N-methyl-D-aspartate autoreceptors respond to low and high agonist concentrations by facilitating, respectively, exocytosis and carrier-mediated release of glutamate in rat hippocampus.
Presynaptic NMDA autoreceptors regulating glutamate release have rarely been investigated. High-micromolar N-methyl-D-aspartate (NMDA) was reported to elicit glutamate release from hippocampal synaptosomes in a Ca(2+)-independent manner by reversal of excitatory amino acid transporters. The aim of this work was to characterize excitatory amino acid release evoked by low-micromolar NMDA from glutamatergic axon terminals. Purified rat hippocampal synaptosomes were prelabelled with [(3)H]D-aspartate ([(3)H]D-ASP) and exposed in superfusion to varying concentrations of NMDA in the presence of 1 microM glycine. The release of [(3)H]D-ASP and also that of endogenous glutamate provoked by 10 microM NMDA were external Ca(2+) dependent and sensitive to the NMDA channel blocker MK-801 but insensitive to the glutamate transporter inhibitor DL-TBOA, which, on the contrary, prevented the Ca(2+)-independent release evoked by 100 microM NMDA. The NMDA (10 microM) response was blocked by 1 nM Zn(2+) and 1 microM ifenprodil, compatible with the involvement of a NR1/NR2A/NR2B assembly, although the presence of two separate receptor populations, i.e., NR1/NR2A and NR1/NR2B, cannot be excluded. This response was strongly antagonized by submicromolar (0.01-1 microM) concentrations of kynurenic acid and was mimicked by quinolinic acid (1-100 microM) plus 1 microM glycine. Finally, the HIV-1 protein gp120 potently mimicked the NMDA co-agonists glycine and D-serine, being significantly effective at 30 pM. In conclusion, glutamatergic nerve terminals possess NMDA autoreceptors mediating different types of release when activated by different agonist concentrations: low-micromolar glutamate would potentiate glutamate exocytosis, whereas higher glutamate concentrations would also provoke carrier-mediated release.
Pharmacology and Toxicology Section, Department of Experimental Medicine, University of Genoa, Viale Cembrano 4, Genoa, Italy.
Elisa
E
Luccini
Veronica
V
Musante
Elisa
E
Neri
Maurizio
M
Raiteri
Anna
A
Pittaluga
D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism / Q000494:pharmacology; D017660:Autoreceptors / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D018698:Glutamic Acid / Q000378:metabolism*; D005998:Glycine / Q000494:pharmacology; D015699:HIV Envelope Protein gp120 / Q000378:metabolism / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D016202:N-Methylaspartate / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000819:agonists / Q000187:drug effects / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013574:Synaptosomes; D050496:Vesicular Glutamate Transport Proteins / Q000187:drug effects / Q000378:metabolism*
D017660:Autoreceptors; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D015699:HIV Envelope Protein gp120; C409466:NR1 NMDA receptor; C120997:NR2A NMDA receptor; D016194:Receptors, N-Methyl-D-Aspartate; D050496:Vesicular Glutamate Transport Proteins; C120673:benzyloxyaspartate; D001224:Aspartic Acid; D018698:Glutamic Acid; D016202:N-Methylaspartate; D005998:Glycine