‘Fight or flight’ – unless internal clocks are disrupted

Here’s how it’s supposed to work: Your brain sends signals to your body to release different hormones at certain times of the day. For example, you get a boost of the hormone cortisol — nature’s built-in alarm system — right before you usually wake up.

But hormone release actually relies on the interconnected activity of clocks in more than one part of the brain. New research from Washington University in St. Louis shows how daily release of glucocorticoids depends on coordinated clock-gene and neuronal activity rhythms in neurons found in two parts of the hypothalamus, the suprachiasmatic nucleus (SCN) and paraventricular nucleus (PVN).

The new study, conducted with freely behaving mice, is published in Nature Communications.

“Normal behavior and physiology depends on a near 24-hour circadian release of various hormones,” said Jeff Jones, who led the study as a postdoctoral research scholar in biology in Arts & Sciences and recently started work as an assistant professor of biology at Texas A&M University. “When hormone release is disrupted, it can lead to numerous pathologies, including affective disorders like anxiety and depression and metabolic disorders like diabetes and obesity.

“We wanted to understand how signals from the central biological clock — a tiny brain area called the SCN — are decoded by the rest of the brain to generate these diverse circadian rhythms in hormone release,” said Jones, who worked with Erik Herzog, the Viktor Hamburger Distinguished Professor in Arts & Sciences at Washington University and senior author of the new study.

The daily timing of hormone release is controlled by the SCN. Located in the hypothalamus, just above where the optic nerves cross, neurons in the SCN send daily signals that are decoded in other parts of the brain that talk to the adrenal glands and the body’s endocrine system.

“Cortisol in humans (corticosterone in mice) is more typically known as a stress hormone involved in the ‘fight or flight’ response,” Jones said. “But the stress of waking up and preparing for the day is one of the biggest regular stressors to the body. Having a huge amount of this glucocorticoid released right as you wake up seems to help you gear up for the day.”

Or for the night, if you’re a mouse.

The same hormones that help humans prepare for dealing with the morning commute or a challenging work day also help mice meet their nightly step goals on the running wheel.

(Image caption: Projections from SCN neurons (magenta) send daily signals to rhythmic PVN neurons (cyan) to regulate circadian glucocorticoid release. Credit: Jeff Jones)

Using a novel neuronal recording approach, Jones and Herzog recorded brain activity in individual mice for up to two weeks at a time.

“Recording activity from identified types of neurons for such a long period of time is difficult and data intensive,” Herzog said. “Jeff pioneered these methods for long-term, real-time observations in behaving animals.”

Using information about each mouse’s daily rest-activity and corticosterone secretion, along with gene expression and electrical activity of targeted neurons in their brains, the scientists discovered a critical circuit between the SCN and neurons in the PVN that produce the hormone that triggers release of glucocorticoids.

Turns out, it’s not enough for the neurons in the SCN to send out daily signals; the ‘local’ clock in the PVN neurons also has to be working properly in order to produce coordinated daily rhythms in hormone release.

Experiments that eliminated a clock gene in the circadian-signal-receiving area of the brain broke the regular daily cycle.

“There’s certain groups of neurons in the SCN that communicate timing information to groups of neurons in the PVN that regulate daily hormone release,” Jones said. “And for a normal hormone rhythm to proceed, you need clocks in both the central pacemaker and this downstream region to work in tandem.”

The findings in mice could have implications for humans down the road, Jones said. Future therapies for cortisol-related diseases and genetic conditions in humans will need to take into account the importance of a second internal clock.

(Image caption: Microscopic image of the mouse suprachiasmatic nucleus, the brain region responsible for controlling circadian rhythms)

A master gear in the circadian clock

A gene called Npas4, already known to play a key role in balancing excitatory and inhibitory inputs in brain cells, appears to also be a master timekeeper for the brain’s circadian clock, new research led by UT Southwestern scientists suggests. The finding, published online in Neuron, broadens understanding of the circadian clock’s molecular mechanisms, which could eventually lead to new treatments for managing challenges such as jet lag, shift work, and sleep disorders.

“To reset the circadian clock, you ultimately need to reset its molecular gears,” said study leader Joseph S. Takahashi, Ph.D., Professor and Chair of Neuroscience at UTSW and a Howard Hughes Medical Institute Investigator. “This study suggests that Npas4 might be one of the most important components for resetting the clock to light.”

For decades, researchers have known that a brain region called the suprachiasmatic nucleus (SCN) is responsible for controlling circadian rhythms, the various cycles of activity that typically run on a 24-hour basis. These rhythms are entrained by light, Dr. Takahashi explained; cells in the SCN respond to signals relayed by the retina, the eye’s light-sensitive tissue. However, the molecular basis of this phenomenon is not well understood.

To better understand how the SCN sets circadian rhythms, the researchers used a technique called single-nucleus sequencing to look at gene activity in individual cells in mice after the animals were exposed to light. Dr. Takahashi and his colleagues found that three different subpopulations of SCN neurons respond to light stimulation. A common thread tying these subtypes together was increased activity in genes that respond to neuronal PAS domain protein 4 (NPAS4), the protein made by the Npas4gene.

When Dr. Takahashi and his colleagues exposed mice engineered to lack Npas4 to light, it dampened the response of hundreds of circadian clock genes. In addition, the animal’s circadian period lengthened about an extra hour, to nearly 25 hours instead of the normal 24. Together, these results suggest that Npas4 is a master regulator of many light-induced genes, a key piece in the puzzle of how the circadian system works, Dr. Takahashi said.

The more researchers learn about the molecular underpinnings of the circadian clock, Dr. Takahashi added, the more they may be able to manipulate it to improve health and well-being – for example, to ease jet lag or help shift workers stay awake or asleep to match their work cycles. It could also lead to new treatments for disorders marked by abnormal sleep/wake cycles.