Exposure to hypoxia inhibits pituitary-testicular activity in the fish Oreochromis mossambicus

Abstract

Fishes are exposed to natural hypoxic conditions during aquacultural practices. Although negative effects of hypoxia are documented on fish reproduction, the mechanism of hypoxia-induced alteration in spermatogenic process is not well understood in continuously breeding fishes. The aim of this investigation was to determine the influence of hypoxia on pituitary-testicular activity in the tilapia Oreochromis mossambicus, a cichlid fish that breeds throughout the year. The fish were exposed to moderate hypoxia (MH, Dissolved oxygen (DO) level, 3.24 ± 0.22 mg/L) and severe hypoxia (SH, DO level, 1.64 ± 0.38 mg/L) for 21 days. No significant differences were noticed in the mean numbers of spermatogonia (Sg), primary spermatocytes (Ps), secondary spermatocytes (Ss), early spermatids (Est) and late spermatids (Lst) between controls and fish subjected to MH, whereas the numbers of these cells were significantly lower in fish exposed to SH. Furthermore, weakly immunoreactive luteinizing hormone (LH) secreting cells in the proximal pars distalis (PPD) of the pituitary gland were concomitant with significant reduction in mean serum levels of LH and testosterone as well as androgen receptor immunoreactivity in the Sertoli cells in the testis of fish exposed to SH compared to other groups. On the other hand, the serum cortisol levels were significantly higher in fish exposed to SH compared with other experimental groups. Taken together, these results indicate that the tilapia is able to withstand the moderate hypoxic condition, whereas exposure to severe hypoxic condition suppresses the process of spermatogenesis through the inhibition of LH secretion, which in turn might suppress the testicular steroidogenesis and androgen receptor expression in the Sertoli cells. The results also suggest a possibility that this inhibition might be mediated through the activation of stress-axis in O. mossambicus.