A Traditional Chinese Herb Formula Makes Ketamine’s Antidepressant Effects Last Longer in Mice

Ketamine can lift severe depression within hours, a property that has made it one of the most closely watched pharmacological discoveries in psychiatry. The problem is durability: its effects typically vanish within a week, and its potential for dissociative side effects and misuse make repeat dosing a fraught solution. A new mouse study published in Pharmaceuticals points to an unexpected partner that might extend ketamine's reach, not another drug, but a four-ingredient Traditional Chinese Medicine formula called Dajianzhong Decoction (DJZT), and the mechanism runs straight through the gut.

Dajianzhong Decoction and Ketamine Combine to Sustain Antidepressant Behavior

Researchers from the Zhang lab and colleagues subjected mice to chronic unpredictable mild stress (CUMS), a standard protocol for inducing depressive-like behavior, measuring outcomes through sucrose preference, forced-swim, and tail-suspension tests. Neither DJZT alone nor ketamine as a rapid antidepressant alone produced meaningful behavioral improvement when measured seven days after the ketamine injection. The combination, however, significantly prolonged the antidepressant response at that same seven-day mark. The mice groomed more, floated less, and showed reduced immobility across multiple tests. That result set the stage for the study's central question: what is the combination doing that neither ingredient achieves on its own?

Gut Microbiota Transplants Reveal the Microbial Engine Behind Recovery

To isolate the microbial contribution, the team performed fecal microbiota transplants (FMT). Antibiotic-treated, pseudo-germ-free mice received stool from CUMS-model donors treated with the combined therapy. Those recipient animals displayed the same antidepressant-like behavioral improvements as the original treated mice, even though they had never received DJZT or ketamine directly. Recipient mice given stool from untreated CUMS animals, by contrast, developed depressive-like behaviors. The gut microbiome of DJZT-plus-ketamine-treated donors was enriched in a bacterial genus called Alistipes, and that population correlated strongly with two short-chain fatty acids (SCFAs): acetic acid and isobutyric acid. CUMS had depleted both metabolites in fecal and serum samples; the combined treatment restored them. These SCFAs and depression mechanisms have previously been linked to gut-brain signaling through free fatty acid receptors, and this study traced that path in detail.

FFAR2-NLRP3-IL-1β Signaling Links Gut Metabolites to Brain Inflammation

Short-chain fatty acids act on a receptor called FFAR2 (also known as GPR43), found on immune and neural cells. In the medial prefrontal cortex (mPFC), a brain region central to mood regulation, CUMS mice showed reduced FFAR2 expression alongside elevated levels of NLRP3 and IL-1β. The NLRP3 inflammasome is a molecular complex that drives the release of pro-inflammatory interleukin-1β; prior work has shown the NLRP3 inflammasome in depression plays a significant role in stress-induced neuroinflammation. Transplanting microbiota from DJZT-ketamine-treated mice restored FFAR2 expression and suppressed both NLRP3 and IL-1β in the mPFC. Pro-inflammatory cytokines IL-6 and TNF-α also fell, microglial activation (measured by Iba-1 protein) declined, and markers of synaptic health improved: the synaptic protein SYP increased, and dendritic spine density in the mPFC rose. The team then administered exogenous acetic acid and isobutyric acid to recipient mice and found that SCFAs alone were sufficient to replicate the anti-inflammatory and antidepressant effects. When they blocked FFAR2 pharmacologically using the antagonist GLPG0974, all of those benefits disappeared, confirming that the receptor sits at the crux of the pathway.

What This Study Cannot Yet Tell Us

The findings are preliminary in important ways. SCFA levels were measured in feces and serum but not in the brain or cerebrospinal fluid, so whether these molecules actually cross the blood-brain barrier in sufficient quantities remains an open question. The cellular location of FFAR2 within the mPFC was not pinpointed, meaning it is unclear whether the receptor's relevant activity happens in neurons, microglia, or another cell type. The study also focused entirely on SCFAs and did not rule out contributions from other gut-derived metabolites. Perhaps most critically for translation, the challenges in gut-brain axis research are substantial: microbiome composition differs enormously between laboratory mice and humans, and FMT-based findings in rodents have a checkered history of not replicating in clinical populations. The untargeted metabolomics analyses also worked with relatively small sample sizes, which limits statistical confidence.

What the Research Needs to Move Forward

The authors flag several clear next steps: larger metabolomics cohorts, direct measurement of SCFAs in brain tissue, strain-level identification of the causal microbes within Alistipes and related genera, and cell-type-specific experiments to localize FFAR2 activity. The broader field is moving toward personalized microbiota treatments for depression that match interventions to individual microbiome profiles, a direction this study supports by showing that the microbial fingerprint of DJZT-ketamine treatment is both identifiable and functionally transferable. If the FFAR2 pathway proves relevant in human tissue, it could become a target in its own right, independent of DJZT, offering a way to extend the benefits of ketamine or other rapid-acting antidepressants without simply raising the dose. For now, the study is best read as a proof-of-concept that the gut microbiome participates actively in the durability of ketamine's effects, and that the mechanism is specific enough to be studied, blocked, and potentially exploited.