Importance of h5-HT1B receptor selectivity for 5-HT terminal autoreceptor activity: an in vivo microdialysis study in the freely-moving guinea-pig

The importance of h5-HT1B receptor selectivity for 5-HT terminal autoreceptor activity was investigated with the selective h5-HT1B receptor ligands SB 219085, SB 220272, SB 224289 and SB 216641. The studies employed measurement of compound affinity and efficacy in vitro and the measurement of extracellular 5-HT in the frontal cortex of the freely-moving guinea-pig using in vivo microdialysis. All compounds had high affinity and selectivity for the h5-HT1B receptor, with SB 224289 the most selective for h5-HT1B over h5-HT1D receptors. Compounds exhibited a range of efficacies at both receptors: SB 224289 and SB 219085 were inverse agonists, SB 220272 was an antagonist and SB 216641 was a partial agonist. SB 220272, SB 216641 and SB 224289 had no effect on extracellular 5-HT following systemic administration, however, SB 219085 produced a significant increase. The SB 219085-induced increase in extracellular 5-HT was attributed to the compounds non-specific releasing properties as it was also demonstrated to increase basal release of [3H]5-HT from pre-loaded guinea-pig cortical slices. The lack of effect of the above h5-HT1B receptor selective compounds and the decrease in extracellular 5-HT elicited by the non-selective compounds GR 127935, GR125743 and methiothepin suggest that antagonism of 5-HT1D receptors may mediate this decrease in 5-HT levels. It is plausible that blockade of 5-HT1D receptors increases 5-HT levels in the raphe, this activates 5-HTtA receptors which results in an overall decrease in terminal 5-HT release. Definitive proof now awaits elucidation of the action of a selective 5-HT1D receptor antagonist.

Behavioural subsensitivity induced by long-term administration of a low dose of haloperidol to rats

This study examines the effects on open-field and stereotyped behaviour of rats of abrupt withdrawal from repeated treatment with a low (0.03 mg kg-1) dose of haloperidol. Single administration of this low dose of haloperidol significantly increased open-field locomotion without modifying apomorphine (0.5 or 2.0 mg kg-1)-induced stereotyped behaviour. Forty-eight hours after abrupt withdrawal from 0.03 mg kg-1 haloperidol (twice daily for 15 days) a significant decrease in locomotion frequency was observed, but no change was observed in apomorphine-induced stereotypy. Our results suggest that dopamine autoreceptor supersensitivity might be evaluated in a behavioural situation of absence of postsynaptic dopamine receptor supersensitivity.

5-Hydroxytryptamine1B and alpha 2 adrenaline receptors in the brain of rats prenatally treated with methylazoxymethanol

Methylazoxymethanol (MAM)-induced cerebral hypoplasia resulted in a significant increase in concentrations of 5-hydroxytryptamine (5-HT, serotonin) and norepinephrine in the frontal cortex, suggesting that these monoaminergic neurons were compressed due to smaller brain volumes. The serotonergic and noradrenergic presynaptic autoreceptors in rat brain are thought to be 5-HT1B receptors and alpha 2-adrenoceptors, respectively. If so, prenatal MAM treatment should increase the density of 5-HT1B and adrenaline alpha 2 receptors in the brain via the compression of noradrenergic and serotonergic axon terminals in the brains of rats with MAM-induced microencephaly. However, neither the densities nor the affinities of 5-HT1B and adrenaline alpha 2 receptors were changed in the MAM rats, suggesting that these presynaptic autoreceptors comprise only a small percentage of the total receptor population. Most 5-HT1B and adrenaline alpha 2 receptors were localized post-synaptically.

Preservation of autoreceptor-mediated increases in dopamine synthesis in aged mice with experimentally-induced parkinsonism

This study examined autoreceptor (i.e haloperidol)-mediated regulation of dopamine synthesis in normal aged mice and aged mice with a partial lesion of the nigrostriatal dopamine system induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). There were no significant differences between haloperidol-induced increases in striatal dihydroxyphenylalanine (DOPA) accumulation between lesioned and unlesioned animals. Striatal DOPA/dopamine ratios were significantly increased in lesioned animals treated with haloperidol as compared to normal lesioned animals treated only with haloperidol. These data suggest that striatal dopamine synthetic capacity and autoreceptor-mediated regulation of dopamine synthesis are relatively preserved in aged mice, even after a moderate nigrostriatal lesion. Aged dopamine neurons appear to retain the ability to increase synthesis and release of dopamine in the presence of an appropriate pharmacological stimulus.

3-[[(4-Aryl-1-piperazinyl)alkyl]cyclohexyl]-1H-indoles as dopamine D2 partial agonists and autoreceptor agonists

A series of arylpiperazines and tetrahydropyridines joined to indoles by semirigid cycloalkyl spacers were prepared. Target compounds were studied for their ability to bind to the DA D2 receptor in vitro and to inhibit dopamine synthesis and spontaneous locomotor activity in rats. Effects of tether length and relative stereochemistry were assessed for a series of 2-pyridylpiperazines. The cyclohexylethyl spacer was found to be the most active in the series. Further studies explored effects of changes in the arylpiperazine and indole portions of the molecule. From these studies trans-2-[[4-(1H-3-indolyl)cyclohexyl]ethyl]-4- (2-pyridinyl)piperazine (30a) was selected for further evaluation. It was characterized as a partial agonist of DA D2 receptors in vitro and caused decreases in dopamine synthesis and release as well as dopamine neuronal firing. Compound 30a was shown to be devoid of behavioral effects associated with postsynaptic DA D2 receptor activation. Furthermore, compound 30a was shown both to decrease DA synthesis and to inhibit Sidman avoidance responding in squirrel monkeys. These findings suggest that DA D2 partial agonists with the appropriate level of intrinsic activity can act to decrease dopamine synthesis and release and may have potential utility as antipsychotic agents.

Pontine acetylcholine release is regulated by muscarinic autoreceptors

Acetylcholine (ACh) in the medial pontine reticular formation (mPRF) originates from the laterodorsal and pedunculopontine tegmental (LDT/PPT) nuclei and contributes to generating rapid eye movement (REM) sleep. The mechanisms controlling mPRF ACh levels are incompletely understood. This study tested the hypothesis that mPRF ACh release is regulated, in part, by muscarinic autoreceptors. The mPRF of intact, halothane-anesthetized cats was dialyzed with Ringer's solution (control) or Ringer's containing the muscarinic antagonist scopolamine, Scopolamine caused a dose-dependent increase in mPRF ACh release and a concomitant decrease in the number of halothane-induced cortical EEG spindles. These data suggest that presynaptic muscarinic receptors, presumed to reside on cholinergic LDT/PPT terminals in the mPRF, play a role in regulating mPRF ACh release, REM sleep and EEG spindles.

Antagonism of cocaine's stimulant effects on local cerebral glucose utilization by the preferential autoreceptor antagonist (+)-AJ 76

(+)-AJ 76 is a stimulant dopamine (DA) antagonist, which has a putative preferential action at DA nerve terminal autoreceptors. Because it is both a mild stimulant and a DA antagonist, it has previously been suggested that (+)-AJ 76 might antagonize both the euphoria and craving associated with cocaine abuse and withdrawal, respectively. To evaluate this hypothesis further, (+)-AJ 76 was evaluated for its ability to affect cocaine-induced changes in regional brain energy metabolism. Using Sokoloff's 2-deoxyglucose autoradiographic technique, (+)-AJ 76 antagonized the stimulant effect of cocaine. Although classical DA antagonists are known to depress regional brain energy metabolism, (+)-AJ 76 by itself had no effect. It is concluded that the results are consistent with the previously stated hypothesis that (+)-AJ 76 might be useful as a pharmacotherapy for treatment of cocaine abuse.

Evidence for presynaptic location of inhibitory 5-HT1D beta-like autoreceptors in the guinea-pig brain cortex

The effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on tritium overflow evoked by high K+ were determined in superfused synaptosomes and slices, preincubated with [3H]5-HT, from guinea-pig brain cortex. In addition, we estimated the potencies of 5-HT receptor ligands in inhibiting specific [3H]5-HT binding (in the presence of 8-hydroxy-2(di-n-propylamino)tetralin and mesulergine to prevent binding to 5-HT1A and 5-HT2C sites) to guinea-pig cortical synaptosomes and membranes. 5-HT receptor agonists inhibited the K(+)-evoked tritium overflow from synaptosomes and slices. In synaptosomes the rank order of potencies was 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl] -1H-indole-3-yl] ethylamine (L-694,247) > 5-carboxamidotryptamine (5-CT) > oxymetazoline (in the presence of idazoxan) > or = 5-HT > sumatriptan > or = 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969). The potencies of the agonists in inhibiting tritium overflow from slices correlated with those in synaptosomes, suggesting that the same site of action is involved in both preparations. In synaptosomes the nonselective antagonist at cloned human 5-HT1D alpha and 5-HT1D beta receptors, methiothepin, shifted the concentration-response curve for 5-CT to the right (apparent pA2: 7.87). In contrast, ketanserin at a concentration which should block the 5-HT1D alpha, but not the 5-HT1D beta, receptor did not alter the inhibitory effect of 5-CT on tritium overflow. In cortical synaptosomes and membranes, [3H]5-HT bound to a single site with high affinity. In competition experiments, 5-HT receptor agonists and antagonists inhibited specific [3H]5-HT binding. In synaptosomes the rank order was L-694,247 > methiothepin > 5-CT > 5-methoxytryptamine > 5-HT > or = sumatriptan > or = oxymetazoline > RU 24969 > ketanserin > ritanserin. A very similar rank order was obtained in cerebral cortical membranes. The potencies of the 5-HT receptor agonists in inhibiting tritium overflow from synaptosomes and slices correlated with their potencies in inhibiting [3H]5-HT binding to synaptosomes and membranes. In conclusion, the 5-HT receptors mediating inhibition of 5-HT release in the guinea-pig cortex are located on the serotoninergic axon terminals and, hence, represent presynaptic inhibitory autoreceptors. The [3H]5-HT binding sites in cerebral cortical synaptosomes and membranes exhibit the pharmacological properties of 5-HT1D receptors. The correlation between the functional responses and the binding data confirms the 5-HT1D character of the presynaptic 5-HT autoreceptors. According to the results of the interaction experiment of ketanserin and methiothepin with 5-CT on 5-HT release, the presynaptic 5-HT autoreceptors can be subclassified as 5-HT1D beta-like.

Antagonists that differentiate between alpha 2A-and alpha 2D-adrenoceptors

Four antagonists were examined for their ability to differentiate alpha 2A-from the orthologous alpha 2D-adrenoceptors. The antagonists were (2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octah ydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one (MK912), 2-[2-(methoxy-1,4-benzodioxanyl)imidazoline (RX 821002), efaroxan and benoxathian. The alpha 2-autoreceptors in rabbit brain cortex were chosen as alpha 2A-and the alpha 2-autoreceptors in guinea-pig brain cortex as alpha 2D-adrenoceptors. Slices of the brain cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically by brief pulse trains (4 pulses, 100 Hz) that led to little, if any, alpha 2-autoinhibition. 5-Bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) was used as an alpha 2-adrenoceptor agonist. UK 14, 304 decreased the stimulation-evoked overflow of tritium. The antagonists shifted the concentration-inhibition curve of UK 14, 304 to the right in an apparently competitive manner. Dissociation constants of the antagonists were calculated from the shifts. MK 912, RX 821002 and efaroxan had markedly higher affinity for (guinea-pig) alpha 2D-adrenoceptors (pKd values 10.0, 9.7 and 9.1, respectively) than for (rabbit) alpha 2A-adrenoceptors (pKd 8.9, 8.2 and 7.6, respectively). Benoxathian had higher affinity for alpha 2A-(pKd 7.4) than for alpha 2D-adrenoceptors (pKd 6.9). Ratios calculated from the Kd values of the four compounds differentiated between alpha 2A and alpha 2D up to 100 fold. It is concluded that MK 912, RX 821002, efaroxan and benoxathian are antagonists with high power to differentiate alpha 2A-from alpha 2D-adrenoceptors.

Serotonin efflux in the rat ventral lateral geniculate nucleus assessed by fast cyclic voltammetry is modulated by 5-HT1B and 5-HT1D autoreceptors

Fast cyclic voltammetry (FCV) was used to measure electrically stimulated monoamine efflux in the rat ventral lateral geniculate nucleus (vLGN). The electrochemical characteristics of the released species resembled 5-HT but not dopamine or noradrenaline. Amine efflux was abolished by the sodium channel blocker tetrodotoxin (0.1 microM), Ro 4-1284 (1.0 microM), the fast-acting reserpine analogue, and removal of Ca2+ from the superfusate. Amine efflux was unaffected by the monoamine oxidase inhibitor clorgyline (0.1 microM). Of paroxetine (0.1 microM), desipramine (50 nM) and vanoxerine (0.5 microM), selective blockers of 5-HT, noradrenaline and dopamine uptake respectively, only paroxetine increased monoamine efflux (to 194 +/- 25%, mean +/- SEM) and prolonged the removal half-life (to 638 +/- 105%). The non-specific 5-HT1 antagonist methiothepin (0.2 microM) increased 5-HT efflux on long (20 pulses at 20 Hz) but not short trains (20 pulses at 100 Hz). When tested on pseudo-one-pulse stimulations (5 pulses, 100 Hz), the selective 5-HT1A agonist 8-OHDPAT (1.0 microM) had no effect. CP 93129 (0.3 microM), the selective 5-HT1B agonist, decreased 5-HT efflux to 37 +/- 4% of control and was antagonised by the 5-HT1B blocker isamoltane (0.5 microM) and by the 5-HT1D/B antagonist GR 127935 (50 nM). The preferential 5-HT1D agonist sumatriptan (0.5 microM) also decreased 5-HT efflux, to 55 +/- 6% and was antagonised by GR 127935 (50 nM) but not isamoltane (0.5 microM). These results suggest that 5-HT released in the vLGN can be measured by FCV. Furthermore, released 5-HT is taken up by the 5-HT transporter and may be under the influence of 5-HT1B and 5-HT1D autoreceptors.

Identification, characterization and pharmacological profile of three metabolites of (R)-(+)-1,2,3,6-tetrahydro-4-phenyl-1-[(3-phenylcyclohexen-1- yl)methyl]pyridine (CI-1007), a dopamine autoreceptor agonist and potential antipsychotic agent

Liquid chromatographic-mass spectrometric (LC-MS) analysis of plasma taken from cynomolgus monkeys dosed orally with (R)-(+)-1,2,3,6-tetrahydro-4-phenyl-1-[(3-phenylcyclohexen-1- yl)methyl]pyridine (1), a dopamine (DA) autoreceptor agonist and potential antipsychotic agent, revealed several metabolites. The molecular masses of three major metabolites suggested that they were mono- and dihydroxylated derivatives of 1. We synthesized compounds 2 and 3, the two possible mono-p-hydroxyphenyl derivatives of 1, along with the bis-p-hydroxyphenyl derivative 4. These compounds coeluted by HPLC with the three hydroxylated metabolites of 1. Compounds 2-4 all had high affinities for DA D2 and D3 receptors and moderate affinities for D4 receptors. Like 1, compound 2 decreased DA synthesis and neuronal firing in rat brain, indicative of DA autoreceptor activation. Compound 2 inhibited exploratory locomotor activity in rodents and was active in the Sidman avoidance test in squirrel monkeys, predictive of antipsychotic activity in humans. Compounds 3 and 4 showed weak activity in all these tests. After squirrel monkeys were dosed with 1 orally at the ED100 dose of the Sidman avoidance test, the plasma concentration of 2 was below the limit of quantitation. Therefore, these metabolites are unlikely to contribute greatly to the potent activity seen with 1 in the Sidman avoidance test.

Evidence for prejunctional muscarinic autoreceptors in human and guinea pig trachea

Functional studies suggest the presence of prejunctional muscarinic autoreceptors on cholinergic nerves in human airways. However, these studies are an indirect method of evaluating changes in neurally evoked acetylcholine (ACh) release. We have investigated the presence of muscarinic autoreceptors in human and guinea pig trachea by comparing the effects of the muscarinic receptor antagonists pirenzepine (M1), methoctramine (M2), 4-DAMP (M3), and rispenzepine (M1/M3) on cholinergic neural contractile responses evoked by electrical field stimulation (EFS) and [3H]ACh release. The M1, M1/M3, or M3 antagonists inhibited the EFS-evoked cholinergic contractile response in a concentration-dependent manner (4-DAMP > rispenzepine > pirenzepine), whereas methoctramine facilitated this response at low concentrations ( < 3 microM). In ACh release studies, the M3 antagonist had no significant effect, whereas pirenzepine, methoctramine, and rispenzepine significantly increased ACh release in guinea pig trachea. In contrast, ACh release was significantly inhibited by the muscarinic agonist oxotremorine M. Methoctramine and the nonselective antagonist ipratropium bromide, but not the M1, M1/M3, or M3 antagonists, significantly increased ACh release in human trachea. These data suggest the presence of an autoinhibitory receptor on cholinergic nerve terminals in human and guinea pig trachea. In addition, the action of ipratropium bromide at the autoinhibitory receptor may limit its use in the treatment of obstructive airways disease.

On the quantitative structure-activity relationships of meta-substituted (S)-phenylpiperidines, a class of preferential dopamine D2 autoreceptor ligands: modeling of dopamine synthesis and release in vivo by means of partial least squares regression

The quantitative structure-activity relationship between physicochemical properties and effects on dopamine (DA) synthesis and release in the rat brain, in a series of meta-substituted (S)-phenylpiperidines, has been investigated by means of partial least squares regression (PLS). The effect on DA synthesis caused by the drugs, in both non-pretreated and reserpine-pretreated rats, was assessed by measurements of tissue levels of L-DOPA accumulated in the striatum following treatment with a decarboxylase inhibitor. Assessment of effects on DA release was performed by analysis of perfusates collected from implanted microdialysis probes. The numerical characterization of the variation in physicochemical features of the phenylpiperidines used in the regression modeling was accomplished by using common tabulated aromatic and aliphatic substituent constants in combination with a set of property descriptors derived from molecular mechanics and semiempirical calculations. It was found that the biochemical responses could be accurately predicted by the regression models based on these molecular feature measures. The molecular features exerting influence on DA synthesis were found to be markedly different from those influencing DA release. This finding is discussed in terms of the possible existence of a dopamine receptor-mediated DA release-controlling mechanism, which may not involve the synthesis regulating DA D2 autoreceptor. Some findings regarding the impact of the piperidine N substituent on agonist properties of the drugs are reported. The regression models were also used for guidance in the search for a phenylpiperidine with a lower intrinsic activity, at the DA D2 type autoreceptor, than the partial DA agonist preclamol (3).

In vivo assessment of striatal dopamine release in the aged male Fischer 344 rat

A microdialysis probe was implanted into the striatum of young (4- to 5-month-old) and aged (26- to 27-month-old) Fischer 344 male rats to assess age-related alterations in striatal dopamine (DA) release. Basal levels of DA and the magnitude of DA response evoked by 50 mM and 100 mM high potassium (K+) in aged rats were similar to those in young rats. Furthermore, K(+)-evoked DA release did not correlate with motor performance within either age group. In contrast, amphetamine (250 microM) evoked-DA release of aged rats was significantly lower than that of young rats. Moreover, the enhancement of K(+)-evoked DA release by oxotremorine (500 microM) was significantly attenuated in aged rats. These results indicate that a putative DA release mechanism and its cholinergic modulation of the aged striatum are impaired.

Autoreceptor presynaptic control of dopamine release from striatum is lost at early stages of manganese poisoning

Manganese (Mn) poisoning in man produces an early psychotic disorder that is later followed by a Parkinson-like syndrome. Since alterations in the brain DA system are thought to be involved, we assessed the presynaptic autoreceptor regulation of K(+)-evoked 3H-DA release from superfused striatal slices of mice treated i.p. with 5 mg Mn/kg weight/day for 2 and 8 weeks. Mn poisoning did not change basal and evoked DA release. In controls, 1 microM apomorphine (APO), a D2-like DA receptor agonist, produced an inhibition of K(+)-evoked 3H-DA release that was blocked by the D2-like DA receptor antagonist, S(-)-sulpiride (1 microM). Yet, APO lost its capacity to inhibit the K(+)-evoked 3H-DA release after 2 weeks of Mn poisoning. After 8 weeks of Mn poisoning, APO was again able to reduce K(+)-evoked 3H-DA release. MK-801 (0.3 microM), a NMDA-glutamate receptor antagonist, could restore APO inhibitory control on DA release lost at week 2 of Mn poisoning. These findings suggest a NMDA-glutamate-receptor-mediated loss of autoreceptor presynaptic control of striatal DA release at early Mn poisoning.

Fluctuations in nucleus accumbens dopamine during cocaine self-administration behavior: an in vivo electrochemical study

High-speed chronoamperometry with Nafion-coated monoamine-sensitive carbon fiber electrodes was used to estimate changes in extracellular dopamine concentration in the nucleus accumbens during cocaine self-administration behavior in rats. In trained animals, time-locked biphasic fluctuations in dopamine-dependent electrochemical signal were found to accompany cocaine self-injections (0.8-0.9 mg/kg/inj). The mean signal gradually increased by the equivalent of 20-30 nM of dopamine during the 60 s preceding the injection, reached a peak value at the lever-press and decreased abruptly by about 20-30 nM for 40-60 s after the injection. This cyclic pattern was repeated with the next lever-press. The post-cocaine signal decreases were most pronounced during the first 30 min of each session, when self-administration behavior was highest (eight to 16 injections), and gradually diminished during the session. In contrast, the pre-injection signal increases became enhanced over time. Lever-presses reinforced by a double cocaine dose were followed by significantly larger and longer lasting signal decreases. These biphasic fluctuations quickly disappeared after several non-reinforced lever-presses. Although experimenter-delivered cocaine injections paced to mimic the pattern of self-administration also induced biphasic signal fluctuations, both the post-drug signal decreases and subsequent pre-injection increases were significantly smaller. It is hypothesized that the increases in signal seen in trained animals are a consequence of cocaine-induced dopamine uptake inhibition following behavior-associated dopamine cell activation. In contrast, the post-cocaine abrupt transient signal depression may be related to a decrease in mesolimbic dopamine release due to inhibition of dopamine cell activity. Signal decreases seen after self-administered procaine suggest that cocaine's local anesthetic action may contribute to this decrease in dopamine release. Additionally, while the latency of response differed somewhat, since apomorphine administration also led to a reduction in signal, autoreceptor activation may also have contributed to the cocaine-induced signal depression. That learning and behavioral mechanisms are also important determinants of the observed cocaine-induced signal changes is suggested by the signal decreases after the first non-reinforced responses, signal differences between self- and passively-administered cocaine and signal increases caused by cocaine-related cues. In light of numerous neuropharmacological studies implicating the significance of the mesolimbic dopamine system in the organization and regulation of goal-directed behaviors, these data suggest that mesolimbic dopamine system activation may mediate motivational and activational components of drug-seeking and drug-taking behavior, while the transient, reward-associated inhibition of the system may be involved in regulating these behaviors.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction of in vivo striatal 5-hydroxytryptamine release by 8-OH-DPAT after inactivation of Gi/G(o) proteins in dorsal raphe nucleus

5-HT1A receptor agonists reduce firing-dependent terminal 5-HT synthesis and release by activating somatodendritic 5-HT1A receptors. We have examined the effects of 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT, 0.1 mg/kg s.c.) on in vivo striatal 5-HT release in conscious rats with somatodendritic 5-HT1A receptors inactivated by the application of pertussis toxin in the dorsal raphe nucleus. The uncoupling of 5-HT1A receptors from hyperpolarizing potassium channels was demonstrated by the inability of the intra-raphe application of citalopram to reduce striatal release (control animals had a 47% reduction, an effect prevented by previous treatment with the 5-HT1A antagonist (-)-tertatolol). Yet 8-OH-DPAT (0.1 mg/kg s.c.) decreased striatal 5-HT release by 66% (peak effect) in pertussis toxin-treated rats, a value comparable to that found in naive animals (74%). This raises the possibility that other 8-OH-DPAT-sensitive serotonergic receptors different from 5-HT1A autoreceptors may be involved in the control of terminal 5-HT release.

Further evidence for the importance of 5-HT1A autoreceptors in the action of selective serotonin reuptake inhibitors

The clinical efficacy of antidepressants that block serotonin (5-hydroxytryptamine, 5-HT) reuptake may be restrained by indirect activation of autoreceptors. In vivo microdialysis in rat hippocampus was used to assess the release-inhibitory properties of the 5-HT reuptake inhibitors citalopram and paroxetine. When reuptake was first blocked by infusing citalopram into the hippocampus, systemic administration of citalopram or paroxetine resulted in a 50-70% decrease in hippocampal 5-HT overflow. This presumably reflected the inhibition of 5-HT release subsequent to reuptake blockade in the raphe nuclei and, in turn, activation of somatodendritic autoreceptors. In support, pretreatment with (+/-)-pindolol or (+)-WAY100135 ((+)-N-tert-butyl-3-(4-(2- methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide dihydrochloride), to block 5-HT1A autoreceptors, abolished the decrease in 5-HT produced by systemic injection of the uptake blockers.

Monoamines modulate the electrically-evoked efflux of 3H-choline from slices of guinea pig nucleus basalis magnocellularis

The influence exerted by monoamines on acetylcholine release was studied in electrically stimulated slices of guinea pig nucleus basalis magnocellularis (nbM) prelabelled with 3H-choline (3H-Ch). Noradrenaline, 30 microM, and clonidine, 1 microM, reduced the evoked 3H-Ch efflux by about 50%, but phenylephrine, 100 microM, did not; idazoxan, 0.1 microM, but not prazosin, 1 microM, antagonized these effects, pointing to the involvement of alpha 2 receptors. Apomorphine, 1 or 30 microM, reduced 3H-Ch efflux from nbM slices as well. The effect was shared by quinpirole, 1 or 10 microM, but not by 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393), 10 microM, and was antagonized by sulpiride, 1 microM, but not by R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin++ +-7-ol (SCH 23390), 1 microM, suggesting the involvement of the D2 receptor subtype. 5-hydroxytryptamine (5-HT) 0.3-30 microM, and alpha-methyl-5-HT, 10 microM, significantly increased 3H-Ch efflux from nbM slices; the 5-HT2 antagonist ritanserin, 1 microM, prevented this response. 2-methyl-5-HT, 1-30 microM, inhibited the evoked 3H-Ch efflux and its effect was prevented by the 5-HT3 antagonist 1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate (MDL 72222), 1 microM. These findings indicate that i) catecholamines inhibit nbM neurons through alpha 2 and D2 receptors and that ii) a complex serotonergic modulation of cholinergic function exists in the nbM, involving the activation of various receptor subtypes, which can mediate opposite responses.

New insights into receptor theory, as provided by an artificial partial agonist made-to-measure

In the present study a mixture of a full agonist (noradrenaline) and a full antagonist (yohimbine) was used to mimic the effects of a partial agonist (clonidine) on alpha 2-autoreceptor-mediated regulation of noradrenaline release in order to learn more about the shape of concentration-response curves in the absence and presence of spare receptors. The sigmoidal shape of the cloud of single experimental data points may be reflected by different curve fits based on either descriptive or mechanistic mathematical models. Only mechanistic models allow the interpretation of the relationship between occupancy of receptors and induced response. The experiments were performed in rat neocortex and in rabbit hippocampus tissue where electrical field stimulation with 4 pulses/100 Hz of slices prelabelled with [3H]noradrenaline elicited the release of noradrenaline. A receptor reserve was found in the rabbit hippocampus and quantified from the concentration-response curve of noradrenaline in this tissue using a mechanistic general response function, developed to reflect the condition of spare receptors. The mixture of noradrenaline and yohimbine, NA-Yoh, (three parts to one part), corrected by the different affinities to the alpha 2-autoreceptors, was designed to mirror the quantified proportion of 75% non-spare and 25% spare alpha 2-autoreceptors. In the spare receptor-free rat cortex NA-Yoh acted like a typical partial agonist, as clonidine, with nearly the same EC50 (= Kd in this case) as the full agonist noradrenaline, but with a maximum effect significantly lower than that of noradrenaline. In the rabbit hippocampus, however, the same maximum effect was obtained with NA-Yoh, noradrenaline and clonidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased acetylcholine release in tracheas from allergen-exposed IgE-immune mice

Increased release of acetylcholine (ACh) from airway parasympathetic nerve endings is one mechanism that may contribute to increases in airway responsiveness in immunoglobulin E (IgE)-immune allergen-exposed animals. We measured ACh released from murine tracheas following electrical field stimulation in vitro. BALB/c mice were immunized by exposure to an aerosol of 1% ovalbumin in sterile phosphate-buffered saline for 20 min/day for 10 days. At this time, levels of ovalbumin-specific IgE were proportionately higher than ovalbumin-specific IgG. As a control, nonimmune mice were similarly exposed to phosphate-buffered saline alone. Forty-eight hours after the last aerosol, tracheas were removed for assessment of either the contractile responses to electrical field stimulation and a cholinergic agonist (methacholine or ACh) or release of ACh produced by electrical field stimulation. ACh in the bath was measured using high-performance liquid chromatography with electrochemical detection. The stimulation frequencies causing one-half the maximal contractile response to electrical field stimulation were 4.1 +/- 0.2 and 2.8 +/- 0.2 Hz (P = 0.0001) for nonimmune and immune mice, respectively, whereas the molar concentrations of methacholine causing one-half of the maximal contractile response did not significantly differ. In addition, the dose-response curves of immune and nonimmune tracheas to ACh were superimposable. A significant increase in ACh release was demonstrated at both 10 and 20 Hz in tracheas from immune mice. ACh release (pmol.g tissue-1.min-1) from nonimmune and immune murine tracheas, respectively, were 140 +/- 8 and 205 +/- 22 (P = 0.013) at 10 Hz and 147 +/- 13 and 227 +/- 14 (P = 0.008) at 20 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT3 receptors which modulate [3H]5-HT release in the guinea pig hypothalamus are not autoreceptors

The 5-HT3 agonist 2-methyl-5-HT had previously been shown to enhance the electrically evoked release of [3H]5-HT from preloaded slices of the guinea pig brain. In the present study, 2-methyl-5-HT (1 microM) was also found to increase the K+ evoked release of [3H]5-HT from preloaded slices of the guinea pig hypothalamus and this effect was blocked by the selective 5-HT3 antagonist ondansetron. In the presence of tetrodotoxin, the enhancement of the K(+)-evoked release of [3H]5-HT by 2-methyl-5-HT in hypothalamus slices was blocked, thus suggesting that the 5-HT3 receptors mediating this effect are not located directly on 5-HT terminals. In agreement with this, 2-methyl-5-HT did not alter the K(+)-evoked release of [3H]5-HT in a synaptosomal preparation of the same brain structure, even at a concentration 10-fold greater than that used in the slices. Taken together, these data indicate that these facilitatory 5-HT3 receptors are not located on 5-HT terminals in the guinea pig hypothalamus and therefore are not autoreceptors.