The Effects of Ethanol on Cone Photoreceptor Function in Zebrafish Larvae

Audra Wilson

doi:10.7273/000003785

Everyone experiences vision on a spectrum depending on the health and functioning of structures within the visual system. The process of vision is mediated by a cascade that begins by light entering the eye and being interpreted by cells that send signals to neighboring cells. These electrical signals lead to the movement of ions and neurotransmitters that form a signal that the brain can ultimately interpret as vision. One of the key structures in the visual transduction cascade are cells that receive the first light stimulus, known as photoreceptor cells. There are two types of photoreceptor cells: rod cells are responsible for vision in dim lighting and cone cells are responsible for color vision and visual acuity. The ubiquitin-proteasome system (UPS) is important in the health of photoreceptor cells and functions to degrade proteins that the cell no longer needs by tagging it with a molecule, ubiquitin, marking it for destruction. Dysfunction of these cells or systems can lead to visual impairments and sometimes blindness. This experiment was designed to test the effects of ethanol on the function of photoreceptor cells. The data will be added to a larger study being performed to examine UPS disturbances in relation to vision loss associated with ethanol. Zebrafish larvae approximately five to eleven days post fertilization (dpf) were used as the animal model to test the functioning of cone photoreceptor cells. At this stage of development, the larvae have visual functioning but only working cone photoreceptor cells are present. The larvae were exposed to 1% ethanol for 72 hours, then given a three-day recovery period. The visual performance of the ethanol-treated and control larvae were examined through a behavioral test known as the optomotor response (OMR). Moving stripes ran perpendicular to a narrow tank in which the larvae were placed. If the larvae’s vision was intact, their innate response would be to swim towards the destination end for the stripes (OMR +). To measure the functioning of the cone photoreceptor cells, a burst of light was directed onto the eye of the zebrafish and the sum electrical response of the retina was recorded graphically on a computer. The compilation of graphs between both groups allowed us to compare ethanol’s effect on the cone cells. Photoreceptor cell function is measured by the electrical response elicited upon light stimulation. When looking at the graph, the large upward deflection is termed the b-wave and represents the signal that is sent from the photoreceptor cells to the next structure in the cascade; the ON bipolar cells. Therefore, dysfunctions in the photoreceptor cells can be inferred by calculating b-wave amplitude variations. The b-wave amplitudes from both groups at varying light intensities were also gathered and graphed on a dose-response curve. After completing the experiment, it was discovered that ethanol produced detrimental effects on the visual system. During the OMR test, most of the ethanol- treated larvae failed to swim towards the stripes (OMR -) indicating visual dysfunction. They also had significantly lower b-wave amplitudes at every light intensity. This led to the conclusion that ethanol produced partial loss of function of the cone photoreceptor cells measured by decrease in bipolar cell activation. This study along with other preliminary data supports the working hypothesis that ethanol exposure leads to UPS disturbances causing visual deficits.

The Effects of Ethanol on Cone Photoreceptor Function in Zebrafish Larvae

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