Abstract
In non-mammalian vertebrates, the pineal gland functions as the central pacemaker that regulates the circadian rhythms of animal behavior and physiology. We generated a transgenic zebrafish line [Tg(Gnat2:gal4-VP16/UAS:nfsB-mCherry)] in which the E. coli nitroreductase 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. By 8 months of age, the Gnat2 promoter-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 with metronidazole. 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.
Highlights
The pineal gland, which contains light-sensitive photoreceptor cells, plays important roles in the regulation of circadian rhythms in animal behavior and physiology [1,2]
We demonstrated that retinal photoreceptor cells are sufficient to entrain the circadian rhythms of behavioral visual sensitivity; cellular signals from pineal photoreceptor cells are required for maintaining the rhythms
We examined the cell types that express the transgene in the pineal gland and retina in young adult animals (2 months old) by co-localization of mCherry with various opsin antibodies
Summary
The pineal gland, which contains light-sensitive photoreceptor cells, plays important roles in the regulation of circadian rhythms in animal behavior and physiology [1,2]. The pineal gland develops during early embryonic stages [3]. The pineal gland is located in the dorsal diencephalon, and it contains both rod and cone photoreceptor cells [4,5,6]. The pineal photoreceptor cells function autonomously with strong rhythmic patterns. In cultured zebrafish pineal glands held in constant darkness, robust day-night rhythms in melatonin release and the expression of serotonin-n-acetyl-transferase are observed, suggesting the involvement of endogenous circadian clocks in pineal photoreceptor cells [9,10]
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