The most confusing result in obesity drug design just got a physical address. Two different brain regions, a Cambridge team reports, explain why turning the GIP receptor on and turning it off both add weight loss.
The work, published July 24 in Nature Metabolism ↗ by a group at the University of Cambridge's Institute of Metabolic Science, is a mouse study. It goes after a puzzle that has hung over the field for years and that a separate mouse experiment we covered on July 8 ↗ could only restate, not solve.
The paradox
GIP is a gut hormone. Its receptor, GIPR ↗, sits beside the GLP-1 receptor as one of the two levers the newest weight-loss drugs pull. The strange part is that the two best-known drugs at that receptor do opposite things and both work. Tirzepatide ↗, sold as Mounjaro and Zepbound, switches the GIP receptor on while also hitting GLP-1. MariTide ↗, Amgen's monthly injection, blocks the GIP receptor while hitting GLP-1. Activate the receptor, people lose weight. Block the same receptor, people also lose weight. Nobody had cleanly shown why a switch and its opposite land in the same place.
What the study did
The Cambridge group, with Jo Edward Lewis as first author and Fiona Gribble and Frank Reimann as the senior authors, deleted the GIP receptor gene in one brain area at a time in mice. One set of animals lost the receptor in the area postrema, a small patch of the brainstem that samples the blood for signals of nausea and fullness. Another set lost it in the hypothalamus, the appetite-control hub deeper in the brain. Then the researchers gave each group a GIP receptor activator, a GIP receptor blocker, and the GLP-1 drug liraglutide ↗, alone and in combination, and watched what happened to body weight and food intake.
The two knockouts came apart cleanly. Mice missing the receptor in the area postrema were partly protected from diet-induced obesity and barely responded to the appetite-suppressing effect of a GIP activator. But liraglutide still worked normally in them, and adding a GIP blocker on top of liraglutide still delivered its usual extra weight loss.
Mice missing the receptor in the hypothalamus showed the mirror image. They still lost their appetite when given a GIP activator, so that side of the drug was intact. But liraglutide worked better than usual in them, and the bonus weight loss from adding a GIP blocker vanished entirely. Take out hypothalamic GIP receptors and the antagonist has nothing left to push on.
Why it matters
Read together, the two experiments split the paradox in two. The appetite-suppressing half of GIP activation, the part tirzepatide leans on, runs through the area postrema in the brainstem. The weight-loss boost you get from blocking GIP instead, the part MariTide leans on, runs through the hypothalamus. They are not two answers to the same question. They are two different circuits, and a drug can recruit either one.
That reframes a debate the industry has treated as winner-take-all. If agonism and antagonism act in separate places, the question stops being which one is correct and becomes which circuit a given drug, dose, or patient should target. The Cambridge team also found that blocking GIP, or removing hypothalamic GIP receptors, made mice more sensitive to weight loss from cagrilintide ↗, an amylin drug Novo Nordisk is developing, which suggests the hypothalamic route is not GLP-1 specific.
The caveats are the usual ones for this kind of work. These are engineered mice, not people, and a receptor deleted from birth in one brain region is a blunter tool than a drug given to an adult. The paper does not claim a clinical dosing rule. What it offers is a map: two brain regions, two mechanisms, one weight-loss endpoint, and a reason the field's most contradictory result was never actually a contradiction.