Local calcium signalling by inositol-1,4,5-trisphosphate in Purkinje cell dendrites

Intracellular Ca(2+) pools in neuronal signalling

Different intracellular pools participate in generating Ca(2+) signals in neuronal cells and in shaping their spatio-temporal patterns. They include the endoplasmic reticulum (endowed with different classes of Ca(2+) channels, with distinct functional properties and highly defined expression patterns in the brain), the Golgi apparatus, and the mitochondria. The release of Ca(2+) from intracellular pools plays an important role in controlling processes such as neurite outgrowth, synaptic plasticity, secretion and neurodegeneration.

Differential neuronal localizations and dynamics of phosphorylated and unphosphorylated type 1 inositol 1,4,5-trisphosphate receptors

Type 1 inositol 1,4,5-trisphosphate receptors are phosphorylated by cyclic-AMP-dependent protein kinase A at serines 1589 and 1755, with serine 1755 phosphorylation greatly predominating in the brain. Inositol 1,4,5-trisphosphate receptor protein kinase A phosphorylation augments Ca(2+) release. To assess type 1 protein kinase A phosphorylation dynamics in the intact organism, we developed antibodies selective for either serine 1755 phosphorylated or unphosphorylated species. Immunohistochemical studies reveal marked variation in localization. For example, in the hippocampus the phosphorylated type 1 inositol 1,4,5-trisphosphate receptor is restricted to CA1, while the unphosphorylated receptor occurs ubiquitously in CA1-CA3 and dentate gyrus granule cells. Throughout the brain the phosphorylated type 1 inositol 1,4,5-trisphosphate receptor is selectively enriched in dendrites, while the unphosphorylated receptor predominates in cell bodies. Focal cerebral ischemia in rats and humans is associated with dephosphorylation of type 1 inositol 1,4,5-trisphosphate receptors, and glutamatergic excitation of cerebellar Purkinje cells mediated by ibogaine elicits dephosphorylation of type 1 inositol 1,4,5-trisphosphate receptors that precedes evidence of excitotoxic neuronal degeneration. We have demonstrated striking variations in regional and subcellular distribution of inositol 1,4,5-trisphosphate receptor phosphorylation that may influence normal physiological intracellular Ca(2+) signaling in rat and human brain. We have further shown that the subcellular distribution of inositol 1,4,5-trisphosphate receptor phosphorylation in neurons is regulated by excitatory neurotransmission, as well as excitotoxic insult and neuronal ischemia-reperfusion. Phosphorylation dynamics of type 1 inositol 1,4,5-trisphosphate receptors may modulate intracellular Ca(2+) release and influence the cellular response to neurotoxic insults.

The endoplasmic reticulum: one continuous or several separate Ca(2+) stores?

The Ca2+ store and sink in the endoplasmic reticulum (ER) is important for Ca2+ signal integration and for conveyance of information in spatial and temporal domains. Textbooks regard the ER as one continuous network, but biochemical and biophysical studies revealed apparently discrete ER Ca2+ stores. Recent direct studies of ER lumenal Ca2+ movements show that this organelle system is one continuous Ca2+ store, which can function as a Ca2+ tunnel. The concept of a fully connected ER network is entirely compatible with evidence indicating that the distribution of Ca2+ -release channels in the ER membrane is discontinuous with clustering in certain localities.

Deficient long-term synaptic depression in the rostral cerebellum correlated with impaired motor learning in phospholipase C beta4 mutant mice

Long-term depression (LTD) at parallel fibre-Purkinje cell synapse of the cerebellum is thought to be a cellular substrate for motor learning. LTD requires activation of metabotropic glutamate receptor subtype 1 (mGluR1) and its downstream signalling pathways, which invariably involves phospholipase Cbetas (PLCbetas). PLCbetas consist of four isoforms (PLCbeta1-4) among which PLCbeta4 is the major isoform in most Purkinje cells in the rostral cerebellum (lobule 1 to the rostral half of lobule 6). We studied mutant mice deficient in PLCbeta4, and found that LTD was deficient in the rostral but not in the caudal cerebellum of the mutant. Basic properties of parallel fibre-Purkinje cell synapses and voltage-gated Ca2+ channel currents appeared normal. The mGluR1-mediated Ca2+ release induced by repetitive parallel fibre stimulation was absent in the rostral cerebellum of the mutant, suggesting that their LTD lesion was due to the defect in the mGluR1-mediated signalling in Purkinje cells. Importantly, the eyeblink conditioning, a simple form of discrete motor learning, was severely impaired in PLCbeta4 mutant mice. Wild-type mice developed the conditioned eyeblink response, when pairs of the conditioned stimulus (tone) and the unconditioned stimulus (periorbital shock) were repeatedly applied. In contrast, PLCbeta4 mutant mice could not learn the association between the conditioned and unconditioned stimuli, although their behavioural responses to the tone or to the periorbital shock appeared normal. These results strongly suggest that PLCbeta4 is essential for LTD in the rostral cerebellum, which may be required for the acuisition of the conditioned eyeblink response.

Calcium dynamics and electrophysiological properties of cerebellar Purkinje cells in SCA1 transgenic mice

Cerebellar Purkinje cells (PCs) from spinocerebellar ataxia type 1 (SCA1) transgenic mice develop dendritic and somatic atrophy with age. Inositol 1,4,5-trisphosphate receptor type 1 and the sarco/endoplasmic reticulum Ca(2+) ATPase pump, which regulate [Ca(2+)](i), are expressed at lower levels in these cells compared with the levels in cells from wild-type (WT) mice. To examine PCs in SCA1 mice, we used whole-cell patch clamp recording combined with fluorometric [Ca(2+)](i) and [Na(+)](i) measurements in cerebellar slices. PCs in SCA1 mice had Na(+) spikes, Ca(2+) spikes, climbing fiber (CF) electrical responses, parallel fiber (PF) electrical responses, and metabotropic glutamate receptor (mGluR)-mediated, PF-evoked Ca(2+) release from intracellular stores that were qualitatively similar to those recorded from WT mice. Under our experimental conditions, it was easier to evoke the mGluR-mediated secondary [Ca(2+)](i) increase in SCA1 PCs. The membrane resistance of SCA1 PCs was 3.3 times higher than that of WT cells, which correlated with the 1.7 times smaller cell body size. Most SCA1 PCs (but not WT) had a delayed onset (about 50--200 ms) to Na(+) spike firing induced by current injection. This delay was increased by hyperpolarizing prepulses and was eliminated by 4-aminopyridine, which suggests that this delay was due to enhancement of the A-like K(+) conductance in the SCA1 PCs. In response to CF stimulation, most PCs in mutant and WT mice had rapid, widespread [Ca(2+)](i) changes that recovered in <200 ms. Some SCA1 PCs showed a slow, localized, secondary Ca(2+) transient following the initial CF Ca(2+) transient, which may reflect release of Ca(2+) from intracellular stores. Thus, with these exceptions, the basic physiological properties of mutant PCs are similar to those of WT neurons, even with dramatic alteration of their morphology and downregulation of Ca(2+) handling molecules.

Synaptically activated calcium responses in dendrites of hippocampal oriens-alveus interneurons

Activation of metabotropic glutamate receptors (mGluRs) by agonists increases intracellular calcium levels ([Ca(2+)](i)) in interneurons of stratum oriens/alveus (OA) of the hippocampus. We examined the mechanisms that contribute to dendritic Ca(2+) increases in these interneurons during agonist activation of mGluRs and during synaptically evoked burst discharges, using simultaneous whole cell recordings and confocal Ca(2+) imaging in rat hippocampal slices. First, we found that the group I/II mGluR agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100 microM) increased dendritic [Ca(2+)](i) and depolarized OA interneurons. Dendritic Ca(2+) responses were correlated with membrane depolarizations, but Ca(2+) responses induced by ACPD were larger in amplitude than those elicited by equivalent somatic depolarization. Next, we used linescans to measure changes in dendritic [Ca(2+)](i) during synaptically evoked burst discharges and somatically elicited repetitive firing in disinhibited slices. Dendritic Ca(2+) signals and electrophysiological responses were stable over repeated trials. Peak Ca(2+) responses were linearly related to number and frequency of action potentials in burst discharges for both synaptic and somatic stimulation, but the slope of the relationship was steeper for responses evoked somatically. Synaptically evoked [Ca(2+)](i) rises and excitatory postsynaptic potentials were abolished by antagonists of ionotropic glutamate receptors. The group I/II mGluR antagonist S-alpha-methyl-4-carboxyphenylglycine (500 microM) produced a significant partial reduction of synaptically evoked dendritic Ca(2+) responses. The mGluR antagonist did not affect synaptically evoked burst discharges and did not reduce either Ca(2+) responses or burst discharges evoked somatically. Therefore ionotropic glutamate receptors appear necessary for synaptically evoked dendritic Ca(2+) responses, and group I/II mGluRs may contribute partially to these responses. Dendritic [Ca(2+)](i) rises mediated by both ionotropic and metabotropic glutamate receptors may be important for synaptic plasticity and the selective vulnerability to excitotoxicity of OA interneurons.

Development and molecular organization of dendritic spines and their synapses

Nearly all excitatory input in the hippocampus impinges on dendritic spines which serve as multifunctional compartments that can, at the very least, selectively isolate and amplify incoming signals. Their importance to normal brain function is highlighted by the severe mental impairment observed in most individuals having poorly developed spines (Purpura, Science 1974;186:1126-1128). Distinct groups of membrane proteins, cytoskeletal elements, scaffolding proteins, and second messenger-related proteins are concentrated particularly in dendritic spines, but their ability to generate, maintain, and coordinately regulate spine structure or function is poorly understood. Here we review the unique molecular composition of dendritic spines along with the factors known to influence dendritic spine development in order to construct a model of dendritic spine development in relation to synaptogenesis.

L-Type calcium channels mediate calcium oscillations in early postnatal Purkinje neurons

Ca(2+) signaling is important in many fundamental neuronal processes including neurotransmission, synaptic plasticity, neuronal development, and gene expression. In cerebellar Purkinje neurons, Ca(2+) signaling has been studied primarily in the dendritic region where increases in local Ca(2+) have been shown to occur with both synaptic events and spontaneous electrical activity involving P-type voltage-gated Ca(2+) channels (VGCCs), the predominant VGCC expressed by Purkinje neurons. Here we show that Ca(2+) signaling is also a prominent feature of immature Purkinje neurons at developmental stages that precede expression of dendritic structure and involves L-type rather than P-type VGCCs. Immature Purkinje neurons acutely dissociated from postnatal day 4-7 rat pups exhibit spontaneous cytoplasmic Ca(2+) oscillations. The Ca(2+) oscillations require entry of extracellular Ca(2+), are blocked by tetrodotoxin, are communicated to the nucleus, and correlate closely with patterns of endogenously generated spontaneous and evoked electrical activity recorded in the neurons. Immunocytochemistry showed that L-, N-, and P/Q-types of VGCCs are present on the somata of the Purkinje neurons at this age. However, only the L-type VGCC antagonist nimodipine effectively antagonized the Ca(2+) oscillations; inhibitors of P/Q and N-type VGCCs were relatively ineffective. Release of Ca(2+) from intracellular Ca(2+) stores significantly amplified the Ca(2+) signals of external origin. These results show that a somatic signaling pathway that generates intracellular Ca(2+) oscillations and involves L-type VGCCs and intracellular Ca(2+) stores plays a prominent role in the Ca(2+) dynamics of early developing Purkinje neurons and may play an important role in communicating developmental cues to the nucleus.

Origin sites of calcium release and calcium oscillations in frog sympathetic neurons

In many neurons, Ca(2+) signaling depends on efflux of Ca(2+) from intracellular stores into the cytoplasm via caffeine-sensitive ryanodine receptors (RyRs) of the endoplasmic reticulum. We have used high-speed confocal microscopy to image depolarization- and caffeine-evoked increases in cytoplasmic Ca(2+) levels in individual cultured frog sympathetic neurons. Although caffeine-evoked Ca(2+) wave fronts propagated throughout the cell, in most cells the initial Ca(2+) release was from one or more discrete sites that were several micrometers wide and located at the cell edge, even in Ca(2+)-free external solution. During cell-wide cytoplasmic [Ca(2+)] oscillations triggered by continual caffeine application, the initial Ca(2+) release that began each Ca(2+) peak was from the same subcellular site or sites. The Ca(2+) wave fronts propagated with constant amplitude; the spread was mostly via calcium-induced calcium release. Propagation was faster around the cell periphery than radially inward. Local Ca(2+) levels within the cell body could increase or decrease independently of neighboring regions, suggesting independent action of spatially separate Ca(2+) stores. Confocal imaging of fluorescent analogs of ryanodine and thapsigargin, and of MitoTracker, showed potential structural correlates to the patterns of Ca(2+) release and propagation. High densities of RyRs were found in a ring around the cell periphery, mitochondria in a broader ring just inside the RyRs, and sarco-endoplasmic reticulum Ca(2+) ATPase pumps in hot spots at the cell edge. Discrete sites at the cell edge primed to release Ca(2+) from intracellular stores might preferentially convert Ca(2+) influx through a local area of plasma membrane into a cell-wide Ca(2+) increase.

Roles of phospholipase Cbeta4 in synapse elimination and plasticity in developing and mature cerebellum

The beta isoforms of phospholipase C (PLCbetas) are thought to mediate signals from metabotropic glutamate receptor subtype 1 (mGluR1) that is crucial for the modulation of synaptic transmission and plasticity. Among four PLCbeta isoforms, PLCbeta4 is one of the two major isoforms expressed in cerebellar Purkinje cells. The authors have studied the roles of PLCbeta4 by analyzing PLCbeta4 knockout mice, which are viable, but exhibit locomotor ataxia. Their cerebellar histology, parallel fiber synapse formation, and basic electrophysiology appear normal. However, developmental elimination of multiple climbing fiber innervation is clearly impaired in the rostral portion of the cerebellar vermis, where PLCbeta4 mRNA is predominantly expressed in the wild-type mice. In the adult, long-term depression is deficient at parallel fiber to Purkinje cell synapses in the rostral cerebellum of the PLCbeta4 knockout mice. The impairment of climbing fiber synapse elimination and the loss of long-term depression are similar to those seen in mice defective in mGluR1, Galphaq, or protein kinase C. Thus, the authors' results strongly suggest that PLCbeta4 is part of a signaling pathway, including the mGluR1, Galphaq and protein kinase C, which is crucial for both climbing fiber synapse elimination in the developing cerebellum and long-term depression induction in the mature cerebellum.

Hippocampal mossy fiber activity evokes Ca2+ release in CA3 pyramidal neurons via a metabotropic glutamate receptor pathway

Mossy fiber activity can evoke Ca2+ release from internal stores in CA3 neurons, but the physiological conditions under which this occurs and the mechanisms underlying the release are not understood. Using rat hippocampal slices we report here that short trains of mossy fiber stimulation activate group I metabotropic glutamate receptors (mGluRs) on CA3 pyramidal neurons and elicit waves of Ca2+ release from inositol 1,4,5-trisphosphate (IP3) sensitive internal stores that propagate from stratum lucidum to the soma and in some cases distally out the dendrites. Activation of mGluR1,5 receptors by an agonist trans-azetidine-2,4-dicarboxylic acid (tADA) applied to stratum lucidum was also sufficient to induce waves of Ca2+ release. This release was blocked by internal heparin, but not by dantrolene, suggesting the involvement of IP3 rather than ryanodine receptors in not only the initial release but also in the maintenance of the propagating waves. Release could be facilitated by Ca2+ influx through voltage-gated Ca2+ channels, which is consistent with the known Ca2+ sensitivity of IP3 receptors.These results provide insight into the mechanisms and conditions of Ca2+ release in CA3 neurons and demonstrate the powerful influence mossy fiber input can have on these neurons.

Calsyntenin-1, a proteolytically processed postsynaptic membrane protein with a cytoplasmic calcium-binding domain

In a screen for proteins released from synapse-forming spinal cord neurons, we found the proteolytically cleaved N-terminal fragment of a transmembrane protein localized in the postsynaptic membrane of both excitatory and inhibitory synapses. We termed this protein calsyntenin-1, because it binds synaptic Ca2+ with its cytoplasmic domain. By binding Ca2+, calsyntenin-1 may modulate Ca2+-mediated postsynaptic signals. Proteolytic cleavage of calsyntenin-1 in its extracellular moiety generates a transmembrane stump that is internalized and accumulated in the spine apparatus of spine synapses. Therefore, the synaptic Ca2+ modulation by calsyntenin-1 may be subject to regulation by extracellular proteolysis in the synaptic cleft. Thus, calsyntenin-1 may link extracellular proteolysis in the synaptic cleft and postsynaptic Ca2+ signaling.

Biochemical networks in nervous systems: expanding neuronal information capacity beyond voltage signals

In addition to synaptically mediated signals that are based on changes in membrane potential, neurons also generate and receive many types of signals that involve biochemical pathways, some of which are independent of voltage. Although networks of biochemical pathways have often been thought of as being only neuromodulatory, recent computational and experimental studies have highlighted how these pathways can also integrate and transfer information themselves. Interactions between biochemical pathways involving positive and negative feedback loops allow biochemical signals to exhibit emergent properties, most notably bistability and oscillations. New and evolving techniques, including real-time imaging of second messengers, hold the promise of illuminating information processing that cannot be detected using microelectrodes, and revealing how 'biochemical integration' might contribute to the computational abilities of the nervous system.

Confocal quality imaging of afferent neurons from semi-thin sections of Drosophila ganglia

The aim of this study was to develop protocols for computer imaging of the thoraco-abdominal ganglion of Drosophila from serial semi-thin sections, in which specific neurons were stained and related to neuropilar structures. The central projections of a subset of transgenically labelled sensory neurons were revealed by immunohistochemistry, while Nomarski optics were used to show motor neuron targets in the neuropil. Digital photomicrographs of each section were aligned and the resultant image stacks rendered into three-dimensional (3D) images that can be rotated in real time. The result is a detailed, in-depth visualization of labelled neurons at a resolution comparable with that in confocal reconstructions, which also allows investigation of their relationships with other components of the neuropil.

Coincidence detection in single dendritic spines mediated by calcium release

Cerebellar long-term depression (LTD) is a calcium-dependent process in which coincident activity of parallel fiber (PF) and climbing fiber (CF) synapses causes a long-lasting decrease in PF synaptic strength onto Purkinje cells. Here we show that pairing CF activation with bursts of PF activity triggers large (>10 microM) calcium signals in Purkinje cell dendrites. When PFs are densely activated, signals span whole dendritic branchlets and are mediated by voltage-dependent calcium entry. When PFs are sparsely activated, however, signals are restricted to single spines and blocked by metabotropic glutamate receptor antagonists. Single-spine signals and sparse-stimulation LTD are also blocked by thapsigargin, indicating that calcium must be released from stores. Single-spine signals and sparse-stimulation LTD are greatest when PF activation precedes the CF activation within 50-200 ms. This timing rule matches the properties of several forms of motor learning, providing a link between behavior and functional properties of cerebellar synaptic plasticity.

Diversity and dynamics of dendritic signaling

Communication between neurons in the brain occurs primarily through synapses made onto elaborate treelike structures called dendrites. New electrical and optical recording techniques have led to tremendous advances in our understanding of how dendrites contribute to neuronal computation in the mammalian brain. The varied morphology and electrical and chemical properties of dendrites enable a spectrum of local and long-range signaling, defining the input-output relationship of neurons and the rules for induction of synaptic plasticity. In this way, diversity in dendritic signaling allows individual neurons to carry out specialized functions within their respective networks.

A neural model of cortico-cerebellar interactions during attentive imitation and predictive learning of sequential handwriting movements

Much sensory-motor behavior develops through imitation, as during the learning of handwriting by children. Such complex sequential acts are broken down into distinct motor control synergies, or muscle groups, whose activities overlap in time to generate continuous, curved movements that obey an inverse relation between curvature and speed. How are such complex movements learned through attentive imitation? Novel movements may be made as a series of distinct segments, but a practiced movement can be made smoothly, with a continuous, often bell-shaped, velocity profile. How does learning of complex movements transform reactive imitation into predictive, automatic performance? A neural model is developed which suggests how parietal and motor cortical mechanisms, such as difference vector encoding, interact with adaptively timed, predictive cerebellar learning during movement imitation and predictive performance. To initiate movement, visual attention shifts along the shape to be imitated and generates vector movement using motor cortical cells. During such an imitative movement, cerebellar Purkinje cells with a spectrum of delayed response profiles sample and learn the changing directional information and, in turn, send that learned information back to the cortex and eventually to the muscle synergies involved. If the imitative movement deviates from an attentional focus around a shape to be imitated, the visual system shifts attention, and may make an eye movement, back to the shape, thereby providing corrective directional information to the arm movement system. This imitative movement cycle repeats until the cortico-cerebellar system can accurately drive the movement based on memory alone. A cortical working memory buffer transiently stores the cerebellar output and releases it at a variable rate, allowing speed scaling of learned movements which is limited by the rate of cerebellar memory readout. Movements can be learned at variable speeds if the density of the spectrum of delayed cellular responses in the cerebellum varies with speed. Learning at slower speeds facilitates learning at faster speeds. Size can be varied after learning while keeping the movement duration constant (isochrony). Context-effects arise from the overlap of cerebellar memory outputs. The model is used to simulate key psychophysical and neural data about learning to make curved movements, including a decrease in writing time as learning progresses; generation of unimodal, bell-shaped velocity profiles for each movement synergy; size and speed scaling with preservation of the letter shape and the shapes of the velocity profiles; an inverse relation between curvature and tangential velocity; and a Two-Thirds Power Law relation between angular velocity and curvature.

Local calcium release in dendritic spines required for long-term synaptic depression

We have used rats and mice with mutations in myosin-Va to evaluate the range and function of IP3-mediated Ca2+ signaling in dendritic spines. In these mutants, the endoplasmic reticulum and its attendant IP3 receptors do not enter the postsynaptic spines of parallel fiber synapses on cerebellar Purkinje cells. Long-term synaptic depression (LTD) is absent at the parallel fiber synapses of the mutants, even though the structure and function of these synapses otherwise appear normal. This loss of LTD is associated with selective changes in IP3-mediated Ca2+ signaling in spines and can be rescued by photolysis of a caged Ca2+ compound. Our results reveal that IP3 must release Ca2+ locally in the dendritic spines to produce LTD and indicate that one function of dendritic spines is to target IP3-mediated Ca2+ release to the proper subcellular domain.

An evaluation of the synapse specificity of long-term depression induced in rat cerebellar slices

Whole-cell excitatory postsynaptic currents (EPSCs) were recorded from single Purkinje cells (PCs) in rat cerebellar slices in response to alternate activation of two separate sets of parallel fibres (PF1 and PF2). Pairing the stimulation of one input (PF1) with PC depolarisation at 1 Hz for 5 min produced varied effects, including a long-term depression (LTD) of subsequent responses, a medium-term potentiation, or no change relative to baseline levels (n = 14). In all but two cases PF2 responses mirrored those in PF1, in both direction and magnitude even though this second pathway was not specifically activated during pairing. Increasing the stimulus strength to evoke larger amplitude EPSCs (> 1000 pA) dramatically increased the proportion of cells that underwent LTD in both PF1 and PF2. LTD in both pathways was postsynaptic calcium dependent. PC depolarisation alone (n = 7) or PF1 stimulation paired with PC hyperpolarisation (n = 6) failed to induce LTD at either site. Pairing PF1 stimulation with climbing fibre (CF) activation at 1 Hz for 5 min produced LTD in the majority of cells regardless of the strength of PF stimulation. LTD under these conditions was not, however, input specific, even at the lowest stimulus strengths. With EPSCs greater than 1000 pA in amplitude, depression was apparent in both pathways even when the duration of PF1 pairing with depolarisation was limited to 1 min. Full expression of LTD in PF2 required stimulation of this pathway to be resumed within a distinct temporal window of conjunctive pairing with PF1. Introducing a delay of 20 min before resumption of PF2 activation preserved the input specificity of synaptic depression. We conclude that pairing either PC depolarisation or CF activation with stimulation of a discrete set of PFs produces LTD that spreads to adjacent synapses on the same PC.

Conservation of localization patterns of IP(3) receptor type 1 in cerebellar Purkinje cells across vertebrate species

The distribution of inositol 1,4,5-trisphosphate (IP(3)) receptor type 1 (IP(3)R1) protein was studied in the adult cerebella of six different vertebrate species, zebrafish, skate, claw frog, rat, hamster, and mouse. The receptor was found at high expression levels in Purkinje cells in all species examined using a subtype-specific polyclonal antiserum against IP(3)R1 and fluorescence immunocytochemistry. The immunoreactivity for IP(3)R1 was found intracellularly at high concentrations in dendrites and somata and at lower levels in axons of these cells. Despite the morphological and functional differences of the cerebella the staining patterns of IP(3)R1 labeling in Purkinje cells was preserved. This is notable because the cerebella were taken from organisms representing a large segment of vertebrate phylogenetic development. The high expression levels of IP(3)R1 in Purkinje cells were found independent of the degree of the formation of fissures and folia and of the degree of branching of Purkinje cell dendrites. The conservation of cerebellar structures not only at the cellular level but more importantly at the molecular level suggests that identical intracellular calcium signaling mechanisms are used in a number of species that represent different areas of phylogenetic development and specialization.

Cyclic nucleotide-gated channels colocalize with adenylyl cyclase in regions of restricted cAMP diffusion

Cyclic AMP is a ubiquitous second messenger that coordinates diverse cellular functions. Current methods for measuring cAMP lack both temporal and spatial resolution, leading to the pervasive notion that, unlike Ca(2+), cAMP signals are simple and contain little information. Here we show the development of adenovirus-expressed cyclic nucleotide-gated channels as sensors for cAMP. Homomultimeric channels composed of the olfactory alpha subunit responded rapidly to jumps in cAMP concentration, and their cAMP sensitivity was measured to calibrate the sensor for intracellular measurements. We used these channels to detect cAMP, produced by either heterologously expressed or endogenous adenylyl cyclase, in both single cells and cell populations. After forskolin stimulation, the endogenous adenylyl cyclase in C6-2B glioma cells produced high concentrations of cAMP near the channels, yet the global cAMP concentration remained low. We found that rapid exchange of the bulk cytoplasm in whole-cell patch clamp experiments did not prevent the buildup of significant levels of cAMP near the channels in human embryonic kidney 293 (HEK-293) cells expressing an exogenous adenylyl cyclase. These results can be explained quantitatively by a cell compartment model in which cyclic nucleotide-gated channels colocalize with adenylyl cyclase in microdomains, and diffusion of cAMP between these domains and the bulk cytosol is significantly hindered. In agreement with the model, we measured a slow rate of cAMP diffusion from the whole-cell patch pipette to the channels (90% exchange in 194 s, compared with 22-56 s for substances that monitor exchange with the cytosol). Without a microdomain and restricted diffusional access to the cytosol, we are unable to account for all of the results. It is worth noting that in models of unrestricted diffusion, even in extreme proximity to adenylyl cyclase, cAMP does not reach high enough concentrations to substantially activate PKA or cyclic nucleotide-gated channels, unless the entire cell fills with cAMP. Thus, the microdomains should facilitate rapid and efficient activation of both PKA and cyclic nucleotide-gated channels, and allow for local feedback control of adenylyl cyclase. Localized cAMP signals should also facilitate the differential regulation of cellular targets.

Critical period for activity-dependent synapse elimination in developing cerebellum

Synapse elimination is considered to be the final step in neural circuit formation, by causing refinement of redundant connections formed at earlier developmental stages. The developmental loss of climbing fiber innervation from cerebellar Purkinje cells is an example of such synapse elimination. It has been suggested that NMDA receptors are involved in the elimination of climbing fiber synapses. In the present study, we probed the NMDA receptor-dependent period of climbing fiber synapse elimination by using daily intraperitoneal injections of the NMDA receptor antagonist MK-801. We found that blockade of NMDA receptors during postnatal day 15 (P15) and P16, but not before or after this period, resulted in a higher incidence of multiple climbing fiber innervation and caused a mild but persistent loss of motor coordination. Neither basic synaptic functions nor cerebellar morphology were affected by this manipulation. Chronic local application of MK-801 to the cerebellum during P15 and P16 also yielded a higher incidence of multiple climbing fiber innervation. During P15-P16, large NMDA receptor-mediated EPSCs were detected at the mossy fiber-granule cell synapse, but not at the parallel fiber-Purkinje cell or climbing fiber-Purkinje cell synapse. It is therefore likely that the NMDA receptors located at the mossy fiber-granule cell synapse mediate signals leading to the elimination of surplus climbing fibers. These results suggest that an NMDA receptor-dependent phase of climbing fiber synapse elimination lasts 2 d at most. During this phase, the final refinement of climbing fiber synapses occurs, and disruption of this process leads to permanent impairment of cerebellar function.

Quantification of spread of cerebellar long-term depression with chemical two-photon uncaging of glutamate

Localized, chemical two-photon photolysis of caged glutamate was used to map the changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors caused by long-term synaptic depression (LTD) in cerebellar Purkinje cells. LTD produced by pairing parallel fiber activity with depolarization was accompanied by a decline in the response of Purkinje cells to uncaged glutamate that accounted for both the time course and magnitude of LTD. This depression of glutamate responses was observed not only at the site of parallel fiber stimulation but also at more distant sites. The amount of LTD decreased with distance and was half-maximal 50 microm away from the site of parallel fiber activity. Estimation of the number of parallel fibers active during LTD induction indicates that LTD modified glutamate receptors not only at active synapses but also at 600 times as many inactive synapses on a single Purkinje cell. Therefore, both active and inactive parallel fiber synapses can undergo changes at a postsynaptic locus as a result of associative pre- and postsynaptic activity.

Regulation of the type III InsP(3) receptor by InsP(3) and ATP

Many hormones and neurotransmitters raise intracellular calcium (Ca(2+)) by generating InsP(3) and activating the inositol 1,4, 5-trisphosphate receptor (InsP(3)R). Multiple isoforms with distinct InsP(3) binding properties () have been identified (). The type III InsP(3)R lacks Ca(2+)-dependent inhibition, a property that makes it ideal for signal initiation (). Regulation of the type III InsP(3)R by InsP(3) and ATP was explored in detail using planar lipid bilayers. In comparison to the type I InsP(3)R, the type III InsP(3)R required a higher concentration of InsP(3) to reach maximal channel activity (EC(50) of 3.2 microM versus 0.5 microM for the types III and I InsP(3)R, respectively). However, the type III InsP(3)R did reach a 2.5-fold higher level of activity. Although activation by InsP(3) was isoform-specific, regulation by ATP was similar for both isoforms. In the presence of 2 microM InsP(3), low ATP concentrations (<6 mM) increased the open probability and mean open time. High ATP concentrations (>6 mM) decreased channel activity. These results illustrate the complex nature of type III InsP(3)R regulation. Enhanced channel activity in the presence of high InsP(3) may be important during periods of prolonged stimulation, whereas allosteric modulation by ATP may help to modulate intracellular Ca(2+) signaling.

Altered calcium homeostasis in cerebellar Purkinje cells of leaner mutant mice

The leaner (tg(la)) mouse mutation occurs in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit, the pore-forming subunit of P/Q-type Ca(2+) channels. This mutation results in dramatic reductions in P-type Ca(2+) channel function in cerebellar Purkinje neurons of tg(la)/tg(la) mice that could affect intracellular Ca(2+) signaling. We combined whole cell patch-clamp electrophysiology with fura-2 microfluorimetry to examine aspects of Ca(2+) homeostasis in acutely dissociated tg(la)/tg(la) Purkinje cells. There was no difference between resting somatic Ca(2+) concentrations in tg(la)/tg(la) cells and in wild-type (+/+) cells. However, by quantifying the relationship between intracellular Ca(2+) elevations and depolarization-induced Ca(2+) influx, we detected marked alterations in rapid calcium buffering between the two genotypes. Calcium buffering values (ratio of bound/free ions) were significantly reduced in tg(la)/tg(la) (584 +/- 52) Purkinje cells relative to +/+ (1,221 +/- 80) cells. By blocking the endoplasmic reticulum (ER) Ca(2+)-ATPases with thapsigargin, we observed that the ER had a profound impact on rapid Ca(2+) buffering that was also differential between tg(la)/tg(la) and +/+ Purkinje cells. Diminished Ca(2+) uptake by the ER apparently contributes to the reduced buffering ability of mutant cells. This report constitutes one of the few instances in which the ER has been implicated in rapid Ca(2+) buffering. Concomitant with this reduced buffering, in situ hybridization with calbindin D28k and parvalbumin antisense oligonucleotides revealed significant reductions in mRNA levels for these Ca(2+)-binding proteins (CaBPs) in tg(la)/tg(la) Purkinje cells. All of these results suggest that alterations of Ca(2+) homeostasis in tg(la)/tg(la) mouse Purkinje cells may serve as a mechanism whereby reduced P-type Ca(2+) channel function contributes to the mutant phenotype.

From form to function: calcium compartmentalization in dendritic spines

Dendritic spines compartmentalize calcium, and this could be their main function. We review experimental work on spine calcium dynamics. Calcium influx into spines is mediated by calcium channels and by NMDA and AMPA receptors and is followed by fast diffusional equilibration within the spine head. Calcium decay kinetics are controlled by slower diffusion through the spine neck and by spine calcium pumps. Calcium release occurs in spines, although its role is controversial. Finally, the endogenous calcium buffers in spines remain unknown. Thus, spines are calcium compartments because of their morphologies and local influx and extrusion mechanisms. These studies highlight the richness and heterogeneity of pathways that regulate calcium accumulations in spines and the close relationship between the morphology and function of the spine.

Elevation of intradendritic sodium concentration mediated by synaptic activation of metabotropic glutamate receptors in cerebellar Purkinje cells

Cerebellar Purkinje cells express both ionotropic glutamate receptors and metabotropic glutamate receptors. Brief tetanic stimulation of parallel fibers in rat and mouse cerebellar slices evokes a slow excitatory postsynaptic current in Purkinje cells that is mediated by the mGluR1 subtype of metabotropic glutamate receptors. The effector system underlying this mGluR1 EPSC has not yet been identified. In the present study, we recorded the mGluR1 EPSC using the whole-cell patch-clamp technique in combination with microfluorometric recordings of the intracellular sodium concentration ([Na+]i) by means of the fluorescent sodium indicator SBFI. The mGluR1 EPSC was induced by local parallel fibre stimulation in the presence of the ionotropic glutamate receptor antagonists NBQX and D-APV and the GABAA receptor antagonists bicuculline or picrotoxin. The mGluR1 EPSC was associated with an increase in [Na+]i that was restricted to a specific portion of the dendritic tree. The mGluR1 EPSC as well as the increase in [Na+]i were inhibited by the mGluR antagonist S-MCPG. In the presence of NBQX, D-APV, pictrotoxin and TTX, bath application of the selective mGluR agonist 3,5-DHPG induced an elevation in [Na+]i which extended over the whole dendritic field of the Purkinje cell. This finding demonstrates that the mGluR1-mediated postsynaptic current leads to a significant influx of sodium into the dendritic cytoplasm of Purkinje cells and thereby provides a novel intracellular signalling mechanism that might be involved in mGluR1-dependent synaptic plasticity at this synapse.

Local current spread in electrically compact neurons of the fly

Analyses of active and passive membrane properties predict an asymmetry in the spread of electrical current through a neuron. Simulated current injection into a large-diameter compartment of a biophysically realistic model neuron causes a local potential shift that can spread throughout the cell. In contrast, causing the same local potential shift in the dendritic tip of the same neuron results in only minimal changes in electrical potential in the rest of the cell. Using calcium as a reporter of electrical activity in neurons in the fly's lobula plate we find that current injected into the thick axon caused depolarization throughout the cell, whereas activation of a dendritic region remained local. These results have important implications for the ability of integrating neurons to perform local computations of synaptic input without additional hardware.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.

Inositol 1,4,5-trisphosphate directs Ca(2+) flow between mitochondria and the Endoplasmic/Sarcoplasmic reticulum: a role in regulating cardiac autonomic Ca(2+) spiking

The signaling role of the Ca(2+) releaser inositol 1,4, 5-trisphosphate (IP(3)) has been associated with diverse cell functions. Yet, the physiological significance of IP(3) in tissues that feature a ryanodine-sensitive sarcoplasmic reticulum has remained elusive. IP(3) generated by photolysis of caged IP(3) or by purinergic activation of phospholipase Cgamma slowed down or abolished autonomic Ca(2+) spiking in neonatal rat cardiomyocytes. Microinjection of heparin, blocking dominant-negative fusion protein, or anti-phospholipase Cgamma antibody prevented the IP(3)-mediated purinergic effect. IP(3) triggered a ryanodine- and caffeine-insensitive Ca(2+) release restricted to the perinuclear region. In cells loaded with Rhod2 or expressing a mitochondria-targeted cameleon and TMRM to monitor mitochondrial Ca(2+) and potential, IP(3) induced transient Ca(2+) loading and depolarization of the organelles. These mitochondrial changes were associated with Ca(2+) depletion of the sarcoplasmic reticulum and preceded the arrest of cellular Ca(2+) spiking. Thus, IP(3) acting within a restricted cellular region regulates the dynamic of calcium flow between mitochondria and the endoplasmic/sarcoplasmic reticulum. We have thus uncovered a novel role for IP(3) in excitable cells, the regulation of cardiac autonomic activity.

Functional coupling of Ca(2+) channels to ryanodine receptors at presynaptic terminals. Amplification of exocytosis and plasticity

Ca(2+)-induced Ca(2+) release (CICR) enhances a variety of cellular Ca(2+) signaling and functions. How CICR affects impulse-evoked transmitter release is unknown. At frog motor nerve terminals, repetitive Ca(2+) entries slowly prime and subsequently activate the mechanism of CICR via ryanodine receptors and asynchronous exocytosis of transmitters. Further Ca(2+) entry inactivates the CICR mechanism and the absence of Ca(2+) entry for >1 min results in its slow depriming. We now report here that the activation of this unique CICR markedly enhances impulse-evoked exocytosis of transmitter. The conditioning nerve stimulation (10-20 Hz, 2-10 min) that primes the CICR mechanism produced the marked enhancement of the amplitude and quantal content of end-plate potentials (EPPs) that decayed double exponentially with time constants of 1.85 and 10 min. The enhancement was blocked by inhibitors of ryanodine receptors and was accompanied by a slight prolongation of the peak times of EPP and the end-plate currents estimated from deconvolution of EPP. The conditioning nerve stimulation also enhanced single impulse- and tetanus-induced rises in intracellular Ca(2+) in the terminals with little change in time course. There was no change in the rate of growth of the amplitudes of EPPs in a short train after the conditioning stimulation. On the other hand, the augmentation and potentiation of EPP were enhanced, and then decreased in parallel with changes in intraterminal Ca(2+) during repetition of tetani. The results suggest that ryanodine receptors exist close to voltage-gated Ca(2+) channels in the presynaptic terminals and amplify the impulse-evoked exocytosis and its plasticity via CICR after Ca(2+)-dependent priming.

L-Type Ca(2+) channel charge movement and intracellular Ca(2+) in skeletal muscle fibers from aging mice

In this work we tested the hypothesis that skeletal muscle fibers from aging mice exhibit a significant decline in myoplasmic Ca(2+) concentration resulting from a reduction in L-type Ca(2+) channel (dihydropyridine receptor, DHPR) charge movement. Skeletal muscle fibers from the flexor digitorum brevis (FDB) muscle were obtained from 5-7-, 14-18-, or 21-24-month-old FVB mice and voltage-clamped in the whole-cell configuration of the patch-clamp technique according to described procedures (Wang, Z.-M., M. L. Messi, and O. Delbono. 1999. Biophys. J. 77:2709-2716). Total charge movement or the DHPR charge movement was measured simultaneously with intracellular Ca(2+) concentration. The maximum charge movement (Q(max)) recorded (mean +/- SEM, in nC microF(-1)) was 53 +/- 3.2 (n = 47), 51 +/- 3.2 (n = 35) (non-significant, ns), and 33 +/- 1.9 (n = 32) (p < 0.01), for the three age groups, respectively. Q(max) corresponding to the DHPR was 43 +/- 3.3, 38 +/- 4.1 (ns), and 25 +/- 3.4 (p < 0.01) for the three age groups, respectively. The peak intracellular [Ca(2+)] recorded at 40 mV (in microM) was 15.7 +/- 0. 12, 16.7 +/- 0.18 (ns), and 8.2 +/- 0.07 (p < 0.01) for the three age groups, respectively. No significant changes in the voltage distribution or steepness of the Q-V or [Ca(2+)]-V relationship were found. These data support the concept that the reduction in the peak intracellular [Ca(2+)] results from a larger number of ryanodine receptors uncoupled to DHPRs in skeletal muscle fibers from aging mammals.

NMDA receptor-mediated subthreshold Ca(2+) signals in spines of hippocampal neurons

We have used rapid confocal microscopy to investigate the mechanism of Ca(2+) signals in individual dendritic spines of hippocampal CA1 pyramidal cells. The experiments focused on the signals that occur during single weak synaptic responses that were subthreshold for triggering postsynaptic action potentials. These Ca(2+) signals were not strongly affected by blocking the EPSPs with the AMPA receptor antagonist CNQX. The signals were also not strongly reduced by blocking T-type voltage-gated Ca(2+) channels (VGCCs) with Ni(2+) or by blocking a broad range of VGCCs with intracellular D890. The spine Ca(2+) signals were blocked by NMDA receptor channel (NMDAR) antagonist and had the voltage dependence characteristic of these channels. Neither ryanodine nor cyclopiazonic acid (CPA), substances known to deplete intracellular Ca(2+) stores, substantially reduced the amplitude of synaptically evoked Ca(2+) signals. CPA slowed the recovery phase of Ca(2+) signals in spines produced by synaptic stimulation or by backpropagating action potentials, suggesting a role of intracellular stores in Ca(2+) reuptake. Thus, we find that Ca(2+) release from intracellular stores is not required to produce spine Ca(2+) signals. We conclude that synaptic Ca(2+) signals in spines are primarily caused by Ca(2+) entry through NMDARs. Although these channels are largely blocked by Mg(2+) at voltages near the resting potential, they can nevertheless produce significant Ca(2+) elevation. The resulting Ca(2+) signals are an integral component of individual evoked or spontaneous synaptic events and may be important in the maintenance of synaptic function.

Alterations in Purkinje cell spines of calbindin D-28 k and parvalbumin knock-out mice

The second messenger Ca2+ is known to act in a broad spectrum of fundamental cell processes, including modifications of cell shape and motility, through the intermediary of intracellular calcium-binding proteins. The possible impact of the lack of the intracellular soluble Ca2+-binding proteins parvalbumin (PV) and calbindin D-28 k (CB) was tested on spine morphology and topology in Purkinje cell dendrites of genetically modified mice. Three different genotypes were studied, i.e. PV or CB single knock-out (PV-/-, CB-/-) and PV and CB double knock-out mice (PV-/-CB-/-). Purkinje cells were microinjected with Lucifer Yellow and terminal dendrites scanned at high resolution with a confocal laser microscope followed by three-dimensional (3-D) reconstruction. The absence of PV had no significant effect on spine morphology, whereas the absence of CB resulted in a slight increase of various spine parameters, most notably spine length. In double knock-out mice, the absence of both PV and CB entailed a doubling of spine length, an increase in spine volume and spine surface, a higher spine density along the dendrites, as well as a more clustered spine distribution. In all three genotypes, a reduction in the number of stubby spines was observed compared with wild-type animals. These results suggest a morphological compensation for the lack of the soluble calcium buffers in the cytoplasm of Purkinje cell dendritic spines. The increase in various spine parameters, particularly volume, may counteract the lack of the calcium buffers, such as to adjust Ca2+-transients at the transitional zone between spines and dendrites.

Impaired calcium release in cerebellar Purkinje neurons maintained in culture

Cerebellar Purkinje neurons demonstrate a form of synaptic plasticity that, in acutely prepared brain slices, has been shown to require calcium release from the intracellular calcium stores through inositol trisphosphate (InsP(3)) receptors. Similar studies performed in cultured Purkinje cells, however, find little evidence for the involvement of InsP(3) receptors. To address this discrepancy, the properties of InsP(3)- and caffeine-evoked calcium release in cultured Purkinje cells were directly examined. Photorelease of InsP(3) (up to 100 microM) from its photolabile caged analogue produced no change in calcium levels in 70% of cultured Purkinje cells. In the few cells where a calcium increase was detected, the response was very small and slow to peak. In contrast, the same concentration of InsP(3) resulted in large and rapidly rising calcium responses in all acutely dissociated Purkinje cells tested. Similar to InsP(3), caffeine also had little effect on calcium levels in cultured Purkinje cells, yet evoked large calcium transients in all acutely dissociated Purkinje cells tested. The results demonstrate that calcium release from intracellular calcium stores is severely impaired in Purkinje cells when they are maintained in culture. Our findings suggest that cultured Purkinje cells are an unfaithful experimental model for the study of the role of calcium release in the induction of cerebellar long term depression.

Clustering of delta glutamate receptors is regulated by the actin cytoskeleton in the dendritic spines of cultured rat Purkinje cells

The interaction of neurotransmitter receptors with the cytoskeleton is an important mechanism for the targeting of receptors to the postsynaptic membrane. Using cytoskeleton-perturbing agents, it was demonstrated that delta glutamate receptors, predominantly expressed on the dendritic spines of cerebellar Purkinje cells, are anchored to the actin cytoskeleton. The number of delta glutamate receptor-immunoreactive clusters was dramatically decreased following treatment of the Purkinje cells with the actin-disrupting agents, cytochalasin D or latrunculin A, without any significant effect on the number of presynaptic contacts of the granule cell axons. The clusters disrupted by latrunculin A were re-established 24 h after removal of the drug. These results suggest that morphological changes in the actin cytoskeleton regulate the delta glutamate receptor clustering on the dendritic spines, and may affect synaptic efficacy and plasticity.

Mapping of IP(3)-mediated Ca(2+) signals in single human neuroblastoma SH-SY5Y cells: cell volume shaping the Ca(2+) signal

Fast confocal laser-scanning microscopy was used to study spatiotemporal properties of IP(3)-mediated Ca(2+) release signals in human SH-SY5Y neuroblastoma cells. [Ca(2+)](i) increases were not affected by ryanodine (30 microgM) or caffeine (10 mM) and largely insensitive to removal of external Ca(2+), indicating predominance of IP(3)-induced Ca(2+) release. Ca(2+) signals evoked by high concentration (10 microM) of the muscarinic agonist carbachol appeared as self-propagating waves initiating in cell processes. At low carbachol concentrations (500 nM) Ca(2+) changes in most cells displayed striking spatiotemporal heterogeneity. The Ca(2+) response in the cell body was delayed and had a smaller amplitude and a slower rise time than that in processes. Ca(2+) changes in processes either occurred in a homogeneous manner throughout the whole process or were sometimes confined to hot spots. Regional differences in surface-to-volume ratio appear to be critical clues that determine the spatiotemporal pattern of intracellular Ca(2+) release signals.

Complex interactions between mGluRs, intracellular Ca2+ stores and ion channels in neurons

Metabotropic glutamate receptors (mGluRs) can increase intracellular Ca2+ concentration via Ins(1,4,5)P3- and ryanodine-sensitive Ca2+ stores in neurons. Both types of store are coupled functionally to Ca2+-permeable channels found in the plasma membrane. The mGluR-mediated increase in intracellular Ca2+ concentration can activate Ca2+-sensitive K+ channels and Ca2+-dependent nonselective cationic channels. These mGluR-mediated effects often result from mobilization of Ca2+ from ryanodine-sensitive, rather than Ins(1,4, 5)P3-sensitive, Ca2+ stores, suggesting that close functional interactions exist between mGluRs, intracellular Ca2+ stores and Ca2+-sensitive ion channels in the membrane.

Using photolabile ligands in drug discovery and development

Photoactivatable ligands are important tools used in drug discovery and drug development. These ligands enable researchers to identify the targets of drugs, to determine the affinity and selectivity of the drug-target interaction, and to identify the binding site on the target. Examples are presented from three fundamentally different approaches: (1) photoaffinity labeling of target macromolecules; (2) photoactivation and release of 'caged ligands'; and (3) photoimmobilization of ligands onto surfaces.

Calcium release from internal stores is required for the generation of spontaneous hyperpolarizations in dopaminergic neurons of neonatal rats

We recently have demonstrated the existence of spontaneous hyperpolarizations in midbrain dopaminergic neurons of neonatal but not adult rats. These events are mediated by the opening of apamin-sensitive K(+) channels after a rise in the intracellular concentration of Ca(2+). They are resistant to tetrodotoxin in most cases and are probably endogenous (i.e., not synaptically activated). Here their mechanism was investigated. Cyclopiazonic acid (10 microM), a specific inhibitor of endoplasmic reticulum Ca(2+) ATPases, reversibly abolished the events. Caffeine, which promotes Ca(2+) release from intracellular stores, had concentration-dependent effects. At 1 mM, it markedly and steadily increased the frequency and the amplitude of the hyperpolarizations. At 10 mM, it induced a transient increase in their frequency followed by their cessation. All these effects were quickly reversible. Ryanodine (10 microM), which decreases the conductance of Ca(2+) release channels, irreversibly blocked the spontaneous hyperpolarizations. Dantrolene (100 microM), a blocker of Ca(2+) release from sarcoplasmic reticulum of striated muscle, did not affect the events. On the other hand, Cd(2+) (100-300 microM), a broad antagonist of membrane voltage-gated Ca(2+) channels, significantly reduced the amplitude and the frequency of the hyperpolarizations. However, when the frequency of the events was increased by 1 mM caffeine, Cd(2+) affected them to a smaller extent, whereas cyclopiazonic acid still abolished them. We conclude that internal stores are the major source of Ca(2+) ions that induce the K(+) channel openings underlying the spontaneous hyperpolarizations of these neurons.

How the basal ganglia use parallel excitatory and inhibitory learning pathways to selectively respond to unexpected rewarding cues

After classically conditioned learning, dopaminergic cells in the substantia nigra pars compacta (SNc) respond immediately to unexpected conditioned stimuli (CS) but omit formerly seen responses to expected unconditioned stimuli, notably rewards. These cells play an important role in reinforcement learning. A neural model explains the key neurophysiological properties of these cells before, during, and after conditioning, as well as related anatomical and neurophysiological data about the pedunculopontine tegmental nucleus (PPTN), lateral hypothalamus, ventral striatum, and striosomes. The model proposes how two parallel learning pathways from limbic cortex to the SNc, one devoted to excitatory conditioning (through the ventral striatum, ventral pallidum, and PPTN) and the other to adaptively timed inhibitory conditioning (through the striosomes), control SNc responses. The excitatory pathway generates CS-induced excitatory SNc dopamine bursts. The inhibitory pathway prevents dopamine bursts in response to predictable reward-related signals. When expected rewards are not received, striosomal inhibition of SNc that is unopposed by excitation results in a phasic drop in dopamine cell activity. The adaptively timed inhibitory learning uses an intracellular spectrum of timed responses that is proposed to be similar to adaptively timed cellular mechanisms in the hippocampus and cerebellum. These mechanisms are proposed to include metabotropic glutamate receptor-mediated Ca(2+) spikes that occur with different delays in striosomal cells. A dopaminergic burst in concert with a Ca(2+) spike is proposed to potentiate inhibitory learning. The model provides a biologically predictive alternative to temporal difference conditioning models and explains substantially more data than alternative models.

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

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

ER transport on actin filaments in squid giant axon: implications for signal transduction at synapse

The smooth endoplasmic reticulum (S-ER) is transported on actin filaments in the giant axon of the squid. The identity of the myosin motors that transport S-ER in the squid giant axon has been determined. Our recent studies have shown that the motor for movement of S-ER vesicles on actin filaments is Myosin-V (1). These findings grew out of a series of studies that began with the initial observation that vesicles in the giant axon of the squid move on both microtubules and actin filaments (2). These initial studies documented the ability of individual vesicles to move from microtubules to actin filaments and led to the development of the dual filament model of vesicle transport (3, 4). The model proposes that long-range movement of vesicles occurs on microtubules and short-range movement on actin filaments. S-ER vesicles were identified as the major population of vesicles in the axon that use myosin-V for movement on actin filaments. The S-ER is the primary site of calcium storage, and it regulates the local cytosolic calcium concentration. Calcium release from the S-ER in neurons couples electrical excitation to signal transduction cascades. The signaling cascades triggered by the release of calcium from S-ER in dendritic spines are postulated to initiate the cellular mechanisms that lead to learning and memory.

New Ca2+-releasing messengers: are they important in the nervous system?

In the nervous system, Ca2+ signalling is determined primarily by voltage-gated Ca2+-selective channels in the plasma membrane, but there is increasing evidence for involvement of intracellular Ca2+ stores in such signalling. It is generally assumed that neurotransmitter-elicited release of Ca2+ from internal stores is primarily mediated by Ins(1,4,5)P3, as originally discovered in pancreatic acinar cells. The more-recently discovered Ca2+-releasing messenger, cyclic ADP-ribose (cADPR), which activates ryanodine receptors, has so far only been implicated in a few cases, and the possible importance of another Ca2+-releasing molecule, nicotinic acid adenine dinucleotide phosphate (NAADP), has been ignored. Recent investigations of the action of the brain-gut peptide cholecystokinin on pancreatic acinar cells have indicated that NAADP and cADPR receptors are essential for Ca2+ release. Tools are available for testing the possible involvement of NAADP and cADPR in neurotransmitter-elicited intracellular Ca2+ release, and such studies could reveal complex mechanisms that control this release in the nervous system.

Protein targeting and calcium signaling microdomains in neuronal cells

Over the last several years, a number of optical imaging, physiological, and molecular studies have clarified the mechanisms underlying differential calcium signaling in the postsynaptic neuron. These studies have revealed the existence of membrane-associated calcium microdomains, which are often specifically coupled to distinct protein signaling pathways. In this review, we discuss how these signaling microdomains are organized and regulated, emphasizing the structural and molecular features of synaptic protein complexes containing the metabotropic and N-methyl-D-aspartate (NMDA) glutamate receptors and the L-type voltage-dependent calcium channels (VDCCs). We conclude with a discussion of how these different signaling complexes may interact with one another, relationships which may be important in orchestrating the complex calcium signaling underlying developmental and activity-dependent changes in synaptic function.

Synergistic release of Ca2+ from IP3-sensitive stores evoked by synaptic activation of mGluRs paired with backpropagating action potentials

Increases in postsynaptic [Ca2+]i can result from Ca2+ entry through ligand-gated channels or voltage-gated Ca2+ channels, or through release from intracellular stores. Most attention has focused on entry through the N-methyl-D-aspartate (NMDA) receptor in causing [Ca2+]i increases since this pathway requires both presynaptic stimulation and postsynaptic depolarization, making it a central component in models of synaptic plasticity. Here, we report that repetitive synaptic activation of metabotropic glutamate receptors (mGluRs), paired with backpropagating action potentials, causes large, wave-like increases in [Ca2+]i predominantly in restricted regions of the proximal apical dendrites and soma of hippocampal CA1 pyramidal neurons. [Ca2+]i changes of several micromolars can be reached by regenerative release caused by the synergistic effect of mGluR-generated inositol 1,4,5-trisphosphate (IP3) and spike-evoked Ca2+ entry acting on the IP3 receptor.

Patch-clamp recording of charge movement, Ca2+ current, and Ca2+ transients in adult skeletal muscle fibers

Intramembrane charge movement (Q), Ca(2+) conductance (G(m)) through the dihydropyridine-sensitive L-type Ca(2+) channel (DHPR) and intracellular Ca(2+) fluorescence (F) have been recorded simultaneously in flexor digitorum brevis muscle fibers of adult mice, using the whole-cell configuration of the patch-clamp technique. The voltage distribution of Q was fitted to a Boltzmann equation; the Q(max), V(1/2Q), and effective valence (z(Q)) values were 41 +/- 3.1 nC/microF, -17.6 +/- 0.7 mV, and 2.0 +/- 0.12, respectively. V(1/2G) and z(G) values were -0.3 +/- 0.06 mV and 5.6 +/- 0.34, respectively. Peak Ca(2+) transients did not change significantly after 30 min of recording. F was fit to a Boltzmann equation, and the values for V(F1/2) and z(F) were 6.2 +/- 0.04 mV and 2.4, respectively. F was adequately fit to the fourth power of Q. These results demonstrate that the patch-clamp technique is appropriate for recording Q, G(m), and intracellular [Ca(2+)] simultaneously in mature skeletal muscle fibers and that the voltage distribution of the changes in intracellular Ca(2+) can be predicted by a Hodgkin-Huxley model.