Daily Rhythmic Expression Patterns ofClock1a,Bmal1, andPer1Genes in Retina and Hypothalamus of the Rainbow Trout,Oncorhynchus Mykiss

Jul 5, 2011Chronobiology international

Daily rhythms of key clock genes in the eye and brain control center of rainbow trout

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Abstract

All clock genes analyzed in the retina and hypothalamus of rainbow trout showed circadian fluctuations.

Gene expression peaked in the hypothalamus with a 3-hour delay for Clock1a and Bmal1, and a 6-hour delay for Per1 compared to the retina.
The highest expression levels in the retina occurred at zeitgeber times 9 (ZT9) for Clock1a and Bmal1, and at ZT21 for Per1.
Rhythmic gene expression patterns were maintained in the retina under constant darkness, while only Clock1a and Per1 maintained cycling in the hypothalamus.
Bmal1 did not exhibit rhythmic cycling in the hypothalamus under constant darkness, suggesting possible dependence on additional clock-gene isoforms or external regulation.
Core circadian clock genes are present in both the retina and hypothalamus of rainbow trout.

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Living organisms show daily rhythms in physiology, behavior, and gene expression, which are due to the presence of endogenous clocks that synchronize biological processes to the 24-h light/dark cycle. In metazoans, generation of circadian rhythmicity is a consequence of specialized tissues known as "master clocks," having different locations among species. A few studies have described clock-gene expression in fish neural tissues, but none of them assessed clock-gene expression in different discrete regions. The present study was designed to explore the presence/absence of circadian clock-gene expression in the rainbow trout (Oncorhynchus mykiss) retina and hypothalamus. Juvenile fish were acclimated to a 12:12 light (L)-dark (D) cycle. Then, retina and hypothalamus were collected from animals kept under LD conditions or constant darkness (DD) for 24 h. Real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assays were performed to quantify expression of the core circadian genes Clock1a, Bmal1, and Per1 as representative members of the circadian oscillator. All clock genes analyzed in the retina and hypothalamus showed circadian fluctuations. However, gene expression peaked in the rainbow trout hypothalamus with a 3-h (Clock1a and Bmal1) or 6-h (Per1) delay relative to that observed in the retina, the latter showing highest expression levels at zeitgeber times 9 (ZT9) for Clock1a and Bmal1, and at ZT21 for Per1. When exposed to DD, the rhythmic gene expression pattern was maintained for all genes in the rainbow trout retina, but only for Clock1a and Per1 in the hypothalamus. Bmal1 failed to cycle under DD, suggesting that hypothalamic clock function might depend on either several clock-gene isoforms or regulation from external inputs. Overall, these data indicate that representative molecular members of the core circadian clock are present in both the retina and hypothalamus of rainbow trout.

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Daily Rhythmic Expression Patterns ofClock1a,Bmal1, andPer1Genes in Retina and Hypothalamus of the Rainbow Trout,Oncorhynchus Mykiss

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