Opposite effects of Met-enkephalin and Leu-enkephalin on a discriminated shock-escape task.

Leu-enkephalin was found to impair and Met-enkephalin to enhance acquisition of a discriminated Y-maze shock-escape task in mice at equivalent doses (100 micrograms/kg, ip). Naloxone (1.0 and 10.0 mg/kg) also enhanced acquisition of the escape response and blocked the impairing actions of Leu-enkephalin. Neither naloxone nor the enkephalins influenced shock-induced locomotor activity in an open field. The results suggest that enkephalin actions on escape conditioning are mediated through opioid receptors.