The bioenergetics of neurotransmitter release

Evidence for a role of presynaptic AMPA receptors in the control of neuronal glutamate release in the rat forebrain

The role of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in controlling the neuronal release of excitatory amino acids has been investigated. Stimulation of presynaptic AMPA receptors by the endogenous agonist L-glutamate, or by (R,S)-AMPA, dose-dependently enhanced the Ca(2+)-dependent, tetrodotoxin-insensitive, electrically-stimulated release of [3H]D-aspartate from rat forebrain slices. This AMPA receptor-mediated response showed marked stereoselectivity with the activity residing solely in the (S)-isomer. (R)-AMPA was inactive in this respect. AMPA-evoked responses were significantly enhanced in the presence of the AMPA receptor desensitization inhibitor, cyclothiazide (10 microM). Moreover, responses to both AMPA and glutamate were inhibited by competitive (NBQX) and non-competitive (GYKI 52466) AMPA receptor-selective antagonists in a dose-dependent manner. These results provide strong support for the existence of presynaptic AMPA receptors acting to enhance the synaptic release of excitatory amino acids in the mammalian forebrain. Such a positive feedback system may play an important functional role in physiological (e.g., long-term potentiation) and/or pathological (e.g., epileptogenesis) processes in the mammalian central nervous system. AMPA-type autoreceptors may provide new targets for drug action.

A protein factor that inhibits ATP-dependent glutamate and gamma-aminobutyric acid accumulation into synaptic vesicles: purification and initial characterization

Glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, is transported into and stored in synaptic vesicles. We have purified to apparent homogeneity a protein from brain cytosol that inhibits glutamate and gamma-aminobutyric acid uptake into synaptic vesicles and have termed this protein "inhibitory protein factor" (IPF). IPF refers to three distinct proteins with relative molecular weights of 138,000 (IPF alpha), 135,000 (IPF beta), and 132,000 (IPF gamma), respectively. Gel filtration and sedimentation data suggest that all three proteins share an elongated structure, identical Stokes radius (60 A), and identical sedimentation coefficient (4.3 S). Using these values and a partial specific volume of 0.716 ml/g, we determined the native molecular weight for IPF alpha to be 103,000. Partial sequence analysis shows that IPF alpha is derived from alpha fodrin, a protein implicated in several diverse cellular activities. IPF alpha inhibits ATP-dependent glutamate uptake into purified synaptic vesicles with an IC50 of approximately 26 nM, while showing no ability to inhibit ATP-independent uptake at concentrations up to 100 nM. Moreover, IPF alpha inhibited neither norepinephrine uptake into chromaffin vesicles nor Na+-dependent glutamate uptake into synaptosomes. However, IPF alpha inhibited uptake of gamma-aminobutyric acid into synaptic vesicles derived from spinal cord, suggesting that inhibition may not be limited to glutamatergic systems. We propose that IPF could be a novel component of a presynaptic regulatory system. Such a system might modulate neurotransmitter accumulation into synaptic vesicles and thus regulate the overall efficacy of neurotransmission.

Localization of voltage-sensitive Ca2+ fluxes and neuropeptide Y immunoreactivity to varicosities in SH-SY5Y human neuroblastoma cells differentiated by treatment with the protein kinase inhibitor staurosporine

The distribution of voltage-sensitive elevations of the level of Ca2+ in untreated SH-SY5Y cells and cells that had been induced to differentiate with staurosporine was investigated by monitoring fura-2 fluorescence in cell suspensions, and by using microfluorometry and quantitative fluorescence imaging on cell bodies and on cellular processes. Cell bodies of both types of cells displayed small Ca2+ elevations, which were composed of transient and sustained components. Elevations were partially sensitive to the L- and N-channel blockers nifedipine (1 microM) and omega-conotoxin GVIA (100 nM) respectively. Up to ten times Ca2+ elevations were observed in varicosities of treated cells than in cell bodies of treated and cells. These elevations were insensitive to compounds known to release Ca2+ from intracellular stores. Elevations of Ca2+ were sustained, and they were insensitive to 5 microM nifedipine, 100 nM omega-agatoxin IVA and 100 nM omega-conotoxin GVIA, and partially sensitive to 2 microM omega-conotoxin GVIA, indicating predominance of non-L-type, non-N-type, non-P-type channel activity. The intracellular localization of neuropeptide Y, a marker of differentiation in these cells, was also investigated by fluorescence immunocytochemistry. Varicosities of treated cells displayed marked fluorescence when viewed in a confocal microscope. These findings show that the varicosities of staurosporine-treated cells exhibit some of the functional properties of nerve terminals. The varicosities resemble boutons en passant nerve endings and they seem to express Ca2+ channels different from those in the cell body.

Properties of 3,4-diaminopyridine-evoked dopamine and acetylcholine release in rabbit caudate nucleus slices: involvement of facilitatory adenosine A2 receptors or nitric oxide?

The 3H-overflow from slices of the rabbit caudate nucleus preincubated with tritiated dopamine (DA), or choline, and then superfused and stimulated twice with 3,4-diaminopyridine (3,4-DAP; 25 microM, 1 min), was explored as an in vitro model for evoked release of DA, or acetylcholine (ACh), respectively. In both cases the 3,4-DAP-evoked 3H-overflow was tetrodotoxin-sensitive and Ca(2+)-dependent and hence most probably represents action potential-induced exocytotic release of DA or ACh, respectively. Using pairs of preferential agonists/antagonists it was shown, that evoked DA release was inhibited via presynaptic D2 autoreceptors (quinpirole/domperidone) and kappa-opioid receptors (U-50488H/norbinaltorphimine). No evidence was found for the presence of presynaptic adenosine A1 or A2 receptors on dopaminergic terminals. Moreover, 3,4-DAP-evoked DA release was unaffected by increased intracellular cyclic AMP levels or by drugs affecting the NO/guanylate cyclase pathway. In a similar manner it was shown that 3,4-DAP-evoked ACh release was inhibited via presynaptic muscarine autoreceptors (oxotremorine/atropine) and dopamine D2 heteroreceptors (quinpirole/domperidone). Again, no evidence for the involvement of the NO/guanylate cyclase system in the modulation of ACh release was found, whereas the presence of inhibitory adenosine A1 receptors, but not of facilitatory A2 receptors, could be clearly established. It is concluded, that 3,4-DAP-evoked 3H-overflow from rabbit caudate nucleus slices preincubated with [3H]DA or [3H]choline, represents a simple and useful in vitro model for action potential-induced DA or ACh release, respectively. Moreover, at least in this model or rabbit brain region, facilitatory adenosine A2 receptors and the NO/guanylate cyclase system seem not to be involved in the release of these transmitters.

Regulation of glutamate transport into synaptic vesicles by chloride and proton gradient

Glutamate uptake into synaptic vesicles is driven by an electrochemical proton gradient formed across the membrane by a vacuolar H+-ATPase. Chloride has a biphasic effect on glutamate transport, which it activates at low concentrations (2-8 mM) and inhibits at high concentrations (>20 mM). Stimulation with 4 mM chloride was due to an increase in the Vmax of transport, whereas inhibition by high chloride concentrations was related to an increase in Km to glutamate. Both stimulation and inhibition by Cl- were observed in the presence of A23187 or (NH4)2SO4, two substances that dissipate the proton gradient (deltapH). With the use of these agents, we show that the transmembrane potential regulates the apparent affinity for glutamate, whereas the deltapH antagonizes the effect of high chloride concentrations and is important for retaining glutamate inside the vesicles. Selective dissipation of deltapH in the presence of chloride led to a significant glutamate efflux from the vesicles and promoted a decrease in the velocity of glutamate uptake. The H+-ATPase activity was stimulated when the deltapH component was dissipated. Glutamate efflux induced by chloride was saturable, and half-maximal effect was attained in the presence of 30 mM Cl-. The results indicate that: (i) both transmembrane potential and deltapH modulate the glutamate uptake at different levels and (ii) chloride affects glutamate transport by two different mechanisms. One is related to a change of the proportions between the transmembrane potential and the deltapH components of the electrochemical proton gradient, and the other involves a direct interaction of the anion with the glutamate transporter.

omega-Agatoxin IVA identifies a single calcium channel subtype which contributes to the potassium-induced release of acetylcholine, 5-hydroxytryptamine, dopamine, gamma-aminobutyric acid and glutamate from rat brain slices

The voltage-dependent calcium channels (VDCCs) involved in K(+)-induced transmitter release have been studied. A maximally effective concentration of the N-type VDCC inhibitor, omega-conotoxin GVIA (GVIA) blocked the release of 5-HT (30%), DA (30%) and ACh (60%) but not that of GABA or glutamate. The O, P and Q-type VDCC inhibitor, omega-agatoxin IVA (Aga IVA, 1 microM), blocked 100% of GABA and glutamate, 70% of DA and about 50% of 5-HT and ACh release. The slopes of the inhibiton curves indicate that it acts on the same, single type of VDCC in all cases. omega-Conotoxin MVIIC (MVIIC) completely inhibited the release of all the transmitters. It is concluded that a single GVIA-insensitive type of VDCC is involved in the K(+)-induced release of all the transmitters and, in addition, N-type VDCCs, with a higher affinity for GVIA than MVIIC, are required for the release of 5-HT, DA and ACh. The non-N-type VDCC is not the O-type as it is not blocked by low (< 10 nM) concentrations of MVIIC. Further resolution of this VDCC into P or Q-type requires more selective antagonists.

Glutamate receptor agonists evoked Ca(2+)-dependent and Ca(2+)-independent release of [3H]D-aspartate from cultured chick retina cells

We studied the release of [3H]D-aspartate evoked by glutamate receptor agonists from monolayer cultures of chick retina cells, and found that activation of the glutamate receptors can evoke both Ca(2+)-dependent and Ca(2+)-independent release of [3H]D-aspartate. In Ca(2+)-free (no added Ca2+) Na+ medium, the agonists of the glutamate receptors induced the release of [3H]D-aspartate with the following rank order of potency: kainate > alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) approximately N-methyl-D-aspartate (NMDA). In media containing 1 mM CaCl2 the release of [3H]D-aspartate evoked by NMDA, kainate and AMPA was increased by about 112 percent, 20 percent and 39 percent, respectively, as compared to the release evoked by the same agonists in Ca(2+)-free medium. NMDA was the most potent agonist in stimulating the Ca(2+)-dependent release of [3H]D-aspartate, possibly by exocytosis, and AMPA was as potent as kainate. The Ca(2+)-dependent release of [3H]D-aspartate evoked by kainate was dependent on the influx of Ca2+ through the receptor associated channel, as well as through the N-(omega-Conotoxin GVIA-sensitive) and L- (nitrendipine-sensitive) type voltage-sensitive Ca2+ channels (VSCC). The exocytotic release of [3H]D-aspartate evoked by AMPA relied exclusively on Ca2+ entry through the L-type VSCC, whereas the effect of NMDA was partially mediated by the influx of Ca2+ through the receptor-associated channel, but not through L- or N-type VSCC. Thus, activation of these different glutamate receptors under physiological conditions is expected to cause the release of cytosolic and vesicular glutamate, and the routes of Ca2+ entry modulating vesicular release may be selectively recruited.

Metabotropic glutamate receptors, transmitter output and fatty acids: studies in rat brain slices

1. The effects of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a non-selective agonist of the metabotropic glutamate receptors (mGluRs), have been studied in rat cortical and striatal slices by measuring the depolarization-induced output of D-[3H]-aspartate (D-[3H]-Asp) and of [3H]-glutamate ([3H]-Glu), neosynthesized from [3H]-glutamine. 2. In cortical slices, 1S,3R-ACPD potentiated the depolarization-induced (KCl, 30 mM) output of both D-[3H]-Asp and [3H]-Glu. The potentiation, obtained at 300 microM 1S,3R-ACPD was 65 +/- 6% for D-[3H]-Asp and 56 +/- 10% for [3H]-Glu. Conversely, in striatal slices, 1S,3R-ACPD reduced the depolarization-induced transmitter output. The reduction, obtained at 300 microM of the agonist, was 60 +/- 8% for D-[3H]-Asp and 50 +/- 5% for neosynthesized [3H]-Glu. 3. Bovine serum albumin (BSA, 15 microM), which is able to bind locally produced fatty acids, completely eliminated the potentiating effect 1S,3R-ACPD had on D-[3H]-Asp output from cortical slices. Low concentrations of arachidonic acid (1-10 microM) or of oleic acid (1-10 microM) added to BSA-containing perfusion medium, restored this potentiating effect. BSA, however, had no effect on the inhibitory action of 1S,3R-ACPD in striatal slices. 4. Bromophenacyl bromide (100 microM), an inhibitor of phospholipase A2, and RG80267 (100 microM), an inhibitor of diacylglycerol lipase, have been shown to inhibit fatty acid production. These compounds prevented the potentiating effect of 1S,3R-ACPD on D-[3H]-Asp-output in cortical slices. 5. Indomethacin (100 microM), an inhibitor of cyclo-oxygenases, plus nordihydroguaiaretic acid (100 microM), an inhibitor of lipoxygenases, increased D-[3H]-Asp output in cortical slices perfused with BSA-containing medium. 6. These experiments suggest that the mGluR-mediated potentiation of transmitter output requires the availability of unsaturated fatty acids, such as arachidonic or oleic acids, in cortical slices. In contrast, the mGluR-induced inhibition of transmitter output is not dependent upon fatty acid availability in striatal slices. The requirement of both unsaturated fatty acids and 1S,3R-ACPD in the facilitation of transmitter exocytosis may play an important role in the regulation of synaptic plasticity.

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.

Transgenic animals as models in the study of the neurobiological role of polyamines

Natural polyamines, putrescine, spermidine and spermine, exhibit a number of neurophysiological and metabolic effects in brain preparations. In the in vitro studies, several specific sites of action have been identified such as ion channels, transmitter release and Ca2+ homeostasis. Polyamines have been linked to the development of neuronal degeneration caused by, for instance, epileptic seizures and stroke. The role of endogenous polyamines in the functioning brain is not clear, however. We review the work carried out using state-of-the-art transgenic animal models for polyamine research. A number of transgenic mouse lines carrying human ornithine decarboxylase, spermidine synthase and S-adenosylmethionine decarboxylase gene have been generated. Of these animals those with ornithine decarboxylase transgene show an extensive and constitutive expression of the enzyme in the brain with an exceedingly high putrescine concentration, a phenotype that is not encountered under physiological conditions. In this article we review the neurometabolic, behavioural and histological data that has been obtained from these transgenic mice.

The use of the human neuroblastoma SH-SY5Y to study the effect of second messengers on noradrenaline release

1. Recent data suggesting that the human neuroblastoma SH-SY5Y is a suitable cell line in which to study the effect of second messengers on NA release are discussed in the context of current views on exocytosis. 2. Release of NA is evoked by depolarization, as well as activation of muscarinic (M3) and bradykinin (B2) receptors in SH-SY5Y cells which have not been differentiated by the addition of growth factors. 3. Evoked release is enhanced by activation of protein kinase C. 4. Activation of protein kinase C decreases the changes in intracellular calcium evoked by carbachol, bradykinin and 100 mM K+. 5. SH-SY5Y express N-type and L-type voltage sensitive Ca2+ channels. L-Type Ca(2+)-channels are coupled to NA release under conditions of weak depolarization. However with strong depolarization (100 mM K+) both L-type and N-type channels are involved. 6. Muscarinic- and neuropeptide Y receptors are coupled to the inhibition of Ca2+ channel activity.

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampal slices: facilitation by endogenous or exogenous nitric oxide

The involvement of nitric oxide (NO) in the evoked release of noradrenaline (NA) was studied in rat hippocampal slices preincubated with [3H]NA and stimulated with 3,4-diaminopyridine (3,4-DAP; 200 microM) for 2 min. The 3,4-DAP-evoked [3H]overflow was enhanced by the NO synthase substrate L-arginine, but not by D-arginine; it was reduced by the NO synthase inhibitor NG-nitro-L-arginine, which also antagonized the effects of L-arginine. The corresponding nitro derivative of D-arginine was inactive and unable to block the effects of L-arginine. Also drugs known to produce NO in-vitro, like sodium nitroprusside (SNP), 3-morpholino-sydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP) enhanced the 3,4-DAP-evoked NA release. The NO scavenger hemoglobin showed no significant effects when given alone, but reduced or abolished, respectively, the facilitatory effects of SNP, or SNAP and L-arginine. The cyclic GMP derivatives 8-Br-cGMP and Sp-8-p-chlorophenylthioguanosine-3',5'-cyclic monophosphorothioate (Sp-8-pCPT-cGMPS) also acted facilitatory, whereas the corresponding Rp-enantiomer of the latter compound was inactive, but antagonized the effect of Sp-8-pCPT-cGMPS. NA release evoked by 3,4-DAP (10 microM) from rat hippocampus synaptosomes was not affected by L-arginine or NG-nitro-L-arginine but slightly increased by SNAP and Sp-8-pCPT-cGMPS. Antagonists at NMDA, non-NMDA and metabotropic glutamate receptors neither affected the 3,4-DAP-evoked NA release nor the facilitatory effect of L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca/calmodulin-dependent kinase II inhibitor KN62 attenuates glutamate release by inhibiting voltage-dependent Ca(2+)-channels

The effect of KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N -methyl-L-tyrosyl]-4-phenylpiperazine), a putative inhibitor of Ca/calmodulin-dependent kinase II (Ca/CaM-K II), on glutamate release from isolated nerve-terminals (synaptosomes) was examined. The drug caused a potent inhibition of KCl- and 4-aminopyridine-evoked glutamate release from isolated nerve-terminals (synaptosomes). Examination of the effect of the inhibitor on Ca(2+)-influx revealed that the diminution of glutamate release could be attributed to a decrease in cytosolic Ca. A direct effect of KN62 on synaptosomal Ca(2+)-channels was confirmed in experiments where Ba, which does not support CaM-dependent processes, was used in place of Ca. Additionally, whole-cell patch-clamping of cerebellar granule neurones directly demonstrated inhibition of Ca-currents by KN62. We therefore suggest that, in cellular systems, conclusions based on the use of KN62 as a Ca/CaM-K II blocker may be ambiguous and should be viewed with caution unless the effect of the drug on Ca-influx has also been quantified. The effect of KN62 on Ca(2+)-influx appears to be specific to slowly-or non-inactivating conductances, and therefore presents KN62 as a potentially useful tool in this context.

Temporal differences in the phosphorylation state of pre- and postsynaptic protein kinase C substrates B-50/GAP-43 and neurogranin during long-term potentiation

The phosphorylation state of two identified neuralspecific protein kinase C substrates (the presynaptic protein B-50 and the postsynaptic protein neurogranin) was monitored after the induction of long term potentiation in the CA1 field of rat hippocampus slices by quantitative immunoprecipitation following 32Pi labeling in the recording chamber. B-50 phosphorylation was increased from 10 to 60 min, but no longer at 90 min after long term potentiation had been induced, neurogranin phosphorylation only at 60 min. Increased phosphorylation was not found when long term potentiation was blocked with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate, when only low frequency stimulation was applied or tetanic stimulation failed to induce long term-potentiation. Our data show that both B-50 and neurogranin phosphorylation are increased following the induction of long term potentiation, thus providing strong evidence for pre- and postsynaptic protein kinase C activation during narrow, partially overlapping, time windows after the induction of long term potentiation.

Dynamic storage of glutamate in rat brain synaptic vesicles

Storage of [3H]glutamate accumulated by highly purified synaptic vesicles from brain was characterized. [3H]Glutamate was lost with single exponential kinetics with a time constant of minutes after synaptic vesicles were diluted into medium that allowed uptake to continue but that contained unlabeled glutamate in place of [3H]glutamate. This [3H]glutamate efflux occurred at similar rates in media containing 50 and 500 microM glutamate, which suggests that it did not depend on the rate of glutamate transport and was independent of the external and internal glutamate concentrations. All efflux was blocked at 0 degrees C. These results imply that glutamate stored in synaptic vesicles turns over with a half-time of minutes, even during active uptake under physiological conditions.

Inhibition by KF17837 of adenosine A2A receptor-mediated modulation of striatal GABA and ACh release

1. The effect of the A2A adenosine receptor agonist, 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS 21680) on the potassium evoked release of [3H]-gamma-aminobutyric acid ([3H]-GABA) from nerve terminals derived from the caudate-putamen and the globus pallidus of the rat was compared. In both preparations CGS 21680 (1 nM) inhibited the [3H]-GABA release evoked by 15 mM KCl but had no effect on that evoked by 30 mM KCl. 2. The ability of CGS 21680 (1 nM) to inhibit the release of [3H]-GABA from striatal nerve terminals was unaffected by the presence of the GABA receptor antagonists, bicuculline (10 microM), phaclofen (100 microM) and 2-hydroxysaclofen (100 microM). Similarly the opioid receptor antagonist, naloxone (10 microM), the adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 40 nM), and the cholinoceptor antagonists, mecamylamine (10 microM) and atropine (100 nM) had no effect on this inhibition. 3. The ability of CGS 21680 (0.1 nM) to stimulate the release of [3H]-acetylcholine ([3H]-ACh) from striatal nerve terminals was unaffected by the presence of bicuculline (10 microM), 2-hydroxysaclofen (100 microM), phaclofen (100 microM), naloxone (10 microM) and DPCPX (4 nM). 4. The novel A2A receptor antagonist, (E)-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine (KF 17837), blocked the CGS 21680 (1 nM)-induced inhibition of [3H]-GABA efflux with an EC50 of approximately 30 nM and also antagonized the CGS 21680 (0.1 nM)-induced stimulation of [3H]-ACh release with an EC50 of approximately 0.3 nM. 5. It is concluded that the A2A adenosine receptor is present on both GABAergic and cholinergic nerve terminals of the rat striatum and that in both the caudate-putamen and the globus pallidus this receptor inhibits [3H]-GABA release. No evidence was seen for a difference in the ligand binding sites of this receptor in the two groups of nerve terminals.

The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors

Rat brain slices were used to study the effects of different metabotropic glutamate receptor ligands on (i) the depolarization (30 mM KCl)-induced outflow of previously taken up D-[3H]aspartate; (ii) the inhibition of forskolin (30 microM)-induced cyclic AMP accumulation; and (iii) the hydrolysis of phosphoinositides. In addition, the localization of mRNAs coding for different metabotropic glutamate receptor subtypes was detected using in situ hybridization. (1S-3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (30-300 microM), a non selective metabotropic glutamate receptor agonist, significantly increased the KCl-induced output of radioactivity from cortical slices, whereas it inhibited the output from striatal slices. Conversely, (1S,3S,4S)-carboxycyclopropylglycine (0.1-1 microM), a relatively selective agonist of the mGluR2 metabotropic glutamate receptor subtype, had an inhibitory effect on the output of D-[3H]aspartate from both cortical and striatal slices and proved to be the most potent metabotropic glutamate receptor agonist in inhibiting cyclic AMP accumulation, but not in stimulating phosphoinositide hydrolysis. Since 2-amino-4-phosphonobutyrate (a mGluR4, mGluR6 and mGluR7 agonist) was not active in any of the assays tested, we hypothesized that the mGluR2 subtype could be involved in these events. Accordingly, mGluR2 mRNA expression was abundant in cortical neurons projecting to the striatum. Our experiments suggest that the stimulation of metabotropic glutamate receptors may either decrease or increase transmitter release depending on the subtype that prevails in the region under study.

Compartmentation of cerebral glutamate in situ as detected by 1H/13C n.m.r

Incorporation of 13C label from either [1-13C]glucose to glutamate C-4 and lactate C-3 or from [2-13C]acetate to glutamate C-4 was monitored in situ in a superfused brain slice preparation by using 1H-detected/13C-edited (1H/13C) n.m.r. spectroscopy. The fractional enrichments of both metabolites were determined by this means in both brain slices and acid extracts of the preparations in order to assess their 1H-n.m.r. detectabilities. The 1H/13C satellite resonances from glutamate C-4 and lactate C-3 in brain tissue were followed from 4 min onwards in the presence of 5 mM [1-13C]glucose. Fractional enrichment of glutamate C-4 in the slice preparations was higher than in their acid extracts throughout the incubation of 100 min; at 30 min the enrichment was 15.9 +/- 0.6% in the slice preparations and 10.6 +/- 0.9% in extracts and at 100 min 24.5 +/- 1.7% compared with 19.7 +/- 0.4%, respectively. In contrast, lactate C-3 reached a steady-state fractional enrichment of approx. 43% by 15 min and there was no difference between the values determined in the slice preparations and the acid extracts. There was a significant difference between the glutamate C-4 fractional enrichments in the brain slices (7.4 +/- 0.6%) and extracts (5.1 +/- 0.3%) after 60 min of incubation with [2-13C]acetate. Thus 13C label from both glucose and exogenous acetate enters a pool of glutamate that is more amenable to 1H n.m.r. detection than total acid-extracted brain biochemical glutamate, whereas lactate is labelled with full 1H n.m.r. visibility. The results are discussed in the light of the biochemical factors that affect glutamate 1H-n.m.r. susceptibility and thus its n.m.r. visibility.

Effects of calcium channel blockers on the kinetics of voltage-dependent changes in synaptosomal calcium concentrations

Synaptosomal preparations from rat cerebral cortex have been used in stopped-flow fluorescence studies to measure rapid changes in intrasynaptosomal calcium concentrations upon depolarization. Synaptosomes were loaded with the fluorescent calcium chelating dye, Fura-2, by incubation with the membrane permeant acetoxymethyl ester derivative. Depolarization by elevated external K+ concentration resulted in a rapid increase in cytoplasmic Ca2+ as measured by a quench in Fura-2 fluorescence when excited at 390 nm. The fluorescence change could be reasonably fit by a single exponential process with an apparent rate of 10-15 s-1 and the magnitude of the response was voltage-dependent, increasing with increasing external K+ over the range of 5-30 mM. The observed quench was blocked by micromolar concentrations of the inorganic calcium channel blockers, Cd2+, Co2+ and La3+. Nimodipine, a dihydropyridine which blocks L-type calcium channels, inhibited only 10-15% of the flux response while nitrendipine had no consistent effect. omega-Conotoxin GVIA, a blocker of N-type channels in many species, had only a small inhibitory effect at high (1-10 microM) concentrations. The response was, however, inhibited by pre-incubation of the synaptosomes with venom of the funnel web spider. Agelenopsis aperta (0.1-300 micrograms/ml). Inhibition was observed with both a purified polyamine fraction (FTX) from the venom (IC50 = 4 nl/ml) and a purified peptide toxin, omega-AgaIVA (IC50 = 30 nM). These results indicate that voltage-dependent Ca2+ uptake by mammalian nerve terminals is mediated primarily by channels that are insensitive to dihydropyridines and omega-conotoxin GVIA but are sensitive to components of funnel web spider venom.

L-trans-pyrrolidine-2,4-dicarboxylate and cis-1-aminocyclobutane-1,3-dicarboxylate behave as transportable, competitive inhibitors of the high-affinity glutamate transporters

The ability of two conformationally restricted analogues of L-glutamate to function as non-transportable inhibitors of plasma membrane L-glutamate transport was investigated in primary cultures of cerebellar granule cells and cortical astrocytes. L-trans-Pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC) and cis-1-aminocyclobutane-1,3-dicarboxylic acid (cis-ACBD) behaved as linear competitive inhibitors of the uptake of D-[3H]aspartate (used as a non-metabolizable analogue of L-glutamate) exhibiting Ki values between 40 and 145 microM; L-trans-PDC being the more potent inhibitor in each preparation. However, both L-trans-PDC and cis-ACBD, over a concentration range of 1 microM-5 mM, dose-dependently stimulated the release of exogenously supplied D-[3H]aspartate from granule cells maintained in a continuous superfusion system. The stimulated release was independent of extracellular calcium ions; essentially superimposable dose-response profiles being obtained in the absence and presence of 1.3 mM CaCl2 and yielding EC50 values of 16-25 microM and 180-220 microM for L-trans-PDC and cis-ACBD, respectively. Stimulated release of D-[3H]aspartate was unaffected by either 300 microM D-(-)-2-amino-5-phosphonopentanoic acid [D-APV; a selective antagonist of the N-methyl-D-aspartate (NMDA) receptor] or by 25 microM 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX; a selective antagonist of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor]. The release of D-[3H]-aspartate following stimulation by either L-trans-PDC or cis-ACBD was however markedly attenuated following substitution in the superfusion medium of sodium ions by choline ions. Taken together, these results support an action of L-trans-PDC and cis-ACBD consistent with that of being competitive substrates rather than non-transportable blockers of the plasma membrane L-glutamate uptake system.

Unchanged exocytotic release of glutamic acid in cortex and neostriatum of the rat during aging

The Ca(2+)-dependent release of glutamate induced by 4-aminopyridine in synaptosomes prepared both from the cerebral cortex and basal ganglia was unchanged in aged rats (27-30 months) when compared to adults rats (3 months). Consistent with the absence of changes in glutamate exocytosis during aging, the rise in the cytosolic free Ca2+ concentration, [Ca2+]c, induced by depolarization in synaptosomes from aged rats was similar to that found in control adult rats. The results suggest that during aging the nerve terminals from the cerebral cortex and basal ganglia maintain an intact ability to release glutamate by exocytosis.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

The calcium channel coupled to the exocytosis of L-glutamate from cerebellar granule cells is inhibited by the spider toxin, Aga-GI

The increase in cytosolic calcium, [Ca2+]c, evoked with 50 mM KCl in cerebellar granule cells consists of four components; (1) a rapidly inactivating transient or spike; (2) a nifedipine-sensitive non-inactivating plateau; (3) an Aga-GI (spider toxin) sensitive non-inactivating plateau; (4) a residual non-inactivating plateau insensitive to nifedipine and Aga-GI. None of these components is blocked by synthetic arginine polyamine toxin, spermine, (+)-MK-801 hydrogen maleate, D(-)-2-amino-5-phosphonopentanoic acid or omega-conotoxin-GVIA. The proposed P-type channel antagonist, omega-agatoxin-IVA, has a limited but non-significant effect on the elevated plateau [CA2+]c.L-type Ca2+ channels are located primarily on the soma whereas the component of the plateau which is blocked specifically by Aga-GI is localized primarily on the cell neurites. The latter component is coupled to the exocytosis of endogenous glutamate evoked with 50 mM KCl.

Multiple Ca2+ channel types coexist to regulate synaptosomal neurotransmitter release

The regulation of excitation-secretion coupling by Ca2+ channels is a fundamental property of the nerve terminal. Peptide toxins that block specific Ca2+ channel types have been used to identify which channels participate in neurotransmitter release. Subsecond measurements of [3H]-glutamate and [3H]dopamine release from rat striatal synaptosomes showed that P-type channels, which are sensitive to the Agelenopsis aperta venom peptide omega-Aga-IVA, trigger the release of both transmitters. Dopamine (but not glutamate) release was also controlled by N-type, omega-conotoxin-sensitive channels. With strong depolarizations, where neither toxin was very effective alone, a combination of omega-Aga-IVA and omega-conotoxin produced a synergistic inhibition of 60-80% of Ca(2+)-dependent dopamine release. The results suggest that multiple Ca2+ channel types coexist to regulate neurosecretion under normal physiological conditions in the majority of nerve terminals. P- and N-type channels coexist in dopaminergic terminals, while P-type and a omega-conotoxin- and omega-Aga-IVA-resistant channel coexist in glutamatergic terminals. Such an arrangement could lend a high degree of flexibility in the regulation of transmitter release under diverse conditions of stimulation and modulation.

Thirty years of synaptosome research

Detached synapses (synaptosomes), first isolated by the author in 1958 and identified as such in 1960, are sealed presynaptic nerve terminals often with a portion of the target cell--sometimes amounting to a complete dendritic spine--adhering to their external surface. They can be prepared in high yield from brain tissue and also in decreasing yield from spinal cord, retina, sympathetic ganglia, myenteric plexus and electric organs. They are sealed structures which, under metabolizing conditions, respire, take up oxygen and glucose, extrude Na+, accumulate K+, maintain a normal membrane potential and, on depolarization, release transmitter in a Ca(2+)-dependent manner. They thus provide an excellent preparation with which to investigate synaptic function without the complications encountered with synapses in situ. They also serve as the parent fraction for preparations of synaptic vesicles and other synaptic components.

Release of cholecystokinin-like immunoreactivity in the frontal cortex of conscious rats as assessed by transcerebral microdialysis: effects of different depolarizing stimuli

The release of cholecystokinin-like immunoreactivity (CCK-LI) from the frontal cortex of freely moving rats has been studied using a transcerebral microdialysis technique coupled to a radioimmunoassay procedure. Basal levels of CCK-LI in the dialysate were above detection limits (2.4 +/- 0.7 pg/20 min; n = 8). High-K+ media evoked CCK-LI overflow in a concentration-dependent manner. The threshold concentration was 50 mM KCl. The peak overflow evoked by 100 mM K+ amounted to 42.7 +/- 2.8 pg/20 min (n = 6); it was totally Ca2+ dependent but insensitive to 1 microM tetrodotoxin. Infusion of 4-aminopyridine (1 mM; 20 min) evoked an overflow of CCK-LI (32 +/- 2.3 pg/20 min; n = 4), which was totally Ca2+ dependent and tetrodotoxin sensitive. Depolarization with 100 micrograms/ml of veratrine (20 min) provoked a CCK-LI overflow (62.2 +/- 10 pg/20 min; n = 6), which was also blocked by tetrodotoxin or by the absence of Ca2+ ions. The CCK-LI material collected under basal conditions or during veratrine infusion consisted essentially of CCK octapeptide sulfate. The veratrine-induced CCK-LI overflow did not change significantly when the infusion time was prolonged to 100 min. A second 20-min stimulus with 100 micrograms/ml of veratrine applied 200 min after a first 20-min stimulus evoked a barely significant CCK-LI overflow. These data suggest that one single 20-min stimulus with 100 micrograms/ml of veratrine may be sufficient to deplete the CCK-LI releasable stores and that > 200 min are required to replenish the depleted CCK-containing vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Barium-evoked glutamate release from guinea-pig cerebrocortical synaptosomes

Ba2+ has multiple effects on presynaptic terminals. The ion inhibits the K+ channels responsible for stabilizing the plasma membrane potential in the same way as previously reported for dendrotoxin and 4-aminopyridine. Secondly, the ion can substitute fully for Ca2+ in supporting KCl-evoked release of glutamate from guinea-pig cerebrocortical synaptosomes. In the latter case, the kinetics of glutamate release in the presence of saturating Ca2+ or Ba2+ are essentially identical. Substantially lower external concentrations of Ba2+ are required to achieve the same release kinetics as with Ca2+. The average internal free Ba2+ concentration attained during KCl depolarization is some 10-fold higher than that for Ca2+. However, because the fura-2 signal reflects predominantly the overflow of divalent cation after dissociation from the release trigger, it is not the valid parameter to compare effectiveness of the cations in triggering glutamate exocytosis. In view of the established inability of Ba2+ to interact with calmodulin, these results are discussed in relation to theories in which Ca2+/calmodulin-dependent protein kinase-mediated phosphorylation is a prerequisite for synaptic vesicle exocytosis.

Vesamicol blocks the recovery, by recycling cholinergic electromotor synaptic vesicles, of the biophysical characteristics of the reserve population

The effect of vesamicol on the ability of recycling cholinergic synaptic vesicles to recover, during a period of post-stimulation rest, the biophysical properties of the reserve pool was studied in prestimulated perfused blocks of the electric organ of the electric ray, Torpedo marmorata, a tissue rich in cholinergic synapses. The effect of the drug was analysed by high-resolution centrifugal density-gradient fractionation in a zonal rotor of the extracted vesicles. The two vesicle fractions were identified by their ATP and acetylcholine content and the recycled vesicles by their acquisition of [3H]acetylcholine derived from [3H]acetate in the perfusate. Vesamicol (10 microM) blocked the uptake of tritiated acetylcholine by recycled vesicles and also prevented them from rejoining the reserve pool. This is consistent with a previously formulated model of the recovery process, whereby the increase in the acetylcholine and ATP content of the recycled vesicles which takes place during a post-stimulus period of rest increases their osmotic load and thus their content of free water. Vesamicol, by blocking acetylcholine uptake, also blocks rehydration of the recycled vesicles and thus the accompanying decrease in their density to the value characteristic of fully charged vesicles.

Studies on the role of B-50 (GAP-43) in the mechanism of Ca(2+)-induced noradrenaline release: lack of involvement of protein kinase C after the Ca2+ trigger

The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca(2+)-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (> 60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10(-8) to 10(-5) M Ca2+. The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca(2+)-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca(2+)-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca(2+)-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10(-5) M), failed to inhibit Ca(2+)-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca(2+)-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca(2+)- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA.

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.

Effect of toxin of Prymnesium patelliferum on neurotransmitter transport mechanisms: development of a sensitive test method

A crude extract of the ichthyotoxic phytoflagellate Prymnesium patelliferum strongly inhibited the uptake of neurotransmitters into isolated nerve endings (synaptosomes) and synaptic vesicles. These systems were about 100 and 10 times more sensitive toward the algal toxin, respectively, than a standard hemolysis assay often used for testing the toxicity of Prymnesium spp. and other ichthyotoxic algae. Prymnesium patelliferum grown in phsophorus-deficient (-P) medium was about five times more active than when grown in phosphorus sufficient (+P) medium. The inhibition ratio between the high-affinity synaptosomal uptake of L-glutamate and gamma-aminobyturic acid (GABA) was 1/2.7 for the -P culture and 1/1.9 for the +P culture. The inhibition ratios for the low-affinity vesicular uptake of L-glutamate, GABA, and dopamine (DA) were 1/5.8/0.3 and 1/1.7/0.2, respectively. The synaptosomal transport of L-glutamate is a rapid, simple, and sensitive test method for toxicity determination of Prymnesium spp. and will be a useful tool in the further isolation and purification of the toxic principles of this and other related algae. It is suggested that the toxin interferes with ion channels or acts as an ionopore.

Storage and release of neurotransmitters

Because synaptic vesicles and secretory granules are simple in composition and easy to purify, many of their protein components have been identified and often sequenced. Attempts are underway to link the small number of membrane proteins to the small number of functions the vesicles perform. The discovery of sequence homologies has helped greatly with this. In addition, techniques that have begun to prove successful involve microinjection, identification of proteins that bind synaptic vesicle proteins, DNA transfection into cells and oocytes, and more recently, in vitro reconstitution of exocytosis, endocytosis, and vesicle biogenesis. Advances in the latter areas have been strongly influenced by the breakthroughs in our knowledge of membrane traffic in nonneuronal cells. The budding reactions involved in making synaptic vesicles and secretory granules resemble in many ways the generation of carrier vesicles from the ER and the Golgi complex. Finally, exocytosis in neurons may closely resemble fusion of carrier vesicles with target organelles in nonneuronal cells, using complexes of peripheral membrane proteins, GTP hydrolysis, and integral membrane proteins with fusogenic domains. The usefulness of in vitro reconstitution, reverse genetics, and the parallels with better understood systems compensates in part for a major weakness in the field, namely the difficulty in obtaining viable mutants that are defective in the storage and release of secretory vesicle content.

Glutamate uptake system in the presynaptic vesicle: glutamic acid analogs as inhibitors and alternate substrates

A variety of naturally occurring amino acids, their isomers, and synthetic analogs were tested for their ability to inhibit uptake of [3H]glutamate into presynaptic vesicles from bovine cerebral cortex. Strongest inhibition (Ki < 1mM) was observed for trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) and erythro-4-methyl-L-glutamic acid (MGlu), while 4-methylene-L-glutamic acid (MeGlu) was only moderately inhibitory (Ki = approximately 3mM), indicating that the synaptic vesicle glutamate translocator has higher affinity for trans-ACPD and MGlu than for glutamate. A few other amino acids, e.g., 4-hydroxyglutamic acid, S-carboxyethyl cysteine, and 5-fluorotryptophan, were slightly inhibitory; all L- and DL-isomers of protein amino acids and longer chain acidic amino acids were without measurable inhibition. Potassium tetrathionate and S-sulfocysteine exhibited strong to moderate noncompetitive or irreversible inhibition. Inhibition by t-ACPD, MGlu, or MeGlu was competitive with glutamic acid. Each of these competitive inhibitors was also taken up by the vesicle preparation in an ATP-dependent manner, as indicated by their being recovered unchanged from filtered vesicles. Similar results were obtained with reconstituted vesicles, while glutamate uptake by partially purified rat synaptosomes was inhibited only by MGlu. These results indicate that the glutamate translocator of presynaptic vesicles has stringent structural requirements distinct from those of the plasma membrane translocator and the metabotropic type of postsynaptic glutamate receptor. They further suggest possible structural requirements of pharmacologically significant compounds that can substitute for glutamic acid in the presynaptic side of glutamatergic synapses, thus serving to moderate or control glutamate excitation and associated excitotoxic effects in these neurons.

A toxin (Aga-GI) from the venom of the spider Agelenopsis aperta inhibits the mammalian presynaptic Ca2+ channel coupled to glutamate exocytosis

Venom of the funnel web spider Agelenopsis aperta was fractionated and screened for activity against the mammalian presynaptic, voltage-dependent Ca2+ channel coupled to glutamate exocytosis. A purified toxin (Aga-GI) from this venom inhibits glutamate exocytosis evoked by elevated potassium or by 4-aminopyridine but is without effect on ionomycin-evoked release. At the same time a partial inhibition of the depolarisation-evoked elevation of cytoplasmic free Ca2+ is seen. The toxin does not inhibit 4-aminopyridine- or potassium-evoked depolarisation, or block Ca(2+)-dependent, potassium-evoked [3H]noradrenaline release. The results indicate that the venom contains a toxin capable of inhibiting the presynaptic voltage-dependent Ca2+ channel coupled to glutamate exocytosis in the mammalian central nervous system. This channel is resistant to block by either omega-conotoxin GVIA or nifedipine. Thus Aga-GI is a novel tool with which to probe this elusive neuronal calcium channel.