[Effect of thyroxine on the hypothalamic concentration of biogenic amines in rats of different ages]

Male rats of 2.0--2.5 and 8--12 months old received intraperitoneally thyroxin T4 (2 mcg/100 g daily during 8 days) or placebo. T4 significantly increased the noradrenaline concentration in the young rats' hypothalamus. No difference was revealed in concentrations of dophamine, 5--OIAA, or serotonin in either group. The data obtained are discussed from the standpoint of a possible mechanism of changing thresholds for the homeostatic inhibition in the hypothalamo-hypophyseal-thyroid system with ageing.