Alpha2-adrenergic receptors in spiral ganglion neurons may mediate protective effects of brimonidine and yohimbine against glutamate and hydrogen peroxide toxicity

Brimonidine, an alpha2-adrenergic receptor (α2-AR) agonist, is thought to be neuroprotective in some types of neurons via the activation of α2-AR. However, it is still unknown whether the α2-ARs exist in cochlear spiral ganglion neurons (SGNs). The authors aimed to demonstrate the presence and localization of α2-ARs in rat-cultured SGNs and to investigate the effect of brimonidine on glutamate- and hydrogen peroxide (H2O2)-induced damage in the primary-cultured rat SGNs. The expression of α2-ARs was determined by reverse transcription-polymerase chain reaction, Western blot analysis and immunofluorescence. Then SGNs were exposed to glutamate or H2O2 respectively with or without brimonidine. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay. Apoptosis was determined by acridine orange and Hoechst 33342/propidium iodide double staining. The protein expressions of α2-ARs, Bax, Bcl-2, Caspase-9, Caspase-3, p-ERK1/2, iNOS, and artemin were determined by Western blot respectively. The cell viability was markedly reduced after exposure of glutamate (1 mM) or H2O2 (300 μM) to SGNs. Treatment with brimonidine protected SGNs against glutamate- or H2O2-induced cell damage, enhanced SGNs survival, decreased the elevation of Bax, Caspase-9, Caspase-3, p-ERK1/2, and artemin triggered by glutamate or H2O2, and altered the expressions of Bcl-2 and iNOS. These protective effects of brimonidine can be reversed by yohimbine. Overall, the study describes the localization of α2-ARs in rat-cultured SGNs and indicates that brimonidine, which may work directly via interaction with α2-ARs, attenuates glutamate- and H2O2-induced damage in SGNs by Caspase-dependent modes as well as Caspase-independent modes.

83Highlights► α2-ARs exist in rat SGNs. ► Brimonidine protects SGNs from apoptosis against glutamate and H2O2 insults. ► This neuroprotection is directly mediated via α2-ARs. ► Caspase-dependent modes and Caspase-independent modes are involved in this neuroprotection.