Effects of mu opioid receptor activation in rat hippocampal slice

The effects of the mu opioid agonist DAGO were examined in the hippocampal slice preparation. The effects of the drug on active and passive properties of neurons in CA1 and CA2 were examined. Intracellular responses of these neurons to afferent stimulation were examined before and after DAGO application. As seen with other mu agonists, DAGO reduced IPSPs and enhanced the duration of EPSPs but not their rise time. In CA2 cells, DAGO also produced a transient hyperpolarization. Further experiments examined the interactions of the opioid peptides with other neurotransmitter systems in the hippocampus. The effects of DAGO were reduced by prior fornix transection. Some effects of DAGO on generation of a secondary afterdischarge are abolished by an NMDA receptor antagonist, 2-APV. The significance of these results to the understanding of functioning of the hippocampal circuit is discussed.