Depression is one of the leading causes of disability worldwide (Friedrich, 2017; Rotarou and Sakellariou, 2018). The pathological mechanisms of depression are not fully understood. Further, current antidepressants produce adverse effects and approximately 40 % of patients respond poorly to them (Cascade et al., 2009; Karrouri et al., 2021). There is a strong need to better understand the disease mechanism and develop more effective treatments for depression (Cui et al., 2024). Many studies suggest that chronic stress is a major risk factor for depression (Juruena, 2014; LeMoult et al., 2020). Stress refers to the organism's response to environmental challenges and is mediated mainly by the hypothalamic-pituitary-adrenal (HPA) axis. During stress, glucocorticoids [cortisol in humans and corticosterone (CORT) in rodents] are released by the HPA axis to subsequently target various organs including the brain (Lucassen et al., 2014; Nicolaides et al., 2014). While acute stress is beneficial, chronic stress has deleterious effects on the brain and contributes significantly to the aetiologies of depression (Cui et al., 2024)

Recently, chronic stress has been found to impair the neuronal structure, connectivity, and function in the brain, which may contribute to the pathophysiological development of depression (Cui et al., 2024; Pittenger and Duman, 2008; Rădulescu et al., 2021). For example, studies have shown that restraint stress for 21 days reduced dendritic length and branching of pyramidal neurons in rat hippocampal CA3 region and medial prefrontal cortex (Radley et al., 2006). Chronic treatment with CORT for 21 days has also been found to decrease axonal and dendritic outgrowth in mouse hippocampal pyramidal neurons (Magarinos et al., 1998; Zhang et al., 2021). In addition, exposure of primary rat hippocampal neurons to CORT or glucocorticoid receptor agonist dexamethasone for 4 and 7 days was found to decrease the length of primary, secondary, and tertiary processes (Levone et al., 2021). Altogether this evidence suggests that chronic stress and chronic CORT treatment impairs dendritic outgrowth and neurotransmission, which might alter brain functionality.

Thioredoxin (Trx) antioxidant system mainly includes Trx, Trx reductase (TrxR) and Trx-interacting protein (Txnip) proteins. These proteins are widely expressed across all body tissue, and play an important role in regulating protein oxidation (Choi and Park, 2023; Yang et al., 2024). Trx is an oxidoreductase that contains the active conserved site cysteine 32- glycine- proline- cysteine 35. Trx can reverse cysteine protein oxidation, which helps maintain the cellular redox balance and protects the cell against oxidative stress (Matsuzawa, 2017; Yang et al., 2024). Trx is maintained in a reduced state by TrxR (Yang et al., 2024). Txnip is an endogenous inhibitor for Trx. Txnip can bind to Trx through Txnip cysteine residue Cys-247 and Trx cysteine residue Cys-32. Binding Txnip to Trx can inhibit Trx reducing activity and cause protein oxidative damage (E.-H. Choi and Park, 2023; Nasoohi et al., 2018; Qayyum et al., 2021).

Previously, our laboratory reported that although chronic CORT treatment had no effect on Trx and TrxR protein levels, this treatment increased Txnip protein levels in primary cultured mouse cerebrocortical neurons (Bharti et al., 2018). This finding suggests that chronic CORT treatment can upregulate Txnip. Cyclic AMP response element-binding protein (CREB) is a transcription factor that plays a significant role in the regulation of neurotrophic factors (Esvald et al., 2020). Recently, our laboratory found that PX12, a Trx pharmacological inhibitor, decreased dendritic outgrowth and CREB phosphorylation, while the Trx mimetic peptide CB3 reversed H2O2-reduced CREB phosphorylation in primary cultured mouse cerebrocortical neurons and human neuroblastoma SH-SY5Y cells (Alejandra Llanes-Cuesta et al., 2024). These findings suggest that Trx regulates dendritic outgrowth of primary mouse cerebrocortical neurons by maintaining the cellular redox balance and thus facilitating CREB phosphorylation. Previously, our laboratory found that mice exposed to chronic unpredictable stress not only induced depressive-like behaviors but also increased Txnip protein levels and protein oxidation in the hippocampus and frontal cortex (Zhou et al., 2019). This evidence suggests that chronic stress may interrupt Trx antioxidant system, contributing to the development of depression and other psychiatric disorders. In the present study, to understand the role of the Trx system in chronic stress-induced depression, we first determined whether chronic CORT treatment regulates neurite outgrowth process in primary cultured mouse cerebrocortical neurons. Second, we determined whether knocking down Txnip in the medial frontal cortex reversed CORT-induced depressive-like behaviors, and cognitive dysfunction in mice.