Maternal diet rich in omega-6 polyunsaturated fatty acids during gestation and lactation produces autistic-like sociability deficits in adult offspring

Abstract

Multiple studies have reported prenatal stress as a potential risk factor for the development of autism spectrum disorder (ASD). In rodents, a significant reduction in sociability is seen in prenatally stressed offspring of genetically stress-susceptible dams. Certain dietary factors that contribute to stress reactivity may, therefore, exacerbate prenatal stress-mediated behavioral changes in adult offspring. Adults with a diet rich in omega-6 polyunsaturated fatty acids (PUFAs) display increased stress reactivity. In the current study, the effects of prenatal diet and prenatal stress on social behavior in adult offspring mice were examined. Pregnant C57BL/6 J dams received either chronic variable stress or no stress, and were also placed on a control diet or a diet rich in omega-6 PUFAs, in a 2 × 2 design. We subsequently tested the adult offspring for sociability, anxiety, and locomotor behaviors using a 3-chambered social approach task, an elevated-plus maze, an open field task and a rotarod task. Results indicated that a maternal diet rich in omega-6 PUFAs during gestation and lactation produce changes in sociability consistent with those observed in ASD. Additionally, offspring exposed to a diet rich in omega-6 PUFAs during gestation and lactation had increased levels of anxiety in the elevated-plus maze. Prenatal stress had no effect on offspring behavior. These findings provide evidence for a possible environmental risk factor that contributes to the production of autistic-like behavior in mice.

Introduction

Autism spectrum disorders (ASD) are a set of neurodevelopmental disorders classified by deficits in social interaction, social communication, and the presence of restrictive and repetitive behaviors [1]. Currently, the prevalence rate of ASD is thought to be as high as one individual in every 88 [2]. While the etiology of ASD is unknown, genetics is thought to play a large role in the disorder [3]. Previous twin studies have shown the monozygotic concordance rate to be approximately 70%, and dizygotic concordance rates to be approximately 5% [4], [5], [6]. While genetics play a large role in the etiology of ASD, evidence suggests that environmental factors also contribute to the disorder. Furthermore, a more recent twin study has suggested that susceptibility to autism has a lesser genetic heritability component and a greater environmental component than previously believed [7].

One environmental risk factor for autism appears to be prenatal stress. Mothers of children with autism have reported high family discord and psychiatric problems during their pregnancies in comparison to mothers of nonautistic children, which suggests prenatal stress may be a potential risk factor of autism [8]. Additionally, a 2005 study surveyed mothers of children with autism, Down syndrome, and control children on their level of stress during pregnancy. Stress was reported to be significantly greater in mothers of children with autism during weeks 21–32 of gestation in comparison to mothers of children with Down syndrome or control children, with a peak at 25–28 weeks of gestation [9]. A Louisiana study found an increase in ASD birth rates among mothers living in proximity to hurricane and tropical storm landfall during months 5 and 6 of gestation. The severity of the hurricane or tropical storm occurring during this 5–6 months period of gestation also correlated with a greater incidence of ASD births [10]. While these studies suggest that prenatal stress serves as a risk factor for autism, many of the pregnancies reported to be associated with stress resulted in children without autism. Therefore, while prenatal stress serves as a risk factor of autism, it alone cannot account for the development of autism in all cases. A recent animal study found that prenatal stress in combination with a stress-susceptible maternal genotype produced autistic-like social deficits in a rodent model of autism [11].

In addition to prenatal stress's role in resulting behavior, prenatal stress has been shown to have significant implications for offspring brain development as well. Maternal stress during gestation is associated with reduced blood flow to the fetus via the placenta [12]. Additionally, fetal cortisol levels are linked to maternal cortisol levels, indicating that stress during gestation increases fetal exposure to cortisol [13]. Stress initiates a cascade of events resulting in elevated glucocorticoids and brain catecholamines [14] that, if present during prenatal brain development, induce marked disruptors of structure and function [15], [16]. Prenatal stress is linked to alterations in proteins and cellular mechanisms involved in neuronal plasticity that influence the expression of trophic factors [17], [18], changes in neurogenesis [19], [20] and NMDA receptor-dependent synaptogenesis [21].

An additional environmental factor that may contribute to the etiology of autism is diet. One dietary variable that has received recent attention is polyunsaturated fatty acids (PUFAs), particularly the decline in omega-3 PUFAs in the modern Western diet [22]. Currently, modern Western diets are rich in omega-6 (n-6) PUFAs and deficient in omega-3 (n-3) PUFAs [23]. The ratio of n-6 to n-3 in the traditional hunter-gatherer diet was approximately 2:1 to 3:1, whereas the ratio in a modern Western diet is approximately 15:1 to 17:1 [23]. In extreme cases, the ratio may even reach 50:1, caused by an excess of n-6 PUFAs and a severe deficiency in n-3 PUFAs [23]. As long-chain PUFAs (LC-PUFAs) are not endogenously made in humans, they must be obtained from the diet [22]. While the level of the n-6 PUFA arachidonic acid (AA) is relatively consistent worldwide, n-3 PUFAs DHA and eicosapentaenoic acid (EPA) are more variable [24], resulting in the n-6:n-3 ratio imbalance. Omega-3 polyunsaturated fatty acids (PUFAs), specifically docosahexaenoic acid (DHA), play a central role in the functioning and development of the brain and central nervous system [22], [24], [25]. During gestation and lactation, infants depend on the mother for a steady supply of LC-PUFAs [24]. A maternal diet deficient in n-3 PUFAs during gestation and lactation has been associated with lower scores on tests of cognitive function [26], [27]. Additionally, it appears that a maternal diet during gestation that is rich in n-6 PUFAs and/or is deficient in n-3 PUFAs leads to impaired neurological functioning, which may manifest in the form of developmental and behavioral disorders [22]. For example, a diet deficient in DHA and rich in AA during development impairs neurogenesis, dendritic arborization, synaptogenesis, selective pruning, and myelination [28]. The n-6 and n-3 fatty acids also regulate carbohydrate and lipid metabolism through effects on gene expression involving steroid regulatory element binding proteins and peroxisomal proliferator activated receptors [28]. Depletion of DHA from the brain and retina results in reduced visual function, cognitive and behavioral abnormalities, altered monaminergic neurotransmitter metabolism, and decreased membrane protein and receptor activities [28].

Diets rich in n-6 PUFAs are known to have an impact on immune function, as their biological derivatives are generally pro-inflammatory [23]. Interestingly, children with autism have a higher observed ratio of n-6 PUFAs to n-3 PUFAs compared to control children [29]. As immune dysregulation has been frequently described in individuals with autism as well as their family members [30], a diet rich in n-6 PUFAs may contribute in this manner to the etiology of autism. Finally, a 2007 study found that individuals with a high n-6:n-3 ratio diet were found to be more susceptible to the psychological and immunological impact of stress than those with a more balanced n-6:n-3 ratio [31]. These studies collectively warrant investigation of the role of PUFAs in the development of ASD.

While other research has investigated the independent effect of prenatal stress on the etiology of autism, none have investigated the importance of prenatal n-3 and n-6 PUFAs on the etiology of autism or have investigated the combined effect of these factors on autistic-like behavior. Therefore, we wished to examine the effect of maternal dietary PUFAs on social behavior in offspring, and the potential interaction with prenatal stress in this phenomenon.