Constant dark-rearing effects on visual adaptation of the zebrafish ERG

Anatomical and electrophysiological studies have shown that zebrafish retinal neurons develop in a sequential manner. Several studies have examined the impact of restricted rearing environments on zebrafish visual development, but the results have been mixed. The purpose of this study was to examine the development of light adaptation properties of the zebrafish electroretinogram (ERG), and examine the effects of constant dark rearing on retinal development. Subjects were zebrafish, Danio rerio, reared under normal lighting conditions and zebrafish reared in constant dark from fertilization to 6 days postfertilization (dpf). Increment threshold functions were obtained from a- and b-wave ERG responses from normally reared subjects at different ages and from animals exposed to early constant dark rearing. Dark-reared subjects were tested immediately following constant dark exposure (6-9 dpf) and after exposure to normal cyclic lighting (11-13 dpf). Adult zebrafish were significantly more sensitive at lower background illuminations than were larvae zebrafish. Also, constant dark rearing had a differential effect on the a- and b-wave response measures. Constant dark rearing raised b-wave threshold uniformly across background illuminations, while only producing higher a-wave thresholds at low levels of illumination. These results are consistent with findings in studies on zebrafish retinal development, and may help explain some of the discrepancies across studies examining the effects of restricted rearing.