukocyte recruitment. The mast cell secretagogue compound 48/80 (i.p. for 15 min) induced a marked venular accumulation of polymorphonuclear leukocytes (PMNL) which was almost abolished by combined histamine1 (H1)- and histamine2 (H2)-receptor blockade. In contrast, the 5-HT-receptor antagonist methysergide was inactive in this regard. Moreover, exogenous 5-HT was less active than exogenous histamine in evoking venular PMNL accumulation (histamine response dose-dependent; 5-HT response bell shaped). Prostaglandin D2 did not cause PMNL accumulation. 3. The venular PMNL response to exogenous histamine peaked between 15 min and 1 h, was still significantly elevated at 2 h, and then returned to prechallenge values after 3 h. At all time points, the histamine-induced PMNL accumulation was nearly abolished by i.v. treatment with the polysaccharide fucoidin (which blocks rolling but not firm adhesion per se), suggesting that the PMNL response to histamine was due to rolling rather than firm adhesion over the entire 3 h period. At no time point did histamine trigger accumulation of mononuclear leukocytes (MNL). 4. To examine the role of histamine-receptors in the histamine-induced PMNL accumulation (i.e. rolling), the animals were pretreated with diphenhydramine (H1-receptor antagonist), cimetidine, or ranitidine (H2-receptor antagonists). Diphenhydramine alone inhibited the venular PMNL response to histamine by 52%, while both H2-receptor antagonists were completely inactive. However, the combination of cimetidine and diphenhydramine reduced the histamine-induced PMNL rolling by 82%. Furthermore, in contrast to an H3-receptor agonist, challenge with either the H1-receptor agonist 2-thiazolylethylamine or two different H2-receptor agonists (impromidine, dimaprit) was sufficient to provoke significant venular PMNL accumulation. 5. Treatment with the nitric oxide-synthase inhibitor L-NAME did not affect the histamine-induced PMNL rolling. On the other hand, 3 h pretreatment with dexamethasone reduced the PMNL response to histamine by 73%, and flow cytometric analysis showed that the dexamethasone treatment almost completely inhibited binding of soluble P-selectin to rat isolated PMNLs. 6. We conclude that initial leukocyte recruitment after mast cell activation in the rat mesentery is critically dependent on histamine release. The cellular response to histamine was specifically due to PMNL rolling, involved activation of both H1- and H2-receptors, and lasted for 2 3 h. Moreover, the histamine-induced PMNL rolling was not dependent on nitric oxide synthesis, but was sensitive to glucocorticoid treatment, possibly via inhibition of expression or function of leukocytic P-selectin ligand(s).

Risperidone inhibits 5-hydroxytryptaminergic neuronal activity in the dorsal raphe nucleus by local release of 5-hydroxytryptamine

1. The effects of risperidone on brain 5-hydroxytryptamine (5-HT) neuronal functions were investigated and compared with other antipsychotic drugs and selective receptor antagonists by use of single cell recording and microdialysis in the dorsal raphe nucleus (DRN). 2. Administration of risperidone (25-400 micrograms kg-1, i.v.) dose-dependently decreased 5-HT cell firing in the DRN, similar to the antipsychotic drug clozapine (0.25-4.0 mg kg-1, i.v.), the putative antipsychotic drug amperozide (0.5-8.0 mg kg-1, i.v.) and the selective alpha 1-adrenoceptor antagonist prazosin (50-400 micrograms kg-1, i.v.). 3. The selective alpha 2-adrenoceptor antagonist idazoxan (10-80 micrograms kg-1, i.v.), in contrast, increased the firing rate of 5-HT neurones in the DRN, whereas the D2 and 5-HT2A receptor antagonists raclopride (25-200 micrograms kg-1, i.v.) and MDL 100,907 (50-400 micrograms kg-1, i.v.), respectively, were without effect. Thus, the alpha 1-adrenoceptor antagonistic action of the antipsychotic drugs might, at least partly, cause the decrease in DRN 5-HT cell firing. 4. Pretreatment with the selective 5-HT1A receptor antagonist WAY 100,635 (5.0 micrograms kg-1, i.v.), a drug previously shown to antagonize effectively the inhibition of 5-HT cells induced by risperidone, failed to prevent the prazosin-induced decrease in 5-HT cell firing. This finding argues against the notion that alpha 1-adrenoceptor antagonism is the sole mechanism underlying the inhibitory effect of risperidone on the DRN cells. 5. The inhibitory effect of risperidone on 5-HT cell firing in the DRN was significantly attenuated in rats pretreated with the 5-HT depletor PCPA (p-chlorophenylalanine; 300 mg kg-1, i.p., day-1 for 3 consecutive days) in comparison with drug naive animals. 6. Administration of risperidone (2.0 mg kg-1, s.c.) significantly enhanced 5-HT output in the DRN. 7. Consequently, the reduction in 5-HT cell firing by risperidone appears to be related to increased availability of 5-HT in the somatodendritic region of the neurones leading to an enhanced 5-HT1A autoreceptor activation and, in turn, to inhibition of firing, and is probably only to a minor extent caused by its alpha 1-adrenoceptor antagonistic action.

Evidence for a 5-HT3 receptor involvement in the facilitation of peristalsis on mucosal application of 5-HT in the guinea pig isolated ileum

1. The 5-HT receptor involved in the effect of mucosal application of 5-HT to facilitate peristalsis was investigated in the isolated guinea pig ileum. 2. An application of 5-HT (3-100 microM) to the mucosal surface (by inclusion of 5-HT in the Krebs-Henseleit solution passing through the lumen of the ileum) caused a concentration related facilitation of peristalsis characterized by a reduction in the peristaltic threshold. 3. Peristalsis was not modified by methiothepine (0.1 microM), ritanserin (0.1 microM), ondansetron (5 microM), granisetron (1 microM) or SB 204070 (0.1 microM) administered alone to the mucosal surface. 4. The concentration-response curve to mucosally applied 5-HT was not altered by the mucosally applied 5-HT1/2 receptor antagonist methiothepine (0.1 microM), the 5-HT2 receptor antagonist ritanserin (0.1 microM) or the 5-HT4 receptor antagonist SB 204070 (0.1 microM). However, the mucosally applied 5-HT3 receptor antagonists ondansetron (5 microM) and granisetron (1 microM) shifted the response curves to mucosally applied 5-HT to the right in a parallel and surmountable manner. The pD2 values in the absence and presence of ondansetron were 5.42 +/- 0.07 and 4.12 +/- 0.10, respectively, (n = 6) and that of granisetron were 5.45 +/- 0.12 and 4.50 +/- 0.10 respectively, (n = 5). 5. Serosally applied ondansetron (5 microM) or granisetron (1 microM) had no effect on the concentration-response curve to mucosally applied 5-HT. However, the serosally applied ondansetron and granisetron antagonised the facilitatory effect of serosally applied 5-HT (10 microM) when administered in the presence of serosally applied SB 204070 (0.1 microM). 6. It is concluded that the facilitatory effect of mucosally applied 5-HT to reduce the peristaltic threshold in the guinea pig ileum is mediated via a 5-HT3 receptor located on the mucosal and not the serosal side of the ileum.

Morphological heterogeneity with normal expression but altered function of G proteins in porcine cultured regenerated coronary endothelial cells

1. Experiments were designed to investigate whether the pertussis toxin-dependent endothelial dysfunction following balloon injury is due to a reduced expression or an insufficient function of G-proteins. 2. Endothelium-dependent responses of porcine coronary arteries were examined in vitro by use of conventional organ chambers. Morphological analysis was performed by isolating and culturing the endothelial cells from these arteries. The expression of Gi-proteins in regenerated endothelial cells was measured by Western blots and immunolabelling. The function of G-proteins was assessed by measuring the GTPase activity of cultured endothelial cells. 3. Eight days following denudation, endothelial regrowth was confirmed by histological examination and by demonstrating the presence of endothelium-dependent relaxations to bradykinin and 5-hydroxytryptamine (5-HT). In primary culture, the regenerated endothelial cells displayed a 'cobblestone' pattern as seen with native endothelial cells. 4. Twenty eight days after denudation, the endothelium-dependent relaxations induced by 5-HT were impaired, but those to bradykinin were maintained. However, the latter were reduced when endothelium-dependent hyperpolarization was prevented. 5. Twenty eight days after denudation, multinucleated giant cells were present in the regenerated but not in the native cultured endothelial cell populations. These regenerated endothelial cells incorporated less tritiated thymidine than native endothelial cells. 6. The intensities of the bands on the immunoblot of the regenerated endothelial cells, when several antibodies against Gi alpha 1/alpha 2/alpha 3 were used, were the same as those obtained in native endothelial cells. The immunolabelling with the same antibodies was similar between the giant cells and the regenerated endothelial cells of normal size. The hydrolysis of GTP was lower in regenerated than in native endothelial cell membranes. 7. In conclusion, endothelium-dependent relaxations mediated by Gi-proteins are impaired in balloon denuded coronary arteries. This dysfunction following regeneration cannot be explained by a reduced expression of Gi proteins but rather reflects an abnormal function of the G-proteins in the regenerated endothelium.

Purification of 5-hydroxytryptamine3 receptors from porcine brain

1. We demonstrate, for the first time, the purification of the 5-hydroxytryptamine3 (5-HT3) receptor from a native tissue source, pig cerebral cortex. 2. From a range of detergents, the non-ionic detergent Triton X-100 was demonstrated to exhibit the least inhibition of [3H]-(S)-zacopride binding to membrane bound 5-HT3 receptors from pig cerebral cortex at concentrations above its critical micellular concentration (CMC). This detergent was therefore selected to solubilize 5-HT3 binding sites from homogenates of pig cerebral cortex. Maximum yield (43.8 +/- 3.7%, mean +/- s.e.mean, n = 13) was obtained with Triton X-100 at 0.4% (22.1 x CMC). Radioligand binding studies with [3H]-(S)-zacopride indicated that the solubilized 5-HT3 receptor displayed near identical pharmacology to the membrane bound receptor (the correlation coefficient (r) between the pKi values of structurally unrelated compounds competing for [3H]-(S)-zacopride binding in the membrane bound and solubilized 5-HT3 receptor preparations was 0.99, Bmax = 20.7 +/- 4.2 fmol mg(-1) protein, Kd = 1.57 +/- 0.53 nM, mean +/- s.e.mean, n = 6). 3. Solubilized (0.4% Triton X-100) 5-HT3 receptors were affinity purified using Affi-Gel 15 coupled to the high affinity 5-HT3 receptor ligand GR119566X. Radioligand binding studies indicated that the pharmacological profile of the affinity purified 5-HT3 receptor, assessed using ligands with a range of affinities spanning 3 orders of magnitude, was similar to that in both crude homogenates (r = 0.85) and solubilized 5-HT3 receptor sites (r = 0.85) from pig brain. The specific activity for the purified 5-HT3 receptor overlapped the theoretical specific activity of the receptor (Bmax = 3.27 +/- 1.41 and 5.35 +/- 2.33 nmol mg(-1) protein, assessed by saturation and competition studies respectively, mean +/- s.e.mean, n = 3-4), which indicated a 60000-100000 fold purification of the membrane bound receptor. 4. Under non-reducing conditions, samples of the affinity purified protein failed to enter a 10% separating gel in SDS-PAGE analysis, indicating a molecular mass for the receptor complex of > 200 kDa. Further investigation of the non-reduced purified protein with a 7.5% separating gel gave a mass for the complex of approximately 279 kDa. Under reducing conditions, SDS-PAGE analysis of the affinity purified 5-HT3 receptor resulted in 3-6 silver stained bands at apparent molecular masses of 37, 44-50, 52, 57-61, 63 and 65-71 kDa (n = 12). Unlike protein bands at 45, 50, 60 and 66 kDa, the bands corresponding to proteins of 52, 57, 63 and 71 kDa consistently gave no reaction with an antiserum specific for the cloned A subunit of the 5-HT3 receptor in both a modified dot blot procedure and a Western blot procedure (n = 2-5). 5. We conclude that we have purified the 5-HT3 receptor from pig brain to homogeneity and suggest this may contain non-5-HT3-A receptor subunit(s).

Enhancement and depression of spinal reflexes by 8-hydroxy-2-(di-n-propylamino)tetralin in the decerebrated and spinalized rabbit: involvement of 5-HT1A- and non-5-HT1A-receptors

1. In decerebrated, spinalized and paralyzed rabbits, intravenous administration of the 5-HT1A-receptor agonists (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 3-300 nmol kg(-1), cumulative) and flesinoxan (22-2200 nmol kg(-1), cumulative) significantly increased the short latency reflex evoked in gastrocnemius medialis motoneurones by electrical stimulation of all myelinated afferents (Abeta and Adelta fibres) of the sural nerve. Reflexes increased to median values of 198% (inter-quartile range (IQR) 148-473%) and 296% (IQR 254-522%) of pre-drug values with the highest doses of 8-OH-DPAT and flesinoxan, respectively. The enhancement of reflexes induced by 5-HT1A-receptor agonists was not reversed by the selective 5-HT1A-receptor antagonist (S)WAY-100135 (2.05 micromol kg[-1]). 2. The effects of 8-OH-DPAT were tested after pretreatment with (S)WAY-100135 (2.05 micromol kg[-1]), its more potent analogue WAY-100635 (185 nmol kg[-1]), and the 5-HT2/5-HT1D-/5-HT7-receptor ligand ritanserin (1.67 micromol kg[-1]). 8-OH-DPAT (300 nmol kg(-1) single dose) significantly increased gastrocnemius reflex responses in the presence of (S)WAY-100135 and WAY-100635, to median values of 260% (IQR 171-295%) and 165% (IQR 136-170%) of pre-drug levels, respectively. These values were not significantly different from each other, or from the effects of 8-OH-DPAT given alone. When 8-OH-DPAT was given after ritanserin, reflexes were a median of 102% (IQR 76-148%) of pre-drug values: i.e. there was no significant increase in responses. Neither WAY-100635 nor ritanserin had any effects on reflexes per se. 3. WAY-100635 (185 nmol kg[-1]) and ritanserin (1.67 micromol kg[-1]) were given after 8-OH-DPAT (300 nmol kg[-1]). The agonist increased reflexes to a median value of 184% (IQR 135-289%), after which WAY-100635 significantly reduced responses to 165% (IQR 130-254%) and ritanserin further decreased reflexes to a median of 107% (IQR 100-154%) of pre-drug levels, i.e. not significantly different from controls. 4. Previous studies have shown that reflexes evoked by large myelinated axons tend to be suppressed, rather than enhanced, by 5-HT1A-receptor agonists. When tested against reflexes evoked by stimulation of the sural nerve at strengths between 1.5 and 2.5 times threshold, 8-OH-DPAT (3-300 nmol kg(-1), cumulative) and flesinoxan (22-2200 nmol kg(-1), cumulative) significantly reduced gastrocnemius responses to median values of 36% (IQR 15-75%) and 17% (IQR 12-38%) of pre-drug levels, respectively. This inhibition was fully reversed by (S)WAY-100135 (2.05 micromol kg[-1]). 5. These data show that drugs that are agonists at 5-HT1A-receptors increase polysynaptic spinal reflexes evoked by moderate to high stimulus intensities and depress responses to very low intensity stimuli. The inhibitory effects of these drugs were mediated through 5-HT1A-receptors as they were abolished by a selective antagonist for these sites. However, the facilitatory effects of 8-OH-DPAT could be completely blocked only by a combination of ritanserin, which has no significant affinity for 5-HT1A-receptors, with WAY-100635. It appears that the enhancement of reflexes by 8-OH-DPAT arises from a combined action at 5-HT1A-receptors and other, ritanserin-sensitive, sites which could be 5-HT1D- or 5-HT7-receptors.

Investigation of the mechanisms underlying the hypophagic effects of the 5-HT and noradrenaline reuptake inhibitor, sibutramine, in the rat

1. Sibutramine is a novel 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor (serotonin-noradrenaline reuptake inhibitor, SNRI) which is currently being developed as a treatment for obesity. Sibutramine has been shown to decrease food intake in the rat. In this study we have used a variety of monoamine receptor antagonists to examine the pharmacological mechanisms underlying sibutramine-induced hypophagia. 2. Individually-housed male Sprague-Dawley rats were maintained on reversed phase lighting with free access to food and water. Drugs were administered at 09 h 00 min and food intake was monitored over the following 8 h dark period. 3. Sibutramine (10 mg kg-1, p.o.) produced a significant decrease in food intake during the 8 h following drug administration. This hypophagic response was fully antagonized by the alpha 1-adrenoceptor antagonist, prazosin (0.3 and 1 mg kg-1, i.p.), and partially antagonized by the beta 1-adrenoceptor antagonist, metoprolol (3 and 10 mg kg-1, i.p.) and the 5-HT receptor antagonists, metergoline (non-selective; 0.3 mg kg-1, i.p.); ritanserin (5-HT2A/2C; 0.1 and 0.5 mg kg-1, i.p.) and SB200646 (5-HT2B/2C; 20 and 40 mg kg-1, p.o.). 4. By contrast, the alpha 2-adrenoceptor antagonist, RX821002 (0.3 and 1 mg kg-1, i.p.) and the beta 2-adrenoceptor antagonist, ICI 118,551 (3 and 10 mg kg-1, i.p.) did not reduce the decrease in food intake induced by sibutramine. 5. These results demonstrate that beta 1-adrenoceptors, 5-HT2A/2C-receptors and particularly alpha 1-adrenoceptors, are involved in the effects of sibutramine on food intake and are consistent with the hypothesis that sibutramine-induced hypophagia is related to its ability to inhibit the reuptake of both noradrenaline and 5-HT, with the subsequent activation of a variety of noradrenaline and 5-HT receptor systems.

Comparison of the effects of sibutramine and other monoamine reuptake inhibitors on food intake in the rat

1. The effects of the potent 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor (serotonin-noradrenaline reuptake inhibitor, SNRI), sibutramine, on the cumulative food intake of freely-feeding male Sprague-Dawley rats during an 8 h dark period were investigated and compared to those of the selective 5-HT reuptake inhibitor (selective serotonin reuptake inhibitor, SSRI), fluoxetine; the selective noradrenaline reuptake inhibitor, nisoxetine; the 5-HT and noradrenaline reuptake inhibitors, venlafaxine and duloxetine; and the 5-HT releaser and 5-HT reuptake inhibitor, (+)-fenfluramine. 2. Sibutramine (3 and 10 mg kg-1, p.o.) and (+)-fenfluramine (1 and 3 mg kg-1, p.o.) produced a significant, dose-dependent decrease in food intake over the 8 h dark period. These responses became apparent within the first 2 h following drug administration. 3. Fluoxetine (3, 10 and 30 mg kg-1, p.o.), and nisoxetine (3, 10 and 30 mg kg-1, p.o.) had no significant effect on food intake during the 8 h dark period. However, a combination of fluoxetine and nisoxetine (30 mg kg-1, p.o., of each) significantly decreased food intake 2 and 8 h after drug administration. 4. Venlafaxine (100 and 300 mg kg-1, p.o.) and duloxetine (30 mg kg-1, p.o.) also significantly decreased food intake in the 2 and 8 h following drug administration. 5. The results of this study demonstrate that inhibition of 5-HT and noradrenaline reuptake by sibutramine, venlafaxine, duloxetine, or by a combination of fluoxetine and nisoxetine, markedly reduces food intake in freely-feeding rats and suggest that this may be a novel approach for the treatment of obesity.

Combined administration of 5-HT2 and thromboxane A2 antagonists: effects on platelet aggregation and isolated cardiac muscle

1. To investigate possible mechanisms underlying the ability of combined administration of a 5-hydroxytryptamine2 (5-HT2) antagonist and a thromboxane A2 antagonist to reduce reperfusion-induced arrhythmias, the effects of these drugs alone and in combination on platelet aggregation and on cardiac muscle were determined. 2. Platelet aggregation was measured in whole blood obtained from anaesthetized rats. Concentrations of 5-HT (10 microM) and the thromboxane A2 mimetic U46619 (1 microM) which did not cause aggregation themselves, enhanced the responses to ADP (0.1 microM) and to collagen (1 microgram ml-1). For example, the response of 1.0 +/- 0.5 omega to ADP alone was increased significantly to 6.4 +/- 1.0 omega by 5-HT, 15.5 +/- 2.8 omega by U46619, and 17.3 +/- 1.3 omega when U46619, 5-HT and ADP were added together. 3. In further experiments blood was obtained from rats which had received either the 5-HT2 antagonist, ICI 170,809 (1 mg kg-1), or the thromboxane A2 antagonist. ICI 192,605 (1 mg kg-1 min-1), or both in combination. When ADP was used as the primary aggregating agent, the ability of U46619 alone, or together with 5-HT, to enhance responses was reduced significantly by ICI 192,605 alone and in combination with ICI 170,809. Similar results were obtained with lower doses of ICI 170,809 (0.3 mg kg-1) and ICI 192,605 (0.3 mg kg-1 min-1). 4. When collagen was used as the primary aggregating agent ICI 170,809 (1 mg kg-1) reduced the response to 5-HT (5.0 +/- 0.8 omega versus 10.9 +/- 1.2 omega in controls), and ICI 192,605 (1 mg kg-1 min-1) reduced the response to U46619 (6.8 +/- 2.5 omega versus 11.2 +/- 2.2 omega in control). The greatest reduction of platelet aggregation was seen in blood from rats which had received both antagonists, with the response to U46619 plus 5-HT plus collagen being 2.7 +/- 0.6 omega compared to 14.2 +/- 1.7 omega in controls. In contrast, there was no significant attenuation of platelet aggregation in blood from rats which had received the lower doses of each antagonist alone. Only the combination of ICI 170,809 (0.3 mg kg-1) and ICI 192,605 (0.3 mg kg-1 min-1) reduced the response to U46619 plus 5-HT plus collagen (7.6 +/- 1.4 omega versus 15.0 +/- 0.5 omega in controls). 5. In rat isolated ventricular muscle preparations, ICI 170,809 increased the effective refractory period; e.g. from 39 +/- 4 to 86 +/- 18 ms, 10 min after adding 30 microM to left papillary muscles. ICI 192,605 did not increase the effective refractory period itself and did not alter the ability of ICI 170,809 to prolong the effective refractory period. In the presence of 100 microM ICI 192,605, ICI 170,809 (30 microM) increased the effective refractory period from 38 +/- 7 to 100 +/- 30 ms. 6. These results indicate that the previously observed antiarrhythmic activity of combined administration of the higher doses of ICI 170,809 and ICI 192,605 is unlikely to be due to direct effects on cardiac muscle but could be a consequence of reduced platelet aggregation.

Nitric oxide-dependent and -independent vascular hyporeactivity in mesenteric arteries of portal hypertensive rats

1. Increased production of nitric oxide (NO) has been suggested to underlie both the vascular hyporeactivity to vasoconstrictors and the splanchnic vasodilatation seen in portal hypertension. This study assessed the role of NO in the vasoconstrictor hyporeactivity of portal vein-ligated (PVL) rats in isolated and in situ perfused mesenteric arterial beds. 2. Isolated perfused mesenteric arteries of PVL rats were significantly less reactive to noradrenaline (NA), methoxamine (METH), arginine vasopressin (AVP) and endothelin-1 (ET-1) than those from sham-operated (Sham) rats. 3. Blockade of NO synthesis with NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) in isolated perfused mesenteric arteries from PVL rats restored the reactivity to bolus injections of AVP and ET-1, but had little effect on the hyporeactivity to NA or METH. Cyclo-oxygenase inhibition with indomethacin (5 microM) likewise did not restore reactivity to METH of isolated perfused mesenteric arteries of PVL rats. 4. The hyporeactivity to METH seen in isolated perfused mesenteric arteries from PVL rats was reduced by low concentrations of AVP (20 nM) or ET-1 (1 nM) which per se caused only a slight increase in perfusion pressure. When L-NAME (100 microM) was combined with AVP (20 nM) or ET-1 (1 nM), respectively, reactivity to METH of isolated perfused mesenteric arteries of PVL rats was restored to the level seen in Sham rats. These effects of AVP and ET-1 were not mimicked by precontracting the vessels with 5-hydroxytryptamine (5 microM). 5. The differential effects of L-NAME and AVP on the hyporesponsiveness to methoxamine and AVP were corroborated by experiments performed with the in situ perfused mesenteric vascular bed preparation. 6. These data indicate that both NO-dependent and NO-dependent mechanisms are involved in the vasoconstrictor hyporesponsiveness of mesenteric arteries from portal hypertensive rats. The hyporeactivity to AVP and ET-1 is mediated by NO whereas the reduced responsiveness to adrenoceptor agonists appears to be predominantly NO-independent AVP and ET-1, in addition, seem to inhibit the NO-independent mechanism of vascular hyporeactivity, since the hyporesponsiveness to METH was reduced in the presence of AVP or ET-1 and abolished by the combination of these peptides with L-NAME.

In vivo evidence for free radical involvement in the degeneration of rat brain 5-HT following administration of MDMA ('ecstasy') and p-chloroamphetamine but not the degeneration following fenfluramine

1. Administration of 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') to several species results in a long lasting neurotoxic degeneration of 5-hydroxytryptaminergic neurones in several regions of the brain. We have now investigated whether this degeneration is likely to be the result of free radical-induced damage. 2. Free radical formation can be assessed by measuring the formation of 2,3- and 2,5-dihydroxybenzoic acid (2,3-DHBA and 2,5-DHBA) from salicylic acid. An existing method involving implantation of a probe into the hippocampus and in vivo microdialysis was modified and validated. 3. Administration of MDMA (15 mg kg-1, i.p.) to Dark Agouti (DA) rats increased the formation of 2,3-DHBA (but not 2,5-DHBA) for at least 6 h. Seven days after this dose of MDMA, the concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was reduced by over 50% in hippocampus, cortex and striatum, reflecting neurotoxic damage. There was no change in the concentration of dopamine or 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. 4. p-Chloroamphetamine (PCA), another compound which produces a neurotoxic loss of cerebral 5-HT content, when given at a dose of 5 mg kg-1 also significantly increased the formation of 2.3-DHBA (but not 2,5-DHBA) in the dialysate for over 4.5 h. post-injection starting 2 h after treatment. 5. In contrast, fenfluramine administration (15 mg kg-1, i.p.) failed to increase the 2,3-DHBA or 2,5-DHBA concentration in the dialysate. A single fenfluramine injection nevertheless also markedly decreased the concentration of 5-HT and 5-HIAA in the hippocampus, cortex and striatum seven days later. 6. When rats pretreated with fenfluramine (15 mg kg-1, i.p.) seven days earlier were given MDMA (15 mg kg-1, i.p.) no increase in 2,3-DHBA was seen in the dialysate from the hippocampal probe. This indicates that the increase in free radical formation following MDMA is occurring in 5-HT neurones which have been damaged by the prior fenfluramine injection. 7. Administration of the free radical scavenging agent alpha-phenyl-N-tert-butyl nitrone (PBN; 120 mg kg-1, i.p.) 10 min before and 120 min after an MDMA (15 mg kg-1, i.p.) injection prevented the acute rise in the 2,3-DHBA concentration in the dialysate and attenuated by 30% the long term damage to hippocampal 5-HT neurones (as indicated by a smaller MDMA-induced decrease in both the concentration of 5-HT and 5-HIAA and also the binding of [3H]-paroxetine). 8. These data indicate that a major mechanism by which MDMA and PCA induce damage to 5-hydroxytryptaminergic neurones in rat brain is by increasing the formation of free radicals. These probably result from the degradation of catechol and quinone metabolites of these substituted amphetamines. In contrast, fenfluramine induces damage by another mechanism not involving free radicals; a proposal supported by some of our earlier indirect studies. 9. We suggest that these different modes of action render untenable the recent suggestion that MDMA will not be neurotoxic in humans because fenfluramine appears safe at clinical doses.

Role of nitric oxide in the contractile response to 5-hydroxytryptamine of the basilar artery from Wistar Kyoto and stroke-prone rats

1. Isolated basilar arteries from spontaneously hypertensive stroke-prone rats (SHRSP) are more sensitive to the contractile effect of 5-hydroxytryptamine (5-HT) than those from normotensive Wistar Kyoto rats (WKY). This has been attributed to a different proportion of 5-HT receptor subtypes mediating these responses. In the present study we have examined if differences in nitric oxide release could also contribute to this difference in sensitivity to 5-HT. 2. At rest, the normalized internal diameter was significantly smaller in SHRSP (297.4 +/- 3.5 microm, n = 88) than in WKY (375.1 +/- 4.0 microm, n = 62, P<0.01) arteries. The contractile response to 100 mM KCl was higher in WKY (3.57 +/- 0.15 mN mm(-1), n = 22) than in SHRSP arteries (2.32 +/- 0.20 mN mm(-1), n = 28, P<0.01). 3. When added on the plateau of contraction to 5-HT (1 microM), acetylcholine (ACh, 3 microM) evoked significant relaxation in all preparations from WKY (n = 20), but only in 15 out of 26 preparations from SHRSP. The mean relaxations were 55.4 +/- 5.2% in WKY and 20.6 +/- 4.6% in SHRSP (as % of the contractile tone evoked by 5-HT: P<0.01). 4. The NO synthase inhibitor N(omega)-nitro-L-arginine (L-NOARG, 0.1 mM) produced a similar increase in tone in both WKY and SHRSP. This tone was equal (in % of the contractile response to 100 mM KCl) to 70.8 +/- 4.4% in WKY (n = 20) and 67.6 +/- 5.9% in SHRSP (n=26) and was reversed by L-arginine (1 mM) and by 1,4-dihydropyridine calcium channel blockers (10 nM nisoldipine, 10 nM lacidipine, 100 nM nifedipine). The L-NOARG-induced tone was absent when the arteries were bathed in phosphate-free Krebs (pH 7.4). 5. EC50 values of 5-HT were about four fold smaller in SHRSP than in WKY arteries (P<0.01). The maximal response to 5-HT (Emax) was higher than 100 mM KCl-contraction in SHRSP but not in WKY arteries. Removal of endothelium produced a shift to the left of the 5-HT curve in WKY, but not in SHRSP arteries. 6. When evoked in phosphate-free Krebs, the contractile responses to 5-HT showed tachyphylaxis, but the responses were reproducible by adding the agonist at 30 min intervals. In such conditions, EC50 values of 5-HT were about two fold smaller in SHRSP than in WKY arteries (P<0.01). In phosphate-free Krebs, the blockade of NO synthase did not change the contractile response to 100 mM KCl; it reduced EC50 and increased Emax of 5-HT in WKY, but not in SHRSP. 7. These results confirm that the sensitivity to 5-HT is higher in basilar artery isolated from SHRSP than in those from WKY. They show that endothelium-dependent vasorelaxation to ACh is impaired in SHRSP. The finding that removal of endothelium or blockade of NO synthase augmented the contractile response to 5-HT in WKY, but not in SHRSP basilar arteries indicates that the difference in responsiveness to 5-HT observed between WKY and SHRSP basilar arteries might be, at least in part, related to dissimilarities in NO release. Furthermore, the L-NOARG-induced contraction sensitive to calcium channel blockers indicates that, in basilar arteries, NO production might lower L-type calcium channel opening and thereby control the tone of the vessels.

Inhibition of 5-hydroxytryptamine-induced phosphoinositide hydrolysis and Ca2+ mobilization in canine cultured tracheal smooth muscle cells by phorbol ester

1. Regulation of the increase in inositol-1,4,5-trisphosphate (IP3) production and intracellular Ca2+ concentration ([Ca2+]i by protein kinase C (PKC) was investigated in canine cultured tracheal smooth muscle cells (TSMCs). Stimulation of TSMCs by 5-hydroxytryptamine (5-HT) caused an initial transient [Ca2+]i peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner. 2. Pretreatment of TSMCs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min blocked the 5-HT-induced IP3 formation and Ca2+ mobilization. This inhibition was reduced after the cells had been incubated with PMA for 8 h, and within 48 h the 5-HT-induced Ca2+ mobilization reached the same extent as control cells. 3. The concentration of PMA that gave half-maximal inhibition of 5-HT-induced increase in [Ca2+]i was 4 nM. Pretreatment of TSMCs with staurosporine (1 microM) of GF109203X (0.1 microM), PKC inhibitors, inhibited the ability of PMA to attenuate 5-HT-induced responses, suggesting that the inhibitory effect of PMA was mediated through the activation of PKC. 4. In parallel with the effect of PMA on 5-HT-induced IP3 formation and Ca2+ mobilization, the translocation and down-regulation of PKC isozymes were determined by Western blot analysis in TSMCs. Analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TSMCs expressed PKC-alpha, beta I, beta II, delta, epsilon, theta and zeta. With PMA treatment of the cells for various times, translocation of PKC-alpha, beta I, beta II, delta, epsilon, and theta from the cytosol to the membrane was seen after 5 min, 30 min, 2 h, and 4 h treatment. However, 24 h treatment caused a partial down-regulation of these PKC isozymes PKC-zeta was not significantly translocated and down-regulated at any of the times tested. 5. In conclusion, these results suggest that activation of PKC may inhibit the receptor-mediated phosphoinositide hydrolysis and consequently attenuate the [Ca2+]i increase or inhibit both responses independently. The translocation of PKC-alpha, beta I, beta II, delta, epsilon, and theta induced by PMA caused an attenuation of 5-HT-stimulated IP3 accumulation and Ca2+ mobilization in TSMCs.

A study of the neurotoxic effect of MDMA ('ecstasy') on 5-HT neurones in the brains of mothers and neonates following administration of the drug during pregnancy

1. It is well established that 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') is neurotoxic and produces long term degeneration of cerebral 5-hydroxytryptamine (5-HT) nerve terminals in many species. Since MDMA is used extensively as a recreational drug by young people, it is being ingested by many women of child bearing age. We have therefore examined the effect of administering high doses of MDMA to rats during pregnancy on the cerebral content of both the dams and the neonates. 2. MDMA (20 mg kg-1, s.c.) was injected twice daily on days 14-17 of the gestation period. The initial dose produced a marked hyperthermic response in the dam which was progressively attenuated in both peak height and area under the curve following further doses of the drug. The body weight of the dams decreased during the period of treatment. 3. There was a modest decrease in litter size (-20%) of the MDMA-treated dams. 4. The concentration of 5-HT and its metabolite 5-HIAA was decreased by over 65% in the hippocampus and striatum and 40% in the cortex of the dams 1 week after parturition. In contrast, the content of 5-HT and 5-HIAA in the dorsal telencephalon of the pups of the MDMA-treated dams was the same as that seen in tissue from pups born to control animals. 5. Administration of MDMA (40 mg kg-1, s.c.) to adult rats increased thiobarbituric acid reacting substances (TBARS) in cortical tissue 3 h and 6 h later, indicating increased lipid peroxidation. No increase in TBARS was seen in the cortical tissue of 7-10 day neonates injected with this dose of MDMA 3 h or 6 h earlier. 6. The data suggest that exposure to MDMA in utero during the maturation phase does not produce damage to 5-HT nerve terminals in the foetal rat brain, in contrast to the damage seen in the brains of the mothers. This may be due to MDMA being metabolized to free radical producing entities in the adult brain but not in the immature brain or, alternatively, to more effective or more active free radical scavenging mechanisms being present in the immature brain.

Effects of delayed treatment with nebracetam on neurotransmitters in brain regions after microsphere embolism in rats

1. The effects of delayed treatment with nebracetam, a novel nootropic drug, on neurotransmitters of brain regions were examined in rats with microsphere embolism-induced cerebral ischaemia. 2. Cerebral ischaemia was induced by administration of 900 microspheres (48 microns) into the internal carotid artery. The rats with stroke-like symptoms were treated p.o. with 30 mg kg-1 nebracetam twice daily. The levels of acetylcholine, dopamine, noradrenaline, 5-hydroxytryptamine (5-HT) and their metabolites in the cerebral cortex, striatum and hippocampus of animals with microsphere embolism were determined by high performance liquid chromatography (h.p.l.c.) on the 3rd and 7th days after the operation. 3. Although the microsphere embolism induced significant changes in most of the neurotransmitters and some of their metabolites in the brain regions, the delayed treatment with nebracetam partially restored only the hippocampal 5-HT and the striatal dopamine metabolite contents on the 3rd day. 4. The hippocampal in vivo 5-HT synthesis, but not the striatal dopamine synthesis, was attenuated in rats with microsphere embolism on the 3rd day, but was restored by treatment with nebracetam. In vivo striatal dopamine turnover rate of the rats with microsphere embolism was inhibited on the 3rd day irrespective of treatment with nebracetam. 5. The present study provides evidence for a possible action of nebracetam on 5-HT metabolism in the ischaemic brain.

Role of 5-ht7 receptors in the long-lasting hypotensive response induced by 5-hydroxytryptamine in the rat

1. The receptor mediating the long-lasting hypotensive effect of intravenous (i.v.) 5-hydroxytryptamine (5-HT) in the rat was originally classified as 5-HT1-like. Since some pharmacological properties of this receptor are closely similar to those for the cloned 5-ht7 receptor, the present study investigated the effects of several 5-HT receptor agonists and antagonists showing high affinity for the cloned 5-ht7 receptor in pithed rats with artificially raised blood pressure. 2. I.v. bolus administration of 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine, lisuride and sumatriptan to bilaterally vagotomized pithed rats pretreated with ketanserin (0.18 mumol kg-1, i.v.), the diastolic blood pressure of which had been raised by a continuous i.v. infusion of methoxamine (60-80 nmol kg-1 min-1), produced dose-dependent hypotensive responses; only 5-HT and 5-CT displayed similar maximum effects. In addition to mimicking the hypotensive action of 5-HT with a lower maximum effect, lisuride strongly antagonized the 5-CT-induced hypotensive responses thus suggesting a partial agonist effect. The rank order of hypotensive agonist potency was 5-CT > > 5-HT > or = 5-methoxytryptamine > or = lisuride > > sumatriptan. 3. In experiments with antagonists, i.v. treatment with metergoline (2.48 mumol kg-1), mesulergine (2.76 mumol kg-1), methysergide (2.13 mumol kg-1), lisuride (0.22 mumol kg-1), methiothepin (0.68 mumol kg-1), mianserin (10.6 mumol kg-1), or the atypical antipsychotic drugs, clozapine (11.0 mumol kg-1) or risperidone (78.0 nmol kg-1), produced significant rightward displacements of the dose-response curve for 5-CT in methoxamine-infused pithed animals pretreated with ketanserin (0.18 mumol kg-1, i.v.); lisuride, methiothepin and risperidone behaved as non-competitive antagonists as they elicited a significant reduction of the maximum effect to 5-CT. In contrast, blockade of 5-HT1, 5-HT3 and 5-HT4 receptors with i.v. propranolol (3.38 mumol kg-1), MDL-72222 (1.59 mumol kg-1) and GR125487 (1.91 mumol kg-1), respectively, did not alter 5-CT-induced hypotensive responses; ketanserin (0.18 mumol kg-1, i.v.) failed to modify the dose-response curve for 5-CT in saline-pretreated animals. Lastly, inhibition of the prostaglandin-forming cyclo-oxygenase and nitric oxide synthase with indomethacin (14 mumol kg-1, i.v.) and NG-nitro-L-arginine methyl ester (L-NAME, 120 mumol kg-1, i.v.), respectively, had no significant effects on 5-CT-induced hypotensive effects. 4. Taken together, the present pharmacological data suggest that the long-lasting vasodepressor action of 5-HT in the rat involves activation of receptors closely similar to the cloned 5-ht7 subtype. Since no evidence for an indirect mechanism could be obtained, these receptors may be primarily located in the vascular smooth muscle of the systemic resistance vessels. These findings represent further evidence favouring the functional role of the 5-ht7 receptor.

Mediation of 5-HT-induced external carotid vasodilatation in GR 127935-pretreated vagosympathectomized dogs by the putative 5-HT7 receptor

1. The vasodilator effects of 5-hydroxytryptamine (5-HT) in the external carotid bed of anaesthetized dogs with intact sympathetic tone are mediated by prejunctional sympatho-inhibitory 5-HT1B/1D receptors and postjunctional 5-HT receptors. The prejunctional vasodilator mechanism is abolished after vagosympathectomy which results in the reversal of the vasodilator effect to vasoconstriction. The blockade of this vasoconstrictor effect of 5-HT with the 5-HT1B/1D receptor antagonist, GR 127935, unmasks a dose-dependent vasodilator effect of 5-HT, but not of sumatriptan. Therefore, the present study set out to analyse the pharmacological profile of this postjunctional vasodilator 5-HT receptor in the external carotid bed of vagosympathectomized dogs pretreated with GR 127935 (20 micrograms kg-1, i.v.). 2. One-minute intracarotid (i.c.) infusions of 5-HT (0.3-30 micrograms min-1), 5-carboxamidotryptamine (5-CT; 0.01-0.3 microgram min-1), 5-methoxytryptamine (1-100 micrograms min-1) and lisuride (3-1000 micrograms min-1) resulted in dose-dependent increases in external carotid blood flow (without changes in blood pressure or heart rate) with a rank order of agonist potency of 5-CT > > 5-HT > or = 5-methoxytryptamine > lisuride, whereas cisapride (100-1000 micrograms min-1, i.c.) was practically inactive. Interestingly, lisuride (mean dose of 85 +/- 7 micrograms kg-1, i.c.), but not cisapride (mean dose of 67 +/- 7 micrograms kg-1, i.c.), specifically abolished the responses induced by 5-HT, 5-CT and 5-methoxytryptamine, suggesting that a common site of action may be involved. In contrast, 1 min i.c. infusions of 8-OH-DPAT (3-3000 micrograms min-1) produced dose-dependent decreases, not increases, in external carotid blood flow and failed to antagonize (mean dose of 200 +/- 33 micrograms kg-1, i.c.) the agonist-induced vasodilator responses. 3. The external carotid vasodilator responses to 5-HT, 5-CT and 5-methoxytryptamine were not modified by intravenous (i.v.) pretreatment with either saline, (+/-)-pindolol (4 mg kg-1) or ritanserin (100 micrograms kg-1) plus granisetron (300 micrograms kg-1), but were dose-dependently blocked by i.v. administration of methiothepin (10 and 30 micrograms kg-1, given after ritanserin plus granisetron), mesulergine (10 and 30 micrograms kg-1), metergoline (1 and 3 mg kg-1), methysergide (1 and 3 mg kg-1) or clozapine (0.3 and 1 mg kg-1). Nevertheless, the blockade of the above responses, not significant after treatment with the lower of the two doses of metergoline and mesulergine, was nonspecific after administration of the higher of the two doses of methysergide and clozapine. 4. Based upon the above rank order of agonist potencies and the antagonism produced by a series of drugs showing high affinity for the cloned 5-ht7 receptor, our results indicate that the 5-HT receptor mediating external carotid vasodilatation in GR 127935-pretreated vagosympathectomized dogs is operationally similar to the putative 5-HT7 receptor mediating relaxation of vascular and non-vascular smooth muscles (e.g. rabbit femoral vein, canine coronary artery, rat systemic vasculature and guinea-pig ileum) as well as tachycardia in the cat.

The haemodynamic effects of subcutaneous sumatriptan, a 5HT1-receptor agonist

AIMS

Previous in vivo studies with sumatriptan, a 5HT1-receptor agonist, have demonstrated vasopressor responses in the pulmonary and systemic arterial circulation. Pulmonary artery wedge pressure (PAWP) also increases after sumatriptan injection, raising the possibility of an additional venoconstrictive action or a negative inotropic effect. The mechanism for the rise in PAWP was investigated in the study.

METHODS

Ten patients undergoing diagnostic coronary arteriography underwent haemodynamic monitoring.

RESULTS

There was a significant rise (P < 0.05) in systemic and pulmonary arterial pressure and total systemic and pulmonary vascular resistance. There was a similar rise (P < 0.05) in PAWP and left ventricular end diastolic pressure (LVEDP). There was no change in cardiac output nor in peak rate of left ventricular pressure rise (dP/dt).

CONCLUSIONS

The sumatriptan induced rise in PAWP and LVEDP appears consequent upon increased afterload although a negative inotropic effect cannot be excluded.

Evidence for the involvement of 5-HT4 receptors in the 5-hydroxytryptamine-induced pattern of migrating myoelectric complex in sheep

1. The effects induced by 5-hydroxytryptamine (5-HT) on gastrointestinal myoelectric activity in conscious sheep were recorded through electrodes chronically implanted and analysed by computer. The 5-HT receptors and the cholinergic neuronal pathways involved in these actions were investigated. 2. The intravenous (i.v.) administration of 5-HT (2, 4 and 8 micrograms kg-1 min-1, 5 min) induced an antral inhibition concomitant with a duodenal activity front that migrated to the jejunum, followed by a period of intestinal inactivity. This myoelectric pattern closely resembled that observed in the phases III and I of the migrating myoelectric complex (MMC) in sheep. The 0.5 microgram kg-1 min-1 dose evoked the same pattern in only two out of the six animals used. Likewise, the 1 microgram kg-1 min-1 dose similarly affected four of the six animals. In addition, a transient stimulation was observed in the antrum and jejunum when the two highest doses were used. 3. The 5-HT1 antagonist, methiothepin (0.1 mg kg-1), the 5-HT2 antagonists, ritanserin (0.1 mg kg-1) and ketanserin (0.3 mg kg-1), the 5-HT3 antagonists, granisetron (0.2 mg kg-1) and ondansetron (0.5 mg kg-1), as well as the 5-HT4 antagonist, GR113808 (0.2 mg kg-1), did not modify the spontaneous gastrointestinal myoelectric activity. However, the cholinoceptor antagonists, atropine (0.2 mg kg-1) and hexamethonium (2 mg kg-1), inhibited gastrointestinal activity. 4. When these antagonists were injected i.v. 10 min before 5-HT (2 or 4 micrograms kg-1 min-1, 5 min), only GR113808, atropine and hexamethonium were able to modify the 5-HT-induced actions, all of them being completely blocked by the three antagonists. 5. Our data show that 5-HT initiates a MMC-like pattern in the gastrointestinal area in sheep through 5-HT4 receptors. Furthermore, these actions are mediated by cholinergic neural pathways involving muscarinic and nicotinic receptors. However, our results do not indicate a role for either 5-HT1, 5-HT2 or 5-HT3 receptors in the 5-HT-induced effects.

Mediation by protein kinases C and A of Go-linked slow responses of enteric neurons to 5-HT

5-HT activates the peristaltic reflex and is the neurotransmitter of a subset of myenteric interneurons. Hyperpolarizing afterpotential (AH)/type 2 neurons respond to 5-HT with a long-lived depolarization that is caused by the inhibition of a Ca(2+)-activated K+ conductance (gKCa). This effect is mediated by a G-protein-coupled receptor, 5-HT1P. 5-HT1P agonists specifically activate G alpha o, the immunoreactivity of which was found to be highly abundant and membrane-associated in almost all enteric neurons. Responses of hyperpolarizing AH/type 2 neurons to 5-HT were inhibited by intracellular injection of GDP beta S or anti-G alpha o Fab fragments but were potentiated and prolonged by intracellular GTP gamma S. Responses to 5-HT were antagonized by pertussis toxin, downregulation of protein kinase C (PKC) and inhibitors of phosphatidylcholine phospholipase C (PC-PLC), PKC (including pseudosubstrate peptides, chelerythrine, and the alpha/beta isoform-specific inhibitor Go 6976), protein kinase A (PKA), and adenylate cyclase. Responses to 5-HT were mimicked by activators of PKC, and 5-HT induced a concentration-dependent increase in the membrane-associated PKC activity in isolated myenteric ganglia. Immunocytochemical studies suggested that the most abundant isoforms of PKC in enteric neurons are alpha and delta. These data suggest that signal transduction of the 5-HT1P-mediated slow response to 5-HT involves activation of PC-PLC by G alpha o to liberate diacylglycerol, which stimulates PKC (most likely alpha). PKC probably activates adenylate cyclase, which through cAMP, activates PKA. Activation of both PKA and PKC lead to closure of gKCa.

Inhibitory action of niflumic acid on noradrenaline- and 5-hydroxytryptamine-induced pressor responses in the isolated mesenteric vascular bed of the rat

1. The effects of niflumic acid, an inhibitor of calcium-activated chloride currents, were compared with the actions of the calcium channel blocker nifedipine on noradrenaline- and 5-hydroxytryptamine (5-HT)-induced pressor responses of the rat perfused isolated mesenteric vascular bed. 2. Bolus injections of noradrenaline (1 and 10 nmol) increased the perfusion pressure in a dose-dependent manner. Nifedipine (1 microM) inhibited the increase in pressure produced by 1 nmol noradrenaline by 31 +/- 5%. Niflumic acid (10 and 30 microM) also inhibited the noradrenaline-induced increase in perfusion pressure and 30 microM niflumic acid reduced the pressor response to 1 nmol noradrenaline by 34 +/- 6%. 3. The increases in perfusion elicited by 5-HT (0.3 and 3 nmol) were reduced by niflumic acid (10 and 30 microM) in a concentration-dependent manner and 30 microM niflumic acid inhibited responses to 0.3 and 3 nmol 5-HT by, respectively, 49 +/- 8% and 50 +/- 7%. Nifedipine (1 microM) decreased the pressor response to 3 nmol 5-HT by 44 +/- 9%. 4. In the presence of a combination of 30 microM niflumic acid and 1 microM nifedipine the inhibition of the pressor effects of noradrenaline (10 nmol) and 5-HT (3 nmol) was not significantly greater than with niflumic acid (30 microM) alone. Thus the effects of niflumic acid and nifedipine were not additive. 5. In Ca-free conditions the transient contractions induced by 5-HT (3 nmol) were not reduced by 30 microM niflumic acid, suggesting that this agent does not inhibit calcium release from the intracellular store or the binding of 5-HT to its receptor. 6. Niflumic acid 30 microM did not inhibit the pressor responses induced by KCl (20 and 60 mumol) which were markedly reduced by 1 microM nifedipine. In addition, 1 microM levcromakalim decreased pressor responses produced by 20 mumol KCl. These data suggest that niflumic acid does not block directly calcium channels or activate potassium channels. 7. It is concluded that niflumic acid selectively reduces a component of noradrenaline- and 5-HT-induced pressor responses by inhibiting a mechanism which leads to the opening of voltage-gated calcium channels. Our data suggest that the Ca(2+)-activated chloride conductance may play a pivotal role in the activation of voltage-gated calcium channels in agonist-induced constriction of resistance blood vessels.

Effects of frusemide on the urinary excretion of dopamine and 5-hydroxytryptamine in normal man

The effects of frusemide on the urinary excretion of dopamine and 5-hydroxytryptamine (5-HT) were investigated in eight healthy male subjects in a randomized, placebo-controlled, cross-over study. Frusemide produced the expected rise in urinary dopamine excretion but it did not affect 5-HT excretion when compared with placebo. The lack of an effect on 5-HT excretion in man contrasts with studies in the rat which have reported a marked increase in 5-HT excretion after administration of this loop diuretic.

gamma-L-glutamyl-5-hydroxy-L-tryptophan, but not gamma-L-glutamyl-L-tryptophan, causes sodium retention in normal man

1. This randomized, placebo-controlled, cross-over study compared the relative effectiveness of gamma-L-glutamyl-5-hydroxy-L-tryptophan (glu-5-HTP) and gamma-L-glutamyl-L-tryptophan (glu-TRP) in terms of their ability to act as substrates for renal 5-hydroxytryptamine (5-HT) synthesis and their actions on urinary sodium excretion. 2. Urinary excretion of 5-HT and sodium were determined before, during and after 1 h intravenous infusion of an equimolar amount (45 nmol kg-1 min-1) of glu-5-HTP or glu-TRP or placebo in nine healthy male subjects. 3. Cumulative urinary 5-HT excretion over the 4 h after the start of glu-5-HTP infusion was 350-fold greater than that after placebo, and this was associated with a reduction in the urinary excretion of sodium. 4. In contrast, the urinary excretion values of 5-HT and sodium after administration of glu-TRP were not significantly different from those observed on the placebo day. 5. The marked increase in urinary 5-HT excretion and the retention of sodium after administration of glu-5-HTP have been demonstrated in previous studies and result from increased intrarenal generation of 5-HT. The absence of a rise in urinary excretion of 5-HT after glu-TRP infusion suggests that there was no significant conversion of this glutamyl compound to 5-HT within the kidney. As a result, there was no effect on urinary sodium excretion.

Pharmacological characterization of RP 62203, a novel 5-hydroxytryptamine 5-HT2 receptor antagonist

1. RP 62203 (2-[3-(4-(4-fluorophenyl)-piperazinyl)propyl]naphto[1,8- ca]isothiazole-1,1-dioxide) is a novel naphtosultam derivative which shows very high affinity for 5-HT2 receptors in the rat cerebral cortex (Ki = 50.0 pM). 2. RP 62203 is relatively selective for this sub-type of 5-hydroxytryptamine (5-HT) receptor, having lower affinity for the 5-HT1A receptor and very low affinity for the 5-HT, receptor. RP 62203 displayed low to moderate affinity for alpha 1-adrenoceptors, dopamine D2 receptors and histamine H1 receptors. 3. In vivo binding experiments demonstrated that oral administration of low doses of RP 62203 led to a long-lasting (greater than 6 h) occupation of cortical 5-HT2 receptors (ID50 = 0.39 mgkg-1). 4. In cortical slices from the neonatal rat, RP 62203 potently inhibited inositol phosphate formation evoked by 5-HT, with an IC50 of 7.76 nM. 5. The activity of neurones in the raphé and their responses to microiontophoretically applied 5-HT were studied with extracellular recording electrodes in the anaesthetized rat. RP 62203 potently and dose-dependently blocked excitations evoked by 5-HT when administered at doses of 0.5-4.0 mg kg-1, i.p. In contrast, neither 5-HT-evoked depressions nor glutamate-evoked excitations of raphé neuronal firing were blocked by RP 62203 at doses as high as 8.0 mg kg-1, i.p. 6. Head twitches induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) could be abolished by low doses of RP 62203 in mice (ED50 = 0.44 mg kg-1, p.o.) and in rats (ED50 = 1.54 p.o.). Similar results were obtained with mescaline and 5-hydroxytryptophan (5-HTP). 7. The potency of RP 62203 was compared with that of three other 5-HT2 receptor antagonists, ritanserin, ICI 169,369 and ICI 170,809. In all models, RP 62203 showed similar activity to ritanserin, whilst either ICI 169,369 or ICI 170,809 was several fold less active. 8. It is concluded that RP 62203 is a potent and selective antagonist at 5-HT2 receptors in the rodent central nervous system.