Synchronous calcium oscillations in cerebellar granule cells in culture mediated by NMDA receptors

Phospholipid Scramblase 1 Controls Efficient Neurotransmission and Synaptic Vesicle Retrieval at Cerebellar Synapses

Phospholipids (PLs) are asymmetrically distributed at the plasma membrane. This asymmetric lipid distribution is transiently altered during calcium-regulated exocytosis, but the impact of this transient remodeling on presynaptic function is currently unknown. As phospholipid scramblase 1 (PLSCR1) randomizes PL distribution between the two leaflets of the plasma membrane in response to calcium activation, we set out to determine its role in neurotransmission. We report here that PLSCR1 is expressed in cerebellar granule cells (GrCs) and that PLSCR1-dependent phosphatidylserine egress occurred at synapses in response to neuron stimulation. Synaptic transmission is impaired at GrC Plscr1 -/- synapses, and both PS egress and synaptic vesicle (SV) endocytosis are inhibited in Plscr1 -/- cultured neurons from male and female mice, demonstrating that PLSCR1 controls PL asymmetry remodeling and SV retrieval following neurotransmitter release. Altogether, our data reveal a novel key role for PLSCR1 in SV recycling and provide the first evidence that PL scrambling at the plasma membrane is a prerequisite for optimal presynaptic performance.

Interleukin-10 restores glutamate receptor-mediated Ca2+-signaling in brain circuits under loss of Sip1 transcription factor

OBJECTIVE

This study aimed to investigate the connection between the mutation of the Sip1 transcription factor and impaired Ca²⁺-signaling, which reflects changes in neurotransmission in the cerebral cortex in vitro.

METHODS

We used mixed neuroglial cortical cell cultures derived from Sip1 mutant mice. The cells were loaded with a fluorescent ratiometric calcium-sensitive probe Fura-2 AM and epileptiform activity was modeled by excluding magnesium ions from the external media or adding a GABA(A) receptor antagonist, bicuculline. Intracellular calcium dynamics were recorded using fluorescence microscopy. To identify the level of gene expression, the Real-Time PCR method was used.

RESULTS

It was found that cortical neurons isolated from homozygous (Sip1^fl/fl) mice with the Sip1 mutation demonstrate suppressed Ca²⁺ signals in models of epileptiform activity in vitro. Wild-type cortical neurons are characterized by synchronous high-frequency and high-amplitude Ca²⁺ oscillations occurring in all neurons of the network in response to Mg²⁺-free medium and bicuculline. But cortical Sip1^fl/fl neurons only single Ca²⁺ pulses or attenuated Ca²⁺ oscillations are recorded and only in single neurons, while most of the cell network does not respond to these stimuli. This signal deficiency of Sip1^fl/fl neurons correlates with a suppressed expression level of the genes encoding the subunits of NMDA, AMPA, and KA receptors; protein kinases PKA, JNK, CaMKII; and also the transcription factor Hif1α. These negative effects were partially abolished when Sip1^fl/fl neurons are grown in media with anti-inflammatory cytokine IL-10. IL-10 increases the expression of the above-mentioned genes but not to the level of expression in wild-type. At the same time, the amplitudes of Ca²⁺ signals increase in response to the selective agonists of NMDA, AMPA and KA receptors, and the proportion of neurons responding with Ca²⁺ oscillations to a Mg²⁺-free medium and bicuculline increases.

CONCLUSION

IL-10 restores neurotransmission in neuronal networks with the Sip1 mutation by regulating the expression of genes encoding signaling proteins.

Calcium-induced apoptosis of developing cerebellar granule neurons depends causally on NGFI-B

Immediate early gene nerve growth factor-induced clone B (NGFI-B), a nuclear receptor important for differentiation and apoptosis, is expressed in mice and rat cerebellum from an early stage of postnatal development. Following apoptotic stimuli NGFI-B translocates to mitochondria to initiate cell death processes. Controlled cell death is critical for correct cerebellar development. Immunohistochemical analysis of NGFI-B in sections of mice cerebella showed NGFI-B to be expressed in granule neurons in vivo at a time (P8-11) when apoptosis is known to occur. The importance of NGFI-B for apoptosis of cultured rat cerebellar granule neurons was investigated by inducing apoptosis with calcium ionophore A23187 (CaI, 0.1μM). Imaging studies of gfp-tagged NGFI-B confirmed that mitochondrial translocation of NGFI-B occurred following treatment with CaI and was reduced by addition of 9-cis-retinoic acid (1μM), a retinoid X receptor (RXR) agonist that prevents dimerization of RXR and NGFI-B that is known to occur before translocation. Consequently, 9-cis-retinoic acid partly reduced cell death. To address the causality of NGFI-B in apoptosis further, knock-down by siRNA was performed and it removed 85% of the NGFI-B protein. This resulted in a complete inhibition of apoptosis after CaI exposure. Together these findings suggest that NGFI-B plays a role in controlling correct cerebellar development.

Calcium oscillations induced by gambierol in cerebellar granule cells

Gambierol is a marine polyether ladder toxin derived from the dinoflagellate Gambierdiscus toxicus. To date, gambierol has been reported to act either as a partial agonist or as an antagonist of sodium channels or as a blocker of voltage-dependent potassium channels. In this work, we examined the cellular effect of gambierol on cytosolic calcium concentration, membrane potential and sodium and potassium membrane currents in primary cultures of cerebellar granule cells. We found that at concentrations ranging from 0.1 to 30 microM, gambierol-evoked [Ca(2+)]c oscillations that were dependent on the presence of extracellular calcium, irreversible and highly synchronous. Gambierol-evoked [Ca(2+)]c oscillations were completely eliminated by the NMDA receptor antagonist APV and by riluzole and delayed by CNQX. In addition, the K(+) channel blocker 4-aminopyridine (4-AP)-evoked cytosolic calcium oscillations in this neuronal system that were blocked by APV and delayed in the presence of CNQX. Electrophysiological recordings indicated that gambierol caused membrane potential oscillations, decreased inward sodium current amplitude and decreased also outward IA and IK current amplitude. The results presented here point to a common mechanism of action for gambierol and 4-AP and indicate that gambierol-induced oscillations in cerebellar neurons are most likely secondary to a blocking action of the toxin on voltage-dependent potassium channels and hyperpolarization of sodium current activation.

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.

The role of the t-SNARE SNAP-25 in action potential-dependent calcium signaling and expression in GABAergic and glutamatergic neurons

BACKGROUND

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, comprised of SNAP-25, syntaxin 1A, and VAMP-2, has been shown to be responsible for action potential (AP)-dependent, calcium-triggered release of several neurotransmitters. However, this basic fusogenic protein complex may be further specialized to suit the requirements for different neurotransmitter systems, as exemplified by neurons and neuroendocrine cells. In this study, we investigate the effects of SNAP-25 ablation on spontaneous neuronal activity and the expression of functionally distinct isoforms of this t-SNARE in GABAergic and glutamatergic neurons of the adult brain.

RESULTS

We found that neurons cultured from Snap25 homozygous null mutant (Snap25-/-) mice failed to develop synchronous network activity seen as spontaneous AP-dependent calcium oscillations and were unable to trigger glial transients following depolarization. Voltage-gated calcium channel (VGCC) mediated calcium transients evoked by depolarization, nevertheless, did not differ between soma of SNAP-25 deficient and control neurons. Furthermore, we observed that although the expression of SNAP-25 RNA transcripts varied among neuronal populations in adult brain, the relative ratio of the transcripts encoding alternatively spliced SNAP-25 variant isoforms was not different in GABAergic and glutamatergic neurons.

CONCLUSION

We propose that the SNAP-25b isoform is predominantly expressed by both mature glutamatergic and GABAergic neurons and serves as a fundamental component of SNARE complex used for fast synaptic communication in excitatory and inhibitory circuits required for brain function. Moreover, SNAP-25 is required for neurons to establish AP-evoked synchronous network activity, as measured by calcium transients, whereas the loss of this t-SNARE does not affect voltage-dependent calcium entry.

Developing Itô stochastic differential equation models for neuronal signal transduction pathways

Mathematical modeling and simulation of dynamic biochemical systems are receiving considerable attention due to the increasing availability of experimental knowledge of complex intracellular functions. In addition to deterministic approaches, several stochastic approaches have been developed for simulating the time-series behavior of biochemical systems. The problem with stochastic approaches, however, is the larger computational time compared to deterministic approaches. It is therefore necessary to study alternative ways to incorporate stochasticity and to seek approaches that reduce the computational time needed for simulations, yet preserve the characteristic behavior of the system in question. In this work, we develop a computational framework based on the Itô stochastic differential equations for neuronal signal transduction networks. There are several different ways to incorporate stochasticity into deterministic differential equation models and to obtain Itô stochastic differential equations. Two of the developed models are found most suitable for stochastic modeling of neuronal signal transduction. The best models give stable responses which means that the variances of the responses with time are not increasing and negative concentrations are avoided. We also make a comparative analysis of different kinds of stochastic approaches, that is the Itô stochastic differential equations, the chemical Langevin equation, and the Gillespie stochastic simulation algorithm. Different kinds of stochastic approaches can be used to produce similar responses for the neuronal protein kinase C signal transduction pathway. The fine details of the responses vary slightly, depending on the approach and the parameter values. However, when simulating great numbers of chemical species, the Gillespie algorithm is computationally several orders of magnitude slower than the Itô stochastic differential equations and the chemical Langevin equation. Furthermore, the chemical Langevin equation produces negative concentrations. The Itô stochastic differential equations developed in this work are shown to overcome the problem of obtaining negative values.

An N-methyl-D-aspartate receptor mediated large, low-frequency, spontaneous excitatory postsynaptic current in neonatal rat spinal dorsal horn neurons

Examples of spontaneous oscillating neural activity contributing to both pathological and physiological states are abundant throughout the CNS. Here we report a spontaneous oscillating intermittent synaptic current located in lamina I of the neonatal rat spinal cord dorsal horn. The spontaneous oscillating intermittent synaptic current is characterized by its large amplitude, slow decay time, and low-frequency. We demonstrate that post-synaptic N-methyl-D-aspartate receptors (NMDARs) mediate the spontaneous oscillating intermittent synaptic current, as it is inhibited by magnesium, bath-applied d-2-amino-5-phosphonovalerate (APV), or intracellular MK-801. The NR2B subunit of the NMDAR appears important to this phenomenon, as the NR2B subunit selective NMDAR antagonist, alpha-(4-hydroxphenyl)-beta-methyl-4-benzyl-1-piperidineethanol tartrate (ifenprodil), also partially inhibited the spontaneous oscillating intermittent synaptic current. Inhibition of spontaneous glutamate release by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or the mu-opioid receptor agonist [D-Ala2, N-Me-Phe4, Gly5] enkephalin-ol (DAMGO) inhibited the spontaneous oscillating intermittent synaptic current frequency. Marked inhibition of spontaneous oscillating intermittent synaptic current frequency by tetrodotoxin (TTX), but not post-synaptic N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), suggests that the glutamate release important to the spontaneous oscillating intermittent synaptic current is dependent on active neural processes. Conversely, increasing dorsal horn synaptic glutamate release by GABAA or glycine inhibition increased spontaneous oscillating intermittent synaptic current frequency. Moreover, inhibiting glutamate transporters with threo-beta-benzyloxyaspartic acid (DL-TBOA) increased spontaneous oscillating intermittent synaptic current frequency and decay time. A possible functional role of this spontaneous NMDAR-mediated excitatory postsynaptic current in modulating nociceptive transmission within the spinal cord is discussed.

Spontaneous synchronized calcium oscillations in neocortical neurons in the presence of physiological [Mg(2+)]: involvement of AMPA/kainate and metabotropic glutamate receptors

Primary cultures of neocortical neurons exhibit spontaneous Ca(2+) oscillations under zero or low extracellular [Mg(2+)] conditions. We find that mature murine neocortical neurons cultured for 9 days also produce spontaneous Ca(2+) oscillations in the presence of physiological [Mg(2+)]. These Ca(2+) oscillations were action potential mediated inasmuch as tetrodotoxin eliminated their occurrence. AMPA receptors were found to regulate the frequency of Ca(2+) oscillations. In contrast, Ca(2+) oscillations were independent of activation of L-type Ca(2+) channels, and NMDA receptors provided only a minor contribution. Release of intracellular Ca(2+) stores was involved in the oscillatory activity since thapsigargin reduced the amplitude and frequency of the oscillations. S-4-carboxyphenylglycine (S)-4CPG), an antagonist of group I metabotropic glutamate receptor (mGluR), also reduced the amplitude of oscillations. In addition, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), a group I mGluR agonist, increased the oscillation frequency, suggesting a critical role for mGluR in the generation of Ca(2+) oscillations. The mGluR-mediated release of intracellular Ca(2+) stores appeared to be mediated by phospholipase C (PLC) since the PLC inhibitor U73122 eliminated the Ca(2+) oscillations. These results indicate that Ca(2+) oscillations in neocortical cultures in the presence of physiologic [Mg(2+)] are primarily initiated by excitatory input from AMPA receptors and involve mobilization of intracellular Ca(2+) stores following activation of mGluR.

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.

Block of glutamate-glutamine cycle between astrocytes and neurons inhibits epileptiform activity in hippocampus

Recurrent epileptiform activity occurs spontaneously in cultured CNS neurons and in brain slices in which GABA inhibition has been blocked. We demonstrate here that pharmacological treatments resulting in either the block of glutamine production by astrocytes or the inhibition of glutamine uptake by neurons suppress or markedly decrease the frequency of spontaneous epileptiform discharges both in primary hippocampal cultures and in disinhibited hippocampal slices. These data point to an important role for the neuron-astrocyte metabolic interaction in sustaining episodes of intense rhythmic activity in the CNS, and thereby reveal a new potential target for antiepileptic treatments.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

NMDA receptor-dependent periodic oscillations in cultured spinal cord networks

Cultured spinal cord networks grown on microelectrode arrays display complex patterns of spontaneous burst and spike activity. During disinhibition with bicuculline and strychnine, synchronized burst patterns routinely emerge. However, the variability of both intra- and interculture burst periods and durations are typically large under these conditions. As a further step in simplification of synaptic interactions, we blocked excitatory AMPA synapses with 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzoquinoxaline-7-sulphonamide (NBQX), resulting in network activity mediated through the N-methyl-D-aspartate (NMDA) receptor (NMDA(ONLY)). This activity was APV sensitive. The oscillation under NMDA(ONLY) conditions at 37 degrees C was characterized by a period of 2.9 +/- 0.3 s (16 separate cultures). More than 98% of all neurons recorded participated in this highly rhythmic activity. The temporal coefficients of variation, reflecting the rhythmic nature of the oscillation, were 3.7, 4.7, and 4.9% for burst rate, burst duration, and interburst interval, respectively [mean coefficients of variation (CVs) for 16 cultures]. The oscillation persisted for at least 12 h without change (maximum observation time). Once established, it was not perturbed by agents that block mGlu receptors, GABA(B) receptors, cholinergic receptors, purinergic receptors, tachykinin receptors, serotonin (5-HT) receptors, dopamine receptors, electrical synapses, burst afterhyperpolarization, NMDA receptor desensitization, or the hyperpolarization-activated current. However, the oscillation was destroyed by bath application of NMDA (20-50 microM). These results suggest a presynaptic mechanism underlying this periodic rhythm that is solely dependent on the NMDA synapse. When the AMPA/kainate synapse was the sole driving force (n = 6), the resulting burst patterns showed much higher variability and did not develop the highly periodic, synchronized nature of the NMDA(ONLY) activity. Network size or age did not appear to influence the reliability of expression of the NMDA(ONLY) activity pattern. For this reason, we suggest that the NMDA(ONLY) condition unmasks a fundamental rhythmogenic mechanism of possible functional importance during periods of NMDA receptor-dominated activity, such as embryonic and early postnatal development.

Astrocytes are required for the oscillatory activity in cultured hippocampal neurons

Synchronous oscillations of intracellular calcium concentration ([Ca2+]i) and of membrane potential occurred in a limited population of glutamatergic hippocampal neurons grown in primary cultures. The oscillatory activity occurred in synaptically connected cells only when they were in the presence of astrocytes. Microcultures containing only one or a few neurons also displayed oscillatory activity, provided that glial cells participated in the network. The glutamate-transporter inhibitors L-trans-pyrrolidine-2, 4-dicarboxylic acid (PDC) and dihydrokainate, which produce an accumulation of glutamate in the synaptic microenvironment, impaired the oscillatory activity. Moreover, in neurons not spontaneously oscillating, though in the presence of astrocytes, oscillations were induced by exogenous L-glutamate, but not by the stereoisomer D-glutamate, which is not taken up by glutamate transporters. These data demonstrate that astrocytes are essential for neuronal oscillatory activity and provide evidence that removal of glutamate from the synaptic environment is one of the major mechanisms by which glial cells allow the repetitive excitation of the postsynaptic cell.

Synaptic and intrinsic mechanisms shape synchronous oscillations in hippocampal neurons in culture

We have detected spontaneous, synchronous calcium oscillations, associated with variations in membrane potential, in hippocampal neurons maintained in primary culture. The oscillatory activity is synaptically driven, as it is blocked by tetrodotoxin, by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by toxins inhibiting neurotransmitter release from presynaptic nerve endings. Neuronal oscillations do not require for their expression the presence of a polyneuronal network and are not primarily influenced by the gamma-aminobutyric acid (GABA(A)) receptor antagonist picrotoxin, suggesting that they entirely rely on glutamatergic neurotransmission. Synaptic and intrinsic conductances shape the synchronized oscillations in hippocampal neurons. The concomitant activation of N-methyl-D-aspartate (NMDA) receptors and voltage-activated L-type calcium channels allows calcium entering from the extracellular medium and sustaining the long depolarization, which shapes every single calcium wave.

Effect of dantrolene on KCl- or NMDA-induced intracellular Ca2+ changes and spontaneous Ca2+ oscillation in cultured rat frontal cortical neurons

Dantrolene has been known to affect intracellular Ca2+ concentration ([Ca2+]i) by inhibiting Ca2+ release from intracellular stores in cultured neurons. We were interested in examining this property of dantrolene in influencing the [Ca2+]i affected by the NMDA receptor ligands, KCl, L-type Ca2+ channel blocker nifedipine, and two other intracellular Ca2(+)-mobilizing agents caffeine and bradykinin. Effect of dantrolene on the spontaneous oscillation of [Ca2+]i was also examined. Dantrolene in microM concentrations dose-dependently inhibited the increase in [Ca2+]i elicited by NMDA and KCl. AP-5, MK-801 (NMDA antagonists), and nifedipine respectively reduced the NMDA and KCl-induced increase in [Ca2+]i. Dantrolene, added to the buffer solution together with the antagonists or nifedipine, caused a further reduction in [Ca2+]i to a degree similar to that seen with dantrolene alone inhibiting the increase in [Ca2+]i caused by NMDA or KCl. At 30 microM, dantrolene partially inhibited caffeine-induced increase in [Ca2+]i whereas it has no effect on the bradykinin-induced change in [Ca2+]i. The spontaneous oscillation of [Ca2+]i in frontal cortical neurons was reduced both in amplitude and in base line concentration in the presence of 10 microM dantrolene. Our results indicate that dantrolene's mobilizing effects on intracellular Ca2+ stores operate independently from the influxed Ca2+ and that a component of the apparent increase in [Ca2+]i elicited by NMDA or KCl represents a dantrolene-sensitive Ca2+ release from intracellular stores. Results also suggest that dantrolene does not affect the IP3-gated release of intracellular Ca2+ and that the spontaneous Ca2+ oscillation is, at least partially, under the control of Ca2+ mobilization from internal stores.

Mechanism of synchronized Ca2+ oscillations in cortical neurons

Dissociated rat cortical neurons reassociate in vitro to form synaptically connected networks. Removal of Mg2+ from the extracellular medium then induces neurons in the network to undergo synchronized oscillations of cytoplasmic calcium. Previous studies have shown that these calcium oscillations involve the activation of NMDA receptors and that the rising phase of each calcium spike is coincident with a brief burst of action potentials (Robinson et al., Jpn. J. Physiol. 43 (Suppl. 1) (1993) S125-130; Robinson et al., J. Neurophysiol. 70 (1993) 1606-1616; Murphy et al., J. Neurosci. 12 (1992) 4834-4845). We have found that these calcium oscillations are dependent on an influx of extracellular calcium but are independent of mobilization of calcium from intracellular stores. The influx of extracellular Ca2+ occurs primarily through L-type voltage-gated calcium channels (VGCCs), since diltiazem inhibits calcium oscillations under all conditions. On the other hand, N-, P/Q-, and T-type VGCCs are not required for calcium oscillations, although inhibitors of these channels may act as partial antagonists. In addition to removal of Mg2+, oscillations can also be induced by the inhibition of voltage-gated K+ channels with 4-aminopyridine (4-AP), a treatment known to increase neurotransmitter release. In the presence of 4-AP, synchronized calcium oscillations become independent of NMDA receptor activation, although they continue to require activation of AMPA/KA receptors. A model for the mechanism of neuronal calcium oscillations and the reason for their synchrony is presented.

Mechanisms for synchronous calcium oscillations in cultured rat cerebellar neurons

Removal of Mg2+ caused oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential in cultured cerebellar granule neurons. Oscillations of [Ca2+]i were synchronous in all the cells, and were restricted to the neurons (immunocytochemically identified) that responded to exogenous N-methyl-D-aspartate (NMDA). Oscillations were blocked by Ca2+ removal, nickel, NMDA receptor antagonists, omega-agatoxin IVA, tetrodotoxin, sodium removal and gamma-aminobutyric acid, but not by dihydropyridines, omega-conotoxin M VIIA or by emptying the intracellular Ca2+ stores with thapsigargin or ionomycin. The upstroke of the [Ca2+]i oscillations coincided in time with an increase in manganese permeability of the plasma membrane. Propagation of the [Ca2+]i wave followed more than one pathway and the spatiotemporal pattern changed with time. Membrane potential oscillations consisted of transient slow depolarizations of approximately 20 mV with faster phasic activity superimposed. We propose that the synchronous [Ca2+]i oscillations are the expression of irradiation of random excitation through a neuronal network requiring generation of action potentials and functional glutamatergic synapses. Oscillations of -Ca2+-i are due to cyclic Ca2+ entry through NMDA receptor channels activated by synaptic release of glutamate, which requires Ca2+ entry through P-type Ca2+ channels activated by action potentials at the presynaptic terminal.

Divalent cation entry in cultured rat cerebellar granule cells measured using Mn2+ quench of fura 2 fluorescence

In this study the rate of Mn2+ quench of fura-2 fluorescence evoked by glutamatergic and cholinergic agonists, depolarization and Ca2+ store modulators was measured in cultured cerebellar granule cells, in order to study their effects on Ca2+ entry in isolation from effects on Ca2+ store release. The rate of fluorescence quench by 0.1 mM Mn2+ was markedly increased by 25 mM K(+)-evoked depolarization or by 200 microM N-methyl-D-aspartate (NMDA), with a significantly greater increase occurring during the rapid-onset peak phase compared to the plateau phase of the K(+)- or NMDA-evoked [Ca2+]i response. The stimulatory effect of NMDA on Mn2+ quench was abolished by dizocilpine (10 microM), but nitrendipine (2 microM), while decreasing the rate of basal quench, did not affect NMDA-stimulated Mn2+ entry. This suggests that nitrendipine may not act on NMDA channels in granule cells, at least under these conditions, and that voltage-operated Ca2+ channels are involved in control quench whereas the NMDA-evoked quench is dependent on entry through the receptor channel. The t1/2 of quench was unaffected by alpha-amino-hydroxyisoxazole propionic acid (200 microM) and carbamyl choline (1 mM). Neither thapsigargin (10 microM) nor dantrolene (30 microM) significantly affected the rate of quench under control or NMDA- or K(+)-stimulated conditions, which confirms that the previously reported inhibitory effects on [Ca2+]i elevations evoked by these agents are due to actions on Ca2+ stores. However, thapsigargin elevated [Ca2+]i in the presence of normal [Ca2+]o but not in nominally Ca(2+)-free medium, indicating that it evokes Ca2+ entry in cerebellar granule cells, probably subsequent to store depletion, which appears to be either too small to be detected by Mn2+ quench or to occur via Mn(2+)-impermeant channels.

Calcium dynamics in the central nervous system

Calcium ions are critically important in many functions of the nervous system from neurotransmitter release to intracellular signal transduction. The large difference between intracellular and extracellular calcium ion concentration ([Ca2+]) highlights the importance of the mechanisms controlling influx and efflux of this ion. Loss of the regulatory ability of these mechanisms and the subsequent increased intracellular calcium levels may be involved in pathological events of brain trauma, stroke, epilepsy and other diseases. Ca2+ dynamics in the CNS ranging from 'waves' to 'spirals' are being studied because of the availability of fluorescent indicators of Ca2+ combined with confocal microscopy. Cellular mechanisms of Ca2+ signal transduction have been extensively reviewed (Tsien and Tsein, 1990; Carafoli, 1992; Berridge, 1993; Berridge and Dupont, 1994; Pozzan et al., 1994; Clapham, 1995; Ghosh and Greenberg, 1995). The aim of this review is to present the types of Ca2+ dynamics observed in the CNS thus far, both in normal brain function as well as in response after injury.

The relationship between synaptogenesis and expression of voltage-dependent currents in cerebellar granule cells in situ

In this work we consider the ontogenetic changes of membrane currents and their relationship with synaptogenesis in cerebellar granule cells. Recordings were performed in whole-cell patch-clamp configuration from cerebellar slices obtained from 4 to 31-day-old rats. Granule cells in the external granular layer, and non-connected granule cells in the internal granular layer expressed outward currents, and inconstantly also small Ca2+ currents, but no fast Na+ currents. Most connected granule cells expressed Ca2+ and Na+ currents. These data indicate that Ca2+ and Na+ current development occurs after synapse formation, while outward (K+) currents begin their development before. Mixed NMDA/non-NMDA synaptic currents were observed at all stages, while synaptic currents with a prominent NMDA component were observed exclusively at immature stages. At P4, ie 1-2 days after the arrival of the first granule cells in the internal granular layer, some granule cells already expressed mature synaptic and voltage-dependent currents, suggesting that establishment of mossy fibre synapses and development of membrane properties takes just 1-2 days to complete. Starting at P4, the probability of activating mossy fibre currents, and sizeable Ca2+ and Na+ currents increased at a similar rate, attaining a plateau level around P20. Average amplitude of Na+ and outward currents decreased until P10 and then increased attaining plateau soon beyond P20. Average amplitude of Ca2+ currents increased monotonically. The time courses of probability and average current amplitude curves are likely explained by changes in the rate of accumulation of migrating granule cells in the internal granular layer, and by changes in granule cell membrane surface extension. These data suggest a relevant role for the process of synapse formation in inducing the expression of new channels in the developing granule cells, which may involve Ca2+ influx through the NMDA channel.