Hippocampal system dysfunction and odor discrimination learning in rats: Impairment of facilitation depending on representational demands

Characteristics of dissociable human learning systems

A number of ways of taxonomizing human learning have been proposed. We examine the evidence for one such proposal, namely, that there exist independent explicit and implicit learning systems. This combines two further distinctions, (1) between learning that takes place with versus without concurrent awareness, and (2) between learning that involves the encoding of instances (or fragments) versus the induction of abstract rules or hypotheses. Implicit learning is assumed to involve unconscious rule learning. We examine the evidence for implicit learning derived from subliminal learning, conditioning, artificial grammar learning, instrumental learning, and reaction times in sequence learning. We conclude that unconscious learning has not been satisfactorily established in any of these areas. The assumption that learning in some of these tasks (e.g., artificial grammar learning) is predominantly based on rule abstraction is questionable. When subjects cannot report the “implicitly learned” rules that govern stimulus selection, this is often because their knowledge consists of instances or fragments of the training stimuli rather than rules. In contrast to the distinction between conscious and unconscious learning, the distinction between instance and rule learning is a sound and meaningful way of taxonomizing human learning. We discuss various computational models of these two forms of learning.

Two functional components of the hippocampal memory system

There is considerable evidence that the hippocampal system contributes both to (1) the temporary maintenance of memories and to (2) the processing of a particular type of memory representation. The findings on amnesia suggest that these two distinguishing features of hippocampal memory processing are orthogonal. Together with anatomical and physiological data, the neuropsychological findings support a model of cortico-hippocampal interactions in which the temporal and representational properties of hippocampal memory processing are mediated separately. We propose that neocortical association areas maintain shortterm memories for specific items and events prior to hippocampal processing as well as providing the final repositories of long-term memory. The parahippocampal region supports intermediate-term storage of individual items, and the hippocampal formation itself mediates an organization of memories according to relevant relationships among items. Hippocampal-cortical interactions produce (i) strong and persistent memories for events, including their constituent elements and the relationships among them, and (ii) a capacity to express memories flexibly across a wide range of circumstances.

Septal co-infusions of glucose with the benzodiazepine agonist chlordiazepoxide impair memory, but co-infusions of glucose with the opiate morphine do not

We have found repeatedly that medial septal (MS) infusions of glucose impair memory when co-infused with the gamma-amino butyric acid (GABA) agonist muscimol. The present experiments sought to determine whether the memory-impairing effects of this concentration of glucose would generalize to another GABA(A) receptor agonist and to an agonist from another neurotransmitter system that is known to impair memory. Specifically, we determined whether the dose of glucose that produces memory deficits when combined with muscimol in the MS would also impair memory when co-infused with the GABA(A) receptor modulator chlordiazepoxide (CDP) or the opiate morphine. Male Sprague-Dawley rats were given MS co-infusions and then 15 min later tested for spontaneous alternation or given shock avoidance training (retention tested 48 h later). The results showed that MS infusions of the higher dose of glucose with morphine did not produce memory deficits, whereas, the performance of rats given MS co-infusions of CDP with glucose was impaired. These findings suggest that the memory-impairing effects of brain glucose administration may involve an interaction with the GABA(A) receptor.

Blockade of GABA(B) receptors completely reverses age-related learning impairment

Impaired cognitive functions are well-described in the aging process. GABA(B) antagonists can facilitate learning and memory in young subjects, but these agents have not been well-characterized in aging. Here we show a complete reversal of olfactory discrimination learning deficits in cognitively-impaired aged Fischer 344 rats using the GABA(B) antagonist CGP55845, such that drug treatment restored performance to that on par with young and cognitively-unimpaired aged subjects. There was no evidence that this improved learning was due to enhanced olfactory detection abilities produced by the drug. These results highlight the potential of targeting GABA(B) receptors to ameliorate age-related cognitive deficits and demonstrate the utility of olfactory discrimination learning as a preclinical model for testing novel therapies to improve cognitive functions in aging.

Cortex and memory: emergence of a new paradigm

Converging evidence from humans and nonhuman primates is obliging us to abandon conventional models in favor of a radically different, distributed-network paradigm of cortical memory. Central to the new paradigm is the concept of memory network or cognit--that is, a memory or an item of knowledge defined by a pattern of connections between neuron populations associated by experience. Cognits are hierarchically organized in terms of semantic abstraction and complexity. Complex cognits link neurons in noncontiguous cortical areas of prefrontal and posterior association cortex. Cognits overlap and interconnect profusely, even across hierarchical levels (heterarchically), whereby a neuron can be part of many memory networks and thus many memories or items of knowledge.

A neurocomputational model of classical conditioning phenomena: a putative role for the hippocampal region in associative learning

Some existing models of hippocampal function simulate performance in classical conditioning tasks using the error backpropagation algorithm to guide learning (Gluck, M.A., and Myers, C.E., (1993). Hippocampal mediation of stimulus representation: a computational theory. Hippocampus, 3(4), 491-516.). This algorithm is not biologically plausible because it requires information to be passed backward through layers of nodes and assumes that the environment provides information to the brain about what correct outputs should be. Here, we show that the same information-processing function proposed for the hippocampal region in the Gluck and Myers (1993) model can also be implemented in a network without using the backpropagation algorithm. Instead, our newer instantiation of the theory uses only (a) Hebbian learning methods which match more closely with synaptic and associative learning mechanisms ascribed to the hippocampal region and (b) a more plausible representation of input stimuli. We demonstrate here that this new more biologically plausible model is able to simulate various behavioral effects, including latent inhibition, acquired equivalence, sensory preconditioning, negative patterning, and context shift effects. In addition, the newer model is able to address some new phenomena including the effect of the number of training trials on blocking and overshadowing.

Criteria for validating mouse models of psychiatric diseases

Animal models of human diseases are in widespread use for biomedical research. Mouse models with a mutation in a single gene or multiple genes are excellent research tools for understanding the role of a specific gene in the etiology of a human genetic disease. Ideally, the mouse phenotypes will recapitulate the human phenotypes exactly. However, exact matches are rare, particularly in mouse models of neuropsychiatric disorders. This article summarizes the current strategies for optimizing the validity of a mouse model of a human brain dysfunction. We address the common question raised by molecular geneticists and clinical researchers in psychiatry, "what is a 'good enough' mouse model"?

Is awareness necessary for true inference?

In transitive inference, participants learn a set of context-dependent discriminations that can be organized into a hierarchy that supports inference. Several studies show that inference occurs with or without task awareness. However, some studies assert that without awareness, performance is attributable to pseudoinference. By this account, inference-like performance is achieved by differential stimulus weighting according to the stimuli's proximity to the end items of the hierarchy. We implement an inference task that cannot be based on differential stimulus weighting. The design itself rules out pseudoinference strategies. Success on the task without evidence of deliberative strategies would therefore suggest that true inference can be achieved implicitly. We found that accurate performance on the inference task was not dependent on explicit awareness. The finding is consistent with a growing body of evidence that indicates that forms of learning and memory supporting inference and flexibility do not necessarily depend on task awareness.

The promnesic effect of G-protein-coupled 5-HT4 receptors activation is mediated by a potentiation of learning-induced spine growth in the mouse hippocampus

Pharmacological modulation of synaptic efficacy is a prominent target in the identification of promnesic compounds. Here, we report that pretraining administration of the serotonin 5-HT(4) receptors (5-HT(4)Rs) partial agonist SL65.0155 enhances simultaneous olfactory discrimination performance and potentiates learning-induced dendritic spine growth in the mouse hippocampus. SL65.0155 does not affect spine density in the pseudo-trained mice and, by itself, does not promote spine growth. Injecting the 5-HT(4) antagonist RS39604 prior to SL65.0155 prevents both the increase in performance and the additional formation of spines, thus confirming the 5-HT(4)Rs specificity of the observed effects. These findings provide evidence that 5-HT(4)Rs stimulation selectively increases experience-dependent structural plasticity in learning-activated hippocampal circuits.

Chronic developmental exposure to phenytoin has long-term behavioral consequences

Anti-epileptic compounds have been linked to several developmental disorders. Specifically, fetal exposure to phenytoin is linked to fetal hydantoin syndrome in humans. We have developed a rat model of fetal hydantoin syndrome in an effort to explore the relationship between drug exposure, development, and learning and memory. Previous studies of this animal model have used various embryological periods of exposure; however, the human syndrome is reported in the offspring of mothers that maintain drug regimens throughout gestation and nursing. To that end, the present study investigated associative learning in rats exposed to therapeutic levels of phenytoin throughout prenatal development and the postnatal pre-weaning period. We used an instrumental appetitive-to-aversive transfer paradigm, which has hippocampal-dependent components, and an avoidance-conditioning paradigm to test simple associative learning and higher-order learning and memory. Compared to controls, we report increased rates of acquisition and performance by the phenytoin group in both the appetitive and the avoidance learning paradigm, and a substantial impairment in avoidance learning following the transfer from appetitive to aversive conditioning. The positive deficit observed with simple associative learning and the negative transfer effect associated with higher order learning suggests that chronic exposure to phenytoin throughout gestation disrupts hippocampal development, which subsequently leads to impaired function in adulthood.

Neural systems for perceptual skill learning

Recent studies have begun to use functional neuroimaging techniques to examine the changes in brain activity that occur as humans learn new skills. This review outlines results from a number of imaging studies examining visual perceptual skill learning. Although the regions engaged during skill learning differ across tasks, a common finding has been increasing activation in the inferior temporal and fusiform gyri as skill is acquired and activation of the caudate nucleus in association with learning. Neuroimaging has great promise for the understanding of learning at the level of large neural populations, but further work is necessary to understand the specificity of learning-related changes and their relation to underlying neurophysiological plasticity.

Olfactory system modulation of hippocampal cell death

The hippocampal dentate gyrus is a major recipient of olfactory input in rodents, via connections from the olfactory (piriform) cortex and the olfactory bulb to the entorhinal cortex. Given this connectivity and the known role of activity in dentate gyrus granule cell survival, the present experiment examined the immediate effects of loss of olfactory input to the hippocampus on apoptosis. Adults rats underwent unilateral or bilateral olfactory bulb ablations (OBX), and allowed to recover 24-72 h before the piriform cortex and hippocampal dentate gyrus were processed for terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling [TUNEL] of apoptotic cells. OBX transiently increased TUNEL-positive cells in the ipsilateral piriform cortex and dentate gyrus. Increased TUNEL-labeling was apparent within 24h in both structures, but was more extensive and prolonged in piriform cortex. The results suggest a trans-synaptic regulation of cell survival through at least two synapses.

Involvement of tissue inhibition of metalloproteinases-1 in learning and memory in mice

Tissue inhibitor of metalloproteinases (TIMP-1) is one of the four-member family (TIMPs-1-4) of multifunctional proteins that inhibit matrix metalloproteinases (MMPs). Its expression in the hippocampus is neuronal-activity-dependent and dramatically induced by stimuli leading to long-term potentiation (LTP), suggesting that TIMP-1 is a candidate plasticity protein potentially involved in learning and memory processes. We tested this hypothesis in a hippocampus-dependent task using the new olfactory tubing maze, with mice carrying a null mutation for TIMP-1 (TIMP-1 KO) and mice overexpressing TIMP-1 (TIMP-1 (tg)). The TIMP-1 KO mice were significantly impaired in making correct odor-reward associations when compared with their respective wild type (WT) littermates, while TIMP-1 overexpressing mice performed better than their WT controls. Both genetically modified mice learned the paradigm and the timing of the task, like their respective WTs, and no olfactory dysfunctioning was observed. These data suggest that TIMP-1 is involved in learning and memory processes related to the hippocampus, and support the hypothesis that the MMP/TIMP ratio, and hence MMP activity, modulates neuronal plasticity in normal learning and memory processes, while altered proteolytic activity could impair cognitive functions.

Hippocampal infusions of pyruvate reverse the memory-impairing effects of septal muscimol infusions

Hippocampal infusions of glucose reverse memory deficits in spontaneous alternation and in a continuous multiple trial inhibitory avoidance task. The current experiments tested whether glucose metabolism may participate in these effects of glucose. Specifically, these experiments determined whether the glycolytic metabolite pyruvate would mimic these effects of glucose. Male Sprague-Dawley rats were given septal infusions of vehicle or the gamma-aminobutyric acid (GABA) receptor agonist muscimol (0.15 nmol for spontaneous alternation or 5 nmol for continuous multiple trial inhibitory avoidance) combined with hippocampal infusions of vehicle or pyruvate (200 nmol) 15 min prior to assessing spontaneous alternation or training in a continuous multiple trial inhibitory avoidance task. The infusions of muscimol decreased percent alternation scores and continuous multiple trial inhibitory avoidance retention latencies tested 48 h after training. More importantly, hippocampal infusions of pyruvate reversed the deficits produced by septal infusions of muscimol on both tasks. These findings show for the first time that hippocampal infusions of pyruvate influence memory and suggest that glucose may affect memory via glycolytic metabolism.

Odor increases [3H]phorbol dibutyrate binding to protein kinase C in olfactory structures of rat brain. Effect of entorhinal cortex lesion

Since protein kinase C (PKC) is known to be activated in the olfactory bulb and in several limbic areas related to odor processing, we determined whether an olfactory stimulus was able to modulate the activity of PKC in animals with bilateral entorhinal cortex lesion. The translocation of PKC from the cytosol to the membrane was studied using the phorbol ester 12,13-dibutyrate ([3H]PDBu) binding in control and bilateral entorhinal cortex (EC) lesioned rats. The lesion of EC per se did not significantly affect [3H]PDBu binding in any of the brain structures analyzed, while odor stimulation induced it in both control and EC-lesioned groups in the external plexiform layer of the olfactory bulb. In contrast, an odor-induced increase of [3H]PDBu binding in internal glomerular layer of the olfactory bulb was only observed in EC lesioned animals. Similar results were obtained in the piriform cortex. In both CA1 and CA3 hippocampal subfields, odor stimulation induced an increase of [3H]PDBu binding in both control and EC-lesioned animals, the increase being potentiated only in CA1 of lesioned rats. The dentate gyrus and the amygdala exhibited a similar pattern of [3H]PDBu binding, showing a significant increase exclusively in EC-lesioned animals after odor stimulation. The results strongly suggest that the EC plays a key role in odor processing. PKC appears to play an important role in responding to the activation of lipid second messengers, which have been described to be involved in the processing of odor stimuli in several structures of the olfactory pathway.

Enhanced odor discrimination and impaired olfactory memory by spatially controlled switch of AMPA receptors

Genetic perturbations of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are widely used to dissect molecular mechanisms of sensory coding, learning, and memory. In this study, we investigated the role of Ca2+-permeable AMPARs in olfactory behavior. AMPAR modification was obtained by depletion of the GluR-B subunit or expression of unedited GluR-B(Q), both leading to increased Ca2+ permeability of AMPARs. Mice with this functional AMPAR switch, specifically in forebrain, showed enhanced olfactory discrimination and more rapid learning in a go/no-go operant conditioning task. Olfactory memory, however, was dramatically impaired. GluR-B depletion in forebrain was ectopically variable ("mosaic") among individuals and strongly correlated with decreased olfactory memory in hippocampus and cortex. Accordingly, memory was rescued by transgenic GluR-B expression restricted to piriform cortex and hippocampus, while enhanced odor discrimination was independent of both GluR-B variability and transgenic GluR-B expression. Thus, correlated differences in behavior and levels of GluR-B expression allowed a mechanistic and spatial dissection of olfactory learning, discrimination, and memory capabilities.

Coordination of multiple memory systems

On the basis of lesions of different brain areas, several neural systems appear to be important for processing information regarding different types of learning and memory. This paper examines the development of pharmacological and neurochemical approaches to multiple memory systems from past studies of modulation of memory formation. The findings suggest that peripheral neuroendocrine mechanisms that regulate memory processing may target their actions toward those neural systems most engaged in the processing of learning and memory. In addition, measurements of acetylcholine release in different memory systems reveals extensive interactions between memory systems, some cooperative and some competitive. These results imply that many neural systems, often characterized as relatively independent, may in fact interact extensively, blurring the dependencies of different memory tasks on specific neural systems.

The role of stimulus ambiguity and movement in spatial navigation: A multiple memory systems analysis of location discrimination

This paper reviews recent findings about how rats navigate by learning to discriminate among locations. The assumption underlying the experiments and their interpretation is that the information required to do this is learned by three independent, parallel memory systems. One system processes cognitive information (or "knowledge"), a second system processes reinforced stimulus-response associations and a third processes Pavlovian conditioned responses in the form of stimulus-affect associations. The information stored in each system produces behavior that, in some cases, results in a location discrimination. The present experiments focus on three factors that influence what each system learns and whether the resulting memory produces behavior that results in a location discrimination. One factor is whether the locations to be discriminated can be identified by unique, unambiguous stimuli or whether they are ambiguously associated with the same stimuli. The second factor is whether the stimuli are observed passively or whether the rats move among them, voluntarily or involuntarily. The third factor is whether or not the rats perform specific reinforced responses in the presence of the stimuli. Instances of co-operative behavioral outputs from memory systems that facilitate location discriminations and of competitive outputs that impede discriminations are described.

Age-dependent effects of moderate chronic ethanol administration on different forms of memory expression in mice

A large number of studies have investigated the effects of chronic ethanol administration (CEA) on performance in different types of learning and memory tasks in adult rodents. As a general rule, CEA has been reported to impair performance, although this depends both on the condition of administration (e.g. duration, presence or not of a withdrawn period) and on task demands (e.g. spatial versus non-spatial). Indeed, either no impairment or even a facilitation of performance have been reported following CEA. However, no study has directly addressed the issue as to whether the effect of CEA depends on the age of subjects. In this study, C57Bl/6 mice of two age ranges (i.e. 2-3- and 16-18-month-old) were given either a solution of ethanol (12% v/v) as their only source of fluid for 5 months (experimental groups) or were pair-fed with an isocaloric solution of dextri-maltose (control groups). Then, they were submitted to a place discrimination task in an 8-arm radial maze. Additionally, mice were tested for long-term retention following a 21-day interval. Confirming our previous findings, the results showed that, with respect to adults (7-8-month-old at the time of testing), aged mice (21-23-month-old) of the control group displayed impaired relational memory but not procedural memory performance. Further they exhibited a higher level of forgetting than adults over the 21-day interval. In the same paradigm, CEA resulted in an overall attenuation of both type of deficit in aged subjects without altering their procedural memory. Furthermore these ethanol-consuming aged mice displayed significantly less levels of forgetting than their age-matched controls. Conversely, in the adult group, CEA resulted in an overall, although, somewhat less selective impairment of relational memory with respect to procedural memory but had no effect on long-term forgetting. While confirming the deleterious effect of CEA on learning and memory processes in adults, our present findings provide evidence that CEA can selectively ameliorate certain cognitive deficits normally associated with ageing.

Preserved anterograde and retrograde memory of rapidly acquired olfactory discrminations after neurotoxic hippocampal lesions

A forced-choice discrimination paradigm was used in two experiments, to evaluate retrograde and anterograde amnesia in rats after hippocampal ablation. In a within-subjects design (Experiment 1), rats were trained on a set of 10 olfactory discriminations 4 weeks before surgery and on a separate set of 10 discriminations 1 week before surgery. In a mixed design (Experiment 2), rats were trained on olfactory discriminations in one of three conditions: condition 1 (10 discriminations at 4 weeks before surgery); condition 2 (10 discriminations at 1 week before surgery); or condition 3 (10 discriminations at 4 weeks before surgery and 10 discriminations at 1 week before surgery). Discriminations in both experiments were rapidly learned, requiring 7-10 trials to reach criterion. After training, half of the rats in each condition received bilateral neurotoxic lesions of the hippocampus, and the other half received sham surgery. One week after surgery, all rats were given a retention test, consisting of a single critical trial for each discrimination. In both experiments, rats with selective hippocampal lesions exhibited preserved retention of these olfactory discriminations with no observable retention gradient. A postoperative acquisition test for two new discriminations indicated that anterograde memory was also preserved, while a postoperative test of spatial learning in the Morris water maze confirmed that the hippocampal lesions impaired spatial learning. Together, these experiments refute the contention that the hippocampus is requisite for (non-spatial) olfactory memory consolidation, storage, or access, despite the condition that the information be rapidly acquired.

Effects of Medial Septal Lesions on Action-Outcome Associations in Rats Under Conditions of Delayed Reinforcement

In operant tasks, control rats maintain high response rates under positive contingencies, when the probability of reinforcement is greater following a response (contingent reinforcement) than during the absence of that response. However, as contingencies approach zero, response rates decrease. In this experiment, under immediate contingent reinforcement, rats with medial septal lesions reduced their response rates, just like controls, when contingencies were shifted from positive toward zero. However, the septal rats were less sensitive to this contingency shift, compared with controls, when there was a 5-s delay between lever presses and contingent reinforcements. This lesion effect appeared to be due to a failure of voluntary response memory, which impaired sensitivity to operant contingencies when there was a delay between action and outcome.

Hippocampal lesions and discrimination performance of mice in the radial maze: sparing or impairment depending on the representational demands of the task

The effects of ibotenate hippocampal lesions on discrimination performance in an eight-arm radial maze were investigated in mice, using a three-stage paradigm in which the only parameter that varied among stages was the way the arms were presented. In the initial learning phase (stage 1), animals learned the valence or reward contingency associated with six (three positive and three negative) adjacent arms of the maze using a successive (go/no-go) discrimination procedure. In the first test phase (stage 2), the six arms were grouped into three pairs, so that on each trial, the subject was faced with a choice between two adjacent arms of opposite valence (concurrent two-choice discrimination). In the second test phase (stage 3), the subject was faced with all six arms simultaneously (six-choice discrimination). Hippocampal-lesioned mice acquired the initial learning phase at a near-normal rate but behaved as if they had learned nothing when challenged with the two-choice discriminations at stage 2. In contrast, they behaved normally when confronted with the six-choice discrimination at stage 3. Detailed examination of within- and between-stage performance suggests that hippocampal-lesioned mice perform as intact mice when presentation of the discriminanda encourages the storage and use of separate representations (i.e., in initial learning and six-choice discrimination testing), but that they fail in test situations that involve explicit comparisons between such separate representations (two-choice discriminations), hence requiring the use of relational representations.

Olfactory discrimination learning deficit in heterozygous reeler mice

Adult mice heterozygous for the 'reeler' mutation (HR mice) have been found not to have a reeler phenotype but to express a number of abnormal traits including reduced sensorimotor gating, lower density of dendritic spines in frontoparietal cortex and hippocampus, and selectively decreased expression of one form of glutamic acid decarboxylase (GAD67). Since reelin is expressed in olfactory areas of the adult mouse brain, we have tested for olfactory functions in HR mice that express only half of the reelin found in wild-type (WT) mice. HR and WT mice were trained to criterion performance of 90% correct in a block of 20 trials on eight distinct simultaneous-cue, two-odor discriminations. HR mice required significantly more training sessions and made more errors than did WT mice in acquiring the first olfactory discrimination. Subsequent discriminations were learned equally rapidly by both HR and WT mice. Memory retention for the final discrimination was tested 1 week after training and was equally good for both groups. Both HR and WT mice showed equivalent sensitivity in discriminating low concentrations of either ethyl acetate or butanol from non-odorized air. Whether or not the olfactory learning deficit observed in HR mice is related to the low dendritic spine density or GAD67 reduction observed in hippocampus and cortex are currently under investigation.

Selective impairment of fornix-transected rats on a new nonspatial, odor-guided task

In the present experiment, sham-operated (SH) and fornix-transected (FX) rats were trained on a new nonspatial, odor-guided task. On each session, eight odor pairs were presented twice. On the first occurrence of a pair, rats were reinforced for pushing the container (go response) in which the olfactory stimuli were placed. On the second occurrence, they were not reinforced and had to refrain from responding (no-go response) to be scored as success. Rats were first trained to criterion on odor pairs made of replicates of the same odor (S pairs). Then they were trained to criterion on pairs made of different odors, each member of the pair overlapping with that of another pair (O pairs) and finally, on pairs of different odors with no overlap (NO pairs). The results showed that the number of sessions to reach criterion was significantly higher in FX than in SH rats during training on O pairs, but not during training on S or on NO pairs. These findings are consistent with the configural (Rudy and Sutherland, 1995: Hippocampus 5:375-389) or relational (Eichenbaum et al., 1994: Behav Brain Sci 17:449-518) account of the hippocampal memory function.

Deterioration in learning and memory of inferential tasks for evaluation of transitivity and symmetry in aged SAMP8 mice

This study examined age-dependent deficits in the learning and memory of inferential tasks, using an established senescence-accelerated mouse model in age-related brain dysfunction (SAMP8) and its genetically related inbred strain (SAMR1). The mice learned two sets of nonspatial odor-odor pairs by association learning successively (i.e., A-->B, X-->Y, then B-->C, Y-->Z). They were tested in transitive inference (i.e., A-->C, X-->Z) and symmetrical inference (i.e., C-->B, Z-->Y). In the probe test of A-->C, X-->Z transitive inference, 1-month-old SAMP8 and control SAMR1 at the same age significantly chose the alternative based on transitive inference, but 4- and 7-month-old SAMP8 performed at a random chance level, in comparison with unambiguous inference by control SAMR1 at the same ages. During the test of C-->B, Z-->Y symmetrical inference, SAMP8 at 1 month of age made errors as frequently as control SAMR1 at the same age, but SAMP8 at 4 and 7 months of age made more errors than SAMR1 at the same ages. At 4 and 7 months of age, SAMP8 made more errors than 1-month-old SAMP8. Control SAMR1 did not show such an age-related deficient. These results indicate that SAMP8 mice have age-related learning and memory deficits in the ability to perform inferential tasks. Age-related hippocampal dysfunction is suggested to be the cause of these age-related deficits in old SAMP8 mice during the performance of inferential tasks mediated by declarative memory.

Fos imaging reveals ageing-related changes in hippocampal response to radial maze discrimination testing in mice

A two-stage radial arm maze (RAM) task has been designed recently to demonstrate a specific age-related memory deficit in mice. It highlights the contrast between normal and deficient memory expression in a spatial discrimination problem depending on how to-be discriminated arms were presented to the animal. This specific deficit has been interpreted as a preferential loss in a relational/declarative form of memory, thereby implying an underlying hippocampal dysfunction. To test this hypothesis, neuronal activation measured by Fos immunostaining was compared in aged (21-23 months) and adult (4-6 months) mice trained in the aforementioned task and killed after a retention session consisting in age-insensitive probe trials, performed 6 days later (6-day RAM). Two comparison conditions were included: (i) repeated locomotor training on a treadmill (TM); (ii) the same RAM training, except for the use of a longer (30 days instead of six) retention interval (30-day RAM). Although all RAM groups displayed similar levels of performance at the end of the experiment, immediately before the mice were killed, significant between-group differences in brain activation were observed. In adult mice, 6-day RAM testing was associated with greater septal and hippocampal (CA1, CA3, DG) Fos expression than the TM condition. Lengthening the retention interval from 6 days to 30 days resulted in a significant decrease in RAM testing-induced Fos expression in most of the septo-hippocampal regions. With respect to adult mice, aged mice displayed reduced Fos expression (except for DG) and a lack of interrelationships between levels of Fos produced in each of the SH regions, in the 6-day RAM testing condition. Conversely, there was no effect of ageing on Fos expression associated with either TM training or 30-day RAM testing. These results are interpreted as reflecting age- (or time-) related alterations in recruiting of brain structures that underlie a relational/declarative form of memory expression.