Reversible and irreversible neuronal damage caused by excitatory amino acid analogues in rat cerebellar slices

Receptor-linked ionic channels mediate N-methyl-D-aspartate neurotoxicity in rat cerebellar slices

In young rat cerebellar slices, histological methods showed that the neurotoxic potency of N-methyl-D-aspartate (NMDA) towards granule cells and intracerebellar nucleus neurons was increased 2- to 3-fold on removal of Mg ions, which have a blocking effect on NMDA-activated ion channels. The depolarizing potency of NMDA on granule cells, recorded using a gap method, was similarly enhanced whereas that of kainate, a non-NMDA receptor agonist, was unchanged. The neurotoxic potency of kainate (towards Golgi cells) was also unaltered by removal of Mg2+. In Mg2+-containing medium, neuronal depolarization induced either by kainate or by high K+ potentiated NMDA toxicity, apparently by reducing the channel block by Mg2+. The results strongly support the hypothesis that excessive Ca2+ influx through NMDA/Mg2+-gated ion channels mediates NMDA toxicity. They also have clear implications regarding the likely mechanism of toxicity of agonists, such as glutamate, able to activate both NMDA and non-NMDA receptors.

Pharmacological protection against the toxicity of N-methyl-D-aspartate in immature rat cerebellar slices

In order to delineate the pharmacological characteristics of the toxicity of N-methyl-D-aspartate (NMDA), slices of cerebellum from 7-day old rats were incubated with NMDA, together with various putative protective agents. These comprised three different groups: (i) a competitive receptor antagonist (kynurenic acid), (ii) direct (cobalt ions, flunarizine) and indirect (taurine) calcium entry blockers, (iii) cyclo-oxygenase inhibitors (indomethacin and acetylsalicylic acid) and a blocker of calcium-activated, neutral proteases (leupeptin). When the slices were incubated for 30 min in medium containing 100 microM NMDA, postmigratory granule cell nuclei were rounded and swollen. After 90 min of recovery in normal medium, the nuclei were pyknotic and the cells were irreversibly injured. As expected, these changes were completely blocked by kynurenate, indicating that NMDA receptors mediate the cell death. Cobalt ions abolished the acute toxicity of NMDA, but after recovery, some granule cell nuclei were swollen. This effect could be attributed to the toxicity of cobalt ions and not to delayed toxicity of NMDA. The other inhibitors of the uptake of calcium, flunarizine and taurine, did neither affect acute nor persistent toxicity of NMDA. These results support the previous finding that the toxicity of NMDA is calcium-dependent and that organic calcium channel blockers are ineffective against NMDA-induced uptake of calcium. Leupeptin had no effect on the toxicity of NMDA, suggesting that calcium-activated proteolysis was not the crucial event in excitotoxic necrosis. Indomethacin, but not acetylsalicylic acid, prevented neuronal degeneration provoked by NMDA, but only in very large concentrations (greater than or equal to 100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Selective loss of Purkinje and granule cell responsiveness to N-methyl-D-aspartate in rat cerebellum during development

Depolarizing responses of Purkinje and granule cells to excitatory amino acid receptor agonists were recorded from rat cerebellar slices at various stages of postnatal maturation using a gap technique. No major developmental changes in relative potency or efficacy of kainate and quisqualate were observed. However, Purkinje and granule neurones both became less responsive to N-methyl-D-aspartate (NMDA) with age, most dramatically so between 14 and 21 days. This transient chemosensitivity to NMDA may reflect a special role of the NMDA receptor system in cerebellar development.

Neurotoxicity of calcium ionophore A23187 in immature rat cerebellar slices

The causal role of Ca2+ in neuronal necrosis is controversial and it has been suggested that neuronal Ca2+ uptake is only a secondary effect to cell death. Here, I address this question directly by studying the morphological effects of calcium ionophore A23187 on immature cerebellar slices. Parasagittal slices were prepared and incubated for 30, 90 or 120 min in physiological saline with or without A23187. In some cases Ca2+ was omitted from the incubation medium. Slices were processed for light microscopy. A23187 produced nuclear changes indicative of cell death that encompassed cells of the external granule cell layer at short incubation times (30 min) and more deeply situated cells at longer times (120 min). This indicates that A23187 diffusion is limited in the slice. The histological changes produced by 30 min exposure to the ionophore could not be reversed by incubation for 90 min in normal medium. Necrosis was never observed when slices were exposed to A23187 in Ca2+-free medium. The results demonstrate that influx of excessive amounts of Ca2+ kills cells of the central nervous system.

Quinolinate mimics neurotoxic actions of N-methyl-D-aspartate in rat cerebellar slices

Incubation of slices of immature rat cerebellum for 30 min with quinolinate (QUIN), an endogenous neurotoxin, resulted in the selective necrosis of granule cells and intracerebellar nucleus neurones. Concentration of QUIN in the millimolar range were needed for these effects. The same neuronal populations were also selectively killed by N-methyl-D-aspartate (NMDA) but the toxic potency of NMDA was 40-fold higher than that of QUIN. Depolarizing responses of granule cells to brief applications of QUIN and NMDA were recorded using a gap method. Dose-response curves to the two compounds appeared parallel but NMDA was 30-fold more potent than QUIN. The depolarizing and toxic actions of QUIN and NMDA were inhibited by the NMDA antagonist, 2-amino-5-phosphonopentanoate. We conclude that the selective toxicity of QUIN in this tissue arises from its activity on NMDA receptors.

Cellular origins of cyclic GMP responses to excitatory amino acid receptor agonists in rat cerebellum in vitro

Incubated slices and freshly dissociated cells from 8-day-old rat cerebellum were used to try to identify the cells that participate in the large increases in cyclic GMP levels that follow activation of excitatory amino acid receptors in this tissue. In the slices, cyclic GMP responses to L-glutamate and related excitants were unaffected by tetrodotoxin and could be replicated by the guanylate cyclase activator nitroprusside. Nitroprusside and the receptor agonists appeared to activate the same pool of the enzyme. Prior destruction of neuroblasts, deep nuclei, or Golgi neurones did not cause loss of responses to L-glutamate. If granule cells were rendered necrotic, however, the cyclic GMP responses to all excitants tested were reduced by greater than or equal to 90%. Substantial losses of responses to veratridine and high K+ levels also occurred, but the nitroprusside-induced elevations were unaffected. In dissociated cell suspensions, the magnitude of responses to receptor agonists, but not those to nitroprusside, was markedly dependent on cell concentration. Responses to L-glutamate were the same in cell suspensions that were Purkinje cell depleted and Purkinje cell enriched. It is concluded that granule cells are primarily involved in the cyclic GMP responses to excitatory amino acids but that the cyclic GMP accumulations occur elsewhere, probably in glial cells.

Proliferative activity in incubated slices of immature rat cerebellum

Slices of 8-day-old rat cerebella were prepared with a tissue chopper, incubated in an oxygenated Krebs-Henseleit solution followed by processing for histology. One micrometer thick sections of resin-embedded slices were stained with Toluidine blue. The external granular layer was shown to maintain its normal proliferative activity up to 3 h of incubation as suggested by the presence of all mitotic phases within this period and by a roughly 50% increase in mitotic index upon a 90-min exposure of incubated slices to 20 microM vinblastine. On this basis the incubated slice of the immature rat cerebellum may serve as a simple tool for the rapid histological testing of short-term mitogenic or cytostatic effects and the vulnerability of a brain germinal layer.

Amino acid neurotoxicity: intracellular sites of calcium accumulation associated with the onset of irreversible damage to rat cerebellar neurones in vitro

Electron microscopy and the combined oxalate-pyroantimonate technique were used to locate calcium in intracerebellar nucleus neurones of rat cerebellar slices subjected to a neurotoxic concentration of N-methyl-D-aspartate. After a sub-lethal exposure period (5 min) calcium pyroantimonate deposits were found in swollen cisterns of the Golgi apparatus and, in lesser amounts, in the nuclei. Deposits were more prominent in the nuclei after a just-lethal exposure (10 min) when they were additionally observed within a population of swollen mitochondria and also apparently free in the dendritic and somatic cytoplasm. The results support the proposal that amino acid neurotoxicity is a consequence of an intracellular Ca2+ overload brought about by excessive Ca2+ influx.

Ionic requirements for neurotoxic effects of excitatory amino acid analogues in rat cerebellar slices

The ionic requirements for the neurotoxic effects of N-methyl-D-aspartate and kainate in incubated slices of developing rat cerebellum were studied using light and electron microscopy. Under normal conditions, 30 min exposure to 100 microM N-methyl-D-aspartate followed by a 90 min recovery period in agonist-free medium resulted in the necrosis of differentiating granule cells and deep nuclear neurons, while the corresponding effect of 100 microM kainate was the death of Golgi cells. Substitution of 96% of the Cl- in the medium with isethionate did not prevent the toxicity of either agonist. However, all the ordinarily vulnerable cells survived and exhibited normal ultrastructure if the slices were exposed to the excitants in a Ca2+-free medium and were subsequently allowed to recover in a Ca2+-containing solution. Prior to this recovery period, granule, Golgi and deep nuclear neurons exposed to N-methyl-D-aspartate were markedly swollen but their mitochondria were hypercontracted and there was no clumping of chromatin or obvious swelling of the rough endoplasmic reticulum or Golgi apparatus, in contrast to observations made on slices exposed to this agonist in normal medium. Substitution of all the Na+ in the medium with a mixture of choline (118 mM) and Tris (25 mM) itself caused necrosis of granule cells and deep nuclear neurons and an intense microvacuolation of Purkinje cells, due, in large part, to high amplitude mitochondrial swelling. A low (25 mM) Na+ medium was well tolerated under control conditions. This medium protected granule cells but not deep nuclear neurons from the toxicity of N-methyl-D-aspartate and failed to prevent kainate-induced death of Golgi cells. It is concluded that the acute neurotoxic effects of the two excitatory amino acid receptor agonists in the slices are dependent on extracellular Ca2+ and are independent of extracellular Cl-. Where apparent, the protective effect of reducing extracellular Na+ on the toxicity of N-methyl-D-aspartate is likely to reflect the involvement of this ion in the primary depolarizing mechanism.

Amino acid neurotoxicity: relationship to neuronal depolarization in rat cerebellar slices

It has long been proposed that the excitatory and toxic properties of acidic amino acid receptor agonists are linked. To test this hypothesis, the depolarizing effects of quisqualate, kainate and N-methyl-D-aspartate in adult and immature rat cerebellar slices have been studied in relation to their neurotoxic effects in the same tissues (reported separately). A "grease-gap" method was used to measure the depolarizing responses of Purkinje cells and granule cells in lobule VI to the agonists. The depolarizing potencies of kainate and quisqualate were apparently similar on both cell types and at both ages studied although maximal responses to kainate were always larger. N-Methyl-D-aspartate was a very weak agonist in the adult slices but was much more effective in the immature tissues, apparently on both Purkinje cells and granule cells. Comparison of the depolarizing effects of the agonists with their neurotoxic effects on Purkinje cells and granule cells suggested that: (a) the ability to depolarize is a required condition for an agonist to be neurotoxic, (b) the magnitude of depolarization, rather than depolarizing potency, is the more pertinent determinant of neurotoxic potency and (c) resistance to the neurotoxicity of an agonist is not necessarily associated with resistance to its depolarizing actions. Histological studies indicated that the neurotoxicity of N-methyl-D-aspartate and kainate in immature cerebellar slices could largely not be replicated by veratridine (50 microM) or high extracellular K+ (124 mM) indicating that receptor-mediated ionic fluxes may be needed in addition to those caused by depolarization. Exposure of the slices to anoxia in the absence of glucose partially reproduced the toxicity of the receptor agonists. Application of ouabain for 30 min caused necrosis of all the cells which are vulnerable to the agonists but spared the cells which are not vulnerable. Profound ionic imbalance thus appears to be a sufficient explanation for amino acid neurotoxicity.

Neurotoxicity of excitatory amino acid receptor agonists in rat cerebellar slices: dependence on calcium concentration

In slices of developing rat cerebellum, a 30-min application of the excitatory amino acid receptor agonist, N-methyl-D-aspartate (NMDA), led to the necrosis of differentiating granule cells and deep nuclear neurones. The corresponding effect of another agonist, kainate, was the death of Golgi cells. The toxic effects of both agonists were prevented if the concentration of calcium in the exposing solution was reduced to 0.3 mM from the control level of 2.5 mM. A lesser reduction (to 1 mM) was enough to prevent 90% of the NMDA-induced necrosis of granule cells. The results indicate that an important component of the acute neurotoxic effects of excitatory amino acids is calcium-dependent and suggest reasons why this may not have been revealed in some previous studies.