D-[3H]aspartate release from hippocampus slices studied in a multiwell system: controlling factors and postnatal development of release.

Medium components and various factors were tested to define optimal conditions for D-[3H]aspartate release. Isolation of the hippocampus and preparation of the slices in a medium without Ca2+ increased the release of D-[3H]aspartate in response to veratridine when subsequently tested in a regular Ca2+ containing medium. Apparently, the absence of Ca2+ during preparation of the slices reduced irreversible damage due to hypoxic conditions which prevail throughout the interval between killing the animal and immersion of the slices in a well oxygenated medium. Substitution of 10 mM Mg2+ for Ca2+ was an efficient procedure to test for Ca2+ dependence of D-[3H]aspartate release induced by veratridine. The inhibition was readily reversible when Ca2+ was readded. Veratridine (50 microM) was superior to high K+ (45 mM) in inducing D-[3H]aspartate release under all conditions tested in slices of mature animals. Furthermore, veratridine-induced release could be completely blocked by tetrodotoxin while K+-induced release was essentially unaffected by this toxin. Postnatal development of the D-[3H]aspartate release induced by veratridine was found to require 40-45 days, whereas release induced by K+ reached about 80% of maximum at postnatal day 22. K+-induced release appears to reach maturation when most hippocampal cells have been formed while veratridine-induced release probably requires completion of the neural circuit, involving also extensive sodium channel formation. These investigations were conveniently performed using a modified plastic culture box in which 24 slice systems can be studied simultaneously.