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 functional and electrophysiological effects of the opioids in guinea pig ileum. Preparations 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 response and the excitatory postsynaptic potentials exhibited by the myenteric neurones when the internodal strands are electrically stimulated. Both opioids dose-dependently inhibited the contractile responses and the fast excitatory postsynaptic potentials (fEPSPs) in S neurones but did not modify the amplitude 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 ileum involves the inhibition of neuronal fEPSPs.