Intracerebroventricular injection of propionic acid, an enteric bacterial metabolic end-product, impairs social behavior in the rat: Implications for an animal model of autism
Introduction
Autism spectrum disorder (ASD) is a class of neurodevelopmental disorders affecting approximately 1 in 166 children (DiCicco-Bloom et al., 2006). Behavioral symptoms associated with ASD include abnormal motor behaviors, repetitive interests and behaviors, cognitive deficits, seizures, and impaired social interactions (Arndt et al., 2005, DiCicco-Bloom et al., 2006). While much research supports a strong multigenetic basis to ASD (DiCicco-Bloom et al., 2006), other evidence suggests that environmental, dietary, and gastrointestinal factors may also contribute to the disorder (Arndt et al., 2005, Horvath et al., 1999, Jyonouchi et al., 2002). There is evidence suggesting that exposure to propionic acid (PPA), 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 bacteria found in the gut (Nyhan et al., 1999, Thompson et al., 1990), may be associated with ASD. Anecdotal evidence from parents of ASD children suggests that ASD symptoms increase when their children ingest foods such as refined wheat and dairy products that contain high levels of PPA as a food preservative (Horvath et al., 1999, Jyonouchi et al., 2002). Research has also found that ASD patients may have elevated levels of Clostridia, an early gut colonizer that produces PPA and other short chain fatty acids (Song et al., 2004). Rat models of propionic acidemia or Huntington's disease that make use of PPA or 3-nitropropionic acid (3NP), a derivative of PPA, have yielded brain markers or behavioral symptoms that resemble certain markers and symptoms of human ASD (Borlongan et al., 1995, Brusque et al., 1999, Shear et al., 2000). These include neuroinflammation, neurodevelopmental delay with cognitive deficits, and impaired motor function (Andres, 2002, Vargas et al., 2004, Zwaigenbaum et al., 2005).
Although most of PPA originally accumulates in the gut, PPA readily crosses the gut–blood and blood–brain barriers and can gain access to the CNS. Here it can cross cell membranes and accumulate within cells, inducing intracellular acidification (Bonnet et al., 2000, Karuri et al., 1993), which can alter neurotransmitter release and inhibit gap junctions, potentially altering neuronal communication and behavior (Cannizzaro et al., 2003, Severson et al., 2003). We recently proposed that alterations of PPA levels or metabolism might be related to some symptoms of ASD, and that administration of PPA might be a means of modeling ASD symptoms in the rat (MacFabe et al., 2007). We found that ICV injections of PPA induced repetitive behaviors, hyperactivity, turning behavior, retropulsion, kindled seizures, widespread oxidative stress, and a neuroinflammatory response (MacFabe et al., 2008), all of which appear consistent with symptoms of ASD (Andres, 2002, Vargas et al., 2004, Zwaigenbaum et al., 2005). To date no study has investigated the effects of PPA on social behavior. Impairments in social behavior, including impairments in play behavior and other forms of social contact, are among the most salient symptoms of ASD (Arndt et al., 2005, DiCicco-Bloom et al., 2006, Zwaigenbaum et al., 2005). Therefore, we examined the effects of PPA on social behavior in the rat using the same route of administration and dose of PPA employed previously (MacFabe et al., 2007).
Section snippets
Subjects
A total of 114 adult male Long-Evans hooded rats obtained from Charles River Laboratories (Quebec, Canada) were used. Prior to surgery for implantation of an indwelling guide cannula for ICV injections, rats weighed between 200 and 250 g, were housed in pairs in standard acrylic cages (26 cm × 48 cm × 21 cm) at a controlled temperature (21 ± 1 °C), and were naïve to all experimental procedures. After surgery rats were housed individually for 14 days to allow recovery. The light/dark cycle was a 12:12 cycle
Mean distance apart, light phase
During the light phase the PPA and SA groups exhibited greater mean distances apart than the PBS control group during most time periods (see Fig. 1A). The PPA and SA groups exhibited comparable mean distances apart during the first 10 min of testing, but by the last 10 min the SA group appeared to be similar to PBS controls, whereas the PPA group exhibited a greater mean distance apart than PBS controls.
These impressions were confirmed by ANOVA, with the finding of a significant time × treatment
Discussion
The present results indicate that PPA consistently impaired rat social behavior measured as distance apart, proximity, and play behavior. With one exception these outcomes were obtained during both light and dark phases of testing. The only exception was the failure of PPA to significantly alter the probability of defense during the dark phase, relative to PBS controls. However, PPA altered the nature of the defenses that were displayed, with more evasive defenses and fewer facing defenses than
Acknowledgements
The authors thank Francis Boon and Yalda Mohammad Asef for their technical assistance. This research was supported by contributions from GoodLife Children's Foundation and Round for a Reason Charities to Derrick Fraser MacFabe, by scholarships from OGS to Sandy Richard Shultz, and by a grant from the Natural Science and Engineering Research Council of Canada to Donald Peter Cain.
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