Effects of TRH on hypothalamic catecholaminergic and serotoninergic neurons

Abstract

1. 1. The in vivo and in vitro effects of TRH on tyrosine hydroxylase activity and on endogenous levels of noradrenaline as an index of noradrenergic neurons activity were studied in rat hypothalamus. The hydroxylation of tryptophan and the liberation of 5-hydroxyindole acetic acid were analyzed as an index of serotoninergic activity. 2. 2. TRH had no effect on tyrosine hydroxylase from hypothalamic homogenates in concentrations up to 100 ng/ml. Neither was the hydroxylation of tyrosine in intact hypothalamus. 3. 3. The TRH (4 μg/kg) decreased 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine accumulation after 10 min, and no effect on serotonin and noradrenaline concentrations was observed. 4. 4. TRH decreased endogenous levels of 5-hydroxyindole acetic acid by 35%. 5. 5. Thus, the results show that TRH or TSH might play a role in a short-loop feedback action involving catecholaminergic and serotonergic nerve terminals that regulate the liberation of TRH.