Vulnerability of the hippocampus to kainate excitotoxicity in the aged, mature and young adult rat

A Novel Rat Infant Model of Medial Temporal Lobe Epilepsy Reveals New Insight into the Molecular Biology and Epileptogenesis in the Developing Brain

Although several adult rat models of medial temporal lobe epilepsy (mTLE) have been described in detail, our knowledge of mTLE epileptogenesis in infant rats is limited. Here, we present a novel infant rat model of mTLE (InfRPil-mTLE) based on a repetitive, triphasic injection regimen consisting of low-dose pilocarpine administrations (180 mg/kg. i.p.) on days 9, 11, and 15 post partum (pp). The model had a survival rate of >80% and exhibited characteristic spontaneous recurrent electrographic seizures (SRES) in both the hippocampus and cortex that persisted into adulthood. Using implantable video-EEG radiotelemetry, we quantified a complex set of seizure parameters that demonstrated the induction of chronic electroencephalographic seizure activity in our InfRPil-mTLE model, which predominated during the dark cycle. We further analyzed selected candidate genes potentially relevant to epileptogenesis using a RT-qPCR approach. Several candidates, such as the low-voltage-activated Ca2+ channel Cav3.2 and the auxiliary subunits β 1 and β 2, which were previously reported to be upregulated in the hippocampus of the adult pilocarpine mTLE model, were found to be downregulated (together with Cav2.1, Cav2.3, M1, and M3) in the hippocampus and cortex of our InfRPil-mTLE model. From a translational point of view, our model could serve as a blueprint for childhood epileptic disorders and further contribute to antiepileptic drug research and development in the future.

Traffic-related air pollution impact on mouse brain accelerates myelin and neuritic aging changes with specificity for CA1 neurons

Traffic-related air pollution (TRAP) is associated with lower cognition and reduced white matter volume in older adults, specifically for particulate matter <2.5-μm diameter (PM2.5). Rodents exposed to TRAP have shown microglial activation and neuronal atrophy. We further investigated age differences of TRAP exposure, with focus on hippocampus for neuritic atrophy, white matter degeneration, and microglial activation. Young- and middle-aged mice (3 and 18 months female C57BL/6J) were exposed to nanoscale-PM (nPM, <0.2 μm diameter). Young mice showed selective changes in the hippocampal CA1 region, with neurite atrophy (-25%), decreased MBP (-50%), and increased Iba1 (+50%), with dentate gyrus relatively unaffected. Exposure to nPM of young mice decreased GluA1 protein (-40%) and increased TNFa mRNA (10×). Older controls had age changes approximating nPM effects on young, with no response to nPM, suggesting an age-ceiling effect. The CA1 selective vulnerability in young mice parallels CA1 vulnerability in Alzheimer's disease. We propose that TRAP-associated human cognitive and white matter changes involve hippocampal responses to nPM that begin at younger ages.

Age- and location-dependent differences in store depletion-induced h-channel plasticity in hippocampal pyramidal neurons

Disruptions of endoplasmic reticulum (ER) Ca(2+) homeostasis are heavily linked to neuronal pathology. Depletion of ER Ca(2+) stores can result in cellular dysfunction and potentially cell death, although adaptive processes exist to aid in survival. We examined the age and region dependence of one postulated, adaptive response to ER store-depletion (SD), hyperpolarization-activated cation-nonspecific (h)-channel plasticity in neurons of the dorsal and ventral hippocampus (DHC and VHC, respectively) from adolescent and adult rats. With the use of whole-cell patch-clamp recordings from the soma and dendrites of CA1 pyramidal neurons, we observed a change in h-sensitive measurements in response to SD, induced by treatment with cyclopiazonic acid, a sarcoplasmic reticulum/ER Ca(2+)-ATPase blocker. We found that whereas DHC and VHC neurons in adolescent animals respond to SD with a perisomatic expression of SD h plasticity, adult animals express SD h plasticity with a dendritic and somatodendritic locus of plasticity in DHC and VHC neurons, respectively. Furthermore, SD h plasticity in adults was dependent on membrane potential and on the activation of L-type voltage-gated Ca(2+) channels. These results suggest that cellular responses to the impairment of ER function, or ER stress, are dependent on brain region and age and that the differential expression of SD h plasticity could provide a neural basis for region- and age-dependent disease vulnerabilities.

Neuroprotection by glutamate receptor antagonists against seizure-induced excitotoxic cell death in the aging brain

We previously have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death among two inbred strains of mice. We have also reported that the age-related increased susceptibility to the neurotoxic effects of seizure-induced injury is regulated in a strain-dependent manner. In the present study, we wanted to begin to determine the pharmacological mechanism that contributes to variability in the response to the neurotoxic effects of kainate. Thus, we compared the effects of the NMDA receptor antagonist, MK-801 and of the AMPA receptor antagonist NBQX on hippocampal damage in the kainate model of seizure-induced excitotoxic cell death in young, middle-aged, and aged C57BL/6 and FVB/N mice, when given 90 min following kainate-induced status epilepticus. Following kainate injections, mice were scored for seizure activity and brains from mice in each age and antagonist group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced injury. Administration of MK-801 significantly reduced the extent of hippocampal damage in young, mature and aged FVB/N mice, while application of NBQX was only effective at attenuating cell death in young and aged mice throughout all hippocampal subfields. Our results suggest that both NMDA and non-NMDA receptors are involved in kainate-induced cell death in the mouse and suggest that aging may differentially affect the ability of neuroprotectants to protect against hippocampal damage. Differences in the effectiveness of these two antagonists could result from differential regulation of glutamatergic neurotransmitter systems or ion channel specificity.

Diffusion weighted magnetic resonance imaging of neuronal activity in the hippocampal slice model

Functional magnetic resonance imaging (fMRI) has become the leading modality for studying the working brain. Being based on measuring the haemodynamic changes after enhanced mass neuronal activity the spatiotemporal resolution of the method is somewhat limited. Alternative MR-based methods for detection of brain activity have been proposed and investigated and studies have reported functional imaging based on diffusion weighted (DW) MRI. The basis for such DW fMRI is believed to be the sensitivity of diffusion weighted MRI to changes in tissue micro-structure. However, it remains unclear whether signal changes observed with these methods reflect cell swelling related to neural activation, residual vascular effects, or a combination of both. Here we present evidence of a detectable, activity-related change in the diffusion weighted MR-signal from the cellular level in live hippocampal slices in the absence of vasculature. Slices are exposed to substances which evoke or inhibit neural activity and the effects are evaluated and compared. The results are also compared to earlier DW fMRI studies in humans.

Progesterone blocks estrogen neuroprotection from kainate in middle-aged female rats

The neuroprotective effects of estrogen in young adult rodents are well established. Less well understood is how estrogen neuroprotection is affected by aging and interactions with progesterone. In this study, we investigated the effects of estrogen and continuous progesterone, both alone and in combination, on hippocampal neuron survival following kainate lesion in 14-month-old female rats entering reproductive senescence. Our results show that ovariectomy-induced hormone depletion did not significantly affect the extent of kainate-induced neuron loss. Treatment of ovariectomized rats with estrogen significantly reduced neuron loss, however this effect was blocked by co-administration of continuous progesterone. Treatment of ovariectomized rats with progesterone alone did not significantly affect kainate toxicity. These results provide new insight into factors that regulate estrogen neuroprotection, which has important implications for hormone therapy in postmenopausal women.

Folate deficiency induces neurodegeneration and brain dysfunction in mice lacking uracil DNA glycosylase

Folate deficiency and resultant increased homocysteine levels have been linked experimentally and epidemiologically with neurodegenerative conditions like stroke and dementia. Moreover, folate deficiency has been implicated in the pathogenesis of psychiatric disorders, most notably depression. We hypothesized that the pathogenic mechanisms include uracil misincorporation and, therefore, analyzed the effects of folate deficiency in mice lacking uracil DNA glycosylase (Ung-/-) versus wild-type controls. Folate depletion increased nuclear mutation rates in Ung-/- embryonic fibroblasts, and conferred death of cultured Ung-/- hippocampal neurons. Feeding animals a folate-deficient diet (FD) for 3 months induced degeneration of CA3 pyramidal neurons in Ung-/- but not Ung+/+ mice along with decreased hippocampal expression of brain-derived neurotrophic factor protein and decreased brain levels of antioxidant glutathione. Furthermore, FD induced cognitive deficits and mood alterations such as anxious and despair-like behaviors that were aggravated in Ung-/- mice. Independent of Ung genotype, FD increased plasma homocysteine levels, altered brain monoamine metabolism, and inhibited adult hippocampal neurogenesis. These results indicate that impaired uracil repair is involved in neurodegeneration and neuropsychiatric dysfunction induced by experimental folate deficiency.

Effect of age on kainate-induced seizure severity and cell death

While the onset and extent of epilepsy increases in the aged population, the reasons for this increased incidence remain unexplored. The present study used two inbred strains of mice (C57BL/6J and FVB/NJ) to address the genetic control of age-dependent neurodegeneration by building upon previous experiments that have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death. We determined if seizure induction and seizure-induced cell death are affected differentially in young adult, mature, and aged male C57BL/6J and FVB/NJ mice administered the excitotoxin, kainic acid. Dose response testing was performed in three to four groups of male mice from each strain. Following kainate injections, mice were scored for seizure activity and brains from mice in each age group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced brain damage. Irrespective of the dose of kainate administered or the age group examined, resistant strains of mice (C57BL/6J) continued to be resistant to seizure-induced cell death. In contrast, aged animals of the FVB/NJ strain were more vulnerable to the induction of behavioral seizures and associated neuropathology after systemic injection of kainic acid than young or middle-aged mice. Results from these studies suggest that the age-related increased susceptibility to the neurotoxic effects of seizure induction and seizure-induced injury is regulated in a strain-dependent manner, similar to previous observations in young adult mice.

Domoic acid preconditioning and seizure induction in young and aged rats

Clinical reports suggest that the elderly are hypersensitive to the neurological effects of domoic acid (DOM). In the present study we assessed DOM-induced seizures in young and aged rats, and seizure attenuation following low-dose DOM pretreatment (i.e. preconditioning). Seizure behaviours following saline or DOM administration (0.5-2mg/kg i.p.) were continuously monitored for 2.5h in naïve and DOM preconditioned rats. Competitive ELISA was used to determine serum and brain DOM concentrations. Dose- and age-dependent increases in seizure activity were evident in response to DOM. Lower doses of DOM in young and aged rats promoted low level seizure behaviours. Animals administered high doses (2mg/kg in young; 1mg/kg in aged) progressed through various stages of stereotypical behaviour (e.g., head tics, scratching, wet dog shakes) before ultimately exhibiting tonic-clonic convulsions. Serum and brain DOM analysis indicated impaired renal clearance as contributory to increased DOM sensitivity in aged animals, and this was supported by seizure analysis following direct intrahippocampal administration of DOM. Preconditioning young and aged animals with low-dose DOM 45-90 min before high-dose DOM significantly reduced seizure intensity. We conclude that age-related supersensitivity to DOM is related to reduced clearance rather than increased neuronal sensitivity, and that preconditioning mechanisms underlying an inducible tolerance to excitotoxins are robustly expressed in both young and aged CNS.

In vivo mapping of temporospatial changes in manganese enhancement in rat brain during epileptogenesis

Mesial temporal lobe epilepsy is associated with structural and functional abnormalities, such as hippocampal sclerosis and axonal reorganization. The temporal evolution of these changes remains to be determined, and there is a need for in vivo imaging techniques that can uncover the epileptogenic processes at an early stage. Manganese-enhanced magnetic resonance imaging may be useful in this regard. The aim of this study was to analyze the temporospatial changes in manganese enhancement in rat brain during the development of epilepsy subsequent to systemic kainate application (10 mg/kg i.p.). MnCl(2) was given systemically on day 2 (early), day 15 (latent), and 11 weeks (chronic phase) after the initial status epilepticus. Twenty-four hours after MnCl(2) injection T1-weighted 3D MRI was performed followed by analysis of manganese enhancement. In the medial temporal lobes, there was a pronounced decrease in manganese enhancement in CA1, CA3, dentate gyrus, entorhinal cortex and lateral amygdala in the early phase. In the latent and chronic phases, recovery of the manganese enhancement was observed in all these structures except CA1. A significant increase in manganese enhancement was detected in the entorhinal cortex and the amygdala in the chronic phase. In the latter phase, the structurally intact cerebellum showed significantly decreased manganese enhancement. The highly differentiated changes in manganese enhancement are likely to represent the net outcome of a number of pathological and pathophysiological events, including cell loss and changes in neuronal activity. Our findings are not consistent with the idea that manganese enhancement primarily reflects changes in glial cells.

Hippocampal mossy fiber sprouting and elevated trkB receptor expression following systemic administration of low dose domoic acid during neonatal development

We have previously reported that serial systemic injections of low-dose (subconvulsive) domoic acid (DOM) during early postnatal development produces changes in both behavior and hippocampal cytoarchitecture in aged rats (17 months) that are similar to those seen in existing animal models of temporal lobe epilepsy. Herein we report further hippocampal changes, consisting of mossy fiber sprouting and associated changes in the trkB receptor population in young adult (3 months) rats, and further, report that these changes show regional variation throughout the septo-temporal axis of the hippocampus. Groups of Sprague Dawley rat pups were injected daily from postnatal day 8-14 with either saline (n = 23) or 20 microg/kg DOM (n = 25), tested for key indicators of neonatal neurobehavioral development, and then left undisturbed until approximately 90 days of age, at which time brain tissue was removed, hippocampi were dissected, fixed and processed using either Timm's stain to visualize hippocampal mossy fiber sprouting (MFS) or trkB immunohistochemistry to visualize full length trkB receptors. Multiple sections from dorsal, mid, and ventral hippocampus were analyzed separately and all measures were conducted using image analysis software. The results indicate significant increases in MFS in the inner molecular layer in treated animals with corresponding changes in trkB receptor density. Further we identified significant increases in trkB receptor density in the hilus of the dentate gyrus and area CA3 and report increased mossy fiber terminal density in the stratum lucidum in treated rats. The magnitude of these changes differed between sections from dorsal, mid, and ventral hippocampus. We conclude that low dose neonatal DOM produces cytoarchitectural changes indicative of abnormal development and/or synaptic plasticity that are progressive with age and show regional variation within the hippocampal formation.

Alzheimer' s disease, oxidative stress and gammahydroxybutyrate

Although the cause of Alzheimer's disease is unknown, oxidative stress, energy depletion, excitotoxicity and vascular endothelial pathology are all considered to play a part in its pathogenesis. In reaction to these adverse events, the Alzheimer brain appears to deploy a highly conserved biological response to tissue stress. Oxidative metabolism is turned down, the expression of antioxidative enzymes is increased and intermediary metabolism is shifted in the direction of the pentose phosphate shunt to promote reductive detoxification, repair and biosynthesis. Gathering evidence suggests that the release of beta-amyloid and the formation of neurofibrillary tangles, the two hallmarks of Alzheimer's disease, are components of this protective response. Gammahydroxybutyrate (GHB), an endogenous short chain fatty acid, may be able to buttress this response. GHB can reduce glucose utilization, shift intermediary metabolism in the direction the pentose phosphate shunt and generate NADPH, a key cofactor in the activity of many antioxidative and reductive enzymes. GHB has been shown to spare cerebral energy utilization, block excitotoxicity and maintain vascular integrity in the face of impaired perfusion. Most important, GHB has repeatedly been shown to prevent the tissue damaging effects of oxidative stress. It may therefore be possible to utilize GHB to strengthen the brain's innate defences against the pathological processes operating in the Alzheimer brain and, in this way, stem the advance of Alzheimer's disease.

Age Influences the Expression of GAP-43 in the Rat Hippocampus following Seizure

BACKGROUND

Normal aging is associated with impairments in learning and memory and motor function. One viable hypothesis is that these changes reflect an age-related decrease in brain plasticity.

OBJECTIVE

The aim of the present study was to identify age-related changes in the time course of expression of the axonal growth associated protein 43 (GAP-43) in a rat model of brain plasticity.

METHODS

We examined by Northern blotting, in situ hybridization, and immunohistochemistry the effects of age on the time course of the expression GAP-43 following pentylenetetrazole-induced seizure in the hippocampus of 3-, 18-, and 28-month-old rats.

RESULTS

In this model of brain plasticity, young rats displayed a decrease in GAP-43 mRNA levels in CA1, CA3, and polymorphic regions, lasting from 10 h to 3 days after seizure. This was followed by recovery, with peak expression between days 10 and 20. The baseline levels of GAP-43 mRNA decreased with age, especially in the CA3 region. Despite lower baseline levels, middle-aged rats showed the same pattern of upregulation of GAP-43 mRNA expression as the young animals. Old rats showed only minimal upregulation, however, and this occurred only in the polymorphic layer. The level GAP-43 protein itself was higher in old control rats than in the other two control groups, a condition that was transiently reversed by seizure activity.

CONCLUSIONS

Middle-aged rats are still capable of a sustained, though diminished, response to seizure activity, while old rats lose this ability. Disruption of the temporal and anatomical coordination of expression of GAP-43 may contribute to the general decline in brain plasticity with age.

Soluble KDI domain of γ1 laminin protects adult hippocampus from excitotoxicity of kainic acid

Recent data indicate that the soluble KDI domain of gamma1 laminin promotes survival and neurite outgrowth of human central neurons in vitro (Liebkind et al.[2003] J Neurosci Res 73:637-643), and seems to neutralize both glia- and myelin-derived signals that hamper regeneration in the central nervous system (CNS) of adult mammals. We show that damage of adult rat neocortical and hippocampal areas by a stereotaxic injection of kainic acid (KA) is prevented by a preceding injection of the soluble KDI domain. In the presence of the KDI domain, both neocortical and hippocampal areas show extensive gliosis but have viable neurons and glial cells, which are absent and the areas fully destroyed after injection of KA alone. This result indicates that the KDI domain of the gamma1 laminin protects the CNS against excitotoxic insults and promotes survival of both neurons and glial cells. The KDI domain may thus be a potential drug to prevent CNS damage induced by neurodegenerative disorders, mechanical injury, or ischemia.

Hippocampal neurotransplantation evaluated in the rat kainic acid epilepsy model

OBJECTIVE

Neurotransplantation has focused on disorders that involve subcortical brain targets. We evaluated the concepts of epileptic focus repair and changes in animal behavior through replacement of lost hippocampal neurons. The safety of hippocampal neurotransplantation was assessed in the rat kainic acid (KA) epilepsy model.

METHODS

Sixty-three rats were studied and classified into six groups: KA plus 40,000 LBS-Neurons (Layton BioScience, Sunnyvale, CA; n = 13); KA plus 80,000 cells (n = 12); KA plus media (n = 9); no-KA plus 40,000 cells (n = 12); no-KA plus 80,000 cells (n = 12); and no-KA plus media (n = 5). Clinical observation (2 h daily) and electroencephalogram recording (3 h every other week) were performed to check for seizures until Week 11 after KA injection. On Week 12, the Morris water maze test was performed to assess spatial learning and memory.

RESULTS

Four rats were excluded because of intracranial hematoma or abscess. In the clinical observation of seizures, the no-KA plus media group had significantly fewer seizures than rats that received KA followed by injection of 40,000 cells, 80,000 cells, or media (P = 0.001, 0.0004, and 0.004, respectively). On electroencephalographic analysis, there was no significant difference between any of the groups. Transplanted rats with KA-induced epilepsy did not have an increased number of seizures. In the Morris water maze test, the hidden platform task showed that the KA plus 80,000 cell group had significantly longer swim latencies than groups with no-KA plus 40,000 cells (P = 0.035) or no-KA plus 80,000 cells (P = 0.015), demonstrating the behavioral deficits caused by KA injection. The probe trial showed no significant difference for the percentage of time in the target quadrant between any of the groups. Histological studies showed that 26 (59%) of 44 transplanted rats had evidence of graft survival.

CONCLUSION

The safety of cortical neurotransplantation was demonstrated, even in an animal model predisposed to epilepsy. We did not find evidence for cessation of seizures or improvement in behavior using this model.

Kainate receptor agonists and antagonists mediate tolerance to kainic acid and reduce high-affinity GTPase activity in young, but not aged, rat hippocampus

Domoic acid acts at both kainic acid (KA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-sensitive glutamate receptors and induces tolerance against subsequent domoic acid insult in young but not aged rat hippocampus. To determine the receptor specificity of this effect, tolerance induction was examined in hippocampal slices from young and aged rats. Slices were preconditioned by exposure to low-dose KA to activate kainate receptors, or the AMPA-receptor selective agonist (S)-5-fluorowillardiine (FW), and following washout, tolerance induction was assessed by administration of high concentrations of KA or FW (respectively). FW preconditioning failed to induce tolerance to subsequent FW challenges, while KA-preconditioned slices were significantly resistant to the effects of high-dose KA. KA preconditioning failed to induce tolerance in aged CA1. Given the lasting nature of the tolerance effect, we examined G-protein-coupled receptor function. A number of ionotropic KA receptor agonists and antagonists significantly reduced constitutive GTPase activity in hippocampal membranes from young but not aged rats. Furthermore, in young CA1, low concentrations of the AMPA/KA blocker GYKI-52466 also induced tolerance to high-dose KA. Our findings suggest that tolerance is triggered by a selective reduction in constitutive KA-sensitive G-protein activity, and that this potential neuroprotective mechanism is lost with age.

Alterations in hippocampal voltage-gated calcium channel alpha 1 subunit expression patterns after kainate-induced status epilepticus in aging rats

Young adult and aged male Fisher 344 rats underwent kainate-induced convulsive status epilepticus (SE) for 4 h prior to sacrifice to determine potential aging-related differences in the effect of prolonged SE on the expression of hippocampal voltage-gated calcium channels (VGCCs). Immunohistochemistry was performed on hippocampal sections using antibodies directed against the alpha1 subunit of class A-D VGCCs. Compared to age-matched controls, SE animals showed a marked loss of alpha1A immunoreactivity (IR) in CA3 and the hilus, which was more prominent in aged animals. Alpha1B-IR was decreased selectively in the stratum lucidum of CA3. Alpha1C-IR was increased on neuronal somata in the pyramidal and granule cell layers of both age groups. In contrast, there was a marked decrease of alpha1C-IR in the neuropil of CA3 stratum pyramidale and portions of CA1, which was more pronounced in aged animals. Alpha1D-IR was decreased in CA3 and the hilus, which was more prominent in aged animals. Nissl staining demonstrated mild somal dysmorphia in the pyramidal cell layer of CA3, which was more apparent in aged animals. Fluoro-Jade B staining was prominent in the stratum pyramidale of CA3 and in the hilus of aged SE animals. These results demonstrated that expression patterns of hippocampal high-threshold VGCC alpha1 subunits were altered variably during prolonged convulsive SE and were associated with prominent early degenerative changes in aged neurons in CA3 and the hilus.

Reduced susceptibility to kainic acid-induced excitoxicity in T-cell deficient CD4/CD8(-/-) and middle-aged C57BL/6 mice

Kainic acid (KA)-induced hippocampal injury is a good model for studying human neurodegenerative diseases. To investigate the roles of immune cells and age related changes in neurodegeneration, we used this model to assess reactions in young and middle-aged wild-type and CD4/CD8(-/-) mice by intranasal administration of KA. We found that CD4/CD8-deficiency resulted in a significant reduction of the severity of clinical signs and pathological changes in KA-treated young, but not in KA-treated middle-aged mice. Middle-aged wild-type mice had a similar reaction to KA insult as young and middle-aged CD4/CD8(-/-) mice. CD4/CD8(-/-) mice exhibited decreased locomotor and rearing activities as they approached to middle-aged state, which was not seen in wild-type mice. In addition, CD4/CD8-deficiency and increased age prevented KA-induced increase of both locomotor and rearing activities. The results suggest that a decline of immunological function is associated with aging, and both of them may contribute to the relative resistance to KA-induced neurotoxicity.

N-methyl-D-aspartate receptor blockade after status epilepticus protects against limbic brain damage but not against epilepsy in the kainate model of temporal lobe epilepsy

Most patients with temporal lobe epilepsy (TLE), the most common type of epilepsy, show pronounced loss of neurons in limbic brain regions, including the hippocampus. The massive neurodegeneration in the hippocampus is known as hippocampal sclerosis, and is considered one of the hallmarks of this type of difficult-to-treat epilepsy. There is a long and ongoing debate on whether this sclerosis is the result of an initial pathological event, such as a status epilepticus (S.E.), stroke or head trauma, which often precedes the development of TLE, or is caused by the spontaneous recurrent seizures (SRS) once epilepsy has developed. At present, pharmacological prevention of limbic sclerosis is not available. In a clinical situation, such prevention would only be possible if delayed cell death developing after an initial pathological event is involved. Assuming that sclerotic brain lesions provoke epileptogenesis and that delayed cell death is involved in these lesions, it should be possible to prevent both the lesions and the epilepsy by a prophylactic treatment after an initial insult such as an S.E. In order to test this hypothesis, we used a rat model of TLE in which limbic brain lesions and epilepsy with SRS develop after a kainate-induced S.E. A single low dose of the N-methyl-D-aspartate (NMDA) receptor blocker dizocilpine (MK-801) significantly reduced the damage in limbic regions, including the hippocampus and piriform cortex, and completely protected several rats from such damage when given after an S.E. of 90 min induced by kainate, strongly suggesting that delayed cell death is involved in the damage. This was substantiated by the use of molecular and immunohistochemical markers of delayed active ("programmed") cell death. However, the neuroprotection by dizocilpine did not prevent the development of SRS after the S.E., suggesting that structures not protected by dizocilpine may play a role in the genesis of SRS or that epileptogenesis is not the consequence of structural lesions in the limbic system. The only brain regions that exhibited neuronal damage in all rats with SRS were the hilus of the dentate gyrus and the mediodorsal thalamus, although treatment with dizocilpine reduced the severity of damage in the latter region. The data indicate that NMDA receptor blockade immediately after a prolonged S.E. is an effective means to reduce the damage produced by a sustained S.E. in several brain regions, including the hippocampus, but show that this partial neuroprotection of the limbic system does not prevent the development of epilepsy.

Exercise increases the vulnerability of rat hippocampal neurons to kainate lesion

Available evidence suggests that regular, moderate-intensity exercise has beneficial effects on neural health, perhaps including neuroprotection. To evaluate this idea further, we compared the severity of kainate-induced neuronal loss in exercised versus sedentary female rats. Stereological estimations of neuron number revealed that rats in the exercise condition exhibited significantly greater neuron loss in hippocampal region CA2/3, suggesting that high levels of physical activity may increase neuronal vulnerability to excitotoxicity.

Age-related changes in tolerance to the marine algal excitotoxin domoic acid

During an incident of toxic mussel poisoning, the epileptogenic excitotoxin domoic acid (DOM) was associated with lasting neurological deficits mainly in older patients (), suggesting supersensitivity to excitotoxins is a feature of brain aging. Here, hippocampal slices from young (3 months) and aged (26-29 months) Sprague Dawley rats were assessed by CA1 field potential analysis before and after preconditioning with DOM. In naïve slices from young animals, DOM produced initial hyperexcitability followed by significant dose-dependent reductions in population spike amplitude during prolonged application. Following toxin washout, only small changes in neuronal activity were evident during a second application of DOM, suggesting that a resistance to the effects of DOM occurs in hippocampal slices which have undergone prior exposure to DOM. This inducible tolerance was not antagonized by the NMDA receptor blockers APV or MK-801, nor was it diminished by the group I, II or III mGluR blockers AIDA, CPPG and EGLU. Likewise, neither the AMPA/KA blocker CNQX nor the VSCC blocker nifedipine were effective in blocking tolerance induction in young slices. Field potential analysis revealed significant age-related reductions in CA1 EPSP strength, population spike amplitude and paired-pulse inhibition, but aged slices did not differ in sensitivity to DOM relative to young. However, aged CA1 failed to exhibit any tolerance to DOM following preconditioning, suggesting that a loss of inducible neuroprotective mechanisms may account for increased sensitivity to excitotoxins during aging.

Delayed sclerosis, neuroprotection, and limbic epileptogenesis after status epilepticus in the rat

PURPOSE

Hippocampal sclerosis and massive neurodegeneration in other parts of the limbic system are considered hallmarks of temporal lobe epilepsy. Using the rat model of kainate-induced status epilepticus, we sought to determine if limbic sclerosis after an excitotoxic insult follows a delayed type of neurodegeneration and is thus accessible to neuroprotective intervention after the insult. Effective pharmacologic neuroprotection after status epilepticus also addresses the old question of whether degenerative morphologic changes after an epilepsy-inducing event like status epilepticus are the primary cause of epileptogenesis (i.e., the development of recurrent spontaneous seizures) during the following weeks.

METHODS

Female Wistar rats after 90 min of generalized status epilepticus were used. Molecular biologic and histologic techniques were used to demonstrate markers of delayed cell death (apoptosis) 48 h after the status. The neuroprotective effects of i.c.v. injections of caspase inhibitors and systemic injections of the anticonvulsant drugs (AEDs) dizocilpine and retigabine after the status epilepticus were studied. The effect of neuroprotective intervention on the development of recurrent spontaneous seizures was investigated by behavioral observation of the rats.

RESULTS

After generalized status epilepticus in Wistar rats, massive sclerosis of the hippocampus and the piriform cortex occurred. TUNEL labeling and electron microscopy revealed that apoptosis is involved in the degenerative processes. Immunohistochemical analysis of the time course of the expression of the proapoptotic protein Bax suggested a maximal induction of apoptosis 24-48 h after the status. Application of caspase inhibitors before or after the status did not reduce lesions, although Bax labeling was reduced. Injection of dizocilpine and to a lower extent also of retigabine after the status prevented limbic neurodegeneration and expression of markers of apoptosis. However, the neuroprotection by dizocilpine did not prevent the development of recurrent spontaneous seizures.

CONCLUSIONS

Prolonged seizure activity can induce delayed sclerosis in the hippocampus and other parts of the limbic system. This delayed cell loss can be prevented by neuroprotective drugs after a status epilepticus. However, the damage in limbic brain regions is not the main reason for limbic epileptogenesis and the occurrence of recurrent spontaneous seizures.

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.

Age-related resistance to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-induced hippocampal lesion

This study compares the effects of acute alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) administration in the hippocampus in adult (3 months) and middle-aged (15 months) rats at 15 days postinjection. Injection of 1 and 2.7 mM AMPA produced dose-dependent neurodegeneration, assessed by Nissl staining; a glial reaction shown by glial fibrillary acidic protein immunocytochemistry; and calcification, revealed by alizarin red staining. Furthermore, at both doses, these alterations were significantly greater in 3-month-old rats. Finally, at AMPA 2.7 mM, no significant changes in the density of hippocampal parvalbumin- or calbindin-immunoreactive neurons or in choline acetyltransferase, glutamate uptake, or GABA uptake activities were found in 15-month-old animals, whereas significant reductions in parvalbumin (-76%) and calbindin (-32%) immunostaining and in GABA uptake (-27%) were observed in 3-month-old animals compared to the respective sham-operated or control animals. In conclusion, this study clearly demonstrates that in rats the vulnerability of hippocampal neurons and the glial and calcification reactions to AMPA-induced injury decreased with age between 3 and 15 months. Our results also indicate that hippocampal cholinergic, glutamatergic, and GABAergic systems show an adaptive response to excitotoxic damage in both adult and middle-aged animals.

Regional and age-related expression of gelatinases in the brains of young and old rats after treatment with kainic acid

Due to the possible detrimental impact of local inflammatory responses in neurodegenerative disease, it was of interest to measure the expression of extracellular matrix-degrading enzymes, a group of proteases that are induced during an inflammatory response, in the brains of old and young animals in a model of neuronal death. Doses of kainic acid were administered that resulted in comparable hippocampal pyramidal neuron loss in young and old F344/BN hybrid rats, even though each age group received widely differing doses. Two matrix metalloproteinases (MMPs), MMP-2 and MMP-9, were differentially induced with respect to time after kainic acid in sensitive brain regions in both young and old rats. However, the elevation of MMP-9 in the temporal lobe 12 h after injection in old rats was significantly greater than that observed in young animals. These results suggest that early and late induction of MMPs may play a role in neuronal death and repair mechanisms, respectively, and that inflammatory mechanisms in the central nervous system (CNS) of old rats are exaggerated compared to young rats.

Mice transgenic for the human Huntington's disease mutation have reduced sensitivity to kainic acid toxicity

The mechanism underlying the pathology of Huntington's disease (HD) is unknown, although there is substantial evidence supporting a role for excitotoxicity. The discovery of abnormal aggregations of protein in the brains of patients with HD, as well as in the brains of transgenic mice modeling this disease, has led to the suggestion that these "inclusions" have a pathogenic role. However, the relationship between inclusion formation and the progressive neurodegeneration in HD remains unclear. Here, we used mice transgenic for the first exon of the HD gene and an expanded CAG repeat (R6/2 line) to examine the role of neuronal intranuclear inclusions in kainic acid (KA) excitotoxicity. Unexpectedly, we found that the toxicity of KA was markedly attenuated in R6/2 mice compared with wild-type mice. In particular, the number and severity of KA-induced seizures in R6/2 mice was significantly reduced. When seizures occurred in 3-week-old R6/2 mice, we found lesions in the CA3 region of the hippocampus. However, neuronal intranuclear inclusions were not induced by KA in 3-week-old mice. Further, in older mice (9 weeks), the pre-existence of inclusions in CA3 neurons did not increase the vulnerability of neurons to KA, since no lesions were seen in 9-week R6/2 mouse brain. Our results suggest that an increased susceptibility to excitotoxic stimuli does not underlie the early phase of the neurological phenotype in R6/2 mice, although a role in later stages is not excluded by our findings. The significance of these findings is discussed in the context of the R6/2 mouse as a model for HD.

Age-dependent increase in infarct volume following photochemically induced cerebral infarction: putative role of astroglia

This study demonstrates that the photochemically induced model of stroke is an extremely viable method of inducing cerebral infarction in old animals. The lesions are reproducible both in terms of location and size and compatible with long-term survival of the animal. With this model we demonstrated, one week following surgery, a significantly larger infarct in rats 20 and 24 months of age compared to 4-month-old rats. The older rats also sustained greater neurologic deficits as assessed on a rotarod task. Older rats also were characterized by a glial response that was far less intense than in young animals. While the precise relationship between glia activation and cerebral damage remains to be determined, it would appear that a better understanding of those factors that contribute to the astrocytic response in the aged rat may be of particular benefit in designing therapeutic strategies aimed at reducing the pathologic consequences of cerebral infarction in elderly humans.

Vulnerability of the dentate gyrus to aging and intracerebroventricular administration of kainic acid

The hippocampal formation is highly vulnerable to the aging process, demonstrating functional alterations in circuitry with aging. Aging may also change the sensitivity of the hippocampal formation to excitotoxic lesions. In this study, using young adult, middle aged, and aged Fischer 344 rats, we evaluated morphometric changes in the dentate gyrus as a function of age and also in response to an administration of an excitotoxin (kainic acid) into the right lateral ventricle. The dentate gyrus was measured for changes in the area of dentate hilus and the dentate granule cell layer, alterations in the width of the dentate granule cell layer, and degree of dentate hilar cell loss. With aging, the hilar area increased in size while the area and width of the dentate granule cell layer remained constant. However, the most striking change with aging was a significant reduction in the number of dentate hilar neurons. Intracerebroventricular kainic acid produced consistent lesions in the entire ipsilateral CA3 region, and the size of CA3 lesion was identical in all three ages of animals. Following the lesion, areas of both the dentate hilus and the granule cell layer were significantly decreased in only young adult and middle aged animals whereas the width of the dentate granule cell layer was significantly increased only in the middle aged group. In contrast, dentate hilar neurons were significantly reduced in all ages of animals with the maximum reductions in neuron number observed in the aged group. Thus, aging in the dentate gyrus is characterized by a significantly decreased number of dentate hilar neurons and also a significantly increased susceptibility of dentate hilar neurons to excitotoxic damage.

Age-related toxicity to lactate, glutamate, and beta-amyloid in cultured adult neurons

The age-related susceptibility of the brain to neurodegenerative disease may be inherent in the susceptibility of individual neurons to various stressors. Neurons were isolated from embryonic, young- and old-aged rat hippocampus, cultured in serum-free medium and exposed to lactic acid, glutamate or beta-amyloid. Yields of isolated adult cells were 1 million cells/hippocampus, 12,000 cells/mg tissue, independent of age. For lactic acidosis, there was a non-significant 10% increment in killing of neuron-like cells from old rats compared to young. For glutamate, there was a 5-10% increment in killing of neuron-like cells from old rats compared to young rats and embryonic neurons. For cells exposed to the toxic fragment of beta-amyloid, A beta (25-35), toxicity was age, dose and time-dependent. Maximum toxicity in cells treated for 1 day with 25 microM A beta (25-35) was 16%, 24%, and 33% for embryonic, young and old cells. Similar results were found for A beta (1-40) (LD50 = 2 microM). These results suggest that aging imparts to individual cells an increased susceptibility to toxic substances relevant to neurodegenerative diseases.

Immunohistochemical localization and quantification of glial fibrillary acidic protein and synaptosomal-associated protein (mol. wt 25000) in the ageing hippocampus following administration of 5,7-dihydroxytryptamine

Responses to injury in the ageing hippocampus were assessed utilizing the synaptic markers glial fibrillary acidic protein and synaptosomal-associated protein (mol. wt 25,000) following administration of the neurotoxin, 5,7-dihydroxytryptamine, into the fimbria-fornix and cingulum bundle to denervate serotonergic afferent input to the dorsal hippocampus. Age-dependent alterations in hippocampal immunohistochemical localization of glial fibrillary acidic protein and synaptosomal-associated protein were evaluated in female Fischer 344 rats following serotonergic deafferentation with 5,7-dihydroxytryptamine. Across the lifespan, as indicated by measurements taken at three, 18, 21 and 29 months, marked increases in glial fibrillary acidic protein, but not synaptosomal-associated protein immunoreactivity, occurred throughout the hippocampus at 21 and 29 months compared to three and 18 months. Following three weeks pretreatment with 5,7-dihydroxytryptamine (20 microg total dose) or vehicle (0.1% ascorbic saline; 2 microl total volume) infused in the fimbria-fornix/cingulum bundle, immunohistochemical analysis demonstrated marked increases of glial fibrillary acidic protein, but not synaptosomal-associated protein, in the 18-month 5,7-dihydroxytryptamine group compared to the 18-month vehicle and 3-month 5,7-dihydroxytryptamine groups. Additionally, a significant increase in glial fibrillary acidic protein concentration was found by enzyme-linked immunosorbent assay in the 18-month 5,7-dihydroxytryptamine group compared to the 18-month vehicle and three-month 5,7-dihydroxytryptamine groups. These results demonstrate that selective neurotoxicant damage of the hippocampal serotonergic system differentially alters the expression of glial fibrillary acidic protein. This approach may provide a valuable tool to determine the ability of the hippocampus to respond to age-related neurodegenerative injury.

[31P]/[1H] nuclear magnetic resonance study of mitigating effects of GYKI 52466 on kainate-induced metabolic impairment in perfused rat cerebrocortical slices

PURPOSE

Kainic acid (KA) has long been used in experimental animals to induce status epilepticus (SE). A mechanistic implication of this is the association between excitotoxicity and brain damage during or after SE. We evaluated KA-induced metabolic impairment and the potential mitigating effects of GYKI 52466 [1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine] in superfused rat cerebral cortical slices.

METHODS

Interleaved [31P]/[1H] magnetic resonance spectroscopy (MRS) was used to assess energy metabolism, intracellular pH (pHi), N-acetyl-L-aspartate (NAA) level, and lactate (Lac) formation before, during, and after a 56-min exposure to 4 mM KA in freshly oxygenated artificial cerebrospinal fluid (oxy-ACSF).

RESULTS

In the absence of GYKI 52466 and during the KA exposure, NAA, PCr, and ATP levels were decreased to 91.1 +/- 0.8, 62.4 +/- 3.9, and 59.1 +/- 4.3% of the control, respectively; Lac was increased to 118.2 +/- 2.1 %, and pH, was reduced from 7.27 +/- 0.02 to 7.13 +/- 0.02. During 4-h recovery with KA-free ACSF, pHi rapidly and Lac gradually recovered, NAA decreased further to 85.5 +/- 0.3%, and PCr and ATP showed little recovery. Removal of Mg2+ from ACSF during KA exposure caused a more profound Lac increase (to 147.1 +/- 4.0%) during KA exposure and a further NAA decrease (to 80.4 +/- 0.5%) during reperfusion, but did not exacerbate PCr, ATP, and pHi changes. Inclusion of 100 microM GYKI 52466 during KA exposure significantly improved energy metabolism: the PCr and ATP levels were above 76.6 +/- 2.1 and 82.0 +/- 2.9% of the control, respectively, during KA exposure and recovered to 101.4 +/- 2.4 and 95.0 +/- 2.4%, respectively, during reperfusion. NAA level remained at 99.8 +/- 0.6% during exposure and decreased only slightly at a later stage of reperfusion.

CONCLUSIONS

Our finding supports the notion that KA-induced SE causes metabolic disturbance and neuronal injury mainly by overexcitation through non-N-methyl-D-aspartate (NMDA) receptor functions.

Aging-associated up-regulation of neuronal 5-lipoxygenase expression: putative role in neuronal vulnerability

Aging is associated with neurodegenerative processes. 5-Lipoxygenase (5-LO), which is also expressed in neurons, is the key enzyme in the synthesis of leukotrienes, inflammatory eicosanoids that are capable of promoting neurodegeneration. We hypothesized that neuronal 5-LO expression can be up-regulated in aging and that this may increase the brain's vulnerability to neurodegeneration. We observed differences in the distribution of 5-LO-like immunoreactivity in various brain areas of adult young (2-month-old) vs. old (24-month-old) male rats. Greater 5-LO-like immunoreactivity was found in old vs. young rats, in particular in the dendrites of pyramidal neurons in limbic structures, including the hippocampus, and in layer V pyramidal cells of the frontoparietal cortex and their apical dendrites. The aging-increased expression of neuronal 5-LO protein appears to be due to increased 5-LO gene expression. Using a quantitative reverse transcription/polymerase chain reaction assay and 5-LO-specific oligonucleotide primers and their mutated internal standards, we observed about a 2.5-fold greater hippocampal 5-LO mRNA content in old rats. 5-LO-like immunoreactivity was also observed in small, nonpyramidal cells, which were positive for glutamic acid decarboxylase or glial fibrillary acid protein. This type of 5-LO immunostaining did not increase in the old rats. Hippocampal excitotoxic injury induced by systemic injection of kainate was greater in old rats. Neuroprotection was observed with the 5-LO inhibitor, caffeic acid. Together, these results suggest that aging increases both neuronal 5-LO expression and neuronal vulnerability to 5-LO inhibitor-sensitive excitotoxicity, and indicate that the 5-LO system might play a significant role in the pathobiology of aging-associated neurodegenerative diseases.

The neuroprotective effects of the recombinant interleukin-1 receptor antagonist rhIL-1ra after excitotoxic stimulation with kainic acid and its relationship to the amyloid precursor protein gene

The cytokine interleukin-1 (IL-1) and its endogenous antagonist (IL-1ra) have important functions in the central nervous system. Recent experimental observations have suggested that recombinant IL-1RA (rhIL-1ra) has neuroprotective properties in ischaemia, excitotoxicity, and trauma. We wished to see what effect rhIL-1ra had on kainic acid-induced neuronal death and to investigate how this might relate to changes in expression of the amyloid precursor protein gene (APP) and glial fibrillary acid protein (GFAP) using in situ hybridization. Wistar rats were treated by intracerebroventricular administration with rhIL-1ra at doses of 10, 20 and 40 microg given 10 min before and 10 min after intraperitoneal kainic acid 10 mg/kg. Behaviour was measured and, after 10 days, the brains were removed for histology and in situ hybridization. There were no anticonvulsant effects on kainic acid-induced wet dog shakes or limbic motor seizures. There were no differences in the effects of rhIL-1ra at all doses tested on hippocampal temperature, blood pressure, blood gases, pH, and glucose in comparison to control. With rhIL-1ra 10 microg given twice, there was significant protection of neurons in the CA1 and CA3 field of the hippocampus and dorsal thalamus, but not in the primary olfactory cortex-amygdaloid region. Small, but insignificant, neuroprotective effects were observed in the same anatomical regions with a dose of 20 microg given twice, and no neuroprotective effects were observed with 40 microg. The enhanced neuronal survival in CA1, CA3 and the dorsal thalamus was associated with preservation of APP 695 mRNA (neuronal form) and lack of stimulation of APP 770 (glial form) and GFAP messages. Where there was no neuroprotection APP 695 mRNA was reduced and stimulation of both APP 770 and GFAP mRNAs was observed. In conclusion, rhIL-1ra has dose- and region-dependent effects on neuronal survival after kainic acid and prevents damage-induced changes in APP and GFAP mRNAs.

Eliprodil, a non-competitive, NR2B-selective NMDA antagonist, protects pyramidal neurons in hippocampal slices from hypoxic/ischemic damage

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is one pathway through which excessive influx of calcium has been suggested to trigger ischemia-induced delayed neuronal death. NMDA receptors are heterooligomeric complexes comprised of both NR1 and NR2A-D subunits, in various combinations. NR2B-containing NMDA complexes exhibit larger, more prolonged conductances than those lacking this subunit. We tested the ability of the non-competitive, NR2B-selective NMDA antagonist eliprodil to (a) protect synaptic transmission in in vitro hippocampal slices from hypoxia, and (b) reduce ischemic delayed neuronal death in hippocampal organotypic slice cultures. Eliprodil markedly improved the recovery of Schaffer collateral-CA1 excitatory postsynaptic potentials following a 15 min hypoxic insult, with an EC50 of approximately 0.5 microM. In contrast to this functional protection, eliprodil did not reduce delayed death of CA1 pyramidal neurons in organotypic hippocampal slice cultures treated with severe hypoxia plus hypoglycemia, though it did potently protect CA3 pyramidal neurons in the same cultures. These data indicate that NMDA receptors containing NR2B subunits may play a role in long-term recovery of hippocampal synaptic function following ischemia/hypoxia. Furthermore, the selective protection of CA3, but not CA1, pyramidal neurons suggests that NR2B-containing NMDA receptors may preferentially contribute to an excitotoxic component of ischemia-induced delayed neuronal death.

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.

Neurotoxicity of Kainate to the Hippocampus is not Accrued by Aging, Stress and Exogenous Corticosterone in Wistar Kyoto and Spontaneously Hypertensive Rats

It has been reported that a high corticosterone milieu can exacerbate various experimental insults to the nervous system, in particular to the hippocampus. However, in many of these studies the above milieu was attained by injecting corticosterone in doses (e.g. 10 mg/rat) producing supraphysiological concentrations. In the present study we have investigated whether high plasma corticosterone levels, such as those associated with aging or stress, potentiate a hippocampal excitotoxic insult. Male Wistar Kyoto (WKY) and Spontaneously Hypertensive Rats (SHR) at the age of 6, 12, 18 and 24 months (only WKY for the oldest age) were used. As in other strains, aging in these rats was marked by an increase in basal plasma corticosterone levels. Rats were infused in the dorsal hippocampus with kainic acid (0.035 µg/hippocampus) and the neuronal injury was evaluated within the areas CA3 and CA4. Results indicated that neither aging nor the hypertensive condition affected kainic acid neurotoxicity. In order to study the effect of stress, rats were stressed twice a day, with alternate types of stressors to avoid possible habituation, 3 days prior to and 3 days following the kainic acid infusion. Using this experimental paradigm the hippocampal damage in stressed rats was of the same degree as in non-stressed controls. In a complementary set of experiments, 6 month old WKY and SHR rats were injected with corticosterone (10 mg/rat s.c.). Four hours after administration plasma corticosterone levels in the range of 60-70 µg/100 ml were found. Moreover, a time-course study showed a plasma corticosterone peak in the range of 240 µg/100 ml. Daily corticosterone administration for 3 days before and 3 days after kainic acid infusion potentiated the hippocampal damage in 6 months old SHR but not in the WKY. These results demonstrate that elevation of corticosterone levels within physiological range does not exacerbate hippocampal kainate neurotoxicity and that pharmacological doses of glucocorticoid hormone, which produces plasma levels well above those observable in any physiopathological condition, might, with some strain dependency, potentiate a hippocampal neurotoxic insult.

Effect of age in rodent models of focal and forebrain ischemia

BACKGROUND AND PURPOSE

The majority of animal experiments examining the nature and treatment of stroke have used relatively young animals ranging in age from 2 to 6 months. However, significant morphological, neurochemical, and behavioral changes occur with aging in rodents particularly during the first 24 months of age. This study examines the effect of age in two models of transient ischemia a forebrain and a focal model in male Wistar rats.

METHODS

We induced forebrain ischemia of 12 minutes duration by bilateral carotid artery occlusion with controlled hypotension at a mean blood pressure of 45 mm Hg and using an intraluminal filament technique, induced focal middle cerebral artery occlusion of 100 minutes duration at a mean blood pressure of 60 mm Hg. Physiological parameters were monitored and maintained within normal limits. On day 7 after ischemia, the rats were perfusion-fixed and the brains removed for quantitative histopathology.

RESULTS

After forebrain ischemia, older rats showed significantly less CA1 neuronal necrosis than the younger group (P < .003), whereas both striatal and neocortical injury were significantly greater in the older group (P < .05). Among animals subjected to focal ischemia, the volume of infarcted tissue and the number of necrotic neurons in the area adjacent to the infarction were both greater in older rats (P < .05).

CONCLUSIONS

This study emphasizes the importance of age in models of forebrain and focal ischemia. The interaction between age-related changes in morphology, neurochemistry, and behavior on the ischemic cascade complicates the interpretation of mechanistic data, and pharmacological effects observed in younger animals may not necessarily translate to an older population.

Age-related decrease in vulnerability to excitatory amino acids in the nucleus basalis

The present study investigated the effects of nucleus basalis magnocellularis (NBM) lesions in young (3 months), adult (9 months), and aged (24 months) rats by injections of either NMDA or AMPA upon performance of a delayed alternation task on a T maze. During phase 1 of testing, the interchoice interval (ICI) was 5 s and each rat was given 10 trials per day during phase 2, the ICI was 30 s across 10 trials per day; during phase 3, the ICI was 5 s across 20 trials per day. Analyses of variance revealed (a) a significant effect of age during phase 1 (i.e., 24-month-old rats performed worse than 3-month-old rats); (b) a significant effect of age and lesion in phase 2 (i.e., the lesions impaired choice accuracy equally in all age groups when the ICIs were 30 s); (c) a significant effect of age and lesions, and a significant interaction in phase 3 (i.e., young rats were more impaired by the lesions than were aged rats.