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We have recently noted that the inner nuclear layer (INL) and the inner plexiform layer (IPL) were significantly thinner in mice homozygous for the nervous defect (nr / nr) than in control (nr /+ or +/+) littermates. Here, we have carried out a series of anatomical studies to further understand these inner retinal changes. At postnatal day (P) 13, there was no difference in the inner retina between nervous and control mice, while a significant difference was observed at P30. Similar changes were not seen in other mouse models of photoreceptor degeneration. There was a significant reduction in the density of cells in the INL that were stained by antibodies against the inhibitory neurotransmitters GABA and glycine. These results indicate that the nervous defect causes a degeneration of one or more sub-types of amacrine cells, in addition to the loss of cerebellar Purkinje cells and retinal photoreceptors that is known to occur in these mutant animals. Finally, evidence is provided that photoreceptors die by an apoptotic pathway in nervous mice., (Copyright 2001 Academic Press.)

The retinas of rd/rd mice with inherited retinal degeneration were examined histologically at postnatal days 60-66, an age when most rod cells already have degenerated and disappeared but when a significant number of cones are still present. We observed an unexpected hemispheric asymmetry and large variability in the number of surviving cones. Significantly more cones survived in the inferior than in the superior hemisphere in most retinas, although in about 15% of animals the hemispheric asymmetry was absent or was reversed. The number of surviving cones was highly variable from animal to animal, ranging from 3-30, a factor of 10, within the superior hemisphere, and from 7-51, a factor greater than 7, in the inferior hemisphere. If the specific hemisphere was ignored, the number ranged from 3-51, a factor of 17. These findings have significance for the examination of cone survival in the late stages of degeneration in this widely studied mutant, including therapeutic studies using transplantation, gene therapy or survival factors, as well as for the identification of surviving cells using cone-specific markers.