Melissa D. Bauman

The amygdala and autism: implications from non-human primate studies.

Brothers (1990) has proposed that the amygdala is an important component of the neural network that underlies social behavior. Kemper and Bauman (1993) identified neuropathology in the amygdala of the postmortem autistic brain. These findings, along with recent functional neuroimaging data, have led Baron‐Cohen et al. (2000) to propose that dysfunction of the amygdala may be responsible, in part, for the impairment of social behavior that is a hallmark feature of autism. Recent data from studies in our laboratory on the effects of amygdala lesions in the adult and infant macaque monkey do not support a fundamental role for the amygdala in social behavior. If the amygdala is not essential for the component processes of social behavior, as seems to be case in both non‐human primates and selected patients with bilateral amygdala damage, then it is unlikely to be the primary substrate for the impaired social behavior of autism. However, damage to the amygdala does have an effect on a monkeys response to normally fear‐inducing stimuli, such as snakes, and removes a natural reluctance to engage novel conspecifics in social interactions. These findings lead to the conclusion that an important role for the amygdala is in the detection of threats and mobilizing an appropriate behavioral response, part of which is fear. Interestingly, an important comorbid feature of autism is anxiety ( Muris et al. 1998). If the amygdala is pathological in subjects with autism, it may contribute to their abnormal fears and increased anxiety rather than their abnormal social behavior.

Activation of the maternal immune system during pregnancy alters behavioral development of rhesus monkey offspring.

BACKGROUND Maternal infection during pregnancy is associated with an increased risk of schizophrenia and autism in the offspring. Supporting this correlation, experimentally activating the maternal immune system during pregnancy in rodents produces offspring with abnormal brain and behavioral development. We have developed a nonhuman primate model to bridge the gap between clinical populations and rodent models of maternal immune activation (MIA). METHODS A modified form of the viral mimic, synthetic double-stranded RNA (polyinosinic:polycytidylic acid stabilized with poly-L-lysine) was delivered to two separate groups of pregnant rhesus monkeys to induce MIA: 1) late first trimester MIA (n = 6), and 2) late second trimester MIA (n = 7). Control animals (n = 11) received saline injections at the same first or second trimester time points or were untreated. Sickness behavior, temperature, and cytokine profiles of the pregnant monkeys confirmed a strong inflammatory response to MIA. RESULTS Behavioral development of the offspring was studied for 24 months. Following weaning at 6 months of age, MIA offspring exhibited abnormal responses to separation from their mothers. As the animals matured, MIA offspring displayed increased repetitive behaviors and decreased affiliative vocalizations. When evaluated with unfamiliar conspecifics, first trimester MIA offspring deviated from species-typical macaque social behavior by inappropriately approaching and remaining in immediate proximity of an unfamiliar animal. CONCLUSIONS In this rhesus monkey model, MIA yields offspring with abnormal repetitive behaviors, communication, and social interactions. These results extended the findings in rodent MIA models to more human-like behaviors resembling those in both autism and schizophrenia.

The Development of Social Behavior Following Neonatal Amygdala Lesions in Rhesus Monkeys

We examined the role of the amygdala in the development of nonhuman primate social behavior. Twenty-four rhesus monkeys received bilateral ibotenic acid lesions of either the amygdala or the hippocampus or received a sham surgical procedure at 2 weeks of age. Subjects were reared with their mothers and were provided daily access to social rearing cohorts. The subjects were weaned at 6 months of age and then observed while paired with familiar conspecifics at 6 and 9 months of age and with unfamiliar conspecifics at 1 year of age. The subjects were also observed during daily cohort socialization periods. Neither amygdala nor hippocampus lesions altered fundamental aspects of social behavior development. All subjects, regardless of lesion condition, developed a species-typical repertoire of social behavior and displayed interest in conspecifics during social encounters. The amygdala lesions, however, clearly affected behaviors related to fear processing. The amygdala-lesioned subjects produced more fear behaviors during social encounters than did control or hippocampus-lesioned subjects. Although the heightened fear response of the amygdala-lesioned subjects was consistent across different testing paradigms and was observed with both familiar and novel partners, it did not preclude social interactions. In fact, the amygdala-lesioned subjects displayed particular social behaviors, such as following, cooing, grunting, presenting to be groomed, and presenting to be mounted more frequently than either control or hippocampus-lesioned subjects. These findings are consistent with the view that the amygdala is not needed to develop fundamental aspects of social behavior and may be more related to the detection and avoidance of environmental dangers.

Maternal antibodies from mothers of children with autism alter brain growth and social behavior development in the rhesus monkey

Antibodies directed against fetal brain proteins of 37 and 73 kDa molecular weight are found in approximately 12% of mothers who have children with autism spectrum disorder (ASD), but not in mothers of typically developing children. This finding has raised the possibility that these immunoglobulin G (IgG) class antibodies cross the placenta during pregnancy and impact brain development, leading to one form of ASD. We evaluated the pathogenic potential of these antibodies by using a nonhuman primate model. IgG was isolated from mothers of children with ASD (IgG-ASD) and of typically developing children (IgG-CON). The purified IgG was administered to two groups of female rhesus monkeys (IgG-ASD; n=8 and IgG-CON; n=8) during the first and second trimesters of pregnancy. Another control group of pregnant monkeys (n=8) was untreated. Brain and behavioral development of the offspring were assessed for 2 years. Behavioral differences were first detected when the macaque mothers responded to their IgG-ASD offspring with heightened protectiveness during early development. As they matured, IgG-ASD offspring consistently deviated from species-typical social norms by more frequently approaching familiar peers. The increased approach was not reciprocated and did not lead to sustained social interactions. Even more striking, IgG-ASD offspring displayed inappropriate approach behavior to unfamiliar peers, clearly deviating from normal macaque social behavior. Longitudinal magnetic resonance imaging analyses revealed that male IgG-ASD offspring had enlarged brain volume compared with controls. White matter volume increases appeared to be driving the brain differences in the IgG-ASD offspring and these differences were most pronounced in the frontal lobes.

Maternal Immune Activation in Nonhuman Primates Alters Social Attention in Juvenile Offspring

BACKGROUND Sickness during pregnancy is associated with an increased risk of offspring neurodevelopmental disorders. Rodent models have played a critical role in establishing causal relationships and identifying mechanisms of altered brain and behavior development in pups prenatally exposed to maternal immune activation (MIA). We recently developed a novel nonhuman primate model to bridge the gap between human epidemiological studies and rodent models of prenatal immune challenge. Our initial results demonstrated that rhesus monkeys given the viral mimic synthetic double-stranded RNA (polyinosinic:polycytidylic acid stabilized with poly-l-lysine) during pregnancy produce offspring with abnormal repetitive behaviors, altered communication, and atypical social interactions. METHODS We utilized noninvasive infrared eye tracking to further evaluate social processing capabilities in a subset of the first trimester MIA-exposed offspring (n = 4) and control animals (n = 4) from our previous study. RESULTS As juveniles, the MIA offspring differed from control animals on several measures of social attention, particularly when viewing macaque faces depicting the fear grimace facial expression. Compared with control animals, MIA offspring had a longer latency before fixating on the eyes, had fewer fixations directed at the eyes, and spent less total time fixating on the eyes of the fear grimace images. CONCLUSIONS In the rhesus monkey model, exposure to MIA at the end of the first trimester results in abnormal gaze patterns to salient social information. The use of noninvasive eye tracking extends the findings from rodent MIA models to more human-like behaviors resembling those in both autism spectrum disorder and schizophrenia.

Maternal Immune Activation and Autism Spectrum Disorder: From Rodents to Nonhuman and Human Primates

A subset of women who are exposed to infection during pregnancy have an increased risk of giving birth to a child who will later be diagnosed with a neurodevelopmental or neuropsychiatric disorder. Although epidemiology studies have primarily focused on the association between maternal infection and an increased risk of offspring schizophrenia, mounting evidence indicates that maternal infection may also increase the risk of autism spectrum disorder. A number of factors, including genetic susceptibility, the intensity and timing of the infection, and exposure to additional aversive postnatal events, may influence the extent to which maternal infection alters fetal brain development and which disease phenotype (autism spectrum disorder, schizophrenia, other neurodevelopmental disorders) is expressed. Preclinical animal models provide a test bed to systematically evaluate the effects of maternal infection on fetal brain development, determine the relevance to human central nervous system disorders, and to evaluate novel preventive and therapeutic strategies. Maternal immune activation models in mice, rats, and nonhuman primates suggest that the maternal immune response is the critical link between exposure to infection during pregnancy and subsequent changes in brain and behavioral development of offspring. However, differences in the type, severity, and timing of prenatal immune challenge paired with inconsistencies in behavioral phenotyping approaches have hindered the translation of preclinical results to human studies. Here we highlight the promises and limitations of the maternal immune activation model as a preclinical tool to study prenatal risk factors for autism spectrum disorder, and suggest specific changes to improve reproducibility and maximize translational potential.

Preliminary evidence of neuropathology in nonhuman primates prenatally exposed to maternal immune activation

Maternal infection during pregnancy increases the risk for neurodevelopmental disorders in offspring. Rodent models have played a critical role in establishing maternal immune activation (MIA) as a causal factor for altered brain and behavioral development in offspring. We recently extended these findings to a species more closely related to humans by demonstrating that rhesus monkeys (Macaca mulatta) prenatally exposed to MIA also develop abnormal behaviors. Here, for the first time, we present initial evidence of underlying brain pathology in this novel nonhuman primate MIA model. Pregnant rhesus monkeys were injected with a modified form of the viral mimic polyI:C (poly ICLC) or saline at the end of the first trimester. Brain tissue was collected from the offspring at 3.5 years and blocks of dorsolateral prefrontal cortex (BA46) were used to analyze neuronal dendritic morphology and spine density using the Golgi-Cox impregnation method. For each case, 10 layer III pyramidal cells were traced in their entirety, including all apical, oblique and basal dendrites, and their spines. We further analyzed somal size and apical dendrite trunk morphology in 30 cells per case over a 30 μm section located 100±10 μm from the soma. Compared to controls, apical dendrites of MIA-treated offspring were smaller in diameter and exhibited a greater number of oblique dendrites. These data provide the first evidence that prenatal exposure to MIA alters dendritic morphology in a nonhuman primate MIA model, which may have profound implications for revealing the underlying neuropathology of neurodevelopmental disorders related to maternal infection.

Emergence of Stereotypies in Juvenile Monkeys (Macaca mulatta) With Neonatal Amygdala or Hippocampus Lesions

The emergence of stereotypies was examined in juvenile rhesus monkeys (Macaca mulatta) who, at 2 weeks of postnatal age, received selective bilateral ibotenic acid lesions of the amygdala (N = 8) or hippocampus (N = 8). The lesion groups were compared to age-matched control subjects that received a sham surgical procedure (N = 8). All subjects were maternally reared for the first 6 months and provided access to social groups throughout development. Pronounced stereotypies were not observed in any of the experimental groups during the first year of life. However, between 1 to 2 years of age, both amygdala- and hippocampus-lesioned subjects began to exhibit stereotypies. When observed as juveniles, both amygdala- and hippocampus-lesioned subjects consistently produced more stereotypies than the control subjects in a variety of contexts. More interesting, neonatal lesions of either the amygdala or hippocampus resulted in unique repertoires of repetitive behaviors. Amygdala-lesioned subjects exhibited more self-directed stereotypies and the hippocampus-lesioned subjects displayed more head-twisting. We discuss these results in relation to the neurobiological basis of repetitive stereotypies in neurodevelopmental disorders, such as autism.

Neonatal Amygdala Lesions Result in Globally Blunted Affect in Adult Rhesus Macaques

The amygdala has been implicated in affective and social processing for more than a century. Animals with damage to the amygdala have altered affective and social behavior patterns, though the precise nature of these behavioral changes depends on a number of factors including lesion technique, age of the subject at the time of lesion, rearing, and housing environments. Interpretations of amygdala lesion studies are further complicated by the potentially confounded nature of affective and social stimuli (e.g., social interactions with a conspecific partner that is consistently aggressive). In the present study, we evaluated affective responding to affectively and socially evocative video stimuli in a group of rhesus macaques that received bilateral amygdala lesions as neonates. The stimuli were produced to vary independently in terms of their affective and social content. The responses of the amygdala-lesioned animals were compared with a group of age-matched controls and a group of animals that had sustained bilateral hippocampus damage as neonates. As compared with control animals, amygdala-lesioned animals had blunted responding to both positive and negative stimuli, regardless of social content, but did differentiate between levels of social content. Taken together, these findings suggest that early amygdala damage permanently compromises affective processing while leaving intact the ability to distinguish between socially meaningful contexts.

The impact of early amygdala damage on juvenile rhesus macaque social behavior

The present experiments continue a longitudinal study of rhesus macaque social behavior following bilateral neonatal ibotenic acid lesions of the amygdala or hippocampus, or sham operations. Juvenile animals (approximately 1.5–2.5 years) were tested in four different social contexts—alone, while interacting with one familiar peer, while interacting with one unfamiliar peer, and in their permanent social groups. During infancy, the amygdala-lesioned animals displayed more interest in conspecifics (indexed by increased affiliative signaling) and paradoxically demonstrated more submission or fear (Bauman, Lavenex, Mason, Capitanio, & Amaral, 2004a, this journal). When these animals were assessed as juveniles, differences were less striking. Amygdala-lesioned animals generated fewer aggressive and affiliative signals (e.g., vocalizations, facial displays) and spent less time in social interactions with familiar peers. When animals were observed alone or with an unfamiliar peer, amygdala-lesioned animals, compared with other subjects, spent more time being inactive and physically explored the environment less. Despite the subtle, lesion-based differences in the frequency and duration of specific social behaviors, there were lesion-based differences in the organization of behavior such that lesion groups could be identified based on the patterning of social behaviors in a discriminant function analysis. The findings indicate that, although overall frequencies of many of the observed behaviors do not differ between groups, the general patterning of social behavior may distinguish the amygdala-lesioned animals.