T-type Ca2+ channels mediate neurotransmitter release in retinal bipolar cells

Transmitter release in neurons is thought to be mediated exclusively by high-voltage-activated (HVA) Ca2+ channels. However, we now report that, in retinal bipolar cells, low-voltage-activated (LVA) Ca2+ channels also mediate neurotransmitter release. Bipolar cells are specialized neurons that release neurotransmitter in response to graded depolarizations. Here we show that these cells express T-type Ca2+ channel subunits and functional LVA Call currents sensitive to mibefradil. Activation of these currents results in Call influx into presynaptic terminals and exocytosis, which we detected as a capacitance increase in isolated terminals and the appearance of reciprocal currents in retinal slices. The involvement of T-type Call channels in bipolar cell transmitter release may contribute to retinal information processing.