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

Influence of diet restriction on NMDA receptor subunits and learning during aging

This study was designed to determine if changes related to aging and diet in the mRNA expression of subunits of the NMDA receptor were associated with changes in binding to NMDA receptors and learning ability in C57Bl/6 mice. Three age groups (3, 15, and 26-27 months old) and 2 diet groups (ad libitum-fed and diet restricted) were used. The old ad libitum-fed mice had significantly poorer performance in a spatial reference memory task than all other groups. Diet restriction slightly spared glutamate binding to NMDA sites and improved zeta1, but not epsilon2, mRNA expression. Significant correlations were found between NMDA-displaceable [(3)H]glutamate binding and both learning ability and epsilon2 and epsilon1 mRNA density in several brain regions. Learning ability in the old mice also correlated with the ratios of mRNA expression for epsilon1 and epsilon2 and/or zeta1 subunits in the parietal cortex and CA1 region of the hippocampus. This suggests that it is the relationship between subunit expression levels that is important for maintaining memory functions in older animals.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains

Correlations between the densities of ionotropic glutamate, GABA(A), and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain-specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight-arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammon's horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor-behavioral correlations between the densities of the GABA(A) receptors and the activity-dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight-arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABA(A) receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks.

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

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

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.

Regional changes in the hippocampal density of AMPA and NMDA receptors across the lifespan of the rat

The current study dissected the fascia dentata (FD) and hilar region from the CA and subicular cell fields of the rat and conducted in vitro determinations of the number of binding sites for N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) glutamate receptors across the lifespan. We determined the density of binding of [3H]-glutamate or [3H]-AMPA to NMDA or AMPA receptor sites, respectively. The changes reported might be due to either a change in receptor number or an alteration in the binding characteristics of the receptor site with aging. We found an age-related decline in the number of NMDA receptors in the CA1, CA3 and subicular cell regions of the hippocampus, but not in the FD/hilar region, and an age-related decline in the number of AMPA receptors in the FD/hilar region, but not in the CA fields. The decline in the number of NMDA or AMPA receptors that occurs with aging was not a continuous or homogeneous process. These changes in receptor number might underlie selected age-associated changes in sensitivity to drugs that influence hippocampal function as well as to changes in NMDA-dependent long-term potentiation. A thorough understanding of the mechanisms underlying changes in glutamate receptor function in discrete brain regions, using combined neurochemical and electrophysiological methods, may ultimately provide insight into the fundamental substrates of age-associated memory disorders related to hippocampal dysfunction.

Preservation of N-methyl-D-aspartate receptor binding sites with age in rat neocortex

This study used [3H]dizocilpine ([3H]MK-801) binding to examine glycine, polyamine, and zinc subsites of the N-methyl-D-aspartate (NMDA) receptor in well-washed membranes derived from the neocortex of Fischer 344/Norwegian brown rats aged 3, 12, 24 and 37 months. [3H]dizocilpine binding in the presence of 100 microM glutamate was enhanced by the addition of 30 microM glycine. Binding in the presence of both glutamate and glutamate plus glycine were unaffected by age. The competitive polyamine site antagonist arcaine inhibited [3H]dizocilpine binding in a dose-dependent fashion and 50 microM spermidine caused a rightward shift in this dose response curve. IC50 values derived from these plots were not significantly affected by age. Similarly, zinc inhibited binding in a dose-dependent fashion and was also unaffected by age. These data indicate that the NMDA receptor is spared in aging.

Neurotoxicity of NMDA antagonists: a glutamatergic theory of schizophrenia based on selective impairment of local inhibitory feedback circuits

Modulation of recurrent inhibition is critical not only for the normal function of highly excitable regions of the brain, especially the limbic system, but may also be a primary determining factor for the viability of neurons in these regions. Standard extracellular and intracellular recordings from in vitro brain slices of rat hippocampi were employed to show that recurrent inhibition onto CA1 neurons can be modulated by N-methyl-D-aspartate (NMDA) antagonists. Besides reducing the amplitude of inhibitory postsynaptic potentials (IPSPs) at resting membrane potential conditions, different NMDA antagonists, including the endogenous substance N-acetyl-L-aspartyl-L-glutamic acid (NAAG), are able to block long-term potentiation (LIP) of recurrent inhibition completely at concentrations that are not sufficient to block LTP of the excitatory drive onto pyramidal neurons. This LTP of recurrent inhibition may play a significant role in stimulus discrimination and learning, as simulated in a biophysical computer model of a basic neuronal circuit. Both the amplitude of the IPSP and LTP of the recurrent inhibitory circuit also undergo developmental changes showing their highest expression and vulnerability to chronic NMDA antagonist injections in juvenile rats. Finally, blocking NMDA receptor-dependent transmission in the recurrent inhibition loop may lead to an overall increased excitability of the neuronal network. This may resemble the positive schizophrenic symptoms observed in man, presumably caused by elevated levels of the endogenous NMDA antagonist NAAG.

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.

Excitatory amino acid receptors in the prefrontal cortex of aging mice

The zones of the prefrontal cortex of Balb/c mice were tested for age-related changes of the ionotropic excitatory amino acid receptors density, together with zones of the dorsal cortex. Kainate, N-methyl-D-aspartate, and amino-3-hydroxy-5-methyloxazole-4-propionate sites were measured by slice receptor binding techniques in cortical zones from animals at the age of 6, 12, 18, and 24 months. An increase of the N-methyl-D-aspartate sites was detected in the medial prefrontal zone of mid-aged animals and was followed by a decrease at old age; a decrease of the N-methyl-D-aspartate and kainate sites was found for the medial dorsal (cingulate) cortex at old age. The age-related changes of receptor densities in the different cortical areas seem unrelated in origin. The sites decrease in the cingulate cortex could affect the transfer of the prefrontal cortex activity toward limbic structures.

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.

Deficits in nitric oxide production after tetanic stimulation are related to the reduction of long-term potentiation in Schaffer-CA1 synapses in aged Fischer 344 rats

In the present study, we investigated whether nitric oxide (NO) production after tetanic stimulation is involved in long-term potentiation (LTP) in Schaffer-CA1 synapses in both young adult and aged rats. The changes in both the population spike amplitude and NO metabolites, nitrite (NO2-) and nitrate (NO3-), in the CA1 region were simultaneously determined before and after tetanic stimulation. Increases in NOx (NO2- plus NO3-) levels in the CA1 region were observed after tetanic stimulation in young adult rats as well as increase in the population spike amplitude. In aged rats, LTP was significantly inhibited compared with that in young adult rats. No increase in NOx level after tetanic stimulation was observed in aged rats. These findings directly demonstrated that NO production might be involved in the process of LTP formation in Schaffer-CA1 synapses of the rat hippocampus, and that the deficiency of hippocampal NO production might be responsible for reduction of LTP formation in aged rats.

Age-related impairment of long-term depression in area CA1 and dentate gyrus of rat hippocampus following developmental lead exposure in vitro

Chronic developmental lead exposure is known to be associated with cognitive dysfunction in children. Impairment of the induction of long-term depression (LTD) has been reported in area CA1 and dentate gyrus (DG) of rat hippocampus following chronic lead exposure. The present study was carried out to investigate age-related alterations of LTD in area CA1 and DG of rat hippocampus following developmental lead exposure in vitro. Neonatal Wistar rats were exposed to lead from parturition to weaning via milk of dams drinking 0.2% lead acetate solution. Field excitatory postsynaptic potentials (EPSPs) were recorded in hippocampal slices at various postnatal ages: postnatal day (PND) 17-23, 27-33, and 57-63. Following low-frequency stimulation (LFS, 900 pulses/1 Hz), the average magnitude of LTD is age related. In the controls, LTD magnitude in area CA1 decreased with age, whereas in DG it increased with age. In the lead-exposed groups, the magnitude of LTD declined during development in both area CA1 and DG. The differences of LTD magnitude between the control and lead-exposed rats were 27.26 +/- 9.15% (PND 17-23), 21.59 +/- 12.93% (PND 27-33), and 16.96 +/- 9.33% (PND 57-63) in area CA1, and were 6.95 +/- 9.26%, 17.60 +/- 3.91%, and 33.63 +/- 10.47% in DG, respectively. These results demonstrated that the lead-induced impairment of LTD magnitude was an age-related decline in area CA1 and an age-related increase in area DG of rat hippocampus. Published by Elsevier Science Inc.

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.

Influence of age on nitric oxide modulatory action on Na(+), K(+)-ATPase activity through cyclic GMP pathway in proximal rat trachea

Age-related changes in the modulatory action of nitric oxide (NO) on cyclic GMP levels and Na(+),K(+)-ATPase activity in the proximal rat trachea were investigated using sodium nitroprusside, 8-bromo-cyclic GMP and okadaic acid. At 24 months, both control activities of Na(+), K(+)-ATPase and Mg(2+)-ATPase were decreased when compared to the segments from 4- and 12-month-old animals. However, cyclic GMP levels were similar among the three ages. Sodium nitroprusside (100 microM) induced stimulation of Na(+),K(+)-ATPase activity in segments from both 4- and 12-month-old animals, but not 24-month-old animals. The effect was specific for Na(+),K(+)-ATPase since Mg(2+)-ATPase activity was unaffected. Sodium nitroprusside induced an increase in nitrates/nitrites and cyclic GMP levels in proximal segments at 4, 12 and 24 months. The 8-bromo-cyclic GMP (100 microM) induced a similar specific stimulation of Na(+),K(+)-ATPase activity in segments from 4- and 12- but not 24-month-old animals. Okadaic acid (1 microM), a phosphatase-1 inhibitor, increased proximal Na(+), K(+)-ATPase but not Mg(2+)-ATPase activity in tissues from 4-, 12- and 24-month-old animals. Our results suggest that aging affects cyclic GMP pathway in proximal rat trachea by an action at the level of the cyclic GMP-dependent protein kinase.

Age-related modifications of effects of ketamine and propofol on rat hippocampal acetylcholine release studied by in vivo brain microdialysis

BACKGROUND

We sometimes encounter impairment of learning and memory after general anesthesia in elderly patients. The aim of this study was to examine age-related modifications of the effects of ketamine and propofol on rat hippocampal acetylcholine (ACh) release because hippocampal cholinergic neurons are supposed to be involved in learning and memory.

METHODS

The experiments were performed on male Wistar young rats (2 months old) and old rats (18 months old), using in vivo brain microdialysis technique under freely moving condition. After initial sampling of three collections, test drugs were administered. The ACh release was determined by the HPLC-ECD method.

RESULTS

In old rats, the hippocampal basal ACh release was significantly lower than in young rats. Ketamine (25 and 50 mg kg(-1) i.p.) increased and propofol (25 and 50 mg kg(-1) i.p.) decreased the hippocampal ACh release in both young and old rats. Furthermore, ketamine 50 mg kg(-1) i.p. (anesthetic dose) produced facilitatory effects on the hippocampal ACh release in young rats (193% of the basal release), while in old rats the same dose of ketamine i.p. produced more pronounced facilitatory effects on the hippocampal ACh release (317% of the basal release). On the other hand, propofol 50 mg kg(-1) i.p. (anesthetic dose) produced inhibitory effects on the hippocampal ACh release in young rats (56% of control) and in old rats (77% of control). Although the maximal inhibitory peak effects of propofol 50 mg kg(-1) i.p. did not differ significantly between young rats and old rats, decrease of the hippocampal ACh release in old rats persisted longer than in young rats.

CONCLUSION

Ketamine produced more pronounced facilitatory effects on the hippocampal ACh release in old rats, as compared with young rats. On the other hand, propofol has inhibitory effects on the hippocampal ACh release in young and old rats. The aging process may suppress the ability to recover from the inhibitory anesthetic state induced by propofol.

Food toxins, ampa receptors, and motor neuron diseases

Environmental chemicals involved in the etiology of human neurodegenerative disorders are challenging to identify. Described here is research designed to determine the etiology and molecular pathogenesis of nerve cell degeneration in two little known corticomotoneuronal diseases with established environmental triggers. Both conditions are toxic-nutritional disorders dominated by persistent spastic weakness of the legs and degeneration of corresponding corticospinal pathways. Lathyrism, a disease caused by dietary dependence on grass pea (Lathyrus sativus), is mediated by a stereospecific plant amino acid (beta-N-oxalylamino-L-alanine) that serves as a potent agonist at the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) subclass of neuronal glutamate receptors. A neurologically similar disorder, konzo ("tied legs"), is found among protein-poor African communities that rely for food on cyanogen-containing cassava roots. Thiocyanate, the principal metabolite of cyanide, is an attractive etiologic candidate for konzo because it selectively promotes the action of glutamate at AMPA receptors. Studies are urgently needed to assess the health effects of cassava and other cyanogenic plants, components of which are widely used as food.

Ischemic preconditioning in 18- to 20-month-old gerbils: long-term survival with functional outcome measures

BACKGROUND AND PURPOSE

In young animals, ischemic preconditioning protects CA1 hippocampal neurons against global ischemia. However, cerebral ischemia occurs most frequently in individuals aged >/=65 years. This study examined the protection provided by ischemic preconditioning in a population of aged (18- to 20-month-old) gerbils.

METHODS

One group of animals was exposed to two 1.5-minute episodes of global ischemia separated by 24 hours and followed 72 hours later by a 5-minute occlusion of both carotid arteries. A second group was given 2 episodes of preconditioning only. Two other groups were exposed to 5 minutes of ischemia or sham surgery. The animals survived 10, 30, or 60 days. Functional and histological assessments were used to determine the extent of protection.

RESULTS

Ten days after ischemia there was >80% protection of CA1 neurons in ischemic preconditioned animals compared with 6% in ischemic gerbils. Nevertheless, these preconditioned animals were impaired in open-field tests of habituation. In addition, CA1 dendritic field potentials were smaller in amplitude compared with those in sham animals. While there was a complete loss of staining for CA1 microtubule-associated protein-2 in ischemic animals, staining in ischemic preconditioned animals was normal. This suggests that dendritic abnormalities per se were not responsible for the observed functional deficits. CA1 cell survival declined to approximately 75% of sham values (P<0.05) at 60 days after ischemia.

CONCLUSIONS

Ischemic preconditioning provided substantial neuroprotection in aged gerbils. Nonetheless, the striking dissociation between histological and functional protection provided by ischemic preconditioning in aged animals emphasizes the need to use functional end points and long-term survival when assessing neuroprotection. Although functional recovery was evident with increasing survival time, CA1 cell death continued, thereby raising the possibility that the level of neuroprotection attained was not permanent.

In vivo studies of the cerebral glutamate receptor/NO/cGMP pathway

Overwhelming evidence indicates that the glutamate/nitric oxide (NO) synthase/soluble guanylyl cyclase system is of primary importance in a variety of physiological and pathological processes of the brain. Most of our knowledge on this neurochemical pathway derives from in vitro and ex vivo studies but the recent improvement of microdialysis techniques combined with extremely sensitive measurements of the amplified end-product cyclic GMP (cGMP) has given new impulses to the investigation of this cascade of events, its modulation by neurotransmitters and its functional relevance, in a living brain. The first reports, appeared in the early 90's, have demonstrated that microdialysis monitoring of cGMP in the extracellular environment of the cerebellum and hippocampus exactly reflects what is expected to occur at the intracellular level; thus, in vivo extracellular cGMP is sensitive to NO-synthase and soluble guanylyl cyclase inhibitors, can be increased by NO-donors or phosphodiesterase blockers and is modulated by glutamate receptor stimulation in a NO-dependent fashion. Since then, other microdialysis studies have been reported showing that the brain NO synthase/guanylyl cyclase pathway is mainly controlled by NMDA, AMPA and metabotropic glutamate receptors but can be also influenced by other transmitters (GABA, acetylcholine, neuropeptides) through polysynaptic circuits interacting with the glutamatergic system. The available data indicate that this technique, applied to freely-moving animals and combined with behavioural tests, could be useful to get a better insight into the functional roles played by NO and cGMP in physiological and pathological situations such as learning, memory formation, epilepsy, cerebral ischemia and neurodegenerative diseases.

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.

Adverse psychological impact, glutamatergic dysfunction, and risk factors for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cell loss and pathological changes in neuronal transmission. In particular, malfunction in glutamatergic activity may be associated with the impairment of memory seen in Alzheimer patients. Both hypoactivation and hyperactivation of glutamatergic systems seem to cause impeded cognitive processing in animals. Rats subjected to rearing in isolation display reduced levels of glutamate in temporal regions accompanied by impaired learning and memory. Similar cognitive deficits are also seen in animals exposed to behavioral stress. Stress appears to have deleterious effects on cognition caused by glutamate neurotoxicity leading to attenuated synaptic activity. It is suggested that stress may represent a potential risk factor for AD. The known risk factors for AD (age, heredity, head trauma, low education, depression) may all be related to glutamatergic dysfunction. Some difficulties with pharmacological approaches based on glutamatergic agonists are discussed. It is suggested that optimal glutamate-mediated neurotransmission throughout life may prevent the occurrence of mental decline associated with AD.

Changes in NMDA receptor/nitric oxide signaling pathway in the brain with aging

The N-methyl-D-aspartate (NMDA) receptor/nitric oxide (NO) signaling pathway plays an important role in neuronal plasticity. Previous studies with in vitro autoradiography showed that the number of NMDA receptor/ion channel complexes in the cerebral cortex and hippocampus is decreased by aging. Confocal laser scanning microscopy reveals circuit-specific alterations of NMDA receptor subunit 1 in the dentate gyrus of aged monkeys. Histochemistry for NADPH diaphorase (NADPH-d), a marker for neurons containing NO synthase (NOS), reveals that the number of NADPH-d-positive neurons in the cerebral cortex and striatum is significantly reduced from that in young rats. In the hippocampus, no age-related changes in NADPH-d staining are reported, while in situ hybridization histochemistry indicates an increase in the level of mRNA for neuronal NOS. NOS activity in the brain also appears to decrease with aging. These results suggest that the function of the NMDA receptor/NO signaling pathway in the brain is impaired by aging, and that dysfunction of this signaling pathway may underlie aging-associated memory impairment in rats.

Aging modulates nitric oxide synthesis and cGMP levels in hippocampus and cerebellum. Effects of amyloid beta peptide

The biological roles of nitric oxide (NO) and cGMP as inter- and intracellular messengers have been intensively investigated during the last decade. NO and cGMP both mediate physiological effects in the cardiovascular, endocrinological, and immunological systems as well as in central nervous system (CNS). In the CNS, activation of the N-methyl-D-aspartic acid (NMDA) type of glutamatergic receptor induces Ca(2+)-dependent NOS and NO release, which then activates soluble guanylate cyclase for the synthesis of cGMP. Both compounds appear to be important mediators in long-term potentiation and long-term depression, and thus may play important roles in the mechanisms of learning and memory. Aging and the accumulation of amyloid beta (A beta) peptides are important risk factors for the impairment of memory and development of dementia. In these studies, the mechanism of basal- and NMDA receptor-mediated cGMP formation in different parts of adult and aged brains was evaluated. The relative activity of the NO cascade was determined by assay of NOS and guanylate cyclase activities. In addition, the effect of the neurotoxic fragment 25-35 of A beta (A beta) peptide on basal and NMDA receptor-mediated NOS activity was investigated. The studies were carried out using slices of hippocampus, brain cortex, and cerebellum from 3- and 28-mo-old rats. Aging coincided with a decrease in the basal level of cGMP as a consequence of a more active degradation of cGMP by a phosphodiesterase in the aged brain as compared to the adult brain. Moreover, a loss of the NMDA receptor-stimulated enhancement of the cGMP level determined in the presence of cGMP-phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) was observed in hippocampus and cerebellum of aged rats. However, this NMDA receptor response was preserved in aged brain cerebral cortex. A significant enhancement of the basal activity of NOS by about 175 and 160% in hippocampus and cerebellum, respectively, of aged brain may be involved in the alteration of the NMDA receptor response. The neurotoxic fragment of A beta, peptide 25-35, decreased significantly the NMDA receptor-mediated calcium, and calmodulim-dependent NO synthesis that may then be responsible for disturbances of the NO and cGMP signaling pathway. We concluded that cGMP-dependent signal transduction in hippocampus and cerebellum may become insufficient in senescent brain and may have functional consequences in disturbances of learning and memory processes. A beta peptide accumulated during brain aging and in Alzheimer disease may be an important factor in decreasing the NO-dependent signal transduction mediated by NMDA receptors.

A comparison of the density of NADPH-diaphorase-reactive neurons in the fascia dentata and Ammon's horn between 6-month and 12-month old dark agouti rats

The present study aimed to assess the developmental progress of the hippocampal nitric oxide (NO) system within adulthood by comparing the density of NO-producing neurons in the fascia dentata and Ammon's horn in two groups of adult male rats using NADPH-diaphorase (NADPH-d) histochemistry. One group comprised 6-month-old rats (early adulthood), and the other 12-month-old rats (middle-adulthood). Areal density (number of neurons per unit area) of NADPH-d positive neurons along the three hippocampal axes (septo-temporal, transverse and radial axes) were subjected to quantitative analyses. There were significant variations in the density of NADPH-d-reactive neurons along the transverse and radial axes of the hippocampus, similar to what have been described previously. Comparison between 6-month and 12-month-old rats indicated a substantial reduction in the density of NADPH-d-reactive neurons in the fascia dentata (69%) and Ammon's horn (54%) of the latter group. This reduction was relatively uniform along the septotemporal and radial axes, but appeared to be more pronounced in the fascia dentata and in the proximal region of Ammon's horn. Our finding showed that the hippocampal NO system can undergo significant changes within adulthood. It further highlighted the possibility that an age-related reduction in the capacity to produce NO may not be directly responsible for the cognitive decline associated with senescence, but rather predisposes neuronal degeneration in later life.

Age-related changes in [3H]MK-801 binding in the Fischer 344 rat brain

In this study we tested the hypothesis that the efficacy of L-glutamate to stimulate [3H]MK-801 binding to the NMDA receptor/channel complex is altered as a function of aging. L-Glutamate, or related excitatory amino acid (EAA), is the endogenous neurotransmitter of the NMDA receptor/channel complex. These studies examined the efficacy and potency with which L-glutamate produces receptor activation, channel opening and subsequent MK-801 binding as a function of increasing age by comparing dose-response curves (EC50 and Emax) from 6-, 12-, and 24-month-old F-344 rats. The number of NMDA receptors, as determined by [3H]MK-801 binding in the presence of a saturating concentration of L-glutamate, was reduced in the inner frontal cortex, entorhinal cortex and the lateral striatum in aged rats when compared with young adults. When a range of L-glutamate concentrations were used, differences in Emax were noted in the same brain regions in addition to several others in aged and middle-aged animals when compared with young-adult animals. No changes in EC50 values were noted in any of the brain regions at either age when compared with young-adults.

Muscarinic agonist-induced burst firing in immature rat olfactory cortex neurons In vitro

Age-related changes in pre-/postsynaptic muscarinic (mAChR) and metabotropic-glutamate (mGluR) responsiveness were studied in slices of olfactory cortex from both immature [postnatal day 16-22 (P16-P22)] and adult (>/=P40) rats, using a conventional intracellular recording technique. In adult neurons, bath application of the mAChR agonist oxotremorine-M (OXO-M; 10 microM), or the selective mGluR agonist 1-aminocyclopentane-1S-3R-dicarboxylic acid (1S,3R-ACPD; 10 microM) evoked sustained membrane depolarizations, increases in input resistance, intense repetitive firing, and the appearance of a slow poststimulus afterdepolarizing potential (sADP). Excitatory postsynaptic potentials (EPSPs) evoked by local electrical stimulation of association fiber terminals were also depressed. In contrast, in neurons from immature slices, the 10 microM OXO-M-induced membrane depolarization was followed by the appearance of spontaneous rhythmic epileptiform activity, which was voltage independent and reversible on drug wash out. Epileptiform bursts were abolished or reduced by coapplication of tetrodotoxin (1 microM), atropine (1 microM), pirenzepine (100-200 nM), the N-methyl-D-aspartate (NMDA) receptor antagonist -amino-5-phosphonovaleric acid (-APV; 100 microM), the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5-20 microM), the anesthetic-sedative barbiturate pentobarbitone (100 microM), or by raising the extracellular Mg2+ concentration, whereas a clear facilitatory effect was exhibited by the selective gamma-aminobutyric acid-A (GABAA) receptor blocker (-)-bicuculline methiodide (10 microM). The epileptogenic effects induced by OXO-M were indistinguishable from those produced by 4-aminopyridine (4-AP; 100-200 microM), although these latter actions were unaffected by atropine. In slices from immature animals, electrical stimulation of layer III association fibers in the presence of 10 microM OXO-M was accompanied by a dramatic prolongation of evoked depolarizing postsynaptic potentials (PSPs), with the appearance of recurrent superimposed spike discharges. This effect was readily reversed on wash out of OXO-M. No comparable age-dependent differences were observed in the nature or time course of 1S,3R-ACPD-evoked pre- (or post)synaptic responses, even in immature cells where muscarinic epileptiform activity had previously been demonstrated. We suggest that the overall susceptibility toward muscarinic-induced epileptiform discharge in immature olfactory cortical neurons may depend on the functional integrity of presynaptic inhibitory mAChRs; additional contributing mechanisms were also considered.

Aging differentially alters forms of long-term potentiation in rat hippocampal area CA1

Long-term potentiation (LTP) of the Schaffer collateral/commissural inputs to CA1 in the hippocampus was shown to consist of N-methyl-D-aspartate receptor (NMDAR) and voltage-dependent calcium channel (VDCC) dependent forms. In this study, the relative contributions of these two forms of LTP in in vitro hippocampal slices from young (2 mo) and old (24 mo) Fischer 344 rats were examined. Excitatory postsynaptic potentials (EPSP) were recorded extracellularly from stratum radiatum before and after a tetanic stimulus consisting of four 200-Hz, 0.5-s trains given 5 s apart. Under control conditions, a compound LTP consisting of both forms was induced and was similar, in both time course and magnitude, in young and old animals. NMDAR-dependent LTP (nmdaLTP), isolated by the application of 10 microM nifedipine (a voltage-dependent calcium channel blocker), was significantly reduced in magnitude in aged animals. The VDCC dependent form (vdccLTP), isolated by the application of 50 microM D,L-2-amino-5-phosphonvalerate (APV), was significantly larger in aged animals. Although both LTP forms reached stable values 40-60 min posttetanus in young animals, in aged animals vdccLTP increased and nmdaLTP decreased during this time. In both young and old animals, the sum of the two isolated LTP forms approximated the magnitude of the compound LTP, and application of APV and nifedipine or genestein (a tyrosine kinase inhibitor) together blocked potentiation. These results suggest that aging causes a shift in synaptic plasticity from NMDAR-dependent mechanisms to VDCC-dependent mechanisms. The data are consistent with previous findings of increased L-type calcium current and decreased NMDAR number in aged CA1 cells and may help explain age-related deficits in learning and memory.

Hypothermic neuroprotection. A global ischemia study using 18- to 20-month-old gerbils

BACKGROUND AND PURPOSE

Previous studies from this laboratory have shown that mild intraischemic or prolonged (i.e., 12 to 24 hours) postischemic hypothermia conveys long-lasting (1 to 6 months) protection against CA1 injury. However, these studies have used young animals (aged approximately 3 to 5 months). Stroke incidence rises sharply in late middle age at a time when changes in brain chemistry could alter the response to neuroprotective treatments. Therefore, we evaluated the efficacy of hypothermia in an older population (aged 18 to 20 months) of gerbils.

METHODS

Three groups of gerbils were exposed to a 5-minute episode of global ischemia or sham occlusion. One group was cooled during ischemia (mean brain temperature of 32 degrees C). A second group was maintained at normothermia (36.4 degrees C) during occlusion and the first hour of reperfusion. Beginning 1.0 hour after occlusion, these gerbils were gradually cooled to 32 degrees C and maintained at this level before gradual rewarming to 37 degrees C at 25 hours after ischemia. The third ischemic group was kept at normothermia during surgery and the first hour of reperfusion. After surgery, all animals were tested for acute (i.e., within 30 hours of ischemia) changes in locomotor activity as well as for chronic (i.e., 5, 10, and 30 days after ischemia) habituation deficits in an open field test.

RESULTS

Both intraischemic and postischemic hypothermia provided robust protection (P < .0001) of hippocampal CA1 neurons when assessed 30 days after ischemia. However, intraischemic hypothermia was more effective than postischemic hypothermia in providing behavioral protection.

CONCLUSIONS

This study demonstrates that both intraischemic and prolonged postischemic hypothermia provide robust and lasting (30-day survival) histological protection against a severe ischemic insult. The extent of behavioral protection with postischemic hypothermia was less than that previously observed in younger animals. This suggests that neuroprotective treatments in young animals may lose efficacy as a result of aging.

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.

NMDA receptor activation in the aged rat: electrophysiological investigations in the CA1 area of the hippocampal slice ex vivo

The effects of aging on activation of N-methyl-D-aspartate (NMDA) receptors were studied in the CA1 field of hippocampal slices from young (2-4 months old) and aged (25-32 months old) Sprague-Dawley rats with the use of ex vivo extra- and intracellular electrophysiological recording techniques. No significant age-related changes of the unitary NMDA-receptor mediated excitatory postsynaptic potentials (EPSPs), recorded from the pyramidal cells after stimulation of the stratum radiatum in a magnesium-free medium and isolated in the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, were found. Simultaneously, the magnitude of synaptic plasticity which involved NMDA receptor activation was not altered. No significant age-related modifications in the mechanisms controlling glutamate release and of postsynaptic NMDA receptor responsiveness were revealed. Considering the 30-40% decrease in NMDA binding sites in the aged hippocampus, our results suggest the occurrence of compensatory mechanisms which are discussed.

Age-dependent differences in survival of striatal somatostatin-NPY-NADPH-diaphorase-containing interneurons versus striatal projection neurons after intrastriatal injection of quinolinic acid in rats

Some authors have reported greater sparing of neurons containing somatostatin (SS)-neuropeptide Y (NPY)-NADPH-diaphorase (NADPHd) than projection neurons after intrastriatal injection of quinolinic acid (QA), an excitotoxin acting at NMDA receptors. Such findings have been used to support the NMDA receptor excitotoxin hypothesis of Huntington's disease (HD) and to claim that intrastriatal QA produces an animal model of HD. Other studies have, however, reported that SS/NPY/NADPHd interneurons are highly vulnerable to QA. We examined the influence of animal age (young versus mature), QA concentration (225 mM versus 50 mM), and injection speed (3 min versus 15 min) on the relative SS/NPY/NADPHd neuron survival in eight groups of rats that varied along these parameters to determine the basis of such prior discrepancies. Two weeks after QA injection, we analyzed the relative survival of neurons labeled by NADPHd histochemistry, SS/NPY immunohistochemistry, or cresyl violet staining (which stains all striatal neurons, the majority of which are projection neurons) in the so-called lesion transition zone (i.e., the zone of 40-60% neuronal survival). We found that age, and to a lesser extent injection speed, had a significant effect on relative SS/NPY/NADPHd interneuron survival. The NADPHd- and SS/NPY-labeled neurons typically survived better than projection neurons in young rats and more poorly in mature rats. This trend was greatly accentuated with fast QA injection. Age-related differences may be attributable to declines in projection neuron sensitivity to QA with age. Since rapid QA injections result in excitotoxin efflux, we interpret the effect of injection speed to suggest that brief exposure to a large dose of QA (with fast injection) may better accentuate the differential vulnerabilities of NADPHd/SS/NPY interneurons and projection neurons than does exposure to the same total amount of QA delivered more gradually (slow injection). These findings reconcile the discordant results found by previous authors and suggest that QA injected into rat striatum does reproduce the neurochemical traits of HD under some circumstances. These findings are consistent with a role of excitotoxicity in HD pathogenesis, and they also have implications for the basis of the more pernicious nature of striatal neuron loss in juvenile onset HD.

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.

Differential subcellular regulation of NMDAR1 protein and mRNA in dendrites of dentate gyrus granule cells after perforant path transection

Unilateral transection of the excitatory perforant path results in the acute deafferentation of a segregated zone on the distal dendrites of hippocampal dentate gyrus granule cells (i.e., outer molecular layer), followed by sprouting, reactive synaptogenesis, and a return of physiological and behavioral function. To investigate cellular mechanisms underlying NMDA receptor plasticity in response to such extensive synaptic reorganization, we quantitatively evaluated changes in intensity levels of NMDAR1 immunofluorescence and NMDAR1 mRNA hybridization within subcellular compartments of dentate gyrus granule cells 2, 5, and 9 d after perforant path lesions. There were no significant changes in either measure at 2 d postlesion. However, at 5 and 9 d postlesion, during the period of axonal sprouting and synaptogenesis, there was an increase in NMDAR1 immunolabeling that was restricted to the dendritic segments of the denervated outer molecular layer and the granule cell somata. In contrast, NMDAR1 mRNA levels at 5 and 9 d postlesion increased throughout the full extent of the molecular layer, including both denervated and nondenervated segments of granule cell dendrites. These findings reveal that NMDAR1 mRNA is one of a limited population of mRNAs that is transported into dendrites and further suggest that in response to terminal proliferation and sprouting, increased mRNA transport occurs throughout the full dendritic extent, whereas increased local protein synthesis is restricted to denervated regions of the dendrites whose afferent activity is perturbed. These results begin to elucidate the dynamic postsynaptic subcellular regulation of receptor subunits associated with synaptic plasticity after denervation.

Effect of age on pentylenetetrazol-kindling and kindling-induced impairments of learning performance

Epileptogenesis during ontogeny may not be linearly related to time. It is known that the behavioral manifestations of seizures are age-dependent, but more research is needed to clarify ontogenetic aspects of epilepsies and related alterations, including cognitive deficits. Kindling is an accepted animal model for the study of the convulsive component of epilepsy and its consequences on behavior. Recently, we demonstrated an impairment in acquisition of a conditioned reaction in young adult kindled rats, using pentylenetetrazol (PTZ) as the kindling stimulus. The present study was undertaken to investigate the dependence on age of alterations in the induction of PTZ kindling in rats. We started the kindling protocol in 4-, 6-, and 8-week- and 6-, 12-, 18-, and 24-month-old rats. The PTZ kindling showed an age-dependent decrease in expression of convulsions. The diminished kindling capacity was already seen in 6-month-old rats. In contrast, kindling-related impairment effects on cognitive functions increased with age. Thus, the correlation between learning impairment and occurrence of tonic-clonic seizures that we had demonstrated in 8-week-old rats was abolished in older rats. On the other hand, when the kindling procedure was started in 6-week-old rats, no impairment was found in fully kindled rats.

Effects of chronic treatment with a cyclic AMP-selective phosphodiesterase inhibitor, rolipram, on excitatory amino acid neurotransmission systems in young and aged rat brains

Rolipram selectively inhibits cyclic AMP-specific phosphodiesterase, and leads to an increase in cyclic AMP levels in the brain. In this study, we investigated the effects of chronic rolipram treatment on excitatory and inhibitory amino acid neurotransmission systems in young and aged Wistar rat brains. We used in vitro autoradiography with [3H]MK-801, [3H]glycine, D[3H]aspartate, and [3H]muscimol to label N-methyl-D-aspartate (NMDA) receptors, glycine modulatory sites, glutamate transport sites, and gamma-aminobutyric acid-A (GABA) receptors, respectively. Rolipram (0.01 or 0.1 mg/kg, per os) or its vehicle (distilled water) was administered once a day for 4 weeks. The highest binding of [3H]MK-801, [3H]glycine, and D-[3H]aspartate was seen in the hippocampus in vehicle-treated rats. No significant differences in these binding activities were seen between young and aged rat brains. [3H]Muscimol binding was the highest in the cerebellum, and decreased in many brain regions in aged rats. The chronic rolipram treatment resulted in (1) an increase in [3H]MK-801 binding in the dentate gyrus in both young and aged rats, (2) remarkable reductions in D-[3H]aspartate binding in many regions of both young and aged rats, and (3) no or minimal changes in [3H]glycine and [3H]muscimol binding. These results suggest that the chronic rolipram treatment modifies the excitatory amino acid neurotransmission system.

In vitro autoradiography of ionotropic glutamate receptors in hippocampus and striatum of aged Long-Evans rats: relationship to spatial learning

Using in vitro autoradiography, we investigated [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, [3H]kainate and [3H]N-methyl-D-aspartate binding in two forebrain regions, the hippocampus and striatum, of young (four months of age) and aged (24-25 months of age) Long-Evans rats that had previously been tested for spatial learning ability in the Morris water maze. Although there was substantial preservation of binding in the aged rats, reductions in binding were present in the aged rats that were specific to ligand and anatomical region. In the hippocampus of aged rats, [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding in CA1 and [3H]kainate binding in CA3 were reduced. In contrast, N-methyl-D-aspartate binding was not significantly different between age groups. There was evidence of sprouting in the dentate gyrus molecular layer of aged rats, indicated by changes in the topography of [3H]kainate binding. Binding density was analysed with respect to patch/matrix compartmentalization in the striatum. The most striking result was a large decrease in N-methyl-D-aspartate binding in aged rats that was not limited to any dorsal/ventral or patch/matrix area of the striatum. Additionally, [3H]kainate binding in striatal matrix was modestly reduced in aged rats. Of these age effects, only N-methyl-D-aspartate binding in the striatum and [3H]kainate binding in the CA3 region of the hippocampus were correlated with spatial learning, with lower binding in the aged rats associated with better spatial learning ability. Age-related alterations in ionotropic glutamate receptors differ with respect to the receptor subtype and anatomical region examined. The age effects were not necessarily indicative of cognitive decline, as only two age-related binding changes were correlated with spatial learning. Interestingly, in these instances, lower binding in the aged rats was associated with preserved spatial learning, suggesting a compensatory reduction in receptor binding in a subpopulation of aged rats.

Ionotropic glutamate receptor subtypes in the aged memory-impaired and unimpaired Long-Evans rat

The comparative quantitative autoradiographic distribution of ionotropic glutamate receptor subtypes were investigated in young adults (six months) and aged (24-25 months) cognitively impaired and unimpaired male Long-Evans rats. Aged rats were behaviorally characterized as either cognitively impaired or unimpaired based upon their performances in the Morris water maze task compared to the young adult controls. The status of the N-methyl-D-aspartate, [125I]dizocilpine maleate, [3H]kainate and amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA, [3H]AMPA) receptor binding sites were then established in these three subgroups of animals as a function of their cognitive performance in the Morris water maze task. The apparent densities of both N-methyl-D-aspartate/[125I]dizocilpine maleate and kainate binding sites were significantly decreased in various regions of the aged rat brain. Marked losses in [125I]dizocilpine maleate binding sites were observed in outer laminae of the frontal, parietal and temporal cortices, and the stratum radiatum of the CA3 subfield of the hippocampus. Interestingly, losses in [125I]dizocilpine maleate binding sites were generally most evident in the cognitively unimpaired aged subgroup, suggesting a possible inverse relationship between losses of this receptor subtype and cognitive performances in the Morris water maze task. The levels of [3H]kainate binding were most significantly diminished in various cortical and hippocampal areas as well as the striatum and septal nuclei of both groups of aged rats. In contrast, the apparent density of [3H]AMPA binding was increased in most hippocampal subfields and the superficial laminae of the occipital cortex of the cognitively impaired vs young adult rats. Changes in [3H]AMPA labeling failed to reach significance in the unimpaired cohort. Taken together, these results show that while losses in [3H]kainate binding were similar in both subgroups of aged rats, differences were seen with respect to cognitive status for both [125I]dizocilpine maleate/N-methyl-D-aspartate and [3H]AMPA binding sites. Decreases in [125I]dizocilpine maleate binding sites were mostly restricted to cortical areas of cognitively unimpaired rats, while increases in the AMPA binding subtype were seen in the memory-impaired subgroup. It would thus appear that changes in N-methyl-D-aspartate and AMPA receptor subtypes may be more critical than alterations in kainate binding sites for the emergence of the functional deficits seen in the aged cognitively impaired rat.

Effect of age in rodent models of focal and forebrain ischemia

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Glutamate receptor binding in the human hippocampus and adjacent cortex during development and aging

Distinct patterns of age-related alterations in NMDA (MK801 binding) and non-NMDA, AMPA (CNQX), and kainate binding have been identified in human hippocampus and parahippocampal gyrus in normal individuals with no evidence of degenerative brain disease ranging in age from 24 gestational weeks to 94 years. Whereas MK801 binding did not alter substantially over this age range, CNQX binding rose from low levels in the fetus to maximum levels between neonate and middle age, and kainate binding declined extensively from the perinatal to adult stage. Following maturity, there were no significant changes in kainate binding, although MK801 binding increased in CA1 and CA3 and CNQX binding declined in several regions, particularly CA2 and subiculum. For each receptor binding the timing of these fluctuations ocurring during development and aging varied within different regions of the dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex examined. The transient peaks of receptor binding are likely to reflect processes of synaptogenesis and pruning and may provide clues regarding the role of the different glutamate receptor subtypes in various pathologies of the hippocampus and adjacent cortex associated with developmental disorders (of genetic origin or due to perinatal trauma or insult). The absence of substantial changes in any subtype examined from middle to old age suggests alterations in transmitter binding to these glutamate receptors are not involved in senescent neurodegeneration.

Effects of aging and chronic nimodipine on hippocampal binding of [3H]CGS 19755

Previous studies have suggested that aging is associated with impaired behavioral performance and with decrements of N-methyl-D-aspartate (NMDA) receptors in the rat hippo-campus. Other studies have indicated that chronic treatment with nimodipine, a Ca2+ channel antagonist, prevents the age-related decline in performance by rats in behavioral tasks. Therefore, we tested whether nimodipine altered binding of [3H]CGS 19755 to hippocampal NMDA receptors in rats whose performance on a 14-unit T maze had been tested previously (14). No significant age difference was observed in [3H]CGS 19755 binding in hippocampi from old Fischer-344 rats (27 months) as compared with mature but not senescent rats (9 months); however, old rats that received chronic treatment with a low dose of nimodipine (20 mg pellets implanted subcutaneously twice during 70 days of treatment) showed higher levels of binding. A high dose of nimodipine (40 mg pellets implanted by the same route and at the same times as the low dose) was without effect on [3H]CGS 19755 binding, although aged rats given this treatment performed better in the maze than rats that received no nimodipine or the low dose. In a second experiment comparing hippocampi of young (4 months) and old (24 months) rats, saturation studies confirmed the lack of an age difference in [3H]CGS 19755 binding. The findings suggest that neither the age-related decline in maze performance nor the enhancement of behavior by nimodipine depend upon changes in hippocampal NMDA receptors.

Age-related changes in excitability and recurrent inhibition in the rat CA1 hippocampal region

In hippocampal slices from male Wistar rats aged 1-34 months, we recorded the synaptic field potential responses of the CA1 neurons to stimulation of Schaffer collaterals. Eight electrophysiological indexes were extracted from input/output curves and compared in 11 age groups from 1 to 30 months. Neuronal excitability presented a U-shaped curve of development with a minimum at approximately 7-8 months of age. There was a significant continuous increase in neuronal excitability, i.e. a decrease in excitatory postsynaptic potential (EPSP) producing both the threshold and half-maximal population spike from middle age (8-10 months) to senescence (30 months). Synaptic efficiency also increased in old rats to reach a maximum during senescence, i.e. both the current for threshold EPSP and that for half-maximal EPSP reached a minimum in senescence, although the earlier developmental patterns of these two indexes were non-linear. The duration of the field EPSP elicited with maximal stimulation presented an abrupt decay after the first month. Aged animals presented a relatively small maximal population spike. Recurrent inhibition was most prominent on neuronal excitability rather than synaptic strength. Measured as the percentage change in the half-maximal EPSP and half-maximal population spike, recurrent inhibition was found to decrease during the first 7-10 months of life and remained small in later development.

Effects of hypothermia on traumatic brain injury in immature rats

Hypothermia is beneficial in adult models of traumatic brain injury (TBI), but it has not been evaluated in an immature animal model. We hypothesized that brief hypothermia applied after TBI would reduce cerebral edema and lesion volume in immature rats. Male Wistar rats (3-4 weeks of age, 90-140 g) were anesthetized, intubated, mechanically ventilated, and subjected to TBI by weight drop onto the exposed right parietal cortex. Hypothermic rats were then cooled to a brain temperature of 32.0 +/- 0.5 degrees C for 4 h, and control rats were maintained at a brain temperature of 37.0 +/- 0.5 degrees C. Cerebral edema (wet - dry weight method) was assessed at 5 days. At 4 h, a reduction of percent brain water in the traumatized hemisphere was observed in hypothermic versus normothermic rats (81.75 +/- 0.60 vs. 82.53 +/- 0.67%; p<0.05), but by 24 h posttrauma, the groups were similar (p = 0.82). Total lesion volume (47.2 +/- 8.5 vs. 44.4 +/- 10.0 mm3; p = 0.51) and necrotic volume (20.2 +/- 6.3 vs. 20.0 +/- 7.9 mm3; p = 0.95) were similar in the hypothermic and normothermic groups. We conclude that in this model, a transient (4-h) application of moderate (32 degrees C) hypothermia reduces the cerebral edema characteristically seen in immature rats at 4 h, but this reduction is not sustained at 24 h. Attenuating or delaying the development of cerebral edema could have important therapeutic relevance after TBI. Transient hypothermia, however, did not reduce lesion volume at 5 days posttrauma.

Effects of aging on light-induced phase-shifting of circadian behavioral rhythms, fos expression and CREB phosphorylation in the hamster suprachiasmatic nucleus

Aging is associated with a variety of alterations in circadian rhythms, including changes in the response to environmental stimuli. The underlying causes for these age-related changes in the circadian system remain unknown. Recent studies have demonstrated that light induces the expression of Fos and phosphorylation of the cyclic-AMP response element-binding protein in the rodent suprachiasmatic nuclei, the location of a master circadian pacemaker in mammals, suggesting that these transcription factors may mediate the effects of light on the circadian clock. The purpose of this study was to determine the effects of aging upon light-induced phase-shifting of circadian locomotor activity rhythms, Fos protein expression and cyclic-AMP response element-binding protein phosphorylation in the suprachiasmatic nuclei. Young (three to four months) and old (18-22 months) male golden hamsters free-running in constant darkness were exposed to 5-min monochromatic light pulses of different irradiance levels, at circadian time 19, after which either steady-state phase shifts of locomotor activity rhythms were measured, or else immunocytochemistry for Fos or for phospho-cyclic-AMP response element-binding protein was performed. Old hamsters were approximately 20 times less sensitive to the phase-shifting effects of light on the activity rhythm, and the photic irradiance threshold for Fos-like immunoreactivity induction in the suprachiasmatic nuclei was elevated when compared to young animals. Aging was also associated with a deficit in cyclic-AMP response element-binding protein phosphorylation by light. These data indicate that there are dramatic changes in light-activated molecular responses in the suprachiasmatic nuclei of old hamsters, and suggest that these molecular changes may underlie age-related changes in the effects of light on the circadian clock system.

Dizocilpine binding to cerebral cortical membranes from developing and ageing mice

The binding of [3H]dizocilpine (MK-801) to the N-methyl-D-aspartate (NMDA)-gated ion channel was characterized in cerebral cortical membranes during the major portion of the mouse life-span (from 7-day- to 22-month-olds). The binding was saturable, consisting of only one component at all ages studied. The maximal binding capacity Bmax was very substantial in 14-day-old mice when compared to adults (3-month-olds), decreasing thereafter during ageing. The binding constant KD remained unchanged during development and increased only slightly in aged mice. Glutamate and glycine potentiated dizocilpine binding concentration-dependently. Their efficacy varied markedly with age. Both glutamate and glycine had considerably less effect on the immature cerebral cortex and in the oldest group of mice (22-month-old) than in young adults. The marked increase in dizocilpine binding sites at the age of 2 weeks coincides with the previously reported transient increase in NMDA binding sites in the cerebral cortex. The weak potentiation of dizocilpine binding by glutamate and glycine in the immature brain could be a factor which protects neurons during this period from excitotoxicity and increased susceptibility to seizures induced by acidic amino acids. The decrease in the number of dizocilpine binding sites during ageing could result partly from the loss of cortical neurons.

Differential effects of aging on binding sites of the activated NMDA receptor complex in mice

The NMDA receptor site has been shown to be vulnerable to the effects of aging. Decreases in binding to the receptor site of up to 50% have been reported in aged animals. The present study was designed to quantitate and compare the effects of aging on multiple binding sites of the NMDA receptor complex in various brain regions. Autoradiography with [3H]glutamate, [3H]CPP, [3H]glycine, [3H]MK801 and [3H]TCP was performed on brain sections from 3, 10 and 28-30 month old C57B1/6 mice. The percent declines between 3 and 28-30 months of age in [3H]-glutamate (15-35% declines) and [3H]CPP (20-42% declines) binding were similar within most cortical regions and the caudate nucleus but [3H]glutamate binding showed less change (0-11% declines) than [3H]CPP (13-27% declines) in the occipital/temporal cortex and hippocampal regions. [3H]MK801 and [3H]TCP binding, stimulated by 10 microM glutamate, exhibited intermediate aging changes between the glycine and NMDA sites, both in percent decline (3-28% and 0-26%, respectively) and in the number of brain regions involved. [3H]Glycine binding, stimulated by 10 microM glutamate, showed no significant overall effect of age (declines ranged from 0-34%). [3H]CPP binding was significantly more affected than [3H]glycine binding in many regions. These results suggest that aging has heterogeneous effects on different sites on the NMDA receptor complex throughout the brain and on NMDA receptor agonist versus antagonist binding in selected brain regions.

Age-related changes in [3H]baclofen binding in mouse cerebellum

1. Specific [3H]baclofen binding to cerebellar membranes from 1-month-old (young), 8-month-old (older) and 20-month-old (aged) mice was lower than that to membranes from 3-month-old mice (mature adult), whereas in cerebral cortical membranes there were no age-related changes in [3H]baclofen binding among the four age groups. 2. Scatchard analysis revealed that the density of cerebellar GABAB receptors significantly decreased during aging. 3. There were no age-related changes in the inhibitory effect of Gpp(NH)p on [3H]baclofen binding. 4. These results suggest that the characteristics of GABAB receptors in the cerebellum change during aging without any alteration in the coupling of the receptor to guanine nucleotide regulatory proteins.

Neuroprotection against NMDA induced cell death in rat nucleus basalis by Ca2+ antagonist nimodipine, influence of aging and developmental drug treatment

In the current study the neuroprotective effect of the L-type calcium channel antagonist nimodipine in rat brain was investigated in N-methyl-D-aspartate-induced neuronal degeneration in vivo. In the present model NMDA was unilaterally injected in the magnocellular nucleus basalis and the neurotoxic impact assessed by measuring cortical cholinergic fibre loss as a percentage of fibre density of the intact control hemisphere. This procedure proved to be a reproducible model in which the degree of damage was almost linearly proportional to the NMDA dose. Neuroprotection by nimodipine was determined in a number of conditions. First, the effect of nimodipine treatment in adult animals starting two weeks prior to neurotoxic injury was compared with neuroprotection provided by perinatal treatment of the mother animals with the calcium antagonist. Surprisingly, the degree of protection was in both cases similar, yielding almost 30% reduction of fibre loss. The neuroprotective effect in adulthood of perinatal nimodipine treatment may be explained by developmentally enhanced calcium binding proteins or persistent developmental changes in calcium channel characteristics. Protection by nimodipine was also investigated in aged, 26 month old rats. Compared to young adult cases, aged animals proved to be less vulnerable to NMDA exposure, while nimodipine application was more potent, thus yielding a reduction of nearly 50% in nerve fibre damage induced by NMDA infusions. Possible mechanisms of differential calcium influx in the various experimental conditions will be discussed.

Reduced hippocampal CA1 Ca(2+)-induced long-term potentiation is associated with age-dependent impairment of spatial learning

Expression of Ca(2+)-induced CA1 long-term potentiation (LTP) was analysed in hippocampal slices obtained from (1) 3-month-old and (2) 18-20-month-old Sprague-Dawley rats selected for their performances in the Morris water maze task. In all slices, a transient (10 min) increase of extracellular Ca2+ concentration (4 mM) caused a long-lasting enhancement of potentials evoked by electrical stimulation of radiatum fibers. However, a significant difference was found in the degree of potentiation among groups. In particular, increases of the CA1 response amplitudes were significantly lower in old rats impaired in spatial learning than in young at 30 (P < 0.05), 60, 90 and 120 min (P < 0.01) after restoring the normal Ca2+ concentration. On the contrary, no differences were observed between young animals and the old ones with good performances in spatial learning. The data suggest that amplitude of CA1 Ca(2+)-induced LTP in old rats is related to spatial learning abilities.

Loss of cortical serotonin2A signal transduction in senescent rats: reversal following inhibition of protein kinase C

Using grease gap recordings, age-related changes in serotonin2A receptors were assessed in sensorimotor regions of the cortex by examining serotonin-induced facilitation of the N-methyl-D-aspartate depolarization in cortical wedges prepared from young adult (3-6 months) and senescent (22-34 months) Fisher 344 rats. Serotonin (10-100 microM) facilitated the N-methyl-D-aspartate depolarization in wedges from young adult rats in a concentration-dependent manner, whereas no facilitation was observed in wedges from senescent rats. Similar results were obtained when +/- 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, a mixed serotonin2A and serotonin2C receptor agonist, was substituted for serotonin. In contrast, agonists at alpha 1A-adrenoceptors, metabotropic glutamate receptors and muscarinic cholinoceptors facilitated the N-methyl-D-aspartate depolarization in wedges from both young adult and senescent rats. Chelerythrine and staurosporine, inhibitors of protein kinase C, but not concanavalin A, myo-inositol or calmodulin antagonists, restored the serotonin facilitation in wedges from senescent animals. In situ hybridization histochemistry revealed that serotonin2A receptor messenger RNA was present in layers II-VI of the cortex, with the highest density of silver grains located in layers III and V of both young adult and senescent rats. Detailed examination of layer V showed that silver grains were significantly higher than background only over pyramidal cells. We conclude that serotonin2A receptors are expressed by pyramidal cells in both young adult and senescent rats and that serotonin acts directly on these receptors to facilitate the N-methyl-D-aspartate depolarization. Moreover, in senescent rats, signal transduction at cortical serotonin2A receptors involved with facilitation of the N-methyl-D-aspartate response is compromised as a result of protein kinase C activation.