Hypothalamus but not liver retains daily expression of clock genes during hibernation in terai tree frog (Polypedates teraiensis)

ABSTRACT

Circadian clock(s) allow an organism to be in synchrony with the surrounding environment and perform daily and seasonal physiological processes, including hibernation, migration, and reproduction. To cope with adverse environmental conditions, organisms have evolved various strategies. Insects undergo diapause, while some higher animals either migrate or hibernate/aestivate during unfavorable environmental conditions. Hibernation is an energy conservation strategy used to cope with adverse environmental conditions. Limited knowledge is available on the physiology of hibernation in non-mammalian vertebrates. Some studies suggest that metabolism is altered during amphibian hibernation, but nothing is known about the circadian clock. In the present study, we investigated daily oscillation of clock genes in the brain and liver of the terai tree frog (Polypedates teraiensis) during two annual phases of life: breeding and hibernation. Adult male terai tree frogs were procured from their natural habitat on the Mizoram University campus (23°N 92°E) during their breeding and hibernation phases. Body mass and testes weight were recorded. Animals were sacrificed at six time points: ZT1, ZT5, ZT9, ZT13, ZT17, and ZT21 (ZT0, zeitgeber time 0, indicates the sunrise time at the respective time of the year; N = 5 frogs per time point). Quantitative real-time polymerase chain reaction (qPCR) was performed for clock genes (Bmal1, Clock, Per2, and Cry2) in the hypothalamus and liver. Our results showed that body and testes weights decreased during hibernation. Further, the hypothalamus retained daily clock gene oscillations during breeding and hibernation. However, the liver lost this daily oscillation during hibernation. The maintained rhythm in hypothalamus in contrast to other hibernating animals might be the result of the fact that these animals hibernate at a higher temperature and might be more alert. As the animals have no food intake during their hibernation season which might be the reason the animals loose their rhythm in liver clock genes. These results suggest that retaining daily clock gene oscillations in the hypothalamic clock could be important for internal time tracking and post-hibernation emergence.

KEYWORDS:

• Terai tree frog
• hibernation
• clock genes
• hypothalamus
• liver

Acknowledgments

We are thankful to our reviewers whose constructive suggestions helped us to improve the quality of the manuscript. Financial support to AKT via SERB (ECR/2016/000626) and DBT New Delhi is greatly acknowledged. We also acknowledge the instrumentation facility supported under DST-FIST program to department of Zoology, Mizoram University.

Declaration of interest

We declare no conflict of interest.

Supplementary material

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Additional information

Funding

This work was supported by the Science and Engineering Research Board [ECR/2016/000626].