Effects of MMP8 inhibitors on chronic unpredictable mild stress-induced neuroinflammation and depressive-like behavior：exploring the underlying molecular mechanisms.

Objective: Studies have demonstrated a close association between alterations in the immune system and major depressive disorder (MDD), with MDD potentially inducing neuroinflammation, hippocampal atrophy, and depression-like behaviors. Matrix metalloproteinase-8 (MMP8) contributes to the onset of depression, but it has not been studier yet. This study aims to investigate the mechanistic role of the MMP8 inhibitor (M8I) in alleviating depression induced by chronic unpredictable mild stress (CUMS) in rats. The objectives include evaluating the ameliorative effects of M8I on CUMS-induced depression-like behaviors and exploring its impacts on the TNF-α/TNFR1/NF-κB signaling pathway, NLRP3 inflammasome activation, expression levels of GFAP and IBA-1, oxidative stress pathways, apoptosis, regulation other neurotransmitters, and acetylcholinesterase(AChE) expression.

Methods: The CUMS model was employed to induce depressive-like behaviors in rats, followed by a one-week treatment with M8I. Behavioral tests were performed to assess depressive-like behaviors. Western blot, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay, immunohistochemistry, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and near-infrared II (NIR-II) imaging were utilized to evaluate the expression levels of proteins involved in the TNF-α/TNFR1/NF-κB signaling pathway, NLRP3 inflammasome, GFAP and IBA-1 expression in astrocytes and microglia, oxidative stress markers (SOD, GSH, PI3K/AKT), apoptosis-related proteins (Bax, Bcl-2), as well as brain neurotransmitters regulation and AChE activity.

Results: CUMS induced depressive-like behaviors in rats, upregulated the expression of TNF-α and its associated proteins (TNFR1, NF-κB), NLRP3 inflammasome-related proteins (p-P38, caspase-1), GFAP, and IBA-1 in the hippocampal tissue, and downregulated the expression of SOD, GSH, and PI3K/AKT. Additionally, it disrupted the balance of apoptosis-related proteins (Bax, Bcl-2), brain neurotransmitters regulation, and AChE activity. Treatment with M8I reversed these alterations; however, certain neurotransmitters, such as norepinephrine and dopamine, did not fully return to normal levels.

Conclusion: M8I effectively alleviates CUMS-induced depressive-like behaviors by modulating the TNF-α/TNFR1/NF-κB signaling pathway, inhibiting the NLRP3 inflammasome activation, and suppressing astrocytes and microglia activation, thereby attenuating inflammatory responses, oxidative stress pathways, and apoptosis-related processes. These findings provide novel insights into M8I as a potential therapeutic strategy for depression and futher elucidate the molecular mechanisms underlying its anti-inflammatory effects.