Neurotrophins induce formation of functional excitatory and inhibitory synapses between cultured hippocampal neurons

Reduced number of functional glutamatergic synapses in hippocampal neurons overexpressing full-length TrkB receptors

Brain-derived neurotrophic factor (BDNF) acutely modulates the efficacy of central glutamatergic synapses via activation of the receptor tyrosine kinase TrkB. On a longer time scale, recent evidence suggests an additional role of TrkB signaling in the formation of excitatory synaptic connections. Here, we have overexpressed full-length TrkB receptors (fl-TrkB) in hippocampal neurons, to investigate the contribution of BDNF signaling to the maturation of glutamatergic synapses. Using patch clamp recordings, we show a three-fold reduction in glutamatergic excitatory autaptic and synaptic current amplitudes in neurons overexpressing fl-TrkB, and application of saturating concentrations of BDNF and NT-4/5 completely reverses this effect. Compatible with these overexpression data, in untransfected neurons, scavenging of endogenous BDNF and NT-4/5 by TrkB-IgGs reduces excitatory autaptic current (EAC) amplitudes. By overexpression of truncated TrkB receptors (TrkB.T1, TrkB.T2) and a chimeric receptor containing only the intracellular domain of fl-TrkB, we show that intra- and extracellular domains of fl-TrkB are necessary to observe the EAC reduction. Labeling of presynaptic terminals with FM 4-64 revealed, that the reduced EAC amplitudes in fl-TrkB overexpressing neurons are accompanied by a two-fold reduction in synapse number. These results suggest, that ligand-independent signaling through fl-TrkB receptors can decrease glutamatergic synaptic strength, if sufficient amounts of BDNF or NT-4/5 are not available.

Visualizing synapse formation in arborizing optic axons in vivo: dynamics and modulation by BDNF

Dynamic developmental changes in axon arbor morphology may directly reflect the formation, stabilization and elimination of synapses. We used dual-color imaging to study, in the live, developing animal, the relationship between axon arborization and synapse formation at the single cell level, and to examine the participation of brain-derived neurotrophic factor (BDNF) in synaptogenesis. Green fluorescent protein (GFP)-tagged synaptobrevin II served as a marker to visualize synaptic sites in individual fluorescently labeled Xenopus optic axons. Time-lapse confocal microscopy revealed that although most synapses remain stable, synapses are also formed and eliminated as axons branch and increase their complexity. Most new branches originated at GFP-labeled synaptic sites. Increasing BDNF levels significantly increased both axon arborization and synapse number, with BDNF increasing synapse number per axon terminal. The ability to visualize central synapses in real time provides insights about the dynamic mechanisms underlying synaptogenesis, and reveals BDNF as a modulator of synaptogenesis in vivo.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

Ca(2+) binding protein frequenin mediates GDNF-induced potentiation of Ca(2+) channels and transmitter release

Molecular mechanisms underlying long-term neurotrophic regulation of synaptic transmission and plasticity are unknown. We report here that long-term treatment of neuromuscular synapses with glial cell line-derived neurotrophic factor (GDNF) potentiates spontaneous and evoked transmitter release, in ways very similar to presynaptic expression of the Ca(2+) binding protein frequenin. GDNF enhances the expression of frequenin in motoneurons, and inhibition of frequenin expression or activity prevents the synaptic action of GDNF. GDNF also facilitates Ca(2+) influx into the nerve terminals during evoked transmission by enhancing Ca(2+) currents. The effect of GDNF on Ca(2+) currents is blocked by inhibition of frequenin expression, occluded by overexpression of frequenin, and is selective to N-type Ca(2+) channels. These results identify an important molecular target that mediates the long-term, synaptic action of a neurotrophic factor.

Protein synthesis-dependent and -independent regulation of hippocampal synapses by brain-derived neurotrophic factor

A fundamental difference between short-term and long-term forms of synaptic plasticity is the dependence on transcription and translation of new genes. Using organotypic cultures of hippocampal slices, we have investigated whether the modulation of synapses by brain-derived neurotrophic factor (BDNF) also requires protein synthesis. Long-term treatment of hippocampal slice cultures with BDNF increased the number of docked vesicles, but not that of reserve pool vesicles, at CA1 excitatory synapses. BDNF also increased the levels of the vesicle proteins synaptophysin, synaptobrevin, and synaptotagmin, without affecting the presynaptic membrane proteins syntaxin and SNAP-25, or the vesicle-binding protein synapsin-I. The increase in synaptophysin and synaptobrevin expression was moderate (2-fold) and occurred within 6 h after BDNF application. In contrast, synaptotagmin expression took 24 h to reach maximum levels (5-fold). The delayed increase in synaptotagmin was blocked by protein synthesis inhibitors, while the early increase in synaptophysin and synaptobrevin was not. Moreover, the BDNF-induced increase of synaptotagmin was blocked by inhibiting the cAMP/protein kinase A (PKA) pathway. However, BDNF did not activate PKA, and application of a PKA activator did not mimic the BDNF effect. Taken together, these results suggest a novel, protein synthesis-dependent form of BDNF modulation that requires cAMP gating.

Potential role of endogenous brain-derived neurotrophic factor in long-term neuronal reorganization of the superior colliculus after bilateral visual deprivation

The role of the brain-derived neurotrophic factor (BDNF), the BDNF receptor (TrkB), and the glutamic acid decarboxylase (GAD67) after neonatal, bilateral nerve deafferentiation during postnatal development was investigated in the rat superior colliculus (SC). BDNF and GAD67 mRNA expression were significantly increased in optic (Op) and intermediate gray (InG) layers at 5, 8, 15, and 21 days after birth, but not in adult animals. However, TrkB mRNA expression was not modified at any time tested. At 15 days, where changes in BDNF and GAD67 mRNAs were more evident, an upregulation of the NMDAR(1A) mRNA glutamate receptor in the Op and InG, a modification in the pattern of synaptic zinc in the superficial layers of SC, and an increased synaptophysin immunoreactivity in the Op was found. This indicates the existence of a synergic mechanism between BDNF and NMDA to determine refinement of connections after the loss of visual input in SC.

Rab3A is required for brain-derived neurotrophic factor-induced synaptic plasticity: transcriptional analysis at the population and single-cell levels

Brain-derived neurotrophic factor (BDNF) modulates synaptic strength in hippocampal neurons, in addition to promoting survival and differentiation. To identify genes involved in trophic regulation of synaptic plasticity, we have used a multidisciplinary approach of differential display and family-specific slot blots in combination with whole-cell patch-clamp recordings of dissociated hippocampal neurons. Three hour exposure to BDNF elicited a 2.6-fold increase in synaptic charge and a concomitant induction of 11 genes as revealed by differential display, including the small GTP-binding vesicular trafficking protein Rab3A and the enzyme guanylate cyclase (GC). Slot blot analysis on a population of neurons confirmed an average of 3.1-fold induction of these clones. In contrast, individual pyramidal-like neurons that were first characterized electrophysiologically in the presence of BDNF and subjected to transcriptional analysis displayed more robust increases (4.8-fold), emphasizing the neuronal heterogeneity. Transcriptional changes of Rab3A and GC were accompanied by translational regulation, shown by Western blot analysis. Furthermore, a number of GC-associated and Rab3A effector molecules were induced by BDNF at either the gene or protein levels. The functional role of Rab3A in BDNF-induced synaptic plasticity was assessed using cells derived from Rab3A knock-out mice. These neurons failed to show an increase in synaptic charge in response to BDNF at 10 min; however a late response to BDNF was detected at 20 min. This late response was similar in time course to that induced by postsynaptic activation of glutamate receptors. Our results demonstrate a requirement for Rab3A and may reveal a temporal distinction between presynaptic and postsynaptic mechanisms of BDNF-induced synaptic plasticity associated with learning and memory.

NT-3 regulates BDNF-induced modulation of synaptic transmission in cultured hippocampal neurons

BDNF and NT-3 can modulate the development and plasticity of central synaptic transmission. Although the expression of NT-3 and BDNF in the rodent hippocampus coincides during perinatal development, little is known about possible functional interactions between both neurotrophins in synaptic development. Here, we have investigated the effects of combined long-term application of NT-3 and BDNF on excitatory glutamatergic (mEPSC) and inhibitory GABAergic miniature synaptic currents (mIPSC) in cultured embryonic hippocampal neurons. Our results show that the BDNF-induced twofold increase in mEPSC frequency is abolished by pre-treatment with NT-3. In addition, the NT-3-induced twofold downregulation of mIPSC frequency is reversed by BDNF. Finally, the BDNF-induced increase in c-fos expression is reduced by 50% after pre-treatment with NT-3. In summary, these data suggest an NT-3 controlled modulation of BDNF signalling in differentiating hippocampal neurons.

Regulation of dendritic spine motility in cultured hippocampal neurons

Regulation of dendritic spine motility was studied in dissociated cultures of the rat and mouse hippocampus, using green fluorescent protein-labeled neurons or neurons loaded with the calcium-sensitive dye Oregon Green-1. Cells were time-lapse-photographed on a confocal laser-scanning microscope at high resolution to detect movements as well as spontaneous fluctuations of intracellular calcium concentrations in their dendritic spines. Active presynaptic terminals attached to the spines were labeled with FM4-64, which marks a subset of synaptophysin-labeled terminals. Dendritic spines were highly motile in young, 4- to 7-d-old cells. At this age, neurons had little spontaneous calcium fluctuation or FM4-64 labeling. Within 2-3 weeks in culture, dendritic spines were much less motile, they were associated with active presynaptic terminals, and they expressed high rates of spontaneous calcium fluctuations. Irrespective of age, and even on the same dendrite, there was an inverse relationship between spine motility and presence of FM4-64-labeled terminals in contact with the imaged spines. Spine motility was blocked by latrunculin, which prevents actin polymerization, and was disinhibited by blockade of action potential discharges with tetrodotoxin. It is proposed that an active presynaptic terminal restricts motility of dendritic spines.

Tracking the estrogen receptor in neurons: implications for estrogen-induced synapse formation

Estrogens (E) and progestins regulate synaptogenesis in the CA1 region of the dorsal hippocampus during the estrous cycle of the female rat, and the functional consequences include changes in neurotransmission and memory. Synapse formation has been demonstrated by using the Golgi technique, dye filling of cells, electron microscopy, and radioimmunocytochemistry. N-methyl-d-aspartate (NMDA) receptor activation is required, and inhibitory interneurons play a pivotal role as they express nuclear estrogen receptor alpha (ERalpha) and show E-induced decreases of GABAergic activity. Although global decreases in inhibitory tone may be important, a more local role for E in CA1 neurons seems likely. The rat hippocampus expresses both ERalpha and ERbeta mRNA. At the light microscopic level, autoradiography shows cell nuclear [3H]estrogen and [125I]estrogen uptake according to a distribution that primarily reflects the localization of ERalpha-immunoreactive interneurons in the hippocampus. However, recent ultrastructural studies have revealed extranuclear ERalpha immunoreactivity (IR) within select dendritic spines on hippocampal principal cells, axon terminals, and glial processes, localizations that would not be detectable by using standard light microscopic methods. Based on recent studies showing that both types of ER are expressed in a form that activates second messenger systems, these findings support a testable model in which local, non-genomic regulation by estrogen participates along with genomic actions of estrogens in the regulation of synapse formation.

BDNF enhances quantal neurotransmitter release and increases the number of docked vesicles at the active zones of hippocampal excitatory synapses

Brain-derived neurotrophic factor (BDNF) is emerging as a key mediator of activity-dependent modifications of synaptic strength in the CNS. We investigated the hypothesis that BDNF enhances quantal neurotransmitter release by modulating the distribution of synaptic vesicles within presynaptic terminals using organotypic slice cultures of postnatal rat hippocampus. BDNF specifically increased the number of docked vesicles at the active zone of excitatory synapses on CA1 dendritic spines, with only a small increase in active zone size. In agreement with the hypothesis that an increased docked vesicle density enhances quantal neurotransmitter release, BDNF increased the frequency, but not the amplitude, of AMPA receptor-mediated miniature EPSCs (mEPSCs) recorded from CA1 pyramidal neurons in hippocampal slices. Synapse number, independently estimated from dendritic spine density and electron microscopy measurements, was also increased after BDNF treatment, indicating that the actions of BNDF on mEPSC frequency can be partially attributed to an increased synaptic density. Our results further suggest that all these actions were mediated via tyrosine kinase B (TrkB) receptor activation, established by inhibition of plasma membrane tyrosine kinases with K-252a. These results provide additional evidence of a fundamental role of the BDNF-TrkB signaling cascade in synaptic transmission, as well as in cellular models of hippocampus-dependent learning and memory.

Molecular heterogeneity of central synapses: afferent and target regulation

Electrophysiological recordings show a functional spectrum even within a single class of synapse, with individual synapses ranging widely in fundamental properties, including release probability, unitary response and effects of previous stimulation on subsequent response. Molecular and cellular biological approaches have shown a corresponding diversity in the complement of ion channels, receptors, scaffolds and signal transducing proteins that make up individual synapses. Indeed, we believe that each individual synapse is unique, a function of presynaptic cell type, postsynaptic cell type, environment, developmental stage and history of activity. We review here the molecular diversity of glutamatergic and GABAergic synapses in the mammalian brain in the context of potential cell biological mechanisms that may explain how individual cells develop and maintain such a mosaic of synaptic connections.

Regulation of spontaneous inhibitory synaptic transmission by endogenous glutamate via non-NMDA receptors in cultured rat hippocampal neurons

The regulation of gamma-aminobutyric acid (GABA)-mediated spontaneous inhibitory synaptic transmission by endogenously released glutamate was studied in cultured rat hippocampal neurons. After 7 days in vitro (DIV), both spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents (sIPSCs) could be detected. After 15 DIV, most postsynaptic spontaneous currents occurred as sEPSC/sIPSC sequences when recorded at a holding voltage of -30 mV. In the presence of the glutamate alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subtype antagonist LY303070, both the frequency and amplitude of sIPSC were strongly and reversibly reduced. The N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP5), had no effect on sIPSC while cyclothiazide strongly increased sIPSC frequency. Under blockade of AMPA receptors, the kainate- and GluR5-selective kainate receptor agonists, (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid) (ATPA) and (S)-5-iodowillardiine (5IWill), induced a large enhancement of the frequency of small-amplitude sIPSC which was blocked by the non-NMDA receptor antagonist, 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX). All of these effects were sensitive to tetrodotoxin (TTX). In the presence of LY303070 and TTX, kainate could induce a small inward current while GluR5 agonists had no effect. In the presence of NMDA and AMPA receptor antagonists, the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC) could restore sIPSC. When NBQX was used as an AMPA antagonist, the stimulatory effect of t-PDC was blocked while the group I metabotropic glutamate agonist, 3,5-dihydroxyphenylglycine (DHPG), induced a strong enhancement of sIPSC. Therefore, both AMPA and kainate receptors can regulate inhibitory synaptic transmission in cultured hippocampal neurons, the former by tonic activation, the latter when the glutamate concentration is increased by impairing glutonate uptake.

BDNF reduces miniature inhibitory postsynaptic currents by rapid downregulation of GABA(A) receptor surface expression

Changes in neurotransmitter receptor density at the synapse have been proposed as a mechanism underlying synaptic plasticity. Neurotrophic factors are known to influence synaptic strength rapidly. In the present study, we found that brain-derived neurotrophic factor (BDNF) acts postsynaptically to reduce gamma-aminobutyric acid (GABA)-ergic function. Using primary cultures of rat hippocampal neurons, we investigated the effects of BDNF on GABAergic miniature inhibitory postsynaptic currents (mIPSCs) and on the localization of GABAA receptors. Application of BDNF (100 ng/mL) led within minutes to a marked reduction (33.5%) of mIPSC amplitudes in 50% of neurons, recorded in the whole-cell patch-clamp mode, leaving frequency and decay kinetics unaffected. This effect was blocked by the protein kinase inhibitor K252a, which binds with high affinity to trkB receptors. Immunofluorescence staining with an antibody against trkB revealed that about 70% of cultured hippocampal pyramidal cells express trkB. In dual labelling experiments, use of neurobiotin injections to label the recorded cells revealed that all cells responsive to BDNF were immunopositive for trkB. Treatment of cultures with BDNF reduced the immunoreactivity for the GABAA receptor subunits-alpha2, -beta2,3 and -gamma2 in the majority of neurons. This effect was detectable after 15 min and lasted at least 12 h. Neurotrophin-4 (NT-4), but not neurotrophin-3 (NT-3), also reduced GABAA receptor immunoreactivity, supporting the proposal that this effect is mediated by trkB. Altogether the results suggest that exposure to BDNF induces a rapid reduction in postsynaptic GABAA receptor number that is responsible for the decline in GABAergic mIPSC amplitudes.

Neurotrophins act at presynaptic terminals to activate synapses among cultured hippocampal neurons

We have recently demonstrated that embryonic E16 hippocampal neurons grown in cultures are unable to form fast synaptic connections unless treated with BDNF or NT-3. This experimental system offers an opportunity to define the roles of neurotrophins in processes leading to formation of functional synaptic connections. We have used ultrastructural and electrophysiological methods to explore the cellular locations underlying neurotrophin action on synaptic maturation. The rate of spontaneous miniature excitatory postsynaptic currents (mEPSCs) evoked by hyperosmotic stimulation was 7-16-fold higher in neurotrophin-treated cells than in controls. In addition, the potent neurotransmitter-releasing drug alpha-latrotoxin was virtually ineffective in the control cells while it stimulated synaptic events in neurotrophin-treated cells. Likewise, the membrane-bound dye FM1-43 was taken up by terminals in neurotrophin-treated cultures five-fold more than in controls. Both the total number and the number of docked synaptic vesicles were increased by neurotrophin treatment. Activation of synaptic responses by neurotrophins occurred even when postsynaptic glutamate receptors and action potential discharges were pharmacologically blocked. These results are consistent with a presynaptic locus of action of neurotrophins to increase synaptic vesicle density which is critical for rapid synaptic transmission. They also suggest that neurotrophins can activate synapses in the absence of pre- and postsynaptic neuronal activity.

Increased synaptic inhibition in dentate gyrus of mice with reduced levels of endogenous brain-derived neurotrophic factor

The aim of this study was to explore the role of endogenous neurotrophins for inhibitory synaptic transmission in the dentate gyrus of adult mice. Heterozygous knockout (+/-) mice or neurotrophin scavenging proteins were used to reduce the levels of endogenous brain-derived neurotrophic factor and neurotrophin-3. Patch-clamp recordings from dentate granule cells in brain slices showed that the frequency, but not the kinetics or amplitude, of miniature inhibitory postsynaptic currents was modulated in brain-derived neurotrophic factor +/- compared to wild-type (+/+) mice. Furthermore, paired-pulse depression of evoked inhibitory synaptic responses was increased in brain-derived neurotrophic factor +/- mice. Similar results were obtained in brain slices from brain-derived neurotrophic factor +/+ mice incubated with tyrosine receptor kinase B-immunoglobulin G, which scavenges endogenous brain-derived neurotrophic factor. The increased inhibitory synaptic activity in brain-derived neurotrophic factor +/- mice was accompanied by decreased excitability of the granule cells. No differences in the frequency, amplitude or kinetics of miniature inhibitory postsynaptic currents were seen between neurotrophin-3 +/- and +/+ mice. From these results we suggest that endogenous brain-derived neurotrophic factor, but not neurotrophin-3, has acute modulatory effects on synaptic inhibition onto dentate granule cells. The site of action seems to be located presynaptically, i.e. brain-derived neurotrophic factor regulates the properties of inhibitory interneurons, leading to increased excitability of dentate granule cells. We propose that through this mechanism, brain-derived neurotrophic factor can change the gating/filtering properties of the dentate gyrus for incoming information from the entorhinal cortex to hippocampus. This will have consequences for the recruitment of hippocampal neural circuitries both under physiological and pathological conditions, such as epileptogenesis.

Brain-derived neurotrophic factor superinduction parallels anti-epileptic--neuroprotective treatment in the pilocarpine epilepsy model

Antiepileptic drugs provide neuroprotection in several animal models of brain damage, including those induced by status epilepticus (SE). The mechanisms involved in this action are unknown, but neurotrophic factors such as brain-derived neurotrophic factor (BDNF) may play a role. In this study we investigated the changes in BDNF levels in rats in which SE had been induced by pilocarpine injection (400 mg/kg i.p.) and continued for several hours (unprotected group). In other animals (protected groups), SE was suppressed after 30 min by intraperitoneal injection of either diazepam (10 mg/kg) + pentobarbital (30 mg/kg) or paraldehyde (0.3 mg/kg). In diazepam + pentobarbital-treated rats the hippocampal damage caused by SE was significantly lower (p < 0.05) than in unprotected animals. In addition, 2 and 24 h after pilocarpine injection, the levels of BDNF mRNA were moderately increased in the unprotected group, but 'superinduced' in protected animals, especially in the neocortex and hippocampus. A time-dependent increase in BDNF immunoreactivity was also found by western blot analysis in rats treated with diazepam + pentobarbital. In contrast, a decrease of BDNF immunoreactivity occurred in the unprotected group. In conclusion, these results show that neuroprotection induced by anti-epileptic drugs in pilocarpine-treated rats is accompanied by strong potentiation of BDNF synthesis in brain regions involved in SE.

Exogenous brain-derived neurotrophic factor regulates the synaptic composition of axonally lesioned and normal adult rat motoneurons

Brain-derived neurotrophic factor has previously been shown to promote survival and axonal regeneration in injured spinal motoneurons and, also, to modulate synaptic transmission and regulate the density of synaptic innervation in a variety of neurons. The present light and electron microscopic study demonstrates synaptotrophic effects of exogenously applied brain-derived neurotrophic factor on the synaptic composition of both normal and axonally lesioned adult rat spinal motoneurons. After L5-L6 ventral root avulsion, a massive loss of all types of boutons occurred on the somata of the lesioned motoneurons which persisted for at least 12 weeks postoperatively. We found that (i) intrathecal infusion of brain-derived neurotrophic factor during the first postoperative week did not prevent the synaptic detachment and activation of glial cells; (ii) prolonged treatment for four weeks restored synaptic covering and significantly reduced microglial reaction; (iii) the synaptotrophic effect remained significant for at least eight weeks after cessation of the treatment; (iv) brain-derived neurotrophic factor mainly supported F-type boutons with presumably inhibitory function, while it had little effect on S-type boutons associated with excitatory action; and (v) in normal unlesioned motoneurons, four weeks of treatment with brain-derived neurotrophic factor induced sprouting of F-type boutons, a loss of S-type boutons and motoneuron atrophy. The present data show that exogenous neurotrophins not only help to restore synaptic circuitry in axonally injured motoneurons, but also strongly influence the synaptic composition in normal motoneurons.

Neuronal cyclin-dependent kinase 5 activity is critical for survival

Cyclin-dependent kinase 5 (Cdk5) null mice exhibit a unique phenotype characterized by perinatal mortality, disrupted cerebral cortical layering attributable to abnormal neuronal migration, lack of cerebellar foliation, and chromatolytic changes of neurons in the brainstem and the spinal cord. Because Cdk5 is expressed in both neurons and astrocytes, it has been unclear whether this phenotype is primarily attributable to defects in neurons or in astrocytes. Herein we report reconstitution of Cdk5 expression in neurons in Cdk5 null mice and its effect on the null phenotype. Unlike the Cdk5 null mice, the reconstituted Cdk5 null mice that express the Cdk5 transgene under the p35 promoter (TgKO mice) were viable and fertile. Because Cdk5 expression is mainly limited to neurons in these mice and rescues the defects in the nervous system of the Cdk5 null phenotype, it clearly demonstrates that Cdk5 activity is necessary for normal development and survival of p35-expressing neurons.

A glia-derived signal regulating neuronal differentiation

Astrocytes are present in large numbers in the nervous system, are associated with synapses, and propagate ionic signals. Astrocytes influence neuronal physiology by responding to and releasing neurotransmitters, but the mechanisms that establish the close interaction between these cells are not defined. Here we use hippocampal neurons in culture to demonstrate that vasoactive intestinal polypeptide (VIP) promotes neuronal differentiation through activity-dependent neurotrophic factor (ADNF), a protein secreted by VIP-stimulated astroglia. ADNF is produced by glial cells and acts directly on neurons to promote glutamate responses and morphological development. ADNF causes secretion of neurotrophin 3 (NT-3), and both proteins regulate NMDA receptor subunit 2A (NR2A) and NR2B. These data suggest that the VIP-ADNF-NT-3 neuronal-glial pathway regulates glutamate responses from an early stage in the synaptic development of excitatory neurons and may also contribute to the known effects of VIP on learning and behavior in the adult nervous system.

Neuronal activity and brain-derived neurotrophic factor regulate the density of inhibitory synapses in organotypic slice cultures of postnatal hippocampus

Hippocampal interneurons inhibit pyramidal neurons through the release of the neurotransmitter GABA. Given the importance of this inhibition for the proper functioning of the hippocampus, the development of inhibitory synapses must be tightly regulated. In this study, the possibility that neuronal activity and neurotrophins regulate the density of GABAergic inhibitory synapses was investigated in organotypic slice cultures taken from postnatal day 7 rats. In hippocampal slices cultured for 13 d in the presence of the GABA(A) receptor antagonist bicuculline, the density of glutamic acid decarboxylase (GAD) 65-immunoreactive terminals was increased in the CA1 area when compared with control slices. Treatment with the glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione decreased the density of GAD65-immunoreactive terminals in the stratum oriens of CA1. These treatments had parallel effects on the density of GABA-immunoreactive processes. Electron microscopic analysis after postembedding immunogold labeling with antibodies against GABA indicated that bicuculline treatment increased the density of inhibitory but not excitatory synapses. Application of exogenous BDNF partly mimicked the stimulatory effect of bicuculline on GAD65-immunoreactive terminals. Finally, antibodies against BDNF, but not antibodies against nerve growth factor, decrease the density of GAD65-immunoreactive terminals in bicuculline-treated slices. Thus, neuronal activity regulates the density of inhibitory synapses made by postnatal hippocampal interneurons, and BDNF could mediate part of this regulation. This regulation of the density of inhibitory synapses could represent a feedback mechanism aimed at maintaining an appropriate level of activity in the developing hippocampal networks.

The role of brain-derived neurotrophic factor receptors in the mature hippocampus: modulation of long-term potentiation through a presynaptic mechanism involving TrkB

The neurotrophin BDNF has been shown to modulate long-term potentiation (LTP) at Schaffer collateral-CA1 hippocampal synapses. Mutants in the BDNF receptor gene trkB and antibodies to its second receptor p75NTR have been used to determine the receptors and cells involved in this response. Inhibition of p75NTR does not detectably reduce LTP or affect presynaptic function, but analyses of newly generated trkB mutants implicate TrkB. One mutant has reduced expression in a normal pattern of TrkB throughout the brain. The second mutant was created by cre-loxP-mediated removal of TrkB in CA1 pyramidal neurons of this mouse. Neither mutant detectably impacts survival or morphology of hippocampal neurons. TrkB reduction, however, affects presynaptic function and reduces the ability of tetanic stimulation to induce LTP. Postsynaptic glutamate receptors are not affected by TrkB reduction, indicating that BDNF does not modulate plasticity through postsynaptic TrkB. Consistent with this, elimination of TrkB in postsynaptic neurons does not affect LTP. Moreover, normal LTP is generated in the mutant with reduced TrkB by a depolarization-low-frequency stimulation pairing protocol that puts minimal demands on presynaptic terminal function. Thus, BDNF appears to act through TrkB presynaptically, but not postsynaptically, to modulate LTP.

Actions of brain-derived neurotrophic factor on evoked and spontaneous EPSCs dissociate with maturation of neurones cultured from rat visual cortex

To address the question of whether brain-derived neurotrophic factor (BDNF) directly enhances excitatory synaptic transmission, we recorded excitatory postsynaptic currents (EPSCs) from solitary neurones cultured on glial microislands for 7-38 days, and observed changes in EPSCs after the application of BDNF. In this preparation the possible action of BDNF on GABAergic inhibition was not involved, and evoked and spontaneous (miniature) EPSCs were derived from the same group of synapses (autapses). The application of BDNF at a concentration of 200 ng ml-1 rapidly enhanced the frequency of miniature EPSCs (mEPSCs) in almost all the neurones tested. On the other hand, the amplitude of mEPSCs did not change at all, suggesting that the site of BDNF action is presynaptic. In contrast to the enhanced frequency of mEPSCs, evoked EPSCs were not potentiated in 61 % of the cells tested. Most of these BDNF-insensitive EPSCs had a peak amplitude larger than 1 nA, while most of the other BDNF-sensitive EPSCs had a smaller amplitude. The former EPSCs had smaller coefficients of variation (CVs) of amplitude, while the latter had larger CVs, suggesting the possibility that the presynaptic release probability for the former groups of EPSCs might have beeen saturated so that the BDNF action was occluded. To test this possibility we applied a low Ca2+ solution to 17 cells and reduced the amplitude of their evoked EPSCs to less than or near to 1 nA. It was found, however, that BDNF did not enhance these EPSCs. Rather, evoked EPSCs of almost all the cells cultured for less than 15 days were enhanced by BDNF, while those of most of the cells cultured for longer than 16 days were not enhanced. These results suggest that BDNF enhances transmitter release from presynaptic sites through its action on the release machinery, which can be differentiated into a BDNF-insensitive form for evoked release and a BDNF-sensitive form for spontaneous release with maturation of synapses.

Neurons generated from adult rat hippocampal stem cells form functional glutamatergic and GABAergic synapses in vitro

Basic fibroblast growth factor-responsive neural stem cells (NSCs) derived from adult rat hippocampus were earlier demonstrated to generate neurons and glia. These stem-cell-derived neurons express GABA, acetylcholinesterase, tyrosine hydroxylase, or calbindin. It has not been clear, however, whether or not these stem-cell-derived neurons are able to form functional synapses. In the present study, we investigated the development of synapse formation by adult hippocampus-derived neural stem cells. NSCs from adult rat hippocampi and primary embryonic rat hippocampal neurons were cocultured on a glial feeder layer. Immunofluorescence studies revealed that some of the NSCs became immunoreactive for microtubule-associated protein 2ab, neurofilament 200, synaptobrevin, or synaptophysin. These cells possessed properties of functional neurons such as action potentials and miniature postsynaptic currents (mPSCs). The elicited mPSCs with rapid kinetics were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX), but not by bicuculline (excitatory mPSCs). The remaining mPSCs had slower kinetics and were blocked by bicuculline, but not by DNQX (inhibitory mPSCs). We considered that the neurons derived from the adult NSCs expressed both non-NMDA glutamate receptors and the GABA(A) receptors and formed functional synapses. Our results demonstrate that adult NSCs can differentiate into neurons with functional glutamatergic and GABAergic synaptic transmission in vitro and support the concept that such neurons could integrate into the neuronal circuitry.

TrkB receptor ligands promote activity-dependent inhibitory synaptogenesis

Organotypic cerebellar cultures derived from newborn mice were simultaneously exposed to activity-blocking agents and neurotrophins for 2 weeks. Activity-blocked explants treated with the TrkB receptor ligands BDNF and neurotrophin-4 (NT-4) developed a full complement of Purkinje cell inhibitory axosomatic synapses, as defined ultrastructurally, and displayed control spontaneous cortical discharge rates after recovery from activity blockade. Otherwise untreated activity-blocked cultures and activity-blocked cultures exposed to the TrkC receptor ligand NT-3 had reduced inhibitory synapse development and persistent cortical hyperactivity after recovery. The added TrkB receptor ligands did not induce axonal sprouting to account for increased inhibitory synaptogenesis. Addition of neurotrophins to untreated cerebellar cultures did not increase the complement of Purkinje cell axosomatic synapses. Exposure of cerebellar cultures to a combination of antibodies to BDNF and NT-4 resulted in reduced inhibitory synapse formation, similar to the effects of activity blockade, indicating the necessity for endogenous neurotrophins for development of the full complement of inhibitory synapses in the presence of neuronal activity. Application of antibodies to BDNF and NT-4 to cerebellar explants exposed to picrotoxin to increase neuronal activity prevented the hyperinnervation of Purkinje cell somata by inhibitory terminals characteristic of cultures exposed to picrotoxin alone. These results are consistent with the concept that TrkB receptor ligands promote inhibitory synaptogenesis. The ability of neurotrophins to substitute for neuronal activity in encouraging development of inhibitory synapses may have therapeutic implications.

Different extrinsic trophic factors regulate neurite outgrowth and synapse formation between identified Lymnaea neurons

The requirement for trophic factors in neurite outgrowth is well established, though their role in synapse formation is yet to be determined. Moreover, the issue of whether the trophic factors mediating neurite outgrowth are also responsible for synapse specification has not yet been resolved. To test whether trophic factors mediating neurite outgrowth and synapse formation between identified neurons are conserved in two molluscan species and whether these developmental processes are differentially regulated by different trophic factors, we used soma-soma and neurite-neurite synapses between identified Lymnaea neurons. We demonstrate here that the trophic factors present in Aplysia hemolymph, although sufficient to induce neurite outgrowth from Lymnaea neurons, do not promote specific synapse formation between excitatory partners. Specifically, the identified presynaptic neuron visceral dorsal 4 (VD4) and postsynaptic neuron left pedal dorsal 1 (LPeD1) were either paired in a soma-soma configuration or plated individually to allow neuritic contacts. Cells were cultured in either Lymnaea brain-conditioned medium (CM) or on poly-L-lysine dishes that were pretreated with Aplysia hemolymph (ApHM), but contained only Lymnaea defined medium (DM; does not promote neurite outgrowth). In ApHM-coated dishes containing DM, Lymnaea neurons exhibited extensive neurite outgrowth, but appropriate excitatory synapses failed to develop between the cells. Instead, inappropriate reciprocal inhibitory synapses formed between VD4 and LPeD1. Similar inappropriate inhibitory synapses were observed in Aplysia hemolymph-pretreated dishes that contained dialyzed Aplysia hemolymph. These inhibitory synapses were novel and inappropriate, because they do not exist in vivo. A receptor tyrosine kinase inhibitor (Lavendustin A) blocked neurite outgrowth induced by both Lymnaea CM and ApHM. However, it did not affect inappropriate inhibitory synapse formation between the neurons. These data demonstrate that neurite outgrowth but not inappropriate inhibitory synapse formation involves receptor tyrosine kinases. Together, our data provide direct evidence that trophic factors required for neurite outgrowth are conserved among two different molluscan species, and that neurite extension and synapse specification between excitatory partners are likely mediated by different trophic factors.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Knocking the NT4 gene into the BDNF locus rescues BDNF deficient mice and reveals distinct NT4 and BDNF activities

To directly compare biological activities of the neurotrophins NT4 and BDNF in vivo, we replaced the BDNF coding sequence with the NT4 sequence in mice (Bdnfnt4-ki). Mice expressing NT4 in place of BDNF were viable, in contrast with BDNF null mutants, which die shortly after birth. Although the Bdnfnt4-ki/nt4-ki and wild-type Bdnf+/+ alleles yielded similar levels of NT4 and BDNF proteins, NT4 supported more sensory neurons than BDNF and promoted functional synapse formation in cultured hippocampal neurons. Homozygous Bdnfnt4-ki/nt4-ki mice showed reduced body weight, infertility and skin lesions, suggesting unique biological activities of NT4 in vivo. The distinct activities of NT4 and BDNF may result partly from differential activation of the TrkB receptor and its down-stream signals.

Stabilization of growing retinal axons by the combined signaling of nitric oxide and brain-derived neurotrophic factor

The pattern of axonal projections early in the development of the nervous system lacks the precision present in the adult. During a developmental process of refinement, mistargeted projections are eliminated while correct projections are retained. Previous studies suggest that during development nitric oxide (NO) is involved in the elimination of mistargeted retinal axons, whereas brain-derived neurotrophic factor (BDNF) may stabilize retinal axon arbors. It is unclear whether these neuromodulators interact. This study showed that NO induced growth cone collapse and retraction of developing retinal axons. This effect was not attributable to NO-induced neurotoxicity. BDNF protected growth cones and axons from the effects of NO. This effect was specific to BDNF, because neither nerve growth factor (NGF) nor neurotrophin-3 (NT-3) prevented NO-induced growth cone collapse and axon retraction. Exposure to both BDNF and NO, but not either factor alone, stabilized growth cones and axons. Stabilized axons exhibited minimal retraction or extension. This response appears to be a new axon "state" and not simply a partial amelioration of the effect of NO, because lower doses of BDNF or NO allowed axon extension. Furthermore, BDNF/NO-induced growth cone stabilization correlated with the appearance of a cytochalasin D-resistant population of actin filaments. BDNF protection from NO likely was mediated locally at the level of the growth cone, because growth cones or individual filopodia in contact with BDNF-coated beads were protected from NO-induced collapse. These findings suggest a cellular mechanism by which some axonal connections are stabilized and some are eliminated during development.

Rapid regulation of brain-derived neurotrophic factor mRNA within eye-specific circuits during ocular dominance column formation

The neurotrophin brain-derived neurotrophic factor (BDNF) has emerged as a candidate retrograde signaling molecule for geniculocortical axons during the formation of ocular dominance columns. Here we examined whether neuronal activity can regulate BDNF mRNA in eye-specific circuits in the developing cat visual system. Dark-rearing throughout the critical period for ocular dominance column formation decreases levels of BDNF mRNA within primary visual cortex, whereas short-term (2 d) binocular blockade of retinal activity with tetrodotoxin (TTX) downregulates BDNF mRNA within the lateral geniculate nucleus (LGN) and visual cortical areas. Brief (6 hr to 2 d) monocular TTX blockade during the critical period and also in adulthood causes downregulation in appropriate eye-specific laminae in the LGN and ocular dominance columns within primary visual cortex. Monocular TTX blockade at postnatal day 23 also downregulates BDNF mRNA in a periodic fashion, consistent with recent observations that ocular dominance columns can be detected at these early ages by physiological methods. In contrast, 10 d monocular TTX during the critical period does not cause a lasting decrease in BDNF mRNA expression in columns pertaining to the treated eye, consistent with the nearly complete shift in physiological response properties of cortical neurons in favor of the unmanipulated eye known to result from long-term monocular deprivation. These observations demonstrate that BDNF mRNA levels can provide an accurate "molecular readout" of the activity levels of cortical neurons and are consistent with a highly local action of BDNF in strengthening and maintaining active synapses during ocular dominance column formation.

Hippocampal stem cells differentiate into excitatory and inhibitory neurons

Stem cell technology promises new and rapid advances in cell therapy and drug discovery. Clearly, the value of this approach will be limited by the differentiated functions displayed by the progeny of stem cells. The foetal and adult central nervous system (CNS) harbour stem cells that can be expanded in vitro and differentiate into immature neurons and glia. Surprisingly, we do not know if neurons derived from stem cells form synapses, a definitive feature of neuronal function. Neuronal differentiation is a complex process and in this paper we establish conditions that permit extensive maturation of neurons in the presence of neurotrophins. These conditions permit the differentiation of rat hippocampal stem cells into both excitatory (glutamatergic) and inhibitory (GABAergic) neurons. The proportion of excitatory and inhibitory synapses was strongly influenced by specific neurotrophins, and these responses reflect the region of origin of the stem cells in the brain. These data show that stem cells can be used to study mechanisms of excitation and inhibition in the nervous system.

Investigation of in vitro toxicity of jet fuels JP-8 and Jet A

The in vitro cytotoxicity and electrophysiological toxicity of Jet Propulsion-8 (JP-8 jet fuel) on four cell types: H4IIE liver cell line, NIH Swiss 3T3 cell line, neuroblastoma x glioma NG108-15 cells, and embryonic hippocampal neurons were investigated. H4IIE cells exposed to Jet A (a commercial fuel) and JP-8 demonstrated identical toxicity with an IC50 of 12.6 +/- 0.4 micrograms/ml for the two fuels. Comparison of H4IIE and NIH/3T3 toxicity to JP-8 revealed that NIH/3T3 cells were more sensitive to JP-8 than H4IIE cells, with an IC50 8.5 +/- 0.1 micrograms/ml. JP-8 exposure for the hippocampal neurons proved to be highly toxic (IC50 of < 2 micrograms/ml), while in contrast, the NG108-15 cells were much less sensitive. Electrophysiological examination of NG108-15 cells showed that administration of JP-8 at 1 microgram/ml did not alter significantly any of the electrophysiological properties. However, exposure to JP-8 at 10 micrograms/ml during a current stimulus of +46 pA decreased the amplitude of the action potential to 83 +/- 7% (n = 4), the rate of rise, dV/dtMAX to 50 +/- 8% (n = 4), and the spiking rate to 25 +/- 11% (n = 4) of the corresponding control levels. These results demonstrate JP-8 induced cytotoxic varies among cell types. The possible mechanisms underlying these observations are presented.

The establishment of GABAergic and glutamatergic synapses on CA1 pyramidal neurons is sequential and correlates with the development of the apical dendrite

We have performed a morphofunctional analysis of CA1 pyramidal neurons at birth to examine the sequence of formation of GABAergic and glutamatergic postsynaptic currents (PSCs) and to determine their relation to the dendritic arborization of pyramidal neurons. We report that at birth pyramidal neurons are heterogeneous. Three stages of development can be identified: (1) the majority of the neurons (80%) have small somata, an anlage of apical dendrite, and neither spontaneous nor evoked PSCs; (2) 10% of the neurons have a small apical dendrite restricted to the stratum radiatum and PSCs mediated only by GABA(A) receptors; and (3) 10% of the neurons have an apical dendrite that reaches the stratum lacunosum moleculare and PSCs mediated both by GABA(A) and glutamate receptors. These three groups of pyramidal neurons can be differentiated by their capacitance (C(m) = 17.9 +/- 0.8; 30.2 +/- 1.6; 43.2 +/- 3.0 pF, respectively). At birth, the synaptic markers synapsin-1 and synaptophysin labeling are present in dendritic layers but not in the stratum pyramidale, suggesting that GABAergic peridendritic synapses are established before perisomatic ones. The present observations demonstrate that GABAergic and glutamatergic synapses are established sequentially with GABAergic synapses being established first most likely on the apical dendrites of the principal neurons. We propose that different sets of conditions are required for the establishment of functional GABA and glutamate synapses, the latter necessitating more developed neurons that have apical dendrites that reach the lacunosum moleculare region.

Target-specific factors regulate the formation of glutamatergic transmitter release sites in cultured neocortical neurons

Synapse formation in the mammalian CNS is thought to involve specific target recognition processes between presynaptic and postsynaptic neurons leading to the establishment of defined neuronal circuits. To study the role of target neuron-specific factors in synaptogenesis, we used cocultures of presynaptic explants and dissociated target neurons from rat neocortex, which enabled us to selectively vary the postsynaptic target neurons. Cocultures containing target neurons that were obtained early during development [embryonic day 16 (E16)] were compared to cocultures containing target neurons that were obtained at a later embryonic stage (E19). Postsynaptic currents (PSCs) were evoked in target neurons by maximal extracellular stimulation in the presynaptic explant. The mean amplitudes of AMPA and NMDA receptor-mediated PSCs were sixfold reduced in E16 target neurons, whereas the mean amplitudes of GABA(A) receptor-mediated PSCs did not differ between E16 and E19 target neurons. This reduction was in part caused by an apparently twofold reduction in mean quantal amplitude, as shown by recording AMPA receptor-mediated miniature PSCs. In addition, a reduced number of glutamatergic release sites in E16 target neurons was revealed by synapsin I immunostaining of dendritic presynaptic terminals. No differences in mean release probability were observed between E16 and E19 target neurons. Thus, the formation of glutamatergic transmitter release sites was strongly influenced by target neuron-specific factors. The formation of functional GABAergic synapses, however, was independent of the type of target neurons, suggesting specific retrograde signaling during the establishment of glutamatergic synapses.

Excitatory synaptogenesis between identified Lymnaea neurons requires extrinsic trophic factors and is mediated by receptor tyrosine kinases

Neurotrophic factors have well established roles in neuronal development and adult synaptic plasticity, but their precise role in synapse formation has yet to be determined. This paper provides the first direct evidence that neurotrophic factors in brain conditioned medium (CM) differentially regulate excitatory and inhibitory synapse formation. Somata of identified presynaptic and postsynaptic neurons were isolated from the CNS of Lymnaea and were cultured in a soma-soma configuration in the presence (CM) or absence [defined medium (DM)] of trophic factors. In DM, excitatory synapses did not form. When they were paired in CM or in DM containing Lymnaea epidermal growth factor (EGF); however, all presynaptic neurons reestablished their specific excitatory synapses, which had electrical properties similar to those seen in vivo. CM-induced formation of excitatory synapses required transcription and de novo protein synthesis, as indicated by the observations that synapse formation was blocked by the protein synthesis inhibitor anisomycin and the protein transcription blocker actinomycin D; the CM factor was inactivated by boiling. They were also blocked by receptor tyrosine kinase inhibitors (lavendustin A, genistein, K252a, and KT5926) but not by inactive analogs (genistin and lavendustin B), suggesting that the effect was mediated by receptor tyrosine kinases. These results, together with our previously published data, demonstrate that trophic factors are required for excitatory, but not inhibitory, synapse formation and extends the role of EGF from cell proliferation, neurite outgrowth, and survival to excitatory synapse formation.

Subcellular localization of full-length and truncated Trk receptor isoforms in polarized neurons and epithelial cells

Neurotrophins affect neuronal development and plasticity via spatially localized effects, yet little is known about the subcellular distribution of the Trk neurotrophin receptors and the impact of this distribution on neurotrophin action. To address this, we examined the subcellular location of full-length TrkB and TrkC tyrosine kinase receptors and truncated TrkB isoforms after transfection of Madin-Darby canine kidney (MDCK) cells, dissociated primary hippocampal neurons, and cortical neurons within intact brain slices. Myc-, herpes virus glycoprotein (HVG)-, or FLAG-derived epitope-tagged receptor isoforms were created to allow their unambiguous identification and localization after transfection. All tagged receptors were appropriately synthesized, and full-length myc-TrkB and myc-TrkC mediated appropriate neurotrophin-signaling events. We found that full-length TrkB receptors were excluded from the apical domain of MDCK cells but that TrkC receptors were present in both apical and basolateral domains. Full-length TrkB and TrkC were found throughout transfected primary cultured hippocampal neurons and transfected neurons in neocortical brain slices and showed no evidence of vectorial sorting. Truncated forms of TrkB were also homogeneously distributed in MDCK cells, dissociated hippocampal neurons, and cortical neurons within slice preparations. Levels of full-length and truncated TrkB were examined in postsynaptic densities; both receptor isoforms were present but only moderately enriched in these structures. Together, these findings suggest that Trk receptors are uniformly distributed in both axonal and dendritic compartments and that local neurotrophin responses are controlled by other mechanisms.

Transgenic brain-derived neurotrophic factor modulates a developing cerebellar inhibitory synapse

Brain-derived neurotrophic factor (BDNF) has been shown to promote synapse formation and maturation in neurons of many brain regions, including inhibitory synapses. In the cerebellum, the Golgi cell-granule cell GABAergic synaptic responses undergo developmental transition from slow-decaying to fast-decaying kinetics, which parallels a developmental increase of GABA(A) receptor alpha6 subunit expression in the cerebellar granule cells. In culture, BDNF accelerates the expression of GABA(A) receptor alpha6 subunit expression in granule cells. Here we examined synaptic GABA(A) response kinetics in BDNF transgenic mice. The mutant mouse, which carries a BDNF transgene driven by a beta-actin promoter, overexpresses BDNF (two- to fivefold increase compared with wild types) in all brain regions. Recordings of the spontaneous GABA(A) responses indicate that the decay time constant of the GABAergic responses decreases during early postnatal development; this transition is accelerated in the BDNF transgenic mouse. The amplitude of the spontaneous GABA(A) responses was also larger in the transgenic mouse than in the wild-type mouse. However, the frequency of the spontaneous GABA(A) responses were not different between the two groups. Our results suggest that BDNF may modulate GABAergic synapse maturation in the cerebellum.

Trophic Factor-Induced Plasticity of Synaptic Connections Between Identified Lymnaea Neurons

Neurotrophic factors participate in both developmental and adult synaptic plasticity; however, the underlying mechanisms remain unknown. Using soma–soma synapses between the identified Lymnaea neurons, we demonstrate that the brain conditioned medium (CM)-derived trophic factors are required for the formation of excitatory but not the inhibitory synapse. Specifically, identified presynaptic [right pedal dorsal 1 (RPeD1) and visceral dorsal 4 (VD4)] and postsynaptic [visceral dorsal 2/3 (VD2/3) and left pedal dorsal 1 (LPeD1)] neurons were soma–soma paired either in the absence or presence of CM. We show that in defined medium (DM—does not contain extrinsic trophic factors), appropriate excitatory synapses failed to develop between RPeD1 and VD2/3. Instead, inappropriate inhibitory synapses formed between VD2/3 and RPeD1. Similarly, mutual inhibitory synapses developed between VD4 and LPeD1 in DM. These inhibitory synapses were termed novel because they do not exist in the intact brain. To test whether DM-induced, inappropriate inhibitory synapses could be corrected by the addition of CM, cells were first paired in DM for an initial period of 12 hr. DM was then replaced with CM, and simultaneous intracellular recordings were made from paired cells after 6–12 hr of CM substitution. Not only did CM induce the formation of appropriate excitatory synapses between both cell pairs, but it also reduced the incidence of inappropriate inhibitory synapse formation. The CM-induced plasticity of synaptic connections involved new protein synthesis and transcription and was mediated via receptor tyrosine kinases. Taken together, our data provide the first direct insight into the cellular mechanism underlying trophic factor-induced specificity and plasticity of synaptic connections between soma–soma paired Lymnaea neurons.

Trophic factor-induced plasticity of synaptic connections between identified Lymnaea neurons

Neurotrophic factors participate in both developmental and adult synaptic plasticity; however, the underlying mechanisms remain unknown. Using soma-soma synapses between the identified Lymnaea neurons, we demonstrate that the brain conditioned medium (CM)-derived trophic factors are required for the formation of excitatory but not the inhibitory synapse. Specifically, identified presynaptic [right pedal dorsal 1 (RPeD1) and visceral dorsal 4 (VD4)] and postsynaptic [visceral dorsal 2/3 (VD2/3) and left pedal dorsal 1 (LPeD1)] neurons were soma-soma paired either in the absence or presence of CM. We show that in defined medium (DM-does not contain extrinsic trophic factors), appropriate excitatory synapses failed to develop between RPeD1 and VD2/3. Instead, inappropriate inhibitory synapses formed between VD2/3 and RPeD1. Similarly, mutual inhibitory synapses developed between VD4 and LPeD1 in DM. These inhibitory synapses were termed novel because they do not exist in the intact brain. To test whether DM-induced, inappropriate inhibitory synapses could be corrected by the addition of CM, cells were first paired in DM for an initial period of 12 hr. DM was then replaced with CM, and simultaneous intracellular recordings were made from paired cells after 6-12 hr of CM substitution. Not only did CM induce the formation of appropriate excitatory synapses between both cell pairs, but it also reduced the incidence of inappropriate inhibitory synapse formation. The CM-induced plasticity of synaptic connections involved new protein synthesis and transcription and was mediated via receptor tyrosine kinases. Taken together, our data provide the first direct insight into the cellular mechanism underlying trophic factor-induced specificity and plasticity of synaptic connections between soma-soma paired Lymnaea neurons.

Transgenic Brain-Derived Neurotrophic Factor Modulates a Developing Cerebellar Inhibitory Synapse

Brain-derived neurotrophic factor (BDNF) has been shown to promote synapse formation and maturation in neurons of many brain regions, including inhibitory synapses. In the cerebellum, the Golgi cell-granule cell GABAergic synaptic responses undergo developmental transition from slow-decaying to fast-decaying kinetics, which parallels a developmental increase of GABAAreceptor α6 subunit expression in the cerebellar granule cells. In culture, BDNF accelerates the expression of GABAA receptor α6 subunit expression in granule cells. Here we examined synaptic GABAA response kinetics in BDNF transgenic mice. The mutant mouse, which carries a BDNF transgene driven by a β-actin promoter, overexpresses BDNF (two- to fivefold increase compared with wild types) in all brain regions. Recordings of the spontaneous GABAA responses indicate that the decay time constant of the GABAergic responses decreases during early postnatal development; this transition is accelerated in the BDNF transgenic mouse. The amplitude of the spontaneous GABAA responses was also larger in the transgenic mouse than in the wild-type mouse. However, the frequency of the spontaneous GABAA responses were not different between the two groups. Our results suggest that BDNF may modulate GABAergic synapse maturation in the cerebellum.

Synaptic Reliability Correlates with Reduced Susceptibility to Synaptic Potentiation by Brain-Derived Neurotrophic Factor

Recent studies have implicated brain-derived neurotrophic factor (BDNF) in use-dependent modification of hippocampal synapses. BDNF can rapidly potentiate synaptic transmission at glutamatergic synapses by enhancing transmitter release. Using simultaneous perforated patch recording from pairs and triplets of glutamatergic hippocampal neurons, we have examined how the initial state of the glutamatergic synapse determines its susceptibility to synaptic modification by BDNF. We found that the degree of synaptic potentiation by BDNF depends on the initial reliability and strength of the synapse: Relatively weak connections were strongly potentiated, whereas the effect was markedly reduced at stronger synapses. The degree of BDNF-induced potentiation strongly correlated with the initial coefficient of variation (CV) of the amplitude of excitatory postsynaptic currents (EPSCs) and inversely correlated with the initial paired–pulse facilitation, suggesting that synapses with lower release probability (Pr) are more susceptible to the action of BDNF. To determine whether saturation of Pr could have masked the potentiation effect of BDNF in the stronger synapses, we lowered the initial Pr either by reducing the extracellular Ca2+ concentration ([Ca2+]o) or by bath application of adenosine. Synapses that were initially strong remained unaffected by BDNF under these conditions of reduced Pr. Thus, the lack of BDNF effect on synaptic efficacy cannot simply be accounted for by saturation of Pr, but rather may be due to intrinsic changes associated with synaptic maturation that might covary with Pr. Finally, the dependence on initial synaptic strength was also found for divergent outputs of the same presynaptic neuron, suggesting that synaptic terminals with different degrees of responsiveness to BDNF can coexist within in the same neuron.

Regulation of Synaptic Function by Neurotrophic Factors in Vertebrates and Invertebrates: Implications for Development and Learning

Recent studies have demonstrated that neurotrophic factors contribute to the molecular events involved in synaptic plasticity, both during vertebrate development and in the mature nervous system. Although it is well established that many of the cellular and molecular mechanisms underlying synaptic plasticity are conserved between invertebrates and vertebrates, there are, as yet, very few neurotrophic factors identified in invertebrate species. Nonetheless, vertebrate neurotrophins can influence invertebrate neuronal growth and plasticity. In addition, homologs of neurotrophic factor receptors have been identified in several invertebrate species. These studies may indicate that the roles of neurotrophins in both developmental and adult plasticity are highly conserved across diverse phyla.

Synaptic reliability correlates with reduced susceptibility to synaptic potentiation by brain-derived neurotrophic factor

Recent studies have implicated brain-derived neurotrophic factor (BDNF) in use-dependent modification of hippocampal synapses. BDNF can rapidly potentiate synaptic transmission at glutamatergic synapses by enhancing transmitter release. Using simultaneous perforated patch recording from pairs and triplets of glutamatergic hippocampal neurons, we have examined how the initial state of the glutamatergic synapse determines its susceptibility to synaptic modification by BDNF. We found that the degree of synaptic potentiation by BDNF depends on the initial reliability and strength of the synapse: Relatively weak connections were strongly potentiated, whereas the effect was markedly reduced at stronger synapses. The degree of BDNF-induced potentiation strongly correlated with the initial coefficient of variation (CV) of the amplitude of excitatory postsynaptic currents (EPSCs) and inversely correlated with the initial paired-pulse facilitation, suggesting that synapses with lower release probability (Pr) are more susceptible to the action of BDNF. To determine whether saturation of Pr could have masked the potentiation effect of BDNF in the stronger synapses, we lowered the initial Pr either by reducing the extracellular Ca2+ concentration ([Ca2+]o) or by bath application of adenosine. Synapses that were initially strong remained unaffected by BDNF under these conditions of reduced Pr. Thus, the lack of BDNF effect on synaptic efficacy cannot simply be accounted for by saturation of Pr, but rather may be due to intrinsic changes associated with synaptic maturation that might covary with Pr. Finally, the dependence on initial synaptic strength was also found for divergent outputs of the same presynaptic neuron, suggesting that synaptic terminals with different degrees of responsiveness to BDNF can coexist within in the same neuron.

Synaptic plasticity in cortical systems

Recent studies indicate that synapse addition and/or loss is associated with different types of learning. Other factors influencing synaptogenesis and synapse loss include neurotrophins, hormones, and the induction of long-term potentiation. An emerging view of synaptic plasticity suggests that local neurotrophin action and synaptically associated protein synthesis may promote synaptic remodelling and changes in receptor expression or activation.