Abstract
Purpose: :
The optokinetic reflex (OKR) is a basic visual reflex exhibited by most vertebrates and plays an important role in stabilizing the eye relative to the visual scene. However, the OKR requires that an animal detect moving stripes and it is possible that fish that fail to exhibit an OKR may not be completely blind. One zebrafish mutant, the no optokinetic response c (nrc) has no OKR under any conditions tested and was reported to be completely blind. Last year we reported that OFF-ganglion cell activity can be recorded in these mutants. To determine whether mutant fish with no OKR such as the nrc mutant can detect simple light increments and decrements we developed a visual-motor behavioral assay.
Methods: :
In this assay, single zebrafish larvae are placed in 80 wells of a 96-well plate allowing simultaneous monitoring of each larva using an automated video-tracking system. The movement of each larva in response to periods of 30 min light ON and 30 min light OFF was recorded and quantified per second.
Results: :
WT fish have a spike of motor activity at light onset, known as the startle response, followed by return to lower-than baseline activity, called a freeze. WT fish also sharply increase their activity after light OFF, and gradually (over minutes) their activity returns to baseline. Nrc mutants respond similarly to light OFF as WT fish, but exhibit a slight reduction in their average activity as compared with WT fish. Motor activity in response to light ON in nrc mutants is delayed and sluggish. There is a slow rise time of the nrc mutant response to light ON as compared with the WT light ON response. The results indicate that nrc fish are not completely blind. Because teleosts can detect light through nonretinal tissues, we confirmed that the behavioral responses to light-intensity changes require intact eyes by using chokh (chk) mutants. The chk mutants completely lack eyes from the earliest stages of development. The chk mutants do not increase their activity to either light increments or decrements showing that the lateral eyes mediate this behavior.
Conclusions: :
Whereas the OKR provides a measure of visual responsiveness, it does not determine the ability of fish to detect changes in light intensity. We have developed a new behavioral test that robustly measures motor activity in response to light onset and offset in zebrafish.
Keywords: vision and action • eye movements: saccades and pursuits • retina