Opiate‐Inhibited Adenylate Cyclase in Rat Brain Membranes Depleted of Gs‐Stimulated Adenylate Cyclase

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Abstract
Opiate agonists inhibit adenylate cyclase in brain membranes, but under normal conditions the maximal inhibition is small (10–15%). When rat brain membranes were preincubated at pH 4.5, washed, and then assayed for adenylate cyclase at pH 7.4, stimulation of activity by agents (fluoride, guanylyl-5′-imidodiphosphate, cholera toxin) that act through the stimulatory GTP-binding coupling protein (Gs) protein was lost. At the same time, inhibition of basal adenylate cyclase by opiate agonists was increased to a maximum of 30–40%. Opiate inhibition was maximal at low magnesium concentrations (M), required guanine nucleotides, and decreased the Vmax, not Km, of the enzyme. Incubation of membranes with per tussis toxin lowered the apparent affinity for agonists in inhibiting activity. The δ opioid agonists were more potent than μ agonists, and the Ke values for naloxone in blocking agonist inhibition were similar for both μ and δ agonists (50–90 nM). These results suggest that inhibition of adenylate cyclase in brain is not mediated by μ opiate receptors, but whether classic high-affinity δ and k receptors are involved with this enzyme cannot be confirmed by these experiments.