ORIGINAL RESEARCH ARTICLE Agouti-related protein prevents self-starvation MJH Kas 1 , G van Dijk 2 , AJW Scheurink 2 and RAH Adan 1 1 Department of Medical Pharmacology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Utrecht, The Netherlands; 2 Department of Neuroendocrinology, University of Groningen, The Netherlands Keywords: anorexia nervosa; melanocortin receptor; fasting; hyperactivity; body temperature Food restriction leads to a paradoxical increase in physical activity and further suppression of food intake, such as observed in anorexia nervosa. 1,2 To understand this pathophysiological process, we induced physical hyperactivity and self-starvation in rats by restricting food in the presence of running wheels. Normally, decreased melanocortin receptor activity will prevent starvation. 3,4 However, we found that self-starvation increased melanocortin receptors in the ventral medial hypothalamus, a brain region involved in eating beha- vior. 5 Suppression of melanocortin receptor activity, via central infusion of Agouti-related protein (AgRP), increased survival rate in these rats by counteracting physical hyperactivity, food intake suppression as well as deregulated body temperature. We conclude that self-starvation may result from insufficient suppression of central melanocortin receptor activity. Molecular Psychiatry (2003) 8, 235–240. doi:10.1038/ sj.mp.4001206 Anorexia nervosa (AN) is a psychiatric disorder distinguished by dramatic undereating. At a certain stage of the disease, food intake suppression is associated with physical hyperactivity leading to extreme body weight loss. 2 The incidence of AN is particularly prevalent among young (post)-pubertal females and AN has a high rate of mortality. While AN is thought to have a psychosocial basis, it obviously has a dramatic impact on neuroendocrine mechan- isms involved in the regulation of energy balance, such as increased activity of hypothalamic–pituitary– adrenocortical (HPA) axis, and reduced plasma levels of leptin. 6,7 A major target-system downstream from leptin signaling in the brain is the central melano- cortin system. Genetic and pharmacological studies have shown that the central melanocortin system plays an important role in the regulation of body weight maintenance. Suppression of melanocortin receptor signaling, such as mutations in the melano- cortin-4 receptor, is associated with central leptin resistance and obesity in both humans 8–10 and rodents. 3,4,11,12 In this study, we investigated whether the opposite is also true: increased melanocortin receptor activity will contribute to self-starvation. One-hour-fed young female rats in cages with run- ning wheels developed physical hyperactivity and suppressed food intake. This resulted in a 30% body weight loss (Figure 1), activation of the HPAaxis (Table 1), and eventually death. 1,13 One-hour-fed rats showed an almost three-fold increase in physical activity (total daily wheel running activity levels were 557671209 rpm/24 h (ad libitum) vs 145127 1230 rpm/24 h). Furthermore, 1-h-fed rats with run- ning wheels ate significantly less than 1-h-fed rats without access to running wheels, showing that running wheel activity induced a relative suppression of food consumption in these animals (Figure 1b). One-hour-fed rats with running wheels consumed similar amounts of food as 0.5-h-fed rats without access to running wheels. However, body weight loss in 1-h-fed rats with running wheels was almost doubled (Figures 1a and b). Thus, both food intake suppression and physical hyperactivity contributed to self-starvation in 1-h-fed rats with running wheels. Food restriction activates central pathways that stimulate food intake and reduce energy expenditure to maintain energy balance. For example, a normal response to fasting, such as increased AgRP levels, 14,15 suppresses melanocortin receptor activity. 16 Suppres- sion of melanocortin receptor activity results in decreased physical activity, increased food intake, and decreased activity of the HPAaxis. 3,4,17–19 We found the opposite: the 1-h-fed rats with running wheels exhibited increased physical activity, sup- pressed food intake and activation of the HPAaxis, typical characteristics of increased melanocortin receptor activity. Therefore, it might be hypothesized that the responses to food restriction of rats with a running wheel are due to a seemingly counterintui- tive increase in central melanocortin activity. Among the brain areas known to be involved in body weight regulation, the ventral medial hypo- thalamus (VMH) showed the most pronounced increase (using 125 I-NDP-MSH binding) in melano- cortin (MC) receptors following 1 week of 0.5- and 1-h daily feeding in rats with or without running wheels (Figure 2c). This increase in MC receptors in the VMH was not a function of body weight loss per se. For example, MC receptors of 2-h-fed rats with running wheels were similar to ad libitum fed animals, but body weight was much lower in the 2-h-fed rats. Furthermore, the increased MC receptor activity in the VMH appeared to be specific, since MC receptors in other regions, for example the habenular nucleus, did not change in response to food restriction (Figure 2b). Thus, MC receptors in the Molecular Psychiatry (2003) 8, 235–240 & 2003 Nature Publishing Group All rights reserved 1359-4184/03 $25.00 www.nature.com/mp