Integrative Neuroimmune Role of the Parasympathetic Nervous System, Vagus Nerve and Gut Microbiota in Stress Modulation: A Narrative Review.

It has been demonstrated that prolonged exposure to stress engenders a plethora of neuropsychiatric, immune and metabolic disorders. However, its pathophysiology transcends the conventional hypothalamic-pituitary-adrenal (HPA) axis. This review addresses the central question of how integrated neural and microbial pathways regulate stress responses and resilience. We present a model in which the parasympathetic nervous system (particularly the vagus nerve) and the gut microbiota interact to form a bidirectional neuroimmune network that modulates the HPA axis, immune function, neurotransmitter balance, and metabolic adaptation. Key molecular pathways include nitric oxide synthesis via the classical nitric oxide synthase (NOS)-dependent and microbiota-mediated nitrate-nitrite routes, inducible nitric oxide synthase (iNOS) regulation, nuclear factor erythroid 2-related factor 2 (Nrf2) signalling, lysosomal function, autophagy and the cholinergic anti-inflammatory reflex. Other pathways include the gamma-aminobutyric acid (GABA) and serotonin (5-HT) systems, NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) signalling, polyamine metabolism and peroxisome proliferator-activated receptor gamma (PPARγ). Intermittent hypoxia training (IHT) enhances mitochondrial function, oxidative stress responses, autonomic balance and gut microbiota composition. This promotes parasympathetic activity and stress resilience that is tailored to the individual. These adaptations support the concept of personalised stress response profiles based on hypoxic adaptability. Clinical implications include combining IHT with vagus nerve stimulation, probiotics, dietary strategies, and stress reduction techniques. Monitoring vagal tone and microbiota composition could also serve as predictive biomarkers for personalised interventions in stress-related disorders. This integrative framework highlights the therapeutic potential of targeting the parasympathetic system and the gut microbiota to modulate stress.