Use of stereoisomers of zacopride to analyze actions of 5-hydroxytryptamine on enteric neurons.
Two subtypes of excitatory 5-hydroxytryptamine (5-HT) receptor, 5-HT1P and 5-HT3, are found on type 2-AH neurons of the guinea pig myenteric plexus. The 5-HT1P receptor mediates a slow and the 5-HT3 receptor a fast depolarization of these cells, however, the role of these receptors in the physiology of the gut is unknown. Renzapride (BRL 24924), a substituted benzamide, has previously been found to antagonize responses of myenteric neurons mediated by both 5-HT1P and 5-HT3 receptors. The effects on myenteric type 2-AH neurons of a structurally similar benzamide, zacopride, which unlike renzapride has S and R stereoisomers, were investigated to gain further insight into 5-HT receptor function. In contrast to renzapride, S-, but not R-zacopride, was found to mimic the 5-HT1P receptor-mediated slow response to 5-HT. Desensitization of 5-HT1P receptors with 5-HT inhibited slow depolarizing responses to S-zacopride, and desensitization with S-zacopride antagonized slow responses to 5-HT. Responses to S-zacopride were also inhibited by renzapride and the 5-HT1P receptor antagonist N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (5-HTP-DP). S-zacopride, like renzapride and 5-HT, presynaptically inhibited nicotinic fast excitatory postsynaptic potentials, an effect that can be mediated by 5-HT1P or 5-HT1A receptors. Both S and R stereoisomers of zacopride antagonized 5-HT3 receptor-mediated fast responses to 5-HT. Unlike 5-HTP-DP, neither zacopride or its stereoisomers nor renzapride inhibited the binding of 5-[3H]HT to 5-HT1P receptors. [3H]zacopride (5-10 nM) was found to bind to a site in the gut from which it could be displaced by a 1,000-fold excess of renzapride and S-zacopride (but not R-zacopride) greater than 5-HTP-DP much greater than the 5-HT3 receptor antagonist ICS 205-930. These observations suggest that, in addition to 5-HT3 receptors, there is a benzamide binding site on myenteric neurons that interacts with, but is distinct from, the 5-HT recognition site of 5-HT1P receptors. Benzamides may affect coupling of the 5-HT1P receptor to its effector.
Wade
P R
PR
Department of Anatomy and Cell Biology, Columbia University College of Physicians and Surgeons, New York, New York 10032.
Mawe
G M
GM
Branchek
T A
TA
Gershon
M D
MD
D000818:Animals; D001549:Benzamides / Q000378:metabolism / Q000494:pharmacology*; D001643:Bridged Bicyclo Compounds / Q000378:metabolism / Q000494:pharmacology*; D019086:Bridged Bicyclo Compounds, Heterocyclic; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009197:Myenteric Plexus / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D011985:Receptors, Serotonin / Q000187:drug effects / Q000502:physiology*; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology*; D012702:Serotonin Antagonists / Q000494:pharmacology*; D013237:Stereoisomerism
D001549:Benzamides; D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D012701:Serotonin; C055971:zacopride