Enriched environment and acceleration of visual system development

Effects of maternal care on the development, emotionality, and reproductive functions in male and female rats

Variations in maternal behavior induce long-lasting effects on behavioral and neuroendocrine responses to stress. The aim of this study was to analyze developmental parameters, reproductive function, and anxiety-related behaviors of male and female rats raised by mothers that naturally display high and low levels of maternal licking behavior. Results showed that an increase in licking behavior received by the pups accelerated their eye opening and reduced fear behavior assessed in the open field test. Additionally, female offspring of high licking (HL) mothers showed decreased ovulation and lordosis intensity. In contrast, males from HL and low licking (LL) mothers did not differ in their reproductive function, suggesting a gender difference in maternal effects. Present results showed that individual differences in maternal behavior appear not only to be predictive of later emotionality and stress-responsivity in the offspring, but can also modulate the reproductive function of females. Maternal genetic factors, differences in the prenatal intrauterine milieu, or a combination of these cannot be excluded to explain the effects observed.

Environmental enrichment effects on development of retinal ganglion cell dendritic stratification require retinal BDNF

A well-known developmental event of retinal maturation is the progressive segregation of retinal ganglion cell (RGC) dendrites into a and b sublaminae of the inner plexiform layer (IPL), a morphological rearrangement crucial for the emergence of the ON and OFF pathways. The factors regulating this process are not known, although electrical activity has been demonstrated to play a role. Here we report that Environmental Enrichment (EE) accelerates the developmental segregation of RGC dendrites and prevents the effects exerted on it by dark rearing (DR). Development of RGC stratification was analyzed in a line of transgenic mice expressing plasma-membrane marker green fluorescent protein (GFP) under the control of Thy-1 promoter; we visualized the a and b sublaminae of the IPL by using an antibody selectively directed against a specific marker of cholinergic neurons. EE precociously increases Brain Derived Neurotrophic Factor (BDNF) in the retina, in parallel with the precocious segregation of RGC dendrites; in addition, EE counteracts retinal BDNF reduction in DR retinas and promotes a normal segregation of RGC dendrites. Blocking retinal BDNF by means of antisense oligos blocks EE effects on the maturation of RGC dendritic stratification. Thus, EE affects the development of RGC dendritic segregation and retinal BDNF is required for this effect to take place, suggesting that BDNF could play an important role in the emergence of the ON and OFF pathways.

Neuroconstructivism

Neuroconstructivism is a theoretical framework focusing on the construction of representations in the developing brain. Cognitive development is explained as emerging from the experience-dependent development of neural structures supporting mental representations. Neural development occurs in the context of multiple interacting constraints acting on different levels, from the individual cell to the external environment of the developing child. Cognitive development can thus be understood as a trajectory originating from the constraints on the underlying neural structures. This perspective offers an integrated view of normal and abnormal development as well as of development and adult processing, and it stands apart from traditional cognitive approaches in taking seriously the constraints on cognition inherent to the substrate that delivers it.

Cholinergic system, rearing environment and trajectory learning during aging in mice

Three, 12- and 20-month-old C57BL6/J mice, reared in standard conditions or in enriched environments, were administered subcutaneously either scopolamine hydrobromide, 0.6 or 1.2 mg kg(-1), or physiological saline (control mice) 15 min before testing their abilities to find an invisible platform in a modified version of the Morris water maze, the starting point being kept unchanged throughout the experiment to allow the aged animals to solve the task. The results demonstrated that: 1) All control mice, whatever their age, were able to learn the platform location, but the number of trials needed to reach the learning criterion (3 consecutive trials in less than 8 s) increased with age; 2) All the scopolamine-treated mice, whatever their age, were also able to learn the platform location. However, compared to age-matched controls, the number of trials needed to reach the learning criterion was greater; 3) Rearing the animals in an enriched environment antagonized the effect of scopolamine, but only in the youngest (3 month-old) mice. All control and scopolamine-treated mice, whatever their age and their rearing environment, remembered, 7 days later, the platform location.

Retinal functional development is sensitive to environmental enrichment: a role for BDNF

Retina has long been considered less plastic than cortex or hippocampus, the very sites of experience-dependent plasticity. Now, we show that retinal development is responsive to the experience provided by an enriched environment (EE): the maturation of retinal acuity, which is a sensitive index of retinal circuitry development, is strongly accelerated in EE rats. This effect is present also in rats exposed to EE up to P10, that is before eye opening, suggesting that factors sufficient to trigger retinal acuity development are affected by EE during the first days of life. Brain derived neurotrophic factor (BDNF) is precociously expressed in the ganglion cell layer of EE with respect to non-EE rats and reduction of BDNF expression in EE animals counteracts EE effects on retinal acuity. Thus, EE controls the development of retinal circuitry, and this action depends on retinal BDNF expression.

Early social enrichment shapes social behavior and nerve growth factor and brain-derived neurotrophic factor levels in the adult mouse brain

BACKGROUND

Early experiences produce persistent changes in brain and behavioral function. We investigate whether being reared in a communal nest (CN), a form of early social enrichment that characterizes the natural ecological niche of many rodent species including the mouse, has effects on adult social/aggressive behavior and nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) levels in mice.

METHODS

The CN consisted of a single nest where three mothers kept their pups together and shared care-giving behavior from birth to weaning (postnatal day 25).

RESULTS

Compared to standard laboratory conditions, in CN condition, mouse mothers displayed higher levels of maternal care. At adulthood, CN mice displayed higher propensity to interact socially and achieved more promptly the behavioral profile of either dominant or subordinate male. Furthermore, CN adult mice showed higher NGF levels, which were further affected by social status, and higher BDNF levels in the brain.

CONCLUSIONS

Our findings indicate that CN, a highly stimulating early social environment, produces differences in social behavior later in life associated with marked changes of neurotrophin levels in selected brain areas, including hippocampus and hypothalamus.

Exposure to a broad range of odorants decreases cell mortality in the olfactory bulb

Experience with multiple odorants during early postnatal development increases the number of cells in the olfactory bulb of rats. In this study, we asked whether at least part of this increase was due to decreased cell death. We selected 30 natural odorants or synthetic odorant mixtures to stimulate a broad area of the bulb during postnatal days 1-15, and counted the number of cells with DNA damage associated with cell death in both the glomerular and the granule cell layers of the main olfactory bulb. Early olfactory enrichment significantly decreased cell death in both bulbar laminae. Thus, olfactory enrichment can spare bulbar cells during early development, possibly leading to increased efficacy in bulb function and enhanced bulbar responses.

Enriched environment during adolescence changes brain-derived neurotrophic factor and TrkB levels in the rat visual system but does not offer neuroprotection to retinal ganglion cells following axotomy

The purpose of the present experiment was to characterize changes in TrkB signaling in the rat visual system resulting from exposure to enriched environment. Female Sprague-Dawley rats were placed in enriched or impoverished conditions for 1, 7 or 28 days. Levels of BDNF protein and its predominant receptor TrkB were examined in the retina, superior colliculus and visual cortex. In the retina, 1 day of enrichment increased full-length TrkB and after 28 days increased BDNF. In the superior colliculus, enrichment for 7 days reduced full-length TrkB and after 28 days increased BDNF and full-length TrkB. One day of enrichment significantly increased BDNF, reduced full-length TrkB and increased truncated TrkB in the visual cortex. Consequently, we further investigated whether exposure to enriched environment and the subsequent changes in BDNF and TrkB translates into a neuroprotective effect on retinal ganglion cells (RGCs) following transection of the optic nerve. Although exogenous intraocular application of BDNF provides neuroprotection to RGCs after axotomy, the endogenous increase in BDNF in the retina after 28 days of enrichment had no effect on RGC survival. While enriched housing conditions offer a model of non-invasive rehabilitation treatment for injury and modulates changes in BDNF and TrkB levels, these molecular changes did not translate into a neuroprotective effect on RGCs following transection of the optic nerve.

Environmental enrichment and voluntary exercise massively increase neurogenesis in the adult hippocampus via dissociable pathways

Environmental enrichment (EE) and voluntary exercise (VEx) have consistently been shown to increase adult hippocampal neurogenesis and improve spatial learning ability. Although it appears that these two manipulations are equivalent in this regard, evidence exists that EE and VEx affect different phases of the neurogenic process in distinct ways. We review the data suggesting that EE increases the likelihood of survival of new cells, whereas VEx increases the level of proliferation of progenitor cells. We then outline the factors that may mediate these relationships. Finally, we provide a model showing that VEx leads to the convergence of key somatic and cerebral factors in the dentate gyrus (DG) to induce cell proliferation. Although insufficient evidence exists to provide a similar model for EE, we suggest that EE-induced cell survival in the DG involves cortical restructuring as a means of promoting survival. We conclude that EE and VEx lead to an increase in overall hippocampal neurogenesis via dissociable pathways, and should therefore, be considered distinct interventions with regard to hippocampal plasticity and associated behaviors.

Early rearing environment impacts cerebellar growth in juvenile salmon

The size and structure of an animal's brain is typically assumed to result from either natural or artificial selection pressures over generations. However, because a fish's brain grows continuously throughout life, it may be particularly responsive to the environmental conditions the fish experiences during development. Salmon are an ideal model system for studying these effects because natural habitats differ significantly from the hatchery environments in which these fish are frequently reared. For example, in the wild, salmon alevins (i.e. yolk-sac fry) are buried in the gravel, while hatchery environments lack this structural component. We show that the simple manipulation of adding stones to a standard rearing tank can dramatically alter the growth of specific brain structures in steelhead salmon alevins(Oncorhynchus mykiss). We found that alevins reared with stones grew brains with significantly larger cerebella than genetically similar fish reared in conventional tanks. This shift to a larger cerebellar size was, in turn, accompanied by changes in locomotory behaviors - behaviors that correlate strongly to the function of this brain region. We next show that hatchery fish reared in a more naturalistic setting in the wild had significantly larger brains than their lab-reared counterparts. However,relative cerebellar volumes were similar between wild-reared alevins and those reared in the complex treatment in the laboratory. Together our results indicate that, within the first three weeks of life, variation in rearing environment can result in brain differences that are commonly attributed to generations of selection. These results highlight the need to consider enrichment strategies when designing captive rearing facilities for both conservation and laboratory use.

The layout of functional maps in area 18 of strabismic cats

Strabismus (or squint) is both a well-established model for developmental plasticity and a frequent clinical symptom. To analyze experience-dependent plasticity of functional maps in the brain we used optical imaging of intrinsic signals to visualize both orientation and ocular dominance domains in cat area 18. In strabismic animals, iso-orientation domains exhibited a pinwheel-like organization, as previously described for area 18 of normally raised animals and for area 17 of both normally raised and strabismic cats. In area 18, mean pinwheel density was similar in the experimental (2.2 pinwheel centers per mm2 cortical surface) and control animals (2.3/mm2 in normally raised animals), but significantly lower than in area 17 of both normally raised and strabismic cats (2.7-3.4/mm2). A comparison of orientation and ocular dominance domains revealed that iso-orientation domains were continuous across the borders of ocular dominance domains and tended to cross these borders at steep angles. Thus, the orientation map does not seem to be modified by experience-dependent changes in afferent activity. Together with our recent observation that strabismus does not enhance the segregation of ocular dominance domains in cat area 18, the present data indicate that the layout of functional maps in area 18 is less susceptible to experience-dependent manipulations than in area 17.

Cellular mechanisms underlying an effect of “early handling” on pCREB and BDNF in the neonatal rat hippocampus

Early experiences have long-term effects on brain function and behavior. However, the precise mechanisms involved still remain elusive. In an effort to address this issue, we employed the model of "early handling", which is known to affect the ability of the adult organism to respond to stressful stimuli, and determined its effects on hippocampal pCREB and BDNF 2, 4, and 8 h later. 8 h following "handling" on postnatal day 1, there was an increase in pCREB and BDNF positive cells in the hippocampus, a brain area which is a specific target of "handling". On the other hand, vehicle injection resulted in decreased pCREB and BDNF in both handled and non-handled animals 2 and 4 h later. The "handling"-induced increase of pCREB and BDNF was cancelled by inhibition of NMDA, AMPA/kainate, GABA-A, 5-HT1A or 5-HT2A/C receptors, as well as L-type voltage-gated Ca(2+) channels. It thus appears that "early handling" activates these neurotransmitter receptors, leading to increased intracellular Ca(2+), phosphorylation of the transcription factor CREB, and increased BDNF expression. BDNF can then exert its morphogenetic effects and thus "imprint" the effects of "handling" on the brain.

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.