Amato, Alessandra; (1994) Ion concentration changes in neurons and glia associated with transmitter uptake and cell growth. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Neural activity is often accompanied by intra- and extracellular ion concentration changes which modulate nerve and glial cells' function. In this study measurement of such concentration changes were used to investigate the properties of the glutamate uptake carrier and the role of calcium in controlling neuronal growth. Glutamate uptake in the nervous system helps to terminate the action of the neurotransmitter glutamate and to keep its extracellular concentration below neurotoxic levels. In amphibian Muller cells the uptake carrier co-transports at least two sodium ions into the cell with each glutamate anion while there is controversy over whether it counter-transports potassium and pH-changmg ions. Efflux of potassium ions on the glutamate transporter was demonstrated directly using K+-sensitive microelectrodes outside whole-cell clamped Muller cells. Activation of glutamate uptake in Muller cells induced an intracellular acidification and an extracellular alkalinization. With ClO4- inside the cell, the uptake current was increased, but the uptake-induced extracellular alkalinization was decreased, and uptake was associated with a rise of ClO4- outside the cell. This data suggest that the uptake carrier transports OH- (or HCO3-) out of the cell, and that ClO4- can compete for transport on this site. Intracellular acidification attributable to uptake of glutamate was also studied m rat hippocampal slices. The spatial distribution of [Ca++]i was measured in growth cones of cultured rat sensory neurons, using imaging fluorescence microscopy, to look for a correlation between growth cone motility and intracellular calcium concentration. Depolarization of the cell membrane induced a rise in [Ca++]i around the edge of the growth cones. At the resting membrane potential, no [Ca++]i gradients were observed, and the [Ca++]i was similar m advancing and in motile-but-non-growing growth cones. However, [Ca++]i was significantly higher in a retracting growth cone. Thus, calcium plays a minor role in controlling neurite outgrowth in cultured rat sensory neurons.

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