While it's well known that circadian clock elements sense and respond to light cycles, much less is known about how daily temperature cycles affect the clock's 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 C (35.6 F) 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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The researchers extracted and analyzed DNA and RNA from the brain tissues. They then injected the genetic material into ovary cells from Chinese hamsters. They could then measure the changes in the regulation and processing of messenger RNA (mRNA). mRNA carries instructions from the DNA inside a cell's nucleus to the rest of the cell, telling the cell that it's time to make more protein.

Surprisingly, the mu opioid receptor genes that carried the A118G variation (such variations in genes are called single-nucleotide polymorphisms) produced less mRNA than did the genes without the variant. In addition, the A118G change caused a ten-fold decrease in protein production inside the hamster ovary cells.

The mu opioid receptor gene is the first of 20 or so genes implicated in drug addiction that Sadee and his colleagues want to study. Those other genes may play a role in addiction to various drugs, including alcohol and nicotine.

"Drug addiction is a complex disorder, one that has a strong genetic component," Sadee said. "It's very hard to prove that there is a causative link between one polymorphism and addiction. But the current study provides strong evidence that there is."

Sadee conducted the study with Ohio State colleagues Ying Zhang, Danxin Wang, Andrew Johnson and Audrey Papp.

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