Understanding How the Centrifugal Visual System Modifies Retinal Output
The centrifugal visual system (CVS) of birds consists of a significant feedback projection from the isthmo-optic nucleus (ION) to the retina, via the retinopetal fibers.
We plan to use a fluorescent anterograde tracer injected into the ION to identify the retinopetal fiber terminals within the retina. This will allow us to identify which cells they are synapsing with in living retinal tissue. We can then use electrophysiology and Ca2+-imaging to learn how stimulating the retinopetal fibers alters the responses of these target retinal cells.
Since it is relatively easy to isolate and record from the retina, this system should provide an excellent model to study visual feedback.
My objective is to use this system to answer two important questions:
- What is the pharmacology of the synapse between the retinopetal fiber terminal and its retinal target cell? (i.e. What neurotransmitters are used and since both retinopetal fiber terminals and their target cells express nitric oxide synthase, what role is nitric oxide playing at this synapse?)
- How does stimulation of the retiopetal fibers modify the light response of the retina.
Why the CVS is important
One of the most important yet least understood aspects of the visual system is the role of descending inputs (or feedback). In the primate visual system, for example, there are ten times more fibers running from primary visual cortex to the LGN than vice versa. Obviously, this feedback system is doing something important, and we need to figure out what this is. Without a reasonable idea of what purpose is being served by this ubiquitous arrangement there seems little hope of understanding the broad design principles of the brain.
The avian visual system may provide a more accessible model in which to study descending input within the visual system. Birds receive extensive feedback to the retina through a pathway known as the centrifugal visual system (CVS). The circuitry of the CVS runs from the retina to the optic tectum then to an area of the midbrain called the isthmo-optic nucleus (ION) and the surrounding region know as the ectopic nucleus (EN). The axons from the ION and EN (called retinopetal fibers) then travel back to the retina where they synapse with two or three classes of amacrine cell.
There are two types of retinopetal fibers in birds, convergent and divergent. The convergent retinopetal fibers (CRFs) provide an interesting system in which to pursue a study of feedback pathways, because retinotopy has been preserved through the entire circuit. In other words CRFs project to a target amacrine cell in the same region of the retina from which they receive input. This fact makes this system particularly interesting, because it suggests that it is not simply changing the general attentional level of the retina. Rather, it is carrying specific, urgent signals to modify the activity of specific areas of the retina. Further evidence for this view is provided by the fact that ION cells become quiet during saccades , and CRFs are amongst the largest diameter myelinated axons found in the optic nerve . Furthermore, each CRF forms a cluster of huge synaptic boutons covering the cell body of a single amacrine cell, presumably another adaptation to allow rapid signaling.
The results of the experiments I am proposing will greatly improve our understanding of the role of feedback pathways within the visual system and provide the background needed to understand what may be happening at other sites of feedback within the visual system (such as the feedback pathway from the visual cortex to the LGN found in mammals).
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