Age-related sensitivity to kainate neurotoxicity

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.

Quantitating silver-stained neurodegeneration: the neurotoxicity of trimethlytin (TMT) in aged rats

This report describes the development of a histoanalytical procedure to measure the degree of neurodegeneration produced by the organometal toxicant trimethyltin (TMT). Based on a previous, non-quantitated experiment we hypothesized that the same dose of TMT would produce greater damage in animals of increasing age. Male rats aged 6, 12, 18, or 24 months at the time of dosing were given either 4.5 mg/kg TMT or saline (i.p.). One month after dosing, rats were perfused and their brains removed and processed to selectively silver-impregnate degenerating cell bodies as well as axon terminals and dendrites. Neurodegeneration was most prominent in the hippocampi (especially CA1 stratum radiatum) of TMT-treated rats, but not in the controls. Computer-assisted counting of the silver grains marking damage indicated greater neurotoxicity from the same dose of TMT when given to the older animals. Thus the grain density in the 6-month-old TMT-treated rats was not significantly elevated from the 6-month-old controls (P>0.10). The 12-month-old TMT-treated rats had significantly increased grain densities compared to their controls (P<0.05), but still larger increases of grain counts were observed in the 18- and 24-month-old rats (both P-values<0.01). Our findings with TMT are similar to previous, but nonquantitative, reports that the neurotoxic effects of kainic acid and methionine sulfoximine were also greater in older rats. An increased sensitivity to neurotoxicants might help explain the apparently spontaneous degeneration of cortical neurons in aging and in the neurological diseases of old age. The method we report here for quantitation of silver grains marking neurodegeneration should be adaptable to a wide range of histologically-based neurotoxicology investigations.

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.

Unilateral GluR2(B) hippocampal knockdown: a novel partial seizure model in the developing rat

Kainic acid (KA) induces status epilepticus in both adult and young rats but with different consequences on pathology and gene expression. In adults, GluR2(B) AMPA subunit expression is markedly reduced in CA3 neurons before neurodegeneration. In pups, the GluR2(B) subunit is sustained, possibly contributing to neuronal survival. Mechanisms underlying the reduced vulnerability of developing neurons to seizures was investigated by examining the effects of unilateral microinfusions of GluR2(B) antisense oligodeoxynucleotides (AS-ODNs) into the hippocampus of young rats in the presence or absence of a subconvulsive dose of KA. GluR2(B) AS-ODN infusions resulted in spontaneous seizure-like behavior, high stimulus intensity population spikes in the absence of long-term potentiation, and neurodegeneration of CA3 neurons lateral to the infusion site. Electroencephalography revealed paroxysmal activity and high-frequency high-amplitude discharges associated with vigorous and continuous scratching, wild running, or bilateral jerking movements. Pups lacking phenotypic behavior exhibited high-rhythmic oscillations and status epilepticus by the dose of KA used. Radiolabeled AS-ODNs accumulated throughout the ipsilateral dorsal hippocampus. GluR2(B) but not GluR1(A) receptor protein was markedly reduced after GluR2(B) knockdown. In contrast, GluR1(A) knockdown reduced GluR1(A) but not GluR2(B) protein without change in behavior or morphology. Therefore, unilateral downregulation of hippocampal GluR2(B) but not GluR1(A) protein reduces the seizure threshold and survival of CA3 neurons in the immature hippocampus, possibly providing a novel partial seizure model in the developing rat.

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.

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.

Effect of pH on domoic acid toxicity in mice

Domoic acid is a shellfish toxin which produces gastrointestinal distress, followed by neurological symptoms such as headache, confusion, disorientation and severe deficits in short-term memory. Domoic acid is an amino acid which contains three carboxylic groups, and one imino group, and its solubility, rate of absorption, and elimination would vary depending on the protonation of these groups at different pH's. We propose that domoic acid toxicity varies with pH of administered domoic acid solution. Domoic acid toxicity was measured in mice as the onset times for scratching behaviour, seizure activity, and death, after the intraperitoneal administration of domoic acid at different pH's. Results of the present study show that the scratching behaviour, seizure activity, and death, occurred at 12, 40, and 55 min, after intraperitoneal administration of domoic acid at pH 3.7. Apparently, the onset times for three types of behaviours were relatively long, and well separated from each other. Domoic acid toxicity was lowest at pH 3.7, and highest at pH 7.4, with intermediate toxicity at other pH's. The onset time of scratching behaviour was not influenced by pH of domoic acid solution at three different doses. In contrast, the onset times for seizure activity, and death were significantly affected by pH of domoic acid, toxicity being higher at pH 7.4 than at pH 3.7. The pH effect on domoic acid toxicity diminished as the dose of domoic acid was increased. In fact, at 14.5 mg/kg domoic acid toxicity was similar at both pH's of 3.7 and 7.4. It is concluded that in vivo toxicity of domoic acid varies depending on pH of the administered solution. The differential toxicity of domoic acid at different pH may be related to its solubility, rate of absorption, and elimination, depending on the degree of protonation of domoic acid molecule. Domoic acid toxicity would also vary depending on the age of animal, receptor sensitivity and density in different regions of brain.

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.

Anatomical studies of DNA fragmentation in rat brain after systemic kainate administration

Rats treated systemically with kainate develop stereotyped epileptic seizures involving mainly limbic structures that may last for hours. This model of limbic status epilepticus has been widely studied using classical neuropathological techniques. We used in situ nick translation histochemistry to examine patterns of DNA fragmentation in this model. We found a stereotyped and reproducible pattern of neuronal populations that demonstrate evidence of DNA fragmentation from 24 h to one week after kainate treatment. Neither blockade of new protein synthesis nor blockade of the N-methyl-D-aspartate-type glutamate receptors significantly altered this response. Moreover, we saw no evidence of the regular internucleosomal cleavage of DNA that produces a characteristic laddered appearance of 180-200 bp DNA fragments after gel electrophoresis in samples obtained from microdissected affected regions. These studies suggest that DNA fragmentation after systemic kainate-induced seizures is not the result of programmed cell death. This assay may be useful for quantitative testing of both neuroprotective agents and mechanistic hypotheses.

Quantitative histological evaluation of neuroprotective compounds

The application of quantitative morphometric methods to neurotoxicology is a relatively recent endeavor, and appropriate techniques are still evolving. However, such methods are essential for subsequent use of neurohistological data in mathematical representations of the risk of exposure to neurotoxicants. It can be predicted that the same methods will also be of great utility in studies of the efficacy of neuroprotective drugs. When the neuropathological conditions to be prevented or reversed are best monitored by neurohistology, quantitative morphometry should be considered as the most direct means to demonstrate the efficacy of a neuroprotective agent. Initially, a decision to choose the most appropriate histological procedure must be made. The rationale for such decisions with regard to several common histochemical techniques was discussed. The appropriate stereological and statistical considerations to be addressed by the sampling strategy were also presented. It is anticipated that quantitative morphometric methods will play an increasingly important role in the evaluation of the efficacy and toxicity of neuroactive compounds.

Effects of excitatory amino acid lesions upon neurokinin B and acetylcholine neurons in the nucleus basalis of the rat

The nucleus basalis magnocellularis (NBM) contains cholinergic neurons that project to the neocortex and is densely innervated by excitatory amino acid-containing terminals. A dysfunction in the balance of excitatory inputs or an alteration in the sensitivity of NBM cells to glutamate may underlie the selective vulnerability to aging. Some large NBM neurons contain neurokinin B (NKB) mRNA. The present study investigated whether alpha-2-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA) differentially destroy NKB-containing, NKB-receptive, or cholinergic NBM cells, and whether this vulnerability is altered by aging. Injections of AMPA or NMDA significantly decreased neocortical ChAT activity, as compared to control levels, across all three age groups, with no interaction between lesion and age group. The results of in situ hybridization histochemistry and NKB receptor studies suggest that NKB-containing neurons in the NBM, and the neurons they innervate, are not vulnerable to NMDA or AMPA in either young or old rats. While NKB mRNA-positive cells were diffusely distributed throughout the basal forebrain, only a small proportion of the large NBM cells contained NKB mRNA. The results suggest that NKB does not extensively colocalize with acetylcholine within the basal forebrain of rats and that NBM NKB neurons do not directly innervate cholinergic cells.

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

Sensitivity to excitotoxic damage was assessed in young adult, mature and aged male Sprague-Dawley rats. Kainic acid was injected into the hippocampus and the size of the hippocampal lesion rated. Intrahippocampal injection of kainic acid produced lesions in aged animals that were significantly smaller than lesions in the young rats (P < 0.05), while lesion size in mature rats was intermediate. Excitotoxic damage was localized primarily to the CA3 region of the hippocampus in the aged rats. Young adult rats had more damage to the hippocampus with involvement of CA1 pyramidal and dentate granule cells. These results suggest that increased age may reduce susceptibility to excitotoxic damage.

Domoic acid-treated cynomolgus monkeys (M. fascicularis): effects of dose on hippocampal neuronal and terminal degeneration

Domoic acid is a tricarboxylic amino acid (structurally related to kainic acid and glutamic acid) that is found in the environment as a contaminant of some seafood. To determine the nature of any neurological damage caused by domoate, as well as the minimum neurotoxic dose, juvenile and adult monkeys were dosed intravenously with domoate at one of a range of doses from 0.25 to 4 mg/kg. When animals were perfused one week later, histochemical staining using a silver method to reveal degenerating axons and cell bodies showed two distinct types of hippocampal lesions. One lesion, termed 'Type A', was a small focal area of silver grains restricted to CA2 stratum lucidum, the site of greatest kainic acid receptor concentration in the brain. Type A lesions occurred over a dose range of 0.5 to 2.0 mg/kg in juvenile animals and 0.5 to 1.0 mg/kg in adult animals. No mortality occurred in any of the juvenile monkeys, but one juvenile animal that received 4.0 mg/kg sustained a second type of lesion, termed 'Type B', characterized by widespread damage to pyramidal neurons and axon terminals of CA4, CA3, CA2, CA1, and subiculum subfields of the hippocampus. Doses of more than 1.0 mg/kg in the adult monkeys either proved lethal or resulted in Type B lesions. Induction of c-fos protein had occurred in the hippocampal dentate gyrus and CA1 regions of moribund animals perfused within hours of their initial dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in [3H]-kainate receptor binding in the hippocampal formation of aged Long-Evans rats

The present study used in vitro autoradiography to examine the density of [3H]-kainate (KA) binding in subregions of the hippocampal formation and certain cortical areas in young (7-8 months) and aged (27-29 months) Long-Evans rats. In addition, the topography of KA binding in the dentate molecular layer was examined for evidence of reactive reorganization in the aged brain. This investigation of age-related changes in [3H]-KA binding included correlations with the animals' spatial learning performance in a Morris water maze. The results showed an age-related decrease in the density of [3H]-KA binding in several regions of the hippocampal formation (CA3, CA1, hilus) and within related cortical areas (subicular complex, entorhinal cortex, perirhinal cortex). In addition, an expanded zone of KA binding in the molecular layer of the dentate gyrus was observed in the aged group. This expansion of KA binding may reflect sprouting due to a loss of perforant path input to the dentate. The results of additional correlational analyses, however, indicated that these changes in the density and topography of [3H]-KA binding were not strongly correlated with a decline in place learning ability.

Melatonin, hydroxyl radical-mediated oxidative damage, and aging: a hypothesis

Melatonin is a very potent and efficient endogenous radical scavenger. The pineal indolamine reacts with the highly toxic hydroxyl radical and provides on-site protection against oxidative damage to biomolecules within every cellular compartment. Melatonin acts as a primary non-enzymatic antioxidative defense against the devastating actions of the extremely reactive hydroxyl radical. Melatonin and structurally related tryptophan metabolites are evolutionary conservative molecules principally involved in the prevention of oxidative stress in organisms as different as algae and rats. The rate of aging and the time of onset of age-related diseases in rodents can be retarded by the administration of melatonin or treatments that preserve the endogenous rhythm of melatonin formation. The release of excitatory amino acids such as glutamate enhances endogenous hydroxyl radical formation. The activation of central excitatory amino acid receptors suppress melatonin synthesis and is therefore accompanied by a reduced detoxification rate of hydroxyl radicals. Aged animals and humans are melatonin-deficient and more sensitive to oxidative stress. Experiments investigating the effects of endogenous excitatory amino acid antagonists and stimulants of melatonin biosynthesis such as magnesium may finally lead to novel therapeutic approaches for the prevention of degeneration and dysdifferentiation associated with diseases related to premature aging.

Cerebral blood flow and ischemia-induced neurotransmitter release in the striatum of aged spontaneously hypertensive rats

BACKGROUND AND PURPOSE

We found age-related vulnerability to cerebral ischemia in the hippocampus and striatum in spontaneously hypertensive rats. Further study revealed that ischemia-induced release of hippocampal taurine, an inhibitory amino acid, was reduced by 40% in aged rats compared with adult rats, which suggested an impaired inhibitory function against excitotoxicity in aged rats. The purpose of this study was to examine whether ischemia-induced neurotransmitter release is altered in the striatum of aged spontaneously hypertensive rats.

METHODS

Five adult (5-6 months) and five aged (18-22 months) female spontaneously hypertensive rats were subjected to 20 minutes of cerebral ischemia induced by bilateral carotid artery occlusions and 120 minutes of recirculation under amobarbital anesthesia (100 mg/kg i.p.). Cerebral blood flow was determined using the hydrogen clearance method, and extracellular concentrations of neurotransmitters were determined with the brain dialysis technique in the striatum.

RESULTS

During ischemia, cerebral blood flow in aged rats decreased to 8.7 +/- 1.2 (mean +/- SEM) mL/100 g per minute (11% of the resting), which was not different from 5.2 +/- 1.7 mL/100 g per minute (8% of the resting) in adult rats, and extracellular dopamine and amino acids (glutamate, aspartate, and taurine) increased by approximately 170- and 10-30-fold, respectively, and returned to baseline after 20-40 minutes of recirculation. These values of neurotransmitters, however, were not different between aged and adult rats during ischemia and reperfusion.

CONCLUSIONS

It is unlikely that a presynaptic mechanism is involved in age-related vulnerability in the striatum of hypertensive rats.

An analysis of the U.S. Environmental Protection Agency neurotoxicity testing guidelines

Few of the more than 65,000 chemicals listed in the Environmental Protection Agency (EPA) inventory have been tested for neurotoxicity. The nervous system may be especially vulnerable to toxicants because many compounds can cross the blood-brain barrier and induce irreversible damage. Additionally, the young, the elderly, and other sensitive populations may be particularly susceptible to neurotoxic injury. The EPA has developed guidelines including neurobehavioral, neuropathological, and neurochemical tests for the identification of possible neurotoxicants. In the present review, tests included in the current EPA guidelines for neurotoxicity testing are described and evaluated. The main benefit of the tests is that regulators are familiar with them, thus facilitating interpretation. Additionally, validation data on these tests are available for many known neurotoxicants. These factors make it difficult to introduce new methods that may include in vitro and other techniques. The current in vivo tests can be costly and prolonged and can involve the use of many laboratory animals, making them inappropriate for generalized use on existing chemicals. It is suggested that alternative tests be incorporated for screening of large numbers of chemicals and that testing priority be given to chemicals on the basis of structure/activity relationships, lipophilicity, bioaccumulation, and extent of exposure.

Basal forebrain cholinergic neurons in aged rat brain are more susceptible to ibotenate-induced degeneration than neurons in young adult brain

Choline acetyltransferase (ChAT) activity, acetylcholinesterase (AChE) activity, and [3H]nicotine binding site density were measured in neocortex from unoperated (control) and nucleus basalis (NB)-lesioned young (2-3 months old) and aged (23-24 months old) rats. In control animals, neither enzyme activities nor the density of nicotine binding sites were altered as a function of age. However, age-related differences were apparent 2 weeks following unilateral infusions of ibotenic acid into the right NB. NB lesion-induced decreases in enzyme activities were significantly greater in ipsilateral neocortices from aged rats; ChAT and AChE activities in young animals decreased by 59 and 53%, respectively, while both enzyme activities in aged rats decreased by 72%. NB lesions decreased significantly the density of nicotine binding sites in ipsilateral neocortices from both young and aged rats; binding decreased by 23-26% in young rats and by 31-34% in aged animals. Results indicate that the basal forebrain cholinergic system in the aged rat is more susceptible to ibotenate-induced degeneration than neurons in young animals.

Kainic acid-induced seizures in aged rats: neurochemical correlates

This study was conducted to assess the functional integrity of the kainate receptor-mediated seizure response in aged rats. Kainic acid was administered systemically to aged female Long-Evans (LE) rats and aged male F344 rats and the proconvulsant actions of kainic acid was compared to adult controls. The effects of kainic acid on brain regional content of monoamines and amino acids was also determined in the aged female LE and adult control rats. The latency to full clonic-tonic seizures was significantly reduced in aged female LE rats, and the number of seizures was significantly increased above that of the controls. There was increased mortality and a reduction in the latency to exhibit wet dog shakes in the aged F344 rats. Studies were also conducted to evaluate the role of ovarian hormones, route of administration, and dose of kainic acid in mediating the enhanced proconvulsant actions of kainic acid in aged rats. The neurochemical studies suggested that kainic acid significantly enhanced the release of ASP, GLU, and norepinephrine (NE) in the aged rats exhibiting clonic-tonic seizures. The adult rats given the same dose of kainic acid (15 mg/kg, IP) did not exhibit any significant change in brain content of monoamines or amino acids except for a reduction in mediobasal hypothalamic NE. An in vitro study was also conducted using brain slices from adult and aged F344 and it was found that aged rats released significantly more ASP than adults in response to kainic acid. These neurochemical findings were discussed in relation to previous studies of age-related alterations in excitatory amino acids (EAAs) and the role of EAA and NE in modulating limbic seizures. This study has clearly demonstrated that aged rats may be more susceptible to the excitotoxic action of EEAs acting through kainetic receptors.