NLRP3 and major depressive disorder: Under pathological conditions, microglia can contribute to depression through a variety of mechanisms, including toll-like receptors (TLRs), nod-like receptor protein 3 (NLRP3) inflammasome, lesion of the HPA axis, metabolism of 5-hydroxytryptamine (5-HT), inflammatory cytokines, and their mediated inflammatory signal pathways, imbalance of M1/M2 polarization, gut microbiota, and microRNA.