Melatonin is a molecule ubiquitous in nature and involved in several physiological functions. In the brain, melatonin is converted to N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and then to N1-acetyl-5-methoxykynuramine (AMK), which has been reported to strongly enhance long-term object memory formation. However, the synthesis of AMK in brain tissues and the underlying mechanisms regarding memory formation remain largely unknown. In the present study, young and old individuals from a melatonin-producing strain, C3H/He mice, were employed. The amount of AMK in the pineal gland and plasma was very low compared with those of melatonin at night; conversely, in the hippocampus, the amount of AMK was higher than that of melatonin. Indoleamine 2, 3-dioxygenase (Ido) mRNA was expressed in multiple brain tissues, whereas tryptophan 2,3-dioxygenase (Tdo) mRNA was expressed only in the hippocampus, and its lysate had melatonin to AFMK conversion activity, which was blocked by the TDO inhibitor. The expression levels of phosphorylated cAMP response element binding protein (CREB) and PSD-95 in whole hippocampal tissue were significantly increased with AMK treatment. Before increasing in the whole tissue, CREB phosphorylation was significantly enhanced in the nuclear fraction. In the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, we found that downregulated genes in hippocampus of old C3H/He mice were more enriched for long-term potentiation (LTP) pathway. Gene set enrichment analysis showed that LTP and neuroactive receptor interaction gene sets were enriched in hippocampus of old mice. In addition, Ido1 and Tdo mRNA expression was significantly decreased in the hippocampus of old mice compared with young mice, and the decrease in Tdo mRNA was more pronounced than Ido1. Furthermore, there was a higher decrease in AMK levels, which was less than 1/10 that of young mice, than in melatonin levels in the hippocampus of old mice. In conclusion, we first demonstrated the Tdo-related melatonin to AMK metabolism in the hippocampus and suggest a novel mechanism of AMK involved in LTP and memory formation. These results support AMK as a potential therapeutic agent to prevent memory decline.

中文翻译：

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N1-乙酰基-5-甲氧基犬尿胺随着年龄的增长在海马中减少，通过 CaMKII/CREB ​​磷酸化改善长期记忆

褪黑激素是自然界中普遍存在的分子，参与多种生理功能。在大脑中，褪黑激素转化为 N1-乙酰基-N2-甲酰基-5-甲氧基犬尿胺 (AFMK)，然后再转化为 N1-乙酰基-5-甲氧基犬尿胺 (AMK)，据报道，AMK 可以强烈增强长期物体记忆的形成。然而，AMK 在脑组织中的合成以及记忆形成的潜在机制仍然很大程度上未知。在本研究中，使用了来自褪黑激素产生品系 C3H/He 小鼠的年轻和年老个体。夜间松果体和血浆中AMK的含量与褪黑激素相比非常低；相反，在海马体中，AMK的含量高于褪黑激素。吲哚胺 2,3-双加氧酶( Ido ) mRNA 在多个脑组织中表达，而色氨酸 2,3-双加氧酶( Tdo ) mRNA 仅在海马中表达，其裂解物具有褪黑素向 AFMK 的转化活性，该活性被TDO 抑制剂。AMK 处理后，整个海马组织中磷酸化 cAMP 反应元件结合蛋白 (CREB) 和 PSD-95 的表达水平显着增加。在整个组织中增加之前，CREB ​​磷酸化在核部分中显着增强。在京都基因与基因组百科全书（KEGG）通路分析中，我们发现老C3H/He小鼠海马中下调的基因在长时程增强（LTP）通路中更加富集。基因集富集分析表明，LTP和神经活性受体相互作用基因集在老年小鼠海马中富集。此外，与年轻小鼠相比，老年小鼠海马中Ido1和Tdo mRNA的表达量显着下降，且Tdo mRNA的下降幅度较Ido1更为明显。此外，与年老小鼠海马中褪黑激素水平相比，AMK 水平下降幅度更大，不到年轻小鼠的 1/10。总之，我们首先证明了海马中与 Tdo 相关的褪黑激素对 AMK 的代谢，并提出了 AMK 参与 LTP 和记忆形成的新机制。这些结果支持 AMK 作为预防记忆衰退的潜在治疗剂。