Localization of calcium in presynaptic nerve terminals. An ultrastructural and electron microprobe analysis

Roles and Sources of Calcium in Synaptic Exocytosis

Calcium ions (Ca2+) play a critical role in triggering neurotransmitter release. The rate of release is directly related to the concentration of Ca2+ at the presynaptic site, with a supralinear relationship. There are two main sources of Ca2+ that trigger synaptic vesicle fusion: influx through voltage-gated Ca2+ channels in the plasma membrane and release from the endoplasmic reticulum via ryanodine receptors. This chapter will cover the sources of Ca2+ at the presynaptic nerve terminal, the relationship between neurotransmitter release rate and Ca2+ concentration, and the mechanisms that achieve the necessary Ca2+ concentrations for triggering synaptic exocytosis at the presynaptic site.

Presynaptic endoplasmic reticulum regulates short-term plasticity in hippocampal synapses

Short-term plasticity preserves a brief history of synaptic activity that is communicated to the postsynaptic neuron. This is primarily regulated by a calcium signal initiated by voltage dependent calcium channels in the presynaptic terminal. Imaging studies of CA3-CA1 synapses reveal the presence of another source of calcium, the endoplasmic reticulum (ER) in all presynaptic terminals. However, the precise role of the ER in modifying STP remains unexplored. We performed in-silico experiments in synaptic geometries based on reconstructions of the rat CA3-CA1 synapses to investigate the contribution of ER. Our model predicts that presynaptic ER is critical in generating the observed short-term plasticity profile of CA3-CA1 synapses and allows synapses with low release probability to operate more reliably. Blocking the ER lowers facilitation in a manner similar to what has been previously characterized in animal models of Alzheimer's disease and underscores the important role played by presynaptic stores in normal function.

New insights into the regulation of synaptic transmission and plasticity by the endoplasmic reticulum and its membrane contacts

Mammalian neurons are highly compartmentalized yet very large cells. To provide each compartment with its distinct properties, metabolic homeostasis and molecular composition need to be precisely coordinated in a compartment-specific manner. Despite the importance of the endoplasmic reticulum (ER) as a platform for various biochemical reactions, such as protein synthesis, protein trafficking, and intracellular calcium control, the contribution of the ER to neuronal compartment-specific functions and plasticity remains elusive. Recent advances in the development of live imaging and serial scanning electron microscopy (sSEM) analysis have revealed that the neuronal ER is a highly dynamic organelle with compartment-specific structures. sSEM studies also revealed that the ER forms contacts with other membranes, such as the mitochondria and plasma membrane, although little is known about the functions of these ER-membrane contacts. In this review, we discuss the mechanisms and physiological roles of the ER structure and ER-mitochondria contacts in synaptic transmission and plasticity, thereby highlighting a potential link between organelle ultrastructure and neuronal functions.

Pleiotropic Mitochondria: The Influence of Mitochondria on Neuronal Development and Disease

Mitochondria play many important biological roles, including ATP production, lipid biogenesis, ROS regulation, and calcium clearance. In neurons, the mitochondrion is an essential organelle for metabolism and calcium homeostasis. Moreover, mitochondria are extremely dynamic and able to divide, fuse, and move along microtubule tracks to ensure their distribution to the neuronal periphery. Mitochondrial dysfunction and altered mitochondrial dynamics are observed in a wide range of conditions, from impaired neuronal development to various neurodegenerative diseases. Novel imaging techniques and genetic tools provide unprecedented access to the physiological roles of mitochondria by visualizing mitochondrial trafficking, morphological dynamics, ATP generation, and ultrastructure. Recent studies using these new techniques have unveiled the influence of mitochondria on axon branching, synaptic function, calcium regulation with the ER, glial cell function, neurogenesis, and neuronal repair. This review provides an overview of the crucial roles played by mitochondria in the CNS in physiological and pathophysiological conditions.

TPR-containing proteins control protein organization and homeostasis for the endoplasmic reticulum

The endoplasmic reticulum (ER) is a complex, multifunctional organelle comprised of a continuous membrane and lumen that is organized into a number of functional regions. It plays various roles including protein translocation, folding, quality control, secretion, calcium signaling, and lipid biogenesis. Cellular protein homeostasis is maintained by a complicated chaperone network, and the largest functional family within this network consists of proteins containing tetratricopeptide repeats (TPRs). TPRs are well-studied structural motifs that mediate intermolecular protein-protein interactions, supporting interactions with a wide range of ligands or substrates. Seven TPR-containing proteins have thus far been shown to localize to the ER and control protein organization and homeostasis within this multifunctional organelle. Here, we discuss the roles of these proteins in controlling ER processes and organization. The crucial roles that TPR-containing proteins play in the ER are highlighted by diseases or defects associated with their mutation or disruption.

The axonal endoplasmic reticulum: One organelle-many functions in development, maintenance, and plasticity

The endoplasmic reticulum (ER) is highly conserved in eukaryotes and neurons. Indeed, the localization of the organelle in axons has been known for nearly half a century. However, the relevance of the axonal ER is only beginning to emerge. In this review, we discuss the structure of the ER in axons, examining the role of ER-shaping proteins and highlighting reticulons. We analyze the multiple functions of the ER and their potential contribution to axonal physiology. First, we examine the emerging roles of the axonal ER in lipid synthesis, protein translation, processing, quality control, and secretory trafficking of transmembrane proteins. We also review the impact of the ER on calcium dynamics, focusing on intracellular mechanisms and functions. We describe the interactions between the ER and endosomes, mitochondria, and synaptic vesicles. Finally, we analyze available proteomic data of axonal preparations to reveal the dynamic functionality of the ER in axons during development. We suggest that the dynamic proteome and a validated axonal interactome, together with state-of-the-art methodologies, may provide interesting research avenues in axon physiology that may extend to pathology and regeneration. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 181-208, 2018.

Toxicological effects of pyrethroids on non-target aquatic insects

The toxicological effects of pyrethroids on non-target aquatic insects are mediated by several modes of entry of pyrethroids into aquatic ecosystems, as well as the toxicological characteristics of particular pyrethroids under field conditions. Toxicokinetics, movement across the integument of aquatic insects, and the toxicodynamics of pyrethroids are discussed, and their physiological, symptomatic and ecological effects evaluated. The relationship between pyrethroid toxicity and insecticide uptake is not fully defined. Based on laboratory and field data, it is likely that the susceptibility of aquatic insects (vector and non-vector) is related to biochemical and physiological constraints associated with life in aquatic ecosystems. Understanding factors that influence aquatic insects susceptibility to pyrethroids is critical for the effective and safe use of these compounds in areas adjacent to aquatic environments.

Physiology and pathology of calcium signaling in the brain

Calcium (Ca(2+)) plays fundamental and diversified roles in neuronal plasticity. As second messenger of many signaling pathways, Ca(2+) as been shown to regulate neuronal gene expression, energy production, membrane excitability, synaptogenesis, synaptic transmission, and other processes underlying learning and memory and cell survival. The flexibility of Ca(2+) signaling is achieved by modifying cytosolic Ca(2+) concentrations via regulated opening of plasma membrane and subcellular Ca(2+) sensitive channels. The spatiotemporal patterns of intracellular Ca(2+) signals, and the ultimate cellular biological outcome, are also dependent upon termination mechanism, such as Ca(2+) buffering, extracellular extrusion, and intra-organelle sequestration. Because of the central role played by Ca(2+) in neuronal physiology, it is not surprising that even modest impairments of Ca(2+) homeostasis result in profound functional alterations. Despite their heterogeneous etiology neurodegenerative disorders, as well as the healthy aging process, are all characterized by disruption of Ca(2+) homeostasis and signaling. In this review we provide an overview of the main types of neuronal Ca(2+) channels and their role in neuronal plasticity. We will also discuss the participation of Ca(2+) signaling in neuronal aging and degeneration.

Urban planning of the endoplasmic reticulum (ER): how diverse mechanisms segregate the many functions of the ER

The endoplasmic reticulum (ER) is the biggest organelle in most cell types, but its characterization as an organelle with a continuous membrane belies the fact that the ER is actually an assembly of several, distinct membrane domains that execute diverse functions. Almost 20 years ago, an essay by Sitia and Meldolesi first listed what was known at the time about domain formation within the ER. In the time that has passed since, additional ER domains have been discovered and characterized. These include the mitochondria-associated membrane (MAM), the ER quality control compartment (ERQC), where ER-associated degradation (ERAD) occurs, and the plasma membrane-associated membrane (PAM). Insight has been gained into the separation of nuclear envelope proteins from the remainder of the ER. Research has also shown that the biogenesis of peroxisomes and lipid droplets occurs on specialized membranes of the ER. Several studies have shown the existence of specific marker proteins found on all these domains and how they are targeted there. Moreover, a first set of cytosolic ER-associated sorting proteins, including phosphofurin acidic cluster sorting protein 2 (PACS-2) and Rab32 have been identified. Intra-ER targeting mechanisms appear to be superimposed onto ER retention mechanisms and rely on transmembrane and cytosolic sequences. The crucial roles of ER domain formation for cell physiology are highlighted with the specific targeting of the tumor metastasis regulator gp78 to ERAD-mediating membranes or of the promyelocytic leukemia protein to the MAM.

Inhibition of mitochondrial Na+-dependent Ca²+ efflux by 17β-estradiol in the rat hippocampus

Our results, as well as those of others, have indicated that 17β-estradiol (E2) exerts its nongenomic effects in neuronal cells by affecting plasma membrane Ca(2+) flux. In neuronal cells mitochondria possess Ca(2+) buffering properties as they both sequester and release Ca(2+). The goal of this study was to examine the rapid non-genomic effect of E2 on mitochondrial Ca(2+) transport in hippocampal synaptosomes from ovariectomised rats. In addition, we aimed to determine if, and to what extent, E2 receptors participated in mitochondrial Ca(2+) transport modulation by E2 in vitro. E2-specific binding and Ca(2+) transport was monitored. At physiological E2 concentrations (0.1-1.5 nmol/L), specific E2 binding to mitochondria isolated from hippocampal synaptosomes was detected with a B(max.) and K(m) of 37.6±2.6 fmol/mg protein and 0.69±0.14 nmol/L of free E2, respectively. The main mitochondrial Ca(2+) influx mechanism is the Ruthenium Red-sensitive uniporter driven by mitochondrial membrane potential. Despite no effect of E2 on Ca(2+) influx, a physiological E2 concentration (0.5 nmol/L) protected mitochondrial membrane potential and consequently Ca(2+) influx from the uncoupling agent carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (1 μmol/L). In neuronal cells the predominant mitochondrial Ca(2+) efflux mechanism is the Na(+)/Ca(2+) exchanger. E2 caused Ca(2+) efflux inhibition (by 46%) coupled with increased affinity of the Na(+)/Ca(2+) exchanger for Na(+). Using E2 receptor (ERα and ERβ) antagonists and agonists, we confirmed ERβ's involvement in E2-induced mitochondrial membrane potential protection as well as Ca(2+) efflux inhibition. In summary, our results indicate that the non-genomic neuromodulatory role of E2 in rat hippocampus is achieved by affecting mitochondrial Ca(2+) transport via, in part, mitochondrial ERβ.

Use-dependent control of presynaptic calcium signalling at central synapses

Voltage-gated Ca(2+) channels activated by action potentials evoke Ca(2+) entry into presynaptic terminals thus briefly distorting the resting Ca(2+) concentration. When this happens, a number of processes are initiated to re-establish the Ca(2+) equilibrium. During the post-spike period, the increased Ca(2+) concentration could enhance the presynaptic Ca(2+) signalling. Some of the mechanisms contributing to presynaptic Ca(2+) dynamics involve endogenous Ca(2+) buffers, Ca(2+) stores, mitochondria, the sodium-calcium exchanger, extraterminal Ca(2+) depletion and presynaptic receptors. Additionally, subthreshold presynaptic depolarization has been proposed to have an effect on release of neurotransmitters through a mechanism involving changes in resting Ca(2+). Direct evidence for the role of any of these participants in shaping the presynaptic Ca(2+) dynamics comes from direct recordings of giant presynaptic terminals and from fluorescent Ca(2+) imaging of axonal boutons. Here, some of this evidence is presented and discussed.

A constitutive, transient receptor potential-like Ca2+ influx pathway in presynaptic nerve endings independent of voltage-gated Ca2+ channels and Na+/Ca2+ exchange

Calcium levels in the presynaptic nerve terminal are altered by several pathways, including voltage-gated Ca(2+) channels, the Na(+)/Ca(2+) exchanger, Ca(2+)-ATPase, and the mitochondria. The influx pathway for homeostatic control of [Ca(2+)](i) in the nerve terminal has been unclear. One approach to detecting the pathway that maintains internal Ca(2+) is to test for activation of Ca(2+) influx following Ca(2+) depletion. Here, we demonstrate that a constitutive influx pathway for Ca(2+) exists in presynaptic terminals to maintain internal Ca(2+) independent of voltage-gated Ca(2+) channels and Na(+)/Ca(2+) exchange, as measured in intact isolated nerve endings from mouse cortex and in intact varicosities in a neuronal cell line using fluorescence spectroscopy and confocal imaging. The Mg(2+) and lanthanide sensitivity of the influx pathway, in addition to its pharmacological and short hairpin RNA sensitivity, and the results of immunostaining for transient receptor potential (TRP) channels indicate the involvement of TRPC channels, possibly TRPC5 and TRPC1. This constitutive Ca(2+) influx pathway likely serves to maintain synaptic function under widely varying levels of synaptic activity.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

A new cytochemical method for ultrastructural localization of Co2+ in rat hippocampal CA1 pyramidal neurons in vitro

This paper describes new cytochemical method for the ultrastructural localization of Co(2+) following blockade of synaptic transmission. In the CA1 region of rat hippocampal slices, electrical stimulation of the Schaffer collaterals elicited field excitatory postsynaptic potentials (fEPSPs). The fEPSPs were completely blocked within 2 min after the addition of Co(2+) (2 mM). The slice was then fixed and precipitated Co(2+) was examined by means of a solution containing 2.5% glutaraldehyde and 10 mM K(3)[Fe(3+)(CN)(6)] in 90 mM NaCl. Electron spectroscopic imaging confirmed Co in the precipitate. The precipitates were found as clusters on the membranes of the fine apical dendrites and their spine heads of CA1 pyramidal neurons. No clustered precipitate was found when slices were treated: (1) without Co(2+); (2) after recovery from the Co(2+)-induced blockade of fEPSPs; (3) without electrical stimulation of the Schaffer collaterals; and (4) with dl-2-amino-5-phosphonopentanoate and 6-cyano-7-nitroquinoxaline-2,3-dione. After administrating glutamate (5 mM) in the presence of tetrodotoxin (1 microM) and Co(2+), precipitates were found on dendritic membranes and spine heads. These results indicate that the Schaffer collaterals stimulation induces the binding of Co(2+) on CA1 pyramidal neuron membrane.

Low- and high-affinity reactions in rapid neurotransmission

Until 1950-1960, most physiologists were reluctant to accept chemical neurotransmission. They believed that chemical reactions were much too slow to account for the speed of synaptic processes. The first breakthrough was to discover the impressive velocity of acetylcholinesterase. Then, nicotinic receptors provided an example of complex ultrarapid reactions: fast activation at a low ligand affinity, then desensitization if the ligand is not rapidly removed. Here, we describe synaptic transmission as a chain of low-affinity rapid reactions, assisted by many slower regulatory processes. For starting quantal acetylcholine release, mediatophores are activated by high Ca2+ concentrations, but they desensitize at a higher affinity if Ca2+ remains present. Several mechanisms concur to the rapid removal of Ca2+ from the submembrane microdomains. Among them, the Ca2+/H+ antiport is a typical low-affinity, high-speed process that certainly contributes to the rapidity of neurotransmission.

Effect of antidepressants on ATP-dependent calcium uptake by neuronal endoplasmic reticulum

This study investigated the effect of tricyclic and atypical antidepressants on adenosine triphosphate (ATP) dependent calcium uptake by the endoplasmic reticulum of lysed synaptosomes from rat brain cortex. Tricyclic antidepressants (imipramine, desipramine, clomipramine, amitriptyline) exhibited no effect in the lower range (0.06 to 2 µM) of drug concentrations, and a concentration-dependent inhibition of calcium uptake in the upper range (6 to 200 µM). A concentration-dependent inhibition was observed for atypical antidepressants (mianserin, desmethylmianserin, venlafaxine, desmethylvenlafaxine, fluoxetine) in both the lower and the upper range of drug concentrations. Since no stimulation of calcium uptake was observed in either concentration range, it appears that the tricyclic and atypical antidepressants tested are not capable of normalizing, through their effect on the endoplasmic reticulum, an overactive calcium signal, which is possibly implicated in the etiology of affective disorders. Also, although only marginal inhibition of calcium uptake is expected at brain concentrations of tricyclics and mianserin–desmethylmianserin that are likely to be encountered during clinical use, a more substantial inhibition could occur with fluoxetine.Key words: adenosine triphosphate-dependent calcium uptake, neuronal endoplasmic reticulum, lysed brain synaptosomes, tricyclic antidepressants, atypical antidepressants.

Structural complexity and functional diversity of endoplasmic reticulum Ca(2+) stores

Considerable evidence, including recent direct observations, suggest that endoplasmic reticulum (ER) Ca(2+) stores in neurons, glia, and other cell types, consists of spatially-distinct compartments that can be individually loaded and unloaded. In addition, sub-plasmalemmal ('junctional') components of the ER (jER) are functionally coupled to the overlying plasmalemmal (PL) microdomains in PL-jER units named 'PLasmERosomes'. The PL microdomains and the jER contain clusters of specific transport proteins that regulate Na(+) and Ca(2+) concentrations in the tiny cytosolic space between the PL and jER. This organization helps the ER to produce the many types of complex local and global Ca(2+) signals that are responsible for the simultaneous control of numerous neuronal and glial functions.

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Calcium stores in hippocampal synaptic boutons mediate short-term plasticity, store-operated Ca2+ entry, and spontaneous transmitter release

Evoked transmitter release depends upon calcium influx into synaptic boutons, but mechanisms regulating bouton calcium levels and spontaneous transmitter release are obscure. To understand these processes better, we monitored calcium transients in axons and presynaptic terminals of pyramidal neurons in hippocampal slice cultures. Action potentials reliably evoke calcium transients in axons and boutons. Calcium-induced calcium release (CICR) from internal stores contributes to the transients in boutons and to paired-pulse facilitation of EPSPs. Store depletion activates store-operated calcium channels, influencing the frequency of spontaneous transmitter release. Boutons display spontaneous Ca2+ transients; blocking CICR reduces the frequency of these transients and of spontaneous miniature synaptic events. Thus, spontaneous transmitter release is largely calcium mediated, driven by Ca2+ release from internal stores. Bouton store release is important for short-term synaptic plasticity and may also contribute to long-term plasticity.

Functional IP3- and ryanodine-sensitive calcium stores in presynaptic varicosities of NG108-15 (rodent neuroblastoma x glioma hybrid) cells

Presynaptic varicosities of the model neuronal cell line NG108-15, a cholinergic neuroblastoma cell x glioma cell hybrid capable of innervating striated myotubes, were examined for the presence of inositol 1,4,5-trisphosphate (IP3)-sensitive and Ca2+-activated (ryanodine-sensitive) Ca2+ stores using confocal microscopic imaging of Ca2+-sensitive fluorescent dye loaded into the cells. Initial demonstration of the presence of IP3 receptors and ryanodine receptors in the NG108-15 varicosities was obtained using immunocytochemistry. Treatment of NG108-15 cells with bradykinin (0.1 microM), whose receptor is linked to IP3 generation, and separately, caffeine (10 mM), an activator of endoplasmic reticulum ryanodine receptors, resulted in substantial increases in [Ca2+]i in the varicosities. K+-evoked changes in [Ca2+]i in the varicosities were reduced (52 %) after emptying the ryanodine-sensitive Ca2+ store using caffeine (10 mM), but were not affected by prior depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM). Bradykinin-induced changes in [Ca2+]i were abolished following depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM) and were reduced (72 %) by prior emptying of the ryanodine-sensitive Ca2+ store with caffeine (10 mM). The same results were obtained when the varicosities of the NG108-15 cells had formed synaptic junctions with co-cultured rat hindlimb myotubes. Taken together, the results suggest that, in the varicosities, activation of the IP3 pathway evoked the release of Ca2+ from the IP3-sensitive store, which, in turn, secondarily induced the release of Ca2+ from the ryanodine-sensitive store via Ca2+-induced Ca2+ release, and that depolarization-induced Ca2+ entry evoked Ca2+-induced Ca2+ release only from the ryanodine-sensitive store. Thus, functional internal Ca2+ stores are inherent components of presynaptic varicosities in this neural cell line.

Functional IP3- and ryanodine-sensitive calcium stores in presynaptic varicosities of NG108-15 (rodent neuroblastoma x glioma hybrid) cells

Presynaptic varicosities of the model neuronal cell line NG108-15, a cholinergic neuroblastoma cell x glioma cell hybrid capable of innervating striated myotubes, were examined for the presence of inositol 1,4,5-trisphosphate (IP3)-sensitive and Ca2+-activated (ryanodine-sensitive) Ca2+ stores using confocal microscopic imaging of Ca2+-sensitive fluorescent dye loaded into the cells. Initial demonstration of the presence of IP3 receptors and ryanodine receptors in the NG108-15 varicosities was obtained using immunocytochemistry. Treatment of NG108-15 cells with bradykinin (0.1 microM), whose receptor is linked to IP3 generation, and separately, caffeine (10 mM), an activator of endoplasmic reticulum ryanodine receptors, resulted in substantial increases in [Ca2+]i in the varicosities. K+-evoked changes in [Ca2+]i in the varicosities were reduced (52 %) after emptying the ryanodine-sensitive Ca2+ store using caffeine (10 mM), but were not affected by prior depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM). Bradykinin-induced changes in [Ca2+]i were abolished following depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM) and were reduced (72 %) by prior emptying of the ryanodine-sensitive Ca2+ store with caffeine (10 mM). The same results were obtained when the varicosities of the NG108-15 cells had formed synaptic junctions with co-cultured rat hindlimb myotubes. Taken together, the results suggest that, in the varicosities, activation of the IP3 pathway evoked the release of Ca2+ from the IP3-sensitive store, which, in turn, secondarily induced the release of Ca2+ from the ryanodine-sensitive store via Ca2+-induced Ca2+ release, and that depolarization-induced Ca2+ entry evoked Ca2+-induced Ca2+ release only from the ryanodine-sensitive store. Thus, functional internal Ca2+ stores are inherent components of presynaptic varicosities in this neural cell line.

Microheterogeneity of calcium signalling in dendrites

Transient changes in the intracellular concentration of free Ca2+ ([Ca2+]i) originating from voltage- or ligand-gated influx and by ligand- or Ca2+-gated release from intracellular stores, trigger or modulate many fundamental neuronal processes, including neurotransmitter release and synaptic plasticity. Of the intracellular compartments involved in Ca2+ clearance, the endoplasmic reticulum (ER) has received the most attention because it expresses Ca2+ pumps and Ca2+ channels, thus endowing it with the potential to act as both an intracellular calcium sink and store. We review here our ongoing work on the role of calcium sequestration into, and release from, ER cisterns and the role that this plays in the generation and termination of free [Ca2+]i transients in dendrites of pyramidal neurons in hippocampal slices during and after synaptic activity. These studies have been approached by combining parallel microfluorometric measurements of free cytosolic [Ca2+]i transients with energy-dispersive X-ray microanalytical measurements of total Ca content within specific dendritic compartments at the electron microscopy level. Our observations support the emerging realization that specific subsets of dendritic ER cisterns provide spatial and temporal microheterogeneity of Ca2+ signalling, acting not only as a major intracellular Ca sink involved in active clearance mechanisms after voltage- and ligand-gated Ca2+ influx, but also as an intracellular Ca2+ source that can be mobilized by a signal cascade originating at activated synapses.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Ca2+ homeostasis in the agonist-sensitive internal store: functional interactions between mitochondria and the ER measured In situ in intact cells

Mitochondria have a well-established capacity to detect cytoplasmic Ca2+ signals resulting from the discharge of ER Ca2+ stores. Conversely, both the buffering of released Ca2+ and ATP production by mitochondria are predicted to influence ER Ca2+ handling, but this complex exchange has been difficult to assess in situ using conventional measurement techniques. Here we have examined this interaction in single intact BHK-21 cells by monitoring intraluminal ER [Ca2+] directly using trapped fluorescent low-affinity Ca2+ indicators. Treatment with mitochondrial inhibitors (FCCP, antimycin A, oligomycin, and rotenone) dramatically prolonged the refilling of stores after release with bradykinin. This effect was largely due to inhibition of Ca2+ entry pathways at the plasma membrane, but a significant component appears to arise from reduction of SERCA-mediated Ca2+ uptake, possibly as a consequence of ATP depletions in a localized subcellular domain. The rate of bradykinin-induced Ca2+ release was reduced to 51% of control by FCCP. This effect was largely overcome by loading cells with BAPTA-AM, highlighting the importance of mitochondrial Ca2+ buffering in shaping the release kinetics. However, mitochondria-specific ATP production was also a significant determinant of the release dynamic. Our data emphasize the localized nature of the interaction between these organelles, and show that competent mitochondria are essential for generating explosive Ca2+ signals.

Embryonic lethality and abnormal cardiac myocytes in mice lacking ryanodine receptor type 2

The ryanodine receptor type 2 (RyR-2) functions as a Ca2+-induced Ca2+ release (CICR) channel on intracellular Ca2+ stores and is distributed in most excitable cells with the exception of skeletal muscle cells. RyR-2 is abundantly expressed in cardiac muscle cells and is thought to mediate Ca2+ release triggered by Ca2+ influx through the voltage-gated Ca2+ channel to constitute the cardiac type of excitation-contraction (E-C) coupling. Here we report on mutant mice lacking RyR-2. The mutant mice died at approximately embryonic day (E) 10 with morphological abnormalities in the heart tube. Prior to embryonic death, large vacuolate sarcoplasmic reticulum (SR) and structurally abnormal mitochondria began to develop in the mutant cardiac myocytes, and the vacuolate SR appeared to contain high concentrations of Ca2+. Fluorometric Ca2+ measurements showed that a Ca2+ transient evoked by caffeine, an activator of RyRs, was abolished in the mutant cardiac myocytes. However, both mutant and control hearts showed spontaneous rhythmic contractions at E9.5. Moreover, treatment with ryanodine, which locks RyR channels in their open state, did not exert a major effect on spontaneous Ca2+ transients in control cardiac myocytes at E9.5-11.5. These results suggest no essential contribution of the RyR-2 to E-C coupling in cardiac myocytes during early embryonic stages. Our results from the mutant mice indicate that the major role of RyR-2 is not in E-C coupling as the CICR channel in embryonic cardiac myocytes but it is absolutely required for cellular Ca2+ homeostasis most probably as a major Ca2+ leak channel to maintain the developing SR.

Activity-dependent calcium sequestration in dendrites of hippocampal neurons in brain slices

Synaptic activity-dependent changes in the spatio-temporal distribution of calcium ions regulate important neuronal functions such as dendritic integration and synaptic plasticity, but the processes that terminate the free Ca2+ transients associated with these changes remain unclear. We have characterized at the electron microscopic level the intracellular compartments involved in buffering free Ca2+ transients in dendritic cytoplasm of CA3 neurons by measuring the larger changes in the concentrations of total Ca that persist for several minutes after neuronal activity. Quantitative energy-dispersive x-ray microanalysis of cryosections from hippocampal slice cultures rapidly frozen 3 min after afferent synaptic activity identified a subset of dendritic endoplasmic reticulum (ER) as a high-capacity Ca2+ buffer. Calcium sequestration by cisterns of this subset of ER was graded, reversible, and dependent on a thapsigargin-sensitive Ca2+-ATPase. Sequestration was so robust that after repetitive high-frequency stimulation the Ca content of responsive ER cisterns increased as much as 20-fold. These results demonstrate that a subpopulation of ER is the major dendritic Ca sequestration compartment in the minutes after neuronal activity.

Activity-dependent calcium sequestration in dendrites of hippocampal neurons in brain slices

Synaptic activity-dependent changes in the spatio-temporal distribution of calcium ions regulate important neuronal functions such as dendritic integration and synaptic plasticity, but the processes that terminate the free Ca2+ transients associated with these changes remain unclear. We have characterized at the electron microscopic level the intracellular compartments involved in buffering free Ca2+ transients in dendritic cytoplasm of CA3 neurons by measuring the larger changes in the concentrations of total Ca that persist for several minutes after neuronal activity. Quantitative energy-dispersive x-ray microanalysis of cryosections from hippocampal slice cultures rapidly frozen 3 min after afferent synaptic activity identified a subset of dendritic endoplasmic reticulum (ER) as a high-capacity Ca2+ buffer. Calcium sequestration by cisterns of this subset of ER was graded, reversible, and dependent on a thapsigargin-sensitive Ca2+-ATPase. Sequestration was so robust that after repetitive high-frequency stimulation the Ca content of responsive ER cisterns increased as much as 20-fold. These results demonstrate that a subpopulation of ER is the major dendritic Ca sequestration compartment in the minutes after neuronal activity.

Intracellular calcium redistribution accompanies changes in total tissue Na+, K+ and water during the first two hours of in vitro incubation of hippocampal slices

Changes of total tissue water, Ca, Na and K contents were monitored in whole transverse hippocampal slices of the guinea-pig during the first 2 h of in vitro incubation. A brief, 75% increase in tissue Ca was noted during the initial 15 min of maintenance, in contrast to a permanent increase of sodium and water contents, coupled to simultaneous decrease of potassium level. The rate of tissue Na, K and water changes comprised a rapid phase at the first 10-20 min, parallel with the increase of the tissue Ca content, and a slow phase during the rest of the incubation period. Development of specific morphological alterations, representative of ischemic/hypoxic lesions and a translocation of calcium from cytoplasm to mitochondria and endoplasmic reticulum during slice maintenance, was also detected by electron microscopy. A two-step mechanism might explain the development of a new steady-state total calcium content of slices. in which the cellular Ca2+ uptake at the beginning of incubation, likely triggered by hypoxic/ ischemic trauma of slice preparation, is followed by a balanced Ca2+ influx, extrusion and sequestration (predominantly into mitochondria and endoplasmic reticulum) during maintenance.

Ultrastructural localization of intracellular calcium stores in Xenopus ovarian follicles as revealed by cytochemistry and X-ray microanalysis

The ultrastructural localization of calcium in full-grown ovarian follicles of Xenopus laevis was demonstrated after fixation in the presence of fluoride ions and by means of energy dispersive X-ray microanalysis. In hormonally untreated follicles (prophase I-arrested oocytes), two calcium sites were detected: follicle cells and oocyte pigment granules. In follicle cells, calcium containing deposits were preferentially associated with macrovilli, which ended by gap junctions. In human chorionic gonadotropin treated follicles (meiotically reinitiated oocytes), deposits were only seen in follicle cells. This is the first report of the cytochemical detection of intracellular Ca2+ in follicle cells of amphibians. The possible involvements of these Ca2+ stores in mediating the hormonal control of meiotic maturation are discussed.

Mitochondrial involvement in post-tetanic potentiation of synaptic transmission

Posttetanic potentiation (PTP) is an essential aspect of synaptic transmission that arises from a persistent presynaptic [Ca2+]i following tetanic stimulation. At crayfish neuromuscular junctions, several inhibitors of mitochondrial Ca2+ uptake and release (tetraphenylphosphonium or TPP+, carbonyl cyanide m-chlorophenylhydrazone or CCCP, and ruthenium red) blocked PTP and the persistence of presynaptic residual [Ca2+]i, while endoplasmic reticulum (ER) Ca2+ pump inhibitors and release channel activators (thapsigargin, 2,5-di-(tert-butyl)-1,4-benzohydroquinone or BHQ, and caffeine) had no effects. PTP apparently results from the slow efflux of tetanically accumulated mitochondrial Ca2+.

Ryanodine-sensitive component of calcium transients evoked by nerve firing at presynaptic nerve terminals

Whether Ca2+ released from stores within the presynaptic nerve terminals also contributes to the Ca2+ elevation evoked by action potentials was tested in intact bullfrog sympathetic ganglia. Intraterminal Ca2+ transients (Delta[Ca2+]i) were evoked by electrical shocks to the presynaptic nerves at 20 Hz and were monitored by fura-2 fluorimetry. Ca2+ released through intraterminal ryanodine-sensitive channels accounted for 46% of the peak Ca2+ elevation. Moreover, in half of the terminals when intraterminal release was blocked by ryanodine, Delta[Ca2+]i reached a plateau at 200 +/- 24 nM. Because 20 Hz is a frequency favorable for the release of a neuropeptide, luteinizing hormone releasing hormone (LHRH) from these presynaptic nerve terminals, and because the threshold level for LHRH release is 186 nM, intraterminal Ca2+ release during nerve firing is likely to play a major role in regulating LHRH release. The intraterminal ryanodine channels were facilitated by caffeine as in other tissue. The releasable ryanodine-sensitive store could elevate the intraterminal [Ca2+] by an amount as high as 1.6 microM at a rate as fast as 250 nM/sec. The store could be refilled within 100 sec after a maximal discharge of its content by 20 Hz firing. Oscillation of [Ca2+]i evoked by 20 Hz nerve firing occurred in normal Ringer solution, in ryanodine, and in caffeine with a periodicity of approximately 10 sec. Besides the facilitatory effects on the ryanodine-sensitive channels, caffeine also had inhibitory effects on Delta[Ca2+]i via its action on a different process.

Ryanodine-sensitive component of calcium transients evoked by nerve firing at presynaptic nerve terminals

Whether Ca2+ released from stores within the presynaptic nerve terminals also contributes to the Ca2+ elevation evoked by action potentials was tested in intact bullfrog sympathetic ganglia. Intraterminal Ca2+ transients (Delta[Ca2+]i) were evoked by electrical shocks to the presynaptic nerves at 20 Hz and were monitored by fura-2 fluorimetry. Ca2+ released through intraterminal ryanodine-sensitive channels accounted for 46% of the peak Ca2+ elevation. Moreover, in half of the terminals when intraterminal release was blocked by ryanodine, Delta[Ca2+]i reached a plateau at 200 +/- 24 nM. Because 20 Hz is a frequency favorable for the release of a neuropeptide, luteinizing hormone releasing hormone (LHRH) from these presynaptic nerve terminals, and because the threshold level for LHRH release is 186 nM, intraterminal Ca2+ release during nerve firing is likely to play a major role in regulating LHRH release. The intraterminal ryanodine channels were facilitated by caffeine as in other tissue. The releasable ryanodine-sensitive store could elevate the intraterminal [Ca2+] by an amount as high as 1.6 microM at a rate as fast as 250 nM/sec. The store could be refilled within 100 sec after a maximal discharge of its content by 20 Hz firing. Oscillation of [Ca2+]i evoked by 20 Hz nerve firing occurred in normal Ringer solution, in ryanodine, and in caffeine with a periodicity of approximately 10 sec. Besides the facilitatory effects on the ryanodine-sensitive channels, caffeine also had inhibitory effects on Delta[Ca2+]i via its action on a different process.

High resolution ultrastructural mapping of total calcium: electron spectroscopic imaging/electron energy loss spectroscopy analysis of a physically/chemically processed nerve-muscle preparation

We report on a procedure for tissue preparation that combines thoroughly controlled physical and chemical treatments: quick-freezing and freeze-drying followed by fixation with OsO4 vapors and embedding by direct resin infiltration. Specimens of frog cutaneous pectoris muscle thus prepared were analyzed for total calcium using electron spectroscopic imaging/electron energy loss spectroscopy (ESI/EELS) approach. The preservation of the ultrastructure was excellent, with positive K/Na ratios revealed in the fibers by x-ray microanalysis. Clear, high-resolution EELS/ESI calcium signals were recorded from the lumen of terminal cisternae of the sarcoplasmic reticulum but not from longitudinal cisternae, as expected from previous studies carried out with different techniques. In many mitochondria, calcium was below detection whereas in others it was appreciable although at variable level. Within the motor nerve terminals, synaptic vesicles as well as some cisternae of the smooth endoplasmic reticulum yielded positive signals at variance with mitochondria, that were most often below detection. Taken as a whole, the present study reveals the potential of our experimental approach to map with high spatial resolution the total calcium within individual intracellular organelles identified by their established ultrastructure, but only where the element is present at high levels.

Effect of chronic morphine treatment on transmitter release from sympathetic varicosities of the mouse vas deferens

1 Transmitter release from sympathetic varicosities of mouse vasa deferentia removed from animals which were chronically treated with morphine for 7 to 9 days has been evaluated. 2 In control preparations increasing the extracellular calcium concentration ([Ca2+]o) from 1 to 2 mM increased transmitter release by 3 fold while increasing [Ca2+]o from 6 to 8 mM increased transmitter release by about 0.9 fold. Introduction of morphine (1.0 microM) produced a uniform decrease in transmitter release, shifting the relationship between transmitter release and [Ca2+]o to the right. 3 Only sympathetic varicosities with probabilities of transmitter release greater than 0.01 were chosen for this study. In these varicosities the decrease in transmitter release induced by morphine in control preparations (bathed in [Ca2+]o 2.0 mM) was not observed following 7 to 9 days of morphine treatment. When the morphine was acutely withdrawn from these preparations transmitter release was more than 6 times the average level of transmitter release from control preparations. 4 The morphine induced increase in facilitation of transmitter release while stimulating with short trains of nerve impulses was not observed when the preparations were removed from animals which had been exposed to morphine for 7 to 9 days. When these preparations were acutely withdrawn from morphine there was a further decrease in the level of facilitation and a significant increase in depression of transmitter release when compared to control. 5 The morphine induced decrease in probability of transmitter release when naive sympathetic varicosities in vitro were bathed with morphine (1 microM) was not observed following chronic morphine treatment of the animals for 7 to 9 days. When the morphine was acutely withdrawn from chronically morphine treated preparations the underlying increase in probabilities of transmitter release of sympathetic varicosities was unmasked.

ATP increases extracellular dopamine level through stimulation of P2Y purinoceptors in the rat striatum

The effect of ATP on release of dopamine (DA) from rat striatum was studied using in vivo microdialysis. ATP increased the striatal extracellular levels of DA dose-dependently. These analogs produced an increase in DA according to this order of potency: 2-methylthio ATP > ATP > or = alpha,beta-methylene ATP > ADP > AMP > adenosine. Adenosine 5'-[beta, gamma imido]-triphosphate had a more prolonged effect on the increase in DA level than ATP. The ATP-induced increase in DA was inhibited by adding suramin, a nonselective P2 purinoceptor antagonist, and reactive blue 2, a P2Y purinoceptor antagonist, but not inhibited by xanthine amine congener, an adenosine receptor antagonist. Pertussis toxin reduced the increase in DA produced by ATP, which suggests that the P2 purinoceptor may be coupled with a G-protein in the rat striatum. Results suggest that P2Y purinoceptors may involve an ATP-induced increase in DA. The ATP-induced release of DA was tetrodotoxin-sensitive, Ca(2+)-dependent and was abolished by omega-conotoxin GVIA, indicating that the opening of voltage-sensitive Na+ channel and the Ca2+ influx through the N-type voltage-dependent calcium channel are both required for the ATP-induced increase in DA. The ATP-induced increase in DA is presumably due to the release of DA via the stimulation of P2Y purinoceptors in the rat striatum.

Comparative methodological investigations on the cytochemical localization of calcium in brain and inner ear of cichlid fish

Four different methods for calcium precipitation are compared in the optic tectum and the inner ear of the cichid fish, Oreochromis mossambicus. Several parameters are investigated concerning their influences on the reaction product. Three procedures (bichromate, fluoride, and oxalate-pyroantimonate) produce fine-grained deposits, often flocculent in the latter method. The fourth method (potassium-pyroantimonate) generates predominantly coarse-grained reaction product. The calcium content of the deposits is always proven with energy-filtering transmission electron microscopy (EFTEM). In both tissues fine-grained reaction product is found in endoplasmic reticulum and synaptic vesicles, and in addition in some mitochondria and at the cytoskeleton. The coarse-grained deposits of the potassium-pyroantimonate method have a more unspecific distribution. This is the only method which produces extracellular deposits in the inner ear, whereas in the optic tectum extracellular precipitates are always present except with the oxalate-pyroantimonate procedure. Two factors have an influence on the reaction product: the duration of fixation and the type of resin. The prolongation of the fixation time up to 24 hours leads to an increase of the reaction product, which also becomes coarse-grained. These observations are corroborated by quantification with image analysis. Furthermore the use of an epoxy resin compared to acrylic resins decreases the amount of reaction product produced. We show that the application of several methods is meaningful in order to understand the calcium properties of the investigated tissue, but it is necessary to optimize a certain method for a given tissue.

Kinetic properties of the sodium-calcium exchanger in rat brain synaptosomes

1. The kinetic properties of the internal Na+ (Na+i)- dependent 45Ca2+ influx and external Na+ (Na+o)-dependent 45Ca2+ efflux were determined in isolated rat brain nerve terminals (synaptosomes) under conditions which the concentrations of internal Na+ ([Na+]i), external Na+ ([Na+]o), external Ca2+ (Ca2+]o), and external K+ ([K+]o) were varied. Both fluxes are manifestations of Na(+)-Ca2+ exchange. 2. Ca2+ uptake was augmented by raising [Na+]i and / or lowering [Na+]o. The increase in Ca2+ uptake induced by removing external Na+ was, in most instances, quantitatively equal to the Na+i-dependent Ca2+ uptake. 3. The Na+i-dependent Ca2+ uptake (measured at 1 s) was activated with an apparent half-maximal [Ca2+]o (KCa(o)) of about 0.23 mM. External Na+ inhibited the uptake in a non- competitive manner: increasing [Na+]o from 4.7 to 96 mM reduced the maximal Na+(i)-dependent Ca2+ uptake but did not affect KCa(o). 4. The inhibition of Ca2+ uptake by Na+o was proportional to ([Na+]o)2, and had a Hill coefficient (nH) of approximately 2.0. The mean apparent half-maximal [Na+]o for inhibition (KI(Na)) was about 60mM, and was independent of [Ca2+]o between 0.1 and 1.2mM; this, too, is indicative of non-competitive inhibition. 5. Low concentrations of alkali metal ions (M+) in the medium, including Na+, stimulated the Na+i-dependent uptake. The external Na+ and K+ concentrations required for apparent half-maximal activation (KM(Na) and KM(K), respectively) were 0.12 and 0.10mM. Thus, the relationship between Ca2+ uptake and [Na+]o was biphasic: uptake was stimulated by [Na+]o < or = 10 mM, and inhibited by higher [Na+]o. 6. The calculated maximal Na+i-dependent Ca2+ uptake (Jmax) was about 1530 pmol (mg protein) -1s-1 at 30 degrees C saturating [Ca2+]o and external M+ concentration ([M+]o), and with negligible inhibition by external Na+. 7. Internal Na+ activated the Ca2+ uptake with an apparent half-maximal concentration (KNa(i)) of about 20 mM and a Hill coefficient, nH, of approximately 3.0. 8. The Jmax for the Na+o-dependent efflux of Ca2+ from 45Ca(2+)-loaded synaptosomes treated with carbonyl cyanide p-trifluormethoxy-phenylhydrazone (FCCP) and caffeine (to release stored Ca2+ and raise the internal Ca2+ concentration ([Ca2+]i) was about 1800-2000 pmol (mg protein -1s-1 at 37 degrees C. 9. When the membrane potential (Vm) was reduced (depolarized) by increasing [K+]o, the Na+i-dependent Ca2+ influx increased, and the Na+o-dependent Ca2+ efflux declined. Both fluxes changed about 2-fold per 60 mV change in Vm. This voltage sensitivity corresponds to the movement of one elementary charge through about 60% of the membrane electric field. The symmetry suggests that the voltage-sensitive step is reversible. 10. The Jmax values for both Ca2P influx and efflux correspond to a Na+-Ca2+ exchange-mediated flux of about 425-575 jumol Ca2P (1 cell water)-' s-' or a turnover of about one quarter of the total synaptosome Ca2P in 1 s. We conclude that the Na+-Ca2P exchanger may contribute to Ca2P entry during nerve terminal depolarization; it is likely to be a major mechanism mediating Ca2P extrusion during subsequent repolarization and recovery.

Ultrastructural distribution of calcium in cutaneous electroreceptor organs of teleost fish

The calcium distribution in the ampullary electroreceptor and the type B electroreceptor organ (gymnarchomast) of Gymnarchus niloticus (Glymnarchidae) and in the tuberous organ of Apteronotus leptorhynchus (gymnotidae) was studied. Endogenous calcium appeared as electron-dense precipitates when the cutaneous organs were pre-fixed with phosphate-buffered glutaraldehyde and postfixed with osmium tetroxide plus potassium bichromate. Calcium precipitates were localized in both intracellular compartments of sensory cells, and afferent nerve fibers. In contrast to sensory cells, small amounts of calcium precipitates were found in the cytoplasm of accessory cells. In sensory cells, electron-dense deposits were apparent mainly in synaptic vesicles near synaptic ribbons, inside vacuoles of the endoplasmic reticulum, and between the layers of the nuclear membrane. Very few deposits were found in mitochondria. Precipitates were also observed within the axons of afferent nerves and between the layers of the myelin sheath. The synaptic cleft was devoid of calcium. Calcium deposits have a specific cellular distribution in electroreceptor organs of teleost fish.

Inhibitory effects of HgCl2 on excitation-secretion coupling at the motor nerve terminal and excitation-contraction coupling in the muscle cell

1. Indirect and direct twitch (0.1-Hz) stimulation of the rat phrenic nerve-diaphragm disclosed that the inhibitory effect of HgCl2, 3.7 x 10(-5) M, on the neuromuscular transmission and in the muscle cell, was accelerated by 10-sec periods of 50-Hz tetanic stimulation every 10 min. This activity-dependent enhancement suggested an inhibitory mechanism of HgCl2 related to the development of fatigue, like membrane depolarization or decreased excitability, decreased availability of transmitter, or interference with the factors controlling excitation-secretion coupling of the nerve terminal, i.e. (Ca2+)0 or (Ca2+)i, and excitation-contraction coupling in the muscle cell, i.e., (Ca2+)i. 2. During both indirect and direct stimulation, HgCl2-induced inhibition was enhanced markedly by pretreatment with caffeine, which releases Ca2+ from endoplasmic and sarcoplasmic reticulum in the nerve terminal and muscle cell, respectively. This caffeine-induced enhancement was completely antagonized by dantrolene, which inhibits the caffeine-induced release. However, dantrolene alone did not antagonize the HgCl2-induced inhibition. 3. Since caffeine depletes the intracellular Ca2+ stores of the smooth endoplasmic reticulum, HgCl2 probably inhibits by binding to SH groups of transport proteins conveying the messenger function of (Ca2+)i. In the muscle cell this leads to inhibition of contraction. In the nerve terminal, an additional enhancement of the HgCl2-induced inhibition, by inhibiting reuptake of choline by TEA and tetanic stimulation, suggested that HgCl2 inhibited a (Ca2+)i signal necessary for this limiting factor in resynthesis of acetylcholine. 4. The (Ca2+)0 signal necessary for stimulus-induced release of acetylcholine was not affected by HgCl2. Hyperpolarization in K(+)-free solution antagonized the inhibitory effect of HgCl2 at indirect stimulation, and Ca(2+)-free solution enhanced the inhibitory effect at direct stimulation. K+ depolarization, membrane electric field increase with high Ca2+, membrane stabilization with lidocaine, and half-threshold stimulation, did not change the inhibitory effect of HgCl CH3HgCl. 1.85 x 10(-5) M, disclosed a synergistic interaction with caffeine during direct, but not during indirect, stimulation.

Exo-endocytotic activity during recovery from a brief tetanic stimulation: a role in calcium extrusion?

Synaptic transmission, metabolism of calcium and ultrastructural changes were investigated at the nerve-electroplaque synapse of Torpedo marmorata during and after a brief tetanic stimulation. Calcium was found to accumulate in stimulated tissue as a function of the number of stimuli; it was subsequently expelled during the recovery period. This period was also accompanied by a marked hydrolysis of energy-rich phosphates (ATP and creatine phosphate). Histochemical localization combined with electron spectroscopic imaging showed calcium deposits in synaptic vesicles and in other substructures. The number of synaptic vesicles containing a calcium deposit transiently increased at the end of activity and declined later during the recovery phase. Rapid cryofixation of the tissue followed by freeze-fracturing revealed membrane openings (pits) in the presynaptic membrane. The density of pits was low in resting tissue; it did not rise during the tetanic stimulation. In contrast, the number of presynaptic pits increased significantly soon after, reaching a maximum value at 1 min after tetanus. These results are discussed in the light of current hypotheses. They suggest that synaptic vesicles play an important role in intraterminal calcium homeostasis. The vesicles might sequester calcium ions in synaptic terminals during activity and expel them afterwards by exocytosis.

A new cytochemical method for the ultrastructural localization of calcium in the central nervous system

We have developed a new cytochemical method for the localization of calcium at the ultrastructural level in the central nervous system (CNS). The method is based on the use of phosphate buffer in the primary fixation followed by a mixture of a complex of chromium(III)-trisoxalate and osmium tetroxide (OsO4) which precipitates calcium and results in the formation of a high electron-dense reaction product. Calcium selectivity was verified by reactions made in test tube, by EGTA treatment of the tissue, by electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS). The technique was found to be reproducible, yielding similar results in acutely prepared hippocampal slices or organotypic cultures fixed by immersion and in brain areas fixed by perfusion. In hippocampal slices, calcium deposits were found to accumulate in different subcellular compartments such as endoplasmic reticulum, mitochondria and synaptic vesicles. Interestingly, electron-dense reaction products were also visualized in smooth endoplasmic reticulum structures localized in presynaptic terminals or post-synaptic spines as well as in synaptic clefts and active zones. This new method may thus be of interest for studying the metabolism of calcium, specifically with regard to synaptic activity, in the CNS.

Aluminum alters calcium transport in plasma membrane and endoplasmic reticulum from rat brain

Calcium is actively transported into intracellular organelles and out of the cytoplasm by Ca2+/Mg(2+)-ATPases located in the endoplasmic reticulum and plasma membranes. We studied the effects of aluminum on calcium transport in the adult rat brain. We examined 45Ca-uptake in microsomes and Ca(2+)-ATPase activity in microsomes and synaptosomes isolated from the frontal cortex and cerebellum of adult male Long-Evans rats. ATP-dependent 45Ca-uptake was similar in microsomes from both brain regions. The addition of 50-800 microM AlCl3 resulted in a concentration-dependent inhibition of 45Ca-uptake. Mg(2+)-dependent Ca(2+)-ATPase activity was significantly lower in synaptosomes compared to microsomes in both frontal cortex and cerebellum. In contrast to the uptake studies, AlCl3 stimulated Mg(2+)-dependent Ca(2+)-ATPase activity in both microsomes and synaptosomes from both brain regions. To determine the relationship between aluminum and Mg2+, we measured ATPase activity in the presence of increasing concentrations of Mg2+ or AlCl3. Maximal ATPase activity was obtained between 3 and 6 mM Mg2+. When we substituted AlCl3 for Mg2+, ATPase activity was also stimulated in a concentration-dependent manner, but to a greater extent than with Mg2+. One interpretation of these data is that aluminum acts at multiple sites to displace both Mg2+ and Ca2+, increasing the activity of the Ca(2+)-ATPase, but disrupting transport of calcium.

Ultrastructural localization of calcium in the chick yolk sac membrane endodermal cells as revealed by cytochemistry and X-ray microanalysis

The yolk sac membrane (YSM) of the chick embryo transports calcium from the yolk into the embryonic circulation during the first half of development, but the intracellular pathway of calcium transport is poorly understood. In the present study, the ultrastructural localization of calcium was investigated in cells of the YSM of 9-day chick embryos. X-ray microanalysis as well as cytochemical techniques performed on yolk sac membrane cells treated with potassium oxalate, potassium ferricyanide and potassium antimonate demonstrated accumulation of calcium in yolk granules, digested yolk products, electron-dense bodies (EDBs; 100-400 nm diameter) and electron-dense granules (EDGs; 30-50 nm diameter). When strontium ions were injected into the yolk, they were incorporated into the endodermal cells and sequestered specifically in EDGs. From these results, we propose that calcium enters the endodermal cells by endocytosis of calcium-containing yolk granules, as well as through calcium channels in the apical cell membrane. In the cytoplasm, digested yolk products, EDBs, and EDGs act as sites of sequestration and accumulation of calcium. Extrusion of intracellular calcium into the extracellular space and embryonic circulation is accomplished by exocytosis of calcium-containing material and via an ion pump in the basal cell membrane.

Calcium buffering and free Ca2+ in rat brain synaptosomes

The effects of K+ depolarization and of stimulation by veratridine on apparent cytosolic free Ca2+ ([Ca2+]cyt) and net Ca2+ accumulation were measured in isolated rat brain presynaptic nerve terminals (synaptosomes). [Ca2+]cyt was determined with fura-2, and Ca2+ accumulation was measured with tracer 45Ca. [Ca2+]cyt was approximately 325 nM in synaptosomes incubated in the normal physiological salt solution under resting conditions. When [K+]o was increased from the normal 5 mM to 30 or 50 mM, 45Ca uptake and [Ca2+]cyt both increased within 1 s. Both increases were directly related to [Ca2+]o for [Ca2+]o = 0.02-1.2 mM; however, the increase in 45Ca uptake greatly exceeded the increase in [Ca2+]cyt. With small Ca2+ loads (< or = 100 mumol/L of cell water, equivalent to the Ca2+ entry during a train of < or = 60 impulses), the 45Ca uptake exceeded the increase in [Ca2+]cyt by a factor of nearly 1,000. This indicates that approximately 99.9% of the entering Ca2+ was buffered and/or sequestered within approximately 1 s. With larger Ca2+ loads, a larger fraction of the entering Ca2+ was buffered; approximately 99.97% of the load was buffered with loads of 250-425 mumol/L of cell water. The ratio between the total Ca2+ entry and the increase in [Ca2+]cyt, the "calcium buffer ratio," beta, was therefore approximately 3,500:1. This ratio was somewhat lower than the ratio of total intraterminal calcium: [Ca2+]cyt, which ranged between approximately 7,300:1 and 12,800:1. When the synaptosomes were activated with 10 microM veratridine ([Ca2+]o = 0.2-0.6 mM), 45Ca influx and [Ca2+]cyt increased progressively for approximately 10 s (beta = 2,700:1-3,050:1) and then leveled off.(ABSTRACT TRUNCATED AT 250 WORDS)