Neuroprotection Against Hypoxic/Ischemic Injury: δ-Opioid Receptors and BDNF-TrkB Pathway.

The delta-opioid receptor (DOR) is one of three classic opioid receptors in the opioid system. It was traditionally thought to be primarily involved in modulating the transmission of messages along pain signaling pathway. Although there were scattered studies on its other neural functions, inconsistent results and contradicting conclusions were found in past literatures, especially in terms of DOR's role in a hypoxic/ischemic brain. Taking inspiration from the finding that the turtle brain exhibits a higher DOR density and greater tolerance to hypoxic/ischemic insult than the mammalian brain, we clarified DOR's specific role in the brain against hypoxic/ischemic injury and reconciled previous controversies in this aspect. Our serial studies have strongly demonstrated that DOR is a unique neuroprotector against hypoxic/ischemic injury in the brain, which has been well confirmed in current research. Moreover, mechanistic studies have shown that during acute phases of hypoxic/ischemic stress, DOR protects the neurons mainly by the stabilization of ionic homeostasis, inhibition of excitatory transmitter release, and attenuation of disrupted neuronal transmission. During prolonged hypoxia/ischemia, however, DOR neuroprotection involves a variety of signaling pathways. More recently, our data suggest that DOR may display its neuroprotective role via the BDNF-TrkB pathway. This review concisely summarizes the progress in this field.