Opiates and opioid peptides modify sensory evoked potentials and synaptic excitability in the rat dentate gyrus

The effects of morphine and the synthetic opioid peptide D-Ala²-MePhe⁴-Met-O-ol-enkephalin (FK 33-824) on averaged (AEPS) potentials evoked by a tone and extracellular synaptic potentials (EPSs) in the perforant path, recorded from the outer molecular layer (OM) of the dentate gyrus, were examined in rats trained to respond in an auditory discrimination task. Potentials evoked by a tone were systematically altered by both peripheral (intraperitoneal) and central (intraeerebroventricular) administration of opioids. The short-latency negative (N₁) component of the average evoked potential was increased in amplitude and the longer-latency negative (N₂) component was decreased in amplitude by administration of opioids. At the same time, perforant path extracellular synaptic potentials were enhanced after administration of opioids. The changes in the average evoked potential and extracellular synaptic potentials in the perforant path were reversed by subsequent administration of naloxone. The significance of these results is discussed in terms of a possible role of endogenous opioid peptides in modulating the synaptic efficacy of the perforant path during the transmission of sensory information to the hippocampus from the entorhinal cortex.