When most people think of the new generation of metabolic peptides, they think of the gut, the pancreas and the liver. Yet some of the most interesting questions these molecules raise are about the brain. Hunger, satiety, craving and reward are all shaped by neural circuits, and the hormone receptors that compounds such as tirzepatide and retatrutide act on are found not only in the digestive system but in key areas of the brain as well. For psychologists and neuroscientists, that overlap has opened a fascinating new line of research.
The gut-brain conversation
Appetite has never been purely a matter of willpower or stomach size. The gut and the brain are in constant communication through nerves, hormones and the bloodstream. After a meal, cells in the intestine release hormones known as incretins, including GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). These hormones help regulate insulin, but they also send signals that contribute to the feeling of fullness.
GLP-1 receptors have been identified in several brain regions involved in feeding behaviour, including the hypothalamus, which helps regulate energy balance, and the brainstem, which processes signals from the gut. That is one reason why compounds acting on these receptors became such a focus of metabolic research: they appeared to influence not just what happens after food is eaten, but the drive to eat in the first place.
From hunger to reward
The more surprising finding came from research into the brain’s reward system. GLP-1 receptors are also present in areas such as the ventral tegmental area and the nucleus accumbens, regions closely tied to dopamine signalling, motivation and the pleasure we take in food. In animal studies, activating these receptors has been shown to reduce the motivation to work for highly palatable food, and in some models to reduce responses to other rewarding substances such as alcohol.
Those findings have made incretin-based compounds a subject of interest well beyond obesity research. Scientists are now exploring how hormonal signals from the body interact with the circuits that govern craving and habit. It is early-stage work, much of it in animal models, and researchers are careful not to overstate it. But it reframes appetite as a conversation between body and brain rather than a simple matter of calories.
Two receptors, then three
Tirzepatide was the first molecule designed to activate both the GLP-1 and GIP receptors. Combining the two pathways proved more effective in clinical research than targeting GLP-1 alone, and it raised new questions about the role GIP plays in the brain. GIP receptors are also found in brain regions linked to appetite, and researchers are still working out how the two signals combine. Tirzepatide is licensed in the UK as the prescription medicine Mounjaro, while research-grade tirzepatide is used separately in laboratory work; this Tirzepatide UK guide explains the difference between the two in more detail.
Retatrutide goes a step further. Developed by Eli Lilly and still investigational, it adds a third target, the glucagon receptor, to the GLP-1 and GIP pair. Glucagon signalling is best known for its effects on energy expenditure and the liver, but its role in the brain is far less understood. Because retatrutide shares much of its structure with tirzepatide, comparing the two allows researchers to ask what that third pathway adds, including to appetite and behaviour. For that reason, retatrutide as a research compound has become one of the most closely studied molecules in the field.
What this might mean for psychology
For psychologists, the interest lies in what these compounds reveal about the biology of motivation. Eating behaviour has long been studied through the lens of emotion, stress, habit and environment. The incretin research adds a physiological layer: hormones released by the gut may be one of the levers that adjusts how rewarding food feels and how strongly we are drawn to it.
That raises intriguing questions. How do hormonal signals interact with emotional eating? Why do some people experience food cravings so differently from others? Could the same circuits that govern food reward help explain other compulsive behaviours? Researchers are investigating all of these, though answers remain some way off and the evidence in humans is still developing.
Stress is a good example of why the topic matters to psychologists. Stress hormones such as cortisol are known to shift food preferences towards energy-dense, highly palatable foods, and the reward circuits involved overlap with those that GLP-1 signalling appears to influence. Researchers are beginning to ask whether gut hormones and stress pathways interact, and whether that interaction helps explain why appetite can change so sharply during periods of anxiety or low mood. Sleep, which also affects appetite hormones, is another piece of the same puzzle. None of these questions has a simple answer yet, but they show how closely physical and psychological explanations of eating behaviour are now intertwined.
Reasons for caution
It is worth being clear about the limits of current knowledge. Much of the brain-related research comes from animal studies, and findings in rodents do not always translate to people. Retatrutide is not approved by any regulator and remains in clinical trials. Effects on mood and behaviour, positive or negative, are an active area of monitoring in clinical research, and regulators continue to review safety data on the wider class of GLP-1 based medicines.
Research-grade versions of these compounds are supplied strictly for in vitro laboratory work. They are not medicines, should never be used outside a properly supervised research setting, and anyone with concerns about appetite, weight or eating behaviour should speak to a GP or a qualified healthcare professional.
A new frontier
Whatever their eventual role in medicine, multi-receptor peptides have already changed how scientists think about appetite. The idea that a single molecule can influence signals in the gut, the liver and the reward centres of the brain makes them powerful tools for understanding the biology of motivation. For psychology, that makes the gut–brain axis one of the most exciting areas to watch in the years ahead.
Compounds discussed in this article are supplied strictly for laboratory research use and are not for human or veterinary use.
Simona LeVey, a psychology graduate from the University of Hertfordshire, has a keen interest in the fields of mental health, wellness, and lifestyle.
