Inhibition of glutamate release by arachidonic acid in rat cerebrocortical synaptosomes

The role of CB1 receptors in psychostimulant addiction

Recent studies have implicated the endocannabinoid (eCB) system in the neuronal mechanisms underlying substance dependence. Here, we review results of studies using cannabinoid receptor subtype 1 (CB1) knockout mice as well as CB1 antagonists to elucidate the role of this neurotransmitter system in psychostimulant addiction. The overall picture is that CB1 receptors appear not to be involved in psychostimulant reward, nor in the development of dependence to such substances. In contrast, the eCB system appears to play a role in the persistence of psychostimulant addiction. In particular, CB1 receptors have been found to play a cardinal role in mediating reinstatement of previously extinguished drug-seeking behavior upon re-exposure to the drug or drug-associated cues. The anatomical loci as well as the neuronal mechanisms of the relapse-preventing effects of CB1 antagonists are still poorly understood, although interactions of the eCB system with afferent glutamatergic and possibly dopaminergic projections to the nucleus accumbens are most likely involved. In addition, CB1 receptors seem to modulate drug-related memories, in line with the hypothesized role of the eCB system in memory-related plasticity. Together, these findings suggest that modulators of the eCB system represent a promising novel type of therapy to treat drug addiction.

PKC-independent inhibition of glutamate exocytosis by arachidonic acid in rat cerebrocortical synaptosomes

In rat cerebrocortical synaptosomes, the addition of 4 beta-phorbol dibutyrate (4 beta-PDBu) and arachidonic acid enhances and decreases, respectively, the glutamate release evoked by 4-aminopyridine. Pretreatment of synaptosomes with 12-O-tetradecanoylphorbol 13-acetate (TPA) or pre-incubation with staurosporine, prevent the stimulatory effect of 4 beta-PDBu, but are without effect on the inhibitory action of arachidonic acid. Moreover, methyl arachidonate, which is not effective as a PKC activator, also strongly inhibits glutamate exocytosis. These results suggest that PKC is not involved in the inhibition of glutamate release by arachidonic acid.

Arachidonic acid inhibits uptake of amino acids and potentiates PKC effects on glutamate, but not GABA, exocytosis in isolated hippocampal nerve terminals

Arachidonic acid (AA), the putative retrograde messenger in long-term potentiation, enhanced extracellular aspartate, glutamate, and GABA levels in rat hippocampus synaptosomes. Whether this effect was determined by stimulating the release and/or inhibiting the uptake of amino acids was further investigated using different experimental conditions. To approach physiological conditions, a static incubation assay was used where both release and uptake occur. Under these conditions, AA dose-dependently (10-25 microM) enhanced basal extracellular amino acid levels in a completely Ca2+-independent way. AA still exerted this effect in the presence of inhibitors of PKC or of AA metabolism. When using the superfusion release assay, in which amino acid uptake cannot occur, no potentiating effect of AA on superfusate amino acid levels was observed. Therefore, AA possibly enhances the extracellular levels of aspartate, glutamate and GABA by inhibiting the uptake of these amino acids and not their efflux. Indeed, AA reduced the Na+-dependent uptake of endogenously released amino acids, which were labelled with traces of tritiated D-aspartate and GABA. When stimulating hippocampus synaptosomes with 4-aminopyridine, AA (2 microM) potentiated the Ca2+-dependent release of glutamate, but not of GABA, synergistically with PKC activation by 4beta-phorbol-12,13-dibutyric acid. In rat hippocampus, AA exerts different presynaptic effects to regulate extracellular amino acid levels, by inhibiting carrier-mediated uptake and, for glutamate, by stimulating exocytosis.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Arachidonic acid regulation of vasopressin release and intracellular Ca2+ in neurohypophysial nerve endings

The effects of arachidonic acid (AA) and arachidonic acid metabolites on vasopressin secretion and on intracellular free calcium concentration ([Ca2+]i) from both intact and streptolysin-O permeabilized isolated nerve endings of the rat neurohypophysis were studied. Arachidonic acid induced a dose-dependent increase in resting vasopressin (AVP) secretion in both intact and streptolysin-O permeabilized nerve endings. Although AA also dose-dependently induced an increase in [Ca2+]i in intact nerve endings, the AA-induced secretory response was largely independent of an increase in [Ca2+]i. Secretory responses in intact nerve endings showed AA-induced secretion to be sustained and that AA-induced vasopressin secretion occurs via exocytosis. Arachidonic acid also dose-dependently potentiated K+-depolarization evoked vasopressin release. The potentiation of secretion occurred despite an AA-induced reduction in K+-evoked Ca2+ influx. In addition, AA reinitiated secretion following a decline in the Ca2+-dependent exocytotic secretory response suggesting a separate secretory mechanism from Ca2+-induced secretion. Inhibition of the metabolic pathways for AA suggested that AA itself mediates the secretory effects and that AA is likely subject to rapid metabolism by lipoxygenases.

cAMP-dependent facilitation of glutamate release by beta-adrenergic receptors in cerebrocortical nerve terminals

We have investigated the presence of a cAMP-protein kinase A-dependent pathway in cerebrocortical nerve terminals and its role in the modulation of glutamate release. The activation of adenylyl cyclase with forskolin enhances intrasynaptosomal cAMP and induces Ca2+-dependent glutamate release. The membrane permeant analogue dibutyryl cAMP mimics this facilitatory effect, whereas the inactive compound 1,9-dideoxyforskolin is without effect. This cAMP-induced facilitation is consistent with the induction of spontaneous action potentials that are abolished by the Na+ channel blocker tetrodotoxin and by reducing nerve terminal excitability with arachidonic acid. We have also demonstrated that a beta-adrenergic receptor is linked to this pathway because isoproterenol increases cAMP levels and glutamate release, and both actions are antagonized by the receptor antagonist propanolol and the protein kinase A inhibitors H89 and 8-chloroadenosine 3',5'-monophosphorothioate ((Rp)-isomer). The finding that the increase in cytoplasmic free Ca2+ concentration induced by synaptic activity reduces the concentration of agonist required to maximally activate adenylyl cyclase suggests that this enzyme may act as a coincidence detector, integrating glutamatergic neurotransmission and noradrenaline release.

Evidence for a selective and electroneutral K+/H(+)-exchange in Saccharomyces cerevisiae using plasma membrane vesicles

The existence of a K+/H+ transport system in plasma membrane vesicles from Saccharomyces cerevisiae is demonstrated using fluorimetric monitoring of proton fluxes across vesicles (ACMA fluorescence quenching). Plasma membrane vesicles used for this study were obtained by a purification/reconstitution protocol based on differential and discontinuous sucrose gradient centrifugations followed by an octylglucoside dilution/gel filtration procedure. This method produces a high percentage of tightly-sealed inside-out plasma membrane vesicles. In these vesicles, the K+/H+ transport system, which is able to catalyse both K+ influx and efflux, is mainly driven by the K+ transmembrane gradient and can function even if the plasma membrane H(+)-ATPase is not active. Using the anionic oxonol VI and the cationic DISC2(5) probes, it was shown that a membrane potential is not created during K+ fluxes. Such a dye response argues for the presence of a K+/H+ exchange system in S. cerevisiae plasma membrane and established the non-electrogenic character of the transport. The maximal rate of exchange is obtained at pH 6.8. This reversible transport system presents a high selectivity for K+ among other monovalent cations and a higher affinity for the K+ influx into the vesicles (exit from cells). The possible role of this K+/H+ exchange system in regulation of internal potassium concentration in S. cerevisiae is discussed.

Mechanisms of glutamate and aspartate release in the ischemic rat cerebral cortex

Elevated levels of glutamate and aspartate have been implicated in the pathogenesis of neural injury and death induced by ischemia. The mechanism(s) whereby they escape into the extracellular environment have been a subject of controversy. This study evaluated the contribution of phospholipases and protein kinases to ischemia-evoked glutamate and aspartate release from the ischemic/reperfused rat cerebral cortex. Changes in the extracellular levels of these amino acids during four-vessel occlusion elicited global cerebral ischemia were examined using a cortical cup technique. Ischemia-evoked amino acid release was compared in control vs. drug treated animals, in which selective inhibitors of phospholipases and protein kinases were applied topically onto the cerebral cortex. The phospholipase inhibitors tested included 4-bromophenacyl bromide, a non-selective inhibitor; 7,7-dimethyleicosadienoic (DEDA), an inhibitor of secretory type phospholipase A2 (PLA2); AACOCF3, an inhibitor of the Ca2(+)-dependent cytoplasmic form of PLA2, HELSS, which inhibits a Ca(2+)-independent cytoplasmic PLA2, and U73122, a selective inhibitor of phospholipase C (PLC). All five phospholipase inhibitors significantly attenuated glutamate and aspartate release into the extracellular milieu, indicating the possibility that several forms of the enzyme are likely to be involved. The protein kinase C (PKC) inhibitor, chelerythrine chloride, also reduced excitatory amino acid efflux, wheres the PKC activator phorbol 12-myristate 13-acetate (PMA) enhanced their release. The non-selective kinase inhibitor, staurosporine, and H-89, which selectively inhibits protein kinase A, did not reduce ischemia-evoked amino acid efflux. These results suggest that ischemia-evoked release of the excitatory transmitters amino acids is a result, in part, of the activation of phospholipases A2 and C, with PKC involvement in the transduction process. Destabilization and deterioration of the plasma membrane, as a consequence of phospholipid hydrolysis, may allow these transmitter amino acids to diffuse down their concentration gradients into the extracellular fluid.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Co-existence and interaction between facilitatory and inhibitory metabotropic glutamate receptors and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals

We have investigated the interaction between facilitatory and inhibitory metabotropic glutamate receptors (mGluRs) and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals from young (3 weeks postnatal) rats. The adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) (1 microM) and the mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) (100 microM) inhibited Ca(2+)-dependent release of glutamate evoked by depolarization of synaptosomes with 30 mM KCl to 33 +/- 6 and 30 +/- 4% of control values, respectively. The CHA and L-AP4 inhibition of release was consistent with the reduction of a component of Ca2+ entry in nerve terminals which was also sensitive to omega-Aga-IVA. When the inhibitory agonists were co-applied at optimal concentrations, no additivity of the inhibitory effects on either glutamate release or [Ca2+]c was observed. The nerve terminals from young rats also exhibit the facilitatory pathway for glutamate release that is observed during 4-aminopyridine-evoked depolarization after stimulation of mGluRs with the agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) in the presence of arachidonic acid (AA). The addition of ACPD or AA alone did not alter the ability of CHA and L-AP4 to reduce the release, however the co-application of AA and ACPD abolished the inhibitory effect induced by CHA and L-AP4 whether alone or in combination. These results indicate the co-existence of the three modulatory pathways of glutamate release and the dominant role of the ACPD/AA activated facilitatory pathway in its interaction with the inhibitory pathways activated by L-AP4 and CHA.

Arachidonic acid as a neurotoxic and neurotrophic substance

In this article we summarize a wide variety of properties of arachidonic acid (AA) in the mammalian nervous system especially in the brain. AA serves as a biologically-active signaling molecule as well as an important component of membrane lipids. Esterified AA is liberated from the membrane by phospholipase activity which is stimulated by various signals such as neurotransmitter-mediated rise in intracellular Ca2+. AA exerts many biological actions which include modulation of the activities of protein kinases and ion channels, inhibition of neurotransmitter uptake, and enhancement of synaptic transmission. AA serves also as a precursor of a variety of eicosanoids, which are formed by oxidative metabolism of AA. AA cascade is activated under several pathological conditions in the brain such as ischemia and seizures, and may be involved in irreversible tissue damage. On the other hand, AA can show beneficial influences on brain tissues and cells in several situations. In a recent study using cultured brain neurons, we have found that AA shows quite distinct actions at a narrow concentration range, such as induction of cell death, promotion of cell survival and enhancement of neurite extension. The neurotoxic action is mediated by free radicals generated by AA metabolism, whereas the neurotrophic actions are exerted by AA itself. The observed in vitro actions of AA might be related to important roles of AA in brain pathogenesis and neural development.

NMDA and carbachol but not AMPA affect differently the release of [3H]GABA in striosome- and matrix-enriched areas of the rat striatum

The effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 10(-3) M), N-methyl-D-aspartate (10(-3) M, in the absence of magnesium or presence of AMPA) and carbachol (10(-3) M) on the release of preloaded [3H]gamma-aminobutyric acid ([3H]GABA) from microdiscs of tissue punched out from sagittal brain slices in striosome- or matrix-enriched areas of the rat striatum have been compared. Although AMPA stimulated similarly the release of [3H]GABA in both striatal compartments, the release of [3H]GABA evoked by either N-methyl-D-aspartate (in the presence of AMPA) or carbachol was more pronounced in matrix- than in striosome-enriched areas. AMPA- and N-methyl-D-aspartate- (in the absence of magnesium) evoked responses were reduced but not abolished in the presence of tetrodotoxin (10(-6) M) in both compartments while the carbachol-evoked release of [3H]GABA was decreased by tetrodotoxin only in the matrix. The interruption of cholinergic transmission by the combined application of atropine (10(-5) M) and pempidine (10(-4) M) was without effect on the AMPA-evoked release of [3H]GABA, but it reduced the N-methyl-D-aspartate- (in the absence of magnesium or presence of AMPA) evoked release of [3H]GABA in both compartments, these reductions being of similar amplitude than those observed with tetrodotoxin.

Effect of arachidonic acid on [3H]D-aspartate outflow in the rat hippocampus

The aim of this study was to investigate the effect of arachidonic acid on [3H]d-aspartate outflow in rat hippocampus synaptosomes and slices. Arachidonic acid 1) increased basal outflow of [3H]d-aspartate in both synaptosomes and slices, and 2) increased K(+)-evoked overflow in slices but not in synaptosomes. The latter effect was dependent (at least in part) on arachidonic acid metabolism, most likely mediated by lipo-oxygenase metabolites and free radical production. It was prevented by nordihydroguairetic acid but not by indomethacin, and was significantly reduced by free radical scavengers (superoxide-dismutase and catalase). This effect was dependent upon stimulation since it could not be observed after a continuous perfusion of arachidonic acid in the absence of stimulation. Furthermore, it was long-lasting since a 30 min perfusion of arachidonic acid was sufficient to exert a significant effect on a stimulation following termination of the application.

Arachidonic acid inhibits sodium currents and synaptic transmission in cultured striatal neurons

In the striatum, dopamine generates arachidonic acid (AA) and induces synaptic depression. Here, we report that Na+ channels are a target for AA in both cultured and acutely isolated striatal neurons. AA depressed veratrine-induced Na+ influx and neurotransmitter release. Whole-cell voltage clamp revealed that peak Na+ currents are depressed, and steady-state inactivation shifts -15 mV in the presence of AA. Furthermore, inactivation was accelerated, and recovery from inactivation was delayed. These actions of AA were not produced by AA metabolites or protein kinase C activation. In addition, AA reduced both the amplitude and frequency of action potentials and depressed spontaneous inhibitory postsynaptic currents without affecting miniature inhibitory postsynaptic currents. These data suggest that AA modulates presynaptic, Na(+)-dependent action potentials, thereby contributing to striatal synaptic depression.

Effect of inhibitors of eicosanoid metabolism on release of [3H]noradrenaline from the human neuroblastoma, SH-SY5Y

Nordihydroguaiaretic acid (NDGA; a lipoxygenase inhibitor), LY-270766 (an inhibitor of 5-lipoxygenase), and the diacylglycerol lipase inhibitor RG 80267 completely eliminated potassium-evoked release of [3H]-noradrenaline ([3H]NA) from the human neuroblastoma clone SH-SY5Y with IC50 values of 10, 15, and 30 microM, respectively. In contrast, these inhibitors only partially inhibited carbachol-evoked release and had little effect on the calcium ionophore A23187-evoked release of NA in this cell line. Arachidonic acid partially inhibited potassium- and A23187-evoked release but did not reverse the inhibition of potassium-evoked release observed in the presence of RG 80267. These studies suggest that arachidonic acid (or its lipoxygenase products) are not important intermediates in the regulation of exocytosis in SH-SY5Y. This conclusion is strengthened by our studies in which SH-SY5Y cells were grown in medium supplemented with bovine serum albumin-linoleic acid (50 microM). Under these conditions there was a selective increase in content of membrane polyunsaturated fatty acids of the omega 6 series, including arachidonic acid; however, these changes did not effect potassium-, veratridine-, carbachol-, or calcium ionophore-evoked release of [3H]NA.

Arachidonic acid and other unsaturated fatty acids alter membrane potential in PC12 and bovine adrenal chromaffin cells

The action of arachidonic acid and other fatty acids on membrane potential in PC12 and bovine chromaffin cells was investigated using a membrane potential-sensitive fluorescent dye. Arachidonic acid (1-40 microM) provoked dose-dependent membrane hyperpolarization, thereby reducing hyperpolarization induced by the K(+)-selective ionophore valinomycin. Other cis-unsaturated fatty acids, but not lipoxygenase products or the saturated fatty acid palmitic acid, also affected membrane potential. Tetraethylammonium blocked the arachidonic acid-induced hyperpolarization. These data suggest that cis-unsaturated fatty acids alter membrane potential in PC12 and bovine chromaffin cells by modulating K+ conductances. Valinomycin-generated hyperpolarization had no effect on agonist-induced Ca2+ influx into bovine chromaffin cells, whereas preincubation with arachidonic acid and other cis-unsaturated fatty acids blocked Ca2+ influx and secretion. We propose a model where internally generated fatty acids act as a feedback to desensitize the stimulated cell via inhibition of receptor-dependent Ca2+ influx and induction of membrane hyperpolarization.

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.

Activation of Protein Kinase C by Phorbol Esters and Arachidonic Acid Required for the Optimal Potentiation of Glutamate Exocytosis

The effects of arachidonic acid and phorbol esters in the Ca(2+)-dependent release of glutamate evoked by 4-aminopyridine (4-AP) in rat cerebrocortical synaptosomes were studied. In the absence of arachidonic acid, high concentrations (500 nM) of 4 beta-phorbol dibutyrate (4 beta-PDBu) were required to enhance the release of glutamate. However, in the presence of arachidonic acid, low concentrations of 4 beta-PDBu (1-50 nM) were effective in potentiating glutamate exocytosis. This potentiation of glutamate release by phorbol esters was not observed with the methyl ester of arachidonic acid, which does not activate protein kinase C. Moreover, pretreatment of synaptosomes with the protein kinase inhibitor staurosporine also prevented the stimulatory effect by arachidonic acid and phorbol esters. These results suggest that the activation of protein kinase C by both arachidonic acid and phorbol esters may play a role in the potentiation of glutamate exocytosis.

High sensitivity of glutamate uptake to extracellular free arachidonic acid levels in rat cortical synaptosomes and astrocytes

By using both synaptosomes and cultured astrocytes from rat cerebral cortex, we have investigated the inhibitory action of arachidonic acid on the high-affinity glutamate uptake systems, focusing on the possible physiological significance of this mechanism. Application of arachidonic acid (1-100 microM) to either preparation leads to fast (within 30 s) and largely reversible reduction in the uptake rate. When either melittin (0.2-1 microgram/ml), a phospholipase A2 activator, or thimerosal (50-200 microM), which inhibits fatty acid reacylation in phospholipids, is applied to astrocytes, both an enhancement in extracellular free arachidonate and a reduction in glutamate uptake are seen. The two effects display similar dose dependency and time course. In particular, 10% uptake inhibition correlates with 30% elevation in free arachidonate, whereas inhibition greater than or equal to 60% is paralleled by threefold stimulation of arachidonate release. In the presence of albumin (1-10 mg/ml), a free fatty acid-binding protein, inhibition by either melittin, thimerosal, or arachidonic acid is prevented and an enhancement of glutamate uptake above the control levels is observed. Our data show that neuronal and glial glutamate transport systems are highly sensitive to changes in extracellular free arachidonate levels and suggest that uptake inhibition may be a relevant mechanism in the action of arachidonic acid at glutamatergic synapses.

Advances in the use of the fluorescent probe fura-2 for the estimation of intrasynaptosomal calcium

Fura-2 has been used to measure intracellular Ca2+ with great success in a variety of cell and subcellular preparations, including synaptosomes. There is, however, a great deal of variability in the reported estimates of resting intrasynaptosomal Ca2+ ([Ca2+]i). Fura-2 AM is highly lipophilic and passes readily across the plasma membrane into the cytoplasm, where it is de-esterified and trapped. The lipophilicity of fura-2, however, promotes the formation of micelles in aqueous media, which may impede the passage of the probe across cell membranes. Our results suggest that some of the variability in the reported [Ca2+]i estimates may be related to fura-2 de-esterification and loading efficiencies. The use of the nonionic detergent pluronic F-127 is recommended to prevent the formation of fura-2 micelles. The use of a detergent is not always an acceptable practice, however, especially in studies in which detergent-lipid interactions may influence membrane parameters. We found that fatty acid free bovine serum albumin (BSA) (0.25%) greatly increases the intrasynaptosomal concentration of the probe, resulting in a significant increase in the signal-to-noise (S/N) ratio. The mechanism appears to be independent of effects of BSA on synaptosomal integrity and directly related to the prevention of fura-2 micelle formation, as evidenced by light spectroscopic scattering measurements. Thus, BSA appears to keep the probe in a form that crosses the synaptic plasma membrane more readily. The effectiveness of BSA in improving the loading of fura-2 into synaptosomes was comparable to the detergent pluronic F-127, making it possible to measure [Ca2+]i without compromising membrane integrity.