Developmental switch in excitability, Ca2+ and K+ currents of retinal ganglion cells and their dendritic structure

In the retina of teleost fish, continuous neuronal development occurs at th e margin, in the peripheral growth zone (PGZ). We prepared tissue slices fr om the retina of rainbow trout that include the PGZ and that comprise a tim e line of retinal development, in which cells at progressive stages of diff erentiation are present side by side. We studied the changes in dendritic s tructure and voltage-dependent Ca2+, Na+, and K+ currents that occur as gan glion cells mature. The youngest ganglion cells form a distinct bulge. Cell s in the bulge have spare and short dendritic trees. Only half express Ca2 currents and then only high-voltage-activated currents with slow inactivat ion (HVAslow). Bulge cells are rarely electrically excitable. They express a mixture of rapidly inactivating and noninactivating K+ currents (IKA and IKdr). The ganglion cells next organize into a transition zone, consisting of a layered structure two to three nuclei thick, before forming the single layered structure characteristic of the mature retina. In the transition z one, the dendritic arbor is elaborately branched and extends over multiple laminae in the inner plexiform layer, without apparent stratification. The arbor of the mature cells is stratified, and the span of the dendritic arbo r is well over five times the cell body's diameter. The electrical properti es of cells in the transition and mature zones differ significantly from th ose in the bulge cells. Correlated with the more elaborate dendritic struct ures are the expression of both rapidly inactivating HVA (HVAfast) and of l ow-voltage-activated (LVA) Ca2+ currents and of a high density of Na+ curre nts that renders the cells electrically excitable. The older ganglion cells also express a slowly activating K+ current (IKsa).