Behavioral activation by enkephalins in mice.

Intraventricular injection of long lasting enkephalin analogues (D--Ala2Leu and Met enkephalin-amides) produced a sustained elevation of psychomotor activity in mice. The motor syndromes were characterized by continual stereotyped activity and were reversed by naloxone pretreatment. Naloxone administered to a separate group of mice reduced the initial activation seen after exposure to a novel environment. The present findings suggest one or more endogenous opiates normally facilitates behavioral excitation in mice.