The neural network that regulates energy balance is responsive to glucocorticoids and insulin and also regulates HPA axis responsivity at a site proximal to CRF neurons.
The structure of a large neural system that responds to and regulates energy balance and that encompasses that PVN and activity of the HPA axis has begun to emerge from these experiments (Fig. 6). Several large loops have been delineated within this context of the maintenance of energy balance. Corticosteroids stimulate both feeding and insulin secretion. The actions of corticosteroids in the periphery are catabolic, causing mobilization of energy stores; their actions in the central nervous system are stimulatory to energy acquisition (food intake). By contrast, the action of insulin in the periphery is anabolic, causing energy storage; its action in the central nervous system is inhibitory to energy acquisition (food intake). At the level of the CNS, insulin inhibits and corticosteroids stimulate expression of NPY mRNA in the arcuate nuclei, and these actions may explain, in part, the reciprocal actions of the hormones on energy acquisition. Thus over the long term, stimulation of insulin secretion by corticosteroids tends to supply an automatic brake on the effects of corticosteroids on feeding. The neural system that controls energy balance and responds to the reciprocal signals of corticosterone and insulin also regulates responsivity to restraint stress in the HPA axis. The low-amplitude ACTH responses to restraint, corticosteroid feedback, and prior stress-induced facilitation that are observed under conditions of relative fasting in the PM can be produced in the AM by a 14-h, overnight fast. By contrast, NPY injected ivt stimulates identical ACTH responses in the AM in fed rats and in rats fasted overnight, suggesting that NPY acts to stimulate CRF secretion at a site closer to the PVN than the stress of restraint, which is filtered through the neural energy balance system. In the periphery, corticosteroids and insulin also have reciprocal effects on energy storage; effects that are opposite those exerted in the CNS on energy acquisition. Thus, together, the two hormones may be construed as a bihormonal system that regulates overall energy balance. Although under normal conditions this system is well designed to accomplish energy balance, and provides a mechanism by which total energy stores may be increased appropriately (e.g., prior to hibernation or migration), it seems probable that under conditions of chronic stress, this regulatory system may be maladaptive. Chronic stress and glucocorticoid treatment cause increases in mean daily concentrations of both corticosteroids and insulin. Increases in the absolute levels of both hormones, with the normal ratio between them maintained, results in remodeling of body energy stores-away from muscle stores and toward fat stores, particularly abdominal fat stores. It seems quite likely that some conditions of abdominal obesity in man may be explained, at least in part, by increased activity in the HPA axis. Because abdominal obesity is associated with cardiovascular diseases, these responses, when they persist, are clearly maladaptive. Exploration of the role and control of the HPA axis in and by the larger neural network that regulates energy balance has to date been instructive. Clearly this work has just begun and is primarily still at the level of phenomenology. However, once the phenomenology is understood, mechanistic work can be performed that will flesh out our understanding of this very large and physiologically essential system.