Neurotrophin-4/5 promotes dendritic outgrowth and calcium currents in cultured mesencephalic dopamine neurons

Ca(2+) currents and their modulation by neurotrophin-4/5 were studied in cultured mesencephalic neurons. Tyrosine hydroxylase-positive neurons consistently had larger somas than tyrosine hydroxylase-negative neurons. Neurons with larger somas were therefore targeted for recording. In both control and neurotrophin-4/5-treated cultured neurons, isolation of Ca(2+) currents in cultured mesencephalic neurons revealed prominent low- and high-voltage-activated currents. These currents were separable based upon their voltage dependence of activation, the response to replacement of Ca(2+) with Ba(2+) and the response to Ca(2+) channel blockers. Replacement of Ca(2+) with Ba(2+) resulted in a slight reduction of low-voltage-activated currents and a significant enhancement of high-voltage-activated currents. Cd(2+) blocked a larger fraction of the high-voltage-activated current than Ni(2+). The synthetic conotoxins SNX-124 and SNX-230 selectively blocked high-voltage-activated currents. Morphological analysis of mesencephalic cultures pretreated with neurotrophin-4/5 revealed an increase in soma size and dendritic length in tyrosine hydroxylase-positive neurons. In agreement with the neurotrophin-4/5 induction of growth, neurotrophin-4/5 also increased cell capacitance in whole-cell recordings. Neurotrophin-4/5 significantly enhanced both low- and high-voltage-activated currents, but normalization for changes in capacitance revealed only a significant increase in high-voltage-activated current density.This study demonstrates the existence of low-voltage-activated and multiple classes of high-voltage-activated calcium currents in cultured mesencephalic neurons. Morphological and physiological data demonstrate that the increases in calcium currents due to neurotrophin-4/5 pretreatment are associated with somatodendritic growth, but an increase in high-voltage-activated Ca(2+) channel expression also occurred.