Erika D. Eggers - US grants

DESCRIPTION (provided by applicant): GABAergic and glycinergic inhibitory pathways in the inner plexiform layer modulate the flow of visual information between bipolar cells and ganglion cells and are an essential element in retinal information processing. In mammalian retina the precise roles of the different inhibitory neurotransmitters, receptor subtypes and circuits are not well understood. Different bipolar cell types separate the visual signal into distinct visual channels, e.g., rod versus cone, on versus off and sustained versus transient signals. Distinct GABAA and GABAC receptors found on bipolar cell terminals have been shown to have different biophysical properties and are present in different proportions on distinct cell types. Glycine receptors have also been shown to be present on bipolar cell terminals, but their functional roles on most bipolar cells is unclear. The goal of the proposed study is to analyze the role of GABAergic and glycinergic inhibition of bipolar cell terminals on spatial and temporal processing in the IPL. We will measure the light-evoked inhibitory signals in different bipolar cell types to determine whether these distinct receptor types mediate unique inhibitory signals and whether the distinct bipolar cell signaling pathways utilize distinct inhibitory signals, which affects transmission from bipolar cells to third order cells

[unreadable] DESCRIPTION (provided by applicant): The retina is a well-organized sensory structure with two main pathways of information transfer. In the vertical pathway photoreceptors sense light and transmit information to bipolar cells, which relay information to ganglion cells, the output of the retina to the brain. Lateral inhibitory pathways, mediated by horizontal cells in the outer retina and amacrine cells in the inner retina, modulate this vertical pathway. Inhibition in the inner retina plays several important roles in retinal signal processing, as it both comprises part of the center-surround receptive field spatial organization of the retina and affects the gain and temporal processing of retinal signaling. Inhibitory inputs come from two distinct amacrine cell sources: GABAergic and glycinergic amacrine cells, and are mediated by three different inhibitory receptors: GABAC, GABAA and glycine receptors. Previous work has suggested that the distributions and kinetics of these inhibitory receptors are important for determining the properties of light-evoked inhibition in the retina. However, little is known about how the neurotransmitter release properties or spatial activation of amacrine cells that provide inhibitory inputs contribute to bipolar cell inhibition. During my mentored research in the lab of Dr. Peter Lukasiewicz, I will determine the role of inhibitory connections between amacrine cells in the spatial regulation of retinal inhibition. For my independent research, I will investigate how distinct neurotransmitter release properties in glycinergic and GABAergic amacrine cells temporally shape inhibition in the retina. Additionally, I will determine if GABAergic and glycinergic amacrine cells, which have distinct spatial extents within the retina, create spatially discrete bipolar cell inhibition. These experiments will determine how the connectivity, release and spatial properties of amacrine cells combine to create the total inhibition in the retina. This will add important knowledge to our understanding of the roles of inhibition in retinal signaling. Additionally, as the retina is an accessible neuronal circuit that can be stimulated physiologically, with light, these experiments will add to our knowledge about how multiple inhibitory inputs contribute to total inhibition in the nervous system. General Statement: To develop approaches to restore normal vision after disease and injury damage the retina, we must first understand how the healthy retina processes visual information. In the proposed research, I will investigate how the properties of modulatory neurons in the retina shape visual signals before they are transmitted to the brain. [unreadable] [unreadable] [unreadable] [unreadable]

When going from a dark movie theater into the bright sunshine, vision is initially overwhelmed by the suddenly bright light levels, but after time we are again able to see well. This process, called light adaptation, involves a change in the sensitivity of neurons in the retina of the eye. Although retinal adaptation to brighter light levels is a crucial function of the visual system, the changes that happen in the retina to enable this adaptation are not well understood. Previous work has suggested that release of the chemical dopamine from retinal neurons is responsible for light adaptation, but the details of how this works are not clear. This study will determine what changes are occurring in the retina to allow adaptation to increasing light levels and determine how dopamine is changing visual signaling. Additionally, since the retina is an easily accessible part of the brain, these experiments can be used as a model for how dopamine changes the responses of groups of neurons. Dopamine signaling is important throughout the brain and in disorders such as Parkinson's disease. As part of this research, educational materials will also be developed with students to explain how light adaptation works and the importance of this process for normal vision. These materials will be used to increase interest in science for high school students, many of whom are underrepresented in science, and the public through outreach at a yearly festival and web interactions.

Light adaptation is a crucial retinal signaling mechanism that allows the visual system to avoid saturation by resetting the gain of neuronal signaling. Light adaptation also increases visual acuity by increasing the response of ganglion cells, the output neurons of the retina, to small light stimuli. Inhibition of bipolar cells, which relay information to ganglion cells, influences ganglion cell spatial resolution. Therefore, light adaptation modulation of bipolar cell inhibition could increase visual acuity. However, the role of inhibition in light adaptation is not known. Modulation of inhibition may be especially important to adaptation of the retinal pathway that responds to the offset of light (OFF), as inhibitory inputs to OFF-bipolar cells switch between dim light rod and bright light cone sources and light adaptation decreases the spatial extent of inhibition to OFF-bipolar cells. This suggests a model where light adaptation narrowing of bipolar cell inhibition underlies the changes in ganglion cell spatial signaling. It is unknown how narrowing of OFF-bipolar cell inhibition will affect OFF-ganglion cell signaling or what mechanisms underlie the OFF-bipolar cell changes. Dopamine is a key neuromodulator of retinal light adaptation. Dopamine-mediated uncoupling of gap junctions and/or increases in inhibitory connections between upstream neurons might narrow OFF-bipolar cell inhibition. However, the role of dopamine modulation of physiological inhibition is not known. These proposed mechanisms for light adaptation of OFF pathway inhibition and their effects on retinal acuity will be tested using a combination of single-cell electrophysiology, morphology, genetic mouse models and optogenetic activation of retinal neurons.

? DESCRIPTION (provided by applicant): Diabetes causes deficits in vision and global retinal activity that happen prior to the end-stage vascular growth and edema of diabetic retinopathy. Several studies have shown that decreases in retinal function predict vascular changes, suggesting that diabetic retinopathy may be a neurovascular disease, where direct damage to retinal neurons contributes to the progression of diabetic retinopathy. However, it is unknown what causes this retinal dysfunction. Decreases measured in human and animal models with electroretinograms are population measurements and thus could be due to deficits in retinal neuronal signaling or loss of retinal neurons. To address this major knowledge gap, we are proposing to investigate the progression of direct neuronal damage in diabetes and differentiate between retinal neuronal dysfunction and retinal cell death as the causes. Previous in vivo studies have identified potential decreases in the activity or survival of retinal photoreceptors that sense light, bipolar cells that receive inputs from photoreceptors, amacrine cells that feedback input onto bipolar cells and ganglion cells that are the output of the retina. As bipolar cells receive inputs from photoreceptors and amacrine cells, changes in the activity or cell loss of bipolar cells or ganglion cells may be an important mechanism of retinal dysfunction in early diabetes. We will use an innovative approach with mouse lines that express fluorescent proteins in specific retinal bipolar cells and the STZ model of diabetes, which kills pancreatic beta cells, to investigate the mechanisms of diabetic retinal damage in vitro over multiple time points. In Aim 1 we will determine which specific retinal neurons have a dysfunctional physiological response in diabetes and test a potential treatment to prevent this dysfunction, using single cell electrophysiology recordings of the responses of targeted bipolar cells and ganglion cells to light. In Aim 2 we will determine what changes in the mechanisms of neuronal signaling explain the physiological changes measured in Aim 1 in the targeted bipolar cells. In Aim 3 we will determine if the diabetes induced changes in neuronal signaling are accompanied or preceded by changes in morphology, survival and/or receptor expression, using the fluorescent protein expression of our targeted bipolar cells and immunohistochemistry techniques. These studies will be the first to make physiological measurements from individual retinal cell types in the intact retina from a diabetic mouse model, where it is possible to determine what neuronal mechanisms have changed due to diabetes. The illustration of the mechanisms of diabetic retinal neuronal damage will test one therapeutic target (dopamine deficiency) and suggest others, such as therapies that reverse changes in neuronal activity or promote neuronal survival that can be used in a time window where retinal damage from diabetes may not be irreversible. This will provide a foundation for future studies to determine if these direct neuronal changes correlate with vascular changes and how preventing neuronal changes would affect the end-stage of diabetic retinopathy.