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.
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 mumol/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 mumol/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 KCl, 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-5-HT 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.(ABSTRACT TRUNCATED AT 250 WORDS)
Craig
D A
DA
Institute of Pharmacology, Syntex Research, Palo Alto, CA 94304.
Eglen
R M
RM
Walsh
L K
LK
Perkins
L A
LA
Whiting
R L
RL
Clarke
D E
DE
D008735:5-Methoxytryptamine / Q000494:pharmacology*; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D004246:Dimethylphenylpiperazinium Iodide / Q000494:pharmacology; D004558:Electric Stimulation; D006168:Guinea Pigs; D006632:Histamine / Q000494:pharmacology; D007082:Ileum / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D009130:Muscle, Smooth / Q000187:drug effects*; D009197:Myenteric Plexus / Q000187:drug effects; D011189:Potassium Chloride / Q000494:pharmacology; D011985:Receptors, Serotonin / Q000187:drug effects*; D012701:Serotonin / Q000031:analogs & derivatives* / Q000494:pharmacology*
D011985:Receptors, Serotonin; D012701:Serotonin; D008735:5-Methoxytryptamine; D004246:Dimethylphenylpiperazinium Iodide; D011189:Potassium Chloride; C045918:2-methyl-5-HT; D006632:Histamine; D002217:Carbachol