The role of ambient light on dopamine signaling and myopia susceptibility Open Access
Landis, Erica G. (Fall 2018)
Abstract
Myopia, or nearsightedness, results in a blurred image of objects at a distance caused by an elongated eye. In recent decades rates of myopia prevalence have risen dramatically. The increases in myopia are likely due to environmental factors during childhood. Research into the growing myopia prevalence has led to new discoveries of how visual experience influences refractive development and myopia. Evidence in both clinical studies and animal models of myopia have indicated that bright light exposure during time outdoors can prevent myopic eye growth. However, the effect of a broad range of ambient light on myopia susceptibility had not been investigated. By housing mice in dim, intermediate, and bright light with and without lens defocus, I was able to test the effect of a wide range of ambient lighting to determine the role each plays on myopia susceptibility in the mouse model. My novel findings show that dim light, in addition to bright, is protective against myopia. To determine the retinal signaling mechanisms behind this protection, dopamine, which had previously been implicated as a “stop signal” in myopic eye growth, and proteins related to dopamine synthesis, packaging, uptake, and degradation were measured in myopic and control mice from each light level. My results show that dopamine dynamics are dependent on an interaction between ambient light and lens defocus. To determine the potential for dopamine to prevent myopia, I measured myopia susceptibility after either pharmacological or transgenic approaches to increasing endogenous dopamine. L-DOPA, a dopamine precursor, completely prevented form deprivation myopia in mice. The clinical applicability of these findings was investigated by analyzing light exposure data from a cohort of children. I showed that non-myopic children spend as much time in dim light as in bright light, supporting the potential of dim light to be used as a preventive therapy for myopia. Together, these findings reveal a more complex effect of ambient light and visual defocus on dopamine signaling and refractive eye growth. Furthermore, these data show that a broad range of ambient light is important for healthy ocular development.
Table of Contents
CHAPTER 1: Introduction.......................................................................................................... 1
1.1 Anatomy and Physiology of the Eye and Retina........................................................... 1
1.1.1 Brief summary of ocular anatomy and visual processing.............................. 1
1.1.2 The neural retina............................................................................................. 4
1.1.3 Neurotransmitters in the retina....................................................................... 7
1.1.4 Dopamine dynamics in the retina................................................................... 8
1.2 Overview of Myopia................................................................................................... 13
1.2.1 Refractive error and myopia as visual disorders.......................................... 13
1.2.2 Current treatments for refractive errors........................................................ 17
1.2.3 Epidemiological understanding of refractive error and myopia................... 19
1.3 The Scientific Problem of Myopia.............................................................................. 23
1.3.1 Dopamine as a ‘stop signal’ in myopic eye growth..................................... 24
1.3.2 Light-driven prevention of myopia in animal models.................................. 30
1.3.3 Connecting light and dopaminergic prevention of myopia.......................... 32
1.3.4 The importance of time outdoors and light in human myopia..................... 35
1.4 Summary of the Work Described in this Dissertation................................................. 40
CHAPTER 2: Ambient Light Alters Myopia Susceptibility in the Mouse Model................ 43
2.1 Abstract....................................................................................................................... 43
2.2 Introduction................................................................................................................. 44
2.3 Methods....................................................................................................................... 47
2.3.1 Animals and experimental design................................................................ 47
2.3.2 Light exposure.............................................................................................. 48
2.3.3 Refractive and ocular measurements............................................................ 49
2.3.4 Lens-induced myopia................................................................................... 51
2.3.5 Circadian rhythms in experimental light levels............................................ 53
2.3.6 Statistical analysis........................................................................................ 53
2.4 Results......................................................................................................................... 54
2.4.1 Scotopic and photopic lighting protects against refractive error changes.... 54
2.4.2 Corneal curvature, not other ocular parameters, is altered by lens defocus under mesopic light 57
2.4.3 C57BL/6J mice can detect all three experimental luminance levels............ 60
2.5 Discussion................................................................................................................... 63
2.5.1 Exposure to scotopic and photopic light prevents lens defocus myopia in the mouse model 63
2.5.2 Potential signaling mechanisms in the retina for coding light and altering refractive development 64
2.5.3 Implications for clinical treatment of myopia.............................................. 66
CHAPTER 3: Dopamine Signaling and Activity are Impacted by Ambient Light Level.... 68
3.1 Abstract....................................................................................................................... 68
3.2 Introduction................................................................................................................. 69
3.3 Methods....................................................................................................................... 71
3.3.1 Animals, light exposure, and lens defocus................................................... 71
3.3.2 Retina collection for HPLC detection of dopamine..................................... 72
3.3.3 Gene expression of dopamine related proteins............................................ 73
3.3.4 Western blot detection of dopamine related proteins................................... 76
3.3.5 Eye cup perfusion system to measure dopamine release............................. 79
3.3.6 Statistical analysis........................................................................................ 82
3.4 Results......................................................................................................................... 82
3.4.1 Retinal dopamine levels and metabolism increases with bright light.......... 82
3.4.2 Light and lens treatment interact to alter gene expression and presence of dopamine related proteins 85
3.4.3 Extracellular dopamine in the retina increases with bright light exposure in normal retinas 89
3.5 Discussion................................................................................................................... 91
3.5.1 Dopamine activity increases with increasing light intensity........................ 91
3.5.2 Lens treatment and light intensity alter dopamine dynamics by changing dopamine related proteins93
3.5.3 Dopamine activity is dysregulated under lens defocus................................ 94
3.5.4 Potential alternative signaling mechanisms mediating scotopic light protection 96
CHAPTER 4: Preventing Myopia by Increasing Endogenous Dopamine............................ 98
4.1 Abstract....................................................................................................................... 98
4.2 Introduction................................................................................................................. 99
4.3 Methods..................................................................................................................... 105
4.3.1 Transgenic and wild-type animals.............................................................. 105
4.3.2 Refractive and ocular measurements.......................................................... 106
4.3.3 L-DOPA and ascorbic acid administration................................................ 107
4.3.4 Form deprivation treatment........................................................................ 107
4.3.5 HPLC detection of dopamine and DOPAC in the retina........................... 108
4.3.6 Contrast sensitivity and visual acuity with VMAT2 overexpression......... 108
4.3.7 Statistical analysis...................................................................................... 109
4.4 Results....................................................................................................................... 110
4.4.1 L-DOPA prevents the effects of form deprivation in WT mice................. 110
4.4.2 Ascorbic acid, not L-DOPA, prevents form deprivation in rd10mice...... 116
4.4.3 Dopamine activity is not altered by VMAT2 over-expression.................. 123
4.4.4 Visual function is not altered by VMAT2 overexpression........................ 125
4.4.5 Altered VMAT2 expression does not alter normal refractive development in mice 127
4.4.6 FDM unaltered with increased VMAT2 expression.................................. 129
4.5 Discussion................................................................................................................. 131
4.5.1 Increasing endogenous dopamine pharmacologically prevents myopia in a mouse model 131
4.5.2 With degenerating retinas, dopamine dynamics are not altered such that increasing DA has no effect on the susceptibility to FDM.................................................................................................................... 131
4.5.3 VMAT2 does not alter retinal dopamine or visual function in mice......... 133
4.5.4 Increasing VMAT2 does not affect refractive development or myopia susceptibility134
4.5.5 Potential clinical applications for increasing endogenous retinal dopamine136
4.6 Summary................................................................................................................... 137
CHAPTER 5: Dim Light Exposure and Myopia in Children............................................... 138
5.1 Abstract..................................................................................................................... 138
5.2 Introduction............................................................................................................... 139
5.3 Methods..................................................................................................................... 142
5.3.1 Participants and data collection.................................................................. 142
5.3.2 Data analysis............................................................................................... 147
5.4 Results....................................................................................................................... 148
5.4.1 Non-myopic children spend more time in photopic and scotopic light on weekends 148
5.4.2 Increased time in outdoor photopic light is correlated with less severe refractive errors in myopia 153
5.5 Discussion................................................................................................................. 155
CHAPTER 6: Conclusions....................................................................................................... 161
6.1 Summary of results.................................................................................................... 161
6.2 Innovating techniques in myopia research................................................................ 166
6.3 Potential retinal mechanisms for light driven protection from myopia.................... 169
6.3.1 Potential role of rod photoreceptors across light levels............................. 170
6.3.2 Gap junctions in light with lens defocus.................................................... 171
6.3.3 Light sensitive retinal ganglion cells and myopia susceptibility................ 175
6.4 Clinical potential....................................................................................................... 176
6.5 Future directions for this work.................................................................................. 178
6.6 Final thoughts............................................................................................................ 180
REFERENCES.......................................................................................................................... 181
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