Fumitaka Kimura

Change of conduction velocity by regional myelination yields constant latency irrespective of distance between thalamus and cortex

The widely spanning sensory cortex receives inputs from the disproportionately smaller nucleus of the thalamus, which results in a wide variety of travelling distance among thalamic afferents. Yet, latency from the thalamus to a cortical cell is remarkably constant across the cortex (typically, ≈2 ms). Here, we found a mechanism that produces invariability of latency among thalamocortical afferents, irrespective of the variability of travelling distances. The conduction velocity (CV) was calculated from excitatory postsynaptic currents recorded from layer IV cells in mouse thalamocortical slices by stimulating the ventrobasal nucleus of the thalamus (VB) and white matter (WM). In adults, the obtained CV for VB to WM (CVVB-WM; 3.28 ± 0.11 m/s) was ≈10 times faster than that of WM to layer IV cells (CVWM-IV; 0.33 ± 0.05 m/s). The CVVB-WM was confirmed by recording antidromic single-unit responses from VB cells by stimulating WM. Exclusion of synaptic delay from CVWM-IV did not account for the 10-fold difference of CV. By histochemical staining, it was revealed that VB to WM was heavily myelinated, whereas in the cortex staining became substantially weaker. We also found that such morphological and physiological characteristics developed in parallel and were accomplished around postnatal week 4. Considering that VB to WM is longer and more variable in length among afferents than is the intracortical region, such an enormous difference of CV makes conduction time heavily dependent on the length of intracortical region, which is relatively constant. Our finding may well provide a general strategy of connecting multiple sites irrespective of distances in the brain.

Long-term potentiation and N-methyl-D-aspartate receptors in the visual cortex of young rats.

1. Long‐term potentiation (LTP) of synaptic transmission following tetanic stimulation of the white matter was studied by recording extracellular field potentials and intracellular synaptic potentials from layer II/III of visual cortical slices from young rats ranging in age from 21 to 40 days. 2. Single shocks applied to the white matter at 0.1 Hz, used as test stimuli, elicited field potentials that consisted of primary and secondary components. The removal of Ca2+ ions from the perfusate allowed identification of the secondary component as originating postsynaptically and the primary one as reflecting a mixture of antidromic and postsynaptic potentials. 3. Tetanic stimulation at 5 Hz for 60 s was delivered to the white matter and field potentials were observed for 20 min to 9 h after the tetanus. LTP was defined as being present when the response displayed more than a 20% increase in amplitude of the Ca2+‐sensitive components 20 min after the tetanus. LTP was induced in twelve of twenty‐three slices tested, and this potentiation lasted throughout the period of observation. The average magnitude of potentiation was 147.8 +/‐ 28.4% of the control value for the twelve slices. 4. Administration of D,L‐2‐amino‐5‐phosphonovalerate (APV), an antagonist selective for N‐methyl‐D‐aspartate (NMDA)‐preferring receptors, slightly reduced the amplitudes of Ca2+‐sensitive components of the field potentials. The average magnitude of reduction was 80.2 +/‐ 15.3% of the pre‐drug control values. In the presence of APV, LTP was induced in only one slice of twelve tested. 5. Stable intracellular recordings were obtained from twenty‐three cells from layer II/III. Excitatory postsynaptic potentials (EPSPs) evoked by white matter stimulation had mean onset and peak latencies of 4.1 and 11.3 ms, respectively. In some cells these fast EPSPs were followed by another slow EPSP with a much longer latency and higher amplitude. Administration of APV revealed further that the fast EPSPs consisted of two components, i.e. early and late components. 6. Tetanization of the white matter induced long‐lasting enhancement of EPSPs in eight of twelve cells tested. In five of these eight cells, fast EPSPs were enhanced in amplitude and in the remaining three cells, slow EPSPs appeared de novo after the tetanus. 7. APV reduced the amplitudes of the fast EPSPs and abolished the slow EPSPs if present. The average magnitude of reduction for the fast EPSPs was 65.6 +/‐ 15.1% and this reduction was due mainly to an elimination of the late component.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine suppresses the spread of excitation in the visual cortex revealed by optical recording: possible differential effect depending on the source of input.

Optical recording with a voltage‐sensitive dye was performed in visual cortical slices of the rat to determine the effect of acetylcholine (ACh) on the spread of excitation. In the presence of ACh, the spread of excitation initiated by stimulation at the white matter/layer VI (WM/VI) was greatly suppressed throughout the cortex, with less suppression in the middle layers. By comparing the effect of ACh with that of 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), the fraction of the synaptic component that was sensitive to ACh was evaluated. ACh suppressed ∼ 40–50% (maximum 55.8%, n = 11) of the initial synaptic component in the superficial and deep layers. In the middle, however, the effect was weakest and only ∼ 20–30% (minimum 20.9%, n = 11) of the initial synaptic component was suppressed. On the basis of histological analysis, the region with the weakest ACh effect extended from upper V to lower II/III. To identify the site of ACh action in terms of pre‐ versus postsynaptic localization, exogenous glutamate was applied. Because ACh did not suppress the excitation induced by glutamate, the site of the ACh action was indicated to be presynaptic. When layer II/III was stimulated instead of WM/VI, the suppression was uniform throughout the cortex. A muscarinic receptor antagonist, atropine, blocked the suppression by ACh. In conclusion, our results indicate the following two points. First, ACh strongly suppresses intracortical connectivity through presynaptic muscarinic receptors. Secondly, in contrast to the intracortical connection, some group(s) of fibres, possibly thalamocortical afferents that arise from white matter and terminate in the middle cortical layers are suppressed much less by ACh. While ACh has been reported to have confusingly diverse effects, e.g. direct depolarization and hyperpolarization as well as synaptic facilitation and suppression, its effect on the propagation of excitation is very clear; suppression on intracortical connection, leaving thalamocortical inputs rather intact. We postulate that cholinergic innervation enables the afferent input to have a relatively dominant effect in the cortex.

Cholinergic modulation of cortical function: A hypothetical role in shifting the dynamics in cortical network

Wide innervation of cholinergic projections throughout the cortex implies that acetylcholine (ACh) plays an essential role in information processing, but how it works is still enigmatic. Experimental as well as theoretical work in the olfactory cortex and hippocampus suggests that ACh, via the muscarinic receptors, serves to shift the dynamics of the cortical networks into a state where afferent influence predominates over intracortical influence. Recent experiments in the visual and somatosensory cortex suggested that this hypothesis could be extended to neocortex. In addition, participation of the nicotinic receptors in regulating the synaptic response in the somatosensory cortex further substantiates this hypothesis. This hypothesis, derived mainly from in vitro work, also seemed to account for results from in vivo experiments without any obvious inconsistencies.

Brain-derived neurotrophic factor-dependent unmasking of “silent” synapses in the developing mouse barrel cortex

Brain-derived neurotrophic factor (BDNF) is a critical modulator of central synaptic functions such as long-term potentiation in the hippocampal and visual cortex. Little is known, however, about its role in the development of excitatory glutamatergic synapses in vivo. We investigated the development of N-methyl-D-aspartate (NMDA) receptor (NMDAR)-only synapses (silent synapses) and found that silent synapses were prominent in acute thalamocortical brain slices from BDNF knockout mice even after the critical period. These synapses could be partially converted to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-containing ones by adding back BDNF alone to the slice or fully converted to together with electric stimulation without affecting NMDAR transmission. Electric stimulation alone was ineffective under the BDNF knockout background. Postsynaptically applied TrkB kinase inhibitor or calcium-chelating reagent blocked this conversion. Furthermore, the AMPAR C-terminal peptides essential for interaction with PDZ proteins postsynaptically prevented the unmasking of silent synapses. These results suggest that endogenous BDNF and neuronal activity synergistically activate AMPAR trafficking into synaptic sites.

Brain-Derived Neurotrophic Factor Regulates the Maturation of Layer 4 Fast-Spiking Cells after the Second Postnatal Week in the Developing Barrel Cortex

Brain-derived neurotrophic factor (BDNF) has been reported to play a critical role in modulating plasticity in developing sensory cortices. In the visual cortex, maturation of neuronal circuits involving GABAergic neurons has been shown to trigger a critical period. To date, several classes of GABAergic neurons are known, each of which are thought to play distinct functions. Of these, parvalbumin (PV)-containing, fast-spiking (FS) cells are suggested to be involved in the initiation of the critical period. Here, we report that BDNF plays an essential role in the normal development of PV–FS cells during a plastic period in the barrel cortex. We found that characteristic electrophysiological properties of PV–FS cells, such as low spike adaptation ratio, reduced voltage sags in response to hyperpolarization, started to develop around the second postnatal week and attained adult level in several days. We also found that immunoreactivity against PV was also acquired after the similar developmental time course. Then, using BDNF(−/−) mice, we found that these electrophysiological as well as chemical properties were underdeveloped or did not appear at all. We conclude BDNF regulates the development of electrophysiological and immunohistochemical characteristics in PV–FS cells. Because BDNF is suggested to regulate the initiation of plasticity, our results strongly indicate that BDNF is involved in the regulation of the critical period by promoting the functional development of PV–FS GABAergic neurons.

Long-term depression but not potentiation is induced in Ca(2+)-chelated visual cortex neurons.

An entry of Ca2+ into postsynaptic sites may play a role in the induction of long-term potentiation (LTP) of synaptic transmission in the visual cortex. To test this hypothesis, a Ca(2+)-chelator was injected into layer II/III neurons of sliced visual cortex obtained from young rats, and excitatory postsynaptic potentials (EPSPs) of these cells to test stimulation of the white matter were observed before and after tetanic stimulation of the same site. To confirm the effectiveness of the tetanus, field potentials reflecting the activities of many cells were recorded with another extracellular electrode. The chelator injection led to long-term depression (LTD) of EPSPs following tetanic stimuli which simultaneously induced LTP of field potentials derived from unchelated cells in most of the slices tested. This suggests that a low concentration of post-synaptic, free Ca2+, when associated with tetanic inputs, may lead to LTD while a rise of Ca2+ may lead to LTP.

Ephrin-A5 and EphA5 Interaction Induces Synaptogenesis during Early Hippocampal Development

Background Synaptogenesis is a fundamental step in neuronal development. For spiny glutamatergic synapses in hippocampus and cortex, synaptogenesis involves adhesion of pre and postsynaptic membranes, delivery and anchorage of pre and postsynaptic structures including scaffolds such as PSD-95 and NMDA and AMPA receptors, which are glutamate-gated ion channels, as well as the morphological maturation of spines. Although electrical activity-dependent mechanisms are established regulators of these processes, the mechanisms that function during early development, prior to the onset of electrical activity, are unclear. The Eph receptors and ephrins provide cell contact-dependent pathways that regulate axonal and dendritic development. Members of the ephrin-A family are glycosyl-phosphatidylinositol-anchored to the cell surface and activate EphA receptors, which are receptor tyrosine kinases. Methodology/Principal Findings Here we show that ephrin-A5 interaction with the EphA5 receptor following neuron-neuron contact during early development of hippocampus induces a complex program of synaptogenic events, including expression of functional synaptic NMDA receptor-PSD-95 complexes plus morphological spine maturation and the emergence of electrical activity. The program depends upon voltage-sensitive calcium channel Ca2+ fluxes that activate PKA, CaMKII and PI3 kinase, leading to CREB phosphorylation and a synaptogenic program of gene expression. AMPA receptor subunits, their scaffolds and electrical activity are not induced. Strikingly, in contrast to wild type, stimulation of hippocampal slices from P6 EphA5 receptor functional knockout mice yielded no NMDA receptor currents. Conclusions/Significance These studies suggest that ephrin-A5 and EphA5 signals play a necessary, activity-independent role in the initiation of the early phases of synaptogenesis. The coordinated expression of the NMDAR and PSD-95 induced by eprhin-A5 interaction with EphA5 receptors may be the developmental switch that induces expression of AMPAR and their interacting proteins and the transition to activity-dependent synaptic regulation.

Brain-derived neurotrophic factor increases inhibitory synapses, revealed in solitary neurons cultured from rat visual cortex

To elucidate chronic actions of brain-derived neurotrophic factor (BDNF) on GABAergic synapses, we examined effects of a long-term application of BDNF for 10-15 days on autapses (synapses) of solitary GABAergic neurons cultured from rat visual cortex. Solitary neuron preparations were used to exclude a possible contamination of BDNF actions on excitatory neurons in dissociated neuron culture or slice preparations. Neurons were confirmed to be GABAergic pharmacologically with bicuculline, a selective antagonist for GABAA receptors and immunocytochemically with antibody against glutamic acid decarboxylase 65, a GABA synthesizing enzyme. To evaluate GABAergic synaptic function, evoked and/or miniature inhibitory postsynaptic currents (IPSCs) were recorded in the whole-cell voltage-clamp mode. The treatment with BDNF at a concentration of 100 ng/ml enhanced the amplitude of evoked IPSCs and the frequency of miniature IPSCs. In contrast, BDNF did not have a detectable effect on the amplitude of miniature IPSCs and the paired pulse ratio of IPSCs evoked by two, successive activations. To evaluate morphological changes, neurons were immunocytochemically stained with antibodies against microtubule-associated protein 2, to visualize somatodendritic region and synapsin I, to visualize presynaptic sites. The quantitative analysis indicated that BDNF increased the area of soma, the numbers of primary dendrites and dendritic branching points, the total length of dendrites and the number of synaptic sites. Such an action of BDNF was seen in both subgroups of GABAergic neurons, parvalbumin-positive and -negative neurons. To visualize functionally active presynaptic sites, neurons were stained with a styryl dye, FM1-43. BDNF increased the number of stained sites that was correlated with the frequency of miniature IPSCs. These results suggest that the chronic treatment with BDNF promotes dendritic and synaptic development of GABAergic neurons in visual cortex.

Actions of excitatory amino acid antagonists on synaptic potentials of layer II/III neurons of the cat's visual cortex

SummaryActions of excitatory amino acid (EAA) antagonists on the responses of cells in layers II/III and IV of the cats visual cortex to stimulation of layer VI and the underlying white matter were studied in slice preparations. Antagonists used were 2-amino-5-phosphonovalerate (APV), a selective antagonist for the N-methyl-D-aspartate (NMDA) type of EAA receptors, and kynurenate, a broadspectrum antagonist for the three types of EAA receptors. In extracellular recordings it was demonstrated that most of the layer II/III cells were sensitive to APV, while the great majority of the layer IV cells were not, By contrast, kynurenate suppressed the responses completely in both layers. Excitatory post-synaptic potentials (EPSPs) evoked by stimulation of layer VI and the while matter were recorded intracellularly from layer II/III neurons. To determine whether the EPSPs were elicited mono- or polysynaptically, the synaptic delay for each EPSP was calculated from a pair of onset latencies of EPSPs evoked by stimulation of the two sites. Forty-two percent of the layer II/III cells were classified as having monosynaptic EPSPs. In 60% of these monosynaptic cells, the rising slope of the EPSPs was reduced by APV while in the other 40%, it was not. In the former (APV-sensitive cells), subtraction of the APV-sensitive component from the total EPSP indicated that the onset latency of the NMDA receptor-mediated component was roughly equal to that of the non-NMDA component. In the latter (APV-resistant cells), only the slowly-decaying component was in part mediated by NMDA receptors. The conduction velocities of the afferent fibers innervating APV-resistant cells were slower than those of the APV-sensitive cells, suggesting that both types of cells are innervated by different types of afferents. The polysynaptic EPSPs of almost all layer II/III cells were sensitive to APV. The subtraction method indicated that the NMDA component had about the same magnitude as the non-NMDA components. When the slices were superfused by a Mg2+-free solution, the EPSPs were potentiated dramatically, but this potentiation was reduced to the control level during the administration of APV. Similarly, APV-sensitive components were potentiated during the administration of bicuculline, a selective antagonist for gamma-aminobutyric acid receptors of A type. These results suggest that NMDA receptors participate, at varying degrees, in excitatory synaptic transmission at most layer II/III cells in the cats visual cortex, and their actions appear to be regulated by intracortical inhibition.