Regulation of the intracellular free calcium concentration in acutely dissociated neurones from rat nucleus basalis

An Inconvenient Truth: Calcium Sensors Are Calcium Buffers

Recent advances in Ca²⁺ imaging have given neuroscientists a tool to follow the activity of large numbers of individual neurons simultaneously in vivo in the brains of animals as they are presented with sensory stimulation, respond to environmental challenges, and engage in behaviors. The Ca²⁺ sensors used to transduce changes in cellular Ca²⁺ into changes in fluorescence must bind Ca²⁺ to produce a signal. By binding Ca²⁺, these sensors can act as buffers, often reducing the magnitude of a Ca²⁺ change severalfold, and producing a proportional slowing of the rates of change. Ca²⁺ probes can thus distort the patterns of activity they are intended to study and modify ongoing Ca²⁺ signaling functions. Recognizing these factors will enhance the use of in vivo Ca²⁺ imaging in the investigation of neural circuit function.

Nutraceuticals against Neurodegeneration: A Mechanistic Insight

The mechanisms underlying neurodegenerative disorders are complex and multifactorial; however, accumulating evidences suggest few common shared pathways. These common pathways include mitochondrial dysfunction, intracellular Ca²⁺ overload, oxidative stress and inflammation. Often multiple pathways co-exist, and therefore limit the benefits of therapeutic interventions. Nutraceuticals have recently gained importance owing to their multifaceted effects. These food-based approaches are believed to target multiple pathways in a slow but more physiological manner without causing severe adverse effects. Available information strongly supports the notion that apart from preventing the onset of neuronal damage, nutraceuticals can potentially attenuate the continued progression of neuronal destruction. In this article, we i) review the common pathways involved in the pathogenesis of the toxicants-induced neurotoxicity and neurodegenerative disorders with special emphasis on Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), Multiple sclerosis (MS) and Amyotrophic lateral sclerosis (ALS), and ii) summarize current research advancements on the effects of nutraceuticals against these detrimental pathways.

REVIEW ■ : The Na-Ca Exchanger in Neurons and Glial Cells

The Na+-Ca 2+ exchanger is a secondary active antiporter found in all excitable cells. This transporter couples transmembrane fluxes of Na+ to opposite fluxes of Ca2+. Under normal conditions, the energy stored in the electrochemical Na+ gradient is used to export Ca 2+ from the cytoplasm, thus contributing to cellular Ca2+ homeostasis, such as termination of Ca2+ transients during synaptic transmission in nerve terminals. The reversible and electrogenic properties of the Na+-Ca2+ exchanger suggest an interesting additional role of controlled Ca2+ entry, e.g., during action potential generation in axons. Moreover, under pathological conditions, such as anoxia/ischemia, the exchanger may function either to help extrude damaging Ca2+ loads entering via other pathways in neurons or mediate Ca2+ overload in axons. Cell geometry will influence the rate and extent of collapse of the Na+ gradient and membrane potential, the two main driving forces acting on the exchanger, which will in turn dictate to what extent and in which direction Ca2+ will be transported. The Na+-Ca2+ exchanger is subject to complex regulatory control by several ions and chemical messengers, and several recently identified isoforms are undoubtedly tailored for specific roles in different regions of the CNS. NEUROSCIENTIST 2:162-171, 1996

Calcium mobilizations in response to changes in the gravity vector in Arabidopsis seedlings: possible cellular mechanisms

Gravity influences the growth direction of higher plants. Changes in the gravity vector (gravistimulation) immediately promote the increase in the cytoplasmic free calcium ion concentration ([Ca(2+)]c) in Arabidopsis (Arabidopsis thaliana) seedlings. When the seedlings are gravistimulated by reorientation at 180°, a transient two peaked (biphasic) [Ca(2+)]c-increase arises in their hypocotyl and petioles. Parabolic flights (PFs) can generate a variety of gravity-stimuli, and enables us to measure gravity-induced [Ca(2+)]c-increases without specimen rotation, which demonstrate that Arabidopsis seedlings possess a rapid gravity-sensing mechanism linearly transducing a wide range of gravitational changes into Ca(2+) signals on a sub-second timescale. Hypergravity by centrifugation (20 g or 300 g) also induces similar transient [Ca(2+)]c-increases. In this review, we propose models for possible cellular processes of the garavi-stimulus-induced [Ca(2+)]c-increase, and evaluate those by examining whether the model fits well with the kinetic parameters derived from the [Ca(2+)]c-increases obtained by applying gravistimulus with different amplitudes and time sequences.

Endogenous calcium buffering capacity of substantia nigral dopamine neurons

Dopamine (DA)-containing cells from the substantia nigra pars compacta (SNc) play a major role in the initiation of movement. Loss of these cells results in Parkinson's disease (PD). Changes in intracellular calcium ion concentration ([Ca(2+)](i)) elicit several events in DA cells, including spike afterhyperpolarizations (AHPs) and subthreshold oscillations underlying autonomous firing. Continuous Ca(2+) load due to Ca(2+)-dependent rhythmicity has been proposed to cause the death of DA cells in PD and normal aging. Because of the physiological and pathophysiological importance of [Ca(2+)](i) in DA cells, we characterized their intrinsic Ca(2+)-buffering capacity (K(S)) in brain slices. We introduced a fluorescent Ca(2+)-sensitive exogenous buffer (200 microM fura-2) and cells were tracked from break-in until steady state by stimulating with a single action potential (AP) every 30 s and measuring the Ca(2+) transient from the proximal dendrite. DA neurons filled exponentially with a tau of about 5-6 min. [Ca(2+)](i) was assumed to equilibrate between the endogenous Ca(2+) buffer and the exogenous Ca(2+) indicator buffer. Intrinsic buffering was estimated by extrapolating from the linear relationships between the amplitude or time constant of the Ca(2+) transients versus [fura-2]. Extrapolated Ca(2+)-transients in the absence of fura-2 had mean peak amplitudes of 293.7 +/- 65.3 nM and tau = 124 +/- 13 ms (postnatal day 13 [P13] to P17 animals). Intrinsic buffering increased with age in DA neurons. For cells from animals P13-P17, K(S) was estimated to be about 110 (n = 20). In older animals (P25-P32), the estimate was about 179 (n = 10). These relatively low values may reflect the need for rapid Ca(2+) signaling, e.g., to allow activation of sK channels, which shape autonomous oscillations and burst firing. Low intrinsic buffering may also make DA cells vulnerable to Ca(2+)-dependent pathology.

Differences of Ca2+ handling properties in identified central olfactory neurons of the antennal lobe

Information processing in neurons depends on highly localized Ca2+ signals. The spatial and temporal dynamics of these signals are determined by a variety of cellular parameters including the calcium influx, calcium buffering and calcium extrusion. Our long-term goal is to better understand how intracellular Ca2+ dynamics are controlled and contribute to information processing in defined interneurons of the insect olfactory system. The latter has served as an excellent model to study general mechanisms of olfaction. Using patch-clamp recordings and fast optical imaging in combination with the 'added buffer approach', we analyzed the Ca2+ handling properties of different identified neuron types in Periplaneta americana's olfactory system. Our focus was on two types of local interneurons (LNs) with significant differences in intrinsic electrophysiological properties: (1) spiking LNs that generate 'normal' Na+ driven action potentials and (2) non-spiking LNs that do not express voltage-activated Na+ channels. We found that the distinct electrophysiological properties from different types of central olfactory interneurons are strongly correlated with their cell specific calcium handling properties: non-spiking LNs, in which Ca2+ is the only cation that enters the cell to contribute to membrane depolarization, had the highest endogenous Ca2+ binding ratio and Ca2+ extrusion rate.

Na+/Ca2+ exchange and regulation of cytoplasmic concentration of calcium in rat cerebellar neurons treated with glutamate

In the present work, the forward and/or reversed Na+/Ca2+ exchange in cerebellar granular cells was suppressed by substitution of Na+o by Li+ before, during, and after exposure to glutamate for varied time and also using the inhibitor KB-R7943 of the reversed exchange. After glutamate challenge for 1 min, Na+o/Li+ substitution did not influence the recovery of low [Ca2+]i in a calcium-free medium. A 1-h incubation with 100 microM glutamate induced in the neurons a biphasic and irreversible [Ca2+]i rise (delayed calcium deregulation (DCD)), enhancement of [Na+]i, and decrease in the mitochondrial potential. If Na+o had been substituted by Li+ before the application of glutamate, i.e. the exchange reversal was suppressed during the exposure to glutamate, the number of cells with DCD was nearly fourfold lowered. However, addition of the Na+/K+-ATPase inhibitor ouabain (0.5 mM) not preventing the exchange reversal also decreased DCD in the presence of glutamate. Both exposures decreased the glutamate-caused loss of intracellular ATP. Glucose deprivation partially abolished protective effects of the Na+o/Li+ substitution and ouabain. KB-R7943 (10 microM) increased 7.4-fold the number of cells with the [Ca2+]i decreased to the basal level after the exposure to glutamate. Thus, reversal of the Na+/Ca2+ exchange reinforced the glutamate-caused perturbations of calcium homeostasis in the neurons and slowed the recovery of the decreased [Ca2+]i in the post-glutamate period. However, for development of DCD, in addition to the exchange reversal, other factors are required, in particular a decrease in the intracellular concentration of ATP.

Identification of functional domains of Mid1, a stretch-activated channel component, necessary for localization to the plasma membrane and Ca2+ permeation

The Saccharomyces cerevisiae MID1 gene product (Mid1) is a stretch-activated Ca(2+)-permeable channel component required for Ca2+ influx and the maintenance of viability of cells exposed to the mating pheromone, alpha-factor. It is composed of 548-amino-acid (aa) residues with four hydrophobic segments, H1 (aa 2-22), H2 (aa 92-111), H3 (aa 337-356) and H4 (aa 366-388). It also has 16 putative N-glycosylation sites. In this study, sequentially truncated Mid1 proteins conjugated with GFP were expressed in S. cerevisiae cells. The truncated protein containing the region from H1 to H3 (Mid1(1-360)-GFP) localized normally in the plasma and endoplasmic reticulum (ER) membranes and complemented the low viability and Ca(2+)-uptake activity of the mid1 mutant, whereas Mid1(1-133)-GFP containing the region from H1 to H2 did not. Mid1(Delta3-22)-GFP lacking the H1 region failed to localize in the plasma membrane. Membrane fractionation showed that Mid1(1-22)-GFP containing only H1 localized in the plasma membrane in the presence of alpha-factor, suggesting that H1 is a signal sequence responsible for the alpha-factor-induced Mid1 delivery to the plasma membrane. The region from H1 to H3 is required for the localization of Mid1 in the plasma and ER membranes. Finally, trafficking of Mid1-GFP to the plasma membrane was dependent on the N-glycosylation of Mid1 and the transporter protein Sec12.

Role of calcium binding proteins in the control of cerebellar granule cell neuronal excitability: experimental and modeling studies

Calcium binding proteins, such as calretinin, are abundantly expressed in distinctive patterns in the central nervous system but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin deficient mice exhibit dramatic alterations in motor coordination and in Purkinje cell firing recorded in vivo through unknown mechanisms. In the present paper, we review the results obtained with the patch clamp recording techniques in acute slice preparation. This data allow us to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mM of the exogenous fast calcium buffer BAPTA is infused in the cytosol to restore the calcium buffering capacity. Furthermore, we propose a mathematical model demonstrating that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium buffering capacity due to the absence of calretinin. We suggest that calcium binding proteins modulate intrinsic neuronal excitability and may therefore play a role in the information processing in the central nervous system.

Reduced mitochondrial buffering of voltage-gated calcium influx in aged rat basal forebrain neurons

Alterations of neuronal Ca(2+) homeostatic mechanisms could be responsible for many of the cognitive deficits associated with aging in mammals. Mitochondrial participation in Ca(2+) signaling is now recognized as a prominent feature in neuronal physiology. We combined voltage-clamp electrophysiology with Ca(2+)-sensitive ratiometric microfluorimetry and laser scanning confocal microscopy to investigate the participation in Ca(2+) buffering of in situ mitochondria in acutely dissociated basal forebrain neurons from young and aged F344 rats. By pharmacologically blocking mitochondrial Ca(2+) uptake, we determined that mitochondria were not involved in rapid buffering of small Ca(2+) influx through voltage-gated Ca(2+) channels (VGCCs) in the somatic compartment. For larger Ca(2+) influx, aged mitochondria showed a significant buffering deficit. Evidence obtained with the potentiometric indicator, JC-1, suggests a significantly reduced mitochondrial membrane potential in aged neurons. These results support the interpretation that there is a fundamental difference in the way young and aged neurons buffer Ca(2+), and a corresponding difference in the quality of the Ca(2+) signal experienced by young and aged neurons for different intensities of cytoplasmic Ca(2+) influx.

Altered neuronal excitability in cerebellar granule cells of mice lacking calretinin

Calcium-binding proteins such as calretinin are abundantly expressed in distinctive patterns in the CNS, but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells, which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin-deficient mice exhibit dramatic alterations in motor coordination and Purkinje cell firing recorded in vivo through unknown mechanisms. In the present study, we used patch-clamp recording techniques in acute slice preparation to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin-deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mm of the exogenous fast-calcium buffer BAPTA is infused in the cytosol to restore the calcium-buffering capacity. A proposed mathematical model demonstrates that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium-buffering capacity resulting from the absence of calretinin. This result suggests that calcium-binding proteins modulate intrinsic neuronal excitability and may therefore play a role in information processing in the CNS.

Altered neuronal excitability in cerebellar granule cells of mice lacking calretinin

Calcium-binding proteins such as calretinin are abundantly expressed in distinctive patterns in the CNS, but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells, which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin-deficient mice exhibit dramatic alterations in motor coordination and Purkinje cell firing recorded in vivo through unknown mechanisms. In the present study, we used patch-clamp recording techniques in acute slice preparation to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin-deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mm of the exogenous fast-calcium buffer BAPTA is infused in the cytosol to restore the calcium-buffering capacity. A proposed mathematical model demonstrates that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium-buffering capacity resulting from the absence of calretinin. This result suggests that calcium-binding proteins modulate intrinsic neuronal excitability and may therefore play a role in information processing in the CNS.

Cellular regulatory mechanisms influencing the activity of the cochlear nucleus: a review

The cochlear nucleus is the site in the auditory pathway where the primary sensory information carried by the fibres of the acoustic nerve is transmitted to the second-order neurones. According to the generally accepted view this transmission is not a simple relay process but is considered as the first stage where the decoding of the auditory information begins. This notion is based on the diverse neurone composition and highly ordered structure of the nucleus, on the complex electrophysiological properties and activity patterns of the neurones, on the activity of local and descending modulatory mechanisms and on the presence of a highly sophisticated intracellular Ca2+ homeostasis. This review puts emphasis on introducing the experimental findings supporting the above statements and on the questions which should be answered in order to gain a better understanding of the function of the cochlear nucleus.

Ca2+ clearance at growth cones produced by crayfish motor axons in an explant culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 microM) and CCCP (10 microM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Ca2+ Clearance at Growth Cones Produced by Crayfish Motor Axons in an Explant Culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 μM) and CCCP (10 μM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Removal of Ca(2+) following depolarization-evoked cytoplasmic Ca(2+) transients in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus

Cytoplasmic [Ca(2+)] ([Ca(2+)](i)) was measured using Fura-2 in pyramidal neurones isolated from the rat dorsal cochlear nucleus (DCN). The kinetic properties of Ca(2+) removal following K(+) depolarization-induced Ca(2+) transients were characterized by fitting exponential functions to the decay phase. The removal after small transients (<82 nM peak [Ca(2+)](i)) had monophasic time course (time constant of 6.43 +/- 0.48 s). In the cases of higher Ca(2+) transients biphasic decay was found. The early time constant decreased (from 3.09 +/- 0.26 to 1.46 +/- 0.11 s) as the peak intracellular [Ca(2+)] increased. The value of the late time constant was 18.15 +/- 1.60 s at the smallest transients, and showed less dependence on [Ca(2+)](i). Blockers of Ca(2+) uptake into intracellular stores (thapsigargin and cyclopiazonic acid) decreased the amplitude of the Ca(2+) transients and slowed their decay. La(3+) (3 mM) applied extracellularly during the declining phase dramatically changed the time course of the Ca(2+) transients as a plateau developed and persisted until the La(3+) was present. When the other Ca(2+) removal mechanisms were available, reduction of the external [Na(+)] to inhibit the Na(+)/Ca(2+) exchange resulted in a moderate increase of the time constants. It is concluded that in the isolated pyramidal neurones of the DCN the removal of Ca(2+) depends mainly on the activity of Ca(2+) pump mechanisms.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 µM), and (iv) nifedipine (2.0 µM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 µM] was 54 ± 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 ± 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 µM). Thapsigargin (2 µM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 µM) produced increases in [Ca2+]i (47 ± 11 nM, n = 6 and 22 ± 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.Key words: calcium homeostasis, intracellular calcium stores, anterior pituitary cells, mitochondria.

Modification of ion channels and calcium homeostasis of basal forebrain neurons during aging

In this paper we review the last several years of work from our lab with attention to changes in the properties of basal forebrain neurons during aging. These neurons play a central role in behavioral functions, such as: attention, arousal, cognition and autonomic activity, and these functions can be adversely affected during aging. Therefore, it is fundamental to define the cellular mechanisms of aging in order to understand the basal forebrain and to correct deficits associated with aging. We have examined changes in the physiological properties of basal forebrain neurons during aging with whole-cell and single-channel patch-clamp, as well as, microfluorimetric measurements of intracellular calcium concentrations. These studies contribute to the understanding of integration within the basal forebrain and to the identification of age-related changes within central mammalian neurons. Although extensive functional/behavioral decline is often assumed to occur during aging, our data support an interpretation of compensatory increases in function for excitatory amino acid receptors, GABA(A) receptors, voltage-gated calcium currents and calcium homeostatic mechanisms. We believe that these changes occur to compensate for decrements accruing with age, such as decreased synaptic contacts, ion imbalances or neuronal loss. The basal forebrain must retain functionality into late aging if senescence is to be productive. Thus, it is critical to recognize the potential cellular and subcellular targets for therapeutic interventions intended to correct age-related behavioral deficits.

The role of mitochondria in presynaptic calcium handling at a ribbon synapse

Mitochondria are thought to be important in clearing calcium from synaptic terminals. It is unclear, however, whether the principal role of mitochondria in pre-synaptic calcium handling is to take up Ca2+ directly or to fuel Ca2+ removal by other mechanisms. We used patch clamp techniques and fluorescence imaging to examine calcium clearance mechanisms, including mitochondrial uptake, in single synaptic terminals of retinal bipolar neurons. We found that extrusion through the ATP-dependent Ca2+ pump of the plasma membrane is the dominant form of Ca2+ removal in the synaptic terminal. Calcium uptake into mitochondria was sometimes evident with large Ca2+ loads but was consistently observed only when plasma membrane extrusion was inhibited. We conclude that mitochondria act primarily as an energy source in clearance of Ca2+ from bipolar cell synaptic terminals.

Supralinear Ca2+ signaling by cooperative and mobile Ca2+ buffering in Purkinje neurons

Endogenous high-affinity Ca2+ buffering and its roles were investigated in mouse cerebellar Purkinje cells with the use of a low-affinity Ca2+ indicator and a high-affinity caged Ca2+ compound. Increases in the cytosolic Ca2+ concentration ([Ca2+]i) were markedly facilitated during repetitive depolarization, resulting in the generation of steep micromolar Ca2+ gradients along dendrites. Such supralinear Ca2+ responses were attributed to the saturation of a large concentration (0.36 mM) of a mobile, high-affinity (dissociation constant, 0.37 microM) Ca2+ buffer with cooperative Ca2+ binding sites, resembling calbindin-D28K, and to an immobile, low-affinity Ca2+ buffer. These data suggest that the high-affinity Ca2+ buffer operates as the neuronal computational element that enables efficient coincidence detection of the Ca2+ signal and that facilitates spatiotemporal integration of the Ca2+ signal at submicromolar [Ca2+]i.

Growth cones exhibit enhanced cell-cell adhesion after neurotransmitter release

Evoked release of acetylcholine and subsequent cell-cell adhesive contacts between growth cones and acetylcholine sensing neurons were observed using cultured neurons dissociated from the diagonal band of Broca of the rat. Stimulation to the soma of the diagonal band of Broca neurons evoked release of acetylcholine from the growth cones. The release of acetylcholine was monitored using whole-cell patch-clamp recording from acetylcholine receptor-rich superior cervical ganglion neuron positioned on the growth cone as a sensor of acetylcholine release. By measuring changes in fluorescence from the growth cone using Ca2(+)-sensitive dye while voltage-clamping the superior cervical ganglion neuron, transient intracellular Ca2+ concentration increase and acetylcholine release from growth cone were recorded simultaneously. Video-enhanced differential interference contrast imaging of the growth cones demonstrated tether formation between the growth cone and superior cervical ganglion cell soma when the superior cervical ganglion cell soma was moved away from the growth cone after acetylcholine release, suggesting formation of adhesive contacts between the growth cone and the sensor neuron. Adhesive contacts between growth cones and sensor neurons were also detected when a high K+ solution or alpha-latrotoxin was applied to the growth cone. Adhesions were also observed between growth cones and latex beads, when growth cones were exposed to high K+ solution. The properties of the adhesive contacts at the growth cone were studied by optically manipulating a latex bead attached to the growth cone surface. These results suggest that growth cones exhibit cell-cell adhesion after neurotransmitter release.

Age-related alterations in caffeine-sensitive calcium stores and mitochondrial buffering in rat basal forebrain

The properties of caffeine- and thapsigargin-sensitive endoplasmic reticulum calcium stores were compared in acutely dissociated basal forebrain neurons from young and aged F344 rats by ratiometric microfluorimetry. The ability of these stores to sequester and release calcium resembles that observed in other central neurons, with an important role of mitochondrial calcium buffering in regulating the response to caffeine. An age-related reduction in the filling state of the stores in resting cells appears to be mediated by increased rapid calcium buffering, which reduces the availability of calcium for uptake into the stores. An age-related decrease in the amplitude of maximal caffeine-induced calcium release was attributed to increased mitochondrial buffering. There were no age-related differences in the sensitivity to caffeine or in the calcium sequestration/release process at the level of the endoplasmic reticulum per se. These findings demonstrate the importance of interactions between cellular calcium buffering mechanisms and provide details regarding age-related changes in calcium homeostasis which have been thought to occur in these and other neurons associated with age-related neuronal dysfunctions.

Mechanisms of neuronal damage in brain hypoxia/ischemia: focus on the role of mitochondrial calcium accumulation

Following a hypoxic-ischemic insult, the collapse of ion gradients results in the inappropriate release of excitatory neurotransmitters. Although excitatory amino acids such as glutamate are the likely extracellular mediators of the ensuing neuronal cell death, the intracellular events occurring downstream of glutamate receptor activation are much less clear. The present review attempts to summarize how Ca2+ overload of neurons following a hypoxic-ischemic insult is neurotoxic. In particular, the interlocked relation between mitochondrial Ca2+ accumulation and subsequent neuronal cell death is examined.

Endogenous calcium buffering in motoneurones of the nucleus hypoglossus from mouse

1. Simultaneous patch clamp and rapid microfluorometric calcium measurements were performed on sixty-five motoneurones in slices of the nucleus hypoglossus in the brainstem of 2- to 6-day-old mice. 2. Hypoglossal motoneurones were particularly vulnerable to mechanical or metabolic stress during isolation of in vitro slice preparations. Therefore, experimental conditions were optimized for functional integrity, as judged by spontaneous rhythmic activity of hypoglossal nerves (XII). 3. Calcium concentrations in the cell soma were monitored with a temporal resolution in the millisecond time domain during depolarizing voltage steps. Ratiometric fluorescence measurements were made using a rapid monochromator (switching tau < 10 ms), a photomultiplier tube and the calcium sensitive dyes fura-2 and mag-fura-5. 4. Dynamics of somatic calcium transients were investigated as a function of the concentration of calcium indicator dye in the cell. Decays of calcium transients were approximated to a single exponential component and decay time constants showed a linear dependence on dye concentration. The extrapolated decay time in the absence of indicator dye was 0.7 +/- 0.2 s, suggesting rapid somatic calcium dynamics under physiological conditions. 5. By a process of back-extrapolation, the 'added buffer' method, a calcium binding ratio of 41 +/- 12 (9 cells) was obtained indicating that 98% of the calcium ions entering a hypoglossal motoneurone were bound by endogenous buffers. 6. Endogenous calcium binding ratios in hypoglossal motoneurones were small compared with those of other neurones with comparable size or geometry. Accordingly, our measurements suggest that the selective vulnerability of hypoglossal motoneurones to calcium-related excitotoxicity might partially result from low concentrations of calcium buffers in these cells.

Intracellular calcium clearance in Purkinje cell somata from rat cerebellar slices

1. The mechanisms governing the return of intracellular calcium (Cai2+) to baseline levels following depolarization-evoked [Ca2+]i rises were investigated in Purkinje cell somata using tight-seal whole-cell recordings and fura-2 microfluorometry, for peak [Ca2+]i ranging from 50 nm to 2 microM. 2. Cai2+ decay was well fitted by a double exponential with time constants of O.6 and 3 s. Both time constants were independent of peak [Ca2+]i but the contribution of the faster component increased with [Ca2+]i. 3. Thapsigargin (10 microM) and cyclopiazonic acid (50 microM) prolonged Cai2+ decay indicating that sarco-endoplasmic reticulum Ca2+ (SERCA) pumps contribute to Purkinje cell Cai2+ clearance. 4. A modest participation in clearance was found for the plasma membrane Ca2+ (PMCA) pumps using 5,6-succinimidyl carboxyeosin (40 microM). 5. The Na(+)-Ca2+ exchanger also contributed to the clearance process, since replacement of extracellular Na+ by Li+ slowed Cai2+ decay. 6. Carbonyl cyanide m-chlorophenylhydrazone (CCCP, 2 microM) and rotenone (10 microM) increased [Ca2+]i and elicited large inward currents at -60 mV. Both effects were also obtained with CCCP in the absence of external Ca2+, suggesting that mitochondrial Ca2+ uptake uncouplers release Ca2+ from intracellular stores and may alter the membrane permeability to Ca2+. These effects were irreversible and impeded tests on the role of mitochondria in Cai2+ clearance. 7. The relative contribution of the clearance systems characterized in this study varied as a function of [Ca2+]i. At 0.5 microM Cai2+, SERCA pumps and the Na(+)-Ca2+ exchanger contribute equally to removal and account for 78% of the process. Only 45% of the removal at 2 microM Cai2+ can be explained by these systems. In this high [Ca2+]i range the major contribution is that of SERCA pumps (21%) and of the Na(+)-Ca2+ exchanger (18%), whereas the contribution of PMCA pumps is only 6%.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

Na+/Ca2+ exchange in catfish retina horizontal cells: regulation of intracellular Ca2+ store function

The role of the Na+/Ca2+ exchanger in intracellular Ca2+ regulation was investigated in freshly dissociated catfish retinal horizontal cells (HC). Ca2+-permeable glutamate receptors and L-type Ca2+ channels as well as inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive intracellular Ca2+ stores regulate intracellular Ca2+ in these cells. We used the Ca2+-sensitive dye fluo 3 to measure changes in intracellular Ca2+ concentration ([Ca2+]i) under conditions in which Na+/Ca2+ exchange was altered. In addition, the role of the Na+/Ca2+ exchanger in the refilling of the caffeine-sensitive Ca2+ store following caffeine-stimulated Ca2+ release was assessed. Brief applications of caffeine (1-10 s) produced rapid and transient changes in [Ca2+]i. Repeated applications of caffeine produced smaller Ca2+ transients until no further Ca2+ was released. Store refilling occurred within 1-2 min and required extracellular Ca2+. Ouabain-induced increases in intracellular Na+ concentration ([Na+]i) increased both basal free [Ca2+]i and caffeine-stimulated Ca2+ release. Reduction of external Na+ concentration ([Na+]o) further and reversibly increased [Ca2+]i in ouabain-treated HC. This effect was not abolished by the Ca2+ channel blocker nifedipine, suggesting that increases in [Na+]i promote net extracellular Ca2+ influx through a Na+/Ca2+ exchanger. Moreover, when [Na+]o was replaced by Li+, caffeine did not stimulate release of Ca2+ from the caffeine-sensitive store after Ca2+ depletion. The Na+/Ca2+ exchanger inhibitor 2',4'-dimethylbenzamil significantly reduced the caffeine-evoked Ca2+ response 1 and 2 min after store depletion.

Immunolocalization of the Na(+)-Ca2+ exchanger in mammalian myelinated axons

Previous studies on the pathophysiology of white matter anoxic injury have revealed that the Na(+)-Ca2+ exchanger is an important mediator of Ca2+ overload. To date, however, the localization of this key Ca2+ transporter in myelinated axons has not been demonstrated. The present study uses immunofluorescence labeling with a monoclonal antibody (R3F1) to the canine cardiac type I Na(+)-Ca2+ exchanger to localize exchanger protein to rat peripheral and central myelinated axons. The indirect immunofluorescence labeling technique was used to study paraformaldehyde fixed frozen cryostat sections of sciatic nerve, optic nerve and spinal cord. Examination of sciatic nerve sections with both conventional and confocal microscopy revealed a staining pattern which suggested both a glial and axonal localization of the exchanger. In the rat optic nerve, positive label was associated with cell bodies and their processes, likely glia, and with numerous finer processes arranged in parallel, running longitudinally. These finer processes likely represent axonal profiles. A similar staining pattern was observed in lateral and dorsal columns from spinal cord. Immunoelectron microscopy of dorsal root axons revealed gold particles associated with the paranodal and internodal myelin, in the axoplasm, and close to the nodal/paranodal axon membrane. The high density of Na(+)-Ca2+ exchanger demonstrated in central and peripheral myelinated mammalian axons supports the importance of this transporter in Ca2+ regulation in these tissues.

Calcium homeostatic mechanisms operating in cultured postnatal rat hippocampal neurones following flash photolysis of nitrophenyl-EGTA

1. We examined Ca2+ homeostatic mechanisms in cultured postnatal rat hippocampal neurones by monitoring the recovery of background-subtracted fluo-3 fluorescence levels at 20-22 degrees C immediately following a rapid increase in Ca2+ levels induced by flash photolysis of the caged Ca2+ compound nitrophenyl-EGTA (NP-EGTA). 2. A variety of methods or drugs were used in attempt to block specifically efflux of Ca2+ by the plasmalemmal Na(+)-Ca2+ exchanger or uptake of Ca2+ into mitochondria. 3. Many of the experimental manipulations produced a decrease in intracellular pH (pHi) measured in sister cultures using the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). Accordingly, in each case, we determined the appropriate amount of the weak base trimethylamine (TMA) required to restore baseline pHi prior to flash photolysis. 4. Blockade of the plasmalemmal Na(+)-Ca2+ exchanger by replacement of external Na+ with either Li+ or N-methyl-D-glucamine (NMDG) markedly reduced pHi but did not affect the rate of recovery of fluo-3 fluorescence intensities once pHi was restored. 5. Inhibition of mitochondrial Ca2+ uptake, using the protonophore carbonyl cyanide m-chloro-phenylhydrazone (CCCP), resulted in a reduction in pHi, which could be restored by the addition of 2 mM TMA. Under these conditions the rate of recovery of Ca2+ levels was significantly slower than in the controls. Similar results were found using the respiratory chain inhibitor rotenone. 6. We conclude that, when the potential effects of changes in pHi are taken into account, mitochondria appear to sequester significant amounts of Ca2+ in the neuronal preparations used.

Distinct contributions of high- and low-voltage-activated calcium currents to afterhyperpolarizations in cholinergic nucleus basalis neurons of the guinea pig

The contributions made by low- (LVA) and high-voltage-activated (HVA) calcium currents to afterhyperpolarizations (AHPs) of nucleus basalis (NB) cholinergic neurons were investigated in dissociated cells. Neurons with somata >25 microM were studied because 80% of them stained positively for choline acetyltransferase and had electrophysiological characteristics identical to those of cholinergic NB neurons previously recorded in basal forebrain slices. Calcium currents of cholinergic NB neurons first were dissected pharmacologically into an amiloride-sensitive LVA and at least five subtypes of HVA currents. Approximately 17% of the total HVA current was sensitive to nifedipine (3 microM), 35% to omega-conotoxin-GVIA (200-400 nM), 10% to omega-Agatoxin-IVA (100 nM), and 20% to omega-Agatoxin-IVA (300-500 nM), suggesting the presence of L-, N-, P-, and Q-type channels, respectively. A remaining current (R-type) resistant to these antagonists was blocked by cadmium (100-200 microM). We then assessed pharmacologically the role that LVA and HVA currents had in activating the apamin-insensitive AHP elicited by a long train of action potentials (sAHP) and the AHP evoked either by a short burst of action potentials or by a single action potential (mAHP) that is known to be apamin-sensitive. During sAHPs, approximately 60% of the hyperpolarization was activated by calcium flowing through N-type channels and approximately 20% through P-type channels, whereas T-, L-, and Q-type channels were not involved significantly. In contrast, during mAHPs, N- and T-type channels played key roles (approximately 60 and 30%, respectively), whereas L-, P-, and Q-type channels were not implicated significantly. It is concluded that in cholinergic NB neurons various subtypes of calcium channels can differentially activate the apamin-sensitive mAHP and the apamin-insensitive sAHP.

Distinct contributions of high- and low-voltage-activated calcium currents to afterhyperpolarizations in cholinergic nucleus basalis neurons of the guinea pig

The contributions made by low- (LVA) and high-voltage-activated (HVA) calcium currents to afterhyperpolarizations (AHPs) of nucleus basalis (NB) cholinergic neurons were investigated in dissociated cells. Neurons with somata >25 microM were studied because 80% of them stained positively for choline acetyltransferase and had electrophysiological characteristics identical to those of cholinergic NB neurons previously recorded in basal forebrain slices. Calcium currents of cholinergic NB neurons first were dissected pharmacologically into an amiloride-sensitive LVA and at least five subtypes of HVA currents. Approximately 17% of the total HVA current was sensitive to nifedipine (3 microM), 35% to omega-conotoxin-GVIA (200-400 nM), 10% to omega-Agatoxin-IVA (100 nM), and 20% to omega-Agatoxin-IVA (300-500 nM), suggesting the presence of L-, N-, P-, and Q-type channels, respectively. A remaining current (R-type) resistant to these antagonists was blocked by cadmium (100-200 microM). We then assessed pharmacologically the role that LVA and HVA currents had in activating the apamin-insensitive AHP elicited by a long train of action potentials (sAHP) and the AHP evoked either by a short burst of action potentials or by a single action potential (mAHP) that is known to be apamin-sensitive. During sAHPs, approximately 60% of the hyperpolarization was activated by calcium flowing through N-type channels and approximately 20% through P-type channels, whereas T-, L-, and Q-type channels were not involved significantly. In contrast, during mAHPs, N- and T-type channels played key roles (approximately 60 and 30%, respectively), whereas L-, P-, and Q-type channels were not implicated significantly. It is concluded that in cholinergic NB neurons various subtypes of calcium channels can differentially activate the apamin-sensitive mAHP and the apamin-insensitive sAHP.

Kinetic studies of Ca2+ binding and Ca2+ clearance in the cytosol of adrenal chromaffin cells

The Ca2+ binding kinetics of fura-2, DM-nitrophen, and the endogenous Ca2+ buffer, which determine the time course of Ca2+ changes after photolysis of DM-nitrophen, were studied in bovine chromaffin cells. The in vivo Ca2+ association rate constants of fura-2, DM-nitrophen, and the endogenous Ca2+ buffer were measured to be 5.17 x 10(8) M-1 s-1, 3.5 x 10(7) M-1 s-1, and 1.07 x 10(8) M-1 s-1, respectively. The endogenous Ca2+ buffer appeared to have a low affinity for Ca2+ with a dissociation constant around 100 microM. A fast Ca2+ uptake mechanism was also found to play a dominant role in the clearance of Ca2+ after flashes at high intracellular free Ca2+ concentrations ([Ca2+]), causing a fast [Ca2+]i decay within seconds. This Ca2+ clearance was identified as mitochondrial Ca2+ uptake. Its uptake kinetics were studied by analyzing the Ca2+ decay at high [Ca2+]i after flash photolysis of DM-nitrophen. The capacity of the mitochondrial uptake corresponds to a total cytosolic Ca2+ load of approximately 1 mM.

High endogenous calcium buffering in Purkinje cells from rat cerebellar slices

1. The ability of Purkinje cells to rapidly buffer depolarization-evoked intracellular calcium changes (delta [Ca2+]i) was estimated by titrating the endogenous buffer against incremental concentrations of the Ca(2+)-sensitive dye fura-2. 2. In cells from 15-day-old rats, pulse-evoked delta [Ca2+]i were stable during the loading with 0.5 mM fura-2 through the patch pipette. In cells from 6-day-old rats, delta [Ca2+]i decreased by approximately 50% during equivalent experiments. This decrease was not related to changes in Ca2+ influx, since the integral of the Ca2+ currents remained constant throughout the recording. 3. Experiments with high fura-2 concentrations (1.75-3.5 mM) were performed in order to obtain for each cell the curve relating delta [Ca2+]i to fura-2 concentration. From this relationship, values for the Ca2+ binding ratio (the ratio of buffer-bound Ca2+ changes over free Ca2+ changes) were calculated. 4. In Purkinje cells from 15-day-old rats, the Ca2+ binding ratio was approximately 2000, an order of magnitude larger than that of other neurones and neuroendocrine cells studied to date. This Ca2+ binding ratio was significantly smaller (approximately 900) in Purkinje cells from 6-day-old rats. 5. We propose that the large Ca2+ binding ratio of Purkinje cells is related to the presence of large concentrations of Ca2+ binding proteins and that these cells regulate their ability to handle Ca2+ loads during development through changes in the concentration of Ca2+ binding proteins.

Bcl-2 potentiates the maximal calcium uptake capacity of neural cell mitochondria

Expression of the human protooncogene bcl-2 protects neural cells from death induced by many forms of stress, including conditions that greatly elevate intracellular Ca2+. Considering that Bcl-2 is partially localized to mitochondrial membranes and that excessive mitochondrial Ca2+ uptake can impair electron transport and oxidative phosphorylation, the present study tested the hypothesis that mitochondria from Bcl-2-expressing cells have a higher capacity for energy-dependent Ca2+ uptake and a greater resistance to Ca(2+)-induced respiratory injury than mitochondria from cells that do not express this protein. The overexpression of bcl-2 enhanced the mitochondrial Ca2+ uptake capacity using either digitonin-permeabilized GT1-7 neural cells or isolated GT1-7 mitochondria by 1.7 and 3.9 fold, respectively, when glutamate and malate were used as respiratory substrates. This difference was less apparent when respiration was driven by the oxidation of succinate in the presence of the respiratory complex I inhibitor rotenone. Mitochondria from Bcl-2 expressors were also much more resistant to inhibition of NADH-dependent respiration caused by sequestration of large Ca2+ loads. The enhanced ability of mitochondria within Bcl-2-expressing cells to sequester large quantities of Ca2+ without undergoing profound respiratory impairment provides a plausible mechanism by which Bcl-2 inhibits certain forms of delayed cell death, including neuronal death associated with ischemia and excitotoxicity.

Effects of injecting calcium-buffer solution on [Ca2+]i in voltage-clamped snail neurons

We have investigated why fura-2 and Ca(2+)-sensitive microelectrodes report different values for the intracellular free calcium ion concentration ([Ca(2+)]i or its negative log, pCa(i)) of snail neurons voltage-clamped to -50 or -60 mV. Both techniques were initially calibrated in vitro, using calcium calibration solutions that had ionic concentrations similar to those of snail neuron cytoplasm. Pressure injections of the same solutions at resting and elevated [Ca(2+)]i were used to calibrate both methods in vivo. In fura-2-loaded cells these pressure injections generated changes in [Ca(2+)]i that agreed well with those expected from the in vitro calibration. Thus, using fura-2 calibrated in vitro, the average resting [Ca(2+)]i was found to be 38 nM (pCa(i) 7.42 +/- 0.05). With Ca(2+)-sensitive microelectrodes, the first injection of calibration solutions always caused a negative shift in the recorded microelectrode potential, as if the injection lowered [Ca2+]i. No such effects were seen on the fura-2 ratio. When calibrated in vivo the Ca(2+)-sensitive microelectrode gave an average resting [Ca2+]i of approximately 25 nM (pCa(i) 7.6 +/- 0.1), much lower than when calibrated in vitro. We conclude that [Ca(2+)]i in snail neurons is approximately 40 nM and that Ca(2+)-sensitive microelectrodes usually cause a leak at the point of insertion. The effects of the leak were minimized by injection of a mobile calcium buffer.

Depolarization-evoked increases in cytosolic calcium concentration in isolated smooth muscle cells of rat portal vein

1. Ca2+ current through voltage-dependent Ca2+ channels (ICa) and intracellular free Ca2+ concentration ([Ca2+]i) were measured simultaneously in rat portal vein smooth muscle cells using conventional whole-cell voltage clamp technique and high temporal resolution microfluorimetry. 2. The relationship between depolarization-evoked ICa and rise in [Ca2+]i was examined. The extracellular Ca2+ concentration dependence and the voltage dependence of the depolarization-evoked increases in ICa and [Ca2+]i were similar. Both ICa and increased [Ca2+]i were blocked to a similar extent by nimodipine and cadmium and augmented by Bay K 8644. Furthermore, the time course of the measured increase in [Ca2+]i, closely followed the increase in [Ca2+]i expected from the time-integrated ICa. These observations suggest that the depolarization-evoked rise in [Ca2+]i was tightly coupled to ICa. 3. The cytosolic Ca2+ buffering capacity, determined as the ratio of the expected increase in [Ca2+]i (from ICa) divided by the measured increase in [Ca2+]i, was over 100. Therefore, less than 1 out of 100 Ca2+ ions entering the cell appears as a free Ca2+. 4. Ryanodine (30 microM), a blocker of the Ca(2+)-induced Ca2+ release mechanism, had little effect on buffering capacity measured over the first 200 ms of the depolarizing voltage clamp pulse. Ryanodine also had little effect on the buffering capacity during 800-1000 ms of the depolarizing voltage clamp pulse. Therefore, it was concluded that there is little Ca(2+)-induced Ca2+ release from the stores in rat portal vein smooth muscle cells during depolarization-evoked Ca2+ entry. 5. During brief depolarizations, the largest [Ca2+]i increase and ICa occurred at 0 mV. However, during steady-state depolarization, the largest increase in [Ca2+]i occurred around -30 mV, and we estimate the peak steady-state ICa to be about 0.6 pA.

Synthesis, storage and release of acetylcholine at and from growth cones of rat central cholinergic neurons in culture

Neurons from the nucleus diagonal band of Broca (DBB) from new born rats protrude neuronal processes and growth cones in culture. Cytochemical observations with the light and electron microscope indicate that growth cones of these neurons take up choline, synthesize acetylcholine (ACh) and store ACh in the vesicles. Electrical stimuli at the soma of DBB neurons evoked inward currents in ACh-sensitive neurons attached to DBB growth cones. These currents were suppressed by TTX, a Ca2+ channel blocker (Cd2+), and an ACh nicotinic antagonist (C6). These results suggest that ACh is synthesized, stored and released from the growth cones of DBB neurons prior to synapse formation.

The use of fura-2 for estimating Ca buffers and Ca fluxes

The compound fura-2 (Grynkiewicz et al., J. Biol. Chem. 260, 3440-3450, 1985) is generally known as an indicator dye for measuring the concentration of free calcium ([Ca2+]) inside living cells. It should be appreciated, however, that this is not what it actually is. More accurately, it is a divalent metal ion chelator which changes its fluorescence properties upon complexation. Thus, [Ca2+] has to be inferred indirectly by means of the law of mass action. As a chelator, fura-2 may influence the quantity of interest, the Ca signal. On the other hand, the chelator action may be used for a number of other purposes, some of them more directly related to its molecular properties: as a chelator, competing with endogenous Ca buffers, it can be used to estimate endogenous buffers and their properties. When present at sufficiently high concentration, such that it outcompetes endogenous buffers, fura-2 reports total Ca changes and is a probe for Ca fluxes across the membrane. Here, theory and methodological considerations of such applications of fura-2 will be summarized and results on Ca buffer and Ca flux measurements derived from various methods will be compared.

Localization and functional significance of the Na+/Ca2+ exchanger in presynaptic boutons of hippocampal cells in culture

Immunocytochemical evidence for localized distribution of the Na+/Ca2+ exchange protein in nerve terminals of cultured hippocampal cells is presented together with results on the functional relevance of the exchanger in the control of [Ca2+]i and of synaptic vesicle recycling. The monoclonal antibody R3F1, directed against an epitope on the intracellular loop of the protein, revealed higher densities of expression in synaptic regions than in other parts of the neurons. Removal of extracellular Na+ produced enhanced and prolonged elevation of [Ca2+]i in nerve terminals during and after electrical stimulation of the cells. Correspondingly, initial rates of exocytosis, measured by fluorescence changes of FM 1-43 during stimulation, were faster in LiCl-containing solution than in NaCl-containing solution. By contrast, endocytosis at 20 s was the same in both solutions.

Na+ dependent Ca2+ influx induced by depolarization in neurons dissociated from rat nucleus basalis

Neurons were acutely dissociated from the rat nucleus basalis, and whole-cell patch clamp recordings were made. Voltage dependent calcium currents (ICa) were recorded and fura-2 microfluorimetric recordings of intracellular free Ca2+ concentration ([Ca2+]i) were made at the same time. In Na(+)-containing solution, a depolarization from -60 to +40 mV evoked the maximal increase in [Ca2+]i, and this decreased to 43% of the maximal with a large depolarization to +120 mV. The [Ca2+]i increase induced by the large depolarization (+20 to +120 mV) was inhibited by perfusion of Na(+)-free external solution, and was less when the recording pipette contained a peptide (PRLLFYKYVYKRYRAGKQRG, named XIP) known to inhibit Na/Ca exchange. These results suggest that the [Ca2+]i increase by the large depolarization is mediated by reverse operation of Na/Ca exchange (Ca2+ inward and Na+ outward).

Analysis of Ca2+ homeostasis in neurons dissociated from rat nucleus basalis

Whole-cell patch-clamp recordings of calcium currents (ICa) and fura-2 microfluorimetric measurements of intracellular free Ca2+ concentration ([Ca2+]i) were made simultaneously in neurons acutely dissociated from rat nucleus basalis. Depolarization activated ICa and caused an increase in [Ca2+]i. The relationship between total Ca2+ influx and the increase in [Ca2+]i was studied. After repolarization, [Ca2+]i recovered to control values within a few seconds. A mathematical model was constructed to simulate the mechanisms underlying [Ca2+]i regulation; the parameters were (1) the rate of Ca2+ influx, (2) the rate of the [Ca2+]i increase by the Ca2+ influx, and (3) the rate of Ca2+ clearance from cytosol due to extrusion across the plasma membrane and sequestration into calcium storing organelles. After an appropriate evaluation of parameter values from the experimental results, the model mimicked the processes of [Ca2+]i increase and recovery. The experimental results and simulations suggest that (1) neurons possess a large Ca2+ buffering capacity, (2) systems for Ca2+ clearance are activated by the Ca2+ influx in a saturable manner, (3) the rate of Ca2+ clearance is relatively small compared to the rate of Ca2+ influx evoked by depolarizations, and (4) the shoulder in the [Ca2+]i recovery phase is due to the asymptote of the Ca2+ clearance rate.

Intracellular calcium and its sodium-independent regulation in voltage-clamped snail neurones

1. We have used both Ca(2+)-sensitive microelectrodes and fura-2 to measure the intracellular free calcium ion concentration ([Ca2+]i or its negative log, pCai) of snail neurones voltage clamped to -50 or -60 mV. Using Ca(2+)-sensitive microelectrodes, [Ca2+]i was found to be approximately 174 nM and pCai, 6.76 +/- 0.09 (mean +/- S.E.M.; n = 11); using fura-2, [Ca2+]i was approximately 40 nM and pCai, 7.44 +/- 0.06 (mean +/- S.E.M., n = 10). 2. Depolarizations (1-20 s) caused an increase in [Ca2+]i which was abolished by removal of extracellular Ca2+, indicating that the rise in [Ca2+]i was due to Ca2+ influx through voltage-activated Ca2+ channels. 3. Caffeine (10-20 mM) caused an increase in [Ca2+]i in the presence or absence of extracellular Ca2+. The effects of caffeine on [Ca2+]i could be prevented by ryanodine. 4. Thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a small increase in resting [Ca2+]i and slowed the rate of recovery from Ca2+ loads following 20 s depolarizations. 5. Neither replacement of extracellular sodium with N-methyl-D-glucamine (NMDG), nor loading the cells with intracellular sodium, had any effect on resting [Ca2+]i or the rate of recovery of [Ca2+]i following depolarizations. 6. The mitochondrial uncoupling agent carbonyl cyanide m-chlorophenylhydrazone (CCmP) caused a small gradual rise in resting [Ca2+]i. Removal of extracellular sodium during exposure to CCmP had no further effect on [Ca2+]i. 7. Intracellular orthovanadate caused an increase in resting [Ca2+]i and prevented the full recovery of [Ca2+]i following small Ca2+ loads, but removal of extracellular sodium did not cause a rise in [Ca2+]i. We conclude that there is no Na(+)-Ca2+ exchanger present in the cell body of these neurones and that [Ca2+]i is maintained by an ATP-dependent Ca2+ pump.

Functional availability of sodium channels modulated by cytosolic free Ca2+ in cultured mammalian neurons (N1E-115)

1. Whole-cell sodium currents (INa) were measured in mouse neuroblastoma cells (N1E-115) at different [Ca2+]i values using appropriate Ca-EGTA buffers in the pipettes. 2. INa was found to be larger at pCa 7 than at pCa 8 or 9 with a ratio of 1:0.65 or 0.55, respectively. The steady-state inactivation (h infinity curve) was independent of [Ca2+]i, thus excluding surface charge effects as a cause of the Ca2+ effect. 3. Recovery of INa from slow inactivation after changing from resting (-30 to -40 mV) to holding potential (-70 mV) occurred in a similar way at all pCa values. The Ca2+ effect appears to be independent of slow inactivation and to occur within the first 2 min of pipette buffer-cytoplasm equilibration. 4. The cell membrane capacitance (Cm) was independent of [Ca2+]i, thus excluding exo- or endocytosis of sodium channel-containing membrane as a cause of the Ca2+ effect. 5. Non-stationary fluctuation analysis was used to determine simultaneously the single channel current (iNa) and the size of INa. At pCa values of 7 and 9, iNa was identical, i.e. 0.59 and 0.58 pA, while INa/Cm differed, i.e. 41.1 and 22.2 pA pF-1, respectively. The peak open probability at 0 mV was about 0.5 for both pCa values indicating that [Ca2+]i controls the fraction of channels available for activation. 6. Since [Ca2+]i in other neurons varies between 30 and 100 nM in the resting and active state, respectively, the present data suggest a modulatory role for [Ca2+]i in neuronal excitability.

Ca2+ regulation in the presynaptic terminals of goldfish retinal bipolar cells

1. To investigate regulation of the intracellular free Ca2+ concentration ([Ca2+]i) in presynaptic terminals, the Ca2+ current (ICa) and [Ca2+]i in axon terminals were simultaneously monitored in acutely dissociated retinal bipolar cells under whole-cell voltage clamp. 2. The recovery phase of the Ca2+ transient, which was evoked by activation of ICa, became slower when the Na(+)-Ca2+ exchanger was suppressed by removing extracellular Na+. 3. Inhibition of the plasma membrane Ca2+ pump produced by raising extracellular pH to 8.4 increased the basal [Ca2+]i and caused incomplete recovery from the Ca2+ transient. These effects were not observed in orthovanadate-loaded bipolar cells. 4. The Ca2+ transient was not significantly affected by ryanodine, caffeine, thapsigargin, Ruthenium Red or FCCP. Internal Ca2+ stores may not participate in shaping the Ca2+ transient. 5. The ratio of the peak amplitude of the Ca2+ transient to the total amount of Ca2+ influx became smaller as the size of the Ca2+ influx increased. This action was not affected by blockage of Ca2+ transporters in the plasma membrane, or by reduction of the rate of Ca2+ influx. The peak amplitude of the Ca2+ transient seemed to be determined by Ca2+ buffering substances with a positive co-operativity.

Potentiation of Ca2+ transients in the presynaptic terminals of goldfish retinal bipolar cells

1. To study a possible contribution of intracellular Ca2+ stores to the presynaptic Ca2+ regulation, the Ca2+ current (ICa) and the intracellular free Ca2+ concentration ([Ca2+]i) were simultaneously monitored in isolated goldfish retinal bipolar cells using the whole-cell voltage clamp procedure and fura-2 fluorimetry. 2. The Ca2+ transient triggered by the activation of ICa was potentiated when [Ca2+]i was increased by applying either a prepulse or a small steady depolarization. The potentiation seemed to be partly due to the release of Ca2+ from intracellular Ca2+ stores. 3. The intracellular Ca2+ release was reversibly inhibited by caffeine but was not affected by ryanodine, suggesting that Ca2+ is released through intracellular Ca2+ channels which differ from ryanodine receptor channels. 4. These results suggest that the intracellular Ca2+ release may contribute to the facilitation of transmitter release.

Calcium homeostasis in the presence of fura-2 in neurons dissociated from rat nucleus basalis: theoretical and experimental analysis of chelating action of fura-2

Intracellular calcium ions (Ca2+) play important roles in cell functions. Measurements of intracellular calcium ion concentration ([Ca2+]i) are often made with the fura-2 fluorescence recording technique in various preparations including neurons. Fura-2 has, however, a Ca(2+)-chelating action which complicates the interpretation of experimental results. In this report the chelating action of intracellular fura-2 was studied by means of computer simulations. The chelating action of an endogenous Ca(2+)-binding protein, calmodulin, was also estimated. Furthermore, whole-cell patch-clamp recordings of calcium currents (ICa) and fura-2 microfluorimetric recordings of [Ca2+]i were simultaneously made from neurons which were acutely dissociated from the rat nucleus basalis. Since Ca2+ influx can be initiated and terminated by using the voltage-clamp technique, the relationship between Ca2+ influx and rapid [Ca2+]i increase was examined. The present theoretical evaluations and experimental results disclosed the relationship between fura-2 and endogenous Ca(2+)-binding proteins; fura-2 at low concentration (10 microM) did not substantially affect the endogenous Ca2+ buffering mechanisms, but at high concentration (200 microM) effectively buffered cytosolic Ca2+ instead of endogenous Ca2+ buffers. Calcium homeostasis in neurons is furthermore discussed.

Cytoplasmic Ca2+ activity regulation as measured by a calcium-activated current

Calcium-activated non-selective cation (CAN) currents were activated by quantitative injections of Ca2+ into voltage clamped bursting neurons of the snails Helix aspersa or Helix pomatia. Membrane potential was held at the potassium equilibrium potential and CAN currents were fit with a rising and falling exponential function. Ca2+ transporters and pumps of the cell membrane, endoplasmic reticulum, and mitochondria were selectively blocked with pharmacological agents. Bath solutions containing 0 Na Ringers, chlorpromazine, Na3VO4, or thapsigargin did not significantly change the CAN current decay constants from those measured in Ringers. External 2,4-dinitrophenol or internal ruthenium red, however, significantly lengthened the CAN current decay constant. It is concluded that mitochondria are the most important sink for sub-membrane Ca2+ activity in the range necessary to effectively activate CAN currents.

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.