BDNF and major depressive disorder: (2) The treatment with L. reuteri NK33 can activate microglial cell infiltration into the hippocampus and induce hippocampal brain-derived neurotrophic factor (BDNF) expression and cAMP-response element binding protein (CREB) phosphorylation in IS-exposed mice as well as LPS-stimulated SH-SY5Y cells by suppressing the activation of the NF-κB pathway and HPA axis, thus suggesting that NK33 alleviated the suppression of NF-κB-mediated BDNF expression in the hippocampus, with the regulation of LPS infiltration into the brain, leading to the attenuation of anxiety and depression.