In the vertebrate retina there exists two types of photoreceptor cells: rods and cones. When excited by light, these cells generate electrical signals that initiate the process of vision. The signals generated by each cell type are distinct with respect to their sensitivity to light, speed and adaptation properties and because this is true, the visual system as a whole can perform over a range of 7 orders of magnitude in light intensity with great reliability and accuracy. Phototransduction involves the activation by light of a complex sequence of enzymatic reactions, the outcome of which is the continuous control of the cytoplasmic concentration of a diffusible second messenger, cyclic GMP which, in turn, regulates the activity of ligand-gated ion-channels in the membrane of the photoreceptors. In parallel with the changes in cGMP, the concentration of Ca also changes and this change feeds-back and regulates the activity of the enzymes of transduction. A major aim of our work is to understand the mechanisms that make the transduction signal so profoundly different in the two types of photoreceptors. At this time, we pursue this aim through studies of the light-dependent changes in cytoplasmic Ca concentration in rods and cones and of the biophysical properties of the cGMP-gated channels, both in native membranes and with recombinant protein.
Retinal development is controlled by factors other than strict genetic lineage. We study the role of electrical activity and electrical- and neurotransmittermediated cell-cell interaction as modulators of the process of neuronal differentiation. We pursue these studies in the retina of rainbow trout. Teleost fish are unique among vertebrates because their eyes continue to grow throughout the life of the animal. New retinal differentiation occurs continually at the edge of the mature retina. This "ring" of newly developing retina is the peripheral germinal zone (PGZ). In the PGZ there is a developmental gradient that extends continuously from undifferentiated stem cells at the margin to the fully mature retina more centrally. Thus, in the PGZ at any given moment, time and distance are equivalent and every possible stage of retinal differentiation is present, side by side. This features provides a unique experimental opportunity not available in any other vertebrate. Developmental changes in retinal cell function can be studied in the same slice of tissue, without the need to compare different animals, each sacrificed at a different time in development.
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Picones A. and J. I. Korenbrot (1995) Permeability and interaction of Ca++ with cGMP-gated channels differ in retinal rod and cone photoreceptors. Biophys. J. 69: 24-31
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