For decades, medications for obesity have been met with modest results, but the arrival of Ozempic and other GLP-1 therapies has changed the game. These medications can produce sustained weight loss of 10 to 15% or more, making them a game-changer in the fight against obesity. However, despite their remarkable effectiveness, scientists still don’t fully understand what these medications do inside the brain.

New research from Yale University 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. They also used genetic techniques that allowed them to selectively eliminate or silence AgRP hunger neurons. This enabled the scientists to test whether those neurons were actually required for semaglutide to produce its lasting effects.

The results were striking. In mice genetically engineered to lack AgRP neurons, GLP-1 drugs were no longer able to sustain weight loss. Additional experiments involving electron microscopy, molecular biology, and electrophysiology revealed another surprise. Rather than being suppressed by semaglutide, the AgRP neurons were activated.

The researchers say the findings point to a more complicated response inside the brain than previously recognized. When GLP-1 treatment creates a calorie deficit, the brain appears to respond by increasing the activity of AgRP hunger neurons. Those same neurons also help coordinate the loss of fat.

In other words, cells traditionally viewed as obstacles to weight loss may actually become part of the biological machinery that allows GLP-1 therapies to maintain it. The finding adds a previously unknown layer to scientists’ understanding of how these medications work.

The experiments were conducted in mice, so further research will be necessary to determine whether the same mechanism operates in humans. Even so, identifying how GLP-1 medications influence the brain could provide an important foundation for developing the next generation of obesity treatments.

‘By identifying a previously unrecognized neural mechanism involved in sustaining weight loss, our work provides new biological insights that could eventually help researchers design therapies that are even more effective or have fewer side effects,’ d’Ávila said.

Other authors from YSM include Roberto Collado-Pérez, a postdoctoral associate; Zhong-Wu Liu, assistant professor adjunct; Joseph Schlessinger, the William H. Prusoff Professor of Pharmacology; and Horvath, the Jean and David W. Wallace Professor of Comparative Medicine.