Elsevier

Brain Research

Volume 1013, Issue 2, 9 July 2004, Pages 194-203
Brain Research

Research report
Multiple vulnerability of photoreceptors to mesopic ambient light in the P23H transgenic rat

https://doi.org/10.1016/j.brainres.2004.04.030Get rights and content

Abstract

The P23H transgenic rat was engineered to mimic a human form of retinal degeneration caused by a mutation in rhodopsin. We have tested whether the P23H transgene influences the vulnerability of photoreceptors to modest variations in ambient light, well within the physiological range. P23H-3 (P23H line 3) and control Sprague–Dawley (SD) rats were raised in cyclic light (12 h light, 12 h dark), with the light phase set at either 5 lx (‘scotopic-reared’) or 40–60 lx (‘mesopic-reared’). Mesopic rearing reduced the length of outer segments (OSs) in both SD and P23H-3 strains, but the shortening was more marked in the P23H-3 strain. Mesopic rearing was associated with thinning of the ONL, again more prominently in the P23H-3. Correspondingly, mesopic rearing increased the rate of photoreceptor death (assessed by TUNEL labelling), the increase occurring during early postnatal life. Mesopic rearing upregulated FGF-2 (basic fibroblast growth factor) levels in photoreceptors and glial fibrillary acidic protein (GFAP) in Müller cells in both SD and P23H-3 strains; again the changes were more marked in the P23H-3. Finally, mesopic rearing decreased the amplitude of the a-wave of the ERG in both strains; again the effect was greater in the P23H-3 strain. The ERG decline induced in both strains by mesopic-rearing can be explained by a reduction of functional OS membrane, due to a combination of photoreceptor death and OS shortening. The P23H-3 transgene makes photoreceptors abnormally vulnerable to modest levels of ambient light, their vulnerability being evident in multiple ways. In humans suffering photoreceptor degeneration from comparable genetic causes, light restriction may preserve the number and the function of photoreceptors.

Introduction

The impact of light on the functional and morphological state of photoreceptors has been studied in both non-degenerative [38], [42], [43] and degenerative strains of rodents [12], [21], [35], [53], and in humans [2], [33], [48]. This study describes the impact of modest levels of ambient light, experienced in a physiological day/night cycle, on photoreceptors in the P23H-3 transgenic rat, in which the transgene (a rhodopsin mutation engineered to mimic a common human form of retinitis pigmentosa) causes a progressive retinal degeneration [31], [40], [47].

The relationship between light experience and photoreceptor viability is complex. Acute exposure to prolonged light can cause near-total photoreceptor degeneration and loss of the ERG [24], [37], [38], [49]. Conversely, dark-rearing (zero light experience) results in a retina which has a maximal population of photoreceptors, and maximal sensitivity to light yet extreme vulnerability to stress [42]. More physiological (cyclic daily) exposure to light levels typical of modest daylight has a ‘conditioning’ effect on the retina, reducing numbers of photoreceptors, outer segment length and the ERG, but upregulating protective mechanisms which make the surviving photoreceptors damage resistant [28], [42]. A brief exposure of the normal retina to bright, potentially damaging light, has a similar conditioning effect [29], [35].

In some degenerative strains, light experience has similar effects, accelerating photoreceptor death and the upregulation of protective mechanisms [26], [34], [35], [39], [56]. In other degenerative strains, however, the progression of the degenerations appears unrelated to ambient lighting [18], [23], [45], [46].

Clinically, the management of light exposure could be a useful therapeutic/management strategy for the RP-affected. Prior studies of light restriction in humans are limited in number and involve patients with degenerations of uncharacterised and presumably diverse causes [2], [33], [48]. Nevertheless, some evidence of a protective effect of light restriction is available [48]. It therefore seemed worth testing the effect of light management in the P23H-3 transgenic strain of rat, in which the transgene was engineered to mimic a rhodopsin mutation which produces an autosomal dominant form of RP common in North America [13].

Section snippets

Animals and light-rearing conditions

P23H-3 homozygous animals were obtained from M.M. LaVail (UCSF School of Medicine, Beckman Vision Centre) and were bred in the University of Sydney Animal Facility. The term P23H designates the mutation engineered in the rhodopsin gene, a proline for histidine substitution at position 23. Line 3 of the P23H transgenics reported by the Beckman Vision Centre was chosen because of its relatively extended course of degeneration [27], [31], [60]. Those used in the present experiments were

Mesopic-rearing shortens outer segments

Representative images from the outer retinal layers of scotopic- and mesopic-reared SD and P23H rats are shown in Fig. 1A–D. The layers of OSs are indicated by double-headed arrows. In the SD rat (Fig. 1A,B,E), mesopic rearing was associated with a small reduction in mean OS length (from 31.1±3.4 to 28.3±4.6; mean±SD), which did not reach significance in our data. In the scotopic-reared P23H-3 rat (Fig. 1C,D,F), OS length was significantly less than in the SD control (24.1 vs. 31.1 μm; P<0.001

Discussion

The present experiments tested the effects of a modest difference in ambient light on the survival, morphology and function of photoreceptors in the transgenic P23H-3 rat. The conditions of ambient illumination chosen are both at the low end of normal diurnal variations; the brighter level is in the mesopic range, and (to give the reader a general impression) too low for comfortable reading. Despite the modest levels of lighting used, rearing in the brighter condition degraded P23H-3

Acknowledgments

This work was supported by the Medical Foundation of the University of Sydney, the National Health and Medical Research Council of Australia and the Sir Zelman Cowen Universities Fund.

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