title: Activation of gamma-aminobutyric acid GAT-1 transporters on glutamatergic terminals of mouse spinal cord mediates glutamate release through anion channels and by transporter reversal.
abstract: The effects of gamma-aminobutyric acid (GABA) on the release of glutamate from mouse spinal cord nerve endings have been studied using superfused synaptosomes. GABA elicited a concentration-dependent release of [3H]D-aspartate ([3H]D-ASP; EC50= 3.76 microM). Neither muscimol nor (-)baclofen mimicked GABA, excluding receptor involvement. The GABA-evoked release was strictly Na+ dependent and was prevented by the GABA transporter inhibitor SKF89976A, suggesting involvement of GAT-1 transporters located on glutamatergic nerve terminals. GABA also potentiated the spontaneous release of endogenous glutamate; an effect sensitive to SKF89976A and low-Na+-containing medium. Confocal microscopy shows that the GABA transporter GAT-1 is coexpressed with the vesicular glutamate transporter vGLUT-1 and with the plasma membrane glutamate transporter EAAT2 in a substantial portion of synaptosomal particles. The GABA effect was external Ca2+ independent and was not decreased when cytosolic Ca2+ ions were chelated by BAPTA. The glutamate transporter blocker DL-TBOA or dihydrokainate inhibited in part (approximately 35%) the GABA (10 microM)-evoked [3H]D-ASP release; this release was strongly reduced by the anion channel blockers niflumic acid and NPPB. GABA, up to 30 microM, was unable to augment significantly the basal release of [3H]glycine from spinal cord synaptosomes, indicating selectivity for glutamatergic transmission. It is concluded that GABA GAT-1 transporters and glutamate transporters coexist on the same spinal cord glutamatergic terminals. Activation of these GABA transporters elicits release of glutamate partially by reversal of glutamate transporters present on glutamatergic terminals and largely through anion channels.
authors.affiliation: Department of Experimental Medicine, Pharmacology and Toxicology Section, University of Genoa, Genoa, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Raiteri
authors.forename: Sara
authors.initials: S
authors.lastname: Stigliani
authors.forename: Laura
authors.initials: L
authors.lastname: Patti
authors.forename: Cesare
authors.initials: C
authors.lastname: Usai
authors.forename: Giovanna
authors.initials: G
authors.lastname: Bucci
authors.forename: Alberto
authors.initials: A
authors.lastname: Diaspro
authors.forename: Maurizio
authors.initials: M
authors.lastname: Raiteri
authors.forename: Giambattista
authors.initials: G
authors.lastname: Bonanno
mesh_terms: D000818:Animals; D000838:Anions / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D027342:Excitatory Amino Acid Transporter 2 / Q000378:metabolism; D005260:Female; D018755:GABA Agonists / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000378:metabolism*; D005998:Glycine / Q000378:metabolism; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000819:agonists / Q000378:metabolism*; D051379:Mice; D009544:Niflumic Acid / Q000494:pharmacology; D009579:Nitrobenzoates / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism*; D013116:Spinal Cord / Q000033:anatomy & histology / Q000187:drug effects / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D050598:Vesicular Glutamate Transport Protein 1; D005680:gamma-Aminobutyric Acid / Q000378:metabolism / Q000494:pharmacology
chemical_list: D000838:Anions; D004791:Enzyme Inhibitors; D027342:Excitatory Amino Acid Transporter 2; D018755:GABA Agonists; D050485:GABA Plasma Membrane Transport Proteins; D007473:Ion Channels; D026901:Membrane Transport Proteins; D009579:Nitrobenzoates; C493414:Slc6a1 protein, mouse; D050598:Vesicular Glutamate Transport Protein 1; C058176:5-nitro-2-(3-phenylpropylamino)benzoic acid; D018698:Glutamic Acid; D009544:Niflumic Acid; D005680:gamma-Aminobutyric Acid; D002118:Calcium; D005998:Glycine