Figure 3. TTX-sensitive Na⁺ channels are expressed predominantly in rd1 AII amacrine cells whereas I_h is predominantly expressed in rd1 ON cone bipolar cells.

A, voltage-clamp recording from an rd1 AII amacrine cell (a) and ON cone bipolar cell (b). Currents were evoked during voltage steps (–120 mV to +40 mV) from a holding potential of –60 mV. Ac, the frequency of small inward spikelets is plotted as a function of voltage for AII amacrine (AII) and ON cone bipolar cells (BC). Ba and b, in current-clamp mode, injecting steady currents depolarized the membrane (as indicated on the left) and evoked small-amplitude spikelets in control but not in the presence of 1 μm TTX. Bc, the frequency of spikelets in control and in TTX is plotted as a function of injected current. Bd and e, sustained spikelet activity persists in the presence of the gap junction blocker MFA (200 μm) upon steady 40 pA depolarizing current injections. Ca, slowly activating inward currents evoked by hyperpolarizing pulses (−81 mV to −131 mV, 50 ms) from a holding of –71 mV in rd1 ON cone bipolar cells in control (a) and in the presence of 1 mm Cs⁺ (b). Cc, the amplitude of the slowly activating phase is plotted against voltage for the two conditions.