The same nerves involved in the fight-or-flight response can cause permanent damage to the cells responsible for producing hair colour in mice, scientists have found.
There is some truth to the long-standing anecdote that your locks can lose colour when you're stressed.
A team of researchers has found that in mice, stressful events trigger damage to the stem cells that are responsible for producing pigment in hair. These stem cells, found near the base of each hair follicle, differentiate to form more specialised cells called melanocytes, which generate the brown, black, red and yellow hues in hair and skin. Stress makes the stem cells differentiate faster, exhausting their number and resulting in strands that are more likely to be transparent — grey.
The study, published Wednesday in Nature, also found that the sympathetic nervous system, which prepares the body to respond to threats, plays an important role in the greying process.
"Normally, the sympathetic nervous system is an emergency system for fight or flight, and it is supposed to be very beneficial or, at the very least, it's effects are supposed to be transient and reversible," said Ya-Chieh Hsu, a stem cell biologist at Harvard University who led the study.
The sympathetic nervous system helps mobilize many biological responses, including increasing the flow of blood to muscles and sharpening mental focus. But the researchers found that in some cases the same system of nerves permanently depleted the stem cell population in hair follicles.
The findings provide the first scientific link between stress and hair greying, Hsu said.
Stress affects the whole body, so the researchers had to do some sleuthing to figure out which physiological system was conveying its effects to hair follicles.
At first, the team hypothesised that stress might cause an immune attack on melanocyte stem cells. They exposed mice to acute stress by injecting the animals with an analogue of capsaicin, the chemical in chile peppers that causes irritation. But even mice that lacked immune cells ended up with grey hair.
Next, the scientists looked at the effects of the stress hormone cortisol. Mice that had their adrenal glands removed so they could not produce cortisol still had hair that turned grey under stress.
The system responsible for the appearance of silvery strands turns out to be the sympathetic nerves that branch out into each hair follicle in the skin.
The researchers found that the sympathetic nerve cells released a neurotransmitter called noradrenaline that was taken up by nearby melanocyte stem cells. Then a series of events unfolded in quick succession: The melanocyte stem cells proliferated and turned into specialized pigment-producing cells, which abandoned their niche near the base of the follicle and left the hair without a source of pigmentation.
"I was amazed by how dramatic this change is," said Mayumi Ito, a biologist at the New York University School of Medicine who was not involved in the study. In her own research on aging mice, the greying process was gradual: The depletion of melanocyte stem cells led first to a few salt and pepper strands and then to grey or white fur, much as humans begin to see more white hair as they get older. But in Hsu's study, acute stress depleted the entire melanocyte stem cell population in mice in just five days.
Her team also found that the greying process could be halted with drugs known as CDK inhibitors, which stop the proliferation of stem cells, or by blocking the release of noradrenaline.
The findings underscore the consequences of triggering a survival mechanism when the situation isn't life-threatening.
"Stress is a normal part of life, but there are situations where stress is helpful and situations where it is detrimental," said Subroto Chatterjee, a biologist at Johns Hopkins University who studies the effects of stress on the cells in blood vessels.
Other studies have shown that stress is just one factor affecting how quickly hair goes grey, Chatterjee said. Genes and diet play a big role as well.
In a 2018 study, Chatterjee and his colleagues found that mice placed on the equivalent of a Western diet — high in fat and cholesterol — not only developed inflamed arteries, they also started going grey and experiencing hair loss. (The team also found a way to halt the process.)
But the new study is an important step toward understanding the role of stress on various tissues.
"If we can know more about how our tissues and stem cells change under stress, we can eventually create treatments that can halt or reverse its detrimental impact," Hsu said.
Written by: Knvul Sheikh
© 2020 THE NEW YORK TIMES