Neuroinflammation, the gut-brain axis, and psychedelic-assisted psychotherapy: A synthesis of emerging paradigms in psychiatric care

Cellular and molecular mediators

The key effectors of neuroinflammation are glial cells. Microglia, the resident macrophages of the CNS, are the principal players.⁷ In a healthy state, they perform homeostatic functions, including synaptic pruning and surveillance of the brain parenchyma. In response to inflammatory signals, they transition to an activated state, undergoing morphological changes and releasing a host of signalling molecules, including pro-inflammatory cytokines.⁷ Astrocytes also play a critical, often dual, role. They can be activated by cytokines such as interleukin-1β (IL−1β) to promote inflammation, but they also have neuroprotective functions.^5,7

This glial activation results in the release of pro-inflammatory cytokines and chemokines, which orchestrate the neuro-immune response. Elevated levels of tumour necrosis factor-alpha (TNF−α), interleukin-6 (IL−6), and IL−1β are consistently found in patients with MDD and anxiety.^5,7,17 These molecules are not passive markers of illness; they actively contribute to pathophysiology by altering neurotransmitter metabolism (e.g., shunting tryptophan away from serotonin synthesis via the kynurenine pathway), dysregulating the hypothalamic-pituitary-adrenal (HPA) axis, and impairing neuroplasticity.^5,8 A summary of these key players is provided in Table 1.

Evidence in psychiatric disorders

Direct evidence for neuroinflammation in psychiatric disorders is provided by in vivo neuroimaging. Positron emission tomography (PET) studies using radioligands for the 18 kDa translocator protein (TSPO), a marker upregulated in activated microglia and astrocytes, have provided some of the most compelling results. Multiple studies have found consistently increased TSPO binding in the brains of individuals with MDD, indicating heightened glial activation.¹⁶ This central inflammation is mirrored by elevated levels of peripheral inflammatory markers, such as C-reactive protein (CRP) and circulating cytokines, which correlate with depression severity and resistance to conventional antidepressants. The link between inflammation and psychiatric symptoms is further supported by the high incidence of depression in patients with chronic inflammatory medical conditions and in those receiving cytokine-based therapies.^5,17 Similar evidence implicates neuroinflammation in schizophrenia, PTSD, and neuropsychiatric symptoms following traumatic brain injury.^6,10

Heterogeneity and conflicting evidence in neuroinflammation

However, the link between inflammation and psychiatric illness is not uniformly consistent, and the evidence is complicated by significant heterogeneity and some conflicting findings. The relationship is now understood to be bidirectional: inflammation can contribute to depressive symptoms, but depression and its associated stressors can also exacerbate inflammatory responses, creating a complex feedback loop that is difficult to disentangle in cross-sectional studies.^18,19

Furthermore, the inflammatory profile is not monolithic across all psychiatric disorders. While MDD is often associated with elevated pro-inflammatory markers, post-mortem and imaging studies have yielded variable results across MDD, schizophrenia, and bipolar disorder, with differences depending on the specific markers, brain regions, disease stage, and medication status.^20,21 For instance, while TSPO PET studies often show increased glial activation in MDD, some studies report reduced TSPO levels in patients with schizophrenia, challenging a simple model where all neuroinflammation is uniformly detrimental.¹⁶ This suggests that the critical factor may be a disruption of glial homeostasis rather than inflammation per se. It is also plausible that inflammation is a causal factor for only a subset of patients, with one estimate suggesting that clinically elevated inflammatory biomarkers are present in approximately 25% of individuals with depression.²² This heterogeneity underscores that inflammation is likely one of several interacting pathophysiological pathways and highlights the need for patient stratification in both research and clinical practice.²¹

Methodological limitations in assessing neuroinflammation

Our ability to study neuroinflammation in vivo is also constrained by methodological limitations, primarily related to neuroimaging. PET imaging of the TSPO has been the most widely used technique, but it has several significant drawbacks.^23,24 First, TSPO is not specific to activated microglia; it is also expressed by other cells, including astrocytes and endothelial cells, which complicates the interpretation of the signal.²⁵ Second, the most common TSPO radioligands suffer from a low signal-to-noise ratio and are affected by a common single-nucleotide polymorphism (rs6971) in the TSPO gene, which alters tracer binding affinity and requires genetic screening of participants.^24,26 Third, and perhaps most importantly, TSPO expression does not differentiate between pro-inflammatory (neurotoxic) and anti-inflammatory (neuroprotective) glial phenotypes, meaning an increased signal could reflect either a damaging or a reparative process.²⁵

The development of alternative PET tracers for other targets, such as cannabinoid receptor 2 (CB2R) and purinergic receptor P2X7R, has so far proven disappointing in clinical studies due to issues like poor signal or lack of observable changes in disease states.²⁷ These limitations mean that our current in vivo picture of neuroinflammation is incomplete and must be interpreted with caution, reinforcing the need for more specific and reliable biomarkers.

The “Leaky gut” Hypothesis in psychiatry

A growing body of evidence indicates that patients with psychiatric disorders, particularly depression, exhibit significant alterations in the composition and diversity of their gut microbiota, a state known as dysbiosis.^2,11,29 This dysbiosis can compromise the integrity of the intestinal epithelial barrier, leading to increased intestinal permeability, colloquially termed “leaky gut”.^1,30 This allows bacterial components, most notably lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria, to translocate from the gut lumen into systemic circulation.³¹ The presence of circulating LPS, a potent endotoxin, triggers a robust systemic inflammatory response, characterised by the release of pro-inflammatory cytokines. These peripheral cytokines can then cross a compromised BBB or signal through it, activating microglia and initiating the neuroinflammatory cascades implicated in psychiatric symptoms.^2,7,11

Furthermore, the gut microbiota directly influences neurochemistry and immunity. Commensal bacteria are capable of synthesising numerous neurotransmitters, including serotonin and gamma-aminobutyric acid (GABA), as well as their precursors, such as tryptophan, thereby directly modulating CNS signaling ^5,12,32

Metabolites produced from the fermentation of dietary fibre, particularly SCFAs like butyrate, are vital for maintaining the integrity of the BBB and exert potent local and systemic anti-inflammatory effects.^11,32 A reduction in SCFA-producing bacteria is a common finding in the gut microbiomes of individuals with depression, further linking gut dysbiosis to impaired BBB function and inflammation.²⁹

The interplay between psychological stress, the HPA axis, and the GBA creates a self-perpetuating inflammatory cycle central to the pathophysiology of many psychiatric disorders. The sequence unfolds as follows: (i) Psychological stress, a primary trigger for psychiatric illness, activates the HPA axis, leading to the release of cortisol.^1,31 (ii) Sustained high levels of cortisol disrupt the gut microbial balance, promoting dysbiosis ^1,33 (iii) This dysbiosis contributes to a “leaky gut,” allowing LPS to enter the bloodstream.^31,34 (iv) Circulating LPS induces systemic inflammation, which in turn drives neuroinflammation within the brain.² (v) Neuroinflammation exacerbates core psychiatric symptoms like anhedonia, anxiety, and cognitive deficits, which are themselves potent psychological stressors.^5,8 This establishes a vicious feedback loop: Stress ↔ HPA Dysregulation ↔ Gut Dysbiosis ↔ Systemic Inflammation ↔ Neuroinflammation ↔ Worsened Psychiatric Symptoms ↔ Increased Stress. An effective therapeutic intervention must be capable of interrupting this cycle at one or more key nodes.

Clinical advances in modulating the GBA: Faecal microbiota transplantation (FMT)

The recognition of the GBA’s role in psychiatric illness has spurred interest in novel therapeutic strategies aimed at directly modulating the gut microbiome. Among the most powerful of these is Faecal Microbiota Transplantation (FMT), a procedure that involves transferring faecal matter from a healthy, screened donor into a patient’s gastrointestinal tract to restore a balanced microbial ecosystem.³⁵ While established as a highly effective treatment for recurrent Clostridium difficile infection, its application in psychiatry is an emerging and promising field.³⁶

Preclinical studies have shown that transplanting microbiota from healthy donors into rodent models of depression can alleviate depressive-like behaviours, increase levels of key neurochemicals like serotonin and BDNF in the hippocampus, and reduce inflammatory markers. Conversely, transplanting microbiota from depressed patients into healthy animals can induce depressive and anxiety-like behaviors.³⁵ These findings provide strong causal evidence for the microbiota’s role in mood regulation. Early-stage human trials are now underway. For example, a small randomised controlled trial (RCT) in patients with MDD found that FMT delivered via enema was feasible, safe, and well-tolerated, with participants showing significant improvements in gastrointestinal symptoms and near-significant improvements in quality of life compared to placebo.³⁷ Another small study using oral frozen FMT capsules as an add-on therapy for MDD reported symptom improvement in two patients at four weeks post-intervention.³⁶ Larger RCTs are currently recruiting for both MDD and obsessive-compulsive disorder (OCD), aiming to evaluate the efficacy of FMT in these conditions rigorously.^38,39 While still in its early days, FMT represents a tangible clinical application of GBA theory, offering a novel, systems-level approach to treating mental illness.

Established neuropsychological mechanisms

The acute psychological effects of psychedelics are primarily mediated by their action as agonists at the serotonin 5-HT2A receptor.^4,40 This receptor activation is thought to underlie their most well-documented neural effects, including the disruption of the DMN—a brain network involved in self-referential thinking that is typically overactive in depression.¹² This temporary “reset” of entrenched neural circuits is believed to increase cognitive flexibility. Concurrently, psychedelics potently promote structural and functional neuroplasticity, including synaptogenesis and neuritogenesis, creating a “window of opportunity” where the brain is more receptive to therapeutic learning and the revision of maladaptive cognitive and emotional patterns.^3,15 The psychotherapeutic container is essential for harnessing this neuroplastic window to facilitate lasting change.^14,41

The immunomodulatory properties of psychedelics

Beyond these profound effects on brain connectivity and psychology, a compelling body of preclinical evidence reveals that classic psychedelics are potent anti-inflammatory and immunomodulatory agents.^42-45 This action is also mediated primarily through the 5-HT2A receptor, which is expressed not only on neurons but also on various immune cells, including microglia, macrophages, and lymphocytes.^40,44 Activation of these peripheral and central 5-HT2A receptors by psychedelics has been shown to robustly inhibit the release of pro-inflammatory cytokines, including TNF−α, IL−6, and IL−1β, and to modulate microglial activity towards a more homeostatic, less inflammatory state.^43-45 In human intestinal tissue models, psilocybin significantly reduced levels of multiple inflammatory markers, including TNF−α, IL−6, and IL−8.⁴⁶ Notably, these anti-inflammatory effects have been observed at sub-behavioural doses in animal models, suggesting that the immunomodulatory action can be dissociated from the hallucinogenic experience.^40,45

This presents an intriguing paradox: the body’s endogenous ligand for the 5-HT2A receptor, serotonin, is generally considered to be a pro-inflammatory mediator.^40,47 Yet psychedelics, acting at the very same receptor, are potently anti-inflammatory.^44,45,48 This can be explained by the pharmacological concept of functional selectivity, or biased agonism. This principle holds that different ligands binding to the same receptor can stabilise distinct receptor conformations, causing it to couple to different intracellular signalling pathways preferentially. The evidence suggests that serotonin binding favours signalling cascades that promote inflammation.

In contrast, the binding of psychedelic compounds stabilises a receptor conformation that preferentially engages pathways that inhibit inflammation, such as by modulating the nuclear factor kappa B (NF−κB) and mechanistic target of rapamycin (mTOR) pathways.^43,44 This elevates psychedelics from simple serotonin mimics to highly sophisticated, biased agonists with specific immunomodulatory functions. A summary of these mechanisms is provided in Table 2.