Abstract
Purpose: :
To assess the number, spatial distribution and outer segment (OS) length of cones in the P23H-3 transgenic rat in two protocols: (1) in the aging retina, when rod loss is slow and continuous, and (2) in the light-stressed retina, when rod loss is rapid.
Methods: :
We studied P23H-3 heterozygote rats raised in scotopic cyclic light (12h dark/12h 5 lux) from postnatal day (P) 10 to P540. At adulthood some animals were transferred to photopic conditions (12h dark/12h 300 lux) for 1 week. Retinas were assessed for cone density, distribution and OS length by wholemount immunohistochemical labelling of LM opsin, the dominant cone opsin in the rat retina.
Results: :
Over the life of the scotopic-reared P23H-3 rat (P10-P540) both the density and OS length of cones was significantly higher in the central retina than in the peripheral retina. During adulthood and until the oldest age examined (P115-P540), cone numbers remained steady at ~2000/mm2 and equal to levels in the non-degenerative retina, while OSs slowly shortened by 23%. Exposure of the adult P23H-3 retina to photopic light for 1 week resulted in no measurable change in cone density, but a 42% reduction in OS length that was most pronounced in the central inferior retina.
Conclusions: :
Whether rods degenerate slowly, or rapidly due to an increase in ambient light levels, cone density in the P23H-3 retina remains stable. By contrast, cone OS length is relatively independent of the status of rods when rod degeneration is slow, but is highly dependent when changes to rods are rapid. These findings suggest that, in humans suffering photoreceptor dystrophies from comparable causes, the maintenance of steady, low ambient light conditions will preserve the integrity of cones by minimizing damage to rods.
Keywords: photoreceptors • opsins • retinal degenerations: cell biology