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 transmit the sensation known as pleasant touch name from the skin to the brain.
Such contact—delivered through hugs, hands, or caresses, for example—triggers a psychological boost known to be important for emotional well-being and healthy development. Identifying the neuropeptide and circuitry that drives the sensation of pleasurable touch could potentially help scientists better understand and treat disorders characterized by avoidance of touch and impaired social development, including autism spectrum disorders.
The study is published April 28 in the journal Science.
“A pleasant sensation of touch is very important in all mammals,” said the lead researcher. Zhou-Feng Chen, PhDdirector of the Center for the Study of Itching and Sensory Disorders at the University of Washington. “One of the main ways babies are fed is through touch. Holding the hand of a dying person is a very powerful and comforting force. Animals groom each other. People kiss and shake hands. Massage therapy reduces pain and stress and may benefit patients with psychiatric disorders.In these experiments with mice, we identified a key neuropeptide and hardwired neural pathway dedicated to this sensation.
Chen’s team found that when they bred mice without the neuropeptide, called prokinectin 2 (PROK2), these mice couldn’t feel pleasant tactile cues but continued to respond normally to itching and other stimuli.
“This is important because now that we know which neuropeptides and receptors only transmit pleasurable tactile sensations, it may be possible to enhance pleasurable tactile signals without interfering with other circuits, which is crucial because pleasurable touch stimulates many hormones in the brain that are essential for social interactions and mental health,” he explained.
Among other findings, Chen’s team found that mice engineered to lack PROK2 or the spinal cord neural circuitry expressing its receptor (PROKR2) also avoided activities such as grooming and showed signs of stress not seen in humans. normal mice. The researchers also found that mice lacking pleasurable tactile sensation from birth had more severe stress responses and exhibited greater social avoidance behavior than mice whose pleasurable tactile response was blocked in adulthood. Chen said the finding highlights the importance of maternal contact in offspring development.
“Mothers like to lick their young, and adult mice groom themselves frequently too, for good reasons, like to help bond emotionally, to sleep, and to relieve stress,” he said. “But these mice avoid it. Even when their cage mates try to groom them, they back away. They also don’t groom other mice. They are removed and isolated.
Scientists generally divide the sense of touch into two parts: discriminative touch and affective touch. Discriminating touch allows the person being touched to sense that touch and identify its location and strength. Affective, pleasant or aversive, touch attaches an emotional value to this touch.
Studying pleasant touch in people is easy because a person can tell a researcher how a certain type of touch feels. Mice, on the other hand, can’t do this, so the research team had to find a way to get the mice to let themselves be touched.
“If an animal doesn’t know you, they usually shy away from any type of contact because they may view it as a threat,” said Chen, Russell D. and Mary B. Shelden professor of anesthesiology and professor of psychiatry. medicine and developmental biology. “Our challenging task was to design experiments that helped overcome the instinctive avoidance of animal touch.”
To get the mice to cooperate — and to find out if they felt such a pleasant touch — the researchers kept the mice away from cagemates for a while, after which the animals were more willing to be stroked with a brush. gentle, like caressed and cared for pets. . After several days of such brushing, the mice were then placed in a two-chamber environment. In one room the animals were brushed. In the other room, there were no stimuli of any kind. When given the choice, the mice went to the chamber where they would be brushed.
Next, Chen’s team began working to identify neuropeptides that were activated by pleasant brushing. They found that PROK2 in sensory neurons and PROKR2 in the spinal cord transmitted pleasant tactile signals to the brain.
In other experiments, they found that the neuropeptide they focused on was not involved in transmitting other sensory signals, such as itching. Chen, whose lab was the first to identify a similar pathway dedicated to itching, said the pleasant touch sensation is transmitted by an entirely different dedicated network.
“Just as we have itch-specific cells and peptides, we have now identified pleasant-touch specific neurons and a peptide to transmit these signals,” he said.