Effects of TRH and a rat TSH preparation on discrete hypothalamic and forebrain catecholamine nerve terminal networks in the hypophysectomized male rat.

A rat pituitary TSH preparation in doses of 10 and 100 micrograms/kg produced rapid and marked increases in dopamine (DA) levels and alpha-methyltyrosine-induced decline consistent with increased DA synthesis and release in the medial and lateral palisade zones (MPZ, LPZ) of the median eminence, and reduced noradrenaline (NA) turnover in the paraventricular hypothalamic nucleus (PA) of the hypophysectomized male rat. The TSH serum levels measured in these rats 2 h after the injection were within the physiological range after the injection of 10 micrograms/kg. TRH given intravenously in a dose of 100 micrograms/kg produced rapid and marked increases of DA release in the MPZ and LPZ of the median eminence and reduction of NA turnover in the PA of the hypophysectomized male rat. The TRH injection did not alter the serum levels of prolactin, TSH, T3 and T4. The results indicate that TRH-TSH-DA interactions take place in the local circuits in the median eminence thus supporting the view that a short and an ultrashort feedback action of rTSH and TRH respectively may exist in the median eminence. The rapid action of the rat TSH preparation as well as of TRH further supports this concept. The rat TSH preparation and TRH produced marked reductions in NA turnover in the PA. These results support the possibility that rat TSH and TRH may, via an action on the hypothalamus, influence the facilitatory noradrenergic mechanism operating at the soma-dendritic level of the TRH immunoreactive neurons projecting to the median eminence. Thus, the existence of a neuronal feedback loop from the medio-basal hypothalamus into the paraventricular hypothalamic nucleus is postulated.