Epileptic properties of leucine- and methionine-enkephalin: comparison with morphine and reversibility by naloxone.

Morphologically similar epileptic seizures were recorded from the cortex of rats after injections into the lateral ventricle of 100 microgram of leucine-enkephalin, methionine-enkephalin, and morphine. Seizures were either greatly attenuated or blocked completely by prior systemic administration of naloxone (10 mg/kg). These findings suggest that such seizures result from an interaction of these compounds with opiate receptors in the brain. The epileptogenic potency of the enkephalins was illustrated by the observation that seizures and other pathological manifestations could still be elicited by doses as low as 10 microgram. Leucine-enkephalin was seen to have greater epiliptic potency than methionine-enkephalin. At doses of 1 microgram both enkephalins typically evoked cortical spindles resembling those seen in drowsy animals. Enkephalin-induced analgesia was seen in only one animal at the 100 microgram dose. Results obtained with repeated injections of morphine suggest that the epileptogenic effect of opiates may be subject to either tolerance or potentiation, depending on the prior occurrence of seizures. A synthesis of the present findings with several other lines of evidence suggests both that endogenous enkephalins play some role in normal mechanisms of reward, and that, when regulatory processes are disturbed, they may contribute as well to the elaboration of certain epileptic phenomena.