Fractional contribution of calcium to the cation current through glutamate receptor channels

Dendritic glutamate receptor channels in rat hippocampal CA3 and CA1 pyramidal neurons

1. Properties of dendritic glutamate receptor (GluR) channels were investigated using fast application of glutamate to outside-out membrane patches isolated from the apical dendrites of CA3 and CA1 pyramidal neurons in rat hippocampal slices. CA3 patches were formed (15-76 microns from the soma) in the region of mossy fibre (MF) synapses, and CA1 patches (25-174 microns from the soma) in the region of Schaffer collateral (SC) innervation. 2. Dual-component responses consisting of a rapidly rising and decaying component followed by a second, substantially slower, component were elicited by 1 ms pulses of 1 mM glutamate in the presence of 10 microM glycine and absence of external Mg2+. The fast component was selectively blocked by 2-5 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the slow component by 30 microM D-2-amino-5-phosphonopentanoic acid (D-AP5), suggesting that the fast and slow components were mediated by the GluR channels of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and NMDA type, respectively. The peak amplitude ratio of the NMDA to AMPA receptor-mediated components varied between 0.03 and 0.62 in patches from both CA3 and CA1 dendrites. Patches lacking either component were rarely observed. 3. The peak current-voltage (I-V) relationship of the fast component was almost linear, whereas the I-V relationship of the slow component showed a region of negative slope in the presence of 1 mM external Mg2+. The reversal potential for both components was close to 0 mV. 4. Kainate-preferring GluR channels did not contribute appreciably to the response to glutamate. The responses to 100 ms pulses of 1 mM glutamate were mimicked by application of 1 mM AMPA, whereas 1 mM kainate produced much smaller, weakly desensitizing currents. This suggests that the fast component is primarily mediated by the action of glutamate on AMPA-preferring receptors. 5. The mean elementary conductance of AMPA receptor channels was about 10 pS, as estimated by non-stationary fluctuation analysis. The permeability of these channels to Ca2+ was low (approximately 5% of the permeability to Cs+). 6. The elementary conductance of NMDA receptor channels was larger, with a main conductance state of about 45 pS. These channels were 3.6 times more permeable to Ca2+ than to Cs+.(ABSTRACT TRUNCATED AT 400 WORDS)

Subthreshold synaptic Ca2+ signalling in fine dendrites and spines of cerebellar Purkinje neurons

The conventional view of synaptic integration is that it results from the simple summation of electrical signals produced by each active synapse innervating a given neuron. However, because synaptic action can go beyond the production of postsynaptic electrical signals, to include intracellular biochemical events such as the generation of second messengers, it is possible that synaptic integration could occur at another level. We have considered this possibility by examining changes in the dendritic concentration of the second messenger, calcium, resulting from subthreshold excitatory synaptic activity in cerebellar Purkinje neurons. We report here clear evidence that such non-electrical synaptic integration occurs and that it takes place in restricted dendritic compartments consisting of spines and adjacent fine dendrites.

Confocal imaging of dendritic Ca2+ transients in hippocampal brain slices during simultaneous current- and voltage-clamp recording

Changes in the intracellular Ca2+ concentration ([Ca2+]i) within CA1 hippocampal pyramidal neurons were imaged using confocal laser scanning microscopy in conjunction with Ca(2+)-sensitive fluorescent indicators. The imaging was performed in thick hippocampal brain slices while simultaneously measuring or controlling electrical activity with sharp microelectrodes or whole-cell patch-clamp electrodes. The combination of imaging and electrophysiology was essential for interpreting the changes in [Ca2+]i. We compared the increases in [Ca2+]i produced by either of two methods--direct depolarization of the cell via the somatic electrode or high-frequency stimulations of synaptic inputs. The increases in [Ca2+]i in the soma and proximal dendrites caused by both methods were of comparable magnitude and they always decayed within seconds in healthy cells. However, the spatial patterns of distal Ca2+ increases were different. Separate sets of synaptic inputs to the same cell resulted in different spatial patterns of [Ca2+]i transients. We isolated and observed what appeared to be a voltage-independent component of the synaptically mediated [Ca2+]i transients. This work demonstrates that the combination of neurophysiology and simultaneous confocal microscopy is well suited for visualizing and analyzing [Ca2+]i changes within highly localized regions of neurons in thick brain slices. The approach should allow further analysis of the relative contribution of voltage- and agonist-dependent influences on [Ca2+]i within neurons throughout the CNS and it raises the possibility of routinely relating subcellular [Ca2+]i changes to structural and functional modifications.

Differential role of two Ca(2+)-permeable non-NMDA glutamate channels in rat retinal ganglion cells: kainate-induced cytoplasmic and nuclear Ca2+ signals

1. The permeability of non-N-methyl-D-aspartate (non-NMDA) glutamate channels to divalent cations and specifically the entry of Ca2+ and subsequent elevations in cytoplasmic and nuclear Ca2+ signals were investigated in cultured neonatal rat retinal ganglion cells using the whole cell patch-clamp technique and Ca2+ imaging with confocal microscopy. In addition, divalent-permeable non-NMDA receptor channels were studied in retinal slices using a Co2+ staining technique. 2. Using Ca2+ (2.5 mM) as the only permeable cation in the external solution, stimulation with 100 microM kainate produced nondesensitizing, nonselective cation currents with either low or high Ca2+ permeability. Both currents were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Neurons with the low divalent-permeable currents (type 1) had reversal potentials of -41.5 +/- 4.4 mV (mean +/- SD), and neurons with the high divalent-permeable currents (type 2) had reversal potentials of -22.6 +/- 5.5 mV. The permeability ratio PCa/PCs was 3.3 for the type 1 currents and 8.5 for the type 2 currents, indicating a 2.5-fold greater permeability to Ca2+ for the type 2 non-NMDA glutamate channels. 3. Both types of non-NMDA glutamate channels showed relatively little selectivity between Ca2+ and Co2+. The type 1 neurons had a slightly higher permeability to Co2+ than to Ca2+, whereas the type 2 neurons were equally permeable to both divalent cations. The type 2 neurons had a much higher permeability for both divalent cations compared with the type 1 neurons. 4. Staining for Co2+ uptake through kainate-stimulated non-NMDA glutamate channels in retinal slices provided additional evidence for the presence of the two ganglion cell populations. Activation of the neurons by kainate in conditions isolating the non-NMDA glutamate channel caused differential uptake of Co2+. In contrast, depolarization in the presence of the non-NMDA antagonist CNQX failed to cause Co2+ influx. 5. Imaging experiments using confocal microscopy showed that kainate stimulation induced an increase in intracellular Ca2+ in both types of retinal ganglion cells, but only the type 2 neurons showed a substantial increase in cytoplasmic and nuclear Ca2+ signals. Kainate-induced Ca2+ signals in the type 2 neurons were almost nine times greater than those of the type 1 neurons. 6. When intracellular Ca2+ stores were depleted by brief treatment with thapsigargin, kainate-induced Ca2+ signals in the type 1 neurons were unchanged. However, in the type 2 neurons kainate no longer induced large Ca2+ signals in the cytoplasm and nucleus, despite normal influx of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium homeostasis in the presence of fura-2 in neurons dissociated from rat nucleus basalis: theoretical and experimental analysis of chelating action of fura-2

Intracellular calcium ions (Ca2+) play important roles in cell functions. Measurements of intracellular calcium ion concentration ([Ca2+]i) are often made with the fura-2 fluorescence recording technique in various preparations including neurons. Fura-2 has, however, a Ca(2+)-chelating action which complicates the interpretation of experimental results. In this report the chelating action of intracellular fura-2 was studied by means of computer simulations. The chelating action of an endogenous Ca(2+)-binding protein, calmodulin, was also estimated. Furthermore, whole-cell patch-clamp recordings of calcium currents (ICa) and fura-2 microfluorimetric recordings of [Ca2+]i were simultaneously made from neurons which were acutely dissociated from the rat nucleus basalis. Since Ca2+ influx can be initiated and terminated by using the voltage-clamp technique, the relationship between Ca2+ influx and rapid [Ca2+]i increase was examined. The present theoretical evaluations and experimental results disclosed the relationship between fura-2 and endogenous Ca(2+)-binding proteins; fura-2 at low concentration (10 microM) did not substantially affect the endogenous Ca2+ buffering mechanisms, but at high concentration (200 microM) effectively buffered cytosolic Ca2+ instead of endogenous Ca2+ buffers. Calcium homeostasis in neurons is furthermore discussed.

Rectification properties and Ca2+ permeability of glutamate receptor channels in hippocampal cells

Excitatory amino acids exert a depolarizing action on central nervous system cells through an increase in cationic conductances. Non-NMDA receptors have been considered to be selectively permeable to Na+ and K+, while Ca2+ influx has been thought to occur through the NMDA receptor subtype. Recently, however, the expression of cloned non-NMDA receptor subunits has shown that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are permeable to Ca2+ whenever the receptor lacks a particular subunit (edited GluR-B). The behaviour of recombinant glutamate receptor channels predicts that Ca2+ would only permeate through receptors that show strong inward rectification and vice versa, i.e. AMPA receptors with linear current-voltage relationships would be impermeable to Ca2+. Using the whole-cell configuration of the patch-clamp technique, we have studied the Ca2+ permeability and the rectifying properties of AMPA receptors, when activated by kainate, in hippocampal neurons kept in culture or acutely dissociated from differentiated hippocampus. Cells were classified according to whether they showed outward rectifying (type I), inward rectifying (type II) or almost linear (type III) current-voltage relationships for kainate-activated responses. AMPA receptors of type I cells (52.2%) were mostly Ca(2+)-impermeable (PCa/PCs = 0.1), while type II cells (6.5%) expressed Ca(2+)-permeable receptors (PCa/PCs = 0.9). Type III cells (41.3%) showed responses with low but not negligible Ca2+ permeability (PCa/PCs = 0.18). The degree of Ca2+ permeability and inward rectification were well correlated in cultured cells, i.e. more inward rectification corresponded to higher Ca2+ permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural basis of ion channel permeation and selectivity

There has been rapid progress in understanding the structural basis of ion selectivity and permeation in both ligand- and voltage-gated channels. Recognition of similarities in overall architecture within a channel class has led to an increasing focus on the specific molecular determinants that endow a channel with its own distinctive character. It has been possible in some cases to identify individual amino acids essential for ion selectivity.

Dendritic calcium dynamics

Further characterization of the biochemical components that contribute to calcium handling, together with advances in optical imaging of ion concentration, are providing quantitative information on the dynamics of calcium in the dendrites of neurons in tissue culture, brain slices and in vivo. It has recently been demonstrated that strong spatial gradients and transient calcium elevations result from the geometry and membrane properties of dendrites. These studies are adding to our understanding of calcium's role in synaptic plasticity and in shaping the electrophysiological properties of neurons.

Glutamate receptor update

The past year has seen significant advances in matching the actions of recombinant glutamate receptors with the actions of native receptors, and in mapping their distribution and regulation. The discovery of a novel RNA editing mechanism for AMPA receptors and a revised view of the transmembrane topology of the NMDA receptor subunit, NR1, are particularly noteworthy. Seven metabotropic glutamate receptor subtypes have been identified with several interesting expression patterns and transduction mechanisms; results from work on these subtypes has led to a provocative model of the ligand-binding site. Functional studies of metabotropic receptors have been enhanced by the development of the first subtype-specific antagonist.

Calcium transients in dendrites of neocortical neurons evoked by single subthreshold excitatory postsynaptic potentials via low-voltage-activated calcium channels

Simultaneous recordings of membrane voltage and concentration of intracellular Ca2+ ([Ca2+]i) were made in apical dendrites of layer 5 pyramidal cells of rat neocortex after filling dendrites with the fluorescent Ca2+ indicator Calcium Green-1. Subthreshold excitatory postsynaptic potentials (EP-SPs), mediated by the activation of glutamate receptor channels, caused a brief increase in dendritic [Ca2+]i. This rise in dendritic [Ca2+]i was mediated by the opening of low-voltage-activated Ca2+ channels in the dendritic membrane. The results provide direct evidence that dendrites do not function as passive cables even at low-frequency synaptic activity; rather, a single subthreshold EPSP changes the dendritic membrane conductance by opening Ca2+ channels and generating a [Ca2+]i transient that may propagate towards the soma. The activation of these Ca2+ channels at a low-voltage threshold is likely to influence the way in which dendritic EPSPs contribute to the electrical activity of the neuron.

Regulation of NMDA channel function by endogenous Ca(2+)-dependent phosphatase

Protein kinases modulate the activity of several ligand-gated ion channels, including the NMDA (N-methyl-D-aspartate) subtype of glutamate receptor. Although phosphorylation and dephosphorylation of glutamate receptors may participate in several lasting physiological and pathological alterations of neuronal excitability, the physiological control of this cycle for NMDA channels has not yet been established. Using cell-attached recordings in acutely dissociated adult rat dentate gyrus granule cells, we now demonstrate that inhibitors of an endogenous serine/threonine phosphatase prolong the duration of single NMDA channel openings, bursts, clusters and superclusters. Okadaic acid, a non-selective phosphatase inhibitor, prolongs channel openings only at a concentration that inhibits the Ca2+/calmodulin-dependent phosphatase 2B (calcineurin), and is ineffective when Ca2+ entry through NMDA channels is prevented. Furthermore, FK506, an inhibitor of calcineurin, mimics the effects of okadaic acid. Thus in adult neurons, calcineurin, activated by calcium entry through native NMDA channels, shortens the duration of channel openings. Simulated synaptic currents were enhanced after phosphatase inhibition, which is consistent with the importance of phosphorylation of the NMDA-receptor complex in the short- and long-term control of neuronal excitability.

Hebbian induction of long-term potentiation of Aplysia sensorimotor synapses: partial requirement for activation of an NMDA-related receptor

Long-term potentiation (LTP) of Aplysia sensorimotor synapses in cell culture can be induced by pairing sensory neuron activity with depolarization of the motorneuron. This pairing-induced LTP is prevented by perfusion with D,L-2-amino-5-phosphononovalerate (APV), a selective antagonist for the N-methyl-D-asparate (NMDA) subclass of glutamate receptors. Repeated pairing of presynaptic activity with postsynaptic depolarization induces LTP comprising both APV-sensitive and APV-insensitive components. Infusing BAPTA, a selective Ca2+ chelator, into the postsynaptic motoneuron completely blocks pairing-induced LTP. These results demonstrate that Aplysia sensorimotor synapses are capable of hebbian LTP-similar to that exhibited by synapses in the mammalian hippocampus - and suggest a role for this type of synaptic plasticity in classical conditioning of the defensive withdrawal reflex of Aplysia.

Confocal laser scanning microscopy reveals voltage-gated calcium signals within hippocampal dendritic spines

The induction of long-term potentiation (LTP) is generally assumed to be triggered by Ca2+ entry into dendritic spines via NMDA receptor-gated channels. A previous computational model proposed that spines serve several functions in this process. First, they compartmentalize and amplify increase in [Ca2+]i. Second, they augment the nonlinear relationship between synaptic strength and the probability or magnitude of LTP induction. Third, they isolate the metabolic machinery responsible for LTP induction from increases in [Ca2+]i produced by voltage-gated Ca2+ channels in the dendritic shaft. Here we examine this last prediction of the model using methods that combine confocal microscopy with simultaneous neurophysiological recordings in hippocampal brain slices. Either of two Ca(2+)-sensitive dyes were injected into CA1 pyramidal neurons. Direct depolarization of the neurons via the somatic electrode produced clear increases in Ca2+ signals within the dendritic spines, a result that was not predicted by the previous spine model. Our new spine model suggests that some of this signal could theoretically result from Ca(2+)-bound dye diffusing from the dendritic shaft into the spine. Dye diffusion alone cannot, however, explain the numerous cases in which the Ca2+ signal in the spine was considerably larger than that in the adjacent dendritic shaft. The latter observations raise the possibility of voltage-gated Ca2+ entry directly into the spine or else perhaps via Ca(2+)-dependent Ca2+ release. The new spine model accommodates these observations as well as several other recent experimental results.

Calcium permeability of nicotinic acetylcholine receptor channels in bovine adrenal chromaffin cells

The fractional contribution of Ca to current flow through neuronal-type nicotinic acetylcholine receptor channels was determined by quantitative fluorescence microfluorimetry using fura-2. The method, which has been applied already to several types of cells and channels is described in detail here. At -70 mV and 2 mM external Ca concentration it was found that Ca contributes 2.5% to the net current. The fractional contribution was found to be voltage dependent, increasing at negative potentials e-fold for a 110 mV potential difference. Total non-specific cation current was found to have a bell-shaped dependence on external Ca concentration peaking at 2 mM.

Glutamate- and AMPA-mediated calcium influx through glutamate receptor channels in medial septal neurons

The Ca(2+)-fraction of the ion current flowing through glutamate receptor channels activated either by glutamate or by AMPA was determined in forebrain neurons of the rat medial septum. By combining whole-cell patch-clamp and fura-2 fluorometric measurements we found that, at negative membrane potentials and at an extracellular free Ca(2+)-concentration of 1.6 mM, the Ca(2+)-fraction of the current activated by glutamate is 5.7%. A pharmacological analysis of responses produced by ionophoretically-released glutamate demonstrated a large contribution of NMDA-receptors but a small contribution of AMPA/kainate receptors to these responses. Interestingly, also AMPA-mediated currents were associated with significant changes in Ca(2+)-sensitive fluorescence. The fractional Ca2+ current of AMPA-induced responses was 1.2 +/- 0.4% (n = 5).