Modulation of spontaneous intracellular Ca^sup 2+^ fluctuations and spontaneous cholinergic transmission in rat chromaffin cells in situ by endogenous GABA acting on GABA^sub A^ receptors

• Published in: Pflügers Archiv 2016-02, Vol.468 (2), p.351
• Main Authors: Tzitzitlini, Alejandre-garcía, Pedro, Segura-chama, Martha, Pérez-armendáriz E, Rodolfo, Delgado-lezama, Arturo, Hernández-cruz
• Format: Article
• Language: English
• ISSN: 0031-6768; 1432-2013
• DOI: 10.1007/s00424-015-1744-y

Using fluorescence [Ca^sup 2+^]^sub i^ imaging in rat adrenal slices, we characterized the effects of agonists and antagonists of the GABA^sub A^ receptor (GABA^sub A^-R) on resting intracellular Ca^sup 2+^ ([Ca^sup 2+^]^sub i^) and spontaneous [Ca^sup 2+^]^sub i^ fluctuations (SCFs) in hundreds of individual chromaffin cells (CCs) recorded simultaneously in situ. Muscimol, a GABA^sub A^-R agonist (20 [mu]M; 25 s), induced an increase of resting [Ca^sup 2+^]^sub i^ in 43±3 % of CCs, a decrease in 26±2 %, and no response in 30±5 %. In Ca^sup 2+^-free external medium, SCFs ceased completely and muscimol failed to elicit [Ca^sup 2+^]^sub i^ rises. All muscimol-induced [Ca^sup 2+^]^sub i^ changes were blocked by the GABA^sub A^-R antagonist bicuculline, suggesting that they result from changes in membrane potential depending on the cell's Cl^sup -^ equilibrium potential. Unexpectedly, bicuculline increased the amplitude and frequency of SCFs in 54 % of CCs, revealing a tonic inhibition of SCFs by ambient GABA acting through GABA^sub A^-R. Mecamylamine (a specific nicotinic cholinergic blocker) decreased basal SCF activity in 18 % of CCs and inhibited bicuculline-induced SCF intensification, suggesting that spontaneous acetylcholine (ACh) release from nerve endings contributes to SCF generation in CCs in situ and that blockade of presynaptic GABA^sub A^-Rs intensifies SCFs in part through the disinhibition of spontaneous cholinergic transmission. Electrophysiological experiments confirmed that spontaneous excitatory postsynaptic currents recorded from CCs in situ were enhanced by bicuculline. To our knowledge, this is the first description of a regulatory effect of endogenous GABA on synaptic currents and SCFs of adrenal CCs. These findings denote a novel GABA-mediated presynaptic and postsynaptic regulatory mechanism of CC activity which may participate in the control of catecholamine secretion.