By: Gregory Brown, MD/PhD Candidate
Circadian (circa: around, dia: 1 day) rhythms are major controllers of our biological routine. This internal clock tells us when to wake up, eat, focus, exercise, sleep, and more. What regulates this clock? The superchiasmatic nucleus.
The superchiasmatic nucleus is involved in maintaining circadian rhythms through multiple inputs and outputs.
The suprachiasmatic nucleus (SCN) is a major regulator of the biological clock. It lies in the skull just behind your eyes. This brain region controls much of our daily biological routine. Damage to the SCN has been found to disrupt or abolish circadian rhythms.
The electrophysiology of the cells in the SCN provides a bases for coherent circadian signals, which enables an overall robust pacemaker function. The molecular basis of the SCN as a circadian pacemaker involves two key gene classes Per and Cry. These work in a feedback loop to provide inherent rhythmicity.
Some of the molecular signaling proteins involved in the circadian clock include vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP). Based on the functions of these peptides, vasoregulation (regulation of blood vessels) is an important part of the circadian cycle. Circadian rhythms can also help maintain normal metabolic function.
Circadian rhythms can be regulated by stimuli such as temperature, sound, food, and social cues. The 24-hr light-dark cycle is important to maintaining circadian rhythm. However, mice with loss of classic photoreceptors maintain normal circadian response to light. In the eye, there are cells whose whole purpose is to observe light and report to the SCN. This helps the body to understand the time of day.
Intrinsically photosensitive retinal ganglion cells can respond to light and release melaopsin, which is important in maintenance of circadian rhythms. Lack of light tells the SCN to release melatonin. This is why melatonin is a natural sleep-aid.
Other factors influencing the SCN include neuropeptide Y, GABA, and serotonergic signaling. The SCN can then send neuroendocrine and synaptic signals to modulate behavior and sensory information. The rhythmicity of the circadian cycle is an important regulator in maintaining the behavioral cycle associated with society.
Astrocytes in the superchiasmatic nucleus regulate the period of circadian rhythms
Astrocytes, the support cells of the brain, are important in regulating many neurological functions. They also play a role in regulating the SCN.
Researchers found that by knocking out Bmal1, a key circadian rhythm gene, from astrocytes in the SCN of mice, the circadian period in the SCN was increased. Per2 rhythms increased from 24.63 hrs to 25.82 hrs. Behavior was also altered. The locomotor period became about a half hour longer from 23.76 to 24.22 hrs. This finding that astrocytes regulate circadian period was validated using an independent technique.
Biological rhythms are vital to maintaining our healthy lifestyle, and leveraging these rhythms can give us better control and understanding into how our body functions throughout the day. Now we know the SCN controls these rhythms.