Contribution of regional brain melanocortin receptor subtypes to elevated activity energy expenditure in lean, active rats.

Physical activity and non-exercise activity thermogenesis (NEAT) are crucial factors accounting for individual differences in body weight, interacting with genetic predisposition. In the brain, a number of neuroendocrine intermediates regulate food intake and energy expenditure (EE); this includes the brain melanocortin (MC) system, consisting of MC peptides as well as their receptors (MCR). MC3R and MC4R have emerged as critical modulators of EE and food intake. To determine how variance in MC signaling may underlie individual differences in physical activity levels, we examined behavioral response to MC receptor agonists and antagonists in rats that show high and low levels of physical activity and NEAT, that is, high- and low-capacity runners (HCR, LCR), developed by artificial selection for differential intrinsic aerobic running capacity. Focusing on the hypothalamus, we identified brain region-specific elevations in expression of MCR 3, 4, and also MC5R, in the highly active, lean HCR relative to the less active and obesity-prone LCR. Further, the differences in activity and associated EE as a result of MCR activation or suppression using specific agonists and antagonists were similarly region-specific and directly corresponded to the differential MCR expression patterns. The agonists and antagonists investigated here did not significantly impact food intake at the doses used, suggesting that the differential pattern of receptor expression may by more meaningful to physical activity than to other aspects of energy balance regulation. Thus, MCR-mediated physical activity may be a key neural mechanism in distinguishing the lean phenotype and a target for enhancing physical activity and NEAT.

Rhythms in Fos expression in brain areas related to the sleep-wake cycle in the diurnal Arvicanthis niloticus.

Most mammals show daily rhythms in sleep and wakefulness controlled by the primary circadian pacemaker, the suprachiasmatic nucleus (SCN). Regardless of whether a species is diurnal or nocturnal, neural activity in the SCN and expression of the immediate-early gene product Fos increases during the light phase of the cycle. This study investigated daily patterns of Fos expression in brain areas outside the SCN in the diurnal rodent Arvicanthis niloticus. We specifically focused on regions related to sleep and arousal in animals kept on a 12:12-h light-dark cycle and killed at 1 and 5 h after both lights-on and lights-off. The ventrolateral preoptic area (VLPO), which contained cells immunopositive for galanin, showed a rhythm in Fos expression with a peak at zeitgeber time (ZT) 17 (with lights-on at ZT 0). Fos expression in the paraventricular thalamic nucleus (PVT) increased during the morning (ZT 1) but not the evening activity peak of these animals. No rhythm in Fos expression was found in the centromedial thalamic nucleus (CMT), but Fos expression in the CMT and PVT was positively correlated. A rhythm in Fos expression in the ventral tuberomammillary nucleus (VTM) was 180 degrees out of phase with the rhythm in the VLPO. Furthermore, Fos production in histamine-immunoreactive neurons of the VTM cells increased at the light-dark transitions when A. niloticus show peaks of activity. The difference in the timing of the sleep-wake cycle in diurnal and nocturnal mammals may be due to changes in the daily pattern of activity in brain regions important in sleep and wakefulness such as the VLPO and the VTM.

Fos expression in the sleep-active cell group of the ventrolateral preoptic area in the diurnal murid rodent, Arvicanthis niloticus.

The ventrolateral preoptic area (VLPO) of the nocturnal laboratory rat receives direct input from the retina and is active during sleep; however, nothing is known about VLPO function in day-active (diurnal) species. In the first study, we used 24-h videotaping of Arvicanthis niloticus, a diurnal murid rodent, to estimate the distribution of sleep and wakefulness across a 12:12 light-dark cycle. Based on behavioral data, A. niloticus were perfused at a time when the animals are inactive (zeitgeber time (ZT) 20) or at a time when they are awake and active (ZT 23). The brains were processed for immunocytochemistry for Fos, an immediate early gene product used as an index of neural activity. Animals had more Fos-immunoreactive (Fos+) cells in the VLPO at ZT 20 than at ZT 23. The pattern of change in Fos expression seen in this area suggest that the VLPO serves the same function in A. niloticus as in rats. Eye injections of cholera toxin (beta subunit) were used to identify the retinal inputs to the VLPO of A. niloticus. In these animals, the VLPO had only very sparse retinal inputs compared to the rat. Together, these results raise the possibility that inputs from the suprachiasmatic nucleus (SCN) or the retina affect neuronal activity in the VLPO differently in rats and A. niloticus, thereby, contributing to differences in their sleep/wake patterns.

Fos expression within vasopressin-containing neurons in the suprachiasmatic nucleus of diurnal rodents compared to nocturnal rodents.

The underlying neural causes of the differences between nocturnal and diurnal animals with respect to their patterns of rhythmicity have not yet been identified. These differences could be due to differences in some subpopulation of neurons within the suprachiasmatic nucleus (SCN) or to differences in responsiveness to signals emanating from the SCN. The experiments described in this article were designed to address the former hypothesis by examining Fos expression within vasopressin (VP) neurons in the SCN of nocturnal and diurnal rodents. Earlier work has shown that within the SCN of the diurnal rodent Arvicanthis niloticus, approximately 30% of VP-immunoreactive (IR) neurons express Fos during the day, whereas Fos rarely is expressed in VP-IR neurons in the SCN of nocturnal rats. However, in earlier studies, rats were housed in constant darkness and pulsed with light, whereas Arvicanthis were housed in a light:dark (LD) cycle. To provide data from rats that would permit comparisons with A. niloticus, the first experiment examined VP/Fos double labeling in the SCN of rats housed in a 12:12 LD cycle and perfused 4 h into the light phase or 4 h into the dark phase. Fos was significantly elevated in the SCN of animals sacrificed during the light compared to the dark phase, but virtually no Fos at either time was found in VP-IR neurons, confirming that the SCN of rats and diurnal Arvicanthis are significantly different in this regard. The authors also evaluated the relationship between this aspect of SCN function and diurnality by examining Fos-IR and VP-IR in diurnal and nocturnal forms of Arvicanthis. In this species, most individuals exhibit diurnal wheel-running rhythms, but some exhibit a distinctly different and relatively nocturnal pattern. The authors have bred their laboratory colony for this trait and used animals with both patterns in this experiment. They examined Fos expression within VP-IR neurons in the SCN of both nocturnal and diurnal A. niloticus kept on a 12:12 LD cycle and perfused 4 h into the light phase or 4 h into the dark phase, and brains were processed for immunohistochemical identification of Fos and VP. Both the total number of Fos-IR cells and the proportion of VP-IR neurons containing Fos (20%) were higher during the day than during the night. Neither of these parameters differed between nocturnal and diurnal animals. The implications of these findings are discussed.

Daily rhythms in Fos activity in the rat ventrolateral preoptic area and midline thalamic nuclei.

The present experiment investigated the expression of the nuclear phosphoprotein Fos over the 24-h light-dark cycle in regions of the rat brain related to sleep and vigilance, including the ventrolateral preoptic area (VLPO), the paraventricular thalamic nucleus (PVT), and the central medial thalamic nucleus (CMT). Immunocytochemistry for Fos, an immediate-early gene product used as an index of neuronal activity, was carried out on brain sections from rats perfused at zeitgeber time (ZT) 1, ZT 5, ZT 12.5, and ZT 17 (lights on ZT 0-ZT 12). The number of Fos-immunopositive (Fos+) cells in the VLPO was elevated at ZT 5 and 12.5 (i.e., during or just after the rest phase of the cycle). Fos+ cell number increased at ZT 17 and ZT 1 in the PVT and CMT, 180 degrees out of phase with the VLPO. A positive correlation was found between the numbers of Fos+ cells in the PVT and CMT, and Fos expression in each thalamic nucleus was negatively correlated with VLPO Fos+ cell number. The VLPO, PVT, and CMT may integrate circadian and homeostatic influences to regulate the sleep-wake cycle.