CHARACTERIZATION OF PUTATIVE AMINO ACID TRANSMITTER RELEASE FROM SLICES OF RAT DENTATE GYRUS

Abstract— Superfused slices of the rat dentate gyrus were employed to study the release of GABA, glutamate and aspartate, which are considered strong neurotransmitter candidates in this region. The introduction of Ca2+ to a Ca2+-free superfusion medium containing a depolarizing agent augmented the e...
Ausführliche Beschreibung

Abstract— Superfused slices of the rat dentate gyrus were employed to study the release of GABA, glutamate and aspartate, which are considered strong neurotransmitter candidates in this region. The introduction of Ca2+ to a Ca2+-free superfusion medium containing a depolarizing agent augmented the efflux of all three amino acids. The response to application of Ca2+ nearly always occurred within 30 s, the shortest interval tested in these studies. The efflux rate reached a peak within 90 s and then declined to a level slightly greater than the prestimulation baseline. The failure to maintain the maximal rate with continued exposure to Ca2+ and depolarizing influences appeared not to result from a reduction in Ca2+ permeability caused by continuous depolarization. Ca2+ also stimulated the efflux of exogenously loaded radiolabeled GABA, glutamate and aspartate, but not proline. Exogenously loaded GABA was more readily released than endogenous GABA. Otherwise the effects of various treatments on their efflux rates were qualitatively similar.Mg2+ inhibited Ca2+-dependent efflux. Ba2+, but not Mg2+, stimulated amino acid efflux in the absence of Ca2+. Extracellular Na+ was not required to support Ca2+-dependent efflux. Addition of Ca2+ to a Ca2+-free medium in the absence of a depolarizing agent released GABA from the slices, but not glutamate or aspartate.K+-enriched medium and the depolarizing alkaloid, veratridine, stimulated both Ca2+-dependent and Ca2+-independent release processes. Na+-free medium enhanced the Ca2+-independent releasing action of elevated K+. Ca2+-independent release was inhibited by raising the Mg2+ concentration by 15 or 30 mM and appeared to be inhibited by Ca2+ as well. Amino acid output in the absence of Ca2+ is probably not directly related to transmission and is considered to result partially from a general increase in membrane permeability induced by depolarization in a Ca2+-free medium and partially from stimulation of carrier-mediated amino acid efflux.These results support previously suggested transmitter roles for GABA, glutamate and aspartate in the rat dentate gyrus. Ausführliche Beschreibung