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-aspartyl3H]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.