GABA Enhances Growth Hormone Expression by Modulating Somatotroph Pit-1 Transcription via Activation of...

Background: Gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the central nervous system (CNS), is also a potent modulator of peripheral endocrine function. We previously demonstrated that dietary GABA supplementation improves growth and fatty acid metabolism in male mice while elevating pituitary growth hormone (GH). However, the mechanisms by which GABA regulates the somatotropic axis remain unclear. Methods: Adolescent mice (3-4 weeks old) were treated with or without GABA in drinking water. Cultured pituitaries and GH3 somatotroph-derived cells were exposed to GABA, Picrotoxin, or STO-609, and protein expression was analyzed by Western blot. Results: GABA treatment increased Pit-1 (POU1F1) protein levels among males in vivo (ctrl: 0.55 ± 0.11; GABA: 1.46 ± 0.16; p = 0.0034) and ex vivo (ctrl: 0.66 ± 0.03; GABA: 1.46 ± 0.14; p = 0.0013), as well as in GH3 cells (ctrl: 1.36 ± 0.12; GABA: 3.05 ± 0.12; p < 0.0001). GH expression was also increased by GABA treatment in ex vivo pituitaries (ctrl: 1.62 ± 0.06; GABA: 1.84 ± 0.01; p = 0.0115) and GH3 cells (ctrl: 0.34 ± 0.08; GABA: 1.35 ± 0.13; p = 0.0006). Mechanistically, GABA, via the GABA_A receptor (GABA_AR), enhanced CaMKK2 pathway activity, as evidenced by increased phosphorylation of CaMKIV (ctrl: 0.86 ± 0.07; GABA: 1.12 ± 0.07; p = 0.0378) and AKT (ctrl: 0.89 ± 0.08; GABA: 1.75 ± 0.23; p = 0.0122). Inhibition of GABA_ARs by picrotoxin (PTX) markedly reduced Pit-1 (GABA: 2.73 ± 0.29; GABA + PTX: 1.76 ± 0.21; p = 0.0351) and GH expression (GABA: 0.17 ± 0.02; GABA + PTX: 0.05 ± 0.02; p = 0.0052). Treatment with CaMKK2 inhibitor STO-609 reduced basal Pit-1 (ctrl: 1.76 ± 0.09; STO-609: 1.25 ± 0.12; p = 0.0157) and GH levels (ctrl: 1.18 ± 0.10; STO-609: 0.50 ± 0.04; p = 0.0006). Ghrelin receptor activation by anamorelin (ANA) increased Pit-1 (ctrl: 0.83 ± 0.8; ANA: 1.59 ± 0.28; p = 0.0425) and GH (ctrl: 0.27 ± 0.03; ANA: 0.66 ± 0.16; p = 0.0497) through a CaMKK2-independent pathway but required basal GABA_AR activity for maximal effect. Conclusions: These findings identify GABA as a modulator of somatotroph hormone expression through a CaMKK2/CaMKIV-dependent cascade and reveal a previously unrecognized interplay whereby the basal GABAergic tone promotes Pit-1 expression, thereby positively regulating ghrelin receptor signaling. This study provides new insights on the cellular mechanisms behind GABA-induced GH synthesis, which may reveal new strategies for modulating the somatotropic axis and help contextualize the variety of reported physiological and cognitive effects of GABA supplementation.