Stable, long-term, spatial memory in young and aged rats achieved with a one day Morris water maze training protocol.

Here, we present data demonstrating that a 1 d Morris water maze training protocol is effective at producing stable, long-term spatial memory in both young (3 mo old) and aged (24 mo old) F344xBN rats. Four trials in each of four sessions separated by a 2.5 h ISI produced robust selective search for the platform 1 and 4 d after training, in both age groups. A 1 h ISI protocol did not produce good retention. Also, compressing the trials into just two sessions separated by a 2.5 h ISI produced limited retention in only young rats.

Inhibiting PKMζ reveals dorsal lateral and dorsal medial striatum store the different memories needed to support adaptive behavior.

Evidence suggests that two regions of the striatum contribute differential support to instrumental response selection. The dorsomedial striatum (DMS) is thought to support expectancy-mediated actions, and the dorsolateral striatum (DLS) is thought to support habits. Currently it is unclear whether these regions store task-relevant information or just coordinate the learning and retention of these solutions by other brain regions. To address this issue, we developed a two-lever concurrent variable-interval reinforcement operant conditioning task and used it to assess the trained rat's sensitivity to contingency shifts. Consistent with the view that these two regions make different contributions to actions and habits, injecting the NMDA antagonist DL-AP5 into the DMS just prior to the shift impaired the rat's performance but enhanced performance when injected into the DLS. To determine if these regions support memory content, we first trained rats on a biased concurrent schedule (Lever 1: VI 40" and Lever 2: VI 10"). With the intent of "erasing" the memory content stored in striatum, after this training we inhibited the putative memory-maintenance protein kinase C isozyme protein kinase Mζ (PKMζ). Infusing zeta inhibitory peptide (ZIP) into the DLS enhanced the rat's ability to adapt to the contingency shift 2 d later, whereas injecting it into the DMS had the opposite effect. Infusing GluR2(3Y) into the DMS 1 h before ZIP infusions prevented ZIP from impairing the rat's sensitivity to the contingency shift. These results support the hypothesis that the DMS stores information needed to support actions and the DLS stores information needed to support habits.

PKMzeta maintains spatial, instrumental, and classically conditioned long-term memories.

How long-term memories are stored is a fundamental question in neuroscience. The first molecular mechanism for long-term memory storage in the brain was recently identified as the persistent action of protein kinase Mzeta (PKMzeta), an autonomously active atypical protein kinase C (PKC) isoform critical for the maintenance of long-term potentiation (LTP). PKMzeta maintains aversively conditioned associations, but what general form of information the kinase encodes in the brain is unknown. We first confirmed the specificity of the action of zeta inhibitory peptide (ZIP) by disrupting long-term memory for active place avoidance with chelerythrine, a second inhibitor of PKMzeta activity. We then examined, using ZIP, the effect of PKMzeta inhibition in dorsal hippocampus (DH) and basolateral amygdala (BLA) on retention of 1-d-old information acquired in the radial arm maze, water maze, inhibitory avoidance, and contextual and cued fear conditioning paradigms. In the DH, PKMzeta inhibition selectively disrupted retention of information for spatial reference, but not spatial working memory in the radial arm maze, and precise, but not coarse spatial information in the water maze. Thus retention of accurate spatial, but not procedural and contextual information required PKMzeta activity. Similarly, PKMzeta inhibition in the hippocampus did not affect contextual information after fear conditioning. In contrast, PKMzeta inhibition in the BLA impaired retention of classical conditioned stimulus-unconditioned stimulus (CS-US) associations for both contextual and auditory fear, as well as instrumentally conditioned inhibitory avoidance. PKMzeta inhibition had no effect on postshock freezing, indicating fear expression mediated by the BLA remained intact. Thus, persistent PKMzeta activity is a general mechanism for both appetitively and aversively motivated retention of specific, accurate learned information, but is not required for processing contextual, imprecise, or procedural information.

Separate neural substrates for skill learning and performance in the ventral and dorsal striatum.

It is widely accepted that the striatum of the basal ganglia is a primary substrate for the learning and performance of skills. We provide evidence that two regions of the rat striatum, ventral and dorsal, play distinct roles in instrumental conditioning (skill learning), with the ventral striatum being critical for learning and the dorsal striatum being important for performance but, notably, not for learning. This implies an actor (dorsal) versus director (ventral) division of labor, which is a new variant of the widely discussed actor-critic architecture. Our results also imply that the successful performance of a skill can ultimately result in its establishment as a habit outside the basal ganglia.

Activation of the infralimbic cortex in a fear context enhances extinction learning.

Activation of the infralimbic region (IL) of the medial prefrontal cortex (mPFC) reduces conditioned fear in a variety of situations, and the IL is thought to play an important role in the extinction of conditioned fear. Here we report a series of experiments using contextual fear conditioning in which the IL is activated with the GABAa antagonist picrotoxin (Ptx) during a single extinction session in the fear context. We investigate the impact of this manipulation on subsequent extinction sessions in which Ptx is no longer present. First, we demonstrate that a single treatment with intra-IL Ptx administered in a conditioned fear context greatly accelerates the rate of extinction on the following days. Importantly, IL-Ptx also enhances extinction to a different fear context than the one in which IL-Ptx was administered. Thus, IL-Ptx primes extinction learning regardless of the fear context in which the IL was initially activated. Second, activation of the IL must occur in conjunction with a fear context in order to enhance extinction; the extinction enhancing effect is not observable if IL-Ptx is administered in a neutral context. Finally, this extinction enhancing effect is specific to the IL for it does not occur if Ptx is injected into the prelimbic region (PL) of the mPFC. The results indicate a novel persisting control of fear induced by activation of the IL and suggest that IL activation induces changes in extinction-related circuitry that prime extinction learning.

Context representations, context functions, and the parahippocampal-hippocampal system.

Psychologists and neurobiologists have a long-standing interest in understanding how the context surrounding the events of our lives is represented and how it influences our behavior. The hippocampal formation emerged very early as a major contributor to how context is represented and functions. There is a large literature examining its contribution that on the surface reveals an array of conflicting outcomes and controversy. This review reveals that these conflicts can be resolved by building Nadel and Willner's dual-process theory of context representations. Two general conclusions emerge: (1) There are two neural systems that can support context representations and functions-a neocortical system composed primarily of perirhinal and postrhinal cortices and a hippocampal system that includes perirhinal, postrhinal, entorhinal cortices, and the hippocampal formation. (2) These two systems are not equivalent-some context representations and functions are uniquely supported by the hippocampal system. These conclusions are discussed in the context of canonical ideas about the special properties of the hippocampal system that enable it to make unique contributions to memory.

Hippocampal and extrahippocampal systems compete for control of contextual fear: role of ventral subiculum and amygdala.

Two neural systems, a hippocampal system and an extrahippocampal system compete for control over contextual fear, and the hippocampal system normally dominates. Our experiments reveal that output provided by the ventral subiculum is critical for the hippocampal system to win this competition. Bilateral electrolytic lesions of the ventral subiculum after conditioning, but not before conditioning, impaired contextual fear conditioning. Reversibly inactivating this region by bilateral injections of muscimol produced the same results-no impairment when the injection occurred prior to conditioning but a significant impairment when this region was inactivated after conditioning. Thus, the extrahippocampal system can support contextual fear conditioning if the ventral subiculum is disabled before conditioning but not if it is disabled after conditioning. Our experiments also reveal that the basolateral region of the amygdala (BLA) is where the two systems compete for associative control of the fear system. To test this hypothesis we reasoned that the extrahippocampal system would also acquire associative control over the fear system, even if the hippocampal system were functional, if the basal level of plasticity potential in the BLA could be increased. We did this by injecting the D1 dopamine agonist, SKF82958, into the BLA just prior to conditioning. This treatment resulted in a significant increase in freezing when the ventral subiculum was disabled prior to the test. These results are discussed in relationship to the idea that D1 agonists increase plasticity potential by increasing the pool of available extrasynaptic GluR1 receptors in the population of neurons supporting acquired fear.

DHPG activation of group 1 mGluRs in BLA enhances fear conditioning.

Group 1 metabotropic glutamate receptors are known to play an important role in both synaptic plasticity and memory. We show that activating these receptors prior to fear conditioning by infusing the group 1 mGluR agonist, (R.S.)-3,5-dihydroxyphenylglycine (DHPG), into the basolateral region of the amygdala (BLA) of adult Sprague-Dawley rats enhances freezing normally supported by a weak footshock. This effect of DHPG was blocked when it was co-infused with either the general group 1 mGluR1 antagonist, (R,S)-1-aminoindan-1,5 dicarboxylic acid (AIDA), or with the selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP). These results support previous findings by Rodrigues and colleagues that mGluR5s in the lateral region of the amygdala make an import contribution to fear conditioning. More importantly, they support the general ideas embedded in the concept of metaplasticity, as per Abraham, and the synaptic-tagging hypothesis per Frey and Morris-that the processes that specify the content of experience can be experimentally separated from those needed to acquire the memory.

Characterization of the sickness response in young and aging rats following E. coli infection.

To more fully characterize the sickness response in young (3 mo) and older (24 mo) rats, we measured core body temperature (CBT), activity level, and body weight changes for 7 days following a peripheral immune challenge with Escherichia coli. CBT increases were delayed and blunted during the 12h following infection in older rats. Indeed, in aging subjects the initial response was hypothermia, but this was followed by a significant and prolonged elevation in CBT lasting 3 days. Young rats, in contrast, generated a rapid and robust CBT elevation lasting just over a day. Activity level was significantly reduced only on the day of E. coli administration in both young and older rats. Body weight loss was equivalent in both age groups one day after E. coli administration, although there was a trend for older rats to continue losing more weight across the next 6 days than in young rats. This is the first study to examine CBTs in young and older rats for a protracted amount of time, thereby revealing that aging rats do have an exaggerated, albeit delayed, fever which is in keeping with other exaggerated sickness behavioral responses observed in aging rodents.

Time course of hippocampal IL-1 beta and memory consolidation impairments in aging rats following peripheral infection.

We previously reported that aging F344XBN rats are more vulnerable to disruptions of memory consolidation processes following an injection of Escherichia coli than are young rats. Furthermore, this disruption was specific to hippocampal-dependent memory. In the present study we examined the time course of the proinflammatory cytokine IL-1 beta in young and old rats following a peripheral injection of E. coli. Compared to young rats, aging rats treated with E. coli showed an exaggerated and prolonged up-regulation of IL-1 beta protein in the hippocampus, but not in hypothalamus, parietal cortex, prefrontal cortex, serum or spleen. Aging rats showed greater hippocampal IL-1 beta protein levels than their young counterparts 4h after E. coli, and these levels remained significantly elevated for 8 but not 14 days after E. coli. In a second experiment, aging rats exhibited anterograde memory consolidation impairments 4 and 8 days after an E. coli injection, but not after 14 days. A third experiment revealed that following an E. coli injection, bacterial clearance from the spleen and peritoneum was not impaired in aged rats, suggesting that elevations in hippocampal IL-1 beta were not mediated by impaired clearance in the periphery in aging rats. These data suggest that the exaggerated and prolonged elevation of IL-1 beta, specifically in the hippocampus, may be responsible for hippocampal-dependent memory impairments observed in aging rats following a bacterial infection.

The role of the dorsal striatum and dorsal hippocampus in probabilistic and deterministic odor discrimination tasks.

Three experiments explored the contribution of the cortico-striatal system and the hippocampus system to the acquisition of solutions to simultaneous instrumental odor discriminations. Inactivation of the dorsal striatum after rats had reached criterion on a three problem probabilistic set of discriminations--A (80%) vs. B (20%), C (67%) vs. D (33%), E(67%) vs. F(33%)--impaired test performance and disrupted performance when the rats were tested with novel cue combinations (C vs. F and E vs. D), where control animals chose C and F. In contrast, inactivating the dorsal hippocampus enhanced performance on this task and on a deterministic discrimination A (100%) vs. B (0%). These results are consistent with the complementary learning systems view, which assumes that the cortico-striatal and hippocampal system capture information in parallel. How this information combines to influence task performance depends on the compatibility of the content captured by each system. These results suggest that the trial-specific information captured by the hippocampal system can be incompatible with the across-trial integration of trial outcomes captured by the cortico-striatal system.

Is there a baby in the bathwater? Maybe: some methodological issues for the de novo protein synthesis hypothesis.

The de novo protein synthesis hypothesis has a long history and will no doubt continue to influence research. Yet, the primary behavioral evidence for this claim continues to come from studies in which amnesia is produced by broad scale protein synthesis inhibitors such as anisomycin. What is remarkable is the uncritical acceptance of the idea that because anisomycin is a protein synthesis inhibitor then it must have produced amnesia because it prevented translation. Several viable alternative interpretations of such experiments are discussed here and it is concluded that there is nothing to be gained by the continued use of broad-scaled antibiotics to address this hypothesis. Moreover, this approach cannot answer two critical and related questions - why must new proteins be synthesized and what are they? A focus on specific proteins such as those synthesized locally and upregulate the translation of other proteins may be a promising approach to answering these questions.

Early-life infection leads to altered BDNF and IL-1beta mRNA expression in rat hippocampus following learning in adulthood.

Neonatal bacterial infection in rats leads to profound hippocampal-dependent memory impairments following a peripheral immune challenge in adulthood. Here, we determined whether neonatal infection plus an immune challenge in adult rats is associated with impaired induction of brain-derived neurotrophic factor (BDNF) within the hippocampus (CA1, CA3, and dentate gyrus) following fear conditioning. BDNF is well characterized for its critical role in learning and memory. Rats injected on postnatal day 4 with PBS (vehicle) or Escherichia coli received as adults either no conditioning or a single 2min trial of fear conditioning. Half of the rats in the conditioned group then received a peripheral injection of 25mug/kg lipopolysaccharide (LPS) and all were sacrificed 1 or 4h later. Basal (unconditioned) BDNF mRNA did not differ between groups. However, following conditioning, neonatal infection with E. coli led to decreased BDNF mRNA induction in all regions compared to PBS-treated rats. This decrease in E. coli-treated rats was accompanied by a large increase in IL-1beta mRNA in CA1. Taken together, these data indicate that early infection strongly influences the induction of IL-1beta and BDNF within distinct regions of the hippocampus, which likely contribute to observed memory impairments in adulthood.

Context preexposure prevents forgetting of a contextual fear memory: implication for regional changes in brain activation patterns associated with recent and remote memory tests.

Contextual fear conditioning was maintained over a 15-day retention interval suggesting no forgetting of the conditioning experience. However, a more subtle generalization test revealed that, as the retention interval increased, rats showed enhanced generalized fear to an altered context. Preexposure to the training context prior to conditioning, however, prevented this enhanced generalized fear from developing. These results support the hypothesis that the memory representation of the context degrades as the memory ages and is responsible for enhanced generalization. The implications of these results for systems consolidation versus forgetting interpretations of regional changes in neural activation patterns that occur as memories age are discussed.

Amygdala regulation of immediate-early gene expression in the hippocampus induced by contextual fear conditioning.

The basolateral nuclei of the amygdala (BLA) are thought to modulate memory storage in other brain regions (McGaugh, 2004). We reported that BLA modulates the memory for both an explored context and for contextual fear conditioning. Both of these memories depend on the hippocampus. Here, we examined the hypothesis that the BLA exerts its modulatory effect by regulating the expression of immediate-early genes (IEGs) in the hippocampus. The main findings of these experiments were: (1) Arc activity-regulated cytoskeletal protein (Arc), an immediate-early gene (also termed Arg 3.1) and c-fos mRNA are induced in the hippocampus after a context exposure, or context plus shock experience, but not after an immediate shock; and (2) BLA inactivation with muscimol attenuated the increase in Arc and c-fos mRNA in the hippocampus associated with contextual fear conditioning but did not influence Arc mRNA associated with context exploration. These results support the hypothesis that the amygdala modulates contextual fear memory by regulating expression of IEGs in the hippocampus.

The role of dorsal hippocampus and basolateral amygdala NMDA receptors in the acquisition and retrieval of context and contextual fear memories.

The authors used 3-phase context preexposure facilitation methodology to study the contribution of N-methyl-D-aspartate (NMDA) receptors in dorsal hippocampus (DH) and the basal lateral region of the amygdala (BLA) to (a) acquisition of the context memory, (b) retrieval of the context memory, (c) acquisition of context-shock association, and (d) retrieval of the context-shock association. The NMDA receptor antagonist D-2-amino-5 phosphonopentanoic acid (D-AP5) was injected into either the DH or BLA prior to (a) the context preexposure phase, (b) the immediate shock phase, or (c) the test for contextual fear. Antagonizing NMDA receptors in the DH impaired the acquisition of the context memory but did not affect its retrieval or retrieval of the fear memory. Antagonizing NMDA receptors with D-AP5 in the BLA impaired acquisition of the context-shock association but had no effect on the expression of fear. However, both DL-AP5 and L-AP5 reduced the expression of fear when they were injected into the amygdala prior to testing for contextual fear.

Characterization of the temporo-spatial effects of chronic bilateral intrahippocampal cannulae on interleukin-1beta.

The implantation of a foreign object in the brain produces an acute neuroinflammatory state in which glia (astrocytes and microglia) may remain chronically activated in response to the inert foreign object. Activated glia can exhibit a sensitized pro-inflammatory response to immunogenic stimuli. This may be relevant to intracranial cannula implantation, which is commonly used to administer substances directly into the brain. If intracranial cannulation activates glia, a subsequent neuroinflammatory stimulus might induce a potentiated pro-inflammatory response, thereby introducing a potential experimental confound. We tested the temporal and spatial responses of interleukin-1beta (IL-1beta) to an acute immune challenge produced by lipopolysaccharide (LPS) in animals with chronic bilateral intrahippocampal cannulae implants (stainless steel). Cannulation increased the gene expression of the microglia activation antigens MHC II and CD11b, but not the astrocyte antigen GFAP. Moreover, this activation was temporally and spatially dependent. In addition, IL-1beta mRNA, but not IL-1beta protein, was significantly elevated in cannulated animals. Administration of LPS, however, significantly potentiated the brain IL-1beta response in cannulated animals, but not in stab wounded or naïve animals. This IL-1beta response was also temporo-spatially dependent. Thus, the pro-inflammatory sequelae of intracranial cannulation should be considered when designing studies of neuroinflammatory processes.

Neonatal infection-induced memory impairment after lipopolysaccharide in adulthood is prevented via caspase-1 inhibition.

We have reported that neonatal infection leads to memory impairment after an immune challenge in adulthood. Here we explored whether events occurring as a result of early infection alter the response to a subsequent immune challenge in adult rats, which may then impair memory. In experiment 1, peripheral infection with Escherichia coli on postnatal day 4 increased cytokines and corticosterone in the periphery, and cytokine and microglial cell marker gene expression in the hippocampus of neonate pups. Next, rats treated neonatally with E. coli or PBS were injected in adulthood with lipopolysaccharide (LPS) or saline and killed 1-24 h later. Microglial cell marker mRNA was elevated in hippocampus in saline controls infected as neonates. Furthermore, LPS induced a greater increase in glial cell marker mRNA in hippocampus of neonatally infected rats, and this increase remained elevated at 24 h versus controls. After LPS, neonatally infected rats exhibited faster increases in interleukin-1beta (IL-1beta) within the hippocampus and cortex and a prolonged response within the cortex. There were no group differences in peripheral cytokines or corticosterone. In experiment 2, rats treated neonatally with E. coli or PBS received as adults either saline or a centrally administered caspase-1 inhibitor, which specifically prevents the synthesis of IL-1beta, 1 h before a learning event and subsequent LPS challenge. Caspase-1 inhibition completely prevented LPS-induced memory impairment in neonatally infected rats. These data implicate IL-1beta in the set of immune/inflammatory events that occur in the brain as a result of neonatal infection, which likely contribute to cognitive alterations in adulthood.

A behavioural characterization of neonatal infection-facilitated memory impairment in adult rats.

We have reported that exposure to bacteria (Escherichia coli) during the neonatal period in rats is associated with impaired memory for a novel context in adulthood. However, impairment is only observed if a peripheral immune challenge (bacterial lipopolysaccharide (LPS)) is administered immediately following context exposure. The goal of the current study was to more fully characterize this phenomenon. In Experiment 1, memory impairment as a result of neonatal infection and subsequent LPS challenge was observed in juvenile rats, indicating that the changes induced by infection occur early on and are then manifest throughout the lifespan. In Experiment 2, infection in juvenile rats did not lead to LPS-induced memory impairment in adulthood, suggesting there is a critical period for early infection-induced alterations. In Experiments 3 and 4, memory for a novel context was impaired in neonatally infected rats, a task that is dependent on the hippocampus, whereas cued memory for a tone, which does not depend on the hippocampus, was not impaired. Furthermore, long-term, but not short-term contextual memory was impaired in adult rats infected as neonates following an LPS challenge either 24 h before or immediately after conditioning. Finally, in Experiment 5, no neonatal group differences were observed in corticosterone or open field behaviour, suggesting that decreased freezing to a conditioned context reflects impaired memory, and not simply hyperactivity or altered stress reactivity. Taken together, we have demonstrated that neonatal infection results in robust hippocampal-dependent memory impairment following an immune challenge in adulthood using a number of conditioning paradigms.

The role of the dorsal hippocampus in the acquisition and retrieval of context memory representations.

It is argued that the hippocampus contributes to contextual fear conditioning by supporting the acquisition of a conjunctive memory representation of context, which associates with shock. This function was examined by studying the context pre-exposure facilitation effect (CPFE). A rat that is shocked immediately after being placed into a context subsequently displays almost no fear of that context. However, if it is pre-exposed to the context the day before immediate shock, it displays significant freezing to that context. By using 5-aminomethyl-3-hydroxysoxazole to temporarily inactivate the dorsal hippocampus (DH) at three different phases of the procedure, which produces the CPFE, we show that the hippocampus is necessary for the following: (1) acquisition of the context memory, (2) retrieval of this memory at the time of immediate shock, and (3) retrieval of the context-shock memory at the time of testing. In contrast, inactivating the DH before a standard contextual shock experience had no effect on contextual fear conditioning. These results support the view that two processes can support contextual fear conditioning: (1) conditioning to the conjunctive representation, which depends on the hippocampus, and (2) conditioning to the features that make up the context, which does not.