Ca _v2 channels mediate low and high voltage-activated calcium currents in Drosophila motoneurons

Different blends of membrane currents underlie distinct functions of neurons in the brain. A major step towards understanding neuronal function, therefore, is to identify the genes that encode different ionic currents. This study combined in situ patch clamp recordings of somatodendritic calcium currents in an identified adult Drosophila motoneuron with targeted genetic manipulation. Voltage clamp recordings revealed transient low voltage-activated (LVA) currents with activation between -60 mV and -70 mV as well as high voltage-activated (HVA) current with an activation voltage around -30 mV. LVA could be fully inactivated by prepulses to -50 mV and was partially amiloride sensitive. Recordings from newly generated mutant flies demonstrated that DmαG (Ca _v3 homolog) encoded the amiloride-sensitive portion of the transient LVA calcium current. We further demonstrated that the Ca _v2 homolog, Dmca1A, mediated the amiloride-insensitive component of LVA current. This novel role of Ca _v2 channels was substantiated by patch clamp recordings from conditional mutants, RNAi knock-downs, and following Dmca1A overexpression. In addition, we show that Dmca1A underlies the HVA somatodendritic calcium currents in vivo. Therefore, the Drosophila Ca _v2 homolog, Dmca1A, underlies HVA and LVA somatodendritic calcium currents in the same neuron. Interestingly, DmαG is required for regulating LVA and HVA derived from Dmca1A in vivo. In summary, each vertebrate gene family for voltage-gated calcium channels is represented by a single gene in Drosophila, namely Dmca1D (Ca _v1), Dmca1A (Ca _v2) and DmαG (Ca _v3), but the commonly held view that LVA calcium currents are usually mediated by Ca _v3 rather than Ca _v2 channels may require reconsideration.