Pineal Photoreceptor Cells Required In Maintaining The Circadian Rhythms Of Behavioral Visual Sensitivity And The Genetic Screening Of A Long Term Memory Mutant In Zebrafish

Circadian clock exist widely in a variety of organisms, such as animals, plants and fugi. Biological rhythm is present in fission of monoplasts, the photosynthesis of plants, breeding and hormone secretion of animals, as well as the molecular level. Circadian rhythm is one of basic characteristics in parts of prokaryotes and all of eukaryotes. Zebrafish (Danio rerio) is a diploid vertebrate with a good balance of complexity and simplicity. Zebrafish with small size, powerful reproductive capacity, embryo in vitro fertilization and development, short cyclogenl and transparent etc., is the ideal modle vertebrates for large-scale genetic research. Meanwhile, zebrafish also becomes a model animal for study in biological rhythm since the molecular mechanism of rhythm regulatory in zebrafish is similar to mammal.In non-mammalian vertebrates, the pineal gland functions as the central pacemaker that regulates the circadian rhythms of animal behavior and physiology. The circadian nature of pineal photoreceptor cells, the day-night fluctuation of opsin expression and the oscillation of retinal sensitivity have been well documented. However, the fundamental mechanisms that regulate the circadian rhythms of visual sensitivity and the possible involvement of pineal central pacemakers in the regulation of the circadian rhythms of visual system functions remain to be examined. We generated a transgenic zebrafish line [Tg(Gnat2:gal4-VP16/UAS:nfsB-mCherry)] in which the E.coli nitroreductase(NTR) is expressed in pineal photoreceptor cells. In developing embryos and young adults, the transgene is expressed in both retinal and pineal photoreceptor cells. During aging, the expression of the transgene in retinal photoreceptor cells gradually diminishes. By8months of age, the Gnat2promoter-driven nitroreductase is no longer expressed in retinal photoreceptor cells, but its expression in pineal photoreceptor cells persists. This provides a tool for selective ablation of pineal photoreceptor cells, i.e., by treatments in transgenic fishes with metronidazole, the NTR enzyme reduces nitroimidazole prodrugs metronidazole into cytotoxins that generate DNA interstrand crosslinking and specifically induce death of pineal photoreceptor. In the absence of pineal photoreceptor cells, the behavioral visual sensitivity of the fish remains unchanged; however, the circadian rhythms of rod and cone sensitivity are diminished. Brief light exposures restore the circadian rhythms of behavioral visual sensitivity.Together, the data suggest that retinal photoreceptor cells respond to environmental cues and are capable of entraining the circadian rhythms of visual sensitivity; however, they are insufficient for maintaining the rhythms. Cellular signals from the pineal photoreceptor cells may be required for maintaining the circadian rhythms of visual sensitivity。Learning and memory, the basis of human cognitive, are mode of advanced activity of central nervous system. Learning is the ability to modify behavioral responses dependent on experience and memory is retention of this change for variable lengths of time, they characterize all animal species. Memory formation further into two distinct phases:short-term memory, and long-term memory. Studies focus on how learning and memory works. However, there are still many mysteries waiting for us to explore in learning and memory as a kind of complex behavior. Finding mutant genes in learning and memory contribute to the study of the mechanism in vertebrate animals. In this study we use zebrafish as a mode animal, isolate a zebrafish mutant in learn and memory named fgt by inhibitory avoidance assay in ENU mutagenesis and large-scale genetic screen. Long-term memory is weak in24hr after training in this fgt zebrafish mutant. The mutant shows normal locomotor activity and normal sensitivity to dark preference. About half of fgt mutant F2(13/30) have bad long-term memory when24hr after training. The expression of IEGs c-fos in half of fgt mutant F2(13/30) after learning in a novel environment increase distinctly from the expression in wild-type fishes, which were in accordance with the behavioral results. These data show that the mutant fgt is a dominant mutant with defect in long-term learning and memory. The identification of fgt mutant may provide a possibility in mechanism and signal pathway of learning and memory.