NMDA depolarizations and long-term potentiation are reduced in the aged rat neocortex

The effects of age and biological sex on the association between I-wave recruitment and the response to cTBS: an exploratory study

The neuroplastic response to continuous theta burst stimulation (cTBS) is inherently variable. The measurement of I-wave latencies has been shown to strongly predict the magnitude and direction of the response to cTBS, whereby longer latencies are associated with stronger long-term depression-like responses. However, potential differences in this association relating to age and sex have not been explored. We performed cTBS and measured I-wave recruitment (via MEP latencies) in 66 participants (31 female) ranging in age from 11 to 78 years. The influence of age and sex on the association between I-wave recruitment and the response to cTBS was tested using linear regression models. In contrast to previous studies, there was not a significant association between I-wave latencies and cTBS response at the group level (p = 0.142, R² = 0.033). However, there were interactions between I-waves and both age and sex when predicting cTBS response. Subgroup analysis revealed that preferential late I-wave recruitment predicted cTBS response in adolescent females, but not in adolescent or adult males or adult females. These data suggest that the generalisability of I-wave measurement in predicting the response to cTBS may be lower than initially believed. Prediction models should include age and sex, rather than I-wave latencies alone, as our findings suggest that, while each factor alone is not a strong predictor, these factors interact to influence the response to cTBS.

Tau-Induced Pathology in Epilepsy and Dementia: Notions from Patients and Animal Models

Patients with dementia present epilepsy more frequently than the general population. Seizures are more common in patients with Alzheimer’s disease (AD), dementia with Lewy bodies (LBD), frontotemporal dementia (FTD) and progressive supranuclear palsy (PSP) than in other dementias. Missense mutations in the microtubule associated protein tau (MAPT) gene have been found to cause familial FTD and PSP, while the P301S mutation in MAPT has been associated with early-onset fast progressive dementia and the presence of seizures. Brains of patients with AD, LBD, FTD and PSP show hyperphosphorylated tau aggregates, amyloid-β plaques and neuropil threads. Increasing evidence suggests the existence of overlapping mechanisms related to the generation of network hyperexcitability and cognitive decline. Neuronal overexpression of tau with various mutations found in FTD with parkinsonism-linked to chromosome 17 (FTDP-17) in mice produces epileptic activity. On the other hand, the use of certain antiepileptic drugs in animal models with AD prevents cognitive impairment. Further efforts should be made to search for plausible common targets for both conditions. Moreover, attempts should also be made to evaluate the use of drugs targeting tau and amyloid-β as suitable pharmacological interventions in epileptic disorders. The diagnosis of dementia and epilepsy in early stages of those diseases may be helpful for the initiation of treatments that could prevent the generation of epileptic activity and cognitive deterioration.

Premature Aging Phenotype in Mice Lacking High-Affinity Nicotinic Receptors: Region-Specific Changes in Layer V Pyramidal Cell Morphology

The mechanisms by which aging leads to alterations in brain structure and cognitive deficits are unclear. Α deficient cholinergic system has been implicated as one of the main factors that could confer a heightened vulnerability to the aging process, and mice lacking high-affinity nicotinic receptors (β2(-/-)) have been proposed as an animal model of accelerated cognitive aging. To date, however, age-related changes in neuronal microanatomy have not been studied in these mice. In the present study, we examine the neuronal structure of yellow fluorescent protein (YFP(+)) layer V neurons in 2 cytoarchitectonically distinct cortical regions in wild-type (WT) and β2(-/-) animals. We find that (1) substantial morphological differences exist between YFP(+) cells of the anterior cingulate cortex (ACC) and primary visual cortex (V1), in both genotypes; (2) in WT animals, ACC cells are more susceptible to aging compared with cells in V1; and (3) β2 deletion is associated with a regionally and temporally specific increase in vulnerability to aging. ACC cells exhibit a prematurely aged phenotype already at 4-6 months, whereas V1 cells are spared in adulthood but strongly affected in old animals. Collectively, our data reveal region-specific synergistic effects of aging and genotype and suggest distinct vulnerabilities in V1 and ACC neurons.

Selective Vulnerabilities of N-methyl-D-aspartate (NMDA) Receptors During Brain Aging

N-methyl-D-aspartate (NMDA) receptors are present in high density within the cerebral cortex and hippocampus and play an important role in learning and memory. NMDA receptors are negatively affected by aging, but these effects are not uniform in many different ways. This review discusses the selective age-related vulnerabilities of different binding sites of the NMDA receptor complex, different subunits that comprise the complex, and the expression and functions of the receptor within different brain regions. Spatial reference, passive avoidance, and working memory, as well as place field stability and expansion all involve NMDA receptors. Aged animals show deficiencies in these functions, as compared to young, and some studies have identified an association between age-associated changes in the expression of NMDA receptors and poor memory performance. A number of diet and drug interventions have shown potential for reversing or slowing the effects of aging on the NMDA receptor. On the other hand, there is mounting evidence that the NMDA receptors that remain within aged individuals are not always associated with good cognitive functioning. This may be due to a compensatory response of neurons to the decline in NMDA receptor expression or a change in the subunit composition of the remaining receptors. These studies suggest that developing treatments that are aimed at preventing or reversing the effects of aging on the NMDA receptor may aid in ameliorating the memory declines that are associated with aging. However, we need to be mindful of the possibility that there may also be negative consequences in aged individuals.

Age-related deficits in a forebrain-dependent task, trace-eyeblink conditioning

Trace-eyeblink conditioning is a forebrain-dependent learning paradigm that has assisted in our understanding of age-related hippocampal neuronal plasticity; however, the hippocampus is not believed to be the permanent site for most long-term-memory storage. Studies in adult subjects have suggested the neocortex as one such site. Whisker plucking studies have further suggested that the ability for plasticity in the neocortex declines with age. Mice were trained in trace- and delay-eyeblink conditioning with whisker or auditory stimulation as the conditioned stimulus to examine possible age-related behavioral and neocortical abnormalities. Whisker stimulation was determined to be a more effective stimulus for examining age-related behavioral abnormalities in C57 mice. Additionally, neocortical barrel expansion, observed in trace conditioned adult mice and rabbits, does not occur in mice conditioned on a delay paradigm or in old mice unable to learn the whisker trace association. Abnormalities in neocortical memory storage in the elderly could contribute to normal age-dependent declines in associative learning abilities.

Comparison of the effects of dose-dependent zinc chloride on short-term and long-term memory in young male rats

The aim of the present study was to evaluate the effects of dose-dependent of zinc chloride on short-term and long-term memory in a shuttle box. Young Wistar rats (94+/-10 g) (age 27-30 days) consumed zinc chloride drinking water in five different doses (20, 30, 50, 70 and 100 mg kg(-1) day(-1)) for two weeks by gavage. After 14 days on experimental diets, a shuttle box used to test short- and long-term memory. Two criteria considering for behavioral test, including latency in entering dark chamber and time spent in the dark chamber. This experiment shows that after 2 weeks oral administration of ZnCl2 with (20, 30 and 50 mg kg(-1) day(-1)) doses, the rat's working (short-term) has been improved (p<0.05). Whereas ZnCl2 with 30 mg kg(-1) day(-1) dose has been more effected than other doses (p<0.001). But rat which received ZnCl2 with 100 mg kg(-1) day(-1), has been shown significant impairment in working memory (p<0.05) and there was no significant difference in reference (long-term) memory for any of groups. In general, this study has demonstrated that zinc chloride consumption with 30 mg kg(-1) day(-1) dose for two weeks was more effective than other doses on short-term memory. But consumption of ZnCl2 with 100 mg kg(-1) day(-1) dose for two week had the negative effect on short-term memory. On the other hand, zinc supplementation did not have an effect on long-term memory.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Expression of groups I and II metabotropic glutamate receptors in the rat brain during aging

Age-dependent changes in the expression of group I and II metabotropic glutamate (mGlu) receptors were studied by in situ hybridization, Western blot analysis and immunohistochemistry. Male Fisher 344 rats of three ages (3, 12 and 25 months) were tested. Age-related increases in mGlu1 receptor mRNA levels were found in several areas (thalamic nuclei, hippocampal CA3) with parallel increases in mGlu1a receptor protein expression. However, a slight decrease in mGlu1a receptor mRNA expression in individual Purkinje neurons and a decline in cerebellar mGlu1a receptor protein levels were detected in aged animals. In contrast, mGlu1b receptor mRNA levels increased in the cerebellar granule cell layer. Although mGlu5 receptor mRNA expression decreased in many regions, its protein expression remained unchanged during aging. Compared to the small changes in mGlu2 receptor mRNA levels, mGlu3 receptor mRNA levels showed substantial age differences. An increased mGlu2/3 receptor protein expression was found in the frontal cortex, thalamus, hippocampus and corpus callosum in aged animals. These results demonstrate region- and subtype-specific, including splice variant specific changes in the expression of mGlu receptors in the brain with increasing age.

Experience-dependent regulation of synaptic zinc is impaired in the cortex of aged mice

Zinc plays an important role in synaptic signaling in the mammalian cerebral cortex. Zinc is sequestered into presynaptic vesicles of subpopulations of glutamatergic neurons and is released by depolarization, in a calcium-dependent manner. As the majority of mechanisms that have been suggested to participate in experience-dependent alterations in synaptic strength in the cerebral cortex implicate signaling by glutamate, it stands to reason that zincergic signaling might also be crucial. Here we show that synaptic zinc is rapidly and dynamically modulated in relation to alterations in sensory input and that this response is highly age-dependent. Juvenile, adult, and aged mice were subjected to whisker removal and levels of staining for synaptic zinc in deprived and non-deprived cortical barrels were quantitatively assessed at post-deprivation times ranging from 3 h to 21 days. In the first 12 h, zinc levels increased slightly, but significantly, in all groups. At later time points, zinc levels increased robustly (23%) in the youngest group by 24 h and remained elevated through 7 days. By contrast, deprivation-induced changes in zinc staining in aged animals, achieved their maximal levels at 12 h (approximately 10%) and steadily declined thereafter. Adult animals revealed a biphasic, intermediate change with time. In all age groups, levels of zinc staining returned to baseline by 21 days after whisker plucking. However, only in juvenile and adult mice did we observe that the level of zinc staining in deprived barrel hollows, was correlated with the length of whiskers as they regrew. Our data suggest that alterations in the regulation of synaptic zinc may be involved with decrements of synaptic plasticity that accompany senescence.

Blockade of ionotropic glutamate receptors produces neuronal apoptosis through the Bax-cytochrome C-caspase pathway: the causative role of Ca2+ deficiency

Blockade of ionotropic glutamate receptors induces neuronal cell apoptosis. We investigated if mitochondria-mediated death signals would contribute to neuronal apoptosis following administration of glutamate antagonists. The administration of MK-801 and CNQX (MK-801/CNQX), the selective antagonists of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors, produced widespread neuronal death in neonatal rat brain and cortical cell cultures. MK-801/CNQX-induced neuronal apoptosis was prevented by zVAD-fmk, a broad inhibitor of caspases, but insensitive to inhibitors of calpain or cathepsin D. Activation of caspase-3 was observed within 6-12 h and sustained over 36 h after exposure to MK-801/CNQX, which cleaved PHF-1 tau, the substrate for caspase-3. Activation of caspase-3 was blocked by high K+ and mimicked by BAPTA-AM, a selective Ca2+ chelator. Reducing extracellular Ca2+, but not Na+, activated caspase-3, suggesting an essential role of Ca2+ deficiency in MK-801/CNQX-induced activation of caspases. Cortical neurons treated with MK-801/CNQX triggered activation of caspase-9, release of cytochrome c from mitochondria, and translocation of Bax into mitochondria. The present study suggests that blockade of ionotropic glutamate receptors causes caspase-3-mediated neuronal apoptosis due to Ca2+ deficiency that is coupled to the sequential mitochondrial death pathway.

Glutamatergic neurotransmission in aging: a critical perspective

The effects of aging on glutamate neurotransmission in the brain is reviewed and evaluated. Glutamate is the neurotransmitter in most of the excitatory synapses and appears to be involved in functions such as motor behaviour, cognition and emotion, which alter with age. However, relatively few studies have been conducted to study the relationship between glutamate and aging of the brain. The studies presented here indicate the existence of a number of changes in the glutamatergic system during the normal process of aging. First, an age-related decrease of glutamate content in tissue from cerebral cortex and hippocampus has been reported, although it may be mainly a consequence of changes in metabolic activity rather than glutamatergic neurotransmission. On the other hand, studies in vitro and in vivo have shown no changes in glutamate release during aging. Since glutamate sampled in most of these studies is the result of a balance between release and uptake processes, the lack of changes in glutamate release may be due to compensatory changes in glutamate uptake. In fact, a reduced glutamate uptake capacity, as well as a loss in the number of high affinity glutamate transporters in glutamatergic terminals of aged rats, have been described. However, the most significant and consistent finding is the decrease in the density of glutamatergic NMDA receptors with age. A new perspective, in which glutamate interacts with other neurotransmitters to conform the substrates of specific circuits of the brain and its relevance to aging, is included in this review. In particular, studies from our laboratory suggest the existence of age-related changes in the interaction between glutamate and other neurotransmitters, e.g. dopamine and GABA, which are regionally specific.

Changes in anesthetic sensitivity and glutamate receptors in the aging canine brain

This study investigated whether the aging process in dogs is associated with an increased sensitivity to inhalation anesthesia and whether age-related changes in glutamate receptors are related to the increased sensitivity. The mean minimum alveolar concentration (MAC) of isoflurane was 1.82 +/- .08% for 2-3 year olds and 1.45 +/- .06% for 11 years olds, indicating that there was an increased potency of isoflurane in the older dogs as compared to the young. These older animals also showed a significant decrease in binding of [3H]glutamate and [3H]dizocilpine ([3H]MK801) to N-methyl-D-aspartate (NMDA) receptors in multiple cortical and hippocampal regions. The density of binding to NMDA receptors in the cortex, using a single concentration of ligand, correlated significantly with individual MAC values. These results demonstrate that dogs experience an increase in anesthetic potency with increased age, similar to humans, and that age-related changes in the NMDA receptor may represent one mechanism underlying this increased sensitivity to anesthesia.

Caloric restriction prevents age-related deficits in LTP and in NMDA receptor expression

A major focus of aging research has been the search for treatments that will prevent or ameliorate the memory deficits associated with aging. One paradigm, lifelong caloric restriction, has been reported to reduce some of the effects of aging. In the current report, we examined the effects of this treatment on age-related deficits in LTP, a putative cellular building block for memory formation. We report here that lifelong caloric restriction completely prevents the age-related deficit in LTP. In addition, we report that there is a dramatic decrease in the expression of the NMDA receptor subunit NR1 in aged rats and this age-related defect is also prevented by caloric restriction. These data provide a molecular and cellular mechanism by which life long caloric restriction may ameliorate some of the cognitive deficits associated with the aging process.

Declines in mRNA expression of different subunits may account for differential effects of aging on agonist and antagonist binding to the NMDA receptor

The purpose of the present study was to determine whether some of the age-related changes that occur in binding to the NMDA receptor complex can be accounted for by changes in subunit expression during the aging process. In situ hybridization for the NMDA subunits zeta1, epsilon1, and epsilon2, and receptor autoradiography, using the agonist glutamate and the competitive antagonist [(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP), were performed on sections from C57Bl/6 mice representing three different age groups (3, 10, and 30 months of age). There was a significant overall decrease between 3 and 30 month olds in the density of mRNA for the zeta1 subunit in the cortex and hippocampus, but only a few individual brain regions exhibited significant declines. The mRNA for the epsilon2 subunit was significantly decreased in a majority of cortical regions and in the dentate granule cells. Emulsion analysis indicated that the change in the density of epsilon2 subunit mRNA in the inner frontal cortex was primarily attributable to a decrease in the amount of messages per cell. Age-related changes in mRNA density of the epsilon2 subunit correlated with changes in NMDA-displaceable [(3)H]glutamate binding, and mRNA density changes in the zeta1 subunit showed a significant relationship with changes in [(3)H]CPP binding in the 30-month-old mice. These results suggest that changes during aging in the expression of different subunits of the NMDA receptor may account for the differential effects of aging on agonist versus antagonist binding to the NMDA binding site.

Age-related changes in expression of hippocalcin and NVP2 in rat brain

Expression of hippocalcin and neural visinin-like calcium-binding protein 2 (NVP2) in aging rat brain was investigated by immunoblot and immunohistochemical analyses. In 3-month old rats, hippocalcin and NVP2 were present at high concentrations in hippocampal and cerebral pyramidal cells and dentate granule cells, with hippocalcin protein levels being five to ten times higher than NVP2 levels. Hippocalcin levels in hippocampus and cerebral cortex decreased by approximately 20% at 24 months. While the number of hippocalcin-positive cells in CA3, dentate gyrus and cerebral cortex were preserved, staining intensity decreased. In contrast, the number and staining intensity of hippocalcin-positive cells in CA1 were maintained. NVP2 levels in hippocampus and cerebral cortex decreased by approximately 30% at 24 months. In cerebral cortex, the number and intensity of NVP2-positive cells decreased. In CA1 through CA3 and in dentate gyrus, NVP2-positive cell numbers were preserved, but staining intensity decreased. In summary, the loss of hippocalcin and NVP2 in aging rat brain may be associated with age-related impairment of postsynaptic functions.

Alteration of NMDA receptor-mediated synaptic responses in CA1 area of the aged rat hippocampus: contribution of GABAergic and cholinergic deficits

Synaptic responses mediated by the N-methyl-D-aspartate receptor (NMDAr) and non-NMDAr activation were compared in CA1 hippocampal region of young (3-4 months old) and aged (25-33 months old) Sprague-Dawley rats with the use of ex vivo extracellular recordings techniques. In aged rats, the amplitude of the NMDAr-mediated field excitatory postsynaptic potentials (fEPSPs) was not altered, whereas their duration was significantly increased. In contrast, the magnitude of non-NMDAr-mediated fEPSPs was significantly smaller. The presynaptic fiber volley was not affected by age. Considering that the depression of non-NMDAr-mediated responses was previously attributed to fewer synaptic contacts between glutamatergic afferent fibers and pyramidal cells in aged animals (see Barnes et al., Hippocampus 1992;2:457-468), the absence of age-related changes in the amplitude of NMDAr-mediated fEPSPs suggests that compensatory mechanisms may occur. The contribution of gamma-aminobutyric acid (GABA) and acetylcholine to these mechanisms was addressed. The NMDAr-mediated fEPSPs were then recorded (1) in young and aged rats before and after blockade of the GABA(B) receptor-mediated inhibition by the specific antagonist CGP 55845 and (2) in young rats after a selective cholinergic denervation of the hippocampus by the immunotoxin 192 IgG-saporin. The results did not indicate statistically relevant age-related effects of CGP 55845. In contrast, the loss of the cholinergic innervation by the immunotoxin induced a significant increase in both the amplitude and duration of the NMDAr-mediated fEPSPs. Our results indicate that the functional properties of the ionotropic glutamate receptor subtypes located on CA1 pyramidal cells are differentially affected by aging and suggest that the cholinergic deficit that occurs during aging may be involved in the maintenance of robust NMDAr-mediated synaptic responses.

Enhanced isoflurane suppression of excitatory synaptic transmission in the aged rat hippocampus

1. The effects of the volatile anaesthetic, isoflurane, were investigated on evoked dendritic field excitatory postsynaptic potentials (f.e.p.s.p.) and antidromic and orthodromic population spikes recorded extracellularly in the CA1 cell layer region in the in vitro hippocampal slice taken from young mature (2-3 months) and old (24-27 months) Fisher 344 rats. 2. Isoflurane depressed the f.e.p.s.ps and the orthodromically-evoked population spikes in both old and young hippocampi. However, the magnitude of the anaesthetic-induced depression was greater in slices taken from old rats compared to those taken from young rats during the application of different isoflurane concentrations (0.5-5%). 3. In the presence of the GABA(A) antagonist, bicuculline methiodide (15 microM), isoflurane suppressed the f.e.p.s.ps to the same extent as was observed in the absence of the GABA(A) antagonist. 4. Orthodromically evoked population spikes were suppressed by isoflurane in a manner quantitatively similar to the suppression of the f.e.p.s.ps. However, antidromic population spikes and presynaptic volleys evoked in young and old slices were resistant to anaesthetic action. In addition, paired pulse facilitation ratio of the evoked dendritic f.e.p.s.ps was not affected in both young and old slices during the application of isoflurane. 5. When slices were exposed to low Ca2+/high Mg2+ solution, isoflurane (1 and 3%) depressed the f.e.p.s.ps in aged slices to the same extent as in young slices. 6. The augmented anaesthetic depression of f.e.p.s.ps in old compared to young hippocampi in the absence and presence of bicuculline, and the lack of anaesthetic effects on antidromic population spikes and presynaptic volleys in old and young slices, suggest that the increased sensitivity of anaesthetic actions in old hippocampi is due to age-induced attenuation of synaptic excitation rather than potentiation of synaptic inhibition. Furthermore, elimination of the increased sensitivity of old slices to anaesthetic actions when the slices were perfused with low Ca2+/high Mg2+ medium, which presumably would decrease intracellular [Ca2+], suggests that the enhanced anaesthetic effects in aged neurones might be related to increased intraneuronal [Ca2+] in the synaptic terminal.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

PCBs reduce long-term potentiation in the CA1 region of rat hippocampus

Prenatal exposure to polychlorinated biphenyls (PCBs) has been associated with a lower IQ in childhood. We have examined the effects of acute exposure to PCB mixtures and two single congeners on synaptic transmission between Schaffer collaterals and CA1 neurons of the rat hippocampus as well as posttetanic potentiation (PTP), paired pulse facilitation (PPF), and long-term potentiation (LTP). PTP and PPF represent transient increases in transmitter release immediately after stimulation, while LTP is a measure of long-term changes in synaptic plasticity that has been related to learning and memory. LTP, but neither PTP nor PPF, was reduced by Aroclor 1016 in a dose-dependent fashion at concentrations that had little effect on general synaptic transmission. The more highly chlorinated Aroclor 1254 at low concentrations specifically blocked LTP, but at higher concentrations also reduced synaptic transmission. The mono-ortho PCB congener 2,4,4'-trichlorobiphenyl and the coplanar congener 3,3',4,4'-tetrachlorobiphenyl also blocked LTP without effect on PTP or PPF. We conclude that PCBs selectively impair the process of LTP in CA1 neurons of the hippocampus.

Pharmacological characterization of ionotropic excitatory amino acid receptors in young and aged rat basal forebrain

Ionotropic glutamate receptors were characterized in acutely dissociated medial septum/nucleus of diagonal band neurons from one- to four-month- and 24-26-month-old male Fischer 344 rats. Whole-cell patch-clamp recordings were used to study glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate and N-methyl-D-aspartate-induced currents. Pharmacological properties of these ionotropic receptors were studied across different age groups by comparing concentration response curves and EC50 for agonist-induced currents, as well as dissociation constants (Kb) for competitive receptor antagonists. Our results suggest that non-N-methyl-D-aspartate receptors on medial septum/nucleus of diagonal band neurons were predominantly of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate type and display biophysical and pharmacological properties similar to other central neurons. However, peak alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents were enhanced in aged (35.0+/-4.4 pA/pF) compared to young cells (16.2+/-1.7 pA/pF, P<0.005), and the EC50 shifted to the right (4.4+/-0.6 in young compared to 8.8+/-1.3 microM in aged, P<0.05). The Kb for 6,7-dinitroquinoxaline-2,3-dione inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents likewise shifted to the right (0.16+/-0.02 in young and 0.29+/-0.04 microM in aged, P<0.05) suggesting an age-related decrease in affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors. N-Methyl-D-aspartate-induced currents were generated in standard physiological solutions with the addition of 1 microM glycine and the removal of Mg2+. The N-methyl-D-aspartate responses were predictably modulated by magnesium and glycine, and were antagonized by the competitive antagonist 2-amino-5-phosphonovaleric acid. No age-related change in N-methyl-D-aspartate maximum, EC50, magnesium sensitivity, glycine sensitivity or Kb for 2-amino-5-phosphonovaleric acid was observed. Overall, our results suggest that ionotropic glutamate receptors in the medial septum/nucleus of diagonal band have a similar pharmacological profile compared to glutamate receptors in other brain regions. More importantly, these data suggest that while medial septum/nucleus of diagonal band cells maintain N-methyl-D-aspartate receptors during ageing, a significant increase in current density and decrease in receptor affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, during this same time period, may provide a mechanism for age-related changes in neuronal plasticity and excitotoxicity in the basal forebrain.

Chronic intraventricular infusion with NGF improves LTP in old cognitively-impaired rats

Aged (21 months) cognitively-impaired male Sprague-Dawley rats received intraventricular infusion of nerve growth factor (NGF) or cytochrome C (Cit C) for 14 or 28 days using miniosmotic pumps and were evaluated either 1 week or 3 months after treatment. Groups of untreated young, aged-impaired and aged non-impaired rats were also evaluated. Under narcose recording and stimulating electrodes were stereotactically implanted in the dentate gyrus and the perforant path. The stimulation intensity was individually adjusted to obtain a half-maximal population spike (P) for test stimuli and a quarter-maximal for tetanization. The amplitude and latency of P and the slope (S) of the field EPSP were determined before and at 2, 5, 15, 30 and 60 min after tetanization at 400 Hz. Paired stimuli at 30 ms interval were also applied before and after tetanization. Aged, cognitively impaired rats showed an absent S potentiation and a delayed P potentiation, both in amplitude and latency, while non-impaired rats behaved like the young controls. Paired pulse inhibition showed no difference among groups before or after tetanization suggesting that the impaired potentiation is not due to an increased retroactive inhibition. NGF treatment ameliorates LTP deficits to levels equivalent to non-impaired rats, while Cit C controls showed no improvement. No differences appear among NGF treated groups, but evidence suggest that the animals evaluated 3 months after treatment developed a stronger potentiation.

Distribution and levels of [125I]IGF-I, [125I]IGF-II and [125I]insulin receptor binding sites in the hippocampus of aged memory-unimpaired and -impaired rats

The insulin-like growth factors (IGF-I and IGF-II) and insulin are localized within distinct brain regions and their respective functions are mediated by specific membrane receptors. High densities of binding sites for these growth factors are discretely and differentially distributed throughout the brain, with prominent levels localized to the hippocampal formation. IGFs and insulin, in addition to their growth promoting actions, are considered to play important roles in the development and maintenance of normal cell functions throughout life. We compared the anatomical distribution and levels of IGF and insulin receptors in young (five month) and aged (25 month) memory-impaired and memory-unimpaired male Long Evans rats as determined in the Morris water maze task in order to determine if alterations in IGF and insulin activity may be related to the emergence of cognitive deficits in the aged memory-impaired rat. In the hippocampus, [125I]IGF-I receptors are concentrated primarily in the dentate gyrus (DG) and the CA3 sub-field while high amounts of [125I]IGF-II binding sites are localized to the pyramidal cell layer, and the granular cell layer of the DG. [125I]insulin binding sites are mostly found in the molecular layer of the DG and the CA1 sub-field. No significant differences were found in [125I]IGF-I. [125I]IGF-II or [125I]insulin binding levels in any regions or laminae of the hippocampus of young vs aged rats. and deficits in cognitive performance did not relate to altered levels of these receptors in aged memory-impaired vs aged memory-unimpaired rats. Other regions. including various cortical areas, were also examined and failed to reveal any significant differences between the three groups studied. It thus appears that IGF-I, IGF-II and insulin receptor sites are not markedly altered during the normal ageing process in the Long Evans rat, in spite of significant learning deficits in a sub-group (memory-impaired) of aged animals. Hence. recently reported changes in IGF-I receptor messenger RNA levels in aged memory-impaired rats are apparently not reflected at the level of the translated protein.

Age-related decrease in the N-methyl-D-aspartateR-mediated excitatory postsynaptic potential in hippocampal region CA1

Glutamatergic fast synaptic transmission is known to be altered with age in a region-specific manner in hippocampus of memory-impaired old rats. In the present experiment, presynaptic fiber potentials and non-N-methyl-D-aspartate (NMDAR) and NMDAR-mediated synaptic responses in CA1 were compared in three ages of behaviorally characterized male F-344 rats. In the CA1 region, old rats showed approximately equivalent reductions in non-NMDAR- and NMDAR-excitatory postsynaptic potential amplitudes for a given size of presynaptic fiber potential. There was no change in magnitude of the presynaptic response itself at any stimulus level. These results are consistent with the hypothesis that there is a reduction in the number of Schaffer collateral synapses per presynaptic axon. This pattern of results in CA1 is very different from what is known to occur at the perforant path-granule cell synapse. In fascia dentata the non-NMDAR-mediated excitatory postsynaptic potential is increased in amplitude, although the NMDAR-mediated excitatory postsynaptic potential is reduced for a given presynaptic input. These data suggest that age-related functional alterations in neurotransmitter receptor subtypes occur differentially between closely-related anatomical subregions.

Aging reduces neostriatal responsiveness to N-methyl-D-aspartate and dopamine: an in vitro electrophysiological study

Excitatory amino acids and dopamine interact to control information flow in the neostriatum. The present study was designed to examine some of the age-induced alterations in the interaction of these two neurotransmitter systems. First, responsiveness of neostriatal neurons to glutamate and N-methyl-D-aspartate was compared in neurons from young and in aged animals. N-Methyl-D-aspartate function was chosen for emphasis because declines in cognitive processes during aging are thought to involve changes in this excitatory amino acid receptor. Second, the age-related changes in dopamine's ability to modulate responses mediated by excitatory amino acid receptors was examined. Specifically, the ability of dopamine to differentially modulate responses induced by N-methyl-D-aspartate and glutamate was assessed. There is considerable evidence for alterations in dopamine receptors and behavioral responses to dopamine in aged animals. It thus becomes important to determine how these alterations are reflected at an electrophysiological level. The responses to application of excitatory amino acid agonists and dopamine as well as changes in synaptic responses mediated by activation of N-methyl-D-aspartate receptors were assessed in 69 neurons obtained from young Fischer 344 rats (3-5 months) and young cats (3-4 years) and 69 neurons obtained from aged Fischer 344 rats (24-26 months) and aged cats (10-16 years) using an in vitro slice preparation. The results indicated that populations of aged neurons from both rats and cats displayed qualitative and quantitative alterations in responses to iontophoretic application of excitatory amino acid receptor agonists. These alterations included lack of response, unusual responses consisting of depolarizations without action potentials or combinations of prepotentials and full amplitude action potentials. Threshold currents for induction of responses were also significantly elevated in neurons from aged animals. Synaptic response components mediated by activation of N-methyl-D-aspartate receptors in aged rats were reduced as well. Exposure to Mg(2+)-free artificial cerebrospinal fluid resulted in marked increases in the size of responses evoked by local stimulation in young neurons from rats. These increases, which are mediated by activation of N-methyl-D-aspartate receptors, were significantly attenuated in aged neurons. The ability of dopamine to modulate responses mediated by activation of excitatory amino acid receptors was reduced in cells from both aged rats and cats. Subpopulations of cells were either unresponsive to dopamine or required higher iontophoretic current intensities to modulate excitatory amino acid-induced responses. The present findings further document age-induced changes in neostriatal electrophysiology indicating that interactions between excitatory amino acids and dopamine appear to be compromised during aging. They emphasize alterations in N-methyl-D-aspartate receptor function and suggest further than the ability of neostriatal neurons to integrate information is altered during aging. The present findings are supported by data from the literature indicating decreases in N-methyl-D-aspartate receptor function during aging. Furthermore, the decreases in excitatory amino acid function during aging suggest that therapeutic interventions designed to prevent or retard the deleterious effects of age in the neostriatum might be directed toward enhancing excitatory amino acid receptor function.

N-methyl-D-aspartate receptor density and membrane fluidity as possible determinants of the decline of passive avoidance performance in aging

The effect of aging on three different parameters possibly relevant for cognition was investigated in female Naval Medical Research Institute mice: a) N-methyl-D-aspartate (NMDA) receptor density, as determined by the specific binding of [3H]MK-801 to forebrain membranes, decreased by 22% in aged (23 mo) and by 19% in middle-aged (12 mo) animals compared with young (3 mo) animals. b) In a passive avoidance acquisition task, the 24-h latency decreased significantly with age; the middle-aged mice also tended to show impairment in this task. c) The fluidity of the forebrain membranes also decreased significantly with age. Again, there was a significant reduction in the middle-aged group. A comparison of these parameters revealed significant correlations between NMDA receptor density and 24-h latency (r = 0.52, p < 0.003) over all three age groups, as well as significant correlations between membrane fluidity and either NMDA receptor density or 24-h latency. These findings do not prove a causal relationship, but are compatible with the hypothesis that changes of membrane fluidity, by decreasing the number of NMDA receptors, affect passive avoidance performance.

Long-term potentiation in the piriform cortex is blocked by lead

1. Long-term potentiation (LTP) is a prolonged increase in synaptic efficacy that is triggered by a brief tetanic stimulation at certain central synapses. LTP is one of the best available model systems available to the neurophysiologist of neuronal plasticity such as that underlying learning and memory. 2. We have studied the susceptibility of LTP to blockade by lead as a test of the hypothesis that the negative effect of lead on intelligence in children may result from interference with this process. LTP was studied in slices of rat piriform cortex. At this site, as in many other central synapses, LTP requires activation of postsynaptic N-methyl-D-aspartate (NMDA) receptors, and we investigated whether lead actions, if any, were mediated via effects on NMDA-activation ion channels or, alternatively, at voltage-activated calcium channels. 3. We find that lead blocks LTP at low micromolar concentrations. However, concentrations of lead that totally block LTP had no apparent effect on either NMDA-activated responses or presynaptic calcium channels, as monitored by transmitter release from presynaptic terminals. 4. While the mechanism of lead blockade of LTP remains to be determined, these observations are consistent with the hypothesis that the cognitive effects of lead neurotoxicity may result from effects on LTP.

Effects of brain tissue hydrolysate on synaptic transmission in the hippocampus

A mix of peptides and amino acids obtained from porcine brain tissue (Cerebrolysin) has been shown to affect passive avoidance behavior in neonatal rats. To identify the active components and mechanisms of action, Cerebrolysin effects were studied in in vitro hippocampal slices. Cerebrolysin induced dose-dependent suppression followed by a small rebound increase of synaptic responses in the CA1 but not dentate gyrus neurons. These actions may be due to peptides present in Cerebrolysin and may contribute to its reported behavioural effects.

The function of the NMDA-receptor during normal brain aging

Age-related changes of N-methyl-D-aspartate (NMDA) receptors have been found in cortical areas and in the hippocampus of many species. On the basis of a variety of experimental observations it has been suggested that the decrease of NMDA-receptor density might be one of the causative factors of the cognitive decline with aging. Based on these findings several strategies have been developed to improve cognition by compensating the NMDA-receptor deficits in aging. The most promising approaches are the indirect activation of glutamatergic neurotransmission by agonists of the glycine site or the restoration of the age-related deficit of receptor density by several nootropics.

Selective reduction of hippocampal dentate frequency potentiation in aged rats with impaired place learning

Induction of posttetanic potentiation (PTP) and long-term potentiation (LTP) was analyzed in hippocampal slices obtained from a) young 6-month-old Sprague-Dawley (SD) rats, all of them performing well in the Morris Maze, and b) aged SD 20-month-old and Fischer 344 24-month-old rats showing different degrees of ability in the same test. After the application of an electrical tetanus 1 s, 100 Hz, 50 microA in the stratum radiatum, no significant differences were found in the percent of induction of both PTP and LTP in the CA1 area of hippocampal slices obtained from rats of different strains and ages. After the application of an electrical tetanus 1 s, 100 Hz, 50 microA in the stratum moleculare, a significant difference was found in the percent of dentate PTP induction in hippocampal slices obtained from rats of different ages. Specifically, dentate PTP induction was significantly (p < 0.01) higher in slices obtained from young SD rats, and from old SD rats with a better performance in the Morris maze, escape latency less than 10 s and 150 cm, than in slices obtained from old SD or Fischer 344 rats that had shown poor performance in the Morris Maze. On the contrary, no significant differences were found in the percent of dentate LTP in hippocampal slices obtained from rats of different strains and ages. The data demonstrate that the induction of hippocampal dentate high-frequency PTP is selectively reduced in old rats with impaired Morris Maze performance.

Glutamatergic treatment strategies for age-related memory disorders

Age-related changes of N-methyl-D-aspartate (NMDA) receptors have been found in cortical areas and in the hippocampus of many species. On the basis of a variety of experimental observations it has been suggested that the decrease of NMDA receptor density might be one of the causative factors of the cognitive decline with aging. Based on these findings several strategies have been developed to improve cognition by compensating the NMDA receptor deficits in aging. The most promising approaches are the indirect activation of glutamatergic neurotransmission by agonists of the glycine site or the restoration of the age-related deficit of receptor density by several nootropics.

Normal aging: regionally specific changes in hippocampal synaptic transmission

Results of electrophysiological investigations of aging in the rodent hippocampus contradict the popular conception of the aging process as one of general deterioration. Such studies have revealed a selective pattern of both degenerative change and functional sparing in different physiological parameters of the same cells. In synaptic transmission, changes have been observed that might even be considered compensatory. The selectivity of the aging process is further demonstrated by the fact that it exhibits clear regional specificity, even among the different subfields of the hippocampus. The future challenges will be to understand both how these specific patterns of age-related neurobiological change arise, and how they lead to the cognitive changes that arise during normal aging.

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Effects of subchronic administration of pyritinol on receptor deficits and phosphatidylinositol metabolism in the brain of the aged mouse

The effect of pyritinol, a commonly used nootropic drug, on receptor properties and function was investigated in different neuronal systems, possibly associated with age-related decline in brain function. Chronic treatment (15 days) of aged (22 months) female NMRI mice with pyritinol (200 mg/kg) restored the reduced density of N-methyl-D-aspartate receptors in the aged mouse brain. Furthermore, the total number of binding sites of the alpha 2-receptor ([3H]yohimbine binding) decreased after treatment with drug, while the number of high-affinity agonist binding sites ([3H]UK 14304 binding) was not changed. In both systems, receptor affinity was not influenced. The densities of other receptors investigated (muscarinic-cholinergic, benzodiazepine and beta-adrenergic) were not altered by treatment with pyritinol. Additionally, the effect of pyritinol on phosphatidylinositol (PI) metabolism was investigated in dissociated neurones from young and aged mice. Muscarinic-cholinergic induced accumulation of phosphatidylinositol and the inositol phosphate response due to activation of G-protein by fluoride was increased in aged animals, treated with drug. The inositolphosphate response after stimulation with pilocarpine was slightly but not significantly increased. The metabolism of phosphatidylinositol in young animals was not altered by treatment with drug. These results support the hypothesis of a nootropic-mediated restoration of age-related brain deficits. Changes caused by pyritinol may be due to beneficial effects on age-related alterations of the properties of the neuronal membrane.

Age-induced changes in electrophysiological responses of neostriatal neurons recorded in vitro

The present studies were undertaken to determine whether the major electrophysiological characteristics of neostriatal neurons are altered during aging. The passive and active membrane properties of 130 neostriatal neurons obtained from young (three to five months, N = 65) and aged (24-26 months, N = 65) Fischer 344 rats were compared using an in vitro slice preparation. The results indicated that in a population of aged neostriatal neurons the majority of the electrophysiological changes that occurred resulted in decreases in cellular excitability. These changes included increased threshold to induce action potentials by intracellular current injection and decreased negativity of membrane potentials at which such action potentials were induced. In addition, there were increases in the amplitude of the action potential afterhyperpolarization and increases in the frequency of occurrence of accommodation when trains of action potentials were induced. These two latter effects can limit the frequency of action potential generation. The thresholds to elicit synaptically evoked depolarizing responses and action potentials were increased. The results also indicated that a number of basic electrophysiological parameters were unchanged by the aging process. These included action potential amplitude, rise time and duration, resting membrane potential, input resistance and time constant. Although thresholds for the induction of synaptic and action potentials by extracellular stimulation were increased, the latency, amplitude and duration of the evoked depolarization remained unchanged. These findings suggest that the ability of neostriatal neurons to integrate spatiotemporal inputs must be severely compromised in this population of aged cells. Furthermore, the present findings, when compared with age-induced electrophysiological alterations in neurons in other brain areas, indicate that age may differentially alter electrophysiological properties of neurons in separate nuclei. Profiles of age-related changes in neurophysiological properties of neurons provide important information that can be related to the contributions of individual neural areas to the behavioral effects of aging.

Age-related changes in kynurenic acid production in rat brain

Two separate in vitro assays were used to examine the biosynthesis of the broad spectrum excitatory amino acid receptor antagonist kynurenic acid (KYNA) during the life span of the adult rat. Assessment of KYNA's anabolic enzyme kynurenine aminotransferase revealed steady increases between 3 and 24 months of age in all five brain regions examined. No changes were observed in the liver. The changes were particularly pronounced in the cortex and in the striatum where enzyme activity increased three-fold during the period studied. KYNA production from its bioprecursor L-kynurenine was also investigated in tissue slices and was found to be significantly enhanced in the cortex and hippocampus of old animals. The effect of depolarizing agents or sodium replacement was virtually identical in tissues from young and old rats. These data, which are in excellent agreement with reports on an age-dependent increase of KYNA concentration in brain tissue, suggest an enhanced KYNA tone in the aged brain. Together with the reported decline in cerebral excitatory amino acid receptor densities with age, increased production of KYNA may play a role in cognitive and memory dysfunction in old animals.

Development of long-term potentiation in the somatosensory cortex of rats of different ages

The age dependence of possible long-term potentiation (LTP) induction in rat somatosensory cortex was studied in in vitro slice experiments. Coronal slices were prepared from the somatosensory cortex of rats of different ages, and excitatory postsynaptic potentials evoked by stimulation of the white matter (0.1 Hz, subthreshold for spike) were recorded intracellularly. In 70% of the slices taken from 2-week-old rats, a moderate potentiation (20-30%) could be induced by either 5 or 100 Hz stimulation. No LTP was observed in younger (1 week) or older (3 weeks) cortex. On the basis of our experiments an important ontogenetic role of increased synaptic efficacy is suggested in a critical developmental period of rats after birth.

Age-related alterations of NMDA-receptor properties in the mouse forebrain: partial restoration by chronic phosphatidylserune treatment

The effect of aging on the properties of N-methyl-D-aspartate (NMDA) receptors in the forebrain of female NMRI mice was investigated using the antagonist [3H]MK-801 as radioligand. Compared to young (3 months) mice, aged (20 months) mice showed changes of the properties of the NMDA receptor at three different levels: (1) the density was reduced by about 35%; (2) the efficacy of L-glutamate and glycine for stimulating specific [3H]MK-801 binding was enhanced, probably because more NMDA receptor-associated ion channels are closed under baseline conditions in the aged brain; (3) the affinity of L-glutamate and glycine to its binding sites at the NMDA receptor complex was also enhanced. Chronic treatment of aged mice with phosphatidylserine (20 mg/kg, i.p., once daily) for three weeks completely normalized enhanced efficacy and affinity of L-glutamate and glycine and elevated NMDA receptor density by approximately 25%. These findings are consistent with the assumptions that deficits of the NMDA receptor are one of the mechanisms of age-related cognitive impairment and that the beneficial effects of phosphatidylserine treatment on cognitive deficits of aged individuals might be partially due to the effects of this drug on age-related NMDA receptor deficits.

Effects of the N-methyl-D-aspartate antagonists on the rise in [Ca2+]i following depolarization in aged rat brain synaptosomes

The effects of non-competitive NMDA antagonists, MK-801 and dextrorphan in relation to the rise in intracellular Ca2+ concentrations ([Ca2+]i) after stimulation with 15 mM K+ in whole brain synaptosomes from young (3 months old) and aged (24 months old) Fisher344 rats were examined. A fluorescent chelating agent, Rhod-2, was employed to monitor any alterations of K(+)-evoked [Ca2+]i. In young rats, the rise in [Ca2+]i following depolarization was affected by neither dextrorphan (1, 10, 100 microM) nor MK-801 (0.1, 1, 10 microM), while in aged rats, 1 microM dextrorphan and 0.1 microM MK-801 brought about a significant increase in [Ca2+]i following depolarization. In low Mg2+ medium, 10 microM MK-801 and 100 microM dextrorphan significantly inhibited the rise in [Ca2+]i after stimulation with 15 mM K+ in young rats, while neither dextrorphan nor MK-801 could affect the rise in [Ca2+]i significantly in aged rats. When 100 microM NMDA was applied in a medium containing 1.2 mM Mg2+, the rise in [Ca2+]i following depolarization was slightly inhibited by 1 microM MK-801 in young rats, but it was not inhibited significantly by dextrorphan. In aged rats, both 100 microM dextrorphan and 10 microM MK-801 strongly inhibited the rise in [Ca2+]i following depolarization in the presence of 100 microM NMDA. Instead of NMDA, when 100 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a non-NMDA receptor agonist, was applied, dextrorphan did not inhibit the rise in [Ca2+]i. In low Mg2+ medium, 100 microM NMDA potentiated the inhibitory effect of 10 microM dextrorphan in young rats, while 100 microM dextrorphan or MK-801 did not show any further inhibition by adding 100 microM NMDA. The addition of 100 microM AMPA did not affect the effect of dextrorphan in a low Mg2+ medium in young rats. These results suggest that NMDA antagonist-mediated [Ca2+]i homeostatic system may alter through aging. In addition, the findings that NMDA potentiated the inhibitory effect of NMDA antagonist, which being further potentiated by aging or lowered extrasynaptosomal Mg2+, indicate the possibility that the Mg2+ block to NMDA receptors might be attenuated through aging.

Effects of aging on quinolinic acid lesions in rat striatum

Several neurologic illnesses in which excitotoxic mechanisms may play a role increase in prevalence with age. In the present study we examined the susceptibility of rats to quinolinic acid striatal lesions at 1, 4 and 20 months of age, and susceptibility to N-methyl-D-aspartate (NMDA) at 1 and 4 months of age. The extent of the lesions was quantitated with measurements of substance P-like immunoreactivity (SPLI) and gamma-aminobutyric acid (GABA). The lesions in the 4- and 20-month-old age groups showed significantly smaller depletions of SPLI and GABA than those in 1-month-old animals. Neuropeptide Y-like immunoreactivity (NPYLI) and somatostatin-like immunoreactivity (SLI) were unchanged in the lesioned striata. NMDA lesions were also attenuated in 4-month- and 12-month-old animals as compared with 1-month-old animals. Uric acid concentrations showed marked dose-dependent increases in the lesioned striatum, and to a lesser extent in the overlying cerebral cortex, in all 3 age groups. There were no changes of SLI, NPYLI or SPLI with aging in the cerebral cortex or hippocampus. Kynurenine and kynurenic acid concentrations showed significant increases with aging in frontal cortex. The present results show a reduced susceptibility of animals to striatal quinolinic acid and NMDA lesions with normal aging. The delayed onset of several neurodegenerative illnesses is therefore unlikely to be due to an increasing susceptibility to excitotoxin lesions with aging.

Local cerebral glucose utilization in the brain of old, learning impaired rats

The local cerebral glucose utilization (LCGU) was measured in 63 different cortical areas and nuclei of the telencephalon, diencephalon and rhombencephalon of young adult (3 to 4-month-old) rats and of 27-month-old Wistar rats, in which learning impairments had been proven by a water maze test. The LCGU was determined by [14C]2-deoxyglucose autoradiography. In the old rats the mean LCGU of all brain regions was significantly reduced by about 10% compared with the young control group; the mean LCGU was 74.2 mumol glucose/(100 g x min) in the young and 66.7 in the old rats. Different degrees of LCGU decrease were found in the different regions. Most of the brain regions with significantly reduced LCGU values in the aged, learning impaired rats were associated with auditory and visual functions, the dopaminergic system, and structures known to be involved in learning and memory processes. Therefore, the regional pattern of LCGU reduction found in the aged, learning impaired rats did not resemble any known pattern found after lesions of a single transmitter system or systemic administration of transmitter agonists or antagonists.