Comparison of 5-HT4 receptors in guinea-pig colon and rat oesophagus: effects of novel agonists and antagonists

5-HT₄ receptors in isolated distal colon myenteric plexus of guinea-pig, mediating contraction of longitudinal smooth muscle, have been further characterized by selective agonists and antagonists. The indole agonists, 5-HT and 5-methoxytryptamine (5-MeOT), were full agonists (relative to 5-HT) with potency values (pEC₅₀) of 8.0 ± 0.1 (n = 50) and 7.8 ± 0.1 (n = 12), respectively. 5-HT₄ receptor agonists of other structural classes, including benzimidazolones (BIMU 1 and BIMU 8), and benzamides ((S)-zacopride, (R)-zacopride, renzapride, SC 49518) were partial agonists with intrinsic activities less than that of 5-HT. In general, the potencies for these compounds at 5-HT₄ receptors in guinea-pig colon were similar to the potencies seen in the rat isolated oesophagus, where 5-HT₄ receptors mediate relaxation.GR 113808 {[1-[2-[(methylsulfonyl)amino]ethyl]-4-piperidinyl]methyl1-methyl-1H-indole-3-carboxylat}, RS 39604 {1-[4-amino-5-chloro-2-(3,5-dimethoxybenzyloxy)phenyl]-3-[1-[2-[(methylsulfonyl)amino] ethyl]-4-piperidinyl]-1-propanone hydrochloride and SB 204070 {(1-n-butyl-4-piperidinyl)methyl 8-amino-7-chloro-1,4-benzodioxane-5-carboxylate} antagonized 5-HT responses with pA₂ values of 9.1 ± 0.1, 9.0 ± 0.2 and 11.0 ± 0.1, respectively. These affinity values were similar to those obtained at 5-HT₄ receptors in isolated rat oesophagus (9.0 ± 0.4, 9.3 ± 0.1 and 10.6 ± 0.1, respectively).Despite these operational similarities between 5-HT₄ receptors in guinea-pig colon and rat oesophagus, several novel compounds have revealed important differences between 5-HT₄ receptors in the two tissues. For example, the substituted benzoate, RS 23597 {3-(piperidine-1-yl) propyl-4-amino-5-chloro-2-methoxybenzoate hydrochloride, acted as a partial agonist (intrinsic activity 0.5) in guinea-pig colon with a potency of 7.6 ± 0.1 (n = 16). In isolated rat oesophagus, however, this compound was a surmountable antagonist (pA₂ = 7.8 ± 0.1) with no intrinsic activity. In contrast, the substituted naphthalimide (S)RS 56532 {(S)6-amino-5-chloro-2-(1-azabicyclo[2, 2, 2]octan-3-yl)2,3-dihydro-1H-benz isoquinoline-1,3-dione hydrochloride}, was a potent (pEC₅₀ = 7.9 ± 0.1), efficacious partial agonist (intrinsic activity = 0.8) in the rat oesophagus. However, in guinea-pig colon, it was a surmountable antagonist with an affinity (pK_B) of 9.4 ± 0.1. Furthermore, several novel, selective, 5-HT₄ compounds also showed opposing patterns of intrinsic activities similar to those described for RS 23597 and (S)RS 56532.It is concluded that these differences are inconsistent with differences in 5-HT₄ receptor reserves, and may suggest that 5-HT₄ receptors in the guinea-pig colon and the rat oesophagus can be operationally distinguished.     

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

Functional 5-HT₄ receptors have been reported to be present in numerous isolated tissue preparations including the rat oesophagus, guineapig ileum, and human colon. The pharmacological properties of the novel, potent and selective 5-HT₄ receptor antagonists SB203186 (1-piperidinyl)ethyl 1H-indole 3-carboxylate, SB205008 (1-butyl-1-methyl4-piperidinylmethyl)-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate iodide, and SB207710 (1-butyl-4-piperidinylmethyl)-8-amino-7-iodo-1,4 benzodioxan-5-carboxylate) were studied in these tissues. The nature of antagonism of the 5-HT-induced effects was investigated on the above isolated tissue preparations.5-HT produced its effect with the following EC₅₀ values: 400 ± 0.4 nM (rat oesophagus, n = 20), 154 ±14 nM (guinea-pig ileum, n = 9) and 144 ± 0.1 nM (hu man colon, n = 9). SB207710 (0.03–1 nM), SB205008 (1.0–10 nM), and SB203186 (10–100 nM) antagonised the 5-HT₄ receptor-mediated relaxations of the carbachol-contracted rat isolated oesophagus against 5-HT with pK_B values of 10.9 ± 0.1, 9.5 ± 0.1, and 9.0 ± 0.1 respectively without effecting the maximum response. On the guinea-pig ileum peristaltic reflex preparation, SB207710 (0.01–1 nM) did not modify the reflex but it behaved as an antagonist of the 5-HT-induced facilitation with a pA₂ value of 9.9 ± 0.2. The agonists DAU 6236 (endo-8-methyl-8-azabicyclo [3.2.1] oct-3-yl 2,3-dihydro-3-ethyl-2-oxo-1H-benzimidazole-1-carboxylate hydrochloride) at 2 M, and SC 53116 (1-S,8-S)-4-amino-5-chloro-N- [(hexahydro-1H-pyrrolizin-1-yl)methyl]-2-methoxy-benzamide hydrochloride at 500 nM facilitated the peristaltic reflex. SB207710 (0.25–5 nM), SB205008 (1–100 nM) and SB203186 (10–1000 nM) failed to modify the spontaneous activity of the circular muscle of the human colon, but caused parallel, dextral shifts in the concentration-effect curves to 5-HT with apparent pK_B values of 10.0 ± 0.3 (0.25 nM), 9.8 ± 0.2 (1 nM), and 8.6 ± 0.3 (10 nM) respectively. Higher concentrations of each antagonist shifted the concentration-effect curves to 5-HT to the right but this antagonism did not appear to be simple competitive. Methysergide (10 M) and ondansetron (10 M) increased the activity of SB207710 (5 nM) in the human colon giving rise to a 4 point (0.25, 0.5, 1,5 nM) Schild slope not significantly different from unity and a pK_B of 10.1 ± 0.1. Cocaine (30 M). did not significantly affect the activity of SB207710.The compounds SB207710, SB205008 and SB 203186 were shown to be high affinity 5-HT₄ receptor antagonists. SB207710 is the most potent 5-HT₄ receptor antagonist described so far and as such, is a potentially useful tool for 5-HT₄ receptor characterisation in functional studies of tissue from man and other species.     

5-Hydroxytryptamine stimulates cyclic AMP formation in the tunica muscularis mucosae of the rat oesophagus via 5-HT4 receptors.

The nature of 5-HT4 receptor coupling in the tunica muscularis mucosae of the rat oesophagus has been studied. 5-HT and renzapride stimulated cyclic AMP formation concentration dependently, with -log EC50 values of 7.1 and 6.8, respectively. Renzapride, relative to 5-HT, acted as a partial agonist. Tropisetron (ICS 205 930) and a novel 5-HT4 antagonist, SDZ 205 557, inhibited 5-HT-induced cyclic AMP production competitively, with pA2 estimates of 6.7 and 7.7, respectively. These data are consistent with the hypothesis that 5-HT4 receptors mediate relaxation of the smooth muscle cells of the tunica muscularis mucosae of rat oesophagus via activation of adenylyl cyclase.     

Piglet sinoatrial 5-HT receptors resemble human atrial 5-HT4-like receptors

Summary 5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT) and the gastrointestinal kinetic benzamides renzapride and cisapride caused tachycardia in spontaneously beating right atria of piglet in the presence of 400 nmol/l(±)-propranolol and 6 mol/l cocaine. The maximum tachycardia caused by agonists, compared to that evoked by 200 mol/l(–)-isoprenaline, was 63% for 5-HT, 50% for 5-CT, 50% for renzapride and 28% for cisapride. The rank order of potency was 5-HT > renzapride > cisapride > 5-CT. The effects of the agonists, but not those of (–)-isoprenaline, were antagonised by 3-tropanyl-1H-indole-3-carboxylic acid (ICS 205930); the pK_B of ICS 205930 (vs 5-HT) was 6.9. These characteristics suggest that piglet sinoatrial 5-HT receptors are similar to so-called 5-HT₄ receptors previously described in mouse colliculi neurons. Piglet sinoatrial 5-HT₄-like receptors resemble the human atrial 5-HT receptors that mediate positive isotropic effects of 5-HT.Send of fprint requests to A. J. Kaumann at the above address     

Involvement of neurokinins in the non-cholinergic response to activation of 5-HT3 and 5-HT4 receptors in guinea-pig ileum.

1. The effects of vasopressin and deamino-8-D-arginine vasopressin (DDAVP, desmopressin) were studied in artery rings (0.8-1 mm in external diameter) obtained from portions of human omentum during the course of abdominal operations (27 patients). 2. In arterial rings under resting tension, vasopressin produced concentration-dependent, endothelium-independent contractions with an EC50 of 0.59 +/- 0.12 nM. The V1 antagonist d(CH2)5Tyr(Me)AVP (1 microM) and the mixed V1-V2 antagonist desGly-d(CH2)5D-Tyr(Et)ValAVP (0.01 microM) displaced the control curve to vasopressin to the right in a parallel manner without differences in the maximal responses. In the presence of indomethacin (1 microM) the contractile response to vasopressin was significantly increased (P < 0.01). 3. In precontracted arterial rings, previously treated with the V1 antagonist, d(CH2)5Tyr(Me)AVP (1 microM), vasopressin produced endothelium-dependent relaxation. This relaxation was reduced significantly (P < 0.05) by indomethacin (1 microM) and unaffected by the V1-V2 receptor antagonist desGly-d(CH2)5D-Tyr(Et)ValAVP (1 microM) or by NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM). 4. The selective V2 receptor agonist, DDAVP, caused endothelium-independent, concentration-dependent relaxations in precontracted arterial rings that were inhibited by the mixed V1-V2 receptor antagonist, but not by the V1 receptor antagonist or by pretreatment with indomethacin or L-NAME. 5. Results from this study suggest that vasopressin is primarily a constrictor of human mesenteric arteries by V1 receptor stimulation; vasopressin causes dilatation only during V1 receptor blockade. The relaxation appears to be mediated by the release of vasodilator prostaglandins from the endothelial cell layer and is independent of V2 receptor stimulation or release of nitric oxide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin 5-HT3, 5-HT4, and 5-HT-M receptors

Serotonin (5-hydroxytryptamine [5-HT]) receptors can be classified either pharmacologically, into 5-HT1, 5-HT2, 5-HT3 and 5-HT4 receptors, or functionally, into G-protein-coupled receptors (5-HT1, 5-HT2, and 5-HT4) and ligand-gated ion channels (5-HT3). This article concentrates on the pharmacology, distribution, receptor-effector coupling, possible subtypes, and species differences of 5-HT3 receptors, which are equivalent to Gaddum and Picarelli's 5-HT-M receptor. Also presented here are some of the prominent features of the recently characterized 5-HT4 receptor. Although pharmacologic similarities have been suggested between 5-HT-M and 5-HT4 receptors (some potent 5-HT3 antagonists are active, with lower potency, at 5-HT4 receptors), it is clear that 5-HT4 receptors are different from 5-HT-M receptors.     

Regional analysis of 5-HT1A and 5-HT2 receptors in the fawn-hooded rat.

The Fawn-Hooded strain of rats exhibits a hemorrhagic disorder, known as platelet storage pool deficiency. In addition to the platelet dysfunction, there is an altered response to certain serotonin drugs. To assess the characteristics of the binding to 5-HT1A and 5-HT2 receptors in this strain, regions of the brain from Fawn-Hooded, Sprague-Dawley and Wistar male rats were examined. The drug [3H]8-OH-DPAT was used to label 5-HT1A receptors and the Kd values for frontal cortex, hippocampus, striatum, hypothalamus and brainstem were similar in all three strains of rat. As with the 5-HT1A receptors, no differences were observed in the Kd values for 5-HT2 receptors, in any of the regions examined, among the three strains. However, the Bmax for the binding of [3H]8-OH-DPAT in the striatum and brainstem of Fawn-Hooded rats was less than in the Sprague-Dawley and Wistar animals. Furthermore, 5-HT2 receptors displayed a greater Bmax value in the striatum and in the frontal cortex of Fawn-Hooded animals, compared to Sprague-Dawley and Wistar rats. These differences in receptors are consistent with previous studies in which Fawn-Hooded rats were found to have altered serotonergic function, relative to Wistar and Sprague-Dawley animals.     

Activation of 5-HT3 receptors in the rat and mouse intestinal tract: a comparative study

This study provides a comprehensive evaluation of 5-HT(3) receptor functional distribution in both the rat and mouse intestinal tract. 5-HT(3A-S) receptor splice variant mRNA was expressed throughout the intestine of the rat and mouse; the 5-HT(3A-L) variant being more common in the rat.5-HT, m-CPB, 1-PBG and 2-methyl-5-hydroxytryptamine (2m5-HT) induced contraction in the jejunum, ileum, proximal colon and distal colon of the rat (pEC(50) range: 2m5-HT, 5.86+/-0.40 to m-CPB, 7.47+/-0.27) and mouse (pEC(50) range: 1-PBG, 5.34+/-0.06 to m-CPB, 6.49+/-0.14) in the presence of nontarget 5-HT receptor antagonists, methysergide (1 muM) and GR125487 (0.1 microM). The rank orders of potency in the four regions of the rat and mouse intestine were concordant with the accepted order and the responses to 5-HT were inhibited by ondansetron (0.1 microM).5-HT(3)-induced contractions to 5-HT were reduced by tetrodotoxin (1 microM). Pargyline (10 muM) and fluoxetine (1 microM) potentiated responses in the rat jejunum. Atropine (0.1 microM) potentiated 5-HT(3)-induced responses in the rat jejunum (E(max) 49-65%), but attenuated responses in most other regions of the rat and mouse (e.g. mouse ileum: E(max) 57-26%). In the rat jejunum, L-NAME (100 microM) mimicked the effect of atropine, hexamethonium (100 microM) suppressed 5-HT(3)-induced responses, but tachykinin receptor antagonists were without effect. It is concluded that functional 5-HT(3) receptors are present in nerves along the length of the rat and mouse intestinal tract. The mouse proximal colon was found to discriminate 5-HT(3) receptor agonist profiles better than any other region in the rat or mouse. The rat jejunum shows evidence of 5-HT uptake and inactivation processes as well as inhibitory nitrergic and nontachykinin excitatory pathways associated with the 5-HT(3)-induced response.     

Molecular, pharmacological and functional diversity of 5-HT receptors

Serotonin (5-hydroxytryptamine, 5-HT) is probably unique among the monoamines in that its effects are subserved by as many as 13 distinct heptahelical, G-protein-coupled receptors (GPCRs) and one (presumably a family of) ligand-gated ion channel(s). These receptors are divided into seven distinct classes (5-HT(1) to 5-HT(7)) largely on the basis of their structural and operational characteristics. Whilst this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity continues to emerge. The challenge for modern 5-HT research has therefore been to define more precisely the properties of the systems that make this incredible diversity possible. Much progress in this regard has been made during the last decade with the realisation that serotonin is possibly the least conservative monoamine transmitter and the cloning of its many receptors. Coupled with the actions of an extremely avid and efficient reuptake system, this array of receptor subtypes provides almost limitless signalling capabilities to the extent that one might even question the need for other transmitter systems. However, the complexity of the system appears endless, since posttranslational modifications, such as alternate splicing and RNA editing, increase the number of proteins, oligomerisation and heteromerisation increase the number of complexes, and multiple G-protein suggest receptor trafficking, allowing phenotypic switching and crosstalk within and possibly between receptor families. Whether all these possibilities are used in vivo under physiological or pathological conditions remains to be firmly established, but in essence, such variety will keep the 5-HT community busy for quite some time. Those who may have predicted that molecular biology would largely simplify the life of pharmacologists have missed the point for 5-HT research in particular and, most probably, for many other transmitters. This chapter is an attempt to summarise very briefly 5-HT receptor diversity. The reward for unravelling this complex array of serotonin receptor--effector systems may be substantial, the ultimate prize being the development of important new drugs in a range of disease areas.     

Do functional relationships exist between 5-HT1A and 5-HT2 receptors?

To investigate the possible functional relationship between 5-HT1 and 5-HT2 receptors, we studied the effects of a nonselective 5-HT agonist (5-MeO DMT), a 5-HT1A-selective (8-OH-DPAT) and a 5-HT1B/5-HT1C-selective (TFMPP) agonist on the head-twitch behavior induced by the putative 5-HT2-selective receptor agonist (+/-)-DOI. In the mouse (+/-)-DOI produced the head-twitch response in a dose-dependent manner and (-)-DOI was twice as potent as the (+) isomer. Selective 5-HT2 antagonists, ketanserin and spiperone, dose-dependently inhibited the (+/-)-DOI-induced head-twitch response. The nonselective and the 5-HT1A-selective agonists also dose-dependently reduced the behavior, whereas 5-HT1B/5-HT1C-selective agonist (TFMPP) failed to affect the (+/-)-DOI-induced response. Taken together with previously published literature data, we propose a 5-HT1A inhibitory action on the 5-HT2 receptor-mediated response when induced by its selective agonist (+/-)-DOI.     

5-HT1-like receptors requiring functional cyclo-oxygenase and 5-HT2 receptors independent of cyclo-oxygenase mediate contraction of the human umbilical artery.

1. The interactions between 5-hydroxytryptamine (5-HT) and the antagonists ketanserin, methysergide and phentolamine were studied in isolated preparations of human umbilical artery (HUA) at physiological oxygen tension (Po2 approximately 15 mmHg) and at high PO2 (approximately 120 mmHg). 2. At physiological Po2 ketanserin, methysergide and phentolamine behaved as silent competitive antagonists of the 5-HT-induced contraction of HUA. pA2 values calculated by Schild analysis were 8.92, 8.52 and 6.37, respectively. 3. At high Po2, 5-HT-induced contractions were antagonised in a biphasic manner by ketanserin (0.1 microM); the response to low but not to high concentrations of 5-HT was resistant to blockade by ketanserin. The ketanserin-resistant component was abolished following cyclo-oxygenase inhibition by indomethacin (1 microM). 4. At high Po2, methysergide behaved as a partial agonist. Methysergide-induced contractions were inhibited but not abolished by indomethacin, and resistant to 5-HT2 receptor and alpha 1-adrenoceptor blockade. 5. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was mimicked by the selective 5-HT1-like receptor agonist 5-carboxamidotryptamine (5-CT): 5-CT was 7 fold more potent than 5-HT. 6. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was antagonised by phentolamine and the selective alpha 2-adrenoceptor antagonist Wy 26703. 7. These results suggest that (i) at physiological Po2 5-HT2 receptors almost exclusively mediate contractions induced by 5-HT, and (ii) at high Po2 the agonist potency order of 5-CT greater than 5-HT greater than methysergide suggests that ketanserin-resistant responses are mediated by 5-HT1-like receptors which require functional cyclo-oxygenase.     

Behavioural evidence for a functional interaction between central 5-HT2 and 5-HT1A receptors.

1. The possibility of 5-HT2 receptor modulation of central 5-HT1A receptor function has been examined using the 5-hydroxytryptamine (5-HT) behavioural syndrome induced by 5-HT1A receptor active drugs in rats. 2. The 5-HT2/5-HTIC antagonist ritanserin (0.1-2 mg kg-1) increased the 5-HT behavioural syndrome induced by submaximally effective doses of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and gepirone. 3. Pretreatment with the 5-HT2/5-HT1C antagonist ICI 170,809 (0.25-5 mg kg-1) also enhanced the behavioural syndrome induced by 8-OH-DPAT or 5-MeODMT. 4. The 5-HT2/alpha 1-adrenoceptor antagonist ketanserin in a low dose (0.25 mg kg-1) significantly increased the 5-HT behavioural syndrome induced by 8-OH-DPAT or 5-MeODMT, while in a higher dose (2.5 mg kg-1) this drug decreased the response. Experiments with prazosin indicate that the higher dose of ketanserin might reduce the 5-HT behavioural syndrome through blockade of alpha 1-adrenoceptors. 5. Ritanserin and ICI 170,809 had no effect on apomorphine-induced stereotypy or hyperactivity, indicating that these drugs do not produce non-specific behavioural activation. 6. Ritanserin and ICI 170,809 inhibited quipazine-induced wet dog shakes at doses similar to those enhancing the 5-HT behavioural syndrome. 7. We suggest that ritanserin, ICI 170,809 and ketanserin enhance 5-HT1A agonist-induced behaviour through blockade of an inhibitory 5-HT2 receptor regulating or coupled to 5-HT1A receptor-mediated function.     

Blockade of human atrial 5-HT4 receptors by GR 113808.

1. The mode of antagonism of 5-hydroxytryptamine (5-HT)-induced positive inotropic effects by the highly selective 5-HT4 receptor antagonist GR 113808 ([1-[2-methylsulphonylamino ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate) was investigated on isolated preparations of human right atrium. 2. GR 113808 caused concentration-dependent (2-100 nM) surmountable antagonism of the effects of 5-HT with a pKB (M) of 8.8. 3. The affinity of GR 113808 for human atrial 5-HT4 receptors, together with its high selectivity for 5-HT4 receptors comprise useful properties for investigating the question of 5-HT4 receptor subtypes.     

Recombinant saphenous vein 5-HT1B receptors of the rabbit: comparative pharmacology with human 5-HT1B receptors

1. The rabbit recombinant saphenous vein 5-hydroxytryptamine_1B (rb 5-HT_1B) receptor stably transfected in rat C6-glial cells was characterized by measuring adenosine 3′:5′-cyclic monophosphate (cyclic AMP) formation upon exposure to various 5-HT receptor ligands. The effects of agonists and antagonists were compared with their effects determined previously at the human cloned 5-HT_1B (h 5-HT_1B) receptor under similar experimental conditions.
2. Intact C6-glial cells expressing rb 5-HT_1B receptors exhibited [³H]-5-carboxamidotryptamine (5-CT) binding sites with a K_d of 0.80±0.13 nM and a B_max between 225 to 570 fmol mg⁻¹ protein. The binding affinities of a series of 5-HT receptor ligands determined in a membrane preparation with [³H]-5-CT or [³H]-N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide (GR 125,743) were similar. With the exception of ketanserin, ligand affinities were comparable to those determined at the cloned h 5-HT_1B receptor site.
3. rb 5-HT_1B receptors were negatively coupled to cyclic AMP formation upon stimulation with 5-HT agonists. Of the several 5-HT agonists tested, 5-CT was the most potent, the potency rank order being: 5-CT>5-HT>zolmitriptan>naratriptan>rizatriptan>sumatriptan>R(+)-8-(hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The maximal responses of these agonists were similar to those induced by 5-HT. The potency of these agonists showed a positive correlation (r²=0.87; P<0.002) with their potency at the cloned h 5-HT_1B receptor subtype.
4. 2′-Methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), methiothepin and ketanserin each behaved as silent, competitive antagonists at rb 5-HT_1B receptors; pK_B values were 8.41, 8.32 and 7.05, respectively when naratriptan was used as an agonist. These estimates accorded with their binding affinities and the potencies found on 5-HT and/or sumatriptan-mediated contraction of isolated rabbit saphenous vein segments.
5. In conclusion, the recombinant saphenous vein 5-HT_1B receptor of the rabbit shares important pharmacological similarities with the cloned h 5-HT_1B receptor. However, ketanserin is a more potent antagonist of rb 5-HT_1B receptors.

     

Evidence for a functional interaction between 5-HT1A and 5-HT2 receptors in rats

 Evidence of a functional interaction between serotonin 5-HT_1A and 5-HT₂ receptor subtypes has been compromised by incomplete experimental designs and conflicting data. To test for such an interaction, combinations of the 5-HT_1A agonist 8-OH-DPAT and the 5-HT₂ agonist DOI were administered to rats prior to testing of locomotor activity in the Behavioral Pattern Monitor (BPM). The BPM is an activity and holeboard chamber that enables analyses of quantitative and qualitative changes in locomotor and investigatory activity. Dose-response studies of 8-OH-DPAT and DOI alone and in the presence of selected doses of the other drug were performed in order to allow isobolographic analysis, which characterizes the relationship of two drugs as either additive (no interaction), supra-additive, or infra-additive. Rats treated with saline, 8-OH-DPAT (6.25, 12.5, 25, or 50 μg/kg SC), DOI (0.15, 0.3, or 0.6 mg/kg SC), or selected combinations of both drugs were tested in the BPM for 1 h. Isobolographic analysis of the effects on locomotor activity revealed that 8-OH-DPAT and DOI interact in an infra-additive manner. Thus, at a functional level, 5-HT_1A and 5-HT₂ receptors interact antagonistically in the modulation of locomotor activity. Received: 15 December 1997 / Final version: 13 February 1998     

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.(ABSTRACT TRUNCATED AT 250 WORDS)     

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-butyl4-piperidinyl) methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate was investigated on isolated preparations of human right atrial appendage. SB 207 710 caused concentration-dependent (0.1–10 nmol/l) surmountable antagonism of the effects of 5-hydroxytryptamine with a pK_B (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.