Development of a radioligand binding assay for 5-HT4 receptors in guinea-pig and rat brain

Stroking human jejunal mucosa induces 5-HT release and Cl- secretion via afferent neurons and 5-HT4 receptors

5-Hydroxytryptamine (5-HT) release and neural reflex pathways activated in response to mucosal stroking were investigated in muscle-stripped human jejunum mounted in modified Ussing chambers. The mucosa was stroked with a brush at 1/s for 1-10 s. Mucosal stroking resulted in a significant increase in the concentration of 5-HT in the mucosal bath within 5 min. It also was associated with a reproducible positive change (Delta) in short-circuit current (Isc), which was abolished by inhibitors of chloride secretion. Capsaicin and hexamethonium significantly inhibited the DeltaIsc but not the release of 5-HT. The DeltaIsc was inhibited by TTX but not by atropine. It was also inhibited by the 5-HT(3,4) receptor antagonist tropisetron (10 microM) and the 5-HT(4,3) receptor antagonist SDZ-205-557 (10 microM) but not by preferential antagonists of 5-HT(1P), 5-HT(2A), or 5-HT3 receptors. These results suggest that mucosal stroking induces release of mucosal 5-HT, which activates a 5-HT4 receptor on enteric sensory neurons, evoking a neuronal reflex that stimulates chloride secretion.

5-CT stimulation of adenylyl cyclase activity in guinea-pig hippocampus: evidence for involvement of 5-HT7 and 5-HT1A receptors

1. A number of compounds, including the selective 5-HT7 receptor antagonist SB-258719, were investigated for their effect on [3H]-5-carboxamidotryptamine (5-CT) radioligand binding and 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membranes, in order to confirm the presence of functionally coupled 5-HT7 receptors in this tissue. 2. The [3H]-5-CT radioligand binding profile was consistent with binding predominantly to 5-HT7 receptors. The affinity of SB-258719 (pKi 7.2+/-0.1) was similar to its reported human 5-HT7 receptor affinity. 3. In the adenylyl cyclase functional assay, 5-CT was a potent and full agonist compared to 5-HT, whereas 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) was a partial agonist (intrinsic activity 0.4+/-0.1). The rank order of potency for agonists (5-CT>5-HT approximately 8-OH-DPAT) was consistent with activation of 5-HT7 receptors. SB-258719 (5 microM) and methiothepin (1 microM) surmountably antagonized the response to 5-CT, consistent with competitive antagonism. The pKB for SB-258719 (7.2+/-0.1) was in good agreement with its reported antagonist potency at the human cloned 5-HT7 receptor. 4. In the functional assay, WAY-100635 (100 nM) and cyanopindolol (1 microM) induced a biphasic 5-CT response curve, consistent with selective antagonism of a component of the response to 5-CT. The estimated pKB values for WAY-100635 and cyanopindolol (9.6 and 8.4 respectively) were in good agreement with their reported 5-HT1A receptor affinities. 5. The data are consistent with the presence of 5-HT7 receptors in guinea-pig hippocampus which are positively coupled to adenylyl cyclase. In addition, 5-HT7 receptor-mediated stimulation of adenylyl cyclase activity in this tissue appears to be augmented by a mechanism involving 5-HT1A receptor activation.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

2,3-Dihydro-2-oxo-1H-benzimidazole-1-carboxamides with selective affinity for the 5-HT(4) receptor: synthesis and structure-affinity and structure-activity relationships of a new series of partial agonist and antagonist derivatives

A series of 2,3-dihydro-2-oxo-1H-benzimidazole-1-carboxamide derivatives bearing a piperazine moiety was synthesized. Their in vitro 5-HT(4), 5-HT(3), and D(2) receptors affinities were evaluated by radioligand binding assay. For selected compounds functional studies at the 5-HT(4) receptor were made by using precontracted (by carbachol) preparations of rat esophageal tunica muscularis mucosae (TMM). The influence of the 3-substituent of the benzimidazole ring, the 4-substituent of the piperazine moiety, and the alkylene spacer was studied. Compounds with an ethyl or a cyclopropyl substituent in the 3-position of the benzimidazole ring showed moderate to high affinity (K(i) = 6.7-75.4 nM) for the 5-HT(4) receptor with selectivity over 5-HT(3) and D(2) receptors and moderate antagonist activity (pK(b) = 6.19-7.73). Compounds with an isopropyl substituent in the 3-benzimidazole position exhibited moderate and selective 5-HT(4) affinity (K(i) >/= 38.9 nM) and a partial agonist activity (5a, i.a. = 0.94) higher than that of the reference compound BIMU 8 (i.a. = 0.70). This reversal of the pharmacological activity due only to a small structural difference might confirm the existence of two binding sites on the 5-HT(4) receptor. In the alkylene spacer, a two-methylene chain is favorable to optimize the affinity and the antagonist or the partial agonist activity. In the ethyl and cyclopropyl series, 5-HT(4) antagonist activity seems to be unrelated to the size of the 4-substituent of the piperazine moiety, whereas a methyl group is optimal for high partial agonist activity in the isopropyl series; however, the presence of a butyl substituent is a favorable pattern for 5-HT(4) antagonism and even causes a reversal of the pharmacological profile in the isopropyl series (5h, pK(b) = 7.94). N-Butyl quaternization of 5a led to an improvement in affinity for the 5-HT(4) receptor and mantained the high partial agonist activity (5r, K(i) = 66.3 nM, i.a. = 0.93).

Characterisation of the 5-HT receptor binding profile of eletriptan and kinetics of [3H]eletriptan binding at human 5-HT1B and 5-HT1D receptors

The affinity of eletriptan ((R)-3-(1-methyl-2-pyrrolidinylmethyl)-5-[2-(phenylsulphonyl )ethyl]-1H-indole) for a range of 5-HT receptors was compared to values obtained for other 5-HT1B/1D receptor agonists known to be effective in the treatment of migraine. Eletriptan, like sumatriptan, zolmitriptan, naratriptan and rizatriptan had highest affinity for the human 5-HT1B, 5-HT1D and putative 5-ht1f receptor. Kinetic studies comparing the binding of [3H]eletriptan and [3H]sumatriptan to the human recombinant 5-HT1B and 5-HT1D receptors expressed in HeLa cells revealed that both radioligands bound with high specificity (>90%) and reached equilibrium within 10-15 min. However, [3H]eletriptan had over 6-fold higher affinity than [3H]sumatriptan at the 5-HT1D receptor (K(D)): 0.92 and 6.58 nM, respectively) and over 3-fold higher affinity than [3H]sumatriptan at the 5-HT1B receptor (K(D): 3.14 and 11.07 nM, respectively). Association and dissociation rates for both radioligands could only be accurately determined at the 5-HT1D receptor and then only at 4 degrees C. At this temperature, [3H]eletriptan had a significantly (P<0.05) faster association rate (K(on) 0.249 min(-1) nM(-1)) than [3H]sumatriptan (K(on) 0.024 min(-1) nM(-1)) and a significantly (P<0.05) slower off-rate (K(off) 0.027 min(-1) compared to 0.037 min(-1) for [3H]sumatriptan). These data indicate that eletriptan is a potent ligand at the human 5-HT1B, 5-HT1D, and 5-ht1f receptors and are consistent with its potent vasoconstrictor activity and use as a drug for the acute treatment of migraine headache.

Identification of SK-951, a novel benzofuran derivative, as an agonist to 5-HT4 receptors

The pharmacological profile of SK-951 ((-)4-amino-N-[2-(1-azabicyclo[3.3.0]octan-5-yl) ethyl]-5-chloro-2,3-dihydro-2-methylbenzo[b]furan-7-carboxamide hemifumarate) was identified in relation to serotonin 5-HT3 and 5-HT4 receptors by the receptor binding assay and functional studies. The receptor binding assay showed that SK-951 bound to the 5-HT3 receptor with a high affinity, to the 5-HT4 receptor with relatively higher affinity and to the muscarinic M2 receptor with a low affinity, but not to dopamine D1 and D2 and serotonin 5-HT1 and 5-HT2 and muscarinic M1 and M3 receptors. SK-951 caused relaxations of tunica muscularis mucosae preparations from rat esophagus which were precontracted with carbachol, and the effects were antagonized by GR113808, a selective 5-HT4 antagonist. In the longitudinal muscle with myenteric plexus (LMMP) preparations from guinea pig ileum, SK-951 enhanced the electrically-stimulated contraction of preparations in which the 5-HT1, 5-HT2 and 5-HT3 receptors were blocked, and it enhanced the electrically-stimulated release of [3H]acetylcholine (ACh). These effects of SK-951 were antagonized by GR113808. SK-951 inhibited the 5-HT3 receptor-mediated contractions. These results indicate that SK-951 possesses properties of an agonist for the 5-HT4 receptor and an antagonist for the 5-HT3 receptor. Thus, SK-951 is a new and potent 5-HT4-receptor agonist and causes contractions of guinea pig ileum mediated by enhancement of ACh release via the 5-HT4 receptor.

The analgesic tropane analogue (+/-)-SM 21 has a high affinity for sigma2 receptors

The analgesic properties of the tropane analogue (+/-)-SM 21 have been attributed to the antagonism of presynaptic m2 receptors resulting in a potentiation of acetylcholine release. However, drugs targeting a number of other neurotransmitter receptors have been shown to enhance acetylcholine release. In the current study, in vitro studies were conducted in order to determine the affinity of (+/-)-SM 21 for serotonin 5-HT3, 5-HT4, and sigma receptors. Our results indicate that (+/-)-SM 21, and its structural congeners, have a relatively high affinity for sigma2 receptors relative to their reported affinity for muscarinic receptors. The higher affinity for sigma2 versus sigma1 receptors indicates that (+/-)-SM 21 may be a suitable lead compound for developing sigma2-selective ligands.

Serotonin-4 receptor antagonists reverse cocaine-induced cardiac arrhythmia

The effect of the 5-HT4 antagonists GR113808A and GR125487D on cocaine-induced cardiac arrhythmia was examined in the rat. Cocaine alone, given i.v. at a rate of 2 mg/kg every 5 min, produced an initial increase in blood pressure followed by a severe drop in pressure and bradycardia. Sustained ventricular fibrillation was noted after 6-12 mg/kg cocaine and quickly progressed to asystole. Pretreatment with both GR113808A and GR125487D antagonized these effects in a dose-dependent manner. When given after the onset of arrhythmia, both drugs reversed the cocaine-induced arrhythmia's. Thus, the 5-HT4 antagonists may be useful in the treatment of cocaine toxicity.

5-HT4 receptor antagonism does not affect motor and reward mechanisms in the rat

5-HT4 receptors are concentrated in areas of the brain which are rich in dopamine neuronal markers, which may suggest that they influence motor and reward processes. We tested this hypothesis by examining the effects of a 5-HT4 receptor antagonist, 8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1,4-dioxan-5-car boxylate hydrochloride (SB-204070-A) on amphetamine- and nicotine-induced locomotor stimulation in intact rats. In rats with unilateral 6-hydroxydopamine-induced lesions of the ascending nigrostriatal dopaminergic projection, SB-204070-A was tested for its effects on amphetamine-induced rotation. SB-204070-A was also tested for its effects on rewarded behaviour maintained by intracranial self-stimulation. SB-204070-A did not alter behaviour under any of these conditions, suggesting a lack of involvement of the 5-HT4 receptor in motor and reward processes.

Inhibition of NMDA-receptor mediated response in the rat medial prefrontal cortical pyramidal cells by the 5-HT3 receptor agonist SR 57227A and 5-HT: intracellular studies

The techniques of intracellular recording and single-electrode voltage-clamp were used to study the effect of serotonin (5-HT) and the selective 5-HT3 receptor agonist SR 57227A on N-methyl-D-aspartic acid (NMDA)-evoked responses in pyramidal cells of the rat medial prefrontal cortex (mPFC) in in vitro brain slice preparations. Bath application of 5-HT or SR 57227A produced a concentration-dependent inhibition of NMDA-induced membrane depolarization, action potentials, and inward current. The depressant action of 5-HT and SR 57227A had a slow onset and showed no signs of receptor desensitization. This action was markedly attenuated or completely blocked by the selective 5-HT3 receptor antagonists granisetron and BRL 46470A, but not other receptor antagonists. In addition to inhibiting NMDA-evoked responses, SR 57227A also depressed significantly pharmacologically isolated, NMDA receptor-mediated, monosynaptic excitatory postsynaptic currents (EPSCs) elicited by electrical stimulation of the forceps minor; this inhibitory action was blocked by BRL 46470A but not other 5-HT receptor antagonists. Perfusion of Ca2+-free or low Ca2+ plus Cd2+ artificial cerebrospinal fluid prevented electrical stimulation-induced EPSCs, but did not affect the inhibitory action of 5-HT and SR 57227A. In conclusion, we demonstrate for the first time that 5-HT and SR 57227A interact with 5-HT3-like receptors to produce a direct inhibitory action on NMDA receptor-mediated response in pyramidal cells of the mPFC.

Behavioral effects of 5-HT2C receptor antagonism in the substantia nigra zona reticulata of the 6-hydroxydopamine-lesioned rat model of Parkinson's disease

Overactivity of the substantia nigra zona reticulata and the medial segment of the globus pallidus are responsible for the generation of symptoms in Parkinson's disease. Reducing the activity of these regions has been shown to be a viable alternative to dopamine replacement in the symptomatic treatment of Parkinson's disease. 5-HT2C receptors in the substantia nigra zona reticulata are excitatory. In this study we have shown that intracerebral infusion of the selective 5-HT2C receptor antagonist SB 206553 (50 nmol) into the substantia nigra zona reticulata has an antiparkinsonian action in the 6-hydroxydopamine-lesioned rat model of Parkinson's disease. SB 206553 did not affect locomotion when injected into the nonparkinsonian substantia nigra. Furthermore, we have demonstrated that systemic administration of selective 5-HT2C receptor antagonists SB 200646A (20 mg/kg) and SB 206553 (20 mg/kg) can potentiate the antiparkinsonian action of the dopamine D2 receptor agonist quinpirole in the 6-hydroxydopamine-lesioned rat. Hence, 5-HT2C receptor antagonists may be useful adjuncts to dopamine agonists in the treatment of Parkinson's disease.

Affinity of mosapride citrate, a new gastroprokinetic agent, for 5-HT4 receptors in guinea pig ileum

We examined the binding affinity of mosapride citrate (mosapride) (4-amino-5-chloro-2-ethoxy-N-[[4-(4-fluorobenzyl)-2-morpholinyl]me thyl] benzamide citrate), a novel gastroprokinetic agent, for the 5-hydroxytryptamine (5-HT) 4 receptors in guinea pig ileum using a selective 5-HT4-receptor radioligand, [3H]GR113808. In membrane preparations from longitudinal muscle with myenteric plexus in guinea pig ileum, specific [3H]GR113808 binding revealed a single saturable site of high affinity (Kd=0.28 +/-0.02 nM, Bmax = 45+/- 3 fmol/mg protein). Mosapride and other 5-HT4-receptor agonists inhibited the specific binding of [3H]GR113808 in guinea pig ileum. The 5-HT4 agonists examined displayed the following inhibition potency order: BIMU-8 > cisapride > mosapride > renzapride > 5-HT > zacopride > metoclopramide. Mosapride exhibited monophasic inhibition of the specific [3H]GR113808 binding in the ileum (Ki value: 84.2 nM). The presence of mosapride (30 nM) significantly increased the Kd value to 0.44+/-0.05 nM in the Scatchard analysis of [3H]GR113808 binding. Bmax of [3H]GR113808, however, was not affected (48 +/-4 fmol/mg protein) by mosapride. As for the affinity of mosapride, the addition of GppNHp (100microM) slightly increased the Ki value to 104 nM. These results indicate that mosapride has an affinity for 5-HT4 receptors in guinea pig ileum in the radioligand binding study.

[11C]S21007, a putative partial agonist for 5-HT3 receptors PET studies. Rat and primate in vivo biological evaluation

We recently labeled with carbon-11, a high affinity, selective, 5-HT3 receptor (5-HT3R) ligand, S21007, for potential positron emission tomography (PET) applications. To evaluate the in vivo binding properties of [11C]S21007, its brain regional distribution, tissue and plasma pharmacokinetics and plasma metabolisation were characterized. To circumvent the problem of highly discrete brain localization of the 5-HT3R (area postrema, hippocampus), we designed an original approach combining high-resolution imaging techniques (ex vivo phosphor plate autoradiography and MRI-guided coronal PET in the rat and baboon, respectively). After i.v. injection of trace amounts of [11C]S21007 to rats, phosphorimager autoradiography failed to reveal in vivo specific binding to, nor selectivity for 5-HT3R-rich areas. PET studies in the baboon showed consistent results, i.e., there was no selective accumulation of [11C]S21007 in the area postrema or hippocampus, and neither displacement nor presaturation with cold S21007 resulted in significant changes in tissue distribution or kinetics of [11C]S21007.

Characterization of desamino-5-[125I]iodo-3-methoxy-zacopride ([125I]MIZAC) binding to 5-HT3 receptors in the rat brain

1. Antagonists at 5-HT3 receptors have shown activity in animal models of mental illness, however, few radiolabeled 5-HT3 ligands are available for preclinical studies. MIZAC, an analogue of the selective 5-HT3 antagonist, zacopride, binds with high affinity (1.3-1.5 nM) to CNS 5-HT3 sites. The authors report here the selectivity of MIZAC for these sites in rat brain homogenates. 2. Ninety-seven percent of total specific binding of [125I]MIZAC (0.1 nM) of was displaced by bemesetron (3 microM), a selective 5-HT3 antagonist. Competition studies using ligands with known affinities for 5-HT3 sites give a high correlation with reported pKi values (r2 0.98). Bemesetron displaceable binding has a regional distribution consistent with that of the 5-HT3 receptor, i.e. highest in cortex and hippocampus, and lowest in striatum and cerebellum. 3. Potent antagonists present at concentrations sufficient to occupy 95% of other 5-HT receptor populations (1A, 1B, 1D, 2A, 2B, 2C, 5A, 5B, 6, and 7) showed minimal ability to displace [125I]MIZAC binding (3 nM). Specificity studies using radioligand binding assays selective for 5-HT4, 5-HT6, and 5-HT7 receptors, and for binding sites of other neurotransmitters indicate a high degree of selectivity of [125I]MIZAC for the 5-HT3 receptor. 4. [125I]MIZAC binds to an apparent low affinity (benzac) site having a unique pharmacology. Low affinity binding was displaceable by benztropine, but not by other muscarinic agents nor inhibitors of dopamine uptake. The regional distribution of the low affinity site differed markedly from that of the high affinity site. The apparent affinity of [125I]MIZAC for the benzac site is two orders of magnitude lower than for the 5-HT3 receptor. Given its high selectivity for 5-HT3 binding sites, [125I]MIZAC appears to be a promising ligand for labeling 5-HT3 receptors in vitro and in vivo.

Synthesis and 5-HT-3 receptor binding of 2- and 3-(halo)alkoxyl substituted (S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chlorobenzamides as potential radioligands

In an effort to develop selective, high-affinity radioligands for the 5-HT-3 receptor, a series of homologues of 5-chloro-2,3-dimethoxy-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide (2b) was prepared in which individual methoxy groups were replaced by ethoxyl, (2-fluoroethoxyl), allyloxyl, propargyloxyl, or (3-iodoallyl)oxyl groups. Affinities for the 5-HT-3 receptor were determined by displacement of the binding of [125I]MIZAC (2a), a selective 5-HT-3 receptor antagonist radioligand, in rat brain homogenates. The 3-substituted homologues were more potent than the lead compound, 2b. The homologue having the largest 3-substituent, i.e., E-(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-3-(3-iodo-2-propenyl)oxy- 2-methoxybenzamide (3b, THIZAC), had one of the highest affinities, Ki 0.08 nM. The 2-substituted homologues were equipotent with 2b, having Ki 0.2-0.3 nM, regardless of the size of the substituent. The corresponding iodoallyl derivative, E-(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-2-(3-iodo-2-propenyl)oxy- 3-methoxybenzamide (4, LIZAC), displayed a Ki of 0.29 nM. Saturation binding of [125I]-4 gave a KD of 0.31 +/- 0.04 nM and a Bmax of 2.36 +/- 0.10 fmol/mg of entorhinal cortex. In vivo biodistribution of [125I]-4 in the rat brain showed increased accumulation in hippocampus relative to that in cerebellum. Both the high-affinity ligands [125I]-3b and [125I]-4 are potentially useful radioligands for studying the 5-HT-3 receptor.

Involvement of 5-hydroxytryptamine7 receptors in inhibition of porcine myometrial contractility by 5-hydroxytryptamine

1 5-Hydroxytryptamine (5-HT; 1 nM - 100 microM) concentration-dependently inhibited the amplitude and frequency of spontaneous contractions in longitudinal and circular muscles of the porcine myometrium. The circular muscle (EC50; 68-84 nM) was more sensitive than the longitudinal muscle (EC50; 1.3-1.44 microM) to 5-HT. To characterize the 5-HT receptor subtype responsible for inhibition of myometrial contractility, the effects of 5-HT receptor agonists on spontaneous contractions and of 5-HT receptor antagonists on inhibition by 5-HT were examined in circular muscle preparations. 2 Pretreatment with tetrodotoxin (1 microM), propranolol (1 microM), atropine (1 microM), guanethidine (10 microM) or L-NAME (100 microM) failed to change the inhibition by 5-HT, indicating that the inhibition was due to a direct action of 5-HT on the smooth muscle cells. 3 5-CT, 5-MeOT and 8-OH-DPAT mimicked the inhibitory response of 5-HT, and the rank order of the potency was 5-CT>5-HT>5-MeOT>8-OH-DPAT. On the other hand, oxymethazoline, alpha-methyl-5-HT, 2-methyl-5-HT, cisapride, BIMU-1, BIMU-8, ergotamine and dihydroergotamine had almost no effect on spontaneous contractions, even at 10-100 microM. 4 Inhibition by 5-HT was not decreased by either pindolol (1 microM), ketanserin (1 microM), tropisetron (10 microM), MDL72222 (1 microM) or GR113808 (10 microM), but was antagonized by the following compounds in a competitive manner (with pA2 values in parentheses): methiothepin (8.05), methysergide (7.92), metergoline (7.4), mianserin (7.08), clozapine (7.06) and spiperone (6.86). 5 Ro 20-1724 (20 microM) and rolipram (10 microM) significantly enhanced the inhibitory response of 5-HT, but neither zaprinast (10 microM) nor dipyridamole (10 microM) altered the response of 5-HT. 6 5-HT (1 nM - 1 microM) caused a concentration-dependent accumulation of intracellular cyclic AMP in the circular muscle. 7 From the present results, the 5-HT receptor, which is functionally correlated with the 5-HT7 receptor, mediates the inhibitory effect of 5-HT on porcine myometrial contractility. This inhibitory response is probably due to an increase in intracellular cyclic AMP through the activation of adenylate cyclase that is positively coupled to 5-HT7 receptors.

A serotonin-4 receptor-like pseudogene in humans

During a search for new G-protein-linked receptors for dopamine and serotonin, we found a serotonin-4 receptor-like pseudogene. This receptor-like pseudogene is intronless, contains an in-frame stop codon following transmembrane-3, and has two one-nucleotide insertions between transmembrane-5 and -6 regions which alter the reading frame. The predicted amino acid sequence of the human pseudogene is about 35% identical with that of the rat serotonin-4 receptor.

The influence of 5-HT2 and 5-HT4 receptor antagonists to modify drug induced disinhibitory effects in the mouse light/dark test

1. The ability of 5-HT2 and 5-HT4 receptor antagonists to modify the disinhibitory profile of diazepam and other agents was investigated in male BKW mice in the light/dark test box. 2. The 5-HT2A/2B/2C receptor antagonists ritanserin, MDL11939 and RP62203 and also methysergide, which failed to modify mouse behaviour when administered alone, caused dose-related enhancements (4 to 8 fold) in the potency of diazepam to disinhibit behavioural responding to the aversive situation of the test box. 3. Ritanserin was shown to enhance the disinhibitory potency of other benzodiazepines, chlordiazepoxide (4 fold), temazepam (10 fold) and lorazepam (10 fold), the 5-HT1A receptor ligands, 8-OH-DPAT (25 fold), buspirone (100 fold) and lesopitron (500 fold), the 5-HT3 receptor antagonists, ondansetron (100 fold) R(+)-zacopride (100 fold) and S(-)-zacopride (greater than a 1000 fold), the substituted benzamides, sulpiride (10 fold) and tiapride (5 to 10 fold) and the cholecystokinin (CCK)A receptor antagonist, devazepide (100 fold). It also reduced the onset of action of disinhibition following treatment with the 5-HT synthesis inhibitor parachlorophenylalanine. Ritanserin failed to enhance the disinhibitory effects of the CCKB receptor antagonist CI-988, the angiotensin AT1 receptor antagonist losarten or the angiotensin converting enzyme inhibitor ceranapril. 4. The 5-HT4 receptor antagonists SDZ205-557, GR113808 and SB204070 caused dose-related reductions in the disinhibitory effect of diazepam, returning values to those shown in vehicle treated controls. The antagonists failed to modify mouse behaviour when administered alone. 5. GR113808 was also shown to cause a dose-related antagonism of the disinhibitory effects of chlordiazepoxide, lorazepam, 8-OH-DPAT, buspirone, lesopitron, ondansetron, R(+)-zacopride, sulpiride, tiapride, devazepide, CI-988, losarten, ceranapril and parachlorophenylalanine. 6. It was concluded that in BKW mice (a) the failure of 5-HT2 and 5-HT4 receptor antagonists when administered alone to modify behaviour in the light/dark test indicates an absence of an endogenous 5-HT tone at the 5-HT2 and 5-HT4 receptors and (b) the enhancement by the 5-HT2 receptor antagonists and attenuation by the 5-HT4 receptor antagonists of drug-induced disinhibition indicates a plurality of 5-HT receptor involvement in the mediation of drug-induced disinhibitory profiles in the mouse.

Comparative receptor mapping of serotoninergic 5-HT3 and 5-HT4 binding sites

The clinical use of currently available drugs acting at the 5-HT4 receptor has been hampered by their lack of selectivity over 5-HT3 binding sites. For this reason, there is considerable interest in the medicinal chemistry of these serotonin receptor subtypes, and significant effort has been made towards the discovery of potent and selective ligands. Computer-aided conformational analysis was used to characterize serotoninergic 5-HT3 and 5-HT4 receptor recognition. On the basis of the generally accepted model of the 5-HT3 antagonist pharmacophore, we have performed a receptor mapping of this receptor binding site, following the active analog approach (AAA) defined by Marshall. The receptor excluded volume was calculated as the union of the van der Waals density maps of nine active ligands (pKi > or = 8.9), superimposed in pharmacophoric conformations. Six inactive analogs (pKi < 7.0) were subsequently used to define the essential volume, which in its turn can be used to define the regions of steric intolerance of the 5-HT3 receptor. Five active ligands (pKi > or = 9.3) at 5-HT4 receptors were used to construct an antagonist pharmacophore for this receptor, and to determine its excluded volume by superimposition of pharmacophoric conformations. The volume defined by the superimposition of five inactive 5-HT4 receptor analogs that possess the pharmacophoric elements (pKi < or = 6.6) did not exceed the excluded volume calculated for this receptor. In this case, the inactivity may be due to the lack of positive interaction of the amino moiety with a hypothetical hydrophobic pocket, which would interact with the voluminous substituents of the basic nitrogen of active ligands. The difference between the excluded volumes of both receptors has confirmed that the main difference is indeed in the basic moiety. Thus, the 5-HT3 receptor can only accommodate small substituents in the position of the nitrogen atom, whereas the 5-HT4 receptor requires more voluminous groups. Also, the basic nitrogen is located at ca. 8.0 A from the aromatic moiety in the 5-HT4 antagonist pharmacophore, whereas this distance is ca. 7.5 A in the 5-HT3 antagonist model. The comparative mapping of both serotoninergic receptors has allowed us to confirm the three-component pharmacophore accepted for the 5-HT3 receptor, as well as to propose a steric model for the 5-HT4 receptor binding site. This study offers structural insights to aid the design of new selective ligands, and the resulting models have received some support from the synthesis of two new active and selective ligands: 24 (Ki(5-HT3) = 3.7 nM; Ki(5-HT4) > 1000 nM) and 25 (Ki(5-HT4) = 13.7 nM; Ki(5-HT3) > 10,000 nM).

Firing frequency modulation of substantia nigra reticulata neurons by 5-hydroxytryptamine

Unitary extracellular recordings were made in in vitro rat brain slices to explore the effects of serotoninergic analogues on the spontaneous activity of substantia nigra reticulata (SNr) neurons. Most SNr neurons exhibited regular spontaneous firing (23.4 +/- 8.9 Hz, mean +/- S.E.M., n = 30) similar to that found in vivo. The most reproducible effect of serotonin (5-HT) was an increase in firing frequency found in 53% of the cells. The effect was concentration dependent and blocked by the 5-HT1/2 antagonist methysergide (1-10 microM) but unaffected by the 5-HT4- and 5-HT1-preferring antagonists DAU 6285 (5 microM) and metiothepin (5 microM), respectively. However, 5-HT also decreased the firing frequency in several neurons. In 19% of the neurons an inhibition was found alone but a biphasic response (inhibition and excitation) was found in another 28% of the neurons. Interestingly, the effect of the 5-HT-uptake inhibitor, duloxetine (100-400 nM), was frequency inhibition. Agonists that mimicked the 5-HT-induced inhibition were of the 5-HT1B-class (25 microM CP 93129 and 25 microM TFMPP). Neither the 5-HT2-antagonist ritanserin (5 microM) nor the GABA(A)-antagonist, bicuculline (30 microM) were able to block the inhibition suggesting that some SNr neurons may be directly inhibited by 5-HT.

Phenylimidazolidin-2-one derivatives as selective 5-HT3 receptor antagonists and refinement of the pharmacophore model for 5-HT3 receptor binding

A possible bioisosterism between the benzamido and the phenylimidazolidin-2-one moieties has been suggested on the basis of the similarity between the molecular electrostatic potential (MEP) of metoclopramide, a D2 receptor antagonist with weak 5-HT3 receptor antagonist properties, and zetidoline, a D2 receptor antagonist. Starting from this premise, a series of phenylimidazolidin-2-one derivatives bearing a basic azabicycloalkyl or an imidazolylalkyl moiety were synthesized and evaluated for 5-HT3 receptor radioligand binding affinity ([3H]-GR 43,694). In vitro 5-HT3 receptor antagonist activity was tested in the guinea pig ileum assay (GPI). A number of high-affinity ligands were shown to be potent 5-HT3 receptor antagonists in vivo as determined by inhibition of the Bezold--Jarisch reflex in the anesthetized rat. In general, the imidazolylalkyl derivatives were found to be more active than azabicycloalkyls. 1-(3,5-Dichlorophenyl)-3-[(5-methyl-1H-imidazol-4-yl)methyl]imidazoli din-2-one (58), in particular, displayed very high affinity for the 5-HT3 receptor (Ki of 0.038 nM) with a Kb of 5.62 nM in the GPI assay, being more potent than the reference compounds (ondansetron, tropisetron, granisetron, and BRL 46,470) tested. 58 showed an ID50 comparable to that of ondansetron (2.2 micrograms/kg i.v.) in the Bezold--Jarisch reflex. A molecular modeling study based on this structurally novel series of compounds allowed the refinement of previously reported 5-HT3 receptor antagonist pharmacophore models.

Formalin-induced nociceptive behavior and edema: involvement of multiple peripheral 5-hydroxytryptamine receptor subtypes

The role of 5-hydroxytryptamine and its receptor subtypes in the development of acute inflammation was investigated using the rat paw formalin test as a model for pain (measured by flinching behavior) and edema formation (measured by plethysmometry). The role of endogenously released 5-hydroxytryptamine was assessed using 5-hydroxytryptamine receptor subtype-selective antagonists co-injected with 2.5% formalin, while the receptor subtypes involved in the inflammatory process were further defined by co-injection of 5-hydroxytryptamine or 5-hydroxytryptamine receptor subtype-selective agonists with 0.5% formalin in anticipation of an augmented response. When co-administered with 2.5% formalin, propranolol, tropisetron or GR113808A, but not ketanserin, effectively blocked nociceptive behavior. In the presence of 0.5% formalin, 5-carboxamidotryptamine, 1-(m-chlorophenyl) biguanide or 5-methoxytryptamine, but not (+/-)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane, augmented the flinching response. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors in peripheral nociception. There may be some dissociation of nociception and edema formation, since no single 5-hydroxytryptamine receptor antagonist inhibited edema formation with 2.5% formalin; however, with 0.5% formalin, edema formation was enhanced by co-administration of 5-hydroxytryptamine, 5-carboxamidotryptamine, (+/-)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane or 5-methoxytryptamine, but not 1-(m-chlorophenyl) biguanide. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine2 and possibly 5-hydroxytryptamine4 receptors in edema formation. These results confirm the involvement of 5-hydroxytryptamine1 and 5-hydroxytryptamine3 receptor subtypes in peripheral nociception associated with acute inflammation and further suggest an involvement of the more recently characterized 5-hydroxytryptamine4 receptor in this process. There appears to be a dissociation in 5-hydroxytryptamine receptors involved in peripheral nociception and edema formation.

Arylcarbamate derivatives of 1-piperidineethanol as potent ligands for 5-HT4 receptors

A series of carbamate derivatives (7) of 2-(1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoates, which have been described as potent agonists and antagonists of 5-HT4 receptors, were synthesized. They were evaluated using radioligand binding assays with [3H]GR 113808, a 5-HT4 receptor selective ligand, in the rat striatum and the electrically stimulated myenteric plexus longitudinal muscle of the guinea pig. In contrast to the previously described ester derivatives, a drop in the affinity for 5-HT4 receptors was observed and the compounds were inactive as agonists in the guinea pig ileum preparation. Unexpectedly, the ortho-substituted carbamates 8b,c (R' = H, RO = MeO or EtO, R" = H) had nanomolar affinity for 5-HT4 receptors (Ki = 8.9 +/- 0.5 and 2.6 +/- 0.4 nM, respectively). As reported previously, the cis- or trans-3,5-dimethyl substitution of piperidine (8n,o) was particularly favorable (Ki = 1.1 +/- 0.6 nM for both isomers). 8c is an antagonist equipotent to the 5-HT4 receptor antagonist SDZ 205-557 (1).

Effects of risperidone, clozapine and haloperidol on extracellular recordings of substantia nigra reticulata neurons of the rat brain

Risperidone has proven to be effective as an antipsychotic drug and has fewer extrapyramidal side-effects than classic neuroleptics. In addition to its dopamine D2 receptor antagonistic properties, this antipsychotic agent is a potent 5-HT2 receptor antagonist. The atypical antipsychotic, clozapine, also possesses both dopamine D2 and 5-HT2 receptor affinity next to affinities for other receptors. To gain an insight in the consequences for basal ganglia activity of treatment with these atypical neuroleptics vs. typical neuroleptics, the effects of cumulative doses of risperidone, clozapine and haloperidol on the firing rate of substantia nigra reticulata neurons were studied. Extracellular recordings were performed in chloralhydrate-anaesthetized male Wistar rats. Both risperidone (50-3200 micrograms/kg i.v.) and clozapine (100-6400 micrograms/kg i.v.) dose dependently decreased substantia nigra reticulata activity maximally to 70% of the basal activity. With both treatments, a dose of 800 micrograms/kg was significantly effective. In contrast, haloperidol (12.5-800 micrograms/kg i.v.) gradually induced a slight increase in substantia nigra reticulata activity, which was identical to the substantia nigra reticulata activity after saline treatment. Therefore, these results indicate that typical and atypical neuroleptics affect differentially the output of the basal ganglia in the substantia nigra reticulata. To evaluate the involvement of 5-HT2 receptors in the effect of risperidone, the 5-HT2 receptor agonist, quipazine (0.5 mg/kg i.p.), was administered 15 min preceding risperidone treatment. A 4-fold higher dose of risperidone was needed to significantly affect the substantia nigra reticulata firing rate. Thus, the 5-HT2 component of the effect of risperidone is, at least partly, responsible for the difference in effect on substantia nigra reticulata neurons in comparison to haloperidol.

BIMU1 increases associative memory in rats by activating 5-HT4 receptors

Olfactory association learning was used to investigate the involvement of 5-HT4 receptors in learning and long-term memory. The behavioral role of the 5-HT4 receptors was studied by using BIMU1 (3-ethyl-2,3-dihydro-N-[endo-8-methyl-8-azabicyclo(3.2.1)oct-3-yl]-2-oxo -1 H-benzimidazole-1-carboxamide, hydrochloride (Boehringer Ingelheim, Italy); a mixed 5-HT4 agonist/5-HT3 antagonist, and GR125487 (1-[2-[methyl sulphonyl)-amino]ethyl]-4-piperidinyl-methyl 5-fluro-2-methoxy-1H-indole-3- carboxylate; Glaxo Group Research, Hertfordshire, U.K.), a specific 5-HT4 antagonist. The intraperitoneal injections of BIMU1 at 1, 5, and 10 mg/kg were followed by an substantial improvement (> 15% in percentage of correct responses at the dose of 10 mg/kg) in associative memory. Difficulty rapidly reversing behavioral responses to previously learned association, 1 month later indicated that the BIMU1 effect at 10 mg/kg was not transient, but correlated to long-term memory. The effects of BIMU1 are most likely to be mediated by 5-HT4 receptors since they were blocked by GR125487 at 10 mg/kg. These data suggest that activation of 5-HT4 receptors may modulate cognitive processes like learning and memory.

Anxiolytic-like actions of the selective 5-HT4 receptor antagonists SB 204070A and SB 207266A in rats

The highly selective 5-HT4 receptor antagonists, SB 204070A (0.001-0.1 mg/kg s.c., 30 min pretest) and SB 207266A (0.01, 1 and 10 mg/kg p.o., 1 hr pre-test), increased time spent in social interaction without affecting locomotor activity, in a rat 15 min social interaction test under high light, unfamiliar conditions. At 1 and 10 mg/kg s.c., SB 204070A was no longer active. These results are consistent with the profile expected of anxiolytic treatments in this procedure. In a rat 5 min elevated x-maze test, SB 204070A (0.01 and 1 mg/kg s.c., 30 min pre-test) significantly increased the percentage of time spent on the open arms. SB 204070A (0.01 mg/kg s.c.) and SB 207266A (1 mg/kg p.o., 1 hr pre-test) also increased percentage entries to the open arms. Neither compound affected locomotion at any dose tested in the procedure. The effects of both compounds in this procedure are also consistent with anxiolysis. Neither SB 204070A (0.1 or 1 mg/kg s.c., 30 min pre-test) nor SB 207266A (0.1 or 1 mg/kg p.o., 1 hr pre-test) affected either unpunished or punished responding, in a rat Geller-Seifter conflict model of anxiety. The maximal efficacy of both SB 204070A and SB 207266A in the rat social interaction test was similar to that of the benzodiazepine anxiolytic chlordiazepoxide (5 mg/kg s.c. or p.o.) used as a positive control, but was considerably less in the elevated x-maze procedure. The results suggest that 5-HT4 receptor antagonists may have modest anxiolytic-like actions in rats.

Modulation of IH by 5-HT in neonatal rat motoneurones in vitro: mediation through a phosphorylation independent action of cAMP

The depolarization of adult and neonatal rat facial and spinal motoneurones by 5-hydroxytryptamine (5-HT) in part involves an enhancement of the hyperpolarization-activated, inward-rectifier, IH. Under experimental conditions which promote this action, 5-HT evokes an inward current which can be mimicked by intracellularly applied adenosine 3',5'-cyclic monophosphate (cAMP) and potentiated by the cAMP-specific phosphodiesterase inhibitor Ro 20-1724. In this study, we show that this action of 5-HT can be blocked by the adenylyl cyclase inhibitors 2'3'-dideoxyadenosine (2',3'-DDA). 5'-adenylimidodiphosphate (AMP-PNP) and SQ-22536 (9-(tetrahydro-2-furyl)adenine), but not by external or internal application of the protein kinase inhibitors H-7, staurosporine and chelerythrine. The most recently cloned 5-HT receptor subtypes, 5-HT4, 5-HT6 and 5-HT7, can all stimulate adenylyl cyclase when activated. In the presence of internal GTP-gamma-S, 5-HT irreversibly enhanced IH. The 5-HT-induced inward current could be reversibly blocked by methysergide, but not by the 5-HT4 receptor antagonist GR-113808A, the 5-HT6 and 5-HT7 antagonist clozapine and the 5-HT1A antagonist WAY-100365. 5-Methoxytryptamine (5-MeOT) and 5-carboxamidotryptamine (5-CT) mimicked the action of 5-HT with a rank order of potency of 5-HT = 5MeOT > 5-CT. Surprisingly, 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH DPAT), a 5-HT1A and 5-HT7 agonist was inactive on facial motoneurones unlike its reported agonist action on spinal motoneurones. It is proposed that cAMP produced by 5-HT-mediated stimulation of adenylyl cyclase acts in a phosphorylation-independent manner, possibly directly, on the IH channel. The 5-HT receptor subtype mediating this response cannot be correlated with any of the classified 5-HT receptor subtypes that stimulate adenylyl cyclase.

5-HT4 receptors improve social olfactory memory in the rat

Serotonin (5-HT) is involved in a large variety of physiological functions and it appears now that it could play a role in cognitive processes through the activation of 5-HT4 receptors. The present study was conducted to determine the effect of BIMU1, a mixed 5-HT4 agonist/5-HT3 antagonist on social olfactory recognition in rats, a behaviour test which has previously been shown to access short-term memory and to be sensitive to cholinergic drugs. This test is based on the investigation of an unfamiliar juvenile by an adult rat during two distinct 5-min presentations. At a 30-min delay after each presentation adults recognized the juvenile, whereas after a 2-hr delay all the adults had forgotten it. When administered intraperitoneally immediately after the first presentation, BIMU1 (10 mg/kg) enhanced short-term memory (i.e. recognition of the juvenile after a 2-hr delay). Ondansetron (10 and 100 micrograms/kg injected intraperitoneally), a 5-HT3 antagonist, had no significant effect on this form of memory. The effect of BIMU1 was antagonized by intraperitoneal injection of GR 125487, a very selective and potent 5-HT4 antagonist. The antagonistic effect was obtained at 1 and 10 mg/kg of GR 125487, but not at 0.1 mg/kg. It is certainly a specific effect on brain 5-HT4 receptors, since we determined a brain concentration of GR 125487 equal to 3.8 x 10(-7) M after the intraperitoneal injection of 10 mg/kg of this drug. This GR 125487 concentration is certainly sufficient to occupy all the 5-HT4 brain receptors (Kd = 10(-10) M) but not to occupy 5-HT3 receptors (Kd > 10(-6) M). The 5-HT4 specificity of the blockade by GR 125487 is further demonstrated by the fact that a 10-fold lower dose of GR 125487 (1 mg/kg) is also effective to inhibit the BIMU1 effect.

New 2-piperazinylbenzimidazole derivatives as 5-HT3 antagonists. Synthesis and pharmacological evaluation

A series of 2-piperazinylbenzimidazole derivatives were prepared and evaluated as 5-HT3 receptor antagonists. Their 5-HT3 receptor affinities were evaluated by radioligand binding assays, and their abilities to inhibit the 5-HT-induced Bezold-Jarisch reflex in anesthetized rats were determined. Compound 7e (lerisetron, pKi = 9.2) exhibited higher affinity for the 5-HT3 receptor than did tropisetron and granisetron, while compound 7q (pKi = 7.5) had very low affinity for this receptor, showing that substitution on the N1 atom of the benzimidazole ring is essential for affinity and activity. The effect of substitution on the aromatic ring of benzimidazole by several substituents in different positions is also discussed. A strong correlation between the 5-HT3 antagonistic activity of the studied compounds and the position of substitution on the aromatic ring was established. Thus, while the 4-methoxy derivative 7m showed weak affinity for the 5-HT3 receptor (pKi = 6.7), the 7-methoxy derivative 7n exhibited the highest affinity (pKi = 9.4). Compounds 7e and 7n are now under further investigation as drugs for the treatment of nausea and emesis evoked by cancer chemotherapy and radiation.

New esters of 4-amino-5-chloro-2-methoxybenzoic acid as potent agonists and antagonists for 5-HT4 receptors

A number of benzoates derived from 4-amino-5-chloro-2-methoxybenzoic acid and substituted 1-piperidineethanol were synthesized and found to be potent 5-HT4 receptor agonists in the electrically-stimulated myenteric plexus and longitudinal muscle of the guinea pig ileum and the rat esophagus muscle. Monosubstitution of the piperidine ring with Me, OH, NH-Ac, or CONH2 groups gave compounds equipotent to 7a (ML 10302), a 5-HT4 receptor agonist previously reported to have nanomolar affinity. 7a,k were as potent as serotonin (5-HT) but had maximal responses which were only 60-80% of that of 5-HT, suggesting a partial agonist profile for these compounds. Binding assays were performed with [3H]GR 113808 in the rat striatum, and several of these compounds were found to have nanomolar affinity for 5-HT4 receptors (7a, Ki = 1.07 +/- 0.5 nM; 7k, Ki = 1.0 +/- 0.3 nM). The introduction of two methyl groups on the piperidine ring brought about a dramatic change in the pharmacological profile of 2-[(cis- and trans-3,5-dimethylpiperidinyl)ethyl]-4-amino-5-chloro-2- methoxybenzoate, 7g,h. 7g (Ki = 0.26 +/- 0.06 nM) inhibited the relaxant action of 5-HT in the rat esophagus muscle with a pA2 value of 8.6. The advantage of the ester function was demonstrated by comparing the activity of several such compounds at 5-HT4 receptors with those of the corresponding amidic derivatives. This difference was less marked when the basic moiety was sterically constrained as in the quinuclidine and tropane moieties. Structural analyses of 7a,g were performed by determining their X-ray crystal structures and by molecular modeling (SYBYL). A relatively limited number of minimum energy conformers was found for both compounds. They were characterized by the cis folded conformation of the ethyl chain and by the orientation of the lone pair of the nitrogen atom pointing out of the molecule as seen in conformationally-constrained benzamides such as zacopride and renzapride. A hypothetical model for the 5-HT4 receptor with two sites for the binding of agonist and antagonist molecules was proposed.

Differential actions of serotonin, mediated by 5-HT1B and 5-HT2C receptors, on GABA-mediated synaptic input to rat substantia nigra pars reticulata neurons in vitro

The ability of serotonin to modulate GABA-mediated synaptic input to substantia nigra pars reticulata (SNr) neurons was investigated with the use of whole-cell patch-clamp recording from slices of rat midbrain. Fast evoked GABA(A) receptor-mediated synaptic currents (IPSCs) were attenuated reversibly approximately 60% by serotonin, which also caused an inward current with reversal potential of -25 mV. This inward current was blocked by the 5-HT2 receptor antagonist ritanserin, whereas the IPSC depression was blocked by the 5-HT1B receptor antagonist pindolol. The amplitude ratio of IPSC pairs (50 msec interpulse interval) was enhanced by serotonin (in ritanserin) and also by the GABA(B) receptor agonist baclofen (which also depressed the IPSC), consistent with a presynaptic site of action in both cases. In contrast, spontaneous tetrodotoxin-sensitive GABA(A) synaptic currents (sIPSCs) were increased in frequency by serotonin (an action that was sensitive to ritanserin, but not pindolol) but reduced in frequency by baclofen. SNr neurons therefore receive inhibitory synaptic input mediated by GABA(A) receptors from at least two distinct sources. One, probably originating from the striatum, may be depressed via presynaptic 5-HT1B and GABA(B) receptors. The second is likely to arise from axon collaterals of SNr neurons themselves and is facilitated by an increase in firing via postsynaptic, somatodendritic 5-HT2C receptor activation, but it is depressed by GABA(B) receptor activation. Thus, serotonin can both depolarize and disinhibit SNr neurons via 5-HT2C and 5-HT1B receptors, respectively, but excitation may be limited by GABA released from axon collaterals.

VB20B7, a novel 5-HT-ergic agent with gastrokinetic activity. I. Interaction with 5-HT3 and 5-HT4 receptors

This study describes the in-vitro interaction of the gastrokinetic agent 2[1-(4-piperonyl)piperazinyl]benzothiazole (VB20B7) with the 5-hydroxytryptamine 5-HT3 and 5-HT4 receptor subtypes, using functional as well as radioligand binding studies. The benzamide derivative cisapride was used as a comparison. In radioligand binding assays VB20B7 showed, like cisapride, a weak affinity at 5-HT3 receptors from rat cerebral cortex. The new compound lacked any affinity at other 5-HT receptors or at dopaminergic D2 receptors, whereas cisapride showed high affinity for the 5-HT4 receptors from guinea-pig hippocampus and moderate affinity at dopaminergic D2 receptors. In the non-stimulated guinea-pig ileum, the concentration-response curves to the specific 5-HT3 agonist 2-Me-5-HT and to 5-HT were shifted to the right by VB20B7. In the rat oesophagus tunica muscularis mucosae preparation (TMM), VB20B7 was evaluated for its activity at 5-HT4 receptors. VB20B7 behaved as a 5-HT4 receptor agonist, inducing a concentration-dependent relaxation of the preparation precontracted with carbachol. In this preparation, VB20B7 and cisapride were able to stimulate adenylate cyclase activity, an effect probably mediated through activation of 5-HT4 receptors, as can be inferred from the blockade by the 5-HT4 antagonist, tropisetron, of the enhanced cAMP formation. However, consistent with the lack of affinity at central 5-HT4 receptors, VB20B7 did not stimulate cAMP formation in guinea-pig hippocampal slices. VB20B7 also caused an increase in the twitch response of the transmurally stimulated guinea-pig ileum, although at a concentration higher than cisapride. This effect was blocked by desensitization of the 5-HT4 receptor with 5-MeOT and also by the 5-HT4 receptor antagonist tropisetron. Both VB20B7 and cisapride increased the K(+)-evoked acetylcholine release in this preparation. The results show that VB20B7 possesses affinity for 5-HT4 receptors located in the rat TMM and guinea-pig ileum preparations, but is devoid of affinity at central 5-HT4 receptors. In addition, VB20B7 shows low to moderate affinity at both central and peripheral (enteric) 5-HT3 receptors. The interaction of VB20B7 with the peripheral 5-HT4 and 5-HT3 receptors may be relevant for the gastrokinetic effects of the new compound.

5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption in rabbit jejunum

The aim of the present study was to determine the 5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption across rabbit jejunum in vitro. A number of agonists and antagonists were used to characterize the receptors through which serotonin inhibits this absorption. The results show that 2.5x10(-6) M 5-HT inhibits the amino acid absorption by about 20%. The 5-HT receptor agonists, alpha-methyl-5-HT (5-HT2), 2-methyl-5-HT (5-HT3) and zacopride (5-HT4) at concentrations 2.5x10(-6) and 2.5x10(-5) M produced 10-30% inhibition on L-leucine intestinal absorption. 5-carboxyamidotryptamine (5-HT1) did not produce any inhibition. The 5-HT antagonists, GR 113808A (5-HT4) at 2.5x10(-6) M and ritanserin (5-HT2) and ondansetron (5-HT3) at 2.5x10(-5) M completely blocked the effect of 5-HT. However, methiothepin (5-HT1) did not produce any effect on serotonin action in the intestinal absorption of amino acid. It can be concluded that 5-HT2, 5-HT3 and 5-HT4 receptors could mediate inhibition of L-leucine absorption across rabbit jejunum.

Localization of 5-HT4 receptor mRNA in rat brain by in situ hybridization histochemistry

Oligonucleotide probes that recognize two cloned splice variants (5-HT4S and 5-HT4L) of 5-HT4 receptors were used to study by in situ hybridization the localization in rat brain of mRNA encoding these receptors. A probe common to both variants reveals high levels of transcripts in olfactory tubercle, some components of the basal ganglia (caudate putamen, ventral striatum), medial habenula and hippocampal formation. Similar patterns of distribution are obtained with probes that recognize each splice variant individually, suggesting that no dramatic differences exist in their respective regional distribution. Comparison of mRNA distribution with receptor distribution as visualized with [125I]SB 207710 indicates that 5-HT4 receptors are localized both somatodendritically in e.g. caudate putamen and on axon terminals in e.g. substantia nigra and globus pallidus.

Interaction of S 21007 with 5-HT3 receptors. In vitro and in vivo characterization

The interaction of S 21007 [5-(4-benzyl piperazin-1-yl)4H pyrrolo [1,2-a]thieno[3,2-e]pyrazine] with serotonin 5-HT3 receptors was investigated using biochemical, electrophysiological and functional assays. Binding studies using membranes from N1E-115 neuroblastoma cells showed that S 21007 is a selective high affinity (IC50 = 2.8 nM) 5-HT3 receptor ligand. As expected of an agonist, S 21007 stimulated the uptake of [14C]guanidinium (EC50 approximately 10 nM) in NG 108-15 cells exposed to substance P, and this effect could be prevented by the potent 5-HT3 receptor antagonist ondansetron. In addition, like 5-HT and other 5-HT3 receptor agonists (phenylbiguanide and 3-chloro-phenylbiguanide), S 21007 (EC50 = 27 microM) produced a rapid inward current in N1E-115 cells. The 5-HT3 receptor agonist action of S 21007 was also demonstrated in urethane-anaesthetized rats as this drug (120 micrograms/kg i.v.) triggered the Bezold-Jarisch reflex (rapid fall in heart rate), and this action could be prevented by pretreatment with the potent 5-HT3 receptor antagonist zacopride. Finally, in line with its 5-HT3 receptor agonist properties, S 21007 also triggered emesis in the ferret. Evidence for 5-HT3 receptor antagonist-like properties of S 21007 was also obtained in some of these experiments since previous exposure to this compound prevented both the 5-HT-induced current in N1E-115 cells and the Bezold-Jarisch reflex elicited by an i.v. bolus of 5-HT (30 micrograms/kg) in urethane-anaesthetized rats. These data suggest that S 21007 is a selective 5-HT3 receptor agonist which can exhibit antagonist-like properties either by triggering a long lasting receptor desensitization or by a partial agonist activity at 5-HT3 receptors in some tissues.

Lesion study of the distribution of serotonin 5-HT4 receptors in rat basal ganglia and hippocampus

The regional distribution of 5-hydroxytryptamine (5-HT4) receptors labelled with [3H]GR113808 was examined in rat basal ganglia and hippocampus after specific lesions. Lesion of serotonin neurons induced by injections of 5,7-dihydroxytryptamine into the dorsal and medial raphe nuclei resulted in increased 5-HT4 receptor binding in most regions examined, compared with controls. More precisely, there was a 78% increase in the rostral but no change in the caudal part of caudate-putamen, and 83% and 54% increases in the shell and core of the nucleus accumbens respectively. In the substantia nigra, the increase in 5-HT4 binding was larger (72%) than that in the globus pallidus (32%). In the hippocampus, 63%, 30% and 28% increases were measured in CA2, CA1 and CA3 respectively. Following lesion of dopamine neurons by intranigral injection of 6-hydroxydopamine, increased 5-HT4 receptor binding was observed in the caudal (59%), but not the rostral part of caudate-putamen, as well as in the globus pallidus (93%). Since no decreases in 5-HT4 receptor density were detected after the dopamine lesion, it was concluded that these receptors are not expressed in dopamine neurons. Kainic acid lesions of the caudate-putamen were associated with dramatic local decreases in 5-HT4 receptor binding on the injected side (-89%), which suggested that striatal neurons express 5-HT4 receptors. Corresponding decreases of 72 and 20% in receptor density were detected in globus pallidus and substantia nigra, consistent with a presumed localization of 5-HT4 receptors on striatal GABA neurons projecting to these regions. In the substantia nigra, the decrease in [3H]GR113808 binding was localized to the pars lateralis, indicating that striatal neurons belonging to the cortico-striato-nigro-tectal pathway, and containing GABA and dynorphin, express 5-HT4 receptors.

Differential actions of serotonin, mediated by 5-HT1B and 5-HT2C receptors, on GABA-mediated synaptic input to rat substantia nigra pars reticulata neurons in vitro

The ability of serotonin to modulate GABA-mediated synaptic input to substantia nigra pars reticulata (SNr) neurons was investigated with the use of whole-cell patch-clamp recording from slices of rat midbrain. Fast evoked GABA(A) receptor-mediated synaptic currents (IPSCs) were attenuated reversibly approximately 60% by serotonin, which also caused an inward current with reversal potential of -25 mV. This inward current was blocked by the 5-HT2 receptor antagonist ritanserin, whereas the IPSC depression was blocked by the 5-HT1B receptor antagonist pindolol. The amplitude ratio of IPSC pairs (50 msec interpulse interval) was enhanced by serotonin (in ritanserin) and also by the GABA(B) receptor agonist baclofen (which also depressed the IPSC), consistent with a presynaptic site of action in both cases. In contrast, spontaneous tetrodotoxin-sensitive GABA(A) synaptic currents (sIPSCs) were increased in frequency by serotonin (an action that was sensitive to ritanserin, but not pindolol) but reduced in frequency by baclofen. SNr neurons therefore receive inhibitory synaptic input mediated by GABA(A) receptors from at least two distinct sources. One, probably originating from the striatum, may be depressed via presynaptic 5-HT1B and GABA(B) receptors. The second is likely to arise from axon collaterals of SNr neurons themselves and is facilitated by an increase in firing via postsynaptic, somatodendritic 5-HT2C receptor activation, but it is depressed by GABA(B) receptor activation. Thus, serotonin can both depolarize and disinhibit SNr neurons via 5-HT2C and 5-HT1B receptors, respectively, but excitation may be limited by GABA released from axon collaterals.

Cloning, expression and pharmacology of the mouse 5-HT(4L) receptor

Since most of our knowledge on pharmacological properties of brain 5-HT4 receptors have been discussed for mouse colliculi neurons, we cloned the corresponding receptor using the RT-PCR approach. As expected, the homology with the already cloned rat 5-HT(4L) receptor was high, revealing only 16 differences at the amino-acid level. One of the differences, proline75 in mouse, alanine75 in the already published rat sequences was not confirmed. Therefore this proline is part of the consensus sequence present in all 5-HT receptor transmembrane domain II (LVMP). Comparing the affinities of 11 agonists and five antagonists for the cloned mouse receptor (5-HT(4L))expressed in LLCPK1 and the corresponding receptor in mouse colliculi shows an excellent correlation. The transfected mouse 5-HT(4L) receptor stimulated cAMP production. When expressed at high density, it exhibited intrinsic activity. In contrast to the previously described distribution, we found that mRNA encoding for both the short (5-HT(4S))and the long form (5-HT(4L)) of 5-HT4 receptors are expressed in all mouse and rat brain areas.

5-HT receptors in mammalian brain: receptor autoradiography and in situ hybridization studies of new ligands and newly identified receptors

In recent years the family of mammalian serotonin receptors has grown to 14 different subtypes, characterized by pharmacological or molecular biological techniques. In parallel, new ligand molecules have been developed for their study. However, selective ligands are not yet available to study every one of them. In addition the degree of selectivity of ligands, hitherto regarded as specific for a particular receptor subtype has been called in question by their affinities for newly discovered receptors. Consequently, a re-evaluation of past ligand receptor autoradiography work is necessary in view of the redefined receptor profiles of these ligands, and the introduction of newly developed ligands. A further difficulty for the characterization of these receptors is the absence of selective antagonist ligands which, for some of the subtypes, have become available only recently. In an attempt to overcome these difficulties we have combined in situ hybridization histochemistry and receptor ligand autoradiography to study the regional and cellular localization of several serotonin receptors in the rodent brain. In addition, for some receptors, we have expanded these studies to primates, including humans. We have found that the distribution of 5-HT1A receptors in monkey brain, labelled with the agonist 3H-8-OH-DPAT and the antagonist 3H-WAY 100635 was very similar at the levels examined, and corresponded well with that observed for the cells containing mRNA coding for this receptor, confirming the somatodendritic localization of 5-HT1A receptors in monkey brain. The labelling conditions to visualize 5-HT1F receptors in guinea pig brain, namely 3H-sumatriptan in the presence of 10(-8) M 5-CT to block 5-HT1D receptors, are suitable for visualizing this receptor, since the results agreed with those observed by in situ hybridization. By using 3H-ketanserin and 3H-mesulergine in parallel with in situ hybridization using the corresponding oligonucleotides, we were able to show that these ligands label respectively 5-HT2A and 5-HT2C binding sites in monkey brain. 5-HT4 receptors were localized in the brain of several species including humans by using 125I-SB 207710. In situ hybridization experiments performed in guinea pig confirmed that 5-HT4 receptors are localized on the terminals of the striatopallidal and striatonigral projections. 5-HT7 binding sites were labelled in rat and guinea pig brains by incubating with 3H-5-CT in the presence of 100 microM WAY 100135 and 250 microM GR 127935; the distribution obtained in both species agreed, in general, with that of the corresponding mRNA coding for them. These results are an illustration of the understanding of our current knowledge of the chemical neuroanatomy of the mammalian 5-HT system.

Identification and characterization of the 5-HT4 receptor in the intestinal tract and striatum of the guinea pig

Receptors for 5-hydroxytryptamine (5-HT) of the 5-HT4 type were investigated in the intestinal tract and the striatum in guinea-pig, in binding studies using the 5-HT4 radioligand, [3H]GR113808. In the intestinal tract, specific binding was observed in preparations of the longitudinal muscle with the myenteric plexus (LMMPs) but not in the whole tissue. Saturable binding of [3H]GR113808 was demonstrated (striatum: Kd = 0.054 +/- 0.002 nM, Bmax = 90.25 +/- 10.44 fmol/mg protein, LMMPs of ileum: Kd = 0.077 +/- 0.016 nM, Bmax = 11.95 +/- 3.24 fmol/mg protein). Selective 5-HT4 receptor agonists and antagonists inhibited binding of [3H]GR113808 with high affinities in LMMPs of the ilcum and which correlated well with their actions on the striatum (r = 0.98), as indicated by the rank order of displacement potencies: SDZ205-557 > LY297524 > cisapride = BIMU8 > 5-HT > mosapride > renzapride > 5-hydroxy-N-methyltryptamine(5-HMT) > (+/-) zacopride > alpha-methyl-5-hydroxytryptamine (alpha-M-5-HT) > 5-methyltryptamine(5-MT) > 5-carboxamidotryptamine (5-CT). The number of binding sites of [3H]GR113808 in the intestine was significantly smaller than that in the brain. In the intestine, a larger number of binding sites was noted in the upper part of the intestine, the rank order being duodenum > jcjunum > ilcum > > colon > rectum. Such data are relevant regarding the potential use of the 5-HT4 receptor ligand to modify motility and secretion in the intestine.

The role of the 5-HT4 receptor in Cl- secretion in human jejunal mucosa

5-Hydroxytryptamine (5-HT) is a mediator of chloride ion (Cl-) secretion in the intestine which can be seen as a rise in short circuit current (Isc) in the Ussing chamber model. We investigated the 5-HT receptor mediating 5-HT-induced Cl- secretion in the human jejunum in vitro. Jejunal segments obtained from patients having gastric bypass surgery for obesity, were stripped of muscularis and mounted in Ussing chambers and short-circuited. The 5-HT receptor agonist-induced change (delta) in Isc was recorded in the presence and and absence of 5-HT receptor antagonists. The rank order of agonist potency was: 5-HT > 5-methoxytryptamine > renzapride (BRL 24924 > alpha-methyl-5-HT >> 2-methyl-5-HT. In the presence of Cl(-)-free media or 100 microM furosemide, 5-HT-induced delta Isc was significantly reduced. It was also antagonized by > or = 1 microM tropisetron (a 5-HT 3/5-HT4 receptor antagonist) and > or = 10 nM GR 113808 (a selective 5-HT4 receptor antagonist) with pA2 values of 6.5 and 7.9, respectively. Another 5-HT4 receptor antagonist, SC 53606 (0.1 microM), antagonized the 5-HT-induced response with a pA2 of 7.3 5-HT1-like/5-HT2 (methysergide), 5-HT1P [N-acetyl-5-hydroxytryptophyl 5-hydroxytryptophan amide (5-HT-DP], 5-HT2A (ketanserin) and 5-HT3 (ondansetron) receptor antagonists and tetrodotoxin, had no significant effect on the EC50 for 5-HT. In conclusion, this study demonstrates that in the human muscle-stripped jejunum in vitro, 5-HT induced change in short circuit current is mediated by a 5-HT4 receptor via a non-neural pathway.

Effects of 5-HT4 receptor antagonists on rat behaviour in the elevated plus-maze test

The anxiolytic-like effects of a variety of 5-HT receptor agonists and antagonists have been intensively studied in animal models. However, no direct effects of agents modulating 5-HT4 receptors have been reported, in spite of their suggestive location in the brain. The objective of the present study was the determination of the effects of two selective 5-HT4 receptor antagonists, SB 204070 [1-butyl-4-piperidinylmethyl)-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate] and GR 113808 [[1-[2-methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl 1-methyl-1 H-indole-3-carboxylate], in the elevated plus-maze test in rats. Results have shown that both 5-HT4 receptor antagonists exhibit an anxiolytic-like profile, although only at the dose of 1.0 mg/kg (s.c.). At this dose, both compounds significantly increased the percentage of time spent in open arms exploration, while other variables evaluated remained unaffected at the dose range tested. Results suggest that 5-HT4 receptor antagonists could have some anxiolytic-like properties, although their effects seem more limited and less consistent than those presented by classic anxiolytics, such as diazepam. However, they are similar to those exhibited by granisetron [endo-1-methyl-N-(9-methyl-9-azabicyclo[3.3.1]non-3-yl)-1 H-indazole-3-carboxamide], a 5-HT3 receptor antagonist.

Distribution of 5-HT4 receptor mRNA in the rat brain

We have analyzed the brain distribution of the rat 5-HT4 receptor mRNA. A receptor specific probe was used for in situ hybridization of rat brain sections. Abundant expression of the 5-HT4 receptor mRNA was observed in the olfactory system, striatum, medial habenula and the hippocampal formation, while faint or no specific signals could be detected in most other areas of the brain. Several brain areas which display strong ligand binding do not contain mRNA, suggesting an axonal localization of the 5-HT4 receptor.

Lack of binding of acetaminophen to 5-HT receptor or uptake sites (or eleven other binding/uptake assays)

The mechanism of analgesic action of acetaminophen (paracetamol) remains unknown. However, a central component distinct from that of the NSAIDs (non-steroidal antiinflammatory drugs) seems likely. A recent report (NeuroReport 6:1546-1548, 1995) suggests the involvement of 5-HT3 receptors. In the present study, we measured the affinity of acetaminophen at 5-HT3, as well as 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2, 5-HT2C, 5-HT4, 5-HT6, 5-HT7 and eleven other receptor sites and at serotonin and norepinephrine reuptake sites. At 10 microM, acetaminophen inhibited less than 10% specific radioligand binding at any site. These findings: (i) suggest that acetaminophen's effect on serotonergic pathways is indirect, and (ii) circumscribe acetaminophen's possible central analgesic mechanism(s).