Kainate-induced apoptotic cell death in hippocampal neurons

Neuronal injury associated with HIV-1: approaches to treatment

Mounting evidence suggests that cognitive dysfunction developing as a result of HIV-1 infection is mediated at least in part by generation of excitotoxins and free radicals in the brain. This syndrome is currently designated HIV-1-associated cognitive/motor complex, was originally termed the AIDS Dementia Complex, and for simplicity, is called AIDS dementia in this review. Recently, brains of patients with AIDS have been shown to manifest neuronal injury and apoptotic-like cell death. How can HIV-1 result in neuronal damage if neurons themselves are only rarely, if ever, infected by the virus? Experiments from several different laboratories have lent support to the existence of HIV- and immune-related toxins in a variety of in vitro and in vivo paradigms. In one recently defined pathway to neuronal injury, HIV-infected macrophages and microglia, or immune-activated macrophages and astrocytes (activated by the shed HIV-1 envelope protein, gp120, or other viral proteins and cytokines), appear to secrete excitants and neurotoxins. These substances may include arachidonic acid, platelet-activating factor, free radicals (NO. and O2.-), glutamate, quinolinate, cysteine, amines, and as yet unidentified factors emanating from stimulated macrophages and reactive astrocytes. A final common pathway for neuronal susceptibility is operative, similar to that observed in stroke and several neurodegenerative diseases. This mechanism involves excessive activation of N-methyl-D-aspartate (NMDA) receptor-operated channels, with resultant excessive influx of Ca2+ and the generation of free radicals, leading to neuronal damage. With the very recent development of clinically tolerated NMDA antagonists, there is hope for future pharmacological intervention.

Kainate-evoked changes in dystrophin messenger RNA levels in the rat hippocampus

Dystrophin and dystroglycan messenger RNAs are expressed in specific brain areas, including regions of the cortex and the hippocampus, and in such neurons dystrophin has been localized to postsynaptic densities. In the present study we examined by in situ hybridization the effect of neuronal activation and neurotoxicity induced by kainate and pentylenetetrazole administered in vivo on dystrophin and dystroglycan expression in the rat brain. Kainate injection resulted in a transient but dramatic decrease in dystrophin transcript levels in the dentate gyrus granule cells, neurons not affected by kainate neurotoxicity, 6 h after injection. There was also a strong, concomitant increase in dystrophin messenger RNA levels in the CA3 subfield. At 24-72 h after kainate injection, the dystrophin transcript in the dentate granule cells returned to control levels, while it decreased gradually in the CA subfields, coinciding with the neurodegeneration observed in these areas. Comparable results were obtained with pan-dystrophin probes and probes specific to the short, G-dystrophin (Dp71) isoform that predominates in the dentate gyrus. This indicates that any dystrophin transcript that might be expressed in these areas responds to kainate in the same manner. In contrast, kainate insult had no significant effect on the dystroglycan messenger RNA levels in these hippocampal areas at 6 h post-injection. At later times. however, there was a gradual decrease in the dystroglycan messenger RNA in those areas which respond to the kainate insult with extensive neuronal death. For comparison, seizures which are not associated with progressive neurodegeneration were induced by pentylenetetrazole: in this situation the dystrophin and dystroglycan messenger RNA levels remained unchanged in all areas of the hippocampal formation. Since activation of glutamate receptors is thought to be involved in some forms of synaptic plasticity in the adult hippocampus, our data indicate that the dystrophin gene behaves as a candidate plasticity-related gene responding to glutamate.

Anoxia during kainate status epilepticus shortens behavioral convulsions but generates hippocampal neuron loss and supragranular mossy fiber sprouting

In rats, this study determined the impact of systemic hypoxia during late kainate-induced status epilepticus on hippocampal neuron loss and mossy fiber sprouting. Non-fasted Sprague Dawley rats were prepared as follows: Naive controls (n=5); rats placed 2 min in a hypoxia chamber (hypoxia only; n=6); rats that seized for more than 6 h from kainic acid (KA-status; 12 mg/kg; i.p.; n=7); and another KA-status group placed into the hypoxia chamber 75 min after the convulsions started (KA-status/hypoxia; n=16). All rats, except for half of the KA-status/hypoxia animals, were perfused 2 weeks later (short-term). The other 8 KA-status/hypoxia rats were perfused after 2 months (long-term). Hippocampal sections were studied for neuron densities and aberrant mossy fiber sprouting at three ventral to dorsal levels. Fascia dentata (FD) mossy fiber sprouting was quantified as an increase in the inner minus outer molecular layer (IML-OML) gray value (GV) difference. Behaviorally, KA-status/hypoxia rats had a shorter duration of convulsive status epilepticus than KA-status animals without anoxia. Hippocampal sections showed that compared to controls: (1) hypoxia-only rats showed no differences in ventral neuron densities and neo-Timm's stained IML-OML GVs; (2) KA-status rats had decreased CA3 densities and a non-significant increase in ventral IML-OML GV differences; and (3) KA-status/hypoxia short-term animals showed decreased hilar, CA3 and CA1 densities and increased ventral IML-OML GV differences. Compared to KA-status/hypoxia short-term rats, long-term animals showed no differences in ventral hippocampal neuron densities, but middle and dorsal sections demonstrated increased IML-OML GV differences and animals were observed to have spontaneous limbic epilepsy. These results indicate that rats exposed to kainate-induced status epilepticus for over 1 h and then a hypoxic insult had a shorter duration of convulsive status, decreased hippocampal neuron densities and greater FD mossy fiber sprouting than controls and the amount of neuronal damage and sprouting was slightly more than animals subjected to 6 h of kainate-induced status. This supports the hypothesis that a physiologic insult during status can shorten the convulsive episode, but still produce hippocampal pathology with a number of clinical and pathologic similarities to human mesial temporal lobe epilepsy (MTLE).

Apoptosis of hippocampal neurons after amygdala kindled seizures

Seizure-induced neuronal damage may involve both excitotoxic and apoptotic (programmed cell death) mechanisms. In the present study, we used an amygdala kindled seizure model to study whether apoptotic cell death occurs. To evaluate apoptosis, we counted the numbers of cells that had DNA fragments labeled at the 3' end with digoxigenin using terminal transferase (ApopTag, Oncor). Additionally, the expression of Bax and Bcl-2, two genes associated with apoptotic cell death, was also measured following kindled seizures. We found that the number of ApopTag-positive cells in the hippocampus increased 30.4% after one kindled seizure and 82.5% after 20 seizures compared to sham controls. The ApopTag-labeled cells could be mainly interneurons of the hippocampal formation, although additional studies are required. Preferential vulnerability of inhibitory interneurons is consistent with previous studies on seizure-induced cell loss. These results, coupled with our findings that the ratio of Bax/Bcl-2 expression is increased in the hippocampus by seizures, suggest that apoptosis of hippocampal interneurons may lead to dysinhibition in the hippocampus and increased seizure susceptibility.

Apoptosis, excitotoxicity, and neuropathology

While a high rate of cell loss is tolerated and even required to model the developing nervous system, an increased rate of cell death in the adult nervous system underlies neurodegenerative disease. Evolutionarily conserved mechanisms involving proteases, Bcl-2-related proteins, p53, and mitochondrial factors participate in the modulation and execution of cell death. In addition, specific death mechanisms, based on specific neuronal characteristics such as excitability and the presence of specific channels or enzymes, have been unraveled in the brain. Particularly important for various human diseases are excessive nitric oxide (NO) production and excitotoxicity. These two pathological mechanisms are closely linked, since excitotoxic stimulation of neurons may trigger enhanced NO production and exposure of neurons to NO may trigger the release of excitotoxins. Depending on the experimental situation and cell type, excitotoxic neuronal death may either be apoptotic or necrotic.

Cell death during development of testis and cerebellum in the mutant mouse weaver

The murine mutation weaver confers early death during development on cells in testes, cerebellum, and midbrain. The results reported here support the hypothesis that the action of weaver is intrinsic to testes and independent of Sertoli cells: germ cells are the only testicular cell type seen to die in weaver homozygotes, while Sertoli cell-dependent development of the blood testis barrier is normal. This report includes characterization of patterns of germ cell death and cerebellar granule cell death in homozygous weavers with respect to that seen during normal development by in situ end-labeling of DNA and high-magnification light microscopy. Comparison of the spatial distribution of dying cells in the weaver's cerebellum with that of dividing cells revealed disarray in the external germinal zone. The results show that cells vulnerable to weaver die by apoptotic and nonapoptotic mechanisms and indicate that weaver-induced cell death is not the consequence of extended naturally occurring developmental cell death, although their timing overlaps. Thus, although the death of cells in each region is likely to be caused by the same mutation, a base pair substitution in the G protein-coupled inwardly rectifying potassium channel 2 gene, the cell death program activated differs depending on cell type.

Hippocampal stimulation produces neuronal death in the immature brain

We re-examined the proposed resistance of the immature brain to seizure-induced damage. In awake, freely moving rat pups, intermittent perforant path stimulation produced selective hippocampal cell loss and reduction in paired-pulse inhibition. During 16 h of stimulation, animals showed frequent wet dog shakes and hind-limb scratching movements but no convulsive motor activity. In situ end-labelling performed 2 h after the end of stimulation showed an intense band of positively-labelled eosinophilic cells with condensed profiles bilaterally in the dentate granule cell layer of stimulated animals. Control animals showed no in situ end-labelling positivity in the dentate gyrus. These cells were not observed 24 h later, suggestive of rapidly scavenged apoptotic cells. One day after the end of stimulation, many necrotic interneurons with eosinophilic cytoplasm and pyknotic nuclei were observed in the hilus of the stimulated dentate gyrus in all rats tested. Hippocampal pyramidal cells in CA1, CA3 and subiculum showed bilateral damage greater on the side of stimulation, and prepiriform cortex sustained bilateral symmetrical lesions. One month after perforant path stimulation, Cresyl Violet staining showed the number of large hilar interneurons (>15 microm) was reduced on the stimulated side (54.1 +/- 12.2) compared to the non-stimulated side (100.5 +/- 10.2 cells, P<0.01). Immunohistochemical analysis showed significant losses in somatostatin (8.5 +/- 1.6 stimulated side, 22.8 +/- 3.8 unstimulated side, P<0.05) and neuropeptide Y (12.8 +/- 3.2 stimulated side, 17.0 +/- 4.1 unstimulated side, P<0.05) immunoreactive cells in the stimulated hilus but no loss of parvalbumin-immunoreactive cells. Significant reductions in paired-pulse inhibition were found after stimulation but there was some return of inhibition by one month. These combined data demonstrate that the immature brain can incur damage as a result of prolonged seizure-like hippocampal activity mimicking status epilepticus in immature rats. The hippocampal damage produced by perforant path stimulation is associated with the immediate loss of physiological inhibition suggesting important modification of excitatory control in an extremely epileptogenic region of the brain.

MRS metabolic markers of seizures and seizure-induced neuronal damage

PURPOSE

Proton magnetic resonance spectroscopy (MRS) was used to identify specific in situ metabolic markers for seizures and seizure-induced neuronal damage. Kainic acid (KA)-induced seizures lead to histopathologic changes in rat brain. The protective effect of cycloheximide treatment against neuronal damage caused by KA-induced seizures was studied, using in situ proton MRS imaging technique.

METHODS

Rats were pretreated with placebo or cycloheximide 1 h before KA injection. Rat brains (n = 25) were scanned at the level of the hippocampus before, during, and 24 h after seizures. Spectra were recorded and the relative ratios of N-acetylaspartate (NAA), choline (cho), and lactate (Lac) to creatine (Cr) were calculated and compared between groups.

RESULTS

A significant increase in Lac ratios was observed in KA-treated rats during and 24 h after seizure onset and this increase was prevented by cycloheximide pretreatment. NAA ratios were significantly higher during the ictal phase following KA treatment and this effect was not affected by cycloheximide pretreatment. Nissl staining confirmed previously reported prevention of KA-induced neuronal loss in CA3 and CA1 areas of the hippocampus by cycloheximide pretreatment.

CONCLUSIONS

Our results suggest that in situ Lac increase is a marker of seizure-induced neuronal damage, whereas N-acetylaspartate (NAA) changes during and after status epilepticus may be a reflection of neuronal activity and damage, respectively.

Apoptosis in neurological disease

Enormous interest in cell death in the past several years has moved apoptosis to the forefront of scientific research. Apoptosis has been found to mediate cell deletion in tissue homeostasis, embryological development, and immunological functioning. It also occurs in pathological conditions, including cancer and acquired immunodeficiency syndrome, and is implicated in neurodegenerative diseases. Claims of neuronal apoptosis induced by various agents and conditions are published regularly, but in many instances the data are questionable because they are incomplete. This review presents a brief history of apoptosis and describes the evidence required before claims of apoptosis are made. Summaries and critiques of important investigations concerning the genetic and biochemical regulation of neuronal apoptosis are presented, as are other studies describing connections between apoptosis and neuronal cell death in physiological and pathological situations. There is a realization that apoptosis can be programmed and is distinguishable from necrotic cell death. Combining apoptosis with programmed cell death produces misleading terminology and confusion over these two forms of cell degeneration. Further investigations into neuronal apoptosis should focus on all of the criteria that the original investigators outlined 25 years ago, to clarify whether apoptosis and/or another form of cell death mediates neuronal degeneration in physiological settings and in neurological diseases such as Alzheimer's disease, Parkinson's disease, epilepsy, and ischemia/stroke.

Long-lasting enhanced expression in the rat hippocampus of NMDAR1 splice variants in a kainate model of epilepsy

Chronic epilepsy is associated with increased excitability which may result from abnormal glutamatergic synaptic transmission involving altered properties of N-methyl-D-aspartate (NMDA) receptors. To date two gene families encoding NMDA receptor subunits have been cloned, NR1 and NR2. Eight NR1 mRNAs are generated by alternative splicing of exons 5, 21 and 22; the NR1-1 to NR1-4 C-terminal variants exist in the a or b version depending on the presence or absence of the domain encoded by exon 5. Epilepsy was induced in rats by unilateral intra-amygdalar injection of kainate and animals were killed from 6 h to 4 months following the injection. Increased NR1 mRNA levels were observed during status epilepticus (6-24 h after the injection), both psilateral and contralateral, while a second wave of NMDAR1 mRNA increase occurred in chronic epileptic animals, between 21 days and 4 months following kainate injection. Our data show: (i) a permanent increase of the NR1-2a and NR1-2b mRNA species (containing exon 22) in all hippocampal fields, both ipsilateral and contralateral, and (ii) an increase of the NR1-3 (a and b) mRNAs (containing exon 21) in the ipsilateral CA1, and NR1-3a mRNA in the ipsilateral dentate gyrus. No long-term changes were observed for the NR1-1 and NR14 splice variants. In the ipsilateral CA3 area a globally decreased mRNA expression was associated with neuronal loss. A possible contribution to the maintenance of the epileptic state by an increased expression of NMDA receptors is discussed.

The role of calcium in apoptosis

One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid redistribution of ionised calcium throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Various physiological stimuli increase [Ca2+]i transiently and, thereby, induce cellular responses. However, under pathological conditions, changes of [Ca2+]i are generally more pronounced and sustained. Marked elevations of [Ca2+]i activate hydrolytic enzymes, lead to exaggerated energy expenditure, impair energy production, initiate cytoskeletal degradation, and ultimately result in cell death. Such Ca(2+)-induced cytotoxicity may play a major role in several diseases, including neuropathological conditions such as chronic neurodegenerative diseases and acute neuronal losses (e.g. in stroke).

Response of postmitotic neurons to X-irradiation: implications for the role of DNA damage in neuronal apoptosis

The molecular changes responsible for inducing neuronal apoptosis are unknown. Rat cortical neurons were treated with x-irradiation 7 d after isolation to test for the role of DNA damage in neuronal death. The response of neurons to x-irradiation was compared with that of astrocytes that had been isolated 3 weeks earlier from newborn rats. At the time of irradiation, the neurons appeared well differentiated morphologically and were predominantly (90-95%) noncycling, based on flow cytometric analysis. There was a similar, linear increase in DNA double-strand breaks with increasing radiation dose in neurons and astrocytes. However, whereas doses as low as 2 Gy induced typical apoptotic changes in neurons, including nuclear fragmentation and/or internucleosomal DNA fragmentation, doses as high as 32 Gy caused little or no apoptosis in astrocytes. Radiation-induced apoptosis of neurons started 4-8 hr after irradiation, was maximal at 12 hr, and was dependent on dose up to 16 Gy. It was prevented when cycloheximide, a protein synthesis inhibitor, was added up to 6 hr after irradiation. In addition to their distinct apoptotic response, neurons rejoined radiation-induced DNA double-strand breaks more slowly than astrocytes. Treatment with benzamide to inhibit ADP-ribosylation and strand break repair increased apoptosis; splitting the dose of radiation to allow increased time for DNA repair decreased apoptosis. These data suggest that DNA damage may induce neuronal apoptosis, that the extent of damage may determine the degree of apoptosis induced, and that slow repair of damage may play a role in the susceptibility of neurons to apoptosis.

Blood-brain barrier disruption, edema formation, and apoptotic neuronal death following cold injury

The temporal pattern of brain edema and apoptosis following cold injury was investigated. Extravasation of Evans blue from the disrupted blood-brain barrier (BBB) maximized immediately after injury and returned to the control level at 24 h. However, water content increased up to 24 h and was maintained at a higher level than the control at 72 h. Apoptotic cells as detected by in situ end labeling were observed in the entire lesion at 24 h. At 72 h after injury, these apoptotic cells were observed in the margin of the lesion, but not in the core. These results suggest that apoptosis contributes to neuronal damage following cold injury and may result from the development of vasogenic edema.

An improved method for detection of apoptosis in tissue sections and cell culture, using the TUNEL technique combined with Hoechst stain

We describe a novel procedure for combining a fluorescent variant of the TUNEL technique with Hoechst 33342 stain (bis-benzimide) to identify apoptosis in tissue sections and cell culture. The biochemical hallmark of apoptosis is internucleosomal DNA cleavage, which gives rise to oligonucleosome-sized fragments (multiples of approximately 180 bp) that can be directly visualised by labelling with biotinylated or digoxygenin-conjugated nucleosides in a reaction that employs terminal deoxynucleotide transferase (TUNEL). TUNEL and Hoechst 33342 have been used separately to identify apoptosis. TUNEL specifically labels dying cells, yet a low background makes comparison of labelled cells with surrounding normal cells difficult and causes disorientation in tissue sections. Hoechst 33342 binds all DNA therefore staining all nuclear material, allowing identification of apoptotic nuclei, but the analysis is laborious. Combining the two fluorescent labels allows the initial recognition of apoptotic cells using the TUNEL technique then, by simply changing the filter, the TUNEL positive nuclei can be compared to surrounding normal nuclei to assess changes in morphology and size. Hoechst 33342 acts as a counterstain, allowing identification of anatomical structures, and permits quantitative comparison between TUNEL positive versus normal cells. We have evaluated the technique using sections of rat embryo, post-axotomy neonatal dorsal root ganglia and adult dorsal root ganglia cell culture.

The anticonvulsant properties of antisense c-fos oligodeoxynucleotides in kainic acid-induced seizures

Evidence has accumulated that the immediate early gene c-fos has important physiological and pharmacological properties in the central nervous system. The role of c-fos in seizures and, in particular, kainic acid-induced seizures, is unclear. It is unknown if c-fos stimulation after kainic acid is a consequence of neuronal activation, or an intrinsic critical component of the metabolic pathways leading to seizure. To elucidate this problem we have pretreated male Wistar rats with antisense c-fos and nonsense c-fos oligodeoxynucleotides 12 h prior to kainic acid 10 mg/kg intraperitoneal. Antisense c-fos inhibited the number of wet dog shakes and the appearance of limbic motor seizures, effects not seen with nonsense or vehicle. The anticonvulsant effects were associated with reduction of both Fos and NGFI-A immunoreactivity and neuroprotection in the hippocampus, thalamus and primary olfactory cortex-amygdaloid region. Four days after antisense c-fos limbic motor seizures were not inhibited, and there was no decrease in Fos or NGFI-A immunoreactivity and no neuroprotection, indicating that the anticonvulsant effects were not secondary to a toxic effect. Sense oligonucleotides had no anticonvulsant effects when given 12 h prior to kainic acid and did not influence immunoreactivity or neuronal survival. In conclusion, these findings suggest a role for c-fos in the generation of kainic acid-induced limbic seizures and neuronal death.

Neuronal apoptosis and necrosis following spinal cord ischemia in the rat

We examined the characteristics of neuronal death induced by ischemia in the spinal cord. Spinal cord ischemia was induced in Long-Evans rats by occlusion of the descending aorta with a 2F Fogarty catheter for 20 min (model 1) or more limited aortic occlusion (15 min) coupled with blood volume reduction (model 2); rats were sacrificed 6 h-7 days later. The animals developed variable paraparesis in model 1 and reliable paraplegia in model 2. The extent of histopathological spinal cord damage, being maximal in the lumbar cord, correlated well with the severity of paraparesis. Two distinct types of spinal cord neuronal death were observed, consistent with necrosis and apoptosis. Neuronal necrosis was seen in gray matter laminae 3-7, characterized by the rapid (6 h) onset of eosinophilia on hematoxylin/eosin-stained sections, and gradual (1-7 days) development of eosinophilic ghosting. Although TUNEL positivity was present, disintegration of membranes and cytoplasmic organelles was seen under electron microscopy. Neuronal apoptosis was seen after 1-2 days in dorsal horn laminae 1-3, characterized by both TUNEL positivity and electron microscopic appearance of nuclear chromatin aggregation and the formation of apoptotic bodies. DNA extracted from the ischemic lumbar cord showed internucleosomal fragmentation (laddering) on gel electrophoresis. These data suggest that distinct spinal cord neuronal populations may undergo necrosis and apoptosis following transient ischemic insults.

Kainate induces the expression of the DNA damage-inducible gene, GADD45, in the rat brain

The expression of the novel growth arrest and DNA damage-inducible gene GADD45 was examined in kainate-induced epileptic brain damage in the rat using in situ hybridization, northern blot analysis, western blot analysis and immunocytochemistry. Systemic administration of kainate resulted in DNA damage and neuronal degeneration in vulnerable neurons of limbic regions, including the amygdala and hippocampal pyramidal layers, as shown by in situ DNA nick end-labelling and histological staining. GADD45 messenger RNA was transiently increased in non-vulnerable neurons (2-8 h after kainate injection) but was persistently elevated in vulnerable neurons (up to 24 h after injection) after kainate injection. GADD45 protein was elevated in both vulnerable and non-vulnerable neurons at 4 h, but levels decreased in vulnerable neurons thereafter, suggesting that translational blockage of GADD45 protein occurred in these cells. GADD45 protein was overexpressed in non-vulnerable neurons up to 72 h after kainate injection. Because GADD45 may participate in the DNA excision repair process and because it has been shown to be overexpressed in neurons that survive focal cerebral ischaemia, these results support the hypothesis that GADD45 may have a protective role in the injured brain.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

Occurrence of apoptosis following cold injury-induced brain edema in mice

Apoptosis has been known to contribute to neuronal death following a variety of brain insults. However, the role of vasogenic brain edema in neuronal apoptosis is unknown. We studied the temporal pattern of brain edema and neuronal apoptosis following cold injury. Cold injury-induced brain edema, which was detected by the increased water content in the injured hemisphere, reached its maximum level at 24 h and remained there at 72 h, whereas the blood-brain barrier breakdown detected by Evans Blue extravasation returned to the control value by 24 h after injury. Terminal deoxynucleotidyl transferase-mediated uridine-5'-triphosphate-biotin nick end labeling (TUNEL)-positive apoptotic cells were scattered in the center of the lesion at 1 h and were dispersed over the cold lesion at 24 h. The number of these TUNEL-positive cells was maximized in the periphery but decreased in the center at 72 h after cold injury. We postulate that secondary neuronal damage occurred not only through necrotic, but also apoptotic pathways, and that apoptotic neuronal death may result from vasogenic edema development and may contribute to the expansion of the lesion in both the acute and delayed phases after cold injury.

Activation of CPP-32 protease in hippocampal neurons following ischemia and epilepsy

Recent in vitro studies indicate an involvement of members of the interleukin-1beta converting enzyme (ICE) family of proteases in programmed neuronal cell death. Cell death of hippocampal neurons in animal models of cerebral ischemia and epilepsy shows morphological features of apoptosis and can be prevented by administration of protein synthesis inhibitors suggesting that de novo synthesis of components of the cell death program is necessary for neuronal apoptosis. In the present study we demonstrate by in situ hybridization analysis that expression of CPP-32, an ICE-related protease, is significantly upregulated in CA1 hippocampal neurons following global ischemia induced by cardiac arrest and in hippocampal neurons of the CA3/CA4 region after kainate-mediated epilepsy, respectively. Moreover, an increase in CPP-32-like proteolytic activity was detected in hippocampal extracts 24 h after ischemia using the fluorogenic CPP-32 substrate Ac-DEVD-AMC. Activation of CPP-32 clearly preceded cell death of hippocampal neurons as assessed by in situ end-labelling of nuclear DNA fragments. These results indicate that CPP-32 protease may be activated at both the transcriptional and post-translational level during neuronal apoptosis and that activation correlates with the selective vulnerability of hippocampal pyramidal neurons to ischemic and epileptic insults.

Protection of rat spinal cord from ischemia with dextrorphan and cycloheximide: effects on necrosis and apoptosis

OBJECTIVE

We examined the characteristics of neuronal cell death after transient spinal cord ischemia in the rat and the effects of an N-methyl-D-aspartate antagonist, dextrorphan, and a protein synthesis inhibitor, cycloheximide.

METHODS

Spinal cord ischemia was induced for 15 minutes in Long-Evans rats with use of a 2F Fogarty catheter, which was passed through the left carotid artery and occluded the descending aorta, combined with a blood volume reduction distal to the occlusion. The rats were killed after 1, 2, and 7 days. Other groups of rats were pretreated with dextrorphan (30 mg/kg, intraperitoneally, n = 7), cycloheximide (30 mg, intrathecally, n = 7), or vehicle (saline solution, n = 12) and killed after 2 days.

RESULTS

This model reliably produced paraplegia and histopathologically distinct morphologic changes consistent with necrosis or apoptosis by light and electron microscopic criteria in different neuronal populations in the lumbar cord. Scattered necrotic neurons were seen in the intermediate gray matter (laminae 3 to 7) after 1, 2, and 7 days, whereas apoptotic neurons were seen in the dorsal horn laminae 1 to 3 after 1 and 2 days. Deoxyribonucleic acid extracted from lumbar cord showed internucleosomal fragmentation (laddering) on gel electrophoresis indicative of apoptosis. The severity of paraplegia in the rats treated with dextrorphan and cycloheximide was attenuated 1 day and 2 days after ischemia. The numbers of both necrotic and apoptotic neurons were markedly reduced in both dextrorphan- and cycloheximide-treated rats.

CONCLUSIONS

The results suggest that both N-methyl-D-aspartate receptor-mediated excitotoxicity and apoptosis contribute to spinal cord neuronal death after ischemia and that pharmacologic treatments directed at blocking both of these processes may have therapeutic utility in reducing spinal cord ischemic injury.

Apoptosis and proliferation of dentate gyrus neurons after single and intermittent limbic seizures

Neuronal apoptosis was observed in the rat dentate gyrus in two experimental models of human limbic epilepsy. Five hours after one hippocampal kindling stimulation, a marked increase of in situ terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) of fragmented DNA was observed in nuclei located within and on the hilar border of the granule cell layer and in the polymorphic region. Forty kindling stimulations with 5-min interval produced higher numbers of labeled nuclei compared with one stimulation. The increase of TUNEL-positive nuclei was prevented by the protein synthesis inhibitor cycloheximide but not affected by the N-methyl-D-aspartate receptor antagonist MK-801. Kainic acid-induced seizures lead to a pattern of labeling in the hippocampal formation identical to that evoked by kindling. A large proportion of cells displaying TUNEL-positive nuclei was double-labeled by the neuron-specific antigen NeuN, demonstrating the neuronal identity of apoptotic cells. Either 1 or 40 kindling stimulations also gave rise to a marked increase of the number of cells double-labeled with the mitotic marker bromodeoxyuridine and NeuN in the subgranular zone and on the hilar border of the dentate granule cell layer. The present data show that single and intermittent, brief seizures induce both apoptotic death and proliferation of dentate gyrus neurons. We hypothesize that these processes, occurring early during epileptogenesis, are primary events in the development of hippocampal pathology in animals and possibly also in patients suffering from temporal lobe epilepsy.

Overexpression of CuZn-superoxide dismutase reduces hippocampal injury after global ischemia in transgenic mice

BACKGROUND AND PURPOSE

The role of copper, zinc-superoxide dismutase (CuZn-SOD) in hippocampal injury after transient global ischemia was studied using transgenic (Tg) mice and wild-type littermates.

METHODS

Global ischemia was induced by bilateral common carotid artery occlusion. The hemisphere with the hypoplastic posterior communicating artery was determined and then the hippocampus in this hemisphere was evaluated qualitatively using a score of 0 to 4 and quantitatively using an image analyzer.

RESULTS

Hippocampal injury was reduced in Tg mice after both 5 and 10 minutes of ischemia. In the 5-minute ischemia group, the mean score of the injury was significantly lower in Tg than nontransgenic (nTg) mice at 3 days. In the 10-minute group, the hippocampal injury was reduced more in Tg than nTg mice at 1 day. Quantitative evaluation by an image analyzer confirmed the qualitative data. Neurons with fragmented DNA were also studied in the hippocampal injury. In the 5-minute group, despite the reduction of the injury in Tg mice, their neurons with fragmented DNA were relatively increased at 1 day. In the 10-minute group, this ratio was almost the same in both nTg and Tg mice.

CONCLUSIONS

CuZn-SOD plays a protective role in the pathogenesis of selective hippocampal injury after brief ischemia, whether the insult is relatively mild or intense. Furthermore, CuZn-SOD may reduce both necrotic and DNA fragmented neuronal death after global ischemia.

Cholinergic deafferentation exacerbates seizure-induced loss of somatostatin-immunoreactive neurons in the rat hippocampus

The loss of somatostatin-immunoreactive neurons and the sprouting of mossy fibers are typical histopathological abnormalities in the hippocampus in experimental and human temporal lobe epilepsy. To investigate whether the development of seizure-induced alterations is regulated by the subcortical afferent pathways to the hippocampus, we lesioned cholinergic, noradrenergic or serotonergic afferent pathways in rats two days after seizures were induced with kainate. Two months later, somatostatin-immunoreactive neurons were counted in the hilus to assess the severity of neuronal damage. Mossy fiber sprouting was analysed from adjacent Timm-stained sections. Kainate-induced seizures caused a loss of hilar somatostatin-immunoreactive neurons in the septal end of the hippocampus, where 63% of the somatostatin-immunoreactive neurons survived. Even more severe damage was found in the temporal end of the hippocampus (only 21% surviving). Cholinergic deafferentation of the hippocampus (using 192-IgG saporin) decreased the overall number of hilar somatostatin-immunoreactive neurons. In control rats that did not receive kainate, 87% (septal end) and 74% (temporal end) of the hilar somatostatin-immunoreactive neurons remained after cholinergic deafferentation. Moreover, seizure-induced damage to hilar somatostatin-immunoreactive neurons was further exacerbated by 192-IgG-saporin, with only 35% of the neurons remaining in the septal end and 14% in the temporal end of the hippocampus. Noradrenergic [using N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] or serotonergic (using 5,7-dihydroxytryptamine) lesions did not affect the number of hilar somatostatin-immunoreactive neurons either in control or in kainate-treated rats. The severity and distribution of seizure-induced mossy fiber sprouting were also not affected by any of the lesions. These data suggest that various subcortical afferent pathways may differentially modulate seizure-induced damage to the hippocampus. Damage to cholinergic neurons results in the loss of hilar somatostatin-immunoreactive neurons and exacerbates the seizure-induced loss of somatostatin-immunoreactive neurons.

Influence of AMPA/kainate receptors on extracellular 5-hydroxytryptamine in rat midbrain raphe and forebrain

1. The regulation of 5-hydroxytryptamine (5-HT) release by excitatory amino acid (EAA) receptors was examined by use of microdialysis in the CNS of freely behaving rats. Extracellular 5-HT was measured in the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), nucleus accumbens, hypothalamus, frontal cortex, dorsal and ventral hippocampus. 2. Local infusion of kainate produced increases in extracellular 5-HT in the DRN and MRN. Kainate infusion into forebrain sites had a less potent effect. 3. In further studies of the DRN and nucleus accumbens, kainate-induced increases in extracellular 5-HT were blocked by the EAA receptor antagonists, kynurenate and 6,7-dinitroquinoxaline-2,3-dione (DNQX). 4. The effect of infusing kainate into the DRN or nucleus accumbens was attenuated or abolished by tetrodotoxin (TTX), suggesting that the increase in extracellular 5-HT is dependent on 5-HT neuronal activity. In contrast, ibotenate-induced lesion of intrinsic neurones did not attenuate the effect of infusing kainate into the nucleus accumbens. Thus, the effect of kainate in the nucleus accumbens does not depend on intrinsic neurones. 5. Infusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate (AMPA) into the DRN and nucleus accumbens induced nonsignificant changes in extracellular 5-HT. Cyclothiazide and diazoxide, which attenuate receptor desensitization, greatly enhanced the effect of AMPA on 5-HT in the DRN, but not in the nucleus accumbens. 6. In conclusion, AMPA/kainate receptors regulate 5-HT in the raphe and in forebrain sites.

Neurotoxic effects of kainic acid on substantia nigra neurons in rat brain slices

Excitatory amino acids (EAAs) have been implicated as mediators of cell death in neurodegenerative diseases involving catecholamine neurons. Few studies, however, have examined the toxic effects of EAAs on identified catecholamine neurons in vitro. We have investigated the neurotoxic effects of kainic acid in a rat brain substantia nigra (SN) slice preparation. Rats (60-80 g) were anesthetised with halothane and killed by cervical dislocation. SN slices, 300 microm thick, were incubated at 35 degrees C in a modified Krebs solution in the presence or absence of kainic acid and then fixed and processed for either immunohistochemistry (IHC) or electron microscopy (EM). In IHC experiments, SN neurons were labeled using antibody to tyrosine hydroxylase (TH) coupled to diaminobenzidine. In control slices, the antibody labeled not only the cell body but also the prolific dendritic arbor of SN neurons. Treatment with 50 microM kainic acid for 15 min or 2 h resulted in loss of TH staining and apparent fragmentation of the dendrites. EM provided ultrastructural evidence for kainic acid-induced degeneration of the dendritic arbor of SN neurons. Typically, the dendritic membrane was broken, or diffuse and collapsed. Ultrastructural damage, including clumping and marginalization of chromatin and vacuolation of the cytoplasm, was also observed in cell bodies. Damage to the dendritic arbor may occur early in the neurotoxic events leading to cell death, preceding the loss of the cell body. Our observations are consistent with the postulated role of EAAs as mediators of catecholamine neuron death.

Kainate-induced hippocampal DNA damage is attenuated in superoxide dismutase transgenic mice

Peripheral administration of kainic acid (KA) can cause cell death in the hippocampus of rodents. This is thought to involve oxidative stress. In the present study, we tested the possibility that KA-induced neuronal cell death might be attenuated in CuZn superoxide dismutase transgenic (SOD-Tg) mice. Acute administration of KA causes animal death in a dose-dependent fashion; this was attenuated in SOD-Tg mice. Similarly, KA caused dose-dependent neuronal cell death in the hippocampus of wild-type mice; this cell death was attenuated in the SOD-Tg mice, in a gene-dosage-dependent fashion, with homozygous mice showing complete protection even at the highest dose (45 mg/kg) of KA used in this study. These results provide further support for the involvement of oxygen-based radicals in the toxic effects of KA.

Apoptosis, neurotrophic factors and neurodegeneration

Apoptosis is an active process of cell death characterized by distinct morphological features, and is often the end result of a genetic programme of events, i.e. programmed cell death (PCD). There is growing evidence supporting a role for apoptosis in some neurodegenerative diseases. This conclusion is based on DNA fragmentation studies and findings of increased levels of pro-apoptotic genes in human brain and in in vivo and in vitro model systems. Additionally, there is some evidence for a loss of neurotrophin support in neurodegenerative diseases. In Alzheimer's disease, in particular, there is strong evidence from human brain studies, transgenic models and in vitro models to suggest that the mode of nerve cell death is apoptotic. In this review we describe the evidence implicating apoptosis in neurodegenerative diseases with a particular emphasis on Alzheimer's disease.

Induction of apoptosis in vitro and in vivo by the cholinergic neurotoxin ethylcholine aziridinium

The patterns of cell death induced by the cholinergic neurotoxin ethylcholine aziridinium have been investigated in vitro and in vivo. In vitro, the drug induced apoptosis both in neuronal SK-N-MC cells (human neuroblastoma cells) and in non-neuronal 293 cells (a human embryonic kidney cell line). Apoptosis was developed maximally between 15 and 24 h of exposure to ethylcholine aziridinium (100 microM). At the ultrastructural level apoptotic cells were characterized by condensation and margination of nuclear chromatin, fragmentation of nuclei and the formation of apoptotic bodies. Inhibition of endonuclease by zinc almost completely prevented the occurrence of apoptosis. The free radical scavenger Tempol effectively inhibited ethylcholine aziridinium-induced apoptosis by 78.6 +/- 10.3% (n=4), whereas cycloheximide and actinomycin D were only partially effective. In vivo, following injection of ethylcholine aziridinium (2 nmol) into the lateral ventricle of rat brain a high incidence of apoptotic cells as verified by in situ tailing was visible in the periventricular tissue. Neurons as well as glia were affected by the neurotoxin. The number of apoptotic cells peaked two to three days after injection of ethylcholine aziridinium and declined thereafter. Up to one week after ethylcholine aziridinium no signs for the induction of apoptosis in the medial septal nucleus were found. This study provides clear evidence that a neurotoxic compound that induces programmed cell death in vitro is likely to have the same capacity in vivo. Yet, in the case of ethylcholine aziridinium, both the in vitro and the in vivo induction of programmed cell death appears to be an additional feature of ethylcholine aziridinium, which may be independent of the well-established degenerative effect of ethylcholine aziridinium on the cholinergic septohippocampal pathway. The present data indicate that ethylcholine aziridinium provides a useful tool to study molecular mechanisms of neuronal apoptosis.

mRNAs encoding urokinase-type plasminogen activator and plasminogen activator inhibitor-1 are elevated in the mouse brain following kainate-mediated excitation

Urokinase-type plasminogen activator (uPA) is an inducible extracellular serine protease implicated in fibrinolysis and in tissue remodeling. Recently, we have localized uPA mRNA strictly in limbic structures and the parietal cortex of the adult mouse brain. Here, we tested whether the systemic treatment of mice with kainic acid (KA), an amino acid inducing limbic seizures, could elevate in the brain mRNAs encoding uPA and its specific inhibitor, plasminogen activator inhibitor-1 (PAI-1), a major antifibrinolytic agent. Brain sections encompassing the hippocampus were tested through in situ hybridization using radiolabeled riboprobes specific for the two mRNA species. The results showed that KA greatly enhanced both mRNA species in sites of limbic structures and cortex. However, in the hypothalamus and brain blood vessels only PAI-1 mRNA was elevated. Those were also the only two locations where PAI-1 mRNA was detected in the non-treated control brain, although at a low level. For both mRNAs, KA enhancement was first evident 2-4 h after treatment, and it was most prolonged in the hippocampal area, where prominent hybridization signals persisted for three days. Here, both mRNAs were initially elevated in the hilar region of the dentate gyrus and in the molecular and oriens layers; however, PAI-1 mRNA became evident throughout the area, while uPA mRNA became especially pronounced in the CA3/CA4 subfield. In the cortex both mRNA types were induced, but only uPA mRNA was elevated in the retrosplenial cortex, and also in the subiculum. In the amygdaloid complex, uPA mRNA was restricted to the basolateral nucleus, whereas PAI-1 mRNA was seen throughout the structure, however, excluding this nucleus. These data show that seizure activity enhances the expression of uPA and PAI-1 genes in the brain; the patterns of enhancement suggest that the protease and its inhibitor may act in brain plasticity in synchrony, however, also independently of each other. Furthermore, the results suggest that by elevating PAI-1 mRNA in brain blood vessels, limbic seizures generate a risk for stroke.

Immediate early gene expression and delayed cell death in limbic areas of the rat brain after kainic acid treatment and recovery in the cold

Systemic injection of kainic acid (KA) results in characteristic behaviors and programmed cell death in some regions of the rat brain. We used KA followed by recovery at 4 degrees C to restrict damage to limbic structures and compared patterns of immediate early gene (IEG) expression and associated DNA binding activity in these damaged areas with that in spared brain regions. Male Wistar rats were injected with KA (12 mg/kg, i.p.) and kept at 4 degrees C for 5 h. This treatment reduced the severity of behaviors and restricted damage (observed by Nissl staining) to the CA1 and CA3 regions of the hippocampus and an area including the entorhinal cortex. DNA laddering, characteristic of apoptosis, was first evident in the hippocampus and the entorhinal cortex 18 and 22 h after KA, respectively. The pattern of IEG mRNA induction fell into three classes: IEGs that were induced in both damaged and spared areas (c-fos, fos B, jun B, and egr-1), IEGs that were induced specifically in the damaged areas (fra-2 and c-jun), and an IEG that was significantly induced by saline injection and/or the cold treatment (jun D). The pattern of immunoreactivity closely followed that of mRNA expression. Binding to the AP-1 and EGR DNA consensus sequences increased in all three regions studied. This study describes a unique modification of the animal model of KA-induced neurotoxicity which may prove a useful tool for dissecting the molecular cascade that ultimately results in programmed cell death.

Isolation of the gene encoding lamp-1, a lysosomal membrane protein, by differential screening in an animal model of status epilepticus

The present study employed differential library screening to identify genes associated with kainic acid (KA)-mediated selective neuronal death. One of the isolated clones was lamp-1, which encodes a major lysosomal membrane protein that is also present in the cell membrane. Following systemic KA treatment, lamp-1 was induced in vulnerable hippocampal and other limbic regions. This effect was blocked by cycloheximide (CHX) pre-treatment. Northern blot analysis also demonstrated the presence of lamp-1 transcripts in non-neural tissues. These findings suggest a novel role for lysosomal membrane proteins as markers of selective neuronal vulnerability.

Induction of tumor suppressor p53 and DNA fragmentation in organotypic hippocampal cultures following excitotoxin treatment

The p53 tumor suppressor gene encodes a cell cycle regulatory protein that is induced by DNA damage and has been implicated in apoptosis. To investigate whether excitotoxic cell death due to kainic acid (KA) and cell death due to N-methyl-D-aspartate (NMDA) share similar molecular mechanisms, we studied p53 expression and DNA fragmentation in organotypic hippocampal slice cultures following excitotoxin treatment. Cellular analyses showed that both p53 induction and DNA fragmentation occurred only in injured neurons following exposure to either excitotoxin. The temporal profiles of these changes demonstrated that p53 induction preceded DNA fragmentation. The extent of regional alterations in p53 expression and DNA fragmentation correlated with drug-related toxicity (i.e., NMDA > KA). These results support the hypothesis that p53 is a marker of neuronal death in the CNS and suggest the possibility that excitotoxin-mediated neuronal death may occur through a p53-dependent pathway.

Strong c-Jun/AP-1 immunoreactivity is restricted to apoptotic cells following intracerebral ibotenic acid injection in developing rats

Strong c-Jun immunoreactivity, as revealed with the antibody c-Jun/activator protein 1 (AP-1) which is raised against the amino acids 91-105 mapping with the amino terminal domain of mouse c-Jun p39, is observed in apoptotic cells, but not in necrotic cells, following intracerebral injection of ibotenic acid in the developing rat brain processed for immunohistochemistry. Immunostaining occurs in the cytoplasm and dendrites, thus suggesting impaired nuclear translocation of c-Jun in apoptotic cells. Western blotting of total brain homogenates, using the same antibody, shows a band at p39 which is more marked in treated animals than in age-matched controls. In addition, increased c-Jun N-terminal kinase 1 (JNK-1) expression, as revealed on Western blots, is found in rats treated with ibotenic acid when compared with controls. In contrast, apoptotic cells are not stained with antibodies to Jun B and Jun D. These results give further support to previous studies showing strong c-Jun expression in apoptotic cells at determinate stages of development, and emphasize that intracellular distribution of c-Jun, possible post-translational modifications of c-Jun due to phosphorylation at specific transactivation sites, and lack of associated Jun B and Jun D expression may differentiate the Jun response in apoptotic cells from other forms of cellular response involving c-Jun which are not associated with cell death.

Is cell death necessary for hippocampal mossy fiber sprouting?

To examine the relationship between cell death and sprouting of the mossy fibers, repeated seizures of the hippocampal-parahippocampal circuit were elicited in anesthetized rats. The presence of mossy fiber growth was assessed with the Timm's stain for zinc. At 4 weeks, after 18 repeated seizures, there was a significant increase in the degree of zinc containing granules in the inner molecular layer of the dentate gyrus. The amount of sprouting was less than that seen four weeks after a single injection of kainic acid. A silver impregnation stain and an assay for damaged DNA were used to detect damaged or dying neurons and immunohistochemistry for a 72 kDa heat shock protein was used to detect any neurons that had suffered potentially injurious stress. The same number of repeated seizures that caused sprouting of the mossy fibers did not cause detectable cell death or severe stress in any cells within the hippocampus, subicular region or adjacent entorhinal cortex. These experiments demonstrate that repeated seizures of the hippocampal-parahippocampal circuits can cause sprouting of mossy fibers in the absence of evidence of cell death. This supports the hypothesis that alterations in intrinsic neural excitability and impulse activity from the dentate gyrus can result in growth of axonal processes in the adult rat brain.

DNA fragmentation and Prolonged expression of c-fos, c-jun, and hsp70 in kainic acid-induced neuronal cell death in transgenic mice overexpressing human CuZn-superoxide dismutase

Kainic acid (KA) neurotoxicity was examined in transgenic (Tg) mice overexpressing human CuZn-superoxide dismutase (SOD-1). The doses of KA required to produce seizures, the severity of the seizures, and the regions damaged were similar in SOD-1 Tg and non-transgenic wild-type mice. Intraperitoneal KA injection induced seizure-related neuronal damage in the CA3 and CA1 regions of the hippocampus and in other regions of the brain in both SOD-1 Tg and wild-type mice. These damaged neurons were labeled with the terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling (TUNEL) technique up to 72 h, although no significant difference in the number of TUNEL-positive neurons was observed between SOD-1 Tg and wild-type mice. In situ hybridization showed that c-fos, c-jun, and hsp70 genes were expressed in the hippocampus, cortex, and other regions of the brain after KA treatment. The expression of these genes was maximal 1 to 4 h following KA treatment but persisted longer in the hippocampus and other regions in SOD-1 Tg compared with wild-type mice; however, cell death in the hippocampus, assessed using cresyl violet staining, was similar in SOD-1 Tg and wild-type mice. The data show that superoxide radicals modulate both immediate early gene and heat shock gene expression after KA-induced seizures. The prolonged expression of c-fos, c-jun, and hsp70 in SOD-1 Tg compared with wild-type mice may indicate that hippocampal neurons survive longer in SOD-1 Tg than in wild-type animals; however, cell death as well as the seizure threshold, seizure severity and the pattern of regional vulnerability were not affected substantially by increased levels of SOD in the brain.

Signal transduction and gene expression in the eye: a contemporary view of the pro-inflammatory, anti-inflammatory and modulatory roles of prostaglandins and other bioactive lipids

Eye tissues respond to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. These rapid events have profound effects on long-term ocular physiology. Activation of phospholipase A2 is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and platelet-activating factor (PAF). PAF accumulates in the cornea in response to injury. It has been shown to stimulate metalloproteinase gene expression in the corneal epithelium, and is, thus, implicated in the extracellular matrix remodeling that accompanies wound healing and ulceration. PAF antagonists confer protection in animal models of acute and chronic anterior segment inflammation, and block the PAF-enhanced glutamate release from retina. The latter effect suggests a role for PAF in glaucomatous neuronal damage. The eicosanoids, in particular the prostaglandins, have long been implicated in the pathophysiology of ocular inflammation and there is pharmacological evidence for their role in the regulation of intraocular pressure. The induction by PAF of the inducible prostaglandin synthase in neurons and in the corneal epithelium provides a link between the actions of these two lipid second messengers. There may be thresholds of lipid second messenger concentrations which govern their activities as physiological, defensive, or harmful.

Epilepsy in the developing brain: lessons from the laboratory and clinic

Children with epilepsy present unique challenges to the clinician. In addition to having differences in clinical and EEG phenomena, children differ from adults in regard to etiological factors, response to antiepileptic drugs (AEDs), and outcome. It is now recognized that the immature brain also differs from the mature brain in the basic mechanisms of epileptogenesis and propagation of seizures. The immature brain is more prone to seizures due to an imbalance between excitation and inhibition. gamma-Aminobutyric acid (GABA), the major CNS inhibitory neurotransmitter in the mature brain, can lead to depolarization in the hippocampal CA3 region in very young rats. There are also age-related differences in response to GABA agonists and antagonists in the substantia nigra, a structure important in the propagation of seizures. These age-related differences in response to GABAergic agents provide further evidence that the pathophysiology of seizures in the immature brain differs from that in the mature brain. Although prolonged seizures can cause brain damage at any age, the extent of brain damage after prolonged seizures is highly age dependent. Far less histological damage and fewer disturbances in cognition result from prolonged seizures in the immature brain than from seizures of similar duration and intensity in mature animals. However, detrimental effects of AEDs may be greater in the immature brain, than in the mature brain. These lessons from the animal laboratory raise questions about the appropriateness of current therapeutic approaches to childhood seizure disorders.

Relationship between GAP-43 expression in the dentate gyrus and synaptic reorganization of hippocampal mossy fibres in rats treated with kainic acid

Kainic acid-induced seizures, in adult rats produce neurodegeneration in the hippocampus followed by sprouting of the mossy fibres in the inner molecular layer of the dentate gyrus and changes in GAP-43 expression in the granule cells. In the present study we observed that 4 days after kainic acid injection a dense plexus of silver-impregnated degenerating terminals detected by Gallyas's method and a decrease of GAP-43 immunostaining was observed in the inner molecular layer of the dentate gyrus indicating deafferentiation of this region. This was associated with the formation of an intense GAP-43 immunostained band in the supragranular layer. MK-801, a non-competitive inhibitor of the NMDA receptor, which partially inhibited the behavioural seizures induced by KA, also protected from the inner molecular layer deafferentation and markedly reduced the expression of GAP-43 mRNA in the granule cells and the intense GAP-43 immunostained band in the supragranular layer, suggesting a relationship among these events. Two months after kainic acid injection the intense supragranular GAP-43 positive band was no longer evident but the whole inner molecular layer appeared more labelled in association with the formation of the collateral sprouting of the mossy fibres in the inner molecular layer as detected by Timm's staining. These effects were also markedly reduced by the pretreatment with MK-801. Taken together, these experiments indicate for the first time a direct relationship between the increase of GAP-43 immunostaining in the inner molecular layer of the dentate gyrus and the collateral sprouting of mossy fibres in this district in response to kainic acid induced seizures. This further supports the hypothesis that the early induction of GAP-43 in granule cells may be one of the molecular mechanisms required for the synaptic reorganization of the mossy fibres.

Comparison of neurodegenerative pathology in transgenic mice overexpressing V717F beta-amyloid precursor protein and Alzheimer's disease

Overexpression of mutated human amyloid precursor protein (hAPP717V-->F) under control of platelet-derived growth factor promoter (PDAPP minigene) in transgenic (tg) mice results in neurodegenerative changes similar to Alzheimer's disease (AD). To clarify the pathology of these mice, we studied images derived from laser scanning confocal and electron microscopy and performed comparisons between PDAPP tg mice and AD. Similar to AD, neuritic plaques in PDAPP tg mouse contained a dense amyloid core surrounded by anti-hAPP- and antineurofilament-immunoreactive dystrophic neurites and astroglial cells. Neurons were found in close proximity to plaques in PDAPP tg mice and, to a lesser extent, in AD. In PDAPP tg mice, and occasionally in AD, neuronal processes contained fine intracellular amyloid fibrils in close proximity to the rough endoplasmic reticulum, coated vesicles, and electron-dense material. Extracellular amyloid fibrils (9-11 nm in diameter) were abundant in PDAPP tg and were strikingly similar to those observed in AD. Dystrophic neurites in plaques of PDAPP tg mouse and AD formed synapses and contained many dense multilaminar bodies and neurofilaments (10 nm). Apoptotic-like figures were present in the tg mice. No paired helical filaments have yet been observed in the heterozygote PDAPP tg mice. In summary, this study shows that PDAPP tg mice develop massive neuritic plaque formation and neuronal degeneration similar to AD. These findings show that overproduction of hAPP717V-->F in tg mice is sufficient to cause not only amyloid deposition, but also many of the complex subcellular degenerative changes associated with AD.

Kainic acid induces apoptosis in neurons

Growing evidence suggests that non-N-methyl-D-aspartate receptor activation may contribute to neuronal death in both acute and chronic neurological diseases. The intracellular processes that mediate this form of neuronal death are poorly understood. We have previously characterized a model of kainic acid neurotoxicity using cerebellar granule cell neurons in vitro and we sought to determine the mechanism of kainic acid-induced neuronal degeneration. We found DNA laddering by agarose gel electrophoresis, cellular DNA fragmentation by in situ end labeling of DNA, and chromatin condensation using a fluorescent DNA intercalating dye, in cerebellar granule cells following exposure to kainic acid (100 microM). Aurintricarboxylic acid protected cerebellar granule cells from kainic acid-induced death. While the morphological and biochemical features of neuronal death induced by kainic acid resembled low K(+)-induced apoptosis in cerebellar granule cells, the time interval from the institution of the death promoting condition to neuronal death was shorter with kainic acid and did not require new protein or RNA synthesis. These results demonstrate that kainic acid receptor activation can induce transcription-independent apoptosis in neurons. This in vitro model should be useful in identifying the intracellular pathways that link kainic acid receptor activation with apoptosis.

Apoptotic morphology of dentate gyrus granule cells following experimental cortical impact injury in rats: possible role in spatial memory deficits

Loss of hippocampal neurons as a result of traumatic brain injury (TBI) is thought to contribute to the observed spatial memory deficits. Using a rodent model of experimental brain injury, we have examined the nature of hippocampal cell death following TBI. Light microscope examination of stained sections showed the presence of a large number of hyperchromatic and dystrophic neurons in the dentate gyrus of the hippocampus. These cells appeared to be undergoing nuclear condensation. Electron microscope examination demonstrated the presence of cell shrinkage, condensed chromatin, nuclear segmentation, and cytoplasmic vacuolization. Detection of a DNA ladder and in situ labeling (TUNEL) were also consistent with the process of apoptosis. However, in some dystrophic neurons these morphologies were also accompanied by the presence of swollen mitochondria and a lack of distinctive rough endoplasmic reticulum which are typically associated with necrosis. These findings show that cortical impact injury produces cell death in the hippocampus which has both apoptotic and necrotic features.

Absence of excitotoxic neuropathology in microencephalic rats after systemic kainic acid administration

We have s.c. injected with kainic acid (12 mg/kg) normal adult rats as well as rats rendered microencephalic by selectively timed administration of the DNA alkylating agent methylazoxymethanol acetate (MAM) to the mother during pregnancy. Histological examination of the brains revealed that normal animals underwent neurodegeneration in brain regions sensitive to kainic acid excitotoxicity, such as the olfactory cortex and the hippocampus, while no damage was apparent in the same regions of microencephalic rats. Evaluation of the neurotoxic outcome consequent to the excitotoxic stimulation, was quantitatively performed by measuring the levels of appropriate neurochemical markers 15 days after kainic acid injection. In normal animals, this resulted in significant decrease (up to 60% in the olfactory cortex and 30% in the hippocampus) of markers related to glutamatergic and GABAergic neurons, whereas in MAM-treated rats the same markers were not significantly affected, thus demonstrating a substantial protection against the excitotoxic insult in the microencephalic condition.

Conditionally immortalized neural progenitor cell lines integrate and differentiate after grafting to the adult rat striatum. A combined autoradiographic and electron microscopic study

Neural progenitor cell lines, generated by conditional immortalization from the embryonic CNS, have previously been shown to survive and integrate after transplantation to the adult brain. The present study was designed to investigate the in vivo differentiation and morphological features of grafted neural progenitors using combined autoradiography and transmission electron microscopy of two temperature-sensitive neural progenitor cell lines, HiB5 and ST14A, labeled with 3H-thymidine prior to grafting. Two weeks after transplantation to the striatum the cells were found dispersed over an area extending about 1.5 mm from the injection site. Labeled cells located within the myelinated fiber bundles of the internal capsule were closely associated with myelinated axons and presented profiles similar to oligodendrocytes, while most of the grafted cells in the grey matter had morphological features of astroglia. Some labeled cells occurred also in close association with small blood vessels, morphologically resembling host pericytes. The results show that the immortalized neural progenitors can differentiate into mature glial cells, including astrocytes, oligodendrocytes and pericytes, after implantation into the adult striatum. The ability of the cells to become fully integrated with the resident glial population suggests that they will be highly useful as vehicles for intracerebral transgene expression in ex vivo gene transfer.

Sustained induction of prostaglandin endoperoxide synthase-2 by seizures in hippocampus. Inhibition by a platelet-activating factor antagonist

Prostaglandin G/H synthase-2 and zif-268 mRNA expression is transiently induced in rat brain by kainic acid (KA)-induced seizures and by a single electroconvulsive shock. Induction of both genes by KA shows neuroanatomical specificity in the order hippocampus > cerebral cortex > striatum > brain stem > cerebellum. Nuclear run-on and Western blotting shows that both genes are transcriptionally activated, and that kainic acid up-regulation of prostaglandin G/H synthase-2 mRNA expression in hippocampus matches increased protein levels. Whereas the magnitude of hippocampal zif-268 mRNA induction is similar in both seizure models, peak induction of prostaglandin G/H synthase-2 mRNA is 7-fold greater in the kainic acid model than in the electroconvulsive shock model and is much more prolonged. Pretreatment of animals by intracerebroventricular injection with the intracellular platelet-activating factor receptor antagonist BN 50730 strongly attenuates kainic acid and electroconvulsive shock induction of prostaglandin G/H synthase-2 expression. The drug partially inhibits electroconvulsive shock induction of zif-268, but is relatively ineffective against kainic acid-induced zif-268 expression. Seizure-induced expression of both genes involves platelet-activating factor, but the mechanisms of induction must be otherwise distinct. The selectively elevated induction of hippocampal prostaglandin G/H synthase-2 by kainic acid correlates with a neuroanatomical region in which the agonist induces neuronal damage.

In vivo elevation of extracellular potassium in the rat amygdala increases extracellular glutamate and aspartate and damages neurons

It is well known that high potassium (K+) solutions introduced by microdialysis into normal brain increase the extracellular concentration of the excitatory amino acid glutamate, and in vitro studies suggest that a high exogenously applied glutamate concentration can produce excitotoxic neuronal death. However, only recently were in vivo studies undertaken to determine whether high-K+ exposure damages neurons. We implanted microdialysis probes into rat amygdalae bilaterally, and after a 2-h baseline period exposed one side to a modified Krebs-Ringer-bicarbonate solution containing 100 mmol/l KCl for 30,50 and 70 min, followed by a 2-h recovery period, and 70 min and 3 h without a recovery period. Of 100.9 +/- 2.0 mmol/l KCl, 12.0 +/- 1.0% was extracted by amygdalar tissue in vivo. Election of the extracellular K+ concentration in the amygdala for 70 min or longer without a recovery period produced extensive neuronal damage and edematous-appearing neuropil in the tissue dialysed, as well as loss of normal neurons. Histological evidence of edema subsided in the groups with a 2-h recovery period. Although the number of damaged neurons was not significantly higher in the group with a 70 min high-K+ exposure and 2-h recovery period, the number of normal neurons was reduced, suggesting cell loss. During 70-min high-K+ exposure, the extracellular glutamate concentration increased to 242-377% of baseline during the first 60 min, and extracellular aspartate rose to 162-213% during the first 50 min; extracellular taurine rose even higher, to 316-567% of baseline, and glutamine fell to 14-27% of baseline. Extracellular serine was decreased at 20, 50 and 70 min of high-K+ exposure; extracellular glycine was unchanged. The elevated extracellular glutamate and aspartate concentrations suggest that exposure of the amygdala to high extracellular K+ may produce cell death through an excitotoxic process, and point the way to future studies to define the specific mechanisms involved.