Touch plays an important role in social behavior. A kind gesture, a hug, a pat on the back strengthens our social relationships. But what happens to our brain as a result of touch?
In his latest study, published in the scientific journal current biologyresearchers at the Institute of Biology at the Faculty of Science at Eötvös Loránd University described the role of a new neural pathway in the brain.
Social behavior requires complex sensory inputs involving multiple senses, for example, touch, sight, hearing, and smell. Psychologists, but also instinctively the everyday person, know the importance of touch, for example, the calming effect of hugs and caresses from 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 identified that information acquired through touch is transmitted to the thalamus of the brain and made conscious in the cerebral cortex, but at the same time, it has been suggested that the brain learns about stimuli that come from our peers in another way, since the pleasant sensation appears even without consciousness.
In order to understand the mechanism of unconscious touch, a study was completed led by Árpád Dobolyi, professor at Eötvös Loránd University, in which Semmelweis University, the Institute of Experimental Medicine and the University of Heidelberg also participated. The first author was David Keller, a PhD student of Árpád Dobolyi.
During their research, they noted that the neural pathway leading directly from the thalamus to the hypothalamus plays an important role in processing tactile information, and this pathway uses parathyroid hormone neuropeptide 2 (PTH2) as a neurotransmitter .
In the future, their results may contribute to the development of therapeutic agents that can help the development of social behaviors.
“The research showed that tactile stimuli from conspecifics are processed in the brain differently than stimuli created by inanimate objects. The two pathways separate in the thalamus area.
“The brain mechanisms activated by conspecifics also directly reach the hypothalamic regions responsible for the activation of behavioral, hormonal and vegetative responses, as well as the feeling of reward,” said Árpád Dobolyi, head of research at the Department of Physiology and Neurobiology. ELTE Institute of Biology.
WHAT HAPPENS IN OUR BRAIN WHEN IT TOUCHES US?
The researchers modeled the social contact of female rats who 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 the regulation of instinctive behaviors in humans. However, it is not known how the information necessary for social behavior reaches the hypothalamus.
According to the research hypothesis, the ascending sensory pathway that carries information about social touch reaches the hypothalamus from the thalamus without a relay to the cerebral cortex. This thalamo-hypothalamic neuronal pathway was previously unknown. At the same time, this input can directly cause hormonal and autonomic changes controlled by the hypothalamus.
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. The activity of these neurons was then experimentally increased or decreased using chemogenetic methods based on viral gene transfer.
These thalamic neurons were found to facilitate friendly social interactions between experimental rats of the same sex, which involve direct contact, that is, physical contact. They then described the outputs of the studied thalamic nucleus and found that neurons in the nucleus project further to the anterior part of the hypothalamus, the so-called preoptic area.
AFTER DISCOVERING NEURAL PATHWAYS, THEY RETURNED TO MOLECULAR MECHANISMS
The researchers also showed that experimentally manipulating the activity of the thalamo-hypothalamic neural pathway also determines how the animals interact with each other. Therefore, this neural pathway plays an important role in the processing of information associated with contact. After that, they looked at the molecules that carry information in the neural pathway, the so-called neurotransmitters. Neuropeptide transmitters play multiple roles in the neural networks responsible for social relationships.
Oxytocin is a prosocial neuropeptide known to promote social interactions, including social contact in rodents.
Other neuropeptides have been shown to play a crucial role in the behavioral response to chronic social isolation.
Parathyroid hormone-like neuropeptide (PTH2) has recently been shown to detect the presence of conspecifics in zebrafish via lateral line organ mechanoreceptors. Surprisingly, the neuropeptide PTH2 was only present in those neurons in the thalamus that were activated during social interaction.
Furthermore, the level of PTH2 in these neurons decreased when the animals were separated from each other. In subsequent experiments, the researchers showed that PTH2 stimulates neurons located in the preoptic area of the hypothalamus. However, when PTH2 was experimentally prevented from binding to its receptors in the preoptic area, physical contact between the animals ceased.
This demonstrated that the peptide neurotransmitter PTH2 conveys important inputs that determine social behavior to the preoptic area from the thalamus. Finally, the researchers showed a similarity in the anatomical structure of the thalamo-hypothalamic neuronal pathway and the distribution of the PTH2 receptor between the rat and human brains.
“This discovery may also be important in the future for the treatment of psychological diseases, since avoiding physical contact is an inherent part of many diseases. If we know these neural pathways and mechanisms, in the long term we will be able to better understand why the ‘avoidance of physical contact, and possibly prevent these processes and influence them favorably for the individual.
Of course, this is still a distant future, but with research we have come closer to understanding how our brain and therefore our body reacts to touch, which brain areas are activated when we are touched,” explained Árpád Dobolyi , head of research at the Department of Physiology and Neurobiology, ELTE Institute of Biology, summarized the results.
About this social neuroscience research news
Author: Sara Bohm
Contact: Sara Bohm – ETLE
Image: The image is in the public domain
Original Research: Closed access
“A thalamo-preoptic pathway promotes social preparation in rodents” by Árpád Dobolyi. current biology
A thalamo-preoptic pathway favors social preparation in rodents
- Social interaction increases activity in the posterior thalamus (PIL)
- Activity of socially labeled PIL neurons drives social grooming behavior
- PIL neurons that express the neuropeptide PTH2 project to the preoptic area (MPOA)
- PTH2 neurons excite MPOA cells and in turn control social grooming
Social contact is an essential component of communication. Little is known about the underlying pathways and mechanisms. Here, we discovered a novel neural pathway from the posterior intralaminar thalamic nucleus (PIL) to the medial preoptic area (MPOA) involved in the control of social grooming.
We found that PIL and MPOA neurons were naturally activated by physical contact between female rats and also by chemogenetic stimulation of PIL neurons. Activity-dependent labeling of PIL neurons was performed in rats experiencing physical social contact.
Chemogenetic activation of these neurons increased social conditioning among familiar rats, as did selective activation of the PIL-MPOA pathway. Neurons projecting from the PIL to the MPOA express parathyroid neuropeptide 2 (PTH2), and central infusion of its receptor antagonist decreased social appropriation.
Finally, we showed a similarity in the anatomical organization of the PIL and the distribution of the PTH2 receptor in the MPOA between rat and human brains. We propose that the discovered neural pathway facilitates physical contact with conspecifics.