5-HT3 and 5-HT4 receptors and cholinergic and tachykininergic neurotransmission in the guinea-pig proximal colon.
The pathways and possible transmitters involved in the contractile response to selective 5-HT3 and 5-HT4 receptor stimulation in the guinea-pig proximal colon were studied. In the presence of methysergide, 5-HT induced contractions, yielding a biphasic concentration-response curve that was changed into a monophasic curve in the presence of the 5-HT3 receptor antagonist, granisetron (1 microM) (low-affinity phase blocked), or the 5-HT4 receptor antagonist, SB 204070 ((1-butyl-4-piperidinyl methyl)-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate) (10 nM) (high-affinity phase blocked) combination of the two antagonists abolished the contraction to 5-HT. The effectiveness and selectivity of both antagonists was confirmed by testing them against contractions in response to the 5-HT3 receptor-selective agonist, 2-methyl-5-HT, and the 5-HT4 receptor-selective agonist, 5-methoxytryptamine. Hexamethonium (100 microM) did not affect the 5-HT3 receptor-mediated contractions, whereas tetrodotoxin (0.3 microM) caused only slight inhibition. Both in the absence and presence of tetrodotoxin, atropine (0.3 microM) inhibited the 5-HT3 receptor-mediated contractions. Hence, the contractions to 5-HT are partly mediated by 5-HT3 receptors that are localized on the nerve endings of the motor neurons. Hexamethonium halved the 5-HT4 receptor-mediated contractions, whereas tetrodotoxin abolished them. The 5-HT4 receptor-mediated contractions were inhibited by atropine (0.3 microM). Thus, the 5-HT4 receptors seem to be localized in the soma of the motor neurons; they also occur on interneurons. The remaining contractions induced by 5-HT3 and 5-HT4 receptor stimulation in the presence of atropine were almost completely inhibited by the tachykinin NK1 receptor antagonist, CP 96345 ((2S,3S)-cis-2-(diphenyl methyl)-N-[(2-methoxy phenyl)-methyl]-1-azabicyclo-[2.2.2]-octan-3-amine) (0.1 microM). CP 96345 also abolished or strongly inhibited contractions in response to substance P (10 nM) and to neurokinin A (30 nM), but neither granisetron nor SB 204070 affected them. Hence, stimulation of either 5-HT3 or 5-HT4 receptors induced contractions that are partially mediated by acetylcholine, and partially by a tachykinin NK1 receptor-stimulating neurotransmitter, probably substance P and/or neurokinin A.
Briejer
M R
MR
Department of Gastrointestinal Pharmacology, Janssen Research Foundation, Beerse, Belgium.
Schuurkes
J A
JA
D000818:Animals; D001713:Biphenyl Compounds / Q000494:pharmacology*; D003106:Colon / Q000187:drug effects* / Q000294:innervation; D004146:Dioxanes / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D017829:Granisetron / Q000494:pharmacology*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007395:Interneurons / Q000378:metabolism; D008297:Male; D009046:Motor Neurons / Q000378:metabolism; D009119:Muscle Contraction; D064729:Neurokinin-1 Receptor Antagonists; D010880:Piperidines / Q000494:pharmacology*; D018040:Receptors, Neurokinin-1 / Q000502:physiology; D011985:Receptors, Serotonin / Q000187:drug effects* / Q000502:physiology; D012701:Serotonin / Q000494:pharmacology*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology*
D001713:Biphenyl Compounds; D004146:Dioxanes; D017829:Granisetron; D064729:Neurokinin-1 Receptor Antagonists; D010880:Piperidines; D018040:Receptors, Neurokinin-1; D011985:Receptors, Serotonin; D012701:Serotonin; D017366:Serotonin Receptor Agonists; C084858:SB 204070A; C066892:CP 96345