Young hippocampal neurons are critical for recent and remote spatial memory in adult mice.

New granule cells are continuously generated throughout adulthood in the mammalian hippocampus. These newly generated neurons become functionally integrated into existing hippocampal neuronal networks, such as those that support retrieval of remote spatial memory. Here, we sought to examine whether the contribution of newly born neurons depends on the type of learning and memory task in mice. To do so, we reduced neurogenesis with a cytostatic agent and examined whether depletion of young hippocampal neurons affects learning and/or memory in two hippocampal-dependent tasks (spatial navigation in the Morris water maze and object location test) and two hippocampal-independent tasks (cued navigation in the Morris water maze and novel object recognition). Double immunohistofluorescent labeling of the birth dating marker 5-bromo-2'deoxyuridine (BrdU) together with NeuN, a neuron specific marker, was employed to quantify reduction of hippocampal neurogenesis. We found that depletion of young adult-generated neurons alters recent and remote memory in spatial tasks but spares non-spatial tasks. Our findings provide additional evidence that generation of new cells in the adult brain is crucial for hippocampal-dependent cognitive functions.

Nucleus accumbens dopamine receptors in the consolidation of spatial memory.

Nucleus accumbens dopamine is known to play an important role in motor activity and in behaviours governed by drugs and natural reinforcers, as well as in non-associative forms of learning. At the same time, activation of D1 and D2 dopamine receptors has been suggested to promote intracellular events related to neural plasticity. Therefore, in this study we wished to investigate the role of the two classes of dopamine receptors within the nucleus accumbens on the consolidation of spatial information. On day 1, CD1 male mice were placed in an open field containing five different objects and, immediately after three sessions of habituation, the animals were focally injected within the nucleus accumbens with either the D1 antagonist SCH 23390 (12.5, 25 or 50 ng/side), or the D2 antagonist sulpiride (25, 50, 75 or 100 ng/side). Twenty-four hours later the ability of mice to discriminate an object displacement was assessed. Both the D1 and the D2 antagonists impaired the ability of mice to detect the spatial change. If the highest doses of the two antagonists were injected 2 h after the end of the last of the habituation sessions, no effect was observed in the reactivity to spatial change examined 24 h later. These data demonstrate that activation of both D1 and D2 receptors within the accumbens is necessary in the early stages of the consolidation of spatial information. The data are discussed in terms of involvement of nucleus accumbens dopamine in information processing in the absence of explicit reinforcers.

NMDA and AMPA antagonist infusions into the ventral striatum impair different steps of spatial information processing in a nonassociative task in mice.

Most of the research on ventral striatal functions has been focused on their role in modulating reward and motivation. More recently, a possible role of this structure in cognitive functions has been suggested. However, very little information is available on the involvement of the nucleus accumbens in the different stages of the consolidation process. In this study, the effect of focal injections of AP-5 and DNQX, competitive antagonists at the NMDA and AMPA receptors, respectively, was examined in a nonassociative task designed to estimate the ability of mice to react to spatial changes. The task consists of placing the animals in an open field containing five objects; after three sessions of habituation, their reactivity to object displacement was examined 24 hr later. AP-5 injections administered after training impaired the ability of mice to detect the spatial novelty but did not affect response when injected 120 min after training or before testing. On the contrary, DNQX did not affect response when administered immediately or 120 min after training but did impair spatial discrimination when administered before training or testing. These data demonstrate a double dissociation between glutamate receptor subtypes, such that accumbens NMDA receptors are important for consolidation and not ongoing discrimination of spatial information, whereas AMPA receptors have an opposite role in these processes.

Effects of lesions to the glutamatergic afferents to the nucleus accumbens in the modulation of reactivity to spatial and non-spatial novelty in mice.

The purpose of this study was to compare the effects of selective lesions of the three main sources of limbic afferents to the nucleus accumbens-fornix, prelimbic cortex and amygdala-with those induced by N-methyl-D-aspartate receptor blockage in this structure, in a non-associative task designed to estimate the ability of rodents to encode spatial and non-spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). Focal administrations of the competitive N-methyl-D-aspartate antagonist DL-2-amino-5-phosphonopentanoic acid (0.1 microg/side) induced a selective impairment in the ability of mice to react to the spatial change. Lesions to the different structures affect the response of mice to spatial and non-spatial novelty in different ways. In particular, while fornix lesions induced a decrease in re-exploration of the displaced objects, prelimbic cortex lesions enhanced the exploration of both displaced and non-displaced objects. Finally, the basolateral amygdala lesions did not induce any impairment in the detection of the displaced objects but decreased the latencies to approach novel objects. It is concluded that N-methyl-D-aspartate receptor blockage in the nucleus accumbens subsumes the effects of the three lesions. Some hypotheses on the role of glutamatergic transmission in the accumbens on information processing are briefly discussed.

Attenuation of emotional and nonemotional memories after their reactivation: role of beta adrenergic receptors.

A memory trace in its active state is susceptible to interference by amnesic agents, such as hypothermia and electroconvulsive shock, and by NMDA receptor antagonists, suggesting that a time-dependent consolidation process occurs each time a memory is reactivated. The role of beta noradrenergic receptors in reconsolidation in rats was examined in both a positively reinforced radial maze task and a footshock-reinforced conditioned emotional response task. For the former, rats were trained over several days in a spatial reference memory task and received a single reactivation trial followed by propranolol. A temporally graded impairment was observed when propranolol treatment occurred after the memory reactivation trial. In the emotional task, memory impairing effects of propranolol were greater when the drug was administered after a reactivation trial than when administered immediately after the initial training. These results suggest that reactivation of memory triggers a beta receptor-dependent cascade of intracellular events, recapitulating that which occurs during initial postacquisition consolidation, thus permitting reorganization of the existing memory as a function of new information in the retrieval environment. This remarkable lability of an active memory trace provides a new basis for pharmacotherapeutic intervention in such syndromes as Posttraumatic Stress Disorder. beta adrenoreceptor antagonists may be promising pharmacological agents for attenuating debilitating memories at the time of their controlled reactivation.

Learning-induced plasticity of N-methyl-D-aspartate receptors is task and region specific.

Changes in binding of [3H]dizocilpine maleate to N-methyl-D-aspartate-sensitive ion channel receptors were evaluated after learning in order to specify brain regions which might be involved in memory formation. Rats were trained in a five-trial session of 40 min, to discriminate among three odours to obtain food reinforcement. Another group was trained in an eight-arm maze to choose always the same three arms to obtain food reinforcement (nine trials over 150 min). In rats killed 30 min after odour discrimination learning, dizocilpine maleate binding was significantly reduced in hippocampal sub-regions CA3, CA1 and fascia dentata and in frontal cortex. After spatial learning, changes in binding were limited to the amygdala, where a decrease was also observed. These results indicate that functional changes occur in specific brain regions after learning and suggest anatomical loci for further study of synaptic changes at a morphological level, after spatial learning or odour discrimination.

Consolidation of memory for odor-reward association: beta-adrenergic receptor involvement in the late phase.

Experimentally naive rats can learn rapidly to discriminate among three odors to obtain food reinforcement. After three massed trials, they show almost errorless performance. This task has proved to be useful in studying time-dependent postacquisition intracellular processes necessary for long-term memory. The present experiments evaluated the temporal dynamics of the role of beta-noradrenergic receptors in long-term consolidation. Rats were implanted with intracerebroventricular cannulae and trained in a single session to find reinforcement in a hole in a sponge impregnated with a particular odor. Injections of the beta-receptor antagonist timolol were made at 5 min, 1, 2, or 5 hr after training. Memory and relearning ability were evaluated 48 hr later. Rats treated with timolol 2 hr after training showed a memory deficit at the retention test, but were able to relearn the task normally. Injections at the earlier or later time points were ineffective. The results reinforce previous observations with systemic injections that beta-noradrenergic receptors are involved in the late phase of memory consolidation and suggest a critical time window during which they are necessary. The time window is compatible with the current view that long-term memory depends on late involvement of the cAMP cascade leading to new protein synthesis necessary for synaptic reorganization.

N-methyl-D-aspartate and dopamine receptor involvement in the modulation of locomotor activity and memory processes.

In this study we report on the effects of N-methyl-D-aspartate (NMDA)- and dopamine (DA)-receptor manipulation on the modulation of one-trial inhibitory avoidance response and the encoding of spatial information, as assessed with a non-associative task. Further, a comparison with the well-known effects of the manipulation of these two receptor systems on locomotor activity is outlined. It is well assessed that NMDA-receptor blockage induces a stimulatory action on locomotor activity similar to that exerted by DA agonists. There is evidence showing that the nucleus accumbens is involved in the response induced by both NMDA antagonists and DA agonists. We show results indicating a functional interaction between these two neural systems in modulating locomotor activity, with D2 DA-receptor antagonists (sulpiride and haloperidol) being more effective than the D1 antagonist (SCH 23390) in blocking MK-801-induced locomotion. A different profile is shown in the effects of NMDA antagonists and DA agonists in the modulation of memory processes. In one-trial inhibitory avoidance response, NMDA antagonists (MK-801 and CPP) impair the response on test day, while DA agonists exert a facilitatory effect; furthermore, sub-effective doses of both D1 (SKF 23390) and D2 (quinpirole) are able to attenuate the impairing effect in a way similar to that induced by NMDA antagonists. The effects of NMDA- and DA-acting drugs on the response to spatial novelty, as assessed with a task designed to study the ability of animals to react to discrete spatial changes, are in good accord with the effects observed on passive avoidance. The results show that NMDA as well as DA antagonists, at low doses, selectively impair the reactivity of mice to spatial changes. In a last series of experiments, the possible role of NMDA receptors located in the nucleus accumbens was investigated regarding reactivity to spatial novelty. The experiments gave apparently contrasting results: while showing an impairing effect of focal administrations of NMDA antagonists in the nucleus accumbens on reactivity to spatial novelty, no effect of ibotenic acid lesions of the same structure was observed.

Detection of object orientation and spatial changes by mice: importance of local views.

This study examines the types of information used by mice to detect changes in their environment. After a period of habituation to an open field containing three bicolored cube-shaped objects, we modified the spatial configuration of the objects or rotated the objects 180 degrees, or both. We noted that whatever the modification, mice reexplored the objects. This shows that mice are able to detect both topological changes and, in particular, changes in the orientation of objects. This is possible only if mice can establish a local view of the objects in relation to a stable element of their environment. A second experiment showed that this stable element is in fact a pattern within the open field and not an element of the distal environment. These two experiments demonstrate the importance of local views in spatial representation for rodents.

N-methyl-D-aspartate receptors in the nucleus accumbens are involved in detection of spatial novelty in mice.

The aim of this study was to investigate the role played by intra-accumbens N-methyl-D-aspartate (NMDA) receptors in spatial information encoding. For this purpose, the effect of local administration of both competitive (AP-5) and non-competitive (MK-801) NMDA antagonists was assessed in a task designed to estimate the ability of rodents to encode spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). The results show that both doses of MK-801 (0.15 and 0.3 microg/side) induced a selective impairment in the capability of mice to detect spatial novelty. A similar effect was obtained by injecting the low dose of the competitive antagonist AP-5 (0.1 microg/side), whereas the high dose (0.15 microg/side) abolished detection of both spatial and object novelty. Taken together, these results show that intra-accumbens injections of low doses of competitive and non-competitive NMDA antagonists can produce selective deficits in processing spatial information resembling those observed after hippocampal damage. Moreover, the fact that pharmacological treatments spare memory processes involved in habituation suggests that NMDA antagonists may interfere with the formation of spatial representations rather than producing memory deficits per se.

Consolidation of memory after its reactivation: involvement of beta noradrenergic receptors in the late phase.

Evidence is growing that the cAMP pathway through the cAMP responsive element binding protein (CREB) transcription factor plays an important role in long-term memory formation (LTM). To study the role of beta-noradrenergic receptors, positively linked to the cAMP second-messenger system, in the dynamics of LTM processes, we used a memory-reactivation paradigm because recent studies in our laboratory confirmed that reactivated memory is labile and undergoes an extended reconsolidation process. In an eight-arm maze, rats were trained to choose the same three baited arms; 24 hr later, memory was reactivated and then the rats were injected intracerebroventricularly at 5 min, 30 min, 60 min, or 5 hr later with the beta-antagonist timolol or with saline. The results showed that injection of timolol induced amnesia only at the 60 min post-reactivation interval, whereas all control groups and groups that were timolol-injected at other post-reactivation intervals displayed optimal retention. The delayed amnesic action of timolol suggests that beta noradrenergic receptors and the cAMP cascade are implicated in the late phase of reprocessing of a remembered event.

Neural cell adhesion molecules play a role in rat memory formation in appetitive as well as aversive tasks.

A polyclonal antibody (R1), raised against chick synaptic membrane glycoproteins and recognizing the neural cell adhesion molecule (NCAM) caused amnesia for avoidance tasks when injected into day-old chicks and adult rats 5.5 h post-training. We investigated the effects of R1 antibody on memory formation in a non-aversive task, where stress is minimal: a massed trial odour discrimination task in rats. Preimmune serum or R1 antibody was injected i.c.v. 5.5 h after the last training session. Forty-eight hours after the training session, control rats showed very good retention whereas R1 antibody injection significantly disrupted retention. The results suggest that glycoproteins recognized by R1 in the rat play a specific role in memory formation for appetitive events as well as in memory formation for aversive situations.

Ibotenic lesions of the nucleus accumbens promote reactivity to spatial novelty in nonreactive DBA mice: implications for neural mechanisms subserving spatial information encoding.

The role of the nucleus accumbens (NA) in forming spatial representations was investigated in C57BL/6 (C57) and DBA/2 (DBA) inbred mice. One week before testing, bilateral excitotoxic lesions were performed in the NA using ibotenic acid. Testing consisted of placing mice in an arena containing 5 objects at a fixed location and, after habituation to the object configuration, examining their reactivity to the displacement (spatial novelty) or the substitution (object novelty) of some of these objects. C57 mice reacted to spatial novelty and DBA mice did not. Both strains, however, reacted to object novelty. The lesion had no effect on C57 mice's performance, but in the DBA mice, it promoted a clear reaction to spatial novelty that was absent in control animals. Radial maze performance also was improved in DBA with NA lesions. Results suggest the NA as a possible site for modulating spatially mediated behaviors in poor-performing subjects.

Habituation and memorization of spatial objects' configurations in mice from weaning to adulthood.

This experiment investigated the development of habituation and memorization capacities of C57BL/6 mice. After a first session on a classic open field, four groups of subjects (3, 4, 5 and 9 weeks of age) were exposed to objects arranged in a pre-defined spatial environment during three exploratory sessions. Subsequently, for the test session, half of the mice was exposed to the previous situation, while the other half was exposed to a novel situation with a different spatial configuration for testing animal's abilities to detect and react to a change in their environment. Analysis showed age-related differences in behavioural habituation patterns. Moreover, contrary to our expectancy based on previous studies, the youngest mice (3 week-old) didn't exhibit significant renewal of exploration of the displaced objects during the test session. This results indicated that the youngest mice react differently than the adult mice when they are confronted to a novel environment and especially seem enable to construct a long-lasting representation of their environment when this representation concerns proximal information. The results are discussed in relation to previous studies conducted on the radial maze and the Morris water maze and it seems that the abilities of the youngest mice to construct a representation of their environment are partially dependent upon the type of information available (i.e. proximal versus distal information).

Use of proximal and distal cues in place navigation by mice changes during ontogeny.

The ontogeny of the ability of C57BL/6 mice to use different cues for spatial learning was examined in several Morris water maze tasks. In the first two studies, three learning procedures were used, in which only distal cues (place learning), only proximal cues (cue learning), or both proximal and distal cues (cue + place learning) were pertinent to localize the platform. The results indicated that whatever the procedure, 22-day-old mice showed the same capabilities as adults. Moreover, in the cue + place-learning procedure, although the distal cues were not necessary to solve the task, both young and adult mice demonstrated the integration of distal information by exhibiting a strong spatial bias during a probe test. However, in the third experiment, it was shown that nonpertinent proximal cues perturbed 22-day-old mice in a place-learning procedure. Taken together, these results suggest that while even the youngest mice show striking spatial navigation abilities, young mice give greater importance to proximal cues for orientation whereas adults preferentially use distal information.

Involvement of glutamatergic and dopaminergic systems in the reactivity of mice to spatial and non-spatial change.

Injections of glutamatergic NMDA as well as dopaminergic antagonists produce selective place- but not cue-learning deficits in associative spatial tasks. The present work was aimed at examining if the blockade of NMDA and dopaminergic receptors interferes with the encoding of spatial information in a non-associative task specifically designed for rodents. CD1 mice injected with MK-801 (0.1 and 0.25 mg/kg), haloperidol (0.04 and 0.08 mg/kg), a combination of the lower doses of each drug (haloperidol: 0.04 mg/kg and MK-801: 0.1 mg/kg) or saline were placed in an open field containing five objects and their reactivity to the displacement (spatial change) or the substitution (non-spatial change) of some of these objects was examined. The results show that saline-injected mice reacted to spatial as to non-spatial change by increasing the time spent exploring the displaced objects or the substituted one. Both doses of MK-801 prevented mice from detecting spatial change but did not affect their reactivity to the novel object. Both doses of haloperidol abolished the reactivity of mice to spatial change but the higher dose of the drug also altered the reaction to non-spatial change. Taken together, the present results indicate that the blockade of dopaminergic or glutamatergic NMDA receptors abolishes the detection of spatial novelty. The well-documented impairing effects of haloperidol and MK-801 on spatial learning may, therefore, be the consequence of a drug-induced inability in forming and/or updating spatial representations. The effects of haloperidol was, however, less specific than that of MK-801, since haloperidol always modified activity together with the response to spatial change and, at the higher dose, abolished the detection of both spatial and non-spatial change. Finally, haloperidol pretreatment was found to enhance the effect of MK-801 thus suggesting a possible interaction between the two systems in modulating these behavioral responses.

Radial maze learning using exclusively distant visual cues reveals learners and nonlearners among inbred mouse strains.

Spatial working memory on the radial maze was studied in 8 groups of isogenic mice. The device and procedure were specially designed to prevent the mice from using a response strategy or taking advantage of olfactory trails or other proximal cues. The results showed that the strains of mice were clearly split between those which succeeded (C57BL/6, DBA/2, CB6F1, B6D2F1) and those which failed (NZB, CBA, C3H/HE, BALB/c) to learn the task. A second experiment established that when more extended training was given, the four strains which had performed poorly in experiment 1 still did not improve their performance. In the conclusion, we discuss the possible reasons for the deficits of nonlearners and emphasize the importance of using proper tools to ensure the unambiguous assessment of the cognitive processes underlying behavioral adaptation.

Ontogeny of orientation and spatial learning on the radial maze in mice.

The development of the orientation capacities of C57BL/6 mice has been studied on the radial maze in several procedures allowed to dissociate the different types of cues used by the mouse for solving the task with two intersession delays (2 and 24 hr). The results of the first two studies show that performance is independent of intersession delay regardless of the age of the subject. Mice as early as 23 days old obtain good performances when they can develop an algorithmic strategy or when they dispose of both proximal and distal cues during learning. At 37 days of age, however, mice can efficiently solve the radial maze task with distal cues alone. However, in the third experiment, 23-day-old mice were able to use distal cues for orientation at the end of the learning session if, at the onset, they also had access to proximal cues. These results suggest that, on weaning, mice use several types of information for task performance and that, as they mature, they turn more often to distal cues for orientation.

Inter-session delay and its effects on performance and retention of spatial learning on a radial maze with mice.

Spatial learning on the radial maze was studied in two inbred strains of mice (C57BL/6 and DBA/2). Five different periods of delay (no delay, 40 min, 2, 8, and 24 h) were inserted between sessions in order to analyze the role of this inter-session delay on training and on retest 1 month later. Results showed that learning profiles and performance levels varied widely with inter-session delay. When the delay was very short (no delay and 40 min), mice of both strains were incapable of learning the task but when the delay was more than or equal to 2 h, the mice succeeded very quickly. The inter-session delay also influenced the performance of mice in the 1-month retention test. C57BL/6 mice obtained good performances in the procedure including a 2-h inter-session delay, while DBA/2 mice obtained good performances with 2- and 8-h delays. These results demonstrate the importance of the procedure in complex spatial learning.

Drug effects in a radial maze designed for dissociation of cues used by mice.

Scopolamine and amphetamine effects were investigated in a new radial maze. Three distinct procedures were designed to dissociate the use, by C57BL/6 mice, of the different cues available: a procedure where only spatial information was available, a procedure in which both spatial cues and olfactory trials were present, and a nonconfinement procedure where mice could use spatial cues, olfactory trials, and/or algorithmic strategies. We found that while scopolamine impaired performance on the maze in all three procedures, amphetamine tended to improve solving of the maze problem, but only in the procedure where spatial cues alone were available. The results are discussed in relation to hypotheses concerning these drug effects.