ROLE OF BROWN ADIPOSE-TISSUE THERMOGENESIS IN CONTROL OF THERMOREGULATORY FEEDING IN RATS - A NEW HYPOTHESIS THAT LINKS THERMOSTATIC AND GLUCOSTATIC HYPOTHESES FOR CONTROL OF FOOD-INTAKE

The hypothesis proposed in this review provides a novel view of both t he control of feeding and the function of brown adipose tissue (BAT) t hermogenesis. It takes into account the episodic nature of feeding in rats allowed free access to food and the necessity for episodic events in the controlling systems which govern initiation and termination of feeding. A feeding episode is proposed to occur during an episode of increased sympathetic nervous system activity that stimulates BAT ther mogenesis and increases body temperature. Two different aspects of sti mulated BAT metabolism, namely increased uptake of glucose and increas ed heat production, evoke initiation and termination of feeding, respe ctively, Initiation is mediated by a transient dip in blood glucose co ncentration caused by stimulated glucose utilization in BAT, Feeding c ontinues while both BAT and core temperature continue to rise. Termina tion is induced by the high level of core temperature brought about by the episode of stimulated BAT thermogenesis. The time between initiat ion and termination determines the size of the meal and depends on the balance between BAT thermogenesis and heat loss, and thus on ambient temperature. The underlying cause of the episodic stimulation of sympa thetic nervous system activity is a decline in core temperature to a l evel recognized by the hypothalamus as needing a burst of increased he at production, Thus, BAT thermogenesis is important in control of meal sire, relating it to thermoregulatory needs. When this function is lo st, as in many obese animal models of obesity, the animal loses its ab ility to remain in energy balance by precisely adjusting its intake in relation to environmental temperature and meal size increases, The hy pothesis also predicts that an increase in endogenous heat production that is not due to BAT thermogenesis will prevent the matching of inta ke to increased expenditure via thermoregulatory feeding, This is seen , for example, in the shivering rat during the early stage of acclimat ion to cold. Feeding is viewed as the outcome of a thermoregulatory ev ent. Rats do not eat to warm up; they start to eat after they have sta rted to warm up and stop eating once they have warmed up. The phenomen on is termed thermoregulatory feeding, to distinguish it from feeding initiated by other stimuli.