Ammonium influx pathways into astrocytes and neurones of hippocampal slices

J. Neurochem. (2010) 115, 1123–1136.AbstractAmmonium (NH4+) is required to maintain pathways involved in shuttling metabolic precursors between astrocytes and neurones. Under hyperammonaemic conditions, increases in the cellular influx of NH4+, and accompanying changes in ion concentrations, may contribute to disruptions in metabolism and neurotransmission. We investigated mechanisms of cellular NH4+ influx in hippocampal slices by measuring acute NH4+/NH3‐evoked changes in intracellular pH (pHi) and sodium ([Na+]i). In both astrocytes and neurones, application of 5 mM NH4Cl for 30–45 min decreased pHi by 0.2–0.3 units, consistent with NH4+ influx. In astrocytes, but not neurones, acidifications were accompanied by [Na+]i increases of 25–30 mM. Glial [Na+]i increases were blocked by bumetanide, suggesting that NH4+/NH3 activated Na+‐dependent, K+, Cl− cotransport. Bumetanide also reduced NH4+/NH3‐evoked acidifications in astrocytes. Neuronal acidifications were insensitive to bumetanide and inhibition of Cl−‐dependent transport and K+ channels, but were prevented by inhibition of Na+,K+‐ATPase with ouabain. Furthermore, ouabain reduced astrocyte acidifications. Our results suggest that following rapid elevation of NH4+, Na+,K+‐ATPase is the major influx pathway for NH4+ in neurones, whereas Na+,K+‐ATPase and Na+‐dependent, K+, Cl− cotransport mediate NH4+ transport into astrocytes. The different mechanisms of NH4+ influx in astrocytes and neurones may contribute to the different susceptibility of both cell types to acute hyperammonaemic conditions.