Effect of starvation on hypothalamic tyrosine hydroxylase activity in adult male rats.

The activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in norepinephrine synthesis, was determined in the mediobasal hypothalamus of adult male rats during acute and semistarvation. 3,4-Dihydroxyphenylalanine (DOPA) formed by TH was measured using high-performance liquid chromatography (HPLC). Acute starvation, as well as 3 weeks of semistarvation on a high-protein low-carbohydrate diet (CST PR) reduced TH activity significantly. Three weeks of a low-protein high-carbohydrate diet (CST KH) did not affect TH activity. While maximal velocity (Vmax) is significantly diminished in acute starvation and in semistarvation with a high-protein low-carbohydrate diet, Kd-values for tyrosine were not changed. These results suggest that TH activity in the brain contributes to decreased norepinephrine (NE) turnover in starvation.