Gap Junction Connections Research Articles

Assessment of transcript and protein levels contributing to cell cycle control and gap junction connections in morphologically variable groups of porcine cumulus-oocyte complexes

Oocytes and somatic cumulus cells are connected by an extensive network of gap junctions. These connections contribute in a major way to oocyte maturation and developmental competence. Cumulus-oocyte complexes (COCs) were cultured in standard porcine IVM culture medium (TCM 199) for 44 h. The morphological classification of COCs is based on the number of cumulus cell layers and the degree of their compaction, as well as on cytoplasm composition (homogenous, heterogeneous). The obtained COCs were divided into four grades according to this classification system. By assessing the activity of glucose-6-phosphate dehydrogenase (G6PDH) using the brilliant cresyl blue (BCB) test, real-time quantitative PCR (RQ-PCR) reaction methods and confocal microscopic observations, we determined the transcript levels of connexins 43 and 45, cyclin dependent kinases (cdk5 and cdk5r), and cdk inhibitors 1 and 3 (p27kip1 and cdkn3) as well as cdk4 protein in morphologically different groups of porcine oocytes isolated from puberal gilts. To assess their nuclear status the completely denuded oocytes were subjected to DAPI staining. We found statistically increased cdkn3, cdk5 and connexin 45 mRNA levels in oocytes graded as I as compared to II, III, and IV. The cdkn1, cdk5r and connexin 43 transcript contents were higher only when comparing between oocytes graded as I, III and IV. The cdk4 protein in oocytes graded I and II is localized mainly in the zona pellucida, although in grade III COCs the expression of this protein is decreased and observed only in the cytoplasm. Grade IV COCs do not demonstrate a significant presence of cdk4 protein. With regards to nuclear maturation, the percentage of MII stage oocytes was significantly (P < 0.05) higher in grade I and II oocytes as compared to grade III and IV oocytes. Our results demonstrate for the first time that cdk4 protein localization and all of the investigated transcript levels are associated with COC morphology and may be related to further maturation ability as well as developmental competence of oocytes.

Signaling networks in somatic cells and oocytes activated during ovulation

The molecular and structural changes in the follicle triggered by the gonadotropin luteinizing hormone (LH) at ovulation require complex interactions between somatic cells and the oocytes. It is well established that the signals originating from the oocytes play an essential role in orchestrating the growth and development of the follicle. Conversely, the exact contribution of local signals in the two compartments to ovulation is less clear. LH causes oocyte meiotic resumption, cumulus expansion, and follicle rupture through direct activation of granulosa cells, but also indirectly by acting on cumulus cells and the oocyte. Recent studies from our and other laboratories have identified some of the components of the signaling network activated at this critical transition. The oocyte meiotic arrest is maintained through the activity of GPCR and adenylyl cyclases endogenous to the oocyte and by diffusion of signals through gap junction connections with the soma. It is possible that cAMP, and more recently, cGMP, diffuses through the gap junction network from somatic cells to the oocyte to maintain meiotic arrest. Recent evidence suggests that the biochemical mechanism by which cGMP maintains meiotic arrest is likely through regulation of the oocyte PDE and therefore, indirectly cAMP. The exact series of events regulating reentry of the oocyte into the cell cycle are still not completely understood, but it is widely accepted that a decrease in cAMP is the primary signal. This decrease in cAMP is caused by an activation of the oocyte PDE but also by closure of the gap junctions. The regulatory mechanisms in the soma that lead to activation of the PDE in the oocytes and gap junction closure are the focus of intense investigation. Several signaling pathways converge on these two final events. LH initially activates cAMP signaling but then the stimulus branches into a number of contiguous and interacting pathways. Activation of the EGF network is one of these pathways and accumulating evidence strongly indicates that this is a component critical for oocyte maturation, cumulus expansion, and follicle rupture. Because these regulatory mechanisms are conserved in rodents and in humans, a better understanding of these signaling networks activated during ovulation will provide new pharmacological opportunities for the manipulation of fertility.

Impact of delays and rewiring on the dynamics of small-world neuronal networks with two types of coupling

We study synchronization transitions and pattern formation on small-world networks consisting of Morris–Lecar excitable neurons in dependence on the information transmission delay and the rewiring probability. In addition, networks formed via gap junctional connections and coupling via chemical synapses are considered separately. For gap-junctionally coupled networks we show that short delays can induce zigzag fronts of excitations, whereas long delays can further detriment synchronization due to a dynamic clustering anti-phase synchronization transition. For the synaptically coupled networks, on the other hand, we find that the clustering anti-phase synchronization can appear as a direct consequence of the prolongation of information transmission delay, without being accompanied by zigzag excitatory fronts. Irrespective of the coupling type, however, we show that an appropriate small-world topology can always restore synchronized activity if only the information transmission delays are short or moderate at most. Long information transmission delays always evoke anti-phase synchronization and clustering, in which case the fine-tuning of the network topology fails to restore the synchronization of neuronal activity.

Intrathecal injection of carbenoxolone, a gap junction decoupler, attenuates the induction of below-level neuropathic pain after spinal cord injury in rats

The most common type of chronic pain following spinal cord injury (SCI) is central neuropathic pain and SCI patients typically experience mechanical allodynia and thermal hyperalgesia. The present study was designed to examine the potential role of astrocyte gap junction connectivity in the induction and maintenance of “below-level” neuropathic pain in SCI rats. We examined the effect of intrathecal treatment with carbenoxolone (CARB), a gap junction decoupler, on SCI-induced bilateral thermal hyperalgesia and mechanical allodynia during the induction phase (postoperative days 0 to 5) and the maintenance phase (days 15 to 20) following T13 spinal cord hemisection. Immunohistochemistry was performed to determine potential SCI-induced changes in spinal astrocyte activation and phosphorylation of the NMDA receptor NR1 subunit (pNR1). CARB administered during the induction period dose-dependently attenuated the development of bilateral thermal hyperalgesia and mechanical allodynia. Intrathecal CARB also significantly reduced the bilateral SCI-induced increase in GFAP-immunoreactive (ir) staining and the number of pNR1-ir cell profiles in the spinal cord dorsal horn compared to vehicle-treated rats. In contrast, CARB treatment during the maintenance phase had no effect on the established thermal hyperalgesia and mechanical allodynia nor on spinal GFAP expression or the number of pNR1-ir cell profiles. These results indicate that gap junctions play a critical role in the activation of astrocytes distant from the site of SCI and in the subsequent phosphorylation of NMDA receptors in the lumbar spinal cord. Both of these processes appear to contribute to the induction of bilateral below-level pain in SCI rats.

Local dynamics of gap-junction-coupled interneuron networks

Interneurons coupled by both electrical gap-junctions (GJs) and chemical GABAergic synapses are major components of forebrain networks. However, their contributions to the generation of specific activity patterns, and their overall contributions to network function, remain poorly understood. Here we demonstrate, using computational methods, that the topological properties of interneuron networks can elicit a wide range of activity dynamics, and either prevent or permit local pattern formation. We systematically varied the topology of GJ and inhibitory chemical synapses within simulated networks, by changing connection types from local to random, and changing the total number of connections. As previously observed we found that randomly coupled GJs lead to globally synchronous activity. In contrast, we found that local GJ connectivity may govern the formation of highly spatially heterogeneous activity states. These states are inherently temporally unstable when the input is uniformly random, but can rapidly stabilize when the network detects correlations or asymmetries in the inputs. We show a correspondence between this feature of network activity and experimental observations of transient stabilization of striatal fast-spiking interneurons (FSIs), in electrophysiological recordings from rats performing a simple decision-making task. We suggest that local GJ coupling enables an active search-and-select function of striatal FSIs, which contributes to the overall role of cortical-basal ganglia circuits in decision-making.

Non-Debye dielectric behavior and near-field interactions in biological tissues: When structure meets function

The Maxwell–Wagner theory of interfacial polarization treats dielectric particles suspended in homogeneous media and subjected to alternating fields as systems of independent induced dipoles, and therefore predicts simple Debye-type dispersion in which both permittivity and conductivity vary between low- and high-frequency plateaus. Unfortunately, Maxwell–Wagner theory cannot replicate its own success when applied to more concentrated and structured (i.e., non-random) systems of cells such as tissues. The effective medium theory, devised to incorporate contributions of the induced dipoles to the average far-field, brought about some improvement, but the general shape of the dielectric spectra remained that of a simple Debye – in contrast with experimental data. Supracellular organization of tissues induces non-Debye behavior due to near-field interactions between neighboring cells. Specifically, biological cells may perturb each other’s electrical environment via dipolar (multipolar) fields or conductance bridges (such as pores and gap junctions) between cytoplasms and across cell membranes. We substantiate this hypothesis by presenting comparatively data from yeast and red blood cell suspensions, blood forming reversible supra-cellular structures, called ‘rouleaux’, breast tissues, as well as from brain treated with anesthetics that are known to affect the gap-junctional connectivity between cells.

Activation of glutamate transporters in the locus coeruleus paradoxically activates descending inhibition in rats

Descending noradrenergic inhibition is an important endogenous pain-relief mechanism which can be activated by local glutamate signaling. In the present study, we examined the effect of glutamate transporter activation by riluzole in the regulation of activity of locus coeruleus (LC) neurons, which provide the major inhibitory descending noradrenergic projection to the spinal cord. Local injection of riluzole into the LC dose-dependently reduced hypersensitivity in rats after L5-L6 spinal nerve ligation (SNL). This anti-hypersensitivity effect of LC-injected riluzole was blocked by intrathecal administration of the α2-adrenoceptor antagonist idazoxan and intra-LC co-injection of the AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the gap-junction blockers, carbenoxolone (CBX) and meclofenamic acid (MEC). In brainstem slices from normal rats, riluzole increased phosphorylated cAMP response element binding protein (pCREB) expressing nuclei in dopamine-β-hydroxylase (DβH) containing cells in the LC. This riluzole-induced pCREB activation in LC neurons was also blocked by CNQX and CBX. In the primary astrocyte culture, riluzole enhanced glutamate-induced glutamate release. Contrary to expectations, these results suggest that activation of glutamate transporters in the LC results in increase of extracellular glutamate signaling, possibly via facilitation of glutamate release from astrocytes, and activation of LC neurons to induce descending inhibition, and that this paradoxical action of glutamate transporters in the LC requires gap-junction connections.

Olfactory ensheathing cell membrane properties are shaped by connectivity

Olfactory ensheathing cells (OECs) have been repeatedly implicated in mediating plasticity, particularly in situ in the olfactory nerve in which they support the extension of olfactory sensory neuron (OSN) axons from the olfactory epithelium to the olfactory bulb (OB). OECs are specialized glia whose processes surround OSN axon fascicles within the olfactory nerve and across the OB surface. Despite their purported importance in promoting axon extension, and following transplants, little is known about either morphology or biophysical properties of OECs in situ. In particular, cell-cell interactions that may influence OEC function are largely unexplored. Here, we studied OEC connectivity and morphology in slice preparations, preserving tissue structure and cell-cell interactions. Our analyses showed that OECs form a matrix of cellular projections surrounding axons, unique among glia, and express high levels of connexin-43. Lucifer Yellow injections revealed selective dye coupling among small subgroups of OECs. Two types of OECs were biophysically distinguished with whole-cell voltage-clamp recordings: (1) with low-input resistance (R(i)), linear current profiles, and frequently dye coupled; and (2) with high R(i), nonlinear current profiles, and infrequent dye coupling. Pharmacological blockade of gap junctions changed OEC membrane properties such that linear OECs became nonlinear. Double recordings indicated that the appearance of the nonlinear current profile was associated with the loss of electrical coupling between OECs. We conclude that the diversity of OEC current profiles can be explained by differences in gap-junction connectivity and discuss implications of this diversity for OEC influences on axon growth and excitability.

THE PHYSIOLOGICAL FOUNDATION OF YOGA CHAKRA EXPRESSION

Abstract.Chakras are a basic concept of yoga but typically are ignored by scientific research on yoga, probably because descriptions of chakras can appear like a fanciful mythology. Chakras are commonly considered to be centers of concentrated metaphysical energy. Although clear physiological effects exist for yoga practices, no explanation of how chakras influence physiological function has been broadly accepted either in the scientific community or among yoga scholars. This problem is exacerbated by the fact that yoga is based on subjective experience, and practitioners often shun objective descriptions. This essay builds on earlier work hypothesizing that intercellular gap junction connections provide a physiological mechanism underlying subtle energy systems described in yoga as well as other disciplines such as acupuncture. Three physical aspects of chakras are distinguished that are integrated through gap junction mechanisms and are proposed to have arisen during embryological development. Furthermore, electrical conductance associated with a high concentration of gap junctions could generate phenomena that, when subjectively experienced, have the radiant qualities attributed to chakras. This theory provides a scientific rationale for previously unexplained details of chakra theory and offers a new orientation to conceptualizing and studying such subjective phenomena.

A mathematical model of vasoreactivity in rat mesenteric arterioles. II. Conducted vasoreactivity

This study presents a multicellular computational model of a rat mesenteric arteriole to investigate the signal transduction mechanisms involved in the generation of conducted vasoreactivity. The model comprises detailed descriptions of endothelial (ECs) and smooth muscle (SM) cells (SMCs), coupled by nonselective gap junctions. With strong myoendothelial coupling, local agonist stimulation of the EC or SM layer causes local changes in membrane potential (V(m)) that are conducted electrotonically, primarily through the endothelium. When myoendothelial coupling is weak, signals initiated in the SM conduct poorly, but the sensitivity of the SMCs to current injection and agonist stimulation increases. Thus physiological transmembrane currents can induce different levels of local V(m) change, depending on cell's gap junction connectivity. The physiological relevance of current and voltage clamp stimulations in intact vessels is discussed. Focal agonist stimulation of the endothelium reduces cytosolic calcium (intracellular Ca(2+) concentration) in the prestimulated SM layer. This SMC Ca(2+) reduction is attributed to a spread of EC hyperpolarization via gap junctions. Inositol (1,4,5)-trisphosphate, but not Ca(2+), diffusion through homocellular gap junctions can increase intracellular Ca(2+) concentration in neighboring ECs. The small endothelial Ca(2+) spread can amplify the total current generated at the local site by the ECs and through the nitric oxide pathway, by the SMCs, and thus reduces the number of stimulated cells required to induce distant responses. The distance of the electrotonic and Ca(2+) spread depends on the magnitude of SM prestimulation and the number of SM layers. Model results are consistent with experimental data for vasoreactivity in rat mesenteric resistance arteries.

Particle tracking model of electrophoretic morphogen movement reveals stochastic dynamics of embryonic gradient

Some developmental events rely on an electrophoretic force to produce morphogenetic gradients. To quantitatively explore the dynamics of this process, we constructed a stochastic model of an early phase of left-right patterning: serotonin movement through the gap junction-coupled blastomeres of the Xenopus embryo. Particle-tracking simulations showed that a left-right gradient is formed rapidly, quickly reaching a final stable level. The voltage difference was critical for producing a morphogen gradient of the right steepness; gap junctional connectivity and morphogen mass determined the timing of the gradient. Endogenous electrophoresis drives approximately 50% of the particles across more than one cell width, and approximately 20% can travel across half the embryo. The stochastic behavior of the resulting gradients exhibited unexpected complexity among blastomeres' morphogen content, and showed how spatiotemporal variability within individual cells resulted in robust and consistent gradients across the embryonic left-right axis. Analysis of the distribution profile of gradient gain values made quantitative predictions about the conditions that result in the observed background level of laterality defects in unperturbed frog embryos. This work provides a general model that can be used to quantitatively analyze the unexpectedly complex dynamics of morphogens in a wide variety of systems.

Role of rat urinary bladder interstitial cells in neurogenic detrusor overactivity

We have previously identified an increase in number and gap junction connectivity (connexin 43) of interstitial cells (ICs) in the urinary bladders of spinal cord transected (SCT) rats. Our data suggest that these ICs drive the enhanced spontaneous contractions responsible for neurogenic detrusor overactivity. In this study, we further characterized the changes in ICs that may contribute to this overactivity. ICs were identified through immunohistochemistry using vimentin and CD34 as IC‐markers. ICs form an extensive network throughout the bladder wall with a dense plexus in the suburothelium. In bladders from SCT rats, there was also a significant increase in the expression of P2Y6 and c‐kit receptors on ICs, particularly in the suburothelium. The amplitude of spontaneous contractions from in vitro tension recordings of SCT rat whole bladders sheet preparations (1.3±0.9 g/mm2, n=9) were decreased by the addition of 30 μM Gleevec (c‐kit receptor inhibitor, 52.5±16.0% reduction, n=6) and 10 μM MRS2578 (P2Y6‐antagonist, 34.8±18.6% reduction, n=4) to the superfusion solution. However, no effect on spontaneous contractions of normal adult bladder preparations was seen with these compounds (0.3±0.2 g/mm2, n=6). These functional results correlated with immunohistochemical data showing increased number of ICs and the involvement of P2Y6 and c‐kit receptors in detrusor overactivity following SCT. In summary, increased functionality of ICs has a contributory role in neurogenic detrusor overactivity and may represent a potential therapeutic target. This research was sponsored by awards from Pfizer Ltd.

Multiscale Modeling of Electrical and Intracellular Activity in the Pancreas: The Islet Tridomain Equations

We derive continuum models of the electrical syncytia of the pancreatic islet of Langerhans using homogenization. Beta-cells in the islets are coupled in their intra- and extracellular potential fields via membrane currents and can exchange calcium (and metabolite) via direct intracellular communication through gap junctional connections. In earlier work, Goel, Sneyd, and Friedman [Multiscale Model. Simul., 5 (2006), pp. 1045–1062] have derived homogenized models of cytosolic calcium and endoplasmic reticulum (ER) calcium in cells. The islet tridomain presented here extends that scheme to three domains: the extracellular space, the cytosol, and the intracellular ER (or sarcoplasmic reticulum (SR)) are each represented at every point in space. We also consider three different models of gap junctions: as pores, as semipermeable membranes, and as thin conductors. We derive the macroscopic equations for all three cases. The islet tridomain equations are different depending on the gap junction model. In additi...

Effects of succinate on ATP-induced Ca2+ waves in the nucleus accumbens

Event Abstract Back to Event Effects of succinate on ATP-induced Ca2+ waves in the nucleus accumbens Tünde Molnár1*, Peter Barabas1, Julianna Kardos1 and Miklos Palkovits2 1 Department of Neurochemistry, Institute of Biomolecular Chemistry, Hungarian Academy of Sciences, Hungary 2 Semmelweis University and Hungarian Academy of Sciences, Hungary We have previously reported a membrane protein that binds the intermediary metabolite of gamma-hydroxybutyric acid (GHB), succinic acid (SUC) and the gap-junction blocker carbenoxolone in nucleus accumbens (NA) synaptosomal subfractions. In the present study, we explored propagating Ca2+ waves induced by extracellular ATP (100 μM) and challenged them by possible inhibitors, intermediary metabolites and their combinations. Confocal laser imaging has been used to detect changes in the number and the fluorescence intensity of cells loaded with the fluorescent indicator of cytosolic Ca2+ ion Fluo-4 acetoxymethyl ester. We report for the NA, that ATP-induced Ca2+ wave propagates in astrocytes that can be decreased by gap junction blockers carbenoxolone (0.1 and 1 mM) and flufenamic acid (1 mM). Conversely the ATP-gated cation channel (P2x) antagonist suramin (0.1 mM) had no effect on the cell number and fluorescence intensity of cells of ATP-induced Ca2+ waves in the NA, suggesting that propagation depends on functional activity of gap-junctions. In addition, concentration-dependent effects of SUC and GHB were described. In low concentration (0.05 mM) SUC and GHB enhanced both the number and the fluorescence intensity of cells, while in higher concentration (0.2 and 2 mM) an inhibitory effect of SUC was observed. Our findings indicate that in the NA, calcium waves propagate via the mechanism involving gap-junctional connections between glutamate receptor containing astrocytes. The facilitatory role in astrocytic Ca2+ wave propagation suggests SUC as a strong candidate for metabolic signalling and will therefore benefit from further study. Conference: 12th Meeting of the Hungarian Neuroscience Society, Budapest, Hungary, 22 Jan - 24 Jan, 2009. Presentation Type: Poster Presentation Topic: Developmental neurobiology and subcortical functions Citation: Molnár T, Barabas P, Kardos J and Palkovits M (2009). Effects of succinate on ATP-induced Ca2+ waves in the nucleus accumbens. Front. Syst. Neurosci. Conference Abstract: 12th Meeting of the Hungarian Neuroscience Society. doi: 10.3389/conf.neuro.01.2009.04.246 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: 13 Mar 2009; Published Online: 13 Mar 2009. * Correspondence: Tünde Molnár, Department of Neurochemistry, Institute of Biomolecular Chemistry, Hungarian Academy of Sciences, Budapest, Hungary, tmolnar@chemres.hu 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 Tünde Molnár Peter Barabas Julianna Kardos Miklos Palkovits Google Tünde Molnár Peter Barabas Julianna Kardos Miklos Palkovits Google Scholar Tünde Molnár Peter Barabas Julianna Kardos Miklos Palkovits PubMed Tünde Molnár Peter Barabas Julianna Kardos Miklos Palkovits 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.

Diverse Mechanisms Underlie Glycinergic Feedback Transmission onto Rod Bipolar Cells in Rat Retina

Synaptic inhibition shapes visual signaling in the inner retina, but the physiology of most amacrine cells, the interneurons that mediate this inhibition, is poorly understood. Discerning the function of most individual amacrine cell types is a daunting task, because few molecular or morphological markers specifically distinguish between approximately two dozen different amacrine cell types. Here, we examine a functional subset of amacrine cells by pharmacologically isolating glycinergic inhibition and evoking feedback IPSCs in a single cell type, the rod bipolar cell (RBC), with brief glutamate applications in the inner plexiform layer. We find that glycinergic amacrine cells innervating RBCs receive excitatory inputs from ON and OFF bipolar cells primarily via NMDA receptors (NMDARs) and Ca2+-impermeable AMPA-type glutamate receptors. Glycine release from amacrine cells is triggered by Ca2+ influx through both voltage-gated Ca2+ (Ca(v)) channels and NMDARs. These intracellular Ca2+signals are amplified by Ca2+-induced Ca2+ release via both ryanodine and IP3 receptors, which are activated independently by Ca2+ influx through Ca(v) channels and NMDARs, respectively. Glycinergic feedback signaling depends strongly, although not completely, on voltage-gated Na+ channels, and the spatial extent of feedback inhibition is expanded by gap junction connections between glycinergic amacrine cells. These results indicate that a diversity of mechanisms underlie glycinergic feedback inhibition onto RBCs, yet they highlight several physiological themes that appear to distinguish amacrine cell function.

Spiral wave drift and complex-oscillatory spiral waves caused by heterogeneities in two-dimensional in vitro cardiac tissues

Understanding spiral reentry wave dynamics in cardiac systems is important since it underlies various cardiac arrhythmia including cardiac fibrillation. Primary cultures of dissociated cardiac cells have been a convenient and useful system for studying cardiac wave dynamics, since one can carry out systematic and quantitative studies with them under well-controlled environments. One key drawback of the dissociated cell culture is that, inevitably, some spatial inhomogeneities in terms of cell types and density, and/or the degree of gap junction connectivity, are introduced to the system during the preparation. These unintentional spatial inhomogeneities can cause some non-trivial wave dynamics, for example, the entrainment dynamics among different spiral waves and the generation of complex-oscillatory spiral waves. The aim of this paper is to quantify these general phenomena in an in vitro cardiac system and provide explanations for them with a simple physiological model having some realistic spatial inhomogeneities incorporated.

Neuronal Networks with Gap Junctions: A Study of Piecewise Linear Planar Neuron Models

The presence of gap junction coupling among neurons of the central nervous systems has been appreciated for some time now. In recent years there has been an upsurge of interest from the mathematical community in understanding the contribution of these direct electrical connections between cells to large-scale brain rhythms. Here we analyze a class of exactly soluble single neuron models, capable of producing realistic action potential shapes, that can be used as the basis for understanding dynamics at the network level. This work focuses on planar piecewise linear models that can mimic the firing response of several different cell types. Under constant current injection the periodic response and phase response curve (PRC) are calculated in closed form. A simple formula for the stability of a periodic orbit is found using Floquet theory. From the calculated PRC and the periodic orbit a phase interaction function is constructed that allows the investigation of phase-locked network states using the theory of weakly coupled oscillators. For large networks with global gap junction connectivity we develop a theory of strong coupling instabilities of the homogeneous, synchronous, and splay states. For a piecewise linear caricature of the Morris–Lecar model, with oscillations arising from a homoclinic bifurcation, we show that large amplitude oscillations in the mean membrane potential are organized around such unstable orbits.

Modulation of osteoblast gap junction connectivity by serum, TNFα, and TRAIL

We studied the effects of serum growth factors and of TNF family proteins on osteoblast gap junction connectivity. Serum starvation of human MG63 osteosarcoma cells or nontransformed osteoblasts decreased connexin43 protein. TNFα or TRAIL reduced connexin43 further. Serum starvation redistributed gap junctions but did not reduce intercellular diffusion. In contrast, TNFα or TRAIL reduced gap junctions on cell processes and decreased intercellular diffusion. Effects of TNFs on connexin43 were mediated by lysosomal proteolysis. Activating analogs of cAMP increased connexin43 protein, but did not block effects of serum starvation, TNFα, or TRAIL on connexin43 protein. Connexin43 and connectivity recovered overnight if stimuli were withdrawn. Surprisingly, connexin43 mRNA increased in serum starvation and with TNFα or TRAIL. Since β-catenin is a binding partner of connexin43, when connexin43 is degraded, β-catenin activation may contribute to a reflexive increase in connexin43 transcription. We conclude that osteoblast connectivity is regulated by a multifactorial system that maintains intercellular connections. Serum starvation, TNFα and TRAIL augmented connexin43 degradation and connexin43 transcription. Cell–cell communication was maintained in serum starvation, which may model response to acute injury, but was sensitive to TNFs. These inflammatory agents mediated selective, reversible removal of connexin43 from cell processes.

Drug development for treatment of cardiac arrhythmias: targeting the gap junctions

reentrant excitation ([12][1]) is likely the most important mechanism for serious life-threatening arrhythmias. Traditionally, a major focus of antiarrhythmic drug development has been to explore the actions of drugs designed to block conduction in reentrant pathways(s). During the latter part of

Neurons Skip a Beat during Fast Ripples

Fast ripples are EEG transients emanating from epileptic foci, but fast-ripple frequencies far exceed maximal neuronal firing rates. In this issue of Neuron, Menendez de la Prida and coworkers propose that out-of-phase firing of a subpopulation of neurons during physiological ripple activity effectively doubles the ripple frequency to produce fast ripples.