STEROID-HORMONES AND RECEPTORS OF THE GABA-A SUPRAMOLECULAR COMPLEX .2. PROGESTERONE AND ESTROGEN INHIBITORY EFFECTS ON THE CHLORIDE-ION CHANNEL RECEPTOR IN DIFFERENT FOREBRAIN AREAS OF THE FEMALE RAT

The inhibitory effect of female sex steroid hormones on the binding of [S-35]t-butylbicyclophosphorothionate [S-35]TBPS to the chloride ion channel receptor in different forebrain areas of the female rat proved to be of a differential nature. The in vivo administration of estradi ol and estradiol + progesterone were responsible for substantially low er chloride ion channel receptor levels in brain areas that contain el evated steroid receptors, such as the medial preoptic area, the cortic o-medial amygdala nucleus, the vertical limb diagonal band-medial sept al nucleus and the cortex lamina V. The administration of progesterone alone reduced receptor levels in the oriens-pyramidalis CA1 layer of the hippocampus, caudate putamen, cortex lamina VI (brain areas that c ontain little if any steroid receptors) and in the lateral and basolat eral amygdala nucleus (brain sites that contain noninducible progester one receptors). On the basis of the progesterone-inhibitory activity o n the chloride ion channel receptors, it was important to investigate whether progesterone per se or whether the potent progesterone metabol ites 3alpha-hydroxy-5alpha-dihydroprogesterone (3alpha,5alpha-THP) and 3beta-hydroxy-5beta-dihydroprogesterone (3beta,5beta-THP) were involv ed in the binding level changes, and to establish the specific brain s ites where these steroid effects occur. In fact, in vitro addition of the 5alpha-reduced progesterone metabolite produced even greater depre ssive effects on [S-35]TBPS binding not only in the same brain areas a s the in vivo progesterone replacement therapy but also in some sites that provided significant receptor level changes following the sequent ial administration of estradiol + progesterone. However, when the 5bet a-reduced metabolite was tested on the binding of [S-35]TBPS to the ch loride ion channel receptor, only the basolateral amygdala nucleus, th e cortex lamina VI and the dorsolateral septal nucleus exhibited chang es. Because the steroid-mediated chloride ion flux is regulated in a G ABA-dependent manner, we next checked for the type of GABA effects on the chloride ion channel receptor levels and found that GABA not only intensified the 3alpha,5alpha-THP inhibitory effects but, together wit h this progesterone metabolite, was also involved in binding changes i n the vertical limb diagonal band-medial septal nucleus. It is interes ting to note that the GABA effects on 5beta-metabolite-induced recepto r changes were not of the enhancing type, but tended, rather, to be in hibitory. Moreover, the addition of the GABA antagonist bicuculline to the in vitro hormone treatment in the presence of GABA, caused a nota ble and specific inhibitory effect only to 3alpha,5alpha-THP-induced c hloride ionophore receptor levels, indicating that this metabolite is probably the main modulator involved in the progesterone-dependent GAB Aergic functions via a specific interaction with the neurosteroid site of the GABA molecule in the various brain areas.