Opiates and the hippocampus: a review of the functional and morphological evidence.

In spite of a small number of opiate receptors, recent electrophysiological studies have shown that exogenously applied opiates have a powerful excitatory effect on the principal neurons of the hippocampus. These studies have spawned a reconsideration of opiate binding sites in the hippocampus and investigations of the presence of endogenous opiates within this limbic area. This literature, as well as the relevant behavioral studies, is reviewed, and questions of the role of hippocampal opiates and their binding sites are raised.