The effect of an N-methyl-D-aspartate lesion in the hippocampus on glial and neuronal marker proteins

Preferential Inhibition of Tonically over Phasically Activated NMDA Receptors by Pregnane Derivatives

Postsynaptic N-methyl-d-aspartate receptors (NMDARs) phasically activated by presynaptically released glutamate are critical for synaptic transmission and plasticity. However, under pathological conditions, excessive activation of NMDARs by tonically increased ambient glutamate contributes to excitotoxicity associated with various acute and chronic neurological disorders. Here, using heterologously expressed GluN1/GluN2A and GluN1/GluN2B receptors and rat autaptic hippocampal microisland cultures, we show that pregnanolone sulfate inhibits NMDAR currents induced by a prolonged glutamate application with a higher potency than the NMDAR component of EPSCs. For synthetic pregnanolone derivatives substituted with a carboxylic acid moiety at the end of an aliphatic chain of varying length and attached to the steroid skeleton at C3, the difference in potency between tonic and phasic inhibition increased with the length of the residue. The steroid with the longest substituent, pregnanolone hemipimelate, had no effect on phasically activated receptors while inhibiting tonically activated receptors. In behavioral tests, pregnanolone hemipimelate showed neuroprotective activity without psychomimetic symptoms. These results provide insight into the influence of steroids on neuronal function and stress their potential use in the development of novel therapeutics with neuroprotective action.

SIGNIFICANCE STATEMENT

Synaptic activation of N-methyl-d-aspartate receptors (NMDARs) plays a key role in synaptic plasticity, but excessive tonic NMDAR activation mediates excitotoxicity associated with many neurological disorders. Therefore, there is much interest in pharmacological agents capable of selectively blocking tonically activated NMDARs while leaving synaptically activated NMDARs intact. Here, we show that an endogenous neurosteroid pregnanolone sulfate is more potent at inhibiting tonically than synaptically activated NMDARs. Further, we report that a novel synthetic analog of pregnanolone sulfate, pregnanolone hemipimelate, inhibits tonic NMDAR currents without inhibiting the NMDAR component of the EPSC and shows neuroprotective activity in vivo without inducing psychomimetic side effects. These results suggest steroids may have a clinical advantage over other known classes of NMDAR inhibitors.

NMDA receptors in the medial prefrontal cortex and the dorsal hippocampus regulate methamphetamine-induced hyperactivity and extracellular amino acid release in mice

The medial prefrontal cortex (mPFC) and the dorsal hippocampus (DHC) play significant roles in stimulant-induced neurobehavioral effects. Methamphetamine (MAP)-induced hyperactivity has been reported to be involved in the regulation of the glutamatergic system. The present study examined whether the glutamatergic and GABAergic systems in the mPFC and DHC were involved in MAP-induced hyperactivity in mice. A combined kainic acid (KA) or N-methyl-d-aspartate (NMDA) lesion and microdialysis technique targeting both the mPFC and DHC were used. The results showed that both KA- and NMDA-induced lesions of the mPFC facilitated MAP-induced hyperactivity, while neither KA- nor NMDA-induced lesions of the DHC had a similar effect. MAP increased the extracellular glutamate (Glu) levels in the mPFC and reduced Glu levels in the DHC. GABA levels in both of these regions were reduced. A KA or NMDA lesion of the mPFC inhibited the Glu reduction in the DHC, and the same lesion of the DHC inhibited the Glu increase in the mPFC induced by MAP. A NMDA lesion of the mPFC blocked GABA reduction in the DHC, but a lesion of DHC enhanced the GABA decrease in the mPFC induced by MAP. Furthermore, a NMDA lesion of DHC increased the vesicular glutamate transporter-2 (VGLUT2) expression in the mPFC following MAP-administration. These findings indicate that glutamatergic as well as GABAergic systems in these two regions are involved in MAP-induced hyperactivity. Moreover, there may be an inhibitory role in these two regions, especially mediated by NMDA receptors, in MAP-induced abnormal behavior and neurotransmission responses.

Normal pressure hydrocephalus triggers intrathecal production of TNF-alpha

Normal pressure hydrocephalus (NPH) is associated with periventricular white matter lesions and demyelination. The aim of the present study was to examine the cerebrospinal fluid (CSF) levels of tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine mediating myelin damage, in patients with NPH. TNF-alpha levels were analyzed by ELISA and measured before and after shunt operation in 35 patients with NPH. The levels of this cytokine were related to the symptomatology and to magnetic resonance imaging (MRI) verified white matter lesions. They were also related to intrathecal levels of sulfatide, a marker for white matter degradation and to levels of neurofilament, a marker for neuronal degeneration. The preoperative levels of TNF-alpha were increased in the CSF of NPH patients compared to controls, and correlated to the levels of sulfatide. The intrathecal TNF-alpha levels were higher in NPH patients with impairment of wakefulness than in those without this symptom. The preoperative TNF-alpha levels were significantly correlated to the improvement of psychometrical test scores, and of wakefulness and to the overall improvement of the patients following shunt operation. Importantly, shunt operation led to complete disappearance of intrathecal TNF-alpha. We conclude that NPH is correlated with intrathecal TNF-alpha production being reversed following shunt operation in parallel with the clinical improvement. The positive correlation between preoperative TNF-alpha and sulfatide levels in the CSF suggest that intrathecal TNF-alpha may contribute to the damage of the white matter known to occur in patients with NPH.

Correlation between intrathecal sulfatide and TNF-alpha levels in patients with vascular dementia

OBJECTIVES

Subcortical vascular dementia (SVD) is associated with white matter lesions and demyelination. The aim of the present study was to examine the cerebrospinal fluid (CSF) levels of TNF-alpha, a proinflammatory cytokine mediating myelin damage, in SVD patients. The intrathecal TNF-alpha levels were related to the clinical symptoms of dementia, as well as to intrathecal levels of sulfatide, a marker of white matter degradation, and of neurofilament, a marker of neuronal degeneration.

METHODS

CSF levels of TNF-a, sulfatide and neurofilament were all analyzed by immunoenzymatic procedures in 17 patients with SVD and in 26 healthy controls.

RESULTS

The intrathecal concentration of TNF-alpha was significantly increased in SVD patients compared to healthy controls (p = 0.0001). The intrathecal levels of TNF-alpha were significantly correlated (r = 0.6, p = 0.02) to the levels of sulfatide, but not to the levels of neurofilament, (r = 0.08, NS).

CONCLUSIONS

We have demonstrated intrathecal production of TNF-alpha in SVD patients. The correlation between TNF-a and sulfatide levels in the CSF suggests that this apoptosis-inducing cytokine leads to the death of oligodendrocytes, thereby contributing to white matter degeneration, a hallmark of SVD.

Redistribution of neurofilaments and accumulation of beta-amyloid protein after brain injury by rotational acceleration of the head

Rotational acceleration of the head, as occurs in falls, car crashes, and sport injuries, may result in diffuse brain damage, with acute and chronic neurological and psychiatric symptoms. The present study addresses the effects of rotational trauma on the neuronal cytoskeleton, which stabilizes perikaryal, dendritic and axonal shape and function. The study focuses upon the distribution of (1) the phosphorylated form of the heavy neurofilament subunit, (2) the light neurofilament subunit, and (3) beta-amyloid, a marker for brain injury. While normally restricted to axons, the phosphorylated heavy neurofilament subunits were drastically decreased in the axons after rotational trauma. Instead, they accumulated in the neuronal perikarya, normally devoid of the phosphorylated subunit. This alteration was seen, not only in the cerebral cortex, but also in the hippocampus, the cervical spinal cord, the cerebellum, the cranial nerves and the pyramidal tract. The distribution of the light subunit of neurofilaments was also altered post trauma. Only a weak beta-amyloid immunoreactivity was detected in the brains of control animals. Promptly after the trauma, a large number of beta-amyloid positive neurons appeared. Intensely co-localized immunoreactivity for the light subunit of neurofilaments and of beta-amyloid was seen 3 days after the rotational trauma axons of in the subcortical white matter and in the granule cell layer of the dentate gyrus as well as in neurons of the hypoglossal nucleus. The reported alterations in the central nervous system neurons are similar to those in the human brain after closed head injury and in chronic degenerative diseases. Regions of importance for social behavior, memory and body movement were affected.

In vivo excitotoxicity induced by ouabain, a Na+/K+-ATPase inhibitor

The susceptibility of immature rat brain to neurotoxicity of N-methyl-D-aspartate (NMDA) has provided a widely used paradigm to study excitotoxicity relevant to acute neurodegenerative diseases such as cerebral ischemia. In this study, excitotoxicity was induced via injection of ouabain (1 mM/0.5 microL), a Na+/K+ -ATPase-inhibitor, into neonatal rat brain and compared with NMDA injection. The aim of the study was to induce excitotoxicity secondary to cellular membrane depolarization, thereby more closely mimicking the pathophysiologic processes of ischemia-induced brain injury where NMDA-receptor overstimulation by glutamate follows, not precedes, membrane depolarization. Na+/K+ -ATPase-inhibition caused an acute, 40% +/- 8% decrease of the apparent diffusion coefficient (ADC) of water, as measured using diffusion-weighted magnetic resonance imaging (MRI), and resulted in infarctlike lesions as measured using T2-weighted MRI and histology up to 2 weeks later. Localized one- and two-dimensional 1H-magnetic resonance spectroscopy (MRS) demonstrated that the early excitotoxic diffusion changes were not accompanied by an overall metabolic disturbance. Furthermore, 31P-MRS demonstrated that energy depletion is not a prerequisite for ADC decrease or excitotoxic cell death. Treatment with the NMDA-antagonist MK-801 (1 mg/kg) attenuated the volume of tissue exhibiting a decreased ADC (P < 0.005), demonstrating that the ouabain-induced injury is indeed excitotoxic in nature. The authors argue that, compared with NMDA-injection, ouabain-induced excitotoxicity elicits more appropriate glutamate-receptor overstimulation and is better suited to detect relevant neuroprotection in that it is more sensitive to attenuation of synaptic glutamate levels.

Comparison of excitotoxic profiles of ATPA, AMPA, KA and NMDA in organotypic hippocampal slice cultures

The excitotoxic profiles of (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propionic acid (ATPA), (RS)-2-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainic acid (KA) and N-methyl-D-aspartate (NMDA) were evaluated using cellular uptake of propidium iodide (PI) as a measure for induced, concentration-dependent neuronal damage in hippocampal slice cultures. ATPA is in low concentrations a new selective agonist of the glutamate receptor subunit GluR5 confined to KA receptors and also in high concentrations an AMPA receptor agonist. The following rank order of estimated EC(50) values was found after 2 days of exposure: AMPA (3.7 mM)>NMDA (11 mM)=KA (13 mM)>ATPA (33 mM). Exposed to 30 microM ATPA, 3 microM AMPA and 10 microM NMDA, CA1 was the most susceptible subfield followed by fascia dentata and CA3. Using 8 microM KA, CA3 was the most susceptible subfield, followed by fascia dentata and CA1. In 100 microM concentrations, all four agonists induced the same, maximal PI uptake in all hippocampal subfields, corresponding to total neuronal degeneration. Using glutamate receptor antagonists, like GYKI 52466, NBQX and MK-801, inhibition data revealed that AMPA excitotoxicity was mediated primarily via AMPA receptors. Similar results were found for a high concentration of ATPA (30 microM). In low GluR5 selective concentrations (0.3-3 microM), ATPA did not induce an increase in PI uptake or a reduction in glutamic acid decarboxylase (GAD) activity of hippocampal interneurons. For KA, the excitotoxicity appeared to be mediated via both KA and AMPA receptors. NMDA receptors were not involved in AMPA-, ATPA- and KA-induced excitotoxicity, nor did NMDA-induced excitotoxicity require activation of AMPA and KA receptors. We conclude that hippocampal slice cultures constitute a feasible test system for evaluation of excitotoxic effects and mechanisms of new (ATPA) and classic (AMPA, KA and NMDA) glutamate receptor agonists. Comparison of concentration-response curves with calculation of EC(50) values for glutamate receptor agonists are possible, as well as comparison of inhibition data for glutamate receptor antagonists. The observation that the slice cultures respond with more in vivo-like patterns of excitotoxicity than primary neuronal cultures, suggests that slice cultures are the best model of choice for a number of glutamate agonist and antagonist studies.

A new nerve guide conduit material composed of a biodegradable poly(phosphoester)

There is a resurgence of interest in the development of degradable and biocompatible polymers for fabrication of nerve guide conduits (NGCs) in recent years. Poly(phosphoester) (PPE) polymers are among the attractive candidates in this context, in view of their high biocompatibility, adjustable biodegradability, flexibility in coupling fragile biomolecules under physiological conditions and a wide variety of physicochemical properties. The feasibility of using a biodegradable PPE, P(BHET-EOP/TC), as a novel NGC material was investigated. Two types of conduits were fabricated by using two batches of P(BHET-EOP/TC) with different weight-average molecular weights (Mw) and polydispersity indexes (PI). The polymers as well as conduits were non-toxic to all six types of cells tested, including primary neurones and neuronally differentiated PC12 cells. After in situ implantation in the sciatic nerve of the rat, two types of conduits triggered a similar tissue response, inducing the formation of a thin tissue capsule composed of approximately eight layers of fibroblasts surrounding the conduits at 3 months. Biological performances of the conduits were examined in the rat sciatic nerve model with a 10 mm gap. Although tube fragmentation, even tube breakage, was observed within less than 5 days post-implantation, successful regeneration through the gap occurred in both types of conduits, with four out of 10 in the Type I conduits (Mw 14,900 and PI 2.57) and 11 out of 12 in the Type II conduits (Mw 18,900 and PI 1.72). The degradation of conduits was further evidenced by increased roughness on the tube surface in vivo under scanning electron microscope and a mass decrease in a time-dependent manner in vitro. The Mw of the polymers dropped 33 and 24% in the Type I and II conduits, respectively, in vitro within 3 months. Among their advantages over other biodegradable NGCs, the PPE conduits showed negligible swelling and no crystallisation after implantation. Thus, these PPE conduits can be effective aids for nerve regeneration with potential to be further developed into more sophisticated NGCs that have better control of the conduit micro-environment for improved nerve regeneration.

Blast exposure causes redistribution of phosphorylated neurofilament subunits in neurons of the adult rat brain

There is little information on threshold levels and critical time factors for blast exposures, although brain damage after a blast has been established both clinically and experimentally. Moreover, the cellular pathophysiology of the brain response is poorly characterized. This study employs a rat model for blast exposure to investigate effects on the neuronal cytoskeleton. Exposure in the range of 154 kPa/198 dB or 240 kPa/202 dB has previously been shown neither to cause visual damage to the brain, nor to affect the neuronal populations, as revealed with routine histology. Here, the brains were investigated immunohistochemically from 2 h to 21 days after blast exposure. A monoclonal antibody was used which detects only the phosphorylated epitope of the heavy subunit of the neurofilament proteins (p-NFH). This epitope is normally restricted to axons, that is, not demonstrable in the perikarya. Eighteen hours after exposure in the 240-kPa/202-dB range, p-NFH immunoreactivity accumulated in neuronal perikarya in layers II-IV of the temporal cortex and of the cingulate and the piriform cortices, the dentate gyrus and the CA1 region of the hippocampus. At the same time, the p-NFH immunoreactivity disappeared from the axons and dendrites of cerebral cortex neurons. The most pronounced immunostaining of neuronal perikarya was found in the hemisphere, which faced the blast source. The perikaryal accumulation of p-NFH was present also at 7 days but the neuronal perikarya had become negative at 21 days, at which time the axons again displayed p-NFH immunoreactivity. Exposure in the range of 154 kPa/198 dB caused similar, although less marked accumulation of p-NFH immunoreactivity in the neuronal perikarya. The findings are interpreted to show a dephosphorylation of NFHs in axons and dendrites and a piling up of p-NFHs in the perikarya due to disturbed axonal transport.

Quantitative immunochemistry on neuronal loss, reactive gliosis and BBB damage in cortex/striatum and hippocampus/amygdala after systemic kainic acid administration

Cell specific markers were quantified in the hippocampus, the amygdala/pyriform cortex, the frontal cerebral cortex and the striatum of the rat brain after systemic administration of kainic acid. Neuron specific enolase (NSE) reflects loss of neurons, glial fibrillary acidic protein (GFAP) reflects reactive gliosis, and brain levels of serum proteins measures blood-brain-barrier permeability. While the concentration of NSE remained unaffected in the frontal cerebral cortex and the striatum, their GFAP content increased during the first three days. In the hippocampus and amygdala, NSE levels decreased significantly. GFAP levels in the hippocampus were unaffected after one day and decreased in the amygdala/pyriform cortex. After that, GFAP increased strikingly until day 9 or, in the case of amygdala/pyriform cortex, even longer. This biphasic time course for GFAP was accompanied by a decrease of S-100 during days 1-9 followed by a significant increase at day 27 above the initial level. The regional differences in GFAP and S-100 could result from the degree of neuronal degeneration, the astrocytic receptor set-up and/or effects on the blood-brain barrier.

Methylmalonic and propionic acids increase the in vitro incorporation of 32P into cytoskeletal proteins from cerebral cortex of young rats through NMDA glutamate receptors

In this study we investigated the effects of methylmalonic acid (MMA) and propionic acid (PA) on the phosphorylation of cytoskeletal proteins of cerebral cortex of rats. Slices of tissue were incubated with 32P-orthophosphate in the presence or absence of glutamate, MMA, PA and ionotropic or metabotropic glutamate receptor agonists. The cytoskeletal fraction was isolated and the radioactivity incorporated into the cytoskeletal proteins was measured. Results demonstrated that the acids, glutamate and NMDA increased the phosphorylation of the proteins studied. However, this effect was not observed for non-NMDA ionotropic agonists or metabotropic agonists. Experiments using glutamate receptor antagonists confirmed that MMA and PA at the same concentrations as found in tissues from propionic or methylmalonic acidemic children increase the phosphorylation of cytoskeletal proteins, possibly via NMDA glutamate receptors. Therefore, it is feasible that these findings may be related to the neurological dysfunction characteristic of these disorders.

The trophic factors S-100beta and basic fibroblast growth factor are increased in the forebrain reactive astrocytes of adult callosotomized rat

S-100 is a calcium-binding protein that is predominantly found in astrocytes of the central nervous system. In the present study, we investigated the temporal and spatial changes of S-100beta immunoreactivity after a stereotaxic mechanical lesion of the adult rat corpus callosum performed with an adjustable wire knife. Rats were killed 7, 14 and 28 days after surgery. S-100beta immunoreactivity was found within the cytoplasm and processes of quiescent putative astrocytes that were observed throughout the gray and white matters of the forebrain of sham-operated rats. Following callosotomy, the S-100beta immunoreactive profiles showed increased size and thick processes, as well as increased amount of S-100beta immunoreactivity. Unbiased stereologic analysis revealed a sustained and widespread increase of the Areal Fraction of S-100beta immunoreactive profiles in the medial and lateral regions of the white matter of callosotomized rats at the studied time-intervals. In the cerebral cortex of callosotomized rats, the estimated total number of S-100beta immunoreactive profiles was also increased 7 and 14 days after the lesion. Since the cellular and temporal changes in S-100beta immunoreactivity were closely similar to those described for basic fibroblast growth factor (bFGF) following brain lesions, we co-localized the S-100beta and bFGF immunoreactivities after callosotomy. bFGF immunoreactivity was found in the nuclei of S-100beta immunoreactive glial profiles throughout the forebrain regions of the sham-operated rats. bFGF immunoreactivity was increased in the nuclei of reactive S-100beta immunoreactive putative astrocytes in the forebrain white matter and in the cerebral cortex of callosotomized rats. These results indicate that after transection of the corpus callosum of adult rats, the reactive astrocytes may exert paracrine trophic actions through S-100beta and bFGF. Interactions between S-100beta and bFGF may be relevant to the events related to neuronal maintenance and repair following brain injury.

Semi-automated analysis of NMDA-mediated toxicity in digitised colour images from rat hippocampus

The evaluation of neuronal cell survival after, for example, mechanical, hypoxic or drug-mediated injury requires the analysis of a high number of histological specimens. Since this is a time-consuming occupation, we have developed a semi-automated analysis routine for the determination of the distribution of live and dead cells. After digitalization of the histological preparations, 8-bit colour bitmaps were assessed using a compiled image-analysis programme of the software package Khoros. In the current study a detailed example of the application of this image-processing approach is described for the investigation of the cell survival after intraventricular application of N-methyl-D-aspartate (NMDA). The samples were prepared as fuchsin acid/toluidine blue stained hippocampal thin slices. The calculated areas of the live and dead cells were highly correlated with manual counts of live and dead cells in the 100 samples examined in this study. Twenty-four hours following NMDA-treatment animals (n = 5) were found to have significantly fewer live and more dead hippocampal cells than the saline-treated animals (n = 5), using either automated or manual examination techniques. The automated technique also revealed that NMDA treatment resulted in a reduction in the density of live cell distribution.

Quantitative localization of NMDAR1 receptor subunit immunoreactivity in inferotemporal and prefrontal association cortices of monkey and human

The cellular and synaptic localization of immunoreactivity for the N-methyl-D-aspartate (NMDA) receptor subunit, NMDAR1, was investigated in inferotemporal and prefrontal association neocortices of monkeys and humans. In all monkey association areas examined, the laminar distribution patterns of NMDAR1 immunoreactivity were similar, and characterized by predominant pyramidal-like neuronal labeling in layers II, III, V and VI and a dense neuropil labeling consisting of intensely stained puncta and fine-caliber processes present throughout layers I-III, and V-VI. Layer IV, in contrast, contained only very lightly immunostained neurons which mostly lacked extensive dendritic staining. The laminar distribution of NMDAR1 immunolabeling in human association cortex was similar to that observed in monkeys. Electron microscopy of monkey areas 46 and TE1 confirmed that intensely immunoreactive asymmetrical postsynaptic densities were present throughout all cell-dense layers of prefrontal and inferotemporal association cortex. Quantitative analyses of the laminar proportions of immunoreactive synapses demonstrated that in both areas examined, the percentages of immunolabeled synapses were mostly similar across superficial layers, layer IV and infragranular layers. Finally, quantitative double-labeling immunofluorescence for non-NMDA receptor subunits or calcium-binding proteins demonstrated that virtually all GluR2/3 or GluR5/6/7-immunoreactive neurons were also labeled for NMDAR1, while regionally-specific subsets of parvalbumin-, calbindin- and calretinin-immunoreactive neurons were co-labeled. These data indicate that in primate association cortex, NMDA receptors are heterogeneously distributed to subsets of functionally distinct types of neurons and subsets of excitatory synapses, suggesting a critical and highly specific role in mediating the activity of excitatory connectivity which converges on cortical association areas.

Ultrastructural localization and comparative distribution of nitric oxide synthase and N-methyl-D-aspartate receptors in the shell of the rat nucleus accumbens

Nitric oxide (NO), the diffusible gas formed by nitric oxide synthase (NOS) has been implicated in the enhanced locomotor activity attributed mainly to increased dopamine release in the shell of the nucleus accumbens (Acb). Furthermore, the release of both NO and dopamine are known to be altered by agonists of N-methyl-D-aspartate (NMDA) type glutamate receptors in this region. We examined the cellular sites of NO synthesis and the sites of potential relevancy for functional associations between neurons containing NOS and the NMDA receptor in the shell of the Acb. This was achieved by dual ultrastructural immunogold and immunoperoxidase labeling of antisera raised against the brain form of NOS and the NMDAR1 subunit of the NMDA receptor in this region of rat brain. NOS-like immunoreactivity (NOS-LI) was seen throughout the cytoplasm of isolated medium-large somata, aspiny dendrites and axon terminals. In 217 NOS-labeled profiles, NMDAR1-like immunoreactivity (NMDAR1-LI) was colocalized in 17% of somata and dendrites. Additionally, 35% of NOS-labeled dendrites apposed glial processes containing NMDAR1-LI, and 29% apposed axon terminals containing NMDARI-LI. NOS-labeled terminals more rarely colocalized NMDAR1 or apposed NMDAR1-labeled glial processes or dendrites. These results provide anatomical evidence that, in the shell of the Acb, NMDA receptors are localized so as to directly modulate the output of neurons producing NO as well as to influence other neurons and glia having the greatest access to the released gas.

Assessment of neurotoxicity and "neuroprotection"

Coronal brain slices allow the study of neurotoxicity and "neuroprotection" under conditions where the differentiation-state and interrelationships of the neurones and glial cells are closer to those occurring in the intact tissue than is the case for co-cultured cell systems. The involvement of glial cells in the excitotoxicity of kainate and the potentiation of this toxicity by inhibition of glutamine synthase can be demonstrated. Longer-term toxicity of kainate may also be compounded by depletion of glutathione levels resulting from inhibition of gamma-glutamylcysteine synthase. The involvement of nitric oxide formation in the toxicity of N-methyl-D-aspartate can also be shown. The neurotoxicity of 1-methyl-4-phenylpyridinium can be readily demonstrated in coronal slice preparations. Taurine affords protection against this neurotoxicity. The possible mechanisms of these effects are considered in terms of the cyclic interrelationships between the different events which can lead to cell death.

Selective distribution of the NMDA-R1 glutamate receptor in astrocytes and presynaptic axon terminals in the nucleus locus coeruleus of the rat brain: an immunoelectron microscopic study

The regional and cellular distribution of the different classes of excitatory amino acid receptors with respect to the noradrenergic neurons of the nucleus locus coeruleus (LC) are unknown. We therefore combined immunoperoxidase labeling for the R1 subunit of the N-methy-D-aspartate (NMDA) receptor with immunogold-silver localization of the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), in single sections through the rat LC to determine the subcellular localization of this glutamate receptor subtype with respect to the noradrenergic neurons. At the light microscopic level, there was light to moderate labeling for the NMDA-R1-like (li) receptor in the caudal pole of the LC and dense labeling in the dorsolateral aspect of the LC adjacent to the superior cerebellar peduncle. In the rostral pole of the LC which is enriched with noradrenergic dendrites, significant overlap between both immunoreactivities could be observed. At the ultrastructural level, immunoperoxidase labeling for NMDA-R1 was selectively distributed in astrocytic processes and within presynaptic axon terminals but was rarely seen in catecholamine-containing somata or dendrites. Peroxidase labeling for NMDA-R1, however, was occasionally observed in dendrites in the rostral pole of the LC. Most of these dendrites lacked detectable levels of TH, although TH immunoreactivity was apparent in the neuropil. Dendrites containing NMDA-R1-li immunoreactivity often received asymmetric (excitatory-type) contacts from unlabeled terminals. NMDA-R1-li-immunoreactive axon terminals usually contained small clear, as well as large dense-core vesicles and were often apposed to unlabeled dendrites, axon terminals and/or glial processes. These results provide the first ultrastructural evidence that NMDA-R1-li immunoreactivity is selectively distributed within astrocytic processes and presynaptic axon terminals within the LC.

Comparative ultrastructural localization of the NMDAR1 glutamate receptor in the rat basolateral amygdala and bed nucleus of the stria terminalis

The N-methyl-D-aspartate (NMDA)-type glutamate receptor in the basolateral amygdala (BLA) has been implicated in activity-dependent plasticity important for cortically evoked acquisition of fear-potentiated startle response. We examined the ultrastructural immunoperoxidase labeling of the R1 subunit of the NMDA receptor in the BLA of adult rats to determine the potential cellular and subcellular sites mediating the effects generated by NMDA activation. The localization was compared with that seen in the bed nucleus of the stria terminalis (BNST), the major efferent pathway from the central nucleus of the amygdala, which has a more pronounced involvement in autonomic function. Electron microscopy established that in the BLA, 68.4% (n = 177) of the profiles showing NMDAR1-like immunoreactivity (NMDAR1-LI) were dendrites, and 19.8% were distal tips of astrocytic processes. In contrast, profiles containing NMDAR1-LI (n = 262) in the BNST were more equally distributed between dendrites (37.4%) and axons (38.2%). The subcellular localization of NMDAR1 immunoreactivity was, however, similar in both regions. Our findings provide the first ultrastructural evidence that glutamate may prominently act through NMDAR1 receptors to elicit postsynaptic actions on intrinsic neurons in the BLA and BNST. The results also indicate that, in the BLA, the NMDAR1 receptor plays an important role in astrocytic function, whereas the receptor is more preferentially a presynaptic modulator in axons which terminate in or pass through the BNST.

Phosphorylated and non-phosphorylated neurofilament proteins: distribution in the rat hippocampus and early changes after kainic acid induced seizures

The regional distribution of neurofilament proteins in the rat hippocampus and their early changes after kainic acid induced seizures were investigated immunocytochemically with antibodies against light weight neurofilament, phosphorylated and non-phosphorylated heavy weight neurofilament. The light weight and non-phosphorylated heavy weight neurofilaments were distributed more unevenly than the phosphorylated neurofilament. The perikarya and processes of pyramidal cells in the CA3 field contained the highest light weight and non-phosphorylated heavy weight neurofilaments, while the perikarya of granule cells contained only few light weight neurofilament and the perikarya of CA1 pyramidal cells were even devoid of immunoreactivity of both light and heavy weight neurofilaments. The fiber staining of the light weight and non-phosphorylated heavy weight neurofilaments, especially the former, was less in the CA1 field and molecular layer of dentate gyrus. The phosphorylated neurofilament immunoreactivity was identified only in axons. Mossy fibers, the axons of granule cells, contained the light weight and phosphorylated heavy weight neurofilaments, but not the non-phosphorylated neurofilament. Seven days after the kainic acid induced seizures, the phosphorylated neurofilament staining was greatly reduced in the CA1 and inner molecular layer of the dentate gyrus, probably resulting from the axonal degeneration of the Schaffer collaterals and the commissural/associational fibers. Furthermore, the nonphosphorylated neurofilament appeared in the mossy fibers of the CA3 stratum lucidum, which normally do not express such immunoreactivity. The results indicate that the neurofilaments are altered following the neuronal degeneration and postlesional plasticity caused by the kainic acid administration. Therefore, the examination of various phosphorylated neurofilaments may offer a comprehensive understanding of major hippocampal pathways, axonal plasticity and the possible roles of neurofilaments in the hippocampus following excitotoxic insults.

Alteration in the glial cell metabolism of glutamate by kainate and N-methyl-D-aspartate

Incubation of coronal slices of rat brain with neurotoxic concentrations of kainate (300 microM) and N-methyl-D-aspartate (NMDA; 500 microM) for 40 min reduced the activity of the glial enzyme, glutamine synthetase, by 33% and 21%, respectively. The immunoreactivity of the neuronal enzyme, gamma gamma-enolase (neuron-specific enolase), was also decreased, but to a lesser extent than glutamine synthetase. Pre-incubation of the slices with L-methionine-S-sulphoximine (500 microM), an irreversible inhibitor of both glutamine synthetase and gamma-glutamylcysteine synthetase, before addition of either kainate or NMDA produced a supra-additive reduction in the activity of the enzyme in both cases. Neither kainate nor NMDA directly inhibited the activity of glutamine synthetase, but kainate did inhibit gamma-glutamylcysteine synthetase, a rate-limiting enzyme of the gamma-glutamyl cycle, which is responsible for maintaining glutathione levels within cells. Pre-incubation of the slices with L-NG-nitroarginine, a competitive inhibitor of nitric oxide synthase, effectively prevented the NMDA-induced reduction in glutamine synthetase and neuron specific enolase, but did not diminish the kainate-induced decrease in the activity of either enzyme. These results provide evidence that NMDA, as well as kainate, indirectly affects the activity of glutamine synthetase in brain slices, yet does so by a different mechanism from kainate. The results are discussed in terms of the possible mode of action of each toxin in inhibiting the glial cell metabolism of glutamate.

Regional and temporal profiles of calcium accumulation and glial fibrillary acidic protein levels in rat brain after systemic injection of kainic acid

Cerebral calcium accumulation and increases in the astroglial intermediate filament protein, glial fibrillary acidic protein (GFAP), have been used as markers of neurotoxic and ischemic brain damage. The present study was aimed at quantitatively investigating the regional and temporal relationship of those indices following a neurotoxic insult. For this purpose, regional changes in 45Ca uptake and GFAP levels, using ELISA, were evaluated in rat brains at both early (several hours) and late time points (up to 6 months) after a single systemic injection of kainic acid (12 mg/kg). After 4 h, limbic brain areas were already heavily labelled by 45Ca. In most investigated brain areas 45Ca accumulation peaked at day 4 (maximum 5 fold increase in amygdala) and returned to normal levels within 1 week (cerebellum, pons/medulla, occipital cortex), 2 weeks (striatum, frontal cortex), 2 or 4 months (limbic brain areas), or remained significantly elevated until 6 months (thalamus). In contrast, in all investigated brain areas, except cerebellum and pons/medulla, GFAP was increased from day 2, reaching maximum levels at day 28 in most limbic structures and remained significantly elevated at the same high level (15 fold increase) in amygdala, or somewhat lower levels in other affected regions (2-7 fold), but not in the thalamus. In all brain areas with 45Ca accumulation, GFAP was increased and the peak responses were highly correlated. Thus, both indices are useful quantitative biochemical markers of acute or subchronic neurotoxicity.

Early evolution and recovery from excitotoxic injury in the neonatal rat brain: a study combining magnetic resonance imaging, electrical impedance, and histology

We explored the therapeutic potentials of two N-methyl-D-aspartate (NMDA) receptor antagonists in vivo using different techniques. NMDA injected into the striatum of neonatal rats (20 nmol/0.5 microliters) induced a rapid increase in the diffusion-weighted (DW) image intensity, spreading over a large part of the ipsilateral hemisphere. Subcutaneous injection of the NMDA receptor antagonist MK-801 (1 mg/kg) or D-(E)-4-(3-phosphono-2-prop-enyl)-2-piperazine-carboxylic acid (D-CPPene; 1.5 mg/kg) reversed both the volume and the grading of the NMDA-induced hyperintensity of DW images, the reversal by MK-801 being more rapid than that by D-CPPene. In the cerebral cortex, there was an inverse relationship between changes in DW image intensity and the size of the extracellular space, assessed by electrical impedance measurements. The reduction of the hyperintense volume in DW images 1 or 2 h after MK-801 or D-CPPene treatment of NMDA-injected animals depended on the type of antagonist used and on the interval between intrastriatal NMDA injection and antagonist treatment. The reduction was 95% when MK-801 was given with a delay of 90 min and decreased to 20% when it was given at 360 min. With D-CPPene, the reduction was 80% after a delay of 30 min and virtually absent when it was administered at 360 min. Quantitative analysis showed significant correlations between the residual hyperintense volume 1 or 2 h after MK-801 or D-CPPene treatment and the final lesion volume, assessed from either T2-weighted images (R = 0.89, p < 0.001) or histology (R = 0.80, p < 0.001) 5 days after the insult. This study illustrates the sensitivity of DW magnetic resonance imaging to monitor in vivo early events after an excitotoxic insult and the effect of putative protective drugs that may counteract the resulting damage.

Neuronal and glial marker proteins in encephalopathy associated with acute liver failure and acute hyperammonemia in the rabbit

Neuronal and glial cell marker proteins were quantified in order to evaluate the possibility of increased proteolysis in the brain of rabbits with acute liver failure and acute hyperammonemia. Acute liver failure was induced by a two-stage devascularization procedure. Acute hyperammonemia was induced by a prolonged infusion of ammonium acetate, which simulates the plasma ammonia level in acute liver failure. Control animals received an infusion of sodium/potassium acetate. After development of severe encephalopathy, the animals were sacrificed (13.7 +/- 1.3 hours for rabbits with acute liver failure and 20.2 +/- 0.8 hours for rabbits with hyperammonemia) (x +/- S.E.M./n = 6) and their brains were dissected into cerebral cortex, hippocampus, cerebellum and brain stem. The total protein content and the concentrations of the neuronal cell marker proteins NSE (neuron specific enolase), NF68 and NF200 (68 kD and 200 kD neurofilament polypeptides) and the glial cell marker proteins GFAP (glial fibrillary acidic protein) and S-100 were determined. Total protein content was decreased in the brain stem in acute hyperammonemia only. The content of neuronal and glial cell markers was not affected in either of the two conditions. However, low molecular weight proteolytic fragments of the NF 68 kD polypeptide were observed in the hippocampus of three out of six animals in both experimental groups. No proteolytic degradation of GFAP was observed. The results show that, in experimental encephalopathy due to acute liver failure and acute hyperammonemia, no major changes occur in the marker proteins. The finding of proteolytic fragments of the NF68 polypeptide indicates that the neuronal population is affected prior to glial alterations. These findings are in agreement with the concept that acute hepatic encephalopathy is reversible and induces only slight structural changes.

Co-variation of free amino acids in human epileptogenic cortex

The concentration of free amino acids was measured in 41 surgically removed samples of human epileptogenic brain and in 7 specimens of non-epileptic brain tissue, removed during surgery for meningiomas, etc. The material was subdivided according to the neuropathological diagnosis: mild cortical dysplasia (MCD), gliosis astrocytoma infiltration and a histologically heterogeneous group. The non-tumoral epileptogenic samples had five times higher than normal concentration of ethanolamine and 50% elevated concentration of glycine. The concentration of other neurotransmitter amino acids did not differ markedly between epileptogenic and non-epileptic samples. The concentration of neurotransmitter amino acids showed a strong correlation with the enzyme neuron specific enolase (NSE) and were low in most samples with astrocytoma infiltration. On the other hand, tyrosine and leucine had higher concentrations in samples with lower NSE concentration. Factor analysis of the amino acids revealed four groups of covarying compounds in the brain samples, first, a neurotransmitter group, including aspartate, glutamate, GABA and phosphoethanolamine. Another group contained ethanolamine, glutamine, glycine and taurine. Factor analysis on corresponding extracellular amino acids showed two groups, the first being a "neurotransmitter" group, containing serine, taurine phosphoethanolamine and ethanolamine in addition to aspartate and glutamate. The other group consisted of asparagine, glycine, alanine, tyrosine, valine, phenylalanine, isoleucine and leucine.

Perchloroethylene-induced reduction in glial and neuronal cell marker proteins in rat brain

Rats were exposed to continuous inhalation of 300 and 600 p.p.m. of perchloroethylene for 4 and 12 weeks. Exposure to 600 p.p.m. for 4 to 12 weeks resulted in a slower increase in brain weight. Brain region weights, total proteins and DNA were decreased in frontal cerebral cortex and brain stem but not in hippocampus after exposure to 600 p.p.m. for 12 weeks. Four marker proteins were measured to monitor the specific neurotoxic effects of perchloroethylene: S-100 protein and glial fibrillary acidic protein as glial cell markers and neurone specific enolase and neurofilament 68 kD polypeptide as neuronal markers. The concentrations of glial and neuronal cytoskeletal proteins (glial fibrillary acidic protein and neurofilament 68 kD polypeptide) were reduced in frontal cerebral cortex. The total tissue contents of glial proteins (S-100 protein and glial fibrillary acidic protein) were decreased in all 3 brain regions investigated (frontal cerebral cortex, hippocampus and brain stem). Neurone specific enolase was unchanged by perchloroethylene exposure. These results indicate that exposure to perchloroethylene reduces the number of brain cells, possibly glial cells, and interferes with the metabolism of cytoskeletal elements in both glial and neuronal cells.

Glial and neuronal marker proteins in the silicone chamber model for nerve regeneration

In the present study, neuronal and Schwann cell marker proteins were used to biochemically characterize the spatiotemporal progress of degeneration/regeneration in the silicone chamber model for nerve regeneration. Rat sciatic nerves were transected and the proximal and distal stumps were inserted into a bridging silicone chamber with a 10-mm interstump gap. Using dot immunobinding assays, S-100 protein and neuronal intermediate filament polypeptides were measured in different parts of the nerve 0-30 days after transection. In the most proximal nerve segment, all the measured proteins were transiently increased. In the proximal and distal stumps adjacent to the transection, the studied proteins were decreased indicating degeneration of the nerve. Within the silicone chamber, the regenerating nerve expressed the Schwann cell S-100 protein already at 7 days, whereas the neurofilament polypeptides appeared later. These observations are corroborated by previous morphological studies. The biochemical method described provides a new and fast approach to the study of nerve regeneration.

Quantitative alterations of S-100 protein and neuron specific enolase in the rat nervous system after chronic 2,5-hexanedione exposure

The regional changes in quantities of the glial S-100 protein and the neuron specific enolase in the rat nervous system have been studied after long-term exposure to 2,5-hexanedione. The wet weights of most of the examined nervous tissues were found to be reduced, with an extensive effect seen in the brain stem. Using dot immunobinding assays, the concentrations of S-100 were found to be increased in most of the examined tissues, but unaffected in the brain stem. The total amount of S-100 per tissue was markedly reduced in the brain stem. The content of neuron specific enolase was reduced only in the brain stem. Thus the effects of 2,5-hexanedione on the nervous system varied regionally. The brain stem was severely atrophied with a reduction of neuronal as well as of glial marker proteins. Other brain regions contained increased glial cell marker proteins as signs of progressive astroglial reactions.

Biphasic changes in NCAM level after an NMDA lesion to the hippocampal formation: a quantitative dot-immunobinding assay

With a quantitative dot-immunobinding assay, the time course changes of neuronal cell adhesion molecule (NCAM) concentrations and total tissue content were monitored in the rat hippocampus after a 40 nmol NMDA injection. A biphasic alteration was observed; a decrease occurred at day 3, an increase at day 30. The time course of changes differed from that of the glial fibrillary acidic protein (GFAP), a marker for reactive astroglial cell, but was similar to that for the markers of sprouting neurites, i.e., low (L) and high (H) molecular weight subunits of the neurofilament polypeptides. It is suggested that NCAM is implicated in the onset of neurite sprouting in the hippocampus after an excitotoxic trauma.

In vivo activation of kainate receptors induces dephosphorylation of the heavy neurofilament subunit

Injection of kainic acid (KA) into the rat hippocampus reduced the phosphorylation-related immunoreactivity of the heavy subunit of neurofilament proteins (NF-H). The effect was demonstrated quantitatively with a dot-immunobinding assay and qualitatively by immunoblotting with monoclonal antibodies against phosphorylation-dependent and nonphosphorylation-related epitopes of NF-H. The KA-induced reduction affected 50% of the phosphorylated NF-H in half of the hippocampus after 48 h. At the same time, the nonphosphorylation-related NF-H immunoreactivity increased as revealed by immunoblotting, indicating a shift from phosphorylated to nonphosphorylated NF-H. The effects on NF-H preceded a decrease in content of the neuron-specific enolase, a soluble neuronal cytoplasmic protein. No alterations of the light subunit of neurofilament proteins occurred, suggesting that KA has a preferential effect on NF-H phosphorylation. N-Methyl-D-aspartate administered similarly did not lead to a rapid dephosphorylation of NF-H. We propose that kainate receptor-mediated dephosphorylation in NF-H is involved in the signal transduction of excitatory amino acids with consequences for neuronal functions dependent on intermediary filament phosphorylation.

Proteolysis of filament proteins in glial and neuronal cells after in vivo stimulation of hippocampal NMDA receptors

An intrahippocampal injection of N-methyl-D-aspartate induced the appearance of degradation products of both the 68 kiloDalton neurofilament protein and the glial fibrillary acidic protein, as revealed by immunoblot techniques. The degradation of these two filament proteins was maximal at 10 days after the lesion. The degradation patterns were similar to those induced with calpains or calcium in vitro. There were no degradation effects on the 200 kD neurofilament protein as tested with both mono- and polyclonal antibodies. Consequently, the neuronal degeneration after excessive activation of NMDA receptors appears to involve calcium activation of proteolytic enzymes. The effects on the glial proteins are probably secondary to neuronal damage but could be related to calcium dependent processes.