Effects of naloxone and opioid agonists on gastric excitatory responses to stimulation of the vagus nerve in cats

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Abstract
1 An investigation was made into the contribution of endogenous opioids to the initial and delayed excitatory response of the stomach induced by stimulation of the vagal trunk in cats. 2 Naloxone (100 to 1000 μg kg−1) had no effect on the initial excitatory response to stimulation of the vagal efferent fibres. However, the same treatment dose-dependently enhanced the delayed excitatory response to stimulation of the vagal afferent fibres. 3 In comparison with the μ-opioid-receptor agonist, [d-Ala2, MePhe4, Gly-ol5]enkephalin and the κ-opioid-receptor agonist dynorphin A (1–13), lower doses of methionine enkephalin ([Met]enkephalin) markedly inhibited the excitation caused by stimulation of the vagal efferent and afferent fibres. The inhibitory effect of [Met]enkephalin was antagonized by naloxone. 4 The δ-opioid-receptor selective agonist [d-Pen2, d-Pen5]enkephalin mimicked the inhibitory effects of [Met]enkephalin and inhibition by [d-Pen2, d-Pen5]enkephalin was antagonized by the δ-opioid-receptor antagonist, ICI 174,864. 5 It is concluded that the inhibitory effects of exogenous opioids on the excitatory response of the stomach to stimulation of the vagal efferent and afferent fibres are mediated, at least in part, by δ-opioid-receptors. Naturally occurring opioids may participate in the inhibition of the delayed gastric excitation to stimulation of the vagal afferent fibres.