A role for hippocampal opioids in long-term functional plasticity.

In the granule cells of the hippocampus, glutamate coexists with opioid peptides derived from the proenkephalin and prodynorphin genes. The functional significance of this coexistence has been unclear but recent evidence suggests that the dynorphins and enkephalins play a crucial role in regulating the efficiency of neurotransmission at granule-cell synapses. Together with evidence that the level of opioid activity in this pathway can change dramatically according to the physiological or pathological state of the tissue, this information focuses attention on granule-cell opioids as primary mediators of hippocampal plasticity.