New in vivo evidence for narcotic agonistic property of leucine-enkephalin.

Administration of leucine-enkephalin or morphine to mice rendered dependent on morphine by pellet implantation inhibited the naloxone-precipitated abstinence syndrome. The withdrawal jumping response was inhibited by morphine or leucine-enkephalin; however, both failed to inhibit withdrawal defecation and rearing behavior. On a molar basis, leucine-enkephalin was half as potent as morphine in inhibiting the abstinence syndrome. New in vivo pharmacological evidence for narcotic agonist-like activity of leucine-enkephalin is provided.