Voltage-dependent Na+ channels are usually expressed in neurons that use spikes as a means of signal coding. Retinal bipolar cells are commonly thought to be nonspiking neurons, a category of neurons in the CNS that uses graded potential for signal transmission. Here we report for the first time voltage-dependent Na+ currents in acutely isolated mammalian retinal bipolar cells with whole cell patch-clamp recordings. Na+ currents were observed in ~45% of recorded cone bipolar cells but not in rod bipolar cells. Both ON and OFF cone bipolar cells were found to express Na+ channels. The Na+ currents were activated at membrane potentials around -50 to -40 mV and reached their peak around -20 to 0 mV. The half-maximal activation and steady-state inactivation potentials were -24.7 and -68.0 mV, respectively. The time course of recovery from inactivation could be fitted by two time constants of 6.2 and 81 ms. The amplitude of the Na+ currents ranged from a few to >300 pA with the current density in some cells close or comparable to that of retinal third neurons. In current-clamp recordings, Na+-dependent action potentials were evoked in Na+-current-bearing bipolar cells by current injections. These findings raise the possibility that voltage-dependent Na+ currents may play a role in bipolar cell function.
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