bluuebunny/crossref_metadata_2025 huggingface dataset:
Four-Dimensional Neuronal Signaling by Nitric Oxide: A Computational Analysis
Authors: Philippides Andrew , Husbands Phil , O'Shea Michael
Published: April 2018

Nitric oxide (NO) is now recognized as a transmitter of neurons that express the neuronal isoform of the enzyme nitric oxide synthase. NO, however, violates some of the key tenets of chemical transmission, which is classically regarded as occurring at points of close apposition between neurons. It is the ability of NO to diffuse isotropically in aqueous and lipid environments that has suggested a radically different form of signaling in which the transmitter acts four-dimensionally in space and time, affecting volumes of the brain containing many neurons and synapses. Although “volume signaling” clearly challenges simple connectionist models of neural processing, crucial to its understanding are the spatial and temporal dynamics of the spread of NO within the brain. Existing models of NO diffusion, however, have serious shortcomings because they represent solutions for “point-sources,” which have no physical dimensions. Methods for overcoming these difficulties are presented here, and results are described that show how NO spreads from realistic neural architectures with both simple symmetrical and irregular shapes. By highlighting the important influence of the geometry of NO sources, our results provide insights into the four-dimensional spread of a diffusing messenger. We show for example that reservoirs of NO that accumulate in volumes of the nervous system where NO is not synthesized contribute significantly to the temporal and spatial dynamics of NO spread.
Intracellular ATP Increases Capsaicin-Activated Channel Activity by Interacting with Nucleotide-Binding Domains
Authors: Kwak Jiyeon , Wang Myeong Hyeon , Hwang Sun Wook , Kim Tae-Yoon , Lee Soon-Youl , Oh Uhtaek
Published: April 2018

Capsaicin (CAP)-activated ion channel plays a key role in generating nociceptive neural signals in sensory neurons. Here we present evidence that intracellular ATP upregulates the activity of capsaicin receptor channel. In inside-out membrane patches isolated from sensory neurons, application of CAP activated a nonselective cation channel (icap). Further addition of ATP to the bath caused a significant increase inicap, with aK1/2of 3.3 mm. Nonhydrolyzable analogs of ATP, adenylimidodiphosphate and adenosine 5′-O-(3-thio)-triphosphate, also increasedicap. Neither Mg2+-free medium nor inhibitors of various kinases blocked the increase inicapinduced by ATP. The enhancing effect of ATP was also observed in inside-out patches of oocytes expressing vanilloid receptor 1, a cloned capsaicin receptor. Single point mutations (D178N, K735R) within the putative Walker type nucleotide-binding domains abolished the effect of ATP. These results show that ATP increasesicapin sensory neurons by direct interaction with the CAP channel without involvement of phosphorylation.
Dopamine D2 Long Receptor-Deficient Mice Display Alterations in Striatum-Dependent Functions
Authors: Wang Yanyan , Xu Rong , Sasaoka Toshikuni , Tonegawa Susumu , Kung Mei-Ping , Sankoorikal Emma-Betty
Published: April 2018

The dopamine D2 receptor (D2) system has been implicated in several neurological and psychiatric disorders, such as schizophrenia and Parkinson's disease. There are two isoforms of the D2 receptor: the long form (D2L) and the short form (D2S). The two isoforms are generated by alternative splicing of the same gene and differ only by 29 amino acids in their protein structures. Little is known about the distinct functions of either D2 isoform, primarily because selective pharmacological agents are not available. We generated D2L receptor-deficient (D2L−/−) mice by making a subtle mutation in the D2 gene. D2L−/− mice (which still express functional D2S) displayed reduced levels of locomotion and rearing behavior. Interestingly, haloperidol produced significantly less catalepsy and inhibition of locomotor activity in D2L−/− mice. These findings suggest that D2L and D2S may contribute differentially to the regulation of certain motor functions and to the induction of the extrapyramidal side effects associated with the use of typical antipsychotic drugs (e.g., haloperidol). Quinpirole induced a similar initial suppression of locomotor activity in both D2L−/− and wild-type mice. In addition, the D2S receptor in the mutant mice functioned approximately equally well as did D2L as an impulse-modulating autoreceptor. This suggests that the functions of these two isoforms are not dependent on the formation of receptor heterodimers. Our findings may provide novel information for potentially developing improved antipsychotic drugs.
Spatial Frequency Maps in Cat Visual Cortex
Authors: Issa Naoum P. , Trepel Christopher , Stryker Michael P.
Published: April 2018

Neurons in the primary visual cortex (V1) respond preferentially to stimuli with distinct orientations and spatial frequencies. Although the organization of orientation selectivity has been thoroughly described, the arrangement of spatial frequency (SF) preference in V1 is controversial. Several layouts have been suggested, including laminar, columnar, clustered, pinwheel, and binary (high and low SF domains). We have reexamined the cortical organization of SF preference by imaging intrinsic cortical signals induced by stimuli of various orientations and SFs. SF preference maps, produced from optimally oriented stimuli, were verified using targeted microelectrode recordings. We found that a wide range of SFs is represented independently and mostly continuously within V1. Domains with SF preferences at the extremes of the SF continuum were separated by no more than ¾ mm (conforming to the hypercolumn description of cortical organization) and were often found at pinwheel center singularities in the cortical map of orientation preference. The organization of cortical maps permits nearly all combinations of orientation and SF preference to be represented in V1, and the overall arrangement of SF preference in V1 suggests that SF-specific adaptation effects, found in psychophysical experiments, may be explained by local interactions within a given SF domain. By reanalyzing our data using a different definition of SF preference than is used in electrophysiological and psychophysical studies, we can reproduce the different SF organizations suggested by earlier studies.
Androgen-Induced Changes in the Response Dynamics of Ampullary Electrosensory Primary Afferent Neurons
Authors: Sisneros Joseph A. , Tricas Timothy C.
Published: April 2018

Male stingrays use their ampullary electroreceptors to locate mates, but the effect of gonadal androgens on electrosensory encoding during the reproductive season is unknown. We tested the hypothesis that gonadal androgens induce neurophysiological changes in the electrosense of male Atlantic stingrays. During the primary androgen increase in wild males, the electrosensory primary afferent neurons show an increase in discharge regularity, a downshift in best frequency (BF) and bandpass, and a greater sensitivity to low-frequency stimuli from 0.01 to 4 Hz. Experimental implants of dihydrotestosterone in male stingrays induced a similar lowered BF and bandpass and increased average neural sensitivity to low-frequency stimuli (0.5–2 Hz) by a factor of 1.5. Primary afferents from long ampullary canals (>3 cm) were more sensitive and had a lower bandpass and BF than did afferents from short canals (<2 cm). We propose that these androgen-induced changes in the frequency response properties of electrosensory afferents enhance mate detection by male stingrays and may ultimately increase the number of male reproductive encounters with females. Furthermore, differences in primary afferent sensitivity among short and long canals may facilitate detection, orientation, and localization of conspecifics during social interactions.
Fine-Tuning an Auditory Synapse for Speed and Fidelity: Developmental Changes in Presynaptic Waveform, EPSC Kinetics, and Synaptic Plasticity
Authors: Taschenberger Holger , von Gersdorff Henrique
Published: April 2018

Fast, precise, and sustained synaptic transmission at high frequency is thought to be crucial for the task of sound localization in the auditory brainstem. However, recordings from the calyx of Held synapse have revealed severe frequency-dependent synaptic depression, which tends to degrade the exact timing of postsynaptic spikes. Here we investigate the functional changes occurring throughout the critical period of synapse refinement from immature calyx terminal [postnatal day 5 (P5)] to after the onset of hearing (P12–P14). Surprisingly, for recordings near physiological temperature (35°C), we find that P14 synapses are already able to follow extremely high input rates of up to 800 Hz. This ability stems in part from a remarkable shortening of presynaptic action potentials, which may lead to a lowering of release probability and decrease in synaptic delays during development. In addition, AMPA receptor-mediated EPSCs as well as quantal synaptic currents acquired progressively faster kinetics, although their mean amplitudes did not change significantly. NMDA receptor-mediated EPSCs, however, diminished with age, as indicated by a 50% reduction in mean amplitude and faster decay kinetics. Finally, the degree of synaptic depression was greatly attenuated with age, presumably because of a 2.5-fold or larger increase in the releasable pool of vesicles, which together with a decreasing release probability produces a fairly constant EPSC amplitude. This finely tuned orchestra of developmental changes thus simultaneously promotes speed while preventing premature vesicle pool depletion during prolonged bouts of firing. A few critical days in postnatal development can thus have a large impact on synaptic function.
Differential Expression of Plasticity-Related Genes in Waking and Sleep and Their Regulation by the Noradrenergic System
Authors: Cirelli Chiara , Tononi Giulio
Published: April 2018

Behavioral studies indicate that the ability to acquire long-term memories is severely impaired during sleep. It is unclear, however, why the highly synchronous discharge of neurons during sleep should not be followed by the induction of enduring plastic changes. Here we show that the expression of phosphorylatedCRE-binding protein,Arc, andBDNF, three genes whose induction is often associated with synaptic plasticity, is high during waking and low during sleep. We also show that the induction of these genes during waking depends on the activity of the noradrenergic system, which is high in waking and low in sleep. These molecular results complement behavioral evidence and provide a mechanism for the impairment of long-term memory acquisition during sleep.
Circadian Clock Resetting by Sleep Deprivation without Exercise in the Syrian Hamster
Authors: Antle M. C. , Mistlberger R. E.
Published: April 2018

Circadian rhythms in several species can be phase-shifted by procedures that stimulate locomotor activity (“exercise”) during the usual sleep period. The role of arousal or sleep loss, independent of activity, in this effect has not been adequately resolved. We show here, using the sleep deprivation procedure of gentle handling, that comparably large phase shifts (up to 240 min advances) of the rest–activity cycle can be induced in Syrian hamsters by 3 hr of behavioral arousal, with minimal locomotion, beginning 6 hr before the usual active period. Horizontal distance traveled during the deprivation procedure averaged ∼0.08 km, compared to 2.5 km typical in exercise studies. Hamsters requiring fewer interventions exhibited larger shifts, suggesting that the level or continuity of spontaneous arousal determines shift size. The circadian rhythm of light-induced c-fos expression in the suprachiasmatic nucleus (SCN) was used as a phase marker to further demonstrate that the clock is reset within 1 hr after a 3 hr deprivation. Sleep deprivation mimicked the effects of exercise on basal c-fos expression in two components of the circadian system, suppressing basal Fos immunoreactivity in the SCN, and increasing Fos in the intergeniculate leaflet. Sleep deprivation without exercise in hamsters can rapidly reset the circadian clock and alter gene expression within the circadian system.
Minocycline, a Tetracycline Derivative, Is Neuroprotective against Excitotoxicity by Inhibiting Activation and Proliferation of Microglia
Authors: Tikka Tiina , Fiebich Bernd L. , Goldsteins Gundars , Keinänen Riitta , Koistinaho Jari
Published: April 2018

Minocycline, a semisynthetic tetracycline derivative, protects brain against global and focal ischemia in rodents. We examined whether minocycline reduces excitotoxicity in primary neuronal cultures. Minocycline (0.02 μm) significantly increased neuronal survival in mixed spinal cord (SC) cultures treated with 500 μmglutamate or 100 μmkainate for 24 hr. Treatment with these excitotoxins induced a dose-dependent proliferation of microglia that was associated with increased release of interleukin-1β (IL-1β) and was followed by increased lactate dehydrogenase (LDH) release. The excitotoxicity was enhanced when microglial cells were cultured on top of SC cultures. Minocycline prevented excitotoxin-induced microglial proliferation and the increased release of nitric oxide (NO) metabolites and IL-1β. Excitotoxins induced microglial proliferation and increased the release of NO metabolites and IL-1β also in pure microglia cultures, and these responses were inhibited by minocycline. In both SC and pure microglia cultures, excitotoxins activated p38 mitogen-activated protein kinase (p38 MAPK) exclusively in microglia. Minocycline inhibited p38 MAPK activation in SC cultures, and treatment with SB203580, a p38 MAPK inhibitor, but not with PD98059, a p44/42 MAPK inhibitor, increased neuronal survival. In pure microglia cultures, glutamate induced transient activation of p38 MAPK, and this was inhibited by minocycline. These findings indicate that the proliferation and activation of microglia contributes to excitotoxicity, which is inhibited by minocycline, an antibiotic used in severe human infections.
Contribution of the Plasmalemma to Ca2+Homeostasis in Hair Cells
Authors: Boyer Catherine , Art Jonathan J. , Dechesne Claude J. , Lehouelleur Jacques , Vautrin Jean , Sans Alain
Published: April 2018

Calcium influx through transduction channels and efflux via plasmalemmal Ca2+-ATPases (PMCAs) are known to contribute to calcium homeostasis and modulate sensory transduction in vertebrate hair cells. To examine the relative contributions of apical and basolateral pathways, we analyzed the calcium dynamics in solitary ciliated and deciliated guinea pig type I and type II vestibular hair cells. Whole-cell patch-clamp recordings demonstrated that these cells had resting potentials near −70 mV and could be depolarized by 10–20 mV by superfusion with high potassium. Fura-2 measurements indicated that ciliated type II cells and deciliated cells of either type had low basal [Ca2+]i, near ∼90 nm, and superfusion with high potassium led to transient calcium increases that were diminished in the presence of Ca2+channel blockers. In contrast, measurements of type I ciliated cells, hair cells with large calyceal afferents, were associated with a higher basal [Ca2+]iof ∼170 nm. High-potassium superfusion of these cells induced a paradoxical decrease in [Ca2+]ithat was augmented in the presence of Ca2+channel blockers. Optical localization of dihydropyridine binding to the kinocilium suggests that they contain L-type calcium channels, and as a result apical calcium influx includes a contribution from voltage-dependent ion channels in addition to entry via transduction channels localized to the stereocilia. Eosin block of PMCA significantly altered both [Ca2+]ibaseline and transient responses only in ciliated cells suggesting that, in agreement with immunohistochemical studies, PMCA is primarily localized to the bundles.
Diverse Types of Interneurons Generate Thalamus-Evoked Feedforward Inhibition in the Mouse Barrel Cortex
Authors: Porter James T. , Johnson Cary K. , Agmon Ariel
Published: April 2018

Sensory information, relayed through the thalamus, arrives in the neocortex as excitatory input, but rapidly induces strong disynaptic inhibition that constrains the cortical flow of excitation both spatially and temporally. This feedforward inhibition is generated by intracortical interneurons whose precise identity and properties were not known. To characterize interneurons generating feedforward inhibition, neurons in layers IV and V of mouse somatosensory (“barrel”) cortexin vitrowere tested in the cell-attached configuration for thalamocortically induced firing and in the whole-cell mode for synaptic responses. Identification as inhibitory or excitatory neurons was based on intrinsic firing patterns and on morphology revealed by intracellular staining. Thalamocortical stimulation evoked action potentials in ∼60% of inhibitory interneurons but in <5% of excitatory neurons. The inhibitory interneurons that fired received fivefold larger thalamocortical inputs compared with nonfiring inhibitory or excitatory neurons. Thalamocortically evoked spikes in inhibitory interneurons followed at short latency the onset of excitatory monosynaptic responses in the same cells and slightly preceded the onset of inhibitory responses in nearby neurons, indicating their involvement in disynaptic inhibition. Both nonadapting (fast-spiking) and adapting (regular-spiking) inhibitory interneurons fired on thalamocortical stimulation, as did interneurons expressing parvalbumin, calbindin, or neither calcium-binding protein. Morphological analysis revealed that some interneurons might generate feedforward inhibition within their own layer IV barrel, whereas others may convey inhibition to upper layers, within their own or in adjacent columns. We conclude that feedforward inhibition is generated by diverse classes of interneurons, possibly serving different roles in the processing of incoming sensory information.
Regulation of Limbic Information Outflow by the Subthalamic Nucleus: Excitatory Amino Acid Projections to the Ventral Pallidum
Authors: Turner Michael S. , Lavin Antonieta , Grace Anthony A. , Napier T. Celeste
Published: April 2018

The subthalamic nucleus (STN), a component of the basal ganglia motor system, sends an excitatory amino acid (EAA)-containing projection to the ventral pallidum (VP), a major limbic system output region. The VP contains both NMDA and AMPA subtypes of EAA receptors. To characterize the physiology of the subthalamic pathway to the VP, and to determine the influence of EAA receptor subtypes,in vivointracellular recordings, andin vivoextracellular recordings combined with microiontophoresis, were made from VP neurons in anesthetized rats. Of the intracellularly recorded neurons, 86% responded to STN stimulation, and these displayed EPSPs with an onset of 8.7 msec, consistent with a monosynaptic input. The EPSPs evoked in spontaneously firing neurons were nearly twice the amplitude of those in nonfiring cells (13.1 vs 6.8 mV, respectively). As neurons were depolarized by current injection, the latency for spiking decreased from 24.2 to 14.2 msec, although EPSP latency was unaffected. Eighty-seven percent of the extracellularly recorded VP neurons responded to STN stimulation with a rapid and robust enhancement of spiking; the response onset, like the EPSP onset, equaled 8.7 msec. Firing rate was enhanced by NMDA in 94% of the STN-excited cells, and AMPA increased firing in 94% as well. The NMDA-selective antagonist AP-5 attenuated 67% of the STN-evoked excitatory responses, and the AMPA-selective antagonist CNQX attenuated 52%. Both antagonists attenuated 33% of responses, and 78% were attenuated by at least one. This evidence suggests that a great majority of VP neurons are directly influenced by STN activation and that both NMDA and non-NMDA receptors are involved. Moreover, the VP response to STN stimulation appears to be strongly dependent on the depolarization state of the neuron.
Neuronal Population Codes and the Perception of Object Distance in Weakly Electric Fish
Authors: Lewis John E. , Maler Leonard
Published: April 2018

Weakly electric fish use an electric sense to navigate and capture prey in the dark. Objects in the surroundings of the fish produce distortions in their self-generated electric field; these distortions form a two-dimensional Gaussian-like electric image on the skin surface. To determine the distance of an object, the peak amplitude and width of its electric image must be estimated. These sensory features are encoded by a neuronal population in the early stages of the electrosensory pathway, but are not represented with classic bell-shaped neuronal tuning curves. In contrast, bell-shaped tuning curves do characterize the neuronal responses to the location of the electric image on the body surface, such that parallel two-dimensional maps of this feature are formed. In the case of such two-dimensional maps, theoretical results suggest that the width of neural tuning should have no effect on the accuracy of a population code. Here we show that although the spatial scale of the electrosensory maps does not affect the accuracy of encoding the body surface location of the electric image, maps with narrower tuning are better for estimating image width and those with wider tuning are better for estimating image amplitude. We quantitatively evaluate a two-step algorithm for distance perception involving the sequential estimation of peak amplitude and width of the electric image. This algorithm is best implemented by two neural maps with different tuning widths. These results suggest that multiple maps of sensory features may be specialized with different tuning widths, for encoding additional sensory features that are not explicitly mapped.
Modulation of Hippocampal and Amygdalar-Evoked Activity of Nucleus Accumbens Neurons by Dopamine: Cellular Mechanisms of Input Selection
Authors: Floresco Stan B. , Blaha Charles D. , Yang Charles R. , Phillips Anthony G.
Published: April 2018

Inputs from multiple sites in the telencephalon, including the hippocampus and basolateral amygdala (BLA), converge on neurons in the nucleus accumbens (NAc), and dopamine (DA) is believed to play an essential role in the amplification and gating of these different limbic inputs. The present study used extracellular single-unit recordings of NAc neurons in combination with chronoamperometric sampling of mesoaccumbens DA efflux to assess the importance of DA in the integration of different limbic inputs to the NAc. Tetanic stimulation of the fimbria potentiated hippocampal-evoked firing activity of NAc neurons and increased DA extracellular levels. Systemic administration of the D1receptor antagonist SCH23390 or the NMDA receptor antagonist CPP abolished the potentiation of hippocampal-evoked activity and produced a D2receptor-mediated suppression of evoked firing. In neurons that received converging input from the hippocampus and BLA, fimbria tetanus potentiated hippocampal-evoked firing activity and suppressed BLA-evoked activity in the same neurons. Both D1and NMDA receptors participated in the potentiation of fimbria-evoked activity, whereas the suppression of BLA-evoked activity was blocked by either D1receptor antagonism with SCH23390 or the adenosine A1antagonist 8-cyclopentyl-1,2-dimethylxanthine. Coincidental tetanus of both the fimbria and BLA resulted in potentiation of both inputs, indicating that DA and adenosine-mediated suppression of BLA-evoked firing was activity-dependent. These data suggest that increases in mesoaccumbens DA efflux by hippocampal afferents to the NAc play a critical role in an input selection mechanism, which can ensure preferential responding to the information conveyed from the hippocampus to the ventral striatum.
Kainate Receptors Depress Excitatory Synaptic Transmission at CA3→CA1 Synapses in the Hippocampus via a Direct Presynaptic Action
Authors: Frerking Matthew , Schmitz Dietmar , Zhou Qiang , Johansen Joshua , Nicoll Roger A.
Published: April 2018

Kainate receptor activation depresses synaptic release of neurotransmitter at a number of synapses in the CNS. The mechanism underlying this depression is controversial, and both ionotropic and metabotropic mechanisms have been suggested. We report here that the AMPA/kainate receptor agonists domoate (DA) and kainate (KA) cause a presynaptic depression of glutamatergic transmission at CA3→CA1 synapses in the hippocampus, which is not blocked by the AMPA receptor antagonist GYKI 53655 but is blocked by the AMPA/KA receptor antagonist CNQX. Neither a blockade of interneuronal discharge nor antagonists of several neuromodulators affect the depression, suggesting that it is not the result of indirect excitation and subsequent release of a neuromodulator. Presynaptic depolarization, achieved via increasing extracellular K+, caused a depression of the presynaptic fiber volley and an increase in the frequency of miniature EPSCs. Neither effect was observed with DA, suggesting that DA does not depress transmission via a presynaptic depolarization. However, the effects of DA were abolished by the G-protein inhibitorsN-ethylmaleimide and pertussis toxin. These results suggest that KA receptor activation depresses synaptic transmission at this synapse via a direct, presynaptic, metabotropic action.
DrosophilaStoned Proteins Regulate the Rate and Fidelity of Synaptic Vesicle Internalization
Authors: Stimson Daniel T. , Estes Patricia S. , Rao Sujata , Krishnan K. S. , Kelly Leonard E. , Ramaswami Mani
Published: April 2018

At an initial step during synaptic vesicle recycling, dynamin and adaptor proteins mediate the endocytosis of synaptic vesicle components from the plasma membrane. StonedA and stonedB, novel synaptic proteins encoded by a singleDrosophilagene, have predicted structural similarities to adaptors and other proteins implicated in endocytosis. Here, we test possible roles of the stoned proteins in synaptic vesicle internalization via analyses of third instar larval neuromuscular synapses in twoDrosophila stoned(stn) mutants,stntsandstn8P1. Both mutations reduce presynaptic levels of stonedA and stonedB, althoughstntshas relatively weak effects. The mutations cause retention of synaptic vesicle proteins on the presynaptic plasma membrane but do not alter the levels or distribution of endocytosis proteins, dynamin, α-adaptin, and clathrin. In addition,stn8P1mutants exhibit depletion and enlargement of synaptic vesicles. To determine whether these defects arise from altered synaptic vesicle endocytosis or from defects in synaptic vesicle biogenesis, we implemented new methods to assess directly the efficiency of synaptic vesicle recycling and membrane internalization atDrosophilanerve terminals. Behavioral and electrophysiological analyses indicate thatstnts, an allele with normal evoked release and synaptic vesicle number, enhances defects in synaptic vesicle recycling shown byDrosophila shitsmutants. A dye uptake assay demonstrates that slow synaptic vesicle recycling instntsis accompanied by a reduced rate of synaptic vesicle internalization after exocytosis. These observations are consistent with a model in which stonedA and stonedB act to facilitate the internalization of synaptic vesicle components from the plasma membrane.
Dynamics of Glycine Receptor Insertion in the Neuronal Plasma Membrane
Authors: Rosenberg Madelaine , Meier Jochen , Triller Antoine , Vannier Christian
Published: April 2018

The exocytosis site of newly synthesized glycine receptor was defined by means of a morphological assay to characterize its export from thetrans-Golgi Network to the plasma membrane. This was achieved by expressing in transfected neurons an α1 subunit bearing an N-terminal tag selectively cleavable from outside the cell by thrombin. This was combined with a transient temperature-induced block of exocytic transport that creates a synchronized exocytic wave. Immunofluorescence microscopy analysis of the cell surface appearance of newly synthesized receptor revealed that exocytosis mainly occurred at nonsynaptic sites in the cell body and the initial portion of dendrites. At the time of cell surface insertion, the receptors existed as discrete clusters. Quantitative analysis showed that glycine receptor clusters are stable in size and subsequently appeared in more distal dendritic regions. This localization resulted from diffusion in the plasma membrane and not from exocytosis of transport vesicles directed to dendrites. Kinetic analysis established a direct substrate-product relationship between pools of somatic and dendritic receptors. This indicated that clusters represent intermediates between newly synthesized and synaptic receptors. These results support a diffusion–retention model for the formation of receptor-enriched postsynaptic domains and not that of a vectorial intracellular targeting to synapses.
Stages of Synapse Development Defined by Dependence on F-Actin
Authors: Zhang Wandong , Benson Deanna L.
Published: April 2018

It has been widely speculated that actin plays a central role in CNS synapse assembly, but such a requirement for actin filaments (F-actin) has not yet been demonstrated experimentally. We used hippocampal neurons grown in culture and the actin depolymerizing agent, latrunculin A, to examine directly the relationship between F-actin and synapse formation and maturation. During the first week in culture, actin depolymerization results in a near complete loss of synapses defined by synaptophysin-labeled vesicle clusters, synaptic vesicle recycling, and ultrastructure. Over the second week in culture, F-actin becomes increasingly stable, but actin depolymerization no longer disrupts basic synaptic structure. There is, however, a reduction in the number and size of synaptophysin-labeled clusters and in the size of vesicle clusters undergoing FM4–64 recycling, suggesting that synaptic vesicle anchoring remains partially dependent on F-actin. By 18 d in culture, synaptophysin clusters and synaptic vesicle recycling are largely resistant to F-actin depolymerization. The decrease in synapse dependence on F-actin correlates well with the acquisition and retention of presynaptic scaffolding proteins such as Bassoon and postsynaptic scaffolding proteins such as those of the postsynaptic density-95 family. Increased activity stabilizes F-actin and its associated proteins at synaptic sites, suggesting a correlation between active synapses, actin stability, and synapse stability. Our findings demonstrate that F-actin is essential for the development and maintenance of young synapses. Because F-actin is also highly regulatable, we propose that F-actin may be a principal target for stabilizing or destabilizing signals that ultimately result in synapse maintenance or elimination.
A Role for the Cytoplasmic Polyadenylation Element in NMDA Receptor-Regulated mRNA Translation in Neurons
Authors: Wells David G. , Dong Xin , Quinlan Elizabeth M. , Huang Yi-Shuian , Bear Mark F. , Richter Joel D. , Fallon Justin R.
Published: April 2018

The ability of neurons to modify synaptic connections based on activity is essential for information processing and storage in the brain. The induction of long-lasting changes in synaptic strength requires new protein synthesis and is often mediated by NMDA-type glutamate receptors (NMDARs). We used a dark-rearing paradigm to examine mRNA translational regulation in the visual cortex after visual experience-induced synaptic plasticity. In this model system, we demonstrate that visual experience induces the translation of mRNA encoding the α-subunit of calcium/calmodulin-dependent kinase II in the visual cortex. Furthermore, this increase in translation is NMDAR dependent. One potential source for newly synthesized proteins is the translational activation of dormant cytoplasmic mRNAs. To examine this possibility, we developed a culture-based assay system to study translational regulation in neurons. Cultured hippocampal neurons were transfected with constructs encoding green fluorescent protein (GFP). At 6 hr after transfection, ∼35% of the transfected neurons (as determined byin situhybridization) expressed detectable GFP protein. Glutamate stimulation of the cultures at this time induced an increase in the number of neurons expressing GFP protein that was NMDAR dependent. Importantly, the glutamate-induced increase was only detected when the 3′-untranslated region of the GFP constructs contained intact cytoplasmic polyadenylation elements (CPEs). Together, these findings define a molecular mechanism for activity-dependent synaptic plasticity that is mediated by the NMDA receptor and requires the CPE-dependent translation of an identified mRNA.
Expression of A53T Mutant But Not Wild-Type α-Synuclein in PC12 Cells Induces Alterations of the Ubiquitin-Dependent Degradation System, Loss of Dopamine Release, and Autophagic Cell Death
Authors: Stefanis Leonidas , Larsen Kristin E. , Rideout Hardy J. , Sulzer David , Greene Lloyd A.
Published: April 2018

α-Synuclein mutations have been identified in certain families with Parkinson's disease (PD), and α-synuclein is a major component of Lewy bodies. Other genetic data indicate that the ubiquitin-dependent proteolytic system is involved in PD pathogenesis. We have generated stable PC12 cell lines expressing wild-type or A53T mutant human α-synuclein. Lines expressing mutant but not wild-type α-synuclein show: (1) disruption of the ubiquitin-dependent proteolytic system, manifested by small cytoplasmic ubiquitinated aggregates and by an increase in polyubiquitinated proteins; (2) enhanced baseline nonapoptotic death; (3) marked accumulation of autophagic–vesicular structures; (4) impairment of lysosomal hydrolysis and proteasomal function; and (5) loss of catecholamine-secreting dense core granules and an absence of depolarization-induced dopamine release. Such findings raise the possibility that the primary abnormality in these cells may involve one or more deficits in the lysosomal and/or proteasomal degradation pathways, which in turn lead to loss of dopaminergic capacity and, ultimately, to death. These cells may serve as a model to study the effects of aberrant α-synuclein on dopaminergic cell function and survival.
Retardation of Cochlear Maturation and Impaired Hair Cell Function Caused by Deletion of All Known Thyroid Hormone Receptors
Authors: Rüsch Alfons , Ng Lily , Goodyear Richard , Oliver Dominik , Lisoukov Igor , Vennström Björn , Richardson Guy , Kelley Matthew W. , Forrest Douglas
Published: April 2018

The deafness caused by early onset hypothyroidism indicates that thyroid hormone is essential for the development of hearing. We investigated the underlying roles of the TRα1 and TRβ thyroid hormone receptors in the auditory system using receptor-deficient mice. TRα1 and TRβ, which act as hormone-activated transcription factors, are encoded by theThraandThrbgenes, respectively, and both are expressed in the developing cochlea. TRβ is required for hearing because TRβ-deficient (Thrbtm1/tm1) mice have a defective auditory-evoked brainstem response and retarded expression of a potassium current (IK,f) in the cochlear inner hair cells. Here, we show that although TRα1 is individually dispensable, TRα1 and TRβ synergistically control an extended array of functions in postnatal cochlear development. Compared withThrbtm1/tm1mice, the deletion of all TRs inThratm1/tm1Thrbtm1/tm1mice produces exacerbated and novel phenotypes, including delayed differentiation of the sensory epithelium, malformation of the tectorial membrane, impairment of electromechanical transduction in outer hair cells, and a low endocochlear potential. The induction ofIK,fin inner hair cells was not markedly more retarded than inThrbtm1/tm1mice, suggesting that this feature of hair cell maturation is primarily TRβ-dependent. These results indicate that distinct pathways mediated by TRβ alone or by TRβ and TRα1 together facilitate control over an extended range of functions during the maturation of the cochlea.
The Role of Kv1.2-Containing Potassium Channels in Serotonin-Induced Glutamate Release from Thalamocortical Terminals in Rat Frontal Cortex
Authors: Lambe Evelyn K. , Aghajanian George K.
Published: April 2018

Serotonin 5-HT2Areceptors have been implicated in psychiatric illness and the psychotomimetic effects of hallucinogens. In brain slices, focal stimulation of 5-HT2Areceptors in rat prefrontal cortex results in dramatically increased glutamate release onto layer V pyramidal neurons, as measured by an increase in “spontaneous” (nonelectrically evoked) EPSCs. This glutamate release is blocked by tetrodotoxin (TTX) and is thought to involve local spiking in thalamocortical axon terminals; however, the detailed mechanism has remained unclear.

Here, we investigate parallels in EPSCs induced by either serotonin or the potassium channel blockers 4-aminopyridine (4-AP) or α-dendrotoxin (DTX). DTX, a selective blocker of Kv1.1-, Kv1.2-, and Kv1.6-containing potassium channels, has been shown to release glutamate in cortical synaptosomes, presumably by inhibiting a subthreshold-activated, slowly inactivating potassium conductance. By comparing DTX with other potassium channel blockers, we found that the ability to induce EPSCs in cortical pyramidal neurons depends on affinity for Kv1.2 subunits. DTX-induced EPSCs are similar to 5-HT-induced EPSCs in terms of sensitivity to TTX and ω-agatoxin-IVA (a blocker of P-type calcium channels) and laminar selectivity. The involvement of thalamocortical terminals in DTX-induced EPSCs was confirmed by suppression of these EPSCs by μ-opiates and thalamic lesions. More directly, DTX-induced EPSCs substantially occlude those induced by 5-HT, suggesting a common mechanism of action. No occlusion by DTX was seen when EPSCs were induced by a nicotinic mechanism. These results indicate that blockade of Kv1.2-containing potassium channels is part of the mechanism underlying 5-HT-induced glutamate release from thalamocortical terminals.
Long-Lasting Potentiation of GABAergic Synapses in Dopamine Neurons after a SingleIn VivoEthanol Exposure
Authors: Melis Miriam , Camarini Rosana , Ungless Mark A. , Bonci Antonello
Published: April 2018

The mesolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) is involved in many drug-related behaviors, including ethanol self-administration. In particular, VTA activity regulating ethanol consummatory behavior appears to be modulated through GABAAreceptors. Previous exposure to ethanol enhances ethanol self-administration, but the mechanisms underlying this phenomenon are not well understood. In this study, we examined changes occurring at GABA synapses onto VTA DA neurons after a singlein vivoexposure to ethanol. We observed that evoked GABAAIPSCs in DA neurons of ethanol-treated animals exhibited paired-pulse depression (PPD) compared with saline-treated animals, which exhibited paired-pulse facilitation (PPF). Furthermore, PPD was still present 1 week after the single exposure to ethanol. An increase in frequency of spontaneous miniature GABAAIPSCs (mIPSCs) was also observed in the ethanol-treated animals. Additionally, the GABABreceptor antagonist (3-aminopropyl)(diethoxymethyl) phosphinic acid shifted PPD to PPF, indicating that presynaptic GABABreceptor activation, likely attributable to GABA spillover, might play a role in mediating PPD in the ethanol-treated mice. The activation of adenylyl cyclase by forskolin increased the amplitude of GABAAIPSCs and the frequency of mIPSCs in the saline- but not in the ethanol-treated animals. Conversely, the protein kinase A (PKA) inhibitorN-[z-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide significantly decreased both the frequency of spontaneous mIPSCs and the amplitude of GABAAIPSCs in the ethanol-treated mice but not in the saline controls. The present results indicate that potentiation of GABAergic synapses, via a PKA-dependent mechanism, occurs in the VTA after a singlein vivoexposure to ethanol, and such potentiation might be a key synaptic modification underlying increased ethanol intake.
Synaptically Released Glutamate Activates Extrasynaptic NMDA Receptors on Cells in the Ganglion Cell Layer of Rat Retina
Authors: Chen Shan , Diamond Jeffrey S.
Published: April 2018

NMDA and AMPA receptors (NMDARs and AMPARs) are colocalized at most excitatory synapses in the CNS. Consequently, both receptor types are activated by a single quantum of transmitter and contribute to miniature and evoked EPSCs. However, in amphibian retina, miniature EPSCs in ganglion cell layer neurons are mediated solely by AMPARs, although both NMDARs and AMPARs are activated during evoked EPSCs. One explanation for this discrepancy is that NMDARs are located outside of the synaptic cleft and are activated only when extrasynaptic glutamate levels increase during coincident release from multiple synapses. Alternatively, NMDARs may be segregated at synapses that either are not spontaneously active or yield miniature EPSCs that are too small to detect. In this study, we examined excitatory, glutamatergic synaptic inputs to neurons in the ganglion cell layer of acute slices of rat retina. EPSCs, elicited by electrically stimulating presynaptic bipolar cells, exhibited both NMDAR- and AMPAR-mediated components. However, spontaneous EPSCs exhibited only an AMPAR-mediated component. The effects of low-affinity, competitive receptor antagonists indicated that NMDARs encounter less glutamate than AMPARs during an evoked synaptic response. Reducing glutamate uptake or changing the probability of release preferentially affected the NMDAR component in evoked EPSCs; reducing uptake revealed an NMDAR component in spontaneous EPSCs. These results indicate that NMDARs are located extrasynaptically and that glutamate transporters prevent NMDAR activation by a transmitter released from a single vesicle and limit their activation during evoked responses.
Microglial Activation and β-Amyloid Deposit Reduction Caused by a Nitric Oxide-Releasing Nonsteroidal Anti-Inflammatory Drug in Amyloid Precursor Protein Plus Presenilin-1 Transgenic Mice
Authors: Jantzen Paul T. , Connor Karen E. , DiCarlo Giovanni , Wenk Gary L. , Wallace John L. , Rojiani Amyn M. , Coppola Domenico , Morgan Dave , Gordon Marcia N.
Published: April 2018

3-4-(2-Fluoro-α-methyl-[1,1′-biphenyl]-4-acetyloxy)-3-methoxyphenyl]-2-propenoic acid 4-nitrooxy butyl ester (NCX-2216), a nitric oxide (NO)-releasing derivative of the cyclooxygenase-1-preferring nonsteroidal anti-inflammatory drug (NSAID) flurbiprofen, dramatically reduced both β-amyloid (Aβ) loads and Congo red staining in doubly transgenic (Tg) amyloid precursor protein plus presenilin-1 mice when administered at 375 ppm in diet between 7 and 12 months of age. This reduction was associated with a dramatic increase in the number of microglia expressing major histocompatibility complex-II antigen, a marker for microglial activation. In contrast, ibuprofen at 375 ppm in diet caused modest reductions in Aβ load but not Congo red staining, suggesting that the effects of this nonselective NSAID were restricted primarily to nonfibrillar deposits. We detected no effects of the cyclooxygenase-2-selective NSAID celecoxib at 175 ppm on amyloid deposition. In short-term studies of 12-month-old Tg mice, we found that the microglia-activating properties of NCX-2216 (7.5 mg · kg−1 · d−1, s.c.) were present after 2 weeks of treatment. Microglia were not activated by NCX-2216 in non-Tg mice lacking Aβ deposits, nor were microglia activated in Tg animals by flurbiprofen (5 mg · kg−1 · d−1) alone. These data are consistent with the argument that activated microglia can clear Aβ deposits. We conclude that the NO-generating component of NCX-2216 confers biological actions that go beyond those of typical NSAIDs. In conclusion, NCX-2216 is more efficacious than ibuprofen or celecoxib in clearing Aβ deposits from the brains of Tg mice, implying potential benefit in the treatment of Alzheimer's dementia.
Complete Deletion of the Neurotrophin Receptor p75NTRLeads to Long-Lasting Increases in the Number of Basal Forebrain Cholinergic Neurons
Authors: Naumann Thomas , Casademunt Elisabeth , Hollerbach Ewald , Hofmann Jutta , Dechant Georg , Frotscher Michael , Barde Yves-Alain
Published: April 2018

Cholinergic neurons innervating cortical structures are among the most affected neuronal populations in Alzheimer's disease. In rodents, they express high levels of the neurotrophin receptor p75NTR. We have analyzed cholinergic septohippocampal neurons of the medial septal nucleus inp75exonIII(partial p75NTRknock-out) andp75exonIV(complete p75NTRknock-out) mice, in their original genetic background and in congenic strains. At postnatal day 15, thep75exonIIImutation leads to a moderate increase (+13%) in these neurons among littermates only after back-crossing in a C57BL/6 background. In contrast, the nullp75exonIVmutation, which prevents expression of both the full-length and the shorter p75NTRisoforms, results in a 28% neuronal increase, independent of genetic background. The incomplete nature of the p75NTRmutation used previously, coupled with difficulties in delineating the mouse medial septum and the impact of the genetic background on cell numbers, all contribute to explain previous difficulties in establishing the role of p75NTRin regulating cholinergic neuron numbers in the mouse forebrain.
The Ras1–Mitogen-Activated Protein Kinase Signal Transduction Pathway Regulates Synaptic Plasticity through Fasciclin II-Mediated Cell Adhesion
Authors: Koh Young-Ho , Ruiz-Canada Catalina , Gorczyca Michael , Budnik Vivian
Published: April 2018

Ras proteins are small GTPases with well known functions in cell proliferation and differentiation. In these processes, they play key roles as molecular switches that can trigger distinct signal transduction pathways, such as the mitogen-activated protein kinase (MAPK) pathway, the phosphoinositide-3 kinase pathway, and the Ral–guanine nucleotide dissociation stimulator pathway. Several studies have implicated Ras proteins in the development and function of synapses, but the molecular mechanisms for this regulation are poorly understood. Here, we demonstrate that the Ras–MAPK pathway is involved in synaptic plasticity at theDrosophilalarval neuromuscular junction. Both Ras1 and MAPK are expressed at the neuromuscular junction, and modification of their activity levels results in an altered number of synaptic boutons. Gain- or loss-of-function mutations in Ras1 and MAPK reveal that regulation of synapse structure by this signal transduction pathway is dependent on fasciclin II localization at synaptic boutons. These results provide evidence for a Ras-dependent signaling cascade that regulates fasciclin II-mediated cell adhesion at synaptic terminals during synapse growth.