Silent Cells Research Articles

Hippocampal Place Fields Emerge upon Single-Cell Manipulation of Excitability During Behavior

The origin of the spatial receptive fields of hippocampal place cells has not been established. A hippocampal CA1 pyramidal cell receives thousands of synaptic inputs, mostly from other spatially tuned neurons; however, how the postsynaptic neuron's cellular properties determine the response to these inputs during behavior is unknown. We discovered that, contrary to expectations from basic models of place cells and neuronal integration, a small, spatially uniform depolarization of the spatially untuned somatic membrane potential of a silent cell leads to the sudden and reversible emergence of a spatially tuned subthreshold response and place-field spiking. Such gating of inputs by postsynaptic neuronal excitability reveals a cellular mechanism for receptive field origin and may be critical for the formation of hippocampal memory representations.

Neural Representations of Location Composed of Spatially Periodic Bands

The mammalian hippocampal formation provides neuronal representations of environmental location, but the underlying mechanisms are poorly understood. Here, we report a class of cells whose spatially periodic firing patterns are composed of plane waves (or bands) drawn from a discrete set of orientations and wavelengths. The majority of cells recorded in parasubicular and medial entorhinal cortices of freely moving rats belonged to this class and included grid cells, an important subset that corresponds to three bands at 60° orientations and has the most stable firing pattern. Occasional changes between hexagonal and nonhexagonal patterns imply a common underlying mechanism. Our results indicate a Fourier-like spatial analysis underlying neuronal representations of location, and suggest that path integration is performed by integrating displacement along a restricted set of directions.

Analysis of Expression of Genes Involved in Immune Response Modulation in Silent Multipotent Mesenchymal Stromal Cells

The expression of some genes modulating the immune response was studied in multipotent mesenchymal stromal cells (MMSC) from the bone marrow of a healthy donor. Non-activated MMSC expressed IL-6 and IL-10, complement H factor, macrophage growth factor, prostaglandin E2 synthase, and indoleamine-2,3-dioxygenase. The expression of all these genes was higher in female MMSC. A close inverse relationship between IL-6 expression in MMSC and male donor age, close relationship between body weight index and fibroblast CFU concentration in female donor bone marrow and between indoleamine-2,3-dioxygenase and macrophage growth factor in MMSC from these donors were detected. The expression of the analyzed genes was higher in MMSC of donors who had no antibodies to cytomegalovirus, herpes simplex virus, and Epstein-Barr virus in the blood. The results demonstrate the MMSC regulation of immune reactions by MMSC at the cell and organism levels.

Abstract 209: Identification of β-catenin lysine 394 as a novel charge button and ubiquitination site by Rad6B

Abstract Clinical evidence suggests that β-catenin accumulation in breast cancer is associated with poor prognosis. Accumulation of β-catenin occurs even though activating mutations are rare. However, the mechanisms responsible for stabilization of β-catenin in breast cancer are not well known. We have previously shown that Rad6B stabilizes β-catenin by polyubiquitin modifications that render β-catenin insensitive to 26S proteasome. Rad6B silencing in breast cancer cells with autocrine Wnt activity inhibits β-catenin polyubiquitination and β-catenin mediated transcriptional activity. In this study, we identify the Rad6B-induced site of β-catenin ubiquitination. To map the lysine(s) of β-catenin that are ubiquitinated by Rad6B, in vitro ubiquitination assays were performed with recombinant Rad6B and extracts of COS7 cells transiently transfected with various lengths of myc-tagged β-catenin. These experiments indicated lysine residues located between amino acids 181-422 of β-catenin as potential targets of Rad6B mediated ubiquitination sites. To identify the Rad6B responsive ubiquitination site(s) in β-catenin, lysines 312, 335, 345, 354 or 394 located in the transcriptional regulatory domain in armadillo repeats (ARM) 5-7 were mutated to arginine as this maintains the charge distribution and minimizes the risk of disturbing the superhelical armadillo domain structure. In vitro ubiquitination assays identified K394 located in ARM 7 as the major site of Rad6B-induced ubiquitination. In vivo ubiquitination assays in Wnt silent MCF-7 breast cancer cells showed impairment of K345R- and K394R-β-catenin ubiquitination. The impact of these mutations on β-catenin mediated reporter transactivation was determined in MCF-7 cells. Whereas K312R, K335R, K345R or K354R mutations caused none to marginal drop in TOP-mediated reporter activation as compared to wild type β-catenin, K394R mutation of β-catenin caused ∼50% drop in TOP/Flash activity. Consistent with the TOP reporter transactivation data, expression of Rad6B, itself a β-catenin/TCF transcriptional target, was reduced in K394R-β-catenin transfected MCF-7 cells. In vitro ubiquitination assays using recombinant wild type or catalytically inert Rad6B and a synthetic 29 amino acid β-catenin peptide encompassing the K394 residue confirmed Rad6B ubiquitination of K394. We observed decreased expression of K394R-β-catenin in vivo. Treatment with the proteasomal inhibitor MG132 failed to rescue its expression, indicating that decreased expression is not due to proteasomal degradation. These data reveal K394 as a novel site of β-catenin ubiquitination and a novel charge button that may be important for maintaining the overall structure and stability of β-catenin. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 209. doi:1538-7445.AM2012-209

Research of SOCS1 silent DC vaccine on laryngocarcinoma therapy

Investigate the specific antitumor mechanism of SOCS1 silent DC vaccine and discuss the prospect of RNAi in the gene therapy for laryngocarcinoma in order to provide novel ideas of DCs clinical applications. Dendritic cells derived from peripheral blood monocytes were cultured in vitro in the presence of GM-CSF, IL-4 and TNF-alpha. The morphological feature of DC was observed with inverted microscope. RNAi vector were transfected into DC. The expression of SOCS1 protein was detected with Western blot. The effective target sequences of siRNA against SOCS1 were screened out. The surface markers of mature DC, including CD83, CD86 and HLA-DR, were detected with flow cytometry. The concentration of IFN-gamma in the supernatant was assayed by ELISA. The proliferative ability of T cell stimulated by DC and the specific killing activity of cytotoxic T lymphocyte (CTL) induced by DC were evaluated by MTT assay. Dendritic cells were obtained successfully. The RNAi vector was proved to be right by sequencing. The expression of SOCS1 decreased significantly under the influence of the 5th interference sequence. SOCS1 silent dendritic cells which were loaded with Hep-2 antigen had high expressions of CD83 (85.61 +/- 0.96)%, CD86 (96.86 +/- 1.20)% and HLA-DR (98.02 +/- 0.94)%. It could also stimulate the proliferation of T cells effectively as well as could increase the production of IFN-gamma, eventually enhanced the specific killing effect of CTL. The killing activity was more higher than that in control group when the effect cells and target cells were mixed up at the ratio of 50:1 (P < 0.01). SOCS1 silent DC vaccines which were loaded with Hep-2 antigen could induce effective and specific anti-laryngocarcinoma immune responses.

Remapping-like changes in place field activity through dopaminergic action

Event Abstract Back to Event Remapping-like changes in place field activity through dopaminergic action Moritz Von Heimendahl1* and Michael Brecht1 1 Humboldt Universität, Bernstein Center for Compuational Neuroscience, Germany The phenomenon of hippocampal remapping has received sustained attention ever since its discovery. Remapping is striking because usually, most place cells in the rodent hippocampus show stable response properties over weeks and more. However, strong changes in the environment lead to global remapping, whereby most place cells change their firing fields drastically or shut down altogether, while new, previously silent cells become active. While the kind of environmental cues that elicit remapping have been studied in detail, it is still unclear what kind of internal signal prompts the otherwise so stable place representations to change all at once. Dopamine is often discussed as a potential novelty signal and here we investigated the effects of dopamine agonists on place cell activity. To this end, we recorded the spatial response properties of pyramidal cells in the dorsal hippocampus of rats while they freely explored an environment. Systemic injection of the nonspecific dopamine agonist apomorphine, but not of saline, led to changes in place field locations of many cells, similar to changes observed in global remapping. In further experiments, the dose-dependence, role of the specific receptor subtypes, and the effect of dopamine antagonists were examined. The finding that dopaminergic action induces remapping-like place field changes shows that global remapping can occur in the absence of environmental changes. Pharmacological manipulation opens up new avenues to analyze the mechanisms underlying global remapping. Acknowledgements This work was supported by Humboldt Universität zu Berlin, the Bernstein Center for Computational Neuroscience Berlin, the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung) (Förderkennzeichen 01GQ1001A), Neurocure, and a European Research Council grant (M.B.). We thank Brigitte Geue and Undine Schneeweiss for technical assistance. Keywords: Dopamine, Hippocampus, remapping Conference: Bernstein Conference 2012, Munich, Germany, 12 Sep - 14 Sep, 2012. Presentation Type: Poster Topic: Learning, plasticity, memory Citation: Von Heimendahl M and Brecht M (2012). Remapping-like changes in place field activity through dopaminergic action. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference 2012. doi: 10.3389/conf.fncom.2012.55.00199 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 11 May 2012; Published Online: 12 Sep 2012. * Correspondence: Dr. Moritz Von Heimendahl, Humboldt Universität, Bernstein Center for Compuational Neuroscience, Berlin, 10115, Germany, moritz@heimendahl.net Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Moritz Von Heimendahl Michael Brecht Google Moritz Von Heimendahl Michael Brecht Google Scholar Moritz Von Heimendahl Michael Brecht PubMed Moritz Von Heimendahl Michael Brecht Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

The Mechanism of Injury-Induced Intracellular Calcium Concentration Oscillations in the Endothelium of Excised Rat Aorta

Endothelial injury is the primary event that leads to a variety of severe vascular disorders. Mechanical injury elicits a Ca²⁺ response in the endothelium of excised rat aorta, which comprises an initial Ca²⁺ release from inositol-1,4,5-trisphosphate (InsP₃)-sensitive stores followed by a long-lasting decay phase due to Ca²⁺ entry through uncoupled connexons. The Ca²⁺ signal may also adopt an oscillatory pattern, the molecular underpinnings of which are unclear. In the light of the role played by Ca²⁺ spiking in tissue regeneration, this study aimed to unveil the mechanisms underlying injury-induced Ca²⁺ oscillations. The latter reversibly ceased upon removal of extracellular Ca²⁺ or addition of the gap junction blockers heptanol, 18 α,β-glycyrrhetinic acid, La³⁺ and Ni²⁺, but were insensitive to BTP-2 and SKF 96365. The spiking response was abolished by inhibiting the Ca²⁺ entry mode of the Na⁺/Ca²⁺ exchanger (NCX). The InsP₃-producing agonist ATP resumed Ca²⁺ oscillations in silent cells, while the phospholipase C inhibitor U73122 suppressed them. Injury-induced Ca²⁺ transients were prevented by the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) blockers thapsigargin and cyclopiazonic acid, while they were unaffected by suramin and genistein. These data show for the first time that the coordinated interplay between NCX-mediated Ca²⁺ entry and InsP₃-dependent Ca²⁺ release contributes to injury-induced intracellular Ca²⁺ concentration oscillations.

Signal propagation and neuronal avalanches analysis in networks of formal neurons

To study neurons with computational tools, one may call upon, at least, two different approaches: (i) Hodgkin-Huxley like neurons [1] (i.e. biological neurons) and (ii) formal neurons (e.g. Hindmarsh-Rose (HR) model [2], Kinouchi-Tragtenberg (KT) model [3], etc). Formal neurons may be represented by ordinary differential equations (e.g. HR), or by maps, which are dynamical systems with continuous state variables and discrete time dynamics (e.g. KT). A few maps had been proposed to describe neurons [3-10]. Such maps provide one with a number of computational advantages [10], since there is no need to set any precision on the integration variable, which leads to better performance in the calculations. An extended KT neuron model, here called KTz model, has been studied in [4] and [5], may be supplied with a Chemical Synapse Map (CSM) in order to study interacting neurons in a lattice, in the framework of coupled map lattices. KTz model presents most of the standard behavior of excitable cells like fast spiking, regular spiking, bursting, plateau action potentials and adaptation phenomena, and the CSM is in good agreement with some standard functions used to model post-synaptic currents, like the alpha function or the double-exponential function [4]. Preliminary results indicate antiferromagnetic oscillatory behavior or plane wave behavior in KTz neurons coupled with inhibitory CSM on a square lattice. Besides, many systems in nature are characterized by complex behavior where large cascades of events, named avalanches, unpredictably alternate with periods of little activity (e.g. snow avalanches, earthquakes, etc). Avalanches are described by power law distributions and when the branching parameter equals to unity, the system is said to be a self-organized critical (SOC) system [13]. These have been observed for neuronal activity in vitro [11,12]. And since both SOC systems and neuronal activity show large variability, long-term stability and memory capabilities, networks of neurons have been proposed to be SOC systems. This hypothesis was tested in [13], where they made comparisons among in vivo recordings using Local Field Potentials in three macaque monkeys performing a short term memory task and three different well-established subsampled SOC models (e.g. Sandpile model, Random Neighbour Sandpile model and Forest Fire model). Some similar comparison has been done in [14] with in vivo data from fourteen rats and a cellular automaton developed by the authors. We claim that still no simulation has been made to detect whether formal or realistic neuron models can evolve naturally to a SOC state, in a full or subsampled network. Our simulations are made with KTz model, which is a formal neuron, but keeps the usual behaviors of living cells, connected through CSM on a square lattice. We divided the work into two parts: (i) the analysis of network itself and how it evolves with time from a given initial state, varying its parameters and (ii) the analysis of the data generated by a network of silent cells, stimulated at random sites, trying to resemble the SOC models above. We compare these second part results with the experimental ones presented in [11-13].

Intracellular Determinants of Hippocampal CA1 Place and Silent Cell Activity in a Novel Environment

For each environment a rodent has explored, its hippocampus contains a map consisting of a unique subset of neurons, called place cells, that have spatially tuned spiking there, with the remaining neurons being essentially silent. Using whole-cell recording in freely moving rats exploring a novel maze, we observed differences in intrinsic cellular properties and input-based subthreshold membrane potential levels underlying this division into place and silent cells. Compared to silent cells, place cells had lower spike thresholds and peaked versus flat subthreshold membrane potentials as a function of animal location. Both differences were evident from the beginning of exploration. Additionally, future place cells exhibited higher burst propensity before exploration. Thus, internal settings appear to predetermine which cells will represent the next novel environment encountered. Furthermore, place cells fired spatially tuned bursts with large, putatively calcium-mediated depolarizations that could trigger plasticity and stabilize the new map for long-term storage. Our results provide new insight into hippocampal memory formation.

An Inside Look at Hippocampal Silent Cells

Hippocampal pyramidal cells can be divided into place cells, which fire action potentials when an animal is in specific locations, and silent cells, which are not spatially selective. In this issue of Neuron, Epsztein etal. find intracellular differences between place and silent cells by using whole-cell recordings in freely moving rats.

The mechanism of injury-induced [Ca2+]i oscillations in the endothelium of excised rat aorta

Endothelial injury is the primary event that leads to a variety of severe vascular disorders. The signal transduction pathway which drives the subsequent healing process is far from being fully elucidated. Mechanical injury elicits a Ca2+ response in the endothelium of intact rat aorta, which comprises an initial Ca2+ release from inositol-1,4,5-trisphosphate-sensitive (IP3Rs) stores followed by a long-lasting decay phase due to Ca2+ entry through uncoupled connexons. In a minor fraction of cells, the Ca2+ signal adopts an oscillatory pattern whose molecular underpinnings are yet to be elucidated. In the light of the role played by repetitive Ca2+ spikes in regulating tissue regeneration, the present study aims at elucidating the mechanisms underlying injury-induced Ca2+ oscillations. The repetitive Ca2+ signal reversibly ceased upon removal of extracellular Ca2+ or addition of the inorganic cations, La3+ and Ni2+. Moreover, the spiking response was abolished by the gap-junction blockers, heptanol and 18 beta-glycyrrhetinic acid and by interfering with the Ca2+ entry-mode mode of the Na+/Ca2+ exchanger (NCX). The InsP3-producing agonist, ATP, resumed Ca2+ oscillations in silent cells, while the phospholipase C inhibitor, U73122, inhibited the oscillatory signal. The latter was also prevented by the SERCA inhibitors, thapsigargin and cyclopiazonic acid acid. These data show that injury-induced [Ca2+]i oscillations require the coordinated interplay between NCX-mediated Ca2+ entry and InsP3-dependent Ca2+ release. Besides directly gating Ca2+ inflow, uncoupled connexons might let Na+ into the cells and stimulate Ca2+ entry through NCX by increasing submembranal Na+ levels.

Opposite Effects of 5-Hydroxymethyluracil on Mitogenic Response of T Cells Stimulated through T-Cell Receptor or through T-Cell Receptor and CD28 Co-Receptor Molecule

The effect of a synthetic analog of "minor" pyrimidine base 5-hydroxymethyluracil (0.01-10 μg/ml) on the mitogenic response of donor peripheral blood T cells was studied in vitro under conditions of lymphocyte stimulation through T-cell receptor (antiCD3 monoclonal antibodies) or through T-cell receptor and CD28 co-receptor molecule (antiCD3 and antiCD28 monoclonal antibodies). 5-hydroxymethyluracil suppressed the mitogenic response of T cells stimulated with antiCD3 antibodies, which was paralleled by an increase in the count of silent cells and decrease in the count of dividing lymphocytes, but not by stimulation of apoptosis of activated cells. Under conditions of integration of stimuli (lymphocyte stimulation with antiCD3 and antiCD28), 5-hydroxymethyluracil stimulated the mitogenic response, which was paralleled by suppression of activation apoptosis and increase in proliferative potential of cells.

Presynaptic and postsynaptic ion channel expression in vestibular nuclei neurons after unilateral vestibular deafferentation

Vestibular compensation refers to the recovery of function occurring after unilateral vestibular deafferentation, but some patients remain uncompensated. Similarly, more than half of the operated chickens compensate three days after unilateral vestibular ganglionectomy (UVG), but the rest remain uncompensated. This review focuses on the studies performed on the principal cells of the chick tangential nucleus after UVG. The tangential nucleus is a major avian vestibular nucleus whose principal cells are all second-order, vestibular reflex projection neurons participating in the vestibuloocular and vestibulocollic reflexes controlling posture, balance, and eye movements. Using whole-cell patch-clamp approach in brain slice preparations, spontaneous spike firing, ionic conductances, and spontaneous excitatory postsynaptic currents (sEPSCs) are recorded in principal cells from controls and operated chickens three days after UVG. In compensated chickens, the proportion of spontaneous spike firing principal cells and their spike discharge rate are symmetric on the lesion and intact sides, with the rates increased over controls. However, in the uncompensated chickens, the spike discharge rate increases on the lesion side, but not on the intact side, where only silent principal cells are recorded. In all the experimental groups, including controls, silent principal cells are distinguished from spontaneous spiking cells by smaller persistent sodium conductances and higher activation thresholds for the fast sodium channel. In addition, silent principal cells on the intact side of uncompensated chickens have larger dendrotoxin-sensitive potassium conductances, with a higher ratio of immunolabeling for surface/cytoplasmic expression of a dendrotoxin-sensitive, potassium channel subunit, Kv1.1. Finally, in compensated chickens, sEPSC frequency is symmetric bilaterally, but in uncompensated chickens sEPSC frequency increased only on the lesion side, where the expression of Kv1.2 decreased in synaptotagmin-labeled terminal profiles on the principal cell bodies. Altogether, the specific sodium and potassium channels important for the development of spike firing pattern and/or presynaptic glutamate release on vestibular reflex projection neurons may be critically involved in changing postsynaptic neuron excitability after vestibular deafferentation.

Mechanisms Underlying Neuronal “hyperactivity” in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive, irreversible neurodegenerative disorder characterized by deposition of amyloid plaques and accumulation of intraneuronal neurofibrillary tangles. Mouse models of the disease, expressing human amyloid precursor protein and/or presenilins with mutations typically causing early onset AD in humans, recapitulate many hallmarks of the human disease. They develop senile plaques and neurofibrillary tangles; exhibit dysregulation of the intracellular Ca2+ homeostasis, brain inflammatory response and memory impairment. Because the dysregulation of the intracellular Ca2+ homeostasis was postulated to act as an important progenitor of AD, we studied intraneuronal Ca2+ dynamics in APP23xPS45 mutant mice. In vivo two-photon Ca2+ imaging in these mice revealed 3 different classes of layer 2/3 cortical neurons: “silent cells” showing no spontaneous Ca2+ transients during a prolonged recording period, neurons with normal frequency of Ca2+ transients (< 4/min) and “hyperactive” cells with unusually high frequency of Ca2+ transients (Busche et al., 2008). Here we studied the mechanisms underlying Ca2+ transients in normal and hyperactive cells. The Ca2+ transients in both cell types had similar amplitudes and kinetics. They were tetrodotoxin-sensitive and thus caused by action potential firing. Moreover, these transients were of synaptic origin because they were completely and reversibly blocked by a mixture of glutamate receptor blockers CNQX and APV. Surprisingly, APV alone was sufficient to block the transients in both cell types, suggesting a key contribution of NMDA receptor-channels. According to our data, the increased frequency of Ca2+ transients in hyperactive cells was not due to an increase in intrinsic excitability but rather to a relative loss of synaptic inhibition. Thus, neuronal hyperactivity in APP23xPS45 mice is caused by a local synaptic rewiring with a relative increase in the excitation vs. inhibition.

Adaptation of chicken vestibular nucleus neurons to unilateral vestibular ganglionectomy

Vestibular compensation refers to the behavioral recovery after a unilateral peripheral vestibular lesion. In chickens, posture and balance deficits are present immediately following unilateral vestibular ganglionectomy (UVG). After three days, most operated chickens begin to recover, but severe deficits persist in others. The tangential nucleus is a major avian vestibular nucleus whose principal cells are vestibular reflex projection neurons. From patch-clamp recordings on brain slices, the percentage of spontaneous spike firing principal cells, spike discharge rate, ionic conductances, and spontaneous excitatory postsynaptic currents (sEPSCs) were investigated one and three days after UVG. Already by one day after UVG, sEPSC frequency increased significantly on the lesion side, although no differences were detected in the percentage of spontaneous spike firing cells or discharge rate. In compensated chickens three days after UVG, the percentage of spontaneous spike firing cells increased on the lesion side and the discharge rate increased bilaterally. In uncompensated chickens three days after UVG, principal cells on the lesion side showed increased discharge rate and increased sEPSC frequency, whereas principal cells on the intact side were silent. Typically, silent principal cells exhibited smaller persistent sodium conductances and higher activation thresholds for the fast sodium channel than spiking cells. In addition, silent principal cells on the intact side of uncompensated chickens had larger dendrotoxin-sensitive potassium conductance, with a higher ratio of Kv1.1 surface/cytoplasmic expression. Increased sEPSC frequency in principal cells on the lesion side of uncompensated chickens was accompanied by decreased Kv1.2 immunolabeling of presynaptic terminals on principal cell bodies. Thus, both intrinsic ionic conductances and excitatory synaptic inputs play crucial roles at early stages after lesions. Unlike the principal cells in compensated chickens which showed similar percentages of spontaneous spike firing cells, discharge rates, and sEPSC frequencies bilaterally, principal cells in uncompensated chickens displayed gross asymmetry in these properties bilaterally.

GABAA- and AMPA-like receptors modulate the activity of an identified neuron within the central pattern generator of the pond snail Lymnaea stagnalis

To examine the neurochemistry underlying the firing of the RPeD1 neuron in the respiratory central pattern generator of the pond snail, Lymnaea stagnalis, we examined electrophysiologically and pharmacologically either "active" or "silent" preparations by intracellular recording and pharmacology. GABA inhibited electrical firing by hyperpolarizing RPeD1, while picrotoxin, an antagonist of GABA(A) receptors, excited silent cells and reversed GABA-induced inhibition. Action potential activity was terminated by 1 mM glutamate (Glu) while silent cells were depolarized by the GluR agonists, AMPA, and NMDA. Kainate exerted a complex triphasic effect on membrane potential. However, only bath application of AMPA desensitized the firing. These data indicate that GABA inhibits RPeD1 via activation of GABA(A) receptors, while Glu stimulates the neuron by activating AMPA-sensitive GluRs.

Increased bursting in layer 2/3 neurones of awake neocortex

Increased bursting in layer 2/3 neurones of awake neocortex

Targeting silent cells

Targeting silent cells

Cortisol and prolactin modulation of caudal neurosecretory system activity in the euryhaline flounder Platichthys flesus

Previous studies have shown roles for cortisol and prolactin in osmoregulatory adaptation to seawater and freshwater, respectively, in euryhaline fish. This study of the European flounder investigated the potential for these hormones to modulate activity of the caudal neurosecretory system (CNSS), which is thought to be involved in physiological adaptation to changing external salinity. Superfusion of isolated CNSS with either cortisol or prolactin (10 μM; 15 min) led to changes in firing activity in neuroendocrine Dahlgren cells, recorded extracellularly. Cortisol evoked a modest increase in overall firing activity, with the response delayed by 4 h after treatment. The response to prolactin was short latency, continued to build up over the subsequent 4-h wash period, and comprised increased firing activity together with recruitment of previously silent Dahlgren cells. Immunoreactivity for glucocorticoid and prolactin receptors was localised to Dahlgren cells. The CNSS expression level for glucocorticoid-2 receptor mRNA, measured by Q-PCR, was significantly lower in fish fully acclimated to freshwater, compared to seawater. No differences were seen between these two states for prolactin receptor mRNA expression. These results provide evidence for a modulatory action of both hormones on the neurosecretory function of the CNSS.

Episodic activity in a heterogeneous excitatory network, from spiking neurons to mean field

Many developing neural systems exhibit spontaneous activity (O'Donovan, Curr Opin Neurobiol 9:94-104, 1999; Feller, Neuron 22:653-656, 1999) characterized by episodes of discharge (active phases) when many cells are firing, separated by silent phases during which few cells fire. Various models exhibit features of episodic behavior by means of recurrent excitation for supporting an episode and slow activity-dependent depression for terminating one. The basic mechanism has been analyzed using mean-field, firing-rate models. Firing-rate models are typically formulated ad hoc, not derived from a spiking network description, and the effects of substantial heterogeneity amongst the units are not usually considered. Here we develop an excitatory network of spiking neurons (N-cell model) with slow synaptic depression to model episodic rhythmogenesis. This N-cell model displays episodic behavior over a range of heterogeneity in bias currents. Important features of the episodic behavior include orderly recruitment of individual cells during silent phases and existence of a dynamical process whereby a small critical subpopulation of intermediate excitability conveys synaptic drive from active to silent cells. We also derive a general self-consistency equation for synaptic drive that includes cell heterogeneity explicitly. We use this mean-field description to expose the dynamical bistability that underlies episodic behavior in the heterogeneous network. In a systematic numerical study we find that the robustness of the episodic behavior improves with increasing heterogeneity. Furthermore, the heterogeneity of depression variables (imparted by the heterogeneity in cellular firing thresholds) plays an important role in this improvement: it renders the network episodic behavior more robust to variations in excitability than if depression is uniformized. We also investigate the effects of noise vs. heterogeneity on the robustness of episodic behavior, especially important for the developing nervous system. We demonstrate that noise-induced episodes are very fragile, whereas heterogeneity-produced episodic rhythm is robust.