Abstract
Purpose: To assess the effects of thymoquinone (TQ) on the integrity of sperm DNA in nicotineinduced sperm impairment in rats.
 Methods: Adult male Sprague Dawley rats were randomized into four equal groups: control group received normal saline orally for 60 days; nicotine group was subcutaneously injected with 5 mg/kg/day nicotine for 30 days and then given normal saline for the next 30 days; TQ group was given normal saline for 30 days followed by TQ at 5 mg/kg/day for 30 days; and nicotine-TQ group received 5 mg/kg of nicotine for 30 days and 5mg/kg of TQ for another 30 days. Sperm DNA breakages were evaluated using Comet assay. The expression levels of protamine 1 (PT1) and transition nuclear protein 2 (Tnp2) genes which are essential for the proper compaction of the sperm DNA were analyzed by quantitative polymerase chain reaction (qPCR).
 Results: Thymoquinone significantly decreased DNA fragmentation in the sperm of nicotine-treated rats. However, there was no change in PT1 gene expression. Tnp2 was downregulated in the nicotine group and slightly upregulated in nicotine-TQ group (p < 0.05).
 Conclusion: The results demonstrate the potential benefits of TQ in improving sperm DNA quality of nicotine-induced male infertility.
Highlights
Drugs and chemicals may adversely affect the male reproductive system
The DNA damage caused by nicotine was evidenced by the increase in the DNA damage profile such as TL, TM, OTM and % tail DNA compared to the control and TQ group (p < 0.05; Table 2)
The findings of this study show that administration of nicotine has harmful effects on sperm DNA quality and integrity in rats
Summary
Drugs and chemicals may adversely affect the male reproductive system. Smoking exerts injurious effects on sperm count, kinetic parameters, morphology and DNA integrity [1]. The adverse effects of cigarette smoking on fertility could be attributed to the content of cigarettes which includes several toxic chemicals, mutagens and carcinogens. Nicotine represents 90 % of the total alkaloids in cigarettes [2] and mimics most of the deleterious effects of cigarette smoking [3]. Histones are replaced by transition nuclear proteins (Tnp). They are replaced with protamines [7]. Disturbance of this could expose DNA to fragmentation and compromise fertility
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