Exercise and brain catecholamine relationships with brown adipose tissue and whole-body oxygen consumption in rats

Brown Adipose Tissue (BAT) is thought to maintain caloric homeostasis by increasing its metabolic rate during periods of energy surfeit or low environmental temperatures and decreasing it during periods of deprivation or high environmental temperatures. The present study investigated the relationship of BAT and whole-body metabolic rate with negative energy balance induced by a minimally stressful exercise regimen. Twenty-four rats exercised in a running wheel for 2 hours per day for 8 weeks for food reinforcement either under a variable ratio or variable time schedule. Whole-body oxygen consumption and the thermic effect of a test meal were then measured, the rats sacrificed, BAT weight and oxygen consumption measured, and the brains assayed for catecholamines. Food intake was found to be positively correlated with expenditure either in the form of exercise, overnight weight loss, or BAT metabolic rate. Brain dopamine concentrations were found to be related to BAT metabolic rate which was in turn related to the amount of weight lost overnight and the amount of food intake. Running was positively correlated with BAT metabolic rate, suggesting that brown adipose tissue is not simply an organ of caloric homeostasis or thermoregulation. BAT, then, appears to be a multi-purpose tool used by a number of systems which regulate body temperature and energy metabolism.