Morphological and Visual Functional Changes in the Pigmented P23H Rhodopsin Transgenic Rat With the Progress of the Retinal Degeneration

Abstract

Abstract: : Purpose: To examine the effects of photoreceptor degeneration on the changes in the morphology and visual function in pigmented P23H rhodopsin transgenic rats. Methods: Pigmented P23H heterozygote (line 1) and Long Evans (LE) control rats were studied from 21 to 120 days by immunocytochemistry and functional assessment. A range of cell–specific antibodies were used to define specific retinal cell types. Retinal sections were examined by confocal microscopy. Functional assessment included the tectal mapping, ERG and OptoMotor responses. Results: Immunostaining showed major morphological changes in outer retinal neurons in pigmented P23H rats compared with normal control LE rats. Already at 21 days old, the photoreceptors were reduced to 10 cell layers compared with 13 cell layers in age–matched LE controls. By 120 days, 3 cell layers remained in P23H rats. Both cone axons and outer and inner segments were shorter. There was reduction of rod spherules and cone pedicles and of horizontal cell axons. The Müller cell processes became disorganized and disrupted: changes were also seen in inner retinal neurons. Tectal mapping studies showed that the thresholds of dark–adapted rod responses were approximately 0.5 log units higher than those recorded in LE animals at 28 and 40 days, whereas cone thresholds were no different from LE rats. ERG revealed that both scotopic a– and b–waves were already decreased in amplitude at 21days, while photopic b–waves showed first signs of decline only at 60 days. The OptoMotor functional assessment showed pigmented P23 performed poorly in comparison with LE rats with peak performance of 0.35 c/d at 18 days then dropping rapidly to become non–responsive by 40 days. Conclusions: This study revealed early rod photoreceptor degeneration, followed by cone changes and abnormalities affecting secondary neurons. All these events developed more slowly than in albino P23H rats. Functional studies showed complex changes with ERG following a pattern expected of the morphology, and threshold responses deteriorating slowly but OptoMotor responses deteriorating faster than expected.

Keywords: retinal degenerations: hereditary • immunohistochemistry • electroretinography: non-clinical