GABA(B) receptors in anterior pituitary cells - Mechanism of action coupled to endocrine effects

The activation of pituitary GABA(B) receptors by the specific agonist baclo fen inhibits pituitary hormone secretion in vitro. Here we studied the mech anism of action of GABA(B) receptors in rat adenohypophysis. Anterior pitui tary cells were obtained by trypsinization and were either plated for hormo nal studies and cAMP determination or incubated in FURA 2AM for calcium mea surements. Baclofen (BACL: 1.10(-5) M) significantly inhibited basal and th yrotropic releasing hormone (TRH)-stimulated (1.10(-7) IM) PRL secretion in anterior pituitary cells from proestrous rats. In the presence of pertussi s toxin (PTX: 150 ng/ml, 20 h), which leads to the uncoupling of the G(i/o) -protein from the receptor, both effects of BACL were abolished while the e ffect of dopamine (DA: 1.10(-8) M), used as an inhibitory control, was redu ced from 70 to 25%. PTX also reversed BACL-induced inhibition of gonadotrop in-releasing hormone (GnRH)-elicited luteinizing hormone (LH) secretion in anterior pituitary cells from 15-day-old female rats. In addition, though w orking in a pituitary mixed cell population, in which only some cell types possess GABA(B) receptors, BACL (1.10(-5) M) attenuated the forskolin-induc ed (0.5 muM) increase in cAMP. This effect was prevented by co-incubation w ith the antagonist 2 hydroxysaclofen and by preincubation with PTX. BACL (5 .10(-5) M) and DA (5.10(-7) M) inhibited basal intracellular calcium concen trations ([Ca2+](i)) in pituitary cells and the effect of the latter was si gnificantly stronger. The effect of BACL on [Ca2+](i) was abolished after p reincubation with PTX. In the presence of the potassium channel blocking ag ents barium (200 muM and 1 mM) and tetraethylammonium (10 mM), BACL was sti ll able to inhibit [Ca2+](i). Blockade of voltage-sensitive calcium channel s (VSCC) with either verapamil (5.10(-6) M) or nifedipine (1.10(-6) M) comp letely abolished the effect of BACL on [Ca2+](i). In the presence of 12.5 m M potassium concentration baclofen significantly inhibited [Ca2+](i). In co nclusion, our results describe the negative coupling of adenohypophyseal GA BA(B) receptors to VSCC through PTX-sensitive G-proteins. These characteris tics suggest a resemblance of these receptors to the typical presynaptic GA BA(B) sites described in the central nervous system. Copyright (C) 2001 S. Karger AG, Basel.