FACILITATION OF MEMORY PROCESSING BY POST-TRIAL MORPHINE - POSSIBLE INVOLVEMENT OF REINFORCEMENT MECHANISMS

Posttrial administration of 40 mg/kg and 100 mg/kg, but not of 1 mg/kg, of morphine hydrochloride facilitates learning of a one-trial passive avoidance task in drug-naive mice. The effect does not depend on the punishing properties of the morphine injection, since an injection of LiCl (a strong punisher) fails to enhance learning in a similar way. After the establishment of tolerance by several morphine administrations, the 100 mg/kg, but not the 40 mg/kg, dose level resulted in memory facilitation. The data are discussed in connection with the hypothesis that morphine acts directly on reinforcement mechanisms by activating the opiate receptor. © 1979 Springer-Verlag.