Research paves the way for therapies that may one day treat jet lag.
While the two are interconnected, our biological clocks and circadian rhythms are not exactly one and the same. The so-called biological clock is, in fact, a group of neurons that form the suprachiasmatic nucleus (SCN) - a very small region in the brain's hypothalamus.
The SCN consists of around 20,000 brain cells and is responsible for aligning our internal bodily states with the external day and night cycle - that is, for our circadian rhythms.
Light is the main factor that influences our circadian rhythms, as signals from our retinas "tell" the body to activate or deactivate genes that control our biological clocks - either making us sleepy, or prompting us to wake up.
Other bodily functions that are influenced by our circadian rhythms include body temperature, the release of hormones, and physiological needs such as thirst or the need to go the bathroom.
When our circadian rhythms get disrupted, however - as they do when we have jet lag - we experience confusion and sometimes even develop health disorders. Severe disruption of our circadian rhythms may lead to sleep and mood disorders, gastrointestinal and cardiovascular problems, and even a higher risk of cancer.
However, new research may help to treat disruptions in our circadian rhythms and restore our biological clocks. The new study - published in the Journal of Physiology and led by Mike Ludwig, professor of neurophysiology at the University of Edinburgh in the United Kingdom - suggests that a group of cells in the retina may hold the key to anti-jet lag therapies.
Vasopressin-expressing cells found in rodents' retina
Ludwig and team performed a set of physiological tests in rats, in which they interfered with the light information signal sent to the rodents' SCN.
The researchers showed that the retina has cells that express vasopressin and directly regulate the circadian rhythm.
Vasopressin is a naturally occurring neurohormone - also called the antidiuretic hormone - that is secreted by our pituitary glands. It serves to regulate blood pressure and blood volume by increasing or decreasing the flow of urine. Vasopressin also generally helps the cells to stay healthy by maintaining a balanced volume of water around them.
Additionally, vasopressin is known to help the SCN control the circadian rhythm. During sleep, vasopressin levels go up, which is probably why many of us do not feel thirsty at night.
Until now, it was only known that the retina sends signals to the SCN, "telling" it that it is either light or dark outside, but the exact mechanism that made this happen was not entirely clear.
The new study, however, reveals for the first time that the retina itself has vasopressin-expressing cells and that these cells communicate directly with the SCN to regulate the body's circadian rhythm.
The findings offer a deeper understanding of our biological clocks and how light controls them. Additionally, the study opens up new avenues for researching eye treatments that could one day help to reset our biological clocks and counter the negative effects of jet lag.
"Our exciting results show a potentially new pharmacological route to manipulate our internal biological clocks. Studies in the future which alter vasopressin signaling through the eye could lead to developing eye drops to get rid of jet lag, but we are still a long way off from this."