Participation of opioid receptor types on post-tetanic twitch inhibition in isolated guinea-pig ileal longitudinal muscle.

The participation of opioid receptor types in the inhibition of the twitch (0.1 Hz, 0.5 msec duration, maximum intensity) following high frequency stimulation (10 Hz, 0.5 msec duration, maximum intensity for 1 min) (post-tetanic twitch inhibition) was investigated in isolated guinea-pig ileal longitudinal muscle using highly selective mu, kappa and delta opioid receptor antagonists. The mu antagonist, beta-funaltrexamine (beta-FNA), produced a concentration-dependent twitch inhibition, which disappeared with thorough washing. The concentration-response curve of the twitch inhibition for morphine was shifted rightward about 10-20-fold by beta-FNA pre-exposure, whereas that for dynorphin 1-13 inhibition remained unchanged. However, after thorough washing of beta-FNA the twitch inhibition of morphine continued during the post-tetanic twitch inhibition for as long as it was tested. The post-tetanic twitch inhibition was partially antagonized by beta-FNA pretreatment and further antagonized by additional exposure to naloxone (NLX). In the presence of nor-binaltorphimine (nor-BNI), a kappa antagonist, which has no effect on the twitch inhibition of morphine, the concentration-response curve of the twitch inhibition of dynorphin 1-13 was shifted rightward approx 100-fold. The post-tetanic twitch inhibition was antagonized by nor-BNI, and no further antagonism was produced by NLX. (Allyl)2-Tyr-Aib-Aib-Phe-Leu (ICI 174,864), a delta antagonist, did not affect the post-tetanic twitch inhibition. These results suggest that both mu and kappa opioid receptors participate in post-tetanic twitch inhibition in isolated guinea-pig ileal myenteric plexus-longitudinal muscle (MPLM).