The beneficial impact of astaxanthin-loaded chitosan nanoparticles on physiological responses, semen quality, testicular structure, and sperm ultrastructure in heat-stressed rabbit bucks

This study investigated the protective effects of Astaxanthin-loaded chitosan nanoparticles (ASX-CNPs) in alleviating heat stress-induced impairments in physiological responses, antioxidant capacity, semen quality, testicular structure, sperm ultrastructure, and fertility in rabbit bucks. Forty adult V-line rabbit bucks were divided into four groups and fed diets containing ASX-CNPs at 0, 50, 100, and 200 mg/kg for 12 weeks. Dietary supplementation with ASX-CNPs significantly improved physiological responses. The 100 mg/kg diet treatment increased total protein concentration, total lipids, total antioxidant capacity, and antioxidant enzymes, including glutathione peroxidase (GPx) and superoxide dismutase (SOD). Additionally, it lowered oxidative stress biomarkers (hydrogen peroxidase and malondialdehyde) and reduced the activities of aspartate aminotransferase and alanine aminotransferase in both blood serum and seminal plasma. This concentration also improved seminal plasma fructose levels, sperm quality (motility, viability, concentration, morphology), and increased the pregnancy rate by 74.70% compared to the control group (p < 0.05). Quadratic regression analysis predicted the optimal ASX-CNPs doses for improving testosterone concentration, reaction time, progressive motility, and intact acrosome ranges between 130 and 150 mg/kg diet. Molecular docking exploration indicated astaxanthin’s antioxidant potential via binding to active sites of GPx, SOD, and Nrf2 target proteins. TEM images revealed that the ASX-CNPs100 and ASX-CNPs200 groups had more sperm cells with normal structures, intact acrosomes, and plasma membranes compared to the control group. Additionally, ASX-CNPs supplementation improved testicular architecture. Overall, dietary supplementation with ASX-CNPs at 100–150 mg/kg appears to be an effective strategy for enhancing heat tolerance and reproductive efficiency in rabbit bucks in hot climates.