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

Decreased number of interneurons and increased seizures in neuropilin 2 deficient mice: implications for autism and epilepsy

PURPOSE

Clinically, perturbations in the semaphorin signaling system have been associated with autism and epilepsy. The semaphorins have been implicated in guidance, migration, differentiation, and synaptic plasticity of neurons. The semaphorin 3F (Sema3F) ligand and its receptor, neuropilin 2 (NPN2) are highly expressed within limbic areas. NPN2 signaling may intimately direct the apposition of presynaptic and postsynaptic locations, facilitating the development and maturity of hippocampal synaptic function. To further understand the role of NPN2 signaling in central nevous system (CNS) plasticity, structural and functional alterations were assessed in NPN2 deficient mice.

METHODS

In NPN2 deficient mice, we measured seizure susceptibility after kainic acid or pentylenetetrazol, neuronal excitability and synaptic throughput in slice preparations, principal and interneuron cell counts with immunocytochemical protocols, synaptosomal protein levels with immunoblots, and dendritic morphology with Golgi-staining.

RESULTS

NPN2 deficient mice had shorter seizure latencies, increased vulnerability to seizure-related death, were more likely to develop spontaneous recurrent seizure activity after chemical challenge, and had an increased slope on input/output curves. Principal cell counts were unchanged, but GABA, parvalbumin, and neuropeptide Y interneuron cell counts were significantly reduced. Synaptosomal NPN2 protein levels and total number of GABAergic synapses were decreased in a gene dose-dependent fashion. CA1 pyramidal cells showed reduced dendritic length and complexity, as well as an increased number of dendritic spines.

DISCUSSION

These data suggest the novel hypothesis that the Sema 3F signaling system's role in appropriate placement of subsets of hippocampal interneurons has critical downstream consequences for hippocampal function, resulting in a more seizure susceptible phenotype.

Blueberry-enriched diet ameliorates age-related declines in NMDA receptor-dependent LTP

NMDA receptor-dependent long-term potentiation (LTP) in the hippocampus is widely accepted as a cellular substrate for memory formation. Age-related declines in the expression of both NMDAR-dependent LTP and NMDAR subunit proteins in the CA1 region of the hippocampus have been well characterized and likely underlie age-related memory impairment. In the current study, we examined NMDAR-dependent LTP in young Fischer 344 rats (4 months old) and aged rats (24 months old) given either a control diet or a diet supplemented with blueberry extract for 6-8 weeks. NMDAR-dependent LTP was evoked by high-frequency stimulation (HFS) in the presence of nifedipine, to eliminate voltage-gated calcium channel LTP. Field excitatory postsynaptic potentials (fEPSPs) were increased by 57% 1 h after HFS in young animals, but this potentiation was reduced to 31% in aged animals. Supplementation of the diet with blueberry extract elevated LTP (63%) in aged animals to levels seen in young. The normalization of LTP may be due to the blueberry diet preventing a decline in synaptic strength, as measured by the slope of the fEPSP for a given fiber potential. The blueberry diet did not prevent age-related declines in NMDAR protein expression. However, phosphorylation of a key tyrosine residue on the NR2B subunit, important for increasing NMDAR function, was enhanced by the diet, suggesting that an increase in NMDAR function might overcome the loss in protein. This report provides evidence that dietary alterations later in life may prevent or postpone the cognitive declines associated with aging.

Alteration of CREB phosphorylation and spatial memory deficits in aged 129T2/Sv mice

Phosphorylation of cAMP-response element binding protein (CREB) is required for hippocampus-dependent long-term memory formation. The present study was designed to determine whether spatial memory deficits in aged mice were associated with alteration of hippocampal CREB phosphorylation. We examined the temporal pattern of CREB activation in 5-6 and 23-24-month-old 129T2/Sv mice trained on a spatial reference memory task in the water maze. Phosphorylated CREB (pCREB), total CREB (t-CREB) and c-Fos immunoreactivity (ir) were measured at four time points after the end of training. In young mice, pCREB-ir was significantly increased 15 and 60 min after training in the CA1 region and dentate gyrus. In aged mice sacrificed 15 min after training, pCREB-ir in these structures was reduced whereas t-CREB-ir remained unchanged compared to respective young-adults. An age-related reduction of c-Fos-ir also occurred selectively in hippocampal CA1 region. Since reduced pCREB-ir in CA1 from the 15 min-aged group strongly correlated with individual learning performance, we suggest that altered CREB phosphorylation in CA1 may account for spatial memory impairments during normal aging.

Sequence reactivation in the hippocampus is impaired in aged rats

The hippocampus is thought to coordinate memory consolidation by reactivating traces from behavioral experience when the brain is not actively processing new input. In fact, during slow-wave sleep, the patterns of CA1 pyramidal cell ensemble activity correlations are reactivated in both young and aged rats. In addition to correlated activity patterns, repetitive track running also creates a recurring sequence of pyramidal cell activity. The present study compared CA1 sequence activity pattern replay in young and old animals during rest periods after behavior. Whereas the young rats exhibited significant sequence reactivation, it was markedly impaired in the aged animals. When the spatial memory scores of all animals were compared with the degree of sequence reactivation, there was a significant correlation. The novel finding that weak replay of temporal patterns has behavioral consequences, strengthens the idea that reactivation processes are integral to memory consolidation.

Synaptic commitment: developmentally regulated reciprocal changes in hippocampal granule cell NMDA and AMPA receptors over the lifespan

Synaptic transmission in hippocampal field CA1 is largely N-methyl-d-aspartate receptor (NMDA(R)) dependent during the early postnatal period. It becomes increasingly mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) receptors until an adult ratio of AMPA to NMDA receptors is achieved. It is shown here that increases in the AMPA receptor (AMPA(R))-mediated field potential response continue over the life span of the F-344 rat at the perforant path-granule cell synapse in the dentate gyrus. In contrast, the NMDA(R)-dependent component of the response decreases with age between 1 and 27 mo, leading to an increase of AMPA(R)/NMDA(R) ratio with age. One possible explanation of this age difference is that the AMPA(R)/NMDA(R) ratio can be modified by experience. To test the idea that the changed ratio is caused by the old rats' longer lives, an intensive 10-mo period of enrichment treatment was given to a group of animals, beginning at 3 mo of age. Compared with animals housed in standard cages, the enrichment treatment did not alter the glutamatergic response ratio measured with field potential recording methods. These data provide support for the conclusion that the observed change with age is developmentally regulated rather than experience dependent. Given the role of the NMDA(R) in synaptic plasticity, these changes suggest a progressive commitment of perforant path synapses to particular weights over the life span. One possible implication of this effect includes preservation of selected memories, ultimately at the expense of a reduced capacity to store new information.

Treatment with dexamethasone and vitamin D3 attenuates neuroinflammatory age-related changes in rat hippocampus

Among the changes which occur in the brain with age is an increase in hippocampal concentration of proinflammatory cytokines like interleukin-1beta (IL-1beta) and an increase in IL-1beta-induced signaling. Here we demonstrate that the increase in IL-1beta concentration is accompanied by an increase in expression of IL-1 type I receptor (IL-1RI) and an age-related increase in microglial activation, as shown by increased expression of the cell surface marker, major histocompatibility complex II (MHCII) and increased MHCII staining. The evidence indicates that these age-related changes were abrogated in hippocampus of aged rats treated with dexamethasone and vitamin D3. Similarly, the age-related increases in activation of the stress-activated protein kinase, c-Jun N-terminal kinase (JNK), as well as caspase-3 and PARP were all attenuated in hippocampal tissue prepared from rats that received dexamethasone and vitamin D3. The data indicate that dexamethasone and vitamin D3 ameliorated the age-related increase in IFNgamma and suggest that IFNgamma may be the trigger leading to microglial activation, since it increases MHCII mRNA and IL-1beta release from cultured glia. In parallel with its ability to decrease microglial activation in vivo, we report that treatment of cultured glia with dexamethasone and vitamin D3 blocked the lipopolysaccharide increased MHCII mRNA and IL-1beta concentration, while the IL-1beta-induced increases in activation of JNK and caspase 3 in cultured neurons were also reversed by treatment with dexamethasone and vitamin D3. The data suggest that the antiinflammatory effect of dexamethasone and vitamin D3 derives from their ability to downreguate microglial activation.

Aging effects on the limits and stability of long-term synaptic potentiation and depression in rat hippocampal area CA1

Altered hippocampal synaptic plasticity may underlie age-related memory impairment. In acute hippocampal slices from aged (22-24 mo) and young adult (1-12 mo) male Brown Norway rats, extracellular excitatory postsynaptic field potentials were recorded in CA1 stratum radiatum evoked by Schaffer collateral stimulation. We used enhanced Ca(2+) to Mg(2+) ratio and paired-pulse stimulation protocol to induce maximum changes in the synaptic plasticity. Six episodes of theta-burst stimulation (TBS) or nine episodes of paired low-frequency stimulation (pLFS) were used to generate asymptotic long-term potentiation (LTP) and long-term depression (LTD), respectively. In addition, long-term depotentiation (LTdeP) or de-depression (LTdeD) from maximal LTP and LTD were examined using two episodes of pLFS or TBS. Multiple episodes of TBS or pLFS produced significant LTP or LTD in aged and young adult rats; this was not different between age groups. Moreover, there was no significant difference in the amount of LTdeP or LTdeD between aged and young adult rats. Our results show no age differences in the asymptotic magnitude of LTP or LTD, rate of synaptic modifications, development rates, reversal, or decay after postconditioning. Thus impairment of the basic synaptic mechanisms responsible for expression of these forms of plasticity is not likely to account for decline in memory function within this age range.

The importance of being subtle: small changes in calcium homeostasis control cognitive decline in normal aging

Aging is a complex, multifactorial process. One of the features of normal aging of the brain is a decline in cognitive functions and much experimental attention has been devoted to understanding this process. Evidence accumulated in the last decade indicates that such functional changes are not due to gross morphological alterations, but to subtle functional modification of synaptic connectivity and intracellular signalling and metabolism. Such synaptic modifications are compatible with a normal level of activity and allow the maintenance of a certain degree of functional reserve. This is in contrast to the changes in various neurodegenerative diseases, characterized by significant neuronal loss and dramatic and irreversible functional deficit. This whole special issue has been initiated with the intention of focusing on the processes of normal brain aging. In this review, we present data that shows how subtle changes in Ca(2+) homeostasis or in the state of various Ca(2+)-dependent processes or molecules, which occur in aging can have significant functional consequences.

Maintenance of superior learning and memory function in NR2B transgenic mice during ageing

Brain ageing represents a general and evolutionarily conserved phenomenon and is marked by gradual declines in cognitive functions such as learning and memory. As a synaptic coincidence detector, the N-methyl-d-aspartate (NMDA) receptor is known to be essential for the induction of synaptic plasticity and memory formation. Here, we test the hypothesis that up-regulation of NR2B expression is beneficial for learning and memory in the aged animals. Our in vitro recordings show that the aged transgenic mice with the forebrain-specific overexpression of the NR2B subunit indeed exhibit more robust hippocampal long-term potentiation (LTP) induced by either high-frequency stimulation or theta-stimulation protocol. Furthermore, those aged NR2B transgenic mice consistently outperform their wild-type littermates in five different learning and memory tests, namely, novel object recognition, contextual and cued fear conditioning, spatial reference memory, and spatial working memory T-maze task. Thus, we conclude that increased expression of NR2B in the forebrain improves learning and memory function in the aged brain.

Synaptic plasticity in early aging

Studies of how aging affects brain plasticity have largely focused on old animals. However, deterioration of memory begins well in advance of old age in animals, including humans; the present review is concerned with the possibility that changes in synaptic plasticity, as found in the long-term potentiation (LTP) effect, are responsible for this. Recent results indicate that impairments to LTP are in fact present by early middle age in rats but only in certain dendritic domains. The search for the origins of these early aging effects necessarily involves ongoing analyses of how LTP is induced, expressed, and stabilized. Such work points to the conclusion that cellular mechanisms responsible for LTP are redundant and modulated both positively and negatively by factors released during induction of potentiation. Tests for causes of the localized failure of LTP during early aging suggest that the problem lies in excessive activity of a negative modulator. The view of LTP as having redundant and modulated substrates also suggests a number of approaches for reversing age-related losses. Particular attention will be given to the idea that induction of brain-derived neurotrophic factor, an extremely potent positive modulator, can be used to provide long periods of normal plasticity with very brief pharmacological interventions. The review concludes with a consideration of how the selective, regional deficits in LTP found in early middle age might be related to the global phenomenon of brain aging.

Calcium chelation improves spatial learning and synaptic plasticity in aged rats

Impaired regulation of intracellular calcium is thought to adversely affect synaptic plasticity and cognition in the aged brain. Comparing young (2-3 months) and aged (23-26 months) Fisher 344 rats, stratum radiatum-evoked CA1 field EPSPs were smaller and long-term potentiation (LTP) was diminished in aged hippocampal slices. Resting calcium, in presynaptic axonal terminals in the CA1 stratum radiatum area, was elevated in aged slices. Loading the slice with the calcium chelator, BAPTA-AM, depressed LTP in young slices, but enhanced this plasticity in old slices. Forty-five minutes following LTP-inducing high frequency stimulation, resting calcium levels were significantly increased in both young and old presynaptic terminals, and significantly reduced by pretreatment with BAPTA-AM. In vivo, intraperitoneal administration of BAPTA-AM prior to training in the reference memory version of the Morris water maze test, significantly improved the acquisition of spatial learning in aged animals, without a significant effect in young rats. These results support the hypothesis that increasing intracellular neuronal buffering power for calcium in aged rats ameliorates age-related impaired synaptic plasticity and learning.

Pre- and postsynaptic contributions to age-related alterations in corticostriatal synaptic plasticity

Aging creates deficits in motor performance related to changes in striatal processing of cortical information. This study describes age-related changes in corticostriatal snaptic plasticity and associated mechanisms, which may contribute to declines in motor behavior. Intracellular recordings revealed an age-related decrease in the expression of paired-pulse, posttetanic, and long-term potentiation (LTP). The age-related difference in LTP was associated with reduced sensitivity to block of N-methyl-D-aspartate (NMDA) receptors in the aged population. These age-related changes could not be explained by increased L-type Ca(2+)channel activity, since block of L-type Ca(2+) channels with nifedipine increased rather than decreased the age-related difference in long-term plasticity. Age-related increases in reactive oxygen species (ROS) modulation were also ruled out, since application of H(2)O(2) produced changes in synaptic function that were opposite to trends seen in aging, and addition of the antioxidant Trolox-C had a larger effect on long-term plasticity in young rats than in older rats. A robust age-related difference in long-term synaptic plasticity was found by studying synaptic plasticity following the blocking of D2 receptors with l-sulpiride, which may involve age-difference in NMDA receptor function. l-sulpiride consistently enabled a slow development of LTP at young (but not aged) corticostriatal synapses. However, No age differences were found in the sensitivity to the addition of the D2 receptor agonist quinpirole. These findings provide evidence for age-induced changes in the release properties of cortical terminals and in the functioning of postsynaptic striatal NMDA receptors, which may contribute to age-related deficits in striatum control of movement.

Neural plasticity in the ageing brain

The mechanisms involved in plasticity in the nervous system are thought to support cognition, and some of these processes are affected during normal ageing. Notably, cognitive functions that rely on the medial temporal lobe and prefrontal cortex, such as learning, memory and executive function, show considerable age-related decline. It is therefore not surprising that several neural mechanisms in these brain areas also seem to be particularly vulnerable during the ageing process. In this review, we discuss major advances in our understanding of age-related changes in the medial temporal lobe and prefrontal cortex and how these changes in functional plasticity contribute to behavioural impairments in the absence of significant pathology.

Reduced immunoreactivities of a vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide receptor (VPAC1 receptor) in the cerebral cortex, hippocampal region, and amygdala of aged rats

In this study, we examined expressional changes of VPAC1 receptor in aged rat brains using an immunohistochemical approach and found that its immunoreactivities are significantly reduced in the cerebral cortex, hippocampal region, and amygdala of aged rats. These results suggest that this reduction could underlie aging-associated memory/learning deficits and several other age-induced functional changes in these areas. However, the functional consequences of these down-regulations require further elucidation.

Age-related disturbance of memory and CREB phosphorylation in CA1 area of hippocampus of rats

In the early process of long-term memory formation, cyclic AMP response element-binding protein (CREB), a transcription factor on which multiple signal transduction pathways converge, has been implicated. We examined whether the age difference in the performance of contextual fear conditioning (CFC) is associated with a change in activation of CREB in the hippocampus which is an important neural structure for long-term memory. The activation of CREB in the hippocampus in young (15 weeks old) and old (120 weeks old) male rats was determined immunohistochemically with an antibody that specifically recognizes the phosphorylated form of CREB (pCREB). Young rats exhibited better performance than old rats with respect to the freezing time in CFC. Phosphorylation of CREB as revealed by the ratio of the pCREB-immunoreactive cell number to the CREB-immunoreactive cell number was increased in the CA1 region, but not in other hippocampal regions following training for CFC. The close relationship between behavioral performance and CREB phosphorylation in the CA1 region suggests that hippocampal CREB is involved in age-related decline of learning and memory.

Long-term potentiation is impaired in middle-aged rats: regional specificity and reversal by adenosine receptor antagonists

Memory loss in humans begins early in adult life and progresses thereafter. It is not known whether these losses reflect the failure of cellular processes that encode memory or disturbances in events that retrieve it. Here, we report that impairments in hippocampal long-term potentiation (LTP), a form of synaptic plasticity associated with memory, are present by middle age in rats but only in select portions of pyramidal cell dendritic trees. Specifically, LTP induced with theta-burst stimulation in basal dendrites of hippocampal field CA1 decayed rapidly in slices prepared from 7- to 10-month-old rats but not in slices from young adults. There were no evident age-related differences in LTP in the apical dendrites. Both the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and a positive AMPA receptor modulator (ampakine) offset age-related LTP deficits. Adenosine produced greater depression of synaptic responses in middle-aged versus young adult slices and in basal versus apical dendrites. These results were not associated with variations in A1 receptor densities and may instead reflect regional and age-related differences in adenosine clearance. Pertinent to this, brief applications of A1 receptor antagonists immediately after theta stimulation fully restored LTP in middle-aged rats. We hypothesize that the build-up of extracellular adenosine during theta activity persists into the postinduction period in the basal dendrites of middle-aged slices and thereby activates the A1 receptor-dependent LTP reversal effect. Regardless of the underlying mechanism, the present results provide a candidate explanation for memory losses during normal aging and indicate that, with regard to plasticity, different segments of pyramidal neurons age at different rates.

Adenylate Cyclase-Mediated Forms of Neuronal Plasticity in Hippocampal Area CA1 Are Reduced With Aging

Beta-adrenergic receptors and the cyclic AMP signaling pathway play an important role in neuronal plasticity and in learning and memory and are known to change with aging. We examined the effects of β-adrenergic stimulation paired with 5-Hz low frequency stimulation (LFS) of Schaffer collateral-commissural afferents on population spike amplitude in area CA1 of hippocampal slices from young (3 mo) and aged (22 mo) Fischer 344 rats. Application of the β-adrenergic agonist isoproterenol (1 μM) for 10 min followed immediately by 3 min LFS produced long-lasting potentiation in young hippocampi, but the magnitude of potentiation in aged rats was significantly attenuated and was not long-lasting. In slices prepared from young rats, long-term potentiation (LTP) induced by this protocol occludes subsequent attempts to produce conventional high frequency stimulation-induced LTP, and vice versa, suggesting that these two forms of potentiation share one or more molecular mechanisms. Age-related differences in response to LFS alone were not observed, but significant differences in response to β-adrenergic stimulation were apparent. Similarly, significant age-related differences in response to direct activation of adenylate cyclase with forskolin (10 μM) were observed. In both age groups, this enhancement produced by isoproterenol or forskolin is only transient, returning to baseline within 60 or 90 min, respectively. Taken together, these studies of adenylate cyclase-mediated forms of potentiation in area CA1 suggest that there is an age-related defect, either upstream or downstream of adenylate cyclase activation, in this important signaling system. Such changes may contribute to the compromised performance on memory tasks that is often observed with normal aging.

The effect of acute stress on LTP and LTD induction in the hippocampal CA1 region of anesthetized rats at three different ages

Not all experiences are memorized equally well. Especially, some types of stress are unavoidable in daily life and the stress experience can be memorized for life. Previous evidence has showed that synaptic plasticity, such as long-term potentiation (LTP) that may be the major cellular model of the mechanism underlying learning and memory, is influenced by behavioral stress. However, the effect of behavioral stress on age-related synaptic plasticity in vivo was primarily known. Here we found that the LTP induction in the hippocampal CA1 region of anesthetized rats obviously showed inverted-U shape related to ages (4, 10 and 74 weeks old rats), but low-frequency stimulation was unable to induce reliable long-term depression (LTD) in these animals. Furthermore, acute elevated platform (EP) stress enabled reliable LTD significantly and completely blocked LTP induction at these ages. Importantly, LTD after exposure to acute EP stress showed similar magnitude over these ages. The present results that stress enables LTD but impairs LTP induction at these three ages strengthen a view that stress experience-dependent LTD (SLTD) may underlie stress form of aberrant memories.

Ca2+ and mitochondria as substrates for deficits in synaptic plasticity in normal brain ageing

Normal brain ageing is associated with a degree of functional impairment of neuronal activity that results in a reduction in memory and cognitive functions. One mechanism proposed to explain the age-dependent changes was the "Ca(2+) hypothesis of ageing" but data accumulated in the last decade revealed a number of inconsistencies. Two important questions were raised: (a) which are, if any, the most reliable age-associated change in neuronal Ca(2+) homeostasis and (b) are these changes primary, and thus determinant of the ageing phenotype, or are they secondary to other changes in the physiology of the aged neurones. After a brief review of the evidence accumulated for the age-induced changes in synaptic plasticity, we assess the proposal that these changes are, ultimately, determined by changes in the metabolic state of the aged neurones, that are manifest particularly after neuronal stimulation. In this context, it appears that the changes in mitochondrial status and function are of primary importance.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Later developments: molecular keys to age-related memory impairment

Age-related memory impairment, a cognitive decline not clearly related to any gross pathology, is progressive and widespread in the population, although not universal. While the mechanisms of learning and memory remain incompletely understood, the study of their molecular mechanisms is already yielding promising approaches toward therapy for such "normal" declines in the efficiency of learning. This review presents the rationale and results for two such approaches. One approach, partial inhibition of the type IV cAMP specific phosphodiesterase, appears to act indirectly. Although little evidence supports an age-related decline in this system, considerable evidence indicates that this approach can facilitate the transition from short-term to long-term memory and thus counterbalance defects in long-term memory, which may be due to other causes. A second approach, inhibition of l-type voltage gated calcium channels (LVGCCs) may be a specific corrective for a molecular pathology of aging, as substantial evidence indicates that an ongoing increase occurs throughout the lifespan in the density of these channels in hippocampal pyramidal cells, with a concomitant reduction in cellular excitability. Because LVGCCs are also crucial to extinction, a paradigm of inhibitory learning, age-related memory impairment may be an unfortunate side effect of a developmental process necessary to the maturation of the ability to suppress inappropriate behavior, an interpretation consistent with the antagonistic pleiotropy theory of aging.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Neuronal ageing from an intraneuronal perspective: roles of endoplasmic reticulum and mitochondria

The nature of brain ageing and the age-dependent decline in cognitive functions remains poorly understood. Physiological brain ageing is characterised by mild mental dysfunctions, whereas age-dependent neurodegeneration, as illustrated by Alzheimer disease (AD), results rapidly in severe dementia. These two states of the aged brain, the physiological and the pathological, are fundamentally different as the latter stems from significant neuronal loss, whereas the former develops without significant neuronal demise. In this paper, we review the changes in neuronal Ca(2+) homeostasis that occur during brain ageing, and conclude that normal, physiological ageing is characterised mainly by a decrease of neuronal homeostatic reserve, defined as the capacity to respond effectively to functional and metabolic stressors, but does not reach the trigger required to induce neuronal death. In contrast, during neurodegenerative states, Ca(2+) homeostasis is affected early during the pathological process and result in significant neuronal demise. We also review recent evidence suggesting that the endoplasmic reticulum (ER) might play an important role in controlling the balance between healthy and pathological neuronal ageing.

Small-conductance, Ca2+-activated K+ channel SK3 generates age-related memory and LTP deficits

Cognitive deficits are among the most devastating changes associated with the aging process. Age-related decrement in performance on learning tasks is correlated with substantial changes in neuronal signal processing in the hippocampus. Here we show that elevated expression of small-conductance Ca2+-activated K+ channels (SK channels) of the SK3 type in hippocampi of aged mice contributes to reduced long-term potentiation (LTP) and impaired trace fear conditioning, a hippocampus-dependent learning task.

Synaptic plasticity preserved with arachidonic acid diet in aged rats

We examined whether synaptic plasticity was preserved in aged rats administered an arachidonic acid (AA) containing diet. Young male Fischer-344 rats (2 mo of age), and two groups of aged rats of the same strain (2 y of age) who consumed either a control diet or an AA ethyl ester-containing diet for at least 3 mo were used. In the Morris water maze task, aged rats on the AA diet had tendency to show better performance than aged rats on the control diet. Long-term potentiation induced by tetanic stimulation was recorded from a 300 microm thick hippocampal slice with a 36 multi-electrode-array positioned at the dendrites of CA1 pyramidal neurons. The degree of potentiation after 1 h in aged rats on the AA diet was comparable as that of young controls. Phospholipid analysis revealed that AA and docosahexaenoic acid were the major fatty acids in the hippocampus in aged rats. There was a correlation between the behavioral measure and the changes in excitatory postsynaptic potential slope and between the physiologic measure and the total amount of AA in hippocampus.

Reduced synaptic plasticity in the lateral perforant path input to the dentate gyrus of aged C57BL/6 mice

Hippocampal slices obtained from C57BL/6 mice (3-25 mo) were used to investigate the effects of aging on excitatory postsynaptic potentials (EPSPs) elicited in dentate gyrus with lateral perforant path stimulation. The maximal amplitude of the EPSP, as well as the degree of paired-pulse facilitation, was significantly reduced in animals aged 12 mo or more compared with younger animals (<12 mo). Although all animals showed equivalent short-term potentiation (STP) in response to high-frequency stimulation, this did not translate into a long-lasting increase in synaptic efficacy in the older animals. A significant degree of long-term potentiation (LTP) of synaptic efficacy was only observed in animals <12 mo of age when measured 30 min after induction. Blocking GABAA-mediated inhibition significantly enhanced STP in younger and older animals; however, a significant degree of LTP was again only observed in slices taken from younger animals. These data indicate that the lateral perforant path input to the dentate gyrus is altered by the aging process, and that this results in a reduction in the capacity of this input to exhibit long-lasting synaptic plasticity.

Long-term potentiation and the ageing brain

Ageing is associated with learning and memory impairments. Data are reviewed that suggest that age-related impairments of hippocampal-dependent forms of memory, may be caused, in part, by altered synaptic plasticity mechanisms in the hippocampus, including long-term potentiation (LTP). To the extent that the mechanisms responsible for LTP can be understood, it may be possible to develop therapeutic approaches to alleviate memory decline in normal ageing.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

Long-term potentiation in aged rats is restored when the age-related decrease in polyunsaturated fatty acid concentration is reversed

Several age-related changes have been identified in rat hippocampus; among these are deficits in glutamate release and long-term potentiation in dentate gyrus. These deficits correlate with a decrease in the concentration of arachidonic acid in hippocampus. In this study, the effects of dietary supplementation for 8 weeks with omega -6 or omega -3 fatty acids were assessed in groups of aged and young rats. The data presented indicate that dietary supplementation in aged rats restored the concentrations of arachidonic acid and docosahexanoic acid in hippocampal preparations to those observed in tissue prepared from young rats. In parallel, aged rats which received the experimental diets sustained long-term potentiation in a manner indistinguishable from young rats. The evidence presented supports the view that an age-related increase in reactive oxygen species production is linked with the decrease in polyunsaturated fatty acids and that a diet enriched in eicosapentanoic acid has antioxidant properties which may play a key role in reversal of the observed age-related deficits.

A hippocampal NR2B deficit can mimic age-related changes in long-term potentiation and spatial learning in the Fischer 344 rat

Aged rats are known to have deficits in spatial learning behavior in the Morris water maze. We have found that aged rats also have deficits in NR2B protein expression and that the protein expression deficit is correlated with their performance in the Morris water maze. To test whether this NR2B deficit was sufficient to account for the behavioral deficit, we used antisense oligonucleotides to specifically knock down NR2B subunit expression in the hippocampus of young rats. NR2B antisense treatment diminished NMDA receptor responses, abolished NMDA-dependent long-term potentiation (LTP), and impaired spatial learning. These data demonstrate the important role of NR2B in LTP and learning and memory and suggest a role for reduced NR2B expression in age-related cognitive decline.

Elevated postsynaptic [Ca2+]i and L-type calcium channel activity in aged hippocampal neurons: relationship to impaired synaptic plasticity

Considerable evidence supports a Ca(2+) dysregulation hypothesis of brain aging and Alzheimer's disease. However, it is still not known whether (1) intracellular [Ca(2+)](i) is altered in aged brain neurons during synaptically activated neuronal activity; (2) altered [Ca(2+)](i) is directly correlated with impaired neuronal plasticity; or (3) the previously observed age-related increase in L-type voltage-sensitive Ca(2+) channel (L-VSCC) density in hippocampal neurons is sufficient to impair synaptic plasticity. Here, we used confocal microscopy to image [Ca(2+)](i) in single CA1 neurons in hippocampal slices of young-adult and aged rats during repetitive synaptic activation. Simultaneously, we recorded intracellular EPSP frequency facilitation (FF), a form of short-term synaptic plasticity that is impaired with aging and inversely correlated with cognitive function. Resting [Ca(2+)](i) did not differ clearly with age. Greater elevation of somatic [Ca(2+)](i) and greater depression of FF developed in aged neurons during 20 sec trains of 7 Hz synaptic activation, but only if the activation triggered repetitive action potentials for several seconds. Elevated [Ca(2+)](i) and FF also were negatively correlated in individual aged neurons. In addition, the selective L-VSCC agonist Bay K8644 increased the afterhyperpolarization and mimicked the depressive effects of aging on FF in young-adult neurons. Thus, during physiologically relevant firing patterns in aging neurons, postsynaptic Ca(2+) elevation is closely associated with altered neuronal plasticity. Moreover, selectively increasing postsynaptic L-VSCC activity, as occurs in aging, negatively regulated a form of short-term plasticity that enhances synaptic throughput. Together, the results elucidate novel processes that may contribute to impaired cognitive function in aging.

17beta-estradiol: effect on CA1 hippocampal synaptic plasticity

An understanding of synaptic plasticity in the mammalian brain has been one of R. F. Thompson's major pursuits throughout his illustrious career. A current series of experiments of significant interest to R. F. Thompson is an examination of the interactions between sex hormones, synaptic plasticity, aging, and stress. This research is contained within a broader project whose aim is to investigate animal models that evaluate estrogen interactions with Alzheimer's disease. This paper reviews the recent results that have led to a better understanding of how the sex hormone estrogen influences synaptic plasticity in an important structure within the mammalian brain responsible for learning and memory: the hippocampus. In this review, a number of experiments have been highlighted that investigate the molecular mechanisms that underlie estrogen's effect on two specific forms of synaptic plasticity commonly studied in neurophysiology and the behavioral neurosciences: long-term potentiation and long-term depression.

Aging and learning-specific changes in single-neuron activity in CA1 hippocampus during rabbit trace eyeblink conditioning

Rabbit trace eyeblink conditioning is a hippocampus-dependent task in which the auditory conditioned stimulus (CS) is separated from the corneal airpuff unconditioned stimulus (US) by a 500-ms empty trace interval. Young rabbits are able to associate the CS and US and acquire trace eyeblink conditioned responses (CRs); however, a subset of aged rabbits show poor learning on this task. Several studies have shown that CA1-hippocampal activity is altered by aging; however, it is unknown how aging affects the interaction of CA1 single neurons within local ensembles during learning. The present study examined the extracellular activity of CA1 pyramidal neurons within local ensembles in aged (29-34 mo) and young (3-6 mo) rabbits during 10 daily sessions (80 trials/session) of trace eyeblink conditioning. A single surgically implanted nonmovable stereotrode was used to record ensembles ranging in size from 2 to 12 separated single neurons. A total of six young and four aged rabbits acquired significant levels of CRs, whereas five aged rabbits showed very few CRs similar to a group of five young pseudoconditioned rabbits. Pyramidal cells (2,159 total) were recorded from these four groups during training. Increases in CA1 pyramidal cell firing to the CS and US were diminished in the aged nonlearners. Local ensembles from all groups contained heterogeneous types of pyramidal cell responses. Some cells showed increases while others showed decreases in firing during the trace eyeblink trial. Hierarchical clustering was used to isolate seven different classes of single-neuron responses that showed unique firing patterns during the trace conditioning trial. The proportion of cells in each group was similar for six of seven response classes. Unlike the excitatory modeling patterns reported in previous studies, three of seven response types (67% of recorded cells) exhibited some type of inhibitory decrease to the CS, US, or both. The single-neuron response classes showed different patterns of learning-related activity across training. Several of the single-neuron types from the aged nonlearners showed unique alterations in response magnitude to the CS and US. Cross-correlation analyses suggest that specific single-neuron types provide more correlated single-neuron activity to the ensemble processing of information. However, aged nonlearners showed a significantly lower level of coincident pyramidal cell firing for all cell types within local ensembles in CA1.

Acute and chronic nicotine exposure reverse age-related declines in the induction of long-term potentiation in the rat hippocampus

Long-term potentiation (LTP) is widely considered to be the cellular substrate of learning and memory. The induction of LTP becomes more difficult with age in parallel with declining learning and memory ability. Because nicotine improves learning and memory in aged rats, we examined the effects of acute and chronic nicotine exposure on age-related declines in LTP induction. We found that acute nicotine exposure lowered the threshold for LTP induction in the aging hippocampus. The effect of nicotine was mimicked by the alpha7 nicotinic acetylcholine receptor (nAChR) antagonist methyllycaconitine and blocked by the non-alpha7 nAChR antagonist dihydro-beta-erythroidine, suggesting that both nicotine-mediated desensitization of alpha7 nAChRs and activation of non-alpha7 nAChRs contribute to the nicotine effect. The non-alpha7 nAChR agonist A85380 that facilitates the induction of LTP in the young hippocampus had no effect, however, suggesting that at least one pathway involving non-alpha7 nAChRs was altered by aging. Chronic nicotine treatment of aged rats also lowered the threshold for LTP induction and acute nicotine exposure lowered the threshold further in the chronic-nicotine-treated aged hippocampus. These results not only suggest that the mechanisms mediated by acute and chronic nicotine exposure are different, but also demonstrate that age-associated declines in LTP induction can be reversed with nicotine treatment.

Protein synthesis in entorhinal cortex and long-term potentiation in dentate gyrus

Despite the concentration of effort in recent years, the mechanisms underlying the expression of long-term potentiation (LTP) in the hippocampus remain elusive, but amidst the uncertainty and sometimes controversy, one consistent finding is emerging; this is that late-phase LTP requires synthesis of proteins. This hypothesis was first proposed by a number of groups who reported that the more persistent components of LTP were blocked by protein synthesis inhibitors, and was supported by a significant literature which indicated that morphological changes accompanied LTP. Recent evidence indicated that the increase in protein synthesis may be cAMP-dependent and that subsequent activation of the transcription factor, CREB, represented one step in the cascade of events leading to protein synthesis. Whether protein synthesis occurs in presynaptic or postsynaptic neurons, or both, is still a subject of debate. Here we present evidence which suggests that LTP in perforant path-granule cell synapses is accompanied by protein synthesis, specifically synthesis of synaptic vesicle proteins, in the entorhinal cortex. We also show that protein synthesis is decreased in the entorhinal cortex of aged rats and a strain of rat which is genetically hypertensive, both of which exhibited impaired LTP. We propose that that the observed increase in protein synthesis in the entorhinal cortex, which accompanied LTP in the dentate gyrus, contributes to the reported changes in morphology in the presynaptic terminal.

NMDA receptor activation in the aged rat hippocampus

Age-related alterations of N-methyl-D-aspartate receptor (NMDAr) activation were investigated in the CA1 field of hippocampal slices from young (3-6 months old) and aged (25-33 months old) Sprague-Dawley rats by using ex vivo extracellular electrophysiological recording techniques. NMDAr-mediated field excitatory postsynaptic potentials (fEPSPs) were induced by electrical stimulation of glutamatergic fibers in a magnesium (Mg(2+))-free medium supplemented with the non-NMDAr antagonist CNQX. The fEPSPs were significantly smaller in aged rats, whereas the response of presynaptic afferent fibers remained unaffected. No significant age-related differences were found in the ability of Mg(2+) to depress the magnitude of NMDAr-mediated fEPSPs. The responsiveness of postsynaptic NMDAr to the agonist was assessed in both groups of animals. No age-related differences were recorded either in the depolarizing effect of bath-applied NMDA or in the magnitude of the depolarization after altering extracellular Mg(2+) concentration. Finally, short-term potentiation (STP) of excitatory transmission was studied in young and aged rats considering the pivotal role of NMDAr in synaptic plasticity. No age-related alterations of the magnitude and the time course of STP in response to 10 or 30Hz conditioning stimulation were found. Because of the decrease in the magnitude of NMDAr-mediated synaptic transmission in aged animals, the absence of obvious modifications of synaptic plasticity suggests the occurrence of compensatory mechanisms that are discussed.

17beta-estradiol suppresses expression of long-term depression in aged rats

It has been recently reported that the female steroid hormone 17beta-estradiol enhances synaptic transmission and the magnitude of long-term potentiation (LTP) in adult rodent hippocampus. Moreover, 17beta-estradiol ameliorates cognitive and memory function in postmenopausal women. Since aging is associated with an alteration of synaptic plasticity (e.g., higher susceptibility to long-term depression [LTD]), we examined whether 17beta-estradiol alters the expression of LTD in aged rats. We now report that the induction of LTD recorded from CA1 hippocampal neurons of aged rats is suppressed by 17beta-estradiol treatment, which produced only a minimal effect in suppressing LTD in adult rats. These results suggest that estrogen may act to improve memory by suppressing forgetfulness via a synaptic mechanism, such as LTD.

Age-related decreases in lymphocyte protein kinase C activity and translocation are reduced by aerobic fitness

This study investigated the effects of advancing age and long-term aerobic fitness on lymphocyte protein kinase C (PKC) activity and translocation. Lymphocytes were obtained from young (20-36 years old) and older (61-78 years old) healthy men who were either aerobically conditioned or deconditioned. Both baseline PKC activity and the response of this enzyme to the direct PKC stimulating agent, phorbol 12-myristate, 13-acetate (PMA) or to the mitogen, phytohaemagglutinin (PHA), were measured in partially purified extracts of cytosolic and membranous fractions of lymphocytes. Basal PKC activity, PMA-induced redistribution of PKC, and PHA-induced enhancement of PKC activity were reduced among older subjects in both lymphocyte cytosolic and membranous fractions. However, the magnitudes of these reductions were smaller among the older subjects who were aerobically fit. Lymphocyte PKC activity and translocation may be biological markers of aging, and the maintenance of aerobic fitness into later life may serve to slow the rate at which activation of this enzyme declines during senescence.

Caloric restriction prevents age-related deficits in LTP and in NMDA receptor expression

A major focus of aging research has been the search for treatments that will prevent or ameliorate the memory deficits associated with aging. One paradigm, lifelong caloric restriction, has been reported to reduce some of the effects of aging. In the current report, we examined the effects of this treatment on age-related deficits in LTP, a putative cellular building block for memory formation. We report here that lifelong caloric restriction completely prevents the age-related deficit in LTP. In addition, we report that there is a dramatic decrease in the expression of the NMDA receptor subunit NR1 in aged rats and this age-related defect is also prevented by caloric restriction. These data provide a molecular and cellular mechanism by which life long caloric restriction may ameliorate some of the cognitive deficits associated with the aging process.

Quantitating silver-stained neurodegeneration: the neurotoxicity of trimethlytin (TMT) in aged rats

This report describes the development of a histoanalytical procedure to measure the degree of neurodegeneration produced by the organometal toxicant trimethyltin (TMT). Based on a previous, non-quantitated experiment we hypothesized that the same dose of TMT would produce greater damage in animals of increasing age. Male rats aged 6, 12, 18, or 24 months at the time of dosing were given either 4.5 mg/kg TMT or saline (i.p.). One month after dosing, rats were perfused and their brains removed and processed to selectively silver-impregnate degenerating cell bodies as well as axon terminals and dendrites. Neurodegeneration was most prominent in the hippocampi (especially CA1 stratum radiatum) of TMT-treated rats, but not in the controls. Computer-assisted counting of the silver grains marking damage indicated greater neurotoxicity from the same dose of TMT when given to the older animals. Thus the grain density in the 6-month-old TMT-treated rats was not significantly elevated from the 6-month-old controls (P>0.10). The 12-month-old TMT-treated rats had significantly increased grain densities compared to their controls (P<0.05), but still larger increases of grain counts were observed in the 18- and 24-month-old rats (both P-values<0.01). Our findings with TMT are similar to previous, but nonquantitative, reports that the neurotoxic effects of kainic acid and methionine sulfoximine were also greater in older rats. An increased sensitivity to neurotoxicants might help explain the apparently spontaneous degeneration of cortical neurons in aging and in the neurological diseases of old age. The method we report here for quantitation of silver grains marking neurodegeneration should be adaptable to a wide range of histologically-based neurotoxicology investigations.

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

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

Deficits in nerve growth factor release and tyrosine receptor kinase phosphorylation are associated with age-related impairment in long-term potentiation in the dentate gyrus

Previous findings have indicated that nerve growth factor may play a role in the expression of long-term potentiation in perforant path-granule cell synapses and that nerve growth factor treatment restores the ability of aged rats to sustain long-term potentiation. In this study, we have attempted to analyse the changes which occur in nerve growth factor release and tyrosine receptor kinase phosphorylation following tetanization in tissue prepared from dentate gyrus of young rats, as well as aged rats which did or did not sustain long-term potentiation. We report that KCl-stimulated nerve growth factor release was significantly increased in slices of the dentate gyrus or whole hippocampus, but not in synaptosomes prepared from the dentate gyrus. KCl-induced nerve growth factor release was also significantly enhanced in slices prepared from tetanized, compared with untetanized, tissue obtained from young rats and aged rats which sustained long-term potentiation; this response was absent in tissue prepared from aged rats which failed to sustain long-term potentiation, perhaps due to the enhanced basal nerve growth factor release observed in this tissue. Tetanization increased tyrosine receptor kinase phosphorylation in the dentate gyrus of young rats and aged rats which sustained long-term potentiation. In parallel with the changes in nerve growth factor release, tyrosine receptor kinase phosphorylation was markedly increased in untetanized tissue, which may contribute to the lack of effect in tetanized tissue prepared from aged rats which failed to sustain long-term potentiation. We observed that nerve growth factor concentration and tyrosine receptor kinase expression were decreased in aged, compared with young, rats. The data suggest that deficits in nerve growth factor release and subsequent signalling may contribute to age-related deficits in long-term potentiation.

Age-related impairment in LTP is accompanied by enhanced activity of stress-activated protein kinases: analysis of underlying mechanisms

The age-related impairment in long-term potentiation in the rat dentate gyrus is coupled with an increase in the proinflammatory cytokine, interleukin-1beta (IL-1beta). It is possible that this increase in IL-1beta might be a consequence of the age-related increase in reactive oxygen species production in hippocampal tissue. In this study we set out to identify the underlying cause of the age-related increase in reactive oxygen species production and to establish whether any consequences of such a change might impact on the ability of aged rats to sustain long-term potentiation (LTP). We report that there was an age-related increase in the activity of superoxide dismutase but no parallel increases in activities of glutathione peroxidase or catalase, while age-related decreases in the concentration of the scavengers, vitamins E and C and glutathione were also observed. We propose that these compromises in antioxidative strategies may result in an increase in reactive oxygen species production. The data described indicate that IL-1beta and H2O2 increase the activity of two stress-activated mitogen-activated protein kinases, c-Jun NH2-terminal kinase (JNK) and p38 in vitro, while age-related increases in both kinases were observed. We propose that the endogenous increase in these parameters which occurs with age induces the increase in activity of the stress-activated kinases, which in turn impacts on the ability of the aged rat to sustain LTP.

Age-related changes in synaptic function: analysis of the effect of dietary supplementation with omega-3 fatty acids

Depolarization-induced transmitter release in synaptosomes prepared from the hippocampus of aged rats is decreased compared with release from young animals. Although the underlying cause of this deficit is not known, some evidence suggests that increased membrane rigidity may contribute to these age-related synaptic changes. One possible consequence of the decreased transmitter release in the hippocampus of aged rats is a reduced ability to sustain long-term potentiation in perforant path-granule cell synapses, a pathway in which maintenance of long-term potentiation and increased glutamate release have been coupled. The observation that there is an age-dependent impairment in long-term potentiation is consistent with this view. If the age-related deficits in release and long-term potentiation are a consequence of increased membrane rigidity, it must be predicted that any manoeuvre which reverses membrane rigidity should reverse these functional deficits. In the present study, we investigated the effect of dietary manipulation of aged rats with omega-3 fatty acids on synaptic function. The data obtained indicate that an eight-week modified feeding schedule reversed the age-related impairments in long-term potentiation and depolarization-induced glutamate transmitter release. We also report that the concentrations of both docosahexanoic acid and arachidonic acid, two main polyunsaturated fatty acids in neuronal membranes, were decreased in the hippocampus of aged rats, and were restored by dietary manipulation. The data are consistent with the hypothesis that these deficits results from a change in membrane composition.

Age-related changes in LTP and antioxidant defenses are reversed by an alpha-lipoic acid-enriched diet

Among the age-related changes identified in rat hippocampus are impairments in LTP and glutamate release. These deficits have been coupled with decreased arachidonic acid concentration. In this study we compared LTP and glutamate release in groups of aged and young rats fed for 8 weeks on a control diet or on a diet enriched in alpha-lipoic acid. Dietary supplementation in aged rats restored hippocampal arachidonic acid concentration to levels observed in tissue prepared from young rats. We observed that aged rats that received the experimental diet sustained LTP in perforant path-granule cell synapses in a manner indistinguishable from young rats whereas the age-related impairment in glutamate release was reversed in synaptosomes prepared from dentate gyrus obtained from these rats. The evidence presented supports the hypothesis that the alpha-lipoic acid-enriched diet has antioxidant properties, because the age-related increase in superoxide dismutase activity and decrease in alpha-tocopherol concentration were reversed. The finding that the age-related increase in interleukin-1 (IL-1)beta concentration was also reversed suggests a possible role for this cytokine in ageing.

Age-related changes in oxidative mechanisms and LTP are reversed by dietary manipulation

Aged rats exhibit an impaired ability to sustain long-term potentiation in dentate gyrus which correlates with a decrease in arachidonic acid concentration. Here we confirm the previous finding that dietary supplementation with arachidonic acid and its precursor, gamma-linolenic acid, reversed the impairment in LTP in aged rats and report that there is a significant correlation between membrane arachidonic acid concentration and response to tetanic stimulation. We observed that age was associated with decreases in the concentration of vitamins C and E and increased activity of superoxide dismutase, indicative of a compromise in antioxidative defenses; these changes were paralleled by increases in interleukin-1beta (IL-1beta) concentration and lipid peroxidation. Dietary manipulation restored polyunsaturated fatty acid concentrations to values observed in tissue prepared from young rats and reversed the age-related changes in vitamins E and C, IL-1beta concentration and superoxide dismutase activity. We propose that these changes reverse the increase in lipid peroxidation and thereby the age-related change in polyunsaturated fatty acids.