Kainic acid stimulation of cerebellar cyclic GMP levels: potentiation by glutamate and related amino acids

Maturational changes in retinal excitatory amino acid receptors

The appearance, kinetics and pharmacological properties of receptors for n-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate (KA), L-glutamate (Glu) and L-aspartate (Asp) was investigated using 3H-ligand binding during the development of chick embryo retina. Receptors for AMPA are maximally concentrated at embryonic day 7 (ED 7) and decline 50% in subsequent days; L-Glu receptors are low until ED 11, and the same is true for Asp and NMDA receptors which increase at ED 14 and 18 respectively. All receptors studied underwent an increase in pharmacological specificity, whereas only AMPA-receptors showed an important change in affinity during ontogeny. Results demonstrate that receptors for excitatory amino acids in the retina suffer maturational changes and suggest that while NMDA and aspartate could interact with the same receptor, AMPA and glutamate seem to bind to different sites.

Excitatory amino acid recognition sites coupled with inositol phospholipid metabolism: developmental changes and interaction with alpha 1-adrenoceptors

Glutamate, aspartate, ibotenate, and quisqualate activate inositol phospholipid hydrolysis in hippocampal slices prepared from brains of 6- to 8-day-old rats. The stimulation by glutamate and aspartate progressively declines during postnatal development and is negligible after the 24th day of life. In contrast, the stimulation of inositol phospholipid hydrolysis by norepinephrine is low in hippocampal slices from newborn animals and increases during development, reaching mature values after the 35th day of life. In adult hippocampal slices, the stimulation of inositol phospholipid hydrolysis elicited by norepinephrine is inhibited by glutamate in a concentration-dependent fashion. This inhibition can also be brought about by aspartate, 2-amino-4-phosphonobutanoate, and L-phosphoserine, a product of endogenous phosphatidylserine hydrolysis.

Coupling of inositol phospholipid metabolism with excitatory amino acid recognition sites in rat hippocampus

Ibotenate, a rigid structural analogue of glutamate, markedly enhances the hydrolysis of membrane inositol phospholipids, as reflected by the stimulation of [3H]inositol monophosphate formation in rat hippocampal slices prelabeled with [3H]inositol and treated with Li+. Quisqualate, homocysteate, L-glutamate, and L-aspartate also induce a significant (albeit weaker) increase in [3H]inositol monophosphate formation, whereas N-methyl-D-aspartate, kainate, quinolinate, and N-acetylaspartylglutamate are inactive. The increase in [3H]inositol monophosphate formation elicited by the above-mentioned excitatory amino acids is potently and selectively antagonized by DL-2-amino-4-phosphonobutyric acid, a dicarboxylic amino acid receptor antagonist. These results suggest that, in the hippocampus, a class of dicarboxylic amino acid recognition sites is coupled with phospholipase C, the enzyme that catalyzes the hydrolysis of membrane inositol phospholipids.

Kainate-glutamate interactions in rat cerebellar slices

The effects of L-glutamate on the potency of kainate for stimulating guanosine 3',5'-cyclic monophosphate (cyclic GMP) accumulation and for killing neurones in incubated slices of immature (8-day) and adult rat cerebellum were investigated. L-glutamate did not potentiate the cyclic GMP responses to kainate in either the adult or the immature tissue (in contrast to a recent report), nor did it alter the pharmacological characteristics of this postsynaptic action of kainate in the immature cerebellum. Slices incubated for 2 h in the presence of L-glutamate displayed pronounced glial swelling and neuronal damage. These effects were concentration-dependent and neurones in the immature cerebellum proved to be about 10 times more susceptible than in the adult. None of the neuronal cell types appeared to be selectively vulnerable to the toxicity of glutamate. At the lower concentrations tested (300 microM in the immature tissue, 3 mM in the adult), neurotoxicity was largely restricted to regions near the cut edges of the slices, indicating that very effective mechanisms limit the diffusion of glutamate within the cerebellum. Kainate caused selective necrosis of Purkinje cells and inhibitory interneurones in slices of adult cerebellum at concentrations between 5 and 20 microM; 30 microM kainate, however, also affected granule cells. The neurotoxic potency of kainate towards all neuronal cell types was significantly lower in the immature cerebellum and was not enhanced by including glutamate in the incubation medium. Similarly, glutamate did not potentiate the neurotoxicity of kainate towards Purkinje cells and inhibitory interneurones or or towards granule cells in adult slices. It is concluded that the availability of glutamate is unlikely to be a factor which limits the neurotoxicity of kainate either in the immature or in the adult cerebellum. The increase in the neurotoxic potency of kainate with cerebellar maturation can be ascribed, more readily, to be developmentally-related appearance of kainate receptors.

Excitatory amino acid receptors and guanosine 3',5'-cyclic monophosphate in incubated slices of immature and adult rat cerebellum

Addition of the excitatory amino acids l-glutamate, l-aspartate and their analogues kainate and N-methyl-d-aspartate to incubated slices of adult rat cerebellum led to large increases in cyclic GMP levels. The order of apparent potencies was kainate greater than N-methyl-d-aspartate greater than glutamate and aspartate. D-alpha-aminoadipate and Mg2-+ inhibited responses to N-methyl-d-aspartate while glutamic acid diethyl ester was most effective against those to glutamate; responses to kainate were least affected by the antagonists. The exicitant amino acids also elicited large elevations of cyclic GMP levels in slices of immature (8 day) cerebellum. Kainate was less effective than in adult but induced two responses distinguishable by their different time courses, concentration dependencies and sensitivity to antagonists. N-methyl-d-aspartate, glutamate and aspartate were 5 to 10-fold more potent than in the adult. Responses to N-methyl-d-aspartate were similarly inhibited by d-alpha-aminoadipate and Mg2+ but those to glutamate were more resistant to glutamic acid diethyl ester than in the adult. It is concluded that the accumulation of cyclic GMP in response to excitant amino acids in the adult cerebellum is mediated via the operation of receptor types showing pharmacological characteristics expected of excitatory amino acid receptors. The actions of kainate in the immature cerebellum appear to be mediated by receptors different from those on which it acts primarily in the adult.