Microelectrophoretic studies concerning the spread of glutamic acid and GABA in brain tissue

The spread of microelectrophoretically applied substances was investigated in the cortex and in the caudate nucleus by means of double-multi-barrelled electrodes with tip separations varying from 12–300 μ by finding the mean distance between neurone and electrode was found to be about 20 μ when neurones with a satisfactory spike/noise ratio were recorded.

Abstract

The spread of microelectrophoretically applied substances was investigated in the cortex and in the caudate nucleus by means of double-multi-barrelled electrodes with tip separations varying from 12–300 μ. Spike activity induced in non-spontaneously firing neurones by application of glutamate and inhibition of spontaneously firing neurones by GABA were interpreted as an effect of the substances diffusing into the immediate neighbourhood of the neurone. This interpretation seems to be acceptable, since in only a small number of tests could an indication for trans-neuronally induced firing be found. The data obtained from dosage-response-curves, when adequately corrected, correspond to curves deduced from the diffusion equation for a diffusion coefficient of about 1.0×10−5 (cm2/sec). The mean threshold dosage for activation of spike activity by glutamate was found to be 0.25 mM. When glutamate was applied from the remote electrode the threshold concentration was achieved with comparatively lower dosages. This discrepancy is interpreted in terms of different areas of distribution. The mean distance between neurone and electrode was found to be about 20 μ when neurones with a satisfactory spike/noise ratio were recorded. This field was found to often be smaller than that occupied by the substances, even at low dosages.