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J. Biol. Chem., Vol. 280, Issue 18, 17823-17830, May 6, 2005

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Molecular and Functional Characterization of the Electroneutral Na/HCO₃ Cotransporter NBCn1 in Rat Hippocampal Neurons^*

Deborah S. Cooper, Nina C. Saxena, Han Soo Yang, Hye Jeong Lee, Anisha G. Moring¶, Amy Lee¶, and Inyeong Choi||

From the Department of Physiology, ^¶Department of Pharmacology, and the Center for Neurodegenerative Disease, Emory University School of Medicine, Atlanta, Georgia 30322

We examined molecular and electrophysiological properties of the electroneutral sodium/bicarbonate cotransporter (NBCn1) that is present in rat hippocampal neurons. By PCR, a deletion variant (NBCn1-E) that lacks 123 amino acids in the cytoplasmic N-terminal domain was found in adult neurons. The previously characterized NBCn1-B, which does not have the deletion, was detected in embryonic neurons. In Xenopus oocytes, NBCn1-E raised the intracellular pH in the presence of HCO₃ without significantly affecting the membrane potential. Despite this electroneutral cotransport activity, the transporter mediated a steady-state current that positively shifted the resting potential by almost 30 mV. The mean reversal potential of the steady-state current was –21.2 mV, close to the resting potential of –21.4 mV. The reversal potential shifted 26 mV in response to a 10-fold increase of external Na⁺ for concentrations above 10 mM. The current activity mediated by the transporter was unaffected by K⁺, Mg²⁺, Ca²⁺, or Cl^–. Stable expression of NBCn1-E in human embryonic kidney cells also evoked an inward current that shifted the resting potentials more positive compared with the sham-transfected controls. In primary cultures of embryonic hippocampal neurons, the NBCn1 protein was localized in somatodendrites and synapses. NBCn1 protein was partially colocalized with the postsynaptic density protein PSD-95. Single-cell PCR showed that NBCn1 mRNA expression was present in both -aminobutyric acid (GABA)ergic and non-GABAergic neurons. We propose that NBCn1 in hippocampal neurons may affect neuronal activity by regulating local pH as well as steady-state inward currents at synapses.

Received for publication, July 29, 2004 , and in revised form, February 17, 2005.

^* This work was supported by National Institutes of Health Grant NS044922 and the Whitehall Foundation (to A. L.) and by the American Heart Association and the University Research Committee Grant at Emory (to I. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^|| To whom correspondence should be addressed: Dept. of Physiology, Emory University, 615 Michael St., Atlanta, GA 30322. Tel.: 404-712-2092; Fax: 404-727-2648; E-mail: ichoi{at}physio.emory.edu.

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