Synaptosomal transport processes

Synaptosomal glutamate uptake in a model of experimental cerebral ischemia

In the present investigation we studied the synaptosomal uptake of glutamate in brain omogenate of Mongolian gerbils submitted to bilateral common carotid occlusion, with and without subsequent return of blood flow. The results show that glutamate uptake after ischemia is reduced by about 35%. The damage appears to be persistent, since return of blood flow restores uptake only slightly. The membrane alterations occurring in ischemia could explain the persistence of glutamate transporter impairment. Besides the blockade of NMDA receptors, the stimulation and/or the protection of the uptake systems for glutamate could be of help in preventing neuronal ischemic damage.

Age-related changes in rat brain monoamines release: peculiarity of dopamine release

Our aim has been to investigate the ability of the rat brain to retain its level of neurotransmitter release over life. We have investigated the neurotransmitter release from the rat brain synaptosomes prelabeled with 3H-DA, 3H-NA, or 3H-5HT, and perfused with Krebs-Ringer medium alone (basal release) or containing a high K+, calcium ionophore, tyramine or amphetamine (evoked release). Brain areas have been dissected of animals 45 days and 4, 6, and 11 months old. The results have shown a gradual reduction of the 3H-NA release evoked by a high K+ from 45 days to 6 months, which is stabilized until 11 months of age. The reduction rate has been relatively different from the brain areas investigated (36% for the frontal cortex and 26% for the hippocampus and cerebellar cortex). A similar reduction has been seen with 3H-5HT released from synaptosomes of the frontal cortex, hippocampus, and striatum. Surprisingly, the 3H-DA release that was evoked by high K+ was greater in rats 11 months old than in younger rats; this effect has been seen in synaptosomes from the caudate and the frontal cortex. The calcium ionophore A23187 has shown a releasing picture similar to a high K+. When we analyzed a nonexocitotic, but probably carrier mediated, release (evoked by tyramine or amphetamine), there was reduced release of all of the above neurotransmitters from 45 days to 11 months of age. We presume that there have been adaptive changes in neurotransmitter evoked release due to changes in Ca++ utilization, as inferred from the results from calcium ionophore experiments and carrier performance.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotoxic effect of 1-methyl-4-phenylpyridinium ion on dopaminergic neurons of the retina of goldfish

Dopaminergic neurons of the goldfish retina were selectively destroyed after a single intravitreal injection of 1-methyl-4-phenylpyridinium ion (MPP+). The ultrastructural analysis of the retina 3 days after toxin administration shows darkening of some retinal neurons present in the inner nuclear layer including their cytoplasmic processes. Both uptake and release of dopamine were reduced in the toxin-injected retina, whereas choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities, as well as the uptake of D-[3H]aspartate were not affected.

Glutamate receptor subtypes in cultured cerebellar neurons: modulation of glutamate and gamma-aminobutyric acid release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.

Continuous glutamate leakage from brain cells is balanced by compensatory high-affinity reuptake transport

The glutamate (and aspartate) uptake blocker threo-3-hydroxyaspartate (20 microM) was added to superfusion fluids employed for in vivo microdialysis of corpus striatum, and to incubation medium for striatal slices (5 microM). In vivo it caused an increase in glutamate and aspartate concentrations in the superfusion fluid. In vitro it caused increases in the levels of glutamate, aspartate, GABA, taurine and glutamine in the incubation fluid. Tetrodotoxin (1 microM) did not influence the rises in glutamate or aspartate. It is concluded from these results that there is a continuous outward leakage of glutamate, and aspartate, from neural cells which is normally balanced by an inward flux due to reuptake processes. This leakage is distinct from synaptic release of these substances due to spike activity, since tetrodotoxin added to striatal slices did not diminish the action of threo-3-hydroxyaspartate. The significance of the findings for mechanisms leading to ischaemic or hypoxic brain damage, and basic mechanisms in epilepsy is discussed.

Are there both low- and high-affinity glutamate transporters in rat cortical synaptosomes?

Kinetics of sodium dependent glutamic acid transport have been studied in rat cortical synaptosomes at sufficiently high glutamic acid concentrations ([G]) to delineate the "low affinity" transporter. Computer optimization techniques were used to fit the data to models which account for the sodium and substrate dependence of uptake. The data fit about equally well models consisting of two carriers (Model 1) or one carrier plus a linear component (Model 2). However, the results of further studies were inconsistent with Model 1, but totally consistent with Model 2. Thus the results are incompatible with the presence of both high- and low-affinity carriers. The carrier model found in previous studies of high affinity glutamate transport predicts the effects of high [G] and [Na] observed in the present study. The biphasic effect of [Na] on velocity of uptake is the logical consequence of the operation of this model. The rate equation for this model has been utilized to define and compute kinetic parameters which characterize the transporter. These kinetic functions are remarkably similar in shape and magnitude to previous estimates from the studies of the high affinity transport (low [G]). The results of other studies by the author which corroborate and expand the predictions of the kinetic model are discussed. These have been combined with the present results to formulate a rather comprehensive model of glutamate function. This model can be used to describe function in terms of mathematical equations and to make predictions from these equations. These equations relate velocity of uptake and the kinetic parameters to sodium and substrate concentration, velocity to membrane potential, distribution ratio to the electrochemical potential, and release to time, compartment sizes, and exchange constants. Such processes as concentration in the presynaptic terminal, depolarization induced release, re-uptake following stimulus induced release, and postsynaptic depolarization are all possible consequences of the operation of this model. The wide applicability of the model to the transport of other substrates in addition to glutamate is discussed.

Rapid release of [3H]dopamine from median eminence and striatal synaptosomes

Release of preaccumulated, tritium-labeled dopamine ([3H]DA) from preparations of isolated nerve terminals (synaptosomes) of rat median eminence (ME) and corpus striatum (CS) was examined over short time intervals (1-20 s). In both preparations, basal efflux of [3H]DA was linear with time. Depolarization with high K+ resulted in an initial rapid release of [3H]DA which stabilized by 20 s, whereas veratridine elicited an increased rate of release over basal levels that was linear over the first 20 s. The calculated rate constants of release for both the initial phase of K+- and the veratridine-stimulated release were approximately threefold greater in CS than in ME synaptosomes. The major component of the high K+-induced release of [3H]DA from both synaptosome preparations increased as a graded function of [Ca2+]o. However, a smaller component, independent of external Ca2+, existed in both ME and CS synaptosomes. Increasing the [Mg2+] in the external solution resulted in a right shift of both the [K+]o and the [Ca2+]o dose-response curves, consistent with actions of Mg2+ on screening surface membrane charges and blocking voltage-dependent Ca2+ channels. In all studies, steady-state uptake of the [3H]DA was about twofold greater into CS than into ME synaptosomes. Moreover, the fraction of incorporated [3H]DA released by stimulation from the CS was much greater than that released from ME synaptosomes. These data are consistent with differences between these two types of dopaminergic terminals with respect to packaging and/or distribution of the accumulated neurotransmitter in intraneuronal pools, as well as marked differences in the apparent kinetics of DA release.

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.

Morphometric and autoradiographic analysis of crude synaptosomal preparations from rat cerebral cortex

Morphometric and autoradiographic studies have been made of a crude synaptosomal preparation, which has been used extensively for membrane transport studies. When filters are used to separate membrane bound structures from incubation medium, the structures which survive filtration are those that are entrapped within the matrix of the filter structure. The population of membrane bound structures differs when one compares pellets of the preparation to sections of loaded 0.45 and 0.65 micron pore size filters. Both the relative numbers of synaptosomes, mitochondria, and other membrane bound structures (OMBS) and the mean size of each of the structures differ for pellet, 0.45 micron, and 0.65 micron filters. The percentage of total membrane bound volume attributable to synaptosomes increases from 28 in the crude preparation to 40 in 0.45 micron filters and 61 in 0.65 micron filters. The total volume of synaptosomes entrapped by differing pore size filters roughly correlates with the amount of substrate uptake. Neither mitochondrial volume nor the volume of other membrane bound structures was found to correlate with uptake. These results indicated that only the synaptosomes contribute measurably to this function. Autoradiographic studies confirm this conclusion. EM autoradiography following loading of the synaptosomal preparation with tritiated glutamate or GABA showed about 81% of the grains to be associated with synaptosomes. It is concluded that crude synaptosomal preparations may be used without further purification for membrane transport studies with unambiguous results.

Kainic acid differentially affects the synaptosomal release of endogenous and exogenous amino acidic neurotransmitters

Presynaptic actions of kainic acid have been tested on uptake and release mechanisms in synaptosome-enriched preparations from rat hippocampus and goldfish brain. Kainic acid increased in a Ca2+-dependent way the basal release of endogenous glutamate and aspartate from both synaptosomal preparations, with the maximum effect (40-80%) being reached at the highest concentration tested (1 mM). In addition, kainic acid potentiated, in an additive or synergic way, the release of excitatory amino acids stimulated by high K+ concentrations. Kainic acid at 1 mM showed a completely opposite effect on the release of exogenously accumulated D-[3H]aspartate. The drug, in fact, caused a marked inhibition of both the basal and the high K+-stimulated release. Kainic acid at 0.1 mM had no clear-cut effect, whereas at 0.01 mM it caused a small stimulation of the basal release. The present results suggest that kainic acid differentially affects two neurotransmitter pools that are not readily miscible in the synaptic terminals. The release from an endogenous, possibly vesiculate, pool of excitatory amino acids is stimulated, whereas the release from an exogenously accumulated, possibly cytoplasmic and carrier-mediated, pool is inhibited or slightly stimulated, depending on the external concentration of kainic acid. Kainic acid, in addition, strongly inhibits the high-affinity uptake of L-glutamate and D-aspartate in synaptic terminals. All these effects appear specific for excitatory amino acids, making it likely that they are mediated through specific recognition sites present on the membranes of glutamatergic and aspartatergic terminals. The relevance of the present findings to the mechanism of excitotoxicity of kainic acid is discussed.

A superfusion apparatus for the examination of neurotransmitter release from synaptosomes

A superfusion apparatus for examining the release of neurotransmitter from synaptosomes is described. The apparatus provides for accurate temperature control, rapid switching and continuous gassing of superfusing buffers, short (i.e. 1-s) pulses of high-potassium-containing buffer for examining depolarized release, rapid collection of superfusing eluate, and low tissue chamber dead volume. Depolarization (high-potassium) evoked release and to some degree spontaneous release of [3H]dopamine from rat striatal synaptosomes are dependent on buffer calcium concentration. The putative calcium channel blocker nickel reduces both spontaneous and high-potassium-evoked [3H]dopamine release.

Depolarisation-dependent protein phosphorylation in rat cortical synaptosomes: factors determining the magnitude of the response

The sequence of molecular events linking depolarisation-dependent calcium influx to the release of neurotransmitters from nerve terminals is unknown; however, calcium-stimulated protein phosphorylation may play a role. In this study the incorporation of phosphate into proteins was investigated using an intact postmitochondrial pellet isolated from rat cerebral cortex. The rate and relative incorporation of label into individual phosphoproteins depended on the prelabelling time and buffer concentrations of calcium and phosphate. After prelabelling for 45 min, depolarisation caused a greater than 20% increase in the labelling of 10 phosphoproteins, and this initial increase was maximal with 41 mM K+ for 5 s, or 30 microM veratridine for 15 s, in the presence of 1 mM calcium. Both agents also led to an initial dephosphorylation of four phosphoproteins. Depolarisation for 5 min led to a significant decrease in the labelling of all phosphoproteins. All of the depolarisation-stimulated changes in protein phosphorylation were calcium-dependent. The depolarisation conditions found to optimally alter the phosphorylation of synaptosomal proteins find many parallels in studies on calcium uptake and neurotransmitter release. However, the uniform responses of such a large number of phosphoproteins to the multitude of depolarisation conditions studied suggest that the changes could equally well relate to recovery events such as biosynthesis of neurotransmitters and regulation of intraterminal metabolic activity.

Critical assessment of noradrenaline uptake in synaptosomal preparations

Synaptosomes and other subcellular organelles were prepared from rat brain using a vertical rotor. The preparation time was reduced by up to 60% compared to conventional techniques. Uptake of [3H]-(-)-noradrenaline into subcellular fractions was characterized. The characteristics of this uptake were dependent on the subcellular composition and anatomical origin. Various methods of correction for energy independent processes were compared, but only sodium ion removal from the medium selectively inhibited the energy dependent uptake mechanism. Kinetic analysis of data revealed that high and low affinity uptake systems were dependent on the fraction under analysis. Noradrenaline uptake was not exclusively localised in noradrenergic terminals. Selective inhibitors of the noradrenaline uptake process (tricyclic antidepressants) inhibited energy dependent uptake completely only in purified synaptosomes prepared from cortex. In whole brain synaptosomal fractions, noradrenaline was partially accumulated into dopaminergic neurones; this uptake process was not inhibited by tricyclic antidepressants.

Neurotransmitter metabolism in rat brain synaptosomes: effect of anoxia and pH

Synaptosomes isolated from the rat cerebral cortex by means of a discontinuous Ficoll gradient carry out net, sodium-dependent, veratridine-sensitive accumulation of gamma-aminobutyric acid (GABA), serotonin, norepinephrine, and dopamine. The intrasynaptosomal contents of the four neurotransmitters are: 30.4 nmol/mg protein, 17.4 pmol/mg protein, 13.5 pmol/mg protein, and 21.2 pmol/mg protein, respectively. Anaerobic preincubation of synaptosomes causes an irreversible decrease in the rates of neurotransmitter accumulation but does not affect the rates of their release. The inhibitory effect of anaerobiosis is enhanced by increased concentration of [H+] (decreased pH) in the medium. The most sensitive is the uptake of dopamine, the least that of serotonin. The rates of neurotransmitter efflux are unaffected by anaerobiosis. Synaptosomes leak catecholamines, GABA, and serotonin into the medium when subjected to anaerobiosis, and reintroduction of oxygen is accompanied by a rapid reaccumulation of all four neurotransmitters. It is concluded that: (1) Responses of synaptosomes to anaerobiosis are remarkably similar to the behavior of intact brain in hypoxia and ischemia. (2) Neurotransmitter uptake systems are more sensitive to short periods of anaerobiosis than either the energy metabolism or ion transport. (3) Some neurotransmitter uptake systems are more easily damaged by anaerobiosis than others.

Measurement of release of endogenous GABA and catabolites of [3H]GABA from synaptosomal preparations using ion-exchange chromatography

Picomole quantities of endogenous GABA in acidified superfusates of synaptosomal preparations have been measured using micro-bore ion-exchange chromatography and post-column formation of the fluorescent iso-indole derivative. Using this technique superfusates have been analyzed directly, without further manipulations, to investigate the release of endogenous GABA. Spontaneous release of GABA was 2-5 pmol/200 microliters superfusate increasing to 20 pmol/200 microliters with potassium stimulation. When gamma-vinyl GABA (RMI 71754), an inhibitor of GABA-T was injected into rats (750 mg/kg) and synaptosomes prepared the potassium-evoked release of GABA was increased 3-fold compared to controls. Chromatographic separations and measurement of release of endogenous and radiolabeled GABA allowed the real specific activity of released GABA to be calculated. Only when 500 microM amino-oxyacetic acid was added during isolation of synaptosomes was the specific activity of released GABA the same as the initial specific activity.

Evidence in favor of a neurotransmitter role of glycine in the rat cerebral cortex

In the present study we analyze whether glycine satisfies some electrophysiological and biochemical criteria to consider it as a putative transmitter in the rat cerebral cortex. Intracellular recordings from rat sensory-motor cortex showed that in 15-20% of the tested neurons glycine hyperpolarized the cell membrane, decreased the firing rate and flattened the evoked EPSP-IPSP sequence by increasing the membrane conductance. The iontophoretic application of strychnine antagonized the block of 'spontaneous' firing and the membrane hyperpolarization induced by glycine. Moreover, in a group of neurons, strychnine decreased the amplitude and duration of the IPSP and brought back the membrane potential to resting values. Previously accumulated [3H]glycine and endogenous glycine were released from cortical synaptosomal preparations by depolarizing stimuli in a Ca2+-dependent way. The release pattern of glycine was qualitatively similar in cortical and in spinal synaptosomes. [14C]Glycine was rapidly synthetized from [14C]serine in cortical synaptosomal preparations, and the newly formed [14C]glycine was released by depolarizing stimuli in a Ca2+-dependent way. It is concluded that glycine, which is generally considered as an inhibitory neurotransmitter in the spinal cord, medulla and pons, may also have a transmitter role in a discrete number of cortical neurons of some mammalian species.U

The Na+,K+-ATPase: a plausible trigger for voltage-independent release of cytoplasmic neurotransmitters

A comparison was made between the releasability of eight neurotransmitters from eight regions of mouse brain in response to either 60 mM-K+ or 20 microM-ouabain, a specific inhibitor of the Na+,K+-ATPase. With few exceptions, all transmitters were released by either or both agents from each brain region examined. Potassium was superior in releasing the biogenic amines and acetylcholine, while the putative amino acid transmitters were generally releasable by both agents. Measurements of tissue depolarization using [3H]-tetraphenylphosphonium uptake indicated that 60 mM-K+ is capable of depolarizing brain tissue above the threshold necessary for initiating an action potential, but 20 microM-ouabain is not. The pattern of release by ouabain coupled with its failure to depolarize brain tissue at 20 microM suggests that inhibition of the Na+,K+-ATPase is capable of releasing cytoplasmic neurotransmitters in a voltage-independent manner.

Inhibition by neurotoxic phospholipases A2 of synaptosomal uptake of gamma-aminobutyric acid

A comparison has been made of the abilities of several neurotoxic and nontoxic phospholipases A2 from snake venoms to inhibit the intake of gamma-aminobutyric acid into synaptosomes from rat cerebral cortex. The neurotoxic phospholipase A2 inhibited GABA uptake more than the nontoxic enzymes did. However, there was a poor correlation between the measured specific enzyme activity of a phospholipase A2 and its ability to inhibit the uptake of GABA.

Effects of in vitro hypoxia and lowered pH on potassium fluxes and energy metabolism in rat brain synaptosomes

Synaptosomes isolated on isosmotic Ficoll density gradients are an effective model for some aspects of neuronal function. They maintain metabolic energy levels ([ATP]/[ADP] [Pi]) and transplasma membrane electrical potentials very similar to those of neurons in the intact brain. The concentration of K+ in the external medium (K+-sensitive electrode), O2 uptake, and cytochrome c reduction (550 nm minus 540 nm) were simultaneously monitored in synaptosomal suspensions. Oxidative metabolism is the primary source of intrasynaptosomal ATP and at pH 7.4 anaerobiosis results in K+ leakage at 4.5 +/- 0.8 nmol/min/mg protein with glucose as substrate and 10.7 +/- 1.9 nmol/min/mg protein with lactate plus pyruvate (10:1) as substrate. Reintroduction of oxygen initiates complete (ouabain-sensitive) reuptake of K+ at initial rates of 35.4 +/- 3.2 nmol/min/mg protein and 18 +/- 1.7 nmol K+/min/mg protein, respectively. The rates of K+ leakage and reuptake fall when the pH is lowered from 7.4 to 6.0 but recover fully if the pH is raised to the original value. The rates of K+ release and uptake decrease when the Na+ concentration in the medium is decreased and increase when the Ca2+ concentration is decreased. The intrasynaptosomal [K+] under aerobic conditions was 77.3 +/- 3 mM and the calculated K+ diffusion potential was -72 mV. Anaerobic incubation of the synaptosomes from up to 20 min and at pH values from 7.4 to 6.0 did not produce irreversible impairment of any of the measured variables. These results suggest that permanent loss of brain function following prolonged hypoxia and ischemia is not due to irreversible damage to the synapses with respect to these parameters but rather to impairment of some other neuronal functions.

Storage and release of noradrenaline in hypothalamic synaptosomes

The noradrenaline storage capacity of vesicles in hypothalamic synaptosomes was measured by incubating them with [3H]noradrenaline under saturating conditions. The normal noradrenaline content is 52% of storage capacity. Incubation or superfusion with 50 mM-potassium causes calsium-dependent release from the vesicles. Such release reduces not only the vesicular content, but also the noradrenaline storage capacity. This suggests that after exocytosis vesicles cannot refill with noradrenaline.

Energy transduction in intact synaptosomes. Influence of plasma-membrane depolarization on the respiration and membrane potential of internal mitochondria determined in situ

A method is described, based on the differential accumulation of Rb+ and methyltriphenylphosphonium, for the simultaneous estimation of the membrane potentials across the plasma membrane of isolated nerve endings (synaptosomes), and across the inner membrane of mitochondria within the synaptosomal cytoplasm. These determinations, together with measurements of respiratory rates, and ATP and phosphocreatine concentrations, are used to define the bioenergetic behaviour of isolated synaptosomes under a variety of conditions. Under control conditions, in the presence of glucose, the plasma and mitochondrial membrane potentials are respectively 45 and 148mV. Addition of a proton translocator induces a 5-fold increase in respiration, and abolishes the mitochondrial membrane potential. The addition of rotenone to inhibit respiration does not affect the plasma membrane potential, and only lowers the mitochondrial membrane potential to 128mV. Evidence is presented that ATP synthesis by anaerobic glycolysis is sufficient under these conditions to maintain ATP-dependent processes, including the reversal of the mitochondrial ATP synthetase. Addition of oligomycin under non-respiring conditions leads to a complete collapse of the mitochondrial potential. Even under control conditions the plasma membrane (Na+ + K+)-dependent ATPase is responsible for a significant proportion of the synaptosomal ATP turnover. Veratridine greatly increases respiration, and depolarizes the plasma membrane, but only slightly lowers the mitochondrial membrane potential. High K+ and ouabain also lower the plasma membrane potential without decreasing the mitochondrial membrane potential. In non-respiring synaptosomes, anaerobic glycolysis is incapable of maintaining cytosolic ATP during the increased turnover induced by veratridine, and the mitochondrial membrane potential collapses. It is concluded that the internal mitochondria must be considered in any study of synaptosomal transport.