Effect of beta-endorphin and naloxone on acquisition, memory, and retrieval of shuttle avoidance and habituation learning in rats.

Naloxone impairs acquisition of shuttle avoidance behavior (0.8 mg/kg IP) and habituation to a rearing response to a tone (1.6 mg/kg IP) in rats. beta-Endorphin (2 microgram/kg IP) has no effect on acquisition, but, when given prior to test sessions, facilitates retrieval of the two tasks. Naloxone has no effect of its own upon retrieval. In addition to these effects, the pretraining administration of beta-endorphin disrupts, and that of naloxone facilitates retention of the two tasks. The results are consistent with the hypothesis that these two forms of learning are state-dependent on the release of beta-endorphin (and, perhaps, of other opiate peptides as well), that this substance is released during training in a sufficient amount for this purpose, and that, in addition, there is a physiological amnesic mechanism mediated by opiate peptides. Furthermore, the results are also consistent with previous observations that beta-endorphin is released from the rat brain during training, but not during test sessions of the two tasks (Izquierdo et al., 1980b). The possibility is discussed that state-dependency and the amnesic effect comprise one single, rather than two separate mechanisms.