COUNTER-ROTATION OF THE ZEBRAFISH (DANIO RERIO) EYE IN RESPONSE TO CHANGES IN LIGHT POSITION.

J.E. Anwah(1) and S.J. Moorman, UNT Health Science Center at Ft. Worth and (1)Viginia Military Institute.

When a normal zebrafish hatchling is tilted around the transverse body axis (tail-up and tail-down, i.e. pitch) the eye maintains a fairly constant orientation with respect to gravity. This reflex counter-rotation of the eye is thought to be initiated by activity in the gravity sensing portion of the equilibrium receptor system. In addition to gravity, zebrafish use the position of the light for purposes of equilibrium orientation. Here we provide evidence that zebrafish also use the position of the light to initiate reflex counter-rotation of the eye. Zebrafish eggs were collected once a week for 5 weeks within 1 hour after they were laid and fertilized. About 50% of those eggs were transferred into a beaker with aquarium water and about 50% were placed in a Rotating Wall Perfused Vessel (RWPV). The RWPV was designed by NASA to simulate a microgravity environment for cells in culture. We have previously shown that zebrafish that hatch from eggs that have been incubated in the RWPV for 96 hours have irreversible vestibular deficits. The zebrafish eggs/hatchlings were maintained in the RWPV for 96 hours. (hatching occurs between 48 and 72 hours after fertilization). Reflex counter-rotation of the eye was observed with the hatchling in a capillary tube mounted on an ophthalmic microscope. The orientation of the eye with respect to gravity was noted while each hatchling was illuminated from above and tilted around a transverse axis and then again while illuminated from the side and tilted around a transverse axis. When illuminated from the side, the eye did not maintain a consistent orientation with respect to gravity. However, when illuminated from above, the eye maintained a consistent orientation with respect to the position of the light. Unlike the other reflex eye movements in zebrafish that are functionally mature by 80 hours post-fertilization, this light-induced counter-rotation of the eye did not reach functional maturity until between 96 and 120 hours post-fertilization. Similar to the other reflex eye movements in zebrafish, the development of this light-induced counter-rotation of the eye did not depend on the animal being exposed to light during development. Light induced counter-rotation of the eye has not been reported in any other animal.

This work was presented at the National Minority Research Symosium in New Orleans, 15-19 October 1997