A novel GABA-mediated corticotropin-releasing hormone secretory mechanism in the median eminence

Corticotropin-releasing hormone (CRH), which is synthesized in the paraventricular nucleus (PVN) of the hypothalamus, plays an important role in the endocrine stress response. The excitability of CRH neurons is regulated by g-aminobutyric acid (GABA)–containing neurons projecting to the PVN. We investigated the role of GABA in the regulation of CRH release. The release of CRH was impaired, accumulating in the cell bodies of CRH neurons in heterozygous GAD67-GFP (green fluorescent protein) knock-in mice (GAD67^+/GFP), which exhibited decreased GABA content. The GABA_A receptor (GABA_AR) and the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC1), but not the K⁺-Cl⁻ cotransporter (KCC2), were expressed in the terminals of the CRH neurons at the median eminence (ME). In contrast, CRH neuronal somata were enriched with KCC2 but not with NKCC1. Thus, intracellular Cl⁻ concentrations ([Cl⁻]_i) may be increased at the terminals of CRH neurons compared with concentrations in the cell body. Moreover, GABAergic terminals projecting from the arcuate nucleus were present in close proximity to CRH-positive nerve terminals. Furthermore, a GABA_AR agonist increased the intracellular calcium (Ca²⁺) levels in the CRH neuron terminals but decreased the Ca²⁺ levels in their somata. In addition, the increases in Ca²⁺ concentrations were prevented by an NKCC1 inhibitor. We propose a novel mechanism by which the excitatory action of GABA maintains a steady-state CRH release from axon terminals in the ME.