Effects of taurine on depolarization-evoked release of amino acids from rat cortical synaptosomes

Effects of taurine on endogenous aspartic acid (Asp), glutamic acid (Glu) and γ-aminobutyric acid (GABA) release has been investigated using synaptosomes prepared from rat cerebral cortex. Although basal release of these amino acids was not affected, taurine inhibited KCl (30 mM)-evoked overflow of Asp, Glu and GABA in a concentration-dependent manner with potencies (IC₅₀) of 1 μM, 0.8 μM and 5 nM, respectively. Taurine (10 μM) maximally inhibited K⁺-evoked Asp, Glu and GABA overflow by 28, 37 and 65%, respectively. Phaclofen (10 μM, a GABA_B receptor antagonist), but not bicuculline (10 μM, a GABA_A receptor antagonist), counteracted the inhibition of GABA overflow, although the inhibition of Asp and Glu overflow was not attenuated. These data suggest that taurine may inhibit GABA release through the activation of presynaptic GABA_B autoreceptors and, at high concentration, also act on Asp- and Glu-nerve terminals to regulate release of excitatory amino acids in rat cortex.     

Patterns of endogenous amino acid release from slices of rat and guinea-pig olfactory cortex

A study has been made of the effects of depolarizing stimuli on the release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and taurine) from in vitro preparations of rat and guinea pig olfactory cortex. Exposure of small cubes of olfactory cortex tissue from either species to potassium chloride (50 mM) was accompanied by a calcium-dependent release of aspartate, glutamate and GABA. A similar release pattern was evoked by protoveratrine A (100 μM) although the release was largely calcium-independent. Neither agent led to increased release of taurine. Electrical stimulation of the excitatory input (lateral olfactory tract) of freshly prepared, synaptically intact olfactory cortex slices of both species induced significant release of aspartate and GABA from the uncut pial surface and of aspartate, GABA and glutamate from the cut surface. Evoked taurine release occurred from both surfaces of rat olfactory cortex slices but no release was detected from guinea pig olfactory cortex slices. These patterns of release were unaffected by changes in stimulus frequency and were mimicked by protoveratrine A (100 μM) applied to one or other surface. Preincubation of slices from rats for 2 h led to loss of tissue amino acids and to changes in their release patterns; the presence of glutamine (5 mM) during preincubation prevented the loss of amino acids but did not alter their pattern of release. Because of the close similarities between both the electrophysiological properties and the patterns of amino acid release it is concluded that there is probably an identity of amino acid neurotransmitters (aspartate, glutamate and GABA) in rat and guinea pig olfactory cortex. The role of taurine in the rat olfactory cortex is unknown but would seem unlikely to be that of a neurotransmitter. The results are discussed: (i) in terms of the cellular origins of the released amino acids; and (ii) with respect to apparent experimental discrepancies which have appeared in the literature^3,10.     

GABA and benzodiazepine-induced modification of [C]l-glutamic acid release from rat spinal cord slices

The spontaneous and potassium-evoked release of [¹⁴C]-label from rat spinal cord slices preloaded with [¹⁴C]l-glutamic acid and its modification by GABA and related drugs, such as flurazepam, was studied as a possible indirect measure of presynaptic inhibition and of the ability of benzodiazepines to augment it. GABA (100 μM) reduced the spontaneous release of [¹⁴C]-label (glutamate) provided that GABA metabolism was blocked by amino-oxyacetic acid (AOAA), but failed to reduce the potassium-evoked release of glutamate, although muscimol (10 μM) had some effect. In contrast, flurazepam (1–100 μM) did not affect spontaneous release but produced some inhibition of the evoked release (through a system insensitive to 10 μM bicuculline). This inhibition became more marked in the presence of both GABA and AOAA, and was then overcome by bicuculline. It is concluded that either some benzodiazeophine receptors must be occupied for GABA to produce an effect on evoked release and/or, that the benzodiazepines can only augment GABA function once a certain amount has been released. Studies of the rapid distribution of [¹⁴C]-label from glutamate, to GABA, glutamine and other amino acids, using high voltage electrophoresis, showed the importance of blocking metabolic pathways in studies of this kind.     

Adenosine–amino acid interactions in the chick brain: a role in passive avoidance learning

The present work describes interactions between adenosine and the amino acids glutamate and GABA in slices of intermediate medial hyperstriatum ventrale (IMHV), an area of the chick brain known to be involved in learning and memory events associated with a one-trial passive avoidance task. In slices derived from the IMHV of untrained chicks, the A₁ receptor agonist N⁶-cyclohexyladenosine (CHA; 10 μM) specifically inhibited glutamate release. Conversely, cyclopentyltheophylline (CPT; 100 μM an A₁ antagonist) increased glutamate release from the slices and blocked the CHA-induced inhibition of glutamate. The A₂ receptor agonist 2-p-(2-carboxylethyl)-phenylamino-5′-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) selectively increased glutamate release when applied at 5 μM while it selectively inhibited GABA release at a lower concentration (10 nM). The addition of NMDA to the medium, resulted in increased adenosine release equivalent to that found following stimulation with 50 mM KCl. Both the NMDA and the KCl-induced increases were eliminated by addition of -2-amino-5 phosphopentanoic acid ( -AP5), an NMDA-receptor antagonist. Slices prepared from the IMHV of chicks following successful training on the task showed enhanced adenosine release 30 min, 1, 3 and 6.5 h after training compared to chicks trained to peck a water-coated bead. The results show that changes in adenosine release from the IMHV accompany memory formation in the chick. We suggest that adenosine–amino acid transmitter interactions potentially via the activation of NMDA receptors, a necessary step in long-term memory formation for the task, may modulate the formation of memory for the one-trial passive avoidance task.     

Failure of carbachol to influence the release of somatostatin from slices of rat cerebral cortex

A[K⁺]-related, Ca²⁺-dependent efflux of immunoreactive somatostatin (IRS) from superfused slices of rat cerebral cortex has been observed; this release paralleled the release of both [¹⁴C]noradrenaline and [¹⁴C]GABA. However IRS release in this preparation was not stimulated by the muscarinic agonist carbachol at low (10 μM) or high (500 μM) concentrations. Furthermore, 100 or 500 μM carbachol did not affect the IRS efflux from rat cortex slices incubated in the presence of 12, 25 or 53 mM K⁺.     

Characterization of the excitatory amino acid receptor-mediated release of [H]acetylcholine from rat striatal slices

The pharmacological nature of the interaction of excitatory amino acids with striatal cholinergic neurons was investigated in vitro. Agonists of excitatory amino acid receptors evoked the release of [³H]acetylcholine from slices of rat striatum, in the presence of magnesium (1.2 mM). Removal of magnesium from the medium markedly increased the release of [³H]acetylcholine evoked by all excitatory amino acid receptor agonists tested, with the exception of kainate. In the absence but not the presence of magnesium, a clear rank order of potency was found: N-methyl-dl-aspartate = ibotenate >l-glutamate >l-aspartate cysteate > kainate = quisqualate.The excitatory amino acid receptor mediating [³H]acetylcholine release resembles the N-methyl-d-aspartate preferring (N-type) receptor, as previously characterized electrophysiologically, according to 3 criteria: (1) rank order of potency of agonists; (2) magnesium-sensitivity; and (3) antagonism by 2-amino-5-phosphonovalerate.The release of [³H]acetylcholine evoked by N-methyl-dl-aspartate was blocked by tetrodotoxin (0.5 μM). Moreover, N-methyl-dl-aspartate failed to evoke [³H]acetylcholine release from slices of hippocampus, where cholinergic afferents, rather than interneurons, are found. These results suggest that excitatory amino acids act at receptors on the dendrites of striatal cholinergic interneurons, giving rise to action potentials and release of acetylcholine from cholinergic nerve terminals.     

GABAergic control of Arg-vasopressin release from suprachiasmatic nucleus slice culture

γ-Aminobutyric acid (GABA) is contained in many neurons in the suprachiasmatic nucleus (SCN), and is considered to be a circadian entraining factor. Arg-vasopressin (AVP)-containing neurons represent one of the output paths from the SCN to other brain areas. We examined the effects of GABA, muscimol (GABA-A agonist), bicuculline (GABA-A antagonist), baclofen (GABA-B agonist) and phaclofen (GABA-B antagonist) on AVP release using SCN slice preparations in culture. SCN slices were prepared from coronally sliced brain tissue and cultured in organic tissue culture dishes with DMEM/N2 medium in a CO₂ (5%) incubator. The culture medium was changed at 3-h intervals until 9 h after 3 h application of each drug. Concentrations of AVP in 1 ml aspirates of the medium were analyzed by EIA. Muscimol (1, 10 μM) increased and bicuculline (1, 10, 100 μM) decreased the AVP release 3–6 h after application. However, baclofen and phaclofen had no apparent effects on AVP release. Riluzole (0.1 mM) and nipecotic acid (1 mM), GABA uptake inhibitors, increased AVP release 3–6 h after application. These results indicate that GABA promotes AVP release mediated by GABA-A receptors in the SCN.     

Kindling increases the K -evoked Ca -dependent release of endogenous GABA in area CA1 of rat hippocampus

The release of endogenous amino acids from hippocampal CA1 subslices under basal conditions and the release evoked by high potassium (50 mM K⁺) depolarization was studied during kindling epileptogenesis. Emphasis was put on the release of the amino acid neurotransmitters γ-aminobutyric acid (GABA) and glutamate. Kindling was induced by tetanic stimulation of the Schaffer-collaterals/commissural fibers of the dorsal hippocampus of the rat. The calcium-dependent GABA release in the presence of high K⁺ was significantly increased (40–46%) in fully kindled animals, 24 h after the last seizure, in comparison to controls. At long-term, 28 days after the last seizure, the calcium-dependent GABA release was still significantly increased (45–49%). An increased release of GABA in kindled animals was still found when GABA uptake was blocked by nipecotic acid. In contrast, no significant alterations were encountered in the basal or high potassium induced release of the excitatory amino acids aspartate and glutamate. These results suggest that kindling epileptogenesis is accompanied by a specific and long-lasting enhancement of GABA exocytosis which may lead to a desensitization of the GABA receptor, and thus determine the increase of seizure sensitivity.     

Stimulation of GABA release from retinal horizontal cells by potassium and acidic amino acid agonists

The release of [³H]GABA from horizontal cells of goldfish retina was studied by biochemical analysis of perfused isolated retina. Retinas were incubated for 15 min in 0.72 μM[³H]GABA, rinsed for 30 min and then perfused with 1 min pulses of increasing concentrations of K⁺ and acidic amino acid agonists under a variety of conditions. Radioactivity in the perfusate was determined by liquid scintillation spectroscopy. The main findings are: (1) virtually all of the [³H]GABA released byl-glutamate (l-Glu) andl-aspartate (l-Asp) and 50% of the K⁺-evoked release, is calcium independent; (2) K⁺-evoked [³H]GABA release is only 10% of that released byl-Glu; (3) threshold [³H]GABA release occurs with 320 μMl-Glu, 1175 μMl-Asp, 4μM quisqualic acid (QA), 4μM kainic acid (KA) and 53 μM N-methyl-dl-aspartate (NMDLA); (4) the quisqualate antagonist glutamic acid diethyl ester (GDEE), has no specific inhibitory action on any of the agonists, whereasd-α-aminoadipic acid (DαAA), an NMDA antagonist, potently inhibits the action of NMDLA andl-Asp; (5) the presence of Mg²⁺, even at 1 mM, totally inhibits NMDLA and also inhibits the action ofl-Glu andl-Asp below 1 mM; (6)d-Asp potentiates the action ofl-Glu by 0.6–0.8 log units and completely inhibits the action ofl-Asp; (7)l-Asp at a ratio of 3:1 potentiates the effect ofl-Glu. From these and other results one concludes that: (a) [³H]GABA release from H1 cells is calcium independent and depends on factors other than passive depolarization, probably sodium; (2) the likely transmitter of red cones isl-Glu acting on quisqualate or kainate receptors, and (3)l-Asp acts predominantly on NMDA receptors and may provide a modulatory role in the outer retina by potentiating the action ofl-Glu.     

Excitatory and inhibitory effects of noradrenaline on synaptic transmission in the rat olfactory cortex slice

An investigation has been made of the effects of noradrenaline on excitatory transmission at the lateral olfactory tract (LOT)-superficial pyramidal cell synapse of the rat olfactory cortex slice by measuring the effects of bath-applied noradrenaline on the amplitudes and latencies of the field potentials evoked on LOT stimulation. Low concentrations of noradrenaline (0.1–5 μM) facilitate transmission whereas higher doses (20–250 μM) depress transmission. Both these effects were completely blocked by non-selective α- and β-adrenoceptor antagonists, by 2-amino-5-phosphonovaleric acid (an antagonist of excitatory amino acid receptors of the N-methyl-d-aspartate type) and by the methylxanthine theophylline. The depressant effects of noradrenaline were mimicked by bath application of GABA or adenosine and specifically antagonized by bicuculline and picrotoxin. In parallel experiments, noradrenaline (100 μM) significantly increased the potassium-evoked release of endogenous aspartate, glutamate and GABA, proposed transmitters of the olfactory cortex, although the effect on GABA release was specifically antagonized by 2-amino-5-phosphonovaleric acid. Noradrenaline (100 μM) also significantly increased the potassium-evoked release ofd-[³H]aspartate, an effect antagonized by a number of α- and β-adrenoceptor antagonists. It is concluded that at low concentrations, noradrenaline facilitates transmission at the LOT-superficial pyramidal cell synapse by increasing excitatory amino acid neurotransmitter release. This effect is mediated by both α- and β-adrenoceptors although the primary site of release is unknown. At higher concentrations of noradrenaline, the increased levels of excitatory transmitters release sufficient endogenous GABA (and possibly adenosine) to cause an overall depression of transmission. These conclusions are supported by the results of a series of experiments in which the effects of noradrenaline on stimulus input-evoked field potential output relationships were assessed. It is not possible to exclude additional direct effects of noradrenaline on membrane excitability.     

Effect ofd- andl-α-aminoadipate on the efflux ofl-aspartate,l-glutamate and γ-aminobutyrate from superfused rat brain slices

d-α-Aminoadipate (d-AA) andl-α-aminoadipate (l-AA) were found to significantly reduce spontaneous efflux of [¹⁴C]l-aspartate from preloaded rat brain slices. Onlyd-AA significantly reduced spontaneous efflux of [¹⁴C]l-glutamate and [³H]γ-aminobutyric acid (GABA);l-AA reduced but not significantly the efflux of these 2 labeled amino acids.d-AA reduced K⁺-stimulated release of [¹⁴C]l-aspartate and [^14]C]l-glutamate significantly, andl-AA that of [³H]GABA significantly. Since bothd-AA andl-AA inhibit the uptake ofl-aspartate,l-glutamate and GABA, their effects on the efflux of these amino acids are more specific. These results also suggest that it is unlikely that the depressant effect ofd-AA, and the excitant effect ofl-AA on neurons when applied locally by iontophoresis are secondary to the accelerated or decelerated release of more specific transmitter amino acids from neighboring cells.     

Adenosine modulation of amino acid release in rat hippocampus during ischemia and veratridine depolarization

This study was undertaken to determine whether endogenous adenosine modulates ‘in vivo’ neurotransmitter amino acid release via its presynaptic receptors. Two conditions were compared: neuronal depolarization by local infusion of veratridine (600 μM), and transient global ischemia by four-vessel occlusion. Both stimuli were applied for 20 min. Extracellular amino acid (glutamate, taurine/GABA, glycine) variations in concentration were determined in the rat hippocampus by microdialysis and HPLC. Modulation of adenosine receptor activity was objectified by continuous local infusion of an adenosine agonist (R-phenylisopropyladenosine R-PIA) or an antagonist (theophylline), starting one hour before stimulation of amino acid release. R-PIA (100 μM) significantly decreased the glutamate release (50%) evoked by veratridine, whereas it did not significantly modify the ischemia-induced glutamate release. In contrast, theophylline did not significantly affect veratridine-induced glutamate release, but it significantly potentiated glutamate efflux (400%) under ischemic conditions. Neither treatment altered the release of the other amino acids. These data suggest that endogenous adenosine appearing in the extracellular space during veratridine-induced depolarization cannot control glutamate release. In contrast, ischemia-induced glutamate release was strongly inhibited by the concomitant increase in extracellular adenosine.     

Net uptake of aspartate by a high-affinity rat cortical synaptosomal transport system

Reuptake of neuroactive amino acids by high affinity transport systems (K_m ≈ 10 μM) is thought to terminate the action of these substances. Since homoexchange can complicate the interpretation of uptake experiments, it is necessary to demonstrate net inward transport of neuroactive amino acids before uptake can be considered as a likely mechanism for transmitter inactivation. When rat cortical synaptosomes are incubated with 10 μM [¹⁴C]l-aspartate, net (chemical) and apparent (radioactive) uptake into the synaptosomal fractions are equivalent. Although there is net aspartate uptake into the synaptosome fraction, aspartate exchange can be demonstrated in a variety of conditions. Net uptake exhibits the characteristics of high-affinity transport systems including Na⁺- and temperature-dependence. Furthermore, KCl (orRbCl) — 1mM — are required for net uptake but not radioactive or apparent uptake. LiCl, NH₄Cl, CsCl, and choline chloride fail to support net uptake. Ouabain (0.1mM) inhibits net uptake to a greater extent than apparent uptake. Although glutamate inhibits aspartate uptake (and vice versa), the net uptake of the combination is greater than that of each alone. The demonstration of net uptake of aspartate by a high-affinity system is consonant with the idea that this system may play a role in its inactivation in the synaptic region.     

Rapid desensitization of presynaptic dopamine autoreceptors during exposure to exogenous dopamine

When nomifensine is employed to inhibit neuronal uptake, exposure to dopamine (DA) (0.1–0.3 μM) or apomorphine (0.01–0.1 μM) inhibited, in a concentration-dependent manner, the electrically evoked release of [³H]dopamine from slices of the rabbit caudate nucleus. Apomorphine inhibited transmitter release independently of the time of exposure to the drug (6–32 min). On the other hand, the inhibitory effect of exogenous dopamine occurred only if a short period (4–12 min) of exposure was employed. In studies on the electrically evoked release of [³H]acetylcholine in slices of the rabbit caudate nucleus there was no evidence for desensitization to apomorphine or exogenous dopamine at the level of the dopamine receptors that inhibit [³H]acetylcholine release. These results indicate that the dopamine autoreceptors modulating [³H]dopamine release in the caudate nucleus become subsensitive after a few minutes of exposure to exogenous dopamine. This effect does not occur at the level of the dopamine receptors which inhibit the release of [³H]acetylcholine.     

Effect of oxidative stress on the release of [H]GABA in cultured chick retina cells

The effect of ascorbate (1.5 mM)/Fe²⁺ (7.5 μM)-induced oxidative stress on the release of pre-accumulated [³H]γ-aminobutyric acid ([³H]GABA) from cultured chick retina cells was studied. Depolarization of control cells with 50 mM K⁺ increased the release of [³H]GABA by 1.01 ± 0.16% and 2.5 ± 0.3% of the total, in the absence and in the presence of Ca²⁺, respectively. Lipid peroxidation increased the release of [³H]GABA to 2.07 ± 0.31% and 3.6 ± 0.39% of the total, in Ca²⁺-free or in Ca²⁺-containing media, respectively. The inhibitor of the GABA carrier, 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711) blocked almost completely the release of [³H]GABA due to K⁺-depolarization in the absence of Ca²⁺, but only 65% of the release occurring in the presence of Ca²⁺ in control and peroxidized cells. Under oxidative stress retina cells release more [³H]GABA than control cells, being the Ca²⁺-independent mechanism, mediated by the reversal of the Na⁺/GABA carrier, the most affected. MK-801 (1 μM), a non-competitive antagonist of the NMDA receptor-channel complex, blocked by 80% the release of [³H]GABA in peroxidized cells, whereas in control cells the inhibitory effect was of 40%. The non-selective blocker of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), inhibited the release of [³H]GABA by 30% and 70% in control and peroxidized cells, respectively. Glycine (5 μM) stimulated [³H]GABA release evoked by 50 mM K⁺-depolarization in control but not in peroxidized cells. The release of -[³H]aspartate (a non-metabolized analog of -glutamate) evoked by 50 mM K⁺, in the absence of Ca²⁺, was significantly higher in peroxidized cells (6.76 ± 0.64% of the total) than in control cells (3.79 ± 0.27% of the total). The results suggest that oxidative stress induced by ascorbate/Fe²⁺ causes an excessive release of endogenous excitatory amino acids upon K⁺-depolarization. The glutamate released may activate NMDA and non-NMDA receptors, raising the intracellular Na⁺ concentration and consequently stimulating the release of [³H]GABA by reversal of the Na⁺/GABA carrier.     

γ-Aminobutyric acid activation ofCl-flux in rat hippocampal slices and its potentiation by barbiturates

γ-Aminobutyric acid (GABA) increases the rate of³⁶Cl- efflux from preloaded rat hippocampal slices in a dose-dependent manner (EC₅₀: 400 μM). This action has the pharmacological specificity expected of activation of GABA receptor in that it is mimicked by the agonists muscimol and 3-aminopropanesulfonic acid, and blocked by the antagonists bicuculline and picrotoxinin. GABA uptake inhibitors, nipecotic acid and 2, 4-diaminobutyric acid, fail to increase³⁶Cl- flux. Pentobarbital produces a dose-dependent activation (EC₅₀ = 1.5mM) of³⁶Cl- efflux with maximal response greater than that of GABA. The effect of pentobarbital can be mimicked by 1, 3-dimethylbutylbarbiturate, secobarbital, (+)hexobarbital but not (−)hexobarbital, and is blocked by bicuculline and picrotoxinin. Pentobarbital and the other active barbiturates also potentiate the action of GABA. Phenobarbital does not have any effect independently or in combination with GABA. It is suggested that GABA increases³⁶Cl- permeability by activation of a postsynaptic receptor which is in turn functionally copupled to a barbiturate receptor.     

Endogenous GABA potentiates the potassium-induced release of dopamine in striatum of the freely moving rat: a microdialysis study

Using microdialysis, a study was made of the effects of an increase of endogenous GABA on basal and potassium-stimulated release of dopamine in striatum of the awake rat. The dopamine metabolites DOPAC and HVA were also measured. Extracellular concentrations of GABA were increased by inhibiting its uptake with nipecotic acid. TTX (10 μM) reduced basal extracellular concentrations of dopamine, and dopamine metabolites, but not GABA. Nipecotic acid (200, 500, and 1000 μM) produced a dose-related increase in basal extracellular concentrations of GABA, but did not change basal extracellular concentrations of dopamine and dopamine metabolites. However, nipecotic acid significantly enhanced the dopamine release produced by perfusion of potassium (50 mM) and also enhanced the extracellular increase of GABA produced by high potassium. These results suggest that an increase of endogenous GABA is facilitating the stimulated release, but not the basal release, of dopamine in the striatum of the awake rat.     

Neostriatal dopaminergic terminals prevent the GABAergic involvement in the μ- and δ-opioid inhibition of KCl-evoked endogenous acetylcholine release

Endogenous acetylcholine (ACh) release from rat neostriatal slices were inhibited by the μ-opioid agonist [d-Ala²,Gly(ol)⁵]-enkephalin (DAGO) both in 6-hydroxydopamine (6-OHDA)-lesioned and non-lesioned neostriatum. However, the δ-opioid agonist [d-Pen², d-Pen⁵]-enkephalin (DPDPE) could not inhibit KCl-evoked ACh release in the 6-OHDA-lesioned striatum. This results suggests that δ-opioid agonist act on dopaminergic terminals to inhibit the cholinergic neurons. In unlesioned rats, GABA_A or GABA_B antagonists (bicuculline or phaclofen, respectively) prevented μ- or δ-opioid inhibition of endogenous ACh release evoked by glutamate, but not by potassium. However, in the 6-OHDA-lesioned side, DAGO inhibition of KCl-evoked ACh release was antagonized by either of the GABA antagonists. Our results suggest that the dopaminergic neurotransmission, favored by KCl, blocks the GABAergic involvement in the μ- and δ-opioid inhibition of endogenous ACh release.     

Effects of neurotransmitter candidates on Ca uptake by cortical slices of rat brain: Stimulatory effect of l-glutamic acid

The effects of neurotransmitter candidates and the characteristics of the stimulatory effect of l-glutamic acid (l-Glu) on⁴⁵Ca uptake by rat brain slices were investigated.⁴⁵Ca uptake was significantly stimulated by acetylcholine, serotonin and especially l-Glu, but not by other neurotransmitter candidates. L-Glu caused dose-dependent stimulation of⁴⁵Ca uptake (L-Glu-stimulated⁴⁵Ca uptake), its effect being half-maximal at 1 μM. The related compounds D-glutamic acid, D, L-α-aminoadipic acid and N-methyl-D, L-glutamic acid (final conc. of 10 μM) also stimulated⁴⁵Ca uptake, but less than 10 μM L-Glu. D, L-α-Methylglutamic acid and L-glutamic acid diethylether (final conc. of 10 μM), which are specific inhibitors of L-Glu, inhibitedL-Glu-stimulated⁴⁵Ca uptake. Mg, Ca-ATPase activity was hardly affected by a concentration of 10 μML-Glu that caused maximal stimulation of⁴⁵Ca uptake.These findings suggest thatL-Glu-stimulated⁴⁵Ca uptake by brain cortical slices is linked toL-Glu receptor.     

Acute effects of lead at central synapses in vitro

The acute effects of lead in the rat CNS in vitro were studied on synaptic transmission in the isolated hemisected spinal cord from newborn rats and on the transport of exogenous GABA, acetylcholine and cis-3-aminocyclohexane car☐ylic acid (ACHC) from slices of cerebral cortex from adult rats. Lead had quite variable effects on monosynaptic reflexes and synaptic potentials. When it occurred, the depression of synaptic transmission by lead (typically at 18.5 μmol/liter of added lead acetate) was reversible provided exposure times were less than 15 min; furthermore, depression could be antagonised by increasing the external calcium concentration. Lead had no effect on the postsynaptic responses of motoneurons to the putative transmittersl-glutamate, GABA and glycine or to eledoisin-related peptide.The effects of lead on uptake and release of exogenous GABA and ACHC were dependent on the perfusion buffer employed: minimal effects were seen in solutions buffered with either phosphate or carbonate. When Tris HCl was used as buffer, lead inhibited the uptake of GABA and potentiated the spontaneous release of GABA with anEC₅₀ = 50 μmol/liter as added lead acetate. In Tris HCl buffer, lead acetate (100 μmol/liter) produced a two-fold enhancement in the spontaneous release of acetylcholine under conditions where choline and acetylcholine re-uptake was blocked by hemicholinium. The availability of free lead cations in solution is highly dependent on the concentrations of other ions (particularly phosphates) and the pH. Under the appropriate conditions, lead can inhibit CNS synaptic function acutely in a manner consistent with lead competing with calcium ions in transmitter release processes as has been established for acetylcholine release at peripheral synapses.