L-aspartate as an amino acid neurotransmitter: mechanisms of the depolarization-induced release from cerebrocortical synaptosomes.

The role of l-aspartate as a classical neurotransmitter of the CNS has been a matter of great debate. In this study, we have characterized the main mechanisms of its depolarization-induced release from rat purified cerebrocortical synaptosomes in superfusion and compared them with those of the well-known excitatory neurotransmitter l-glutamate. High KCl and 4-aminopyridine were used as depolarizing agents. At 15 mM KCl, the overflows of both transmitters were almost completely dependent on external Ca2+. At 35 and 50 mM KCl, the overflows of l-aspartate, but not those of l-glutamate, became sensitive to dl-threo-β-benzyloxyaspartic acid (dl-TBOA), an excitatory amino acid transporter inhibitor. In the presence of dl-TBOA, the 50 mM KCl-evoked release of l-aspartate was still largely external Ca2+-dependent. The dl-TBOA insensitive, external Ca2+-independent component of the 50 mM KCl-evoked overflows of l-aspartate and l-glutamate was significantly decreased by the mitochondrial Na+/Ca2+ exchanger blocker CGP 37157. The Ca2+-dependent, KCl-evoked overflows of l-aspartate and l-glutamate were diminished by botulinum neurotoxin C, although to a significantly different extent. The 4-aminopyridine-induced l-aspartate and l-glutamate release was completely external Ca2+-dependent and never affected by dl-TBOA. Superimposable results have been obtained by pre-labeling synaptosomes with [3H]d-aspartate and [3H]l-glutamate. Therefore, our data showing that l-aspartate is released from nerve terminals by calcium-dependent, exocytotic mechanisms support the neurotransmitter role of this amino acid.