High affinity [3H]glutamate uptake systems in normal and audiogenic seizure-susceptible mice

Plasticity of excitatory amino acid transporters in experimental epilepsy

PURPOSE

To examine the relationship between seizures and excitatory amino acid transporter (EAAT) activity and whether up-regulation of EAAT activity alters epileptogenicity.

METHODS

In this study, we exposed rat hippocampal slices to different convulsants before measuring EAAT activity. Rats were exposed to the EAAT inhibitor pyrrolidine-2,4-dicarboxylic acid (PDC) before entorhinal cortex/hippocampal slices were obtained. These slices were exposed to low-Mg2+ buffer while electrophysiological recordings were obtained from the entorhinal cortex. mGluR III acting agents were used to study whether activation of mGluR III could regulate EAAT activity and if this regulation could overcome the effects on EAAT activity induced by the convulsants.

RESULTS

Veratridine, kainic acid (KA), and pilocarpine reduced EAAT activity in rat hippocampal slices. L-2-Amino-4-phosphonobutyric acid (an mGluR III agonist) restored EAAT activity and reduced epileptiform activity to near control levels. The saturation curve for glutamate uptake in slices from KA-seized rats killed 2 hours after the first forelimb clonus was displaced to the left, suggesting a compensatory change for the enhanced excitation. On the other hand, rats injected with the EAAT inhibitor PDC (by intracerebroventricular injection) had more severe KA-induced seizures and N-methyl-D-aspartate epileptiform activity than control rats. Furthermore, hippocampal slices from KA- or KA+PDC-treated rats exposed to low Mg2+ reduced their firing rate to nearly zero once they returned to normal solution, whereas their control counterparts continued to fire, although at a lower rate.

CONCLUSIONS

These results suggest a significant contribution of EAATs in some experimental epilepsy models and point to their short-term regulation by mGluR III as a possible source of their plasticity.

Deficiency of a protein-repair enzyme results in the accumulation of altered proteins, retardation of growth, and fatal seizures in mice

L-Asparaginyl and L-aspartyl residues in proteins are subject to spontaneous degradation reactions that generate isomerized and racemized aspartyl derivatives. Proteins containing L-isoaspartyl and D-aspartyl residues can have altered structures and diminished biological activity. These residues are recognized by a highly conserved cytosolic enzyme, the protein L-isoaspartate(D-aspartate) O-methyltransferase (EC 2.1.1.77). The enzymatic methyl esterification of these abnormal residues in vitro can lead to their conversion (i.e., repair) to normal L-aspartyl residues and should therefore prevent the accumulation of potentially dysfunctional proteins in vivo as cells and tissues age. Particularly high levels of the repair methyltransferase are present in the brain, although enyzme activity is present in all vertebrate tissues. To define the physiological relevance of this protein-repair pathway and to determine whether deficient protein repair would cause central nervous system dysfunction, we used gene targeting in mouse embryonic stem cells to generate protein L-isoaspartate(D-aspartate) O-methyltransferase-deficient mice. Analyses of tissues from methyltransferase knockout mice revealed a striking accumulation of protein substrates for this enzyme in the cytosolic fraction of brain, heart, liver, and erythrocytes. The knockout mice showed significant growth retardation and succumbed to fatal seizures at an average of 42 days after birth. These results suggest that the ability of mice to repair L-isoaspartyl- and D-aspartyl-containing proteins is essential for normal growth and for normal central nervous system function.

Possible regulation of high-affinity glutamate uptake in synaptosomes of normal and epileptic mice

Glutamate (Glu) uptake is the primary mechanism for its removal from the synapse. In genetic audiogenic seizures (AGS), Glu uptake is elevated prior to the appearance of seizures. Increased Glu uptake is also observed in synaptosomes from normal mice preincubated with lithium or nitroarginine, an NO synthase inhibitor. Pertussis and cholera toxins cause a marked reduction in Glu uptake. In contrast, neither lithium nor nitroarginine affected Glu uptake by synaptosomes from genetic epileptic mice. Arachidonic acid inhibits Glu uptake, whereas synaptosomes from epileptic mouse brain appear to be more sensitive to arachidonic acid as indicated by a shift of the inhibition curve to the left. These observations are indicative of the possible regulation of Glu uptake by second messengers and its alteration in genetic epilepsy.

Altered GABAergic and glutamatergic transmission in audiogenic seizure-susceptible mice

The C57BL/10 SPS/sps mouse mutant are audiogenic seizure-susceptible. The enzymatic activities of glutamate decarboxylase (GAD), GABA aminotransferase (GABA-T), alanine aminotransferase (ALA-T), aspartate aminotransferase (ASP-T), and glutamate dehydrogenase (GDH) of whole brain supernatant are significantly reduced in these epileptic mice. GABA uptake is decreased in cortex, midbrain, and pons medulla. Previous studies showed the presence of two sodium-dependent GLU uptake systems in normal (SPS/SP) mice. Glutamate Umax by System 1 is significantly decreased in these mice, whereas the Umax value for System 2 is significantly increased in the epileptic mice.