Effects of tetrodotoxin, calcium and magnesium on the release of amino acids from slices of guinea-pig cerebral cortex.

Abstract

Abstract— Tetrodotoxin, Ca2+‐deprivation and high‐Mg2+ were used in an effort to identify the portion of the evoked release of endogenous amino acids, labelled via metabolism of [14C]‐glucose, and several exogenous labelled amino acids, that came from nerve terminals when slices of guinea pig cerebral cortex were superfused with glucose‐free solutions and stimulated electrically.With some exceptions, spontaneous release of labelled amino acids was decreased by 2 μm‐tetrodotoxin but increased in Ca2+‐free medium and in solutions containing an extra 24 mm‐MgCl2. Tetrodotoxin suppressed 85–90% of the stimulated release of almost all labelled amino acids, but had a smaller effect on the release of endogenous 14C‐labelled threonine‐serine‐glutamine (unseparated). In Ca2+‐free solution, the stimulated release of endogenous 14C‐labelled glutamate, aspartate and GABA was suppressed by 80–90%, but that of endogenous 14C‐labelled threonine‐serine‐glutamine was unaffected as was most of the release of the other labelled amino acids. In medium containing an extra 24mM‐MgCl2, the stimulated release of endogenous 14C‐labelled glutamate, aspartate and GABA was suppressed by 75‐85%, that of exogenous labelled aspartate and GABA by 50–65%, but the release of the other labelled amino acids was unaffected. The control stimulated releases of endogenous 14C‐labelled glutamate, aspartate and GABA were much larger than those of other labelled amino acids but were reduced by tetrodotoxin, Ca2+‐deprivation and high‐Mg2+ to a level similar to that of the control stimulated releases of the other labelled amino acids.These results suggest that almost all of the stimulated release of endogenous 14C‐labelled glutamate, aspartate and GABA came from nerve terminals while those of the other labelled amino acids came from other tissue elements. In addition, they are in accord with a transmitter role for glutamate, aspartate and GABA in cerebral cortex.