Age-related decreases of the N-methyl-D-aspartate receptor complex in the rat cerebral cortex and hippocampus

Age-related changes in the NMDA receptor/nitric oxide/cGMP pathway in the hippocampus and cerebellum of freely moving rats subjected to transcerebral microdialysis

The N-methyl-D-aspartate (NMDA) receptor/nitric oxide synthase/guanylate cyclase pathway was studied during aging by monitoring extracellular cGMP in the rat hippocampus and cerebellum during in vivo microdialysis. In the hippocampus the basal cGMP efflux decreased by 50% from 3 to 12 months of age, whereas it remained constant with age in the cerebellum. Locally perfused NMDA (1 mM) evoked remarkable cGMP responses in 3-month-old rats; in the hippocampus the cGMP production was already dramatically reduced at 12 months, whereas in the cerebellum a similar impairment occurred much later (24 months). The nitric oxide donor S-nitroso-N-penicillamine (1 mM) elicited cGMP responses which slightly decreased from 3 to 12-24 months in the hippocampus, while no significant decrement with age could be seen in the cerebellum. Local perfusion of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 1 mM) produced large increases in hippocampal cGMP levels. The response decreased at 12 and 24 months, apparently in parallel with the fall in the basal level of cGMP. No significant differences across ages were observed following IBMX infusion in the cerebellum. The decreases in basal outflow and in the NMDA-evoked cGMP response seen in the aged hippocampus were not compensated for by supplying L-arginine. Infusion of D-serine (1 mM) enhanced (150-200%) extracellular cGMP in the cerebellum with no age-related differences. The activity in vitro of hippocampal nitric oxide synthase at 24 months was 33% lower than at 3 months, whereas the cerebellar enzyme did not show any age-related decay.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Chronic treatment with the uncompetitive NMDA receptor antagonist memantine influences the polyamine and glycine binding sites of the NMDA receptor complex in aged rats

Receptor binding studies on rat cortical membranes were used to characterize the NMDA receptor in aged rats (22 months) treated for 20 months with a memantine containing diet delivering 30 mg/kg/day in comparison to aged and young/adult rats treated with control-diet. Spatial memory impairing effects of (+)-MK-801 (0.16 mg/kg) in the radial maze was not altered within the course of memantine-treatment (up to 16 months). However, chronic memantine-treatment significantly increased the number of [3H]MK-801 binding sites and the affinity of [3H]glycine. A non-significant trend to such changes was also seen in aged-control rats. Glycine-dependent [3H]MK-801 binding (functional binding under non-equilibrium conditions at a fixed L-glutamate concentration) revealed that a decreased ability of glycine to stimulate channel opening in aged rats was partially attenuated by the long-term memantine treatment. Furthermore, an increased ability of spermidine to enhance [3H]MK-801 binding in aged-control rats was even more pronounced in the aged memantine-treated group. Together these findings may indicate that changes in functional receptor-channel properties during the process of aging occur prior to a detectable loss of binding sites and that memantine enhances an endogenous compensatory mechanism triggered by glutamatergic hypofunction which is suggested to take place in aging.

Stable maintenance of glutamate receptors and other synaptic components in long-term hippocampal slices

Cultured hippocampal slices retain many in vivo features with regard to circuitry, synaptic plasticity, and pathological responsiveness, while remaining accessible to a variety of experimental manipulations. The present study used ligand binding, immunostaining, and in situ hybridization assays to determine the stability of AMPA- and NMDA-type glutamate receptors and other synaptic proteins in slice cultures obtained from 11 day postnatal rats and maintained in culture for at least 4 weeks. Binding of the glutamate receptor ligands [3H]AMPA and [3H]MK-801 exhibited a small and transient decrease immediately after slice preparation, but the binding levels recovered by culture day (CD) 5-10 and remained stable for at least 30 days in culture. Autoradiographic analyses with both ligands revealed labeling of dendritic fields similar to adult tissue. In addition, slices at CD 10-20 expressed a low to high affinity [3H]AMPA binding ratio that was comparable with that in the adult hippocampus (10:1). AMPA receptor subunits GluR1 and GluR2/3 and an NMDA receptor subunit (NMDAR1) exhibited similar postcutting decreases as that exhibited by the ligand binding levels, followed by stable recovery. The GluR4 AMPA receptor subunit was not evident during the first 10 CDs but slowly reached detectable levels thereafter in some slices. Immunocytochemistry and in situ hybridization techniques revealed adult-like labeling of subunit proteins in dendritic processes and their mRNAs in neuronal cell body layers. Long-term maintenance was evident for other synapse-related proteins, including synaptophysin, neural cell adhesion molecule isoforms (NCAMs), and an AMPA receptor related antigen (GR53), as well as for certain structural and cytoskeletal components (e.g., myelin basic protein, spectrin, microtubule-associated proteins). In summary, following an initial and brief depression, many synaptic components were expressed at steady-state levels in long-term hippocampal slices, thus allowing the use of such a culture system for investigations into mechanisms of brain synapses.

Glycine release from hippocampal slices in developing and ageing mice: modulation by glutamatergic receptors

The release of preloaded [3H]glycine from hippocampal slices in developing and ageing mice (from 7 days to 22 months) was characterized using a superfusion system. The release was Ca(2+)-independent in each age group studied. The basal release and the responses to potassium stimulation were fairly constant during the whole life span. The release was potentiated by the glutamate receptor agonists kainate, N-methyl-D-aspartate (NMDA), tetrazolylglycine, quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) in developing mice, but only kainate was effective in adult and aged animals. The kainate effect was not modified by the antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in adult and old mice, indicating that glutamatergic systems may not be involved in the release. On the other hand, hippocampal glycine release in immature mice seems to be subject to regulation by both NMDA and non-NMDA (kainate and AMPA) receptors. The potentiations by NMDA and AMPA were antagonized by dizocilpine (MK-801) and CNQX, respectively. The modulation of glycine release by glutamatergic receptors could be of importance in the regulation of synaptic glycine levels in the developing hippocampus.

Reduced bicuculline response and GABAA agonist binding in aged rat hippocampus

Extracellular field recordings from CA1 pyramidal cells in the rat hippocampal slice preparation were used to examine the effects of age on gamma-aminobutyric acid (GABA)-mediated recurrent inhibition. The actions of bicuculline (1-100 microM), a GABAA antagonist, were assessed in slices from young (1-3 months) and aged (26 months) Fischer 344 rats. Pre-drug population spike amplitudes were smaller in slices from aged rats. Bicuculline increased population spike amplitudes in slices from both age groups, but slices from young rats were more sensitive to the antagonist. Bicuculline also produced multiple population spikes in slices from both age groups, however the increase in population spike burst durations was much greater in slices from young rats than in slices from aged rats. Agonist radiolabeled GABAA binding site density was significantly decreased in hippocampal tissue from aged rats. Our results suggest there is a reduction in GABAergic inhibition in hippocampal slices from aged rats, possibly mediated by a decrease in GABAA receptors.

Age-related changes in expression of AMPA-selective glutamate receptor subunits: is calcium-permeability altered in hippocampal neurons?

Age-related decline of cognition and memory, in humans and other animals, appears to be associated with neuronal loss. Experimental and clinical evidence has shown that the hippocampal formation is one of the brain regions most vulnerable to the ageing process. Because excess of glutamate is neurotoxic to hippocampal neurons, abnormalities in glutamate neurotransmitter function may play a crucial role in neurodegenerative disorders, especially in conjunction with brain ageing. We have used in situ hybridization to study the expression of the two major alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective glutamate receptor subunits, involved in the control of calcium permeability in the young adult and aged rat hippocampus. We show that the levels of messenger RNA encoding the AMPA-selective glutamate receptor subunit-1 (GluR1 or GluRA) and AMPA-selective glutamate receptor subunit-2 (GluR2 or GluRB) are highest in the dentate gyrus, followed by the CA1 and CA3 hippocampal subfields. We also show that the levels of both messenger RNAs decrease differentially with age in all subfields of the hippocampus. Finally, the GluR1/GluR2 messenger RNA ratios increase in the aged hippocampus, particularly in the CA3 subfield, suggesting that altered calcium homeostasis may contribute to age-related neuronal death.

Age-dependent loss of NMDA receptors in hippocampus, striatum, and frontal cortex of the rat: prevention by acetyl-L-carnitine

Acute i.p. administration of Acetyl-L-Carnitine (ALCAR), a component of several biological systems, has been found to modify spontaneous and evoked electrocortical activity in young rats, and, in the old rats, to improve learning ability and to increase the number of NMDA receptors in the whole brain. The present study was aimed at ascertaining the effect of chronic treatment with ALCAR added to drinking water on age-related changes in the different brain areas of rats. In twenty-four-month-old rats, ALCAR treatment for six months significantly impeded the decline in the number of NMDA receptors within the hippocampus, the frontal cortex and the striatum compared to the adult animal. This finding thus confirms the previously reported positive effect of ALCAR on the brain NMDA receptor system.

New pharmacological strategies for cognitive enhancement using a rat model of age-related memory impairment

We have developed the Stone maze paradigm for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Evidence produced thus far clearly implicates both the cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results have been very inconsistent regarding the cognitive enhancing abilities of cholinomimetics for use in Alzheimer's disease, new classes of cholinesterase inhibitors may offer greater therapeutic efficacy. The use of glycine and polyamine agonists appears to be a viable strategy for positive modulation of the NMDA receptor. In addition, an approach that combines stimulation both of cholinergic and glutamatergic systems may have greater potential than agonism of either separately. Manipulation of signal transduction events might also have potential for cognitive enhancement. The influx of Ca2+ through the NMDA receptor stimulates production of NO via the action of NOS. By using NARG to block NOS activity, we have demonstrated in rats that NO production appears to influence learning in the Stone maze. We are currently exploring the age-related changes in NOS activity in specific brain regions of rats to determine if loss in the NO generating system is related to age-related memory impairment observed in the Stone maze. In addition, we are exploring pharmacological strategies for inducing NO production; however, because of the potential neurotoxicity for NO overstimulation, this strategy will present some obstacles. The identification of NO as a simple molecule serving vital physiological functions but representing potential for neurotoxicity presents an important unifying area for neurobiological investigations searching for mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease.

NMDA receptor function is enhanced in the hippocampus of aged rats

The density and functional activity of the N-methyl-D-aspartate (NMDA)-sensitive glutamate receptor was examined in various brain areas of 3-, 18- and 24-month-old rats. The total numbers of binding sites for the NMDA receptor antagonists [3H]CGP 39653 and [3H]MK 801 binding sites were decreased in the hippocampus, cerebral cortex and striatum of 18- and 24-month-old rats, relative to 3-month-old animals. In the hippocampus of 18-month-old rats, the reduced number of NMDA receptors was associated with an increased sensitivity of [3H]MK 801 binding to the stimulatory action of glycine and glutamate. Thus, 10 microM glycine and 10 microM glutamate increased [3H]MK 801 binding in the hippocampus of 18-month-old rats by 75 and 160%, respectively; in 3-month-old animals, the same concentration of these amino acids increased binding by 37 and 95%, respectively. The sensitivity of [3H]MK 801 binding to glycine and glutamate was not increased in the cerebral cortex and striatum of aged rats. Moreover, an increased efficacy of glycine and glutamate in stimulating the binding of [3H]MK 801 in the hippocampus was no longer apparent in the 24-month-old rats. The increased sensitivity of [3H]MK 801 binding to glycine and glutamate in the hippocampus of 18-month-old rats may reflect an increase in NMDA receptor activity to compensate for the decrease in receptor number.

Acetyl-L-carnitine affects aged brain receptorial system in rodents

Acetyl-L-carnitine (ALCAR), the acetyl ester of carnitine, is regarded as a compound of considerable interest because of its capacity to counteract several physiological and pathological modifications typical of brain ageing processes. In particular, it has been demonstrated that ALCAR can counteract the age-dependent reduction of several receptors in the central nervous system of rodents, such as the NMDA receptorial system, the Nerve Growth Factor (NGF) receptors, those of glucocorticoids, neurotransmitters and others, thereby enhancing the efficiency of synaptic transmission, which is considerably slowed down by ageing. The present review thus postulates the importance of ALCAR administration in preserving and/or facilitating the functionality of carnitines, the concentrations of which are diminished in the brain of old animals.

Rodent models of memory dysfunction in Alzheimer's disease and normal aging: moving beyond the cholinergic hypothesis

The Stone maze paradigm has been developed for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Results from several studies have demonstrated the involvement of both cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results of clinical studies on the cognitive enhancing abilities of cholinomimetics for treatment of memory impairment in Alzheimer's disease have been inconsistent, new classes of cholinesterase inhibitors offer greater potential for therapeutic efficacy. The physostigimine derivative, phenserine, appears to have marked efficacy for improving learning performance of aged rats or of young rats treated with scopolamine in the Stone maze. Declines in markers of glutamatergic neurotransmission in Alzheimer's disease and in normal aging suggest that pharmacological manipulation of this system might also prove beneficial for cognitive enhancement. Treatment with glycine and/or polyamine agonists is suggested as a strategy for activating the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In addition, the use of combined pharmacological activation of cholinergic and glutamatergic systems is suggested. Manipulation of signal transduction events should also be considered as a strategy for cognitive enhancement. The influx of Ca2+ through the channel formed by the NMDA receptor stimulates the production of the oxyradical, nitric oxide (NO*), via the action of nitric oxide synthase (NOS). Compounds that inhibit NOS activity impair acquisition in the Stone maze, suggesting an involvement of NO*. Thus, strategies for inducing NO* production to enhance cognitive performance may be beneficial. Because of the potential neurotoxicity for NO*, this strategy is not straightforward. Although many new directions beyond the cholinergic hypothesis can be suggested, each has its potential benefits which must be weighed against its risks. Nonetheless, an important unifying area for neurobiological research examining mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease, might emerge from the identification of NO* as a simple molecule serving vital physiological functions but representing potential for neurotoxicity.

Chronic nimodipine treatment in aged rats: analysis of motor and cognitive effects and muscarinic-induced striatal dopamine release

Nimodipine is a calcium channel blocker reported to have beneficial effects on treatment of ischemic damage as well as the potential for retarding aspects of brain and behavioral aging when provided chronically to rats. We treated aged male F-344 rats (24 months) with nimodipine in SC pellets in the following doses: 0 (controls), 20 mg (low-dose), or 40 mg (high-dose) replenished after 6 weeks. After 3 months of treatment, surviving rats and a group of young controls (6 months) were tested in a behavioral battery involving exploratory activity in an open field and in a runwheel cage as well as motor abilities required for remaining on an inclined screen, suspended from a wire, and balanced on a rotorod. Rats were also pretrained for one-way active avoidance in a straight runway before being trained in a 14-unit T maze. During 20 trials rats were required to negotiate each of 5 maze segments within 10 s to avoid foot shock (0.8 mA). Nimodipine treatment produced no significant effects on body weight, food intake, or survival of aged rats. Analysis of behavioral results indicated significant age-related decline in performance of all tasks except in open-field behavior. Nimodipine treatment had no significant effects on behavioral performance of aged rats except in maze learning. Rats on the high-dose regimen performed significantly better than aged controls in the maze. The results indicate that chronic nimodipine treatment of aged rats had no toxic effects and might be beneficial for preventing age-related decline in learning performance.

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.

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.

Release of 3H-noradrenaline by excitatory amino acids from rat mediobasal hypothalamus and the influence of aging

The present study was designed to analyze the effects of glutamate (GLU) and its agonists on the release of noradrenaline (NA) from the mediobasal region of rat hypothalamus (MBH). Slices from hypothalamus were loaded in vitro with 3H-NA and thereafter exposed to GLU and the glutamate agonists N-methyl-D-aspartic acid (NMDA) and kainate (KA), in superfusion chambers. GLU evoked a significant 3H-NA release in a concentration-dependent manner. The EC50 was 35 mM. 6-Cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a non-NMDA selective antagonist, and amino-7-phosphonoheptanoic acid (AP 7), a NMDA selective antagonist, both decreased the GLU-evoked response to about 50% of its value. NMDA, superfused in Mg(2+)-free Krebs-Ringer, exhibited a greater potency than GLU with an EC50 = 124 microM. KA was also able to evoke 3H-NA release, although overall responses to KA were lower than those of NMDA. The maximal response to KA was a 36% increase of release at a concentration of 200 microM. The effect of KA was blunted by CNQX. NMDA-induced 3H-NA release was progressively altered with age. In old rats (16-18 months) and middle-aged rats (10 months), responses to 200 microM NMDA were decreased respect to young (4 months) male rats. These results show that NMDA and KA receptors mediate the excitatory effects of GLU on NA release from nerve terminals in the MBH and suggest that GLU, in association with NA, participates in the complex mechanisms that regulate neuroendocrine functions.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The potent free radical scavenger alpha-lipoic acid improves memory in aged mice: putative relationship to NMDA receptor deficits

alpha-Lipoic acid (alpha-LA) improved longer-term memory of aged female NMRI mice in the habituation in the open field test at a dose of 100 mg/kg body weight for 15 days. In a separate experiment, no such effect could be found for young mice. alpha-LA alleviated age-related NMDA receptor deficits (Bmax) without changing muscarinic, benzodiazepine, and alpha 2-adrenergic receptor deficits in aged mice. The carbachol-stimulated accumulation of inositol monophosphates was not changed by the treatment with alpha-LA. These results give tentative support to the hypothesis that alpha-LA improves memory in aged mice, probably by a partial compensation of NMDA receptor deficits. Possible modes of action of alpha-LA based on its free radical scavenger properties are discussed in relation to the membrane hypothesis of aging.

Strychnine-insensitive glycine binding to cerebral cortical membranes in developing and ageing mice

The strychnine-insensitive binding of [3H]glycine was characterized in purified cerebral cortical membranes from mice aged from 7 days to 22 months. The binding was saturable, exhibiting only one component during the whole life-span studied. The binding constant KD did not change during development and ageing, whereas the maximal binding capacity Bmax, calculated per protein content, increased up to the age of two weeks and then again in ageing animals (18- and 22-month-olds). The binding was similarly inhibited by the antagonists 7-chlorokynurenate, 3-amino-1-hydroxypyrrolidin-2-one (HA-966) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in 7-day-, 3-month- and 12-month-old mice. The inhibition caused by glycine, L-serine and beta-alanine also remained unaltered during the whole life-span. beta-Alanine was a noncompetitive inhibitor. The alterations in the maximal binding capacities during development and ageing could be of importance in the regulation of NMDA receptors, which have been implicated in synaptic potentiation, developmental processes and various pathological conditions.

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.

Phencyclidine-binding sites in mouse cerebral cortex during development and ageing: effects of inhibitory amino acids

The binding of N-[1-(2-thienyl)cyclohexyl]-[3H]piperidine ([3H]TCP) to the phencyclidine-binding sites in the N-methyl-D-aspartate (NMDA) receptor complex-associated ion channel was characterized in cerebral cortical membranes from 3-day-old to 24-month-old mice. The binding was saturable, exhibiting only one binding component during the whole life-span studied. The maximal binding capacity Bmax, calculated per protein content, decreased during postnatal development until 3 months of age, remaining thereafter constant in ageing mice, thus indicating the greatest availability of phencyclidine-binding sites in the immature cerebral cortex. The binding constant KD increased during the first postnatal week, remained thereafter unchanged and increased again during the second year of life, indicating a decreased affinity of the receptor sites for the ligand. The general properties of the binding; potentiation by glutamate and NMDA, as well as by glycine in a strychnine-insensitive manner, prevailed during development and ageing, certain of these effects being however less pronounced in the immature brain. Taurine and beta-alanine stimulated TCP binding, acting probably at the glycine modulatory site. The actions of these inhibitory amino acids were weak and inconsistent when compared to that of glycine. Since NMDA receptors have been suggested to be involved in neuronal plasticity and learning and memory processes, these modifications in the properties of cortical phencyclidine-binding sites might be of importance in the regulation of excitatory amino acid functions during development and ageing.

Age-related changes in excitatory amino acid receptors in two mouse strains

The present study examined the binding of [3H]-L-glutamate to NMDA receptors, [3H]-kainate to kainate receptors, and [3H]-AMPA to AMPA/quisqualate receptors in the brains of C57Bl and BALB/c mice as a function of increasing age. Significant decreases in binding to NMDA receptors occurred with increasing age (3 to 30 months) in a majority of cortical and hippocampal brain regions from the C57Bl and BALB/c strains of mice. Significant decreases in binding to kainate and AMPA receptors were found in the inner frontal and parietal cortices and stratum lacunosum/moleculare of CA1 in both strains. These regions also exhibited the greatest percent decline in NMDA binding sites with aging. The loss of NMDA receptors in the stratum lacunosum/moleculare of CA1 was greater in the BALB/c mice than the C57Bl strain. These results demonstrate that a few brain regions have age-associated reductions in all three ionotropic EAA receptors; however, the NMDA receptor appears to be selectively vulnerable to the aging process throughout much of the cerebral cortex and hippocampus.

Effects of aging on NMDA and MK801 binding sites in mice

[3H]MK801 binding was significantly reduced in three cortical and two subcortical regions in 30-month-old C57Bl mice, as compared to 3-month-olds. NMDA binding sites showed significant reductions with aging in sixteen of nineteen brain regions. These results suggest that, in a majority of cortical and hippocampal regions, decreases in binding to NMDA sites with aging may be due to factors other than cell loss.

Age-related decrease of the NMDA receptor-mediated noradrenaline release in rat hippocampus and partial restoration by D-cycloserine

The release of [3H]noradrenaline ([3H]NA) evoked by N-methyl-D-aspartate (NMDA) from superfused rat hippocampus synaptosomes was monitored during aging. The maximal effects of NMDA decreased with age from 50% (1.5 months) to 10% enhancement (24 months). Quisqualic acid (100 microM) also enhanced [3H]NA release. Its effect decreased with age with a pattern partly different from that of NMDA. Glycine (1 microM) potentiated the [3H]NA releasing effect of 100 microM NMDA. Unexpectedly, the potentiation which amounted to 50% at 1.5 months, reached almost 200% and 300% in the 18- and 24-month-old rats, respectively, thus compensating in part for the age-related loss of the NMDA-induced effect. Concentration-response relationships for glycine at 3 vs. 24 months suggest that the glycine receptor is superresponsive in the aged brain. This may be due to more efficient glycine removal or/and to impaired release since uptake of the amino acid was increased by 350% in 24- vs. 3-month-old rats, while the K(+)-evoked tritium release from synaptosomes prelabeled with [3H]glycine was decreased. D-Cycloserine, although about 10 times less potent than glycine, strongly enhanced the NMDA-evoked [3H]NA release and may prove useful in cognitive deficits associated with aging and dementia.

Glutamate: its role in learning, memory, and the aging brain

L-Glutamate is the most abundant of a group of endogenous amino acids in the mammalian central nervous system which presumably function as excitatory neurotransmitters and under abnormal conditions may behave as neurotoxins. As neurotransmitters, these compounds are thought to play an important role in functions of learning and memory. As neurotoxins, they are believed to be involved in the pathogenesis of a variety of neurodegenerative disorders in which cognition is impaired. Moreover, brain structures which are considered anatomical substrata for learning and memory may be particularly vulnerable to the neurotoxic actions of these excitatory amino acids, especially in the elderly who are also the segment of the population most susceptible to impairments of mnemonic function. This paper is a review of data concerning the role of excitatory amino acids in the processes of learning and memory and in the pathogenesis and treatment of disorders thereof.

Acetyl-L-carnitine: behavioral, electrophysiological, and neurochemical effects

Aged rats were chronically administered acetyl-L-carnitine (AC) for 10 months. During this period they were tested on learning and sensorimotor tasks and were then subsequently tested electrophysiologically to assess induction and decay rates of long-term synaptic enhancement (LTE) in the hippocampus. Four groups were tested: young controls (4 mo-con), middle-aged controls (16 mo-con), old controls (24 mo-con), and old AC-treated rats (24 mo-AC). After completion of electrophysiological testing, each rat was sacrificed and investigated for age- or drug-related changes in three neurotransmitter markers; including, NMDA-sensitive glutamate receptors, high affinity choline uptake, and adenosine receptor number in the neocortex, hippocampus or caudate nucleus. Aging impaired spatial learning and there was a robust positive correlation between NMDA receptors in the hippocampus and acquisition of the spatial learning task. Induction of hippocampal LTE was reduced in 24 mo-AC rats and NMDA receptor number and high-affinity choline uptake in the frontal cortex was increased. Several suggestions are offered to explain the action of AC on these neurobiological parameters in old rats.

Effects of age on L-glutamate-induced depolarization in three hippocampal subfields

The effects of aging on the translation of L-glutamate-induced depolarization into hippocampal neuronal firing frequency were studied in vitro. L-glutamate was iontophoretically-applied to the somatic region of extracellularly recorded single units. In none of the three principal hippocampal subfields (fascia dentata, CA3, and CA1) were there any effects of age on neuronal sensitivity to L-glutamate. Because there are pronounced, region-specific age effects on AMPA sensitivity (3), these results are in agreement with the conclusions of other investigators that the depolarization caused by exogenously applied L-glutamate probably exerts its effects through nonsynaptic mechanisms. These mechanisms, however, which lead to powerful depolarization and action potentials in hippocampal cells, are unaffected by age.

Hippocampal plasticity induced by primed burst, but not long-term potentiation, stimulation is impaired in area CA1 of aged Fischer 344 rats

The effect of two types of electrical stimulation designed to induce long-lasting plasticity of the Schaffer/commissural inputs to CA1 pyramidal neurons was investigated using in vitro hippocampal slices made from young (3-6 month) and old (24-27 month) Fischer 344 rats. The first stimulation paradigm, primed burst (PB) stimulation, consisted of a total of five physiologically patterned stimuli: a single priming pulse followed 170 ms later by a burst of four pulses at 200 Hz. The second stimulation paradigm, long-term potentiation (LTP) stimulation, consisted of a 200 Hz/1 second train (a total of 200 stimuli). Primed burst and LTP stimulation were equally effective at inducing a lasting increase in the population spike recorded from slices made from young rats. However, the enhancement of population spike amplitude produced by PB, but not LTP, stimulation was significantly less in slices made from old rats. These results suggest that the capacity of the hippocampus to demonstrate long-lasting synaptic plasticity is not altered with age, but that engaging plasticity-inducing mechanisms becomes more difficult. Furthermore, these data suggest that physiologically patterned paradigms for inducing long-lasting synaptic plasticity may more accurately assess the functional status of hippocampal memory encoding mechanisms than does conventional LTP stimulation.

Changes in calcium's role as a messenger during aging in neuronal and nonneuronal cells

Alterations in calcium transport appear to be functionally significant. Treatment with drugs that promote calcium uptake partially reverse some of the age-related deficits in calcium-dependent processes. Thus, the relevance of decreased calcium coupled receptor binding is supported by the ability of 3,4-diaminopyridine to promote acetylcholine release by forebrain slices from aged mice. This drug also reduces the age-related depression in synaptosomal calcium uptake in aged rats and mice. 3,4-Diaminopyridine also reverses the age-related deficit in calcium transport, the age-related deficits in the tight rope test, and 8 arm maze performance. 3,4-Diaminopyridine is also effective in nonexcitable tissues, such as cultured skin fibroblasts; it increases the decreased cytosolic-free calcium. Depressed cell spreading of fibroblasts can be reversed by treatment of cells with the calcium ionophore A23187 which promotes calcium influx. 4-Aminopyridine, a similarly related compound, partially reverses short-term memory deficits in patients with Alzheimer's disease. Tetrahydroaminoacridine, an aminopyridine analog with anticholinesterase properties, produces clinical improvement in behavioral deficits due to Alzheimer's disease. Only recently has the aging brain become a subject of intense study. Evidently, the neurobiology of aging needs to develop its own theories to account for the unique aspects of brain aging as well as integrate them with the peripheral changes. An exciting but unexplored area of research in the aging brain concerns the coupling between calcium and the final end product, the induction of genes. Still unknown are the molecular events that set these processes in motion. In addition, whether conditions such as dietary restriction that increase longevity in certain rodents also retard age-related changes in calcium remains to be determined.

Region-specific age effects on AMPA sensitivity: electrophysiological evidence for loss of synaptic contacts in hippocampal field CA1

The effects of aging on the responsiveness of hippocampal neurons to iontophoretic application of L-glutamate and AMPA were studied in vitro. There were no effects of age on neuronal responses to L-glutamate; however, CA1 pyramidal cells of old rats, but not granule cells in the fascia dentata, showed both a smaller reduction in extracellularly-recorded synaptic responses following application of AMPA (presumably mediated by depolarization), and smaller extracellular "DC" fields (measured by subtracting the DC potentials at the dendrite and soma following AMPA application in the dendrites). To examine the cellular bases of this age-related alteration in AMPA sensitivity, two additional electrophysiological approaches were used: (1) measurement of the amplitude ratios of extracellular EPSP and fiber potential components of the Schaffer collateral-CA1 response; (2) measurement of intracellularly recorded unitary EPSPs and quantal analysis of their fluctuations. The interpretations that would be placed on four hypothetical possible outcomes of such experiments are outlined and assessed in relation to the experimental data. The pattern of results obtained in the present experiments supports the following conclusions: In old rats, individual Schaffer collateral synapses do not appear to have altered AMPA receptor properties, as neither the mean size of the unitary synaptic response nor the apparent quantal size differs between age groups; however, the data do support the conclusion that there are fewer synapses per Schaffer collateral branch in old versus young CA1 pyramidal cells.

Mouse telencephalon exhibits an age-related decrease in glutamate (AMPA) receptors but no change in nerve terminal markers

The central excitatory amino acid receptor selective for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) was examined in brain tissue from mice at 3 and 25 months after birth. Antibodies against the rat GluR-A glutamate receptor subunit (selective for kainate and AMPA) labeled a mouse brain component of about M(r) 100,000. Telencephalic tissue from the older group of mice exhibited 31% less immunoreactivity towards this component as compared with that from the young group. Binding of [3H]AMPA also decreased with age in the telencephalon to an extent which was similar to the loss of receptor immunoreactivity. Scatchard analysis revealed that this reduction is due to a decrease in receptor density and not to a change in binding affinity. In contrast, there were only small age-related changes in AMPA receptor immunoreactivity and binding levels in the brain stem and cerebellum. Binding to dopamine, serotonin, or GABA receptors was not significantly reduced in the older mice. Since the nerve terminal markers synaptophysin and the SV2 glycoprotein were not detectably different in the two groups of mice, the age-related reduction in AMPA receptors is not likely to be due to a general decrease in synaptic density. These data suggest that glutamatergic neurotransmission mediated by AMPA-type receptors is selectively impaired with aging in the telencephalon.

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.

[3H]MK-801 binding to the NMDA receptor complex, and its modulation in human frontal cortex during development and aging

[3H]MK-801 binding was found to decline with age in well washed membranes from human frontal cortex taken from an age series from 24 weeks gestation to 100 years old. The decline was significant under basal conditions (no added modulators) (P less than 0.01), and highly significant under stimulation with glutamate, glycine and spermidine alone and in combination (P less than 0.001). Scatchard analysis in the presence of glutamate and glycine showed this decline was due to a loss in the number of [3H]MK-801 binding sites rather than a change in the affinity of the binding site. There was a highly significant age related reduction in the attenuation of [3H]MK-801 binding by zinc (P less than 0.001). In foetal and neonatal cases up to 7 weeks of age spermidine behaved in an antagonistic manner, inhibiting rather than stimulating [3H]MK-801 binding, when alone or in the presence of glutamate and glycine. The changes in influence of glutamate, glycine, spermidine and zinc on [3H]MK-801 binding during development and aging were not due to other pre- or postmortem factors. The reverse effect of spermidine in the foetal and neonatal cases has therapeutic implications in the treatment of neonates with antiischaemic agents whose action involves the polyamine site.

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.

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.

Excitatory amino acid receptors in brains of rats with methylazoxymethanol-induced microencephaly

We used methylazoxymethanol-acetate (MAM), a potent alkylating agent, to produce microencephaly in offspring by injecting it into pregnant rats on day 15 of gestation. Binding activities of central excitatory amino acid receptors were examined in Triton-treated membranes prepared from brains of adult offspring with MAM-induced microencephaly (MAM rats). MAM rats exhibited approximately 40-50% reductions of the wet weights of the cerebral cortex, hippocampus and striatum compared to those in controls. In the cortex and hippocampus of MAM-rats, total bindings of [3H]glutamate (Glu) (which is sensitive to N-methyl-D-aspartate (NMDA) receptor), and strychnine-insensitive [3H]glycine (Gly) and (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801; a noncompetitive antagonist of NMDA receptor), were reduced to approximately 40% of those in controls. Similarly, in both regions of MAM rats, total bindings of [3H]kainate and DL-alpha-amino-3-[3H]hydroxy-5-methylisoxazole-4-propionic acid (an agonist of quisqualate receptors), were reduced to approximately 35-50% of those in controls. However, total bindings of these radioligands in the striatum of MAM rats were more than 65% of those in controls, despite the significant loss of striatum mass. However, specific bindings of radioligands in the striatum of MAM rats were elevated by more than 60% of those in controls, and Scatchard analysis revealed that elevations of [3H]Glu, [3H]Gly and [3H]MK-801 bindings were due to a significant increase in the densities of binding sites, with their affinities remaining unaltered. Spatial recognition ability examined by an 8-armed radial maze task was markedly impaired compared to those in controls. These results suggest that the proliferation of neurons bearing excitatory amino acid receptors (EAA) in the striatum is less affected by MAM treatment on day 15 of gestation than that in the cortex and hippocampus in spite of drastic weight loss in these brain regions. The significant reduction of EAA receptors in the cortex and hippocampus may be involved in the impairment of spatial memory observed in MAM-treated rats.

Reduced density of NMDA receptors and increased sensitivity to dizocilpine-induced learning impairment in aged rats

About 20 min prior to training in a shock-motivated 14-unit T-maze, young (3-4 months) and aged (24-25 months) male Fischer-344 rats were given s.c. injections of either saline or dizocilpine (MK-801, 0.02 or 0.04 mg/kg), a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. The aged rats showed a dose-dependent impairment in maze performance. Deficiencies were manifested as increases in errors, in runtime from start to goal, and in the number and duration of shocks received. In contrast, young rats exhibited no detrimental effects of dizocilpine on maze performance. Analysis of [3H]glutamate binding in these rats revealed a marked age-related decline in NMDA receptor binding in hippocampus. A significant correlation was observed between errors in the maze and hippocampal [3H]-glutamate binding, but the correlation was positive, i.e., rats that made the most errors had the highest level of NMDA receptor binding. Thus, compared to young rats, aged rats were more sensitive to the behavioral effects of NMDA receptor antagonism and they showed a hippocampal loss of [3H]glutamate in binding, which may be related to the increased sensitivity to dizocilpine. The positive correlation between poor maze performance and NMDA receptor binding suggests that the behaviors assessed involve complex interactions between NMDA receptors and other neuronal systems in the hippocampus.

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.