Izquierdo I, Dias R D
Neurosci Lett. 1981 Mar 10;22(2):189-93. doi: 10.1016/0304-3940(81)90086-0.
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.
