Neurotrophins regulate dendritic growth in developing visual cortex

BDNF-dependent accumulation of palmitoleic acid in CNS neurons

Although it is known that brain-derived neurotrophic factor (BDNF) plays a critical role in neuronal survival and differentiation, its effect on lipid homeostasis is poorly understood. To understand them, we here investigated the effect of BDNF on the fatty acid composition of primary neurons. A detailed analysis of the fatty acid composition of BDNF-stimulated primary neurons revealed that BDNF treatment led to a significant and selective increase in intracellular palmitoleic acid (PLO) levels. Correspondingly, BDNF induced the expression of the enzyme responsible for PLO synthesis [stearoyl-CoA desaturase-1]. In addition, this increase was suppressed by K252a, an inhibitor for tropomyosin-related kinase (Trk) receptors, indicating that BDNF-dependent increase in the PLO was mediated through the activation of TrkB. Further, PLO in culture media was reduced by BDNF treatment. This result suggested that BDNF suppressed extracellular release of PLO. Taken together, these data indicate that BDNF increases intracellular PLO both by activating its biosynthesis and by suppressing its extracellular release.

Regulation of dendritic branching by Cdc42 GAPs

Nerve cells form elaborate, highly branched dendritic trees that are optimized for the receipt of synaptic signals. Recent work published in this issue of Genes & Development by Rosario and colleagues (pp. 1743-1757) shows that a Cdc42-specific GTPase-activating protein (NOMA-GAP) regulates the branching of dendrites by neurons in the top layers of the mouse cortex. The results raise interesting questions regarding the specification of arbors in different cortical layers and the mechanisms of dendrite branching.

Brain-derived neurotrophic factor and androgen interactions in spinal neuromuscular systems

Neurotrophic factors and steroid hormones interact to regulate a variety of neuronal processes such as neurite outgrowth, differentiation, and neuroprotection. The coexpression of steroid hormone and neurotrophin receptor mRNAs and proteins, as well as their reciprocal regulation provides the necessary substrates for such interactions to occur. This review will focus on androgen brain-derived neurotrophic factor (BDNF) interactions in the spinal cord, describing androgen regulation of BDNF in neuromuscular systems following castration, androgen manipulation, and injury. Androgens interact with BDNF during development to regulate normally-occurring motoneuron death, and in adulthood, androgen-BDNF interactions are involved in the maintenance of several features of neuromuscular systems. Androgens regulate BDNF and trkB expression in spinal motoneurons. Androgens also regulate BDNF levels in the target musculature, and androgenic action at the muscle regulates BDNF levels in motoneurons. These interactions have important implications for the maintenance of motoneuron morphology. Finally, androgens interact with BDNF after injury, influencing soma size, dendritic morphology, and axon regeneration. Together, these findings provide further insight into the development and maintenance of neuromuscular systems and have implications for the neurotherapeutic/neuroprotective roles of androgens and trophic factors in the treatment of motoneuron disease and recovery from injury.

Imipramine induces brain-derived neurotrophic factor mRNA expression in cultured astrocytes

Depression is one of the most prevalent and livelihood-threatening forms of mental illnesses and the neural circuitry underlying depression remains incompletely understood. Recent studies suggest that the neuronal plasticity involved with brain-derived neurotrophic factor (BDNF) plays an important role in the recovery from depression. Some antidepressants are reported to induce BDNF expression in vivo; however, the mechanisms have been considered solely in neurons and not fully elucidated. In the present study, we evaluated the effects of imipramine, a classic tricyclic antidepressant drug, on BDNF expression in cultured rat brain astrocytes. Imipramine dose-dependently increased BDNF mRNA expression in astrocytes. The imipramine-induced BDNF increase was suppressed with inhibitors for protein kinase A (PKA) or MEK/ERK. Moreover, imipramine exposure activated transcription factor cAMP response element binding protein (CREB) in a dose-dependent manner. These results suggested that imipramine induced BDNF expression through CREB activation via PKA and/or ERK pathways. Imipramine treatment in depression might exert antidepressant action through BDNF production from astrocytes, and glial BDNF expression might be a target of developing novel antidepressants.

Ankyrin repeat-rich membrane spanning protein (kidins220) is required for neurotrophin and ephrin receptor-dependent dendrite development

Dendrites are the primary sites on neurons for receiving and integrating inputs from their presynaptic partners. Defects in dendrite development perturb the formation of neural circuitry and impair information processing in the brain. Extracellular cues are important for shaping the dendritic morphogenesis, but the underlying molecular mechanisms are not well understood. In this study, we examined the role of ARMS (ankyrin repeat-rich membrane spanning protein), also known as Kidins220 (kinase D-interacting substrate of 220 kDa), previously identified as a downstream target of neurotrophin and ephrin receptors, in dendrite development. We report here that knockdown of ARMS/Kidins220 by in utero electroporation impairs dendritic branching in mouse cerebral cortex, and silencing of ARMS/Kidins220 in primary rat hippocampal neurons results in a significant decrease in the length, number, and complexity of the dendritic arbors. Overexpression of cell surface receptor tyrosine kinases, including TrkB and EphB2, in ARMS/Kidins220-deficient neurons can partially rescue the defective dendritic phenotype. More importantly, we show that PI3K (phosphoinositide-3-kinase)- and Akt-mediated signaling pathway is crucial for ARMS/Kidins220-dependent dendrite development. Furthermore, loss of ARMS/Kidins220 significantly reduced the clustering of EphB2 receptor signaling complex in neurons. Our results collectively suggest that ARMS/Kidins220 is a key player in organizing the signaling complex to transduce the extracellular stimuli to cellular responses during dendrite development.

Alterations of neocortical development and maturation in autism: insight from valproic acid exposure and animal models of autism

Autism spectrum disorder (ASD) is a behaviourally defined brain disorder affecting approximately 1 in 88 children. Many pathological studies have shown that ASD is frequently associated with grey and white matter changes that can be described by their deviations from the normal trajectory of cortical maturation. For example, during the early (i.e. <2 years) postnatal period there is marked and selective tissue overgrowth in the higher-order temporal and frontal networks involved in emotional, social, and communication functions. In this focused review we first summarize some basic principles of neocortical neural organization and how they are disrupted in ASD. We will then highlight some of the potential mechanisms by which the normal developmental trajectory and organization of neocortical networks can be altered based on animal studies of valproic acid, a teratogen widely used in animal models of ASD. We argue that the trajectory of postnatal cerebral neocortex development may be influenced by several cellular and molecular mechanisms that may all converge to produce a neuropathology characterized by premature or accelerated neuronal growth.

Differential brain-derived neurotrophic factor expression in limbic brain regions following social defeat or territorial aggression

Syrian hamsters readily form dominant-subordinate relationships under laboratory conditions. Winning or losing in agonistic encounters can have striking, long-term effects on social behavior, but the mechanisms underlying this experience-induced behavioral plasticity are unclear. The present study tested the hypothesis that changes in brain-derived neurotrophic factor (BDNF) may at least in part mediate this plasticity. Male hamsters were paired for 15-min using a resident-intruder model, and individuals were identified as winners or losers on the basis of their behavior. BDNF was examined with in situ hybridization 2 hr after treatment during the consolidation period of emotional learning. Losing animals had significantly more BDNF mRNA in the basolateral (BLA) and medial (MeA) nuclei of the amygdala when compared with winning animals as well as novel cage and home cage controls. Interestingly, winning animals had significantly more BDNF mRNA in the dentate gyrus of the dorsal hippocampus than did losing animals, novel, and home cage controls. No conflict-related changes in BDNF mRNA were observed in several other regions including the bed nucleus of the stria terminalis and central amygdala. Next, we demonstrated that K252a, a Trk receptor antagonist, significantly reduced the acquisition of conditioned defeat when administered within the BLA. These data support a model in which BDNF-mediated plasticity within the BLA supports learning of submission or subordinate social status in losing animals, whereas BDNF-mediated plasticity within the hippocampus may instantiate aspects of winning such as control of a territory in dominant animals.

The impact of poverty on the development of brain networks

Although the study of brain development in non-human animals is an old one, recent imaging methods have allowed non-invasive studies of the gray and white matter of the human brain over the lifespan. Classic animal studies show clearly that impoverished environments reduce cortical gray matter in relation to complex environments and cognitive and imaging studies in humans suggest which networks may be most influenced by poverty. Studies have been clear in showing the plasticity of many brain systems, but whether sensitivity to learning differs over the lifespan and for which networks is still unclear. A major task for current research is a successful integration of these methods to understand how development and learning shape the neural networks underlying achievements in literacy, numeracy, and attention. This paper seeks to foster further integration by reviewing the current state of knowledge relating brain changes to behavior and indicating possible future directions.

Dendritic BDNF synthesis is required for late-phase spine maturation and recovery of cortical responses following sensory deprivation

Sensory experience in early postnatal life shapes neuronal connections in the brain. Here we report that the local synthesis of brain-derived neurotrophic factor (BDNF) in dendrites plays an important role in this process. We found that dendritic spines of layer 2/3 pyramidal neurons of the visual cortex in mutant mice lacking dendritic Bdnf mRNA and thus local BDNF synthesis were normal at 3 weeks of age, but thinner, longer, and more closely spaced (morphological features of immaturity) at 4 months of age than in wild-type (WT) littermates. Layer 2/3 of the visual cortex in these mutant animals also had fewer GABAergic presynaptic terminals at both ages. The overall size and shape of dendritic arbors were, however, similar in mutant and WT mice at both ages. By using optical imaging of intrinsic signals and single-unit recordings, we found that mutant animals failed to recover cortical responsiveness following monocular deprivation (MD) during the critical period, although they displayed normally the competitive loss of responsiveness to an eye briefly deprived of vision. Furthermore, MD still induced a loss of responsiveness to the closed eye in adult mutant mice, but not in adult WT mice. These results indicate that dendritic BDNF synthesis is required for spine pruning, late-phase spine maturation, and recovery of cortical responsiveness following sensory deprivation. They also suggest that maturation of dendritic spines is required for the maintenance of cortical responsiveness following sensory deprivation in adulthood.

Altered balance of proteolytic isoforms of pro-brain-derived neurotrophic factor in autism

Defects in synaptic development and plasticity may lead to autism. Brain-derived neurotrophic factor (BDNF) plays a critical role in synaptogenesis and synaptic plasticity. BDNF is synthesized as a precursor, pro-BDNF, which can be processed into either a truncated form or into mature BDNF. Previous studies reported increased BDNF-immunoreactive protein in autism, but the mechanism of this increase has not been investigated. We examined BDNF mRNA by real-time reverse transcription-polymerase chain reaction and BDNF protein by Western blotting and enzyme-linked immunosorbent assay in postmortem fusiform gyrus tissue from 11 patients with autism and 14 controls. BDNF mRNA levels were not different in the autism versus control samples, but total BDNF-like immunoreactive protein, measured by enzyme-linked immunosorbent assay, was greater in autism than in controls. Western blotting revealed greater pro-BDNF and less truncated BDNF in autism compared with controls. These data demonstrate that increased levels of BDNF-immunoreactive protein in autism are not transcriptionally driven. Increased pro-BDNF and reduced truncated BDNF are consistent with defective processing of pro-BDNF to its truncated form. Distortion of the balance among the 3 BDNF isoforms, each of which may exhibit different biological activities, could lead to changes in connectivity and synaptic plasticity and, hence, behavior. Thus, imbalance in proteolytic isoforms is a possible new mechanism for altered synaptic plasticity leading to autism.

Kidins220/ARMS as a functional mediator of multiple receptor signalling pathways

An increasing body of evidence suggests that several membrane receptors--in addition to activating distinct signalling cascades--also engage in substantial crosstalk with each other, thereby adjusting their signalling outcome as a function of specific input information. However, little is known about the molecular mechanisms that control their coordination and integration of downstream signalling. A protein that is likely to have a role in this process is kinase-D-interacting substrate of 220 kDa [Kidins220, also known as ankyrin repeat-rich membrane spanning (ARMS), hereafter referred to as Kidins220/ARMS]. Kidins220/ARMS is a conserved membrane protein that is preferentially expressed in the nervous system and interacts with the microtubule and actin cytoskeleton. It interacts with neurotrophin, ephrin, vascular endothelial growth factor (VEGF) and glutamate receptors, and is a common downstream target of several trophic stimuli. Kidins220/ARMS is required for neuronal differentiation and survival, and its expression levels modulate synaptic plasticity. Kidins220/ARMS knockout mice show developmental defects mainly in the nervous and cardiovascular systems, suggesting a crucial role for this protein in modulating the cross talk between different signalling pathways. In this Commentary, we summarise existing knowledge regarding the physiological functions of Kidins220/ARMS, and highlight some interesting directions for future studies on the role of this protein in health and disease.

Dendritic structural plasticity

Dendrites represent the compartment of neurons primarily devoted to collecting and computating input. Far from being static structures, dendrites are highly dynamic during development and appear to be capable of plastic changes during the adult life of animals. During development, it is a combination of intrinsic programs and external signals that shapes dendrite morphology; input activity is a conserved extrinsic factor involved in this process. In adult life, dendrites respond with more modest modifications of their structure to various types of extrinsic information, including alterations of input activity. Here, the author reviews classical and recent evidence of dendrite plasticity in invertebrates and vertebrates and current progress in the understanding of the molecular mechanisms that underlie this plasticity. Importantly, some fundamental questions such as the functional role of dendrite remodeling and the causal link between structural modifications of neurons and plastic processes, including learning, are still open.

Brain-derived neurotrophic factor (BDNF) reverses the effects of rapid eye movement sleep deprivation (REMSD) on developmentally regulated, long-term potentiation (LTP) in visual cortex slices

Work in this laboratory demonstrated a role for rapid eye movement sleep (REMS) in critical period (CP), postnatal days (P) 17-30, synaptic plasticity in visual cortex. Studies in adolescent rats showed that REMS deprivation (REMSD) reinitiates a developmentally regulated form of synaptic plasticity that otherwise is observed only in CP animals. Subsequent work added that REMSD affects inhibitory mechanisms that are thought to be involved in terminating the CP. Neurotrophins are implicated in the synaptic plasticity that underlies CP maturation and also final closure of the CP in visual cortex. Expression of brain-derived neurotrophic factor (BDNF) is dependent upon neuronal activity, and REMSD may block BDNF expression. We propose that REMS contributes to the maturation of visual cortex through regulation of BDNF expression and consequent, downstream increase in cortical inhibitory tone. In this study, osmotic minipumps delivered BDNF into visual cortex on one side of brain. The opposite hemisphere was not implanted and served as an internal control. We tested the hypothesis that BDNF is blocked by REMSD in late-adolescent rats and investigated whether replacing BDNF prevents induction of LTPWM-III by theta burst stimulation (TBS). We also assessed relative inhibitory tone in visual cortex with paired-pulse stimulation (PPS) in animals that were similarly REMSD- and BDNF-infused. After REMSD, both hemispheres were prepared in parallel for in vitro synaptic plasticity studies (LTPWM-III or PPS). In visual cortex of REMSD rats on the side receiving BDNF infusions (8 of 8 animals), TBS consistently failed to induce LTPWM-III. In contrast, LTPWM-III was obtained (5 of 5 animals) in the matched, non-infused hemisphere, as expected in rats of this age. REMSD animals that were unilaterally infused with saline produced LTPWM-III in both hemispheres. PPS studies in another group of REMSD animals that were unilaterally BDNF-infused displayed age-appropriate inhibition of the second response on the BDNF-infused side (5/5), whereas on the non-infused side facilitation was observed (3/3). Intracortical infusion of BDNF in REMSD adolescent rats appears to restore neurochemical processes necessary for termination of the CP for developmentally regulated synaptic plasticity in visual cortex. The results suggest that REMSD blocks BDNF expression and also maturation of inhibitory processes in adolescent visual cortex. These data support REMS' function in brain development.

Divergent roles of p75NTR and Trk receptors in BDNF's effects on dendritic spine density and morphology

Activation of TrkB receptors by brain-derived neurotrophic factor (BDNF) followed by MAPK/ERK signaling increases dendritic spine density and the proportion of mature spines in hippocampal CA1 pyramidal neurons. Considering the opposing actions of p75(NTR) and Trk receptors in several BDNF actions on CNS neurons, we tested whether these receptors also have divergent actions on dendritic spine density and morphology. A function-blocking anti-p75(NTR) antibody (REX) did not affect spine density by itself but it prevented BDNF's effect on spine density. Intriguingly, REX by itself increased the proportion of immature spines and prevented BDNF's effect on spine morphology. In contrast, the Trk receptor inhibitor k-252a increased spine density by itself, and prevented BDNF from further increasing spine density. However, most of the spines in k-252a-treated slices were of the immature type. These effects of k-252a on spine density and morphology required neuronal activity because they were prevented by TTX. These divergent BDNF actions on spine density and morphology are reminiscent of opposing functional signaling by p75(NTR) and Trk receptors and reveal an unexpected level of complexity in the consequences of BDNF signaling on dendritic morphology.

Oxytocin and dendrite remodeling in the hypothalamus

For most people, their quality of life depends on their successful interdependence with others, which requires sophisticated social cognition, communication, and emotional bonds. Across the lifespan, new bonds must be forged and maintained, and conspecific menaces must be managed. The dynamic nature of the human social landscape suggests ongoing specific alterations in neural circuitry across several brain systems to subserve social behavior. To discover the biological mechanisms that contribute to normal social activities, animal models of social behavior have been developed. One valuable model system has been female rat sexual behavior, which is governed by cyclic variation of ovarian hormones. This behavior is modulated by the neuropeptide oxytocin (OT) through its actions in the hypothalamic ventromedial nucleus (VMH). The fluctuation of this behavior is associated with dendrite remodeling, like several other examples of behavioral plasticity. This review compares hormone-induced plasticity in the VMH with other examples of dendrite plasticity across the mammalian nervous system, namely the neurobehavioral paradigms of environmental enrichment, chronic stress, and incentive sensitization, which affect the neocortex, hippocampal formation, and ventral striatum, respectively. This comparison suggests that the effects of ovarian hormones on VMH neurons in rats, given the simple dendritic arbor and short time course for dendrite remodeling, provide a dual opportunity for mechanistic and functional studies that will shed light on i) the neural actions of OT that regulate social behavior and, ii) behaviorally relevant dendrite regulation in a variety of brain structures. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Glia determine the course of brain-derived neurotrophic factor-mediated dendritogenesis and provide a soluble inhibitory cue to dendritic growth in the brainstem

Cardiorespiratory control neurons in the brainstem nucleus tractus solitarius (NTS) undergo dramatic expansion of dendritic arbors during the early postnatal period, when functional remodeling takes place within the NTS circuitry. However, the underlying molecular mechanisms of morphological maturation of NTS neurons are largely unknown. Our previous studies point to the neurotrophin brain-derived neurotrophic factor (BDNF), which is abundantly expressed by NTS-projecting primary sensory neurons, as a candidate mediator of NTS dendritogenesis. In the current study, we used neonatal rat NTS neurons in vitro to examine the role of BDNF in the dendritic development of neurochemically identified subpopulations of NTS neurons. In the presence of abundant glia, BDNF promoted NTS dendritic outgrowth and complexity, with the magnitude of the BDNF effect dependent on neuronal phenotype. Surprisingly, BDNF switched from promoting to inhibiting NTS dendritogenesis upon glia depletion. Moreover, glia depletion alone led to a significant increase in NTS dendritic outgrowth. Consistent with this result, astrocyte-conditioned medium (ACM), which promoted hippocampal dendritogenesis, inhibited dendritic growth of NTS neurons. The latter effect was abolished by heat-inactivation of ACM, pointing to a diffusible astrocyte-derived negative regulator of NTS dendritic growth. Together, these data demonstrate a role for BDNF in the postnatal development of NTS neurons, and reveal novel effects of glia on this process. Moreover, previously documented dramatic increases in NTS glial proliferation in victims of sudden infant death syndrome (SIDS) underscore the importance of our findings and the need to better understand the role of glia and their interactions with BDNF during NTS circuit maturation. Furthermore, while it has previously been demonstrated that the specific effects of BDNF on dendritic growth are context-dependent, the role of glia in this process is unknown. Thus, our data carry important implications for mechanisms of dendritogenesis likely beyond the NTS.

Stress leads to contrasting effects on the levels of brain derived neurotrophic factor in the hippocampus and amygdala

Recent findings on stress induced structural plasticity in rodents have identified important differences between the hippocampus and amygdala. The same chronic immobilization stress (CIS, 2h/day) causes growth of dendrites and spines in the basolateral amygdala (BLA), but dendritic atrophy in hippocampal area CA3. CIS induced morphological changes also differ in their temporal longevity- BLA hypertrophy, unlike CA3 atrophy, persists even after 21 days of stress-free recovery. Furthermore, a single session of acute immobilization stress (AIS, 2h) leads to a significant increase in spine density 10 days, but not 1 day, later in the BLA. However, little is known about the molecular correlates of the differential effects of chronic and acute stress. Because BDNF is known to be a key regulator of dendritic architecture and spines, we investigated if the levels of BDNF expression reflect the divergent effects of stress on the hippocampus and amygdala. CIS reduces BDNF in area CA3, while it increases it in the BLA of male Wistar rats. CIS-induced increase in BDNF expression lasts for at least 21 days after the end of CIS in the BLA. But CIS-induced decrease in area CA3 BDNF levels, reverses to normal levels within the same period. Finally, BDNF is up regulated in the BLA 1 day after AIS and this increase persists even 10 days later. In contrast, AIS fails to elicit any significant change in area CA3 at either time points. Together, these findings demonstrate that both acute and chronic stress trigger opposite effects on BDNF levels in the BLA versus area CA3, and these divergent changes also follow distinct temporal profiles. These results point to a role for BDNF in stress-induced structural plasticity across both hippocampus and amygdala, two brain areas that have also been implicated in the cognitive and affective symptoms of stress-related psychiatric disorders.

Network, cellular, and molecular mechanisms underlying long-term memory formation

The neural network stores information through activity-dependent synaptic plasticity that occurs in populations of neurons. Persistent forms of synaptic plasticity may account for long-term memory storage, and the most salient forms are the changes in the structure of synapses. The theory proposes that encoding should use a sparse code and evidence suggests that this can be achieved through offline reactivation or by sparse initial recruitment of the network units. This idea implies that in some cases the neurons that underwent structural synaptic plasticity might be a subpopulation of those originally recruited; However, it is not yet clear whether all the neurons recruited during acquisition are the ones that underwent persistent forms of synaptic plasticity and responsible for memory retrieval. To determine which neural units underlie long-term memory storage, we need to characterize which are the persistent forms of synaptic plasticity occurring in these neural ensembles and the best hints so far are the molecular signals underlying structural modifications of the synapses. Structural synaptic plasticity can be achieved by the activity of various signal transduction pathways, including the NMDA-CaMKII and ACh-MAPK. These pathways converge with the Rho family of GTPases and the consequent ERK 1/2 activation, which regulates multiple cellular functions such as protein translation, protein trafficking, and gene transcription. The most detailed explanation may come from models that allow us to determine the contribution of each piece of this fascinating puzzle that is the neuron and the neural network.

Regulation of brain-derived neurotrophic factor exon IV transcription through calcium responsive elements in cortical neurons

Activity-dependent transcription of brain-derived neurotrophic factor (BDNF) has been studied as an important model to elucidate the mechanisms underlying numerous aspects of neuroplasticity. It has been extensively emphasized that Ca²⁺ influx through different routes may have significantly different effects on BDNF transcription. Here, we examined the regulatory property of the major calcium responsive elements (CaRE) in BDNF promoter IV in cultured rat cortical neurons. BDNF promoter IV, as well as CaRE1 and CaRE3, was significantly activated by Ca²⁺ influx through L-type voltage-gated calcium channel (L-VGCC) or NMDA receptor (NMDAR). However, the L-VGCC- and NMDAR-mediated activation of CaRE was differentially regulated by different Ca²⁺-stimulated protein kinases. Specifically, PKA, CaMKI, and CaMKIV activity were required for L-VGCC-, but not NMDAR-mediated CaRE1 activation. CaMKI activity was required for NMDAR- but not L-VGCC-mediated CaRE3 activation. Surprisingly, the activation of CaRF, a previously identified transcription factor for CaRE1, was stimulated via L-VGCC but not NMDAR, and required MEK, PI3K and CaMKII activity. These results suggest a new working model that activity-dependent BDNF IV up-regulation may be coordinately mediated by CaRE1 and CaRE3 activity, which show different responses to Ca²⁺-stimulated kinases. Our data also explain how the individual cis-element in BDNF promoter is distinctively coupled to different Ca²⁺ routes.

Transcriptional regulation of neuronal polarity and morphogenesis in the mammalian brain

The highly specialized morphology of a neuron, typically consisting of a long axon and multiple branching dendrites, lies at the core of the principle of dynamic polarization, whereby information flows from dendrites toward the soma and to the axon. For more than a century, neuroscientists have been fascinated by how shape is important for neuronal function and how neurons acquire their characteristic morphology. During the past decade, substantial progress has been made in our understanding of the molecular underpinnings of neuronal polarity and morphogenesis. In these studies, transcription factors have emerged as key players governing multiple aspects of neuronal morphogenesis from neuronal polarization and migration to axon growth and pathfinding to dendrite growth and branching to synaptogenesis. In this review, we will highlight the role of transcription factors in shaping neuronal morphology with emphasis on recent literature in mammalian systems.

HDAC activity is required for BDNF to increase quantal neurotransmitter release and dendritic spine density in CA1 pyramidal neurons

Molecular mechanisms involved in the strengthening and formation of synapses include the activation and repression of specific genes or subsets of genes by epigenetic modifications that do not alter the genetic code itself. Chromatin modifications mediated by histone acetylation have been shown to be critical for synaptic plasticity at hippocampal excitatory synapses and hippocampal-dependent memory formation. Considering that brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity and behavioral adaptations, it is not surprising that regulation of this gene is subject to histone acetylation changes during synaptic plasticity and hippocampal-dependent memory formation. Whether the effects of BDNF on dendritic spines and quantal transmitter release require histone modifications remains less known. By using two different inhibitors of histone deacetylases (HDACs), we describe here that their activity is required for BDNF to increase dendritic spine density and excitatory quantal transmitter release onto CA1 pyramidal neurons in hippocampal slice cultures. These results suggest that histone acetylation/deacetylation is a critical step in the modulation of hippocampal synapses by BDNF. Thus, mechanisms of epigenetic modulation of synapse formation and function are novel targets to consider for the amelioration of symptoms of intellectual disabilities and neurodegenerative disorders associated with cognitive and memory deficits.

Neuronal growth cone retraction relies on proneurotrophin receptor signaling through Rac

Growth of axons and dendrites is a dynamic process that involves guidance molecules, adhesion proteins, and neurotrophic factors. Although neurite extension is stimulated by the neurotrophin nerve growth factor (NGF), we found that the precursor of NGF, proNGF, induced acute collapse of growth cones of cultured hippocampal neurons. This retraction was initiated by an interaction between the p75 neurotrophin receptor (p75NTR) and the sortilin family member SorCS2 (sortilin-related VPS10 domain-containing receptor 2). Binding of proNGF to the p75NTR-SorCS2 complex induced growth cone retraction by initiating the dissociation of the guanine nucleotide exchange factor Trio from the p75NTR-SorCS2 complex, resulting in decreased Rac activity and, consequently, growth cone collapse. The actin-bundling protein fascin was also inactivated, contributing to the destabilization and collapse of actin filaments. These results identify a bifunctional signaling mechanism by which proNGF regulates actin dynamics to acutely modulate neuronal morphology.

BDNF polymorphism-dependent OFC and DLPFC plasticity differentially moderates implicit and explicit bias

This study examined the role of orbitofrontal cortex (OFC) and dorsolateral prefrontal cortex (DLPFC) plasticity in controlling implicit and explicit social biases. Normal controls and patients with varied OFC and DLPFC lesion size and single nucleotide polymorphisms (SNPs) in the brain-derived neurotrophic factor (BDNF) gene, which promotes (methionine-valine [Met/Val] SNP) or stifles (valine-valine [Val/Val] SNP) plasticity in damaged PFC regions, completed measures of implicit and explicit social bias. Patients and controls demonstrated comparable levels of implicit bias, but patients with Met/Val SNPs exhibited less implicit bias when they had smaller OFC lesions compared with Val/Val patients with similar size lesions and those with large OFC lesions. Both patients and controls demonstrated patterns of explicit bias consistent with hypotheses. Patients with Met/Val SNPs exhibited less explicit bias when they had smaller DLPFC lesions sizes compared with Val/Val patients with similar size lesions and those with large DLPFC lesions. OFC lesion size and BDNF SNP type did not moderate explicit bias; DLPFC lesion size and BDNF SNP type did not moderate implicit bias (nor did other medial or lateral regions). Findings suggest that plasticity within specific PFC regions modulates the type and degree of social bias that individuals' exhibit.

Seizures in early life suppress hippocampal dendrite growth while impairing spatial learning

Impaired learning and memory are common in epilepsy syndromes of childhood. Clinical investigations suggest that the developing brain may be particularly vulnerable to the effects of intractable seizure disorders. Magnetic resonance imaging (MRI) studies have demonstrated reduced volumes in brain regions involved in learning and memory. The earlier the onset of an epilepsy the larger the effects seem to be on both brain anatomy and cognition. Thus, childhood epilepsy has been proposed to interfere in some unknown way with brain development. Experiments reported here explore these ideas by examining the effects of seizures in infant mice on learning and memory and on the growth of CA1 hippocampal pyramidal cell dendrites. Fifteen brief seizures were induced by flurothyl between postnatal days 7 and 11 in mice that express green fluorescent protein (GFP) in hippocampal pyramidal cells. One to 44days later, dendritic arbors were reconstructed to measure growth. Spatial learning and memory were also assessed in a water maze. Our results show that recurrent seizures produced marked deficits in learning and memory. Seizures also dramatically slowed the growth of basilar dendrites while neurons in littermate control mice continued to add new dendritic branches and lengthen existing branches. When experiments were performed in older mice, seizures had no measureable effects on either dendrite arbor complexity or spatial learning and memory. Our results suggest that the recurring seizures of intractable childhood epilepsy contribute to associated learning and memory deficits by suppressing dendrite growth.

Cellular and molecular responses of cultured neurons to stressful stimuli

Synaptic function is critical for the brain to process experiences dictated by the environment requiring change over the lifetime of the organism. Experience-driven adaptation requires that receptors, signal transduction pathways, transcription and translational mechanisms within neurons respond rapidly over its lifetime. Adaptive responses communicated through the rapid firing of neurons are dependent upon the integrity and function of synapses. These rapid responses via adaptation underlie the organism's ability to perceive, learn, remember, calculate and plan. Glutamate, the endogenous neurotransmitter required for physiological excitation in the brain, is critically involved in neuronal adaptive responses and in the pathophysiology of neurodegenerative disorders. Using neuronal experimental systems, we will discuss how compounds with low dose effects mediated via glutamate receptors can result either in a neuroprotective or neurotoxic response. Because the brain has evolved to respond rapidly to environmental cues, exposure of neurons to stressful stimuli can result in a pivotal response toward either synaptic adaptation or dysfunction and neuronal cell death. Understanding how neurons adapt to stressful stimuli will provide important clues toward the development of strategies to protect the brain against neurodegeneration.

Medium spiny neurons of the neostriatal matrix exhibit specific, stereotyped changes in dendritic arborization during a critical developmental period in mice

In mice, the matrix compartment of the striatum (caudate/putamen) undergoes major developmental changes during the second postnatal week, including the establishment of corticostriatal and nigrostriatal afferents, the maturation of parvalbumin-positive interneurons and the appearance of perineuronal nets. It is not known if any of these events influence the dendritic structure of medium spiny neurons, the principal output cells of the striatum. To determine whether any measurable changes in the dendrites of matrix medium spiny neurons occur during this important developmental period, we labeled individual cells at different time points flanking the second postnatal week. These cells exhibit distinct dendritic morphologies from the earliest postnatal time points examined. Furthermore, our data show that the dendritic arbors of these neurons change in length, branch points, diameter and tortuosity, regardless of morphological type. The increase in dendritic length is accompanied by a decrease in the number of branch points that occur in different, but consistent, parts of the dendritic arbor. All of these changes are most pronounced during the second postnatal week, coinciding with a number of developmental events considered important for consolidating circuitry within the striatal matrix. Our results further support the critical importance of this early postnatal period in striatal development.

Spatial segregation of BDNF transcripts enables BDNF to differentially shape distinct dendritic compartments

BDNF is produced from many transcripts that display distinct subcellular localization, suggesting that spatially restricted effects occur as a function of genetic and physiological regulation. Different BDNF 5' splice variants give a restricted localization in the cell body or the proximal and distal compartments of dendrites; however, the functional consequences are not known. Silencing individual endogenous transcripts or overexpressing BDNF-GFP transcripts in cultured neurons demonstrated that whereas some transcripts (1 and 4) selectively affected proximal dendrites, others (2C and 6) affected distal dendrites. Moreover, segregation of BDNF transcripts resulted in a highly selective activation of the BDNF TrkB receptor. These studies indicate that spatial segregation of BDNF transcripts enables BDNF to differentially shape distinct dendritic compartments.

BDNF-promoted increases in proximal dendrites occur via CREB-dependent transcriptional regulation of cypin

Alterations in dendrite branching and morphology are present in many neurodegenerative diseases. These variations disrupt postsynaptic transmission and affect neuronal communication. Thus, it is important to understand the molecular mechanisms that regulate dendritogenesis and how they go awry during disease states. Previously, our laboratory showed that cypin, a mammalian guanine deaminase, increases dendrite number when overexpressed and decreases dendrite number when knocked down in cultured hippocampal neurons. Here, we report that exposure to brain-derived neurotrophic factor (BDNF), an important mediator of dendrite arborization, for 72 h but not for 24 h or less increases cypin mRNA and protein levels in rat hippocampal neurons. BDNF signals through cypin to regulate dendrite number, since knocking down cypin blocks the effects of BDNF. Furthermore, BDNF increases cypin levels via mitogen-activated protein kinase and transcription-dependent signaling pathways. Moreover, the cypin promoter region contains putative conserved cAMP response element (CRE) regions, which we found can be recognized and activated by CRE-binding protein (CREB). In addition, exposure of the neurons to BDNF increased CREB binding to the cypin promoter and, in line with these data, expression of a dominant negative form of CREB blocked BDNF-promoted increases in cypin protein levels and proximal dendrite branches. Together, these studies suggest that BDNF increases neuronal cypin expression by the activation of CREB, increasing cypin transcription leading to increased protein expression, thus identifying a novel pathway by which BDNF shapes the dendrite network.

Experience-dependent structural plasticity in the adult human brain

Contrary to assumptions that changes in brain networks are possible only during crucial periods of development, research in the past decade has supported the idea of a permanently plastic brain. Novel experience, altered afferent input due to environmental changes and learning new skills are now recognized as modulators of brain function and underlying neuroanatomic circuitry. Given findings in experiments with animals and the recent discovery of increases in gray and white matter in the adult human brain as a result of learning, the old concept of cognitive reserve, that is the ability to reinforce brain volume in crucial areas and thus provide a greater threshold for age-dependent deficits, has been reinforced. The challenge we face is to unravel the exact nature of the dynamic structural alterations and, ultimately, to be able to use this knowledge for disease management. Understanding normative changes in brain structure that occur as a result of environmental changes and demands is pivotal to understanding the characteristic ability of the brain to adapt.

Retrolinkin cooperates with endophilin A1 to mediate BDNF-TrkB early endocytic trafficking and signaling from early endosomes

Both retrolinkin and its interaction partner endophilin A1 are required for BDNF-induced dendrite outgrowth of cultured hippocampal neurons. They function sequentially in an early endocytic trafficking pathway for BDNF-activated TrkB, which provides spatiotemporal control of downstream ERK signaling from endosomes.

Brain-derived neurotrophic factor (BDNF) binds to its cell surface receptor TrkB to regulate differentiation, development, synaptic plasticity, and functional maintenance of neuronal cells. Binding of BDNF triggers TrkB dimerization and autophosphorylation, which provides docking sites for adaptor proteins to recruit and activate downstream signaling molecules. The molecular mechanisms underlying BDNF–TrkB endocytic trafficking crucial for spatiotemporal control of signaling pathways remain to be elucidated. Here we show that retrolinkin, a transmembrane protein, interacts with endophilin A1 and mediates BDNF-activated TrkB (pTrk) trafficking and signaling in CNS neurons. We find that activated TrkB colocalizes and interacts with the early endosome marker APPL1. Both retrolinkin and endophilin A1 are required for BDNF-induced dendrite development and acute extracellular signal-regulated kinase activation from early endosomes. Suppression of retrolinkin expression not only blocks BDNF-triggered TrkB internalization, but also prevents recruitment of endophilin A1 to pTrk vesicles trafficking through APPL1-positive endosomes. These findings reveal a novel mechanism for BDNF–TrkB to regulate signaling both in time and space through a specific membrane trafficking pathway.

Developmental sculpting of dendritic morphology of layer 4 neurons in visual cortex: influence of retinal input

Dendritic morphology determines the kinds of input a neuron receives, having a profound impact on neural information processing. In the mammalian cerebral cortex, excitatory neurons have been ascribed to one of two main dendritic morphologies, either pyramidal or stellate, which differ mainly on the extent of the apical dendrite. Developmental mechanisms regulating the emergence and refinement of dendritic morphologies have been studied for cortical pyramidal neurons, but little is known for spiny stellate neurons. Using biolistics to label single cells on acute brain slices of the ferret primary visual cortex, we show that neurons in layer 4 develop in a two-step process: initially, all neurons appear pyramidal, growing a prominent apical dendrite and few small basal dendrites. Later, a majority of these neurons show a change in the relative extent of basal and apical dendrites that results in a gradual sculpting into a stellate morphology. We also find that ∼ 22% of neurons maintain the proportionality of their dendritic arbors, remaining as pyramidal cells at maturity. When ferrets were deprived of retinal input at early stages of postnatal development by binocular enucleation, a significant proportion of layer 4 spiny neurons failed to remodel their apical dendrites, and ∼ 55% remained as pyramidal neurons. Our results demonstrate that cortical spiny stellate neurons emerge by differential sculpting of the dendritic arborizations of an initial pyramidal morphology and that sensory input plays a fundamental role in this process.

Cooperation between BDNF and glutamate in the regulation of synaptic transmission and neuronal development

Ample evidence supports a role of brain-derived neurotrophic factor (BDNF) in the survival and differentiation of selective populations of neurons in the peripheral and central nervous systems. In addition to its trophic actions, BDNF exerts acute effects on synaptic transmission and plasticity. In particular, BDNF enhances excitatory synaptic transmission through pre- and postsynaptic mechanisms. In this regard, BDNF enhances glutamate release, the frequency of miniature excitatory postsynaptic currents (mEPSCs), NMDA receptor activity and the phosphorylation of NMDA receptor subunits. Our recent studies revealed a novel cooperative interaction between BDNF and glutamate in the regulation of dendritic development. Indeed, we found that the effects of BDNF on dendritic growth of cortical neurons require both the stimulation of cAMP response element-binding protein (CREB) phosphorylation by BDNF and the activation of the CREB-regulated transcription coactivator 1 (CRTC1) by glutamate. Together, these studies highlight the importance of the cooperation between BDNF and glutamate in the regulation of synaptic transmission and neuronal development.

Regulation of protein prenyltransferase in central neurons

Geranylgeranyltransferase I (GGT) is a protein prenyltransferase that mediates lipid modification of some proteins such as Rho family small GTPases. Since the activation of Rho GTPases mediates tumorgenesis and metastasis, GGT has become an attractive target for anti-tumor drug design. Although GGT is extensively expressed in the brain, the function of GGT in central nervous system (CNS) is totally unknown. We have previously shown that GGT was involved in neuromuscular synaptogenesis. In this study, we report that neuronal activity- and brain-derived neurotropic factor (BDNF)-dependent dendritic morphogenesis requires activation of GGT. Furthermore, GGT was activated by depolarization or BDNF in cultured neurons or in hippocampus of the mice under novelty exploration test, suggesting that neuronal activity activates GGT in vitro and in vivo. In this addendum, we further discuss the significance of this study and the possible implication to the field.

Fluoxetine regulates the expression of neurotrophic/growth factors and glucose metabolism in astrocytes

RATIONALE

The pharmacological actions of most antidepressants are ascribed to the modulation of serotonergic and/or noradrenergic transmission in the brain. During therapeutic treatment for major depression, fluoxetine, one of the most commonly prescribed selective serotonin reuptake inhibitor (SSRI) antidepressants, accumulates in the brain, suggesting that fluoxetine may interact with additional targets. In this context, there is increasing evidence that astrocytes are involved in the pathophysiology of major depression.

OBJECTIVES

The aim of this study was to examine the effects of fluoxetine on the expression of neurotrophic/growth factors that have antidepressant properties and on glucose metabolism in cultured cortical astrocytes.

RESULTS

Treatment of astrocytes with fluoxetine and paroxetine, another SSRI antidepressant, upregulated brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), and VGF mRNA expression. In contrast, the tricyclic antidepressants desipramine and imipramine did not affect the expression of these neurotrophic/growth factors. Analysis of the effects of fluoxetine on glucose metabolism revealed that fluoxetine reduces glycogen levels and increases glucose utilization and lactate release by astrocytes. Similar data were obtained with paroxetine, whereas imipramine and desipramine did not regulate glucose metabolism in this glial cell population. Our results also indicate that the effects of fluoxetine and paroxetine on glucose utilization, lactate release, and expression of BDNF, VEGF, and VGF are not mediated by serotonin-dependent mechanisms.

CONCLUSIONS

These data suggest that, by increasing the expression of specific astrocyte-derived neurotrophic factors and lactate release from astrocytes, fluoxetine may contribute to normalize the trophic and metabolic support to neurons in major depression.

Antidepressant drugs transactivate TrkB neurotrophin receptors in the adult rodent brain independently of BDNF and monoamine transporter blockade

Background

Antidepressant drugs (ADs) have been shown to activate BDNF (brain-derived neurotrophic factor) receptor TrkB in the rodent brain but the mechanism underlying this phenomenon remains unclear. ADs act as monoamine reuptake inhibitors and after prolonged treatments regulate brain bdnf mRNA levels indicating that monoamine-BDNF signaling regulate AD-induced TrkB activation in vivo. However, recent findings demonstrate that Trk receptors can be transactivated independently of their neurotrophin ligands.

Methodology

In this study we examined the role of BDNF, TrkB kinase activity and monoamine reuptake in the AD-induced TrkB activation in vivo and in vitro by employing several transgenic mouse models, cultured neurons and TrkB-expressing cell lines.

Principal Findings

Using a chemical-genetic TrkB^F616A mutant and TrkB overexpressing mice, we demonstrate that ADs specifically activate both the maturely and immaturely glycosylated forms of TrkB receptors in the brain in a TrkB kinase dependent manner. However, the tricyclic AD imipramine readily induced the phosphorylation of TrkB receptors in conditional bdnf^−/− knock-out mice (132.4±8.5% of control; P = 0.01), indicating that BDNF is not required for the TrkB activation. Moreover, using serotonin transporter (SERT) deficient mice and chemical lesions of monoaminergic neurons we show that neither a functional SERT nor monoamines are required for the TrkB phosphorylation response induced by the serotonin selective reuptake inhibitors fluoxetine or citalopram, or norepinephrine selective reuptake inhibitor reboxetine. However, neither ADs nor monoamine transmitters activated TrkB in cultured neurons or cell lines expressing TrkB receptors, arguing that ADs do not directly bind to TrkB.

Conclusions

The present findings suggest that ADs transactivate brain TrkB receptors independently of BDNF and monoamine reuptake blockade and emphasize the need of an intact tissue context for the ability of ADs to induce TrkB activity in brain.

A new aspect of the TrkB signaling pathway in neural plasticity

In the central nervous system (CNS), the expression of molecules is strictly regulated during development. Control of the spatiotemporal expression of molecules is a mechanism not only to construct the functional neuronal network but also to adjust the network in response to new information from outside of the individual, i.e., through learning and memory. Among the functional molecules in the CNS, one of the best-studied groups is the neurotrophins, which are nerve growth factor (NGF)-related gene family molecules. Neurotrophins include NGF, brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4/5 in the mammal. Among neurotrophins and their receptors, BDNF and tropomyosin-related kinases B (TrkB) are enriched in the CNS. In the CNS, the BDNF-TrkB signaling pathway fulfills a wide variety of functions throughout life, such as cell survival, migration, outgrowth of axons and dendrites, synaptogenesis, synaptic transmission, and remodeling of synapses. Although the same ligand and receptor, BDNF and TrkB, act in these various developmental events, we do not yet understand what kind of mechanism provokes the functional multiplicity of the BDNF-TrkB signaling pathway. In this review, we discuss the mechanism that elicits the variety of functions performed by the BDNF-TrkB signaling pathway in the CNS as a tool of pharmacological therapy.

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.

Phase advance of the light-dark cycle perturbs diurnal rhythms of brain-derived neurotrophic factor and neurotrophin-3 protein levels, which reduces synaptophysin-positive presynaptic terminals in the cortex of juvenile rats

In adult rat brains, brain-derived neurotrophic factor (BDNF) rhythmically oscillates according to the light-dark cycle and exhibits unique functions in particular brain regions. However, little is known of this subject in juvenile rats. Here, we examined diurnal variation in BDNF and neurotrophin-3 (NT-3) levels in 14-day-old rats. BDNF levels were high in the dark phase and low in the light phase in a majority of brain regions. In contrast, NT-3 levels demonstrated an inverse phase relationship that was limited to the cerebral neocortex, including the visual cortex, and was most prominent on postnatal day 14. An 8-h phase advance of the light-dark cycle and sleep deprivation induced an increase in BDNF levels and a decrease in NT-3 levels in the neocortex, and the former treatment reduced synaptophysin expression and the numbers of synaptophysin-positive presynaptic terminals in cortical layer IV and caused abnormal BDNF and NT-3 rhythms 1 week after treatment. A similar reduction of synaptophysin expression was observed in the cortices of Bdnf gene-deficient mice and Ca(2+)-dependent activator protein for secretion 2 gene-deficient mice with abnormal free-running rhythm and autistic-like phenotypes. In the latter mice, no diurnal variation in BDNF levels was observed. These results indicate that regular rhythms of BDNF and NT-3 are essential for correct cortical network formation in juvenile rodents.

Gene expression profiling of pituitary melanotrope cells during their physiological activation

The pituitary melanotrope cells of the amphibian Xenopus laevis are responsible for the production of the pigment-dispersing peptide α-melanophore-stimulating hormone, which allows the animal to adapt its skin color to its environment. During adaptation to a dark background the melanotrope cells undergo remarkable changes characterized by dramatic increases in cell size and secretory activity. In this study we performed microarray mRNA expression profiling to identify genes important to melanotrope activation and growth. We show a strong increase in the expression of the immediate early gene (IEG) c-Fos and of the brain-derived neurotrophic factor gene (BDNF). Furthermore, we demonstrate the involvement of another IEG in the adaptation process, Nur77, and conclude from in vitro experiments that the expression of both c-Fos and Nur77 are partially regulated by the adenylyl cyclase system and calcium ions. In addition, we found a steady up-regulation of Ras-like product during the adaptation process, possibly evoked by BDNF/TrkB signaling. Finally, the gene encoding the 105-kDa heat shock protein HSPh1 was transiently up-regulated in the course of black-background adaptation and a gene product homologous to ferritin (ferritin-like product) was >100-fold up-regulated in fully black-adapted animals. We suggest that these latter two genes are induced in response to cellular stress and that they may be involved in changing the mode of mRNA translation required to meet the increased demand for de novo protein synthesis. Together, our results show that microarray analysis is a valuable approach to identify the genes responsible for generating coordinated responses in physiologically activated cells.

Automated Sholl analysis of digitized neuronal morphology at multiple scales: Whole cell Sholl analysis versus Sholl analysis of arbor subregions

The morphology of dendrites and the axon determines how a neuron processes and transmits information. Neurite morphology is frequently analyzed by Sholl analysis or by counting the total number of neurites and branch tips. However, the time and resources required to perform such analysis by hand is prohibitive for the processing of large data sets and introduces problems with data auditing and reproducibility. Furthermore, analyses performed by hand or using course-grained morphometric data extraction tools can obscure subtle differences in data sets because they do not store the data in a form that facilitates the application of multiple analytical tools. To address these shortcomings, we have developed a program (titled "Bonfire") to facilitate digitization of neurite morphology and subsequent Sholl analysis. Our program builds upon other available open-source morphological analysis tools by performing Sholl analysis on subregions of the neuritic arbor, enabling the detection of local level changes in dendrite and axon branching behavior. To validate this new tool, we applied Bonfire analysis to images of hippocampal neurons treated with 25 ng/ml brain-derived neurotrophic factor (BDNF) and untreated control neurons. Consistent with prior findings, conventional Sholl analysis revealed that global exposure to BDNF increases the number of neuritic intersections proximal to the soma. Bonfire analysis additionally uncovers that BDNF treatment affects both root processes and terminal processes with no effect on intermediate neurites. Taken together, our data suggest that global exposure of hippocampal neurons to BDNF results in a reorganization of neuritic segments within their arbors, but not necessarily a change in their number or length. These findings were only made possible by the neurite-specific Sholl data returned by Bonfire analysis.

Post-treatment with amphetamine enhances reinnervation of the ipsilateral side cortex in stroke rats

Amphetamine (AM) treatment has been shown to alter behavioral recovery after ischemia caused by embolism, permanent unilateral occlusion of the common carotid and middle cerebral arteries, or unilateral sensorimotor cortex ablation in rats. However, the behavioral results are inconsistent possibly due to difficulty controlling the size of the lesion before treatment. There is also evidence that AM promotes neuroregeneration in the cortex contralateral to the infarction; however, the effects of AM in the ipsilateral cortex remain unclear. The purpose of this study was to employ T2-weighted imaging (T2WI) to establish controlled criteria for AM treatment and to examine neuroregenerative effects in both cortices after stroke. Adult rats were anesthetized, and the right middle cerebral artery was ligated for 90 min to generate lesions in the ipsilateral cortex. Animals were separated into two equal treatment groups (AM or saline) according to the size of infarction, measured by T2WI at 2days after stroke. AM or saline was administered to stroke rats every third day starting on day 3 for 4weeks. AM treatment significantly reduced neurological deficits, as measured by body asymmetry and Bederson's score. T2WI and diffusion tensor imaging (DTI) were used to examine the size of infarction and axonal reinnervation, respectively, before and following treatment on days 2, 10 and 25 after stroke. AM treatment reduced the volume of tissue loss on days 10 and 25. A significant increase in fractional anisotropy ratio was found in the ipsilateral cortex after repeated AM administration, suggesting a possible increase in axonal outgrowth in the lesioned side cortex. Western analysis indicated that AM significantly increased the expression of synaptophysin ipsilaterally and neurofilament bilaterally. AM also enhanced matrix metalloproteinase (MMP) enzymatic activity, determined by MMP zymography in the lesioned side cortex. qRT-PCR was used to examine the expression of trophic factors after the 1st and 2nd doses of AM or saline injection. The expression of BDNF, but not BMP7 or CART, was significantly enhanced by AM in the lesioned side cortex. In conclusion, post-stroke treatment with AM facilitates behavioral recovery, which is associated with an increase in fractional anisotropy activity, enhanced fiber growth in tractography, synaptogenesis, upregulation of BDNF, and MMP activity mainly in the lesioned cortex. Our data suggest that the ipsilateral cortex may be the major target of action in stroke brain after AM treatment.

The role of the Met66 brain-derived neurotrophic factor allele in the recovery of executive functioning after combat-related traumatic brain injury

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, promotes survival and synaptic plasticity in the human brain. The Val66Met polymorphism of the BDNF gene interferes with intracellular trafficking, packaging, and regulated secretion of this neurotrophin. The human prefrontal cortex (PFC) shows lifelong neuroplastic adaption implicating the Val66Met BDNF polymorphism in the recovery of higher-order executive functions after traumatic brain injury (TBI). In this study, we examined the effect of this BDNF polymorphism on the recovery of executive functioning after TBI. We genotyped a sample of male Vietnam combat veterans consisting of a frontal lobe lesion group with focal penetrating head injuries and a non-head-injured control group for the Val66Met BDNF polymorphism. The Delis-Kaplan Executive Function System as a standardized psychometric battery was administrated to examine key domains of executive functions. The results revealed that the Met allele but not the hypothesized Val allele promotes recovery of executive functioning. Overall, the Met66 carriers in the lesion group performed as well as the Met66 carriers in the control group. The Met66 allele accounted for 6.2% of variance for executive functioning independently of other significant predictors including preinjury intelligence, left hemisphere volume loss, and dorsolateral PFC volume loss. The findings point to different mechanisms of the Val66Met BDNF gene in complex phenotypes under normal and pathological conditions. A better understanding of these mechanisms could be instrumental in the development and application of effective therapeutic strategies to facilitate recovery from TBI.