Social touch and its newly discovered neural pathway

Overview: A neural pathway leading directly from the thalamus to the hypothalamus plays a critical role in processing touch information, a new study reveals.

Source: MEAT

Touch plays an important role in social behavior. A friendly gesture, a hug, a pat on the back strengthens our social relationships. But what happens in our brains as a result of touch?

In their latest study, published in the scientific journal Current Biologythe researchers from the Institute of Biology of the Faculty of Science at Eötvös Loránd University described the role of a new neural pathway in the brain.

Social behavior requires complex sensory input involving several senses, such as touch, sight, hearing, and smell. Psychologists, but also instinctively ordinary people, know the importance of touch, for example the calming effect of the hugs and caresses of our loved ones and friends.

The physical contact of touch is also important in social relationships, just think of the grooming behavior of monkeys, or an appreciative pat on the back.

Neurobiologists have already established that the information obtained through touch is transmitted in the thalamus of the brain and becomes conscious in the cerebral cortex, but at the same time it has been suggested that the brain learns about the stimuli of our peers in a different way, since the pleasant feeling appears even without consciousness.

To learn more about the mechanism of touch without consciousness, a study was conducted led by Árpád Dobolyi, a professor at Eötvös Loránd University, which also included Semmelweis University, the Institute for Experimental Medicine and the University of Heidelberg. The first author was David Keller, PhD student of Árpád Dobolyi.

During their research, they pointed out that the neural pathway leading directly from the thalamus to the hypothalamus plays an important role in the processing of touch information, and this pathway uses the parathyroid hormone 2-neuropeptide (PTH2) as a neurotransmitter.

In the future, their results may contribute to the development of therapeutic agents that can aid the development of social behavior.

“The research showed that tactile stimuli from congeners are processed in the brain in a different way than stimuli created by inanimate objects. The two pathways separate in the area of ​​the thalamus.

“The brain mechanisms activated by conspecifics also directly reach the hypothalamic regions responsible for triggering behavioral, hormonal and vegetative responses, as well as the sense of reward” – said Árpád Dobolyi, head of research at the Department of Physiology and Neurobiology, ELTE Institute of Biology.

WHAT HAPPENS IN OUR BRAINS WHEN WE GET CONNECTED?

The researchers modeled social contact of female rats that were littermates. The hypothalamus, located in the lower part of the brain, below the thalamus, is the main regulatory center for social behavior in rodents, because in this species the role of the cerebral cortex is not dominant. At the same time, the hypothalamus probably also plays a role in regulating instinctive behavior in humans. However, it is not known how information needed for social behavior gets to the hypothalamus.

According to the hypothesis of the research, the ascending sensory pathway with information about social touch reaches the hypothalamus from the thalamus without relays in the cerebral cortex. This thalamo-hypothalamic neural pathway was previously unknown. At the same time, such input can directly lead to hypothalamic-controlled hormonal and autonomic changes.

As a first step, the researchers showed that neurons in a hitherto little-known area of ​​the thalamus are selectively activated in response to social contact. Subsequently, the activity of these neurons was experimentally increased or decreased using chemogenetic methods based on viral gene transfer.

These thalamic neurons were found to enable friendly social interactions between same-sex experimental rats involving direct, that is, physical contact. They then described the exits of the studied thalamic nucleus and found that the neurons of the nucleus extend most towards the anterior part of the hypothalamus, the so-called preoptic area.

AFTER DISCOVERING THE NEURAL ROADS, THEY FORCES THE MOLECULAR MECHANISMS

The researchers further proved that the experimental manipulation of the activity of the thalamo-hypothalamic neural pathway also determines how much the animals interact with each other. Thus, this neural pathway plays an important role in processing information related to contact. Then the information-carrying molecules in the neural pathway, the so-called neurotransmitters, were examined. Neuropeptide transmitters play multiple roles in the neuronal networks responsible for social relationships.

Oxytocin is a prosocial neuropeptide known to promote social interactions, including social contact in rodents.

Psychologists, but also instinctively ordinary people, know the importance of touch, for example the calming effect of the hugs and caresses of our loved ones and friends. Image is in the public domain

Other neuropeptides have been shown to play a critical role in the behavioral response to chronic social isolation.

The parathormone-like neuropeptide (PTH2) has recently been shown to detect conspecific presence in zebrafish via the mechanoreceptors of the lateral line organ. Surprisingly, the PTH2 neuropeptide was only present in those neurons of the thalamus that were activated during social interaction.

In addition, the level of PTH2 in these neurons decreased when the animals were separated from each other. In further experiments, the researchers showed that PTH2 stimulates neurons located in the preoptic region of the hypothalamus. However, when PTH2 was experimentally prevented from binding to its receptors in the preoptic region, physical contact between the animals ceased.

This proved that the PTH2 peptide neurotransmitter transmits important social behavioral inputs to the preoptic area from the thalamus. Finally, the researchers showed a similarity in the anatomical structure of the thalamo-hypothalamic neural pathway and the distribution of the PTH2 receptor between the brains of rats and humans.

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This shows a blurred blue figure in the shape of a person

“This discovery could also be important for the treatment of mental illness in the future, as avoiding physical contact is an inherent part of many diseases. Knowing these neural pathways and mechanisms will allow us to better understand, in the long run, why avoiding physical contact develops, potentially preventing and beneficially influencing these processes for the individual.

Of course, this is still a distant future, but the research has brought us closer to understanding how our brains and therefore our bodies react to touch, which areas of the brain are activated when we are touched,” Árpád Dobolyi, head of research at the department Physiology and Neurobiology, ELTE Institute of Biology, summarized the results.

About this social neuroscience research news

Author: Sara Bohm
Source: MEAT
Contact: Sara Bohm – ETLE
Image: The image is in the public domain

Original research: Closed access.
“A thalamo-preoptic pathway promotes social grooming in rodents” by Árpád Dobolyi. Current Biology


Abstract

A thalamo-preoptic pathway promotes social grooming in rodents

Highlights

  • Social interaction increases activity in the posterior thalamus (PIL)
  • The activity of socially tagged PIL neurons stimulates social grooming behavior
  • PTH2-neuropeptide-expressing PIL neurons project to the preoptic region (MPOA)
  • The PTH2 neurons excite MPOA cells and in turn control social grooming

Overview

Social touch is an essential part of communication. Little is known about the underlying pathways and mechanisms. Here we discovered a novel neuronal pathway from the posterior intralaminar thalamic nucleus (PIL) to the medial preoptic area (MPOA) involved in the control of social grooming.

We found that the neurons in the PIL and MPOA were naturally activated by physical contact between female rats and also by the chemogenetic stimulation of PIL neurons. The activity-dependent tagging of PIL neurons was performed in rats that had physical social contact.

The chemogenetic activation of these neurons enhanced social grooming among known rats, as well as the selective activation of the PIL-MPOA pathway. Neurons projecting from the PIL to the MPOA express the neuropeptide parathyroid hormone 2 (PTH2), and the central infusion of its receptor antagonist reduced social grooming.

Finally, we showed a similarity in the anatomical organization of the PIL and the distribution of the PTH2 receptor in the MPOA between the rat and human brain. We propose that the discovered neuronal pathway allows physical contact with conspecifics.

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