Itching can be contagious and the reflex pathway behind it has been identified in mice

The contagious itch may have evolved as a survival mechanism, such as alerting other animals in the group that a parasite might be circulating

By Myriam Fauzia

Does watching someone else relieve a nagging itch spark the same impulse in you? A study in mice suggests that the contagious itch may have evolved as a reflex to protect animals from threats.

In 2017, Zhou Feng Chen at the University of Washington in Missouri and colleagues found the suprachiasmatic nucleus (SCN) in the brain of a mouse lights up when he watches other rodents scratching. This coincides with the release of a chemical messenger called gastrin-releasing peptide (GRP), first associated with itching in 2007.

Now, Chen and his colleagues have identified the SCN as a midpoint between the light-capturing cells in the retina of the eyes and a structure in the center of the brain called the paraventricular nucleus of the thalamus (PVT).

To better understand how the SCN receives contagious itch signals, the team referred to their previous research, which suggested that retinal ganglion cells, a type of light-capturing neuron, were involved.

In the latest study, the researchers injected a virus into the SCNs of a group of mice. This virus had been genetically engineered to infect target neurons in the SCN and other neurons closely related to those in the SCN. This revealed that most of the connections of SCN neurons were with retinal ganglion cells.

Then, these neurons were inhibited by genetic modification and by injection of chemical compounds into the eyes of 10 mice.

Chen and his colleagues then observed the itching behavior of these mice after the rodents were exposed to half an hour of images of other mice scratching rigorously.

They found that inhibiting these retinal ganglion cells prevented the mice from scratching.

The researchers also found that this contagious itch signaling did not appear to pass through the usual visual pathway seen in primates and rodents, which involves the visual cortex.

Light normally activates these neurons, but when light was directed through the mice’s brains to where their ganglion cells meet the visual cortex, there was no itching.

When light shone at the interface of the cells with the SCN, the contagious itching resumed.

In the same experiment, the researchers searched for other GRP-responsive neurons, finding abundance in mouse PVTs, the same region of the brain that interprets incoming stimuli and translates them into behavioral responses.

When these neurons were inhibited, the contagious itch behavior of the mice also stopped.

Additionally, Chen and her colleagues identified a chemical called pituitary adenylate cyclase-activating peptide, which light-sensitive ganglion cells release in the SCN, apparently triggering contagious itch.

“We discovered a visual pathway that is subcortical, which is mediated by unconscious behavior,” says Chen. Unlike most visual stimuli, which are first processed by the brain’s visual cortex, this pathway appears to be a reflex, he says.

He can have evolved as a survival mechanism that allows animals to react quickly to incoming threats by observing the behavior of others, Chen explains. For example, scratching could indicate the presence of a parasite.

But whether this pathway explains contagious itching in humans has yet to be seen, says Gil Yosipovich at the University of Miami Miller School of Medicine in Florida.

Human brain circuitry appears to be much more complex than that of mice, Yosipovitch says. Interpreting visual stimuli in humans also typically relies on a pathway that involves the visual cortex, he says.

If this contagious itch pathway doesn’t apply to humans, Chen says the findings could help us understand other types of contagious behaviors, such as our emotional responses.

“Emotional contagion is very interesting because it concerns the evolution and origin of empathy, [which] is so fundamental to human society,” he says.

Journal reference: Cell reports, DOI: 10.1016/j.celrep.2022.111444