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

Confocal imaging of dendritic Ca2+ transients in hippocampal brain slices during simultaneous current- and voltage-clamp recording

Changes in the intracellular Ca2+ concentration ([Ca2+]i) within CA1 hippocampal pyramidal neurons were imaged using confocal laser scanning microscopy in conjunction with Ca(2+)-sensitive fluorescent indicators. The imaging was performed in thick hippocampal brain slices while simultaneously measuring or controlling electrical activity with sharp microelectrodes or whole-cell patch-clamp electrodes. The combination of imaging and electrophysiology was essential for interpreting the changes in [Ca2+]i. We compared the increases in [Ca2+]i produced by either of two methods--direct depolarization of the cell via the somatic electrode or high-frequency stimulations of synaptic inputs. The increases in [Ca2+]i in the soma and proximal dendrites caused by both methods were of comparable magnitude and they always decayed within seconds in healthy cells. However, the spatial patterns of distal Ca2+ increases were different. Separate sets of synaptic inputs to the same cell resulted in different spatial patterns of [Ca2+]i transients. We isolated and observed what appeared to be a voltage-independent component of the synaptically mediated [Ca2+]i transients. This work demonstrates that the combination of neurophysiology and simultaneous confocal microscopy is well suited for visualizing and analyzing [Ca2+]i changes within highly localized regions of neurons in thick brain slices. The approach should allow further analysis of the relative contribution of voltage- and agonist-dependent influences on [Ca2+]i within neurons throughout the CNS and it raises the possibility of routinely relating subcellular [Ca2+]i changes to structural and functional modifications.

Differential role of two Ca(2+)-permeable non-NMDA glutamate channels in rat retinal ganglion cells: kainate-induced cytoplasmic and nuclear Ca2+ signals

1. The permeability of non-N-methyl-D-aspartate (non-NMDA) glutamate channels to divalent cations and specifically the entry of Ca2+ and subsequent elevations in cytoplasmic and nuclear Ca2+ signals were investigated in cultured neonatal rat retinal ganglion cells using the whole cell patch-clamp technique and Ca2+ imaging with confocal microscopy. In addition, divalent-permeable non-NMDA receptor channels were studied in retinal slices using a Co2+ staining technique. 2. Using Ca2+ (2.5 mM) as the only permeable cation in the external solution, stimulation with 100 microM kainate produced nondesensitizing, nonselective cation currents with either low or high Ca2+ permeability. Both currents were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Neurons with the low divalent-permeable currents (type 1) had reversal potentials of -41.5 +/- 4.4 mV (mean +/- SD), and neurons with the high divalent-permeable currents (type 2) had reversal potentials of -22.6 +/- 5.5 mV. The permeability ratio PCa/PCs was 3.3 for the type 1 currents and 8.5 for the type 2 currents, indicating a 2.5-fold greater permeability to Ca2+ for the type 2 non-NMDA glutamate channels. 3. Both types of non-NMDA glutamate channels showed relatively little selectivity between Ca2+ and Co2+. The type 1 neurons had a slightly higher permeability to Co2+ than to Ca2+, whereas the type 2 neurons were equally permeable to both divalent cations. The type 2 neurons had a much higher permeability for both divalent cations compared with the type 1 neurons. 4. Staining for Co2+ uptake through kainate-stimulated non-NMDA glutamate channels in retinal slices provided additional evidence for the presence of the two ganglion cell populations. Activation of the neurons by kainate in conditions isolating the non-NMDA glutamate channel caused differential uptake of Co2+. In contrast, depolarization in the presence of the non-NMDA antagonist CNQX failed to cause Co2+ influx. 5. Imaging experiments using confocal microscopy showed that kainate stimulation induced an increase in intracellular Ca2+ in both types of retinal ganglion cells, but only the type 2 neurons showed a substantial increase in cytoplasmic and nuclear Ca2+ signals. Kainate-induced Ca2+ signals in the type 2 neurons were almost nine times greater than those of the type 1 neurons. 6. When intracellular Ca2+ stores were depleted by brief treatment with thapsigargin, kainate-induced Ca2+ signals in the type 1 neurons were unchanged. However, in the type 2 neurons kainate no longer induced large Ca2+ signals in the cytoplasm and nucleus, despite normal influx of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Nuclear calcium transport and the role of calcium in apoptosis

The last decade has been the rapid development of research investigating the molecular mechanisms whereby hormones, peptide growth factors and cytokines regulate cell metabolism, differentiation and proliferation. One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid Ca2+ redistribution throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Ca2+ signals are required for a number of cellular functions, including the activation of nuclear processes such as gene transcription and cell cycle events. The latter requires that appropriate Ca2+ signals elicited in response to agonists be transduced across the nuclear envelope. It has generally been assumed that small molecules, metabolites and ions could diffuse freely across the nuclear envelope. Nevertheless, several findings during the past few years have suggested that nuclear pore permeability can be regulated and that ion transport systems and ion-selective channels may exist in the nuclear membranes and regulate intranuclear processes. Intranuclear Ca2+ fluctuations can affect chromatin organization, induce gene expression and also activate cleavage of nuclear DNA by nucleases during programmed cell death or apoptosis. The possible mechanisms involved in nuclear Ca2+ transport and the regulation of nuclear Ca(2+)-dependent enzymes in apoptosis are discussed in the following sections.

Propagation of cytosolic calcium waves into the nuclei of hepatocytes

The temporal and spatial organization of [Ca2+] changes within the nucleus of Fura-2 loaded hepatocytes maintained in primary culture has been investigated. Vasopressin stimulation induced oscillatory waves of cytosolic free [Ca2+] increase, which propagated freely through the nuclear region. Based on the amplitude of the Fura-2 signals from this region, the morphology of the hepatocyte nucleus and the rapid penetration of the nucleus by injected Fura-2, it can be concluded that the nuclear Ca2+ responses reflect changes occurring within the nucleoplasm. Intranuclear Ca2+ increases occurred as waves that appear to be directed by the Ca2+ waves passing through the surrounding cytoplasm. The apparent velocity of Ca2+ waves was higher in the nucleoplasm than in the cytoplasm (19.5 +/- 2.9 versus 11.0 +/- 1.1 microns/s). The nucleoplasm does not contain vesicular Ca2+ stores that might be released by Ins(1,4,5)P3. However, the nuclear envelope functions as a Ca2+ store that is sensitive to mobilization by Ins(1,4,5)P3. We conclude that the [Ca2+] in the nucleoplasm of the hepatocyte is close to equilibrium with the cytosolic [Ca2+] and that oscillatory waves of cytosolic [Ca2+] are closely paralleled by similar [Ca2+] changes in the nucleoplasm. The nuclear envelope is a component of the intracellular Ins(1,4,5)P3-sensitive Ca2+ storage pool and may serve as a reservoir for [Ca2+] elevations within the nucleus.

Calcium signals in and around the nucleus in sea urchin eggs

A transient rise in cytoplasmic Ca2+ activity in the sea urchin egg occurs during fertilization due to calcium release from an intracellular store. Using a combination of the Ca2+ sensitive dye Calcium Green dextran and the Ca2+ insensitive dye tetramethylrhodamine dextran we have obtained confocal ratio images of free cytoplasmic calcium distribution during the fertilization calcium wave. We can also trigger calcium release using calcium-releasing agonists such as InsP3, ryanodine and cADP-ribose. Calcium levels are in all cases similar within nucleus and in the cytoplasm. A striking result from confocal calcium imaging is that the fertilization calcium wave is not the only spatio-temporal calcium signal observed after fertilization. In fact, a second calcium wave propagates through the egg as pronuclear migration begins; this wave also originates at the point of sperm entry. A global calcium increase is also recorded during the fusion of the male and female pronuclei. We conclude that calcium concentrations in the nucleus are similar to those in the cytoplasm during these calcium transients, that a remnant at the point of sperm entry can originate a second propagating calcium wave and that a global calcium transient occurs at the time of pronuclear fusion.

A UV laser-scanning confocal microscope for the measurement of intracellular Ca2+

Modifications to the optics of a conventional confocal laser-scanning microscope were made to allow imaging intracellular Ca(2+)-dependent fluorescence with a UV laser (351 or 364 nm). Modifications included: (1) a chromatic compensation lens in the laser path; (2) the design of a practically achromatic relay lens; (3) a longer tube length for the objective; and (4) highly reflective mirrors maximizing fluorescence measurement. This UV laser-scanning confocal microscope (UV-CLSM) yielded a lateral resolution of < 0.3 micron and an axial resolution of < 1.5 microns and a relevant field size of 100 microns in diameter for a 40X objective). The effects of varying the focal length of a compensation lens, the degree of the correction for the coverglass thickness of objective and the detector aperture size on the quality of image formation were examined. Finally, UV-CLSM revealed optical sections of fine and complex structures of bullfrog sympathetic neurones loaded with a Ca(2+)-sensitive fluorescent probe. Changes in intracellular free Ca2+ distribution in response to high [K+] or caffeine were demonstrated. In addition, an increase in the intracellular concentration of caffeine applied externally was clearly imaged in space and time and distinguished from a resultant rise in [Ca2+]i. Thus, the UV-CLSM developed is suitable for ratiometric intracellular Ca2+ measurements and other biological studies.

Regulation of nitric oxide synthase activity in cortical slices by excitatory amino acids and calcium

Nitric oxide synthase (NOS) activity was determined in adult rat frontal cortex and hippocampus by measuring the conversion of L-[3H]arginine to L-[3H]citrulline. N-methyl-D-aspartate (NMDA), but not kainate or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), stimulated NOS activity. This effect was concentration dependent (EC50 approximately 30 microM) and was inhibited by tetrodotoxin, EGTA, N omega-nitro-L-arginine (NOARG), Mg2+, phencyclidine, and (cis)-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). NOS activity was increased to an even greater extent by the calcium ionophores ionomycin and A23187 and by depolarization with 50 mM K+. Interestingly, neither caffeine nor 1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), drugs that would be expected to increase intracellular Ca2+ concentration by release of Ca2+ from intracellular ryanodine- and inositol-1,4,5-trisphosphate-sensitive stores, respectively, had any significant effect on NOS activity. It is concluded that NOS can be activated by NMDA binding to a classic NMDA glutamate receptor subtype as well as by depolarization or other agents that increase the influx of extracellular Ca2+. The paradoxical lack of effect of caffeine, as well as the inhibitory effect of tetrodotoxin, are discussed.

Local communication within dendritic spines: models of second messenger diffusion in granule cell spines of the mammalian olfactory bulb

Dendritic spines are generally believed to play a role in modulating synaptically induced electrical events. In addition, they may also confine second messengers and thus topologically limit the distance over which second messenger cascades may be functionally significant. In order to address this possibility, computer simulations of transient second messenger concentration changes were performed. The results show the importance of spine morphology and binding and extrusion mechanisms in controlling second messenger transients. In the presence of intrinsic cytoplasmic binding sites and kinetic rates similar to that expected for calcium, second messengers were confined to the spine head. In the absence of binding/extrusion mechanisms, the size and time course of the input transient to the spine head influenced the second messenger transients that might be seen at the base of the spine neck and in other spines. Large and/or sustained increases in second messenger concentration in the spine head were communicated to the spine base and to other spine heads. The results emphasize the importance of a knowledge of breakdown pathways, concentrations and kinetics of binding sites, and extrusion mechanisms for understanding the dynamics of local chemical changes for dendritic spine function.

Pulsed laser imaging of rapid Ca2+ gradients in excitable cells

Excitable cells are thought to respond to action potentials by forming short lived and highly localized Ca2+ gradients near sites of Ca2+ entry or near the site of Ca2+ release by intracellular stores. However, conventional imaging techniques lack the spatial and temporal resolution to capture these gradients. Here we demonstrate the use of pulsed-laser microscopy to measure Ca2+ gradients with submicron spatial resolution and millisecond time resolution in two preparations where the Ca2+ signal is thought to be fast and highly localized: adrenal chromaffin cells, where the entry of Ca2+ through voltage dependent Ca2+ channels triggers exocytotic fusion; and skeletal muscle fibers, where intracellular Ca2+ release from the sarcoplasmic reticulum initiates contraction. In chromaffin cells, Ca2+ gradients developed over 10-100 ms and were initially restricted to discrete submembrane domains, or hot spots, before developing into complete rings of elevated Ca2+ concentration. In frog skeletal muscle large, short-lived (approximately 6 ms) Ca2+ gradients were observed within individual sarcomeres following induction of action potentials. The pulsed laser imaging approach permits, for the first time, the capture and critical examination of rapid Ca2+ signaling events such as those underlying excitation-secretion and excitation-contraction coupling.

Nuclear pore complex ion channels (review)

It is currently thought that nuclear pore complexes (NPCs) primarily govern nucleocytoplasmic interactions via selective recognition and active transport of macromolecules. However, in various nuclear preparations, patch-clamp and fluorescence, luminiscence and ion microscopy support classical microelectrode measurements indicating that monoatomic ion flow across the nuclear envelope (NE) is strictly regulated. Gating of large conductance nuclear envelope ion channels (NICs) somewhat resembles that of gap junctional channels. In other respects, NICs are distinct in that they require cytosolic factors, are blocked by wheat germ agglutinin and are blocked and/or modified by antibodies to epitopes of NPC glycoproteins. Therefore, NIC activity, recorded as electrical current/conductance is likely to be intrinsic to NPCs. This observation suggests a potential use for the patch-clamp technique in establishing the mechanisms underlying nuclear pore gating in response to cytosolic and nucleosolic factors such as transcription and growth factors, oncogene and proto-oncogene products and receptors for retinoids, steroids and thyroid hormone. NIC activity may also be useful in evaluating the mechanisms of nuclear import of foreign nucleic acid material such as that contained in virons and viroids. Finally, in consideration to the electrophysiological data accumulated so far, the study of nuclear pore ion channel activity may help our understanding of other important issues such as cell suicide, programmed cell death or apoptosis.

Effects of rapid buffers on Ca2+ diffusion and Ca2+ oscillations

Based on realistic mechanisms of Ca2+ buffering that include both stationary and mobile buffers, we derive and investigate models of Ca2+ diffusion in the presence of rapid buffers. We obtain a single transport equation for Ca2+ that contains the effects caused by both stationary and mobile buffers. For stationary buffers alone, we obtain an expression for the effective diffusion constant of Ca2+ that depends on local Ca2+ concentrations. Mobile buffers, such as fura-2, BAPTA, or small endogenous proteins, give rise to a transport equation that is no longer strictly diffusive. Calculations are presented to show that these effects can modify greatly the manner and rate at which Ca2+ diffuses in cells, and we compare these results with recent measurements by Allbritton et al. (1992). As a prelude to work on Ca2+ waves, we use a simplified version of our model of the activation and inhibition of the IP3 receptor Ca2+ channel in the ER membrane to illustrate the way in which Ca2+ buffering can affect both the amplitude and existence of Ca2+ oscillations.

Calcium homeostasis in identified rat gonadotrophs

1. Whole-cell voltage clamp was used in conjunction with the fluorescent Ca2+ indicator indo-1 to measure extracellular Ca2+ entry and intracellular Ca2+ concentrations ([Ca2+]i) in rat gonadotrophs identified with the reverse haemolytic plaque assay. 2. Depolarizations to potentials more positive than -40 mV elicited inward Ca2+ current (ICa) and transient elevations of [Ca2+]i. 3. The relationship between [Ca2+]i elevations and Ca2+ entry with different Ca2+ buffer concentrations in the pipette showed that endogenous Ca2+ buffers normally bind approximately 99% of the Ca2+ entering the cell. 4. With [Ca2+]i elevations less than 500 nM, decay of [Ca2+]i could be approximated by an exponential whose time constant increased with the concentration of exogenous Ca2+ buffers. 5. Inhibitors of intracellular Ca(2+)-ATPases, thapsigargin, cyclopiazonic acid (CPA) and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (BHQ), caused [Ca2+]i to rise. Application of BHQ during [Ca2+]i oscillations induced by gonadotrophin-releasing hormone (GnRH) terminated the oscillation in a slowly decaying elevation. BHQ slowed the decay of depolarization-induced [Ca2+]i elevations about 3-fold. 6. Taking into account the Ca2+ buffering properties of the cytoplasm permitted estimation of the fluxes and rate constants for Ca2+ movements in gonadotrophs. The intracellular store is a major determinant of Ca2+ homeostasis in gonadotrophs.

Calcium and calcium-binding proteins in the nucleus

Calcium has long been known to play a role as a key cytoplasmic second messenger, but until relatively recently its possible involvement in nuclear signal transduction and the regulation of nuclear events has not been extensively studied. Evidence revealing the presence of transmembrane nuclear Ca2+ gradients and a variety of intranuclear Ca2+ binding proteins has fueled renewed interest in this key ion and its involvement in cell-cycle timing and division, gene expression, and protein activation. This review will offer an overview of the current state of knowledge and theory regarding calcium orchestration of nuclear functions and events and discuss possible future directions in this field of study.

Nuclear Ca2+: physiological regulation and role in apoptosis

The last decade has seen the rapid development of research investigating the molecular mechanisms whereby hormones, peptide growth factors and cytokines regulate cell metabolism, differentiation and proliferation. One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid Ca2+ redistribution throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Ca2+ signals are required for a number of cellular processes including the activation of nuclear processes such as gene transcription and cell cycle events. The latter require that appropriate Ca2+ signals elicited in response to agonists be transduced across the nuclear envelope. It has generally been assumed that small molecules, metabolites and ions could freely diffuse across the nuclear envelope. Nevertheless several findings during the past few years have suggested that nuclear pore permeability can be regulated and that ion transport systems and ion-selective channels may exist on the nuclear membranes and regulate intranuclear processes. Intranuclear Ca2+ fluctuations can affect chromatin organization, induce gene expression and also activate cleavage of nuclear DNA by nucleases during programmed cell death or apoptosis. The possible mechanisms involved in nuclear Ca2+ transport and the control of nuclear Ca(2+)-dependent enzymes in apoptosis is discussed below.

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, induced whole-cell currents through Ca2+ channels (ICa) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 microM) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 microM) reduced the peak ICa by 42% and the late ICa by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+]i) from 0.1 to 2.21 microM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca2+]o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca2+ channels in cat chromaffin cells. It seems, however, that though extracellular Ca2+ entry through both channel types leads to similar increments of averaged [Ca2+]i, the control of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segregation of L-type Ca2+ channels to localized "hot spots" in the plasmalemma of chromaffin cells where exocytosis occurs.

To boldly glow ... applications of laser scanning confocal microscopy in developmental biology

The laser scanning confocal microscope (LSCM) is now established as an invaluable tool in developmental biology for improved light microscope imaging of fluorescently labelled eggs, embryos and developing tissues. The universal application of the LSCM in biomedical research has stimulated improvements to the microscopes themselves and the synthesis of novel probes for imaging biological structures and physiological processes. Moreover the ability of the LSCM to produce an optical series in perfect register has made computer 3-D reconstruction and analysis of light microscope images a practical option.

Confocal microscopy to analyze cytosolic and nuclear calcium in cultured vascular cells

With the development of calcium-sensitive fluorescent dyes and videomicroscopic imaging, several investigators have located the changes in intracellular calcium in the cytoplasm, in the perinuclear region, and possibly in the nucleus. However, the presence of calcium in the nucleus is often difficult to ascertain because the fluorescence derived from the perinuclear area interferes with that of the nucleus. We have used confocal microscopy together with two calcium-sensitive dyes [acetoxymethyl esters of fluo 3 (fluo 3-AM) and rhod 2 (rhod 2-AM)] to analyze the cytosolic and nuclear calcium distribution in vascular smooth muscle and endothelial cells studied at rest and after stimulation with receptor-dependent (angiotensin, vasopressin) and receptor-independent (KCl) stimuli. With fluo 3-AM, the baseline fluorescence was located in the cytoplasm but was slightly higher in the nucleus. With all stimuli, the fluorescence intensity increased in both compartments but remained more pronounced within the nucleus. Yet, after calibration, the cytosolic calcium concentration was greater than that of the nucleus at rest and was equally high after stimulation, suggesting different properties of fluo 3 in the cytosol and in the nucleus. With rhod 2-AM, baseline fluorescence was low in the nucleus and high in the cytosol. Cell stimulation caused an initial increase in cytosolic calcium with no change in the nucleus followed by a rise in both compartments. Thus the stimulation of vascular cells is associated with marked increases in cytosolic and nuclear calcium. Fluo 3-AM seems to be a better indicator of nuclear calcium than rhod 2-AM. The increases in nuclear calcium induced by angiotensin II and vasopressin may contribute to their cell proliferative effect.

Calcium-induced calcium release in cerebellar Purkinje cells

Depolarization-induced intracellular Ca2+ rises were measured in fura-2-loaded, voltage-clamped Purkinje cells. The peak Ca2+ rise increased more than linearly with voltage step duration, suggesting the presence of Ca(2+)-induced Ca2+ release. In cells from young animals, in which Ca2+ currents could be satisfactorily recorded, a supralinear relation was also found between peak Ca2+ rise and Ca2+ current integral. Responses to long pulses were inhibited in cells dialyzed with 20 microM ruthenium red and potentiated in cells bathed in the presence of 20 microM ryanodine. Upon repetitive depolarization, increasing Ca2+ rises were elicited by successive voltage pulses, probably because of a potentiating effect of residual Ca2+. Altogether, the results indicate an important contribution of Ca(2+)-induced Ca2+ release to Ca2+ signals of Purkinje cells.

Calcium concentration changes in the calyciform nerve terminal of the avian ciliary ganglion after tetanic stimulation

A study has been made of the changes in calcium concentration in the calyciform nerve terminal ([Ca]c) and in the neurone soma ([Ca]s) of avian ciliary ganglion cells following tetanic stimulation of the nerve terminal. Dissociated ciliary neurones were loaded with the calcium indicator Fura-2 and digital imaging techniques used to determine the spatial and temporal distribution of calcium in the cells during post-tetanic potentiation (PTP) and long-term potentiation (LTP). Stimulation of the calyciform terminal with an extracellular electrode at 10 Hz for 2 s increased both [Ca]s and [Ca]s over 3-fold, with the [Ca] increasing for each impulse in the facilitatory train. The increase in [Ca]s could be prevented by allowing the terminal to degenerate in culture before stimulation. Stimulation of the calyciform terminal with a long tetanus of 30 Hz for 20 s gave an over 4-fold increase in both [Ca]c and [Ca]s by the end of the train. Analysis of the decline in [Ca]c after the train showed that it disappeared from the calyx along a double exponential time course with time constants of about 1 min and 50 min, respectively. These times are similar to those of PTP and LTP in the ganglia, and are almost independent of the extracellular calcium level. In order to determine whether the influx of calcium ions during a tetanus was through N-type calcium channels, these were blocked with adenosine (100 microM). Adenosine blocked the increase in both [Ca]s and [Ca]c that normally accompanies a tetanus. Thapsigargin (200 nM) did not affect [Ca]c or [Ca]s, but blocked transient increases in [Ca] caused by caffeine (10 mM) in both 3 mM and Ca2+ free bath solutions. These results are discussed in relation to the role of intracellular calcium in initiating LTP after a tetanus to the nerve terminals.

Intracellular calcium mobilization and neurite outgrowth in mammalian neurons

Cultured adult rat dorsal root ganglion (DRG) neurons were used to study depolarization-induced Ca2+ mobilization and the effects of intracellular Ca2+ depletion on neurite outgrowth. Cytoplasmic and nuclear Ca2+ signals were visualized in dissociated DRG neurons using confocal scanning laser microscopy and the Ca2+ indicator dye fluo-3. The depolarization-induced Ca2+ signals were highest in neurons during the first few days in culture, prior to neurite extension; during this time nuclear signals exceeded those of the cytoplasm severalfold. After several days in culture, neurons began to arborize, depolarization-induced Ca2+ signals became attenuated, and nuclear signals no longer exceeded those of the cytoplasm. Elevated Ca2+ signals were dependent upon both Ca2+ influx and intact intracellular Ca2+ stores, indicating that the signals are generated by calcium-induced calcium release (CICR). Thapsigargin, an endoplasmic reticulum Ca2+ ATPase inhibitor, depleted intracellular Ca2+ stores and blocked the induction of the large nuclear Ca2+ signals. Treating DRG neurons briefly with thapsigargin (200 nM for 20 min) shortly after plating reduced subsequent neuritogenesis, implying that intact Ca2+ stores are necessary for initiating neurite outgrowth. Immunostaining of DRG neurons with antibodies to Ca2+/calmodulin-dependent kinase II (CaM kinase II) demonstrated that this enzyme is present in the nucleus at early times in culture. These observations are consistent with the idea that CICR triggered by Ca2+ entry subsequent to depolarization may elicit neurite outgrowth by activating nuclear enzymes appropriate for such outgrowth.

The nucleus is insulated from large cytosolic calcium ion changes

Extracellular events regulate functions in the cell nucleus by means of calcium ions acting through effector enzymes. Recently, the traditional view of the nuclear pore as freely permeable to small ions has been questioned as a result of reports that nuclear calcium can be regulated independently of cytosolic calcium. We have used confocal microscopy of fluorescent Ca2+ indicators to investigate the Ca2+ dynamics between cytosol and nucleus in neurons. We find that a previously reported amplification of Ca2+ changes in the nucleus is a measurement artefact. Small changes of cytosolic Ca2+ cause equally rapid changes in nuclear Ca2+, consistent with the free diffusion of Ca2+ through nuclear pores. In contrast, large cytosolic Ca2+ increases (above 300 nM) are attenuated in the nucleus. Our results show the nuclear envelope shapes but does not block the passage of Ca2+ signals from cytosol to nucleus.

Open states of nuclear envelope ion channels in cardiac myocytes

Prevalent nucleocytoplasmic transport theory views flow of monoatomic ions as completely unrestricted, resulting from the presence of large diameter pore complexes (NPCs) that perforate, but hold together, the two separate membranes of the nuclear envelope (NE). However, three lines of investigations indicate that, at least in some cell types, monoatomic ion flow is restricted. (i) Patch clamp reveals quantized, ion channel-like activity in several NE preparations; activity thought to result from nuclear ion channels (NICs) connected to NPCs. (ii) Ratiometric fluorescence microscopy demonstrates that ions, as well as small molecules relevant to signal transduction, do distribute as if there is a NE barrier. (iii) Electron microscopy shows that NPCs contain material that behaves like a plug. NICs' large conductance (up to 1,000 pS) makes them a major determinant of nuclear ion concentrations which, in turn, influence nuclear processes. Therefore, NICs are an important modulating force of gene and transcriptional activities--two major determinants of gene expression. As nuclear processes may take from seconds (e.g., signaling) to minutes (e.g., transcription), the time the channels dwell in the ion-conducting open state is relevant to understanding NICs' role in nuclear function. Consequently, dwell-times and lifetimes of open NIC states were studied in 61 patch-clamped adult mouse cardiac myocyte nuclei. Upon voltage stimulation, NICs opened to main states of large conductance (281 +/- 198 pS, range = 120-490 pS, n = 55) and wide-range mean dwell-times (approximately 100 msec, 1-10 sec, and min). Closed states (0 pS) also had widely distributed mean dwell-times (approximately 100 msec, 1-10 sec, and min). Putative open substates (37 +/- 11 pS, range = 25-50, pS, n = 6) of high bursting frequency (< 1 msec) were observed without intervening main states (approximately 5% of patches). Fast (approximately 0.1 msec) and slow (approximately 10 msec) state-transitions were also detected. These observations suggest a role of NICs in mediating cytoplasmic signal control of cardiomyocyte gene expression.

Brief increases in intracellular Ca2+ activate K+ current and non-selective cation current in rat nucleus basalis neurons

Neurons were acutely dissociated from the rat nucleus basalis, and membrane currents (whole-cell patch-clamp) and intracellular free Ca2+ concentrations (Fura-2) were measured simultaneously from large neurons (approximately 25 microns in diameter). A brief depolarization from -60 to 0 mV for 200 ms evoked an increase in intracellular free calcium and a slow outward tail current (72 +/- 8 pA, n = 30). The outward current reversed polarity at -75.5 +/- 2.7 mV (n = 14). The tail current declined and the intracellular calcium recovered its resting level exponentially with time-constants of 1.0 +/- 0.1 s and 2.5 +/- 0.2 s, respectively (n = 17). In neurons loaded with Cs-gluconate, a similar depolarizing pulse evoked a similar increase in intracellular free calcium, but this was now followed by an inward tail current (118 +/- 8 pA, n = 44). The inward tail current reversed polarity at -27.8 +/- 3.8 mV (n = 7), and was suppressed by removal of external sodium ions. Neither outward nor inward tail currents were observed, when the external solution was calcium-free or when the pipette solution contained EGTA (10 mM). These results indicate that a depolarization causes a calcium entry and that this consequently increases both K+ conductance and non-selective cation conductance.

Fluorescence imaging of intracellular Ca2+

The measurement of intracellular Ca2+ concentrations ([Ca2+]i) is of critical importance, because many cellular functions are tightly regulated by [Ca2+]i. The fluorescent indicator, fura-2, has been used frequently to measure [Ca2+]i because of its sensitivity and specificity, and because it can be loaded into living cells with little disruption of function. Most importantly, the peak excitation wavelength of fura-2 changes when it binds Ca2+. As a consequence, measurements of fluorescence at two excitation wavelengths can be used to obtain an estimate of [Ca2+]i that is independent of dye concentration and cell thickness. Fura-2 acetoxymethyl ester (AM) is a lipid-soluble derivative that is often used because of its ability to pass through cell membranes. There are, however, several problems with the use of fura-2 AM such as intracellular compartmentation and incomplete deesterification. The availability of low-light-level cameras and computer hardware for the digitization of fluorescent images has made quantitative fluorescence microscopy possible. This technique has shown a striking spatial heterogeneity of [Ca2+]i in a variety of cell types, and has revealed substantial new information on dynamic intracellular biochemistry and signal transduction. However, the current imaging technology is not fully developed because of dye and instrumentation limitations. Further development of techniques and new probes are required to improve temporal and spatial resolution.

Nuclear and cytoplasmic Ca2+ signals in developing rat dorsal root ganglion neurons studied in excised tissue

Confocal microscopy and the Ca(2+)-sensitive fluorescent dye fluo-3 were used to study subcellular Ca2+ signals in embryonic, neonatal, and adult dorsal root ganglion (DRG) neurons in excised dorsal root ganglia. Optical images obtained from isolated whole embryonic and neonatal ganglia revealed a marked variability in the resting Ca2+ signals of different neurons as compared to signals in adult neurons which were uniformly faint. Many of the embryonic and neonatal neurons displayed nuclear Ca2+ signals at rest which were larger than those in the cytoplasm. Embryonic DRG neurons showed a significant increase in nuclear and cytoplasmic fluorescence in response to depolarization with elevated extracellular potassium or electrical stimulation. A single brief electrical stimulus was sufficient to elicit nuclear Ca2+ signals in a subset of the embryonic neurons. The depolarization-induced Ca2+ signals were blocked by removal of extracellular Ca2+, but not by treatment with 2,5-di (tert-butyl)-1,4 benzohydroquinone (DTBHQ), a compound which depletes intracellular Ca2+ stores. The intensity of the depolarization-induced Ca2+ signals declined significantly between the late embryonic (E18-E20) and early postnatal time periods (P0-P1). The nuclear and cytoplasmic Ca2+ signals of the embryonic DRG neurons in the excised tissue preparation occur at a time of intense target innervation, suggesting a role for Ca2+ signals in the development and maturation of rat DRG neurons.

Caffeine-induced calcium release from internal stores in cultured rat sensory neurons

Free intracellular calcium concentration ([Ca2+]in) was recorded at 22 degrees C by means of Indo-1 or Fura-2 single-cell microfluorometry in cultured dorsal root ganglion neurons obtained from neonatal rats. The resting [Ca2+]in in dorsal root ganglion neurons was 73 +/- 21 nM (mean +/- S.D., n = 94). Fast application of 20 mM caffeine evoked [Ca2+]in transient which reached a peak of 269 +/- 64 nM within 5.9 +/- 1.1 s. After reaching the peak the [Ca2+]in level started to decline in the presence of caffeine and for 87.2 +/- 10.6 s cytoplasmic calcium returned to an initial resting value. In 40% of neurons tested [Ca2+]in decreased to subresting levels following the washout of caffeine (the so-called post-caffeine undershoot). On average, the undershoot level was 19 +/- 2.5 nM below the resting [Ca2+]in value. Prolonged exposure of caffeine depleted the caffeine-sensitive stores of releasable Ca2+; the degree of this depletion depended on caffeine concentration. The depletion of the caffeine-sensitive internal stores to some extent was linked to calcium extrusion via La(3+)-sensitive plasmalemmal Ca(2+)-ATPases. The stores could be partially refilled by the uptake of cytoplasmic Ca2+, but the complete recovery of releasable Ca2+ content of the caffeine-sensitive pools required the additional calcium entry via voltage-operated calcium channels. Caffeine-evoked [Ca2+]in transients were effectively blocked by 10 microM ryanodine, 5 mM procaine, 10 microM dantrolene or 0.5 mM Ba2+, thus sharing the basic properties of the Ca(2+)-induced-Ca2+ release from endoplasmic reticulum. Pharmacological manipulation with caffeine-sensitive stores interfered with the depolarization-induced [Ca2+]in transients. In the presence of low caffeine concentration (0.5-1 mM) in the extracellular solution the rate of rise of the depolarization-triggered [Ca2+]in transients significantly increased (by a factor 2.15 +/- 0.29) suggesting the occurrence of Ca(2+)-induced Ca2+ release. When the caffeine-sensitive stores were emptied by prolonged application of caffeine, the amplitude and the rate of rise of the depolarization-induced [Ca2+]in transients were decreased. These facts suggest the involvement of internal caffeine-sensitive calcium stores in the generation of calcium signal in sensory neurons.

Mechanisms of calcium release and propagation in cardiac cells. Do studies with confocal microscopy add to our understanding?

Laser-scanning confocal microscopy (LSCM) has a number of recognised advantages over other techniques of light microscopy for the study of cell and tissue structure. These include increased image spatial resolution, and even more importantly, removal of out-of-focus information from 2-dimensional images of 3-dimensional structures. Moreover, these features have also recently proved to be of immense benefit when coupled with ion-sensitive fluorescent probes, in the study of second messenger systems in relation to cell function. This review summarises the contribution that recent studies with LSCM have made to our understanding of the important patho-physiological state, spontaneous Ca(2+)-release (SCR) in isolated cardiac myocytes, and the relationship of this phenomenon to the induction of abnormal cell automaticity or cardiac arrhythmia. In some components of SCR and propagation, our existing knowledge has only been confirmed by recent results, while in others facets of this complex process, our understanding is being greatly enhanced by LSCM.

Characterization of Ca2+ signals induced in hippocampal CA1 neurones by the synaptic activation of NMDA receptors

1. A combination of confocal microscopy, whole-cell patch-clamp recording, intracellular dialysis and pharmacological techniques have been employed to study Ca2+ signalling in CA1 pyramidal neurones, within rat hippocampal slices. 2. In the soma of CA1 neurones, depolarizing steps applied through the patch-pipette resulted in transient increases in the fluorescence emitted by the Ca2+ indicator fluo-3. The intensity of the fluorescence transients was proportional to the magnitude of the Ca2+ currents recorded through the pipette. Both the somatic fluorescence transients and the voltage-activated Ca2+ currents ran down in parallel over a period of between approximately 15-45 min. The fluorescence transients were considered, therefore, to be caused by increases in cytosolic free Ca2+. 3. Under current-clamp conditions, high-frequency (tetanic) stimulation (100 Hz, 1 s) of the Schaffer collateral-commissural pathway led to compound excitatory postsynaptic potentials (EPSPs) and somatic Ca2+ transients. The somatic Ca2+ transients were sensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 100 microM). These transients, but not the EPSPs, disappeared with a time course similar to that of the run-down of voltage-gated Ca2+ currents. Tetanus-induced somatic Ca2+ transients could not be elicited under voltage-clamp conditions. 4. Fluorescence images were obtained from the dendrites of CA1 pyramidal neurones starting at least 30 min after obtaining whole-cell access to the neurone. Measurements were obtained only after voltage-gated Ca2+ channel activity had run down completely. 5. Tetanic stimulation of the Schaffer collateral-commissural pathway resulted in compound EPSPs and excitatory postsynaptic currents (EPSCs), under current- and voltage-clamp, respectively. In both cases, these were invariably associated with dendritic Ca2+ transients. In cells voltage-clamped at -35 mV, the fluorescent signal increased on average 2-fold during the tetanus and decayed to baseline values with a half-time (t1/2) of approximately 5 s. 6. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) partially reduced the tetanus-induced EPSC without affecting the Ca2+ transients. In contrast, AP5, which also depressed the EPSC, substantially reduced or eliminated the Ca2+ transients. 7. In normal (i.e. 1 mM Mg(2+)-containing) medium, NMDA receptor-mediated synaptic currents displayed the typical region of negative slope conductance in the peak I-V relationship (between -90 and -35 mV). The dendritic tetanus-induced Ca2+ transients also displayed a similar anomalous voltage dependence, decreasing in size from -35 to -90 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell-permeant Ca2+ chelators reduce early excitotoxic and ischemic neuronal injury in vitro and in vivo

We report the characterization of the first successful treatment of neuronal ischemic injury in vivo by cell-permeant Ca2+ chelators. The chelators attenuated glutamate-induced intracellular Ca2+ increases and neurotoxicity in neuronal explant cultures. When infused intravenously in rats, permeant fluorescent BAPTA analogs accumulated in neurons in several brain regions. BAPTA-AM, infused in vivo, reduced Ca(2+)-dependent spike frequency adaptation and post-spike train hyperpolarizations in CA1 neurons taken from treated animals. This effect was reproduced by direct injections of BAPTA into untreated neurons. The effects of three different chelators (BAPTA, 5,5'-difluoro BAPTA, and 4,4'-difluoro BAPTA) on Ca(2+)-dependent membrane excitability varied with their Ca2+ affinity. When the chelators' permeant forms were used to treat rats prior to the induction of focal cortical ischemia, they were highly neuroprotective, as gauged by significant reductions in cortical infarction volumes and neuronal sparing. The chelators' protective effects in vivo also reflected their affinity for Ca2+. This report provides the most direct evidence to date that intracellular Ca2+ excess triggers early neurodegeneration in vivo and contributes a novel therapeutic approach to neuronal ischemia of potential clinical utility.

Kainate elicits elevated nuclear calcium signals in retinal neurons via calcium-induced calcium release

Intracellular Ca2+ was imaged in cultured neonatal rat retinal neurons using the Ca(2+)-sensitive dye fluo-3 and confocal scanning laser microscopy. Depolarization via elevation of bath K+ concentration resulted in large cytoplasmic and nuclear Ca2+ signals; responses in the nucleus exceeded those of the cytoplasm. Glutamate or kainate application elicited the same intracellular pattern of elevated Ca2+ signals. Kainate stimulation was blocked by the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and greatly reduced by removing Ca2+ from the bath and adding ethylene glycol-bis (beta-amino-ethyl ether) N,N,N',N'-tetraacetic acid (EGTA). Kainate was equally effective in eliciting Ca2+ signals when bath Na+ was replaced with equimolar concentrations of choline, or in the presence of the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV). Caffeine treatment significantly reduced the kainate-induced intracellular Ca2+ response. These results suggest that Ca2+ can enter through the kainate receptor of retinal neurons and amplify the Ca2+ signals in the cytoplasm and nucleus by releasing Ca2+ from intracellular stores.

Spatial and temporal control of intracellular free Ca2+ in chick sensory neurons

Digital imaging of fura-2 fluorescence and the voltage-clamp technique were combined to study cytoplasmic free Ca2+ concentration, [Ca]i, in neurons cultured from chick dorsal root ganglia. Depolarizing pulses raised [Ca]i to a new steady-state level which was achieved earlier in neurites than in the soma. The rise in [Ca]i during stimulated bursting or rhythmic activity was also faster in neurites. After stimulation [Ca]i recovered monoexponentially in the soma and biexponentially in neurites. Application of 50 mM KCl produced membrane depolarization and a concomitant increase of [Ca]i. During wash-out [Ca]i often declined to an intermediate steady-state level at which it stayed for several minutes. Thereafter the resting level of [Ca]i was quickly restored. [Ca]i recovery was delayed after treating the cell with 2 microM thapsigargin, an inhibitor of the Ca2+ pump of internal Ca2+ stores. Caffeine (10 mM) transiently increased [Ca]i. A second caffeine application produced smaller [Ca]i changes due to the prior depletion of Ca2+ stores, which could be replenished by brief exposure to KCl. Thapsigargin (2 microM) transiently increased [Ca]i both in the standard and Ca(2+)-free solution. [Ca]i transients due to caffeine and thapsigargin started in the cell interior, in contrast to [Ca]i changes evoked by membrane depolarization, which were noticed first at the cell edge. Caffeine and thapsigargin induced a transient inward current which persisted in the presence of 1 mM La3+ and in Ca(2+)-free solutions, but which was greatly diminished in Na(+)-free solutions. The effects of caffeine and thapsigargin were mutually exclusive both in the generation of [Ca]i transients and in the inward current induction.

Calcium clamp in single nerve cells

Free calcium concentration in isolated single neurons was clamped using a new technical approach based on a feed-back connection between the Fura-2 fluorescence signal measuring the intracellular Ca2+ concentration ([Ca2+]i) and iontophoretic current injecting Ca2+ into the cell. Beginning of [Ca2+]i clamping at a level above the basal one triggered fast (few seconds) current transients equal to injection of 36 +/- 20 microM Ca2+ (for a 0.1 microM change of [Ca2+]i), representing the filling of a fast cytosolic buffer. Continuation of clamping required very small clamping currents (corresponding to injection of 0.39 +/- 0.20 microM.s-1 Ca2+). This value increased proportionally to the magnitude of the change of [Ca2+]i above basal level, indicating the activation of calcium-dependent mechanisms for Ca2+ removal from the cytosol. The described approach allowed measurement, under physiological conditions, of the capacitative and kinetic properties of different Ca-regulating systems functioning in a single nerve cell as well as other types of cells.

Restricted ion flow at the nuclear envelope of cardiac myocytes

Flow of small ions across the nuclear envelope (NE) is thought to occur without restriction through large diameter nuclear pore complexes (NPCs). However, investigations with electron and fluorescence microscopy, and with patch-clamp and microelectrode electrophysiology, suggest that in many animal and plant cell types small ions move through a barrier having the signature of large conductance nuclear ion channels (NICs). As nucleocytoplasmic transport and gene activity are regulated by cytoplasmic signals and as it has recently been shown by this investigator that cardiac NICs are sensitive to cAMP-dependent processes (1), it was considered relevant to further investigate the effects of various cytosolic signals on NIC activity. Ion species substitution demonstrated that K+ is the major species responsible for NIC currents. The Na-channel blocker tetrodotoxin (TTX, 100 microM) and the Ca-channel blocker diltiazem (100 microM) had no effect, indicating no relation of NICs to Na- or Ca-channels in transit to the cell surface membrane. Zn2+ (100 microM) blocked NIC activity, suggesting a dual role in nucleocytoplasmic transport and gene function. GTP did not produce measurable effect. However, its nonhydrolyzable analogue GTP-gamma-S (10 microM) suppressed NIC activity, suggesting a role for GTP hydrolysis in NIC function. Deoxynucleotides (dNTPs, 200 microM) produced a transient increase in NIC activity, pointing to a modulation of NIC function by nucleic acid substrates. These results indicate a role for NICs in mediating: (a) control of gene activity by transduction and other cytosolic signals, and (b) nuclear demands and response to such signals.

Characteristics of Ca2+ release induced by Ca2+ influx in cultured bullfrog sympathetic neurones

1. A rise in intracellular Ca2+ ([Ca2+]i) and a Ca2+ current (ICa) induced by a depolarizing pulse were simultaneously recorded by fura-2 or indo-1 fluorescence and whole-cell patch clamp techniques in cultured bullfrog sympathetic ganglion cells. 2. [Ca2+]i (calculated from the ratio of fura-2 fluorescences excited at 380 and 340 nm and recorded with a photomultiplier at > 492 nm) rose regeneratively (in most cells) during a command pulse (from -60 to 0 mV, 100 ms), continued to rise thereafter, peaked at 666 ms (on average) and decayed slowly with a half-decay time of 22.8 s. 3. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording indo-1 fluorescences at two wavelengths (peaked at 410 and 475 nm) with a confocal microscope demonstrated that [Ca2+]i beneath the cell membrane rose much faster than that in the deeper cytoplasm. The time course of the spatial integral of [Ca2+]i, however, corresponded well with that recorded with fura-2 fluorescence using a photomultiplier. 4. [Ca2+]i measured by fura-2 fluorescence ratio using a photomultiplier did not increase during a strong depolarizing pulse (-60 to +80 mV), but sometimes rose after the pulse. A depolarization-induced rise in [Ca2+]i ([Ca2+]i transient) was blocked in a Ca(2+)-free, EGTA solution, reduced by lowering the extracellular Ca2+ concentration ([Ca2+]o) to 0.45 or 0.9 mM and enhanced by raising [Ca2+]o to 7.2 or 14.4 nM. 5. The extracellular Ca2+ dependence was non-linear when long depolarizing pulses (up to 500 ms) were applied; the amplitude of [Ca2+]i transient/Ca2+ entry (unit [Ca2+]i transient) increased with an increase in Ca2+ entry. 6. Increasing the duration of depolarization (-50 or -60 to 0 mV) from 20 to 500 ms enhanced asymptotically the integral of ICa (due to inactivation), and progressively the magnitude of [Ca2+]i transients, leading to the apparent non-linear dependence of unit [Ca2+]i transient on Ca2+ entry as well as on the duration of membrane depolarization. The peak time of [Ca2+]i transient was unchanged for pulse durations up to 300 ms, but prolonged with an increase in pulse duration to 500 ms. 7. Inhibitors of Ca2+ release from intracellular Ca2+ reservoirs, dantrolene (10 microM) and ryanodine (50 microM), blocked the [Ca2+]i transient to 56 and 30%, respectively, of the control. 8. The higher the basal [Ca2+]i level, the greater was the magnitude of the [Ca2+]i transients.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of gene expression in hippocampal neurons by distinct calcium signaling pathways

Calcium ions (Ca2+) act as an intracellular second messenger and can enter neurons through various ion channels. Influx of Ca2+ through distinct types of Ca2+ channels may differentially activate biochemical processes. N-Methyl-D-aspartate (NMDA) receptors and L-type Ca2+ channels, two major sites of Ca2+ entry into hippocampal neurons, were found to transmit signals to the nucleus and regulated gene transcription through two distinct Ca2+ signaling pathways. Activation of the multifunctional Ca(2+)-calmodulin-dependent protein kinase (CaM kinase) was evoked by stimulation of either NMDA receptors or L-type Ca2+ channels; however, activation of CaM kinase appeared to be critical only for propagating the L-type Ca2+ channel signal to the nucleus. Also, the NMDA receptor and L-type Ca2+ channel pathways activated transcription by means of different cis-acting regulatory elements in the c-fos promoter. These results indicate that Ca2+, depending on its mode of entry into neurons, can activate two distinct signaling pathways. Differential signal processing may provide a mechanism by which Ca2+ controls diverse cellular functions.

Spatial heterogeneity of caffeine- and inositol 1,4,5-trisphosphate-induced Ca2+ transients in isolated snail neurons

Inositol 1,4,5-trisphosphate- and caffeine-induced Ca2+ release was examined in neurons isolated from the mollusc Helix pomatia using Ca2+ indicator fura-2 and fluorescent digital-imaging microscopy technique. Extracellular application of caffeine caused a fast and pronounced augmentation of [Ca2+]i whose amplitude and kinetics differ in the centre of the cell and near its membrane. Mean values of caffeine-induced increase of [Ca2+]i were 0.97 +/- 0.11 microM at the periphery and 0.53 +/- 0.13 microM in the centre. The rates of rise and relaxation of caffeine-evoked [Ca2+]i transients were faster near the membrane. Pressure injection of inositol, 1,4,5-trisphosphate into the same neurons produced an abrupt and significant increase of [Ca2+]i in the centre (mean value of inositol 1,4,5-trisphosphate-induced elevation = 0.55 +/- 0.11 microM) while the response was smaller or even absent near the cellular membrane. Inositol 1,4,5-trisphosphate- and caffeine-induced Ca2+ transients did not affect each other. The data obtained indicate that in snail neurons these two calcium pools are not overlapping and at least some part of the caffeine-sensitive store is located close to the cellular membrane and that the inositol 1,4,5-trisphosphate-sensitive one is located in the centre of the cell.

Intracellular calcium mobilization by inositol 1,4,5-trisphosphate: intracellular movements and compartmentalization

Intracellular calcium ion (Ca2+) changes in NIH-3T3 fibroblasts responding to inositol 1,4,5-trisphosphate (IP3) injections have been monitored using high resolution digital imaging of the calcium indicator Fura-2. Ester loaded and microinjected indicator report radically different patterns of Ca2+ change during the IP3 response. These differences arise from intracellular compartmentalization of the ester loaded indicator which can seriously distort reported Ca2+ levels. Prominent among these aberrant responses is a signal in which Ca2+ levels in the cell nucleus appear to exceed those in the rest of the cell, and an apparent slowing of the Ca2+ recovery time-course throughout the cell when temperature is increased. Similar behavior is observed in other cell types. Judicious use of both loading techniques can provide information on Ca2+ movements into organelles that might otherwise escape detection. The Ca2+ rise normally measured in bulk or integrated single cell measurements is a complex mix of cytosol/nucleus and organellar changes. Much, if not all, of the observable organellar change is an accumulation, not release, of Ca2+ following the IP3 injection. The Golgi apparatus is a conspicuous early site for this accumulation, and mitochondria show a large, temperature sensitive uptake that is capable of limiting the maximal Ca2+ change during the response.

Receptor-mediated calcium signal playing a nuclear third messenger in the activation of antigen-specific B cells

We have studied receptor-mediated calcium signals in antigen-specific B cells (trinitrophenol-specific B cell clone, TP67.21) using a confocal fluorescence microscope with an argon ion laser (488 nm) and a He-Cd laser (325 nm). Confocal fluorescence images of fluo-3 loaded B cells, excited by an argon ion laser, became much brighter and more nonhomogeneous than those before antigen stimulation. Time-dependent fluorescence changes in intensities were abrupt and quite similar to the patterns of the intracellular calcium ion concentration [Ca2+]i observed by a conventional fluorescence microscope using fura-2. From the morphological patterns of the calcium images, the parts of the bright fluorescence seemed to belong to the nucleus in B cells. To confirm the above events we measured the confocal fluorescence images of the nucleus. From the fluorescence images of co-loaded Hoechst 33342 (a DNA-specific fluorescent probe), which excited by a He-Cd laser, the brighter parts of the fluo-3 fluorescence intensities were identified to the nucleus in B cells. This suggested the possibility that the increased intranuclear calcium ions may play a nuclear third messenger in B cells.

Caffeine response in pyramidal neurons freshly dissociated from rat hippocampus

The effect of caffeine on the CA1 pyramidal neurons freshly dissociated from rat hippocampus was investigated with nystatin-perforated patch technique under voltage-clamp condition. Caffeine evoked a transient outward current (Icaffeine) in a concentration-dependent manner at a holding potential of -40 mV. The activation and inactivation of Icaffeine were accelerated with increasing caffeine concentration. The reversal potential for Icaffeine was close to K+ equilibrium potential. The Icaffeine was not blocked by apamin and 4-aminopyridine but suppressed by charybdotoxin, tetraethylammonium, quinine and Ba2+. Thus, the pharmacological characteristics of Icaffeine were similar to those of Ca(2+)-activated K+ current having a large conductance (IC), which generates a fast afterhyperpolarization (a.h.p.). Icaffeine was depressed by pretreatment with a membrane-permeant Ca2+ chelator (BAPTA-AM) and by depletion of the Ca(2+)-induced Ca2+ release (CICR) pool with ryanodine. A blocker of CICR sites, procaine, potently depressed the Icaffeine. In the absence of the extracellular Ca2+, an application of 10 mM caffeine depleted the caffeine-sensitive Ca2+ pools. Icaffeine recovered in an exponential fashion in the presence of the extracellular Ca2+. It was concluded that rat hippocampal pyramidal neurons have a caffeine-sensitive Ca2+ pool. Furthermore, the Ca2+ released from the pool evokes K+ current similar to IC current and hyperpolarizes the neurons.

Differential control of free calcium and free zinc levels in isolated bovine liver nuclei

Modulation of intracellular signal transduction by Ca2+ and possibly Zn2+ is based on an effective homoeostatic control of the corresponding free ion concentrations. We used the fluorescent indicator fura-2 to monitor concentrations of free Ca2+ and free Zn2+ in nuclei isolated from bovine liver. The nuclei showed an ATP-stimulated accumulation of intranuclear free Ca2+, which was inhibited in the presence of the Ca(2+)-pump inhibitor thapsigargin. Furthermore, uptake and intranuclear levels of free Zn2+ were measured after incubation with different extranuclear Zn2+ concentrations. There was no stimulating effect of ATP on Zn2+ uptake. Our data suggest that the levels of intranuclear free Ca2+ and free Zn2+ are controlled differentially. A distinct regulation of free ion levels in the nucleus may contribute to the specific control of nuclear events associated with gene transcription and cell differentiation.