The release of preloaded D-[3H]
aspartate, an unmetabolizable analogue of
L-glutamate, was studied in superfused hippocampal slices from 7-day-old and 3-month-old (adult) mice under various cell-damaging conditions, including
hypoxia,
hypoglycemia,
ischemia, oxidative stress and the presence of
free radicals and metabolic
poisons. The release was generally markedly enhanced in most of the above conditions, the responses being greater in adults than in developing mice. The presence of dinitrophenol had the most pronounced effect at both ages, followed by NaCN- and
free-radical-containing media and
ischemia.
Hypoxia did not affect release in the immature hippocampus. Under most conditions K+ stimulation (50 mM) was still able markedly to enhance
D-aspartate release. This potentiation under cell-damaging conditions in both adult and developing hippocampus signifies that increased
L-glutamate release contributes to excitotoxicity and subsequent cell death. The mechanisms of
ischemia-induced release of
D-aspartate were analyzed in the adult hippocampus using
ion channel inhibitors and modified superfusion media. The induced release proved to be partly Ca(2+)-dependent and partly Ca(2+)-independent. The results obtained with Na+ omission and homo- and heteroexchange with
D-aspartate and
L-glutamate demonstrated that a part of the release in normoxia and
ischemia is mediated by the reversal of Na(+)-dependent
glutamate transporters. The Na+ channel blockers
amiloride and
riluzole reduced the
ischemia-induced release, also indicating the involvement of Na+ channels. In addition to this, the enhanced release of
D-aspartate may comprise a swelling-induced component through
chloride channels.