Evaluation of the analgesic activity of morphine, enkephalins and their synthetic analogues.

In the rabbit, a morphine-like analgesic activity of natural and synthetic enkephalins has been demonstrated using the method of the return of corneal anesthesia due to conjunctival instillation of procaine. FK 33824 is the most effective drug in producing analgesia when administered intracerebroventricularly, whereas morphine is much more active when injected i.v. The effect of morphine and enkephalins is readily reversed by nalorphine (10 mg/kg, i.v.) indicating an involvement of opiate receptors in the CNS.