A potential antioxidant role for melatonin and AFMK in plasma, ovarian fluid, and eggs during reproduction in rainbow trout

Melatonin (Mel; N-acetyl-5-methoxytryptamine) is recognized in fish as both a biological time-keeper and regulator of many physiological processes, including reproduction. Beyond its endocrine functions, Mel acts as an antioxidant, either by directly scavenging reactive radicals and forming N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK), or by influencing the activity of antioxidant enzymes. This study examined whether Mel contributes to the protection of post-ovulated eggs of farmed rainbow trout against oxidative stress. Mel and AFMK levels were measured in plasma, ovarian fluid, and eggs, while the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione-S-transferase (GST) were determined in eggs. Plasma Mel concentrations did not significantly differ among pre-ovulation, ovulation, and post-ovulation stages, nor did they correlate with ovarian fluid levels. AFMK was undetectable in both plasma and ovarian fluid, whereas both Mel and AFMK were consistently present and positively correlated in eggs. This suggests that Mel directly reacts with reactive oxygen species within the eggs, resulting in AFMK formation. Mel concentrations in eggs did not correlate with the activities of SOD, CAT, or GST, implying that Mel primarily contributes to the antioxidant defense of post-ovulated eggs through non-enzymatic radical scavenging. Mel and AFMK levels were lower in eggs from four-year-old females compared to three-year-old, whereas SOD activity showed the opposite trend, suggesting a compensatory upregulation of enzymatic defense mechanisms in response to an age-related Mel decline. The concentration of Mel in eggs was approximately three times higher than in plasma or ovarian fluid, suggesting possible local synthesis within oocytes.