SUMMARY
Coadministration of antagonists of N-methyl-D-aspartate(NMDA) receptor and opioids has been shown to prevent development of opiate tolerance in animal and clinical studies, but its cellular and molecular mechanisms are not understood. In this study, the effect of NMDA on ␦-opioid receptor (DOR)-mediated signal transduction was investigated in neuroblastoma ϫ glioma NG108 -15 cells that functionally express both DOR and NMDA receptors. Acute incubation of NG108 -15 cells with NMDA, a specific agonist of NMDA receptor, significantly attenuated the ability of DOR agonist [D-Pen 2 , D-Pen 5 ]-enkephalin (DPDPE) to inhibit forskolin-stimulated cAMP production. The attenuation caused by NMDA was dosedependent, and the EC 50 of DPDPE increased 100-fold (from 4.6 nM to 500 nM) after NMDA treatment. The NMDA effect on responsiveness of ␦-opioid receptors to DPDPE could be blocked by ketamine, a NMDA receptor-specific antagonist. This NMDA attenuation effect on DOR activity was also observed in neuronal primary cell cultures from fetal mouse brain but not in the Chinese hamster ovary cell line stably transfected with DOR alone. Interestingly, NMDA pretreatment reduced the cellular response to epinephrine but not to that of prostaglandin E 1 in NG108 -15 cells, which suggests differential modulation of NMDA on different G protein-coupled receptors. Pretreatment of NG108 -15 cells with ketamine along with DPDPE greatly attenuated DPDPE-induced acute desensitization of DOR. Furthermore, the specific inhibitors of protein kinase C, either chelerythrine chloride or Gö 6979, effectively blocked the NMDA effect, which indicates the involvement of protein kinase C in the process. In conclusion, the activation of NMDA receptors can attenuate acute responsiveness of DOR in neuronal cells, whereas its blockage leads to reduction of DOR desensitization. These results have thus provided an insight into cross-talk between NMDA and opioid signal transduction.Chronic exposure to an opioid agonist leads to drug tolerance, which is characterized by a decrease in analgesic efficacy in vivo and by reduced functions of opioid receptors at the cellular level (1, 2). Significant reduction of opioid responsiveness after acute opioid treatment was also documented and is functionally defined as receptor desensitization (3). Although they are known to involve phosphorylation, internalization, and down-regulation of opioid receptors, uncoupling of the opioid receptor/G protein system, as well as adaptations in the cAMP signal transduction cascade, the molecular mechanisms underlying opioid tolerance and receptor desensitization are not yet fully understood (1-8).The evidence accumulated in recent years suggests the existence of interactions between the signal transduction systems of excitatory amino acid receptors and opioid receptors. On one hand, opioid peptides functionally and directly interact with the NMDA receptor (9). On the other hand, various antagonists of NMDA receptors profoundly attenuate opioid tolerance in animal experiments,...