Nucleus-specific effects of meal duration on daily profiles of Period1 and Period2 protein expression in rats housed under restricted feeding.
Restricted feeding (RF) schedules provide a cycle of fasting and feeding each day and induce circadian rhythms in food-anticipatory activity. In addition, daily rhythms in the expression of circadian clock genes, such as rhythms in Period1 (PER1) or Period2 (PER2), are also shifted in many brain areas that are important for the regulation of motivation and emotion. In order to differentiate brain areas that respond to the time of food presentation from areas that are sensitive to the degree of restriction, the present study compared RF schedules that provided rats with either a 2 h-meal (2hRF) or a 6 h-meal (6hRF) each day. As expected, 2hRF was associated with less food-consumption, more weight-loss, and more food-anticipatory running-wheel activity than 6hRF. In association with these metabolic and behavioral differences, the daily pattern of PER1 and PER2 expression in the dorsomedial hypothalamic nucleus (DMH), which has been proposed to be integral to the generation and/or maintenance of food-anticipatory activities, peaked earlier in the 2hRF group and later in the 6hRF group. Because both RF groups exhibited approximately synchronous food-anticipatory activity, but phase shifted rhythms of PER1 and PER2 expression in the DMH, it suggests that the phase of food-anticipatory activity is not directly regulated by this brain area. Next, daily rhythms of PER2 expression in the limbic forebrain responded to each RF schedule in a nucleus-specific manner. In some brain areas, the amplitude of the PER2 rhythm was differentially adjusted in response to 2hRF and 6hRF, while other areas, responded similarly to both RF schedules. These findings demonstrate that daily rhythms of clock gene expression can be modulated by the motivational state of the animal, as influenced by meal duration, weight loss and food-consumption.