Modulation of spontaneous intracellular Ca²⁺ fluctuations and spontaneous cholinergic transmission in rat chromaffin cells in situ by endogenous GABA acting on GABAA receptors.
Using fluorescence [Ca(2+)]i imaging in rat adrenal slices, we characterized the effects of agonists and antagonists of the GABAA receptor (GABAA-R) on resting intracellular Ca(2+) ([Ca(2+)]i) and spontaneous [Ca(2+)]i fluctuations (SCFs) in hundreds of individual chromaffin cells (CCs) recorded simultaneously in situ. Muscimol, a GABAA-R agonist (20 μM; 25 s), induced an increase of resting [Ca(2+)]i in 43 ± 3 % of CCs, a decrease in 26 ± 2 %, and no response in 30 ± 5 %. In Ca(2+)-free external medium, SCFs ceased completely and muscimol failed to elicit [Ca(2+)]i rises. All muscimol-induced [Ca(2+)]i changes were blocked by the GABAA-R antagonist bicuculline, suggesting that they result from changes in membrane potential depending on the cell's Cl(-) 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 GABAA-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 GABAA-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.
Departamento Neurociencia Cognitiva, Instituto de Fisiología Celular, UNAM. Circuito de la Investigación Científica s/n, Ciudad Universitaria, México, D.F., C.P. 04510, México.
Alejandre-García
AG
Tzitzitlini
Laboratorio Nacional de Canalopatías from Instituto de Fisiología Celular, México, México.|Unidad de Investigación en Medicina Experimental, México, México.
Segura-Chama
SC
Pedro
Departamento de Biología Celular y Tisular, from Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F., C.P. 04510, Mexico.
Pérez-Armendáriz E
PA
Martha
Departamento de Fisiología Biofísica y Neurociencias, from Centro de Investigación y Estudios Avanzados del IPN, Ave. IPN 2508, México City, D.F., México.
Delgado-Lezama
DL
Rodolfo
Departamento Neurociencia Cognitiva, Instituto de Fisiología Celular, UNAM. Circuito de la Investigación Científica s/n, Ciudad Universitaria, México, D.F., C.P. 04510, México. ahernan@ifc.unam.mx.|Laboratorio Nacional de Canalopatías from Instituto de Fisiología Celular, México, México. ahernan@ifc.unam.mx.
Hernández-Cruz
HC
Arturo
D000313:Adrenal Medulla / Q000166:cytology / Q000378:metabolism; D000818:Animals; D020013:Calcium Signaling; D018680:Cholinergic Antagonists / Q000494:pharmacology*; D019439:Chromaffin Cells / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D018756:GABA Antagonists / Q000494:pharmacology; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000378:metabolism*; D055366:Synaptic Potentials; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D018680:Cholinergic Antagonists; D018756:GABA Antagonists; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid
Adrenal medulla gland; Chromaffin cell; Endogenous GABA; GABAA receptors; Intracellular calcium