Blockade by pertussis toxin of the opioid effect on guinea pig ileum. Contractility and electrophysiological neuronal recording

The aim of the present study was to analyse the correlation between functio nal and electrophysiological effects of the opioids in guinea pig ileum. Pr eparations of guinea pig ileum myenteric plexus-longitudinal muscle strips were used to compare the effect of two opioids, morphine (a mu-agonist) and U-50,488H (a kappa-agonist) on the electrically-induced contractile respon se and the excitatory postsynaptic potentials exhibited by the myenteric ne urones when the internodal strands are electrically stimulated. Both opioid s dose-dependently inhibited the contractile responses and the fast excitat ory postsynaptic potentials (fEPSPs) in S neurones but did not modify the a mplitude of the slow postsynaptic potentials. Pertussis toxin pretreatment was able to antagonise the effects of both morphine and U-50,448H. From our results it can be suggested that the effect of the opioids on guinea pig i leum involves the inhibition of neuronal fEPSPs. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.