Orexins (hypocretins) are neuropeptides synthesized in the central nervous system exclusively by neurons of the lateral hypothalamus. Orexin-containing neurons have widespread projections and have been implicated in complex physiological functions including feeding behavior, sleep states, neuroendocrine function, and autonomic control. Two orexin receptors (OX(1)R and OX(2)R) have been identified, with distinct expression patterns throughout the brain, but a systematic examination of orexin receptor expression in the brain has not appeared. We used in situ hybridization histochemistry to examine the patterns of expression of mRNA for both orexin receptors throughout the brain. OX(1)R mRNA was observed in many brain regions including the prefrontal and infralimbic cortex, hippocampus, paraventricular thalamic nucleus, ventromedial hypothalamic nucleus, dorsal raphe nucleus, and locus coeruleus. OX(2)R mRNA was prominent in a complementary distribution including the cerebral cortex, septal nuclei, hippocampus, medial thalamic groups, raphe nuclei, and many hypothalamic nuclei including the tuberomammillary nucleus, dorsomedial nucleus, paraventricular nucleus, and ventral premammillary nucleus. The differential distribution of orexin receptors is consistent with the proposed multifaceted roles of orexin in regulating homeostasis and may explain the unique role of the OX(2)R receptor in regulating sleep state stability.

Activation of melanocortin-4-receptors (MC4Rs) reduces body fat stores by decreasing food intake and increasing energy expenditure. MC4Rs are expressed in multiple CNS sites, any number of which could mediate these effects. To identify the functionally relevant sites of MC4R expression, we generated a loxP-modified, null Mc4r allele (loxTB Mc4r) that can be reactivated by Cre-recombinase. Mice homozygous for the loxTB Mc4r allele do not express MC4Rs and are markedly obese. Restoration of MC4R expression in the paraventricular hypothalamus (PVH) and a subpopulation of amygdala neurons, using Sim1-Cre transgenic mice, prevented 60% of the obesity. Of note, increased food intake, typical of Mc4r null mice, was completely rescued while reduced energy expenditure was unaffected. These findings demonstrate that MC4Rs in the PVH and/or the amygdala control food intake but that MC4Rs elsewhere control energy expenditure. Disassociation of food intake and energy expenditure reveals unexpected divergence in melanocortin pathways controlling energy balance.

Ghrelin is a hormone that stimulates growth hormone secretion and signals energy insufficiency via interaction with its receptor, the growth hormone secretagogue receptor (GHSR). The GHSR is located in both the central nervous system and the periphery. Its distribution in the CNS, as assessed by in situ hybridization histochemistry (ISHH), has been described previously in a few mammalian species, although these studies were limited by either the detail provided or the extent of the regions examined. In the present study, we systematically examined the distribution of GHSR mRNA in the adult rat and mouse brains and cervical spinal cords by using ISHH with novel cRNA probes specific for the mRNA encoding functional GHSR (the type 1a variant). We confirmed GHSR mRNA expression in several hypothalamic nuclei, many of which have long been recognized as playing roles in body weight and food intake. GHSR also was found in several other regions previously unknown to express GHSR mRNA, including many parasympathetic preganglionic neurons. Additionally, we found GHSR mRNA within all three components of the dorsal vagal complex, including the area postrema, the nucleus of the solitary tract, and the dorsal motor nucleus of the vagus. Finally, we examined the co-expression of GHSR with tyrosine hydroxylase and cholecystokinin, and demonstrate a high degree of GHSR mRNA expression within dopaminergic, cholecystokinin-containing neurons of the substantia nigra and ventral tegmental area.