The effects of a lengthy period of environmental diversity on well-fed and previously undernourished rats. II. Synapse-to-neuron ratios

Decreased environmental complexity during development impairs habituation of reinforcer effectiveness of sensory stimuli

Previous research has shown that rats reared in simple/impoverished environments demonstrate greater repetitive responding for sensory reinforcers (e.g., light onset). Moreover, the brains of these rats are abnormally developed, compared to brains of rats reared in more complex/enriched environments. Repetitive behaviors are commonly observed in individuals with developmental disorders. Some of these repetitive behaviors could be maintained by the reinforcing effects of the sensory stimulation that they produce. Therefore, rearing rats in impoverished conditions may provide an animal model for certain repetitive behaviors associated with developmental disorders. We hypothesize that in rats reared in simple/impoverished environments, the normal habituation process to sensory reinforcers is impaired, resulting in high levels of repetitive behaviors. We tested the hypothesis using an operant sensory reinforcement paradigm in rats reared in simple/impoverished (IC), standard laboratory (SC), and complex/enrichened conditions (EC, treatments including postnatal handling and environmental enrichment). Results show that the within-session habituation of the reinforcer effectiveness of light onset was slower in the IC and SC rats than in the EC rats. A dishabituation challenge indicated that within-session decline of responses was due to habituation and not motor fatigue or sensory adaptation. In conclusion, rearing rats in simple/impoverished environments, and comparing them to rats reared in more complex/enriched environments, may constitute a useful approach for studying certain repetitive behaviors associated with developmental disorders.

Environmental Enrichment Rescues Binocular Matching of Orientation Preference in the Mouse Visual Cortex

Neural circuits are shaped by experience during critical periods of development. Sensory deprivation during these periods permanently compromises an organism's ability to perceive the outside world. In the mouse visual system, normal visual experience during a critical period in early life drives the matching of individual cortical neurons' orientation preferences through the two eyes, likely a key step in the development of binocular vision. Here, in mice of both sexes, we show that the binocular matching process is completely blocked by monocular deprivation spanning the entire critical period. We then show that 3 weeks of environmental enrichment (EE), a paradigm of enhanced sensory, motor, and cognitive stimulation, is sufficient to rescue binocular matching to the level seen in unmanipulated mice. In contrast, 6 weeks of conventional housing only resulted in a partial rescue. Finally, we use two-photon calcium imaging to track the matching process chronically in individual cells during EE-induced rescue. We find that for cells that are clearly dominated by one of the two eyes, the input representing the weaker eye changes its orientation preference to align with that of the dominant eye. These results thus reveal ocular dominance as a key driver of the binocular matching process, and suggest a model whereby the dominant input instructs the development of the weaker input. Such a mechanism may operate in the development of other systems that need to integrate inputs from multiple sources to generate normal neuronal functions.SIGNIFICANCE STATEMENT Critical periods are developmental windows of opportunity that ensure the proper wiring of neural circuits, as well as windows of vulnerability when abnormal experience could cause lasting damage to the developing brain. In the visual system, critical period plasticity drives the establishment of binocularly matched orientation preferences in cortical neurons. Here, we show that binocular matching is completely blocked by monocular deprivation during the critical period. Moreover, environmental enrichment can fully rescue the disrupted matching, whereas conventional housing of twice the duration results in a partial rescue. We then use two-photon calcium imaging to track individual cells chronically during the EE-induced recovery, and reveal important insights into how appropriate function can be restored to the nervous system after the critical period.

Alteration of Energy Metabolism and Antioxidative Processing in the Hippocampus of Rats Reared in Long-Term Environmental Enrichment

Environmental enrichment (EE) is a typical experimental method that promotes levels of novelty and complexity that enhance experience-dependent neuroplasticity and cognitive behavior function in laboratory animals. Early EE is associated with resilience in the face of later-life challenges. Since increased synaptic activity enhances endogenous neuronal antioxidant defenses, we hypothesized that long-term EE beginning at an early stage may alter the levels of oxidative stress. We investigated global protein expression and oxidative stress in hippocampal proteins from rats nurtured for a 6-month EE beginning in the prenatal period. The analysis of protein expression was carried out using 2-dimensional gel electrophoresis with matrix-associated laser desorption ionization time-of-flight mass spectrometry. Proteins with altered expression were involved in energy metabolism (phosphoglycerate mutase 1, α-enolase isoform 1, adenylate kinase 1, and triose phosphate isomerase) and antioxidant enzymes (superoxide dismutase 1, glutathione S-transferase ω type 1, peroxiredoxin 5, DJ-1, and glial maturation factor β). Using Western blot assays, some of the proteins with altered expression and NADPH oxidase 2 were confirmed to be decreased. Further confirmation was demonstrated with attenuated expression of 7,8-dihydro-8-oxo-deoxyguanine, a DNA oxidative stress marker, in the hippocampus of EE group rats. Our data demonstrate that a long-term EE program beginning in the prenatal and early postnatal phase of development modulates energy metabolism and reduced oxidant stress possibly through enhanced synaptic activity. We provide evidence that EE can be developed as a tool to protect the brain from oxidative stress-induced injury.

Toward a neurology of loneliness

Social isolation has been recognized as a major risk factor for morbidity and mortality in humans for more than a quarter century. The brain is the key organ of social connections and processes, however, and the same objective social relationship can be experienced as caring and protective or as exploitive and isolating. We review evidence that the perception of social isolation (i.e., loneliness) impacts brain and behavior and is a risk factor for broad-based morbidity and mortality. However, the causal role of loneliness on neural mechanisms and mortality is difficult to test conclusively in humans. Mechanistic animal studies provide a lens through which to evaluate the neurological effects of a member of a social species living chronically on the social perimeter. Experimental studies show that social isolation produces significant changes in brain structures and processes in adult social animals. These effects are not uniform across the brain or across species but instead are most evident in brain regions that reflect differences in the functional demands of solitary versus social living for a particular species. The human and animal literatures have developed independently, however, and significant gaps also exist. The current review underscores the importance of integrating human and animal research to delineate the mechanisms through which social relationships impact the brain, health, and well-being.

Vibrissa-signaled eyeblink conditioning induces somatosensory cortical plasticity

Whisker deflection conditioned stimuli (CS) were demonstrated to activate physiologically and anatomically defined barrels in the contralateral somatosensory cortex and to support trace-eyeblink conditioned responses when paired with corneal airpuff unconditioned stimuli in rabbits. Analysis of cytochrome-oxidase-stained somatosensory whisker-associated cortical barrels revealed a row-specific expansion of the conditioned compared with the nontrained hemisphere. This expansion was not evident in pseudo-conditioned rabbits, suggesting that this expansion of conditioned cortical barrels in response to a hippocampal- and forebrain-dependent learning task (trace conditioning) is associative rather than activity dependent. Using whisker stimulation as a CS in the well studied eyeblink conditioning paradigm will facilitate characterizing sensory cortical involvement in controlling and modulating an associatively learned response at the neural systems and cellular level.

The morphology of bi-directional experience-dependent cortical plasticity: a meta-analysis

Describing the neural mechanisms underlying learning and memory continues to be an intensive area of study within neuroscience. Of specific interest are changes in synaptic number and structure in the neocortex, which may play a distinct role in learning and memory. As such, characterizing the structural correlates of neocortical learning and memory may be critical to understanding the link between synaptic structure and function. Towards this understanding, a meta-analysis was conducted on several well-researched paradigms of behavioral plasticity, categorized by those which enhance or deprive plasticity-inducing experience (PIE). Results revealed several distinct groups. Several variables (spine size, density of multisynaptic terminals, vesicular content) showed distinct dynamics under enhanced vs. deprived PIE, but changed consistently within these categories, regardless of the manipulation. A second set of variables (i.e., density of excitatory, inhibitory, excitatory spinuous, and inhibitory spinuous synapses) showed the same qualitative changes following both enhanced and impoverished PIE. A third group (total synapse density, total basilar branches, apical spine density, total postsynaptic density size, and total bouton size) showed significant heterogeneity that could not be accounted for by partitioning enhancement and deprivation of PIE. However, this variance was accounted for by the modality and duration of the manipulation, the delay between this manipulation and sacrifice, and the stereological/methodological rigor of the study. These data, along with suggestions for future investigation based on gaps in the literature may go far towards the goal of relating neural structure and function.

Brain area- and isoform-specific inhibition of synaptic plasticity by apoE4

The allele E4 of apolipoprotein E4 (apoE4), the most prevalent genetic risk factor of Alzheimer's disease (AD), inhibits the improvements in learning and memory which result from exposure of apoE transgenic mice to environmental stimulation (ES). In the present study, we investigated the extent to which these cognitive deficits are associated with distinct presynaptic, postsynaptic and axonal impairments and whether these effects are brain area-specific. Exposure to an enriched environment of young mice transgenic for human apoE3, which is the AD benign apoE allele, increased the levels of the presynaptic protein synaptophysin and of the dendritic marker MAP-2 in the hippocampus and entorhinal cortex, whereas the corresponding levels of these proteins in the apoE4 transgenic mice were unaffected by the enriched environment. In contrast, the levels of synaptophysin and MAP-2 in the motor cortex were elevated by environmental stimulation in both the apoE3 and the apoE4 transgenic mice. These findings show that apoE4 inhibits synaptic plasticity following environmental stimulation and that this effect is both isoform- and brain area-specific.

Synaptic plasticity in thalamic nuclei enhanced by motor skill training in rat with transient middle cerebral artery occlusion

The goal of this study was to determine if synaptic plasticity in the thalamus of rats subjected to stroke could be altered by motor training. Transient occlusion of right middle cerebral artery in adult female Sprague-Dawley rats (n = 35) was induced with an intraluminal filament followed by three training conditions, 1. motor skill training on Rota-rod requiring balance and coordination skills, 2. simple exercise on treadmill, and 3. nontrained controls. Synaptic plasticity in brain was evaluated by synapotophysin immunocytochemistry at 14 or 28 days after training procedures. Infarct volume was determined in Nissl stained sections. Both at 14 and 28 days after Rota-rod training, intense synaptophysin immunoreactivity was present in the right but not the left mediodorsal and ventromedial nuclei of thalamus of ischemic rats. In treadmill-trained animals, however, similarly intense synaptic plasticity in these two thalamic nuclei was seen only at 28 days. Immunostaining was found also in other brain regions adjacent to or remote from infarct site. The data suggest that motor training, particularly motor skill training involving balance and coordination, facilitates a uniquely lateralized synaptogenesis in the thalamus.

Functional improvement after motor training is correlated with synaptic plasticity in rat thalamus

The goals of this study were to determine whether functional outcome after motor training in rats was linked to synaptic plasticity in thalamus, and whether the Rota-rod apparatus, widely used to test motor function, could be used as an easy and quantitative motor skill training procedure. Adult female Sprague-Dawley rats (n = 39) were evaluated under three training conditions: 1. Movement requiring balance and coordination skills on Rota-rod; 2. simple exercise on treadmill; 3. nontrained controls. Motor function was evaluated by a series of motor tests (foot fault placing, parallel bar crossing, rope and ladder climbing) before and 14 or 28 days after training procedure. Synaptic strength in brain was assessed by synaptophysin immunocytochemistry. After 14 days of training, Rota-rod-trained animals significantly (p < 0.01) improved motor performance, compared to treadmill and nontrained animals. Animals with up to 28 days of simple exercises on the treadmill did not show a significantly improved performance on most motor tasks, except for an improvement in foot fault placing. Intensive synaptophysin immunoreactivity was present in the right but not the left mediodorsal and ventromedial nuclei of thalamus in Rota-rod-trained rats at 14 and 28 days, and in treadmill-trained rats at 28 days. The data suggested that functional outcome is effectively improved by motor skill training rather than by simple exercises, and this may be related, at least partially, to uniquely lateralized synaptogenesis in the thalamus. Both Rota-rod and treadmill could be quantitatively used in rats for motor training of different complexity.

The interstitial nuclei of the human anterior hypothalamus: an investigation of variation with sex, sexual orientation, and HIV status

The interstitial nuclei of the human anterior hypothalamus (INAH1-4) have been considered candidates for homology with the sexually dimorphic nucleus of the preoptic area of the rat. Volumetric sexual dimorphism has been described for three of these nuclei (INAH1-3), and INAH3 has been reported to be smaller in homosexual than heterosexual men. The current study measured the INAH in Nissl-stained coronal sections in autopsy material from 34 presumed heterosexual men (24 HIV- and 10 HIV+), 34 presumed heterosexual women (25 HIV- and 9 HIV+), and 14 HIV+ homosexual men. HIV status significantly influenced the volume of INAH1 (8% larger in HIV+ heterosexual men and women relative to HIV- individuals), but no other INAH. INAH3 contained significantly more neurons and occupied a greater volume in presumed heterosexual males than females. No sex difference in volume was detected for any other INAH. No sexual variation in neuronal size or density was observed in any INAH. Although there was a trend for INAH3 to occupy a smaller volume in homosexual men than in heterosexual men, there was no difference in the number of neurons within the nucleus based on sexual orientation.

Neural consequences of environmental enrichment

Neuronal plasticity is a central theme of modern neurobiology, from cellular and molecular mechanisms of synapse formation in Drosophila to behavioural recovery from strokes in elderly humans. Although the methods used to measure plastic responses differ, the stimuli required to elicit plasticity are thought to be activity-dependent. In this article, we focus on the neuronal changes that occur in response to complex stimulation by an enriched environment. We emphasize the behavioural and neurobiological consequences of specific elements of enrichment, especially exercise and learning.

Visual input regulates the expression of basic fibroblast growth factor and its receptor

Emerging evidence indicates that the expression of trophic factors in the brain is regulated in an activity-dependent manner, which suggests an involvement of trophic factors in events controlled by input activity. We have investigated the possibility that visual sensory input impacts the expression of basic fibroblast growth factor and its receptor in the brain. Rats were maintained for seven days in darkness and then re-exposed to normal illumination for 0, 1, 3 or 6 h. We assessed relative levels of basic fibroblast growth factor and fibroblast growth factor receptor messenger RNAs using nuclease protection assays, and examined possible changes in the phenotypic expression of basic fibroblast growth factor and its receptor using immunohistochemistry. There was a significant decrease in levels of basic fibroblast growth factor and fibroblast growth factor receptor messenger RNAs as a result of dark rearing, and levels of messenger RNAs increased progressively with light re-exposure. Changes in messenger RNAs were observed primarily in the cerebral cortex (caudal portion) and were accompanied by alterations in the staining intensity and density of cells exhibiting basic fibroblast growth factor and fibroblast growth factor receptor phenotypes. Regulation of the basic fibroblast growth factor system by sensory input suggests that basic fibroblast growth factor, and perhaps other trophic factors, are mediators of the effects of experience on the structure and function of the CNS.

Homosexuality and the human genome project: private and public choices

Recent scientific research which offers evidence of genetic and biologic influence in homosexuality has created serious concerns. The intent of this article is to offer suggestions based in principles of bioethics in which perceived negative outcomes may be diminished and the positive qualities of the research enhanced. For a portion of the general population the concerns expressed in this article could be alleviated through public discussion and exposure to the findings and theories of the academic and scientific communities. For another portion of the population, however, additional safeguards against misuse of screening tests and somatic cell interventions may be advisable through efforts initiated by researchers themselves, general public policies, and additional medical policies. While these efforts are recommended as short term goals for the separate scientific and social paradigms of homosexuality, it is proposed that an equally important and related debate involves the subjects of disease, normality and the value of diversity. It is suggested that while it is imperative that the behavioral and biological sciences recognize the limitations of their separate approaches, the reductionist approach itself limits our understanding of what essentially are questions of attraction and relationships. In conclusion, homosexuality should be understood from the perspective of autonomy as every person's right to experience a full and meaningful life.

Learning-dependent synaptic modifications in the cerebellar cortex of the adult rat persist for at least four weeks

Several experiments have demonstrated increased synapse number within the cerebellar cortex in association with motor skill learning but not with motor activity alone. The persistence of these synaptic changes in the absence of continued training was examined in the present experiment. Adult female rats were randomly allocated to either an acrobatic condition (AC) or a motor activity condition (MC). The AC animals were trained to traverse a complex series of obstacles, and each AC animal was pair-matched with an MC animal that traversed an obstacle-free runway. These animals were further assigned to one of three training conditions. Animals in the EARLY condition were trained for 10 consecutive days before being killed, animals in the DELAY, condition received the same 10 d of training followed by a 28 d period without training, and animals in the CONTINUOUS condition were trained for the entire 38 d. Unbiased stereological techniques were used to obtain estimates of the number of synapses per Purkinje cell within the cerebellar paramedian lobule. Results showed the AC animals to have significantly more synapses per Purkinje cell than the MC animals in all three training conditions. There were no differences in the number of synapses per Purkinje cell among the EARLY, DELAY, and CONTINUOUS conditions. These data demonstrate that both the motor skills and the increases in synapse number presumed to support them persist in the absence of continued training.

Physical exercise induces FGF-2 and its mRNA in the hippocampus

Clinical and experimental evidence indicate that physical activity has a positive impact on brain function; however, the molecular bases for how exercise affects the structure and function of the brain are largely unknown. We have investigated the influences of variable periods of voluntary wheel-running on the expression of basic fibroblast growth factor and its mRNA in various brain regions. Nuclease protection assays revealed that the hippocampus was the only region examined exhibiting changes in FGF-2 mRNA as a result of exercise. FGF-2 mRNA increased to reach a peak by the 4th night of wheel-running. FGF-2 immunoreactivity, normally located in the perinuclear area of astrocytes, following exercise became stronger and appeared to spread to the cytoplasm and processes of astrocytes. Quantification of the FGF-2-immunoreactive astrocytes showed an increase in density between 2 and 4 nights of running in discrete regions of the hippocampus. These results demonstrate that exercise regulates FGF-2 expression and suggest that growth factors are likely mediators of the positive effects of exercise on the brain.

How to count synapses unbiasedly and efficiently at the ultrastructural level: proposal for a standard sampling and counting protocol

After almost 40 years, there is still no consensus on criteria for identifying different types of synapse seen in electron microscopical thin sections or on methods for counting them unbiasedly in 3D. This review proposes a procedure which meets these aims and could be adopted as a standard best-practice sampling and counting convention. It deals exclusively with unbiased stereological methods for counting particles in 3D space because these are efficient and applicable to arbitrary particles regardless of their size, shape and orientation. Methods based on individual sections are excluded because arbitrary particles cannot be counted unbiasedly with such sections. Model-based methods (e.g. treating synaptic membrane densities as circular disks) are excluded because they are not unbiased in general and now have limited (mainly historical) interest only. For unbiased counting, the absolute minimum requirement is a pair of parallel sections (dissector). The following protocol is recommended for future studies on synapse number: (1) use para(membrane) densities as synaptic counting units, (2) do not qualify definition of the counting unit by reference to a minimum number of synaptic vesicle profiles, (3) sample and count synapses unbiasedly using the dissector, and (4) in preference convert number per volume into absolute number or, in this is not possible, estimate a synapse-to-neuron ratio.

Effects of early protein malnutrition and environmental stimulation upon the reactivity to diazepam in two animal models of anxiety

In order to investigate the effects of early protein malnutrition and environmental stimulation upon the response to the anxiolytic properties of diazepam, two animal models of anxiety (elevated plus-maze and light-dark transition tests) were used. Rats were malnourished by feeding their dams a 6% protein diet during the lactation period (0-21 days of age) while well-nourished controls received a 16% protein diet. From 21 to 70 days of age all rats received a balanced lab chow diet. Environmental stimulation consisted of 3-min daily handling from birth to 70 days of age. Additional stimulation was provided from 21 to 70 days of age by rearing the rats in an enriched living cage. Eight groups of rats were studied in a 2 (malnourished or well-nourished) x 2 (stimulated or nonstimulated) x 2 (diazepam or vehicle) design. At 70 days of age, independent groups of rats treated with diazepam (2.5 mg/kg, IP) or vehicle were submitted to testing in the elevated plus-maze or light-dark transition procedures. The results showed that both diazepam and environmental stimulation reduced anxiety in the elevated plus-maze; stimulation changed the anxiolytic response to diazepam and the two diet conditions altered differentially the response to both pharmacological and stimulation procedures. These results suggest that environmental stimulation can affect differentially the behavioral response of malnourished and well-nourished rats treated with diazepam.

Quantitative aspects of the GABA circuitry in the primary visual cortex of the adult rat

The number and size of synaptic contacts made by GABA-immunoreactive axonal boutons were estimated in each layer of the primary visual cortex (area Oc1M) of adult rats by using the dissector method. Immunoreactivity for GABA was detected with the postembedding immunogold technique on ultrathin sections. Targets of GABA synaptic contacts were also identified to predict the sites of GABA influence in the rat visual cortex. For the total cortical depth, 82 million out of an overall population of 666 million synaptic contacts per mm3 of tissue (or 1 in 8 contacts, 12%) were GABA. Layer IV averaged 62% more GABA contacts per unit volume than did any other cortical layer. Consequently, these represented a larger proportion (1 in 6, 17%) of the overall population of layer IV synaptic contacts. This higher number of GABA contacts was not due to a greater density of GABA boutons, but to an increased number of contacts made by each layer IV GABA bouton (mean of 1.4 contacts per bouton compared to 1.1 in other cortical layers). The total area occupied by the contacts on an average GABA bouton was similar in all layers; the higher number of contacts per GABA bouton in layer IV being compensated for by their smaller size. This observed constancy in the area of synaptic contacts suggests the presence of one or more regulatory mechanisms maintaining optimal numbers of the different macromolecules forming the synaptic contacts. The increased density of GABA contacts in layer IV compared to other cortical layers was due to their greater number targeting distal regions of the dendritic tree. Since layer IV receives the vast majority of thalamocortical terminals and since these axons preferentially target dendritic spines, the specific arrangement of GABA synaptic contacts in this layer could be designed to exert a precise inhibition near the site of the thalamic input and thus serve as the structural basis for the strong GABA-related hyperpolarization that followed the excitatory response after physiological stimulations of the thalamocortical pathway.

Heterogeneity of synaptic density in the adult pigmented rabbit's visual cortex

In the adult pigmented rabbit, synaptic density in the lateral and medial part of the visual cortex was estimated along the projection area of the visual streak. A higher synaptic density distribution was observed in the lateral cortex (projection area of the nasal visual field) than in the medial cortex (projection area of the temporal visual field). This shows that there is a higher synaptic density in the region of the visual cortex receiving input from a retinal area with a high ganglion cell concentration than the area of the cortex receiving input from the retina with a low concentration of such cells. The regions of the visual cortex with higher and lower synaptic densities are the areas having a higher and lower magnification factor respectively.

Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. III. Origins and frequency of occurrence of the terminals

The cell bodies of the layer II/III pyramidal cells in rat visual cortex receive three morphologically distinct types of axon terminals. These axon terminals all form symmetric synapses and have been termed large, medium-sized, and dense axon terminals. The present study shows that each of these different kinds of axon terminals contains gamma-aminobutyric acid (GABA) which suggests that they are inhibitory. From an analysis of the profiles of 50 cell bodies it is calculated that the average layer II/III pyramidal cell has 65 axosomatic synapses, of which 43 are formed by medium-sized terminals, 10 by large terminals, and 12 by dense terminals. Comparison of these different kinds of axon terminals with labelled axon terminals of known origin suggests that the medium-sized terminals are derived from smooth multipolar cells with unmyelinated axons, and that at least some of the dense terminals originate from bipolar cells that contain vasoactive intestinal polypeptides. The source of the large axon terminals is not known, but it is suggested that they originate from multipolar non-pyramidal cells with myelinated axons. Since the initial axon segments of these same neurons receive GABAergic axon terminals from chandelier cells, at least four different types of neurons provide inhibition to the cell bodies and axons of layer II/III pyramidal cells. This serves as an illustration of the complexity of the neuronal circuits in which pyramidal cells are involved.

Dendritic morphology of pyramidal neurones of the visual cortex of the rat: III. Spine distributions

The vast majority of excitatory synaptic inputs to neocortical pyramidal cells terminate on dendritic spines, which can thus serve as markers, visible by light microscopy, for the locations of these synapses. The aim of this study was to provide estimates of the total numbers and distributions of spines on the dendrites of individual pyramidal neurones from layers 2/3 and 5 of the visual cortex of the rat. High magnification camera lucida drawings were made of dendritic segments lying close to the plane of section and the number of spines per unit length of dendrite calculated for each. These spine densities were used to estimate the numbers of spines on the other dendritic segments and the results were entered to a computer program that calculated various statistics. Mean total numbers of spines per cell were 7,965 +/- 2,723 (S.D.) for layer 2/3 cells, 8,647 +/- 3,097 for slender layer 5 cells, and 14,932 +/- 3,371 for thick layer 5 cells; these figures are in good agreement with previous stereological estimates. For all cell classes, 70% or more of spines were located on the basal and apical oblique dendrites. The distribution of spines with respect to cortical layers was also explored. Most cells had most of their spines in the layer containing the soma, but there were differences within and between cell classes. Layer 2/3 cells showed a progressive reduction in the proportion of their spines in layers 1 and 2 with increasing depth of their soma in the cortex. Thick layer 5 cells had substantial contributions from layers 4, 3, 2, and especially layer 1. Slender layer 5 cells had small contributions from layers 6 and 4, but relatively few spines in layers 3 and 2. The distribution of spines with path distance from the soma was explored by estimating the numbers of spines contained within a series of concentric shells centred on the soma. All cells showed a rapid increase in the number of spines per shell for the proximal 100 micrograms or so, followed by a sharp decline to approximately 250 micrograms, beyond which the number remained relatively constant until the end of the terminal arbor. In each case, the majority of spines were located within a path length of 150 micrograms from the soma.

Neuronal and synaptic measurements in the visual cortex of adult rats after undernutrition during normal or artificial rearing

It is possible that the reported effects of early life undernutrition on brain morphology may be due to alterations in mother-infant interactions and not directly to undernutrition. We have investigated this possibility by comparing artificially reared with mother-reared rats. Four groups of black-and-white hooded male rats were reared. These consisted of mother reared control (MRC), mother reared undernourished (MRU), artificially reared control (ARC) and artificially reared undernourished (ARU). Artificially reared rats were raised in isolation away from their mothers from 5 to 21 days of postnatal age. They were fitted with a gastric cannula through which 'milk' was infused automatically. The period of undernutrition lasted from 5 to 25 postnatal days, following which the animals were fed ad libitum until 312 days of age. Rats from each group were then killed by perfusion with buffered 2.5% glutaraldehyde. Pieces of visual cortex from each rat were postfixed in osmium tetroxide and embedded in resin. Stereological procedures at the light and electron microscopical levels were used to estimate the synapse-to-neuron ratios in cortical layers II to IV. Both MRC and ARC rats had about 7000 synapses per neuron. However, this ratio was about 8300 in MRU rats whilst it was only about 5000 in ARU animals. The rearing x nutrition interaction was statistically significant at the 0.1% level. These changes in the synapse-to-neuron ratio were mainly due to alterations in the numerical densities of the synapses rather than that of neurons. These results demonstrate that environmental isolation, as a result of artificial rearing procedures, and concurrent undernutrition during the first three weeks of postnatal life, interact with one another to produce marked morphological changes in the adult rat brain. However, environmental isolation was not, by itself, sufficient to cause permanent changes in interneuronal connectivity.

A multivariate statistical summary of synaptic plasticity measures in rats exposed to complex, social and individual environments

Multivariate analysis of variance, canonical discriminant analysis, factor analysis, and stepwise multiple regression were applied to 36 variables representing measures of occipital cortical synaptic, cellular, and vascular morphology in rats reared in complex, social, and individual housing environments. The results indicate differential expression of coordinated vascular and cellular metabolic processes across the three environments.

Effect of the richness of the environment on the cat visual cortex

In a recent study of the cat visual cortex, it was shown that there are interindividual differences in the numerical density (Nv) of symmetrical synapses associated with flat vesicles (FS synapses) but not of asymmetrical synapses associated with round vesicles (RA synapses). Since many of the environment-sensitive properties of visual cortex neurons are GABA-dependent, it was suggested that the interindividual differences in FS synapses might be due to environmental factors. To verify this possibility we estimated the Nv of both types of synapses in two groups of six cats, paired by litter and by sex, and raised either in isolation or in a colony from the time of weaning to the age of 8 months. We also measured the Nv of neurons and the thickness of the cortex and made some gross anatomical measurements. The brains of animals raised in the enriched environment are 7% heavier, and their total body weight is 10% greater: The brain-to-body-weight ratio remains unchanged. The total length of the brain is not affected, but the length and width of the cerebral hemispheres are each 5% greater in the enriched cats. As in comparable rat studies, the thickness of the cortex is 4% greater, but in the present study this difference is not significant. The numerical density of neurons is diminished by 17% in enriched animals. This is probably due to a wider separation of neuronal cell bodies in a larger cortical volume, rather than to a loss of neurons. There are no significant changes in the numerical density of RA synapses between the two milieux, but there are nearly twice as many FS synapses per mm3 of tissue in the impoverished cortex. The coefficient of variation of FS synapses, which in the previous study was on the order of 30%, has been reduced to 10% and 7% in enriched and impoverished cats, respectively. We conclude that environmental conditions can lead to selective interindividual differences in the Nv of FS synapses, as seen in our previous study of animals whose rearing conditions were not controlled. The average diameter of RA synaptic profiles is not affected by the environment but FS synapses are 25% wider in the enriched animals. Because of the smaller neuronal Nv in enriched animals, there are, in fact, 18% more RA synapses and 34% fewer FS synapses per neuron in the enriched condition.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of neurons and glia in the visual cortex (area 17) of the adult albino rat: a quantitative description

The neuronal and glial cell composition of the rat visual cortex (area 17) has been determined quantitatively using stereological techniques. The volume numerical densities (number of cells per mm3 of cortex) of neurons and of the principal glial cell types (astroglia, oligodendroglia, and microglia) were calculated from tangential semithin resin sections spaced at regular intervals 50 micron apart throughout the entire depth of the visual cortex. From measurements of cortical and laminar thickness the separate volume numerical densities of neurons and glial cells were derived for each lamina in the cortex. In addition, the absolute numbers of cells in each lamina under 1 mm2 of cortical surface were calculated. The mean cortical volume numerical density of neurons was 60,020 +/- 3840/mm3 (mean +/- SEM; n = 8), and 49,040 +/- 2610/mm3 for the combined glial cell types. Astroglia, oligodendroglia, and microglia were present in a ratio of 6:3:1 respectively. It was determined from neuronal and glial somatic volume estimates that the somata of these cells occupied approximately 13.5% of unit cortical volume, with 81.3% of the unit volume being occupied by cortical neuropil. Using previously published reports that described the laminar composition of neurons in terms of the relative proportions of pyramidal and non-pyramidal cells, the laminar volume numerical densities for these neuronal categories have been derived. In addition, it has been estimated that under 1 mm2 of cortical surface there are 79,500 pyramidal and 7790 non-pyramidal neurons distributed throughout layers 1-6 of the rat visual cortex.

A comparison of the number of neurons in individual laminae of cortical areas 17, 18 and posteromedial suprasylvian (PMLS) area in the cat

The binocular region of area 17 (17B) has a greater number of neurons under a given unit of cortical surface (NC, number per column) than either the monocular region of area 17 (17M), area 18 or the posteromedial suprasylvian area (PMLS). The latter three areas follow the general principle of basic uniformity in the number of neurons under given units of cortical surface formulated by Rockel et al. This basic uniformity is maintained in layers I and II. The NC of other layers varies. The greatest differences are found in layer IV where the NC of each area is significantly different from that of each other area. Though the difference between layer IV of 17M and of either 18 and PMLS is largely offset by changes in the third layer, layers V (PMLS) and VI (18 and PMLS) also contribute to the compensation. The compensation between 18 and PMLS is due entirely to changes in layer VI. It is important to note that there are statistically demonstrable interindividual differences in the neuronal NC of the cats used in the present study. We suggest that these may be due to age, breeding, or rearing conditions, probably the latter.

Evidence for active synapse formation or altered postsynaptic metabolism in visual cortex of rats reared in complex environments

Animals placed in complex environments develop greater numbers of visual cortex synapses per neuron than animals housed in standard cages. Increased numbers of synapses could theoretically arise from (i) active formation of new synapses, or (ii) selective stabilization of constitutively produced synapses. The postsynaptic location of polyribosomal aggregates appears to be an indicator of newly forming synapses. In developmental synaptogenesis and adult reactive (to injury) synaptogenesis, polyribosomes are more frequently found at spine synapses and are more likely to appear in the spine head and stem. In the visual cortex of rats from complex environments, there was a greater frequency of spine synapses associated with polyribosomes, relative to rats from individual or group cages. Furthermore, a greater percentage of these spines had polyribosomes in the head and stem region. This suggests that synapses in this region may be actively induced by neural activity arising from the complex environment experience.

The effects of a 30 day period of environmental diversity on well-fed and previously undernourished rats: neuronal and synaptic measures in the visual cortex (area 17)

Black and white Lister hooded rats were undernourished from the 16th day of gestation until 25 postnatal days of age. These previously undernourished rats and a set of well-fed rats were later subjected to 30 days of environmental diversity, i.e., environmental enrichment or isolation. Two separate experiments were carried out. In experiment 1, the environmental diversity lasted from 85 to 115 days of age and in experiment 2, from 35 to 65 days of age. At the end of the period of environmental diversity, all rats were killed by perfusion with 2% phosphate-buffered glutaraldehyde. Small pieces of tissue from the right visual cortex were embedded in Spurr's resin. For each rat two blocks of resin-embedded tissue were randomly selected, and from these semithin sections (0.5 micron) were cut and stained with toluidine blue. Photomicrographs of cortical layers II and III were taken from these sections and used to estimate the numerical density of neurons. Ultrathin (ca. 70 nm) sections of the same region of the cortex were cut and stained with lead citrate. These sections were used to estimate the synaptic disc diameter and numerical density. Synapse-to-neuron ratios were calculated from the estimates of synaptic and neuronal numerical densities. In experiment 1, well-fed rats raised in enriched environments had a significantly smaller neuronal numerical density and a greater synaptic disc diameter than well-fed rats raised in an impoverished environment. In experiment 2, neither the well-fed nor previously undernourished rats showed significant effects of environmental treatment on any of the features studied. The statistical interaction between nutrition and environment was not significant for any of the features in either experiment.