Neuroscience in recession?

As the global financial downturn continues, its impact on neuroscientists - both on an individual level and at the level of their research institute - becomes increasingly apparent. How is the economic crisis affecting neuroscience funding, career prospects, international collaborations and scientists' morale in different parts of the world? Nature Reviews Neuroscience gauged the opinions of a number of leading neuroscientists: the President of the Society for Neuroscience, the President Elect of the British Neuroscience Association, the former President of the Japan Neuroscience Society, the President of the Federation of European Neuroscience Societies and the Director of the US National Institute of Mental Health. Their responses provide interesting and important insights into the regional impact of the global financial downturn, with some causes for optimism for the future of neuroscience research.

Curtailing effect of awakening on visual responses of cortical neurons by cholinergic activation of inhibitory circuits.

Visual responsiveness of cortical neurons changes depending on the brain state. Neural circuit mechanism underlying this change is unclear. By applying the method of in vivo two-photon functional calcium imaging to transgenic rats in which GABAergic neurons express fluorescent protein, we analyzed changes in visual response properties of cortical neurons when animals became awakened from anesthesia. In the awake state, the magnitude and reliability of visual responses of GABAergic neurons increased whereas the decay of responses of excitatory neurons became faster. To test whether the basal forebrain (BF) cholinergic projection is involved in these changes, we analyzed effects of electrical and optogenetic activation of BF on visual responses of mouse cortical neurons with in vivo imaging and whole-cell recordings. Electrical BF stimulation in anesthetized animals induced the same direction of changes in visual responses of both groups of neurons as awakening. Optogenetic activation increased the frequency of visually evoked action potentials in GABAergic neurons but induced the delayed hyperpolarization that ceased the late generation of action potentials in excitatory neurons. Pharmacological analysis in slice preparations revealed that photoactivation-induced depolarization of layer 1 GABAergic neurons was blocked by a nicotinic receptor antagonist, whereas non-fast-spiking layer 2/3 GABAergic neurons was blocked only by the application of both nicotinic and muscarinic receptor antagonists. These results suggest that the effect of awakening is mediated mainly through nicotinic activation of layer 1 GABAergic neurons and mixed nicotinic/muscarinic activation of layer 2/3 non-fast-spiking GABAergic neurons, which together curtails the visual responses of excitatory neurons.

Reciprocal Homosynaptic and heterosynaptic long-term plasticity of corticogeniculate projection neurons in layer VI of the mouse visual cortex.

Most neurons in layer VI of the visual cortex project to the dorsal lateral geniculate nucleus (dLGN). These corticogeniculate projection neurons (CG cells) receive top-down synaptic inputs from upper layers (ULs) and bottom-up inputs from the underlying white matter (WM). Use-dependent plasticity of these synapses in layer VI of the cortex has received less attention than in other layers. In the present study, we used a retrograde tracer injected into dLGN to identify CG cells, and, by analyzing EPSPs evoked by electrical stimulation of the UL or WM site, examined whether these synapses show long-term synaptic plasticity. Theta-burst stimulation induced long-term potentiation (LTP) of activated synapses (hom-LTP) and long-term depression (LTD) of nonactivated synapses (het-LTD) in either pathway. The paired-pulse stimulation protocol and the analysis of coefficient variation of EPSPs suggested postsynaptic induction of these changes except UL-induced het-LTD, which may be presynaptic in origin. Intracellular injection of a Ca(2+)-chelator suggested an involvement of postsynaptic Ca(2+) rise in all types of long-term plasticity. Pharmacological analysis indicated that NMDA receptors and type-5 metabotropic glutamate receptors are involved in WM-induced and UL-induced plasticity, respectively. Analysis with inhibitors and/or in transgenic mice suggested an involvement of cannabinoid type 1 receptors and calcineurin in UL-induced and WM-induced het-LTD, respectively. These results suggest that hom-LTP and het-LTD may play a role in switching the top-down or bottom-up regulation of CG cell function and/or in maintaining stability of synaptic transmission efficacy through different molecular mechanisms.

NSF workshop report: discovering general principles of nervous system organization by comparing brain maps across species.

Efforts to understand nervous system structure and function have received new impetus from the federal Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative. Comparative analyses can contribute to this effort by leading to the discovery of general principles of neural circuit design, information processing, and gene-structure-function relationships that are not apparent from studies on single species. We here propose to extend the comparative approach to nervous system 'maps' comprising molecular, anatomical, and physiological data. This research will identify which neural features are likely to generalize across species, and which are unlikely to be broadly conserved. It will also suggest causal relationships between genes, development, adult anatomy, physiology, and, ultimately, behavior. These causal hypotheses can then be tested experimentally. Finally, insights from comparative research can inspire and guide technological development. To promote this research agenda, we recommend that teams of investigators coalesce around specific research questions and select a set of 'reference species' to anchor their comparative analyses. These reference species should be chosen not just for practical advantages, but also with regard for their phylogenetic position, behavioral repertoire, well-annotated genome, or other strategic reasons. We envision that the nervous systems of these reference species will be mapped in more detail than those of other species. The collected data may range from the molecular to the behavioral, depending on the research question. To integrate across levels of analysis and across species, standards for data collection, annotation, archiving, and distribution must be developed and respected. To that end, it will help to form networks or consortia of researchers and centers for science, technology, and education that focus on organized data collection, distribution, and training. These activities could be supported, at least in part, through existing mechanisms at NSF, NIH, and other agencies. It will also be important to develop new integrated software and database systems for cross-species data analyses. Multidisciplinary efforts to develop such analytical tools should be supported financially. Finally, training opportunities should be created to stimulate multidisciplinary, integrative research into brain structure, function, and evolution.

Cell type-specific, presynaptic LTP of inhibitory synapses on fast-spiking GABAergic neurons in the mouse visual cortex.

Properties and plasticity of inhibitory synapses on fast-spiking (FS) GABAergic (FS-GABA) interneurons in layer II/III of the mouse visual cortex were examined in cortical slices by whole-cell recordings of IPSCs or IPSPs evoked by activation of presynaptic FS or non-FS GABAergic interneurons. Unitary IPSCs (uIPSCs) evoked by action potentials of FS-GABA neurons have shorter onset latency, faster rising slope, higher peak amplitude, and faster decay time than those evoked by action potentials of non-FS-GABA neurons. Tetanic activation of presynaptic FS-GABA neurons induced long-term potentiation (LTP) of uIPSCs, whereas that of presynaptic non-FS-GABA neurons did not induce LTP, indicating that long-term plasticity of inhibitory synapses on FS-GABA neurons is pathway specific. For further analysis of inhibitory synaptic plasticity, IPSPs evoked by electrical stimulation of an adjacent site in the cortex were recorded from FS-GABA neurons. Theta burst stimulation induced LTP of IPSPs in 12 of 14 FS-GABA neurons. The paired-pulse stimulation protocol and coefficient of variation analysis indicated that this form of LTP may be presynaptic in origin. Filling postsynaptic cells with a Ca(2+) chelator did not block the induction of LTP, suggesting no involvement of postsynaptic Ca(2+) rise. Also, this form of LTP was dependent neither on metabotropic glutamate receptors nor voltage-gated Ca(2+) channels of the L and T types. Further pharmacological analysis indicated that voltage-gated Ca(2+) channels other than the P/Q type, such as N and R types, were not involved in LTP, suggesting that P/Q-type channels are a candidate for factors inducing LTP of inhibitory synapses between FS-GABA neurons.

Laminar-specific maturation of GABAergic transmission and susceptibility to visual deprivation are related to endocannabinoid sensitivity in mouse visual cortex.

The developmental period when neuronal responses are modified by visual experience is reported to start and end earlier in layer 4 than in layer 2/3 of the visual cortex, and the maturation of GABAergic inhibitory circuits is suggested to determine the timing of this period. Here, we examine whether the laminar difference in such timing corresponds to a difference in the time course of the functional maturation of GABAergic synaptic transmission to star pyramidal and pyramidal cells in layers 4 and 2/3, respectively, of the mouse visual cortex and whether the development of the strength of GABAergic transmission is affected by visual deprivation in a laminar-specific manner. Our analysis of developmental changes in inhibitory postsynaptic currents of star pyramidal and pyramidal cells evoked by electrical stimulation of afferents or action potentials of fast-spiking GABAergic neurons revealed that there was a sequential maturation of GABAergic function from layers 4 to 2/3. The maturation of inhibition in layer 4 occurred at postnatal week 3, which preceded by 1 week that of layer 2/3. Visual deprivation by dark rearing arrested the functional development of GABAergic transmission in layer 2/3, whereas dark rearing was not so effective in layer 4. GABAergic synapses in layer 2/3 were sensitive to an agonist for cannabinoid type 1 receptors and not normally matured in receptor knock-out mice, whereas those in layer 4 were not so. These results suggest laminar-specific maturation of inhibition and susceptibility to visual deprivation, which may be related to the laminar difference in sensitivity to endocannabinoids.

Difference in binocularity and ocular dominance plasticity between GABAergic and excitatory cortical neurons.

Neuronal circuits in the cerebral cortex consist mainly of glutamatergic/excitatory and GABAergic/inhibitory neurons. In the visual cortex, the binocular responsiveness of neurons is modified by monocular visual deprivation during the critical period of postnatal development. Although GABAergic neurons are considered to play a key role in the expression of the critical period, it is not known whether their binocular responsiveness and ocular dominance plasticity are different from those of excitatory neurons. Recently, the end of the critical period was found to be not strict so that cortical neurons in the adult still have some ocular dominance plasticity. It is not known, however, which type of neurons or both maintain such plasticity in adulthood. To address these issues, we applied in vivo two-photon functional Ca(2+) imaging to transgenic mice whose GABAergic neurons express a yellow fluorescent protein called Venus. We found that GABAergic neurons are more binocular than excitatory neurons in the normal visual cortex, and both types of neurons show the same degree of modifiability to monocular visual deprivation during the critical period, but the modifiability of GABAergic neurons is stronger than that of excitatory neurons after the end of the critical period.

Efhc1 deficiency causes spontaneous myoclonus and increased seizure susceptibility.

Mutations in EFHC1 gene have been previously reported in patients with epilepsies, including those with juvenile myoclonic epilepsy. Myoclonin1, also known as mRib72-1, is encoded by the mouse Efhc1 gene. Myoclonin1 is dominantly expressed in embryonic choroid plexus, post-natal ependymal cilia, tracheal cilia and sperm flagella. In this study, we generated viable Efhc1-deficient mice. Most of the mice were normal in outward appearance, and both sexes were found to be fertile. However, the ventricles of the brains were significantly enlarged in the null mutants, but not in the heterozygotes. Although the ciliary structure was found intact, the ciliary beating frequency was significantly reduced in null mutants. In adult stages, both the heterozygous and null mutants developed frequent spontaneous myoclonus. Furthermore, the threshold of seizures induced by pentylenetetrazol was significantly reduced in both heterozygous and null mutants. These observations seem to further suggest that decrease or loss of function of myoclonin1 may be the molecular basis for epilepsies caused by EFHC1 mutations.

Antibody to varicella-zoster virus immediate-early protein 62 augments allodynia in zoster via brain-derived neurotrophic factor.

Varicella-zoster virus (VZV) expresses immediate-early protein 62 (IE62), and zoster is associated with neuropathic pain. Brain-derived neurotrophic factor (BDNF) is involved in the neuronal mechanism underlying pain hypersensitivity. Zoster is associated with prodrome and the robust production of booster antibody to VZV. We hypothesized that the intrathecal production of antibody to IE62 cross-reacting with BDNF and the nerve injury by skin lesions may augment allodynia in zoster by enhancing BDNF activity. One of three monoclonal antibodies against the 268-556 peptide of IE62 recognized BDNF. Immunological cross-reactivity between IE62 and BDNF and the effects of anti-IE62 monoclonal antibody (anti-IE62 MAb) cross-reactivity with BDNF on BDNF activity in cultured neurons were examined. Anti-IE62 MAb and anti-BDNF MAbs recognized the 414-429 peptide of IE62 and the BDNF dimer. Anti-IE62 MAb significantly augmented BDNF-related transcription in neurons and the morphological development of spinal dorsal horn neurons. Sera from patients recognized IE62 and BDNF and enhanced BDNF activity in neurons. The effect of anti-IE62 antibody on mechanical allodynia was characterized by the threshold of allodynia using von Frey filaments in a spinal nerve injury (SNI) in mice. The administration of anti-IE62 MAb to or immunization with cross-reacting IE62 protein to mice significantly enhanced mechanical allodynia on the side with SNI but not on the uninjured side. Anti-IE62 antibody augmented BDNF activity in neurons and allodynia in mice with SNI. The intrathecal production of anti-IE62 antibody augmenting BDNF activity and peripheral nerve injury by zoster may participate in the pathogenesis of allodynia in zoster.

Regulation of excitability and plasticity by endocannabinoids and PKA in developing hippocampus.

The activity-dependent strengthening and weakening of synaptic transmission are hypothesized to be the basis of not only memory and learning but also the refinement of neural circuits during development. Here we report that, in the developing CA1 area of the hippocampus, endocannabinoid (eCB)-mediated heterosynaptic long-term depression (LTD) of glutamatergic excitatory synaptic transmission is associated with PKA-mediated homosynaptic long-term potentiation (LTP). This form of LTD was dominant at postnatal days 2-10 (P2-P10), attenuated during development, and finally disappeared in the mature hippocampus. Heterosynaptic LTD of excitatory postsynaptic currents in the developing hippocampus was expressed presynaptically, spread to neighboring neurons, and was mediated by eCBs. Heterosynaptic LTD of field excitatory postsynaptic potentials was associated with a decrease in fiber volley amplitude with a similar time course. Depression of fiber volleys was blocked by K(+) channel blockers, suggesting the involvement of the decrease in presynaptic excitability in heterosynaptic LTD. In the P2-P5 hippocampus, eCBs also attenuate LTP and fiber volleys in homosynaptic pathways and help to prevent too much excitability in the neonatal hippocampus where the GABAergic system is poorly developed and even excitatory. In the hippocampus older than P6 (P > 6), however, LTP is protected from eCB-mediated depression by PKA activated at presynaptic sites by high-frequency stimulation, serving to highlight PKA-mediated LTP by weakening inactive synapses even in adjacent cells. Thus, eCBs and PKA make synapses plastic without changing excitability homeostasis in the developing hippocampus.

Roles of endocannabinoids in heterosynaptic long-term depression of excitatory synaptic transmission in visual cortex of young mice.

Tetanic stimulation of one of two afferent pathways converging to neurons in the visual cortex induces long-term depression (LTD) of synaptic transmission in the other, nonactivated pathway under a certain condition. This form of synaptic plasticity called heterosynaptic LTD (hetero-LTD) was not systematically investigated in previous studies, whereas homosynaptic LTD has been extensively studied. To determine whether hetero-LTD is induced in visual cortical slices of mice and, if so, through what mechanisms, we recorded EPSPs evoked in layer II/III neurons by alternating test stimulation of two sites in layer IV at 0.05 Hz. After theta-burst stimulation of one site, EPSPs evoked by test stimulation of the other site were depressed for a long time in most of the neurons, whereas homosynaptic long-term potentiation was induced at activated synapses. Such a hetero-LTD was induced in most mice at postnatal day 7-20 (P7-P20), but not induced in mice at P35-P41. Tests using the paired-pulse stimulation protocol and coefficient of variation analysis suggested that hetero-LTD was expressed at presynaptic sites. Pharmacological analysis indicated that this form of LTD was induced through activation of the type 5 of metabotropic glutamate receptors, not through the NMDA type of glutamate receptors. Additional analysis using a cannabinoid type 1 receptor agonist and an antagonist suggested that endocannabinoids (eCBs) are involved in this type of LTD. Moreover, results suggest that brain-derived neurotrophic factor, which may be released from strongly activated presynaptic sites, prevents eCBs from suppressing the release of transmitters from these sites.

Metabotropic glutamate receptor type 5-dependent long-term potentiation of excitatory synapses on fast-spiking GABAergic neurons in mouse visual cortex.

Long-term potentiation (LTP) of excitatory synapses on GABAergic neurons in layer II/III of visual cortical slices was examined in GAD67-GFP knock-in mice by whole-cell recordings of EPSPs evoked by layer IV stimulation. Theta burst stimulation (TBS) paired with postsynaptic depolarization induced LTP in 14 of 19 fast-spiking GABAergic (FS-GABA) neurons, whereas only in 6 of 17 non-FS GABAergic neurons. The mean magnitude of LTP in the former cell group was larger than that in the latter. The paired-pulse stimulation protocol and coefficient of variation analysis indicated that LTP of excitatory synapses on FS-GABA neurons may be postsynaptic in origin. Filling postsynaptic cells with a Ca2+-chelator blocked the induction of LTP, suggesting an involvement of postsynaptic Ca2+ rise. The developmental analysis of LTP indicated that almost the same magnitude of LTP was induced after postnatal day 17 to the young adulthood, suggesting no age dependence after eye opening. This form of LTP was dependent neither on NMDA receptors nor voltage-gated Ca2+ channels (L and T types). An antagonist for type 5 metabotropic glutamate receptors (mGluR5) blocked this form of LTP, whereas an antagonist for mGluR1 was not effective. An agonist for mGluR1/5 induced potentiation of EPSPs of FS-GABA neurons in concentration- and use-dependent manners. This potentiation and TBS-induced LTP occluded each other. Further pharmacological analyses suggested that this form of LTP at FS-GABA neurons is induced through an activation of mGluR5, which triggers Ca2+ release from internal stores via activations of phospholipase C and inositol triphosphate.

GABAergic neurons are less selective to stimulus orientation than excitatory neurons in layer II/III of visual cortex, as revealed by in vivo functional Ca2+ imaging in transgenic mice.

Most neurons in the visual cortex are selectively responsive to visual stimulation of a narrow range of orientations, and GABAergic neurons are considered to play a role in the formation of such orientation selectivity. This suggests that response properties of GABAergic neurons may be different from those of excitatory neurons. This view remains unproved, however. To address this issue, we applied in vivo two-photon functional Ca2+ imaging to transgenic mice, in which GABAergic neurons express enhanced green fluorescent protein. Astroglia were stained by an astrocyte-specific dye. The three types of cells, GABAergic neurons, excitatory neurons, and astrocytes, in layer II/III of the visual cortex were differentially identified by using different wavelengths of excitation light and a dichroic mirror for emitted fluorescence, and their responses to moving visual stimuli at different orientations were measured with changes in the intensity of fluorescence of a Ca2+-sensitive dye. We found that almost all GABAergic neurons have orientation-insensitive responses, whereas most of excitatory neurons have orientation-selective responses.

A local reduction in cortical GABAergic synapses after a loss of endogenous brain-derived neurotrophic factor, as revealed by single-cell gene knock-out method.

To address questions of whether brain-derived neurotrophic factor (BDNF) released from active excitatory neurons acts locally only on GABAergic presynaptic terminals contacting these neurons or generally also on GABAergic terminals contacting other inactive neurons, we developed a single-cell gene knock-out method in organotypic slice culture of visual cortex of floxed BDNF transgenic mice. A biolistic transfection of Cre recombinase with green fluorescence protein (GFP) plasmids to layer II/III of the cortex resulted in loss of BDNF in a single neuron or a small number of neurons, which expressed GFP at 13-14 d in vitro. Analysis with in situ hybridization and immunohistochemistry confirmed that neurons expressing GFP lacked BDNF mRNA and protein, respectively. Analysis with immunohistochemistry using antibody against GABA synthesizing enzyme showed that the number of GABAergic terminals on the soma of BDNF knock-out neurons was smaller than that of neighboring control neurons. Morphological analysis indicated that there was no significant difference in the soma size and branch points and length of dendrites between the BDNF knock-out and control neurons. Recordings of miniature IPSCs (mIPSCs) showed that the frequency of mIPSCs of BDNF knock-out neurons was lower than that of control neurons, although the amplitude was not significantly different, suggesting the smaller number of functional GABAergic synapses on whole the BDNF knock-out neuron. The present results suggest that BDNF released from postsynaptic target neurons promotes the formation or proliferation of GABAergic synapses through its local actions in layer II/III of visual cortex.

Bidirectional trafficking of prostaglandin E2 receptors involved in long-term potentiation in visual cortex.

Although prostaglandin E2 (PGE2) has a broad spectrum of biological activities that have been confirmed by previous studies, the roles of PGE2 in synaptic plasticity such as long-term potentiation (LTP) in the CNS have yet to be characterized in detail. The present results of electrophysiological and biochemical studies indicated that PGE2 is actually produced in acute visual cortex slices in response to theta-burst stimulation (TBS) and is involved postsynaptically in TBS-induced LTP. RNA interference (RNAi) for PGE2 receptor subtypes EP2 and EP3, which are known to upregulate and downregulate the level of cAMP, respectively, induced significant decreases and increases of LTP, respectively. Moreover, analysis of the localization of receptor subtypes at the membrane surface or cytosol showed that stimuli such as TBS regulate the trafficking of EP2 and EP3 between the membrane and cytosol of the postsynapses, rising up to and leaving the membrane, respectively, resulting in increased and decreased expression of EP2 and EP3 at the membrane, respectively. Increased activation of EP2 and decreased activation of EP3 by PGE2 synergistically induce an increase in cAMP level, which may induce LTP. This causes activation of CREB (cAMP response element-binding protein) in the postsynaptic cells, which may be involved in the maintenance of LTP. These observations indicate that in TBS-induced LTP of the visual cortex, PGE2 is released from the postsynaptic cells and then activates PGE2 receptors at the postsynaptic membranes, which is regulated by trafficking of the differential PGE2 receptor subtypes in an activity-dependent bidirectional manner.

Parvalbumin-expressing interneurons can act solo while somatostatin-expressing interneurons act in chorus in most cases on cortical pyramidal cells.

Neural circuits in the cerebral cortex consist primarily of excitatory pyramidal (Pyr) cells and inhibitory interneurons. Interneurons are divided into several subtypes, in which the two major groups are those expressing parvalbumin (PV) or somatostatin (SOM). These subtypes of interneurons are reported to play distinct roles in tuning and/or gain of visual response of pyramidal cells in the visual cortex. It remains unclear whether there is any quantitative and functional difference between the PV → Pyr and SOM → Pyr connections. We compared unitary inhibitory postsynaptic currents (uIPSCs) evoked by electrophysiological activation of single presynaptic interneurons with population IPSCs evoked by photo-activation of a mass of interneurons in vivo and in vitro in transgenic mice in which PV or SOM neurons expressed channelrhodopsin-2, and found that at least about 14 PV neurons made strong connections with a postsynaptic Pyr cell while a much larger number of SOM neurons made weak connections. Activation or suppression of single PV neurons modified visual responses of postsynaptic Pyr cells in 6 of 7 pairs whereas that of single SOM neurons showed no significant modification in 8 of 11 pairs, suggesting that PV neurons can act solo whereas most of SOM neurons may act in chorus on Pyr cells.

NMDA receptor-dependent synaptic translocation of insulin receptor substrate p53 via protein kinase C signaling.

The activity-dependent remodeling of postsynaptic structure is a fundamental process underlying learning and memory. Insulin receptor substrate p53 (IRSp53), a key player in cytoskeletal dynamics, is enriched in the postsynaptic density (PSD) fraction, but its significance in synaptic functions remains unclear. We report here that IRSp53 is accumulated rapidly at the postsynaptic sites of cultured hippocampal neurons after glutamate or NMDA stimulation in an actin cytoskeleton-dependent manner. Pharmacological profiles showed that a PKC inhibitor, but not other kinase inhibitors, specifically suppressed the synaptic translocation of IRSp53 in response to NMDA, and the selective activation of PKC with phorbol ester markedly induced the synaptic translocation. Reverse transcriptase-PCR and Western blotting showed that IRSp53-S is the major isoform expressed in cultured hippocampal neurons. The synaptic targeting of IRSp53-S was found to be mediated through N-terminal coiled-coil domain and the PDZ (PSD-95/Discs large/zona occludens-1)-binding sequence at its C-terminal end and regulated by the PKC phosphorylation of its N terminus. In electrophysiological experiments, overexpression of IRSp53-S wild type and IRSp53-S mutant that is spontaneously accumulated at the postsynaptic sites enhanced the postsynaptic function as detected by an increased miniature EPSC amplitude. These data suggest that IRSp53 is involved in NMDA receptor-linked synaptic plasticity via PKC signaling.

Long-term depression is not induced by low-frequency stimulation in rat visual cortex in vivo: a possible preventing role of endogenous brain-derived neurotrophic factor.

Low-frequency stimulation (LFS) at 1 Hz for 15 min is an effective protocol to induce homosynaptic long-term depression (LTD) in visual cortical slices. It is reported that LFS becomes ineffective when brain-derived neurotrophic factor (BDNF) is applied to slices. It is not known, however, whether such a protocol induces LTD in visual cortex in vivo, and whether endogenous BDNF has the same or similar action. To address these questions, we recorded field potentials of rat visual cortex evoked by stimulation of lateral geniculate nucleus, white matter, or cortical layer IV. We found that LFS did not induce LTD of cortical responses in vivo. To test the possibility that spontaneous activity from retinas would interfere with the induction of LTD, both eyes were removed or inactivated by tetrodotoxin. LTD was not induced in these conditions either. To test whether the difference in temperature between the two preparations is a factor for the discrepancy, the temperature of slices was increased from 31 to 37 degrees C. LTD was induced in slices at either temperature. Then, we hypothesized that endogenous BNDF and its receptors, TrkB, prevent the induction of LTD. To test this, we infused the cortex with an inhibitor of Trk receptor tyrosine kinases, anti-TrkB IgG1, anti-BDNF, and anti-neurotrophin 4/5 antibodies. LTD was induced when the BDNF-TrkB system was blocked. In slices, the level of phosphorylation of Trks was found to decrease with time. These results indicate that activation of TrkB signal pathway prevents LFS from inducing synaptic depression in visual cortex in vivo.

Inhibitory but not excitatory cortical neurons require presynaptic brain-derived neurotrophic factor for dendritic development, as revealed by chimera cell culture.

To address questions of whether endogenous BDNF acts differentially on inhibitory and excitatory neurons, and through what routes, we used chimera culture of cerebral cortical neurons derived from BDNF-/- mice and another type of transgenic mice that express green fluorescence protein and BDNF. Presynaptic BDNF transferred to both types of neurons, GABA-synthesizing enzyme-positive and -negative neurons. The latter neurons were confirmed to be glutamatergic with immunocytochemistry. Dendritic development of the former inhibitory neurons was promoted by endogenous BDNF transferred from presynaptic, excitatory neurons. In contrast, dendritic development of excitatory neurons was not related to the presence or absence of presynaptic BDNF, suggesting that BDNF acts on inhibitory neurons through an anterograde, transsynaptic route so as to promote dendritic development, whereas this is not the case in excitatory neurons.

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) microm/s, was slower than that of the moving puncta in axons (0.73 +/- 0.26 microm/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.