GABA control of GnRH release from the ewe hypothalamus in vitro: sensitivity to oestradiol

The present study examines the involvement of GABA(A or B) receptors in gonadotrophin-releasing hormone (GnRH) release in vitro and determines whether oestradiol modulates gamma-aminobutyric acid (GABA)-GnRH interaction. Within 10 min after ewe killing, hypothalamic slices were dissected and placed in oxygenated Minimum Essential Media (MEM)-alpha at 4 degrees C; within 2 h, slices were singly perifused at 37 degrees C with oxygenated MEM-alpha (0.15 ml/min), with or without oestradiol (24 pg/ml). After 4 h equilibration, fractions were collected for 4 h interposed with a 10 min exposure to specific GABA(A or B) receptor ligands (0.1-10 mM). The GABA(A or B) agonists (muscimol or baclofen) did not greatly influence GnRH release. However, GnRH increased (p < 0.05) after exposure to 10 mM GABA(A or B) antagonists (bicuculline or CGP52432, respectively). The GABA(A) antagonist stimulated greater sustained GnRH release (p < 0.05) in the absence of oestradiol than in its presence. The bioactivity of the released GnRH was studied using a hypothalamus-pituitary sequential double-chamber perifusion. Only after exposure of hypothalamic slices to the GABA(A) antagonist, did the hypothalamic eluate stimulate luteinizing hormone release from pituitary fragments (p < 0.05) confirming that the GABA(A) antagonist stimulated release of biologically active GnRH. In summary, GnRH release from the hypothalamus is predominantly under GABA(A) receptor inhibitory control and this is attenuated in the presence of oestradiol.