Retinal development and photoreceptor synaptic ultrastructure are altered by abnormal rearing light regimes

The effects of light rearing regimen on distal retinal development and photoreceptor ultrastructure were investigated using light and electron microscopy. Zebrafish larvae were reared in constant light, control/cyclic light (14 hr light/10 hr dark), or constant dark conditions until 4 or 8 days postfertilization (dpf). Gross retinal morphology was not altered by light rearing conditions; however, ultrastructural differences were noted both within and between age groups. Significant differences were seen in photoreceptor outer segments (OS) and synaptic ribbons, the size of cone photoreceptor mitochondria, and postsynaptic horizontal cell spinules. Larvae reared in constant dark displayed reduced pigment dispersion; OS development was delayed and cone mitochondria were smaller at 4dpf, two results that reversed by 8dpf. Photoreceptor terminals of larvae reared in all treatment conditions displayed anchored synaptic ribbons with arciform densities and no significant differences in ribbon number. Ribbons were 3040% longer in photoreceptor terminals within the constant light treatment. The number of horizontal cell spinules invaginating into cone terminals varied and the spinule-to-ribbon ratio was higher in control and constant light-reared tissue by more than 2x at 4dpf. By 8dpf, this ratio was significantly highest in retinas reared in control/cyclic light conditions. Taken together, these results show that abnormal light rearing conditions affect synaptic structure in distal retina. These changes suggest a mechanism for the physiological and behavioral deficits reported in zebrafish larvae grown under constant light and/or dark conditions.