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

Excitatory amino acid treatment of the ventromedial globus pallidus enhances dopamine utilization in the prefrontal cortex of the rat via the thalamic mediodorsal nucleus

Infusion of a low dose (5 microM) of the cell-selective chemical excitant quisqualic acid (QUIS) into rostral ventromedial globus pallidus (GP) had no immediate effect on DA utilization (assessed as [DOPAC]:[DA] and [HVA]:[DA] ratios) in either the medial bank of the prefrontal cortex (FCx) or the agranular insular cortex (AgCx). In contrast, a larger dose (630 microM) of another excitant sodium ibotenate (IBO) produced an immediate bilaterally symmetrical increase in both indices of DA utilization in FCx. There was also a marked trend towards a bilateral increase in these indices of DA utilization in AgCx. In order to determine whether these effects on cortical DA utilization are mediated by a direct cortical route or via the thalamic mediodorsal nucleus (lateral division, MDL), infusions of IBO into GP were repeated in animals with a 1-week-old N-methyl-D-aspartate lesion of MDL. The increase in DA utilization of FCx following infusion of IBO into GP was abolished, although the trend towards increased DA utilization in AgCx was still maintained. Since MDL innervates FCx but not AgCx and since we have previously shown that MDL lesions alone have no effect on DA utilization in either cortical region, the present results suggest that the changes in cortical DA utilization are probably mediated via MD. Thus in addition to the well-documented control exerted by the thalamus over brain DA function, this has now been extended in the present study to include GP, which projects both directly and indirectly to the thalamus.

Synaptic activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors in the mossy fibre pathway in adult and immature rat cerebellar slices

The participation of excitatory amino acid receptors in mossy fibre-granule cell synapses in lobule VIa of adult and immature rat cerebellar slices was investigated using an extracellular grease-gap technique. For the immature slices, the age selected (14 days after birth) was one at which the sensitivity of granule cells to exogenous N-methyl-D-aspartate is much higher than in the adult. The principal synaptic potentials observed after low-frequency electrical stimulation of the white matter resembled closely those found to be centred in the granule cell layer in field potential studies in the cat in vivo. They comprised a short latency negative potential, a slow negative wave and, in the adult, a further late negative wave. In the adult, with 1.2 mM Mg2+ in the perfusing solution, none of these potentials was significantly affected by the N-methyl-D-aspartate antagonist, 2-amino-5-phosphonovalerate, but they were all markedly inhibited by the broad spectrum antagonist, kynurenate, and, more potently, by the selective non-N-methyl-D-aspartate receptor blocker, 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline. After removal of Mg2+, which has a blocking action on the ion channels associated with N-methyl-D-aspartate receptors, the size of all the potentials increased. The increase in the short latency potential was insensitive to 2-amino-5-phosphonovalerate but a component of the slow negative wave (and of the late negative wave) was reduced back to control levels by the antagonist. Application of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (10 microM) in Mg2+-free solution revealed, in near isolation, a slow wave (latency to peak, 28 ms) which could be abolished by 2-amino-5-phosphonovalerate. In the immature slices, bathed in normal (Mg2+-containing) medium, 2-amino-5-phosphonovalerate caused a small reduction in the short latency potential and inhibited a component of the slow negative wave which could, again, be observed in relative isolation after perfusion of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline. Removal of Mg2+ increased the amplitudes of the short latency potential and the slow negative wave in a manner which was sensitive to 2-amino-5-phosphonovalerate and increased the size of the slow, 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline-resistant wave. It is concluded that glutamate is likely to be the transmitter released by mossy fibres, at least those innervating lobule VIa.(ABSTRACT TRUNCATED AT 400 WORDS)

A quantitative study of the actions of excitatory amino acids and antagonists in rat hippocampal slices

1. A quantitative pharmacological investigation of the actions of excitatory amino acids on hippocampal CA1 neurones has been made using a new slice preparation developed for grease gap recording; d.c. potential was measured across a grease barrier placed between alvear fibres and the bathing medium. 2. In Mg2+-free perfusate, N-methyl-D-aspartate (NMDA, 1-100 microM), quisqualate (1-500 microM), kainate (1-200 microM) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1-100 microM) caused dose-dependent depolarizations. 3. The dose-response relationships were fitted to logistic expressions. The maximum responses to AMPA, NMDA and kainate were similar; their respective EC50 values were 5, 13 and 23 microM. Quisqualate had a smaller maximum; its EC50 value was 10 microM. The slopes of the dose-response relationships were different for the 4 agonists; the order of steepness of the slopes was NMDA greater than AMPA greater than kainate greater than quisqualate. 4. Similar amino acid-induced depolarizations were observed in slices of just the CA1 region or in whole slices bathed in tetrodotoxin. Isolated alvear fibres, however, were insensitive to the excitatory amino acids. 5. D-2-Amino-5-phosphonovalerate (APV, 50 microM) selectively and reversibly antagonized responses induced by NMDA (apparent pA2 = 5.21). 6. Kynurenic acid (1 mM) reversibly depressed responses to the three agonists tested. The dose-ratios for antagonism of AMPA, kainate and quisqualate were 6.9, 5.6 and 4.6 respectively. 7. This preparation has a different sensitivity profile to agonists from those of previously reported preparations of spinal cord, neocortex and cerebellum. The greater sensitivity to NMDA may be due to the higher density of NMDA receptors in the hippocampus. The effects of the antagonists, APV and kynurenate, are similar to those found in other brain areas.

N-methyl-D-aspartate raises cytosolic calcium concentration in rat cerebellar granule cells in culture

The effects of glutamate receptor agonists on cytosolic calcium concentration in freshly isolated cerebellar granule cells and cells maintained in short term culture were determined using the fluorescent indicator fura-2. In freshly isolated cells depolarisation with high-K+ or addition of N-methyl-D-aspartate (NMDA) had no or only small effects on cytosolic calcium. After cells had been grown in monolayer culture for 20 h marked increases in cytosolic calcium concentration in response to 25 mm K+ or NMDA were detected. Other glutamate agonists had no (quisqualate) or small (kainate) effects on cytosolic calcium concentration. NMDA elevated cytosolic calcium concentration in the absence or presence of Mg2+ both before and after depolarisation by high K+. The results suggest that the effect of NMDA on granule cell differentiation and survival could be mediated by elevation of cytosolic calcium levels.

Properties of quisqualate-sensitive L-[3H]glutamate binding sites in rat brain as determined by quantitative autoradiography

Quisqualate, a glutamate analogue, displaced L-[3H]glutamate binding in a biphasic manner, corresponding to "high-affinity" and "low-affinity" binding sites. High-affinity quisqualate sites were termed "quisqualate-sensitive L-[3H]glutamate" binding sites. Quisqualate-sensitive L-[3H]glutamate binding was regionally distributed, with the highest levels present in the cerebellar molecular layer. This binding was stimulated by millimolar concentrations of chloride and calcium. The stimulatory effects of calcium required the presence of chloride ions, whereas chloride's stimulatory effects did not require calcium. All of the L-[3H]glutamate binding stimulated by chloride/calcium was quisqualate sensitive and only weakly displaced by N-methyl-D-aspartate, L-aspartate, or kainate. At high concentrations (1 mM), the anion blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid both reduced, by 41 and 43%, respectively, the stimulatory effects of chloride. At concentrations of 100 microM, kynurenate, L-aspartate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and L-2-amino-4-phosphonobutyric acid (L-APB) failed to displace quisqualate-sensitive L-[3H]glutamate binding in the cerebellar molecular layer. In the presence of KSCN, however, 100 microM AMPA displaced 44% of binding. Quisqualate-sensitive L-[3H]glutamate binding was not sensitive to freezing, and, in contrast to other chloride- and calcium-dependent L-[3H]glutamate binding sites that have been reported, quisqualate-sensitive binding observed by autoradiography was enhanced at 4 degrees C compared with 37 degrees C. Quisqualate-sensitive L-[3H]glutamate binding likely represents binding to the subclass of postsynaptic neuronal glutamate receptors known as quisqualate receptors, rather than binding to previously described APB receptors, chloride-driven sequestration into vesicles, or binding to astrocytic membrane binding sites.

Excitatory amino acid neurotoxicity in the hippocampal slice preparation

The effect of sustained activation of excitatory amino acid receptors on neuronal survival was studied using slices of adult rat hippocampus and light and electron microscopy. Kainate, N-methyl-D-aspartate, quisqualate, and ibotenate all produce signs of severe neurotoxicity within 90 min. Neuronal damage occurs in the form of perikaryal and dendritic swelling, cytoplasmic and nucleoplasmic disintegration, and plasma and nuclear membrane ruffling and collapse. The toxicity is restricted to intrinsic neuronal somata, dendrites and spines, while afferent axons, boutons and glia are spared. Although damage is generally distributed throughout all areas of hippocampus, kainate has little effect on pyramidal neurons in the CA2 region. Quantitative analysis of neuronal survival indicates that agonists induce dose-dependent damage over concentration ranges known to be excitatory. Based on selective antagonism by DL-aminophosphonoheptanoate and the patterns of damage produced by each, N-methyl-D-aspartate, kainate, and quisqualate trigger neurotoxicity by acting on distinct receptor classes. It is concluded that, in hippocampal slices, excitatory amino acids induce neurotoxicity in a similar manner to their actions in vivo. The results support the hypothesis that hippocampal neurotoxicity is initiated by excessive excitation, and provide another example of the capacity of adult hippocampal neurons for rapid structural modification.

Noise and single channels activated by excitatory amino acids in rat cerebellar granule neurones

1. Glutamate-receptor ion channels in rat cerebellar granule cells maintained in explant cultures have been investigated with patch-clamp methods. Properties of these channels were determined from noise analysis of whole-cell currents and from noise and single-channel currents recorded in outside-out membrane patches. 2. Glutamate (10-20 microM) evoked two types of response. Some granule cells gave small inward currents accompanied by clear increases in current noise ('large noise' responses), whereas other cells gave larger inward currents and small noise increases ('small noise' responses). 3. A mean single-channel conductance (gamma) of 46.6 pS was estimated for glutamate from four 'large noise' cells. A mean gamma value of 8.4 pS was estimated for seven other 'large noise' cells. The results suggest that in these latter cells glutamate activated both large (approximately equal to 50 pS) and small conductance (approximately equal to 140 fS) channels. 4. Applications of aspartate (10-30 microM) or N-methyl-D-aspartate (NMDA, 10-30 microM) produced small inward currents and large increases in noise; gamma noise = 48.5 pS (aspartate) and 46.7 pS (NMDA). 5. Large single-channel currents were evoked by glutamate, aspartate and NMDA in outside-out patches. The mean conductance values obtained for the largest amplitude openings were: gamma(glutamate) = 49.5 pS, gamma(aspartate) = 51.5 pS, and gamma(NMDA) = 53.0 pS. For each agonist, these 50 pS openings comprised 75-85% of the completely resolved currents in each patch. Openings to 40 and 30 pS conductance levels accounted for 10-15% and 3-7% of the total, and the presence of apparently direct transitions between these levels and the 50 pS level suggests they are sublevels of the same multi-conductance channels. 6. A mean channel conductance of 22.9 pS was estimated from noise evoked by quisqualate (10-30 microM). Single-channel currents were examined in four patches. In two, quisqualate evoked predominantly small currents of two amplitudes, gamma = 8.4 pS and 16.5 pS; some 50 pS openings were also present. In the other two patches, most openings were 50 pS events. 7. Granule cells gave inward currents to kainate (10-30 microM), and a mean conductance of 3.1 pS was estimated from kainate noise. In patches in which aspartate or NMDA produced mainly 50 pS openings, more than 74% of the single-channel currents evoked by kainate were of smaller amplitude, with mean conductances of gamma = 8.1 and 15.1 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Patch-clamp recording of amino acid-activated responses in "organotypic" slice cultures

Patch-clamp recording techniques were used to study the properties of amino acid-activated channels in cultured "organotypic" slices from rat cerebellum and hippocampus. Hippocampal pyramidal cells responded to the three main glutamatergic agonists, N-methyl-D-aspartate (N-Me-D-Asp), quisqualate, and kainate, whereas Purkinje cells responded only to quisqualate and kainate. Analysis of single-channel events recorded in outside-out patches from hippocampal neurons showed large conductance events (50 pS), which occurred more frequently in the presence of glycine. These events could be produced by N-Me-D-Asp and also, at low frequency, by quisqualate. On the other hand, 50-pS events were never observed in Purkinje neurons. This supports the hypothesis that N-Me-D-Asp and "non-N-Me-D-Asp" receptors are distinct molecular entities. Comparison of whole-cell and outside-out patch recordings from Purkinje cells revealed a clear spatial segregation of gamma-aminobutyric acid (GABA) and glutamate receptors: although GABA receptors are found at high density in somatic membrane, quisqualate and kainate receptors are mostly extrasomatic. The results show that organotypic slice cultures are amenable to patch-clamp methods. They also show that, in these cultures, amino acids receptors have specific distribution patterns according to cell type and to region within a cell.

The cellular neurobiology of neuronal development: the cerebellar granule cell

Cerebellar granule cells in vivo and in vitro have been widely used in the study of the cellular neurobiology of neuronal development. We have described the basic neuroanatomical data on the granule cell in the developing and mature cerebellum. The importance of the cytoskeleton in determining the morphology of the granule cell and in process outgrowth and cell migration has been described. Extensive information is now available on the composition of the granule cell cytoskeleton. Cell surface glycoproteins are thought to be involved in the control of cell adhesion and cellular interactions during development. A number of surface molecules belonging to either the N-CAM or the Ng-CAM groups of glycoproteins have been studied in detail in the cerebellum. The role of these proteins in cell adhesion and in granule cell-astroglial interactions during granule cell migration has been reviewed. The survival and differentiation of neurones is controlled by soluble trophic factors. Several factors have been described which act as trophic factors for granule cells in vitro and may do the same in vivo. The numerous studies that have been carried out on the cerebellar granule cell have allowed us to describe certain aspects of the cellular neurobiology of this class of neurones as an example with general significance for the understanding of neuronal differentiation and function.

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.

Glutamate receptor subtypes in cultured cerebellar neurons: modulation of glutamate and gamma-aminobutyric acid release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.

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.

Glutamate acting on NMDA receptors stimulates neurite outgrowth from cerebellar granule cells

The effect of endogenous glutamate on neurite outgrowth from cerebellar granule cells in culture was examined. Neurite outgrowth was inhibited by enzymatic removal of endogenous glutamate from the culture medium. The broad-spectrum glutamate receptor antagonist kynurenate also inhibited neurite outgrowth from granule cells in serum-containing and serum-free cultures; the inhibition by kynurenate was reversed by exogenous glutamate. Neurite outgrowth was inhibited to the same extent by the NMDA receptor antagonist APV. These results indicate that endogenous glutamate, possibly released by granule cells themselves, stimulated neurite outgrowth through activation of the NMDA class of glutamate receptors. Activation of NMDA receptors on developing neurons may be an important mechanism for the regulation of neuronal growth and differentiation.

Autoradiographic localization of cerebellar excitatory amino acid binding sites in the mouse

We have investigated the cellular localization of cerebellar excitatory amino acid binding sites in normal mice, in mice deficient in granule cells and, perhaps, stellate, basket and Golgi cells (granuloprival mice) and in mice lacking Purkinje cells. In the molecular layer of normal mouse cerebellum, the quisqualate-sensitive binding sites were the predominant type of excitatory amino acid receptor and there were relatively few N-methyl-D-aspartate or kainate-sensitive binding sites. The granule cell layer of normal mice contained a mixture of all 3 types, the N-methyl-D-aspartate-sensitive binding sites being predominant. In the molecular layer of granuloprival mice, the number of quisqualate-sensitive binding sites was increased to 214% of control (P less than 0.01), whereas N-methyl-D-aspartate-sensitive binding sites were decreased to 62% of control (P less than 0.001) and kainate-sensitive binding sites were unchanged. In the granule cell layer of these mice, quisqualate-sensitive binding sites were increased to 200% (P less than 0.01), N-methyl-D-aspartate-sensitive binding sites were decreased to 47% (P less than 0.001) and kainate-sensitive binding sites were decreased to 49% (P less than 0.01 of their respective control values. In the molecular layer of mice lacking Purkinje cells, quisqualate-sensitive binding sites were reduced to 29% (P less than 0.001) of control and N-methyl-D-aspartate-sensitive binding sites were unchanged. In the granule cell layer of these mice, neither quisqualate nor N-methyl-D-aspartate-sensitive binding sites were changed. These results suggest that (1) quisqualate-sensitive binding sites are located principally on dendrites of Purkinje cells and that they up-regulate after deafferentation; (2) N-methyl-D-aspartate-sensitive binding sites are located on granule cells and, perhaps, stellate, basket and Golgi cells, and (3) kainate binding sites are located on cell bodies of granule and, perhaps, Golgi cells.

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.

Extracellular overflow of glutamate, aspartate, GABA and taurine in the cortex and basal ganglia of fetal lambs during hypoxia-ischemia

Extracellular levels of excitatory and inhibitory amino acids were measured in the cortex and striatum of asphyxiated fetal lambs. The fetus was exteriorized from the anesthetized ewe and dialysis probes were placed in the parietal cortex and caudate nucleus. Cerebral blood flow was measured with Xe-clearance. Cortical somatosensory-evoked potentials and electroencephalogram (EEG) were continuously recorded. Asphyxia was induced by clamping the umbilical cord or by graded compression of the maternal aorta. Asphyxia accompanied by elevated cerebral blood flow resulted in a moderate rise in extracellular amino acid levels. During extreme asphyxia, i.e. abolished evoked potentials and reduced cerebral blood flow, marked extracellular elevations of glutamate (3- to 11-fold), aspartate (3- to 7-fold), gamma-aminobutyric acid (GABA) (3- to 5-fold) and taurine (3- to 18-fold) occurred, the higher values representing striatum. Excessive levels of excitatory amino acids may exert injurious effects on immature neurons during such hypoxic-ischemic states.

Functional evaluation of glutamate receptor subtypes in cultured cerebellar neurones and astrocytes

Micromolar concentrations of kainic acid and quisqualic acid released [3H]D-aspartate preaccumulated by cerebellar granule cells in culture. The effect of kainate was selectively antagonized by kynurenic acid and, less effectively, by PDA. Kainate and quisqualate also increased [3H]GABA release from a subpopulation of cultured cerebellar astrocytes. Kynurenic acid selectively blocked the effect of kainic acid. NMDA and no D-aspartate or GABA releasing effect. These results suggest the existence of two different excitatory amino acid sites active on neurotransmitter amino acid release in both cerebellar granule cells and astrocytes.

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.

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.

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

Slice preparations of the developing rat cerebellum were used to investigate the light and electron microscopic correlates of reversible and irreversible neuronal injury caused by the neurotoxic excitatory amino acid receptor agonists, kainate and N-methyl-D-aspartate. The slices were examined after various periods of exposure to the agonists (up to 30 min) with or without a 90 min recovery period in agonist-free medium. N-Methyl-D-aspartate (100 microM) caused necrosis of deep nuclear neurons and differentiating granule cells, the exposure times necessary to induce non-recoverable damage (leading to necrosis), being, respectively, 10 min and 20-30 min. Exposure periods of only 2-4 min with kainate (100 microM) were needed for Golgi cells to subsequently undergo necrosis. Other cell types (Purkinje, granule and deep nuclear neurons) were altered histologically by kainate but most recovered fully from 30 min exposures. Before the recovery period, the worst affected of these cells (deep nuclear neurons) displayed increased cytoplasmic and nuclear electron density and microvacuolation due to swelling of Golgi cisterns but little or no chromatin clumping or mitochondrial expansion. The neurons which were injured irreversibly by the agonists within 30 min displayed, near the time of lethal injury, increased cytoplasmic and nuclear electron lucency, marked focal aggregation of chromatin and swelling of Golgi apparatus. Mitochondrial swelling did not appear to precede lethal injury and even after exposure times sufficient, or more than sufficient, to lead to necrosis, large numbers of mitochondria remained in a condensed configuration. The significance of the histological changes is discussed and they are compared with those occurring in other pathological conditions. The time scales required for the receptor agonists to induce irreversible cellular lesions would be consistent with this being a process which is responsible for acute neuronal necrosis in the brain.

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.

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.

In vitro neurotoxicity of excitatory acid analogues during cerebellar development

The neurotoxic effects of the selective excitatory amino acid receptor agonists, quisqualate, kainate and N-methyl-D-aspartate, were studied in slice preparations of cerebellum from rats at different stages of postnatal development. With increasing age, (i) Purkinje cells became more vulnerable to kainate and quisqualate but remained insensitive to N-methyl-D-aspartate (up to 300 microM); (ii) Golgi cells became more sensitive to kainate, quisqualate and N-methyl-D-aspartate; (iii) granule cells became more sensitive to kainate, less sensitive to N-methyl-D-aspartate and remained unaffected by quisqualate (up to 100 microM), and (iv) basket and stellate cells and, up to 14 days of age, neurones of the deep cerebellar nuclei, became more vulnerable to kainate and quisqualate, but their sensitivity to N-methyl-D-aspartate stayed the same. The neurotoxicity of N-methyl-D-aspartate, but not that of kainate in 8-day-old cerebellar slices was prevented by 2-amino-5-phosphonovaleric acid; tetrodotoxin did not affect the toxicity of the agonists in 8-day-old or adult slices. The results with kainate are consistent with other studies indicating an insensitivity of the immature brain to its neurotoxic effects, but suggest that this property is not a peculiarity of kainate. Alterations in excitatory potency can explain some of the observed developmental changes. However, other observations cannot readily be accounted for on the basis of either changes in excitatory potency, the functional maturation of cerebellar circuits, changes in synaptic density, or the developmental appearance of Ca2+ channels in susceptible cells, suggesting that additional factors play an important role in the neurotoxic effects of the excitants.