Vitamin D and omega-3 fatty acids control the synthesis and effect of serotonin. Part 2: Significance for ADHD, bipolar disorder, schizophrenia and impulsive behavior

Summary:

Serotonin regulates a variety of brain functions and behaviors. Here we summarize previous findings that serotonin regulates executive functions, sensory control and social behavior and that attention deficit hyperactivity disorder, bipolar disorder, schizophrenia and impulsive behavior share defects in these functions. Until now, it was unclear why supplementation with omega-3 fatty acids and vitamin D improves cognitive function and behavior in these brain disorders. Here we propose mechanisms by which serotonin synthesis, release and function in the brain are modulated by vitamin D and the two marine omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Serotonin in the brain is synthesized from tryptophan by tryptophan hydroxylase 2, which is transcriptionally activated by the vitamin D hormone. An insufficient supply of vitamin D (~70% of the population) and omega-3 fatty acids is widespread, suggesting that serotonin synthesis in the brain is not optimal. We propose mechanisms by which EPA increases serotonin release from presynaptic neurons by reducing E2-series prostaglandins and DHA influences the action of serotonin receptors by increasing cell membrane fluidity in postsynaptic neurons. We propose a model that inadequate vitamin D, EPA or DHA levels, in conjunction with genetic factors and at key time points during development, leads to impaired serotonin activation and function and may be an underlying mechanism contributing to neuropsychiatric disorders and depression. This model suggests that optimizing the intake of vitamin D and marine omega-3 fatty acids may help prevent and modulate the severity of brain dysfunction.

Patrick, R. P., Ames, B. N. Vitamin D and the omega-3 fatty acids control serotonin synthesis and action, part 2: relevance for ADHD, bipolar disorder, schizophrenia, and impulsive behavior. FASEB J. 29, 2207-2222 (2015). www.fasebj.org

As a neurotransmitter, hormone and brain morphogen, serotonin plays a crucial role in brain function (1). Serotonin is concentrated in specific brain regions known to regulate social cognition and decision-making, collectively referred to as the “social brain” (2-4). There is a wealth of evidence for the link between serotonin and social behavior (5, 6). For example, polymorphisms in the serotonin transporter gene have been associated with social behavioral disorders such as aggression, impulsivity, anxiety, psychopathology and personality disorder (7-11). Experimental lowering of serotonin levels in the brains of normal people has a wide range of behavioral consequences: impulsive behavior, learning and memory impairment, poor long-term planning, inability to resist short-term gratification, and deficits in social behavior characterized by impulsive aggression or lack of altruism (5, 12-14). Since social behavior is disrupted in many brain disorders such as autism spectrum disorders (ASD), attention deficit hyperactivity disorder (ADHD), bipolar disorder, schizophrenia, impulsive behavior disorder, depression and anxiety, understanding the biological mechanisms that regulate the serotonin pathway is important to understand how social cognition and decision making are disrupted in these disorders.

Despite the wealth of data establishing a link between serotonin and social behavior, the specific factors that make a person more susceptible to social-cognitive disorders and mental illness remain unclear. One challenge in this area of research is the complexity of the interactions that manifest themselves in a particular disorder. Neuropsychiatric disorders are multifactorial and are probably influenced by a complex interaction between genetics, nutrition and environment (15). Significant progress has been made in identifying genetic variations associated with psychiatric disorders, and for some there is a common genetic etiology (16). Less is known about how nutritional deficiencies can interact with genetic signaling pathways such as the serotonin pathway, which are important for brain development, social cognition and decision-making, and how gene-environment interactions can trigger mental illness. Billions of neurons are involved in cognitive functions, working together with numerous biochemical pathways and the associated enzymes. Many of these enzymes require micronutrients, essential vitamins and minerals as cofactors for optimal function. It is therefore to be expected that suboptimal function caused by micronutrient deficiencies could impair the functions of proteins and enzymes involved in brain function. Several factors, including micronutrient deficiency, exercise, inflammation, and stress, have been shown to influence the serotonin pathway, as we will discuss here, and consequently affect social behavior (schematic representation of the serotonin pathway in Fig. 1) (17-21). In a previous article (Part 1 of this series), we demonstrated a biological mechanism by which the vitamin D hormone regulates serotonin synthesis in a tissue-specific manner and how aberrant serotonin production during fetal and neonatal development may play a central causal role in ASD (22).

Here we synthesize evidence from the literature that serotonin regulates executive functions, impulsivity, sensory control, and social behavior, and hypothesize that ASD, ADHD, bipolar disorder, schizophrenia, and impulsive behavior all exhibit defects in these functions due to dysfunction in the serotonin pathway. We build on our previous findings on how the vitamin D hormone is instrumental in controlling serotonin synthesis in the brain via tryptophan hydroxylase 2 (TPH2), which contains a vitamin D response element (VDRE) that coincides with activation, and how this relates to psychiatric disorders (22). We propose mechanisms by which the marine omega-3 fatty acids eicosapentaenoic acid (EPA) increases serotonin release from presynaptic neurons by reducing E2-series prostaglandins, and docosahexaenoic acid (DHA) influences serotonin action by increasing membrane fluidity and thus the accessibility of serotonin receptors in postsynaptic neurons. Because vitamin D insufficiency and low dietary intake of omega-3 fatty acids are common, we hypothesize that suboptimal intake of these micronutrients contributes to serotonin pathway dysfunction and, in combination with genetic factors, exacerbates serotonin system dysfunction, leading to defects in executive function, impulse control, sensory gating, and prosocial behavior and promoting neuropsychiatric disorders. These brain disorders are more common in men, which we believe is due to the protective effect of oestrogen, which increases serotonin synthesis in the female brain. In conclusion, we suggest that supplementation with vitamin D and omega-3 fatty acids may help prevent mental illness and/or mitigate the severity of brain dysfunction.

ROLE OF SEROTONIN IN NEUROPSYCHIATRIC DISORDERS

Serotonin regulates executive functions and sensory control

Executive function is modulated by serotonin and is essential for planning and decision making; the latter involves weighing the expected gains, losses and probabilities of each of these outcomes (19, 23). Serotonin levels in the brain have been experimentally reduced by the administration of branched-chain amino acids, which compete strongly with tryptophan during transport across the blood-brain barrier and cause serotonin levels to drop sharply (referred to below as acute tryptophan deficiency) (23). Acute tryptophan deficiency in healthy subjects, which lowers serotonin levels in the brain, impairs the decision-making process by altering the ability to recognize the magnitude of differences between immediate and long-term rewards (19). Tryptophan deficiency also increases the tendency to choose the less likely outcome, similar to amphetamine drug users and individuals with damage to the prefrontal cortex that impairs executive function (24). Another key component of decision-making is the ability to forgo short-term gratification in order to benefit in the long term. Serotonin deficiency in normal people leads to a shift in behavior toward lack of impulse control and short-term gratification at the expense of long-term benefits (13, 25-27). A tryptophan deficiency leads to increased activity in the ventral striatum, the part of the brain responsible for short-term decisions (28). In contrast, tryptophan intake activates the dorsal striatum, which is responsible for long-term decision making (28). It therefore appears that both decision-making and impulsive decisions are controlled by serotonin.

Sensory gating, i.e. the brain’s ability to filter out foreign sensory impressions, also depends on serotonin levels. Defects in sensory control lead to sensory overload with irrelevant information and thus to cognitive fragmentation, which is associated with numerous psychopathological disorders (21-26). Acute tryptophan deficiency in normal individuals leads to impaired sensory gating, suggesting that serotonin plays an important role in this process (29, 30). Defects in sensory control can also impair executive function and decision-making. In general, the data support the concept that low serotonin levels lead to impaired executive function and sensory control.

Serotonin regulates social behavior and impulsivity

Serotonin plays an important role in inhibiting impulsive aggression against oneself, including suicide, and aggression against others (5, 31). A reduction in serotonin levels in the brain of normal people leads to a rejection of cooperative behavior in favor of short-term gains and to antisocial behavior, increased uncontrolled aggressive behavior, feelings of anger, quarrelsome behavior and self-harm (5, 20, 32-35). In adolescents with ADHD, a reduction in serotonin levels in the brain led to an increase in aggressive behavior (36). A reduction in serotonin levels in the brain has also been shown to lead to a loss of inhibitory behavior towards adverse consequences, which has been associated with relapse (5, 12, 14, 33). In contrast, an acute increase in serotonin levels in the brain makes people more inclined to harm others, suggesting that serotonin may play a role in moral behavior (20). Tryptophan supplementation has been shown to reduce social anxiety and argumentative behavior in normal individuals and in irritable individuals, and therefore may improve overall social behavior (37-40). In addition, tryptophan supplementation reduced impulsivity and increased social cooperation in boys with disruptive behavior (41). In addition, tryptophan supplementation can reduce aggression and the need for antipsychotics in schizophrenics (42, 43). These studies suggest a causal role for serotonin in the regulation of social behavior.

Description Figure 1: Metabolic pathways of tryptophan. Stress and inflammation activate the enzymes indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO), which metabolize tryptophan to kynurenine and remove it from transport into the brain. Alternatively, tryptophan can be metabolized by the enzyme tryptophan hydroxylase 1 (TPH1), which uses tetrahydrobiopterin (BH4) and iron as cofactors to produce 5-hydroxytryptophan (5HTP). 5HTP is converted to 5HT by the enzyme aromatic L-amino acid decarboxylase (AAAD), which uses pyridoxal-5-phosphate (P5P) as a cofactor. 5HT is metabolized by the enzyme N-acetyltransferase (AANAT) to form the sleep hormone melatonin. In order to form serotonin (5HT) in the brain, tryptophan must first be transported across the blood-brain barrier. This transport depends on the ratio between tryptophan and branched-chain amino acids, which are strongly inferior to tryptophan when transported across the blood-brain barrier. Sport alleviates this competition by increasing the uptake of branched-chain amino acids in the muscles and thus increasing the availability of tryptophan for the brain. In the brain, tryptophan is converted into 5HTP by the rate-limiting enzyme tryptophan hydroxylase 2 (TPH2). 5HTP is metabolized by AAAD to form 5HT in the brain. 5HT is converted by the enzyme monoamine oxidase (MAO) to the inactive serotonin metabolite 5-hydroxyindoleacetic acid (5HIAA).

In non-human primates, low serotonin levels in the brain also lead to severe aggressive behavior and lack of impulse control (44). In mice, a lack of tryptophan in the diet leads to increased killing behavior, which can be improved by increasing the serotonin level in the brain (45, 46). Mutant mice that have a non-functional or switched off tph2 and thus have no serotonin synthesis in the brain show exaggerated aggressive behavior and compulsive behavior compared to control mice (47-49). This data shows that a lack of serotonin in the brain leads to increased aggression.

Polymorphisms in serotonin-related genes are associated with mental disorders

ASD, ADHD, bipolar disorder, schizophrenia and impulsive behavior disorders have significant overlap, as they all have impairments in executive function, sensory control and social behavior. These impairments are characterized by a variety of phenotypes, including poor long-term planning skills, impulsivity, poor attention switching, emotional dysregulation, impaired sensory gating, poor social skills, impulsive aggression toward self and others, and depression (50-58). Since serotonin plays a key role in regulating many of these executive and behavioral functions, impaired serotonin signaling is likely a common cause of these psychopathological disorders. In fact, serotonin levels in the brain have been shown to be low in ASD, ADHD, bipolar disorder, schizophrenia, and impulsive behavior disorders (22, 59-61). The concentration of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the cerebrospinal fluid is a biomarker for low serotonin levels in the brain. Low levels of 5-HIAA are associated with harmful behaviors such as aggression, violent suicide, impulsive homicide, and homicide recidivism, and have also been found in individuals with depression (62-68).

Polymorphisms in the TPH2 gene and other serotonin-related genes provide further evidence that aberrant serotonin levels in the brain are associated with increased susceptibility to ASD, ADHD, bipolar disorder, schizophrenia, and impulsive behavior, including aggression toward self and others (22, 69-75). These polymorphisms are also associated with aggression, depression and anxiety, all of which are common psychological abnormalities in these psychiatric disorders (76-78). Suicide is closely associated with impulsive behavior, and polymorphisms in tryptophan hydroxylase and other serotonin-related pathways have been linked to increased suicide attempts (79, 80). In a recent study, 58% of suicide attempt patients had vitamin D deficiency, and their vitamin D levels were significantly lower than those of healthy individuals and patients with depression but without suicidality (81). In this context, a lower daily sunshine duration was also associated with a higher frequency of suicide regardless of the time of year (82, 83).

THE ROLE OF VITAMIN D IN NEUROPSYCHIATRIC DISORDERS

Vitamin D regulates serotonin

Vitamin D is first converted into 25-hydroxyvitamin D [25 (OH)D3], which is the main stable circulating form of vitamin D, and then into the biologically active steroid hormone 1,25-dihydroxyvitamin D (84). We have recently proposed a mechanism describing how the vitamin D hormone, which appears to control more than 900 genes, is a major regulator of serotonin synthesis in the brain through TPH2, which contains a VDRE that coincides with activation (22, 85). We identified two different VDREs in the regulatory regions of TPH2 and TPH1, the two genes responsible for the conversion of tryptophan to serotonin in the brain and other tissues, respectively (22). We proposed that VDREs would respond in an inverse manner to vitamin D hormone, with TPH2 being transcriptionally activated in the brain and TPH1 being repressed in tissues outside the blood-brain barrier (22). This proposal was based on the evidence that the VDRE sequence alone can determine whether vitamin D hormone activates or represses gene transcription (86). New biochemical evidence confirms our proposal by showing that vitamin D hormone activates the expression of TPH2 in cultured neuronal cells (M. Haussler, personal communication, July 19, 2014; see note in the reading sample).

Vitamin D deficiency

The exact blood level of 25(OH)D3, which is defined as vitamin D deficiency, is still somewhat controversial. Based on the classical function of vitamin D, which is the maintenance of bone homeostasis, vitamin D deficiency has been defined by the Institute of Medicine as a 25(OH)D3 serum concentration <20 ng/ml (87). The current guidelines for an adequate vitamin D supply are >30 ng/ml (87).

According to the U.S. National Health and Nutrition Examination Survey, vitamin D sufficiency (30-60 ng/mL) declined from ~60% to 30% for Caucasians, from 10% to 5% for African Americans, and from 24% to 6% for Latinos between 1994 and 2004, suggesting that more than half of the U.S. population has inadequate levels of this important vitamin D hormone (88, 89). Currently, ~70% of adults and 67% of children aged 1-11 years in the United States do not have adequate vitamin D levels, even when fortification and supplementation are taken into account (89-91). The epidermal synthesis of vitamin D requires exposure to UVB radiation, which comes from the sun. However, the use of sunscreen and high levels of melanin, the skin’s brown pigment, block UVB radiation and thus impair the skin’s ability to synthesize vitamin D (84, 92, 93). In addition, the skin’s exposure to UVB is lower if you live in northern latitudes (84). A modest amount of vitamin D can be obtained from food, such as seafood, which is the relatively richest food source (94). Some foods have been fortified with vitamin D, including milk (100 IU per 8 oz) and orange juice (100 IU per 8 oz), but these amounts are not enough to achieve an adequate vitamin D status of 30 ng/ml. In addition, dairy products are a poor choice for fortification for the approximately 50 million Americans who are lactose intolerant, including 75% of African Americans (95).

Vitamin D can alter the severity of brain dysfunction

There may be a very important interaction between genetics and vitamin D hormone that could play a role in influencing the severity of mental illness. Individuals with polymorphisms in serotonin-related genes are already predisposed to dysregulation of serotonin synthesis or metabolism; therefore, any additional reduction in serotonin synthesis as a result of inadequate vitamin D levels may exacerbate defects in executive function, sensory gating, and impulsive behavior. Low serum 25(OH)D3 concentrations have been shown to be associated with an increased risk of ASD, ADHD, bipolar disorder, schizophrenia, antisocial behavior, and impulsive behavior (22, 96-101). There appears to be an interaction between polymorphisms in serotonin-related genes and the season of birth: Individuals with bipolar disorder or schizophrenia who also have polymorphisms in the TPH gene or the gene that codes for the serotonin transporter have an increased risk of mental illness if they were born in the winter/spring months (102). The interaction between vitamin D deficiency and defects in serotonin-related genes has also been demonstrated in mice: Mice that already have reduced serotonin synthesis due to a polymorphism in their TPH2 gene are highly sensitive to vitamin D deficiency in adulthood and, as a result, exhibit profound defects in cognitive function and behavior when vitamin D is restricted (103, 104). Presumably, these mice exhibit more pronounced behavioral disorders due to a further attenuation of serotonin synthesis as a result of vitamin D deficiency. These studies emphasize the role that the vitamin D hormone plays in influencing the severity of brain dysfunction in combination with genetic factors that influence serotonin levels in the brain.

The timing of vitamin D deficiency can exacerbate brain dysfunction

In addition to genetic factors, the timing of vitamin D deficiency and the developmental stage at which it occurs also affect the severity of executive function, sensory gating and social behavior disorders. Gestational vitamin D deficiency in rats leads to later impairments in latent inhibition (which is related to sensory gating), attentional processing and impulsive behavior (105, 106). Consistent with these findings, mutant mice lacking a functional vitamin D receptor exhibit defects in sensory gating and abnormal social behavior, including social neglect and impulsive behavior (107, 108). However, when vitamin D is restricted only in adulthood, the behavioral abnormalities, including impulsive behavior and impaired attention processing, are more subtle than the effects of lifelong vitamin D deficiency (107-109). These subtle effects of vitamin D deficiency in adulthood contrast with the more severe behavioral effects observed in mice already predisposed to low serotonin synthesis (104). Thus, the severity of behavioral abnormalities in response to vitamin D deficiency is more pronounced when the deficiency occurs during development, but is also exacerbated by genetic factors that also affect the serotonin system.

A lack of vitamin D during early development can lead to disorders of social perception, decision-making and brain morphology, which are comparable to many brain disorders. This may be partly due to the important role that vitamin D and serotonin play in the structure and wiring of the brain (110, 111). In rats, vitamin D deficiency during pregnancy leads to a 200% increase in the volume of the lateral ventricles, a slightly smaller width of the neocortex, increased cell proliferation, reduced differentiation and a decrease in neurotrophic factors (112, 113). In humans, vitamin D deficiency during pregnancy has been shown to lead to an enlargement of the neonatal ventricles by up to 28% (114). It is known that enlargement of the cerebral lateral ventricles is associated with ASD, ADHD and schizophrenia (115). Assuming that this aberrant brain morphology plays a role in the etiology of these brain disorders, it is plausible that the same ventricular enlargement, if caused by early vitamin D deficiency during pregnancy, can also cause the same diseases if the right genetic and environmental background is present.

There is clear evidence that low vitamin D levels during pregnancy and neonatal development are a risk factor for schizophrenia and psychosis, in some cases increasing the risk sixfold (97, 98, 116-119). In addition, lack of vitamin D supplementation during pregnancy is associated with an increased risk of schizophrenia in male children (120). Low vitamin D levels have been associated with an increased risk of psychotic experiences in both childhood and adolescence (121, 122). A meta-analysis found that prevalence rates for schizophrenia increase significantly with increasing latitude; however, lighter skin color, a factor in improving vitamin D status, and high fish consumption were both associated with protection against schizophrenia (99). These data suggest that vitamin D may play an important role in shaping the structure of the developing brain and in reducing psychosis and schizophrenia.

Vitamin D supplementation: a simple solution?

Vitamin D supplementation during early brain development may reduce the risk of neuropsychiatric disorders, and supplementation later in life may improve brain dysfunction. This could be mediated in part by the ability of vitamin D to activate TPH2 and thus increase serotonin synthesis (22) (M. Haussler, personal communication, July 19, 2014). Low vitamin D levels are common in ASD, ADHD, bipolar disorder, schizophrenia, and impulsive behavior (22, 81, 96, 122-125). For this reason, many people at risk of or already diagnosed with one of these disorders would benefit from vitamin D supplementation. In fact, vitamin D supplementation in the first year of life reduced the incidence of schizophrenia by 77% (120). This is of particular importance as there is a wide range of vitamin D insufficiency in pregnant women in the United States (up to 91%). The degree of insufficiency varies by state, perhaps due to differences in sun exposure (126). About 50% of mothers taking prenatal vitamins and their newborns had inadequate vitamin D levels, while supplementation with 4000 IU/day, the upper tolerable intake level, was safe and most effective in achieving adequate vitamin D concentrations without toxicity (127, 128). Vitamin D supplementation has also been shown to improve inattention, hyperactivity and impulsivity in children and adults with ADHD (129, 130). Given the widespread vitamin D deficiency, particularly in people with brain dysfunction, these data suggest that vitamin D supplementation of 4000 IU could eliminate vitamin D deficiency and help reduce the risk of psychiatric disorders and improve brain function. Further clinical studies investigating this will provide more information.

OMEGA-3 FATTY ACIDS REGULATE SEROTONIN FUNCTION

Omega-3 fatty acids influence behavior

Long-chain marine omega-3 fatty acids in the brain consist mainly of C22 n-3 DHA and some C20 n-3 EPA (131). Blood concentrations of EPA and DHA have been found to be low in individuals with a variety of psychiatric disorders such as ASD, ADHD, bipolar disorder, schizophrenia, suicide attempts, and other impulsive behaviors; supplementation has been shown to be beneficial in reducing the severity of symptoms (22, 132-136). Compared to healthy people, schizophrenics have significantly lower DHA levels in the orbitolfrontal cortex region of the brain, where serotonin is concentrated (137). Epidemiological studies indicate that ω-3 deficiency may be a risk factor for bipolar disorder: Plasma DHA concentrations are significantly reduced in patients with bipolar disorder (135). Suicidal thoughts are common in people with bipolar disorder and depression and have been linked to a lack of omega-3 and serotonin in the brain (138, 139). Randomized controlled trials have shown that supplementation with several grams of EPA and DHA improves depression, suicidal thoughts and behaviors (132, 140). Supplementation with omega-3 fatty acids from fish oil has been shown to improve cognitive function, including language skills, concentration, motor skills, schizophrenic symptoms, and aggressive and impulsive behavior (132, 141). Intervention studies have shown that supplementation with omega-3 fatty acids improves aggression, anger, hostility, antisocial behavior, anxiety and impulsivity in normal school children, juvenile offenders, adolescents, prison inmates and drug addicts (142, 143). Although many recent meta-analyses show a clear benefit of omega-3 fatty acids in the treatment of depression, there is some heterogeneity between clinical trials (144-149). Plausible explanations for the contradictory results include genetic variations, dietary omega-3 fatty acids and different doses of EPA and DHA in different formulations, as EPA appears to play a more important role (150, 151). A large clinical trial in which the omega-3 fatty acid concentrations in the red blood cells are measured and various EPA and DHA doses are tested in comparison with placebo should help to clarify the situation.

EPA regulates the release of serotonin

We hypothesize that an important mechanism by which omega-3 fatty acids modulate serotonin function is through the regulation of serotonin release in the presynaptic neuron (Fig. 2). Serotonin release is inhibited by the prostaglandins of the E2 series, which are formed from arachidonic acid, an omega-6 fatty acid produced from linoleic acid in animals (152, 153). EPA inhibits the formation of prostaglandins of the E2 series and inhibits the formation of arachidonic acid in both young and old people (154, 155). In rats fed an arachidonic acid-rich diet, E2-series prostaglandins were elevated in the hippocampus, which was attenuated by feeding EPA (156). Since the prostaglandins of the E2 series inhibit serotonin release and EPA inhibits the formation of these prostaglandins, it seems likely that EPA is important in the brain for normal serotonin release. Indeed, plasma omega-3 levels in humans are positively correlated with the serotonin metabolite 5-HIAA in cerebrospinal fluid (157). Dietary surveys in the United States show that the average intake of linoleic acid (omega-6 fatty acid), α-linolenic acid (omega-3 fatty acid), EPA and DHA in adults is ~12-20, 1.4-2.0, 0.03-0.06 and 0.05-0.10 g/d, respectively (158). These data suggest that most adults do not consume enough EPA and DHA in their diet.

EPA inhibits inflammation and depression

The prostaglandins of the E2 series are hormone-like signaling molecules that play an important role in promoting inflammation, in particular by inducing the production of proinflammatory cytokines such as the interleukins IL-6 and IL-1β and TNF (159). Inflammatory cytokines produced in the periphery can cross the blood-brain barrier and cause neuroinflammation. Endotoxin injection has been shown to cause depression and inhibition of verbal and non-verbal memory in humans, triggering an immune response and the production of pro-inflammatory cytokines (160). Similarly, intravenous injection of the inflammatory cytokine IFN-γ causes depressive symptoms in humans; however, depression is ameliorated by supplementation with a high dose of EPA (161). In addition, individuals with gene polymorphisms in serotonin-related genes have been shown to have an even higher risk of inflammation-related depression as a result of intravenous injection of IFN-γ (162). Although a link has been established between depression and inflammation, the mechanism is not yet known. We think it is likely that the depression that occurs as a result of inflammation is due to the inhibition of serotonin release, as serotonin also plays an important role in mood. Since serotonin regulates a variety of cognitive functions and social behaviors in addition to mood, the inhibition of the pro-inflammatory prostaglandins of the E2 series by EPA has very important therapeutic effects on serotonin.

DHA regulates the function of serotonin receptors

We propose another mechanism that omega-3 fatty acids influence the serotonin system through DHA-mediated regulation of serotonin receptor function, which depends on cell membrane fluidity. DHA is the most abundant fatty acid in the brain and accounts for 30% of the fatty acid content (163-167). The fluidity of the cell membrane depends on the amount of cholesterol, which reduces membrane fluidity, and on the omega-3 fatty acids in the membrane phospholipids, which increase membrane fluidity. The DHA composition in the lipid membrane is required for adequate membrane fluidity (167-170). Cholesterol is strictly regulated in the brain, while the fatty acid composition is influenced by dietary factors. The serotonin receptor is a G-protein-coupled receptor that crosses the cell membrane seven times and is strongly influenced by the composition of the lipid membrane (170-172). When the membrane becomes less fluid, the binding of serotonin to its receptor decreases significantly, as serotonin receptors are less accessible (173, 174). This effect is not limited to the serotonin receptors, but also affects the dopamine receptors and other neurotransmitter receptors (175). The role of DHA in membrane fluidity has also been shown to be important for synaptosomal membranes that regulate neurotransmission (176, 177). Omega-3 fatty acid deficiency has been associated with decreased serotonergic neurotransmission, and DHA deficiency decreases serotonin concentration in the frontal cortex (178, 179). Since DHA is important for cell membrane fluidity and serotonin receptor function is dependent on cell membrane fluidity, this suggests that DHA may be important for serotonin receptor function.

Description Figure 2: Micronutrient regulation of serotonin metabolism. A) Tryptophan is transported across the blood-brain barrier, and adequate levels of vitamin D enable normal tryptophan metabolism by increasing the expression of tryptophan hydroxylase 2 (TPH2) to produce serotonin (5HT). A sufficient level of eicosapentaenoic acid (EPA) enables the release of 5HT by the presynaptic neuron. A sufficient content of docosahexaenoic acid (DHA) enables the binding of 5HT to the serotonin receptor (5HTR) in the postsynaptic nerve cell. This enables normal serotonin neurotransmission and executive functions, sensory control and prosocial behavior. B) If the vitamin D status is insufficient, TPH2 is not well expressed and little serotonin is produced. Insufficient EPA status leads to inhibition of 5HT release from the presynaptic neuron. Insufficient DHA status alters the accessibility of the serotonin receptor, resulting in less 5HT binding to the serotonin receptor of the postsynaptic neuron. This leads to abnormal serotonin neurotransmission and poor executive function, poor sensory control and impulsive behavior.

Omega-3 fatty acids regulate neurodevelopment through serotonin

Omega-3 fatty acids play a very important role during brain development, among other things by regulating the serotonin system. Reduced intake of EPA and DHA during neurodevelopment leads to reduced serotonin synthesis, storage, release and receptor function (164). Omega-3 fatty acid deficiency also affects the structure and wiring of the developing brain, as it is associated with a decrease in neurogenesis, dendritic arborization, synaptogenesis, selective pruning, and myelination (164, 165). Perinatal omega-3 deficiency in rats led to a 65% decrease in serotonin levels in the prefrontal cortex and correlated with a 29% decrease in mRNA expression of tph2 (180). In contrast, supplemental administration of fish oil during gestation and early development increased serotonin levels in the prefrontal cortex of rats and reduced the stress-induced decrease in serotonin levels (181, 182). Supplementation with omega-3 fatty acids can be very important during pregnancy, as the fetus must obtain all of its omega-3 fatty acids from the mother via the placenta (183). Pregnant and breastfeeding women therefore have a considerable need for DHA for their fetus. In the third trimester of pregnancy, most DHA accumulates in the human brain, as the brain experiences a growth spurt during this time (184). However, pregnant women consume even less omega-3 fatty acids than the general population, which is already deficient in omega-3 fatty acids, due to concerns about mercury in seafood, the best dietary source of omega-3 fatty acids (184). These data suggest that the intake of omega-3 fatty acids from the sea during pregnancy and early development can modulate the serotonin system.

PROTECTIVE ROLE OF ESTROGEN IN NEUROPSYCHIATRIC DISORDERS

ASD, ADHD, schizophrenia, and impulsive behavior disorders are more common in men than in women, while bipolar disorder is equally common (22, 185-188). We hypothesize that this sex difference in ASD is due to the fact that estrogen, a steroid hormone, can substitute for vitamin D hormone in activating the TPH2 gene, thereby increasing serotonin levels (22). Since estrogen significantly increases the expression of TPH2 in the brain, serotonin levels would also increase (189-193). We therefore assume that estrogen would also protect against other neuropsychiatric disorders. Consistent with this proposal, rats, mice and humans exhibit higher tryptophan hydroxylase activity in females compared to males (194-199).

Estrogen increases serotonin synthesis and thus has a protective effect on impairments of learning, memory, impulse control and sensory gating, which are experimentally induced in acute tryptophan deficiency (200-204). For example, the negative effects of tryptophan deficiency on verbal memory are mitigated in women treated with oestrogen (201). Estrogen probably also has positive effects on social behavior, as shown by the fact that women are less aggressive, commit fewer violent crimes and are less likely to commit suicide (78). Acute tryptophan deficiency in women during the luteal phase of their menstrual cycle, when estrogen levels are low, causes them to be more aggressive than at other times of the menstrual cycle when estrogen is abundant (205). Similarly, in postmenopausal women, when estrogen levels are low, acute tryptophan deficiency has significant effects on cognitive function and emotional regulation, decreases working memory, and causes hyperactivation of the amygdala, which can be reversed by administration of estrogen (200). These data are consistent with the hypothesis that estrogen activation of TPH2, and consequently the increase in brain serotonin levels, is a mechanism by which women are somewhat protected from many of the impairments associated with neuropsychiatric disorders, including executive function, sensory gating, and disruption of social behavior.

Although estrogen may have a general protective role against many neuropsychiatric disorders in women, the decline in estrogen levels in the postpartum and postmenopausal period may make women vulnerable to mental illness, particularly bipolar disorder, when these biochemical changes occur. In the first four months after birth, estrogen levels drop precipitously by 100- to 1000-fold, potentially exacerbating the effects of already low vitamin D, tryptophan, and omega-3 fatty acid levels (206-208). The postpartum period has been shown to trigger the first onset of bipolar disorder or postpartum psychosis, and during this time there is a risk of infanticide and maternal suicide, as well as other problems with cognitive dysfunction (209, 210). Misdiagnosis of bipolar disorder as postpartum depression is common (211). These data suggest that the postpartum period is a unique situation in which estrogen, vitamin D, tryptophan and omega-3 levels are particularly low, which may create an environment for the manifestation of bipolar disorder.

DISCUSSION

We hypothesize that serotonergic dysfunction is a common denominator in a variety of neuropsychiatric disorders, including ASD, ADHD, bipolar disorder, schizophrenia, impulsive behavior disorders and depression. This proposal is based on evidence that executive function, sensory control, and prosocial behavior are regulated by serotonin and that serotonin levels are low and polymorphisms in serotonin-related genes are common in many of these disorders. We hypothesize that an underlying mechanism is that the vitamin D hormone regulates serotonin synthesis and thus modulates the severity of the above-mentioned disorders. We also provide evidence for mechanisms by which EPA regulates the release of serotonin by inhibiting the production of E2-series prostaglandins and DHA controls serotonin function by increasing the fluidity of the neuronal cell membrane. The mechanism we propose explains how vitamin D and marine omega-3 fatty acids work together to improve cognitive function, health and behavior. This synergy can be explained in part by their effects on the serotonin system: Vitamin D regulates serotonin synthesis, EPA influences serotonin release, and DHA improves the accessibility of membrane-bound serotonin receptors. This also partly explains why supplementation with vitamin D, EPA and DHA improves some behaviors associated with ADHD, bipolar disorder, schizophrenia and impulsive behavior by controlling serotonin production and function. Although many intervention studies with vitamin D, EPA and DHA have shown apparent benefits, larger clinical trials need to be conducted to determine effective doses for these various disorders.

It is also shown how estrogen can compensate for the disturbances in sensory control and executive functions when serotonin is experimentally lowered. We propose that this effect may be caused by the ability of estrogen to activate TPH2, which explains the lower prevalence of psychiatric disorders in women. The role of the activating VDRE in TPH2 offers a new explanation for why the vitamin D hormone is required for normal serotonin synthesis in the brain and how low vitamin D levels might influence the course and development of neuropsychiatric disorders. Similarly, the ability of oestrogen to increase the expression of TPH2 is one explanation for why women are better protected against mental illness.

Because vitamin D regulates the synthesis of serotonin, EPA regulates the release of serotonin from neurons, and DHA regulates serotonin receptor function, adequate vitamin D and ω-3 fatty acid status would be critical to prevent defects in executive function, impulse control, sensory control, and prosocial behavior, especially in individuals with a polymorphism in a serotonin-related gene (Fig. 2A). Therefore, inadequate vitamin D and omega-3 fatty acid status in combination with genetic factors causing dysfunction in the serotonin pathway can exacerbate the defects and trigger mental illness (Fig. 2B). The timing of the onset of vitamin D and/or omega-3 deficiency in combination with a genetic predisposition to serotonin dysfunction is also likely to be an important factor in the onset of mental illness. Indeed, changes in the migration of GABAergic interneurons during brain development, which is regulated by serotonin, are a key determinant of susceptibility to psychiatric disorders such as schizophrenia and autism (111). This may partly explain why neonatal vitamin D status is associated with the risk of schizophrenia (98, 120).

Other environmental factors, including stress hormones and inflammatory cytokines, also regulate tryptophan metabolism. Stress hormones and inflammatory cytokines activate the rate-limiting enzymes indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase, causing tryptophan to be metabolized to kynurenine instead of serotonin (212). This means that stress and inflammation act like a tryptophan trap and prevent tryptophan from being transported into the brain for serotonin synthesis by TPH2 (Fig. 1). In the context of prenatal stress, this would mean that less maternal serotonin is available to shape the developing brain, which has been shown to cause abnormal brain development in mice (110). In addition, prenatal stress has been shown to cause abnormal migration of GABAergic interneurons and disruption of serotonin neurons in the developing brain, both of which are associated with an increased risk of schizophrenia (213, 214). Early stress events also decrease the expression of TPH2, which reduces serotonin production in the brain and leads to anxious behavior in mice (215). The effects of stress on tryptophan metabolism also lead to a positive feedback loop, which ultimately causes a reduction in serotonin production and release in the brain. This is due to the fact that stress hormones lower serotonin levels and low serotonin levels lead to anxious behavior, which in turn leads to the production of more stress hormones, setting off a vicious cycle. In individuals who have polymorphisms in serotonin-related genes, stressful events and micronutrient deficiencies can be the perfect storm to trigger mental illness. For this reason, it is essential to break this vicious circle in order to normalize serotonin levels and brain function while improving behavior.

Dietary regulators of the serotonin pathway, including vitamin D, EPA and DHA, are an easy way to optimize serotonin synthesis and function in the brain. Tryptophan and 5-hydroxytryptophan may be other methods of increasing serotonin levels in the brain and have been shown to have a positive effect on mood and reducing anxiety (37-39, 216). However, a potential problem with ingesting tryptophan and 5-hydroxytryptophan is that they can be immediately converted to serotonin in the gastrointestinal tract, which reduces bioavailability for transport to the brain and is known to cause inflammation (216, 217). Exercise, which increases tryptophan transport into the brain and thus serotonin production, is another simple solution to break the vicious circle (Fig. 1).

Exercise increases tryptophan transport across the blood-brain barrier by reducing competition with branched-chain amino acids, as these are preferentially absorbed by the muscles (218).

Many people with mental illness are deficient in many micronutrients, especially vitamin D and ω-3 fatty acids (219). This may explain why supplementation with these essential micronutrients has been shown to be effective in treating symptoms associated with ADHD, bipolar disorder, schizophrenia, impulsive behavior, depression, and obsessive-compulsive disorder (142, 220). In addition, supplementation with vitamin D and ω-3 fatty acids would be a safer therapeutic treatment than serotonin-increasing drugs, which often have negative side effects (221). Adequate daily therapeutic doses of ω-3 fatty acids from fish oil appear to be ≥2 g EPA and 1 g DHA per day (132). We hypothesize that supplementation with vitamin D, omega-3 fatty acids and other key micronutrients to achieve sufficiently high serum levels will boost serotonin production and function in the brain, thereby improving cognitive function and limiting impulsive behavior. However, guidelines for vitamin D sufficiency are based on its classical role in bone homeostasis, and it is unclear whether these guidelines are sufficient to maintain non-classical functions of the vitamin D hormone in other tissues, including activation of TPH2 in the brain. Other micronutrients that influence the serotonin pathway also appear to be important, such as vitamin B6 and iron, two cofactors involved in serotonin synthesis (Fig. 1). About 8% of the U.S. population is deficient in vitamin B6; some preliminary evidence suggests that vitamin B6 may also promote moderate improvement in some behaviors (90, 222, 223). Iron deficiency is also common in 16% of menstruating women and 29% of low-income women; iron supplementation has also been shown to help improve some behaviors (224).

Vitamin D and omega-3 supplementation are practical measures and of great therapeutic importance, as there is a massive and widespread vitamin D and omega-3 deficiency in the United States and in certain population groups (90, 225). It is likely that even relatively small dietary deficiencies in several micronutrients can have a cumulative detrimental effect on the nervous system, thereby impairing cognitive function and behavior. Our findings could have important therapeutic implications for individuals with impulsive aggression towards themselves, as in the case of suicide, and aggression towards others. Prisons in the United States are full of violent offenders in whom impulsive-aggressive behaviors are unusually common (226). Decreased serotonin synthesis in an individual has also been shown to play a causal role in relapse, suggesting that improvement in poor behaviors is to some degree dependent on serotonin levels (5). This is of great importance for violent offenders who need to be rehabilitated and suggests that optimizing their micronutrient intake through supplementation with vitamin D, EPA and DHA may help to increase serotonin production and function and thus reduce recidivism. In general, people who are prone to short-term decisions and impulsive behavior may benefit from supplementation with vitamin D and omega-3 fatty acids. Since vitamin D and omega-3 fatty acid deficiencies are widespread, it is possible that a significant portion of the population has subclinical deficits in serotonin production and function.

Increasing vitamin D and omega-3 fatty acid levels in the general population through supplementation could therefore lead to a simultaneous increase in serotonin levels and serotonin function in the brain, thereby improving normal cognitive function, propensity for prosocial behavior and limiting impulsive behaviors.

R.P.P. is grateful for support from the David and Annette Jorgensen Foundation and the Children’s Hospital Oakland Research Institute-Ames Supporting Foundation for the earlier part of this project. The authors would like to thank Henry Wheeler Jr. for the generous support of our laboratory. The authors would like to thank Giovanna Ferro-Luzzi Ames, Sam Barondes, Georganne Garfinkel, Mark Haussler, Ron Krauss, Joyce McCann, Daniel Patrick, Margie Profet and Robert Ryan for comments and suggestions on the manuscript.

Note added to the evidence: Furthermore, in mice administered calcitriol, the hormonally active metabolite of vitamin D, the expression of Tph2 and the serotonin metabolite (5-HIAA) was increased in the prefrontal cortex and hippocampus, demonstrating that the vitamin D hormone indeed activates Tph2 in the brain (227). Remarkably, these mice did not have higher serotonin levels, but did have more 5-HIAA, suggesting that at normal serotonin levels, additional vitamin D enhances serotonin degradation and does not raise serotonin levels above a physiologically normal concentration (227).