Two different responses of hippocampal pyramidal cells to application of gamma‐amino butyric acid.

1. Extra‐ and intracellular recordings were made from CA1 cells in hippocampal slices in vitro. The effects of ionophoretically applied GABA on somatic and dendritic regions were studied. 2. Ionophoresis of GABA at dendritic sites gave a reciprocal effect by inhibiting the effect of excitatory synapses close to the dendritic application, while facilitating those lying further away. For example, GABA delivered to the mid‐radiatum dendritic region reduced the population spike generated by a radiatum volley, while facilitating the population spike evoked by oriens fibre stimulation. Similarly, when single cells were recorded from, mid‐apical dendritic delivery of GABA abolished the synaptically driven discharges evoked by fibres terminating at this part of the dendritic tree, but facilitated the responses to input from fibres terminating on the basal dendrites of the same cell. 3. With intracellular recording two effects were observed. Applied near the soma, GABA induced a hyperpolarization associated with an increased membrane conductance. When applied to dendrites, GABA caused a depolarization also associated with an increased membrane conductance. Both types of GABA applications could inhibit cell discharges, although in some cases the depolarizing response could facilitate other excitatory influences or cause cell firing by itself. 4. Both the hyperpolarizing and depolarizing GABA responses persisted after blockade of synaptic transmission by applying a low calcium high magnesium solution, indicating mediation via a direct effect upon the cell membrane. 5. The reversal potential for the hyperpolarizing GABA effect was similar to the equilibrium potential for the i.p.s.p. evoked from alveus or orthodromically, and was 10‐12 mV more negative than the resting potential. The size of the depolarizing response was also dependent upon the membrane potential. By extrapolation an estimated equilibrium potential was calculated as about ‐40 mV. 6. Our results support the idea that the hyperpolarizing basket cell inhibition at the soma is mediated by the release of GABA. This hyperpolarizing response causes a general inhibition of firing. The dendritic effects of GABA, however, seem to represent another type of inhibition, which by shunting synaptic currents makes possible a selective inhibitory influence on afferents synapsing locally while facilitating more remotely placed excitatory synapses. We propose the term discriminative inhibition for this postulated new type of control of pyramidal cell discharges.