Morphine receptors as regulators of adenylate cyclase activity

Morphine inhibits adenylate cyclase (EC 4.6.1.1) activity of neuroblastoma times glioma hybrid cells. The inhibition is stereospecific and is reversed by the antagonist, naloxone. The relative affinities of narcotics for the opiate receptor agree well with their effectiveness as inhibitors of adenylate cyclase. Morphine-sensitive and -insensitive cell lines were found, and the degree of sensitivity was shown to be dependent upon the abundance of narcotic receptors. Thus, morphine receptors are functionally coupled to adenylate cyclase. A molecular mechanism for narcotic addiction and tolerance is proposed.