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

Norepinephrine and isoproterenol increase the phosphorylation of synapsin I and synapsin II in dentate slices of young but not aged Fisher 344 rats

A number of recent reports have suggested that norepinephrine (NE) produces a form of synaptic enhancement that resembles long-term potentiation (LTP). LTP, thought to be an electrophysiological correlate of memory, in part involves an augmentation of transmitter release. Although the effects of NE have not been unequivocally linked to LTP, it is clear that NE can produce increased transmitter release in the dentate gyrus of the hippocampus. The purpose of this study was to determine whether NE was capable of enhancing the phosphorylation of synapsin I and synapsin II, two homologous phosphoproteins thought to be involved in modulation of neurotransmitter release. NE (10 microM) and isoproterenol (250 nM) produced an increase in the phosphorylation of synapsin I and synapsin II in dentate slices from young rats. Phosphorylation site analysis of synapsin I, performed by limited proteolysis, indicated that NE and isoproterenol increased the phosphorylation of synapsin I at sites modified by Ca2+/calmodulin-dependent protein kinase II as well as cAMP-dependent protein kinase. These data demonstrate that NE stimulates the phosphorylation of synapsin I at its Ca2+/calmodulin-dependent protein kinase II site, which is a site that has been shown to regulate the effect of synapsin I on neurotransmitter release. We have also examined the effects of NE and isoproterenol on synapsin phosphorylation in dentate slices prepared from aged animals. Such animals have previously been shown to exhibit deficits in NE sensitivity as well as significant impairment in their ability to exhibit LTP. Neither NE nor isoproterenol stimulated synapsin phosphorylation in slices prepared from aged animals. Interestingly, the basal level of phosphorylation of the synapsin proteins was higher in slices prepared from aged animals. This higher basal level of phosphorylation may underlie the failure of aged animals to exhibit NE-stimulated increases in phosphorylation of the synapsin proteins. We hypothesize that the beta-adrenergic agonist-stimulated phosphorylation of synapsin I and synapsin II in young rats plays a role in the increase in transmitter release produced by NE in the dentate. Thus, the failure of the aged rats to show such phosphorylation may underlie, in part, their failure to exhibit normal responsiveness to NE. Moreover, these deficits in synapsin phosphorylation may also play some role in the deficits in plasticity seen in aged rats.

Age-related changes in the synaptic plasticity of rat superior cervical ganglia

A few seconds of tetanic preganglionic stimulation of rat sympathetic ganglia results in potentiation of cholinergic synaptic transmission lasting several hours. However, ganglia from aged (28-32 months) rats did not develop as much potentiation as did ganglia from young (3-6 months) rats. This potentiation appears to decay exponentially but in two phases. The early component decays in 15 minutes, a time course consistent with the phenomenon of post-tetanic potentiation (PTP). The later component decays over several hours and is a form of long-term potentiation (LTP). A double exponential decay model was employed to quantitatively resolve the decay of potentiation and allow quantitative comparison of the decay parameters in both aged and young rats. It is clear that the magnitude and duration of PTP was the same in both age groups. However both the magnitude and decay time-constant for LTP were 30-50% smaller in the aged group of rats. Several agents which mimic cyclic AMP or stimulate cyclic AMP production in the ganglion enhance nicotinic transmission for several hours. However, these agents, 8-bromo cAMP, forskolin, isoproterenol, and secretin were equally effective in potentiating transmission in ganglia from both young and aged rats. Previous studies demonstrated that stimulus induced LTP and cyclic AMP induced potentiation was the result of enhanced release of acetylcholine (ACh) and not from increased postsynaptic responsiveness. Presumably these agents act through the same pathway and by the same mechanism that generates LTP following preganglionic tetany. These observations demonstrate that there is a selective age-dependent decline in the capacity for sympathetic ganglia to generate long-term changes in synaptic efficacy.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Binding activities of central excitatory amino acid receptors were examined in Triton-treated membrane preparations of the cerebral cortex and hippocampus from brains of rats at 2, 7 and 29 months after birth. Aged rats exhibited a significant reduction of [3H]glutamate (Glu) binding displaceable by N-methyl-D-aspartate (NMDA), as well as strychnine-insensitive [3H]glycine binding in both central structures, as compared with those in young rats. Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate (MK-801), a non-competitive NMDA antagonist used to label the activated state of ion channels linked to NMDA-sensitive receptors, also decreased with aging irrespective of the experimental conditions employed. Scatchard analysis revealed that reduction of both [3H]Glu binding and [3H]MK-801 binding were due to a significant decrease in the densities of binding sites with aging, with their affinities being unaltered. Binding of [3H]D,L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which is a specific agonist for quisqualate-sensitive receptors, was unchanged with aging when determined in the absence of 100 mM potassium thiocyanate (KSCN). However, AMPA binding determined in the presence of added KSCN was about 25% reduced in both brain regions of aged rats. Binding of [3H]kainate to kainate-sensitive receptors was unchanged with aging. These results suggest that glutaminergic neurotransmission mediated by NMDA-sensitive receptors may be selectively impaired with aging in the hippocampus and cerebral cortex among 3 different subclasses of excitatory amino acid receptors in the brain.

Altered synaptic plasticity in Drosophila memory mutants with a defective cyclic AMP cascade

Synaptic transmission was examined in Drosophila mutants deficient in memory function. These mutants, dunce and rutabaga, are defective in different steps of the cyclic adenosine 3',5'-monophosphate (cAMP) cascade. In both dunce and rutabaga larvae, voltage-clamp analysis of neuromuscular transmission revealed impaired synaptic facilitation and post-tetanic potentiation as well as abnormal responses to direct application of dibutyryl cAMP. In addition, the calcium dependence of transmitter release was shifted in dunce. The results suggest that the cAMP cascade plays a role in synaptic facilitation and potentiation and indicate that synaptic plasticity is altered in Drosophila memory mutants.

The aging brain: protein phosphorylation as a target of changes in neuronal function

There is evidence that senescence affects neurotransmission at different levels. In particular, this review summarizes the studies on age-dependent modifications in protein phosphorylation, which represents the final pathway in the action of transmitters and hormones at neuronal level. Cyclic AMP-dependent protein kinase and protein kinase C have been reported to be modified during aging in various cerebral areas; the changes may involve either enzyme activity or substrate availability. These findings can be related to the alterations in neurotransmitter function and synaptic efficiency observed in the senescent brain. The activity of the other types of protein kinases (tyrosine-, cGMP-, calcium/calmodulin-dependent) during aging needs to be explored. An emerging point is the role of protein phosphorylation in the transfer of membrane signals to the nucleus, for the activation or disactivation of specific genes responsible for long-term neuronal events. Along this view, alterations in protein kinase pathway during senescence would ultimately affect gene expression, resulting in long term modifications of cell function. The reviewed literature opens the perspective of restoring some of the deficits associated with senescence by modulating protein phosphorylation pathway.

A 1-heteroaryl-4-piperidinyl-methyl pyrrolidinone, BMY 21502, delays the decay of hippocampal synaptic potentiation in vitro

The effects of the substituted pyrrolidinone, BMY 21502, on the properties of cell membranes, synaptic transmission and synaptic plasticity, were assessed in area CA1 of hippocampal slices from the rat. Application of the compound to the bath had no consistent direct effects on parameters of the cell membrane or evoked synaptic potentials, at concentrations of less than 30 microM. In a blind experimental design, BMY 21502 at 1.0 and 10 microM, but not 25 microM, significantly delayed the decay of long-term potentiation in slices obtained from young animals; in slices obtained from very old rats (2.5-3.2 yr), 10 microM BMY 21502 significantly delayed decay of long-term potentiation. Therefore BMY 21502 was active in a physiological model that may predict of cognitive enhancement.

Regulation of phorbol ester binding and protein kinase C activity in aged rat brain

Protein kinase C (PKC) function was analyzed in aged male Sprague-Dawley rat brain using two different approaches: the binding of [3H]-phorbol-12,13-dibutyrate and the in vitro phosphorylation of histone H1. In cortex the binding was decreased while in cerebellum no age-related modifications were observed. In hippocampus the binding capacity was increased in old animals and the affinity decreased. The kinase activity in both soluble and particulate fractions was decreased in cortex, increased in hippocampus and unmodified in cerebellum. The area selective, age-dependent modifications in neuronal PKC may sustain short- and long-term regional changes of neuronal excitability.

Markers for biogenic amines in the aged rat brain: relationship to decline in spatial learning ability

The major goal of the study was to evaluate the relationship of brain aging to individual differences in functional decline in rats. Forebrain choline-acetyltransferase (ChAT) and monoamines, including their metabolites, were examined in young and aged male Long-Evans rats in relation to their spatial learning ability. Aged rats that were unimpaired on a spatial learning task exhibited few changes in neurochemistry relative to the young group: each change in this subgroup was also evident in the remaining aged animals that were behaviorally impaired. Additional changes in neurochemical measures only found in the behaviorally impaired aged animals included decreased ChAT in the basal forebrain, striatum, and frontal cortex. A cluster analysis using the 15 neurochemical measures that were sensitive to aging yielded groupings of aged animals that differed with respect to their spatial learning ability, but not in their cue learning latencies. In this analysis the activity of ChAT in the basal forebrain and striatum appeared to be the best predictors of spatial learning impairment.

Aging-related prolongation of calcium spike duration in rat hippocampal slice neurons

Calcium (Ca) spike potentials were investigated in cesium-loaded, tetrodotoxin (TTX)-treated CA1 pyramidal cells in hippocampal slices from young-mature and aged rats. The duration of single Ca spike potentials was prolonged in cells from aged rats, indicating that previously observed age-related changes in Ca-dependent mechanisms (e.g. in the K-mediated afterhyperpolarization and in frequency potentiation) may result from an age-related increase of voltage-dependent Ca conductance. Since we recently found that Ca spike duration in hippocampus can be modulated strongly by a form of Ca-dependent inactivation of Ca current, spike inactivation paradigms also were examined. However, following 5- or 10-s-long depolarizing pulses, or during a 2-Hz train of elicited Ca spikes, there were no age differences in percent inactivation. These results do not support (but do not fully rule out) the possibility that impaired Ca-dependent inactivation underlies the increase in the Ca spike with aging. Conceivably, this prolongation of voltage-dependent Ca influx could have implications for our understanding of normal and abnormal brain aging.

Effects of aging on the dynamics of information processing and synaptic weight changes in the mammalian hippocampus

It is clear that the properties of LTE make it a plausible mechanism for associative information storage at some synapses in the central nervous system. While many of the factors that regulate LTE's induction and expression have been discovered and a strong case is being developed for its role in learning and memory processes, until we understand more clearly the mechanisms underlying both the expression and maintenance of LTE, an understanding of its change with age will be difficult. Judging by the progress that has been made over the past several years in uncovering some of the molecular events that are critical for LTE's expression, one may be optimistic that answers will be forthcoming reasonably soon. Of particular importance to aging mammals, such answers may provide insights into why older organisms show faster forgetting. This may have a profound impact on therapeutic strategies for memory disorders in both normal and pathological conditions of aging.

Synaptic aging as revealed by changes in membrane potential and decreased activity of Na+,K(+)-ATPase

Age-related changes in the membrane potential of nerve terminals were investigated by monitoring the accumulation of tritium-labeled triphenylmethylphosphonium ion, [3H]TPMP+, in mouse cortical synaptosomes. The resting membrane potential became less negative with advancing age, that is, it changed from -64.5 +/- 0.8 to -58.1 +/- 2.3 mV between 6 and 27 months of age. The intrasynaptosomal potassium concentration was found to decrease concomitantly by 13% in aged mice (56.6 +/- 0.9 mM) as compared to young-adult mice (64.9 +/- 0.5 mM). The ouabain-sensitive Na+,K(+)-ATPase activity of synaptic plasma membranes decreased in late senescence to 82% of the adult level. To examine the correlation with the decreased Na+,K(+)-ATPase activity, the membrane lipid composition was analyzed. Among the membrane phospholipids, only the content of phosphatidylcholine decreased in the course of senescence. The changes in the Na+,K(+)-ATPase activity were found to be positively correlated with the changes in the phospholipid content, and more specifically with the changes in the phosphatidyl-choline content. These results suggest that age-related alterations in the microenvironment constructed by phospholipids may decrease the activity of Na+,K+-ATPase, resulting in neuronal ion imbalance and decreased membrane potential. This might be responsible in part for altered functions of nerve terminals in aging brain.

Nuclear pore complex frequency in CA1 pyramidal cells of the aging rat

The frequency and the diameter of nuclear pore complexes, and the nuclear perimeter, were studied in CA1 pyramidal cells of the hippocampi from 3-, 9-, 24-, and 30-month-old rats (Fischer 344). No changes with age in any of these parameters were observed. This finding is discussed in terms of varied responses of different brain areas to the effects of aging.

Comparison of retention performance between young rats with fimbria-fornix lesions and aged rats in a 14-unit T-maze

Young (3-months) and aged (24-months) male F-344 rats were pretrained in one-way active avoidance in a straight runway for 3 days. Then two 10-trial daily sessions were given in a 14-unit T-maze in which the response requirement was to negotiate each of 5 maze segments within 10 s to avoid footshock. One day or one week after acquisition, bilateral electrolytic lesions were made in the fimbria-fornix of young rats (1-day lesion or 1-week lesion). Corresponding sham operations were made for remaining young rats (1-day sham or 1-week sham). Aged animals did not receive any surgical treatment. One week after surgery, a 10-trial retention test was conducted to assess the lesion effects on retention and to manipulate the interval between acquisition and lesions. Aged animals were tested in the maze 1 week after acquisition. Results revealed that rats with fimbria-fornix lesions exhibited significant impairment compared to sham-operated groups on all retention performance measures including errors, runtime, number of shocks, duration of shock, and alternation errors. The number of errors and alternation errors of lesioned animals were still higher than those of sham-operated animals at the second half of the retention test, whereas other non-cognitive measures for lesioned animals recovered to control levels. The interval between acquisition training and lesions had no influence on retention performance. Although performance of aged rats during acquisition and retention trials was significantly worse than that of young controls and lesioned animals, a similar recovery pattern during retention testing was found for young rats with fimbria-fornix lesions and aged rats, i.e. both groups showed significant declines in non-cognitive measures with less decline in cognitive measures. These results suggest that the fimbria-fornix is partially involved in retention of 14-unit T-maze performance and that the age-related retention deficit observed in this task may be related to impaired transmission through this pathway.

The role of calcium regulation in brain aging: reexamination of a hypothesis

Studies of the central nervous system have a long history; however, it is only recently that we have begun to understand brain function in health and disease states. And, the topic of the aging brain has become a subject of intense study for a short period. At present, the process of normal aging is relatively poorly understood. Although there are a number of theories of aging, no single theory appears to account for most age-dependent brain changes. This review provides a re-evaluation of the "Calcium Hypothesis of Brain Aging" in light of new evidence which supports the proposition that cellular mechanisms, which maintain the homeostasis of cytosol Ca2+ concentration, play a key role in brain aging; and that sustained changes in [Ca2+]i homeostasis provide the final common pathway for age-associated brain changes. This revision of the calcium hypothesis suggests that there is a complex interaction between the amount of [Ca2+]i perturbation and the duration of such deregulation of Ca2+ homeostasis and it proposes that a small disturbance in Ca2+ homeostasis with a sustained increase in [Ca+]i over a long period has similar cell injuring consequences as that produced by a large increase in [Ca2+]i over a shorter period. Although there are several alternative mechanisms through which the regulation of cytosol [Ca2+]i can be disrupted (such as changes in ion channels, extrusion pumps, and sequestration), this review focuses on disruptions in energy metabolism and changes in the structure and function of membranes as the most likely antecedent events which lead to disruption of Ca2+ homeostasis. The principle purpose of this review is to identify scientific opportunities and stimulate further research into cellular mechanisms of brain aging.

Diminished calcium currents in aged hippocampal dentate gyrus granule neurones

Voltage-dependent calcium currents in hippocampal dentate gyrus granule neurons of young adult (4-6 months) and aged (24-26 months) Fischer 344 rats have been examined using single-electrode voltage-clamp techniques. Calcium currents, especially a high threshold slowly inactivating L-type current, were significantly depressed in neurons obtained from aged animals. Furthermore, these age-dependent changes could be reversed by intracellular injection of the calcium chelator, ethylene glycol bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). These results are in agreement with the hypothesis that aging results in a persistent increase in the free cytoplasmic calcium concentration in mammalian central neurons.

Leg flexion conditioning in the rat: its advantages and disadvantages as a model system of age-related changes in associative learning

Twelve- and 28-month-old Fischer 344 rats of both sexes received five 60-trial sessions of Pavlovian conditioning in which the CS was a 75 dB, 10,000 Hz tone, and the US was a 0.5-mA, 0.5-sec duration footshock. Right foreleg flexion was measured as the conditioned response (CR). Other animals received a random sequence of unpaired tones and footshock and served as pseudoconditioning control groups. Interstimulus intervals (ISIs) of 1.5 and 3.5 sec were studied. The longer ISI resulted in higher rates of responding in both the conditioning and pseudoconditioning groups. However, with the exception of the young males, all animals showed significantly higher levels of responding in the conditioning groups. Females also showed faster acquisition and higher levels of responding than males. A significant sex by age by sessions interaction occurred, suggesting that old males may be somewhat retarded in acquiring the leg flexion CR compared to the other groups of animals. Old males were also slower to reach a criterion of 5 successive CRs than either young males or young or old females.

Neuroendocrine effects on memory in aged rodents and humans

Considerable evidence indicates that epinephrine regulates memory storage processing in young animals. Recent findings suggest that hyperglycemia subsequent to epinephrine release or injection may mediate the hormone's effects on memory. This paper reviews findings demonstrating that epinephrine and glucose treatments attenuate age-related memory impairments in rodents and humans. Additional results suggest that, in aged human and animal subjects, poor glucose regulation predicts memory performance of individual subjects.

Age-dependent alterations in hippocampal synaptic plasticity: relation to memory disorders

In this paper, we review the evidence indicating that the common disturbance in recent memory associated with aging is a consequence of functional and structural impairment in the hippocampal formation. In the Fischer 344 rat, an experimental model of the human age-related memory disorder was developed. The majority of aged rats of this strain show impaired performance in the 8-arm radial maze in a manner typical of young rats with bilateral hippocampal lesions. Aged animals also exhibit rapid decay of LTP and slower kindling of the perforant path-dentate synapse. Furthermore, quantitative morphometric analysis of the hippocampal synaptic architecture revealed that aged, memory-impaired rats had a specific loss of perforated axospinous synapses in the middle third of the dentate gyrus molecular layer; the extent of loss was directly related to the degree of memory dysfunction. Most important was the fact that the electrophysiological and morphological abnormalities did not appear in equally old animals with good memory.

Aging and the physiology of spatial memory

Evidence for age-related changes in spatial memory in rodents and humans is presented, along with data that suggest that the hippocampal formation is necessary for normal performance on spatial tasks in both species. An examination of the electrophysiological characteristics of this structure in rats suggests that the changes that occur with age in the hippocampus are selective, but that at least two primary types of alterations contribute to the spatial cognitive impairment seen in these animals. These include a deficit in the ability to maintain synaptic enhancement and a reduction in the accuracy of information processing ability of single hippocampal neurons.

Hippocampal neurobiological mechanisms of age-related memory dysfunction

Studies are reviewed which indicate that hippocampal frequency potentiation (the growth of neural responses during repetitive synaptic stimulation) is impaired in aged rats, and that this impairment may be important in learning and memory deficits found in these aged animals. Intracellular recording and ultrastructural studies suggest that both hippocampal frequency potentiation and the age deficit in such potentiation are synaptic processes (probably presynaptic), and that the deficit may be due to an age-related increase in calcium influx during depolarization. The latter could in some way result from alterations in the function of a Ca-mediated inactivation of Ca current mechanism recently found in hippocampal neurons. Since major hippocampal changes occur with aging in both rodents and humans, it seems possible that these data are also relevant to human brain aging. Consequently, it is suggested that Alzheimer's disease results from an acceleration of normal age-related neuronal calcium conductance changes by some unknown process (e.g., viruses, aluminum, genetic factors, etc.), leading to a rapid deterioration of brain structure.

Use of tissue culture models to study neuronal regulatory trophic and toxic factors in the aged brain

Dementia is believed to result from the loss of selective neurons within the brain, but approaches for systematic study of that degenerative process are hampered by the complexity of the neuronal milieu. Tissue culture models provide a means to reduce dramatically the variables inherent in the study of neuronal plasticity. Three levels of complexity can be described: cellular and molecular diversity; primary and secondary interconnections; and finally, the dynamics influenced by age. The following review discusses the advantages and disadvantages of tissue culture models for the detailed study of neuronal trophic and toxic factors. Our selection of factors is broadened to include ions, intermediate metabolites, antioxidants, steroids, neuropeptides, gangliosides, metals, neurotransmitters, brain extracts, and protein molecules. Most of these factors have been shown to be altered in the aged brain, to have a significant effect on cultured neurons, or both. This multilevel analysis provides the reader with an overview of the events regulating neuronal survival, differentiation and death. An understanding of these basic questions is necessary to sequence the molecular events resulting in neuronal death.

Alterations of group Ia-motoneuron monosynaptic EPSPs in aged cats

The fashion in which the aging process affects synaptic transmission was explored in old cats. The synapses between Ia-afferent fibers and spinal cord alpha-motoneurons were studied employing intracellular recording techniques. The amplitude and time course of Ia-motoneuron monosynaptic excitatory postsynaptic potentials (Ia-EPSPs) recorded from motoneurons of the gastrocnemius medialis motor pool were determined in adult (1-3 years of age) and in old (14-15 years of age) cats. These Ia-EPSPs were evoked by the stimulation of the gastrocnemius medialis nerve in pentobarbital-anesthetized cats. No statistically significant change was found in the amplitude of these synaptic potentials in the old cats compared with the adult controls. In contrast, their time course was markedly modified; both rise-time and half-width were significantly longer in the old cats. In addition, the rate-of-rise of these Ia-EPSPs was significantly slower in the old cats. No breaks or inflections were observed on the rising phase of the potentials in the adult cats, whereas they were present in a subpopulation of potentials in the old cats. There was also a reduction, in the old cats, in the conduction velocity of the peripheral portion of the Ia-afferent fibers. These data indicate that Ia synaptic transmission is considerably affected during old age. It is suggested that these effects in aged cats may be due to modifications in presynaptic as well as postsynaptic processes.

Reversed ethanol effects on potassium conductances in aged hippocampal dentate granule neurons

The effect of low dose (20 mM) ethanol superfusion on the membrane and synaptic properties of dentate granule neurons was studied in hippocampal slices from young-mature (6-8 months) and old (25-29 months) Fischer-344 rats. In young neurons, ethanol hyperpolarized the resting membrane potential (RMP) and prolonged the post-spike train afterhyperpolarization (AHP). By contrast, ethanol depolarized old neurons and decreased their AHPs, in addition to reducing IPSP amplitudes and spike frequency adaptation. These effects can be explained by ethanol-enhancing potassium conductance (gK) in young neurons and diminishing gK in old neurons.

Age-related change in the multiple unit activity of the rat brain parietal cortex and the effect of centrophenoxine

In this study, spontaneous multiple unit activity (MUA, action potentials derived simultaneously from a number of neurons in a given brain region) was recorded through electrodes chronically implanted in the parietal cerebral cortex of the rats of 1, 3, 6, 9, 12, and 26 months of age (cross-sectional study). Electrophysiological recordings were obtained from unrestrained conscious rats using standard techniques. The results indicated that multiple unit activity was decreased with aging (senescence). Maximum firing rate (MUA counts) was found at the age of 3 months. At 6 months of age, the MUA was decreased by about 30%, while during 6 to 12 months of age the activity seemed to remain unchanged. At 26 months of age the firing rate was, however, further decreased (about 40%). Centrophenoxine administration led to an increase in MUA in the rats of 12 and 26 months of age. The results, thus, further showed that centrophenoxine, a nootropic drug known for its antiaging effects in experimental animals as well as in humans, also manifested beneficial effects electrophysiologically. The data presented in this work are new and significant, since although age effects on gross electrophysiological signals (EEG, evoked potentials, etc.) are known, the aging changes in cellular level electrophysiological signals (action potentials) have not been generally studied particularly in conscious animals.

Impaired hippocampal plasticity in experimental chronic hydrocephalus

Schaffer collateral responses recorded from CA1 pyramidal cell layers in the hippocampus were analysed in kaolin-induced hydrocephalic rats. Other features of this rat model of chronic hydrocephalus were evaluated from measurements of the intracranial pressure, Na-fluorescein migration from the ventricle to the cerebral parenchyma, and behavioural changes. The results indicated that: (1) the conduction velocity of the Schaffer collaterals was unchanged; (2) the threshold to elicit population spikes was decreased; (3) the paired-pulse facilitation of EPSP tended to increase; (4) the paired-pulse inhibition of population spikes was unchanged; and (5) the long-term potentiation of population spikes was clearly attenuated in the hydrocephalic rats. The changes seen in the long-term potentiation were less pronounced in rats which underwent kaolin injection but did not develop hydrocephalus. The above findings suggest that disturbance in the postsynaptic integration processes, rather than axonal conduction or synaptic transmission, are more important for the production of the neurological deficits seen in chronic hydrocephalus. Electron microscopic observations supporting this inference are also briefly described.

Dendritic spine loss in hippocampus of aged rats. Effect of brain phosphatidylserine administration

Dendritic spine density of pyramidal cells in region CA1 of the hippocampus has been evaluated in young (3 months), old (27 months) and old phosphatidylserine (BC-PS)-treated rats. BC-PS (50 mg/kg, suspended in tap water) was administered daily, starting at the age of 3 months until 27 months. Spine density was analyzed on Golgi-stained pyramidal neurons by a computerized analysis system. In 27-month-old rats, spine density showed with respect to 3-month-old animals, a significant decrease in both basal and apical dendrites (p less than 0.01; one-way ANOVA), with a mean loss of 12.11% in the basal dendrites and of 10.64% in the apical ones. In 27-month-old rats treated with BC-PS, values of spine density were not statistically different when compared to those of 3-month-old animals. The mechanisms underlying the beneficial effect of BC-PS treatment on neuronal connectivity might be explained on the basis of its pharmacological actions on neuronal membranes [9], neurotransmission [43] and/or interaction with NGF [7].

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

Changes in the nuclear pore complexes of the dentate granule cells in aged rats

In 3-, 9-, 24-, and 30-month-old male rats (Fischer 344), the nuclear perimeter and the density and diameter of nuclear pore complexes in the granule cells of the dentate fascia were studied. Whereas the nuclear perimeter and the diameter of nuclear pore complexes did not change as a function of age, there was a significant loss of them at 24 months (20%), compared with the third month. This change suggests that the nucleocytoplasmic communication may be impaired with age which would adversely affect protein synthesis, and could explain the loss of the postsynaptic sites of the dentate fascia of aged rats.

Neurophysiological alterations in caudate neurons in aged cats

These neurophysiological studies provide information on the alterations in functional capacity of neurons in the aging caudate nucleus (Cd) of the cat. The major finding is that there is a marked loss of excitation in the Cd during the aging process. This loss is most apparent in animals 11-14 years of age but is demonstrable in animals 6-7 years of age. Extracellular recording techniques were used to test the ability of Cd neurons to respond to activation of two of their major inputs, the precruciate cortex (CX) and the substantia nigra (SN). Types of responses that were evoked in both 1-3- and 11-14-year groups were similar and consisted of excitation, excitation followed by inhibition of action potentials or inhibition alone without preceding excitation. The frequency of occurrence of these responses was altered in the aged animals when either input was stimulated. In 1-3-year-old cats CX stimulation evoked initially excitatory responses in 75% of the cells tested while in 11-14-year-old cats excitatory responses occurred in 62% of the cells. When the SN was stimulated the decrease in initial excitation was greater (69% in 1-3- vs 35% in 11-14-year groups). In all aged animals but not in 1-3-year-old cats stimulation thresholds were higher (39-79%) for evoking excitatory responses than for evoking inhibitory responses. In order to assess synaptic security, the ability of Cd neurons to respond to iterative stimulation was determined. Distributions of the minimum interval necessary to evoke two excitatory responses were constructed. There was a marked increase in the proportion of longer intervals in the aged animals indicating that the synaptic response was less secure. There was a tendency for more of the responses in aged animals to have shorter latencies. This result was probably due to loss of less secure longer latency responses that are mediated via multisynaptic pathways. These findings indicate that there are functional changes in a population of Cd neurons in aged cats that impair their ability to process information.

Redistribution of synaptic vesicles during long-term potentiation in the hippocampus

Synaptic vesicles were quantified 20 min after the induction of long-term potentiation (LTP) in the Schaffer-commissural system of the hippocampus. With LTP, significant increases were found in vesicles attached to the active zone membrane, and in the percentage of vesicles adjacent to the active zone. In addition, as others have reported, overall vesicle density was decreased and spine area was increased. These results suggest that an increased probability of vesicle release may contribute to brain LTP.

Colchicine-induced lesion of rat hippocampal granular cells prevents conditioned active avoidance with perforant path stimulation as conditioned stimulus, but not conditioned emotion

Successful acquisition of active avoidance by rats with low frequency (15 cps) stimulation of the perforant path as a conditioning stimulus is correlated with a slowly developing long-term enhancement of perforant path-granular cell synapses. After selective destruction of granular cells of the stimulated side by unilateral microinjection of 1.6 micrograms/0.2 microliter colchicine into the dentate area, field potentials could no longer be evoked by test stimuli and animals subsequently failed to acquire the conditioned active avoidance with perforant path stimulation as a CS. However, colchicine-treated animals showed the same development of conditioned emotional responses as saline controls and they could also successfully be conditioned with light and tone as the CS. These results suggest that the granular cells are necessarily involved in the conditioning pathway for the active avoidance with perforant path stimulation as the CS. Other targets of the perforant path, e.g., ipsi- and contralateral CA1 pyramidal cells and contralateral granular cells, or antidromic activation of the entorhinal cortex seem an insufficient substitute for granular cells in the pathway for this conditioned active avoidance, but would probably participate in the conditioned emotional responses. The results additionally support our hypothesis, that post-conditioning LTP in granular cell synapses contribute to the acquisition and/or the storage of a memory trace.

Prenatal protein malnutrition affects synaptic potentiation in the dentate gyrus of rats in adulthood

Long-term potentiation (LTP) was studied in the dentate gyrus of anesthetized normal and prenatally protein malnourished rats in adulthood. LTP was initiated by high-frequency stimulation of the perforant path. Potentiation of both population excitatory postsynaptic potential (EPSP) slope and population spike was studied at various times after conditioning out to 5 h. The results indicate that prenatal protein malnutrition has a differential effect on LTP. Although potentiation of the population spike was relatively unaffected, prenatal protein malnutrition did lead to a significant reduction in potentiation of the population EPSP. Several possibilities are proposed as to the cause of the differential effect.

Calcium distribution in dendritic spines of the dentate fascia varies with age

Calcium distribution in dendritic spines of the dentate fascia was studied as a function of age with the oxalate-pyroantimonate precipitation technique. In postnatal ages P3, P9, P24 and P30 spines were analyzed as to the presence of the spine apparatus (SA) and as to the presence of Ca2+ deposits within the SA and within the spine cytoplasm. The percentage of spines with SA-containing precipitates declined significantly between P3 and P24. Conversely, the percentage of spines with precipitates in the spine cytoplasm was significantly increased by P24. In the absence of an SA loss, this result suggests an age-related decrease in the Ca2+-sequestering capacity by the SA. These parameters were improved by P30 so that they approximated the values of P3. Such a seeming amelioration could be attributed to the fact that the mortality rate in rats sharply increases by P24, so that animals surviving this age represent a selected population in which a compensatory growth of spines has occurred and has secured functionally valid connections.

Loss of perforated synapses in the dentate gyrus: morphological substrate of memory deficit in aged rats

Most, but not all, aged rats exhibit a profound deficit in spatial memory when tested in a radial maze--a task known to depend on the integrity of the hippocampal formation. In this study, animals were divided into three groups based on their spatial memory capacity: young adult rats with good memory, aged rats with impaired memory, and aged rats with good memory. Memory-impaired aged animals showed a loss of perforated axospinous synapses in the dentate gyrus of the hippocampal formation in comparison with either young adults or aged rats with good memory. This finding suggests that the loss of perforated axospinous synapses in the hippocampal formation underlies the age-related deficit in spatial memory.

Age-impaired impulse flow from nucleus basalis to cortex

Recent studies have renewed interest in the role of acetylcholine (ACh) in the cognitive changes associated with ageing and dementia. Deficits in cortical choline acetyltransferase (ChAT) in Alzheimer's disease have been consistently demonstrated, while other research has suggested a connection between deterioration of cortical ACh fibres and dementia. However, despite clear biochemical and anatomical evidence for a fall in ACh in dementia, results of therapeutic trials with cholinergic agonists, precursors and cholinesterase inhibitors have been inconsistent. Such findings suggest that cortical cholinergic disorders are not wholly a function of simple biochemical change; alterations of impulse flow along cholinergic fibres could well be as debilitating. An important extrinsic source of cortical ACh innervation derives from neurones diffusely located in rat basal forebrain, denoted the nucleus basalis (NB). We have now investigated the impulse conduction properties of cortically projecting, putatively cholinergic NB axons in adult and aged rats and have found that conduction latencies from NB to frontal cortex are significantly longer (by 51%) in aged animals. In addition, systematic analysis varying cortical stimulation depth revealed that these longer latencies are due entirely to decreased conduction velocities in the subcortical fibre projections. Indeed, intracortical velocities were virtually identical in the two groups. Our results indicate that ageing occasions a decrease in the temporal fidelity of impulse flow in the cholinergic input to the cortex from the NB, a previously overlooked but potentially important element in cognitive deficits that occur with age.

Spatial memory and visual evoked potentials in young and old rats after housing in an enriched environment

The effects of aging and of housing in an enriched environment on performance in an 8-arm radial maze were evaluated in young adult (7-8 months) and old (30-33 months) male Brown-Norway rats, using a procedure in which the rats were confined for 8 s to the central platform of the maze between consecutive choices. Although the old rats attained a level of performance which was clearly above change, they were shown to perform worse than the young rats. No performance differences were found between differentially housed rats of the same age group. In a second experiment recovery cycles of visual evoked potentials were determined in the same rats by using paired flashes with an interstimulus time of 400, 300, 200, or 100 ms. Recovery was consistently smaller in the old rats as compared to the young ones. No correlation could be demonstrated, however, between radial maze performance or housing condition and recovery functions of the visual evoked potentials. This finding indicates that a decline in visual sensitivity cannot readily explain the impaired radial maze performance of old rats. Evidence which suggests that age-related hippocampal changes play a major role in the radial maze performance deficit is discussed.

Prolonged Ca2+-dependent afterhyperpolarizations in hippocampal neurons of aged rats

The possibility that calcium is elevated in brain neurons during aging was examined by quantifying afterhyperpolarizations induced by spike bursts in CAl neurons of hippocampal slices from young and aged rats. The afterhyperpolarizations result from Ca2+-dependent K+ conductance increases and are blocked in medium low in Ca2+ and prolonged in medium high in Ca2+. The afterhyperpolarization and associated conductance increases were considerably prolonged in cells from aged rats, although inhibitory postsynaptic potentials did not differ with age. Since elevated intracellular Ca2+ can exert deleterious effects on neurons, the data suggest that altered Ca2+ homeostasis may play a significant role in normal brain aging.

Chronically elevating plasma Mg2+ improves hippocampal frequency potentiation and reversal learning in aged and young rats

One prediction of the hypothesis that the capacity for hippocampal frequency potentiation is relevant to behavioral plasticity is tested in this study, by chronically elevating magnesium in intact aged and young rats. Elevated extracellular Mg2+ specifically improves frequency potentiation in hippocampal slices, and chronic alterations in plasma Mg2+ can increase brain Mg2+ in intact animals. Aged and young rats on a diet that elevated plasma Mg2+ exhibited stronger frequency potentiation under urethane anesthesia, and showed improved maze reversal learning.

The effects of retroactive and proactive interference on learning and memory in old and young rats

The effects of interference on learning and memory in old and young rats were compared using a visual discrimination task. In Experiment I, discrimination training was followed by one of three interference treatments and finally by retesting on the discrimination task. There were no age differences in original learning but old rats were significantly impaired in remembering the visual discrimination when a high-interference treatment involving similar stimuli was introduced between original learning and retesting. In Experiment II, old rats were impaired on discrimination learning when the high-interference treatment was administered before discrimination training. Analysis of response patterns showed that the exaggerated susceptibility of old rats to interference effects resulted in a general behavioral inflexibility similar to that observed in young adult rats with damage to the hippocampal region.

Long-term potentiation of hippocampal synaptic transmission affects rate of behavioral learning

Electrical stimulation techniques were used to produce a long-lasting potentiation of synaptic transmission in the hippocampus of naïve rabbits. Animals were then classically conditioned. Long-term potentiation of the hippocampus before training increased the rate at which animals subsequently learned the conditioning task. This result has significance for potential cellular mechanisms of associative learning.

Effect of hyperbaric oxygen on GABA transport in rat brain synaptosomes

Initial velocities of uptake of GABA have been measured in rat brain synaptosomes from animals which had been exposed to oxygen at high pressure (OHP) and compared to similar measurements in normobaric controls. For hypothalamus, no changes in GABA uptake occurred subsequent to exposure to OHP. For cortical synaptosomes, however, exposure to OHP resulted in a decreased velocity of GABA uptake at all combinations of [Na] and [GABA] used. The OHP data were found to fit the same transport model as found previously for control data. Thus, OHP exposure did not alter the basic mechanism by which sodium and GABA interact with the carrier in the process of transport. However, the constants which quantitate the model were changed by OHP exposure. As a consequence, the several kinetic parameters which are calculated from the model change in the OHP animals. These kinetic parameters are compared to similar calculations for both normobaric control animals and normobaric aged animals. Although the effects of OHP do not precisely parallel the effects of aging, the alterations in kinetic parameters are in several ways similar in the aged and OHP animals.

Loss of place specificity in hippocampal complex spike cells of senescent rat

Firing characteristics of "place" cells in dorsal CA1 of hippocampus were recorded from 5 young (10-14 months) and 5 old (25-29 months) Fischer-344 rats. Animals were trained to obtain food reward on a radial 8-arm maze. Entry to the arms was controlled by the experimenter so that all 8-arm were visited in random sequence from trial to trial. For each cell, 8 such trials were given (64 arm choices) in order that statistical reliability could be obtained for firing rates over the maze surface. Single unit activity and the animal's position on the maze were continuously monitored by digital computer. Twenty-seven cells from each age group were studied in this way. No statistically significant differences were found between age groups in unit spike height, width or firing rates. A large, statistically significant difference, however, was found in both spatial specificity and reliability of firing patterns from trial to trial. These results are discussed in terms of a possible deficit in spatial information processing in the older animals.

Behavioral changes in aging Aplysia: a model system for studying the cellular basis of age-impaired learning, memory, and arousal

The marine mollusc Aplysia californica was used to examine the effects of age on simple forms of learning, memory, and arousal. We have found that aging impairs the long-term retention of habituation and prevents the acquisition of sensitization in the siphon withdrawal reflex. In addition, aging reduces arousal as evident in the heart rate component of the response to food stimuli. Our results are similar to the age-dependent alterations in the capacity for behavioral plasticity that have been reported in a variety of vertebrates, including man. These similarities suggest that the mechanisms underlying the effects of age on behavior and its modification may share common features across phyla and therefore might be studied to advantage in Aplysia whose central nervous system is especially accessible to cell biological approaches.