Stimulation of [3H]gamma-aminobutyric acid release by calcium chelators in synaptosomes

Depolarization-evoked GABA release from myenteric plexus is partially coupled to L-, N-, and P/Q-type calcium channels

1. There are many evidences suggesting that gamma-aminobutyrate (GABA) is an important neurotransmitter and/or neuromodulator in the gut. 2. Using the myenteric plexus-longitudinal muscle preparation from the guinea pig ileum, we investigated the evoked release of [3H] GABA from enteric neurons by electrical pulses or high KCl, which occurs in a calcium-dependent and -independent way. In addition, using selective calcium channel blockers, we report the participation of distinct subtypes of calcium channels in the evoked release, showing a minor participation of L- and Q-type calcium channels, while N- and P-type have a participation of approximately 15%, each. However, regardless of the combination of Ca2+ channel blockers, we did not observe an inhibition greater than 50% of the calcium-dependent component of [3H] GABA release. 3. Thus, while the observed Ca2+-independent release mostly probable occur via reversal of the membrane GABA transporter, in our conditions, a considerable portion of the Ca2+-dependent evoked release of [3H] GABA is not coupled to L-, N-, or P/Q-type calcium channels, suggesting the involvement of intracellular calcium stores or other ways of getting calcium across the membrane.

An ultrastructural analysis of tissue surrounding a microdialysis probe

Microdialysis is a widely used in vivo sampling technique commonly used to monitor extracellular levels of a variety of molecules including neurotransmitters and metabolites. To facilitate interpretation of microdialysis results, this study critically examines changes in synaptic morphology induced by microdialysis. Tissue surrounding microdialysis probes was examined using light and electron microscopy at three distances from the probe tract. Microdialysis probes were implanted into rat striatum, and after 40 h of post-operative recovery were perfused with a modified Ringer's solution. Light microscope analysis revealed tissue disruption up to 1.4 mm from the probe site. Axonal damage indicative of non-excitotoxic insult was also seen as far away from the probe as was examined. The presence of dark-degenerating neurons was also noted and estimates of neuronal densities revealed loss up to 400 microm from the probe tract. This study, the first qualitative ultrastructural investigation of neuropil surrounding the probe site, indicated swollen processes up to 1.4 mm from the probe tract. Swollen mitochondria and bloated endoplasmic reticulum suggest intracellular chemical disruption. Tissue damage resulting in synaptic and neuronal disruption may affect neurotransmitter efflux or extracellular concentrations of metabolites.

Regulation of the gamma-aminobutyric acid transporter activity by protein phosphatases in synaptic plasma membranes

The influence of the phosphorylation dephosphorylation states on the gamma-aminobutyric acid (GABA) transporter activity of synaptic plasma membranes (SPM) was studied by using either specific phosphatase inhibitors or activators. Calyculin A and okadaic acid (phosphatase 1 and phosphatase 2A inhibitors) inhibited the GABA uptake by isolated SPM vesicles, whereas cyclosporin A (phosphatase 2B inhibitor) had a stimulatory effect (approximately 10%) which was higher (approximately 38%) when all these drugs were present in the reaction medium. On the other hand, intravesicular Ca2+, up to about 10 microM, inhibited the GABA uptake (approximately 50%) in a manner which appeared to be facilitated in the presence of PP1 and PP2A inhibitors and this inhibition was relieved by the calmodulin antagonist W-7. We also observed that isolated SPM vesicles contain both Ca(2+)-independent phosphatase activity that is significantly inhibited by PP1 and PP2A inhibitors, and Ca(2+)-dependent phosphatase activity that is abolished in the presence of the PP2B inhibitor, cyclosporin A. These results indicate that regulation of the SPM GABA transporter is determined by the internally localized Ca-calmodulin-dependent phosphatase activity (calcineurin), and that other phosphorylated sites, sensitive to PP1 and PP2A inhibitors, potentiate either the positive or negative effects exerted by those internal sites when they are in their phosphorylated or dephosphorylated states, respectively.

Regulation of [gamma-3H]aminobutyric acid transport by Ca2+ in isolated synaptic plasma membrane vesicles

We studied the effect of Ca2+ on the transport of the gamma-aminobutyric acid (GABA) by synaptic plasma membrane (SPM) vesicles isolated from sheep brain cortex and observed that intravesicular Ca2+ inhibits the [3H]GABA accumulation in a concentration-dependent manner. This inhibitory effect of Ca2+ exhibited two distinct components: one in the micromolar range of Ca2+ concentration, and the other in the millimolar range. Previous EGTA washing of the membranes, or incorporation of trifluoperazine into the vesicular space reduced the inhibitory action of Ca2+, particularly at low Ca2+ (1-5 microM). Okadaic acid (1 microM) also relieved the Ca2+ inhibition at low, but not at high Ca2+ concentrations (1 mM), whereas the calpain inhibitor I did not alter the effect of the low Ca2+, but it partially reduced (approximately 28%) the effect of Ca2+ in the millimolar range. The results indicate that the GABA transporter is regulated by low Ca2+ concentration (microM) and probably its effect is mediated by the (Ca2+ x calmodulin)-stimulated phosphatase 2B (calcineurin). In contrast, the GABA uptake inhibition observed at high Ca2+ concentrations (1 mM) is less specific, and probably it is partially related to the proteolytic activity of membrane bound calpain II.

Glycine and GABAA receptor-mediated chloride fluxes in synaptoneurosomes from different parts of the rat brain

Strychnine-sensitive, inhibitory glycine receptors have not until lately been considered to play a significant role in neurotransmission in mammalian forebrain regions. In order to investigate the role of glycine as a neurotransmitter in brain we have measured glycine induced chloride fluxes in different adult rat forebrain areas using synaptoneurosomes and a chloride-sensitive fluorescent indicator. The results have been compared to those obtained with GABA. The synaptoneurosomes from every brain area investigated responded to both glycine and GABA with chloride fluxes in a picrotoxin sensitive manner. The effect of glycine was inhibited by strychnine, which had no effect on the GABA-induced Cl-flux. Bicuculline inhibited the effect of GABA, but had no effect on the glycine-induced Cl-flux. Addition of GABA did not affect the response to glycine and vice versa. The endogenous content of glycine and GABA in the synaptoneurosome preparations was about the same and synaptoneurosomes from every brain area investigated released both glycine and GABA upon depolarisation with KCl. The depolarisation induced release of both GABA and glycine was partly Ca(2+)-dependent and partly Ca(2+)-independent. These results indicate that glycine can induce inhibitory Cl- fluxes distinct from GABA induced fluxes in every investigated brain area and that glycine can be released upon depolarisation.

Transmitter release by non-receptor activation of the alpha-subunit of guanine nucleotide regulatory protein in rat striatal slices

The effects of 5 mM NaF + 10 microM AlCl3, a direct activator of guanine nucleotide-binding proteins (G proteins), on the release of [3H]dopamine ([3H]DA), [3H]gamma-aminobutyric acid ([3H]GABA), and [3H]acethylcholine ([3H]ACh) were investigated in slices of rat striatum. When the tissue was exposed to NaF + AlCl3 the release of [3H]DA, [3H]GABA, and [3H]ACh was enhanced significantly. In a calcium-free solution the release of [3H]GABA and [3H]DA was increased by NaF+AlCl3 much more than in the presence of [Ca2+]o. In slice preparations taken from reserpinized animals, in which the vesicular storage of [3H]DA was therefore prevented, NaF + AlCl3 had no effect on [3H]DA release. HPLC analysis of the radioactivity of the perfusate showed that, in the presence of NaF + AlCl3, the content of dihydroxyphenylacetic acid (DOPAC) in perfusate samples increased significantly, while in pargyline-treated animals only the DA content was increased. Inhibition of DA carriers by nomifensine or low temperature prevented the effect of NaF + AlCl3. N-ethylmaleimide (NEM) preincubation did not modify the effect of NaF + AlCl3 on [3H]DA release Neomycin (0.1 mM), a phospholipase C (PLC) inhibitor, significantly decreased the effect of NaF + AlCl3 on [3H]DA and [3H]GABA release. The internal concentration of Ca2+ in synaptosomes was enhanced by NaF + AlCl3 in normal solution. However, [Ca2+]i was not influenced by NaF + AlCl3 in Ca(2+)-free medium. It is concluded that a non-receptor-mediated activation, by NaF + AlCl3, of the alpha-subunit of a G protein, results in a [Ca2+]o-independent release of DA and GABA, but not that of ACh.

Characterization of the carrier-mediated [3H]GABA release from isolated synaptic plasma membrane vesicles

Synaptic plasma membrane (SPM) vesicles were isolated under conditions which preserve most of their biochemical properties. Therefore, they appeared particularly useful to study the cytoplasmic GABA release mechanism through its neuronal transporter without interference of the exocytotic mechanism. In this work, we utilized SPM vesicles isolated from sheep brain cortex to investigate the process of [3H]GABA release induced by ouabain, veratridine and Na+ substitution by other monovalent cations (K+, Rb+, Li+, and choline). We observed that ouabain is unable to release [3H]GABA previously accumulated in the vesicles and, in our experimental conditions, it does not act as a depolarizing agent. In contrast, synaptic plasma membrane vesicles release [3H]GABA when veratridine is present in the external medium, and this process is sensitive to extravesicular Na+ and it is inhibited by extravesicular Ca2+ (1mM) under conditions which appear to permit its entry. However, veratridine-induced [3H]GABA release does not require membrane depolarization, since this drug does not induce any significant alteration in the membrane potential, which is determined by the magnitude of the ionic gradients artificially imposed to the vesicles. The substitution of Na+ by other monovalent cations promotes [3H]GABA release by altering the Na+ concentration gradient and the membrane potential of SPM vesicles. In the case of choline and Li+, we observed that the fraction of [3H]GABA released relatively to the total amount of neurotransmitter released by K+ or Rb+ is about 28% and 68%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The striatonigral dynorphin pathway of the rat studied with in vivo microdialysis--I. Effects of K(+)-depolarization, lesions and peptidase inhibition

Extracellular levels of dynorphin B were analysed with in vivo microdialysis in the neostriatum and substantia nigra of halothane-anaesthetized rats. Dopamine and its metabolites, 3,4-dihydroxyphenyl-acetic acid and homovanillic acid, as well as GABA were simultaneously monitored. Chromatographic analysis revealed that the dynorphin B-like immunoreactivity measured in perfusates collected under basal and K(+)-depolarizing conditions co-eluted with synthetic dynorphin B. Dynorphin B, GABA and dopamine levels were Ca(2+)-dependently increased by K(+)-depolarization, while 3,4-dihydroxyphenylacetic acid and homovanillic acid levels were decreased. Dopamine and its metabolites, but not dynorphin B or GABA levels, were significantly decreased after a unilateral 6-hydroxydopamine injection into the left medial forebrain bundle. In contrast, following a unilateral injection of ibotenic acid into the striatum, dynorphin B and GABA levels were decreased by > 50% in striatum and substantia nigra on the lesioned side, whereas no significant changes were observed in basal dopamine levels. The inclusion of the peptidase inhibitor captopril (50-500 microM) into the nigral perfusion medium produced a concentration-dependent increase in nigral extracellular levels of dynorphin B. In the striatum, a delayed increase in dynorphin B and GABA levels could be observed following the nigral captopril administration, but this effect was not concentration-dependent. Thus, we demonstrate that extracellular levels of dynorphin B, dopamine and GABA can simultaneously be monitored with in vivo microdialysis. Extracellular dynorphin B appears to originate from neurons, since the levels were (i) increased in a Ca(2+)-dependent manner by K(+)-depolarization, and (ii) decreased by a selective lesion of the striatum, known to contain cell bodies of dynorphin neurons in the striatonigral pathway. Furthermore, (iii) the increase in nigral dynorphin B levels by peptidase inhibition suggests the presence of clearance mechanisms for the released dynorphin peptides.

Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release

The effects of glutamate agonists and their selective antagonists on the Ca(2+)-dependent and independent releases of [3H]GABA from rat coronal hippocampal slices were studied in a superfusion system. The Ca(2+)-dependent release evoked by glutamate, kainate and N-methyl-D-aspartate (NMDA) gradually declined with time despite the continuous presence of the agonists. Quisqualate (QA) caused a sustained release which exhibited no tendency to decline within the 20-min period of stimulation. This release was enhanced in Ca(2+)-free medium. The release evoked by QA in Ca(2+)-containing medium was significantly inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK-801) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), showing that QA activates NMDA receptors directly or indirectly through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. The inhibition of MK-801 was slightly diminished and that of CNQX totally abolished in Ca(2+)-free medium. Verapamil inhibited the QA-activated release in both Ca(2+)-containing and Ca(2+)-free media. The effect of QA but not that of AMPA was blocked in Ca(2+)-free medium by L(+)-2-amino-3-phosphonopropionate (L-AP3), a selective antagonist of the metabotropic glutamate receptor. It is suggested that the sustained release of GABA is also mediated partly by activation of metabotropic receptors and mobilization of Ca2+ form intracellular stores.

On the origin of striatal cholecystokinin release: studies with in vivo microdialysis

In the present study, extracellular levels of the neuropeptide cholecystokinin (CCK), of the monoamine dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and of the excitatory amino acids glutamate and aspartate were simultaneously monitored by microdialysis in the neostriatum of halothane-anesthetized rats under basal and K(+)-depolarizing conditions. Extracellular CCK and dopamine levels, but not glutamate and aspartate levels, were decreased by perfusion with a Ca(2+)-free medium, under both basal and K(+)-depolarizing conditions. HPLC revealed that the majority of the CCK-like immunoreactivity in the perfusates coeluted with CCK octapeptide. Striatal extracellular CCK levels were decreased by decortication plus callosotomy, with a parallel decrease in glutamate levels. Striatal extracellular levels of dopamine, DOPAC, and HVA were significantly decreased in animals treated previously with a unilateral 6-hydroxydopamine injection into the medial forebrain bundle. In these animals, however, the effect of decortication plus callosotomy on CCK and glutamate levels was not further augmented. Thus, this study supports the hypothesis of a neuronal origin of extracellular CCK and dopamine monitored with microdialysis in the striatum of the rat, and also supports the idea of a partly contralateral origin of corticostriatal CCK and glutamate inputs.

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.

Pergolide presynaptically inhibits calcium-stimulated release of gamma-aminobutyric acid

The release of gamma-aminobutyric acid (GABA) in rat dorsolateral striatum was studied using in vivo microdialysis. Dialysis was conducted 2 days after probe implantation in awake, freely moving rats using a modified Ringer solution. Calcium induced a reversible increase in GABA release that was abolished by tetrodotoxin but was only slightly attenuated by a maximally effective dose of pergolide, a D2 receptor agonist. It was thus concluded that pergolide inhibits calcium-stimulated release of GABA presynaptically by a mechanism distinct from that of tetrodotoxin.

Characterization of extracellular histamine in the striatum and bed nucleus of the stria terminalis of the rat: an in vivo microdialysis study

The intracerebral microdialysis technique, coupled with a sensitive radioenzymatic assay, was employed to study histamine release in the striatum and in the bed nucleus of the stria terminalis (BNST) in conscious, freely moving rats. In these brain regions, extracellular histamine concentrations decreased by 20% when calcium was omitted from the perfusion solution. Extracellular histamine was insensitive to the addition of tetrodotoxin to the perfusion medium. In striatum, extracellular histamine concentrations declined in an apparent biexponential manner after the administration of alpha-fluoromethylhistidine, an inhibitor of histamine synthesis. The half-lives for the disappearance of histamine were 32 min and 7.7 h, indicating the presence of at least two histamine pools. Histidine loading resulted in a nearly twofold increase in histamine outflow in striatum. In the BNST, yohimbine increased the extracellular histamine content by 50%, suggesting that histamine release is subject to alpha 2-adrenergic regulation in vivo. The extent to which histamine detected in cerebral microdialysis samples is of neurogenic origin remains to be established.

Incubation of tissue slices in the absence of Ca2+ and Mg2+ can cause nonspecific damage

Superfusion of striatal slices with a medium deficient in Ca2+ and Mg2+ caused a large and sustained increase in release of lactate dehydrogenase, a finding indicative of the disruption of plasma membranes. This was associated with an efflux of dopamine (DA) and the depletion of DA from the tissue. In addition, whereas DA efflux was stimulated by either D-amphetamine (10 microM) or L-glutamate (10 mM) in the absence of Ca2+, these effects were greatly reduced when Mg2+ also was withdrawn from the buffer. These results suggest that (a) incubation in a Ca2+/Mg2(+)-free buffer disrupts plasma membranes, (b) this disruption affects dopaminergic neurons as well as those of other striatal elements, and (c) the failure of a treatment to stimulate DA release in a Ca2+/Mg2(+)-free buffer cannot be used as a test of Ca2+ dependence.

An in vivo microdialysis characterization of extracellular dopamine and GABA in dorsolateral striatum of awake freely moving and halothane anaesthetised rats

This study describes the results of a systematic characterization of extracellular dopamine (DA) and gamma-aminobutyric acid (GABA) recovered from dorsolateral striatum using in vivo microdialysis in rats following acute (2.5 h) and chronic (1 day, 2 day and 4 day) implantation of the probe. The voltage and calcium dependence of DA and GABA overflow was characterised by perfusion with the sodium channel blocker tetrodotoxin (TTX 10-6M) and with Ca2(+)-free Ringers perfusion medium. In addition, the effect of halothane anaesthesia on the responsiveness of these neurotransmitter substances to TTX and Ca2(+)-free perfusion medium was investigated. Perfusion with TTX decreased basal DA levels by at least 60% in all groups. The TTX-induced decrease was most profound in halothane-anaesthetised rats, 24 h after implantation of the probe. Responsiveness of GABA to TTX infusion was different between the groups. In acutely implanted halothane-anaesthetised rats basal GABA levels were unaltered by perfusion with TTX while in the remaining groups at least a 35% reduction was observed. In awake rats 2 days following implantation of the probe removal and replacement of the Ca2+ from the perfusion medium resulted in a reversible reduction of basal DA by 87%. In addition, basal GABA levels were decreased by 52%. This decrease was delayed and was not reversed 1.5 h after the Ca2(+)-free perfusion medium was replaced with normal perfusion medium although basal GABA levels returned to pre-experimental levels by the following day.(ABSTRACT TRUNCATED AT 250 WORDS)

Chelation of endogenous membrane calcium inhibits gamma-aminobutyric acid uptake in synaptosomes

In a previous work, we have demonstrated that calcium chelators induce the release of gamma-aminobutyric acid (GABA) from synaptosomes in a Na+ -dependent manner and that this release is blocked by cations such as Mg2+, La3+, and ruthenium red. In the present study, we show that treatment of synaptosomes with 0.1 mM EGTA in the absence of both Ca2+ and Mg2+ inhibits the sodium-dependent high-affinity uptake of [3H]GABA by about 50%. This inhibition increased to about 65% with 1.5 mM EGTA, and it was completely prevented by an excess of Ca2+ or by 1.2 mM Mg2+. In contrast, when EDTA was used as a chelator, Mg2+ was unable to reverse the inhibition. The inhibitory effect of 0.1 mM EGTA was also prevented by 250 microM La3+ or by 20 microM ruthenium red. In the absence of chelators and the presence of Ca2+ and Mg2+, 50 microM and 200 microM La3+ inhibited GABA uptake by about 20 and 50%, respectively, whereas 20 microM ruthenium red produced a nonsignificant 25% inhibition and nifedipine was without effect. It is concluded that the membrane-surface negative charges, probably those of the sialic acid molecules that have been implicated in the functioning of the GABA carrier, must be neutralized by endogenous Ca2+ or by another cation in order to permit the adequate function of the transporter. The inhibition by La3+ in the absence of the chelators could be explained by a binding of this cation to the Na+ sites on the GABA carrier.

Excitatory amino acid-evoked release of [3H]GABA from hippocampal neurons in primary culture

We investigated the release of gamma-[2,3-3H(N)]aminobutyric acid ([3H]GABA) from hippocampal neurons in primary cell culture. [3H]GABA release was stimulated by the excitatory amino acid neurotransmitter glutamate as well as by N-methyl-D-aspartate (NMDA) and kainate. Cell depolarization induced by raising [K+]o or by veratridine also stimulated [3H]GABA release. NMDA-induced release was completely blocked by 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP+), Mg2+ and Zn2+ whereas the release induced by glutamate and kainate was much less susceptible to inhibition by these substances. Furthermore, removal of external Ca2+ inhibited NMDA-induced release, but not that induced by glutamate, kainate, veratridine or 50 mM K+. Removal of external Na+ reduced [3H]GABA release evoked by all stimuli, but to different extents. All of the excitatory amino acids tested increased [Ca2+]i within hippocampal neurons as assessed by fura-2 based microspectrofluorimetry. This increase in [Ca2+]i was completely dependent on the presence of external Ca2+. These results suggest that Ca2+-dependent and -independent forms of GABA release from hippocampal interneurons may occur. [3H]GABA release evoked by glutamate, kainate, veratridine or 50 mM K+, appeared to be mediated by the reversal of electrogenic, Na+-coupled GABA uptake. Release was inhibited by nipecotic acid, an inhibitor of the Na+-coupled GABA uptake system. However, release induced by NMDA may also include a Ca2+-dependent component.

Characterization of gamma-aminobutyric acid and dopamine overflow following acute implantation of a microdialysis probe

The present study characterized the voltage and calcium dependence of gamma-aminobutyric acid and dopamine overflow after the acute implantation of a microdialysis probe. Probes were implanted in dorsolateral striatum and globus pallidus. Experiments were performed under light halothane anesthesia. Basal, extracellular levels of GABA were not affected by tetrodotoxin (TTX) and were increased to 140 percent of basal values by calcium free Ringer. Basal, extracellular levels of dopamine were reduced to 14 percent of basal values by the addition of TTX and to 30 percent of basal values by the removal of calcium from the Ringer solution. The results suggest that in this in vivo preparation basal extracellular dopamine is largely of vesicular origin while GABA is not.

Release of D-[3H]aspartate and [14C]GABA in rat hippocampus slices: effects of fatty acid-free bovine serum albumin and Ca2+ withdrawal

Extended incubation of hippocampus slices in the presence of fatty acid-free bovine serum albumin (FAF-BSA) strongly enhanced the release of D-[3H]aspartate and [14C]GABA induced by veratridine. Saturation of the FAF-BSA with oleic acid abolished the enhancing effect. Spontaneous release and K+-induced release were not significantly changed by the addition of FAF-BSA. Amino-oxyacetic acid in the medium enhanced the veratridine-induced release of D-[3H] aspartate. The spontaneous release of [14C]GABA was greatly increased by Ca2+ withdrawal. With the further addition of EGTA the spontaneous release in the absence of Ca2+ increased more than 8-fold over the measured in the presence of 1.5 mM Ca2+. The enhanced release caused by Ca2+ withdrawal was totally blocked by tetrodotoxin. The toxin was effective even when added after the spontaneous release in the absence of Ca2+ was already proceeding at a high rate. The veratridine-induced release of [14C]GABA was also considerably augmented by Ca2+ withdrawal. D-[3H]aspartate release, studied simultaneously with [14C]GABA by double labeling, did not show enhanced spontaneous release upon Ca2+ withdrawal. The findings provide evidence that the enhanced [14C]GABA release caused by Ca2+ withdrawal is mediated by voltage-dependent Na+ channels.

VIP release from enteric nerves is independent of extracellular calcium

The release of endogenous vasoactive intestinal polypeptide (VIP) from enteric nerves of isolated rat ileum and the role of extracellular calcium on the release mechanism have been investigated. Evaluation of simultaneous release of endogenous acetylcholine (ACh) and adenosine 5'-triphosphate (ATP) from enteric nerves was used to establish the reliability of the technique. Electrical field stimulation of the ileal preparation induced an increase in the release of endogenous ACh, ATP and VIP. The evoked, but not the basal, release of these substances was blocked by tetrodotoxin (TTX), indicating that the release was a result of nerve stimulation. However, while increase in release of ACh and ATP during nerve stimulation was suppressed in Ca2+-free medium and by the addition of the Ca2+ channel blocker cadmium, nerve-mediated VIP release was unaffected. Further, while K+-depolarization induced release of ACh and ATP from the ileal preparations, it did not lead to an increase in the release of VIP. These results demonstrate that, unlike ACh and ATP release, release of endogenous VIP from enteric nerves is independent of extracellular calcium. The implications of these results in terms of the mechanism of transmitter release in the gastrointestinal tract are discussed.

Neurotransmitters of the cerebellar glomeruli: uptake and release of labeled gamma-aminobutyric acid, glycine, serotonin and choline in a purified glomerulus fraction and in granular layer slices

We have studied some properties of the uptake and release of labeled gamma-aminobutyric acid (GABA), glycine, serotonin and choline in a purified fraction of glomeruli and in slices of the granular layer of the rat cerebellum. The uptake of both GABA and glycine into the glomerulus particles was dependent on the presence of Na+ in the medium. In contrast, the uptake of both serotonin and choline was Na+-independent. In slices of the granular layer also a slight Na+-dependence was observed for both serotonin and choline uptake; imipramine and hemicholinium partially inhibited the uptake of serotonin and choline, respectively. Choline uptake into the glomerulus particles showed two components, with apparent Km values of 16.8 and 102 microM. GABA release was stimulated by K+-depolarization about 100% (peak stimulation) and this value was reduced to 50% when Ca2+ was omitted. The release of glycine was stimulated more rapidly and notably than GABA (200%) and this stimulation was completely abolished in the absence of Ca2+. Serotonin release from the glomerulus particles was only slightly stimulated by depolarization, but this stimulation was strictly Ca2+-dependent. In slices of the granular layer, this stimulation was considerably larger (about 40%) and it was also almost totally dependent on Ca2+. In contrast, after loading with labeled choline the release of radioactivity from both the glomerulus particles and the cerebellar slices was not stimulated at all by K+-depolarization, either in the presence or in the absence of Ca2+. Most of the radioactivity released spontaneously corresponded to choline, and only a small proportion (8-14%) to acetylcholine. From the results of the release experiments and taking into account the pertinent data from the literature, it is concluded that GABA and glycine are probably the transmitters of different populations of Golgi axon terminals, whereas serotonin might be the transmitter of at least a certain population of the mossy fiber giant terminals, in the rat cerebellar glomeruli. In contrast, acetylcholine does not seem to have any transmitter role in the synaptic structures of the glomeruli.

Characterization of the actions of toxins II-9.2.2 and II-10 from the venom of the scorpion Centruroides noxius on transmitter release from mouse brain synaptosomes

Toxic peptides II-9.2.2 and II-10, purified from Centruroides noxius venom, bear highly homologous N-terminal amino acid sequences, and both toxins are lethal to mice. However, only toxin II-10 is active on the voltage-clamped squid axon, selectively decreasing the voltage-dependent Na+ current. Here, we have tested toxins II-9 and II-10 on synaptosomes from mouse brain: both toxins increased the release of gamma-[3H]aminobutyric acid ([3H]GABA). Their effect was completely blocked by tetrodotoxin or by the absence of external Na+. Also, both toxins increased Na+ permeability in isolated nerve terminals. Besides the observation that toxin II-9 is active on synaptosomes, the effect of toxin II-10 in this preparation is opposite to that observed in the squid axon. Thus, our results reflect functional differences between the populations of Na+ channels in mouse brain synaptosomes and in the squid axon. The release of GABA evoked by these toxins from synaptosomes required external Ca2+ and was blocked by Ca2+ channel blockers (verapamil and Co2+). This latter observation is in sharp contrast to the releasing action of veratrine, which evoked release even in the absence of external Ca2+. Furthermore, the action of both C. noxius toxins was potentiated by veratrine, a result suggesting they have different mechanisms of action. Among drugs that release neurotransmitters by increasing Na+ permeability, it is noteworthy that scorpion toxins are the only ones yet reported to have a strict requirement for external Ca2+.

Compartmentation and release of exogenous GABA in sheep brain synaptosomes

Exogenous tritiated gamma-aminobutiric acid ([3H]GABA) is retained in two compartments in sheep cortex synaptosomes, corresponding to cytoplasmic and vesicular spaces, assuming that freeze-thawing the synaptosomes loaded with [3H]GABA releases the cytoplasmic [3H]GABA (81 +/- 3.9%), and that subsequent solubilization of the synaptosomes with 1% sodium cholate releases the vesicular [3H]GABA (19 +/- 3.9%). Depolarization of synaptosomes with 40 mM K+ in a Na+-medium, in the absence of Ca2+, releases 20.3 +/- 2.7% of the [3H]GABA retained in the synaptosomes. The [3H]GABA released under these conditions comes predominantly from the cytoplasm. The presence of 1 mM Ca2+ during depolarization releases an additional 13% (a total of about 33.5 +/- 9.9%) of the releasable [3H]GABA, and the [3H]GABA release which is Ca2+-dependent also comes mostly from the cytoplasmic compartment. When choline replaces external Na+, the [3H]GABA release is absolutely Ca2+-dependent, and the [3H]GABA released also comes mostly from the cytoplasmic pool. Therefore, it appears that [3H]GABA taken up by synaptosomes is accumulated mostly in the cytoplasmic compartment from which it is released upon depolarization. The technique described permits distinguishing the effect of different factors on the two pools of accumulated [3H]GABA.

Taurine efflux from brain slices: potassium-evoked release is greater from immature than mature brain tissue

The stimulated release of taurine was severalfold greater from slices of immature than mature brain. The release had a strikingly slow time course at all ages, the slower the younger were the animals. It is concluded that taurine is an important regulatory of neuronal activity in the developing brain.

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

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

Stimulus-evoked efflux of GABA from preloaded slices of the rabbit oviduct

The effect of electrical and chemical stimulation on the efflux of [3H]gamma-aminobutyric acid (GABA) from preloaded slices of the rabbit oviduct was examined. Electrical field stimulation significantly increased the outflow of [3H]GABA. This effect could not be prevented by tetrodotoxin or by the removal of Ca2+ from the medium. High K+ concentrations, veratrine and ethylenediamine also evoked a remarkable elevation in the efflux. The release induced by veratrine was completely abolished in a Ca2+-free medium or in the presence of tetrodotoxin, while the release evoked by high K+ or ethylenediamine was resistant to both conditions. These findings indicate that GABA can be released from the oviduct under the effect of depolarizing stimuli, raising the possibility of a physiological interaction between oviductual GABA and its receptors. The characteristics of oviductal GABA efflux differ from those of neuronal and glial GABA release.

Differential calcium dependence of gamma-aminobutyric acid and acetylcholine release in mouse brain synaptosomes

The dependence of gamma-aminobutyric acid (GABA) and acetylcholine (ACh) release on Ca2+ was comparatively studied in synaptosomes from mouse brain, by correlating the influx of 45Ca2+ with the release of the transmitters. It was observed that exposure of synaptosomes to a Na+-free medium notably increases Ca2+ entry, and this condition was used, in addition to K+ depolarization and the Ca2+ ionophore A23187, to stimulate the influx of Ca2+ and the release of labeled GABA and ACh. The effect of ruthenium red (RuR) on these parameters was also investigated. Of the three experimental conditions used, the absence of Na+ in the medium proved to be the most efficient in increasing Ca2+ entry. RuR inhibited by 60-70% the influx of Ca2+ stimulated by K+ depolarization but did not affect its basal influx or its influx stimulated by the absence of Na+ or by A23187. The release of ACh was stimulated by K+ depolarization, absence of Na+ in the medium, and A23187 in a strictly Ca2+-dependent manner, whereas the release of GABA was only partially dependent on the presence of Ca2+ in the medium. The extent of stimulation of ACh release was related to the extent of Ca2+ entry, whereas no such correlation was observed for GABA. In the presence of Na+, RuR did not affect the release of the transmitters induced by A23187. In the absence of Na+, paradoxically RuR notably enhanced the release of both ACh and GABA induced by A23187, in a Ca2+-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of previously incorporated gamma-[3H]aminobutyric acid in rabbit caudate nucleus slices

The release of gamma-aminobutyric acid (GABA) was studied in slices of the head of the rabbit caudate nucleus. The slices were preincubated with [3H]GABA and then superfused. Aminooxyacetic acid was present throughout. Both the tritium in the slices and that in the superfusate consisted practically entirely of [3H]GABA. Stimulation for 2 min by electrical field pulses of 3 ms width and 9 V/cm voltage drop (36 mA current strength) at 5 or 20 Hz elicited an overflow of [3H]GABA that amounted to 0.23 or 0.47% of the tritium content of the tissue, respectively, and was diminished by 85% in the presence of tetrodotoxin. At higher current strength, less of the stimulation-evoked overflow was tetrodotoxin-sensitive. cis-1,3-Aminocyclohexane carboxylic acid diminished the uptake of [3H]GABA into the tissue but did not change the percentage released by electrical stimulation. Ca2+ withdrawal greatly accelerated basal [3H]GABA efflux and almost abolished the response to stimulation. Nipecotic acid 10-1,000 microM enhanced both the basal and (up to eightfold) the stimulation-evoked overflow. The method described allows us to elicit electrically a quasiphysiological, i.e., Ca2+-dependent and tetrodotoxin-sensitive, neuronal release of [3H]GABA. Nipecotic acid diverts released [3H]GABA from reuptake to overflow.

Further characterization of [3H]gamma-aminobutyric acid release from isolated neuronal growth cones: role of intracellular Ca2+ stores

We have recently shown that growth cones isolated from neonatal rat forebrain possess uptake and release mechanisms for the neurotransmitter gamma-aminobutyric acid. About half of the K+-induced release of [3H]gamma-aminobutyric acid from isolated growth cones is dependent on extracellular Ca2+. The remaining component of the [3H]gamma-aminobutyric acid release is unaffected by removal of extracellular Ca2+ and is resistant to blockade by the voltage-sensitive Ca2+-channel blocker methoxyverapamil. In the present series of experiments we have used caffeine to assess the possible role of intracellular stores of Ca2+ in supporting that component of the K+-induced release of [3H]gamma-aminobutyric acid from isolated growth cones that is independent of extracellular Ca2+. We have chosen caffeine because of its well established effect of releasing Ca2+ from smooth endoplasmic reticulum in muscle. We found that caffeine can release [3H]gamma-aminobutyric acid from isolated growth cones. This effect persists in Ca2+-free medium, in the presence of methoxyverapamil and in the absence of Na+. Furthermore, isobutylmethylxanthine could not substitute for caffeine suggesting that the caffeine effect is not due to phosphodiesterase inhibition and the subsequent rise in intracellular cyclic nucleotides. A combination of the mitochondrial poisons, Antimycin A and sodium azide had no effect on the release of [3H]gamma-aminobutyric acid induced either by caffeine or by high K+. We conclude that caffeine causes the release of Ca2+ from a non-mitochondrial store within the growth cone and that this Ca2+ store supports that component of the K+-induced release of [3H]gamma-aminobutyric acid that is independent of extracellular Ca2+.

Mechanism of the calcium-dependent stimulation of transmitter release by 4-aminopyridine in synaptosomes

The mechanism of the Ca2+-dependent stimulation of neurotransmitter release by 4-aminopyridine in synaptosomes was studied. The stimulation of gamma-[3H]aminobutyric acid and [3H]acetylcholine release by 4-aminopyridine was not significantly affected either by tetrodotoxin or by the absence of Na+ in the medium, whereas the toxin notably inhibited the release of both transmitters induced by veratridine. On the other hand, the release of labeled gamma-aminobutyric acid induced by 4-aminopyridine was inhibited by both La3+ and ruthenium red, two blockers of Ca2+ transport in synaptosomes. In other experiments, 4-aminopyridine had only a slight stimulatory effect, if any, on the influx of 45Ca2+ into synaptosomes, under both resting and K+-depolarizing conditions. Ruthenium red inhibited the stimulation by K+ of the 45Ca2+ uptake, and 4-aminopyridine did not antagonize this inhibition. We conclude that the transmitter-releasing action of 4-aminopyridine in synaptosomes does not involve an excitatory effect on the membrane which may result in the opening of voltage-sensitive Na+ channels. 4-Aminopyridine does not seem to act either by enhancing Ca2+ entry into the synaptosomes. It is proposed that 4-aminopyridine facilitates the coupling between Ca2+ binding and transmitter secretion at the presynaptic membrane.

Binding of lanthanum ions and ruthenium red to synaptosomes and its effects on neurotransmitter release

A technique for studying the binding of La3+ to synaptosomes in a double-beam spectrophotometer, using murexide as indicator, is described. The binding of La3+ was very rapid and Scatchard plots revealed two components, with KD values of 0.6 and 27 microM in a Na+-free medium (sucrose medium) and 2.3 and 63 microM in an ionic medium containing 135 mM Na+. The binding of the cationic dye ruthenium red (RuR) showed only one site, with a KD of 3.7 microM. La3+ binding was partially inhibited by RuR and vice versa, and La3+ was also capable of partially displacing RuR previously bound to the synaptosomes, particularly in the sucrose medium. The release of labeled gamma-aminobutyric acid (GABA) stimulated by K+ depolarization was inhibited by La3+ concentrations at or above 1 microM, in the ionic medium, whereas in the sucrose medium 2.5 microM or higher La3+ concentrations notably stimulated the spontaneous release of both GABA and glutamic acid. It is concluded that La3+ and RuR share at least one type of binding site, which is probably the high-affinity La3+ site. Since both La3+ and RuR at low concentrations have been shown to block the depolarization-induced Ca2+ entry in synaptosomes, this site might be related to the voltage-dependent Ca2+ entry involved in neurotransmitter release.

Effects of drugs on neurotransmitter release: experiments in vivo and in vitro

Calcium ions play a fundamental role in the release of transmitters in the nervous system. Therefore, drugs capable of modifying Ca2+ transport are useful tools for studying the mechanisms of such release in vivo and in vitro. In this article the action of some of these drugs on motor behavior, as well as on Ca2+ uptake and neurotransmitter release in synaptosomes, is reviewed. Ruthenium red (RuR) inhibits Ca2+ uptake and transmitter release in synaptosomes, and produces flaccid paralysis when injected intraperitoneally (IP) and convulsions after intracranial administration. Drugs which stimulate the Ca2+-dependent transmitter release in synaptosomes, such as 4-aminopyridine, antagonize the paralysis produced by RuR. Lanthanum ions also inhibit Ca2+ uptake and neurotransmitter release in synaptosomes, but no paralysis was observed after La2+ IP injection. However, this cation blocks the binding of RuR to the presynaptic membrane, and prevents the RuR-induced paralysis. Veratridine and the Ca2+ chelator EGTA were used to demonstrate in synaptosomes that besides the Ca2+-dependent mechanism of release of the central inhibitory transmitter gamma-aminobutyric acid (GABA), there seems to be a strictly Na+-dependent process which is not shared by other transmitters such as acetylcholine or dopamine.

Release of preloaded taurine and hypotaurine from astrocytes in primary culture: stimulation by calcium-free media

The spontaneous and stimulated release of taurine and hypotaurine from astrocytes in primary cultures were investigated. Spontaneous efflux was slow, less than one half of preloaded labeled taurine and hypotaurine still remaining in the cells after a 60-min efflux period. The release processes of both amino acids were in principle similar. No homo- or heteroexchange with extracellularly added taurine, hypotaurine or GABA could be detected, and depolarizing potassium concentrations failed to stimulate taurine or hypotaurine release. On the other hand, omission of calcium ions from medium increased efflux of taurine and hypotaurine about three- and twofold, respectively, in both high-K+ and normal-K+ media.