Antiapoptotic action of delta opioid peptide [D-Ala2, D-Leu5]enkephalin against oxygen-glucose deprivation in brain slices

Background

The delta opioid peptide [D-Ala2, D-Leu5]enkephalin (DADLE) has been shown to play a role in neuronal protection against hypoxic ischemia. However, the cellular mechanisms of these actions of DADLE on neurons are not totally clear. Being an in-vitro model of brain ischemia, oxygen-glucose deprivation (OGD) injury in rat brain slices has the advantages of both in-vivo and in-vitro models, and therefore can imitate damages induced by brain I/R injury in intact animals. In the present study, we examined the protective mechanism(s) of DADLE against apoptosis using a rat brain slice model. In addition, we determined whether δ-opioid receptors are unique and have a specific role in neuroprotection against OGD injury by activating the MAPK pathway, specifically through δ2-opioid receptors.

Methods

The brain slices were injured by OGD, and then incubated with different concentrations of DADLE. Selective δ2-opioid antagonist or selective inhibitor of ERK kinase was co-incubated with or without DADLE. The effects of DADLE against apoptosis in neurons were measured by the following biochemical and morphological assays: the LDH release, RT-PCR, western blot, and TUNEL staining.

Results

This protective effect of DADLE is U-shaped as demonstrated in the dose–response curves. It was found that micromolar concentrations of DADLE are antiapoptotic, whereas micromolar concentrations of DADLE lack apparent protective effects. The protective effect of DADLE can be attenuated by a selective δ2-opioid antagonist. The treatment of cells with PD98059, a selective inhibitor of ERK kinase (MEK), blocked both the protective effect of DADLE and the ERK phosphorylation induced by DADLE.

Conclusion

Our results suggest, therefore, that endogenous opioid peptides may, at low concentrations, promote cell survival via the MEK–ERK pathway, perhaps through δ2-opioid receptors.