Analysis of rapid calcium signals in synaptosomes

A constitutive, transient receptor potential-like Ca2+ influx pathway in presynaptic nerve endings independent of voltage-gated Ca2+ channels and Na+/Ca2+ exchange

Calcium levels in the presynaptic nerve terminal are altered by several pathways, including voltage-gated Ca(2+) channels, the Na(+)/Ca(2+) exchanger, Ca(2+)-ATPase, and the mitochondria. The influx pathway for homeostatic control of [Ca(2+)](i) in the nerve terminal has been unclear. One approach to detecting the pathway that maintains internal Ca(2+) is to test for activation of Ca(2+) influx following Ca(2+) depletion. Here, we demonstrate that a constitutive influx pathway for Ca(2+) exists in presynaptic terminals to maintain internal Ca(2+) independent of voltage-gated Ca(2+) channels and Na(+)/Ca(2+) exchange, as measured in intact isolated nerve endings from mouse cortex and in intact varicosities in a neuronal cell line using fluorescence spectroscopy and confocal imaging. The Mg(2+) and lanthanide sensitivity of the influx pathway, in addition to its pharmacological and short hairpin RNA sensitivity, and the results of immunostaining for transient receptor potential (TRP) channels indicate the involvement of TRPC channels, possibly TRPC5 and TRPC1. This constitutive Ca(2+) influx pathway likely serves to maintain synaptic function under widely varying levels of synaptic activity.

Sub-second cellular dynamics: time-resolved electron microscopy and functional correlation

Subcellular processes, from molecular events to organellar responses and cell movement, cover a broad scale in time and space. Clearly the extremes, such as ion channel activation are accessible only by electrophysiology, whereas numerous routine methods exist for relatively slow processes. However, many other processes, from a millisecond time scale on, can be "caught" only by methods providing appropriate time resolution. Fast freezing (cryofixation) is the method of choice in that case. In combination with follow-up methodologies appropriate for electron microscopic (EM) analysis, with all its variations, such technologies can also provide high spatial resolution. Such analyses may include, for example, freeze-fracturing for analyzing restructuring of membrane components, scanning EM and other standard EM techniques, as well as analytical EM analyses. The latter encompass energy-dispersive x-ray microanalysis and electron spectroscopic imaging, all applicable, for instance, to the second messenger, calcium. Most importantly, when conducted in parallel, such analyses can provide a structural background to the functional analyses, such as cyclic nucleotide formation or protein de- or rephosphorylation during cell stimulation. In sum, we discuss many examples of how it is practically possible to achieve strict function-structure correlations in the sub-second time range. We complement this review by discussing alternative methods currently available to analyze fast cellular phenomena occurring in the sub-second time range.

Age-related changes in the subtypes of voltage-dependent calcium channels in rat brain cortical synapses

Age-related changes in the relative contribution of voltage-dependent calcium channel (VDCC) subtypes to depolarization-induced Ca(2+) influx and in the density of VDCC subtypes in cortical synapses were investigated using synaptosomes and their membrane preparations from brain cortices of Wistar rats. The relative contribution of VDCC subtypes to Ca(2+) influx was determined by measuring the inhibition of depolarization-induced Ca(2+) influx with four VDCC subtype-specific peptide blockers. In adult rat synaptosomes, L-, N-, P- and Q-type channels accounted for 24, 32, 27 and 12% of the total Ca(2+) influx, respectively. Brain aging significantly reduced the relative contributions of N- and P-type channels and increased the contribution of the channels resistant to the four blockers used. The densities of VDCC subtypes, determined by binding experiments using radiolabeled PN200 -110, omega-conotoxin GVIA and omega-conotoxin MVIIC, were found to be significantly decreased in aged synaptic plasma membranes. On the contrary, the dissociation constants of the blockers were not changed except for PN200-110-sensitive L-type channels. These results suggest that aging alters the relative contributions of each VDCC subtype to depolarization-induced Ca(2+) influx and decreases the number of VDCCs in rat brain cortical synapses. These changes in VDCCs may lead to age-related hypofunction of synaptic neurotransmission in brain cortices.

Inhibition of neuronal Ca(2+) influx by gabapentin and subsequent reduction of neurotransmitter release from rat neocortical slices

Cytosolic calcium ion concentrations ([Ca(2+)](i)) were measured in rat neocortical synaptosomes using fura-2, and depolarization of synaptosomal membranes was induced by K(+) (30 mM). The release of the endogenous excitatory amino acids glutamate and aspartate was evoked by K(+) (50 mM) and determined by HPLC. The release of [(3)H]-noradrenaline from rat neocortical synaptosomes or slices was evoked by K(+) (15 and 25 mM) and measured by liquid scintillation counting. Gabapentin produced a concentration-dependent inhibition of the K(+)-induced [Ca(2+)](i) increase in synaptosomes (IC(50)=14 microM; maximal inhibition by 36%). The inhibitory effect of gabapentin was abolished in the presence of the P/Q-type Ca(2+) channel blocker omega-agatoxin IVA, but not by the N-type Ca(2+) channel antagonist omega-conotoxin GVIA. Gabapentin (100 microM) decreased the K(+)-evoked release of endogenous aspartate and glutamate in neocortical slices by 16 and 18%, respectively. Gabapentin reduced the K(+)-evoked [(3)H]-noradrenaline release in neocortical slices (IC(50)=48 microM; maximal inhibition of 46%) but not from synaptosomes. In the presence of the AMPA receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2, 3-dioxo-6-nitro-1,2,3,4-tetrahydro[f]quinoxaline-7-sulphonamide (NBQX), gabapentin did not reduce [(3)H]-noradrenaline release. Gabapentin did, however, cause inhibition in the presence of the NMDA receptor antagonist DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849). Gabapentin is concluded to reduce the depolarization-induced [Ca(2+)](i) increase in excitatory amino acid nerve terminals by inhibiting P/Q-type Ca(2+) channels; this decreased Ca(2+) influx subsequently attenuates K(+)-evoked excitatory amino acid release. The latter effect leads to a reduced activation of AMPA receptors which contribute to K(+)-evoked noradrenaline release from noradrenergic varicosities, resulting in an indirect inhibition of noradrenaline release.

Biphasic modulation of intracellular Ca2+ concentration by interleukin-1beta in cortical synaptosomes: involvement of a pertussis toxin-sensitive G-protein and mitogen-activated protein kinase

The modulation of intracellular Ca2+ concentration ([Ca2+]i) by the pro-inflammatory cytokine interleukin-1beta (IL-1beta) was assessed in synaptosomes loaded with the Ca2+-sensitive dye, Fura-2AM. IL-1beta was found to exert a biphasic effect on the KCl-induced rise in [Ca2+]i, extending an inhibitory effect at lower (3.5 ng/ml) concentrations, and a stimulatory effect at high (100 ng/ml) concentrations. The inhibitory action of IL-1beta on [Ca2+]i was sensitive to pertussis toxin (PTX; 2 microg/ml), indicating a role for a PTX-sensitive G-protein, but was unaffected by the p42 mitogen-activated protein kinase kinase (MAP kinase kinase) inhibitor, PD 098059 (2 microM). In contrast, the stimulatory action of higher concentrations of IL-1beta on [Ca2+]i was blocked by PD 098059 and unaffected by PTX. We conclude that the biphasic actions of IL-1beta on the KCl-induced rise in [Ca2+]i are mediated through activation of alternative second messenger pathways.

Activity-dependent changes in voltage-dependent calcium currents and transmitter release

Voltage-dependent Ca2+ channels are important in the regulation of neuronal structure and function, and as a result, they have received considerable attention. Recent studies have begun to characterize the diversity of their properties and the relationship of this diversity to their various cellular functions. In particular, Ca2+ channels play a prominent role in depolarization-secretion coupling, where the release of neurotransmitter is very sensitive to changes in voltage-dependent Ca2+ currents. An important feature of Ca2+ channels is their regulation by electrical activity. Depolarization can selectively modulate the properties of Ca2+ channel types, thus shaping the response of the neuron to future electrical activity. In this article, we examine the diversity of Ca2+ channels found in vertebrate and invertebrate neurons, and their short- and long-term regulation by membrane potential and Ca2+ influx. Additionally, we consider the extent to which this activity-dependent regulation of Ca2+ currents contributes to the development and plasticity of transmitter releasing properties. In the studies of long-term regulation, we focus on crustacean motoneurons where activity levels, Ca2+ channel properties, and transmitter releasing properties can be followed in identified neurons.

Voltage-sensitive Ca2+ channels in rat striatal synaptosomes: role on the [Ca2+]i responses to membrane depolarization

The fluorescent Ca2+ indicator Indo-1 was used to study the effect of depolarization evoked by KCl or 4-aminopyridine (4-AP) on the intracellular free calcium concentration responses (delta[Ca2+]i) in rat striatal synaptosomes. Depolarization of the synaptosomes with [KCl] > 7.5 mM induced a rapid increase of the [Ca2+]i followed by a decay towards a plateau. The size of the [Ca2+]i response varied sigmoidally with the synaptosomal membrane potential, with a transition potential of -27.3 mV. Depolarization with 4-AP evoked a dose-dependent sustained increase of the [Ca2+]i. Nitrendipine, omega-Conotoxin GVIA (omega-CgTx) and omega-Agatoxin IVA (omega-Aga IVA) were used to evaluate the relative role of L-, N-, P- and possibly Q-type voltage-sensitive Ca2+ channels (VSCCs) on the [Ca2+]i changes evoked by each of the two depolarizing agents. Nitrendipine caused only about 10% inhibition of the effect of either agent on the [Ca2+]i, suggesting that the L-type VSCCs have a modest contribution. The omega-CgTx decreased the response to KCl and 4-AP by 15 and 30%, respectively, but the latter effect may be partially due to a non-specific effect on Na+ channels. The omega-Aga IVA reduced the response to 4-AP by 26.5%, and this effect was additive to that of omega-CgTx, further suggesting that the striatal nerve terminals possess P- and/or Q-type, in addition to N-type Ca2+ channels. Neomycin (0.35 mM), tentatively used as an antagonist of the P-type channels, had a potent effect, decreasing the response to K(+)-depolarization and to 4-AP by, respectively, 32.5 and 48.5%. It is suggested that at the concentration used the antibiotic also partially blocks VSCCs which do not belong to the L-, N-, P- or Q-type VSCCs. We conclude that striatal nerve endings are equipped with at least four to five pharmacologically distinct classes of VSCCs, which are sensitive to well known antagonists of the L-, N-, P-, and Q-type VSCCs.

Involvement of class A calcium channels in the KCl induced Ca2+ influx in hippocampal synaptosomes

Using Ca2+ channel toxins, we determined the types of voltage-sensitive calcium channels activated by two levels of KCl depolarization in hippocampal synaptosomes. The increase in the intracellular Ca2+ concentration ([Ca2+]i) induced by 30 mM KCl was equally sensitive to either omega-agatoxin IVA (omega-Aga IVA) or to omega-conotoxin MVIIC (omega-CgTx MVIIC), and the inhibition produced by these two peptides was not additive. The present results indicate that omega-Aga IVA and omega-CgTx MVIIC do not distinguish between two different VSCC in hippocampal synaptosomes and that they both inhibit a channel with the alpha 1A subunit which is present in the rat hippocampus.

Relation of [Ca2+]i to dopamine release in striatal synaptosomes: role of Ca2+ channels

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [3H]dopamine ([3H]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [3H]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca(2+)-dependent [3H]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [3H]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.

Modulation of dopamine and noradrenaline release and of intracellular Ca2+ concentration by presynaptic glutamate receptors in hippocampus

1. We studied the release of [3H]-dopamine and [3H]-noradrenaline (NA) from hippocampal synaptosomes induced by glutamate receptors and the associated Ca2+ influx through Ca2+ channels. The release of tritiated neurotransmitters was studied by use of superfusion system and the intracellular free Ca2+ concentration ([Ca2+]i) was determined by a fluorimetric assay with Indo-1 as a probe for Ca2+. 2. Presynaptic glutamate receptor activation induced Ca(2+)-dependent release of [3H]-dopamine and [3H]-NA from rat hippocampal synaptosomes. Thus, L-glutamate induced the release of both neurotransmitters in a dose-dependent manner (EC50 = 5.62 microM), and the effect of 100 microM L-glutamate was inhibited by 83.8% in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX), but was not affected by 1 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801). 3. Other glutamate receptor agonists also stimulated the Ca(2+)-dependent release of [3H]-dopamine and [3H]-NA as follows: N-methyl-D-aspartate (NMDA), at 200 microM, released 3.65 +/- 0.23% of the total 3H catecholamines, and this effect was inhibited by 81.2% in the presence of 1 microM MK-801; quisqualate (50 microM), S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA) (100 microM) or kainate (100 microM) released 1.57 +/- 0.26%, 1.93 +/- 0.17% and 2.09 +/- 0.22%, of the total 3H catecholamines, respectively. 4. The ionotropic glutamate receptor agonist, AMPA, induced an increase in the [Ca2+]i which was inhibited by 58.6% in the presence of 10 microM CNQX. In contrast, the increase in [Ca2+]i due to stimulation by glutamate was not sensitive to CNQX or MK-801.5. Nitrendipine, at I JAM, did not inhibit the neurotransmitter release induced by AMPA, but, both 0.5 micro M -conotoxin GVIA (w-CgTx) and 100 nM w-Aga IVA reduced catecholamine release to 49.03 +/- 3.79% and 46.06 +/- 10.51% of the control, respectively. In the presence of both toxins the release was reduced to 12.58 +/- 4.64% of the control.6. The results indicate that activation of presynaptic glutamate receptors of the NMDA and non-NMDA type induces the release of [3H]-dopamine and [H]-NA from rat hippocampal synaptosomes and that the release induced by AMPA involves the activation of N- and P-type Ca2" channels which allow the influx of Ca2" that triggers the 3H catecholamines release.

Ca(2+)-dependent inactivation of P-type calcium channels in nerve terminals

Rapid Ca2+ signals evoked by K+ depolarization of rat cerebral cortical synaptosomes were measured by dual-channel Ca2+ spectrofluorometry coupled to a stopped-flow device. Kinetic analysis of the signal rise phase at various extracellular Ca2+ concentrations revealed that the responsible voltage-dependent Ca2+ channels, previously identified as P-type Ca2+ channels, inactivate owing to the rise in intracellular Ca2+ levels. At millimolar extracellular Ca2+ concentrations the channels were inactivated very rapidly and the rate was dependent on the high influx rate of Ca2+, thus limiting the Ca2+ signal amplitudes to 500-600 nM. A slower, probably voltage-dependent regulation appears to be effective at lower Ca2+ influx rates, leading to submaximal Ca2+ signal amplitudes. The functional feedback regulation of calcium channels via a sensor for intracellular Ca2+ levels appears to be responsible for the different inhibition characteristics of Cd2+ versus omega-agatoxin IVa.