Activity-Dependent Activation of TrkB Neurotrophin Receptors in the Adult CNS

A TrkB and TrkC partial agonist restores deficits in synaptic function and promotes activity-dependent synaptic and microglial transcriptomic changes in a late-stage Alzheimer's mouse model

INTRODUCTION

Tropomyosin related kinase B (TrkB) and C (TrkC) receptor signaling promotes synaptic plasticity and interacts with pathways affected by amyloid beta (Aβ) toxicity. Upregulating TrkB/C signaling could reduce Alzheimer's disease (AD)-related degenerative signaling, memory loss, and synaptic dysfunction.

METHODS

PTX-BD10-2 (BD10-2), a small molecule TrkB/C receptor partial agonist, was orally administered to aged London/Swedish-APP mutant mice (APPL/S) and wild-type controls. Effects on memory and hippocampal long-term potentiation (LTP) were assessed using electrophysiology, behavioral studies, immunoblotting, immunofluorescence staining, and RNA sequencing.

RESULTS

In APPL/S mice, BD10-2 treatment improved memory and LTP deficits. This was accompanied by normalized phosphorylation of protein kinase B (Akt), calcium-calmodulin-dependent kinase II (CaMKII), and AMPA-type glutamate receptors containing the subunit GluA1; enhanced activity-dependent recruitment of synaptic proteins; and increased excitatory synapse number. BD10-2 also had potentially favorable effects on LTP-dependent complement pathway and synaptic gene transcription.

DISCUSSION

BD10-2 prevented APPL/S/Aβ-associated memory and LTP deficits, reduced abnormalities in synapse-related signaling and activity-dependent transcription of synaptic genes, and bolstered transcriptional changes associated with microglial immune response.

HIGHLIGHTS

Small molecule modulation of tropomyosin related kinase B (TrkB) and C (TrkC) restores long-term potentiation (LTP) and behavior in an Alzheimer's disease (AD) model. Modulation of TrkB and TrkC regulates synaptic activity-dependent transcription. TrkB and TrkC receptors are candidate targets for translational therapeutics. Electrophysiology combined with transcriptomics elucidates synaptic restoration. LTP identifies neuron and microglia AD-relevant human-mouse co-expression modules.

Localization of truncated TrkB and co-expression with full-length TrkB in the cerebral cortex of adult mice

Brain-derived neurotrophic factor (BDNF), one of the neurotrophins, and its specific receptor TrkB, are abundantly distributed in the central nervous system (CNS) and have a variety of biological effects, such as neural survival, neurite elongation, neural differentiation, and enhancing synaptic functions. Currently, there are two TrkB subtypes: full-length TrkB (TrkB-FL), which has a tyrosine kinase in the intracellular domain, and TrkB-T1, which is a tyrosine kinase-deficient form. While TrkB-FL is a typical tyrosine kinase receptor, TrkB-T1 is a main form expressed in the CNS of adult mammals, but its function is unknown. In this study, we performed fluorescent staining of the cerebral cortex of adult mice, by using TrkB-T1 antiserum and various antibodies of marker molecules for neurons and glial cells. We found that TrkB-T1 was expressed not only in neurons but also in astrocytes. In contrast, little expression of TrkB-T1 was found in oligodendrocytes and microglia. TrkB-T1 was expressed in almost all of the cells expressing TrkB-FL, indicating the direct interaction between TrkB subtypes. These findings suggest that a part of various functions of BDNF-TrkB signaling might be due to the interaction and cellular localization of TrkB subtypes in the cerebral cortex.

Synaptotagmin-4 induces anhedonic responses to chronic stress via BDNF signaling in the medial prefrontal cortex

Stressful circumstances are significant contributors to mental illnesses such as major depressive disorder. Anhedonia, defined as loss of the ability to enjoy pleasure in pleasurable situations, including rewarding activities or social contexts, is considered a key symptom of depression. Although stress-induced depression is associated with anhedonia in humans and animals, the underlying molecular mechanisms of anhedonic responses remain poorly understood. In this study, we demonstrated that synaptotagmin-4 (SYT4), which is involved in the release of neurotransmitters and neurotrophic factors, is implicated in chronic stress-induced anhedonia. Employing chronic unpredictable stress (CUS), we evaluated two subpopulations of mice, susceptible (SUS, anhedonic) and resilient (RES, nonanhedonic), based on sucrose preference, which was strongly correlated with social reward. The FosTRAP (targeted recombination in active populations) system and optogenetic approach revealed that neural activity in the medial prefrontal cortex (mPFC) was significantly associated with CUS-induced anhedonic behavioral phenotypes. By conducting weighted gene coexpression network analysis of RNA sequencing data from the mPFC of SUS and RES mice, we identified Syt4 as a hub gene in a gene network that was unique to anhedonia. We also confirmed that Syt4 overexpression in the mPFC was pro-susceptible, while Syt4 knockdown was pro-resilient; the pro-susceptible effects of SYT4 were mediated through a reduction in brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B (TrkB) signaling in the mPFC. These findings suggest that SYT4-BDNF interactions in the mPFC represent a crucial regulatory mechanism of anhedonic susceptibility to chronic stress.

A TrkB and TrkC partial agonist restores deficits in synaptic function and promotes activity-dependent synaptic and microglial transcriptomic changes in a late-stage Alzheimer's mouse model

Introduction

TrkB and TrkC receptor signaling promotes synaptic plasticity and interacts with pathways affected by amyloid-β (Aβ)-toxicity. Upregulating TrkB/C signaling could reduce Alzheimer's disease (AD)-related degenerative signaling, memory loss, and synaptic dysfunction.

Methods

PTX-BD10-2 (BD10-2), a small molecule TrkB/C receptor partial agonist, was orally administered to aged London/Swedish-APP mutant mice (APP L/S ) and wild-type controls (WT). Effects on memory and hippocampal long-term potentiation (LTP) were assessed using electrophysiology, behavioral studies, immunoblotting, immunofluorescence staining, and RNA-sequencing.

Results

Memory and LTP deficits in APP L/S mice were attenuated by treatment with BD10-2. BD10-2 prevented aberrant AKT, CaMKII, and GLUA1 phosphorylation, and enhanced activity-dependent recruitment of synaptic proteins. BD10-2 also had potentially favorable effects on LTP-dependent complement pathway and synaptic gene transcription.

Conclusions

BD10-2 prevented APP L/S /Aβ-associated memory and LTP deficits, reduced abnormalities in synapse-related signaling and activity-dependent transcription of synaptic genes, and bolstered transcriptional changes associated with microglial immune response.

Oxidative Stress and Natural Antioxidants: Back and Forth in the Neurological Mechanisms of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by the progressive degeneration of neuronal cells. With the increase in aged population, there is a prevalence of irreversible neurodegenerative changes, causing a significant mental, social, and economic burden globally. The factors contributing to AD are multidimensional, highly complex, and not completely understood. However, it is widely known that aging, neuroinflammation, and excessive production of reactive oxygen species (ROS), along with other free radicals, substantially contribute to oxidative stress and cell death, which are inextricably linked. While oxidative stress is undeniably important in AD, limiting free radicals and ROS levels is an intriguing and potential strategy for deferring the process of neurodegeneration and alleviating associated symptoms. Therapeutic compounds from natural sources have recently become increasingly accepted and have been effectively studied for AD treatment. These phytocompounds are widely available and a multitude of holistic therapeutic efficiencies for treating AD owing to their antioxidant, anti-inflammatory, and biological activities. Some of these compounds also function by stimulating cholinergic neurotransmission, facilitating the suppression of beta-site amyloid precursor protein-cleaving enzyme 1, α-synuclein, and monoamine oxidase proteins, and deterring the occurrence of AD. Additionally, various phenolic, flavonoid, and terpenoid phytocompounds have been extensively described as potential palliative agents for AD progression. Preclinical studies have shown their involvement in modulating the cellular redox balance and minimizing ROS formation, displaying them as antioxidant agents with neuroprotective abilities. This review emphasizes the mechanistic role of natural products in the treatment of AD and discusses the various pathological hypotheses proposed for AD.

BDNF as a Mediator of Antidepressant Response: Recent Advances and Lifestyle Interactions

Conventional antidepressants are widely employed in several psychiatric and neurologic disorders, yet the mechanisms underlying their delayed and partial therapeutic effects are only gradually being understood. This narrative review provides an up-to-date overview of the interplay between antidepressant treatment and Brain-Derived Neurotrophic Factor (BDNF) signaling. In addition, the impact of nutritional, environmental and physiological factors on BDNF and the antidepressant response is outlined. This review underlines the necessity to include information on lifestyle choices in testing and developing antidepressant treatments in the future.

The noradrenergic system is necessary for survival of vulnerable midbrain dopaminergic neurons: implications for development and Parkinson's disease

The cause of midbrain dopaminergic (mDA) neuron loss in sporadic Parkinson's disease (PD) is multifactorial, involving cell autonomous factors, cell-cell interactions, and the effects of environmental toxins. Early loss of neurons in the locus coeruleus (LC), the main source of ascending noradrenergic (NA) projections, is an important feature of PD and other neurodegenerative disorders. We hypothesized that NA afferents provide trophic support for vulnerable mDA neurons. We demonstrate that depriving mDA neurons of NA input increases postnatal apoptosis and decreases cell survival in young adult rodents, with relative sparing of calbindin-positive subpopulations known to be resistant to degeneration in PD. As a mechanism, we propose that the neurotrophin brain-derived neurotrophic factor (BDNF) modulates anterograde survival effects of LC inputs to mDA neurons. We demonstrate that the LC is rich in BDNF mRNA in postnatal and young adult brains. Early postnatal NA denervation reduces both BDNF protein and activation of TrkB receptors in the ventral midbrain. Furthermore, overexpression of BDNF in NA afferents in transgenic mice increases mDA neuronal survival. Finally, increasing NA activity in primary cultures of mDA neurons improves survival, an effect that is additive or synergistic in the presence of different concentrations of BDNF. Taken together, our results point to a novel mechanism whereby LC afferents couple BDNF effects and NA activity to provide anterograde trophic support for vulnerable mDA neurons. Early loss of NA activity and anterograde neurotrophin support may contribute to degeneration of vulnerable neurons in PD and other neurodegenerative disorders.

Norepinephrine upregulates the expression of tyrosine hydroxylase and protects dopaminegic neurons against 6-hydrodopamine toxicity

As a classic neurotransmitter in the brain, norepinephrine (NE) also is an important modulator to other neuronal systems. Using primary cultures from rat ventral mesencephalon (VM) and dopaminergic cell line MN9D, the present study examined the neuroprotective effects of NE and its effects on the expression of tyrosine hydroxylase (TH). The results showed that NE protected both VM cultures and MN9D cells against 6-hydroxydopamine-caused apoptosis, with possible involvement of adrenal receptors. In addition, treatment with NE upregulated TH protein levels in dose- and time-dependent manner. Further experiments to investigate the potential mechanisms underlying this NE-induced upregulation of TH demonstrated a marked increase in protein levels of the brain-derived neurotrophic factor (BDNF) and the phosphorylated extracellular signal-regulated protein kinase 1 and 2 (pERK1/2) in VM cultures treated with NE. In MN9D cells, a significantly increase of TH and pERK1/2 protein levels were observed after their transfection with BDNF cDNA or exposure to BDNF peptides. Treatment of VM cultures with K252a, an antagonist of the tropomyosin-related kinase B, blocked the upregulatory effects of NE on TH, BDNF and pERK1/2. Administration of MEK1 & MEK2 inhibitors also reversed NE-induced upregulation of TH and pERK1/2. Moreover, ChIP assay showed that treatment with NE or BDNF increased H4 acetylation in the TH promoter. These results suggest that the neuroprotection and modulation of NE on dopaminergic neurons are mediated via BDNF and MAPK/ERK pathways, as well as through epigenetic histone modification, which may have implications for the improvement of therapeutic strategies for Parkinson's disease.

Regulatory role of NGFs in neurocognitive functions

Nerve growth factors (NGFs), especially the prototype NGF and brain-derived neurotrophic factor (BDNF), have a diverse array of functions in the central nervous system through their peculiar set of receptors and intricate signaling. They are implicated not only in the development of the nervous system but also in regulation of neurocognitive functions like learning, memory, synaptic transmission, and plasticity. Evidence even suggests their role in continued neurogenesis and experience-dependent neural network remodeling in adult brain. They have also been associated extensively with brain disorders characterized by neurocognitive dysfunction. In the present article, we aimed to make an exhaustive review of literature to get a comprehensive view on the role of NGFs in neurocognitive functions in health and disease. Starting with historical perspective, distribution in adult brain, implied molecular mechanisms, and developmental basis, this article further provides a detailed account of NGFs’ role in specified neurocognitive functions. Furthermore, it discusses plausible NGF-based homeostatic and adaptation mechanisms operating in the pathogenesis of neurocognitive disorders and has presents a survey of such disorders. Finally, it elaborates on current evidence and future possibilities in therapeutic applications of NGFs with an emphasis on recent research updates in drug delivery mechanisms. Conclusive remarks of the article make a strong case for plausible role of NGFs in comprehensive regulation of the neurocognitive functions and pathogenesis of related disorders and advocate that future research should be directed to explore use of NGF-based mechanisms in the prevention of implicated diseases as well as to target these molecules pharmacologically.

Repeated social defeat and the rewarding effects of cocaine in adult and adolescent mice: dopamine transcription factors, proBDNF signaling pathways, and the TrkB receptor in the mesolimbic system

RATIONALE

Repeated social defeat (RSD) increases the rewarding effects of cocaine in adolescent and adult rodents.

OBJECTIVE

The aim of the present study was to compare the long-term effects of RSD on the conditioned rewarding effects of cocaine and levels of the transcription factors Pitx3 and Nurr1 in the ventral tegmental area (VTA), the dopamine transporter (DAT), the D2 dopamine receptor (D2DR) and precursor of brain-derived neurotrophic factor (proBDNF) signaling pathways, and the tropomyosin-related kinase B (TrkB) receptor in the nucleus accumbens (NAc) in adult and adolescent mice.

METHODS

Male adolescent and young adult OF1 mice were exposed to four episodes of social defeat and were conditioned 3 weeks later with 1 mg/kg of cocaine. In a second set of mice, the expressions of the abovementioned dopaminergic and proBDNF and TrkB receptor were measured in VTA and NAc, respectively.

RESULTS

Adolescent mice experienced social defeats less intensely than their adult counterparts and produced lower levels of corticosterone. However, both adult and adolescent defeated mice developed conditioned place preference for the compartment associated with this low dose of cocaine. Furthermore, only adolescent defeated mice displayed diminished levels of the transcription factors Pitx3 in the VTA, without changes in the expression of DAT and D2DR in the NAc. In addition, stressed adult mice showed a decreased expression of proBDNF and the TrkB receptor, while stressed adolescent mice exhibited increased expression of latter without changes in the former.

CONCLUSION

Our findings suggest that dopaminergic pathways and proBDNF signaling and TrkB receptors play different roles in social defeat-stressed mice exposed to cocaine.

Muscle Contraction Regulates BDNF/TrkB Signaling to Modulate Synaptic Function through Presynaptic cPKCα and cPKCβI

The neurotrophin brain-derived neurotrophic factor (BDNF) acts via tropomyosin-related kinase B receptor (TrkB) to regulate synapse maintenance and function in the neuromuscular system. The potentiation of acetylcholine (ACh) release by BDNF requires TrkB phosphorylation and Protein Kinase C (PKC) activation. BDNF is secreted in an activity-dependent manner but it is not known if pre- and/or postsynaptic activities enhance BDNF expression in vivo at the neuromuscular junction (NMJ). Here, we investigated whether nerve and muscle cell activities regulate presynaptic conventional PKC (cPKCα and βI) via BDNF/TrkB signaling to modulate synaptic strength at the NMJ. To differentiate the effects of presynaptic activity from that of muscle contraction, we stimulated the phrenic nerve of rat diaphragms (1 Hz, 30 min) with or without contraction (abolished by μ-conotoxin GIIIB). Then, we performed ELISA, Western blotting, qRT-PCR, immunofluorescence and electrophysiological techniques. We found that nerve-induced muscle contraction: (1) increases the levels of mature BDNF protein without affecting pro-BDNF protein or BDNF mRNA levels; (2) downregulates TrkB.T1 without affecting TrkB.FL or p75 neurotrophin receptor (p75) levels; (3) increases presynaptic cPKCα and cPKCβI protein level through TrkB signaling; and (4) enhances phosphorylation of cPKCα and cPKCβI. Furthermore, we demonstrate that cPKCβI, which is exclusively located in the motor nerve terminals, increases activity-induced acetylcholine release. Together, these results show that nerve-induced muscle contraction is a key regulator of BDNF/TrkB signaling pathway, retrogradely activating presynaptic cPKC isoforms (in particular cPKCβI) to modulate synaptic function. These results indicate that a decrease in neuromuscular activity, as occurs in several neuromuscular disorders, could affect the BDNF/TrkB/PKC pathway that links pre- and postsynaptic activity to maintain neuromuscular function.

Clivorine, an otonecine pyrrolizidine alkaloid from Ligularia species, impairs neuronal differentiation via NGF-induced signaling pathway in cultured PC12 cells

BACKGROUND

Pyrrolizidine alkaloids (PAs) are commonly found in many plants including those used in medical therapeutics. The hepatotoxicities of PAs have been demonstrated both in vivo and in vitro; however, the neurotoxicities of PAs are rarely mentioned.

PURPOSE

In this study, we aimed to investigate in vitro neurotoxicities of clivorine, one of the PAs found in various Ligularia species, in cultured PC12 cells.

STUDY DESIGN

PC12 cell line was employed to first elucidate the neurotoxicity and the underlying mechanism of clivorine, including cell viability and morphology change, neuronal differentiation marker and signaling pathway.

METHODS

PC12 cells were challenged with series concentrations of clivorine and/or nerve growth factor (NGF). The cell lysates were collected for MTT assay, trypan blue staining, immunocytofluorescent staining, qRT-PCR and western blotting.

RESULTS

Clivorine inhibited cell proliferation and neuronal differentiation evidenced by MTT assay and dose-dependently reducing neurite outgrowth, respectively. In addition, clivorine decreased the level of mRNAs encoding for neuronal differentiation markers, e.g. neurofilaments and TrkA (NGF receptor). Furthermore, clivorine reduced the NGF-induced the phosphorylations of TrkA, protein kinase B and cAMP response element-binding protein in cultured PC12 cells.

CONCLUSION

Taken together, our results suggest that clivorine might possess neurotoxicities in PC12 cells via down-regulating the NGF/TrkA/Akt signaling pathway. PAs not only damage the liver, but also possess neurotoxicities, which could possibly result in brain disorders, such as depression.

BDNF isoforms: a round trip ticket between neurogenesis and serotonin?

The brain-derived neurotrophic factor, BDNF, was discovered more than 30 years ago and, like other members of the neurotrophin family, this neuropeptide is synthetized as a proneurotrophin, the pro-BDNF, which is further cleaved to yield mature BDNF. The myriad of actions of these two BDNF isoforms in the central nervous system is constantly increasing and requires the development of sophisticated tools and animal models to refine our understanding. This review is focused on BDNF isoforms, their participation in the process of neurogenesis taking place in the hippocampus of adult mammals, and the modulation of their expression by serotonergic agents. Interestingly, around this triumvirate of BDNF, serotonin, and neurogenesis, a series of recent research has emerged with apparently counterintuitive results. This calls for an exhaustive analysis of the data published so far and encourages thorough work in the quest for new hypotheses in the field. BDNF is synthetized as a pre-proneurotrophin. After removal of the pre-region, proBDNF can be cleaved by intracellular or extracellular proteases. Mature BDNF can bind TrkB receptors, promoting their homodimerization and intracellular phosphorylation. Phosphorylated-TrkB can activate three different signaling pathways. Whereas G-protein-coupled receptors can transactivate TrkB receptors, truncated forms can inhibit mBDNF signaling. Pro-BDNF binds p75(NTR) by its mature domain, whereas the pro-region binds co-receptors.

α-Linolenic Acid, A Nutraceutical with Pleiotropic Properties That Targets Endogenous Neuroprotective Pathways to Protect against Organophosphate Nerve Agent-Induced Neuropathology

α-Linolenic acid (ALA) is a nutraceutical found in vegetable products such as flax and walnuts. The pleiotropic properties of ALA target endogenous neuroprotective and neurorestorative pathways in brain and involve the transcription factor nuclear factor kappa B (NF-κB), brain-derived neurotrophic factor (BDNF), a major neuroprotective protein in brain, and downstream signaling pathways likely mediated via activation of TrkB, the cognate receptor of BDNF. In this review, we discuss possible mechanisms of ALA efficacy against the highly toxic OP nerve agent soman. Organophosphate (OP) nerve agents are highly toxic chemical warfare agents and a threat to military and civilian populations. Once considered only for battlefield use, these agents are now used by terrorists to inflict mass casualties. OP nerve agents inhibit the critical enzyme acetylcholinesterase (AChE) that rapidly leads to a cholinergic crisis involving multiple organs. Status epilepticus results from the excessive accumulation of synaptic acetylcholine which in turn leads to the overactivation of muscarinic receptors; prolonged seizures cause the neuropathology and long-term consequences in survivors. Current countermeasures mitigate symptoms and signs as well as reduce brain damage, but must be given within minutes after exposure to OP nerve agents supporting interest in newer and more effective therapies. The pleiotropic properties of ALA result in a coordinated molecular and cellular program to restore neuronal networks and improve cognitive function in soman-exposed animals. Collectively, ALA should be brought to the clinic to treat the long-term consequences of nerve agents in survivors. ALA may be an effective therapy for other acute and chronic neurodegenerative disorders.

Methamphetamine differentially affects BDNF and cell death factors in anatomically defined regions of the hippocampus

Methamphetamine exposure reduces hippocampal long-term potentiation (LTP) and neurogenesis and these alterations partially contribute to hippocampal maladaptive plasticity. The potential mechanisms underlying methamphetamine-induced maladaptive plasticity were identified in the present study. Expression of brain-derived neurotrophic factor (BDNF; a regulator of LTP and neurogenesis), and its receptor tropomyosin-related kinase B (TrkB) were studied in the dorsal and ventral hippocampal tissue lysates in rats that intravenously self-administered methamphetamine in a limited access (1h/day) or extended access (6h/day) paradigm for 17days post baseline sessions. Extended access methamphetamine enhanced expression of BDNF with significant effects observed in the dorsal and ventral hippocampus. Methamphetamine-induced enhancements in BDNF expression were not associated with TrkB receptor activation as indicated by phospho (p)-TrkB-706 levels. Conversely, methamphetamine produced hypophosphorylation of N-methyl-d-aspartate (NMDA) receptor subunit 2B (GluN2B) at Tyr-1472 in the ventral hippocampus, indicating reduced receptor activation. In addition, methamphetamine enhanced expression of anti-apoptotic protein Bcl-2 and reduced pro-apoptotic protein Bax levels in the ventral hippocampus, suggesting a mechanism for reducing cell death. Analysis of Akt, a pro-survival kinase that suppresses apoptotic pathways and pAkt at Ser-473 demonstrated that extended access methamphetamine reduces Akt expression in the ventral hippocampus. These data reveal that alterations in Bcl-2 and Bax levels by methamphetamine were not associated with enhanced Akt expression. Given that hippocampal function and neurogenesis vary in a subregion-specific fashion, where dorsal hippocampus regulates spatial processing and has higher levels of neurogenesis, whereas ventral hippocampus regulates anxiety-related behaviors, these data suggest that methamphetamine self-administration initiates distinct allostatic changes in hippocampal subregions that may contribute to the altered synaptic activity in the hippocampus, which may underlie enhanced negative affective symptoms and perpetuation of the addiction cycle.

Neurotrophins and psychiatric disorders

Increasing number of studies has during the last decade linked neurotrophic factors with the pathophysiology of neuropsychiatric disorders and with the mechanisms of action of drugs used for the treatment of these disorders. In particular, brain-derived neurotrophic factor BDNF and its receptor TrkB have been connected with the pathophysiology in mood disorders, and there is strong evidence that BDNF signaling is critically involved in the recovery from depression with both pharmacological and psychological means. Neurotrophins play a central role in neuronal plasticity and network connectivity in developing adult brain, and recent evidence links plasticity and network rewiring with mood disorders and their treatment. Therefore, neurotrophins should not be seen as happiness factors but as critical tools in the process where brain networks are optimally tuned to environment, and it is against this background that the effects of neurotrophins on neuropsychiatric disorders should be looked at.

Brain-derived neurotrophic factor promotes adaptive plasticity within the spinal cord and mediates the beneficial effects of controllable stimulation

Brain-derived neurotrophic factor (BDNF) has been characterized as a potent modulator of neural plasticity in both the brain and spinal cord. The present experiments use an in vivo model system to demonstrate that training with controllable stimulation increases spinal BDNF expression and engages a BDNF-dependent process that promotes adaptive plasticity. Spinally transected rats administered legshock whenever one hind limb is extended (controllable stimulation) exhibit a progressive increase in flexion duration. This simple form of response-outcome (instrumental) learning is not observed when shock is given independent of leg position (uncontrollable stimulation). Uncontrollable electrical stimulation also induces a lasting effect that impairs learning for up to 48 h. Training with controllable shock can counter the adverse consequences of uncontrollable stimulation, to both prevent and reverse the learning deficit. Here it is shown that the protective and restorative effect of instrumental training depends on BDNF. Cellular assays showed that controllable stimulation increased BDNF mRNA expression and protein within the lumbar spinal cord. These changes were associated with an increase in the BDNF receptor TrkB protein within the dorsal horn. Evidence is then presented that these changes play a functional role in vivo. Application of a BDNF inhibitor (TrkB-IgG) blocked the protective effect of instrumental training. Direct (intrathecal) application of BDNF substituted for instrumental training to block both the induction and expression of the learning deficit. Uncontrollable stimulation also induced an increase in mechanical reactivity (allodynia), and this too was prevented by BDNF. TrkB-IgG blocked the restorative effect of instrumental training and intrathecal BDNF substituted for training to reverse the deficit. Taken together, these findings outline a critical role for BDNF in mediating the beneficial effects of controllable stimulation on spinal plasticity.

The antidepressant-like effects of glutamatergic drugs ketamine and AMPA receptor potentiator LY 451646 are preserved in bdnf⁺/⁻ heterozygous null mice

Accumulating evidence suggests that biogenic amine-based antidepressants act, at least in part, via regulation of brain-derived neurotrophic factor (BDNF) signaling. Biogenic amine-based antidepressants increase BDNF synthesis and activate its signaling pathway through TrkB receptors. Moreover, the antidepressant-like effects of these molecules are abolished in BDNF deficient mice. Glutamate-based drugs, including the NMDA antagonist ketamine, and the AMPA receptor potentiator LY 451646, mimic the effects of antidepressants in preclinical tests with high predictive validity. In humans, a single intravenous dose of ketamine produces an antidepressant effect that is rapid, robust and persistent. In this study, we examined the role of BDNF in expression of the antidepressant-like effects of ketamine and an AMPA receptor potentiator (LY 451646) in the forced swim test (FST). Ketamine and LY 451646 produced antidepressant-like effects in the FST in mice at 45 min after a single injection, but no effects were observed one week after a single ketamine injection. As previously reported, the effects of imipramine in the forced swim test were blunted in heterozygous BDNF knockout (bdnf(+/-)) mice. However ketamine and LY 451646 produced similar antidepressant-like responses in wildtype and bdnf(+/-) mice. Neither ketamine nor LY 451646 significantly influenced the levels BDNF or TrkB phosphorylation in the hippocampus when assessed at 45 min or 7 days after the drug administration. These data demonstrate that under the conditions tested, neither ketamine nor the AMPA-potentiator LY 451656 activate BDNF signaling, but produce a characteristic antidepressant-like response in heterozygous bdnf(+/-) mice. These data indicate that unlike biogenic amine-based agents, BDNF signaling does not play a pivotal role in the antidepressant effects of glutamate-based compounds. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Exogenous gangliosides increase the release of brain-derived neurotrophic factor

Gangliosides are lipophilic compounds found in cell plasma membranes throughout the brain that play a role in neuronal plasticity and regeneration. Indeed, absence or abnormal accumulation of gangliosides has been shown to lead to neurological disorders. Experimental data have shown that exogenous gangliosides exhibit properties similar to the neurotrophins, a family of neurotrophic factors that are important in the survival and maintenance of neurons and prevention of neurological diseases. Brain-derived neurotrophic factor (BDNF) is the most abundant of the neurotrophins. This work was done to reveal the neurotrophic mechanism of exogenous gangliosides. In particular, we examined whether gangliosides promote the release of BDNF. Rat hippocampal neurons or human neuroblastoma cells were transduced with a recombinant adenovirus expressing BDNF-flag to facilitate detection of BDNF. Release of BDNF was then determined by Western blot analysis and a two-site immunoassay of culture medium. The depolarizing agent KCl was used as a comparison. In hippocampal neurons, both GM1 ganglioside and KCl evoked within minutes the release of mature BDNF. In human cells, GM1 and other gangliosides released both mature BDNF and pro-BDNF. The effect of gangliosides was structure-dependent. In fact, GT1b preferentially released mature BDNF whereas GM1 released both mature and pro-BDNF. Ceramide and sphingosine did not modify the release of BDNF. This work provides additional experimental evidence that exogenous gangliosides can be used to enhance the neurotrophic factor environment and promote neuronal survival in neurological diseases. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Neurotrophins and cortical development

The mammalian cerebral cortex requires the proper formation of exquisitely precise circuits to function correctly. These neuronal circuits are assembled during development by the formation of synaptic connections between hundreds of thousands of differentiating neurons. Although the development of the cerebral cortex has been well described anatomically, the cellular and molecular mechanisms that guide neuronal differentiation and formation of connections are just beginning to be understood. Moreover, despite evidence that coordinated patterns of activity underlie reorganization of brain circuits during critical periods of development, the molecular signals that translate activity into structural and functional changes in connections remain unknown. Recently, the neurotrophins have emerged as attractive candidates not only for regulating neuronal differentiation in the developing brain, but also for mediating activity-dependent synaptic plasticity. The neurotrophins meet many of the criteria required for molecular signals involved in neuronal differentiation and plasticity. They are present in the cerebral cortex during development and their expression is regulated by synaptic activity. In turn, the neurotrophins themselves strongly influence both short-term synaptic plasticity and long-term potentiation and depression. In addition to their functional effects, the neurotrophins also profoundly regulate the structural changes that underlie axonal and dendritic differentiation. Finally, the neurotrophins have been implicated in mediating synaptic competition required for activity-dependent plasticity during the critical period. This chapter presents and discusses the rapidly accumulating evidence that the neurotrophins are critical for neuronal differentiation and that they may be involved in activity-dependent synaptic refinement in the developing cerebral cortex.

Expression of TrkB subtypes in the adult monkey cerebellar cortex

BDNF and its specific receptor TrkB are concerned with synaptic plasticity as well as maintenance of the nervous system. TrkB has three subtypes: full-length TrkB (TK+), which has a tyrosine kinase containing intracellular domain, and two truncated TrkBs (TK-; T1 and T2), which lack tyrosine kinases. To understand the molecular interaction among these subtypes, we investigated the expression and distribution of BDNF, TK+, and T1 in the adult monkey cerebellum by single and double immunohistochemistry and Western blot analysis. We observed by single immunohistochemistry that BDNF, TK+, and T1 are distributed in almost all the somata and dendrites of Purkinje and granule cells. In the double-stained sections, three kinds of regions were observed: TK+ >T1; TK+ =T1; TK+ <T1. Moreover, three types of TrkB dimers (TK+/TK+ homodimer, TK+/TK- heterodimer, and TK-/TK- homodimer) were induced by stimulating with exogenous BDNF. These observations suggest that the functions of BDNF may be modified by interaction among subtypes of TrkB in each region of the Purkinje cells.

BDNF and activity-dependent synaptic modulation

It is widely accepted that neuronal activity plays a pivotal role in synaptic plasticity. Neurotrophins have emerged recently as potent factors for synaptic modulation. The relationship between the activity and neurotrophic regulation of synapse development and plasticity, however, remains unclear. A prevailing hypothesis is that activity-dependent synaptic modulation is mediated by neurotrophins. An important but unresolved issue is how diffusible molecules such as neurotrophins achieve local and synapse-specific modulation. In this review, I discuss several potential mechanisms with which neuronal activity could control the synapse-specificity of neurotrophin regulation, with particular emphasis on BDNF. Data accumulated in recent years suggest that neuronal activity regulates the transcription of BDNF gene, the transport of BDNF mRNA and protein into dendrites, and the secretion of BDNF protein. There is also evidence for activity-dependent regulation of the trafficking of the BDNF receptor, TrkB, including its cell surface expression and ligand-induced endocytosis. Further study of these mechanisms will help us better understand how neurotrophins could mediate activity-dependent plasticity in a local and synapse-specific manner.

The developing synapse: construction and modulation of synaptic structures and circuits

Synapse formation and stabilization in the vertebrate central nervous system is a dynamic process, requiring bi-directional communication between pre- and postsynaptic partners. Numerous mechanisms coordinate where and when synapses are made in the developing brain. This review discusses cellular and activity-dependent mechanisms that control the development of synaptic connectivity.

Activity-dependent transfer of brain-derived neurotrophic factor to postsynaptic neurons

Neurotrophins such as brain-derived neurotrophic factor (BDNF) are thought to be transferred from post- to presynaptic neurons and to be involved in the formation and plasticity of neural circuits. However, direct evidence for a transneuronal transfer of BDNF and its relation to neuronal activity remains elusive. We simultaneously injected complementary DNAs of green fluorescent protein (GFP)-tagged BDNF and red fluorescence protein into the nucleus of single neurons and visualized expression, localization, and transport of BDNF in living neurons. Fluorescent puncta representing BDNF moved in axons in the anterograde direction, though some moved retrogradely, and transferred to postsynaptic neurons in an activity-dependent manner.