Melatonin promotes in vitro sperm capacitation in buffalo (Bubalus bubalis)

Melatonin, the main hormone secreted by the pineal gland, plays many roles in reproduction. In ram spermatozoa, melatonin administration increases plasminogen activator activity (Tsantarliotou MP et al. 2007 Theriogenology 69, 458-465), known to be involved in sperm capacitation and acrosome reaction (Taitzoglou IA et al. 1996 Mol. Androl. 8, 187-197). The aim of this study was to evaluate the efficiency of melatonin to induce buffalo in vitro sperm capacitation, indirectly assessed by estimating the capability of spermatozoa to acrosome-react. Frozen-thawed semen from 4 different bulls was pooled and treated by swim-up in order to select only the motile population. Spermatozoa (n = 829) were assessed immediately after swim- up separation, to evaluate the incidence of acrosomal loss in non-treated cells (time 0). The remaining spermatozoa were incubated in the absence of capacitating agents (negative control; n = 513), in the presence of 0.01 mM heparin (positive control; n = 775), 10 μM melatonin (n = 684), 100 μM melatonin (n = 751), and 1 mM melatonin (n = 650), for 2 h. Sperm were then exposed for 15 min to 60 μg mL-1 of lysophosphatidylcholine, an agent known to induce acrosome reaction (AR) only on capacitated spermatozoa. Trypan blue was used first to differentiate live from dead spermatozoa and the dried smears were then fixed in 37% formaldehyde and stained with Giemsa for acrosome evaluation by microscopic examination. The percentage of acrosome-reacted spermatozoa in each group was used to assess the efficiency of capacitation under different incubation conditions. The experiment was repeated 4 times. Differences among groups were analyzed by chi-square test. Acrosomal loss was observed only in 2.1% of the sperm population at time 0. Interestingly, sperm treatment with both heparin and the different concentrations of melatonin resulted in a significantly higher incidence of AR compared to the negative control (24.4, 20.5, 20.0, 23.6 v. 8.0% for the positive control, 10 μM melatonin, 100 μM melatonin, 1 mM melatonin, and the negative control, respectively; P < 0.01). These results demonstrated that melatonin determines capacitation of buffalo spermatozoa in vitro. Furthermore, the effect of melatonin was comparable to that of heparin, that is, the capacitating agent currently used in the IVF system. The capacitating effect was observed at all the tested concentrations, and viability was not affected. This suggests to extend the range of concentrations to test in future studies, in order to identify the optimal dose. Moreover, considering the seasonality of the species and the great differences in fertility attitude of buffalo bulls, it would be interesting to investigate the capacitation effect of melatonin in relation to both the season and the bull