The sodium-bicarbonate cotransporter NBCn1 (SLC4A7) is an acid-base transporter that normally moves Na⁺and HCO₃⁻into the cell. This membrane protein is sensitive to cellular and systemic pH changes. We examined NBCn1 expression and localization in the brain and its response to chronic metabolic acidosis. Two new NBCn1 antibodies were generated by immunizing a rabbit and a guinea pig. The antibodies stained neurons in a variety of rat brain regions, including hippocampal pyramidal neurons, dentate gyrus granular neurons, posterior cortical neurons, and cerebellar Purkinje neurons. Choroid plexus epithelia were also stained. Double immunofluorescence labeling showed that NBCn1 and the postsynaptic density protein PSD-95 were found in the same hippocampal CA3 neurons and partially colocalized in dendrites. PSD-95 was pulled down from rat brain lysates with the GST/NBCn1 fusion protein and was also coimmunoprecipitated with NBCn1. Chronic metabolic acidosis was induced by feeding rats with normal chow or 0.4 M HCl-containing chow for 7 days. Real-time PCR and immunoblot showed upregulation of NBCn1 mRNA and protein in the hippocampus of acidotic rats. NBCn1 immunostaining was enhanced in CA3 neurons, posterior cortical neurons, and cerebellar granular cells. Intraperitoneal administration of N-methyl-d-aspartate caused neuronal death determined by caspase-3 activity, and this effect was more severe in acidotic rats. Administering N-methyl-d-aspartate also inhibited NBCn1 upregulation in acidotic rats. We conclude that NBCn1 in neurons is upregulated by chronic acid loads, and this upregulation is associated with glutamate excitotoxicity.

Postmortem studies reveal that the brain pH in schizophrenia patients is lower than normal. The exact cause of this low pH is unclear, but increased lactate levels due to abnormal energy metabolism appear to be involved. Schizophrenia patients display distinct changes in mitochondria number, morphology, and function, and such changes promote anaerobic glycolysis, elevating lactate levels. pH can affect neuronal activity as H⁺ binds to numerous proteins in the nervous system and alters the structure and function of the bound proteins. There is growing evidence of pH change associated with cognition, emotion, and psychotic behaviors. Brain has delicate pH regulatory mechanisms to maintain normal pH in neurons/glia and extracellular fluid, and a change in these mechanisms can affect, or be affected by, neuronal activities associated with schizophrenia. In this review, we discuss the current understanding of the cause and effect of decreased brain pH in schizophrenia based on postmortem human brains, animal models, and cellular studies. The topic includes the factors causing decreased brain pH in schizophrenia, mitochondria dysfunction leading to altered energy metabolism, and pH effects on the pathophysiology of schizophrenia. We also review the acid/base transporters regulating pH in the nervous system and discuss the potential contribution of the major transporters, sodium hydrogen exchangers (NHEs), and sodium-coupled bicarbonate transporters (NCBTs), to schizophrenia.

To understand the mechanism for ion transport through the sodium/bicarbonate transporter SLC4A4 (NBCe1), we examined amino acid residues, within transmembrane domains, that are conserved among electrogenic Na/HCO₃ transporters but are substituted with residues at the corresponding site of all electroneutral Na/HCO₃ transporters. Point mutants were constructed and expressed in Xenopus oocytes to assess function using two-electrode voltage clamp. Among the mutants, D555E (charge-conserved substitution of the aspartate at position 555 with a glutamate) produced decreasing HCO₃⁻ currents at more positive membrane voltages. Immunohistochemistry showed D555E protein expression in oocyte membranes. D555E induced Na/HCO₃-dependent pH recovery from a CO₂-induced acidification. Current-voltage relationships revealed that D555E produced an outwardly rectifying current in the nominally CO₂/HCO₃⁻-free solution that was abolished by Cl⁻ removal from the bath. In the presence of CO₂/HCO₃⁻, however, the outward current produced by D555E decreased only slightly after Cl⁻ removal. Starting from a Cl⁻-free condition, D555E produced dose-dependent outward currents in response to a series of chloride additions. The D555E-mediated chloride current decreased by 70% in the presence of CO₂/HCO₃⁻. The substitution of Asp⁵⁵⁵ with an asparagine also produced a Cl⁻ current. Anion selectivity experiments revealed that D555E was broadly permissive to other anions including NO₃⁻. Fluorescence measurements of chloride transport were done with human embryonic kidney HEK 293 cells expressing NBCe1 and D555E. A marked increase in chloride transport was detected in cells expressing D555E. We conclude that Asp⁵⁵⁵ plays a role in HCO₃⁻ selectivity.

In this study, we examined the effect of bicarbonate transporters on ammonium/ammonia uptake in the medullary thick ascending limb cell line ST‐1. Cells were treated with 1 mm ouabain and 0.2 mm bumetanide to minimize carrier‐mediated NH₄⁺ transport, and the intracellular accumulation of ¹⁴C‐methylammonium/methylammonia (¹⁴C‐MA) was determined. In CO₂/HCO₃⁻‐free solution, cells at normal pH briefly accumulated ¹⁴C‐MA over 7 min and reached a plateau. In CO₂/HCO₃⁻ solution, however, cells markedly accumulated ¹⁴C‐MA over the experimental period of 30 min. This CO₂/HCO₃⁻‐dependent accumulation was reduced by the bicarbonate transporter blocker, 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonate (DIDS; 0.5 mm). Replacing Cl⁻ with gluconate reduced the accumulation, but the reduction was more substantial in the presence of DIDS. Incubation of cells at pH 6.8 (adjusted with NaHCO₃ in 5% CO₂) for 24 h lowered the mean steady‐state intracellular pH to 6.96, significantly lower than 7.28 for control cells. The presence of DIDS reduced ¹⁴C‐MA accumulation in control conditions but had no effect after acidic incubation. Immunoblotting showed that NBCn1 was upregulated after acidic incubation and in NH₄Cl‐containing media. The Cl⁻–HCO₃⁻ exchanger AE2 was present, but its expression remained unaffected by acidic incubation. Expressed in Xenopus oocytes, NBCn1 increased carrier‐mediated ¹⁴C‐MA transport, which was abolished by replacing Na⁺. Two‐electrode voltage clamp of oocytes exhibited negligible current after NH₄Cl application. These results suggest that DIDS‐sensitive HCO₃⁻ extrusion normally governs NH₄⁺/NH₃ uptake in the medullary thick ascending limb cells. We propose that, in acidic conditions, DIDS‐sensitive HCO₃⁻ extrusion is inactivated, while NBCn1 is upregulated to stimulate NH₄⁺ transport.