Multiple components of synaptosomal [3H]-gamma-aminobutyric acid release resolved by a rapid superfusion system

Neurotransmitters and Behavioral Alterations Induced by Nickel Exposure

BACKGROUND

Nickel ions (Ni2+) are a heavy metal with wide industrial uses. Environmental and occupational exposures to Ni are potential risk factors for brain dysfunction and behavioral and neurological symptoms in humans.

METHODS

We reviewed the current evidence about neurochemical and behavioral alterations associated with Ni exposure in laboratory animals and humans.

RESULTS

Ni2+ exposure can alter (both inhibition and stimulation) dopamine release and inhibit glutamate NMDA receptors. Few reports claim an effect of Ni2+ at the level of GBA and serotonin neurotransmission. At behavioral levels, exposure to Ni2+ in rodents alters motor activity, learning and memory as well as anxiety and depressive-like symptoms. However, no analysis of the dose-dependent relationship has been carried out regarding these effects and the levels of the Ni2+ in the brain, in blood or urine.

CONCLUSION

Further research is needed to correlate the concentration of Ni2+ in biological fluids with specific symptoms/deficits. Future studies addressing the impact of Ni2+ under environmental or occupational exposure should consider the administration protocols to find Ni2+ levels similar in the general population or occupationally exposed workers.

Carrier-mediated GABA Release: Is There a Functional Role?

Neurotransmitters are normally released from neurons via calcium-dependent exocytosis of synaptic vesicles. However, after blockade of vesicular release by removal of calcium, or treatment with tetanus toxin, neurotransmitter release can still occur. In the case of GABA, nonvesicular release results from reversal of its uptake transporter, found on both neurons and glia. These GABA transporters are sodium-dependent and electrogenic, and therefore can be induced to operate in reverse by cell depolarization or by breakdown of the sodium gradient. Although demonstrated biochemically, less is known about whether this form of release occurs in vivo or whether it results in electrophysiological effects. Because conditions that favor reversal of the GABA transporter occur during high-frequency firing, nonvesicular GABA release may occur with excessive neuronal activity, such as during seizures. NEUROSCIENTIST 3:151-157, 1997

Disturbances in the secretion of neurotransmitters in IA-2/IA-2beta null mice: changes in behavior, learning and lifespan

Islet-associated protein 2 (IA-2) and IA-2beta are major autoantigens in type 1 diabetes and transmembrane proteins in dense core secretory vesicles (DCV) of neuroendocrine cells. The deletion of these genes results in a decrease in insulin secretion. The present study was initiated to test the hypothesis that this deletion not only affects the secretion of insulin, but has a more global effect on neuroendocrine secretion that leads to disturbances in behavior and learning. Measurement of neurotransmitters showed that norepinephrine, dopamine and 5-HT were significantly decreased in the brain of double knockout (DKO) mice (P<0.05 to <0.001). In tests evaluating anxiety-like behavior and conditioned-learning, the DKO mice showed a highly significant increase in anxiety-like behavior (P<0.01 to <0.001) and impairment of conditioned learning (P<0.01) as compared to WT mice. The DKO mice also displayed an increase in spontaneous and induced seizures (P<0.01) and age-related death. Contrary to the generally held view that IA-2 and IA-2beta are expressed exclusively in DCV, subcellular fractionation studies revealed that IA-2beta, but not IA-2, co-purifies with fractions rich in synaptic vesicles (SV), and that the secretion of dopamine, GABA and glutamate from the synaptosomes of the DKO mice was significantly decreased as was the number of SV (P<0.01). Taken together, these findings show that IA-2beta is present in both DCV and SV, and that the deletion of IA-2/IA-2beta has a global effect on the secretion of neurotransmitters. The impairment of secretion leads to behavioral and learning disturbances, seizures and reduced lifespan.

Nonvesicular inhibitory neurotransmission via reversal of the GABA transporter GAT-1

GABA transporters play an important but poorly understood role in neuronal inhibition. They can reverse, but this is widely thought to occur only under pathological conditions. Here we use a heterologous expression system to show that the reversal potential of GAT-1 under physiologically relevant conditions is near the normal resting potential of neurons and that reversal can occur rapidly enough to release GABA during simulated action potentials. We then use paired recordings from cultured hippocampal neurons and show that GABAergic transmission is not prevented by four methods widely used to block vesicular release. This nonvesicular neurotransmission was potently blocked by GAT-1 antagonists and was enhanced by agents that increase cytosolic [GABA] or [Na(+)] (which would increase GAT-1 reversal). We conclude that GAT-1 regulates tonic inhibition by clamping ambient [GABA] at a level high enough to activate high-affinity GABA(A) receptors and that transporter-mediated GABA release can contribute to phasic inhibition.

Neuregulin-1 Enhances Depolarization-Induced GABA Release

Neuregulin-1 (NRG1), a regulator of neural development, has been shown to regulate neurotransmission at excitatory synapses. Although ErbB4, a key NRG1 receptor, is expressed in glutamic acid decarboxylase (GAD)-positive neurons, little is known about its role in GABAergic transmission. We show that ErbB4 is localized at GABAergic terminals of the prefrontal cortex. Our data indicate a role of NRG1, both endogenous and exogenous, in regulation of GABAergic transmission. This effect was blocked by inhibition or mutation of ErbB4, suggesting the involvement of ErbB4. Together, these results indicate that NRG1 regulates GABAergic transmission via presynaptic ErbB4 receptors, identifying a novel function of NRG1. Because both NRG1 and ErbB4 have emerged as susceptibility genes of schizophrenia, these observations may suggest a mechanism for abnormal GABAergic neurotransmission in this disorder.

Nicotine enhancement of dopamine release by a calcium-dependent increase in the size of the readily releasable pool of synaptic vesicles

A major factor underlying compulsive tobacco use is nicotine-induced modulation of dopamine release in the mesolimbic reward pathway (Wise and Rompre, 1989). An established biochemical mechanism for nicotine-enhanced dopamine release is by activating presynaptic nicotinic acetylcholine receptors (nAChRs) (Wonnacott, 1997). Prolonged application of 10(-7) to 10(-5) m nicotine to striatal synaptosomes promoted a sustained efflux of [3H]dopamine. This nicotine effect was mediated by non-alpha7 nAChRs, because it was blocked by 5 mum mecamylamine but was resistant to 100 nm alpha-bungarotoxin (alphaBgTx). Dopamine release was diminished by omitting Na+ or by applying peptide calcium channel blockers, indicating that nAChRs trigger release by depolarizing the nerve terminals. However, because alpha7 receptors rapidly desensitize in the continuous presence of agonists, a repetitive stimulation protocol was used to evaluate the possible significance of desensitization. This protocol produced a transient increase in [3H]dopamine released by depolarization and a significant increase in the response to hypertonic solutions that measure the size of the readily releasable pool (RRP) of synaptic vesicles. The nicotine-induced increase in the size of the readily releasable pool was blocked by alphaBgTx and by the calmodulin antagonist calmidazolium, suggesting that Ca2+ entry through alpha7 nAChRs specifically enhances synaptic vesicle mobilization at dopamine terminals. Thus, nicotine enhances dopamine release by two complementary actions mediated by discrete nAChR subtypes and suggest that the alpha7 nAChR-mediated pathway is tightly and specifically coupled to refilling of the RRP of vesicles in dopamine terminals.

Role of the GABA transporter in epilepsy

The GABA transporter plays a well-established role in reuptake of GABA after synaptic release. The anticonvulsant effect of tiagabine appears to result largely from blocking this reuptake. However, there is another side to the GABA transporter, contributing to GABA release by reversing in response to depolarization. We have recently shown that this form of GABA release is induced by even small increases in extracellular [K+], and has a powerful inhibitory effect on surrounding neurons. This transporter-mediated GABA release is enhanced by the anticonvulsants gabapentin and vigabatrin. The latter drug also potently increases ambient [GABA], inducing tonic inhibition of neurons. Here we review the evidence in support of a physiological role for GABA transporter reversal, and the evidence that it is increased by high-frequency firing. We postulate that the GABA transporter is a major determinant of the level of tonic inhibition, and an important source of GABA release during seizures. These recent findings indicate that the GABA transporter plays a much more dynamic role in control of brain excitability than has previously been recognized. Further defining this role may lead to a better understanding of the mechanisms of epilepsy and new avenues for treatment.

A biochemical approach to study sub-second endogenous release of diverse neurotransmitters from central nerve terminals

Exocytosis in central nerve terminals is rapidly triggered by the influx of calcium through high voltage sensitive Ca2+ -channels. Mainly due to their small size, studies in which neurotransmitter release from these terminals was determined at the sub-second time-scale are still rather limited. Here we describe the use of a pneumatic rapid mixing device, allowing application of short (> or = 50 ms) K+ -depolarizing pulses to purified nerve terminals, synaptosomes, to trigger endogenous release of different transmitter types. A consistent, Ca2+ -dependent exocytotic release of the amino acid transmitters, glutamate and GABA, from synaptosomes purified from rat and mouse brain was observed after 100 ms depolarization. For determination of amino acid release after longer depolarizations (> 100 ms), transporter blockers had to be added to prevent clearance of the vesicularly released transmitters. Ca2+ -dependent release of the neuropeptide cholecystokinin occured only after 250 ms depolarization. In addition, the time-courses of amino acid and cholecystokinin release were clearly different. The fast Ca2+ -dependent release of all transmitters was selectively and strongly inhibited by the P/Q-type Ca2+ -channel blocker omega-Agatoxin IVA. In conclusion, this approach allows direct measurement of Ca2+ -dependent release of diverse endogenous neurotransmitters from central nerve terminals upon depolarization pulses at a physiologically relevant, sub-second, time scale.

GABA transaminase inhibition induces spontaneous and enhances depolarization-evoked GABA efflux via reversal of the GABA transporter

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant gamma-vinyl GABA (vigabatrin; 0.05-100 microm) resulted in a large leak current that was blocked by bicuculline (50 microm). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 microm). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA(A) receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 microm vigabatrin, at which time it reached an equivalent conductance of 9.0 +/- 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mm) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K(+)](o) from 3 to as little as 6 mm. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

Ca(2+) regulation of the carrier-mediated gamma-aminobutyric acid release from isolated synaptic plasma membrane vesicles

The regulation of the carrier-mediated gamma-aminobutyric acid (GABA) efflux was studied in isolated synaptic plasma membrane (SPM) vesicles, which are particularly useful to study neurotransmitter release without interference of the exocytotic machinery. We investigated the effect of micromolar intravesicular Ca(2+) on the GABA release from SPM vesicles under conditions of basal release (superfusion with 150 mM NaCl), homoexchange (superfusion with 500 microM GABA) and K(+) depolarization-induced release (superfusion with 150 mM KCl). We observed that, in the presence of intravesicular Ca(2+) (10 microM), the maximal velocity (J(max)) of K(+) depolarization-induced GABA release is decreased by about 64%, and this effect was abolished in the presence of the channel blocker, La(3+). In contrast, the other mechanisms were not significantly altered by these cations. In agreement with our earlier results, inhibition of GABA uptake by intravesicular Ca(2+) was also observed by determining the kinetic parameters (K(0.5) and J(max)) of influx into the SPM vesicles. Under these conditions, the J(max) of GABA uptake was 17.4 pmol/s per mg protein, whereas in control experiments (absence of Ca(2+)), this value achieved 25.5 pmol/s per mg protein. The inhibitory effect of Ca(2+) on translocation of GABA across SPM appears to be mediated by calcium/calmodulin activation of protein phosphatase 2B (calcineurin), since it was completely relieved by W7 (calmodulin antagonist) and by cyclosporin A (calcineurin inhibitor). These results show that the GABA transport system, operating either in forward or backward directions, requires phosphorylation of internally localized calcineurin-sensitive sites to achieve maximal net translocation velocity.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Rotating disk electrode voltammetric measurements of dopamine transporter activity: an analytical evaluation

Rotating disk electrode (RDE) voltammetry at glassy carbon electrodes in 300- to 500-microL volumes has been shown in the literature by this laboratory and others to be useful in measuring dopamine and norepinephrine inward transport and release in preparations from rat brain and in expression systems. However, an analytical evaluation of the technique has not been made, and it is presented here in studies in striatal suspensions and human embryonic kidney cells expressing the human transporter for dopamine. The RDE was found to be able to measure dopamine and its metabolites with linear responses over the range of physiologically relevant concentrations with practical limits of quantification in the 10 to 50 nM range without signal and/or signal to noise enhancements. Response times of the technique were found to vary between 20 and 60 ms depending on rotation rate. Release and inward transport velocities were shown to be kinetically resolved. The glassy carbon electrode was found to be useful for several years at physiological temperatures without significant changes in electrochemical surface area, residual current, or response factors. Some sources of error in the measurement of release and inward transport values of velocity were noted and described.

Matching kinetics of synaptic vesicle recycling and enhanced neurotransmitter influx by Ca2+ in brain plasma membrane vesicles

Using native plasma membrane vesicle suspensions from the rat cerebral cortex under conditions designed to alter intravesicular [Ca2+], we found that Ca2+ induced 47 +/- 5% more influx of [3H]GABA, [3H]D-aspartate and [3H]glycine at 37 degrees C with half-times 1.7 +/- 0.5, 1.3 +/- 0.4 and 1.3 +/- 0.4 min, respectively. We labelled GABA transporter sites with the uptake inhibitor, [3H]-(R,S)-N-[4,4-bis(3-methyl-2-thienyl)but-3-en-1-yl]nipecotic acid and found that Ca2+ induced a partial dissociation of the bound inhibitor from GABA transporter sites with a similar half-time. By means of rapid kinetic techniques applied to native plasma membrane vesicle suspensions, containing synaptic vesicles stained with the amphipathic fluorescent styryl membrane probe N-(3-triethylammoniumpropyl)-4-[4-(dibutylamino)styryl]pyrid inium dibromide, we have measured the progress of the release and reuptake of synaptic vesicles in response to Ca2+ and high-[K+] depolarization in the 0.0004-100 s range of time. Synaptic vesicle exocytosis, strongly influenced by external [Ca2+], appeared with the kinetics accelerated by depolarization. These results are consistent with the potential involvement of Ca2+ in taking low-affinity transporters to the plasma membrane surface via exocytosis.

Regulation of neurotransmitter release by endogenous nitric oxide in striatal slices

This study sought to determine the potential role of nitric oxide (NO) in N-methyl-D-aspartate (NMDA)-stimulated efflux of [14C] gamma-aminobutyric acid (GABA) and [3H]acetylcholine from striatal slices in vitro. In Mg2+-free buffer, NMDA-stimulated [14C]GABA and [3H]acetylcholine release were inhibited by the guanylate cyclase inhibitor, 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and, to a lesser extent, by the nitric oxide synthase inhibitor, nitroarginine (N-Arg). Since reversal of catecholamine transporters previously has been implicated in the mechanism underlying NO-induced catecholamine release, we used the GABA transport inhibitor, 1-(2-(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine-carboxylic acid hydrochloride (NNC-711), to address the role of GABA transport in NArg-sensitive NMDA-induced release. NNC-711 inhibited NMDA-stimulated [14C]GABA efflux by 50%, confirming our previous report that NMDA-stimulated GABA release is partially dependent on reversal of the transporter. The effect of N-Arg in the presence of NNC-711 was similar to its effect in the absence of the transport inhibitor, suggesting that reversal of the transporter is not involved in the NO component of NMDA-stimulated [14C]GABA release. These data suggest that glutamatergic transmission through striatal NMDA receptors is partially mediated through activation of the NO/guanylate cyclase pathway and that this mechanism may contribute to the tetrodotoxin sensitivity of NMDA-induced release of GABA and acetylcholine in the striatum.

(+/-)-kavain inhibits the veratridine- and KCl-induced increase in intracellular Ca2+ and glutamate-release of rat cerebrocortical synaptosomes

The action of (+/-)-kavain on the veratridine, monensin and KCl-depolarization evoked increase in free cytosolic Ca2+ concentration ([Ca2+]i), and its influence on the release of endogenous glutamate from rat cerebrocortical synaptosomes were investigated. [Ca2+]i was fluorimetrically determined employing FURA as the Ca2+ sensitive fluorophore, and glutamate was detected by a continuous enzyme-linked fluorimetric assay. The incubation of synaptosomes in the presence of (+/-)-kavain up to a concentration of 500 mumol/l affected neither basal [Ca2+]i nor spontaneous release of glutamate, but dose-dependently reduced both veratridine-elevated [Ca2+]i (IC50 = 63.2 mumol/l) and glutamate-release (IC500 = 116.4 mumol/l). The inhibition of these parameters, attained with 500 mumol/l(+/-)-kavain, could be overcome by inducing an artificial Na+ influx, using monensin as a Na+ ionophore, An application of (+/-)-kavain after veratridine caused a decrease in veratridine-elevated [Ca2+]i, which was similar to the action of tetrodotoxin (TTX) with regard to time course, half-life of [Ca2+]i decline and the final steady state level of [Ca2+]i. Concomitantly, veratridine-induced glutamate-release was blocked. The results indicate that specific inhibition of voltage-dependent Na+ channels is a primary target of (+/-)-kavain, thus preventing a [Na+]i provoked increase in [Ca2+]i and glutamate-release. However, pathways related to the elevation of [Ca2+]i by [Na+]i itself, and the processes involved in normalization of elevated [Ca2+]i and glutamate-release downstream to enhanced [Ca2+]i, seems to be unaffected by (+/-)-kavain. Using KCl-depolarized synaptosomes, 400 mumol/l (+/-)-kavain reduced, in analogy to Aga-GI toxin, KCl-evoked [Ca2+]i and diminished the part of glutamate-exocytosis which is related to external Ca2+ to about 75% of control. At a concentration of 150 mumol/l, which is above the IC50 value necessary to block voltage-dependent Na+ channels, (+/-)-kavain affected neither basal nor the KCl-induced increase in [Ca2+]i. These results might suggest that (+/-)-kavain at concentrations sufficient to block Na+ channels completely. moderately inhibits the non-inactivating Ca2+ channels located on mammalian presynaptic nerve endings.

Gabapentin potentiates the conductance increase induced by nipecotic acid in CA1 pyramidal neurons in vitro

The anticonvulsant gabapentin (1-(aminomethyl)cyclohexane acetic acid) has been found to be effective for treatment of partial seizures, but the mechanism of action is unknown. Recent evidence from the rat optic nerve suggests that gabapentin may enhance promoted release of GABA, which is thought to be due to reverse operation of the GABA transporter. We have used whole-cell patch clamp recordings from CA1 pyramidal neurons in hippocampal slices to directly measure currents induced by nipecotic acid (NPA) during exposure to gabapentin. Under control conditions, pressure microejection of NPA increased whole-cell conductance with a reversal potential equal to the chloride equilibrium potential. This response was mimicked by GABA application, and blocked by bicuculline. The response to NPA was also present after blockade of synaptic transmission in the presence of calcium-free solution. These results are consistent with NPA promoting nonvesicular release of GABA from neighboring neurons or glia via reverse operation of the GABA uptake system, which then activated GABAA receptors on the recorded neurons. In control solution, the response to NPA slowly decreased over 45 min to approximately 50% of the initial response, consistent with GABAA receptor 'rundown'. However, in the presence of gabapentin there was a slow increase in the response, reaching approximately 170% of the control level after 45 min of gabapentin exposure. These results demonstrate that gabapentin enhances the promoted release of GABA by more than three-fold. The potentiation of the NPA response may be due to gabapentin increasing cytosolic GABA in neighboring cells via a delayed metabolic effect, and would have the functional effect of increasing neuronal inhibition during periods of hyperexcitability.

Role of GABAB receptors in intracellular Ca2+ homeostasis and possible interaction between GABAA and GABAB receptors in regulation of transmitter release in cerebellar granule neurons

The expression of GABAB receptors in cultured mouse cerebellar granule cells was investigated in binding experiments using [3H](S,R)-baclofen as well as in functional assessment of the ability of (R)-baclofen to interact with depolarization (15-40 mM KCl) coupled changes in intracellular Ca2+ homeostasis and neurotransmitter release. In the latter case a possible functional coupling between GABAA and GABAB receptors was investigated. The binding studies showed that the granule cells express specific binding sites for (R)-baclofen. The number of binding sites could be increased by exposure of the cells to the GABAA receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) during the culture period. Pretreatment of the neurons with pertussis toxin showed that the GABAB receptors are coupled to G-proteins. This coupling was, however, less pronounced when the cells had been cultured in the presence of THIP. When 45Ca2+ uptake was measured or the intracellular Ca2+ concentration ([Ca2+]i) determined using the fluorescent Ca2+ chelator Fluo-3 it could be demonstrated that culturing the neurons in THIP influences intracellular Ca2+ homeostasis. Moreover, this homeostasis was found to be functionally coupled to the GABAB receptors as (R)-baclofen inhibited depolarization-induced increases in 45Ca2+ uptake and [Ca2+]i. (R)-Baclofen also inhibited K(+)-induced transmitter release from the neurons as monitored by the use of [3H]D-aspartate which labels the neurotransmitter pool of glutamate. Using the selective GABAA receptor agonist isoguvacine it could be demonstrated that the GABAB receptors are functionally coupled to GABAA receptors in the neurons leading to a disinhibitory action of GABAB receptor agonists.

Modulation of GABA flux across rat brain membranes resolved by a rapid quenched incubation technique

The progress and inhibition of [3H]GABA influx in native plasma membrane vesicles from the rat cerebral cortex was studied on a subsecond to minute time scale under different conditions by applying a rapid quenched incubation technique. In the absence of Ca2+ ([Ca2+]free = 10(-8) M), the progress of influx followed by the addition of 10 nM [3H]GABA to the membrane vesicle suspension with time (500 ms to 15 min) can be described by a first-order rate equation giving an overall rate constant, k, of 3.93 +/- 0.48 x 10(-3) s-1 and equilibrium influx value, INFe, of 8.84 +/- 0.41 pmol [3H]GABA/mg protein. In the presence of Ca2+ ([Ca2+]free = 2.4 x 10(-3) M) a significant increase in the INFe value was observed (k = 4.64 +/- 0.41 x 10(-3) s-1 and INFe = 13.9 +/- 0.40 pmol [3H]GABA/mg protein). Multiplicity of GABA transporters was indicated in the time-dependent inhibition of [3H]GABA influx by different uptake blockers. In the absence of Ca2+, depolarization (75 mM KCl) inhibited the influx of [3H]GABA into the vesicles by approximately 70% and initiated the efflux from vesicles loaded with [3H]GABA. Different uptake blockers inhibited the Ca(2+)-independent translocation of [3H]GABA in both directions with similar specificities.

The use of gadolinium to investigate the relationship between Ca2+ influx and glutamate release in rat cerebrocortical synaptosomes

Gadolinium chloride is a potent blocker of voltage-sensitive Ca2+ channels. When added to fura-2 loaded rat cortical synaptosomes 1 min before depolarization with 33 mM KCl, it causes a dose-dependent inhibition of the resulting rise in intrasynaptosomal free Ca2+ with an IC50 of approximately 10 microM. The effect of GdCl3 on intrasynaptosomal free Ca2+ is not accompanied by an equivalent effect on Ca(2+)-dependent glutamate release. In the presence of 100 microM GdCl3, 33 mM KCl does not produce a detectable change in the fura-2 signal but Ca(2+)-dependent glutamate release is only reduced by around 12%. Using BaCl2, which is less effectively buffered in synaptosomes than Ca2+, we have shown that there is residual KCl-stimulated divalent cation entry into synaptosomes in the presence of 100 microM GdCl3. These data, combined with those from other laboratories, strengthen the argument for localized Ca2+ entry through Ca2+ channels linked to neurotransmitter release from synaptosomes and add to the evidence that the channels may exhibit considerable neurotransmitter specificity.

Na+ influx through Ca2+ channels can promote striatal GABA efflux in Ca(2+)-deficient conditions in response to electrical field depolarization

Electrical field depolarization releases gamma-aminobutyric acid (GABA) in rat striatal slices in the absence of external Ca2+. omega-Conotoxin GVIA (omega-CgTx; 1-50 nM), a neuronal Ca2+ channel blocker, inhibits electrically evoked efflux of newly taken up [3H]GABA in a concentration-dependent manner in either normal or Ca(2+)-free medium. This suggests that ion influx occurs through Ca2+ channels in the absence of external Ca2+ and contributes to the efflux of GABA. Reducing external Na+ concentration to 27.25 mM (low [Na+]o medium) by equimolarly substituting choline chloride for sodium chloride has differential effects on electrically evoked GABA efflux depending on the external Ca2+ concentrations. In normal Ca2+ medium, electrically evoked GABA efflux increases whereas, in Ca(2+)-free medium, it is greatly inhibited when [Na+]o is reduced to 27.25 mM. In low [Na+]o medium, GABA efflux is largely tetrodotoxin (TTX)-sensitive, however, spike firing evoked by antidromic stimulation of striatal cells is inhibited. In Na(+)-free medium, resting GABA efflux increases 17-fold whereas evoked GABA efflux diminishes. In Ca(2+)-free medium, 70 min of incubation with 1-2-bis-(1-aminophenoxy)ethane-N,N,N',N' tetraacetoxy methyl ester (BATPA-AM, 1 microM), an intracellular calcium chelator, increases both resting GABA efflux and electrically evoked GABA overflow by approximately 100%. These results suggest that: (1) in Ca(2+)-free conditions, Na+ permeability of cells increases via Ca2+ channels and this profoundly affects GABA efflux. (2) Electrical field depolarization is likely to release GABA by directly depolarizing axon terminals. (3) Ca(2+)-independent GABA efflux is not promoted by an increase in intracellular free Ca2+ concentration via Na+/Ca2+ exchange processes from internal pools.

Pharmacological characterisation of the calcium channels coupled to the plateau phase of KCl-induced intracellular free Ca2+ elevation in chicken and rat synaptosomes

The effect of various blockers of voltage operated calcium channels (VOCCs) was studied on the non-inactivating, plateau phase of KCl-induced intracellular free Ca2+ ([Ca2+]i) elevation in rat cortical and chicken forebrain synaptosomes. In chicken synaptosomes, omega-CgTx GVIA (0.1 nM to 1 microM) and omega-CgTx MVIIA (0.1 nM to 1 microM), both selective blockers of N-type Ca2+ channels, produced a concentration-dependent inhibition of the plateau phase of [Ca2+]i elevation. omega-CgTx GVIA (IC50 value 28 nM) was more potent than omega-CgTx MVIIA (IC50 value 78 nM), but at submaximal concentrations, took longer to reach its maximum effect (20 min for omega-CgTx GVIA; 10 min for omega-CgTx MVIIA). At 1 microM, the highest concentration tested, each toxin blocked > 85% of [Ca2+]i elevation. The effect of omega-CgTx GVIA on the extent and time-course of inhibition of [Ca2+]i elevation was maintained in a Na(+)-free, choline substituted, medium. omega-Aga IVA (300 nM), a selective blocker of P-type calcium channels, inhibited 28 +/- 5% of [Ca2+]i elevation. The effect of a combination of submaximal inhibitory concentrations of omega-CgTx GVIA (100 nM) and omega-Aga IVA (300 nM) was less than additive. In rat synaptosomes, omega-CgTx GVIA (1 microM) and omega-CgTx MVIIA (1 microM), blocked only 18 +/- 5% and 17 +/- 4% of the plateau phase of free Ca2+ elevation, respectively. omega-Aga IVA produced a concentration-dependent inhibition of [Ca2+]i elevation in this preparation. Threshold inhibition was observed at 1 nM, and maximum inhibition (64 +/- 8%) at 1 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the release of Met-enkephalin from isolated nerve terminals: release kinetics and cation-dependence

The release of the neuropeptide Met-enkephalin (Met-ENK) from isolated nerve terminals (synaptosomes) of the rat forebrain was characterized with respect to the subcellular distribution, the release upon addition of various stimulatory agents, the release kinetics, the cation-dependence of release and the relationship between Met-ENK release and elevations of the intraterminal free Ca(2+)-concentration ([Ca]i). A highly specific radioimmunoassay was developed for determination of Met-ENK (H-Tyr-Gly-Gly-Phe-Met-OH). Truncated and elongated forms of Met-ENK, Leu-enkephalin, beta-endorphin and dynorphin displayed negligible cross-reactivity. Met-ENK-like immunoreactivity (Met-ENK-LI) is enriched in the purified synaptosomal fraction of rat forebrain homogenates and is released in a strictly Ca(2+)-dependent manner upon chemical depolarization or stimulation with the Ca2+ ionophore, ionomycin. A correlation exists between the release of Met-ENK-LI and the elevations of [Ca]i. Barium ions are able to replace Ca2+ in triggering Met-ENK-LI release. The release of Met-ENK-LI is initiated within 20 s after depolarization and is terminated after 3-5 min, although depolarization and [Ca]i elevation are maintained. At this time, > 90% of the initial Met-ENK-LI is still present inside the synaptosomes. Repolarization and renewed stimulation again evokes Ca(2+)-dependent release of this retained Met-ENK-LI. It is concluded that Met-ENK release from isolated nerve terminals is exocytotic, and that exocytosis is terminated by a regulatory mechanism in synaptosomes after 3-5 min of depolarization, a process which can be reversed by repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel large-conductance Ca(2+)-activated potassium channel and current in nerve terminals of the rat neurohypophysis

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Terminal membrane currents were studied using the 'whole-cell' patch clamp technique and channels were studied using inside-out and outside-out patches. 2. In physiological solutions, but with 7 mM 4-aminopyridine (4-AP), depolarizing voltage clamp steps from different holding potentials (-90 or -50 mV) elicited a fast, inward current followed by a slow, sustained, outward current. This outward current did not appear to show any steady-state inactivation. 3. The threshold for activation of the outward current was -30 mV and the current-voltage relation was 'bell-shaped'. The amplitude increased with increasingly depolarized potential steps. The outward current reversal potential was measured using tail current analysis and was consistent with that of a potassium current. 4. The sustained potassium current was determined to be dependent on the concentration of intracellular calcium. Extracellular Cd2+ (80 microM), a calcium channel blocker, also reversibly abolished the outward current. 5. The current was delayed in onset and was sustained over the length of a 150 ms-duration depolarizing pulse. The outward current reached a peak plateau and then decayed slowly. The decay was fitted by a single exponential with a time constant of 9.0 +/- 2.2 s. The decay constants did not show a dependence on voltage but rather on intracellular Ca2+. The time course of recovery from this decay was complex with full recovery taking > 190 s. 6. 4-AP (7 mM), dendrotoxin (100 nM), apamin (40-80 nM), and charybdotoxin (10-100 nM) had no effect on the sustained outward current. In contrast Ba2+ (200 microM) and tetraethylammonium inhibited the current, the latter in a dose-dependent manner (apparent concentration giving 50% of maximal inhibition (IC50) = 0.51 mM). 7. The neurohypophysial terminal outward current recorded here corresponds most closely to a Ca(2+)-activated K+ current (IK(Ca)) and not to a delayed rectifier or IA-like current. It also has properties different from that of the Ca(2+)-dependent outward current described in the magnocellular neuronal cell bodies of the hypothalamus. 8. A large conductance channel is often observed in isolated rat neurohypophysial nerve terminals. The channel had a unit conductance of 231 pS in symmetrical 150 mM K+.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium channels coupled to glutamate release identified by omega-Aga-IVA

Presynaptic calcium channels are crucial elements of neuronal excitation-secretion coupling. In mammalian brain, they have been difficult to characterize because most presynaptic terminals are too small to probe with electrodes, and available pharmacological tools such as dihydropyridines and omega-conotoxin are largely ineffective. Subsecond measurements of synaptosomal glutamate release have now been used to assess presynaptic calcium channel activity in order to study the action of peptide toxins from the venom of the funnel web spider Agelenopsis aperta, which is known to inhibit dihydropyridine and omega-conotoxin-resistant neuronal calcium currents. A presynaptic calcium channel important in glutamate release is shown to be omega-Aga-IVA sensitive and omega-conotoxin resistant.

Ca(2+)-dependent release of [3H]GABA in cultured chick retina cells

Depolarization by K+ (50 mM) of cultured chick retina cells released 1.14 +/- 0.28% of the accumulated [3H] gamma-aminobutyric acid (GABA) in the absence of Ca2+, but when 1.0 mM Ca2+ was present, the internal free calcium ion concentration [Ca2+]i rose by about 750 nM and the [3H]GABA release about doubled to a value of 2.22 +/- 0.2% of the total [3H]GABA. Nitrendipine (0.1 microM), a blocker of the L-type Ca2+ channels, blocked the [Ca2+]i response to K+ depolarization by about 65%, and the omega-Conotoxin GVIA (omega-CgTx) (0.5 microM), a blocker of the N-type of Ca2+ channels, inhibited by 27% the [Ca2+]i rise due to K+ depolarization. Parallel experiments showed that nitrendipine inhibits [3H]GABA release to the level observed in the absence of Ca2+, whereas omega-CgTx did not inhibit significantly the release of [3H]GABA. The results also show that the release of [3H]GABA due to K(+)-depolarization in the absence of Ca2+ can be totally blocked by 1-(2-(((Diphenylmethylene) amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711), an inhibitor of the GABA carrier. However, in the presence of Ca2+, NNC-711 blocks the release only by about 66%, corresponding to the Ca(2+)-independent release. Thus, it is concluded that [3H]GABA is released in chick retina cells by the exocytotic mechanism, which is Ca(2+)-dependent, and by reversal of the carrier, which is Ca(2+)-independent, in much the same way as has been found for other GABAergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effect of depolarizing agents on the Ca(2+)-independent and Ca(2+)-dependent release of [3H]GABA from sheep brain synaptosomes

The purpose of the present study was to compare the effects of several depolarizing agents on both the membrane potential and on the release of [3H] gamma-aminobutyric acid (GABA) from sheep brain cortex synaptosomes. We examined the effects of KCl, 4-aminopyridine (4-AP), veratridine, ouabain and tetraphenylphosphonium cation (TPP+) on Ca(2+)-independent (carrier-mediated) and Ca(2+)-dependent (exocytotic) release. We found that, in the absence of Ca2+, KCl at 40 mM releases 7.57 +/- 0.65%, veratridine at 50 microM releases 45.85 +/- 2.48%, ouabain at 1 mM releases 8.62 +/- 0.93% and TPP+ at 1 mM releases 4.09 +/- 0.37% of the total accumulated neurotransmitter, provided that the external medium contains Na+. These are about the maximal values of release obtained with each depolarizing agent in a Na+ medium and in the absence of Ca2+. Replacing external Na+ with choline blocks the release observed in the presence of the depolarizing agents in the absence of Ca2+, and this divalent ion can increase [3H]GABA release only for K+ or 4-AP. Synaptosomal depolarization requires Na+ except for K+ depolarization. Furthermore, although Ca2+ stimulates the release of [3H]GABA due to K+ depolarization (13.56 +/- 0.44%) or due to 4-AP (4.26 +/- 0.51%), it inhibits the release due to the other depolarizing agents. The amount of [3H]GABA released by 4-AP in Na+ medium (4.26 +/- 0.51%) is similar to that induced by KCl in the presence of Ca2+ in the absence of Na+ (3.39 +/- 0.29%) which represents only exocytotic release. This suggests that the Ca(2+)-dependent exocytotic release of [3H]GABA can be specifically induced by 4-AP in a Na+ medium, or by KCl in the absence of Na+, as reported by us earlier. The observation that Ca2+ inhibits the Ca(2+)-independent release is of interest because it suggests that Ca2+ may modulate the release of cytoplasmic GABA probably by inhibiting the carrier-mediated release of GABA. It is of interest as to whether Ca2+ regulation depends on intracellular Ca2+.

Inhibition of transporter mediated gamma-aminobutyric acid (GABA) release by SKF 89976-A, a GABA uptake inhibitor, studied in a primary neuronal culture from chicken

The effect of SKF 89976-A, a lipophilic non-substrate inhibitor of the gamma-aminobutyric acid (GABA) transporter, on the release of radioactive GABA and D-aspartate has been studied. Neuronal cultures from 8 day old chick embryos, grown for six days, served as a model. The cultures were incubated with [3H] D-aspartate and [14C] GABA with the subsequent addition of high or low concentrations of SKF 89976-A. Finally the cultures were exposed to differently composed media for either 30 or 300 seconds. The release was quantified, using liquid scintillation counting. The efflux of [3H] D-aspartate and [14C] GABA was increased by [K+] and time, and a minimum value was obtained at [Ca2+] 1.05 mM. The release of both [3H] D-aspartate and [14C] GABA was inhibited by SKF 89976-A. The obtained results indicate that transporter mediated processes are the major mechanisms of transmitter release in the investigated model.

Perfusion of Immobilized Isolated Nerve Terminals as a Model for the Regulation of Transmitter Release: Release of Different, Endogenous Transmitters, Repeated Stimulation, and High Time Resolution

To study the release of neurotransmitters, i.e., the recruitment of transmitters for release and the regulation of the release process, isolated nerve terminals (synaptosomes) of the rat forebrain were immobilized in Sephadex gel inside a perfusion chamber. In this way, the following were achieved: (a) A very limited pressure stress was exerted on the synaptosomes, so that these remained viable for long periods (greater than 30 min) inside the chamber and did not elute from the chamber, which allowed long-term experiments with repeated stimulations; (b) estimation of the release of various endogenous transmitters, both in a Ca(2+)-dependent (exocytotic) and Ca(2+)-independent manner; (c) a step-like stimulation with depolarizing agents (rise time, 3-4 s) and a high time resolution (600-ms sampling); and (d) negligible reuptake of transmitter into the terminals or extracellular breakdown. It is concluded that this perfusion setup helps to provide new insights in the presynaptic stimulus-secretion coupling, co-transmission, and the exo-endocytosis cycle.

Presynaptic modulation of amino acid release from synaptosomes

Using synaptosomes prepared from whole rat brain, the spontaneous, calcium-independent, and calcium-dependent release of glutamate and GABA was assessed. Time intervals of 1-30 seconds were studied. Spontaneous release of glutamate (but not GABA) was elevated by 10 microM NMDA or AMPA by thirty seconds. This stimulation was partially calcium-dependent. Calcium-dependent release induced by 30 mM KCl was biphasic, confirming previous findings. This release was stimulated at all time periods by the presence of 10 microM NMDA or AMPA in an antagonist-sensitive manner. These data suggest that glutamate and GABA are released from vesicular stores in rat synaptosomes and that some of this release is modulated by presynaptic glutamate receptors.

Rapid kinetics of potassium-evoked release of acetylcholine from rat brain synaptosomes: analysis by rapid superfusion

The rapid kinetics of spontaneous and evoked [3H]acetylcholine efflux from synaptosomes was investigated using the technique of rapid superfusion. Synaptosomes were isolated from whole rat brain and the intraterminal pool of acetylcholine was radiolabeled by preincubation with [3H]choline. Synaptosomes were retained within the superfusion system on filter disks and superfused with Krebs-bicarbonate buffer, pH 7.4, at flow rates of 0.3-0.5 ml/s. These experimental conditions provided a mixing half-life of 119 ms and efficiency of superfusion of greater than 85%. The kinetics of tritium efflux was followed on the second and subsecond time scales by collection of serial 4.8-s and 50-ms samples for a total of 67.2 and 1.0 s, respectively. Superfusion for 48 s with isoosmotic Krebs buffer containing 10, 20, 30, 50, 75, and 100 mM potassium ion stimulated concentration-dependent tritium release. All of potassium-evoked release, but only 17% of spontaneous release, was calcium-dependent. Kinetic analysis of net (total minus spontaneous) potassium-stimulated release revealed a single calcium-dependent component of release that fit a single exponential function with a half-life of 12.7 s. Analysis of the area under the tritium efflux curves observed on the millisecond time scale revealed that 0.111, 0.550, and 0.614% net tritium release was evoked by superfusion for 750 ms with isoosmotic buffer containing 20, 50, and 100 mM KCl, respectively. Consistent with the results observed on the second time scale, a small fraction of spontaneous release and all of potassium-evoked release observed on the millisecond time scale were calcium-dependent. These data indicate that the technique of rapid superfusion can be utilized for the direct investigation of spontaneous and evoked [3H]acetylcholine release, as well as the factors that regulate this release from brain synaptosomes on the second and millisecond time scales.

On the inhibition of GABA release from a preparation of rat brain nerve-endings by SKF 89976-A, a GABA uptake inhibitor

Fractions enriched in nerve-terminals were made in order to investigate the release of gamma-aminobutyric acid (GABA). A P2-preparation made from whole rat brain was pre-labeled with [3H]GABA and thereafter incubated in high concentrations (1 mM) of the GABA uptake inhibitor SKF 89976-A. The nerve-terminals were thereafter incubated in depolarizing concentrations of KCl and finally the medium and nerve-terminals separated by a filtration procedure. The amount of [3H]GABA present on the filter and in the supernatant was measured by liquid scintillation counting and used to assess the release rate. SKF 89976-A reduced the KCl-induced efflux of [3H]GABA from the nerve-terminals. The inhibition was as effective against KCl-induced [3H]GABA efflux in the presence as well as in the absence of Ca2+. Our finding may demonstrate an inhibition of the inside-out activity of the GABA transporter.

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

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

Calcium as a coagonist of inositol 1,4,5-trisphosphate-induced calcium release

Inositol 1,4,5-trisphosphate (IP3)-induced calcium release from intracellular stores is a regulator of cytosolic-free calcium levels. The subsecond kinetics and regulation of IP3-induced calcium-45 release from synaptosome-derived microsomal vesicles were resolved by rapid superfusion. Extravesicular calcium acted as a coagonist, potentiating the transient IP3-induced release of calcium-45. Thus, rapid elevation of cytosolic calcium levels may trigger IP3-induced calcium release in vivo. Extravesicular calcium also produced a more slowly developing, reversible inhibition of IP3-induced calcium-45 release. Sequential positive and negative feedback regulation by calcium of IP3-induced calcium release may contribute to transients and oscillations of cytosolic-free calcium in vivo.

Differential release of amino acids, neuropeptides, and catecholamines from isolated nerve terminals

We have investigated transmitter release from small and large dense-core vesicles in nerve terminals isolated from guinea pig hippocampus. Small vesicles are found in clusters near the active zone, and large dense-core vesicles are located at ectopic sites. The abilities of Ca2+ channel activation and uniform elevation of Ca2+ concentration (with ionophores) to evoke secretion of representative amino acids, catecholamines, and neuropeptides were compared. For a given increase in Ca2+ concentration, ionophore was less effective than Ca2+ channel activation in releasing amino acids, but not in releasing cholecystokinin-8. Titration of the average Ca2+ concentration showed that the Ca2+ affinity for cholecystokinin-8 secretion was higher than that for amino acids. Catecholamine release showed intermediate behavior. It is concluded that neuropeptide release is triggered by small elevations in the Ca2+ concentration in the bulk cytoplasm, whereas secretion of amino acids requires higher elevations, as produced in the vicinity of Ca2+ channels.

Transmitter glutamate release from isolated nerve terminals: evidence for biphasic release and triggering by localized Ca2+

The kinetics of Ca2(+)-dependent release of glutamate from guinea-pig cerebrocortical synaptosomes evoked by KCl or 4-aminopyridine are investigated using a continuous fluorimetric assay. Release by both agents is biphasic, with a rapid phase complete within 2 s followed by a more extensive slow phase with a half-maximal release in 52 s for KCl-evoked release and greater than 120 s for 4-aminopyridine-evoked release. The two phases of glutamate release may reflect a dual localization of releasable vesicles at the active zone and in the bulk cytoplasm. Decreasing depolarization depresses the extent rather than increasing the time for half-maximal Ca2(+)-dependent release. Both the fast and the slow phases of glutamate release require external Ca2+ and cytoplasmic ATP. KCl depolarization produces a transient "spike" of cytoplasmic free Ca2+ [( Ca2+]c), which recovers to a plateau; the major component of glutamate release occurs during this plateau. Predepolarization in the absence of added external Ca2+, to inhibit transient Ca2+ channels, does not affect the subsequent glutamate release evoked by Ca2+ readdition. Thus, release involves primarily noninactivating Ca2+ channels. For a given increase in [Ca2+]c, KCl and 4-aminopyridine cause equal release of glutamate, while ionomycin releases much less glutamate. This lowered efficiency is not due to ATP depletion. It is concluded that glutamate exocytosis is evoked by localized Ca2+ entering through noninactivating voltage-dependent Ca2+ channels and that nonlocalized Ca2+ entry with ionomycin is inefficient.

Activity-dependent recruitment of endogenous glutamate for exocytosis

The Ca(2+)-dependent release of the neurotransmitter glutamate from purified nerve terminals (synaptosomes) of the rat hippocampus was studied in a rapid perfusion apparatus. The response of the terminals was investigated with respect to the kinetics and duration of the release of endogenous glutamate upon brief and sustained stimulation and upon repetitive stimulation. The synaptosomes were stimulated by sustained chemical depolarization (0.5-3 min 30 mM K+). The cellular levels of glutamate, free Ca2+ and ATP in the nerve terminals were measured. The Ca(2+)-dependent release of glutamate showed an immediate elevation upon K(+)-depolarization. When the stimulation was maintained, a prolonged phase of glutamate release was observed. After 3 min, the Ca(2+)-dependent release stopped, although K(+)-depolarization was still effective. When synaptosomes were stimulated again after a relatively short stimulation period (30 s), the second response was similar to the previous one. After a longer stimulation period, maintained until termination of release, the second response did not show the immediate initial elevation of Ca(2+)-dependent glutamate release. Only 30 s after stimulation the release developed with a time profile comparable to the first response. This initial lack of response was not due to low cytosolic levels of glutamate or ATP or to changes in cellular Ca(2+)-buffering. It can be concluded that the capacity to release glutamate after brief depolarizations is fully restored during the repolarization period. However, if stimulation periods are of long duration (until termination of release), this capacity is no longer fully restored, especially with respect to a fast component of release. New glutamate is recruited only during the subsequent depolarization and with a delay.(ABSTRACT TRUNCATED AT 250 WORDS)

Two types of calcium channels coexist in peptide-releasing vertebrate nerve terminals

The properties of the Ca2+ channels mediating transmitter release in vertebrate neurons have not yet been described with voltage-clamp techniques. Several types of voltage-dependent Ca2+ channels are known to exist on neuronal somata, but the small size and inaccessibility of most vertebrate nerve endings have precluded direct characterization of the presynaptic channels. However, large nerve endings, which release the peptides oxytocin and vasopressin in a Ca2(+)-dependent manner, can be dissociated from the rat neurohypophysis. Using both single-channel and whole-cell patch-clamp techniques, we have characterized two types of Ca2+ channels that coexist in these terminals. One is a large-conductance, high-threshold, dihydropyridine-sensitive channel that contributes a slowly inactivating current. The second is a smaller conductance channel, which is also activated at high thresholds, but underlies a rapidly inactivating, dihydropyridine-insensitive current. Both types of Ca2+ channels may participate in the peptide release process.

A superfusion system designed to measure release of radiolabeled neurotransmitters on a subsecond time scale

A new method for subsecond measurement of release of neurotransmitters from nerve terminal preparations (e.g., synaptosomes) in vitro is described. Synaptosomes were prelabeled with [3H]GABA via a Na-dependent GABA uptake system. The prelabeled nerve terminals are retained on small glass fiber filters in a superfusion chamber accessed by three high speed, solenoid-driven valves. Microcomputer-programmed circuitry controls the timing of valve operation. Each valve controls the delivery of a separate solution to the chamber, permitting rapid and independent control of membrane potential, [Ca2+]e, and drug delivery. The minimal dead volume of the chamber and the relatively high solution flow rate afford time resolution for release of at least 60 ms. This time resolution was necessary to observe the most rapid of at least three components of GABA release.