Effects of micro‐iontophoretic administration of magnesium and calcium on neurones in the central nervous system of cats

Ca2+ and Mg2+ released from five‐barrelled micropipettes by currents of 5 to 80 nA caused a deceleration of the firing rate of neurones discharging spontaneously, or excited by the simulataneous administration of glutamate or acetylcholine, in the cerebral cortex, cuneate nucleus and spinal cord. Excitation of cells in the cuneate nucleus, caused by stimulation of sense organs, was also suppressed by Ca2+ and Mg2+. Synaptic transmission of volleys, evoked by electrical stimulation, from thalamus to cortical cells, from superficial radial nerve to cells in the cuneate nucleus, and from ventral root via recurrent collaterals to Renshaw cells, was partially or completely blocked by Ca2+ and Mg2+ (60 to 200 nA). When given together, the depressant effect of the two ions summed. There was no evidence of enhanced synpatic transmission under the influence of Ca2+. It is concluded that in the central nervous system a small elevation of the concentration of Ca2+ or of Mg2+ has a powerful postsynaptic depressant effect.