Differential expression of voltage-activated calcium currents in zebrafish retinal ganglion cells

We report a study on the characterization of voltage-activated calcium currents (I(Ca)) in retinal ganglion cells (RGCs) and the topographic distribution of RGCs that express different types of I(Ca) in zebrafish retinas. In acutely isolated zebrafish RGCs, both high-voltage-activated (HVA; peak activation potential +7.4 +/- 1.1 mV) and low-voltage-activated (LVA; peak activation potential -33.0 +/- 1.2 mV) I(Ca) were recorded. HVA I(Ca) were recorded in all of the tested RGCs, whereas LVA I(Ca) were recorded in approximately one-third of the tested cells. In RGCs that expressed both HVA and LVA I(Ca), the two currents were readily separated by depolarizing the cell membrane to different voltages from different holding potentials. Among RGCs that expressed LVA I(Ca), some cells expressed large LVA I(Ca) (up to 130 pA), whereas others expressed small LVA I(Ca) (approximately 20 pA). RGCs that expressed large and small LVA I(Ca) were designated as class I and class II cells, respectively, and RGCs that expressed only HVA I(Ca) were designated as class III cells. The topographic distribution of cell classes was similar in various areas of the retina. In the nasal-ventral retina, for example, class III cells outnumbered class I and class II cells by 10.8- and 2.6-fold, respectively. In the temporal and dorsal retinas, the density of class III cells slightly decreased, whereas the density of class I and class II cells increased. The differential expression of I(Ca) in RGCs may correlate with the development and function of the retina.