GPR55 and obesity due to melanocortin 4 receptor deficiency: Our data suggests that GPR18 has a role in skeletal muscle metabolism, just like the traditional cannabinoid receptors CB1 and CB2, and the other putative cannabinoid receptor GPR55, all of which have previously been shown to be expressed in skeletal muscle [8,17] and all of which have a role in obesity [7,16,32].