Activation of A-Type γ-Aminobutyric Acid Receptors Excites Gonadotropin-Releasing Hormone Neurons

Abstract

Gamma-aminobutyric acid (GABA), acting through GABA(A) receptors (GABA(A)R), is hypothesized to suppress reproduction by inhibiting GnRH secretion, but GABA actions directly on GnRH neurons are not well established. In green fluorescent protein-identified adult mouse GnRH neurons in brain slices, gramicidin-perforated-patch-clamp experiments revealed the reversal potential (E(GABA)) for current through GABA(A)Rs was depolarized relative to the resting potential. Furthermore, rapid GABA application elicited action potentials in GnRH neurons but not controls. The consequence of GABA(A)R activation depends on intracellular chloride levels, which are maintained by homeostatic mechanisms. Membrane proteins that typically extrude chloride (KCC-2 cotransporter, CLC-2 channel) were absent from the GT1-7 immortalized GnRH cell line and GnRH neurons in situ or were not localized to the proper cell compartment for function. In contrast, GT1-7 cells and some GnRH neurons expressed the chloride-accumulating cotransporter, NKCC-1. Patch-clamp experiments showed that blockade of NKCC hyperpolarized E(GABA) by lowering intracellular chloride. Regardless of reproductive state, rapid GABA application excited GnRH neurons. In contrast, bath application of the GABA(A)R agonist muscimol transiently increased then suppressed firing; suppression persisted 4-15 min. Rapid activation of GABA(A)R thus excites GnRH neurons whereas prolonged activation reduces excitability, suggesting the physiological consequence of synaptic activation of GABA(A)R in GnRH neurons is excitation.

Prenatal androgens alter GABAergic drive to gonadotropin-releasing hormone neurons: Implications for a common fertility disorder

“…Both frequency and size of GABAergic PSCs were increased in PNA females, suggesting that one mechanism for reproductive dysfunction in these animals may be altered GABAergic drive to GnRH neurons. Interestingly, LH was also elevated in PNA mice, providing further physiological support for electrophysiological evidence suggesting an excitatory action of GABA directly at the GnRH neuron is possible (26,27,29).…”

“…GABA inhibits most mature neurons, but this is controversial for GnRH neurons. GnRH neurons maintain elevated chloride (29), as do other neurons derived from olfactory placode (30), and may thus be excited by GABA A R activation. Studies using functional approaches to examine GnRH neurons differ as to whether GABA excites or inhibits these neurons (refs.…”

Steroid modulation of GABAA receptor-mediated transmission in the hypothalamus: Effects on reproductive function

“…It should be noted, however, that they report values of oxytocin release from the SON of PN34 rats of >300 pg; a value comparable to that from PN9 neonates. This late adolescent age is well beyond the expected developmental shift in E Cl in non-GnRH hypothalamic neurons (Han et al, 2002;DeFazio et al, 2002), and suggests that other factors, such as the posttranslational status of the GABA A receptor, are also likely to contribute to age-specific changes in allopregnanolone sensitivity.…”

“…A very interesting facet with respect to GABAergic control of GnRH neuron activity is that the developmental change in the chloride reversal potential (E Cl ), which causes GABA to switch from imparting depolarizing to hyperpolarizing effects, is dramatically delayed in GnRH neurons. Han et al (2002) report that this developmental change does not occur until approximately the time of pubertal onset (postnatal day 31; PN31), while DeFazio et al (2002) do not observe a developmental shift at all and believe that GABA continues to depolarize GnRH neurons even into adulthood. While the discrepancy between these two reports might result from sampling differences as there is a rostral-caudal gradient in the expression of the potassium-chloride cotransporter KCC2 in female mice (Leupen et al, 2003), the common finding is that GABA continues to depolarize GnRH neurons well beyond the age (~PN14) where GABA switches to a hyperpolarizing action in non-GnRH neurons in the hypothalamus/basal forebrain (Han et al, 2002;Defazio et al, 2002).…”

Membrane-initiated estrogen signaling in hypothalamic neurons

“…The positive feedback is believed to be by an action of E2 in the anteroventral periventricular (AVPV) nucleus (Herbison, 1998, Han et al, 2005, Smith et al, 2006, Wintermantel et al, 2006. Neurons in the AVPV express kisspeptin, a neuropeptide encoded by the Kiss gene, GABA and opioid peptides, all of which are important for regulation of GnRH neurosecretion (Simerly et al, 1988, Wagner et al, 2001a, Jackson and Kuehl, 2002, DeFazio et al, 2002, Smith et al, 2006, Christian and Moenter, 2007. The AVPV area expresses high levels of ERα and also ERβ, and the actions of the gonadal steroids are mediated, in part, via the nuclearinitiated signaling (genomic) mechanism (Shughrue et al, 1997, Wintermantel et al, 2006.…”