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PubMed1984

Kinetics of irreversible inhibition of choline transport in synaptosomes by ethylcholine mustard aziridinium.

Ethylcholine mustard aziridinium (ECMA) inhibits choline transport in synaptosomes at a half-maximal concentration of about 20 microM. The rate of inhibition falls off rapidly after 10 min and the concentration dependency reaches a plateau at about 100 microM. The inhibition is not removed by washing the synaptosomes, and choline and hemicholinium-3 protect the carrier against attack by the mustard. Choline efflux, particularly that stimulated by choline in the medium (transactivation) is also inhibited by the aziridinium compound. Similarly choline influx activated by preloaded internal choline is inhibited by ECMA. The mustard can enter the synaptosomes in an active form but most of the carrier is alkylated when facing the outside. Prior depolarization of the synaptosomes causes an increase in the rate of inhibition by ECMA which is proportionally about the same as the increase in choline influx also caused by depolarization. At low ECMA concentrations the rate of inhibition is that of a first-order reaction with the carrier but at high ECMA concentrations the translocation of the carrier to the outward-facing conformation controls the rate of inhibition. Using a model of choline transport with some simplifying assumptions it is possible to estimate the amount of carrier; cholinergic synaptosomes carry about six times the concentration of carrier found in noncholinergic ones. In noncholinergic synaptosomes the carrier faces predominately out, the reverse in cholinergic ones. The rate constant of carrier translocation is increased by combination with choline some six- to sevenfold to about 3.5 min-1. The rate constant of ECMA attack on the carrier is about 440 M-1 sec-1.

Animals↗

PubMedDate not supplied

Changes in synaptosomal pH and rates of oxygen radical formation induced by chlordecone.

The resting pH of 7.14 +/- 0.02 within rat cortical synaptosomes is elevated in vitro by the insecticide chlordecone, in a dose-dependent manner. Chlordecone also reduces the rate of oxygen radical formation within synaptosomes. Both of these changes can also be demonstrated following in vivo treatment of rats with chlordecone (75 mg/kg body wt). Although chlordecone increases the permeability of the plasma membrane, the increase in pH observed is unlikely to be caused by this, since in vivo administration of chlordecone does not appreciably alter membrane order as evaluated by both a lipophilic probe, and a probe with an ionic segment. Another xenobiotic agent, methyl mercuric chloride, and a free radical generating system, an ascorbic acid-ferrous sulfate mixture, did not modulate synaptosomal pH, although membrane permeability was increased. Other evidence of the ability of synaptosomes to maintain homeostasis was the failure of mitochondrial inhibitors to significantly reduce pH. The drop in synaptosomal pH effected by amiloride, an inhibitor of Na+/H+ exchange, and the transient rise in pH caused by ammonium chloride further suggested that synaptosomes may be a good model in the study of the regulation of intracellular pH. The elevation of cytosolic pH, and depression of oxygen radical formation by chlordecone, may result from both the attenuation of respiratory metabolism and an impaired capacity of the plasma membrane to maintain ionic gradients.

Amiloride↗

PubMed1979

Choline and ethanolamine glycerophospholipid synthesis in isolated synaptosomes of rat brain.

Substantial activities of cholinephosphotransferase (EC 2.7.8.2) and ethanolaminephosphotransferase (EC 2.7.8.1) were found with lysed synaptosomes but not with intact synaptosomes isolated from adult rat brains. Synaptosomal and non-synaptosomal microsomal transferases were similar in kinetic properties. Substantial activities of synaptosomal transferases have not been described previously. Part of the glycerophospholipids in synaptosomal membranes may be synthesized in the nerve ending in addition to the glycerophospholipids supplied by axonal transport. The synthesis of the alkylacyl type of choline and ethanolamine glycerophospholipids was moderately inhibited by 1 mM ATP and 1 microM cyclic AMP. This synthesis was also inhibited by more than 50% by 1 mM norepinephrine and to a lesser extent by 5 mM hydroxytryptamine and 1 mM acetylcholine. Cyclic AMP may mediate the effects of biogenic amines. The relative synthesis of different glycerophospholipid classes and the relative proportion of alkylacyl type (plasmalogen precursors) and diacyl type of glycerophospholipids may be influenced by the levels of adenine nucleotides and/or biogenic amines. Elevated cyclic AMP levels will decrease the synthesis of plasmalogen precursors.

Adenine Nucleotides↗

PubMed1987

Binding ability of Clostridium botulinum neurotoxin to the synaptosome upon treatment of various kinds of the enzymes.

The binding ability of Cl. botulinum neurotoxin to synaptosomes upon treatment with various enzymes (neuraminidase, trypsin, and beta-bungarotoxin containing phospholipase A2 activity) was studied. When synaptosomes were treated with neuraminidase, their ability to bind toxin decreased; trypsin and beta-bungarotoxin had slightly week or no effect. The decrease in toxin-binding ability of synaptosomes was paralleled by a release of sialic acid from the synaptosomes by the neuraminidase treatment. The toxin-binding ability of synaptosomes treated with neuraminidase was lower than untreated ones at a high concentration of sodium chloride. The binding of the toxin to synaptosomes occurred at least at the two types of structural sites, one site which contained sialic acid, and other site which was sensitive to high ionic strength. It may be possible that another binding state except these is present at the synapse.

Animals↗

PubMed1989

Effect of toluene on rat synaptosomal phospholipid methylation and membrane fluidity.

This study investigated the effects of toluene (1 g/kg, 1 hr, i.p.) on rat synaptosomal phospholipid methylation (PLM), phospholipid composition, and membrane fluidity. Toluene significantly decreased basal PLM (35%) in studies using [3H]methionine [( 3H]Met) as the methyl donor; this was reflected by similar decreases in phosphatidylmonomethylethanolamine (PME) (30%). No effects were observed in either PLM reactions that used [3H]adenosylmethionine [( 3H]AdoMet) as methyl donor, or AdoMet synthetase, suggesting that toluene preferentially affects PLM reactions that derive methyl groups from [3H]Met. Also, toluene decreased synaptosomal phosphatidylethanolamine (PE) (24%), the initial substrate for PLM, and the addition of PE back to PE-depleted synaptosomes restored methyltransferase activity. Agonist-stimulated PLM using norepinephrine (NE) demonstrated that agonist-receptor coupling returned PLM to control values in synaptosomes from toluene-treated rats. NE-stimulated PLM was also blocked by propranolol (PRO), suggesting a role for toluene in receptor-mediated events. Membrane fluidity studies demonstrated that in vivo administration of toluene increased the outer synaptosomal membrane fluidity, whereas in vitro administration of toluene had no effect. Our observations support a positive relationship between increased PLM activity and increased outer, not core, membrane fluidity. These data demonstrate that specific toluene-phospholipid interactions occur in synaptosomes, resulting in altered membrane composition, function and fluidity.

Animals↗

PubMed1991

A comparative study of histamine and K+ effects on (Ca(2+)-Mg2+)-ATPase activity in synaptosomes.

Histamine (10(-4) M) and 60 mM K+, but not 60 mM Na+ or 60 mM choline+, increased the maximal synaptosomal (Ca(2+)-Mg2+)-ATPase activity by 15 and 36% respectively and decreased the extrasynaptosomal Ca2+ concentration necessary to reach it. Histamine and K+ enhanced the synaptosomal (Ca(2+)-Mg2+)-ATPase activity in a concentration-dependent manner. In synaptic plasma membranes histamine (10(-4) M) and 60 mM choline+ were not able to alter the enzymatic activity, however 60 mM K+ and 60 mM Na+ elevated (Ca(2+)-Mg2+)-ATPase activity by 20 and 15%, respectively, without altering the affinity for Ca2+. Histamine effects in synaptosomes were mediated by H2 receptor stimulation. 3-Isobutyl-1-methyl-xanthine (10(-4) M) potentiated (15%) the maximal histamine effect. The slow Ca2+ channel antagonists verapamil and diltiazem, both at 10(-6) M, completely inhibited K+ effects in synaptosomes, however histamine effects were only blocked by verapamil. The data suggest that K+ and histamine effects on synaptosomal (Ca(2+)-Mg2+)-ATPase activity are mediated by increases of intrasynaptosomal Ca2+ levels. Moreover, histamine effects on synaptosomal enzyme activity were mediated by cAMP.

1-Methyl-3-isobutylxanthine↗

PubMed1992

Prevention by Ginkgo biloba extract (EGb 761) and trolox C of the decrease in synaptosomal dopamine or serotonin uptake following incubation.

Prolonged incubation of synaptosomes in Krebs-Ringer oxygenated medium in the presence of ascorbic acid (10(-4) M) led, after 20 min, to a decrease in [3H]dopamine (DA) (synaptosomes prepared from the striatum) and [3H]serotonin (5HT) (synaptosomes prepared from the cortex) uptake. The decrease was progressive and uptake was virtually abolished after a 60 min incubation period. A concentration-dependent (from 5 x 10(-6) M) role of ascorbic acid in the decrease of [3H]DA or [3H]5HT uptake was demonstrated. This decrease was potentiated by Fe2+ ions and prevented by the ferrous chelating agent desferrioxamine. Thus, the progressive decrease in synaptosomal uptake of either [3H]DA or [3H]5HT could depend on the generation of free radicals by the association of ascorbic acid with Fe2+ ions. The decrease in synaptosomal uptake was prevented, in a concentration-dependent manner, by the Ginkgo biloba extract EGb 761 (4-16 micrograms/mL) and the vitamin E analog trolox C (10(-4) M). The terpenic fraction of EGb 761, Bn 52063 (up to 0.5 microgram/mL), did not prevent the reduction of [3H]amine uptake. In contrast, the flavonoidic fraction, Cp 202, was effective (from 1 microgram/mL) and its efficacy was shared by the flavonoid quercetin (from 0.1 microgram/mL). The prolongation of the ability of synaptosomes to take up [3H]amine elicited by EGb 761, in particular its flavonoidic fraction, as well as by trolox C could be due to their free radical scavenger properties.

Animals↗

PubMed1996

Chlorpromazine stimulatory effect on iron uptake by rat brain synaptosomes.

Clinical long-term neuroleptic administration induces extrapyramidal motor side-effects, of which tardive dyskinesia is the most important. Experimentally, dopamine D2 supersensitivity is observed after phenothiazine and butyrophenone treatment. Neuroleptic-induced tardive dyskinesia and D2 modulation have been linked to impaired iron homeostasis in the central nervous system. Increased nonheme iron levels found in the basal ganglia of patients with extrapyramidal symptomology support the connection between iron and neuronal dopaminergic modulation. We now report the effect of chlorpromazine on iron uptake by synaptosomes of rat brain from two different iron donors: [55Fe]citrate and [55Fe]transferrin. Iron uptake from both donors by cortical synaptosomes was stimulated by Ca2+ and enhanced by chlorpromazine in a saturable fashion. Synaptosomes from the striatum also showed increased (60%) iron uptake from [55Fe]citrate in the presence of chlorpromazine. Chlorpromazine stimulated iron uptake by cortical synaptosomes more efficiently than Ca2+, at physiological levels, from both [55Fe]transferrin (50%) and [55Fe]citrate (68%). Calcium potentiated the effect of chloropromazine upon cortical synaptosomal iron uptake from [55Fe]citrate, but had no apparent effect on the uptake from [55Fe]transferrin. Chlorpromazine-stimulated iron uptake from the latter was observed without addition of Ca2+. Moreover, fluorescence measurement of Ca2+ uptake by cortical synaptosomes showed intensified uptake in the presence of 50 microM chlorpromazine (42%). Visible spectral studies of chlorpromazine in the presence of Fe(3+)-citrate and diferric-transferrin did not reveal iron displacement by chlorpromazine from either of the two donors. These data suggest that chlorpromazine may increase iron uptake by neurons, and may be involved in the development of tardive dyskinesia and other extrapyramidal disorders.

Animals↗

PubMed1981

Internal K+ and Na+ concentrations and the volume of hypothalamic synaptosomes.

Hypothalamic synaptosomes, prepared by discontinuous sucrose density gradient centrifugation, were suspended in media of varying osmolarity and ionic composition and the internal concentrations of K+ and Na+ were measured. The intrasynaptosomal volume was determined using [14C]inulin, 35SO4 or [14C]sucrose as extracellular markers. When the synaptosomes were suspended in 0.32 M sucrose the distribution volume of [14C]inulin and 35SO4 were similar. However, when the medium contained 140 mM NaCl and 5 mM KCl the 35SO4 space was equal to the total water space while the distribution volume of [14C]inulin corresponded to 35-51% of the total water space. [14C]sucrose distributed in a larger volume than did [14C]inulin, presumably due to intracellular permeation of sucrose. Using inulin as an extracellular marker the synaptosome volume was found to be inversely proportionate to the tonicity of the medium. [14C]inulin was considered a suitable marker of the extrasynaptosomal space and was used when determining intrasynaptosomal K+ and Na+ concentrations. The internal concentration of K+ was considerably higher than the external indicating that the synaptosomes were able to retain K+ against a concentration gradient. The synaptosomes gained Na+ when transferred to media containing 140 mM NaCl. The internal concentrations of K+ and Na+ were unaffected by glucose and elevated temperature and was only moderately changed during 90 min of incubation. The equilibrium potentials for K+ and Na+ were -68 and 13 mV respectively when the medium consisted of 0.03 M sucrose containing 140 mM NaCl, 5 mM KCl. It is concluded that hypothalamic synaptosomes can maintain a high transmembrane concentration gradient for K+ whereas the membrane is rather easily penetrated by Na+.

Animals↗

PubMed1993

Flunarizine inhibits both calcium-dependent and -independent release of glutamate from synaptosomes and cultured neurones.

Flunarizine, an established Ca2+ channel antagonist, blocks both exocytotic glutamate release from mammalian cultured cerebellar granule cells and isolated presynaptic nerve endings (synaptosomes) prepared from two distinct areas of the mammalian brain. This blockade of release displays the same flunarizine concentration dependency in synaptosomes in the presence or absence of Ca2+, with total inhibition at a concentration of 10 microM. In cultured neurones, a selective effect on the L-channel-coupled component of the KCl-evoked rise in intracellular Ca2+, [Ca2+]c, can be demonstrated between flunarizine concentrations of 100 nM and 10 microM, while at concentrations above 10 microM, the remaining residual and transient components are affected. In synaptosomes, flunarizine blocks the KCl-evoked elevation in [Ca2+]c in a concentration-dependent manner. Additionally, 10 microM flunarizine directly antagonises ouabain-induced tetrodotoxin (TTX)-sensitive Na+ influx, glutamate, aspartate and GABA release from synaptosomes, whilst inhibiting veratridine-induced Ca(2+)-independent TTX-sensitive Na+ influx and glutamate release at 15 microM and 10 microM in cells and synaptosomes, respectively. In both cultured neurones and synaptosomes, the ability of flunarizine to block both neurotransmitter and cytoplasmic glutamate release is due to a direct antagonism of both voltage dependent Ca2+ channels and tetrodotoxin-sensitive Na+ channels.

Animals↗

PubMed1979

Presynaptic noradrenergic alpha-receptors and modulation of 3H-noradrenaline release from rat brain synaptosomes.

The depolarization (15 mM K+)-induced release of 3H-NA from superfused rat brain synaptosomes and the effects of alpha-noradrenergic drugs thereon were studied. Noradrenaline (NA; in the presence of the uptake inhibitor desipramine) reduced synaptosomal 3H-NA release. Reduction of the concentration of calcium ions in the medium during K+ stimulation greatly enhanced the sensitivity of 3H-NA release to alpha-receptor-mediated inhibition. Under these conditions NA dose-dependently inhibited 3H-NA release from synaptosomes obtained from cortex or hypothalamus, but did not affect 3H-NA release from striatal (i.e dopaminergic) synaptosomes. Adrenaline, clonidine and oxymetazoline potently inhibited 3H-NA release from cortex synaptosomes at concentrations in the nanomolar range. Phentolamine by itself did not affect synaptosomal 3H-NA release, but antagonized the inhibitory effects of both noradrenaline and adrenaline. The data obtained further substantiate the hypothesis that the alpha-receptors mediating a local negative feedback control of NA release are localized on the varicosities of central noradrenergic neurons, Furthermore, noradrenergic nerve terminals in the hypothalamus appear to be less senstive to alpha-receptor-mediated presynaptic inhibition than those in the cortex.

Adrenergic alpha-Agonists↗

PubMed1993

Evidence of an exocytotic-like release of [3H]5-hydroxytryptamine induced by d-fenfluramine in rat hippocampal synaptosomes.

The monoamine releasing activity of d-fenfluramine was investigated with an in vitro model consisting of synaptosomes preloaded with the 3H-neurotransmitter and extensively washed in a superfusion apparatus before a 3-min exposure to d-fenfluramine. With this model, the drug-induced release is real and is not confused by inhibition of reuptake by the drug. d-Fenfluramine (0.5 microM) induced only [3H]5-hydroxytryptamine ([3H]5-HT) release from hippocampal synaptosomes whereas 10 microM also induced some overflow from hippocampal synaptosomes preloaded with [3H]noradrenaline or from striatal synaptosomes preloaded with [3H]dopamine, although the overflow was much lower than from 5-HTergic synaptosomes. We then focused on the [3H]5-HT release induced by 0.5 microM d-fenfluramine, which was previously shown to be Ca2+ dependent. The same finding was confirmed in the present study with other experimental protocols, indicating the requirement for extracellular Ca2+ ions. By measuring [3H]5-HT uptake into rat hippocampal synaptosomes we confirmed that Ca(2+)-ions are not required for the function of the 5-HT uptake carrier or for its interaction with d-fenfluramine. d-Fenfluramine-induced [3H]5-HT release was not altered by 1 microM nitrendipine (blocking the L-type Ca2+ channels) but was slightly decreased (20%) by 0.5 microM omega-conotoxin (blocking the N-type Ca2+ channels). It was also inhibited by 0.5 microM clonidine, interacting with alpha 2-adrenergic heteroreceptors, and by 10 nM tetanus toxin, known to affect the exocytosis of different neurotransmitters including 5-HT. These compounds had very similar effects on the Ca(2+)-dependent, exocytotic release of [3H]5-HT induced by depolarization, i.e. by 15 mM K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1983

Subcellular fractionation of striatum: sedimentation properties of dopaminergic synaptosomes.

Linear sucrose density gradient centrifugation of a crude synaptosomal-mitochondrial preparation of rat striatum was performed at 82,500g for 7.5, 15 and 30 min and 1, 4 and 20 h. After centrifugation various marker enzyme activities were measured throughout the gradients, viz. tyrosine hydroxylase (TH) and DOPA decarboxylase (DD) as markers of dopaminergic synaptosomes, lactate dehydrogenase (LDH) as a general synaptosomal marker and monoamine oxidase (MAO) as a mitochondrial marker. At all centrifugation times the distribution patterns of TH and DD activity coincided almost perfectly. Notable differences were found between the sedimentation properties of these TH/DD-containing particles and LDH-containing particles: TH and DD were symmetrically distributed in the gradient much sooner than LDH, at all centrifugation times the top of the TH and DD curves was lying deeper in the gradient than the highest LDH activity, and TH and DD became enriched in the gradients to a much greater extent than LDH. It is concluded that rat striatal dopaminergic synaptosomes form a relatively homogeneous population of particles sedimenting faster into the gradients than the bulk of striatal synaptosomes does. This distinct sedimentation behaviour of the dopaminergic synaptosomes can be usefully applied for analytical purposes.

Animals↗

PubMed1985

Changes induced by corticosterone and adrenalectomy in synaptosomal and platelet uptake and binding of 5-HT. The relationship to [3H]imipramine binding.

A comparative study of the effect of adrenalectomy, treatment with corticosterone, and imipramine on platelet an synaptosomal uptake was undertaken. One day after adrenalectomy, uptake of 5-hydroxytryptamine (5-HT) in the platelet and the hypothalamus was significantly decreased with an increase in the apparent Km, but the uptake of 5-HT of the cerebral synaptosomes was unchanged. In this group, the Kd of low affinity binding of 5-HT was also increased in the hypothalamic synaptosomes and in the platelet preparations. Treatment with corticosterone (5 mg/kg, i.m.) restored the decrease in the uptake of 5-HT induced by adrenalectomy in the hypothalamic synaptosomes and in the platelets, and significantly increased the uptake of 5-HT of these fractions in sham-operated rats. The binding of 5-HT was unchanged by acute treatment with corticosterone. The effectiveness of imipramine varied with the preparation and treatment group. The IC50 of the cerebral synaptosomal preparation was greater than that of the hypothalamic synaptosomal preparation. Using this latter preparation, the IC50 for imipramine in adrenalectomised, sham-operated and corticosterone-treated rats were found to be 0.04, 0.09 and 0.25 microM, respectively. These changes in sensitivity to imipramine were not reflected in the binding of [3H]imipramine which was unchanged.

Adrenalectomy↗

PubMed1986

Effects of beta-bungarotoxin and Naja naja atra snake venom phospholipase A2 on acetylcholine release and choline uptake in synaptosomes.

Beta-bungarotoxin is a potent presynaptically acting snake venom toxin that exhibits phospholipase A2 activity. We compared the effects of beta-bungarotoxin and a less toxic snake venom phospholipase A2 on synaptosomal 3H-acetylcholine release and 3H-choline uptake. The purpose of these experiments was to study the mode by which beta-bungarotoxin inhibits 3H-acetylcholine release in this preparation. Under non-depolarizing conditions, both beta-bungarotoxin and Naja naja atra phospholipase A2 stimulated 3H-acetylcholine release from a synaptosomal fraction preloaded with 3H-choline. Beta-bungarotoxin was more potent, but less efficacious, than N. naja atra phospholipase A2. In contrast, both toxins inhibited 3H-acetylcholine release from the synaptosomal fraction incubated with 3H-choline after toxin exposure. In agreement with the results obtained by monitoring acetylcholine release, beta-bungarotoxin and N. naja atra phospholipase A2 appeared to block 3H-choline uptake into the synaptosomal fraction non-competitively. Although the toxins may cause the release of unlabeled choline from synaptosomes, the block of labeled choline uptake could not be explained by decreased specific activity of 3H-choline in the bathing medium. Therefore, beta-bungarotoxin and N. naja atra phospholipase A2 block 3H-acetylcholine release from synaptosomes indirectly by inhibiting the uptake of 3H-choline necessary for 3H-acetylcholine synthesis. In comparing these results using 3H-choline to those in the literature obtained with deuterated choline, there appears to be a difference in apparent toxin action that relates to the type of label (3H or 2H) attached to choline.

Acetylcholine↗

PubMed1992

Purification and characterization of iotrochotin, a novel toxin from the Caribbean sponge Iotrochota birotulata, which selectively permeabilizes synaptosomes.

A protein termed iotrochotin (IOT) was isolated from the exudate of the Caribbean sponge Iotrochota birotulata using as an assay its stimulation of the release of radioactivity from synaptosomes preloaded with [3H]choline. Sephadex G-50 chromatography of the exudate produced one peak, with a mol. wt of approximately 18,000, which was further resolved into two active fractions by anion exchange chromatography. The more tightly bound of the two fractions was characterized and referred to as IOT. The action of IOT was essentially complete by 0.5-1.0 min and was independent of the Ca2+ or Na+ content of the incubation mixture. Released radioactivity included about 50% each of [3H]acetylcholine and [3H]choline. Release of radioactivity increased as a function of IOT concentration and then reached a maximum. Extrapolated asymptotic release was nearly equal to that obtained by lysing the synaptosomes. IOT also released radioactivity from synaptosomes which had been preincubated with other tritiated neurotransmitters or with 2-[3H]deoxy-D-glucose. Lactate dehydrogenase and choline acetyltransferase activities were not released from synaptosomes by treatment with IOT, but were released by digitonin. IOT therefore releases some of the smaller molecular weight components of synaptosomes, but does not permeabilize the synaptosomal membrane in the same way as digitonin treatment.

Animals↗

PubMed1995

Depletion of cytosolic GSH decreases the ATP levels and viability of synaptosomes from aged mice but not from young mice.

The effect of glutathione depletion on the viability of freshly isolated synaptosomes from whole brain was investigated in young and aged mice. Aging did not influence the GSH and ATP levels and the viability of these synaptosomes. However depletion of glutathione caused by the cytosolic glutathione inhibitor diethyl maleate (1 mM) resulted in a significant decline, after 60 min of incubation, in ATP levels and viability in the synaptosomes from aged mice but not in those from young mice. When synaptosomes were incubated in the presence of the mitochondrial glutathione inhibitor ethacrynic acid (0.2 mM) there was a similar decline in glutathione, ATP levels and synaptosomal viability, both in young and aged mice. These results emphasize the relative importance of the cytosolic glutathione pool for the maintenance of the plasma membrane integrity in synaptosomes from aged mice.

Adenosine Triphosphate↗

PubMed1984

Changes in free-calcium levels and pH in synaptosomes during transmitter release.

The free cytoplasmic Ca2+ concentration [( Ca2+]i) in rat brain synaptosomes estimated by the indicator quin 2 is 104 +/- 8 nM (S.D.) in artificial cerebrospinal fluid (1.2 mM Ca2+), but decreases at lower Ca2+ concentrations in the medium. The presence of quin 2 in the synaptosomes does not affect either the spontaneous release of transmitter (gamma-aminobutyric acid) or the release induced by K+ depolarisation. In quin 2-loaded synaptosomes, depolarisation by K+ causes an abrupt increase in [Ca]i (less than 2-fold) that is approximately proportional to the extent of depolarisation, whereas depolarisation by veratrine alkaloids produces a slow rise in [Ca]i. The increase in [Ca]i produced by K+ depolarisation does not occur in the absence of Ca2+ in the medium. The data are consistent with a direct correlation between [Cai] and transmitter release in functional synaptosomes. The pH in synaptosomes estimated by the indicator quene 1 is 7.04 +/- 0.07 and is stable in media containing 5 mM bicarbonate. The pH in synaptosomes was decreased by protoveratrine but not by K+ depolarisation.

Aminoquinolines↗