The bridging role of gut microbiota-derived metabolites in neuropathic pain comorbid with anxiety

Lipopolysaccharide (LPS), a key component of the outer membrane of Gram-negative bacteria, is effectively contained when the intestinal barrier is intact. However, when gut dysbiosis leads to abnormal proliferation of Gram-negative bacteria or impaired intestinal barrier function, LPS translocates into systemic circulation, becoming a core immune mediator linking gut disturbances to central nervous system inflammation (Brandsma et al., 2019; Weiss and Hennet, 2017).

As a potent immune stimulant, LPS activates Toll-like receptor 4 (TLR4), widely distributed in the gut, immune cells, and the central nervous system (Ramakrishna et al., 2019). This triggers downstream signaling pathways like NF-κB, driving the massive release of pro-inflammatory cytokines such as TNF-α and IL-1β (Linnerbauer et al., 2020; Woodburn et al., 2021). These peripheral inflammatory mediators can cross the blood-brain barrier (BBB) via the circulatory system, directly activating microglia and astrocytes, thereby inducing sustained neuroinflammation. Activated glial cells further release chemokines like CXCL1 and pro-inflammatory mediators, creating an inflammatory microenvironment. This environment leads to significant changes in synaptic plasticity: enhancing excitatory transmission from glutamatergic neurons while inhibiting the inhibitory regulation by GABAergic interneurons, collectively contributing to central sensitization (Ji et al., 2018).

The abnormal activation of the immune system and a persistent low-grade inflammatory state have long been recognized as core pathogenic mechanisms of mood disorders like depression and anxiety (Guo et al., 2023). Within this framework, gut-derived LPS, as a potent immune stimulant, provides a key model for studying how peripheral immune activation affects brain function and behavior. Its action chain likely begins with changes in the gut microenvironment. Gut dysbiosis, especially the abnormal proliferation of Gram-negative bacteria, not only directly increases LPS levels but also disrupts intestinal barrier integrity. This “leaky gut” state allows LPS, other microbial metabolites, and neuroactive substances from the gut to enter systemic circulation (Shu et al., 2023). The LPS-TLR4-NF-κB pathway releases large amounts of pro-inflammatory factors peripherally. These peripherally produced inflammatory mediators can activate the brain’s resident immune cells–microglia and astrocytes–via multiple routes (BBB, vagal afferent fibers), inducing central neuroinflammation (Anand et al., 2022; Lu et al., 2023; Sun et al., 2021). Neuroinflammation further affects key brain regions for mood regulation, including the amygdala, prefrontal cortex, and hippocampus, inducing anxiety (Gholami-Mahtaj et al., 2022; Wang et al., 2018; Zheng et al., 2021).

Notably, this process is not unidirectional. On one hand, gut-derived LPS can enter the bloodstream and influence brain function. On the other hand, states like anxiety and stress activate the core stress-regulating system, the hypothalamic-pituitary-adrenal (HPA) axis, promoting elevated adrenal cortisol levels. High cortisol directly damages the intestinal barrier, increasing its permeability, reducing mucus secretion, and downregulating tight junction protein expression, thereby exacerbating structural and functional damage (de Weerth, 2017). Simultaneously, neural and endocrine changes induced by stress alter gut motility, digestive secretion, and local immune status, further disrupting microbial balance and promoting an increase in Gram-negative bacteria and release of more LPS (Zhang H. et al., 2023; Zhang et al., 2024). This LPS re-enters circulation, triggering chronic systemic low-grade inflammation, which in turn feeds back to the brain, forming a self-reinforcing interference loop within the “brain-gut axis.”

A key common pathological basis for the comorbidity of chronic pain and mood disorders is central inflammation (Candelli et al., 2021; Mohammad and Thiemermann, 2020). Clinical studies confirm that LPS can activate systemic immunity, elevate TNF-α, IL-6, and IL-8 levels, thereby exacerbating pain sensitivity and anxiety symptoms (Wegner et al., 2014). Mechanistically, the frequent co-occurrence of pain and anxiety stems from LPS triggering a shared pathological process via the gut-brain axis: it directly activates the LPS/TLR4/NF-κB pathway, causing systemic and central immune responses that directly worsen pain perception; it simultaneously disrupts HPA axis function, leading to persistently abnormal cortisol secretion. The high-cortisol environment induces neuroinflammation in the hippocampus and amygdala and inhibits the action of protective factors like IL-10, synchronously generating anxiety-like behaviors. Therefore, pain and anxiety symptoms induced by LPS are not simply sequential cause and effect but parallel manifestations of the same immune-endocrine disruption across sensory and emotional dimensions (Mahdirejei et al., 2023).

However, the latest evidence does not unidirectionally support the pro-inflammatory role of LPS. LPS from Rhodobacter sphaeroides is a highly specific TLR4 antagonist and can regulate nociception-related factors in the dorsal root ganglion, thereby exerting analgesic effects (Jurga et al., 2018). This indicates that the effect of LPS is not absolutely harmful; its impact may depend on factors like its source, target sites, and microenvironment. Future research needs more in-depth critical discussion on such paradoxical findings, e.g., systematically comparing the structure-function relationships of LPS from different sources, clarifying its bidirectional regulatory role at physiological versus pathological concentrations, and exploring its differential effects at various nodes of the gut-brain axis (e.g., gut, peripheral nerves, spinal cord, brain). This comprehensive, dialectical perspective is crucial for accurately understanding the complex role of LPS in NP-anxiety comorbidity.

Short-chain fatty acids (SCFAs) are a class of microbial metabolites primarily composed of acetate, propionate, and butyrate. They are produced in the gut by specific microbial fermentation of dietary fiber: acetate is mainly produced by Bacteroides spp. and Bifidobacterium spp. (Tsukuda et al., 2021); propionate is predominantly produced by Bacteroides spp. and Prevotella spp. (Killingsworth et al., 2020); butyrate is mainly produced by Firmicutes, specifically clostridial clusters (Singh et al., 2022). These three SCFAs activate shared or unique free fatty acid receptors (FFARs), such as FFAR2 (GPR43) and FFAR3 (GPR41), collectively maintaining gut homeostasis, enhancing the intestinal barrier, and regulating systemic and neural immunity (Fusco et al., 2023; Mishra et al., 2020).

Short-chain fatty acids levels are closely related to NP. A study in the NP rat models revealed an increased Firmicutes/Bacteroidetes ratio accompanied by significantly reduced acetate and butyrate levels. Lactobacillus plantarum supplementation can exert analgesic effects by restoring SCFA levels, promoting anti-inflammatory IL-10 secretion, and inducing macrophage polarization from pro-inflammatory M1 to anti-inflammatory M2 phenotype (Huang et al., 2024). Therefore, SCFAs are not only potential biomarkers for NP, but the anti-inflammatory and immunomodulatory mechanisms they mediate are also core targets for intervening in NP and related comorbidities.

New research finds different SCFAs have varying effects on immune and neural cells:

Acetate is a key microbe-derived molecule driving microglial maturation and functional regulation. It can cross the blood-brain barrier (Fock and Parnova, 2023) and regulate microglial homeostasis and function by modulating histone modifications, influencing the progression of neuroinflammation-related diseases (Erny et al., 2021; Lynch et al., 2021).

The neuroprotective effect of propionate primarily relies on its targeting of FFAR3 and regulation of the epigenetic state. Under neuroinflammatory conditions, propionate specifically activates FFAR3, thereby inducing hyperacetylation of histone H3. This epigenetic remodeling not only directly enhances the resistance of neurons and glial cells to oxidative stress but also promotes the expression of neuroregeneration markers like growth-associated protein-43 by regulating related gene expression, supporting neural repair (Grüter et al., 2023).

Butyrate is the primary energy source for colonocytes and is crucial for maintaining intestinal barrier integrity (Geirnaert et al., 2017). Systemically, it exerts immunomodulatory effects by activating the aryl hydrocarbon receptor (AhR) and G protein-coupled receptors (e.g., GPR109a, GPR43) (Singh et al., 2014). These signaling pathways promote the production of the anti-inflammatory factor IL-10, enhance regulatory T cell function, and inhibit Th1/Th17 cell differentiation and the release of pro-inflammatory mediators like IL-17, thereby alleviating systemic low-grade inflammation (Kasubuchi et al., 2015). Within the central nervous system, butyrate directly inhibits excessive microglial activation by activating the GPR109A receptor, upregulating PPAR-γ, and inhibiting the TLR4/NF-κB signaling pathway. It corrects M1/M2 polarization imbalance, reduces pro-inflammatory factor release, demonstrating direct neuroprotective properties (Wang et al., 2022; Wei et al., 2023).

In summary, SCFAs, through their microbiota-specific production, multi-receptor-mediated signaling, and epigenetic regulation, constitute an integrative protective network from gut to brain. In NP states, abnormal or insufficient SCFA levels may weaken this endogenous anti-inflammatory and neuroprotective barrier.

Short-chain fatty acids produced by gut microbiota metabolism possess the ability to modulate brain cognition and behavior (Mirzaei et al., 2021). Clinical studies show that the abundance of SCFA-producing bacteria is significantly reduced in the guts of anxiety patients, and the severity of anxiety disorder symptoms is positively correlated with Firmicutes abundance (Jiang et al., 2018; Ketel et al., 2024). This is because FFAR2 and FFAR3 are expressed not only in the enteric nervous system (ENS) but also in the portal vein nerves and sensory ganglia, forming the structural basis for gut-brain signal transmission (Dicks, 2022). Among these, FFAR3 in the ENS can directly transmit signals generated by SCFAs to the central nervous system (CNS), thereby influencing brain function and behavior (Nøhr et al., 2013). Blocking this pathway induces gut leakiness, neuroinflammation, and anxiety-like behaviors, while butyrate supplementation effectively alleviates these abnormalities (Mishra et al., 2024). Mechanistically, SCFAs not only inhibit systemic and neuroinflammatory responses by regulating microglial activity (Fusco et al., 2023), but also promote the synthesis of neurotransmitters like 5-HT and GABA, directly modulating mood circuits (Nankova et al., 2014; Palepu et al., 2024). This indicates that SCFAs maintain emotional homeostasis by mediating gut-brain dialogue and exerting anti-inflammatory and neuromodulatory functions; their deficiency or impaired signaling is an important mechanism in the development of anxiety.

Gut dysbiosis not only leads to the accumulation of harmful metabolites (e.g., LPS) but also directly causes reduced synthesis of beneficial metabolites like SCFAs. This metabolic imbalance constitutes an important pathological basis for NP-anxiety comorbidity (Park and Kim, 2021). As key messengers of the gut-brain axis, SCFAs exert neuroprotective effects through the following multiple mechanisms: first, enhancing the structural and functional integrity of both the intestinal barrier and the blood-brain barrier (Herrera et al., 2024); second, regulating gene transcription by inhibiting histone deacetylase (HDAC) activity, and specifically activating FFAR2/FFAR3, thereby inhibiting classical inflammatory pathways like NF-κB, exerting potent anti-inflammatory effects (Kopczyñska and Kowalczyk, 2024; Zhang K. et al., 2023). These mechanisms collectively regulate systemic and neural immune homeostasis, effectively alleviating neuroinflammation-driven pain sensitization and anxiety-like behaviors.

More importantly, SCFAs can act as epigenetic regulators, directly or indirectly modulating the expression of key neurotrophic factors like brain-derived neurotrophic factor (BDNF) in the brain via the “microbiota-gut-brain axis” (Dalile et al., 2019). This may be a core mechanism through which they improve neuroplasticity and intervene in comorbidity. BDNF itself is a key molecule in pain-emotion comorbidity: it activates microglia via purinergic receptors P2×4R and P2×7R, promoting neuroinflammation and other mechanisms that trigger central sensitization, forming the basis of pain (Hu et al., 2022; Rajamanickam et al., 2024). This signal further ascends, participating in the development of anxiety comorbidity by modulating excitability in emotional brain regions like the thalamus, cortex, and limbic system (Pezet et al., 2002; Smith, 2024).

In NP-anxiety comorbidity, SCFAs act as both protectors of the gut and blood-brain barriers and as upstream core regulators. The deficiency of SCFAs not only weakens their direct neuroprotective effects but may also promote the occurrence and development of NP-anxiety comorbidity at multiple levels by disrupting the normal regulation of pathways like BDNF. However, it remains unclear how SCFAs precisely regulate BDNF and other signaling molecules to mediate gut-brain dialogue and influence NP-anxiety comorbidity progression, and whether they can become potential intervention targets for this comorbidity. Future research is needed to clarify these points.

Approximately 90% of bile acids (BAs) are synthesized from cholesterol in the liver and subsequently transported to the intestine, where they are metabolized by gut microbiota (Chávez-Talavera et al., 2017). The biological functions of BAs are primarily realized through binding to the farnesoid X receptor (FXR) and the G protein-coupled bile acid receptor 5 (TGR5) (Jiang X. et al., 2025). Receptor activation can enhance the inhibitory function of GABAergic neurons, thereby inhibiting abnormal activation of glial cells like microglia in the spinal dorsal horn, downregulating activation of their downstream ERK signaling pathway, and ultimately reducing neuronal sensitization and pain (Wu et al., 2023).

However, the opposite occurs in systemic metabolic disease states like diabetes. Gut dysbiosis-induced bile acid metabolism abnormalities lead to abnormal activation of the TGR5 - cyclic adenosine monophosphate (cAMP) - cAMP response element-binding protein (CREB) signaling pathway (Chen et al., 2024). Activated CREB binds to the promoter region of the transient receptor potential vanilloid type 1 (TRPV1) channel gene, directly upregulating TRPV1 transcription and protein expression (Cenac et al., 2015). The TRPV1 protein is enriched at nerve terminals (Tominaga et al., 1998); when stimulated by heat, protons, or endogenous lipid mediators, the channel opens more readily, causing massive calcium influx (Sanz-Salvador et al., 2012), triggering neuronal depolarization and high-frequency action potential firing, i.e., peripheral sensitization (Bujak et al., 2019; Clapham, 2003). TRPV1 activation further activates protein kinase C (PKC), which can directly modulate the function of TRPV1 and other ion channels (e.g., sodium channels) through phosphorylation, forming a positive feedback loop that continuously amplifies pain signals (Gao et al., 2024). This pathway not only provides a new mechanistic explanation for diseases like diabetic peripheral neuropathic pain but also suggests that targeting TGR5 or interrupting its downstream signaling may become a potential therapeutic strategy for alleviating such NP.

Bile acids synthesized by hepatocytes are metabolized by gut microbiota into secondary BAs, including deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) (Chand et al., 2017). Studies have found that abnormal elevation of specific bile acids and their derivatives is associated with anxiety (Wang et al., 2024). For example, gut dysbiosis can lead to increased serum LCA levels, inducing liver inflammation. The resulting inflammatory cytokines may cross the blood-brain barrier, exacerbating neuroinflammation in the brain and leading to anxiety-like behaviors (Weng et al., 2024). Furthermore, bile acids can enter the CNS via the circulatory system. TGR5 is widely expressed in the CNS, enabling BAs to directly regulate neural function (Joyce and O’Malley, 2022). Taurodeoxycholic acid can directly activate central TGR5 receptors, exerting protective effects by inhibiting neuroinflammation, oxidative stress, and endoplasmic reticulum stress (Lenin et al., 2023). Conversely, TGR5 gene deletion leads to decreased serum and hippocampal serotonin (5-HT) levels in mice and induces anxiety- and depression-like behaviors (Castellanos-Jankiewicz et al., 2021; Tao et al., 2024). Therefore, TGR5 is both a key molecule mediating neuroprotection, and its functional loss is a direct cause of mood disorders. Its ultimate effect likely depends highly on factors such as the specificity of the activating ligand and the cellular and brain regional microenvironment (Chen et al., 2023). Thus, simply defining it as a “protective pathway” may not be comprehensive. Although existing evidence demonstrates the critical role of the BA-TGR5 pathway in anxiety regulation, its downstream molecular mechanisms require further study.

The comorbidity of NP and anxiety stems from the synergistic effects of abnormal primary bile acid synthesis and signaling pathway dysregulation (Ridlon and Bajaj, 2015; Zhang J. et al., 2023). In pathological states, excessive specific bile acids or receptor function imbalance can simultaneously activate glial cells in the spinal dorsal horn, hippocampus, and amygdala, releasing pro-inflammatory factors and creating widespread central neuroinflammation (Chen et al., 2025; Shi et al., 2025). Delving into the molecular mechanism, bile acids can induce phosphorylation and ubiquitination of the NLRP3 inflammasome by activating the TGR5-cAMP-protein kinase A (PKA) axis, thereby directly inhibiting the activation of this key inflammatory complex (Guo et al., 2016). Therefore, bile acid signaling dysregulation weakens the physiological inhibition of the NLRP3 inflammasome, consequently synchronously exacerbating neuroinflammatory responses in both the spinal cord and limbic system; conversely, restoring signaling balance can curb this shared pathway from the upstream. This inflammatory process not only directly lowers the pain threshold, causing hyperalgesia/allodynia but also simultaneously impairs hippocampal neuroplasticity, disrupts amygdala fear/anxiety-related neural circuits, and affects the balance of key neurotransmitter systems like 5-HT and GABA (Huang et al., 2015; Romanazzi et al., 2023). Additionally, the bile acid-TGR5-cAMP-CREB-TRPV1 pathway may also be co-activated in this process, exacerbating neuroinflammation and further participating in the regulation of molecular events related to pain and emotion. In summary, the bile acid-TGR5-cAMP signaling pathway is a key shared signaling node in NP-anxiety comorbidity. Activation of this pathway can directly regulate neuronal excitability in brain regions integrating pain and emotion. More critically, this signal, by acting on different downstream molecules, triggers a shared and diffusible neuroinflammatory pathway, thereby contributing to the comorbid state of NP and anxiety (Zhong et al., 2023).

Serotonin (5-HT) is an important gastrointestinal signaling molecule with distinct synthetic sources and functional divisions in the central and peripheral systems: in the CNS, 5-HT is primarily synthesized by raphe nucleus neurons in the brainstem, involved in mood, sleep, and anxiety regulation (Jones et al., 2020); in the periphery, 90%–95% of 5-HT is synthesized by enterochromaffin cells (Gershon and Tack, 2007), its release directly regulated by gut microbiota like lactobacilli (Ling et al., 2022; Lu et al., 2021), and it mainly participates in gastrointestinal function regulation (Bulbring and Lin, 1958).

The vagus nerve is a core pathway for gut-brain communication, originating from the medulla oblongata and widely distributed in organs like the gastrointestinal tract (Dicks, 2023). Vagus nerve activation can significantly alter neurotransmitter levels, thereby affecting digestion, immunity, and gut microbiota composition and metabolism, forming the basis of neural regulation (Shin et al., 2019). Under pathological conditions, gut-derived 5-HT activates 5-HT3 and 5-HT4 receptors on vagal afferent fibers, uploading nociceptive information to the CNS (Brito et al., 2017; Nemoto et al., 2001). Simultaneously, 5-HT also participates in the formation of hyperalgesia by acting on 5-HT1 and 5-HT2 receptors in the spinal cord (Alfaro-Rodríguez et al., 2024; Sagalajev et al., 2015), while activation of 5-HT4 and 5-HT1A receptors in the ENS exhibits analgesic and neuroprotective properties (Bianco et al., 2016). Gut dysbiosis may disrupt this balance, leading to enhanced abnormal vagal afferent input and weakened spinal analgesic function, exacerbating NP. This indicates the functional duality of 5-HT in NP (Liu et al., 2020). Notably, these receptors likely act synergistically; differences in NP etiology and dynamic regulation of 5-HT levels by gut microbiota metabolites may influence the predominant expression and function of different 5-HT receptor subtypes (Zhang et al., 2025), and detailed mechanisms require further investigation.

γ-aminobutyric acid is an inhibitory neurotransmitter that also plays an important role in maintaining ENS structure and functional regulation, participating in the control of gastric acid secretion, gastric emptying, intestinal motility, and pain perception, thus possessing dual functions as both a neurotransmitter and an endocrine modulator (Chen et al., 2022). Studies show that gut microbiota such as Lactobacillus, Bifidobacterium, Bacteroides, and B. fragilis have the ability to synthesize GABA (Strandwitz et al., 2019). GABA and its receptors are widely distributed in the ENS (Auteri et al., 2015), involved in regulating the sensitivity of vagal and spinal afferent nerves (Hyland and Cryan, 2010). Vagus nerve activation can transmit gut signals to the brainstem, subsequently activating GABAergic descending inhibitory pathways in the brain. This pathway further synergizes with the 5-HT system, collectively enhancing inhibition on pain-transmitting neurons in the spinal cord or trigeminal spinal nucleus, thereby achieving pain modulation (Cornelison et al., 2020).

Although gut-derived GABA itself has difficulty crossing the BBB (Janeczko et al., 2007), a small amount may enter the CNS via specific transporters on the BBB (Takanaga et al., 2001), regulating uptake processes at nerve terminals and glial cells (Brown et al., 2003). Furthermore, gut microbiota may achieve cross-system regulation through other means. For example, Akkermansia muciniphila, Parabacteroides merdae, and Parabacteroides distasonis can indirectly influence GABAergic signaling and thus pain perception by modulating peripheral GABA/glutamate metabolic balance (Olson et al., 2018; Pokusaeva et al., 2017). This suggests that the gut microbiota-GABA signal may achieve central regulation through direct transport or other indirect pathways; its specific molecular mechanisms require in-depth elucidation (Boonstra et al., 2015).

Tryptophan (TRP) and its key metabolite 5-HT are central to anxiety regulation. In 5-HT biosynthesis, TRP is first catalyzed by tryptophan hydroxylase (TPH) into 5-hydroxytryptophan, then converted to 5-HT via aromatic L-amino acid decarboxylase. TPH1 is mainly present peripherally (e.g., in enterochromaffin cells), while TPH2 is present in central neurons (Qu et al., 2024). Gut microbiota can control the shunt of TRP metabolism toward the kynurenine versus 5-HT synthesis pathways, promoting the entry of BBB-permeable tryptophan and 5-hydroxytryptophan into the CNS, thereby indirectly regulating brain 5-HT levels (Clarke et al., 2013; Deng et al., 2021). Fluctuations in 5-HT levels, particularly in the amygdala, hippocampus and striatum, are closely related to anxiety symptoms (Šimić et al., 2021). Disruption of this regulatory network may be an important mechanism causing anxiety. Studies show that Toxoplasma gondii infection can ultimately lead to reduced 5-HT levels and neuroinflammation in key brain regions like the amygdala by disrupting gut microbiota, thereby inducing anxiety-like behaviors (Luo et al., 2024). Conversely, probiotic intervention demonstrates the potential to improve mood by positively modulating this pathway. For instance, Lactobacillus paracasei PS23 can elevate 5-HT levels in the hippocampus and striatum and effectively alleviate anxiety behaviors (Liao et al., 2019).

Dysfunction of the GABAergic system has been confirmed to be closely associated with the occurrence of anxiety. Gut microbiota (e.g., lactobacilli, bifidobacteria) possess the ability to synthesize GABA (Włodarczyk et al., 2020) or influence GABA synthesis by regulating levels of its key precursor, glutamine (Zhu et al., 2024). Additionally, gut microbiota can stimulate vagal afferents, transmitting peripheral signals to the brainstem, subsequently regulating GABA receptor expression and function in the limbic system, enhancing central inhibitory neurotransmission, thereby alleviating anxiety (Bravo et al., 2011). Conversely, in pathological states, gut dysbiosis may lead to abnormal elevation of short-chain fatty acids (especially butyrate), which can interfere with normal inhibitory signal transmission by competitively binding to GABA receptors, inducing anxiety-like behaviors (Dicks, 2023).

This indicates that gut dysbiosis can promote anxiety by reducing GABA synthesis, producing receptor-antagonist metabolites, and weakening vagal transmission, leading to central GABAergic inhibitory function deficits; however, supplementing specific probiotics can reverse these pathway alterations, restoring inhibitory function and synaptic efficacy, ultimately alleviating anxiety-like behaviors (Isik et al., 2025; Ma et al., 2021).

Increasing evidence suggests that dysfunctional interaction between vagal signaling and gut microbiota-derived mediators is a key link connecting chronic pain and mood disorders (Kurhaluk et al., 2025). Its core mechanism begins with gut dysbiosis and the resulting microenvironmental changes, relying on the vagus nerve to transmit signals to the brain. Ultimately, they collectively regulate the development of pain and anxiety by modulating the expression of specific brain receptors, influencing neuroinflammation and neuroplasticity. Severing this nerve blocks the central regulatory effects of both 5-HT and GABA (Lee et al., 2025; Zou et al., 2024). However, their modes of action differ. 5-HT is primarily released locally in the gut and initiates signal upload to the nucleus tractus solitarius via direct activation of 5-HT receptors on vagal nerve endings, with projections to brain regions like the prefrontal cortex, hippocampus, and amygdala–a relatively direct pathway (Lee et al., 2025). The role of the vagus nerve in GABAergic gut-brain communication is more indirect and complex: it may influence signal transmission by modulating the local gut environment or vagal nerve function, and its effects are significantly context-dependent. Studies find that in pathological states like Salmonella infection, the vagus nerve may continuously transmit pro-anxiety signals. Probiotics can induce changes in GABA and its receptor levels in the blood and brain limbic system via the vagus nerve, alleviating pain and anxiety (Bravo et al., 2011; Royo et al., 2023). This indicates that the vagus nerve is not merely a simple signal conduit but an active, functionally selective pathway (Faraji et al., 2025). It is responsible for precisely uploading signals from gut microbiota-derived metabolites and induced immune/neuroendocrine signals to the brainstem, subsequently influencing neurotransmitter receptor expression and functional remodeling in brain regions like the limbic system, ultimately achieving regulation of NP and anxiety (Cryan et al., 2019; Yu and Hsiao, 2021).