Calcium mobilization from mitochondria in synaptic transmitter release

Differential modulation of short-term plasticity at hippocampal mossy fiber and Schaffer collateral synapses by mitochondrial Ca2

Presynaptic mitochondrial Ca2+ plays a critical role in the regulation of synaptic transmission and plasticity. The presynaptic bouton of the hippocampal mossy fiber (MF) is much larger in size than that of the Schaffer collateral (SC) synapse. Here we compare the structural and physiological characteristics of MF and SC presynaptic boutons to reveal functional and mechanistic differences between these two synapses. Our quantitative ultrastructural analysis using electron microscopy show many more mitochondria in MF presynaptic bouton cross-section profiles compared to SC boutons. Consistent with these results, post-tetanic potentiation (PTP), a form of presynaptic short-term plasticity dependent on mitochondrial Ca2+, is reduced by inhibition of mitochondrial Ca2+ release at MF synapses but not at SC synapses. However, blockade of mitochondrial Ca2+ release results in reduction of PTP at SC synapses by disynaptic MF stimulation. Furthermore, inhibition of mitochondrial Ca2+ release selectively decreases frequency facilitation evoked by short trains of presynaptic stimulation at MF synapses, while having no effect at SC synapses. Moreover, depletion of ER Ca2+ stores leads to reduction of PTP at MF synapses, but PTP is unaffected by ER Ca2+ depletion at SC synapses. These findings show that MF and SC synapses differ in presynaptic mitochondrial content as well as mitochondrial Ca2+ dependent synaptic plasticity, highlighting differential regulatory mechanisms of presynaptic plasticity at MF and SC synapses.

Mitochondrial Dysfunction and Synaptic Transmission Failure in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder, in which multiple risk factors converge. Despite the complexity of the etiology of the disease, synaptic failure is the pathological basis of cognitive impairment, the cardinal sign of AD. Decreased synaptic density, compromised synaptic transmission, and defected synaptic plasticity are hallmark synaptic pathologies accompanying AD. However, the mechanisms by which synapses are injured in AD-related conditions have not been fully elucidated. Mitochondria are a critical organelle in neurons. The pivotal role of mitochondria in supporting synaptic function and the concomitant occurrence of mitochondrial dysfunction with synaptic stress in postmortem AD brains as well as AD animal models seem to lend the credibility to the hypothesis that mitochondrial defects underlie synaptic failure in AD. This concept is further strengthened by the protective effect of mitochondrial medicine on synaptic function against the toxicity of amyloid-β, a key player in the pathogenesis of AD. In this review, we focus on the association between mitochondrial dysfunction and synaptic transmission deficits in AD. Impaired mitochondrial energy production, deregulated mitochondrial calcium handling, excess mitochondrial reactive oxygen species generation and release play a crucial role in mediating synaptic transmission deregulation in AD. The understanding of the role of mitochondrial dysfunction in synaptic stress may lead to novel therapeutic strategies for the treatment of AD through the protection of synaptic transmission by targeting to mitochondrial deficits.

Ammonium chloride alters neuronal excitability and synaptic vesicle release

Genetically encoded pH-sensors are widely used in studying cell membrane trafficking and membrane protein turnover because they render exo-/endocytosis-associated pH changes to fluorescent signals. For imaging and analysis purposes, high concentration ammonium chloride is routinely used to alkalize intracellular membrane compartments under the assumption that it does not cause long-term effects on cellular processes being studied like neurotransmission. However, pathological studies about hyperammonemia have shown that ammonium is toxic to brain cells especially astrocytes and neurons. Here, we focus on ammonium’s physiological impacts on neurons including membrane potential, cytosolic Ca²⁺ and synaptic vesicles. We have found that extracellularly applied ammonium chloride as low as 5 mM causes intracellular Ca²⁺-increase and a reduction of vesicle release even after washout. The often-used 50 mM ammonium chloride causes more extensive and persistent changes, including membrane depolarization, prolonged elevation of intracellular Ca²⁺ and diminution of releasable synaptic vesicles. Our findings not only help to bridge the discrepancies in previous studies about synaptic vesicle release using those pH-sensors or other vesicle specific reporters, but also suggest an intriguing relationship between intracellular pH and neurotransmission.

Selective vulnerability of motoneuron and perturbed mitochondrial calcium homeostasis in amyotrophic lateral sclerosis: implications for motoneurons specific calcium dysregulation

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder characterized by the selective degeneration of defined subgroups of motoneuron in the brainstem, spinal cord and motor cortex with signature hallmarks of mitochondrial Ca(2+) overload, free radical damage, excitotoxicity and impaired axonal transport. Although intracellular disruptions of cytosolic and mitochondrial calcium, and in particular low cytosolic calcium ([Ca(2+)]c) buffering and a strong interaction between metabolic mechanisms and [Ca(2+)]i have been identified predominantly in motoneuron impairment, the causes of these disruptions are unknown. The existing evidence suggests that the mutant superoxide dismutase1 (mtSOD1)-mediated toxicity in ALS acts through mitochondria, and that alteration in cytosolic and mitochondria-ER microdomain calcium accumulation are critical to the neurodegenerative process. Furthermore, chronic excitotoxcity mediated by Ca(2+)-permeable AMPA and NMDA receptors seems to initiate vicious cycle of intracellular calcium dysregulation which leads to toxic Ca(2+) overload and thereby selective neurodegeneration. Recent advancement in the experimental analysis of calcium signals with high spatiotemporal precision has allowed investigations of calcium regulation in-vivo and in-vitro in different cell types, in particular selectively vulnerable/resistant cell types in different animal models of this motoneuron disease. This review provides an overview of latest advances in this field, and focuses on details of what has been learned about disrupted Ca(2+) homeostasis and mitochondrial degeneration. It further emphasizes the critical role of mitochondria in preventing apoptosis by acting as a Ca(2+) buffers, especially in motoneurons, in pathophysiological conditions such as ALS.

Selective vulnerability of motoneuron and perturbed mitochondrial calcium homeostasis in amyotrophic lateral sclerosis: implications for motoneurons specific calcium dysregulation

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder characterized by the selective degeneration of defined subgroups of motoneuron in the brainstem, spinal cord and motor cortex with signature hallmarks of mitochondrial Ca²⁺ overload, free radical damage, excitotoxicity and impaired axonal transport. Although intracellular disruptions of cytosolic and mitochondrial calcium, and in particular low cytosolic calcium ([Ca²⁺]_c) buffering and a strong interaction between metabolic mechanisms and [Ca²⁺]_i have been identified predominantly in motoneuron impairment, the causes of these disruptions are unknown. The existing evidence suggests that the mutant superoxide dismutase1 (mtSOD1)-mediated toxicity in ALS acts through mitochondria, and that alteration in cytosolic and mitochondria-ER microdomain calcium accumulation are critical to the neurodegenerative process. Furthermore, chronic excitotoxcity mediated by Ca²⁺-permeable AMPA and NMDA receptors seems to initiate vicious cycle of intracellular calcium dysregulation which leads to toxic Ca²⁺ overload and thereby selective neurodegeneration. Recent advancement in the experimental analysis of calcium signals with high spatiotemporal precision has allowed investigations of calcium regulation in-vivo and in-vitro in different cell types, in particular selectively vulnerable/resistant cell types in different animal models of this motoneuron disease. This review provides an overview of latest advances in this field, and focuses on details of what has been learned about disrupted Ca²⁺ homeostasis and mitochondrial degeneration. It further emphasizes the critical role of mitochondria in preventing apoptosis by acting as a Ca²⁺ buffers, especially in motoneurons, in pathophysiological conditions such as ALS.

Effective killing of leukemia cells by the natural product OSW-1 through disruption of cellular calcium homeostasis

3β,16β,17α-Trihydroxycholest-5-en-22-one 16-O-(2-O-4-methoxybenzoyl-β-D-xylopyranosyl)-(1→3)-2-O-acetyl-α-L-arabinopyranoside (OSW-1) is a natural product with potent antitumor activity against various types of cancer cells, but the exact mechanisms of action remain to be defined. In this study, we showed that OSW-1 effectively killed leukemia cells at subnanomolar concentrations through a unique mechanism by causing a time-dependent elevation of cytosolic Ca(2+) prior to induction of apoptosis. A mechanistic study revealed that this compound inhibited the sodium-calcium exchanger 1 on the plasma membrane, leading to an increase in cytosolic Ca(2+) and a decrease in cytosolic Na(+). The elevated cytosolic Ca(2+) caused mitochondrial calcium overload and resulted in a loss of mitochondrial membrane potential, release of cytochrome c, and activation of caspase-3. Furthermore, OSW-1 also caused a Ca(2+)-dependent cleavage of the survival factor GRP78. Inhibition of Ca(2+) entry into the mitochondria by the uniporter inhibitor RU360 or by cyclosporin A significantly prevented the OSW-1-induced cell death, indicating the important role of mitochondria in mediating the cytotoxic activity. The extremely potent activity of OSW-1 against leukemia cells and its unique mechanism of action suggest that this compound may be potentially useful in the treatment of leukemia.

Synaptic mitochondrial pathology in Alzheimer's disease

SIGNIFICANCE

Synaptic degeneration, an early pathological feature in Alzheimer's disease (AD), is closely correlated to impaired cognitive function and memory loss. Recent studies suggest that involvement of amyloid-beta peptide (Aβ) in synaptic mitochondrial alteration underlies these synaptic lesions. Thus, to understand the Aβ-associated synaptic mitochondrial perturbations would fortify our understanding of synaptic stress in the pathogenesis of AD.

RECENT ADVANCES

Increasing evidence suggests that synaptic mitochondrial dysfunction is strongly associated with synaptic failure in many neurodegenerative diseases including AD. Based on recent findings in human AD subjects, AD animal models, and AD cellular models, synaptic mitochondria undergo multiple malfunctions including Aβ accumulation, increased oxidative stress, decreased respiration, and compromised calcium handling capacity, all of which occur earlier than changes seen in nonsynaptic mitochondria before predominant AD pathology. Of note, the impact of Aβ on mitochondrial motility and dynamics exacerbates synaptic mitochondrial alterations.

CRITICAL ISSUES

Synaptic mitochondria demonstrate early deficits in AD; in combination with the role that synaptic mitochondria play in sustaining synaptic functions, deficits in synaptic mitochondria may be a key factor involved in an early synaptic pathology in AD.

FUTURE DIRECTIONS

The importance of synaptic mitochondria in supporting synapses and the high vulnerability of synaptic mitochondria to Aβ make them a promising target of new therapeutic strategy for AD.

Cell signaling and mitochondrial dynamics: Implications for neuronal function and neurodegenerative disease

Nascent evidence indicates that mitochondrial fission, fusion, and transport are subject to intricate regulatory mechanisms that intersect with both well-characterized and emerging signaling pathways. While it is well established that mutations in components of the mitochondrial fission/fusion machinery can cause neurological disorders, relatively little is known about upstream regulators of mitochondrial dynamics and their role in neurodegeneration. Here, we review posttranslational regulation of mitochondrial fission/fusion enzymes, with particular emphasis on dynamin-related protein 1 (Drp1), as well as outer mitochondrial signaling complexes involving protein kinases and phosphatases. We also review recent evidence that mitochondrial dynamics has profound consequences for neuronal development and synaptic transmission and discuss implications for clinical translation.

Amyloid-β as a modulator of synaptic plasticity

Alzheimer's disease is associated with synapse loss, memory dysfunction, and pathological accumulation of amyloid-β (Aβ) in plaques. However, an exclusively pathological role for Aβ is being challenged by new evidence for an essential function of Aβ at the synapse. Aβ protein exists in different assembly states in the central nervous system and plays distinct roles ranging from synapse and memory formation to memory loss and neuronal cell death. Aβ is present in the brain of symptom-free people where it likely performs important physiological roles. New evidence indicates that synaptic activity directly evokes the release of Aβ at the synapse. At physiological levels, Aβ is a normal, soluble product of neuronal metabolism that regulates synaptic function beginning early in life. Monomeric Aβ40 and Aβ42 are the predominant forms required for synaptic plasticity and neuronal survival. With age, some assemblies of Aβ are associated with synaptic failure and Alzheimer's disease pathology, possibly targeting the N-methyl-D-aspartic acid receptor through the nicotinic acetylcholine receptor, mitochondrial Aβ alcohol dehydrogenase, and cyclophilin D. But emerging data suggests a distinction between age effects on the target response in contrast to the assembly state or the accumulation of the peptide. Both aging and Aβ independently decrease neuronal plasticity. Our laboratory has reported that Aβ, glutamate, and lactic acid are each increasingly toxic with neuron age. The basis of the age-related toxicity partly resides in age-related mitochondrial dysfunction and an oxidative shift in mitochondrial and cytoplasmic redox potential. In turn, signaling through phosphorylated extracellular signal-regulated protein kinases is affected along with an age-independent increase in phosphorylated cAMP response element-binding protein. This review examines the long-awaited functional impact of Aβ on synaptic plasticity.

Synaptic mitochondria in synaptic transmission and organization of vesicle pools in health and disease

Cell types rich in mitochondria, including neurons, display a high energy demand and a need for calcium buffering. The importance of mitochondria for proper neuronal function is stressed by the occurrence of neurological defects in patients suffering from a great variety of diseases caused by mutations in mitochondrial genes. Genetic and pharmacological evidence also reveal a role of these organelles in various aspects of neuronal physiology and in the pathogenesis of neurodegenerative disorders. Yet the mechanisms by which mitochondria can affect neurotransmission largely remain to be elucidated. In this review we focus on experimental data that suggest a critical function of synaptic mitochondria in the function and organization of synaptic vesicle pools, and in neurotransmitter release during intense neuronal activity. We discuss how calcium handling, ATP production and other mitochondrial mechanisms may influence synaptic vesicle pool organization and synaptic function. Given the link between synaptic mitochondrial function and neuronal communication, efforts toward better understanding mitochondrial biology may lead to novel therapeutic approaches of neurological disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and psychiatric disorders that are at least in part caused by mitochondrial deficits.

Effects of imaging conditions on mitochondrial transport and length in larval motor axons of Drosophila

The distribution of mitochondria is sensitive to physiological stresses and changes in metabolic demands. Consequently, it is important to carefully define the conditions facilitating live imaging of mitochondrial transport in dissected animal preparations. In this study, we examined Schneider's and the haemolymph-like solutions HL3 and HL6 for their suitability to image mitochondrial transport in motor axons of dissected Drosophila melanogaster larvae. Overall, mitochondrial transport kinetics in larval motor axons appeared similar among all three solutions. Unexpectedly, HL3 solution selectively increased the length of mitochondria in the context of the net-direction of transport. We also found that mitochondrial transport is sensitive to the extracellular Ca(2+) but not glutamate concentration. High concentrations of extracellular glutamate affected only the ratio between motile and stationary mitochondria. Our study offers a valuable overview of mitochondrial transport kinetics in larval motor axons of Drosophila under various conditions, guiding future studies genetically dissecting mechanisms of mitochondrial transport.

Association between mitochondrial DNA 10398A>G polymorphism and the volume of amygdala

Mitochondrial calcium regulation plays a number of important roles in neurons. Mitochondrial DNA (mtDNA) is highly polymorphic, and its interindividual variation is associated with various neuropsychiatric diseases and mental functions. An mtDNA polymorphism, 10398A>G, was reported to affect mitochondrial calcium regulation. Volume of hippocampus and amygdala is reportedly associated with various mental disorders and mental functions and is regarded as an endophenotype of mental disorders. The present study investigated the relationship between the mtDNA 10398A>G polymorphism and the volume of hippocampus and amygdala in 118 right-handed healthy subjects. The brain morphometry using magnetic resonance images employed both manual tracing volumetry in the native space and voxel-based morphometry (VBM) in the spatially normalized space. Amygdala volume was found to be significantly larger in healthy subjects with 10398A than in those with 10398G by manual tracing, which was confirmed by the VBM. Brain volumes in the other gray matter regions and all white matter regions showed no significant differences associated with the polymorphism. These provocative findings might provide a clue to the complex relationship between mtDNA, brain structure and mental disorders.

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.

Abeta oligomers induce neuronal oxidative stress through an N-methyl-D-aspartate receptor-dependent mechanism that is blocked by the Alzheimer drug memantine

Oxidative stress is a major aspect of Alzheimer disease (AD) pathology. We have investigated the relationship between oxidative stress and neuronal binding of Abeta oligomers (also known as ADDLs). ADDLs are known to accumulate in brain tissue of AD patients and are considered centrally related to pathogenesis. Using hippocampal neuronal cultures, we found that ADDLs stimulated excessive formation of reactive oxygen species (ROS) through a mechanism requiring N-methyl-d-aspartate receptor (NMDA-R) activation. ADDL binding to neurons was reduced and ROS formation was completely blocked by an antibody to the extracellular domain of the NR1 subunit of NMDA-Rs. In harmony with a steric inhibition of ADDL binding by NR1 antibodies, ADDLs that were bound to detergent-extracted synaptosomal membranes co-immunoprecipitated with NMDA-R subunits. The NR1 antibody did not affect ROS formation induced by NMDA, showing that NMDA-Rs themselves remained functional. Memantine, an open channel NMDA-R antagonist prescribed as a memory-preserving drug for AD patients, completely protected against ADDL-induced ROS formation, as did other NMDA-R antagonists. Memantine and the anti-NR1 antibody also attenuated a rapid ADDL-induced increase in intraneuronal calcium, which was essential for stimulated ROS formation. These results show that ADDLs bind to or in close proximity to NMDA-Rs, triggering neuronal damage through NMDA-R-dependent calcium flux. This response provides a pathologically specific mechanism for the therapeutic action of memantine, indicates a role for ROS dysregulation in ADDL-induced cognitive impairment, and supports the unifying hypothesis that ADDLs play a central role in AD pathogenesis.

The lysosome or lysosome-related organelle may serve as a Ca2+ store in the boutons of hippocampal pyramidal cells

Boutons are specialised presynaptic compartments that lie along the axons of central neurons. Release of neurotransmitter from boutons is tightly regulated by the level of intracellular calcium [Ca2+]i. A rise in Ca2+ level may be generated in several ways; entry of extracellular Ca2+ via voltage gated calcium channels (VGCCs), entry via ligand-operated channels (LOCs) or the release of Ca2+ from intracellular Ca2+ stores. The role of Ca2+ stores in boutons remains poorly understood, despite recent work indicating that the release of Ca2+ from the endoplasmic reticulum (ER) may contribute to transmitter release. In this study we assess whether the lysosome or a closely related organelle functions as a Ca2+ store in the boutons of hippocampal pyramidal neurones. Lysosomes are small acidic organelles more commonly known for their role in degrading redundant cellular constituents. Using a fluorescent lysosomal marker, we show that lysosomes are located in the axons of hippocampal CA3 neurones. Selective pharmacological lysis of the lysosomes with glycyl-phenylalanine 2-naphthylamide (GPN) generates rapid, highly focal Ca2+ transients within the axon and increases the frequency of spontaneous miniature excitatory post-synaptic currents (mEPSCs), revealing that the organelle contains Ca2+ at a concentration sufficient to evoke transmitter release. Confocal laser scanning microscopy, combined with electrophysiology is used to monitor the action potential evoked increases in [Ca2+]i in boutons. We show that disruption of lysosomes compromises action potential evoked [Ca2+]i but this effect is occluded if the ER is discharged. Conversely, disruption of the lysosome does not appear to impact on the capacity of the ER to release Ca2+. These results suggest that the lysosome may serve as a Ca2+ store within hippocampal boutons, with a Ca2+ signalling role that is unique from that of the ER.

Mitochondria: more than just a powerhouse

Pioneering biochemical studies have long forged the concept that the mitochondria are the 'energy powerhouse of the cell'. These studies, combined with the unique evolutionary origin of the mitochondria, led the way to decades of research focusing on the organelle as an essential, yet independent, functional component of the cell. Recently, however, our conceptual view of this isolated organelle has been profoundly altered with the discovery that mitochondria function within an integrated reticulum that is continually remodeled by both fusion and fission events. The identification of a number of proteins that regulate these activities is beginning to provide mechanistic details of mitochondrial membrane remodeling. However, the broader question remains regarding the underlying purpose of mitochondrial dynamics and the translation of these morphological transitions into altered functional output. One hypothesis has been that mitochondrial respiration and metabolism may be spatially and temporally regulated by the architecture and positioning of the organelle. Recent evidence supports and expands this idea by demonstrating that mitochondria are an integral part of multiple cell signaling cascades. Interestingly, proteins such as GTPases, kinases and phosphatases are involved in bi-directional communication between the mitochondrial reticulum and the rest of the cell. These proteins link mitochondrial function and dynamics to the regulation of metabolism, cell-cycle control, development, antiviral responses and cell death. In this review we will highlight the emerging evidence that provides molecular definition to mitochondria as a central platform in the execution of diverse cellular events.

Calcium ions and integration in neural circuits

Integration in the nervous system is achieved by signal processing within dynamic functional ensembles formed by highly complex neuronal-glial cellular circuits. The interactions between electrically excitable neuronal networks and electrically non-excitable glial syncytium occur through either chemical transmission, which involves the release of transmitters from presynaptic terminals or from astroglial cells, or via direct intercellular contacts, gap junctions. Calcium ions act as a universal intracellular signalling system, which controls many aspects of neuronal-glial communications. In neurones, calcium signalling events regulate the exocytosis of neurotransmitters and establish the link between excitation of postsynaptic cells and integrative intracellular events, which control synaptic strength, expression of genes and memory function. In glial cells metabotropic receptor mediated release of calcium ions from the intracellular endoplasmic reticulum calcium store provide specific form of glial excitability. Glial calcium signals ultimately result in vesicular secretion of "glio" transmitters, which affect neuronal networks thus closing the glial-neuronal circuits. Cellular signalling through calcium ions therefore can be regarded as a molecular mechanism of integration in the nervous system.

Na+-dependent sources of intra-axonal Ca2+ release in rat optic nerve during in vitro chemical ischemia

The contribution of intracellular stores to axonal Ca2+ overload during chemical ischemia in vitro was examined by confocal microscopy. Ca2+ accumulation was measured by fluo-4 dextran (low-affinity dye, KD approximately 4 microM) or by Oregon Green 488 BAPTA-1 dextran (highaffinity dye, KD approximately 450 nM). Axonal Na+ was measured using CoroNa Green. Ischemia in CSF containing 2 mM Ca2+ caused an approximately 3.5-fold increase in fluo-4 emission after 30 min, indicating a large axonal Ca2+ rise well into the micromolar range. Axonal Na+ accumulation was enhanced by veratridine and reduced, but not abolished, by TTX. Ischemia in Ca2+-free (plus BAPTA) perfusate resulted in a smaller but consistent Ca2+ increase monitored by Oregon Green 488 BAPTA-1, indicating release from intracellular sources. This release was eliminated in large part when Na+ influx was reduced by replacement with N-methyl-D-glucamine (NMDG+; even in depolarizing high K+ perfusate), Li+, or by the application of TTX and significantly increased by veratridine. Intracellular release also was reduced significantly by neomycin or 1-(6-[(17beta-methoxyestra-1,3,5 [10]-trien-17-yl) amino] hexyl)-1H-pyrrole-2,5-dione (U73122 [GenBank]) (phospholipase C inhibitors), heparin [inositol trisphosphate (IP3) receptor blocker], or 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157; mitochondrial Na+/Ca2+ exchange inhibitor) as well as ryanodine. Combining CGP37157 with U73122 [GenBank] or heparin decreased the response more than either agent alone and significantly improved electrophysiological recovery. Our conclusion is that intra-axonal Ca2+ release during ischemia in rat optic nerve is mainly dependent on Na+ influx. This Na+ accumulation stimulates three distinct intra-axonal sources of Ca2+: (1) the mitochondrial Na+/Ca2+ exchanger driven in the Na+ import/Ca2+ export mode, (2) positive modulation of ryanodine receptors, and (3) promotion of IP3 generation by phospholipase C.

Pharmacological consequences of oxidative stress in ocular tissues

The eye is a unique organ because of its constant exposure to radiation, atmospheric oxygen, environmental chemicals and physical abrasion. That oxidative stress mechanisms in ocular tissues have been hypothesized to play a role in diseases such as glaucoma, cataract, uveitis, retrolental fibroplasias, age-related macular degeneration and various forms of retinopathy provides an opportunity for new approaches to their prevention and treatment, In the anterior uvea, both H2O2 and synthetic peroxides exert pharmacological/toxicological actions tissues of the anterior uvea especially on the sympathetic nerves and smooth muscles of the iris-ciliary bodies of several mammalian species. Effects produced by peroxides require the presence of trace amounts of extracellular calcium and the functional integrity of mitochondrial calcium stores. Arachidonic acid metabolites appear to be involved in both the excitatory action of peroxides on sympathetic neurotransmission and their inhibitory effect on contractility of the iris smooth muscle to muscarinic receptor activation. In addition to the peroxides, isoprostanes (products of free radical catalyzed peroxidation of arachidonic acid independent of the cyclo-oxygenase enzyme) can also alter sympathetic neurotransmission in anterior uveal tissues. In the retina, both H2O2 and synthetic peroxides produced an inhibitory action on potassium depolarization induced release of [3H] D-aspartate, in vitro and on the endogenous glutamate and glycine concentrations in vivo. Effects caused by peroxides in the retina are mediated, at least in part, by second messengers such as nitric oxide, prostaglandins and isoprostanes. The ability of H2O2 to alter the integrity of neurotransmitter pools from sympathetic nerves in the anterior uvea and glutaminergic nerves in the retina could underlie its role in the etiology of glaucoma.

Long-term in vivo modulation of synaptic efficacy at the neuromuscular junction of Rana pipiens frogs

Prolonged changes in motor neurone activity can result in long-term changes in synaptic transmission. We investigated whether mechanisms commonly thought to be involved in determining synaptic efficacy of vertebrate motor neurones are involved in these long-term changes. The nerve supplying the cutaneous pectoris muscle was chronically stimulated via skin surface electrodes in freely moving frogs for 5-7 days. Chronic stimulation induced a 50% reduction in evoked endplate potential (EPP) amplitude at stimulated neuromuscular junctions (NMJs). These changes appear to be presynaptic since miniature EPP (mEPP) amplitude was unchanged while mEPP frequency was decreased by 46% and paired-pulse facilitation was increased by 26%. High frequency facilitation (40 Hz, 2 s) was also increased by 89%. Moreover, stimulated NMJs presented a 92% decrease in synaptic depression (40 Hz, 2 s). An increase in mitochondrial metabolism was observed as indicated by a more pronounced labelling of active mitochondria (Mitotracker) in stimulated nerve terminals, which could account for their greater resistance to synaptic depression. NMJ length visualized by alpha-bungarotoxin staining of nAChRs was not affected. Presynaptic calcium signals measured with Calcium Green-1 were larger in stimulated NMJs at low frequency (0.2 Hz) and not different from control NMJs at higher frequency (40 Hz, 2 s and 30 s). These results suggest that some mechanisms downstream of calcium entry are responsible for the determination of synaptic output, such as a down-regulation of some calcium-binding proteins, which could explain the observed results. The possibility of a change in frequenin expression, a calcium-binding protein that is more prominently expressed in phasic synapses, was, however, refuted by our results.

Effect of intracellular pH on depolarization-evoked calcium influx in human sperm

Human sperm are endowed with putative voltage-dependent calcium channels (VDCC) that produce measurable increases in intracellular calcium concentration ([Ca2+]i) in response to membrane depolarization with potassium. These channels are blocked by nickel, inactivate in 1–2 min in calcium-deprived medium, and are remarkably stimulated by NH4Cl, suggesting a role for intracellular pH (pHi). In a previous work, we showed that calcium permeability through these channels increases approximately onefold during in vitro “capacitation,” a calcium-dependent process that sperm require to fertilize eggs. In this work, we have determined the pHidependence of sperm VDCC. Simultaneous depolarization and pHialkalinization with NH4Cl induced an [Ca2+]iincrease that depended on the amount of NH4Cl added. VDCC stimulation as a function of pHishowed a sigmoid curve in the 6.6–7.2 pHirange, with a half-maximum stimulation at pH ∼7.00. At higher pHi(≥7.3), a further stimulation occurred. Calcium release from internal stores did not contribute to the stimulating effect of pHibecause the [Ca2+]iincrease induced by progesterone, which opens a calcium permeability pathway that does not involve gating of VDCC, was unaffected by ammonium. The ratio of pHi-stimulated-to-nonstimulated calcium influx was nearly constant at different test depolarization values. Likewise, depolarization-induced calcium influx in pHi-stimulated and nonstimulated cells was equally blocked by nickel. In our capacitating conditions pHiincreased 0.11 pH units, suggesting that the calcium influx stimulation observed during sperm capacitation might be partially caused by pHialkalinization. Additionally, a calcium permeability pathway triggered exclusively by pHialkalinization was detected.