Retrograde amnesia caused by Met-, Leu- and des-Try-Met-enkephalin in the rat and its reversal by naloxone.

Immediate post-training i.p. injection of Met-enkephalin, Leu-enkephalin or des-Tyr-Met-enkephalin, caused retrograde amnesia for a shuttle avoidance task in rats. The effect was linearly related to the log of the dose between 0.32 and 2.0 microgram/kg, and the potency of the three compounds was similar (ED50 0.35-0.45 microgram/kg). Naloxone (0.2 mg/kg) antagonized the amnestic effect of the peptides. Since the log dose-response curves were displaced to the right in quasi-parallel fashion, and Lineweaver-Burk plots of the data showed an intersection very close to the origin of the horizontal axis, the antagonism between naloxone and the three peptides appears to be competitive.