d - , but not m - and k - , opioid receptor activation protects neocortical neurons from glutamate - induced excitotoxic injury

Recent observations from our laboratory have led us to hypothesize that d-opioid receptors may play a role in neuronal protection against hypoxic / ischemic or glutamate excitotocity. To test our hypothesis in this work, we used two independent methods, i.e., ‘‘same field quantification’’ of morphologic criteria and a biochemical assay of lactate dehydrogenase (LDH) release (an index of cellular injury). We used neuronal cultures from rat neocortex and studied whether (1) glutamate induces neuronal injury as a function of age and (2) activation of opioid receptors (d, m and k subtypes) protects neurons from glutamate-induced injury. Our results show that glutamate induced neuronal injury and cell death and this was dependent on glutamate concentration, exposure period and days in culture. At 4 days, glutamate (up to 10 mM, 4 h-exposure) did not cause apparent injury. After 8–10 days in culture, neurons exposed to a much lower dose of glutamate (100 mM, 4 h) showed substantial neuronal injury as assessed by morphologic criteria (.65%, n523, P,0.01) and LDH release (n516, P,0.001). Activation of d-opioid receptors with 10 mM DADLE reduced glutamate-induced injury by almost half as assessed by the same criteria (morphologic criteria, n521, P,0.01; LDH release, n516, P,0.01). Naltrindole (10 mM), a d-opioid receptor antagonist, completely blocked the DADLE protective effect. Administration of mand k-opioid receptor agonists (DAMGO and U50488H respectively, 5–10 mM) did not induce appreciable neuroprotection. Also, mor k-opioid receptor antagonists had no appreciable effect on the glutamate-induced injury. This study demonstrates that activation of neuronal d-opioid receptors, but not mand k-opioid receptors, protect neocortical neurons from glutamate excitotoxicity.  2000 Elsevier Science B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors Topic: Opioid receptors