Globally, around 350 million people suffer from depression (Schmaal et al., 2017). Major depressive disorder (MDD) is a chronic, debilitating psychiatric illness, and poses an enormous emotional and economic burden, with an estimated annual cost of $326.2 billion in the United States (Greenberg et al., 2021). The key challenge remains finding antidepressants with greater response rate and fewer side effects, particularly for treatment-resistant depression (TRD), which affects approximately 30.9 % of patients with a diagnosis of depression (Zhdanava et al., 2021).

Due to ethical and practical challenges in studying the human brain, translational models represent a key tool for unraveling neurobiological mechanisms underlying depression and discovering potential novel molecular drug targets (Pešić et al., 2016). During the past few decades, several animal models of depression have been established, including chronic mild stress, social defeat, early life stress, learned helplessness, Wistar Kyoto rat and model based on chronic administration of corticosterone or adrenocorticotropic hormone (ACTH) (Kolasa and Faron-Gorecka., 2023). Notably, it was observed that Wistar Kyoto rat strain harbors a genetic landscape relevant for TRD, whereas chronic exposure to ACTH injections produces a phenotype resistant to tricyclic antidepressants. ACTH model was first introduced in 2002, when Kitamura et al. (2002) discovered that treatment with ACTH, as one of the key mediators of the systemic stress and hypothalamic-pituitary-adrenal (HPA) axis, blocks antidepressant-like activity of imipramine in a behavioral paradigm. Hyperactivity of the HPA axis is one of the core features of depressive phenotype and further exploration of the complex interplay between stressful life events, polymorphisms in HPA-related genes and their epigenetic modifications, and clinical presentation, may lead to development of personalized and more effective therapeutic strategies for MDD (Baumeister et al., 2016).

A discovery that a single subananesthetic dose of ketamine evokes antidepressant effect was a major breakthrough in neuroscience (Berman et al., 2000). Ketamine was initially developed as a dissociative anesthetic agent in the 1960s, yet, over the past two decades, a plethora of preclinical and clinical evidence has demonstrated that ketamine exerts rapid antidepressant effect, in particular in patients with a diagnosis of TRD (Abbar et al., 2022; Berman et al., 2000; DiazGranados et al., 2010; Lapidus et al., 2014; Li and Vlisides, 2016; Pešić et al., 2016; Price et al., 2014; Zarate Jr et al., 2006). The proposed molecular mechanism of its antidepressant activity involves non-competitive inhibition of glutamatergic N-methyl-d-aspartate (NMDA) receptors expressed on GABAergic interneurons (Berman et al., 2000; Weckmann et al., 2017; Zanos and Gould, 2018). Importantly, in 2019 esketamine was approved, in combination with oral antidepressant, for TRD and for patients with a diagnosis of MDD with suicidal ideation (Turner, 2019).

A variety of hypotheses has been considered in an attempt to unravel neurobiology of depression (Nestler et al., 2002). Beyond monoamine hypothesis, hyperactivity of the HPA axis, decline in neurogenesis and impaired synaptic plasticity, a plethora of evidence suggest an important role of oxidative stress in depressive psychopathology (Duman et al., 2016; Lee and Han, 2019; Masi and Brovedani, 2011). Oxidative stress has been recognized and described as a pathological mechanism implicated in the development of cancer, diabetes, cardiovascular, neurodegenerative and psychiatric disease, including MDD (Black et al., 2015; Liu et al., 2020; Miljević et al., 2018). Namely, oxidative stress is a phenomenon, emerging when the balance between free radicals and antioxidants is disrupted (Pizzino et al., 2017). Reactive oxygen (ROS) and nitrogen species (RNS) are normally produced during aerobic metabolism and play important physiological roles in cognitive functions, immune response, differentiation, proliferation, senescence and apoptosis (Bardaweel et al., 2018; Chiste et al., 2015; Rauf et al., 2023). However, excessive formation of free radicals and/or diminished antioxidant defense can lead to cytotoxicity, by provoking oxidative damage to biomolecules - lipids, proteins and nucleic acids (Black et al., 2015). Several studies have suggested that ketamine enhances the activity of antioxidant enzymes, such as superoxide dismutase (SOD) and glutathione peroxidase (GPx) (Liang et al., 2018). Conversely, it has been proposed recently that excessive formation of ROS and mitochondrial damage may be related to ketamine-induced neurotoxicity, which has been associated with its abuse (De Carvalho et al., 2019). Combined, literature data are ambiguous and the effect of ketamine on ROS metabolism remains elusive.

The aim of the present study was to explore potential behavioral and antioxidant activity of a single subanesthetic dose of ketamine, by evaluating the results of the forced swim test (FST) and plasma oxidative stress biomarkers, in rats chronically exposed to ACTH injections. Furthermore, we sought to investigate potential protective role of ketamine against oxidative DNA damage in stress-related conditions, by assessing ex vivo DNA damage in rat's peripheral blood lymphocytes (PBLs).