researchers uncover the biological pathways that transmit pleasant touch

Researchers at Washington University School of Medicine in St. Louis uncover how the body transfers the sensation of pleasant touch from the skin to our brains – at least as mouse bodies do.

Photo credit Dimitris Vetsikas.

Who here doesn’t love a gentle caress? Known as “gentle touch,” this type of physical stimulus is known to support our emotional state and protect our mental health and overall balanced development. But how exactly this sensation is transmitted through the body was not known. Finding out more about the neural circuits and neuropeptides — chemical messengers that transmit signals between nerve cells — that transmit this sensation between the skin and the brain can help researchers understand and treat disorders associated with touch avoidance.

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“A pleasant sense of touch is very important in all mammals,” said study leader Zhou-Feng Chen, PhD, director of the Center for the Study of Itch & Sensory Disorders at Washington University. “An important way babies are nurtured is through touch. Holding the hand of a dying person is a very powerful, comforting force. Animals groom each other. People hug and shake hands. Massage therapy reduces pain and stress and may benefit patients with psychiatric disorders.”

“In these experiments in mice, we identified an important neuropeptide and hardwired neuronal pathway dedicated to this sensation.”

Our sense of touch is academically divided into two parts: discriminative touch and affective touch. The first is the sense that allows us to determine that something is coming into contact with our body, with what force this is happening and at what exact location this is happening. The second – which can be “pleasant” or “repulsive” – ​​also carries a strong emotional value to the sensation of touch.

While studying affective touch in humans is easy because participants can report the sensation, it is extremely difficult to do the same with laboratory animals. Researchers can only indirectly infer an animal’s emotional response to a particular touch by observing its behavior.

To determine this, Chen’s team placed their lab mice in individual mouse cages, separate from the rest of the mouse community (these animals are usually housed in large groups as they are very social). The lack of physical interaction made the mice more willing to be petted by the researchers with a soft brush, which evokes a similar sensation to being petted or groomed.

After several days of brushing, the mice were placed in an experimental environment where they could choose to go into one of two chambers. In one they would be brushed; in the other no additional stimulus would be delivered. The mice practically always chose the cleaning room. This step established that the mice enjoyed the sensation and would seek it out (strongly suggesting they were comfortable).

Next, the team worked to identify potential neuropeptide candidates for mediating that pleasurable touch produced by brushing. The search led them to the prokineticin 2 (PROK2) molecule in sensory neurons and the prokineticin receptor 2 (PROKR2) molecule in the spinal cord, which relayed this sensation to the brain. By breeding genetically engineered mice lacking PROK2, the team found that these animals could not perceive pleasant touch, but continued to respond normally to other touch stimuli, such as itching. Mice lacking PROKR2 in their spinal cord also avoided activities like grooming and showed signs of stress not seen in control mice.

Further experiments confirmed that the PROK2 pathway is a special pathway for pleasant touch and does not transmit other types of physical stimuli.

“This is important because now that we know which neuropeptide and which receptor transmit only pleasant touch sensations, it may be possible to amplify pleasant touch signals without disrupting other circuitry, which is crucial since pleasant touch boosts multiple hormones in the brain.” necessary for social interactions and mental health,” Chen explained.

Mice that lacked the ability to sense pleasant touch from birth responded more strongly to stress and exhibited much more social avoidance behavior than mice whose sense of pleasant touch was blocked in adulthood. This underscores the importance of maternal touch for the healthy development of offspring, Chen says.

“Mothers like to lick their puppies, and adult mice often groom each other for good reasons, e.g. B. to promote emotional bonding, sleep and stress reduction,” he said. “But these mice avoid it. Even when their cagemates try to groom them, they withdraw. They don’t clean other mice either. They are withdrawn and isolated.”

The paper “Molecular and neural basis of delicious touch sensation” was published in the journal Science.