The Synaptic and Circuit Functions of Vitamin D in Neurodevelopment Disorders

Abstract

Vitamin D deficiency/insufficiency is a public health issue around the world. According to epidemiological studies, low vitamin D levels have been associated with an increased risk of some neurodevelopmental disorders, including autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD). Animal models reveal that vitamin D has a variety of impacts on the synapses and circuits in the brain. A lack of vitamin D affects the expression of synaptic proteins, as well as the synthesis and metabolism of various neurotransmitters. Depending on where vitamin D receptors (VDRs) are expressed, vitamin D may also regulate certain neuronal circuits through the endocannabinoid signaling, mTOR pathway and oxytocin signaling. While inconsistently, some data suggest that vitamin D supplementation may be able to reduce the core symptoms of ASD and ADHD. This review emphasizes vitamin D’s role in the synaptic and circuit mechanisms of neurodevelopmental disorders including ASD and ADHD. Future application of vitamin D in these disorders will depend on both basic research and clinical studies, in order to make the transition from the bench to the bedside.

Introduction

One of the liposoluble vitamins, vitamin D, exists in two distinct forms: vitamin D₂ and vitamin D₃. While vitamin D₂ is obtained from diet, vitamin D₃ is primarily produced from 7-dehydrocholesterol (7-DHC) in the skin by ultraviolet B (UVB) radiation.¹ Vitamin D₂ or vitamin D₃ is first hydroxylated to 25-hydroxyvitamin D₂ [25(OH)D₂] or 25-hydroxyvitamin D₃ [25(OH)D₃] by sterol 27-hydroxylase (CYP27A1) in the liver. 25-hydroxyvitamin D, or 25(OH)D, is the collective name for the hydroxylated vitamin D. It is converted to 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] in the kidney by the second hydroxylation of 1, α-hydroxylase (CYP27B1).^2,3 By binding to vitamin D receptors (VDRs), 1,25(OH)₂D₃ maintains the calcium and phosphorus homeostasis, as well as controls the bone metabolism, which has been discussed in great detail elsewhere.^3–6 1,25(OH)₂D₃ is metabolized by 24-hydroxylase (CYP24A1) into 24,25-dihydroxyvitamin D₃ [24,25(OH)₂D₃] in the kidney, where it is excreted from the body.³ Figure 1 summarizes the metabolic process and roles of vitamin D in maintaining the balance of calcium and phosphorus. Remarkably, recent studies have demonstrated that 1,25(OH)₂D₃ can influence a wide range of biological functions, including cell proliferation and differentiation, immune responses, and brain development.^7,8 As a result, 1,25(OH)₂D₃ has been proposed as a neurosteroid hormone.^9–12 In addition, epidemiological data and animal experiments have revealed a link between the lack of vitamin D and the occurrence of certain neurodevelopmental disorders.^6,13 However, little is known about the molecular mechanisms that underlie vitamin D’s influence on these diseases. The purpose of this review is to provide a comprehensive overview about the synaptic and circuit functions of vitamin D in the neurodevelopmental disorders like autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD).

Figure 1.

The pathways for the synthesis of vitamin D and its classical functions. VitD₂ is obtained from diet, and VitD₃ is primarily produced from 7-DHC in the skin by UVB radiation. VitD₂ and VitD₃ are hydroxylated to 25(OH)D by CYP27A1 in the liver. 25(OH)D is converted to 1,25(OH)₂D3, an active form, by CYP27B1 in the kidney. 1,25(OH)₂D₃ modulates the absorption of calcium and phosphorus, and alters bone formation and resorption by binding to VDRs. 1,25(OH)₂D₃ promotes the absorption of calcium and phosphorus in the intestine, and the reabsorption of calcium in the renal tubules. It can also directly regulate bone metabolism. Through these effects on target organs, vitamin D helps to maintain the homeostasis of calcium and phosphorus in the blood circulation.

Abbreviations: 7-DHC, 7-dehydrocholesterol; UVB, ultraviolet B; VitD₂, vitamin D₂; VitD₃, vitamin D₃; 25(OH)D, 25-hydroxyvitamin D; 1,25(OH)₂D₃, 1,25-dihydroxyvitaminD₃; CYP27A1, sterol 27-hydroxylase; CYP27B1, 1, α-hydroxylase.

The Effects of Vitamin D on Synaptic Functions

The presence of vitamin D and its nuclear receptors (VDRs), as well as its metabolism enzymes (CYP27A1, CYP27B1 and CYP24A1) in the brain has been systematically reviewed elsewhere.^14–16 All of the VDRs, CYP27B1 and CYP24A1 have been identified in neurons and glia cells throughout life, raising the notion that vitamin D might be involved in the fundamental functions of mammalian brains.^15,17 These functions, such as learning, memory, cognition, and behavioral processes, all rely on the connection of neurons.¹⁸ The information transferred in the neural network is largely performed through synaptic transmission, which includes both electrical and chemical synapses.¹⁹ According to the literature, vitamin D participates in multiple processes that regulate synaptic transmission, particularly the chemical synapses.^6,13,20 First, the absence of vitamin D increased cholesterol levels in the presynaptic membrane and vesicles, which altered the synaptic membrane’s fusion properties and, as a result, the efficiency of transmitter release.²¹ On the other hand, vitamin D supplementation could partially restore the capability of vesicle fusion.²² In addition, microarray sequencing revealed that vitamin D affected the transcription of proteins involved in the neurotransmitter release, including proteins in synaptic vesicles such as solute carrier family 17 member 6 (SLC17A6),^23,24 proteins involved in exocytosis such as synaptojanin1 (synj1), complexin2, synaptotagmin1 (syt1), synaptotagmin2 (syt2), synaptotagmin10 (syt10), and synaptic vesicle glycoprotein 2c (SV2C),^23,24 as well as proteins in the active zones such as double C2 gamma (DOC2G), synapsin2, and synapsin3.^23–25 While the majority of the mRNA alterations in synaptic proteins revealed by sequencing were not validated, increased expression of syt2, synj1, and complexin2 were verified by polymerase chain reaction (PCR) or immunohistochemical (IHC) assays.^23,26 Additionally, vitamin D could modulate synchronized transmitter release by either directly increasing the activity of L-type voltage-dependent calcium channels (LVDCCs)²⁷ or by promoting the expression of calcium sensors such as syt1and syt2 in the brain.²⁸ Therefore, vitamin D could potentially exert both immediate and long-term effects on synapses.

In addition to the expression of presynaptic release machinery, vitamin D could modulate the expression of transporters, receptors, as well as enzymes for the synthesis and metabolism of neurotransmitters like glutamate,^22,29,30 GABA,^29,31–33 glycine,³² dopamine,^33–37 serotonin,^33,37,38 and catecholamines.³⁹

1. Transporters: Vitamin D deficiency reduced the expression of excitatory amino acid transporters (EAATs) and GABA transporters 3 (GAT3), which in turn caused the dysfunction of glutamate and GABA reuptake systems.²⁹ In addition, supplementing with vitamin D led to an increased expression of dopamine transporter gene-solute carrier family 6 member 3 (SLC6A3).³⁶

2. Receptors: Vitamin D deficiency reduced the mRNA expression of GABA receptors.³¹ On the other hand, vitamin D supplementation increased the expression of dopamine receptor D2 (DRD2).^33,35,36

3. The synthesis and metabolism enzymes of neurotransmitters: Vitamin D deficiency decreased the expression of glutamate synthetase 1 and GABA transmitter synthetase, which consisted of glutamate decarboxylase 65 and 67 (GAD65 and GAD67).^30,32,33 Furthermore, vitamin D deficiency reduced the expression of catechol-O-methyltransferase (COMT), leading to decreased dopamine metabolism.³⁴ Vitamin D supplementation upregulated the expression of dopamine transmitter synthase-tyrosine hydroxylase (TH).^33,36 In addition, the first and rate-limiting enzyme in the biosynthesis of serotonin, tryptophan hydroxylase 2 (TPH2), could also be enhanced by vitamin D.^33,37

Taken together, these findings suggested that vitamin D affected the process of synaptic transmission in various ways, likely by combining genomic and nongenomic mechanisms. Notably, vitamin D responsive elements (VDREs) could be identified in the promoter regions of certain genes, including syt1, syt2 and TPH2.^23,37,40,41 Many vitamin D responsive genes, however, lack the VDRE sequence.^40,41 Therefore, more VDRE sequences not now documented may exist, or some sequences may not directly respond to the VDR signaling.⁴² In addition, vitamin D could induce growth factors like nerve growth factor (NGF), glial cell derived neurotrophic factor (GDNF) and growth associated protein 43 (GAP43), which could enhance the growth and development of synapses and neurons.^25,31,43 The effects of vitamin D on synaptic functions are summarized in Table 1. These findings suggested potential mechanisms by which inadequate vitamin D negatively impacted brain functions.

Table 1.

The Effects of Vitamin D on Synaptic Functions

Synaptic Functions	Effects on Synaptic Components		Experimental Models	Experimental Findings	References
Synaptic transmission	Presynaptic release machinery	Membrane lipids	Animals with vitamin D deficiency	Vitamin D deficiency increased cholesterol levels, which affected the fusion properties of the lipid bilayers and resulted in reduced vesicle release.	[21,22]
		Synaptic proteins	Animals with vitamin D deficiency or supplementation	Vitamin D deficiency decreased the expression of synaptic proteins such as syt1. Vitamin D supplementation promoted the expression of synaptic proteins such as synj1, syt1 and SLC17A6, and enhanced synaptic plasticity.	[23–25]
	Transmitters	Glutamate	Animals with vitamin D deficiency	Vitamin D deficiency reduced the expression of glutamate transporters and glutamate synthetase.	[22,29,30]
		GABA	Animals with vitamin D deficiency or supplementation	Vitamin D deficiency reduced the expression of transporters (GAT3) and enzymes for GABA (GAD65, GAD67). Vitamin D supplementation promoted the expression of GAD67.	[29,31–33]
		Glycine	Animals with vitamin D deficiency	Glycine level was significantly higher in vitamin D deficient animals.	[32]
		Dopamine	Animals with vitamin D deficiency or supplementation.	Vitamin D deficiency reduced the expression of COMT and Vitamin D supplementation upregulated the expression of TH, both of that were the key enzymes in dopamine synthesis pathway. Vitamin D supplementation increased the expression of dopamine transporters (SLC6A3) and dopamine receptors (DRD2).	[33–37]
		Serotonin	Supplementation of vitamin D to animals and cultured serotonergic neurons	Vitamin D increased the expression of TPH2, a key enzyme in serotonin synthesis, and reduced the expression of MAO-A, an enzyme for serotonin degradation.	[33,37,38]
		Catecholamine	Animals with vitamin D deficiency	Vitamin D deficiency increased the NE concentration in cortex.	[39]
Synapse development	Neurotrophic factors	NGF, GDNF, GAP43	Primary cultured hippocampal or cortical neurons	Vitamin D treatment increased the expression of neurotrophic factors	[25,31,43]

Abbreviations: syt1, synaptotagmin1; synj1, synaptojanin1; SLC17A6, solute carrier family 17 member 6; GABA, gamma-aminobutyric acid; GAT3, GABA transporters 3; GAD65, glutamate decarboxylase 65; GAD67, glutamate decarboxylase 67; COMT, catechol-O-methyltransferase; TH, tyrosine hydroxylase; SLC6A3, solute carrier family 6 member 3; DRD2, dopamine receptor D2; TPH2, tryptophan hydroxylase 2; MAO-A, monoamine oxidase A; NE, norepinephrine; NGF, nerve growth factor; GDNF, glial cell derived neurotrophic factor; GAP43, growth associated protein 43.

The Effects of Vitamin D on the Cognitive Function and Behaviors

There is growing evidence that vitamin D influences on cognition and behaviors in a variety of manners.^4,44,45 The cortex and hippocampus, two crucially important brain areas for cognition, learning and memory, both have VDRs.^16,19 According to epidemiological studies, low vitamin D levels have been linked to cognitive impairment.^20,46–49 For instance, inadequate vitamin D levels (25(OH)D<30ng/mL) were associated with poorer cognitive performance in individuals older than 60.^46–48 Furthermore, daily 800IU vitamin D oral administration for a period of 12 months could improve the cognitive function and reduce amyloid beta (Aβ)-related biomarkers in patients with Alzheimer’s disease.⁴⁹ Nevertheless, some randomized clinical trials (RCTs) found no correlation between vitamin D supplementation and cognitive improvement.^50–52 The inconsistent results might be due to diverse research designs, varied intervention doses and different analysis of confounding factors. Large multicenter RCTs will be necessary in the future to provide more reliable clinical evidence.

Studies using animal models may also provide crucial biological explanations for how vitamin D influences cognition and behaviors. Unfortunately, systemic ablation of VDR or CYP27B1 caused severe rickets and osteomalacia in mice.^53–55 Therefore, the mice’s motor dysfunction made it impossible to draw proper conclusions from standard cognitive and social behavioral assessments. Thus, studies using developmental vitamin D (DVD) deficient animal models, adult vitamin D (AVD) deficient animal models and vitamin D supplementation animal models will be used in the following part to discuss how vitamin D affects cognition and behaviors.

In the DVD deficient model, female rodents (rats or mice) were fed a vitamin D deficient diet for 3–4 weeks before and during mating, as well as throughout pregnancy.^56–58 At the same time, they were kept without UV light to prevent vitamin D synthesis from the skin.^59–61 Therefore, the offsprings were deficient in vitamin D since the fertilized egg-stage until birth, and in some cases, until the time of weaning.^59–61 In AVD deficient animal models, rodents around 4 months old were fed with a vitamin D-deficient diet for 10 weeks.^32,62 At the same time, these mice were housed in an environment without UV light.^32,62 In this way, the animals were deficient in vitamin D due to restricted dietary intake and limited vitamin D₃ production from the skin. In vitamin D supplementation animal models, mice were fed with the vitamin D₃-enriched diet for several months, and the calcium and phosphorus levels in sera were carefully monitored to be stable.^23,63,64 Excellent reviews have elaborated these vitamin D-related animal models.^6,20,65 Hereby, we focused on the effects of vitamin D on cognition and behaviors from studies using these animal models and summarized the key points in Table 2.

Table 2.

The Effects of Vitamin D on Cognition and Behaviors

DVD Deficient Animal Models	AVD Deficient Animal Models	Animals Supplemented with Vitamin D
Behaviors	Behaviors	Behaviors
Reduced grooming of pups.58	Increased sensitivity to environmental changes and excessive activity.31	Improved cognitive functions and learning ability in aging rats.23,24,63,66
Altered frequency of ultrasonic vocalization.57,58	Cognitive impairment and spatial learning deficit.62,67	Increased sociability in NS-PTEN knockout mice.68
Excessive activity.69–71
Increased frequency of self-grooming.72
Increased impulsive behaviors.73,74
Reduced social behaviors.58
Molecular mechanisms	Neuronal mechanisms	Neuronal mechanisms
Decreased NGF and GDNF.55	Decreased glutamate and glutamine.31	Increased late hippocampal neurogenesis.63,66
	Increased GABA and glycine.31	Decreased levels of pS6 and pAKT (downstream targets of mTOR), which led to the reduction of abnormal dendritic spines in NS-PTEN knockout mice.68
Morphology
Thinner cortical layers and larger lateral ventricles.55,58
Reduced size of hippocampi and smaller lateral ventricles.75

Abbreviations: DVD, developmental vitamin D deficient animal models; AVD, adult vitamin D deficient animal models; NGF, nerve growth factor; GDNF, glial cell derived neurotrophic factor; pAKT, phospho-AKT; Ps6, phospho-S6; mTOR, mammalian target of rapamycin; NS-PTEN knockout mice, neural subset-specific phosphatase and tensin homolog knockout mice.

The Potential Effects of Vitamin D in the Etiology Behind ASD and ADHD

The optimal range of serum 25(OH)D concentrations is between 30 and 90ng/mL. Vitamin D deficiency is defined as a serum 25(OH)D level below 10ng/mL, and vitamin D insufficiency as a level between 10 and 30ng/mL.⁷⁶ Currently, vitamin D deficiency or insufficiency is a major global health issue.^77–88 Obese people, people of color, and those individuals who live in high altitudes are more likely to have vitamin D insufficiency or deficiency.^{78–80,84,89} Additionally, children and pregnant women are particularly vulnerable to have vitamin D deficiency or insufficiency.^77,90 A multi-center cross-sectional study conducted in England indicated that up to 14% children under the age of seven were vitamin D deficient.⁹¹ Notably, a number of studies have suggested a strong correlation between low vitamin D levels during pregnancy and a higher likelihood of being diagnosed with neurodevelopmental disorders.^85–87 Here, we will provide an overview of recent findings on the functions of vitamin D in the physiological mechanisms in neurodevelopmental disorders, taking ASD and ADHD as two examples.

The Role of Vitamin D in ASD

ASD is a neurodevelopmental disorder characterized by social impairment, restricted interests and repetitive behaviors.⁹² ASD affects about 1% of people worldwide.^93,94 According to a recent meta-analysis, children and adolescents with ASD had considerably lower vitamin D concentrations than the controls.⁹⁵ Additionally, children with insufficient vitamin D levels (<30ng/mL) displayed more severe core symptoms.^96,97 Besides, some studies suggested that vitamin D supplementation could alleviate the core symptoms of ASD.^96,98 In a clinical trial conducted in 2016, vitamin D supplementation, a dosage of 150000IU/per month i.d. plus a dosage of 400IU/per day orally, was given to ASD children (mean age of 5.1 years old) for three months, and their symptoms were significantly alleviated.⁹⁸ However, Kerley et al reported that ASD children (N = 40, mean age of 7.1 years old) treated with 2000 IU of vitamin D per day for 20 weeks did not show any significant improvement when compared to the placebo group in a double-blind RCT.⁹⁹ The discrepancy of the therapeutic effects reported by these studies might be due to difference not only in sample sizes but also the ages of treatment, since the therapeutic effect of vitamin D could be related to the plasticity of the nervous system.^95,100 Therefore, vitamin D might be more effective for younger patients. Another reason for the diversity of outcomes could be that ASD is a heterogeneous population, and vitamin D might only have an impact on one fraction of the patients. The precise ASD subgroup sensitive to vitamin D remains to be identified. The majority of clinical studies in the literature are based on observations and are unable to address the causality link between the lack of vitamin D and ASD. The exact role that vitamin D plays in the pathogenesis of ASD is still unclear.¹⁰¹ Here, we summarized the potential synaptic and circuit mechanisms through which inadequate vitamin D contributed to the etiology of ASD.

Vitamin D Regulates the Synaptic Functions

One hypothesis about ASD pathophysiology is the disruption of synaptic functions.^102,103 According to autopsy findings, ASD patients’ brains had an abnormally high density of dendritic spines and irregularly shaped spines.^104–106 Mutations in ASD-risk genes like shank3, neuroligin3, neurexin1 and sapap3 have been associated with aberrant dendritic spine formation in animal models.^107,108 As was previously mentioned in this review, vitamin D modulated a variety of synaptic proteins, such as SLC17A6, synj1 and syt1.^23–25,30 Among them, SLC17A6 is a ASD-risk gene.¹⁰⁹ Studies showed that vitamin D regulated the expression of growth factors NGF and GDNF in vitro, which were essential for the formation and development of synapses.^8,9,43,110 In addition, high vitamin D dosages could promote the expression of synaptic proteins such as synj1 and syt1.²³ Remarkably, vitamin D was shown to rescue the ASD-like behaviors in animal models.^111,112 For example, mice that had phosphatase and tensin homolog (PTEN) selectively deleted from the granule cells of hippocampus displayed an osteoporosis phenotype as well as impairments similar to autism.^113,114 These mice became more sociable after receiving a vitamin D-enriched treatment (vitamin D₃ 20000IU/kg per day, orally) for 5 weeks.⁶⁸ Moreover, vitamin D administration was found to decrease the levels of phospho-AKT (pAKT) and phospho-S6 (pS6), both of which were the downstream molecules of mammalian target of rapamycin (mTOR).⁶⁸ The mTOR signaling is an important pathway for synaptic growth and pruning.^106,115,116 These results raise the question of whether vitamin D deficiency-related synaptic dysfunctions can contribute to the development of ASD. More studies are required to answer this question in the future.

Vitamin D Could Modulate the Excitation and Inhibition Balance

Another theory for the etiology of ASD from a neuroscience perspective is an excitation to inhibition (E/I) imbalance.¹¹⁷ ASD animal models demonstrated abnormalities in glutamatergic and GABAergic activities, which lead to an E/I imbalance in the brain.^{111,118–120} Recent work illustrated how vitamin D could potentially modulate the ratio of excitation to inhibition by regulating the synthesis of neurotransmitters.^22,29,45 Vitamin D-deficient animals had lower levels of dopamine and glutamate, while having higher amounts of glycine and GABA.^29,32,34 Mechanistically, a lack of vitamin D prevented glutamate and GABA transporters from being expressed, which would have led to a possible E/I imbalance in the brain.²⁹ Further experimental research is necessary to determine whether inadequate vitamin D directly contributes to the pathogenesis of ASD through affecting the E/I balance.

The Roles of Vitamin D in the Neural Circuits Involved in the Core Symptoms of ASD

Diagnosing mental disorders such as ASD is mainly based on symptoms.⁹² However, these classifications, which are based on clinical manifestations, may not fully capture the fundamental mechanisms underlying mental diseases. Thus, the “Research Domain Criteria (RDoC)” was introduced as a new classification system for the research on mental disorders.¹²¹ The RDoC conceptualized mental illness as brain disorders that could be addressed by altered function of neural circuits.^121,122 The focus of this concept was on researching the functional abnormalities of the brain circuits underlying the symptoms rather than the disease itself.^121,122 The main characteristics of ASD are social deficit, restricted interests and repetitive behaviors.⁹² In the past decade, the research on the relevant neural circuits has made significant progress.¹²³ In the section below, we will discuss the potential roles of vitamin D in ASD, focusing on the underlying neural circuits.

1. Recent studies have suggested that abnormalities in the social-reward circuitry may contribute to the social deficit in ASD patients.^124,125 And this system is mostly involved in basolateral amygdala (BLA), nucleus accumbens (NAC), dorsal anterior cingulate cortex (ACC), hypothalamus and midbrain.^126–129 VDRs were found in the aforementioned brain regions.^16,130,131 According to neuroimaging studies, ASD patients exhibited a feature of reduced activity in the BLA-NAC reward circuit.¹³² Interestingly, it was found that increasing 2-arachidonoylglycerol (2-AG), an endocannabinoid signal, might reduce presynaptic glutamate release in the BLA-NAC pathway, thereby alleviating the social avoidance in Shank3^−/− model mice.¹³³ Vitamin D deprivation lowered cannabinoid receptor expression in the spinal cord as well as 2-AG in the intestines of mice.¹³⁴ These findings raise the possibility that vitamin D deficiency may modulate the level of 2-AG in the BLA-NAC circuitry, contributing to the onset of ASD. However, more experiments are needed for the direct evidence supporting this hypothesis.

2. In addition, an important feature of ASD is the impairment in social functioning, particularly a lack of empathy.¹³⁵ The ACC-BLA circuit is one of the brain networks implicated in emotional empathy.^136,137 In contrast to healthy controls, children with ASD showed decreased connectivity between the amygdala and ACC, according to the functional magnetic resonance imaging (fMRI).¹³⁸ This reduced connectivity was correlated with the degree of social deficits.¹³⁹ The BLA and ACC brain regions were found to express VDRs, suggesting a biological basis of vitamin D to act in ACC-BLA circuit.^16,131 Additionally, vitamin D deficiency was linked to a thinner cingulate cortex.^140,141 These indirect evidences imply that vitamin D deficiency may impair the morphology of the cingulate gyrus, which in turn may affect the function of this brain region.

3. The instability of the cortico-striatal circuit has been proposed as the primary cause of repetitive behaviors manifested by ASD patients.^142–144 Children with idiopathic ASD showed cortico-striatal hyperconnectivity in fMRI, and this functional connectivity feature was related to the overactivation of mTOR signaling pathway.¹⁴⁵ Vitamin D supplementation could reduce pAKT and pS6 levels in the mTOR signaling in mice.⁶⁸ These results indicate that vitamin D may alter the clinical manifestations of ASD by modulating relevant neural circuits. However, there are still a lot of unanswered questions regarding how inadequate vitamin D contributes to the onset and development of ASD. For example, there is still a lack of experimental evidence to support vitamin D’ direct action on the ACC-BLA circuit. Application of latest technique progress in neuroscience, such as the use of optogenetics and pharmacogenetics might help in exploring these questions.¹³⁸

Vitamin D and Oxytocin/Vasopressin Signaling

The paraventricular (PVN) and supraventricular nuclei of the hypothalamus produce the hormones oxytocin and vasopressin, which have been linked to social behaviors.^146,147 According to a 12-week RCT, oxytocin treatment improved the social performance in patients with ASD (mean age 10.3 years, n = 35).¹⁴⁸ Another study reported that children with ASD (aged 9.6–12.9 years, n = 30) who received a 4-week intranasal vasopressin treatment showed a decrease in anxiety symptoms and repetitive behaviors.¹⁴⁹ But according to a different placebo-controlled clinical trial, ASD children (aged 3–17 years, n = 277) who received intranasal oxytocin once a day for 24 weeks did not show any significant improvement in social or cognitive assessments when compared to the control group.¹⁵⁰ The discrepancy in the therapeutic effects reported by these studies might be due to variations in medication delivery methods, treatment ages, and training program compliance.

Despite the fact that the clinical outcomes were controversial, oxytocin has been demonstrated to reduce the abnormal social behaviors in the animal models of ASD.^151–153 In Shank3^−/− model mice, oxytocin supplementation could activate endogenous oxytocin neurons in PVN, and thus alleviate their social deficit.¹⁵¹ It is interesting to note that in the hypothalamus, VDRs partially co-localize with vasopressin and oxytocin receptors.¹⁶ In addition, the presence of VDREs in the genes encoding oxytocin precursor proteins, oxytocin receptors and vasopressin receptors suggests that vitamin D can regulate their transcripts.¹⁵⁴ Furthermore, VDRs are expressed in pro-opiomelanocortin (POMC) neurons in the arcuate nucleus (ARC) of the hypothalamus, and POMC could be directly stimulated by vitamin D.¹⁵⁵ Interestingly, POMC neurons project to the oxytocin-secreting PVN neurons.¹⁵⁶ These results imply that vitamin D may play a role in the social process by stimulating POMC neurons, which in turn activates oxytocin secretion.

All of the aforementioned evidence together provided the experimental foundation for hypothetic link between vitamin D deficiency/insufficiency and ASD susceptibility. It has been proposed that vitamin D exerted multi-dimensional effects on the synapses and circuits. However, the exact synaptic and circuit mechanisms through which vitamin D contributes to the development of ASD remain to be further investigated. In addition, whether vitamin D can be administered as a supplement to treat ASD needs to be determined.

The Role of Vitamin D in ADHD

ADHD is a neurodevelopmental disorder characterized by hyperactivity, inattention and impulsivity performance.^92,157,158 It affects 8~12% of children worldwide.¹⁵⁹ Although ADHD is highly inheritable, many biological and environmental factors, such as food additives, lead pollution, prenatal and postnatal toxicant exposures, and low birth weight, have been identified as risk factors.^160–162

In recent years, numerous clinical studies suggest that vitamin D may be an environmental risk factor for ADHD.^163–166 According to a meta-analysis, children with ADHD had serum 25(OH)D concentrations lower than healthy controls.¹⁶⁶ When compared to children with sufficient vitamin D, children with vitamin D insufficiency had a 2.57-fold higher risk of developing ADHD than children.¹⁶⁶ Prospective studies have revealed a negative correlation between the severity of ADHD symptoms and maternal 25(OH)D levels.¹⁶⁴ The incidence of ADHD-like symptoms in children decreased by 11% for every 10ng/mL increase in maternal 25(OH)D levels.¹⁶³ Interestingly, there is growing evidence suggesting that vitamin D supplementation could help reduce the symptoms of ADHD.¹⁶⁷ In addition, treating ADHD patients with a methylphenidate and vitamin D combination was more effective than using methylphenidate alone.^168–170 Another study, however, reported ADHD children (aged 5–12 years, n = 54) who received 1440 IU of vitamin D daily for eight weeks did not show any improvement from baseline.¹⁷¹ This outcome diversity between these studies might be due to different sample sizes and large individual variations. Therefore, current evidence is not sufficient to conclude that vitamin D supplementation could reduce ADHD-related aberrant behaviors. Understanding the neuronal mechanisms will help address the question, and the following mechanisms have been proposed according to the literature:

Alterations of Dopaminergic and Serotoninergic Pathways in Vitamin D Deficient or Supplemented Animals

ADHD susceptibility may be increased by altered gene expression in dopaminergic pathways, including those encoding the dopamine transporter (SLC6A3), DRD2, dopamine D4 receptor (DRD4), dopamine D5 receptor (DRD5) and COMT.^35,172 Genes related to dopamine metabolic pathways, such as DRD2, COMT, TH, and SLC6A3, significantly decreased in vitamin D-deficient mice.^34,36,154 In addition, ADHD has also been linked to the dysregulation of serotoninergic system, including the serotonin transporter (SERT), 5-hydroxytryptamine (5-HT), and monoamine oxidase A (MAO-A).^173,174 Interestingly, vitamin D was shown to regulate the genes involved in the serotoninergic pathways, including 5-HT, SERT and MAO-A.^38,175

The Link Between Inadequate Vitamin D and the Dysfunctions in ADHD-Related Neural Circuits

1. Impairment in cognition: Response inhibition, which relies on circuits from frontal cortex to striatum and from frontal cortex to subthalamic circuits, was significantly impaired in ADHD to various degrees.^176–178 Animals without enough vitamin D exhibited a phenotype of increased impulsive behavior as a result of impaired response inhibition.^73,179 These indirect evidences suggested that the lack of vitamin D might have an impact on the cortical-striatal and cortical-subthalamic circuits, leading to impulsive behaviors. This hypothesis needs to be verified through a maneuver on the specific circuit.

2. Impairment in executive function: Many studies suggested that the primary cause of executive dysfunction in ADHD is the impairment in frontal cortex.^180–182 Furthermore, the frontal cortices of ADHD patients showed volume reduction as well as disruption in the networks.^183,184 The DVD model rodents, on the other hand, had cortex that was thinner,⁵⁶ implying a possible link between vitamin D deficiency/insufficiency and the executive dysfunction of ADHD. Experiments using techniques that can trace the brain circuitry underpinning executive function will be valuable to fully address the mechanisms.

Table 3 provides an overview of vitamin D’s effects on the ASD and ADHD. These findings offered potential rationales for how vitamin D affects the neurodevelopmental disorders. However, it is still unclear whether vitamin D deficiency/insufficiency directly contributes to the etiology of ADHD. Future studies will be required to further clarify the causal linkages, including animal experiments, prospective cohort studies and intervention trials.

Table 3.

The Effects of Vitamin D in ASD and ADHD

Diseases	ASD	ADHD
Clinical evidence	1. Vitamin D deficiency or insufficiency was prevalent among children with ASD.95–97	1. Vitamin D deficiency or insufficiency was prevalent among children with ADHD.166–169
	2. Maternal or neonatal vitamin D deficiency or insufficiency had been associated to an increased incidence of ASD.86,87	2. Vitamin D insufficiency during pregnancy was associated with a higher risk of childhood ADHD.166
	3. Vitamin D supplementation reduced the hyperactivity and irritability in children with ASD.96,98 See also.99	3. Vitamin D supplementation could alleviate inattentive behaviors in children with ADHD.171–173 See also.174
Potential mechanisms (From animal experiments)	1. Vitamin D regulated the expression of ASD-risk gene SLC17A6.23,109 2. Vitamin D deficiency might cause an E/I imbalance in the brain.29,32,34 3. Vitamin D deficiency might modulate neural circuits related to the core symptoms of ASD, such as BLA-NAC, ACC-BLA, and cortico-striatal circuits.134,136,137 4. The promoter of oxytocin receptor and vasopressin receptors have VDRE.157	1. Vitamin D deficiency caused the anomalous expression of neurotransmitters in dopaminergic pathways.34,36,157 2. Vitamin D deficiency increased impulsive behaviors, likely due to the dysfunction of response inhibition.181,182

Abbreviations: ASD, autism spectrum disorder; ADHD, attention-deficit hyperactivity disorder; E/I, excitation and inhibition; BLC-NAC circuits, the basolateral amygdala to the nucleus accumbens circuits; ACC-BLA circuitry, the adjacent dorsal anterior cingulate cortex to the basolateral amygdala; VDRE, vitamin D responsive elements.

Conclusions and Future Directions

As a neurosteroid hormone, vitamin D exerts multi-dimensional influence on the nervous system. It regulates synaptic transmission and synapse growth, as well as influences cognition and behaviors (Figure 2). Numerous epidemiological, molecular, and animal studies have revealed a link between vitamin D deficiency/insufficiency and an increased risk of ASD and ADHD. On the other hand, some studies demonstrated that vitamin D supplementation could reduce the symptoms in children with ASD and ADHD. Animal studies indicated that vitamin D might influence social process-related neural circuits like BLA-NAC and ACC-BLA pathways. Moreover, vitamin D might reduce the repetitive and aberrant social behaviors in ASD via regulating the mTOR pathway and oxytocin pathway. In addition, the prefrontal cortex circuits, as well as the dopaminergic and serotonergic pathways, which are frequently linked to the etiology of ADHD, may be impacted by inadequate vitamin D. More direct evidence on how vitamin D might affect the onset and progress of these disorders mechanistically is still missing. Nevertheless, vitamin D has the potential to be a treatment for neurodevelopmental disorders such as ASD and ADHD. It has the benefits including high safety, little side effects, and low cost. However, the precise therapeutic dose and effects, treatment duration and age of intervention for vitamin D remain to be determined. More clinical evidence is required before vitamin D can be extensively applied as a treatment strategy for ASD and ADHD. Most importantly, understanding how vitamin D contributes to the neurodevelopmental disorders will provide a solid foundation for the transition from the bench to the bedside.

Figure 2.

The functions of vitamin D in the nervous system and its contribution to the development of ASD and ADHD. Vitamin D participates in a variety of brain functions, including synaptic functions, cognition and behaviors. Vitamin D deficiency affected the synthesis and metabolism of many neurotransmitters, including glutamate, GABA, and dopamine. On the other hand, vitamin D supplementation could promote synaptic growth by increasing neurotrophic factors such as NGF, GDNF and GAP43. Additionally, vitamin D increased the expression of synaptic proteins such as synj1, syt2, SLC17A6 and complexin2. Vitamin D supplementation reduced the growth of abnormal dendritic spines through decreasing the levels of pS6 and pAKT, which were mTOR’s downstream targets of. Animals with vitamin D deficiency displayed altered brain morphology, decreased social interactions, and impaired learning abilities. In addition, taking vitamin D supplements not only improved social learning ability but also increased sociability. In clinical studies, inadequate vitamin D had been associated to an increased risk of neurodevelopmental disorders like ASD and ADHD. Vitamin D might play a role in the development of ASD through regulating neural circuits (BLA-NAC; ACC-BLA; cortico-striatal), E/I balance, and the oxytocin pathway. In ADHD, vitamin D might have an impact on the response inhibition and executive functions, probably through regulating dopaminergic pathway, serotoninergic pathway, and the circuit from frontal cortex to striatum.

Abbreviations: NGF, nerve growth factor; GDNF, glial cell derived neurotrophic factor; GAP43, growth associated protein 43; synj1, synaptojanin1; syt2, synaptotagmin2; SLC17A6, solute carrier family17 member6; pS6, phospho-S6; pAKT, phospho-AKT; ASD, autism spectrum disorder; ADHD, attention-deficit hyperactivity disorder; E/I, excitation and inhibition; BLA-NAC, the projections from basolateral amygdala to nucleus accumbens; ACC-BLA, the projections from anterior cingulate cortex to basolateral amygdala; 2-AG, 2-arachidonoylglycerol; VDR, vitamin D receptor; VDRE, vitamin D responsive element; OXTR, oxytocin receptor; DRD2, dopamine D2 receptor; COMT, catechol-O-methyltransferase; TH, tyrosine hydroxylase; SLC6A3, solute carrier family 6 member 3; 5-HT, 5-hydroxytryptamine; SERT, serotonin transporter; MAO-A, monoamine oxidase A.

Funding Statement

This study was supported by National Natural Science Foundation of China (NO. 31960170) and Hainan Province Science and Technology Special Fund (NO. ZDYF2020216) to L.X.; Hainan Province Science and Technology Special Fund (NO. ZDYF2021SHFZ088) and Hainan Major Science and Technology Projects (NO. ZDKJ2019010) to W.X.; Hainan Graduate Students Innovation Projects (NO. HYYS2021B19) to X.S.Y.; The open grant of NHC Key Laboratory of Tropical Disease Control, Hainan Medical University (NO. 2021NHCTDCKFKT22014) to P.C.R. The study also received financial support from Hainan Province Clinical Medical Center Grant (NO. QWYH202175) and the Excellent Talent Team of Hainan Province (NO. QRCBT202121).

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors report no conflicts of interest in this work.