Control of IRBP mRNA expression differs between zebrafish and goldfish

The expression of the zebrafish interphotoreceptor retinoid binding protein (IRBP) mRNA is under circadian control (Rajendran et al., IOVS Suppl. #24, 1995), but the location of this expression and the mechanism of its control is not understood. Purpose: To determine which photoreceptor subtype(s) express IRBP mRNA and to compare IRBP m RNA expression between goldfish and zebrafish. Methods: All fish were er trained to a 12 hr/12hr-Light/Dark (LD) cycle. mRNA levels were determined every three hours during this LD cycle by northern hybridization with species-specific IRBP probes. Zebrafish IRBP mRNA levels were also measured for seven days in constant darkness after LD entrainment. In situ hybridization of zebrafish and goldfish eyes collected at noon and midnight was also performed. Results: In situ hybridization shows that IRBP mRNA is localized to cones of adult zebrafish and goldfish retinas. The rhythm of zebrafish IRBP expression in a LD cycle persists in constant darkness for two days, tapering off to an intermediate level by the third day. Unlike zebrafish, the levels of goldfish IRBP mRNA remain constant in LD. Conclusions: IRBP mRNA expression in adult goldfish and zebrafish is localized to the cones. However, zebrafish IRBP mRNA expression is regulated by the circadian oscillator, while goldfish IRBP mRNA expression is not.