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

Selective and functional 5-hydroxytryptamine4 receptor antagonism by SB 207266

1. The pharmacology of a novel 5-HT4 receptor antagonist, SB 207266 has been evaluated in vitro in the guinea-pig distal colon longitudinal muscle myenteric plexus (LMMP) and in vivo in the dog Heidenhain pouch. 2. SB 207266 is a highly potent antagonist of 5-HT-evoked, cholinergically-mediated contractions in the guinea-pig distal colon. Low concentrations (0.1-10 nM) produced a parallel shift to the right of the concentration-effect curve (apparent pA2 10.6 +/- 0.1) with no significant effect on the maximum response. With higher concentrations of SB 207266 (30 nM and above) the maximum response to 5-HT was reduced. 3. The antagonism seen with SB 207266 cannot be attributed to a non-selective effect since high concentrations (1 microM) had no effect on cholinergically-mediated contractions evoked by the nicotinic receptor agonist DMPP in the same preparation. 4. SB 207266 is not an irreversible antagonist since the effects of the compound were reversible upon washing of the tissue. 5. In the dog Heidenhain pouch, oral (0.1-100 micrograms kg-1) and intravenous (0.1-100 micrograms kg-1) administration of SB 207266 produced a dose-dependent antagonism of the contractions evoked by a bolus intravenous injection of 5-HT. An ID50 for SB 207266 of 1.3 micrograms kg-1 was obtained following i.v. administration and 9.6 micrograms kg-1 following oral administration. 6. The antagonistic effects of SB 207266 (0.1-100 micrograms kg-1) in the dog Heidenhain pouch were long lasting since, following oral administration, the response to 5-HT was reduced for at least 135 min. 7. SB 207266 is a highly potent, highly selective and orally active 5-HT4 receptor antagonist. This compound is the first orally active amide to be identified in this class of antagonists and as such is an important new tool in the evaluation of 5-HT4 receptor function both in vitro and in vivo.

[3H]5-hydroxytryptamine labels the agonist high affinity state of the cloned rat 5-HT4 receptor

We have used the cloned rat 5-HT4 receptor, and determined that the single protein product produced is able to bind both [3H]5-HT and [3H]GR113808 ([1-[(2-methyl sulphonyl) amino] ethyl-4-piperidinyl] methyl-1-methyl-1H-indole-3-carboxylate) with high affinity. The affinities of agonists for the [3H]5-HT (agonist)-labelled receptor were significantly higher than for the [3H]GR113808 (antagonist)-labelled receptor. Furthermore, [3H]5-HT binding was reduced by addition of guanyl nucleotides. These results strongly support the hypothesis that the 5-HT4 receptor displays two interconvertible affinity states (high and low) for agonists, characteristic of many G protein coupled receptors. [3H]5-HT, at the concentration employed, therefore labels the agonist high affinity state of the 5-HT4 receptor in systems in which high densities of this receptor are found.

Central cholinergic antinociception induced by 5HT4 agonists: BIMU 1 and BIMU 8

The antinociceptive effect of two 5-HT4 agonists, BIMU 1 and BIMU 8, were examined in mice and rats by using the hot-plate, abdominal constriction and paw-pressure tests. In both species, BIMU 1 (10-20 mg kg-1 s.c. and 40-60 mg kg-1 p.o. in mice; 20 mg kg-1 i.p. in rats) and BIMU 8 (20-30 mg kg-1 s.c. and 60 mg kg-1 p.o. in mice; 20 mg kg-1 i.p. in rats), produced significant antinociception which was prevented by atropine (5 mg kg-1 i.p.), hemicholinium-3 (1 microgram per mouse i.c.v.), SDZ 205-557 (10 mg kg-1 i.p.), GR 125487 (20 mg kg-1 i.p.) but not by naloxone (1 mg kg-1 i.p.), CGP 35348 (100 mg kg-1 i.p.) and reserpine (2 mg kg-1 i.p.). Moreover, BIMU 1 and BIMU 8 increase of pain threshold, is abolished by nucleus basalis magnocellularis (NBM) lesions in rats. SDZ 205-557 and GR 125487 which totally antagonized BIMU 1 and BIMU 8 antinociception did not modify morphine (7 mg kg-1 s.c.) or baclofen (4 mg kg-1 s.c.) antinociception. Intracerebroventricular injection in mice of BIMU 1 (3 micrograms per mouse) and BIMU 8 (10 micrograms per mouse), doses which were largely ineffective by parenteral routes, induces an antinociception whose intensity equaled that obtainable s.c., i.p. or p.o. In the antinociceptive dose-range, neither 5HT4 agonist impaired mice motor coordination evaluated by rota-rod test. On the basis of the above data, it can be postulated that BIMU 1 and BIMU 8 exerted an antinociceptive effect mediated by a central amplification of cholinergic transmission.

5-HT3 and 5-HT4 receptor-mediated facilitation of the emptying phase of the peristaltic reflex in the marmoset isolated ileum

1. The patterns of intestinal motility and the effect of an increase in intraluminal pressure were studied in vitro on segments obtained from the marmoset small intestine. 2. Segments obtained from the distal half of the marmoset small intestine exhibited segmentation, consisting of narrow annular contractions (lasting for 2-3 s) interposed between the relaxed segments of varying length. The subsequent contractions occurred slightly distal to the previous contraction, with ring-like contractions appearing to move in the aboral direction. Such segmentation was infrequent or absent in the segments obtained from the proximal half of the small intestine. An increase in intraluminal pressure inhibited segmentation and finally produced peristalsis in most of the tissues. 3. The influence of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on the threshold of the peristaltic reflex was investigated in the segments obtained from the distal half of the intestine after segmentation had subsided. The effect of drug application to the serosal surface was measured as a change in threshold pressure required to trigger the peristaltic reflex. A facilitation was defined in two ways (a) as a reduction in the threshold pressure required to trigger the reflex and (b) in those tissues that failed to respond with peristalsis on raising intraluminal pressure to the maximum attainable (1 kPa), as a reduction in threshold pressure compared to this value. 4. 5-HT (7.85 +/- 0.19), 5-methoxytryptamine (7.79 +/- 0.24), 5-carboxamidotryptamine (6.66 +/- 0.13) and 2-methyl-5-HT (6.24 +/- 0.16) caused a concentration related facilitation of the peristaltic reflex, the pD2 values (mean +/- s.e.mean) being shown in parentheses. 5. The concentration-response curves to both 5-HT and 5-methoxytryptamine were dextrally shifted in a surmountable manner in the presence of GR 113808 (30 nM). pD2 values for 5-HT and 5-methoxytryptamine were significantly decreased to 6.98 +/- 0.24 and 6.83 +/- 0.36 respectively in the presence of GR 113808 (30 nM). 6. In the presence of a high concentration of (10 microM) 5-methoxytryptamine the subsequent addition of 2-methyl-5-HT (3-10 microM) but not 5-methoxytryptamine (10 microM) facilitated peristalsis; the effect of 3 microM 2-methyl-5-HT was significantly decreased by 2 microM ondansetron. 7. It is concluded that the facilitation of the peristaltic reflex in the marmoset intestine induced by 5-HT at submicromolar concentrations involves a 5-HT4 receptor stimulation with an additional 5-HT3 receptor activation at higher concentrations.

Comparison of the densities of 5-HT4 receptors, beta 1- and beta 2-adrenoceptors in human atrium: functional implications

We measured in human atrium the density of 5-HT4 receptors, labelled with [125I]-SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1, 4-benzodioxan-5-carboxylate), and compared it with the density of beta1- and beta2-adrenoceptors, labelled with (-)-[125I]-cyanopindolol. [125I]-SB 207710 (5-1200 pmol/l) labelled a small population of saturable binding sites (Bmax approximately 4 fmol/mg protein) with a pK(D) of 9.7 and with 5-HT4 receptor characteristics, as assessed with competing ligands. The density of atrial binding sites with 5-HT4 receptor characteristics was 10 and 5 times lower, respectively, than the density of beta 1- and beta 2-adrenoceptors. We suggest that the small 5-HT4 receptor population may in part explain why the positive inotropic effects of 5-HT are smaller than those of catecholamines mediated through beta 1- and beta 2-adrenoceptors.

Fenfluramine-induced activation of the immediate-early gene c-fos in the striatum: possible interaction between serotonin and dopamine

DL-Fenfluramine, a serotonin (5-HT) releasing agent, induces rapid expression of Fos-like immunoreactivity (Fos-LI) in the striatum as well as in other brain structures receiving a dense 5-HT innervation. Fenfluramine-induced Fos-LI expression in the striatum may result directly from the activation of 5-HT receptors or may be the result of interactions between dopamine (DA) and 5-HT neurotransmitter systems. To discriminate between these two possibilities, various groups of rats were pretreated with different 5-HT antagonists or a DA D1 antagonist, 20 min before fenfluramine administration. Animals were killed 60 min later. In the striatum, fenfluramine-induced expression of Fos-LI was almost completely blocked by SCH 23390, methysergide and S(-)-propranolol. The immediate-early gene response to fenfluramine was only slightly affected by pretreatment with the 5-HT2A/2C antagonist ritanserin. Fenfluramine was also administered to sham-operated and to unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. In the 6-OHDA-lesioned rats, fen-fluramine-induced Fos-LI was decreased by 60% on the DA denervated side compared to the intact side and to sham-operated rats. To further probe the possibility of a direct activation of Fos-LI by 5-HT receptor subtypes, we evaluated the expression of Fos-LI after the administration of different 5-HT agonists. Our results demonstrate that neither 8-OH-DPAT, CGS-12066B, RU 24969 nor phenyl-biguanide was able to reproduce the effects of fenfluramine. Only a high dose of DOI (8.5 mg/kg) produced a moderate expression of Fos-LI in the dorsomedial part of the striatum. This contrasted with the Fos-LI expression in other brain areas where 8-OH-DPAT and DOI (2.5 and 8.5 mg/kg) reproduced the effects of the 5-HT releasing agent. Our results suggest that the release of 5-HT by fenfluramine induced Fos-LI expression predominantly in a striatal region related to associative functions and, that this c-fos response may be under the control of both 5-HT and DA. Moreover, the mechanism by which fenfluramine induces c-fos expression in the striatum differs from other brain regions.

5-HT4 receptor-mediated modulation of 5-HT release in the rat hippocampus in vivo

1. In the present study, the ability of the 5-hydroxytryptamine, receptor (5-HT4 receptor) to modulate the release of 5-HT in the hippocampus of freely-moving rats was investigated by the in vivo microdialysis technique. 2. The 5-HT4 receptor agonist, renzapride (1.0-100 microM, administered via the microdialysis probe) increased extracellular hippocampal levels of 5-HT in concentration-dependent manner (approximately 200% maximal increase). The ability of renzapride (100 microM, administered via the microdialysis probe) to elevate extracellular levels of 5-HT remained in the presence of the selective 5-HT reuptake blocker, paroxetine (1.0 microM, administered via the microdialysis probe). Furthermore, another 5-HT4 receptor agonist 5-methoxytryptamine (5-MeOT; 10 microM, administered via the microdialysis probe, in the presence of the non-5-HT4 5-HT receptor antagonists pindolol (10 microM) and methysergide (10 microM)) maximally elevated extracellular levels of 5-HT by approximately 450% in the rat hippocampus. The elevation of extracellular 5-HT levels induced by either renzapride (100 microM) or 5-MeOT (10 microM) was completely prevented by combined administration of the selective 5-HT4 receptor antagonist, GR113808 (100 nM, administered via the microdialysis probe). GR113808 (100 nM, administered via the microdialysis probe) administered alone, however, reduced extracellular hippocampal 5-HT levels by some 60%. 3. Systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.1 mg kg-1, s.c.) reduced extracellular levels of 5-HT in the rat hippocampus by approximately 40%. Prior administration of 8-OH-DPAT (0.1 mg kg-1, s.c.), with an associated reduction of extracellular hippocampal 5-HT levels by approximately 40-50%, however, failed to prevent a subsequent elevation of extracellular levels of 5-HT induced by renzapride (100 microM, administered via the microdialysis probe). 4. Systemic administration of the 5-HT4 receptor agonist, renzapride (0.25 and 1.0 mg kg-1, i.p.) increased extracellular levels of 5-HT in the hippocampus in a dose-dependent manner. The higher dose of renzapride increasing extracellular 5-HT levels by some 200%. The selective 5-HT4 receptor antagonist, GR125487D (1.0-100 micrograms kg-1, i.p.) caused a dose-dependent reduction in extracellular levels of 5-HT in the hippocampus (maximally approximately 80% reduction). Prior administration of GR125487D (10 micrograms kg-1, i.p.) prevented the elevation of extracellular levels of 5-HT induced by renzapride (1.0 mg kg-1, i.p.). 5. In conclusion, the present study provides evidence that activation of the 5-HT4 receptor facilitates 5-HT release in the rat hippocampus in vivo.

Pharmacological comparison between [3H]GR 113808 binding sites and functional 5-HT4 receptors in neurons

5-HT4 receptors positively coupled to adenylyl cyclase and possessing unique pharmacological properties were first described in mouse colliculi neurons using functional studies. The recent introduction of a radiolabeled 5-HT4 receptor antagonist, [3H]GR 113808 [1-[2-(methylsulphonylamino)ethyl]4-piperidinyl]methyl-1-methyl-in dole-3 carboxylate] having high specificity and affinity allowed the pharmacological comparison between the specific binding sites identified with this compound and the functional 5-HT4 receptors in the same preparation, the colliculi neurons. We show here that [3H]GR 113808 binding is saturable in this preparation and reveals a homogeneous population of sites with a pKd value of 9.5 +/- 0.2 and a Bmax of 75 +/- 23 fmol/mg protein. Seventeen agonists and six antagonists with molecules structurally related either to indoles, benzamides or benzimidazolones and previously known as 5-HT4 receptor ligands, were tested for their ability to compete with [3H]GR 113808 binding sites and to stimulate or inhibit 5-HT-stimulated adenylyl cyclase activity. Highly significant correlations were obtained between the affinities of either agonists or antagonists for [3H]GR 113808 binding sites and their potencies for functional 5-HT4 receptors (r = 0.87 and 0.99, respectively). In addition, we also found good correlations between the Kd of several 5-HT4 receptor ligands determined in cell membranes of mouse colliculi neurons and their Kd determined in previous studies in guinea-pig striatum (0.95) and in human caudate (0.97). [3H]GR 113808 binding studies demonstrated that the 50% decrease in 5-HT-stimulated cAMP accumulation which followed a 5 min exposure period with 5-HT (10 microM) was not accompanied by any significant decrease in the number of binding sites. Longer exposure periods with 5-HT resulted in a decrease in [3H]GR 113808 binding sites which started to be significant after 30 min.

Biochemical profile of YM992, a novel selective serotonin reuptake inhibitor with 5-HT2A receptor antagonistic activity

YM992, (S)-2-[[(7-fluoro-4-indanyl)oxy]methyl]morpholine monohydrochloride, exhibited the biochemical profile of a selective serotonin (5-HT) reuptake inhibitor (SSRI) with 5-HT2A receptor antagonistic activity. YM922 showed the same high affinity as fluoxetine against the 5-HT reuptake site (Ki = 21 nM) and a similar affinity to that of crazodone against the 5-HT2A receptor (Ki = 86 nM). In other receptor binding studies, an affinity for the adrenergic alpha 1 receptor (Ki = 200 nM) and 5-HT2C receptor (Ki = 680 nM) was observed. In a monoamine uptake study, YM992 showed a selective 5-HT uptake inhibition (IC50 = 0.15 microM), but only very weakly inhibited both noradrenaline (NA) and dopamine (DA) uptake (IC50 = 3.1 microM (NA), > 10 microM (DA)). YM992 was also found to potently inhibit the aggregation of human platelets (IC50 = 1.9 microM), revealing antagonistic activity for the 5-HT2A receptor in vitro. Enhanced serotonergic neurotransmission, in particular that mediated by the 5-HT1A receptor, has recently been reported to be important in the long-term treatment of depressive disorders with antidepressants. In addition, some 5-HT1A receptor-mediated responses are known to be potentiated by co-administration of 5-HT2A receptor antagonists. Thus, YM992, having both selective 5-HT reuptake inhibition and 5-HT2A antagonistic activity, might show potent therapeutic activity as a novel antidepressant in comparison with conventional SSRIs.

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.

Excitation of rat substantia nigra pars reticulata neurons by 5-hydroxytryptamine in vitro: evidence for a direct action mediated by 5-hydroxytryptamine2C receptors

Single-unit extracellular and whole-cell patch clamp recording were used to study the actions of exogenously applied 5-hydroxytryptamine on substantia nigra pars reticulata neurons in parasaggital slices of rat midbrain. Seventy-six per cent of substantia nigra pars reticulata cells (254/334) recorded extracellularly were excited by 5-hydroxytryptamine (EC50 = 9.56 microM); in the remainder, inhibitions (13.5%), biphasic responses (4.2%) or lack of response (6.3%) were observed. Using whole-cell patch recording, 5-hydroxytryptamine (10 microM) caused either an inward current (9/9 cells) or a depolarization (3/3 cells) at membrane potentials in the range -50 to -90 mV, which was resistant to tetrodotoxin (4/4 cells), indicating that the predominant, excitatory action of 5-hydroxytryptamine was due to a direct action on substantia nigra pars reticulata neurons. The 5-hydroxytryptamine excitation (recorded extracellularly) was reduced to 24 +/- 6% of control values by methysergide (0.1 microM) and to 17 +/- 5% of control by ketanserin (10 microM), but was unaffected by the 5-hydroxytryptamine antagonists spiperone (0.1 microM), yohimbine (0.1 microM), pindolol (1 microM), GR113808A (1 microM) or ICS 205930 (10 microM). In addition, the 5-hydroxytryptamine excitation was mimicked by the 5-hydroxytryptamine2C receptor--preferring agonist alpha-methyl 5-hydroxytryptamine (10 microM), but the agonists CP93, 129 (0.1-1 microM) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (0.1-1 microM) were without effect. Taken together, this pharmacology indicated involvement of the 5-hydroxytryptamine2C receptor in the 5-hydroxytryptamine excitation, while other candidate receptors known to be present in rat substantia nigra pars reticulata (5-hydroxytryptamine1B, 5-hydroxytryptamine2A and 5-hydroxytryptamine4) could be excluded from consideration. While in accord with current information on the location of 5-hydroxytryptamine receptor subtypes in substantia nigra pars reticulata, and the consequence of activation of neuronal 5-hydroxytryptamine2C receptors, these results contrast with data from in vivo experiments which suggest that the net effect of 5-hydroxytryptamine is to inhibit substantia nigra pars reticulata neurons. The reason for this apparent discrepancy may lie in detailed consideration of the microcircuitry of the substantia nigra pars reticulata. This may lead to a re-evaluation of the influence of 5-hydroxytryptamine on this basal ganglia output relay nucleus, and its role in motor control and the gating of generalized seizure activity.

Localization of serotonin-4 receptors in the striatonigral pathway in rat brain

Rats were injected unilaterally with 6-hydroxydopamine either in the medial forebrain bundle or in the dorsolateral substantia nigra. Another group was injected unilaterally with kainate in the striatum. The loss of neurons was assessed by a reduction in tyrosine hydroxylase-like immunoreactivity for dopaminergic neurons, and choline acetyltransferase-like and glutamate decarboxylase-like immunoreactivities for cholinergic and GABAergic neurons, respectively. Brain sections also were analysed by autoradiography on 20 micron sections with the radio-iodinated serotonin-4 receptor antagonist [125I]SB 207710 [Brown A. M. et al. (1993) Br. J. Pharmac. 110, 10P]. Kainate injections in the striatum resulted in loss of choline acetyltransferase- and glutamate decarboxylase-like immunoreactive cell bodies in this area. There was also a decrease in glutamate decarboxylase-like immunoreactivity on the ipsilateral side in the substantia nigra and entopeduncular nucleus. These changes were accompanied by substantial (> 50%) decreases in [125I]SB 207710 binding in both the ipsilateral striatum (confined to the lesioned area) and substantia nigra, with no change in either the nucleus accumbens or the globus pallidus. There was also significant loss of [125I]SB 207710 binding in the ipsilateral entopeduncular nucleus. 6-Hydroxydopamine lesions placed either in the medial forebrain bundle or in the substantia nigra failed to decrease [125I]SB 207710 binding in any of these areas, although there was total loss of tyrosine hydroxylase-like immunoreactive terminals in the striatum and cell bodies in the nigra. We conclude that serotonin-4 receptors are present on projection neurons, both on their perikarya in the striatum and terminals in the nigra and entopeduncular nucleus. It is likely that these receptors are located on the GABAergic projection neurons and possibly on cholinergic and GABAergic interneurons. However, serotonin-4 receptors are not located on dopaminergic neurons, either on their cell bodies in the substantia nigra or terminals in the striatum.

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

The tritiated specific antagonist [3H]GR 113808 was used to study the brain regional distribution of 5-HT4 binding sites in rat during development. In adult brain, high densities of binding sites were found in the limbic system (islands of Calleja, olfactory tubercle, fundus striati, ventral pallidum, septum, hippocampus and amygdala) and in the striato-nigro-tectal pathway. Only low densities of binding sites were found in the prenatal brain, except in the brainstem. Marked density increases were observed during the second and third postnatal week in most forebrain regions. This ontogenic pattern indicates that 5-HT4 receptors probably exert a minor role in developmental processes, while, in the adult, the expression of 5-HT4 receptors in limbic areas strongly suggests a role for these receptors in emotional functions. The synchronous appearance of 5-HT4 receptors and cholinergic markers also indicates that these receptors might control acetylcholine release. This hypothesis has been confirmed by functional evidence and further supports the view that 5-HT4 drugs could improve cognitive functions in diseases with impaired cholinergic transmission (e.g., Alzheimer's disease).

Characterization of [3H]GR 113808 binding to 5-HT4 receptors in brain tissues from patients with neurodegenerative disorders

[3H]GR 113808 binding studies in guinea-pig brain tissue revealed a temperature dependency and pharmacological specificity consistent with labelling of a 5-HT4 receptor. Detailed comparison of competition data between brain regions from human and guinea-pig suggest minor differences in the rank order of affinity. Studies in brain tissue from patients with Alzheimer's Huntington's and Parkinson's disease have revealed specific deficits in 5-HT4 receptor densities.

5-HT3 and 5-HT4 receptors in terminal regions of the mesolimbic system

In this brief review, we present the evidence for the regulation of the mesolimbic dopamine system by 5-HT3 and 5-HT4 receptors. A range of studies show good evidence that 5-HT3 receptor antagonists reduce raised mesolimbic dopamine activity by blocking 5-HT3 receptors in terminal parts of the mesolimbic dopamine system. Few studies have been conducted on the effects of 5-HT4 receptors on dopamine systems. However, it is clear that 5-HT4 receptors are present in relatively high density in areas of the brain that contain dopamine and preliminary studies show that 5-HT4 receptors may regulate the release of this transmitter.

Effect of serotonin (5-HT)3-receptor antagonists YM060, YM114 (KAE-393), ondansetron and granisetron on 5-HT4 receptors and gastric emptying in rodents

We investigated the effects of YM060 [(R)-5-[(1-methyl-3-indolyl)carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazol e hydrochloride] and YM114 (KAE-393) [(R)-5-[(2,3-dihydro-1-indolyl)-carbonyl]-4,5,6,7-tetrahydro-1H- benzimidazole hydrochloride] on 5-HT4 receptors and gastric emptying in normal and cisplatin-treated rats and compared results with those for ondansetron and granisetron. YM060, YM114, ondansetron and granisetron dose-dependently inhibited the specific binding of [3H]-GR113808 ([[1-[(2-methylsulphonyl)amino]ethyl]-4-piperidin-yl]methyl 1-methyl-1H-indole-3-carboxylate) in guinea pig striatum, with pKi values of 5.53, 5.13, 5.21 and 5.63, respectively. According to the pKi values reported in 5-HT3-receptor binding of [3H]GR65630 to rat cortical membranes, the affinity of YM060, YM114, ondansetron and granisetron for 5-HT4 receptors was approximately 5, 5, 3.5 and 3.5 log units lower than that for 5-HT3 receptors, respectively. In the guinea pig longitudinal muscle with myenteric plexus and rat esophageal tunica muscularis mucosae, YM060 and YM114 showed neither 5-HT4-agonistic nor antagonistic properties. Although ondansetron produced concentration-dependent increases in the magnitude of the twitch response in longitudinal muscle, it did not possess 5-HT3- and 5-HT4-agonistic activity. Granisetron antagonized 5-HT-induced relaxation of the rat esophagus with an apparent pA2 value of 5.39. Intravenous YM060, YM114, ondansetron and granisetron significantly enhanced gastric emptying of glass beads and improved cisplatin-induced slowing of gastric emptying in rats. These results indicate that the selectivity of YM060 and YM114 for 5-HT3 receptors is higher than that of ondansetron and granisetron and that these 5-HT3 antagonists have gastroprokinetic activity in normal and cisplatin-treated rats without affecting 5-HT4 receptors.

Pharmacological profile of (-)HT-90B, a novel 5-HT1A receptor agonist/5-HT2 receptor antagonist

1. HT-90B ((-)-N-([2-(8-methyl-l, 4-benzodioxane-2-ylmethyl)amino]ethyl) tricyclo[3,3,1,1(3.7)] decane-1-carboxamide) had high affinities for the 5-HT1A (Ki = 0.18 nM) and 5-HT2 (Ki = 9.2 nM) receptors. 2. HT-90B inhibited forskolin activated adenylate cyclase in rat hippocampal membranes as a 5-HT1A full agonist (IC50 = 2 nM), and the potency of the drug was higher than that of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a standard 5-HT1A agonist. 3. In the serotonin syndrome test, HT-90B behaved as a weak partial 5-HT1A agonist in reserpinized rats. 4. 5-HT2 receptor-mediated potentiation of rabbit platelet aggregation by serotonin (5-HT) was reduced by HT-90B (IC50 = 1.73 microM). 5. Head twitch response induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 agonist, was inhibited by HT-90B in mice. 6. It is concluded that HT-90B has potent 5-HT1A receptor agonist as well as 5-HT2 receptor antagonist properties in vitro and in vivo.

The 5-HT4 receptor antagonist, GR113808, reduces ethanol intake in alcohol-preferring rats

The present study evaluated the effect of the selective 5-HT4 receptor antagonist, GR113808, on ethanol intake in alcohol-preferring rats. Rats were offered 10% ethanol 2 h/day. In the first experiment, rats had food and water ad lib and 10% ethanol was offered from 1800 to 2000 h. In the second experiment, food was freely available, 10% ethanol was offered 2 h/day, from 1800 to 2000 h, and water was offered for 4 h, from 1800 to 2200 h. In both experiments GR113808 was subcutaneously injected at doses of 1, 3, or 10 mg/kg for 4 consecutive days, 5 min before access to ethanol. From the first day of administration, GR113808 significantly reduced the volitional ethanol intake in water sated rats at the three doses tested. In water-deprived rats, it reduced ethanol intake at 3 and 10 mg/kg, without modifying total fluid and food intake. In both experiments the effect of GR113808 remained rather stable during the 4 days of administration. The present findings, showing that the 5-HT4 receptor antagonist, GR113808, selectively reduces ethanol intake in alcohol-preferring rats, suggest that 5-HT4 receptors may play a role in alcohol intake control.

Intranigral GR-113808, a selective 5-HT4 receptor antagonist, attenuates morphine-stimulated dopamine release in the rat striatum

GR-113808, a potent and selective 5-HT4 receptor antagonist, was infused through a microdialysis probe into the striatum and nucleus accumbens of awake rats, and basal and morphine-stimulated extracellular concentrations of dopamine (DA) were measured in these regions. At 1 and 10 microM GR-113808 did not affect the extracellular concentrations of DA in either region and 100 microM significantly reduced dialysate DA only in the striatum. A subcutaneous dose of 5 mg/kg morphine significantly raised extracellular concentrations of DA in the striatum and nucleus accumbens from 60 to 120 min after injection and the effect was not modified by 10 microM GR-113808 infused through the probe 20 min before and for 60 min after morphine. Bilateral injections of GR-113808 (1, 2.5 and 10 micrograms/0.5 microliter) in the substantia nigra pars compacta did not affect dialysate DA in the striatum, except for a significant increase 120 min after the injection of 10 micrograms but the highest dose of GR-113808 prevented the increase of striatal DA caused by 5 mg/kg morphine s.c. The results suggest that 5-HT4 receptors in the substantia nigra modulate the activity of the dopaminergic nigrostriatal system only when the neurons are activated.

Pharmacological characterization of two novel and potent 5-HT4 receptor agonists, RS 67333 and RS 67506, in vitro and in vivo

1. The pharmacology of two novel 5-HT4 receptor agonists, RS 67333 (1-(4-amino-5-chloro-2-methoxy-phenyl)-3-[1(n-butyl)-4-piperidinyl]-1- propanone HCl) and RS 67506 (1-(4-amino-5-chloro-2-methoxy-phenyl)-3-[1-(2-methyl sulphonylamino)ethyl-4-piperidinyl]-1-propanone HCl) have been assessed in vitro and in vivo. 2. RS 67333 and RS 67506 exhibited affinities (pKi = 8.7 and 8.8, respectively) for the 5-HT4 binding sites, labelled with [3H]-GR 113808, in guinea-pig striatum. The Hill coefficients from these displacement curves were not significantly different from unity. The compounds exhibited lower affinities (< 6.0) at several other receptors including 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C, dopamine D1, D2 and muscarinic M1-M3 receptors. However, RS 67333 and RS 67506 did exhibit affinities for the sigma 1 (pKi = 8.9 and 7.9, respectively) and sigma 2 (pKi = 8.0 and 7.3, respectively) binding sites. 3. At the 5-HT4 receptor mediating relaxation of the carbachol-precontracted oesophagus, RS 67333 and RS 67506 acted as potent (pEC50 8.4 and 8.6, respectively), partial agonists (intrinsic activities, with respect to 5-HT were 0.5 and 0.6, respectively) with respect to 5-HT. Relaxant responses to RS 67333 or RS 67506 were surmountably antagonized by GR 11308 (10 nM), with apparent affinities (pKB) of 9.1 and 9.0, respectively. RS 67333 and RS 67506 induced dose-dependent increases in heart rate of the anaesthetized micropig (ED50 4.9 and 5.4 micrograms kg-1, i.v.), with maximal increases of 35 and 47 beats min-1, respectively. 4. RS 67333 and RS 67506, therefore, acted as potent, partial 5-HT4 receptor agonists in vitro and in vivo. These compounds, by virtue of their high potency and selectivity, may have some utility in elucidating the physiological role of 5-HT4 receptors.

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.

New players in the 5-HT receptor field: genes and knockouts

In recent years, cloning has revealed the existence of no less than 14 mammalian 5-HT receptors, as well as evidence that isoforms of the 5-HT4 and 5-HT7 receptors are generated by alternative splicing. In addition, molecular biology techniques have provided new tools with which to study the function of 5-HT receptors. For example, new technologies are emerging that will allow the generation of either inducible or tissue-specific knockouts (or both). In this review José Lucas and René Hen focus on the characteristics of the most recently cloned receptors and the contribution of molecular biology to the understanding of 5-HT receptor function.

Labelling with [125I]-SB 207710 of a small 5-HT4 receptor population in piglet right atrium: functional relevance

1. We investigated the affinity of SB 207710 for sinoatrial 5-HT4 receptors and the density of right atrial 5-HT4 receptors with [125I]-SB 207710 in right atria of new-born piglets. 2. SB 207710 (1-100 nM) antagonized the 5-HT-evoked tachycardia surmountably with a pKB of 9.8. 3. [125I]-SB 207710 (5-1500 pM) labelled a small population of saturable binding sites with a pKD of 10.1 and with 5-HT4 receptor characteristics. The density of atrial binding sites with 5-HT4 receptor characteristics was 174 and 22 times lower respectively than those of atrial beta 1- and beta 2-adrenoceptors, labelled with (-)-[125I]-cyanopindolol. 4. We suggest that the small 5-HT4 receptor population may in part explain why the maximal tachycardia caused by 5-HT is smaller than that caused by catecholamines.

RS 39604: a potent, selective and orally active 5-HT4 receptor antagonist

1. Selective antagonism of 5-HT4 receptors may provide therapeutic benefit in certain disorders of the myocardium, alimentary and lower urinary tract. We now report on RS 39604, a novel and selective 5-HT4 receptor antagonist and compare its pharmacological properties with those of SB 204070. 2. In guinea-pig striatal membranes, both RS 39604 and SB 204070 inhibited specific binding of [3H]-GR 113808 in a concentration-dependent manner yielding pKi estimates of 9.1 and 10.9, respectively. RS 39604 displayed a low affinity (pKi < 6.5) for 5-HT1A, 5-HT2C, 5-HT3, alpha 1c, D1, D2, M1, M2, AT1, B1 and opioid mu receptors and moderate affinity for sigma 1, (pKi = 6.8) and sigma 2 (pKi = 7.8) sites. 3. In the rat isolated oesophagus, precontracted with carbachol, RS 39604 (30-300 nM) behaved as a competitive antagonist towards 5-HT-induced relaxation (pA2 = 9.3; Schild slope = 1.0). We and others have shown previously that SB 204070 behaves as an unsurmountable antagonist in this preparation (pA2 approximately 10.5). In the guinea-pig isolated ileal mucosa, RS 39604 (30 nM) antagonized 5-MeOT-induced increase in short-circuit current (pA2 = 9.1). 4. In anaesthetized vagotomized micropigs, RS 39604, administered by the i.v. or intraduodenal (i.duod.) route, produced dose-dependent inhibition of 5-HT-induced tachycardia (ID50 = 4.7 micrograms kg-1, i.v. and 254.5 micrograms kg-1, i.duod). At maximal doses of 30 micrograms kg-1, i.v. and 6 mg kg-1, i.duod., the inhibitory effects of RS 39604 lasted for more than 6 h. In this preparation, SB 204070 was as potent as RS 39604by the i.v. route but was inactive by the intraduodenal route at doses up to 3 mg kg-1.5. In conscious mice, RS 39604, administered by the i.p. or p.o. route, produced dose-depend entinhibition of 5-hydroxytryptophan (5-HTP)-induced diarrhoea (ID50= 81.3 microg kg-1, i.p. and 1.1 mg kg-1,p.o.). In this assay, SB 204070 was inactive by the oral route at doses up to 30 mg kg-1.6. In anaesthetized guinea-pigs, RS 39604 antagonized the contractile effect of 5-HT in the proximal colon by producing parallel, dextral displacement of the dose-response curve to 5-HT. The mean dose ratios to 5-HT at 0.1 mg kg-1, i.v., 1 mg kg-1, i.v. and 10 mg kg-1, i.duod. were 4.6, 30.7 and 10.8,respectively. SB 204070 behaved as an unsurmountable antagonist in this assay.7. In a model of visceral pain in conscious rats, RS 39604 (0.01-1 mg kg-1, i.v.) did not affect colorectal distension-induced increases in arterial pressure whereas morphine (1 mg kg-1, i.v.) produced significant inhibition of the response, implying that 5-HT4 receptors are not involved in nociception in this model.8. The data suggest that RS 39604 is a high affinity and selective 5-HT4 receptor antagonist that is orally active and long-lasting in vivo. It is concluded that RS 39604 may be the preferable probe to use for investigating the physiological and pathophysiological role of 5-HT4 receptors in vivo.

Investigation of the 5-hydroxytryptamine receptor mediating the "transient" short-circuit current response in guinea-pig ileal mucosa

5-Hydroxytryptamine (5-HT) stimulated an increase in short-circuit current (ISC) in guinea-pig isolated ileal mucosa over a wide concentration range (0.1 nM-0.1 mM). The concentration-response relationship was biphasic, consisting of a high potency phase (0.1 nM-1 microM) and a low potency phase (3-10 microM). Stimulation of ISC observed at the high potency phase tended to be sustained while responses at the low potency phase (3-10 microM) contained two components, an initial "transient" response followed by a "maintained" response. Both the high potency phase (maximum stimulation approximately 30 microA cm-2) and the low potency phase (maximum stimulation approximately 80 microA cm-2) 5-HT response were antagonized by tetrodotoxin (TTX, 0.3 microM) and atropine (1 microM). However, another low potency (3 microM-0.1 mM, maximum stimulation approximately 30 microA cm-2) component of the 5-HT response was revealed in the presence of TTX or atropine. In the presence of methysergide (1 microM), the concentration-response relationship of 5-HT was still biphasic and tropisetron (0.1 and 10 microM) antagonized both phases of the 5-HT response. In the presence of methysergide, the high potency phase 5-HT response was mimicked by 5-methoxytryptamine (5-MeOT) and the selective 5-HT4 agonist SC-53116 but not by BIMU 8. The potent 5-HT4 antagonist GR 113808 antagonized the response to 5-MeOT in a surmountable manner with an affinity estimate of 9.6 +/- 0.3 (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative study of the affinities of the 5-HT3 receptor antagonists, YM060, YM114 (KAE-393), granisetron and ondansetron in rat vagus nerve and cerebral cortex

The 5-HT3 receptor blocking properties of YM060, YM114 (KAE-393), granisetron and ondansetron were examined in the vagus nerve and cerebral cortex of rats. 5-HT and 2-methyl-5-HT induced dose-dependent depolarizations of rat isolated vagus nerve with EC50 values of 2.53 (1.93-3.33) x 10(-6) and 4.03 (2.87-5.66) x 10(-6) M, respectively. YM060, YM114 and granisetron dose-dependently antagonized the depolarization of the rat vagus nerve induced by 5-HT, with decreases in the slope and maximal response at higher concentrations. Apparent pA2 values for these antagonists were 10.27 +/- 0.09, 10.12 +/- 0.16 and 9.44 +/- 0.40, respectively. Ondansetron produced a clear rightward shift of the concentration-response curve to 5-HT. The pA2 value was 8.63 (8.23-9.68). YM060 and YM114 at up to 10(-5) M produced no significant depression of the depolarizing responses to DMPP and GABA. YM060, YM114, granisetron and ondansetron displaced specific binding of [3H]GR65630 to rat cortical membranes with pKi values of 10.48 (10.41-10.57), 10.24 (10.18-10.28), 9.15 (9.02-9.28) and 8.70 (8.64-8.77), respectively. An excellent correlation (r = 0.97) was obtained between pA2 values in the vagus nerve and pKi values in the cerebral cortex. YM060, YM114, granisetron and ondansetron showed low affinities for 5-HT1A, 5-HT2 receptor, adrenergic alpha 1, alpha 2, dopamine D2, muscarinic M2, mu-opioid, benzodiazepine and histamine H1 receptors. These results support the possibility that the same type of 5-HT3 receptor occurs in rat vagus nerve and cerebral cortex.

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.

Serotonin receptors: subtypes, functional responses and therapeutic relevance

Recent, rapid progress in the molecular biology of serotonin (5-HT) receptors requires conceptual re-thinking with respect to receptor classification. Thus, based on operational criteria (agonist and antagonist rank order), as well as transduction mechanisms involved and the structure of the receptor protein, the Nomenclature Committee of the Serotonin Club has proposed the following classification and nomenclature: the main receptor types 5-HT1 to 5-HT4, recombinant receptors (e.g. 5-ht5 to 5-ht7) and 'orphan' receptors. The aim of the present review is to discuss the events leading to this classification, the criteria for and functional responses mediated by various 5-HT receptors, as well as the therapeutic possibilities with 5-HT ligands.

Enhancement of cyclic AMP accumulation mediated by 5-HT after chronic amitriptyline treatment in NG 108-15 cells

1. The effects of chronic in vitro administration of amitriptyline, a tricyclic antidepressant, on 5-hydroxytryptamine (5-HT) receptor-mediated adenylyl cyclase activity was studied in the neuroblastoma x glioma hybrid cell line, NG 108-15. 2. Treatment of NG 108-15 cells with 8 microM amitriptyline for 3 days increased forskolin-stimulated (0.1 microM) adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation. Addition of 5-HT (0.1-100 microM) increased forskolin-stimulated cyclic AMP accumulation in amitriptyline-treated cells in a concentration-dependent manner. However, 5-HT did not affect forskolin-stimulated cyclic AMP accumulation in untreated cells. 3. The 5-HT4 receptor agonist, 5-methoxytryptamine, significantly enhanced forskolin-stimulated cyclic AMP accumulation in amitriptyline-treated cells. In contrast, amitriptyline treatment failed to modify 8-hydroxy-2-(di-n-propylamine) tetralin-induced inhibition of forskolin-stimulated cyclic AMP accumulation. 4. Pretreatment of cells with pertussis toxin did not affect the 5-HT-induced enhancement of cyclic AMP accumulation. 5. The 5-HT-induced enhancement of cyclic AMP accumulation in amitriptyline-treated cells was attenuated by the 5-HT4 receptor antagonists, GR 113808 and ICS 205-930, with relatively low potency. However, spiperone, SCH 23390, and pindolol were completely ineffective against this 5-HT-induced enhancement. 6. Chronic treatment with amitriptyline did not modify the cyclic AMP production stimulated by prostaglandin E1 or cholera toxin. This treatment also had no effect on GTP gamma S-, NaF-, and Mn(2+)-stimulated cyclic AMP accumulation in isolated cell membranes. 7. Chronic treatment with the 5-HT receptor antagonists, pindolol or ICS 205-930, did not inhibit the 5-HT-induced enhancement of cyclic AMP accumulation.8. Chronic treatment with other antidepressant drugs, imipramine, mianserin or paroxetine, elicited the 5-HT-induced enhancement of cyclic AMP accumulation.9. Taken together, these results suggest that chronic amitriptyline treatment of NG 108-15 cells causes 5-HT to enhance forskolin-stimulated cyclic AMP accumulation by enhancing 5-HT receptor-mediated stimulation of adenylyl cyclase and not by reducing 5-HT-mediated inhibition of adenylyl cyclase. The 5-HT-induced enhancement of cyclic AMP accumulation in amitriptyline-treated cells may result from changes at the level of the 5-HT receptor rather than at the level of G, proteins or adenylyl cyclase. It is unlikely that this enhancement of cyclic AMP accumulation is caused by long-term antagonism of the 5-HT receptor by amitriptyline.

Evidence for 5-HT4 receptor subtype involvement in the enhancement of striatal dopamine release induced by serotonin: a microdialysis study in the halothane-anesthetized rat

The present study, using the in vivo intracerebral microdialysis method, investigated the role of different serotonin receptor subtypes in the control of dopamine (DA) release exerted by serotonin (5-HT) in the striatum of halothane-anesthetized rats. Striatal dialysate DA content was reduced following the blockade of voltage-dependent Na+ channels by tetrodotoxin or by the removal of Ca2+ from the perfusion medium, and increased following depolarization with K+ ions. These findings demonstrate that under our experimental conditions, DA content reflects the neuronal origin of the neurotransmitter release. Drugs were locally applied by means of the microdialysis probe. One, 2.5 and 5 microM 5-HT significantly enhanced DA release in a concentration-dependent manner up to 157, 253 and 446% of basal values respectively. The effect induced by 1 microM 5-HT was not blocked by 10 microM (-)pindolol, a 5-HT1 receptor antagonist, 1 microM ketanserin or 10 microM cinanserin, both 5-HT2A antagonists. One or 10 microM ondansetron (GR 38032F), a selective 5-HT3 antagonist, were also ineffective. In contrast, 10 or 100 microM DAU 6285, a 5-HT3/4 antagonist, significantly reduced the effect of 5-HT on DA release (-20% and -60% respectively). Moreover, 100 microM BIMU 8, a selective 5-HT4 agonist, enhanced DA release (+85%) and this effect was reduced by 100 microM DAU 6285 (-40%). These results demonstrate that in vivo 5-HT exerts a facilitatory influence on striatal DA release and that the 5-HT4, but not the 5-HT1, 5-HT2 or 5-HT3, receptor subtype is implicated, at least partially, in this effect.

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.

5-Hydroxytryptamine (5-HT)4 receptors in post mortem human brain tissue: distribution, pharmacology and effects of neurodegenerative diseases

1. The distribution, pharmacology and effects of neurodegenerative diseases on 5-HT4 receptors in human brain have been characterized in vitro. 2. The 5-HT4 receptor in post mortem human brain tissue was specifically labelled with [3H]-GR 113808. In human putamen, this ligand labelled a homogeneous population of sites, with an apparent affinity (-log Kd) of 10.1 and a density (Bmax) of 5.73 fmol mg-1 tissue. The pharmacology of this site was characterized by use of a series of displacing ligands, and the following rank order of apparent affinities (with mean +/- s.d. -log Ki values in parentheses) was generated: GR113808 (10.05 +/- 0.04) > SDZ 205,557 (8.65 +/- 0.08) > DAU 6285 (7.95 +/- 0.04) > BIMU-1 (7.81 +/- 0.06) > DAU 6215 (7.42 +/- 0.23) > tropisetron (7.39 +/- 0.23) > 5-HT (7.32 +/- 1.00) > BIMU-8 (7.25 +/- 0.04) > (R)-zacopride (5.82 +/- 0.04). The Hill coefficients were not significantly different from unity, consistent with an interaction at a single site. A comparison of the affinities of these compounds with those obtained from guinea-pig striatum indicated no evidence of species differences. 3. The regional distribution of 5-HT4 receptors was assessed by determining the density of binding sites for [3H]-GR 113808. The distribution were as follows (with mean +/- s.d. Bmax values, fmol mg-1 tissue, in parentheses): caudate nucleus (8.7 +/- 1.5), lateral pallidum (8.6 +/- 5.5), putamen (5.7 +/- 3.0), medial pallidum (3.8 +/- 0.9), temporal cortex (2.6 +/- 0.6), hippocampus (2.4 +/- 0.8), amygdala (2.3 +/-1.1), frontal cortex (1.7 +/- 0.5), cerebellar cortex (<1.0). In these studies, the affinities of GR 113808 were not significantly different.4. The density of 5-HT4 receptors selected from regions of post mortem brains of patients with Parkinson's disease, Huntington's disease and Alzheimer's disease were compared to age-matched controls. In Parkinson's disease, there was no significant difference between control or patient values(mean +/- s.d. Bmax values, fmol mg-1 tissue; putamen, control 4.74 +/- 0.07, patient 5.86 +/- 1.48; substantia nigra, control 4.21 +/- 2.56, patient 5.57 +/- 0.10). In Huntington's disease, there was a significant decrease in putamen (control 5.33 +/- 1.08, patient 2.68 +/- 1.08), while in Alzheimer's disease, there was a marked loss of receptors in hippocampus (control 2.34 +/- 0.62, patient 0.78 +/- 0.61), in frontal cortex (control,1.76 +/- 0.19, patient 1.30 +/- 0.22). Receptor density in temporal cortex showed a decrease, but did not achieve statistical significance (control 2.06 +/- 0.21, patient 1.44 +/- 0.64).5. These data suggest a heterogeneous distribution of 5-HT4 receptors in human brain, with high to moderate densities in basal ganglia and limbic structures. These receptors may not be principally co-localized on dopaminergic cell bodies or terminals, given the lack of change observed in Parkinson's disease. The loss of 5-HT4 receptors in the putamen in Huntington's disease raises the possibility of their presence on intrinsic striatal GABAergic or cholinergic neurones. The marked loss of receptors in hippocampal and cortical regions in the brains from patients with Alzheimer's disease is consistent with a role for the 5-HT4 receptor in cognitive processing.

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.

Do human atrial 5-HT4 receptors mediate arrhythmias?

Since their first description five years ago, knowledge about human atrial 5-HT4 receptors has increased considerably. Progress has been facilitated by the advent of selective antagonists with high affinity for human atrial 5-HT4 receptors. The receptors have been detected in both right and left atrium where they mediate increases in contractile force. Human sinoatrial 5-HT4 receptors may mediate the tachycardia caused by 5-HT and cisapride, and 5-HT elicits arrhythmias via 5-HT4 receptors in human atrium. In this article, Alberto Kaumann suggests that 5-HT may be involved in the genesis of atrial fibrillation and associated thromboembolic stroke and that both the arrhythmia and stroke could be prevented by inhibiting 5-HT4 receptors.

5-Hydroxytryptamine receptor subtypes in vertebrates and invertebrates

In the last few years, molecular biology has led to the cloning and characterization of several 5-HT receptors (serotonin receptors) in vertebrates and in invertebrates. These studies have allowed identification not only of 5-HT receptors already described but also of novel subtypes. The molecular cloning of 13 different mammalian receptor subtypes revealed an unexpected heterogeneity among 5-HT receptors. Except for the 5-HT3 receptors which are ligand-gated ion channel receptors, all the other 5-HT receptors belong to the large family of receptors interacting with G proteins. Based on their amino acid sequence homology and coupling to second messengers these receptors can be divided into distinct families: the 5-HT1 family contains receptors that are negatively coupled to adenylate cyclase: the 5-HT2 family includes receptors that stimulate phospholipase C; the adenylyl cyclase stimulatory receptors are a heterogeneous group including the 5-HT4 receptor which has not yet been cloned, the Drosophila 5-HTdro1 receptor and two mammalian receptors tentatively named 5-HT6 and 5-HT7 receptors. The 5-HT5A and 5-HT5B receptors might constitute a new family of 5-HT receptors whose effectors are unknown. This review focusses on the molecular characteristics of the cloned 5-HT receptors such as their structure, their effector systems and their distribution within the central nervous system. The existence of a large number of receptors with distinct signalling properties and expression patterns might enable a single substance like 5-HT to generate simultaneously a large panel of effects in many brain structures. The availability of the genes encoding these receptors has already allowed a partial characterization of their structure-function relationship and will probably allow in the future a dissection of the contribution of each of these receptor subtypes to physiology and behaviour.