GABA Control of GnRH Release from the Ewe Hypothalamus In Vitro: Sensitivity to Oestradiol

Abstract

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.

Understanding the trade-off between the environment and fertility in cows and ewes

“…Basal GnRH increases in the presence of oestradiol ( Ghuman et al., 2006 ) and an adrenergic agonist increases release of GnRH that is prolonged by oestradiol ( Ghuman et al., 2008b ). In contrast, a GABA-A antagonist results in greater GnRH secretion that is higher in the absence of oestradiol ( Ghuman et al., 2008c ) concurring with in vivo studies ( Scott and Clarke, 1993 ). It is suggested that higher concentrations of oestradiol eventually decrease GABA turnover which in turn facilitates activation of GnRH neurones.…”

Clinical Applications of Neuroendocrinology in Female Farm Animals

“…GABA neurons, as well as a dense plexus of GABA fibers are located within the ewe POA. In this area, GABA may directly control GnRH perikarya as GABA terminals synapse directly on GnRH cell bodies in the ewe and GABAA receptor mRNA co-localizes with GnRH neurons [28,29]. In ewe, about 44% neurons synthesizing GABA co-localize ERα in the POA and these account for 30% of all ERα-positive cells in this area [30].…”