Major depressive disorder (MDD) and anxiety disorders, which will be referred here as depression and anxiety, are devastating and highly prevalent clinical entities that constitute one of the leading causes of disability worldwide ^[1][2][1,2]. These disorders often occur concomitantly and their symptoms frequently overlap in individuals, and patients with these two comorbidities often present with a higher severity and duration of symptoms [3]. Therefore, the diagnosis and treatment of these disorders remain a challenge in the clinical setting ^[1][4][1,4]. Despite being different disorders, the etiology of depression and anxiety involves similar factors, such as genetic predispositions, environmental aspects, and several biological mechanisms ^[1][2][5][1,2,5]. Among the main biological mechanisms implicated in the pathophysiology of these disorders, compelling evidence has pointed to neuroinflammation as a key factor in the onset and progression of these disorders [6]. Notably, other biological mechanisms that have been implicated in depression and anxiety, such as gut dysbiosis, impaired neurogenesis, and monoaminergic dysfunction, may be triggered by a neuroinflammatory process, opening new perspectives for studying molecular targets and neuroprotective agents against these mood disturbances ^[4][7][8][9][4,7,8,9]. In this regard, in recent years, vitamin D has gained prominence due to its antioxidant, anti-inflammatory, pro-neurogenic, and neuromodulatory properties that appear to be fundamental to its antidepressant and anxiolytic effects ^{[10][11][12][13]}[10,11,12,13].

2. Neuroinflammation as a Key Pathophysiological Mechanism Related to Mood Disorders

Neuroinflammation is a complex process that comprises a defense mechanism in the central nervous system (CNS), protecting and restoring the structure and function of the brain against infection and injury, through the modulation of neurogenesis, axonal regeneration, and remyelination of neural cells [14]. However, chronic and exacerbated inflammatory responses can produce harmful effects in the brain. These inflammatory processes may involve inflammation-related signaling molecules, microbiota, as well as immune and brain cells ^[7][15][7,15]. Microglia, nervous-system-specific immune cells, are of special interest to neuroinflammatory responses. Under normal conditions, microglia assume a phenotype defined by a ramified morphology and highly motile processes for constant monitoring of the brain parenchyma (M2 phenotype). After an insult, promoted by pathogen-associated molecular patterns and/or damage-associated molecular patterns that interact with pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), microglia retract their processes and adopt an ameboid form (M1 phenotype) ^{[16][17][18][19]}[16,17,18,19]. In addition to morphological changes, the binding of these molecular patterns to these receptors induces the priming of NLRP3 [nucleotide-binding oligomerization domain (NOD)-, leucine-rich repeats (LRR)- and pyrin domain-containing protein 3] and pro-interleukin (IL)-1β expression via nuclear factor kappa B (NF-kB) and myeloid differentiation primary response 88 (Myd88) pathways ^[17][20][21][17,20,21]. Noteworthily, the expression of NLRP3 may be inhibited under certain conditions, particularly upon activation of nuclear factor erythroid 2-related factor 2 (Nrf2), the main regulator of the antioxidant response ^[22][23][22,23]. Subsequently, different stimuli capable of promoting mitochondrial dysfunction, calcium and potassium ion flux, reactive oxygen species (ROS) production, and lysosomal damage activate the NLRP3 inflammasome, promoting the autoproteolytic activation of pro-caspase-1 [20]. Caspase-1 can subsequently cleave pro-interleukin-1β and pro-interleukin-18 into their active forms, interleukin-1β (IL-1β) and interleukin-18 (IL-18), respectively ^[17][24][17,24]. In addition, activation of NLRP3 can lead to gasdermin D-mediated formation of membrane pores and subsequently pyroptosis [17]. Mediators released by activated microglia induce astrocyte polarization [25]. This polarization contributes to the impairment of signaling pathways that play a crucial role in neuronal survival and synaptic plasticity, such as the brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) signaling pathway [26]. Chronic activation of astrocytes also results in increased levels of chemokines, such as chemokine ligand 2, which interact with peripheral immune cell receptors to induce infiltration of macrophages and monocytes from the circulation into the CNS and resulting in increased blood–brain barrier permeability ^[27][28][27,28]. Interestingly, neuroinflammatory-process-derived pro-inflammatory cytokines, such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), cause an increase in the activity of the enzyme indoleamine 2,3-dioxygenase in astrocytes, microglia, and inflammatory cells. The activation of this enzyme increases the formation of quinolinic acid, an N-methyl-D-aspartate (NMDA) receptor agonist, through the kynurenine pathway, contributing to glutamatergic system disturbance and compromising the synthesis of serotonin by depleting tryptophan ^{[8][29][30][31]}[8,29,30,31]. Within this scenario, it is worth noting that more than 90% of the body’s serotonin is produced in the gut, particularly by enterochromaffin cells, and alterations in the intestinal microbiota triggered by inflammatory processes have been shown to directly compromise the synthesis of this monoamine ^[32][33][32,33]. Indeed, it is important to note that changes in gut microbiota have been reported to impair the efficacy of antidepressants, such as fluoxetine [34].

3. Preclinical Studies: Effects of Vitamin D in Models of Depression and Anxiety

In recent years, several preclinical studies have been conducted to investigate the possible antidepressant and anxiolytic effects of vitamin D. Of note, chronic administration of cholecalciferol [5 mg/kg for 14 days; subcutaneous (s.c.) administration] elicited an antidepressant-like effect in the forced swim test in ovariectomized Wistar rats ^[35][92]. In addition, cholecalciferol supplementation (5 mg/kg for 14 days; s.c.) was capable of attenuating anxiety-like behaviors in the elevated plus-maze and the light–dark box tests in ovariectomized Wistar rats ^[10][36][10,93]. Based on these findings, further studies have been conducted to better understand the mechanisms underlying the anxiolytic and antidepressant effects of vitamin D in animal models ^[37][38][94,98]. Camargo et al. (2018) reported that cholecalciferol [2.5 µg/kg, orally by mouth (p.o.)], administered once a day in the last 7 days of chronic corticosterone administration (20 mg/kg, p.o., for 21 days), exerted an antidepressant-like effect in male mice subjected to the splash test and tail suspension test. Additionally, in this study, cholecalciferol treatment attenuated the increase in protein carbonyl and nitrite levels induced by corticosterone in the brain, suggesting that vitamin D₃ has an antidepressant-like effect by, in part, modulating oxidative stress ^[37][94]. The administration of cholecalciferol (100 IU/kg, p.o.) for 7 days also abolished the depressive-like behavior in the tail suspension test induced by chronic corticosterone administration in female mice ^[39][95]. In this study, Aa significant decrease in ROS production in the hippocampus was observed after treatment with cholecalciferol, both in control and corticosterone-exposed mice, reinforcing the notion that the antidepressant-like effect of this vitamin may involve the modulation of oxidative stress ^[39][95]. More recently, a study by Neis et al. (2022) showed that repeated administration of cholecalciferol for 7 days (2.5 μg/kg, p.o.) abolished chronic unpredictable stress-induced depressive-like behavior in the tail suspension test, as well as a reduction in serotonin levels in the prefrontal cortex of female mice. Moreover, reinforcing the involvement of the serotonergic system in the antidepressant-like effect of cholecalciferol, in this study, the administration of the serotonin synthesis inhibitor p-chlorophenylalanine methyl ester was effective in abolishing the reduction in immobility time in the tail suspension test elicited by cholecalciferol ^[40][96]. The repeated administration of a low dose of cholecalciferol (2.5 μg/kg, p.o.) also caused an antidepressant-like effect and was effective in reducing the immunocontent of proteins that form the NLRP3 inflammasome, such as ASC [apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD)], caspase-1, and thioredoxin-interacting protein (TXNIP) in the hippocampus of male mice ^[41][57]. Calcitriol treatment (100 ng/kg, p.o.) for 10 weeks in ovariectomized female Sprague–Dawley rats was also effective in producing neuroprotective effects by regulating the adenosine monophosphate (AMP)-activated protein kinase (AMPK)/NF-kB signaling pathway. Moreover, calcitriol treatment reduced the pro-inflammatory cytokines IL-1β, IL-6, and TNF-α, as well as iNOS and cyclooxygenase-2 (COX-2) levels in the hippocampus ^[42][97]. This suggests that the modulation of the NLRP3 inflammasome-driven pathway may underlie, at least in part, the antidepressant-like effect of this vitamin. More recently, Bakhtiari-Dovvombaygi et al. (2021) also reported that the anti-inflammatory and antioxidant effects displayed by pretreatment with vitamin D₃ (10,000 IU/kg for 28 days) in male rats underlie the ability of this vitamin to abrogate anxiety- and depressive-like behaviors induced by chronic unpredictable mild stress (CUMS) in the elevated plus-maze and forced swimming test. Indeed, these protective effects of vitamin D were accompanied by a decrease in cortical malondialdehyde and IL-6 levels, as well as an increase in total thiol levels and enhanced SOD and catalase activity ^[38][98]. Interestingly, another study observed that following 4 weeks of CUMS, the occurrence of depressive-like behaviors was associated with an increase in 1,25(OH)₂D and VDR expression in the hippocampus of rats, suggesting a compensatory mechanism, by which vitamin D may protect against the development of depressive-like behaviors ^[43][99]. In addition to attenuating depressive- and anxiety-like behaviors through its anti-inflammatory and antioxidant properties, the modulation of neurotrophic factors has also been shown to contribute to the protective properties of vitamin D ^[44][76]. Xu et al. used male C57BL/6 mice to show that calcitriol (25 μg/kg/day for 4 weeks; i.c.v.) is effective in acting as an antidepressant in a post-stroke depression model by up-regulating VDR and BDNF expression ^[45][75]. In an ovariectomized Wistar rat model of depression induced by CUMS, vitamin D₃ (5 mg/kg for 4 weeks; s.c.) treatment was able to reverse depression-like behaviors in the sucrose preference test and the forced swimming test by increasing BDNF and NT-3/NT-4 levels in the hippocampus ^[44][76]. Although these studies showed that vitamin D supplementation results in an increase in pro-neurogenic neurotrophins, such as BDNF, Groves et al. observed that vitamin D deficiency in BALB/c mice was associated with depressive-like behaviors without compromising hippocampal neurogenesis ^[46][100]. Therefore, further studies are needed to elucidate whether neurogenesis is critical for the anxiolytic and antidepressant effects of this vitamin.

4. Clinical Studies: Effects of Vitamin D in Depression and Anxiety

Several studies have reported that vitamin D supplementation improves symptoms of depression and anxiety associated with various medical conditions, including type II diabetes, Crohn’s disease, ulcerative colitis, and obesity ^{[47][48][49][50][51]}[101,102,103,104,105]. However, the potential therapeutic effects of vitamin D in individuals primarily diagnosed with depression or anxiety remain controversial. For example, vitamin D supplementation (1600 IU for 6 months) was shown to significantly improve anxiety symptoms, but not depressive symptoms, in patients with vitamin D deficiency ^[52][106]. Likewise, supplementation with 2800 IU of vitamin D in patients with depression did not promote a significant reduction in Hamilton D-17 scores ^[53][107]. On the other hand, supplementation with 50,000 IU of vitamin D for 2 weeks was able to improve depression severity, as assessed with the Beck Depression Inventory-II (BDI-II), although no changes in serotonin levels were detected ^[54][108]. However, in another study, cholecalciferol treatment (50,000 IU for 3 months) significantly increased serum serotonin levels, while decreasing BDI scores in women with moderate, severe, and extreme depression. Interestingly, among men, an improvement in the severity of depressive symptoms with vitamin D supplementation was only observed in those diagnosed with severe depression ^[55][83]. Beneficial effects of vitamin D (50,000 IU for 8 weeks) supplementation have also been observed in older adults (over 60 years of age) with depression ^[56][109]. However, lower doses of vitamin D (400 IU daily for 2 years) were not able to improve depressive symptoms ^[57][110]. Finally, a single dose of vitamin D (300,000 IU) was reported as an effective and safe intervention in MDD with concurrent vitamin D deficiency ^[58][59][111,112]. In patients with depression, the daily administration of 1500 IU vitamin D₃ plus 20 mg fluoxetine for 8 weeks was superior to fluoxetine alone ^[60][113]. Another study reported that vitamin D supplementation (50,000 IU once/week for 3 months) in combination with standard of care improved the severity of anxiety in individuals diagnosed with Generalized Anxiety Disorder by increasing serotonin concentrations and decreasing the levels of the inflammatory biomarker neopterin ^[61][114]. Overall, although there is compelling clinical evidence pointing to the benefits of vitamin D for the management of depression and anxiety, it is important to note that divergent results have also been obtained ^[62][46]. Multiple factors may contribute to these discrepant results, including differences in the doses of vitamin D used, treatment time, serum 25-hydroxyvitamin D levels at baseline, nutritional condition at the onset of the treatment, age and sex of the individuals, as well as the presence of comorbidities that may influence the efficacy of vitamin D supplementation. Additionally, the heterogeneity observed in clinical studies may also be associated with genetic polymorphisms that may affect vitamin D efficacy ^[63][64][65][115,116,117].