Ammonium influx pathways into astrocytes and neurones of hippocampal slices

Ammonium (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 (pH(i)) and sodium ([Na+](i)). In both astrocytes and neurones, application of 5 mM NH4Cl for 30-45 min decreased pH(i) 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.