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W N Ross

Publications and source records attributed to W N Ross.

At least 19 recordsLinked to original sources
PubMed2001

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.

Action Potentials↗
PubMed2000

Inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release evoked by metabotropic agonists and backpropagating action potentials in hippocampal CA1 pyramidal neurons.

We examined the properties of [Ca(2+)](i) changes that were evoked by backpropagating action potentials in pyramidal neurons in hippocampal slices from the rat. In the presence of the metabotropic glutamate receptor (mGluR) agonists t-ACPD, DHPG, or CHPG, spikes caused Ca(2+) waves that initiated in the proximal apical dendrites and spread over this region and in the soma. Consistent with previously described synaptic responses (Nakamura et al., 1999a), pharmacological experiments established that the waves were attributable to Ca(2+) release from internal stores mediated by the synergistic effect of receptor-mobilized inositol 1,4, 5-trisphosphate (IP(3)) and spike-evoked Ca(2+). The amplitude of the changes reached several micromoles per liter when detected with the low-affinity indicators fura-6F, fura-2-FF, or furaptra. Repetitive brief spike trains at 30-60 sec intervals generated increases of constant amplitude. However, trains at intervals of 10-20 sec evoked smaller increases, suggesting that the stores take 20-30 sec to refill. Release evoked by mGluR agonists was blocked by MCPG, AIDA, 4-CPG, MPEP, and LY367385, a profile consistent with the primacy of group I receptors. At threshold agonist concentrations the release was evoked only in the dendrites; threshold antagonist concentrations were effective only in the soma. Carbachol and 5-HT evoked release with the same spatial distribution as t-ACPD, suggesting that the distribution of neurotransmitter receptors was not responsible for the restricted range of regenerative release. Intracellular BAPTA and EGTA were approximately equally effective in blocking release. Extracellular Cd(2+) blocked release, but no single selective Ca(2+) channel blocker prevented release. These results suggest that IP(3) receptors are not associated closely with specific Ca(2+) channels and are not close to each other.

Action Potentials↗
PubMed1999

Weak effect of neuromodulators on climbing fiber-activated [Ca(2+)](i) increases in rat cerebellar Purkinje neurons.

The effect of several neuromodulators (carbachol (CCh), serotonin (5-HT), noradrenaline (NE), and dopamine (DA)) on the climbing fiber (CF)-induced [Ca(2+)](i) increase in the dendrites of cerebellar Purkinje cells was examined in slices from the rat cerebellum. Purkinje cells were filled with the Ca(2+) indicator bis-fura-2 with patch electrodes on the soma. [Ca(2+)](i) changes were measured from regions of interest in the dendrites with a high speed camera. Changes evoked by one or three responses were measured in control conditions and with neuromodulators added to the bath. None of these four classic modulators caused a significant change in the CF-induced [Ca(2+)](i) amplitude. Buspirone, a partial 5-HT(1A) agonist and a weak DA receptor antagonist caused a small (10-15%) reduction in the response.

Animals↗
PubMed1999

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.

Action Potentials↗
PubMed1999

Serotonin modulates spike backpropagation and associated [Ca2+]i changes in the apical dendrites of hippocampal CA1 pyramidal neurons.

The effect of serotonin (5-HT) on somatic and dendritic properties was analyzed in pyramidal neurons from the CA1 region in slices from the rat hippocampus. Bath-applied 5-HT (10 microM) hyperpolarized the soma and apical dendrites and caused a conductance increase at both locations. In the dendrites (200-300 microm from the soma) trains of antidromically activated, backpropagating action potentials had lower peak potentials in 5-HT than in normal artificial cerebrospinal fluid. Spike amplitudes were about the same in the two solutions. Similar results were found when the action potentials were evoked synaptically with stimulation in the stratum oriens. In the soma, spike amplitudes increased in 5-HT, with only a small decrease in the peak potential. Calcium concentration measurements, made with bis-fura-2 injected through patch electrodes, showed that the amplitude of the [Ca2+]i changes was reduced at all locations in 5-HT. The reduction of the [Ca2+]i change in the soma was confirmed in slices where cells were loaded with fura-2-AM. The reduction at the soma in 5-HT, where the spike amplitude increased, suggests that the reduction is due primarily to direct modulation of Ca2+ channels. In the dendrites, the reduction is due to a combination of this channel modulation and the lowering of the peak potential of the action potentials.

Action Potentials↗
PubMed1997

Muscarinic modulation of spike backpropagation in the apical dendrites of hippocampal CA1 pyramidal neurons.

In pyramidal neurons from the CA1 region of the rat hippocampus, Na+-dependent action potentials backpropagate over the dendrites in an activity-dependent manner. Consequently, later spikes in a train have smaller amplitudes when recorded in the apical arbors. We studied the effect of the cholinergic agonist carbachol (CCh) on this pattern of activity when spikes were evoked synaptically or antidromically in the transverse slice preparation. Concentrations as low as 1 microM were effective in reversing the modulation, making the amplitude of all spikes in a train equal and independent of the frequency of spike firing. CCh did not change the propagation of the first spike in a train. These effects of CCh were blocked by 1 microM atropine, showing that only muscarinic receptors were involved. The effects of CCh on the pattern of spike propagation were observed in the proximal and middle dendrites, but recordings in the distal dendrites (>300 micron from the soma) showed that CCh did not boost the amplitude in this region. Intracellular BAPTA (10 mM) or EGTA (10 mM) had no effect on activity-dependent backpropagation but blocked the effect of CCh. Backpropagating spikes caused increases in [Ca2+]i at all dendritic locations. In the middle and distal dendrites these increases normally peaked at the time of the first few large action potentials. In association with the enhancement of spike backpropagation, CCh increased the amplitude and duration of the train-evoked [Ca2+]i changes. These effects of CCh on dendritic spike potentials and associated [Ca2+]i changes may be important in modulating synaptic integration and plasticity in these neurons.

Action Potentials↗
PubMed1997

Spatial distribution of synaptically activated sodium concentration changes in cerebellar Purkinje neurons.

The spatial distribution of Na(+)-dependent events in guinea pig Purkinje cells was studied with a combination of high-speed imaging and simultaneous intracellular recording. Individual Purkinje cells in sagittal cerebellar slices were loaded with either fura-2 or the Na+ indicator sodium binding benzofuran isophthalate (SBFI) with sharp electrodes or patch electrodes on the soma or dendrites. [Na+]i changes were detected in response to climbing fiber and parallel fiber stimulation. These changes were located both at the anatomically expected sites of synaptic contact in the dendrites and in the somatic region. The variation in time course of these [Na+]i changes in different locations implies that Na+ enters at the synapse and diffuses rapidly to locations of lower initial [Na+]i. The synaptically activated somatic [Na+]i changes probably reflect Na+ entry through voltage-sensitive Na+ channels because they were detected only when regenerative potentials were recorded in the soma. [Na+]i changes in response to antidromically or intrasomatically evoked Na+ action potentials also were confined to the cell body. These observations are in agreement with other evidence that Na+ spikes are generated in the somatic region of the Purkinje neuron and spread passively into the dendrites. Plateau potentials, evoked by depolarizing pulses to the soma or dendrites, caused [Na+]i changes only in the soma, indicating that the noninactivating Na+ channels contributing to this potential also were concentrated in this region. The climbing fiber-activated [Na+]i changes were blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that these changes were not due to direct stimulation of the Purkinje neuron or activation of metabotropic receptors. Direct depolarization of the soma or dendrites never caused dendritic [Na+]i increases, suggesting that the climbing fiber-activated [Na+]i changes in the dendrites are due to Na+ entry through ligand-gated channels. A climbing fiber-like regenerative potential could be recorded in the soma after anode break stimulation, parallel fiber activation, or depolarizing pulses to the soma. The [Na+]i changes evoked by all of these potentials were confined to the cell body region of the Purkinje cell. [Ca2+]i changes in the dendrites evoked by the anode break potential were small relative to climbing fiber-activated changes, suggesting that a Ca2+ spike was not evoked by this response. The anode break and directly responses were blocked by tetrodotoxin. These results suggest that the somatically recorded climbing fiber response is predominantly a Na(+)-dependent event, consisting of a few fast action potentials and a slower regenerative response activating the same channels as the Na+ plateau potential.

Animals↗
PubMed1997

Activity-dependent [Ca2+]i changes in guinea pig vagal motoneurons: relationship to the slow afterhyperpolarization.

Vagal motoneurons in slices from the guinea-pig brain stem were injected with the fluorescent [Ca2+]i indicators fura-2, furaptra, or Calcium Green-1. Spike-induced fluorescence changes were measured in the soma and dendrites and simultaneously the long-lasting afterhyperpolarization was recorded with a sharp microelectrode in the soma. Na+ spikes or Ca2+ spikes increased [Ca2+]i (measured as a change in indicator fluorescence) in all locations in the soma and dendrites. Each spike in a train of action potentials caused a step increase in fluorescence of about equal amplitude when nonsaturating indicators were used. Peak changes at all locations occurred at the time of the last action potential. Transients measured with low concentrations of Calcium Green-1 or furaptra had a recovery time constant of approximately 500-1,500 ms in the cell body. The recovery time course was faster in the dendrites than in the soma. The norepinephrine-sensitive, slow afterhyperpolarization (sAHP) had a time to peak of approximately 800 ms and a recovery time constant of 2-5 s, much longer than the recovery time course of the fluorescence changes. Some of these experiments were repeated on pyramidal neurons from the CA1 region of the rat hippocampus with similar results. In both cell types, the data suggest that the time course of neither the rising phase nor the falling phase of the sAHP, nor the underlying conductance, directly reflects the time course of the [Ca2+]i change. The mechanism connecting the parameters remains unclear. One possibility is that an additional second messenger system is involved.

Action Potentials↗
PubMed1996

IPSPs modulate spike backpropagation and associated [Ca2+]i changes in the dendrites of hippocampal CA1 pyramidal neurons.

1. We studied the effects of synaptic inhibition on backpropagating Na+ spikes in the apical dendrites of CA1 pyramidal neurons in transverse slices from the rat hippocampus. Action potentials were evoked synaptically by stimulation in the stratum radiatum or antidromically by stimulation in the alveus. 2. Inhibitory postsynaptic potentials, evoked by stimulation in the stratum lacunosum moleculare, reduced the amplitude of single spikes in the distal dendrites but did not change the amplitudes in the somatic or proximal regions. Inhibition also reduced the spike-associated [Ca2+]i changes in the distal dendrites but had little effect on the changes in the proximal part of the cell. Both of these results are consistent with inhibition converting actively backpropagating spikes into passively spreading potentials at some point in the arbor. 3. In most cells, the spike amplitude reduction in the distal dendrites was blocked by bicuculline methiodide (10 microM) and inhibition was most effective when evoked in a time window < 10 ms preceding the action potential. This suggests that the amplitude reduction was due to a conductance shunt activated by gamma-aminobuturic acid-A (GABAA) receptors. Synaptically evoked GABAB responses were detected but usually did not block spike propagation. 4. Direct hyperpolarization in the distal dendrites was also effective in blocking antidromically evoked spike backpropagation but probably does not contribute when the action potentials are evoked synaptically. 5. This effect of inhibition is different from its usual function in synaptic integration because spike generation and propagation down the axon are not significantly affected. This kind of inhibition might be important in regulating transient [Ca2+]i changes in the dendrites including individual dendritic branches.

Animals↗
PubMed1995

Optical recording from cerebellar Purkinje cells using intracellularly injected voltage-sensitive dyes.

We evaluated several techniques for their ability to record membrane potential changes with voltage-sensitive dyes introduced into CNS neurons in the brain slice preparation. Using a probe designed for intracellular application, JPW1114, we found that iontophoresis or pressure pulses could not push the lipophilic dye through electrodes whose resistance was sufficiently high to produce good electrical recordings in cerebellar Purkinje neurons. However, properly selected patch electrodes could introduce the dye into the cell and still give good electrical records. Using this technique we recorded depolarizing and hyperpolarizing transients and climbing fiber responses using either a single photodiode or a fast, cooled CCD camera. While these results are promising, there are still problems due to the slow diffusion of the dye in the dendrites and a low sensitivity which requires signal averaging to acquire traces with a good signal to noise ratio.

Action Potentials↗
PubMed1995

Frequency-dependent propagation of sodium action potentials in dendrites of hippocampal CA1 pyramidal neurons.

1. The propagation of antidromically activated action potentials in CA1 pyramidal neurons was examined with intrasomatic and intradendritic electrical recording and optical measurements using the fluorescent calcium indicator Calcium Green-1. 2. In somatic recordings, trains of 40 action potentials, activated at rates up to 100 Hz, showed modest amplitude reduction. Recordings in the apical dendrites, 150 microns from the soma, showed smaller initial amplitudes and much greater decrement during trains. Higher frequencies caused a greater rate of reduction with a lower final amplitude. 3. Calcium concentration changes ([Ca2+]i), measured with the fluorescent indicator Calcium Green-1 and a fast, cooled charge coupled device (CCD) camera, were detected over the entire length of the apical dendrites in response to single antidromic action potentials, although the changes in distal dendrites were smaller. These changes were rapid, decaying to half-amplitude in < 150 ms in distal dendritic locations. 4. Trains of action potentials at all frequencies up to 100 Hz caused transient [Ca2+]i, increases for each spike at 150 microns from the soma. In the last 100 microns of the distal branches, only the first few spikes caused a [Ca2+]i increase for frequencies above approximately 40 Hz. These patterns could be matched with a simple model of calcium influx and removal, where later spikes in a train brought in less calcium than earlier spikes. 5. These results show that the action-potential amplitude and the spatial extent of their propagation in the dendrites is frequency dependent.

Action Potentials↗
PubMed1995

IPSPs strongly inhibit climbing fiber-activated [Ca2+]i increases in the dendrites of cerebellar Purkinje neurons.

The interaction between the excitatory climbing fiber (CF) response and stellate cell inhibition was studied in guinea pig Purkinje cells in sagittal slices from the cerebellar vermis. Sharp microelectrode recordings from the soma or dendrites were combined with high-speed fluorescence imaging of intracellularly injected fura-2. In this way both the electrical responses and the associated [Ca2+]i changes could be monitored at the same time. Usually simultaneously activated inhibition caused almost no change to the somatically recorded CF response. However, the inhibition caused a strong reduction in the CF-associated [Ca2+]i increase which normally was widespread in the dendrites. This effect was graded; stronger inhibition caused a larger and more widespread reduction in the [Ca2+]i change that was greatest in the more distal dendrites. Sometimes the reduction was over 90% in the distal dendrites and occasionally it was localized to only a single dendritic branch. Both the inhibitory postsynaptic potential (IPSP) and the associated reduction in the CF-induced [Ca2+]i change were blocked by bicuculline, a GABAA receptor antagonist. Dendritic recordings showed that each CF response evoked a 2-3 msec wide action potential. The amplitude of this action potential was reduced in a graded manner by the IPSP in parallel with the reduction in the [Ca2+]i change. Varying the time between the activation of the IPSP and the CF response showed that both the reduction in the [Ca2+]i change and the action potential amplitude occurred in a narrow time window of about 8-10 msec, about the rise time of the IPSP. Together these results indicate that the CF response activates a fast dendritic Ca2+ spike that causes most of the [Ca2+]i increase, both of which can be blocked by an inhibitory shunting conductance. This interaction provides a means whereby Ca(2+)-dependent dendritic mechanisms can be modulated without affecting the immediate output of the Purkinje cell.

Action Potentials↗
PubMed1994

A model for dendritic Ca2+ accumulation in hippocampal pyramidal neurons based on fluorescence imaging measurements.

1. High-speed fluorescence imaging was used to measure intracellular Ca2+ concentration ([Ca2+]i) changes in hippocampal neurons injected with the Ca(2+)-sensitive indicator fura-2 during intrasomatic and synaptic stimulation. The results of these experiments were used to construct a biophysical model of [Ca2+]i dynamics in hippocampal neurons. 2. A compartmental model of a pyramidal neuron was constructed incorporating published passive membrane properties of these cells, three types of voltage-gated Ca2+ channels characterized from adult hippocampal neurons, voltage-gated Na+ and K+ currents, and mechanisms for Ca2+ buffering and extrusion. 3. In hippocampal pyramidal neurons imaging of Na+ entry during electrical activity suggests that Na+ channels, at least in sufficient density to sustain action potentials, are localized in the soma and the proximal part of the apical dendritic tree. The model, which incorporates this distribution, demonstrates that action potentials attenuate steeply in passive distal dendritic compartments or distal dendritic compartments containing Ca2+ and K+ channels. This attenuation was affected by intracellular resistivity but not membrane resistivity. 4. Consistent with fluorescence imaging experiments, a non-uniform distribution of Ca2+ accumulation was generated by Ca2+ entry through voltage-gated Ca2+ channels opened by decrementally propagating Na+ action potentials. Consequently, the largest increases in [C2+]i were produced in the proximal dendrites. Distal voltage-gated Ca2+ currents were activated by broad, almost isopotential action potentials produced by reducing the overall density of K+ channels. 5. Simulations of subthreshold synaptic stimulation produced dendritic Ca2+ entry by the activation of voltage-gated Ca2+ channels. In the model these Ca2+ signals were localized near the site of synaptic input because of the attenuation of synaptic potentials with distance from the site of origin and the steep voltage-dependence of Ca2+ channel activation. 6. These simulations support the hypotheses generated from experimental evidence regarding the differential distribution of voltage-gated Ca2+ and Na+ channels in hippocampal neurons and the resulting voltage-gated Ca2+ accumulation from action and synaptic potentials.

Animals↗
PubMed1993

Spatial distribution of Ca2+ influx in turtle Purkinje cell dendrites in vitro: role of a transient outward current.

1. Intracellular recordings were made from Purkinje cells in a slice preparation of the turtle cerebellum. Simultaneously, changes in [Ca2+]i in all regions of the cell were detected with high-speed fluorescence imaging of injected fura-2. Cells were stimulated either intrasomatically or synaptically. In addition, the cells were polarized locally with an external electrical field aligned parallel to the soma-dendritic axis. 2. The soma, smooth dendrites, and spiny dendrites displayed voltage-dependent changes in [Ca2+]i. Changes in the somatic region were correlated with Na+ spike firing and local depolarization. Small [Ca2+]i changes in the spiny dendrites were correlated with graded potentials and larger changes with Ca2+ action potentials. Individual Ca2+ spike transients sometimes occurred separately in different dendritic regions demonstrating localized firing. 3. The amplitude and spatial extent of spike-related [Ca2+]i transients were increased with intrasomatic depolarizing prestimulus membrane potentials and reduced by hyperpolarizing prestimulus potentials. This dependence and the latency to Ca2+ spike activation were strongly reduced by 4-aminopyridine (4-AP). These results suggest that a transient A-like current regulates the generation of Ca2+ spikes and the localization of Ca2+ influx in turtle Purkinje cell dendrites. 4. Both electric field depolarization and intrasomatic depolarization affected the generation of Ca2+ spikes and [Ca2+]i signals in a similar manner. Strong field stimulation could evoke focal depolarization at the tips of the spiny dendrites and cause local Ca2+ spike generation near the pial surface. When both stimuli were used, their effects were additive. 5. Climbing fiber (CF) or parallel fiber (PF) stimulation were associated with the generation of dendritic Ca2+ transients. In some experiments the PF-induced Ca2+ transients were confined to a small part of the spiny dendrites. The spatial distribution and the amplitude of these transients were influenced by somatic depolarization or field stimulation in a manner similar to their effect on directly evoked Ca2+ spikes and consistent with the involvement of a transient outward current in the control of the synaptically induced Ca2+ influx. 6. These results suggest that the intrinsic potassium conductances dynamically modulate spatial integration and influence the compartmentalization of Ca2+ spikes and [Ca2+]i changes in the dendrites.

Animals↗
PubMed1993

Dendritic excitability in CNS neurons: insights from dynamic calcium and sodium imaging in single cells.

We measured changes in [Ca2+]i and [Na+]i in the dendrites of cerebellar Purkinje cells and hippocampal pyramidal cells using high speed imaging of the fluorescence changes of intracellularly injected fura-2 and SBFI. These transients were detected in synchrony with intracellular recordings of membrane potential. In this way rapid calcium or sodium transients could be associated with specific electrical events. Using this technique we could determine the spatial distribution and source of transients evoked either intrasomatically or synaptically and could relate them to electrically recorded sodium and calcium spikes.

Animals↗
PubMed1993

Dynamics of intracellular free calcium concentration in the presynaptic arbors of individual barnacle photoreceptors.

At photoreceptor synapses, transmitter release is continuous and graded. At this type of synapse, the control of presynaptic [Ca2+]i and calcium's role in releasing transmitter might be different than at terminals invaded by all-or-none action potentials. To examine this possibility, we measured the spatial and temporal changes of [Ca2+]i in response to depolarization of individual photoreceptor terminals of the barnacle Balanus nubilus, which had been injected with the Ca2+ indicator Fura-2. Depolarizing pulses produced voltage-dependent Ca2+ entry that was confined to the tips of the arbor where the release sites are located. At increasing distances from the tips, the rate of [Ca2+]i increase was slower and the peak [Ca2+]i occurred later, suggesting that Ca2+ entered the tips and diffused back into the larger processes of the arbor. Consistent with this result, a stable gradient of [Ca2+]i was observed at maintained depolarizations, with the highest values at the tips of the arbor. Removal of external Na+ did not affect the time course of Ca2+ decline in the terminal, indicating that Na+/Ca2+ exchange was not the primary mechanism for restoring [Ca2+]i to basal levels. Computer simulations, assuming only Ca2+ entry at the arbor's tips and diffusion of Ca2+ away from the entry site, qualitatively reproduced these observations. The threshold for Ca2+ entry was near -60 mV, and entry was maintained during prolonged depolarizations, in agreement with previous experiments showing that Ca2+ channels in the terminal region do not inactivate. The time course of the measured [Ca2+]i change in the terminal paralleled voltage changes due to a Ca(2+)-activated K+ conductance, which senses [Ca2+]i just under the membrane. This parallelism is expected since the release sites are located on processes of small-enough diameter to permit radial equilibration of [Ca2+]i within the time course of physiological voltage changes. Therefore, the optical measurements reflect the mean level of [Ca2+]i under the membrane. Whether this mean concentration is also the value at the sites that trigger exocytosis will depend on how close the Ca2+ channels are to these sites.

Action Potentials↗
PubMed1992

The spread of Na+ spikes determines the pattern of dendritic Ca2+ entry into hippocampal neurons.

The dendrites of many types of neurons contain voltage-dependent Na+ and Ca2+ conductances that generate action potentials (see ref. 1 for review). The function of these spikes is not well understood, but the Ca2+ entry stimulated by spikes probably affects Ca(2+)-dependent processes in dendrites. These include synaptic plasticity, cytotoxicity and exocytosis. Several lines of evidence suggest that dendritic spikes occur within subregions of the dendrites. To study the mechanism that govern the spread of spikes in the dendrites of hippocampal pyramidal cells, we imaged Ca2+ entry with Fura-2 (ref. 9) and Na+ entry with a newly developed Na(+)-sensitive dye. Our results indicate that Ca2+ entry into dendrites is triggered by Na+ spikes that actively invade the dendrites. The restricted spatial distribution of Ca2+ entry seems to depend on the spread of Na+ spikes in the dendrites, rather than on a limited distribution of Ca2+ channels. In addition, we have observed an activity-dependent process that modulates the invasion of spikes into the dendrites and progressively restricts Ca2+ entry to more proximal dendritic regions.

6-Cyano-7-nitroquinoxaline-2,3-dione↗