Diabetes is a metabolic disease characterized by hyperglycemia due to defects in insulin production/function of insulin. Chronic hyperglycemia can cause damage to the organs, including the kidneys, eyes, nerves and heart. Multiple pathogenic processes contribute to the onset of diabetes, including the autoimmune destruction of pancreatic β-cells leading to insulin deficiency, as well as various abnormalities causing insulin resistance, while deficient insulin function can also lead to abnormalities in metabolism. Insufficient secretion of insulin and tissue responses to insulin often coexist, making the primary cause of hyperglycemia unclear (1).

Reactive oxygen species (ROS) are molecules containing chemically active oxygen that are produced within living systems. They are natural by-products of oxygen metabolism in all aerobic organisms (2,3). Elevated levels of ROS result in oxidative stress, which serves an important role in damaging cellular components, such as lipids, proteins and DNA (4). The antioxidant defense system plays a vital role in protecting biological systems by mitigating the detrimental effects of ROS. Numerous antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase, glutathione reductase, catalase (CAT) and paraoxonase, actively contribute to this protective mechanism (5). Apart from the enzymatic antioxidants, the non-enzymatic antioxidant defense system [including ascorbate, tocopherols, retinol, carotenoids, reduced glutathione (GSH), melatonin, polyphenols, ceruloplasmin and carnosine, among others] is equally important in the regulation of ROS levels and preservation of normal cellular function (6). Elevated levels of oxygen free radicals are associated with lipid peroxidation, non-enzymatic protein glycation and glucose oxidation, all of which contribute to the development of diabetes mellitus (DM) and its complications (2).

Cellular and animal model studies have revealed that essential oils possess notable anti-inflammatory, antioxidant and anticancer properties (7,8). Researchers are increasingly interested in essential oils due to their natural phenolic content and potential antioxidant and free radical scavenging activities. Essential oils from basil, cinnamon, clove, nutmeg, oregano and thyme have demonstrated significant radical-scavenging and antioxidant effects in the DPPH radical assay at room temperature (9). However, it is important to note that the free radical scavenging ability of these essential oils is not solely attributed to their phenolic components, as monoterpene alcohols, ketones, aldehydes, hydrocarbons and ethers also contribute to their antioxidant activity (10).

Linalool (LIN), an acyclic monoterpene alcohol, is a naturally occurring compound found in aromatic plants and is widely used as a fragrance ingredient in various products. Its versatile application extends from decorative cosmetics, fine fragrances and toiletries to non-cosmetic items, such as household cleaners and detergents (11,12). LIN has been reported to possess pharmacological properties, including sedative, analgesic, anti-inflammatory, antioxidant, antimicrobial and antitumor effects (13,14).

The interest in the antioxidant properties of essential oils has increased recently (15,16). There is a growing trend to substitute synthetic antioxidants with natural compounds, leading to the emergence of their potential use as natural additives (17). Cinnamomum osmophloeum (Lauraceae) oil exhibited 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity (IC₅₀, 29.7 µg/ml), which was associated with its major component, LIN (73%) (18). Despite the association between the antioxidant activity of essential oils and the presence of major compounds, such as LIN, in certain plants, van Zyl et al (19) showed that LIN alone has minimal antioxidant activity (IC₅₀ >648 µM) against the DPPH radical.

LIN has been reported to reduce oxidative stress-induced damage by inhibiting superoxide anion and hydroxyl radical formation under in vitro conditions (20). In particular, studies have indicated that its antioxidant effect is mediated by the prevention of H₂O₂-induced decreases in cell viability, the reduction of LDH release, excessive ROS production, apoptosis and G₂/M phase cell cycle arrest (21), as well as the inhibition of NF-κB activation (20).

Streptozotocin (STZ), also known as streptozocin, was identified in 1959 as a natural antibiotic originating from Streptomyces achromogenes (22). The toxic effects of STZ on pancreatic β-cells, known as its diabetogenic action, were first reported in 1963 (23,24). The STZ molecule, with a molecular formula of C₈H₁₅N₃O₇ and a molecular weight of ~265 g/mol, comprises two components: i) A glucopyranosyl group, which aids its absorption by pancreatic β-cells through glucose transporter 2; and ii) a nitrosourea group, which is responsible for the destruction of pancreatic β-cells (25). STZ has been observed to induce diabetic conditions in animal studies due to its selective destruction of insulin-producing β-cells within the pancreatic islets (23). The detailed description of this effect in preclinical rat laboratory models has encouraged the use of STZ to induce diabetes in laboratory animals for research purposes (26).

Numerous studies have reported the diverse biological activities of LIN (27-31). While most of these studies were conducted in vitro or in animal models using different administration methods (i.e. intraperitoneally and inhalation) there is a lack of consistent clinical investigations specifically focused on LIN. Considering that processes such as oxidative stress and inflammation are involved in the Considering that processes such as oxidative stress and inflammation are involved in the pathogenesis of diabetes (32), the antioxidant properties of LIN may support a potential activity of this phytochemical against oxidative stress in diabetes. Therefore, the present study aimed to investigate the antioxidant activity of LIN in an STZ-induced diabetic rat model.

Diabetes is a chronic metabolic disorder characterized by elevated blood sugar levels, resulting from either insufficient insulin production or ineffective insulin utilization by the body. It is a complex disease with various complications (such as retinopathy, nephropathy, hypertension, coronary heart disease and neuropathy) that affect multiple organ systems, in particular the eyes, kidneys, heart and nerves (39). Oxidative stress is a condition that occurs when there is an imbalance between the production of ROS and the body's ability to neutralize them with antioxidants. ROS are highly reactive molecules that contain oxygen and can cause damage to cells and tissues if their levels become too high (40). Persistently high blood glucose levels, known as hyperglycemia, are a hallmark of diabetes. Elevated glucose levels can lead to increased production of ROS through processes such as glucose autoxidation and the formation of advanced glycation end products. These reactions generate ROS as byproducts, which can promote oxidative stress (41,42). To counteract the effects of oxidative stress in diabetes, herbal-based therapeutic strategies to enhance the antioxidant defense and reduce ROS production are being investigated (43). Numerous reports have provided various results on the antidiabetic effects of plant extracts containing LIN (44-47). However, studies on the antioxidant effects of LIN in diabetic rats are scarce.

In the present study, a significant increase in the blood glucose concentration of STZ-induced diabetic rats compared with the CONTROL group was observed. In STZ-treated groups, blood glucose concentrations should typically exceed 200 mg/dl in non-fasting animals, while blood glucose levels should be >150 mg/dl in fasting diabetic animals (33). The statistically significant difference between the STZ-treated and CONTROL groups was one of the most important points in this study, as it confirmed the successful establishment of the diabetic model. Typically, it has been reported that within 3 weeks following STZ injection, >50% of animals develop severe hyperglycemia, with blood glucose concentrations ranging from 300-600 mg/dl. Therefore, a 21-day period was considered for the LIN treatment (48).

However, during the 21-day study period, no significant decrease in the blood glucose levels was observed in the DM + LIN group. Despite LIN administration, the blood glucose concentrations remained elevated and were not statistically different from those in the DM group. This lack of a significant difference suggests that LIN did not exert a notable hypoglycemic effect under the conditions of the present study, indicating the necessity of further investigation into its potential therapeutic benefits for diabetes management. It was concluded that LIN alone, at a dose of 100 mg/kg, did not play an effective role in the control of diabetic hyperglycemia. Lee et al (49) reported that indigenous cinnamon leaf essential oil containing ~40% LIN significantly decreased fasting blood glucose and fructosamine levels at all doses tested (12.5, 25 and 50 mg/kg) and increased plasma and pancreatic insulin levels under fasting conditions in the STZ-induced diabetic rat model. The study attributed the effect of LIN in the leaf essential oil from indigenous cinnamon to its hypoglycemic and pancreas-protective effects. LIN, being the major component of cinnamon leaf essential oil, was suggested to contribute to the hypoglycemic effect observed in diabetic rats. Additionally, LIN was reported to reverse the increase in hepatic pro-inflammatory cytokine levels in diabetic rats (50). These findings suggest that LIN may exert its effects through anti-inflammatory and antioxidant mechanisms, contributing to its potential therapeutic benefits in diabetes management (49). Deepa and Venkatraman (50) reported an improvement in blood glucose levels through inhibition of gluconeogenesis in a study in which 25 mg/kg LIN was administered for 45 days in diabetic rats. Although the results of the aforementioned studies are different from those obtained in the present study, it is hypothesized that these differences may be attributed to the particular diabetes model used, the dose and duration of LIN administration, the enantiomer differences of LIN (51) or the type of experimental animal used.

In the liver tissue of DM group, a decrease in CAT activity was exhibited. Similarly, a notable reduction in serum CAT activity was observed. Both liver and serum CAT activities were significantly increased in DM + LIN and LIN groups compared with those in the DM group Altınok-Yipel et al (36) reported that LIN increased CAT activity in rats with liver damage induced by CCl₄. Consistent with this study, the present results demonstrated that LIN can restore impaired CAT activity under stress. The restoration of serum CAT levels suggests that LIN exhibits a systemic effect and can ameliorate oxidative imbalances throughout the body. The results of the present study highlight the efficacy of LIN in combating systemic oxidative stress, as reflected in the serum and its potential regulatory effects on various systems in the organism. This may indicate that the antioxidant capacity of LIN is beneficial for the general oxidative status of the organism beyond the cellular level.

It was found that SOD levels in the liver tissue did not show statistically significant differences among the groups. The level of SOD activity in the LIN group was similar to that in the CONTROL group, suggesting that LIN had no effect on SOD activity under normal physiological conditions. Despite the increase in serum SOD levels in the DM + LIN group compared with those in the DM group, the effects of LIN on SOD activity in the liver were not as pronounced as those in the serum. This suggests that the antioxidant effects of LIN may differ between target tissues. The results of the current study are in accordance with those of previous studies (52-54), which indicated that there were no substantial fluctuations in the tissue activity of SOD in individuals with DM. However, other studies have reported both elevated (55) and decreased (56) levels of SOD in patients with DM.

Antioxidants help protect cells by neutralizing free radicals caused by oxidative stress (57). Reversely, oxidative stress, by increasing the formation of free radicals in cells, depletes antioxidant molecules, such as GSH (58). Both in the serum and liver, no significant effect of LIN treatment on GSH levels was observed in diabetic rats. LIN is a compound with antioxidant properties, and its antioxidant activity may protect GSH in cells from the harmful effects of oxidative stress. However, it has also been indicated that LIN improves GSH levels in brain tissue depending on the time of administration. For example, it was reported that LIN was more effective when it was administered to rats 3 days before or at the same time as the acrylamide-induced neurotoxicity model establishment, while no effect was observed when LIN was administered 3 days after the model was established (59). Even if this model and the diabetes model of the presented study are not the same, LIN was administered 48 h after the onset of diabetes in the present study. Therefore, its efficacy may have varied depending on the time of administration.

MDA is a by-product of lipid peroxidation, indicative of oxidative damage to cell membrane lipids (60). In the present study, a significant increase in serum MDA levels were observed in the diabetic group (DM) compared with the CONTROL group, as well as in the LIN-treated diabetic group (DM + LIN) compared with the CONTROL group. These findings confirm that diabetes induction substantially elevated serum MDA levels, resulting in increased oxidative stress. The results showed that the antioxidant effects of LIN were significantly effective in reducing MDA levels, especially in the liver of diabetic rats. The notable reduction in liver MDA levels in LIN-treated diabetic rats compared with untreated diabetic rats demonstrates the potential therapeutic benefits of LIN in mitigating oxidative stress associated with diabetes. However, the absence of significant differences in serum MDA levels between the treated and untreated diabetic groups, as well as between the non-diabetic groups, indicates that the effects of LIN may be more pronounced in the liver tissue than in the serum. This differential impact suggests that the antioxidant properties of LIN may be tissue specific. Further research is required to elucidate the precise mechanisms through which LIN modulates oxidative stress and to explore its potential applications in other tissues and diseases.

The present study assessed the antioxidant properties of LIN in DM model. In STZ-induced diabetic rats, LIN treatment alone failed to control hyperglycemia. However, a significant restoration of CAT activity was observed in both the liver and serum in diabetic rats treated with LIN. Nonetheless, there was no significant change in liver SOD activity. Additionally, LIN was observed to have a reducing effect on MDA levels, an important indicator of diabetes-related oxidative stress. It was also found that LIN did not affect GSH levels in either the serum or liver in diabetic rats. In summary, the present results suggest that LIN has the potential to reduce diabetes-associated oxidative stress. Overall, the current study may indicate that LIN has a regulatory effect on the overall oxidative status of the organism by reducing liver MDA levels, an important indicator of diabetes-related oxidative stress, while increasing the activity of antioxidant enzymes, such as liver and serum CAT activity, and serum SOD activity in diabetic rats. This suggests that LIN may help in mitigating oxidative damage associated with diabetes.

One of the limitations of the present study is the absence of histopathologic tissue evaluation. Although blood glucose levels and other biochemical parameters were thoroughly assessed, a detailed examination of pancreatic or other relevant tissue samples was not performed. Histopathologic analysis could have provided valuable insights into the cellular and structural changes associated with STZ-induced diabetes and the potential protective effects of LIN. This limitation restricts the understanding of the underlying mechanisms at the tissue level and the broader implications of the present findings. Future studies should incorporate comprehensive histopathologic evaluations to complement biochemical data and offer a more holistic understanding of the therapeutic potential of LIN in diabetic models.

These findings suggest that LIN has potential antioxidant effects in the context of diabetes. Further research is needed to understand the mechanisms underlying these effects and determine the optimal dosage of LIN for diabetes management.