Developmental maturation of passive electrical properties in retinal ganglion cells of rainbow trout.

We investigated the electrotonic and anatomical features of the dendritic arbor in developing retinal ganglion cells (RGCs). Cell anatomy was studied by filling individual cells with fluorescent, membrane-bound dyes and using computer-assisted image reconstruction. Electrotonic properties were characterized through an analysis of charging membrane currents measured with tight-seal electrodes in the whole-cell mode. We studied developing RGCs in the peripheral growth zone (PGZ) of a fish retina. The PGZ presents a developmental time-line ranging from pluripotent, proliferating cells at the extreme edge, to mature, fully developed retina more centrally. In the PGZ, RGCs mature through three histologically distinct zones (in developmental sequence): bulge, transition and mature zones. In the most peripheral three-quarters of the bulge zone, cells have rounded somas, lack dendritic extensions and some are coupled so that membrane-bound dyes traverse from one cell to its immediate neighbours. In the more central quarter of the bulge, cells' dendrites are few, short and of limited branching. In the transition zone dendritic arbors becomes progressively more expansive and branched and we present a morphometric analysis of these changes. Regardless of the size and branching pattern of the developing RGC dendritic arbor, the ratio of the diameters of parent and progeny dendrites at any branching nodes is well described by Rall's 3/2 power law. Given this anatomical feature, the RGC passive electrical properties are well described by an equivalent electrical circuit consisting of an isopotential cell body in parallel with a single equivalent cylinder of finite length. We measured the values of the electrical parameters that define this equivalent circuit in bulge, transition and mature RGCs. As RGCs develop the electrical properties of their dendritic arbor change in an orderly and tightly regulated manner, not randomly. Electrically, dendritic arbors develop along either of two distinct modes, but only these modes: isoelectrotonic and isometric. In isoelectrotonic growth, electrotonic properties are constant regardless of the absolute dimensions of the dendritic arbor or its branching geometry. These cells maintain unvarying relative synaptic efficacy independently of the size or pattern of their dendritic arbor. In isometric growth, in contrast, electronic properties change, but the ratio of the changing electrotonic length to electrotonic diameter is constant. In these cells relative synaptic efficacy decreases linearly as dendrites extend.