Regulation of neural responses in the canine pyloric sphincter by opioids

Abstract

1. Regulation of excitatory and inhibitory junction potentials (e.j.ps and i.j.ps) by opioid peptides was studied in isolated muscle strips from the pyloric sphincter of the dog. 2. Methionine enkephalin (MetEnk; 10(-10) to 10(-6) M) and [D-Ala2, D-Leu5] enkephalin (DADLE; 10(-11) to 10(-7) M), a delta-specific opioid agonist, inhibited i.j.ps and e.j.ps recorded from cells in the myenteric and submucosal regions of the circular muscle layer. These compounds had no effect on resting potential or slow wave activity suggesting that the effects on junction potentials were not due to direct effects on smooth muscle cells. 3. MetEnk and DADLE caused similar effects on junction potentials in preparations in which the myenteric plexus was removed, suggesting that opioids inhibit pre-junctional effects on nerve fibres within the muscularis externa. 4. Inhibition of junction potentials by MetEnk and DADLE was blocked by approximately the same extent by naloxone (10(-6) M) and ICI 174,864 (10(-6) M), a delta-specific antagonist. 5. MetEnk and DADLE blocked a portion of the i.j.p. that was sensitive to arginine analogues; after treatment with N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), MetEnk and DADLE had no further effect on i.j.ps. These data suggest that opioids regulate nitric oxide-dependent neurotransmission. 6. Naloxone (10(-6) M) alone had no effect on i.j.ps elicited by short trains of electrical field stimuli. 7. I.j.p. amplitude was reduced after a period of conditioning stimulation (2 min, 30 Hz, 30 V). Naloxone blocked the post-stimulation inhibition. Repetitive stimulation at high frequencies (30 Hz) resulted in sustained hyperpolarization. Naloxone increased the amplitude of the hyperpolarization responses elicited by high frequency stimulation.8. These results show that e.j.ps and i.j.ps in the canine pylorus are inhibited by opioids. A portion of the inhibitory effects appears to be mediated via delta receptors.9. Although pyloric muscles are richly innervated by nerves containing opioid peptides, brief trains of stimuli do not appear to release concentrations of opioids that are effective in regulating junction potentials. Higher frequency stimulation (or longer durations of stimulation) appear to be necessary to release concentrations of opioids that are effective in modulating the amplitude of junction potentials.