G9a belongs to a superfamily of histone methyltransferases [81] and seems to play an extensive role in pain modulation as it was also associated to changes in the expression of Oprm1 (encoding for the opioid receptor MOR) and cannabinoid receptors 1 and 2 (CB1 and CB2) following nerve injury in rats and mice [82,83]. Here, CNR1 is linked to injury.