Diurnal expression of functional and clock-related genes throughout the rat HPA axis: system-wide shifts in response to a restricted feeding schedule

Food-entrainable circadian oscillators in the brain

Circadian rhythms in mammalian behaviour and physiology rely on daily oscillations in the expression of canonical clock genes. Circadian rhythms in clock gene expression are observed in the master circadian clock, the suprachiasmatic nucleus but are also observed in many other brain regions that have diverse roles, including influences on motivational and emotional state, learning, hormone release and feeding. Increasingly, important links between circadian rhythms and metabolism are being uncovered. In particular, restricted feeding (RF) schedules which limit food availability to a single meal each day lead to the induction and entrainment of circadian rhythms in food-anticipatory activities in rodents. Food-anticipatory activities include increases in core body temperature, activity and hormone release in the hours leading up to the predictable mealtime. Crucially, RF schedules and the accompanying food-anticipatory activities are also associated with shifts in the daily oscillation of clock gene expression in diverse brain areas involved in feeding, energy balance, learning and memory, and motivation. Moreover, lesions of specific brain nuclei can affect the way rats will respond to RF, but have generally failed to eliminate all food-anticipatory activities. As a consequence, it is likely that a distributed neural system underlies the generation and regulation of food-anticipatory activities under RF. Thus, in the future, we would suggest that a more comprehensive approach should be taken, one that investigates the interactions between multiple circadian oscillators in the brain and body, and starts to report on potential neural systems rather than individual and discrete brain areas.

Sexually dimorphic response of the hypothalamo-pituitary-adrenal axis to chronic alcohol consumption and withdrawal

In males, long-term alcohol consumption provokes neurochemical changes in the medial parvocellular division of the PVN (PVNmp) that are partially reversed by withdrawal. Because gonadal steroids modulate the activity of the hypothalamo-pituitary-adrenal axis, we analyzed the possibility that the repercussions of chronic alcohol consumption and withdrawal on the anatomy and neurochemistry of the PVNmp might differ between the sexes. Male and female Wistar rats were examined after ingesting a 20% alcohol solution for 6 months or after 2 months of withdrawal from 6 months of alcohol consumption. The levels of gonadal steroids and the basal concentrations of corticosterone were also evaluated. Chronic alcohol consumption and withdrawal did not alter the global cytoarchitectonic features of the PVNmp in rats of both sexes. However, alcohol consumption was associated with a decrease in the number of vasopressin (VP) neurons only in females and of corticotropin releasing hormone (CRH) neurons in males and females. Further, the response to withdrawal was sexually dimorphic because in males there was a partial recovery of the number of CRH neurons whereas in females there was a further loss of VP and CRH neurons. Corticosterone levels were unchanged by alcohol consumption, but they were decreased by withdrawal in females. Alcohol consumption and withdrawal did not alter estrogen and progesterone concentrations in females, but decreased testosterone levels in males. These findings show that the response of CRH and VP neurons to excess alcohol is gender-specific, with females being more vulnerable during alcohol consumption and, most notably, after withdrawal.

Reduced anticipatory locomotor responses to scheduled meals in ghrelin receptor deficient mice

Ghrelin, an orexigenic hormone produced by the stomach, is secreted in anticipation of scheduled meals and in correlation with anticipatory locomotor activity. We hypothesized that ghrelin is directly implicated in stimulating locomotor activity in anticipation of scheduled meals. To test this hypothesis, we observed 24 h patterns of locomotor activity in mice with targeted mutations of the ghrelin receptor gene (GHSR KO) and wild-type littermates, all given access to food for 4 h daily for 14 days. While wild type (WT) and GHSR KO mice produced increases in anticipatory locomotor activity, anticipatory locomotor activity in GHSR KO mice was attenuated (P<0.05). These behavioral measures correlated with attenuated levels of Fos immunoreactivity in a number of hypothalamic nuclei from GHSR KO placed on the same restricted feeding schedule for 7 days and sacrificed at ZT4. Interestingly, seven daily i.p. ghrelin injections mimicked hypothalamic Fos expression patterns to those seen in mice under restricted feeding schedules. These data suggest that ghrelin acts in the hypothalamus to augment locomotor activity in anticipation of scheduled meals.