Subcellular calcium transients visualized by confocal microscopy in a voltage-clamped vertebrate neuron

Calcium-activated potassium currents in mammalian neurons

1. Influx of calcium via voltage-dependent calcium channels during the action potential leads to increases in cytosolic calcium that can initiate a number of physiological processes. One of these is the activation of potassium currents on the plasmalemma. These calcium-activated potassium currents contribute to action potential repolarization and are largely responsible for the phenomenon of spike frequency adaptation. This refers to the progressive slowing of the frequency of discharge of action potentials during sustained injection of depolarizing current. In some cell types, this adaptation is so marked that despite the presence of depolarizing current, only a single spike (or a few spikes) is initiated. Following cessation of current injection, slow deactivation of calcium-activated potassium currents is also responsible for the prolonged hyperpolarization that often follows. 2. A number of macroscopic calcium-activated potassium currents that can be separated on the basis of kinetic and pharmacological criteria have been described in mammalian neurons. At the single channel level, several types of calcium-activated potassium channels also have been characterized. While for some macroscopic currents the underlying single channels have been unambiguously defined, for other currents the identity of the underlying channels is not clear. 3. In the present review we describe the properties of the known types of calcium-activated potassium currents in mammalian neurons and indicate the relationship between macroscopic currents and particular single channels.

Cholinergic stimulation enhances cytosolic calcium ion accumulation in mouse hippocampal CA1 pyramidal neurones during short action potential trains

Acetylcholine is a regulatory cofactor for numerous activity-dependent processes of central nervous system development and plasticity in which increases in cytosolic calcium ion concentration ([Ca(2+)](cyto) couple membrane excitation to cellular changes. We examined how cholinergic receptor activation affects temporal and spatial aspects of increases in [Ca(2+)](cyto) during short trains of action potentials in hippocampal CA1 pyramidal neurones. Membrane-impermeant Ca(2+)-sensitive dye was introduced into the cytosol during whole-cell recordings, and Ca(2+)-dependent fluorescence was recorded from somatic, nuclear and proximal dendrite regions with high temporal resolution. In all neuronal compartments, the cholinergic agonist carbachol (5 microM) increased resting [Ca(2+)](cyto) and the maximum [Ca(2+)](cyto) attained during a short action potential train. Carbachol also slowed the recovery of [Ca(2+)](cyto) towards resting levels. The largest increases in peak cytosolic Ca(2+) concentration (delta [Ca(2+)](cyto) were seen in the dendrite and apical cell body, while relaxations of the carbachol-induced increase in delta [Ca(2+)](cyto) showed greater prolongation in the nucleus and basal cell body. Most significantly, the difference between Ca(2+) signals recorded before and during exposure to carbachol consistently showed a monotonic rise and smooth fall in all cell compartments, suggesting that the increase in [Ca(2+)](cyto) associated with each action potential was not altered by carbachol. Consistent with this view, changes in Ca(2+) signalling were not accompanied by changes in action potential waveforms. The effects of carbachol were partially reversed by simultaneous exposure to atropine, or partially inhibited by inclusion of heparin in the intracellular solution, indicating the involvement of muscarinic acetylcholine receptors and InsP(3)-sensitive Ca(2+)-release channels. Our data indicate that carbachol-induced slowing of [Ca(2+)]cyto relaxations after each action potential results in enhanced accumulation of Ca(2+) in the cytosol in the absence of changes in action potential-driven Ca(2+) entry. By modulating the time course of Ca(2+) signals, cholinergic stimulation may regulate the activation of Ca(2+)-dependent intracellular processes dependent on patterns of [Ca(2+)](cyto) changes.

Modes of propagation of Ca(2+)-induced Ca2+ release in bullfrog sympathetic ganglion cells

How depolarization-induced Ca2+ entry or caffeine activates Ca(2+)-induced Ca2+ release (CICR) in the cytoplasm and nucleoplasm was studied by recording intracellular Ca2+ ([Ca2+]i) with a confocal microscope in cultured bullfrog sympathetic ganglion cells. The amplitude and propagation speed of voltage pulse-induced rises in [Ca2+]i were greater in the submembrane (< 5 microns depth) region than in the core region, and delayed and smaller, but significant, in the nucleus. Ryanodine and dantrolene reduced the rises in [Ca2+]i in both the cytoplasm and nucleus. A rapid application of high K+ solution induced global rises in [Ca2+]i in both the cytoplasm and nucleoplasm, which were decreased by dantrolene. Caffeine produced a slow, small rise in [Ca2+]i which grew into a global, regenerative rise both in the cytoplasm and nucleoplasm with some inward gradient in the cytoplasm. Each of the high [Ca2+]i phases during caffeine-induced [Ca2+]i oscillation began in the submembrane region, while low [Ca2+]i phases started in the core region. These results suggest that CICR activated by Ca2+ entry or caffeine occurs predominantly in the submembrane region causing an inwardly spreading Ca2+ wave or [Ca2+]i oscillations, and that the nuclear envelope can cause CICR in the nucleoplasm, which is delayed due to Ca2+ diffusion barrier at the nuclear pores.

Dissection of mitochondrial Ca2+ uptake and release fluxes in situ after depolarization-evoked [Ca2+](i) elevations in sympathetic neurons

We studied how mitochondrial Ca2+ transport influences [Ca2+](i) dynamics in sympathetic neurons. Cells were treated with thapsigargin to inhibit Ca2+ accumulation by SERCA pumps and depolarized to elevate [Ca2+(i); the recovery that followed repolarization was then examined. The total Ca2+ flux responsible for the [Ca2+](i) recovery was separated into mitochondrial and nonmitochondrial components based on sensitivity to the proton ionophore FCCP, a selective inhibitor of mitochondrial Ca2+ transport in these cells. The nonmitochondrial flux, representing net Ca2+ extrusion across the plasma membrane, has a simple dependence on [Ca2+](i), while the net mitochondrial flux (J(mito)) is biphasic, indicative of Ca+) accumulation during the initial phase of recovery when [Ca2+](i) is high, and net Ca2+ release during later phases of recovery. During each phase, mitochondrial Ca2+ transport has distinct effects on recovery kinetics. J(mito) was separated into components representing mitochondrial Ca2+ uptake and release based on sensitivity to the specific mitochondrial Na(+)/Ca2+ exchange inhibitor, CGP 37157 (CGP). The CGP-resistant (uptake) component of J(mito) increases steeply with [Ca2+](i), as expected for transport by the mitochondrial uniporter. The CGP-sensitive (release) component is inhibited by lowering the intracellular Na(+) concentration and depends on both intra- and extramitochondrial Ca2+ concentration, as expected for the Na(+)/Ca2+ exchanger. Above approximately 400 nM [Ca2+](i), net mitochondrial Ca2+ transport is dominated by uptake and is largely insensitive to CGP. When [Ca2+](i) is approximately 200-300 nM, the net mitochondrial flux is small but represents the sum of much larger uptake and release fluxes that largely cancel. Thus, mitochondrial Ca2+ transport occurs in situ at much lower concentrations than previously thought, and may provide a mechanism for quantitative control of ATP production after brief or low frequency stimuli that raise [Ca(2+)](i) to levels below approximately 500 nM.

Quantitative analysis of mitochondrial Ca2+ uptake and release pathways in sympathetic neurons. Reconstruction of the recovery after depolarization-evoked [Ca2+]i elevations

Rate equations for mitochondrial Ca2+ uptake and release and plasma membrane Ca2+ transport were determined from the measured fluxes in the preceding study and incorporated into a model of Ca2+ dynamics. It was asked if the measured fluxes are sufficient to account for the [Ca2+]i recovery kinetics after depolarization-evoked [Ca2+]i elevations. Ca2+ transport across the plasma membrane was described by a parallel extrusion/leak system, while the rates of mitochondrial Ca2+ uptake and release were represented using equations like those describing Ca2+ transport by isolated mitochondria. Taken together, these rate descriptions account very well for the time course of recovery after [Ca2+]i elevations evoked by weak and strong depolarization and their differential sensitivity to FCCP, CGP 37157, and [Na+]i. The model also leads to three general conclusions about mitochondrial Ca2+ transport in intact cells: (1) mitochondria are expected to accumulate Ca2+ even in response to stimuli that raise [Ca2+]i only slightly above resting levels; (2) there are two qualitatively different stimulus regimes that parallel the buffering and non-buffering modes of Ca2+ transport by isolated mitochondria that have been described previously; (3) the impact of mitochondrial Ca2+ transport on intracellular calcium dynamics is strongly influenced by nonmitochondrial Ca2+ transport; in particular, the magnitude of the prolonged [Ca2+]i elevation that occurs during the plateau phase of recovery is related to the Ca2+ set-point described in studies of isolated mitochondria, but is a property of mitochondrial Ca2+ transport in a cellular context. Finally, the model resolves the paradoxical finding that stimulus-induced [Ca2+]i elevations as small as approximately 300 nM increase intramitochondrial total Ca2+ concentration, but the steady [Ca2+]i elevations evoked by such stimuli are not influenced by FCCP.

Measurement and Interpretation of Cytoplasmic [Ca2+] Signals From Calcium-Indicator Dyes

Ca2+-indicator dyes are widely used in biology yet difficult to characterize inside cells. Studies in skeletal muscle fibers provide important information about indicator behavior and about Ca2+ signaling within the cytoplasm.

Dual responses of CNS mitochondria to elevated calcium

Isolated brain mitochondria were examined for their responses to calcium challenges under varying conditions. Mitochondrial membrane potential was monitored by following the distribution of tetraphenylphosphonium ions in the mitochondrial suspension, mitochondrial swelling by observing absorbance changes, calcium accumulation by an external calcium electrode, and oxygen consumption with an oxygen electrode. Both the extent and rate of calcium-induced mitochondrial swelling and depolarization varied greatly depending on the energy source provided to the mitochondria. When energized with succinate plus glutamate, after a calcium challenge, CNS mitochondria depolarized transiently, accumulated substantial calcium, and increased in volume, characteristic of a mitochondrial permeability transition. When energized with 3 mM succinate, CNS mitochondria maintained a sustained calcium-induced depolarization without appreciable swelling and were slow to accumulate calcium. Maximal oxygen consumption was also restricted under these conditions, preventing the electron transport chain from compensating for this increased proton permeability. In 3 mM succinate, cyclosporin A and ADP plus oligomycin restored potential and calcium uptake. This low conductance permeability was not effected by bongkrekic acid or carboxyatractylate, suggesting that the adenine nucleotide translocator was not directly involved. Fura-2FF measurements of [Ca(2+)](i) suggest that in cultured hippocampal neurons glutamate-induced increases reached tens of micromolar levels, approaching those used with mitochondria. We propose that in the restricted substrate environment, Ca(2+) activated a low-conductance permeability pathway responsible for the sustained mitochondrial depolarization.

In vitro status epilepticus causes sustained elevation of intracellular calcium levels in hippocampal neurons

Calcium ions and calcium-dependent systems have been implicated in the pathophysiology of status epilepticus (SE). However, the dynamics of intracellular calcium ([Ca2+]i) levels during SE has not yet been studied. We have employed the hippocampal neuronal culture (HNC) model of in vitro SE that produces continuous epileptiform discharges to study spatial and dynamic changes in [Ca2+]i levels utilizing confocal laser scanning microscopy and the calcium binding dye, indo-1. During SE, the average [Ca2+]i levels increased from control levels of 150-200 nM to levels of 450-600 nM. This increased [Ca2+]i was maintained for the duration of SE. Following SE, [Ca2+]i levels gradually returned to basal values. The duration of SE was shown to affect the ability of the neuron to restore resting [Ca2+]i levels. Both N-methyl-D-aspartate (NMDA) receptor-gated and voltage-gated Ca2+ channels (VGCCs) contributed to the increased calcium entry during SE. Moreover, this elevation in [Ca2+]i occurred in both the nucleus and cytosol. These results provide the first dynamic measurement of [Ca2+]i during prolonged electrographic seizure discharges in an in vitro SE model and suggest that prolonged epileptiform discharges give rise to abnormal sustained increases in [Ca2+]i levels that may play a role in the neuronal cell damage and long-term plasticity changes associated with SE.

Dendritic Ca(2+)-activated K(+) conductances regulate electrical signal propagation in an invertebrate neuron

Activity-dependent changes in the short-term electrical properties of neurites were investigated in the anterior pagoda (AP) cell of leech. Imaging studies revealed that backpropagating Na(+) spikes and synaptically evoked EPSPs caused Ca(2+) entry through low-voltage-activated Ca(2+) channels that are distributed throughout the neurites. Voltage-clamp recordings from the soma revealed a TEA-sensitive outward current that was reduced when Ca(2+) entry was blocked with Co(2+) or when the intracellular concentration of free Ca(2+) was reduced by a high-affinity Ca(2+) buffer. Ca(2+) released in the neurite from a caged Ca(2+) compound caused a hyperpolarization of the membrane potential. These data imply that the AP cell expresses Ca(2+)-activated K(+) conductances, and that these conductances are present in the neurites. When the Ca(2+)-activated K(+) current was reduced through the block of Ca(2+) entry, backpropagating Na(+) spikes and synaptically evoked EPSPs increased in amplitude. Hence, the activity-dependent changes in the intracellular [Ca(2+)] together with the Ca(2+)-activated K(+) conductances participate in the regulation of dendritic signal propagation.

Confocal laser scanning microscopy of calcium dynamics in living cells

Confocal laser scanning microscopy (CLSM) is widely used to monitor intracellular calcium levels in living cells loaded with calcium-sensitive fluorophores. This review examines the basic advantages and limitations of CLSM in in vivo imaging analyses of calcium dynamics. The benefits of utilizing ratioed images and dextran-conjugated fluorophores are addressed, and practical aspects of handling confocal datasets are outlined. After considering some relatively new microscopical methods that can be used in conjunction with conventional CLSM, possible future applications of confocal techniques in analyses of intracellular calcium dynamics are discussed.

Controlling the urge for a Ca(2+) surge: all-or-none Ca(2+) release in neurons

Changes in the intracellular calcium concentration ([Ca2+]i) convey signals that are essential to the life and death of neurons. Ca(2+)-induced Ca(2+)-release (CICR), a process in which a modest elevation in [Ca2+]i is amplified by a secondary release of Ca2+ from stores within the cell, plays a prominent role in shaping neuronal [Ca2+]i signals. When CICR becomes regenerative, an explosive increase in [Ca2+]i generates a Ca2+ wave that spreads throughout the cell. A discrete threshold controls activation of this all-or-none behavior and cellular context adjusts the threshold. Thus, the store acts as a switch that determines whether a given pattern of electrical activity will produce a local or global Ca2+ signal. This gatekeeper function seems to control some forms of Ca(2+)-triggered plasticity in neurons.

Activation of nuclear calcium dynamics by synaptic stimulation in cultured cortical neurons

L-type voltage-sensitive Ca2+ channels (VSCCs) are enriched on the neuronal soma and trigger gene expression during synaptic activity. To understand better how these channels regulate somatic and nuclear Ca2+ dynamics, we have investigated Ca2+ influx through L-type VSCCs following synaptic stimulation, using the long-wavelength Ca2+ indicator fluo-3 combined with laser scanning confocal microscopy. Single synaptic stimuli resulted in rapid Ca2+ transients in somatic cytoplasmic compartments (<5 ms rise time). Nuclear Ca2+ elevations lagged behind cytoplasmic levels by approximately 60 ms, consistent with a dependence on diffusion from a cytoplasmic source. Pharmacological experiments indicated that L-type VSCCs mediated approximately 50% of the nuclear and somatic (cytoplasmic) Ca2+ elevation in response to strong synaptic stimulation. In contrast, relatively weak excitatory postsynaptic potentials (EPSPs; approximately 15 mV) or single action potentials were much less effective at activating L-type VSCCs. Antagonist experiments indicated that activation of the NMDA-type glutamate receptor leads to a long-lasting somatic depolarization necessary to activate L-type VSCCs effectively during synaptic stimuli. Simulation of action potential and somatic EPSP depolarization using voltage-clamp pulses indicated that nuclear Ca2+ transients mediated by L-type VSCCs were produced by sustained depolarization positive to -25 mV. In the absence of synaptic stimulation, action potential stimulation alone led to elevations in nuclear Ca2+ mediated by predominantly non-L-type VSCCs. Our results suggest that action potentials, in combination with long-lived synaptic depolarizations, facilitate the activation of L-type VSCCs. This activity elevates somatic Ca2+ levels that spread to the nucleus.

Depolarization-induced mitochondrial Ca accumulation in sympathetic neurons: spatial and temporal characteristics

Several lines of evidence suggest that neuronal mitochondria accumulate calcium when the cytosolic free Ca(2+) concentration ([Ca(2+)](i)) is elevated to levels approaching approximately 500 nM, but the spatial, temporal, and quantitative characteristics of net mitochondrial Ca uptake during stimulus-evoked [Ca(2+)](i) elevations are not well understood. Here, we report direct measurements of depolarization-induced changes in intramitochondrial total Ca concentration ([Ca](mito)) obtained by x-ray microanalysis of rapidly frozen neurons from frog sympathetic ganglia. Unstimulated control cells exhibited undetectably low [Ca](mito), but high K(+) depolarization (50 mM, 45 sec), which elevates [Ca(2+)](i) to approximately 600 nM, increased [Ca](mito) to 13.0 +/- 1.5 mmol/kg dry weight; this increase was abolished by carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP). The elevation of [Ca](mito) was a function of both depolarization strength and duration. After repolarization, [Ca](mito) recovered to prestimulation levels with a time course that paralleled the decline in [Ca(2+)](i). Depolarization-induced increases in [Ca](mito) were spatially heterogeneous. At the level of single mitochondria, [Ca](mito) elevations depended on proximity to the plasma membrane, consistent with predictions of a diffusion model that considers radial [Ca(2+)](i) gradients that exist early during depolarization. Within individual mitochondria, Ca was concentrated in small, discrete sites, possibly reflecting a high-capacity intramitochondrial Ca storage mechanism. These findings demonstrate that in situ Ca accumulation by mitochondria, now directly identified as the structural correlate of the "FCCP-sensitive store, " is robust, reversible, graded with stimulus strength and duration, and dependent on spatial location.

Impulse encoding across the dendritic morphologies of retinal ganglion cells

Nerve impulse entrainment and other excitation and passive phenomena are analyzed for a morphologically diverse and exhaustive data set (n = 57) of realistic (3-dimensional computer traced) soma-dendritic tree structures of ganglion cells in the tiger salamander (Ambystoma tigrinum) retina. The neurons, including axon and an anatomically specialized thin axonal segment that is observed in every ganglion cell, were supplied with five voltage- or ligand-gated ion channels (plus leakage), which were distributed in accordance with those found in a recent study that employed an equivalent dendritic cylinder. A wide variety of impulse-entrainment responses was observed, including regular low-frequency firing, impulse doublets, and more complex patterns involving impulse propagation failures (or aborted spikes) within the encoder region, all of which have been observed experimentally. The impulse-frequency response curves of the cells fell into three groups called FAST, MEDIUM, and SLOW in approximate proportion as seen experimentally. In addition to these, a new group was found among the traced cells that exhibited an impulse-frequency response twice that of the FAST category. The total amount of soma-dendritic surface area exhibited by a given cell is decisive in determining its electrophysiological classification. On the other hand, we found only a weak correlation between the electrophysiological group and the morphological classification of a given cell, which is based on the complexity of dendritic branching and the physical reach or "receptive field" area of the cell. Dendritic morphology determines discharge patterns to dendritic (synaptic) stimulation. Orthodromic impulses can be initiated on the axon hillock, the thin axonal segment, the soma, or even the proximal axon beyond the thin segment, depending on stimulus magnitude, soma-dendritic membrane area, channel distribution, and state within the repetitive impulse cycle. Although a sufficiently high dendritic Na-channel density can lead to dendritic impulse initiation, this does not occur with our "standard" channel densities and is not seen experimentally. Even so, impulses initiated elsewhere do invade all except very thin dendritic processes. Impulse-encoding irregularities increase when channel conductances are reduced in the encoder region, and the F/I properties of the cells are a strong function of the calcium- and Ca-activated K-channel densities. Use of equivalent dendritic cylinders requires more soma-dendritic surface area than real dendritic trees, and the source of the discrepancy is discussed.

DREAM is a Ca2+-regulated transcriptional repressor

Fluxes in amounts of intracellular calcium ions are important determinants of gene expression. So far, Ca2+-regulated kinases and phosphatases have been implicated in changing the phosphorylation status of key transcription factors and thereby modulating their function. In addition, direct effectors of Ca2+-induced gene expression have been suggested to exist in the nucleus, although no such effectors have been identified yet. Expression of the human prodynorphin gene, which is involved in memory acquisition and pain, is regulated through its downstream regulatory element (DRE) sequence, which acts as a location-dependent gene silencer. Here we isolate a new transcriptional repressor, DRE-antagonist modulator (DREAM), which specifically binds to the DRE. DREAM contains four Ca2+-binding domains of the EF-hand type. Upon stimulation by Ca2+, DREAM's ability to bind to the DRE and its repressor function are prevented. Mutation of the EF-hands abolishes the response of DREAM to Ca2+. In addition to the prodynorphin promoter, DREAM represses transcription from the early response gene c-fos. Thus, DREAM represents the first known Ca2+-binding protein to function as a DNA-binding transcriptional regulator.

Oxygen/glucose deprivation in hippocampal slices: altered intraneuronal elemental composition predicts structural and functional damage

Effects of oxygen/glucose deprivation (OGD) on subcellular elemental composition and water content were determined in nerve cell bodies from CA1 areas of rat hippocampal slices. Electron probe x-ray microanalysis was used to measure percentage water and concentrations of Na, P, K, Cl, Mg, and Ca in cytoplasm, nucleus, and mitochondria of cells exposed to normal and oxygen/glucose deficient medium. As an early (2 min) consequence of OGD, evoked synaptic potentials were lost, and K, Cl, P, and Mg concentrations decreased significantly in all morphological compartments. As exposure to in vitro OGD continued, a negative DC shift in interstitial voltage occurred ( approximately 5 min), whereas general elemental disruption worsened in cytoplasm and nucleus (5-42 min). Similar elemental changes were noted in mitochondria, except that Ca levels increased during the first 5 min of OGD and then decreased over the remaining experimental period (12-42 min). Compartmental water content decreased early (2 min), returned to control after 12 min of OGD, and then exceeded control levels at 42 min. After OGD (12 min), perfusion of hippocampal slices with control oxygenated solutions (reoxygenation) for 30 min did not restore synaptic function or improve disrupted elemental composition. Notably, reoxygenated CA1 cell compartments exhibited significantly elevated Ca levels relative to those associated with 42 min of OGD. When slices were incubated at 31 degreesC (hypothermia) during OGD/reoxygenation, neuronal dysfunction and elemental deregulation were minimal. Results show that in vitro OGD causes loss of transmembrane Na, K, and Ca gradients in CA1 neurons of hippocampal slices and that hypothermia can obtund this damaging process and preserve neuronal function.

Imaging neuronal calcium fluorescence at high spatio-temporal resolution

A rapid fluorescence imaging system was developed and utilised to investigate the time-course of intracellular calcium concentration ([Ca2+]i) gradients generated by action potentials in CA1-CA3 pyramidal cells within brain slices of the rat hippocampus. The system, which is based on a fast commercial CCD camera, can acquire hundreds of 128 x 128 pixel images in sequence, with minimal inter-frame interval of 2.5 ms (400 frames/s) and 12 bit/pixel accuracy. By synchronising patch clamp recordings with image capture, the timing of transmembrane potential variation, ionic Ca2+ current and Ca2+ diffusion were resolved at the limit of the relaxation time for the dye-Ca2+ binding reaction (approximately 5 ms at room temperature). Numerical simulations were used to relate measured fluorescence transients to the spatio-temporal distribution of intracellular Ca2+ gradients. The results obtained indicate that dye reaction-diffusion contributes critically to shaping intracellular ion gradients.

Cellular mechanisms of hypoxic injury in the developing brain

The susceptibility of the developing brain to hypoxia should depend on the lipid composition of the brain cell membrane; the rate of lipid peroxidation; the presence of antioxidant defenses; and the development and modulation of the excitatory neurotransmitter receptors such as the N-methyl-D-aspartate (NMDA) receptor, the intracellular Ca++ and intranuclear Ca++-dependent mechanisms. In addition to the developmental status of these cellular components, the response of these potential mechanisms to hypoxia determines the fate of the hypoxic brain cell in the developing brain. In the fetal guinea pig and newborn piglet models, studies have demonstrated that brain tissue hypoxia results in brain cell membrane damage as evidenced by increased membrane lipid peroxidation and decreased Na+,K+-ATPase activity. Using electron spin resonance spectroscopy of alpha-phenyl-N-tert-butyl-nitrone spin-adducts, studies from our laboratory have demonstrated that tissue hypoxia results in increased free radical generation in the cortex of fetal guinea pigs and newborn piglets. We have also shown that brain tissue hypoxia modifies the N-methyl-D-aspartate receptor-ion channel, recognition and modulatory sites. Furthermore, a higher increase in NMDA receptor agonist-dependent Ca++ in synaptosomes of hypoxic as compared to normoxic fetuses was demonstrated. The increase in intracellular Ca++ may activate several enzymatic pathways such as phospholipase A2 and metabolism of arachidonic acid by cyclooxygenase and lipoxygenase, conversion of xanthine dehydrogenase to xanthine oxidase by proteases and activation of nitric oxide synthase. Using specific inhibitors of each of these enzymes such as cyclooxygenase (indomethacin), lipoxygenase (nordihydroguaiaretic acid), xanthine oxidase (allopurinol) and nitric oxide synthase (N-nitro-L-arginine), studies have shown that these enzyme reactions result in oxygen free radical generation, membrane lipid peroxidation and cell membrane dysfunction in the hypoxic brain. We suggest that, during hypoxia, the increased intracellular Ca++ may lead to an increased intranuclear Ca++ concentration and alter nuclear events including transcription of specific genes responsible for programmed cell death. In view of the developmental studies presented, the susceptibility of the fetal brain to hypoxia appears to increase with brain development as gestation approaches term.

The influence of sarcoplasmic reticulum Ca2+ concentration on Ca2+ sparks and spontaneous transient outward currents in single smooth muscle cells

Localized, transient elevations in cytosolic Ca2+, known as Ca2+ sparks, caused by Ca2+ release from sarcoplasmic reticulum, are thought to trigger the opening of large conductance Ca2+-activated potassium channels in the plasma membrane resulting in spontaneous transient outward currents (STOCs) in smooth muscle cells. But the precise relationships between Ca2+ concentration within the sarcoplasmic reticulum and a Ca2+ spark and that between a Ca2+ spark and a STOC are not well defined or fully understood. To address these problems, we have employed two approaches using single patch-clamped smooth muscle cells freshly dissociated from toad stomach: a high speed, wide-field imaging system to simultaneously record Ca2+ sparks and STOCs, and a method to simultaneously measure free global Ca2+ concentration in the sarcoplasmic reticulum ([Ca2+]SR) and in the cytosol ([Ca2+]CYTO) along with STOCs. At a holding potential of 0 mV, cells displayed Ca2+ sparks and STOCs. Ca2+ sparks were associated with STOCs; the onset of the sparks coincided with the upstroke of STOCs, and both had approximately the same decay time. The mean increase in [Ca2+]CYTO at the time and location of the spark peak was approximately 100 nM above a resting concentration of approximately 100 nM. The frequency and amplitude of spontaneous Ca2+ sparks recorded at -80 mV were unchanged for a period of 10 min after removal of extracellular Ca2+ (nominally Ca2+-free solution with 50 microM EGTA), indicating that Ca2+ influx is not necessary for Ca2+sparks. A brief pulse of caffeine (20 mM) elicited a rapid decrease in [Ca2+]SR in association with a surge in [Ca2+]CYTO and a fusion of STOCs, followed by a fast restoration of [Ca2+]CYTO and a gradual recovery of [Ca2+]SR and STOCs. The return of global [Ca2+]CYTO to rest was an order of magnitude faster than the refilling of the sarcoplasmic reticulum with Ca2+. After the global [Ca2+]CYTO was fully restored, recovery of STOC frequency and amplitude were correlated with the level of [Ca2+]SR, even though the time for refilling varied greatly. STOC frequency did not recover substantially until the [Ca2+]SR was restored to 60% or more of resting levels. At [Ca2+]SR levels above 80% of rest, there was a steep relationship between [Ca2+]SR and STOC frequency. In contrast, the relationship between [Ca2+]SR and STOC amplitude was linear. The relationship between [Ca2+]SR and the frequency and amplitude was the same for Ca2+ sparks as it was for STOCs. The results of this study suggest that the regulation of [Ca2+]SR might provide one mechanism whereby agents could govern Ca2+ sparks and STOCs. The relationship between Ca2+ sparks and STOCs also implies a close association between a sarcoplasmic reticulum Ca2+ release site and the Ca2+-activated potassium channels responsible for a STOC.

Phospholipid signalling in the nucleus. Een DAG uit het leven van de inositide signalering in de nucleus

Diverse methodologies, ranging from activity measurements in various nuclear subfractions to electron microscopy, have been used to demonstrate and establish that many of the key lipids and enzymes responsible for the metabolism of inositol lipids are resident in nuclei. PtdIns(4)P, PtdIns(4,5)P2 and PtdOH are all present in nuclei, as well as the corresponding enzyme activities required to synthesise and metabolise these compounds. In addition other non-inositol containing phospholipids such as phosphatidylcholine constitute a significant percentage of the total nuclear phospholipid content. We feel that it is pertinent to include this lipid in our discussion as it provides an alternative source of 1, 2-diacylglycerol (DAG) in addition to the hydrolysis of PtdIns(4, 5)P2. We discuss at length data related to the sources and possible consequences of nuclear DAG production as this lipid appears to be increasingly central to a number of general physiological functions. Data relating to the existence of alternative pathways of inositol phospholipid synthesis, the role of 3-phosphorylated inositol lipids and lipid compartmentalisation and transport are reviewed. The field has also expanded to a point where we can now also begin to address what role these lipids play in cellular proliferation and differentiation and hopefully provide avenues for further research.

Increase of intracellular Ca2+ and relocation of E-cadherin during experimental decompaction of mouse embryos

To determine the role of intracellular Ca2+ in compaction, the first morphogenetic event in embryogenesis, we analyzed preimplantation mouse embryos under several decompacting conditions, including depletion of extracellular Ca2+, blocking of Ca2+ channels, and inhibition of microfilaments, calmodulin, and intracellular Ca2+ release. Those treatments induced decompaction of mouse morulae and simultaneously induced changes in cytosolic free Ca2+ concentration and deregionalization of E-cadherin and fodrin. When morulae were allowed to recompact, the location of both proteins recovered. In contrast, actin did not change its cortical location with compaction nor with decompaction-recompaction. Calmodulin localized in areas opposite to cell-cell contacts in eight-cell stage embryos before and after compaction. Inhibition of calmodulin with trifluoperazine induced its delocalization while morulae decompacted. A nonspecific rise of intracellular free Ca2+ provoked by ionomycin did not affect the compacted shape. Moreover, the same decompacting treatments when applied to uncompacted embryos did not produce any change in intracellular Ca2+. Our results demonstrate that in preimplantation mouse embryos experimentally induced stage-specific changes of cell shape are accompanied by changes of intracellular free Ca2+ and redistribution of the cytoskeleton-related proteins E-cadherin, fodrin, and calmodulin. We conclude that intracellular Ca2+ specifically is involved in compaction and probably regulates the function and localization of cytoskeleton elements.

Hydrogen peroxide-induced DNA damage is independent of nuclear calcium but dependent on redox-active ions

In cells undergoing oxidative stress, DNA damage may result from attack by .OH radicals produced by the Fenton reaction, and/or by nucleases activated by nuclear calcium. In the present study, the participation of these two mechanisms was investigated in HeLa cells. Nuclear-targeted aequorin was used for selectively monitoring Ca2+ concentrations within the nuclei ([Ca2+]n), in conjunction with the cell-permeant calcium chelator bis-(o-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM), the lipid-soluble broad-spectrum metal chelator with low affinity for Ca2+ and Mg2+ N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), and the high-affinity iron/copper chelator 1, 10-phenanthroline (PHE). In Ca2+-containing medium, H2O2 induced extensive DNA strand breaks and an increase in [Ca2+]n that was almost identical to that observed in the cytosol ([Ca2+]c). In cells bathed in Ca2+-free/EGTA medium, in which the increases in [Ca2+]n and [Ca2+]c due to H2O2 were significantly reduced, similar levels of DNA fragmentation also occurred. In cells preloaded with BAPTA/AM or TPEN, the small increase of [Ca2+]n normally elicited by H2O2 in Ca2+-free medium was completely buffered, and DNA damage was largely prevented. On the other hand, pretreatment with PHE did not affect the calcium response in the nuclei, but completely prevented DNA strand breakage induced by H2O2. Re-addition of 100 microM CuSO4 and 100 microM FeSO4 to TPEN- and PHE-treated cells prior to H2O2 challenge reversed the effect of TPEN and PHE, whereas 1 mM was necessary to negate the effect of BAPTA/AM. The levels of DNA strand breakage observed, however, did not correlate with the amounts of 8-hydroxy 2'-deoxyguanosine (8-OHdG): H2O2 did not produce 8-OHdG, whereas PHE alone slightly increased 8-OHdG levels. CuSO4 and FeSO4 enhanced the effects of PHE, particularly in the presence of H2O2. Exposure of cells to a mixture of CuSO4/FeSO4 also resulted in a significant increase in 8-OHdG levels, which was prevented in cells preloaded with BAPTA/AM. Similar results were obtained in a cell-free system using isolated calf thymus DNA exposed to CuSO4/FeSO4, regardless of whether H2O2 was present or not. These results suggest that BAPTA/AM prevents H2O2-induced DNA damage by acting as an iron/copper chelator. These data also indicate that caution must be exercised in using Ca2+ chelating agents as evidence for a role in cellular Ca2+ levels in experimental conditions in which transition-metal-ion-mediated oxidant production is also occurring.

Nuclear and cytosolic calcium changes in osteoclasts stimulated with ATP and integrin-binding peptide

Cytosolic calcium modulates the activity of osteoclasts, large multinucleate cells that resorb bone. Nuclear events, such as gene transcription, are also calcium-regulated in these cells, and fluorescence imaging has suggested that calcium signals produced by some stimuli are specifically targeted to, or amplified within, osteoclast nuclei. We used two alternative techniques of dye loading to examine the changes of intracellular calcium induced in rat osteoclasts by three stimuli. Osteoclasts loaded with the calcium indicator Fura-2 by the acetoxymethyl (AM) ester technique appeared to display marked nuclear calcium amplification. During stimulation with integrin-binding peptides, ATP, or high extracellular calcium, fluorescence ratios recorded from the nuclei rose higher than did ratios recorded from extranuclear regions. In contrast, nuclear calcium amplification was not observed after AM loading in the presence of the anion transport inhibitor sulfinpyrazone, nor in osteoclasts injected with Fura-2 conjugated to a high MW dextran. In these cells, nuclear fluorescence ratios were equal to the extranuclear values at all times: upon stimulation by an agonist, the nuclear and cytosolic calcium concentrations increased by the same amount. The calcium changes seen in stimulated osteoclasts can no longer be taken as evidence for the general validity of the phenomenon of nuclear calcium amplification.

Overview of voltage-dependent calcium channels

Voltage-dependent calcium channels couple electrical signals to cellular responses in excitable cells. Calcium channels are crucial for excitation-secretion coupling in neurons and endocrine cells, and excitation-contraction coupling in muscle. Several pharmacologically and kinetically distinct calcium channel types have been identified at the electrophysiological and molecular levels. This review summarizes the basic properties of voltage-dependent calcium channels, including mechanisms of ion permeation and block, gating kinetics, and modulation by G proteins and second messengers.

A role for local calcium gradients upon hypoxic injury in human umbilical vein endothelial cells (HUVEC)

Upon hypoxic injury, bleb formation is an early event of cell damage observed in a variety of cell types. Although a rise in cytosolic free Ca2+ ([Ca2+]i) has been considered to be involved in this process, the exact relationship between these phenomena remains ill-defined. In order to examine the relationship between bleb formation, and [Ca2+]i or nuclear free Ca2+ ([Ca2+]n), we analyzed [Ca2+]i and [Ca2+]n in HUVEC during hypoxic injury using confocal laser scanning microscopy. [Ca2+]i and [Ca2+]n were measured using Fluo-3, and cell viability and mitochondrial membrane potential were assessed by the exclusion of propidium iodide (PI) and rhodamine 123, respectively. After the initiation of hypoxia, [Ca2+]i and [Ca2+]n rose gradually up to 15 min reaching peak values of 447 +/- 62 and 516 +/- 105 nM, respectively, which was accompanied by a decrease in rhodamine 123 fluorescence and an increase in PI-stained cells. Bleb formation was observed after [Ca2+]i and [Ca2+]n had reached their peak values and the number of blebs increased thereafter. Confocal z-sectioning images revealed a localized increase in [Ca2+]i at the bleb forming site and this localized elevation in [Ca2+]i was observed before bleb formation in the corresponding area. In conclusion, bleb formation induced by hypoxic stress appears to involve Ca(2+)-dependent reactions that are linked to a regional elevation of [Ca2+]i.

Temperature dependencies of Ca2+ current, Ca(2+)-activated Cl- current and Ca2+ transients in sensory neurones

We recorded Ca2+ current (ICa) and Ca(2+)-activated Cl- current (ICl(Ca)) in isolated chick dorsal root ganglion neurons. At room temperature, ICl(Ca) is activated by Ca2+ influx (e.g. ICa) or by caffeine-stimulated release of Ca2+ via ryanodine receptors. Warming from room temperature to 37 degrees C increased the amplitude of ICa as well as the amplitude and rate of deactivation of ICl(Ca) activated by Ca2+ influx. In contrast, the activation of ICl(Ca) by caffeine-stimulated release of Ca2+ from intracellular stores abruptly failed between 19 and 28 degrees C. Warning from 22 to 37 degrees C reduced the amplitude of [Ca2+]i transients (measured with Indo-1) in chick neurons by more than 50% and reduced [Ca2+]i transients in mouse neurons by more than 40%. We investigated the role of mitochondria in these phenomena using carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) to inhibit mitochondrial Ca2+ uptake. 1-4 microM FCCP slowed the deactivation of ICa-activated ICl(Ca) at 20 degrees C and at 36 degrees C, having a greater effect at the higher temperature. In the presence of FCCP, the rate of deactivation of ICl(Ca) was relatively insensitive to temperature in this protocol. In contrast, FCCP had little effect on ICl(Ca) activated by caffeine at warmer temperatures (> 22 degrees C) but prolonged ICl(Ca) at cooler temperatures (< 22 degrees C). Thus, we find that warming reduces the ability of Ca2+ release to raise [Ca2+]i increases the effect of mitochondria on the deactivation of ICl(Ca) if ICl(Ca) is activated by Ca2+ influx, and reduces the effect of mitochondria if ICl(Ca) is activated by caffeine-stimulated Ca2+ release.

Specificity in the interaction of HVA Ca2+ channel types with Ca2+-dependent AHPs and firing behavior in neocortical pyramidal neurons

Intracellular recordings and organic and inorganic Ca2+ channel blockers were used in a neocortical brain slice preparation to test whether high-voltage-activated (HVA) Ca2+ channels are differentially coupled to Ca2+-dependent afterhyperpolarizations (AHPs) in sensorimotor neocortical pyramidal neurons. For the most part, spike repolarization was not Ca2+ dependent in these cells, although the final phase of repolarization (after the fast AHP) was sensitive to block of N-type current. Between 30 and 60% of the medium afterhyperpolarization (mAHP) and between approximately 80 and 90% of the slow AHP (sAHP) were Ca2+ dependent. Based on the effects of specific organic Ca2+ channel blockers (dihydropyridines, omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC), the sAHP is coupled to N-, P-, and Q-type currents. P-type currents were coupled to the mAHP. L-type current was not involved in the generation of either AHP but (with other HVA currents) contributes to the inward currents that regulate interspike intervals during repetitive firing. These data suggest different functional consequences for modulation of Ca2+ current subtypes.

Regulation of nuclear calcium uptake by inositol phosphates and external calcium

Factors affecting Ins(1,3,4,5)P4-mediated nuclear Ca2+ uptake are investigated, which include Ins(1,3,4,5)P4 receptor ligand specificity and free external Ca2+ concentrations. Among various inositol phosphates examined, Ins(1,3,4,5)P4, Ins(3,4,5,6)P4, and Ins(1,3,4,5,6)P5 can also stimulate 45Ca2+ influx into isolated rat liver nuclei by activating the Ins(1,3,4,5)P4 receptor-mediated Ca2+ uptake into the nucleus. The EC50 values of these polyphosphates range between 200 and 300 nM, which are 3-4 folds higher than that of Ins(1,3,4,5)P4. It is plausible that these polyphosphates in conjunction with Ins(1,3,4,5)P4 take part in the regulation of nuclear Ca2+ uptake in view of their intracellular levels during cell activation. Moreover, the inositol phosphate-induced Ca2+ uptake is facilitated by increasing Ca2+ levels in the uptake milieu, suggesting a possible link between cytosolic and nuclear Ca2+ signals through the Ins(1,3,4,5)P4 receptor.

Calcium regulation of nuclear pore permeability

The nuclear envelope is an integral part of the structural framework of the nucleus, and is involved in organizing intranuclear events. It serves as a selective barrier, actively transporting proteins required for normal nuclear function and exporting RNA. The movement of molecules across the nuclear envelope is critical for cellular homeostasis, and it allows cells to respond to external events. The only known pathway for direct communication between the cytoplasm and the nucleoplasm of a cell is through the nuclear pore complex. In the past decade, rapid advances have been made in elucidating the structure and function of the nuclear pore complex. Yet, researchers are just beginning to identify some of the regulatory mechanisms controlling transport through the pore complex. The nucleus is surrounded by a Ca2+ storage compartment, which sequesters and releases Ca2+ in response to intracellular second messengers, Recent evidence suggests that the nuclear Ca2+ store may indirectly regulate passive diffusion through the nuclear pore complex. The evidence for Ca2+ regulation of the nuclear pore complex will be discussed, along with the introduction of the simplest, testable model to describe the observations.

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.

5-HT3 receptors induce rises in cytosolic and nuclear calcium in NG108-15 cells via calcium-induced calcium release

A complete understanding of how excitatory ligand-gated ion channels regulate intracellular Ca2+ in nerve cells remains to be elucidated. Laser-scanning confocal microscopy was used here to measure Ca2+ changes in the neuroblastoma x glioma hybrid cell line NG108-15, employed as a model nerve cell line, upon activation by the 5-HT3 receptor, a serotonin-activated ligand-gated ion channel. Addition of the 5-HT3 agonist 1-m-(chlorophenyl)-biguanide (mCPBG) induced increases in [Ca2+]i in both the cytoplasm and the nuclei of the NG108-15 cells. Using high-time resolution line scanning, no delay was evident between the mCPBG-induced rise in cytosolic [Ca2+]i and the rise in nuclear [Ca2+]i. The agonist-induced responses were completely blocked by addition of EGTA to chelate external Ca2+ and by addition of the 5-HT3 receptor antagonist tropisetron or the L-type Ca2+ channel blocker nitrendipine. Caffeine, but not thapsigargin, treatment significantly reduced the mCPBG-induced responses in the nucleus and the cytoplasm, both to the same extent. We conclude that, upon 5-HT3 receptor activation, Ca2+ enters the cells through voltage-gated Ca2+ channels and then triggers the release of Ca2+ from ryanodine-sensitive intracellular stores, greatly amplifying the increases in Ca2+ in the cytoplasm and the nucleus.

All-or-none Ca2+ release from intracellular stores triggered by Ca2+ influx through voltage-gated Ca2+ channels in rat sensory neurons

Ca2+-induced Ca2+ release (CICR) from intracellular stores amplifies the Ca2+ signal that results from depolarization. In neurons, the amplification has been described as a graded process. Here we show that regenerative CICR develops as an all-or-none event in cultured rat dorsal root ganglion neurons in which ryanodine receptors have been sensitized to Ca2+ by caffeine. We used indo-1-based microfluorimetry in combination with whole-cell patch-clamp recording to characterize the relationship between Ca2+ influx and Ca2+ release. Regenerative release of Ca2+ was triggered when action potential-induced Ca2+ influx increased the intracellular Ca2+ concentration ([Ca2+]i) above threshold. The threshold was modulated by caffeine and intraluminal Ca2+. A relative refractory period followed CICR. The pharmacological profile of the response was consistent with Ca2+ influx through voltage-gated Ca2+ channels triggering release from ryanodine-sensitive stores. The activation of a suprathreshold response increased more than fivefold the amplitude and duration of the [Ca2+]i transient. The switch to a suprathreshold response was regulated very precisely in that addition of a single action potential to the stimulus train was sufficient for this transformation. Confocal imaging experiments showed that CICR facilitated propagation of the Ca2+ signal from the plasmalemma to the nucleus. This all-or-none reaction may serve as a switch that determines whether a given electrical signal will be transduced into a local or widespread increase in [Ca2+]i.

Interest of image processing in cell biology and immunology

Microscopy is a basic tool for cell biologists. Recent progress of electronics and computer science made powerful methodologies for digital processing of microscopic images easily available. These methods allowed impressive increase of the power of conventional microscopy. Dramatic image enhancement may be achieved by combination of filtering techniques, computer-based deblurring and contrast enhancement. Quantitative treatment of digitized images allows absolute determination of the density of different components of the observed sample, including antigens, intracellular calcium and pH. Morphometric studies are also greatly facilitated by image processing techniques. The capture of fast phenomena may be performed by transfer of small portion of microscopic images into computer memory as well as particular use of confocal microscopy. Finally, improved display of experimental data through coded colors or other procedures may enhance the amount of information that can be conveyed by visual examination of microscopical images. The purpose of the present review is to describe the basic principles of image processing and exemplify the power of this approach with a variety of illustrated applications to conventional, fluorescence or electron microscopy as well as confocal microscopy.

Mechanisms by which cell geometry controls repetitive impulse firing in retinal ganglion cells

Models for generating repetitive impulse activity were developed based on multicompartmental representations of ganglion cell morphology in the amphibian retina. Each model includes five nonlinear ion channels and one linear (leakage) channel. Compartmental distribution of ion channel type and density was designed to simulate whole cell recording experiments carried out in the intact retina-eyecup preparation. Correspondence between the model and physiology emphasized channel-specific details in the impulse waveform, based on phase plot analysis, frequency versus current (F/I) properties, and interspike trajectories for current injected into the soma, as well as the ability to conduct impulses in both orthodromic and antidromic directions. Two general types of model are developed, including equivalent cylinder representations and more realistic compartmentalizations of dendritic morphology. These multicompartmental models include representations for dendritic trees, soma, axon hillock, a thin axonal segment, and axon distal to thin segment. A large number of compartments (</=800) representing a single neuron were employed to ensure that maximum voltage differences between neighboring compartments during the steepest rates of change of membrane potential were acceptably small. Leakage conductance varied from 3 to 8 microS/cm2. The results establish that intercompartmental currents, due to inhomogeneous morphology, dominate membrane currents in the interspike intervals and thus play a major role in determining the impulse spacing and the information carried by impulse trains. Variations in input resistance are far less important than the degree to which ion channels are present in the dendritic compartments for the regulation of F/I properties. Cell geometry, including the thin axonal segment, places significant constraints on the location of ion channels required to support impulse initiation and propagation in both the ortho- and antidromic directions. The site of impulse initiation varies greatly and depends on the stimulus magnitude. Models that conform to physiological constraints also show irregular firing, particularly for near threshold stimulation of the soma, due to multiple sites of impulse initiation. Such behavior could represent an asset to the cells for conveying information under conditions of low contrast stimulation. Multiple spike initiation zones also can provide retinal ganglion cells with a variety of response characteristics, including spike doublets, depending on the level of cell activation. Increasing the diameter of the dendritic equivalent cylinder reduces the impulse frequency (F/I) response. Over a restricted range of ion channel densities in the dendritic tree, phase locking between dendritic membrane oscillations and somatic spiking can occur with dendritic stimulation, and mathematical chaos can be demonstrated when sufficiently thin dendritic processes are present. We conclude that cell morphology is the primary factor in determining firing patterns and the impulse frequency response of a given cell and that differences in channel density distribution across a population of cells plays, at most, a secondary role in this function. This conclusion applies to both synaptic activation and electrode stimulation of the soma.

Nuclear calcium and the regulation of the nuclear pore complex

In eukaryotic cells the nucleus and its contents are separated from the cytoplasm by the nuclear envelope. Macromolecules, as well as smaller molecules and ions, can cross the nuclear envelope through the nuclear pore complex. Molecules greater than approx. 60 kDa and containing a nuclear localization signal are actively transported across the nuclear membranes, but there has been little evidence for regulatory mechanisms for smaller molecules and ions. Recently, diffusion across the nuclear envelope has been observed to be regulated by nuclear cisternal Ca2+ concentrations. Following depletion of Ca2+ from the nuclear store by inositol 1,4,5-trisphosphate or Ca2+ chelators, a fluorescent 10 kDa marker molecule was no longer able to enter the nucleus. Distinct conformational states of the nuclear pore complexes depended on the Ca2+ filling state of the nuclear envelope, supporting the assumption that a switch in the conformation of the nuclear pore complex may control the transport of intermediate-sized molecules across the nuclear envelope. Thus nuclear Ca2+ stores may regulate the conformational state of the nuclear pore complex, and thereby passive diffusion of molecules between the cytosol and the nucleoplasm. The physiological significance of this finding is currently unknown.

Does the nuclear envelope contain two types of ligand-gated Ca2+ release channels?

The nuclear envelope is composed of two membranes deliminating a perinuclear space which displays functional properties similar to those of a Ca2+-storing compartment. ATP-driven Ca2+ uptake and InsP3-induced Ca2+ release processes have been described in isolated nuclei. Recently, it was reported that cADP-ribose and InsP3 can trigger a nucleoplasmic Ca2+ increase. It was hypothesized that the inner nuclear membrane possesses Ca2+ channels that are regulated by ryanodine or InsP3. Radio-ligand binding assays and Western blot experiments were performed in order to investigate their presence in sheep cardiac and rat liver nuclear envelopes. Ryanodine receptors (RyR) were not detected in liver nuclear envelopes by either binding assay or Western blot analysis. However, cardiac nuclear envelopes were found to retain a very low level of specific ryanodine binding, which was not detected on immuno-blots obtained with three types of isoform-specific RyR antibodies. In contrast, nuclear InsP3-binding sites were consistently detected in both cardiac and liver nuclear envelopes. Altogether, these results provide evidence for the major contributor InsP3-gated Ca2+ channels in control of Ca2+ release from the perinuclear space in liver and cardiac cells.

Parallel changes in nuclear and cytosolic calcium in mouse pancreatic beta-cells

In the neuroendocrine pancreatic beta-cell, elevations in intracellular Ca2+ lead to insulin secretion and the initiation of gene transcription. However, the relationship between cytosolic and nuclear Ca2+ in these cells is unknown. The Ca2+ permeability of the nuclear membrane would therefore determine if Ca2+ could play a direct role in Ca2+-dependent nuclear processes. Using confocal fluorescence microscopy with the ratiometric Ca2+ indicator indo-1 and carefully correcting for compartmentalized indicator, we now demonstrate that there is no difference between the nuclear Ca2+ concentration and the cytosolic Ca2+ concentration ([Ca2+]c) in the resting beta-cell. Slow Ca2+ oscillations induced by glucose, fast oscillations induced by glucagon-like peptide-1 and responses to potassium and carbachol all indicate that changes in cytosolic Ca2+ are reflected within the nucleus. We conclude that there are no restrictions on Ca2+ entry into the nucleus of the pancreatic beta-cell subsequent to increases in [Ca2+]c. This implies that any signal involved in increasing [Ca2+]c, and thereby insulin release, may also promote nuclear Ca2+-induced gene transcription.

Activity-dependent [Ca2+]i changes in guinea pig vagal motoneurons: relationship to the slow afterhyperpolarization

Vagal motoneurons in slices from the guinea-pig brain stem were injected with the fluorescent [Ca2+]i indicators fura-2, furaptra, or Calcium Green-1. Spike-induced fluorescence changes were measured in the soma and dendrites and simultaneously the long-lasting afterhyperpolarization was recorded with a sharp microelectrode in the soma. Na+ spikes or Ca2+ spikes increased [Ca2+]i (measured as a change in indicator fluorescence) in all locations in the soma and dendrites. Each spike in a train of action potentials caused a step increase in fluorescence of about equal amplitude when nonsaturating indicators were used. Peak changes at all locations occurred at the time of the last action potential. Transients measured with low concentrations of Calcium Green-1 or furaptra had a recovery time constant of approximately 500-1,500 ms in the cell body. The recovery time course was faster in the dendrites than in the soma. The norepinephrine-sensitive, slow afterhyperpolarization (sAHP) had a time to peak of approximately 800 ms and a recovery time constant of 2-5 s, much longer than the recovery time course of the fluorescence changes. Some of these experiments were repeated on pyramidal neurons from the CA1 region of the rat hippocampus with similar results. In both cell types, the data suggest that the time course of neither the rising phase nor the falling phase of the sAHP, nor the underlying conductance, directly reflects the time course of the [Ca2+]i change. The mechanism connecting the parameters remains unclear. One possibility is that an additional second messenger system is involved.

Visualization of calcium influx through channels that shape the burst and tonic firing modes of thalamic relay cells

Thalamic neurons have two firing modes: "tonic" and "burst." During burst mode, both low-threshold (LT) and high-threshold (HT) calcium channels are activated, while in tonic mode, only the HT-type of calcium channel is activated. The calcium signals associated with each firing mode were investigated in rat thalamic slices using whole cell patch clamping and confocal calcium imaging. Action potentials were induced by direct current injection into thalamic relay cells loaded with a fluorescent calcium indicator. In both tonic and burst firing modes, large calcium signals were recorded throughout the soma and proximal dendrites. To map the distribution of the channels mediating these calcium fluxes, LT and HT currents were independently activated using specific voltage-clamp protocols. We focused on the proximal region of the cell (up to 50 microm from the soma) because it appeared to be well clamped. For a voltage pulse of a given size, the largest calcium signals were observed in the proximal dendrites with smaller signals occurring in the soma and nucleus. This was true for both LT and HT signals. Rapid imaging, using one-dimensional linescans, was used to more precisely localize the calcium influx. For both LT and HT channels, calcium influx occurred simultaneously throughout all imaged regions including the soma and proximal dendrites. The presence of sizable calcium signals in the dendrites, soma, and nucleus during both firing modes, and the presence of LT calcium channels in the proximal dendrite where sensory afferents synapse, have implications for both the electrical functioning of relay cells and the transmission of sensory information to cortex.

Nucleoplasmic and cytoplasmic differences in the fluorescence properties of the calcium indicator Fluo-3

The fluorescent indicator Fluo-3 is widely used to monitor the calcium concentration ([Ca2+]) in the cytoplasm and nucleus of various cells. Estimates of nuclear [Ca2+] are based on the assumption of identical behavior of Fluo-3 in different cellular compartments. The assumption is not valid if the fluorescence properties of the dye are altered by the nuclear environment, independent of the [Ca2+]. To determine the effects of the nucleoplasm on the behavior of Fluo-3, we applied laser scanning confocal microscopy and spectrophotometry to measure fluorescence intensity as well as emission and absorbance spectra of the Ca2+ indicator, Fluo-3. Spectra were measured in intact Xenopus oocytes, neuroblastoma cells, and cytoplasmic and nucleoplasmic homogenates. The fluorescence signal in intact cells loaded with Fluo-3 was approximately 2-times higher in the nucleus when compared to the cytoplasm. The fluorescence intensity of Fluo-3 in nucleoplasmic homogenates was higher than in cytoplasmic homogenates or internal buffers even when [Ca2+] was clamped. Despite identical [Ca2+], pH, and temperature, the emission and absorbance spectra of Fluo-3 from nuclear homogenates displayed a higher fluorescence at each wavelength measured when compared to spectra from cytoplasmic homogenates or internal buffer solutions, and saturated above 100 nM. These findings demonstrate that the composition of the nucleoplasm changes the fluorescence properties of the calcium indicator Fluo-3. Consequently, analysis of nuclear calcium dynamics must take into account the distinct behavior of Fluo-3 in different cellular compartments.

Ca(2+)-induced Ca2+ release phenomena in mammalian sympathetic neurons are critically dependent on the rate of rise of trigger Ca2+

The role of ryanodine-sensitive intracellular Ca2+ stores present in nonmuscular cells is not yet completely understood. Here we examine the physiological parameters determining the dynamics of caffeine-induced Ca2+ release in individual fura 2-loaded sympathetic neurons. Two ryanodine-sensitive release components were distinguished: an early, transient release (TR) and a delayed, persistent release (PR). The TR components shows refractoriness, depends on the filling status of the store, and requires caffeine concentrations > or = 10 mM. Furthermore, it is selectively suppressed by tetracaine and intracellular BAPTA, which interfere with Ca(2+)-mediated feedback loops, suggesting that it constitutes a Ca(2+)-induced Ca(2+)-release phenomenon. The dynamics of release is markedly affected when Sr2+ substitutes for Ca2+, indicating that Sr2+ release may operate with lower feedback gain than Ca2+ release. Our data indicate that when the initial release occurs at an adequately fast rate, Ca2+ triggers further release, producing a regenerative response, which is interrupted by depletion of releasable Ca2+ and Ca(2+)-dependent inactivation. A compartmentalized linear diffusion model can reproduce caffeine responses: When the Ca2+ reservoir is full, the rapid initial Ca2+ rise determines a faster occupation of the ryanodine receptor Ca2+ activation site giving rise to a regenerative release. With the store only partially loaded, the slower initial Ca2+ rise allows the inactivating site of the release channel to become occupied nearly as quickly as the activating site, thereby suppressing the initial fast release. The PR component is less dependent on the store's Ca2+ content. This study suggests that transmembrane Ca2+ influx in rat sympathetic neurons does not evoke widespread amplification by CICR because of its inability to raise [Ca2+] near the Ca2+ release channels sufficiently fast to overcome their Ca(2+)-dependent inactivation. Conversely, caffeine-induced Ca2+ release can undergo considerable amplification especially when Ca2+ stores are full. We propose that the primary function of ryanodine-sensitive stores in neurons and perhaps in other nonmuscular cells, is to emphasize subcellular Ca2+ gradients resulting from agonist-induced intracellular release. The amplification gain is dependent both on the agonist concentration and on the filling status of intracellular Ca2+ stores.

Calcium in close quarters: microdomain feedback in excitation-contraction coupling and other cell biological phenomena

Researchers have made good progress in unraveling the molecular mechanisms of excitation-contraction (EC) coupling in striated muscle. Despite this progress, paradoxes abound. In skeletal muscle, the existence of a mechanical coupling between membrane charge movement and activation of sarcoplasmic reticulum (SR) release channels is essentially established, but the contribution of Ca(2+)-induced Ca2+ release (CICR) to the transient and steady-state components of Ca2+ release remains controversial. In cardiac muscle, the role of CICR as the primary mechanism of EC coupling is well established, but the stability and tight coupling between membrane Ca2+ current and release are paradoxical. Answers may lie in microdomain issues, and the examination of discrete elementary release events, although quantitative treatments are needed. This review explores the theoretical and experimental methods used and the observations made in the study of microdomain Ca2+.

Nicotine and sympathetic neurotransmission

Nicotine increases heart rate, myocardial contractility, and blood pressure. These nicotine-induced cardiovascular effects are mainly due to stimulation of sympathetic neurotransmission, as nicotine stimulates catecholamine release by an activation of nicotine acetylcholine receptors localized on peripheral postganglionic sympathetic nerve endings and the adrenal medulla. The nicotinic acetylcholine receptor is a ligand-gated cation channel with a pentameric structure and a central pore with a cation gate, which is essential for ion selectivity and permeability. Binding of nicotine to its extracellular binding site leads to a conformational change of the central pore, which results in the influx of sodium and calcium ions. The resulting depolarization of the sympathetic nerve ending stimulates calcium influx through voltage-dependent N-type calcium channels, which triggers the nicotine-evoked exocytotic catecholamine release. In the isolated perfused guinea-pig heart, cardiac energy depletion sensitizes cardiac sympathetic nerves to the norepinephrine-releasing effect of nicotine, as indicated by a leftward shift of the concentration-response curve, a potentiation of maximum transmitter release, and a delay of the tachyphylaxis of nicotine-evoked catecholamine release. This sensitization was also shown to occur in the human heart under in vitro conditions. Through the intracardiac release of norepinephrine, nicotine induces a beta-adrenoceptor-mediated increase in heart rate and contractility, and an alpha-adrenoceptor-mediated increase in coronary vasomotor tone. The resulting simultaneous increase in oxygen demand and coronary resistance has a detrimental effect on the oxygen balance of the heart, especially in patients with coronary artery disease. Sensitization of the ischemic heart to the norepinephrine-releasing effect of nicotine may be a trigger for acute cardiovascular events in humans, such as acute myocardial infarction and/or life-threatening ventricular tachyarrhythmias.