Opioid enhancement of calcium oscillations and burst events involving NMDAreceptors and L-type calcium channels in cultured hippocampal neurons

Opioid receptor agonists are known to alter the activity of membrane ionic conductances and receptor-activated channels in CNS neurons and, via these mechanisms, to modulate neuronal excitability and synaptic transmission. In neuronal-like cell lines opioids also have been reported to induce intrace llular Ca2+ signals and to alter Ca2+ signals evoked by membrane depolariza tion; these effects on intracellular Ca2+ may provide an additional mechani sm through which opioids modulate neuronal activity. However, opioid effect s on resting or stimulated intracellular Ca2+ levels have not been demonstr ated in native CNS neurons. Thus, we investigated opioid effects on intrace llular Ca2+ in cultured rat hippocampal neurons by using fura-2- based micr oscopic Ca2+ imaging. The opioid receptor agonist D-Ala(2)-N-Me-Phe(4),Gly- ol(5)-enkephalin (DAMGO; 1 mu M) dramatically increased the amplitude of sp ontaneous intracellular Ca2+ oscillations in the hippocampal neurons, with synchronization of the Ca2+ oscillations across neurons in a given field. T he effects of DAMGO were blocked by the opioid receptor antagonist naloxone (1 mM) and were dependent on functional NMDA receptors and L-type Ca2+ cha nnels. In parallel whole-cell recordings, DAMGO enhanced spontaneous, synap tically driven NMDA receptor-mediated burst events, depolarizing responses to exogenous NMDA and current-evoked Ca2+ spikes. These results show that t he activation of opioid receptors can augment several components of neurona l Ca2+ signaling pathways significantly and, as a consequence, enhance intr acellular Ca2+ signals. These results provide evidence of a novel neuronal mechanism of opioid action on CNS neuronal networks that may contribute to both short- and long-term effects of opioids.