Glutamate is the endogenous amino acid selectively released by rat hippocampal mossy fiber synaptosomes concomitantly with prodynorphin-derived peptides.

The release of endogenous amino acids from depolarized rat hippocampal mossy fiber synaptosomes was investigated to assess the possible role(s) of glutamate and aspartate in mediating the excitatory mossy fiber synaptic input. The relative proportions of prodynorphin-derived peptides concomitantly released with amino acids were also determined to further characterize the biochemical basis for mossy fiber synaptic transmission. Of the 18 amino acids shown to be present in superfusate fractions by liquid chromatographic analysis, only glutamate was released at a significantly enhanced rate from K(+)-stimulated (35 mM KCl) mossy fiber nerve endings. The rates of glutamate and aspartate release were increased by 360 +/- 27% and 54 +/- 12% over baseline, respectively. However, the K(+)-evoked release of glutamate was substantially more Ca2(+)-dependent (80%) than was the release of aspartate (49%). The veratridine (45 microM)-evoked release of both acidic amino acids was entirely blocked by 1 microM tetrodotoxin. Depolarization (45 mM KCl) also stimulated the release of the four prodynorphin (Dyn) products examined, in a rank order of Dyn B much greater than Dyn A(1-17) greater than Dyn A(1-8) much greater than Dyn A(1-13), with Dyn B efflux increasing by more than 5-fold over baseline values. These results suggest that the predominant excitatory amino acid in hippocampal mossy fiber synaptic transmission may be glutamate and that this synaptic input may be modulated by at least four different products of prodynorphin processing.