Figure 3.

Brain to microbiota communication under chronic stress. The HPA axis is activated under chronic stress. The final product of the HPA axis, cortisol, directly activates skin microbes. Cortisol activates the amygdala, promoting central sensitization to itch. The amygdala also promotes CRH signaling to the brain stem (PAG), altering the “descending itch modulatory system”. Prolonged exposure to cortisol, NE, and ACTH is associated with increased growth and biofilm genesis and augmented virulence of the skin microbiota. Ach: acetylcholine; ACTH: adrenocorticotropic hormone; CRH: corticotropin-releasing-hormone; HPA: hypothalamic–pituitary–adrenal axis; 5-HT: serotonin; NE: norepinephrine; PNS: parasympathetic nervous system; RVM: rostral ventromedial medulla; SNS: sympathetic nervous system; VLPAG: ventrolateral periaqueductal grey matter