Amygdala modulation of multiple memory systems: hippocampus and caudate-putamen

The amygdala, reward and emotion

Recent research provides new insights into amygdala contributions to positive emotion and reward. Studies of neuronal activity in the monkey amygdala and of autonomic responses mediated by the monkey amygdala show that, contrary to a widely held view, the amygdala is just as important for processing positive reward and reinforcement as it is for negative. In addition, neuropsychological studies reveal that the amygdala is essential for only a fraction of what might be considered 'stimulus-reward processing', and that the neural substrates for emotion and reward are partially nonoverlapping. Finally, evidence suggests that two systems within the amygdala, operating in parallel, enable reward-predicting cues to influence behavior; one mediates a general, arousing effect of reward and the other links the sensory properties of reward to emotion.

How emotion strengthens the recollective experience: a time-dependent hippocampal process

Emotion significantly strengthens the subjective recollective experience even when objective accuracy of the memory is not improved. Here, we examine if this modulation is related to the effect of emotion on hippocampal-dependent memory consolidation. Two critical predictions follow from this hypothesis. First, since consolidation is assumed to take time, the enhancement in the recollective experience for emotional compared to neutral memories should become more apparent following a delay. Second, if the emotion advantage is critically dependent on the hippocampus, then the effects should be reduced in amnesic patients with hippocampal damage. To test these predictions we examined the recollective experience for emotional and neutral photos at two retention intervals (Experiment 1), and in amnesics and controls (Experiment 2). Emotional memories were associated with an enhancement in the recollective experience that was greatest after a delay, whereas familiarity was not influenced by emotion. In amnesics with hippocampal damage the emotion effect on recollective experience was reduced. Surprisingly, however, these patients still showed a general memory advantage for emotional compared to neutral items, but this effect was manifest primarily as a facilitation of familiarity. The results support the consolidation hypothesis of recollective experience, but suggest that the effects of emotion on episodic memory are not exclusively hippocampally mediated. Rather, emotion may enhance recognition by facilitating familiarity when recollection is impaired due to hippocampal damage.

Arousal regulation, emotional flexibility, medial amygdala function, and the impact of early experience: comments on the paper of Lewis et al

The balance between optimal levels of emotional arousal and cognitive performance reflects the integration of several dopaminergically and adrenergically regulated neural systems. The amygdalar system is a key region for gating stimulation to cortical regions and the medial amygdala appears to play an especially key role in mediating the fear response. More generally, these arousal regulatory neural systems are key to frustration or stress impact prefrontal cortical function. Further, the threshold for when the level of stress is overwhelming and hence impairs cognitive function reflects minimally genetic and experiential influence. An important interface between Drs. Lewis and Davis's work is how early experience, especially through early parenting, may set the threshold of responsiveness for these arousal regulatory neural systems.

Differential time-dependent effects of emotion on recollective experience and memory for contextual information

Emotion has been suggested to slow forgetting via a mechanism that enhances memory consolidation. Here, we investigate whether this time dependent process influences the subjective experience of recollection as well as the ability to retrieve specific contextual details of the study event. To do so we examined recognition for emotional and neutral pictures at two retention intervals and collected remember/know reports and reports about the task that had been performed with the item during encoding. Recollective experience was enhanced for emotional compared to neutral photos after a 24-h delay, but not immediately after encoding. In contrast, memory for the task performed during encoding did not differ between emotional and neutral photos at either time point. The findings indicate that emotion slows the effects of forgetting on the recollective experience associated with studied events, without necessarily slowing the forgetting of specific contextual details of those events.

Empirical tests of the functional significance of amygdala-based modulation of hippocampal representations: evidence for multiple memory consolidation pathways

This series of experiments evaluated the effects of amygdala damage on the acquisition and long-term retention of variants of the water task, and tested the hypothesis that the amygdala is an essential neural system for consolidation of hippocampal memories. In Experiment 1, rats with large, neurotoxic lesions of the amygdala (AMYG) showed normal acquisition on the standard spatial version of the water task, as well as normal retention and decay rate profiles on the 24-h and 30-day retention probes. In Experiment 2, AMYG rats showed normal one-trial place learning abilities and could retain this one-trial information over a 24 h delay. Experiment 3 showed that the amygdala lesions used in this study were functionally significant because AMYG rats, from Experiment 2, showed impairments in a discriminative fear conditioning to context paradigm. Experiment 4 was a critical test of the idea that the amygdala is a decisive locus for consolidation of hippocampal memories. AMYG rats were trained to sub-asymptotic levels of performance on the standard version of the water task. Following each training session, the subjects were given a post-training peripheral injection of D-amphetamine. A probe test revealed that normal subjects and AMYG rats showed similar post-training memory improvement effects. Taken together, the results show that hippocampal memory consolidation processes do not require amygdala modulation. Arguments for an alternative view are presented suggesting that there are multiple memory consolidation pathways, one of which may depend on amygdala neural circuitry.

Stress modulates the use of spatial versus stimulus-response learning strategies in humans

Animal studies provided evidence that stress modulates multiple memory systems, favoring caudate nucleus-based "habit" memory over hippocampus-based "cognitive" memory. However, effects of stress on learning strategy and memory consolidation were not differentiated. We specifically address the effects of psychosocial stress on the applied learning strategy in humans. We designed a spatial learning task that allowed differentiating spatial from stimulus-response learning strategies during acquisition. In 13 subsequent trials, participants (88 male and female students) had to locate a "win" card out of four placed at a fixed location in a 3D model of a room. Relocating one cue in the last trial allowed inferring the applied learning strategy. Half of them participated first in the "Trier Social Stress Test." Salivary cortisol and heart rate measurements were taken. Stressed participants used a stimulus-response strategy significantly more often than controls. Subsequent verbal report revealed that spatial learners had a more complete awareness of response options than stimulus-response learners. Importantly, learning performance was not affected by stress. Taken together, stress prior to learning facilitated simple stimulus-response learning strategies in humans-at the expense of a more cognitive learning strategy. Depending on the context, we consider this as an adaptive response.

The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

We have reviewed research on the effects of stress on LTP in the hippocampus, amygdala and prefrontal cortex (PFC) and present new findings which provide insight into how the attention and memory-related functions of these structures are influenced by strong emotionality. We have incorporated the stress-LTP findings into our "temporal dynamics" model, which provides a framework for understanding the neurobiological basis of flashbulb and traumatic memories, as well as stress-induced amnesia. An important feature of the model is the idea that endogenous mechanisms of plasticity in the hippocampus and amygdala are rapidly activated for a relatively short period of time by a strong emotional learning experience. Following this activational period, both structures undergo a state in which the induction of new plasticity is suppressed, which facilitates the memory consolidation process. We further propose that with the onset of strong emotionality, the hippocampus rapidly shifts from a "configural/cognitive map" mode to a "flashbulb memory" mode, which underlies the long-lasting, but fragmented, nature of traumatic memories. Finally, we have speculated on the significance of stress-LTP interactions in the context of the Yerkes-Dodson Law, a well-cited, but misunderstood, century-old principle which states that the relationship between arousal and behavioral performance can be linear or curvilinear, depending on the difficulty of the task.

Blunted hippocampal, but not striatal, acetylcholine efflux parallels learning impairment in diencephalic-lesioned rats

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate the dynamic role of hippocampal and striatal acetylcholine (ACh) efflux across acquisition of a nonmatching-to-position (NMTP) T-maze task. Changes in ACh efflux were measured in rats at different time points in the acquisition curve of the task (early=day 1, middle=day 5, and late=day 10). Overall, the control group had higher accuracy scores than the PTD group in the latter sessions of NMTP training. During the three microdialysis sampling points, all animals displayed significant increases in ACh efflux in both hippocampus and striatum, while performing the task. However, on day 10, the PTD group showed a significant behavioral impairment that paralleled their blunted hippocampal--but not striatal--ACh efflux during maze training. The results support selective diencephalic-hippocampal dysfunction in the PTD model. This diencephalic-hippocampal interaction appears to be critical for successful episodic and spatial learning/memory.

Chronic administration of UMP ameliorates the impairment of hippocampal-dependent memory in impoverished rats

We have previously shown that chronic, but not acute, dietary supplementation with CDP-choline prevents the hippocampal-dependent memory deficits manifested by aged rats and by rats reared under impoverished environmental conditions. In rats, dietary CDP-choline is rapidly metabolized into cytidine and choline; the cytidine is then readily converted to uridine, which enters the brain and, via conversion to UTP and CTP, increases brain levels of membrane phosphatides. Hence, we have assessed whether administering a uridine source (UMP) instead of CDP-choline can also ameliorate the memory deficits in rats reared under impoverished environmental conditions. At weaning, 32 male Sprague-Dawley rats were exposed to either enriched (EC) or impoverished (IC) conditions for 3 mo. Concurrently, IC and EC rats were given access to either a control diet or a diet supplemented with 0.1% UMP. Rats were then assessed for learning and memory skills using 2 versions of the Morris water maze, the hidden platform version that assesses hippocampal-dependent cognitive memory processing, and the visible platform version that assesses striatal-dependent habit memory. As expected, exposure to the impoverished environment impaired hippocampal-dependent, but not striatal-dependent learning and memory. Supplementation with UMP prevented this cognitive dysfunction, as had been observed with supplemental CDP-choline. These results suggest that IC rats do not use and/or remember their spatial strategies for task solving as well as EC rats, and that long-term dietary supplementation with UMP alleviates this dysfunction.

Estrogen modulates learning in female rats by acting directly at distinct memory systems

Physiologically high levels of circulating estradiol enhance the use of place learning and impair the use of response learning to find food on a land maze. These two types of learning are impaired by lesions of distinct neuronal structures, i.e. the hippocampus and striatum, respectively. Moreover, it has been shown in male rats that compromising hippocampal function can promote the use of response learning, while compromising striatal function can promote place learning. These findings suggest an ongoing competition between the hippocampus and striatum during cognition, such that intact functioning of one structure somehow obstructs the relative participation of the other. The goal of this study was to determine if estrogen's opposing effects on place and response learning in female rats are due to direct actions, either independent or interacting, at the hippocampus and striatum. We infused 0.5 microM 17beta-estradiol 3-sulfate sodium or vehicle bilaterally into the dorsal hippocampus or dorsolateral striatum of ovariectomized young adult female rats, 48, 24 and 2 h before training. Rats were tested on one of three appetitive tasks in a Y-maze: place learning, response learning, or response learning with reduced visual cues (cue-poor condition). Intrahippocampal estradiol infusions enhanced place learning, reversing a cannula-induced impairment, whereas intrastriatal infusions had no effects on place learning. Estradiol infusions into neither structure significantly affected response learning when extramaze cues were visible. However, in the response task, cue-poor condition, intrastriatal but not intrahippocampal infusions impaired learning. These data demonstrate that estrogen modulates place and response learning at the hippocampus and striatum respectively, most likely through independent actions at these two structures.

The neural substrates of amphetamine conditioned place preference: implications for the formation of conditioned stimulus-reward associations

Associations formed between conditioned stimuli and drug reward are major contributors in human drug addiction. To better understand the brain changes that accompany this process, we used immunohistochemistry for c-Fos (a neuronal activity marker), synaptophysin (a marker for synaptogenesis) and tyrosine kinase B receptor (a neurotrophic factor receptor that mediates synaptic plasticity) to investigate the neural substrates of amphetamine-induced conditioned place preference in rats. Conditioned place preference was induced by both 1.0 mg/kg and 0.3 mg/kg doses of amphetamine. Furthermore, amphetamine conditioning increased the density of c-Fos-immunoreactive cells and these cells were fully colocalized with the tyrosine kinase B receptor in the dentate gyrus, CA1 field and basolateral amygdala. Amphetamine conditioning increased the density of synaptophysin-immunoreactive varicosities in all brain regions studied, except the nucleus accumbens shell and dorsolateral striatum. The degree of conditioned place preference was highly correlated with c-Fos-immunoreactive cell density in the basolateral amygdala and with the density of synaptophysin-immunoreactive varicosities in all mesolimbic regions studied. The latter correlation was particularly impressive for the ventral pallidum and basolateral amygdala. The formation of conditioned stimulus-amphetamine reward associations is accompanied by tyrosine kinase B receptor expression in the basolateral amygdala and dentate gyrus, CA1 and CA3 fields of the hippocampus. These data therefore suggest that the formation of conditioned stimulus-reward associations requires, at least in part, activation of amygdalar-hippocampal circuits.

Selective excitotoxic lesions of the hippocampus and basolateral amygdala have dissociable effects on appetitive cue and place conditioning based on path integration in a novel Y-maze procedure

The hippocampus and amygdala are thought to be functionally distinct components of different learning and memory systems. This functional dissociation has been particularly apparent in pavlovian fear conditioning, where the integrity of the hippocampus is necessary for contextual conditioning, and of the amygdala for discrete cue conditioning. Their respective roles in appetitive conditioning, however, remain equivocal mainly due to the lack of agreement concerning the operational definition of a 'context'. The present study used a novel procedure to measure appetitive conditioning to spatial context or to a discrete cue. Following selective excitotoxic lesions of the hippocampus (HPC) or basolateral amygdala (BLA), rats were initially trained to acquire discrete CS-sucrose conditioning in a Y-maze apparatus with three topographically identical chambers, the chambers discriminated only on the basis of path integration. The same group of animals then underwent 'place/contextual conditioning' where the CS presented in a chamber assigned as the positive chamber was paired with sucrose, but the same CS presented in either of the other two chambers was not. Thus, spatial context was the only cue that the animal could use to retrieve the value of the CS. HPC lesions impaired the acquisition of conditioned place preference but facilitated the acquisition of cue conditioning, while BLA lesions had the opposite effect, retarding the acquisition of cue conditioning but leaving the acquisition of conditioned place preference intact. Here we provide strong support for the notion that the HPC and BLA subserve complementary and competing roles in appetitive cue and contextual conditioning.

Establishing an agenda for translational research on PTSD

Animal research on brain mechanisms involved in psychiatric disorders presents an enormous challenge because it is impossible to precisely model symptoms of a human disorder in a rat or mouse. Nevertheless, there are uses for animal models as long as the limitations are recognized. Animal research related to posttraumatic stress disorder (PTSD) points to acute and chronic stressors, such as restraint or immobilization as being the most relevant stimuli to study how neural and endocrine systems are affected, both immediately and long term. Of particular relevance are the onset and duration of effects of stressors on brain areas subserving emotional memories, such as the amygdala, prefrontal cortex, and hippocampus. The hippocampus plays a role in memory and in vegetative functions of the body. The hippocampus receives input from the amygdala and its function in spatial memory is altered by amygdala activity. Repeated stress in the rat suppresses dentate gyrus neurogenesis and causes dendrites of hippocampal and medial prefrontal cortical neurons to shrink. Conversely, it causes basolateral amygdala neurons to increase in dendritic complexity and sprout new synapses. Repeated stress also increases fear and aggression, reduces spatial memory, and alters contextual fear conditioning. Antidepressants and mood stabilizers have diverse effects on these processes. New data indicate that a single stress episode can cause a delayed alteration in synapse formation in the basolateral amygdala without changing dendritic length and branching. Further studies are examining the structural changes in prefrontal cortex and hippocampus as a result of single traumatic stressors, which may reflect the functional interactions with the amygdala. Together with mechanistic studies of the role of adrenal glucocorticoids and catecholamines, these results may tell us how the brain is shaped by acute and repeated uncontrollable stress in ways that then can be investigated in human anxiety disorders.

A double dissociation of dorsal and ventral hippocampal function on a learning and memory task mediated by the dorso-lateral striatum

The objectives of this research were to further delineate the neural circuits subserving proposed memory-based behavioural subsystems in the hippocampal formation. These studies were guided by anatomical evidence showing a topographical organization of the hippocampal formation. Briefly, perpendicular to the medial/lateral entorhinal cortex division there is a second system of parallel circuits that separates the dorsal and ventral hippocampus. Recent work from this laboratory has provided evidence that the hippocampus incidentally encodes a context-specific inhibitory association during acquisition of a visual discrimination task. One question that emerges from this dataset is whether the dorsal or ventral hippocampus makes a unique contribution to this newly described function. Rats with neurotoxic lesions of the dorsal or ventral hippocampus were assessed on the acquisition of the visual discrimination task. Following asymptotic performance they were given reversal training in either the same or a different context from the original training. The results showed that the context-specific inhibition effect is mediated by a circuit that includes the ventral but not the dorsal hippocampus. Results from a control procedure showed that rats with either dorso-lateral striatum damage or dorsal hippocampal lesions were impaired on a tactile/spatial discrimination. Taken together, the results represent a double dissociation of learning and memory function between the ventral and dorsal hippocampus. The formation of an incidental inhibitory association was dependent on ventral but not dorsal hippocampal circuitry, and the opposite dependence was found for the spatial component of a tactile/spatial discrimination.

Factors that determine the non-linear amygdala influence on hippocampus-dependent memory

Stressful experiences are known to either improve or impair hippocampal-dependent memory tasks and synaptic plasticity. These positive and negative effects of stress on the hippocampus have been largely documented, however little is known about the mechanism involved in the twofold influence of stress on hippocampal functioning and about what factors define an enhancing or inhibitory outcome. We have recently demonstrated that activation of the basolateral amygdala can produce a biphasic effect, enhancement or inhibition, of hippocampal synaptic plasticity, depending on the timing of activation (priming or spaced activation). A key question is under which conditions do the effects of amygdala activation on hippocampus dependent memory functions change from improvement to impairment of learning and memory. In this chapter we suggest that hippocampal outcome of amygdala activation may be critically dependent on four main factors: (1) The intensity of amygdala activation, (2) the temporal relation between the activation of the amygdala and the hippocampus dependent memory function, (3) the duration of amygdala activation, and (4) the contextual input during the processing of the information.

Emotion, motivation, and the brain: Reflex foundations in animal and human research

This review will focus on a motivational circuit in the brain, centered on the amygdala, that underlies human emotion. This neural circuitry of appetitive/approach and defensive/avoidance was laid down early in our evolutionary history in primitive cortex, sub-cortex, and mid-brain, to mediate behaviors basic to the survival of individuals and the propagation of genes to coming generations. Thus, events associated with appetitive rewards, or that threaten danger or pain, engage attention and prompt information gathering more so than other input. Motive cues also occasion metabolic arousal, anticipatory responses, and mobilize the organism to prepare for action. Findings are presented from research with animals, elucidating these psychophysiological (e.g., cardiovascular, neuro-humoral) and behavioral (e.g., startle potentiation, "freezing") patterns in emotion, and defining their mediating brain circuits. Parallel results are described from experiments with humans, showing similar activation patterns in brain and body in response to emotion cues, co-varying with participants' reports of affective valence and increasing emotional arousal.

Emotion and Cognition: Insights from Studies of the Human Amygdala

Traditional approaches to the study of cognition emphasize an information-processing view that has generally excluded emotion. In contrast, the recent emergence of cognitive neuroscience as an inspiration for understanding human cognition has highlighted its interaction with emotion. This review explores insights into the relations between emotion and cognition that have resulted from studies of the human amygdala. Five topics are explored: emotional learning, emotion and memory, emotion's influence on attention and perception, processing emotion in social stimuli, and changing emotional responses. Investigations into the neural systems underlying human behavior demonstrate that the mechanisms of emotion and cognition are intertwined from early perception to reasoning. These findings suggest that the classic division between the study of emotion and cognition may be unrealistic and that an understanding of human cognition requires the consideration of emotion.

Effects of stress and corticosterone on activity and plasticity in the amygdala

The basolateral amygdala (BLA) has been repeatedly shown to mediate the effects of stress on memory-related processes. However, the way in which stress influences BLA itself has not been fully explored. We studied the effects of stress and corticosterone (CORT) on activity and plasticity in the BLA in the rat, using the electrophysiological procedure of long-term potentiation (LTP) induction in vivo. Rats were exposed to an acute elevated-platform stress or administered vehicle or 5 mg/kg, 10 mg/kg, or 25 mg/kg of CORT systemically, after which they were anesthetized and prepared for field potential recording in the BLA, in response to stimulation of the entorhinal cortex. The elevated platform stress enhanced baseline responses in BLA and plasma CORT but inhibited amygdalar LTP. Systemic injections of CORT enhanced baseline responses in BLA in a dose-dependent manner but did not influence amygdalar LTP. Posttetanic potentiation (PTP) was similarly reduced in CORT- and vehicle-injected groups, possibly because of an additional stress from the injection, thus implying that PTP and LTP in the amygdala differentially react to stress. These results suggest that the increase in amygdalar baseline activity following the exposure to stress may be mediated by the concomitant increase in plasma CORT. However, the suppression of amygdalar LTP is not a result of elevated levels of CORT, suggesting that activity and plasticity in the amygdala might be mediated by different mechanisms.

NMDA receptor antagonists antagonize the facilitatory effects of post-training intra-basolateral amygdala NMDA and physostigmine on passive avoidance learning

In the present study, the effects of post-training intra-basolateral amygdala (BLA) injection of an N-methyl-d-aspartate (NMDA) receptor agonist and competitive or noncompetitive antagonists, on memory retention of passive avoidance learning was measured in the presence and absence of physostigmine in rats. Intra-BLA administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) did not affect memory retention, although higher doses of the drug (10(-3), 10(-2) and 10(-1) microg/rat) increased memory retention. The greatest response was obtained with 10(-1) microg/rat of the drug. The different doses of the competitive NMDA receptor antagonist DL-AP5 (1, 3.2 and 10 microg/rat) and noncompetitive NMDA receptor antagonist MK-801 (0.5, 1 and 2 microg/rat) decreased memory retention in rats dose dependently. Both competitive and noncompetitive NMDA receptor antagonists reduced the effect of NMDA (10(-2) microg/rat). In another series of experiments, intra-BLA injection of physostigmine (2, 3 and 4 microg/rat) improved memory retention. Post-training co-administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) and physostigmine (1 microg/rat), doses which were ineffective when given alone, significantly improved the retention latency. The competitive and noncompetitive NMDA receptor antagonists, DL-AP5 and MK-801, decreased the effect of physostigmine (2 microg/rat). Atropine decreased memory retention by itself and potentiated the response to DL-AP5 and MK-801. It may be concluded that amygdalar NMDA receptor mechanisms interact with cholinergic systems in the modulation of memory.

Differential induction of c-Jun and Fos-like proteins in rat hippocampus and dorsal striatum after training in two water maze tasks

Research examining the neuroanatomical bases of memory in mammals suggests that the hippocampus and dorsal striatum are parts of independent memory systems that mediate "cognitive" and stimulus-response "habit" memory, respectively. At the molecular level, increasing evidence indicates a role for immediate early gene (IEG) expression in memory formation. The present experiment examined whether acquisition of cognitive and habit memory result in differential patterns of IEG protein product expression in these two brain structures. Adult male Long-Evans rats were trained in either a hippocampal-dependent spatial water maze task, or a dorsal striatal-dependent cued water maze task. Ninety minutes after task acquisition, brains were removed and processed for immunocytochemical procedures, and the number of cells expressing Fos-like immunoreactivity (Fos-like-IR) and c-Jun-IR in sections from the dorsal hippocampus and the dorsal striatum were counted. In the dorsal hippocampus of rats trained in the spatial task, there were significantly more c-Jun-IR pyramidal cells in the CA1 and CA3 regions, relative to rats that had acquired the cued task, yoked controls (free-swim), or naïve (home cage) rats. Relative to rats receiving cued task training and control conditions, increases in Fos-like IR were also observed in the CA1 region of rats trained in the spatial task. In rats that had acquired the cued task, patches of c-Jun-IR were observed in the posteroventral striatum; no such patches were evident in rats trained in the spatial task, yoked-control rats, or naïve rats. The results demonstrate that IEG protein product expression is up-regulated in a task-dependent and brain structure-specific manner shortly after acquisition of cognitive and habit memory tasks.

Contributions of the Amygdala to Emotion Processing: From Animal Models to Human Behavior

Research on the neural systems underlying emotion in animal models over the past two decades has implicated the amygdala in fear and other emotional processes. This work stimulated interest in pursuing the brain mechanisms of emotion in humans. Here, we review research on the role of the amygdala in emotional processes in both animal models and humans. The review is not exhaustive, but it highlights five major research topics that illustrate parallel roles for the amygdala in humans and other animals, including implicit emotional learning and memory, emotional modulation of memory, emotional influences on attention and perception, emotion and social behavior, and emotion inhibition and regulation.

Acetylcholine release in hippocampus and striatum during testing on a rewarded spontaneous alternation task

The present experiment tested male Sprague-Dawley rats for spontaneous alternation performance in a food-rewarded Y-shaped maze. Microdialysis samples, later assessed for acetylcholine concentration, were collected from the hippocampus and striatum of each rat prior to and during testing; testing sessions lasted 20 min. Early in testing, rats alternated at a rate of 72%. Alternation scores increased throughout the 20-min testing session and reached 93% during the last 5 min. The behavioral findings suggest that, during testing, rats changed the basis for their performance from a spatial working memory strategy to a persistent turning strategy. ACh release in both hippocampus and striatum increased at the onset of testing. Increases in ACh release in the striatum began at 18% above baseline during the first 5 min of testing and steadily increased reaching 58% above baseline during the final 5 min. The progressive rise of striatum ACh release during testing occurred at about the time rats adopted a persistent turning strategy. In contrast, ACh release in the hippocampus increased by 50% with the onset of testing and remained at this level until declining slightly during the last 5 min of testing. The relative changes in ACh release in the striatum and hippocampus resulted in a close negative relationship between the ratio of ACh release in the hippocampus/striatum and alternation scores.

Metabolism and Receptor Binding of Serotonin in Brain Structures During Performance of a Conditioned Passive Avoidance Response

The levels of serotonin and its metabolite 5-hydroxyindoleacetic acid, monoamine oxidase activity, and the specific binding of the radioligand [3H]serotonin were measured in the prefrontal cortex, striatum, amygdaloid complex, hippocampus, and periacqueductal gray matter of the midbrain in rats at different time points after training to a conditioned passive avoidance reaction. Changes in serotoninergic activity were found to be characteristic only for the process of reproducing the conditioned reaction. The metabolism and serotonin receptor binding in these brain structures did not change immediately after the training period or one day after this, or in conditions of failure to reproduce the reaction because of amnesia, or in untrained animals. The involvement of the brain serotoninergic system in the process of performing the conditioned reaction was found to demonstrate a spatial-structural selectivity: the metabolism and receptor binding of serotonin changed in the amygdaloid complex, periacqueductal gray matter, and the striatum, while no changes were seen in the hippocampus or prefrontal cortex. All three brain structures showed decreases in [3H]serotonin receptor binding of. Serotonin levels did not change, though the amygdaloid complex and periacqueductal gray matter showed increases in oxidative deamination of serotonin and increases in the active transport of the metabolite, while the striatum showed decreases in serotonin catabolism. The differences in the catabolism of this neurotransmitter suggest that the decrease in serotonin receptor binding in these brain structures depends on different synaptic processes--presynaptic in the striatum and postsynaptic in the amygdaloid complex and periacqueductal gray matter. It is concluded that the decrease in the functional activity of serotoninergic transmission in the amygdaloid complex and periacqueductal gray matter is one of the mechanisms involved in activation of the emotiogenic system triggering the process of reproduction of the memory trace.

Post-training intra-amygdala amphetamine injections given during acquisition of a stimulus–response (S–R) habit task enhance the expression of stimulus–reward learning: Further evidence for incidental amygdala learning

The effect of post-training intra-amygdala amphetamine injections was examined on the acquisition and expression of a visual discrimination task. Rats were trained to enter four lit arms for food (stimulus-response) and avoid unlit arms on an eight-arm radial maze visual discrimination task. Post-training intra-amygdala amphetamine injections (10 microg) were given for 4 consecutive days during the mid-point of training (days 20-23). The number of lit arm entries was used as a measure of stimulus-response habit learning 24 h after each injection. Twenty-four hours after the last injection, a transfer test was run to assess the effect of the same post-training manipulation. This transfer test assessed the amount of time spent in the lit arms and was used as a measure of stimulus-reward learning. Compared to saline-injected rats, rats that received post-training amphetamine spent more time in lit as opposed to dark arms during the transfer test. This occurred in the absence of an increase in the number of correct arm entries during visual discrimination training. This suggests that post-training amphetamine strengthened a stimulus-reward association that did not immediately affect behavioral output. This association may reflect a mnemonic representation stored in an ensemble of amygdala neurons.

Differential effects of amphetamines-induced neurotoxicity on appetitive and aversive Pavlovian conditioning in mice

The abuse of substituted amphetamines such as methamphetamine (METH) and 3,4-methylenedioxymethamphetamine (MDMA/Ecstasy) can result in neurotoxicity, manifested as the depletion of dopamine (DA) and 5-hydroxytriptamine (5-HT; serotonin) axon terminal markers in humans and animal models. Human METH and MDMA users exhibit impairments in memory and executive functions, which may be a direct consequence of the neurotoxic potential of amphetamines. The objective of this study was to investigate the influence of amphetamines-induced neurotoxicity on Pavlovian learning. Using mouse models of selective DA neurotoxicity (METH; 5 mg/kg x 3), selective 5-HT neurotoxicity (fenfluramine /FEN; 25 mg/kg x 4) and dual DA and 5-HT neurotoxicity (MDMA; 15 mg/kg x 4), appetitive and aversive conditioning were investigated. Dopaminergic neurotoxicity significantly impaired METH and cocaine conditioned place preference (CPP), but had no effect on LiCl-induced conditioned place aversion (CPA). In contrast, serotonergic neurotoxicity significantly enhanced CPP, and had no effect on CPA. Dual dopaminergic/serotonergic neurotoxicity had no apparent effect on CPP; however, CPA was significantly attenuated. Postmortem analysis revealed that significantly diminished levels of DA and 5-HT markers persisted in the striatum, frontal cortex, hippocampus, and amygdala. These findings suggest that amphetamines-induced dopaminergic and serotonergic neurotoxicity exert opposing influences on the affective state produced by subsequent drug reward, while dual dopaminergic/serotonergic neurotoxicity impairs associative learning of aversive conditioning. Furthermore, results revealed that amphetamines-induced DA and 5-HT neurotoxicity modulates appetitive Pavlovian conditioning similar to other DA and 5-HT neurotoxins. Modulation of Pavlovian conditioning by amphetamines-induced neurotoxicity may be relevant to compulsive drug-seeking behavior in METH and MDMA abusers.

Intraneuronal Abeta causes the onset of early Alzheimer's disease-related cognitive deficits in transgenic mice

Progressive memory loss and cognitive dysfunction are the hallmark clinical features of Alzheimer's disease (AD). Identifying the molecular triggers for the onset of AD-related cognitive decline presently requires the use of suitable animal models, such as the 3xTg-AD mice, which develop both amyloid and tangle pathology. Here, we characterize the onset of learning and memory deficits in this model. We report that 2-month-old, prepathologic mice are cognitively unimpaired. The earliest cognitive impairment manifests at 4 months as a deficit in long-term retention and correlates with the accumulation of intraneuronal Abeta in the hippocampus and amygdala. Plaque or tangle pathology is not apparent at this age, suggesting that they contribute to cognitive dysfunction at later time points. Clearance of the intraneuronal Abeta pathology by immunotherapy rescues the early cognitive deficits on a hippocampal-dependent task. Reemergence of the Abeta pathology again leads to cognitive deficits. This study strongly implicates intraneuronal Abeta in the onset of cognitive dysfunction.

Long-term effects of amygdala GABA receptor blockade on specific subpopulations of hippocampal interneurons

Growing evidence indicates that the amygdala modulates hippocampal functions. To test the hypothesis that this modulation may involve long-lasting effects on interneuronal networks in the hippocampus, changes in the expression of neurochemical markers specific for different interneuronal subpopulations were assessed in adult rats 96 h following acute infusion of low doses of the GABAA receptor antagonist picrotoxin into the amygdala. The numerical density (Nd) of somata showing immunoreactivity (IR) for parvalbumin (PVB) was decreased in dentate gyrus (DG) and the CA4-2 region, while that of calretinin (CR)-IR was decreased in DG and CA2. The Nd of calbindin D28k (CB)-IR somata was decreased in CA3-2. The densities of axon terminals arising from PVB-IR and cholecystokinin (CCK)-IR basket neurons were also altered, with those of CCK-IR terminals increased across all sectors, while PVB-IR terminals were decreased only in the CA region. Increases in CCK-IR terminals were paralleled by increases of terminals with IR for the 65-kD isoform of glutamate decarboxylase (GAD65). Mixed-effects statistical models, adapted specifically for these analyses, indicated that perturbations of amygdalar inputs to the hippocampus significantly alter the drive that hippocampal PVB-, CR-, and CB-IR neurons within the dentate gyrus/CA4 region exercise on CCK-IR terminals within the same region as well as in CA3-1. These results suggest that amygdalar modulation of specific neuronal subpopulations may induce lasting and far-reaching changes in the hippocampus during normal functioning, as well as in diseases involving a disruption of amygdalar activity. In particular, changes in specific interneuronal markers within selective hippocampal sectors detected in the present results are strikingly similar to those reported in this region in schizophrenia. These similarities suggest that, in this disease, a disruption of GABAergic transmission within the amygdala may play a significant role in the induction of abnormalities in the hippocampus.

Learning-induced activation of transcription factors among multiple memory systems

Experimental evidence for multiple memory systems grew initially from reports that integrity of the medial temporal lobes is necessary for some, but not all, types of memory formation. A primary inference from many studies of multiple memory systems is that they operate independently during encoding, storage, and retrieval of information. An accumulation of recent evidence, however, suggests that multiple memory systems may interact under some conditions. At the cellular level of analysis, it is accepted widely that protein synthesis is necessary for the formation of long-term memory and recent efforts have focused on the mechanisms by which learning-induced gene transcription and translation are regulated. The present review examines learning-induced activation of transcription factors among multiple memory systems. The results indicate that studies of transcriptional regulation, in conjunction with other experimental approaches, can provide complementary lines of evidence to further understanding of the extent to which multiple memory systems are independent or interactive.

Amygdala and "emotional" modulation of the relative use of multiple memory systems

The basolateral amygdala modulates the cognitive and habit memory processes mediated by the hippocampus and caudate nucleus, respectively. The present experiments used a plus-maze task that can be acquired using either hippocampus-dependent "place" learning or caudate-dependent "response" learning to examine whether peripheral or intra-basolateral amygdala injection of anxiogenic drugs would bias rats towards the use of a particular memory system. In Experiment 1, adult male Long-Evans rats were trained to swim from the same start point to an escape platform located in a consistent goal arm, and received pre-training peripheral injections of the alpha(2)-adrenoceptor antagonists yohimbine (2.5 or 5.0 mg/kg), RS 79948-197 (0.05, 0.1, or 0.2 mg/kg), or vehicle. On a drug-free probe trial from a novel start point administered 24h following acquisition, vehicle treated rats predominantly displayed hippocampus-dependent place learning, whereas rats previously treated with yohimbine (2.5, 5.0 mg/kg) or RS 79948-197 (0.1 mg/kg) predominantly displayed caudate-dependent response learning. In Experiment 2, rats receiving pre-training intra-basolateral amygdala infusions of RS 79948-197 (0.1 microg/0.5 microl) also predominantly displayed response learning on a drug-free probe trial. The findings indicate (1) peripheral injections of anxiogenic drugs can influence the relative use of multiple memory systems in a manner that favors caudate-dependent habit learning over hippocampus-dependent cognitive learning, and (2) intra-basolateral amygdala infusion of anxiogenic drugs is sufficient to produce this modulatory influence of emotional state on the use of multiple memory systems.

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.

Dietary CDP-choline supplementation prevents memory impairment caused by impoverished environmental conditions in rats

We previously showed that dietary cytidine (5')-diphosphocholine (CDP-choline) supplementation could protect against the development of memory deficits in aging rats. In the present study, younger rats exposed to impoverished environmental conditions and manifesting hippocampal-dependent memory impairments similar to those observed in the aging rodents were given CDP-choline, and its effects on this cognitive deficit were assessed. Male Sprague-Dawley rats reared for 3 mo in impoverished (IC) or enriched environmental (EC) conditions concurrently received either a control diet or a diet supplemented with CDP-choline (approximately 500 mg/kg/d). After 3 mo, rats were trained to perform spatial and cued versions of the Morris water maze, and their rates of acquisition and retention were compared. Impoverished rats exhibited a selective deficit in hippocampal-dependent spatial memory which could be ameliorated by feeding them CDP-choline. The CDP-choline had no memory-enhancing effect in enriched rats, nor did it prevent the memory impairment of impoverished rats if the animals consumed it for the initial or final months instead of for the entire 3-mo period. These findings indicate that long-term dietary CDP-choline supplementation can ameliorate the hippocampal-dependent memory impairment caused by impoverished environmental conditions in rats, and suggest that its actions result, in part, from a long-term effect such as enhanced membrane phosphatide synthesis, an effect shown to require long-term dietary supplementation with CDP-choline.

Demonstration of nondeclarative sequence learning in mice: development of an animal analog of the human serial reaction time task

In this paper, we demonstrate nondeclarative sequence learning in mice using an animal analog of the human serial reaction time task (SRT) that uses a within-group comparison of behavior in response to a repeating sequence versus a random sequence. Ten female B6CBA mice performed eleven 96-trial sessions containing 24 repetitions of a 4-trial sequence. During the 12th session, the repeating sequence was replaced with the random sequence halfway through the session. Reaction time (RT) to respond to an illuminated nose-poke was recorded, and performance was compared at the halfway point in each session to test for any change in behavior. For learning effect, RTs decreased over the no-switch repeating-sequence sessions. For interference effect, behavior did not change appreciably at the halfway point during the last repeating-sequence session. However, RTs deteriorated significantly after the switch from repeating to random sequences halfway through session 12. The mice demonstrated a robust interference effect when switched from repeating to random sequences. This pattern of behavior in humans performing the SRT is interpreted as evidence of nondeclarative sequence learning. The similarity between the human and mouse SRTs will enable more direct comparisons of mouse-human nondeclarative memory behavior and will provide a useful behavioral end-point in mouse-models of basal ganglia dysfunction.

Phosphodiesterase type 5 inhibition improves early memory consolidation of object information

The nitric oxide (NO)-cyclic GMP (cGMP) signaling pathway is assumed to play an important role in processes underlying learning and memory. We used phosphodiesterase type 5 (PDE5) inhibitors to study the role of cGMP in object- and spatial memory. Our results and those reported in other studies indicate that elevated hippocampal cGMP levels are required to improve the memory performance of rodents in object recognition and passive avoidance learning, but not in spatial learning. The timing of treatment modulates the effects on memory and strongly supports a role for cGMP in early stages of memory formation. Alternative explanations for the improved memory performance of PDE5 inhibitors are also discussed. Immunocytochemical studies showed that in vitro slice incubations with PDE5 inhibitors increase NO-stimulated cGMP levels mainly in hippocampal varicose fibers. Reviewing the available data on the localization of the different components of the NO-cGMP signaling pathway, indicates a complex interaction between NO and cGMP, which may be independent of each other. It is discussed that further studies are needed, immunocytochemical and behavioral, to better understand the cGMP-mediated molecular mechanisms underlying memory formation.

Axonal connections from posterior paralaminar thalamic neurons to basomedial amygdaloid projection neurons to the lateral entorhinal cortex in rats

Stimulation of amygdaloid nuclei and emotionally relevant stimuli are known to influence the induction and maintenance of long-term potentiation in the hippocampal formation and the formation of long-term declarative memories. Because the thalamic projection from the posterior paralaminar thalamic nuclei is an important sensory afferent projection to amygdaloid nuclei mediating the fast acquisition of fear-potentiated behavior, we were interested in verifying whether this projection establishes synaptic contacts on amygdala neurons that project to the hippocampal formation. Thalamic afferents were labeled with the anterograde tracer Phaseolus vulgaris leucoagglutinin and amygdalo-hippocampal neurons were identified by injection of the retrograde tracer Fluorogold into the lateral entorhinal cortex. A massive overlap of both projection systems was observed especially in the anterior basomedial nucleus of the amygdala. Light microscopic examination revealed that single anterogradely labeled boutons were in close apposition to retrogradely labeled neurons suggesting synaptic contacts. The occurrence of such synaptic contacts was confirmed with electron microscopy. However, despite the massive overlap of anterogradely labeled axons and retrogradely labeled neurons observed at the light microscopic level, electron microscopy revealed that only 10% of all labeled profiles make direct contacts on each other; anterogradely labeled boutons predominantly contacted unlabeled profiles but synapses with direct contact between labeled profiles were rare. Altogether the findings demonstrate that the thalamic connection with the basomedial nucleus of the amygdala may represent an anatomical substrate for modulating amygdala output to the hippocampal formation.

Amygdala stimulation modulates hippocampal synaptic plasticity

Experience-dependent synaptic plasticity is a fundamental feature of neural networks involved in the encoding of information, and the capability of synapses to express plasticity is itself activity-dependent. Here, we introduce a "low-frequency burst stimulation" protocol, which can readily induce both long-term potentiation (LTP) and long-term depression (LTD) at in vivo medial perforant path-dentate gyrus synapses. By varying stimulation parameters, we were able to build a stimulus-response map of synaptic plasticity as a LTP-LTD continuum. The response curve displayed a bidirectional shift toward LTP and LTD, depending on the degree and timing of neural activity of the basolateral amygdala. The range of this plastic modulation was also modified by past activity of the basolateral amygdala, suggesting that the amygdala can arrange its ability to regulate the dentate plastic responses. The effects of the BLA activation were replicated by stimulation of the lateral perforant path and, hence, BLA stimulation may recruit the lateral entorhinal cortex. These results represent a high-order dimension of heterosynaptic modulations of hippocampal synaptic plasticity.

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.

How arousal modulates memory: Disentangling the effects of attention and retention

Emotion may influence memory both by altering attention and perception during encoding and by affecting memory retention. To date, studies have focused on the enhancement of memory consolidation by arousal. However, they have failed to rule out a role for attention. To specifically link memory enhancement of arousing material to modulation of memory retention, we examined recognition of neutral and arousing words at two time points and under conditions that manipulate attention during encoding. Participants were briefly presented with an arousing or neutral word at the periphery, while fixating on a central word. Recognition of peripheral words was assessed either immediately or after 24 h. Whereas recognition of neutral words became worse over time, recognition of arousing words remained the same and was better than neutral word recognition at delay. The results indicate that arousal supports slower forgetting even when the difference in attentional resources allocated to stimuli is minimized.

Sleep deprivation impairs spatial memory and decreases extracellular signal-regulated kinase phosphorylation in the hippocampus

Loss of sleep may result in memory impairment. However, little is known about the biochemical basis for memory deficits induced by sleep deprivation. Extracellular signal-regulated kinase (ERK) is involved in memory consolidation in different tasks. Phosphorylation of ERK is necessary for its activation and is an important step in mediating neuronal responses to synaptic activities. The aim of the present study was to determine the effects of total sleep deprivation (TSD) on memory and ERK phosphorylation in the brain. Rats were trained in Morris water maze to find a hidden platform (a spatial task) or a visible platform (a nonspatial task) after 6 h TSD or spontaneous sleep. TSD had no effect on spatial learning, but significantly impaired spatial memory tested 24 h after training. Nonspatial learning and memory were not impaired by TSD. Phospho-ERK levels in the hippocampus were significantly reduced after 6 h TSD compared to the controls and returned to the control levels after 2 h recovery sleep. Total ERK1 and ERK2 were slightly increased after 6 h TSD and returned to the control levels after 2 h recovery sleep. These alterations were not observed in the cortex after TSD. Protein phosphotase-1 and mitogen-activated protein kinase phosphatase-2, which dephosphorylates phospho-ERK, were also measured, but they were not altered by TSD. The impairments of both spatial memory and ERK phosphorylation indicate that the hippocampus is vulnerable to sleep loss. These results are consistent with the idea that decreased ERK activation in the hippocampus is involved in sleep deprivation-induced spatial memory impairment.

Post-treatment, but not pre-treatment, with the selective cyclooxygenase-2 inhibitor celecoxib markedly enhances functional recovery from kainic acid-induced neurodegeneration

We have investigated the role of inflammation in the excitotoxicity induced by overstimulation of glutamate receptors using kainic acid, an important tool for studying functions related to excitatory amino acid transmission and for producing neuronal death, especially in areas CA1 and CA3 of the hippocampus. We hypothesised that by inhibiting one of the major components of the neuroinflammation response, after kainic acid injection, that there would be less inflammation and therefore a reduction in cell loss, an enhancement of cognitive function (using spatial learning and object exploration tasks) or both. We examined brain-derived neurotrophic factor levels, expecting that there would be a correlation between its level and subsequent recovery. Our results confirmed our hypothesis: the kainic acid injected-rats treated with celecoxib (after kainic injection) performed better in the spatial and non-spatial tasks than the kainic acid-treated group. However, there was not any improvement if celecoxib was given before kainic acid treatment, underlining also the importance of the production of prostaglandin at the beginning of inflammation.

Intra-amygdala muscimol injections impair freezing and place avoidance in aversive contextual conditioning

Rats were trained by shocking them in a closed compartment. When subsequently tested in the same closed compartment with no shock, normal rats showed an increased tendency to freeze. They also showed an increased tendency to actively avoid the compartment when given access to an adjacent neutral compartment for the first time. Amygdala inactivation with bilateral muscimol injections before training attenuated freezing and eliminated avoidance during the test. Rats trained in a normal state and given intra-amygdala muscimol injections before the test did not freeze or avoid the shock-paired compartment. This pattern of effects suggests that amygdala inactivation during training impaired acquisition of a conditioned response (CR) due either to inactivation of a neural substrate essential for its storage or to elimination of a memory modulation effect that facilitates its storage in some other brain region(s). The elimination of both freezing and active avoidance by amygdala inactivation during testing suggests that neither of these behaviors is the CR. The possibility that the CR is a set of internal responses that produces both freezing and avoidance as well as other behavioral effects is discussed.

The induction of long-term potentiation at amygdalo-hippocampal synapses in vivo

Electrical stimulation of the basolateral amygdala (BLA) evoked synaptic potentials in the dentate gyrus (DG) of the hippocampus in anesthetized rats. To determine if this pathway possesses synaptic plasticity, we investigated the impact of several conditions of high-frequency stimulation on BLA-DG synaptic potentials in these rats. Application of two trains of 100-pulse, 100-Hz stimulation or theta-burst stimulation to the BLA reproducibly induced long-term potentiation (LTP) of BLA-DG synaptic potentials. Paired-pulse facilitation was unchanged during LTP, suggesting that postsynaptic mechanisms are involved in the expression of LTP. In addition, the induction of LTP was not affected by the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate, suggesting that activation of NMDA receptors is not required. This novel form of LTP should be a valuable model for elucidating neural mechanisms underlying the formation of emotional memory.

The amygdala modulates the consolidation of memories of emotionally arousing experiences

Converging findings of animal and human studies provide compelling evidence that the amygdala is critically involved in enabling us to acquire and retain lasting memories of emotional experiences. This review focuses primarily on the findings of research investigating the role of the amygdala in modulating the consolidation of long-term memories. Considerable evidence from animal studies investigating the effects of posttraining systemic or intra-amygdala infusions of hormones and drugs, as well as selective lesions of specific amygdala nuclei, indicates that (a) the amygdala mediates the memory-modulating effects of adrenal stress hormones and several classes of neurotransmitters; (b) the effects are selectively mediated by the basolateral complex of the amygdala (BLA); (c) the influences involve interactions of several neuromodulatory systems within the BLA that converge in influencing noradrenergic and muscarinic cholinergic activation; (d) the BLA modulates memory consolidation via efferents to other brain regions, including the caudate nucleus, nucleus accumbens, and cortex; and (e) the BLA modulates the consolidation of memory of many different kinds of information. The findings of human brain imaging studies are consistent with those of animal studies in suggesting that activation of the amygdala influences the consolidation of long-term memory; the degree of activation of the amygdala by emotional arousal during encoding of emotionally arousing material (either pleasant or unpleasant) correlates highly with subsequent recall. The activation of neuromodulatory systems affecting the BLA and its projections to other brain regions involved in processing different kinds of information plays a key role in enabling emotionally significant experiences to be well remembered.

Shifts in preferred learning strategy across the estrous cycle in female rats

The current status of the effects of ovarian steroids on learning and memory remains somewhat unclear, despite a large undertaking to evaluate these effects. What is emerging from this literature is that estrogen, and perhaps progesterone, influences learning and memory, but does so in a task-dependent manner. Previously, we have shown that ovariectomized rats given acute treatments of estrogen acquire allocentric or "place" tasks more easily than do rats deprived of estrogen, but acquire egocentric or "response" learning tasks more slowly than do those deprived of hormone, suggesting that estrogen treatment may bias the strategy a rat is able to use to solve tasks. To determine if natural fluctuations in ovarian hormones influence cognitive strategy, we tested whether strategy use fluctuated across the estrous cycle in reproductively intact female rats. We found that in two tasks in which rats freely choose the strategy used to solve the task, rats were more likely to use place strategies at proestrous, that is, when ovarian steroids are high. Conversely, estrous rats were biased toward response strategies. The data suggest that natural fluctuations in ovarian steroids may bias the neural system used and thus the cognitive strategies chosen during learning and memory.

Enhancement of inhibitory avoidance and conditioned taste aversion memory with insular cortex infusions of 8-Br-cAMP: involvement of the basolateral amygdala

There is considerable evidence that in rats, the insular cortex (IC) and amygdala are involved in the learning and memory of aversively motivated tasks. The present experiments examined the effects of 8-Br-cAMP, an analog of cAMP, and oxotremorine, a muscarinic agonist, infused into the IC after inhibitory avoidance (IA) training and during the acquisition/consolidation of conditioned taste aversion (CTA). Posttraining infusion into the IC of 0.3 microg oxotremorine and 1.25 microg 8-Br-cAMP enhanced IA retention. Infusions of 8-Br-cAMP, but not oxotremorine, into the IC enhanced taste aversion. The experiments also examined whether noradrenergic activity in the basolateral amygdala (BLA) is critical in enabling the enhancement of CTA and IA memory induced by drug infusions administered into the IC. For both CTA and IA, ipsilateral infusions of beta-adrenergic antagonist propranolol administered into the BLA blocked the retention-enhancing effect of 8-Br-cAMP or oxotremorine infused into the IC. These results indicate that the IC is involved in the consolidation of memory for both IA and CTA, and this effect requires intact noradrenergic activity into the BLA. These findings provide additional evidence that the BLA interacts with other brain regions, including sensory cortex, in modulating memory consolidation.

Amygdala inactivation blocks expression of conditioned memory modulation and the promotion of avoidance and freezing

Rats were exposed to shock-paired cues immediately after training on an appetitive preference task. Elevated levels of freezing in and active avoidance of the shock-paired compartment were observed, and memory for the appetitive task was improved when tested 24 hr later. Intra-amygdala muscimol injected before the posttraining exposure eliminated freezing, avoidance, and memory modulation. The blockade of both freezing and active avoidance, which involve competing behavioral tendencies, makes it unlikely that the amygdala itself generates either behavior. The elimination of conditioned memory modulation suggests that conditioned neurohormonal responses were blocked. These conditioned internal responses may comprise the intervening variable of "conditioned fear" and may promote observable behaviors, the form of which is determined by the environment in which they occur.

Behavioural parameters in aged rats are related to LTP and gene expression of ChAT and NMDA-NR2 subunits in the striatum

Striatal parameters were assessed for their relevance to age-related behavioural decline. Forty aged rats (28-30 months) were tested in the water maze and open field. Of these, seven superior and seven inferior learners were compared with each other in terms of levels of in vitro short- and long-term potentiation (STP and LTP), and gene expression of choline acetyltransferase (ChAT) as well as of the NMDA-NR2A-C subunits assessed by quantitative RT-PCR. Results revealed that the superior as compared with the inferior learners had higher levels of ChAT mRNA in the striatum. For the superior group, ChAT mRNA was correlated with escape on to the cued platform in the water maze, whereas level of LTP was predictive of place learning in the water maze and rearing activity in the open field. For the inferior group, expression of NR2A and NR2B was positively correlated with place learning and probe trial performance in the water maze. The results show that individual differences in various behaviours of aged rats were accounted for by variability in striatal parameters, i.e. LTP, ChAT and NMDA-NR2 subunit mRNA. Notably, the correlations found were heterogeneous amid the groups, e.g. variability in place learning was explained by variability in levels of LTP in the superior learners, but in levels of NR2A-B mRNA in the inferior group.

The amygdala modulates hippocampus-dependent context memory formation and stores cue-shock associations

Preexposing rats to the context facilitates subsequent contextual fear conditioning. This effect depends on the hippocampus (J. W. Rudy, R. M. Barrientos, & R. C. O'Reilly, 2002). The authors report that inactivating the basolateral region of the amygdala (BLA) by injecting muscimol, a GABAA agonist, before or after preexposure reduced this effect. In contrast, bilateral injections of anisomycin, a protein synthesis inhibitor, into BLA did not impair the consolidation of the context memory. However, when injected after fear conditioning, anisomycin impaired consolidation of both contextual and auditory-cue fear conditioning. Results are consistent with 2 ideas about the amygdala's contribution to memory: (a) It modulates memory formation in other regions of the brain, and (b) it is a storage site for cue-shock associations.

Emotional tagging of memory formation--in the search for neural mechanisms

Memory-related areas, such as the hippocampus, should be able to sort out the more significant from the less relevant aspects of an experience in order to transform only the earlier into long-term memory. We have recently suggested the Emotional Tagging concept, according to which the activation of the amygdala in emotionally arousing events mark the experience as important and aids in enhancing synaptic plasticity in other brain regions. Here, we review evidence from both human and animal studies that lend support to the Emotional Tagging hypothesis and to the central role the amygdala may play in its formation. We further speculate on potential neural mechanisms that may underlie emotional tagging. Long-term memory formation is considered to involve lasting alterations in synaptic efficacy, known as synaptic plasticity. It has been suggested that two factors are crucial for obtaining a synapse-specific long-term plasticity: (a) the successful activation of a synapse-specific, protein synthesis-independent tag, and (b) the activation of synapse-non-specific protein synthesis. The activation of protein synthesis can then induce lasting plasticity only in those synapses marked by a tag. Interestingly and relevant to the Emotional Tagging hypothesis, it has been recently shown that the activation of the amygdala could transform transient into long-lasting plasticity. These recent findings seem to fit well with the Emotional Tagging hypothesis. It seems reasonable to assume that the activation of the amygdala triggers neuromodulatory systems, which in turn reduce the threshold for the activation of the synaptic tag, and by this facilitate the transformation of early- into late-phase memory.