Vitamin C (VitC), or ascorbic acid, is a water-soluble vitamin that acts as an antioxidant and enzyme cofactor. Some mammals, including humans and guinea pigs are incapable to synthesize VitC and require dietary intake of VitC. VitC is maintained at high concentrations in the brain and is an essential micronutrient for brain function [1]. VitC deficiency leads to neuropsychiatric scurvy in humans, which is characterized by depression and cognitive impairment [2]. On the other hand, oral administration of VitC induces an antidepressant-like effect during chronic stress in rodents [3]. In acutely hospitalized patients, short-term therapy with VitC has been shown to improve mood and reduce psychological distress [4,5]. A combination of VitC and fluoxetine, an antidepressant drug, improves pediatric major depressive disorder symptoms in humans [6]. Although the molecular connection between VitC and depression remains incompletely understood, VitC may mitigate the risk of depression by preserving the biosynthesis of brain monoamines, such as dopamine and norepinephrine [7]. For example, VitC has been shown to act as a cofactor for the enzyme dopamine β-hydroxylase, which converts dopamine to norepinephrine [8].

The brain is susceptible to oxidative imbalance due to its high energy demand and oxygen consumption. VitC is a powerful antioxidant that scavenges free radicals and reactive oxygen species [9]. VitC deficiency increases oxidative stress indicators such as superoxide generation and lipid peroxidation in the brain [10], [11], [12], [13]. Because oxidative damage results in the dysfunction of the biomolecules, it has been postulated that VitC protects normal brain function by preventing the accumulation of oxidative stress. However, during the early stages of VitC deficiency, the brain retains higher VitC concentrations than other organs, probably because the brain has strong homeostatic mechanisms to maintain VitC levels [13,14]. In fact, when guinea pigs are maintained on a low VitC diet for 6 months, brain VitC levels decrease by half, yet there is no evidence of elevated oxidative stress [15]. In senescence marker protein-30 knockout mice, which cannot synthesize VitC, increased oxidative stress becomes evident only after severe deficiency when the brain is nearly depleted of VitC [12]. Therefore, it remains unclear how mild VitC deficiency, more relevant to human health and disease, affects brain function.

VitC deficiency potentially impacts peripheral organs, which may subsequently alter brain function through humoral mediators, which are substances circulating in the bloodstream, including hormones and nutrients. For example, previous studies have shown that VitC deficiency stimulates the secretion of glucocorticoids from the adrenal gland [16], [17], [18]. Glucocorticoids (cortisol in humans and corticosterone in rodents) are the major steroid hormones that regulate metabolic, immune, and behavioral processes. The hypothalamic–pituitary–adrenal (HPA) axis is an endocrine system crucial for responding to physical and psychological stressors through the release of glucocorticoids [19]. In response to stress, the brain activates neurons within the hypothalamus, prompting the release of corticotropin-releasing hormone (CRH). CRH stimulates the pituitary gland, inducing the production of pro-opiomelanocortin (POMC), a precursor of adrenocorticotropic hormone (ACTH). ACTH then targets the adrenal cortex, resulting in the production of glucocorticoids. Glucocorticoids bind to the glucocorticoid receptor expressed in various tissues. Upon ligands binding, the glucocorticoid receptor in the cytoplasm translocates to the nucleus, where it activates or represses the transcription of target genes [20]. The brain is one of the main targets of their effects, and abnormal secretion of glucocorticoids has been proposed to be deleterious and linked to depression and anxiety [21], [22], [23]. Nevertheless, the potential involvement of glucocorticoids in the aberrant brain function resulting from VitC deficiency has yet to be investigated.

The brain contains various cell types, including neurons and non-neuronal cells. In neurons, VitC plays a crucial role in modulating neuronal transmission and cellular metabolism [14,24]. In contrast to the well-studied function of VitC in neurons, the influence of VitC deficiency on non-neuronal cells is less clear. In this study, we sought to examine how VitC deficiency impacts brain function using Osteogenic Disorder Shionogi (ODS) rats, which like humans, are unable to synthesize VitC because of a mutation in the GULO gene [25,26]. Transcriptomic analysis of the brain revealed that a short-term (two weeks) VitC deficiency changed the expression of genes, mainly in non-neuronal cells, including microglia. Bioinformatic analysis indicated that increased serum glucocorticoid levels contributed to the transcriptomic changes in the brain during VitC deficiency. Furthermore, VitC deficiency induced microglial activation and decreased adult neurogenesis in the hippocampus, which may lead to impaired brain function.