Understanding the Retina
The retina, a thin sheet of neural tissue at the back of the eye, is our window on the visual world. It does far more than simply detect light. It is a highly organized outgrowth of the central nervous system with a mind boggling diversity of cell types and specialized synaptic circuits. Because the retina can easily be isolated and studied in vitro without disrupting its normal architecture and function, it has become one of the most thoroughly characterized regions of the mammalian nervous system.
Our lab is particularly interested in the output cells of the retina — the retinal ganglion cells. A major challenge for studies of the early visual system is to understand the remarkable diversity of ganglion cells, which comprise roughly two dozen distinct types. The broad goal of the lab is to understand how retinal synaptic circuits generate different sorts of light-evoked responses in different ganglion cell types, where in the brain these distinctive signals are sent, and what role they play in visual perception and behavior.
Melanopsin and Ganglion Cell Photoreceptors
In recent years, our primary focus has been on intrinsically photosensitive retinal ganglion cells (ipRGCs). Discovered in this lab just over a decade ago, ipRGCs are unique among retinal ganglion cells in their expression of the photopigment melanopsin, which enables them to respond to light in the absence of rod and cone input. We are exploring the diversity, development, gene expression, brain targets, and functional roles of this mysterious new class of retinal photoreceptors. Originally believed to belong to a single type, ipRGCs have now been shown to comprise at least five distinct types, differing in their levels of melanopsin expression as well as in their structure, function and central projections. By exploring the diversity of ipRGCs, we hope to develop a more complete understanding of the full range of contributions made by melanopsin and ganglion-cell photoreceptors to visual behavior and perception. Recently, we have begun to analyze the patterns of gene expression in specific types of ipRGCs. These studies will help to clarify the nature of phototransduction and synaptic signaling in these cells, and should provide new genetic tools for studying them. For example, the unique molecular signatures of each type should open the door to selective labeling of single subtypes for further study in vitro, to tracing their projections to the brain, or to activating or silencing them selectively in living animals to study their contributions to visual behavior.
The role of ipRGCs in reflexive responses to environmental light
ipRGCs were originally thought to send their axons exclusively to subcortical centers mediating reflexive responses to environmental light. These include the suprachiasmatic nucleus of the hypothalamus, which is the brain's circadian pacemaker. Connections from ipRGC input to this brain center are crucial for synchronizing the biological clock to the rising and setting of the sun. Another such target, within a midbrain region called the pretectum, is responsible for the constriction of the pupil in bright light. Other targets of ipRGCs have been implicated in photic pain or the modulation of levels of the hormone melatonin. Still other targets have no known function as yet. We are working to understand the full range of brain circuits and functional roles served by these diverse ipRGC outputs.
The role of ipRGCs in cortical vision and conscious perception of brightness
Once thought to innervate only ‘non-image-forming’ brain targets involved in visual reflexes, ipRGCs have recently been shown to contribute to pathways reaching the visual cortex, the seat of conscious visual perception. It remains unclear what role these connections make to perceptual function. However, we known that ipRGCs encode the absolute intensity of light better than other ganglion cells, so a reasonable guess is that they contribute to the perception of brightness. We are beginning to study cells in the input pathways to cortex that receive ipRGC input to determine whether these inputs underlie a cortical representation of stimulus brightness.
Contributions of ipRGCs to development of the visual system
ipRGCs are the first functional photoreceptors in the mammalian eye. We recently discovered that ipRGCs play a key role in shaping the spontaneous spiking behavior of other ganglion cells in the early postnatal period. Such activity is apparently crucial for the normal wiring of the maturing visual system and for the refinement of precise brain maps of the visual world. We are working on the mechanisms through with ipRGCs affect other retinal neurons and the implications of such communication for developmental processes.
We use a wide array of methods, almost exclusively in mice, to address these questions. To study the functional properties of neurons cells in whole retina, or slices of retina or brain, we use patch clamp recording. This method allows us to study single cells in great depth, characterizing their direct and synaptically driven light responses and correlating this with cell morphology through dye filling. We also use extracellular multielectrode array recordings to characterize large numbers of ganglion cells simultaneously studied in vitro, or to study neurons in the central visual pathways in awake mice in vivo. We use immunohistochemistry, axon transport tracing and confocal microscopy to work out the chemical identity and synaptic connections specific visual neurons. We use genetically modified mice and powerful viral tools to visualize or manipulate very specific populations of retinal neurons. Functional imaging methods including multiphoton microscopy allow us to visualize the behavior of specific populations of retinal cells in response to a wide array of stimuli. We use deep sequencing of RNA harvested from purified populations of retinal neurons to assess their patterns of gene expression.
Berson, D.M., Dunn, F.A., and Takao, M. (2002). Phototransduction by retinal ganglion cells that set the circadian clock. Science 295, 1070–1073.
Ecker, J.L., Dumitrescu, O.N., Wong, K.Y., Alam, N.M., Chen, S.-K., LeGates, T., Renna, J.M., Prusky, G.T., Berson, D.M., and Hattar, S. (2010). Melanopsin-expressing retinal ganglion-cell photoreceptors: cellular diversity and role in pattern vision. Neuron 67, 49–60.