Scientists have discovered in mice the brain secrets of hugging and caressing: a neuronal circuit and a chemical messenger regulate the sensations that occur when emotional contact with others. Without those circuits, social isolation and stress prevail.
By studying mice, scientists at Washington University School of Medicine in St. Louis have identified a neural circuit and a neuropeptide, a chemical messenger that carries signals between nerve cells, that convey the sensation known as pleasant skin touch to the brain
Pleasant touch, which is caused by hugging, holding hands or caressing another person, triggers a psychological impulse that is important for emotional well-being and healthy development.
Identifying the neuropeptide and circuitry that drives the sensation of pleasant touch may help scientists better understand and treat disorders characterized by contact avoidance and impaired social development, including autism spectrum disorder. The study is published in the journal Science.
“The pleasant sensation to the touch is very important in all mammals”, explains the main researcher, Zhou Feng ChenPhD, director of the Center for the Study of Sensory Disorders at the University of Washington.
“An important way to nurture babies is through touch. Holding the hand of a dying person is a very powerful comforting force. The animals groom each other. People hug and shake hands. Massage therapy reduces pain and stress and may be of benefit to patients with psychiatric disorders. In these mouse experiments, we have identified a key neuropeptide and a hardwired neuronal pathway dedicated to this sensation.”
Chen’s team found that when they bred mice without the neuropeptide, called prokinecticin 2 (PROK2), these mice could not feel pleasant touch cues but still reacted normally to itching and other stimuli.
“This is important because now that we know which neuropeptide and receptor transmit only pleasurable tactile sensations, it is possible to enhance pleasurable tactile signals without interfering with other circuitry, which is crucial because pleasurable touch stimulates various hormones in the brain that are essential for social interactions and mental health”, adds Zhou-Feng Chen.
Among other findings, Chen’s team found that mice engineered to lack PROK2, or the spinal cord neural circuitry that expresses its receptor (PROKR2), also avoided activities such as grooming and showed signs of stress not seen in normal mice. .
The researchers also observed that mice that lacked a pleasurable touch sensation from birth had more severe stress responses and exhibited greater social avoidance behavior than mice whose pleasurable touch response was blocked in adulthood. Chen said the finding underscores the importance of maternal contact in the development of offspring.
“Mothers like to lick their pups, and adult mice groom each other frequently, too, for good reasons, such as aiding in emotional bonding, sleep and stress relief,” he said. “But these mice avoid it. Even when their cage mates try to groom them, they walk away. They also do not groom other mice. They are withdrawn and isolated.”
matter of tact
Scientists usually divide the sense of touch into two parts: discriminative touch Y affective touch. Discriminative touch allows the one being touched to detect that touch and identify its location and strength. When it is affective, pleasurable or aversive, touch acquires an emotional value.
Studying pleasurable touch in people is easy, because a person can tell a researcher how they feel about a certain type of touch. Mice, on the other hand, can’t do that, so the research team had to figure out how to get the mice to let themselves be touched.
“If an animal doesn’t know you, it will usually stay away from any kind of contact because it can see you as a threat,” says Chen. “Our difficult task was to design experiments that would help overcome the animals’ instinctive avoidance of touch.”
To get the mice to cooperate, and to find out if they experienced touching as pleasurable, the researchers kept the mice apart from their cage mates for a period of time, after which the animals were more willing to be stroked with a soft brush. similar to pets being petted and groomed.
After several days of brushing, the mice were placed in a two-chamber environment. The animals were brushed in a chamber. In the other chamber, there was no stimulus of any kind. Given a choice, the mice went into the chamber where they would be brushed.
Chen’s team then began work to identify the neuropeptides that were activated by pleasurable brushing. They discovered that PROK2 in sensory neurons and PROKR2 in the spinal cord transmitted pleasing tactile signals to the brain.
In further experiments, they found that the neuropeptide they had zeroed in on was not involved in the transmission of other sensory signals, such as itching.
Chen, whose lab was the first to identify a similar dedicated pathway for itching, said that the sensation of tactile sensation is transmitted through an entirely different dedicated network.
“Just as we have cells and peptides specific for itch, we have now identified neurons specific for pleasurable touch and a peptide to transmit those signals,” he said.
Molecular and neural basis of pleasant touch sensation. Benlong Liu et al. Science, 28 Apr 2022; Vol 376, Issue 6592; pp. 483-491. DOI: 10.1126/science.abn2479