Mu and kappa opioid agonists elevate brain stimulation threshold for escape by inhibiting aversion

Rats were trained to press a lever to escape electrical stimulation of the nucleus reticularis gigantocellularis and to obtain stimulation of the lateral hypothalamus. Morphine sulfate and ethylketocyclazocine (EKC) both elevated the intensity of stimulation required to sustain escape at doses which did not affect self-stimulation. Parallel dose-response lines were obtained for the two opioid agonists but the effect of EKC was more resistant to naloxone antagonism. These results suggest that both mu-and chi-sub-types of opiate receptor mediate the inhibition of supraspinally-elicited aversion.