Understanding how ageing impacts ganglion cell susceptibility to injury in glaucoma.

Glaucoma is a leading cause of blindness worldwide, with a marked increase in prevalence with advancing age. Due to the multifactorial nature of glaucoma pathogenesis, dissecting how ageing impacts upon glaucoma risk requires analysis and synthesis of evidence from a vast literature. While there is a wealth of human clinical studies examining glaucoma pathogenesis and why older patients have increased risk, many aspects of the disease such as adaptations of retinal ganglion cells to stress, autophagy and the role of glial cells in glaucoma, require the use of animal models to study the complex cellular processes and interactions. Additionally, the accelerated nature of ageing in rodents facilitates the longitudinal study of changes that would not be feasible in human clinical studies. This review article examines evidence derived predominantly from rodent models on how the ageing process impacts upon various aspects of glaucoma pathology from the retinal ganglion cells themselves, to supporting cells and tissues such as glial cells, connective tissue and vasculature, in addition to oxidative stress and autophagy. An improved understanding of how ageing modifies these factors may lead to the development of different therapeutic strategies that target specific risk factors or processes involved in glaucoma.

Measuring the Full-Field Electroretinogram in Rodents.

Electroretinography allows for noninvasive functional assessment of the retina and is a mainstay for preclinical studies of retinal function in health and disease. The full-field electroretinogram is useful for a variety of applications as it returns a functional readout from each of the major cell classes within the retina: photoreceptors, bipolar cells, amacrine cells, and retinal ganglion cells. Rodent models are commonly employed in ocular degeneration studies due to the fast throughput of these mammalian species and the conservation of the electroretinogram from the preclinic to the clinic. Here we describe approaches for in vivo electroretinography in rodent models.

Retinal Assessment Using In Vivo Electroretinography and Optical Coherence Tomography in Rodent Models of Diabetes.

Electroretinography and optical coherence tomography imaging allow for non-invasive quantitative assessment of the retina. These approaches have become mainstays for identifying the very earliest impact of hyperglycemia on retinal function and structure in animal models of diabetic eye disease. Moreover, they are essential for assessing the safety and efficacy of novel treatment approaches for diabetic retinopathy. Here, we describe approaches for in vivo electroretinography and optical coherence tomography imaging in rodent models of diabetes.

A Model of Glaucoma Induced by Circumlimbal Suture in Rats and Mice.

The circumlimbal suture is a technique for inducing experimental glaucoma in rodents by chronically elevating intraocular pressure (IOP), a well-known risk factor for glaucoma. This protocol demonstrates a step-by-step guide on this technique in Long Evans rats and C57BL/6 mice. Under general anesthesia, a "purse-string" suture is applied on the conjunctiva, around the equator and behind the limbus of the eye. The fellow eye serves as an untreated control. Over the duration of our study, which was a period of 8 weeks for rats and 12 weeks for mice, IOP remained elevated, as measured regularly by rebound tonometry in conscious animals without topical anesthesia. In both species, the sutured eyes showed electroretinogram features consistent with preferential inner retinal dysfunction. Optical coherence tomography showed selective thinning of the retinal nerve fiber layer. Histology of the rat retina in cross-section found reduced cell density in the ganglion cell layer, but no change in other cellular layers. Staining of flat-mounted mouse retinae with a ganglion cell specific marker (RBPMS) confirmed ganglion cell loss. The circumlimbal suture is a simple, minimally invasive and cost-effective way to induce ocular hypertension that leads to ganglion cell injury in both rats and mice.

Systemic hypertension is not protective against chronic intraocular pressure elevation in a rodent model.

High intraocular pressure is the most well documented glaucoma risk factor; however many patients develop and/or show progression of glaucoma in its absence. It is now thought that in some instances, ocular perfusion pressure (blood pressure - intraocular pressure) may be as important as intraocular pressure alone. Thus, systemic hypertension would be protective against glaucoma. Epidemiological studies, however, are inconclusive. One theory of why hypertension may not protect against elevated intraocular pressure in spite of increasing ocular perfusion pressure is that with time, morphological changes to the vasculature and autoregulatory failure outweigh the benefits of improved perfusion pressure, ultimately leading to poor retinal and optic nerve head blood supply. In this study we showed the presence of increased wall:lumen ratio and wall area of the ophthalmic artery in rats with chronic hypertension in addition to failure of retinal autoregulation in response to acute modification of ocular perfusion pressure. Subsequently we found that in spite of dramatically increasing ocular perfusion pressure, chronic systemic hypertension failed to protect retinal structure and function from a rodent model of glaucoma.

Age-related changes in the response of retinal structure, function and blood flow to pressure modification in rats.

Age-related changes to the balance between the pressure inside the eye (intraocular pressure, IOP) and the pressure inside the brain (intracranial pressure, ICP) can modify the risk of glaucoma. In this study, we consider whether the optic nerve in older rat eyes is more susceptible to acute IOP and ICP modification. We systematically manipulate both ICP and IOP and quantify their effects on ganglion cell function (electroretinography, ERG), optic nerve structure (optical coherence tomography, OCT) and retinal blood flow (Doppler OCT). We show that ganglion cell function in older eyes was more susceptible to a higher optic nerve pressure difference (ONPD = IOP - ICP). This age-related susceptibility could not be explained by poorer blood flow with elevated ONPD. Rather, as ONPD increased the retinal nerve fibre layer showed greater compression, and the retinal surface showed less deformation in older eyes. Our data suggest that age-related changes to connective tissues in and around the rat optic nerve make it less flexible, which may result in greater strain on ganglion cell axons. This may account for greater functional susceptibility to higher optic nerve pressure differences in older rat eyes. Further studies in a species with a well-developed lamina cribrosa are needed to determine the clinical importance of these observations.

Retinal biomarkers provide "insight" into cortical pharmacology and disease.

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.