Calcium signalling in granule neurones studied in cerebellar slices

The Thermodynamically Expensive Contribution of Three Calcium Sources to Somatic Release of Serotonin

The soma, dendrites and axon of neurons may display calcium-dependent release of transmitters and peptides. Such release is named extrasynaptic for occurring in absence of synaptic structures. This review describes the cooperative actions of three calcium sources on somatic exocytosis. Emphasis is given to the somatic release of serotonin by the classical leech Retzius neuron, which has allowed detailed studies on the fine steps from excitation to exocytosis. Trains of action potentials induce transmembrane calcium entry through L-type channels. For action potential frequencies above 5 Hz, summation of calcium transients on individual action potentials activates the second calcium source: ryanodine receptors produce calcium-induced calcium release. The resulting calcium tsunami activates mitochondrial ATP synthesis to fuel transport of vesicles to the plasma membrane. Serotonin that is released maintains a large-scale exocytosis by activating the third calcium source: serotonin autoreceptors coupled to phospholipase C promote IP3 production. Activated IP3 receptors in peripheral endoplasmic reticulum release calcium that promotes vesicle fusion. The Swiss-clock workings of the machinery for somatic exocytosis has a striking disadvantage. The essential calcium-releasing endoplasmic reticulum near the plasma membrane hinders the vesicle transport, drastically reducing the thermodynamic efficiency of the ATP expenses and elevating the energy cost of release.

Neuritin promotes neurite and spine growth in rat cerebellar granule cells via L-type calcium channel-mediated calcium influx

Neuritin is a neurotrophic factor that is activated by neural activity and neurotrophins. Its major function is to promote neurite growth and branching; however, the underlying mechanisms are not fully understood. To address this issue, this study investigated the effects of neuritin on neurite and spine growth and intracellular Ca²⁺ concentration in rat cerebellar granule neurons (CGNs). Incubation of CGNs for 24 h with neuritin increased neurite length and spine density; this effect was mimicked by insulin and abolished by inhibiting insulin receptor (IR) or mitogen‐activated protein kinase kinase/extracellular signal‐regulated kinase (ERK) activity. Calcium imaging and western blot analysis revealed that neuritin enhanced the increase in intracellular Ca²⁺ level induced by high K⁺, and stimulated the cell surface expression of Ca_V1.2 and Ca_V1.3 α subunits of the L‐type calcium channel, which was suppressed by inhibition of IR or mitogen‐activated protein kinase kinase/ERK. Treatment with inhibitors of L‐type calcium channels, calmodulin, and calcineurin (CaN) abrogated the effects of neuritin on neurite length and spine density. A similar result was obtained by silencing nuclear factor of activated T cells c4, which is known to be activated by neuritin in CGNs. These results indicate that IR and ERK signaling as well as the Ca²⁺/CaN/nuclear factor of activated T cells c4 axis mediate the effects of neuritin on neurite and spine growth in CGNs.

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Cover Image for this issue: doi: 10.1111/jnc.14195.

Spontaneous calcium waves in granule cells in cerebellar slice cultures

Multiple regions in the CNS display propagating correlated activity during embryonic and postnatal development. This activity can be recorded as waves of increased calcium concentrations in spiking neurons or glia cells, and have been suggested to be involved in patterning, axonal guidance and establishment of synaptic transmission. Here, we used calcium imaging in slice cultures of the postnatal cerebellum, and observe spontaneous propagating calcium waves in NeuN-positive granule-like cells. Wave formation was blocked by TTX and the AMPA antagonist NBQX, but persisted after NMDA receptor blockade with MK-801. Whole-cell recordings during wave formation showed cyclic EPSP barrages with an amplitude of 10-20 mV concurrent with wave activity. Local non-propagating putative transglial waves were also present in the cultures, and could be reproduced by pressure application of ATP. We hypothesize, that the propagating wave activity is carried through the tissue by axonal collaterals formed by neighboring granule cells, and further suggest that the correlated activity may be related to processes that ensure correct postnatal wiring of the cerebellar circuits.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

Effects of calretinin on Ca2+ signals in cerebellar granule cells: implications of cooperative Ca2+ binding

Calretinin is thought to be the main endogenous calcium buffer in cerebellar granule cells (GrCs). However, little is known about the impact of cooperative Ca(2+) binding to calretinin on highly localized and more global (regional) Ca(2+) signals in these cells. Using numerical simulations, we show that an essential property of calretinin is a delayed equilibration with Ca(2+). Therefore, the amount of Ca(2+), which calretinin can accumulate with respect to equilibrium levels, depends on stimulus conditions. Based on our simulations of buffered Ca(2+) diffusion near a single Ca(2+) channel or a large cluster of Ca(2+) channels and previous experimental findings that 150 μM 1,2-bis(o-aminophenoxy) ethane-N, N, N', N'-tetraacetic acid (BAPTA) and endogenous calretinin have similar effects on GrC excitability, we estimated the concentration of mobile calretinin in GrCs in the range of 0.7-1.2 mM. Our results suggest that this estimate can provide a starting point for further analysis. We find that calretinin prominently reduces the action potential associated increase in cytosolic free Ca(2+) concentration ([Ca(2+)]( i )) even at a distance of 30 nm from a single Ca(2+) channel. In spite of a buildup of residual Ca(2+), it maintains almost constant maximal [Ca(2+)]( i ) levels during repetitive channel openings with a frequency less than 80 Hz. This occurs because of accelerated Ca(2+) binding as calretinin binds more Ca(2+). Unlike the buffering of high Ca(2+) levels within Ca(2+) nano/microdomains sensed by large conductance Ca(2+)-activated K(+) channels, the buffering of regional Ca(2+) signals by calretinin can never be mimicked by certain concentration of BAPTA under all different experimental conditions.

Calcium-independent phospholipase A2 mediates store-operated calcium entry in rat cerebellar granule cells

Store-operated Ca(2+) entry (SOCE) has been extensively studied in non-neuronal cells, such as glial cells and smooth muscle cells, in which Ca(2+)-independent phospholipase A(2) (iPLA(2)) has been shown to play a key role in the regulation of SOCE channels. In the present study, we have investigated the role of iPLA(2) for store-operated Ca(2+) entry in rat cerebellar granule neurons in acute brain slices using confocal Ca(2+) imaging. Depletion of Ca(2+) stores by cyclopiazonic acid (CPA) induced a Ca(2+) influx, which could be inhibited by SOCE channel blockers 2-aminoethoxy-diphenylborate (2-APB) and 3,5-bistrifluoromethyl pyrazole derivative (BTP2), but not by the voltage-operated Ca(2+) channel blocker diltiazem and by the Na+ channel blocker tetrodotoxin. The inhibitors of iPLA(2), bromoenol lactone (BEL) and 1,1,1-trifluoro-2-heptadecanone, and the selective suppression of iPLA(2) expression by antisense oligodeoxynucleotides, inhibited CPA-induced Ca(2+) influx. Calmidazolium, which relieves the block of inhibitory calmodulin from iPLA(2), elicited a Ca(2+) influx similar to CPA-induced Ca(2+) entry. The product of iPLA(2), lysophosphatidylinositol, elicited a 2-APB- and BTP2-sensitive, but BEL-insensitive, Ca(2+) influx. Spontaneous Ca(2+) oscillations in granule cells in acute brain slices were reduced after inhibiting iPLA(2) activity or by blocking SOCE channels. The results suggest that depletion of Ca(2+) stores activates iPLA(2) to trigger Ca(2+) influx by the formation of lysophospholipids in these neurons.

Computational study of non-homogeneous distribution of Ca(2+) handling systems in cerebellar granule cells

The spatiotemporal distribution of cytosolic free calcium concentration ([Ca(2+)](i)) in cerebellar granule cells (GrCs) is thought to be critical in defining the occurrence and direction of long-term changes in synaptic strength at cerebellar mossy fiber-GrC synapses. Despite this, the mechanisms responsible for shaping Ca(2+) transients in GrCs are not well understood. To investigate the interplay between Ca(2+) entry, extrusion, buffering and dendritic morphology in shaping Ca(2+) elevations in GrCs, we developed a model of Ca(2+) regulation in these cells and examined the requirements for reproducing fluorescence responses to depolarization and synaptic stimulation previously described in the literature. Two conclusions can be drawn from our simulation results. First, a significant progressive decrease in the amplitudes of depolarization-evoked fluorescence transients from the dendritic endings (digits) toward the soma of GrCs, can be reproduced in the model only if the density of Ca(2+) channels is considerably higher or the concentration of endogenous buffers is much lower in the digits than in the parent dendrites. In contrast, heterogeneities in the distribution of Ca(2+) pumps or in cytosolic fractional volume cannot account for the formation of [Ca(2+)](i) gradients in GrCs. Second, much lower amplitudes of fluorescence transients induced by depolarization and synaptic stimulation than expected from typical measurements of Ca(2+) and NMDA receptor-mediated currents can be reconciled with a pronounced slowing of the decay of fluorescence responses in the digits of GrCs after introducing a high-affinity Ca(2+) indicator if a high-capacity immobile Ca(2+) buffer (presumably plasma membrane-associated) is suggested to be present in the soma and apical part of digits. Mitochondria also are likely to modulate synaptically evoked Ca(2+) responses in GrCs. The alternative hypotheses are thoroughly discussed and research avenues for their testing in future experiments are proposed.

Calcium-induced calcium release contributes to somatic secretion of serotonin in leech Retzius neurons

We analyzed the contribution of calcium (Ca2+)-induced Ca2+ release to somatic secretion in serotonergic Retzius neurons of the leech. Somatic secretion was studied by the incorporation of fluorescent dye FM1-43 upon electrical stimulation with trains of 10 impulses and by electron microscopy. Quantification of secretion with FM1-43 was made in cultured neurons to improve optical resolution. Stimulation in the presence of FM1-43 produced a frequency-dependent number of fluorescent spots. While a 1-Hz train produced 19.5+/-5.0 spots/soma, a 10-Hz train produced 146.7+/-20.2 spots/soma. Incubation with caffeine (10 mM) to induce Ca2+ release from intracellular stores without electrical stimulation and external Ca2+, produced 168+/-21.7 spots/soma. This staining was reduced by 49% if neurons were preincubated with the Ca2+- ATPase inhibitor thapsigargin (200 nM). Moreover, in neurons stimulated at 10 Hz in the presence of ryanodine (100 microM) to block Ca2+-induced Ca2+ release, FM1-43 staining was reduced by 42%. In electron micrographs of neurons at rest or stimulated at 1 Hz in the ganglion, endoplasmic reticulum lay between clusters of dense core vesicles and the plasma membrane. In contrast, in neurons stimulated at 20 Hz, the vesicle clusters were apposed to the plasma membrane and flanked by the endoplasmic reticulum. These results suggest that Ca2+-induced Ca2+ release produces vesicle mobilization and fusion in the soma of Retzius neurons, and supports the idea that neuronal somatic secretion shares common mechanisms with secretion by excitable endocrine cells.

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.

Granule neurons in cerebellum express distinct splice variants of the inositol trisphosphate receptor that are modulated by calcium

Primary cultures of granule cells (GC) from rat cerebellar cortex were used to determine whether bioelectric activity, via a Ca(2+)/calmodulin-dependent kinase (CaMK) signaling cascade, modulates expression and exon selection in the inositol trisphosphate receptor type 1 (IP(3)R1). IP(3)R1 contains or lacks three exons (S1, S2, and S3) that are regulated in a regionally and temporally specific manner. The neuronal, or long, form of IP(3)R1 is distinguished from peripheral tissues by inclusion of the S2 exon. Although previous studies indicated that IP(3)R1 are undetectable in the cerebellar granular layer in vivo, receptor protein and mRNA are induced in cultured GC grown in medium supplemented with 25 mM KCl or NMDA, two trophic agents that promote long-term survival, compared with GC grown in 5 mM KCl. IP(3)R1 induction in response to 25 mM KCl or NMDA is attenuated by coaddition of voltage-sensitive calcium channel or NMDA receptor antagonists, respectively. Actinomycin D, CaMK, and calcineurin antagonists likewise suppress induction. Unlike the major variants of IP(3)R1 in Purkinje neurons, which lack S1 and S3, GC grown with trophic agents express mRNA containing these exons. Both neuronal types contain S2. Evidence obtained using mutant mice with Purkinje cell lesions, laser-microdissected GC neurons from slices, and explant cultures indicates that GC predominantly express the S1-containing variant of IP(3)R1 in vivo.

Endoplasmic reticulum Ca(2+) homeostasis and neuronal death

The endoplasmic reticulum (ER) is a universal signalling organelle, which regulates a wide range of neuronal functional responses. Calcium release from the ER underlies various forms of intracellular Ca(2+) signalling by either amplifying Ca(2+) entry through voltage-gated Ca(2+) channels by Ca(2+)-induced Ca(2+) release (CICR) or by producing local or global cytosolic calcium fluctuations following stimulation of metabotropic receptors through inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The ER Ca(2+) store emerges as a single interconnected pool, thus allowing for a long-range Ca(2+) signalling via intra-ER tunnels. The fluctuations of intra-ER free Ca(2+) concentration regulate the activity of numerous ER resident proteins responsible for post-translational protein folding and modification. Disruption of ER Ca(2+) homeostasis results in the developing of ER stress response, which in turn controls neuronal survival. Altered ER Ca(2+) handling may be involved in pathogenesis of various neurodegenerative diseases including brain ischemia and Alzheimer dementia.

Coexpression of functional P2X and P2Y nucleotide receptors in single cerebellar granule cells

The present study describes the presence and expression of functional nucleotide receptors, both ionotropic and metabotropic, in highly purified cultures of cerebellar granule neurons. Microfluorimetric experiments have been carried out to record specific [Ca(2+)](i) transients in individual granule neurons after challenge with diverse nucleotides. Although great heterogeneity was found in nucleotide responses in single cells, these responses all became modified during the course of granule cell differentiation, not only at the level of the number of responding cells, but also in the magnitude of the response to nucleotides. These in vitro developmental changes were more significant in metabotropic responses to pyrimidine nucleotides, UTP and UDP, which were down- and upregulated, respectively, during the time in culture. At least two types of ADP-specific receptors seem expressed in different granule cell subpopulations responding to 2MeSADP, as the specific P2Y(1) antagonist MRS-2179 inhibited Ca(2+) responses in only one of these populations. The great diversity of metabotropic responses observed was confirmed by the RT-PCR expression of different types of P2Y receptors in granule cell cultures: P2Y(1), P2Y(4), P2Y(6), and P2Y(12). Similarly, ionotropic nucleotide responses were confirmed by the presence of specific messengers for different P2X subunits, and by immunolabeling studies (P2X(1), P2X(2), P2X(3), P2X(4) and P2X(7)). Immunolabeling reflected great variety in the P2X subunit distribution along the granule neuron cytoarchitecture, with P2X(2), P2X(3) and P2X(4) present at somatodendritic locations, and P2X(1), P2X(7), and P2X(3), located at the axodendritic prolongations. The punctuated labeling pattern obtained for P2X(3) and P2X(7) subunits is particularly notable, as it presents a high degree of colocalization with synaptophysin, a specific marker of synaptic vesicles, suggesting specialized localization and function in granule neurons.

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

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

Multiple modes of GABAergic inhibition of rat cerebellar granule cells

Cerebellar granule cells are inhibited phasically by GABA released synaptically from Golgi cells, but are inhibited more powerfully by tonic activity of high affinity alpha 6 subunit-containing GABAA receptors. During development the tonic activity is generated by the accumulation of GABA released by action potentials, but in the adult the tonic activity is independent of action potentials. Here we show that in adult rats the tonic activation of GABAA receptors is produced by non-vesicular transmitter release and is reduced by the activity of GAT-1 and GAT-3 GABA transporters, demonstrating that alterations of GABA uptake will modulate information flow through granule cells. Acetylcholine (ACh) evokes a large Ca2+-dependent but action potential-independent release of GABA, which activates alpha 6 subunit-containing GABAA receptors. These data show that three separate modes of transmitter release can activate GABAA receptors in adult cerebellar granule cells: action potential-evoked exocytotic GABA release, non-vesicular release, and ACh-evoked Ca2+-dependent release independent of action potentials. The relative magnitudes of the inhibitory charge transfers generated by action potential-evoked release (during high frequency stimulation of the mossy fibres), tonic inhibition and superfused ACh are 1:3:12, indicating that tonic and ACh-mediated inhibition may play a major role in regulating granule cell firing.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

Spontaneous network activity of cerebellar granule neurons: impairment by in vivo chronic cannabinoid administration

Synchronized activity of neuronal networks has been proposed to be essential for cerebellar function. To examine the occurrence and organization of spontaneous neuronal activity in the cerebellum in vivo, we imaged mouse cerebellar slices loaded with the intracellular Ca2+ concentration indicator, fura-2. Recordings were then analysed statistically to identify correlated network activity. Ca2+ imaging revealed consistent spontaneous correlated network activity of granule cells (GC), which often occurred in clusters of coactivated GC. The number of spontaneously active GC, their activation frequency and correlation, were controlled by glutamate and GABA ionotropic receptors. These findings indicate that distinctive patterns of correlated activity between GC networks may be relevant for cerebellar circuit function. Cannabinoid antagonist-precipitated delta9-tetrahydrocannabinol (THC) withdrawal impaired motor coordination. Given that the cerebellum has been suggested recently to be a main substrate for cannabinoid withdrawal, we used imaging of spontaneous network activity to examine whether GC, which contain CB1 cannabinoid receptors, respond to chronic THC treatment and withdrawal. Acute administration of THC had no effect on patterns of spontaneous GC network activity. In contrast, chronic THC administration severely impaired GC activity and network coordination. Incubation of cerebellar slices, from chronically THC-treated mice, with the cannabinoid antagonist, SR141716A increased the number and network correlation of active GC. These data provide physiological evidence of the involvement of cerebellar circuits in the adaptive changes occurring during chronic THC exposure and withdrawal.

The endoplasmic reticulum and neuronal calcium signalling

The endoplasmic reticulum (ER) is a multifunctional signalling organelle regulating a wide range of neuronal functional responses. The ER is intimately involved in intracellular Ca(2+) signalling, producing local or global cytosolic calcium fluctuations via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The CICR and IICR are controlled by two subsets of Ca(2+) release channels residing in the ER membrane, the Ca(2+)-gated Ca(2+) release channels, generally known as ryanodine receptors (RyRs) and InsP(3)-gated Ca(2+) release channels, referred to as InsP(3)-receptors (InsP(3)Rs). Both types of Ca(2+) release channels are expressed abundantly in nerve cells and their activation triggers cytoplasmic Ca(2+) signals important for synaptic transmission and plasticity. The RyRs and InsP(3)Rs show heterogeneous localisation in distinct cellular sub-compartments, conferring thus specificity in local Ca(2+) signals. At the same time, the ER Ca(2+) store emerges as a single interconnected pool fenced by the endomembrane. The continuity of the ER Ca(2+) store could play an important role in various aspects of neuronal signalling. For example, Ca(2+) ions may diffuse within the ER lumen with comparative ease, endowing this organelle with the capacity for "Ca(2+) tunnelling". Thus, continuous intra-ER Ca(2+) highways may be very important for the rapid replenishment of parts of the pool subjected to excessive stimulation (e.g. in small compartments within dendritic spines), the facilitated removal of localised Ca(2+) loads, and finally in conveying Ca(2+) signals from the site of entry towards the cell interior and nucleus.

Glutamate-induced excitotoxicity in retina: neuroprotection with receptor antagonist, dextromethorphan, but not with calcium channel blockers

The purpose of our studies was to evaluate different strategies for possible neuroprotection in glutamate-induced neurotoxicity in the retina. In a first set of experiments we attempted to determine if dextrorphan antagonism of glutamate action on NMDA receptors would protect against excitotoxic injury associated with secondary damage seen after surgical laser treatment in retina. In a second set of experiments, the effects of different calcium channel blockers in an in-vitro model of N-methyl-D-aspartate (NMDA)-induced retinal ganglion cell excitotoxicity that utilized rabbit retinal explants were evaluated. Dextrorphan infusion prior to laser treatment of rabbit retina produced a significant decrease in the area of neural retinal damage. We attribute the apparent dextrorphan protection to attenuation of glutamate mediated excitotoxicity secondary to laser induced cell death. Preincubation of rabbit retinal explants with verapamil, nimodipine or omega-conotoxin MVIIA did not cause a significant change in NMDA induced cell death in the ganglion cell layer.

Omega-conotoxin GVIA potently inhibits the currents mediated by P2X receptors in rat DRG neurons

We examined effects of omega-conotoxin previously known as a selective blocker of N-type calcium channels, on the adenosine triphosphate (ATP)-induced currents in the rat dorsal root ganglion neurons. These neurons express at least two types of ionotropic purinoreceptors: P2X3 receptors that have very rapid desensitization kinetics and P2X2/X3 heterooligomeric receptor, which exhibits slow desensitization. We have found that omega-conotoxin GVIA potently inhibits the inward currents mediated by both receptor types. This effect was specific for the receptor subtypes: the IC(50) value for responses evoked by 10 microM ATP was 21.2 +/- 1.7 nM for the P2X3 receptor-mediated responses and 3.84 +/- 0.43 microM for slower responses mediated by P2X2/X3 heteropolymers. The efficacy of another type of omega-conotoxin, MVIIC, is much lower: at 10 microM the latter toxin inhibited the rapidly desensitizing response by 65% and the slowly desensitizing response by 18%. The effects of both toxins were reversible and independent on the membrane potential. Omega-Conotoxin GVIA shifted the dose dependence for the agonistic action of ATP on P2X3 receptors to higher concentrations without producing any effect on the kinetics of the response. It is suggested that omega-conotoxin allosterically modulates the receptor properties, rather than competes for the agonist binding site.

The endoplasmic reticulum and mitochondria as elements of the mechanism of intracellular signaling in the nerve cell

Experimental data obtained in our laboratory from studies of intracellular signals arising within nerve cells during excitation are summarized. Measurements of transmembrane ion currents in conditions of fixed membrane potential and intracellular free Ca ion concentrations, using fluorescent probes, yielded the time and spatial characteristics of transient elevations in the Ca concentration (the "calcium signal") in various types of mouse and rat neurons. These studies showed that intracellular structures-the endoplasmic reticulum and mitochondria-had significant roles in forming these signals; these structures can take up Ca from the cytosol and liberate Ca into the cytosol; the contribution of these processes was extremely variable, depending on the internal organization of different functional types of neurons.

Developmental regulation of intracellular calcium by N-methyl-D-aspartate and noradrenaline in rat visual cortex

The effects of N-methyl-D-aspartate and noradrenaline on intracellular Ca2+ concentration in slices of rat visual cortex were studied using a fluorescent indicator, Fura-2. Bath application of N-methyl-D-aspartate (1-100 microM) increased intracellular Ca2+ concentration in a dose-dependent manner, especially in layers II/III. Noradrenaline (1-100 microM) also increased intracellular Ca2+ concentration in a dose-dependent manner, especially in layers I and IV. However, the maximum increase in intracellular Ca2+ concentration after 100 microM noradrenaline application was less than half of that after 100 microM N-methyl-D-aspartate application in slices obtained from animals in the sensitive period. The effect of noradrenaline was most prominent in slices of the sensitive period, whereas the N-methyl-D-aspartate-induced intracellular Ca2+ concentration response decreased with age. Additive effects from application of both N-methyl-D-aspartate and noradrenaline on intracellular Ca2+ concentration were found only in the neonatal stage. Pharmacological experiments showed that alpha1-adrenergic receptors play a major role in the noradrenaline-induced intracellular Ca2+ concentration response, although both alpha2- and beta-adrenergic receptors were also partially involved. The release of Ca2+ from intracellular storage underlay the early phase of the noradrenaline-induced intracellular Ca2+ concentration response, while extracellular Ca2+ influxes contributed to the sustained phase. Experiments using a gliotoxin, fluorocitric acid, suggested that the function of glial cells is involved in the noradrenaline-induced increase of intracellular Ca2+ concentration. The larger intracellular Ca2+ concentration response to noradrenaline during the sensitive period may modulate the increase in intracellular Ca2+ concentration by N-methyl-D-aspartate to maintain a higher level of cortical plasticity during this period.

Single synaptic events evoke NMDA receptor-mediated release of calcium from internal stores in hippocampal dendritic spines

We have used confocal microscopy to monitor synaptically evoked Ca2+ transients in the dendritic spines of hippocampal pyramidal cells. Individual spines respond to single afferent stimuli (<0.1 Hz) with Ca2+ transients or failures, reflecting the probability of transmitter release at the activated synapse. Both AMPA and NMDA glutamate receptor antagonists block the synaptically evoked Ca2+ transients; the block by AMPA antagonists is relieved by low Mg2+. The Ca2+ transients are mainly due to the release of calcium from internal stores, since they are abolished by antagonists of calcium-induced calcium release (CICR); CICR antagonists, however, do not depress spine Ca2+ transients generated by backpropagating action potentials. These results have implications for synaptic plasticity, since they show that synaptic stimulation can activate NMDA receptors, evoking substantial Ca2+ release from the internal stores in spines without inducing long-term potentiation (LTP) or depression (LTD).

Developmental changes in purinergic calcium signalling in rat neocortical neurones

The changes in cytoplasm free calcium concentration ([Ca2+]i transients) were measured in Fura-2/AM loaded pyramidal neurones of sensorimotor cortex in acutely prepared slices isolated from 14 days (P14) and 30 (P30) days old rats. Ni2+ (50 microM) diminished the [Ca2+]i transients evoked by 50 mM KCl bath application by 47%+/-8% in neurones of the P14 group and only by 15%+/-6% in those of P30 group (P<0.002). Nifedipine and verapamil in concentration 100 microM reduced the calcium transients amplitude triggered by depolarization to about the same extent in both groups of neurons-on average by 50% and 35%, respectively. The amplitude of [Ca2+]i transients induced by application of 100 microM ATP reached 103+/-6 nM in P14 neurones and 72+/-8 nM in the P30 ones. The ATP-[Ca2+]i induced transient could be evoked in Ca2+-free external solution, indicating the presence of metabotropic (P2y) purinoreceptors. Almost all (90%) P14 neurones were endowed with such receptors. At the same time only 1/3 of the tested (n=42) P30 neurones presented responses to ATP applications in Ca2+-free solution. The share of ionotropic (P2x) purinoreceptors in generation of calcium signal was the same in both groups of neurons. No caffeine-induced Ca2+-release has been observed in the P14 neurons. To the contrary, in 28 cells from 42 investigated neurones of the P30 group application of 40 mM caffeine for 10 s induced considerable [Ca2+]i transients, which did not disappear in calcium-free solution. A conclusion is made about substantial changes in the expression of Ca2+-handling mechanisms which are occurring in neocortical neurones at the third-fourth week of postnatal development.

Iono- and metabotropically induced purinergic calcium signalling in rat neocortical neurons

ATP receptor-mediated Ca2+ concentration changes were recorded from neocortical neurones in brain slices from 2 week-old rats. To measure the cytoplasmic concentration of Ca2+ ([Ca2+]i) slices were incubated with fura-2/AM, and the microfluorimetry system was focused on an individual cell. During transients the intracellular level of [Ca2+]i in the majority of neocortical neurones (98 of 102) varied in the concentration range of ATP 5-2000 microM between 41. 3+/-5 and 163+/-7 nM. The rank order of efficacy for purinoreceptor agonists in concentration 100 microM was: ATPgammaS>ATP>ADP>>AMP approximately Adenosine approximately alpha,beta-methylene ATP>UTP. 10 microM PPADS, a P2-purinoreceptor antagonist, reduced the ATP-induced [Ca2+]i response by 26%+/-4%. After elimination of calcium from extracellular solution the first ATP-induced [Ca2+]i transient decreased to 65+/-8%, suggesting the participation of metabotropic P2y triggered Ca-release in the generation of the transient. Elevation of cytosolic Ca2+ by activation of plasmalemmal Ca2+ channels failed to potentiate such release indicating the absence of effective reloading of the corresponding stores. No Ca2+-induced Ca2+-release has been observed in the investigated neurons.

Activation of P2-purino-, alpha 1-adreno and H1-histamine receptors triggers cytoplasmic calcium signalling in cerebellar Purkinje neurons

The cytoplasmic calcium concentration ([Ca2+]i) was measured from Purkinje neurons in acutely prepared cerebellar slices. Neurons were loaded with calcium indicator Fura-2 by 40-min slice incubation in Tyrode solution containing 5 microM Fura-2/AM and 0.02% pluronic-F127. Bath applications of ATP (100 microM), epinephrine (10 microM) and histamine (100 microM) triggered a transient increase of [Ca2+]i in Purkinje neurons. ATP-induced [Ca2+]i elevation in Purkinje neurons was mimicked by ADP, but not AMP or adenosine pointing to the involvement of P2Y metabotropic purinoreceptors. Epinephrine-triggered [Ca2+]i responses were blocked by the selective alpha 1-antagonist prazosin and were mimicked by the alpha 1-adrenoreceptor agonist phenylephrine, and were not affected by beta- and alpha 2-adrenoreceptor agonists (isoproterenol and clonidine) and antagonists (propranolol and yohimbine). Histamine-induced [Ca2+]i responses demonstrated specific sensitivity to selective H1 antagonist chlorpheniramine, and were not sensitive to H2 and H3 histamine receptors modulators. The [Ca2+]i responses to all three agonists persisted in Ca(2+)-free extracellular media and were blocked by slice preincubation with thapsigargin (500 nM). We conclude that cerebellar Purkinje neurons are endowed with metabotropic P2 gamma purinoreceptors, alpha 1-adrenoreceptors and H1 histamine receptors which mediate the generation of intracellular [Ca2+]i signals via activation of Ca2+ release from inositol-1,4,5-trisphosphate-sensitive intracellular stores.

Calcium homeostasis in aged neurones

Mechanisms of cytoplasmic calcium homeostasis were investigated in peripheral and central neurones isolated from neonatal, adult and old Wistar rats and in granule neurones in acutely prepared cerebellar slices of adult and old CBA mice. The cytoplasmic calcium concentration ([Ca2+]i) was measured by either indo-1-or fura-2-based microfluorimetry. The resting [Ca2+]i was significantly higher in senile neurones. The depolarization-induced [Ca2+]i transients were markedly altered in old neurones when compared with adult ones: the age-associated changes in stimulus-evoked [Ca2+]i signalling comprised of (i) significant decrease of the amplitudes of [Ca2+]i transients; (ii) prolongation of the rising phase and (iii) prominent deceleration of the recovery of the [Ca2+]i elevation towards the resting level after the end of depolarization. The amplitudes of calcium release from caffeine/Ca(2+)-sensitive endoplasmic reticulum calcium stores became significantly smaller in old central neurones, whereas they remained unaffected in peripheral neurones. Based on our observations we can conclude that ageing of the nervous system is associated with significant changes in mechanisms of [Ca2+]i homeostasis in individual neurones. These changes lead to a stable increase in the resting [Ca2+]i and to a substantial prolongation of stimulus-evoked [Ca2+]i signals. We could suggest also that the ability of the old neurones to handle Ca2+ loads is diminished, which may determine higher vulnerability of aged neurones to excess of calcium ions.

Calcium-induced calcium release in neurones

Neurones express several subtypes of intracellular Ca2+ channels, which are regulated by cytoplasmic calcium concentration ([Ca2+]c) and provide the pathway for Ca(2+)-induced Ca2+ release (CICR) from endoplasmic reticulum Ca2+ stores. The initial studies of CICR which employed several pharmacological tools (and in particular caffeine and ryanodine) demonstrated that: (i) caffeine induces intracellular calcium release in various peripheral and central neurones; and (ii) inhibition of CICR affects the parameters of depolarization-triggered [Ca2+]c responses. Experiments with caffeine demonstrated also that Ca2+ release from internal pools was incremental, suggesting the coexistence of several subpopulations of Ca2+ release channels with different sensitivity to caffeine. The CICR availability in neurones is controlled by both the Ca2+ content of the internal stores and the basal [Ca2+]c. Direct comparison of transmembrane Ca2+ influx with plasmalemmal Ca2+ current and [Ca2+]c elevation performed on sympathetic, sensory and cerebellar Purkinje neurones revealed the gradual activation of CICR. The efficacy of CICR may be regulated by the newly discovered second messenger cADP ribose (cADPR), although the mechanism of signal transduction involving cADPR is still unknown. CICR in neurones may be important in creation of local [Ca2+]c signals and could be involved in a regulation of numerous neuronal functions.