Ammonium influx pathways into astrocytes and neurones of hippocampal slices

Abstract

Ammonium (NH(4) (+) ) is required to maintain pathways involved in shuttling metabolic precursors between astrocytes and neurones. Under hyperammonaemic conditions, increases in the cellular influx of NH(4) (+) , and accompanying changes in ion concentrations, may contribute to disruptions in metabolism and neurotransmission. We investigated mechanisms of cellular NH(4) (+) influx in hippocampal slices by measuring acute NH(4) (+) /NH(3) -evoked changes in intracellular pH (pH(i) ) and sodium ([Na(+) ](i) ). In both astrocytes and neurones, application of 5 mM NH(4) Cl for 30-45 min decreased pH(i) by 0.2-0.3 units, consistent with NH(4) (+) 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 NH(4) (+) /NH(3) activated Na(+) -dependent, K(+) , Cl(-) cotransport. Bumetanide also reduced NH(4) (+) /NH(3) -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 NH(4) (+) , Na(+) ,K(+) -ATPase is the major influx pathway for NH(4) (+) in neurones, whereas Na(+) ,K(+) -ATPase and Na(+) -dependent, K(+) , Cl(-) cotransport mediate NH(4) (+) transport into astrocytes. The different mechanisms of NH(4) (+) influx in astrocytes and neurones may contribute to the different susceptibility of both cell types to acute hyperammonaemic conditions.