INVOLVEMENT OF CAMP-DEPENDENT PROTEIN-KINASE IN MU-OPIOID MODULATION OF NMDA-MEDIATED SYNAPTIC CURRENTS

We have previously reported dual effects of mu-opioids on N-methyl-D-a spartate (NMDA)-receptor-mediated synaptic events in the hippocampal d entate gyrus: an indirect facilitating effect via suppression of CABAe rgic interneurons (disinhibition) and a direct inhibitory effect in th e presence of gamma-aminobutyric acid-A (GABA(A)) antagonists. The cel lular mechanism underlying the inhibitory effect of mu-opioids remains to be determined. In the present study we examine the role of adenosi ne 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) in mu-opioid-induced inhibition of NMDA currents in rat hippocampal slic es. NMDA-receptor-mediated excitatory postsynaptic currents (NMDA EPSC s) were evoked by stimulating the lateral perforant path and were reco rded from dentate granule cells with the use of whole cell voltage-cla mp techniques in the presence of the GABA(A) antagonist and a non-NMDA type of glutamate receptor antagonist. Two selective mu-agonists, [N- MePhe(3), D-Pro(4)]-morphiceptin and [D-Ala(2), N-MePhe(4), Gly-ol(5)] -enkephalin, induced dose-dependent inhibition of NMDA EPSCs in a conc entration range of 0.3-10 mu M. This inhibitory effect could be comple tely reversed by the opioid antagonists naloxone or prevented by a sel ective mu-antagonist cyprodime, but was not affected by removal of Mg2 + from the external perfusion medium. Intracellular application of per tussis toxin (PTX) into the granule cell via whole cell recording pipe ttes completely prevented mu-opioid-induced reduction in NMDA currents , suggesting that a postsynaptic mechanism involving PTX-sensitive G p roteins might be responsible for the inhibitory action of mu-opioids. Further studies were conducted to identify the intracellular messenger s that coupled with G proteins and transduced the effect of mu-opioids in granule cells. The adenylate cyclase activator forskolin was found to enhance NMDA-receptor-mediated synaptic responses and to reverse t he inhibitory effect of mu-opioids. Sp-cAMPS, a specific PKA activator , also enhanced NMDA EPSCs, whereas the PKA inhibitor Rp-cAMPS reduced NMDA EPSCs and occluded further inhibition of the current by mu-opioi ds. These findings strongly suggest that NMDA receptor function is sub ject to the modulation by PKA, and that mu-opioids can inhibit NMDA cu rrents through suppression of the cAMP cascade in the postsynaptic neu ron. Combined with our previous findings, the present results also ind icate that mu-opioids can modulate NMDA-receptor-mediated synaptic act ivity in a complex manner. The net effect of mu-opioids in the dentate gyrus may depend on the interplay between its disinhibitory action, w hich facilitates NMDA-receptor-mediated responses, and its inhibitory action on the cAMP cascade.