Depletion of microglial BDNF increases susceptibility to the behavioral and synaptic effects of chronic unpredictable stress

In the medial prefrontal cortex (PFC), chronic stress reduces synaptic expression of glutamate receptors, leading to decreased excitatory signaling from layer V pyramidal neurons and working memory deficits. One key element driving these changes is a reduction in brain-derived neurotrophic factor (BDNF) signaling. BDNF is a potent mediator of synaptic growth and deficient BDNF signaling has been linked to stress susceptibility. Prior studies indicated that neurons are the primary source of BDNF, but more recent work suggests that microglia are also an important source of BDNF. Adding to this, our work showed that 14 days of chronic unpredictable stress (CUS) reduced Bdnf transcript in PFC microglia, evincing its relevance in the effects of stress. To explore this further, we utilized transgenic mice with microglia-specific depletion of BDNF (Cx3cr1Cre/+:Bdnffl/fl) and genotype controls (Cx3cr1Cre/+:Bdnf+/+). In the following experiments, mice were exposed to a shortened CUS paradigm (7 days) to determine if microglial Bdnf depletion promotes stress susceptibility. Analyses of PFC microglia revealed that Cx3cr1Cre/+:Bdnffl/fl mice had shifts in phenotypic markers and gene expression. In a separate cohort, synaptoneurosomes were collected from the PFC and western blotting was performed for synaptic markers. These experiments showed that Cx3cr1Cre/+:Bdnffl/fl mice had baseline deficits in GluN2B, and that 7 days of CUS additionally reduced GluN2A levels in Cx3cr1Cre/+:Bdnffl/fl mice, but not genotype controls. Behavioral and cognitive testing showed that this coincided with exacerbated stress effects on temporal object recognition in Cx3cr1Cre/+:Bdnffl/fl mice. These results indicate that microglial BDNF promotes glutamate receptor expression in the PFC. As such, mice with deficient microglial BDNF had increased susceptibility to the behavioral and cognitive consequences of stress.