Delta opioid receptors stimulation with [d-Ala2, d-Leu5] enkephalin does not provide neuroprotection in the hippocampus in rats subjected to forebrain ischemia

It has been reported that delta opioid agonists can have neuroprotective efficacy in the central nervous system. This study was conducted to test the hypothesis that a delta opioid receptor (DOR) agonist, [d-Ala2, d-Leu5] enkephalin (DADLE), can improve neuron survival against experimental forebrain ischemia in rats. Using male rats (n = 125), intraperitoneal injection of DADLE (0, 0.25, 1, 4, 16 mg kg−1) was performed 30 min before ischemia. Ten minutes interval forebrain ischemia was provided by the bilateral carotid occlusion combined with hypotension (35 mmHg) under isoflurane (1.5%) anesthesia. All animals were neurologically and histologically evaluated after a recovery period of 1 week. As histological evaluation, percentages of ischemic neurons in the CA1, CA3, dentate gyrus (DG) were measured. During the recovery period, 27 rats died because of apparent upper airway obstruction, seizure, or unidentified causes. There were no differences in the motor activity score among the groups. Ten minutes forebrain ischemia induced approximately 75, 20, and 10% neuronal death in the CA1, CA3, and DG, respectively. Any doses of DADLE did not attenuate neuronal injury in the hippocampus after ischemia. Pre-ischemic treatment of DORs agonism with DADLE did not provide any neuroprotection to the hippocampus in rats subjected to forebrain ischemia.