Detection of brain-derived neurotrophic factor in a vesicular fraction of brain synaptosomes

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.

Untranslated regions of brain-derived neurotrophic factor mRNA control its translatability and subcellular localization

Brain-derived neurotrophic factor (BDNF) promotes neuronal survival and growth during development. In the adult nervous system, BDNF is important for synaptic function in several biological processes such as memory formation and food intake. In addition, BDNF has been implicated in development and maintenance of the cardiovascular system. The Bdnf gene comprises several alternative untranslated 5' exons and two variants of 3' UTRs. The effects of these entire alternative UTRs on translatability have not been established. Using reporter and translating ribosome affinity purification analyses, we show that prevalent Bdnf 5' UTRs, but not 3' UTRs, exert a repressive effect on translation. However, contrary to previous reports, we do not detect a significant effect of neuronal activity on BDNF translation. In vivo analysis via knock-in conditional replacement of Bdnf 3' UTR by bovine growth hormone 3' UTR reveals that Bdnf 3' UTR is required for efficient Bdnf mRNA and BDNF protein production in the brain, but acts in an inhibitory manner in lung and heart. Finally, we show that Bdnf mRNA is enriched in rat brain synaptoneurosomes, with higher enrichment detected for exon I-containing transcripts. In conclusion, these results uncover two novel aspects in understanding the function of Bdnf UTRs. First, the long Bdnf 3' UTR does not repress BDNF expression in the brain. Second, exon I-derived 5' UTR has a distinct role in subcellular targeting of Bdnf mRNA.

Impaired Phasic Discharge of Locus Coeruleus Neurons Based on Persistent High Tonic Discharge-A New Hypothesis With Potential Implications for Neurodegenerative Diseases

The locus coeruleus (LC) is a small brainstem nucleus with widely distributed noradrenergic projections to the whole brain, and loss of LC neurons is a prominent feature of age-related neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). This article discusses the hypothesis that in early stages of neurodegenerative diseases, the discharge mode of LC neurons could be changed to a persistent high tonic discharge, which in turn might impair phasic discharge. Since phasic discharge of LC neurons is required for the release of high amounts of norepinephrine (NE) in the brain to promote anti-inflammatory and neuroprotective effects, persistent high tonic discharge of LC neurons could be a key factor in the progression of neurodegenerative diseases. Transcutaneous vagal stimulation (t-VNS), a non-invasive technique that potentially increases phasic discharge of LC neurons, could therefore provide a non-pharmacological treatment approach in specific disease stages. This article focuses on LC vulnerability in neurodegenerative diseases, discusses the hypothesis that a persistent high tonic discharge of LC neurons might affect neurodegenerative processes, and finally reflects on t-VNS as a potentially useful clinical tool in specific stages of AD and PD.

Role of Region-Specific Brain Decellularized Extracellular Matrix on In Vitro Neuronal Maturation

Recent advancements in tissue engineering suggest that biomaterials, such as decellularized extracellular matrix (ECM), could serve to potentiate the localization and efficacy of regenerative therapies in the central nervous system. Still, what factors and which mechanisms are required from these ECM-based biomaterials to exert their effect are not entirely understood. In this study, we use the brain as a novel model to test the effects of particular biochemical and structural properties by evaluating, for the first time, three different sections of the brain (i.e., cortex, cerebellum, and remaining areas) side-by-side and their corresponding decellularized counterparts using mechanical (4-day) and chemical (1-day) decellularization protocols. The three different brain subregions had considerably different initial conditions in terms of cell number and growth factor content, and some of these differences were maintained after decellularization. Decellularized ECM from both protocols was used as a substrate or as soluble factor, in both cases showing good cell attachment and growth capabilities. Interestingly, the 1-day protocol was capable of promoting greater differentiation than the 4-day protocol, probably due to its capacity to remove a similar amount of cell nuclei, while better conserving the biochemical and structural components of the cerebral ECM. Still, some limitations of this study include the need to evaluate the response in other biologically relevant cell types, as well as a more detailed characterization of the components in the decellularized ECM of the different brain subregions. In conclusion, our results show differences in neuronal maturation depending on the region of the brain used to produce the scaffolds. Complex organs such as the brain have subregions with very different initial cellular and biochemical conditions that should be considered for decellularization to minimize exposure to immunogenic components, while retaining bioactive factors conducive to regeneration. [Figure: see text] Impact statement The present study offers new knowledge about the production of decellularized extracellular matrix scaffolds from specific regions of the porcine brain, with a direct comparison of their effect on in vitro neuronal maturation. Our results show differences in neuronal maturation depending on the region of the brain used to produce the scaffolds, suggesting that it is necessary to consider the initial cellular content of the source tissue and its bioactive capacity for the production of an effective regenerative therapy for stroke.

Brain-derived Neurotrophic Factor and TrkB Levels in Mice that Lack Vesicular Zinc: Effects of Age and Sex

In certain neurons, zinc ions are stored in synaptic vesicles by zinc transporter 3 (ZnT3). Vesicular zinc can then be released synaptically to modulate myriad targets. In vitro evidence indicates that these targets may include brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB). But the effects of vesicular zinc on BDNF and TrkB in the intact brain are unclear. Studies of mice that lack ZnT3 - and, as a result, vesicular zinc - have shown abnormalities in BDNF and TrkB levels, but results have been mixed and are therefore difficult to interpret. This might be caused by differences in the age or sex of mice tested. In the present study, we measured BDNF and TrkB levels in the hippocampus and neocortex, comparing wild type and ZnT3 knockout mice of both sexes at two ages (5 and 12 weeks). We also examined BDNF mRNA expression and protein levels at an intermediate age (8-10 weeks). We found that, regardless of age or sex, BDNF and TrkB protein levels did not differ between wild type and ZnT3 knockout mice. There were sex-specific differences in BDNF protein and mRNA expression, however. BDNF protein levels increased with age in female mice but not in males. And in females, but not males, ZnT3 KO mice exhibited great hippocampal BDNF mRNA expression than wild type mice. We conclude that, at least in naïve mice housed under standard laboratory conditions, elimination of vesicular zinc does not affect BDNF or TrkB protein levels.

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.

Loss of Adaptive Myelination Contributes to Methotrexate Chemotherapy-Related Cognitive Impairment

Activity-dependent myelination is thought to contribute to adaptive neurological function. However, the mechanisms by which activity regulates myelination and the extent to which myelin plasticity contributes to non-motor cognitive functions remain incompletely understood. Using a mouse model of chemotherapy-related cognitive impairment (CRCI), we recently demonstrated that methotrexate (MTX) chemotherapy induces complex glial dysfunction for which microglial activation is central. Here, we demonstrate that remote MTX exposure blocks activity-regulated myelination. MTX decreases cortical Bdnf expression, which is restored by microglial depletion. Bdnf-TrkB signaling is a required component of activity-dependent myelination. Oligodendrocyte precursor cell (OPC)-specific TrkB deletion in chemotherapy-naive mice results in impaired cognitive behavioral performance. A small-molecule TrkB agonist rescues both myelination and cognitive impairment after MTX chemotherapy. This rescue after MTX depends on intact TrkB expression in OPCs. Taken together, these findings demonstrate a molecular mechanism required for adaptive myelination that is aberrant in CRCI due to microglial activation.

Effects of Sulfide Flavors on AHL-Mediated Quorum Sensing and Biofilm Formation of Hafnia alvei

In this study, 10 different sulfide flavor compounds commonly used as food additives were screened for antiquorum-sensing activity. Among these, diallyl disulfide (DADS) and methyl 2-methyl-3-furyl disulfide (MMFDS) were found to exert the strongest inhibition against violacein production in Chromobacterium violaceum 026, the tested biosensor strain. DADS and MMFDS also inhibited the growth of Hafnia alvei H4, yielding MIC values of 48 and 41.6 mM, respectively. In addition, DADS and MMFDS also inhibited the ability of H. alvei H4 to produce acyl-homoserine lactone as demonstrated by the reduced level of C6-HSL in the supernatant of DADS-treated culture. At concentrations corresponding to 1/4 MIC, DADS, and MMFDS inhibited the swarming ability of H. alvei H4 by 73.50% and 76.43%, respectively, while having virtually no effect on cell growth. The same concentrations of DADS and MMFDS also completely inhibited the formation of biofilm. These antiquorum sensing effects of DADS and MMFDS involved changes in the expression of the quorum-sensing genes luxI and luxR. Quantitative RT-PCR analysis showed that the mRNA levels of both genes were significantly reduced by DADS and MMDFS at concentrations below their MICs. However, further test using a mutant strain of H. alvei lacking luxR (ΔluxR) revealed significant reduction in luxI mRNA level upon treatment of the strain with DADS or MMDFS, but no change in luxR mRNA level occurred when a luxI-lacking mutant (ΔluxI) was treated with these compounds. The result therefore suggested that the antiquorum-sensing effect of DADS and MMFDS against H. alvei H4 might operate mainly through the inhibition of luxI expression in the cells.

PRACTICAL APPLICATION

The sulfide flavors compounds used in this paper are commonly used in food processing in China and are listed in the national standard of Chinese food additives GB2760-2014. The application of sulfide flavors in food processing can enhance aroma and prevent food spoilage.

Impaired TrkB Signaling Underlies Reduced BDNF-Mediated Trophic Support of Striatal Neurons in the R6/2 Mouse Model of Huntington's Disease

The principal projection neurons of the striatum are critically dependent on an afferent supply of brain derived neurotrophic factor (BDNF) for neurotrophic support. These neurons express TrkB, the cognate receptor for BDNF, which activates signaling pathways associated with neuronal survival and phenotypic maintenance. Impairment of the BDNF-TrkB pathway is suspected to underlie the early dysfunction and prominent degeneration of striatal neurons in Huntington disease (HD). Some studies in HD models indicate that BDNF supply is reduced, while others suggest that TrkB signaling is impaired earlier in disease progression. It remains important to determine whether a primary defect in TrkB signaling underlies reduced neurotrophic support and the early vulnerability of striatal neurons in HD. Using the transgenic R6/2 mouse model of HD we found that prior to striatal degeneration there are early deficits in striatal protein levels of activated phospho-TrkB and the downstream-regulated protein DARPP-32. In contrast, total-TrkB and BDNF protein levels remained normal. Primary neurons cultured from R6/2 striatum exhibited reduced survival in response to exogenous BDNF applications. Moreover, BDNF activation of phospho-TrkB and downstream signal transduction was attenuated in R6/2 striatal cultures. These results suggest that neurotrophic support of striatal neurons is attenuated early in disease progression due to defects in TrkB signal transduction in the R6/2 model of HD.

Fibrinogen α-chain-derived peptide is upregulated in hippocampus of rats exposed to acute morphine injection and spontaneous alternation testing

Fibrinogen is a secreted glycoprotein that is synthesized in the liver, although recent in situ hybridization data support its expression in the brain. It is involved in blood clotting and is released in the brain upon injury. Here, we report changes in the extracellular levels of fibrinogen α-chain-derived peptides in the brain after injections of saline and morphine. More specifically, in order to assess hippocampus-related working memory, an approach pairing in vivo microdialysis with mass spectrometry was used to characterize extracellular peptide release from the hippocampus of rats in response to saline or morphine injection coupled with a spontaneous alternation task. Two fibrinopeptide A-related peptides derived from the fibrinogen α-chain – fibrinopeptide A (ADTGTTSEFIEAGGDIR) and a fibrinopeptide A-derived peptide (DTGTTSEFIEAGGDIR) – were shown to be consistently elevated in the hippocampal microdialysate. Fibrinopeptide A was significantly upregulated in rats exposed to morphine and spontaneous alternation testing compared with rats exposed to saline and spontaneous alternation testing (P < 0.001), morphine alone (P < 0.01), or saline alone (P < 0.01), respectively. The increase in fibrinopeptide A in rats subjected to morphine and a memory task suggests that a complex interaction between fibrinogen and morphine takes place in the hippocampus.

Pre- and postsynaptic twists in BDNF secretion and action in synaptic plasticity

Overwhelming evidence collected since the early 1990's strongly supports the notion that BDNF is among the key regulators of synaptic plasticity in many areas of the mammalian central nervous system. Still, due to the extremely low expression levels of endogenous BDNF in most brain areas, surprisingly little data i) pinpointing pre- and postsynaptic release sites, ii) unraveling the time course of release, and iii) elucidating the physiological levels of synaptic activity driving this secretion are available. Likewise, our knowledge regarding pre- and postsynaptic effects of endogenous BDNF at the single cell level in mediating long-term potentiation still is sparse. Thus, our review will discuss the data currently available regarding synaptic BDNF secretion in response to physiologically relevant levels of activity, and will discuss how endogenously secreted BDNF affects synaptic plasticity, giving a special focus on spike timing-dependent types of LTP and on mossy fiber LTP. We will attempt to open up perspectives how the remaining challenging questions regarding synaptic BDNF release and action might be addressed by future experiments. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Failure of axonal transport induces a spatially coincident increase in astrocyte BDNF prior to synapse loss in a central target

Failure of anterograde transport to distal targets in the brain is a common feature of neurodegenerative diseases. We have demonstrated in rodent models of glaucoma, the most common optic neuropathy, early loss of anterograde transport along the retinal ganglion cell (RGC) projection to the superior colliculus (SC) is retinotopic and followed by a period of persistence of RGC axon terminals and synapses through unknown molecular pathways. Here we use the DBA/2J mouse model of hereditary glaucoma and an acute rat model to demonstrate that retinotopically focal transport deficits in the SC are accompanied by a spatially coincident increase in brain-derived neurotrophic factor (BDNF), especially in hypertrophic astrocytes. These neurochemical changes occur prior to loss of RGC synapses in the DBA/2J SC. In contrast to BDNF protein, levels of Bdnf mRNA decreased with transport failure, even as mRNA encoding synaptic structures remained unchanged. In situ hybridization signal for Bdnf mRNA was the strongest in SC neurons, and labeling for the immature precursor pro-BDNF was very limited. Subcellular fractionation of SC indicated that membrane-bound BDNF decreased with age in the DBA/2J, while BDNF released from vesicles remained high. These results suggest that in response to diminished axonal function, activated astrocytes in the brain may sequester mature BDNF released from target neurons to counter stressors that otherwise would challenge survival of projection synapses.

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

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

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.

Brain-derived neurotrophic factor and epilepsy--a missing link?

It has been known for some time that brain-derived neurotrophic factor (BDNF) is critical to normal development of the CNS, and more recently, studies also have documented the ability of BDNF to modify adult CNS structure and function. Therefore, it is no surprise that BDNF has been linked to diseases, such as epilepsy, which may involve abnormal cortical development or altered brain structure and function after maturity. This review evaluates the evidence, particularly from recent studies, that BDNF contributes to the development of temporal lobe epilepsy (TLE).

Neuronal plasticity: a link between stress and mood disorders

Although stress represents the major environmental element of susceptibility for mood disorders, the relationship between stress and disease remains to be fully established. In the present article we review the evidence in support for a role of neuronal plasticity, and in particular of neurotrophic factors. Even though decreased levels of norepinephrine and serotonin may underlie depressive symptoms, compelling evidence now suggests that mood disorders are characterized by reduced neuronal plasticity, which can be brought about by exposure to stress at different stages of life. Indeed the expression of neurotrophic molecules, such as the neurotrophin BDNF, is reduced in depressed subjects as well as in experimental animals exposed to adverse experience at early stages of life or at adulthood. These changes show an anatomical specificity and might be sustained by epigenetic mechanisms. Pharmacological intervention may normalize such defects and improve neuronal function through the modulation of the same factors that are defective in depression. Several studies have demonstrated that chronic, but not acute, antidepressant treatment increases the expression of BDNF and may enhance its localization at synaptic level. Antidepressant treatment can normalize deficits in neurotrophin expression produced by chronic stress paradigms, but may also alter the modulation of BDNF under acute stressful conditions. In summary, there is good agreement in considering neuronal plasticity, and the expression of key proteins such as the neurotrophin BDNF, as a central player for the effects of stress on brain function and its implication for psychopathology. Accordingly, effective treatments should not limit their effects to the control of neurotransmitter and hormonal dysfunctions, but should be able to normalize defective mechanisms that sustain the impairment of neuronal plasticity.

Does elevated intraocular pressure reduce retinal TRKB-mediated survival signaling in experimental glaucoma?

Reduced retrograde transport of neurotrophins (NT) and their receptors has been hypothesized to contribute directly to retinal ganglion cell (RGC) loss in glaucoma. However, strategies of supplementing NT and NT receptors have failed to avert ultimate RGC death in experimental glaucoma. This study examines the response of major components of the NT system and their interacting proteins in a rat glaucoma model. Unilateral chronic intraocular pressure (IOP) elevation was produced by episcleral vein injection of hypertonic saline (N = 99). Retinas were collected and grouped by extent of optic nerve injury. Quantitative reverse transcription PCR, western blot analysis and immunohistochemistry were used to determine mRNA and protein levels and protein localization. Out of three RGC-specific Brn3 proteins (Brn3a, b, and c), only Brn3a was significantly downregulated at the message level to 35 +/- 4% of fellow values with the severest nerve injury. With IOP elevation, no significant alterations were found in retinal mRNA levels for BDNF, NGF, NT-4/5 or NT-3. The abundance of mature retinal BDNF protein was not significantly affected by elevated IOP, while proBDNF protein decreased linearly with increasing injury grade (r(2) = 0.50). In retinas with the severest nerve injury, TrkB and TrkC receptor mRNA levels significantly declined to 67 +/- 9% and 44 +/- 5% of fellow values, respectively. However, the levels of TRKB protein and its phosphorylated form were unchanged. Message level for p75(NTR) was linearly upregulated up to 219 +/- 26% with increasing injury (r(2) = 0.46), but no alteration was detected at protein level. The mRNA expression of p75(NTR) apoptosis adaptor proteins NADE, NRIF, and Lingo1 were significantly downregulated in retinas with the greatest nerve injury. A positive correlation was found between injury extent and message levels for Jun (r(2) = 0.23) as well as Junb (r(2) = 0.27), and RGC labeling of activated JUN protein increased. Atf3 mRNA levels demonstrated a positive linear correlation to the extent of injury (r(2) = 0.53), resulting in a nearly five-fold increase (482 +/- 76%) in eyes with the greatest nerve damage. Among downstream pro-survival signaling components, Erk5 mRNA expression was linearly upregulated (r(2) = 0.32) up to 157 +/- 15% of fellow values in retinas with the severest nerve injury (p < 0.01). A slight positive correlation was found between NF-kappaB message levels and injury extent (r(2) = 0.12). Bcl-xl mRNA levels in the most severely injured retinas were significantly reduced to 83 +/- 7% by elevated IOP exposure. Message levels for Erk1/2, Akt1-3 or Bcl2 appeared unaffected. Elevated IOP did not alter mRNA levels of pro-apoptotic Bim, Bax, or p53. This study demonstrates that elevated IOP exposure does not result in a dramatic decrease in retinal levels of either BDNF or its receptor, TrkB. It shows that the responses of NT pathways to elevated IOP are complex, particularly with regard to the role of p75(NTR) and Atf3. A better understanding of the roles of these proteins in IOP-induced injury is likely to suggest informed strategies for neuroprotection in glaucoma.

Ampakines cause sustained increases in brain-derived neurotrophic factor signaling at excitatory synapses without changes in AMPA receptor subunit expression

Recent demonstrations that positive modulators of AMPA-type glutamate receptors (ampakines) increase neuronal brain-derived neurotrophic factor (BDNF) expression have suggested a novel strategy for treating neurodegenerative diseases. However, reports that AMPA and BDNF receptors are down-regulated by prolonged activation raise concerns about the extent to which activity-induced increases in BDNF levels can be sustained without compromising glutamate receptor function. The present study constitutes an initial test of whether ampakines can cause enduring increases in BDNF content and signaling without affecting AMPA receptor (AMPAR) expression. Prolonged (12-24 h) treatment with the ampakine CX614 reduced AMPAR subunit (glutamate receptor subunit (GluR) 1-3) mRNA and protein levels in cultured rat hippocampal slices whereas treatment with AMPAR antagonists had the opposite effects. The cholinergic agonist carbachol also depressed GluR1-3 mRNA levels, suggesting that AMPAR down-regulation is a global response to extended periods of elevated neuronal activity. Analyses of time courses and thresholds indicated that BDNF expression is influenced by lower doses of, and shorter treatments with, the ampakine than is AMPAR expression. Accordingly, daily 3 h infusions of CX614 chronically elevated BDNF content with no effect on GluR1-3 concentrations. Restorative deconvolution microscopy provided the first evidence that chronic up-regulation of BDNF is accompanied by increased activation of the neurotrophin's TrkB-Fc receptor at spine synapses. These results show that changes in BDNF and AMPAR expression are dissociable and that up-regulation of the former leads to enhanced trophic signaling at excitatory synapses. These findings are encouraging with regard to the feasibility of using ampakines to tonically enhance BDNF-dependent functions in adult brain.

Amphetamine-induced locomotion and gene expression are altered in BDNF heterozygous mice

Administration of amphetamine overstimulates medium spiny neurons (MSNs) by releasing dopamine and glutamate from afferents in the striatum. However, these afferents also release brain-derived neurotrophic factor (BDNF) that protects striatal MSNs from overstimulation. Intriguingly, all three neurochemicals increase opioid gene expression in MSNs. In contrast, striatal opioid expression is less in naive BDNF heterozygous (BDNF(+/-)) vs. wild-type (WT) mice. This study was designed to determine whether partial genetic depletion of BDNF influences the behavioral and molecular response to an acute amphetamine injection. An acute injection of amphetamine [5 mg/kg, intraperitoneal (i.p.)] or saline was administered to WT and BDNF(+/-) mice. WT and BDNF(+/-) mice exhibited similar locomotor activity during habituation, whereas BDNF(+/-) mice exhibited more prolonged locomotor activation during the third hour after injection of amphetamine. Three hours after amphetamine injection, there was an increase of preprodynorphin mRNA in the caudate putamen and nucleus accumbens (Acb) and dopamine D(3) receptor mRNA levels were increased in the Acb of BDNF(+/-) and WT mice. Striatal/cortical trkB and BDNF, and mesencephalic tyrosine hydroxylase mRNA levels were only increased in WT mice. These results indicate that BDNF modifies the locomotor responses of mice to acute amphetamine and differentially regulates amphetamine-induced gene expression.

BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory?

It is generally believed that late-phase long-term potentiation (L-LTP) and long-term memory (LTM) require new protein synthesis. Although the full complement of proteins mediating the long-lasting changes in synaptic efficacy have yet to be identified, several lines of evidence point to a crucial role for activity-induced brain-derived neurotrophic factor (BDNF) expression in generating sustained structural and functional changes at hippocampal synapses thought to underlie some forms of LTM. In particular, BDNF is sufficient to induce the transformation of early to late-phase LTP in the presence of protein synthesis inhibitors, and inhibition of BDNF signaling impairs LTM. Despite solid evidence for a critical role of BDNF in L-LTP and LTM, many issues are not resolved. Given that BDNF needs to be processed in Golgi outposts localized at the branch point of one or few dendrites, a conceptually challenging problem is how locally synthesized BDNF in dendrites could ensure synapse-specific modulation of L-LTP. An interesting alternative is that BDNF-TrkB signaling is involved in synaptic tagging, a prominent hypothesis that explains how soma-derived protein could selectively modulate the tetanized (tagged) synapse. Finally, specific roles of BDNF in the acquisition, retention or extinction of LTM remain to be established.

Locomotor exercise alters expression of pro-brain-derived neurotrophic factor, brain-derived neurotrophic factor and its receptor TrkB in the spinal cord of adult rats

Previous evidence indicates that locomotor exercise is a powerful means of increasing brain-derived neurotrophic factor (BDNF) and its signal transduction receptor TrkB mRNA levels, immunolabeling intensity and number of BDNF- and TrkB-immunopositive cells in the spinal cord of adult rats but the contribution of specific cell types to changes resulting from long-term activity is unknown. As changes in BDNF protein distribution due to systemic stimuli may reflect either its in-situ synthesis or its translocation from other sources, we investigated where BDNF and TrkB mRNA are expressed in the spinal lumbar segments. We report on the cell types defined by size, BDNF mRNA levels and number of cells with TrkB transcripts in sedentary and exercised animals following 28 days of treadmill walking. In the majority of cells, exercise increased perikaryonal levels of BDNF mRNA but did not affect TrkB transcript levels. Bidirectional changes in a number of TrkB mRNA-expressing cells occurred in small groups of ventral horn neurons. An increase in BDNF transcripts was translated into changes in pro-BDNF and BDNF levels. A 7-day walking regimen increased BDNF protein levels similarly to 28-day treadmill walking. Our observations indicate that long- and short-term locomotor activity of moderate intensity produce stimuli sufficient to recruit a majority of spinal cells to increased BDNF synthesis, suggesting that continuous tuning of pro-BDNF and BDNF levels permits spinal networks to undergo trophic modulation not requiring changes in TrkB mRNA supply.

Costorage of BDNF and neuropeptides within individual dense-core vesicles in central and peripheral neurons

Some central and peripheral neurons synthesize brain-derived neurotrophic factor (BDNF), and, after anterograde transport, release it at synapses. By immunocytochemistry, we examined, in rat and mouse, the subcellular localization of BDNF and BDNF/peptide coexistence, under normal conditions or after intrathecal infusion of nerve growth factor. In dorsal root ganglion neurons and afferent terminals, and in the parabrachial projection to amygdala, we show that BDNF is costored in individual dense-core vesicles (DCVs) with the neuropeptides calcitonin gene-related peptide (CGRP) and substance P. At both locations, nerve endings costoring all three peptides were fairly rare. Remarkably however, costorage occurred in a stoichiometric ratio of 0.7 BDNF:1 CGRP:1 substance P, and DCVs contained 31 (spinal cord) -36 (amygdala) times the amount of BDNF detected in agranular vesicles. This is the first direct demonstration in peripheral and central neurons from two different mammals, that a growth factor is selectively packaged together with neuropeptide transmitters within individual DCVs. It provides structural bases for differential release upon stimulation, and has important implications for understanding BDNF transmitter function.

Truncated TrkB-T1 regulates the morphology of neocortical layer I astrocytes in adult rat brain slices

By altering their morphology, astrocytes, including those involved in the maintenance and plasticity of neurons and in clearance of transmitter, play important roles in synaptic transmission; however, the mechanism that regulates the morphological plasticity of astrocytes remains unclear. Recently, we reported that T1, a subtype of TrkB (a family of BDNF-specific receptors), altered astrocytic morphology through the control of Rho GTPases in primary astrocyte cultures. In this study, we extended this observation to investigate acute neocortical slices from adult rats. T1 siRNA-expression vectors were electroporated into astrocytes in neocortical layer I of living rats. In both normal slices and control vector-electroporated slices, BDNF induced the elongation of the astrocytic processes and increased the branching of processes in slices after 1 h incubation. In contrast, in T1 siRNA-electroporated slices, no such significant morphological changes were observed in the astrocytes. In addition, the number of synaptophysin+ sites in contact with GFAP+ processes increased in a BDNF-T1-dependent manner without the increase in the total synaptophysin+ sites. Therefore, the present study provides evidence of the regulation of layer I astrocytic morphology by the BDNF-T1 signal in adult rat neocortical slices.

Neurotrophins in the dentate gyrus

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

Dense core vesicles resemble active-zone transport vesicles and are diminished following synaptogenesis in mature hippocampal slices

Large dense core vesicles (approximately 100 nm) contain neuroactive peptides and other co-transmitters. Smaller dense core vesicles (approximately 80 nm) are known to contain components of the presynaptic active zone and thought to transport and deliver these components during developmental synaptogenesis. It is not known whether excitatory axons in area CA1 contain such dense core vesicles, and whether they contribute to synaptic plasticity of mature hippocampus. Serial section electron microscopy was used to identify dense core vesicles in presynaptic axons in s. radiatum of area CA1 in adult rat hippocampus. Comparisons were made among perfusion-fixed hippocampus and hippocampal slices that undergo synaptogenesis during recovery in vitro. Dense core vesicles occurred in 26.1+/-3.6% of axonal boutons in perfusion fixed hippocampus, and in only 17.6+/-4.5% of axonal boutons in hippocampal slices (P<0.01). Most of the dense core vesicle positive boutons contained only one dense core vesicle, and no reconstructed axonal bouton had more than a total of 10 dense core vesicles in either condition. Overall the dense core vesicles had average diameters of 79+/-11 nm. These small dense core vesicles were usually located near nonsynaptic membranes and rarely occurred near the edge of a presynaptic active zone. Their size, low frequency, locations, and decrease following recuperative synaptogenesis in slices are novel findings that merit further study with respect to small dense core vesicle content and possible contributions to synapse assembly and plasticity in the mature hippocampus.

BDNF heterozygous mice demonstrate age-related changes in striatal and nigral gene expression

TrkB receptors mediate the effects of BDNF on striatal medium spiny neurons and mesencephalic dopamine neurons. The effect of partial BDNF gene deletion on locomotor activity and the gene expression of these neurons was evaluated at 3, 12, and 24 months of age in BDNF heterozygous (BDNF(LacZ/neo+)) and wildtype mice. BDNF(LacZ/neo+) mice displayed less spontaneous horizontal activity than wildtypes at 3 and 24 months of age. Whereas striatal preproenkephalin and preprodynorphin mRNA and mesencephalic tyrosine hydroxylase mRNA levels were significantly lower at all ages in BDNF(LacZ/neo+) mice, GAD67 mRNA was only lower at 24 months. In contrast, BDNF(LacZ/neo+) mice expressed more trkB mRNA in the striatum at 3 months and less at 24 months of age than wildtypes. Total striatal cell number in the two genotypes was not different at 12 months of age, whereas Golgi staining revealed that the spine density on distal dendrites of medium spiny neurons was less in BDNF(LacZ/neo+) mice than in wildtypes at 24 months of age. These data indicate that endogenous BDNF is required to maintain the normal phenotype and functioning of striatal projection neurons and mesencephalic dopamine neurons and that exaggerated dysfunction of these neurons and a concomitant decline in locomotor behavior occurs during aging.

Age-dependent interactive changes in serotonergic and noradrenergic cortical axon terminals in F344 rats

In the frontal cortex of aging rats, we found an increase in sprouting of the noradrenergic (NA) axons originated from the locus coeruleus (LC). The serotonergic (5-HT) axons originating from the dorsal raphe (DR) share the same cortical area and their age-dependent changes and interactions with NA axons were still unclear. To compare quantitatively the extent of axonal sprouting of DR and LC neurons in the frontal cortex, we extracellularly recorded from both DR and LC neurons in the same animals and antidromically stimulated 32 cortical sites (a pair of stimulating electrodes was moved at 100-mum intervals from 500 to 2000 microm in depth). In addition, to examine the effects of degeneration of 5-HT axons on NA axons, and vice versa, we used specific neurotoxins for 5-HT (PCA) or NA (DSP-4) axons. We also used noradrenaline uptake inhibitor (maprotiline) to verify the effects of NA on degeneration of 5-HT axons. Results suggested that 5-HT axons sprouted between 15 and 17 months of age and noradrenaline accelerated the age-dependent change of 5-HT axons.

Receptor tyrosine kinase B-mediated excitatory synaptogenesis

The receptor tyrosine kinase B, TrkB, is the high-affinity receptor for brain-derived neurotrophic factor (BDNF). Much evidence supports a role for TrkB signaling in excitatory synapse formation. There have been a number of recent advances in understanding the cell biology of TrkB-mediated excitatory synaptogenesis. The predominant mechanism by which TrkB supports excitatory synaptogenesis appears to be due to cell-autonomous signaling in both pre- and postsynaptic cells. This signaling appears to contribute to the growth and stabilization processes necessary for the net formation of synapses during development. Further, the molecular mechanisms by which TrkB contributes to these growth and stabilization processes are beginning to be elucidated.

The serotonergic and noradrenergic effects of antidepressant drugs are anticonvulsant, not proconvulsant

Contrary to existing evidence, convulsant liability of the antidepressants has been attributed to noradrenergic and serotonergic increments. This is a classic case of confusing treatment effects with the manifestations of illness. In fact, the remarkable anticonvulsant effectiveness of antidepressant-induced noradrenergic and serotonergic activation has been ignored. Some antidepressant drugs such as the specific serotonin reuptake inhibitor (SSRI) fluoxetine may be devoid of convulsant liability entirely, while having distinct anticonvulsant properties. Some authorities advance the notion that the seizure predisposition of patients with epilepsy increases risks for antidepressant-induced seizures. However, evidence does not support this contention. Instead, data increasingly support the concept that noradrenergic and serotonergic deficiencies contribute to seizure predisposition. Indeed, the antidepressants have the potential to overcome seizure predisposition in epilepsy. Whereas therapeutic doses of antidepressants elevate noradrenergic and serotonergic transmission, larger doses can activate other biological processes that may be convulsant.

Presence of neurotrophic factors in skeletal muscle correlates with survival of spinal cord motor neurons

To determine which combination of skeletal muscle-derived neurotrophic factors may be important for the survival of specific subpopulations of developing spinal cord motor neurons, we used Myf5 and MyoD (myogenic regulatory factors) knockouts, containing differentially committed myogenic precursor cells (MPCc) and immunohistochemistry against several muscle-secreted neurotrophic factors. At the peak of motor neuron cell death, skeletal muscle development is delayed in the back and body wall muscles of Myf5-/- embryos and in the limb muscles of MyoD-/- embryos. We hypothesized that, if the skeletal muscle was indeed an important source of survival factors for motor neurons, the back, the abdominal wall, and the forelimb MPCs of Myf5-/- or MyoD-/- embryos should produce at least some neurotrophic factors necessary for the survival of motor neurons. In this report, we demonstrate that (1) different MPCs lacking Myf5, MyoD, or Myf5/MyoD have different capabilities in providing factors potentially required for the survival of motor neurons and intramuscular nerve branching, (2) MPCs in double-mutant embryos do not contain neurotrophic factors in the absence of myogenic specification, and (3) different subpopulations of MPCs contain different combinations of neurotrophic factors potentially required for the survival of the specific subpopulations of innervating motor neurons.

Localization of brain-derived neurotrophic factor to distinct terminals of mossy fiber axons implies regulation of both excitation and feedforward inhibition of CA3 pyramidal cells

Hippocampal dentate granule cells directly excite and indirectly inhibit CA3 pyramidal cells via distinct presynaptic terminal specializations of their mossy fiber axons. This mossy fiber pathway contains the highest concentration of brain-derived neurotrophic factor (BDNF) in the CNS, yet whether BDNF is positioned to regulate the excitatory and/or inhibitory pathways is unknown. To localize BDNF, confocal microscopy of green fluorescent protein transgenic mice was combined with BDNF immunohistochemistry. Approximately half of presynaptic granule cell-CA3 pyramidal cell contacts were found to contain BDNF. Moreover, enhanced neuronal activity virtually doubled the percentage of BDNF-immunoreactive terminals contacting CA3 pyramidal cells. To our surprise, BDNF was also found in mossy fiber terminals contacting inhibitory neurons. These studies demonstrate that mossy fiber BDNF is poised to regulate both direct excitatory and indirect feedforward inhibitory inputs to CA3 pyramdal cells and reveal that seizure activity increases the pool of BDNF-expressing granule cell presynaptic terminals contacting CA3 pyramidal cells.

Difference in trafficking of brain-derived neurotrophic factor between axons and dendrites of cortical neurons, revealed by live-cell imaging

Background

Brain-derived neurotrophic factor (BDNF), which is sorted into a regulated secretory pathway of neurons, is supposed to act retrogradely through dendrites on presynaptic neurons or anterogradely through axons on postsynaptic neurons. Depending on which is the case, the pattern and direction of trafficking of BDNF in dendrites and axons are expected to be different. To address this issue, we analyzed movements of green fluorescent protein (GFP)-tagged BDNF in axons and dendrites of living cortical neurons by time-lapse imaging. In part of the experiments, the expression of BDNF tagged with cyan fluorescent protein (CFP) was compared with that of nerve growth factor (NGF) tagged with yellow fluorescent protein (YFP), to see whether fluorescent protein-tagged BDNF is expressed in a manner specific to this neurotrophin.

Results

We found that BDNF tagged with GFP or CFP was expressed in a punctated manner in dendrites and axons in about two-thirds of neurons into which plasmid cDNAs had been injected, while NGF tagged with GFP or YFP was diffusely expressed even in dendrites in about 70% of the plasmid-injected neurons. In neurons in which BDNF-GFP was expressed as vesicular puncta in axons, 59 and 23% of the puncta were moving rapidly in the anterograde and retrograde directions, respectively. On the other hand, 64% of BDNF-GFP puncta in dendrites did not move at all or fluttered back and forth within a short distance. The rest of the puncta in dendrites were moving relatively smoothly in either direction, but their mean velocity of transport, 0.47 ± 0.23 (SD) μm/s, was slower than that of the moving puncta in axons (0.73 ± 0.26 μm/s).

Conclusion

The present results show that the pattern and velocity of the trafficking of fluorescence protein-tagged BDNF are different between axons and dendrites, and suggest that the anterograde transport in axons may be the dominant stream of BDNF to release sites.

Activity regulates positive and negative neurotrophin-derived signals to determine axon competition

Developmental axon competition plays a key role in sculpting neural circuitry. Here, we have asked how activity and neurotrophins could interact to select one axon over another. Using compartmented cultures of sympathetic neurons, we show that, in the presence of NGF, local depolarization confers a competitive growth advantage on the depolarized axon collaterals and at the same time disadvantages the growth of unstimulated axons from the same and competing neurons. Depolarization mediates the competitive advantage by activating a CaMKII-MEK pathway, which converges to enhance local NGF-mediated downstream growth signals. Patterned electrical stimulation also acts via this pathway to enhance NGF-promoted axonal growth. In contrast, the competitive disadvantage is due to BDNF secreted from and acting on the unstimulated, competing axons through p75NTR. Thus, activity regulates both positive and negative neurotrophin-derived signaling cascades to confer a competitive growth advantage on one axon versus another, thereby providing a cellular mechanism for developmental axon selection.

Brain-derived neurotrophic factor mRNA and protein are targeted to discrete dendritic laminas by events that trigger epileptogenesis

Dendritic targeting of mRNA and local protein synthesis are mechanisms that enable neurons to deliver proteins to specific postsynaptic sites. Here, we demonstrate that epileptogenic stimuli induce a dramatic accumulation of BDNF mRNA and protein in the dendrites of hippocampal neurons in vivo. BDNF mRNA and protein accumulate in dendrites in all hippocampal subfields after pilocarpine seizures and in selected subfields after other epileptogenic stimuli (kainate and kindling). BDNF accumulates selectively in discrete dendritic laminas, suggesting targeting to synapses that are active during seizures. Dendritic targeting of BDNF mRNA occurs during the time when the cellular changes that underlie epilepsy are occurring and is not seen after intense stimuli that are non-epileptogenic, including electroconvulsive seizures and high-frequency stimulation. MK801, an NMDA receptor antagonist that can prevent epileptogenesis but not acute seizures, prevents the dendritic accumulation of BDNF mRNA, indicating that dendritic targeting is mediated via NMDA receptor activation. Together, these results suggest that dendritic accumulation of BDNF mRNA and protein plays a critical role in the cellular changes leading to epilepsy.

BDNF-induced recruitment of TrkB receptor into neuronal lipid rafts: roles in synaptic modulation

Brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity but the underlying signaling mechanisms remain unknown. Here, we show that BDNF rapidly recruits full-length TrkB (TrkB-FL) receptor into cholesterol-rich lipid rafts from nonraft regions of neuronal plasma membranes. Translocation of TrkB-FL was blocked by Trk inhibitors, suggesting a role of TrkB tyrosine kinase in the translocation. Disruption of lipid rafts by depleting cholesterol from cell surface blocked the ligand-induced translocation. Moreover, disruption of lipid rafts prevented potentiating effects of BDNF on transmitter release in cultured neurons and synaptic response to tetanus in hippocampal slices. In contrast, lipid rafts are not required for BDNF regulation of neuronal survival. Thus, ligand-induced TrkB translocation into lipid rafts may represent a signaling mechanism selective for synaptic modulation by BDNF in the central nervous system.

Brain-derived neurotrophic factor

Since the purification of BDNF in 1982, a great deal of evidence has mounted for its central roles in brain development, physiology, and pathology. Aside from its importance in neural development and cell survival, BDNF appears essential to molecular mechanisms of synaptic plasticity. Basic activity-related changes in the central nervous system are thought to depend on BDNF modification of synaptic transmission, especially in the hippocampus and neocortex. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders.

Exacerbated status epilepticus and acute cell loss, but no changes in epileptogenesis, in mice with increased brain-derived neurotrophic factor signaling

Several studies suggest that brain-derived neurotrophic factor (BDNF) can exacerbate seizure development during status epilepticus (S.E.) and subsequent epileptogenesis in the adult brain. On the other hand, evidence exists for the protective effect of BDNF. To study this controversy, we induced S.E. with kainate in transgenic mice with increased BDNF signaling due to trkB overexpression. Transgenic mice experienced a more severe S.E. than wild type animals did. Furthermore, they had increased acute hippocampal neuronal loss when assessed at 48 h after S.E. The effect of trkB overexpression on the development of epilepsy, chronic neuronal death, mossy fiber sprouting, and neurogenesis were studied at 4.5 months after kainate-induced S.E. No differences were found in the rate of epileptogenesis, severity of epilepsy, or cellular markers of network reorganization between transgenic and wild type mice. No differences between genotypes were observed in TUC-4 staining, indicating no effect of trkB overexpression to immature neuron numbers. Instead, in Cresyl Violet-stained preparations, the highest density of neurons was found in untreated transgenic mice suggesting a favorable effect of trkB overexpression on the survival of neurons in the hippocampus. Our data support the role of BDNF and trkB signaling in seizure generation and acute cellular damage after S.E. Long-term outcome was not, however, exacerbated by trkB overexpression.

The Role of BDNF in Epilepsy and Other Diseases of the Mature Nervous System

The neurotrophin brain-derived neurotrophic factor (BDNF) is ubiquitous in the central nervous system (CNS) throughout life. In addition to trophic effects on target neurons, BDNF appears to be part of a general mechanism for activity-dependent modification of synapses in the developing and adult nervous system. Thus, diseases of abnormal trophic support (such as neurodegenerative diseases) and diseases of abnormal excitability (such as epilepsy and central pain sensitization) can be related in some cases to abnormal BDNF signaling. For example, various studies have shown that BDNF is upregulated in areas implicated in epileptogenesis, and interference with BDNF signal transduction inhibits the development of the epileptic state. Further study of the cellular and molecular mechanisms by which BDNF influences cell survival and excitability will likely provide novel concepts and targets for the treatment of diverse CNS diseases.

Expression of BDNF mRNA in substantia nigra is dependent on target integrity and independent of neuronal activation

We have analyzed the regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the nigrostriatal system following neurotoxin ablation of striatal targets by means of kainate (KA) or quinolinic acid (QA) injections. Loss of nigral target cells in the striatum was accompanied by significant induction of BDNF mRNA levels in the ipsilateral substantia nigra (SN) at 12 and 24 h post lesion. Dual tyrosine hydroxylase (TH) and BDNF mRNA in situ hybridization (ISH) confirmed the dopaminergic nature of the BDNF mRNA expressing cells. Analysis of neuronal activity in terms of cFos mRNA expression demonstrated intense induction of this marker in the ipsilateral SN pars reticulata (SNPR), but not in SN pars compacta. Dual glutamic acid decarboxylase (GAD) and cFos mRNA ISH confirmed this view. Colchicine injections into the medial forebrain bundle to specifically disrupt neuronal trafficking between SN and striatum induced BDNF mRNA levels in the ipsilateral SNPC, thus demonstrating that nigral expression of BDNF mRNA is dependent of striatal target tissue. In addition, we found significant elevations of BDNF in the subthalamic nucleus following striatal excitotoxic lesion, which may bring novel roles of BDNF in the basal ganglia complex.

Pro-region of neurotrophins: role in synaptic modulation

Neurotrophins are synthesized first as precursors, followed by maturation through proteolytic removal of the "pro" region. Since pro- and mature neurotrophins elicit opposite functional effects by differential interactions with Trks and p75 receptors, extracellular cleavage represents a new way to control the synaptic functions of neurotrophins. A single nucleotide mutation in the pro-region appears to affect synaptic targeting and activity-dependent secretion of BDNF in hippocampal neurons. These results demonstrate new mechanisms by which neurotrophins regulate synaptic plasticity and memory function.

Nerve growth factor overexpression and autocrine loop in breast cancer cells

We show here that nerve growth factor (NGF), the canonical neurotrophic factor, is synthesized and released by breast cancer cells. High levels of NGF transcript and protein were detected in breast cancer cells by reverse transcription-PCR, Western blotting, ELISA assay and immunohistochemistry. Conversely, NGF production could not be detected in normal breast epithelial cells at either the transcriptional or protein level. Confocal analysis indicated the presence of NGF within classical secretion vesicles. Breast cancer cell-produced NGF was biologically active, as demonstrated by its ability to induce the neuronal differentiation of embryonic neural precursor cells. Importantly, the constitutive growth of breast cancer cells was strongly inhibited by either NGF-neutralizing antibodies or K-252a, a pharmacological inhibitor of NGF receptor TrkA, indicating the existence of an NGF autocrine loop. Together, our data demonstrate the physiological relevance of NGF in breast cancer and its potential interest as a marker and therapeutic target.

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.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

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.

Cellular mechanisms regulating activity-dependent release of native brain-derived neurotrophic factor from hippocampal neurons

Brain-derived neurotrophic factor (BDNF) plays a critical role in activity-dependent modifications of neuronal connectivity and synaptic strength, including establishment of hippocampal long-term potentiation (LTP). To shed light on mechanisms underlying BDNF-dependent synaptic plasticity, the present study was undertaken to characterize release of native BDNF from newborn rat hippocampal neurons in response to physiologically relevant patterns of electrical field stimulation in culture, including tonic stimulation at 5 Hz, bursting stimulation at 25 and 100 Hz, and theta-burst stimulation (TBS). Release was measured using the ELISA in situ technique, developed in our laboratory to quantify secretion of native BDNF without the need to first overexpress the protein to nonphysiological levels. Each stimulation protocol resulted in a significant increase in BDNF release that was tetrodotoxin sensitive and occurred in the absence of glutamate receptor activation. However, 100 Hz tetanus and TBS, stimulus patterns that are most effective in inducing hippocampal LTP, were significantly more effective in releasing native BDNF than lower-frequency stimulation. For all stimulation protocols tested, removal of extracellular calcium, or blockade of N-type calcium channels, prevented BDNF release. Similarly, depletion of intracellular calcium stores with thapsigargin and treatment with dantrolene, an inhibitor of calcium release from caffeine-ryanodine-sensitive stores, markedly inhibited activity-dependent BDNF release. Our results indicate that BDNF release can encode temporal features of hippocampal neuronal activity. The dual requirement for calcium influx through N-type calcium channels and calcium mobilization from intracellular stores strongly implicates a role for calcium-induced calcium release in activity-dependent BDNF secretion.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.