Microglia-Dependent BDNF Signaling in the Dentate Gyrus Underlies the Antidepressant Effects of Gardiquimod, a Toll-Like Receptor 7 Agonist, in Chronically Stressed Mice.

Accumulating evidence suggests that pharmacological restoration of microglial homeostasis in the hippocampus may be a promising strategy for treating depression. In this study, we evaluated whether gardiquimod (GDQ), a selective Toll-like receptor 7 (TLR7) agonist, produces antidepressant effects in mice subjected to chronic unpredictable stress (CUS). A single intraperitoneal injection of GDQ at 1 or 1.5 mg/kg, but not 0.5 mg/kg, improved depression-related behaviors within 5 h of administration. Time-course analyses showed that the antidepressant efficacy of GDQ (1.5 mg/kg) appeared between 5 and 8 h, persisted for up to 7 days, and diminished by 14 days after a single dose. Notably, a second GDQ injection at 14 days restored the behavioral improvements, indicating sustained responsiveness to the drug. Mechanistically, the antidepressant effects of GDQ were abolished by both pharmacological inhibition (minocycline) and genetic depletion (PLX3397) of microglia, highlighting the necessity of these cells. Furthermore, GDQ reversed the CUS-induced reduction in brain-derived neurotrophic factor (BDNF) protein levels in the dentate gyrus in a microglia-dependent manner. The critical role of BDNF signaling was confirmed by three complementary approaches: intra-hippocampal infusion of a BDNF-neutralizing antibody, genetic disruption of activity-dependent BDNF release via the Val68Met knock-in mutation, and pharmacological blockade of the TrkB receptor with K252a. Each intervention abolished the behavioral effects of GDQ. Together, these findings identify GDQ as a promising candidate for antidepressant development and highlight the restoration of microglia-supported BDNF signaling in the dentate gyrus as a key mechanism underlying TLR7-mediated mood regulation.