Histone methyltransferase G9a diminishes expression of cannabinoid CB1 receptors in primary sensory neurons in neuropathic pain.

Type-1 cannabinoid receptors (CB1Rs) are expressed in the dorsal root ganglion (DRG) and contribute to the analgesic effect of cannabinoids. However, the epigenetic mechanism regulating the expression of CB1Rs in neuropathic pain is unknown. G9a (encoded by the Ehmt2 gene), a histone 3 at lysine 9 (H3K9) methyltransferase, is a key chromatin regulator responsible for gene silencing. In this study, we determined G9a's role in regulating CB1R expression in the DRG and in CB1R-mediated analgesic effects in an animal model of neuropathic pain. We show that nerve injury profoundly reduces mRNA levels CB1Rs but increases the expression of CB2 receptors in the rat DRG. Chromatin-immunoprecipitation results indicated increased enrichment of H3K9me2, a G9a-catalyzed repressive histone mark, at the promoter regions of the CB1R genes. G9a inhibition in nerve-injured rats not only upregulates CB1R expression level in the DRG but also potentiated the analgesic effect of a CB1R agonist on nerve injury-induced pain hypersensitivity. Furthermore, in mice lacking Ehmt2 in DRG neurons, nerve injury failed to reduce CB1R expression in the DRG and to decrease the analgesic effect of the CB1R agonist. Moreover, nerve injury diminished the inhibitory effect of the CB1R agonist on synaptic glutamate release from primary afferent nerves to spinal cord dorsal horn neurons in wild-type mice, but not in mice lacking Ehmt2 in DRG neurons. Our findings reveal that nerve injury diminishes the analgesic effect of CB1R agonists through G9a-mediated CB1R downregulation in primary sensory neurons.