As documented in the previous studies and verified in the present study, a decrease in the level of antioxidant enzymes can lead to increased oxidative stress. According to the present results, melatonin treatment could increase the expression of antioxidant enzymes (SOD, GPX, and CAT) and significantly decrease the MDA level in the surgery groups. Although melatonin increased the levels of SOD, GPX, and CAT in all melatonin groups, the oxidative stress induced by surgery seemed to be so high that melatonin could not completely modulate the activity of these antioxidant enzymes. Anesthesia seemed to lead to less oxidative stress because the activities of antioxidant enzymes significantly increased in the anesthesia groups that received melatonin. In addition, in the melatonin-treated groups, melatonin led to a decrease in the level of MDA, which had reached its peak after 3 days.
There are also some other surgical procedures, such as OHE in dogs and abdominal hysterectomy in women, that have been reported to increase lipid peroxidation 24 h after surgery [3, 21]. In Sakundech et al.’s (2020) study, the MDA level reached its peak 3 days after OHE in dogs but the CAT level did not differ significantly between treatment days. They also reported that the pain score reached its maximum after 3 days due to inflammation caused by tissue injury. Similar results were reported by Quarterone et al. (2017), who compared ovariohysterectomized and ovariectomized dogs and rats regarding postoperative analgesia (Quarterone et al., 2017). Sakundech et al. (2020) also concluded that due to the oxidative stress caused by OHE, the ovariohysterectomized dogs should receive some treatment that can protect their bodies from free radicals [2, 22].
Furthermore, Serin et al. (2008) observed that OHE and anesthesia, induced by a combination of Ketamine-Xylazine, could lead to an increase in the oxidative stress markers (Glutathione and MDA). The toxicity of drugs commonly used in anesthesia can induce oxidative stress . Gunay (1999) reported a significant decrease in antioxidative capacity in dogs after enflurane anesthesia . In another study, it was observed that the serum level of vitamin E and beta-carotene significantly decreased whereas the serum level of vitamin A and MDA significantly increased during enflurane anesthesia in dogs .
According to Aengwanich et al. (2019), male dogs were not under oxidative stress after castration due to the non-significant changes in the oxidative stress markers, such as MDA and CAT. However, they concluded that antioxidant treatment may be beneficial for the male dogs in their study because they observed a decrease in total antioxidant capacity (TAC) on days 3, 7, and 10 compared to day 14 . OHE under ketamine-xylazine anesthesia was found to increase lipid peroxidation and decreased antioxidant enzyme activity in rats . The activities of SOD, GPX, and CAT decreased in bitches affected with pyometra due to oxidative stress, but their activities increased after OHE .
The post-spaying period in dogs is similar to the post-menopausal period in women. Six weeks after surgical menopause, the decrease in the blood estrogen may lead to some changes in oxidant and antioxidant status. It has also been shown that estrogen therapy, alone or in combination with progesterone, in perimenopausal and postmenopausal women can improve the antioxidant status. Moreover, some studies have suggested that estrogen can lead to higher concentrations of essential trace elements, of which some are antioxidants, in the body and that estrogen itself may serve as an antioxidant and thus prevent the oxidation of lipoproteins [26–28]. Based on these studies, those women who underwent surgical menopause showed increased oxidative stress while in those who retained their ovaries, estrogen levels increased after surgery. Accordingly, it can be stated that estrogen is a potential endogenous protector against oxidative stress even in postmenopausal women.
Several studies have reported an increase in the oxidative stress biomarkers after OHE [2, 23]. Therefore, the researchers should find a way that minimizes the harmful effects of free radicals. Melatonin has been shown in numerous studies to have antioxidant properties [17, 29]. Mogheiseh et al. (2019) found that dogs were under oxidative stress after castration and that melatonin improved the antioxidant status of intact and castrated dogs .
Gautier and et al. (2020) investigated the physiologic effects of hyperbaric oxygen (HBO) therapy on healthy dogs after OHE. They found that while this therapy, at the dose used in their study, had no adverse effects on the treated group, it was ineffective in reducing postoperative inflammation and improving the oxidative status in them. In their study on the effects of flunixin meglumine (FM) and meloxicam on postoperative and oxidative stress after OHE, Yilmaz et al. (2014) reported that while FM reduced postoperative stress, it had no influence on the oxidative stress status . Salavati et al. (2018) examined the effects of melatonin treatment on the levels of sexual hormones, serotonin, and cortisol in intact and castrated male dogs. Their results showed that melatonin increased the level of serotonin but decreased the level of cortisol in intact and castrated dogs . Taheri et al. (2019) also studied the effects of melatonin administration on the levels of thyroid hormones, leptin, and ghrelin in intact and castrated male dogs. They observed that melatonin could regulate metabolic hormones and mitigate the metabolic side-effects of castration . Furthermore, melatonin has been shown to regulate the activation or inhibition of several transcription factors related to antioxidant response, which can justify the antioxidative effects of this substance .