The mechanisms underlying CNS arousal in response to homeostatic pressures are not known. In this study, we pitted two forces for CNS arousal against each other (circadian influences vs. restricted food availability) and measured the neuronal activation that occurs in a behaviorally defined group of animals that exhibited increased arousal in anticipation of feeding restricted to their normal sleeping time. The number of c-FOS؉ neurons was significantly increased only in the ventromedial nucleus of the hypothalamus (VMH) in these mice, compared with control animals whose feeding was restricted to their normal active and feeding time (P < 0.01). Because the activation of VMH neurons coincides with the earliest signs of behavioral arousal preceding a change in meal time, we infer that VMH activation is involved in the increased arousal in anticipation of food.T he activation of behavior is driven by homeostatically regulated variables, such as hunger and circadian rhythms. The problem of exactly how these two types of influences interact to modulate behavior has been stated (1) but not solved. Mechanisms for changes in CNS arousal have remained controversial.We pitted two forces for CNS arousal against each other (food availability vs. circadian influences) and searched for the first neuronal activation that occurs in animals as they began to change their activation of behavior from a circadian light-driven rhythm to one dictated by restricted food availability.The present study (i) takes into account the individual differences in food anticipatory activity and links those to neuronal activation; (ii) pits the homeostatic drive for feeding against the circadian drive to rest during the light period, thus enabling a cell-by-cell dissection of these two pathways; (iii) has a control group that is exposed to the same restricted feeding paradigm as the test group, for the same number of days, with the difference that control animals receive their daily meal during their behaviorally active period; (iv) conceptualizes the problem as one of generalized CNS arousal; and (v) examines animals' brains as close to the development of the food anticipatory activity as possible. This design was intended to identify the earliest neuronal changes, and therefore the most likely to be causing these behavioral changes. ResultsShifted animals were significantly more active in the 3-h period preceding the shifted food presentation time, compared with controls. In fact, running wheel revolutions were increased from 314 Ϯ 151 in nonshifted animals to 1,768 Ϯ 398 in shifted animals (P Ͻ 0.01) (Figs. 1 and 2 and Table 1).We examined neuronal activation, as measured by c-FOS expression, in every neuronal group that could be conceived, based on the literature (see Discussion), as mediating these changes in the timing of the activation of behavior [16 regions: medial preoptic area (MPA), ventrolateral preoptic nucleus (VLPO), medial part of the medial preoptic nucleus (MPOM), lateral hypothalamus (LH), subparaventricular zone (sPVZ), paraven...
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