Upregulation of BDNF mRNA expression in the barrel cortex of adult mice after sensory stimulation

BDNF-TrkB signaling orchestrates the buildup process of local sleep

Sleep debt accumulates during wakefulness, leading to increased slow wave activity (SWA) during sleep, an encephalographic marker for sleep need. The use-dependent demands of prior wakefulness increase sleep SWA locally. However, the circuitry and molecular identity of this "local sleep" remain unclear. Using pharmacology and optogenetic perturbations together with transcriptomics, we find that cortical brain-derived neurotrophic factor (BDNF) regulates SWA via the activation of tyrosine kinase B (TrkB) receptor and cAMP-response element-binding protein (CREB). We map BDNF/TrkB-induced sleep SWA to layer 5 (L5) pyramidal neurons of the cortex, independent of neuronal firing per se. Using mathematical modeling, we here propose a model of how BDNF's effects on synaptic strength can increase SWA in ways not achieved through increased firing alone. Proteomic analysis further reveals that TrkB activation enriches ubiquitin and proteasome subunits. Together, our study reveals that local SWA control is mediated by BDNF-TrkB-CREB signaling in L5 excitatory cortical neurons.

Hearing Vocalizations during First Social Experience with Pups Increase Bdnf Transcription in Mouse Auditory Cortex

While infant cues are often assumed to innately motivate maternal response, recent research highlights how the neural coding of infant cues is altered through maternal care. Infant vocalizations are important social signals for caregivers, and evidence from mice suggests that experience caring for mouse pups induces inhibitory plasticity in the auditory cortex (AC), though the molecular mediators for such AC plasticity during the initial pup experience are not well delineated. Here, we used the maternal mouse communication model to explore whether transcription in AC of a specific, inhibition-linked, memory-associated gene, brain-derived neurotrophic factor (Bdnf) changes due to the very first pup caring experience hearing vocalizations, while controlling for the systemic influence of the hormone estrogen. Ovariectomized and estradiol or blank-implanted virgin female mice hearing pup calls with pups present had significantly higher AC exon IV Bdnf mRNA compared to females without pups present, suggesting that the social context of vocalizations induces immediate molecular changes at the site of auditory cortical processing. E2 influenced the rate of maternal behavior but did not significantly affect Bdnf mRNA transcription in the AC. To our knowledge, this is the first time Bdnf has been associated with processing social vocalizations in the AC, and our results suggest that it is a potential molecular component responsible for enhancing future recognition of infant cues by contributing to AC plasticity.

BDNF and its Role in the Alcohol Abuse Initiated During Early Adolescence: Evidence from Preclinical and Clinical Studies

Brain-derived neurotrophic factor (BDNF) is a crucial brain signaling protein that is integral to many signaling pathways. This neurotrophin has shown to be highly involved in brain plastic processes such as neurogenesis, synaptic plasticity, axonal growth, and neurotransmission, among others. In the first part of this review, we revise the role of BDNF in different neuroplastic processes within the central nervous system. On the other hand, its deficiency in key neural circuits is associated with the development of psychiatric disorders, including alcohol abuse disorder. Many people begin to drink alcohol during adolescence, and it seems that changes in BDNF are evident after the adolescent regularly consumes alcohol. Therefore, the second part of this manuscript addresses the involvement of BDNF during adolescent brain maturation and how this process can be negatively affected by alcohol abuse. Finally, we propose different BNDF enhancers, both behavioral and pharmacological, which should be considered in the treatment of problematic alcohol consumption initiated during the adolescence.

Long-lasting Postnatal Sensory Deprivation Alters Dendritic Morphology of Pyramidal Neurons in the Rat Hippocampus: Behavioral Correlates

The role of normal sensory inputs in the development of sensory cortices is well known, however, their impacts on the hippocampus, an integrator of sensory modalities with important roles in cognitive functions, has received much less attention. Here, we applied a long-term sensory deprivation paradigm by trimming the rats' whiskers bilaterally, from postnatal day 3 to 59. Female sensory-deprived (SD) rats showed more on-wall rearing and visits to the center of the open-field box, shorter periods of grooming, less defecation and less anxiety-like behaviors in the elevated plus-maze compared to controls, who had their intact whiskers brushed. Passive avoidance memory retention was sex-dependently impaired in the female SD rats. In the radial arm maze, however, reference spatial memory was impaired only in the male SD rats. Nonetheless, working memory errors increased in both sexes of SD rats. Besides depletion of CA1 and CA3 pyramidal neurons in SD rats, Sholl analysis of Golgi-Cox stained neurons revealed that prolonged sensory deprivation has retracted the arborization of CA1 basal dendrites in SD group, while solely female SD rats had diminished CA1 apical dendrites. Sholl analysis of CA3 neurons in SD animals also disclosed significantly more branched apical dendrites in males and basal dendrites in females. Sensory deprivation also led to a considerable spine loss and variation of different spine types in a sex-dependent manner. Our findings suggest that experience-dependent structural plasticity is capable of spreading far beyond the manipulated sensory zones and the inevitable functional alterations can be expressed in a multifactorial sex-dependent manner.

Neuronal Activity Patterns Regulate Brain-Derived Neurotrophic Factor Expression in Cortical Cells via Neuronal Circuits

During development, cortical circuits are remodeled by spontaneous and sensory-evoked activity via alteration of the expression of wiring molecules. An intriguing question is how physiological neuronal activity modifies the expression of these molecules in developing cortical networks. Here, we addressed this issue, focusing on brain-derived neurotrophic factor (BDNF), one of the factors underlying cortical wiring. Real-time imaging of BDNF promoter activity in organotypic slice cultures revealed that patterned stimuli differentially regulated the increase and the time course of the promoter activity in upper layer neurons. Calcium imaging further demonstrated that stimulus-dependent increases in the promoter activity were roughly proportional to the increase in intracellular Ca²⁺ concentration per unit time. Finally, optogenetic stimulation showed that the promoter activity was increased efficiently by patterned stimulation in defined cortical circuits. These results suggest that physiological stimulation patterns differentially tune activity-dependent gene expression in developing cortical neurons via cortical circuits, synaptic responses, and alteration of intracellular calcium signaling.

Genetic and activity dependent-mechanisms wiring the cortex: Two sides of the same coin

The cerebral cortex is responsible for the higher-order functions of the brain such as planning, cognition, or social behaviour. It provides us with the capacity to interact with and transform our world. The substrates of cortical functions are complex neural circuits that arise during development from the dynamic remodelling and progressive specialization of immature undefined networks. Here, we review the genetic and activity-dependent mechanisms of cortical wiring focussing on the importance of their interaction. Cortical circuits emerge from an initial set of neuronal types that engage in sequential forms of embryonic and postnatal activity. Such activities further complement the cells' genetic programs, increasing neuronal diversity and modifying the electrical properties while promoting selective connectivity. After a temporal window of enhanced plasticity, the main features of mature circuits are established. Failures in these processes can lead to neurodevelopmental disorders whose treatment remains elusive. However, a deeper dissection of cortical wiring will pave the way for innovative therapies.

Role of DNA Methyl-CpG-Binding Protein MeCP2 in Rett Syndrome Pathobiology and Mechanism of Disease

Rett Syndrome (RTT) is a severe, rare, and progressive developmental disorder with patients displaying neurological regression and autism spectrum features. The affected individuals are primarily young females, and more than 95% of patients carry de novo mutation(s) in the Methyl-CpG-Binding Protein 2 (MECP2) gene. While the majority of RTT patients have MECP2 mutations (classical RTT), a small fraction of the patients (atypical RTT) may carry genetic mutations in other genes such as the cyclin-dependent kinase-like 5 (CDKL5) and FOXG1. Due to the neurological basis of RTT symptoms, MeCP2 function was originally studied in nerve cells (neurons). However, later research highlighted its importance in other cell types of the brain including glia. In this regard, scientists benefitted from modeling the disease using many different cellular systems and transgenic mice with loss- or gain-of-function mutations. Additionally, limited research in human postmortem brain tissues provided invaluable findings in RTT pathobiology and disease mechanism. MeCP2 expression in the brain is tightly regulated, and its altered expression leads to abnormal brain function, implicating MeCP2 in some cases of autism spectrum disorders. In certain disease conditions, MeCP2 homeostasis control is impaired, the regulation of which in rodents involves a regulatory microRNA (miR132) and brain-derived neurotrophic factor (BDNF). Here, we will provide an overview of recent advances in understanding the underlying mechanism of disease in RTT and the associated genetic mutations in the MECP2 gene along with the pathobiology of the disease, the role of the two most studied protein variants (MeCP2E1 and MeCP2E2 isoforms), and the regulatory mechanisms that control MeCP2 homeostasis network in the brain, including BDNF and miR132.

Microglial Gi-dependent dynamics regulate brain network hyperexcitability

Microglial surveillance is a key feature of brain physiology and disease. We found that Gi-dependent microglial dynamics prevent neuronal network hyperexcitability. By generating Mg^PTX mice to genetically inhibit Gi in microglia, we showed that sustained reduction of microglia brain surveillance and directed process motility induced spontaneous seizures and increased hypersynchrony upon physiologically evoked neuronal activity in awake adult mice. Thus, Gi-dependent microglia dynamics may prevent hyperexcitability in neurological diseases.

Consecutive Treatment with Brain-Derived Neurotrophic Factor and Electrical Stimulation Has a Protective Effect on Primary Auditory Neurons

Degeneration of neurons, such as the inner ear spiral ganglion neurons (SGN), may be decelerated or even stopped by neurotrophic factor treatment, such as brain-derived neurotrophic factor (BDNF), as well as electrical stimulation (ES). In a clinical setting, drug treatment of the SGN could start directly during implantation of a cochlear implant, whereas electrical stimulation begins days to weeks later. The present study was conducted to determine the effects of consecutive BDNF and ES treatments on SGN density and electrical responsiveness. An electrode drug delivery device was implanted in guinea pigs 3 weeks after deafening and five experimental groups were established: two groups received intracochlear infusion of artificial perilymph (AP) or BDNF; two groups were treated with AP respectively BDNF in addition to ES (AP + ES, BDNF + ES); and one group received BDNF from the day of implantation until day 34 followed by ES (BDNF ⇨ ES). Electrically evoked auditory brainstem responses were recorded. After one month of treatment, the tissue was harvested and the SGN density was assessed. The results show that consecutive treatment with BDNF and ES was as successful as the simultaneous combined treatment in terms of enhanced SGN density compared to the untreated contralateral side but not in regard to the numbers of protected cells.

White matter structure and myelin-related gene expression alterations with experience in adult rats

White matter (WM) plasticity during adulthood is a recently described phenomenon by which experience can shape brain structure. It has been observed in humans using diffusion tensor imaging (DTI) and myelination has been suggested as a possible mechanism. Here, we set out to identify molecular and cellular changes associated with WM plasticity measured by DTI. We combined DTI, immunohistochemistry and mRNA expression analysis and examined the effects of somatosensory experience in adult rats. First, we observed experience-induced DTI differences in WM and in grey matter structure. C-Fos mRNA expression, a marker of cortical activity, in the barrel cortex correlated with the MRI WM metrics, indicating that molecular correlates of cortical activity relate to macroscale measures of WM structure. Analysis of myelin-related genes revealed higher myelin basic protein (MBP) mRNA expression. Higher MBP protein expression was also found via immunohistochemistry in WM. Finally, unbiased RNA sequencing analysis identified 134 differentially expressed genes encoding proteins involved in functions related to cell proliferation and differentiation, regulation of myelination and neuronal activity modulation. In conclusion, macroscale measures of WM plasticity are supported by both molecular and cellular evidence and confirm that myelination is one of the underlying mechanisms.

mGluR5 Tunes NGF/TrkA Signaling to Orient Spiny Stellate Neuron Dendrites Toward Thalamocortical Axons During Whisker-Barrel Map Formation

Neurons receive and integrate synaptic inputs at their dendrites, thus dendritic patterning shapes neural connectivity and behavior. Aberrant dendritogenesis is present in neurodevelopmental disorders such as Down's syndrome and autism. Abnormal glutamatergic signaling has been observed in these diseases, as has dysfunction of the metabotropic glutamate receptor 5 (mGluR5). Deleting mGluR5 in cortical glutamatergic neurons disrupted their coordinated dendritic outgrowth toward thalamocortical axons and perturbed somatosensory circuits. Here we show that mGluR5 loss-of-function disrupts dendritogenesis of cortical neurons by increasing mRNA levels of nerve growth factor (NGF) and fibroblast growth factor 10 (FGF10), in part through calcium-permeable AMPA receptors (CP-AMPARs), as the whisker-barrel map is forming. Postnatal NGF and FGF10 expression in cortical layer IV spiny stellate neurons differentially impacted dendritic patterns. Remarkably, NGF-expressing neurons exhibited dendritic patterns resembling mGluR5 knockout neurons: increased total dendritic length/complexity and reduced polarity. Furthermore, suppressing the kinase activity of TrkA, a major NGF receptor, prevents aberrant dendritic patterning in barrel cortex of mGluR5 knockout neurons. These results reveal novel roles for NGF-TrkA signaling and CP-AMPARs for proper dendritic development of cortical neurons. This is the first in vivo demonstration that cortical neuronal NGF expression modulates dendritic patterning during postnatal brain development.

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

Aspects of excitatory/inhibitory synapses in multiple brain regions are correlated with levels of brain-derived neurotrophic factor/neurotrophin-3

Appropriate synapse formation during development is necessary for normal brain function, and synapse impairment is often associated with brain dysfunction. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are key factors in regulating synaptic development. We previously reported that BDNF/NT-3 secretion was enhanced by calcium-dependent activator protein for secretion 2 (CADPS2). Although BDNF/NT-3 and CADPS2 are co-expressed in various brain regions, the effect of Cadps2-deficiency on brain region-specific BDNF/NT-3 levels and synaptic development remains elusive. Here, we show developmental changes of BDNF/NT-3 levels and we assess disruption of excitatory/inhibitory synapses in multiple brain regions (cerebellum, hypothalamus, striatum, hippocampus, parietal cortex and prefrontal cortex) of Cadps2 knockout (KO) mice compared with wild-type (WT) mice. Compared with WT, BDNF levels in KO mice were reduced in young/adult hippocampus, but increased in young hypothalamus, while NT-3 levels were reduced in adult cerebellum and young hippocampus, but increased in adult parietal cortex. Immunofluorescence of vGluT1, an excitatory synapse marker, and vGAT, an inhibitory synapse marker, in adult KO showed that vGluT1 was higher in the cerebellum and parietal cortex but lower in the hippocampus, whereas vGAT was lower in the hippocampus and parietal cortex compared with WT. Immunolabeling for both vGluT1 and vGAT was increased in the parietal cortex but vGAT was decreased in the cerebellum in adult KO compared with WT. These data suggest that CADPS2-mediated secretion of BDNF/NT-3 may be involved in development and maturation of synapses and in the balance between inhibitory and excitatory synapses.

A comprehensive review of genetic and epigenetic mechanisms that regulate BDNF expression and function with relevance to major depressive disorder

Major depressive disorder (MDD) is a mood disorder that affects behavior and impairs cognition. A gene potentially important to this disorder is the brain derived neurotrophic factor (BDNF) as it is involved in processes controlling neuroplasticity. Various mechanisms exist to regulate BDNF's expression level, subcellular localization, and sorting to appropriate secretory pathways. Alterations to these processes by genetic factors and negative stressors can dysregulate its expression, with possible implications for MDD. Here, we review the mechanisms governing the regulation of BDNF expression, and discuss how disease-associated single nucleotide polymorphisms (SNPs) can alter these mechanisms, and influence MDD. As negative stressors increase the likelihood of MDD, we will also discuss the impact of these stressors on BDNF expression, the cellular effect of such a change, and its impact on behavior in animal models of stress. We will also describe epigenetic processes that mediate this change in BDNF expression. Similarities in BDNF expression between animal models of stress and those in MDD will be highlighted. We will also contrast epigenetic patterns at the BDNF locus between animal models of stress, and MDD patients, and address limitations to current clinical studies. Future work should focus on validating current genetic and epigenetic findings in tightly controlled clinical studies. Regions outside of BDNF promoters should also be explored, as should other epigenetic marks, to improve identification of biomarkers for MDD.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

The Role of BDNF in Age-Dependent Changes of Excitatory and Inhibitory Synaptic Markers in the Human Prefrontal Cortex

Reduced brain-derived neurotrophic factor (BDNF) may underlie age-related synaptic loss, in turn contributing to cerebral atrophy, cognitive decline, and increased risk for psychiatric disorders. However, the specific contribution of BDNF to the age-related expression changes in synaptic markers and their temporal trajectories remain uncharacterized. Using microarray data from orbitofrontal cortex of control subjects (n=209; 16-96 years), we identified genes whose expression positively correlates with BDNF (r>0.575; n=200 genes) and analyzed them for enriched biological pathways. qPCR was performed to measure the expression level of transcript variants of BDNF, NTRK2, and selected BDNF-coexpressed genes in younger and older subjects. We confirmed age-related downregulation of BDNF and show 78 of the top 200 BDNF-coexpressed genes are associated with synaptic function. Both excitatory and inhibitory synaptic genes show decreased expression with age and are positively correlated with BDNF and NTRK2 expression and negatively correlated with dominant-negative truncated NTRK2 level. Results were validated at the RNA level in an independent cohort and at the protein level for selected findings. We next tested the causal link between the correlative human findings using mice with conditional blockade of BDNF/NTRK2 signaling. Blockade of NTRK2 activity in adult mice recapitulate the age-like pattern in the expression of markers for inhibitory presynaptic but notably not for excitatory synaptic genes. Together, these findings suggest that age-dependent decrease in BDNF signaling may cause synaptic alterations through an initial and preferential effect on GABA presynaptic genes. These results have implications for neuropsychiatric disorders characterized by accelerated aging molecular profiles, such as major depression.

In Search of the Motor Engram: Motor Map Plasticity as a Mechanism for Encoding Motor Experience

Motor skill acquisition occurs through modification and organization of muscle synergies into effective movement sequences. The learning process is reflected neurophysiologically as a reorganization of movement representations within the primary motor cortex, suggesting that the motor map is a motor engram. However, the specific neural mechanisms underlying map plasticity are unknown. Here the authors review evidence that 1) motor map topography reflects the capacity for skilled movement, 2) motor skill learning induces reorganization of motor maps in a manner that reflects the kinematics of acquired skilled movement, 3) map plasticity is supported by a reorganization of cortical microcircuitry involving changes in synaptic efficacy, and 4) motor map integrity and topography are influenced by various neurochemical signals that coordinate changes in cortical circuitry to encode motor experience. Finally, the role of motor map plasticity in recovery of motor function after brain damage is discussed.

Multidimensional characterisation of biomechanical structures by combining Atomic Force Microscopy and Focused Ion Beam: A study of the rat whisker

Understanding the heterogeneity of biological structures, particularly at the micro/nano scale can offer insights valuable for multidisciplinary research in tissue engineering and biomimicry designs. Here we propose to combine nanocharacterisation tools, particularly Focused Ion Beam (FIB) and Atomic Force Microscopy (AFM) for three dimensional mapping of mechanical modulus and chemical signatures. The prototype platform is applied to image and investigate the fundamental mechanics of the rat face whiskers, a high-acuity sensor used to gain detailed information about the world. Grazing angle FIB milling was first applied to expose the interior cross section of the rat whisker sample, followed by a "lift-out" method to retrieve and position the target sample for further analyses. AFM force spectroscopy measurements revealed a non-uniform pattern of elastic modulus across the cross section, with a range from 0.8GPa to 13.5GPa. The highest elastic modulus was found at the outer cuticle region of the whisker, and values gradually decreased towards the interior cortex and medulla regions. Elemental mapping with EDS confirmed that the interior of the rat whisker is dominated by C, O, N, S, Cl and K, with a significant change of elemental distribution close to the exterior cuticle region. Based on these data, a novel comprehensive three dimensional (3D) elastic modulus model was constructed, and stress distributions under realistic conditions were investigated with Finite Element Analysis (FEA). The simulations could well account for the passive whisker deflections, with calculated resonant frequency as well as force-deflection for the whiskers being in good agreement with reported experimental data. Limitations and further applications are discussed for the proposed FIB/AFM approach, which holds good promise as a unique platform to gain insights on various heterogeneous biomaterials and biomechanical systems.

TrkB-Mediated Neuroprotective and Antihypoxic Properties of Brain-Derived Neurotrophic Factor

The neuroprotective and antihypoxic effects of brain-derived neurotrophic factor (BDNF) on dissociated hippocampal cultures in a hypoxia model were investigated. These experiments demonstrate that 10 minutes of normobaric hypoxia increased the number of dead cells in primary culture, whereas a preventive application of BDNF increased the number of viable cells. Spontaneous bioelectrical and calcium activity in neural networks was analyzed using multielectrode arrays and functional intravital calcium imaging. The results indicate that BDNF affects the functional parameters of neuronal networks in dissociated hippocampal cultures over the 7-day posthypoxic period. In addition, the effects of k252a, an antagonist of tropomyosin-related kinase B (TrkB), on functional bioelectrical activity during and after acute hypoxia were investigated. It was shown that the protective effects of BDNF are associated with binding to the TrkB receptor. Finally, intravital fluorescent mRNA probes were used to study the role of NF-κB1 in the protective effects of BDNF. Our experiments revealed that BDNF application stimulates NF-κB1 mRNA synthesis in primary dissociated hippocampal cells under normal conditions but not in hypoxic state.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

The activity-dependent transcription factor NPAS4 regulates domain-specific inhibition

A heterogeneous population of inhibitory neurons controls the flow of information through a neural circuit. Inhibitory synapses that form on pyramidal neuron dendrites modulate the summation of excitatory synaptic potentials and prevent the generation of dendritic calcium spikes. Precisely timed somatic inhibition limits both the number of action potentials and the time window during which firing can occur. The activity-dependent transcription factor NPAS4 regulates inhibitory synapse number and function in cell culture, but how this transcription factor affects the inhibitory inputs that form on distinct domains of a neuron in vivo was unclear. Here we show that in the mouse hippocampus behaviourally driven expression of NPAS4 coordinates the redistribution of inhibitory synapses made onto a CA1 pyramidal neuron, simultaneously increasing inhibitory synapse number on the cell body while decreasing the number of inhibitory synapses on the apical dendrites. This rearrangement of inhibition is mediated in part by the NPAS4 target gene brain derived neurotrophic factor (Bdnf), which specifically regulates somatic, and not dendritic, inhibition. These findings indicate that sensory stimuli, by inducing NPAS4 and its target genes, differentially control spatial features of neuronal inhibition in a way that restricts the output of the neuron while creating a dendritic environment that is permissive for plasticity.

Interneurons containing somatostatin are affected by learning-induced cortical plasticity

The maintenance of neural circuit stability is a dynamic process that requires the plasticity of many cellular and synaptic components. By changing the excitatory/inhibitory balance, inhibitory GABAergic plasticity can regulate excitability, and contribute to neural circuit function and refinement in learning and memory. Increased inhibitory GABAergic neurotransmission has been shown in brain structures involved in the learning process. Previously, we showed that classical conditioning in which tactile stimulation of one row of vibrissae (conditioned stimulus, CS) was paired with a tail shock (unconditioned stimulus, UCS) in adult mice results in the increased density of GABAergic interneurons and increased expression of glutamic acid decarboxylase (GAD)-67 in barrels of the "trained" row cortical representation. In inhibitory neurons of the rat cortex GAD co-localizes with several proteins and peptides. We found previously that the density of the parvalbumin (GAD+/Prv+)-containing subpopulation is not changed after conditioning. In the present study, we examined GABAergic somatostatin (Som)-, calbindin (CB)- and calretinin (CR)-positive interneurons in the cortical representation of "trained" vibrissae after training. Cells showing double immunostaining for GAD/Som, GAD/CR and GAD/CB were counted in the barrels representing vibrissae activated during the training and in control, untouched rows. We found a substantial increase of GAD/Som-containing cells in the trained row representation. No changes in the density of GAD/CR or GAD/CB neurons were observed. These results suggest that Som-containing interneurons are involved in learning-induced changes in the inhibitory cortical network.

Shaping brain connections through spontaneous neural activity

An overwhelming number of observations demonstrate that neural activity and genetic programs interact to specify the composition and organization of neural circuits during all stages of development. Spontaneous neuronal activities have been documented in several developing neural regions in both invertebrates and vertebrates, and their roles are mostly conserved among species. Among these roles, Ca(2+) spikes and levels of electrical activity have been shown to regulate neurite growth, axon extension and axon branching. Here, we review selected findings concerning the role of spontaneous activity on circuit development.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Lesion-induced synaptic plasticity in the somatosensory cortex of prenatally stressed rats

Prenatal stress exposure causes long-lasting impairments of the behavioral and neuroendocrine responses to later stressors of the offspring. Although mechanisms underlying these effects remain largely unknown, abnormalities in the neuronal plasticity might be responsible for neurobiological alterations. This study used the whisker-to-barrel pathway as a model system to investigate the effects of prenatal stress on lesion-induced plasticity of neurons. Pregnant rats were subjected to immobilization stress during the trigeminal neurogenesis period, corresponding to gestational days 12 to 17, for three hours a day. After birth, the middle row (C) whisker follicles of pups from the control and stressed groups were electrocauterized. Ten days later, tangentially sectioned cortical hemispheres were stained with cytochrome oxidase histochemistry to calculate the volumes of each barrel row (A-E) in both lesioned and intact sides of the cortex, using stereological methods. The adrenal to body weight ratios were significantly increased in stressed animals, when compared to the controls. The pattern and total volume of the barrel subfield remained unaltered, but the lesion-induced map plasticity index, calculated as the D/C ratio, decreased in stressed animals. In addition, the BDNF (Brain Derived Neurotrophic Factor), NT-3 (neurotrophin-3) and the cyclic AMP response element binding protein (CREB) phosphorylation levels in tissue homogenates of the barrel cortices were measured using the ELISA method. In prenatally stressed animals, the BDNF and NT-3 levels were reduced on the lesioned side, but significant CREB activation was observed on the intact side of the barrel cortex. Taken together, the results show that prenatal stress exposure negatively affects critical period plasticity by reducing the expansion of active barrels following peripheral whisker lesion. These changes arise independent of CREB phosphorylation and appear to be mediated by reduced levels of neurotrophins.

Activity-regulated genes as mediators of neural circuit plasticity

Modifications of neuronal circuits allow the brain to adapt and change with experience. This plasticity manifests during development and throughout life, and can be remarkably long lasting. Evidence has linked activity-regulated gene expression to the long-term structural and electrophysiological adaptations that take place during developmental critical periods, learning and memory, and alterations to sensory map representations in the adult. In all these cases, the cellular response to neuronal activity integrates multiple tightly coordinated mechanisms to precisely orchestrate long-lasting, functional and structural changes in brain circuits. Experience-dependent plasticity is triggered when neuronal excitation activates cellular signaling pathways from the synapse to the nucleus that initiate new programs of gene expression. The protein products of activity-regulated genes then work via a diverse array of cellular mechanisms to modify neuronal functional properties. Synaptic strengthening or weakening can reweight existing circuit connections, while structural changes including synapse addition and elimination create new connections. Posttranscriptional regulatory mechanisms, often also dependent on activity, further modulate activity-regulated gene transcript and protein function. Thus, activity-regulated genes implement varied forms of structural and functional plasticity to fine-tune brain circuit wiring.

Neurotoxic (+)-methamphetamine treatment in rats increases brain-derived neurotrophic factor and tropomyosin receptor kinase B expression in multiple brain regions

Methamphetamine (MA) is an abused stimulant which can result in cognitive deficits and monoamine depletions. Animal models of neurotoxic MA exposure show reductions in dopamine, serotonin, and their associated transporters. MA abuse can result in long-term attention, working memory, and executive function deficits in humans and deficits in route-based egocentric learning, novel object recognition, and novel odor preference in rodents. MA has also been shown to affect brain-derived neurotrophic factor (BDNF) in humans and rodents. This experiment examined the effects of a MA binge dosing regimen (10 mg/kg x 4 at 2 h intervals, s.c.) in Sprague-Dawley rats on BDNF, tropomyosin receptor kinase B (TrkB), and tyrosine hydroxylase (TH) mRNA expression, and plasma corticosterone. Tissues were collected 1, 7, and 24 h following the last MA dose. Expression of BDNF and TrkB mRNA was analyzed using in situ hybridization with cRNA probes. Frontal, parietal, and entorhinal cortical BDNF mRNA expression were increased by MA exposure at all time-points. Increases in BDNF mRNA were also seen in the hippocampal CA1, prefrontal cortex (PFC), piriform cortex, and locus coeruleus but only at specific times. TrkB mRNA expression was modified in several subregions of the hippocampus as well as in PFC and striatum. TH mRNA was increased at the 1 h time-point in the substantia nigra pars compacta with no differences noted at the other times. Corticosterone levels were increased at all three time-points. The findings suggest that BDNF and its receptor may be upregulated as a compensatory mechanism after MA exposure.

Genetic influences on neural plasticity

Neural plasticity refers to the capability of the brain to alter function or structure in response to a range of events and is a crucial component of both functional recovery after injury and skill learning in healthy individuals. A number of factors influence neural plasticity and recovery of function after brain injury. The current review considers the impact of genetic factors. Polymorphisms in the human genes coding for brain-derived neurotrophic factor and apolipoprotein E have been studied in the context of plasticity and stroke recovery and are discussed here in detail. Several processes involved in plasticity and stroke recovery, such as depression or pharmacotherapy effects, are modulated by other genetic polymorphisms and are also discussed. Finally, new genetic polymorphisms that have not been studied in the context of stroke are proposed as new directions for study. A better understanding of genetic influences on recovery and response to therapy might allow improved treatment after a number of forms of central nervous system injury.

Brain plasticity and genetic factors

Brain plasticity refers to changes in brain function and structure that arise in a number of contexts. One area in which brain plasticity is of considerable interest is recovery from stroke, both spontaneous and treatment-induced. A number of factors influence these poststroke brain events. The current review considers the impact of genetic factors. Polymorphisms in the human genes coding for brain-derived neurotrophic factor (BDNF) and apolipoprotein E (ApoE) have been studied in the context of plasticity and/or stroke recovery and are discussed here in detail. Several other genetic polymorphisms are indirectly involved in stroke recovery through their modulating influences on processes such as depression and pharmacotherapy effects. Finally, new genetic polymorphisms that have not been studied in the context of stroke are proposed as new directions for study. A better understanding of genetic influences on recovery and response to therapy might allow improved treatment after stroke.

Morphological properties of the action-observation cortical network in adolescents with low and high resistance to peer influence

Children with high resistance to peer influences differ from their low-resistance counterparts in the degree of functional connectivity in fronto-parietal and prefrontal cortical networks. Here we explored the possibility that the degree of morphological similarities across the same cortical regions also varies as a function of this behavioral trait. Using structural magnetic-resonance (MR) images, we measured cortical thickness in a total of 295 adolescents (12 to 18 years of age). We found that inter-regional correlations in cortical thickness increased with the resistance to peer influence (RPI); this was especially the case, in female adolescents, in the premotor and prefrontal networks. We also observed significant differences between the adolescents with high and low RPI scores in their general intelligence and the scores of positive youth development. We suggest that these morphological findings might reflect differences, between adolescents with high vs. low resistance to peer influences, in a repeated and concurrent engagement of these networks in social context.

Experience-dependent plasticity mechanisms for neural rehabilitation in somatosensory cortex

Functional rehabilitation of the cortex following peripheral or central nervous system damage is likely to be improved by a combination of behavioural training and natural or therapeutically enhanced synaptic plasticity mechanisms. Experience-dependent plasticity studies in the somatosensory cortex have begun to reveal those synaptic plasticity mechanisms that are driven by sensory experience and might therefore be active during behavioural training. In this review the anatomical pathways, synaptic plasticity mechanisms and structural plasticity substrates involved in cortical plasticity are explored, focusing on work in the somatosensory cortex and the barrel cortex in particular.

BDNF-TrkB signaling interacts with the GABAergic system to inhibit rhythmic swallowing in the rat

Brain-derived neurotrophic factor (BDNF) acts as an anorexigenic factor in the dorsal vagal complex (DVC) of the adult rat brain stem. The DVC contains the premotoneurons controlling swallowing, a motor component of feeding behavior. Although rats with transected midbrain do not seek out food, they are able to swallow and to ingest food. Because BDNF and tropomyosin-related kinase B (TrkB) receptors are expressed in the DVC, this study hypothesized that BDNF could modify the activity of premotoneurons involved in swallowing. Repetitive electrical stimulation of the superior laryngeal nerve (SLN) induces rhythmic swallowing that can be recorded with electromyographic electrodes inserted in sublingual muscles. We show that a microinjection of BDNF in the swallowing network induced a rapid, transient, and dose-dependant inhibition of rhythmic swallowing. This BDNF effect appeared to be mediated via TrkB activation, since it no longer occurred when TrkB receptors were antagonized by K-252a. Interestingly, swallowing was inhibited when subthreshold doses of BDNF and GABA were coinjected, suggesting a synergistic interaction between these two signaling substances. Moreover, BDNF no longer had an inhibitory effect on swallowing when coinjected with bicuculline, a GABAA receptor antagonist. This blockade of BDNF inhibitory effect on swallowing was reversible, since it reappeared when BDNF was injected 15 min after bicuculline. Finally, we show that stimulation of SLN induced a decrease in BDNF protein within the DVC. Together, our results strongly suggest that BDNF inhibits swallowing via modulation of the GABAergic signaling within the central pattern generator of swallowing.

Neurotrophins and electrical stimulation for protection and repair of spiral ganglion neurons following sensorineural hearing loss

Exogenous neurotrophins (NTs) have been shown to rescue spiral ganglion neurons (SGNs) from degeneration following a sensorineural hearing loss (SNHL). Furthermore, chronic electrical stimulation (ES) has been shown to retard SGN degeneration in some studies but not others. Since there is evidence of even greater SGN rescue when NT administration is combined with ES, we examined whether chronic ES can maintain SGN survival long after cessation of NT delivery. Young adult guinea pigs were profoundly deafened using ototoxic drugs; five days later they were unilaterally implanted with an electrode array and drug delivery system. Brain derived neurotrophic factor (BDNF) was continuously delivered to the scala tympani over a four week period while the animal simultaneously received ES via bipolar electrodes in the basal turn (i.e., turn 1) scala tympani. One cohort (n=5) received ES for six weeks (i.e., including a two week period after the cessation of BDNF delivery; ES(6)); a second cohort (n=5) received ES for 10 weeks (i.e., a six week period following cessation of BDNF delivery; ES(10)). The cochleae were harvested for histology and SGN density determined for each cochlear turn for comparison with normal hearing controls (n=4). The withdrawal of BDNF resulted in a rapid loss of SGNs in turns 2-4 of the deafened/BDNF-treated cochleae; this was significant as early as two weeks following removal of the NT when compared with normal controls (p<0.05). Importantly, there was not a significant reduction in SGNs in turn 1 (i.e., adjacent to the electrode array) two and six weeks after NT removal, as compared with normal controls. This result suggests that chronic ES can prevent the rapid loss of SGNs that occurs after the withdrawal of exogenous NTs. Implications for the clinical delivery of NTs are discussed.

Cre recombinase-mediated gene deletion in layer 4 of murine sensory cortical areas

Thalamocortical input to layer 4 carries the major ascending sensory information to the mammalian sensory cortex and is crucial for the function and plasticity of sensory cortical areas. Here we report identification of a Six3-cre transgene that is selectively expressed in layer 4 of sensory cortical areas but not in the thalamus. In the mature somatosensory cortex Cre recombinase expressed from the transgene is able to mediate gene deletion in the overwhelming majority of layer 4 neurons, including GABAergic interneurons. The gene deletion in layer 4 mainly occurs during the first postnatal week. This cre transgene therefore provides a useful tool for examining the role of proteins expressed in layer 4 neurons.

Relapse to cocaine seeking increases activity-regulated gene expression differentially in the prefrontal cortex of abstinent rats

RATIONALE

Alterations in the activity of the prefrontal and orbitofrontal cortices of cocaine addicts have been linked with re-exposure to cocaine-associated stimuli.

OBJECTIVES

Using an animal model of relapse to cocaine seeking, the present study investigated the expression patterns of four different activity-regulated genes within prefrontal cortical brain regions after 22 h or 15 days of abstinence during context-induced relapse.

MATERIALS AND METHODS

Rats self-administered cocaine or received yoked-saline for 2 h/day for 10 days followed by 22 h or 2 weeks of abstinence when they were re-exposed to the self-administration chamber with or without levers available to press for 1 h. Brains were harvested and sections through the prefrontal cortex were processed for in situ hybridization using radioactive oligonucleotide probes encoding c-fos, zif/268, arc, and bdnf.

RESULTS

Re-exposure to the chamber in which rats previously self-administered cocaine but not saline, regardless of lever availability, increased the expression of all genes in the medial prefrontal and orbitofrontal cortices at both time points with one exception: bdnf mRNA was significantly increased in the medial prefrontal cortex at 22 h only if levers previously associated with cocaine delivery were available to press. Furthermore, re-exposure of rats to the chambers in which they received yoked saline enhanced both zif/268 and arc expression selectively in the orbitofrontal cortex after 15 days of abstinence.

CONCLUSIONS

These results support convergent evidence that cocaine-induced changes in the prefrontal cortex are important in regulating drug seeking following abstinence and may provide additional insight into the molecular mechanisms involved in these processes.

In peripheral nerve regeneration environment enriched with activity stimulating factors improves functional recovery

Enriched environment stimulates brain plasticity processes after brain lesion. Less is known about the influence of enriched environment with activity stimulating factors as determinants of functional outcome after peripheral nerve repair. BDNF (brain-derived neurotrophic factor) plays a role in activity-dependent neuronal plasticity and changes in motor cortex in rats learning complex motor skills. Our study aimed to elucidate if enriched environment influences functional results after peripheral nerve repair. The results in this rat sciatic nerve transection and repair model showed that environment enriched with activity stimulating factors can improve functional results.

Modified Sensory Processing in the Barrel Cortex of the Adult Mouse After Chronic Whisker Stimulation

Chronic stimulation of a mystacial whisker follicle for 24 h induces structural and functional changes in layer IV of the corresponding barrel, with an insertion of new inhibitory synapses on spines and a depression of neuronal responses to the stimulated whisker. Under urethane anesthesia, we analyzed how sensory responses of single units are affected in layer IV and layers II & III of the stimulated barrel column as well as in adjacent columns. In the stimulated column, spatiotemporal characteristics of the activation evoked by the stimulated whisker are not altered, although spontaneous activity and response magnitude to the stimulated whisker are decreased. The sensitivity of neurons for the deflection of this whisker is not altered but the dynamic range of the response is reduced as tested by varying the amplitude and repetition rate of the deflection. Responses to deflection of nonstimulated whiskers remain unaltered with the exception of in-row whisker responses that are depressed in the column corresponding to the stimulated whisker. In adjacent nonstimulated columns, neuronal activity remains unaltered except for a diminished response of units in layer II/III to deflection of the stimulated whisker. From these results we propose that an increased inhibition within the stimulated barrel reduced the magnitude of its excitatory output and accordingly the flow of excitation toward layers II & III and the subsequent spread into adjacent columns. In addition, the period of uncorrelated activity between pathways from the stimulated and nonstimulated whiskers weakens synaptic inputs from in-row whiskers in the stimulated barrel column.

Do first order and higher order regions of the thalamic reticular nucleus have different developmental timetables?

The thalamic reticular nucleus (TRN) can been subdivided into sectors based on thalamic and cortical input. Additionally, in carnivores the visual sector of the TRN can be subdivided into first order (perigeniculate nucleus: PGN) and higher order (TRN) regions. This report examines whether TRN development reflects the nature of its higher order visual connections. 170 cells from 12 kittens aged between postnatal day 0 (P0) and P125 were fully analysed after single cell injections in 400-500 microm fixed brain slices. TRN cells have a period of exuberant dendritic branching that peaks between P3 and P12, around the time of eye opening (P7), followed by branch pruning until P68. Similarly, most dendritic appendages are added between P12 and P22 followed by pruning, which is also largely complete by P68. Most branch points occur within the first 10-30% of the dendritic arbor, peaking between 10 and 20% (roughly equivalent to 100 mum from the soma), while appendages were concentrated between 20 and 30% of the arbour; appendages tend to be distributed over a larger proportion of the arbor up to P14 compared to later ages. TRN and PGN maturation were not significantly different. The present data suggest that clear distinctions cannot be made between the maturation of first and higher order pathways and indicate that GABAergic cells of the ventral thalamus may mature earlier than relay cells of the dorsal thalamus. Furthermore, dendritic development in the TRN may be less dependent on extrinsic factors than an intrinsic growth pattern or factors other than a functional hierarchy within the visual pathway.

Developmental synaptic plasticity at the thalamocortical input to barrel cortex: mechanisms and roles

The thalamocortical (TC) input to layer IV provides the major pathway for ascending sensory information to the mammalian sensory cortex. During development there is a dramatic refinement of this input that underlies the maturation of the topographical map in layer IV. Over the last 10 years our understanding of the mechanisms of the developmental and experience-driven changes in synaptic function at TC synapses has been greatly advanced. Here we describe these studies that point to a key role for NMDA receptor-dependent synaptic plasticity, a role for kainate receptors and for a rapid maturation in GABAergic inhibition. The expression mechanisms of some of the forms of neonatal synaptic plasticity are novel and, in combination with other mechanisms, produce a layer IV circuit that exhibits functional properties necessary for mature sensory processing.

Neurotrophins in the dentate gyrus

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

Clinically relevant doses of fluoxetine and reboxetine induce changes in the TrkB content of central excitatory synapses

We have studied the effect of low doses of two widely used antidepressants, fluoxetine (Flx) and reboxetine (Rbx), on excitatory synapses of rat brain cortex and hippocampus. After 15 days of Flx treatment (0.67 mg/kg/day), its plasma level was 20.7+/-5.6 ng/ml. Analysis of postsynaptic densities (PSDs) by immunoblotting revealed no changes in the glutamate receptor subunits GluR1, NR1, NR2A/B, mGluR1alpha nor in the neurotrophin receptor p75(NTR). However, the brain-derived neurotrophic factor (BDNF) receptor TrkB decreased by 42.8+/-6%, and remained decreased after 6 weeks of treatment. The BDNF and TrkB content in homogenates of cortex and hippocampus began to rise at 9 and 15 days, respectively, and remained high for up to 6 weeks. Similar results were obtained following chronic Rbx administration at 0.128 mg/kg/day. We propose that BDNF, whose synthesis is increased by antidepressants, and which is in part released at synaptic sites, binds to TrkB in PSDs, leading to the internalization of the BDNF-TrkB complex and, thus, to a decrease of TrkB in the PSDs. This was paralleled by greater levels of phosphorylated (ie activated) TrkB in the light membrane fraction, that contains signaling endosomes. The retrograde transport of endocyted BDNF/TrkB complexes from spines to cell bodies, where it activates the synthesis of more BDNF, is a protracted process, potentially requiring several cycles of TrkB/BDNF complex endocytosis and transport. This positive feedback mechanism may help explain the time-lag between drug administration and its therapeutic effect, that is, the antidepressant drug paradox.

Tactile stimulation-induced rapid elevation of the synaptophysin mRNA expression level in rat somatosensory cortex

Synaptophysin is an integral membrane protein abundant in the synaptic vesicle and is found in nerve terminals throughout the brain. It was recently suggested that synaptophysin is also involved in the modulation of activity-dependent synapse formation. In this study, we examined at the individual level whether tactile stimulation selectively influenced the synaptophysin mRNA expression level in the somatosensory cortex of rats. Anesthetized rats were caressed on the back by an experimenter's palms for 20 min and the mRNA expression levels in the somatosensory and the visual cortices 5 min afterwards were determined using quantitative PCR methodology. The synaptophysin mRNA expression level was selectively higher in the experimental group than in the control group in the somatosensory cortex but not in the visual cortex. This suggests that the mRNA expression level of synaptophysin induced by neuronal activity is related to the regulation of synapse formation or remodeling or both.

Analysis of BDNF in brain structures of inbred mice with different phenotypes of mental and stress reaction

The content of BDNF was measured in cerebral structures of intact C57Bl/6 and BALB/c mice in winter and spring. The level of cerebral neurotrophic factor in laboratory mice depended on genetic characteristics and chronobiological factors.

Exposure to music in the perinatal period enhances learning performance and alters BDNF/TrkB signaling in mice as adults

Music has been suggested to have a beneficial effect on various types of performance in humans. However, the physiological and molecular mechanism of this effect remains unclear. We examined the effect of music exposure during the perinatal period on learning behavior in adult mice, and measured the levels of brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), which play critical roles in synaptic plasticity. In addition, we measured the levels of 3-phosphoinositide-dependent protein kinase-1 (PDK1) and mitogen-activated protein kinase (MAPK), downstream targets of two main pathways in BDNF/TrkB signaling. Music-exposed mice completed a maze learning task with fewer errors than the white noise-exposed mice and had lower levels of BDNF and higher levels of TrkB and PDK1 in the cortex. MAPK levels were unchanged. Furthermore, TrkB and PDK1 protein levels in the cortex showed a significant negative correlation with the number of errors on the maze. These results suggest that perinatal exposure of mice to music has an influence on BDNF/TrkB signaling and its intracellular signaling pathway targets, including PDK1, and thus may induce improved learning and memory functions.

Electrical forepaw stimulation during reversible forebrain ischemia decreases infarct volume

BACKGROUND AND PURPOSE

Functional stimulation is accompanied by increases in regional cerebral blood flow which exceed metabolic demands under normal circumstances, but it is unknown whether functional stimulation is beneficial or detrimental in the setting of acute ischemia. The aim of this study was to determine the effect of forepaw stimulation during temporary focal ischemia on neurological and tissue outcome in a rat model of reversible focal forebrain ischemia.

METHODS

Sprague-Dawley rats were prepared for temporary occlusion of the right middle cerebral artery (MCA) using the filament model. Cerebral blood flow in the MCA territory was continuously monitored with a laser-Doppler flowmeter. Subdermal electrodes were inserted into the dorsal forepaw to stimulate either the forepaw ipsilateral or contralateral to the occlusion starting 1 minute into ischemia and continuing throughout the ischemic period. A neurological evaluation was undertaken after 24 hours of reperfusion, and animals were then euthanized and brain slices stained with 2,3,5-triphenyltetrazolium chloride. Cortical and striatal damage was measured separately.

RESULTS

The cortical and striatal infarct volumes were both significantly reduced in the contralateral stimulated group compared with the ipsilateral stimulated group (48% total reduction). There were no statistically significant differences in the neurobehavioral scores between the 2 groups, or in the laser-Doppler flow measurements from the MCA core.

CONCLUSIONS

Functional stimulation of ischemic tissue may decrease tissue damage and improve outcome from stroke. Although the precise mechanism of this effect remains to be determined, functional stimulation could readily be translated to clinical practice.

Regulation of cpg15 expression during single whisker experience in the barrel cortex of adult mice

Regulation of gene transcription by neuronal activity is thought to be key to the translation of sensory experience into long-term changes in synaptic structure and function. Here we show that cpg15, a gene encoding an extracellular signaling molecule that promotes dendritic and axonal growth and synaptic maturation, is regulated in the somatosensory cortex by sensory experience capable of inducing cortical plasticity. Using in situ hybridization, we monitored cpg15 expression in 4-week-old mouse barrel cortex after trimming all whiskers except D1. We found that cpg15 expression is depressed in the deprived barrels and enhanced in the barrel column corresponding to the spared D1 whisker. Changes in cpg15 mRNA levels first appear in layer IV, peak 12 h after deprivation, and then decline rapidly. In layers II/III, changes in cpg15 expression appear later, peak at 24 h, and persist for days. Induction of cpg15 expression is significantly diminished in adolescent as well as adult CREB knockout mice. cpg15's spatio-temporal expression pattern and its regulation by CREB are consistent with a role in experience-dependent plasticity of cortical circuits. Our results suggest that local structural and/or synaptic changes may be a mechanism by which the adult cortex can adapt to peripheral manipulations.

TrkB signaling regulates the developmental maturation of the somatosensory cortex

In the rodent central nervous system, the region of the cortex that responds to facial whisker stimulation is anatomically segregated into discrete regions called barrels. Each barrel is made up of layer IV cortical neurons that receive input from a separate whisker via innervation from the thalamus. It has been shown that neurotrophins play important roles in the development and plasticity of thalamic axon innervation into the visual and retrosplenial cortex. We now extend those findings to the investigation of the role of neurotrophin signaling in barrel cortex formation. We show that the neurotrophin receptor TrkB is expressed in the thalamus and cortex during the time of cortical innervation. The two TrkB ligands, brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4), are expressed in the cortex at this time. Mice lacking TrkB demonstrate a developmental delay in the segregation of thalamic axons within barrels. In TrkB mutants, thalamic axons are abnormally uniform within layer IV of the cortex at postnatal day 4 compared to their control littermates, but show clear segregation into barrels 2 days later. This phenotype is recapitulated in BDNF mutant mice, but not in NT-4 mutant mice. These results demonstrate that BDNF is the sole TrkB ligand responsible for this phenotype. Analysis of conditional knockout mice that lack TrkB within the cortex, and not the thalamus, does not show a delay in thalamic axon segregation. These results indicate that TrkB expression in thalamic axons is important for the appropriate timing of barrel cortex development.