Effects of Melatonin on Spermatozoa Activity and Associated Mechanisms in Heat-Stressed Rats

In animals, heat stress (HS) disrupts spermatogenesis, reducing sperm quality and, in severe cases, potentially inducing the loss of male reproductive function. Melatonin confers significant resistance to oxidative stress and apoptosis; however, its specific effects on rat spermatocytes and the mechanism underlying its anti-HS effects remain inadequately explored. Therefore, this study aimed to analyze the effects of melatonin at different concentrations on sperm cell activity in heat-stressed rats. Modeling heat stress injury, sperm viability and density assay, sperm plasma membrane integrity analysis, and oxidative stress assay of testicular tissue were conducted. The results revealed that HS caused sperm cell injury. However, the intraperitoneal injection of melatonin effectively improved spermatozoa quality, and a dose of 1 mM significantly alleviated the HS-induced damage. Moreover, HS increased the levels of oxidative and endoplasmic reticulum (ES) stress in rat testicular tissues, inducing germ cell apoptosis and pathological changes. Similarly, melatonin treatment improved sperm cell viability and density, inhibited germ cell apoptosis, and reduced oxidative and ES stress levels. Overall, melatonin effectively reduced the adverse effects of HS on rat sperm cells, and an intraperitoneal injection of 1 mM (0.6966 mg) melatonin facilitated the normal production of spermatozoa. Notably, its mechanism may involve reduced ES and oxidative stress levels in testicular tissues, increased expression of the anti-apoptotic protein Bcl-2, and inhibition of germ cell apoptosis.