Exogenous melatonin and photoperiodic regulation of melatonin membrane receptors, steroidogenesis and testicular histomorphology in sexually mature guinea fowl (Numida meleagris)

Abstract

Seasonal reproduction in birds is regulated by photoperiod and melatonin through receptor-mediated signaling. However, their combined effects on testicular steroidogenic pathways, melatonin receptor expression and antioxidant defence-related gene expression remain poorly understood in poultry species such as the male helmeted guinea fowl ( Numida meleagris ). This study investigated the interactive effects of photoperiod and exogenous melatonin on testicular morphology and the transcript expression of genes associated with steroidogenic pathways, androgen signaling, melatonin receptor expression and antioxidant defence-related transcript expression in sexually mature male guinea fowl. Sixty sexually mature guinea cocks (10–12 months old) were randomly assigned to six experimental groups ( n = 10 per group) exposed to control [12 L:12D], short-day [8 L:16D], or long-day [16 L:8D] photoperiods, with or without daily intramuscular administration of melatonin (1 mg/bird body weight) for eight weeks. Quantitative real-time PCR revealed photoperiod- and melatonin-dependent modulation of steroidogenic, androgen signalling, melatonin receptor and antioxidant defence-related transcripts. The transcript levels of steroidogenic acute regulatory protein ( StAR ) and cytochrome P450 side-chain cleavage enzyme ( Cyp11a1/P450 scc ) were significantly decreased under long-day conditions, particularly in melatonin-treated birds. Likewise, expression of melatonin receptor isoforms ( Mel-1α and Mel-1β ) decreased under long-day conditions following melatonin treatment. 3β-hsd expression varied with photoperiod, whereas Cyp19a1 (aromatase) expression was consistently suppressed by melatonin. Ar mRNA expression increased under long-day conditions but was reduced by melatonin under short-day exposure. Among the antioxidant defence-related genes, Sod and Gpx expression showed no significant changes across treatment groups, whereas Cat expression was significantly higher in non-melatonin-treated birds than in melatonin-treated birds ( P < 0.01). Histological examination of the testes revealed Leydig cell hypertrophy characterized by compact cellular organization, prominent nuclei, dense cytoplasmic granulation and a marked preponderance of lipid droplets within the cytoplasm in birds exposed to the long-day [16 L:8D] photoperiod with melatonin treatment. Photoperiod and exogenous melatonin interactively influence Leydig cell morphology and modulate testicular steroidogenic, androgen signaling, melatonin receptor and antioxidant defence-related gene expression in the guinea fowl. These findings provide new insights into the molecular responses associated with photoperiodic regulation of avian reproduction and identify melatonin as an important modulator of testicular responses to seasonal photoperiodic changes. Strategic manipulation of photoperiod and melatonin may therefore have potential for influencing reproductive performance in guinea fowl under variable environmental lighting conditions.