Blocking and activating the same hormone receptor can both promote weight loss because they influence different parts of the brain. In mice, researchers found that GIPR agonists suppress appetite through the area postrema in the brainstem, while GIPR antagonists enhance the weight-loss effects of GLP-1 and amylin-based drugs by acting through the hypothalamus. The findings help explain a long-standing puzzle in obesity research and could guide the design of future combination therapies.
For years, one of the biggest mysteries in obesity research has been how two seemingly opposite strategies can produce a similar outcome. Drugs that activate the glucose-dependent insulinotropic polypeptide receptor (GIPR) can help people lose weight, yet treatments that block the very same receptor have also shown added weight-loss benefits when combined with other medications.
At first glance, that contradiction seems impossible. If turning a receptor on works, why would turning it off also help?
A new study suggests the answer lies not in the receptor itself, but in where it is located in the brain.
Rather than acting on the same cells, the researchers found that GIPR agonists and antagonists appear to influence separate neural circuits with distinct roles in controlling appetite and body weight. That discovery offers a biological explanation for results that have puzzled scientists through both laboratory and clinical studies.
A closer look at two different brain regions
The research focused on two areas known to contain neurons carrying GIPR.
One was the area postrema, a structure in the brainstem that sits outside the blood-brain barrier and is exposed directly to circulating hormones. The other was the hypothalamus, a major control center involved in hunger, metabolism, and energy balance.
To determine how each region contributes to weight regulation, the researchers genetically removed the GIPR gene from either the area postrema or the hypothalamus in mice while leaving the receptor intact elsewhere. They then tested how these animals responded to several experimental treatments, including a long-acting GIPR agonist, the GLP-1 receptor agonist liraglutide, a GIPR-blocking peptide, and the amylin analogue cagrilintide.
This approach allowed the team to isolate the role of each brain region without eliminating GIPR signaling throughout the entire body.
The brainstem proved essential for appetite suppression
The experiments showed that the area postrema played a central role in the appetite-reducing effects of GIPR activation.
When mice lacked GIPR in this brainstem region, the long-acting GIPR agonist largely lost its ability to suppress food intake. The drug no longer reduced eating during normal feeding, after overnight fasting, or during extended monitoring in metabolic chambers. Chronic treatment also failed to lower body weight or reduce fat mass in these mice.
Additional experiments strengthened this conclusion.
The researchers observed that the GIPR agonist activated neurons in the area postrema, producing a sharp increase in cFos, a marker of neuronal activity. Removing GIPR from this region greatly reduced that response. Real-time imaging of living brain slices also showed that the agonist directly increased cellular signaling in GIPR-expressing neurons of the area postrema, confirming that these cells respond immediately to the hormone.
The same brain region also appeared responsible for another well-known effect of GIPR agonists.
Previous studies had shown that activating GIPR can reduce nausea-like behaviors in mice. In this study, the agonist prevented mice from developing an avoidance response after exposure to the appetite-regulating hormone PYY, but only when GIPR remained intact in the area postrema. Removing the receptor from this region eliminated that protective effect.
Together, these findings suggest that the area postrema contains the neural circuitry responsible for both appetite suppression and reduced food aversion following GIPR activation.
The hypothalamus told a very different story
Removing GIPR from the hypothalamus produced an entirely different pattern.
These mice still responded normally to the GIPR agonist. Their food intake fell after treatment, glucose tolerance improved, and the drug continued to activate neurons within the hypothalamus.
But differences emerged when the animals received liraglutide, a widely used GLP-1 receptor agonist.
Compared with control animals, mice lacking hypothalamic GIPR lost even more weight during liraglutide treatment. That suggested that removing GIPR from this brain region somehow made the animals more sensitive to GLP-1 therapy.
The contrast became even clearer when researchers combined liraglutide with a GIPR antagonist.
In normal mice, adding the antagonist produced greater weight loss than liraglutide alone. However, this extra benefit disappeared completely in mice lacking hypothalamic GIPR.
That result indicates that the weight-loss boost produced by GIPR antagonism depends specifically on receptors located in the hypothalamus rather than in the brainstem.
The effect also extended beyond GLP-1 drugs
The researchers wondered whether this sensitizing effect might apply only to GLP-1 therapies or whether it represented a broader feature of hypothalamic GIPR signaling.
To investigate, they tested cagrilintide, an amylin-based obesity drug that works through a different hormonal pathway.
In normal mice, combining a GIPR antagonist with cagrilintide produced greater weight loss than cagrilintide alone. Likewise, mice lacking hypothalamic GIPR experienced stronger and more sustained weight loss after cagrilintide treatment than control animals.
These results suggest that blocking hypothalamic GIPR may increase the effectiveness of more than one class of anti-obesity medication.
Ruling out another possible explanation
Because GLP-1 is naturally produced by specialized preproglucagon (Ppg) neurons in the nucleus tractus solitarius, the researchers investigated whether these brainstem cells explained the interaction between GIPR antagonism and GLP-1 therapies.
They selectively eliminated these neurons and repeated the weight-loss experiments.
Although removing the cells caused greater weight gain when mice switched to a high-fat diet, it did not reduce the effectiveness of liraglutide, the GIPR antagonist, or the combination of both.
This suggests that the enhanced weight loss caused by GIPR antagonism does not require these brainstem GLP-1-producing neurons.
Helping solve a long-standing puzzle
The study offers a framework for understanding why two opposite approaches to the same receptor can both improve weight loss.
Instead of acting through identical pathways, GIPR agonists and GIPR antagonists appear to recruit separate groups of neurons located in different parts of the brain.
Activation of GIPR relies on neurons in the area postrema, where signaling suppresses appetite and reduces food avoidance. Blocking GIPR, meanwhile, depends on receptors in the hypothalamus, where it increases the weight-loss response to both GLP-1 receptor agonists and amylin receptor agonists.
This distinction helps reconcile findings from recent obesity treatments that combine GLP-1 therapy with either GIPR activation or GIPR blockade, despite those strategies appearing biologically contradictory.
Important questions remain
The researchers caution that the work was conducted in mice, and the experiments used targeted genetic modifications and experimental drug treatments designed for laboratory research.
They also note that, although the study consistently produced statistically significant results, several experiments involved relatively small groups of animals. Even so, the effects were reproducible across multiple experimental approaches.
Many details also remain unresolved. The precise neuronal networks connecting the area postrema and hypothalamus are still unknown, and the study did not identify exactly how these brain regions communicate with the broader circuits that regulate hunger and body weight.
Future research will be needed to map those pathways and determine whether selectively targeting these distinct neural populations can improve obesity treatments. If those mechanisms prove relevant in people, they could help explain why apparently opposite drug strategies both succeed—and perhaps inspire new therapies that combine them even more effectively.
Publication details
Distinct brain regions mediate regulation of food intake in response to GIPR agonism and antagonism, Nature Metabolism (2026). DOI: 10.1038/s42255-026-01575-z
