Optophysiological characterisation of retinal ganglion cell function by ultrahigh-resolution optical coherence tomograph

Retinal imaging using laser based techniques now plays a key role in the assessment of eye diseases such as glaucoma, optic neuropathy, diabetic retinopathy and macular degeneration that remain important causes of vision loss and blindness in the UK. There is still a pressing need to increase the resolution with which the retina is imaged since this will facilitate early disease diagnosis and improve the accuracy with which retinal disease can be monitored. Among the various imaging modalities that have been developed, optical coherence tomography (OCT) has the greatest potential to image structure and function of the retina at cellular resolutions. Since early detection allows for early treatment, developments in imaging techniques should translate into improved treatment and the prevention of vision loss. Recently, we have shown that it is possible to detect activity of small regions of the retina by OCT based techniques but we do not know whether this can be used to determine the functional integrity of the cells that comprise the retina. In this study we will explore the ability of functional ultrahigh resolution fUHR-OCT to characterize, functionally, the properties of single retinal ganglion cells in the living (in vivo) retina. These are the cells that connect the eye to the brain; if we can determine their physiological properties, we have the potential to develop novel devices for the detection of retinal disease in its earliest stage. Just as important will be our ability to determine the effects of treatment on retinal function- this will allow us to shorten time needed to evaluate novel treatment for preservation of retinal ganglion cell function (neuroprotection). If successful, we envisage that the techniques developed in this study will also be suited to determine the functional characteristics of other retinal layers and therefore play an important role in the assessment of patients with diseases such as macular degeneration and genetic disorders of the retinal pigment epithelium (e.g. retinitis pigmentosa).

Objectives: To establish a method for the in vivo imaging and quantification of retinal ganglion function by ultrahigh resolution functional optical coherence tomography (fUHR-OCT). This would, for the first time allow the in vivo, non-invasive, assessment of retinal ganglion cell function at the single cell level.
Methodology: fUHR-OCT is an imaging modality that derives high resolution, spatially distinct images of neural activity from the interferometric analysis of tissues imaged with low coherence laser light. Using an animal preparation (paralysed and anaesthetized tree shrews), fUHR-OCT will be used to quantify, optically the function of single retinal ganglion cells (Optophysiology). OCT has such high optical signal sensitivity that it is able to detect subtle optical changes that follow alterations in the plasma membrane potential. Light stimuli will be generated using the measurement beam path of OCT to drive selected retinal ganglion cells. Extracellular recordings will be made from the optic tract to provide a gold standard measure of retinal ganglion cell activation against which the optical signals recorded by f-UHR-OCT will be compared. In selected animals the morphological assessment of retinal ganglion cells will also be obtained following injections of rhodamine dextran into the lateral geniculate nucleus for the retrograde transport to retinal ganglion cells; rhodamine will be released by illumination of selected cells to provide complete intracellular filling and delineation of dendritic morphology to complement the physiological characterization.
Application and exploitation of results: For the first time, fUHR-OCT should allow the in vivo, non-invasive quantification of retinal ganglion cell function at the level of the single cell. The technique will be a major advance for the early detection of retinal ganglion cell disease and dysfunction and will be invaluable for deriving new endpoints for the evaluation of neuroprotective strategies for the prevention of retinal ganglion cell death in diseases such glaucoma and optic neuropathy. The results from this study will lay an important foundation for the functional assessment of neurons in outer retinal layers such as the outer nuclear and photoreceptor layers.