Involvement of periaqueductal gray matter in intestinal effect of centrally administered morphine

Microinjections of morphine in the rat periaqueductal gray matter (PAG) inhibited intestinal transit in linear relation to the log of the dose administered (in the range from 5 to 20 micrograms/rat). This linear regression was parallel with that obtained on intracerebroventricular (i.c.v.) or intraperitoneal (i.p.) administration of morphine and the intracerebral (i.c.) route was calculated to be 4 times more potent than the i.c.v. route and 189 times more potent than the i.p. route. Monolateral electrolytic lesions into the PAG abolished the intestinal effect of i.c.v. morphine to a large extent. The relevance of other brain areas and the type of opiate receptors involved in this central effect of morphine are discussed.