Feeding behavior and social stimulation activate intermingled but distinct brain circuits, and activating one circuit can inhibit the other, according to a new study published in Nature.
The researchers demonstrated in mice that direct stimulation of fewer than two dozen nerve cells, or neurons, linked to social interaction was enough to suppress the animals' drive to feed themselves -- a finding with potential clinical significance for understanding and treating eating disorders such as anorexia.
The researchers made these findings by developing a technique for teasing apart separate but closely intertwined sets of neurons in the brain.
"We know social situations can inhibit the urge to eat," the senior author said. "One example is the behavior of people at different levels of dominance in a social hierarchy. You're not going to dive into that plate of ribs when you're dining in the presence of royalty."
Anorexia is another example. "People with anorexia report that a powerful driver, at the disorder's onset, was feedback from others indicating they'd be rewarded for restricting their food intake," said the senior author.
The group focused on a part of the brain called the orbitofrontal cortex, a sheet of cells that, in both mice and humans, lies on the brain's outer surface toward the front of the organ. This brain region, which is similar in the two species, has been shown in human imaging studies to be active when subjects are wishing for, seeking, obtaining and consuming food, or when they're socially engaged.
Exploring the interactions of feeding and social drives was guaranteed to be tricky."It's not as if there is a cluster of 'feeding' neurons' and another cluster of 'social' neurons sitting in two neatly labeled clumps in the orbitofrontal cortex, so you can just position an electrode in one or the other cluster and find out all you need to know," the senior author said. The neurons driving and responding to these different activities are interspersed, scarce and scattered throughout the orbitofrontal cortex like sprinkles on a cupcake. Plus, they all look pretty much the same.
So the researchers designed a sophisticated system for simultaneously stimulating and monitoring activity in multiple designated neurons. This let them determine which orbitofrontal-cortex neurons were active during feeding-associated or social activities, or both, or neither. The technology also allowed them to stimulate on the order of 20 neurons identified as dedicated to one or the other activity and watch what behavior resulted.
The scientists inserted the gene for improved opsin into the orbitofrontal cortex of mice, along with another gene that causes neurons to fluoresce in proportion to their activity. A tiny lens at the tip of the optical fiber guided light across numerous targeted neurons with near simultaneity, causing as many as two dozen designated neurons to fire together.
During the experiments that followed, the mice were constrained by an apparatus that kept their heads comfortably fixed in place. In one set of experiments, mice were exposed to a spout that occasionally issued a drop of a high-calorie solution, which could be readily licked up. For each mouse, the researchers recorded which orbitofrontal-cortex neurons among the several hundred in their field of view lit up during this activity.
Optogenetically stimulating just 20 feeding-responsive neurons enhanced the mice's licking activity in the presence of the high-calorie solution, tying those neurons causally to feeding behavior.
To identify social-responsive neurons in the mice's orbitofrontal cortex, the scientists introduced juvenile mice -- which older mice perceive as nonthreatening potential buddies and set about sniffing -- and tracked activity levels in the neurons in the field of view. They were able to identify specific neurons responsive to the exploratory social interaction.
Optogenetically stimulating social-responsive orbitofrontal neurons in the presence of a caloric reward reduced the amount of time the mice spent licking the solution. So did the natural-stimulation equivalent: exposure to juvenile mice. The more the social interaction, the less the interest in calories.
While the mice in this study weren't a disease model, the senior author noted the findings' potential clinical significance.
"We've been able to pinpoint otherwise indistinguishable orbitofrontal-cortex neurons involved in feeding and social drive states," the senior author said. "A key goal was to know which neurons actually matter for behavior. Now that we do, we can examine them more closely to look for, say, surface-protein markers or wiring differences that distinguish them from one another. If there are any distinctions like that, it will deepen our understanding of how competing drives are negotiated among neuronal cell types in the cerebral cortex -- and could even lead to pharmaceutical interventions that reduce social inhibition of food consumption among people with anorexia."
http://med.stanford.edu/news/all-news/2019/01/when-activated-social-brain-circuits-inhibit-feeding-behavior-mice.html
https://www.nature.com/articles/s41586-018-0866-8
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