The biological clock controls the circadian rhythms of a wide range of physiological and behavioral processes, from fluctuating hormone levels to sleep–wake cycles and feeding patterns. While its well known that circadian clock elements sense and respond to light cycles, much less is known about how daily temperature cycles affect the clocks timing mechanism in vertebrates. In the open-access journal PLoS Biology, Kajori Lahiri, Nicholas Foulkes, and their colleagues study temperature related responses at the genetic and molecular level in zebrafish. This genetically tractable model organism is especially suited to this task because adults, larvae, and even embryos can tolerate a wide range of core body temperatures (being cold-blooded animals) that can be manipulated simply by changing the water temperature. Temperature variations of as little as 2 ºC (35.6 ºF) can reset the zebrafish clock, Lahiri et al. show, and precise shifts in temperature trigger significant changes in the expression of specific clock genes. More explicitly, clock genes per4, cry2a, cry3, and clock1 showed rhythmic expression under temperature cycles when animals were raised in the dark, and the expression profiles during the high temperature phase matched those seen during a light phase when animals experienced light-dark cycles.
Zebrafish cell lines also proved valuable tools for studying temperature response, showing a similar pattern of clock gene expression during cycles of small temperature changes and continued entrainment of clock gene expression even after the cells were exposed to constant temperature. Acute temperature shifts can also trigger significant changes in clock gene expression (transcript levels of per4 and cry3 dropped after a temperature increase and spiked after a temperature decrease; cry2 showed the opposite response)--changes wrought by temperature-dependent shifts in the behavior of transcriptional regulators, as in the case of per4.
Altogether these results show that temperature can regulate the circadian clock in this vertebrate. If the temperature-induced transcriptional responses described here operate in other temperature-related responses, they may shed light on how temperature affects other biological systems as well, including mammals.
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