Faculty and Research Activities

Gustav A Engbretson, Ph.D. Associate Professor of Ophthalmology 3258 Weiskotten Hall Upstate Medical University 750 East Adams Street Syracuse, NY 13210

Education and Clinical Training

Research Program and Department Affiliations

Research Abstract

Cellular Neurobiology of the Parietal Eye. With B. Knox and M. Cornwall-Brady.

What is the molecular nature of the parietal eye photopigment? The parietal eye is a dorsal Midline photoreceptive organ of pineal origin. It reaches its structural zenith in lizards, being organized as a photoreceptor layer and a ganglion cell layer with an interposed plexiform layer. Glial cells, in many ways similar to retinal Muller cells, span this sensory epithelium radially. The photoreceptors resemble retinal cones with their extradiscal space continuous with extracellular space, and apical oil droplets in some species. We showed that the photoreceptor response to light is unique among vertebrate photoreceptors. Individual photoreceptor cells have a depolarizing mechanism, with a spectral sensitivity maximal at about 495 nm, and a 1.2 - 2 log unit less sensitive hyperpolarizing mechanism most sensitive at about 440 nm. These two mechanisms appear to modulate the same light-sensitive conductance which has recently been shown in another laboratory to be cGMP gated. We have developed a 3-state model that predicts the steady-state response of the cells to light. We are presently working to incorporate the temporal aspects of the response into the model.

The presence of antagonistic photoresponses in a single vertebrate photoreceptor raises the question of how the transduction mechanism functions. How are the two pathways that converge on cGMP isolated and modulated? What is the nature of the photopigment(s)? We are addressing the latter question. The retinal and pineal visual pigments of the lizard Anolis carolinensis have been characterized and two have been proposed as those present in the parietal eye. The electrophysiologically measured spectral sensitivities of the parietal eye photoreceptors, however, do not match those of the proposed photopigments. This suggests that the parietal eye photopigment is unique. A cDNA library of the parietal eye is being constructed. We will use subtractive cloning techniques to isolate genes expressed in the parietal eye and not in the retina. mRNA of these genes will be injected into Xenopus oocytes and the expressed visual pigment(s) will be investigated electrophysiologically.

The long term goal of the research program is to express the parietal eye photopigment in a cell line and produce sufficient amounts to characterize its molecular nature. Besides the potential better understanding of the evolution of vertebrate photopigments that an understanding of this unique photopigment may render, there are potential uses of such a photopigment in molecular computing. A readily expressible bistable photopigment may be present in the parietal eye. Such a photopigment could play a role in commercial applications.

This profile was last updated on 06/01/2005

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