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CAVEAT LECTOR
THE AGING OF THE NMDA RECEPTOR COMPLEX
Department of Anatomy & Neurobiology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523
Received 4/15/98 Accepted 4/20/98
TABLE OF CONTENTS
2.2. Subunits
2.3. Learning and Memory
3.2. Subunit Expression
3.3. Electrophysiology
3.4. Transmitter release
3.5. Learning and Memory
3.6. Aging Interventions
1. ABSTRACT
N-methyl-D-aspartate (NMDA) receptors are present at high density in the cerebral cortex and hippocampus and play an important role in learning and memory. These receptors are negatively affected by the aging process, but this effect does not appear to be uniform throughout the cortex and hippocampus. This review discusses the age-associated changes that occur in the different binding sites of the NMDA receptor complex, in the expression of subunits that comprise the complex, in the electrophysiological properties of the receptor, and in the ability of NMDA to stimulate the release of other transmitters. Spatial memory and some types of passive avoidance memory tasks have been shown to involve NMDA receptors. Aged animals show deficiencies in the performance of these tasks, as compared to young, and some studies have identified an association between lower densities of NMDA receptor binding and poor memory performance. A number of drug and diet interventions have shown potential for reversing or slowing the effects of aging on the NMDA receptor. These studies suggest that the development of treatments that are aimed at preventing or reversing the effects of aging on the NMDA receptor will aid in preventing the memory declines that are associated with aging.
2. INTRODUCTION
Aging causes functional declines in many organs of the body, including the brain. One of the earliest cognitive dysfunctions that humans experience is a decline in learning and memory performance. This deterioration is detectable already in the fifth decade of life (1). These declines in memory can range in severity from "benign senescent forgetfulness" (2), in which individuals have trouble accessing new and old information (3), to the degenerative disorder, Alzheimer's disease, which induces dementia and severe declines in cognitive functions (4). A better understanding of the underlying causes of these memory declines during aging is necessary for the development of appropriate treatments or preventions for memory dysfunction as we grow older. These treatments may also be beneficial in delaying some of the symptoms of Alzheimer’s Disease. One subtype of the glutamate receptors, the N-methyl-D-aspartate (NMDA) receptor, is expressed in high density in cortical and hippocampal regions and is very important in the initiation steps of learning and memory (5). NMDA receptors are involved in the performance of spatial, working, and passive avoidance memory tasks and in long-term potentiation (LTP), a cellular phenomenon that is believed to be involved in at least some types of memory. The NMDA receptors appear to be more vulnerable to the aging process than other glutamate receptors (6,7) and show declines in their binding densities, electrophysiological functions, and influence on other transmitter systems. The evidence suggests that these changes in NMDA receptor function should have an impact on learning and memory abilities and, in fact, several studies that demonstrate aging changes in the NMDA receptor also show a correlation between these changes and memory performance. This review will present the normal features and functions of the NMDA receptor complex, discuss the changes that have been reported in the NMDA receptor and its related functions during aging, and suggest future directions for improving or preventing age-related changes in learning and memory processes by targeting the NMDA receptor complex. Some of this information has been reviewed previously (8). This review represents an expansion of certain topics and an update on the progress achieved since 1994.
2.1. NMDA Receptor Complex
The NMDA receptor complex is a large protein assemblage that has multiple binding sites for different ligands, including an NMDA binding site, a strychnine-insensitive glycine binding site, and a binding site within the channel for certain noncompetitive antagonists; each of which can bind several different compounds (figure 1) (5,9). The NMDA site also binds L-glutamate and L-aspartate as endogenous agonists and D-2-amino-5-phosphonopentanoic acid (AP5), [(±)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP), CGP39653, and CGS19755 as antagonists. The glycine site binds serine and D-cycloserine, which act as agonists, and 7-chlorokynurenic acid (7-Cl-KYNA) is one antagonist for this site (10). Within the channel, non-competitive antagonists, such as (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohept-5,10-imine maleate (MK801), ketamine, phencyclidine (PCP) and 1-(1-thienyl-cyclohexyl)piperidine (TCP); can bind. There are also binding sites on the receptor complex for polyamines, protons, redox reagents, zinc and magnesium that can modulate the activity of the receptor (figure 1) (5,9). These different binding sites and their interactions have been exploited to assess the effects of aging on the NMDA receptor complex.
Figure 1. Binding sites associated with the NMDA receptor complex. Other ligands used in the binding or functional studies referenced in this review are indicated in parentheses. Diagram adapted from Corsi, et al. (9).
The NMDA receptor has an absolute requirement of co-agonism for channel activity; both glutamate and glycine must occupy their binding sites for channel activation (9,11). However, agonist binding alone is insufficient to activate the channel because at hyperpolarized potentials the channel pore is blocked by magnesium (12). Repetitive synaptic activation, leading to neuronal depolarization, will relieve this block, allowing calcium to enter the neuron (12). This influx of calcium plays an important role in the induction of LTP, which may ultimately be expressed as learning (13,14).
2.2. Subunits
The functional subunits of the NMDA receptor complex have been cloned for rats (15-18), mice (19-22) and humans (23-25). There are two families of subunits identified for the NMDA receptor, in rats and humans they are termed the NMDAR1 (NR1; zeta1 for mice) and NMDAR2 (NR2; epsilon for mice) families. There is a 99% amino acid homology between the rat and human NR1 and the mouse zeta1 subunit (21-24,26). The NR1 subunit has the same distribution as NMDA-displaceable [3H]glutamate binding throughout the cortex and hippocampus (15,21,26). This subunit appears to be necessary and sufficient for the formation of functional channels and, in homomeric receptors, can respond to glutamate, glycine, and MK801 (16,17,20,21), suggesting the presence of these binding sites on the NR1 subunit. Mutational analysis also suggests that the glycine site is associated with the NR1 subunit (27).
There are at least four members of the NR2 family of subunits that show high homology between species (21,22,24) and are designated as NR2A-D for the rat (16,17) and human (24) and as epsilon1-4 for the mouse (19-21). The NR2A-D subunits each enhance the activity of the receptor when coupled with the NR1 subunit (16). The subtypes within this family of subunits confer different agonist/antagonist affinities to NR1/NR2 heteromeric receptors (20,22), as well as producing different gating behaviors, responses to Mg++, and I/V curves (16,17). They also differ from each other and the NR1 subunit in distribution and developmental patterns of mRNA expression (16,17,20,21,28,29). The different spatiotemporal expressions of these subunits suggest that multiple NMDA receptor populations exist in the brain and that they differ both within and between brain regions.
2.3. Learning and Memory
The NMDA receptor appears to play an integral role in memory. Much of the evidence has been derived from studies on rodents performing spatial reference memory tasks; functional NMDA receptors have been shown, with antagonists and subunit-specific knockout mice, to be necessary for successful performance in the Morris water maze (30-34). In addition, correlations have been seen between NMDA-displaceable [3H]glutamate binding in prefrontal/frontal and hippocampal regions and reference memory performance in the Morris water maze (35,36). NMDA receptors also appear to be involved in some forms of passive avoidance learning (37) and in working memory functions; NMDA antagonists inhibit performance of spatial working memory tasks when a delay is induced between choices (38) and NMDA application to the prefrontal cortex of macaques increases the short term memory retention time (39). Long term potentiation (LTP) is a sustained increase in the efficiency of synaptic transmission that typically is induced by high-frequency stimulation (14,40). Antagonists of the NMDA receptor and knockouts of the NR1 gene block the initiation of LTP in both the hippocampus (14,33,34,41,42) and neocortex (43) and, in some studies, this has been associated with declines in spatial memory performance (33,34). These studies all demonstrate an important role for NMDA receptors in memory processes and suggest that detrimental changes to the NMDA receptor during the aging process may explain, at least in part, the memory declines that people and animals experience during the aging process.
3. Changes in NMDA Receptors and related functions during Aging
3.1. Receptor Binding
The most consistent finding, with respect to binding studies investigating the effect of aging on the NMDA receptor complex, is that binding to the NMDA binding site by agonist (L-glutamate) and/or antagonists (CPP, CGS19755, and CGP39653) decreases with increasing age in the cerebral cortex and hippocampus, regions important to memory processing (44,45). This age-related decline in binding density to the NMDA binding site has been documented in Fischer 344 (35,46-51; but see (52)), Long-Evans (53), Wistar (54) and Sprague-Dawley (55) rats, in C57Bl/6 and BALB/c mice (56,57), and in rhesus monkeys (58). The changes reported for the older animals in these studies range from a 14% to a 63% decrease in binding to the NMDA site relative to the densities present in young adult animals. The concentration response studies that have been performed on homogenates and membrane preparations demonstrate that the decrease in binding is due to a decrease in the maximum binding (Bmax), indicating that the binding changes during aging are due to a decrease in the total number of binding sites (46,50,53-55). Most of these studies found no age-related changes in the affinity of the agonist or antagonist for the NMDA binding site, but a decrease in the affinity (increased KD) of glutamate for the NMDA binding site with increasing age was seen in our autoradiographic concentration response studies in C57Bl/6 mice (59). A small increase in KD also was reported in Sprague-Dawley rats, but this difference from young did not reach significance (55). It is possible that the ability to analyze individual brain regions with autoradiography allowed us to see affinity changes within populations of neurons that were not detectable in studies with combined brain regions. Although studies differ in the percent decline in binding detected and the issue of changes in total binding sites versus changes in affinity is not fully resolved, we can conclude that the NMDA binding site is negatively affected by the aging process in multiple mammalian species.
The age-associated changes in the NMDA binding site do not, however, appear to be homogeneous across brain regions. The cerebral cortex of aged rats and mice, in most studies, showed greater decreases in binding to the NMDA binding site than the hippocampus (35,46,50,56,57). However, there are two rat studies in which the change in the hippocampus was equal to or slightly larger than the change in the cortex (54,58). There also appears to be a difference between the cerebrum and hippocampus in the effects of aging on agonist versus antagonist binding. Studies in Long-Evans rats in which there are significant declines with increased age in antagonist binding with [3H]CPP in the hippocampus (60), but no significant change in [3H]glutamate binding to NMDA sites in the same region (61), support our findings in C57Bl/6 mice in which antagonist binding is more affected by aging than agonist binding in the hippocampus (57). The percent declines during aging in [3H]glutamate and [3H]CPP binding in cerebral cortical regions of C57Bl/6 mice, however, are more equivalent to each other (57). These results show that the effects of aging on the NMDA binding site are not uniform throughout the brain.
The effects of aging on the glycine binding site appear to be more variable than the changes in the NMDA binding site. We saw no overall age-related changes in [3H]glycine binding in C57Bl/6 mice and there was a significant difference between the percent decline in [3H]glycine binding and antagonist binding to the NMDA binding site in many cortical and hippocampal brain regions (57). Aged NMRI mice manifest a 130% increase in [3H]glycine Bmax between 3 and 22 months of age in the cortex (62). Significant decreases of 27-49% in [3H]glycine binding occur in Fischer 344 rats with increasing age in both cortex and hippocampus (49,50), but Sprague-Dawley rat cortex exhibits no change in Bmax for [3H]glycine binding during aging (63). This variability in age-associated changes in the glycine binding site may be influenced by both strain/species differences and differences in experimental manipulations. The interpretation of these glycine binding studies also may be complicated by the existence of different populations of receptors in cortical neurons with differing glycine affinities (64), which may be differentially affected by aging.
The binding site within the channel of the NMDA receptor complex is often used to reflect the function of the receptor because an open channel is necessary for entrance of the ligands (65,66). Autoradiographic examination of glutamate-stimulated [3H]MK801 and [3H]TCP binding in different ages of mice showed that there is a decrease in binding of both of these ligands with increasing age (57). This decrease is significant in less brain regions than the age-related changes in [3H]glutamate binding to the NMDA site, but there appears to be more change with age in the channel binding site in the cerebral cortex (up to 24% declines in binding) than in the hippocampus (up to 14% declines), similar to the results with other binding sites on the complex (57). A decrease in the Bmax (22-35% decreases from Bmax values in young) for [3H]MK801 and [3H]TCP binding sites has been seen with increased age in the cortex and hippocampus of NMRI mice and in Fischer 344 and Wister rats (50,54,67,68). This decrease in Bmax has also been reported in the senescence-accelerated prone mouse (69). These results from the cortex of rodents fit well with changes in human frontal cortex, in which a 36% drop in Bmax for [3H]MK801 was seen between 10-20 year olds and people in the tenth decade of their life (70). No changes in [3H]MK801 binding were detectable across ages in the hippocampus, entorhinal cortex or cerebellum of humans (71), which appears to fit with the higher susceptibility to aging changes in this receptor in the cortex, as compared to the hippocampus, in C57Bl/6 mice (57). Only Sprague-Dawley rats show a non-significant increase in both Bmax and KD during aging (63). In the majority of mammals studied, therefore, it appears that there is a decrease in the number of channel binding sites, although the percentage change may be less than for the NMDA binding site, suggesting that it may not be a simple case of decreased numbers of receptor complexes.
Age-related changes in the ability of glutamate and glycine binding sites to influence binding within the channel have also been reported. The ability of glutamate and glycine to enhance [3H]MK801 binding in the frontal cortex is reduced from a 44% increase in young adults to a 35% increase in 80-100 year old humans (70). In C57Bl/6 mice, low micromolar concentrations of glycine exacerbate the age difference in [3H]MK801 binding in the cortex and hippocampus, while higher concentrations appear similar to no glycine stimulation, with respect to differences between age groups (72). Glutamate and glycine both show increases in affinity (lower EC50) and stimulate a higher percent increase in [3H]MK801 binding in the forebrain of aged NMRI mice, as compared to young. However, it is interesting that the absolute increase stimulated in young and old is relatively similar to each other and the age-related difference in binding did not change substantially between control and glutamate or glycine stimulation (67). In this respect, these glycine results are similar to ours with a high concentration of glycine (72). Spermine enhancement of [3H]MK801 binding disappears by 80 years of age in humans and zinc inhibition also declines with increased age (70). These changes suggest that, although the direction of change may differ between species and strains, aging appears to alter the affinities of certain binding sites on the receptor complex or changes the ability of the binding sites to influence the channel.
An issue has been raised as to whether the changes in binding, expressed as moles / mg protein, are a true reflection of changes in the number of NMDA receptors or whether there is a change in the total protein during aging. Two reports on Fischer 344 rats indicate that the total protein increases in the hippocampus with increasing age (48,52). Correction for this increase in protein entirely eliminated the significant changes in binding, seen when expressed as pmole/mg protein, to sites on the NMDA receptor in one study (52), but the other still had a significant 51% drop in [3H]glutamate binding to NMDA sites (48). We found no age-related differences in total protein per dry weight of tissue using combined cortex and hippocampus from C57Bl/6 mice (56) and there is no change in protein levels in human frontal cortex between different ages (70). It is possible that this increase in protein content is limited to the hippocampus, but Castorina, et al. (46,73) report no change with age in protein content of the hippocampus of Fischer 344 rats. This issue remains unresolved.
The age-associated declines in binding to one or more sites on the NMDA receptor complex and changes in the interactions between sites are present in species ranging from rodents to primates, including humans. Much of the data points to losses in total binding sites, but it is not clear whether this reflects losses of the entire receptor complex, a selective loss of certain subunits, or both. The studies that show alterations in the affinity of certain binding sites argue for an alteration in subunit composition in the remaining complexes. Whatever the cause, it seems likely that these binding site changes alter the functions associated with the NMDA receptor, both at an individual receptor level and with functions that require multiple NMDA receptors to be active together to produce an outcome. In addition, given the importance of this receptor to learning and memory processes, it also seems probable that the binding site changes are, at least in part, responsible for some of the age-related declines in memory.
3.2. Subunit Expression
Although there are many studies that have documented a decline in binding to NMDA receptors during the aging process, almost no follow-up has been done to determine the cause for the change. The effect of aging on subunit expression is only beginning to be elucidated. A decrease in the expression of the NR1 protein occurs in aged macaque monkeys, as compared to young adults, on the distal dendrites of the dentate granule cells (74). The mRNA expression for NR1 also is decreased in the hypothalamus of middle-aged Sprague-Dawley rats, which may play a role in reproductive senescence (75). One set of splice variants of the NR1 subunit, plus or minus the N terminal 21 amino acid insert, however, show no change in ratio or percent of 6 month old expression in either cortex or hippocampus from Wistar rats by 24 months of age. Aged C57Bl/6 mice showed a decrease in the mRNA expression of the epsilon2 (rat NR2B) subunit within most cortical regions and the dentate granule cells, as compared to young adult mice (76). The zeta1 (rat NR1) subunit mRNA density decreased overall in the cortex and hippocampus with increasing age, but few regions showed significant changes, and the epsilon1 (rat NR2A) subunit showed no significant changes (76). Protein expression of the subunits in this strain of mice also exhibited decreases with increasing age in the zeta1 and epsilon2 subunits, but not the epsilon1 subunit (76). The functional significance of these changes is as yet unclear, but subunit specific declines during aging could alter the composition of the NMDA receptor complex and potentially alter the pharmacology of the binding sites for glycine and ifenprodil, a non-competitive NMDA antagonist, (20,77-79) and/or physiological properties such as desensitization (17,80). Further knowledge about alterations in the subunits of the NMDA receptor during the aging process and the functional consequence may be the key to preventing the changes that aging induces in this receptor.
3.3. Electrophysiology
Changes with aging in the electrophysiological characteristics of the NMDA receptor appear to be highly region-specific. Intracellular responses to applied NMDA in the frontoparietal cortex show a reduced sensitivity in the slices from old Fischer 344 rats, as compared to young, and evidence of long-term potentiation in this same region, using a protocol that produced LTP in the young, is absent in the old rats (81). NMDA responses in both the dentate gyrus and in the CA1 region of the hippocampus, determined by measuring the input / output characteristics following electrical stimulations of axons, show decreased excitatory post-synaptic potentials for a given presynaptic fiber volley in aged Fischer 344 rats (82,83), but NMDA-induced inhibition of the extracellular field potential shows no age-related difference in dose in the CA1 regions of Sprague-Dawley rats (84). Certain aspects of LTP in the hippocampus are altered as rats age; the enhancement reaches maximum slower and the decay rate is faster in older, as compared to younger, rats (85,86). This could be due to NMDA receptor changes or to alterations up- or down-stream from these receptors. NMDA-dependent LTP induction is normal in the CA1 region of old rats when post-synaptic cells are depolarized by intracellular injection of current (87) or when high frequency tetanization is used (84,88-90). Shankar and coworkers (90) suggest that the maintenance of LTP is caused by a decrease in the NMDA-induced LTP and a compensatory increase in calcium channel dependent LTP. Shorter trains of stimulation (88) and primed burst stimulation, a threshold stimulation paradigm (89), for LTP do show a decline in both short-term potentiation (STP) and LTP magnitude in the CA1 region with increasing age. Barnes and colleagues hypothesize that there are fewer synapses per axon in the CA1 region of aged animals and that the deficiencies in NMDA receptor-dependent LTP are only seen with peri-threshold stimulation protocols (82). These results suggest that, although the aged hippocampus is still capable of producing NMDA-dependent LTP under maximal stimulation conditions, it is not as good as the young hippocampus at lower stimulation rates, which are likely to be more physiological. The electrophysiological changes in NMDA receptor function support the binding results showing a greater effect of aging on cortical NMDA receptors than hippocampal (35,46,50,56,57).
3.4. Transmitter release
There is evidence for a decline during aging in NMDA receptor functions that are down-stream from the receptor. NMDA-stimulated release of norepinephrine is diminished in middle-aged and old Fischer 344 and Wistar rats, as compared to young adults (91,92). One group reported a greater percent increase with age in glycine potentiation of NMDA-stimulated release of norepinephrine in middle-aged and old Wistar rats, but again the absolute increase over baseline stimulated by glycine appeared to be the same between the different age groups (92). Dopamine release stimulated by NMDA in the striatum and NMDA-induced inhibition of phosphoinositide hydrolysis in the hippocampus are also reduced with increasing age in Fischer 344 rats. In the senescence-accelerated prone mice (SAM P/8), there is a sharp decline in acetylcholine release stimulated by NMDA in whole brain slices after 2 months of age. In the resistant mice (SAM R/1), the same levels of release are maintained between 2 and 14 months of age. NMDA-stimulated release of leutinizing (LH) and gonadotropin releasing (GnRH) hormones shows decreases by middle age in the hypothalamus of female Sprague-Dawley and Wistar rats (75,93). This decrease in function is associated with a decrease in the mRNA expression of the NR1 subunit, which suggests that the NMDA receptor contributes to the process of reproductive aging (75). These results demonstrate that changes to the NMDA receptor complex during aging probably contribute to age-related dysfunctions that occur in other transmitter systems.
3.5. Learning and Memory
The role of NMDA receptors in learning and memory in the behaving animal has been predominantly studied by Morris and his associates with the use of the Morris water maze in a reference spatial memory task (32,34,94,95). This task (6,96-98) and others, including the Barnes circular platform maze and Olton radial arm maze (99), also have been used extensively in aging research to demonstrate and characterize the age-related declines that occur in spatial memory performance in rodents. Aged humans also show declines in performance (30-80% decreases from young performance) in problems involving spatial memory (100-104), so aging rodents appear to be appropriate models for this age-related human dysfunction. Rodent studies utilizing the water maze have been very effective in relating changes in NMDA receptor binding during aging to memory dysfunctions. Several studies show that lower binding of agonist or antagonist to the NMDA binding site in the prefrontal/frontal cortex and/or hippocampus is associated with poorer performance in spatial memory tasks utilizing the water maze (35,36,53). However, negative correlations, in which lower binding was associated with better performance, have also been reported in the striatum for the water maze task (61) and in the hippocampus for a non-spatial complex maze task (48). The reasons for this "interference" of NMDA receptors in memory processing have not yet been elucidated. Even more curious is the fact that the aged animals in the complex maze were more sensitive to MK801 administration, as manifested by a worsening of performance that was not seen in treated young rats (48). There is also evidence for a role for NMDA receptors in passive avoidance retention memory (37) and there is a significant correlation between [3H]MK801 Bmax and passive avoidance latency during aging; high binding was associated with better retention (68). These results suggest that aging interventions aimed at retaining or improving NMDA receptor function should be beneficial to relieving some age-related memory dysfunctions.
3.6. Aging Interventions
Many different interventions have been used successfully in vivo to attenuate the effect of aging on the NMDA receptor complex. Acetyl-L-carnitine (ALCAR), a compound that demonstrates multiple anti-aging effects in the brain when administered systemically to the animal (see review (105)), improves antagonist binding to the NMDA binding site, slightly in the hippocampus and frontal cortex and significantly in the striatum (46). ALCAR also decreases the age-related difference in NMDA-displaceable [3H]glutamate binding, slightly in the hippocampus (55) and significantly in the anterior cortex (35). In the latter study, there was a slight improvement in spatial memory performance associated with the aged animals that received ALCAR treatment (35). Other cognitive enhancing and free radical scavenging drugs, such as memantine (63), phosphatidylserine (67), piracetam (106), pyrintol (107), and alpha-lipoic acid (108); all produce significant increases in the binding of [3H]MK801 in aged rodents. Mementine, a non-competitive NMDA receptor antagonist, also increases glycine affinity and glycine- and spermine-stimulated [3H]MK801 binding in old rodents, as compared to non-treated old animals, but does not improve spatial memory performance significantly in a radial maze task (63). Bifemalane hydrochloride, a drug used for cerebrovascular diseases, increases CPP binding in most cortical, hippocampal and thalamic regions studied in aged rats (51). D-cycloserine, a partial glycine agonist, improves the NMDA-stimulated norepinephrine release in older treated rats over non-treated, age matched controls (92). Dietary restriction is an aging intervention that has been shown to improve memory performance, particularly in spatial memory tasks (109-111). Diet restriction produces a slight sparing effect on [3H]glutamate binding to the NMDA site in older C57Bl/6 mice (6) and this effect is correlated with spatial memory performance in the water maze (36). These interventions offer some hope for correcting the effects of aging on the NMDA receptor complex. A more in depth examination of how these improvements in the NMDA receptor relate to improvements in receptor function and memory processing is needed before deciding which of these represent the optimal treatment.
4. PERSPECTIVE
There is clear evidence that aging negatively affects some of the binding sites on the NMDA receptor complex. Whether this is a matter of receptor complex loss, overall subunit changes, or alterations within subpopulations of receptors still needs to be determined. More effort is also needed toward determining the underlying cause of the alterations. If different populations of receptors are involved, it behooves us to focus more on subdivisions of the cerebral cortex and hippocampus in order to better understand the regional and cellular changes that are occurring. The declines in electrophysiological output and interactions with other transmitter systems and the correlations with behavioral dysfunctions indicate that the binding changes in the NMDA receptor have a functional consequence to the behaving animal and suggest that further examination of interventions that act on the NMDA receptor will be beneficial in preventing or reversing the memory declines that occur as we grow older.
5. ACKNOWLEDGMENTS
The author would like to thank Dr. Scott Nelson for his helpful comments on the manuscript. The author’s work was supported by NIH FIRST award AG10607 and NIH RCDA AG00659.
6. REFERENCES
1. Albert M. S. & H. H. Funkenstein: The effects of age: Normal variation and its relation to disease. In: Diseases of the Nervous System: Clinical Neurobiology. Eds: Asbury A. K., McKhann G. M., McDonald W. I., Saunders, Philadelphia (1992)
2. Kral V. A.: Senescent forgetfulness: benign and malignant. Can Med Assoc J 86, 257-260 (1962)
3. Jolles J.: Cognitive, emotional and behavioral dysfunctions in aging and dementia. Prog Brain Res 70, 15-39 (1986)
4. Terry R. D. & R. Katzman: Senile dementia of the Alzheimer's type. Ann Neurol 14, 497-506 (1983)
5. Cotman C. W., R. J. Bridges, J. S. Taube, A. S. Clark, J. W. Geddes & D. T. Monaghan: The role of the NMDA receptor in central nervous system plasticity and pathology. J NIH Res 1, 65-74 (1989)
6. Magnusson K. R.: Influence of dietary restriction on ionotropic glutamate receptors during aging in C57Bl mice. Mech Ageing Dev 95, 187-202 (1997)
7. Magnusson K. R.: The effects of age and dietary restriction on metabotropic glutamate receptors. J Gerontol 52A, B291-B299 (1997)
8. Muller W. E., K. Scheuer & S. Stoll: Glutamatergic treatment strategies for age-related memory disorders. Life Sci 55, 2147-2153 (1994)
9. Corsi M., P. Fina & D. G. Trist: Co-agonism in drug-receptor interaction: illustrated by the NMDA receptors. Trends Pharmacol Sci 17, 220-222 (1996)
10. Hood W. F., E. T. Sun, R. P. Compton & J. B. Monaghan: 1-aminocyclobutane-1-carboxylate (ACBC): A specific antagonist of the N-methyl-D-aspartate receptor coupled glycine receptor. Eur J Pharmacol 161, 281-282 (1989)
11. Johnson J. W. & P. Ascher: Glycine potentiates the NMDA response in cultured mouse brain neurones. Nature 325, 529-533 (1987)
12. Ascher P. & J. W. Johnson: The NMDA receptor, its channel, and its modulation by glycine. In: The NMDA Receptor. Eds: Collingridge G. L.,Watkins J. C., Oxford University Press, Oxford, UK 177-205 (1994)
13. Bliss T. V. P. & G. L. Collingridge: A synaptic model of memory: Long-term potentiation in the hipppocampus. Nature 361, 31-39 (1993)
14. Bashir Z. I., N. Berretta, Z. A. Bortolotto, K. Clark, C. H. Davies, B. G. Frenguelli, J. Harvey, B. Potier & G. L. Collingridge: NMDA receptors and long-term potentiation in the hippocampus. In: The NMDA Receptor. Eds: Collingridge G. L.,Watkins J. C., Oxford University Press, Oxford, UK 294-312 (1994)
15. Moriyoshi K., M. Masu, T. Ishii, R. Shigemoto, N. Mizuno & S. Nakanishi: Molecular cloning and characterization of the rat NMDA receptor. Nature 354, 31-37 (1991)
16. Ishii T., K. Moriyoshi, H. Sugihara, K. Sakurada, H. Kadotani, M. Yokoi, C. Akazawa, R. Shigemoto, N. Mizuno, M. Masu & S. Nakanishi: Molecular characterization of the family of the N-methyl-D-aspartate receptor subunits. J Biol Chem 268, 2836-2843 (1993)
17. Monyer H., R. Sprengel, R. Schoepfer, H. A., M. Higuchi, H. Lomeli, N. Burnashev, B. Sakmann & P. H. Seeburg: Heteromeric NMDA receptors: Molecular and functional distinction of subtypes. Science 256, 1217-1221 (1992)
18. Seeburg P. H.: The molecular biology of mammalian glutamate receptor channels. Trends Pharmacol Sci 14, 297-302 (1993)
19. Ikeda K., M. Nagasawa, H. Mori, K. Araki, K. Sakimura, M. Watanabe, Y. Inoue & M. Mishina: Cloning and expression of the epsilon4 subunit of the NMDA receptor channel. FEBS Lett 313, 34-38 (1992)
20. Kutsuwada T., N. Kashiwabuchi, H. Mori, K. Sakimura, E. Kushiya, K. Araki, H. Meguro, H. Masaki, T. Kumanishi, M. Arakawa & M. Mishina: Molecular diversity of the NMDA receptor channel. Nature 358, 36-41 (1992)
21. Meguro H., H. Mori, K. Araki, E. Kushiya, T. Kutsuwada, M. Yamazaki, T. Kumnainshi, M. Arakawa, K. Sakimura & M. Mishina: Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs. Nature 357, 70-74 (1992)
22. Yamazaki M., H. Mori, K. Araki, K. J. Mori & M. Mishina: Cloning, expression and modulation of a mouse NMDA receptor subunit. FEBS Lett 300, 39-45 (1992)
23. Karp S. J., M. Masu, T. Eki, K. Ozawa & S. Nakanishi: Molecular cloning and chromosomal localization of the key subunit of the human N-methyl-D-aspartate receptor. J Biol Chem 268, 3728-3733 (1993)
24. Le Bourdelles B., K. A. Wafford, J. A. Kemp, G. Marshall, C. Bain, A. S. Wilcox, J. M. Sikela & P. J. Whiting: Cloning, functional coexpression, and pharmacological characterization of human cDNAs encoding NMDA receptor NR1 and NR2A subunits. J Neurochem 62, 2091-2098 (1994)
25. Zimmer M., T. M. Fink, Y. Franke, P. Lichter & J. Spiess: Cloning and structure of the gene encoding the human N-methyl-D-aspartate receptor (NMDAR1). Gene 159, 219-223 (1995)
26. Nakanishi S.: Molecular diversity of glutamate receptors and implications for brain function. Science 258, 597-603 (1992)
27. Kuryatov A., B. Laube, H. Betz & J. Kuhse: Mutational analysis of the glycine-binding site of the NMDA receptor: Structural similarity with bacterial amino acid-binding proteins. Neuron 12, 1291-1300 (1994)
28. Monyer H., N. Burnashev, D. J. Laurie, B. Sakmann & P. H. Seeburg: Developmental and regional expression in the rat brain and functional properties of four NMDA receptors. Neuron 12, 529-540 (1994)
29. Sheng M., J. Cummings, L. A. Roldan, Y. N. Jan & L. Y. Jan: Changing subunit composition of heteromeric NMDA receptors during development of rat cortex. Nature 368, 144-147 (1994)
30. Alessandri B., K. Battig & H. Welzl: Effects of ketamine on tunnel maze and water maze performance in the rat. Behav Neural Biol 52, 194-212 (1989)
31. Heale V. & C. Harley: MK801 and AP5 impair acquisition, but not retention, of the Morris milk maze. Pharmacol Biochem Behav 36, 145-149 (1990)
32. Morris R. G. M. & M. Davis: The role of NMDA receptors in learning and memory. In: The NMDA Receptor. Eds: Collingridge G. L.,Watkins J. C., Oxford University Press, Oxford, UK 340-375 (1994)
33. Tsien J. Z., P. T. Huerta & S. Tonegawa: The essential role of hippocampal CA1 NMDA receptor-dependent synaptic plasticity in spatial memory. Cell 87, 1327-1338 (1996)
34. Morris R. G. M., E. Anderson, G. S. Lynch & M. Baudry: Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5. Nature 319, 774-776 (1986)
35. Davis S., A. L. Markowska, G. L. Wenk & C. A. Barnes: Acetyl-L-carnitine: Behavioral, electrophysiological, and neurochemical effects. Neurobiol Aging 14, 107-115 (1993)
36. Shiarella K. T. & K. R. Magnusson: Glutamate receptor binding correlates with spatial memory performance in the aging C57Bl mice. Soc Neurosci Abstr 21, 237.5 (1995)
37. Mondadori C. & L. Weiskrantz: NMDA receptor blockers facilitate and impair learning via different mechanisms. Behav Neural Biol 60, 205-210 (1993)
38. Li H. B., K. Matsumoto, M. Tohda, M. Yamamoto & H. Watanabe: NMDA antagonists potentiate scopolamine-induced amnesic effect. Behav Brain Res 83, 225-228 (1997)
39. Dudkin K. N., V. K. Kruchinin & I. V. Chueva: Neurophysiological correlates of improvements in cognitive characteristrics in monkeys during modification of NMDA-ergic structures of the prefrontal cortex. Neurosci Behav Physiol
26, 545-551 (1996)
40. Collingridge G. L. & T. V. P. Bliss: NMDA receptors - their role in long-term potentiation. Trends Neurosci. 10, 288-293 (1987)
41. Bashir Z., S. Alford, S. Davies, A. Randall & G. Collingridge: Long-term potentiation of NMDA receptor-mediated synaptic transmission in the hippocampus. Nature 349, 156-158 (1991)
42. Harris E. W., A. Ganong & C. W. Cotman: Long-term potentiation in the hippocampus involves activation in N-methyl-D-aspartate receptors. Brain Res 323, 132-137 (1984)
43. Artola A. & W. Singer: NMDA receptors and developmental plasticity in visual neocortex. In: The NMDA Receptor. Eds: Collingridge G. L.,Watkins J. C., Oxford University Press, Oxford, UK 313-339 (1994)
44. Eichenbaum H., C. Stewart & R. G. M. Morris: Hippocampal representation in place learning. J Neurosci 10, 3531-3542 (1990)
45. Ammassari-Teule M. & S. Gozzo: Selective effects of hippocampal and frontal cortex lesions on a spatial learning problem in two inbred strains of mice. Behav Brain Res 5, 189-197 (1982)
46. Castorina M., A. M. Ambrosini, L. Pacifici, M. T. Ramacci & L. Angelucci: Age-dependent loss of NMDA receptors in hippocampus, striatum, and frontal cortex of the rat: Prevention by acetyl-L-carnitine. Neurochem Res 19, 795-798 (1994)
47. Clark A. S., K. R. Magnusson & C. W. Cotman: In vitro autoradiography of hippocampal exciatory amino acid binding in aged Fischer 344 rats: Relationship to performance on the Morris water maze. Beh Neurosci 106, 324-335 (1992)
48. Ingram D. K., P. Garofalo, E. L. Spangler, C. R. Mantione, I. Odano & E. D. London: Reduced density of NMDA receptors and increased sensitivity to dizocilpine-induced learning impairment in aged rats. Brain Res 580, 273-280 (1992)
49. Kito S., R. Miyoshi & T. Nomoto: Influence of age on NMDA receptor complex in rat brain studied in in vitro autoradiography. J Histochem Cytochem 38, 1725-1731 (1990)
50. Tamaru M., Y. Yoneda, K. Ogita, J. Shimizu & Y. Nagata: Age-related decreases of the N-methyl-D-aspartate receptor complex in the rat cerebral cortex and hippocampus. Brain Res 542, 83-90 (1991)
51. Ogawa N., K. Mizukawa, K. Haba, M. Asanuma & A. Mori: Effects of chronic bifemelane hydrochloride administration on receptors for N-methyl-D-aspartate in the aged-rat brain. Neurochem Res 17, 687-691 (1992)
52. Bonhaus D. W., W. B. Perry & J. O. McNamara: Decreased density, but not number, of N-methyl-D-aspartate, glycine and phencyclidine binding sites in hippocampus of senescent rats. Brain Res 532, 82-86 (1990)
53. Pelleymounter M. A., G. Beatty & M. Gallagher: Hippocampal 3H-CPP binding and spatial learning deficits in aged rats. Psychobiology 18, 298-304 (1990)
54. Serra M., C. A. Ghiani, M. C. Foddi, C. Motzo & G. Biggio: NMDA receptor function is enhanced in the hippocampus of aged rats. Neurochem Res 19, 483-487 (1994)
55. Fiore L. & L. Rampello: L-acetylcarnitine attenuates the age-dependent decrease of NMDA-sensitive glutamate receptors in rat hippocampus. Acta Neurol 11, 346-350 (1989)
56. Magnusson K. R. & C. W. Cotman: Age-related changes in excitatory amino acid receptors in two mouse strains. Neurobiol Aging 14, 197-206 (1993)
57. Magnusson K. R.: Differential effects of aging on binding sites of the activated NMDA receptor complex in mice. Mech Ageing Dev 84, 227-243 (1995)
58. Wenk G. L., L. C. Walker, D. L. Price & L. C. Cork: Loss of NMDA, but not GABA-A, binding in the brains of aged rats and monkeys. Neurobiol Aging 12, 93-98 (1991)
59. Magnusson K. R. & G. Sammonds: The aging of the NMDA receptor in C57Bl mice involves changes in agonist affinity. Soc Neurosci Abstr 22, 491.7 (1996)
60. Pelleymounter M. A., M. Smith & M. Gallagher: Spatial learning impairments in aged rats trained with a salient configuration of stimuli. Psychobiology 15, 248-254 (1987)
61. Nicolle M. M., J. L. Bizon & M. Gallagher: In vitro autoradiography of ionotropic glutamate receptors in hippocampus and striatum of aged Long-Evans rats: Relationship to spatial learning. Neuroscience 74, 741-756 (1996)
62. Saransaari P. & S. S. Oja: Strychnine-insensitive glycine binding to cerebral cortical membranes in developing and ageing mice. Mech Ageing Dev 72, 57-66 (1993)
63. Bresink I., W. Danysz, C. G. Parsons, P. Tiedtke & E. Mutschler: Chronic treatment with uncompetitive NMDA receptor antagonist memantine influences the polyamine and glycine binding sites of the NMDA receptor complex in aged
rats. J Neural Trans 10, 11-26 (1995)
64. Kew J. N. C., J. G. Richards, V. Mutel & J. A. Kemp: Developmental changes in NMDA receptor glycine affinity and ifenprodil sensitivity reveal three distinct populations of NMDA receptors in individual rat cortical neurons. J Neurosci 18, 1935-1943 (1998)
65. Reynolds I. J., S. N. Murphy & R. J. Miller: 3H-labeled MK-801 binding to the excitatory amino acid receptor complex from rat brain is enhanced by glycine. Proc Natl Acad Sci 84, 7744-7748 (1987)
66. Ransom R. W. & N. L. Stec: Cooperative modulation of [3H]MK-801 binding to the N-methyl-D-aspartate receptor-ion channel complex by L-glutamate, glycine, and polyamines. J Neurochem 51, 830-836 (1988)
67. Cohen S. A. & W. E. Muller: Age-related alterations of NMDA-receptor properties in the mouse forebrain: Partial restoration by chronic phosphatidylserine treatment. Brain Res 584, 174-180 (1992)
68. Scheuer K., S. Stoll, U. Paschke, R. Weigel & W. E. Muller: N-methyl-D-aspartate receptor density and membrane fluidity as possible determinants of the decline of passive avoidance performance in aging. Pharm Biochem Behav 50, 65-70 (1995)
69. Kitamura Y., X.-H. Zhao, T. Ohnuki, M. Takei & Y. Nomura: Age-related changes in transmitter glutamate and NMDA receptor/channels in the brain of senescence-accelerated mouse. Neurosci Lett 137, 169-172 (1992)
70. Piggott M. A., E. K. Perry, R. H. Perry & J. A. Court: [3H]MK-801 binding to the NMDA receptor complex, and its modulation in human frontal cortex during development and aging. Brain Res 588, 277-286 (1992)
71. Perry E. K., M. A. Piggott, J. A. Court, M. Johnson & R. H. Perry: Transmitters in the developing and senescent human brain. Ann NY Acad Sci 695, 69-72 (1993)
72. Magnusson K. R.: Glycine enhances binding to the NMDA receptor complex in aged mice, but does not correct the aging change. J Gerontol 51A, B141-B147 (1996)
73. Castorina M., A. M. Ambrosini, A. Giuliani, L. Pacifici, M. T. Ramacci & L. Angelucci: A cluster analysis study of acetyl-L-carnitine effect on NMDA receptors in aging. Exp Gerontol 28, 537-548 (1993)
74. Gazzaley A. H., S. J. Siegel, J. H. Kordower, E. J. Mufson & J. H. Morrison: Circuit-specific alterations of N-methyl-D-aspartate receptor subunit 1 in the dentate gyrus of aged monkeys. Proc Natl Acad Sci USA 93, 3121-3125 (1996)
75. Zuo Z. Z., V. B. Mahesh, P. L. Zamorano & D. W. Brann: Decreased gonadotropin-releasing hormone neurosecretory response to glutamate agonists in middle-aged female rats on proestrus afternoon: A possible role in reproductive aging? Endocrinology 137, 2334-2338 (1996)
76. Magnusson K. R. & G. E. Sammonds: Age-related changes in the expression of NMDA receptor subunits. FASEB J 12, #4365 (1998)
77. Priestley T., P. Laughton, J. Myers, B. le Bourdelles, J. Kerby & P. J. Whiting: Pharmacological properties of recombinant human N-methyl-D-aspartate receptors comprising NR1a/NR2A and NR1a/NR2B subunit assemblies expressed in permanently transfected mouse fibroblast cells. Mol Pharm 48, 841-848 (1995)
78. Gallagher M. J., H. Huang, D. B. Pritchett & D. R. Lynch: Interactions between ifenprodil and the NR2B subunit of the N-methyl-D-aspartate receptor. J Biol Chem 271, 9603-9611 (1996)
79. Williams K.: Ifenprodil discriminates subtypes of the N-methyl-D-aspartate receptor: Selectivity and mechanisms at recombinant heteromeric receptors. Mol Pharmacol 44, 851-859 (1993)
80. Seeburg P. H., H. Monyer, R. Sprengel & N. Burnashev: Molecular biology of NMDA receptors. In: The NMDA Receptor. Eds: Collingridge G. L.,Watkins J. C., Oxford University Press, Oxford, UK 147-157 (1994)
81. Baskys A., J. Reynolds & P. Carlen: NMDA depolarizations and long-term potentiation are reduced in the aged rat neocortex. Brain Research 530, 142-146 (1990)
82. Barnes C. A., G. Rao & J. Shen: Age-related decrease in the N-methyl-D-aspartateR-mediated excitatory postsynaptic potential in hippocampal region CA1. Neurobiol Aging 18, 445-452 (1997)
83. Rao G., C. A. Barnes, B. L. McNaughton & J. Shen: Age-related decrease in the NMDA-mediated EPSP in FD. Soc Neurosci Abstr 20, 1207 (1994)
84. Billard J. M., A. Jouvenceau, Y. Lamour & P. Dutar: NMDA receptor activation in the aged rat: Electrophysiological investigations in the CA1 area of the hippocampal slice ex vivo. Neurobiol Aging 18, 535-542 (1997)
85. Barnes C. A. & B. L. McNaughton: An age comparison of the rates of acquisition and forgetting of spatial information in relation to long-term enhancement of hippiocampal synapses. Behav Neurosci 99, 1040-1048 (1985)
86. Barnes C. A.: Memory deficits associated with senescence: A neurophysiological and behavioral study in the rat. J Comp Physiol Psychol 93, 74-104 (1979)
87. Barnes C. A., G. Rao & B. L. McNaughton: Functional integrity of NMDA-dependent LTP induction mechanisms across the lifespan of F-344 rats. Learning Memory 3, 124-137 (1996)
88. Deupree D. L., D. A. Turner & C. L. Watters: Spatial performance correlates with in vitro potentiation in young and aged Fischer 344 rats. Brain Res 554, 1-9 (1991)
89. Moore C. I., M. D. Browning & G. M. Rose: Hippocampal plasticity induced by primed burst, but not long-term potentiation, stimulation is impaired in area CA1 of aged Fischer 344 rats. Hippocampus 3, 57-66 (1993)
90. Shankar S., T. J. Teyler & N. Robbins: Aging differentially alters forms of long-term potentiation in rat hippocampal area CA1. J Neurophys 79, 334-341 (1998)
91. Gonzales R. A., L. M. Brown, T. W. Jones, R. D. Trent, S. L. Westbrook & S. W. Leslie: N-methyl-D-aspartate mediated responses decrease with age in Fischer 344 rat brain. Neurobiol Aging 12, 219-225 (1991)
92. Pittaluga A., E. Fedele, C. Risiglione & M. Raiteri: Age-related decrease of the NMDA receptor-mediated noradrenaline release in rat hippocampus and partial restoration by D-cycloserine. Eur J Pharm 231, 129-134 (1993)
93. Arias P., S. Carbone, B. Szwarcfarb, C. Feleder, M. Rodruguez, P. Scacchi & J. A. Moguilevsky: Effects of aging on N-methyl-D-aspartate (NMDA)- induced GnRH and LH release in female rats. Neurosci Lett 740, 234-238 (1996)
94. Morris R. G. M.: Synaptic plasticity and learning: Selective impariment of learning in rats and blockade of long-term potentiation in vivo by the N-methyl-D-aspartate receptor antagonist AP5. J Neurosci 9, 3040-3057 (1989)
95. Bannerman D. M., M. A. Good, S. P. Butcher, M. Ramsay & R. G. M. Morris: Distinct components of spatial learning revealed by prior training and NMDA receptor blockade. Nature 378, 182-186 (1995)
96. Rapp P., R. Rosenberg & M. Gallagher: An evaluation of spatial information processing in aged rats. Behav Neurosci 101, 3-12 (1987)
97. Gage F., S. Dunnett & A. Bjorklund: Spatial learning and motor deficits in aged rats. Neurobiol Aging 5, 43-48 (1984)
98. Gallagher M., R. Burwell & M. Burchinal: Severity of spatial learning impairment in aging: Development of a learning index for performance in the Morris water maze. Behav. Neurosci. 107, 618-626 (1993)
99. Barnes C. A., L. Nadel & W. K. Honig: Spatial memory deficit in senescent rats. Canad. J. Psychol. 34, 29-39 (1980)
100. Evans G. W. P., P. L. M. Brennan, M. A. M. Skorpanich & D. B. Held: Cognitive mapping and elderly adults: Verbal and location memory for urban landmarks. J Gerontol 39, 452-457 (1984)
101. Cherry K. E. & D. C. Park: Individual difference and contextual variables influence spatial memory in younger and older animals. Psychol Aging 8, 517-526 (1993)
102. Kirasic K. C. & M. R. Bernicki: Acquisition of spatial knowledge under conditions of temporospatial discontunity in young and elderly adults. Psychol Res 52, 76-79 (1990)
103. Moore T. E., B. Richards & J. Hood: Aging and the coding of spatial information. J Gerontol 39, 210-212 (1984)
104. Sharps M. J. & E. S. Gollin: Memory for object locations in young and elderly animals. J Gerontol 42, 336-341 (1987)
105. Castorina M. & L. Ferraris: Acetyl-L-carnitine affects aged brain receptorial system in rodents. Life Sci 54, 1205-1214 (1994)
106. Cohen S. A. & W. E. Muller: Effects of piracetam on N-methyl-D-aspartate receptor properties in the aged mouse brain. Pharmacology 47, 217-222 (1993)
107. Hartmann H., S. A. Cohen & W. E. Muller: Effects of subchronic administration of pyritinol on receptor deficits and phosphatidylinositol metabolism in the brain of the aged mouse. Neuropharmacology 32, 119-125 (1993)
108. Stoll S., H. Hartmann, S. A. Cohen & W. E. Muller: The potent free radical scavenger alpha-lipoic acid improves memory in aged mice: Putative relationship to NMDA receptor deficits. Pharmacol Biochem Behav 46, 799-805 (1993)
109. Algeri S., L. Biagini, A. Manfridi & N. Pitsikas: Age-related ability of rats kept on a life-long hypocaloric diet in a spatial memory test. Longitudinal observations. Neurobiol Aging 12, 277-282 (1991)
110. Idobro F., K. Nandy, D. I. Mostofsky, L. Blatt & L. Nandy: Dietary restriction: Effects on radial maze learning and lipofuscin pigment deposition in the hippocampus and frontal cortex. J Gerontol 6, 355-362 (1987)
111. Pitsikas N. & S. Algeri: Deterioration of spatial and nonspatial reference and working memory in aged rats: Protective effect of life-long calorie restriction. Neurobiol Aging 13, 369-373 (1992)