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

Abstract

Recent observations from our laboratory have led us to hypothesize that δ-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 (δ, μ and κ 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 μM, 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 δ-opioid receptors with 10 μM 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 μM), a δ-opioid receptor antagonist, completely blocked the DADLE protective effect. Administration of μ- and κ-opioid receptor agonists (DAMGO and U50488H respectively, 5–10 μM) did not induce appreciable neuroprotection. Also, μ- or κ-opioid receptor antagonists had no appreciable effect on the glutamate-induced injury. This study demonstrates that activation of neuronal δ-opioid receptors, but not μ- and κ-opioid receptors, protect neocortical neurons from glutamate excitotoxicity.