HCAR2 and narcolepsy: 4-hydroxybutanoic acid not only exerts central nervous system depressant effects and is used clinically in narcolepsy and alcohol dependence, but also modulates innate immunity by upregulating cathelicidin LL-37, activating GPR109A, and inducing MAP kinase/NF-κB pathways, thereby increasing resistance to microbial infections [38,39].