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

Effect of a series of 5-HT4 receptor agonists and antagonists on steroid secretion by the adrenal gland in vitro

We have previously shown that serotonin (5-hydroxytryptamine, 5-HT) stimulate corticosterone and aldosterone secretion from perifused frog adrenal gland in vitro through activation of 5-HT4 receptors. In the present study, we have used this model to investigate the effect of newly discovered 5-HT4 receptor agonists and antagonists on corticosteroid secretion. Serotonin, the benzamide derivatives (R,S)-zacopride ((R,S)-4-amino-N-(1- azabicyclo[2.2.2]oct-3-yl)-5-chloro-2-methoxybenzamide, HCI) and its enantiomers, the azabicycloalkyl benzimidazole derivatives 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, HCl) and BIMU 8 (endo-N-(8-methyl-8- azabicyclo-[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1H- benzimidazole-1-carboxamide, HCl) were all capable of enhancing corticosterone and aldosterone secretion in a dose-dependent manner. Serotonin was the most potent stimulator of steroidogenesis (EC50 = 1.5 x 10(-7) M) while the potency of the benzamide and the benzimidazolone derivatives was approximately 10 times lower. The rank order of efficacy of the different 5-HT4 receptor agonists was: (S)-zacopride > BIMU 8 = (R,S)-zacopride > BIMU 1 = (R)-zacopride = 5-HT. The stimulatory effects of 5-HT and the benzimidazolone derivatives on corticosteroid secretion were not additive, suggesting that they activated the same receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of [3H]5-hydroxytryptamine from the intermediate area of rat thoracic spinal cord is modulated by presynaptic autoreceptors

Serotonin (5-HT) nerve terminals innervate sympathetic preganglionic neurons of the intermediolateral cell column (IML); however, neither the depolarization-induced release of 5-HT nor the presence of presynaptic modulatory autoreceptors have been directly studied in this system. We used in vitro superfusion of the microdissected intermediate area (including the intermediolateral cell column, intercalated nucleus, and central autonomic nucleus) of the rat thoracic spinal cord to measure basal and stimulated release of preloaded [3H]5-HT. Elevated K+ evoked a concentration- and Ca(2+)-dependent release of [3H]5-HT. Exogenous 5-HT and the 5-HT1B agonist, CGS-12066B, both decreased the K(+)-stimulated release of [3H]5-HT. A 5-HT1B antagonist (methiothepin) blocked the 5-HT- and the CGS-12066B-induced inhibition of K(+)-evoked release of [3H]5-HT. A 5-HT1A antagonist (NAN-190) did not alter the inhibitory actions of exogenous 5-HT. Moreover, a 5-HT1A agonist (8-OH-DPAT), a 5-HT2A/2C agonist [(+/-)-DOI hydrochloride), and a 5-HT3 agonist (2-methyl-5-HT) did not alter the K(+)-evoked release of [3H]5-HT. These data demonstrate that 5-HT is released from the intermediate area of the rat thoracic spinal cord. The 5-HT receptor subtype involved in the inhibition of the evoked release of [3H]5-HT is of the 5-HT1B subtype. These findings may help clarify the complex role of 5-HT in spinal regulation of the sympathetic nervous system.

Antagonistic properties of McNeil-A-343 at 5-HT4 and 5-HT3 receptors

1. This study describes the in vitro interaction of the muscarinic ligand McNeil-A-343 with two 5-hydroxytryptamine (5-HT) receptor subtypes, the 5-HT4 and 5-HT3 receptors, using functional as well as radioligand binding studies. 2. In the rat oesophageal muscularis mucosae, precontracted with carbachol, McNeil-A-343 was a competitive antagonist (pA2 6.2) of the 5-HT4 receptor which mediates the relaxation induced by 5-HT. The compound per se relaxed the oesophagus at high concentration only (> or = 10 microM), an effect unchanged by desensitization of the 5-HT4 receptor with 10 microM 5-methoxytryptamine. In the same preparation in the absence of tone, McNeil-A-343 displaced the carbachol concentration-response curve to the right, yielding an apparent affinity (pA2) of 4.9 for muscarinic receptors. 3. In the rat isolated superior cervical ganglion preparation, after blockade of muscarinic and nicotinic receptors, McNeil-A-343 caused a concentration-dependent depolarization that was unaffected by 100 nM ondansetron. The concentration-fast depolarization curve to 5-HT, mediated by the 5-HT3 receptor, was displaced to the right by McNeil-A-343, which showed an apparent affinity (pA2) of 4.8 for the 5-HT3 subtype. 4. In binding studies, McNeil-A-343 recognized a single population of 5-HT4 receptors in pig caudate nucleus, with a pKI of 5.9. The binding affinity of McNeil-A-343 for 5-HT3 receptors in NG 108-15 cells was approximately four times lower (pKI 5.3). Binding affinities (pKI) for muscarinic receptor subtypes in rat tissues were 5.3 (M1, cortex), 5.2 (M2, heart) and 4.9 (M3, submandibular glands), respectively. 5. McNeil-A-343 is an antagonist at 5-HT4 and 5-HT3 receptors; the interaction of the compound with these receptor subtypes (notably the 5-HT4) occurs in a range of concentrations which generally overlaps that relevant to the interaction with muscarinic receptors.

5-Hydroxytryptamine receptors with a 5-HT6 receptor-like profile stimulating adenylyl cyclase activity in pig caudate membranes

This study deals with the characterization of 5-hydroxytryptamine (5-HT, serotonin) receptors positively linked to adenylyl cyclase in membranes from pig brain caudate. 5-HT and related agonists induced a concentration-dependent stimulation of adenylyl cyclase activity in pig caudate membranes, with the following rank order of potency (mean pEC50 values): 5-HT (7.1) > or = 5-methoxytryptamine (6.9) > 5-carboxamidotryptamine (5.6) > sumatriptan (< 5). Maximal stimulation by 5-HT averaged 35 pmol cyclic AMP/min/mg protein over a basal activity of 159 pmol cyclic AMP/min/mg protein. 5-Methoxytryptamine and 5-carboxamidotryptamine had similar efficacies to that of 5-HT, whereas sumatriptan was about half efficacious. Other compounds known as agonists at some 5-HT receptors were weakly potent (mean pEC50 values < 5). They include the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), the 5-HT4 receptor agonist, renzapride and the 5-HT2 receptor agonist, (1-(2,5-dimethoxy-4-iodophenyl)-2 aminopropane) (DOI). In antagonist studies, methiothepin (0.1 and 1 mumol/l) shifted the 5-HT curve to the right with no depression of the Emax, yielding pKB values of 7.4-8.0. Clozapine (1 mumol/l) also produced surmountable antagonism of 5-HT-induced effects (pKB 6.9). Ketanserin (10 mumol/l) weakly antagonized 5-HT (pKB 5.0). The 5-HT4 receptor antagonists, tropisetron (ICS 205-930) and SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester), each at 1 mumol/l, did not significantly alter the concentration-response curve of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of 5-hydroxytryptamine-induced depolarizations in rat isolated vagus nerve

An additional component of the depolarization induced by 5-hydroxytryptamine (5-HT) in the rat isolated vagus nerve has recently been attributed to activation of 5-HT4 receptors. To confirm and extend this finding, extracellular recordings of D.C. potentials were made using the 'grease-gap' technique during continuous superfusion of the isolated nerve. Beginning at 1 nM, 5-HT induced small depolarizations that displayed a slow onset. At concentrations > or = 1 microM, large depolarizations with rapid onset were elicited. In the presence of the 5-HT3 receptor antagonists, granisetron or ondansetron, 5-HT responses were diminished and exhibited an increased latency to peak. These small, slow depolarization were not reduced by 5-HT1 or 5-HT2 receptor antagonists, but were potently inhibited by the 5-HT4 receptor antagonist GR 113808 (pA2 = 9.3), and mimicked by 5-methoxytryptamine (pEC50 = 5.3). 5-HT4-mediated responses were larger at 37 degrees C than at 31 degrees C, but also showed marked diminution with repeated 5-HT applications at concentrations greater than 1 microM. Conversely, 5-HT3 receptor responses were potentiated at lower temperatures (< or = 31 degrees C). Consistent with the reported positive coupling of 5-HT4 receptors to adenylyl cyclase, forskolin and 8-Br-cAMP produced slowly developing depolarizations which were qualitatively similar to 5-HT4 receptor activation. Pre-depolarization of nerves with 10 microM forskolin or 300 microM 8-Br-cAMP diminished the effect of 5-HT4 receptors. This study has confirmed the presence of 5-HT4 receptors on the vagus nerve of the rat and defined some conditions that optimize their pharmacological isolation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Blockade of human atrial 5-HT4 receptors by SB 207710, a selective and high affinity 5-HT4 receptor antagonist

The mode of antagonism of 5-hydroxytryptamine-induced positive inotropic effects by the highly selective 5-HT4 receptor antagonist SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate) was investigated on isolated preparations of human right atrial appendage. SB 207710 caused concentration-dependent (0.1-10 nmol/l) surmountable antagonism of the effects of 5-hydroxytryptamine with a pKB (mol/l) of 10.1. Due to its high selectivity and affinity, SB 207710 could be a powerful tool for the comparison of human atrial 5-HT4 receptors with 5-HT4 receptors of other organs of man and other species.

Gastroprokinetic properties of the benzimidazolone derivative BIMU 1, an agonist at 5-hydroxytryptamine4 and antagonist at 5-hydroxytryptamine3 receptors

We have investigated the in vivo motor stimulating and gastroprokinetic properties of the azabicycloalkyl benzimidazolone derivative BIMU 1 (3-ethyl-2,3-dihydro-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2-oxo-1H- benzimidazole-1-carboxamide hydrochloride) and its binding profile at 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors, in an attempt to assess the serotonergic mechanism underlying its prokinetic action. BIMU 1 dose-dependently (0.01-0.3 mg/kg i.v.) increased the motility of a denervated pouch of canine stomach. This excitatory action was sensitive to muscarinic blockade. A similar stimulatory effect was exerted by the benzamidic prokinetic agent cisapride (0.03-0.3 mg/kg i.v.) but not by the 5-HT3 receptor antagonist ondansetron (up to 1 mg/kg i.v.). The significance for propulsive efficacy of the motor stimulating activity of BIMU 1 was evaluated in a model of gastric emptying of liquids in the conscious dog. The emptying rate of a non-caloric liquid meal instilled through a gastric fistula was accelerated by both BIMU 1 (0.01-1 mg/kg i.v. and 0.1-3 mg/kg p.o.) and cisapride (0.03-1 mg/kg i.v. and 0.3-10 mg/kg p.o.). Ondansetron (1 mg/kg i.v.) did not show any effect. The activity of the 5-HT4 receptor antagonist DAU 6285 was evaluated in the gastric emptying model per se and in interaction experiments on the accelerating action of BIMU 1 (0.3 mg/kg i.v.).(ABSTRACT TRUNCATED AT 250 WORDS)

Receptors for 5-hydroxytryptamine: current perspectives on classification and nomenclature

With the increasing number of 5-HT receptors recently identified, using molecular biology techniques, the classification of 5-HT receptors is under review. An integrated approach is proposed to include operational and transductional as well as structural criteria for definitive receptor characterization. On this basis the existence of as many as seven classes of 5-HT receptor are recognized although only the 5-HT1, 5-HT2 and 5-HT3 receptor classes are well defined.

Regional distribution and ontogeny of 5-HT4 binding sites in rodent brain

We have investigated the regional distribution of 5-hydroxytryptamine4 (5-HT4) receptor binding sites in the adult guinea pig, rat and mouse brain using the specific 5-HT4 antagonist [3H]GR113808 as a radioligand. The developmental changes in the expression of these binding sites were also investigated quantitatively in the rat brain (gestational days 16 and 19; postnatal days 1, 3, 7, 9, 12 and 21). In order to compare previously obtained data on primary cultures, semi-quantitative analysis was also performed during mouse brain ontogeny (postnatal days 1, 7 and 11). The main finding of this study is that 5-HT4 receptors have comparable, wide and heterogeneous distributions in the adult brain of the species investigated, with densities reaching adult levels between the second and third postnatal week in most regions of the rat and mouse brain. In contrast, a progressive loss of 5-HT4 binding sites is observed in the pons, whereas a transient peak of receptor expression is seen during the second postnatal week in the globus pallidus and substantia nigra pars lateralis. The developmental pattern of 5-HT4 receptor distribution suggests, except in latter regions, that these receptors probably exert a minor role in developmental processes. In the adult, high densities of [3H]GR113808 binding sites are present in various regions belonging to limbic system (islands of Calleja, olfactory tubercle, fundus striati, ventral pallidum, septal region, hippocampus, amygdala), or known to be components of different pathways, such as the hippocampo-habenulo-interpeduncular and the striato-nigro-tectal pathways. While the regional distributions of [3H]GR113808 binding sites were identical in the mouse and rat, some differences were observed in the guinea-pig, in particular in the globus pallidus, substantia nigra and interpeduncular nucleus. The expression of 5-HT4 receptors in limbic areas is highly suggestive of a role for these receptors in emotional processes, whereas their expression in the striato-nigral-tectal pathway might be indicative of a role in the control of visuo-motor activity.

(R) and (S) RS 56532: mixed 5-HT3 and 5-HT4 receptor ligands with opposing enantiomeric selectivity

The pharmacological properties of the (R) and (S) enantiomers of RS 56532 have been studied in vitro and in vivo. In radioligand binding studies at 5-HT4 receptors in guinea-pig striatum, (S) RS 56532 exhibited a higher affinity than (R) RS 56532 (-log Ki = 7.6 and 6.5, respectively). (S) RS 56532 acted as a potent agonist at 5-HT4 receptors mediating relaxation of rat oesophageal muscularis mucosae (-log EC50 = 7.9) while (R) RS 56532 acted as a weaker agonist at this receptor (-log EC50 < 6.0). These data suggest that at 5-HT4 receptors, the enantiomeric selectivity of RS 56532 was (S) > (R). In binding studies at 5-HT3 receptors in rat cortex, (R) RS 56532, conversely, exhibited a higher affinity than (R) RS 56532 (-log Ki = 9.1 and 8.0, respectively). At 5-HT3 receptors in guinea-pig isolated ileum, (R) RS 56532 exhibited an affinity (-log KB) of 7.9, whereas (S) RS 56532 (1 nM-1 microM) was inactive. No agonism was observed at ileal 5-HT3 receptors with either enantiomers. These data suggest that at 5-HT3 receptors in rat and guinea-pig, both enantiomers acted as antagonists, with (R) > (S) RS 56532. At the non-5-HT3, high affinity '(R) zacopride' site, (R) RS 56532 exhibited a higher affinity than (S) RS 56532 (-log Ki = 6.1 and 4.9). This site was insensitive to potent 5-HT3 antagonists such as (R) YM 060 or ondansetron. However, it was recognized with relatively high affinity (-log Ki = 7.5) by the (R), but not (S) enantiomer, of RS 42358 (-log Ki = 4.7). Since (S) RS 42358 is a high affinity 5-HT3 receptor antagonist, these data further highlight the dissimilarity between the 5-HT3 receptor and the '(R) zacopride' site. The '(R) zacopride' site also appeared to be pharmacologically distinct from the 5-HT4 receptor, since 5-HT4 ligands such as renzapride, SDZ 205,557 or RS 23597-190 exhibited low affinities. The enantiomeric selectivity of (R) and (S) RS 56532 in vivo was consistent with findings in vitro. At 5-HT4 receptors mediating tachycardia in the pig, 5-HT induced a dose-dependent tachycardia (ED50 = 3 micrograms kg-1, i.v.; maximum response = 90-100 beats min-1). (S) RS 56532 increased heart rate by 88 min-1 with a potency of (ED50) of 3 micrograms kg-1, i.v.(ABSTRACT TRUNCATED AT 400 WORDS)

5-HT1D as well as 5-HT1A autoreceptors modulate 5-HT release in the guinea-pig dorsal raphé nucleus

5-Hydroxytryptamine (5-HT) release was measured by fast cyclic voltammetry in guinea-pig dorsal raphé nucleus slices. Release was reproducibly evoked by a single 0.1 msec pulse of electrical stimulation. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH DPAT) produced a concentration-related inhibition of the stimulated 5-HT release, with 50% inhibition at 47 nM. This inhibition was competitively antagonized by N-tert-butyl 3-4-(2-methoxypheny)piperazin-1-yl-2- phenylpropanamide dihydrochloride [(+/-)WAY 100135], a selective 5-HT1A receptor antagonist (pA2 = 7.9). The 5-HT1D receptor agonist sumatriptan also produced a concentration-related inhibition of 5-HT release, with 50% inhibition at 40 nM. The effect of sumatriptan on 5-HT release was antagonized by the 5-HT1D receptor antagonist 2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyli c acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR127935) (pA2 = 8.7). Both (+/-)WAY 100135 and GR127935 increased the 5-HT release evoked by a train of 5 pulses at 1 Hz, suggesting that they were antagonizing the feedback of endogenously released 5-HT onto its autoreceptors. These findings demonstrate for the first time the presence of functional 5-HT1D as well as 5-HT1A autoreceptors in the guinea-pig dorsal raphé nucleus.

Identification of serotonin 5-HT4 recognition sites in the porcine caudate nucleus by radioligand binding

Specific binding for the serotonin 5-HT4 receptor (5-HT4R) radioligand [3H]GR 113808 was identified in pig caudate nucleus and characterized by serotonin subtype selective drugs. Binding was inhibited by serotonin and by synthetic indoles, benzamides and benzimidazolones known to characterize the 5-HT4R in functional tests. Rank order of potency of 5-HT4R antagonists was: GR 125487 (Ki, 0.19 nM) > GR 113808 >> SC 53606 > SDZ 205,557 > RS 235971/190 > DAU 6285 > tropisetron > DAU 6215. GR 125487 and GR 113808 were highly selective with respect to the 5-HT3 receptor (5-HT3R). Rank order of potency of 5-HT4R agonists was: SC 53116 (Ki, 21 nM) > BIMU 1 > cisapride > BIMU 8 > serotonin > renzapride > S-zacopride > metoclopramide > R-zacopride > 5-methoxytryptamine >> 5-carboxamidotryptamine. BIMU 8, renzapride, metoclopramide and the zacopride enantiomers gave shallow competition curves. The agonists were substantially less selective than the antagonists with respect to the 5-HT3R. With only two exceptions, SCH 23390 and metergoline, which bound with sub-microM affinity to the 5-HT4R, binding was not inhibited by compounds selective for other G-protein-coupled or channel-gated receptors. Highly significant correlations in affinities of compounds for 5-HT4R in caudata of pigs, guinea pigs and humans were found suggesting no difference among mammalian species.

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.

Identification and characterization of serotonin 5-HT4 receptor binding sites in human brain: comparison with other mammalian species

Specific binding for the 5-HT4-selective radioligand [3H]GR 113808 has been identified in human and calf brain membranes. Using human tissue the distribution of the binding was heterogeneous throughout different brain regions, being highest in the caudate nucleus. For this region a Kd value of 0.59 +/- 0.08 nM and a Bmax of 225 +/- 2.6 fmol/mg were obtained. Other regions with substantial densities were the lenticular nucleus, the substantia nigra, the hippocampus and the frontal cortex, whereas no binding could be detected in the cerebellum. The ability of several standard compounds in displacing the radioligand was compatible with the labelling of 5-HT4 receptors. Correlation analysis showed no significant differences amongst data obtained for these compounds using human, calf and guinea-pig membranes.

Evidence for an inhibitory 5-HT4 receptor in urinary bladder of rhesus and Cynomolgus monkeys

1. The present study shows that 5-hydroxytryptamine (5-HT) inhibits electrically-evoked contractions of isolated urinary bladder strips from Rhesus and Cynomolgus monkeys via activation of 5-HT4 receptors. 2. 5-HT (0.1 nM-10 microM) produced concentration-dependent inhibition of the contractile response to electrical stimulation yielding a pEC50 of 7.8 (Rhesus monkey) and 7.6 (Cynomolgus monkey). This action of 5-HT was mimicked by 5-methoxytryptamine, renzapride and BIMU 8, each of which behaved as a full agonist relative to 5-HT. However, the potency estimate for BIMU 8 (pEC50 = 6.5) in Cynomolgus monkey was low, relative to 5-HT, indicating a possible heterogeneity of 5-HT4 receptors. 3. The inhibitory action of 5-HT was resistant to antagonism by methysergide (1 microM) and ondansetron (5 microM), thereby eliminating a role for 5-HT1, 5-HT2 and 5-HT3 receptors. The 5-HT4 receptor antagonists, GR 113808 (10 nM), DAU 6285 (1-10 microM) and RS 23597-190 (1 microM), produced parallel, dextral displacements of the concentration-effect curves to 5-HT and other related agonists with affinity estimates in agreement with those defined previously in other 5-HT4 receptor assay systems. 4. Experiments using direct electrical stimulation of bladder smooth muscle indicate that the 5-HT4 receptors are located post-junctionally. 5. The inhibitory action of 5-HT in isolated urinary bladder of monkey differs from the excitatory effect of 5-HT in urinary bladder of man. Species variation and its implications for the development of therapeutic agents are discussed.

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.

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.