Delayed neurotoxicity of excitatory amino acids in vitro

An investigation of the mechanisms of delayed neurodegeneration caused by direct injection of quinolinate into the rat striatum in vivo

Injection of the N-methyl-D-aspartate receptor agonist quinolinate, or N-methyl-D-aspartate itself, into the rat brain produces neurodegeneration which can be prevented by N-methyl-D-aspartate receptor antagonists administered up to 5 h after excitotoxin injection. The present study was designed to investigate aspects of the mechanisms involved in this delayed form of neurodegeneration. Following its injection into the rat striatum, extracellular levels of [3H]quinolinate were monitored using a microdialysis probe located 1 mm from the site of injection. Peak concentrations were observed 10-20 min after injection and [3H]quinolinate levels decayed in a biexponential fashion, the initial component having an apparent t1/2 of 13.7 +/- 5.2 min (n = 3). Estimations of the extracellular concentrations of quinolinate after an injection of 200 nmol indicated a peak level of 13.7 +/- 6.0 mM (n = 3) at 10-20 min which declined to 1.2 +/- 0.13 mM (n = 3) by 2 h and substantial levels were present up to 5 h, the period over which N-methyl-D-aspartate receptor antagonists are effective in this model. Administration of dizocilpine at 1, 2, 3 or 5 h after injection of 100, 200 or 400 nmol quinolinate resulted in a similar temporal profile of neuroprotection, as assessed by measuring the activities of choline acetyltransferase and glutamate decarboxylase in striatal homogenates, which was independent of the degree of neurodegeneration produced by the different excitotoxin doses. Overall, these results suggest that the neuronal degeneration caused by quinolinate in vivo is critically dependent upon events occurring after the initial peak of excitoxin levels in the extracellular space.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical distribution of calcium-activated neutral proteinases and endogenous CANP inhibitor in the rabbit hippocampus

Intracellular accumulation of Ca2+ after brain ischemia is regarded as one of the principal causes of neuronal death, but details of the intracellular events occurring after Ca2+ accumulation have not yet been described. We propose that a calcium-activated neutral proteinase which can degrade neuronal cytoskeletal proteins might link Ca2+ accumulation and irreversible injury of the neuronal intracellular structure. First, therefore, we examined the distribution of calcium-activated neutral proteinase in normal brains. Immunohistochemical distribution of calcium-activated neutral proteinases (CANP) with high and low sensitivity to Ca2+ (muCANP and mCANP) and of endogenous CANP inhibitor was investigated in the dorsal hippocampus of the rabbit. muCANP-immunoreactivity was detected in almost all of the pyramidal cells and granule cells and in some other neurons. A full-length staining from perikarya to dendrites was shown in muCANP-positive neurons. mCANP-immunoreactivity was found mainly in four kinds of hippocampal interneurons: 1) basket cells in the stratum oriens of Ammon's horn, 2) pyramidal basket cells at the boundary of pyramidal cell layer and stratum oriens, 3) polymorphic cells in the hilar region of dentate gyrus, and 4) pyramidal or fusiform basket cells at the inner boundary of the granule cell layer and the hilar region. The distribution of these four kinds of neurons was similar to that of parvalbumin-containing GABAergic neurons. CANP inhibitor immunoreactivity was confined to pyramidal cells in the CA3-CA3c region and some hilar neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Culture of mature hippocampus slices for 4 days in a newly developed medium: preservation of transmitter release and leucine incorporation into protein

A procedure and a medium for sustaining mature hippocampus slices in vitro for 4 days are described. The ionic composition of the medium, in which the slices were incubated for 1 h of recovery following preparation, strongly affected their ability, 4 days later, to take up and to release D-[3H]aspartate and [14C]GABA. A medium deficient in Na+ and Ca2+ proved best for recovery of the fresh slices prior to transfer to culture medium. The newly developed CSF-like culture medium was the best among several media tested in maintaining the potential of the slices for uptake and for induced release of D-[3H]aspartate and [14C]GABA. Glutamine, present in most culture media, appeared to be particularly toxic. Relative to fresh slices, the slices after 4 days in culture maintained 118% and 97% of the uptake of D-[3H]aspartate and of [14C]GABA respectively. K(+)-induced release of D-[3H]aspartate and of [14C]GABA was 104% and 82% of the respective values in fresh slices. Under the optimal culture conditions worked out, the slices also regained a considerable capacity for incorporation of labelled leucine into protein, which was low in fresh slices.

Effects of lactacidosis on glial cell volume and viability

Effects of severe lactacidosis were analyzed in vitro by employment of C6 glioma cells and astrocytes from primary culture. The cells were suspended in a physiological medium, which was rendered acidotic by addition of lactic acid in rising concentrations. A pH range of 7.4-4.2 was studied under maintenance of isotonicity and a normal electrolyte concentration of the medium. Cell swelling was quantified by flow cytometry using an advanced Coulter system with hydrodynamic focusing. The method was also utilized for assessment of cell viability by exclusion of the fluorescent dye propidium iodide. The volume of C6 glioma cells was found to increase if the pH was titrated to pH 6.8 or below. From this level downward, the extent of cell swelling depended on the degree of acidosis and the duration of exposure. For example, lactacidosis of pH 6.2 for 60 min led to an increase in cell size to 124.5% of normal, while pH 5.0 or 4.2 led to a cell size of 151.1 or 190.9%, respectively. A comparative analysis of the acidosis-induced cell swelling was made by using sulfuric acid. Swelling of C6 glioma at a given pH was only half of what was found when using lactic acid. This indicates specific swelling-inducing properties of lactic acid, while cell viability was not differently affected by both acids. Of the C6 glioma cells, 89.1% were viable under control conditions at pH 7.4. The viability remained unchanged down to pH 6.2. At pH 5.6, viability remained normal for 30 min, but it decreased to 73.4% after 60 min. Further lowering of pH to 5.0 or 4.6 respectively, decreased the number of viable cells to 47.8 or 40.3%. At pH 4.2 only 21.1% of the cells were surviving 1 h of lactacidosis. Cell swelling from lactacidosis could be largely inhibited by replacement of Na+ and bicarbonate ions in the medium by choline chloride and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer, suggesting an involvement of the Na+/H+ and Cl-/HCO3- antiporters in the swelling process. Omission of Na+ and bicarbonate was, however, associated with reduced viability of the glial cells in acidosis. The swelling response of astrocytes obtained from primary culture was similar to that of C6 glioma. Lactic acid was also more effective in inducing cell swelling than sulfuric acid at the same level of acidosis. In astrocytes, viability at, e.g., pH 5.6 appeared to be more affected by lactic than by sulfuric acid.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotoxic effects of excitatory amino acids in the mouse spinal cord: quisqualate and kainate but not N-methyl-D-aspartate induce permanent neural damage

Despite extensive evidence for the neurotoxic effects of excitatory amino acids (EAA) in the brain little is known about their neurotoxic action in the spinal cord. In this study we attempted to produce differential lesions of spinal neurons by pretreating mice, intrathecally, with high concentrations of the EAA: N-methyl-D-aspartate (NMDA), quisqualate and kainate. Pharmacological, behavioral and histological consequences were examined 1, 3, 7 and, in some cases, 30 days after pretreatment. A single, intrathecal, injection of high concentrations of quisqualate and kainate but not NMDA, resulted in damage to spinal cord neurons. The highest concentrations of these agonists produced, in some animals, a massive, non-selective destruction of neurons within the lumbar spinal cord, accompanied by complete paralysis of the hindlimbs. Pretreatment with lower concentrations of intrathecal kainate or quisqualate produced damage to spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal cord to EAA, as seen from the decrease in biting behavior elicited by intrathecal EAA. The neurotoxic effects of quisqualate were completely blocked by the quisqualate/kainate receptor antagonist glutamylaminomethylsulphonate (GAMS), but not the NMDA antagonist 2-amino-5-phosphovalerate. GAMS attenuated the effects of kainate only partially.(ABSTRACT TRUNCATED AT 250 WORDS)

Gangliosides attenuate the delayed neurotoxicity of aspartic acid in vitro

The neurotoxic effects of L-aspartate were evaluated in rat cerebellar granule cell cultures. Acute (15 min) exposure to L-aspartate produced a time-dependent, delayed degeneration of neuronal cell bodies and neurites (LD50 about 40 microM) over 24 h. Aspartate neurotoxicity was prevented by competitive and non-competitive N-methyl-D-aspartate (NMDA) antagonists, but not by non-NMDA antagonists, suggesting a major involvement of NMDA receptors in this neuronal injury. Gangliosides, including GM1, were also effective in attenuating the cytotoxicity of L-aspartate. The neurotoxic potential of L-aspartate may thus contribute to pathologies involving the action of endogenous excitatory amino acids.

N-methyl-D-aspartate receptor activation and Ca2+ account for poor pyramidal cell structure in hippocampal slices

The CA1 pyramidal cells appear damaged in micrographs of guinea pig hippocampal slices incubated in normal physiological buffer at 36-37 degrees C. This is remedied if slices are incubated in modified buffers for the first 45 min. Cell morphology is improved if this buffer is devoid of added Ca2+ and much improved if it contains N-methyl-D-aspartate (NMDA) receptor antagonists or 0 mM Ca2+ and 10 mM Mg2+. The cells then appear similar to CA1 pyramidal cells in situ. These findings support the notion that NMDA receptor activation and Ca2+, acting in the period immediately after slice preparation, permanently damage CA1 pyramidal cells in vitro.

Mechanistic distinctions between excitotoxic and acidotic hippocampal damage in an in vitro model of ischemia

Excitotoxicity is believed to underlie the selective loss of vulnerable neurons after transient ischemia, while lactic acidosis seems to be the principal feature and probable cause of tissue infarcts. Primary hippocampal cultures containing both neurons and astrocytes derived from fetal rats were used to examine the relative contributions of and interactions between excitotoxic and acidotic cell injury. Hypoxia-induced damage was energy dependent and involved the N-methyl-D-aspartate (NMDA) receptor. Glucose above 1 mM could completely protect against hypoxia-induced injury in a pH range of 7.4-6.5, while the NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (500 microM) during the posthypoxic period provided only partial protection in the absence of glucose. Astrocyte cultures were undamaged by ischemic-like treatment in this pH range, suggesting that hypoxia-induced cell death in mixed cultures was restricted to neurons. Lowering the extracellular pH to 7.0 and 6.5 caused no neuronal damage in normoxic controls, but in each case provided significant protection against hypoxic neuronal injury. In contrast, a second type of neurotoxicity was observed after a 6-h exposure to pH 6.0, while exposure to pH 5.5 was required to kill astrocytes. This acidotic damage appeared to be energy independent and did not involve the NMDA receptor. These results suggest that excitotoxic neuron death has an energetic component and that acidosis may produce both protective and damaging effects in the hippocampus during ischemic insults.

Kynurenate does not reduce infarct size after middle cerebral artery occlusion in spontaneously hypertensive rats

The effects of the excitatory amino acid receptor antagonist, kynurenate, were investigated in spontaneously hypertensive rats after middle cerebral artery occlusion. Kynurenate did not significantly modify either the infarct volume, measured 48 h after occlusion, or the neurological score. The absence of a neuroprotective effect of kynurenate in this study, which contrasts with results in normotensive rats, is suggested to be due to impaired collateral circulation in spontaneously hypertensive rats.

Intracellular and extracellular changes of [Ca2+] in hypoxia and ischemia in rat brain in vivo

Changes in intra- and extracellular free calcium concentration were evaluated with ion-selective microelectrodes during periods of anoxia and ischemia in three different regions of intact rat brain. Recordings stable for at least 2 min and in most cases for 4-6 min were chosen for analysis. Under normoxic conditions neuronal [Ca2+]i varied between less than 10(-8) and 10(-7) M from cell to cell but no systematic regional differences were observed. Elimination of O2 or interruption in blood flow caused, within 30-60 s, slight intracellular alkalinization followed by a small rise in [Ca2+]i, a mild degree of hyperpolarization, and disappearance of electrical activity in the cortex, in that order. It is postulated that a decline in cellular energy levels, as manifested by H+ uptake associated with creatine phosphate hydrolysis, leads to an increase in [Ca2+]i, which activates Ca2(+)-dependent K+ channels and consequently enhances gK. 2-4 min later there was a sudden, large rise in [K+]e, a fall in [Ca2+]e and a rapid elevation of [Ca2+]i. The magnitude of the latter was greatest in a high proportion of hippocampal neurons in area CA1 and some cortical cells, while it was smallest and relatively delayed in thalamic neurons. In the hippocampus area CA1 increases in [Ca2+]i to as much as 6-8 x 10(-4) were observed; some of these could be reversed when O2 or blood flow were restored to normal. Pretreatment of animals with ketamine and MK-801, antagonists of excitatory amino acid transmitters, markedly slowed and decreased the rises in [Ca2+]i. The effects of the two agents were most pronounced in the hippocampus. It is concluded that the receptor-operated channels are largely responsible for Ca2+ entry into certain cells during hypoxia/ischemia. This pathway may be of primary importance in parts of the hippocampus and cortex, regions of the brain that are particularly vulnerable to O2 deprivation and which receive high glutamatergic input and have an abundance of excitatory amino acid receptors.

The calcium channel blocker nifedipine attenuates slow excitatory amino acid neurotoxicity

High concentrations of potent N-methyl-D-aspartate (NMDA) agonists can trigger degeneration of cultured mouse cortical neurons after an exposure of only a few minutes; in contrast, selective non-NMDA agonists or low levels of NMDA agonists require exposures of several hours to induce comparable damage. The dihydropyridine calcium channel antagonist nifedipine was used to test whether this slow neurotoxicity is mediated by a calcium influx through voltage-gated channels. Nifedipine had little effect on the widespread neuronal degeneration induced by brief exposure to high concentrations of NMDA but substantially attenuated the neurotoxicity produced by 24-hour exposure to submaximal concentrations of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate, kainate, or quinolinate. Calcium ion influx through dihydropyridine-sensitive, voltage-dependent calcium channels may be an important step in the neuronal injury induced by the prolonged activation of NMDA or non-NMDA glutamate receptors.

Low calcium-induced release of glutamate results in autotoxicity of cerebellar granule cells

Primary cultures of cerebellar rat granule neurons were grown for 18-22 days in vitro in the absence of antibiotics. When the cultures were placed in a low calcium (no EGTA) balanced salt solution at room temperature, rapid cell death occurred usually within 30 min of placing cells in the buffer. Changes in the cells were evident within 10 min and included an apparent cellular granulation with a partial loss of cell body birefringence at 10 x magnification which was complete by 30 min. This rapid death was prevented by (1) replacing chloride in the buffer with acetate; (2) increasing the osmolarity of the buffer by 30% with sucrose; (3) the addition of the selective excitatory amino acid (EAA) antagonist, 2-amino-7-phosphonoheptanoic acid (APH, 200 microM) but not by the selective kainate-quisqualate antagonist, glutamylaminomethylsulfonic acid (GAMS, 400 microM); or (4) the addition of one of the following calcium channel antagonists, verapamil (400 microM) diltiazem (150 microM) or lanthanum (5 microM). Placing cells in low calcium buffer resulted in a 3.7- and 3.2-fold increase in the non-selective secretion of aspartate and glutamate (as well as other amino acids) over baseline secretion (same buffer except containing 2.5 mM calcium). This increase was partially prevented by verapamil, but not by APH or chloride deletion. Verapamil only partially prevented the efflux of glutamate in buffer containing 1 mM EGTA. These results indicate that placing cells in low calcium buffer results in neurotoxicity secondary to both the influx of chloride and water in conjunction with the efflux of amino acids, some of which stimulate an excitatory amino acid receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinolinate and kainate neurotoxicity in neostriatal cultures is potentiated by co-culturing with neocortical neurons

It has been suggested that a disorder in the regulation of excitatory amino acids (EAA) may underlie the loss of neostriatal neurons seen in Huntington's disease. The role of neocortical afferent fibers in determining the EAA sensitivity of neostriatal neurons was assessed by comparing EAA toxicity in co-cultures of neocortex and neostriatum with that of neostriatum alone. In cultures of neostriatum alone, EAAs produced only modest neuronal losses. Kainate, which tended to be the most potent excitotoxin, produced a loss of approximately 30% of the neurons after a 5-min exposure at a 1-mM concentration. In co-cultures, the sensitivity of neostriatal neurons to EAA toxicity was dramatically enhanced; toxicity was increased about two-fold for kainate and quinolinate at millimolar concentrations and as much as 8-fold for quinolinate at micromolar concentrations. The effects of EAA co-incubation with the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid, suggested that the toxic actions of quinolinate, but not kainate, were mediated largely by NMDA receptors.

The calcium channel blocker nifedipine attenuates slow excitatory amino acid neurotoxicity

High concentrations of potent N-methyl-D-aspartate (NMDA) agonists can trigger degeneration of cultured mouse cortical neurons after an exposure of only a few minutes; in contrast, selective non-NMDA agonists or low levels of NMDA agonists require exposures of several hours to induce comparable damage. The dihydropyridine calcium channel antagonist nifedipine was used to test whether this slow neurotoxicity is mediated by a calcium influx through voltage-gated channels. Nifedipine had little effect on the widespread neuronal degeneration induced by brief exposure to high concentrations of NMDA but substantially attenuated the neurotoxicity produced by 24-hour exposure to submaximal concentrations of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate, kainate, or quinolinate. Calcium ion influx through dihydropyridine-sensitive, voltage-dependent calcium channels may be an important step in the neuronal injury induced by the prolonged activation of NMDA or non-NMDA glutamate receptors.

Redox modulation of NMDA receptor-mediated toxicity in mammalian central neurons

Acute neurological injury from hypoxia-ischemia, hypoglycemia, and trauma is thought to be predominantly mediated by activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor in the brain and the subsequent influx of calcium ions through receptor-operated channels. Several chronic degenerative diseases, such as Huntington's disease and the amyotrophic lateral sclerosis-Parkinsonism-dementia complex found on Guam, may share a similar pathogenesis due to a glutamate-like toxin. This laboratory recently reported that exposure to a reducing agent, such as dithiothreitol (DTT), selectively increases ionic current flow through NMDA-activated channels in several types of central neurons; conversely, oxidizing agents reverse this effect. To investigate the novel influence of redox modulation on NMDA neurotoxicity, in the present in vitro study we monitored survival of an identified central neuron, the retinal ganglion cell, approximately 24 h after a brief exposure to DTT. To determine the degree of killing specifically related to activation of the NMDA receptor, 2-amino-5-phosphonovalerate (APV, a selective NMDA antagonist) was added to sibling cultures. APV-preventable, glutamate-induced death was increased 70 +/- 9% with DTT treatment. This effect was totally blocked by the concomitant addition of an oxidizing agent, 5,5-dithiobis-2-nitrobenzoic acid (DTNB). These findings suggest that the enhanced killing following chemical reduction with DTT is mediated at the NMDA receptor site, and that the redox state of the NMDA receptor is crucial for the survival of neurons facing glutamate-related injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum and depolarizing agents cause acute neurotoxicity in cultured cerebellar granule cells: role of the glutamate receptor responsive to N-methyl-D-aspartate

The life span of neonatal rat cerebellar granule cells, grown in basal minimal Eagle's medium containing 10% (vol/vol) fetal calf serum, was extended to 21-30 days by weekly supplementation with glucose. Addition of 1% fetal calf serum to the culture at 14 days killed 85% of the cells within 1 hr. This lethal effect could be prevented by the N-methyl-D-aspartate (NMDA) receptor antagonists dibenzocyclohepteneimine (MK-801) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP). These findings suggested that the glutamate in the serum caused the dramatic neuronal death through action on the NMDA receptor. Indeed, a 5-min incubation in a Locke physiological salt solution containing 20 microM glutamate and 5 microM glycine killed 55-90% of the cells. This acute toxicity could be prevented by a lyso-GM1 ganglioside with N-acetylated sphingosine. The relatively low glutamate content of the sera analyzed suggests that factors in addition to glycine potentiate serum neurotoxicity. The above noted antagonists of the NMDA receptor also greatly reduced the lethal effect of depolarization by 90 mM KCl or 10 microM veratridine. Therefore, it is likely that the toxicity of the depolarizing agents is mediated by glutamate released from the cells. It is concluded that survival of cerebellar neurons in primary culture may be strongly affected by unsuspected neurotoxic phenomena elicited by brief action of a rather low glutamate concentration.

Mild acidosis protects hippocampal neurons from injury induced by oxygen and glucose deprivation

Hippocampal neurons are extremely sensitive to ischemic injury; two plausible mechanisms have been implicated in mediating such damage. The first involves overexposure of neurons to excitatory N-methyl-D-aspartate (NMDA) receptor agonists, which mobilize damaging concentrations of intracellular calcium; the second involves the generation of damaging tissue acidosis. A recent report shows that exposure to pH 6.6 can block NMDA-induced calcium currents in hippocampal neurons. This suggests that moderate acidity might protect against NMDA-mediated neurotoxicity and ischemic injury in vivo. We have observed such projection in vitro using primary hippocampal cultures. At an extracellular pH of 7.4, 6 h of glucose-free anoxia caused delayed and profound damage to neurons which was partially attenuated by the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV). Dropping the pH to 6.5 provided virtually complete protection against this insult. Thus, acidosis need not be viewed exclusively as a damaging component of ischemic insults.

Spinal neuronal pathology associated with continuous intrathecal infusion of N-methyl-D-aspartate in the rat

Continuous intrathecal infusion of N-methyl-D-aspartate (NMDA) at the level of the lumbar enlargement of the spinal cord in middle-aged rats produced dose-dependent toxicity of spinal cord neuronal systems. Toxicity was enhanced by coadministration of glycine, but was significantly reduced when NMDA was co-administered with the competitive inhibitor DL-2-amino-5-phosphovaleric acid or the noncompetitive inhibitor MgSO4. The toxic effects of NMDA were manifest most dramatically and at the lowest concentrations in the neuropil, while neuronal loss was obvious at higher concentrations. The distribution and intensity of reactive astrocytosis was consistent with the known regional and subcellular distribution of NMDA receptors in the spinal cord of rats. The increase in ribosomes and rough endoplasmic reticulum observed in anterior horn cells suggested an increase of cell metabolism reflecting either a nonspecific response to injury or a specific increase in cell metabolism secondary to sustained activation of NMDA receptors. The present studies implicate excitatory amino acid receptors of the NMDA type in producing toxicity to selected neuronal populations of the spinal cord. This model provides a system for studies of the protective effects and rescue of neuronal populations susceptible to the toxic effects of excitatory amino acids.

Neurotoxicity of kainic acid in the rat cochlea during early developmental stages

The neurotoxic effect of kainic acid (KA) was investigated by electron microscopy in rat cochleas at two developmental stages: 17 days of gestation (17 G) and postnatal day 1 (PN 1). In each animal, one cochlea was injected with 1 nmol KA diluted into 2 ml artificial perilymph, while the other cochlea was only injected with artificial perilymph as a control. Ten minutes later, the cochleas were perfused with fixative, removed and processed for electron microscopy. The KA injection resulted in marked swelling of the majority of afferent fibers, i.e. the peripheral processes of spiral ganglion neurons. In the 17 G cochlea, swollen fibers were traced from the perikarya to the undifferentiated otocyst epithelium. Following birth, swollen afferents in the PN 1 cochlea were in contact with both inner (IHCs) and outer hair cells (OHCs), which were now differentiated. At both stages of development, a subclass of small afferent nerves were unaffected. At PN 1, the KA-insensitive afferents only contacted the OHCs. These fibers probably belong to the spiral system of afferents and are related to type II spiral ganglion cells. Conversely, KA-sensitive afferents probably belong to the radial system, related to type I spiral ganglion cells. This system is specific for IHCs in adult cochleas and appears to innervate both IHCs and OHCs at early developmental stages. These findings also indicate that KA neurotoxicity appears very early in the cochlea, at a prenatal time (17 G) before the presynaptic partners of afferent terminals (namely the IHCs) are differentiated.(ABSTRACT TRUNCATED AT 250 WORDS)

Thalamic retrograde degeneration following cortical injury: an excitotoxic process?

Traumatic or stroke-like injuries of the cerebral cortex result in the rapid retrograde degeneration of thalamic relay neurons that project to the damaged area. Although this phenomenon has been well documented, neither the basis for the relay neuron's extreme sensitivity to axotomy nor the mechanisms involved in the degenerative process have been clearly identified. Physiological and biochemical studies of the thalamic response to cortical ablation indicate that pathological overexcitation might contribute to the degenerative process. The responses of thalamic projection neurons, protoplasmic astrocytes, and inhibitory thalamic reticular neurons in adult mice were examined from one to 120 days following ablation of the somatosensory cortex as part of an investigation of the role of excitotoxicity in thalamic retrograde degeneration. The responses of thalamic neurons to cortical ablation were compared with those produced by intracortical injection of the convulsant excitotoxin kainic acid, since the degeneration of neurons in connected brain structures distant to the site of kainic acid injection is also thought to occur via an excitotoxic mechanism. Within two days after either type of cortical injury, protoplasmic astrocytes in affected regions of the thalamic ventrobasal complex and the medial division of the posterior thalamic nuclei became reactive and expressed increased levels of immunohistochemically detectable glial fibrillary acidic protein. Within the affected regions of the ventrobasal complex an increased intensity of puncta positive for glutamate decarboxylase immunoreactivity, presumably due to an increase in its content within the terminals of the reciprocally interconnected thalamic reticular neurons, was also evident. These immunohistochemically detectable alterations in the milieu of the damaged thalamic neurons preceded the disappearance of the affected relay neurons by at least two days following cortical ablation and by seven to 10 days following intracortical kainic acid injection. Regions of the thalamus containing reactive astrocytes corresponded very closely to the regions undergoing retrograde degeneration. Protoplasmic astrocytes in these areas remained intensely reactive up to 60 days after cortical injury. Levels of glutamate decarboxylase were only transiently elevated in the degenerating regions of the ventrobasal complex following cortical ablation and returned to normal by 14 days. Increased glutamate decarboxylase immunoreactivity was transiently seen through the entire ventrobasal complex following intracortical kainic acid injection but was markedly more intense in degenerating regions. These patterns of labeling did not return to normal until 50 days after intracortical kainic acid injection, well after the death of the relay neurons. Cortical ablation and intracortical kainic acid injection produce similar alterations in thalamic neuronal and glial populations.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of excitatory amino acid induced cytotoxicity in cultured neurons

The neurotoxicity of the excitatory amino acids (EAAs) L-glutamate (L-glu), L-aspartate (L-asp), N-methyl-D-aspartate (NMDA), kainate (KA), quisqualate (QA) and RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionate (AMPA) was followed as a function of development in primary cultures of cerebral cortex neurons and cerebellar granule cells. These two types of neurons express, respectively, glutamate receptor subtypes with sensitivity to all of these excitatory amino acids or only to glutamate and aspartate. None of the EAAs were toxic in cerebral cortex neurons at 2 days in culture, whereas at culture day 4 the neurons became sensitive to glutamate, at day 5 to KA followed by sensitivity to QA at day 6, and finally to NMDA, L-asp and AMPA at day 7. The rank order of potency of the EAAs was in cerebral cortex neurons cultured for 12 days: L-asp (ED50 = 0.5 microM) = L-glu (ED50 = 1 microM) greater than AMPA (ED50 = 10 microM) greater than NMDA (ED50 = 65 microM) greater than QA = KA (ED50 = 100 microM). Cerebellar granule cells were insensitive to all of the EAAs at 3 and 5 days in culture but at day 8 the cells became sensitive to toxicity induced by L-glu (ED50 = 70 microM) and L-asp (ED50 = 30 microM). In order to determine ED50 values for L-asp and L-glu accurately, media in these experiments also contained 500 microM of the glutamate uptake inhibitor L-aspartate-beta-hydroxamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreases in excitatory synaptic transmission and increases in recurrent inhibition in the rat dentate gyrus after transient cerebral ischemia

Excitatory transmission along the perforant path from the entorhinal cortex to the granule cells of the dentate gyrus was evaluated two days after 10 min of transient cerebral ischemia in the rat. The amplitude of the population spike, and the amplitude and the initial slope of the population excitatory postsynaptic potential (EPSP) evoked by the perforant path stimulation were measured across a range of stimulus intensities, and were compared with control values. Inhibitory interactions were evaluated using the paradigm of paired pulse stimulation, comparing the amplitude of the population spike evoked by the second pulse of a pair to the initial spike. The maximal values of the initial slope of the population EPSP and the population spike were reduced in the ischemic group. Also, the extent of paired pulse inhibition was greater in the ischemic group. These results suggest that: (1) excitatory synaptic transmission along the perforant path is impaired in the postischemic period, (2) inhibition of the dentate granule cells is enhanced in this period. These results are not consistent with the hypothesis that there is a hyperactivation of the tri-synaptic circuit in the chronic postischemic period that accounts for the excitotoxic death of CA1 neurons.

Monosialoganglioside GM1 protects against anoxia-induced neuronal death in vitro

Excitatory dicarboxylic amino acid neurotransmitters, particularly glutamate, have been implicated in mediating neuronal cell injury in brain ischemia-anoxia, epilepsy, and stroke. Glutamate neurotoxicity has been demonstrated in several in vitro models, as well as its prevention by a variety of agents, including several sialic acid-containing glycosphingolipid species, gangliosides. We have now examined ganglioside effects in anoxic exposed cultures of granule cells from Postnatal Day 8 rat cerebellum. Cells between 10 and 12 days in vitro were placed into an anoxic atmosphere or subjected to a chemical model of anoxia by a pulse exposure to rotenone. Widespread neuronal degeneration of neuronal cell bodies and their associated neurite network was seen the following day. These effects on cell vitality at the morphological level were quantitatively confirmed by measuring the photometric reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide to a blue formazan product. This neuronal injury was abolished by the specific N-methyl-D-aspartate receptor noncompetitive antagonists Mg2+, phencyclidine and MK-801, suggesting that this subtype of glutamate receptor is involved in the pathogenesis of anoxic granule cell injury. Pretreatment for 30 to 60 min or more or concurrent treatment with ganglioside GM1 largely prevented the ensuing neuronal death (ED50 = 25 microM), even 4 days later. Degeneration induced by exogenous glutamate was equally reduced. Asialo GM1 (lacking sialic acid) was ineffective. These results are consistent with the observed beneficial effects of the gangliosides in ischemic brain injury models in vivo.

The olfactory system and Alzheimer's disease

Alzheimer's disease (AD) is considered to be the number one health problem and seems to be reaching epidemic proportion in the USA. The cause of AD is not known, a reliable animal model of the disease has not been found and appropriate treatment of this dementia is wanting. The present review focuses on the possibility that a virus or exogenous toxic materials may gain access to the CNS using the olfactory mucosa as a portal of entry. Anterograde and retrograde transport of the virus/zeolites to olfactory forebrain regions, which receive primary and secondary projections from the main olfactory bulb (MOB) and which, in turn, project centrifugal axons to the MOB, may initiate cell degeneration at such loci. Pathological changes may, thus, be initially confined to projecting and intrinsic neurons localized in cortical and subcortical olfactory structures; arguments are advanced which favor the view that excitotoxic phenomena could be mainly responsible for the overall degenerative picture. Neurotoxic activity may follow infection by the virus itself, be facilitated by loss of GABAergic terminals in olfactory cortex, develop following repeated episodes of physiological long term potentiation (which unmasks NMDA receptors) or be due to excessive release, faculty re-uptake or altered glutamate receptor sensitivity. Furthermore, a reduction in central inhibitory inputs to the MOB might then result in disinhibition of mitral/tufted neurons and enhance the excitotoxic phenomena in the MOB projecting field. Within this context, and in line with recent studies, it is believed that pathology begins at cortical (mainly olfactory) regions, basal forebrain neurons being secondarily affected due to retrograde degeneration. In addition, failure to produce a critical level of neurotrophic factors by a damaged MOB and olfactory cortex, could adversely affect survival of basal cholinergic neurons which innervate both regions. Support for these hypothesis is provided, first, by recent reports on pathological findings in AD brains which seem to involve preferentially the olfactory and entorhinal cortices, the olfactory amygdala and the hippocampus, all of which receive primary or secondary projections from the MOB; secondly, by the presence of severe olfactory deficits in the early stages of the disease, mainly of a discriminatory nature, which points to a malfunction of central olfactory structures.

Halothane anaesthesia reverses the neuroprotective effect of ketamine against ibotenic acid toxicity in the rat hippocampus

The neurodegenerative effect of ibotenic acid injected into the rat hippocampus was unaffected by the anaesthetics halothane or pentobarbital, apart from a trend to an increased toxicity at higher doses of pentobarbital (60-72 mg/kg). Its toxicity was substantially blocked only by high anaesthetic doses of the indirect acting N-methyl-D-aspartic acid antagonist, ketamine (150-180 mg/kg, i.p.). This is in contrast to its previous reported ability to protect at low concentrations in vitro. On the other hand, the protective effect of ketamine was modified under halothane or pentobarbital anaesthesia. Thus, under halothane anaesthesia, ketamine at 60 mg/kg, i.p. caused a large increase in ibotenic acid-induced neuronal death.

Cultured astroglia release a neuronotoxic activity that is not related to the excitotoxins

Neuronal death after brain injury is thought to be in part the result of the activity of the excitotoxins, a family of excitatory amino acids which are released by neurones. We have also described an astroglial cell-derived neuronotoxic activity of low molecular weight whose release can be induced by depolarizing events such as an increase in extracellular potassium concentration. We study here the relationship between this astroglia-derived neuronotoxic activity present in astroglia-conditioned medium (ACM) and the excitotoxins. Using a colorimetric assay of neuronal survival, we show that the ACM neuronotoxic activity, is able to induce the death of all types of neurones tested, including those which are insensitive to excitotoxins. Furthermore, the ACM neuronotoxic activity does not require for its action the extracellular ionic composition which is needed for the activity of excitotoxins. Finally, the ACM neuronotoxic activity is not blocked by competitive or non-competitive antagonists of the various classes of excitotoxin receptors. Those data demonstrate that the astroglia-derived neuronotoxic activity is not related to the excitotoxins. Still, because astrocytes can also be depolarized by members of the excitotoxin family, the possibility exists that the release of astroglia-derived neuronotoxic activity would follow the rise in extracellular excitatory amino acid concentration during nervous system injury.

The role of dihydropyridine-sensitive voltage-gated calcium channels in potassium-mediated neuronal survival

The survival of isolated neurons from chick embryo ciliary, sympathetic, and dorsal root ganglia is greatly enhanced by concentrations of extracellular potassium that significantly depolarize the neurons (ED50 = 20-25 mM). The survival-promoting effect of elevated potassium on each of these 3 types of neurons appears to be the result of the opening of voltage-gated calcium channels. The dihydropyridine, Bay K 8644, which increases calcium influx through L-type voltage-gated calcium channels in neurons, strongly potentiated the survival-promoting action of elevated potassium (ED50 = 10.8 +/- 7.0 nM). In contrast, chemically closely related dihydropyridines, PN200-110 (ED50 = 0.33 +/- 0.15 nM) and nitrendipine (ED50 = 1.3 +/- 0.3 nM), which block calcium influx through the same voltage-gated channels, completely inhibited potassium-mediated neuronal survival. Chemically different agents that also block calcium influx through voltage-gated channels also inhibited potassium-mediated neuronal survival: the phenylalkylamine verapamil (ED50 = 0.78 +/- 0.38 microM), the benzothiazepine diltiazem (ED50 = 1.7 microM), and the inorganic ion cadmium (ED50 = 5.8 microM). These calcium-channel blockers are not simply toxic to neurons, since they did not inhibit neuronal survival mediated by the neurotrophic proteins, nerve growth factor, basic fibroblast growth factor, or ciliary neurotrophic factor, also suggesting that voltage-gated calcium channels are not involved in the action of these factors. These results suggest that neuronal survival in elevated potassium in ciliary, sympathetic, and dorsal root ganglion neurons is the result of calcium influx through dihydropyridine-sensitive, L-type voltage-gated calcium channels. These findings are discussed in relation to the neuronal toxicity of excitatory amino acids which is also thought to occur through increased calcium influx.

Change in calcium permeability caused by quinolinic acid in cultured rat hippocampal neurons

The calcium permeability of receptor channels activated by quinolinic acid (QUIN) in cultured rat hippocampal neurons was investigated using the whole-cell voltage-clamp method. In Na+-free, 10 mM Ca2+ solution with the internal solution containing 165 mM Cs+, QUIN elicited prominent inward currents at -60 mV, and the reversal potential of the QUIN-induced current was -5.8 +/- 1.2 mV, indicating that QUIN-activated channels are highly permeable to Ca2+ (permeability ratio PCa2+/PCs+ = 5.9). This result was substantiated by microfluorometry using fura-2, which revealed that QUIN caused a marked increase in the intracellular Ca2+ concentration even after the voltage-dependent Ca2+ channels had been suppressed by La3+.

Intracellular calcium and neurotoxic events

Calcium is important in many intracellular regulatory processes. However, the maintenance of low levels of this cation within the cytosol is essential for maintenance of cell viability, in view of the large concentration gradient of ionic calcium across the plasma membrane. The expenditure of energy is needed to maintain intracellular calcium concentration [Ca2+]i at normal levels. In addition, the integrity of the limiting membrane is also vital for this function. Thus, any disruption of membrane characteristics or of mitochondrial anabolic processes may lead to deleterious levels of [Ca2+]i. The toxicity of a wide range of unrelated agents may, therefore, be in part due to elevation of cytosolic calcium. This general event may synergize with the more selective harmful properties of a compound, thus adversely affecting cell metabolism. The capacity now exists to measure levels of [Ca2+]i in isolated cells or organelles such as synaptosomes. The use of such in vitro models can be of value in the evaluation of the neurotoxic potential of compounds. This method, in conjunction with the use of pharmacological agents known to act at specific sites, and with the use of radioactive calcium in translocation studies, also has utility in the delineation of the biochemical mode of action of neurotoxic agents.

Excitatory amino acid-induced toxicity in chick retina: amino acid release, histology, and effects of chloride channel blockers

Acute excitotoxicity in embryonic chick retina and the ability of Cl- channel blockers to prevent toxicity were evaluated by measurement of endogenous amino acid release and histology. Treatment of retina with kainate, quisqualate, or N-methyl-D-aspartate resulted in a large dose-dependent release of gamma-aminobutyric acid and taurine, moderate release of glutamine and alanine, and no measurable release of glutamate or aspartate. Concentrations inducing maximal gamma-aminobutyric acid release were 50 microM quisquaalate, 100 microM kainate, and 100 microM N-methyl-D-aspartate. Treatment with 1 mM glutamate resulted in significant gamma-aminobutyric acid release, as well as an elevation in medium aspartate levels. Typical excitotoxic retinal lesions were produced by the agonists and, at the lower concentrations tested, revealed a regional sensitivity. There was a positive correlation between the amount of gamma-aminobutyric acid release and the extent of tissue swelling, suggesting that release may be secondary to toxic cellular events. Omission of Cl- completely blocked cytotoxic effects due to kainate or glutamate. Likewise, addition of the Cl-/bicarbonate anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonate at 600 microM protected retina from cytotoxic damage from all excitotoxic analogs and restored amino acid levels to baseline values. Furosemide, which blocks Na+/K+/2Cl- cotransport, was only minimally effective in reducing amino acid release induced by the agonists. Consistent with the latter, histological examination showed the continued presence of the lesion but with general reduction of cellular edema. These results indicate that although influx of Cl- is a central component of the acute excitotoxic phenomenon, mechanisms other than passive Cl- flux may be involved.

Cerebral excess release of neurotransmitter amino acids subsequent to reduced cerebral glucose metabolism in early-onset dementia of Alzheimer type

A massive cerebral release of amino acids and ammonia was found in early-onset dementia of Alzheimer type. Aspartate and glycine were liberated in high concentrations, whereas glutamate remained rather unchanged. This excess cerebral protein catabolism is due to a 44% reduction in cerebral glucose metabolism. Whereas glutamate and other glucoplastic amino acids may substitute glucose, elevated aspartate may contribute to neuronal damage. The results are discussed with respect to a possible neuronal insulin/insulin receptor deficiency.

Selective neuronal vulnerability and the distribution of N-methyl-D-aspartate (NMDA) receptors

N-Methyl-D-Aspartate (NMDA) receptors are believed to play a critical role in excitotoxic cell death in the CNS. The distribution of NMDA-preferring binding sites is compared here with the patterns of selective neuronal death observed in Alzheimer's disease and following transient ischemia. The distribution of NMDA receptors, by itself, is unable to account for the characteristic patterns of selective neuronal vulnerability observed in conjunction with these types of neuropathology.

Hundred-fold increase in neuronal vulnerability to glutamate toxicity in astrocyte-poor cultures of rat cerebral cortex

In cultures of rat cerebral cortex in which astrocyte proliferation was stringently suppressed, glutamate neurotoxicity occurred at glutamate concentrations similar to those which are normally found in the extracellular space in the hippocampus. Concentrations of glutamate one hundred-fold higher were required to produce neurotoxicity in the presence of abundant astrocytes. This suggests that the sensitivity of central neurons to glutamate toxicity may be dependent upon astrocyte function.

Hippocampal unit activity after transient cerebral ischemia in rats

Single unit activity of CA1 and CA3 neurons in the hippocampus was recorded in rats 1, 2, or 3 days after 10 minutes of transient cerebral ischemia induced by the clamping of both carotid arteries combined with hypotension. In addition, paired pulse inhibition/facilitation of the CA1 population spike was examined on Day 2 using two successive stimuli of the contralateral CA3 region delivered at various intervals. On Day 1, the mean +/- SEM firing rate in the CA1 region was 0.91 +/- 0.42/sec (n = 5), which was not significantly different from the control value of 0.98 +/- 0.26/sec (n = 5). Firing rate increased on Days 2 and 3 to 3.96 +/- 0.69/sec (n = 5), and 6.49 +/- 0.89/sec (n = 5), respectively. In the CA3 region, the mean +/- SEM firing rate of 1.18 +/- 0.27/sec in the five control rats sharply dropped to 0.14 +/- 0.11/sec in the five Day 1 rats and gradually increased to 0.45 +/- 0.11/sec in the five Day 3 rats. Histologic examination of these rats revealed ischemic changes restricted to CA1 neurons on Days 2 and 3. The paired-pulse experiment showed no significant difference between six control and six Day 2 rats in the inhibition of the second population spike with interstimulus intervals of less than 400 msec. At interstimulus intervals of greater than 500 msec there was facilitation of the second spike, which lasted 5 seconds in Day 2 rats. This facilitation was not observed in control rats. Because CA3 neurons constitute the main input to CA1 pyramidal cells, decreased activity of CA3 neurons indicates less excitatory input to CA1 neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitter regulation of neuronal outgrowth, plasticity and survival

Molecules used for communication in mature nervous systems also play important roles in development, maintenance and plasticity of individual neurons. This paper reviews the evidence that neurotransmitters, in addition to their mediation of trans-synaptic information coding, can induce a spectrum of effects on neuronal cytoarchitecture, ranging from neurite sprouting to dendritic pruning and even cell death. Such profound alterations may well constitute a part of the normal functioning and structuring of the nervous system as well as contribute to severe pathological processes.

Aspartate neurotoxicity on cultured cortical neurons

L-aspartate neurotoxicity was quantitatively characterized in murine cortical cell cultures. Five-minute exposure to 30 microM-3 mM L-aspartate resulted in concentration-dependent (ED50 about 190 microM) neuronal destruction over the next 10 hr; glia were not injured. D-aspartate and L-aspartate were roughly equipotent neurotoxins. Ion substitution experiments suggested that L-aspartate neurotoxicity is comprised of both acute, sodium-dependent "excitotoxicity" and delayed, calcium-dependent degeneration, with the latter predominant under conditions of brief exposure. Aspartate neurotoxicity could be attenuated by D-2-amino-5-phosphonovalerate (D-APV), dextrorphan, ketamine, and kynurenate, but not by L-glutamate diethyl ester or gamma-D-glutamylaminomethyl sulfonate, consistent with principal involvement of N-methyl-D-aspartate receptors. D-APV and dextrorphan produced different effects on the aspartate concentration-toxicity relation; the former drug was consistent with a competitive and the latter with a noncompetitive mechanism of antagonism.

Differential dependence on Ca2+ of N-methyl-D-aspartate and quisqualate neurotoxicity in young rat hippocampal slices

Exposure of slices of young (8 days old) rat hippocampus to 100 microM N-methyl-D-aspartate (NMDA) for 20 min followed by 90 min recovery, resulted in widespread, oedematous necrosis of all classes of neurones. The NMDA antagonist, D,L-2-amino-5-phosphonovalerate (APV) or omission of Ca2+ from the exposing solution prevented this cell death, but a large reduction in Cl- was ineffective. Quisqualate (100 microM, 20 min) led to a different pathological pattern characterised most strikingly by large numbers of cells undergoing 'dark cell degeneration'. Numerically, the neurones were affected in the order CA3 greater than CA1 greater than dentate granule cells. Quisqualate toxicity was not prevented by APV nor by reducing Ca2+ or Cl-. It is concluded that, as in cerebellar slices (but unlike in cultures of hippocampal neurones) NMDA toxicity in hippocampal slices is Ca2+-dependent and Cl- -independent. However, quisqualate exerts its pathological effects through a different mechanism. This mechanism may be primarily metabolic rather than ionic.

Calcium dependency of N-methyl-D-aspartate toxicity in slices from the immature rat hippocampus

The literature on ionic requirements for excitotoxicity is largely contradictory. Depending on the experimental paradigms, it has been concluded that either Ca2+ or Na+ and Cl- mediate excitotoxicity. In the present study, the dependence on Ca2+ of N-methyl-D-aspartate-induced damage to neurons in immature rat hippocampal slices was investigated with light microscopy. In addition N-methyl-D-aspartate-induced cell damage was followed by measurement of release of lactate dehydrogenase from slices. When incubated in N-methyl-D-aspartate-containing (100 microM) buffer for 30 min, hippocampal neurons displayed fine chromatin aggregation and swelling of neuronal nuclei and neuropil. Slices incubated in standard medium for 90 min after exposure to N-methyl-D-aspartate contained a large number of neurons that failed to recover from the initial lesion. The acute edema was at least as severe in slices incubated in N-methyl-D-aspartate-containing, Ca2+-free buffer. In contrast, clumping of the chromatin could not be observed. CA1 neurons recovered completely from the acute changes, and granule cells recovered to some extent. While omission of Ca2+ had no obvious morphological effects on the tissue in its own right, the efflux of lactate dehydrogenase was significantly increased after incubation in Ca2+-free medium. Slices exposed to N-methyl-D-aspartate released approximately twice as much lactate dehydrogenase as controls 1-5 h after the exposure, and the same rate of release was seen if Ca2+ was absent during N-methyl-D-aspartate treatment. The morphological results suggest that N-methyl-D-aspartate toxicity is Ca2+-dependent in pyramidal cells whereas the toxicity in granule cells is partly Ca2+-independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Colocalization of taurine- and cysteine sulfinic acid decarboxylase-like immunoreactivity in the hippocampus of the rat

It is proposed that taurine is an inhibitory neurotransmitter/neuromodulator in the CNS. The present study localized taurine-containing neurons within the rat hippocampus with the use of a monoclonal antibody against conjugated taurine (Tau2) in conjunction with an antiserum against cysteine sulfinic acid decarboxylase (CSADC), a synthesizing enzyme for taurine. Taurine-like immunoreactivity Tau-LI) and CSADC-LI were colocalized in neurons of the dentate gyrus, CA1(/CA2), CA3, and CA4. Of all the cells examined, pyramidal basket cells within the granule cell layer of the dentate gyrus were most intensely stained with both Tau2 and CSADC. Granule cells were also double-labeled with Tau-LI and CSADC-LI. Cell nuclei and dendrites in the CA1 region stained more intensely with Tau2 than somata. CSADC-LI was colocalized with Tau-LI within these neurons. Light staining with both Tau2 and the CSADC antiserum was inconsistently present in CA3 and CA4 neurons and was found to be highly dependent on the type of fixation and delay to fixation. Tau-LI was more consistently present in increased numbers of neurons in CA3 when glutaraldehyde was added to the paraformaldehyde fixative solution. Hippocampi which were immersion-fixed in paraformaldehyde following a 0-, 6-, or 24-hour postmortem delay exhibited a lack of Tau2 staining in the CA3 region in the majority of animals studied, similar to some paraformaldehyde perfusion-fixed rats. These studies suggest that taurine was present in the majority of neurons within the major cell layers of the rat hippocampus, but Tau-LI was more easily lost from neurons in the CA3 region following delay to fixation. The localization of Tau-LI in excitatory neurons such as granule cells and pyramidal cells is not consistent with its proposed inhibitory transmitter role. However, the prominent Tau2 staining in dendrites of the CA1 region provides anatomical support for the hypothesis that taurine may be released from dendrites in the CA1 region and may function as a neuromodulator of calcium flux in these pyramidal neurons.

Valproate doubles the anoxic survival time of normal developing mice: possible relevance to valproate-induced decreases in cerebral levels of glutamate and aspartate, and increases in taurine

We have previously reported that chronic administration of valproate in developing mice decreased brain aspartic and glutamic acid levels and increased the brain taurine content. The direction of the valproate-induced changes in the cerebral levels of these neurotransmitter amino acids - excitatory in the case of aspartate and glutamate, inhibitory in the case of taurine - appeared relevant to the mechanism of its anticonvulsant action. Since the neuropathology of hypoxia-ischemia also appears to be mediated by release of glutamate/aspartate at the synapse, the valproate-induced reduction of the levels of these neuroexcitatory/neurotoxic amino acids suggested that valproate might increase the tolerance of young mice to anoxia. A doubling of the length of survival of the intact animal in an atmosphere of pure nitrogen gas and a three-fold increase in the duration of respiratory activity (gasping) of the isolated head after chronic administration of valproate support the speculation.

Glial uptake of excitatory amino acids influences neuronal survival in cultures of mouse hippocampus

Several reports have described apparently normal survival and development of hippocampal and spinal cord culture preparations grown in Ham's F-12 medium, which contains 100 microM each of L-glutamate and L-aspartate. As at this concentration these amino acids are neurotoxic, some adaptive mechanism must occur to allow neuronal survival. We have investigated the mechanism underlying such adaptation. Dissociated cultures of mouse hippocampal neurons were grown in either Eagle's minimum essential medium or Ham's F-12 medium, supplemented with 5% horse serum. Analysis of neuronal density in cultures stained for neuron-specific enolase showed that although large numbers of neurons were present in mature cultures grown in either medium, neuronal survival in cultures grown continuously in F-12 was reduced to 41% compared to controls grown in Eagle's minimum essential medium. Physiological studies showed that those neurons which survived in F-12 did not lose their sensitivity to excitatory amino acids. In addition, the acute application of fresh, serum-free F-12 to 10-14-day-old cultures grown in either minimum essential medium or F-12 was highly neurotoxic. Three lines of evidence suggest that glial uptake of amino acids, and reduction of the extracellular concentration of glutamate and aspartate below neurotoxic levels, rather than receptor desensitization underlies the adaptive mechanism allowing neuronal survival. First, application of fresh F-12 produced large depolarizations, and profound neuronal swelling in cultures grown in F-12; however, after several hours swelling reversed suggesting a slow onset of the adaptive process. Second, pressure application of conditioned F-12 obtained from sister cultures also elicited depolarizations in neurons grown in F-12, but the amplitude of the underlying inward current was 25-30% of that produced by fresh F-12, suggesting a loss of potency of F-12 exposed to prolonged contact with hippocampal cultures. Third, measurement by high performance liquid chromatography showed reduction of aspartate concentrations to around 10% of those present in fresh F-12, within 24 h after exposing glial cell cultures to fresh F-12. It is concluded that cellular uptake mechanisms for amino acids have a strong impact on excitotoxicity in vitro, and most likely play an important role in protecting neurons from the potentially damaging action of high concentrations of excitatory transmitters in vivo. In addition our experiments help to explain the mechanisms permitting neuronal survival in cultures grown in Ham's F-12 medium, which when applied acutely to mature cultures is strikingly neurotoxic.

Neuronal death in vitro: parallelism between survivability of hippocampal neurones and sustained elevation of cytosolic Ca2+ after exposure to glutamate receptor agonist

Hippocampal neurones isolated from rat embryos were maintained on glial monolayers in a medium containing no L-glutamate (Glu). The administration of Glu for a limited period induced a massive death (loss) of neurones. The degree of neuronal loss increased with time after exposure to Glu. The extent of neuronal loss assessed 24 h after exposure to Glu was dependent upon the concentration Glu and on the duration of the exposure. An increase in concentration of external Ca2+ during the exposure to Glu enhanced the extent of loss. By contrast, an increment in concentration of environmental Mg2+ reduced the loss. The inhibitor of spike firing, tetrodotoxin (TTX) and the suppressor of Ca2+ entry, nitrendipine, both decreased the extent of loss, when delivered prior to Glu. The toxicity of Glu became progressively more apparent with further days of culture. The cytosolic concentration of Ca2+ ([Ca2+]i) in single hippocampal neurones was monitored by microscopic fluorometry under conditions equivalent to those in the death assay. The time required for the recovery of [Ca2+]i from the level elevated by exposure to Glu to pre-stimulus levels closely paralleled the degree of neuronal loss; i.e. high doses of Glu, long periods of exposure, high concentrations of external Ca2+, low concentrations of external Mg2+, and extended days of culture all retarded [Ca2+]i recovery, while TTX and nitrendipine accelerated it. These findings show that neuronal death resulting from an extraneous excitation (excitotoxicity) can be analyzed in vitro. Furthermore, substantial support has been provided to the hypothesis that excitotoxicity has an underlying mechanism, an excess loading of Ca2+ in neuronal cytoplasm.

Central mammalian neurons normally resistant to glutamate toxicity are made sensitive by elevated extracellular Ca2+: toxicity is blocked by the N-methyl-D-aspartate antagonist MK-801

It is widely held that a glutamate-like toxin that resembles N-methyl-D-aspartate may be responsible for the death of nerve cells seen after severe neurological insults including stroke, seizures, and degenerative disorders, such as Huntington disease, Alzheimer disease, and the amyotrophic lateral sclerosis-parkinsonism-dementia complex found on Guam. One puzzling fact about these maladies is the differential vulnerability of specific groups of neurons peculiar to each condition. We report here that an identified population of central neurons, rat retinal ganglion cells, are resistant to the neurotoxic effects of millimolar concentrations of glutamate under otherwise normal culture conditions. Patch-clamp experiments show that this resistance is associated with a very small ionic current response to N-methyl-D-aspartate. Varying the ionic milieu by increasing the extracellular Ca2+ concentration, however, results in a striking increase in glutamate-induced cell death in this population. Under these conditions, Mg2+ or the amino acid antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo-(alpha,gamma)-cyclohepten-5 ,10-imine maleate], blockers of N-methyl-D-aspartate receptor-coupled ion channels, completely abrogate the lethal effects of glutamate. These findings strongly suggest that Ca2+ entry through N-methyl-D-aspartate-activated channels is responsible for this type of neuronal death and suggest strategies that may be clinically useful in the treatment of various neurological disorders.

Tricyclic antidepressants block N-methyl-D-aspartate receptors: similarities to the action of zinc

1. Using the radioligand [3H]-MK801, we have examined drug interactions with the phencyclidine recognition site of the N-methyl-D-aspartate receptor. 2. The tricyclic antidepressants desmethylimipramine and imipramine inhibited [3H]-MK801 binding with IC50 values of 7.4 and 22.5 microM, respectively. Other related tricyclic antidepressants and neuroleptics were also effective but less potent. 3. Desmethylimipramine, imipramine and chlorimipramine slowed the dissociation rate of [3H]-MK801 in a similar manner to Zn2+. Phencyclidine and related compounds had no effect on the dissociation rate of [3H]-MK801. 4. Desmethylimipramine, imipramine and ketamine also prevented the Ca2+ influx into cultured cortical neurones of the rat produced by N-methyl-D-aspartate. 5. As the actions of tricyclic antidepressants in this system are not competitive with respect to N-methyl-D-aspartate, glycine or MK-801, and as they slow the dissociation of [3H]-MK801, we conclude that tricyclic antidepressants may be acting at the Zn2+ recognition site on the N-methyl-D-aspartate receptor.

Quisqualate, high calcium concentration and zero-chloride prevent kainate-induced toxicity of cerebellar granule cells

Kainic acid (KA; 100 microM), results in the death of all cultured rat cerebellar granule cells (18-22 days in vitro) within 30 min. Changes in the cells are evident within 2 min of applying the excitatory amino acid (EAA) and include an apparent cellular granulation with a loss of cell body birefringence at 10 X magnification. Quisqualic acid (QA; 25 microM) completely prevents this KA-induced neurotoxicity. In addition, cells are protected from toxicity by increasing calcium concentrations to 10 mM. Moreover, following a 30 min exposure and after washing the cells free of these compounds, cells placed in culture media remain alive 24 h later. Interestingly, neurons die when placed in a balanced salt solution which lacks calcium even when no KA is present. This death is also dependent on the presence of chloride and is prevented with the non-selective EAA antagonist, kynurenic acid, but is not prevented by QA. Collectively, these data suggest that the activation of the EAA receptor by KA in cerebellar granule cells is at least partially regulated by calcium and chloride and is suppressed by QA. Furthermore, placing granule cells in zero-calcium results in neuronal death which appears to be mediated by EAA mechanisms.

The excitatory amino acid antagonist kynurenic acid administered after hypoxic-ischemia in neonatal rats offers neuroprotection

The neuroprotective effect of kynurenic acid, an unspecific antagonist of excitatory amino acid receptors, was evaluated in a model of hypoxic-ischemia in neonatal rats. One-week-old rats were subjected to ligation of the left carotid artery and exposure to 7.7% O2/92.3% N2 for 2 h. Kynurenic acid (300 mg/kg) was administered i.p. immediately after the period of hypoxic-ischemia in one group (n = 32) and compared with saline-treated (n = 27). After 2 weeks the rats were sacrificed and the brain damage evaluated by comparing the weight of the lesioned and unlesioned hemispheres. In rats receiving kynurenic acid the reduction in weight of the lesioned hemisphere was 25.4 +/- 3.3% as compared to 37.8 +/- 3.6% in saline-treated controls (P less than 0.001). The results suggest that excitatory amino acids are involved in the development of postischemic damage in the immature brain.