Investigation of the cues underlying myopia and their association with retinal neuronal nitric oxide synthase in the guinea pig eye

by Hannah Bowrey

Institution: University of Newcastle
Degree: PhD
Year: 2015
Keywords: myopia; retinal nNOS; ocular disease; vision impairment
Record ID: 1062143
Full text PDF: http://hdl.handle.net/1959.13/1059900


Research Doctorate - Doctor of Philosophy (PhD) Background and Aims: Myopia occurs when distant objects are imaged in front of the retina and thus appear blurred. It typically develops if the growth of the eye is too great for its decreasing optical power during emmetropisation. High myopia is associated with serious ocular disease and is recognised as a leading cause of vision impairment globally. The cause(s) of the extraordinary increases in the prevalence of myopia, particularly in East Asia, are unknown. Animal models have demonstrated that myopia can be induced by depriving the eye of visual detail (form deprivation, FD) and when retinal defocus is imposed with lenses. Exposure to high light levels or outdoor activity can slow or delay the onset of myopia, suggesting retinal light signals may be involved. An important light signal present in the retina is nitric oxide (NO). NO is critically involved in light adaptation and its related enzyme, neuronal NO synthase (nNOS), is an important mediator of the choroidal response to myopia recovery in the chick. In the mammalian retina, the expression of nNOS in the guinea pig eye is changed in opposite directions depending on the direction of ocular growth. The guinea pig provides a useful mammalian model of myopia, due to a rapid response to both FD and lensimposed defocus. In this thesis, these paradigms were extended to further investigate the association between retinal nNOS and features underlying myopia. Methods: In Chapter 2, the effect of spatial frequency deprivation was studied in 71 guinea pigs. Bangerter occlusion foils (0, 0.2, 0.4, 0.6, 0.8) or a ND filter (0.1 and 0.6) were worn on one eye by six day-old guinea pigs. After 7 days, refractive error (RE) was measured using streak refraction in cyclopleged eyes, and ocular length measured with A-scan ultrasound. nNOS expression in these animals was investigated in Chapter 3. In Chapter 4, the effect of wearing a spectacle lens with peripheral defocus was studied in 83 guinea pigs. Animals wore a single vision lens (0 Diopter (D), +4 D or –4 D), or a concentric bifocal lens with 5 mm plano centers and either positive (+4/0 D) or negative (–4/0 D) power in the periphery. Lenses were 12 mm in diameter and worn on one eye for 10 days. RE was measured centrally and at 30° in the nasal and temporal retina, and eye shape was measured using high resolution images of sectioned eyes. Chapter 5 detailed the associated changes in retinal nNOS expression. In both Chapters 3 and 5, the localisation of nNOS was investigated by immunohistochemistry in whole-mount retinas. Density counts of nNOS-labeled Type I amacrine cells (ACs) and displaced ACs were made in 48 x 1 mm samples across the entire retina. Finally, in Chapter 6, 57 seven day-old guinea pigs received 3 daily intraocular injections of an NO substrate (L-arginine; 0, 0.1, 0.3, 1.3 μmol), in one eye wearing a diffuser for 3 days, or at the same time in untreated animals (L-arginine; 0, 0.3 μmol). RE and ocular length were measured to ascertain the efficacy of…