Figure 6. Propagation of GABA-induced hyperpolarization at the axon regulates AP generation.

A, DAB staining of recorded neurons. Simultaneous recording from the soma and axon bleb were performed in a pyramidal neuron (left), and GABA was applied to the axon trunk (right). The axon length was 239 µm in this case. The distance between the iontophoresis site and the soma was 117 µm. Scale bar: 100 µm (left); 50 µm (right). B, The sign of the effect of GABA (hyperpolarization or depolarization) depended on the V _m. Top, traces were taken from the bleb. Bottom, traces were the corresponding responses at the soma. The V_m was clamped through somatic DC current injection. Asterisk indicates application of GABA to the main trunk. C, Left, application of GABA to the axon increased the amplitude but decreased the half-width of propagating APs. GABA iontophoresis hyperpolarized the V _m by 2.3±0.4 mV (n = 7). Right, similar results were obtained when V _m was hyperpolarized by 2.8±0.3 mV (n = 5) through DC current injection. *, P<0.05; **, P<0.01, paired t-test. D, Example traces showing activation of axonal GABA_A receptors reduced firing probability and frequency. The distances between the iontophoresis site and the soma were 100 µm (distal axon) and 18 µm (AIS). E, Left, repetitive firing recorded at an axon bleb induced by 400 pA DC current injection at the soma before (black) and after (red) GABA application to the axon trunk. The arrow indicates GABA iontophoresis. Middle, instantaneous firing frequency of APs decreased after GABA application (same data as shown in the left). Right, group data showing a decrease in the mean frequency of APs after GABA iontophoresis at the axon trunk. **, P<0.01, paired t-test. Low-Cl⁻ ICS was used in these experiments.