EFFECTS OF OPIOID AND ALPHA-2-ADRENOCEPTOR AGONISTS ON THE ISOLATED ILEUM OF MORPHINE-DEPENDENT GUINEA-PIGS DURING WITHDRAWAL AND AFTER CLONIDINE TREATMENT

The present study was undertaken to investigate the effect of clonidine administration to opiatedependent guinea-pigs after morphine withdrawal on subsequent twitch responses of the longitudinal muscle-myenteric plexus preparations to electrical field stimulation. The results indicate that clonidine, administered immediately after morphine removal, causes tolerance to the inhibition exerted by opioid and alpha(2)-adrenoceptor agonists on the electrically-evoked twitches. Such a finding suggests that the mechanism of action of clonidine involves not only its well-known effects on locus coeruleus neurons but also that it has specific actions on the myenteric plexus. This work shows the existence of interactions between opioid and alpha(2)-adrenoceptor on the cholinergic neurons present in the isolated ileum.