Interaction of azelastine with adenosine receptors in guinea pig trachea

The effects of azelastine, 8-phenyltheophylline, NDGA, atropine and mepyramine on PIA-induced contraction and relaxation of isolated guinea pig tracheal chains were investigated. Atropine (1 nM) and mepyramine (1 microM) had no effect on PIA-induced relaxation whereas 8-phenyltheophylline (5 microM) caused strong inhibition of PIA-induced relaxation, indicating that the latter effect is mediated by stimulation of extracellular adenosine receptors. NDGA (0.5 microM) caused potentiation of PIA-induced relaxation. Azelastine (10 nM-1 microM) caused dose-dependent potentiation of PIA-induced relaxation. In another model for investigation of extracellular adenosine receptors, namely the negative inotropic effect in the electrically driven isolated guinea pig atrium, the action of PIA was fully reversed by the addition of 8-phenyltheophylline. In contrast, the negative inotropic effect of azelastine was not reversed by 8-phenyltheophylline, indicating that azelastine does not act on extracellular adenosine receptors. The negative inotropic effect of azelastine can be reversed by addition of calcium as for verapamil. It is concluded that the calcium-antagonistic and perhaps antiallergic properties of azelastine are responsible for the potentiation of extracellular adenosine receptor mediated relaxation by azelastine. Since asthmatics show increased hyperreagibility (bronchospasm) to inhalation of adenosine, the inhibition of PIA-induced contraction by azelastine indicates that the drug may be worthwhile in the treatment bronchial hyperreactivity in asthmatic patients.     

Competitive antagonism by recognised 5-HT2 receptor antagonists at 5-HT1C receptors in pig choroid plexus

Summary The effects of several antagonists, known to interact with 5-HT₂ receptors (ritanserin, LY 53857, ICI 169,369, methysergide, mesulergine and ketanserin), were tested against 5-HT-stimulated production of inositol phosphate in pig choroid plexus, a 5-HT_1C receptor model. These antagonists produced dextral shifts of the concentration response curve to 5-HT in a parallel manner, without depressing significantly the maximal response. The following pA₂ values (in parentheses) were obtained: mesulergine (8.88), methysergide (8.85), LY 53857 (8.69), ritanserin (8.69), ICI 169,369 (7.86), and ketanserin (6.57). These pA₂ values were in good agreement with the pK_D values determined in radioligand binding studies performed in pig choroid plexus with [³H]mesulergine. The present data demonstrate that several drugs described as 5-HT₂ receptor selective antagonists (e.g. ritanserin, LY 53857 and ICI 169,369) are also potent, competitive and surmountable antagonists at 5-HT_1C receptors. Thus, the results provide further evidence for the pharmacological similarity of 5-HT_1C and 5-HT₂ receptors. However, in contrast to the situation described with methysergide, ritanserin and LY 53857 in several 5-HT₂ receptor models, none of these antagonists acted in a non-competitive or unsurmountable fashion at 5-HT_1C receptors. These results suggest, but do not firmly rule out, that at least in the presence of the drugs tested in the present study, 5-HT_1C receptors in the choroid plexus do not undergo an allosteric modulation; these findings are apparently in contrast to a model proposed previously for 5-HT₂ receptors (Kaumann and Frenken 1985, Naunyn-Schmiedeberg's Arch Pharmacol 328: 295–300) Send offprint requests to P. Schoeffter at the above address     

Prokinetic benzamides stimulate peristaltic activity in the isolated guinea pig ileum by activation of 5-HT4 receptors.

Substituted benzamides such as metoclopramide, cisapride, zacopride, renzapride or BRL 20627, stimulate intestinal motility in various species. As they are antagonists at 5-HT3 and agonists at 5-HT4 receptors and as both mechanisms could potentially contribute to their gastrointestinal prokinetic effect, the underlying mechanism is unclear. To clarify this, the effect of some substituted benzamides on gut motility was investigated in the isolated guinea pig ileum using the Trendelenburg technique, in which pressure-induced peristaltic contractions are measured. All benzamides stimulated the peristaltic reflex with the rank order of potency: renzapride greater than cisapride greater than BRL 20627 greater than (+/-)-zacopride greater than metoclopramide. ICS 205-930, granisetron and 2-methyl-5-HT did not change the peristaltic response. 5-HT and 5-methoxytryptamine potently mimicked the effect of the benzamides. The effect of 5-HT was not blocked by ICS 205-930 (10(-7) M). These results indicate that the Trendelenburg preparation is suitable for the investigation of intestinal prokinetic effects of the substituted benzamides. Furthermore, the results suggest that the intestinal effect of benzamides results from activation of 5-HT4 receptors rather than from blockade of 5-HT3 receptors.     

5-HT 1D receptors mediate SKF 99101H-induced hypothermia in the guinea pig

The selective, brain penetrant, 5-HT(1D) receptor agonist SKF 99101H (10-30 mg/kg i.p.) caused a dose-related fall in rectal temperature in guinea pigs which lasted longer than 2 h. Sumatriptan (1.0-100 mg/kg i.p.), a selective 5-HT(1D) agonist which does not penetrate the brain, did not produce hypothermia, suggesting that peripheral mechanisms are not critically involved in the response. The hypothermia induced by SKF 99101H (30 mg/kg i.p.) was dose-dependently blocked by the 5-HT(1D) receptor antagonists GR 127935 (0.01-1 mg/kg i.p.) and GR 125743 (0.01-3 mg/kg i.p.), confirming the role of 5-HT(1D) receptors. Mianserin (0.3-10.0 mg/kg i.p.) and granisetron (0.1-3.0 mg/kg i.p.) were inactive, suggesting that 5-HT(2A/2B/2C) or 5-HT( 3) receptors play no significant role in the generation of the hypothermic response. Nor was the hypothermia reversed by prazosin (0.03-1.0 mg/kg i.p.), idazoxan (0.03-1.0 mg/kg i.p.) or scopolamine (0.01-0.3 mg/kg i.p.), thereby excluding mediation by α(1)- and α(2)-adrenoceptors and muscarinic receptors. WAY 100635 (0.1-1.0 mg/kg) significantly potentiated the effect of SKF 99101H. The antagonists, when given alone, had no effect on body temperature, with the exception of prazosin (0.1 and 1.0 mg/kg). Three days of treatment with parachloroamphetamine (30 mg/kg i.p.) depleted forebrain 5-HT by ～ 75% in frontal cortex, hypothalamus, hippocampus and striatum, but failed to alter the hypothermic response to SKF 99101H. The hypothermia is, therefore, unlikely to be mediated by 5-HT(1D) receptors located on 5-HT neurons. SKF 99101H-induced hypothermia in the guinea pig may serve as a useful model for investigation of centrally acting 5-HT( 1D) receptor antagonists.     

5-Hydroxytryptamine receptors,especially the 5-HT4 receptor,in guinea pig urinary bladder

The function of 5-hydroxytryptamine (5-HT) receptors, especially the 5-HT4 receptor, in the urinary bladder were examined in preparations isolated from the guinea pig by in vitro receptor autoradiography and determinations of mechanical activity and acetylcholine (ACh) release. Specific [125I]SB207710 binding sites were detected evenly throughout the urinary bladder. 5-HT (3 x 10(-8)-10(-4) M) caused contractions of strips of the urinary bladder, in a concentration dependent manner. Ketanserin antagonized the 5-HT-induced contractions, while granisetron and SB204070 antagonized the contractions induced by high concentrations of 5-HT. Atropine inhibited the contractions induced by high concentrations of 5-HT. Ketanserin prevented the 5-HT-induced contractions in the presence of atropine, but granisetron and SB204070 did not affect the contractions under such a condition. 5-HT enhanced the electrically-stimulated (5 Hz, 0.5 ms) outflow of [3H]acetylcholine from strips preloaded with [3H]choline, and the enhancement was antagonized by granisetron and SB204070. Thus, the contractile response to 5-HT was mediated by activations of 5-HT2, 5-HT3 and 5-HT4 receptors. The 5-HT2 receptor may be a property of high affinity to 5-HT and located on the smooth muscle cells. The 5-HT4 as well as 5-HT3 receptor may be a property of low affinity to 5-HT and located on the cholinergic neurons.     

5-Methoxytryptamine and 2-methyl-5-hydroxytryptamine-induced desensitization as a discriminative tool for the 5-HT3 and putative 5-HT4 receptors in guinea pig ileum

Summary Agonist-induced desensitization has been utilized to discriminate and independently isolate the neuronal excitatory receptors to 5-hydroxytryptamine (5-HT) in the guinea pig ileum (5-HT₃ and putative 5-HT₄ receptors). Electrically stimulated longitudinal muscle myenteric plexus preparations, and non-stimulated segments of whole ileum were used. Exposure to 5-methoxytryptamine (10 mol/l) inhibited completely responses to 5-HT at the putative 5-HT₄ receptor without affecting 5-HT₃-mediated responses. Conversely, exposure to 2-methyl-5-HT (10 mol/l) inhibited completely responses to 5-HT at the 5-HT₃ receptor without affecting putative 5-HT₄-mediated responses. The inhibition with 5-methoxytryptamine and 2-methyl-5-HT, either alone or in combination, appeared selective as responses to KCI, DMPP, carbachol, histamine, and substance P were unaffected or only very slightly modified. Furthermore, the pA₂ values for ICS 205–930 at the putative 5-HT₄ (pA₂ = 6.2 to 6.5) and 5-HT₃ (pA₂ = 7.6 to 8.1) receptors (estimated in the presence of 2-methyl-5HT and 5-methoxytryptamine, respectively) were consistent with those estimated in the absence of desensitization.5-Methoxytryptamine, but not 2-methyl-5-HT, suppressed completely but reversibly the concentration-effect curve to renzapride, suggesting that responses to this agent are mediated exclusively via agonism at the putative 5-HT₄ receptor.It is concluded that 5-methoxytryptamine and 2-methyl-5-HT can be utilized as selective probes to discriminate the putative 5-HT₄ receptor from the 5-HT₃ receptor in guinea pig ileum. This finding is of importance as no selective antagonist exists for the putative 5-HT₄ receptor. Furthermore, the presently described method of agonist-induced desensitization and 5-HT receptor discrimination may be useful for the identification and characterization of the putative 5-HT₄ receptor in other tissues and species. Send offprint requests to D. E. Clarke at the above address     

A single channel mutation alters agonist efficacy at 5-HT3A and 5-HT3AB receptors

BACKGROUND AND PURPOSE

5-HT₃ receptors are composed of 5-HT₃A subunits (homomeric receptors), or combinations of 5-HT₃A and other 5-HT₃ receptor subunits (heteromeric receptors, the best studied of which are 5-HT₃AB receptors). Here we explore the effects of partial agonists at 5-HT₃A and 5-HT₃AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation.

EXPERIMENTAL APPROACH

Wild type and mutant 5-HT₃A and 5-HT₃AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [³H]granisetron binding.

KEY RESULTS

Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT₃A and 5-HT₃AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT₃A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT₃AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6''S substitution in the 5-HT₃A subunit decreased EC₅₀ and increased R_max of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT₃A_T6''SB receptors, where it became a full agonist (EC₅₀ = 7 nM) with an EC₅₀ 58-fold less than 5-HT (EC₅₀ = 0.4 μM) and no longer caused inhibition of subsequent agonist responses.

CONCLUSIONS AND IMPLICATIONS

These results indicate that a mutation in the pore lining domain in both 5-HT₃A and 5-HT₃AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.     

Physiological relevance of constitutive activity of 5-HT2A and 5-HT2C receptors

It is generally accepted that seven-transmembrane receptors have the capacity to regulate cellular signaling systems in the absence of occupancy by a ligand (i.e. the receptors display constitutive activity). Drugs can increase (agonists), decrease (inverse agonists) or not change (antagonists) receptor activity towards a cellular effector. Moreover, some drugs (protean ligands) have multiple pharmacological properties (e.g. agonism towards one response and inverse agonism towards another response coupled to the same receptor and measured from the same cells, simultaneously). In this article, we describe response-dependent constitutive activity and ligand pharmacology for 5-HT2A and 5-HT2C receptors in vitro. Moreover, we provide evidence that 5-HT2A and 5-HT2C receptor constitutive activity is physiologically relevant in vivo and suggest that strong consideration should be given to the impact of constitutive receptor activity on disease and the therapeutic potential of inverse agonism.     

5-HT2 receptors

5-HT(2) receptors are G-protein coupled receptors that currently comprise three subtypes: 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors. The subtypes are related in their molecular structure, amino acid sequence and signaling properties. 5-HT(2A) and 5-HT(2C) receptors have a widespread distribution and function in the central nervous system. 5-HT(2A)and 5-HT(2C) receptor antagonism is a property of certain antipsychotic and antidepressant drugs. 5-HT(2B) receptors have a restricted expression in the central nervous system. They have an important role in embryogenesis and in the periphery. In this article, selected aspects of 5-HT(2) receptor research are reviewed for each subtype under three main headings : (i) genes, protein structure and receptor signaling; (ii) receptor localization with emphasis on the CNS and (iii) compounds. The general discussion reflects on the reasons for the limited success in the clinic of 5-HT(2) receptor subtype selective drugs.     

Modulation by 5-HT1A receptors of the 5-HT2 receptor-mediated tachykinin-induced contraction of the rat trachea in vitro

1. In the Fisher 344 rat, tachykinins have been shown to cause the release of 5-hydroxytryptamine (5-HT) from airway mast cells, which then causes direct smooth muscle activation as well as the release of acetylcholine from cholinergic nerves. The aim of the present study was to examine the modulatory effects of 5-HT receptors on the neurokinin A (NKA)-induced release of endogenous 5-HT and airway smooth muscle contraction in the isolated Fisher 344 rat trachea.
2. The selective 5-HT₂ receptor antagonist ketanserin (0.1 μM) produced an almost complete inhibition of the contractions caused by NKA (n=4, P<0.0001, two-way ANOVA), and a significant rightward shift of the concentration-response curve to 5-HT (n=8, P<0.001, two-way ANOVA).
3. The partial agonist for 5-HT_1A receptors, 8-OH-DPAT (1 μM), and the full agonist for 5-HT₁ receptors, 5-CT (0.3 μM), potentiated the submaximal contractions induced by the 5-HT₂ receptor agonist α-methyl-5-HT (0.1 μM) (n=4; P<0.005 and P<0.05, respectively). 8-OH-DPAT (1 μM), as well as the 5-HT_1A receptor antagonists pMPPI, SDZ 216525 and NAN-190 (0.1 μM each), caused significant inhibition of the tracheal contractions induced both by NKA (10  nM–3  μM) and 5-HT (10 nM–10 μM) (n=4–10). This suggests that activation of 5-HT_1A receptors potentiates the 5-HT₂ receptor-mediated contractions.
4. SDZ 216525 (0.1 μM) significantly reduced the maximal contraction produced by 1 μM NKA (n=10, P<0.001), without affecting the release of endogenous 5-HT. These data rule out the involvement of a 5-HT_1A receptor-mediated positive feedback mechanism of the 5-HT release from mast cells.
5. Even in the presence of atropine (1 μM), 8-OH-DPAT (1 μM) further reduced the maximal NKA-induced contraction (n=4, P<0.0001), while the contractions of the rat isolated trachea induced by electrical field stimulation and the concentration-response curve to carbachol were unaffected by pMPPI (0.1 μM), SDZ 216525 (0.1 μM), NAN-190 (0.1 μM) and 8-OH-DPAT (1 μM) (n=4–6). These data demonstrate that the 5-HT_1A receptor-mediated potentiation of contractile responses is not due to non-specific inhibition of airway smooth muscle contraction or to modulation of postganglionic nerve activation.
6. The selective 5-HT_1B/1D receptor antagonist GR 127935, the selective 5-HT₃ receptor antagonist tropisetron and the selective 5-HT₄ receptor antagonists SB 204070 and GR 113808 (0.1 μM each) had no effect on the concentration-response curve for NKA (n=6–10), ruling out the involvement of 5-HT_1B/1D, 5-HT₃ and 5-HT₄ receptors.
7. The α-adrenoreceptor antagonist phentolamine (1 μM) had no effect on the 5-HT-induced contractions (n=4), ruling out the involvement of α-adrenoreceptors.
8. In conclusion, the tachykinin-induced contraction of the F334 rat isolated trachea is mediated by the stimulation of 5-HT₂ receptors. Activation of 5-HT_1A receptors located on airway smooth muscle potentiates the direct contractile effects of 5-HT₂ receptor activation. The 5-HT_1B/1D, 5-HT₃ and 5-HT₄ receptors are not involved in the NKA-induced contraction of rat airways.

     

GR46611 potentiates 5-HT1A receptor-mediated locomotor activity in the guinea pig.

5-HT(1B/D) receptor agonists such as GR46611 (3-[3-(2-Dimethylaminoethyl)-H-indol-5-yl]-N-(4-methoxybenzyl)acrylamide ) are known to lower body temperature in guinea pigs. Although stimulation of their functional analogs in rats, the 5-HT1B receptor induces hyperlocomotion, this effect has yet to be demonstrated with 5-HT(1B/D) receptor agonists in the guinea pig. Previous studies have shown that 5-HT1A agonists increase locomotor activity in guinea pigs. The current study set out to examine the effects of 5-HT(1B/D) receptor stimulation on locomotor activity in the guinea pig and to examine the interaction between 5-HT1A and 5-HT(1B/D) receptor stimulation on locomotor activity in that species. The full agonist at 5-HT1A receptors, 8-OH-DPAT (R(+)-8-Hydroxy-dipropylaminotetralin HBr) dose-dependently increased locomotor activity in guinea pigs (0.3-1.25 mg kg(-1) s.c.), as to a lesser extent, did the partial agonist, buspirone (8-[4-[4-(2-Pyramidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5 ]decane-7,9-dione HCl) (5.0-20.0 mg kg(-1) s.c.). The 5-HT(1B/D) receptor agonist GR46611 had no effect on locomotor activity in guinea pigs at doses up to 40 mg kg(-1) s.c. 8-OH-DPAT-induced behavioural activation was reversed by the selective 5-HT1A receptor antagonist WAY100635 (N-[-2-[4-(-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyrinidyl) cyclo hexanocarboxamide trihydro-chloride), with a minimum effective dose of 0.006 mg kg(-1), but not by the 5-HT(1B/D) receptor antagonist GR127935 (2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyli c acid [4-methoxy-3-(4-methyl-piperazin-1-yl)phenyl]-amide) (0.25-1.0 mg kg(-1)). GR46611, at doses that were without effect given alone (0.5-2.5 mg kg(-1)), significantly enhanced the locomotor response to subthreshold doses of 8-OH-DPAT (0.5 mg kg(-1)) and buspirone (10 mg kg(-1)). The effect of GR46611 on 8-OH-DPAT-induced hyperactivity was reversed by pretreatment with GR127935 and with WAY 100635 indicating that activation of both receptors was required for the expression of locomotor hyperactivity. These findings suggest that activation of 5-HT(1B/D) receptors alone may not stimulate locomotor activity but it does potentiate the locomotion induced by 5-HT1A receptor stimulation in guinea pigs.     

Inverse agonist activity of sarpogrelate, a selective 5-HT2A-receptor antagonist, at the constitutively active human 5-HT2A receptor

Mutations producing constitutively active G-protein coupled receptors have been found in the pathophysiology of several diseases, implying that inverse agonists at the constitutively active receptors may have preferred therapeutic applications. Because of the involvement of 5-HT(2A) receptors in mediating many cardiovascular diseases, constitutively active mutants of the 5-HT(2A) receptor may be responsible for the disease states. Thus, the purpose of the present study was to investigate the inverse agonist activity of sarpogrelate, a selective 5-HT(2A)-receptor antagonist, and its active metabolite, M-1; and we compared their activities with those of other 5-HT(2A)-receptor antagonists such as ritanserin, ketanserin, and cyproheptadine. Using a constitutively active mutant (C322K) of the human 5-HT(2A) receptor, we demonstrated that like other 5-HT(2A)-receptor antagonists, sarpogrelate acts as a potent inverse agonist by significantly reducing basal inositol phosphate levels. However, there were no significant differences between sarpogrelate and other 5-HT(2A)-receptor antagonists for their inverse agonist activity. Compared with the wild type receptor, mutant receptor displayed significantly higher affinity for 5-HT and lower affinity for sarpogrelate. These results indicate that stabilization of the inactive conformation of the 5-HT(2A) receptor may be a key component of the mechanism of action of sarpogrelate.     

Antagonism at serotonin 5-HT2A receptors modulates functional activity of frontohippocampal circuit

Rationale  

Several second-generation antipsychotics are characterised by a significant antagonistic effect at serotonin 5-HT_2A receptors (5-HT_2AR), a feature that has been associated with lower incidence of extra-pyramidal symptoms and a putative amelioration of positive and negative symptoms experienced by schizophrenic patients. However, the neurofunctional substrate of 5-HT_2A antagonism and its exact contribution to the complex pharmacological profile of these drugs remain to be elucidated.     

Relationship between intrinsic activity of beta-adrenoceptor agonist and amount of spare receptors in guinea pig taenia caecum

Beta-adrenoceptor mechanisms were studied in the isolated guinea-pig taenia caecum. Isoprenaline, desisopropylprocaterol and carteolol relaxed guinea-pig taenia caecum, and their intrinsic activities (mean +/- S.E.) were 1.0, 0.95 +/- 0.03 and 0.61 +/- 0.08, respectively. The pD2 values (mean +/- S.E.) were 8.59 +/- 0.06 for isoprenaline, 7.94 +/- 0.21 for desisopropylprocaterol and 6.35 +/- 0.14 for carteolol. The beta-adrenoceptors in guinea-pig taenia caecum were irreversibly inactivated by photoaffinity labeling with isoprenaline (6 X 10(-6) M). Photoinactivation of beta-adrenoceptors caused a considerable parallel shift in the dose-response curve of isoprenaline. The decline of the dose-response curves of desisopropylprocaterol and carteolol without a preceding shift was observed after photoinactivation. Isoprenaline, desisopropylprocaterol and carteolol increased cyclic AMP levels in guinea-pig taenia caecum, and the pD2 values were 7.43, 6.97 and 5.96, respectively. When differences between the pD2 values for test drugs obtained from the mechanical responses and from the increases of cyclic AMP levels were calculated, the differences for isoprenaline was larger than those for desisopropylprocaterol and carteolol. The dose-response curves for relaxation and cyclic AMP level by isoprenaline were both shifted to the right as a result of prior incubation with carteolol. These results suggest that there are spare receptors for isoprenaline, but few for desisopropylprocaterol and carteolol in the smooth muscle.     

5-HT autoreceptors in the regulation of 5-HT release from guinea pig raphe nucleus and hypothalamus

5-HT autoreceptors involved in the regulation of 5-HT release in the guinea pig dorsal raphe nucleus have been studied in comparison with those in the hypothalamus. In vitro release was measured in slices of raphe and hypothalamus prelabelled with [³H]5-HT, superfused with Krebs solution and depolarized electrically. The non-selective 5-HT receptor agonist, 5-carboxamidotryptamine (5-CT) (0.1–10 nM for raphe; 1–100 nM for hypothalamus) and antagonist, methiothepin (10–1000 nM), decreased and increased, respectively, the release of [³H]5-HT evoked by electrical stimulation in either of these regions when given alone. The selective 5-HT_1B/D receptor antagonist, GR127935 (100–1000 nM), and the 5-HT_1D receptor antagonist, ketanserin (300–1000 nM), had no significant effect on this release in either of these regions. Methiothepin and GR127935 (100–1000 nM) shifted to the right the concentration-effect curve of 5-CT in both the raphe and the hypothalamus. At 300 nM, ketanserin shifted to the right the concentration-effect curve of 5-CT in the raphe but did not modify the 5-CT curve in the hypothalamus. In microdialysis experiments ketanserin, applied locally at 10 μM, increased the extracellular levels of 5-HT in the dorsal raphe nucleus of the freely moving guinea pig, whereas 5-HT levels were unchanged in the hypothalamus. Ketanserin at 1 μM did not affect the decrease in 5-HT output induced by the selective 5-HT_1B/D receptor agonist, naratriptan (used at 10 μM in raphe and 0.1 μM in hypothalamus), in the raphe or the hypothalamus. In the raphe, WAY100635, a 5-HT_1A receptor antagonist, at 1 μM, did not prevent naratriptan (10 μM) from reducing the extracellular levels of 5-HT. These results suggest that, in the conditions used in this study, the release of 5-HT in the dorsal raphe nucleus is possibly modulated in part by 5-HT_1B receptors but essentially the control is through 5-HT receptors whose subtype is still to be determined. In the hypothalamus, however, it is clear that only 5-HT_1B receptors are involved in the modulation of 5-HT neurotransmission.     

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

1. Marked heterogeneity among species exists in the esophageal response to pharmacological agents. The present study compared the response to serotonin in esophagus from the rat, guinea pig, rabbit and dog.2. The esophagus from all four species contracted to carbamylcholine and to PGF₂α; responses to serotonin were the most variable among species.3. Serotonin contracted the guinea pig and rabbit esophagus; an effect blocked by LY53857 (10⁻⁷M) and ketanserin (10⁻⁷M), consistent with 5-HT₂ receptor activation mediating this contraction.4. Serotonin neither contracted nor relaxed the canine esophagus and relaxed the rat esophagus via 5-HT₄ receptor activation as determined by antagonism with ICS 205–930 (-log K_B = 6.4), metoclopramide (-log K_B = 6.7) and its ester congener SDZ 205–557 (-log K_B = 7.9). Two methylene homologs of SDZ 205–557 also had high 5-HT₄ receptor affinity (-log K_B = 7.7).5. Thus, in guinea pig and rabbit esophagus, serotonin induced a contraction mediated by 5-HT₂ receptors; and serotonin neither contracted nor relaxed the canine esophagus. In rat esophagus, serotonin induced a relaxation mediated by activation of 5-HT₄ receptors.     

Ergometrine--a partial agonist at 5-HT receptors in the uterus isolated from the oestrogen-primed rat

The mechanism of action of ergometrine has been studied in isolated uterus from the oestrogen-primed non-pregnant rat. Ergometrine (30 nM-1 microM) induced spasm and was antagonised selectively by methysergide and ICI 169,369, a proposed competitive antagonist at 5-hydroxytryptamine (5-HT2) receptors. The pA2 values of ICI 169,369 against 5-HT and ergometrine were not significantly different. Ergometrine (0.1-10 microM) was also a selective antagonist of 5-HT with no effect against acetylcholine or potassium chloride. It is suggested that ergometrine is a partial agonist involving 5-HT receptors in rat uterus.