Elsevier

Behavioural Brain Research

Volume 217, Issue 1, 2 February 2011, Pages 47-54
Behavioural Brain Research

Research report
Effects of the enteric bacterial metabolic product propionic acid on object-directed behavior, social behavior, cognition, and neuroinflammation in adolescent rats: Relevance to autism spectrum disorder

https://doi.org/10.1016/j.bbr.2010.10.005Get rights and content

Abstract

Recent evidence suggests that a variety of environmental factors, including dietary and gastrointestinal agents, may contribute to autism spectrum disorders (ASD). Here we administered propionic acid (PPA), a short chain fatty acid that is used as a food preservative and also is a metabolic end-product of enteric bacteria in the gut, to adolescent (41 ± 4 days) male rats in a study of restricted/repetitive behavior, social behavior, and cognition. The goal was to further evaluate the effects of PPA in young rodents. PPA (4 μl of 0.26 M solution) was administered intracerebroventricularly prior to each behavioral test. Rats treated with PPA displayed restricted behavioral interest to a specific object among a group of objects, impaired social behavior, and impaired reversal in a T-maze task compared to controls given phosphate buffered saline. Immunohistochemical analysis of brain tissue from PPA rats revealed reactive astrogliosis and activated microglia, indicating an innate neuroinflammatory response.

These findings are consistent with our earlier findings of ASD-relevant behavioral and brain events in adult rats given PPA, and support further study of effects of PPA in young rodents by establishing similar effects in adolescent animals.

Research highlights

▶ Effects of propionic acid (ICV) in an adolescent rat model of ASD were studied. ▶ Propionic acid increased restricted/repetitive behavior toward objects. ▶ Propionic acid impaired social behavior in a novel rat–novel object choice test. ▶ Propionic acid impaired reversal learning, but not acquisition, in a T-maze. ▶ Results broaden the relevance of the propionic acid rat model of ASD.

Introduction

The prevalence of autism spectrum disorders (ASD) is approximately 1 in 110 children [17]. Behavioral symptoms of ASD include restrictive and/or repetitive interests and behaviors, impaired social behavior, cognitive impairment, and convulsions, suggesting broad neurodevelopmental abnormality in ASD [7], [22]. Although there is a strong genetic component to the etiology of ASD [22], [43], recent research suggests that ASD can be exacerbated by a number of environmental factors in sensitive sub-populations [28]. Recent studies suggest a link between dietary factors or gastrointestinal disturbances and ASD symptoms, but the exact mechanisms by which such factors might contribute to ASD are not clear [7], [29], [32]. Some clinical studies have also found that a subset of ASD patients have high levels of Clostridia or Bacterioidetes in the gut, which produce propionic acid (PPA) and other fatty acids by anaerobic fermentation of dietary carbohydrates and some amino acids [26], [54]. PPA is a short chain fatty acid that is endogenous to the human body as both an intermediary of fatty acid metabolism and a metabolic endproduct of enteric gut bacteria such as clostridia and propionibacteria [4], [40], [58], [67]. Parents of some ASD children report that ASD symptoms are exacerbated when the children crave and consume processed wheat or dairy products that contain PPA as a food preservative [29], [32]. Rat models of propionic acidemia based on administration of PPA or 3-nitropropionic acid (3NP), a derivative of PPA, have revealed behavioral symptoms and brain markers consistent with human ASD, including developmental delay with cognitive impairments, and neuroinflammation [6], [12], [48], [59]. Consistent with such effects on brain and behavior, PPA readily crosses the gut–blood and blood–brain barriers by both passive and active means [33], thus potentially gaining access to the brain where it can accumulate in cells and alter multiple neurophysiological processes, including neurotransmitter release, gene expression, mitochondrial function, immune modulation, gap junction gating, and ultimately behavior [15], [35], [47].

There is a need for a defined set of behavioral tasks relevant to the symptoms of ASD in animal models of the disorder (see [20], [53]). Earlier studies with PPA in our laboratory found that intraventricular administration of PPA to adult rats induced repetitive behaviors, impairments in cognition and social behavior, and brain events including epileptiform spiking in neocortex, hippocampus and caudate nucleus, seizures with convulsions, increases in oxidative stress markers, reductions in glutathione, alterations of brain phospholipids/acylcarnitines and an innate neuroinflammatory response [35], [36], [51], [52], [57]. These outcomes appear to be consistent with ASD behavioral symptoms and brain events [6], [18], [20], [22], [59]. Adolescence is a key developmental period, with exacerbation of many ASD associated symptoms [42]. As part of a systematic investigation of PPA with young rats, this study examined the effects of PPA in adolescent rats. Data were obtained using a test of interaction with multiple objects to study restricted/repetitive behavior, a test of object vs. rat interaction to study social impairment, a T-maze test of cognition, and a measure of locomotor activity. We hypothesized that PPA treatment would increase restricted/repetitive behaviors, and impair social behavior and cognition [51], [52] in adolescent rats. At the completion of the study brain tissue was examined using neuropathological markers for innate neuroinflammation [59].

Section snippets

Subjects

Long–Evans male hooded rats were obtained at age 26 days from Charles River Laboratories (Quebec, Canada) and housed in groups of 3 or 4 at 21 ± 1 °C in acrylic cages (26 cm × 48 cm × 21 cm) for 8 days for acclimation to the animal colony, with lights on from 7:00 to 19:00 h and access to food (LabDiet RMH 3000) and water ad libitum. Post-surgical housing was individual for 7 days to allow recovery. Procedures complied with Canadian Council on Animal Care guidelines and were approved by the University

Object-directed behavior

Preliminary analysis revealed no consistency within or across groups in the specific novel object that was interacted with the most. Therefore for graphing and analysis purposes, for each rat the objects were rank ordered from 1 through 3 on each behavioral measure to indicate greater-to-less interaction/interest in the objects. Thus, Object 1 was, for each rat, the object that the rat interacted with the most, Object 2 was the object that the rat interacted with second most, and Object 3 was

Discussion

The results show that PPA treatment increased restrictive/repetitive behaviors in an object choice test, impaired social behavior in a rat vs. object choice test, and impaired reversal learning in a T-maze task in adolescent rats relative to PBS controls. PPA significantly increased GFAP immunoreactivity in hippocampal areas CA1/CA2 and in white matter adjacent to hippocampus, and there was a nonsignificant trend for similar increases in CD68 immunoreactivity in the same brain structures. These

Acknowledgements

We thank Karen Jameson and Lisa Tichenoff for excellent technical assistance in immunohistochemistry quantification. This research was supported by GoodLife Children's Foundations (to DFM) and The Natural Sciences and Engineering Research Council of Canada (to DPC).

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