Scientists say they have discovered the specific brain circuitry that causes overeating, according to a study published in the journal Science.
Researchers from the University of North Carolina (UNC) School of Medicine say the discovery could provide insight into the cause of obesity, as well as lead to treatments for anorexia nervosa, bulimia nervosa and binge eating.
The researchers note that 60 years ago, it was found that scientists could electrically stimulate brain cells in the region of a mouse’s brain – called the lateral hypothalamus – causing the mouse to eat even when it was not hungry.
This latest research builds on this discovery, as the team found they could trigger hunger in mice by stimulating a specific area of the brain using fiber optic cables.
The research team focused on one specific cell type – gaba neurons. These are present in the bed nucleus of the stria terminalis, also known as the BNST. This is an “outcropping” of the amygdala – an area of the brain linked to emotion.
The BNST also forms a link between the amygdala and the lateral hypothalamus – the area of the brain linked to eating, sexual behavior and aggression. The researchers explain that the gaba neurons in the BNST consist of a cell body and a long strand with branched synapses, able to transmit electronic signals into the lateral hypothalamus.
The research team wanted to stimulate these synapses using an “optogenetic technique.” This is a process that allows BNST cells to be stimulated by shining light on the synapses.
Since brain cells do not generally respond to light, the team used genetically engineered proteins from algae that are sensitive to light and delivered these into the brains of mice using genetically engineered viruses.
From this, the proteins are only expressed in the BNST cells, meaning they are expressed in the synapses connected to the hypothalamus.
Using fiber optic cables implanted into the brains of the mice, the researchers were able to shine light through the cables, onto the BNST synapses.
Even though these specially bred mice had been well fed prior to the experiment, when the BNST synapses were activated through the light, the mice appeared ravenously hungry and even had a strong preference for high-fat foods.
“They would essentially eat up to half their daily caloric intake in about 20 minutes,” explains Garret Stuber, assistant professor in the Department of psychiatry and Department of Cell Biology and Physiology at UNC.
“This suggests that this BNST pathway could play a role in food consumption and pathological conditions such as binge eating.”
The researchers note that stimulating the BNST also caused the mice to show behaviors linked to reward, suggesting that activated BNST cells enhanced the pleasure of eating.
However, when the light on the BNST was deactivated, the mice showed little interest in food, even if they had been food-deprived.
The researchers say their findings suggest that if a person has “faulty wiring” within BNST cells, this could potentially interfere with hunger or satiety cues, leading to the development of eating disorders, or causing people to eat or avoid food when they are hungry.
Prof. Garret Stuber says:
“The study underscores that obesity and other eating disorders have a neurological basis.
With further study, we could figure out how to regulate the activity of cells in a specific region of the brain and develop treatments.”
He adds that in these further studies, the team would like to observe the normal function of these cell types and determine how they fire electrical signals when animals are feeding or hungry.
“We want to understand their genetic characteristics – what genes are expressed. For example, if we find cells that become really activated after binge eating, can we look at the gene expression profile to find out what makes those cells unique from other neurons.”
He said a successful outcome from this could lead to potential targets for the development of drugs able to treat specific populations of patients who suffer from eating disorders.