Environmental enrichment: effects on stereotyped behavior and regional neuronal metabolic activity

Do housing-induced changes in brain activity cause stereotypic behaviours in laboratory mice?

Abnormal repetitive stereotypic behaviours (SBs) (e.g. pacing, body-rocking) are common in animals with poor welfare (e.g. socially isolated/in barren housing). But how (or even whether) poor housing alters animals' brains to induce SBs remains uncertain. To date, there is little evidence for environmental effects on the brain that also correlate with individual SB performance. Using female mice from two strains (SB-prone DBA/2s; SB-resistant C57/BL/6s), displaying two forms of SB (route-tracing; bar-mouthing), we investigated how housing (conventional laboratory conditions vs. well-resourced 'enriched' cages) affects long-term neuronal activity as assessed via cytochrome oxidase histochemistry in 13 regions of interest (across cortex, striatum, basal ganglia and thalamus). Conventional housing reduced activity in the cortex and striatum. However, DBA mice had no cortical or striatal differences from C57 mice (just greater basal ganglia output activity, independent of housing). Neural correlates for individual levels of bar-mouthing (positive correlations in the substantia nigra and thalamus) were also independent of housing; while route-tracing levels had no clear neural correlates at all. Thus conventional laboratory housing can suppress cortico-striatal activity, but such changes are unrelated to SB (since not mirrored by congruent individual and strain differences). Furthermore, the neural correlates of SB at individual and strain levels seem to reflect underlying predispositions, not housing-mediated changes. To aid further work, hypothesis-generating model fit analyses highlighted this unexplained housing effect, and also suggested several regions of interest across cortex, striatum, thalamus and substantia nigra for future investigation (ideally with improved power to reduce risks of Type II error).

Reduction of restricted repetitive behavior by environmental enrichment: Potential neurobiological mechanisms

Restricted repetitive behaviors (RRB) are one of two diagnostic criteria for autism spectrum disorder and common in other neurodevelopmental and psychiatric disorders. The term restricted repetitive behavior refers to a wide variety of inflexible patterns of behavior including stereotypy, self-injury, restricted interests, insistence on sameness, and ritualistic and compulsive behavior. However, despite their prevalence in clinical populations, their underlying causes remain poorly understood hampering the development of effective treatments. Intriguingly, numerous animal studies have demonstrated that these behaviors are reduced by rearing in enriched environments (EE). Understanding the processes responsible for the attenuation of repetitive behaviors by EE should offer insights into potential therapeutic approaches, as well as shed light on the underlying neurobiology of repetitive behaviors. This review summarizes the current knowledge of the relationship between EE and RRB and discusses potential mechanisms for EE's attenuation of RRB based on the broader EE literature. Existing gaps in the literature and future directions are also discussed.

Genetic Analysis of the Stereotypic Phenotype in Peromyscus maniculatus (deer mice)

Peromyscus maniculatus, including the laboratory stock BW, have been used as a model organism for autism spectrum disorder and obsessive-compulsive disorder because of the high occurrence of stereotypy. Several studies have identified neurological and environmental components of the phenotype; however, the heritability of the phenotype has not been examined. This study characterizes the incidence and heritability of vertical jumping stereotypy (VS) and backflipping (BF) behavior in the BW stock of the Peromyscus Genetic Stock Center, which are indicative of autism spectrum disorders. In addition, interspecies crosses between P. maniculatus and P. polionotus were also performed to further dissect genetically stereotypic behavior. The inheritance pattern of VS suggests that multiple genes result in a quantitative trait with low VS being dominant over high VS. The inheritance pattern of BF suggests that fewer genes are involved, with one allele causing BF in a dominant fashion. An association analysis in BW could reveal the underlying genetic loci associated with stereotypy in P. maniculatus, especially for the BF behavior.

Putative neural consequences of captivity for elephants and cetaceans

The present review assesses the potential neural impact of impoverished, captive environments on large-brained mammals, with a focus on elephants and cetaceans. These species share several characteristics, including being large, wide-ranging, long-lived, cognitively sophisticated, highly social, and large-brained mammals. Although the impact of the captive environment on physical and behavioral health has been well-documented, relatively little attention has been paid to the brain itself. Here, we explore the potential neural consequences of living in captive environments, with a focus on three levels: (1) The effects of environmental impoverishment/enrichment on the brain, emphasizing the negative neural consequences of the captive/impoverished environment; (2) the neural consequences of stress on the brain, with an emphasis on corticolimbic structures; and (3) the neural underpinnings of stereotypies, often observed in captive animals, underscoring dysregulation of the basal ganglia and associated circuitry. To this end, we provide a substantive hypothesis about the negative impact of captivity on the brains of large mammals (e.g., cetaceans and elephants) and how these neural consequences are related to documented evidence for compromised physical and psychological well-being.

The neurobiology of environmentally induced stereotypic behaviours in captive animals: assessing the basal ganglia pathways and cortico-striatal-thalamo-cortical circuitry hypotheses

The neurobiology of environmentally induced stereotypic behaviours (SBs) (e.g., pacing in zoo carnivores, crib-biting in horses, tail chasing in dogs) is hypothesized to involve altered functioning within the basal ganglia (‘Basal Ganglia (BG) Pathways Hypotheses’) and/or between the basal ganglia and cortex (‘Cortico-Striatal-Thalamo-Cortical (CSTC) Circuits Hypotheses’). We review four decades of relevant studies, critically assessing support for both hypotheses. Currently no BG Pathways or CSTC Circuits hypothesis is fully supported. While some results are partially consistent with some hypotheses (decreased subthalamic nucleus activity in deer mice and C58 mice); others (nucleus accumbens activity in mink and C57 mice) seem to reflect individual differences in SB, but not environmental effects. Yet others can be tentatively rejected: neither elevated striatal dopamine nor the cortico-striatal connection of the sensorimotor circuit seem to be involved for most species studied to date. Further research is now important for understanding the impact of captivity on animals’ functioning.

Pharmacological targeting of striatal indirect pathway neurons improves subthalamic nucleus dysfunction and reduces repetitive behaviors in C58 mice

Repetitive behaviors (e.g., stereotypic movements, compulsions, rituals) are common features of a number of neurodevelopmental disorders. Clinical and animal model studies point to the importance of cortical-basal ganglia circuitry in the mediation of repetitive behaviors. In the current study, we tested whether a drug cocktail (dopamine D2 receptor antagonist + adenosine A2A receptor agonist + glutamate mGlu5 positive allosteric modulator) designed to activate the indirect basal ganglia pathway would reduce repetitive behavior in C58 mice after both acute and sub-chronic administration. In addition, we hypothesized that sub-chronic administration (i.e. 7 days of twice-daily injections) would increase the functional activation of the subthalamic nucleus (STN), a key node of the indirect pathway. Functional activation of STN was indexed by dendritic spine density, analysis of GABA, glutamate, and synaptic plasticity genes, and cytochrome oxidase activity. The drug cocktail used significantly reduced repetitive motor behavior in C58 mice after one night as well as seven nights of twice-nightly injections. These effects did not reflect generalized motor behavior suppression as non-repetitive motor behaviors such as grooming, digging and eating were not reduced relative to vehicle. Sub-chronic drug treatment targeting striatopallidal neurons resulted in significant changes in the STN, including a four-fold increase in brain-derived neurotrophic factor (BDNF) mRNA expression as well as a significant increase in dendritic spine density. The present findings are consistent with, and extend, our prior work linking decreased functioning of the indirect basal ganglia pathway to expression of repetitive motor behavior in C58 mice and suggest novel therapeutic targets.

Habit learning in hierarchical cortex-basal ganglia loops

How do the multiple cortico-basal ganglia-thalamo-cortical loops interact? Are they parallel and fully independent or controlled by an arbitrator, or are they hierarchically organized? We introduce here a set of four key concepts, integrated and evaluated by means of a neuro-computational model, that bring together current ideas regarding cortex-basal ganglia interactions in the context of habit learning. According to key concept 1, each loop learns to select an intermediate objective at a different abstraction level, moving from goals in the ventral striatum to motor in the putamen. Key concept 2 proposes that the cortex integrates the basal ganglia selection with environmental information regarding the achieved objective. Key concept 3 claims shortcuts between loops, and key concept 4 predicts that loops compute their own prediction error signal for learning. Computational benefits of the key concepts are demonstrated. Contrasting with former concepts of habit learning, the loops collaborate to select goal-directed actions while training slower shortcuts develops habitual responses.

Neurophysiological correlates of stereotypic behaviour in a model carnivore species

Stereotypic behaviour (SB) is common in animals housed in farm, zoo or laboratory conditions, including captive Carnivora (e.g. wild ursids and felids). Neurobiological data on housing-induced SBs come from four species (macaques, two rodent species, and horses), and suggest basal ganglia (BG) dysfunction. We investigated whether similar patterns occur in Carnivora via a model, American mink, because their SB is distinctive in form and timing. We raised 32 males in non-enriched (NE) or enriched (E) cages for 2 years, and assessed two forms of SB: 1) Carnivora-typical locomotor-and-whole-body ('loco') SBs (e.g. pacing, weaving); 2) scrabbling with the forepaws. Neuronal activity was analysed via cytochrome oxidase (CO) staining of the dorsal striatum (caudate; putamen), globus pallidus (externus, GPe; internus, GPi), STN, and nucleus accumbens (NAc); and the GPe:GPi ratio (GPr) calculated to assess relative activation of direct and indirect pathways. NE mink stereotyped more, and had lower GPr CO-staining indicating relatively lower indirect pathway activation. However, no single BG area was affected by housing and nor did GPr values covary with SB. Independent of housing, elevated NAc CO-staining predicted more loco SB, while scrabbling, probably because it negatively correlated with loco SB, negatively covaried with NAc CO-staining in NE subjects. These results thus implicate the NAc in individual differences in mink SB. However, because they cannot explain why NE subjects showed more SB, they provide limited support for the BG dysfunction hypothesis for this species' housing-induced SB. More research is therefore needed to understand how barren housing causes SB in captive Carnivora.

Reduction of repetitive behavior by co-administration of adenosine receptor agonists in C58 mice

Repetitive behaviors are diagnostic for autism spectrum disorder (ASD) and commonly observed in other neurodevelopmental disorders. Currently, there are no effective pharmacological treatments for repetitive behavior in these clinical conditions. This is due to the lack of information about the specific neural circuitry that mediates the development and expression of repetitive behavior. Our previous work in mouse models has linked repetitive behavior to decreased activation of the subthalamic nucleus, a brain region in the indirect and hyperdirect pathways in the basal ganglia circuitry. The present experiments were designed to further test our hypothesis that pharmacological activation of the indirect pathway would reduce repetitive behavior. We used a combination of adenosine A1 and A2A receptor agonists that have been shown to alter the firing frequency of dorsal striatal neurons within the indirect pathway of the basal ganglia. This drug combination markedly and selectively reduced repetitive behavior in both male and female C58 mice over a six-hour period, an effect that required both A1 and A2A agonists as neither alone reduced repetitive behavior. The adenosine A1 and A2A receptor agonist combination also significantly increased the number of Fos transcripts and Fos positive cells in dorsal striatum. Fos induction was found in both direct and indirect pathway neurons suggesting that the drug combination restored the balance of activation across these complementary basal ganglia pathways. The adenosine A1 and A2A receptor agonist combination also maintained its effectiveness in reducing repetitive behavior over a 7-day period. These findings point to novel potential therapeutic targets for development of drug therapies for repetitive behavior in clinical disorders.

Sociability deficits after prenatal exposure to valproic acid are rescued by early social enrichment

Background

Autism spectrum disorder (ASD) is characterized by impaired social interactions and repetitive patterns of behavior. Symptoms appear in early life and persist throughout adulthood. Early social stimulation can help reverse some of the symptoms, but the biological mechanisms of these therapies are unknown. By analyzing the effects of early social stimulation on ASD-related behavior in the mouse, we aimed to identify brain structures that contribute to these behaviors.

Methods

We injected pregnant mice with 600-mg/kg valproic acid (VPA) or saline (SAL) at gestational day 12.5 and evaluated the effect of weaning their offspring in cages containing only VPA animals, only SAL animals, or mixed. We analyzed juvenile play at PD21 and performed a battery of behavioral tests in adulthood. We then used preclinical PET imaging for an unbiased analysis of the whole brain of these mice and studied the function of the piriform cortex by c-Fos immunoreactivity and HPLC.

Results

Compared to control animals, VPA-exposed animals play less as juveniles and exhibit a lower frequency of social interaction in adulthood when reared with other VPA mice. In addition, these animals were less likely to investigate social odors in the habituation/dishabituation olfactory test. However, when VPA animals were weaned with control animals, these behavioral alterations were not observed. Interestingly, repetitive behaviors and depression-related behaviors were not affected by social enrichment. We also found that VPA animals present high levels of glucose metabolism bilaterally in the piriform cortex (Pir), a region known to be involved in social behaviors. Moreover, we found alterations in the somatosensory, motor, and insular cortices. Remarkably, these effects were mostly reversed after social stimulation. To evaluate if changes in glucose metabolism in the Pir correlated with changes in neuronal activity, we measured c-Fos immunoreactivity in the Pir and found it increased in animals prenatally exposed to VPA. We further found increased dopamine turnover in the Pir. Both alterations were largely reversed by social enrichment.

Conclusions

We show that early social enrichment can specifically rescue social deficits in a mouse model of ASD. Our results identified the Pir as a structure affected by VPA-exposure and social enrichment, suggesting that it could be a key component of the social brain circuitry.

Electronic supplementary material

The online version of this article (10.1186/s13229-018-0221-9) contains supplementary material, which is available to authorized users.

Subthalamic nucleus pathology contributes to repetitive behavior expression and is reversed by environmental enrichment

Repetitive motor behaviors are common in neurodevelopmental, psychiatric and neurological disorders. Despite their prevalence in certain clinical populations, our understanding of the neurobiological cause of repetitive behavior is lacking. Likewise, not knowing the pathophysiology has precluded efforts to find effective drug treatments. Our comparisons between mouse strains that differ in their expression of repetitive behavior showed an important role of the subthalamic nucleus (STN). In mice with high rates of repetitive behavior, we found significant differences in dendritic spine density, gene expression and neuronal activation in the STN. Taken together, these data show a hypoglutamatergic state. Furthermore, by using environmental enrichment to reduce repetitive behavior, we found evidence of increased glutamatergic tone in the STN with our measures of spine density and gene expression. These results suggest the STN is a major contributor to repetitive behavior expression and highlight the potential of drugs that increase STN function to reduce repetitive behavior in clinical populations.

Pacing stereotypies in laboratory rhesus macaques: Implications for animal welfare and the validity of neuroscientific findings

Stereotypic behaviours are commonly observed in captive animals and are usually interpreted as a sign of poor welfare. Stereotypies have also been linked with brain abnormalities. However, stereotypies are a heterogeneous class of behaviours and mounting evidence indicates that different stereotypies can have different causes, and can be linked to different affective states. As a consequence, the implications of a specific stereotypy in a specific species cannot be safely inferred from evidence on other stereotypies or species. Here we review what is known about pacing behaviour in laboratory rhesus macaques, a common stereotypy in this species. Our review highlights the current lack of understanding of the causal factors underlying pacing behaviour. According to current knowledge, the welfare of pacing macaques could be either better, worse or equivalent to that of non-pacing individuals. It is also unclear whether pacing results from brain abnormalities. Since rhesus macaques are widely used as a model of healthy humans in neuroscience research, determining if pacing behaviour reflects an abnormal brain and/or poor welfare is urgent.

The development of repetitive motor behaviors in deer mice: Effects of environmental enrichment, repeated testing, and differential mediation by indirect basal ganglia pathway activation

Little is known about the mechanisms mediating the development of repetitive behaviors in human or animals. Deer mice reared with environmental enrichment (EE) exhibit fewer repetitive behaviors and greater indirect basal ganglia pathway activation as adults than those reared in standard cages. The developmental progression of these behavioral and neural circuitry changes has not been characterized. We assessed the development of repetitive behavior in deer mice using both a longitudinal and cohort design. Repeated testing negated the expected effect of EE, but cohort analyses showed that progression of repetitive behavior was arrested after 1 week of EE and differed significantly from controls after 3 weeks. Moreover, EE reductions in repetitive behavior were associated with increasing activation of indirect pathway nuclei in males across adolescence, but not females. These findings provide the first assessment of developmental trajectories within EE and support indirect pathway mediation of repetitive behavior in male deer mice.

How does environmental enrichment reduce repetitive motor behaviors? Neuronal activation and dendritic morphology in the indirect basal ganglia pathway of a mouse model

Repetitive motor behaviors are observed in many neurodevelopmental and neurological disorders (e.g., autism spectrum disorders, Tourette syndrome, fronto-temporal dementia). Despite their clinical importance, the neurobiology underlying these highly stereotyped, apparently functionless behaviors is poorly understood. Identification of mechanisms that mediate the development of repetitive behaviors will aid in the discovery of new therapeutic targets and treatment development. Using a deer mouse model, we have shown that decreased indirect basal ganglia pathway activity is associated with high levels of repetitive behavior. Environmental enrichment (EE) markedly attenuates the development of such aberrant behaviors in mice, although mechanisms driving this effect are unknown. We hypothesized that EE would reduce repetitive motor behaviors by increasing indirect basal ganglia pathway function. We assessed neuronal activation and dendritic spine density in basal ganglia of adult deer mice reared in EE and standard housing. Significant increases in neuronal activation and dendritic spine densities were observed only in the subthalamic nucleus (STN) and globus pallidus (GP), and only for those mice that exhibited an EE-induced decrease in repetitive motor behavior. As the STN and GP lie within the indirect pathway, these data suggest that EE-induced attenuation of repetitive motor behaviors is associated with increased functional activation of the indirect basal ganglia pathway. These results are consistent with our other findings highlighting the importance of the indirect pathway in mediating repetitive motor behaviors.

Repetitive Behavior in Neurodevelopmental Disorders: Clinical and Translational Findings

Repetitive behavior refers to a highly heterogeneous set of responses associated with a wide range of conditions, including normative development. Treatment studies for aberrant repetitive behavior are limited although one promising approach involves conceptualizing such behavior as a generalized inflexibility or lack of variability in responding. Relatively little is known about the neurobiological mechanisms that mediate the development and expression of repetitive behavior, information critical to the design of effective pharmacotherapies, early interventions, and prevention strategies. We will review clinical findings in repetitive behavior as well as findings from animal models highlighting environmental factors and the role of cortical-basal ganglia circuitry in mediating the development and expression of these behaviors. Findings from animal models have included identification of a specific neural pathway important in mediating repetitive behavior. Moreover, pharmacological studies that support the importance of this pathway have led to the identification of novel potential therapeutic targets. Expanding the evidence base for environmental enrichment-derived interventions and focusing on generalized variability in responding will aid in addressing the broader problem of rigidity or inflexibility.

Repetitive motor behavior: further characterization of development and temporal dynamics

Repetitive behaviors are diagnostic for autism spectrum disorders, common in related neurodevelopmental disorders, and normative in typical development. In order to identify factors that mediate repetitive behavior development, it is necessary to characterize the expression of these behaviors from an early age. Extending previous findings, we characterized further the ontogeny of stereotyped motor behavior both in terms of frequency and temporal organization in deer mice. A three group trajectory model provided a good fit to the frequencies of stereotyped behavior across eight developmental time points. Group based trajectory analysis using a measure of temporal organization of stereotyped behavior also resulted in a three group solution. Additionally, as the frequency of stereotyped behavior increased with age, the temporal distribution of stereotyped responses became increasingly regular or organized indicating a strong association between these measures. Classification tree and principal components analysis showed that accurate classification of trajectory group could be done with fewer observations. This ability to identify trajectory group membership earlier in development allows for examination of a wide range of variables, both experiential and biological, to determine their impact on altering the expected trajectory of repetitive behavior across development. Such studies would have important implications for treatment efforts in neurodevelopmental disorders such as autism.

The use of the puzzle box as a means of assessing the efficacy of environmental enrichment

Environmental enrichment can dramatically influence the development and function of neural circuits. Further, enrichment has been shown to successfully delay the onset of symptoms in models of Huntington's disease (1-4), suggesting environmental factors can evoke a neuroprotective effect against the progressive, cellular level damage observed in neurodegenerative disorders. The ways in which an animal can be environmentally enriched, however, can vary considerably. Further, there is no straightforward manner in which the effects of environmental enrichment can be assessed: most methods require either fairly complicated behavioral paradigms and/or postmortem anatomical/physiological analyses. This protocol describes the use of a simple and inexpensive behavioral assay, the Puzzle Box (5-7) as a robust means of determining the efficacy of increased social, sensory and motor stimulation on mice compared to cohorts raised in standard laboratory conditions. This simple problem solving task takes advantage of a rodent's innate desire to avoid open enclosures by seeking shelter. Cognitive ability is assessed by adding increasingly complex impediments to the shelter's entrance. The time a given subject takes to successfully remove the obstructions and enter the shelter serves as the primary metric for task performance. This method could provide a reliable means of rapidly assessing the efficacy of different enrichment protocols on cognitive function, thus paving the way for systematically determining the role specific environmental factors play in delaying the onset of neurodevelopmental and neurodegenerative disease.

Self-injurious behaviour in autistic children: a neuro-developmental theory of social and environmental isolation

RATIONALE

Self-injurious behaviour is not one of the three core symptoms that define autism. However, children on the autism spectrum appear to be particularly vulnerable. Afflicted children typically slap their faces, punch or bang their heads, and bite or pinch themselves. These behaviours can be extremely destructive, and they interfere with normal social and educational activities. However, the neurobiological mechanisms that confer vulnerability in children with autism have not been adequately described.

OBJECTIVES

This review explores behavioural and neurobiological characteristics of children with autism that may be relevant for an increased understanding of their vulnerability for self-injurious behaviour.

METHODS

Behavioural characteristics that are co-morbid for self-injurious behaviour in children with autism are examined. In addition, the contributions of social and environmental deprivation in self-injurious institutionalized orphans, isolated rhesus macaques, and additional animal models are reviewed.

RESULTS

There is extensive evidence that social and environmental deprivation promotes self-injurious behaviour in both humans (including children with autism) and animal models. Moreover, there are multiple lines of convergent neuroanatomical, neurophysiological, and neurochemical data that draw parallels between self-injurious children with autism and environmentally deprived humans and animals.

CONCLUSIONS

A hypothesis is presented that describes how the core symptoms of autism make these children particularly vulnerable for self-injurious behaviour. Relevant neurodevelopmental pathology is described in cortical, limbic, and basal ganglia brain regions, and additional research is suggested.

The cell-autonomous role of excitatory synaptic transmission in the regulation of neuronal structure and function

The cell-autonomous role of synaptic transmission in the regulation of neuronal structural and electrical properties is unclear. We have now employed a genetic approach to eliminate glutamatergic synaptic transmission onto individual CA1 pyramidal neurons in a mosaic fashion in vivo. Surprisingly, while electrical properties are profoundly affected in these neurons, as well as inhibitory synaptic transmission, we found little perturbation of neuronal morphology, demonstrating a functional segregation of excitatory synaptic transmission from neuronal morphological development.

Effects of environmental enrichment on repetitive behaviors in the BTBR T+tf/J mouse model of autism

Lower order and higher order repetitive behaviors have been documented in the BTBR T+tf/J (BTBR) mouse strain, a mouse model that exhibits all three core behavioral domains that define autism. The purpose of this study was to evaluate the effectiveness of environmental enrichment for reducing repetitive behaviors in BTBR mice. Lower order behaviors were captured by assaying the time and sequence of grooming, while higher order behaviors were measured using pattern analysis of an object exploration task from digital recordings. Baseline scores were established at 7 weeks of age, followed by 30 days of housing in either a standard or enriched cage. As expected, BTBR mice spent significantly more time grooming and had a more rigid grooming sequence than control C57BL/6J mice did at baseline. After 30 days of enrichment housing, BTBR mice demonstrated a significant reduction in time spent grooming, resulting in levels that were lower than those exhibited by BTBR mice in standard housing. However, no changes were noted in the rigidity of their grooming sequence. In contrast to previous findings, there was no difference in repetitive patterns of exploration at baseline between BTBR and C57BL/6J mice in the object exploration test. Subsequently, enrichment did not significantly alter the number of repetitive patterns at posttest. Overall, the results suggest that environmental enrichment may be beneficial for reducing the time spent engaging in lower order repetitive behaviors, but may not change the overall quality of the behaviors when they do manifest.

The Possibilities and Limitations of Animal Models for Psychiatric Disorders

In the search for novel treatments for psychiatric disorders, many compounds that have shown promising pharmacological properties in disease models have failed to induce benefit in patients. There is good reason to believe that the preclinical approaches routinely used in drug discovery often provide an overly optimistic picture of clinical potential. Here we discuss some of the factors that we believe lead to erroneous decision-making, including: false interpretations of the behavioural significance of drug effects in the model species; fundamental flaws in aspects of experimental design and analysis; and misconceptions about the criteria that need to be applied before a model can be said to be validated. Only by focusing on well-constructed biological hypotheses of drug action in conjunction with reliable neurochemical, electrophysiological and behavioural assays that can be demonstrated to engage clinically relevant brain circuits will the chances of clinical success be improved. As psychiatric disorders come to be viewed less descriptively and more mechanistically as developmental disorders in brain circuits, incorporating biomarkers – measured biological variables that can indicate a normal or abnormal biological etiological process – will become the essential key to improving model development and validation, and target assessment and refinement.

Further characterization of repetitive behavior in C58 mice: developmental trajectory and effects of environmental enrichment

Aberrant repetitive behaviors are commonly observed in a variety of neurodevelopmental, neurological, and neuropsychiatric disorders. Little is known about the specific neurobiological mechanisms that underlie such behaviors, however, and effective treatments are lacking. Valid animal models can aid substantially in identifying pathophysiological factors mediating aberrant repetitive behavior and aid in treatment development. The C58 inbred mouse strain is a particularly promising model, and we have further characterized its repetitive behavior phenotype. Compared to C57BL/6 mice, C58 mice exhibit high rates of spontaneous hindlimb jumping and backward somersaulting reaching adult frequencies by 5 weeks post-weaning and adult temporal organization by 2 weeks post-weaning. The development of repetitive behavior in C58 mice was markedly attenuated by rearing these mice in larger, more complex environments. In addition to characterizing repetitive motor behavior, we also assessed related forms of inflexible behavior that reflect restricted and perseverative responding. Contrary to our hypothesis, C58 mice did not exhibit increased marble burying nor did they display reduced exploratory behavior in the holeboard task. The C58 strain appears to be a very useful model for the repetitive motor behavior characteristic of a number of clinical disorders. As an inbred mouse strain, studies using the C58 model can take full advantage of the tool kit of modern genetics and molecular neuroscience. This technical advantage makes the model a compelling choice for use in studies designed to elucidate the etiology and pathophysiology of aberrant repetitive behavior. Such findings should, in turn, translate into effective new treatments.

Cage-induced stereotypies, perseveration and the effects of environmental enrichment in laboratory mice

When kept in barren and restrictive cages, animals frequently develop stereotypic behaviour patterns that are characterized by high repetition rates, conspicuous invariance and an apparent lack of function. Although millions of animals are affected, the underlying causes and mechanisms are still unclear. Growing evidence suggests that cage-induced stereotypies may reflect pathological dysfunction within basal ganglia circuitry expressed by perseverative behaviour. In order to assess whether variation in stereotypy performance and variation in perseverative behaviour may have a common cause in ICR CD-1 mice, we assessed the effects of environmental enrichment on both phenomena. We raised 48 female ICR CD-1 mice in standard or enriched cages from three weeks to either 6 or 11 months of age and measured stereotypy level in the home cage and perseveration on an extinction task. We further examined whether enriched rearing conditions (early enrichment) protect mice from the developing stereotypies later in life and whether stereotypies developed in barren cages would persist in an enriched environment (late enrichment) by transferring standard mice to enriched cages and vice versa for 14 weeks after completion of the extinction task. We found no evidence for a causal relation between stereotypy and perseveration in mice. However, transfer to enriched cages reduced stereotypy levels significantly both at 6 and 11 months of age indicating that stereotypies had not become established yet. Finally, we found that removing enrichments at both ages did not induce higher stereotypy levels, thereby confirming earlier reports of a neuroprotective effect of early enrichment.

Recurrent perseveration correlates with abnormal repetitive locomotion in adult mink but is not reduced by environmental enrichment

We analysed the relationship between abnormal repetitive behaviour (ARB), the presence/absence of environmental enrichment, and two types of behavioural disinhibition in farmed American mink, Neovison vison. The first type, recurrent perseveration, the inappropriate repetition of already completed responses, was assessed using three indices of excessive response repetition and patterning in a bias-corrected serial two-choice guessing task. The second type, disinhibition of prepotent responses to reward cues, a form of impulsivity, was tested in a locomotive detour task adapted from primate reaching tasks: subjects were required to walk around, rather than directly into, a transparent barrier behind which food was visible. In older adult females, recurrent perseveration positively predicted pre-feeding abnormal repetitive locomotion (ARL) in Non-enriched housing. High-ARL subjects also performed repeated (same-choice) responses more rapidly than low-ARL animals, even when statistically controlling for alternated (different-choice) response latency. Mink performed much less ARL following transfer to Enriched housing, but there was no corresponding change in recurrent perseveration. Thus, elevated recurrent perseveration is not sufficient for frequent ARL; and while captive environments do determine ARL frequency, in mink, they do not necessarily do so by modifying levels of perseveration. Disinhibition of prepotent responses to reward cues, meanwhile, did not predict ARL. In a separate sample of differentially housed young adults, neither type of behavioural disinhibition predicted ARL, and again, whether or not housing was enriched did not affect behavioural disinhibition despite affecting ARL. Thus, the relationship between recurrent perseveration and ARB may only develop with age; longitudinal studies are now required for confirmation.

Development of repetitive behavior in a mouse model: roles of indirect and striosomal basal ganglia pathways

Restricted repetitive behaviors (stereotypy, compulsions, rituals) are diagnostic for autism spectrum disorder and common in related neurodevelopmental disorders. Despite their prevalence in clinical populations, underlying mechanisms associated with the development of these behaviors remain poorly understood. We examined the role of the indirect basal ganglia pathway in the development of stereotypy using deer mice. We measured neuronal metabolic activity in the subthalamic nucleus (STN) and other relevant brain regions using cytochrome oxidase (CO) histochemistry at three developmental time-points. Although no differences were observed in STN across development, significant differences were found when mice were grouped by developmental trajectory. At 6 weeks post-weaning, significantly lower CO activity in STN was found in those trajectory groups that developed high levels of repetitive behavior versus the trajectory group that did not, suggesting the development of stereotypy is associated with decreased indirect basal ganglia pathway activity. In addition, we tested the hypothesis that preferential activation of striatal striosomes relative to surrounding matrix would be associated with the development of stereotypy. No differences in the relative activation of these striatal compartments were observed across development or among trajectory groups. Our results point to dynamic changes in the indirect pathway associated with the development of repetitive behavior and extends our prior work linking reduced indirect pathway activation to stereotypy in adult deer mice.

Using animal models of enriched environments to inform research on sensory integration intervention for the rehabilitation of neurodevelopmental disorders

The field of behavioral neuroscience has been successful in using an animal model of enriched environments for over five decades to measure the rehabilitative and preventative effects of sensory, cognitive and motor stimulation in animal models. Several key principles of enriched environments match those used in sensory integration therapy, a treatment used for children with neurodevelopmental disorders. This paper reviews the paradigm of environmental enrichment, compares animal models of enriched environments to principles of sensory integration treatment, and discusses applications for the rehabilitation of neurodevelopmental disorders. Based on this review, the essential features in the enriched environment paradigm which should be included in sensory integration treatment are multiple sensory experiences, novelty in the environment, and active engagement in challenging cognitive, sensory, and motor tasks. Use of sensory integration treatment may be most applicable for children with anxiety, hypersensitivity, repetitive behaviors or heightened levels of stress. Additionally, individuals with deficits in social behavior, social participation, or impairments in learning and memory may show gains with this type of treatment.

Indirect basal ganglia pathway mediation of repetitive behavior: attenuation by adenosine receptor agonists

Repetitive behaviors are diagnostic for autism and common in related neurodevelopmental disorders. Despite their clinical importance, underlying mechanisms associated with the expression of these behaviors remain poorly understood. Our lab has previously shown that the rates of spontaneous stereotypy in deer mice (Peromyscus maniculatus) were negatively correlated with enkephalin content, a marker of striatopallidal but not striatonigral neurons. To investigate further the role of the indirect basal ganglia pathway, we examined neuronal activation of the subthalamic nucleus (STN) using cytochrome oxidase (CO) histochemistry in high- and low-stereotypy mice. CO activity in STN was significantly lower in high-stereotypy mice and negatively correlated with the frequency of stereotypy. In addition, exposure to environmental enrichment, which attenuated stereotypy, normalized the activity of STN. Co-administration of the adenosine A(2A) receptor agonist CGS21680 and the A(1) receptor agonist CPA attenuated stereotypy dose-dependently. The significant reduction associated with the lowest dose of the drug combination tested was due to its effects on mice with lower baseline levels of stereotypy. Higher doses of the drug combination were required to show robust behavioral effects, and presumably requisite activation of the indirect pathway, in high-stereotypy mice. These findings support that decreased indirect pathway activity is linked to the expression of high levels of stereotypy in deer mice and that striatal A(1) and A(2A) receptors may provide promising therapeutic targets for the treatment of repetitive behaviors in neurodevelopmental disorders.

Behavioural exposure and sleep do not modify corticospinal and intracortical excitability in the human motor system

OBJECTIVE

Behavioural exposure and sleep may bidirectionally modify the excitability of cortical networks including those in the motor cortex. Here we tested whether the excitability of intracortical inhibitory and excitatory networks within the primary motor cortex exhibited changes suggestive of a time of day influence.

METHODS

Short-interval intracortical inhibition (SICI) and facilitation (ICF), and input-output curves (IO curves) were investigated using transcranial magnetic stimulation (TMS). Recordings were made from the resting right first dorsal interosseous (FDI) muscle in 10 healthy subjects on three occasions: 9A.M. and 4P.M. of the same day, and 9A.M. of the following day.

RESULTS

There was no significant change in any of the measures across the three assessments.

CONCLUSIONS

These findings provide evidence that time of day does not significantly influence corticospinal and intracortical excitability in the primary motor cortex.

SIGNIFICANCE

These results provide no support for the hypothesis that synapses within the motor cortex undergo potentiation due to daytime use and behavioural experiences. Additionally, these findings provide evidence that measurement of motor cortical excitability is not systematically biased by time-of-day dependent variability and thus does not pose a confound in studies assessing corticospinal excitability longitudinally.

Effects of access to voluntary wheel running on the development of stereotypy

Stereotyped motor behaviors are a common consequence of environmental restriction in a wide variety of species. Although environmental enrichment has been shown to substantially reduce stereotypy levels, the various components of enrichment have not been evaluated independently to determine which is responsible for this effect. Exercise, particularly voluntary wheel running, is a promising candidate based on several lines of behavioral and neurobiological evidence. To test the hypothesis that access to wheel running will reduce stereotyped motor behavior, we reared deer mice from weaning with continuous access to either a functional running wheel or a locked wheel. We assessed running behavior throughout this time period and stereotypy levels in a test context at 30 and 45 days post-weaning. We found that exercise did not significantly affect stereotypy level nor was there an association between wheel running and stereotypy. Thus, exercise alone, unlike environmental enrichment, does not prevent the development of stereotypy. These results have important implications for animal welfare.

Assessing the validity of current mouse genetic models of obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD is a major cause of disability; however, the genetic factors and pathophysiological mechanisms underlying this complex, heterogeneous disorder remain largely unknown. During the past decade, a number of putative mouse genetic models of OCD have been developed for the purpose of studying the neural mechanisms underlying this disorder and developing novel treatments. This review presents and evaluates these experimental preparations to date. Models using knockout or transgenic approaches, as well as those examining variation in genetically diverse populations, are evaluated and discussed.

Early behavioral intervention, brain plasticity, and the prevention of autism spectrum disorder

Advances in the fields of cognitive and affective developmental neuroscience, developmental psychopathology, neurobiology, genetics, and applied behavior analysis have contributed to a more optimistic outcome for individuals with autism spectrum disorder (ASD). These advances have led to new methods for early detection and more effective treatments. For the first time, prevention of ASD is plausible. Prevention will entail detecting infants at risk before the full syndrome is present and implementing treatments designed to alter the course of early behavioral and brain development. This article describes a developmental model of risk, risk processes, symptom emergence, and adaptation in ASD that offers a framework for understanding early brain plasticity in ASD and its role in prevention of the disorder.

The application of Russell and Burch 3R principle in rodent models of neurodegenerative disease: the case of Parkinson's disease

Currently, the accepted ethical standards for the regulation of animal experimentation are provided by the 3R principle (Replacement, Reduction and Refinement). The development of alternative methods to the use of animals (Replacement), the design of adequate experimental protocols to reduce the number of animals (Reduction), the application of refinement practices (Refinement) are all aspects to be considered to ensure ethical and scientific validity to animal experimentation. This review intends to address these issues, using experimental research on Parkinson's disease (PD) as a paradigmatic example of the use of animal models to improve knowledge on a devastating human pathology. In particular, current rodent models of PD and their validity are reviewed and discussed, and methodologies that may ultimately reduce animal's suffering emphasized. Although procedures referring to with 3R principle can be traced in the literature reviewed, they are not considered yet an important part of the methodological information. The formal inclusion in scientific papers of a section devoted to 3Rs may increase knowledge and eventually adherence to this principle by scientists.

Procedural learning and cognitive flexibility in a mouse model of restricted, repetitive behaviour

Restricted, repetitive behaviours (e.g., stereotypies, compulsions, rituals) in neurodevelopmental disorders have been linked to alterations in cortico-basal ganglia circuitry. Cognitive processes mediated by this circuitry (e.g., procedural learning, executive function) are likely to be impaired in individuals exhibiting high rates of repetitive behaviour. To test this hypothesis, we assessed both procedural learning and cognitive flexibility (reversal learning) using a T-maze task in deer mice (Peromyscus maniculatus) exhibiting various rates of repetitive behaviour (vertical jumping and backward somersaulting). These mice exhibited high rates of stereotypy when reared in standard rodent cages, and such behaviour was significantly attenuated by housing them in larger more complex environments. Mice reared in complex environments exhibited significantly better procedural and reversal learning than standard caged mice. Thus, early experience associated with the prevention and attenuation of stereotypy was associated with better striatally mediated learning and cognitive flexibility. Stereotypy score was significantly correlated with the number of errors made in reversal learning, and interacted with housing condition to affect overall cognitive performance. Our findings support the applicability of the deer mouse model of spontaneous stereotypy to a wider range of restricted, repetitive behaviour (e.g., insistence on sameness) typical of neurodevelopmental disorders.

Is exposure to enriched environment beneficial for functional post-lesional recovery in temporal lobe epilepsy?

Exposure to enriched environment has been shown to induce robust neuronal plasticity in both intact and injured adult central nervous system, including up-regulation of multiple neurotrophic factors, enhanced neurogenesis in the dentate gyrus of the hippocampus, and improved spatial learning and memory function. Neuronal plasticity, though mostly adaptive and abnormal, also occurs during certain neurodegenerative conditions such as the temporal lobe epilepsy (TLE). The TLE is characterized by hippocampal neurodegeneration, aberrant mossy fiber sprouting, spontaneous recurrent motor seizures, cognitive deficits, and abnormally enhanced neurogenesis during the early phase and dramatically declined neurogenesis during the chronic phase of the disease. As environmental enrichment has been found to be beneficial for treating animal models of Alzheimer's, Parkinson's, and Huntington's diseases, there is considerable interest in determining the efficacy of this strategy for preventing or treating chronic TLE after the initial precipitating brain injury. This review first discusses the proof of principle behind the potential application of the environmental enrichment strategy for preventing or treating TLE after brain injury. The subsequent chapters confer the portrayed beneficial effects of enrichment for functional post-lesional recovery in TLE and the possible complications which may arise from housing epilepsy-prone or epileptic rats in enriched environmental conditions. The final segment discusses studies that are essential for further understanding the efficacy of this approach for preventing or treating TLE.

Broad activation of the glomerular layer enhances subsequent olfactory responses

Early olfactory experience with a specific odorant enhances the subsequent response of the glomerular layer of the rat olfactory bulb to that same odorant. Because different odorants activate different glomerular layer regions, it seemed plausible that experience with a large number of odorants might result in enhanced glomerular activation during subsequent exposure to both the previously experienced odorants and the novel odorants evoking activity in regions that overlapped with those previously stimulated by different odorants. To this end, 7 odorants were selected using our glomerular response data archive that together stimulated much of the glomerular layer (alpha-phellandrene, benzaldehyde, L-carvone, decanal, pentanol, santalol, and valeric acid). Young rats were exposed to a different odorant each day for 7 days, and this cycle was repeated 3 times from postnatal days 1-21. The [(14)C]2-deoxyglucose technique was used to measure neural activity in response to both previously experienced and novel odorants. The 2 novel odorants (alpha-ionone and L-menthone) activate regions of the glomerular layer that overlap with those stimulated by the 7 enrichment odorants. Our results indicate that early experience with multiple odorants results in increased responsiveness both to previously experienced odorants and to novel odorants that stimulate previously activated regions of the bulb.

Animal models of restricted repetitive behavior in autism

Restricted, repetitive behavior, along with deficits in social reciprocity and communication, is diagnostic of autism. Animal models relevant to this domain generally fall into three classes: repetitive behavior associated with targeted insults to the CNS; repetitive behavior induced by pharmacological agents; and repetitive behavior associated with restricted environments and experience. The extant literature provides potential models of the repetitive behavioral phenotype in autism rather than attempts to model the etiology or pathophysiology of restricted, repetitive behavior, as these are poorly understood. This review focuses on our work with deer mice which exhibit repetitive behaviors associated with environmental restriction. Repetitive behaviors are the most common category of abnormal behavior observed in confined animals and larger, more complex environments substantially reduce the development and expression of such behavior. Studies with this model, including environmental enrichment effects, suggest alterations in cortical-basal ganglia circuitry in the development and expression of repetitive behavior. Considerably more work needs to be done in this area, particularly in modeling the development of aberrant repetitive behavior. As mutant mouse models continue to proliferate, there should be a number of promising genetic models to pursue.

Environmental complexity and central nervous system development and function

Environmental restriction or deprivation early in development can induce social, cognitive, affective, and motor abnormalities similar to those associated with autism. Conversely, rearing animals in larger, more complex environments results in enhanced brain structure and function, including increased brain weight, dendritic branching, neurogenesis, gene expression, and improved learning and memory. Moreover, in animal models of CNS insult (e.g., gene deletion), a more complex environment has attenuated or prevented the sequelae of the insult. Of relevance is the prevention of seizures and attenuation of their neuropathological sequelae as a consequence of exposure to a more complex environment. Relatively little attention, however, has been given to the issue of sensitive periods associated with such effects, the relative importance of social versus inanimate stimulation, or the unique contribution of exercise. Our studies have examined the effects of environmental complexity on the development of the restricted, repetitive behavior commonly observed in individuals with autism. In this model, a more complex environment substantially attenuates the development of the spontaneous and persistent stereotypies observed in deer mice reared in standard laboratory cages. Our findings support a sensitive period for such effects and suggest that early enrichment may have persistent neuroprotective effects after the animal is returned to a standard cage environment. Attenuation or prevention of repetitive behavior by environmental complexity was associated with increased neuronal metabolic activity, increased dendritic spine density, and elevated neurotrophin (BDNF) levels in brain regions that are part of cortical-basal ganglia circuitry. These effects were not observed in limbic areas such as the hippocampus.

Stereotypies and Other Abnormal Repetitive Behaviors: Potential Impact on Validity, Reliability, and Replicability of Scientific Outcomes

Normal behavior plays a key role in facilitating homeostasis, especially by allowing the animal to control and modify its environment. Captive environments may interfere with these behavioral responses, and the resulting stress may alter many physiological parameters. Abnormal behaviors indicate that an animal is unable to adjust behaviorally to the captive environment and, hence, may be expressing abnormal physiology. Therefore, captive environments may affect the following aspects of an experiment: validity, by introducing abnormal animals into experiments; reliability, by increasing interindividual variation through the introduction of such individuals; and replicability, by altering the number and type of such individuals between laboratories. Thus, far from increasing variability, enrichment may actually improve validity, reliability, and replicability by reducing the number of abnormal animals introduced into experiments. In this article, the specific example of abnormal repetitive behaviors (ARBs) is explored. ARBs in captive animals appear to involve the same mechanisms as ARBs in human psychiatry, which reflect underlying abnormalities of brain function. ARBs are also correlated with a wide range of behavioral changes that affect experimental outcomes. Thus, ARBs in laboratory animals may compromise validity, reliability, and replicability, especially in behavioral experiments; and enrichments that prevent ARB may enhance validity, reliability, and replicability. Although many links in this argument have been tested experimentally, key issues still remain in the interpretation of these data. In particular, it is currently unclear (1) whether or not the differences in brain function seen in animals performing ARB are abnormal, (2) which common behavioral paradigms are affected by ARB, and (3) whether enrichment does indeed improve the quality of behavioral data. Ongoing and future work addressing these issues is outlined.

Environmental enrichment: effects on stereotyped behavior and neurotrophin levels

The present study evaluated whether environmental enrichment-related effects on the development of stereotyped behavior in deer mice were associated with alterations in neurotrophin levels. Deer mice were reared in enriched or standard cage conditions for 60 days. The mice were then tested in automated photocell detectors and classified as either stereotypic or nonstereotypic. This testing paradigm yielded four behaviorally distinct groups: enriched stereotypic, enriched nonstereotypic, standard cage stereotypic, and standard cage nonstereotypic. The motor cortex, striatum, and hippocampus were dissected, and the levels of brain-derived neurotrophin factor (BDNF) and nerve growth factor (NGF) in each brain region were analyzed using Promega ELISA kits. There were no differences in either NGF or BDNF in either the motor cortex or the hippocampus. In the striatum, the enriched nonstereotypic mice exhibited significantly more BDNF than the enriched stereotypic, the standard cage nonstereotypic, or the standard cage stereotypic mice. There were no differences in NGF in the striatum. These results provide evidence that the enrichment-related prevention of stereotyped behavior in deer mice is associated with increased BDNF in the striatum.

Animal models of anxiety disorders

Animal models may be useful in investigating the fundamental mechanisms underlying psychiatric disorders, and may contribute to the development of new medications. A computerized literature search was used to collect studies on recently developed animal models for anxiety disorders. Particular cognitive-affective processes (eg, fear conditioning, control of stereotypic movements, social submissiveness, and trauma sensitization) may be particularly relevant to understanding specific anxiety disorders. Delineation of the phenomenology and psychobiology of these processes in animals leads to a range of useful models of these conditions. These models demonstrate varying degrees of face, construct, and predictive validity.

Environmental enrichment: effects on stereotyped behavior and dendritic morphology

We evaluated whether environmental enrichment-related effects on the development of stereotyped behavior in deer mice were associated with alterations in dendritic morphology. Deer mice were reared under enriched or standard housing conditions and then tested in automated photocell detectors and classified as stereotypic or nonstereotypic. Dendritic morphology was assessed in layer V pyramidal neurons of the motor cortex, medium spiny neurons of the dorsolateral striatum, and granule cells of the dentate gyrus using Golgi-Cox histochemistry. Enriched nonstereotypic mice exhibited significantly higher dendritic spine densities in the motor cortex and the striatum than enriched stereotypic or standard-cage mice. Significant increases in dendritic arborization following environmental enrichment also were observed. These results suggest that the enrichment-related prevention of stereotyped behavior is associated with increased dendritic spine density.