This work was supported by research grants from the National Health and Medical Research Council of Australia and the Robert and Helen Kleberg Foundation*

Abstract

Control injections of saline in or into the 3rd ventricle (2 .mu.l) failed to modify concentrations of GH [growth hormone] or TSH [thyrotropin] in plasma of conscious, free-moving, ovariectomized (OVX) or OVX, estrogen- and progesterone-treated (OEP) rats. Intraventricular injection of GABA in doses ranging from 0.1-8 .mu.mol induced a rapid elevation in plasma GH within 10 min of injection which was dose related and the elevation persisted for the 60 min duration of the study in both OVX and OEP rats. This effect was completely blocked by the GABA antagonist, bicuculline, which by itself had no effect on plasma GH in doses of 1 or 2 mmol i.p. GABA administration lowered plasma TSH, which became apparent within 10 min in OVX rats and persisted for the duration of the experiment and appeared to be dose related at the initial sampling time of 10 min. The inhibitory effect of GABA was less prominent in OEP rats. Bicuculline, which by itself had no effect on plasma TSH, completely blocked the inhibitory effect of GABA in OVX rats. The effects of GABA were mediated centrally since 4 .mu.mol GABA given i.v. or in vitro incubation of hemipituitaries with varying doses of GABA had no effect on either hormone. GABA can stimulate release of GH by a hypothalamic action whereas it can inhibit the release of TSH also by an action on the hypothalamus. Since bicuculline did not modify the levels of GH or TSH, it is not clear whether GABA has a physiological role in control of the 2 hormones.