bluuebunny/crossref_metadata_2025 huggingface dataset:
An Alternate Pathway for Visual Signal Integration into the Hypothalamo-Pituitary Axis: Retinorecipient Intergeniculate Neurons Project to Various Regions of the Hypothalamus and Innervate Neuroendocrine Cells Including Those Producing Dopamine
Authors: Horvath Tamas L.
Published: April 2018

Using tract tracing and immunocytochemistry, this study explored the connectivity between lateral geniculate efferents and neurons of the hypothalamus, including those producing dopamine, that have direct access to fenestrated capillaries. It was also determined whether the intergeniculate neurons that give rise to hypothalamic projections are targeted by retinal axons.

Within the hypothalamus,Phaseolus vulgarisleucoagglutinin-labeled, lateral geniculate efferents were observed in the suprachiasmatic nucleus, subparaventricular area, periventricular nuclei, medial preoptic areas, and between the arcuate and ventromedial nuclei. In these sites, intergeniculate efferents contacted populations of neurons that were retrogradely labeled from fenestrated capillaries by the intraperitoneal injection of fluorogold. Hypothalamic dopamine neurons, a population of which was neuroendocrine, were also synaptic targets of lateral geniculate efferents. After injection of the retrograde tracer fluorogold into these hypothalamic projection sites in parallel with bilateral enucleation, retrogradely labeled perikarya were restricted to the intergeniculate leaflet. All of the labeled perikarya contained infolded nuclei, and their distal dendrites were frequently found to be contacted by degenerated, retinal fibers.

This study provides morphological evidence for a signaling pathway from the retina through the intergeniculate leaflet to hypothalamic cells that participate in neuroendocrine regulations. These observations raise the possibility that visual signals independent of the circadian clock may also influence the hypothalamo-pituitary axis. In light of the overlapping distribution of intergeniculate and suprachiasmatic efferents in the hypothalamus and their similar relationship with neuroendocrine cells, it is suggested that integration of circadian and visual signals can occur outside of the suprachiasmatic nucleus to regulate endocrine rhythms.
Practice-Related Improvements in Somatosensory Interval Discrimination Are Temporally Specific But Generalize across Skin Location, Hemisphere, and Modality
Authors: Nagarajan Srikantan S. , Blake David T. , Wright Beverly A. , Byl Nancy , Merzenich Michael M.
Published: April 2018

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10–15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.
Gaze-Centered Remapping of Remembered Visual Space in an Open-Loop Pointing Task
Authors: Henriques Denise Y. P. , Klier Eliana M. , Smith Michael A. , Lowy Deborah , Crawford J. Douglas
Published: April 2018

Establishing a coherent internal reference frame for visuospatial representation and maintaining the integrity of this frame during eye movements are thought to be crucial for both perception and motor control. A stable headcentric representation could be constructed by internally comparing retinal signals with eye position. Alternatively, visual memory traces could be actively remapped within an oculocentric frame to compensate for each eye movement. We tested these models by measuring errors in manual pointing (in complete darkness) toward briefly flashed central targets during three oculomotor paradigms; subjects pointed accurately when gaze was maintained on the target location (control paradigm). However, when steadily fixating peripheral locations (static paradigm), subjects exaggerated the retinal eccentricity of the central target by 13.4 ± 5.1%. In the key “dynamic” paradigm, subjects briefly foveated the central target and then saccaded peripherally before pointing toward the remembered location of the target. Our headcentric model predicted accurate pointing (as seen in the control paradigm) independent of the saccade, whereas our oculocentric model predicted misestimation (as seen in the static paradigm) of an internally shifted retinotopic trace. In fact, pointing errors were significantly larger than were control errors (p≤ 0.003) and were indistinguishable (p≥ 0.25) from the static paradigm errors. Scatter plots of pointing errors (dynamic vs static paradigm) for various final fixation directions showed an overall slope of 0.97, contradicting the headcentric prediction (0.0) and supporting the oculocentric prediction (1.0). Varying both fixation and pointing-target direction confirmed that these errors were a function of retinotopically shifted memory traces rather than eye position per se. To reconcile these results with previous pointing experiments, we propose a “conversion-on-demand” model of visuomotor control in which multiple visual targets are stored and rotated (noncommutatively) within the oculocentric frame, whereas only select targets are transformed further into head- or bodycentric frames for motor execution.
Myelin Galactolipids Are Essential for Proper Node of Ranvier Formation in the CNS
Authors: Dupree Jeffrey L. , Coetzee Timothy , Blight Andrew , Suzuki Kinuko , Popko Brian
Published: April 2018

The vertebrate myelin sheath is greatly enriched in the galactolipids galactocerebroside (GalC) and sulfatide. Mice with a disruption in the gene that encodes the biosynthetic enzyme UDP-galactose:ceramide galactosyl transferase (CGT) are incapable of synthesizing these lipids yet form myelin sheaths that exhibit major and minor dense lines with spacing comparable to controls. These CGT mutant mice exhibit a severe tremor that is accompanied by hindlimb paralysis. Furthermore, electrophysiological studies reveal nerve conduction deficits in the spinal cord of these mutants. Here, using electron microscopic techniques, we demonstrate ultrastructural myelin abnormalities in the CNS that are consistent with the electrophysiological deficits. These abnormalities include altered nodal lengths, an abundance of heminodes, an absence of transverse bands, and the presence of reversed lateral loops. In contrast to the CNS, no ultrastructural abnormalities and only modest electrophysiological deficits were observed in the peripheral nervous system. Taken together, the data presented here indicate that GalC and sulfatide are essential in proper CNS node and paranode formation and that these lipids are important in ensuring proper axo-oligodendrocyte interactions.
Functional Cooperation of β1-Integrins and Members of the Ig Superfamily in Neurite Outgrowth Induction
Authors: Treubert Ullrich , Brümmendorf Thomas
Published: April 2018

Neurite outgrowth is a central aspect of the ontogenetic formation of neural networks and is regulated by distinct groups of cell surface molecules. One protein involved in neurite elongation and fasciculation is the neural Ig superfamily member F11/contactin. We have shown previously that F11 promotes neurite extension of chick tectal neurons by interaction with the tectal receptor NrCAM, a member of the L1 subgroup of the Ig superfamily. By contrast, it does not induce outgrowth of retinal neurons despite the fact that these cells also express NrCAM, suggesting that in retinal cells the F11–NrCAM interaction alone is not sufficient to induce neurite extension. In this report we present a novel image analysis procedure to quantify neurite outgrowth and use it to demonstrate that F11 enhances the fibronectin-induced outgrowth response of embryonic retinal neurons. We reveal that NrCAM is the neuronal receptor mediating the enhanced outgrowth of retinal neurons, whereas the related F11-binding molecule NgCAM is not involved. Furthermore, we provide evidence that a β1-integrin may represent the fibronectin-dependent receptor that cooperates indirectly with the F11–NrCAM pathway. Our results support the concept of a combinatorial labeling of cells in nervous system histogenesis by different classes of cell surface proteins, in particular by integrins and molecules of the Ig superfamily.
Successful Verbal Encoding into Episodic Memory Engages the Posterior Hippocampus: A Parametrically Analyzed Functional Magnetic Resonance Imaging Study
Authors: Fernández Guillén , Weyerts Helga , Schrader-Bölsche Michael , Tendolkar Indira , Smid Henderikus G. O. M. , Tempelmann Claus , Hinrichs Hermann , Scheich Henning , Elger Christian E. , Mangun George R. , Heinze Hans-Jochen
Published: April 2018

The medial temporal lobe (MTL) is essential for episodic memory encoding, as evidenced by memory deficits in patients with MTL damage. However, previous functional neuroimaging studies have either failed to show MTL activation during encoding or they did not differentiate between two MTL related processes: novelty assessment and episodic memory encoding. Furthermore, there is evidence that the MTL can be subdivided into subcomponents serving different memory processes, but the extent of this functional subdivision remains unknown. The aim of the present functional magnetic resonance imaging (fMRI) study was to investigate the role of the MTL in episodic encoding and to determine whether this function might be restricted to anatomical subdivisions of the MTL. Thirteen healthy volunteers performed a word list learning paradigm with free recall after distraction. Functional images acquired during encoding were analyzed separately for each participant by a voxel-wise correlation (Kendall’s tau) between the time series of the T2*-signal intensity and the number of subsequently recalled words encoded during each particular scan. Of the 13 participants, 11 showed voxel clusters with statistically significant, positive correlations in the posterior part of the hippocampus. Across participants, an ANOVA on the number of voxels with significant, positive correlations within individually defined volumes of interest confirmed a statistically significant difference in activation for anterior versus posterior regions of the hippocampus. However, no differences between left and right hippocampal activation were revealed. Thus, these findings demonstrate that successful encoding into episodic memory engages neural circuits in the posterior part of the hippocampus.
Interaction between Duration of Activity and Time Course of Recovery from Slow Inactivation in Mammalian Brain Na+Channels
Authors: Toib Amir , Lyakhov Vladimir , Marom Shimon
Published: April 2018

NaII and NaIIA channels are the most abundant voltage-gated channels in neonatal and adult cortex, respectively. The relationships between activity and availability for activation of these channels were examined using theXenopusexpression system. The main point of this work is that the time constant (τ) of recovery from the unavailable (inactivated) pool is related to the duration (t) of previous activation by a power law: τ(t) =p·tD, with a scaling powerDcongruent to 0.8 and 0.5 for NaII and NaIIA, respectively, andpas a constant kinetic setpoint. These relationships extend from tens of milliseconds to several minutes and are intrinsic to the channel protein. Coexpression of β1 auxiliary subunit, together with the α subunit of the NaIIA channel, modulates the constant kinetic setpoint but not the scaling power of the latter. The power law scaling between activity and availability is not a universal property of ion channels; unlike that of voltage-gated sodium channels, the rate of recovery from slow inactivation of the ShakerB channel is virtually insensitive to the duration of previous stimuli. It is suggested that the power law scaling described here can act as a molecular memory mechanism that preserves traces of previous activity, over a wide range of time scales, in the form of modulated reaction rates. This mechanism should be considered when theorizing about the dynamics of threshold and firing patterns of neurons.
Myelin-Associated Glycoprotein Is a Myelin Signal that Modulates the Caliber of Myelinated Axons
Authors: Yin Xinghua , Crawford Thomas O. , Griffin John W. , Tu Pang-hsien , Lee Virginia M.-Y. , Li Chumei , Roder John , Trapp Bruce D.
Published: April 2018

Myelination increases neuronal conduction velocity through its insulating properties and an unidentified extrinsic effect that increases axonal caliber. Although it is well established that demyelination can cause axonal atrophy, the myelin molecule that regulates axonal caliber is not known. Loss of the structural proteins of compact peripheral nervous system (PNS) myelin, P0protein, and myelin basic protein does not lead to axonal atrophy. This study demonstrates that mice with a null mutation of the myelin-associated glycoprotein (MAG) gene have a chronic atrophy of myelinated PNS axons that results in paranodal myelin tomaculi and axonal degeneration. Absence of MAG was correlated with reduced axonal calibers, decreased neurofilament spacing, and reduced neurofilament phosphorylation. Because axonal atrophy and degeneration in MAG-deficient mice occur in the absence of inflammation, hypomyelination, significant demyelination–remyelination, or gain of function mutations, these data support a functional role for MAG in modulating the maturation and viability of myelinated axons.
Distinct Requirements for Evoked and Spontaneous Release of Neurotransmitter Are Revealed by Mutations in theDrosophilaGeneneuronal-synaptobrevin
Authors: Deitcher David L. , Ueda Atsushi , Stewart Bryan A. , Burgess Robert W. , Kidokoro Yoshi , Schwarz Thomas L.
Published: April 2018

Two modes of vesicular release of transmitter occur at a synapse: spontaneous release in the absence of a stimulus and evoked release that is triggered by Ca2+influx. These modes often have been presumed to represent the same exocytotic apparatus functioning at different rates in different Ca2+concentrations. To investigate the mechanism of transmitter release, we have examined the role of synaptobrevin/VAMP, a protein involved in vesicular docking and/or fusion. We generated a series of mutations, including null mutations, inneuronal-synaptobrevin(n-syb), the neuronally expressed synaptobrevin gene inDrosophila. Mutant embryos completely lackingn-sybform morphologically normal neuromuscular junctions. Electrophysiological recordings from the neuromuscular junction of these mutants reveal that the excitatory synaptic current evoked by stimulation of the motor neuron is abolished entirely. However, spontaneous release of quanta from these terminals persists, although its rate is reduced by 75%. Thus, at least a portion of the spontaneous “minis” that are seen at the synapse can be generated by a protein complex that is distinct from that required for an evoked synaptic response.
Deficiency in Protein l-Isoaspartyl Methyltransferase Results in a Fatal Progressive Epilepsy
Authors: Yamamoto Akihiro , Takagi Hideyuki , Kitamura Daisuke , Tatsuoka Hozumi , Nakano Hirotake , Kawano Hitoshi , Kuroyanagi Hidehito , Yahagi Yu-ichi , Kobayashi Shin-ichiro , Koizumi Ken-ichi , Sakai Tsuyoshi , Saito Ken-ichi , Chiba Tanemichi , Kawamura Koki , Suzuki Katsushi , Watanabe Takeshi , Mori Hiroshi , Shirasawa Takuji
Published: April 2018

Proteinl-isoaspartyl methyltransferase (PIMT) is suggested to play a role in the repair of aged protein spontaneously incorporated with isoaspartyl residues. We generated PIMT-deficient mice by targeted disruption of thePIMTgene to elucidate the biological role of the genein vivo. PIMT-deficient mice died from progressive epileptic seizures with grand mal and myoclonus between 4 and 12 weeks of age. An anticonvulsive drug, dipropylacetic acid (DPA), improved their survival but failed to cure the fatal outcome.l-Isoaspartatate, the putative substrate for PIMT, was increased ninefold in the brains of PIMT-deficient mice. The brains of PIMT-deficient mice started to enlarge after 4 weeks of age when the apical dendrites of pyramidal neurons in cerebral cortices showed aberrant arborizations with disorganized microtubules. We conclude that methylation of modified proteins with isoaspartyl residues is essential for the maintenance of a mature CNS and that a deficiency in PIMT results in fatal progressive epilepsy in mice.
Behavioral Studies of Sound Localization in the Cat
Authors: Populin Luis C. , Yin Tom C. T.
Published: April 2018

Using the magnetic search coil technique to measure eye and ear movements, we trained cats by operant conditioning to look in the direction of light and sound sources with their heads fixed. Cats were able to localize noise bursts, single clicks, or click trains presented from sources located on the horizontal and vertical meridians within their oculomotor range. Saccades to auditory targets were less accurate and more variable than saccades to visual targets at the same spatial positions. Localization accuracy of single clicks was diminished compared with the long-duration stimuli presented from the same sources. Control experiments with novel auditory targets, never associated with visual targets, demonstrated that the cats localized the sound sources using acoustic cues and not from memory.

The role of spectral features imposed by the pinna for vertical sound localization was shown by the breakdown in localization of narrow-band (one-sixth of an octave) noise bursts presented from sources along the midsagittal plane. In addition, we show that cats experience summing localization, an illusion associated with the precedence effect. Pairs of clicks presented from speakers at (±18°,0°) with interclick delays of ±300 μsec were perceived by the cat as originating from phantom sources extending from the midline to approximately ±10°.
Distribution of the Tetrodotoxin-Resistant Sodium Channel PN3 in Rat Sensory Neurons in Normal and Neuropathic Conditions
Authors: Novakovic Sanja D. , Tzoumaka Elda , McGivern Joseph G. , Haraguchi Miki , Sangameswaran Lakshmi , Gogas Kathleen R. , Eglen Richard M. , Hunter John C.
Published: April 2018

The novel sodium channel PN3/α-SNS, which was cloned from a rat dorsal root ganglion (DRG) cDNA library, is expressed predominantly in small sensory neurons and may contribute to the tetrodotoxin-resistant (TTXR) sodium current that is believed to be associated with central sensitization in chronic neuropathic pain states. To assess further the role of PN3, we have used electrophysiological,in situhybridization and immunohistochemical methods to monitor changes in TTXRsodium current and the distribution of PN3 in normal and peripheral nerve-injured rats. (1) Whole-cell patch-clamp recordings showed that there were no significant changes in the TTXRand TTX-sensitive sodium current densities of small DRG neurons after chronic constriction injury (CCI) of the sciatic nerve. (2) Additionally,in situhybridization showed that there was no change in the expression of PN3 mRNA in the DRG up to 14 d after CCI. PN3 mRNA was not detected in sections of brain and spinal cord taken from either normal or nerve-injured rats. (3) In contrast, immunohistochemical studies showed that major changes in the subcellular distribution of PN3 protein were caused by either CCI or complete transection of the sciatic nerve. The intensity of PN3 immunolabeling decreased in small DRG neurons and increased in sciatic nerve axons at the site of injury. The alteration in immunolabeling was attributed to translocation of presynthesized, intracellularly located PN3 protein from neuronal somata to peripheral axons, with subsequent accumulation at the site of injury. The specific subcellular redistribution of PN3 after peripheral nerve injury may be an important factor in establishing peripheral nerve hyperexcitability and resultant neuropathic pain.
Hippocampal Lesions Disrupt an Associative Mismatch Process
Authors: Honey R. C. , Watt A. , Good M.
Published: April 2018

Novel assays were used to assess inter alia whether the hippocampus is involved in detecting novelty per se or in an associative mismatch process. During training, rats received two audiovisual sequences (tone–left constant light and click–left flashing light). In both sham-operated control rats and those with excitotoxic hippocampal lesions, novel visual targets provoked an orienting response that habituated during training. Moreover, like sham-operated rats, rats with hippocampal lesions acquired associations between the elements of two audiovisual sequences. However, subsequent test trials in which the auditory stimuli preceding the visual targets were switched (click–left constant light and tone–left flashing light) provoked renewed orienting to the visual targets in sham-operated rats but not in hippocampal rats. These results support the view that hippocampal damage results in a failure to detect (or act on) mismatches that are generated when an auditory stimulus associatively evokes the memory of one visual stimulus and a different (familiar) visual stimulus is present in the environment.
Action Potentials Are Required for the Lateral Transmission of Glycinergic Transient Inhibition in the Amphibian Retina
Authors: Cook Paul B. , Lukasiewicz Peter D. , McReynolds John S.
Published: April 2018

Transient lateral inhibition (TLI), the suppression of responses of a ganglion cell to light stimuli in the receptive field center by changes in illumination in the receptive field surround, was studied in light-adapted mud puppy and tiger salamander retinas using both eyecup and retinal slice preparations. In the eyecup, TLI was measured in on–off ganglion cells as the ability of rotating, concentric windmill patterns of 500–1200 μm inner diameter to suppress the response to a small spot stimulus in the receptive field center. Both the suppression of the spot response and the hyperpolarization produced in ganglion cells by rotation of the windmill were blocked in the presence of 2 μmstrychnine or 500 nmtetrodotoxin (TTX), but not by 150 μmpicrotoxin. In the slice preparation in which GABA-mediated currents were blocked with picrotoxin, IPSCs elicited by diffuse illumination were blocked by strychnine and strongly reduced by TTX. The TTX-resistant component was probably attributable to illumination of the receptive field center. TTX had a much greater effect in reducing the glycinergic inhibition elicited by laterally displaced stimulation versus nearby focal electrical stimulation. Strychnine enhanced light-evoked excitatory currents in ganglion cells, but this was not mimicked by TTX. The results suggest that local glycinergic transient inhibition does not require action potentials and is mediated by synapses onto both ganglion cell dendrites and bipolar cell terminals. In contrast, the lateral spread of this inhibition (at least over distances >250 μm) requires action potentials and is mainly onto ganglion cell dendrites.
Gαs-Induced Neurodegeneration inCaenorhabditis elegans
Authors: Berger Allison J. , Hart Anne C. , Kaplan Joshua M.
Published: April 2018

We describe a genetic model for neurodegeneration in the nematodeCaenorhabditis elegans. Constitutive activation of the GTP-binding protein Gαsinduces neurodegeneration. Neuron loss occurs in two phases whereby affected cells undergo a swelling response in young larvae and subsequently die sometime during larval development. Different neural cell types vary greatly in their susceptibility to Gαs-induced cytotoxicity, ranging from 0 to 88% of cells affected. Mutations that prevent programmed cell death do not prevent Gαs-induced killing, suggesting that these deaths do not occur by apoptosis. Mutations in three genes protect against Gαs-induced cell deaths. Theacy-1gene is absolutely required for neurodegeneration, and the predicted ACY-1 protein is highly similar (40% identical) to mammalian adenylyl cyclases. Thus, Gs-induced neurodegeneration is mediated by the second messenger cAMP. Mutations in theunc-36andeat-4genes are partially neuroprotective, which indicates that endogenous signaling modulates the severity of the neurotoxic effects of Gαs. These experiments define an intracellular signaling cascade that triggers a necrotic form of neurodegeneration.
The Expression of Two Splice Variants of the Kv3.1 Potassium Channel Gene Is Regulated by Different Signaling Pathways
Authors: Liu Si-qiong J. , Kaczmarek Leonard K.
Published: April 2018

The Kv3.1 potassium channel gene gives rise to two different channel proteins, Kv3.1a and Kv3.1b, by alternative splicing of nuclear RNA. During development the levels of Kv3.1b mRNA (but not Kv3.1a) substantially increase in rat cerebellum after postnatal day 8. The molecular mechanism underlying the differential regulation of the two transcripts is not known. Usingin vitroslices of cerebellum, we have found that basic fibroblast growth factor (bFGF) upregulates both Kv3.1a and Kv3.1b at this developmental stage, but that depolarization by elevated potassium concentrations is without effect. Combined treatment with bFGF and depolarization, however, prevents the increase in Kv3.1a transcripts and selectively increases Kv3.1b mRNA levels. A protein kinase C (PKC) inhibitor blocks the increase in Kv3.1a mRNA levels induced by bFGF alone but does not affect the increase in Kv3.1b mRNA. Measurement of nuclear protein kinase C activity shows that bFGF activates this enzyme and that depolarization blocks this activation. In contrast to these findings at postnatal day 8, bFGF fails to alter Kv3.1 transcripts in slices from adult animals, and PKC activity is enhanced rather than suppressed by depolarization. Our results indicate that different signaling pathways regulate Kv3.1a and Kv3.1b expression and suggest that Kv3.1a mRNA levels may be modulated by neuronal activity.
A Ca2+-Independent Receptor for α-Latrotoxin, CIRL, Mediates Effects on Secretion via Multiple Mechanisms
Authors: Bittner Mary A. , Krasnoperov Valery G. , Stuenkel Edward L. , Petrenko Alexander G. , Holz Ronald W.
Published: April 2018

α-Latrotoxin (α-Ltx), a component of black widow spider venom, stimulates secretion from nerve terminals and from PC12 cells. In this study we examine the effects of expression of a newly cloned Ca2+-independent receptor for α-Ltx (CIRL) on secretion from bovine chromaffin cells. We first characterized the effect of α-Ltx on secretion from untransfected cells. α-Ltx, by binding in a Ca2+-independentmanner to an endogenous receptor, causes subsequent Ca2+-dependent secretion from intact cells. The stimulation of secretion is correlated with Ca2+influx caused by the toxin. In permeabilized cells in which the Ca2+concentration is regulated by buffer, α-Ltx also enhances Ca2+-dependent secretion, indicating a direct role of the endogenous receptor in the secretory pathway. Expression of CIRL increased the sensitivity of intact and permeabilized cells to the effects of α-Ltx, demonstrating that this protein is functional in coupling to secretion. Importantly, in the absence of α-Ltx, the expression of CIRL specifically inhibited the ATP-dependent component of secretion in permeabilized cells without affecting the ATP-independent secretion. This suggests that this receptor modulates the normal function of the regulated secretory pathway and that α-Ltx may act by reversing the inhibitory effects of the receptor.
The Role of an α Subtype M2-M3His in Regulating Inhibition of GABAAReceptor Current by Zinc and Other Divalent Cations
Authors: Fisher Janet L. , Macdonald Robert L.
Published: April 2018

Sensitivity of GABAAreceptors (GABARs) to inhibition by zinc and other divalent cations is influenced by the α subunit subtype composition of the receptor. For example, α6β3γ2L receptors are more sensitive to inhibition by zinc than α1β3γ2L receptors. We examined the role of a His residue located in the M2-M3extracellular domain (rat α6 H273) in the enhanced zinc sensitivity conferred by the α6 subtype. The α1 subtype contains an Asn (N274) residue in the equivalent location. GABA-activated whole-cell currents were obtained from L929 fibroblasts after transient transfection with expression vectors containing GABAAreceptor cDNAs. Mutation of α1 (α1(N274H)) or α6 (α6(H273N)) subtypes did not alter the GABA EC50of αβ3γ2L receptors. α1(N274H)β3γ2L receptor currents were as sensitive to zinc as α6β3γ2L receptor currents, although α6(H273N)β3γ2L receptor currents had the reduced zinc sensitivity of α1β3γ2L receptor currents. We also examined the activity of other inhibitory divalent cations with varying α subtype dependence: nickel, cadmium, and copper. α6β3γ2L receptor currents were more sensitive to nickel, equally sensitive to cadmium, and less sensitive to copper than α1β3γ2L receptor currents. Studies with α1 and α6 chimeric subunits indicated that the structural dependencies of the activity of some of these cations were different from zinc. Compared with α6β3γ2L receptor currents, α6(H273N)β3γ2L receptor currents had reduced sensitivity to cadmium and nickel, but the sensitivity to copper was unchanged. Compared with α1β3γ2L receptor currents, α1(N274H)β3γ2L receptor currents had increased sensitivity to nickel, but the sensitivity to cadmium and copper was unchanged. These findings indicate that H273 of the α6 subtype plays an important role in determining the sensitivity of recombinant GABARs to the divalent cations zinc, cadmium, and nickel, but not to copper. Our results also suggest that the extracellular N-terminal domain of the α1 subunit contributes to a regulatory site(s) for divalent cations, conferring high sensitivity to inhibition by copper and cadmium.
Electrical Properties of Frog Saccular Hair Cells: Distortion by Enzymatic Dissociation
Authors: Armstrong Cecilia E. , Roberts William M.
Published: April 2018

Although it is widely accepted that the electrical resonance seen in many types of auditory and vestibular hair cells contributes to frequency selectivity in these sensory systems, unexplained discrepancies in the frequency (f) and sharpness (Q) of tuning have raised serious questions. For example, enzymatically dissociated hair cells from bullfrog (Rana catesbeiana) sacculus resonate at frequencies well above the range of auditory and seismic stimuli to which the sacculus is most responsive. Such disparities, in addition to others, have led to the proposal that electrical resonance alone cannot account for frequency tuning. Using grassfrog (Rana pipiens) saccular hair cells, we show that the reported discrepancies infandQin this organ can be explained by the deleterious effects of enzyme (papain) exposure during cell dissociation. In patch-clamp studies of hair cells in a semi-intact epithelial preparation, we observed a variety of voltage behaviors with frequencies of 35–75 Hz. This range is well below the range of resonant frequencies observed in enzymatically dissociated hair cells and more in tune with the frequency range of natural stimuli to which the sacculus is maximally responsive. The sharpness of tuning also agreed with previous studies using natural stimuli. In contrast to results from enzymatically dissociated hair cells, both a calcium-activated K+(KCa) current and a voltage-dependent K+(KV) current contributed to the oscillatory responses of hair cells in the semi-intact preparation. The properties of the KCaand the Ca2+current were altered by enzymatic dissociation. KVand a small-conductance calcium-activated K+current were apparently eliminated.
Regulation of Presynaptic NMDA Responses by External and Intracellular pH Changes at Developing Neuromuscular Synapses
Authors: Chen Yu-Hwa , Wu Mei-Lin , Fu Wen-Mei
Published: April 2018

NMDA receptors play important roles in synaptic plasticity and neuronal development. The functions of NMDA receptors are modulated by many endogenous substances, such as external pH (pHe), as well as second messenger systems. In the present study, the nerve–muscle cocultures ofXenopusembryos were used to investigate the effects of both external and intracellular pH (pHi) changes on the functional responses of presynaptic NMDA receptors. Spontaneous synaptic currents (SSCs) were recorded from innervated myocyte using whole-cell recordings. Local perfusion of NMDA at synaptic regions increased the SSC frequency via the activation of presynaptic NMDA receptors. A decrease in pHefrom 7.6 to 6.6 reduced NMDA responses to 23% of the control, and an increase in pHefrom 7.6 to 8.6 potentiated the NMDA responses in increasing SSC frequency. The effect of NMDA on intracellular Ca2+concentration ([Ca2+]i) was also affected by pHechanges: external acidification inhibited and alkalinization potentiated [Ca2+]iincreases induced by NMDA. Intracellular pH changes of single soma were measured by ratio fluorometric method using 2,7-bis (carboxyethyl)-5,6-carboxyfluorescein (BCECF). Cytosolic acidification was used in which NaCl in Ringer’s solution was replaced with weak organic acids. Acetate and propionate but not methylsulfate substitution caused intracellular acidification and potentiated NMDA responses in increasing SSC frequency, intracellular free Ca2+concentration, and NMDA-induced currents. On the other hand, cytosolic alkalinization with NH4Cl did not significantly affect these NMDA responses. These results suggest that the functions of NMDA receptors are modulated by both pHeand pHichanges, which may occur in some physiological or pathological conditions.
Reversal of Age-Related Alterations in Synaptic Plasticity by Blockade of L-Type Ca2+Channels
Authors: Norris Christopher M. , Halpain Shelley , Foster Thomas C.
Published: April 2018

The role of L-type Ca2+channels in the induction of synaptic plasticity in hippocampal slices of aged (22–24 months) and young adult (4–6 months) male Fischer 344 rats was investigated. Prolonged 1 Hz stimulation (900 pulses) of Schaffer collaterals, which normally depresses CA3/CA1 synaptic strength in aged rat slices, failed to induce long-term depression (LTD) during bath application of the L-channel antagonist nifedipine (10 μm). When 5 Hz stimulation (900 pulses) was used to modify synaptic strength, nifedipine facilitated synaptic enhancement in slices from aged, but not young, adult rats. This enhancement was pathway-specific, reversible, and impaired by the NMDA receptor (NMDAR) antagonistdl-2-amino-5-phosphonopentanoic acid (AP5). Induction of long-term potentiation (LTP) in aged rats, using 100 Hz stimulation, occluded subsequent synaptic enhancement by 5 Hz stimulation, suggesting that nifedipine-facilitated enhancement shares mechanisms in common with conventional LTP. Facilitation of synaptic enhancement by nifedipine likely was attributable to a reduction (∼30%) in the Ca2+-dependent K+-mediated afterhyperpolarization (AHP), because the K+channel blocker apamin (1 μm) similarly reduced the AHP and promoted synaptic enhancement by 5 Hz stimulation. In contrast, apamin did not block LTD induction using 1 Hz stimulation, suggesting that, in aged rats, the AHP does not influence LTD and LTP induction in a similar way. The results indicate that, during aging, L-channels can (1) facilitate LTD induction during low rates of synaptic activity and (2) impair LTP induction during higher levels of synaptic activation via an increase in the Ca2+-dependent AHP.
Calcium-Sensitive Particulate Guanylyl Cyclase as a Modulator of cAMP in Olfactory Receptor Neurons
Authors: Moon Cheil , Jaberi Parham , Otto-Bruc Annie , Baehr Wolfgang , Palczewski Krzysztof , Ronnett Gabriele V.
Published: April 2018

The second messengers cAMP and inositol-1,4,5-triphosphate have been implicated in olfaction in various species. The odorant-induced cGMP response was investigated using cilia preparations and olfactory primary cultures. Odorants cause a delayed and sustained elevation of cGMP. A component of this cGMP response is attributable to the activation of one of two kinetically distinct cilial receptor guanylyl cyclases by calcium and a guanylyl cyclase-activating protein (GCAP). cGMP thus formed serves to augment the cAMP signal in a cGMP-dependent protein kinase (PKG) manner by direct activation of adenylate cyclase. cAMP, in turn, activates cAMP-dependent protein kinase (PKA) to negatively regulate guanylyl cyclase, limiting the cGMP signal. These data demonstrate the existence of a regulatory loop in which cGMP can augment a cAMP signal, and in turn cAMP negatively regulates cGMP production via PKA. Thus, a small, localized, odorant-induced cAMP response may be amplified to modulate downstream transduction enzymes or transcriptional events.
High Constitutive NF-κB Activity Mediates Resistance to Oxidative Stress in Neuronal Cells
Authors: Lezoualc’h Frank , Sagara Yutaka , Holsboer Florian , Behl Christian
Published: April 2018

Selected clones of the sympathetic precursor-like cell line PC12 (rCl8) are resistant to oxidative cell death induced by the Alzheimer’s disease-associated amyloid β protein (Aβ) and hydrogen peroxide (H2O2). Here, we show that the transcriptional activity and DNA binding activity of the redox-sensitive transcription factor NF-κB and its nuclear expression are constitutively increased in rCl8 cells compared with their nonresistant parental PC12 cell (PC12p) counterpart. Suppression of the transcriptional activity of NF-κB in rCl8 cells with the synthetic glucocorticoid dexamethasone or by direct overexpression of a super-repressor mutant form of IκBα, a specific inhibitor of NF-κB, reversed the oxidative stress resistance phenotype of these cells and ultimately led to increased cell death after the challenge with H2O2. Dexamethasone treatment also caused an increase in the protein level of IκBα. Our data show that an increased baseline of NF-κB activity may mediate the resistance of these cells of neuronal origin to oxidative stress. Therefore, the presented model may help to identify possible neuronal target genes of NF-κB and to further elucidate the molecular basis of the differential sensitivity of neurons in neurodegenerative conditions associated with an increased oxidative burden, such as in Alzheimer’s disease.
GABAergic Cells Are the Major Postsynaptic Targets of Mossy Fibers in the Rat Hippocampus
Authors: Acsády László , Kamondi Anita , Sı́k Attila , Freund Tamás , Buzsáki György
Published: April 2018

Dentate granule cells communicate with their postsynaptic targets by three distinct terminal types. These include the large mossy terminals, filopodial extensions of the mossy terminals, and smalleren passantsynaptic varicosities. We examined the postsynaptic targets of mossy fibers by combiningin vivointracellular labeling of granule cells, immunocytochemistry, and electron microscopy. Single granule cells formed large, complex “mossy” synapses on 11–15 CA3 pyramidal cells and 7–12 hilar mossy cells. In contrast, GABAergic interneurons, identified with immunostaining for substance P-receptor, parvalbumin, and mGluR1a-receptor, were selectively innervated by very thin (filopodial) extensions of the mossy terminals and by smallen passantboutons in both the hilar and CA3 regions. These terminals formed single, often perforated, asymmetric synapses on the cell bodies, dendrites, and spines of GABAergic interneurons. The number of filopodial extensions and small terminals was 10 times larger than the number of mossy terminals. These findings show that in contrast to cortical pyramidal neurons, (1) granule cells developed distinct types of terminals to affect interneurons and pyramidal cells and (2) they innervated more inhibitory than excitatory cells. These findings may explain the physiological observations that increased activity of granule cells suppresses the overall excitability of the CA3 recurrent system and may form the structural basis of the target-dependent regulation of glutamate release in the mossy fiber system.
Patchy Distribution of NMDAR1 Subunit Immunoreactivity in Developing Visual Cortex
Authors: Trepel Christopher , Duffy Kevin R. , Pegado Victor D. , Murphy Kathryn M.
Published: April 2018

Development of ocular dominance columns is dependent on patterned retinal activity, and yet patterned activity alone cannot explain all aspects of cortical column development. Features intrinsic to the cortex have been proposed to interact with activity to guide the patterning of cortical columns (Jones et al., 1991), and the NMDA receptor, because of its role in experience-dependent plasticity, is an obvious candidate. Using immunohistochemical techniques, we found a transiently patchy distribution of the NMDA receptor 1 (NMDAR1) subunit in kitten visual cortex. Regularly spaced patches of NMDAR1-immunoreactive neurons were found at the top of the cortical plate in the developing visual cortex at 2 weeks of age. At 4–5 weeks of age, the radial extent of the NMDAR1 patches spanned the supragranular layers, and by 12 weeks of age, this nonuniform pattern of NMDAR1 immunostaining was no longer apparent. Monocular visual experience prevented the expression of the NMDAR1 patches, but just 4 d of subsequent binocular visual experience was sufficient to promote expression of the patches. Furthermore, the NMDAR1 patches tended to be associated with the borders of ocular dominance columns. These results suggest that the degree of plasticity associated with NMDA-mediated mechanisms is elevated in local regions across the tangential extent of the visual cortex and that the NMDAR1 patches may participate in sculpting the overall arrangement of visual cortical columns.
Response to Change Is Facilitated by a Three-Neuron Disinhibitory Pathway in the Tiger Salamander Retina
Authors: Roska Botond , Nemeth Erik , Werblin Frank S.
Published: April 2018

Most retinal ganglion cells respond only transiently, for ∼150 msec at the onset and termination of a light flash. The responses are transient because it has been shown that bipolar-to-ganglion cell transmission is truncated after 150 msec by a feedback inhibition to bipolar cell terminals. The feedback inhibition itself must be delayed by ∼150 msec to allow the initial bipolar–ganglion cell transmission. This study identifies a three-component serial synaptic pathway from glycinergic amacrine cells to GABAergic amacrine cells to bipolar cell terminals as one source of this delay. We used perforated and whole-cell patch-clamp recordings to measure the timing of light responses in amacrine, bipolar, and ganglion cells under control and glycine and GABA receptor-blocked conditions. Our results suggest that, after a light flash, a population of glycinergic amacrine cells responds first, inhibiting a population of GABAergic amacrine cells for ∼150 msec. The GABAergic amacrine cells feed back to bipolar terminals, but only after the 150 msec delay, allowing the bipolar terminals to excite ganglion cells for the first 150 msec. Blocking the glycinergic amacrine cell activity with strychnine allows the GABAergic system to become active earlier. GABAergic amacrine cells then inhibit release from bipolar cells earlier. Under these conditions, the ganglion cell response to change would be decreased.
Region-Specific and Calcium-Dependent Increase in Dialysate Choline Levels by NMDA
Authors: Zapata Agustí , Capdevila Jordi L. , Trullas Ramon
Published: April 2018

NMDA receptor-induced excitotoxicity has been hypothesized to mediate abnormal choline (Cho) metabolism that is involved in alterations in membrane permeability and cell death in certain neurodegenerative disorders. To determine whether NMDA receptor overactivation modulates choline metabolismin vivo, we investigated the effects of NMDA on interstitial choline concentrations using microdialysis. Perfusion of NMDA by retrodialysis increased dialysate choline (∼400%) and reduced dialysate acetylcholine (Ach) (∼40%). Choline levels remained increased for at least 2.5 hr, but acetylcholine returned to pretreatment values 75 min after NMDA perfusion. The NMDA-evoked increase in dialysate choline was calcium and concentration dependent and was prevented with 1 mmAP-5, a competitive NMDA antagonist, but was not altered by mepacrine, a phospholipase A2inhibitor. NMDA increased extracellular choline levels four- to fivefold in prefrontal cortex and hippocampus, produced a slight increase in neostriatum, and did not modify dialysate choline in cerebellum. Perfusion with NMDA for 2 hr produced a delayed, but not acute, reduction in choline acetyltransferase activity in the area surrounding the dialysis probe. Consistent with a lack of acute cholinergic neurotoxicity evoked by this treatment, basal acetylcholine levels were unaltered by 2 hr of continuous NMDA perfusion. Prolonged NMDA perfusion produced a 34% decrease in phosphatidylcholine content in the lipid fraction of the tissue surrounding the dialysis probe. These results show that NMDA modulates choline metabolism, eliciting a receptor-mediated, calcium-dependent, and region-specific increase in extracellular choline from membrane phospholipids that is not mediated by phospholipase A2and precedes delayed excitotoxic neuronal cell death.
Suppression of Sprouting: An Early Function of NMDA Receptors in the Absence of AMPA/Kainate Receptor Activity
Authors: Lin Shuh-Yow , Constantine-Paton Martha
Published: April 2018

Electrophysiological studies have documented the existence of synapses showing only NMDA ionotropic glutamate receptor function that are therefore presumably “silent” at resting membrane potentials. Silent synapses are more prevalent in young than in older neurons, and NMDA receptor activity at such contacts may facilitate the appearance of functional AMPA receptors. However, it is uncertain whether such silent synapses actually have a function in young neurons independent of AMPA receptor induction. Using a newly characterized culture system for neurons from larvalXenopustecta, we show that blocking NMDA receptors or preventing changes in intracellular free Ca2+concentration with BAPTA AM significantly increases neurite sprouting and elongation in contacted but not in isolated neurons. Blocking AMPA/KA receptors or Na+-dependent action potentials does not mimic this effect. Moreover, in these young neurons, NMDA receptor-dependent Ca2+responses to glutamate measured with confocal fluo-3 imaging are retained during AMPA/KA receptor blockade. The data suggest that many of the young contacts in these cultures are active even though they use only NMDA ionotropic glutamate receptors. Calcium influx through the NMDA receptor at these contacts seems to reduce neurite motility. This effect should lead to the accumulation of glutamatergic inputs on NMDA receptor-expressing dendrites, which could facilitate the onset of AMPA/KA receptor function and the action potential-dependent phase of synaptogenesis.