Synaptosomal transport of radiolabel from N-acetyl-aspartyl-[3H]glutamate suggests a mechanism of inactivation of an excitatory neuropeptide.
This study was undertaken to explore in synaptosomal preparations the disposition of N-acetyl-aspartyl-glutamate (NAAG), an endogenous acidic dipeptide neurotransmitter candidate. Radiolabel from N-acetyl-aspartyl[3H]glutamate was taken up rapidly into an osmotically sensitive compartment by rat brain synaptosomal preparations in a sodium-, temperature-, and time-dependent manner. HPLC analysis of the accumulated radiolabel indicated that the bulk of the tritium cochromatographed with glutamic acid and not with NAAG. In contrast, [14C]NAAG, labeled on the N-terminal acetate, was not taken up by the synaptosomal preparation. All effective inhibitors of synaptosomal, Na+-dependent [3H]glutamate uptake were found to exhibit similar potency in inhibiting uptake of tritium derived from [3H]NAAG. However, certain alpha-linked acidic dipeptides, structurally similar to NAAG, as well as the potent convulsant quisqualic acid inhibited synaptosomal transport of [3H]NAAG but were ineffective as inhibitors of [3H]glutamate transport. Together with a demonstration of disparities between the regional accumulation of radiolabel from [3H]NAAG and high-affinity [3H]glutamate uptake, these data suggest the presence in brain of a specific peptidase targeting carboxy-terminal glutamate-containing dipeptides that may be coupled to the Na+-dependent glutamate transporter. These findings provide a possible mechanism for NAAG inactivation subsequent to its release from nerve endings.
D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D004151:Dipeptides / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D007700:Kinetics; D008297:Male; D010069:Oxadiazoles / Q000494:pharmacology; D016318:Quisqualic Acid; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000494:pharmacology; D013574:Synaptosomes / Q000378:metabolism*; D014316:Tritium
D000596:Amino Acids; D004151:Dipeptides; D005971:Glutamates; D010069:Oxadiazoles; D014316:Tritium; C027172:isospaglumic acid; D018698:Glutamic Acid; D016318:Quisqualic Acid; D012964:Sodium