Sex-dependent pronociceptive role of spinal α -GABA receptor and its epigenetic regulation in neuropathic rodents.

Extrasynaptic α -subunit containing GABA (α -GABA ) receptors participate in chronic pain. Previously, we reported a sex difference in the action of α -GABA receptors in dysfunctional pain. However, the underlying mechanisms remain unknown. The aim of this study was to examine this sexual dimorphism in neuropathic rodents and the mechanisms involved. Female and male Wistar rats or ICR mice were subjected to nerve injury followed by α -GABA receptor inverse agonist intrathecal administration, L-655,708. The drug produced an antiallodynic effect in nerve-injured female rats and mice, and a lower effect in males. We hypothesized that changes in α -GABA receptor, probably influenced by hormonal and epigenetic status, might underlie this sex difference. Thus, we performed qPCR and western blot. Nerve injury increased α -GABA mRNA and protein in female dorsal root ganglia (DRG) and decreased them in DRG and spinal cord of males. To investigate the hormonal influence over α -GABA receptor actions, we performed nerve injury to ovariectomized rats and reconstituted them with 17β-estradiol (E2). Ovariectomy abrogated L-655,708 antiallodynic effect and E2 restored it. Ovariectomy decreased α -GABA receptor and estrogen receptor α protein in DRG of neuropathic female rats, while E2 enhanced them. Since DNA methylation might contribute to α -GABA receptor down-regulation in males, we examined CpG island DNA methylation of α -GABA receptor coding gene through pyrosequencing. Nerve injury increased methylation in male, but not female rats. Pharmacological inhibition of DNA methyltransferases increased α -GABA receptor and enabled L-655,708 antinociceptive effect in male rats. These results suggest that α -GABA receptor is a suitable target to treat chronic pain in females.