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 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 (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 microM, 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 microM 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 microM), 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 microM) did not induce appreciable neuroprotection. Also, mu- or kappa-opioid receptor antagonists had no appreciable effect on the glutamate-induced injury. This study demonstrates that activation of neuronal delta-opioid receptors, but not mu- and kappa-opioid receptors, protect neocortical neurons from glutamate excitotoxicity.