Morphine applied to the mesencephalic central gray suppresses brain stimulation induced escape

In the rat, microinjections of morphine (1.5 to 15 nmoles) into the dorsal part of the mesencephalic central gray (CG) were found to suppress both escape responding induced by electrical stimulations applied to either the medial hypothalamus (MH) or the CG and behavioral responsiveness to peripheral nociceptive stimulations. The time course of these two effects proved quite similar (Experiments 1 and 2). A systemic injection of naloxone reversed--in a dose dependent manner--the effects of morphine on the centrally induced escape responses (Experiment 3). The possibility that microinjections of morphine decrease both responsiveness to peripheral nociceptive stimulation and the reactivity of higher structures involved in the generation of aversive effects is discussed.