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1 Department of Anesthesia and Critical Care, The University of Chicago, Chicago, Illinois 60637; 3 Departments of Neurobiology, Pharmacology and Physiology, The University of Chicago, Chicago, Illinois 60637; 2 Department of Anesthesiology and Pharmacology, Vanderbilt University, Nashville, Tennessee 37232-2520
Submitted 27 June 2003; accepted in final form 25 August 2003
GABA is the primary inhibitory neurotransmitter in the adult mammalian brain. However, in neonatal animals, activation of Cl–-permeable GABA receptors is excitatory and appears to depend on the expression of a Na+-K+-2Cl– cotransporter (NKCC) that elevates intracellular Cl– levels, leading to a depolarized Cl– equilibrium potential (ECl). The change from excitation to inhibition appears to involve the expression of the K+/Cl– co-transporter, KCC2, which lowers intracellular Cl– levels resulting in a hyperpolarized ECl. In this study, we show that bovine chromaffin cells from 4- to 5-mo-old animals are excited by GABA. Activation of GABAA receptors depolarizes the cells, opens voltage-dependent Ca2+ channels, elevates [Ca2+]i, and promotes the release of catecholamines. Blockade of voltage-dependent Ca2+ channels prevents the elevation of [Ca2+]i by GABA. The extrapolated anion reversal potential in these cells is approximately –28 mV, indicating a resting intracellular anion concentration of approximately 50 mM. Expression of KCC2 protein was not detected in the juvenile chromaffin cells. In contrast, clear expression of NKCC1 was observed. Blockade of NKCC1 should reduce the intracellular Cl– concentration and hyperpolarize ECl. Bumetanide, an NKCC1 blocker, reduced the elevation of [Ca2+]i by GABA. In some cells, activation of GABAA receptors inhibits responses to excitatory neurotransmitters, even though GABA itself is depolarizing. Co-activation of cholinergic and GABAA receptors in chromaffin cells produced elevations in [Ca2+]i that were comparable to those produced by cholinergic receptors alone. Our data showing the selective expression of chloride co-transporters and the resulting strongly depolarized anion reversal potential may help explain how activation of GABAA receptors causes sufficient excitation to elicit catecholamine release from chromaffin cells.
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