Figure 3.
Current-clamp analysis of Na channel activity in AII amacrines. A, Positive current injection elicits membrane depolarization and small spikelets. Note that spontaneous spikelets are observed, at a lower frequency, at VREST (−47 mV) in the absence of injected current. Both evoked and spontaneous spikelets are blocked by TTX (500 nm). B, When a slowly varying current ramp is injected into the AII (−75 to 75 pA in 2 s), spikelets are evident at potentials near −60 mV (black). At potentials depolarized to VREST, spikelets are not seen, likely because of accumulated inactivation of Na channels, but membrane noise is increased. Spikelets and increased membrane noise are not observed in the presence of TTX. C, Deviations in VM around the baseline are plotted against membrane potential to illustrate the voltage range in which spikelets are generated, and membrane noise is increased by the TTX-sensitive Na conductance. D, TTX hyperpolarizes VM from −50 ± 1 to −53 ± 1 mV (n = 27), indicating that some GNa is open at rest. Gray and orange markers are observations from individual cells; black bars are means ± SEM. E, TTX increased τM at a potential depolarized to VREST (−45 mV) but not when VM was −80 mV, below the activation threshold of GNa (n = 9).