Exposure to Ebselen Changes Glutamate Uptake and Release by Rat Brain Synaptosomes

We investigated effects of Ebselen, diphenyl diselenide (PhSe)₂ and diphenyl ditelluride (PhTe)₂ on [³H]glutamate uptake and release by brain synaptosomes. Ebselen after acute exposure inhibited K⁺-stimulated [³H]glutamate release by brain synaptosomes. (PhSe)₂ and (PhTe)₂ did not change [³H]glutamate release by brain synaptosomes. Ebselen, (PhSe)₂ and (PhTe)₂ had no significantly effects on [³H]glutamate uptake after acute exposure. In vitro, Ebselen (100 M) inhibited [³H]glutamate release and uptake. (PhSe)₂ had no significant effect, while (PhTe)₂ (100 M) inhibited [³H]glutamate uptake by brain synaptosomes. In vitro, (PhSe)₂, (PhTe)₂ and Ebselen caused a significant inhibition of [³H]glutamate uptake by brain synaptic vesicles in vitro. The results demonstrated that organochalcogenides have a rather complex effect on glutamate homeostasis depending on the compound and the schedule of exposition. We propose that the neuroprotective action of Ebselen can be related, in addition to its glutathione peroxidase-like and antilipoperoxidative activity, to a direct interaction with the glutamatergic system by reducing K^ï-evoked glutamate release.     

Organochalcogens Affect the Glutamatergic Neurotransmission in Human Platelets

Blood platelets have repeatedly been suggested as an excellent model for various aspects of the synaptic apparatus. Considering that organochalcogens affect some parameters of glutamatergic neurotransmission in rats, in the current study we evaluated the effect of diphenyl diselenide (PhSe)2, diphenyl ditelluride (PhTe)2, and Ebselen on glutamatergic neurotransmission in human platelets. (PhTe)2 and (PhSe)2 caused a significant inhibition, but Ebselen did not interfere in Na-independent glutamate binding. Dithiothreitol (DTT) did not completely prevent the [3H]glutamate binding inhibition caused by 100 microM (PhTe)2. (PhSe)2, (PhTe)2, and Ebselen (100 microM) significantly inhibited [3H]glutamate uptake, whereas organochalcogens at 1 and 10 microM had no significant effect on the [3H]glutamate uptake in human platelets. In this study, platelets were demonstrated to be a suitable model for neurotoxicological research, and, to the best of our knowledge, this is the first report documenting the toxic effects of organochalcogens in human platelets.     

Diphenyl Ditelluride Induces Anxiogenic-Like Behavior in Rats by Reducing Glutamate Uptake

Anxiety-related disorders are a common public health issue. Several lines of evidence suggest that altered glutamatergic neurotransmission underlies anxiety. The present study evaluated the effect of diphenyl ditelluride [(PhTe)₂] exposure on the behavioral performance of rats and examined whether the behavioral effects could be attributed to changes in the modulation of glutamatergic function. Rats were exposed to (PhTe)₂ (subcutaneously) during 8 weeks—final dose one third LD₅₀ (124 μg/kg). The testing schedule included elevated plus-maze, open-field, T-maze, rotorod, and Morris water maze tests. Synaptosomal basal [³H] glutamate release and uptake were also evaluated. The time spent in the open arm and the ratio of time spent in the open arm/total were decreased in the (PhTe)₂ group. Furthermore, the [³H] glutamate uptake was decreased in this experimental group. The results suggest that exposure to (PhTe)₂ did not change motor abilities whereas it may result in anxiogenic-like behavior, induced by changes in the glutamatergic system at the pre-synaptic level.     

Effects of Linalool on Glutamate Release and Uptake in Mouse Cortical Synaptosomes

Linalool, a monoterpene compound prevalent in essential oil of plant species traditionally used as sedatives, has been characterized as anticonvulsant in several experimental models. Linalool inhibits the binding of [³H]glutamate and [³H]dizocilpine to brain cortical membranes, indicating a participation of the glutamatergic transmission its mechanism of action. In this study, we investigated the effects of linalool on [³H]glutamate release (basal and potassium-stimulated) and [³H]glutamate uptake in mice cortical synaptosomes. Linalool significantly reduced potassium-stimulated glutamate release as well as glutamate uptake, not interfering with basal glutamate release. The data indicates that linalool may interfere with several relevant elements of the glutamatergic transmission, including detriment of the K⁺-stimulated glutamate release.     

<Emphasis Type="Italic">In vivo</Emphasis> Quinolinic Acid Increases Synaptosomal Glutamate Release in Rats: Reversal by Guanosine

Glutamate, the main excitatory neurotransmitter in the mammalian central nervous system (CNS), plays important role in brain physiological and pathological events. Quinolinic acid (QA) is a glutamatergic agent that induces seizures and is involved in the etiology of epilepsy. Guanine-based purines (GBPs) (guanosine and GMP) have been shown to exert neuroprotective effects against glutamatergic excitotoxic events. In this study, the influence of QA and GBPs on synaptosomal glutamate release and uptake in rats was investigated. We had previously demonstrated that QA “in vitro” stimulates synaptosomal L-[³H]glutamate release. In this work, we show that i.c.v. QA administration induced seizures in rats and was able to stimulate synaptosomal L-[³H]glutamate release. This in vivo neurochemical effect was prevented by i.p. guanosine only when this nucleoside prevented QA-induced seizures. I.c.v. QA did not affect synaptosomal L-[³H]glutamate uptake. These data provided new evidence on the role of QA and GBPs on glutamatergic system in rat brain.     

In vitro neuropharmacological evaluation of penitrem-induced tremorgenic syndromes: importance of the GABAergic system

The effects of the fungal neurotoxin penitrem A on the GABAergic and glutamatergic systems in rat brain were evaluated. Penitrem A inhibited binding of the GABA_A-receptor ligand [³H]TBOB to rat forebrain and cerebellar membrane preparations with IC₅₀ (half maximal inhibitory concentration) values of 11 and 9 μM, respectively. Furthermore, penitrem A caused a concentration-dependent increase of [³H]flunitrazepam and [³H]muscimol binding in rat forebrain, but not in cerebellar preparations. The stimulation of [³H]flunitrazepam binding by penitrem A was abolished by the addition of GABA. In cerebellar preparations, a different pharmacological profile was found, with penitrem A allosterically inhibiting [³H]TBOB binding by interacting with a bicuculline-sensitive site. Moreover, penitrem A inhibited the high affinity uptake of GABA and glutamate into cerebellar synaptosomes with IC₅₀ values of 20 and 47 μM, respectively. The toxin showed no effect on NMDA or AMPA glutamate receptor binding. In conclusion, our results suggest that penitrem A exerts region-specific effects in the brain, leading to positive modulation of GABA_A-receptor function in forebrain. Conversely, penitrem A may act as a bicuculline-like convulsant in cerebellum.     

Effect of Low pH on Glutamate Uptake and Release in Isolated Presynaptic Endings from Rat Brain

The effect of acidification of the incubation medium on the membrane potential and glutamate uptake and release was studied in isolated presynaptic neuronal endings (synaptosomes) from rat brain. Using the fluorescent probe diS-C₃-(5), a rapid depolarization of plasma membrane was detected at pH 6.0, most probably as a result of the inhibition of the sodium pump and potassium channel blockade. The membrane potential decrease did not result in increase of basal efflux of glutamate. Glutamate release following K⁺-induced depolarization was decreased upon lowering pH to 6.0. Acidosis inhibited mainly calcium-dependent (vesicular) release of glutamate and did not significantly reduce [¹⁴C]glutamate uptake. This inhibition of glutamate release but not of glutamate uptake may be a mechanism of the protective effect of acidosis during brain ischemia.     

Glutamic Acid and γ-Aminobutyric Acid Modulate Each Other's Release Through Heterocarriers Sited on the Axon Terminals of Rat Brain

Abstract: The effects of γ-aminobutyric acid (GABA) on the spontaneous release of endogenous glutamic acid (Glu) or aspartic acid (Asp) and the effects of Glu on the release of endogenous GABA or [³H]GABA were studied in superfused rat cerebral cortex synaptosomes. GABA increased the outflow of Glu (EC₅₀17.2 μM) and Asp (EC₅₀ 18.4 μM). GABA was not antagonized by bicuculline or picrotoxin. Neither muscimol nor (-)-baclofen mimicked GABA. The effects of GABA were prevented by GABA uptake inhibitors and were Na⁺ dependent. Glu enhanced the release of [³H]GABA (EC₅₀ 11.5 μM) from cortical synaptosomes. Glu was not mimicked by the glutamate receptor agonists N-methyl-d -aspartic, kainic, or quisqualic acid. The Glu effect was decreased by the Glu uptake inhibitor D-threo-hydroxyaspartic acid (THA) and it was Na⁺ sensitive. Similarly to Glu, D-Asp increased [³H]GABA release (EC₅₀ 9.9 μM), an effect blocked by THA. Glu also increased the release of endogenous GABA from cortex synaptosomes. In this case the effect was in part blocked by the (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaiine-2, 3-dione, whereas the 6-cyano-7-nitroquinoxaline- 2, 3-dione-insensitive portion of the effect was prevented by THA. GABA increased the [³H]D-Asp outflow (EC₅₀ 13.7 μM) from hippocampal synaptosomes in a muscimol-, (-)- baclofen-, bicuculline-, and picrotoxin-insensitive manner. The GABA effect was abolished by blocking GABA uptake and was Na⁺ dependent. Glu increased the release of [³H]- GABA from hippocampal synaptosomes (EC₅₀ 7.1 μM) in an N-methyl-d -aspartic acid-, kainic acid-, or quisqualic acid-insensitive way. The effect of Glu was prevented by THA and was Na⁺ dependent. As in the cortex, the effect of Glu was mimicked by D-Asp in a THA-sensitive manner. It is proposed that high-affinity GABA or Glu heterocarriers are sited respectively on glutamatergic or GA- BAergic nerve terminals in rat cerebral cortex and hippocampus. The uptake of GABA may modulate Glu and Asp release, whereas the uptake of Glu may modulate the release of GABA. The existence of these heterocarriers is in keeping with the reported colocalization of GABA and Glu in some cortical and hippocampal neurons. Preliminary data suggest that these mechanisms may also be present in rat cerebellum and spinal cord.     

NMDA and Non-NMDA Receptor-Mediated Release of [3H]GABA from Granule Cell Dendrites of Rat Olfactory Bulb

Abstract: We have studied the effect of glutamate and the glutamatergic agonists N-methyl-d -aspartate (NMDA), kainate, and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) on [³H]GABA release from the external plexiform layer of the olfactory bulb. The GABA uptake blocker nipecotic acid significantly increased the basal [³H]GABA release and the release evoked by a high K⁺ concentration, glutamate, and kainate. The glutamate uptake blocker pyrrolidine-2,4-dicarboxylate (2,4-PDC) inhibited by 50% the glutamate-induced [³H]GABA release with no change in the basal GABA release. The glutamatergic agonists NMDA, kainate, and AMPA also induced a significant [³H]GABA release. The presence of glycine and the absence of Mg²⁺ have no potentiating effect on NMDA-stimulated release; however, when the tissue was previously depolarized with a high K⁺ concentration, a significant increase in the NMDA response was observed that was potentiated by glycine and inhibited by the NMDA receptor antagonist 2-amino-5-phosphonoheptanoic acid (AP-7). The kainate and AMPA effects were antagonized by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not by AP-7. The glutamate effect was also inhibited by CNQX but not by the NMDA antagonist 2-amino-5-phosphonopentanoic acid (AP-5); nevertheless, in the presence of glycine, [³H]GABA release evoked by glutamate was potentiated, and this response was significantly antagonized by AP-5. Tetrodotoxin inhibited glutamate- and kainate-stimulated [³H]GABA release but not the NMDA-stimulated release. The present results show that in the external plexiform layer of the olfactory bulb, glutamate is stimulating GABA release through a presynaptic, receptor-mediated mechanism as a mixed agonist on NMDA and non-NMDA receptors; glutamate is apparently also able to induce GABA release through heteroexchange.     

Cerebral Alanine Transport and Alanine Aminotransferase Reaction: Alanine as a Source of Neuronal Glutamate

Abstract: Alanine transport and the role of alanine amino-transferase in the synthesis and consumption of glutamate were investigated in the preparation of rat brain synaptosomes. Alanine was accumulated rapidly via both the high-and low-affinity uptake systems. The high-affinity transport was dependent on the sodium concentration gradient and membrane electrical potential, which suggests a cotransport with Na⁺. Rapid accumulation of the Na⁺-alanine complex by synaptosomes stimulated activity of the Na⁺/K⁺ pump and increased energy utilization; this, in turn, activated the ATP-producing pathways, glycolysis and oxidative phosphorylation. Accumulation of Na⁺ also caused a small depolarization of the plasma membrane, a rise in [Ca²⁺]₁, and a release of glutamate. Intra-synaptosomal metabolism of alanine via alanine aminotransferase, as estimated from measurements of N fluxes from labeled precursors, was much slower than the rate of alanine uptake, even in the presence of added oxoacids. The velocity of [¹⁵N]alanine formation from [¹⁵N]glutamine was seven to eight times higher than the rate of [¹⁵N]glutamate generation from [¹⁵N]alanine. It is concluded that (a) overloading of nerve endings with alanine could be deleterious to neuronal function because it increases release of glutamate; (b) the activity of synaptosomal alanine aminotransferase is much slower than that of glutaminase and hence unlikely to play a major role in maintaining [glutamate] during neuronal activity; and (c) alanine aminotransferase might serve as a source of glutamate during recovery from ischemia/hypoxia when the alanine concentration rises and that of glutamate falls.     

GABA FLUXES IN PRESYNAPTIC NERVE ENDINGS FROM IMMATURE RATS

Abstract— Several parameters of GABA Auxes across the synaptosomal membrane were studied using synaptosomes prepared from the brain of immature (8-day-old) rats. The following aspects of GABA carrier-mediated transport were similar in immature and mature synaptosomes: (1) magnitude of [³H]GABA accumulation; (2) GABA homoexchange in normal ionic conditions; (3) GABA homoexchange in the presence of cationic fluxes (Na⁺ and Ca²⁺ influx, K⁺ efflux) characteristic of physiological depolarization. As in adult synaptosomes (Levi & Raiteri , 1978), in these conditions the stoichiometry of GABA homoexchange was in the direction of net outward transport (efflux > influx). The essential differences between the behaviour of 8-day-old and adult synaptosomes were the following: (1) β-alanine (a glial uptake inhibitor) inhibited GABA uptake in immature synaptosomes (the inhibition being greater in crude than in purified preparations) and was without a significant effect in adult synaptosomes. DABA and ACHC (two neuronal uptake inhibitors) depressed GABA uptake more efficiently in purified than in crude immature synaptosomes, but were as effective in crude and purified nerve endings from adult animals. The data suggest a greater uptake of GABA in the‘gliosomes’contaminating the synaptosomal preparations from immature animals. (2) In immature synaptosomes prelabelled with [³H]GABA the specific radioactivity of the GABA released spontaneously or by heteroexchange (with 300 μm -OH-GABA) was the same as that present in synaptosomes, while in adult synaptosomes OH-GABA released GABA with increased specific radioactivity. The data suggest a homogeneous distribution of the [³H]GABA taken up within the endogenous GABA pool in immature, but not in mature synaptosomes. (3) In immature synaptosomes the release of GABA (radioactive and endogenous) induced by depolarization with high KC was not potentiated by Ca²⁺, unless the synaptosomes had been previously depleted of Na⁺ These data suggest that, although a Ca²⁺ sensitive pool of GABA may be present, this pool is not susceptible to being released in normal conditions, probably because the high intrasynaptosomal Na⁺ level prevents a sufficient depolarization. The possible significance of these findings in terms of functional activity of GABAergic neurotransmission in the immature brain is discussed.     

Repeated Restraint Stress Alters Hippocampal Glutamate Uptake and Release in the Rat

Glutamatergic mechanisms are thought to be involved in stress-induced changes of brain function, especially in the hippocampus. We hypothesized that alterations caused by the hormonal changes associated with chronic and acute stress may affect glutamate uptake and release from hippocampal synaptosomes in Wistar rats. It was found that [3H]glutamate uptake and release by hippocampal nerve endings, when measured 24 h after 1 h of acute restraint, presented no significant difference. The exposure to repeated restraint stress for 40 days increased neuronal presynaptic [3H]glutamate uptake as well as basal and K+-stimulated glutamate release when measured 24 h after the last stress session. Chronic treatment also caused a significant decrease in [3H]glutamate binding to hippocampal membranes. We suggest that changes in the glutamatergic system are likely to take part in the mechanisms involved in nervous system plasticity following repeated stress exposure.     

Characterization of Separated Cell Types from the Developing Rat Cerebellum: Transport of Glutamate and Aspartate by Preparations Enriched in Purkinje Cells, Granule Neurones, and Astrocytes

Preparations of structurally preserved cerebellar perikarya (cells) were found to express high-affinity transport systems for glutamate but not for certain putative transmitter substances (including monoamines, glycine and taurine) and non-transmitter amino acids. The characteristics of the high-affinity glutamate transport system were similar to those of other preparations of brain tissue: [³H]glutamate uptake by the cells was Na⁺-dependent and was inhibited competetively by other acidic amino acids. The rank order of apparent affinities of the carrier for acidic amino acids was L-aspartate > L-glutamate > D-aspartate ? D-glutamate (the affinity for D-glutamate being over two orders of magnitude lower than for the other three amino acids). Comparison of high-affinity [³H]glutamate uptake in preparations enriched in different cell types showed that although the affinities are similar (2-4 fiM), the rate is outstandingly high in astrocytes (V_max 18 nmol/min per mg protein). Significantly, uptake into the putatively glutamatergic granule cells was very low. These observations were supported by autoradiographic findings which showed that the predominant sites of [3H]glutamate uptake in cerebellar cultures enriched in interneurones are the astrocytes. Furthermore, the V_max in cultures enriched in astrocytes was as high as that in separated astrocytes. Thus, it seems that the principal cell type involved in acidic amino acid uptake in the cerebellum is the astrocyte, and this must be taken into consideration when high-affinity uptake is used as a marker for glutamatergic transmitter systems. Furthermore, the selective cellular distribution of glutamate transport sites, together with the uneven distribution of enzymes related to glutamate metabolism observed previously, indicates that a metabolic interaction takes place between the different cell types, supporting the current hypothesis on metabolic compartmentation in the brain.     

Uptake and metabolism of glutamate and aspartate by astroglial and neuronal preparations of rat cerebellum

Astrocytes, neuronal perikarya and synaptosomes were prepared from rat cerebellum. Kinetics of high and low affinity uptake systems of glutamate and aspartate, nominal rates of¹⁴CO₂ production from [U–¹⁴C]glutamate, [U–¹⁴C]aspartate and [1–¹⁴C]glutamate and activities of enzymes of glutamate metabolism were studied in these preparations. The rate of uptake and the nomial rate of production of¹⁴CO₂ from these amino acids was higher in the astroglia than neuronal perikarya and synaptosomes. Activities of glutamine synthetase and glutamate dehydrogenase were higher in astrocytes than in neuronal perikarya and synaptosomes. Activities of glutaminase and glutamic acid decarboxylase were observed to be highest in neuronal perikarya and synaptosomes respectively. These results are in agreement with the postulates of theory of metabolic compartmentation of glutamate while others (presence of glutaminase in astrocytes and glutamine synthetase in synaptosomes) are not. Results of this study also indicated that (i) at high extracellular concentrations, glutamate/aspartate uptake may be predominantly into astrocytes while at low extracellular concentrations, it would be into neurons (ii) production of -ketoglutarate from glutamate is chiefly by way of transamination but not by oxidative deamination in these three preparations and (iii) there are topographical differences glutamate metabolism within the neurons.     

Comparison of Effects of DL-Threo-β-Benzyloxyaspartate (DL-TBOA) and L-Trans-Pyrrolidine-2,4-Dicarboxylate (t-2,4-PDC) on Uptake and Release of [3H]D-Aspartate in Astrocytes and Glutamatergic Neurons

Uptake and release processes in cerebellar astrocytes and granule neurons (glutamatergic) for glutamate were investigated by the use of [³H]D-aspartate, a non-metabolizable glutamate analog. The effects of DL-threo--benzyloxyaspartate (DL-TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC) on uptake and release of [³H]D-aspartate were studied. Both compounds inhibited potently uptake of [³H]D-aspartate in neurons and astrocytes (IC₅₀ values 10-100 M), DL-TBOA being slightly more potent than t-2,4-PDC. Release of preloaded [³H]D-aspartate from neurons or astrocytes could be stimulated by addition of excess t-2,4-PDC whereas addition of DL-TBOA had no effect on [³H]D-aspartate efflux. Moreover, DL-TBOA inhibited significantly the depolarization-induced (55 mM KCl) release of preloaded [³H]D-aspartate in the neurons. The results reflect the fact that DL-TBOA is not transported by the glutamate carriers while t-2,4-PDC is a substrate which may heteroexchange with [³H]D-aspartate. It is suggested that DL-TBOA may be used to selectively inhibit depolarization coupled glutamate release mediated by reversal of the carriers.     

Calcium-dependent [3H]acetylcholine release and muscarinic autoreceptors in rat cortical synaptosomes during development

A number of presynaptic cholinergic parameters (high affinity [³H]choline uptake, [³H]acetylcholine synthesis, [³H]acetylcholine release, and autoinhibition of [³H]acetylcholine release mediated by muscarinic autoreceptors) were comparatively analyzed in rat brain cortex synaptosomes during postnatal development. These various functions showed a differential time course during development. At 10 days of age the release of [³H]acetylcholine evoked by 15 mM KCl from superfused synaptosomes was Ca²⁺-dependent but insensitive to the inhibitory action of extrasynaptosomal acetylcholine. The muscarinic autoreceptors regulating acetylcholine release were clearly detectable only at 14 days, indicating that their appearance may represent a criterion of synaptic maturation more valuable than the onset of a Ca²⁺-dependent release.     

Chronic Stress and Lithium Treatments Alter Hippocampal Glutamate Uptake and Release in the Rat and Potentiate Necrotic Cellular Death After Oxygen and Glucose Deprivation

This study was undertaken to evaluate the effects of chronic variate stress and lithium treatment on glutamatergic activity and neuronal vulnerability of rat hippocampus. Male Wistar rats were simultaneously treated with lithium and submitted to a chronic variate stress protocol during 40?days, and afterwards the hippocampal glutamatergic uptake and release, measured in slices and synaptosomes, were evaluated. We observed an increased synaptosomal [(3)H]glutamate uptake and an increase in [(3)H]glutamate stimulated release in hippocampus of lithium-treated rats. Chronic stress increased basal [(3)H]glutamate release by synaptosomes, and decreased [(3)H]glutamate uptake in hippocampal slices. When evaluating cellular vulnerability, both stress and lithium increased cellular death after oxygen and glucose deprivation (OGD). We suggest that the manipulation of glutamatergic activity induced by stress may be in part responsible for the neuroendangerment observed after stress exposure, and that, in spite of the described neuroprotective effects of lithium, it increased the neuronal vulnerability after OGD.     

Uptake and release for glutamine and glutamate in a crude synaptosomal fraction from rat brain

[14C]Glutamine uptake in a crude synaptosomal (P2) fraction, (representing the sum of [¹⁴C]glutamine accumulated and [¹⁴C]glutamate formed by hydrolysis), is distinct from glutamate uptake. Glutamine uptake is Na⁺-independent and unaffected by the Na⁺–K⁺-ATPase inhibitor ouabain, whereas glutamate uptake is Na⁺-dependent and inhibited by ouabain. The uptake of both glutamine and glutamate is unaffected by the gamma-glutamyltransferase inhibitor, Acivicin. This indicates that glutamine uptake is not mediated by a carrier, as distinct from that of glutamate, and also not linked to gamma-glutamyl-transferase. Na⁺ affects the distribution of glutamine-derived glutamate by increasing the synaptosomal content and reducing that of the medium. When glutamate release from synaptosomes preloaded with [¹⁴C]glutamate is measured by superfusion technique in order to prevent reuptake, Na⁺ has been found to inhibit release in a non-depolarizing medium (Ringer buffer with no Ca²⁺) of the [¹⁴C]glutamate as well as of endogenous glutamate. The specific activity of the [¹⁴C]glutamine-derived glutamate in the incubation medium is much higher than that in the synaptosomes, indicating that there exists a readily releasable pool of newly formed glutamate in addition to another pool. The latter glutamate pool is partially reduced by Na⁺.Special Issue Dedicated to Dr. Abel Lajtha.     

Effects of Chronic Restraint Stress and Estradiol Replacement on Glutamate Release and Uptake in the Spinal Cord from Ovariectomized Female Rats

Glutamate is an excitatory neurotransmitter involved in neuronal plasticity and neurotoxicity. Chronic stress produces several physiological changes on the spinal cord, many of them presenting sex-specific differences, which probably involve glutamatergic system alterations. The aim of the present study was to verify possible effects of exposure to chronic restraint stress and 17β-estradiol replacement on [³H]-glutamate release and uptake in spinal cord synaptosomes of ovariectomized (OVX) rats. Female rats were subjected to OVX, and half of the animals received estradiol replacement. Animals were subdivided in controls and chronically stressed. Restraint stress or estradiol had no effect on [³H]-glutamate release. The chronic restraint stress promoted a decrease and 17β-estradiol induced an increase on [³H]-glutamate uptake, but the uptake observed in the restraint stress +17β-estradiol group was similar to control. Furthermore, 17β-estradiol treatment caused a significant increase in the immunocontent of the three glutamate transporters present in spinal cord. Restraint stress had no effect on the expression of these transporters, but prevented the 17β-estradiol effect. We suggest that changes in the glutamatergic system are likely to take part in the mechanisms involved in spinal cord plasticity following repeated stress exposure, and that 17β-estradiol levels may affect chronic stress effects in this structure.