Recent studies had reported that compounds that stimulate microglia could be developed as potential drugs for the treatment of depression due to their reversal effect on depression-like behaviors in chronically stressed mice. Zymosan A is a cell wall preparation of Saccharomyces cerevisiae composed of β-glucans. Based on its immuno-stimulatory activities, we hypothesized that zymosan A might have a therapeutic effect on depression. Our results showed that a single injection of zymosan A 5 h before behavioral tests at a dose of 1 or 2 mg/kg, but not at a dose of 0.5 mg/kg, reversed chronic unpredictable stress (CUS)-induced depression-like behaviors in mice in the tail suspension test, forced swimming test, and sucrose preference test. Time-dependent analysis showed that the antidepressant effect of zymosan A (2 mg/kg) in CUS mice became statistically significant at 5 and 8 h, but not at 3 h, and persisted for at least 7 days. Fourteen days after a single injection of zymosan A, no antidepressant effect was observed anymore. However, the disappeared antidepressant effect of zymosan A was restored by a second zymosan A injection (2 mg/kg, 5 h) 14 days after the first zymosan A injection. Stimulation of microglia was essential for the antidepressant effect of zymosan A because pre-inhibition of microglia by minocycline or pre-depletion of microglia by PLX3397 prevented the antidepressant effect of zymosan A. Based on these effects of zymosan A, zymosan A administration could be developed as a new strategy for the treatment of depression.
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