The GLP-1 Mood Mystery: Why a Diabetes Drug's Antidepressant Effect Runs Through Your Gut

GLP-1 analogues, also known as GLP-1 receptor agonists (GLP-1RAs), are widely prescribed for the treatment of type 2 diabetes and obesity. These agents mimic the action of the endogenous hormone glucagon-like peptide-1 (GLP-1) by binding to GLP-1 receptors expressed on the surface of multiple cell types throughout the body. Activation of these receptors enhances glucose-dependent insulin secretion, suppresses glucagon release, and promotes satiety through central nervous system signalling, thereby improving glycaemic control and facilitating weight loss.

However, evidence regarding the neuropsychiatric effects of GLP-1 remains inconsistent. Both preclinical and clinical studies investigating the effects of GLP-1 administration in individuals with diabetes and obesity have produced conflicting findings. While some studies have reported an association with depressive symptoms and suicidal ideation or behaviour, others have demonstrated improvements in mood and reductions in depressive symptoms, highlighting the need for further investigation into their effects on mental health.

A persistent low mood characterises major depressive disorder (MDD) and is among the leading causes of disability worldwide. Emerging evidence suggests that gut microbiota dysbiosis may contribute to the pathogenesis of MDD through the disruption of the gut–brain axis. As the gastrointestinal tract is the primary site of action of GLP-1 receptor agonists, and the gut microbiota play a key role in regulating mood and behaviour, it remains unclear whether GLP-1 analogue administration influences depressive symptoms through modulation of the gut–brain axis.

In a study published in Cell Host & Microbe, researchers compared the levels of GLP-1 in the blood of healthy people and people with major depressive disorder (MDD). They found that individuals with MDD had much lower levels of GLP-1. The same pattern was also seen in mice exposed to chronic unpredictable stress, a commonly used model for studying depression. When these mice were treated with liraglutide (a GLP-1 analogue in Victoza), their depression-like behaviours improved.

The researchers then wanted to find out whether these beneficial effects depended on the GLP-1 receptor, which is the normal mode of action of GLP-1. To test this, they gave the stressed mice both liraglutide and a drug that blocks the GLP-1 receptor. If liraglutide worked only through this receptor, blocking it should have prevented the improvement in depression-like behaviours. However, the treatment remained effective even when the receptor was blocked, suggesting that liraglutide may reduce depressive symptoms through a different mechanism. The alleviation of depression like symptoms was similarly observed in GLP-1 receptor knock-out mice.

To understand how liraglutide produces its antidepressant effects, the researchers attached a fluorescent tag to the drug so they could track where it travelled in the body. They found that liraglutide was widely distributed throughout different tissues, but the highest levels were found in the intestine.

Because the drug accumulated in the gut, the researchers wondered whether its antidepressant effects depended on the gut microbiome- the community of bacteria and other microorganisms that live in the digestive tract. To test this idea, they used a combination of broad-spectrum antibiotics to remove most of the gut bacteria in mice. In these mice, liraglutide no longer reduced depression-like behaviours. The same result was seen in germ-free mice. These findings suggest that a healthy gut microbiome is necessary for liraglutide to produce its antidepressant effects.

To understand how liraglutide affects the community of bacteria living in the gut, researchers used a technique called 16S rRNA sequencing, which allows scientists to identify and compare different bacterial species within a complex microbiome. They found that liraglutide changed the composition of gut bacteria in mice exposed to chronic unpredictable stress (CUS). In particular, treatment increased the abundance of a bacterium called Lactobacillus delbrueckii.

The researchers then grew Lactobacillus delbrueckii in the laboratory with increasing amounts of liraglutide and found that the drug directly promoted the growth of this bacterium. Further analysis of the molecules produced by the bacteria showed increased activity in pathways linked to serine and phosphoenolpyruvate, which are involved in bacterial growth and metabolism. Adding these molecules back to bacterial cultures further enhanced bacterial growth, suggesting they may help explain how liraglutide supports the expansion of this beneficial bacterium.

The researchers also examined changes in small molecules produced by the body and gut microbes using a technique called untargeted metabolomics. Using fecal microbiota transplant (FMT) experiments (transferring gut bacteria from liraglutide-treated donors into new CUS mice), they found increased levels of a molecule called 2-arachidonoylglycerol (2-AG) in both the blood and faeces.
Colonising antibiotic-treated CUS mice with L. delbrueckii alone (no liraglutide) increased 2-AG and improved behaviour. Blocking the microbiota with antibiotics prevented liraglutide from raising 2-AG, confirming the bacteria are the necessary middle step. Directly injecting 2-AG into mice (at effective doses) also reproduced the antidepressant effect, showing that once you have elevated 2-AG, you don't even need the drug or the bacteria.

Using c-Fos mapping (a marker of recent neuronal activity), the team found CUS mice had hyperactive neurons in several brain regions, but only two- the basolateral amygdala (BLA) and dorsomedial hypothalamus (DMH) were normalized by FMT from liraglutide-treated donors, by L. delbrueckii colonisation, or by 2-AG injection. Finally, blocking the cannabinoid receptor CB1 (with AM251) completely erased the antidepressant benefits of FMT or bacterial colonization, confirming that 2-AG works through classic endocannabinoid signalling to calm these overactive stress circuits.

Science is increasingly showing us that mental health isn't confined to the brain, it's shaped by metabolism, by the trillions of microbes in our gut, and by small molecules. This fascinating research shows that there may be more levers to pull for depression treatment than we thought.

However, since this study was conducted exclusively in male subjects to minimise hormonal variability, similar research in females is needed to determine whether the same mechanisms apply and how they might shift across different reproductive life stages.

Reference

Bian L, Cai Y, Zhang Y, Shen L, Wang H, Gao F, Cai N, Chen W, You C, Yang Y, Wang F, Yuan Y, Han B, Yao H. Microbiota-driven gut-brain signaling underlies antidepressant effects of a GLP-1 analog. Cell Host Microbe. 2026 Jun 10;34(6):1000-1017.e5. doi: 10.1016/j.chom.2026.05.003. PMID: 42269582.