Interaction of morphine, etorphine and enkephalins with dopamine-stimulated adenylate cyclase of monkey amygdala

Abstract

Adenylate cyclase activity (AC) of homogenates of monkey amygdaloid nucleus was approximately doubled in the presence of dopamine (10 micrometer). Morphine, etorphine, and several enkephalin analogs (met-enkephalin, D-ala2-met-enkephalin, and D-met2, pro5-enkephalinamide) were capable of inhibiting the stimulation of AC produced by dopamine (90-100% with etorphine or D-ala2-met-enkephalin). Unlike morphine and etorphine, the peptides exhibited bell-shaped dose-response curves for this inhibition with maximal effects at approximately 1 X 10(-7) M, but negligible effects at 1 X 10(-5) M. Under the conditions studied, only etorphine inhibited basal AC. Naloxone antagonized the inhibitory effects of each of the opioids tested, and dextrorphan, an inactive L-(+)-opiate, failed to inhibit the dopamine response. Together these data indicate that the effects were mediated via the classically described stereospecific opiate receptor. The relative order of potency (etorphine greater than enkephalins greater than morphine) was similar to that previously reported for the binding affinities of these drugs in rat brain homogenates. The influence of narcotic agents on dopamine stimulated AC was eliminated by either freezing the amygdaloid tissue or preincubating the homogenate at 4 degrees C; the dopamine responses, however, could still be elicited. The narcotic receptor interaction with the adenylate cyclase thus appears to be distinct from and more labile than that of the dopamine receptor. Gpp(NH)p-stimulated AC was not inhibited by morphine. It is postulated that the inhibition involves interaction of opiate receptors with catalytic units of dopamine-stimulated AC, but not with other cyclase species which may provide the major component of Gpp(NH)p-stimulated activity in amygdala.