Melatonin Alleviated Cadmium Induced Microbiota-Gut-Brain Disorder in Adult Zebrafish: Insights from Transcriptomic and Microbiome Analysis

Cadmium (Cd), a toxic metal, poses significant threats to ecological and human health due to its neurotoxic and intestinal toxicity effects. However, the mechanisms by which Cd disrupts brain-gut axis interactions remain unclear, and strategies to mitigate these effects are limited. Melatonin (MT), known for its anti-inflammatory and antioxidant properties, has shown promise in counteracting heavy metal toxicity. This study investigated the protective mechanisms of MT against Cd-induced toxicity in adult zebrafish using histopathological analysis, 16S rRNA sequencing, RNA-sequencing, and qRT-PCR. Results showed that MT significantly alleviated Cd-induced structural damage in brain spongiosa and restored intestinal villi integrity. 16S rRNA sequencing revealed that MT reduced pathogenic bacteria and increased beneficial bacteria in the gut microbiota. Transcriptomic analysis identified 31 differentially expressed genes (DEGs) in brain, KEGG enrichment analysis showed these DEGs are associated with neurodegenerative diseases pathways. Concurrently, 8 DEGs in intestinal were linked to oxidative phosphorylation signaling pathways. Correlation analysis showed pathogenic Legionella and Aeromonas were positively correlated with htr2b, il21r.2, il2rb, il21r.2, cyp46a1.3, cyp2ad3, cyp46a1.3 in brain, Candidatus_Protochlamydia was positively correlated with il7r, drd3 in intestinal, those are down regulated DEGs, whereas beneficial Acinetobacter and Achromobacter were positively correlated with cyp2x8 in intestinal, this is up regulated DEG. These suggests that Legionella, Candidatus_Protochlamydia, Achromobacter and Acinetobacter may be key bacterial that mediate the MT reduction in neurotoxicity and immunotoxicity induced by Cd. These findings highlight MT's potential to mitigate Cd-induced toxicity by modulating the gut microbiota, offering therapeutic insights for reducing Cd toxicity risks in aquaculture.