Yale study in mice says semaglutide needs brain hunger neurons to sustain fat loss
A mouse study suggests semaglutide's lasting fat loss depends on brain hunger neurons it activates rather than quiets, a finding that does not yet change what is known about the drugs in people.
Semaglutide, the drug sold as Ozempic, Wegovy and Rybelsus, appears to need the brain's hunger neurons to keep fat off — cells the drug activates instead of quiets, according to a Yale-led study in female mice. Mice engineered to lack those neurons did not sustain weight loss on GLP-1 drugs, the team reported August 4 in the Proceedings of the National Academy of Sciences [1][2].
The findings complicate the common explanation that GLP-1 medications work mainly by suppressing appetite. Agouti-related peptide (AgRP) neurons have long been described as hunger cells that stimulate eating and work against weight loss [1]. In the new experiments, semaglutide recruited them instead [1].
The study began with a gap in researchers' understanding. GLP-1 therapies such as semaglutide can produce sustained weight loss of 10 to 15% or more, and earlier generations of weight loss drugs can reduce appetite nearly as effectively without producing the same lasting loss [1]. That difference led the Yale team to suspect semaglutide does more than make animals eat less [1].
Researchers gave semaglutide to mice and tracked body weight, food intake, metabolism and energy expenditure. Using genetic techniques, they selectively eliminated or silenced AgRP neurons in some animals. In mice without those neurons, GLP-1 drugs could no longer sustain weight loss [1][2]. Additional work using electron microscopy, molecular biology and electrophysiology showed the AgRP neurons were activated during treatment rather than suppressed [1].
"This completely changes how we think about the mechanism involved in these medications and provides new insight into the biology underlying their long-term effects," said Mateus d'Ávila, a Ph.D. candidate in neuroscience in Tamas Horvath's lab at Yale School of Medicine and the study's first author [1][2].
The experiments were done in mice, so whether the same mechanism operates in humans is not yet known [1]. Whether it applies to males of any species is also unestablished: the paper's title specifies female mice [1].
Outside researchers said the work underlines how much remains unknown. "These drugs are very mysterious," said Michael Russo, M.D., a board-certified bariatric surgeon at MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, California. He said the overall message is that "we don't fully understand how GLP-1s affect weight loss in the brain," while the drugs' effectiveness is established [2].
Why it matters for patients
The study does not change the approved uses or dosing of semaglutide, and whether the finding will affect how doctors prescribe GLP-1 drugs has not been reported. It is a mouse study; no human data on this mechanism has been published in these reports.
If the brain handles appetite and fat loss through partly separate circuits, as these results suggest, that offers one possible explanation for why semaglutide's effects last longer than older appetite-suppressing drugs even though both reduce how much animals eat [1]. The Yale authors said the work could eventually help researchers design therapies that are more effective or have fewer side effects, but no such drug has been tested in people on the basis of this finding [1][2].
Whether men and women taking semaglutide respond through the same brain circuit is not addressed by these reports. The paper's experiments were limited to female mice [1].
What happens next
The next-generation obesity drug retatrutide is not approved by the FDA, and the agency has raised concerns about illicit, unapproved versions of it. Its manufacturer, Eli Lilly, expects to file for approval in early 2027 [2]. Tirzepatide (Mounjaro, Zepbound), which acts on the GLP-1 and GIP receptors, may already be more effective than first-generation GLP-1s such as liraglutide, according to the surgeon quoted by Forbes Health [2]. Whether the AgRP mechanism applies to tirzepatide or retatrutide has not been reported.
Whether the Yale team or other researchers will test the mechanism in people has not been reported, and no timeline for human studies has been disclosed [1][2].
Sources
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