Hippocampal markers of age-related memory dysfunction: behavioral, electrophysiological and morphological perspectives

Cholinergic facilitation of trace eyeblink conditioning in aging rabbits

The hippocampus is importantly involved in learning and memory, and is severely impacted by aging. In in vitro hippocampal slices, both the post-burst afterhyperpolarization (AHP) and spike-frequency accommodation are reduced in hippocampal pyramidal neurons after hippocampally-dependent trace eyeblink conditioning, indications of increased cellular excitability. The AHP results from the activation of outward potassium currents, including sI(AHP) and muscarine-sensitive I(M). The AHP is significantly increased in aging hippocampal neurons, potentially contributing to age-associated learning deficits. Compounds which reduce the AHP and spike-frequency accommodation could facilitate learning in normal aging or in age-associated dementias such as Alzheimer's disease. The cholinesterase inhibitor metrifonate enhances trace eyeblink conditioning by aging rabbits and reduces the AHP and accommodation in hippocampal CA1 neurons in a dose-dependent manner. These reductions are mediated by muscarinic cholinergic transmission as they are blocked by atropine. Hippocampal neurons from metrifonate treated but behaviorally naive rabbits were more excitable and not desensitized to the effects of metrifonate since the AHP and accommodation were further reduced when metrifonate was bath applied to the neurons. These observations suggest that the facilitating effect of chronic metrifonate on acquisition of hippocampally dependent tasks is mediated at least partially by increasing the baseline excitability of CA1 pyramidal neurons. The issue of whether learning can be facilitated with muscarinic cholinergic agonists, in addition to cholinesterase inhibitors, was addressed by training aging rabbits during intravenous treatment with the M1 agonist CI1017. A dose-dependent enhancement of acquisition was observed, with rabbits receiving 1.0 or 5.0 mg/ml CI1017 showing comparably improved learning rates as those receiving 0.5 mg/ml or vehicle. Sympathetic side effects, mainly excess salivation, were seen with the 5.0 mg/ml dose. Post-training evaluations suggested that the effective doses of CI1017 were enhancing responsivity to the tone conditioned stimulus. These studies suggest that muscarinic cholinergic neurotransmission is importantly involved in associative learning; that learning in aging animals may be facilitated by enhancing cholinergic transmission; and that the facilitation may be mediated through actions on hippocampal neurons.

Norepinephrine promotes long-term potentiation in the adult rat hippocampus in vitro

We previously found a reduction in the ability of a single 100 Hz x 1 sec tetanus to induce long-term potentiation (LTP) in the CA1 region of hippocampal slices prepared from adult animals. To determine whether this reduction in LTP generation results from changes in neuromodulator function, we examined the ability of several neuromodulators to promote LTP in slices prepared from mature rats. Although acetylcholine, N-methyl-D-aspartate, and an agonist at metabotropic glutamate receptors failed to promote LTP, administration of norepinephrine allowed robust LTP. The effects of norepinephrine were mimicked by an alpha1-adrenergic agonist and were blocked by an alpha1-receptor antagonist. Beta-adrenergic agonists and antagonists were ineffective. These results suggest that norepinephrine acting via alpha1-adrenoceptors may be an important cofactor in promoting lasting synaptic plasticity in the adult central nervous system and that changes in adrenergic function may contribute to maturation- or aging-associated changes in memory function.

The mosaic of brain glial hyperactivity during normal ageing and its attenuation by food restriction

Food restriction of adult rodents increases lifespan, with commensurate attenuation of age-related pathological lesions in many organs, as well as attenuation of normal ageing changes that are distinct from gross lesions. Previous work showed that chronic food restriction attenuated age-associated astrocyte and microglial hyperactivity in the hippocampal hilus, as measured by expression of glial fibrillary acidic protein and major histocompatibility complex II antigen (OX6). Here, we examined other markers of astrocyte and microglial activation in gray and white matter regions of ad libitum-fed (Brown Norway x Fischer 344) F1 male rats aged three and 24 months and chronic food-restricted rats aged 24 months. In situ hybridization and immunohistochemical techniques evaluated glial expression of glial fibrillary acidic protein, apolipoprotein E, apolipoprotein J (clusterin), heme oxygenase-1, complement 3 receptor (OX42), OX6 and transforming growth factor-beta1. All markers were elevated in the corpus callosum during ageing and were attenuated by food restriction, but other regions showed marked dissociation of the extent and direction of changes. Astrocytic activation, as measured with glial fibrillary acidic protein expression (coding and intron-containing RNA, immunoreactivity), increased with age in the corpus callosum, basal ganglia and hippocampus. Generally, food restriction attenuated the age-related increase in glial fibrillary acidic protein messenger RNA and immunoreactivity. Food restriction also reduced the age-related increase in apolipoprotein J and E messenger RNA and heme oxygenase-1 immunoreactivity in the basal ganglia and corpus callosum. However, astrocytes in the hilus of the hippocampus showed an age-related decrease in apolipoprotein J and E messenger RNA, which was further intensified by food restriction. The age-associated microglial activation measured by OX6 and OX42 immunoreactivity was reduced by food restriction in most subregions. The localized subsets of glial age changes and effects of food restriction comprise a mosaic of ageing consistent with the regional heterogeneity of ageing changes reported by others. In particular, age has a differential effect on astrocytic and microglial hyperactivity in gray versus white matter areas. The evident mosaic of glial ageing and responses to food restriction suggests that multiple mechanisms are at work during ageing.

MK-801 improves retention in aged rats: implications for altered neural plasticity in age-related memory deficits

Alterations in N-methyl-d-aspartate receptor (NMDAR)-dependent synaptic plasticity, characteristic of aged rodents, may contribute to impaired memory with advanced age. The purpose of the current research was to examine whether NMDARs contribute to rapid forgetting on a spatial memory task. Aged (22-24 months) and adult (3-6 months) male Fischer 344 rats received 18 training trials, over a period of 3 to 4 h, on the spatial version of the Morris water maze. Immediately after training, a standard free-swim probe trial was administered to assess the acquisition of spatial bias, which was determined by the percent of time spent in the goal quadrant and the number of platform crossings. Rats then received injections of the noncompetitive NMDAR antagonist, (+)-10, 11-dihydro-5methyl-5H-dibenzo(a,b)cycloheptene-5,10 imine (MK-801, 0. 05 mg/kg, i.p.), or a vehicle injection of equal volume. Approximately 24 h later, rats were administered a second free-swim probe trial to assess retention of spatial bias. All age/drug groups exhibited a spatial bias on the acquisition probe, with adults generally outperforming the aged rats. On the retention probe, this spatial bias continued to be shown by adult rats, regardless of treatment. For the aged group, in contrast, only MK-801-injected rats maintained a spatial bias on the retention probe, suggesting that NMDAR activity may be involved in rapid forgetting during aging. Because blockade of NMDARs also may impair new learning, which may, in turn, protect previously stored information from retroactive interference, rats in a second experiment received post-training injections of scopolamine (0.05 mg/kg), a compound known to inhibit learning. However, scopolamine did not enhance retention in the aged group, consistent with the hypothesis that MK-801 influenced memory in aged rats through its actions on NMDAR-dependent synaptic plasticity.

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.

Hippocampal neuron and synaptophysin-positive bouton number in aging C57BL/6 mice

A loss of hippocampal neurons and synapses had been considered a hallmark of normal aging and, furthermore, to be a substrate of age-related learning and memory deficits. Recent stereological studies in humans have shown that only a relatively minor neuron loss occurs with aging and that this loss is restricted to specific brain regions, including hippocampal subregions. Here, we investigate these age-related changes in C57BL/6J mice, one of the most commonly used laboratory mouse strains. Twenty-five mice (groups at 2, 14, and 28-31 months of age) were assessed for Morris water-maze performance, and modern stereological techniques were used to estimate total neuron and synaptophysin-positive bouton number in hippocampal subregions at the light microscopic level. Results revealed that performance in the water maze was largely maintained with aging. No age-related decline was observed in number of dentate gyrus granule cells or CA1 pyramidal cells. In addition, no age-related change in number of synaptophysin-positive boutons was observed in the molecular layer of the dentate gyrus or CA1 region of hippocampus. We observed a significant correlation between dentate gyrus synaptophysin-positive bouton number and water-maze performance. These results demonstrate that C57BL/6J mice do not exhibit major age-related deficits in spatial learning or hippocampal structure, providing a baseline for further study of mouse brain aging.

Environmental influences on cognitive decline in aged rats

Two experiments are reported in which young and old rats, housed in an impoverished (IE), enriched (EE), or standard (SE), environment, were tested on a series of complex, blind-alley mazes. In Experiment 1, 3-months exposure to IE exacerbated age differences in maze performance, relative to the differences between young and old rats in EE and SE. Old rats in the EE and SE conditions did not differ from each other. In Experiment 2, rats were raised for an additional 3 months in either IE or EE before further maze testing. The main findings were that the maze performance of old rats, transferred from IE to EE, improved significantly, whereas the performance of old rats, transferred from SE or EE to IE, declined. These results indicated that the deleterious effects of an impoverished environment on learning and memory are, at least partly, reversible, and that experience in a stimulating environment can protect old rats from the adverse effects of relocation to a deprived environment. Taken together, the results highlight the impact of environmental influences on cognitive function in old age, and emphasize the need to consider nonbiological factors in understanding the process of cognitive aging.

Effects of nerve growth factor on brain glutathione-related enzymes from aged rats

Neurotrophins, like the nerve growth factor (NGF), trigger a variety of biological effects in their targets. Stimulating effects on antioxidant defenses have been postulated to underlie neurotrophic influence on neuron survival and maintenance. To test whether NGF is capable of inducing changes in glutathione-related enzymes in the aged cognitively impaired brain, glutathione reductase (GRD), glutathione S-transferase (GST) and total glutathione peroxidase (GPX) activities were measured in the striatum, septum, hippocampus and frontal cortex of four Sprague-Dawley rat groups: young (2 months old), aged (20 months old) untreated, aged cytochrome c-treated, and aged NGF-treated (icv delivery, 34 micrograms during 28 days). All the aged rats utilized in the study were memory impaired according to their performance in the Morris water maze test. These aged rats showed increases in the activities of septal and hippocampal GST, as well as, in the hippocampal, striatal and cortical GPX. These increases could be interpreted as compensatory responses to cope with the oxidative damage that has been accumulated by the aged brain. The increases in hippocampal and cortical GPX activity were attenuated by NGF treatment, whereas the neurotrophin induced an increase in GRD activity in the striatum of aged rats. These results point out GRD and GPX as possible targets of the neurotrophic effects.

Long-term treatment with calcitriol (1,25(OH)2 vit D3) retards a biomarker of hippocampal aging in rats

Based on a literature implicating altered calcium homeostasis in brain aging and Alzheimer's Disease (AD) and evidence of decreased vitamin D action in AD subjects, the possibility was tested that calcitriol (1,25(OH)2 vitamin D3), the active form of vitamin D3, might reduce markers of brain aging in rats. Animals were treated 5x weekly for prolonged periods (6-12 months) with either calcitriol in doses sufficient to elevate serum calcium and phosphate (20 ng/rat), calcitonin (1.5 IU/rat) or vehicle, in three separate long-term experiments on aging rats. New stereological methods (physical disector) of cell counting were used to evaluate neuronal density, a reliable biomarker of hippocampal aging in rats. In two experiments utilizing Brown-Norway x F344 hybrid rats (BN x F344), 8 months and 12 months of chronic treatment with calcitriol resulted in a higher density of CA1 neurons in the middle regions of the hippocampus, compared to vehicle or calcitonin treatment. However, one study with aging F344 rats was terminated early because of extensive strain-specific pathology and no effect of calcitriol on neuronal density was observed. These studies suggest that, under some conditions, hormonal treatments that regulate calcium homeostasis can modulate markers of brain aging.

Glucose treatment attenuates spatial learning and memory deficits of aged rats on tests of hippocampal function

Groups of old and young rats were administered three tests of spatial learning and memory that are known to be sensitive to hippocampal dysfunction: the radial arm maze (RAM), spatial non-matching-to-sample (SNMTS), and a spatial vs. local cue-preference task. Old rats performed worse than young rats on the RAM and SNMTS tasks; on the cue-preference task, young rats were biased to use spatial cues, whereas old rats exhibited strong preferences for distinct, local cues. Peripheral injections of glucose (100 mg/kg) improved performance by old rats on the RAM and SNMTS, which correlated with measures of glucose metabolism. Glucose treatment did not affect old rats performance on the cue-preference task. There was evidence that glucose-treatment improved performance of young rats in the RAM test, but not the other tests. The results extend the range of tasks on which glucose-induced cognitive enhancement has been demonstrated in aged rats, and provides further evidence that memory loss resulting from hippocampal dysfunction is especially amenable to glucose treatment.

Nerve growth factor improves evoked potentials and long-term potentiation in the dentate gyrus of presenile rats

Chronic infusion of nerve growth factor (NGF, 1.2 microg/day) for 14 days to presenile rats (17 months at the beginning of treatment) that showed an initial cognitive impairment led to an improved long-term potentiation in the dentate gyrus. Both the relative increase of the slope of the population excitatory postsynaptic potential and that of the population spike were enhanced by NGF pretreatment after long-term potentiation induction at 400 Hz. The treatment was also able to increase the diminished baseline amplitude of the population spike, an effect not seen when the treatment was applied to older animals [Bergado, J., Fernández, C.I., Gómez-Soria, A., González, O., 1997a. Chronic intraventricular infusion with NGF improves LTP in old cognitively-impaired rats. Brain Res. 770, 1-9] stressing the importance of an early start of trophic therapy to achieve better results.

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.

Age-related activation of microglia and astrocytes: in vitro studies show persistent phenotypes of aging, increased proliferation, and resistance to down-regulation

Astrocytes and microglia from cerebral cortex of 3-, 6-, 12-, and 24-month-old F344 male rat donors showed progressively greater proliferation during primary culture. Microglia from aging donor brains exhibited an amoeboid-like morphology and express antigens characteristic of an activated state (e.g., major histocompatibility complex class II). Moreover, microglia from aging donors were less sensitive to several types of regulators. Granulocyte-macrophage colony stimulating factor stimulated proliferation in microglia from young, but not aging brains. Transforming growth factor (TGF)-beta1 inhibited astrocytic and microglial proliferation in cultures from young, but not aging donors. Similarly, the inhibition of lipopolysaccharide-induced NO production by TGF-beta1 in microglia was impaired in cultures from 12-month (middle-age) brains. Another aging change detected by middle age, increased glial fibrillary acidic protein (GFAP) expression, also persisted in astrocytes from 12- to 24-month-old brains, as evaluated by increased activity of a 5'-upstream GFAP promoter construct. Thus, both microglia and astrocytes originated from aging cerebral cortex maintain in vitro at least some of the activated phenotypes of aging glia that are observed in vivo. This new in vitro cell model may allow efficient analysis of glial age changes.

Calcium and neuronal ageing

Brain ageing is associated with a marked decline in mental faculties. One hypothesis postulates that sustained changes in the regulation of intracellular Ca2+ concentration, [Ca2+]i, are the major cause of neuronal degeneration. This 'calcium hypothesis' is supported by demonstrations of the impairment in aged neurones of molecular cascades that regulate [Ca2+]i. However, the number of direct measurements of [Ca2+]i in senescent neurones is limited, and the hypothesis cannot be regarded as fully confirmed. Furthermore, physiological brain ageing, at least in certain regions, need not necessarily be a degenerative process accompanied by neuronal loss. Pharmacological manipulation of Ca2+ entry has been shown to be effective in increasing some aspects of cognitive function of the aged brain. Therefore, further exploration of Ca2+ homeostasis and signalling might reveal the mechanisms involved in the age-dependent decline in neuronal performance, and might aid the search for new therapeutic treatments.

Glial and neuronal cell numbers and cytochrome oxidase activity in CA1 and CA3 during postnatal development and aging of the rat

Stereological methods (neuron and glial cell numbers) and histochemical methods (cytochrome c oxidase) were used to study postnatal development and aging of the CA1 and CA3 hippocampal areas in male rats. No changes were observed in 10 microns sections in the neuronal population of areas CA1 and CA3 in any of the groups (14 days, 21 days, adult-90 days and elderly-22 months). Statistical differences were found in the number of glial cells in both the CA1 and CA3 areas. An increase was observed in cytochrome oxidase (CO) activity in the CA1 area in the 14 day old rats compared to the other groups while in area CA3 this parameter increased in the 14 and 21 day old groups and the group of adult rats. No significant changes in CO activity were found in the elderly rats in both areas. These results are discussed in the light of those recorded in other areas of the limbic system.

Is activation of the metabotropic glutamate receptors impaired in the hippocampal CA1 area of the aged rat?

The effects of aging on activation of metabotropic glutamate (mGlu) receptors were studied in the CA1 field of hippocampal slices from young (3- to 4-month-old) and aged (24- to 27-month-old) Sprague-Dawley rats with the use of ex vivo electrophysiological recording techniques. The depolarization of membrane potential, the increase in input resistance, and the blockade of the afterhyperpolarization induced in pyramidal cells of young rats by bath application of the mGlu receptor agonist (+/-)-trans-1-aminocyclopentate-1,3-dicarboxylic acid were not altered in aged animals. No age-related changes of the depressive effects of the mGlu receptor agonist were found on either the excitatory glutamatergic postsynaptic potential or the GABA-mediated inhibitory postsynaptic potentials induced by the stimulation of the stratum radiatum. The magnitude of synaptic plasticity involving mGlu receptor activation, although weaker, was not significantly altered in aged rats. This absence of age-related effects on activation of mGlu receptors may be important in understanding the possible origins of the alterations in neuronal plasticity which occur in brain aging.

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

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Life and death of neurons in the aging brain

Neurodegenerative disorders are characterized by extensive neuron death that leads to functional decline, but the neurobiological correlates of functional decline in normal aging are less well defined. For decades, it has been a commonly held notion that widespread neuron death in the neocortex and hippocampus is an inevitable concomitant of brain aging, but recent quantitative studies suggest that neuron death is restricted in normal aging and unlikely to account for age-related impairment of neocortical and hippocampal functions. In this article, the qualitative and quantitative differences between aging and Alzheimer's disease with respect to neuron loss are discussed, and age-related changes in functional and biochemical attributes of hippocampal circuits that might mediate functional decline in the absence of neuron death are explored. When these data are viewed comprehensively, it appears that the primary neurobiological substrates for functional impairment in aging differ in important ways from those in neurodegenerative disorders such as Alzheimer's disease.

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

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

NOR activity in hippocampal areas during the postnatal development and ageing

The silver staining of the nucleolar organizer regions (Ag-NORs) was used in order to estimate the biosynthetic activity of three hippocampal areas (dentate gyrus, CA1 and CA3) during postnatal development and ageing. 32 Wistar rats were used and 4 groups were formed according to the age of the animals (14, 21, 90 days and 23 months). Several Ag-NOR parameters such as mean Ag-NOR area and the ratio between Ag-NOR and nuclear areas per neuronal cell were quantified using an image analysis system. High values of these parameters are associated with a high rate of rRNA transcription. In this way, the neural biosynthetic activity in all regions studied decreased as the older ages are reached. Differences between areas are shown with the dentate gyrus and CA1 areas decreasing faster. The different activity among these areas is discussed, taking into account the particular affect on these areas of some injuries and the ageing process. Our results support the hypothesis of NOR loss as a main cause of ageing as reported by other authors.

Calcium channel density and hippocampal cell death with age in long-term culture

The expression of voltage-gated calcium (Ca2+) channel activity in brain cells is known to be important for several aspects of neuronal development. In addition, excessive Ca2+ influx has been linked clearly to neurotoxicity both in vivo and in vitro; however, the temporal relationship between the development of Ca2+ channel activity and neuronal survival is not understood. Over a period spanning 28 d in vitro, progressive increases in high voltage-activated whole-cell Ca2+ current and L-type Ca2+ channel activity were observed in cultured hippocampal neurons. On the basis of single-channel analyses, these increases seem to arise in part from a greater density of functionally available L-type Ca2+ channels. An increase in mRNA for the alpha1 subunit of L-type Ca2+ channels occurred over a similar time course, which suggests that a change in gene expression may underlie the increased channel density. Parallel studies showed that hippocampal neuronal survival over 28 d was inversely related to increasing Ca2+ current density. Chronic treatment of hippocampal neurons with the L-type Ca2+ channel antagonist nimodipine significantly enhanced survival. Together, these results suggest that age-dependent increases in the density of Ca2+ channels might contribute significantly to declining viability of hippocampal neurons. The results also are analogous to patterns seen in neurons of aged animals and therefore raise the possibility that long-term primary neuronal culture could serve as a model for some aspects of aging changes in hippocampal Ca2+ channel function.

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.

Murine models of brain aging and age-related neurodegenerative diseases

In the past, structural changes in the brain with aging have been studied using a variety of animal models, with rats and nonhuman primates being the most popular. With the rapid evolution of mouse genetics, murine models have gained increased attention in the neurobiology of aging. The genetic contribution of age-related traits as well as specific mechanistic hypotheses underlying brain aging and age-related neurodegenerative diseases can now be assessed by using genetically-selected and genetically-manipulated mice. Against this background of increased demand for aging research in mouse models, relatively few studies have examined structural alterations with aging in the normal mouse brain, and the data available are almost exclusively restricted to the C57BL/6 strain. Moreover, many older studies have used quantitative techniques which today can be questioned regarding their accuracy. Here we review the state of knowledge about structural changes with aging in outbred, inbred, genetically-selected, and genetically-engineered murine models. Moreover, we suggest several new opportunities that are emerging to study brain aging and age-related neurodegenerative diseases using genetically-defined mouse models. By reviewing the literature, it has become clear to us that in light of the rapid progress in genetically-engineered and selected mouse models for brain aging and age-related neurodegenerative diseases, there is a great and urgent need to study and define morphological changes in the aging brain of normal inbred mice and to analyze the structural changes in genetically-engineered mice more carefully and completely than accomplished to date. Such investigations will broaden knowledge in the neurobiology of aging, particularly regarding the genetics of aging, and possibly identify the most useful murine models.

Metrifonate treatment enhances acquisition of eyeblink conditioning in aging rabbits

The cholinergic system is known to show deterioration during aging and Alzheimer's disease. In response, a therapeutic approach to Alzheimer's disease has been to attempt to compensate for the decrease in central cholinergic function by potentiating the activity of the remaining intact cholinergic cells with cholinesterase inhibitors. In this study treatment with the long-lasting cholinesterase inhibitor metrifonate enhanced acquisition of eyeblink conditioning in aging rabbits without producing interfering side effects. The effects of metrifonate on central and peripheral cholinesterase activity were evaluated, as was the involvement of plasma atropine esterase activity on the central and peripheral response to metrifonate. Results demonstrate that metrifonate can produce predictable, dose-dependent ChE inhibition. Associative learning in the aging rabbit was improved by metrifonate-induced steady state ChE inhibition within a range of 30-80%. Metrifonate was behaviorally effective in the absence of the severe side effects which typically plague cholinesterase inhibitors, suggesting that metrifonate is a possible treatment for the cognitive deficits resulting from normal aging and Alzheimer's disease.

Aging changes in voltage-gated calcium currents in hippocampal CA1 neurons

Previous current-clamp studies in rat hippocampal slice CA1 neurons have found aging-related increases in long-lasting calcium (Ca)-dependent and Ca-mediated potentials. These changes could reflect an increase in Ca influx through voltage-gated Ca channels but also could reflect a change in potassium currents. Moreover, if altered Ca influx is involved, it is nuclear whether it arises from generally increased Ca channel activity, lower threshold, or reduced inactivation. To analyze the basis for altered Ca potentials, whole-cell voltage-clamp studies of CA1 hippocampal neurons were performed in nondissociated hippocampal slices of adult (3- to 5-month-old) and aged (25- to 26-month-old) rats. An aging-related increase was found in high-threshold Ca and barium (Ba) currents, particularly in the less variable, slowly inactivating (late) current at the end of a depolarization step. Input resistance of neurons did not differ between age groups. In steady-state inactivation and repetitive-pulse protocols, inactivation of Ca and Ba currents was not reduced and, in some cases, was slightly greater in aged neurons, apparently because of larger inward current. The current blocked by nimodipine was greater in aged neurons, indicating that some of the aging increase was in L-type currents. These results indicate that whole-cell Ca currents are increased with aging in CA1 neurons, apparently attributable to greater channel activity rather than to reduced inactivation. The elevated Ca influx seems likely to play a role in impaired function and enhanced susceptibility to neurotoxic influences.

Unbiased stereological estimation of the total number of synapses in a brain region

Modern stereological methods have been used to make unbiased estimates of the total number of synapses in the striatum radiatum of the hippocampal CA1 region of five rabbits. The approach used involved a two stage analysis and is generally applicable to all parts of the nervous system. During the first stage of the analysis, the reference volume was estimated by point counting, at the light microscope level, according to the Cavalieri principle. During the second stage, the numerical density of synapses was estimated with dissectors at the electron microscopic level. The total number of synapses was calculated as the product of the numerical density and the volume of the region. The sampling with points and dissectors was carried out in all three dimensions of the entire CA1 region in a manner that ensured that all parts of the region and all synapses within it had equal probabilities of being sampled. An analysis of the precision of the estimate of total synapse number has been performed in terms of the variances of volume and synaptic numerical density at different levels of sampling, i.e. at the level of points, sections, individual animals and group of animals. Detailed descriptions of the procedures used to estimate the total number of synapses, evaluate the precision of the estimates, and optimize the sampling scheme are provided.

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.

Trace eyeblink conditioning in rabbits demonstrates heterogeneity of learning ability both between and within age groups

Rabbits 2 to 41 months of age were conditioned in the 500 ms trace eyeblink paradigm to cross-sectionally define the age of onset and the severity of age-associated impairments in acquisition of this relatively difficult hippocampally dependent task. Using a strict behavioral criterion of 80% conditioned responses (CRs), age-associated learning impairments were significant by 24 months of age. Among rabbits that successfully reached this criterion, impairments in acquisition plateaued at 30 months of age. However, the proportion of severely impaired rabbits (that failed to reach the 80% criterion) continued to increase age dependently. Using an easier criterion of 8 out of 10 CRs, behavioral impairments were not detected until 30 months of age, and cases of severe impairment (failure to reach criterion) were rare. Additional controls demonstrated that the deficits observed were not attributable to nonassociative changes that might have artifactually skewed the data. Even severely impaired 36-month-old rabbits were able to reach a criterion of 80% CRs when switched from a trace to a delay conditioning task that is not hippocampally dependent. The results are discussed in terms of operationally defining and predicting behavioral effects of aging, hypothetical neural mechanisms, and efficient experimental design.