Melatonin-induced disruption of spermatogenesis and hypophyseal function in the Mozambique tilapia (Oreochromis mossambicus)

Although melatonin (MLT) is well known for its role in regulating circadian rhythms and reproductive functions in mammals, its influence on spermatogenesis in fish is not yet clearly understood. In this study, we examined the impact of MLT on the hypophysis-testicular axis in Oreochromis mossambicus, a cichlid fish species that breeds round the year. Daily intraperitoneal administration of MLT at higher dose (2 mg/0.1 mL saline) for 21 days caused a substantial decline in the numbers of spermatogonia A, primary and secondary spermatocytes, as well as early and late spermatids compared to the control group. This was accompanied by a significant reduction in Sertoli cell count, androgen receptor immunoreactivity in the testis, and serum testosterone levels. Additionally, the high-dose MLT group exhibited a marked decrease in both the percent area and intensity of luteinizing hormone (LH) immunolabelling within the proximal pars distalis of the hypophysis (PPD). In contrast, treatment with a low dose of MLT (0.5 mg/0.1 mL saline) had no marked effect on LH and androgen receptor immunoreactivity and spermatogenesis, except for a significant decrease in the number of secondary spermatocytes. These findings reveal that elevated MLT levels can disrupt spermatogenesis in Oreochromis mossambicus by affecting germ cells at different stages of development. The observed impairment is likely mediated through the suppression of testicular steroidogenic output and androgen receptor protein expression, concomitant with a reduction in LH content in the PPD.