Bipolar cells of the magnocellular pathway exhibit large Na_{v} currents and T-type Ca_{v} currents. *A–C*, Confocal projections showing examples of DB3a (*A*), DB3b (*B*), and DB4 (*C*) cells overlayed on a transmitted light image of the same field. Retinal layers are indicated to the left of the panels. *D–F*, Leak-subtracted currents activated by 100 ms long depolarizing voltage steps from −65 to +15 mV in 5 mV increments. *V*_{h} = −70 mV. Traces are averages from *n* = 20 (DB3a; *D*), *n* = 13 (DB3b; *E*), and *n* = 34 (DB4; *F*). The round symbols show the time points used for the *I–V* relations in the right-hand panels (*J–L*). Note the presence of inward currents at the start of the voltage steps. *G*, Top, Data from *D* showing the inward currents on an expanded timescale (black traces). These currents were completely blocked by application of the Na_{V} channel blocker, TTX (red traces, *n* = 4). Bottom, *V*_{h} = −90 mV; average currents activated by voltage steps from −80 mV to −30 mV in 10 mV increments (black traces, *n* = 10). The inward currents are completely blocked by application of TTX (red traces, *n* = 2). *H*, Top, Data from *E* showing inward currents on an expanded timescale. Bottom, *V*_{h} = −90 mV; average inward currents activated by voltage steps from −80 mV to −30 mV in 10 mV increments (*n* = 10). *I*, Top, Data from *F* showing control inward currents on an expanded timescale (black traces). Inward currents are completely blocked by application of TTX (red traces, *n* = 8). Middle and bottom, *V*_{h} = −90 mV; average currents activated by voltage steps from −80 to −30 mV in 10 mV increments in control (middle, black, *n* = 8), 0.5 μm TTX (middle, red, *n* = 5), mibefradil (MB, bottom, blue, *n* = 3), or mibefradil plus TTX (bottom, green, *n* = 2). Note that the TTX-resistant inward current is blocked with the T-type Ca_{V} channel blocker mibefradil. *J–L*, Maximum inward current amplitudes (±SEM) near the onset of the voltage steps for DB3a (*J*), DB3b (*K*), and DB4 (*L*) cells. The red data points in *J* and *L* were recorded in TTX. The green data points in *L* were recorded in the presence of mibefradil plus TTX. The smooth lines through the data points fit to a Boltzmann activation function. The triangles in *K* and *L* show the maximum inward current recorded from a *V*_{h} = −90 mV. Note that for clarity, only alternate traces from *D* to *F* are displayed in the top panels of *G* to *I*. GCL, Ganglion cell layer. Scale bar: *C* (for *A–C*), 10 μm. Error bars represent ±1 SEM.

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