The presence of GABA_A (but likely not GABA_B) receptors in the axon.

<p>A, Reversal potential of GABA responses (I_GABA) in the axon bleb. Left, representative currents induced by GABA application at different holding potentials (from –100 to –40 mV). At –60 mV (near reversal potential), GABA application induced no obvious change in baseline current (gray). Right, I-V curve of the GABA-induced responses shown on the left. B, I_GABA could be blocked by GABA_A receptor blocker PTX. Left, example traces before (black), during (gray) and after (Wash, dashed line) the bath application of PTX (25 µM). V_hold = –50 mV, GABA was applied via iontophoresis. Middle, time course of the effect of PTX. Right, group data showing the change of I_GABA during (n = 6) and after (n = 3) PTX application. The dashed line indicates 100% of control. C, Left, currents evoked by puffing baclofen (200 µM), a GABA_B receptor agonist, to the soma (16 psi, 15 ms). Right, no response was observed when baclofen was applied to the axon trunk (16 psi, 20 ms). D, Group data showing that GABA-induced currents at the axon blebs could not be blocked by the GABA_B receptor antagonist CGP 35348 (100 µM); however, PTX could diminish these responses. Different symbols indicate different cells.