Modulation of spontaneous intracellular Ca super(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.351-365
• Main Authors: Tzitzitlini, Alejandre-Garcia, Pedro, Segura-Chama, Martha, Perez-Armendariz E, Rodolfo, Delgado-Lezama, Arturo, Hernandez-Cruz
• Format: Article
• Language: English
• ISSN: 0031-6768; 1432-2013
• DOI: 10.1007/s00424-015-1744-y

Using fluorescence [Ca super(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 super(2+) ([Ca super(2+)] sub(i)) and spontaneous [Ca super(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 super(2+)] sub(i) in 43 plus or minus 3 % of CCs, a decrease in 26 plus or minus 2 %, and no response in 30 plus or minus 5 %. In Ca super(2+)-free external medium, SCFs ceased completely and muscimol failed to elicit [Ca super(2+)] sub(i) rises. All muscimol-induced [Ca super(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 super(-) 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.