The vagus nerve in thermoregulation and energy metabolism.

The vagus nerve may indirectly influence thermoregulation by modulation of energy balance: its afferent fibers convey signals that represent information on feeding state, resulting in either depression or stimulation of metabolic processes. A regulated metabolic depression can be detected in the background of fasting-induced hypometabolism and hypothermia. In its development (besides humoral signals) vagally transmitted neural signals of gastrointestinal and hepatoportal origin are important. These signals are related to hunger, to decrease of mechanical/chemical stimuli from the gut, to decline of blood glucose; they alter discharge rates of vagal afferents and activity of the nucleus of the solitary tract, eliciting inhibition of metabolic rate and enhancement of food intake. In this hunger-related metabolic inhibition the nucleus of the solitary tract is in interaction with hypothalamic nuclei, that contribute to neuropeptide changes characterized by high neuropeptide Y activity (with energy-conserving type of regulation) and depressed cholecystokinin and corticotropin releasing hormone activities (with depressed energy-expenditure). In postalimentary states the hypermetabolism and hyperthermia are due to opposite changes in metabolic regulation. Satiety-related stimulatory signals of abdominal origin, transmitted via hepatic vagal afferents to the nucleus of the solitary tract, contribute to enhancement of sympathetic activity and stress-responsiveness, leading to hypermetabolism and hyperthermia. Depressed neuropeptide Y release and enhanced cholecystokinin and corticotropin releasing hormone activities participate in the central regulatory changes, and in the high energy-expenditure. The biological role of these vagal functions is not directly the regulation of body temperature, rather the regulation of energy balance and energy content in the body.