Restraining action of GABA on estradiol-induced LH surge in the rat: GABA activity in brain nuclei and effects of GABA mimetics in the medial preoptic nucleus.

The relationship between GABA dynamics and LH release was studied on day 2 after subcutaneous estrogen implant in short-term ovariectomized rats. GABA accumulation, used as an index of GABA turnover, was determined in the medial preoptic nucleus (MPN), medial (MS) and lateral (LS) septal nuclei, median eminence-mediobasal hypothalamus (MBH) and locus ceruleus (LC). Measurements of GABA were performed at two different times of day (11.00 and 15.00 h), 3 h after intraperitoneal administration of gamma-vinyl-GABA (GVG), an irreversible inhibitor of GABA transaminase. Either morning or afternoon ovariectomized rats (OVX) showed a significant increase in GABA accumulation after GVG treatment in all the areas studied. Estrogen-treated OVX rats showed in the morning a lower GABA accumulation in the MPN, MBH and LC, and GABA levels remained unchanged in the LS and MS. In the afternoon, the MPN and LS showed a lower rate of GABA accumulation whereas in the MBH and LC the GABA increase was not observed. In contrast the MS showed a rate of GABA accumulation similar as in the OVX rats. Local administration in the MPN of 20 micrograms GVG, or GABA-A receptor stimulation by muscimol (50 ng), prior to the increase in plasma LH levels, prevented the occurrence of the estradiol-induced LH surge. The effect of muscimol was reversed by bicuculline (30 ng), a GABA-A receptor antagonist. Bicuculline in low doses lacked effect by itself. In conclusion, these results strongly suggest that a decreased GABAergic activity in MPN, MBH and LC precedes the estradiol-evoked LH surges in ovariectomized rats. Moreover, that in septal nuclei, a low GABAergic activity takes place well before the occurrence of plasma LH increase.(ABSTRACT TRUNCATED AT 250 WORDS)