Different types of environmental enrichment have discrepant effects on spatial memory and synaptophysin levels in female mice

Environmental enrichment paradigms that incorporate cognitive stimulation, exercise, and motor learning benefit memory and synaptic plasticity across the rodent lifespan. However, the contribution each individual element of the enriched environment makes to enhancing memory and synaptic plasticity has yet to be delineated. Therefore, the current study tested the effects of three of these elements on memory and synaptic protein levels. Young female C57BL/6 mice were given 3h of daily exposure to either rodent toys (cognitive stimulation) or running wheels (exercise), or daily acrobatic training for 6 weeks prior to and throughout behavioral testing. Controls were group housed, but did not receive enrichment. Spatial working and reference memory were tested in a water-escape motivated radial arm maze. Levels of the presynaptic protein synaptophysin were then measured in frontoparietal cortex, hippocampus, striatum, and cerebellum. Exercise, but not cognitive stimulation or acrobat training, improved spatial working memory relative to controls, despite the fact that both exercise and cognitive stimulation increased synaptophysin levels in the neocortex and hippocampus. These data suggest that exercise alone is sufficient to improve working memory, and that enrichment-induced increases in synaptophysin levels may not be sufficient to improve working memory in young females. Spatial reference memory was unaffected by enrichment. Acrobat training had no effect on memory or synaptophysin levels, suggesting a minimal contribution of motor learning to the mnemonic and neuronal benefits of enrichment. These results provide the first evidence that different elements of the enriched environment have markedly distinct effects on spatial memory and synaptic alterations.     

GSK-3β activity in the hippocampus is required for memory retrieval

Several molecules were recently found to be important for the memory retrieval process in the hippocampus; however, the mechanisms underlying the memory retrieval remain poorly understood. GSK-3β has been implicated in the control of synaptic plasticity and memory formation. Here, we investigated the relationship between hippocampal GSK-3β activity and memory retrieval using behavioral and Western blotting methods. We found that GSK-3β was activated in the hippocampus after a retention session in the passive avoidance task. An intrahippocampal injection of the GSK-3β inhibitor, SB 216763, before the retention session blocked memory retrieval (but not reconsolidation) without affecting locomotor activity. These results suggest that GSK-3β activation would be essential for memory retrieval in the hippocampus.     

Phase-dependent synaptic changes in the hippocampal CA1 field underlying extinction processes in freely moving rats

Recent studies focus on the functional significance of a novel form of synaptic plasticity, low-frequency stimulation (LFS)-induced synaptic potentiation in the hippocampal CA1 area. In the present study, we elucidated dynamic changes in synaptic function in the CA1 field during extinction processes associated with context-dependent fear memory in freely moving rats, with a focus on LFS-induced synaptic plasticity. Synaptic transmission in the CA1 field was transiently depressed during each extinction trial, but synaptic efficacy was gradually enhanced by repeated extinction trials, accompanied by decreases in freezing. On the day following the extinction training, synaptic transmission did not show further changes during extinction retrieval, suggesting that the hippocampal synaptic transmission that underlies extinction processes changes in a phase-dependent manner. The synaptic potentiation produced by extinction training was mimicked by synaptic changes induced by LFS (0.5 Hz) in the group that previously received footshock conditioning. Furthermore, the expression of freezing during re-exposure to footshock box was significantly reduced in the LFS application group in a manner similar to the extinction group. These results suggest that LFS-induced synaptic plasticity may be associated with the extinction processes that underlie context-dependent fear memory. This hypothesis was supported by the fact that synaptic potentiation induced by extinction training did not occur in a juvenile stress model that exhibited extinction deficits. Given the similarity between these electrophysiological and behavioral data, LFS-induced synaptic plasticity may be related to extinction learning, with some aspects of neuronal oscillations, during the acquisition and/or consolidation of extinction memory.     

Role of the somatostatin system in contextual fear memory and hippocampal synaptic plasticity

Somatostatin has been implicated in various cognitive and emotional functions, but its precise role is still poorly understood. Here, we have made use of mice with somatostatin deficiency, based upon genetic invalidation or pharmacologically induced depletion, and Pavlovian fear conditioning in order to address the contribution of the somatostatin system to associative fear memory. The results demonstrate an impairment of foreground and background contextual but not tone fear conditioning in mice with targeted ablation of the somatostatin gene. These deficits were associated with a decrease in long-term potentiation in the CA1 area of the hippocampus. Both the behavioral and the electrophysiological phenotypes were mimicked in wild-type mice through application of the somatostatin-depleting substance cysteamine prior to fear training, whereas no further deficits were observed upon application in the somatostatin null mutants. These results suggest that the somatostatin system plays a critical role in the acquisition of contextual fear memory, but not tone fear learning, and further highlights the role of hippocampal synaptic plasticity for information processing concerning contextual information.     

Cannabinoid CB1 receptor deficiency increases contextual fear memory under highly aversive conditions and long-term potentiation in vivo

The cannabinoid receptor type 1 (CB1) is abundantly expressed in the central nervous system where it negatively controls the release of several neurotransmitters. CB1 activity plays a crucial role in learning and memory and in synaptic plasticity. In the present study, the role of CB1 was investigated in three different hippocampus-dependent memory tasks and in in vivo hippocampal synaptic plasticity in knockout (CB1-ko) and wildtype mice. There was no difference in short-term and long-term social and object recognition memory between CB1-ko and wildtype mice. In contrast, in background contextual fear conditioning CB1-ko mice showed enhanced freezing levels in the conditioning context and increased generalised contextual fear after a high-intensity conditioning foot shock of 1.5 mA, but not after 0.7 mA. In in vivo field potential recordings in the dentate gyrus, CB1-ko mice displayed a decreased paired-pulse facilitation of the populations spikes, suggesting an altered inhibitory synaptic drive onto hippocampal granule cells. Furthermore, CB1-ko mice displayed significantly higher levels of in vivo long-term potentiation (LTP) in the dentate gyrus. In conclusion, CB1 deficiency leads to enhanced contextual fear memory and altered synaptic plasticity in the hippocampus, supporting the key role of endocannabinoid signalling in learning and memory, in particular following highly aversive encounters.     

Reelin supplementation enhances cognitive ability, synaptic plasticity, and dendritic spine density

Apolipoprotein receptors belong to an evolutionarily conserved surface receptor family that has intimate roles in the modulation of synaptic plasticity and is necessary for proper hippocampal-dependent memory formation. The known lipoprotein receptor ligand Reelin is important for normal synaptic plasticity, dendritic morphology, and cognitive function; however, the in vivo effect of enhanced Reelin signaling on cognitive function and synaptic plasticity in wild-type mice is unknown. The present studies test the hypothesis that in vivo enhancement of Reelin signaling can alter synaptic plasticity and ultimately influence processes of learning and memory. Purified recombinant Reelin was injected bilaterally into the ventricles of wild-type mice. We demonstrate that a single in vivo injection of Reelin increased activation of adaptor protein Disabled-1 and cAMP-response element binding protein after 15 min. These changes correlated with increased dendritic spine density, increased hippocampal CA1 long-term potentiation (LTP), and enhanced performance in associative and spatial learning and memory. The present study suggests that an acute elevation of in vivo Reelin can have long-term effects on synaptic function and cognitive ability in wild-type mice.     

Long-term plasticity of intrinsic excitability: learning rules and mechanisms

Spatio-temporal configurations of distributed activity in the brain is thought to contribute to the coding of neuronal information and synaptic contacts between nerve cells could play a central role in the formation of privileged pathways of activity. Synaptic plasticity is not the exclusive mode of regulation of information processing in the brain, and persistent regulations of ionic conductances in some specialized neuronal areas such as the dendrites, the cell body, and the axon could also modulate, in the long-term, the propagation of neuronal information. Persistent changes in intrinsic excitability have been reported in several brain areas in which activity is elevated during a classical conditioning. The role of synaptic activity seems to be a determinant in the induction, but the learning rules and the underlying mechanisms remain to be defined. We discuss here the role of synaptic activity in the induction of intrinsic plasticity in cortical, hippocampal, and cerebellar neurons. Activation of glutamate receptors initiates a long-term modification in neuronal excitability that may represent a parallel, synergistic substrate for learning and memory. Similar to synaptic plasticity, long-lasting intrinsic plasticity appears to be bidirectional and to express a certain level of input or cell specificity. These nonsynaptic forms of plasticity affect the signal propagation in the axon, the dendrites, and the soma. They not only share common learning rules and induction pathways with the better-known synaptic plasticity such as NMDA receptor dependent LTP and LTD, but also contribute in synergy with these synaptic changes to the formation of a coherent engram.     

Hippocampal long-term potentiation, memory, and longevity in mice that overexpress mitochondrial superoxide dismutase

Superoxide has been shown to be critically involved in several pathological manifestations of aging animals. In contrast, superoxide also can act as a signaling molecule to modulate signal transduction cascades required for hippocampal synaptic plasticity. Mitochondrial superoxide dismutase (SOD-2 or Mn-SOD) is a key antioxidant enzyme that scavenges superoxide. Thus, SOD-2 may not only prevent aging-related oxidative stress, but may also regulate redox signaling in young animals. We used transgenic mice overexpressing SOD-2 to study the role of mitochondrial superoxide in aging, synaptic plasticity, and memory-associated behavior. We found that overexpression of SOD-2 had no obvious effect on synaptic plasticity and memory formation in young mice, and could not rescue the age-related impairments in either synaptic plasticity or memory in old mice. However, SOD-2 overexpression did decrease mitochondrial superoxide in hippocampal neurons, and extended the lifespan of the mice. These findings increase our knowledge of the role of mitochondrial superoxide in physiological and pathological processes in the brain.     

Transgenic mice expressing a truncated form of CREB-binding protein (CBP) exhibit deficits in hippocampal synaptic plasticity and memory storage

Deletions, translocations, or point mutations in the CREB-binding protein (CBP) gene have been associated with Rubinstein-Taybi Syndrome; a human developmental disorder characterized by retarded growth and reduced mental function. To examine the role of CBP in memory, transgenic mice were generated in which the CaMKII alpha promoter drives expression of an inhibitory truncated CBP protein in forebrain neurons. Examination of hippocampal long-term potentiation (LTP), a form of synaptic plasticity thought to underlie memory storage, revealed significantly reduced late-phase LTP induced by dopamine-regulated potentiation in hippocampal slices from CBP transgenic mice. However, four-train induced late-phase LTP is normal. Behaviorally, CBP transgenic mice exhibited memory deficits in spatial learning in the Morris water maze and deficits in long-term memory for contextual fear conditioning, two hippocampus-dependent tasks. Together, these results demonstrate that CBP is involved in specific forms of hippocampal synaptic plasticity and hippocampus-dependent long-term memory formation.     

Contextual memory deficits observed in mice overexpressing small conductance Ca2+-activated K+ type 2 (KCa2.2, SK2) channels are caused by an encoding deficit

Hippocampal-dependent synaptic plasticity and memory are modulated by apamin-sensitive small conductance Ca2+-activated K+ (SK) channels. Transgenic mice overexpressing SK2 channels (SK2+/T mice) exhibit marked deficits in hippocampal memory and synaptic plasticity, as previously reported. Here, we examined whether SK2 overexpression affects the encoding or retention of contextual memory. Compared with wild-type littermates, SK2+/T mice exhibited significantly less context-dependent freezing 10 min and 24 h after conditioning. Interestingly, this contextual memory impairment was eliminated if SK2+/T mice were permitted longer pre-exposure to the conditioning chamber. These data support converging evidence that SK2 channels restrict the encoding of hippocampal memory.     

Environmental enrichment modifies the PKA-dependence of hippocampal LTP and improves hippocampus-dependent memory

cAMP-dependent protein kinase (PKA) is critical for the expression of some forms of long-term potentiation (LTP) in area CA1 of the mouse hippocampus and for hippocampus-dependent memory. Exposure to spatially enriched environments can modify LTP and improve behavioral memory in rodents, but the molecular bases for the enhanced memory performance seen in enriched animals are undefined. We tested the hypothesis that exposure to a spatially enriched environment may alter the PKA dependence of hippocampal LTP. Hippocampal slices from enriched mice showed enhanced LTP following a single burst of 100-Hz stimulation in the Schaffer collateral pathway of area CA1. In slices from nonenriched mice, this single-burst form of LTP was less robust and was unaffected by Rp-cAMPS, an inhibitor of PKA. In contrast, the enhanced LTP in enriched mice was attenuated by Rp-cAMPS. Enriched slices expressed greater forskolin-induced, cAMP-dependent synaptic facilitation than did slices from nonenriched mice. Enriched mice showed improved memory for contextual fear conditioning, whereas memory for cued fear conditioning was unaffected following enrichment. Our data indicate that exposure of mice to spatial enrichment alters the PKA dependence of LTP and enhances one type of hippocampus-dependent memory. Environmental enrichment can transform the pharmacological profile of hippocampal LTP, possibly by altering the threshold for activity-dependent recruitment of the cAMP-PKA signaling pathway following electrical and chemical stimulation. We suggest that experience-dependent plasticity of the PKA dependence of hippocampal LTP may be important for regulating the efficacy of hippocampus-based memory.     

Translocation and activation of Rac in the hippocampus during associative contextual fear learning

Small G proteins including Rac are mediators of changes in neuronal morphology associated with synaptic plasticity. Previous studies in our laboratory showed that Rac is highly expressed in the adult mouse hippocampus, a brain area that exhibits robust synaptic plasticity and is crucial for the acquisition of memories. In this study, we investigated whether Rac was involved in NMDA receptor-dependent associative fear learning in the area CA1 of adult mouse hippocampus. We found that Rac translocation and activation was increased in the hippocampus following associative fear conditioning in mice, and that these increases are blocked by intraperitoneal injection of the NMDA receptor channel blocker MK801 at the acquisition stage. Our data indicate that NMDA receptor-dependent associative fear learning alters Rac localization and function in the mouse hippocampus.     

Enriching the environment of alphaCaMKIIT286A mutant mice reveals that LTD occurs in memory processing but must be subsequently reversed by LTP

alphaCaMKII(T286A) mutant mice lack long-term potentiation (LTP) in the hippocampal CA1 region and are impaired in spatial learning. In situ hybridization confirms that the mutant mice show the same developmental expression of alphaCaMKII as their wild-type littermates. A simple hypothesis would suggest that if LTP is a substrate for learning, then enriching the environment should cause learning-dependent changes in wild-type mice that have LTP. Such changes would not be seen in LTP-deficient alphaCaMKII(T286A) mutants. Excitatory synaptic currents in CA1 neurons, recorded with patch clamp in brain slices, revealed that enrichment induces an increase in glutamate release probability and a decreased miniature current amplitude. Confocal microscopy also showed dendritic spine density to be reduced. However, contrary to the hypothesis above, these enrichment-induced changes occur only in the mutant mice and are not detectable in wild-type littermates. We suggest that enrichment induces alphaCaMKII-independent changes in both wild-type and mutant mice. Such changes may be subsequently reversed in wild-type animals via alphaCaMKII-dependent mechanisms, such as LTP. Reversal of plasticity has long been hypothesized to be essential for the hippocampus to maintain its role in memory processing. The inability to reverse plasticity in alphaCaMKII(T286A) mutant mice would then result in impairment of spatial learning.     

A role for ERK MAP kinase in physiologic temporal integration in hippocampal area CA1

Recent studies demonstrate a requirement for the Extracellular signal Regulated Kinase (ERK) mitogen-activated protein kinase (MAPK) cascade in both the induction of long-lasting forms of hippocampal synaptic plasticity and in hippocampus-dependent associative and spatial learning. In the present studies, we investigated mechanisms by which ERK might contribute to synaptic plasticity at Schaffer collateral synapses in hippocampal slices. We found that long-term potentiation (LTP) induced with a pair of 100-Hz tetani does not require ERK activation in mice whereas it does in rats. However, in mice, inhibition of ERK activation blocked LTP induced by two LTP induction paradigms that mimicked the endogenous θ rhythm. In an additional series of studies, we found that mice specifically deficient in the ERK1 isoform of MAPK showed no impairments in tests of hippocampal physiology. To investigate ERK-dependent mechanisms operating during LTP-inducing stimulation paradigms, we monitored spike production in the cell body layer of the hippocampus during the period of θ-like LTP-inducing stimulation. θ-burst stimulation (TBS) produced a significant amount of postsynaptic spiking, and the likelihood of spike production increased progressively over the course of the three trains of TBS independent of any apparent increase in Excitatory Post-Synaptic Potential (EPSP) magnitude. Inhibition of ERK activation dampened this TBS-associated increase in spiking. These data indicate that, for specific patterns of stimulation, ERK may function in the regulation of neuronal excitability in hippocampal area CA1. Overall, our data indicate that the progressive increase in spiking observed during TBS represents a form of physiologic temporal integration that is dependent on ERK MAPK activity.     

Long-term enrichment enhances the cognitive behavior of the aging neurogranin null mice without affecting their hippocampal LTP

Neurogranin (Ng), a PKC substrate, is abundantly expressed in brain regions important for cognitive functions. Deletion of Ng caused severe deficits in spatial learning and LTP in the hippocampal CA1 region of mice. These Ng-/- mice also exhibit deficits in the amplification of their hippocampal signaling pathways critical for learning and memory. A short-term exposure to an enriched environment failed to improve their behavioral performances. Here, we showed that a long-term enrichment protocol for the aging mice was beneficial to the Ng-/- as well as Ng+/+ and Ng+/- mice in preventing age-related cognitive decline. Enrichment also caused an increase in the hippocampal CREB level of all three genotypes and Ng level of Ng+/+ and Ng+/- mice, but not that of alphaCaMKII or ERK. Interestingly, hippocampal slices of these enriched aging Ng-/- mice, unlike those of Ng+/+ and Ng+/- mice, did not show enhancement in the high frequency stimulation (HFS)-induced LTP in the CA1 region. It appears that the learning and memory processes in these enriched aging Ng-/- mice do not correlate with the HFS-induced LTP, which is facilitated by Ng. These results demonstrated that long-term enrichment for the aging Ng-/- mice may improve their cognitive function through an Ng-independent plasticity pathway.