Mammals respond to reduced food availability by becoming more wakeful and active, yet the central pathways regulating arousal and instinctual motor programs (such as food seeking) according to homeostatic need are not well understood. We demonstrate that hypothalamic orexin neurons monitor indicators of energy balance and mediate adaptive augmentation of arousal in response to fasting. Activity of isolated orexin neurons is inhibited by glucose and leptin and stimulated by ghrelin. Orexin expression of normal and ob/ob mice correlates negatively with changes in blood glucose, leptin, and food intake. Transgenic mice, in which orexin neurons are ablated, fail to respond to fasting with increased wakefulness and activity. These findings indicate that orexin neurons provide a crucial link between energy balance and arousal.

The lateral hypothalamus (LH) is implicated in the behavioral actions of drugs of abuse, but the cellular and molecular basis of this role is unclear. Recent identification of neuropeptides localized in LH neurons has allowed for more specific studies of LH function. The LH-specific peptide orexin (hypocretin) has been shown to be important in arousal and sleep regulation. However, orexin cells of the LH project broadly throughout the brain such that orexin may influence other behaviors as well. In this study, we show that orexin neurons, and not nearby LH neurons expressing melanin-concentrating hormone (MCH), have mu-opioid receptors and respond to chronic morphine administration and opiate antagonist-precipitated morphine withdrawal. cAMP response element-mediated transcription is induced in a subset of orexin cells, but not MCH cells, after exposure to chronic morphine or induction of withdrawal. Additionally, c-Fos and the orexin gene itself are induced in orexin cells in the LH during morphine withdrawal. Finally, we show that orexin knock-out mice develop attenuated morphine dependence, as indicated by a less severe antagonist-precipitated withdrawal syndrome. Together, these studies support a role for the orexin system in molecular adaptations to morphine, and demonstrate dramatic differences in molecular responses among different populations of LH neurons.

Temporal restriction of feeding can phase-shift behavioral and physiological circadian rhythms in mammals. These changes in biological rhythms are postulated to be brought about by a food-entrainable oscillator (FEO) that is independent of the suprachiasmatic nucleus. However, the neural substrates of FEO have remained elusive. Here, we carried out an unbiased search for mouse brain region(s) that exhibit a rhythmic expression of the Period genes in a feedingentrainable manner. We found that the compact part of the dorsomedial hypothalamic nucleus (DMH) demonstrates a robust oscillation of mPer expression only under restricted feeding. The oscillation persisted for at least 2 days even when mice were given no food during the expected feeding period after the establishment of foodentrained behavioral rhythms. Moreover, refeeding after fasting rapidly induced a transient mPer expression in the same area of DMH. Taken in conjunction with recent findings (i) that behavioral expression of food-entrainable circadian rhythms is blocked by cell-specific lesions of DMH in rats and (ii) that DMH neurons directly project to orexin neurons in the lateral hypothalamus, which are essential for proper expression of food-entrained behavioral rhythms, the present study suggests that DMH plays a key role as a central FEO in the feeding-mediated regulation of circadian behaviors.circadian rhythm ͉ feeding ͉ mouse ͉ obesity ͉ period F or animals to survive effectively, the timing of feedingrelated behaviors should be appropriately coordinated both with environmental conditions, such as food availability and risk of predation, and internal physiological states, such as gastrointestinal function and energy balance. The circadian oscillator in the suprachiasmatic nucleus (SCN), which is regarded as the master clock in mammals, orchestrates multiple circadian rhythms in the organism and is regulated according to environmental light͞dark cues conveyed from the eye (light-entrainable oscillator) (1, 2). However, when food availability is restricted to a single period scheduled at a fixed time of the day [restricted feeding (RF)] animals adapt to this condition within a few days by feeding during the period of food availability and increasing food-seeking activity in the preceding hours [food-anticipatory activity (FAA)] (3, 4). Such anticipatory behaviors are often accompanied by increases in body temperature, adrenal corticosterone secretion, and gastrointestinal motility. These RFinduced biological rhythms persist even when SCN function is physically or genetically ablated, indicating the presence of a food-entrainable oscillator (FEO), that is separate from and independent of SCN.Expression of certain molecular components of circadian clocks, such as Per1 and Per2, has been found oscillating (usually entrained by the SCN master clock) in many peripheral tissues and brain regions outside of the SCN (1, 2, 5-8). Previous studies demonstrated that RF entrains and shifts the circadian oscillators in the peripheral tissues and certain non-...