Dopamine stimulates release of thyrotrophin-releasing hormone from perfused intact rat hypothalamus via hypothalamic D2-receptors

We have studied the effect of dopamine together with agonist and antagonist drugs of different specificities on the release of TRH from the perfused, intact hypothalamus of the adult rat in vitro. Dopamine produced a dose-related stimulatory effect on TRH release with maximal effect being achieved at 1 mumol/l (increase over basal, 118 +/- 16.5 (S.E.M.) fmol TRH; P less than 0.001 vs basal). This effect was mimicked by the specific D2-agonist drugs bromocriptine (0.1 mumol/l) and LY 171555 (0.1 mumol/l) (increase over basal values, 137.5 +/- 13.75 fmol and 158.6 +/- 10.7 fmol respectively; P less than 0.001 vs basal), but not by the D1-agonist SKF 38393A. The stimulatory effect of dopamine (1 mumol/l) was blocked in a stereospecific manner by the active (D) but not by the inactive (L) isomers of the dopamine antagonist butaclamol. Similar blockade was achieved with the specific D2-antagonist domperidone (0.01 mumol/l) whereas the D1-antagonist SCH 23390 was only effective when used at a concentration 100 times greater. Lower concentrations (0.01 mumol/l) of this D1-antagonist did not block the stimulatory effect of dopamine. High-performance liquid chromatography characterization of the material secreted within the hypothalamus showed one single peak of immunoreactive material which coeluted with synthetic TRH. These data suggest that dopamine exerts a stimulatory role in the control of hypothalamic TRH release by acting at specific D2-receptors.