LEP and obesity due to melanocortin 4 receptor deficiency: Indeed, the signalling isoform of the leptin receptor, ObRb, is most highly expressed in the arcuate nucleus of the hypothalamus (Baskin et al. 1999; Mercer et al. 1996), and central intracerebroventricular infusion of leptin into obese ob/ob and diet-induced obesity (DIO) mice causes a reduction in food intake and body weight in comparison to vehicle-treated controls; an effect that is absent in db/db mice (Campfield et al. 1995).