Depolarization-induced changes in neurite elongation and intracellular Ca2+ in isolated Helisoma neurons.

This study focuses on the effects of K+ depolarization on neurite elongation of identified Helisoma neurons isolated into culture. Application of K+ to the external medium caused a dose-dependent suppression of neurite elongation. Lower concentrations of K+ were associated with a slowing in the rate of neurite elongation, whereas higher concentrations produced neurite retraction. Surprisingly, the effects of K+ depolarization were transient, and neurite elongation rates recovered towards control levels within 90 min even though the neurons remained in high-K+ solution. Identified neurons differed in the magnitude of their response to K+ depolarization; neurite elongation of buccal neuron B4 was inhibited at 5 mM K+, but elongation in B5 and B19 was not affected until concentrations of 25 mM. Electrophysiologically, K+ application evoked a brief period (5-10 s) of action potential activity that was followed by a steady-state membrane depolarization lasting 2 h or more. The changes in neurite elongation induced by K+ depolarization occurred in isolated growth cones severed from their neurites and were blocked by application of calcium antagonists. Intracellular free Ca2+ levels in growth cones of B4 and B19 increased and then decreased during the 90-min depolarization, corresponding to the changes in elongation. B4 and B19 showed differences in the magnitude, time course, and spatial distribution of the Ca2+ change during depolarization, reflecting their different sensitivities to depolarization.     

Low- and high-voltage-activated calcium currents: their relationship to the site of neurotransmitter release in an identified neuron of Helisoma

In this study we have characterized two types of Ca2+ currents in identified neuron B5 of Helisoma and have examined the relationship between these currents and neurotransmitter release. Neuron B5 contains low-voltage-activated (LVA) and high-voltage-activated (HVA) Ca2+ currents. These currents have distinct electrophysiological and pharmacological properties. To gain access to the site of neurotransmitter release, we used a model system in which somata that do not extend neurites assume the role of neurotransmitter release. Before somata gain the ability to release neurotransmitter, they contain LVA and HVA Ca2+ currents. After 3 days of culture, when spherical somata have gained the secretory capacity, only the HVA Ca2+ current is present. Experiments were also performed when neurite extension was permitted. These data indicate that neurons with processes have a differential distribution of Ca2+ currents. The soma, which does not release neurotransmitter, contains both LVA and HVA Ca2+ currents, while distal secretory processes contain only HVA current.     

Synchronized Ca2+ signals mediated by Ca2+ action potentials in the hippocampal neuron network in vitro

Periodic, synchronized Ca2+ signals appeared 30-120 min after the application of tetrodotoxin, 4-aminopyridine and Cs+, and became stable in interval (6-47s) for hours. The Ca2+ signals were accompanied by excitatory or inhibitory postsynaptic potentials (excitatory postsynaptic currents (EPSCs) for the former) and blocked by the simultaneous application of 6-cyano-7-nitroquinoxaline-2,3-dione and 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid or treatment with Ca2+ -free solution, nicardipine, or omega-conotoxin MVIIC (omegaCTX), but not with ryanodine, caffeine, thapsigargin or CPP alone. Nicardipine largely, but omegaCTX less, blocked Ca2+ action potentials or voltage pulse-induced Ca2+ currents at the cell soma, while omegaCTX completely blocked autaptic EPSCs. Ca2+ signals within a neuron occurred almost simultaneously in the cell soma and all the processes (> 200 microm), while the latency between Ca2+ signals of neighbouring neurons varied over hundreds of ms like that of Ca2 action potential induction from EPSPs. Ca2+ signals propagated in random directions throughout neural circuits. Thus, when Na+ and K+ channels are blocked, Ca2+ action potentials spontaneously occur somewhere in a neuron, eventually propagate via the cell soma to the presynaptic terminals and activate excitatory synaptic transmission, causing synchronized Ca2+ signals. The results further suggest that the axon of hippocampal neurones have the potential ability to convey coded information via Ca2+ action potentials.     

Ca2+ waves in PC12 neurites: a bidirectional, receptor-oriented form of Ca2+ signaling

Spatial and temporal aspects of Ca2+ signaling were investigated in PC12 cells differentiated with nerve growth factor, the well known nerve cell model. Activation of receptors coupled to polyphosphoinositide hydrolysis gave rise in a high proportion of the cells to Ca2+ waves propagating non decrementally and at constant speed (2-4 microns/s at 18 degrees C and approximately 10-fold faster at 37 degrees C) along the neurites. These waves relied entirely on the release of Ca2+ from intracellular stores since they could be generated even when the cells were incubated in Ca(2+)-free medium. In contrast, when the cells were depolarized with high K+ in Ca(2+)-containing medium, increases of cytosolic Ca2+ occurred in the neurites but failed to evolve into waves. Depending on the receptor agonist employed (bradykinin and carbachol versus ATP) the orientation of the waves could be opposite, from the neurite tip to the cell body or vice versa, suggesting different and specific distribution of the responsible surface receptors. Cytosolic Ca2+ imaging results, together with studies of inositol 1,4,5-trisphosphate generation in intact cells and inositol 1,4,5-trisphosphate-induced Ca2+ release from microsomes, revealed the sustaining process of the waves to be discharge of Ca2+ from the inositol 1,4,5-trisphosphate- (and not the ryanodine-) sensitive stores distributed along the neurites. The activation of the cognate receptor appears to result from the coordinate action of the second messenger and Ca2+. Because of their properties and orientation, the waves could participate in the control of not only conventional cell activities, but also excitability and differential processing of inputs, and thus of electrochemical computation in nerve cells.     

Beta-subunits promote the expression of Ca(V)2.2 channels by reducing their proteasomal degradation

The β-subunits of voltage-gated calcium channels regulate their functional expression and properties. Two mechanisms have been proposed for this, an effect on gating and an enhancement of expression. With respect to the effect on expression, β-subunits have been suggested to enhance trafficking by masking an unidentified endoplasmic reticulum (ER) retention signal. Here we have investigated whether, and how, β-subunits affect the level of Ca(V)2.2 channels within somata and neurites of cultured sympathetic neurons. We have used YFP-Ca(V)2.2 containing a mutation (W391A), that prevents binding of β-subunits to its I-II linker and found that expression of this channel was much reduced compared with WT CFP-Ca(V)2.2 when both were expressed in the same neuron. This effect was particularly evident in neurites and growth cones. The difference between the levels of YFP-Ca(V)2.2(W391A) and CFP-Ca(V)2.2(WT) was lost in the absence of co-expressed β-subunits. Furthermore, the relative reduction of expression of Ca(V)2.2(W391A) compared with the WT channel was reversed by exposure to two proteasome inhibitors, MG132 and lactacystin, particularly in the somata. In further experiments in tsA-201 cells, we found that proteasome inhibition did not augment the cell surface Ca(V)2.2(W391A) level but resulted in the observation of increased ubiquitination, particularly of mutant channels. In contrast, we found no evidence for selective retention of Ca(V)2.2(W391A) in the ER, in either the soma or growth cones. In conclusion, there is a marked effect of β-subunits on Ca(V)2.2 expression, particularly in neurites, but our results point to protection from proteasomal degradation rather than masking of an ER retention signal.     

Ca2+ influx and neurite growth in response to purified N-cadherin and laminin

The signaling mechanisms underlying neurite growth induced by cadherins and integrins are incompletely understood. In our experiments, we have examined these mechanisms using purified N-cadherin and laminin (LN). We find that unlike the neurite growth induced by fibroblastic cells expressing transfected N-cadherin (Doherty, P., and F.S. Walsh. 1992. Curr. Opin. Neurobiol. 2:595-601), growth induced by purified N- cadherin in chick ciliary ganglion (CG), sensory, or forebrain neurons is not sensitive to inhibition by pertussis toxin. Using fura-2 imaging of single cells, we show that soluble N-cadherin induces Ca2+ increases in CG neuron cell bodies, and, importantly, in growth cones. In contrast, N-cadherin can induce Ca2+ decreases in glial cells. N- cadherin-induced neuronal Ca2+ responses are sensitive to Ni2+, but are relatively insensitive to diltiazem and omega-conotoxin. Similarly, neurite growth induced by purified N-cadherin is inhibited by Ni2+, but is unaffected by diltiazem and conotoxin. Soluble LN also induced small Ca2+ responses in CG neurons. LN-induced neurite growth, like that induced by N-cadherin, is insensitive to diltiazem and conotoxin, but is highly sensitive to Ni2+ inhibition. K+ depolarization experiments suggest that voltage-dependent Ca2+ influx pathways in CG neurons (cell bodies and growth cones) are largely blocked by the combination of diltiazem and Ni2+. Our results demonstrate that cadherin signaling involves cell type-specific Ca2+ changes in responding cells, and in particular, that N-cadherin can cause Ca2+ increases in neuronal growth cones. Our findings are consistent with the current idea that distinct neuronal transduction pathways exist for cell adhesion molecules compared with integrins, but suggest that the involvement of Ca2+ signals in both of these pathways is more complex than previously appreciated.     

Electrically induced changes in Ca2+ in Helisoma neurons: Regional and neuron-specific differences and implications for neurite outgrowth

The present experiments addressed the questions of how electrical stimulation influenced the magnitude, time course, and regional levels of free intracellular calcium of different identified neurons. The calcium concentration in the growth cones, neurites and cell bodies of Helisoma buccal neurons B4 and B19 was measured while somata were electrically stimulated via an intracellular electrode. The findings showed that calcium levels in B4 and B19 increased monotonically with increasing stimulation frequency. However, the range of calcium levels evoked by electrical stimulation differed significantly for each type of neuron. The greater increase in calcium concentration in B4 was correlated with its longer duration action potential compared to B19. The increase in calcium concentration was much smaller in the cell bodies than in the growth cones and neurites. Extending the duration of the B19 action potential produced a sixfold increase in the change in calcium concentration at 2 Hz stimulation. Under conditions where the electrical stimulation produced a calcium concentration of < 160 nM, the elevated level of free intracellular calcium remained constant. When calcium concentration increased above 200 nM in both identified neurons, an initial peak concentration was followed by a decline to a lower concentration suggesting increased calcium buffering occurring above 200 nM. By correlating the calcium concentration data herein with growth data from a previous study, we suggest that specific calcium levels that influence neurite outgrowth may differ widely between neurons. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 150–162, 1997.     

Multiple, coordinated Ca2+ -release events underlie the inositol trisphosphate-induced local Ca2+ spikes in mouse pancreatic acinar cells.

Ca2+ wave initiation and non-propagating Ca2+ spikes occur as a result of localized Ca2+ release from the more sensitive intracellular Ca2+ stores. Using high spatial and temporal Ca2+ -imaging techniques we have investigated inositol 1,4,5 triphosphate (InsP3)-induced local Ca2+ spiking, which occurs at the site of Ca2+ wave initiation in pancreatic acinar cells. The spatial and temporal organization of a single spike suggested discrete hot spots of Ca2+ release. Further analysis of long trains of Ca2+ spikes demonstrated that these hot spots showed regenerative Ca2+ -release events which were consistently active from spike to spike. Regions adjacent to these hot spots also showed regenerative Ca2+ -release events of similar amplitude but with a much lower frequency of occurrence. We conclude that the InsP3-induced non-propagating Ca2+ spikes can be devolved into smaller components of release. Our results are consistent with a model of coordinated activity of pacemaker hot spots of Ca2+ release that recruit and entrain active Ca2+ -release events from surrounding regions.     

Pulsed laser imaging of rapid Ca2+ gradients in excitable cells.

Excitable cells are thought to respond to action potentials by forming short lived and highly localized Ca2+ gradients near sites of Ca2+ entry or near the site of Ca2+ release by intracellular stores. However, conventional imaging techniques lack the spatial and temporal resolution to capture these gradients. Here we demonstrate the use of pulsed-laser microscopy to measure Ca2+ gradients with submicron spatial resolution and millisecond time resolution in two preparations where the Ca2+ signal is thought to be fast and highly localized: adrenal chromaffin cells, where the entry of Ca2+ through voltage dependent Ca2+ channels triggers exocytotic fusion; and skeletal muscle fibers, where intracellular Ca2+ release from the sarcoplasmic reticulum initiates contraction. In chromaffin cells, Ca2+ gradients developed over 10-100 ms and were initially restricted to discrete submembrane domains, or hot spots, before developing into complete rings of elevated Ca2+ concentration. In frog skeletal muscle large, short-lived (approximately 6 ms) Ca2+ gradients were observed within individual sarcomeres following induction of action potentials. The pulsed laser imaging approach permits, for the first time, the capture and critical examination of rapid Ca2+ signaling events such as those underlying excitation-secretion and excitation-contraction coupling.     

Postnatal developmental changes in Ca2+ homeostasis in supraoptic magnocellular neurons

Supraoptic magnocellular neurons (SMNs) undergo dramatic changes in morphological and electrical properties during postnatal development. We investigated the developmental change in Ca2+ homeostasis in SMNs. The decay rate of Ca2+ transients markedly increased during the third postnatal week (PW3) to an adult level. This increase in the Ca2+ decay rate was paralleled by hypertrophy of the SMN somata. Activity of Na+/Ca2+ exchanger (Na/CaX) and sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) was quantified as a decrement in the Ca2+ decay rate caused by extracellular [Na+] reduction and that by thapsigargin, respectively. SERCA activity was negligible during PW2, and markedly increased during PW3. SERCA activity and soma size remained stable thereafter. Na/CaX activity was a major Ca2+-clearance mechanism (CCM) during PW2, increased further during PW3, but was negligible in mature SMNs (PW10). In parallel with the decrease in Na/CaX activity, endogenous Ca2+ buffering capacity declined, resulting that the apparent Ca2+ decay rate remained relatively constant between PW4 and PW10. Replacement of intracellular K+ with Li+ had no effect on Na/CaX activity, suggesting that NCX rather than NCKX comprises Na/CaX. These findings indicate a developmental shift in the balance of CCMs from Ca2+ extrusion via NCX toward Ca2+ sequestration into endoplasmic reticulum via SERCA.     

Synaptically activated increases in Ca2+ concentration in hippocampal CA1 pyramidal cells are primarily due to voltage-gated Ca2+ channels.

Changes in intracellular Ca2+ concentration ([Ca2+]i) in the soma and dendrites of hippocampal CA1 pyramidal neurons were measured using intracellularly injected fura-2. A large component of the [Ca2+]i elevation caused by high frequency stimulation of the Schaffer collaterals was correlated with the Na+ spikes triggered by the excitatory postsynaptic potentials (EPSPs). These spikes were generated in the soma and proximal dendrites and stimulated Ca2+ entry through voltage-gated Ca2+ channels. Suppressing spikes by hyperpolarizing the soma or by injecting QX-314 revealed a smaller nonspike component of Ca2+ entry. A substantial fraction of this component was mediated by the action of the EPSPs on voltage-gated Ca2+ channels, because it persisted in 2-amino-5-phosphonovaleric acid and because it was usually reduced when Ca2+ channel activity was suppressed by hyperpolarization. Ca2+ entry through the N-methyl-D-aspartate receptor channel could not be detected with certainty, perhaps because it was highly localized.     

Ca2+ transients control CNS neuronal migration

In the developing CNS, postmitotic neurons exhibit dynamic changes in the mode, direction and rate of migration as they traverse different cortical layers, but the mechanisms underlying this process is largely unknown. Recent studies show that the changes in Ca2+ transient frequency play a central role in controlling the neuronal cell migration in a cortical layer-specific manner. In this article, we will first describe how granule cells migrate through different terrains of the developing cerebellar cortex. We will then present how such migration of granule cells is controlled by altering the Ca2+ transient frequency in their somata. Finally, we will discuss how the loss of Ca2+ transients triggers the completion of granule cell migration at their final destination.     

Ca2+ transients are not required as signals for long-term neurite outgrowth from cultured sympathetic neurons

A method for clamping cytosolic free Ca2+ ([Ca2+]i) in cultures of rat sympathetic neurons at or below resting levels for several days was devised to determine whether Ca2+ signals are required for neurite outgrowth from neurons that depend on Nerve Growth Factor (NGF) for their growth and survival. To control [Ca2+]i, normal Ca2+ influx was eliminated by titration of extracellular Ca2+ with EGTA and reinstated through voltage-sensitive Ca2+ channels. The rate of neurite outgrowth and the number of neurites thus became dependent on the extent of depolarization by KCl, and withdrawal of KCl caused an immediate cessation of growth. Neurite outgrowth was completely blocked by the L type Ca2+ channel antagonists nifedipine, nitrendipine, D600, or diltiazem at sub- or micromolar concentrations. Measurement of [Ca2+]i in cell bodies using the fluorescent Ca2+ indicator fura-2 established that optimal growth, similar to that seen in normal medium, was obtained when [Ca2+]i was clamped at resting levels. These levels of [Ca2+]i were set by serum, which elevated [Ca2+]i by integral of 30 nM, whereas the addition of NGF had no effect on [Ca2+]i. The reduction of [Ca2+]o prevented neurite fasciculation but this had no effect on the rate of neurite elongation or on the number of extending neurites. These results show that neurite outgrowth from NGF-dependent neurons occurs over long periods in the complete absence of Ca2+ signals, suggesting that Ca2+ signals are not necessary for operating the basic machinery of neurite outgrowth.     

Initial stages of neural regeneration in Helisoma trivolvis are dependent upon PLA2 activity

Neuronal regeneration after damage to an axon tract requires the rapid sealing of the injured plasma membrane and the subsequent formation of growth cones that can lead regenerating processes to their appropriate target. Membrane sealing and growth cone formation are Ca(2+)-dependent processes, but the signaling pathways activated by Ca(2+) to bring about these effects remain poorly understood. An in vitro injury model was employed in which neurites from identified snail neurons (Helisoma trivolvis) were transected with a glass microknife, and the formation of new growth cones from the distal portions of transected neurites was recorded at defined times after transection. This study presents three main results. First, phospholipase A(2) (PLA(2)), a calcium-activated enzyme, is necessary for membrane sealing in vitro. Second, PLA(2) activity is also required for the formation of a new growth cone after the membrane has sealed successfully. Thus, PLA(2) plays a dual role by affecting both growth cone formation and membrane sealing. Third, the injury-induced activation of PLA(2) by Ca(2+) controls growth cone formation through the production of leukotrienes, secondary metabolites of PLA(2) activity. Taken together, these results suggest that the injury-induced Ca(2+) influx acts via PLA(2) and leukotriene production to assure growth cone formation. These findings indicate that events that cause an inhibition of PLA(2) or lipoxygenases, enzymes that produce leukotrienes, could result in the inability of neurites to regenerate.     

Depolarization-induced changes in neurite elongation and intracellular Ca2+ in isolated Helisoma neurons

This study focuses on the effects of K⁺ depolarization on neurite elongation of identified Helisoma neurons isolated into culture. Application of K⁺ to the external medium caused a dose-dependent suppression of neurite elongation. Lower concentrations of K⁺ were associated with a slowing in the rate of neurite elongation, whereas higher concentrations produced neurite retraction. Surprisingly, the effects of K⁺ depolarization were transient, and neurite elongation rates recovered towards control levels within 90 min even though the neurons remained in high-K⁺ solution. Identified neurons differed in the magnitude of their response to K⁺ depolarization; neurite elongation of buccal neuron B4 was inhibited at 5 mM K⁺, but elongation in B5 and B19 was not affected until concentrations of 25 mM. Electrophysiologically, K⁺ application evoked a brief period (5–10 s) of action potential activity that was followed by a steady-state membrane depolarization lasting 2 h or more. The changes in neurite elongation induced by K⁺ depolarization occurred in isolated growth cones severed from their neurites and were blocked by application of calcium antagonists. Intracellular free Ca²⁺ levels in growth cones of B4 and B19 increased and then decreased during the 90-min depolarization, corresponding to the changes in elongation. B4 and B19 showed differences in the magnitude, time course, and spatial distribution of the Ca²⁺ change during depolarization, reflecting their different sensitivities to depolarization.     

Local Ca2+ signals in cellular signalling

Local Ca2+ rises and propagated Ca2+ signals represent different patterns that are differentially decoded for fine tuning cellular signalling. This Ca2+ concentration plasticity is absolutely required to allow adaptation to different needs of the cells ranging from contraction or increased learning to proliferation and cell death. A wide diversity of molecular structures and specific location of Ca2+ signalling molecules confer spatial and temporal versatility to the Ca2+ changes allowing specific cellular responses to be elicited. Various types of local Ca2+ signals have been described. Ca2+ spikes correspond to Ca2+ signals spanning several micrometers but displaying limited propagation into a cell leading to regulation of cellular functions in one particular zone of this cell. This is of particular relevance in cells presenting distinct morphological specializations, i.e. apical versus basal sites or dendritic versus somatic/axonal sites. More stereotyped elementary Ca2+ events (denominated Ca2+ sparks or Ca2+ puffs depending on the type of endoplasmic reticulum Ca2+ release channel involved) are highly confined and non-propagated Ca2+ rises which are observed in the close neighbouring of the Ca2+ channels. These elementary Ca2+ events play a major role in controlling cellular excitability. Elementary Ca2+ events involve Ca2+ release channels such as the ryanodine receptors (RyRs) and the inositol 1,4,5-trisphosphate receptors (InsP3Rs). The molecular bases underlying the various local Ca2+ release events will be discussed by reviewing the channels and particularly the different isoforms of RyRs and InsP3Rs and their role in inducing localized Ca2+ responses. These calcium release events are controlled by various second messengers and are regulated by Ca2+ channel-associated proteins, intra-luminal Ca2+ content of the endoplasmic reticulum (ER) and other Ca2+ organelles. We will discuss on how the control of local cellular Ca2+ content may account for cellular functions in physiological and physiopathological conditions.     

Imaging of cytosolic Ca2+ transients arising from Ca2+ stores and Ca2+ channels in sympathetic neurons

Changes in cytosolic free Ca2+ concentration [( Ca2+]i) due to Ca2+ entry or Ca2+ release from internal stores were spatially resolved by digital imaging with the Ca2+ indicator fura-2 in frog sympathetic neurons. Electrical stimulation evoked a rise in [Ca2+]i spreading radially from the periphery to the center of the soma. Elevated [K+]o also increased [Ca2+]i, but only in the presence of external Ca2+, indicating that Ca2+ influx through Ca2+ channels is the primary event in the depolarization response. Ca2+ release or uptake from caffeine-sensitive internal stores was able to amplify or attenuate the effects of Ca2+ influx, to generate continued oscillations in [Ca2+]i, and to persistently elevate [Ca2+]i above basal levels after the stores had been Ca2(+)-loaded.     

Early differentiation of vertebrate spinal neurons in the absence of voltage-dependent Ca2+ and Na+ influx

The development of the action potential and responses to neurotransmitters have been described for a population of embryonic spinal neurons developing in vivo. A comparable pattern is seen for spinal neurons developing in dissociated cell culture. The impulse appears very early in this developmental sequence, and the action potential involves a large inward Ca2+ current. Since Ca2+ is a ubiquitous intracellular regulator, we questioned whether a large influx of Ca2+ is necessary for the subsequent differentiation of membrane properties. Embryonic Xenopus neurons grown in normal culture medium do not make Ca2+- or Na+-dependent action potentials in their cell bodies in a Ca2+-free saline containing tetrodotoxin (TTX). To achieve a chronic blockade of impulse activity, neurons were grown in a medium in which Ca2+ was replaced by Mg2+, and to which 1 mM EGTA was added. In some instances TTX was present. Neurons grown in these experimental culture media extend neurites more rapidly than controls. Action potentials cannot be elicited from neurons when examined in experimental medium. However, examination in saline reveals that the change in the ionic dependence of the impulse is indistinguishable from that observed in neurons grown in normal medium. Furthermore, the time of onset of responses to GABA is unaffected by this experimental treatment. Thus the expression of Ca2+- and Na+-dependent action potentials seems not to play a part in the early differentiation of these membrane properties. However, the later development of GABA sensitivity is reduced.     

Ca2+-calmodulin-dependent phosphodiesterase (PDE1): current perspectives

Ca2+-calmodulin-dependent phosphodiesterases (PDE1), like Ca2+-sensitive adenylyl cyclases (AC), are key enzymes that play a pivotal role in mediating the cross-talk between cAMP and Ca2+ signalling. Our understanding of how ACs respond to Ca2+ has advanced greatly, with significant breakthroughs at both the molecular and functional level. By contrast, little is known of the mechanisms that might underlie the regulation of PDE1 by Ca2+ in the intact cell. In living cells, Ca2+ signals are complex and diverse, exhibiting different spatial and temporal properties. The potential therefore exists for dynamic changes in the subcellular distribution and activation of PDE1 in relation to intracellular Ca2+ dynamics. PDE1s are a large family of multiply-spliced gene products. Therefore, it is possible that a cell-type specific response to elevation in [Ca2+]i can occur, depending on the isoform of PDE1 expressed. In this article, we summarize current knowledge on Ca2+ regulation of PDE1 in the intact cell and discuss approaches that might be undertaken to delineate the responses of this important group of enzymes to changes in [Ca2+]i.     

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.