Mechanisms of the anorexia of aging in the Brown Norway rat.

Aging is associated with a loss of the ability to maintain homeostasis in response to physiologic and environmental disturbances. Age-related dysregulation of food intake and energy balance appears to be the result of impaired responsiveness of hypothalamic integrative circuitry to metabolic cues, which can lead to lack of appropriate food intake (the anorexia of aging) and thus to inappropriate weight loss in response to acute or chronic illness or other stressors. Using the Brown Norway (BN) male rat model, we have shown that old animals fail to appropriately increase food intake after the metabolic challenge of a 72 h fast, resulting in the failure to re-gain lost body weight upon refeeding. Leptin levels increase with adiposity and age, and remain elevated above levels of young animals even after a 72 h fast, suggesting that hyperleptinemia may be influencing the energy balance dysregulation. It is unclear whether this age-related response is due to a failure of the network of hypothalamic neurons to appropriately integrate hormonal and neural inputs, or due to a failure of the neurons to produce the appropriate neuropeptides. We hypothesize that sequential, age-related alterations in the expression patterns of neuropeptides that maintain melanocortinergic tone, and in the hormone mediators that inform the system of the state of energy balance, result in a diminished ability to maintain energy homeostasis with increasing age. We have undertaken a number of interventional approaches to test this hypothesis, including manipulations of the hormones ghrelin, insulin and testosterone, and direct application of neuropeptides to the central nervous system in these animals.