In chemical synapses undergoing high frequency stimulation, vesicle components can be retrieved from the plasma membrane via a clathrin-independent process called activity-dependent bulk endocytosis (ADBE). Alix (ALG-2-interacting protein X/PDCD6IP) is an adaptor protein binding to ESCRT and endophilin-A proteins which is required for clathrin-independent endocytosis in fibroblasts. Alix is expressed in neurons and concentrates at synapses during epileptic seizures. Here, we used cultured neurons to show that Alix is recruited to presynapses where it interacts with and concentrates endophilin-A during conditions triggering ADBE. Using Alix knockout (ko) neurons, we showed that this recruitment, which requires interaction with the calcium-binding protein ALG-2, is necessary for ADBE. We also found that presynaptic compartments of Alix ko hippocampi display subtle morphological defects compatible with flawed synaptic activity and plasticity detected electrophysiologically. Furthermore, mice lacking Alix in the forebrain undergo less seizures during kainate-induced status epilepticus and reduced propagation of the epileptiform activity. These results thus show that impairment of ADBE due to the lack of neuronal Alix leads to abnormal synaptic recovery during physiological or pathological repeated stimulations.

In chemical synapses undergoing high frequency stimulation, vesicle components can be retrieved from the plasma membrane via a clathrin-independent process called activity dependent bulk endocytosis (ADBE). Alix (ALG-2 interacting protein X)/ PDCD6IP) is an adaptor protein binding to ESCRT and endophilin-A proteins and thereby driving deformation and fission of endosomal and cell surface membranes. In fibroblasts, Alix is required for clathrin-independent endocytosis. Here, using electron microscopy, we show that synapses from mice lacking Alix have subtle defects in presynaptic compartments, translating into flawed synaptic plasticity. Using cultured neurons, we demonstrate that Alix is required for ADBE. We further demonstrate that in order to perform ADBE, Alix must be recruited to synapses by the calcium-binding protein ALG-2 and interact with endophilin-A. Finally, we show that mutant mice lacking Alix in the forebrain undergo less seizures during kainate-induced status epilepticus. Furthermore, propagation of the epileptiform activity to the contralateral side of kainate injection is reduced. These results thus highlight Alix ko mice as an invaluable model to study the exact role of ADBE at synapses undergoing physiological or pathological stimulations.

In the central nervous system, neurons are organized in specific neural networks with distinct electrical patterns, input integration capacities, and intracellular dynamics. In order to better understand how neurons process information, it is crucial to keep the complex organization of brain circuits. However, performing subcellular investigations with high spatial and temporal resolution in vivo is technically challenging, especially in fine structures, such as axonal projections. Here, we present an on-a-chip system that combines a microfluidic platform with a dedicated matrix of electrodes to study activity-dependent dynamics in the physiological context of brain circuits. Because this system is compatible with high-resolution video-microscopy, it is possible to simultaneously record intracellular dynamics and electrical activity in presynaptic axonal projections and in their postsynaptic neuronal targets. Similarly, specific patterns of electrical activity can be applied to both compartments in order to investigate how intrinsic and network activities influence intracellular dynamics. The fluidic isolation of each compartment further allows the selective application of drugs at identified sites to study activity-dependent synaptic transmission. This integrated microfluidic/microelectrode array (microMEA) platform is a valuable tool for studying various intracellular and synaptic dynamics in response to neuronal activity in a physiologically relevant context that resembles in vivo brain circuits.

Studying intracellular dynamics in neurons is crucial to better understand how brain circuits communicate and adapt to environmental changes. In neurons, axonal secretory vesicles underlie various functions from growth during development to plasticity in the mature brain. Similarly, transport of mitochondria, the power plant of the cell, regulates both axonal development and synaptic homeostasis. However, because of their submicrometric size and rapid velocities, studying the kinetics of these organelles in projecting axons in vivo is technically challenging. In parallel, primary neuronal cultures are adapted to study axonal transport but they lack the physiological organization of neuronal networks, which in turn may bias observations. We previously developed a microfluidic platform to reconstruct a physiologically-relevant and functional corticostriatal network in vitro that is compatible with high-resolution videorecording of axonal trafficking. Here, using this system we report progressive changes in axonal transport kinetics of both dense core vesicles and mitochondria that correlate with network development and maturation. Interestingly, axonal flow of both types of organelles change in opposite directions, with rates increasing for vesicles and decreasing for mitochondria. Overall, our observations highlight the need for a better spatiotemporal control for the study of intracellular dynamics in order to avoid misinterpretations and improve reproducibility.

Event Abstract Back to Event Integration of Micro Electrode Array with compartimentalized microfluidics for the analysis of reconstructed neuronal junctions Benoit Charlot1*, Eve Moutaux2, Fabrice Bardin1, 3, Eugene Malyshev1, Frederic Saudou2, 4 and Maxime Cazorla2 1 UMR5214 Institut d'Electronique et des Systèmes (IES), France 2 INSERM U1216 Grenoble Institut des Neurosciences (GIN), France 3 University of Nîmes, France 4 Centre Hospitalier Universitaire de Grenoble, France Compartmentalized circuits are the most employed microfluidic devices in neuroscience. These circuits allow the in-vitro reconstitution of neuronal circuits using primary cultures of different neuronal population. These systems can be used to build thousands of specific neuronal junctions and to study intracellular dynamics using selective markers and live fluorescent reporters. In this work we added a dedicated Micro Electrode Array that fits the microfluidics design to record and control electrical activity of pre- and postsynaptic neurons while simultaneously monitoring intracellular dynamics. Because microfluidics are fluidically isolated, each compartment can be independently manipulated to determine their contribution to network functions. Microfluidic circuit are based on the design of Taylor et al. 2010 (3) for the reconstruction of cortico-cortical networks in which each compartment (presynaptic, synaptic and postsynaptic) is identified (space compartmentalization) and in which the progression from axonal growth to synapse regulation is controlled (time compartmentalization). The device shown in Figure 1(a,b,c) consists of two opposite neuronal chambers that communicate via an intermediate synaptic chamber through long microchannels on one side (to select axons only) and short ones on the other side (to allow dendrites to reach the synaptic chamber). This configuration allows the reconstruction of physiological axodendritic contacts between two cortical populations such as those found between cortical layers in vivo. In order to extend the functional analysis of reconstructed cortico-cortical networks, we developed a specific Micro Electrode Array (MEA) substrate that fits to the microfluidic geometry to stimulate and record pre- and post-synaptic neurons (Figure1(c)). Presynaptic electrodes were disposed at the entrance of axonal channels in order to stimulate the axon initiation segment that will generate physiological action potentials and trigger release of neurotransmitter in the synaptic chamber. Postsynaptic electrodes are located under the cell bodies to concomitantly stimulate or record electrical activity of target neurons. Combined with high-resolution fluorescence videomicroscopy and fast calcium imaging (Figure1(e)), this system allows monitoring intracellular dynamics in response to different patterns of neuronal activity. The MEA have been produced specifically to be included in microfluidic circuit. They are built in clean room with simple, efficient and robust microfabrication process that emphasis on repetability. The microfabrication process for building the MEA chips (Figure1(a)) are made of a two-step lithographic process on top of 4.9 x 4.9cm, 170µm thin glass substrates. The first step consists in the deposition of micro electrodes by a lift-off of 10/200nm Ti/Pt metal over an AZ 2020 patterned photoresist. Next, a 500nm thick Silicon Nitride (Si3N4) passivation layer is deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) process at 280°C. A second photolithographic step is done to define openings in the Si3N4 passivation layer on the microelectrodes and connexion pads. These openings are made by Reactive Ion etching with a mix of CHF3 and 02 plasma. This process has been stabilized in order to produces hundreds of devices with a fabrication yield of more than 80%. The microfluidic circuit is made of PDMS using a standard dual thickness SU8 on silicon process that is aligned on top of the MEA and bonded by oxygen plasma activation of the surface. Microelectrodes show electrical impedances of 235 and 165kΩ (for 50 and 30µm diameter) in PBS 1X medium. The MEA-microfluidic platform is connected to a 60-channels MEA workstation (MultiChannel Systems) and is then mounted on an inverted confocal spinning-disc videomicroscope (CSU-W1, Yokogawa). Rat primary neurons are cultured into their respective chambers and are infected with lentiviruses to express markers of intracellular dynamics such as the fast calcium indicator GCaMP6f. Spontaneous electrical activity and calcium dynamics in pre- and post-synaptic neurons can be followed throughout network maturation (Day 4 to 21). Specific stimulation protocols mimicking different patterns of neuronal activity can then be applied to study calcium dynamics in presynaptic axons and in postsynaptic dendrites. Synaptic transmission between the two populations can also be assessed by applying selective drugs in the synaptic compartment (Figure 2(d,e)). By combining space-time compartmentalization of neuronal populations, electrophysiological stimulation and recording, and high-resolution videomicroscopy, our integrated platform allows to decipher the cellular events that are involved in synaptic transmission and plasticity within neuronal networks. This device can be applied to virtually any type of neuronal circuits, in normal and pathological conditions. Figure 1 Acknowledgements This work is funded by grants from Agence Nationale pour la Recherche (ANR-12-PDOC-0004-01-BATMAN, M. C.), and Labex NUMEV (ANR-10-LABX-20, B.C.) References [1] J.M. Jang, J. Lee, H. Kim, N.L. Jeon, W. Jung, "One-photon and two-photon stimulation of neu-rons in a microfluidic culture system", Lab Chip 16, 1684-1690 (2016). [2] M. Peyrin et al., "Axon diodes for the reconstruction of oriented neuronal networks in microflu-idic chambers", Lab Chip 11, 3663-3673 (2011). [3] A.M. Taylor, D.C. Dieterich, H.T. Ito, S.A. Kim, E.M. Schuman,"Microfluidic local perfusion chambers for the visualization and manipulation of synapses", Neuron 66, 57-68 (2010). Keywords: Microfluidics, Micro electrode arrays (MEAs), calcium imaging, Electrophysiology, microfabrication Conference: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays, Reutlingen, Germany, 4 Jul - 6 Jul, 2018. Presentation Type: Oral Presentation Topic: Microelectrode Array Technology Citation: Charlot B, Moutaux E, Bardin F, Malyshev E, Saudou F and Cazorla M (2019). Integration of Micro Electrode Array with compartimentalized microfluidics for the analysis of reconstructed neuronal junctions. Conference Abstract: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays. doi: 10.3389/conf.fncel.2018.38.00039 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Mar 2018; Published Online: 17 Jan 2019. * Correspondence: Dr. Benoit Charlot, UMR5214 Institut d'Electronique et des Systèmes (IES), Montpellier, France, benoit.charlot@um2.fr Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Benoit Charlot Eve Moutaux Fabrice Bardin Eugene Malyshev Frederic Saudou Maxime Cazorla Google Benoit Charlot Eve Moutaux Fabrice Bardin Eugene Malyshev Frederic Saudou Maxime Cazorla Google Scholar Benoit Charlot Eve Moutaux Fabrice Bardin Eugene Malyshev Frederic Saudou Maxime Cazorla PubMed Benoit Charlot Eve Moutaux Fabrice Bardin Eugene Malyshev Frederic Saudou Maxime Cazorla Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Huntington's disease (HD), a devastating neurodegenerative disorder, strongly affects the corticostriatal network, but the contribution of pre- and postsynaptic neurons in the first phases of disease is unclear due to difficulties performing early subcellular investigations in vivo. Here, we have developed an on-a-chip approach to reconstitute an HD corticostriatal network in vitro, using microfluidic devices compatible with subcellular resolution. We observed major defects in the different compartments of the corticostriatal circuit, from presynaptic dynamics to synaptic structure and transmission and to postsynaptic traffic and signaling, that correlate with altered global synchrony of the network. Importantly, the genetic status of the presynaptic compartment was necessary and sufficient to alter or restore the circuit. This highlights an important weight for the presynaptic compartment in HD that has to be considered for future therapies. This disease-on-a-chip microfluidic platform is thus a physiologically relevant in vitro system for investigating pathogenic mechanisms and for identifying drugs.

Cocaine exposure alters brain-derived neurotrophic factor (BDNF) expression in the brain. BDNF signaling through TrkB receptors differentially modulates cocaine self-administration, depending on the brain regions involved. In the present study, we determined how brain-wide inhibition of TrkB signaling affects cocaine intake, the motivation for the drug, and reinstatement of drug taking after extinction. To overcome the inability of TrkB ligands to cross the blood-brain barrier, the TrkB antagonist cyclotraxin-B was fused to the nontoxic transduction domain of the tat protein from human immunodeficiency virus type 1 (tat-cyclotraxin-B). Intravenous injection of tat-cyclotraxin-B dose-dependently reduced cocaine intake, motivation for cocaine (as measured under a progressive ratio schedule of reinforcement), and reinstatement of cocaine taking in rats allowed either short or long access to cocaine self-administration. In contrast, the treatment did not affect operant responding for a highly palatable sweet solution, demonstrating that the effects of tat-cyclotraxin-B are specific for cocaine reinforcement. Cocaine self-administration increased TrkB signaling and activated the downstream Akt pathway in the nucleus accumbens, and had opposite effects in the prefrontal cortex. Pretreatment with tat-cyclotraxin-B normalized protein levels in these two dopamine-innervated brain regions. Cocaine self-administration also increased TrkB signaling in the ventral tegmental area, where the dopaminergic projections originate, but pretreatment with tat-cyclotraxin-B did not alter this effect. Altogether, our data show that systemic administration of a brain-penetrant TrkB antagonist leads to brain region-specific effects and may be a potential pharmacological strategy for the treatment of cocaine addiction.

Deficits of auditory scene analysis accompany Alzheimer's disease (AD). However, the functional neuroanatomy of spatial sound processing has not been defined in AD. We addressed this using a “sparse” fMRI virtual auditory spatial paradigm in 14 patients with typical AD in relation to 16 healthy age-matched individuals. Sound stimulus sequences discretely varied perceived spatial location and pitch of the sound source in a factorial design. AD was associated with loss of differentiated cortical profiles of auditory location and pitch processing at the prescribed threshold, and significant group differences were identified for processing auditory spatial variation in posterior cingulate cortex (controls > AD) and the interaction of pitch and spatial variation in posterior insula (AD > controls). These findings build on emerging evidence for altered brain mechanisms of auditory scene analysis and suggest complex dysfunction of network hubs governing the interface of internal milieu and external environment in AD. Auditory spatial processing may be a sensitive probe of this interface and contribute to characterization of brain network failure in AD and other neurodegenerative syndromes.

Most of the cellular or molecular studies in HD used so far separated cultures of striatal or cortical neurons. However, in the brain these neurons are connected and form a particular network that is defective in HD. The polarised nature of neurons and the size and density of synapses complicates the manipulation and visualisation of specific events taking place in axons or dendrites and of specific synaptic transmission within the cortico-striatal network. To overcome these limitations, we developed several microfluidic systems compatible with high-resolution videomicroscopy and connected to microelectrode arrays (MEA) to reconstitute and identify each component of the corticostriatal network. The microfluidic system directs the formation of identified synapses separately between cortical axons and striatal dendrites and soma. In parallel, a multielectrode substrate monitors and controls presynaptic and postsynaptic activity independently. Using this multicomplex system we are investigating how the trafficking of synaptic vesicles or mitochondria along axons is regulated by presynaptic and postsynaptic patterns in the corticostriatal network in health and HD. In addition, the system allows modifying the genetic status of the cortical or striatal neurons as a way to selectively investigate how disease neurons differentially affect pre or post-synaptic events in HD and overall alter synapse function.

Current therapies for treating movement disorders such as Parkinson's disease are effective but limited by undesirable and intractable side effects. Developing more effective therapies will require better understanding of what causes basal ganglia dysregulation and why medication-induced side effects develop. Although basal ganglia have been extensively studied in the last decades, its circuit anatomy is very complex, and significant controversy exists as to how the interplay of different basal ganglia nuclei process motor information and output. We have recently identified the importance of an underappreciated collateral projection that bridges the striatal output direct pathway with the indirect pathway. These bridging collaterals are extremely plastic in the adult brain and are involved in the regulation of motor balance. Our findings add a new angle to the classical model of basal ganglia circuitry that could be exploited for the development of new therapies against movement disorders. In this Scientific Perspective, we describe the function of bridging collaterals and other recent discoveries that challenge the simplicity of the classical basal ganglia circuit model. We then discuss the potential implication of bridging collaterals in the pathophysiology of Parkinson's disease and schizophrenia. Because dopamine D2 receptors and striatal neuron excitability have been found to regulate the density of bridging collaterals, we propose that targeting these projections downstream of D2 receptors could be a possible strategy for the treatment of basal ganglia disorders. © 2015 International Parkinson and Movement Disorder Society.

Structural plasticity in the adult brain is essential for adaptive behavior. We have found a remarkable anatomical plasticity in the basal ganglia of adult mice that is regulated by dopamine D2 receptors (D2Rs). By modulating neuronal excitability, striatal D2Rs bidirectionally control the density of direct pathway collaterals in the globus pallidus that bridge the direct pathway with the functionally opposing indirect pathway. An increase in bridging collaterals is associated with enhanced inhibition of pallidal neurons in vivo and disrupted locomotor activation after optogenetic stimulation of the direct pathway. Chronic blockade with haloperidol, an antipsychotic medication used to treat schizophrenia, decreases the extent of bridging collaterals and rescues the locomotor imbalance. These findings identify a role for bridging collaterals in regulating the concerted balance of striatal output and may have important implications for understanding schizophrenia, a disease involving excessive activation of striatal D2Rs that is treated with D2R blockers.

Several lines of evidence indicate that brain-derived neurotrophic factor (BDNF) plays a key role as a central pronociceptive modulator of pain, acting through postsynaptic TrkB receptors that trigger intracellular signaling cascades leading to central sensitization. The overall aim of this study was to investigate to what extent BDNF could participate in the generation and maintenance of trigeminal neuropathic pain. The results showed that acute intracisternal administration of nanogram doses of BDNF in naive mice elicited long-lasting, dose-related, cold allodynic responses to topical application of acetone onto vibrissal pad skin. The systemic administration of cyclotraxin-B (CTX-B), a new TrkB receptor antagonist, or propentofylline, an inhibitor of glial activation, was able to either prevent or reverse the effects of intracisternal BDNF on cold nociception. In addition, the blockade of TrkB receptor by CTX-B inhibited the mechanisms that either initiate or maintain cold allodynia in the ipsilateral vibrissal pad skin after unilateral constriction of the infraorbital nerve. These observations raise the possibility that BDNF is capable on its own of conveying many features of the signaling mechanisms that underlie central sensitization caused by nerve constriction.Perspective: Although further studies are necessary to examine in detail the mechanisms underlying the strong anti-allodynic action of CTX-B, this compound may represent an interesting lead for the development of novel therapeutic strategies aimed at preventing and/or suppressing central sensitization associated with neuropathic pain. (C) 2012 by the American Pain Society

Structural plasticity in the adult brain is essential for adaptive behaviors and is thought to contribute to a variety of neurological and psychiatric disorders. Medium spiny neurons of the striatum show a high degree of structural plasticity that is modulated by dopamine through unknown signaling mechanisms. Here, we demonstrate that overexpression of dopamine D2 receptors in medium spiny neurons increases their membrane excitability and decreases the complexity and length of their dendritic arbors. These changes can be reversed in the adult animal after restoring D2 receptors to wild-type levels, demonstrating a remarkable degree of structural plasticity in the adult striatum. Increased excitability and decreased dendritic arborization are associated with downregulation of inward rectifier potassium channels (Kir2.1/2.3). Downregulation of Kir2 function is critical for the neurophysiological and morphological changes in vivo because virally mediated expression of a dominant-negative Kir2 channel is sufficient to recapitulate the changes in D2 transgenic mice. These findings may have important implications for the understanding of basal ganglia disorders, and more specifically schizophrenia, in which excessive activation of striatal D2 receptors has long been hypothesized to be of pathophysiologic significance.

The interaction of brain-derived neurotrophic factor (BDNF) with its tropomyosin-related kinase receptor B (TrkB) is involved in fundamental cellular processes including neuronal proliferation, differentiation and survival as well as neurotransmitter release and synaptic plasticity. TrkB signaling has been widely associated with beneficial, trophic effects and many commonly used psychotropic drugs aim to increase BDNF levels in the brain. However, it is likely that a prolonged increased TrkB activation is observed in many pathological conditions, which may underlie the development and course of clinical symptoms. Interestingly, genetic and pharmacological studies aiming at decreasing TrkB activation in rodent models mimicking human pathology have demonstrated a promising therapeutic landscape for TrkB inhibitors in the treatment of various diseases, e.g. central nervous system (CNS) disorders and several types of cancer. Up to date, only a few selective and potent TrkB inhibitors have been developed. As such, the use of crystallography and in silico approaches to model BDNF-TrkB interaction and to generate relevant pharmacophores represent powerful tools to develop novel compounds targeting the TrkB receptor.

BACKGROUND AND PURPOSEBy interacting with trkB receptors, brain-derived neurotrophic factor (BDNF) triggers various signalling pathways responsible for neurone survival, differentiation and modulation of synaptic transmission. Numerous reports have implicated BDNF and trkB in the pathogenesis of various central nervous system affections and in cancer, thus representing trkB as a promising therapeutic target. In this study, we used an antibody-based approach to search for trkB-selective functional reagents.EXPERIMENTAL APPROACHSix commercially available polyclonal and monoclonal antibodies were tested on recombinant and native, human and rodent trkB receptors. Functional and pharmacological characterization was performed using a modified version of the KIRA-elisa method and radioligand binding studies. Western blot analyses and neurite outgrowth assays were carried out to determine the specificity and selectivity of antibody effects. The survival properties of one antibody were further assessed on cultured neurones in a serum-deprived paradigm.KEY RESULTSThe functional trkB-selective antibodies showed distinct pharmacological profiles, ranging from partial agonists to antagonists, acting on trkB receptors through allosteric modulations. The same diversity of effects was observed on the mitogen-activated protein kinase signalling pathway downstream of trkB and on the subsequent neurite outgrowth. One antibody with partial agonist activity demonstrated cell survival properties by activating the Akt pathway. Finally, these antibodies were functionally validated as true trkB-selective ligands because they failed activating trkA or trkC, and contrary to BDNF, none of them bind to p75NTR.CONCLUSIONS AND IMPLICATIONSThese trkB-selective antibodies represent a novel class of pharmacological tools to explore the pathophysiological roles of trkB and its potential therapeutic relevance for the treatment of various disorders.

The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) have emerged as key mediators in the pathophysiology of several mood disorders, including anxiety and depression. However, therapeutic compounds that interact with TrkB receptors have been difficult to develop. Using a combination of structure-based in silico screening and high-capacity functional assays in recombinant and neuronal cells, we identified a low-molecular weight TrkB ligand (ANA-12) that prevented activation of the receptor by BDNF with a high potency. ANA-12 showed direct and selective binding to TrkB and inhibited processes downstream of TrkB without altering TrkA and TrkC functions. KIRA-ELISA analysis demonstrated that systemic administration of ANA-12 to adult mice decreased TrkB activity in the brain without affecting neuronal survival. Mice administered ANA-12 demonstrated reduced anxiety- and depression-related behaviors on a variety of tests predictive of anxiolytic and antidepressant properties in humans. This study demonstrates that structure-based virtual screening strategy can be an efficient method for discovering potent TrkB-selective ligands that are active in vivo. We further propose that ANA-12 may be a valuable tool for studying BDNF/TrkB signaling and may constitute a lead compound for developing the next generation of therapeutic agents for the treatment of mood disorders.

Brain-derived neurotrophic factor (BDNF) is expressed in the mammalian pituitary gland, in both the anterior and intermediate lobes, where its functional significance is unknown. Melanotrope cells in the intermediate pituitary lobe of the amphibian Xenopus laevis also produce BDNF, which co-exists in secretory granules with α-melanophore-stimulating hormone (α-MSH), a peptide that causes pigment dispersion in dermal melanophores during adaptation of the toad to a dark background. Xenopus melanotropes are highly plastic, undergoing very strong growth to support the high biosynthesis and release of α-MSH in black-adapted animals. In this study we have tested our hypothesis that this enhanced growth of the melanotrope is maintained by autocrine release of BDNF. Furthermore, since the extracellular-regulated kinase (ERK) pathway is a major component of BDNF signaling in neuronal plasticity, we investigated its involvement in melanotrope cell growth. For these purposes melanotropes were treated for 3 days in vitro, with either an anti-BDNF serum or a recombinant tropomyosin-receptor kinase B (TrkB) receptor fragment to eliminate released BDNF, or with the ERK inhibitor U0126. We also applied a novel inhibitor of the TrkB receptor, cyclotraxin-B, to test this receptor's involvement in melanotrope cell growth regulation. All treatments markedly reduced melanotrope cell growth. Therefore, we conclude that autocrine release of BDNF and subsequent TrkB-dependent ERK-mediated signaling is important for melanotrope cell growth during its physiologically induced activation.

In the last decades, few mechanistically novel therapeutic agents have been developed to treat mental and neurodegenerative disorders. Numerous studies suggest that targeting BDNF and its TrkB receptor could be a promising therapeutic strategy for the treatment of brain disorders. However, the development of potent small ligands for the TrkB receptor has proven to be difficult. By using a peptidomimetic approach, we developed a highly potent and selective TrkB inhibitor, cyclotraxin-B, capable of altering TrkB-dependent molecular and physiological processes such as synaptic plasticity, neuronal differentiation and BDNF-induced neurotoxicity. Cyclotraxin-B allosterically alters the conformation of TrkB, which leads to the inhibition of both BDNF-dependent and -independent (basal) activities. Finally, systemic administration of cyclotraxin-B to mice results in TrkB inhibition in the brain with specific anxiolytic-like behavioral effects and no antidepressant-like activity. This study demonstrates that cyclotraxin-B might not only be a powerful tool to investigate the role of BDNF and TrkB in physiology and pathology, but also represents a lead compound for the development of new therapeutic strategies to treat brain disorders.