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.     

Dissociation between spontaneously emitted and apomorphine-induced stereotypy in Peromyscus maniculatus bairdii

Stereotyped behavior is repetitive, topographically invariant motor activity that lacks an obvious function. We have previously characterized the spontaneous and persistent stereotypies that occur in deer mice housed in standard laboratory cages. Providing these animals with enriched environments markedly reduces their vulnerability to develop stereotypic behavioral repertoires, thus enabling us to generate behaviorally distinct (stereotypic and nonstereotypic) mice of the same species. As stereotypic behaviors are readily induced by systemic administration of a dopamine (DA) agonist, the present study tested whether apomorphine would induce stereotypies in environmentally enriched (nonstereotypic) deer mice that were topographically similar to the stereotypies that are spontaneously emitted by standard-caged (stereotypic) deer mice. The effects of apomorphine were also evaluated in the standard-caged (stereotypic) deer mice. DA agonist-induced behaviors in nonstereotypic mice included stereotypies that were largely topographically distinct from spontaneously emitted stereotypies; apomorphine failed to produce statistically significant elevations in two of the three stereotypic behaviors typical of standard-caged deer mice. Furthermore, there was no evidence of increased DA receptor sensitivity in stereotypic mice. Thus, environmentally related stereotypy is distinct from systemically administered DA agonist-induced stereotypy, and is not exacerbated by such drug treatment. The results obtained do provide support, however, for a limited involvement of the DA system in the mediation of these behaviors.     

Cortical regulation of dopamine depletion-induced dendritic spine loss in striatal medium spiny neurons

The proximate cause of Parkinson's disease is striatal dopamine depletion. Although no overt toxicity to striatal neurons has been reported in Parkinson's disease, one of the consequences of striatal dopamine loss is a decrease in the number of dendritic spines on striatal medium spiny neurons (MSNs). Dendrites of these neurons receive cortical glutamatergic inputs onto the dendritic spine head and dopaminergic inputs from the substantia nigra onto the spine neck. This synaptic arrangement suggests that dopamine gates corticostriatal glutamatergic drive onto spines. Using triple organotypic slice cultures composed of ventral mesencephalon, striatum, and cortex of the neonatal rat, we examined the role of the cortex in dopamine depletion-induced dendritic spine loss in MSNs. The striatal dopamine innervation was lesioned by treatment of the cultures with the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) or by removing the mesencephalon. Both MPP+ and mesencephalic ablation decreased MSN dendritic spine density. Analysis of spine morphology revealed that thin spines were preferentially lost after dopamine depletion. Removal of the cortex completely prevented dopamine depletion-induced spine loss. These data indicate that the dendritic remodeling of MSNs seen in parkinsonism occurs secondary to increases in corticostriatal glutamatergic drive, and suggest that modulation of cortical activity may be a useful therapeutic strategy in Parkinson's disease.     

A rodent model of spontaneous stereotypy: initial characterization of developmental, environmental, and neurobiological factors.

Stereotypies are patterns of motor behavior that are repetitive, excessive, topographically invariant, and that lack any obvious function or purpose. In humans, stereotyped behaviors are associated with psychiatric, neurological, and developmental disorders. In animals, stereotypy has been frequently associated with adverse environmental circumstances and often related to alterations in striatal dopamine. To assess the development of stereotyped behaviors and to test the hypothesis that these behaviors are associated with environmental restriction, deer mice were housed in either standard laboratory cages or larger, enriched cages, and the development of stereotypy was followed from weaning over a 17-week period. Standard-caged deer mice engaged in stereotyped behaviors at a higher rate and developed these behaviors more quickly when compared to animals in enriched caging. Additionally, enriched caging was associated with higher rates of patterned running, whereas jumping and backward somersaulting were typically observed in standard cages. In addition, there was a significant effect of litter, but no effect of sex or cage, on the time to develop stereotypy. No differences were found in the density of either striatal D1 or D2 dopamine receptors or the concentration of striatal dopamine or its metabolites as a function of rearing condition or as a function of whether the animals developed stereotypy. These results characterize the development of stereotypies in this species, demonstrate the importance of environmental conditions in the genesis of stereotypy, and suggest that alterations in striatal dopamine content or dopamine receptor density do not account for the expression of stereotyped behaviors in this model.     

Quantitative analysis of age-related dendritic changes in medium spiny I (MSI) striatal neurons of C57BL/6N mice

Our study used quantitative morphometric analysis and Golgi staining methods to evaluate postnatal changes in the dendritic architecture of MSI neurons of the striatum between 1 and 30 months of age. Morphological changes and chronological age were also correlated with functional testing in order to identify subpopulations of aged mice with dendritic alterations that may be more characteristic of a motor deficit rather than the normal aging process. We found that the overall size of the dendritic arbor of MSI neurons in the rostral striatum remained stable with age, while caudal MSI neurons exhibited a significant elongation of terminal dendritic segments between 25 and 30 months of age. In addition, our correlation analysis of motor performance and chronological age found that neither striatal-motor deficits nor their associated anatomical correlates were inevitable consequences of senescence but were characteristic for a select subpopulation of aged mice with striatal-motor deficits. We found that mice that tested poorly on the balance rod had a significant increase in the number of MSI neurons with small dendritic arbors in various stages of atrophic degeneration. Conversely, 30-month-old mice that had no functional impairment showed no significant change in the number of neurons with atrophic dendrites. These data reinforce the premise that the correlation of structure and function plays an important role in the analysis of an aging population since data may vary based on the number of functionally impaired or unimpaired mice that make up an experimental group.     

Short-term exposure to an enriched environment enhances dendritic branching but not brain-derived neurotrophic factor expression in the hippocampus of rats with ventral subicular lesions

Environmental enrichment promotes structural and behavioral plasticity in the adult brain. We have evaluated the efficacy of enriched environment on the dendritic morphology and brain-derived neurotrophic factor (BDNF) expression in the hippocampus of ventral subicular-lesioned rats. Bilateral ventral subicular lesion has significantly reduced the dendritic architecture and spine density of hippocampal pyramidal neurons. The lesioned rats exposed to enriched housing for 10 days showed a significant degree of morphological plasticity in terms of enhanced dendritic branching and spine density. However, the BDNF expression in the hippocampus remained unchanged following subicular lesion and following environmental enrichment. We suggest the participation of other neurotrophic factors in mediating the synaptic plasticity events following exposure to environmental enrichment in ventral subicular-lesioned rats.     

Physiological variability in brain-derived neurotrophic factor expression predicts dendritic spine density in the mouse dentate gyrus

Dendritic spines are the predominant sites of excitatory neurotransmission in the adult brain, and brain-derived neurotrophic factor (BDNF) is a well-characterized determinant of dendritic spine number and morphology. The relationship between BDNF expression and dendritic spine number is particularly evident in the hippocampus, where environmental conditions that enhance hippocampal BDNF levels also promote local increases in dendritic spine density. However, the relationship between physiological variability in hippocampal BDNF expression and spine number has yet to be assessed. To determine whether natural variability in BDNF expression is associated with hippocampal dendritic spine number, correlations between BDNF protein levels and dendritic spine density among Golgi-impregnated neurons in the hippocampal dentate gyrus and CA1 subfields were assessed in adult male C57Bl/6J mice. In the dentate gyrus, but not in the apical oblique dendrites of CA1 pyramidal cells, BDNF protein expression was significantly correlated with dendritic spine density. This observation suggests that there may be a subregionally specific relationship between hippocampal BDNF expression and the density of spines.     

Chronic cocaine treatment alters dendritic arborization in the adult motor cortex through a CB1 cannabinoid receptor-dependent mechanism

The CB1 cannabinoid receptors modulate the addictive processes associated with different drugs of abuse, including psychostimulants. Mice lacking CB1 receptors exhibit an important attenuation of the reinforcing responses produced by cocaine in an operant self-administration paradigm. We have investigated the effect of chronic cocaine treatment on dendrite structure and spine density of the principal cortical neuron, the pyramidal neuron, in CB1 knockout mice and wild type littermates. Layer III pyramidal cells of the motor cortex were injected intracellularly in fixed cortical slices and their morphometric parameters analyzed. Under basal conditions, the field area of the dendritic arbors was more extensive and dendritic spine density was higher in wild type mice than in CB1 knockout mice. Chronic treatment of cocaine diminished the size and length of the basal dendrites and spine density on pyramidal cells from wild type mice. However, the total number of spines in the pyramidal cells of CB1 knockout mice augmented slightly following chronic cocaine treatment, although no changes in the morphology of the dendritic arbor were observed. Our data demonstrate that microanatomy and synaptic connectivity are affected by cocaine, the magnitude and nature of these changes depend on the presence of CB1 receptors.     

Alterations of dendritic protrusions over the first postnatal year of a mouse: an analysis in layer VI of the barrel cortex

Dendritic spines are small protrusions that serve as the principal recipients of excitatory inputs onto cortical pyramidal cells. Alterations in spine and filopodia density and morphology correlate with both developmental maturity and changes in synaptic strength. In order to better understand the developmental profile of dendritic protrusion (dendritic spines + filopodia) morphology and density over the animal’s first postnatal year, we used the Golgi staining technique to label neurons and their dendritic protrusions in mice. We focused on quantifying the density per length of dendrite and categorizing the morphology of dendritic protrusions of layer VI pyramidal neurons residing in barrel cortex using the computer assisted reconstruction program Neurolucida. We classified dendritic protrusion densities at seven developmental time points: postnatal day (PND) 15, 30, 60, 90, 180, 270, and 360. Our findings suggest that the dendritic protrusions in layer VI barrel cortex pyramidal neurons are not static, and their density as well as relative morphological distribution change over time. We observed a significant increase in mushroom spines and a decrease in filopodia as the animals matured. Further analyses show that as the animal mature there was a reduction in pyramidal cell dendritic lengths overall, as well as a decrease in overall protrusion densities. The ratio of apical to basilar density decreased as well. Characterizing the profile of cortical layer VI dendritic protrusions within the first postnatal year will enable us to better understand the relationship between the overall developmental maturation profile and dendritic spine functioning.     

小胶质细胞M1型极化诱发小鼠焦虑样行为及其对前额叶神经元树突棘密度的影响

目的:焦虑样行为与脑内小胶质细胞M1型极化关系密切,但M1型小胶质细胞如何作用于神经元导致焦虑样行为并不清楚,因此本研究的目的在于观察小鼠脑内小胶质细胞向M1型极化诱发产生焦虑样行为,并探讨其对神经元树突棘的影响。方法:实验组采用脂多糖(lipopolysaccharide,LPS)脑内注射诱导C57小鼠小胶质细胞向M1型极化,通过旷场实验和高架十字迷宫实验观察小鼠的行为学改变;应用免疫荧光染色方法观察实验组和对照组小鼠的内侧前额叶皮质(medial prefrontal cortex,m PFC)内一氧化氮合酶(i NOS)与小胶质细胞标志物iba-1的表达变化,通过Western Blot方法观察实验组和对照组小鼠的m PFC内i NOS、致炎因子白介素1(IL-1)β、肿瘤坏死因子(TNF)-α的表达变化,以i NOS、IL-1β和TNF-α表达量均显著增加作为小胶质细胞向M1型极化的标志。采用高尔基(Golgi)染色观察m PFC内神经元树突棘的变化。结果:(1)实验组小鼠mPFC内小胶质细胞i NOS、IL-1β和TNF-α表达量均升高,提示小胶质细胞向M1型极化。(2)实验组小鼠在旷场内的中央活动距离和中央活动时间比对照组均明显减少(P0.001);实验组小鼠在高架十字迷宫开臂停留时间比正常组明显减少(P0.05)。(3)Golgi染色结果显示实验组小鼠mPFC内神经元树突棘的密度相较于对照组增多(P0.05)。结论:mPFC内小胶质细胞M1型极化可诱发小鼠产生焦虑样行为,并伴随mPFC内神经元树突棘密度增多,提示脑内小胶质细胞M1型极化可诱导焦虑样行为,可能与内侧前额叶皮质锥体神经元树突棘可塑性改变相关。     

Dendritic morphology of callosal and ipsilateral projection neurons in monkey prefrontal cortex

Subpopulations of cortical pyramidal neurons have been distinguished based on the projection target of their principal axons or by their dendritic morphology. In this study, we sought to test the hypothesis that pyramidal neurons in monkey prefrontal cortex that furnish callosal or ipsilateral projections have distinctive dendritic morphologies. Retrogradely-labeled, Fast Blue-containing callosal and ipsilateral neurons were intracellularly filled with Lucifer Yellow, immunoconverted, and reconstructed. Quantitative measurements of the size and complexity of the dendritic arbor, including total dendritic length, horizontal extent, number of branch points, maximum branch order, and number of segments, as well as spine density, were made. In general, callosal neurons had larger and more complex dendritic arbors for both apical and basilar dendritic trees than did ipsilateral neurons. The greatest difference was in total dendritic length; the apical and basilar trees of callosal neurons were 34 and 25% longer, respectively. In addition, spine density was also significantly greater on the apical and basilar dendrites of callosal neurons. These findings could not be explained by differences in somal size or completeness of dendritic filling between callosal and ipsilateral neurons.Our observations support the hypothesis that callosal and ipsilateral neurons differ on a number of measures of dendritic size and complexity. Furthermore, these findings imply that these two subpopulations of pyramidal cells differ in the number and perhaps types of excitatory inputs that they receive. Finally, differences in the dendritic morphology of callosal and ipsilateral neurons have implications for understanding the functional attributes of these two populations of cells, as well as for the characterization of pyramidal neurons in human disease states.     

Dendritic spine abnormalities in the occipital cortex of C57BL/6 Fmr1 knockout mice.

Fragile X syndrome (FXS) is the most common form of inherited mental retardation. Observed neuropathologies associated with FXS include abnormal length, morphology, and density of dendritic spines, reported in individuals with FXS and in Fmr1 knockout (KO) mice, an animal model of FXS. To date, however, these neuropathologies have been studied in Fmr1 KO mice bred in a FVB background (a strain with genetic mutations that complicate interpretation of results) and findings have been inconsistent. Here, Golgi-Cox impregnation was used to investigate length, morphology, and density of dendritic spines on layer V pyramidal neurons in visual cortices of Fmr1 KO and wildtype (WT) mice bred in a C57BL/6 background. We report that spine abnormalities in these animals parallel abnormalities reported in humans with FXS, perhaps to a greater degree than KO mice bred in an FVB background. Specifically, Fmr1 KO mice bred in a C57BL/6 background exhibited significantly more longer dendritic spines and fewer shorter spines, as well as more spines with immature-appearing morphology and fewer with mature-appearing morphology than WT littermates. Spine length abnormalities were demonstrated to be largely independent of spine morphology abnormalities, as the length phenotype was observed in KOs even within a morphological category. Fmr1 KO mice also had a greater overall spine density than WTs. These findings provide powerful support for the essence of the dendritic spine abnormalities in the absence of FMRP, now found to be largely consistent with human data across two mouse backgrounds.     

Postweaning social isolation enhances morphological changes in the neonatal ventral hippocampal lesion rat model of psychosis

Neonatal ventral hippocampal (nVH) lesions in rats have been widely used as a neurodevelopmental model that mimics schizophrenia-like behaviors. Recently, we reported that nVH-lesions result in significant decreases in both length of dendrites and dendritic density of spines of pyramidal neurons of the prefrontal cortex (PFC) and in the density of dendritic spines of medium spiny neurons of the nucleus accumbens (NAcc). Moreover, postweaning social isolation induces major decreases in dendritic spiny density of PFC neurons. We investigated here the comparative dendritic morphology of PFC pyramidal neurons and NAcc medium spiny neurons in nVH rats, following social isolation after weaning (8 weeks). Morphological characteristics of dendrites were measured using the Golgi-Cox procedure followed by a Sholl analysis. Social isolation (SI) by itself induced decreases in dendritic length and dendritic spine density of the NAcc. In socially isolated nVH-lesion rats decrease in dendritic length in PFC and NAcc neurons were exacerbated whereas an increase in spine density of medium spiny neurons was observed in the NAcc. These results indicate that nVH-lesions alter dendritic morphology of NAcc and PFC neurons. These anatomical modifications in both structures may be relevant to behaviors observed in schizophrenia.     

Acute stress-induced neuronal plasticity in the corticoid complex of 15-day-old chick,Gallus domesticus

Several studies conducted on chicken have shown that a single stress exposure may impair or improve memory as well as learning processes. However, to date, stress effects on neuronal morphology are poorly investigated wherefore it was of interest to evaluate this further in chicks. Thus, the present study aims to investigate the role of single acute stress (AS) of 24 h food and water deprivation in neuronal plasticity in terms of spine density of the corticoid complex (CC) in 15-day-old chick, Gallus domesticus, by using three neurohistological techniques: Cresyl Violet, Golgi Colonnier, and Golgi Cox technique. The dorsolateral surface of the cerebral hemisphere is occupied by CC which can be differentiated into two subfields: an intermediate corticoid (CI) subfield (arranged in layers) and a dorsolateral corticoid (CDL) subfield. Based on different criteria such as soma shape, dendritic branching pattern, and dendritic spine density, two main moderately spinous groups of neuronal cells were observed in the CC, namely, projection neurons (comprising of multipolar and pyramidal neurons) and stellate neurons. In the present study, the stellate neurons have shown a significant decrease as well as an increase in their spine density in both CI and CDL subfields, whereas the multipolar neurons had shown a significant increase in their spine density in the CDL region only. The present study shows that AS induces neuronal plasticity in terms of spine density in both CI and CDL neurons. The morphological changes in the form of decreased dendritic branches due to stress have been observed in the CI region in comparison to CDL region, which could be linked to more effect of stress in this region. The avian CDL corresponds to the entorhinal cortex of mammals on the basis of neuronal morphology and bidirectional connections between adjacent areas. The projection neurons increase their branches and also their spine number to cope with the stress effects, while the stellate neurons show contrasting effect in their spine density. Therefore, this study will establish that slight modifications in natural stimuli or environmental changes faced by the animal may affect their dorsolateral forebrain which shows neuronal plasticity that help in the development of an adaptive capacity of the animal to survive under changing environmental conditions.     

Spontaneous Stereotypy in an Animal Model of Down Syndrome: Ts65Dn Mice

Stereotyped behaviors (e.g., body rocking) occur at high rates in individuals with mental retardation (e.g., Down syndrome). To determine if spontaneous stereotypy occurs in a murine model of Down syndrome, the home cage behavior of Ts65Dn and control mice was monitored during the dark cycle. Motor activity was further assessed in novel automated test chambers, with acoustic startle and rotor rod paradigms providing additional environmental challenges. Spontaneous stereotypy (repetitive jumping and cage top twirling) was observed in the home cage in approximately half of the Ts65Dn mice, compared with approximately 10% of diploid controls. Repetitive jumping was observed exclusively in the Ts65Dn mice. In the open field, although no differences were found between Ts65Dn and control mice, stereotypic Ts65Dn mice exhibited significantly less locomotor activity and rearing relative to control and nonstereotypic Ts65Dn mice. Ts65Dn mice attained significantly lower rotor rod speeds but did not differ from controls in the amplitude of the acoustic startle response. These environmental challenges did not increase stereotypy over home cage rates but induced stereotypy in two additional animals. The Ts65Dn model may aid in identifying genes associated with the development and expression of stereotypy.     

Dendritic spine and dendritic field characteristics of layer V pyramidal neurons in the visual cortex of fragile-X knockout mice

Fragile-X syndrome is a common form of mental retardation resulting from the inability to produce the fragile-X mental retardation protein. The specific function of this protein is unknown; however, it has been proposed to play a role in developmental synaptic plasticity. Examination of human brain autopsy material has shown that fragile-X patients exhibit abnormalities in dendritic spine structure and number, suggesting a failure of normal developmental dendritic spine maturation and pruning in this syndrome. Similar results using a knockout mouse model have previously been described; however, it was noted in retrospect that the mice used in that study may have carried a mutation for retinal degeneration, which may have affected cell morphology in the visual cortex of those animals. In this study, dendritic spines on layer V pyramidal cells of visual cortices, taken from fragile-X knockout and wild-type control mice without the retinal degeneration mutation and stained using the Golgi-Cox method, were investigated for comparison with the human condition. Quantitative analyses of the lengths, morphologies, and numbers of dendritic spines, as well as amount of dendritic arbor and dendritic branching complexity, were conducted. The fragile-X mice exhibited significantly more long dendritic spines and significantly fewer short dendritic spines than control mice. Similarly, fragile-X mice exhibited significantly more dendritic spines with an immature-like morphology and significantly fewer with a more mature type morphology. However, unlike the human condition, fragile-X mice did not exhibit statistically significant dendritic spine density differences from controls. Fragile-X mice also did not demonstrate any significant differences from controls in dendritic tree complexity or dendritic arbor. Long dendritic spines with immature morphologies are characteristic of early development or a lack of sensory experience. These results are similar to those found in the human condition and further support a role for the fragile-X mental retardation protein specifically in normal dendritic spine developmental processes. They also support the validity of these mice as a model of fragile-X syndrome.     

Control of hippocampal dendritic spine morphology through ephrin-A3/EphA4 signaling

Communication between glial cells and neurons is emerging as a critical parameter of synaptic function. However, the molecular mechanisms underlying the ability of glial cells to modify synaptic structure and physiology are poorly understood. Here we describe a repulsive interaction that regulates postsynaptic morphology through the EphA4 receptor tyrosine kinase and its ligand ephrin-A3. EphA4 is enriched on dendritic spines of pyramidal neurons in the adult mouse hippocampus, and ephrin-A3 is localized on astrocytic processes that envelop spines. Activation of EphA4 by ephrin-A3 was found to induce spine retraction, whereas inhibiting ephrin/EphA4 interactions distorted spine shape and organization in hippocampal slices. Furthermore, spine irregularities in pyramidal neurons from EphA4 knockout mice and in slices transfected with kinase-inactive EphA4 indicated that ephrin/EphA4 signaling is critical for spine morphology. Thus, our data support a model in which transient interactions between the ephrin-A3 ligand and the EphA4 receptor regulate the structure of excitatory synaptic connections through neuroglial cross-talk.     

Neuronal D-serine regulates dendritic architecture in the somatosensory cortex

D-Serine, which is synthesized by the enzyme serine racemase (SR), is a co-agonist at the N-methyl-D-aspartate receptor (NMDAR). In an animal model of NMDAR hypofunction, the constitutive SR knockout (SR-/-) mouse, pyramidal neurons in primary somatosensory cortex (S1) have reductions in the complexity, total length, and spine density of apical and basal dendrites. We wondered whether the dendritic pathology required deprivation of D-serine throughout development or reflected the loss of D-serine only in adulthood. To address this question, we used mice homozygous for floxed SR in which we bred CaMKIICre2834, which is expressed in forebrain glutamatergic neurons starting at 3-4 weeks post-partum (nSR-/-). Our prior studies demonstrated that the majority of cortical SR is expressed in glutamatergic neurons. We found that similar to SR-/- mice, pyramidal neurons in S1 of nSR-/- also had significantly reduced dendritic arborization and spine density, albeit to a lesser degree. S1 neurons of nSR-/- mice had reduced total basal dendritic length that was accompanied by less complex arborization. These characteristics were unaltered in the apical dendritic compartment. In contrast, spine density on S1 neurons was significantly reduced on apical, but not basal dendrites of nSR-/- mice. These results demonstrate that in adulthood neuronally derived D-serine, which is required for optimal activation of post-synaptic NMDAR activity, regulates pyramidal neuron dendritic arborization and spine density. Moreover, they highlight the glycine modulatory site (GMS) of the NMDAR as a potential target for therapeutic intervention in diseases characterized by synaptic deficits, like schizophrenia.