From the article:
The researchers examined blood-brain barrier cells in depressed stressed mice, resilient stressed mice, and control mice. Their observations show that the epigenetic processes that allow the expression of the claudin-5 gene are more readily activated in resilient mice. They also observed that the resilient mice produce less of one of the proteins that inhibit expression of the claudin-5 gene.
Conversely, depressed stressed mice express more of an enzyme called HDAC1 that triggers a loss of claudin-5. “When a chemical compound is used to block HDAC1, the depressive mice produce more claudin-5 and their social interactions spontaneously increase,” says Professor Ménard.