Estimating intracellular Ca2+ concentrations and buffering in a dendritic inhibitory hippocampal interneuron

Intracellular Calcium Responses Encode Action Potential Firing in Spinal Cord Lamina I Neurons

Maladaptive plasticity of neurons in lamina I of the spinal cord is a lynchpin for the development of chronic pain, and is critically dependent on intracellular calcium signaling. However, the relationship between neuronal activity and intracellular calcium in these neurons is unknown. Here we combined two-photon calcium imaging with whole-cell electrophysiology to determine how action potential firing drives calcium responses within subcellular compartments of male rat spinal cord lamina I neurons. We found that single action potentials generated at the soma increase calcium concentration in the somatic cytosol and nucleus, and these calcium responses invade dendrites and dendritic spines by active backpropagation. Calcium responses in each compartment were dependent on voltage-gated calcium channels, and somatic and nuclear calcium responses were amplified by release of calcium from ryanodine-sensitive intracellular stores. Grouping single action potential-evoked calcium responses by neuron type demonstrated their presence in all defined types, as well as a high degree of similarity in calcium responses between neuron types. With bursts of action potentials, we found that calcium responses have the capacity to encode action potential frequency and number in all compartments, with action potential number being preferentially encoded. Together, these findings indicate that intracellular calcium serves as a readout of neuronal activity within lamina I neurons, providing a unifying mechanism through which activity may regulate plasticity, including that seen in chronic pain.SIGNIFICANCE STATEMENT Despite their critical role in both acute pain sensation and chronic pain, little is known of the fundamental physiology of spinal cord lamina I neurons. This is especially the case with respect to calcium dynamics within these neurons, which could regulate maladaptive plasticity observed in chronic pain. By combining two-photon calcium imaging and patch-clamp electrophysiological recordings from lamina I neurons, we found that action potential firing induces calcium responses within the somatic cytosol, nucleus, dendrites, and dendritic spines of lamina I neurons. Our findings demonstrate the presence of actively backpropagating action potentials, shifting our understanding of how these neurons process information, such that calcium provides a mechanism for lamina I neurons to track their own activity.

Buffer mobility and the regulation of neuronal calcium domains

The diffusion of calcium inside neurons is determined in part by the intracellular calcium binding species that rapidly bind to free calcium ions upon entry. It has long been known that some portion of a neuron's intracellular calcium binding capacity must be fixed or poorly mobile, as calcium diffusion is strongly slowed in the intracellular environment relative to diffusion in cytosolic extract. The working assumption was that these immobile calcium binding sites are provided by structural proteins bound to the cytoskeleton or intracellular membranes and may thereby be relatively similar in composition and capacity across different cell types. However, recent evidence suggests that the immobile buffering capacity can vary greatly between cell types and that some mobile calcium binding proteins may alter their mobility upon binding calcium, thus blurring the line between mobile and immobile. The ways in which immobile buffering capacity might be relevant to different calcium domains within neurons has been explored primarily through modeling. In certain regimes, the presence of immobile buffers and the interaction between mobile and immobile buffers have been shown to result in complex spatiotemporal patterns of free calcium. In total, these experimental and modeling findings call for a more nuanced consideration of the local intracellular calcium microenvironment. In this review we focus on the different amounts, affinities, and mobilities of immobile calcium binding species; propose a new conceptual category of physically diffusible but functionally immobile buffers; and discuss how these buffers might interact with mobile calcium binding partners to generate characteristic calcium domains.

A single-compartment model of calcium dynamics in nerve terminals and dendrites

This introduction describes a single-compartment model of calcium dynamics that has been applied to fluorescence measurements of intracellular free calcium concentration ([Ca(2+)]i) changes in neurons. The model describes intracellular calcium handling under simplified conditions, for which analytical expressions for the amplitude and the time constants of [Ca(2+)]i changes can be explicitly derived. In particular, it reveals the dependence of the measured [Ca(2+)]i changes on the calcium indicator concentration. Applied to experimental data from small cells or subcellular compartments, the model equations have been extremely useful for obtaining quantitative information about essential parameters of Ca(2+) influx, buffering, and clearance. We illustrate also several changes that occur when the basic assumptions do not hold (e.g., when calcium diffusion, dye saturation, or kinetic effects become significant). Finally, we discuss how the changes in calcium dynamics, which are explained by the model, have been exploited for measuring properties of calcium-driven reactions, such as those regulating short-term synaptic enhancement, vesicle recycling, and adaptation.

Modulation of the GABAergic pathway for the treatment of fragile X syndrome

Fragile X syndrome (FXS) is the most common genetic cause of intellectual disability and the most common single-gene cause of autism. It is caused by mutations on the fragile X mental retardation gene (FMR1) and lack of fragile X mental retardation protein, which in turn, leads to decreased inhibition of translation of many synaptic proteins. The metabotropic glutamate receptor (mGluR) hypothesis states that the neurological deficits in individuals with FXS are due mainly to downstream consequences of overstimulation of the mGluR pathway. The main efforts have focused on mGluR5 targeted treatments; however, investigation on the gamma-aminobutyric acid (GABA) system and its potential as a targeted treatment is less emphasized. The fragile X mouse models (Fmr1-knock out) show decreased GABA subunit receptors, decreased synthesis of GABA, increased catabolism of GABA, and overall decreased GABAergic input in many regions of the brain. Consequences of the reduced GABAergic input in FXS include oversensitivity to sensory stimuli, seizures, and anxiety. Deficits in the GABA receptors in different regions of the brain are associated with behavioral and attentional processing deficits linked to anxiety and autistic behaviors. The understanding of the neurobiology of FXS has led to the development of targeted treatments for the core behavioral features of FXS, which include social deficits, inattention, and anxiety. These symptoms are also observed in individuals with autism and other neurodevelopmental disorders, therefore the targeted treatments for FXS are leading the way in the treatment of other neurodevelopmental syndromes and autism. The GABAergic system in FXS represents a target for new treatments. Herein, we discuss the animal and human trials of GABAergic treatment in FXS. Arbaclofen and ganaxolone have been used in individuals with FXS. Other potential GABAergic treatments, such as riluzole, gaboxadol, tiagabine, and vigabatrin, will be also discussed. Further studies are needed to determine the safety and efficacy of GABAergic treatments for FXS.

Tuning local calcium availability: cell-type-specific immobile calcium buffer capacity in hippocampal neurons

It has remained difficult to ascribe a specific functional role to immobile or fixed intracellular calcium buffers in central neurons because the amount of these buffers is unknown. Here, we explicitly isolated the fixed buffer fraction by prolonged whole-cell patch-clamp dialysis and quantified its buffering capacity in murine hippocampal slices using confocal calcium imaging and the "added-buffer" approach. In dentate granule cells, the calcium binding ratio (κ) after complete washout of calbindin D28k (Cb), κfixed, displayed a substantial value of ∼100. In contrast, in CA1 oriens lacunosum moleculare (OLM) interneurons, which do not contain any known calcium-binding protein(s), κfixed amounted to only ∼30. Based on these values, a theoretical analysis of dendritic spread of calcium after local entry showed that fixed buffers, in the absence of mobile species, decrease intracellular calcium mobility 100- and 30-fold in granule cells and OLM cells, respectively, and thereby strongly slow calcium signals. Although the large κfixed alone strongly delays the spread of calcium in granule cells, this value optimizes the benefits of additionally expressing the mobile calcium binding protein Cb. With such high κfixed, Cb effectively increases the propagation velocity to levels seen in OLM cells and, contrary to expectation, does not affect the peak calcium concentration close to the source but sharpens the spatial and temporal calcium gradients. The data suggest that the amount of fixed buffers determines the temporal availability of calcium for calcium-binding partners and plays a pivotal role in setting the repertoire of cellular calcium signaling regimens.

Comparison of Ca2+ transients and [Ca2+]i in the dendrites and boutons of non-fast-spiking GABAergic hippocampal interneurons using two-photon laser microscopy and high- and low-affinity dyes

Using two-photon laser microscopy, high- and low-affinity dyes and patch clamp electrophysiology, we successfully measured somatic stimulation-evoked Ca(2+) transients simultaneously in the dendrites and axonal boutons of the same non-fast-spiking GABAergic interneurons in acute slice preparations obtained from hippocampal area CA1. The advantage of the acute preparation is that both neuronal connections and anatomy are maintained. Calculated as unperturbed values, the amplitudes of Ca(2+) transients and changes in [Ca(2+)]i in response to somatic single or burst stimulation were much higher in boutons (428 nM/AP) than in dendrites (49 nM/AP), leading to the conclusion that the much greater influx of Ca(2+) observed in terminals might be due to a higher density of N-type voltage-sensitive Ca(2+) channels compared to the L-type channels present in dendrites. Whereas the decay of Ca(2+) transients recorded in dendrites was primarily mono-exponential, the decay in boutons was bi-exponential, as indicated by an initial fast phase, followed by a much slower reduction in fluorescence intensity. The extrusion of Ca(2+) was much faster in boutons than in dendrites. To avoid saturation effects and the flawed conversion of fluorescence measures of [Ca(2+)]i, we assessed the limits of [Ca(2+)] measurements (which ranged between 6 and 82% of the applied dye saturation) when high- and low-affinity dyes were applied at different concentrations. When two APs were delivered at a high frequency (>3 Hz) of stimulation, the low-affinity indicators OGB-6F (KD = 3.0 μM) and OGB-5N (KD = 20 μM) were able to accurately reflect the changes in ΔF/F produced by the consecutive APs. There was no difference in the endogenous buffer capacity (κE), which can shape Ca(2+) signals, calculated in dendrites (κE = 354) or boutons (κE = 458).

Plasticity of NMDA receptor-mediated excitatory postsynaptic currents at perforant path inputs to dendrite-targeting interneurons

Synaptic plasticity of NMDA receptors (NMDARs) has been recently described in a number of brain regions and we have previously characterised LTP and LTD of glutamatergic NMDA receptor-mediated EPSCs (NMDAR-EPSCs) in granule cells of dentate gyrus. The functional significance of NMDAR plasticity at perforant path synapses on hippocampal network activity depends on whether this is a common feature of perforant path synapses on all postsynaptic target cells or if this plasticity occurs only at synapses on principal cells. We recorded NMDAR-EPSCs at medial perforant path synapses on interneurons in dentate gyrus which had significantly slower decay kinetics compared to those recorded in granule cells. NMDAR pharmacology in interneurons was consistent with expression of both GluN2B- and GluN2D-containing receptors. In contrast to previously described high frequency stimulation-induced bidirectional plasticity of NMDAR-EPSCs in granule cells, only LTD of NMDAR-EPSCs was induced in interneurons in our standard experimental conditions. In interneurons, LTD of NMDAR-EPSCs was associated with a loss of sensitivity to a GluN2D-selective antagonist and was inhibited by the actin stabilising agent, jasplakinolide. While LTP of NMDAR-EPSCs can be readily induced in granule cells, this form of plasticity was only observed in interneurons when extracellular calcium was increased above physiological concentrations during HFS or when PKC was directly activated by phorbol ester, suggesting that opposing forms of plasticity at inputs to interneurons and principal cells may act to regulate granule cell dendritic integration and processing.

Dendritic calcium mechanisms and long-term potentiation in cortical inhibitory interneurons

Calcium (Ca(2+) ) is a major second messenger in the regulation of different forms of synaptic and intrinsic plasticity. Tightly organized in space and time, postsynaptic Ca(2+) transients trigger the activation of many distinct Ca(2+) signaling cascades, providing a means for a highly specific signal transduction and plasticity induction. High-resolution two-photon microscopy combined with highly sensitive synthetic Ca(2+) indicators in brain slices allowed for the quantification and analysis of postsynaptic Ca(2+) dynamics in great detail. Much of our current knowledge about postsynaptic Ca(2+) mechanisms is derived from studying Ca(2+) transients in the dendrites and spines of pyramidal neurons. However, postsynaptic Ca(2+) dynamics differ considerably among different cell types. In particular, distinct rules of postsynaptic Ca(2+) signaling and, accordingly, of Ca(2+) -dependent plasticity operate in GABAergic interneurons. Here, I review recent progress in understanding the complex organization of postsynaptic Ca(2+) signaling and its relevance to several forms of long-term potentiation at excitatory synapses in cortical GABAergic interneurons.

Evaluating nanoparticle sensor design for intracellular pH measurements

Particle-based nanosensors have over the past decade been designed for optical fluorescent-based ratiometric measurements of pH in living cells. However, quantitative and time-resolved intracellular measurements of pH in endosomes and lysosomes using particle nanosensors are challenging, and there is a need to improve measurement methodology. In the present paper, we have successfully carried out time-resolved pH measurements in endosomes and lyosomes in living cells using nanoparticle sensors and show the importance of sensor choice for successful quantification. We have studied two nanoparticle-based sensor systems that are internalized by endocytosis and elucidated important factors in nanosensor design that should be considered in future development of new sensors. From our experiments it is clear that it is highly important to use sensors that have a broad measurement range, as erroneous quantification of pH is an unfortunate result when measuring pH too close to the limit of the sensitive range of the sensors. Triple-labeled nanosensors with a pH measurement range of 3.2-7.0, which was synthesized by adding two pH-sensitive fluorophores with different pK(a) to each sensor, seem to be a solution to some of the earlier problems found when measuring pH in the endosome-lysosome pathway.

A voltage-sensitive dye-based assay for the identification of differentiated neurons derived from embryonic neural stem cell cultures

BACKGROUND

Pluripotent and multipotent stem cells hold great therapeutical promise for the replacement of degenerated tissue in neurological diseases. To fulfill that promise we have to understand the mechanisms underlying the differentiation of multipotent cells into specific types of neurons. Embryonic stem cell (ESC) and embryonic neural stem cell (NSC) cultures provide a valuable tool to study the processes of neural differentiation, which can be assessed using immunohistochemistry, gene expression, Ca(2+)-imaging or electrophysiology. However, indirect methods such as protein and gene analysis cannot provide direct evidence of neuronal functionality. In contrast, direct methods such as electrophysiological techniques are well suited to produce direct evidence of neural functionality but are limited to the study of a few cells on a culture plate.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we describe a novel method for the detection of action potential-capable neurons differentiated from embryonic NSC cultures using fast voltage-sensitive dyes (VSD). We found that the use of extracellularly applied VSD resulted in a more detailed labeling of cellular processes compared to calcium indicators. In addition, VSD changes in fluorescence translated precisely to action potential kinetics as assessed by the injection of simulated slow and fast sodium currents using the dynamic clamp technique. We further demonstrate the use of a finite element model of the NSC culture cover slip for optimizing electrical stimulation parameters.

CONCLUSIONS/SIGNIFICANCE

Our method allows for a repeatable fast and accurate stimulation of neurons derived from stem cell cultures to assess their differentiation state, which is capable of monitoring large amounts of cells without harming the overall culture.

High-density expression of Ca2+-permeable ASIC1a channels in NG2 glia of rat hippocampus

NG2 cells, a fourth type of glial cell in the mammalian CNS, undergo reactive changes in response to a wide variety of brain insults. Recent studies have demonstrated that neuronally expressed acid-sensing ion channels (ASICs) are implicated in various neurological disorders including brain ischemia and seizures. Acidosis is a common feature of acute neurological conditions. It is postulated that a drop in pH may be the link between the pathological process and activation of NG2 cells. Such postulate immediately prompts the following questions: Do NG2 cells express ASICs? If so, what are their functional properties and subunit composition? Here, using a combination of electrophysiology, Ca2+ imaging and immunocytochemistry, we present evidence to demonstrate that NG2 cells of the rat hippocampus express high density of Ca2+-permeable ASIC1a channels compared with several types of hippocampal neurons. First, nucleated patch recordings from NG2 cells revealed high density of proton-activated currents. The magnitude of proton-activated current was pH dependent, with a pH for half-maximal activation of 6.3. Second, the current-voltage relationship showed a reversal close to the equilibrium potential for Na+. Third, psalmotoxin 1, a blocker specific for the ASIC1a channel, largely inhibited proton-activated currents. Fourth, Ca2+ imaging showed that activation of proton-activated channels led to an increase of [Ca2+]i. Finally, immunocytochemistry showed co-localization of ASIC1a and NG2 proteins in the hippocampus. Thus the acid chemosensor, the ASIC1a channel, may serve for inducing membrane depolarization and Ca2+ influx, thereby playing a crucial role in the NG2 cell response to injury following ischemia.

Cell type-specific expression of acid-sensing ion channels in hippocampal interneurons

Acid-sensing ion channels (ASICs), a member of the degenerin/epithelial Na+ channel superfamily, are widely expressed in the mammalian CNS. Accumulating evidence suggests that ASIC current density is higher in GABAergic interneurons than that in glutamatergic pyramidal neurons (PNs) in the hippocampus. Such differential expression of ASICs in cortical networks is thought to be a key element for seizure termination. However, GABAergic interneurons are highly diverse; it is unclear whether the functional expression of ASICs differs in distinct GABAergic interneuron subtypes. Moreover, the subunit composition of ASICs in individual GABAergic interneurons remains unknown. By combining patch-clamp recording and single-cell reverse transcription (RT)-PCR analysis, we correlated ASIC currents with their gene expression in acute rat hippocampal slices. The results yielded several surprising findings. First, ASIC current density of oriens lacunosum-moleculare (O-LM) cells in the CA1 region, a classical type of dendrite-targeting interneuron, is 6 times greater than that of fast-spiking basket cells (BCs) in the dentate gyrus, a major class of soma-targeting interneuron. Second, the recovery of ASICs from desensitization is slowest in BCs, intermediate in PNs, and fastest in O-LM cells. Third, the tarantula venom psalmotoxin 1, the specific blocker for ASIC1a homomers, inhibits ASIC currents in BCs but not in O-LM cells. Finally, single-cell RT-PCR analysis reveals coexpression of ASIC1a and ASIC2 subunit transcripts in O-LM cells, whereas only ASIC1a subunit transcript is detected in most BCs. Thus, differential expression of ASICs in inhibitory microcircuits likely contributes to the distinct roles of GABAergic interneurons in normal physiology and pathophysiology.

Abnormal Excitability of Oblique Dendrites Implicated in Early Alzheimer's: A Computational Study

The integrative properties of cortical pyramidal dendrites are essential to the neural basis of cognitive function, but the impact of amyloid beta protein (abeta) on these properties in early Alzheimer's is poorly understood. In animal models, electrophysiological studies of proximal dendrites have shown that abeta induces hyperexcitability by blocking A-type K+ currents (I(A)), disrupting signal integration. The present study uses a computational approach to analyze the hyperexcitability induced in distal dendrites beyond the experimental recording sites. The results show that back-propagating action potentials in the dendrites induce hyperexcitability and excessive calcium concentrations not only in the main apical trunk of pyramidal cell dendrites, but also in their oblique dendrites. Evidence is provided that these thin branches are particularly sensitive to local reductions in I(A). The results suggest the hypothesis that the oblique branches may be most vulnerable to disruptions of I(A) by early exposure to abeta, and point the way to further experimental analysis of these actions as factors in the neural basis of the early decline of cognitive function in Alzheimer's.