Calcium uptake related to K+-depolarization and Na+/Ca2+ exchange in sheep brain synaptosomes

Transport of gatifloxacin involves Na+/Ca2+ exchange and excludes P-glycoprotein and multidrug resistance associated-proteins in primary cultured rat brain endothelial cells

The characteristics of gatifloxacin transport across blood brain barrier were investigated using primary cultured rat brain microvessel endothelial cells (rBMECs) as an in vitro model. Gatifloxacin uptake by rBMECs was time-, temperature- and energy-dependent. Gatifloxacin uptake by rBMECs was not influenced by P-glycoprotein (P-GP) inhibitor cyclosporine A or multidrug resistance associated-proteins (MRPs) inhibitor probenecid. However, verapamil inhibited the uptake in a concentration-dependent manner. Transendothelial transport study showed that transport of gatifloxacin across rBMEC monolayer was bidirectional, verapamil concentration-dependently inhibited transport from the apical to basolateral side, but did not significantly affect transport from basolateral to apical side. Gatifloxacin uptake was decreased in Ca(2+)-deprived medium but increased in Mg(2+)-deprived medium significantly. Furthermore, organic Ca(2+) channel blockers nifedipine and diltiazem had no effect on gatifloxacin uptake, but inorganic Ca(2+) channel blockers Ni(2+) and Mg(2+) inhibited the gatifloxacin uptake. The present study suggests that gatifloxacin transport across rBMECs involves a Na(+)/Ca(2+) exchange mechanism and extracellular Ca(2+) but not P-GP and MRPs.

The use of antioxidants to prevent glutamate-induced derangement of calcium ion metabolism in rat cerebral cortex synaptosomes

Glutamate is shown to induce increases in intracellular Ca2+ concentrations ([Ca2+]i), increases in 45Ca2+ influx, decreases in the activity of Na+,K+-ATPase activity, and activation of the Na+/Ca2+ exchanger in rat cerebral cortex synaptosomes. NMDA receptor antagonists virtually prevented these effects. Preincubation of synaptosomes with alpha-tocopherol, superoxide dismutase, and ganglioside GM1 normalized [Ca2+]i, 45Ca2+ influx, and Na+,K+-ATPase activity in rat cerebral cortex synaptosomes exposed to glutamate. Glutamate and GM1 activated the Na+/K+ exchanger, and their effects were additive. Calcium ions entering cerebral cortex nerve cells via NMDA receptors during exposure to high glutamate concentrations appeared to be only the trigger for the processes activating free-radical reactions. Activation of these reactions led to increases in Ca2+ influx into cells, decreases in Na+,K+-ATPase activity, and significant increases in [Ca2+]i, though this could be prevented by antioxidants and gangliosides.

The adenosine receptor agonist, APNEA, increases calcium influx into rat cortical synaptosomes through N-type channels associated with A2a receptors

N6-2-(4-aminophenyl)ethyladenosine (APNEA) is a nonselective adenosine receptor agonist known to have a high affinity for the adenosine A1 and A3 receptors. It was found to be able to dose-dependently increase the sustained (4 min) Ca2+ influx into rat cortical synaptosomes while 2-chloro-N6-(3-iodobenzyl)-adenosine-5-N-methyluronamide (Cl-IB-MECA), a selective A3 agonist has no effect. However, this effect of APNEA was not affected by the presence of 8-cyclopentyl-1,3-dimethylxanthine (CPT), a selective A1 antagonist; but instead completely abolished by 8-(3-chlorostyryl)caffeine (CSC), a selective A2a antagonist, or omega-conotoxin GVIA. These results show that in the rat cortex, presynaptic A2a receptors can mediate neurotransmitter release by increasing Ca2+ influx through the N-type calcium channels. A1 and A3 receptors appear not to be involved.

The Na+-Ca2+ exchange inhibitor KB-R7943 inhibits high K+-induced increases in intracellular Ca2+ concentration and [3H]noradrenaline release in the human neuroblastoma SH-SY5Y

The effects of the Na+-Ca2+ exchange inhibitor 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943) on depolarization-induced Ca2+ signal and [3H]noradrenaline release were examined in SH-SY5Y cells. KB-R7943 at 10 microM significantly inhibited high K+-induced increase in intracellular Ca2+ concentration. KB-R7943 also inhibited high K+-evoked release of [3H]noradrenaline from the cells. These findings suggest that the Na+-Ca2+ exchanger in the reverse mode is involved at least partly in depolarization-induced transmitter release.

Relationships between ATP depletion, membrane potential, and the release of neurotransmitters in rat nerve terminals. An in vitro study under conditions that mimic anoxia, hypoglycemia, and ischemia

BACKGROUND AND PURPOSE

It is known that the extracellular accumulation of glutamate during anoxia/ischemia is responsible for initiating neuronal injury. However, little information is available on the release of monoamines and whether the mechanism of its release resembles that of glutamate, which may itself influence the release of monoamines by activating presynaptic receptors. This study was designed to characterize the release of both amino acids and monoamines under chemical conditions that mimic anoxia, hypoglycemia, and ischemia.

METHODS

The contents of synaptosomes in adenine nucleotides (ATP, ADP, and AMP), amino acids (aspartate, glutamate, taurine, and gamma-aminobutyric acid), and monoamines (dopamine, noradrenaline, and 5-hydroxytryptamine) were measured by high-performance liquid chromatography, after the synaptosomes were subjected to anoxia (KCN + oligomycin), hypoglycemia (2 mmol/L 2-deoxyglucose in glucose-free medium), and ischemia (anoxia plus hypoglycemia).

RESULTS

The anoxia- and ischemia-induced release or noradrenaline, dopamine, 5-hydroxytryptamine, and glutamate correlated well with ATP depletion. The correlation observed between glutamate levels and the release of dopamine and 5-hydroxytryptamine in ischemic conditions suggests a functional linkage between the two transmitter systems. However, the antagonists of presynaptic glutamate receptors failed to alter the amount of monoamines released. The inhibition of Na+,K+-ATPase by ouabain had an effect similar to that produced by ischemia.

CONCLUSIONS

The decrease in Na+ and K+ gradients resulting from the energy depletion of the synaptosomes under ischemic conditions or resulting from the inhibition of Na+, K+-ATPase by ouabain promotes the reversal of the neurotransmitter transporters. The decrease in uptake of neurotransmitters may also contribute to the rise in the extracellular concentration of different transmitters observed during brain ischemia.

Rate of Na+/Ca2+ exchange across the plasma membrane of synaptosomes measured using the fluorescence of chlorotetracycline. Implications to calcium homeostasis in synaptic terminals

It is shown that the fluorescence of chlorotetracycline (CTC) can be used to continuously monitor Ca2+ fluxes mediated by the Na+/Ca2+-exchanger of the plasma membrane of synaptosomes. The kinetics of Ca2+ uptake can be followed from the kinetics of the increase of CTC fluorescence with external Ca2+ concentrations in the micromolar range. Since the fluorescence of CTC is not sensitive to Ca2+ concentration below 20 microM this avoids any significant contribution of Ca2+ flux through Ca2+ channels to CTC fluorescence. By replacing KCl by choline chloride in the buffer to avoid plasma membrane depolarization it is shown that the amplitude of the CTC fluorescence change is dependent upon the Na(+)-gradient preimposed across the plasma membrane, and the rate constant of the kinetic process is dependent upon the Ca2+ concentration. The rate constant of the Ca2+ influx measured with depolarized and non-depolarized synaptic plasma membrane vesicles at 37 degrees C and pH 7.4 were 0.55 +/- 0.10 and 0.25 +/- 0.02 min-1, respectively. The overall rate of Na+/Ca2+ exchange calculated under conditions close to physiological Na+ and Ca2+ gradients and membrane resting potential ranged from 15 to 25% of the activity of the plasma membrane Ca2+ pump under these experimental conditions. The results also point out that membrane depolarization increases approx. 2-fold the rate of Na+/Ca2+ exchange in synaptic plasma membrane vesicles.

Contribution of plasma membrane and endoplasmic reticulum Ca(2+)-ATPases to the synaptosomal [Ca2+]i increase during oxidative stress

In the present study we analyzed the effect of ascorbate (0.8 mM)/Fe2+ (2.5 microM)-induced membrane lipid peroxidation on the levels of intracellular free calcium,[Ca2+]i and on the possible mechanisms involved in the perturbation of intracellular calcium homeostasis during oxidative stress. For this purpose, the influence of the ascorbate/iron oxidant system on the plasma membrane and endoplasmic reticulum Ca(2+)-dependent ATPases of brain cortical synaptosomes was studied. In addition, the influence of the peroxidative process on the uptake of calcium (45Ca2+) and on the Na+/Ca2+ exchange activity at the plasma membrane was evaluated. After ascorbate/Fe(2+)-induced membrane lipid peroxidation of the order of 18.05 +/- 4.20 nmol TBARS/mg protein, an increase in [Ca2+]i occurred, under basal or depolarizing conditions (30 mM KCl), which was dependent on the extracellular calcium concentration. Thus, for 1 and 3 mM extracellular calcium concentration, an increase of the resting [Ca2+]i values of 19.8% and 33.7% was observed, while after the K(+)-depolarization the enhancement of the [Ca2+]i was 18.4% and 29.5%, respectively. The Na+/Ca2+ exchange activity and the time-dependent influx of 45Ca2+ observed in basal conditions and after the 30 mM K(+)-depolarization, were not affected under the peroxidative conditions. The Ca(2+)-dependent ATPase activity of the synaptosomal plasma membrane was significantly depressed following peroxidation of membrane lipids, decreasing the V(max) by 48.1%, without significant changes in the affinity of the enzyme for calcium (K(m) for Ca2+ was 0.54 +/- 0.04 microM in control conditions and 0.56 +/- 0.034 microM in peroxidized conditions). The Ca(2+)-ATPase activity of the endoplasmic reticulum was also affected during ascorbate/iron-induced oxidative stress; thus, an inhibition of 45.2% was observed 5 min after adding ATP. These data suggest that the increase in synaptosomal [Ca2+]i due to oxidative stress may result from the inhibition of the plasma membrane and the endoplasmic reticulum membrane Ca(2+)-ATPase activities, probably as a result of the alteration of the lipid environment required for the maximal activity of these membrane enzymes. The consequent increase in [Ca2+]i may be responsible for the injury of the nervous tissue observed during several pathological conditions in which free radical generation seems to be involved.

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.

Membrane lipid peroxidation induces changes in gamma-[3H]aminobutyric acid transport and calcium uptake by synaptosomes

In the present study, we analyze the effect of Fe2+/ascorbate-induced lipid peroxidation on Ca(2+)-dependent and Ca(2+)-independent release and on the uptake of gamma-[3H]aminobutyric acid (GABA) by sheep brain synaptosomes. In addition, we study the effect of lipid peroxidation on the levels of cytosolic calcium and on the uptake of calcium (45Ca2+). After membrane lipid peroxidation, a decrease in the uptake of GABA is observed. After ascorbate/Fe(2+)-induced membrane lipid peroxidation, a significant decrease in [3H]GABA release in response to K(+)-depolarization occurs, in the absence and in the presence of Ca2+. The influx of 45Ca2+ induced by K(+)-depolarization is significantly depressed under peroxidative conditions, while basal calcium uptake is inhibited to a much lesser degree. The levels of free ionic calcium [Ca2+]i, as determined by the fluorescent dye Indo-1, are increased after synaptosomes were submitted to the ascorbate/Fe2+ oxidative stress. It is concluded that membrane lipid peroxidation induces a decrease in Ca(2+)-dependent and Ca(2+)-independent efflux of accumulated [3H]GABA in response to elevated K+ pulses (60 mM) and in the depolarization-induced calcium influx, while free ionic calcium levels increase. The Ca(2+)-dependent efflux is interpreted to reflect stimulus-secretion coupling process and the Ca(2+)-independent efflux may reflect membrane transport processes. Thus, the results suggest a possible relationship between a reduced calcium movement across the membrane, the decrease in neurotransmitters uptake and release and oxidative stress.

Effect of some stereoisomeric tricyclic antidepressants on 45Ca uptake in synaptosomes from rat hippocampus

The present study has examined the inhibition of synaptosomal 45calcium uptake by trimipramine, oxaprotiline and doxepin, and their stereoisomers, in synaptosomes from the rat hippocampus. No significant difference in potency could be established for inhibition of net depolarization-induced 45calcium uptake for any pair of antipodes, and the IC50 values for calcium channel blockade were in the vicinity of 30 microM for this group of compounds. Trimipramine, doxepin and oxaprotiline also inhibited the 45calcium uptake mediated by Na(+)-Ca2+ exchange, with IC50 values of 71 microM, 110 microM, and 100 microM, respectively. The similar potency of doxepin isomers for inhibition of voltage-dependent calcium channels is in harmony with their reported similar potency in the clinic. A slight difference in potency is reported between the isomers of oxaprotiline in the behavioral despair test in rats, and the dextrorotatory isomer of trimipramine is reported to be a much more potent antidepressant than the levorotatory isomer: these order of potencies do not correspond perfectly with the similar potency of the antipodes against voltage-dependent calcium channels. The present study of stereoisomeric tricyclic antidepressants therefore fails to provide unequivocal support for the hypothesis that calcium channel blockade by tricyclic antidepressants is involved in their therapeutic effect.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Na+ influx-induced decrease of (Na+ + K+)-ATPase activity in rat brain slices: role of Ca2+

Treatment of rat brain slices with veratrine and monensin decreased (Na+ + K+)-ATPase activity in the membranes in a dose-dependent manner. The effect of monensin, like that of veratrine, was accompanied by a decrease of maximal binding sites for ouabain. The inhibitory effect of monensin on the enzyme activity was dependent on external Ca2+ at low concentrations, but not at a high concentration. The decreased enzyme activity induced by monensin was restored by subsequent incubation of the slices in a Ca(2+)-free medium containing 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. The effect of monensin at a low concentration on enzyme activity was antagonized by amiloride (1 mM), bepridil (5 microM), quinacrine (30 microM) or verapamil (30 microM), but not by nifedipine (1 microM) or omega-conotoxin (1 microM). Furthermore, the inhibitory effect of monensin at a high concentration under Ca(2+)-free conditions was blocked by BAPTA-AM (30 microM) and by bepridil (100 microM) or diazepam (500 microM), inhibitors of mitochondrial Na(+)-Ca2+ exchange. Inhibitors of calmodulin, protein kinase C, phospholipase A2 and calpain did not affect the monensin-induced decrease of enzyme activity. Dithiothreitol (10 mM) blocked the effect of monensin on enzyme activity but did not affect the ionophore-induced influx of Ca2+ in the slices.

Synaptosomal [Ca2+]i as influenced by Na+/Ca2+ exchange and K+ depolarization

The modulation of the intrasynaptosomal concentration of Ca2+, [Ca2+]i, by Na+/Ca2+ exchange was studied using Indo-1 fluorescence. The electrochemical gradient of Na+ was manipulated by substituting Li+ or choline for Na+ in the external medium and, then, the influx of 45Ca2+ and the [Ca2+]i were measured. It was found that the increase in [Ca2+]i induced by K+ depolarization is lower if the value of [Ca2+]i has been previously raised by Na+/Ca2+ exchange, suggesting that Ca2+ entering by Na+/Ca2+ exchange reduces the Ca2+ entering by voltage-dependent calcium channels. Our results show that a value of [Ca2+]i of about 650 nM induced by Na+/Ca2+ exchange reduces by 50% the Ca2+ entering due to K+ depolarization and no Ca2+ enters through the channels if the [Ca2+]i is previously raised above about 800 nM. Furthermore, predepolarization of the synaptosomes in a Ca-free medium also inhibits by at least 40% the [Ca2+]i rise through Ca2+ channels. Thus, the results suggest that both predepolarization and [Ca2+]i rise due to Na+/Ca2+ exchange decrease the Ca2+ entering by voltage-sensitive Ca2+ channels. The Ca2+ entering by Na+/Ca2+ exchange might contribute to the regulation of neurotransmitter release. Our results also show that the presence of Li+ in the external medium decreases the buffering capacity of synaptosomes, probably by releasing Ca2+ from mitochondria by Li+/Ca2+ exchange.

Regulation of carrier-mediated and exocytotic release of [3H]GABA in rat brain synaptosomes

In this study we investigated the role of external monovalent cations, and of intracellular Ca2+ concentration ([Ca2+]i) in polarized and depolarized rat cerebral cortex synaptosomes on the release of [3H]-gamma-aminobutyric acid (3H-GABA). We found that potassium-depolarization, in the absence of Ca2+, of synaptosomes loaded with 3H-GABA releases 7.4 +/- 2.1% of the accumulated neurotransmitter, provided that the external medium contains Na+, and an additional 19.0 +/- 2.5% is released upon adding 1.0 mM CaCl2 to the exterior. The Ca(2+)-independent release component does not occur in a choline medium and it is only 3.4 +/- 0.8% of the 3H-GABA accumulated in a Li+ medium, but both ions support the Ca(2+)-dependent release of 3H-GABA (13.4 +/- 0.6% in choline and 15.4 +/- 1.5% in Li+), which suggests that the exocytotic release is independent of the external monovalent cation present, whereas the carrier-mediated release specifically requires Na+ outside. Furthermore, previous release of the cytosolic 3H-GABA due to predepolarization in the absence of Ca2+ does not influence the amount of 3H-GABA subsequently released by exocytosis due to Ca2+ addition (19.1 +/- 2.5% or 19.1 +/- 1.1%, respectively). In choline or Li+ medium, the value of the [Ca2+]i is raised by Na+/Ca2+ exchange to 663 +/- 75 nM or 782 +/- 54 nM, respectively, within three minutes after adding 1.0 mM Ca2+, in the absence of depolarization, and parallel release experiments show no release of 3H-GABA in the choline medium, but a substantial release (7.1 +/- 2.1%) of 3H-GABA occurs in the Li+ medium without depolarization. Subsequent K(+)-depolarization shows normal Ca(2+)-dependent release of 3H-GABA in the choline medium (14.1 +/- 2.0%) but only 8.6 +/- 1.1% release in the Li+ medium, which suggests that raising the [Ca2+]i by Na+/Ca2+ exchange, without depolarization, supports some exocytotic release in Li+, but not in choline media. The role of [Ca2+]i and of membrane depolarization in the release process is discussed on the basis of the results obtained and other relevant observations which suggest that both Ca2+ and depolarization are essential for optimal exocytotic release of GABA.

Changing cation levels (Mg2+, Ca2+, Na+) alters the release of glutamate, GABA and other substances from the guinea pig cochlea

We examined the effects of changes in cation levels (increased Mg2+ concentration combined with low Ca2+ concentration, and two low concentrations of Na+) on the perilymph levels of gamma-aminobutyric acid (GABA), glutamate (Glu), aspartate (Asp) and other substances. Artificial perilymph solutions containing normal (5 mM) and high (50 mM) levels of K+ were perfused through the perilymphatic compartment of the guinea pig cochlea to examine basal release (5 mM K+) and depolarization-induced release (50 mM K+). Each of the two K+ concentrations were contained in four different solutions: [I] normal artificial perilymph (NARP; NaCl, 137 mM; CaCl2, 2 mM; MgCl2, 1 mM;); [II] high Mg2+ (20 mM)/low Ca2+ (0.1 mM) (HMgLCa); [III] low Na+ (117 mM; LNa), and [IV] very low Na+ (NaCl, 0 mM; VLNa). The effluent was collected and assayed for eighteen primary amines by HPLC. Compared with NARP, the HMgLCa group had an increase in the high K(+)-induced release of Asp and Glu with no change in GABA. VLNa increased the normal K+ levels of Asp, Glu and GABA up to those observed with high K+ in NARP. VLNa increased the high K+ levels of Asp and Glu over fivefold compared with the high K+ levels in NARP, but decreased GABA. We ascribe the results to an interference with either a Na(+)-dependent uptake processes or a Na+/Ca2+ exchange carrier.

Conditions restricting depolarization-dependent calcium influx in synaptosomes reveal a graded response of P96 dephosphorylation and a transient dephosphorylation of P65

Temporal changes in the phosphorylation level of synaptosomal phosphoproteins following depolarization of synaptosomes were investigated under conditions restricting calcium influx. High-K+ depolarization in media of low [Na+]o (32 mM during preincubation and depolarization) at pH 6.5 resulted in a pronounced fall in the cytosolic free calcium concentration transient, and in a reduction in the initial K(+)-stimulated 45Ca2+ uptake and endogenous acetylcholine release relative to the values obtained with control synaptosomes (preincubated and depolarized in Na(+)-based media). This reduction was paralleled by a decrease in the rate of dephosphorylation of the synaptosomal protein P96. A slower dephosphorylation of P96 also was observed on exposure to 20 microM veratridine at 0.5 mM external calcium. Our results indicate that, similar to synapsin I phosphorylation, P96 dephosphorylation shows a graded response to the amount of calcium entering the presynaptic terminal. Depolarization of synaptosomes under conditions restricting the influx of calcium revealed a transient dephosphorylation (reversed within 10 s) of the phosphoprotein P65. The possible significance of this finding to the process of neurotransmitter release is discussed.

Release of gamma-[3H]aminobutyric acid from synaptosomes: effect of external cations and of ouabain

In the present study we have investigated the effect of cations and ouabain on Ca(2+)-independent and Ca(2+)-dependent release of gamma-[3H]aminobutyric acid ([3H]GABA) from sheep brain synaptosomes. The presence of Na+ in the external medium is essential for the Ca(2+)-independent release induced by K+ or ouabain. Thus, in the absence of Ca2+, ouabain or K+ causes the release of [3H]GABA provided that Na+ is present in the external medium. Under K(+)-depolarizing conditions, in a Na+ medium, either ouabain or Ca2+ further increases the [3H]GABA release induced by depolarization, but their effects are not additive. The presence of external Na+ is not required for the Ca(2+)-dependent release of [3H]GABA due to K+ depolarization, and this release, which occurs in a choline medium, is not modified by ouabain. Under these conditions (choline medium) K(+)-depolarization dependent release is absolutely dependent on external Ca2+, which suggests that this release of [3H]GABA occurs only by exocytosis, without the carrier-mediated efflux which normally co-exists with exocytosis due to K(+)-depolarization in a Na+ medium. It is likely that the release induced by ouabain or K+ involves the membrane carrier which responds to changes in membrane potential.

Phosphatidylserine vesicles increase Ca2+ uptake by rat brain synaptosomes

Phosphatidylserine (PS) vesicles incorporated into rat brain synaptosomes increased total Ca2+ uptake. Total Ca2+ uptake was resolved in three components: K+ depolarization-induced Ca2+ uptake, Na+/Ca2+ exchange, and passive Ca2+ entry, which were differently affected by PS depending on the amount of incorporated phospholipid. K+ depolarization-induced Ca2+ uptake was stimulated by 0.05-0.10 mumol PS/mg protein while 0.10-0.30 mumol PS/mg protein increased Na+/Ca2+ exchange activity and passive Ca2+ entry but not K+ depolarization-induced Ca2+ uptake. High amounts of incorporated PS also increased passive Rb+ uptake.

Calcium influx in resting conditions in a preparation of peptidergic nerve terminals isolated from the rat neurohypophysis

1. Calcium accumulation in a preparation of nerve terminals isolated from the rat neurohypophysis was measured both in rapid (10-60 s) and long-term (up to 60 min) uptake experiments, by use of 45Ca2+ as radiotracer and ion-exchange chromatography as separation method. Unless otherwise stated all experiments have been performed in the absence from the incubation media of secretagogues or depolarizing agents. 2. The uptake of 45Ca2+ in nerve terminals was linear up to 30-45 s, with an apparent initial rate of uptake of 0.98 nmol Ca2+ (mg protein)-1 min-1. 3. The level of 45Ca2+ accumulation was sensitive to manipulations of electrochemical gradient for Na+ across the plasma membrane. Alterations of extracellular concentrations of Na+ affected secretory activity to a larger extent than manipulations of internal Na+. These effects were not qualitatively dependent on the nature of the replacement for Na+. 4. Removal of extracellular Na+ induced a significant increase of both the level of 45Ca2+ accumulation and of the apparent initial rate of uptake. The concentration for half-maximal stimulatory effect was 40 mM-Na+. 5. The analysis of the stimulatory effect of high extracellular K+ on the 45Ca2+ accumulation reveals at least two components: a depolarization and an intrinsic K+ effect. 6. Sodium channel inhibitors (TTX, 1.25 microM) decreased significantly the level of 45Ca2+ accumulation, an effect which was evident from the first minute of exposure to the drug. 7. A specific L-type Ca2+ channel blocker (nicardipine) inhibited 45Ca2+ uptake, in a dose-dependent manner. Simultaneous addition of both TTX and nicardipine (20 microM) decreases the 45Ca2+ uptake up to 50%. 8. In conclusion, the uptake of Ca2+ in isolated peptidergic nerve terminals, incubated in resting conditions, is mediated by at least three pathways: a TTX-sensitive and a nicardipine (dihydropyrine)-sensitive pathway and through a Na(+)-Ca2+ exchange-dependent mechanism. The principal route of Ca2+ entry appears to be through TTX-sensitive channels.

Characterization of ouabain-induced phosphoinositide hydrolysis in brain slices of the neonatal rat

The effect of the Na/K-ATPase inhibitor ouabain on phosphoinositide (Ptdlns) hydrolysis was studied in rat brain cortical slices. Ouabain induced a dose-dependent accumulation of inositol phosphates (InsPs) which was much higher in neonatal rats (1570 +/- 40% of basal) than in adult animals (287 +/- 18% of basal). For this reason, all experiments were conducted with 7 day-old rats. Strophantidin caused a similar stimulation of Ptdlns hydrolysis, although it was less potent than ouabain. The order of potency for ouabain-stimulated InsPs accumulation in brain areas was hippocampus greater than cortex greater than brainstem greater than cerebellum. The effect of ouabain was not blocked by antagonists for the muscarinic, alpha1 -adrenergic and glutamate receptors. Also ineffective were the K+ channel blockers 4-aminopyridine and tetraethylammonium, the sodium channel blocker tetrodotoxin, and the calcium channel blocker verapamil, whereas the Na/Ca exchanger blocker amiloride partially antagonized the effect of ouabain. The accumulation of InsPs induced by ouabain was additive to that of carbachol and norepinephrine, as well as to that induced by high K+ and veratrine, but not to that of glutamate. Removal of Na+ ions from the incubation buffer completely prevented the accumulation of InsPs induced by ouabain. The effect of ouabain was also dependent upon extracellular calcium and was under negative feedback control of protein kinase C. Despite the higher effect of ouabain on Ptdlns hydrolysis of immature rats, the density of [3H]ouabain binding sites, as well as the activity of Na/K-ATPase were higher in adult animals. Furthermore, a poor correlation was found between ouabain-stimulated Ptdlns hydrolysis and [3H]ouabain binding in brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of receptor-mediated inhibition of 45Ca accumulation into synaptosomes

1. The effects of alpha 2-adrenoceptor and kappa-opiate receptor activation on 45Ca accumulation into rat cortical synaptosomes were examined. 2. Clonidine (1 microM) and U50488H (1 microM) significantly reduced 45Ca accumulation under both resting (5 mM K+) and depolarizing (15-30 mM K+) conditions. 3. The inhibitory effects of the agonists on 45Ca accumulation into synaptosomes were enhanced in the presence of the Na(+)-Ca2+ exchange inhibitor sodium orthovanadate (vanadate, 2 mM), and were not present in mitochondrial preparations. 4. When the agonists were used together, their inhibitory effects were not additive but were, in fact, attenuated. 5. In the presence of the alpha 2-adrenoceptor antagonist idazoxan (1 microM), the inhibitory effect of U50488H on 45Ca accumulation was enhanced. A similar increase in the inhibitory effectiveness of clonidine was observed in the presence of naloxone (20 microM). 6. When synaptosomes were pretreated with 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM), dibutyrylcyclic AMP (db-cyclic AMP, 10 microM) or 8-bromo-cyclic AMP (8Br-cyclic AMP, 10 microM), the inhibitory effects of clonidine and U50488H were abolished, suggesting that a decrease in cyclic AMP production is part of the receptor-effector coupling mechanism of both receptor systems. 7. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.05 microM) increased 45Ca accumulation but did not alter the inhibitory effects of clonidine or U50488H, thus showing that the effects of the agonists are not mediated by protein kinase C. 8. We conclude that alpha 2-adrenoceptor and Kappa-opiate receptor activation dramatically reduce 45Ca influx through Ca21 channels (e.g., by 50%), that there is a functional antagonism between the two receptor systems and that in both cases, the receptor effector mechanism involves a decrease in cyclic AMP production.

Influence of isolation media on synaptosomal properties: intracellular pH, pCa, and Ca2+ uptake

Preparations of synaptosomes isolated in sucrose or in Na(+)-rich media were compared with respect to internal pH (pHi), internal Ca2+ concentration ([Ca2+]i), membrane potential and 45Ca2+ uptake due to K+ depolarization and Na+/Ca2+ exchange. We found that synaptosomes isolated in sucrose media have a pHi of 6.77 +/- 0.04 and a [Ca2+]i of about 260 nM, whereas synaptosomes isolated in Na(+)-rich ionic media have a pHi of 6.96 +/- 0.07 and a [Ca2+]i of 463 nM, but both types of preparations have similar membrane potentials of about -50 mV when placed in choline media. The sucrose preparation takes up Ca2+ only by voltage sensitive calcium channels (VSCC'S) when K(+)-depolarized, while the Na(+)-rich synaptosomes take up 45Ca2+ both by VSCC'S and by Na+/Ca2+ exchange. The amiloride derivative 2',4'-dimethylbenzamil (DMB), at 30 microM, inhibits both mechanisms of Ca2+ influx, but 5-(N-4-chlorobenzyl)-2',4' dimethylbenzamil (CBZ-DMB), at 30 microM, inhibits the Ca2+ uptake by VSCC'S, but not by Na+/Ca2+ exchange. Thus, DMB and CBZ-DMB permit distinguishing between Ca2+ flux through channels and through Na+/Ca2+ exchange. We point out that the different properties of the two types of synaptosomes studied account for some of the discrepancies in results reported in the literature for studies of Ca2+ fluxes and neurotransmitter release by different types of preparations of synaptosomes.

Metal cations as synaptosomal calcium blockers in studies with Fura-2

Metal ions are often used to block calcium channels in various tissues, including synaptosomes. In the present study, Fura-2 was used to determine the effectiveness of various metal ions as calcium channel blockers in rat brain synaptosomes in vitro. In buffer solutions, La3+ and Cd2+ increased the Fura-2 fluorescence in a manner similar to Ca2+. Ni2+ and Mn2+ appeared to be fluorescence quenching cations, and Sr2+ and Co2+ had little effect on the fluorescence of Fura-2. In suspensions of synaptosomes under resting conditions, Cd2+, Ni2+ and Mn2+ were found to be not suitable for use in synaptosome studies. On the other hand, La3+ and Co2+ had little effect on the Fura-2 fluorescence of resting synaptosomes, and under depolarizing conditions, La3+ and Co2+ decreased the Fura-2 fluorescence. These results, therefore, suggest that La3+ and Co2+ may be suitable as calcium channel blockers in synaptosome studies.

Phosphoinositide hydrolysis induced by depolarization and sodium channel activation in mouse cerebrocortical slices

Carbachol, a muscarinic receptor agonist and the sodium channel-activating agents, scorpion venom, veratridine, batrachotoxin and aconitine, were shown to stimulate the formation of [3H]inositol phosphates in [3H]inositol-labelled miniprisms, obtained from the cerebral cortex of the mouse. The inositol response to the Na+ channel-activating agents was inhibited by the sodium channel blocker tetrodotoxin (TTX), while the response induced by carbachol was partially resistant to TTX. The response to scorpion venom and the TTX-insensitive portion of the response to carbachol was additive, indicating different mechanisms. The presence of high potassium (K+) induced hydrolysis of inositide in a TTX-insensitive manner and was not additive with that resulting from sodium channel activators, thus indicating a common mechanism. The addition of large concentrations of magnesium to block the release of acetylcholine, did not inhibit the inositol response to high K+ or to veratridine. Calcium channel blockers such as nickel or cobalt, or the dihydropyridine calcium (Ca2+) channel activator BAY K 8644 and the calcium channel blocker nifedipine, nimodipine or PN-200 110 had little effect. Monensin, a sodium ionophore, stimulated the turnover of phosphatidylinositol at non-depolarizing concentrations and the omission of Na+ ions inhibited the response to sodium channel agents and to high K+. Thus, membrane potential and gradients of K+, Na+ and Ca2+ are all important factors determining the final effect on the turnover of phosphatidylinositol. The data are consistent with a model in which all these factors impinge on the Na+/Ca2+ exchanger regulating internal Ca2+ that, in turn, activates phospholipase C.

Alterations in calcium antagonist receptors and sodium-calcium exchange in cardiomyopathic hamster tissues

The Syrian cardiomyopathic (CM) hamster (BIO 14.6) develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy, and, eventually, cardiac dilatation and congestive heart failure. Several lines of evidence implicate cellular calcium overload as an important etiologic factor. We previously reported an increased number of receptors for calcium antagonist drugs, which block voltage-dependent calcium channels, in heart, skeletal muscle, and brain tissue of these hamsters in the early necrotic stage of the disease. To better characterize the pathophysiological significance of this abnormality we evaluated calcium antagonist receptor binding and Na+-Ca2+ exchange in CM and control hamsters at different stages of disease as documented by quantitative histopathologic assessment. In CM hamsters as young as 10 days, an age previously thought to be before the onset of disease, we identified cardiac myocyte hypertrophy, a twofold increase in calcium antagonist receptor binding in heart and brain, and a 50% increase in skeletal muscle. Overt histological lesions were present in skeletal muscle at 25 days and in heart between 28-30 days. The size of cardiac lesions increased over time and changed from necrotic foci with cellular infiltration to fibrotic or calcified lesions by 360 days. Myocardial cellular hypertrophy persisted through the late stages of the disease (360 days), but increased calcium antagonist binding was present in heart only to 6 months of age, in skeletal muscle to 90 days, and in brain to 30 days. Na+-Ca2+ exchange in heart was normal until 15 days and then increased by 400% at 30 days suggesting that this augmentation might be a secondary response to the earlier increase in calcium antagonist receptors. At 360 days cardiac Na+-Ca2+ exchange was decreased by 50%, likely reflecting progressive cardiac damage. The increase in calcium antagonist receptors in CM animals as young as 10 days supports the hypothesis that abnormalities in voltage-dependent calcium channels play a role in the pathophysiology of CM hamsters.

Further characterization of phasic calcium influx in rat cerebrocortical synaptosomes: inferences regarding calcium channel type(s) in nerve endings

Under conditions minimizing the contribution of Na+/Ca2+ exchange to calcium entry in synaptosomes, the K+ depolarization-dependent calcium influx (JCa) is a single exponential function of time. JCa activates and slowly inactivates at membrane potentials positive to -50 mV, a result indicating the involvement of moderate voltage-activating, slowly inactivating calcium channels. Calcium channels in synaptosomes are characterized by stronger sensitivity to blockage by Cd2+ than Co2+, insensitivity to dihydropyridine calcium antagonists or the agonist Bay K 8644, and weak, partial sensitivity to the peptide toxin omega-conotoxin GVIA. These characteristics suggest that voltage-sensitive calcium channels in rat cerebrocortical synaptosomes are dissimilar from the somatic T, N, or L channel types. JCa is not affected by treatment of synaptosomes with the adenylate cyclase activator forskolin, the membrane permeant dibutyryl-cyclic AMP, or the kinase C activator phorbol 12-myristate 13-acetate diester, results suggesting that calcium channels in synaptosomes are not directly modulated by protein kinase A- or C-mediated phosphorylation.

'Slow' K+-stimulated Ca2+ influx is mediated by Na+-Ca2+ exchange: a pharmacological study

K+-stimulated 45Ca2+ influx was measured in rat brain presynaptic nerve terminals that were predepolarized in a K+-rich solution for 15 s prior to addition of 45Ca2+. This 'slow' Ca2+ influx was compared to influx stimulated by Na+ removal, presumably mediated by Na+-Ca2+ exchange. The K+-stimulated Ca2+ influx in predepolarized synaptosomes, and the Na+-removal-dependent Ca2+ influx were both saturating functions of the external Ca2+ concentration; and both were half-saturated at 0.3 mM Ca2+. Both were reduced about 50% by 20 microM Hg2+, 20 microM Cu2+ or 0.45 mM Mn2+. Neither the K+-stimulated nor the Na+-removal-dependent Ca2+ influx was inhibited by 1 microM Cd2+, La3+ or Pb2+, treatments that almost completely inhibited K+-stimulated Ca2+ influx in synaptosomes that were not predepolarized. The relative permeabilities of K+-stimulated Ca2+, Sr2+ or Ba2+ influx in predepolarized synaptosomes (10:3:1) and the corresponding selectivity ratio for Na+-removal-dependent divalent cation uptake (10:2:1) were similar. These results strongly suggest that the K+-stimulated 'slow' Ca2+ influx in predepolarized synaptosomes and the Na+-removal-dependent Ca2+ influx are mediated by a common mechanism, the Na+-Ca2+ exchanger.

Reduction of K+-stimulated 45Ca2+ influx in synaptosomes with age involves inactivating and noninactivating calcium channels and is correlated with temporal modifications in protein dephosphorylation

The voltage-dependent calcium uptake in rat brain synaptosomes was measured under conditions in which [Ca2+]o/[Na+]i exchange was minimized to characterize the voltage-sensitive calcium channels from rats of different ages. In solutions of CaCl2 concentrations of less than 500 microM, the initial (5-s) calcium uptake declined by approximately 20-50% in 12- and 24-month-old rats relative to 3-month-old adults. Depolarization of synaptosomes from 3-month-old rats in a calcium-free medium or in the presence of 0.5 mM CaCl2 led to an exponential decline of the calcium uptake rate after 20 s (voltage- or voltage-and-calcium-dependent inactivation) to approximately 66 and 34% of the initial value with a t1/2 of 1.6 or 0.7 s, respectively. The presence of 1 microM nifedipine resulted in a 15-25% reduction of 45Ca2+ uptake rates, which appeared to affect noninactivating calcium channels, but addition of the calcium channel agonist Bay K 8644 was without effect. In 24-month-old rats, inactivation of 45Ca2+ uptake in calcium-free media was nondetectable, and in the presence of 0.5 mM CaCl2, the rate and extent of inactivation were also much lower than in 3-month-old animals (the t1/2 was 0.9 s, and the calcium uptake rate at 20 s was 55% of its initial value). Moreover, the presence of 1 microM nifedipine was without effect on initial calcium uptake or inactivation in synaptosomes from 24-month-old rats. These results indicate that the decrease in calcium channel-mediated 45Ca2+ uptake involves an inhibition or block of both dihydropyridine-resistant and -sensitive calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the involvement of Na+/Ca2+ exchange in the stimulation of inositol phospholipid hydrolysis by sodium channel activation and depolarization

Amiloride, an inhibitor of the Na+/Ca2+ exchanger, blocked the hydrolysis of inositol phospholipids in mouse cerebrocortical slices induced by the sodium channel activator veratridine, by KCl, or by the sodium ionophore monensin; there was no inhibition by A 23187, a Ca2+ ionophore, or by serotonin. It is concluded that agents that increase intracellular Na2+ stimulate inositide hydrolysis by an indirect effect via Na+/Ca2+ exchange.

Stimulation of D2 dopamine receptors decreases intracellular calcium levels in rat anterior pituitary cells but not striatal synaptosomes: a flow cytometric study using indo-1

An important question is whether all D2 dopamine (DA) receptors employ the same signal transduction mechanisms. Anterior pituitary cells and striatal synaptosomes, which possess pharmacologically similar D2 DA receptors, were compared with respect to the effect of D2 DA receptor stimulation on free intracellular Ca2+ levels [( Ca2+]i). Flow cytometry, in combination with either the fluorescent calcium indicator indo-1 or fluorescent voltage-sensitive dyes, was used to measure [Ca2+]i and to detect changes in membrane potential. In subpopulations of anterior pituitary cells, increases in [Ca2+]i were produced by elevated K+, veratridine, thyrotropin-releasing hormone, and BAY K 8644. These increases were blocked by nifedipine, suggesting the involvement of L-type voltage-sensitive calcium channels (VSCC's). In 10-15% of the cells, D2 agonists decreased resting [Ca2+]i, reversed stimulus-induced increases in [Ca2+]i, and caused a hyperpolarization. In striatal synaptosomes, elevated K+ and veratridine also increased [Ca2+]i. However, the K+-induced increase was eliminated if choline was substituted for Na+ in the medium, suggesting that Ca2+ entry in response to sustained K+ depolarization resulted from reversal of Na+/Ca2+ exchange. Nifedipine and verapamil inhibited K+-induced increases in [Ca2+]i only at concentrations greater than 10 microM, while omega-conotoxin had no effect. D2 agonists had no effect on resting or stimulated [Ca2+]i but did hyperpolarize 10-20% of the synaptosomes, indicating that D2 DA receptors are functional in this preparation. The ability of pituitary but not striatal D2 DA receptors to modulate [Ca2+]i may reflect the fact that the two systems differ with respect to pathways for Ca2+ influx.

Na+-Ca2+ exchange in squid optic nerve membrane vesicles is activated by internal calcium

The role of intracellular Ca2+ as essential activator of the Na+-Ca2+ exchange carrier was explored in membrane vesicles containing 67% right-side-out and 10% inside-out vesicles, isolated from squid optic nerves. Vesicles containing 100 microM free calcium exhibited a 2-fold increase in the initial rate of Na+i-dependent Ca2+ uptake as compared with vesicles where intravesicular calcium was chelated by 2 mM EGTA or 10 mM HEDTA. The activatory effect exerted by intravesicular Ca2+ on the reverse mode of Na+-Ca2+ exchange (i.e. Na+i-Ca2+o exchange) is saturated at about 100 microM Ca2+i and displays an apparent K 1/2 of 12 microM. Intravesicular Ca2+ produced activation of Na+i-Ca2+i exchange activity rather than an increase in Ca2+ uptake due to Ca2+-Ca2+ exchange. The presence of Ca2+i was essential for the Na+i-dependent Na+ influx, a partial reaction of the Na+-Ca2+ exchanger. In fact, the Na+ influx levels in vesicles loaded with 2 mM EGTA were close to those expected from diffusional leak while in vesicles containing Ca2+i an additional Na+-Na+ exchange was measured. The results suggest that in nerve membrane vesicles Ca2+ at the inner aspect of the membrane acts as an activator of the Na+-Ca2+ exchange system.

Long-term storage of functional, isolated nerve endings by slow freezing and rapid thawing

Nerve endings (synaptosomes) were isolated from homogenized rat brain corpora striata following centrifugation on discontinuous sucrose gradients. The synaptosomes (in 0.8 M sucrose) were (i) slowly frozen by placing the tube containing the suspension in a freezer at -10 degrees C for 1 h followed by (ii) swirling in a mixture of acetone and dry ice for 15 min and (iii) were stored in liquid nitrogen for up to 6 weeks. Freshly isolated synaptosomes and synaptosomes from the same preparation that were frozen for 2, 4, or 6 weeks and rapidly thawed in a water bath at 37 degrees C were re-equilibrated with a physiological salt solution and assayed for their ability to accumulate Ca and to release transmitter (dopamine) upon depolarization in high K medium. K-dependent Ca uptake gradually declined to approximately 1/3 the value observed with freshly isolated synaptosomes after 6 weeks of storage. K-stimulated dopamine release (only from intact synaptosomes) was normal over the entire period of storage. It is concluded that synaptosomes retain their physiological properties when stored frozen for a few weeks and that cold storage may be a useful technique for experiments requiring lengthy or repeated assays or accumulation of material.

Measurement of the cytosolic sodium ion concentration in rat brain synaptosomes by a fluorescence method

A method for the measurement of the cytosolic Na+ concentration in intact synaptosomes is described. This method makes use of a pH sensitive dye (BCECF) that can be loaded into the cytosol and a relatively specific ionophore (monensin) that can exchange Na+ for H+ across the synaptosomal membrane. By setting conditions such that there is no electrochemical potential difference for H+ across the membrane (no membrane potential and pHi = pHo), addition of ionophore would induce a H+ flux only if there is a concentration difference for Na+. Thus, when there is no fluorescence change (no cytosolic pH change) extracellular [Na+] equals intrasynaptosomal [Na+]. The intrasynaptosomal [Na+] concentration was determined to be 7 +/- 3 mM (n = 5; mean +/- S.E.). The results obtained with this fluorescence method are compared with estimates obtained by atomic absorption spectrometry. Limitations and applications of the method are discussed.

Effects of kainic acid lesions on calcium uptake and dopamine release in nerve endings isolated from rat striata

Rats were injected intrastriatally with kainic acid and the viability of dopaminergic terminals two days following the injection was determined by comparing voltage dependent calcium uptake and dopamine release in isolated nerve endings. Evoked dopamine release remained normal following the lesion, but the initial rate of potassium stimulated calcium entry decreased by approximately 1/3. These results suggest that the excitotoxic lesion caused by acute intrastriatal injection of kainic acid results in retention of functional dopaminergic terminals.

Differential effects of age on the pathways of calcium influx into nerve terminals

Calcium accumulation by synaptosomes decreases during ageing and this is partly due to an impaired calcium uptake by mitochondria (Brain Research, 378 (1986) 36-48). In the present work we have sought to define that effect of age on the pathways of K+-stimulated calcium influx. The plasma membrane potential of synaptosomes incubated at different K+ concentrations in choline-based or sodium-based media monitored with TPP+ did not change significantly with age. 45Ca uptake was reduced by around 20% in 24-vs 3-month-old rats at high K+ concentrations in both choline- and sodium-based media. However, the internal free calcium concentration in K+-depolarized synaptosomes estimated by the quin-2 method was found to be higher in 24- than in 3-month-old rats. When the apparent calcium permeabilities (P'Ca) in choline-based media were calculated from the corresponding calcium uptake values, membrane potentials and internal calcium concentration, it was found that the P'Ca values from old rats were only slightly lower than those of adults over the whole range of membrane potentials. The contribution of the Na/Ca exchanger to 45Ca uptake was estimated at different voltages by subtracting the normalized calcium uptake values obtained in choline media from those in Na media. The 'estimated' Na/Ca exchange was found to decrease markedly with age. Our results suggest that under our experimental conditions the apparent calcium permeability of synaptosomes is only modestly decreased during ageing. However, the operation of 45Ca/Na exchange is markedly reduced maybe as a result of alterations of the exchanger itself or due to changes in the concentration of internal Na or other ions.

Effects of 1,1,1-trichloroethane on synaptosomal calcium accumulation in mouse brain

Addition of 1,1,1-trichloroethane (TCE), in vitro to synaptosomes isolated from the mouse cerebellum and cerebral cortex inhibited the slow phase of K+-stimulated 45Ca2+ influx and the net 45Ca2+ influx (delta k). In the brain stem, however, TCE increased the fast and slow phases of calcium uptake under depolarizing conditions and also delta k. The non-depolarized calcium influx was not altered by TCE added in vitro. Two hours after injection of TCE (2.4 g/kg) the calcium accumulation in the presence of high K+ was lowered in the cerebellar synaptosomes, while it was increased in brain stem synaptosomes. TCE administered in vivo did not alter the calcium influx into cerebrocortical synaptosomes nor did it affect the non-depolarization-induced 45Ca2+ influx. Thus, these data indicate that TCE may influence voltage-dependent calcium channels in mouse brain synaptosomes.

Cation effects on taurine release from brain slices: comparison to GABA

The effects of cations on the spontaneous and potassium-stimulated taurine release from mouse cerebral cortex slices were assessed with an emphasis on the as yet unestablished calcium dependence of the stimulated release. Spontaneous and stimulated GABA release was analyzed for comparison. A depolarizing concentration (50 mM) of potassium ions caused an approximately 3.5-fold increase in taurine release and the omission of sodium a 6-fold enhancement. GABA release was increased by the same stimuli about 20- and 34-fold, respectively. Omission of calcium ions greatly enhanced basal taurine and GABA release when the medium was supplemented with the calcium chelator ethylenediaminetetraacetate. The potassium stimulation was then abolished, however, with taurine even more readily than with GABA. Magnesium and calcium ions had antagonistic effects on the stimulated release, more clearly with taurine than with GABA. Verapamil abolished the potassium stimulation of both taurine and GABA release, the latter being more sensitive. Although the stimulated taurine release was less in magnitude and had a slower time course than the GABA release, the results are not at variance with the possible neurotransmitter role of taurine.

Glycinergic systems in the brain stem of developing and adult mice: effects of taurine

The release of glycine from slices of the brain stem and binding of strychnine to brain stem membranes were characterized in adult and developing mice. Spontaneous glycine efflux was markedly facilitated by homoexchange with exogenous glycine and moderately by heteroexchange with taurine. Potassium stimulation released more glycine from brain stem slices from adult than from 7-day-old mice. Potassium-stimulated glycine release was also potentiated by glycine and by the novel anticonvulsant taurine derivatives. One population of strychnine-binding sites was found in both mature and immature brain stem. The number of binding sites increased with age, whereas the affinity of the sites for strychnine remained the same. The glycine inhibition was stronger in adult than in developing mice. In the presence of taurine the affinity for strychnine decreased without any change in the maximal binding capacity, suggesting a competitive type of inhibition. The binding constant and maximal binding capacity of strychnine increased in the presence of NaCl (200 mM) both in adult and 7-day-old mice. The calculated IC50 values for displacement of strychnine binding by glycine, taurine and beta-alanine were higher in the presence than in the absence of sodium. The results show that the evoked release of glycine and the number of binding sites of strychnine increase during postnatal development in the mouse but that their characteristics do not change.

Effects of Ca2+ channel blockers on Ca2+ translocation across synaptosomal membranes

The binding of [3H]nimodipine to purified synaptic plasma membranes (SPM) isolated from sheep brain cortex was characterized, and the effects of nimodipine, nifedipine, and (+)-verapamil on the [3H]nimodipine binding were compared to the effects on 45Ca2+ translocation under conditions that separate 45Ca2+ fluxes through Ca2+ channels from 45Ca2+ uptake via Na+/Ca2+ exchange. [3H]Nimodipine labels a single class of sites in SPM, with a KD of 0.64 +/- 0.1 nM, a Bmax of 161 +/- 27 fmol X mg-1 protein, and a Hill slope of 1.07, at 25 degrees C. Competition of [3H]nimodipine binding to purified SPM with unlabelled Ca2+ channel blockers shows that: nifedipine and nimodipine are potent competitors, with IC50 values of 4.7 nM and 5.9 nM, respectively; verapamil and (-)-D 600 are partial competitors, with biphasic competition behavior. Thus, (+)-verapamil shows an IC50 of 708 nM for the higher affinity component and the maximal inhibition is 50% of the specific binding, whereas for (-)-verapamil the IC50 is 120 nM, and the maximal inhibition is 30%; (-)-D 600 is even less potent than verapamil in inhibiting [3H]nimodipine binding (IC50 = 430 nM). However, (+)-verapamil, nifedipine, and nimodipine are less potent in inhibiting depolarization-induced 45Ca2+ influx into synaptosomes in the absence of Na+/Ca2+ exchange than in competing for [3H]nimodipine binding. Thus, (+)-verapamil inhibits Ca2+ influx by 50% at about 500 microM, whereas it inhibits 50% of the binding at concentrations 200-fold lower, and the discrepancy is even larger for the dihydropyridines. The Na+/Ca2+ exchange and the ATP-dependent Ca2+ uptake by SPM vesicles are also inhibited by the Ca2+ channel blockers verapamil, nifedipine, and d-cis-diltiazem, with similar IC50 values and in the same concentration range (10(-5)-10(-3) M) at which they inhibit Ca2+ influx through Ca2+ channels. We conclude that high-affinity binding of the Ca2+ blockers by SPM is not correlated with inhibition of the Ca2+ fluxes through channels in synaptosomes under conditions of minimal Na+/Ca2+ exchange. Furthermore, the relatively high concentrations of blockers required to block the channels also inhibit Ca2+ translocation through the Ca2+-ATPase and the Na+/Ca2+ exchanger. In this study, clear differentiation is made of the effects of the Ca2+ channel blockers on these three mechanisms of moving Ca2+ across the synaptosomal membrane, and particular care is taken to separate the contribution of the Na+/Ca2+ exchange from that of the Ca2+ channels under conditions of K+ depolarization.

gamma-Aminobutyric acid release from synaptosomes as influenced by Ca2+ and Ca2+ channel blockers

We studied the correlation between the high affinity binding of Ca2+ channel blockers to purified synaptic plasma membranes (SPM) and the effect of these drugs in blocking the 45Ca2+ uptake and the release of [3H]gamma-aminobutyric acid [( 3H]GABA) by preloaded synaptosomes. The Ca2+ channel blocker binding sites were characterized by studying the binding of the dihydropyridine, [3H]nimodipine, and of the phenylalkylamine, (-)-[3H]desmethoxyverapamil, to purified SPM isolated from sheep brain cortex synaptosomes. The purified SPM had high affinity binding sites for both Ca2+ channel blockers. The binding parameters were similar to those previously reported for whole brain homogenates: KD = 0.64 nM and Bmax = 160 fmol/mg of protein for [3H]nimodipine, and KD = 7.9 nM and Bmax = 1,500 fmol/mg of protein for (-)-[3H]desmethoxyverapamil. The Ca2+ channel blockers inhibited the release of [3H]GABA induced by K+ depolarization in the presence or in the absence of Ca2+. The Ca2+-dependent component of [3H]GABA release was inhibited by verapamil, (-)-D 600, d-cis-diltiazem, nifedipine and PY 108-86 with IC50 values of 2.2 X 10(-5) M, 6.3 X 10(-5) M, 3 X 10(-4) M, greater than 10(-4) M and 3 X 10(-5) M, respectively. Furthermore, the Ca2+ channel blockers also inhibited the Ca2+-independent [3H]GABA release which occurred in the presence, but not in the absence, of external Na+. The Ca2+ channel blockers at concentrations which inhibited [3H]GABA release inhibited the entry of Ca2+ through the Ca2+ channels and also the entry of Ca2+ by Na+/Ca2+ exchange. We conclude that the concentrations of Ca2+ blockers necessary to block Ca2+ uptake through the Ca2+ channels and by Na+/Ca2+ exchange coincide with the concentrations at which they inhibit [3H]GABA release, but that their effect on the relationship between Ca2+ uptake and [3H]GABA release is different for the various blockers. The effects of the drugs on Ca2+ movements and [3H]GABA release are not specifically mediated through the high affinity binding of the drugs since relatively high concentrations were necessary (greater than 10(-5) M) for the effects reported here.