SDZ 205-557, a selective, surmountable antagonist for 5-HT4 receptors in the isolated guinea pig ileum

The Multiplicity of Serotonin Receptors: Uselessly Diverse Molecules or an Embarrassment of Riches?

A large number of 5-HT receptors (>15) have been identified by molecular cloning technology over the past 10 years. This review briefly summarizes available information regarding the functional and therapeutic implications of serotonin receptor diversity for neurology and psychiatry. 5-HT receptors are divided into seven main families: 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6, and 5-HT7. Several families (e.g., 5-HT1 family) have many members (e.g., 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F), each of which is encoded by a distinct gene product. In addition to the genomic diversity of 5-HT receptors, splice variants and editing isoforms exist for many of the 5-HT receptors, making the family even more diverse. Evidence that is summarized in this review suggests that 5-HT receptors represent novel therapeutic targets for a number of neurologic and psychiatric diseases including migraine headaches, chronic pain conditions, schizophrenia, anxiety, depression, eating disorders, obsessive compulsive disorder, pervasive developmental disorders, and obesity-related conditions (Type II diabetes, hypertension, obesity syndromes). It is possible that sub-type-selective serotonergic agents may revolutionize the treatment for a number of medical, psychiatric, and neurological disorders.

5-HT4 receptors exert a frequency-related facilitatory control on dorsal raphé nucleus 5-HT neuronal activity

We investigated, using single-unit recordings in chloral hydrate-anaesthetized rats, the role of serotonin 4 (5-HT4) receptors in the control of dorsal raphé nucleus (DRN) 5-HT neuron activity. About one-half (36) of the 76 neurons recorded were affected by either the preferential 5-HT4 agonist cisapride (500 and 1000 micro g/kg, i.v.) or the selective 5-HT4 antagonist, GR 125487 (200- 2000 micro g/kg, i.v.). Responding neurons displayed a significantly higher mean basal firing rate (1.93 +/- 0.1 Hz) than non-responders (1.31 +/- 0.1 Hz). The firing rate of responding 5-HT neurons was enhanced dose-dependently by cisapride (+47 and +94% at 500 and 1000 micro g/kg, respectively), an effect abolished by GR 125487 (500 micro g/kg) and reduced by the 5-HT4 antagonist, SDZ 205557 (500 micro g/kg, i.v). Conversely, GR 125487 induced a dose-dependent inhibition of responders activity, which was almost completely suppressed at the dose of 2000 micro g/kg. In a separate set of experiments, the selective 5-HT4 agonist, prucalopride (500 micro g/kg, i.v), increased the firing activity (+35%) of 5-HT neurons displaying a high basal firing rate; subsequent injection of GR 125487 (500 micro g/kg, i.v.) suppressed this effect. These results indicate that 5-HT4 receptors exert both a tonic and a phasic, positive, frequency-related control on DRN 5-HT neuronal activity. The existence of such a control might open new avenues for therapeutic research in the antidepressant field.

5-Hydroxytryptamine mediated contractions in isolated preparations of equine ileum and pelvic flexure: pharmacological characterization of a new 5-HT(4) agonist

The effects of 5-hydroxytryptamine (5-HT), HTF 919, a new 5-HT(4) agonist, and the antagonists SB 203-186 (5-HT(4)) and tropisetron (5-HT(3)) on intestinal motility were tested in vitro on isolated preparations of horse ileum and pelvic flexure. Concentration-response curves were created by cumulative application of the agonists with or without preincubation of the antagonists. The 5-HT preparation induced a concentration-dependent contraction in equine ileum and pelvic flexure. The results indicate that 5-HT receptors are present in all parts of equine intestine investigated in this study. Tropisetron was found to act as a noncompetitive antagonist in all locations of the equine intestine. SB 203-106 was confirmed as an antagonist to 5-HT in the equine ileum circular muscle, in pelvic flexure circular and longitudinal muscle. Nevertheless, a discernible increase of smooth muscle contractions caused by HTF 919 could only be observed in pelvic flexure. In accordance with an earlier study in the guinea pig, in the equine gut HTF 919 acted as a partial agonist for the 5-HT(4) receptor with an affinity constant in the nanomolar range. It is concluded that 5-HT receptors, and especially their subtypes, may represent a promising target for the treatment and prevention of gastrointestinal (GI) motility disorders in horses.

The pharmacokinetics of the novel promotile drug, tegaserod, are similar in healthy subjects-male and female, elderly and young

BACKGROUND

Tegaserod (HTF 919) is a selective 5-HT4 receptor partial agonist in development for the treatment of irritable bowel syndrome.

AIM

This study aimed to assess the effect of age and gender on the single-dose pharmacokinetics of tegaserod.

METHODS

In a parallel-group, open-label study, a single dose of tegaserod (12 mg) was administered to four groups of healthy young male, young female, elderly male and elderly female subjects (n=10 per group). Blood samples were collected from 0 to 24 h postdose. Non-compartmental pharmacokinetics evaluation and statistical analysis (ANOVA and Wilcoxon signed ranks test for tmax) were performed.

RESULTS

Tegaserod was well tolerated in all groups. There was no effect of age or gender on tmax and Cmax. Gender did not affect AUC0-infinity and AUC0-tz; there was a statistically significant effect of age on these parameters. AUC0-infinity and AUC0-tz in the elderly were greater than in the young (AUC0-infinity ratio 1.37, P < 0.001; AUC0-tz ratio 1.23, P=0.029). This increase in exposure is judged not to be clinically relevant because it is within the variability in the pharmacokinetics parameters of tegaserod and because the dose-response relationship of tegaserod is relatively shallow.

CONCLUSIONS

No dose adjustment for age or gender is recommended in tegaserod therapy.

Roles of 5-HT receptors in the release and action of secretin on pancreatic secretion in rats

5-Hydroxytryptamine (serotonin, 5-HT) is a hormone and neurotransmitter regulating gastrointestinal functions. 5-HT receptors are widely distributed in gastrointestinal mucosa and the enteric nervous system. Duodenal acidification stimulates not only the release of both 5-HT and secretin but also pancreatic exocrine secretion. We investigated the effect of 5-HT receptor antagonists on the release of secretin and pancreatic secretion of water and bicarbonate induced by duodenal acidification in anesthetized rats. Both the 5-HT(2) receptor antagonist ketanserin and the 5-HT(3) receptor antagonist ondansetron at 1-100 microg/kg dose-dependently inhibited acid-induced increases in plasma secretin concentration and pancreatic exocrine secretion. Neither the 5-HT(1) receptor antagonists pindolol and 5-HTP-DP nor the 5-HT(4) receptor antagonist SDZ-205,557 affected acid-evoked release of secretin or pancreatic secretion. None of the 5-HT receptor antagonists affected basal pancreatic secretion or plasma secretin concentration. Ketanserin or ondansetron at 10 microg/kg or a combination of both suppressed the pancreatic secretion in response to intravenous secretin at 2.5 and 5 pmol x kg(-1) x h(-1) by 55-75%, but not at 10 pmol x kg(-1) x h(-1). Atropine (50 microg/kg) significantly attenuated the inhibitory effect of ketanserin on pancreatic secretion but not on the release of secretin. These observations suggest that 5-HT(2) and 5-HT(3) receptors mediate duodenal acidification-induced release of secretin and pancreatic secretion of fluid and bicarbonate. Also, regulation of pancreatic exocrine secretion through 5-HT(2) receptors may involve a cholinergic pathway in the rat.

Chronotropic action of 5-hydroxytryptamine (5-HT) on colonic migrating motor complexes (CMMCs) in the isolated mouse colon

The effects of 5-hydroxytryptamine (5-HT) and related drugs on colonic migrating motor complexes (CMMCs) were evaluated in isolated colons from the heterozygotes of pie-bald lethal mice. 5-HT produced a dose-related increase in the frequency of CMMCs without any change in the amplitude or duration of the CMMC contractions themselves. The 5-HT(2) agonist, alpha-methyl 5-HT, (100 nM-1 microM) increased the frequency of CMMCs whilst the 5-HT(3) agonist, 2-methyl 5-HT, did so at 10 microM. The 5-HT(4) agonist, 5-methoxy dimethyl tryptamine oxalate did not alter the frequency of CMMCs in the concentration range 1 nM-10 microM. The 5-HT(3) receptor antagonist, ondansetron, increased the interval between CMMCs in the concentration range 100 nM-1 microM, whilst the 5-HT(1) receptor antagonist, methiothepin, the 5-HT(2) receptor antagonist, cyproheptadine and the 5-HT(4) receptor antagonist, SDZ 205 557, had no significant effects on the interval between CMMCs in the concentration range 1 nM-10 microM. The effects of 5-HT did not appear to be altered by the presence of ondansetron (1 microM) or cyproheptadine (1 microM). However, in the presence of ondansetron (1 microM), the further addition of cyproheptadine (1 microM) effectively abolished CMMCs. Furthermore, in the combined presence of these antagonists the effects of 5-HT were severely diminished. It is suggested that the frequency of CMMCs may be under the influence of endogenously released 5-HT in this preparation

Facilitation of acetylcholine release by SK-951, a benzofuran derivative, via the 5-hydroxytryptamine4 receptor in guinea pig stomach

Facilitation of acetylcholine (ACh) release by 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-carboxami de hemifumarate), a benzofuran derivative, via the 5-hydroxytryptamine (5-HT)4 receptor in guinea pig stomach was examined by in vitro receptor autoradiography and functional studies. [125I]SB207710 binding was detected in the myenteric plexus of the gastric corpus. High densities of binding sites were observed in the myenteric plexus and a moderate density in the muscle layer. SK-951 inhibited the binding of [125I]SB207710, a specific 5-HT4-receptor ligand, as in the case of SB204070, a specific 5-HT4-receptor antagonist, thus indicating the presence of 5-HT4 receptors in guinea pig stomach. SK-951 as well as 5-HT enhanced the electrically stimulated twitch contractions of gastric corpus strips, which were sensitive to tetrodotoxin and atropine, and enhanced electrically stimulated release of ACh from corporal strips, which was tetrodotoxin-sensitive and Ca2+-dependent. The enhancements of twitch contractions and ACh release by SK-951 were antagonized by GR113808, a selective 5-HT4-receptor antagonist. Thus, SK-951 binds to 5-HT4 receptors of the guinea pig gastric corpus and may accelerate gastric motility due to facilitation of ACh release.

Benzimidazole derivatives. Part 1: Synthesis and structure-activity relationships of new benzimidazole-4-carboxamides and carboxylates as potent and selective 5-HT4 receptor antagonists

New benzimidazole-4-carboxamides 1-16 and -carboxylates 17-26 were synthesized and evaluated for binding affinity at serotonergic 5-HT4 and 5-HT3 receptors in the CNS. Most of the synthesized compounds exhibited moderate-to-very high affinity (in many cases subnanomolar) for the 5-HT4 binding site and no significant affinity for the 5-HT3 receptor. SAR observations and structural analyses (molecular modeling, INSIGHT II) indicated that the presence of a voluminous substituent in the basic nitrogen atom of the amino moiety and a distance of ca. 8.0 A from this nitrogen to the aromatic ring are of great importance for high affinity and selectivity for 5-HT4 receptors. These results confirm our recently proposed model for recognition by the 5-HT4 binding site. Amides 12-15 and esters 24 and 25 bound at central 5-HT4 sites with very high affinity (Ki = 0.11-2.9 nM) and excellent selectivity over serotonin 5-HT3, 5-HT2A, and 5-HT1A receptors (Ki > 1000-10,000 nM). Analogues 12 (Ki(5-HT4) = 0.32 nM), 13 (Ki(5-HT4) = 0.11 nM), 14 (Ki(5-HT4) = 0.29 nM) and 15 (Ki(5-HT4) = 0.54 nM) were pharmacologically characterized as selective 5-HT4 antagonists in the isolated guinea pig ileum (pA2 = 7.6, 7.9, 8.2 and 7.9, respectively), with a potency comparable to the 5-HT4 receptor antagonist RS 39604 (pA2 = 8.2). The benzimidazole-4-carboxylic acid derivatives described in this paper represent a novel class of potent and selective 5-HT4 receptor antagonists. In particular, compounds 12-15 could be interesting pharmacological tools for the understanding of the role of 5-HT4 receptors.

Effect of TKS159, a novel 5-hydroxytryptamine4 agonist, on gastric contractile activity in conscious dogs

A novel 5-hydroxytryptamine (5-HT)4 receptor agonist, TKS159, ¿4-amino-5-chloro-2-methoxy-N-[(2S,4S)-1-ethyl-2- hydroxymethyl-4-pyrrolidinyl] benzamide), has recently been developed as a gastroprokinetic drug. Cisapride is already used clinically to increase gastric contractions. The stimulatory effects of TKS159 and cisapride on gastric contractions were examined using force transducers chronically implanted on the vagally denervated pouch (Heidenhain pouch) and the vagally innervated main stomach in conscious dogs. Contractile activity was analysed by computer and expressed as a motor index. Intravenous administration of TKS159 or cisapride significantly increased the motor index in both the main stomach and the Heidenhain pouch during the fed and fasted states. Pharmacological characterization in the fasted state revealed that the contraction-stimulating activity of TKS159 and cisapride on the stomach was significantly inhibited by atropine, hexamethonium and a 5-HT4 receptor antagonist, SDZ 205-557. Granisetron (a 5-HT3 receptor antagonist) significantly inhibited cisapride-induced, but not TKS159-induced gastric contractions. The plasma motilin concentration was significantly increased after cisapride, but not after TKS159 injection. In conclusion, TKS159 has a contractile-stimulating effect on both the innervated and the denervated stomach. It is likely that a cholinergic pathway and 5-HT4 receptors are involved in producing the contractions, although other mechanisms cannot be excluded. Cisapride has almost the same characteristics, but the present findings suggest the involvement of motilin and 5-HT3 receptors in the effects of cisapride.

5-Hydroxytryptamine-induced secretion by rat jejunum in-vitro involves several 5-hydroxytryptamine receptor subtypes

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced anion secretion by rat jejunum have been investigated by testing the effects of selective agonists and antagonists in-vitro using both intact and stripped intestinal sheets. In both intact and stripped jejunum 5-HT and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, induced concentration-dependent increases in the short-circuit current (SCC), although 5-methoxytryptamine induced a smaller maximum response. In intact sheets 1-phenylbiguanide, a selective 5-HT3 agonist, induced a response that was similar in magnitude to that of 5-methoxytryptamine, but in stripped preparations it had little effect. Tetrodotoxin inhibited the response of intact jejunum to 5-HT (by 86%) and 5-methoxytryptamine (by 85%) and abolished the response to 1-phenylbiguanide. In stripped sheets inhibition of the 5-HT response by tetrodotoxin was reduced to 27%. Desensitization to 1-phenylbiguanide reduced the response to 5-HT in intact but not stripped sheets whereas, in contrast, desensitization to 5-methoxytryptamine inhibited the 5-HT response in stripped sheets but was without effect in intact sheets. Mianserin, a 5-HT1, 5-HT2 and 5-HT3 antagonist, and renzapride, a 5-HT1 and 5-HT3 antagonist, both reduced the maximum response to 5-HT, but 5-HTP-DP, a 5-HT1 antagonist, was without effect. The 5-HT3 antagonist granisetron reduced the response to 5-HT in intact, but not in stripped sheets. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine in both preparations, but did not alter the response to 5-HT. It is concluded that 5-HT-induced jejunal secretion involves more than one 5-HT receptor subtype, with both neural and non-neural mechanisms contributing to the response.

Effects of ondansetron on electrically evoked contraction in rat stomach fundus: possible involvement of 5-HT2B receptors

This study examined the effects of ondansetron, an antagonist of the 5-hydroxytryptamine (5-HT3) receptor without 5-HT4 receptor agonistic activity. on electrically evoked contractions and acetylcholine release in rat stomach fundus strips. Ondansetron (10(-8)-10(-4) M) produced a concentration-dependent increase in the magnitude of the electrically evoked contraction, while it inhibited the release of acetylcholine induced by electrical field stimulation. The stimulatory effect of ondansetron (10(-6) M) on electrically evoked contractions was antagonized by yohimbine, a 5-HT2B receptor antagonist, but not by SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-[diethylamino] ethyl ester), a 5-HT4 receptor antagonist. In the presence of tetrodotoxin, ondansetron (10(-7)-10(-5) M) enhanced the contractions induced by acetylcholine. This stimulatory effect of ondansetron on acetylcholine-induced contractions was antagonized by yohimbine. These data suggest that ondansetron potentiates the contractile response to acetylcholine in the rat stomach fundus through the activation of 5-HT2B receptors.

Several receptor subtypes contribute to 5-hydroxytryptamine-induced secretion by rat ileum in-vitro

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced secretion by rat ileum were investigated in-vitro using selective agonists and antagonists. 5-HT induced a dose-dependent increase in the short-circuit current (SCC) generated by both intact and stripped sheets of rat ileum. 1-Phenylbiguanide, a selective 5-HT3 agonist, and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, also increased the SCC. In intact sheets 5-HT was more effective than either 1-phenylbiguanide or 5-methoxytryptamine, whereas in stripped sheets 5-HT and 5-methoxytryptamine were equipotent, with 1-phenylbiguanide having little effect. Tetrodotoxin abolished the response of intact sheets to 1-phenylbiguanide but only reduced the responses to 5-HT and 5-methoxytryptamine by 57% and 54%, respectively. This inhibition was reduced to 25% in stripped sheets. The 5-HT3 antagonist granisetron abolished the response to 1-phenylbiguanide, but did not alter the effects of 5-HT. Ketanserin, a 5-HT2 antagonist, had a small effect on the actions of 5-methoxytryptamine in intact, but not stripped, sheets and no effect on the response to 5-HT in either preparation. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine, but had less effect on the response to 5-HT. Desensitization to 1-phenylbiguanide inhibited the response to 5-HT in intact, but not stripped sheets, whereas desensitization to 5-methoxytryptamine abolished the 5-HT response in stripped sheets, but induced only 42% inhibition in intact sheets. Previous exposure to a combination of both 1-phenylbiguanide and 5-methoxytryptamine abolished the 5-HT-induced increase in SCC in both preparations. The findings suggest that 5-HT-induced ileal secretion involves more than one 5-HT receptor subtype and that both neural and non-neural mechanisms contribute to the response.

Investigation into the contractile response of melatonin in the guinea-pig isolated proximal colon: the role of 5-HT4 and melatonin receptors

1. The interaction of melatonin (N-acetyl-5-methoxytryptamine) with 5-hydroxytryptamine4 (5-HT4) receptors and/or with melatonin receptors (ML1, ML2 sites) has been assessed in isolated strips of the guinea-pig proximal colon. In the same preparation, the pharmacological profile of a series of melatonin agonists (2-iodomelatonin, 6-chloromelatonin, N-acetyl-5-hydroxytryptamine (N-acetyl-5-HT), 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT)) was investigated. 2. In the presence of 5-HT1/2/3 receptor blockade with methysergide (1 microM) and ondansetron (10 microM), melatonin (0.1 nM-10 microM), 5-HT (1 nM-1 microM) and the 5-HT4 receptor agonist, 5-methoxytryptamine (5-MeOT: 1 nM-1 microM) caused concentration-dependent contractile responses. 5-HT and 5-MeOT acted as full agonists with a potency (-log EC50) of 7.8 and 8.0, respectively. The potency value for melatonin was 8.7, but its maximum effect was only 58% of that elicited by 5-HT. 3. Melatonin responses were resistant to atropine (0.1 microM), tetrodotoxin (0.3 microM), and to blockade of 5-HT4 receptors by SDZ 205,557 (0.3 microM) and GR 125487 (3, 30 and 300 nM). The latter antagonist (3 nM) inhibited 5-HT-induced contractions with an apparent pA2 value of 9.6 GR 125487 antagonism was associated with 30% reduction of the 5-HT response maximum. Contractions elicited by 5-HT were not modified when melatonin (1 and 10 nM) was used as an antagonist. 4. Like melatonin, the four melatonin analogues concentration-dependently contracted colonic strips. The rank order of agonist potency was: 2-iodomelatonin (10.8) > 6-chloromelatonin (9.9) > or = N-acetyl-5-HT (9.8) > or = 5-MCA-NAT (9.6) > melatonin (8.7), an order typical for ML2 sites. In comparison with the other agonists, 5-MCA-NAT had the highest intrinsic activity. 5. The melatonin ML1B receptor antagonist luzindole (0.3, 1 and 3 microM) had no effect on the concentration-response curve to melatonin. Prazosin, an alpha-adrenoceptor antagonist possessing moderate/ high affinity for melatonin ML2 sites did not affect melatonin-induced contractions at 0.1 microM. Higher prazosin concentrations (0.3 and 1 microM) caused a non-concentration-dependent depression of the maximal response to melatonin without changing its potency. Prazosin (0.1 and 1 microM) showed a similar depressant behaviour towards the contractile responses to 5-MCA-NAT. 6. In the guinea-pig proximal colon, melatonin despite some structural similarity with the 5-HT4 receptor agonist 5-MeOT, does not interact with 5-HT4 receptors (or with 5-HT1/2/3 receptors). As indicated by the rank order of agonist potencies and by the inefficacy of luzindole, the most likely sites of action of melatonin are postjunctional ML2 receptors. However, this assumption could not be corroborated with the use of prazosin as this 'ML2 receptor antagonist' showed only a non-concentration-dependent depression of the maximal contractile response to both melatonin and 5-MCA-NAT. Further investigation with the use of truly selective antagonists at melatonin ML2 receptors is required to clarify this issue.

Pharmacological characterization of the 5-hydroxytryptamine receptor mediating relaxation in the rat isolated ileum

1 The aim of the present study was to investigate a 5-HT4 receptor involvement in the mediation of a 5-HT-induced relaxation response in the rat isolated ileum in vitro. 2 Ileal segments were taken at regular intervals from the ileo-caecal junction to duodenum. 5-HT (1 microM) induced a relaxation or contraction response in segments taken from the terminal ileum: the relaxation decreased and finally disappeared as contractions dominated in the proximal tissues. The 5-HT-induced relaxations were enhanced in the terminal segments and the contractions attenuated in both terminal and proximal segments, in the presence of methysergide (1 microM) and atropine (0.1 microM). 3 In the presence of methysergide (1 microM) and atropine (0.1 microM), a cumulative addition of 5-HT (0.01-1 microM) induced a concentration-dependent relaxation in the terminal (1-20 cm from the ileo-ceacal junction) ileal segments which at higher concentrations of 5-HT (3-30 microM) reverted to contraction. 4 The rank order of potency of indole agonists in inducing a concentration-related relaxation response in tissues of the terminal ileum (pretreated with pargyline (100 microM) and in the presence of methysergide (1 or 100 microM) and atropine (0.1 microM) was 5-hydroxytryptamine (6.97 +/- 0.06), 5-methoxytryptamine (6.50 +/- 0.07), alpha-methyl-5-hydroxytryptamine (5.53 +/- 0.17), 5-carboxamidotryptamine (5.51 +/- 0.12) and 2-methyl-5-hydroxytryptamine (< 5), the pEC50 values (mean +/- s.e.mean) being shown in parentheses. 5 Pretreatment of tissues with pargyline (100 microM) selectively enhanced the potency of 5-methoxytryptamine by a factor of 19 but failed to modify the potency of the other indole agonists. 6 The 5-HT4 receptor antagonists, tropisetron, SDZ 205-557 and GR 113808 antagonized the relaxation response to 5-HT (in the presence of methysergide (1 or 10 microM) and atropine (0.1 microM)) with pKB values (95% CL) of 6.09 (5.94-6.24), 7.0 (6.9-7.09) and 8.95 (8.81-9.1) respectively. Apparent pKB values estimations for tropisetron (1 microM) and GR 113808 (10 nM) using the agonists 5-methoxytryptamine and 5-carboxamidotryptamine were 6.37 +/- 0.31, 5.91 +/- 0.38 and 8.83 +/- 0.11, 8.82 +/- 0.22 respectively. 7 Tropisetron (10 microM), SDZ 205-557 (3 microM) and GR 113808 (10-100 nM) caused an increase in basal tone of the rat terminal ileum when administered in the presence of methysergide and atropine. 8 The relaxation response to 5-HT in the rat terminal ileum was not antagonized by ritanserin (1 microM), ondansetron (1 microM) or N omega-nitro-L-arginine methyl ester (100 microM) and with only a twofold dextral shift of the concentration-response curve by tetrodotoxin (1 microM). 9 It is concluded that the relaxant response to 5-HT in the terminal region of the ileum is mediated directly at the smooth muscle; a ranked indole agonist potency and selective antagonism by 5-HT4 receptor antagonists tropisetron, SDZ 205-557 and GR 113808 indicate a 5-HT4 receptor involvement in the relaxation response.

Ability of 5-HT4 receptor ligands to modulate rat striatal dopamine release in vitro and in vivo

1. The ability of 5-HT4 (5-hydroxytryptamine4) receptor ligands to modify dopamine release from rat striatal slices in vitro and in the striatum of freely moving rats was assessed by the microdialysis technique. 2. The release of dopamine from slices of rat striatum continually perfused with Krebs buffer was enhanced by 5-HT4 receptor agonists; 5-HT (10 microM), 5-methoxytryptamine (5-MeOT; 10 microM), renzapride (10 microM) and (S)-zacopride (10 microM) maximally increased dopamine release by 133 +/- 5, 214 +/- 25, 232 +/- 29 and 264 +/- 69%, respectively (mean +/- s.e.mean, n = 3-8). The drug-induced responses were maximal within the first 2 min of drug application, and subsequently declined. The non-selective 5-HT3/5-HT4 receptor antagonist, SDZ205-557 (10 microM), failed to modify basal dopamine release from striatal slices but completely antagonized the (S)-zacopride (10 microM)-induced increase in dopamine release. 3. To allow faster drug application, the modulation of dopamine release from rat striatal slices in a static release preparation was also investigated. The 5-HT4 receptor agonist, renzapride (10 microM) also enhanced dopamine release in this preparation (maximal increase = 214 +/- 35%, mean +/- s.e.mean, n = 14), whilst a lower concentration of renzapride (3 microM) was less effective. The renzapride-induced response was maximal within the first 2 min of drug application, before declining. In this preparation, the stimulation of dopamine release by renzapride (10 microM), was completely antagonized by the selective 5-HT4 receptor antagonist, GR113808 (100 nM). In addition, both the Na+ channel blocker, tetrodotoxin (100 nM) and the non-selective protein kinase A inhibitor, H7 (100 nM) completely prevented the stimulation of dopamine release induced by renzapride (10 microM). 4. In vivo microdialysis studies demonstrated that the 5-HT4 receptor agonists, 5-MeOT (10 microM), renzapride (100 microM) and (S)-zacopride (100 microM) maximally elevated extracellular levels of dopamine in the striatum by 220 +/- 20, 161 +/- 10 and 189 +/- 53%, respectively (mean +/- s.e.mean, n = 5-9). A lower concentration of renzapride (10 microM) was less effective. The elevation of extracellular striatal dopamine levels induced by either renzapride (100 microM) or (S)-zacopride (100 microM) were completely antagonized by the non-selective 5-HT4 receptor antagonist, SDZ205-557 (100 microM). In addition, the elevation of extracellular levels of dopamine induced by either 5-MeOT (10 microM) or renzapride (100 microM) was completely prevented by the selective 5-HT4 receptor antagonist, GR113808 (1 microM) and the renzapride (100 microM)-induced response was also completely prevented by the non-selective protein kinase A inhibitor, H7 (1 microM). In this in vivo preparation, both GR113808 (1 microM) and H7 (1 microM), when perfused alone, reduced extracellular levels of dopamine. 5. In conclusion, the present study provides evidence that the 5-HT4 receptor facilitates rat striatal dopamine release in vitro and in vivo.

Blockade of human and porcine myocardial 5-HT4 receptors by SB 203186

We investigated the blockade of the positive inotropic effects of 5-hydroxytryptamine (5-HT) by SB 203 186 (piperidinoethyl-indole-3-carboxylate hydrochloride) and its affinity for 5-HT4 receptors of human right atrium and piglet left atrium. We also compared the blocking effects of SB 203 186 against 5-HT-evoked tachycardia in anaesthetised adult Yucatan minipigs as well as new-born Camborough piglets. SB 203 186 caused competitive antagonism of the positive inotropic effects of 5-HT in electrically paced atrial preparations of man (pKB = 8.9) and piglet (pKB = 8.5) at concentrations (up to 0.3 micromol/l) which were devoid of depressant or stimulant effects. The affinity of SB 203 186 for atrial 5-HT4 receptors was 30-160 times higher than that of tropisetron. 5-HT caused tachycardia with similar potency and efficacy in Yucatan minipigs and new-born Camborough piglets. SB 203 186 (0.1-3 mg/kg, i.v.) surmountably antagonised 5-HT-evoked tachycardia in anaesthetised Yucatan minipigs or new-born Camborough piglets with similar potency. The blocking potency of SB 203 186 in Yucatan minipigs was 17 times higher than that of tropisetron. Intraduodenally administered SB 203 186 (0.3-3 mg/kg) to new-born Camborough piglets produced blockade of 5-HT-evoked tachycardia which was maximal after 20 min and lasted for more than 3 h with 0.3 mg/kg. The antagonism produced by the SB 203 186 administration in new-born Camborough piglets was dose-related and threefold greater through the intravenous route than through the intraduodenal route. We conclude that SB 203 186 is an antagonist with nanomolar affinity for both human and porcine atrial 5-HT4 receptor. The in vivo results demonstrate that the sinoatrial 5-HT4 receptors function is similar in new-born Camborough piglets and adult Yucatan minipigs. Both porcine breeds are valid models for human atrial 5-HT4 receptors as demonstrated with the antagonist SB 203 186.

Evidence that the secretory response of rat intestine to 5-hydroxytryptamine in-vivo involves more than one 5-hydroxytryptamine-receptor subtype

The transintestinal potential difference (PD) across rat mid-small intestine and proximal colon was measured in-vivo. The 5-hydroxytryptamine (5-HT)-induced increase in PD, which reflects a stimulation of electrogenic C1 secretion, was mimicked by both 2-methyl-5-hydroxytryptamine (2-CH3-5-HT), an agonist at 5-HT3 receptors, and 5-methoxytryptamine (5-MT), an agonist that lacks affinity for 5-HT3 receptors. The 5-HT3 antagonist granisetron caused a marked inhibition of the response to 2-CH3-5-HT in both regions, but only produced a small inhibition of the small intestinal response to 5-HT, with a more pronounced effect in the colon. The failure of granisetron to produce a marked antagonism of the 5-HT-induced rise in the transintestinal PD, coupled with the ability of 5-MT to induce a secretory response, indicates that 5-HT3 receptors are not the only ones involved in the stimulation of C1 secretion. The 5-HT2 antagonist ketanserin failed to influence the response to 5-HT in either the small intestine or the colon, but it did inhibit the action of 5-MT, having a much greater effect in the small intestine. In the presence of granisetron however, ketanserin also inhibited the small intestinal response to 5-HT, having only a minimal effect in the colon. This suggests that 5-HT2 receptors can also play a role in the activation of C1 secretion. These observations suggest that both 5-HT2 and 5-HT3 receptors contribute to the stimulation of electrogenic C1 secretion by 5-HT, with 5-HT2 receptors playing a more prominent role in the small intestine and 5-HT3 receptors being more important in the colon.

5-HT4 receptor antagonist affinities of SB207710, SB205008, and SB203186 in the human colon, rat oesophagus, and guinea-pig ileum peristaltic reflex

Functional 5-HT4 receptors have been reported to be present in numerous isolated tissue preparations including the rat oesophagus, guinea-pig ileum, and human colon. The pharmacological properties of the novel, potent and selective 5-HT4 receptor antagonists SB203186 (1-piperidinyl)ethyl 1H-indole 3-carboxylate), SB205008 (1-butyl-1-methyl-4-piperidinylmethyl)-8-amino-7-chloro-1,4-ben zodioxan-5- carboxylate iodide), and SB207710 (1-butyl-4-piperidinylmethyl)-8-amino-7-iodo-1,4 benzo-dioxan-5-carboxylate) were studied in these tissues. The nature of antagonism of the 5-HT-induced effects was investigated on the above isolated tissue preparations. 5-HT produced its effect with the following EC50 values: 400 +/- 0.4 nM (rat oesophagus, n = 20), 154 +/- 14 nM (guinea-pig ileum, n = 9) and 144 +/- 0.1 nM (human colon, n = 9). SB207710 (0.03-1 nM), SB205008 (1.0-10 nM), and SB203186 (10-100 nM) antagonised the 5-HT4 receptor-mediated relaxations of the carbachol-contracted rat isolated oesophagus against 5-HT with pKB values of 10.9 +/- 0.1, 9.5 +/- 0.1, and 9.0 +/- 0.1 respectively without effecting the maximum response. On the guinea-pig ileum peristaltic reflex preparation, SB207710 (0.01-1 nM) did not modify the reflex but it behaved as an antagonist of the 5-HT-induced facilitation with a pA2 value of 9.9 +/- 0.2.(ABSTRACT TRUNCATED AT 250 WORDS)

Preparation and in vitro pharmacology of 5-HT4 receptor ligands. Partial agonism and antagonism of metoclopramide analogous benzoic esters

Alicyclic ester analogues of the gastroprokinetic benzamide metoclopramide (1) and its ester congener SDZ 205557 (2), a 5-HT4 receptor antagonist, were prepared by O-alkylation of 4-amino-5-chloro-2-methoxybenzoate with N-(2-chloroethyl) substituted alicyclic amines. The bromo and iodo analogue of compound 13b (2-(1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoate) were obtained by halogenation of dechloro-13b with N-halogenated succinimides. The series was evaluated in functional in vitro assays with regard to affinity for serotoninergic 5-HT4, 5-HT3 and muscarinic M3 receptors. The affinities for 5-HT3 and M3 receptors were below 6.0 (pKB or pA2). On 5-HT4 receptors in guinea-pig ileal longitudinal muscle and rat oesophagus, the majority of compounds revealed partial 5-HT4 receptor agonism susceptible to blockade by SDZ 205557, a reference 5-HT4 receptor antagonist (pKB = 7.25-7.73 (guinea-pig ileum) and 7.09-7.43 (rat oesophagus)). The relative agonist potency was in the range of 5-303% (5-HT: 100%). Compound 13b and its bromo analogue 17 were the most potent esters of the series. The enantiomers of 13g ((R)- and (S)-2-(2-methyl-1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoate) interacted stereoselectively with 5-HT4 receptors and displayed enantiomeric potency ratios (R)/(S) of 4.3-8.7. There was an excellent correlation between (a) antagonist affinity on guinea-pig ileum and rat oesophagus, (b) relative agonist potency on guinea-pig ileum and rat oesophagus, and (c) between antagonist affinity and relative agonist potency within each assay (r2 > 0.91). The new compounds may serve as academic tools in evaluating the functional role of 5-HT4 receptors. The selective partial 5-HT4 receptor agonists presented in this paper may be useful to restore physiological motility and secretion in the gut with reduced or absent propensity to elicit tachycardia and desensitization of the intestinal target receptor.

Differences in response to 5-HT4 receptor agonists and antagonists of the 5-HT4-like receptor in human colon circular smooth muscle

1. In isolated circular smooth muscle strips of human colon 5-hydroxytryptamine (5-HT) produced a concentration-related inhibition of spontaneous motility. 2. The azabicycloalkyl benzimidazolones, BIMU 8 and BIMU 1, which have 5-HT4 receptor stimulant properties, inhibited motility with EC50 values of 0.76 microM and 3.19 microM respectively and their Emax values were not significantly different from 5-HT (EC50, 0.13 microM). 3. The 5-HT4 receptor antagonist, DAU 6285 (1-10 microM), displaced the 5-HT concentration-response curve to the right in a parallel concentration-dependent manner without depressing the maximum. The Schild plot was linear and the slope did not differ significantly from unity giving a pA2 value of 6.32. 4. The high affinity selective 5-HT4 receptor antagonist, GR 113808, at a concentration of 3 nM displaced the 5-HT concentration-response curve in a parallel manner giving an apparent pKB estimate of 8.9 +/- 0.24. However, higher concentrations of 10-100 nM GR 113808 did not result in a further significant displacement of the 5-HT concentration-response curve and there was no suppression of Emax. 5. GR 113808 (10 nM) also caused a parallel displacement of the concentration-response curve to the 5-HT4 receptor agonist, 5-methoxytryptamine (5-MeOT) giving apparent pKB values ranging from 8.3-9.3. 6. GR 113808 (3-100 nM) failed to displace 5-HT or 5-MeOT concentration-response curves in tissue strips from 3 patients out of a total of 10 patients studied in whom the response to 5-HT and 5-MeOT was normal. 7. The 5-HT4 receptor antagonist, SDZ 205-557 (0.3-10 microM), had no significant effect on 5-HT-induced inhibition of spontaneous motility.8. The present results are discussed in the light of variability of response to GR 113808 and SDZ205-557 in other tissues.9. Overall, our data indicate that human colon circular smooth muscle can be regarded as a site in which 5-HT4-like receptors are present but it is as yet unclear whether these results are also an indication of receptor variation.

A comparative study of functional 5-HT4 receptors in human colon, rat oesophagus and rat ileum

1. The pharmacological properties of 5-hydroxytryptamine (5-HT), the 5-HT4 receptor agonists, DAU 6236 and SC 53116 and the 5-HT4 receptor antagonist, GR 1130808, were studied in the rat oesophagus, rat ileum and human colon. 2. 5-HT relaxed the longitudinal muscle of the rat oesophagus and rat ileum and the circular muscle of the human colon. Absolute values of relaxation were measured and showed the order of the maximum responses, rat oesophagus >> human colon > rat ileum with EC50 values of 189 +/- 15 nM, 157 +/- 4 nM, 306 +/- 72 nM, respectively. 5-HT also inhibited the spontaneous contractions of the human colon with an EC50 value of 119 +/- 1 nM. The effect of 5-HT on the human colon was not affected by methysergide (10 microM) or ondansetron (1 microM). 3. The use of the uptake and metabolism inhibitors, cocaine (30 microM) and pargyline (100 microM), did not increase the potency of 5-HT in the rat oesophagus or human colon. In the rat oesophagus, cocaine (30 microM) produced a reduction in carbachol-induced tone of 22.2 +/- 0.6% and reduced the 5-HT maximum effect by 52.0 +/- 0.4%. 4. The compounds, DAU 6236 and SC 53116, showed a different pattern of potencies and efficacies in the rat oesophagus, rat ileum and human colon compared to 5-HT. DAU 6236 relaxed the human colonic circular muscle with an EC50 value of 129 +/- 16 nM but its efficacy was less than that of 5-HT. DAU 6236 (1 microM) also antagonized the 5-HT-induced relaxation of the human colon with a dose-ratio of 9.9. In the rat oesophagus and rat ileum, DAU 6236 was inactive in the majority of tissues. In the minority of oesophagus tissues that did respond the EC50 value was 1.2 +/- 0.7 microM. DAU 6236 also antagonized the effect of 5-HT in the rat oesophagus in a non-surmountable fashion. SC 53116 relaxed the rat oesophagus with an EC50 value of 91 +/- 4 nM, with an efficacy less than that observed to 5-HT; however, at 200 nM it did not antagonize the 5-HT-induced relaxation of the rat oesophagus. SC 53116 showed no agonist activity in the rat ileum and human colon, but at 1 microM it did antagonize the effect of 5-HT in the human colon with a dose-ratio of 11.3 +/- 0.3. 5. GR 113808 competitively antagonized the 5-HT4 receptor-mediated relaxation of the rat oesophagus with a pA2 value of 8.59 (8.18-9.00) against 5-HT and 9.05 (8.79-9.31) against SC 53116. GR 113808(0.01 microM) also antagonized the 5-HT-induced relaxation of human colonic circular muscle with an apparent pA2 value of 9.02 +/- 0.12. However at 1 microM the apparent pA2 value was significantly lower than that measured at 0.01 and 0.1 microM. GR 113808 (0.01 microM) antagonized the 5-HT4 receptor-mediated relaxation of the rat ileum with an apparent pA2 value of 9.30 +/- 0.21.6. In conclusion, these studies have shown that the human colon, rat oesophagus and rat ileum contain functional 5-HT4 receptors. However, the 5-HT4 receptor agonists displayed differences in these tissues making it necessary to be cautious when extrapolating from animal to human tissue. This emphasizes the importance of the use of human tissue in the development of therapeutic drugs.

Do 5-HT4 receptors mediate the intestinal secretory response to 5-HT in rat in-vivo?

The involvement of the recently characterized 5-HT4 receptor in the actions of 5-hydroxytryptamine (5-HT) on jejunal, ileal and colonic electrogenic ion secretion was investigated in the rat in-vivo. 5-HT and the 5-HT1-, 5-HT2- and 5-HT4-receptor agonist 5-methoxytryptamine (5-MeOT), induced a rise in transintestinal PD in all regions of the gut. However, the 5-HT4-receptor agonists renzapride and cisapride had no effect. Furthermore, the 5-HT4-receptor antagonists SDZ 205-557 (2-diethylaminoethyl-[2-methoxy-4-amino-5-chloro] benzoate), tropisetron and SB 204070 ([1-butyl-4-piperidinylmethyl]-8-amino-7-chloro-1,4- benzodioxan-5-carboxylate hydrochloride) did not affect the secretory response to either 5-HT or 5-MeOT in the jejunum, but did cause a small inhibition in the ileum and colon. It is concluded that 5-HT4 receptors do not make a contribution to the electrically monitored 5-HT intestinal secretory response in the rat jejunum in-vivo, but may play a small role in the ileum and colon.

The interaction of antidepressant drugs with central and peripheral (enteric) 5-HT3 and 5-HT4 receptors

1. A combined study of receptor binding in central neuronal cell membranes and functional responses in isolated segments of guinea-pig small intestine allowed characterization of the interaction of four antidepressant drugs with central and peripheral 5-HT3 and 5-HT4 receptors. 2. Clomipramine, paroxetine and fluoxetine inhibited [3H]-DAU 6215 binding to 5-HT3 recognition sites in NG 108-15 cells with IC50 values in the range 1.3-4 microM. Litoxetine had an IC50 of 0.3 microM. The specific binding of [3H]-GR 113808 to 5-HT4 recognition sites in pig striatal membranes was inhibited by all four antidepressants with negligible potency (IC50 values > or = 20 microM). 3. In whole ileal segments, concentration-response curves to 5-HT were biphasic, with the high- and low-potency phases involving 5-HT4 and 5-HT3 receptors, respectively. Curves to 2-methyl-5-hydroxytryptamine (2-methyl-5-HT: a 5-HT3 receptor agonist) and 5-methoxytryptamine (5-MeOT: a 5-HT4 receptor agonist) were monophasic. All antidepressants were used at concentrations lacking anticholinoceptor properties, as demonstrated in both electrically stimulated longitudinal muscle-myenteric plexus preparations (LMMPs) and in unstimulated LMMPs following addition of acetylcholine (100 nM). 4. Fluoxetine (0.1-1 microM) and litoxetine (0.3-3 microM) antagonized both the high- and low-potency phases of the 5-HT curve. Schild analysis for the low-potency phase yielded pA2 estimates of 6.6 +/- 0.3 (Schild slope of 1.1) and of 6.6 +/- 0.1 (Schild slope of 1.1), respectively. At higher concentrations (3 microM), fluoxetine markedly inhibited the 5-HT response maximum. Clomipramine (10-300 nM) inhibited, by a mechanism independent of concentration, both phases of the 5-HT curve with a reduction of the maximum response. Paroxetine (1 microM) was ineffective on the high-potency phase, but caused a rightward shift of the low-potency phase (pKB: 6.1 +/- 0.01). 5. Responses to 2-methyl-5-HT were inhibited by 1 microM fluoxetine (pKB: 5.4 +/- 0.02). Like clomipramine(30 and 100 nM), litoxetine (1 and 3 microM) produced rightward displacements of 2-methyl-5-HT-induced contractions, which were virtually independent of antidepressant concentration (pKB values: 6.0 +/- 0.02 and 5.5 +/- 0.01, respectively). At higher concentrations, fluoxetine (3 microM) and clomipramine (300 nM)markedly reduced the 2-methyl-5-HT response maximum. Paroxetine (1 micro M) was ineffective.6. Responses to 5-MeOT were shifted to the right by fluoxetine (0.1-1 micro M) and litoxetine (1 and 3 microM)in a concentration-dependent manner. At higher concentrations, fluoxetine (3 microM) markedly reduced the 5-MeOT response maximum, an effect also observed with 100 and 300 nM clomipramine. Paroxetine(1 microM) was ineffective.7. In unstimulated LMMPs, the excitatory effects evoked by 5-HT, 2-methyl-5-HT and 5-MeOT and the antagonism produced by 300 nM clomipramine were comparable to those obtained in whole ileal segments. This suggests that 5-HT contained in the mucosa of whole preparations does not interfere with agonist-induced contractile responses and with the inhibitory effect of antidepressant drugs.8. In conclusion, our results show that clomipramine, fluoxetine, paroxetine and litoxetine possess low to moderate potency/affinity at both central and peripheral (enteric) 5-HT3 receptors. In contrast, all four antidepressants are virtually ineffective at central 5-HT4 receptors. Inhibition of 5-HT4 receptor mediated ileal contractions by fluoxetine, litoxetine and clomipramine may result from allostericant agonism or, more likely, from post-receptor blockade of second messenger generation. The interaction of antidepressants with central and peripheral 5-HT3 and 5-HT4 receptors may be relevant for both potential therapeutic action and adverse effects at gastrointestinal level.

Antagonistic activities of N-3389, a newly synthesized diazabicyclo derivative, at 5-HT3 and 5-HT4 receptors

The antagonistic activities of compound N-3389 (endo-3,9-dimethyl-3,9- diazabicyclo[3,3,1]non-7-yl 1H-indazole-3-carboxamide dihydrochloride) at 5-HT3 and 5-HT4 receptors were examined using in vitro and in vivo assays. N-3389 showed potent 5-HT3 receptor antagonistic activities in a radioligand binding assay (pKi = 8.77), against 2-methyl-5-HT (2-Me-5-HT)-induced bradycardia in rats (ED50 = 0.73 micrograms/kg i.v., 38 micrograms/kg p.o.) and against 2-Me-5-HT-induced contraction in longitudinal muscle myenteric plexus preparations of guinea-pig ileum (IC50 = 3.2 x 10(-8) M). As a preliminary to investigating the effect of N-3389 on 5-HT4 receptors, we examined the contraction induced by 5-HT in guinea-pig ileum preparations. We confirmed that 5-HT (10(-8)-10(-5) M) induced biphasic contractions in the preparations. Furthermore, 5-HT3 receptor antagonism inhibited the late phase of the contraction induced by high concentrations of 5-HT (3 x 10(-6)-10(-5) M), whereas 5-HT4 receptor antagonism inhibited the early phase of the contraction induced by low concentrations of 5-HT (10(-8)-10(-6) M). N-3389 (10(-7)-10(-5) M) inhibited both phases of contraction induced by 5-HT. In addition, N-3389 (3 x 10(-7)-3 x 10(-6) M) was found to inhibit the increase of electrically stimulated twitch responses induced by 5-HT (10(-8) M) longitudinal muscle myenteric plexus preparation of the guinea-pig ileum. These results suggest that N-3389 acts as a 5-HT3 and 5-HT4 receptor antagonist.

5-HT4 receptors in rat but not guinea pig, rabbit or dog esophageal smooth muscle

1. Marked heterogeneity among species exists in the esophageal response to pharmacological agents. The present study compared the response to serotonin in esophagus from the rat, guinea pig, rabbit and dog. 2. The esophagus from all four species contracted to carbamylcholine and to PGF2 alpha; responses to serotonin were the most variable among species. 3. Serotonin contracted the guinea pig and rabbit esophagus; an effect blocked by LY53857 (10(-7 M) and ketanserin (10(-7) M), consistent with 5-HT2 receptor activation mediating this contraction. 4. Serotonin neither contracted nor relaxed the canine esophagus and relaxed the rat esophagus via 5-HT4 receptor activation as determined by antagonism with ICS 205-930 (-log KB = 6.4), metoclopramide (-log KB = 6.7) and its ester congener SDZ 205-557 (-log KB = 7.9). Two methylene homologs of SDZ 205-557 also had high 5-HT4 receptor affinity (-log KB = 7.7). 5. Thus, in guinea pig and rabbit esophagus, serotonin induced a contraction mediated by 5-HT2 receptors; and serotonin neither contracted nor relaxed the canine esophagus. In rat esophagus, serotonin induced a relaxation mediated by activation of 5-HT4 receptors.

Stimulant effects of 5-hydroxytryptamine on guinea pig stomach preparations in vitro

5-Hydroxytryptamine (5-HT) contracts and relaxes isolated stomach preparations. This study attempts to characterise receptors involved in the contractile response using electrically stimulated circular muscle strips from guinea pig stomach. Electrically induced contractions were abolished by atropine and tetrodotoxin. 5-HT enhanced contractions in corpus and fundus strips with pEC50% values (-log10 of the concentrations causing a 50% increase in twitch height) of 9.6 and 9.1, respectively. 5-Carboxamidotryptamine and 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), 5-HT1A receptor agonists, and alpha-methyl-5-HT, an agonist at 5-HT2 receptors, reduced contractions. The 5-HT3 receptor agonist, 2-methyl-5-HT, increased contractions. The effect of 2-methyl-5-HT but not of 5-HT was antagonized by the 5-HT3 receptor antagonist, tropisetron (10(-7) M). The 5-HT3 receptor antagonists, tropisetron, MDL 72222 (1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate), grainsetron and ondansetron, did not modify twitch responses at concentrations below 10(-7) M. Renzapride and metoclopramide, agonists at 5-HT4 receptors, increased contractions and this effect was inhibited by the 5-HT4 receptor antagonist SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester) with a pA2 of 7.4. The effect of 5-HT at a submaximal concentration of 10(-8) M was blocked by SDZ 205-557 (10(-6) M). It is concluded that electrically induced contractions in guinea pig stomach strips are enhanced by activation of 5-HT3- and 5-HT4 receptors and are diminished by 5-HT1 receptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

A pharmacological analysis of receptors mediating the excitatory response to 5-hydroxytryptamine in the guinea-pig isolated trachea

1. Experiments were carried out to characterize the receptors mediating the indirect excitatory response to 5-hydroxytryptamine (5-HT) in the guinea-pig isolated trachea. 2. 5-HT caused concentration-dependent contractions of tracheal strips, and the resulting concentration-response curve was biphasic in nature. The first phase was obtained with agonist concentrations in the range of 0.01-3 nM and achieved a maximum which was 30% of the total 5-HT response, while the second phase was in the range 10 nM-1 microM. 3. Atropine (0.1 microM) and tetrodotoxin (TTX: 0.3 microM) significantly reduced both phases of the 5-HT curve. Morphine (10 microM), which can act to inhibit neuronal acetylcholine release, abolished the first phase and reduced the second phase. This suggests that the first phase is mainly neurogenic (cholinergic) in nature, while the second phase is in part neurogenic and in part due to direct activation of the effector cells. 4. The 5-HT2A receptor antagonist, ketanserin (0.01, 0.1 microM) markedly depressed the first phase and shifted the second phase to the right in a parallel manner, with some depression of the 5-HT response maximum. The less selective (5-HT1/5-HT2A) antagonist, methiothepin (0.1 microM) mimicked the action of ketanserin, albeit with less potency. Concomitant administration of ketanserin and methiothepin (each at 0.1 microM) produced an antagonism similar to that caused by ketanserin (0.1 microM) alone. 5. The 5-HT3 receptor antagonists, ondansetron (0.1 microM) and granisetron (0.01 microM) slightly but significantly inhibited the first phase of the 5-HT curve without altering the second phase. SDZ 205,557(0.3 MicroM), a 5-HT4 receptor antagonist, was ineffective.6. Our results suggest that neural 5-HT2A and, to a lesser extent, 5-HT3 receptor subtypes mediate the first phase of the 5-HT curve in the guinea-pig trachea. The second phase is mediated by 5-HT2Areceptors, which are probably located at both the neural and muscular level. No evidence for the participation of 5-HT1 receptors in the 5-HT response has been obtained.

Mediation by 5-HT1D receptors of 5-hydroxytryptamine-induced contractions of rabbit middle and posterior cerebral arteries

1. 5-Hydroxytryptamine (5-HT) receptor-mediated contraction of endothelium denuded rabbit middle (MCA) and posterior (PCA) cerebral arteries was characterized by use of selective agonists and antagonists for different 5-HT receptor subtypes. 2. 5-HT and various 5-HT receptor agonists contracted the arteries with the following rank order of potency in MCA: 5-carboxamidotryptamine (5-CT) > 5-HT > 5-methoxytryptamine (5-MeOT) > sumatriptan > alpha-methyl-5-HT (alpha-Me-5-HT) >> 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and in PCA: 5-CT > 5-HT > sumatriptan > 5-MeOT > alpha-Me-5-HT >> 8-OH-DPAT. With few exceptions, the maximal contractile responses of these agonists were similar to that induced by 5-HT. 3. The selective antagonists of 5-HT2A/2C (ketanserin), 5-HT4 (SDZ 205-557) and 5-HT1A/1B (S-(-)-propranolol) sites were devoid of inhibitory effect on 5-HT-mediated contraction in both MCA and PCA, thus excluding activation of the corresponding receptors. 4. In both arteries, the contraction-response curve to 5-HT was unaffected by the 5-HT3 receptor antagonist, ICS 205-930 (0.01 and 0.1 microM) whilst a small (3 and 6 fold displacement) was seen with MDL 72222 (0.1 and 1 microM). 5. The mixed 5-HT1-like/5-HT2A receptor antagonist, methiothepin (0.001-0.1 microM), was a potent antagonist of 5-HT-induced contractions in both arteries, giving pA2 values of 9.4 +/- 0.7 and 9.6 +/- 0.8 in MCA and PCA, respectively. 6. Rauwolscine (O.1-10 MicroM) and yohimbine (0.3, 3 MicroM) inhibited contractions to 5-HT in a competitive manner, pA2 values of 7.1 +/- 0.6 and 6.7 +/-0.6 were determined for rauwolscine in MCA and PCA,respectively. An apparent pA2 value of 6.9 +/-0.2 was calculated for yohimbine (3 MicroM) in both MCA and PCA.7. In conclusion, these results suggest that the contractile response to 5-HT in rabbit isolated MCA and PCA is predominantly mediated by the 5-HTID receptor subtype, although a small contribution by 5-HT3 receptors cannot be excluded.

The effects of SB 204070, a highly potent and selective 5-HT4 receptor antagonist, on guinea-pig distal colon

1. The pharmacology of a novel 5-HT4 receptor antagonist, SB 204070 has been evaluated in the guinea-pig isolated distal colon longitudinal muscle-myenteric plexus (LMMP). 2. SB 204070 is a highly potent antagonist of 5-HT-evoked cholinergically-mediated contractions in the guinea-pig distal colon. Low concentrations (10-100 pM) produced a shift to the right of the curve (apparent pA2 10.8 +/- 0.1) with no significant effect on the maximum response. With higher concentrations of SB 204070 (300 pM and above), the maximum response to 5-HT was reduced. 3. When tested against the partial 5-HT4 receptor agonist, BIMU 1, SB 204070 was active at similar low concentrations (10 pM and above) but produced a reduction in maximum, with no prior shift to the right of the curve, at all concentrations tested (10-300 pM). 4. The antagonism seen with SB 204070 is unlikely to be due to a non-selective effect since high concentrations (10 nM and 1 microM) of the compound had no effect on cholinergically-mediated contractions evoked by the nicotinic receptor agonist, DMPP, in the same preparation. SB 204070 is unlikely to be an irreversible antagonist since the effects of the compound could be reversed upon washing of the tissue. 5. Radioligand binding studies show that SB 204070 has a greater that 5000 fold selectivity for the 5-HT4 receptor over 5-HT1A, 5-HT1D, 5-HT1E, 5-HT2A, 5-HT2C, 5-HT3, GABAA, BDZ, TBPS, A1 adenosine receptors, alpha 1, alpha 2, beta 1, beta 2 adrenoceptors and D1, D2 and D3 dopamine receptors. 6. SB 204070 is a highly potent, highly selective 5-HT4 receptor antagonist and as such is an important new tool in evaluating the functional role of the 5-HT4 receptor.

Drugs affecting serotonin receptors

A greater understanding of the various serotonin receptor subtypes has led to a clearer appreciation of the role of serotonin in gastrointestinal motility, sensation and secretion. Serotonin is definitely involved in the aetiopathogenesis of cisplatin-induced emesis and carcinoid diarrhoea. The application of serotonergic drugs in clinical therapeutics for gut disturbances is presently dominated by the use of 5-HT3 antagonists for acute chemotherapy-induced nausea and vomiting, and the use of substituted benzamides which are 5-HT4 agonists stimulating gut motor function through 5-HT4 neuronal receptors. The best-studied 5-HT4 agonist is cisapride, which has been shown to stimulate motility at several levels of the gut. Cisapride is approved for healing and maintenance treatment of reflux oesophagitis and is used in several countries for the alleviation of symptoms consistent with regional stasis, from dyspepsia to constipation. Carcinoid diarrhoea is a prototypic disease associated with deranged serotonin metabolism, and a rationale for using 5-HT3 or 5-HT4 antagonists is based on the recent appreciation of the important role of impaired gut motor function in carcinoid diarrhoea. In the future, greater understanding of the serotonin receptor subtypes and their role in gut disorders may lead to novel approaches to alleviate increased visceral perception of functional gastrointestinal disorders, to correct changes in colonic capacitance, or to alter gastrointestinal motility that contributes to diarrhoea or constipation. However, at the present time, it must be stressed that these uses are still at an experimental stage and that careful validation and proper controlled studies are still required.

GR113808: a novel, selective antagonist with high affinity at the 5-HT4 receptor

1. The 5-HT4 receptor has only recently been identified but has yet to be cloned. This paper describes the pharmacology of a potent and selective 5-HT4 receptor antagonist, GR113808, which will be useful in the further characterization of this receptor. 2. On the guinea-pig ascending colon, GR113808 (1 nM-0.1 microM) behaved as an antagonist of 5-hydroxytryptamine (5-HT)-induced contraction, producing rightward displacements of the concentration-effect curve to 5-HT and a concentration-related depression of the maximum effect. However, the compound had no effect on cholecystokinin (CCK-8)-induced contraction in concentrations up to 1 microM. 3. In the guinea-pig colon preparation, onset and offset of the antagonism by GR113808 of 5-HT-induced contraction was examined. Incubation of the tissues for either 15 min, 30 min or 60 min produced similar rightward displacements of the concentration-effect curves to 5-HT, with no increase in the degree of depression of the maxima with increasing time of incubation. Experiments examining offset of antagonism (0.01 microM) demonstrated that washout for 30 min was required to reverse fully the effects of the antagonist. 4. Potency estimates in the colon for GR113808 were made by determining approximate pA2 values (30 min) using the Gaddum equation. The values obtained were 9.2, 9.7 and 9.2 when tested against the agonists 5-HT, 5-methoxytryptamine and R,S-zacopride respectively. 5. On the carbachol-contracted tunica muscularis mucosae preparation of the rat thoracic oesophagus, GR113808 behaved as an antagonist of 5-HT-induced relaxation, producing no reduction in maximum response. Analysis of these data yielded a pA2 of 9.3. GR1 13808 also antagonised the relaxant effects of 5-methoxytryptamine (pA2 = 9.0) and R,S-zacopride (pA2 = 9.4). The compound had no effect on isoprenaline-induced relaxation of the carbachol-contracted oesophagus at a concentration of 1 MicroM.6. In tests of selectivity, GR113808 had only low affinity for 5-HT3 receptors (pKi = 6.0) and had no functional activity at either 5-HT2 or 5-HT1-like receptors on vascular smooth muscle preparations. In a range of binding assays, GRi 13808 was shown to have no appreciable affinity for any other receptor type investigated.7. In the anaesthetized piglet, GRI13808 was a potent antagonist of 5-methoxytryptamine-induced tachycardia (mean DRo = 97.2 microg kg-1 h-1). The compound was ineffective against isoprenaline-induced tachycardia.8. The present results are discussed in comparison with those for existing antagonists at the 5-HT4receptor. The results of this study indicate that GRI13808 will be a valuable antagonist for studying 5-HT4 receptor mechanisms in vitro and in vivo and validate its use as a radioligand for determining 5-HT4 receptor distribution.

The guinea-pig distal colon--a sensitive preparation for the investigation of 5-HT4 receptor-mediated contractions

1. Experiments were designed to characterize pharmacologically the contractile responses to 5-hydroxytryptamine (5-HT) in the guinea-pig isolated distal colon longitudinal muscle-myenteric plexus preparation (LMMP). 2. In the presence of methiothepin (100 nM) and granisetron (1 microM), 5-HT (10 pM-10 nM) produced concentration-dependent contractile responses of the guinea-pig distal colon LMMP, with a pEC50 of 9.2 +/- 0.08. 3. Responses to 5-HT were mimicked by a series of tryptamine analogues, with the following rank order of potency; 5-HT > 5-MeOT >> 5-CT > tryptamine > 2-Me-5-HT. All were found to be full agonists. 4. Responses to 5-HT were also mimicked by a series of substituted benzamide analogues. Their rank order of potency was 5-HT > renzapride > cisapride > (S)-zacopride > (R)-zacopride > metoclopramide. All were full agonists relative to 5-HT. 5. The benzimidazolone derivatives, BIMU 1 and BIMU 8 were approximately equipotent partial agonists (intrinsic activities of 0.8 +/- 0.07 and 0.5 +/- 0.08 respectively) in the guinea-pig distal colon. 6. Tropisetron produced a rightward displacement of the 5-HT concentration-effect curve, yielding an apparent pA2 of 6.4 +/- 0.1. The slope of the Schild plot (1.3 +/- 0.1) was significantly greater than unity. 7. SDZ 205,557 produced a concentration-dependent shift to the right of the 5-HT concentration-response curve, yielding an estimated pA2 of 7.8 +/- 0.1 and a slope which did not significantly deviate from unity. SDZ 205 557 produced similar pKB estimates (7.3-7.9) when tested against 5-MeOT,renzapride and 5-CT, indicating a common site of action.8. The pharmacological profile of the 5-HT-evoked contractions of the guinea-pig distal colon LMMPare consistent with activity at the 5-HT4 receptor. Furthermore, of the models of this receptor described in the literature, the guinea-pig distal colon appears to be the most sensitive model to date, making it a useful tool in the investigation of 5-HT4 receptor-mediated events.

5-HT4 receptor mediated facilitation of the emptying phase of the peristaltic reflex in the guinea-pig isolated ileum

1. The influence of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on the emptying phase (circular muscle contraction) of the peristaltic reflex was investigated in the guinea-pig isolated ileum. 2. The effect of drug application to the serosal surface was measured as the changes in threshold pressure required to trigger the peristaltic reflex and the interval between the peristaltic strokes. A facilitation or inhibition of peristalsis was defined as a reduction or increase in threshold pressure respectively. 3. Peristalsis was not modified by the inclusion of methysergide (1 microM) and/or ondansetron (2 microM) in the bathing medium. 5-HT (0.1-1.0 microM) caused a facilitation of the peristaltic reflex; the response curve to 5-HT was not altered by the presence of methysergide (1 microM) and ondansetron (2 microM). 4. In the presence of methysergide (1 microM) plus ondansetron (2 microM), 5-HT (7.36 +/- 0.06), 5-methoxytryptamine (7.01 +/- 0.17), 5-carboxamidotryptamine (5.43 +/- 0.06), renzapride (6.09 +/- 0.17), (S)-zacopride (5.99 +/- 0.11), (R)-zacopride (5.61 +/- 0.13) and metoclopramide (4.8 +/- 0.65) caused a concentration-related facilitation of the peristaltic reflex, the pEC50 values (mean +/- s.e.mean) being shown in parentheses. 2-Methyl-5-HT was ineffective up to 10 microM. 5. The administration of SDZ 205-557 (1 microM) alone failed to modify the peristaltic reflex, but caused a parallel dextral shift in the concentration-effect curve to 5-HT (apparent pKB 7.38 +/- 0.30). It failed to modify the effect of acetylcholine to enhance the peristaltic reflex. 6. It is concluded that the rank order of potency of the 5-HT agonists from the indole and substituted benzamide series to facilitate the emptying phase of the peristaltic reflex in the guinea-pig ileum closely correlates with their published actions as 5-HT4 agonists in other systems. An agonist action on the 5-HT4 receptor is also supported by the potency of the 5-HT3/5-HT4 antagonist SDZ 205-557 (but not the 5-HT3 antagonist ondansetron) to inhibit the effects of 5-HT.

Characterization of the 5-HT4 receptor mediating tachycardia in piglet isolated right atrium

1. In order to explore whether 5-HT4 receptor subtypes exist, we have characterized further the 5-HT4 receptor that mediates tachycardia in the piglet isolated right atrium. All experiments were carried out in the presence of propranolol (400 nM) and cocaine (6 microM). We used tryptamine derivatives, substituted benzamides and benzimidazolone derivatives as pharmacological tools. 2. Tachycardia responses to 5-hydroxytryptamine (5-HT) were mimicked by other tryptamine derivatives with the following order of potency: 5-HT > 5-methoxytryptamine alpha-methyl-5-HT = bufotenine bufotenine > 5-carboxamidotryptamine = tryptamine (after treatment with pargyline) > 5-methoxy-N,N-dimethyltryptamine > 2-methyl-5-HT. 3. The substituted benzamides were all partial agonists relative to 5-HT except (-)-zacopride which was a full agonist. The stimulant potency order was renzapride > cisapride = (-)-zacopride > metoclopramide > (+)-zacopride. 4. The benzimidazolone derivatives had contrasting effects. BIMU 8 (endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl(eth yl- 2-oxo-1H-benzimidazole-1-carboxamide hydrochloride) was a full agonist relative to 5-HT whilst BIMU 1 (endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-3-ethyl-2-oxo - 1H-benzimidazole-1-carboxamide hydrochloride) was a partial agonist with low intrinsic activity compared to 5-HT but had similar potency. We estimated a pKB of 7.9 for BIMU 1 antagonism of 5-HT-induced tachycardia. DAU 6215 (N-endo-8-methyl-8-azabicyclo[3.2.1]-oct-3-yl)-2,3-dihydro-2-oxo-lH-benzimidazole-l-carboxamide, hydrochloride) had no chronotropic activity and was found to be a simple competitive antagonist with a pKB of 7.15.SB 203186 (1-piperidinyl)ethyl lH-indole 3-carboxylate) was a potent antagonist with a pKB of 8.3.The affinity of SB 203186 was approximately 20 times higher than that of tropisetron (ICS 205-930;pKB= 6.9) and DAU 6215 (pKB= 7.0). GR1 13808 (([1-[2-[methylsulphonyl amino]ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate) and SDZ 205-557 ((2-diethylaminoethyl)2-methoxy-4-amino-5-chloro-benzoate) also antagonized 5-HT-induced tachycardia but not by simple competitive blockade.6. The sinoatrial 5-HT4 receptor in the piglet has a pharmacological profile that correlates well with 5-HT4 receptors characterized in rat oesophagus, guinea-pig ileum and colon, mouse embryonic colliculi neurones and human atrium.

RS 23597-190: a potent and selective 5-HT4 receptor antagonist

1. The pharmacological properties of RS 23597-190 (3-(piperdine-1-yl)-propyl-4-amino-5-chloro-2-methoxy benzoate hydrochloride) have been studied in vitro and in vivo. 2. RS 23597-190 competitively antagonized 5-HT4 receptor-mediated relaxations of rat, carbachol precontracted oesophageal muscularis mucosae, (pA2 = 7.8 +/- 0.1; Schild slope = 1.2 +/- 0.2). Affinity estimates (-log KB) at 5-HT4 receptors using either renzapride or SC-53116 as agonists yielded a -log KB value of 8.0 +/- 0.01. In contrast, RS 23597-190 failed to antagonize contractile responses to 5-HT of guinea-pig ileal 5-HT3 receptors, even at concentrations up to 10 microM. 3. Increases in short-circuit current, induced by 5-HT, were studied in guinea-pig ileal mucosal sheets. Concentration-response curves to 5-HT were biphasic, with the high potency phase to 5-HT inhibited by RS 23597-190 and mimicked by 5-methoxytryptamine. The -log KB value for RS 23597-190 at the high potency phase was 7.3 confirming that 5-HT4 receptors mediated the high potency phase. 4. In rat isolated vagus nerve, 5-HT elicited a slow, maintained depolarization at low concentrations and a rapid, transient depolarization at higher concentrations. The high potency, slow depolarizing phase to 5-HT was abolished selectively in the presence of 1 microM RS 23597-190 and the low potency phase was abolished selectively in the presence of 1 microM ondansetron. These data confirm that 5-HT4 and 5-HT3 receptors mediated slow and fast depolarization responses, respectively. 5. At 5-HT3 binding sites in membranes from NG 108-15 cells, labelled by [3H]-quipazine, RS 23597-190 exhibited an apparent affinity (- log Ki) of 5.7 +/- 0.1. At 5-HT3 receptors in membranes from rat cerebral cortex, labelled by [3H]-RS 42358-197, the apparent affinity (- log Ki) of RS 23597-190 was also 5.7 +/- 0.1. In both studies, Hill coefficients were not significantly different from unity. At 5-HT1A, 5-HT2,muscarinic M1, M2, M3, M4 and dopamine D1 and D2 receptors, RS 23597-190 exhibited low apparent affinities, with all - log Ki values less than 5.5.6. Intravenous infusion of RS 23597-190 in the conscious, restrained rat antagonized the von Bezold Jarisch reflex induced by 2-methyl 5-HT, with an ID50 of 300 microg kg-1 min-1, i.v. In the anaesthetized,bilaterally vagotomized micropig, RS 23597-190 (6 mg kg-1, i.v.) antagonized 5-HT-induced tachycardia with a half-life of 77 (63-99) min. Transient arrhythmic effects were noted after administration of the compound.7. In conclusion, RS 23597-190 acts as a high affinity, selective competitive antagonist at 5-HT4 receptors. Thus, the compound appears to be a useful tool for 5-HT4 receptor identification in vitro. In vivo, the compound is rapidly metabolized in pigs such that 5-HT4 blockade is not maintained. However,in the rat, when given by infusion, RS 23597-190 antagonizes 5-HT3 mediated responses, at doses consistent with a low affinity 5-HT3 receptor. These data suggest that, under appropriate experimental conditions, RS 23597-190 may also be used in vivo to characterize further 5-HT4 receptor function.

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

1. The 5-HT4 receptor antagonist, GR113808, has been radiolabelled to a high specific activity with tritium. 2. Characterization of specific [3H]-GR113808 binding in homogenates of guinea-pig striatum and hippocampus revealed a single site of high affinity (Kd values 0.20 and 0.13 nM respectively). 3. [3H]-GR113808 binding was reversible and displayed rapid kinetics such that association and dissociation were complete within 3 min. 4. Specific [3H]-GR113808 binding was potently and stereoselectively inhibited by agonists and antagonists acting at the 5-HT4 receptor but not by compounds selective for other 5-HT receptors or other neurotransmitter receptors. 5. Autoradiographic analysis revealed a discrete localization in both guinea-pig and rat brain with high concentrations of binding in brain areas such as the striatum, substantia nigra and olfactory tubercle. 6. [3H]-GR113808 binding to homogenates of guinea-pig striatum meets the criteria for labelling of the 5-HT4 receptor and, as such, represents the first characterization of this receptor in a radioligand binding assay.

Cisapride and a structural analogue, R 76,186, are 5-hydroxytryptamine4 (5-HT4) receptor agonists on the guinea-pig colon ascendens

The purpose of this study was to investigate whether the effects of cisapride and its close structural analogue R 76,186 on the isolated guinea-pig colon ascendens, are mediated through 5-HT4 receptors. Both cisapride and R 76,186 induced contractions in a concentration-dependent fashion, giving monophasic concentration-response curves (cisapride: EC50 = 1.1 x 10(-7) M, maximum effect = 40.3% of methacholine-induced (3 x 10(-7) M) contractions; R 76,186: EC50 = 2.4 x 10(-8) M, maximum effect = 52.1%). Blockade of either 5-HT2 or 5-HT3 receptors did not affect the responses to cisapride. However, tropisetron (in 5-HT4 receptor-blocking concentrations), and DAU 6285 and SDZ 205-557, two novel selective 5-HT4 receptor antagonists, depressed the concentration-response curve to cisapride (to about 50%), and the curve to R 76,186 was shifted to the right. The apparent pA2 values were 6.6 (tropisetron), 6.3 (DAU 6285), and 7.5 (SDZ 205-557). However, none of these antagonisms was purely competitive as higher concentrations of these antagonists depressed the curve to R 76,186. Desensitization of the 5-HT4 receptor with 5-methoxytryptamine (5-MeOT) inhibited the responses to cisapride, and abolished those to R 76,186. The contractions to cisapride and R 76,186 were sensitive to mutual antagonism, depressing their concentration-response curves.

CONCLUSIONS

Both cisapride and R 76,186 mediate their contractile effects in the guinea-pig colon ascendens through agonism at the 5-HT4 receptor, though cisapride also uses a non-5-HT mechanism. R 76,186 is a selective and potent 5-HT4 receptor agonist.

The action of SDZ 205,557 at 5-hydroxytryptamine (5-HT3 and 5-HT4) receptors

1. The interaction of the novel antagonist, SDZ 205,557 (2-methoxy-4-amino-5-chloro benzoic acid 2-(diethylamino) ethyl ester), at 5-HT3 and 5-HT4 receptors has been assessed in vitro and in vivo. 2. In guinea-pig hippocampus and in the presence of 0.4 microM 5-carboxamidotryptamine, 5-HT4-mediated stimulation of adenylyl cyclase was competitively antagonized by SDZ 205,557, with a pA2 value of 7.5, and a Schild slope of 0.81. In rat carbachol-contracted oesophagus, 5-HT4-receptor mediated relaxations were surmountably antagonized by SDZ 205,557 with a similar pA2 value (7.3). This value was agonist-independent with the exception of (R)-zacopride, against which a significantly lower value (6.4) was observed. 3. In functional studies of 5-HT3 receptors, SDZ 205,557 exhibited an affinity of 6.2 in guinea-pig ileum compared with 6.9 at binding sites labelled by [3H]-quipazine in NG108-15 cells. In the anaesthetized, vagotomized micropig, SDZ 205,557 produced only a transient blockade of 5-HT4-mediated tachycardia. This contrasted with tropisetron, which was active for over 60 min after administration. The half-lives for the inhibitory responses of SDZ 205,557 and tropisetron were 23 and 116 min, respectively. 4. In conclusion, SDZ 205,557 has similar affinity for 5-HT3 and 5-HT4 receptors. The apparent selectivity observed in guinea-pig is due to the atypical nature of the 5-HT3 receptor in this species. The short duration of action of this novel antagonist may complicate its use in vivo. SDZ 205,557 should, therefore, be used with appropriate caution in studies defining the 5-HT4 receptor.

5-HT4 receptors, present in piglet atria and sensitive to SDZ 205-557, are absent in papillary muscle

Putative 5-HT4 receptors were investigated in isolated piglet left atria and papillary muscles. In atrial tissues, 5-hydroxytryptamine (5-HT) was a potent positive inotropic agent with a pD2 of 6.7. Its effects were antagonised in a selective and competitive manner by the 5-HT4 receptor antagonist, SDZ 205-557, with a pA2 of 7.3. In contrast to atrial tissues, piglet papillary muscles were virtually unresponsive to 5-HT 10(-8)-10(-3) M, suggesting that functional cardiac 5-HT4 receptors are not present in ventricular tissue.