Differential effects of environmental enrichment on behavior and learning of male and female Ts65Dn mice, a model for Down syndrome

Nurturing brain plasticity: impact of environmental enrichment

Environmental enrichment (EE) is known to profoundly affect the central nervous system (CNS) at the functional, anatomical and molecular level, both during the critical period and during adulthood. Recent studies focusing on the visual system have shown that these effects are associated with the recruitment of previously unsuspected neural plasticity processes. At early stages of brain development, EE triggers a marked acceleration in the maturation of the visual system, with maternal behaviour acting as a fundamental mediator of the enriched experience in both the foetus and the newborn. In adult brain, EE enhances plasticity in the cerebral cortex, allowing the recovery of visual functions in amblyopic animals. The molecular substrate of the effects of EE on brain plasticity is multi-factorial, with reduced intracerebral inhibition, enhanced neurotrophin expression and epigenetic changes at the level of chromatin structure. These findings shed new light on the potential of EE as a non-invasive strategy to ameliorate deficits in the development of the CNS and to treat neurological disorders.

Enduring effects of environmental enrichment from weaning to adulthood on pituitary-adrenal function, pre-pulse inhibition and learning in male and female rats

Environmental enrichment (EE) increases stimulation and provides richer sensory, cognitive and motor opportunities through the interaction with the social and physical environment. EE produces a wide range of neuroanatomical, neurochemical and behavioural effects in several animal species. However, the effects of EE have mainly been studied shortly after the treatment, so its long-lasting effects remain to be elucidated. Thus, we studied in male and female Sprague-Dawley rats the enduring effects of EE on tasks that measured emotional reactivity, social exploration and memory, sensorimotor gating and learning. After weaning, rats reared in EE were housed in single-sex groups of 12-14 in enriched cages during 12 weeks, whereas control rats were housed in single-sex groups of 2-3 animals in standard cages. Then, all rats were housed in pairs and successively exposed to different tests between 4 and 60 weeks post-EE. The results indicated that animals of both sexes reared in EE gained less weight during the enrichment period; differences disappeared in females during the post-EE period, but were maintained intact in males. Rats reared in EE showed an altered daily pattern of corticosterone and a lower hormone response to a novel environment (hole board, HB), although no differences in ACTH were found. EE resulted in more exploratory behaviour in the HB and higher number of entries in the open arms of the elevated plus maze (with no changes in the time spent in the open arms), suggesting a greater motivation to explore. Unexpectedly, rats reared in EE showed reduced pre-pulse inhibition (PPI), a measure of sensorimotor gating, suggesting lower capability to filter non-relevant information compared with control rats. EE increased social exploratory behaviour towards juvenile rats and social discrimination in males, but decreased social discrimination in females. Finally, in the Hebb-Williams maze, rats reared in EE showed better performance in terms of reduced number of errors and shorter distances travelled in the mazes. It is concluded that EE exposure from weaning to adulthood has important and long-lasting consequences on physiological and behavioural variables, most of them similar in both sexes, although sex differences in response to the EE are also reported.

Impaired spatial learning strategies and novel object recognition in mice haploinsufficient for the dual specificity tyrosine-regulated kinase-1A (Dyrk1A)

BACKGROUND

Pathogenic aneuploidies involve the concept of dosage-sensitive genes leading to over- and underexpression phenotypes. Monosomy 21 in human leads to mental retardation and skeletal, immune and respiratory function disturbances. Most of the human condition corresponds to partial monosomies suggesting that critical haploinsufficient genes may be responsible for the phenotypes. The DYRK1A gene is localized on the human chromosome 21q22.2 region, and has been proposed to participate in monosomy 21 phenotypes. It encodes a dual-specificity kinase involved in neuronal development and in adult brain physiology, but its possible role as critical haploinsufficient gene in cognitive function has not been explored.

METHODOLOGY/PRINCIPAL FINDINGS

We used mice heterozygous for a Dyrk1A targeted mutation (Dyrk1A+/-) to investigate the implication of this gene in the cognitive phenotypes of monosomy 21. Performance of Dyrk1A+/- mice was assayed 1/ in a navigational task using the standard hippocampally related version of the Morris water maze, 2/ in a swimming test designed to reveal potential kinesthetic and stress-related behavioral differences between control and heterozygous mice under two levels of aversiveness (25 degrees C and 17 degrees C) and 3/ in a long-term novel object recognition task, sensitive to hippocampal damage. Dyrk1A+/- mice showed impairment in the development of spatial learning strategies in a hippocampally-dependent memory task, they were impaired in their novel object recognition ability and were more sensitive to aversive conditions in the swimming test than euploid control animals.

CONCLUSIONS/SIGNIFICANCE

The present results are clear examples where removal of a single gene has a profound effect on phenotype and indicate that haploinsufficiency of DYRK1A might contribute to an impairment of cognitive functions and stress coping behavior in human monosomy 21.

Sleep and EEG features in genetic models of Down syndrome

Down syndrome is characterized by a host of behavioral abnormalities including sleep disturbances. Sleep and EEG was studied at the age of 3 months in two mouse models of the condition, Ts65Dn and Ts1Cje, carrying one extra copy of partially overlapping segments of the mmu chromosome 16 (equivalent to the human chromosome 21). We found that the Ts65Dn mice showed increased waking amounts at the expense of non-REM sleep, increased theta power during sleep and a delayed sleep rebound after sleep deprivation. In contrast, Ts1Cje had limited sleep and EEG abnormalities, showing only a delayed sleep rebound after sleep deprivation and no difference in theta power. We previously found that mice over-expressing the human APPwt transgene, a gene triplicated in Ts65Dn but not Ts1Cje, also show increased wake and theta power during sleep. These results demonstrate abnormalities in sleep and EEG in Ts65Dn mice and underscore a possible correlation between App overexpression and hippocampal theta oscillations.

Chronic pentylenetetrazole but not donepezil treatment rescues spatial cognition in Ts65Dn mice, a model for Down syndrome

The most commonly used model of Down syndrome, the Ts65Dn (TS) mouse, is trisomic for most of the region of MMU16 that is homologous to HSA21. This mouse shares many phenotypic characteristics with people with Down syndrome including behavioral and cognitive alterations. The objective of this study was to analyze the ability of two drugs that improve cognition in different experimental models, the acetylcholinesterase inhibitor donepezil and the non-competitive GABA(A) antagonist pentylenetetrazole (PTZ), to improve the cognitive deficits found in TS mice. The drugs were administered p.o. to TS and CO mice for 8 weeks and a behavioral characterization was performed. Sensorimotor abilities, including vision, hearing, strength and motor coordination, as well as locomotor activity in the home cage, were not modified by any chronic treatment in TS and CO mice. TS mice showed altered equilibrium in the aluminium rod, and this effect was larger under PTZ treatment. This result may indicate a potential adverse effect of PTZ in Ts65Dn mice. Learning and memory were evaluated in TS and CO mice after both treatments in the Morris water maze. Donepezil administration did not modify learning and memory in animals of any genotype. On the other hand, PTZ administration rescued TS performance in the Morris water maze.

Effects of chronic administration of SGS-111 during adulthood and during the pre- and post-natal periods on the cognitive deficits of Ts65Dn mice, a model of Down syndrome

The Ts65Dn mouse is the most commonly used model of Down syndrome. This mouse shows many phenotypic characteristics present in people with Down syndrome, including behavioral and cognitive deficits. SGS-111 is a novel analogue of the nootropic piracetam, which prevents oxidative damage and apoptosis in both normal and Down syndrome human cortical neurons. In this work we tested the ability of chronic administration of SGS-111 to adult Ts65Dn mice to reverse the cognitive deficit found in these mice. Moreover, since oxidative stress has been reported as early as the fetal stage, SGS-111 was also administered to pregnant Ts65Dn females from the day of conception throughout the pregnancy and to Ts65Dn pups during their entire life (5 months), from birth to the end of the behavioral testing period. A characterization of the effects of SGS-111 treatment on Ts65Dn and control mice sensorimotor abilities, motor coordination, spontaneous activity, activity in the open field, exploration, anxiety and spatial and non-spatial short- and long-term learning and memory was performed. The behavioral characterization showed that chronic administration of the antioxidant SGS-111 reduced the hyperactivity shown by Ts65Dn mice in their home cage, in the open field and in the hole board test. SGS-111 administration during adulthood improved performance in the first session in the Morris water maze in control mice, and when administered during the pre- and post-natal periods, improved spatial learning in the control mice but not in Ts65Dn mice. Chronic SGS-111 administration failed to affect behavior and cognition in Ts65Dn mice.

Both increases in immature dentate neuron number and decreases of immobility time in the forced swim test occurred in parallel after environmental enrichment of mice

A direct relation between the rate of adult hippocampal neurogenesis in mice and the immobility time in a forced swim test after living in an enriched environment has been suggested previously. In the present work, young adult mice living in an enriched environment for 2 months developed considerably more immature differentiating neurons (doublecortin-positive, DCX(+)) than control, non-enriched animals. Furthermore, we found that the more DCX(+) cells they possessed, the lower the immobility time they scored in the forced swim test. This DCX(+) subpopulation is composed of mostly differentiating dentate neurons independently of the birthdates of every individual cell. However, variations found in this subpopulation were not the result of a general effect on the survival of any newborn neuron in the granule cell layer, as 5-bromo-2-deoxyuridine (BrdU)-labeled cells born during a narrow time window included in the longer lifetime period of DCX(+) cells, were not significantly modified after enrichment. In contrast, the survival of the mature population of neurons in the granule cell layer of the dentate gyrus in enriched animals increased, although this did not influence their performance in the Porsolt test, nor did it influence the dentate gyrus volume or granule neuronal nuclei size. These results indicate that the population of immature, differentiating neurons in the adult hippocampus is one factor directly related to the protective effect of an enriched environment against a highly stressful event.

Fostering and environmental enrichment ameliorate anxious behavior induced by early weaning in Balb/c mice

Postnatal stimuli affect many aspects of physiological and behavioral development. In mice, earlier weaning augments anxiety, putatively as a result of removing mother-pup interactions during the weaning period. Here, we examined the ameliorating effects of social and environmental enrichment on anxiety related to early weaning. Mice weaned at postpartum day 14 were fostered by virgin females, who displayed some nursing behavior during the 1-week fostering period. In elevated plus-maze tests, 10-week-old pups reared with a foster mother spent more time in the open arms than early-weaned mice, and entered into the open arms at a rate between that of normally- and early-weaned mice. Subsequently, the mice from each rearing group were transferred into either standard housing or housing enriched with toys that were changed periodically. Elevated plus-maze tests were conducted again when the mice were 18 and 26 weeks old. The enriched environment increased the duration of time spent in the open arms, but the magnitude of the effect varied with the rearing condition. Furthermore, mice that lived in the enriched environment showed lower activity than those kept in standard housing. These results suggest that fostering after early weaning attenuates increases in anxiety levels, and maternal care during this period may be important in the development of an offspring's emotionality. Environmental stimuli in adulthood may act to blunt the effects deprivation in early life.

Trisomy for the Down syndrome 'critical region' is necessary but not sufficient for brain phenotypes of trisomic mice

Trisomic Ts65Dn mice show direct parallels with many phenotypes of Down syndrome (DS), including effects on the structure of cerebellum and hippocampus. A small segment of Hsa21 known as the 'DS critical region' (DSCR) has been held to contain a gene or genes sufficient to cause impairment in learning and memory tasks involving the hippocampus. To test this hypothesis, we developed Ts1Rhr and Ms1Rhr mouse models that are, respectively, trisomic and monosomic for this region. Here, we show that trisomy for the DSCR alone is not sufficient to produce the structural and functional features of hippocampal impairment that are seen in the Ts65Dn mouse and DS. However, when the critical region is returned to normal dosage in trisomic Ms1Rhr/Ts65Dn mice, performance in the Morris water maze is identical to euploid, demonstrating that this region is necessary for the phenotype. Thus, although the prediction of the critical region hypothesis was disproved, novel gene dosage effects were identified, which help to define how trisomy for this segment of the chromosome contributes to phenotypes of DS.

Functional recovery in rats with chronic spinal cord injuries after exposure to an enriched environment

BACKGROUND/OBJECTIVE

The objective of this study was to determine the effect of environmental enrichment on the sensorimotor function of rats with chronic spinal cord injuries.

DESIGN

Adult Sprague-Dawley rats received a contusive injury of moderate severity at vertebral level T8 using a weight-drop device. Three months after injury, 1 randomized group (n = 16) of rats was placed in an enriched environment, whereas the control group (n = 16) remained housed in standard laboratory cages (2/cage).

METHODS

Animals were placed in an enriched environment for 4 weeks beginning at 3 months after injury. The enriched environment consisted of a large cage (5-6 rats/cage) with access to items such as tubes, ramps, and running wheel, with items changed daily.

MAIN OUTCOME MEASURES

Functional evaluation consisted of the open field Basso, Beattie and Bresnahan (BBB) locomotor test and the tests that form the combined behavioral score (CBS). The CBS includes motor score, toe spread, placing, withdrawal, righting, inclined plane, hot plate, and swim tests. Behavioral testing was repeated 7 times before and after the period of intervention.

RESULTS

The group placed in the enriched environment scored significantly better on the BBB (ANOVA repeated-measures, P < 0.01) test and CBS (ANOVA repeated-measures, P < 0.01).

CONCLUSIONS

Environmental enrichment results in significant functional improvement in animals with spinal cord injury even with a substantial delay in initiating treatment after injury. The features of an enriched environment that may be responsible for the improvement include social interactions, exercise, and novel items in an interesting environment. These findings suggest a continued plasticity of the chronically injured rat spinal cord and a possible therapeutic intervention for people with spinal cord injury.

Chronobiometry of Behavioral Activity in the Ts65Dn Model of Down Syndrome

Disruption of the sleep-wake cycle has been reported among individuals with Down syndrome (DS). Here we studied behavioral rhythms in adult male and female Ts65Dn mice, a model of DS. The overall behavioral activity of Ts65Dn and diploid (2N) littermates as defined by total movements (TM), movement time (MT), ambulatory movement time (AMT), time spent in center of arena (CT), jumps (JFP), rotational behavior (TURNS), and wheel-running activity (WRA) was recorded under a 12 h:12 h light-dark photocycle. During the light phase, Ts65Dn mice exhibited higher TM, MT, CT, JFP, and WRA compared to 2N littermates. During the dark phase, Ts65Dn and 2N mice differed only in CT and WRA, with the Ts65Dn group engaging in higher levels of both. There were no gender differences for any of the behavioral variables studied. Non-linear least-squares (Cosinor) analysis of the distribution of total behavioral activity (TM) indicated that Ts65Dn mice exhibited a slightly higher mean oscillation (i.e., mesor), but significantly lower amplitude in comparison to 2N mice, suggesting that levels of TM were elevated in trisomic mice but were relatively constant throughout the photocycle. The peak of the Ts65Dn TM rhythm was significantly phase-advanced, occurring approximately 4 h earlier than 2N mice. Overall, Ts65Dn mice were hyperactive and differed significantly in daily patterns of specific behaviors from those of 2N littermates. To control for the potential confound of retinal degeneration in Ts65Dn and 2N mice, we compared and found no difference between the TM rhythm parameters of 2N and non-retinally degenerate C57/129Sv mice, suggesting that abnormal behavioral rhythmicity in Ts65Dn mice may not due to the absence of rod and cone photoreceptors. These results serve as a starting point for further investigations into the physiological basis of sleep-wake disturbances in DS patients.

Enriched environments, experience-dependent plasticity and disorders of the nervous system

Behavioural, cellular and molecular studies have revealed significant effects of enriched environments on rodents and other species, and provided new insights into mechanisms of experience-dependent plasticity, including adult neurogenesis and synaptic plasticity. The demonstration that the onset and progression of Huntington's disease in transgenic mice is delayed by environmental enrichment has emphasized the importance of understanding both genetic and environmental factors in nervous system disorders, including those with Mendelian inheritance patterns. A range of rodent models of other brain disorders, including Alzheimer's disease and Parkinson's disease, fragile X and Down syndrome, as well as various forms of brain injury, have now been compared under enriched and standard housing conditions. Here, we review these findings on the environmental modulators of pathogenesis and gene-environment interactions in CNS disorders, and discuss their therapeutic implications.

Effects of daily environmental enrichment on memory deficits and brain injury following neonatal hypoxia-ischemia in the rat

Environmental enrichment (EE) results in improved learning and spatial memory, as well as attenuates morphological changes resulting from cerebral ischemia in adult animals. This study examined the effects of daily EE on memory deficits in the water maze and cerebral damage, assessed in the hippocampus and cerebral cortex, caused by neonatal hypoxia-ischemia. Male Wistar rats in the 7th postnatal day were submitted to the Levine-Rice model of neonatal hypoxia-ischemia (HI), comprising permanent occlusion of the right common carotid artery and a period of hypoxia (90 min, 8%O(2)-92%N(2)). Starting two weeks after the HI event, animals were stimulated by the enriched environment (1h/day for 9 weeks); subsequent to the stimulation, performance of animals in the water maze was assessed. HI resulted in spatial reference and working memory impairments that were completely reversed by EE. Following the behavioral study, animals were killed and the hippocampal volume and cortical area were estimated. There was a significant reduction of both hippocampal volume and cortical area, ipsilateral to arterial occlusion, in HI animals; environmental stimulation had no effect on these morphological measurements. Presented data indicate that stimulation by the daily environmental enrichment recovers spatial memory deficits caused by neonatal hypoxia-ischemia without affecting tissue atrophy in either hippocampus or cortex.

alphaCaMKII autophosphorylation: a fast track to memory

Alpha Ca(2+)/calmodulin-dependent kinase II (alphaCaMKII), the major synaptic protein in the forebrain, can switch into a state of autonomous activity upon autophosphorylation. It has been proposed that alphaCaMKII autophosphorylation mediates long-term memory (LTM) storage. However, recent evidence shows that synaptic stimulation and behavioural training only transiently increase the autonomous alphaCaMKII activity, implicating alphaCaMKII autophosphorylation in LTM formation rather than storage. Consistent with this, mutant mice deficient in alphaCaMKII autophosphorylation can store LTM after a massed training protocol, but cannot form LTM after a single trial. Here, we review evidence that the role of alphaCaMKII autophosphorylation is in fact to enable LTM formation after a single training trial, possibly by regulating LTM consolidation-specific transcription.

Anxiety and panic responses to a predator in male and female Ts65Dn mice, a model for Down syndrome

Hyperactivity is a feature frequently reported in behavioral studies on the Ts65Dn (TS) mouse, the most widely accepted model of Down syndrome, when tested in anxiety-provoking situations such as the plus-maze and the open-field tests. Although this behavior could be considered as an expression of reduced anxiety, it has been considered as a consequence of a lack of behavioral inhibition and/or reduced attention. This study addressed anxiety and panic behavior of male and female TS mice by evaluating serum biochemical parameters and behavioral responses to a predator in the Mouse Defense Test Battery. Flight, risk assessment, defensive threat/attack and escape attempts were measured during and after rat confrontation. When confronted to a rat, male TS mice showed similar biochemical and behavioral responses as control mice. However, female control and TS mice presented lower serum adrenocorticotropic hormone (ACTH) levels under basal conditions and higher corticosterone levels after predator exposure than male mice. Thus, there was a larger increase in ACTH and corticosterone levels after predator exposure with respect to the undisturbed condition in females than in males. In addition, TS females showed some alterations in defensive behaviors after predator exposure. The results emphasize the need to consider gender as a confounding factor in the behavioral assessment of TS mice.

Mouse Models of Cognitive Disorders in Trisomy 21: A Review

Trisomy 21 (TRS21) is the most frequent genetic cause of mental retardation. Although the presence of an extra copy of HSA21 is known to be at the origin of the syndrome, we do not know which 225 HSA21 genes have an effect on cognitive processes. Mouse models of TRS21 have been developed using syntenies between HSA21 and MMU16, MMU10 and MMU17. Available mouse models carry extra fragments of MMU16 or of HSA21 that cover all of HSA21 (chimeric HSA21) or MMU16 (Ts16); some carry large parts of MMU16 (Ts65Dn, Ts1Cje, Ms1Cje), while others have reduced contiguous fragments covering the D21S17-ETS2 region or single transfected genes. This offers a nest design strategy for deciphering cognitive (learning, memory and exploration) and associated brain abnormalities involving each of these chromosomal regions. This review confirms the crucial but not exclusive contribution of the D21S17-ETS2 region encompassing 16 genes to cognitive disorders.

Pitfalls And Hopes in Down Syndrome Therapeutic Approaches: In the Search for Evidence-Based Treatments

Trisomy 21 or Down syndrome (DS) is a complex syndrome, of genetic origin with multiple and variable neurobiological and neuropsychological manifestations. DS patients have consistent signs of brain damage along their lives, but understanding the biology of DS is complicated due to the extraordinary heterogeneity of the phenotypic signs. Thus, treatment of DS mental retardation poses significant challenges for clinicians and scientists. The review addresses the classical pharmacological and environmental treatments and also critically reviews the new possibilities that are emerging from the exciting advances in gene or cell therapy. We describe some of the most recent developments in the field and give a sense of the prospects for future prevention and therapy.

Preparation of Animals for Research--Issues to Consider for Rodents and Rabbits

This article provides details to consider when preparing to use animals in biomedical research. The stress of transport and receipt of animals into a new environment mandate the need for a period of stabilization and acclimation. This allotment of time often occurs in conjunction with the quarantine period and permits a stress "recovery" period. Discussions in the article include specific effects of the environment on the animal, such as housing and environmental enrichment. Suggestions are offered regarding how to minimize the effects of procedures and equipment through the use of preconditioning techniques. Guidelines for these techniques and for acclimation should be instituted by the institutional animal care and use committee. Stress and distress are placed in perspective as they relate to the preparation of laboratory animals for research.

Behavioral, cognitive and biochemical responses to different environmental conditions in male Ts65Dn mice, a model of Down syndrome

Ts65Dn mouse is the most widely accepted model for Down syndrome. We previously showed that environmental enrichment improved spatial learning in female but deteriorated it in male Ts65Dn mice. This study analyzed the factors contributing to the disturbed cognition of male Ts65Dn mice after enriched housing, by allocating male control and Ts65Dn mice in four conditions after weaning: small (n = 2-3) and large group (n = 8-10) housing, and enriched housing in small (2-3) and large groups (8-10). Learning, aggressive behavior, anxiety-like behavior and biochemical correlates of stress were evaluated when Ts65Dn and control mice were 4-5 months old. Environmental enrichment in large mixed colonies of Ts65Dn and diploid littermates disturbed behavioral and learning skills of Ts65Dn mice in the Morris water maze. ACTH and testosterone levels were not modified in any group of mice. Ts65Dn and control mice subjected to enriched housing in large groups and Ts65Dn mice housed in large groups showed higher corticosterone levels. Aggressive behavior was evaluated by measuring the number of attacks performed in the presence of an intruder. Ts65Dn mice performed less attacks than controls in all conditions, especially after enriched housing, indicating subordination. In the plus maze, cognitive aspects (i.e. risk assessment) and motor components (open arm avoidance) of anxiety behavior were evaluated; no difference in any condition was found. It is suggested that an excess of social and/or physical stimulation in Ts65Dn mice may affect cognition by disturbing the emotional and behavioral components of the learning process.

Exploratory activity and motor coordination in wild-type SOD1/SOD1 transgenic mice

SOD1 is one of several overexpressed genes in trisomy 21. In order to dissect possible genetic causes of the syndrome, wild-type SOD1/SOD1 transgenic mice were compared to FVB/N non-transgenic controls at 5 months of age in tests of exploratory activity and motor coordination. Wild-type SOD1/SOD1 transgenic mice had fewer stereotyped movements in an open-field and fell sooner from a rotorod than controls. In contrast, wild-type SOD1/SOD1 transgenic mice had fewer falls on a wire suspension test. There was no intergroup difference for ambulatory movements in the open-field, exploration of the elevated plus-maze, emergence from a small compartment, and motor coordination on a stationary beam. These results indicate that homozygous mice expressing human SOD1 are impaired in their ability to adjust their posture in response to a moving surface and make fewer small-amplitude movements without any change in general exploratory activity.

Cell proliferation is reduced in the dentate gyrus of aged but not young Ts65Dn mice, a model of Down syndrome

Reduced number of neurons is a common feature in Down's syndrome (DS) brains. Since reduced neuronal number also occurs in the dentate gyrus of Ts65Dn mice (TS), a model for DS, hippocampal cell proliferation and survival were analyzed in young and old TS mice. For evaluating proliferation and survival, half of the mice were sacrificed 1 day, and the other half 30 days after the last bromodeoxyuridine injection, respectively. No difference was found in the number of proliferating or surviving cells of young TS and control mice. An age-associated decline in total cell number and density has been found in both genotypes, this decline being more pronounced in TS animals. Thus, aged TS mice showed reduced cell proliferation and density of surviving cells compared to CO mice. Due to the putative involvement of newborn cells in the dentate gyrus in learning processes, the reduced proliferative capacity found in TS mice could be involved in the cognitive problems found in this model of Down syndrome.

Impaired sustained attention and error-induced stereotypy in the aged Ts65Dn mouse: a mouse model of Down syndrome and Alzheimer's disease

This study compared performance of 15- to 17-month-old Ts65Dn mice to that of littermate controls on an automated sustained attention task in which the location, onset time, and duration of brief visual cues varied unpredictably. Ts65Dn mice committed more omission errors than controls, particularly on trials with the briefest cues. Videotape data revealed that the trisomic mice attended less than controls during the period before cue presentation and engaged in stereotypic jumping and grooming immediately after making an error. These findings reveal that Ts65Dn mice are impaired in sustaining attention and exhibit heightened reactivity to committing an error, and support the validity of this mouse model for studying Down syndrome and Alzheimer's disease. The attention task, coupled with the videotape analyses of task performance, provides a useful paradigm for studying attention and reactivity to errors in mice.

Plasticity of nonneuronal brain tissue: roles in developmental disorders

Neuronal and nonneuronal plasticity are both affected by environmental and experiential factors. Remodeling of existing neurons induced by such factors has been observed throughout the brain, and includes alterations in dendritic field dimensions, synaptogenesis, and synaptic morphology. The brain loci affected by these plastic neuronal changes are dependent on the type of experience and learning. Increased neurogenesis in the hippocampal dentate gyrus is a well-documented response to environmental complexity ("enrichment") and learning. Exposure to challenging experiences and learning opportunities also alters existing glial cells (i.e., astrocytes and oligodendrocytes), and up-regulates gliogenesis, in the cerebral cortex and cerebellum. Such glial plasticity often parallels neuronal remodeling in both time and place, and this enhanced morphological synergism may be important for optimizing the functional interaction between glial cells and neurons. Aberrant structural plasticity of nonneuronal elements is a contributing factor, as is aberrant neuron plasticity, to neurological and developmental disorders such as epilepsy, autism, and mental retardation (i.e., fragile X syndrome). Some of these nonneuronal pathologies include abnormal cerebral and cerebellar white matter and myelin-related proteins in autism; abnormal myelin basic protein in fragile X syndrome (FXS); and abnormal astrocytes in autism, FXS, and epilepsy. A number of recent studies demonstrate the possibility of using environmental and experiential intervention to reduce or ameliorate some of the neuronal and nonneuronal abnormalities, as well as behavioral deficits, present in these neurological and developmental disorders.

Potential for Unintended Consequences of Environmental Enrihment for Laboratory Animals and Research Results

Many aspects of the research animal's housing environment are controlled for quality and/or standardization. Of recent interest is the potential for environmental enrichment to have unexpected consequences such as unintended harm to the animal, or the introduction of variability into a study that may confound the experimental data. The effects of enrichment provided to nonhuman primates, rodents, and rabbits are described to illustrate that the effects can be numerous and may vary by strain and/or species. Examples of parameters measured where no change is detected are also included because this information provides an important counterpoint to studies that demonstrate an effect. In addition, this review of effects and noneffects serves as a reminder that the provision of enrichment should be evaluated in the context of the health of the animal and research goals on a case-by-case basis. It should also be kept in mind that the effects produced by enrichment are similar to those of other components of the animal's environment. Although it is unlikely that every possible environmental variable can be controlled both within and among research institutions, more detailed disclosure of the living environment of the subject animals in publications will allow for a better comparison of the findings and contribute to the broader knowledge base of the effects of enrichment.

Impoverished rearing environment alters metabotropic glutamate receptor expression and function in the prefrontal cortex

Rearing rats in impoverished (IC) and enriched (EC) environmental conditions alters synaptic plasticity and cognitive processes. Metabotropic glutamate receptors (mGluRs) are known to play a key role in synaptic and behavioral plasticity. In the present study, the effect of rearing conditions on the expression of mGluR proteins in the prefrontal cortex (PFC) was assessed by immunoblotting. A significant difference in the content of prefrontal mGluR1 and mGluR5 (ie group I) and mGluR2/3 (ie group II) was observed between IC and EC rats. To functionally characterize this difference, in vivo microdialysis was used to verify differences in mGluR regulation of extracellular glutamate in the PFC. The results indicate that the capacity of group I and II mGluRs to elevate extracellular glutamate levels was significantly blunted in the PFC of IC rats compared to either EC subjects, or rats reared in normal environmental conditions (ie NIH standards). Group II mGluR receptors regulate performance in a forced T-maze spatial memory task that involves the PFC, and IC rats demonstrated deficits in this task relative to EC rats. These data suggest that reduced mGluR transmission in the PFC produced by impoverished, relative to enriched, rearing environments may contribute to cognitive deficits.

Experimental parameters affecting the Morris water maze performance of a mouse model of Down syndrome

The Ts65Dn mouse is the most studied and genetically the most complete animal model of Down syndrome (DS) available. These mice display many DS-like features, including performance deficits in different behavioral tasks, motor dysfunction, and age-dependent loss of cholinergic markers in the basal forebrain. At present, the only robust data demonstrating a behavioral deficit potentially associated with learning and memory in Ts65Dn mice less than 6 months old have come from studies that used some variation of the Morris water maze task. However, the specific features of the water maze deficits seen in these animals are still poorly defined. This study is an initial attempt to bridge this knowledge gap. We investigated three major factors potentially influencing the performance of Ts65Dn mice in the water maze: (1) order in which the test is executed; (2) age of the animals; and (3) levels of aversiveness associated with the test. Measurements of plasma corticosterone levels and core body temperature after swimming were also carried out in additional subsets of mice. Overall, we found that the behavioral phenotype of Ts65Dn mice was milder than previously described in the literature. Additionally, Ts65Dn mice were significantly more responsive to potential stressors and more prone to swim-induced hypothermia than euploid control animals. More studies are needed to tease out further the potential effects of confounding factors on the performance of Ts65Dn mice.

Recovery from brain injury in animals: relative efficacy of environmental enrichment, physical exercise or formal training (1990-2002)

In the 1960s, it was shown for the first time that enriched housing enhances functional recovery after brain damage. During the 1970s and 1980s, many findings similar to this initial one have been reported, enlarging greatly its generality. Over the last 13 years, many different kinds of brain damage were modelled in animals or even directly studied in humans. Overall, these recent studies corroborated earlier findings, although occasional exceptions were reported. Other critical data, obtained mainly in intact animals, showed that enriched housing increases neurogenesis in the adult hippocampus. Recent evidence that this neurogenesis is involved in hippocampal-dependent learning supports the original interpretation of the enrichment effects as being the result of an accumulation of informal learning experiences (e.g., [. Heredity, environment, brain biochemistry, and learning. In: Current Trends in Psychological Theory. University of Pittsburgh Press, Pittsburgh, pp. 87-110;. Brain changes in response to experience. Sci. Am. 226, 22-29]). Other components of enriched environment, such as physical exercise, may have additive effects with those of training. The comparison of the relative effectiveness of enriched experience, of physical exercise and of training on structural and/or functional assessments of recovery, shows that training/learning is generally more effective than physical exercise and that enriched experience is a more potent therapy than either of these two other treatments. The combination of enriched experience with some other neurosurgical and/or neuropharmacological treatments may further improve its therapeutic effectiveness. Finally, other recent reports emphasize that the treatment parameters may be changed in order to approximate clinical/rehabilitation conditions and, nevertheless, remain effective.

The influences of standard laboratory cages on rodents and the validity of research data

Standard cages for laboratory animals are often small, minimalist and barren. Such cages can compromise animal welfare, indicating that there are welfare-based reasons for improving their designs. However, a second issue, that is, whether animals from standard laboratory housing and husbandry conditions provide valid research data, also indicates that cage designs and husbandry methods need to be improved. This paper reviews various influences of standard laboratory cage design and husbandry. These include their effects on the repeatability of studies, models of neuro-degenerative disease, sensory development, physiology, and behaviour, the effects of standard social housing and standard handling, and the effects of maternal experience on the responses of offspring. These studies show that the development and responses of animals from standard laboratory housing and husbandry conditions are often unrepresentative and idiosyncratic, indicating that data are likely to have reduced external validity. An underlying question is whether animals from standard, barren laboratory cages are ‘abnormal’ and therefore might not provide valid baseline data. In terms of animal welfare, these studies indicate that standard laboratory housing may sometimes be associated both with reduced welfare and with reduced benefits gained from research. It is suggested that in a similar manner to the use of production measures when assessing cages for production animals, laboratory cages could be assessed in terms of their suitability to provide valid research data.

Understanding mental retardation in Down's syndrome using trisomy 16 mouse models

Mental retardation in Down's syndrome, human trisomy 21, is characterized by developmental delays, language and memory deficits and other cognitive abnormalities. Neurophysiological and functional information is needed to understand the mechanisms of mental retardation in Down's syndrome. The trisomy mouse models provide windows into the molecular and developmental effects associated with abnormal chromosome numbers. The distal segment of mouse chromosome 16 is homologous to nearly the entire long arm of human chromosome 21. Therefore, mice with full or segmental trisomy 16 (Ts65Dn) are considered reliable animal models of Down's syndrome. Ts65Dn mice demonstrate impaired learning in spatial tests and abnormalities in hippocampal synaptic plasticity. We hypothesize that the physiological impairments in the Ts65Dn mouse hippocampus can model the suboptimal brain function occuring at various levels of Down's syndrome brain hierarchy, starting at a single neuron, and then affecting simple and complex neuronal networks. Once these elements create the gross brain structure, their dysfunctional activity cannot be overcome by extensive plasticity and redundancy, and therefore, at the end of the maturation period the mind inside this brain remains deficient and delayed in its capabilities. The complicated interactions that govern this aberrant developmental process cannot be rescued through existing compensatory mechanisms. In summary, overexpression of genes from chromosome 21 shifts biological homeostasis in the Down's syndrome brain to a new less functional state.

Down's syndrome: a genetic disorder in biobehavioral perspective

Down's syndrome is a genetic disorder that can lead to mental retardation of varying degrees. How this chromosomal abnormality causes mental retardation remains an open question. This paper reviews what is currently known about the neural and cognitive features of Down's syndrome, noting the growing evidence of disproportionate impairment of specific systems such as the hippocampal formation, the prefrontal cortex and the cerebellum. The development of animal models of these defects offers a way of ultimately connecting the genetic disorder to its cognitive consequences.

Handling and environmental enrichment do not rescue learning and memory impairments in alphaCamKII(T286A) mutant mice

Environmental enrichment and postnatal handling have been shown to improve learning and memory in the Morris water maze, and to rescue impairments caused by genetic modification, age or genetic background. Mice with a targeted point mutation that prevents autophosphorylation at threonine-286 of the alpha-isoform of the Ca2+/calmodulin-dependent kinase II have impaired hippocampus-dependent and -independent strategy learning and memory in the water maze. We have investigated whether these impairments can be rescued with a combination of postnatal handling and environmental enrichment in a hybrid genetic background. Severe impairments were seen in acquisition and probe trials in both enriched and nonenriched mutants, indicating that enrichment did not rescue the learning and memory impairments. However, enrichment did rescue a specific performance deficit; enhanced floating behaviour, in the mutants. In summary, we have shown the lack of autophosphorylation of the alpha-isoform of the Ca2+/calmodulin-dependent kinase II prevents enrichment-induced rescues of strategy learning and memory impairments. Furthermore, we have established that there are enrichment mechanisms that are independent of this autophosphorylation.

Modeling behavior: the quest to link mechanisms to function

T. Dobzhansky (1973) has been credited with saying: 'nothing in biology makes sense, except in the light of evolution'. The evolutionary conservation of gene function, as well as remarkable conservation of elemental behavioral mechanisms, guarantees that much of what we learn in one organism will inform our understanding of behavior in all animals, including humans. This insight has permitted behavior-geneticists to choose organisms based on experimental tractability for a given scientific question. IBANGS as a society has clearly embraced this Dobzhanskian worldview. As a result, the intellectual synergy of cross-species behavior-genetic analysis was palpable at the IBANGS meeting in Tours, France.