Research ReportMalformation of the superior olivary complex in an animal model of autism
Highlights
► The superior olive complex (SOC) is located in the brainstem and functions in hearing. ► Auditory dysfunction is a common feature of autism. ► In autism, the superior olive has fewer neurons and abnormal neuronal morphology. ► Prenatal exposure to valproic acid (VPA) is an animal model of autism. ► In animals prenatally exposed to VPA there are fewer SOC neurons which have abnormal morphology.
Introduction
Autism is a neurodevelopmental disorder characterized by varying degrees of social and communicative impairments and restricted, repetitive behaviors (Allen, 1988, Wing, 1997). Neuronal dysmorphology appears to be a hallmark of the autistic brain, which includes alterations in neuronal number, packing density, cell body size and dendrite branching (Bauman and Kemper, 1985, Gaffney et al., 1988, Kulesza and Mangunay, 2008, Kulesza et al., 2011, Palmen et al., 2004, Piven et al., 1992, Ritvo et al., 1986, Schumann and Amaral, 2006). Further, the majority of individuals with autism demonstrate some degree of auditory dysfunction, ranging from deafness and increased thresholds to hyperacusis and difficulty listening in the presence of background noise (Alcantara et al., 2004, Gravel et al., 2006, Greenspan and Wieder, 1997, Kellerman et al., 2005, Khalfa et al., 2004, Lepistö et al., 2005, Roper et al., 2003, Rosenhall et al., 1999, Szelag et al., 2004, Teder-Salejarvi et al., 2005, Tharpe et al., 2006, Tomchek and Dunn, 2007). Additionally, studies of the auditory brainstem response (ABR) in autistic individuals implicate dysfunction in lower brainstem centers (Gillberg et al., 1983, Klin, 1993, Kwon et al., 2007, Maziade et al., 2000, McClelland et al., 1992, Rosenhall et al., 2003, Rumsey et al., 1984, Skoff et al., 1980). These findings, taken together, suggest involvement of the lower auditory brainstem in the hearing deficits that are so common in autism.
The superior olivary complex (SOC) is a conglomerate of nuclei within the lower brainstem which functions in localization of sound sources, encoding temporal features of sound and descending modulation of the cochlear nucleus and cochlea for listening in background noise (see reviews by Heffner and Masterton, 1990, Oliver, 2000, Schofield, 2010, Schwartz, 1992, Spangler and Warr, 1991, Thompson and Schofield, 2000). The SOC includes the medial and lateral superior olives (MSO and LSO, respectively), the superior paraolivary nucleus (SPON) and the medial, ventral and lateral nuclei of the trapezoid body (MNTB, VNTB and LNTB respectively). In support of auditory brainstem dysfunction in autism, we have recently reported consistent and significant malformations in the human SOC in a series of fourteen postmortem human autistic specimens (Kulesza and Mangunay, 2008, Kulesza et al., 2011). In the autistic SOC, we find significant alterations in cell body size, shape, orientation and significantly fewer neurons. We propose that these malformations contribute to the auditory deficits observed in autism and provide the foundation for a systematic examination of the auditory system in a controlled model of autism. Finally, although we have documented significant alterations in neuronal morphology and number, the precise implication of this dysmorphology on circuitry and physiology is unclear.
Prenatal exposure to valproic acid (VPA; an antiepileptic with known teratogenic qualities) has been implicated in autism in humans and reproduces morphological and functional malformations associated with autism in rats (Bromley et al., 2008, Christianson et al., 1994, Ingram et al., 2000, Manent et al., 2007, Williams et al., 2001). Thus, prenatal exposure to VPA is considered an animal model of autism (Alsdorf and Wyszynski, 2005, Arndt et al., 2005, Ingram et al., 2000, Kuwagata et al., 2009, Rodier et al., 1996). Specifically, prenatal VPA-exposure has been shown to reduce the number of neurons in brainstem motor nuclei and cerebellar vermis, increase the complexity of pyramidal cell dendritic arborizations and affect axonal projections (Ingram et al., 2000, Rinaldi et al., 2008a, Rinaldi et al., 2008b, Rodier et al., 1996, Snow et al., 2008). Additionally, the developmental migration of serotoninergic and dopaminergic neurons and forebrain expression of enkephalin are disrupted following prenatal exposure to VPA (Dufour-Rainfray et al., 2010, Kuwagata et al., 2009, Schneider et al., 2007). Finally, a number of authors have demonstrated behavioral anomalies in prenatally VPA-exposed rats and mice (Dufour-Rainfray et al., 2010, Murawski et al., 2009, Narita et al., 2010, Roullet et al., 2010, Schneider et al., 2007, Stanton et al., 2007, Vorhees, 1987). Thus, our hypothesis is that prenatal VPA exposure will replicate the malformations we have described in the SOC of the autistic brain. Our objective here is to demonstrate the feasibility of prenatal VPA exposure as an animal model of autism in the auditory system of rats.
Section snippets
Results
Overall, prenatal VPA-exposure results in significant dysmorphology in each nucleus of the rat SOC. However, the basic organization of the SOC in VPA-exposed animals was generally intact; all SOC nuclei could be identified in each of the experimental animals (Fig. 1A and B). However, we observe a significant decrease in the total number of neurons in the SOC of VPA-exposed animals (Fig. 2A and B). The six control SOC nuclei combined averaged 17,562 ± 395 (standard error) neurons while the same
General comments
This work constitutes the first detailed morphological investigation of the auditory brainstem in an animal model of autism. We demonstrate that prenatal exposure to VPA on E12.5 in rats results in fewer neurons and disrupted neuronal morphology in the SOC. These results are of major clinical significance, as we have described similar dysmorphology and loss of neurons in the SOC of children, adolescents and adults diagnosed with autism (Kulesza and Mangunay, 2008, Kulesza et al., 2011).
Implications and etiology
In the
Specimens
This study is based on the analysis of six SOC nuclei in seven control and eight experimental rat brainstems. Experimental animals were exposed to valproic acid (VPA) in the following manner: timed pregnant control females were injected with a 600 mg/kg solution of VPA on day 12.5 of gestation. Control and exposed rats, between the ages of 90 and 120 days, were anesthetized with pentobarbital (80 mg/kg) and perfused through the ascending aorta with a solution of normal saline followed by a
Acknowledgments
The authors would like to thank Dr. Bertalan Dudas and the LECOM Research Collective for continued support and Dr. Mark Stanton (University of Delaware) for his assistance and insight in the prenatal VPA exposure model.
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2022, Brain ResearchCitation Excerpt :Accordingly, timed in utero exposure to VPA is a biologically relevant and validated animal model of ASD (rodents: Rodier et al., 1996; Mabunga et al., 2015; primates: Zhao et al., 2019). In fact, many of the morphological changes identified in the auditory brainstem of human subjects with ASD are also found in VPA-exposed rodents (human: Kulesza and Mangunay, 2008; Kulesza et al., 2011; Lukose et al., 2015; Mansour and Kulesza, 2021a; rodent: Lukose et al., 2011; Zimmerman et al., 2018; Mansour et al., 2021; Mansour and Kulesza, 2021b). Specifically, in utero VPA exposure results in significantly fewer neurons throughout the auditory brainstem, reduced axonal projections to the inferior colliculus and medial geniculate, reduced expression of the calcium binding protein calbindin (CB) and calretinin (CR), and abnormal responses to pure tone stimuli (Lukose et al., 2011; Dubiel and Kulesza, 2015; Zimmerman et al., 2018; Mansour et al., 2019; Mansour and Kulesza, 2021b).
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2022, Brain ResearchCitation Excerpt :It has also been reported that the expression level of potassium channels, such as Kv10.2, in the hippocampus of VPA-treated rats is reduced, which is associated with a high-frequency neuronal activity (Wang et al. 2019). Exposure to VPA during pregnancy has also been shown to induce morphological changes and hypocellularity in several brain areas in offspring (Snow et al., 2008; Lukose et al., 2011; Hara et al., 2012; Yang et al., 2016; Hajisoltani et al., 2019; Sandhya et al.,2012; Gogolla et al., 2009). Previous studies have shown a correlation between neuronal morphology and intrinsic neuronal excitability, so that an increase in the surface area of neuronal soma dampens intrinsic excitability (Matovic et al., 2020), whereas a decrease in the cell surface area, which is associated with increased input resistance induces neuronal hyperexcitability (Pilarski et al., 2011).
Degraded cortical temporal processing in the valproic acid-induced rat model of autism
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