Hypothalamic paraventricular nucleus: interaction between alpha 2-noradrenergic system and circulating hormones and nutrients in relation to energy balance.

Extensive evidence suggests that norepinephrine (NE) in the brain is active in the control of eating behavior. Central injection studies demonstrate a stimulatory effect of NE on food intake, a response which is mediated by alpha 2-noradrenergic receptors located in the medial hypothalamus, in particular the paraventricular nucleus (PVN). Activation of these PVN receptors stimulates ingestion specifically of carbohydrate-rich foods, and this response is believed to reflect the role of endogenous NE in controlling natural appetite for this macronutrient. This alpha 2-noradrenergic system in the PVN appears to be physiologically activated at the onset of the animals' active cycle, when there is a natural peak in preference for carbohydrate. At this time, the adrenal hormone corticosterone, which is known to play a major role in carbohydrate metabolism, is found to interact positively with NE in the potentiation of carbohydrate ingestion. Circulating glucose also influences the activity of PVN alpha 2-noradrenergic receptors at this time, and, moreover, alpha-noradrenergic stimulation of the PVN produces an increase in circulating levels of both corticosterone and glucose. This and other evidence has led to the hypothesis that NE in the PVN, through the activation of glucocorticoid- and glucose-sensitive alpha 2-receptor sites, is physiologically active in energy homeostasis, most particularly at the onset of the animal's active cycle. Specifically, this neurotransmitter in the PVN evokes a state of energy conservation. This state involves adjustments in carbohydrate ingestion as well as metabolism, that allow animals to maintain energy reserves by anticipating or responding to a depletion.