Effects of uptake carrier blockers SK & F 89976-A and L-trans-PDC on in vivo release of amino acids in rat hippocampus

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

The effects of local perfusion of DAMGO on extracellular GABA and glutamate concentrations in the rostral ventromedial medulla

Electrophysiological data suggest an involvement of rostral ventromedial medulla (RVM) GABA and glutamate (GLU) neurons in morphine analgesia. Direct evidence that extracellular concentrations of GABA or GLU are altered in response to mu opioid receptor (MOP-R) activation is, however, lacking. We used in vivo microdialysis to investigate this issue. Basal GABA overflow increased in response to intra-RVM perfusion of KCl (60 mmol/L). Reverse microdialysis of the MOP-R agonist D-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) (20-500 micromol/L) produced a concentration-dependent decrease of RVM GABA overflow. Behavioral testing revealed that concentrations that decreased GABA levels increased thermal withdrawal thresholds. A lower agonist concentration that did not increase GABA failed to alter thermal thresholds. DAMGO did not alter GLU concentrations. However, KCl also failed to modify GLU release. Since rapid, transporter-mediated uptake may mask the detection of changes in GLU release, the selective excitatory amino acid transporter inhibitor pyrrolidine-2,4-dicarboxylic acid (tPDC, 0.6 mmol/L) was added to the perfusion medium for subsequent studies. tPDC increased GLU concentrations, confirming transport inhibition. KCl increased GLU dialysate levels in the presence of tPDC, demonstrating that transport inhibition permits detection of depolarization-evoked GLU overflow. In the presence of tPDC, DAMGO increased GLU overflow in a concentration-dependent manner. These data demonstrate that MOP-R activation decreases GABA and increases GLU release in the RVM. We hypothesize that the opposing effects of MOP-R on GLU and GABA transmission contribute to opiate antinociception.

Dissociation between hippocampal neuronal loss, astroglial and microglial reactivity after pharmacologically induced reverse glutamate transport

The inflammatory central nervous system response that involves activated microglia and reactive astrocytes may both heal and harm neurons, as inflammatory mediators lead to neuroprotection or excitation at one dose but to injury at a different concentration. To investigate these complex cellular interactions, L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a selective substrate inhibitor of glutamate transport, was infused during 14 days in the rat hippocampus at three different rates: 5, 10 and 25 nmol/h. A microglial reaction appeared at the 5 nmol/h PDC rate in absence of astroglial reaction and neuronal loss. Microgliosis and neuronal death were observed at PDC 10 nmol/h in absence of astrogliosis and calcium precipitation, whereas all four aspects were present at the highest rate. This dissociation between neuronal loss and astroglial reactivity took place in presence of a permanent microglial reaction. These data suggest a specific response of microglia to PDC whose neuronal effects may differ with the infused dose.

Characteristics of GABA release in mouse brain stem slices under normal and ischemic conditions

GABA is known to be the inhibitory neurotransmitter in the majority of brain stem nuclei. The release of GABA has been extensively studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release in the brain stem have not been systemically characterized. The properties of preloaded [3H]GABA were now investigated in mouse brain stem slices, using a superfusion system. The basal release was enhanced by K+ stimulation (50 mM K+) and under various cell-damaging conditions (ischemia, hypoglycemia, the presence of free radicals and metabolic poisons). No K+-stimulated release was discernible in the absence of Ca2+, indicating that the release was at least partly Ca2+-dependent. Moreover, the release was increased when Na+ or Cl- was omitted from the superfusion medium. GABA and beta-alanine stimulated the release, confirming the involvement of the reversed function of GABA transporters. Incubation of the slices with the anion channel inhibitors diisothiocyanostilbene and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate and with the Cl- uptake inhibitor 9-anthracenecarboxylic acid also reduced GABA release, demonstrating that a part of it comprises leakage through anion channels. All these mechanisms were involved in the ischemia-induced GABA release, which was over 4-fold greater than the release in normoxia. Contrary to the other brain areas, GABA release in the brain stem was not affected by ionotropic glutamate receptors but may be modulated by metabotropic receptors. This ischemia-induced GABA release might constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Effects of theanine, r-glutamylethylamide, on neurotransmitter release and its relationship with glutamic acid neurotransmission

t Theanine, r-glutamylethylamide, is one of the major amino acid components in green tea and many researchers have compared theanine's effects with glutamic acid because the chemical structure is similar. In the previous study, we demonstrated that theanine can pass brain-blood barrier and may play as an agonist or an antagonist of some receptors. In this study, we investigated the effects of theanine on neurotransmitter release in the rat brain striatum by in vivo brain microdialysis and examined whether theanine affected glutamate transporters by comparing it with a glutamate transporter blocker, L-trans-Pyrrolidine-2,4-dicarboxylic acid (L-trans-2,4-PDC). Because we investigated whether the effects of theanine is similar to L-trans-2,4-PDC on the brain neurotransmission, we measured dopamine release and some amino acids release which are known as excitatory or inhibitory neurotransmitters from neurons by theanine or L-trans-2,4-PDC perfusion into the rat brain striatum. L-trans-2,4-PDC or theanine perfusion into the brain striatum caused dopamine release from dopaminergic neurons. In addition, L-trans-2,4-PDC perfusion increased glutamic acid, aspartic acid and, whereas theanine perfusion prevented aspartic acid release and increased glycine release. These results suggested that the mechanism of dopamine release caused by theanine is different from glutamate transporter blockers or glutamic acid. Further, L-trans-2,4-PDC cause excitatory neurotransmission, whereas theanine may inhibit excitatory neurotransmission and cause inhibitory neurotransmission via glycine receptors.

Differential effects of the substrate inhibitor l-trans-pyrrolidine-2,4-dicarboxylate (PDC) and the non-substrate inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) of glutamate transporters on neuronal damage and extracellular amino acid levels in rat brain in vivo

The extracellular concentration of glutamate is highly regulated by transporter proteins, due to its neurotoxic properties. Dysfunction or reverse activation of these transporters is related to the extracellular accumulation of excitatory amino acids and neuronal damage associated with ischemia and hypoglycemia. We have investigated by microdialysis the effects of the substrate and the non-substrate inhibitors of glutamate transporters, l-trans-2,4-pyrrolidine dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA), respectively, on the extracellular levels of amino acids in the rat hippocampus in vivo. In addition, we have studied the effect of both inhibitors on neuronal damage after direct administration into the hippocampus and striatum. Electroencephalographic activity was recorded after the intrahippocampal infusion of DL-TBOA or PDC. Microdialysis administration of 500 microM DL-TBOA into the hippocampus increased 3.4- and nine-fold the extracellular levels of aspartate and glutamate, respectively. Upon stereotaxic administration it induced neuronal damage dose-dependently in CA1 and dentate gyrus, and convulsive behavior. Electroencephalographic recording showed the appearance of limbic seizures in the hippocampus after DL-TBOA infusion. In the striatum it also induced dose-dependent neuronal damage. These effects were prevented by the i.p. administration of the glutamate receptor antagonists (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-iminemaleate and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline. In contrast to dl-TBOA, PDC (500 microM) induced a more discrete elevation of excitatory amino acids levels (2.6- and three-fold in aspartate and glutamate, respectively), no neuronal damage or behavioral changes, and no alterations in electroencephalographic activity. The differential results obtained with DL-TBOA and PDC might be attributed to their distinct effects on the extracellular concentration of amino acids. Results are relevant to the understanding of the role of glutamate transporters in amino acid removal or release and the induction of excitotoxic cell death.

GABAergic Modulation of the Activity of Globus Pallidus Neurons in Primates: In Vivo Analysis of the Functions of GABA Receptors and GABA Transporters

Neurons in the external and internal segment of the globus pallidus (GPe and GPi, respectively) receive substantial GABAergic inputs from the striatum and through axon collaterals of neighboring pallidal neurons. The effects of GABA on pallidal activity depend on the synaptic localization of GABA receptors and the distribution and activity of GABA transporters (GATs). To explore the contribution of GABA receptors and transporters to pallidal function, we recorded the activity of single neurons in GPe or GPi before, during, and after local microinjections of GABAergic compounds in awake rhesus monkeys. Activation of GABAA or GABAB receptors with muscimol or baclofen, respectively, inhibited pallidal activity. These effects were reversed by concomitant infusion of the respective GABA receptor antagonists, gabazine and CGP-55845. Given alone, the antagonists were without consistent effect. Application of the selective GAT-1 inhibitor, SKF-89976A, and the semiselective GAT-3 blocker, SNAP-5114, decreased pallidal activity. Both GAT inhibitors increased GABA levels in the pallidum, as measured by microdialysis. Electron microscopic observations revealed that these transporters are located on glial processes and unmyelinated axonal segments, but rarely on terminals. Our results indicate that activation of GABAA and GABAB receptors inhibits neuronal activity in both segments of the pallidum. GAT-1 and GAT-3 are involved in the modulation of endogenous GABA levels and may be important in regulating the extrasynaptic levels of GABA. Together with previous evidence that a considerable proportion of pallidal GABA receptors are located outside the synaptic cleft, our experiments strongly support the importance of extrasynaptic GABAergic transmission in the primate pallidum.

Local injection of a glutamate uptake inhibitor into the ventral tegmental area produces sensitization to the behavioural effects of d-amphetamine

Circumstantial evidence suggests that sensitization to the behavioral effects of d-amphetamine is mediated by increased glutamate levels in the ventral tegmental area. To test this directly, the present study examined whether increasing glutamate levels in the ventral tegmental area with a glutamate uptake inhibitor is sufficient, in the absence of d-amphetamine administration, to elicit sensitization to a subsequent d-amphetamine challenge. Rats were treated bilaterally once a day for 2 days with either intra-ventral tegmental area L-trans-pyrollidine-2,4-dicarboxylic acid (50 nmol), saline, L-trans-pyrollidine-2,4-dicarboxylic acid coadministered with the competitive N-methyl-d-aspartate antagonist (+/-)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid; CPP, 0.5 nmol), or CPP alone (0.5 nmol; all 1.0 microl/side). Following a 2 day withdrawal period, all rats were administered systemic d-amphetamine (1 mg/kg, i.p.). Repeated intra-ventral tegmental area injection of L-trans-pyrollidine-2,4-dicarboxylic acid sensitized animals to the behavioral effects of a systemic d-amphetamine challenge, an action which was blocked by co-administration of CPP. The results directly implicate ventral tegmental area glutamate in the process of sensitization to d-amphetamine. Furthermore, they demonstrate that inhibition of glutamate uptake produces the neuroadaptations necessary to induce sensitization, adding support to the contention that d-amphetamine sensitizes by modulating glutamate uptake.

Presenilin-1 and intracellular calcium stores regulate neuronal glutamate uptake

Glutamate uptake by high affinity glutamate transporters is essential for preventing excitotoxicity and maintaining normal synaptic function. We have discovered a novel role for presenilin-1 (PS1) as a regulator of glutamate transport. PS1-deficient neurons showed a decrease in glutamate uptake of approximately 50% compared to wild-type neurons. Gamma-secretase inhibitor treatment mimicked the effects of PS1 deficiency on glutamate uptake. PS1 loss-of-function, accomplished by PS1 deficiency or gamma-secretase inhibitor treatment, caused a corresponding decrease in cell surface expression of the neuronal glutamate transporter, EAAC1. PS1 deficiency is known to reduce intracellular calcium stores. To explore the possibility that PS1 influences glutamate uptake via regulation of intracellular calcium stores, we examined the effects of treating neurons with caffeine, thapsigargin, and SKF-96365. These compounds depleted intracellular calcium stores by distinct means. Nonetheless, each treatment mimicked PS1 loss-of-function by impairing glutamate uptake and reducing EAAC1 expression at the cell surface. Blockade of voltage-gated calcium channels, activation and inhibition of protein kinase C (PKC), and protein kinase A (PKA) all had no effect on glutamate uptake in neurons. Taken together, these findings indicate that PS1 and intracellular calcium stores may play a significant role in regulating glutamate uptake and therefore may be important in limiting glutamate toxicity in the brain.

Characterisation of the actions of group I metabotropic glutamate receptor subtype selective ligands on excitatory amino acid release and sodium-dependent re-uptake in rat cerebrocortical minislices

In this study we have tested the effects of a wide range of metabotropic glutamate receptor ligands on (i) depolarisation-evoked efflux of pre-accumulated d-[3H]aspartic acid (d-[3H]asp) from rapidly superfused rat cerebrocortical minislices, and (ii) Na+-dependent uptake of d-[3H]asp into cerebrocortical tissue. Transient elevations in extracellular K+ produced concentration-dependent increases in d-[3H]asp efflux. A submaximally effective concentration (50 mm) was used in all subsequent experiments. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; EC50 17.8 microm], the group I mGlu-selective agonist (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG; EC50 0.5 microm] and the mGlu5 receptor subtype-selective agonist (RS)-2-chloro-5-hydroxyphenylglycine [(RS)-CHPG; EC50 7.3 microm] all concentration-dependently potentiated high K+-evoked d-[3H]asp efflux in the absence of effects on basal outflow of radiolabel. At concentrations selective for mGlu1 receptors, the antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid [(RS)-AIDA; 10-300 microm]; (+)-2-methyl-4-carboxyphenylglycine [LY367385; 1-100 microm] and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylate ethyl ester [CPCCOEt, 1-30 microm] all failed to inhibit responses to (S)-3,5-DHPG. However, the broad-spectrum mGlu receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; IC50 88.5 microm] together with the recently described mGlu5-selective antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP; IC50 0.6 microm), 6-methyl-2-(phenyl-azo)-3-pyridinol (SIB-1757; IC50 4.4 microm) and (E)-2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893; IC50 3.1 microm), at mGlu5-selective concentrations, all powerfully and concentration-dependently inhibited (S)-3,5-DHPG-evoked responses. Two selective excitatory amino acid (EAA) uptake inhibitors, l-trans-2,4-pyrrolidine dicarboxylate (l-trans-2,4-PDC; IC50 229 microm) and dl-threo-beta-benzyloxyaspartate (dl-TBOA; IC50 665 microm) both inhibited the Na+-dependent uptake of d-[3H]asp into cerebrocortical minislices. Importantly, none of the mGlu ligands utilized in the present study significantly inhibited d-[3H]asp uptake at concentrations shown to potentiate K+-evoked efflux. These data demonstrate for the first time that mGlu5 ligands modulate extracellular EAA concentrations by a direct effect on mGlu5-type autoreceptors on EAA nerve terminals as they evoke clear changes in EAA release in the absence of any effects on EAA uptake. Selective mGlu5 receptor antagonists that show high potency and good central bioavailability may provide novel classes of neuroprotective agents for the treatment of brain disorders associated with abnormal EAAergic neurotransmission.

Light-induced changes in glutamate release from isolated rat retina is regulated by cyclic guanosine monophosphate

Isolated rat retina was preloaded with [(14)C]glutamate and subsequently superfused to follow release of glutamate (Glu). After 20 min of superfusion in the dark, exposure of the [(14)C]Glu preloaded rat retina to a single train of white light pulses reduced Glu efflux significantly in the absence as well as in the presence of low (4 microM) and high (0.5 mM) concentrations of the Glu uptake inhibitor trans-L-pyrrolidine-2,4-dicarboxylate (t-PDC). The dark-light response was the highest in the presence of 4 microM t-PDC by establishing a plateau at 75% +/- 7% of the tonic Glu release in the dark (100%). Displaying transient to saturating responses with increasing relative luminance, time series of four trains of white light pulses arrived at a plateau of 85% +/- 10%. The cyclic guanosine monophosphate (cGMP) phosphodiesterase inhibitor Zaprinast (200 microM) antagonized the effect of the light series, leading to a plateau of 115% +/- 9%. Exposure of the retina to the guanylyl cyclase inhibitor LY83583 (30 and 100 microM) showed fast, transient responses characterized by peaks at 90% +/- 1% and 80% +/- 3%, respectively.

A kainate receptor increases the efficacy of GABAergic synapses

Brain functions are based on the dynamic interaction of excitatory and inhibitory inputs. Spillover of glutamate from excitatory synapses may diffuse to and modulate nearby inhibitory synapses. By recording unitary inhibitory postsynaptic currents (uIPSCs) from cell pairs in CA1 of the hippocampus, we demonstrated that low concentrations of Kainate receptor (KAR) agonists increased the success rate (P(s)) of uIPSCs, whereas high concentrations of KAR agonists depressed GABAergic synapses. Ambient glutamate released by basal activities or stimulation of the stratum radiatum increases the efficacy of GABAergic synapses by activating presynaptic KARs, which facilitate Ca(2+)-dependent GABA release. The results suggest that glutamate released from excitatory synapses may also function as an intermediary between excitatory and inhibitory synapses to protect overexcitation of local circuits.

Effects of vigabatrin on brain GABA+/CR signals in patients with epilepsy monitored by 1H-NMR-spectroscopy: responder characteristics

PURPOSE

Vigabatrin (VGB) is a new antiepileptic drug that increases the human brain gamma-aminobutyric acid (GABA) level by irreversibly inhibiting GABA transaminase. Although some patients respond to VGB with a significant seizure reduction, others do not. The aim of this study was to identify possible responders before or in an early phase of VGB treatment by measuring the GABA and homocarnosine contaminated with macromolecules/creatine and phosphocreatine ratio (GABA+/Cr) signal by means of proton-nuclear magnetic resonance (1H NMR) spectroscopy.

METHODS

Measurements were performed immediately before and after a titration period of 1 month (2 g/day during the past 2 weeks). A third measurement followed a maintenance period of 3 months (2 or 3 g/day). In 14 patients with drug-resistant temporal lobe epilepsy and 3 patients with occipital lobe epilepsy, GABA+/Cr was measured in the ipsilateral (i.e., epileptogenic) hemisphere and contralateral (i.e., nonepileptogenic) hemisphere in a volume of 8 cm3.

RESULTS

Depending on the therapeutic efficacy of VGB, we defined three groups: (a) full responders (n = 7), (b) nonresponders (n = 7), and (c) partial responders (n = 3). The nonresponders had no significant change in the GABA+/Cr signal during the treatment compared with baseline. The full responders had a significant increase of the GABA+/Cr signal during the whole treatment phase and a lower ipsilateral level at baseline. The partial responders had also a lowered ipsilateral GABA+/Cr signal at baseline and an increase during treatment but a decrease when the seizures started again.

CONCLUSIONS

Responders to VGB could be identified by a lower ipsilateral baseline GABA+/Cr signal and a steeper increase during VGB treatment. However, it was not possible to predict the duration of the response (full versus partial responder) with these criteria.

Oxygen-independent real-time monitoring of distinct biphasic glutamate release using dialysis electrode in rat striatum during anoxia: in vivo evaluation of glutamate release and reversed uptake

Using a dialysis electrode, previous studies showed a clear biphasic release of glutamate during anoxia and ischemia. In this study, we examined two hypotheses: (1) glutamate is of vesicular origin and its release is thus Ca2+- and ATP-dependent in the first phase, while in the second phase glutamate is derived primarily from the metabolic pool, and (2) reversed glutamate uptake, due to electrogenic stoichiometry, produces the second phase during anoxic insult in the rat brain. A dialysis electrode continuously perfused with glutamate oxidase and ferrocene-conjugated bovine serum albumin (BSA) optimized the time resolution of monitoring, allowing quantitative oxygen-independent, real-time measurement of the extracellular glutamate concentration ([Glu]e) during anoxia. [Glu]e dynamics were analyzed during anoxia by combining the dialysis electrode with focal microinjection of substances inducing glutamate release. Following anoxia in the rat brain, a sharp and rapid [Glu]e elevation took place (first phase). The [Glu]e elevation then shifted, continuing a gently sloping rise throughout the anoxic period (second phase). This first phase disappeared with intracranial administration of either Co2+ or omega-conotoxin. The second phase rise increased with focal microinjection of KCl (300 mM, 1 microL) and decreased with NaCl (300 mM, 1 microL), ultimately reaching a plateau in both cases. Preloading with a novel glutamate transporter inhibitor (tPDC) decreased both the first and second phases of [Glu]e elevation. This dialysis electrode system provides data supporting in vivo evidence that the peak of the first phase of [Glu]e elevation is derived from the "neurotransmitter pool," while the second phase is derived from the neuronal and glial "metabolic pool," which is, at least, partly related to a "reversed uptake" mechanism in the anoxic rat brain.

Synaptosomes still viable after 25 years of superfusion

Superfused synaptosomes have been utilized in studies of neurotransmitter release during 25 years. This review summarizes the aspects of neurotransmission that have been and could be successfully investigated with this technique. The major aim of the article is to draw attention on the versatility of superfused synaptosomes and to suggest how the system could be exploited in clarifying several aspects of synaptic neurochemistry including neurotransmitter transport, receptor localization, receptor-receptor interactions, functional aspects of multi-sited receptor complexes, receptor heterogeneity and mechanisms of neurotransmitter exocytosis-endocytosis.

Compensatory change in EAAC1 glutamate transporter in rat hippocampus CA1 region during kindling epileptogenesis

Functional and molecular changes in glutamate transporters during kindling epileptogenesis were investigated in hippocampus CA1-region of rats. In control animals total glutamate transporter activity was indicated by the stimulatory effect of the high-affinity transporter blocker L-trans-pyrrolidine-2,4-dicarboxylate on extracellular glutamate and aspartate concentrations, as measured by in vivo microdialysis. This blocker-induced elevation was absent already early during epileptogenesis. CA1 levels of the glutamate transporter subtypes GLAST and GLT-1, analyzed by quantitative immunoblotting, did not change during kindling epileptogenesis. However, the 60% decrease in EAAC-1 level observed in age-matched controls was fully compensated for in kindled animals 4-5 weeks after the last generalized seizure. These results indicate a compensatory change of the neuronal EAAC-1 glutamate transporter in CA1 region during kindling epileptogenesis, which may be the consequence of a decrease in total transporter activity.

Lamotrigine and carbamazepine affect differently the release of D-[3H]aspartate from mouse cerebral cortex slices: involvement of NO

The effects of lamotrigine and carbamazepine on the release of preloaded D-[3H]aspartate and the involvement of nitric oxide were studied with mouse cerebral cortical slices in a superfusion system. Lamotrigine inhibited the veratridine-evoked release, whereas the K+-stimulated release was attenuated more strongly by carbamazepine than by lamotrigine. These effects were accentuated by the N-methyl-D-aspartate receptor antagonist L-2-amino-5-phosphonovalerate and the nitric oxide synthase inhibitor L-nitroarginine, but diminished by the nitric oxide donor sodium nitroprusside. The results show that in addition to the blockade of voltage-sensitive Na+ (and Ca2+) channels, NO-mediated mechanisms are probably involved in the anticonvulsant actions of carbamazepine and, in particular, those of lamotrigine.

Changes in the extracellular profiles of neuroactive amino acids in the rat striatum at the asymptomatic stage of hepatic failure

Rats were treated with a hepatotoxin thioacetamide (TAA) and examined 21 days later, when they showed moderate fatty metamorphosis of the liver and morphological changes in brain indicative of excitotoxic neuronal damage, but no evident biochemical or neurophysiological symptoms of hepatic encephalopathy (HE). High-performance liquid chromatography (HPLC) analysis of extracellular amino acids in striatal microdialysates of TAA-treated rats revealed a significant increase in the excitatory amino acids glutamate (Glu) and aspartate (Asp) and their amino acid metabolites glutamine (Gln) and alanine (Ala). Microdialysis in the presence of 50 mM K+ triggered in TAA-treated rats an accumulation of Asp and Glu, and diminished the accumulation of Gln. These effects were virtually absent in control rats. None of the treatments affected the accumulation of the nontransmitter amino acid leucine (Leu). The above changes mirror those previously described in symptomatic HE and are likely to contribute to excitotoxic damage. The basal microdialysate content of taurine (Tau), an amino acid with antioxidant and volume regulatory properties, was 60% lower in TAA-treated rats than in control rats despite its increased blood-to-brain transport. The decrease in extracellular Tau may thus reflect Tau redistribution to adjacent central nervous system (CNS) cells manifesting a cell-protective response. Stimulation with 50 mM K+ increased extracellular Tau in control rats by 182% and in TAA-treated rats by 322%. Stimulation with 100 microM N-methyl-D-aspartate (NMDA) increased extracellular Tau in control rats by 27 % and in TAA-treated rats by as much as 250%. The increase of K+- or NMDA-dependent Tau release may reflect improved cell volume regulation and neuroprotection and contribute to attenuation of neurologic symptoms in rats with liver failure.

Regional differences in the effects of glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid on extracellular amino acids and dopamine in rat brain: an in vivo microdialysis study

The effects of the glutamate transporter inhibitor, trans-PDC, on extracellular amino acids, were investigated in the frontal cortex, striatum, hippocampus and cerebellum of rats using in vivo microdialysis. Trans-PDC infusion (0.1, 1, 10 mM) dose-dependently increased Glu and Asp levels, and these increases in the cerebellum were smaller than those in other brain regions. A small but significant dose-dependent increase was observed for Gly and Tau. However, high extracellular Glu induced by trans-PDC was not sufficient to increase extracellular DA in the striatum and frontal cortex.

Coupling on-line brain microdialysis, precolumn derivatization and capillary electrophoresis for routine minute sampling of O-phosphoethanolamine and excitatory amino acids

In previous papers, we described the analysis of excitatory amino acids (EAAs) and catecholamines in microdialysis samples using capillary electrophoresis with laser-induced fluorescence detection (CE-LIFD). In the present paper, we report that an automated analysis of such samples can be easily achieved by on-line coupling of the microdialysis probe with a continuous flow derivatization system and a commercially available CE-LIFD apparatus. Because of the short analysis time (less than 2 min) and high separation efficiency (100-200,000 theoretical plates), high temporal resolution of microdialysis (minute range) is preserved as compared to off-line systems, while both EAAs and O-phosphoethanolamine (PEA) can be simultaneously detected. This new method has been applied to the measurement of these compounds in microdialysis samples from hippocampal slice cultures and striatum of anesthetized rats. Extracellular concentrations of EAAs, but not PEA, increased during perfusion of a solution containing high K+ or a glutamate uptake inhibitor. However, after in vitro ischemia on hippocampal slices, both EAAs and PEA concentrations increased, but with different temporal patterns.

A presynaptic N-methyl-D-aspartate autoreceptor in rat hippocampus modulating amino acid release from a cytoplasmic pool

A possible role of the N-methyl-D-aspartate receptor (NMDA-R) as a presynaptic autoreceptor was investigated using Percoll-purified hippocampus nerve terminals (synaptosomes). This preparation contained only a neglectable amount of postsynaptic structures. Two main effects of NMDA were observed. First, NMDA dose-dependently (10-100 microM) and in the absence of Mg2+, stimulated basal release of aspartate and glutamate, but not of GABA. MK801 (10 microM), an open NMDA-R-channel blocker, reduced this effect even below control levels, indicating endogenous NMDA-R activation. By superfusing synaptosomes, which prevents a tonic receptor occupation, also basal GABA release was stimulated by NMDA. The NMDA-induced potentiation of amino acid superfusate levels was blocked both by MK801 and Mg2+ (1 mM), was slow in onset and returned to baseline after NMDA-removal. The NMDA-effect was also found in the absence of extracellular Ca2+, suggesting that amino acids were released from a non-vesicular (cytoplasmic) pool. Secondly, in KCl-depolarized synaptosomes exposed to 1 mM Mg2+, NMDA did not affect the release of the amino acids. MK801, however, reduced the KCl-evoked Ca2+-independent release of aspartate and glutamate, but not of GABA. L-trans-PDC, the selective inhibitor of the glutamate/aspartate transporter, prevented this MK801-effect, suggesting a coupling between NMDA-Rs and these transporters. These data provide evidence for a presynaptic NMDA autoreceptor in rat hippocampus. We speculate on the role of this NMDA-R to depolarize the presynaptic membrane by Na+-entry, which may induce reversal of amino acid transporters and thereby releasing amino acids from a cytoplasmic pool.

Microdialysis monitoring of extracellular glutamate combined with the simultaneous recording of evoked field potentials in hippocampal organotypic slice cultures

These experiments combined extracellular electrophysiological multirecordings from hippocampal organotypic slice cultures with application of drugs to and sampling of extracellular fluid from a restricted region of the slice using a microdialysis probe. Glutamate (Glu) concentrations were monitored in 0.5 or 2 min microdialysis samples, while evoked field potentials responses (EvFPR) in the CA1 region of the hippocampus (stimulation in the CA3 area) were simultaneously recorded using a multi-electrodes array (Physiocard). Glu was assayed by capillary electrophoresis with laser-induced fluorescence detection combined with a continuous flow derivatization of dialysates. The performance of this combined approach was demonstrated by monitoring extracellular Glu concentrations and EvFPR after K+ induced depolarisation, Glu uptake blockade by trans-pyrrolidine-2,4-dicarboxylic acid (PDC), and electrical stimulation. Such an approach allows a global monitoring of the neuronal functioning with a fine time resolution (up to 30 s) on a simple in vitro brain slice model, to be used as a complement to conventional in vivo microdialysis studies.

Expression cloning of GABA(B) receptors uncovers similarity to metabotropic glutamate receptors

GABA (gamma-amino-butyric acid), the principal inhibitory neurotransmitter in the brain, signals through ionotropic (GABA(A)/ GABA(c)) and metabotropic (GABA(B)) receptor systems. Here we report the cloning of GABA(B) receptors. Photoaffinity labelling experiments suggest that the cloned receptors correspond to two highly conserved GABA(B) receptor forms present in the vertebrate nervous system. The cloned receptors negatively couple to adenylyl cyclase and show sequence similarity to the metabotropic receptors for the excitatory neurotransmitter L-glutamate.