The role of opioid peptides in memory and learning.

Evidence is discussed which points to the existence of a physiologic amnesic mechanism mediated by beta-endorphin and perhaps by other opioid peptides as well. This mechanism is triggered by various forms of training and by either painful or painless stimulation. It may operate through the inhibition of central dopaminergic and beta-adrenergic systems that modulate the memory consolidation process. This amnesic mechanism in unrelated to the regulation of pain perception, and operates at opioid peptide levels several orders of magnitude below those that are needed to cause analgesia or other effects. In addition, shuttle avoidance and habituation learning seem to be dependent on a state induced by the release of beta-endorphin. It is possible that this may be related to the amnesic properties of this substance. Therefore, it appears that the endogenous opioid peptides may exert their primary function in the modulation of memory processes.