For the first time, researchers have identified, in the neurobiological pathways of the skin,a link with touch and the pleasure felt, observable in the affected areas of the brain. realizing their study on mice, the scientists published their results in the Cell magazine. The neurons in the skin therefore react to caresses and the information received is sent directly to the pleasure centers, towards the brain. This discovery paves the way for possible therapies, based on touchTo fightagainst anxiety and depression and restore social ties with people with this type of disorder, or autism. The skin has sensory cells allowing us to recognize textures, assess temperature and react to different mechanical stimuli. But so far the pleasure system at the level of these same cells was not proven (source 1).
“We weren’t sure that this image of social touch was entirely correct. We investigated whether there were any tactile neurons specifically adapted to gratifying touch,” said study researcher Dr. Abdus-Saboor.
This new discovery is the culmination of a four-year collaboration between nearly 20 scientists from three different institutions, to study these cells more closely. The technique used is called “optogenetics”. It consists of change individual cell types so that they can be activated when researchers illuminate them with specific colors of light. This technique is particularly suitable for studying the functions of specific populations of cells. Upon realizing this, they will make an unexpected discovery.
“We found that by activating this little-studied population of tactile sensory cells in the mouse back, the animals lowered their backs and assumed this dorsiflexion posture,” said Dr. Elias, a member of the study scientists.
For rodents, such posture is a key signature of sexual receptivitywhich normally requires the physical attentions from another mouse. The scientists then genetically modified mice so that certain touch cells, the “Mrgprb4”, are sensitive when illuminated by blue light. Once this mutation was made, they observed the mice realize round trips to be illuminated by the blue light, installed on one side of the observation chamber. This indicates that the animals felt Mrgprb4 sensory cell activation behind them like a reward.
“This is the first documented example showing that a specific behavior can be generated or supported by these ‘Mrgprb4’ neurons,” Dr. Abdus-Saboor said.
To get to the bottom of their theory, the scientists then eliminated these ‘Mrgprb4’ cells, using genetic techniques. They were able to observe a collapse of touch-related reward circuitsand therefore a decrease in sexual response in mice deprived of these cells
“Sexual receptivity just collapsed. We then knew for sure that these cells were important for social touch in natural encounters,” says Dr. Elias.
Dr. Elias wanted, using fiber photometry, a technique to see the brain’s reward neurons, to check if they were lighting up in response to pleasant stimuli. By activating the ‘Mrgprb4’ cells, effectively, the relevant neurons fired.
How does the passage of the sensory message from the skin to the brain take place?
The skin and the brain are linked. But how ? Through the spinal cord, and in particular certain cells called ‘GPR83’. These latter have for the role of detecting the gratifying stimuli arriving on our skinto send the message to the brain.
“It gave us the idea that these GPR83 neurons are probably a conduit connecting the skin to the brain,” Dr. Abdus-Saboor said.
Through further experiments, the team was able to trace the circuits of touch between the skin and the brain in greater depth and detail than had been achieved before. One of the main conclusions is that the brainstem neurons studied are linked to even deeper places in the brain. This is a crucial connection to observe, since these two areas of the brain are already known to be associated experience reward and pleasure.
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