It is believed that the road to a man’s heart is via his stomach, but a new study suggests that when it comes to sex, food may be the second thing on his mind.
Although the new study was performed on a mouse, it could aid us in resolving the issue of “why diets don’t work” in humans.
A new study in the journal Cell Metabolism today reveals that hungry mice tend to prioritize social interaction with the opposite sex when their brains are stimulated with leptin, an appetite-suppressing hormone.
The researchers’ findings suggest that the answer to the age-old question lies in the leptin hormone’s influence on the brain.
As explained by senior author Tatiana Korotkova, “we can only pursue one behavior at a time, so our brain has to somehow compute what will be the most rewarding behavior, or what is our most urgent need.”
The primary function of the hormone leptin, which is mainly produced by adipose cells, is to regulate long-term energy balance. This crucial hormone serves as a major indicator of the body’s energy status and has a significant impact on appetite, satiety, and motivated behaviors aimed at maintaining energy reserves.
Korotkova’s team studied and activated mice neurons in the lateral hypothalamus, one of the major brain “feeding regions,” to clarify the order of basic activities including eating, drinking, socializing, and mating. They focused on neurons that had leptin receptors and neurons that make neurotensin, two hormones associated with hunger and thirst. Unexpectedly, scientists discovered that these neurons also had a role in regulating social behavior and supporting mice in striking a balance between their social and nutritional demands.
“We were astonished to find that the lateral hypothalamus links feeding and drinking to social behaviors,” adds first author Anne Petzold.
When leptin receptor neurons are activated, mice favor social contact above acute hunger or thirst. This makes biological sense since one has to be able to disregard hunger or thirst in order to participate in mating because mating partners are not always available.
Researchers used small microscopes to watch the activity of individual brain cells while mice roamed around an enclosure and did different things.
“It was a huge advantage that we could record the activity of neurons in a freely behaving animal,” points out Korotkova. “We could really see how neuronal activity changes during particular behaviors, and we could track and change the activity of individual cells with a high time precision.”
The team compared the behavior of mice with unlimited access to food with that of “acutely hungry” mice (whose food had been taken away overnight) and “chronically hungry” mice to see how the mice’s priorities changed depending on how hungry they were (whose food had been restricted for 5 days). The researchers point out that in the wild when food is sometimes scarce, this “chronic hunger” may also manifest itself.
Researchers discovered that mice with leptin receptor neurons suppressed while eating and activated when interacting with mice of the opposite sex (possible mates), but not with mice of the same sex.
The next step was to selectively excite neurons using light and chemical signals, and then watch for any behavioral changes in the mice as a result of this stimulation.
When mice were full, leptin stimulation didn’t change their behavior much, because they were more interested in socializing than eating. However, when researchers turned on the leptin receptor neurons of acutely hungry mice, their priorities changed. They moved slower toward food, ate less, and spent more time talking to potential mates.
Leptin stimulation, on the other hand, was unable to counteract the increased hunger of chronically food-restricted mice because their appetites and priorities were unaffected by leptin activation.
“So, we have this system which can only regulate moderate hunger, but not strong hunger,” adds Korotkova. “This circuit might contribute to why diets don’t work: it’s not a problem to reduce your food intake for a short time, but it doesn’t work if you try to do it for longer.”
Yet, when the neurotensin neurons were stimulated, the researchers saw increased drinking behavior at the price of interacting with mice of the same sex and possible mates.
“We usually think about neurons having a particular function, but we found that one cell can actually encode multiple different stimuli,” adds Korotkova. “This makes a lot of sense biologically because behaviors need to be coordinated, and it’s much more efficient to coordinate behaviors with the same cell than by many different cell types somehow communicating with each other.”
Next, Korotkova adds, they would “like to understand how the activity of these cells changes during the progression of obesity or the development of eating disorders.”
Image Credit: Getty and Petzold et al.
