Chronobiology

Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms.

In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members (which may also be influenced or reset by external factors).

A circadian cycle was first observed in the 18th century in the movement of plant leaves by the French scientist Jean-Jacques d'Ortous de Mairan.

By arranging the selected species in a circular pattern, he designed a clock that indicated the time of day by the flowers that were open at each given hour.

In 2002, Hattar and his colleagues showed that melanopsin plays a key role in a variety of photic responses, including pupillary light reflex, and synchronization of the biological clock to daily light-dark cycles.

Using a Beta-galactosidase assay, they found that these RGC axons exit the eyes together with the optic nerve and project to the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals.

Hattar concluded that melanopsin is the photopigment in a small subset of RGCs that contributes to the intrinsic photosensitivity of these cells and is involved in their non-image forming functions, such as photic entrainment and pupillary light reflex.

[12] Hattar and his co-workers discovered that, even among the subtypes of ipRGC, there can be designated sets that differentially control circadian versus pupillary behavior.

This demonstrated that the M1 ipRGC consist of molecularly distinct subpopulations that innervate different brain regions and execute specific light-induced functions.

[13] This isolation of a 'labeled line' consisting of differing molecular and functional properties in a highly specific ipRGC subtype was an important first for the field.

It also underscored the extent to which molecular signatures can be used to distinguish between RGC populations that would otherwise appear the same, which in turn facilitates further investigation into their specific contributions to visual processing.

In a study published in 2012, the Hattar Laboratory was able to show that deviant light cycles directly induce depression-like symptoms and lead to impaired learning in mice, independent of sleep and circadian oscillations.

Chronic administration of the antidepressant Fluoxetine lowered corticosterone levels in T7 mice and reduced depression-like behavior while leaving their circadian rhythms unaffected.

In addition, T7 mice exhibited impaired hippocampal long-term potentiation (LTP) when subjected to theta burst stimulation (TBS).

[19] More recently, light therapy and melatonin administration have been explored by Alfred J. Lewy (OHSU), Josephine Arendt (University of Surrey, UK) and other researchers as a means to reset animal and human circadian rhythms.

Additionally, the presence of low-level light at night accelerates circadian re-entrainment of hamsters of all ages by 50%; this is thought to be related to simulation of moonlight.

[20] In the second half of 20th century, substantial contributions and formalizations have been made by Europeans such as Jürgen Aschoff and Colin Pittendrigh, who pursued different but complementary views on the phenomenon of entrainment of the circadian system by light (parametric, continuous, tonic, gradual vs. nonparametric, discrete, phasic, instantaneous, respectively[21]).

During restricted feeding, it takes over control of such functions as activity timing, increasing the chances of the animal successfully locating food resources.