Circadian Rhythms

Circadian rhythms are endogenous and self sustaining in all animals and plants. These rhythms are present in the absence of environment clues such as light, temperature and social clues. In absence of clues, animals free run in constant darkness due to programmed genetic interactions. Some of the genes involve in this processes are Per, Clock and Cry. The expressions of these genes are tightly regulated at molecular level by proteins which bind to promoters and repressors to create a rhythm throughout the day. For example, bmal and clock bind to ebox region to produce cry and mper proteins (Hong and Chong, 2007). These proteins are concentration dependant which means high level binds to repressor region to avoid further transcription. Such oscillations work on close to 24 hour cycle in animals and plants. These processes occur without any environmental clues. In case the environment clues are introduced to animals, they tend to synchronize internal clock with external signals. One such example of synchronization is shown in dorsophilia which increase Tim protein at night and the presence of external light decrease the production of Tim protein. This results into phase delay in dorsophilia (Leuloup and Goldbeter, 2001).

The idea of phase advance and delay were first proposed by Aschoff and Pittendrigh (1960), but subsequent genetic studies have shown exact genes involved in phase delay and advance occurs due to over or under production of proteins as described in dorsophilia studies. Many knock out studies have shown that disruption of genes involve in circadian rhythm have created arrythmicity in animals. Low-Zeddies and Takahashi (2001), created clock mutants which were arrhythmic when exposed to dark condition. The period of clock mutants were greater when compared to wildtype mice. The mutant also showed higher phase-shifts hours and lower circadian amplitude.

Although clock expression has been important to understand rhythm, the initial information from retinohypothalamic tract to core or ventro-lateral region of the SCN has been a prime focus of the recent studies. It is widely known that information from ventrolateral region of SCN communicates with other regions of the SCN. Buhr and Yoo (2010), show ventrolateral and dorsomedial neuronal connection exists and this connection has a role in circadian rythm. Their data shows that tetrodoxin can make SCN temperature incompensated due to inhibition of signal from core to shell regions. Similarly, vasoactive intestinal peptide and peptide histidine iso-leucine are expressed in SCN when light information travel from retinohypothalamic tract.