Melatonin improves testicular hemodynamics and sperm quality in rams subjected to mild testicular heat stress

Melatonin is a potent free-radical scavenger, with anti-inflammatory, anti-oxidative, and anti-apoptotic effects. The objective was to determine whether melatonin promoted testicular blood flow and protected sperm quality in rams after mild heat stress (HS; scrotal neck insulation). Twelve yearling Dorset rams with good semen quality were housed indoors (∼18-20 °C). Once weekly for 2 wk, Doppler indices (resistive index [RI] and pulsatility index [PI]) were measured in the supratesticular artery and semen collected by electroejaculation. Then, rams were randomly allocated into two equal groups, and given either 36 mg melatonin in 1 ml corn oil SQ under the ear (MEL), or only corn oil (CONT). At 15 d after treatment, all rams were subjected to mild HS for 96 h, with blood flow measurements and semen collection done once weekly for 7 wk. There were group, week and group∗week interaction effects (P < 0.005) for total and progressive sperm motility (CASA); total sperm abnormalities and acrosome integrity had effects of group, week and group∗week interaction effects (P < 0.00); and there were group and week effects for RI and PI (P < 0.005), with no significant differences before treatment. Changes in total and progressive motility and sperm abnormalities were evident at Week 1 post-HS in CONT rams, but MEL mitigated (P ˂ 0.05) these effects from Weeks 2-7. Furthermore, both PI and RI were reduced (P ˂ 0.05; i.e., significant increase in blood flow) in MEL versus CONT rams most weeks after HS. In MEL rams, sperm motility and total abnormalities had recovered at Weeks 5 and 6, respectively, whereas CONT rams had not completely recovered by Week 7. There was no difference (P < 0.05) between MEL and CONT groups in scrotal subcutaneous temperatures in the 4-d intervals before, during and after HS. In conclusion, melatonin significantly improved testicular blood flow and protected sperm motility and morphology in rams exposed to testicular HS. Therefore, melatonin has potential for mitigating effects of testicular HS under field conditions.