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

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