Nose–brain axis: A bridge from the nasal cavity to the central nervous system

The nose-brain axis is a direct pathway linking the nasal cavity to the central nervous system. Odors, as well as exogenous substances such as pathogens, inflammatory mediators, and drugs, can enter the cranial cavity through pathways including the olfactory nerve, trigeminal nerve, and humoral routes, thereby enabling signal transmission and material exchange from the peripheral nasal cavity to the central nervous system. In recent years, advances in multimodal visualization technologies have made it possible to dynamically monitor the nose-brain axis from the molecular level to the tissue level, providing important means for revealing its functional characteristics and pathological changes. Owing to the existence of the nose-brain axis, nasal inflammation can, through neuro-immune interactions, activate central microglia and astrocytes and induce neuroinflammation, thus promoting the onset and progression of central nervous system diseases. In addition, the nose-brain axis offers a unique route for the treatment of central nervous system disorders. Intranasal drug delivery can bypass the blood-brain barrier, act directly on the central nervous system, increase intracranial drug bioavailability, and produce rapid effects, providing new ideas for treating cross-system diseases. This review systematically summarizes the anatomical pathways of the nose-brain axis, visualization monitoring technologies, and mechanisms by which nasal inflammation affects the central nervous system. It also reviews advances in intranasal drug delivery for emotional disorders, migraine, Parkinson's disease, and Alzheimer's disease, aiming to provide new strategies for studying the mechanisms by which nasal inflammation influences the central nervous system and for cross-system targeted therapy.