Synaptic plasticity in cortical systems

Molecular pharmacological dissection of short- and long-term memory

1. It has been discussed for over 100 years whether short-term memory (STM) is separate from, or just an early phase of, long-term memory (LTM). The only way to solve this dilemma is to find out at least one treatment that blocks STM while keeping LTM intact for the same task in the same animal. 2. The effect of a large number of treatments infused into the hippocampus, amygdala, and entorhinal, posterior parietal or prefrontal cortex on STM and LTM of a one-trial step-down inhibitory avoidance task was studied. The animals were tested at 1.5 h for STM, and again at 24 h for LTM. The treatments were given after training. 3. Eleven different treatments blocked STM without affecting LTM. Eighteen treatments affected the two memory types differentially, either blocking or enhancing LTM alone. Thus, STM is separate from, and parallel to the first hours of processing of, LTM of that task. 4. The mechanisms of STM are different from those of LTM. The former do not include gene expression or protein synthesis; the latter include a double peak of cAMP-dependent protein kinase activity, accompanied by the phosphorylation of CREB, and both gene expression and protein synthesis. 5. Possible cellular and molecular events that do not require mRNA or protein synthesis should account for STM. These might include a hyperactivation of glutamate AMPA receptors, ribosome changes, or the exocytosis of glycoproteins that participate in cell addition.

Patterns of Nogo mRNA and protein expression in the developing and adult rat and after CNS lesions

Nogo-A is a neurite growth inhibitor involved in regenerative failure and restriction of structural plasticity in the adult CNS. Three major protein products (Nogo-A, -B, and -C) are derived from the nogo gene. Here we describe the embryonic and postnatal expression of the three Nogo isoforms in the rat by in situ hybridization and immunohistochemistry. Northern and Western blot analysis indicated that Nogo-A is predominantly expressed in the nervous system with lower levels also present in testis and heart. In CNS myelin, confocal and immunoelectron microscopy revealed that Nogo-A is expressed in oligodendrocyte cell bodies and processes and localized in the innermost adaxonal and outermost myelin membranes. Additionally, we find Nogo-A to be expressed by projection neurons, in particular during development, and by postmitotic cells in the developing cortex, spinal cord, and cerebellum. The expression levels of Nogo-A/B were not changed significantly after traumatic lesions to the cortex or spinal cord. Nogo-B showed widespread expression in the central and peripheral nervous systems and other peripheral tissues. Nogo-C was mainly found in skeletal muscle, but brain and heart were also found to express this isoform. The localization of Nogo-A in oligodendrocytes fits well with its role as a myelin-associated inhibitor of regenerative fiber growth and structural plasticity. However, expression of Nogo-A in other tissues and, in particular, in neurons and the widespread expression of the two shorter isoforms, Nogo-B and -C, suggest that the Nogo family of proteins might have function(s) additional to the neurite growth-inhibitory activity.

Have we been hebbing down the wrong path?

In this issue of Neuron, Stepanyants, Hof, and Chklovskii derive a simple mathematical formula to calculate, from measurable anatomical parameters, the capacity for neuronal wiring plasticity involving local synaptic rearrangements. Their work provides a deeper understanding of the potential contribution of structural plasticity to learning and memory.

Central mechanisms of motor skill learning

Recent studies have shown that frontoparietal cortices and interconnecting regions in the basal ganglia and the cerebellum are related to motor skill learning. We propose that motor skill learning occurs independently and in different coordinates in two sets of loop circuits: cortex-basal ganglia and cortex-cerebellum. This architecture accounts for the seemingly diverse features of motor learning.

Expression of c-Fos, ICER, Krox-24 and JunB in the whisker-to-barrel pathway of rats: time course of induction upon whisker stimulation by tactile exploration of an enriched environment

Modified tactile information has been shown to induce adaptive plasticity in the somatosensory cortex of rat. The cellular mechanisms resulting in plastic neuronal responses, however, are largely unknown. Inducible transcription factors have been proposed as one major link in the cascade from modified input to altered neuronal structure and function. We investigated the spatial and temporal patterns of transcription factor induction in the rat whisker-to-barrel pathway by placing the animals in a novel, enriched environment while having clipped sets of whiskers on one side of the face. Such stimulation resulted not only in a specific c-Fos induction in brainstem barrelettes and thalamic barreloids, but also in the barrel-related cortical columns, each with different time courses. In the barrel cortex, c-Fos and Krox-24 immunostaining showed a rapid induction with peak levels at 1 h and a return to basal levels after 14 h. JunB was induced after 1 h of exploration, declined at 6 h and returned to basal levels after this time point. The inducible cyclic AMP early repressor (ICER), a transcription factor of the cAMP signaling pathway, showed a maximum after 6 h, decreased slowly, but elevated levels were still detectable after 5 days. Our data demonstrate that upon whisker stimulation by exploration of a novel, enriched environment, (i) subcortical relay stations in the whisker-to-barrel pathway are able to express elevated levels of c-Fos and (ii) in the barrel cortex c-Fos, JunB, Krox-24 and ICER are differentially regulated in the temporal domain.

Dietary essential fatty acids and brain function: a developmental perspective on mechanisms

Brain development is a complex interactive process in which early disruptive events can have long-lasting effects on later functional adaptation. It is a process that is dependent on the timely orchestration of external and internal inputs through sophisticated intra- and intercellular signalling pathways. Long-chain polyunsaturated fatty acids (LCPUFA), specifically arachidonic acid and docosahexaenoic acid (DHA), accrue rapidly in the grey matter of the brain during development, and brain fatty acid (FA) composition reflects dietary availability. Membrane lipid components can influence signal transduction cascades in various ways, which in the case of LCPUFA include the important regulatory functions mediated by the eicosanoids, and extend to long-term regulation through effects on gene transcription. Our work indicates that FA imbalance as well as specific FA deficiencies can affect development adversely, including the ability to respond to environmental stimulation. For example, although the impaired water-maze performance of mice fed a saturated-fat diet improved in response to early environmental enrichment, the brains of these animals showed less complex patterns of dendritic branching. Dietary n-3 FA deficiency influences specific neurotransmitter systems, particularly the dopamine systems of the frontal cortex. We showed that dietary deficiency of n-3 FA impaired the performance of rats on delayed matching-to-place in the water maze, a task of the type associated with prefrontal dopamine function. We did not, however, find an association over a wider range of brain DHA levels and performance on this task. Some, but not all, studies of human infants suggest that dietary DHA may play a role in cognitive development as well as in some neurodevelopmental disorders; this possibility has important implications for population health.

Cholinergic nerve terminals establish classical synapses in the rat cerebral cortex: synaptic pattern and age-related atrophy

This study addresses the issue of whether cholinergic varicosities in the cerebral cortex establish 'classical synapses' or whether they communicate with their targets non-synaptically by 'volume transmission'. Most recent studies in the neocortex have suggested that acetylcholine acts non-synaptically, however in the present study we provide ultrastructural evidence that suggests synaptic mechanisms prevail. This conclusion is based upon our ultrastructural observations that cholinergic boutons--as revealed by immunoreactivity for the specific cholinergic market, vesicular acetylcholine transporter--establish a high percentage of classical synapses in layer V of the rat parietal cortex. Furthermore, the combination of this approach with the intracellular labeling of large pyramidal neurons on slice preparations revealed significant incidences of cholinergic contacts abutting preferentially on dendritic shafts. Finally, we have gathered information suggesting that cholinergic boutons undergo atrophy with aging which could be related to the well-known cholinergic and cognitive decline. These results illustrate that the cholinergic terminations in the neocortex establish proper synaptic connections and that they experience important age-dependent atrophy.

Motor learning-dependent synaptogenesis is localized to functionally reorganized motor cortex

The regional specificity and functional significance of learning-dependent synaptogenesis within physiologically defined regions of the adult motor cortex are described. In comparison to rats in a motor activity control group, rats trained on a skilled reaching task exhibited an areal expansion of wrist and digit movement representations within the motor cortex. No expansion of hindlimb representations was seen. This functional reorganization was restricted to the caudal forelimb area, as no differences in the topography of movement representations were observed within the rostral forelimb area. Paralleling the physiological changes, trained animals also had significantly more synapses per neuron than controls within layer V of the caudal forelimb area. No differences in the number of synapses per neuron were found in either the rostral forelimb or hindlimb areas. This is the first demonstration of the co-occurrence of functional and structural plasticity within the same cortical regions and provides strong evidence that synapse formation may play a role in supporting learning-dependent changes in cortical function.

Brain-derived neurotrophic factor enhances neuronal translation by activating multiple initiation processes: comparison with the effects of insulin

The effects of neurotrophic factors on translational activation were investigated in cortical neurons. Brain-derived neurotrophic factor (BDNF) increased protein synthesis within 30 min, whereas insulin produced a weaker enhancement of protein synthesis. BDNF-triggered protein synthesis was inhibited by LY294002, PD98059, and rapamycin, whereas the effect of insulin was unaffected by PD98059. To explore the mechanisms underlying this effect, the protein phosphorylation cascades that lead to the activation of translation initiation in neurons were examined. BDNF induced the phosphorylation of both eukaryote initiation factor (eIF) 4E and its binding protein (eIF4E-binding protein-1). The former reaction was inhibited by PD98059, whereas the latter was inhibited by LY294002 or rapamycin. In agreement, BDNF induced the phosphorylation of mammalian TOR (target of rapamycin) and enhanced its kinase activity toward eIF4E-binding protein-1. In contrast, insulin failed to activate MAPK and did not induce the phosphorylation of eIF4E. Since BDNF and insulin increased the activity of eIF2B and eIF2, the only difference between them was eIF4E phosphorylation. Thus, this may explain the lower activity of insulin in potentiating neuronal protein synthesis. These results suggest strongly that BDNF simultaneously activates multiple signaling cascades consisting of phosphatidylinositol 3-kinase, mammalian TOR, and MAPK to enhance translation initiation in neurons.

Spatial long-term memory is related to mossy fiber synaptogenesis

Structural synaptic changes have been suggested to underlie long-term memory formation. In this work, we investigate if hippocampal mossy fiber synaptogenesis induced by water maze overtraining can be related with long-term spatial memory performance. Rats were trained in a Morris water maze for one to five identical daily sessions and tested for memory retrieval 1 week and 1 month after training. After the last test session, the rat brains were obtained and processed for Timm's staining to analyze mossy fiber projection. The behavioral results showed that with more training, animals showed a better performance in the memory tests, and this performance positively correlates with Timm's staining in the stratum oriens. Furthermore, with the use of the NMDA antagonist MK801 before, but not after acquisition, water maze spatial memory was impaired. Increased Timm's staining in the stratum oriens was observed in the animals treated with MK801 after acquisition but not in those treated before. Finally, we observed that mossy fiber synaptogenesis occurs mainly in the septal region of the dorsal hippocampus, supporting the idea that this anterior region is important for spatial memory. Altogether, these results suggest that mossy fiber synaptogenesis can be related with spatial long-term memory formation.

Dendritic spine hypoplasticity and downregulation of reelin and GABAergic tone in schizophrenia vulnerability

In this review, we will first present a brief overview of the current understanding of: (a) the biology of reelin; (b) the putative reelin signaling pathways via integrin receptor stimulation; (c) the cytosolic adapter protein DAB1, which appears to be operative in the transduction of reelin's pleiotropic actions in embryonic, adolescent, and adult brain; (d) the regulation of GABAergic function, including some aspects of GABAergic system development; and (e) dendritic spine function and its role in the regulation of synaptic plasticity. We argue that a downregulation of reelin expression occurring in prefrontal cortex and in every brain structure of schizophrenia patients so far studied may be associated with a decrease in dendritic spine expression that in turn may provide an important reduction of cortical function as documented by the downregulation of glutamic acid decarboxylase67 (GAD67) expression, which might be secondary to a reduction of GABAergic axon terminals. This hypothesis is supported by a genetic mouse model of reelin haploinsufficiency that replicates the above-described dendritic and presynaptic GABAergic defects documented in schizophrenia brains.

Impaired water maze learning performance without altered dopaminergic function in mice heterozygous for the GDNF mutation

Exogenous glial cell line-derived neurotrophic factor (GDNF) exhibits potent survival-promoting effects on dopaminergic neurons of the nigrostriatal pathway that is implicated in Parkinson's disease and also protects neurons in forebrain ischemia of animal models. However, a role for endogenous GDNF in brain function has not been established. Although mice homozygous for a targeted deletion of the GDNF gene have been generated, these mice die within hours of birth because of deficits in kidney morphogenesis, and, thus, the effect of the absence of GDNF on brain function could not be studied. Herein, we sought to determine whether adult mice, heterozygous for a GDNF mutation on two different genetic backgrounds, demonstrate alterations in the nigrostriatal dopaminergic system or in cognitive function. While both neurochemical and behavioural measures suggested that reduction of GDNF gene expression in the mutant mice does not alter the nigrostriatal dopaminergic system, it led to a significant and selective impairment of performance in the spatial version of the Morris water maze. A standard panel of blood chemistry tests and basic pathological analyses did not reveal alterations in the mutants that could account for the observed performance deficit. These results suggest that endogenous GDNF may not be critical for the development and functioning of the nigrostriatal dopaminergic system but it plays an important role in cognitive abilities.

Associative learning elicits the formation of multiple-synapse boutons

The formation of new synapses has been suggested to underlie learning and memory. However, previous work from this laboratory has demonstrated that hippocampus-dependent associative learning does not induce a net gain in the total number of hippocampal synapses and, hence, a net synaptogenesis. The aim of the present work was to determine whether associative learning involves a specific synaptogenesis confined to the formation of multiple-synapse boutons (MSBs) that synapse with more than one dendritic spine. We used the behavioral paradigm of trace eyeblink conditioning, which is a hippocampus-dependent form of associative learning. Conditioned rabbits were given daily 80-trial sessions to a criterion of 80% conditioned responses in a session. During each trial, the conditioned stimulus (tone) and the unconditioned stimulus (corneal airpuff) were presented with an intervening trace interval of 500 msec. Brain tissue was taken for morphological analyses 24 hr after the last session. Unbiased stereological methods were used for obtaining estimates of the total number of MSBs in the stratum radiatum of hippocampal subfield CA1. The results showed that the total number of MSBs was significantly increased in conditioned rabbits as compared with pseudoconditioned or unstimulated controls. This conditioning-induced change, which occurs without a net synaptogenesis, reflects a specific synaptogenesis resulting in MSB formation. Models of the latter process are proposed. The models postulate that it requires spine motility and may involve the relocation of existing spines from nonactivated boutons or the outgrowth of newly formed spines for specific synaptogenesis with single-synapse boutons activated by the conditioning stimulation.

Forced limb-use effects on the behavioral and neurochemical effects of 6-hydroxydopamine

Rats with unilateral depletion of striatal dopamine (DA) show marked preferential use of the ipsilateral forelimb. Previous studies have shown that implementation of motor therapy after stroke improves functional outcome (Taub et al., 1999). Thus, we have examined the impact of forced use of the impaired forelimb during or soon after unilateral exposure to the DA neurotoxin 6-hydroxydopamine (6-OHDA). In one group of animals, the nonimpaired forelimb was immobilized using a cast, which forced exclusive use of the impaired limb for the first 7 d after infusion. The animals that received a cast displayed no detectable impairment or asymmetry of limb use, could use the contralateral (impaired) forelimb independently for vertical and lateral weight shifting, and showed no contralateral turning to apomorphine. The behavioral effects were maintained throughout the 60 d of observation. In addition to the behavioral sparing, these animals showed remarkable sparing of striatal DA, its metabolites, and the expression of the vesicular monoamine transporter, suggesting a decrease in the extent of DA neuron degeneration. Behavioral and neurochemical sparing appeared to be complete when the 7 d period of immobilization was initiated immediately after 6-OHDA infusion, only partial sparing was evident when immobilization was initiated 3 d postoperatively, and no sparing was detected when immobilization was initiated 7 d after 6-OHDA treatment. These results suggest that physical therapy may be beneficial in Parkinson's disease.

Induction of long-term depression in cerebellar Purkinje cells requires a rapidly turned over protein

Evidence is presented indicating that the induction of long-term depression (LTD) in Purkinje cells (PCs) requires a rapidly turned over protein(s) during a critical time period within 15 min after the onset of LTD-inducing stimulation and that synthesis of this protein is maintained by mRNAs supplied via transcription. LTD was induced in granule cell axon (GA)-to-PC synapses by stimulation of these synapses at 1 Hz for 5 min in conjunction with the climbing fibers (CFs) forming synapses on the same PCs and represented by a persistent reduction in the GA-induced excitatory postsynaptic potentials (EPSPs). Not only a prolonged but also a brief (5 min) pulse application of translational inhibitors (anisomycin, puromycin, or cycloheximide) effectively blocked the LTD induction. Pulses applied during the period from 30 min before to 10 min after the onset of conjunctive stimulation blocked the LTD induction, but those applied 15 min after were ineffective. The three translational inhibitors blocked the LTD induction similarly, suggesting that the effect is due to their common action of inhibiting protein synthesis. Infusion of a mRNA cap analogue (7-methyl GTP) into PCs also blocked LTD induction, ensuring that the postsynaptic protein synthesis within PCs is required for LTD induction. Transcriptional inhibitors, actinomycin D and 5,6-dichloro-l-beta-D-ribofuranosyl-benzimidazole, also blocked the LTD induction, but this effect was apparent when 5-min pulses of the transcriptional inhibitors preceded the conjunctive stimulation by 30 min or more. This time lag of 30 min is presumed to be required for depletion of the protein(s) required for LTD induction. The presently observed effects of translational and transcriptional inhibitors on the LTD induction are of temporal characteristics corresponding to their depressant effects on the type-1 metabotropic glutamate-receptor (mGluR1)-mediated slow EPSPs in PCs as we have reported recently. An antagonist of mGluR1s [(RS)-1-aminoindan-1,5-dicarboxylic acid], however, did not block LTD induction when it was applied during the 10-min period following conjunctive stimulation, where translational inhibitors effectively blocked LTD induction. This discrepancy in time course suggests that the rapidly turned over protein(s) required for LTD induction is involved in a process occurring downstream of the activation of mGluR1s.

Nerve growth factor induces light adaptive cellular and synaptic plasticity in the outer retina of fish

Recent evidence suggests that neurotrophins can be involved in short-term synaptic plasticity in parts of the central nervous system. In the present study, the possible role of nerve growth factor (NGF) in inducing morphologic (cellular and subcellular) changes in the outer retina of carp was assessed. The effects of NGF on cone photomechanical movements (PMMs) and horizontal cell (HC) spinule formation were measured. NGF-induced cone contraction and formation of HC spinules in the dark-adapted retina were consistent with its role in light adaptation. These effects were dose dependent in the range of 5--250 nM. Because cone contraction and HC spinule formation have previously been shown to be controlled by dopamine (DA), nitric oxide (NO), or both, the possibility that the effects of NGF could be occurring by means of release of DA and/or NO was tested. Haloperidol (HAL), a nonspecific DA receptor blocker, or 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium (cPTIO), a NO scavenger, was applied in combination with NGF to dark-adapted eyecups. The results showed that both HAL and cPTIO significantly blocked the effects of NGF on cone PMMs and HC spinule formation. In conclusion, (1) NGF represents a novel light-adaptive signalling mechanism in the outer retina of fish; and (2) NGF-induced cone contraction and HC spinule formation in the retina together with our previous observation would suggest that the effects of NGF may be mediated through NO by means of DA.

Spontaneous corticospinal axonal plasticity and functional recovery after adult central nervous system injury

Although it is believed that little recovery occurs after adult mammalian spinal cord injury, in fact significant spontaneous functional improvement commonly occurs after spinal cord injury in humans. To investigate potential mechanisms underlying spontaneous recovery, lesions of defined components of the corticospinal motor pathway were made in adult rats in the rostral cervical spinal cord or caudal medulla. Following complete lesions of the dorsal corticospinal motor pathway, which contains more than 95% of all corticospinal axons, spontaneous sprouting from the ventral corticospinal tract occurred onto medial motoneuron pools in the cervical spinal cord; this sprouting was paralleled by functional recovery. Combined lesions of both dorsal and ventral corticospinal tract components eliminated sprouting and functional recovery. In addition, functional recovery was also abolished if dorsal corticospinal tract lesions were followed 5 weeks later by ventral corticospinal tract lesions. We found extensive spontaneous structural plasticity as a mechanism correlating with functional recovery in motor systems in the adult central nervous system. Experimental enhancement of spontaneous plasticity may be useful to promote further recovery after adult central nervous system injury.

Enhanced CREB phosphorylation in immature dentate gyrus granule cells precedes neurotrophin expression and indicates a specific role of CREB in granule cell differentiation

Differentiation and maturation of dentate gyrus granule cells requires coordinated interactions of numerous processes. These must be regulated by protein factors capable of integrating signals mediated through diverse signalling pathways. Such integrators of inter and intracellular physiological stimuli include the cAMP-response element binding protein (CREB), a leucine-zipper class transcription factor that is activated through phosphorylation. Neuronal activity and neurotrophic factors, known to be involved in granule cell differentiation, are major physiologic regulators of CREB function. To examine whether CREB may play a role in governing coordinated gene transcription during granule cell differentiation, we determined the spatial and temporal profiles of phosphorylated (activated) CREB throughout postnatal development in immature rat hippocampus. We demonstrate that CREB activation is confined to discrete, early stages of granule cell differentiation. In addition, CREB phosphorylation occurs prior to expression of the neurotrophins BDNF and NT-3. These data indicate that in a signal transduction cascade connecting CREB and neurotrophins in the process of granule cell maturation, CREB is located upstream of neurotrophins. Importantly, CREB may be a critical component of the machinery regulating the coordinated transcription of genes contributing to the differentiation of granule cells and their integration into the dentate gyrus network.

DNA recombination as a possible mechanism in declarative memory: a hypothesis

The durability of declarative memory suggests that it has either a chemical or a structural basis. Current models of long-term memory are based on the general assumption that traces of memory are stored by structural modifications of synaptic connections, resulting in alterations in the patterns of neural activity. Changes in gene expression, regulated at both the transcriptional and the translational levels, are considered essential for structural synaptic modifications. Here we present an alternative hypothesis stating that permanent memory has a chemical rather than a structural basis. We suggest that the mechanism of memory coding in the brain is similar to that in the immune system so that the permanence of memories in the nervous system is ensured at the genomic level by a somatic recombination mechanism. Thus, we hypothesize that traces of permanent declarative memory might present within cerebral neurons in the form of novel proteins coded by the modified genes. This discussion is intended to provide evidence in support of a DNA recombination mechanism for memory storage in the brain and to stimulate further research working toward the evaluation of this hypothesis.

Development of NMDA and non-NMDA receptor-mediated excitatory synaptic transmission in geniculocortical and corticocortical connections in organotypic coculture preparations

Development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated excitatory synaptic transmission was studied in the visual cortex using organotypic slice cocultures. A slice of visual cortex (VC) dissected from newborn rats was cocultured with either a chunk of embryonic lateral geniculate nucleus (LGN) or another VC. During 7-38 days in vitro (DIV), geniculocortical monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded from layer IV neurons in response to stimulation of the LGN in cocultures of the VC with the LGN. Similarly, corticocortical monosynaptic EPSPs were recorded from layers II/III and V/VI neurons in cocultures of two VCs when stimulating the partner VC. The initial slopes of the non-NMDA and NMDA receptor-mediated components of the EPSPs, which were dissociated pharmacologically, were assessed and compared among three different culture stages, early (7-11 DIV), middle (12-15 DIV) and late (17-38 DIV). With progression of the culture stage, the non-NMDA component tended to increase in both the geniculocortical and corticocortical connections. In contrast, the NMDA component exhibited distinct developmental changes. The NMDA component in layer IV neurons, which receive geniculate inputs, showed a transient increase in the middle stage. In the corticocortical connection, the magnitude of the NMDA component was large in the early stage and maintained through all culture stages in layer V/VI cells, whereas in layer II/III cells it decreased sharply by the late stage. Our results suggest that glutamatergic transmission in the visual cortex develops differently in the geniculocortical and corticocortical connections.

Exploration of a novel environment leads to the expression of inducible transcription factors in barrel-related columns

Tactile information acquired through the vibrissae is of high behavioral relevance for rodents. Numerous physiological studies have shown adaptive plasticity of cortical receptive field properties due to stimulation and/or manipulation of the whiskers. However, the cellular mechanisms leading to these plastic processes remain largely unknown. Although genomic responses are anticipated to take place in this sequel, virtually no data so far exist for freely behaving animals concerning this issue. Thus, adult rats were placed overnight in an enriched environment and most of them were also subjected to clipping of different sets of whiskers. This type of stimulation led to a specific and statistically significant increase in the expression of the protein products of the inducible transcription factors c-Fos, JunB, inducible cyclic-AMP early repressor and Krox-24 (also frequently named Zif268 or Egr-1), but not c-Jun. The response was found in columns of the barrel cortex corresponding to the stimulated vibrissae; it displayed a layer-specific pattern. However, no induction of transcription factors was observed in the subcortical relay stations of the whisker-to-barrel pathway, i.e. the trigeminal nuclei and the ventrobasal complex. These results strongly suggest that a coordinated transcriptional response is initiated in the barrel cortex as a consequence of processing of novel environmental stimuli.

Neurogenesis in the adult brain: death of a dogma

For over 100 years a central assumption in the field of neuroscience has been that new neurons are not added to the adult mammalian brain. This perspective examines the origins of this dogma, its perseverance in the face of contradictory evidence, and its final collapse. The acceptance of adult neurogenesis may be part of a contemporary paradigm shift in our view of the plasticity and stability of the adult brain.

New neurochemical markers for psychosis: a working hypothesis of their operation

Reelin (Reln) is expressed in specific GABAergic neurons in layer I and II of neocortex, and is secreted into the extracellular matrix where it surrounds dendrites, spines and neurite arborizations, and binds to integrin receptors located on post-synaptic densities of apical dendritic spines. Experiments in rodents (including wild type or reeler heterozygous mice) and non-human primates suggest the Reln secreted in the extracellular matrix of neocortex, via integrin receptors, modulates the function of the adaptor protein DAB1(drosophila disable-gene) homologous product) thereby participating in dynamic processes associated with plasticity changes in dendrites, dendritic spines and their synapses. A local protein synthesis at dendritic spines (ie the activity regulated cytoskeleton associated protein, Arc) probably acts as a signal for plastic modulatory activities in synapses operative in neural group interactions. A research strategy directed toward identifying specific neurochemical markers operative in the etiopathology of psychotic disorders lead to the identification of a downregulation (30-50%) of Reln and glutamic acid decarboxylase 67(GAD67) expression in prefrontal cortex and other brain areas of schizoprenia and bipolar disorder patients with psychosis. These downregulations were not due to neuronal damage, postmortem interval, or antipsychotic medication. The dysfunction of GABAergic interneurons observed in psychotic brains in combination with reduced Reln expression and downregulation of Reln-integrin receptor interaction, may provide an explanation for the reported decrease in neuropile expression including dendritic spine density reduction, in neocortex of schizophrenia patients. This downregulation of neuropile plasticity may be a factor to be considered in the etiology of the disintegration of consciousness, which is one of the primary signs of psychosis.

Cognition. The holey grail of general intelligence

Exactly what is human intelligence and how should we measure it? Such questions have plagued us since the time of Plato, who first proposed that the seat of intelligence is in the brain. In a Perspective, Sternberg discusses new findings (Duncan et al.) that purport to show that the seat of general intelligence is located in the brain's frontal lobes. But, as Sternberg points out, measures of intelligence are only as good as the tests you use and the conclusions that you draw from them.

Discussion of developmental plasticity: factors affecting cognitive outcome after pediatric traumatic brain injury

Current research on plasticity has altered the over simplistic view of greater capacity in the developing brain after injury. In another paper in this issue, Dennis provides a model to elucidate the complexity of the multiple factors that influence recovery after brain injury in children. The authors present a brief summary of findings from their longitudinal research in neurobehavioral recovery after traumatic brain injury in children and adolescents that elaborates on the framework of Dennis. The discussion highlights the psychobiological factors that interact to define developmental plasticity and outlines promising directions for future research to elucidate and promote long-term recovery in pediatric brain-injured populations.

Conceptualizing functional neuroplasticity

There are at least four major forms of functional neuroplasticity that can be studied in humans: homologous area adaptation, cross-modal reassignment, map expansion, and compensatory masquerade. Homologous area adaptation is the assumption of a particular cognitive process by a homologous region in the opposite hemisphere. Cross-modal reassignment occurs when structures previously devoted to processing a particular kind of sensory input now accepts input from a new sensory method. Map expansion is the enlargement of a functional brain region on the basis of performance. Compensatory masquerade is a novel allocation of a particular cognitive process to perform a task. By focusing on these four forms of functional neuroplasticity, several fundamental questions about how functional cooperation between brain regions is achieved can be addressed.

Spatial learning-induced increase in the argyrophilic nucleolar organizer region of dorsolateral telencephalic neurons in goldfish

Spatial learning and memory related morphological changes in the argyrophilic nucleolar organizer region (AgNOR) of telencephalic neurons in goldfish were quantitatively evaluated by means of AgNOR neurohistochemical stain. The AgNORs and nuclei of nerve cells of two different telencephalic regions of goldfish trained in a spatial task or submitted to a similar non-contingent behavioral procedure (control group) were morphometrically evaluated. Results show that the area of AgNORs in goldfish dorsolateral telencephalic neurons increased significantly in the spatial learning group but not in control group. This effect seems to be highly specific as it did not appear in the dorsolateral area of the control group neither in the dorsomedial area of both groups. As the size of AgNORs in the nerve cell nuclei reflect the level of transcriptive activity, these morphological changes could be revealing increased protein synthesis in goldfish dorsolateral telencephalic neurons related with learning and memory. These findings could contribute to determining the subregions of the teleost telencephalon implicated in spatial learning and could indicate that the AgNOR staining technique would be a useful tool in assesing learning and memory related neuronal activity.

Chronic n-3 polyunsaturated fatty acid deficiency alters dopamine vesicle density in the rat frontal cortex

We studied the effects of a chronic deficiency in n-3 polyunsaturated fatty acids (n-3 PUFA) on the vesicle dopaminergic compartment in the frontal cortex of rats. Electronic micrographic analysis showed that the synaptic density and the clear vesicle density were similar in deficient and control rats. However, dopaminergic immunolabeling revealed a significantly decreased number of gold-labeled vesicles in the dopaminergic presynaptic terminals of the deficient rats. These findings demonstrate that dopamine cortical vesicles are specifically decreased in n-3 PUFA deficiency. The mechanism leading to this modification could involve several abnormalities (vesicle turn-over, membrane fluidity, vesicular monoamine transporter). This reduction in the dopaminergic vesicle pool constitutes the first structural support for the previously described modifications of dopamine metabolism in the frontal cortex. Such changes in dopamine neurotransmission could be involved in behavioral abnormalities occurring in n-3 PUFA deficient rats.

Colocalization of integrin receptors and reelin in dendritic spine postsynaptic densities of adult nonhuman primate cortex

The expression of telencephalic reelin (Reln) and glutamic acid decarboxylase mRNAs and their respective cognate proteins is down-regulated in postmortem brains of schizophrenia and bipolar disorder patients. To interpret the pathophysiological significance of this finding, immunoelectron microscopic experiments are required, but these cannot be carried out in postmortem human brains. As an alternative, we carried out such experiments in the cortex of rats and nonhuman primates. We found that Reln is expressed predominantly in layer I of both cortices and is localized to bitufted (double-bouquet), horizontal, and multipolar gamma-aminobutyric acid-ergic interneurons, which secrete Reln into extracellular matrix. Reln secretion is mediated by a constitutive mechanism that depends on the expression of a specific signal peptide present in the Reln carboxy-terminal domain. Extracellular matrix Reln is found to aggregate in proximity of postsynaptic densities expressed in apical dendrite spines, which include also the alpha(3) subunit of integrin receptors. Most pyramidal neurons of various cortical layers express the mouse-disabled 1 (Dab1) protein, which, after phosphorylation by a soluble tyrosine kinase, functions as an adapter protein, probably mediating a modulation of cytoskeleton protein expression. We hypothesize that the decrease of neuropil and dendritic spine density reported to exist in the neocortex of psychiatric patients may be related to a down-regulation of Reln-integrin interactions and the consequent decrease of cytoskeleton protein turnover.

Colocalization of integrin receptors and reelin in dendritic spine postsynaptic densities of adult nonhuman primate cortex

The expression of telencephalic reelin (Reln) and glutamic acid decarboxylase mRNAs and their respective cognate proteins is down-regulated in postmortem brains of schizophrenia and bipolar disorder patients. To interpret the pathophysiological significance of this finding, immunoelectron microscopic experiments are required, but these cannot be carried out in postmortem human brains. As an alternative, we carried out such experiments in the cortex of rats and nonhuman primates. We found that Reln is expressed predominantly in layer I of both cortices and is localized to bitufted (double-bouquet), horizontal, and multipolar gamma-aminobutyric acid-ergic interneurons, which secrete Reln into extracellular matrix. Reln secretion is mediated by a constitutive mechanism that depends on the expression of a specific signal peptide present in the Reln carboxy-terminal domain. Extracellular matrix Reln is found to aggregate in proximity of postsynaptic densities expressed in apical dendrite spines, which include also the alpha(3) subunit of integrin receptors. Most pyramidal neurons of various cortical layers express the mouse-disabled 1 (Dab1) protein, which, after phosphorylation by a soluble tyrosine kinase, functions as an adapter protein, probably mediating a modulation of cytoskeleton protein expression. We hypothesize that the decrease of neuropil and dendritic spine density reported to exist in the neocortex of psychiatric patients may be related to a down-regulation of Reln-integrin interactions and the consequent decrease of cytoskeleton protein turnover.

Brain plasticity and stroke rehabilitation. The Willis lecture

Neuronal connections and cortical maps are continuously remodeled by our experience. Knowledge of the potential capabilityof the brain to compensate for lesions is a prerequisite for optimal stroke rehabilitation strategies. Experimental focal cortical lesions induce changes in adjacent cortex and in the contralateral hemisphere. Neuroimaging studies in stroke patients indicate altered poststroke activation patterns, which suggest some functional reorganization. To what extent functional imaging data correspond to outcome data needs to be evaluated. Reorganization may be the principle process responsible for recovery of function after stroke, but what are the limits, and to what extent can postischemic intervention facilitate such changes? Postoperative housing of animals in an enriched environment can significantly enhance functional outcome and can also interact with other interventions, including neocortical grafting. What role will neuronal progenitor cells play in future rehabilitation-stimulated in situ or as neural replacement? And what is the future for blocking neural growth inhibitory factors? Better knowledge of postischemic molecular and neurophysiological events, and close interaction between basic and applied research, will hopefully enable us to design rehabilitation strategies based on neurobiological principles in a not-too-distant future.