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Many aspects of normal behavior, and physiology for animals and humans depend on the correct functioning of the body’s circadian clocks.
This is how it works: Your brain sends signals for your body to release certain hormones at specific times during the day. You might get an increase in cortisolnature's alarm system right before you normally wake up.
Hormone release is actually dependent on interconnected activity between clocks in different parts of the brain. Washington University in St. Louis has revealed that daily release of glucocorticoids is dependent on coordinated clock-gene activity rhythms in neuronal activity rhythms found in the two hypothalamus parts, the suprachiasmatic and paraventricular nucleus.
Nature Communications published the new study with naturally behaving mice on Oct. 1.
Jeff Jones, a postdoctoral researcher in biology in Arts & Sciences who recently began work as an assistant professor in biology at Texas A&M University, said that normal behavior and physiology depend on a 24-hour circadian release. Hormone disruption can cause a variety of pathologies, including anxiety, depression, and metabolic disorders such as diabetes and obesity.
Jones was a senior author on the study and worked alongside Erik Herzog, Viktor Hamburger Distinguished Professor of Arts & Sciences at Washington University.
The SCN controls the daily timing of hormone releases. The SCN is located in the hypothalamus just above the crossing point of the optic nerves. It sends daily signals to other brain parts that decode them.
Jones stated that cortisol (corticosterone for mice and humans) is more commonly known as a stresshormone involved in the fight or flight response. The body is most stressed by the stress of getting up in the morning and getting ready for the day. This glucocorticoid is released immediately after you wake up to aid in your preparation for the day.
Or, for the night, if your mouse.
The same hormones that prepare humans for a difficult work day or morning commute also help mice to reach their running goals at night.
Jones and Herzog used a novel method of recording brain activity in mice. They did this for as long as two weeks.
Herzog stated that it is hard and expensive to record activity for long periods of time from the identified types of neurons. These methods were pioneered by Jeff for long-term, real time observations of animals behaving.
The scientists identified a critical link between the SCN (the brain's receptor for corticosterone) and the neurons in the PVN, which produce the hormone that causes the release of glucocorticoids. They used information from each mouse's rest-activity and corticosterone production.
It turns out that it is not enough for neurons in the SCN send out daily signals. The 'local' clock within the PVN neurons must also be functioning properly to produce coordinated hormone release rhythms.
Experiments that eliminated a clock gene in the circadian-signal-receiving area of the brain broke the regular daily cycle.
Jones explained that there are certain neurons in the SCN which communicate timing information to the PVN neurons that regulate daily hormone releases. "In order for normal hormone rhythms to occur, clocks must be in sync with the clocks in this downstream region and the central pacemaker.
Jones stated that the findings from mice could have implications on humans in the future. The importance of an internal clock will be a key consideration in future therapies for cortisol-related conditions and genetic conditions in humans.
Learn more about how brain rhythms can be fine-tuned to regulate behavior
More information: The paraventricular nucleus of the Circadian neuron entrains and sustains daily rhythms in glucosecorticoids, Nature Communications (2021). Journal information: Nature Communications Circadian neurons in the paraventricular nucleus entrain and sustain daily rhythms in glucocorticoids,(2021). DOI: 10.1038/s41467-021-25959-9
