Release of glutamate, aspartate, and gamma-aminobutyric acid from isolated nerve terminals

Enhanced Cortical Metabolic Activity in Females and Males of a Slow Progressing Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder with selective degeneration of motor neurons in the central nervous system. The pathophysiology of ALS is not well understood. We have used ¹H-[¹³C]-NMR spectroscopy together with an administration of [1,6-¹³C₂]glucose and [2-¹³C]acetate in female and male SOD1^G37R mice to assess neuronal and astroglial metabolic activity, respectively, in the central nervous system in ALS condition. The female (p = 0.0008) and male (p < 0.0001) SOD1^G37R mice exhibited decreased forelimb strength when compared with wild-type mice. There was a reduction in N-acetylaspartylglutamate level, and elevation in myo-inositol in the spinal cord of female and male SOD1^G37R mice. The transgenic male mice exhibited increased acetate oxidation in the spinal cord (p = 0.05) and cerebral cortex (p = 0.03), while females showed an increase in the spinal cord (p = 0.02) only. As acetate is transported and preferentially metabolized in the astrocytes, the finding of increased rate of acetate oxidation in the transgenic mice is suggestive of astrocytic involvement in the pathogenesis of ALS. The rates of glucose oxidation in glutamatergic (p = 0.0004) and GABAergic neurons (p = 0.0052) were increased in the cerebral cortex of male SOD1^G37R mice when compared with the controls. The female mice showed an increase in glutamatergic (p = 0.039) neurometabolic activity only. The neurometabolic activity was unperturbed in the spinal cord of either sex. These data suggest differential changes in neurometabolic activity across the central nervous system in SOD1^G37R mice.

Associations between polygenic risk scores and amplitude of low-frequency fluctuation of inferior frontal gyrus in schizophrenia

Schizophrenia (SCZ) is a serious and complex mental disorder with high heritability. Polygenic risk score (PRS) is a useful tool calculating the accumulating effects of multiple common genetic variants of schizophrenia. The amplitude of low-frequency fluctuation (ALFF) is an efficient index to reflect spontaneous, intrinsic neuronal activity. Aberrant ALFF of brain regions were reported in schizophrenia frequently, but the relationship between PRS and ALFF has not been studied. In the present study, we compared PRS and ALFF in 101 schizophrenia patients and 106 age-matched healthy controls to test their associations with schizophrenia. Then, the correlation of PRS with ALFF was measured to reveal the effect of polygenic risk on brain activity in schizophrenia. We found that schizophrenia patients showed significant differences in PRS and ALFF compared with controls. Twenty-six brain regions showed significant difference of ALFF between schizophrenia cases and controls, of which left inferior frontal gyrus, triangular part (IFGtriang.L) showed increased activity in schizophrenia. PRS-SCZ was positively correlated with ALFF in IFGtriang.L in 57 non-chronic patients. Genes involved in synaptic organization and transmission, especially in glutamatergic synapse, were highly enriched in PRS-SCZ genes, suggesting the dysfunction of synapses in schizophrenia. These results help to understand the molecular mechanism underlying schizophrenia and related brain dysfunction.

Chlorogenic Acid Decreases Glutamate Release from Rat Cortical Nerve Terminals by P/Q-Type Ca2+ Channel Suppression: A Possible Neuroprotective Mechanism

The glutamatergic neurotransmitter system has received substantial attention in research on the pathophysiology and treatment of neurological disorders. The study investigated the effect of the polyphenolic compound chlorogenic acid (CGA) on glutamate release in rat cerebrocortical nerve terminals (synaptosomes). CGA inhibited 4-aminopyridine (4-AP)-induced glutamate release from synaptosomes. This inhibition was prevented in the absence of extracellular Ca²⁺ and was associated with the inhibition of 4-AP-induced elevation of Ca²⁺ but was not attributed to changes in synaptosomal membrane potential. In line with evidence observed through molecular docking, CGA did not inhibit glutamate release in the presence of P/Q-type Ca²⁺ channel inhibitors; therefore, CGA-induced inhibition of glutamate release may be mediated by P/Q-type Ca²⁺ channels. CGA-induced inhibition of glutamate release was also diminished by the calmodulin and Ca²⁺/calmodilin-dependent kinase II (CaMKII) inhibitors, and CGA reduced the phosphorylation of CaMKII and its substrate, synapsin I. Furthermore, pretreatment with intraperitoneal CGA injection attenuated the glutamate increment and neuronal damage in the rat cortex that were induced by kainic acid administration. These results indicate that CGA inhibits glutamate release from cortical synaptosomes by suppressing P/Q-type Ca²⁺ channels and CaMKII/synapsin I pathways, thereby preventing excitotoxic damage to cortical neurons.

Complex I Controls Mitochondrial and Plasma Membrane Potentials in Nerve Terminals

Reductions in the activities of mitochondrial electron transport chain (ETC) enzymes have been implicated in the pathogenesis of numerous chronic neurodegenerative disorders. Maintenance of the mitochondrial membrane potential (Δψm) is a primary function of these enzyme complexes, and is essential for ATP production and neuronal survival. We examined the effects of inhibition of mitochondrial ETC complexes I, II/III, III and IV activities by titrations of respective inhibitors on Δψm in synaptosomal mitochondria. Small perturbations in the activity of complex I, brought about by low concentrations of rotenone (1-50 nM), caused depolarisation of Δψm. Small decreases in complex I activity caused an immediate and partial Δψm depolarisation, whereas inhibition of complex II/III activity by more than 70% with antimycin A was required to affect Δψm. A similarly high threshold of inhibition was found when complex III was inhibited with myxothiazol, and inhibition of complex IV by more than 90% with KCN was required. The plasma membrane potential (Δψp) had a complex I inhibition threshold of 40% whereas complex III and IV had to be inhibited by more than 90% before changes in Δψp were registered. These data indicate that in synaptosomes, both Δψm and Δψp are more susceptible to reductions in complex I activity than reductions in the other ETC complexes. These findings may be of relevance to the mechanism of neuronal cell death in Parkinson's disease in particular, where such reductions in complex I activity are present.

Advances in Imaging Brain Metabolism

Metabolism is central to neuroimaging because it can reveal pathways by which neuronal and glial cells use nutrients to fuel their growth and function. We focus on advanced magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) methods used in brain metabolic studies. ¹⁷O-MRS and ³¹P-MRS, respectively, provide rates of oxygen use and ATP synthesis inside mitochondria, whereas ¹⁹F-MRS enables measurement of cytosolic glucose metabolism. Calibrated functional MRI (fMRI), an advanced form of fMRI that uses contrast generated by deoxyhemoglobin, provides maps of oxygen use that track neuronal firing across brain regions. ¹³C-MRS is the only noninvasive method of measuring both glutamatergic neurotransmission and cell-specific energetics with signaling and nonsignaling purposes. Novel MRI contrasts, arising from endogenous diamagnetic agents and exogenous paramagnetic agents, permit pH imaging of glioma. Overall, these magnetic resonance methods for imaging brain metabolism demonstrate translational potential to better understand brain disorders and guide diagnosis and treatment.

Isolation of Synaptosomes, Synaptic Plasma Membranes, and Synaptic Junctional Complexes

Isolation of synaptic nerve terminals or synaptosomes provides an opportunity to study the process of neurotransmission at many levels and with a variety of approaches. For example, structural features of the synaptic terminals and the organelles within them, such as synaptic vesicles and mitochondria, have been elucidated with electron microscopy. The postsynaptic membranes are joined to the presynaptic "active zone" of transmitter release through cell adhesion molecules and remain attached throughout the isolation of synaptosomes. These "post synaptic densities" or "PSDs" contain the receptors for the transmitters released from the nerve terminals and can easily be seen with electron microscopy. Biochemical and cell biological studies with synaptosomes have revealed which proteins and lipids are most actively involved in synaptic release of neurotransmitters. The functional properties of the nerve terminals, such as responses to depolarization and the uptake or release of signaling molecules, have also been characterized through the use of fluorescent dyes, tagged transmitters, and transporter substrates. In addition, isolated synaptosomes can serve as the starting material for the isolation of relatively pure synaptic plasma membranes (SPMs) that are devoid of organelles from the internal environment of the nerve terminal, such as mitochondria and synaptic vesicles. The isolated SPMs can reseal and form vesicular structures in which transport of ions such as sodium and calcium, as well as solutes such as neurotransmitters can be studied. The PSDs also remain associated with the presynaptic membranes during isolation of SPM fractions, making it possible to isolate the synaptic junctional complexes (SJCs) devoid of the rest of the plasma membranes of the nerve terminals and postsynaptic membrane components. Isolated SJCs can be used to identify the proteins that constitute this highly specialized region of neurons. In this chapter, we describe the steps involved in isolating synaptosomes, SPMs, and SJCs from brain so that these preparations can be used with new technological advances to address many as yet unanswered questions about the synapse and its remarkable activities in neuronal cell communication.

Human presynaptic receptors

Presynaptic receptors are sites at which transmitters, locally formed mediators or hormones inhibit or facilitate the release of a given transmitter from its axon terminals. The interest in the identification of presynaptic receptors has faded in recent years and it may therefore be justified to give an overview of their occurrence in the autonomic and central nervous system; this review will focus on presynaptic receptors in human tissues. Autoreceptors are presynaptic receptors at which a given transmitter restrains its further release, though in some instances may also increase its release. Inhibitory autoreceptors represent a typical example of a negative feedback; they are tonically activated by the respective endogenous transmitter and/or are constitutively active. Autoreceptors also play a role under pathophysiological conditions, e.g. by limiting the massive noradrenaline release occurring during congestive heart failure. They can be used for therapeutic purposes; e.g., the α₂-adrenoceptor antagonist mirtazapine is used as an antidepressant and the inverse histamine H₃ receptor agonist pitolisant has been marketed as a new drug for the treatment of narcolepsy in 2016. Heteroreceptors are presynaptic receptors at which transmitters from adjacent neurons, locally formed mediators (e.g. endocannabinoids) or hormones (e.g. adrenaline) can inhibit or facilitate transmitter release; they may be subject to an endogenous tone. The constipating effect of the sympathetic nervous system or of the antihypertensive drug clonidine is related to the activation of inhibitory α₂-adrenoceptors on postganglionic parasympathetic neurons. Part of the stimulating effect of adrenaline on the sympathetic nervous system during stress is related to its facilitatory effect on noradrenaline release via β₂-adrenoceptors.

Selective cerebral perfusion prevents abnormalities in glutamate cycling and neuronal apoptosis in a model of infant deep hypothermic circulatory arrest and reperfusion

Deep hypothermic circulatory arrest is often required for the repair of complex congenital cardiac defects in infants. However, deep hypothermic circulatory arrest induces neuroapoptosis associated with later development of neurocognitive abnormalities. Selective cerebral perfusion theoretically provides superior neural protection possibly through modifications in cerebral substrate oxidation and closely integrated glutamate cycling. We tested the hypothesis that selective cerebral perfusion modulates glucose utilization, and ameliorates abnormalities in glutamate flux, which occur in association with neuroapoptosis during deep hypothermic circulatory arrest. Eighteen infant male Yorkshire piglets were assigned randomly to two groups of seven (deep hypothermic circulatory arrest or deep hypothermic circulatory arrest with selective cerebral perfusion for 60 minutes at 18℃) and four control pigs without cardiopulmonary bypass support. Carbon-13-labeled glucose as a metabolic tracer was infused, and gas chromatography-mass spectrometry and nuclear magnetic resonance were used for metabolic analysis in the frontal cortex. Following 2.5 h of cerebral reperfusion, we observed similar cerebral adenosine triphosphate levels, absolute levels of lactate and citric acid cycle intermediates, and carbon-13 enrichment among three groups. However, deep hypothermic circulatory arrest induced significant abnormalities in glutamate cycling resulting in reduced glutamate/glutamine and elevated γ-aminobutyric acid/glutamate along with neuroapoptosis, which were all prevented by selective cerebral perfusion. The data suggest that selective cerebral perfusion prevents these modifications in glutamate/glutamine/γ-aminobutyric acid cycling and protects the cerebral cortex from apoptosis.

Apigenin, a natural flavonoid, inhibits glutamate release in the rat hippocampus

The purpose of this study was to examine the effect and mechanism of apigenin, a natural flavonoid, on glutamate release in the rat hippocampus. In rat hippocampal nerve terminals (synaptosomes), apigenin inhibited glutamate release and the elevation of the cytosolic free Ca(2+) concentration evoked by 4-aminopyridine, whereas it had no effect on 4-aminopyridine-mediated depolarization and Na(+) influx. The apigenin-mediated inhibition of evoked glutamate release was prevented by chelating the extracellular Ca(2+) ions and blocking Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel activity. Furthermore, we determined that gamma-aminobutyric acid type A (GABAA) receptors are present in the hippocampal nerve terminals because they are colocalized with the presynaptic marker synaptophysin. However, the effect of apigenin on 4-aminopyridine-evoked glutamate release from synaptosomes was unaffected by the GABAA receptor antagonists SR95531 and bicuculline. Furthermore, in slice preparations, whole-cell patch-clamp experiments showed that apigenin reduced the frequency of spontaneous excitatory postsynaptic currents without affecting their amplitude, suggesting a presynaptic mechanism. On the basis of these results, we suggested that apigenin exerts its presynaptic inhibition probably by reducing Ca(2+) entry mediated by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels, thereby inhibiting glutamate release from the rat hippocampal nerve terminals.

Acacetin inhibits glutamate release and prevents kainic acid-induced neurotoxicity in rats

An excessive release of glutamate is considered to be a molecular mechanism associated with several neurological diseases that causes neuronal damage. Therefore, searching for compounds that reduce glutamate neurotoxicity is necessary. In this study, the possibility that the natural flavone acacetin derived from the traditional Chinese medicine Clerodendrum inerme (L.) Gaertn is a neuroprotective agent was investigated. The effect of acacetin on endogenous glutamate release in rat hippocampal nerve terminals (synaptosomes) was also investigated. The results indicated that acacetin inhibited depolarization-evoked glutamate release and cytosolic free Ca²⁺ concentration ([Ca²⁺]C) in the hippocampal nerve terminals. However, acacetin did not alter synaptosomal membrane potential. Furthermore, the inhibitory effect of acacetin on evoked glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker known as ω-conotoxin MVIIC. In a kainic acid (KA) rat model, an animal model used for excitotoxic neurodegeneration experiments, acacetin (10 or 50 mg/kg) was administrated intraperitoneally to the rats 30 min before the KA (15 mg/kg) intraperitoneal injection, and subsequently induced the attenuation of KA-induced neuronal cell death and microglia activation in the CA3 region of the hippocampus. The present study demonstrates that the natural compound, acacetin, inhibits glutamate release from hippocampal synaptosomes by attenuating voltage-dependent Ca²⁺ entry and effectively prevents KA-induced in vivo excitotoxicity. Collectively, these data suggest that acacetin has the therapeutic potential for treating neurological diseases associated with excitotoxicity.

Glutamatergic function in the resting awake human brain is supported by uniformly high oxidative energy

Rodent (13)C magnetic resonance spectroscopy studies show that glutamatergic signaling requires high oxidative energy in the awake resting state and allowed calibration of functional magnetic resonance imaging (fMRI) signal in terms of energy relative to the resting energy. Here, we derived energy used for glutamatergic signaling in the awake resting human. We analyzed human data of electroencephalography (EEG), positron emission tomography (PET) maps of oxygen (CMR(O2)) and glucose (CMR(glc)) utilization, and calibrated fMRI from a variety of experimental conditions. CMR(glc) and EEG in the visual cortex were tightly coupled over several conditions, showing that the oxidative demand for signaling was four times greater than the demand for nonsignaling events in the awake state. Variations of CMR(O2) and CMR(glc) from gray-matter regions and networks were within ±10% of means, suggesting that most areas required similar energy for ubiquitously high resting activity. Human calibrated fMRI results suggest that changes of fMRI signal in cognitive studies contribute at most ±10% CMR(O2) changes from rest. The PET data of sleep, vegetative state, and anesthesia show metabolic reductions from rest, uniformly >20% across, indicating no region is selectively reduced when consciousness is lost. Future clinical investigations will benefit from using quantitative metabolic measures.

Recruitment of GABA(A) receptors and fearfulness in chicks: modulation by systemic insulin and/or epinephrine

One-day-old chicks were individually assessed on their latency to peck pebbles, and categorized as low latency (LL) or high latency (HL) according to fear. Interactions between acute stress and systemic insulin and epinephrine on GABA(A) receptor density in the forebrain were studied. At 10 days of life, LL and HL chicks were intraperitoneally injected with insulin, epinephrine or saline, and immediately after stressed by partial water immersion for 15 min and killed by decapitation. Forebrains were dissected and the GABA(A) receptor density was measured ex vivo by the (3)[H]-flunitrazepam binding assay in synaptosomes. In non-stressed chicks, insulin (non-hypoglycemic dose) at 2.50 IU/kg of body weight incremented the Bmax by 40.53% in the HL chicks compared to saline group whereas no significant differences were observed between individuals in the LL subpopulation. Additionally, insulin increased the Bmax (23.48%) in the HL group with respect to the LL ones, indicating that the insulin responses were different according to the anxiety of each category. Epinephrine administration (0.25 and 0.50mg/kg) incremented the Bmax in non-stressed chicks, in the LL group by about 37% and 33%, respectively, compared to ones injected with saline. In the stressed chicks, 0.25mg/kg bw epinephrine increased the Bmax significantly in the HL group by about 24% compared to saline, suggesting that the effect of epinephrine was only observed in the HL group under acute stress conditions. Similarly, the same epinephrine doses co-administered with insulin increased the receptor density in both subpopulations and also showed that the highest dose of epinephrine did not further increase the maximum density of GABA(A)R in HL chicks. These results suggest that systemic epinephrine, perhaps by evoking central norepinephrine release, modulated the increase in the forebrain GABA(A) receptor recruitment induced by both insulin and stress in different ways depending on the subpopulation fearfulness.

Glutamate and GABA synthesis, release, transport and metabolism as targets for seizure control

The synthesis, release, reuptake, and metabolism of the excitatory and inhibitory neurotransmitters glutamate and GABA, respectively, are tightly controlled. Given the role that these two neurotransmitters play in normal and abnormal neurotransmission, it is important to consider the processes whereby they are regulated. This brief review is focused entirely on the metabolic aspects of glutamate and GABA synthesis and neurotransmission. It describes in limited detail the synthesis, release, reuptake, metabolism, cellular compartmentation and pharmacology of the glutamatergic and GABAergic synapse. This review also provides a summary and brief description of the pathologic and phenotypic features of the various genetic animal models that have been developed in an effort to provide a greater understanding of the role that each of the aforementioned metabolic processes plays in controlling excitatory and inhibitory neurotransmission and how their use will hopefully facilitate the development of safer and more efficacious therapies for the treatment of epilepsy and other neurological disorders.

Ligand-specific deactivation time course of GluN1/GluN2D NMDA receptors

N-methyl-D-aspartate (NMDA) receptors belong to the family of ionotropic glutamate receptors that mediate a majority of excitatory synaptic transmission. One unique property of GluN1/GluN2D NMDA receptors is an unusually prolonged deactivation time course following the removal of L-glutamate. Here we show, using x-ray crystallography and electrophysiology, that the deactivation time course of GluN1/GluN2D receptors is influenced by the conformational variability of the ligand-binding domain (LBD) as well as the structure of the activating ligand. L-glutamate and L-CCG-IV induce significantly slower deactivation time courses compared with other agonists. Crystal structures of the isolated GluN2D LBD in complex with various ligands reveal that the binding of L-glutamate induces a unique conformation at the backside of the ligand-binding site in proximity to the region at which the transmembrane domain would be located in the intact receptors. These data suggest that the activity of the GluN1/GluN2D NMDA receptor is controlled distinctively by the endogenous neurotransmitter L-glutamate.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Rose Bengal analogs and vesicular glutamate transporters (VGLUTs)

Vesicular glutamate transporters (VGLUTs) allow the loading of presynaptic glutamate vesicles and thus play a critical role in glutamatergic synaptic transmission. Rose Bengal (RB) is the most potent known VGLUT inhibitor (Ki 25 nM); therefore we designed, synthesized and tested in brain preparations, a series of analogs based on this scaffold. We showed that among the two tautomers of RB, the carboxylic and not the lactonic form is active against VGLUTs and generated a pharmacophore model to determine the minimal structure requirements. We also tested RB specificity in other neurotransmitter uptake systems. RB proved to potently inhibit VMAT (Ki 64 nM) but weakly VACHT (Ki>9.7 microM) and may be a useful tool in glutamate/acetylcholine co-transmission studies.

Aripiprazole and its human metabolite OPC14857 reduce, through a presynaptic mechanism, glutamate release in rat prefrontal cortex: possible relevance to neuroprotective interventions in schizophrenia

Aripiprazole is a novel atypical antipsychotic drug with neuroprotective properties. As excessive glutamate release is now considered to be part of the pathophysiology of schizophrenia, the objective of this study was to use an in vitro assay system to investigate the effect of aripiprazole and its human metabolite OPC14857 on the release of endogenous glutamate from isolated nerve terminals (synaptosomes), freshly prepared from rat prefrontal cortex. Both aripiprazole and OPC13857 potently inhibited 4-aminopyridine (4-AP)-evoked glutamate release in a concentration-dependent manner. Inhibition of glutamate release by aripiprazole and OPC13857 was associated with a reduction of 4AP-evoked Na+ influx and depolarization, as well as downstream elevation of cytoplasmic free calcium concentration mediated via N- and P/Q-type voltage-dependent Ca2+ channels (VDCCs). Release induced by direct Ca2+ entry with Ca2+ ionophore (ionomycin) was unaffected by aripiprazole or OPC13857, indicating that the inhibitory effect of aripiprazole or OPC13857 is not due to directly interfering with the release process at some point subsequent to Ca2+ influx. In addition, the dopamine D2 receptor antagonist haloperidol and the 5-HT 1A receptor antagonist WAY100635 all effectively blocked the aripiprazole or OPC13857-mediated inhibition of 4-AP-evoked glutamate release. Moreover, aripiprazole or OPC13857 modulation of 4-AP-evoked glutamate release appears to involve a protein kinase A (PKA) signaling cascade, insofar as pretreatment of synaptosomes with the PKA inhibitor H89 suppressed the inhibitory effect of aripiprazole or OPC13857. Together, these results suggest that aripiprazole and its human metabolite OPC14857 inhibit glutamate release from rat prefrontocortical nerve terminals, likely by the activation of dopamine D2 and 5-HT 1A receptors, which subsequently results in the reduction of nerve terminal excitability and downstream VDCC activation through a signaling cascade involving PKA. These actions of aripiprazole may contribute to its neuroprotective effect in excitotoxic injury.

Synaptic vesicle-bound pyruvate kinase can support vesicular glutamate uptake

Glucose metabolism is essential for normal brain function and plays a vital role in synaptic transmission. Recent evidence suggests that ATP synthesized locally by glycolysis, particularly via glyceraldehyde 3-phosphate dehydrogenase/3-phosphoglycerate kinase, is critical for synaptic transmission. We present evidence that ATP generated by synaptic vesicle-associated pyruvate kinase is harnessed to transport glutamate into synaptic vesicles. Isolated synaptic vesicles incorporated [(3)H]glutamate in the presence of phosphoenolpyruvate (PEP) and ADP. Pyruvate kinase activators and inhibitors stimulated and reduced PEP/ADP-dependent glutamate uptake, respectively. Membrane potential was also formed in the presence of pyruvate kinase activators. "ATP-trapping" experiments using hexokinase and glucose suggest that ATP produced by vesicle-associated pyruvate kinase is more readily used than exogenously added ATP. Other neurotransmitters such as GABA, dopamine, and serotonin were also taken up into crude synaptic vesicles in a PEP/ADP-dependent manner. The possibility that ATP locally generated by glycolysis supports vesicular accumulation of neurotransmitters is discussed.

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 17beta-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 17beta-estradiol induced an increase on [3H]-glutamate uptake, but the uptake observed in the restraint stress +17beta-estradiol group was similar to control. Furthermore, 17beta-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 17beta-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 17beta-estradiol levels may affect chronic stress effects in this structure.

Partial inhibition of complex I activity increases Ca-independent glutamate release rates from depolarized synaptosomes

Mitochondria have been implicated in the pathogenesis of several neurodegenerative disorders and, in particular, complex I (NADH:ubiquinone oxidoreductase, EC 1.6.5.3) activity has been shown to be partially reduced in postmortem studies of the substantia nigra of Parkinson's disease patients. The present study examines the effect of partial inhibition of complex I activity on glutamate release from rat brain synaptosomes. Following a 40% inhibition of complex I activity with rotenone, it was found that Ca(2+)-independent release of glutamate increased from synaptosomes depolarized with 4-aminopyridine. Highest rates of glutamate release were found to occur between 60-90% complex I inhibition. A similar pattern of increase was shown to occur in synaptosomes depolarized with KCl. The increase in glutamate release was found to correlate to a significant decrease in ATP. Inhibition of complex I activity by 40% was also shown to cause a significant collapse in mitochondrial membrane potential (Deltapsi(m)). These results suggest that partial inhibition of complex I activity in in situ mitochondria is sufficient to significantly increase release of glutamate from the pre-synaptic nerve terminal. The relevance of these results in the context of excitotoxicity and the pathogenesis of neurodegenerative disorders is discussed.

Drug assessment based on detection of L-glutamate released from C6 glioma cells using an enzyme-luminescence method

Monitoring of excitation activity of nerve cells is very useful for not only brain research but also assessment of the effects of various chemicals, including drugs and toxins. We previously reported a novel enzyme-luminescence method for real-time monitoring of l-glutamate release from C6 glioma cells with high levels of sensitivity ( approximately 10 nM) and temporal resolution (<1 s) using a luminescence plate reader. In the present study, we tested the applicability of this novel system for assessment of effects of drugs in vitro. Several drugs (e.g., veratridine and 4-aminopyridine) were administered to C6 glioma cells for inducing glutamate release. Moreover, antagonists of voltage-dependent Ca (2+) channels (e.g., nifedipine, flunarizine, and NiCl 2) and Na (+) channels (e.g., carbamazepine and lidocaine) were applied separately for evaluating the effects of these chemicals on glutamate release from the cells. The combined effect of carbamazepine and lidocaine was also investigated by using our method, and the combined effect was found to be more potent than that of single drug administration. These results indicated that the glutamate release from C6 cells was modulated by these drugs in a way similar to that found by using several conventional analytical techniques. We therefore conclude that the developed monitoring system for real-time detection of dynamic l-glutamate release from cells could be very useful for application to assessment of drugs acting on the nervous system.

Acute stress or systemic insulin injection increases flunitrazepam sensitive-GABAA receptor density in synaptosomes of chick forebrain: Modulation by systemic epinephrine

Interactions between acute stress and systemic insulin and epinephrine on GABAA receptor density in the forebrain were studied. Here, 10 day-old chicks were intraperitoneally injected with insulin, epinephrine or vehicle and then immediately stressed by partial water immersion for 15 min and killed by decapitation. Non-stressed controls were similarly injected, then returned to their rearing boxes for 15 min and then killed. Forebrains were dissected and GABAA receptor density was measured ex vivo in synaptosomes by 3[H]-flunitrazepam binding assay. In non-stressed chicks, insulin at 1.25, 2.50 and 5.00 IU/kg of body weight (non-hypoglycemic doses) increased Bmax by 33, 53 and 44% compared to saline, respectively. A similar increase of 41% was observed in receptor density after stress. However, the insulin effect was not additive to the stress-induced increase suggesting that both effects occur through similar mechanisms. In contrast, epinephrine, at 0.25 and 0.5 mg/kg did not induce any changes in Bmax in non-stressed chicks. Nevertheless, after stress these doses increased the receptor density by about 13 and 27%, respectively. Similarly, the same epinephrine doses co-administered with insulin (2.50 IU/kg), increased the receptor density by about 20% compared to insulin alone. These results suggest that systemic epinephrine, perhaps by evoking central norepinephrine release, modulates the increase in forebrain GABAA receptor binding induced by both insulin and stress.

Ginsenosides Rg1 and Rb1 enhance glutamate release through activation of protein kinase A in rat cerebrocortical nerve terminals (synaptosomes)

We examined the effect of ginsenoside Rg1 or Rb1, the active ingredients of ginseng, on the release of endogenous glutamate from glutamatergic nerve terminals purified from rat cerebral cortex. Result showed that the Ca(2+)-dependent release of glutamate evoked by 4-aminopyridine was facilitated by ginsenoside Rg1 or Rb1 in a concentration-dependent manner. Sequential experiments reveal that ginsenoside Rg1 or Rb1-mediated facilitation of glutamate release (i) results from an enhancement of vesicular exocytosis; (ii) is not due to an alternation of synaptosomal excitability; (iii) is associated with an increase in Ca(2+) influx through presynaptic N- and P/Q-type voltage-dependent Ca(2+) channels; (iv) appears to involve a protein kinase A pathway. These results conclude that ginsenoside Rg1 or Rb1 exerts their presynaptic facilitatory effect, likely through the activation of protein kinase A, which subsequently enhances Ca(2+) entry to cause an increase in evoked glutamate release from rat cortical synaptosomes. This finding might provide important information regarding the action of ginseng in the central nervous system.

Mechanisms of glutamate release elicited in rat cerebrocortical nerve endings by 'pathologically' elevated extraterminal K+ concentrations

Extracellular [K+] can increase during some pathological conditions, resulting into excessive glutamate release through multiple mechanisms. We here investigate the overflow of [3H]D-aspartate ([3H] D-ASP) and of endogenous glutamate elicited by increasing [K+] from purified rat cerebrocortical synaptosomes. Depolarization with [K+] <or= 15 mmol/L provoked [3H] D-ASP and glutamate overflows almost totally dependent on external Ca2+. Consistent with release by exocytosis, the overflow of [3H] D-ASP evoked by 12 mmol/L K+ was sensitive to clostridial toxins. The overflows evoked by 35/50 mmol/L K+ remained external Ca2+-dependent by more than 50%. The Ca2+-independent components of the [3H] D-ASP overflows evoked by [K+] > 15 mmol/L were prevented by the glutamate transporter inhibitors DL-threo-beta-benzyloxyaspartate (DL-TBOA) and dihydrokainate. Differently, the overflows of endogenous glutamate provoked by [K+] > 15 mmol/L were insensitive to both inhibitors; the external Ca2+-independent glutamate overflow caused by 50 mmol/L KCl was prevented by bafilomycin, by chelating intraterminal Ca2+, by blocking the mitochondrial Na+/Ca2+ exchanger and, for a small portion, by blocking anion channels. In contrast to purified synaptosomes, the 50 mmol/L K+-evoked release of endogenous glutamate or [3H]D-ASP was inhibited by DL-TBOA in crude synaptosomes; moreover, it was external Ca2+-insensitive and blocked by DL-TBOA in purified gliosomes, suggesting that carrier-mediated release of endogenous glutamate provoked by excessive [K+] in CNS tissues largely originates from glia.

Evidence that α7 nicotinic receptor modulates glutamate release from mouse neocortical gliosomes

The presence of nicotinic receptors on astrocytes in human and rat brain has been previously demonstrated however their possible functional role is still poorly understood. In this study we investigated on the presence of nicotinic receptors on gliosomes, purified from mouse cortex, and on their role in eliciting glutamate release. Epibatidine significantly increased basal release of [3H]D-aspartate and of endogenous glutamate from mouse gliosomes but not from synaptosomes. This effect was prevented by methyllycaconitine, alpha-bungarotoxin and mecamylamine but not by dihydro-beta-erythroidine. Epibatidine provoked also a significant increase of calcium concentration in gliosomes but not in synaptosomes; the increase in [Ca2+]i induced by epibatidine and KCl in gliosomes was very similar to each other. The present results indicate that alpha7 nicotinic receptors exist on mouse cortical glial particles and stimulate glutamate release.

Unexpected inhibitory regulation of glutamate release from rat cerebrocortical nerve terminals by presynaptic 5-hydroxytryptamine-2A receptors

Presynaptic 5-HT(2A) receptor modulation of glutamate release from rat cerebrocortical nerve terminals (synaptosomes) was investigated by using the 5-HT(2A/2C) receptor agonist (+/-)-1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI). DOI potently inhibited 4-aminopyridine (4AP)-evoked glutamate release. Involvement of presynaptic 5-HT(2A) receptors in this modulation of 4AP-evoked release was confirmed by blockade of the DOI-mediated inhibition by the 5-HT(2A) receptor antagonist ketanserin but not by the 5-HT(2C) receptor antagonist RS102221. Inhibition of glutamate release by DOI was associated with a reduction of 4AP-evoked depolarization and downstream elevation of cytoplasmic free calcium concentration ([Ca(2+)](C)) mediated via P/Q- and N-type voltage-dependent Ca(2+) channels (VDCCs). In contrast to the DOI effect on 4AP-evoked release, the agonist had no effect on high external [K(+)] (30 mM)-induced (KCl) stimulation of VDCCs or glutamate release. Likewise, release mediated by direct Ca(2+) entry with Ca(2+) ionophore (ionomycin) or by hypertonic sucrose was unaffected by DOI. Mechanistically, DOI modulation of 4AP-evoked glutamate release appeared to involve a phospholipase C/protein kinase C signaling cascade, insofar as pretreatment of synaptosomes with the phospholipase C inhibitor U73122 or protein kinase C inhibitors Ro320432 or GF109203X all effectively occluded the inhibitory effect of the agonist. Together, these results suggest that presynaptic 5-HT(2A) receptors present on glutamatergic terminals effect an unexpected depression of glutamate release by negatively modulating nerve terminal excitability and downstream VDCC activation through a signaling cascade involving phospholipase C/protein kinase C. These observations invoke presynaptic inhibitory 5-HT(2A) receptor function as a potential target for drugs to mitigate the effects of excessive glutamatergic transmission.

The Effect of Cyclohexyladenosine on the Penischemic Increases of Hippocampal Glutamate and Glycine in the Rabbit

We investigated the ability of N6-cyclohexyladenosine (CHA), a potent and selective agonist of the adenosine A1 receptor, to attenuate elevations of levels of extracellular hippocampal glutamate and glycine that result from episodes of transient global cerebral ischemia (TGCI). A total of 30 New Zealand white rabbits were randomly assigned to receive 0 (n = 5), 0.1 (n = 8), 1.0 (n = 6), 10 (n = 6), or 100 (n = 5) microM CHA. The drug was dissolved in artificial CSF (vehicle) and administered via a microdialysis probe placed stereotactically into the dorsal hippocampus. A second microdialysis probe placed into the contralateral hippocampus of each animal was perfused with vehicle alone. Ten minutes of TGCI was induced by neck tourniquet inflation and deliberate hypotension from 0 to 10 min. Microdialysis samples were collected as follows: every 20 min preischemia (at -80, -60, -40, -20, and 0 min); every 5 min during ischemia and in the immediate reperfusion period (at 5, 10, 15, and 20 min); and every 20 min for the remainder of the reperfusion period (at 40, 60, and 80 min). Samples were then analyzed for their concentration of glutamate and glycine by HPLC. Following 10 min of ischemia, glutamate levels increased to a peak of 3.28 +/- 0.55 times baseline and returned to preischemic levels by 40 min, i.e., during reperfusion. Glycine concentrations increased to 5.41 +/- 0.91 times over baseline and remained elevated for the duration of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential glutamate dehydrogenase (GDH) activity profile in patients with temporal lobe epilepsy

PURPOSE

Pathophysiologic mechanisms underlying temporal lobe epilepsy (TLE) are still poorly understood. One major hypothesis links alterations in energy metabolism to glutamate excitotoxicity associated with seizures in TLE. The purpose of this study was to determine whether changes in the activities of enzymes critical in energy and neurotransmitter metabolism contributed to the alterations in metabolic status leading to the excitotoxic effects of glutamate.

METHODS

Activities of four key enzymes involved in energy metabolism and glutamate cycling in the brain [aspartate aminotransferase (AAT), citrate synthase (CS), glutamate dehydrogenase (GDH), and lactate dehydrogenase (LDH)] were measured in anterolateral temporal neocortical and hippocampal tissues obtained from three different groups of medically intractable epilepsy patients having either mesial, paradoxical, or mass lesion-associated temporal lobe epilepsy (MTLE, PTLE, MaTLE), respectively.

RESULTS

We found that GDH activity was significantly decreased in the temporal cortex mainly in the MTLE group. A similar trend was recognized in the hippocampus of the MTLE. In all three patient groups, GDH activity was considerably lower, and AAT and LDH activities were higher in cortex of MTLE as compared with the corresponding activities in hippocampus (p<0.05). In the MTLE cortex and hippocampus, GDH activities were negatively correlated with the duration since the first intractable seizure.

CONCLUSIONS

Our results support the hypothesis suggesting major alteration in GDH activity mainly in the MTLE group. It is proposed that significant alterations in the enzyme activities may be contributing to decreased metabolism of glutamate, leading to its accumulation.

Stimulation of excitatory amino acid release from adult mouse brain glia subcellular particles by high mobility group box 1 protein

The multifunctional protein high mobility group box 1 (HMGB1) is expressed in hippocampus and cerebellum of adult mouse brain. Our aim was to determine whether HMGB1 affects glutamatergic transmission by monitoring neurotransmitter release from glial (gliosomes) and neuronal (synaptosomes) re-sealed subcellular particles isolated from cerebellum and hippocampus. HMGB1 induced release of the glutamate analogue [(3)H]d-aspartate form gliosomes in a concentration-dependent manner, whereas nerve terminals were insensitive to the protein. The HMGB1-evoked release of [(3)H]d-aspartate was independent of modifications of cytosolic Ca(2+) , but it was blocked by dl-threo-beta-benzyloxyaspartate (dl-TBOA), an inhibitor of glutamate transporters. HMGB1 also stimulated the release of endogenous glutamate in a Ca(2+)-independent and dl-TBOA-sensitive manner. These findings suggest the involvement of carrier-mediated release. Moreover, dihydrokainic acid, a selective inhibitor of glutamate transporter 1 (GLT1), does not block the effect of HMGB1, indicating a role for the glial glutamate-aspartate transporter (GLAST) subtype in this response. We also demonstrate that HMGB1/glial particles association is promoted by Ca(2+). Furthermore, although HMGB1 can physically interact with GLAST and the receptor for advanced glycation end products (RAGE), only its binding with RAGE is promoted by Ca(2+). These results suggest that the HMGB1 cytokine could act as a modulator of glutamate homeostasis in adult mammal brain.

Single and combined effects of carbamazepine and vinpocetine on depolarization-induced changes in Na+, Ca2+ and glutamate release in hippocampal isolated nerve endings

The single and combined effects of carbamazepine and vinpocetine on the release of the excitatory amino acid neurotransmitter glutamate, on the rise in internal Na+ (Na(i), as determined with SBFI), and on the rise in internal Ca2+ (Ca(i), as determined with fura-2) induced by an increased permeability of presynaptic Na+ channels, with veratridine, or by an increased permeability of presynaptic Ca2+ channels with high K+, were investigated in isolated hippocampal nerve endings. The present study shows that carbamazepine and vinpocetine, both inhibit dose dependently the release of preloaded [3H]Glu induced by veratridine. However, carbamazepine is two orders of magnitude less potent than vinpocetine. The calculated IC(50)'s for carbamazepine and vinpocetine to inhibit veratridine-induced [3H]Glu release are 200 and 2 microM, respectively. Consistently 150 microM carbamazepine and 1.5 microM vinpocetine reduce the veratridine-induced rise in Na(i) in a similar extent. The single effects of carbamazepine and of vinpocetine on the presynaptic Na+ channel mediated responses, namely the rise in Na(i) and the release of Glu induced by veratridine, are additive. Responses that depend on the entrance of external Ca2+ via presynaptic Ca2+ channels, such as the release of [3H]Glu and the rise in Ca(i) induced by high K+, are insensitive to 300 microM carbamazepine and slightly reduced by 5 microM vinpocetine. It is concluded that the additive effects of carbamazepine, which is one of the most common antiepileptic drugs, and vinpocetine that besides its known neuroprotective action and antiepileptic potential is a memory enhancer, may perhaps be advantageous in the treatment of epileptic patients.

Neuronal-glial glucose oxidation and glutamatergic-GABAergic function

Prior 13C magnetic resonance spectroscopy (MRS) experiments, which simultaneously measured in vivo rates of total glutamate-glutamine cycling (V(cyc(tot))) and neuronal glucose oxidation (CMR(glc(ox), N)), revealed a linear relationship between these fluxes above isoelectricity, with a slope of approximately 1. In vitro glial culture studies examining glutamate uptake indicated that glutamate, which is cotransported with Na+, stimulated glial uptake of glucose and release of lactate. These in vivo and in vitro results were consolidated into a model: recycling of one molecule of neurotransmitter between glia and neurons was associated with oxidation of one glucose molecule in neurons; however, the glucose was taken up only by glia and all the lactate (pyruvate) generated by glial glycolysis was transferred to neurons for oxidation. The model was consistent with the 1:1 relationship between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) measured by 13C MRS. However, the model could not specify the energetics of glia and gamma-amino butyric acid (GABA) neurons because quantitative values for these pathways were not available. Here, we review recent 13C and 14C tracer studies that enable us to include these fluxes in a more comprehensive model. The revised model shows that glia produce at least 8% of total oxidative ATP and GABAergic neurons generate approximately 18% of total oxidative ATP in neurons. Neurons produce at least 88% of total oxidative ATP, and take up approximately 26% of the total glucose oxidized. Glial lactate (pyruvate) still makes the major contribution to neuronal oxidation, but approximately 30% less than predicted by the prior model. The relationship observed between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) is determined by glial glycolytic ATP as before. Quantitative aspects of the model, which can be tested by experimentation, are discussed.

Facilitation of glutamate release by nicotine involves the activation of a Ca2+/calmodulin signaling pathway in rat prefrontal cortex nerve terminals

The effect of nicotine on evoked glutamate release from isolated nerve terminals (synaptosomes) from rat prefrontal cortex was examined. We found that nicotine significantly potentiated 4-aminopyridine (4AP)-evoked glutamate release, and this potentiatory effect was mimicked by the selective alpha7 nicotinic receptor agonist choline and was blocked by the selective alpha7 nicotinic receptor antagonist methyllycaconitine, indicating its mediation by alpha7 nicotinic receptors. Examination of the effect of nicotine on cytosolic [Ca(2+)] revealed that the potentiation of glutamate release was associated with an increase in voltage-dependent Ca(2+) influx through N- and P/Q-type Ca(2+) channels. The potentiatory effect of nicotine on Ca(2+) influx seems to be attributed to its increasing synaptosomal excitability because nicotine significantly increased depolarization-evoked increase in the intrasynaptosomal free Na(+) concentration and 4AP-evoked depolarization of the synaptosomal plasma membrane potential. Also, Ca(2+) ionophore ionomycin-induced glutamate release was enhanced by nicotine, and this action was blocked by methyllycaconitine. These results suggest that nicotine exerts its potentiatory effect presynaptically, likely through the activation of alpha7 nicotinic receptors, resulting in Na(+) influx and local depolarization, which subsequently enhances the Ca(K+) entry through voltage-dependent N-and P/Q-type Ca(2+) channels as well as the vesicular release machinery to cause an increase in evoked glutamate release from rat prefrontocortical nerve terminals. Moreover, in this release potentiation may involve an activation of Ca(2+)/calmodulin signaling pathway as nicotine-mediated potentiation of 4AP- and ionomycin-evoked glutamate release were significantly attenuated by KN62, a selective inhibitor of Ca(2+)/calmodulin-dependent kinase II.

α2-Adrenoceptor-mediated modulation of the release of GABA and noradrenaline in the rat substantia nigra pars reticulata

The control of movement by the basal ganglia is influenced by inputs from diverse brain structures. Unfortunately, the mechanisms of modulation are poorly defined. Based on neuroanatomical evidence for alpha2A and alpha2C subtypes of alpha2 adrenergic receptors within this region, we hypothesize that noradrenergic alpha2-receptors can influence transmitter release in the SNr. To test this hypothesis we examined the effect of the alpha 2 adrenergic agonist, clonidine, and antagonist, rauwolscine, on the efflux of [3H]-GABA and [3H]-noradrenaline from brain slices of the rat substantia nigra pars reticulata. At low concentrations (10 nM), rauwolscine caused an 84.2 +/- 18.51% (p < 0.01) increase in KCl-evoked GABA release. At higher concentrations, rauwolscine caused a dose-dependent return to basal levels. Rauwolscine also enhanced basal GABA efflux after KCl washout with a similar biphasic concentration-dependence. Surprisingly, clonidine also enhanced [3H]-GABA release but had no effect on KCl-evoked [3H]-GABA release at concentrations which inhibited [3H]-NA efflux. These effects were potentiated by the GABA re-uptake inhibitor nipecotic acid. Together, our data indicate an important role for noradrenergic modulation in the SNr. The enhancing effect of both the alpha2 adrenoceptor agonist and antagonist on GABA release, while appearing paradoxical, can be rationalised by actions at distinct subsets of alpha2 adrenoceptors, using a simple model where alpha2A adrenoceptors are localized on the terminals of noradrenergic afferents impinging upon alpha2C adrenoceptor-containing GABAergic striato-nigral neurones.

P2X(7) receptors exert a permissive role on the activation of release-enhancing presynaptic alpha7 nicotinic receptors co-existing on rat neocortex glutamatergic terminals

Adenosine triphosphate (ATP) has been reported to enhance the release of glutamate by acting at P2X presynaptic receptors. Acetylcholine (ACh) can elicit glutamate release through presynaptic nicotinic cholinergic receptors (nAChRs) of the alpha7 subtype situated on glutamatergic axon terminals, provided that the terminal membrane is weakly depolarized. Considering that ATP and ACh are co-transmitters, we here investigate on the possibility that P2X and nAChRs co-exist and interact on the same glutamatergic nerve endings using purified rat neocortex synaptosomes in superfusion. ATP evoked Ca(2+)-dependent release of pre-accumulated D-[(3)H]aspartate ([(3)H]D-ASP) as well as of endogenous glutamate; (-)-nicotine, inactive on its own, potentiated the ATP-evoked release. The ATP analogue benzoylbenzoylATP (BzATP) behaved like ATP, but was approximately 30 times more potent; the potentiation of the BzATP-evoked release was blocked by methyllycaconitine or alpha-bungarotoxin. Adding inactive concentrations of (-)-nicotine, epibatidine or choline together with inactive concentrations of BzATP resulted in significant elevation of the [(3)H]D-ASP release mediated by alpha7 nAChRs. To conclude, P2X(7) receptors and alpha7 nAChRs seem to co-exist and interact on rat neocortex glutamatergic terminals; in particular, P2X(7) receptors exert a permissive role on the activation of alpha7 nAChRs, suggesting that ATP may not only evoke glutamate release on its own, but may also regulate the release of the amino acid elicited by ACh.

Presynaptic α7 and non-α7 nicotinic acetylcholine receptors modulate [3H]d-aspartate release from rat frontal cortex in vitro

The presynaptic nicotinic modulation of glutamatergic transmission in the CNS has been associated with activation of the alpha7 subtype of nicotinic acetylcholine receptor (nAChR) in sub-cortical regions, whereas in the frontal cortex, non-alpha7 nAChRs have been implicated. The aim of this investigation was to directly characterise nAChR-evoked release of excitatory amino acids from rat frontal cortex, by monitoring the release of [3H]D-aspartate from superfused synaptosomes or minces. Co-administration of a nAChR agonist with a depolarising stimulus enhanced [3H]D-aspartate release above the effect of depolarising agent alone. This enhancement was blocked by the nicotinic antagonist mecamylamine. Other experiments revealed that in the absence of a depolarising stimulus, the nAChR agonists nicotine, epibatidine and anatoxin-a could evoke the release of [3H]D-aspartate in a Ca2+- and concentration-dependant manner. Differential sensitivity to the alpha7- and beta2*-selective nAChR antagonists alpha-bungarotoxin (alpha-Bgt) and dihydro-beta-erythroidine (DHbetaE) implicated two nAChR subtypes (alpha7 and beta2*), and this was supported by using the subtype-selective agonists choline (10 mM; alpha7 selective, blocked by alpha-Bgt but not by DHbetaE) and 5-Iodo-A-85380 (10 nM; beta2*-selective, blocked by DHbetaE but not by alpha-Bgt). Immunocytochemistry showed that alpha-Bgt labelling was associated with structures immunopositive for vesicular glutamate transporters, in both frontal cortex sections and synaptosome preparations, supporting the presence of alpha7 nAChR on glutamatergic terminals in rat frontal cortex.

GABA and glutamate systems as therapeutic targets in depression and mood disorders

Advances made in diverse areas of neuroscience suggest that neurotransmitter systems, additional to the monoaminergic, contribute to the pathophysiology of mood disorders. This ever accruing body of preclinical and clinical research is providing increased recognition of the contribution made by amino acid neurotransmitters to the neurobiology of mood disorders. This review examines evidence supporting the role of GABA and glutamate in these processes and explores the potential to target these systems in the development of novel compounds; the viability of these agents for treatment-related co-morbidities will also be considered.

Inhibition of Vesicular Glutamate Uptake by Rose Bengal-Related Compounds: Structure–Activity Relationship

Synaptic vesicular accumulation of glutamate is a vital initial step in glutamate transmission. We have previously shown that Rose Bengal, a polyhalogenated fluorescein analog, is a potent inhibitor of glutamate uptake into synaptic vesicles. Here, we report the structural features of Rose Bengal required for this inhibition. Various Rose Bengal-related compounds, with systematic structural variations, were tested. Results indicate that the four iodo groups and the phenyl group attached to the xanthene moiety are critical for potent inhibitory activity. Replacement of these groups with two iodo groups and an alkyl group, respectively, results in substantial reduction in potency. Of further interest in creating high potency is the critical nature of the oxygen atom which links the two benzene rings of xanthene. Thus, the phenyl group and multiple iodo groups, as well as the bridging oxygen of xanthene, are crucial elements of Rose Bengal required for its potent inhibitory action.

Activation of neuropeptide Y Y1 receptors inhibits glutamate release through reduction of voltage-dependent Ca2+ entry in the rat cerebral cortex nerve terminals: Suppression of this inhibitory effect by the protein kinase C-dependent facilitatory pathway

Neuropeptide Y (NPY) is known to regulate the presynaptic glutamate release and neuronal responses to excitatory neurotransmission. The aim of this study was to investigate the effect of NPY on the release of endogenous glutamate from rat cerebrocortical nerve terminals (synaptosomes). NPY inhibited the Ca2+-dependent glutamate release evoked by 4-aminopyridine, and this inhibitory effect was mediated via NPY Y1 receptors, because it was mimicked by the specific NPY Y1 receptor agonist [Leu31 Pro34] NPY and blocked by the NPY Y1 receptor antagonist GR 231118. The inhibitory action of NPY was not due to it decreasing synaptosomal excitability or directly interfering with the release process at some point subsequent to Ca2+ influx, because NPY did not alter the 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential or ionomycin and hypertonic solution-induced glutamate release. Examination of the effect of NPY on the cytosolic [Ca2+] revealed that the inhibition of glutamate release could be attributed to a reduction in voltage-dependent Ca2+ influx. Consistent with this, the NPY-mediated inhibition of glutamate release was completely abolished in synaptosomes pretreated with N- and P/Q-type Ca2+ channel blocker, omega-conotoxin MVIIC. Moreover, NPY-mediated inhibition of 4-aminopyridine-evoked glutamate release was insensitive to KT 5720 and Ro32-0432 but was suppressed when protein kinase C was stimulated with phorbol ester. Together, these results suggest that NPY acting predominantly on NPY Y1 receptors inhibits glutamate release from rat cerebrocortical synaptosomes, likely by a mechanism involving direct coupling of receptors to N- and P/Q-type Ca2+ channels, and this coupling is subject to regulation by protein kinase C-dependent pathway. This implies that selective ligand for NPY receptors may be of value for treatment of conditions characterized by excessive glutamate release in the cerebral cortex.

Oxidative stress affects synaptosomal gamma-aminobutyric acid and glutamate transport in diabetic rats: the role of insulin

Evidence suggests that oxidative stress is involved in the pathophysiology of diabetic complications and that insulin has a neuroprotective role in oxidative stress conditions. In this study, we evaluated the in vitro effect of insulin in the susceptibility to oxidative stress and in the transport of the amino acid neurotransmitters gamma-aminobutyric acid (GABA) and glutamate in a synaptosomal fraction isolated from male type 2 diabetic Goto-Kakizaki (GK) rat brain cortex. The ascorbate/Fe(2+)-induced increase in thiobarbituric acid reactive substances (TBARSs) was similar in Wistar and GK rats and was not reverted by insulin (1 micromol/l), suggesting that other mechanisms, rather than a direct effect in membrane lipid peroxidation, may mediate insulin neuroprotection. Diabetes did not affect GABA and glutamate transport, despite the significant decrease in membrane potential and ATP/ADP ratio, and insulin increased the uptake of both GABA and glutamate in GK rats. Upon oxidation, there was a decrease in the uptake of both neurotransmitters and an increase in extrasynaptosomal glutamate levels and in ATP/ADP ratio in GK rats. Insulin treatment reverted the ascorbate/Fe(2+)-induced decrease in GABA accumulation, with a decrease in extrasynaptosomal GABA. These results suggest that insulin modulates synaptosomal GABA and/or glutamate transport, thus having a neuroprotective role under oxidizing and/or diabetic conditions.

Brain-derived neurotrophic factor facilitates glutamate and inhibits GABA release from hippocampal synaptosomes through different mechanisms

Brain-derived neurotrophic factor (BDNF) has an acute excitatory effect on rat hippocampal synaptic transmission. To compare the action of BDNF upon the release of excitatory and inhibitory neurotransmitters in the hippocampus, we studied the effect of acutely applied BDNF on the K+-evoked glutamate and on the K+-evoked gamma-aminobutyric acid (GABA) release from rat hippocampal nerve terminals (synaptosomes). The acute application of BDNF (30-100 ng/ml) enhanced the K+-evoked [3H]glutamate release. This effect involved tyrosine-kinase B (TrkB) receptor phosphorylation and Ca2+ entry into synaptosomes through voltage-sensitive calcium channels, since it was abolished by K252a (200 nM), which prevents TrkB-mediated phosphorylation, and by CdCl2 (0.2 mM), a blocker of voltage-sensitive calcium channels. In contrast, BDNF (3-100 ng/ml) inhibited K+-evoked [3H]GABA release from hippocampal synaptosomes. This action was also mediated by phosphorylation of the TrkB receptor, but was independent of Ca2+ entry into synaptosomes through voltage-sensitive calcium channels. Blockade of transport of GABA with SKF 89976a (20 microM) prevented the inhibitory action of BDNF upon GABA release, indicating that BDNF influences the activity of GABA transporters. It is concluded that BDNF influences in an opposite way, through distinct mechanisms, the release of glutamate and the release of GABA from hippocampal synaptosomes.

Mechanisms underlying the riluzole inhibition of glutamate release from rat cerebral cortex nerve terminals (synaptosomes)

We have examined the effect of riluzole, a neuroprotective agent with anticonvulsant properties, on the release of endogenous glutamate from rat cerebrocortical synaptosomes using an on-line enzyme-coupled fluorometric assay. Riluzole inhibited the calcium-dependent release of glutamate that was evoked by exposing cerebrocortical synaptosomes to the potassium channel blocker 4-aminopyridine, and this presynaptic inhibition was concentration-dependent. Riluzole did not alter either 4-aminopyridine-evoked depolarization of the synaptosomal membrane potential or ionomycin-mediated glutamate release, indicating that riluzole-mediated inhibition of glutamate release is not due to a decrease in synaptosomal excitability or a direct effect on the exocytotic machinery. Examination of the effect of riluzole on Ca2+ influx revealed that the diminution of glutamate release could be attributed to a reduction in cytosolic calcium. A possible effect of riluzole on synaptosomal calcium channels was confirmed in experiments where synaptosomes pretreated with P/Q-type calcium channel blocker omega-agatoxin IVA, which abolished the riluzole-mediated inhibition of glutamate release. In addition, pretreatment of synaptosomes with either the Gi/Go protein inhibitor pertussis toxin or the GABAB receptor agonist baclofen, completely prevented the inhibitory effect of riluzole on 4-aminopyridine-evoked glutamate release. It is concluded that riluzole exerts their presynaptic inhibition, likely through a reduction in the calcium influx mediated by P/Q-type calcium channels, and thereby inhibits the release of glutamate from rat cerebrocortical nerve terminals. This release inhibition may involve a pertussis toxin-sensitive G protein signalling pathway. This finding provides further support that presynaptic calcium channel blockade concomitant with inhibition of glutamate release could be an important mechanism underlying the therapeutic actions of this drug.

Characterization of the participation of sodium channels on the rise in Na+ induced by 4-aminopyridine (4-AP) in synaptosomes

The participation of voltage-sensitive Na+ channels (VSSC) on the changes on internal (i) Na+, K+, Ca2+, and on DA, Glu, and GABA release caused by different concentrations of 4-AP was investigated in striatum synaptosomes. TTX, which abolished the increase in Na(i) (as determined with SBFI), induced by 0.1 mM 4-AP only inhibited by 30% the rise in Na(i) induced by 1 mM 4-AP. One millimolar 4-AP markedly decreased the fluorescence of the K+ indicator dye PBFI but 0.1 mM 4-AP did not. Like 1 mM 4-AP, ouabain decreased PBFI fluorescence and increased a considerable fraction of Na(i) in a TTX-insensitive manner. In contrast with the different TTX sensitivity of the rise in Na(i) induced by 0.1 and 1 mM 4-AP, the rise in Ca(i) (as determined with fura-2) induced by the two concentrations of 4-AP was markedly inhibited by TTX, as well as by omega-agatoxin in combination with omega-conotoxin GVIA, indicating that only the TTX-sensitive fraction of the rise in Na(i) induced by 4-AP is linked with the activation of presynaptic Ca2+ channels. It is concluded that the TTX-sensitive fraction of neurotransmitter release evoked by 4-AP is released by exocytosis, and the TTX insensitive fraction involves reversal of the neurotransmitters transporters. This contrasts with the exocytosis evoked by high K+ that is unchanged by TTX and with the neurotransmitter-transporter-mediated release evoked by veratridine, which is highly TTX sensitive and does not require activation of Ca2+ channels.

Guanine derivatives modulate L-glutamate uptake into rat brain synaptic vesicles

Glutamate uptake into synaptic vesicles is driven by a proton electrochemical gradient generated by a vacuolar H(+)-ATPase and stimulated by physiological concentrations of chloride. This uptake plays an important role in glutamatergic transmission. We show here that vesicular glutamate uptake is selectively inhibited by guanine derivatives, in a time- and concentration-dependent manner. Guanosine, GMP, GDP, guanosine-5'-O-2-thiodiphosphate, GTP, or 5'-guanylylimidodiphosphate (GppNHp) inhibited glutamate uptake in 1.5 and 3 min incubations, however, when incubating for 10 min, only GTP or GppNHp displayed such inhibition. By increasing ATP concentrations, the inhibitory effect of GTP was no longer observed, but GppNHp still inhibited glutamate uptake. In the absence of ATP, vesicular ATPase can hydrolyze GTP in order to drive glutamate uptake. However, 5mM GppNHp inhibited ATP hydrolysis by synaptic vesicle preparations. GTP or GppNHp decreased the proton electrochemical gradient, whereas the other guanine derivatives did not. Glutamate saturation curves were assayed in order to evaluate the specificity of inhibition of the vesicular glutamate carrier by the guanine derivatives. The maximum velocity of the initial rate of glutamate uptake was decreased by all guanine derivatives. These results indicate that, although GppNHp can inhibit ATPase activity, guanine derivatives are more likely to be acting through interaction with vesicular glutamate carrier.

Metabolic flux analysis as a tool for the elucidation of the metabolism of neurotransmitter glutamate

A flux analysis model for the metabolism of neurotransmitter glutamate is constructed, in order to study functional aspects of its metabolism. This work is based on the potassium [K(+)] evoked neurotransmitter glutamate released, as measured in a series of experiments of superfused rat or mouse brain preparations. These measurements are combined with data reported, concerning the metabolism of glutamate and its precursors, glutamine and glucose in rat cerebral cells in vivo. The proposed stoichiometry of the specific reaction network renders the model solvable. The classification procedure establishes that the measured fluxes are all balanceable and all non-measured fluxes can be calculated. The system is well posed with a condition number of 7.8536. The results emphasize the importance of phosphate activated glutaminase and aspartate aminotransferase in the metabolism of neurotransmitter glutamate. Reported data on the rate of the malate-aspartate shuttle, as well as the anaplerotic flux of the glial pyruvate carboxylase reaction are in agreement with the estimations calculated from the proposed model.