Morphine applied to the ventral tegmentum differentially affects centrally and peripherally induced aversive effects.

In the rat, a microinjection of 15 nmoles of morphine into the ventral tegmentum (VT) was found to suppress escape responding induced by electrical stimulation applied to the dorsal part of the mesencephalic central gray. This suppressant effect could be reversed by a systemic injection of naloxone and was unlikely to be due to gross motor impairment since morphine injected into the VT provoked a behavioral activation. A similar microinjection of morphine into the VT did not induce analgesic effects since it did not affect the reaction thresholds to a nociceptive stimulus. The mechanisms underlying these differential effects of morphine applied to the VT are discussed.