Environmental enrichment rescues protein deficits in a mouse model of Huntington's disease, indicating a possible disease mechanism

Neurotrophins in the dentate gyrus

Since the discovery of nerve growth factor (NGF) in the 1950s and brain-derived neurotrophic factor (BDNF) in the 1980s, a great deal of evidence has mounted for the roles of neurotrophins (NGF; BDNF; neurotrophin-3, NT-3; and neurotrophin-4/5, NT-4/5) in development, physiology, and pathology. BDNF in particular has important roles in neural development and cell survival, as well as appearing essential to molecular mechanisms of synaptic plasticity and larger scale structural rearrangements of axons and dendrites. Basic activity-related changes in the central nervous system (CNS) are thought to depend on BDNF modulation of synaptic transmission. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders. In this chapter, I review neurotrophin structure, signal transduction mechanisms, localization and regulation within the nervous system, and various potential roles in disease. Modulation of neurotrophin action holds significant potential for novel therapies for a variety of neurological and psychiatric disorders.

Adult Hippocampal Neurogenesis, Synaptic Plasticity and Memory: Facts and Hypotheses

The demonstration that progenitor cells in regions of the adult mammalian brain such as the dentate gyrus of the hippocampus can undergo mitosis and generate new cells that differentiate into functionally integrated neurons throughout life has marked a new era in neuroscience. In recent years, a wide range of investigations has been directed at understanding the physiological mechanisms and functional relevance of this form of brain plasticity. Our current knowledge of adult hippocampal neurogenesis indicates that the production of new cells in the brain follows a multi-step process during which newborn cells are submitted to various regulatory factors that influence cell proliferation, maturation, fate determination and survival. As details of the dynamics of morphological maturation and functional integration of newborn neurons in corticohippocampal circuits have become clearer, an increasing number of studies have examined how environmental and/or behavioural factors can modulate neurogenesis and affect hippocampal-dependent learning and memory. In this article we present an overview of recent literature that relates neurogenesis to hippocampal function on the basis of correlative studies investigating the modulation of neurogenesis by learning and behavioural experience, and the consequences of the loss of hippocampal neurogenesis for memory function. We also highlight experimental evidence that immature neurons exhibit unique electrophysiological characteristics and therefore may constitute a specific cell population particularly inclined to undergo activity-dependent plasticity. Moreover, we review recent work that reveals an unsuspected mechanistic link between synaptic plasticity and the proliferation and survival of new hippocampal neurons. From the present background of research, we argue that the incorporation of functional adult-generated neurons into existing neural networks provides a higher capacity for plasticity, which may favour the encoding and storage of certain types of memories. Depending on their birth date and maturation stage, new neurons might be implicated in the encoding/storage process of the task at hand or may help future learning experience. Finally, we highlight critical issues to be addressed in order to decipher the exact contribution of newly generated neurons to cognitive functions.

Oral uridine pro-drug PN401 is neuroprotective in the R6/2 and N171-82Q mouse models of Huntington's disease

Previously, uridine pro-drug 2',3',5'-tri-O-acetyluridine (PN401) was shown to be protective in the mitochondrial complex II inhibitor 3-nitropropionic acid model of Huntington's disease (HD). In this study, PN401 increased survival and improved motor function on the rotarod in both R6/2 and N171-82Q polyglutamine repeat mouse models of HD. PN401 significantly decreased neurodegeneration in both the piriform cortex and striatum although PN401 decreased huntingtin protein aggregates only in the striatum. Cortical and striatal brain-derived neurotrophic factor (BDNF) protein levels were reduced in the +/- compared to the -/- N171-82Q mice and PN401 treatment significantly increased cortical BDNF in both +/- and -/- mice, but PN401 did not affect striatal BDNF. These results suggest that PN401 may have beneficial effects in the treatment of neurodegenerative diseases such as HD.

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.

Neurogenesis in diseases of the central nervous system

Neurogenesis is altered in ageing, and diseases of the central nervous system (CNS) such as neurodegenerative disorders. We discuss the process of neurogenesis, its relevance for disorders of the CNS, the dynamic nature of neurogenesis, how and why it may be abnormal in ageing, and disease, and possibilities to ameliorate abnormal neurogenesis in disease.

Strenuous, intensive, long-term exercise does not prevent or delay the onset of Huntington's disease

Recent work in mice has suggested that an enriched environment can delay disease onset in models of Huntington's disease (HD). The component of an enriched environment most likely able to be translated to humans is increased exercise. But here I note that a recently reported case of HD in a marathon runner who had, for many years before becoming symptomatic with HD, been running at an advanced amateur/semiprofessional level, suggests that in humans exercise is not able to prevent or even delay onset of HD. In general study of marathon runners may be useful to study if exercise can prevent or delay onset of genetic diseases.

Environmental enrichment enhances neurogranin expression and hippocampal learning and memory but fails to rescue the impairments of neurogranin null mutant mice

Environmental enrichment is known to enhance hippocampal neurogenesis and cognitive functions. Neurogranin (Ng), a specific substrate of protein kinase C (PKC), is abundantly expressed in brain regions important for cognitive functions. Deletion of Ng in mice causes severe deficits in spatial learning and long-term potentiation (LTP) in the hippocampal CA1 region. These Ng-/- mice, as compared with Ng+/+, respond poorly after treatment of their hippocampal slices with agents that activate signaling molecules important for learning and memory, including Ca2+/calmodulin-dependent protein kinase II (alphaCaMKII), PKC, protein kinase A (PKA), extracellular signal-regulated kinase (ERK), and cAMP response element-binding protein (CREB). In the present study, adult mice were housed in either regular home cages (control group) or more spacious cages with an exercise wheel and change of toys twice per week (enriched group) for at least 3 weeks. Enriched Ng+/+ and Ng+/- mice showed enhanced LTP in the hippocampal CA1 after high-frequency stimulation, but Ng-/- mice were affected only minimally. Behaviorally, the enriched Ng+/+ and Ng+/-, but not Ng-/- mice, performed significantly better than their respective control cohorts in Morris water maze and in step-down fear conditioning. Enriched Ng+/- mice also showed improvement in the radial arm maze. Quantitative immunoblot analyses showed that the enriched groups of all three genotypes exhibited elevated hippocampal levels of alphaCaMKII and CREB, but not ERK. Interestingly, enrichment caused a significant increase in hippocampal Ng levels both in Ng+/+ and Ng+/- mice that seemed to contribute to their improved LTP and behavioral performances. These results suggest that Ng gates the neuronal signaling reactions involved in learning and memory. During environmental enrichment, these Ng-regulated reactions are also critical for the enhancement of synaptic plasticity and cognitive functions.

Neurogenesis in the R6/1 transgenic mouse model of Huntington's disease: effects of environmental enrichment

Previous work has demonstrated that the transgenic R6/1 mouse model of Huntington's disease has decreased proliferation of neural precursor cells (NPCs) in the dentate gyrus of the hippocampus. This study therefore examined the survival and differentiation of NPCs in presymptomatic and symptomatic R6/1 mice and the effects of environmental enrichment on these variables. Here it is demonstrated that the survival of bromodeoxyuridine-positive (BrdU+) NPCs in the dentate gyrus is decreased in the transgenic mice. In addition, the number of doublecortin-positive (DCX+) cells is greatly reduced in these mice, as is the total number of new mature neurons, while the proportion of BrdU+ cells differentiating into mature neurons was not significantly different between genotypes. Furthermore, the DCX+ cells in the R6/1 mice had smaller and irregular-shaped somas, shorter neurites, and migrated a shorter distance into the granular cell layer compared with wild-type mice. Older symptomatic mice housed in an enriched environment had an increased number of BrdU+ and DCX+ cells as well as longer neurites and increased migration of DCX+ cells. There was no significant difference between genotypes or environments in the number of BrdU+ cells in the subventricular zone. These results suggest that decreased neurogenesis might be responsible, in part, for the hippocampal deficits observed in these mice and that environmental enrichment produces morphological changes in newborn granule neurons in both wild-type and R6/1 mice, which could underlie some of the beneficial effects of enrichment.

Differential effects of voluntary physical exercise on behavioral and brain-derived neurotrophic factor expression deficits in Huntington's disease transgenic mice

Huntington's disease is a fatal neurodegenerative disorder caused by a mutation of the huntingtin gene and involves progressive motor abnormalities (including chorea), cognitive deficits (dementia) as well as psychiatric symptoms. We have previously demonstrated that environmental enrichment slows the onset and progression of Huntington's disease in transgenic mice. Here, we investigated the effects of enhanced physical exercise on disease progression and brain-derived neurotrophic factor expression. Standard-housed Huntington's disease mice developed phenotypic rear-paw clasping by 16 weeks of age, displayed abnormal rearing behavior, deficits in motor co-ordination and of spatial working memory. Huntington's disease mice with access to running wheels exhibited delayed onset of rear-paw clasping, normalized levels of rearing behavior and amelioration of the cognitive deficits. However, in contrast to our previous environmental enrichment studies, there was no rescue of motor coordination deficits in wheel-running Huntington's disease mice. An abnormal accumulation of brain-derived neurotrophic factor protein in the frontal cortex of Huntington's disease mice was unaffected by running. Striatal and hippocampal brain-derived neurotrophic factor protein levels were unchanged. Brain-derived neurotrophic factor mRNA levels were reduced in the anterior cortex, striatum and hippocampus of Huntington's disease mice, and only striatal deficits were ameliorated by running. Overall, we show that voluntary physical exercise delays the onset of Huntington's disease and the decline in cognitive ability. In addition, our results reveal that some aspects of hippocampal dependent memory are not entirely reliant on sustained hippocampal brain-derived neurotrophic factor expression.

Abnormalities of neurogenesis in the R6/2 mouse model of Huntington's disease are attributable to the in vivo microenvironment

Huntington's disease (HD) is an autosomal dominant neurodegenerative condition characterized by movement disorders, psychiatric disturbance, and cognitive decline. There are no treatments to halt or reverse the disease. Mammalian neurogenesis persists into adulthood in the subventricular zone (SVZ) and dentate gyrus (DG) of the hippocampus. In 2001, our laboratory published the hypothesis that neurogenesis is impaired in neurodegenerative diseases and that this may contribute to disease progression. Since then, it has been shown that neurogenesis is reduced in the DG of transgenic HD mice but increased in the SVZ of HD patients. We sought to characterize neurogenesis further. We found that, in the DG of the transgenic R6/2 mouse model of HD, newborn cell proliferation and morphology, but not differentiation or survival, was compromised. In R6/2 mice, neurogenesis failed to upregulate in the DG in response to seizures. Basal SVZ neurogenesis was similar between R6/2 mice and their wild-type littermates. There was no difference in the in vitro growth of adult neural precursor cells (NPCs) between genotypes. These results suggest that abnormal neurogenesis in the R6/2 mouse is not attributable to an intrinsic impairment of the NPC itself but is attributable to the environment in which the cell is located.

Early development of aberrant synaptic plasticity in a mouse model of Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by progressive motor, psychiatric and cognitive decline. Marked neuronal loss occurs in the cortex and striatum. HD is inherited in an autosomal dominant fashion and caused by a trinucleotide repeat expansion (CAG) in the gene encoding the protein huntingtin. Predictive genetic testing has revealed early cognitive deficits in asymptomatic gene carriers at a time when there is little evidence for cell death, suggesting that impaired cognition results from a cellular or synaptic deficit, such as aberrant synaptic plasticity. Altered hippocampal long-term potentiation has been reported in mouse models of HD; however, the relationship between synaptic dysfunction and phenotype progression has not previously been characterized. We examined the age-dependency of aberrant hippocampal synaptic plasticity in the R6/1 mouse model of HD. Long-term depression (LTD) is a developmentally regulated form of plasticity, which normally declines by early adulthood. Young R6/1 mice follow the same pattern of LTD expression as controls, in that they express LTD in the first weeks of life, and then lose the ability with age. Unlike controls, R6/1 synapses later regain the ability to support LTD. This is associated with nuclear localization of mutant huntingtin, but occurs months prior to the formation of nuclear aggregates. We present the first detailed description of a progressive derailment of a functional neural correlate of cognitive processing in HD.

Ageing and neuronal vulnerability

Everyone ages, but only some will develop a neurodegenerative disorder in the process. Disease might occur when cells fail to respond adaptively to age-related increases in oxidative, metabolic and ionic stress, thereby resulting in the accumulation of damaged proteins, DNA and membranes. Determinants of neuronal vulnerability might include cell size and location, metabolism of disease-specific proteins and a repertoire of signal transduction pathways and stress resistance mechanisms. Emerging evidence on protein interaction networks that monitor and respond to the normal ageing process suggests that successful neural ageing is possible for most people, but also cautions that cures for neurodegenerative disorders are unlikely in the near future.

GENE–ENVIRONMENT INTERACTIONS, NEURONAL DYSFUNCTION AND PATHOLOGICAL PLASTICITY IN HUNTINGTON'S DISEASE

Huntington's disease (HD) is a fatal autosomal dominant disorder in which there is progressive neurodegeneration producing motor, cognitive and psychiatric symptoms. The dynamic mutation that causes the disease is common to numerous other brain disorders, which may share similar pathogenic mechanisms. Much progress has been made in the past decade in understanding how a trinucleotide (CAG) repeat expansion, encoding an expanded polyglutamine tract in the huntingtin protein, induces dysfunction at molecular and cellular levels. The present review integrates various lines of experimental evidence in an attempt to move towards a unifying mechanistic framework, which may explain the pathogenesis of HD, from molecular through to neuronal network and behavioural levels. Recent evidence, using transgenic mouse models, also suggests that environmental factors can modify the onset and progression of HD. The effects of specific environmental manipulations are discussed in the context of gene-environment interactions and experience-dependent plasticity in the healthy and diseased brain, particularly the cerebral cortex.

The use of the R6 transgenic mouse models of Huntington's disease in attempts to develop novel therapeutic strategies

Huntington's disease (HD) is a genetic neurodegenerative disorder. Since identification of the disease-causing gene in 1993, a number of genetically modified animal models of HD have been generated. The first transgenic mouse models, R6/1 and R6/2 lines, were established 8 years ago. The R6/2 mice have been the best characterized and the most widely used model to study pathogenesis of HD and therapeutic interventions. In the present review, we especially focus on the characteristics of R6 transgenic mouse models and, in greater detail, describe the different therapeutic strategies that have been tested in these mice. We also, at the end, critically assess the relevance of the HD mouse models compared with the human disease and discuss how they can be best used in the future.

Reduced hippocampal neurogenesis in R6/2 transgenic Huntington's disease mice

We investigated whether cell proliferation and neurogenesis are altered in R6/2 transgenic Huntington's disease mice. Using bromodeoxyuridine (BrdU), we found a progressive decrease in the number of proliferating cells in the dentate gyrus of R6/2 mice. This reduction was detected in pre-symptomatic mice, and by 11.5 weeks, R6/2 mice had 66% fewer newly born cells in the hippocampus. The results were confirmed by immunohistochemistry for the cell cycle markers Ki-67 and proliferating cell nuclear antigen (PCNA). We did not observe changes in cell proliferation in the R6/2 subventricular zone, indicating that the decrease in cell proliferation is specific for the hippocampus. This decrease corresponded to a reduction in actual hippocampal neurogenesis as assessed by double immunostaining for BrdU and the neuronal marker neuronal nuclei (NeuN) and by immunohistochemistry for the neuroblast marker doublecortin. Reduced hippocampal neurogenesis may be a novel neuropathological feature in R6/2 mice that could be assessed when evaluating potential therapies.

Cognitive disorders and neurogenesis deficits in Huntington's disease mice are rescued by fluoxetine

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeat encoding an extended polyglutamine tract in the huntingtin protein. Affected individuals display progressive motor, cognitive and psychiatric symptoms (including depression), leading to terminal decline. Given that transgenic HD mice have decreased hippocampal cell proliferation and that a deficit in neurogenesis has been postulated as an underlying cause of depression, we hypothesized that decreased hippocampal neurogenesis contributes to depressive symptoms and cognitive decline in HD. Fluoxetine, a serotonin-reuptake inhibitor commonly prescribed for the treatment of depression, is known to increase neurogenesis in the dentate gyrus of wild-type mouse hippocampus. Here we show that hippocampal-dependent cognitive and depressive-like behavioural symptoms occur in HD mice, and that the administration of fluoxetine produces a marked improvement in these deficits. Furthermore, fluoxetine was found to rescue deficits of neurogenesis and volume loss in the dentate gyrus of HD mice.

To be or not to be (inflamed) – is that the question in anti-inflammatory drug therapy of neurodegenerative disorders?

A sustained inflammatory reaction is present in acute (e.g. stroke) and chronic (e.g. Alzheimer's disease, Parkinson's disease and multiple sclerosis) neurodegenerative disorders. Inflammation, which is fostered by both residential glial cells and blood-circulating cells that infiltrate the diseased brain, probably starts as a time- and site-specific defense mechanism that could later evolve into a destructive and uncontrolled reaction. In this article, we review the crucial dichotomy of brain inflammation, where failure to resolve an acute beneficial response could lead to a vicious and anarchic state of chronic activation. The possible use of non-steroidal anti-inflammatory drugs for the management of neurodegenerative diseases is discussed in light of recent data demonstrating a neuroprotective role of local innate and adaptive immune responses. Novel therapeutic approaches must rely on potentiation of endogenous anti-inflammatory pathways, identification of early markers of neuronal deterioration and a combination treatment involving immune modulation and anti-inflammatory therapies.

Novel therapeutic targets for Huntington’s disease

Huntington's disease (HD) is a fatal autosomal-dominant disorder involving progressive motor, cognitive and psychiatric symptoms. HD is one of a large family of neurodegenerative diseases caused by a trinucleotide (CAG) repeat mutation, encoding an expanded tract of glutamines in the disease protein. HD was one of the first neurological disorders for which accurate transgenic models were created, allowing mechanisms of pathogenesis to be explored at molecular, cellular and behavioural levels. In the last decade, the understanding of molecular and cellular changes which occur in HD prior to onset of symptoms, and at early and late stages of disease progression, has been greatly expanded. A wide range of potential molecular targets for therapeutic intervention have been identified, associated with a variety of cellular processes including gene transcription, protein trafficking, protein degradation, protein-protein interactions, glutamatergic synaptic transmission, presynaptic signalling, postsynaptic signalling, synaptic plasticity, dopaminergic and neurotrophic modulation of synaptic function, experience-dependent neurogenesis, mitochondrial function and oxidative metabolism. Presymptomatic testing for the HD gene mutation necessitates future development of novel therapeutics aimed at delaying onset of symptoms, as well as slowing or reversing disease progression.

Environmental enrichment and neurodegenerative diseases

Recent reports on experimental models of neurodegeneration in mice have strengthened the notion that environmental enrichment (EE) is beneficial, in terms of delayed onset and progression, to a variety of neurodegenerative diseases. These studies also revealed interesting mechanistic understandings as to how EE might function. While it is generally assumed that EE elicits transcriptional and translational events that on the whole tend to be neuroprotective and neurogenic, fairly specific changes that appear to target the underlying pathological causes of disease in these various mouse models have been noted. These include a possible restoration of brain-derived neurotrophic factor striatal transport in the R6/1 Huntington's mice and an elevation in the levels of amyloid-degrading enzyme neprilysin in the APPswe/PS1DeltaE9 Alzheimic mice. An elevation in glial-derived neurotrophic factor coupled to a reduction in dopamine transporter may underlie beneficial effects in mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonian symptoms. How all these findings would translate to disease settings in human patients are unclear, but they do provide useful leads for further clinical and paraclinical investigations.

Nature, nurture and neurology: gene-environment interactions in neurodegenerative disease. FEBS Anniversary Prize Lecture delivered on 27 June 2004 at the 29th FEBS Congress in Warsaw

Neurodegenerative disorders, such as Huntington's, Alzheimer's, and Parkinson's diseases, affect millions of people worldwide and currently there are few effective treatments and no cures for these diseases. Transgenic mice expressing human transgenes for huntingtin, amyloid precursor protein, and other genes associated with familial forms of neurodegenerative disease in humans provide remarkable tools for studying neurodegeneration because they mimic many of the pathological and behavioural features of the human conditions. One of the recurring themes revealed by these various transgenic models is that different diseases may share similar molecular and cellular mechanisms of pathogenesis. Cellular mechanisms known to be disrupted at early stages in multiple neurodegenerative disorders include gene expression, protein interactions (manifesting as pathological protein aggregation and disrupted signaling), synaptic function and plasticity. Recent work in mouse models of Huntington's disease has shown that enriching the environment of transgenic animals delays the onset and slows the progression of Huntington's disease-associated motor and cognitive symptoms. Environmental enrichment is known to induce various molecular and cellular changes in specific brain regions of wild-type animals, including altered gene expression profiles, enhanced neurogenesis and synaptic plasticity. The promising effects of environmental stimulation, demonstrated recently in models of neurodegenerative disease, suggest that therapy based on the principles of environmental enrichment might benefit disease sufferers and provide insight into possible mechanisms of neurodegeneration and subsequent identification of novel therapeutic targets. Here, we review the studies of environmental enrichment relevant to some major neurodegenerative diseases and discuss their research and clinical implications.

Optimising plasticity: environmental and training associated factors in transplant-mediated brain repair

With progressively ageing populations, degeneration of nerve cells of the brain, due to accident or disease, represents one of the major problems for health and welfare in the developed world. The molecular environment in the adult brain promotes stability limiting its ability to regenerate or to repair itself following injury. Cell transplantation aims to repair the nervous system by introducing new cells that can replace the function of the compromised or lost cells. Alternatives to primary embryonic tissue are actively being sought but this is at present the only source that has been shown reliably to survive grafting into the adult brain and spinal cord, connect with the host nervous system, and influence behaviour. Based on animal studies, several clinical trials have now shown that embryonic tissue grafts can partially alleviate symptoms in Parkinson's disease, and related strategies are under evaluation for Huntington's disease, spinal cord injury, stroke and other CNS disorders. The adult brain is at its most plastic in the period following injury, offering a window of opportunity for therapeutic intervention. Enriched environment, behavioural experience and grafting can each separately influence neuronal plasticity and recovery of function after brain damage, but the extent to which these factors interact is at present unknown. To improve the outcome following brain damage, transplantation must make use of the endogenous potential for plasticity of both the host and the graft and optimise the external circumstances associated with graft-mediated recovery. Our understanding of mechanisms of brain plasticity subsequent to brain damage needs to be associated with what we know about enhancing intrinsic recovery processes in order to improve neurobiological and surgical strategies for repair at the clinical level. With the proof of principle beginning to emerge from clinical trials, a rich area for innovative research with profound therapeutic application, even broader than the specific context of transplantation, is now opening for investigation.

Development of novel therapies for Huntington's disease: hope and challenge

Huntington's disease (HD) is an autosomal dominant neurological disease. It is a fatal neurological disorder affecting 5-10 out of 10,000 people. While there are intensive research efforts focusing on uncovering molecular mechanisms of the pathogenesis of HD, a number of studies have begun to look for effective therapies for HD. There is a large body of encouraging news on novel therapeutic developments. The present paper reviews drugs used for symptomatic treatment of HD and experimental therapies targeting HD molecular pathology.

Mandatory "Enriched" Housing of Laboratory Animals: The Need for Evidence-based Evaluation

Environmental enrichment for laboratory animals has come to be viewed as a potential method for improving animal well-being in addition to its original sense as a paradigm for learning how experience molds the brain. It is suggested that the term housing supplementation better describes the wide range of alterations to laboratory animal housing that has been proposed or investigated. Changes in the environments of animals have important effects on brain structure, physiology, and behavior--including recovery from illness and injury--and on which genes are expressed in various organs. Studies are reviewed that show how the brain and other organs respond to environmental change. These data warrant caution that minor cage supplementation intended for improvement of animal well-being may alter important aspects of an animal's physiology and development in a manner not easily predicted from available research. Thus, various forms of housing supplementation, although utilized or even preferred by the animals, may not enhance laboratory animal well-being and may be detrimental to the research for which the laboratory animals are used.

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.

A neural signaling triumvirate that influences ageing and age-related disease: insulin/IGF-1, BDNF and serotonin

The ageing process and its associated diseases all involve perturbed energy metabolism, oxidative damage, and an impaired ability of the organism and its cells to cope with adversity. We propose that some specific signaling pathways in the brain may be important determinants of health during ageing. Among such specific signaling modalities are those activated in neurons by insulin-like growth factors (IGFs), brain-derived neurotrophic factor (BDNF) and serotonin. This triumvirate may be particularly important because of their cooperative influence on energy metabolism, food intake, stress responses and cardiovascular function. The health benefits to the periphery and central nervous system of dietary restriction and exercise may be mediated by this triumvirate of signals in the brain. At the molecular level, BDNF, serotonin and IGFs can all stimulate the production of proteins involved in cellular stress adaptation, growth and repair, neurogenesis, learning and memory and cell survival. The importance of this triumvirate is emphasized when it is seen that their general roles in energy metabolism, stress adaptation and disease resistance are conserved among diverse organisms consistent with important roles in the ageing process.

Brain-derived neurotrophic factor

Since the purification of BDNF in 1982, a great deal of evidence has mounted for its central roles in brain development, physiology, and pathology. Aside from its importance in neural development and cell survival, BDNF appears essential to molecular mechanisms of synaptic plasticity. Basic activity-related changes in the central nervous system are thought to depend on BDNF modification of synaptic transmission, especially in the hippocampus and neocortex. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders.

Dendritic spine pathology and deficits in experience-dependent dendritic plasticity in R6/1 Huntington's disease transgenic mice

Huntington's disease (HD) is a fatal neurodegenerative disease caused by a CAG repeat expansion coding for an expanded polyglutamine tract in the huntingtin protein. Dendritic abnormalities occur in human HD patients and in several transgenic mouse models of the disease. In this study, we examine, for the first time, dendrite and spine pathology in the R6/1 mouse model of HD, which mimics neurodegeneration seen in human HD. Enriching the environment of HD transgenic mice delays the onset of symptoms, so we also examine the effects of enrichment on dendrite pathology. Golgi-impregnated tissue from symptomatic R6/1 HD mice reveals a decrease in dendritic spine density and dendritic spine length in striatal medium spiny neurons and cortical pyramidal neurons. HD also causes a specific reduction in the proportion of bifurcated dendritic spines on basal dendrites of cortical pyramidal neurons. No differences in soma size, recurving distal dendrites, or dendritic branching were observed. Although home-cage environmental enrichment from 1 to 8 months of age increases spine density in wild-type mice, it has no effect on the spine pathology in HD mice. These results show that dendritic spine pathology in R6/1 HD mice resembles degenerative changes seen in human HD and in other transgenic mouse models of the disease. We thus provide further evidence that the HD mutation disrupts the connectivity in both neostriatum and cerebral cortex, which will contribute to motor and cognitive disease symptoms. Furthermore, we demonstrate that Huntington's disease pathology interferes with the normal plastic response of dendritic spines to environmental enrichment.