OPIOID RECEPTOR-DEPENDENT LONG-TERM POTENTIATION AT THE LATERAL PERFORANT PATH-CA3 SYNAPSE IN RAT HIPPOCAMPUS

The involvement of opioid receptors in the induction of long-term pote ntiation (LTP) was investigated in the lateral and medial perforant pa th projections to area CA3 of the hippocampus in anesthetized rats. Th e opioid receptor antagonist naloxone (10 nmol), applied to the hippoc ampal CA3 region 10 min prior to tetanization, blocked the induction l ateral perforant path-CA3 LTP induced by high-frequency stimulation. B y contrast, LTP induction in medial perforant path-CA3 was not attenua ted by a 10 nmol quantity of naloxone. (+)-Naloxone (10 nmol), the ina ctive stereoisomer of naloxone, was without effect on the induction of lateral perforant path-CA3 LTP. Naloxone applied 1 h following LTP in duction did not reverse established lateral perforant path-CA3 LTP, in dicating that opioid receptors are involved in the induction but not t he maintenance of LTP in this pathway. LTP of medial perforant path re sponses developed immediately, while LTP of lateral perforant path res ponses was slow to develop. The latter pattern is similar to the time course of the development of LTP observed at the mossy fiber-CA3 synap se and suggests that lateral and medial perforant path synapses may us e distinct mechanisms of both induction and expression of LTP. These d ata extend previous findings demonstrating opioid receptor-dependent m echanisms of LTP induction at both the mossy fiber-CA3 synapse and the lateral perforant path-dentate gyrus synapse. We suggest that lateral perforant path and mossy fiber synapses may utilize similar, opioid r eceptor-dependent, mechanisms of LTP induction and expression.