bluuebunny/crossref_metadata_2025 huggingface dataset:
Total Number and Ratio of Excitatory and Inhibitory Synapses Converging onto Single Interneurons of Different Types in the CA1 Area of the Rat Hippocampus
Authors: Gulyás Attila I. , Megı́as Manuel , Emri Zsuzsa , Freund Tamás F.
Published: April 2018

The least known aspect of the functional architecture of hippocampal microcircuits is the quantitative distribution of synaptic inputs of identified cell classes. The complete dendritic trees of functionally distinct interneuron types containing parvalbumin (PV), calbindin D28k(CB), or calretinin (CR) were reconstructed at the light microscopic level to describe their geometry, total length, and laminar distribution. Serial electron microscopic reconstruction and postembedding GABA immunostaining was then used to determine the density of GABA-negative asymmetrical (excitatory) and GABA-positive symmetrical (inhibitory) synaptic inputs on their dendrites, somata, and axon initial segments. The total convergence and the distribution of excitatory and inhibitory inputs were then calculated using the light and electron microscopic data sets.

The three populations showed characteristic differences in dendritic morphology and in the density and distribution of afferent synapses. PV cells possessed the most extensive dendritic tree (4300 μm) and the thickest dendrites. CR cells had the smallest dendritic tree (2500 μm) and the thinnest shafts. The density of inputs as well as the total number of excitatory plus inhibitory synapses was several times higher on PV cells (on average, 16,294) than on CB (3839) or CR (2186) cells. The ratio of GABAergic inputs was significantly higher on CB (29.4%) and CR (20.71%) cells than on PV cells (6.4%). The density of inhibitory terminals was higher in the perisomatic region than on the distal dendrites.

These anatomical data are essential to understand the distinct behavior and role of these interneuron types during hippocampal activity patterns and represent fundamental information for modeling studies.
A Putative Transcription Factor with Seven Zinc-Finger Motifs Identified in the Developing Suprachiasmatic Nucleus by the Differential Display PCR Method
Authors: Maebayashi Yoshiro , Shigeyoshi Yasufumi , Takumi Toru , Okamura Hitoshi
Published: April 2018

The suprachiasmatic nucleus (SCN) is a mammalian central circadian pacemaker. This nucleus develops in the last stage of fetal life and matures to make strong synaptic connections within 2 weeks of postnatal life to establish strong oscillation characteristics. To identify factors that initiate the circadian oscillation, we applied a differential display PCR method to developing SCN, and isolated a gene with seven zinc-finger motifs,Lot1, which encodes a gene that appeared at a very high level in the SCN during the early postnatal days.Lot1mRNA first appeared at postnatal day 1 (P1) at a very high level, and the signal in the SCN continued to be very high until P10 and thereafter rapidly decreased until P20 and was expressed at a very faint level during adulthood.Lot1mRNA expression was observed only in neurons of the dorsomedial SCN throughout the course of development. During the developmental stage,Lot1mRNA expression shows a circadian rhythm with a peak in the day time and a trough at night time in both light–dark and constant dark conditions. These observations imply thatLot1is the first identified putative transcription factor expressed only in the period of active synaptogenesis in the SCN, whereLot1might play a role in establishing autonomous oscillation.
A Molecular Link between Inward Rectification and Calcium Permeability of Neuronal Nicotinic Acetylcholine α3β4 and α4β2 Receptors
Authors: Haghighi Ali Pejmun , Cooper Ellis
Published: April 2018

Many nicotinic acetylcholine receptors (nAChRs) expressed by central neurons are located at presynaptic nerve terminals. These receptors have high calcium permeability and exhibit strong inward rectification, two important physiological features that enable them to facilitate transmitter release. Previously, we showed that intracellular polyamines act as gating molecules to block neuronal nAChRs in a voltage-dependent manner, leading to inward rectification. Our goal is to identify the structural determinants that underlie the block by intracellular polyamines and govern calcium permeability of neuronal nAChRs. We hypothesize that two ring-like collections of negatively charged amino acids (cytoplasmic and intermediate rings) near the intracellular mouth of the pore mediate the interaction with intracellular polyamines and also influence calcium permeability. Using site-directed mutagenesis and electrophysiology on α4β2and α3β4receptors expressed inXenopusoocytes, we observed that removing the five negative charges of the cytoplasmic ring had little effect on either inward rectification or calcium permeability. However, partial removal of negative charges of the intermediate ring diminished the high-affinity, voltage-dependent interaction between intracellular polyamines and the receptor, abolishing inward rectification. In addition, these nonrectifying mutant receptors showed a drastic reduction in calcium permeability. Our results indicate that the negatively charged glutamic acid residues at the intermediate ring form both a high-affinity binding site for intracellular polyamines and a selectivity filter for inflowing calcium ions; that is, a common site links inward rectification and calcium permeability of neuronal nAChRs. Physiologically, this molecular mechanism provides insight into how presynaptic nAChRs act to influence transmitter release.
Kv4.2 mRNA Abundance and A-Type K+Current Amplitude Are Linearly Related in Basal Ganglia and Basal Forebrain Neurons
Authors: Tkatch Tatiana , Baranauskas Gytis , Surmeier D. James
Published: April 2018

A-type K+currents are key determinants of repetitive activity and synaptic integration. Although several gene families have been shown to code for A-type channel subunits, recent studies have suggested that Kv4 family channels are the principal contributors to A-type channels in the somatodendritic membrane of mammalian brain neurons. If this hypothesis is correct, there should be a strong correlation between Kv4 family mRNA and A-type channel protein or aggregate channel currents. To test this hypothesis, quantitative single-cell reverse transcription-PCR analysis of Kv4 family mRNA was combined with voltage-clamp analysis of A-type K+currents in acutely isolated neurons. These studies revealed that Kv4.2 mRNA abundance was linearly related to A-type K+current amplitude in neostriatal medium spiny neurons and cholinergic interneurons, in globus pallidus neurons, and in basal forebrain cholinergic neurons. In contrast, there was not a significant correlation between estimates of Kv4.1 or Kv4.3 mRNA abundance and A-type K+current amplitudes. These results argue that Kv4.2 subunits are major constituents of somatodendritic A-type K+channels in these four types of neuron. In spite of this common structural feature, there were significant differences in the voltage dependence and kinetics of A-type currents in the cell types studied, suggesting that other determinants may create important functional differences between A-type K+currents.
Intermediate Zone Cells Express Calcium-Permeable AMPA Receptors and Establish Close Contact with Growing Axons
Authors: Métin Christine , Denizot Jean-Pierre , Ropert Nicole
Published: April 2018

Recent studies have shown that cells in the intermediate zone (IZ) of the embryonic neocortex originate in the basal telencephalon and migrate tangentially in the cortical wall (Anderson et al., 1997;Tamamaki et al., 1997; Wichterle et al., 1999). We had previously observed growing cortical axons closely apposed to calbindin-positive, tangentially oriented cells in the IZ (Métin and Godement, 1996), and it has been shown that neurites in the IZ express a glutamate transporter (Furuta et al., 1997). To test if glutamate released by corticofugal growth cones could influence the tangential IZ cells, we characterized the glutamate receptors expressed by IZ cells using patch-clamp techniques, histochemical labeling, and immunostaining on slices of embryonic mice forebrain. We show that tangential IZ cells express inwardly rectifying kainate responses, but not NMDA responses, and accumulate cobalt after AMPA receptor activation. We conclude that IZ cells express calcium-permeable AMPA receptors. This property correlates with our observation that the GluR2 subunit is not expressed in the IZ.

AMPA receptors are activated by a millimolar concentration of glutamate. To know whether this high level of glutamate could occur at the surface of IZ cells, we examined contacts made by corticofugal growth cones and calbindin-positive IZ cells using electron microscopy. We show vesicle-containing neurites tightly apposed to calbindin-positive IZ cells over remarkably long length. This suggests that glutamate released by growing corticofugal axons could reach high concentrations close to AMPA receptors of tangential IZ cells and efficiently activate them to control the intracellular calcium in embryonic IZ cells.
Fos Protein Expression and Cocaine-Seeking Behavior in Rats after Exposure to a Cocaine Self-Administration Environment
Authors: Neisewander Janet L. , Baker David A. , Fuchs Rita A. , Tran-Nguyen Ly T. L. , Palmer Art , Marshall John F.
Published: April 2018

To examine neuronal activation associated with incentive motivation for cocaine, cocaine-seeking behavior (operant responding without cocaine reinforcement) and Fos expression were examined in rats exposed to saline and cocaine priming injections and/or a self-administration environment. Rats were first trained to self-administer cocaine or received yoked saline administration (“control”). They then received 21 daily exposures to either the self-administration environment (“extinction”) or a different environment (“no extinction”) without cocaine available. Extinction training, used to decrease incentive motivation for cocaine elicited by the self-administration environment, decreased cocaine-seeking behavior elicited by both the environment and the cocaine priming injection. Exposure to the self-administration environment enhanced Fos expression in the no extinction group relative to control and extinction groups in the anterior cingulate, basolateral amygdala, hippocampal CA1 region, dentate gyrus, nucleus accumbens shell and core, and central gray area, regardless of whether or not priming injections were given. The priming injections enhanced Fos expression in the ventral tegmental area, caudate putamen, substantia nigra pars reticulata, entorhinal cortex, central amygdala, lateral amygdala, arcuate nucleus, and central gray area, regardless of group. Thus, these changes likely reflect an unconditioned effect from either cocaine or injection stress. The priming injections also enhanced Fos expression in the anterior cingulate, but only in cocaine-experienced groups, suggesting that this enhancement reflects an experience-dependent motivational effect of the priming injections. The results suggest that different neural circuits may be involved in the incentive motivational effects of cocaine-paired environmental stimuli versus priming injections and that the anterior cingulate may be part of a common pathway for both.
GABACReceptors Control Adaptive Changes in a Glycinergic Inhibitory Pathway in Salamander Retina
Authors: Cook Paul B. , Lukasiewicz Peter D. , McReynolds John S.
Published: April 2018

We studied the role of GABA in adaptive changes in a lateral inhibitory system in the tiger salamander retina. In dark-adapted retinal slice preparations picrotoxin caused a slow enhancement of glycine-mediated IPSCs in ganglion cells. The enhancement of glycinergic IPSCs developed slowly over the course of 5–20 min, even though picrotoxin blocked both GABAAand GABACreceptors within a few seconds. The slow enhancement of glycinergic IPSCs by picrotoxin was much weaker in light-adapted preparations. The slow enhancement of glycinergic inhibitory inputs was not produced by bicuculline, indicating that it involved GABACreceptors. The responses of ganglion cells to direct application of glycine were not enhanced by picrotoxin, indicating that the enhancement was not caused by an action on glycine receptors. In dark-adapted eyecup preparations picrotoxin caused a slow enhancement of glycinergic IPSPs and transient lateral inhibition produced by a rotating windmill pattern, similar to the effect of light adaptation. The results suggest that the glycinergic inhibitory inputs are modulated by an unknown substance whose synthesis and/or release is inhibited in dark-adapted retinas by GABA acting at GABACreceptors.
Relationship of Activity in the Subthalamic Nucleus–Globus Pallidus Network to Cortical Electroencephalogram
Authors: Magill Peter J. , Bolam J. Paul , Bevan Mark D.
Published: April 2018

One of the functions of the excitatory subthalamic nucleus (STN) is to relay cortical activity to other basal ganglia structures. The response of the STN to cortical input is shaped by inhibition from the reciprocally connected globus pallidus (GP). To examine the activity in the STN–GP network in relation to cortical activity, we recorded single and multiple unit activity in STN and/or GP together with cortical electroencephalogram in anesthetized rats during various states of cortical activation.

During cortical slow-wave activity (SWA), STN and GP neurons fired bursts of action potentials at frequencies that were similar to those of coincident slow (∼1 Hz) and spindle (7–14 Hz) cortical oscillations. Spontaneous or sensory-driven global activation was associated with a reduction of SWA and a shift in STN–GP activity from burst- to tonic- or irregular-firing. Rhythmic activity in STN and GP neurons was lost when the cortex was inactivated by spreading depression and did not resume until SWA had recovered.

Although rhythmic STN–GP activity was correlated with SWA, the phase relationships of activities of neurons within the STN and GP and between the nuclei were variable. Even when neurons displayed synchronous bursting activity, correlations on the millisecond time scale, which might indicate shared synaptic input, were not observed.

These data indicate that (1) STN and GP activity is intimately related to cortical activity and hence the sleep–wake cycle; (2) rhythmic oscillatory activity in the STN–GP network in disease states may be driven by the cortex; and (3) activity of the STN–GP network is regulated in space in a complex manner.
β Subunits Modulate Alternatively Spliced, Large Conductance, Calcium-Activated Potassium Channels of Avian Hair Cells
Authors: Ramanathan Krishnan , Michael Timothy H. , Fuchs Paul A.
Published: April 2018

Electrical tuning confers frequency selectivity onto sensory hair cells in the auditory periphery of frogs, turtles, and chicks. The resonant frequency is determined in large part by the number and kinetics of large conductance, calcium-activated potassium (BK) channels. BK channels in hair cells are encoded by the alternatively splicedslogene and may include an accessory β subunit. Here we examine the origins of kinetic variability among BK channels by heterologous expression of avian cochlearslocDNAs. Four alternatively spliced forms of theslo-α gene from chick hair cells were co-expressed with accessory β subunits (from quail cochlea) by transient transfection of human embryonic kidney 293 cells. Addition of the β subunit increased steady-state calcium affinity, raised the Hill coefficient for calcium binding, and slowed channel deactivation rates, resulting in eight functionally distinct channels. For example, a naturally occurring splice variant containing three additional exons deactivated 20-fold more slowly when combined with β. Deactivation kinetics were used to predict tuning frequencies and thus tonotopic location if hair cells were endowed with each of the expressed channels. All β-containing channels were predicted to lie within the apical (low-frequency) 30% of the epithelium, consistent with previousin situhybridization studies. Individualslo-α exons would be found anywhere within the apical 70%, depending on the presence of β, and other alternative exons. Alternative splicing of theslo-α channel message provides intrinsic variability in gating kinetics that is expanded to a wider range of tuning by modulation with β subunits.
Tachykinin-Related Peptide and GABA-Mediated Presynaptic Inhibition of Crayfish Photoreceptors
Authors: Glantz Raymon M. , Miller Clyde S. , Nässel Dick R.
Published: April 2018

Off-axis illumination elicits lateral inhibition at the primary visual synapse in crustacea and insects. The evidence suggests that the inhibitory action is presynaptic (i.e., on the photoreceptor terminal) and that the amacrine neurons of the lamina ganglionaris (the first synaptic layer) may be part of the inhibitory pathway. The neurotransmitters and the synaptic mechanisms are unknown. We show by immunocytochemistry that GABA and a tachykinin-related peptide (TRP) are localized in the amacrine neurons of the crayfish lamina ganglionaris. Indirect evidence suggests that GABA and TRP may be colocalized in these neurons. The extensive processes of the amacrine neurons occupy lamina layers containing the terminals of photoreceptors. Application of exogenous GABA and TRP to photoreceptor terminals produces a short-latency, dose-dependent hyperpolarization with a decay time constant on the order of a few seconds. TRP also exhibits actions that evolve over several minutes. These include a reduction of the receptor potential (and the light-elicited current) by ∼40% and potentiation of the action of GABA by ∼100%. The mechanisms of TRP action in crayfish are not known, but a plausible pathway is a TRP-dependent elevation of intracellular Ca2+that reduces photoreceptor sensitivity in arthropods. Although the mechanisms are not established, the results indicate that in crayfish photoreceptors TRP displays actions on two time scales and can exert profound modulatory control over cell function.
Expression of Kv1 Potassium Channels in Mouse Hippocampal Primary Cultures: Development and Activity-Dependent Regulation
Authors: Grosse Gisela , Draguhn Andreas , Höhne Lysann , Tapp Rosemarie , Veh Ruediger W. , Ahnert-Hilger Gudrun
Published: April 2018

Excitability and discharge behavior of neurons depends on the highly variable expression pattern of voltage-dependent potassium (Kv) channels throughout the nervous system. To learn more about distribution, development, and activity-dependent regulation of Kv channel subunit expression in the rodent hippocampus, we studied the protein expression of members of the Kv1 subfamily in mouse hippocampusin situand in primary cultures.

In adult hippocampus, Kv1 (1–6) channel α-subunits were present, whereas at postnatal day 2, none of these proteins could be detected in CA1–CA3 and dentate gyrus. Kv1.1 was the first channel to be observed at postnatal day 6. The delayed postnatal expression and most of the subcellular distribution observed in hippocampal sections were mimicked by cultured hippocampal neurons in which Kv channels appeared only after 10 daysin vitro. This developmental upregulation was paralleled by a dramatic increase in total K+current, as well as an elevated GABA release in the presence of 4-aminopyridine. Thus, the developmental profile, subcellular localization, and functionality of Kv1 channels in primary culture of hippocampus closely resembles thein situsituation.

Impairing secretion by clostridial neurotoxins or blocking activity by tetrodotoxin inhibited the expression of Kv1.1, Kv1.2, and Kv1.4, whereas the other Kv1 channels still appeared. This activity-dependent depression was only observed before the initial appearance of the respective channels and lost after they had been expressed.

Our data show that hippocampal neurons in culture are a convenient model to study the developmental expression and regulation of Kv1 channels. The ontogenetic regulation and the activity-dependent expression of Kv1.1, Kv1.2, and Kv1.4 indicate that neuronal activity plays a crucial role for the development of the mature Kv channel pattern in hippocampal neurons.
Disruption of the Olfactoretinal Centrifugal Pathway May Relate to the Visual System Defect innight blindness bMutant Zebrafish
Authors: Li Lei , Dowling John E.
Published: April 2018

We describe here a dominant mutation,night blindness b(nbb), which causes an age-related visual system defect in zebrafish. At 4–5 months of age, dark-adaptednbb+/−mutants show abnormal visual threshold fluctuations when measured behaviorally. Light sensitizes the animals; thus early dark adaptation ofnbb+/−fish is normal. After 2 hr of dark adaptation, however, visual thresholds ofnbb+/−mutants are raised on average 2–3 log units, and rod system function is not detectable. Electroretinograms recorded fromnbb+/−mutants are normal, but ganglion cell thresholds are raised in prolonged darkness, suggesting an inner retinal defect. The visual defect ofnbb+/−mutants may be likely caused by an abnormal olfactoretinal centrifugal innervation; innbb+/−mutants, the olfactoretinal centrifugal projection to the retina is disrupted, and the number of retinal dopaminergic interplexiform cells is reduced. A similar visual defect as shown bynbb+/−mutants is observed in zebrafish in which the olfactory epithelium and olfactory bulb have been excised. Homozygousnbbfish display an early onset neural degeneration throughout the CNS and die by 7–8 d of age.
Effects of Dopamine Depletion on Visual Sensitivity of Zebrafish
Authors: Li Lei , Dowling John E.
Published: April 2018

The visual sensitivity of zebrafish in which the retinal dopaminergic interplexiform cells (DA-IPCs) were destroyed by 6-hydroxydopamine was measured behaviorally. During the first 6–8 min of dark adaptation, visual thresholds of DA-IPC-depleted animals were similar to those of control animals. Thereafter, their visual thresholds were elevated so that by 14–18 min of dark adaptation, they were 2–3 log units above those of control animals. In DA-IPC-depleted animals, the electroretinogram was normal in terms of light sensitivity and waveform, but the light threshold for eliciting a ganglion cell discharge was raised by 1.8 log units as compared with control animals. No obvious rod system function was detected in DA-IPC-depleted animals as measured behaviorally. Partial rescue of the behavioral visual sensitivity loss in DA-IPC-depleted animals occurred when dopamine or a long-acting dopamine agonist (2-amino-6, 7-dihydroxy-1, 2, 3, 4-tetrahydronaphthalene hydrobromide) were injected intraocularly. Our data suggest that the principal visual defect shown by DA-IPC-depleted animals is attributable to effects occurring in the inner retina, mainly on rod signals. We also show that dopamine is involved in mediating the effect of the circadian clock on visual sensitivity.
Brain-Derived Neurotrophic Factor Acutely Inhibits AMPA-Mediated Currents in Developing Sensory Relay Neurons
Authors: Balkowiec Agnieszka , Kunze Diana L. , Katz David M.
Published: April 2018

Brain-derived neurotrophic factor (BDNF) is expressed by many primary sensory neurons that no longer require neurotrophins for survival, indicating that BDNF may be used as a signaling molecule by the afferents themselves. Because many primary afferents also express glutamate, we investigated the possibility that BDNF modulates glutamatergic AMPA responses of newborn second-order sensory relay neurons. Perforated-patch, voltage-clamp recordings were made from dissociated neurons of the brainstem nucleus tractus solitarius (nTS), a region that receives massive primary afferent input from BDNF-containing neurons in the nodose and petrosal cranial sensory ganglia. Electrophysiological analysis was combined in some experiments with anterograde labeling of primary afferent terminals to specifically analyze responses of identified second-order neurons. Our data demonstrate that BDNF strongly inhibits AMPA-mediated currents in a large subset of nTS cells. Specifically, AMPA responses were either completely abolished or markedly inhibited by BDNF in 73% of postnatal day (P0) cells and in 82% of identified P5 second-order sensory relay neurons. This effect of BDNF is mimicked by NT-4, but not NGF, and blocked by the Trk tyrosine kinase inhibitor K252a, consistent with a requirement for TrkB receptor activation. Moreover, analysis of TrkB expression in culture revealed a close correlation between the percentage of nTS neurons in which BDNF inhibits AMPA currents and the percentage of neurons that exhibit TrkB immunoreactivity. These data document a previously undefined mechanism of acute modulation of AMPA responses by BDNF and indicate that BDNF may regulate glutamatergic transmission at primary afferent synapses.
Three Levels of Lateral Inhibition: A Space–Time Study of the Retina of the Tiger Salamander
Authors: Roska Botond , Nemeth Erik , Orzo Laszlo , Werblin Frank S.
Published: April 2018

The space–time patterns of activity generated across arrays of retinal neurons can provide a sensitive measurement of the effects of neural interactions underlying retinal activity. We measured the excitatory and inhibitory components associated with these patterns at each cellular level in the retina and further dissected inhibitory components pharmacologically. Using perforated and loose patch recording, we measured the voltages, currents, or spiking at 91 lateral positions covering ∼2 mm in response to a flashed 300-μm-wide bar. First, we showed how the effect of well known lateral inhibition at the outer retina, mediated by horizontal cells, evolved in time to compress the spatial representation of the stimulus bar at ON and OFF bipolar cell bodies as well as horizontal cells. Second, we showed, for the first time, how GABACreceptor mediated amacrine cell feedback to bipolar terminals compresses the spatial representation of the stimulus bar at ON bipolar terminals over time. Third, we showed that a third spatiotemporal compression exists at the ganglion cell layer that is mediated by feedforward amacrine cells via GABAAreceptors. These three inhibitory mechanisms, via three different receptor types, appear to compensate for the effects of lateral diffusion of activity attributable to dendritic spread and electrical coupling between retinal neurons. As a consequence, the width of the final representation at the ganglion cell level approximates the dimensions of the original stimulus bar.
Peptide Cotransmitter Release from Motorneuron B16 inAplysia californica: Costorage, Corelease, and Functional Implications
Authors: Vilim Ferdinand S. , Cropper Elizabeth C. , Price David A. , Kupfermann Irving , Weiss Klaudiusz R.
Published: April 2018

Many neurons contain multiple peptide cotransmitters in addition to their classical transmitters. We are using the accessory radula closer neuromuscular system ofAplysia,which participates in feeding in these animals, to define the possible consequences of multiple modulators converging on single targets. How these modulators are released onto their targets is of critical importance in understanding the outcomes of their modulatory actions and their physiological role. Here we provide direct evidence that the partially antagonistic families of modulatory peptides, the myomodulins and buccalins, synthesized by motorneuron B16 are costored and coreleased in fixed ratios. We show that this release is calcium-dependent and independent of muscle contraction. Furthermore, we show that peptide release is initiated at the low end of the physiological range of motorneuron firing frequency and that it increases with increasing motorneuron firing frequency. The coordination of peptide release with the normal operating range of a neuron may be a general phenomenon and suggests that the release of peptide cotransmitters may exhibit similar types of regulation and plasticity as have been observed for classical transmitters. Stimulation paradigms that increase muscle contraction amplitude or frequency also increase peptide release from motor neuron B16. The net effect of the modulatory peptide cotransmitters released from motorneuron B16 would be to increase relaxation rate and therefore allow more frequent and/or larger contractions to occur without increased resistance to antagonist muscles. The end result of this modulation could be to maximize the efficiency of feeding.
Fine Structure of Parvocellular Receptive Fields in the Primate Fovea Revealed by Laser Interferometry
Authors: McMahon Matthew J. , Lankheet Martin J. M. , Lennie Peter , Williams David R.
Published: April 2018

Optical blurring in the eye prevents conventional physiological techniques from revealing the fine structure of the small parvocellular receptive fields in the primate foveain vivo. We explored the organization of receptive fields in macaque parvocellular lateral geniculate nucleus cells by using sinusoidal interference fringes formed directly on the retina to measure spatial frequency tuning at different orientations. Most parvocellular cells in and near the fovea respond reliably to spatial frequencies up to and beyond 100 cycles/° of visual angle, implying center input arising mainly from a single cone. Temporal frequency and contrast response characteristics were also measured at spatial frequencies up to 130 cycles/°. We compared our spatial frequency data with the frequency responses of model receptive fields that estimate the number, configuration, and weights of cones that feed the center and surround. On the basis of these comparisons, we infer possible underlying circuits. Most cells had irregular spatial frequency–response curves that imply center input from more than one cone. The measured responses are consistent with a single cone center together with weak input from nearby cones. By exposing a fine structure that cannot be discerned by conventional techniques, interferometry allows functional measurements of the early neural mechanisms in spatial vision.
Neurocan Is Upregulated in Injured Brain and in Cytokine-Treated Astrocytes
Authors: Asher Richard A. , Morgenstern Daniel A. , Fidler Penny S. , Adcock Kathryn H. , Oohira Atsuhiko , Braistead Janet E. , Levine Joel M. , Margolis Richard U. , Rogers John H. , Fawcett James W.
Published: April 2018

Injury to the CNS results in the formation of the glial scar, a primarily astrocytic structure that represents an obstacle to regrowing axons. Chondroitin sulfate proteoglycans (CSPG) are greatly upregulated in the glial scar, and a large body of evidence suggests that these molecules are inhibitory to axon regeneration. We show that the CSPG neurocan, which is expressed in the CNS, exerts a repulsive effect on growing cerebellar axons. Expression of neurocan was examined in the normal and damaged CNS. Frozen sections labeled with anti-neurocan monoclonal antibodies 7 d after a unilateral knife lesion to the cerebral cortex revealed an upregulation of neurocan around the lesion. Western blot analysis of extracts prepared from injured and uninjured tissue also revealed substantially more neurocan in the injured CNS. Western blot analysis revealed neurocan and the processed forms neurocan-C and neurocan-130 to be present in the conditioned medium of highly purified rat astrocytes. The amount detected was increased by transforming growth factor β and to a greater extent by epidermal growth factor and was decreased by platelet-derived growth factor and, to a lesser extent, by interferon γ. O-2A lineage cells were also capable of synthesizing and processing neurocan. Immunocytochemistry revealed neurocan to be deposited on the substrate around and under astrocytes but not on the cells. Astrocytes therefore lack the means to retain neurocan at the cell surface. These findings raise the possibility that neurocan interferes with axonal regeneration after CNS injury.
Increase in Neurite Outgrowth Mediated by Overexpression of Actin Depolymerizing Factor
Authors: Meberg Peter J. , Bamburg James R.
Published: April 2018

Growth cone motility is regulated by changes in actin dynamics. Actin depolymerizing factor (ADF) is an important regulator of actin dynamics, and extracellular signal-induced changes in ADF activity may influence growth cone motility and neurite extension. To determine this directly, we overexpressed ADF in primary neurons and analyzed neurite lengths. Recombinant adenoviruses were constructed that express wild-typeXenopusADF/cofilin [XAC(wt)], as well as two mutant forms of XAC, the active but nonphosphorylatable XAC(A3) and the less active, pseudophosphorylated XAC(E3). XAC expression was detectable on Western blots 24 hr after infection and peaked at 3 d in cultured rat cortical neurons. Peak expression was ∼75% that of endogenous ADF. XAC(wt) expression caused a slight increase in growth cone area and filopodia but decreased filopodia numbers on neurite shafts. At maximal XAC levels, neurite lengths increased >50% compared with controls infected with a green fluorescent protein-expressing adenovirus. Increased neurite extension was directly related to the expression of active XAC. Expression of the XAC(E3) mutant did not increase neurite extension, whereas expression of the XAC(A3) mutant increased neurite extension but to a lesser extent than XAC(wt), which was partially phosphorylated. XAC expression had minimal, if any, impact on F-actin levels and did not result in compensatory changes in the expression of endogenous ADF or actin. However, F-actin turnover appeared to increase based on F-actin loss after treatment with drugs that block actin polymerization. These results provide direct evidence that increased ADF activity promotes process extension and neurite outgrowth.
Release-Independent Short-Term Synaptic Depression in Cultured Hippocampal Neurons
Authors: Brody David L. , Yue David T.
Published: April 2018

Short-term synaptic plasticity may dramatically influence neuronal information transfer, yet the underlying mechanisms remain incompletely understood. In autapses (self-synapses) formed by cultured hippocampal neurons, short-term synaptic depression (STD) had several unusual features. (1) Reduction of neurotransmitter release probability with Cd2+, a blocker of voltage-gated calcium channels, did not change depression. (2) Lowering [Ca2+]oand/or raising [Mg2+]ohad little effect on STD in cells with strong baseline depression, but in cells with more modest baseline depression, it reduced the depression. (3) Random variations in the size of initial EPSCs did not influence successive EPSC sizes. These findings were inconsistent with release-dependent mechanisms, such as vesicle depletion, post-synaptic receptor desensitization, and autoreceptor inhibition. Instead, other results suggested that changes in action potentials (APs) contributed to depression. The somatic APs declined in amplitude with repetitive stimulation, and modest reduction of AP amplitudes with tetrodotoxin inhibited EPSCs. Notably, tetrodotoxin also increased depression. Similar changes in axonal APs could produce STD in at least two ways. First, decreasing presynaptic spike amplitudes could reduce calcium entry and release probability. Alternatively, APs could fail to propagate through some axonal branches, reducing the number of active synapses. To explore these possibilities, we derived the expected variance of EPSCs for the two scenarios. Experimentally, the variance increased and then decreased on average with successive responses during trains of APs, confirming a unique prediction from the conduction failure scenario. Thus, STD had surprising properties, incompatible with commonly postulated mechanisms but consistent with AP conduction failure at axonal branches.
Regulation of Long-Term Potentiation by H-Ras through NMDA Receptor Phosphorylation
Authors: Manabe Toshiya , Aiba Atsu , Yamada Atsushi , Ichise Taeko , Sakagami Hiroyuki , Kondo Hisatake , Katsuki Motoya
Published: April 2018

The proto-oncogenerasplays a critical role in cell proliferation and differentiation. However,rasgenes are abundantly expressed in the adult CNS, although neuronal cells normally do not proliferate. Recently, several lines of evidence implicated the involvement of Ras signaling pathway in synaptic plasticity. To explore the role of the Ras proteins in the CNS, we generated knock-out mice lacking the H-rasgene and then used them to study the roles of Ras in synaptic transmission and plasticity. An investigation of protein phosphorylation and synaptic transmission in H-rasnull mutant mice has shown that the NMDA receptor is a final target molecule of the Ras protein pathway in the CNS. In the H-rasnull mutant hippocampus, the tyrosine phosphorylation of NR2A (ε1) and NR2B (ε2) subunits of NMDA receptors is increased, and, correspondingly, NMDA synaptic responses are selectively enhanced. In addition, long-term potentiation is markedly enhanced in mutant mice, most likely because of a selective enhancement of NMDA synaptic responses. Therefore, although Ras proteins have been implicated in cell proliferation and differentiation, the regulation of activity-dependent synaptic plasticity in the adult animals by downregulation of the phosphorylation of the NMDA receptor may be another major and pivotal role for H-Ras protein.
Thrombomodulin as a New Marker of Lesion-Induced Astrogliosis: Involvement of Thrombin through the G-Protein-Coupled Protease-Activated Receptor-1
Authors: Pindon Armelle , Berry Martin , Hantaı̈ Daniel
Published: April 2018

Because injury of the CNS causes an astrogliosis, characterized by cell swelling and proliferation, similar to the effects of the serine protease thrombin on astrocytes, we hypothesized that a high level of thrombin at the site of injury might initially induce an astrocyte reaction and later increase the expression of its specific inhibitor, thrombomodulin. Thrombomodulin could then stabilize the astroglial scar through its adhesive properties.

Here, we studied thein vivoinjury response of astrocytes in the anterior medullary velum of adult rat by immunostaining andin situhybridization of thrombomodulin. Thrombomodulin was poorly expressed on astrocytes in normal tissue, increased up to 2 d after injury, and was still highly expressed at 6 d.

To check that thrombin had a direct effect on thrombomodulin expression by astrocytes, we used brain cortical astrocyte primary cultures treated with either thrombin or the agonist peptide thrombin receptor-activating peptide-6, known to activate directly the thrombin G-protein-coupled receptor (GPCR) protease-activated receptor-1 (PAR-1). Modification of thrombomodulin expression was studied by Western blotting and quantitative reverse transcription-PCR. There was a dose-dependent increase in thrombomodulin after 48 hr of treatment, with gene expression peaking at 24 hr but falling to control levels by 48 hr.

Together, these results show the following: (1) injury increases astrocyte thrombomodulin expression; (2) thrombin might mediate thrombomodulin expression via the specific receptor PAR-1; and (3) serine proteases, their inhibitors, and the new family of GPCR, PARs, are active on astrogliosis.
Morphofunctional Plasticity in the Adult Hypothalamus Induces Regulation of Polysialic Acid–Neural Cell Adhesion Molecule through Changing Activity and Expression Levels of Polysialyltransferases
Authors: Soares Sylvia , von Boxberg Ysander , Ravaille-Veron Michèle , Vincent Jean-Didier , Nothias Fatiha
Published: April 2018

Polysialic acid–neural cell adhesion molecule (PSA-NCAM) expression in the adult nervous system is restricted to regions retaining a capacity for morphological plasticity. For the female rat hypothalamoneurohypophysial system (HNS), we have previously shown that lactation induces a dramatic decrease in PSA-NCAM, while leaving the level of total NCAM protein unchanged. Here, we wanted to elucidate the molecular mechanisms leading to a downregulation of PSA, thereby stabilizing newly established synapses and neurohemal contacts that accompany the increased activity of oxytocinergic neurons.

First, we show that the overall specific activity of polysialyltransferases present in tissue extracts from supraoptic nuclei decreases by ∼50% during lactation. So far, two polysialyltransferase enzymes, STX and PST, have been characterized for their capacity to transfer PSA onto NCAMin vitro. Using a competitive RT-PCR on RNA extracts from the HNS, we demonstrate furthermore a significant decrease in the expression levels of both STX and PST mRNAs in lactating versus virgin animals. Interestingly, this downregulation of NCAM polysialylation is not correlated with the post-transcriptional regulation of variable alternative spliced exon splicing, in contrast to neural development. The control of polysialylation via a regulation of both enzyme activity and expression underlines the important role of this post-translational modification of NCAM in morphofunctional plasticity in adult brain.
Distinct Ionotropic GABA Receptors Mediate Presynaptic and Postsynaptic Inhibition in Retinal Bipolar Cells
Authors: Shields Colleen R. , Tran My N. , Wong Rachel O. L. , Lukasiewicz Peter D.
Published: April 2018

Ionotropic GABA receptors can mediate presynaptic and postsynaptic inhibition. We assessed the contributions of GABAAand GABACreceptors to inhibition at the dendrites and axon terminals of ferret retinal bipolar cells by recording currents evoked by focal application of GABA in the retinal slice. Currents elicited at the dendrites were mediated predominantly by GABAAreceptors, whereas responses evoked at the terminals had GABAAand GABACcomponents. The ratio of GABACto GABAA(GABAC:GABAA) was highest in rod bipolar cell terminals and variable among cone bipolars, but generally was lower in OFF than in ON classes. Our results also suggest that the GABAC:GABAAcould influence the time course of responses. Currents evoked at the terminals decayed slowly in cell types for which the GABAC:GABAAwas high, but decayed relatively rapidly in cells for which this ratio was low. Immunohistochemical studies corroborated our physiological results. GABAAβ2/3 subunit immunoreactivity was intense in the outer and inner plexiform layers (OPL and IPL, respectively). GABACρ subunit labeling was weak in the OPL but strong in the IPL in which puncta colocalized with terminals of rod bipolars immunoreactive for protein kinase C and of cone bipolars immunoreactive for calbindin or recoverin. These data demonstrate that GABAAreceptors mediate GABAergic inhibition on bipolar cell dendrites in the OPL, that GABAAand GABACreceptors mediate inhibition on axon terminals in the IPL, and that the GABAC:GABAAon the terminals may tune the response characteristics of the bipolar cell.
Slow Death of Postnatal Hippocampal Neurons by GABAAReceptor Overactivation
Authors: Xu Wanyan , Cormier Robert , Fu Tao , Covey Douglas F. , Isenberg Keith E. , Zorumski Charles F. , Mennerick Steven
Published: April 2018

Neurotransmitters can have both toxic and trophic functions in addition to their role in neural signaling. Surprisingly, chronic blockade of GABAAreceptor activity for 5–8 din vitroenhanced survival of hippocampal neurons, suggesting that GABAAreceptor overactivation may be neurotoxic. Potentiating GABAAreceptor activity by chronic treatment with the endogenous neurosteroid (3α,5α)-3-hydroxypregnan-20-one caused massive cell loss over 1 week in culture. Other potentiators of GABAAreceptors, including benzodiazepines, mimicked the cell loss, suggesting that potentiating endogenous GABA activity is sufficient to produce neuronal death. Neurosteroid-treated neurons had lower resting intracellular calcium levels than control cells and produced smaller calcium rises in response to depolarizing challenges. Manipulating intracellular calcium levels with chronic elevated extracellular potassium or with the calcium channel agonist Bay K 8644 protected neurons. The results may have implications for the mechanisms of programmed cell death in the developing CNS as well as implications for the long-term consequences of chronic GABAmimetic drug use during development.
DNA Methyltransferase Contributes to Delayed Ischemic Brain Injury
Authors: Endres Matthias , Meisel Andreas , Biniszkiewicz Detlev , Namura Shobu , Prass Konstantin , Ruscher Karsten , Lipski Andreas , Jaenisch Rudolf , Moskowitz Michael A. , Dirnagl Ulrich
Published: April 2018

DNA methylation is important for controlling the profile of gene expression and is catalyzed by DNA methyltransferase (MTase), an enzyme that is abundant in brain. Because significant DNA damage and alterations in gene expression develop as a consequence of cerebral ischemia, we measured MTase activityin vitroand DNA methylationin vivoafter mild focal brain ischemia. After 30 min middle cerebral artery occlusion (MCAo) and reperfusion, MTase catalytic activity and the 190 kDa band on immunoblot did not change over time. However, [3H]methyl-group incorporation into DNA increased significantly in wild-type mice after reperfusion, but not in mutant mice heterozygous for a DNA methyltransferase gene deletion (DnmtS/+).DnmtS/+mice were resistant to mild ischemic damage, suggesting that increased DNA methylation is associated with augmented brain injury after MCA occlusion. Consistent with this formulation, treatment with the MTase inhibitor 5-aza-2′-deoxycytidine and the deacetylation inhibitor trichostatin A conferred stroke protection in wild-type mice. In contrast to mild stroke, however, DNA methylation was not enhanced, and reduceddnmtgene expression was not protective in an ischemia model of excitotoxic/necrotic cell death. In conclusion, our results demonstrate that MTase activity contributes to poor tissue outcome after mild ischemic brain injury.
Selective Presynaptic Propagation of Long-Term Potentiation in Defined Neural Networks
Authors: Tao Hui-zhong W. , Zhang Li I. , Bi Guo-qiang , Poo Mu-ming
Published: April 2018

Induction of long-term potentiation (LTP) of the synaptic connection between two hippocampal glutamatergic neurons in a neural network formed in cell culture resulted in a specific pattern of potentiation at other connections within the network. We found that potentiation propagated from the site of induction retrogradely to glutamatergic or GABAergic synapses received by the dendrites of the presynaptic neuron and laterally to those made by its axonal collaterals onto other glutamatergic cells. In contrast, synapses made by the same presynaptic neuron onto GABAergic cells were not affected, and there was no postsynaptic lateral or forward propagation to other synapses received or made by the postsynaptic neuron. In addition, there was no secondary propagation to synapses not directly associated with the presynaptic neuron. Both induction and propagation of LTP required correlated spiking of the postsynaptic cell as well as the activation of the NMDA subtype of glutamate receptors. Such selective propagation suggests the existence of a long-range cytoplasmic signaling within the presynaptic neuron, leading to a specific pattern of coordinated potentiation along excitatory pathways in a neural network.
c-Raf Regulates Cell Survival and Retinal Ganglion Cell Morphogenesis during Neurogenesis
Authors: Pimentel Belén , Sanz Carmen , Varela-Nieto Isabel , Rapp Ulf R. , De Pablo Flora , de la Rosa Enrique J.
Published: April 2018

The signaling cascade Ras/Raf/mitogen-activated protein kinases modulates cell proliferation, differentiation, and survival, all key cellular processes during neural development. To better define thein vivorole of Raf during chick retinal neurogenesis, we interfered with Raf-dependent signaling during days 4.5 to 7.5 of embryonic development by expressing a dominant negative mutant of c-Raf (ΔRaf), which blocks Ras-dependent Raf activation, and by overexpressing wild-type c-Raf. ΔRaf expression induced an increase in cell death by apoptosis, whereas it did not affect overall cell proliferation and differentiation. In parallel, the number of Islet-1/2-positive and TUJ1-positive retinal ganglion cells were diminished in their definitive layer, whereas there was an increase in the number of mislocated Islet-1/2-positive cells. This disturbed morphogenesis correlated with a disruption of the optic fiber layer. Conversely, c-Raf overexpression caused moderate opposite effects on apoptosis. These results framein vivoearly neurogenesis processes in which c-Raf is essential.