Evidence for a cerebellar role in reduced exploration and stereotyped behavior in autism.

BACKGROUND: Although limited environmental exploration in autism is an obvious behavioral feature and may be a manifestation of "restricted interests" as described in DSM-IV criteria, there have been no behavioral or neurobiological studies of this important aspect of the disorder. Given consistent reports of cerebellar abnormality in autism, combined with animal research showing a relationship between exploration and the cerebellum, this study aimed to test the possible link between cerebellar abnormality and exploration in autism. METHODS: The relationship between visuospatial exploration, stereotyped motor movements, and magnetic resonance imaging measures of the cerebellar vermis, whole brain volume, and frontal lobes in 14 autistic and 14 normal children was investigated. Children were exposed to a large room with several exploration containers and instructed to play. Exploration behavior was videotaped and scored for percentage of time engaged in exploration, number of containers explored, as well as stereotyped movements. RESULTS: Children with autism spent significantly less time in active exploration and explored fewer containers overall than normal children. Measures of decreased exploration were significantly correlated with the magnitude of cerebellar hypoplasia of vermal lobules VI-VII in the autistic children, but no relationship to vermis size was found with normal control children. Further, measures of rates of stereotyped behavior were significantly negatively correlated with area measures of cerebellar vermis lobules VI-VII and positively correlated with frontal lobe volume in the autism sample. CONCLUSIONS: Reduced environmental exploration and repetitive behavior may have particularly important developmental consequences for children with autism because it may lead them to miss learning opportunities that fall outside their scope of interest. Our findings represent the first documented link between the restricted range of interests and stereotyped behaviors pathognomonic of autism and particular neuroanatomic sites.     

The vestibulo-ocular reflex as a model system for motor learning: what is the role of the cerebellum?

Motor systems are under a continuous adaptive process to maintain behavior throughout developmental changes and disease, a process called motor learning. Simple behaviors with easily measurable inputs and outputs are best suited to understand the neuronal signals that contribute to the required motor learning. Considering simple behaviors, the vestibulo-ocular reflex (VOR) allows quantification of its input and motor output and its neural circuitry is among the best documented. The main candidates for plastic change are the cerebellum and its target neurons in the brainstem. This review focuses on recent data regarding the involvement of the cerebellum in VOR motor learning. Learning can be divided into that acutely acquired over a period of hours and that chronically acquired over longer periods. Both acute and chronic learning have three phases named acquisition, consolidation, and retention. The cerebellar role in retention is disputed, but there is a consensus on the need of an intact cerebellum for acquisition. Data from neuronal recording, lesion studies and transgenic mouse experiments is complex but suggests that the signal representation in the cerebellum contains aspects of both motor output and sensory input. The cerebellum apparently uses different mechanisms for acute and chronic learning as well as for increases and decreases in VOR gain. Recent studies also suggest that the signal content in the cerebellum changes following learning and that the mechanisms used for chronic adaptation involve not only changes in a head velocity component but also in the efference copy of an eye movement command signal reaching Purkinje cells. This data leads to a new conceptual framework having implications for developing theories on the role of the cerebellum in motor learning and in the search for plastic elements within the VOR circuitry. For chronic learning we hypothesize that changes in the head velocity information traveling through the circuitry occur in parallel with changes in the integrator pathway and the efference copy pathway. We further propose that these changes are necessary to maintain the broadband characteristics of the learned behavior.     

Fetal exposure to teratogens: evidence of genes involved in autism

Environmental challenges during the prenatal period can result in behavioral abnormalities and cognitive deficits that appear later in life such as autism. Prenatal exposure to valproic acid, ethanol, thalidomide and misoprostol has been shown to be associated with an increased incidence of autism. In addition, rodents exposed in utero to some of these drugs show autism-like abnormalities, including brain changes and lifelong behavior dysfunction. Our aim is to summarize current understanding of the relationship between in utero exposure to these drugs and autism in humans and in autism-like animal model phenotypes. It also highlights the importance of these models to understanding the neurobiology of autism, particularly in the identification of susceptibility genes. These drugs are able to modulate the expression of many genes involved in processes such as proliferation, apoptosis, neuronal differentiation and migration, synaptogenesis and synaptic activity. It seems essential to focus research on genes expressed during early neurodevelopment which may be the target of mutations or affected by drugs such as those included in this review.     

The Cerebellum and Neurodevelopmental Disorders

Cerebellar dysfunction is evident in several developmental disorders, including autism, attention deficit-hyperactivity disorder (ADHD), and developmental dyslexia, and damage to the cerebellum early in development can have long-term effects on movement, cognition, and affective regulation. Early cerebellar damage is often associated with poorer outcomes than cerebellar damage in adulthood, suggesting that the cerebellum is particularly important during development. Differences in cerebellar development and/or early cerebellar damage could impact a wide range of behaviors via the closed-loop circuits connecting the cerebellum with multiple cerebral cortical regions. Based on these anatomical circuits, behavioral outcomes should depend on which cerebro-cerebellar circuits are affected. Here, we briefly review cerebellar structural and functional differences in autism, ADHD, and developmental dyslexia, and discuss clinical outcomes following pediatric cerebellar damage. These data confirm the prediction that abnormalities in different cerebellar subregions produce behavioral symptoms related to the functional disruption of specific cerebro-cerebellar circuits. These circuits might also be crucial to structural brain development, as peri-natal cerebellar lesions have been associated with impaired growth of the contralateral cerebral cortex. The specific contribution of the cerebellum to typical development may therefore involve the optimization of both the structure and function of cerebro-cerebellar circuits underlying skill acquisition in multiple domains; when this process is disrupted, particularly in early development, there could be long-term alterations of these neural circuits, with significant impacts on behavior.     

Organizational principles and microcircuitry of the cerebellum

The cerebellum is known to influence motor behavior and to enable smooth, coordinated movements. Recent evidence also suggests that the cerebellum contributes to non-motor behavior, including components of cognition and regulation of affective state.This review summarizes the organization and circuitry of the cerebellum as a basis for understanding newly emerging concepts about the function of this neuronal system. The cerebellum consists of several divisions with separate functions. One region is associated with the vestibular system and another with brainstem and spinal cord. A third region, the cerebrocerebellum, has extensive interconnections with cerebral cortex and is likely to be involved in motor coordination and regulation of non-motor behavior. The cerebellar cortex is made up of radial modules of interconnected neurons. The Purkinje cell is the principle integrating neuron and focal point of each module. Other neuron types include the granule cell and three inhibitory interneurons. The Purkinje cell integrates excitatory inputs from climbing and parallel fibers, while its axon modulates activity of neurons in the deep nuclei, which represents the final outflow from cerebellum to other parts of the brain. Cerebellar circuitry exhibits a strong parasagittal organization based on climbing fiber input and the distributions of neuronal proteins and neuronal vulnerability to insults. The combination of this parasagittal circuitry with the mediolateral course of parallel fibers results in a Cartesian coordinate system which is likely to be a crucial factor in its signal processing function. Although numerous details of cerebellar microcircuitry, synaptic transmission and signal transduction have been determined, the functional contribution of cerebellar signalling to brain function remains highly enigmatic.     

Evidence for a deficit in procedural learning in children and adolescents with autism: implications for cerebellar contribution.

To examine the hypothesis that abnormalities in those cognitive functions for which cerebellar components have been implicated contribute to the pathophysiology of autism, tests of judgment of explicit time intervals and procedural learning were administered to 11 participants with autism and 17 age-and-IQ-matched controls. Results indicated that the group with autism demonstrated significant impairments in procedural learning compared with the group of controls. No significant difference in judgment of explicit time intervals was found. The data suggest that deficits in procedural learning may contribute to the cognitive and behavioral phenotype of autism; these deficits may be secondary to abnormalities in cerebellar-frontal circuitry.     

Effects of Chronic Cerebellar Stimulation on Chronic Limbic Seizures in Monkeys

No influences of chronic cerebellar stimulation were found in 10 different controlled experiments in 5 different monkeys with chronic alumina-induced psychomotor seizures. The stimulation parameters used were comparable to those used in human epileptics, and continuous daily EEG and behavioral monitoring allowed all seizures to be measured for daily frequency and duration over the several weeks of the experiments. Nocturnal seizures were similarly quantified in 3 monkeys to verify that cerebellar stimulation did not affect them. Motor cortex potentials evoked by cerebellar pulses confirmed that the stimulations were activating the cerebellum throughout the experiments, and measures of electrode access resistance and impedance verified that the electrodes remained in contact with the cerebellum. In one monkey given phenobarbital medication, interictal morbid behavior appeared to be improved by chronic stimulation of either cerebellum or dorsolateral frontal cortex, thus indicating an arousal influence of brain stimulation not due to cerebellum per se.     

Amygdala Kindling of Forebrain Seizures and the Occurrence of Brainstem Seizures in Genetically Epilepsy-Prone Rats

Forebrain seizures were kindled in rats by daily electrical stimulation of the amygdala. Genetically epilepsy-prone rats scoring 9 (GEPR-9s) on the seizure severity scale during audiogenic seizure (AGS) screening (“brainstem seizure-experienced”) required fewer stimulations to achieve fully kindled seizures (forelimb clonus with rearing and falling) than control rats. AGS-naive GEPR-9s required an intermediate number of stimulations, indicating a role for both genetic predisposition and previous acoustically evoked brainstem seizure experience. Other forebrain kindling indices such as afterdischarge thresholdlduration and seizure latencylduration also involved genetic as well as phenotypic (previous seizure experience) factors. In most GEPR-9s in both groups, severe brainstem seizures occurred after forebrain stimulation. The occurrence of brainstem seizures had a random nature and was not related to the sequence of kindling-dependent forebrain seizure progression. The lack of a difference in the occurrence of brainstem seizures between seizure-experienced and AGS-naïve GEPR-9s suggest that genetic predisposition is the major factor in forebrain seizure-induced activation of brainstem seizure circuitry. This brainstem seizure activity appears to model pertinent aspects of secondary generalization observed in human partial seizures.     

Gabrb3 gene deficient mice exhibit impaired social and exploratory behaviors, deficits in non-selective attention and hypoplasia of cerebellar vermal lobules: a potential model of autism spectrum disorder

OBJECTIVE: GABA(A) receptors play an important regulatory role in the developmental events leading to the formation of complex neuronal networks and to the behaviors they govern. The primary aim of this study was to assess whether gabrb3 gene deficient (gabrb3(-/-)) mice exhibit abnormal social behavior, a core deficit associated with autism spectrum disorder. METHODS: Social and exploratory behaviors along with non-selective attention were assessed in gabrb3(-/-), littermates (gabrb3(+/+)) and progenitor strains, C57BL/6J and 129/SvJ. In addition, semi-quantitative assessments of the size of cerebellar vermal lobules were performed on gabrb3(+/+) and gabrb3(-/-) mice. RESULTS: Relative to controls, gabrb3(-/-) mice exhibited significant deficits in activities related to social behavior including sociability, social novelty and nesting. In addition, gabrb3(-/-) mice also exhibited differences in exploratory behavior compared to controls, as well as reductions in the frequency and duration of rearing episodes, suggested as being an index of non-selective attention. Gabrb3(-/-) mice also displayed significant hypoplasia of the cerebellar vermis compared to gabrb3(+/+) mice. CONCLUSIONS: The observed behavioral deficits, especially regarding social behaviors, strengthens the face validity of the gabrb3 gene deficient mouse as being a model of autism spectrum disorder.     

Repetitive behavior and increased activity in mice with Purkinje cell loss: a model for understanding the role of cerebellar pathology in autism

Repetitive behaviors and hyperactivity are common features of developmental disorders, including autism. Neuropathology of the cerebellum is also a frequent occurrence in autism and other developmental disorders. Recent studies have indicated that cerebellar pathology may play a causal role in the generation of repetitive and hyperactive behaviors. In this study, we examined the relationship between cerebellar pathology and these behaviors in a mouse model of Purkinje cell loss. Specifically, we made aggregation chimeras between Lc/+ mutant embryos and +/+ embryos. Lc/+ mice lose 100% of their Purkinje cells postnatally due to a cell‐intrinsic gain‐of‐function mutation. Through our histological examination, we demonstrated that Lc/+?+/+ chimeric mice have Purkinje cells ranging from zero to normal numbers. Our analysis of these chimeric cerebella confirmed previous studies on Purkinje cell lineage. The results of both open‐field activity and hole‐board exploration testing indicated negative relationships between Purkinje cell number and measures of activity and investigatory nose‐poking. Additionally, in a progressive‐ratio operant paradigm, we found that Lc/+ mice lever‐pressed significantly less than +/+ controls, which led to significantly lower breakpoints in this group. In contrast, chimeric mice lever‐pressed significantly more than controls and this repetitive lever‐pressing behavior was significantly and negatively correlated with total Purkinje cell numbers. Although the performance of Lc/+ mice is probably related to their motor deficits, the significant relationships between Purkinje cell number and repetitive lever‐pressing behavior as well as open‐field activity measures provide support for a role of cerebellar pathology in generating repetitive behavior and increased activity in chimeric mice.     

Effects of kainic acid lesions of the cerebellar interpositus and dentate nuclei on amygdaloid kindling in rats

Some neurophysiological studies suggest that the cerebellum could participate in epileptic activity. Therefore, to study the participation of the main efferent projections from the cerebellum to the forebrain, we injected small doses of kainic acid (KA) into the deep cerebellar nuclei to selectively injure neighboring cells while avoiding fiber lesions. Uninjured fibers were confirmed using histological findings and by assessing the number of cells in the main cerebellar afferents, compared with controls. Under such conditions, we found that dentate and interpositus nuclei lesions interfere with seizure expression, both at early kindling acquisition and at the kindled stage. We hypothesize that the cerebellar effect on epilepsy drives skeletal motor responses, mainly in generalized seizures when the thalamus and neocortex are affected.     

Cerebellar Nicotinic Cholinergic Receptors are Intrinsic to the Cerebellum: Implications for Diverse Functional Roles

Although recent studies have delineated the specific nicotinic subtypes present in the mammalian cerebellum, very little is known about their location or function within the cerebellum. This is of increased interest since nicotinic receptors (nAChRs) in the cerebellum have recently been implicated in the pathology of autism spectrum disorders. To begin to better understand the roles of these heteromeric nAChRs in the cerebellar circuitry and their therapeutic potential as targets for drug development, we used various chemical and stereotaxic lesion models in conjunction with slice electrophysiology to examine how specific heteromeric nAChR subtypes may influence the surrounding cerebellar circuitry. Using subunit-specific immunoprecipitation of radiolabeled nAChRs in the cerebella following N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride, p-chloroamphetamine, and pendunculotomy lesions, we show that most, if not all, cerebellar nicotinic receptors are present in cells within the cerebellum itself and not in extracerebellar afferents. Furthermore, we demonstrate that the β4-containing, but not the β2-containing, nAChRs intrinsic to the cerebellum can regulate inhibitory synaptic efficacy at two major classes of cerebellar neurons. These tandem findings suggest that nAChRs may present a potential drug target for disorders involving the cerebellum.     

Reward processing in autism: a thematic series

This thematic series presents theoretical and empirical papers focused on understanding autism from the perspective of reward processing deficits. Although the core symptoms of autism have not traditionally been conceptualized with respect to altered reward-based processes, it is clear that brain reward circuitry plays a critical role in guiding social and nonsocial learning and behavior throughout development. Additionally, brain reward circuitry may respond to social sources of information in ways that are similar to responses to primary rewards, and recent clinical data consistently suggest abnormal behavioral and neurobiologic responses to rewards in autism. This thematic series presents empirical data and review papers that highlight the utility of considering autism from the perspective of reward processing deficits. Our hope is that this novel framework may further elucidate autism pathophysiology, with the ultimate goal of yielding novel insights with potential therapeutic implications.     

Brainstem seizure severity regulates forebrain seizure expression in the audiogenic kindling model

PURPOSE: Although sound-induced (audiogenic) seizures in the genetically epilepsy-prone rat (GEPR) initially occur independent of the forebrain, repeated audiogenic seizures recruit forebrain seizure circuits in a process referred to as audiogenic kindling. In GEPR-3s, audiogenic kindling results in facial and forelimb (F&F) clonic seizures that are typical of forebrain seizures. However, in GEPR-9s, audiogenic kindling produces posttonic all-limb clonus not usually observed during forebrain seizures. We hypothesized that the more severe brainstem seizures of the GEPR-9 prevent the expression of F&F clonic seizures during audiogenic kindling. Therefore attenuation of audiogenic seizures during audiogenic kindling in GEPR-9s should allow F&F clonic seizures to be expressed. Likewise, intensifying audiogenic seizure severity in GEPR-3s should inhibit audiogenically kindled F&F clonic seizures. We have tested this hypothesis in the present study. METHODS: Lesions of the superior colliculus or treatment with low-dose phenytoin were used to suppress audiogenic seizure severity in GEPR-9s. Depletion of brain serotonin was used to increase the seizure severity in GEPR-3s. All GEPRs were then subjected to audiogenic kindling. Behavioral and electrographic seizures were assessed. RESULTS: Suppression of audiogenic seizure severity during audiogenic kindling in GEPR-9s increased the incidence forebrain seizure behavior. Kindled GEPR-9s that continued to display full tonic seizures did not exhibit forebrain convulsions, but did show posttonic clonus and forebrain seizure activity in the EEG. GEPR-3s chronically depleted of brain serotonin, along with displaying tonic brainstem seizures, tended to display less severe forebrain seizures during audiogenic kindling. CONCLUSIONS: These findings support the concept that severe brainstem seizures prevent the behavioral expression of forebrain seizures in audiogenically kindled GEPR-9s. It appears that the severe brainstem seizure of the GEPR-9 does not allow the forebrain seizure to manifest its typical behavioral concomitants despite electrographic evidence that spike-wave discharge is occurring in the forebrain.     

Developmental Effects of Oxytocin Neurons on Social Affiliation and Processing of Social Information

Hormones regulate behavior either through activational effects that facilitate the acute expression of specific behaviors or through organizational effects that shape the development of the nervous system thereby altering adult behavior. Much research has implicated the neuropeptide oxytocin (OXT) in acute modulation of various aspects of social behaviors across vertebrate species, and OXT signaling is associated with the developmental social deficits observed in autism spectrum disorders (ASDs); however, little is known about the role of OXT in the neurodevelopment of the social brain. We show that perturbation of OXT neurons during early zebrafish development led to a loss of dopaminergic neurons, associated with visual processing and reward, and blunted the neuronal response to social stimuli in the adult brain. Ultimately, adult fish whose OXT neurons were ablated in early life, displayed altered functional connectivity within social decision-making brain nuclei both in naive state and in response to social stimulus and became less social. We propose that OXT neurons have an organizational role, namely, to shape forebrain neuroarchitecture during development and to acquire an affiliative response toward conspecifics.SIGNIFICANCE STATEMENT Social behavior is developed over the lifetime of an organism and the neuropeptide oxytocin (OXT) modulates social behaviors across vertebrate species, and is associated with neuro-developmental social deficits such as autism. However, whether OXT plays a role in the developmental maturation of neural systems that are necessary for social behavior remains poorly explored. We show that proper behavioral and neural response to social stimuli depends on a developmental process orchestrated by OXT neurons. Animals whose OXT system is ablated in early life show blunted neuronal and behavioral responses to social stimuli as well as wide ranging disruptions in the functional connectivity of the social brain. We provide a window into the mechanisms underlying OXT-dependent developmental processes that implement adult sociality.     

Social Behavior and Autism Traits in a Sex Chromosomal Disorder: Klinefelter (47XXY) Syndrome

Although Klinefelter syndrome (47,XXY) has been associated with psychosocial difficulties, knowledge of the social behavioral phenotype is limited. We examined specific social abilities and autism traits in Klinefelter syndrome. Scores of 31 XXY men on the Scale for Interpersonal Behavior and the Autism Spectrum Questionnaire were compared to 24 and 20 control men respectively. XXY men reported increased distress during social interactions and less engagement in specific social behaviors. In the XXY group, levels of autism traits were significantly higher across all dimensions of the autism phenotype. These findings call for a clinical investigation of vulnerability to autism in Klinefelter syndrome. Klinefelter syndrome might serve as a model for studying a role of the X chromosome in social behavioral dysfunction and autism-like behavior.     

Modeling early cortical serotonergic deficits in autism

Autism is a developmental brain disorder characterized by deficits in social interaction, language and behavior. Brain imaging studies demonstrate increased cerebral cortical volumes and micro- and macro-scopic neuroanatomic changes in children with this disorder. Alterations in forebrain serotonergic function may underlie the neuroanatomic and behavioral features of autism. Serotonin is involved in neuronal growth and plasticity and these actions are likely mediated via serotonergic and glutamatergic receptors. Few animal models of autism have been described that replicate both etiology and pathophysiology. We report here on a selective serotonin (5-HT) depletion model of this disorder in neonatal mice that mimics neurochemical and structural changes in cortex and, in addition, displays a behavioral phenotype consistent with autism. Newborn male and female mice were depleted of forebrain 5-HT with injections of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the bilateral medial forebrain bundle (mfb). Behavioral testing of these animals as adults revealed alterations in social, sensory and stereotypic behaviors. Lesioned mice showed significantly increased cortical width. Serotonin immunocytochemistry showed a dramatic long-lasting depletion of 5-HT containing fibers in cerebral cortex until postnatal day (PND) 60. Autoradiographic binding to high affinity 5-HT transporters was significantly but transiently reduced in cerebral cortex of 5,7-DHT-depleted mice. AMPA glutamate receptor binding was decreased at PND 15. We hypothesize that increased cerebral cortical volume and sensorimotor, cognitive and social deficits observed in both 5-HT-depleted animals and in individuals with autism, may be the result of deficiencies in timely axonal pruning to key cerebral cortical areas.     

Hipnic modulation of cerebellar information processing: implications for the cerebro-cerebellar dialogue

Recent evidence indicates that during the sleep-waking cycle the forebrain and the cerebellum show parallel changes of their operating capabilities and suggest that cooperation between these two structures plays a different role in the different behavioral states. In particular, a high degree of cerebro-cerebellar cooperation is expected in waking and in paradoxical sleep when enhanced information processing within the cerebellum and the cortex is associated with effective reciprocal cerebro-cerebellar signal transmission. We first speculate that during waking, a state in which a wide range of behaviors is produced by the interaction with the external world, the cerebellum might assist the cortex to develop the neural dynamic patterns which underlie behaviors and that this could be accomplished via cerebellar modulation of both short- and long-range cortical synchronization. In particular, we propose that the cerebellum might favour the automatic triggering of the patterns already acquired, when requested by the context, as well as the acquisition of novel patterns, when found to be of adaptive value, and might even modulate the access to consciousness of brain operations, if producing unpredicted results, by regulating pattern complexity. This proposal is based on the experimental evidence that oscillatory activity may flow within the cerebro-cerebellar loops and that stimulation or lesion of the cerebellar structures affects cortical synchronization. Then we report evidence indicating that during paradoxical sleep, when brain activation occurs in the absence of sensory inflow and motor output, cerebro-cerebellar cooperation mainly favours consolidation of newly acquired waking patterns and/or savings of old patterns from disruption possibly through a non-utilitarian replay process. Finally, we propose that cerebro-cerebellar cooperation weakens during slow wave sleep, given that in this sleep state neuronal activity and excitability decrease both in the cerebellum and in the forebrain and cerebello-cortical signal transmission is at least partially gated at the thalamic level.     

Oculomotor abnormalities parallel cerebellar histopathology in autism

OBJECTIVE: To investigate cerebellar function in autism by measuring visually guided saccades. METHODS: A visually guided saccade task was performed by 46 high-functioning individuals with autism with and without delayed language acquisition, and 104 age and IQ matched healthy individuals. RESULTS: Individuals with autism had increased variability in saccade accuracy, and only those without delayed language development showed a mild saccadic hypometria. Neither autistic group showed a disturbance in peak saccade velocity or latency. CONCLUSIONS: The observed saccadic abnormalities suggest a functional disturbance in the cerebellar vermis or its output through the fastigial nuclei, consistent with reported cerebellar histopathology in autism. The pattern of mild hypometria and variable saccade accuracy is consistent with chronic rather than acute effects of cerebellar vermis lesions reported in clinical and non-human primate studies, as might be expected in a neurodevelopmental disorder. The different patterns of oculomotor deficits in individuals with autism with and without delayed language development suggest that pathophysiology at the level of the cerebellum may differ depending on an individual's history of language development.