ZEBRAFISH VISUAL SENSITIVITY IS REGULATED BY A CIRCADIAN CLOCK

We have recently developed a behavioral assay, based on the escape res ponse of fish to a threatening object, to analyze quantitatively the v isual sensitivity of zebrafish. During the course of dark adaptation, we measure the threshold light intensity required to evoke an escape r esponse. Under a normal light-dark (LD) cycle, thresholds for both the cone and rod systems are considerably lower in late afternoon hours t han in early morning hours. Over a period of 24 h, zebrafish are most sensitive to visual stimuli prior to light off and least sensitive pri or to light on. Under conditions of constant illumination, this rhythm of visual sensitivity persists for several days but is gradually lost . In constant light (LL), the rhythm persists 1-2 days; thereafter, vi sual thresholds at all times of the day converge at a level similar to thresholds measured in late afternoon hours in control animals. In co nstant darkness (DD), the rhythm persists at least 5 days; thereafter, it dampens to a level about a half-log unit less sensitive to that me asured in the late afternoon hours in control animals. These data sugg est that visual sensitivity in zebrafish is regulated by an endogenous circadian clock which functions to decrease the visual sensitivity.