Ketamine preserves perineuronal nets in chronic stressed mice brain through the inhibition of microglia activation.

Deeper understanding of ketamine's mechanism of action would contribute to the discovery of novel therapeutic targets with fast-onset actions. To gain insight on the mechanism underlying the antidepressant-like effects of ketamine, we focused on perineuronal nets (PNNs), an extracellular matrix structure that surrounds fast-spiking parvalbumin-positive interneurons and regulates synaptic plasticity, whose integrity is known to be compromised under stress-induced depressive conditions. We first performed the description of the plastic remodeling of PNNs in the Chronic restraint stress (CRS) mice treated or un-treated with ketamine, by quantifying the number of WFA, marker of PNNs, parvalbumin (PV) and c-Fos positive cells, as a surrogate of neuronal activity. And then, we investigated the the transcripts of a number of proteins involved in the formation or degradation of PNNs with or without ketamine in CRS mice. Next, we evaluated the expression of IBA1, a microglial marker, in the hippocampus and medial prefrontal cortex after CRS treated or un-treated with ketamine. We found that ketamine effectively alleviated the animals' depression like behavior as well as attenuates CRS-induced reduction of WFA-positive cells in the hippocampus and medial prefrontal cortex of mice. Although ketamine treatment had little or no effect on the number of PV, c-Fos positive cells and the transcripts of proteins involved in the formation or degradation of PNNs. Notably, ketamine treatment lead to remarkably reduced the number of IBA1-positive cells in the hippocampus and medial prefrontal cortex after CRS. Our findings suggest that PNNs is characterized by region-specific changes in chronic stress mouse brain and provide extensive evidence that ketamine exposure initiates microglia to remodel PNN, instead of the PNN accumulation and degradation enzymes.