Abstract
Transmitter release in neurons is thought to be mediated exclusively by high-voltage-activated (HVA) Ca 2+ channels. However, we now report that, in retinal bipolar cells, low-voltage-activated (LVA) Ca 2+ 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 Ca 2+ channel subunits and functional LVA Ca 2+ currents sensitive to mibefradil. Activation of these currents results in Ca 2+ 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 Ca 2+ channels in bipolar cell transmitter release may contribute to retinal information processing.
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