- Title
- The role of cell coupling in the control of myopia in the guinea pig retina
- Creator
- Myles, William Edward
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Background: Myopia (short-sightedness) results when the length of the eye is too long for the power of its optics. The rate of eye elongation is regulated by the retina in response to visual cues, however the specific mechanisms underlying this response remain elusive. Dopamine, retinoic acid, and nitric oxide are all implicated in the mechanisms underlying myopia, and each regulates retinal gap-junction permeability suggesting retinal coupling changes may be involved. This thesis investigated the changes in retinal cell coupling in myopic guinea pig eyes and under conditions when myopia was inhibited. This thesis then investigated the effect of inhibiting retinal cell coupling on the formation of experimental myopia. Methods: Retinal cell coupling in several cell-types was assessed using a variation of the cut-loading technique which included the kinetics of dye-diffusion through a network of coupled neurons. Models describing the effective diffusion (De) and coupling coefficients (kj) were validated in highly coupled a-type horizontal cells (aHCs) in guinea pig retinae following cut-loading with increasing dye-diffusion times, in different lighting environments and in the presence of non-specific gap-junction inhibitors (Meclofenamic Acid (MFA) or 1-Octanol). Cell coupling was investigated in retinae extracted from young guinea pigs after eyes were treated to induce myopia, or after recovery from myopia or after receiving pharmacological treatments for myopia. Myopia was induced with either a negative lens (-6D) or optical diffuser (form deprivation, ‘FD’) worn in front of one eye for 2 weeks. Myopia recovery was induced by subsequently removing the negative lens from a lens-wearing animal for 72 hours. Myopia treatments were delivered concurrently with FD, either in the form of daily (1% atropine, 0.15% atropine or saline) or twice daily eye drops (Nepafenac or saline) or via intravitreal injection (10µL MFA or vehicle). Myopia was assessed by measuring refractive error using a NIDEK autorefractor in cyclopleged eyes, eye length using high frequency A-scan ultrasonography, and corneal power using a custom infrared keratometer. Eye health was assessed using optical coherence tomography (OCT) and fundus photography. Results: In normal dark-adapted retina, aHCs had a mean coupling coefficient (kj) of 0.038 cells^2 .s^−1 and an effective diffusion coefficient (De) of 1.38 cm^2 .s^−1 . These measurements were consistent across all retinal incubation times. Coupling between aHCs was highest in retinae adapted to scotopic lighting and decreased in photopic environments and was also readily inhibited in vitro by MFA (max 96% reduction) and 1-Octanol (max 96% reduction). In the superior retina, coupling between aHCs was bidirectionally associated with the sign of ocular defocus in lens-wearing myopic (increased coupling) and myopia recovery animals (decreased coupling). Average coupling in cells in the inner nuclear layer was not affected by myopia but increased in the inferior retina during myopia recovery. Coupling between aHCs was similarly increased in the superior retina of FD guinea pigs, but no such difference was observed in FD animals receiving 1% atropine eye drops, in which myopia was reduced by 63%. Both 0.15% and 1% atropine reduced ocular length in FD eyes. Coupling between type-1 nNOS amacrine cells was unaffected by FD or atropine, whereas coupling between displaced nNOS amacrine cells decreased in the inferior retina extracted from guinea pigs treated with FD, FD+Saline and FD+1% atropine, but not in FD+0.15% atropine treated animals. Coupling between aHCs was reduced in vivo by up to 73% for at least two hours by a single dose of MFA (3.73mM). Inclusion of the surfactant polysorbate-60 was required to rectify the restricted diffusion of MFA in the guinea pig vitreous due to the presence of hyaluronic acid. Inhibition of retinal coupling by daily intravitreal injection of MFA significantly reduced the extent of myopia by 45% compared with FD and vehicle injected FD animals, due to shrinkage of the vitreous and anterior chambers which eliminated ocular elongation. Concurrently, daily MFA reversed the increased aHC coupling observed in FD animals. Both vehicle and MFA injected eyes showed increased corneal power, increased lens thickness, and decreased anterior chambers, indicating this was due to intravitreal injections per se with a net effect of mild myopia (-3.7D and -4.5D respectively). Eyes receiving FD+MFA treatment only developed -5D of myopia (90% of which was the artefact from injection per se) compared with -9.3D in FD+Vehicle or -9.1D in FD animals. Importantly, MFA did not inhibit normal eye growth. Lastly, twice daily application of eye drops containing Nepafenac (a COX2 inhibitor structurally similar to MFA) inhibited myopia by 36% compared with FD animals receiving saline drops by reducing growth in the anterior chamber and reducing corneal power. Interestingly, Nepafenac and its active form Amfenac also were not as effective as MFA at inhibiting retinal cell coupling. Conclusions: This thesis highlights the importance of retinal cell-coupling in the mechanisms underlying myopia in a mammalian eye. The finding of bidirectional changes in aHCs suggests not only that cell coupling changes occur very early in the photoreceptor to ganglion cell signal flow, affecting downstream receptive field sizes, but implicates these cells in at least partially coding the retinal sign of defocus.
- Subject
- myopia; retina; cell coupling; gap junctions; treatment; guinea pig
- Identifier
- http://hdl.handle.net/1959.13/1507042
- Identifier
- uon:55950
- Rights
- This thesis is currently under embargo and will be available from 06.12.2024, Copyright 2023 William Edward Myles
- Language
- eng
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