The effect of chronic treatment with morphine via pellet implantation on the sensitivity of the longitudinal smooth muscle-myenteric plexus of the guinea pig ileum to the contractile effects of γ-aminobutyric acid (GABA(A))-receptor agonists was assessed. GABA and muscimol elicited concentration-dependent contractions of the longitudinal smooth muscle which were due to the release of acetylcholine because the contractile effects were markedly attenuated by atropine (10 nM). The contractile action of GABA agonists does not involve an intermediate step mediated by nicotinic receptors because the concentration-response curves for GABA were unaffected by hexamethonium (1 mM). Bicuculline (10 μM) produced nearly equivalent rightward shifts of the concentration-response curves for both GABA and muscimol, indicating mediation of the contractile effects of these agents by GABA(A) receptors. Chronic exposure to morphine via pellet implantation did not alter the sensitivity of this preparation to either GABA or muscimol. This is in contrast to the development of supersensitivity of the longitudinal smooth muscle-myenteric plexus to other excitatory agonists (nicotine, 5-hydroxytryptamine and potassium), which accompanies the development of tolerance to opioids. GABA induces depolarization of myenteric neurons that is observed most prominently in AH neurons and rarely in S neurons. The stimulatory effects of nicotine and of GABA were inhibited by morphine (a predominantly μ opioid agonist) and by U50,488H (a predominantly κ opioid agonist). The results are discussed within the context that supersensitivity to neuronal stimulants of the myenteric plexus in morphine-tolerant preparations is limited to substances which depolarize S neurons.
|Number of pages||8|
|Journal||Journal of Pharmacology and Experimental Therapeutics|
|Publication status||Published - 1 Dec 1991|