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Neurochemical and electrophysiological studies on the inhibitory effect of ammonium ions on synaptic transmission in slices of rat hippocampus: Evidence for a postsynaptic action

1990; Elsevier BV; Volume: 37; Issue: 2 Linguagem: Inglês

10.1016/0306-4522(90)90403-q

1873-7544

Pengcheng Fan, Jean‐Claude Lavoie, N. L. O. Lê, John C. Szerb, Roger F. Butterworth,

Ion channel regulation and function

Abstract To elucidate the mechanisms involved in the inhibition of synaptic transmission by ammonium ions, the effects of NH 4 Cl on glutamate release and on synaptic transmission from Schaffer collaterals to CA1 pyramidal cells were measured in fully submerged slices of rat hippocampus. The large, Ca 2+ -dependent release of glutamate evoked by electrical-field stimulation or by 56 mM K + was not reduced by 5 mM NH 4 Cl. In contrast, 5 mM NH 4 Cl decreased the smaller, field stimulation-induced release of glutamate observed in the presence of low concentrations of Ca 2+ (0.1 mM), as well as the spontaneous release of glutamate both in normal and low Ca 2+ . Unlike the Ca 2+ -dependent release of glutamate, synaptic transmission was reversibly depressed even by 1 mM NH 4 Cl. Firing of CA1 pyramidal cells evoked by iontophoretically applied glutamate was significantly inhibited by 2 or 5 mM NH 4 Cl. This depression was increased in the presence of 25 μ M bicuculline. Results suggest that ammonium ions do not depress the Ca 2+ -dependent release of glutamate originating from synaptic vesicles, which is involved in synaptic transmission. Rather, ammonium ions inhibit synaptic transmission by a postsynaptic action, a conclusion strengthened by the inhibitory effect of NH 4 Cl on glutamate-induced firing. However, NH 4 Cl may inhibit the formation of cytoplasmic glutamate, the source of spontaneous and Ca 2+ -independent release.