Calcium-Dependent Evoked Release of N[3H]Acetylaspartylglutamate from the Optic Pathway

N-Acetylaspartylglutamate (NAAG) is a neuropeptide localized to several putative glutamatergic neuronal systems, including the rodent optic pathway. To determine whether the peptide is released by depolarization, the superior colliculus of the rat was perfused with 2 microCi of 3H]NAAG, then with Krebs-bicarbonate buffer for 1 h, using a microdialysis system. Subsequently, 10-min fractions were collected and analyzed by HPLC for 3H]NAAG. Addition of 100 microM veratridine resulted in a several-fold increase in the evoked release of 3H]NAAG that was virtually abolished by coperfusion with Ca2+-free Krebs buffer containing 1 mM EGTA. When 3H]glutamate was used as the precursor, veratridine depolarization resulted in only an 80% increase in the release of 3H]NAAG. Prior enucleation of the right eye reduced the spontaneous release of 3H]NAAG by 50%, and the veratridine-evoked release by greater than 85%, from the left superior colliculus. These results suggest that NAAG is released upon depolarization and may serve as a neurotransmitter/neuromodulator in the optic tract.