Neural mechanisms in the responses to amino acid deficiency.

Food intake is rapidly and reliably reduced when animals are offered diets that result in an essential amino acid deficiency, such as those used in the imbalanced amino acid diet (IMB) paradigm. There seem to be at least three phases in the responses of rats to IMB: 1) In order to respond to a dietary challenge, the animals must first recognize that challenge. The available data suggest that before the behavioral effects occur, a decline in the concentration of an essential amino acid is sensed in a specific brain area, the prepyriform cortex. This recognition phase is associated with localized decreases in the concentrations of the limiting amino acid, norepinephrine and cyclic AMP and with altered protein synthesis. 2) Subsequent to recognition of the deficiency, a conditioned taste aversion develops, mediated in part by serotonin at the level of the vagus. 3) Finally, in the absence of a choice, the animals adapt to an IMB (but not a diet devoid of one or more essential amino acids) in approximately 1 wk. Damage to certain extrahypothalamic brain areas or liver denervation accelerates adaptation to IMB, suggesting both central and peripheral control in the adaptation phase. The resulting behavioral responses provide adaptive advantage to an animal in the selection of a diet with an appropriate balance of amino acids.