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Neuroscience Graduate Program at UCSF
Phototransduction and Vision in Primates
The light-evoked responses of photoreceptors play a key role in shaping our visual experience. We would like to understand the physical processes that determine our abilities to detect dim light, discriminate between different colors and to achieve optimal visual acuity and temporal discrimination. To study the earliest stages of phototransduction we use suction electrodes to measure photocurrent in single rod and cone outer segments of human and monkey eyes and compare this to corresponding measures of human visual perception. The brightness of colored lights and the appearance of color mixtures could be predicted from the measured wavelength dependence of the responses in single rods and cones. Our ability to detect very dim light corresponded to the signal-to-noise ratio of rod electrical activity. After exposure to bright lights, the rod signal-to-noise ratio recovered slowly, in a fashion similar to the timecourse of human dark adaptation and the fading of visual afterimages.
The photocurrent response leads to a change in membrane voltage; the voltage is further modified by voltage-activated conductances and synaptic interactions. To look at these later stages of phototransduction we use patch electrodes to record the photovoltage of primate photoreceptors. We have found that rods act as independent light detectors, unlike the rods of cold-blooded vertebrates which extensively coupled to each other through gap junctions. Primate cones, however, were found to receive input from surrounding rods. This input would be expected to degrade the spatial resolution of cone vision. We are currently exploring the pharmacological mechanisms of the coupling and investigating the effects of light adaptation. We are also investigating on the deactivation steps of phototransduction.
After the photoreceptors, the next step in visual transduction is the excitation of retinal bipolar cells. In the primate, there are at least 12 subtypes of bipolar cells with differing synaptic connections and neurochemistry. Almost nothing is known about the electrical response properties of these cells. We expect that different subtypes are tuned to extract different aspects of the visual scene. In what manner are bipolar cells specialized? How are single photon responses processed? How is information about color encoded by bipolar cell wiring? To address these questions we are making whole-cell recordings in bipolar cells from primate and mouse retina.
Link to Publications via PubMed
Schnapf, J.L., Nunn, B.J., Meister, M., and Baylor, D.A. (1990). Visual transduction in cones of the monkey Macaca fascicularis. Journal of Physiology 427:681-713.
Kraft, T.W., Schneeweis, D.M., and Schnapf, J.L. (1993). Visual transduction in human rod photoreceptors. Journal of Physiology 464:747-765.
Schneeweis, D.M., and Schnapf, J.L. (1995). Photovoltage of rods and cones in the macaque retina. Science 268:1053-1056.
Kraft, T.W. and Schnapf, J.L. (1998). Aberrent photon responses in rods of the macaque monkey. Visual Neuroscience 15:153-159.
Schnapf, J.L. & Schneeweis, D.M. (1999). Electrophysiology of primate cone photoreceptors. in Color Vision: From Genes to Perception. Ed. Sharpe, L. & Genenfurter, K., Cambridge:Cambridge University Press.
Schneeweis, D.M., and Schnapf, J.L. (1999). Photovoltage of macaque cone photoreceptors: Adaptation, noise and kinetics. Journal of Neuroscience 19:1203-1216.
Schneeweis D.M. & Schnapf J.L. (2000). Noise and light adaptation in rods of the macaque monkey. Visual Neuroscience 17: 659-666.
Verweij, J., Hornstein, E., & Schnapf, J.L. (2003). Surround antagonism in macaque cone photoreceptors. Journal of Neuroscience 23: 10249-10257.
Hornstein, E., Verweij, J., & Schnapf, J.L. (2004). Electrical coupling between red and green cones in primate retina. Nature Neuroscience 7: 745-750.
Hornstein, E., Verweij, J., Li, P.H., & Schnapf, J.L. (2005). Gap-junctional coupling and absolute sensitivity of photoreceptors in macaque retina. Journal of Neuroscience 25: 11201-11209.
Packer, O.S, Verweij, J, Li, P.H., Schnapf, J. L., & Dacey, D.M. (2010). Blue-yellow opponency found in primate S cones. Journal Neuroscience 30: 568-572.
Julie Schnapf, Ph.D.
Phone
415-476-6758
Office Address
UCSF Box 0730
10 Kirkham Street, room K-227
San Francisco, CA 94143
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