Anatomic Estimation of the Superior Colliculus in Aged Retinal Degenerate Rats

Abstract

Purpose: : To examine whether retinal degeneration-induced visual deprivation causes alterations in the superficial superior colliculus (SC), size and population of SC neurons and retinal ganglion cell (RGC) axonal input into the SC. Evaluation was in S334 ter-line-3 rats, a model comparable to some forms of human retinitis pigmentosa.

Methods: : Morphological characterization of SC neurons was performed in aged RD rats (500-600 day old) and age matched controls (normal Copenhagen rats). For this, left and right SC were extracted from the golgi-impregnated brain. Vibratome sections were stained and imaged under a Nikon FXA microscope. Various neuronal types commonly found in the SC surface were identified. The total number of individual neurons present in a 1mm square area in the SC surface was calculated and their mean soma size was measured. To evaluate the RGC axonal projections into the SC, anterograde tracing using choleratoxin β- subunit (CTB) was employed.

Results: : Morphological evaluation revealed the presence of six major neuronal types in the SC of normal Copenhagen rats. All the above neuronal types were present in the SC of aged RD rats. Morphometric analysis revealed significant differences (P<0.05, student t test) between normal and RD’s in the size of horizontal neurons (Copenhagen rats = 0.04±0.001 um vs RD = 0.02±0.002 um). In aged RD rats, the population of horizontal neurons (per square mm SC area) was significantly reduced (1.22±0.22 in normal Copenhagen rats vs 0.64±0.15 in RD rats, P<0.05). CTB labeling revealed the existence of RGC axons in the SC of aged RD rats although the majority (60%) of the ganglion cells in the retina were degenerated.

Conclusions: : This study demonstrates that major neuronal types that are present in the SC of normal Copenhagen rats are preserved in the SC of RD rats even in the advanced stages of the disease. Selective deformation of SC neurons in aged RD rats was observed as evidenced by changes in the size and population of horizontal neurons. The above changes may influence the properties of the visual responses reaching the SC of RD animals, and may have implications for therapeutic strategies such as retinal cell replacement or retinal prostheses if similar changes occur in human visual areas.

Keywords: retina • degenerations/dystrophies • superior colliculus/optic tectum