BLOCKADE OF DOPAMINE (DA) RECEPTORS WITH PIMOZIDE AND PITUITARY-HORMONE RELEASE IN RESPONSE TO INTRA-VENTRICULAR INJECTION OF GAMMA-AMINOBUTYRIC ACID (GABA) IN CONSCIOUS OVARIECTOMIZED RATS

To determine the role of dopamine in the action of intraventricular γ-aminobutyric acid (GABA), dopamine receptors were blocked by s.c. injection of pimozide and the effect of intraventricular GABA was evaluated in ovariectomized, conscious female rats. Systemic injection of pimozide alone or the vehicle had no effect on plasma hormone levels except in the case of prolactin which was markedly elevated by pimozide. Similarly injection of 2 μl of saline into the third ventricle failed to alter any of the hormone levels. The elevation in LH and growth hormone induced in pimozide-treated rats by intraventricular injection of 4 μM of GABA in 2 μl of 0.9% NaCl was similar to that observed in animals not treated with pimozide in concurrent studies; however, the suppression of plasma TSH induced GABA in intact animals was completely blocked in the pimozide-treated rats. On the other hand, the elevation of prolactin which occurred in intact animals following intraventricular injection of this dose of GABA was reversed and a lowering of prolactin followed intraventricular injection of GABA in the pimozide-treated animals. It is concluded that dopaminergic systems are probably not involved in mediation of the effects of GABA on GH and LH release but that the inhibition of TSH release induced by GABA may be mediated by a stimulation of dopaminergic neurons which then inhibit TRH release. A speculative explanation is given for the ability of pimozide to reverse the action of GABA on plasma prolactin. © 1979.