Murine studies have demonstrated the existence of obesity-resistant (OR) model rats that have high intrinsic SPA and enhanced orexin-induced SPA [134,135] and high-activity (HA) model rats (identified by their endogenous SPA) that have high resistance to obesity following being fed with a high-energy diet [136]. The gene discussed is HCRT; the disease is obesity due to melanocortin 4 receptor deficiency.