Impaired synaptic potentiation processes in the hippocampus of aged, memory-deficient rats

Activation of p42 mitogen-activated protein kinase by arachidonic acid and trans-1-amino-cyclopentyl-1,3- dicarboxylate impacts on long-term potentiation in the dentate gyrus in the rat: analysis of age-related changes

Maintenance of long-term potentiation in perforant path-granule cell synapses is associated with an increase in glutamate release, which we have suggested relies on an interaction between arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD). Evidence suggests that this interaction is dependent on stimulation of tyrosine kinase, which phosphorylates and activates phospholipase Cgamma. In this study, we demonstrate that arachidonic acid and ACPD stimulate tyrosine phosphorylation of a protein of about 40,000 mol. wt and further analysis, using a specific antibody, suggested that this may be extracellular signal-regulated kinase, one member of the family of mitogen-activated protein kinases. Activity of extracellular signal-regulated kinase was increased by arachidonic acid and ACPD in vitro, but it was also increased by induction of long-term potentiation in perforant path-granule cell synapses. A role for extracellular signal-regulated kinase in long-term potentiation was supported by the observation that expression of long-term potentiation, as well as the associated increases in endogenous glutamate release and extracellular signal-regulated kinase activation, were inhibited by pretreatment with the mitogen-activated protein kinase inhibitor, PD98059, while PD98059 pretreatment inhibited the interaction between arachidonic acid and ACPD on glutamate release. An age-related decrease in extracellular signal-regulated kinase activity was observed in the dentate gyrus, and there was no evidence of increased extracellular signal-regulated kinase activity or endogenous glutamate release in tissue prepared from aged rats in which long-term potentiation was compromised. The evidence is consistent with the view that increased activation of extracellular signal-regulated kinase plays a role in long-term potentiation, and that activation of this kinase relies on the interaction between arachidonic acid and ACPD.

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.

Increased calcium buffering in basal forebrain neurons during aging

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

Development of a transient increase in recurrent inhibition and paired-pulse facilitation in hippocampal CA1 region

Paired-pulse recurrent inhibition (RI) of population spike (PS) and facilitation (PPF) of field excitatory postsynaptic potential (EPSP) were studied in the CA1 region of hippocampal slices taken from Wistar rats aged from 9 days to 16 months. The comparison of three different paired-pulse protocols revealed the antidromic-orthodromic (A-O) stimulation as the most reliable in quantifying the strength of fast (peaking at 10 ms) and slow (peaking at 200 ms) components of recurrent inhibition. Fast RI, present but weak at 9 days, progressively increased to reach its maximal strength at 30 days, declining in adult (2 m) and middle-aged (16 m) animals. Slow RI was replaced by facilitation at 9 days while it was absent at 15 days. It reached adult values at 30 days. A reduction of the test response at interpulse interval (IPI) of 2-4 ms was strong in developing and adult animals, but was significantly decreased in 16 m. At maximal stimulation PPF was expressed as an enhancement of the slow rather than the fast phase of the EPSP and was particularly strong with a prominent N-methyl-D-aspartate dependent component. A very characteristic selectivity for a prominent PPF at stimulation frequency of 5 Hz appeared first at the 18th day and increased gradually to reach a maximum at the 30th day, after which it declined to very low values in middle-aged animals. A similar developmental pattern was observed in slices taken from rats reared in complete darkness, suggesting a strong innate origin. The ability of hippocampal circuits for plastic gating of information appears to be transiently enhanced at the completion of the first postnatal month as it can be exercised at a wider part of the frequency spectrum, with maximal inhibition and potentiation especially at the frequency of theta rhythm.

Dietary supplementation with vitamin E reverses the age-related deficit in long term potentiation in dentate gyrus

Long term potentiation (LTP) in dentate gyrus is impaired in aged rats, and this has been associated with an age-related decrease in membrane arachidonic acid concentration. In this study, we considered whether the trigger for this age-related decrease in arachidonic acid might be increased lipid peroxidation stimulated by the proinflammatory cytokine, interleukin-1beta. Groups of aged and young rats were fed on a control diet or a diet supplemented with alpha-tocopherol and assessed for their ability to sustain LTP. Aged rats fed on the control diet exhibited an impaired ability to sustain LTP and analysis of tissue prepared from these rats exhibited increased interleukin-1beta, increased lipid peroxidation, and decreased membrane arachidonic acid concentration compared with young rats fed on either diet. Aged rats fed on the supplemented diet sustained LTP in a manner indistinguishable from young rats, and the age-related increases in interleukin-1beta and lipid peroxidation and the decrease in membrane arachidonic acid concentration were all reversed. We propose that interleukin-1beta may be the trigger that induces these age-related changes and may therefore be responsible for the deficit in long term potentiation in aged rats. The observation that alpha-tocopherol reverses these changes is consistent with the hypothesis that some age-related changes in hippocampus might derive from oxidative stress.

Evidence that increased hippocampal expression of the cytokine interleukin-1 beta is a common trigger for age- and stress-induced impairments in long-term potentiation

Several cytokines and their receptors are identified in brain; one of these is the proinflammatory cytokine interleukin-1beta that is synthesized and released from neurons and glia in response to stress or insult. Among the actions of interleukin-1beta is its ability to inhibit long-term potentiation in the hippocampus in vitro, an action that mimics one of the consequences of stress and age. It has been shown that the concentration of interleukin-1beta in brain tissue is increased in neurodegenerative conditions, and recent evidence from our laboratory has indicated an increase in the concentration of interleukin-1beta in the hippocampus of aged rats. These observations led us to consider that the underlying common cause of impaired long-term potentiation in aged and stressed rats might be increased endogenous interleukin-1beta concentration in hippocampus. The data presented here indicate that there was an inverse relationship between concentration of interleukin-1beta in the dentate gyrus and long-term potentiation in perforant path-->granule cell synapses in aged rats, stressed rats, and rats pretreated with interleukin-1beta. The evidence suggested that the cytokine induces formation of reactive oxygen species that triggers lipid peroxidation in vivo, as well as in vitro, and that these changes lead to depletion of membrane arachidonic acid that correlates with impaired long-term potentiation. We propose that three theories of aging, the glucocorticoid theory, the membrane theory, and the free radical theory, constitute three facets of age with one underlying trigger: an increase in the endogenous concentration of interleukin-1beta in hippocampus.

Age-associated changes in Ca(2+)-dependent processes: relation to hippocampal synaptic plasticity

Altered calcium (Ca2+) homeostasis is thought to play a key role in aging and neuropathology resulting in memory deficits. Several forms of hippocampal synaptic plasticity are dependent on Ca2+, providing a potential link between altered Ca2+ homeostasis and memory deficits associated with aging. The current study reviews evidence for Ca2+ dysregulation during aging which could interact with Ca(2+)-dependent synaptic plasticity. The authors suggest that changes in Ca2+ regulation could adjust the thresholds for synaptic modification, favoring processes for depression of synaptic strength during aging.

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.

Role of temporal summation in age-related long-term potentiation-induction deficits

Hippocampal long-term potentiation (LTP) is reduced in aged relative to young F-344 rats when peri-threshold stimulation protocols (several stimulus pulses at 100-200 Hz) are used. The present study was designed to examine the possibility that this LTP-induction deficit is caused by a reduced overlap of Schaffer-collateral inputs onto CA1 pyramidal cells (input cooperativity). This reduced input cooperativity would decrease the levels of postsynaptic depolarization during LTP induction, which might account for the age-related LTP deficit. Both behavioral data (Morris Water Maze) and electrophysiological data (intracellular recordings from hippocampal slices) were collected from adult and aged F-344 rats. To counter the effects of reduced input cooperativity, stimulus intensities were adjusted to elicit baseline excitatory postsynaptic potentials (EPSPs) of equivalent amplitude in aged and young rats. Contrary to expectations, however, an age-related LTP-induction deficit was still observed. Further evaluation of the electrophysiological data revealed that temporal summation of multiple EPSPs during high-frequency stimulation was impaired in the aged rats. Thus, despite the equalization across age groups of the baseline EPSP amplitudes, the cells of aged rats were less depolarized during the LTP-inducing stimulation than were those of young rats. This reduced total depolarization was not an artifact of the higher stimulus intensity used on aged animals, nor was it caused by a failure of aged rats' CA1 afferents to follow high-frequency stimulation. The present data therefore suggest that there is a deficit in the ability of aged rats' synapses to provide the sustained depolarization necessary to active the LTP-induction cascade.

The ability of aged rats to sustain long-term potentiation is restored when the age-related decrease in membrane arachidonic acid concentration is reversed

The ability of aged rats to sustain long-term potentiation in the dentate gyrus of the hippocampus is impaired and this impairment correlates with decreased release of glutamate and a decrease in membrane arachidonic acid concentration. Twenty-two-month-old rats receiving a diet supplemented with arachidonic acid and its precursor, gamma-linolenic acid, sustained long-term potentiation in a manner indistinguishable from four-month-old controls. Dietary supplementation also restored arachidonic acid concentrations in membranes prepared from hippocampus of these aged animals to levels observed in hippocampus of four-month-old rats. Glutamate release stimulated by depolarization was similar in dentate gyrus prepared from young rats and aged rats which received the experimental diet, but was markedly reduced in aged animals which received the control diet. In addition, the synergism between arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate, on glutamate release, which was observed in hippocampal synaptosomes prepared from four-month-old rats, was also observed in hippocampal preparations obtained from aged rats which had been fed with the experimental diet, but was absent in hippocampal preparations obtained from aged animals which were fed with control diet. Thus, reversing the age-related decrease in membrane arachidonic acid concentration restored ability of aged animals to sustain long-term potentiation and reversed age-related changes in glutamate release.

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.

Changes in protein synthesis and synthesis of the synaptic vesicle protein, synaptophysin, in entorhinal cortex following induction of long-term potentiation in dentate gyrus: an age-related study in the rat

We have examined protein synthesis in entorhinal cortex following induction of long-term potentiation (LTP) in perforant path-granule cell synapses. The data presented here indicate that there was an increase in [35S]methionine labelling of TCA-precipitated proteins and [35S]methionine labelling of synaptophysin in the ipsilateral entorhinal cortex 40 min after induction of LTP in dentate gyrus. Intraventricular injection of both the NMDA antagonist, D-amino-phosphonovalerate, and the protein synthesis inhibitor, anisomycin reduced protein synthesis though the decrease caused by anisomycin was much more profound. Both agents blocked induction of LTP and the increase in protein synthesis and synaptophysin synthesis which accompanied LTP. These data indicate a close coupling of increased protein synthesis in the entorhinal cortex and expression of LTP in the dentate gyrus. This coupling was further suggested by the absence of an LTP-associated increase in protein synthesis in aged animals, in which LTP was markedly attenuated. The possibility that these changes impact on morphological changes which accompany LTP is discussed.

Selective alteration of long-term potentiation-induced transcriptional response in hippocampus of aged, memory-impaired rats

Normal human aging is associated with selective changes in cognition that are attributable, in part, to dysfunction of hippocampal pathways. Rodents also exhibit age-dependent hippocampal dysfunction that results in spatial memory deficits and a correlated reduction in the maintenance of long-term potentiation (LTP). Although suprathreshold stimulus protocols result in normal LTP induction in aged rats, the ability to sustain this increase in synaptic efficacy is reduced in the old animals. The maintenance phase of LTP is known to be dependent on rapid, transcriptional events, and recent studies have identified signal transduction mechanisms that link glutamate-induced responses at the synapse with transcriptional responses at the nucleus. To examine the integrity of these signaling pathways in aged hippocampus, we monitored the induction of a panel of immediate early genes (IEGs) that are known to be transcriptionally activated after LTP-inducing stimuli, using a "reverse Northern" strategy. Here we report that a broad representation of IEGs are similarly induced in awake, behaving young adult and aged, memory-impaired rats. This indicates a general preservation of these presumptive signaling pathways during the aging process. Induced levels of c-fos mRNA, however, are significantly higher in the aged animals. These observations suggest that age-dependent hippocampal dysfunction may be associated with a selective change in the dynamic activity of signaling pathways upstream of c-fos, possibly involving calcium regulation.

Increased susceptibility to induction of long-term depression and long-term potentiation reversal during aging

Homosynaptic long-term depression (LTD) and reversal of long-term potentiation (LTP) were examined extracellularly at CA3-CA1 synapses in stratum radiatum of slices from adult (6-9 months) and aged (20-24 months) Fischer 344 rats. Prolonged low-frequency stimulation (LFS) (900 pulses/1 Hz) of the Schaffer collaterals depressed the initial slope of the excitatory postsynaptic potential (EPSP) in aged but not adult rats. LTD at aged synapses was pathway-specific, persistent, and sensitive to the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5). Adult slices exhibited AP5-sensitive LTD in high [Ca2+] medium, whereas LTD in aged slices was blocked by high [Mg2+], suggesting that differences in Ca2+ regulation may underlie susceptibility to LTD. Despite age-related differences in LTD induction, no age difference in LTP magnitude was revealed. Additionally, LFS delivered 60 min after LTP induction resulted in similar LTP reversal for both age groups. Susceptibility differences to LTP reversal were indicated after multiple short-duration LFS bursts (30 pulses/1 Hz), with each burst separated by 10 min. Aged synapses exhibited significant reversal after a single burst and complete reversal after three LFS episodes. In adult slices, LTP reversal appeared after the fourth burst, and at no time was LTP depressed to initial baseline levels. This study provides the first characterization of homosynaptic LTD/LTP reversal in the aged animal and demonstrates that one form of plasticity, depression attributable to LFS, is increased during aging.

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.

Chronic RU486 treatment reduces age-related alterations of mouse hippocampal function

The present study investigates the protective effect of a chronic blockade of the glucocorticoid receptor (type II) by a single weekly SC injection (20 mg/kg) of RU486 (a potent antiglucocorticoid) from mid-age (12 months old) until senescence (20 to 22 months old) on perturbations of some electrophysiological parameters classically observed in CA1 hippocampal slices of aged BALB/c mice. In this CA1 hippocampal area, no electrophysiological difference was observed at a stimulation frequency of 0.3 Hz. However, an important age-related effect was observed in not-treated animals concerning the three phases of the synaptic response during and after 4 Hz repetitive stimulation ith impairment of the frequency potentiation (FP). Interestingly, this electrophysiological disturbance disappeared completely in aged animals treated previously with RU486. Furthermore, a 10 microM CORT bath application had no effect in CA1 of aged animals, while it produced the classical type II-mediated population spike (PS) decrease in adult animals. This PS amplitude decrease was maintained in aged animals previously treated with RU486. These electrophysiological findings suggest an important type II-mediated glucocorticoid action on age-related alterations of hippocampal function.

Temperature change-induced potentiation: a comparative study of facilitatory mechanisms in aged and young rat hippocampal slices

The effect of temperature changes in the medium on the evoked potentials of pyramidal neurons in response to the electrical stimulation of Schaffer collaterals was investigated in both young (one to two months) and aged (24-28 months) rat hippocampal slices. Temperature increase was shown to reduce, and subsequent temperature decrease to enhance, the population spike amplitude in both young and aged rats. Temperature decrease produced a long-lasting (> 30 min) and important potentiation (180%) of population spike amplitude in young but not in aged rats. Long-term post-tetanic potentiation was maintained in young but suppressed in aged rats by temperature changes. The impairment of temperature-induced potentiation in aged rats is suggested as a tool for studying promnesic drugs.

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.

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

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

Age-dependent changes in second messenger and rolipram receptor systems in the gerbil brain

Age-related alterations in binding sites of major second messengers and a selective adenosine 3',5'-cyclic monophosphate (cyclic-AMP) phosphodiesterase (PDE) in the gerbil brain were analysed by receptor autoradiography. [3H]Phorbol 12,13-dibutyrate (PDBu), [3H]inositol 1,4,5-trisphosphate (IP3), [3H]forskolin, [3H]cyclic-AMP, and [3H]rolipram were used to label protein kinase C (PKC), IP3 receptor, adenylate cyclase, cyclic-AMP dependent protein kinase (PKA), and Ca2+/calmodulin-independent cyclic-AMP PDE, respectively. In middle-aged gerbils (16 months old), [3H]PDBu binding was significantly reduced in the hippocampal CA1 sector, thalamus, substantia nigra, and cerebellum, compared with young animals (1 month old). [3H]IP3 binding revealed significant elevations in the nucleus accumbens, hippocampal CA1 sector, dentate gyrus, and a significant reduction in cerebellum of middle-aged gerbils. [3H]Forskolin binding in middle-aged animals was significantly increased in the nucleus accumbens and hilus of dentate gyrus, but was diminished in the substantia nigra and cerebellum. On the other hand, in middle-aged animals, [3H]cyclic-AMP binding revealed a significant elevation only in the hippocampal CA3 sector, whereas [3H]rolipram binding showed a significant reduction in the thalamus and cerebellum. Thus, the age-related alteration in these binding sites showed different patterns among various brain regions in middle-aged gerbils indicating that the binding sites of PKC, IP3, and adenylate cyclase are more markedly affected by aging than those of PKA and cyclic-AMP PDE and that the hippocampus and cerebellum are more susceptible to these aging processes than other brain regions. The findings suggest that intracellular signal transduction is affected at an early stage of senescence and this may lead to neurological deficits.

Membrane arachidonic acid concentration correlates with age and induction of long-term potentiation in the dentate gyrus in the rat

We examined the induction and maintenance of long-term potentiation (LTP) in vivo in the dentate gyrus of 4-month-old and 22-month-old urethane-anaesthetized rats. High-frequency stimulation of the perforant path induced an immediate increase in the slope of the population excitatory postsynaptic potential (EPSP), which was sustained in the 4-month-old animals for the duration of the experiment (45 min post-tetanus). In the 22-month-old group, the mean slope of the population EPSP decreased almost to baseline by the end of the experiment. Examination of the individual records indicated that LTP was sustained for the duration of the experiment in half of the 22-month-old animals, while in the others only post-tetanic potentiation was observed. Membrane arachidonic acid concentration was reduced in aged compared with young animals and was lowest in the subgroup of aged animals which failed to sustain LTP. Potassium-stimulated, calcium-dependent release of glutamate was also decreased in aged compared with young animals, but LTP was associated with an increase in glutamate release in the 4-month-old group and 22-month-old subgroup in which LTP was successfully sustained; no change was observed in the 22-month-old group in which LTP was not sustained. The results indicate a correlation between membrane arachidonic acid concentration, glutamate release and ability to sustain LTP in aged animals.

NMDA receptor function is enhanced in the hippocampus of aged rats

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

Physical Activity, Age, and Cognitive-Neuropsychological Function

Findings from three research paradigms that employed aerobic exercise as an independent variable were used to test the hypothesis that aerobic exercise improves cognitive-neuropsychological functioning. The research paradigms were animal intervention studies, cross-sectional human studies, and human intervention studies. Results from studies of animals, usually rodents, provide consistent evidence that aerobic fitness is associated with improved neurobiological and behavioral functioning. Cross-sectional studies with humans indicate a strong positive association between physical activity level and cognitive-neuropsychological performance. However, results from these studies must be interpreted cautiously, as individuals who elect to exercise or not exercise may differ on other variables that could influence cognitive-neuropsychological performance. To date, human intervention studies have not consistently demonstrated cognitive-neuropsychological improvements following exercise training. To satisfactorily test the exercise/cognition hypothesis with humans, carefully controlled intervention studies that last longer than those previously employed are needed.

Normal aging: regionally specific changes in hippocampal synaptic transmission

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

Environmental influences on the central nervous system and their implications for the aging rat

Two methods of providing environmental stimulation that were introduced in the 1950s have guided much research on neurobehavioural plasticity. These are neonatal handling and environmental enrichment. Neonatal handling has been shown to permanently affect behaviour and endocrine responses. Recently this manipulation has been shown to have important influences on the aging individual, protecting the hippocampus from age-related dysfunction and neuronal loss. These effects are mediated, in part, by keeping glucocorticoid levels low. This has been characterised by, among other things, elevated expression of glucocorticoid receptors in the hippocampus. Earlier studies have failed to present convincing evidence for differences in hormonal variables between animals housed in enriched and impoverished environments, and have not consistently reported changes in the hippocampus. Recent data from our laboratories have shown that adult animals housed in enriched environments had, like neonatally-handled rats, higher expression of the gene encoding glucocorticoid receptors in the hippocampus. Taken together with the induction of NGF and NGFIA gene expression in the hippocampus of enriched animals, these results implicate genes encoding transcription factors and glucocorticoid receptors in the cascade of events leading to environmentally induced cerebral changes. In addition, these results suggest that environmental enrichment in adulthood, like neonatal handling, may have the potential to protect the aging hippocampus from glucocorticoid neurotoxicity.

B-50/GAP-43 phosphorylation in hippocampal slices from aged rats: effects of phosphatidylserine administration

Phosphorylation of the presynaptic protein B-50/GAP-43, a substrate of protein kinase C (PKC), has been implicated in neuronal mechanisms related to learning and memory. We evaluated both basal (5 mM KCl) and stimulated (30 mM KCl) B-50/GAP-43 phosphorylation in 32P-prelabeled hippocampal slices obtained from adult and senescent male Sprague-Dawley rats. The in situ B-50/GAP-43 phosphorylation was assayed by quantitative immunoprecipitation. There was no age-related difference in B-50/GAP-43 basal phosphorylation. However, B-50/GAP-43 phosphorylation in depolarized slices from aged rats was significantly decreased relative to that of adult animals. Aged rats were treated with either tris buffer or sonicated suspension of phosphatidylserine (PS) in tris buffer (15 mg/kg IP for 7 and 17 days). PS did not affect basal and high K(+)-induced B-50/GAP-43 phosphorylation in the 7-day treatment. However, after 17 days, PS restored the K(+)-induced B-50/GAP-43 phosphorylation. It is proposed that repeated PS administrations might be beneficial to the age-induced deterioration of endogenous B-50/GAP-43 phosphorylation by acting on Ca++ homeostatic mechanisms and/or PKC.

Age-related decline in multiple unit action potentials of CA3 region of rat hippocampus: correlation with lipid peroxidation and lipofuscin concentration and the effect of centrophenoxine

Changes in lipid peroxidation, lipofuscin concentration, and multiple unit activity (MUA recorded in conscious animals) in the CA3 region were studied in the hippocampus of male Wistar rats aged 4, 8, 16, and 24 months. The lipid peroxidation and lipofuscin concentration were increased with age. The MUA, however, declined with age. Correlational analyses were performed for the four age groups to determine the relationship between the age-associated decline in MUA with the age-related alterations in lipid peroxidation and lipofuscin concentrations. The age-related increase in lipid peroxidation correlated positively with the age-associated increase in lipofuscin concentration. The age-related increases in lipid peroxidation and lipofuscin concentration correlated negatively with the changes in MUA. Since lipid peroxidation may affect neuronal electrophysiology, our data suggested that age-related increase in lipid peroxidation may contribute to an age-associated decline in neuronal electrical activity. Centrophenoxine effects were studied on the three above-mentioned age-associated changes in the hippocampus. The drug had no effect on all three parameters in 4- and 8-month-old rats. In 16- and 24-month-old rats, however, the drug significantly increased the MUA but concomitantly decreased lipofuscin concentration and lipid peroxidation. Correlational analyses of the data on MUA, lipid peroxidation and lipofuscin concentration from the centrophenoxine-treated animals showed that the drug-induced diminution in both lipofuscin and lipid peroxidation was significantly correlated with the drug-induced increase in MUA. The differential effect of the drug in younger (4-8 months) and older (16-24 months) animals indicated that the stimulation of MUA was clearly associated with concomitant decrease in lipid peroxidation and lipofuscin concentration.

Age-related alterations in potentiation in the CA1 region in F344 rats

F344 rats of various ages (2-3 months, 15-16 months, and 24-25 months) were tested on a spatial memory task. The 15- and 24-month-old rat groups showed impaired acquisition and retention of the memory task, compared to the young animals. Extracellular field potential recordings in the CA1 region were subsequently performed in vitro, using hippocampal slices from both these tested rats and similar but untested F344 young and aged rats. Findings included: a) a positive correlation between baseline dendritic EPSP slope values and retention scores across age groups; b) a more rapid decay of both somatic and dendritic short-term potentiation in aged slices; c) decreased somatic but not dendritic long-term potentiation overall in aged slices, regardless of bath Mg2+ level; and d) decreased paired-pulse facilitation in slices from aged rats bathed in 4.0 mM Mg2+ media compared to young controls. These findings suggest an age-related alteration in both presynaptic and postsynaptic potentiation mechanisms, which may relate to the poor spatial memory acquisition and retention in the aged rats. These age-related differences point to substantial changes in neuronal signal processing capabilities and local circuit function in the hippocampus as a function of aging.

Similar effects of aging and corticosterone treatment on mouse hippocampal function

Cumulative exposure to corticosterone (CORT) during the lifespan plays an important role in the hippocampal aging process, and similar disturbances have been observed in chronic stress. However, there is little information on the electrophysiological changes observed in these two situations at the hippocampal level. The present study investigates the electrophysiological changes observed in control conditions and after a 10 microM CORT bath application on hippocampal slices taken from control adult BALB/c mice, from adult animals subjected to chronic overexposure to corticosterone (20 mg/kg/day during 3 months) and from aged animals. No electrophysiological difference was observed in the CA1 area of chronically CORT treated and aged groups compared to the control group. Conversely, the input/output curves from the dentate area showed a similar, statistically significant right shift in these two groups compared to the control group. Furthermore, in control subjects, a 10 microM CORT bath application produced the classical population spike amplitude decrease. However, in slices taken from chronically CORT-treated and aged mice, this effect did not occur in the CA1 while it was replaced by a population spike amplitude increase in the dentate. This increase was blocked by spironolactone. These electrophysiological alterations may indicate that a part of the aged-induced functional disturbances is mediated by glucocorticoids, and may progressively lead to impairment of neuroendocrine functions and behavioral adaptation.

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

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

Age-related alterations in second-messenger systems in the gerbil brain: autoradiographic analysis and effect of vinconate

Age-related alterations in binding sites of major second-messenger systems in the gerbil brain were analyzed by receptor autoradiography using [3H]phorbol 12,13-dibutyrate (PDBu), and [3H]inositol 1,4,5-trisphosphate (IP3). In middle-aged gerbils (16 months old), the reduction of PDBu-binding sites was noticed in various brain regions compared with young animals (4 weeks old). By contrast, the elevation of IP3-binding sites was seen in most of brain regions except for the reduction in the cerebellum in the middle-aged animals. Chronic treatment with vinconate partly regulated the age-related changes in the binding sites of these second messengers. The result demonstrates the age-related alterations in the binding sites of an intracellular second-messenger system. Furthermore, they suggest that vinconate partly modulates the age-related alterations in the bindings of second messengers.

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

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

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

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

Differential effects of age on subpopulations of hippocampal theta cells

The possible contribution of age-related changes in the firing properties of hippocampal theta cells to spatial learning deficits was addressed in the present study. The behavioral correlates of theta cells in strata oriens, pyramidale, and granulosum were compared as young and old rats performed a radial maze spatial working memory task. Behaviorally, the old animals made significantly more errors on the maze and required more time to solve the task than did young animals. Firing rates were compared in four different locomotion states: still, running radially inward and radially outward, and forward motion. The discharge rates of theta cells in strata pyramidale and granulosum were significantly modulated by these movements in both age groups. Stratum oriens theta cells recorded from young animals, on the other hand, were not movement-sensitive, while similar cells from old animals demonstrated exaggerated responses to movement. In old animals, the mean discharge rates were higher in stratum granulosum and lower in stratum oriens than in the young rats. The discharge rates of cells in stratum pyramidale did not differ between age groups. These region specific changes in the firing characteristics of hippocampal theta cells are likely to have important consequences for information processing in this structure.

Aging reverts to juvenile conditions the synaptic connectivity of cerebral cortical pyramidal shafts

Quantitative analysis of the total number and distribution of dendritic spines along the apical shafts of layer V cerebral cortical pyramids has been performed on aging rats (90-120 to 1,135 days old) and on rats during the period of early and late development (10-80 days). As expected from previous work, present results show that the total number of dendritic spines along the shafts increase from 10 to 80 days, after which it starts to gradually decrease until the last age studied (1,135 days). The quantitative analysis of the effect of aging on the relative decrease of dendritic spines shows that this decrease starts being homogeneous along the whole length of the apical shafts and that from a certain age onwards, estimated according to present results in 400 days, this effect is significantly more pronounced in layers IV and III-II than in deep layers. Furthermore, the comparison made between the distribution of dendritic spines along the apical shafts of pyramidal neurons of old and young animals has shown that aging produces a regression of this distribution to juvenile conditions.

Individual differences in the cognitive and neurobiological consequences of normal aging

Defining the neural basis of age-related cognitive dysfunction is a major goal of current research on aging. Compelling evidence from laboratory animals and humans indicates that aging does not inevitably lead to cognitive decline. Conducting neurobiological investigations in subjects that have previously undergone behavioral characterization has therefore emerged as a promising strategy for identifying those alterations in brain structure and function that are specifically associated with age-related cognitive impairment.

Hippocampal nerve growth factor levels are related to spatial learning ability in aged rats

Brain nerve growth factor (NGF) was determined in two groups of aged rats: 'good' and 'poor' performers. The animals were selected out of a population of 40 aged rats (26-28 months old) trained in a spatial learning task. Animals performing well in the test had significantly higher NGF in the hippocampus when compared to 'poor' performers. No differences in the levels of NGF were found in the cortex, septum and cerebellum. The results implicate hippocampal NGF in cognitive functioning of aged rats, and suggests that the forebrain cholinergic neuronal atrophy which has been observed in cognitively impaired aged rats may be due to reduced availability of target-derived NGF.

Prenatal protein malnutrition alters behavioral state modulation of inhibition and facilitation in the dentate gyrus

We have examined the effects of prenatal protein malnutrition on interneuronally mediated inhibition and facilitation in the dentate gyrus of the rat using the paired-pulse technique. Field potentials were recorded in the dentate gyrus in response to paired stimuli delivered to the perforant path. The paired-pulse index (PPI) was used as a measure of the net short-term facilitation or interneuronally mediated inhibition effective at the time of the paired-pulse test and was computed by dividing the amplitude of the second population spike (p2) by the amplitude of the first population spike (p1). PPIs were classified according to p1 in order to compare PPIs between behavioral states and dietary treatments since population spike amplitudes in the dentate gyrus vary in relation to behavioral state. Testing was performed during 4 behavioral states: slow-wave sleep (SWS), paradoxical sleep (REM), immobile waking (IW) and exploratory locomotion (AW) using interpulse intervals (IPI) from 20 to 400 ms. The magnitude and duration of interneuronally mediated inhibition was significantly increased in prenatal protein malnourished animals when compared with controls. Paired-pulse tests performed using an IPI of 20 ms under the high p1 (p1 greater than median) condition showed significantly smaller PPIs in prenatal protein malnourished rats regardless of behavioral state. For IPIs greater than 20 ms PPIs were consistently smaller in prenatal protein malnourished rats during SWS and IW. These data indicate that both the magnitude and duration of interneuronally mediated inhibition are increased in prenatally malnourished rats. No consistent diet-related differences were found during AW and REM using IPIs greater than 20 ms because interneuronally mediated inhibition was relatively suppressed during these behavioral states for both dietary groups. There was no consistent behavioral state modulation of paired-pulse facilitation (IPI = 40 to 80 ms) or late inhibition (IPI = 400 ms) in either diet group. In addition, a new relation between PPI and IPI was found under the low p1 (p1 greater than median) condition. During AW the PPIs observed using IPIs of 40 and 50 ms were smaller than those observed using IPIs of 30 and 60 ms. This depression interrupts what is generally considered the "facilitatory" phase of paired-pulse response and may indicate an interaction between perforant path stimulation and hippocampal theta rhythm which is masked when p1 amplitude is high.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium as sculptor and destroyer of neural circuitry

This paper examines the hypothesis that intracellular calcium plays guiding roles in the formation and adaptive modification of neural circuits in development and adult plasticity and that imbalances in calcium regulation lead to the degeneration of neural circuits in aging and disease. The neuronal growth cone is the motile structure largely responsible for the generation of neuroarchitecture. Studies of developing neurons in culture demonstrated that environmental signals believed to play key roles in neural development (i.e., neurotransmitters and growth factors) regulate growth cones by altering neuronal calcium-regulating systems. Different components of neurite outgrowth (i.e., neurite elongation and growth cone motility) are based upon different cytoskeletal systems (microtubules and microfilaments) which are differentially affected by calcium. In addition, cytoskeleton-associated proteins such as tau and microtubule-associated protein 2 (MAP2) are likely candidates for regulation by calcium. "Natural" neuronal death in development may occur as the result of growth factor deficiency or excess excitatory activity leading to sustained elevations in intracellular calcium levels. With aging and in disease, a loss of calcium homeostasis may underlie the aberrant neurodegeneration that occurs. For example, neurons subjected to conditions (e.g., glutamate and beta-amyloid) that cause sustained rises in intracellular calcium exhibit changes in the cytoskeleton similar to those seen in neurofibrillary tangles of Alzheimer's disease and related disorders. Taken together, the data suggest that cellular systems for calcium homeostasis are integral to both the adaptive and aberrant neuroarchitectural changes that occur throughout the lifespan of the nervous system.

Recognition memory deficits in a subpopulation of aged monkeys resemble the effects of medial temporal lobe damage

The present study examined individual differences in recognition memory function in a group of Old World monkeys (Macaca mulatta). Four young (9-11 years) and 10 aged (22-33 years) monkeys were tested in the same delayed-nonmatching-to-sample (DNMS) recognition memory procedure that has been widely used to study the effects of experimental hippocampal lesions in young subjects. Animals were first trained to a 90% correct learning criterion in the DNMS task using a 10-second delay between the sample and recognition phase of each trial. The memory demands of the task were then increased by gradually extending the retention interval from 15 seconds to 10 minutes. Three of the aged monkeys performed as accurately as young subjects at all delays. The remaining aged monkeys performed well at the shortest delays (15 and 30 seconds), but progressively greater impairments emerged across delays of 60 seconds, 2 minutes, and 10 minutes. These results suggest that recognition memory is only compromised in a subpopulation of aged monkeys. Moreover, aged monkeys that are impaired in the DNMS task exhibit the same delay-dependent pattern of deficits that is the hallmark of memory dysfunction resulting from medial temporal lobe damage.

Dietary restriction does not alter retinal aging in the Fischer 344 rat

In the present study, the effects of long-term dietary restriction (60% of the calories in the ad lib diet, beginning at 16 weeks of age) on quantitative morphometric measures and histopathologic indications of aging have been investigated in the retina of Fischer 344 male rats. The animals were maintained by the NIA Biomarkers Program, National Center for Toxicological Research. Group size ranged from 8 to 15 rats. A gradual thinning of the outer nuclear layer (ONL) of photoreceptor nuclei occurred with aging in control ad lib groups. The restricted diet did not affect retinal aging in 18-, 21-, 26-, or 27-month-old rats, as judged by photoreceptor cell death, ONL thickness, and pattern of cell loss. Retinal thickness (RT) was unaffected by restricted diet, except in the 21-month-old group; in that group, the RT was reduced significantly in thickness as compared to ad lib animals. These results are in contrast to studies of dietary restriction on most nonneuronal markers of aging and suggest that a different mechanism may modulate at least some aspects of brain aging.

Spatial performance correlates with in vitro potentiation in young and aged Fischer 344 rats

Young adult (2-4 months old) and aged (24-26 months old) Fischer 344 (F344) rats were trained for spatial behavior (locating a hidden escape platform) in a circular water maze. The aged rats showed deficits in both the acquisition and retention of the learned response. Following the behavioral training, hippocampal slices from the rats were prepared. Potentiation of CA1 extracellular, somatic field potentials was studied in vitro following either a short stimulus train (4 pulses) or a longer train (50 pulses). Slices from the aged rats showed less short-term potentiation (124.8 +/- 4.9% baseline, mean +/- S.E.M.) at 1 min following the short train in comparison to slices from the young rats (151.8 +/- 7.5%, P less than 0.05). However, following the longer train, no differences were found between the groups in the degree of either short-term (measured at 1 min after stimulation) or long-term potentiation (measured at 60 min). The amount of potentiation seen at various time points after either train correlated with the behavioral measure of retention. These results indicate that F344 rats exhibit age-related behavioral deficits, and age-related synaptic potentiation deficits in response to short stimulation trains. The correlation between the degree of potentiation (both short-term and long-term) and retention of a behavioral task adds strength to the hypothesis that potentiation mechanisms may underlie memory processes.

Functional elongation of CA1 hippocampal neurons with aging in Fischer 344 rats

Dendritic function of CA1 pyramidal cells was measured during intracellular recording in vitro and correlated with in vivo behavior in Fischer 344 rats. The aged rats (greater than 26 months) were significantly impaired on a water maze test of hippocampal behavioral function. CA1 neurons from these aged rats demonstrated an elevated action potential threshold compared to the young rats. Electrotonic length (L, in lambda), calculated independently from physiological transients and electrotonic cell reconstructions, was significantly longer in neurons from aged rats (L = 0.73 +/- 0.02 lambda; mean +/- SEM) than in neurons from young rats (L = 0.66 +/- 0.02 lambda). Analysis of proximal and distal synaptic potentials pointed to a more distal electrotonic siting of all dendritic synapses in the aged neurons. Thus, electrical lengthening of dendrites, alterations in synaptic location and decreased excitability in neurons from aged rats with behavioral impairment may represent an endpoint of neuronal reactive mechanisms in response to the aging process.