Effect of naloxone and morphine on various forms of memory in the rat: Possible role of endogenous opiate mechanisms in memory consolidation

Naloxone, morphine, and morphine plus naloxone were injected IP into rats immediately after training in several different behavioral tasks, and their effect on retention was evaluated in test sessions carried out 1 or 7 days later. Naloxone (0.4 mg/kg) enhanced and morphine (1.0 mg/kg) depressed retention for a standard shuttle avoidance task acquired with contiguous tone-footshock presentations. Retention of this behavior is known to result from the independent operation of four memory channels (Izquierdo and Elisabetsky, 1978). In consequence, the effect of naloxone on each of those channels was investigated. The channel which stores responses acquired through an avoidance mode and makes them available for retrieval in the same mode, was facilitate by as little as 0.2 mg/kg of the drug. The other channels were sensitive only to 0.4 or 0.8 mg/kg naloxone. Retention of the habituation of a rearing response to a tone from one day to the next was also enhanced by naloxone (0.8 mg/kg) and depressed by morphine (1.0 mg/kg), and, again, there was an antagonistic interaction between the two drugs. The results suggest a role for endogenous opiate substances as general inhibitory modulators of memory consolidation, point to a differential sensitivity of the various memory channels to naloxone (therefore, presumably to endogenous opiate modulation), and demonstrate that the post-training effect of naloxone and morphine on memory (or the role of endogenous opiates thus suggested) does not depend on the presence of foot shocks in the training session to become manifest.