For decades, medications for obesity typically delivered only modest reductions in body weight. That changed with the arrival of Ozempic and other GLP-1 therapies, which can produce sustained weight loss of 10 to 15% or more. Yet despite their remarkable effectiveness, scientists still do not fully understand what these medications are doing inside the brain.
New research from Yale has now uncovered an unexpected mechanism that challenges a long-standing view of the brain circuits involved in hunger and weight control. Scientists had generally thought that agouti-related peptide (AgRP) neurons, which are well known for stimulating hunger, worked mainly against weight loss. The new findings suggest something very different.
During treatment with GLP-1 drugs such as Ozempic, these neurons appear to be recruited to help maintain fat loss. "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, opening an avenue for the development of more efficient drugs," said Mateus d'Ávila, a Ph.D. candidate in neuroscience working in Tamas Horvath's lab in the Department of Comparative Medicine at Yale School of Medicine (YSM) and first author of the study. The research was published in the journal Proceedings of the National Academy of Sciences (PNAS).
Semaglutide, the active ingredient in GLP-1 medications such as Ozempic, has emerged as one of the most effective drugs ever developed to treat obesity. What has remained less clear is why its effects are so powerful and persistent. Earlier generations of weight loss medications can reduce appetite nearly as effectively as semaglutide, yet they do not produce the same level of sustained weight loss.
That difference led the Yale team to suspect that semaglutide must be doing more than simply making people or animals eat less. One widely discussed explanation had been that GLP-1 medications cause weight loss by decreasing the activity of neurons responsible for promoting hunger. However, researchers had not directly tested the role of AgRP neurons during chronic GLP-1 treatment in vivo.
The Yale team set out to identify what was missing from that explanation. By studying how the brain responds and adapts during treatment, they hoped to uncover biological targets that could eventually lead to more effective obesity therapies. Using a mouse model, the researchers combined several experimental approaches while tracking body weight, food consumption, metabolism, and energy expenditure during semaglutide treatment.
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