Ding Liu (right), a postdoctoral investigator in the laboratory of Catherine Dulac (left).
Veasey Conway/Harvard Staff Photographer
Health
Nourishment, hydration — and a companionable face
Health practitioners regard social interaction as a fundamental human necessity. New research has uncovered its neurological foundations.
Healthcare and medical experts now perceive social ties as an essential human requirement on par with nourishing food and adequate shelter. Indeed, the U.S. Surgeon General brought attention to social disconnection as a significant public health issue in 2023.
Nonetheless, the processes underlying how feelings of loneliness or inherent social needs are represented in the brain remain ambiguous. A groundbreaking study published in Nature, “A Hypothalamic Circuit Underlying the Dynamic Control of Social Homeostasis,” delves into this need’s neurological underpinnings, revealing the mechanisms that regulate the longing for companionship.
“Recent investigations, ours included, propose that social requirements are just as vital for the well-being of animals as other [fundamental] needs,” remarked Ding Liu, a postdoctoral researcher in the Catherine Dulac Lab and Nao Uchida Lab within the Department of Molecular and Cellular Biology and the Center for Brain Science, who directed the research.
Furthermore, an inability to partake in meaningful social engagements constitutes one of the most crippling dimensions of mental health disorders such as autism, depression, and schizophrenia, stated Dulac, the Samuel W. Morris University Professor.
She highlighted that although scientists do not fully grasp why this occurs, obtaining a mechanistic comprehension of how the brain manages the desire for social interaction will yield essential insights into healthy versus diseased states of the brain associated with social contexts.
What if the longing for social engagement was less about seeking pleasure and more about evading discomfort — similarly to hunger and thirst?
In their quest to understand the need for social interaction, Liu and his colleagues flipped the traditional narrative. “A wave of research has addressed the gratifying aspect of social behavior,” cited Liu, referencing substances like dopamine, oxytocin, and serotonin that are generated during social interactions and instill feelings that reinforce such engagements.
Instead, they posed the question: What if the urge for social interaction wasn’t about pursuing happiness but rather about avoiding unhappiness – akin to hunger and thirst? Researchers have pinpointed neurons that trigger drives associated with negative experiences.
“For instance, if we are in pursuit of the ‘hunger neurons,’ we should focus on the neurons that are activated during periods of food deprivation rather than during feeding,” clarified Liu.
Considering the hypothesis that the requirement for social conduct might resemble that for hunger or thirst, the team concentrated on neural activity within the hypothalamus, where neurons regulating these other needs reside.
Aiming to pinpoint the neurons responsive to isolation, the researchers devised a scenario. By isolating mice for several days, they discerned two separate phases: the deprivation phase (when the mice were solitary) and the reunion phase (when the animals reunited).
They then monitored gene expression related to activity and conducted in vivo calcium imaging to detect which neurons activated during “social seeking” (the deprivation phase) and “social satiety” (the reunion phase).
A complicating factor in their findings was the realization that prolonged deprivation alters mice’s responses. “If you isolate the mice for over four weeks, they begin to develop an aversion to social engagement,” he stated. Isolation becomes normalized, and having companionship feels disruptive, he hypothesized.
The researchers also examined how sensory experiences affect social needs in mice. In one experiment, mice were physically separated from their littermates yet could still see, hear, and smell them through a perforated barrier. Yet the outcomes resembled those of the social isolation study, indicating that tactile stimulation is critical for satisfying social needs.
To delve deeper into this aspect, the researchers set up a tactile preference experiment, allowing mice to choose between a tunnel lined with soft fabric or one of bare plastic. The mice exhibited a marked preference for the fabric tunnel following social isolation.
This, he noted, could have direct implications for humans. “For human beings, tactile interaction is a major component of social behavior as well,” he explained. “We embrace one another, shake hands, and in close relationships, we engage in even more touch-oriented interactions.”
In today’s context, where increased interactions take place via screens rather than face-to-face, such research may unveil important insights into human responses. For individuals “overwhelmed by the digital realm,” Liu conjectured, “tactile interaction is a missing element.”
“Investigating the reasons behind our need to socialize aids in deciphering the biological and psychological foundations of human conduct,” expressed Mostafizur Rahman, a postdoc in the Dulac Lab and one of the authors of the paper. “By examining these origins, we can gain a better understanding of how social connections impact our mental well-being and relationships with others.”
Dulac concluded: “Our revelation of analogous neural circuit structures that encode social necessity and physiological needs like those for nourishment, hydration, and sleep directly underscores how vital social interactions are for leading healthy lives.”
This research was partially supported by grants from the National Institutes of Health.