title: gamma-Hydroxybutyrate binds to the synaptic site recognizing succinate monocarboxylate: a new hypothesis on astrocyte-neuron interaction via the protonation of succinate.
abstract: Succinate (SUC), a citrate (CIT) cycle intermediate, and carbenoxolone (CBX), a gap junction inhibitor, were shown to displace [3H]gamma-hydroxybutyrate ([3H]GHB), which is specifically bound to sites present in synaptic membrane subcellular fractions of the rat forebrain and the human nucleus accumbens. Elaboration on previous work revealed that acidic pH-induced specific binding of [3H]SUC occurs, and it has been shown to have a biphasic displacement profile distinguishing high-affinity (K(i,SUC) = 9.1 +/- 1.7 microM) and low-affinity (K(i,SUC) = 15 +/- 7 mM) binding. Both high- and low- affinity sites were characterized by the binding of GHB (K(i,GHB) = 3.9 +/- 0.5 microM and K(i,GHB) = 5.0 +/- 2.0 mM) and lactate (LAC; K(i,LAC) = 3.9 +/- 0.5 microM and K(i,LAC) = 7.7 +/- 0.9 mM). Ligands, including the hemiester ethyl-hemi-SUC, and the gap junction inhibitors flufenamate, CBX, and the GHB binding site-selective NCS-382 interacted with the high-affinity site (in microM: K(i,EHS) = 17 +/- 5, K(i,FFA) = 24 +/- 13, K(i,CBX) = 28 +/- 9, K(i,NCS-382) = 0.8 +/- 0.1 microM). Binding of the Na+,K+-ATPase inhibitor ouabain, the proton-coupled monocarboxylate transporter (MCT)-specific alpha-cyano-hydroxycinnamic acid (CHC), and CIT characterized the low-affinity SUC binding site (in mM: K(i,ouabain) = 0.13 +/- 0.05, K(i,CHC) = 0.32 +/- 0.07, K(i,CIT) = 0.79 +/- 0.20). All tested compounds inhibited [3H]SUC binding in the human nucleus accumbens and had K(i) values similar to those observed in the rat forebrain. The binding process can clearly be recognized as different from synaptic and mitochondrial uptake or astrocytic release of SUC, GHB, and/or CIT by its unique GHB selectivity. The transient decrease of extracellular SUC observed during epileptiform activity suggested that the function of the synaptic target recognizing protonated succinate monocarboxylate may vary under different (patho)physiological conditions. Furthermore, we put forward a hypothesis on the synaptic activity-regulated signaling between astrocytes and neurons via SUC protonation.
authors.affiliation: Department of Neurochemistry, Institute of Biomolecular Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: Tünde
authors.initials: T
authors.lastname: Molnár
authors.forename: Péter
authors.initials: P
authors.lastname: Barabás
authors.forename: László
authors.initials: L
authors.lastname: Héja
authors.forename: Erzsébet Kútiné
authors.initials: EK
authors.lastname: Fekete
authors.forename: Bálint
authors.initials: B
authors.lastname: Lasztóczi
authors.forename: Pál
authors.initials: P
authors.lastname: Szabó
authors.forename: Gabriella
authors.initials: G
authors.lastname: Nyitrai
authors.forename: Edit
authors.initials: E
authors.lastname: Simon-Trompler
authors.forename: Ferenc
authors.initials: F
authors.lastname: Hajós
authors.forename: Miklós
authors.initials: M
authors.lastname: Palkovits
authors.forename: Julianna
authors.initials: J
authors.lastname: Kardos
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D004347:Drug Interactions; D006801:Humans; D066298:In Vitro Techniques; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D009714:Nucleus Accumbens / Q000166:cytology* / Q000187:drug effects; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D012978:Sodium Oxybate / Q000378:metabolism* / Q000493:pharmacokinetics; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism; D013386:Succinates / Q000378:metabolism* / Q000493:pharmacokinetics; D013569:Synapses / Q000187:drug effects / Q000648:ultrastructure; D013574:Synaptosomes / Q000187:drug effects; D014316:Tritium / Q000493:pharmacokinetics
chemical_list: D013386:Succinates; D014316:Tritium; D012978:Sodium Oxybate