Assessment of possible transneuronal changes in the retina of rats with inherited retinal dystrophy: Cell size, number, synapses, and axonal transport by retinal ganglion cells

In pigmented RCS rats with inherited retinal dystrophy, most photoreceptor cells disappear between postnatal days 20 and 100. We have examined the time course of the degeneration of photoreceptor nuclei and synapses and determined whether transneuronal changes occur in the inner nuclear layer (INL), inner plexiform layer (IPL), and retinal ganglion cells following loss of photoreceptor cells in these animals. Electron microscopic photomontages of the entire thickness of the IPL of dystrophic (RCS-p+) and control (RCS-rdy+ p+) rats 334 to 515 days old were prepared, and synapses were counted and identified as either conventional (amacrine) or ribbon (bipolar) types. Neither the incidence of synapses in the IPL nor the ratio of conventional to ribbon synapses differed in the dystrophic and control retinas. Ganglion cell diameter, perimeter, area, and density were measured from drawings of wholemount preparations of dystrophic and control rats 105 days and older. Diameter, perimeter, area and number of ganglion cells were not significantly different in the two genotypes. Anterograde axonal transport was measured by studying the displacement of labeled material as it traveled along ganglion cell axons and accumulated in the superior colliculus. The normal and dystrophic rats showed no significant difference in (1) the rates of rapidly moving components (approximately 110-180 mm/day) and slowly moving components (1.7-2.5 mm/day) or (2) the amount of radioactive material transported to the superior colliculus. The absence of transneuronal changes in retinal ganglion cells of RCS rats contrasts with results obtained earlier in rd mice (Graftstein et al., '72). Unlike the RCS rat, retinal degeneration in rd mice occurs before the maturation of the retina. We hypothesize that the ganglion cells may be more affected by loss of input early in development, and, therefore, ganglion cells of retinal dystrophic rats are less affected despite little or no synaptic input for several months. Furthermore, any reduction in the electrical activity of retinal ganglion cells that might follow loss of photoreceptor cells does not result in a significantly decreased rate of axonal transport.