Auditory thalamus, dorsal hippocampus, basolateral amygdala, and perirhinal cortex role in the consolidation of conditioned freezing to context and to acoustic conditioned stimulus in the rat

A reversal of the normal pattern of parahippocampal response to neutral and fearful faces is associated with reality distortion in schizophrenia

BACKGROUND

Individuals with schizophrenia demonstrate impaired recognition of facial expressions and may misattribute emotional salience to otherwise nonsalient stimuli. The neural mechanisms underlying this deficit and the relationship with different symptoms remain poorly understood.

METHODS

We used event-related functional magnetic resonance imaging to measure neural responses to neutral, mildly fearful, and prototypically fearful facial expressions. The sample included 15 medicated individuals with chronic schizophrenia (SZ) and 11 healthy control individuals (CON), matched for gender (all male), age, and years of education.

RESULTS

A repeated measures 3 x 2 analysis of variance (ANOVA) revealed a significant interaction between expression intensity and group in right parahippocampal gyrus (p < .01). Individuals with chronic schizophrenia demonstrated a decrease, whereas CON showed an increase, in right parahippocampal gyrus response to increasingly fearful expressions. Between-group comparison revealed greater activation in SZ than CON in right parahippocampal gyrus to neutral faces. The reality distortion dimension, but not neuroleptic medication dose, was positively associated with the right parahippocampal gyral and right amygdalar response to neutral faces in SZ.

CONCLUSIONS

An abnormally increased parahippocampal response to neutral faces was positively associated with reality distortion in SZ. This may underlie the previously reported finding of a misattribution of emotional salience to nonsalient social stimuli in schizophrenia.

Different molecular cascades in different sites of the brain control memory consolidation

To understand cognition, it is important to understand how a learned response becomes a long-lasting memory. This process of memory consolidation has been modeled extensively using one-trial avoidance learning, in which animals (or humans) establish a conditioned response by learning to avoid danger in just one trial. This relies on molecular events in the CA1 region of the hippocampus that resemble those involved in CA1 long-term potentiation (LTP), and it also requires equivalent events to occur with different timings in the basolateral amygdala and the entorhinal, parietal and cingulate cortex. Many of these steps are modulated by monoaminergic pathways related to the perception of and reaction to emotion, which at least partly explains why strong and resistant consolidation is typical of emotion-laden memories. Thus memory consolidation involves a complex network of brain systems and serial and parallel molecular events, even for a task as deceptively simple as one-trial avoidance. We propose that these molecular events might also be involved in many other memory types in animals and humans.

Importance of early environment in the development of post-traumatic stress disorder-like behaviors

A number of clinical studies in which early adversities were defined retrospectively, demonstrated that early adverse experiences increased the morbidity rate of post-traumatic stress disorder (PTSD) in later life. However, no prospective studies have yet been conducted to elucidate whether early adversity affects the risk or severity of PTSD. Thus, we examined whether early adversity would strengthen the severity of PTSD symptoms in later life by using neonatal isolation (NI) and single prolonged stress (SPS) as an animal model of PTSD. We measured anxiety-like behavior in the elevated plus maze (EPM), contextual freezing in the contextual fear (CF) test, and analgesia in the flinch-jump and hot-plate tests in four groups of adult rats (sham, NI, SPS, and NI+SPS). NI significantly enhanced the SPS-induced decrease in the percentage of open arm time and open arm entries in the EPM, enhanced the SPS-induced increase in contextual freezing, and strengthened SPS-induced analgesia, without any changes in locomotor activity in the open field locomotor test. In addition, we examined the effect of environmental enrichment (EE). Repeated exposure to EE ameliorated the NI-induced enhancement of contextual freezing, but not anxiety-like behavior or analgesia, in response to SPS. The results of the present study demonstrated that while early adversity strengthened PTSD-like symptoms, EE alleviated the enhanced contextual freezing by NI and SPS. These findings suggest that early adversity may worsen dysfunction of the amygdala and hippocampus in PTSD, and an early intervention may alleviate the early adversity-mediated enhancement of hippocampal dysfunction in PTSD.

Dorsal hippocampus inhibition disrupts acquisition and expression, but not consolidation, of cocaine conditioned place preference

Cocaine abusers may experience drug craving upon exposure to environmental contexts where cocaine was experienced. The dorsal hippocampus (DHC) is important for contextual conditioning, therefore the authors examined the specific role of the DHC in cocaine conditioned place preference (CPP). Muscimol was used to temporarily inhibit the DHC and was infused before conditioning sessions or tests for CPP to investigate acquisition and expression of cocaine CPP, respectively. To investigate consolidation, rats received intra-DHC muscimol either immediately or 6 hr after conditioning sessions. Inhibition of DHC, but not the overlying cortex, disrupted acquisition and expression of cocaine CPP. It is interesting to note that there was no effect of post-conditioning DHC inhibition. The findings suggest that the DHC is important for both acquisition and recall, but not consolidation, of context-cocaine associations.

Beta-adrenoceptor mediated inhibition of long-term reward-related memory reconsolidation

Well-consolidated fear-related memories, once retrieved, are susceptible to disruption and require reconsolidation in order to be maintained. We examined whether reactivated reward-related memories are also susceptible to interference by evaluating the effect of propranolol (PROP), a beta-adrenergic antagonist known to impair reconsolidation of fear-related memories, on context-induced sucrose seeking. PROP administration upon reactivation reduced sucrose seeking behaviour 3 weeks post-training, indicating that reconsolidation of reward-related memories can be disrupted after a long post-training interval.

Age-related naturally occurring depression of hippocampal neurogenesis does not affect trace fear conditioning

New neuron production throughout adulthood in granule cell layer (GCL) of rat hippocampus is a well-known phenomenon. A role of new neurons in hippocampal learning has been proposed, but the question is still open. A reduction of neural precursor proliferation in GCL of 2-month-old rats to about 20%, induced by the cytostatic agent methylazoxymethanol, was found to cause impairment in trace conditioning, suggesting a role of immature neurons in this kind of hippocampus-dependent learning (Shors et al., Hippocampus 2002;12:578-584). Neurogenesis decreases with increasing age. In this study, neural precursor proliferation and newborn cell survival were evaluated in GCL of adult rats within a range of ages following development and preceding old age. In 5-month-old rats, neural precursor proliferation was reduced to 57% and newborn cell survival was reduced to 40% in comparison to rats of 2 months of age; in 12-month-old rats, the decrease was to 5 and 4%, respectively. Consistently, the density of immature neurons decreased to 41 and 13% in 5- and 12-month-old rats, respectively. The role of neurogenesis in trace fear conditioning was studied in this natural model of neurogenesis depression. No impairment in trace fear conditioning was found both in 5- and 12-month-old rats in comparison to 2-month-old rats, notwithstanding the decrease of neurogenesis that is marked in 12-month-old rats. This finding shows that a lower rate of neurogenesis is sufficient for learning in 12-month-old rats in comparison to young rats.

Effects of lesions to the dorsal and ventral hippocampus on defensive behaviors in rats

This study investigated the role of the hippocampus in both unconditioned and conditioned defensive behaviors by examining the effects of pretraining ibotenic acid lesions to the dorsal and ventral hippocampus in male Long-Evans hooded rats exposed to three types of threat stimuli: cat-odor, a live cat and footshock. Defensive behaviors were assessed during exposure to cat-odor and a live cat, and immediately following the presentation of footshock. Conditioned defensive behaviors were also assessed in each context 24 h after initial threat exposure. During both unconditioned and conditioned trials, dorsal hippocampal lesions failed to significantly alter any behavioral measure in each test of defense. In contrast, ventral hippocampal lesions significantly reduced unconditioned defensive behaviors during exposure to cat-odor without producing any observable effects during cat exposure. Furthermore, ventral lesions significantly attenuated conditioned defensive behaviors following the administration of footshock and during re-exposure to each context. These results suggest a specific role for the ventral, not dorsal, hippocampus in modulating anxiety-like behaviors in certain animal models of defense.

Differential involvement of the hippocampus, anterior cingulate cortex, and basolateral amygdala in memory for context and footshock

Extensive evidence from contextual fear conditioning experiments suggests that the hippocampus is involved in processing memory for contextual information. Evidence also suggests that the rostral anterior cingulate cortex (rACC) may be selectively involved in memory for nociceptive stimulation. In contrast, many findings indicate that the basolateral amygdala (BLA) is more broadly involved in modulating the consolidation of different kinds of information. To investigate further the differential involvement of these brain regions in memory consolidation, the present experiments used a modified inhibitory avoidance training procedure that took place on 2 sequential days to separate context training from footshock training. Male Sprague-Dawley rats were implanted with unilateral cannulae aimed at the (i) hippocampus, (ii) rACC, or (iii) BLA, and given infusions of the muscarinic cholinergic agonist oxotremorine (OXO) immediately after either context training (day 1) or footshock training in that context (day 2). OXO enhanced retention when infused into the hippocampus after context, but not footshock, training. Conversely, OXO infusions enhanced memory when administered into the rACC immediately after footshock, but not context, training. Lastly, intra-BLA OXO infusions enhanced retention when administered after either context or footshock training. These findings are consistent with evidence that the hippocampus and rACC play selective roles in memory for specific components of training experiences. Additionally, they provide further evidence that the BLA is more liberally involved in modulating memory consolidation for various aspects of emotionally arousing experiences.

Membrane excitability and fear conditioning in cerebellar Purkinje cell

In a previous study it has been demonstrated that fear conditioning is associated with a long-lasting potentiation of parallel fiber to Purkinje cell synaptic transmission in vermal lobules V and VI. Since modifications of intrinsic membrane properties have been suggested to mediate some forms of memory processes, we investigated possible changes of Purkinje cell intrinsic properties following the same learning paradigm and in the same cerebellar region. By means of the patch clamp technique, Purkinje cell passive and active membrane properties were evaluated in slices prepared from rats 10 min or 24 h after fear conditioning and in slices from control naïve animals. None of the evaluated parameters (input resistance, inward rectification, maximal firing frequency and the first inter-spike interval, post-burst afterhyperpolarization, action potential threshold and amplitude, action potential afterhyperpolarization) was significantly different between the three studied groups also in those cells where parallel fiber-Purkinje cell synapse was potentiated. Our results show that fear learning does not affect the intrinsic membrane properties involved in Purkinje cell firing. Therefore, at the level of Purkinje cell the plastic change associated with fear conditioning is specifically restricted to synaptic efficacy.

The perirhinal cortex of the rat is necessary for spatial memory retention long after but not soon after learning

Many observations in humans and experimental animals support the view that the hippocampus is critical immediately after learning in order for long-term memory formation to take place. However, exactly when the medial temporal cortices adjacent to the hippocampus are necessary for this process to occur normally is not yet well known. Using a spatial task, we studied whether the perirhinal cortex of rats is necessary to establish representations in long-term memory. Results showed that, in a spatial task sensitive to hippocampal lesions, control and perirhinal lesioned rats can both learn at the same rate (Experiment 1). Interestingly, a differential involvement of the perirhinal cortex in memory retention was observed as time passes after learning. Thus, 24 days following the end of learning, lesioned and control rats remembered the task perfectly as measured by a retraining test. In contrast, 74 days after the learning the perirhinal animals showed a profound impairment in the retention of the spatial information (Experiment 2). Taken together, these results suggest that the perirhinal region is critical for the formation of long-term spatial memory. However, its contribution to memory formation and retention is time-dependent, it being necessary only long after learning takes place and not during the phase immediately following acquisition.

Effects of unconditioned and conditioned aversive stimuli in an intense fear conditioning paradigm on synaptic plasticity in the hippocampal CA1 area in vivo

Repeated vivid recalls or flashbacks of traumatic memories and memory deficits are the cardinal features of post-traumatic stress disorder (PTSD). The underlying mechanisms are not fully understood yet. Here, we examined the effects of very strong fear conditioning (20 pairings of a light with a 1.5-mA, 0.5-s foot shock) and subsequent reexposure to the conditioning context (chamber A), a similar context (chamber B), and/or to the fear conditioned stimulus (CS) (a light) on synaptic plasticity in the hippocampal CA1 area in anesthetized Sprague-Dawley rats. The conditioning procedure resulted in very strong conditioned fear, as reflected by high levels of persistent freezing, to both the contexts and to the CS, 24 h after fear conditioning. The induction of long-term potentiation (LTP) was blocked immediately after fear conditioning. It was still markedly impaired 24 h after fear conditioning; reexposure to the conditioning chamber A (CA) or to a similar chamber B (CB) did not affect the impairment. However, presentation of the CS in the CA exacerbated the impairment of LTP, whereas the CS presentation in a CB ameliorated the impairment so that LTP induction did not differ from that of control groups. The induction of long-term depression (LTD) was facilitated immediately, but not 24 h, after fear conditioning. Only reexposure to the CS in the CA, but not reexposure to either chamber A or B alone, or the CS in chamber B, 24 h after conditioning, reinstated the facilitation of LTD induction. These data demonstrate that unconditioned and conditioned aversive stimuli in an intense fear conditioning paradigm can have profound effects on hippocampal synaptic plasticity, which may aid to understand the mechanisms underlying impairments of hippocampus-dependent memory by stress or in PTSD.

Fos expression and task-related neuronal activity in rat cerebral cortex after instrumental learning

Learning has been shown to induce changes in neuronal gene expression and to produce development of task-specific neuronal activity. The connection between these two features of neuronal plasticity remains of a great interest. To address this issue we compared distribution of c-Fos expressing and task-related neurons in the rat cerebral cortex following instrumental learning of appetitive lever-press task. The number of Fos-positive neurons was determined immunohistochemically in the retrosplenial and the motor cortex of naive ("control" group), newly trained ("acquisition" group) and overtrained ("performance" group) animals. A significant activation of c-Fos expression was observed in the neurons of the retrosplenial but not motor cortex in the "acquisition" group rats, as compared with the "control" and "performance" groups. In accordance with this c-Fos expression difference, the retrosplenial cortex of the trained animals contained significantly more neurons with lever-press-related activity than the motor cortex. Therefore, the two examined cortical areas showed a parallel between experience-dependent induction of c-Fos and development of task-related neuronal activity. These data support a notion that learning-induced activation of c-Fos is associated with long-term neurophysiological changes produced by training.

Distinct Influences of Neonatal Epidermal Growth Factor Challenge on Adult Neurobehavioral Traits in Four Mouse Strains

Epidermal growth factor (EGF) receptor (ErbB1) signals regulate dopaminergic development and function and are implicated in schizophrenia. We evaluated genetic effects on neurobehavioral changes induced by neonatal EGF administration, using four mouse strains. Subcutaneous EGF administration increased phosphorylation of brain ErbB1 in all strains, although DBA/2 and C57BL/6 mice had lower basal phosphorylation. Neonatal EGF treatment differentially influenced physical and behavioral/cognitive development, depending on mouse strain. Prepulse inhibition was decreased in DBA/2 and C57BL/6 mice but not C3H/He and ddY mice. Locomotor activity was accelerated in DBA/2 mice, but reduced in ddY mice. EGF treatment enhanced fear-learning performance with a tone cue in DBA/2 mice, but decreased performance with tone and context cues in C3H/He and ddY mice, respectively. The strain-dependent behavioral sensitivity was correlated with basal ErbB1 phosphorylation. Genetic components regulating brain ErbB1 signaling strongly influence the direction and strength of behavioral responses stemming from the neonatal neurotrophic perturbation.

Hippocampal lesions facilitate instrumental learning with delayed reinforcement but induce impulsive choice in rats

BACKGROUND

Animals must frequently act to influence the world even when the reinforcing outcomes of their actions are delayed. Learning with action-outcome delays is a complex problem, and little is known of the neural mechanisms that bridge such delays. When outcomes are delayed, they may be attributed to (or associated with) the action that caused them, or mistakenly attributed to other stimuli, such as the environmental context. Consequently, animals that are poor at forming context-outcome associations might learn action-outcome associations better with delayed reinforcement than normal animals. The hippocampus contributes to the representation of environmental context, being required for aspects of contextual conditioning. We therefore hypothesized that animals with hippocampal lesions would be better than normal animals at learning to act on the basis of delayed reinforcement. We tested the ability of hippocampal-lesioned rats to learn a free-operant instrumental response using delayed reinforcement, and what is potentially a related ability -- the ability to exhibit self-controlled choice, or to sacrifice an immediate, small reward in order to obtain a delayed but larger reward.

RESULTS

Rats with sham or excitotoxic hippocampal lesions acquired an instrumental response with different delays (0, 10, or 20 s) between the response and reinforcer delivery. These delays retarded learning in normal rats. Hippocampal-lesioned rats responded slightly less than sham-operated controls in the absence of delays, but they became better at learning (relative to shams) as the delays increased; delays impaired learning less in hippocampal-lesioned rats than in shams. In contrast, lesioned rats exhibited impulsive choice, preferring an immediate, small reward to a delayed, larger reward, even though they preferred the large reward when it was not delayed.

CONCLUSION

These results support the view that the hippocampus hinders action-outcome learning with delayed outcomes, perhaps because it promotes the formation of context-outcome associations instead. However, although lesioned rats were better at learning with delayed reinforcement, they were worse at choosing it, suggesting that self-controlled choice and learning with delayed reinforcement tax different psychological processes.

The H3 receptor protean agonist proxyfan enhances the expression of fear memory in the rat

Consolidation of fear memory requires neural changes to occur in the basolateral amygdala (BLA), including modulation of histaminergic neurotransmission. We previously demonstrated that local blockade or activation of histamine H3 receptors in the BLA impaired or ameliorated, respectively, retention of fear memory. The histamine H3 receptor is a G-protein-coupled receptor (GPCR) displaying high constitutive activity that regulates histamine neurons in the brain. Proxyfan is a high-affinity histamine H3 receptor protean agonist exhibiting the full spectrum of pharmacological activities, from full agonist to full inverse agonist depending on the competition between constitutively active and quiescent H3 receptors in a given tissue or brain region. Therefore, protean agonists are powerful tools to investigate receptor conformation and may be useful in designing specific compounds selective for the various receptor conformations. In the present study we examined the effect of post-training, systemic or intra-BLA injections of proxyfan on contextual fear memory. Rats receiving intra-BLA, bilateral injections of 1.66 ng proxyfan immediately after fear conditioning showed stronger memory for the context-footshock association, as demonstrated by longer freezing assessed at retention performed 72 hr later compared to controls. Comparable results were obtained when doses as low as 0.04 mg/kg of proxyfan were injected systemically. Hence, our results suggest that proxyfan behaves as an H3 receptor agonist with a low level of constitutive activity of the H3 receptor in the rat BLA.

Anterior perirhinal cortex kindling produces long-lasting effects on anxiety and object recognition memory

Temporal lobe epilepsy (TLE) is frequently accompanied by memory impairments and, although their bases are unknown, most research has focused on the hippocampus. The present study investigated the importance of another medial temporal lobe structure, the perirhinal cortex (Prh), in changes in memory in TLE using kindling as a model. Rats were kindled twice daily with anterior Prh stimulation until three fully generalized seizures were evoked. Beginning 7 days later and on successive days, rats were tested in an elevated plus maze, a large circular open field, an open field object exploration task and a delayed-match-to-place task in a water maze in order to assess anxiety-related and exploratory behaviour, object recognition memory and spatial cognition. Kindling increased anxiety-related behaviour in both the elevated plus and open field mazes and disrupted spontaneous object recognition but spared all other behaviours tested. These results are consistent with other findings indicating a greater role for the Prh in object memory and emotional behaviour than in spatial memory and contrast with the selective disruption of spatial memory produced by dorsal hippocampal kindling. The site-selectivity of the behavioural disruptions produced by kindling indicates that such effects are probably mediated by changes particular to the site of seizure initiation rather than to changes in the characteristic circuitry activated by limbic seizure generalization. Further investigation of the behavioural effects of Prh kindling may be useful for studying the mechanisms of mnemonic and affective dysfunction associated with TLE and offer insights into bases for variability in such dysfunction across patients.

Lasting increases in basolateral amygdala activity after emotional arousal: implications for facilitated consolidation of emotional memories

Manipulations that reduce or enhance the activity of basolateral amygdala (BLA) neurons in the minutes to hours after training have been shown to respectively impair or facilitate retention on the inhibitory avoidance task. Although this suggests that BLA activity is altered after emotional arousal, such changes have not been directly demonstrated. To test this, we devised a feline analog of the inhibitory avoidance task and recorded BLA unit activity before and after a single inescapable footshock. Single-unit recordings revealed that the firing rate of many BLA neurons gradually increased after the footshock, peaking 30-50 min post-shock and then subsiding to baseline levels 2 h later. During this period of increased activity, the discharges of simultaneously recorded BLA cells were more synchronized than before the shock. Although it was known that pairing innocuous (conditioned stimulus, CS) and noxious stimuli modifies the responsiveness of BLA neurons to the CS, our results constitute the first demonstration that emotional arousal produces lasting increases in the spontaneous firing rates of BLA neurons. We propose that these changes in BLA activity may promote Hebbian interactions between coincident but spatially distributed activity patterns in BLA targets, facilitating the consolidation of emotional memories.

Conceptually driven pharmacologic approaches to acute trauma

Secondary prevention of posttraumatic stress disorder (PTSD) entails intervening in the aftermath of a traumatic event to forestall the development of PTSD. There has been little psychopharmacologic research in this area. This is surprising, given that PTSD is the mental disorder with the most clearly identified cause and onset. In a translational model of PTSD's pathogenesis presented herein: A traumatic event (unconditioned stimulus) overstimulates endogenous stress hormones (unconditioned response); these mediate an overconsolidation of the event's memory trace; recall of the event in response to reminders (conditioned stimulus); releases further stress hormones (conditioned response); these cause further overconsolidation; and the overconsolidated memory generates PTSD symptoms. Noradrenergic hyperactivity in the basolateral amygdala is hypothesized to mediate this cycle. Preventing pre-synaptic norepinephrine release with alpha2-adrenergic agonists or opioids, or blocking post-synaptic norepinephrine receptors with beta-adrenergic antagonists such as propranolol, reduces hormonally enhanced memories and fear conditioning. Two controlled studies of trauma victims presenting to emergency rooms suggest that posttrauma propranolol reduces subsequent PTSD, as does one naturalistic clinical study of morphine treatment of burned children. Cortisol both enhances memory consolidation and reduces memory retrieval, leading to mixed predictions. Two controlled studies of intensive care unit patients found that cortisol reduced PTSD. One study did not find benzodiazepines effective in preventing PTSD. Selective serotonin reuptake inhibitors, antiepileptics, and alpha2-adrenergic agonists have yet to be tried.

Learning and memory

Learning and memory processes are thought to underlie a variety of human psychiatric disorders, including generalised anxiety disorder and post-traumatic stress disorder. Basic research performed in laboratory animals may help to elucidate the aetiology of the respective diseases. This chapter gives a short introduction into theoretical and practical aspects of animal experiments aimed at investigating acquisition, consolidation and extinction of aversive memories. It describes the behavioural paradigms most commonly used as well as neuroanatomical, cellular and molecular correlates of aversive memories. Finally, it discusses clinical implications of the results obtained in animal experiments in respect to the development of novel pharmacotherapeutic strategies for the treatment of human patients.

The histamine H3 receptor as a novel therapeutic target for cognitive and sleep disorders

Histamine H3 receptor pharmacology, functions and biochemistry are far from being fully understood; however, progress is being made. Activation of this Gi/GO-protein-coupled receptor affects cognition, the sleep-wake cycle, obesity and epilepsy, which are physiological and pathological conditions that are the main focus of research into the therapeutic potential of selective H3 receptor ligands. This heterogeneity of targets can be reconciled partially by the fact that the histamine system constitutes one of the most important brain-activating systems and that H3 receptors regulate the activity of histamine and other neurotransmitter systems. Furthermore, the H3 receptor shows functional constitutive activity, polymorphisms in humans and rodents with a differential distribution of splice variants in the CNS, and potential coupling to different intracellular signal transduction mechanisms. In light of the genetic, pharmacological and functional complexity of the H3 receptor, the importance of the histamine system as a therapeutic target to control the sleep-wake cycle and cognitive disorders will be discussed.

Psychopharmacological possibilities in the acute disaster setting

This article focuses on possible psychopharmacological interventions in the immediate post disaster setting. As there is little evidence for the efficacy or effectiveness of such interventions-given the difficulty in performing randomized, double-blind, placebo controlled studies with these populations-the article will delineate the neurobiological basis for pathological sequelae and theoretical drug interventions targeting putative disease mechanisms.

New vistas on amygdala networks in conditioned fear

It is currently believed that the acquisition of classically conditioned fear involves potentiation of conditioned thalamic inputs in the lateral amygdala (LA). In turn, LA cells would excite more neurons in the central nucleus (CE) that, via their projections to the brain stem and hypothalamus, evoke fear responses. However, LA neurons do not directly contact brain stem-projecting CE neurons. This is problematic because CE projections to the periaqueductal gray and pontine reticular formation are believed to generate conditioned freezing and fear-potentiated startle, respectively. Moreover, like LA, CE may receive direct thalamic inputs communicating information about the conditioned and unconditioned stimuli. Finally, recent evidence suggests that the CE itself may be a critical site of plasticity. This review attempts to reconcile the current model with these observations. We suggest that potentiated LA outputs disinhibit CE projection neurons via GABAergic intercalated neurons, thereby permitting associative plasticity in CE. Thus plasticity in both LA and CE would be necessary for acquisition of conditioned fear. This revised model also accounts for inhibition of conditioned fear after extinction.

Parahippocampal gyrus in first episode psychotic disorders: a structural magnetic resonance imaging study

Neuropathological abnormalities in schizophrenia have been demonstrated in the parahippocampal gyrus (PHG). Only a few studies on first-episode neuroleptic-naive schizophrenia patients have been done using in vivo neuroimaging techniques. The authors examined the PHG morphology using structural MRI in neuroleptic-naive subjects with first episode psychoses. Volumetric measurements of PHG and intracranial volume (ICV) were obtained on subjects with schizophrenia and schizoaffective disorders (SCZ; n = 33), nonschizophrenia psychotic disorders (NSCZ; n = 11) and matched healthy subjects (HS; n = 43). The subjects were rated on the Brief Psychiatric Rating Scale (BPRS). Group differences and clinical correlations of ICV-adjusted PHG volumes were examined. Left PHG was significantly different across the groups consisting of SCZ, NSCZ and HS. PHG was larger in NSCZ compared to SCZ but not relative to HS. Bilaterally, PHG was no different between SCZ and HS. In pooled psychotic patients, the PHG volume negatively correlated with total positive symptom, delusion and conceptual disorganization scores on BPRS. Patients with delusions had relatively smaller PHG compared to nondelusional subjects. Observed differences in PHG volume in first-episode neuroleptic-naive patients suggest that these observations are not confounded by illness chronicity or medication effects. Significant association of PHG volume with psychotic symptoms suggests that PHG pathology plays an important role in the etiopathology of psychosis and its symptoms.

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.

Long-Term Synaptic Changes Induced in the Cerebellar Cortex by Fear Conditioning

To better understand learning mechanisms, one needs to study synaptic plasticity induced by behavioral training. Recently, it has been demonstrated that the cerebellum is involved in the consolidation of fear memory. Nevertheless, how the cerebellum contributes to emotional behavior is far from known. In cerebellar slices at 10 min and 24 hr following fear conditioning, we found a long-lasting potentiation of the synapse between parallel fibers and Purkinje cells in vermal lobules V-VI, but not in the climbing fiber synapses. The mechanism is postsynaptic, due to an increased AMPA response. In addition, in hotfoot mice with a primary deficiency of the parallel fiber to Purkinje cell synapse, cued (but not contextual) fear conditioning is affected. We propose that this synapse plays an important role in the learned fear and that its long-term potentiation may represent a contribution to the neural substrate of fear memory.

Adaptive and maladaptive psychobiological responses to severe psychological stress: implications for the discovery of novel pharmacotherapy

Post-traumatic stress disorder (PTSD) is one of the few DSM-IV diagnoses contingent upon a psychosocial stressor. In this context, there is an urgent need to acquire a better understanding of both the adaptive and maladaptive psychobiological responses to traumatic stress. Preclinical investigators have utilized a variety of animal models to identify the behavioral and neurobiological features of the organism's response to stress. However, given the complexity of the healthy and pathological human response to physiological and psychological stress, the extent to which the animal data is immediately transferable to human remains to be fully determined. This review draws upon preclinical and clinical literature to examine the transformation of an adaptive human stress response into a maladaptive and debilitating mental disorder. An integrative psychobiological model for PTSD is presented, linking psychological processes and behavioral patterns with current findings in neurocircuitry, neurochemistry and psychophysiology. The implications of this model for the discovery of novel pharmacological approaches to the treatment of severe psychological distress are discussed.

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.

Temporary inactivation of the perirhinal cortex by muscimol injections block acquisition and expression of fear-potentiated startle

The present study examined the role of the perirhinal cortex (PRh) in aversive information processing and emotional learning. Specifically, we studied the effects of temporary inactivation of the PRh on acquisition and expression of conditioned fear as measured by fear-potentiated startle in rats, as well as on shock sensitization of startle. Temporary inactivation of the PRh was induced by local injections of the GABAA agonist muscimol (0.0, 1.1, 2.2, 4.4 nmol/0.5 micro L). Muscimol injections into the PRh blocked both the expression and acquisition of fear-potentiated startle, as well as shock sensitization of startle. Shock sensitivity was not affected by muscimol injections, indicating that the observed blockade of acquisition and shock sensitization was not caused by a disruption in the perception of shock. Taken together, the present data show that the PRh is critical for the processing of aversive information and is necessary for the expression of emotional learning.

Hippocampal mGluR1 and consolidation of contextual fear conditioning

The effects of post-training intra-hippocampal injections of group I mGluR agonists and antagonists, were examined in the contextual fear test, in rats. It was found that (S)-3,5-dihydroxyphenylglycine (DHPG) (a mGluR1-5 agonist) decreased, and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (a mGluR1 antagonist) increased fear conditioning (a freezing reaction), examined 24h after conditioning session. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) (a mGluR5 agonist), and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (a mGluR5 antagonist) did not cause any effect. In the immunocytochemical study, the post-conditioning administration of AIDA decreased the c-Fos induction in the dentate gyrus and CA1 layer of the hippocampus proper, 2h after exposure of animals to the aversive context, and 24h after conditioning session. It is suggested that overactivation of glutamatergic transmission in the critical for memory trace formation structure and period of time, may result in an attenuation of memory consolidation. On the other hand, reduction of an exaggerated glutamatergic tone can facilitate learning and memory processes. The immunocytochemical study and factor analysis of experimental data revealed that hippocampal mGlu1 receptors significantly influence the memory consolidation in a way dependant on the level of glutamatergic activity. Furthermore, they indicate that changes of glutamatergic activity within brain limbic structures can affect the threshold for the induction of the long-term neuronal plasticity, involved in some forms of learning and memory.

Efferent connections of “posterodorsal” auditory area in the rat cortex: Implications for auditory spatial processing

We examined efferent connections of the cortical auditory field that receives thalamic afferents specifically from the suprageniculate nucleus (SG) and the dorsal division (MGD) of the medial geniculate body (MG) in the rat [Neuroscience 117 (2003) 1003]. The examined cortical region was adjacent to the caudodorsal border (4.8-7.0 mm posterior to bregma) of the primary auditory area (area Te1) and exhibited relatively late auditory response and high best frequency, compared with the caudal end of area Te1. On the basis of the location and auditory response property, the cortical region is considered identical to "posterodorsal" auditory area (PD). Injections of biocytin in PD revealed characteristic projections, which terminated in cortical areas and subcortical structures that play pivotal roles in directed attention and space processing. The most noticeable cortical terminal field appeared as dense plexuses of axons in area Oc2M, the posterior parietal cortex. Small terminal fields were scattered in area frontal cortex, area 2 that comprises the frontal eye field. The subcortical terminal fields were observed in the pontine nucleus, the nucleus of the brachium inferior colliculus, and the intermediate and deep layers of the superior colliculus. Corticostriatal projections targeted two discrete regions of the caudate putamen: the top of the middle part and the caudal end. It is noteworthy that the inferior colliculus and amygdala virtually received no projection. Corticothalamic projections terminated in the MGD, the SG, the ventral zone of the ventral division of the MG, the ventral margin of the lateral posterior nucleus (LP), and the caudodorsal part of the posterior thalamic nuclear group (Po). Large terminals were found in the MGD, SG, LP and Po besides small terminals, the major component of labeling. The results suggest that PD is an auditory area that plays an important role in spatial processing linked to directed attention and motor function. The results extend to the rat findings from nonhuman primates suggesting the existence of a posterodorsal processing stream for auditory spatial perception.

Distinct roles of the different ionotropic glutamate receptors within the nucleus accumbens in passive-avoidance learning and memory in mice

Research on the role of the nucleus accumbens in behaviour has been largely focused on the functions of this structure in conditioning to appetitive stimuli. It has been suggested that a network comprising the nucleus accumbens and its convergent inputs might mediate dissociable functions in the acquisition, the consolidation and the retrieval of information. However, findings related to a role of this structure in aversive conditioning are somewhat contradictory, and its involvement in this form of learning is still under debate. Moreover, very little evidence is available on the step of information processing mediated by the accumbens. Thus the purpose of this study was to investigate the effects of the blockade of the AMPA and NMDA glutamate receptors, which have been suggested to mediate the transmission of information from the limbic system to this structure, on a classical aversive conditioning task - the one-trial step through inhibitory avoidance paradigm (24 h interval between training and testing). Intra-accumbens focal injections of AP-5 and DNQX (NMDA and AMPA antagonists, respectively) were performed immediately after training, before training and before testing in mice. The NMDA antagonist (37.5, 75 and 150 ng per side) impaired animal performance only if administered immediately after but not before training or before testing. Conversely, DNQX (0.5, 1.0 and 5.0 ng per side) reduced the step through latencies when administered before training and before testing. These findings suggest that NMDA receptor activation within the accumbens is necessary in formation but not expression of memory for inhibitory avoidance. AMPA receptors, instead, are necessary for the acquisition and the expression but not consolidation of inhibitory avoidance memory.

Improvement in fear memory by histamine-elicited ERK2 activation in hippocampal CA3 cells

Consolidation of associative memories appears to require extracellular signal-related kinase2 (ERK2) activation, which is modulated by several factors, including neurotransmitter receptor stimulation. Here we show that in vitro stimulation of either H2 or H3 histaminergic receptors activates ERK2 in hippocampal CA3 pyramidal cells. In behaving animals, bilateral posttraining injections into the dorsal hippocampus of histamine H2 or H3 receptor agonists improve memory consolidation after contextual fear conditioning. Local administration of U0126, a selective inhibitor of ERK kinase, prevents memory improvements exerted by the agonists, without causing any behavioral effect per se. This is the first evidence of a positive correlation between ERK phosphorylation and memory improvement. Moreover, we demonstrate that the brain histaminergic system regulates hippocampal ERK cascade. Finally, our data indicate that early ERK2 hippocampal activation is not required for the expression of long-term fear memories.

Role of the neocortex in consolidation of fear conditioning memories in rats

The aim of the present work was to investigate, by means of the reversible inactivation technique, the distinct roles of three neocortical sites, the Prefrontal (PFC), Frontal (FC) and Parietal (PAC) cortices, during memory consolidation of conditioned freezing to a tone, a defined conditioned stimulus (CS), and to the training context. Reversible tetrodotoxin inactivations were performed at increasing post-acquisition delays (0.25, 24 or 96 h). Retention testing was always performed 72 h after the inactivation procedure in order to avoid any possible influence on the retrieval phase. It was found that none of the three sites are involved in context freezing consolidation, but that FC and PAC are differentially involved in the consolidation of memory to the CS. FC inactivation was followed by retention impairment only when performed immediately after acquisition while PAC inactivation was followed by amnesia when performed both immediately after acquisition and 24 h later, but not when performed 96 h later. PFC inactivation was not followed by retention impairment at any of the employed delays. These findings show that some neocortical structures are involved in auditory-cued fear conditioning during the initial phases of the consolidation process.

Role of the basolateral amygdala in memory consolidation

Typically, emotionally charged events are better remembered than neutral ones. This paper reviews data indicating that the amygdala is responsible for this facilitation of memory by emotional arousal. Pharmacological and behavioral studies have shown that the release of adrenal stress hormones facilitates memory consolidation. The available evidence suggests that this effect depends on a central action of stress hormones involving the release of the neuromodulators noradrenaline (NA) and acetylcholine in the basolateral complex of the amygdala (BLA). Indeed, BLA lesions block the memory modulating effects of stress hormones. Moreover, microdialysis studies have revealed that BLA concentrations of NA and acetylcholine are transiently (2h) elevated following emotionally arousing learning episodes. Last, post-learning intra-BLA injections of beta-adrenergic or muscarinic receptor antagonists reduce retention. These results have led to the hypothesis that NA and acetylcholine increase the activity of BLA neurons in the hours after the learning episode. In turn, the BLA would facilitate synaptic plasticity in other brain structures, believed to constitute the storage sites for different types of memory. Consistent with this, post-learning treatments that reduce or enhance the excitability of BLA neurons respectively decrease or improve long-term retention on various emotionally charged learning tasks. However, a number of issues remain unresolved. Chief among them is how the BLA facilitates synaptic plasticity elsewhere in the brain. The present review concludes with a consideration of this issue based on recent advances in our understanding of the BLA. Among other possibilities, it is suggested that rhythmic BLA activity at the theta frequency during arousal as well as the uniform conduction times of BLA axons to distributed rhinal sites may promote plasticity in co-active structures of the temporal lobe.

Post-training intra-basolateral amygdala infusions of norepinephrine enhance consolidation of memory for contextual fear conditioning

Post-training infusions of drugs, including noradrenergic agonists and antagonists, into the basolateral amygdala (BLA) influence the consolidation of memory for training in several tasks, including inhibitory avoidance. There is, however, conflicting evidence concerning whether post-training intra-BLA drug infusions modulate the consolidation of contextual fear conditioning (CFC). In the present study, norepinephrine (NE) was infused bilaterally into the BLA of male Sprague Dawley rats immediately after training on two CFC tasks: a Y-maze and a straight alley. Post-training intra-BLA infusions enhanced memory of CFC training in the Y-maze, as assessed by percentage of time spent freezing and shock arm entrance latencies. Post-training intra-BLA infusions of NE enhanced 48 hr retention of CFC training in the straight alley, as assessed by shock compartment entrance latencies and the number of shocks required to learn to avoid entering the shock compartment. These findings indicate that the consolidation of memory for CFC, like that for inhibitory avoidance training, is influenced by post-training neuromodulatory influences within the BLA. Thus, the findings provide additional evidence consistent with the hypothesis that the BLA has a general role in modulating memory consolidation.

Spared anterograde memory for shock-probe fear conditioning after inactivation of the amygdala

Previous studies have shown that amygdala lesions impair avoidance of an electrified probe. This finding has been interpreted as indicating that amygdala lesions reduce fear. It is unclear, however, whether amygdala-lesioned rats learn that the probe is associated with shock. If the lesions prevent the formation of this association, then pretraining reversible inactivation of the amygdala should impair both acquisition and retention performance. To test this hypothesis, the amygdala was inactivated (tetrodotoxin; TTX; 1 ng/side) before a shock-probe acquisition session, and retention was tested 4 d later. The data indicated that, compared with rats infused with vehicle, rats infused with TTX received more shocks during the acquisition session, but more importantly, were not impaired on the retention test. In Experiment 2, we assessed whether the spared memory on the retention test was caused by overtraining during acquisition. We used the same procedure as in Experiment 1, with the exception that the number of shocks the rats received during the acquisition session was limited to four. Again the data indicated that amygdala inactivation did not impair performance on the retention test. These results indicate that amygdala inactivation does not prevent the formation of an association between the shock and the probe and that shock-probe deficits during acquisition likely reflect the amygdala's involvement in other processes.

The amygdaloid complex: anatomy and physiology

A converging body of literature over the last 50 years has implicated the amygdala in assigning emotional significance or value to sensory information. In particular, the amygdala has been shown to be an essential component of the circuitry underlying fear-related responses. Disorders in the processing of fear-related information are likely to be the underlying cause of some anxiety disorders in humans such as posttraumatic stress. The amygdaloid complex is a group of more than 10 nuclei that are located in the midtemporal lobe. These nuclei can be distinguished both on cytoarchitectonic and connectional grounds. Anatomical tract tracing studies have shown that these nuclei have extensive intranuclear and internuclear connections. The afferent and efferent connections of the amygdala have also been mapped in detail, showing that the amygdaloid complex has extensive connections with cortical and subcortical regions. Analysis of fear conditioning in rats has suggested that long-term synaptic plasticity of inputs to the amygdala underlies the acquisition and perhaps storage of the fear memory. In agreement with this proposal, synaptic plasticity has been demonstrated at synapses in the amygdala in both in vitro and in vivo studies. In this review, we examine the anatomical and physiological substrates proposed to underlie amygdala function.

Auditory thalamic nuclei projections to the temporal cortex in the rat

Thalamocortical projections from the auditory thalamic nuclei were examined systematically in the rat, including those from the dorsal division (MGD) of the medial geniculate body (MG), which were less clearly determined in previous studies. Injections of biocytin confined in each thalamic nucleus revealed characteristic features of projections in terms of cortical areas and layers of termination. In contrast to exclusively selective projections to cortical area Te1 from the ventral division (MGV) of the MG, diffuse and selective terminations were observed in the projections from the dorsal (MGD) and medial divisions (MGM) of the MG and the suprageniculate nucleus (SG). Diffuse termination was continuous in layer I or VI of the temporal cortex, while selective termination was in layers III and IV of discrete cortical areas. In addition to diffuse termination in the upper half of layer I of cortical areas Te1, Te2d and Te3v, the MGD and SG projections formed plexuses of axons selectively in lower layer III and layer IV of Te2d and Te3v. The SG projections targeted further the dorsal bank of the perirhinal cortex (PRh), while the MGD projections targeted in part the ventral fringe of Te1. The MGM projections terminated diffusely in layer VI of Te1 and Te3v, and selectively in lower layer III and layer IV of the rostral part of Te3v. Diffuse projections to layers I and VI from the SG and MGM extended in cortical regions over the dorsal fringe of Te1. Selective dense projections to middle cortical layers of Te2d and Te3v (especially its rostral part) indicate the existence of auditory areas, which could be involved in cross-modal interaction with visual and somatosensory system, respectively. Diffuse projections are supposed to bind information processings in these areas and the primary auditory area (Te1).

Regulation of contextual fear conditioning by baseline and inducible septo-hippocampal cyclin-dependent kinase 5

In this work, we confirm the novel role of cyclin-dependent kinase (Cdk) 5 in associative learning by demonstrating that injection of the Cdk5 inhibitor butyrolactone I into the lateral septum or hippocampus profoundly impaired context-dependent fear conditioning of C57BL/6J mice. However, unlike the inducible up-regulation of Cdk5 and its regulator p35 observed in Balb/c mice, high baseline levels, which were not affected by fear conditioning, were found in C57BL/6J mice. Surprisingly, microinjections of butyrolactone I into the lateral septum or hippocampus significantly decreased baseline Cdk5 activity within the entire septo-hippocampal circuitry, suggesting a functional link between septal and hippocampal Cdk5 activity. Significantly higher levels of the transcription factor Sp4 in the septo-hippocampal system of C57BL/6J mice may account for the high baseline Cdk5/p35 production. On the other hand, the stronger cFos production observed in the lateral septum of fear conditioned Balb/c mice may be responsible, at least in part, for the inducible up-regulation of Cdk5 in this strain. These results suggest that the role of Cdk5 in memory consolidation is strain independent and functionally related to the septo-hippocampal circuitry. However, the molecular regulation of baseline and inducible Cdk5 protein might be different among individual mouse strains and possibly other species.

Perirhinal cortex lesions impair context aversion learning

Rats with perirhinal cortex lesions were compared with sham controls on a conditional discrimination in which saccharin was paired with LiCl in context 1, but paired with saline in context 2. Perirhinal-lesioned rats were slightly slower to acquire the discrimination but reached control levels by the end of acquisition. Both groups showed transfer to familiar tap water, consuming less in context 1 than in context 2. Unlike sham rats, perirhinal rats failed to show an aversion to context 1 on a place choice test. These data provide neuroanatomical support for the postulated difference between Pavlovian conditioning and conditional learning.

The place of the hippocampus in fear conditioning

Pavlovian fear conditioning is a phenomenon amenable to laboratory analysis of the neurobiology of fear and the investigation of neural mechanisms of learning and memory. Investigators have made much progress in delineating the neurocircuitry and neurochemistry of fear conditioning. The place of the hippocampus in context fear remains a controversial issue. In this review, we examine the evidence that the hippocampus plays a role in fear conditioning. We then critically examine hypotheses concerning its exact role in learning and memory for cued and context fear conditioning.

New life in an old idea: the synaptic plasticity and memory hypothesis revisited

The notion that changes in synaptic efficacy underlie learning and memory processes is now widely accepted, although definitive proof of the synaptic plasticity and memory hypothesis is still lacking. This article reviews recent evidence relevant to the hypothesis, with particular emphasis on studies of experience-dependent plasticity in the neocortex and hippocampus. In our view, there is now compelling evidence that changes in synaptic strength occur as a consequence of certain forms of learning. A major challenge will be to determine whether such changes constitute the memory trace itself or play a less specific supporting role in the information processing that accompanies memory formation.

Sensitivity to instrumental contingency degradation is mediated by the entorhinal cortex and its efferents via the dorsal hippocampus

Previous studies have shown that electrolytic lesions of the dorsal hippocampus render the instrumental performance of rats insensitive to selective degradation of the action-outcome contingency (Corbit and Balleine, 2000). In the present experiments, we sought to replicate this finding and to examine the effects of excitotoxic lesions. In the first three experiments, rats with either electrolytic or NMDA lesions of the dorsal hippocampus and sham-lesioned controls were trained to press two levers, each of which delivered a unique food outcome, before their sensitivity to outcome devaluation and degradation of the instrumental contingency was assessed. Although we were able to replicate our original finding that electrolytic lesions of the dorsal hippocampus render rats insensitive to selective degradation of the instrumental contingency, NMDA lesions of the dorsal hippocampus had no effect. Neither lesion had any detectable effect on sensitivity to outcome devaluation. In experiment 4, we assessed the possibility that the effect of the electrolytic lesion resulted from damage to fibers originating in the retrohippocampal region (including both entorhinal cortex and subiculum) by examining the impact of bilateral NMDA-induced lesions of the retrohippocampus on the same tasks. Importantly, this lesion produced a deficit similar to that observed after electrolytic hippocampal lesions. The final experiment used a disconnection procedure to assess more directly whether damage to efferents from the retrohippocampal region, rather than the dorsal hippocampus itself, can account for the observed deficit. The data from these tests suggest that the deficits observed previously after electrolytic hippocampal lesions were the result of damage to entorhinal efferents.

Nucleus accumbens dopamine and learned fear revisited: a review and some new findings

A role for the nucleus accumbens (NAcc) and its dopamine (DA) innervation in fear and fear learning is supported by a large body of evidence, which has challenged the view that the NAcc is solely involved in mediating appetitive processes. Unfortunately, due to conflicting findings in the aversive conditioning literature the role of the NAcc in aversive conditioning remains unclear. This review focuses on the results of recent in vivo microdialysis studies that have examined the release of NAcc DA during Pavlovian aversive conditioning. In addition, we present additional new findings, which re-examine the involvement of NAcc DA in aversive conditioning. DA release was measured in the NAcc core using in vivo microdialysis during discrete cue Pavlovian aversive conditioning in four experiments. In all cases no change in DA levels was observed either during training or in response to the CS presentations despite robust behavioural evidence of discrete cue Pavlovian aversive conditioning. These findings contrast with some previous studies that show that primary and conditioned aversive stimuli increase DA release in the NAcc. We suggest that the inconsistencies in the literature might be due to procedural differences in the measurement of aversive conditioning, and the precise location of the probe in the NAcc region. Hence, rather than discount an involvement of NAcc DA in affective processes, we propose that functionally dissociable sub-regions of the NAcc may contribute to different aspects of Pavlovian aversive learning.

Memory consolidation and the amygdala: a systems perspective

The basolateral region of the amygdala (BLA) plays a crucial role in making significant experiences memorable. There is extensive evidence that stress hormones and other neuromodulatory systems activated by arousing training experiences converge in regulating noradrenaline-receptor activity within the BLA. Such activation of the BLA modulates memory consolidation via BLA projections to many brain regions involved in consolidating lasting memory, including the hippocampus, caudate nucleus, nucleus basalis and cortex. Investigation of the involvement of BLA projections to other brain regions is essential for understanding influences of the amygdala on different aspects and forms of memory.

Activation of histaminergic H3 receptors in the rat basolateral amygdala improves expression of fear memory and enhances acetylcholine release

The basolateral amygdala (BLA) is involved in learning that certain environmental cues predict threatening events. Several studies have shown that manipulation of neurotransmission within the BLA affects the expression of memory after fear conditioning. We previously demonstrated that blockade of histaminergic H3 receptors decreased spontaneous release of acetylcholine (ACh) from the BLA of freely moving rats, and impaired retention of fear memory. In the present study, we examined the effect of activating H3 receptors within the BLA on both ACh release and expression of fear memory. Using the microdialysis technique in freely moving rats, we found that the histaminergic H3 agonists R-alpha-methylhistamine (RAMH) and immepip, directly administered into the BLA, augmented spontaneous release of ACh in a similar manner. Levels of ACh returned to baseline on perfusion with control medium. Rats receiving intra-BLA, bilateral injections of the H3 agonists at doses similar to those enhancing ACh spontaneous release, immediately after contextual fear conditioning, showed stronger memory for the context-footshock association, as demonstrated by longer freezing assessed at retention testing performed 72 h later. Post-training, bilateral injections of 15 ng oxotremorine also had a similar effect on memory retention, supporting the involvement of the cholinergic system. Thus, our results further support a physiological role for synaptically released histamine, that in addition to affecting cholinergic transmission in the amygdala, modulates consolidation of fear memories

Cerebellar role in fear-conditioning consolidation

Some cerebellar structures are known to be involved in the memorization of several conditioned responses. The role of the interpositus nucleus (IN) and the vermis (VE) in fear-conditioning consolidation was investigated by means of a combined behavioral and neurophysiological technique. The IN and VE were subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats that underwent acoustic conditioned stimulus (CS) and context fear training. TTX was injected in different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after the stereotaxic TTX administration. This schedule ensures that there is no interference with normal cerebellar function during either the acquisition or the retrieval phase so that any amnesic effect may be due only to consolidation disruption. Our results show that IN functional integrity is necessary for acoustic CS fear response memory formation up to the 96-h after-acquisition delay. VE functional integrity was shown to be necessary for memory formation of both context (up to the 96-h after-acquisition delay) and acoustic CS (up to the 192-h after-acquisition delay) fear responses. The present findings help to elucidate the role of the cerebellum in memory consolidation and better define the neural circuits involved in fear memories.