δ-, but not μ- and κ-, opioid receptor activation protects neocortical neurons from glutamate-induced excitotoxic injury

Recent observations from our laboratory have led us to hypothesize that delta -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 held 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 (delta, mu and kappa 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 muM, 4 h) showed substantial neuronal injury as assessed by morphologic criteria (>65%, n = 23, P < 0.01) and LDH release (n = 16, P < 0.001). Activation of delta -opioid receptors with 10 muM DADLE reduced glutamate-induced injury by almost half as assessed by the same criteria (morphologic criteria, n = 21, P < 0.01; LDH release, n = 16, P < 0.01). Naltrindole (10 muM), a delta -opioid receptor antagonist, completely blocked the DADLE protective effect. Administration of mu- and kappa -opioid receptor agonists (DAMGO and U50488H respectively, 5-10 muM) did not induce appreciable neuroprotection. Also, gamma- or kappa -opioid receptor antagonists had no appreciable effect on the glutamate-induced injury. This study demonstrates that activation of neuronaI delta -opioid receptors, but not gamma- and kappa -opioid receptors, protect neocortical neurons from glutamate excitotoxicity. (C) 2000 Elsevier Science B.V. All rights reserved.