Activation of A-type gamma-aminobutyric acid receptors excites gonadotropin-releasing hormone neurons.
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.
DeFazio
R Anthony
RA
Department of Internal Medicine, University of Virginia, Charlottesville, VA 22908, USA.
Heger
Sabine
S
Ojeda
Sergio R
SR
Moenter
Suzanne M
SM
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002461:Cell Line, Transformed; D002712:Chlorides / Q000032:analysis; D004553:Electric Conductivity; D005260:Female; D015870:Gene Expression; D007987:Gonadotropin-Releasing Hormone / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D006096:Gramicidin; D049452:Green Fluorescent Proteins; D006706:Homeostasis; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008164:Luminescent Proteins / Q000235:genetics; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008822:Mice, Transgenic; D009118:Muscimol / Q000494:pharmacology; D009474:Neurons / Q000737:chemistry / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011301:Preoptic Area / Q000737:chemistry; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000502:physiology*; D011993:Recombinant Fusion Proteins / Q000032:analysis; D049994:Sodium Potassium Chloride Symporter Inhibitors; D028021:Sodium-Potassium-Chloride Symporters / Q000032:analysis / Q000235:genetics; D064506:Solute Carrier Family 12, Member 2; D027981:Symporters; D005680:gamma-Aminobutyric Acid / Q000008:administration & dosage; D000096922:K Cl- Cotransporters
D002712:Chlorides; D007987:Gonadotropin-Releasing Hormone; D006096:Gramicidin; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins; D009118:Muscimol; D012333:RNA, Messenger; D011963:Receptors, GABA-A; D011993:Recombinant Fusion Proteins; D049994:Sodium Potassium Chloride Symporter Inhibitors; D028021:Sodium-Potassium-Chloride Symporters; D064506:Solute Carrier Family 12, Member 2; D027981:Symporters; D005680:gamma-Aminobutyric Acid; D000096922:K Cl- Cotransporters; C578878:Slc12a2 protein, mouse; C578879:Slc12a2 protein, rat