Distribution of GABA sensitive areas on hippocampal pyramidal cells

In isolated transverse hippocampal slices GABA (gamma-amino-butyric acid) was applied iontophoretically to various parts of the dendritic tree of CA1 pyramidal cells. Indices of GABA effects were reduced amplitude of orthodromically driven CA1 population spikes and inhibition of single CA1 units driven by orthodromic stimulation or by application of L-glutamate. Weak iontophoretic currents of GABA (3-6 nA, backing current -3 nA) effectively reduced the amplitude of the population spike and arrested unit activity when applied in a position close to the soma. The effect was halved when GABA was applied 25-30 micrometer away at right angles to the main dendritic axis. In the direction of the main dendritic axis, GABA was affective as far as 250 and 150 micrometer from the soma in the apical and basal dendritic directions, respectively, corresponding to about 50% of the total dendritic length. The best effect was usually found at a depth corresponding to that of the recording electrode, probably because the main dendritic axis was parallel to the slice surface. The sharp localization of GABA sensitivity when applied in the pyramidal layer supports earlier evidence that GABA mediates the basket cell inhibition on the soma of the pyramidal hippocampal cells. In the dendritic tree, GABA may also have an inhibitory function with an effectiveness matching that of the soma application.