Mechanisms of AMPA Neurotoxicity in Rat Brain Slices

Attenuation of AMPA-induced neurotoxicity by a calpain inhibitor

The effects of a membrane-permeable inhibitor of calpain, Cbz-Val-Phe-H, were examined in an in vitro model of neurotoxicity. Cerebellar slices from young rats were treated with the glutamate receptor agonist, amino-3-hydroxy-5-methyl-4-isoazole propionic acid (AMPA), and cytotoxicity was quantified using conventional histological techniques. Slices treated with AMPA exhibited damage to 83.0% of cerebellar Purkinje cells. In contrast, only 23.6% of Purkinje cells were damaged in slices treated with Cbz-Val-Phe-H and AMPA. These findings indicate that calcium-activated proteolysis is a critical event in AMPA-induced toxicity, and provide evidence that calpain inhibitors are capable of attenuating this form of excitotoxic damage in the central nervous system.

Excitatory amino acid-mediated cytotoxicity and calcium homeostasis in cultured neurons

A large body of evidence suggests that disturbances of Ca2+ homeostasis may be a causative factor in the neurotoxicity induced by excitatory amino acids (EAAs). The route or routes by which an increase in intracellular calcium concentration ([Ca2+]i) is mediated in vivo are presently not clarified. This may partly reflect the complexity of intact nervous tissue in combination with the relative unspecific action of the available "calcium antagonists," e.g., blockers of voltage-sensitive calcium channels. By using primary cultures of cortical neurons as a model system, it has been found that all EAAs stimulate increases in [Ca2+]i but via different mechanisms. By using the drug dantrolene, it has been shown that 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionate (AMPA) apparently exclusively stimulates Ca2+ influx through agonist-operated calcium channels and voltage-operated calcium channels. Increased [Ca2+]i due to exposure to kainate (KA) is for the major part caused by influx, as in the case of AMPA, but a small part of the increase in [Ca2+]i may be attributed to a release of Ca2+ from intracellular stores. Quisqualate (QA) stimulates Ca2+ release from an intracellular store that is independent of Ca2+ influx; presumably this store is activated by inositol phosphates. The increase in [Ca2+]i due to exposure to glutamate or N-methyl-D-aspartate (NMDA) may be compartmentalized into three components, one of which is related to influx and the other two to Ca2+ release from internal stores. Only one of the latter stores is dependent on Ca2+ influx with regard to release of Ca2+, whereas the other is activated by some other second messengers or, alternatively, directly coupled to the receptor. In muscles dantrolene is known to inhibit Ca2+ release from the sarcoplasmic reticulum, and also in neurons dantrolene inhibits an equivalent release from one or more hitherto unidentified internal Ca2+ pool(s). By using this drug it has been possible to show to what extent these Ca2+ stores are involved in the toxicity observed subsequent to exposure to the EAAs. It turned out that dantrolene, even under conditions allowing Ca2+ influx, inhibited toxicity induced by QA, NMDA, and glutamate, whereas that induced by AMPA or KA was unaffected. In combination with the findings that dantrolene inhibited release from the intracellular stores activated by QA, NMDA, and glutamate, it may be concluded that Ca2+ influx per se is not the primary event causing toxicity following exposure to these EAAs in these neurons. However, it may certainly be involved in the cases of toxicity induced by AMPA and KA.(ABSTRACT TRUNCATED AT 400 WORDS)

Mg2+ antagonizes alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced brain damage and convulsions

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an agonist of the non-N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor, was used to imitate glutamate-induced brain injury. A single intracerebroventricular injection of AMPA (9 nmol; 1.7 micrograms) induced convulsive reactions and heavy neurodegeneration in the hippocampal formation. MgSO4 (600 mg/kg), administered 20 min prior to or simultaneously with AMPA exposure, was able to protect completely against this non-NMDA-induced neurotoxicity. Magnesium is suggested to be a hopeful therapeutic principle for glutamate-mediated brain disorders.

Mechanisms of drug actions against neuronal damage caused by ischemia--an overview

1. Oxygen and energy deficits induces a cascade of pathological processes which lead to neuronal dysfunction and cell death. 2. The pathogenesis of ischemia-induced neuronal damage includes a disturbed calcium homeostasis, an excessive release of EAA and an enhanced formation of free oxygen radicals. 3. Calcium antagonists inhibit Ca2+ influx into the neuronal cell via VSCC. 4. Glutamate antagonists reduce intracellular Ca2+ concentration by inactivation of NMDA receptor-associated calcium channels (NMDA antagonists) or AMPA/quisqualate receptor-linked sodium channels (non-NMDA antagonists). 5. Furthermore, oxygen radical scavengers can avoid neuronal damage. 6. Agonists of the adenosinergic and serotonergic transmitter systems contribute to neuroprotection by hyperpolarization of the neuronal membrane due to an increase of K+ permeability.

Regulatory behaviour, exploration and locomotion following NMDA or 6-OHDA lesions in the rat nucleus accumbens

The effects of bilateral, N-methyl-D-aspartate (NMDA)-induced lesions of the nucleus accumbens (N.Acc.) on regulatory and behavioural responding were studied in rats and compared with the effects of bilateral 6-hydroxydopamine (6-OHDA) lesions. After postoperative body weight, food and water intake had been monitored for a period of 4 weeks, rats were tested in an exploration-choice box. Spontaneous locomotion and the locomotor and stereotypy responses to different doses of dopaminergic agonists were measured subsequently. Detailed assessment of NMDA-induced lesion volumes showed that on average 81.53% of total N.Acc. area was damaged, depending on excitotoxin dose. Tyrosine hydroxylase immunohistochemistry was used to confirm loss of mesolimbic dopamine neurones following 6-OHDA. Analysis of the behavioural data showed that NMDA N.Acc. lesions significantly enhanced exploratory behaviour, spontaneous locomotor activity and the locomotor response to a low dose of D-amphetamine. By comparison, 6-OHDA lesions did not affect exploration and spontaneous locomotion but significantly attenuated the locomotor response to a low dose of D-amphetamine. Regulatory responses were unaffected 28 days after surgery, although NMDA-lesioned rats took longer to recover from postoperative hypodipsia. The results suggest that NMDA N.Acc. lesions induce a deficit in the control of general locomotor output and are consistent with the hypothesis that the N.Acc. functions as an interface between sensory input and locomotor output and that it is needed to channel activity levels appropriately.

The non-NMDA glutamate antagonist NBQX blocks the local hippocampal toxicity of kainic acid, but not the diffuse extrahippocampal damage

The neuronal lesion caused locally by the injection of 0.47 nmol kainic acid into the dorsal hippocampus was greatly reduced by the co-administration of 190 nmol 2,3-dihydro-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX). Protection was particularly marked for the neurons present in the CA3 and dentate hilar regions which are the neurons most vulnerable to kainic acid. On the other hand, systemic administration of NBQX (3 doses of 30 mg/kg i.p.) was completely ineffective in blocking neuronal loss in the CA3 and hilar regions. Furthermore, neither hippocampal nor systemic NBQX could prevent the diffuse neuronal damage to other regions in the limbic system outside of the dorsal hippocampus.