Kainic acid selectively stimulates the release of endogenous excitatory acidic amino acids

Abstract

Kainic acid (KA) exhibits both neuroexcitatory and neurotoxic actions when applied to the brain. Whereas the neuroexcitatory actions are direct, the neurotoxic effects require the integrity of excitatory afferents. In addition, specific receptors for KA have been demonstrated on both pre- and postsynaptic neuronal elements. In the current study, both KA and KCl are shown to stimulate selectively and in a Ca++-dependent manner the efflux of endogenous Glu and Asp from hippocampal, striatal and cerebellar slices incubated in vitro. Unlike the release of other amino acids, the efflux induced by KCl, but not by KA, is attenuated by tetrodotoxin, suggesting that the action of KA is direct. Furthermore, the action of KA is not mimicked by GABAergic, cholinergic, dopaminergic or serotonergic agonists or antagonists, nor by the excitotoxins N-methyl-DL-aspartic acid or ibotenic acid. Both dihydrokainic acid and allokainic acid, which have low affinity for the KA receptor, also fail to stimulate Glu and Asp efflux. In the case of cerebella from 10-day-old rats, which lack a mature parallel fiber system, KA stimulated the efflux of several amino acids including Asp, but not Glu. Similarly, in slices prepared from adult granulo-prival mice, KA caused an efflux of Asp comparable with that observed in intact cerebellum, whereas the release of Glu was reduced by 75%. On the basis of these studies, the primary source of KA-stimulated Glu efflux in the cerebellar slices appears to be the granule cell-parallel fiber system. A hypothesis is proposed that KA stimulates the release of excitatory amino acids, independent of impulse flow, by activating presynaptic receptors located on excitatory nerve terminals. This additional site of action of KA at excitatory afferents may contribute to the potent but uneven neurotoxic effects of this compound.