Enkephalin hydrolysis in plasma is highly correlated with escape performance in the rat.

A very high correlation was found in rats between latency to escape on the first trial of an active avoidance task and the rate at which [leu]enkephalin is hydrolyzed in plasma. In addition, the rate of [leu]enkephalin hydrolysis is significantly altered following the first training trial. The results suggest that a regulatory enzyme system exists for [leu]enkephalin in plasma and that this system may be important for modulating behavior.