[3H]-Mesulergine labels 5-HT7 sites in rat brain and guinea-pig ileum but not rat jejunum.
1. The primary aim of this investigation was to determine whether binding sites corresponding to the 5-HT7 receptor could be detected in smooth muscle of the rat jejunum. Binding studies in rat brain (whole brain minus cerebellum) and guinea-pig ileal longitudinal muscle were also undertaken in order to compare the binding characteristics of these tissues. Studies were performed using [3H]-mesulergine, as it has a high affinity for 5-HT7 receptors. 2. In the rat brain and guinea-pig ileum, pKD values for [3H]-mesulergine of 8.0 +/- 0.04 and 7.9 +/- 0.11 (n = 3) and Bmax values of 9.9 +/- 0.3 and 21.5 +/- 4.9 fmol mg(-1) protein were obtained respectively, but no binding was detected in the rat jejunum. [3H]-mesulergine binding in the rat brain and guinea-pig ileum was displaced with the agonists 5-carboxamidotryptamine (5-CT) > 5-hydroxytryptamine (5-HT) > or = 5-methoxytryptamine (5-MeOT) > sumatriptan and the antagonists risperidone > or = LSD > or = metergoline > ritanserin > > pindolol. 3. Despite the lack of [3H]-mesulergine binding in the rat jejunum, functional studies undertaken revealed a biphasic contractile response to 5-HT which was partly blocked by ondansetron (1 microM). The residual response was present in over 50% of tissues studied and was found to be inhibited by risperidone > LSD > metergoline > mesulergine = ritanserin > pindolol, but was unaffected by RS 102221 (3 microM), cinanserin (30 nM), yohimbine (0.1 microM) and GR 113808 (1 microM). In addition, the agonist order of potency was 5-CT > 5-HT > 5-MeOT > sumatriptan. 4. In conclusion, binding studies performed with [3H]-mesulergine were able to detect 5-HT7 sites in rat brain and guinea-pig ileum, but not in rat jejunum, where a functional 5-HT7-like receptor was present.
Department of Pharmaceutical Biology and Pharmacology, Victorian College of Pharmacy, Monash University (Parkville Campus), Australia.
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Hemedah
I M
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Coupar
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FJ
Mitchelson
D008735:5-Methoxytryptamine / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive / Q000187:drug effects; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004873:Ergolines / Q000378:metabolism* / Q000494:pharmacology; D005260:Female; D016166:Free Radical Scavengers / Q000494:pharmacology; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D007583:Jejunum / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D017294:Ondansetron / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D044405:Receptor, Serotonin, 5-HT2C; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000031:analogs & derivatives / Q000494:pharmacology; D012702:Serotonin Antagonists / Q000378:metabolism* / Q000494:pharmacology; D017366:Serotonin Receptor Agonists / Q000494:pharmacology; D013141:Spiro Compounds / Q000494:pharmacology; D013449:Sulfonamides / Q000494:pharmacology; D018170:Sumatriptan / Q000494:pharmacology; D014316:Tritium
C107821:8-(5-(5-amino-2,4-dimethoxyphenyl)-5-oxopentyl)-1,3,8-triazaspiro(4.5)decane-2,4-dione; D004873:Ergolines; D016166:Free Radical Scavengers; D044405:Receptor, Serotonin, 5-HT2C; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; D013141:Spiro Compounds; D013449:Sulfonamides; C104425:serotonin 7 receptor; D014316:Tritium; D012701:Serotonin; D008735:5-Methoxytryptamine; D017294:Ondansetron; D018170:Sumatriptan; C047087:5-carboxamidotryptamine; C031649:mesulergine