Effects of modulatory agents on neurally-mediated responses of trout intestinal smooth musclein vitro

Abstract

Mediators and mechanisms responsible for the inhibitory modulation of trout intestinal smooth muscle were examined using a series of putative mediators and substances known to modulate neurotransmission in mammalian systems. Frequency response relationships to transmural stimulation and concentration response relationships to 5-hydroxytryptamine, carbachol, and substance P were established on paired segments of rainbow trout intestinein vitro in the presence and absence of putative modulatory agents. Modulation of neurally-mediated contractions of trout intestine was achieved with dibutyryl cyclic AMP and forskolin, agents that increase intracellular levels of cyclic AMP. The effect appears to be at the level of the smooth muscle, since the adenylate cyclase activator, forskolin, inhibited muscarinic and serotoninergic contractions as well as transmurally stimulated contractions. Substance P-induced contractions were unaffected by forskolin. The endogenous agonists/neurotransmitters which would increase cyclic AMP levels in rainbow trout intestinal smooth muscle are as yet unknown. The effects do not appear to be modulated by vasoactive intestinal peptide (VIP), calcitonin, calcitonin gene-related peptide (CGRP), or agents that activate β-adrenoceptors. Prostaglandin E2 (PGE2) and α2-adrenergenic agonists are possible agents which will decrease contractility of the smooth muscle. They were only active in the proximal intestine and on transmurally stimulated contractions. The effects of both PGE2 and α2-agonists appear to be prejunctional, decreasing release of contractile neurotransmitters in the enteric nervous system.

Ontogeny of the gut motility control system in zebrafishDanio rerioembryos and larvae

“…Nilsson and Fänge, 1969;Edwards, 1972;Thorndyke and Holmgren, 1990;Burka et al, 1996;Aronsson and Holmgren, 2000). Atropine is a general muscarinic receptor antagonist and blocks the effect of acetylcholine.…”

Actions and pharmacokinetic properties of the α2-adrenergic agents, medetomidine and atipamezole, in rainbow trout (Oncorhynchus mykiss)

“…This corresponds to a molar atipamezole: medetomidine ratio of 4: 1. Information on the general effects of a2 agonist and antagonist drugs in fish is limited, although a2 agonists have been reported to stimulate contraction of rainbow trout stomach smooth muscle (Kitazawa et al 1986(Kitazawa et al , 1988 and inhibit contractions of intestinal smooth muscle in vitro (Burka et al 1996), implying modulatory roles of a2adrenoceptors in gastrointestinal motility.…”