Exogenous leptin alleviates glutamate-excitotoxic injury caused by cerebral ischemia-reperfusion in mice by affecting the expression of glutamate transporters

Ischemic stroke is characterized by high morbidity and mortality and a lack of effective therapeutic interventions. Leptin plays an important role in regulating oxidative stress, angiogenesis, hematopoiesis, etc. Although recent studies have found a neuroprotective effect of leptin, little is known about its role in cerebral ischemia. This study explores the possible roles and potential preventative mechanisms of leptin in cerebral ischemia-reperfusion injury (CIRI). An in vivo middle cerebral artery occlusion/reperfusion (MCAO/R) mouse model was used to replicate the CIRI model, low (0.5 mg/kg), medium (1 mg/kg) and high (2 mg/kg) concentrations of leptin were injected intraperitoneally immediately after inserting the embolic line. After 1.5 h of ischemia and 24 h of reperfusion, we examined the neural function of the mice, collected brain tissue for histological examination, and screened for the optimal concentrations of leptin intervention. On this basis, we observed the changes of cortical apoptosis injury, intracellular calcium fluorescence intensity and astrocyte glial fibrillary acidic protein (GFAP) expression and morphological changes. In addition, we also tested the expression of transporters and metabolism-related enzymes (VGLUT-1, VGLUT-2, GLAST, GLT-1, GS, ATP1α1), the expression of inflammatory factors and the content of glutamate (Glu). Compared with the I/R group, we found that leptin improved neurological deficits, reduced the area of infarcts, maintained the normal morphology of astrocytes (AST), downregulated the expression of VGLUT-1, and upregulates the expression of GLT-1 and GLAST, thereby reducing the content of Glu in the synaptic cleft. Our studies suggest that leptin may have a neuroprotective effect by decreasing the excitotoxicity of glutamate.

In Vitro Study of L-Glutamate and L-Glutamine Transport in Retinal Pericytes: Involvement of Excitatory Amino Acid Transporter 1 and Alanine-Serine-Cysteine Transporter 2

L-Glutamate (L-Glu) is known to be a relaxant of pericytes and to induce changes in microcirculatory hemodynamics. Since the concentration of L-Glu which induces the dilation of retinal capillaries is reported to be high compared with the estimated concentration in the retinal interstitial fluid, it is hypothesized that some systems involving concentrative L-Glu release are present in retinal pericytes. The purpose of this study was to investigate the existence of L-Glu-storing systems, which contribute to autocrine L-Glu release, in retinal pericytes using conditionally immortalized rat retinal pericytes (TR-rPCT1 cells), which express mRNAs of L-Glu-synthesizing enzymes from L-glutamine (L-Gln). TR-rPCT1 cells express the mRNAs of vesicular L-Glu transporter 1 (VGLUT1), indicating that L-Glu in the cytoplasm is taken up into VGLUT1-expressing vesicles of retinal pericytes. L-Glu and L-Gln are taken up into TR-rPCT1 cells via Na(+)-dependent saturable process(es) with a Km value of 22.4 µM and 163 µM, respectively. The [(3)H]L-Glu uptake was inhibited by ca. 50% in the presence of D-aspartate, a substrate of excitatory amino acid transporter (EAAT) subtypes, whereas substrates of alanine-serine-cysteine transporter (ASCT) subtypes exhibited only a weak inhibitory effect on [(3)H]L-Glu uptake compared with D-aspartate. Regarding the L-Gln uptake by TR-rPCT1 cells, the inhibitory effect of ASCT substrates on the [(3)H]L-Gln uptake was stronger than that of substrates of other neutral amino acid transport systems. Consequently, it was determined that EAAT1 and ASCT2 play a role in the transport of L-Glu and L-Gln, respectively, from retinal interstitial fluid to the cytoplasm of retinal pericytes.

The glutamatergic system as a target for neuropathic pain relief

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The understanding of glutamatergic transmission in the nervous system has been greatly expanded with the discovery and investigation of the family of ionotropic and metabotropic glutamate receptors (mGluRs). Metabotropic glutamate receptors are localized at nerve terminals, postsynaptic sites and glial cells and thus, they can influence and modulate the action of glutamate at different levels in the synapse. Moreover, there is substantial evidence of glial participation in glutamate nociceptive processes and neuropathic pain. Metabotropic glutamate receptors have been shown to play a role in neuropathic pain, which is one of the most troublesome illnesses because the therapy is still not satisfactory. Recently, the development of selective mGluR ligands has provided important tools for further investigation of the role of mGluRs in the modulation of chronic pain processing. This paper presents a review of the literature of glutamate receptors in neuropathic pain and the role of glia in these effects. Specifically, pharmacological interventions aimed at inhibiting group I mGluRs and/or potentiating group II and III mGluR-mediated signalling is discussed. Moreover, we introduce data about the role of glutamate transporters. They are responsible for the level of glutamate in the synaptic cleft and thus regulate the effects of all three groups of mGluRs and, in consequence, the activity of this system in nociceptive transmission. Additionally, the question of how the modulation of the glutamatergic system influences the effectiveness of analgesic drugs used in neuropathic pain therapy is addressed.

Bovine neuronal vesicular glutamate transporter activity is inhibited by ergovaline and other ergopeptines

l-Glutamate (Glu) is a major excitatory neurotransmitter responsible for neurotransmission in the vertebrate central nervous system. Vesicular Glu transporters VGLUT1 and VGLUT2 concentrate (50mM) Glu [Michaelis constant (measuring affinity), or K(m),=1 to 4mM] into synaptic vesicles (SV) for subsequent release into the synaptic cleft of glutamatergic neurons. Vesicular Glu transporter activity is dependent on vacuolar H(+)-ATPase function. Previous research has shown that ergopeptines contained in endophyte-infected tall fescue interact with dopaminergic and serotoninergic receptors, thereby affecting physiology regulated by these neuron types. To test the hypothesis that ergopeptine alkaloids inhibit VGLUT activity of bovine cerebral SV, SV were isolated from cerebral tissue of Angus-cross steers that were naive to ergot alkaloids. Immunoblot analysis validated the enrichment of VGLUT1, VGLUT2, synaptophysin 1, and vacuolar H(+)-ATPase in purified SV. Glutamate uptake assays demonstrated the dependence of SV VGLUT-like activity on the presence of ATP, H(+)-gradients, and H(+)-ATPase function. The effect of ergopeptines on VGLUT activity was evaluated by ANOVA. Inhibitory competition (IC(50)) experiments revealed that VGLUT-mediated Glu uptake (n=9) was inhibited by ergopeptine alkaloids: bromocriptine (2.83±0.59μM)<ergotamine (20.5±2.77μM)<ergocornine (114±23.1μM)<ergovaline (137±6.55μM). Subsequent ergovaline kinetic inhibition analysis (n=9; Glu=0.05, 0.10, 0.50, 1, 2, 4, 5mM) demonstrated no change in apparent K(m). However, the maximum reaction rate (V(max)) of Glu uptake was decreased when evaluated in the presence of 50, 100, and 200μM ergovaline, suggesting that ergovaline inhibited SV VGLUT activity through a noncompetitive mechanism. The findings of this study suggest cattle with fescue toxicosis may have a decreased glutamatergic neurotransmission capacity due to consumption of ergopeptine alkaloids.

Use of the hydantoin isostere to produce inhibitors showing selectivity toward the vesicular glutamate transporter versus the obligate exchange transporter system x(c)(-)

Evidence was acquired prior to suggest that the vesicular glutamate transporter (VGLUT) but not other glutamate transporters were inhibited by structures containing a weakly basic α-amino group. To test this hypothesis, a series of analogs using a hydantoin (pK(a)∼9.1) isostere were synthesized and analyzed as inhibitors of VGLUT and the obligate cystine-glutamate transporter (system x(c)(-)). Of the hydantoin analogs tested, a thiophene-5-carboxaldehyde analog 2l and a bis-hydantoin 4b were relatively strong inhibitors of VGLUT reducing uptake to less than 6% of control at 5mM but few inhibited system x(c)(-) greater than 50% of control. The benzene-2,4-disulfonic acid analog 2b and p-diaminobenzene analog 2e were also good hydantoin-based inhibitors of VGLUT reducing uptake by 11% and 23% of control, respectively, but neither analog was effective as a system x(c)(-) inhibitor. In sum, a hydantoin isostere adds the requisite chemical properties needed to produce selective inhibitors of VGLUT.

Intercellular glutamate signaling in the nervous system and beyond

Most intercellular glutamate signaling in the nervous system occurs at synapses. Some intercellular glutamate signaling occurs outside synapses, however, and even outside the nervous system where high ambient extracellular glutamate might be expected to preclude the effectiveness of glutamate as an intercellular signal. Here, I briefly review the types of intercellular glutamate signaling in the nervous system and beyond, with emphasis on the diversity of signaling mechanisms and fundamental unanswered questions.

Conformationally-restricted amino acid analogues bearing a distal sulfonic acid show selective inhibition of system x(c)(-) over the vesicular glutamate transporter

A panel of amino acid analogs and conformationally-restricted amino acids bearing a sulfonic acid were synthesized and tested for their ability to preferentially inhibit the obligate cysteine-glutamate transporter system x(c)(-) versus the vesicular glutamate transporter (VGLUT). Several promising candidate molecules were identified: R/S-4-[4'-carboxyphenyl]-phenylglycine, a biphenyl substituted analog of 4-carboxyphenylglycine and 2-thiopheneglycine-5-sulfonic acid both of which reduced glutamate uptake at system x(c)(-) by 70-75% while having modest to no effect on glutamate uptake at VGLUT.