**Background:** Polyethylene microplastics (PE-MP) are ubiquitous environmental pollutants that accumulate in the gastrointestinal tract after ingestion, potentially disrupting the intestinal barrier. The intestinal barrier comprises a mucus layer (mainly Muc2 mucin), intestinal epithelial cells, and tight junction proteins (e.g., occludin, ZO-1, claudin-1) regulated by myosin light chain kinase (MLCK). Melatonin, a pineal hormone with anti-inflammatory and antioxidant properties, has shown protective effects in gastrointestinal diseases. This study aimed to evaluate the effects of oral PE-MP exposure on intestinal barrier components and to determine whether melatonin can alleviate PE-MP-induced damage.
**Methods:** Forty-nine adult male albino rats were divided into seven groups (n=7 each): control (saline+ethanol or corn oil), melatonin (5 mg/kg/day), low-dose PE-MP (3.75 mg/kg/day), high-dose PE-MP (15 mg/kg/day), low-dose PE-MP + melatonin, and high-dose PE-MP + melatonin. All treatments were given by oral gavage for 5 weeks. Jejunal tissues were collected for histopathology (H&E, Alcian blue), transmission electron microscopy (TEM), quantification of PE-MP particles, qPCR for Muc2, occludin, ZO-1, and MLCK mRNA, immunohistochemistry for claudin-1 and cleaved caspase-3, and ELISA for IL-1β and TNF-α.
**Key Results:** PE-MP particles were detected in jejunal tissues in a dose-dependent manner. PE-MP exposure caused villous degeneration, shortening, epithelial atrophy, inflammatory cell infiltration, and decreased goblet cell mucin secretion (Alcian blue area %, goblet cell count, and diameter). Muc2 mRNA was significantly downregulated in both PE-MP groups vs. controls (p<0.0001). Tight junction proteins were disrupted: occludin and ZO-1 mRNA were significantly downregulated, MLCK mRNA was upregulated, and claudin-1 immunoreactivity was reduced in PE-MP groups (p<0.01 to p<0.0001). IL-1β levels were significantly elevated in both PE-MP groups vs. controls (p<0.001 to p<0.0001), while TNF-α was significantly decreased only in the high-dose PE-MP group (p<0.01). Cleaved caspase-3 immunoreactivity was significantly increased in PE-MP groups (p<0.001 to p<0.0001). Melatonin co-treatment significantly improved histopathology, increased mucin secretion and Muc2 mRNA, upregulated occludin and ZO-1 mRNA, downregulated MLCK mRNA, restored claudin-1 expression, reduced IL-1β, normalized TNF-α in the high-dose group, and decreased cleaved caspase-3 (all p<0.05 to p<0.0001). TEM confirmed improved enterocyte ultrastructure and tight junctions with melatonin.
**Clinical Implications:** This study demonstrates that oral PE-MP exposure at environmentally relevant doses disrupts the intestinal barrier by impairing mucus secretion, tight junction proteins, and promoting inflammation and apoptosis. Melatonin effectively counteracts these damaging effects, suggesting its potential as a therapeutic agent to protect against MP-induced intestinal barrier dysfunction. Given the widespread human exposure to microplastics, these findings highlight the need for further research on MP toxicity and the protective role of melatonin. The study also underscores the importance of considering MP type, size, dose, and duration when evaluating intestinal effects.