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-b-benzyloxy aspartic 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.
Cavallero
Anna
A
Department of Experimental Medicine, Section of Pharmacology and Toxicology, University of Genoa, Genoa, Italy.
Marte
Antonella
A
Fedele
Ernesto
E
D000818:Animals; D001224:Aspartic Acid / Q000737:chemistry / Q000378:metabolism*; D016764:Cell Polarity / Q000502:physiology; D002540:Cerebral Cortex / Q000737:chemistry / Q000378:metabolism*; D018846:Excitatory Amino Acids / Q000737:chemistry / Q000378:metabolism; D008297:Male; D018377:Neurotransmitter Agents / Q000737:chemistry / Q000378:metabolism*; D011189:Potassium Chloride / Q000737:chemistry; D051381:Rats; D017207:Rats, Sprague-Dawley; D013574:Synaptosomes / Q000737:chemistry / Q000378:metabolism*
D018846:Excitatory Amino Acids; D018377:Neurotransmitter Agents; D001224:Aspartic Acid; D011189:Potassium Chloride