Complementary roles for the amygdala and hippocampus in aversive conditioning to explicit and contextual cues

Context-dependent effects of hippocampal damage on memory in the shock-probe test

We assessed the role of the hippocampus in anterograde memory, using the shock-probe test. Rats with sham or neurotoxic lesions of the hippocampus were given a shock-probe acquisition session during which each time they contacted a probe they received a shock; 24 h later, the rats were given a second shock-probe session to test their retention, but in this instance the probe was not electrified. Rats were tested in either the same context as the one used during acquisition or in a different context. The hippocampal lesions impaired avoidance of the probe and burying on the retention test, suggesting that the lesions induced anterograde amnesia. However, the impairment was context dependent. The hippocampal lesions impaired avoidance only when the rats were tested in the context in which they received the conditioning. The results of the shock-probe test suggest that the anterograde amnesia following hippocampal lesions is due mainly to an inability to associate the context with the shock more than to an inability to associate the probe with shock.

Electrolytic lesions to nucleus accumbens core and shell have dissociable effects on conditioning to discrete and contextual cues in aversive and appetitive procedures respectively

The nucleus accumbens (n. acc.) has been implicated in conditioning to both discrete and contextual cues but its precise role is as yet controversial because conflicting patterns of effect have been reported. These inconsistencies may relate to the extent to which the lesions used encroach on different subfields of n. acc. and the use of different task variants. The present study compared the effects of selective lesions of shell and core subfields of nucleus accumbens (n. acc.) across aversive and appetitive trace conditioning variants. In both experiments, an auditory stimulus was contiguous with footshock or food, or presented at a trace interval. A continuous flashing light in each case provided an experimental background stimulus. Conditioning to the cues provided by the experimental chambers was also assessed. Rats with electrolytic lesions to the n. acc. shell and core showed different patterns of effect in aversive (Experiment 1) and appetitive (Experiment 2) variants of this procedure. In Experiment 1, the core lesion reduced the difference between trace and contiguously conditioned groups, in responding to the discrete noise stimulus. However, neither lesion had any detectable effect on contextual conditioning. In Experiment 2, the shell lesion clearly increased contextual conditioning, selectively in the trace conditioned group, but neither lesion had any detectable effect on discrete cue conditioning. Thus, whilst the shell and core lesions produced dissociable effects on discrete cue and contextual conditioning, the conclusions to be drawn depend on the procedural variant in use.

Relational memory for object identity and spatial location in rats with lesions of perirhinal cortex, amygdala and hippocampus

Previous studies dissociate medial temporal lobe regions using non-relational object versus relational spatial tasks. We compared a relational object identity task to the commonly used, relational spatial Morris water task. Lesions of perirhinal cortex, amygdala and hippocampus led to impaired performance on only the relational object preference task. Rats with perirhinal cortex and amygdala lesions performed normally on the Morris water task, but showed reduced perseveration in the correct quadrant on the probe trial. Rats with hippocampal damage were impaired on all measures of the Morris water task. Our findings demonstrate that perirhinal and amygdala damage creates impairments for relational tasks that rely on information processed by these structures (object identity and stimulus valence, respectively). In addition, these structures contribute non-essentially to performance of relational spatial tasks. The hippocampus is critical for all tasks that require the use of relational representations, regardless of whether the disambiguating information is provided by object identity or spatial arrangements. The current pattern of results suggests that the previous object-spatial dissociations among medial temporal lobe regions may be due to the relational nature of the spatial tasks versus the non-relational nature of the object tasks. Further, they illustrate that discrete dissociations among different types of processing may be an oversimplification.

The ventral striatum in off-line processing: ensemble reactivation during sleep and modulation by hippocampal ripples

Previously it has been shown that the hippocampus and neocortex can spontaneously reactivate ensemble activity patterns during post-behavioral sleep and rest periods. Here we examined whether such reactivation also occurs in a subcortical structure, the ventral striatum, which receives a direct input from the hippocampal formation and has been implicated in guidance of consummatory and conditioned behaviors. During a reward-searching task on a T-maze, flanked by sleep and rest periods, parallel recordings were made from ventral striatal ensembles while EEG signals were derived from the hippocampus. Statistical measures indicated a significant amount of reactivation in the ventral striatum. In line with hippocampal data, reactivation was especially prominent during post-behavioral slow-wave sleep, but unlike the hippocampus, no decay in pattern recurrence was visible in the ventral striatum across the first 40 min of post-behavioral rest. We next studied the relationship between ensemble firing patterns in ventral striatum and hippocampal ripples-sharp waves, which have been implicated in pattern replay. Firing rates were significantly modulated in close temporal association with hippocampal ripples in 25% of the units, showing a marked transient enhancement in the average response profile. Strikingly, ripple-modulated neurons in ventral striatum showed a clear reactivation, whereas nonmodulated cells did not. These data suggest, first, the occurrence of pattern replay in a subcortical structure implied in the processing and prediction of reward and, second, a functional linkage between ventral striatal reactivation and a specific type of high-frequency population activity associated with hippocampal replay.

Glutamate receptor antagonist infusions into the basolateral and medial amygdala reveal differential contributions to olfactory vs. context fear conditioning and expression

The basolateral amygdala's involvement in fear acquisition and expression to visual and auditory stimuli is well known. The involvement of the basolateral and other amygdala areas in fear acquisition and expression to stimuli of other modalities is less certain. We evaluated the contribution of the basolateral and medial amygdala to olfactory and to context fear and fear conditioning by infusing into these areas the NMDA receptor antagonist AP5, the AMPA/kainate receptor antagonist NBQX, or vehicle prior to either odor-shock pairings or fear-potentiated startle testing. Pre-training AP5 infusions into the basolateral amygdala disrupted fear conditioning to the odor but not the context conditioned stimulus (CS). Pre-test NBQX infusions disrupted fear-potentiated startle to the odor but not context CS. Neither compound blocked fear conditioning when infused into the medial amygdala prior to training, but pre-test NBQX infusions did block fear-potentiated startle. The results confirm and extend recent findings suggesting a role for the basolateral amygdala in olfactory fear and fear conditioning, reveal an unexpected dissociation of the basolateral amygdala's involvement in discrete cue versus context fear and fear conditioning, and implicate for the first time the medial amygdala in fear-potentiated startle.

Differential Encoding of Behavior and Spatial Context in Deep and Superficial Layers of the Neocortex

Rodent hippocampal activity is correlated with spatial and behavioral context, but how context affects coding in association neocortex is not well understood. The cellular distribution of the neural activity-regulated immediate-early gene Arc was used to monitor the activity history of cells in CA1, and in deep and superficial layers of posterior parietal and gustatory cortices (which encode movement and taste, respectively), during two behavioral epochs in which spatial and behavioral context were independently manipulated while gustatory input was held constant. Under conditions in which the hippocampus strongly differentiated behavioral and spatial contexts, deep parietal and gustatory layers did not discriminate between spatial contexts, whereas superficial layers in both neocortical regions discriminated well. Deep parietal cells discriminated behavioral context, whereas deep gustatory cortex neurons encoded the two conditions identically. Increased context sensitivity of superficial neocortical layers, which receive more hippocampal outflow, may reflect a general principle of neocortical organization for memory retrieval.

Animal cognition: defining the issues

The assessment of cognitive functions in rodents represents a critical experimental variable in many research fields, ranging from the basic cognitive neurosciences to psychopharmacology and neurotoxicology. The increasing use of animal behavioral tests as 'assays' for the assessment of effects on learning and memory has resulted in a considerable heterogeneity of data, particularly in the field of behavioral and psycho pharmacology. The limited predictive validity of changes in behavioral performance observed in standard animal tests of learning and memory indicates that a renewed effort to scrutinize the validity of these tests is warranted. In humans, levels of processing (effortful vs. automatic) and categories of information (procedural vs. episodic/declarative) are important variables of cognitive operations. The design of tasks that assess the recall of 'episodic' or 'declarative' information appears to represent a particular challenge for research using laboratory rodents. For example, the hypothesis that changes in inspection time for a previously encountered place or object are based on the recall of declarative/episodic information requires substantiation. In order to generalize findings on the effects of neuronal or pharmacological manipulations on learning and memory, obtained from one species and one task, to other species and other tasks, the mediating role of important sets of variables which influence learning and memory (e.g. attentional, affective) needs to be determined. Similar to the view that a neuronal manipulation (e.g. a lesion) represents a theory of the condition modeled (e.g. a degenerative disorder), an animal behavioral task represents a theory of the behavioral/cognitive process of interest. Therefore, the test of hypotheses regarding the validity of procedures used to assess cognitive functions in animals is an inherent part of the research process.

A comparison and evaluation of the predictions of relational and conjunctive accounts of hippocampal function

Relational and conjunctive memory theory each postulate that the hippocampus participates in the formation of long-term memory representations comprised of associations between multiple elements. The goals of the current work were to clarify and contrast these theories by outlining the nature of the representations that are spared vs. impaired following hippocampal damage according to each theoretical perspective. Relational theory predicts that hippocampal lesions will impair performance on tasks that require the formation of new long-term representations in which distinct elements must be regarded in relation to all other elements. Representations that remain intact despite hippocampal damage include separate representations of distinct individual elements or multiple stimuli fused into a static "blend" such as several elements viewed from one vantage point. Additionally, the relational account predicts that rapid incidental online processing of the relations can be achieved through structures other than the hippocampus, but this information will not be stored. In contrast, conjunctive theory predicts that hippocampal damage will impair the rapid formation of unitary representations that contain features of elements and their relative relationships bound in an inflexible manner. Deficits in the rapid formation of these conjunctive representations result in impaired performance on tasks that require rapid incidental stimulus binding. However, intact formation of conjunctive representations can occur over multiple trials in the service of problem solving. Using these theoretical frameworks, recent findings from the human and nonhuman animal literature are reexamined in order to determine whether one theory better accounts for current findings. We discuss empirical studies that serve as "critical experiments" in addressing the relational vs. conjunctive debate, and find that the predictions of relational theory are supported by existing findings over those from the conjunctive account.

Hippocampal and amygdalar involvement in discriminatory place learning

A conflict task was developed that simultaneously examines place aversion learning and fear-motivated context discrimination. The task superimposed Pavlovian discriminative fear conditioning on an appetitively motivated instrumental response (alternation). Rats were trained to alternate along a high-walled, diamond-shaped runway between two chambers for food. On half of the trials, a tone CS signaled the fact that a fixed section at the apex of the runway was electrified. Both the tone and the shock were turned on at the beginning of, and remained on for the duration of, each tone trial. A new trial was initiated at the time the animal entered the subsequent food chamber. Therefore, during a tone trial, in order to attain additional food reinforcement, the animal had to cross over the electrified region at the runway apex. Behavioral performance of rats with small lesions of the amygdala or dorsal hippocampus (DH) was compared with that of sham-operated controls. All groups displayed significant discriminative responding, hesitating more on tone trials while in areas of the runway adjacent to the shock region. Animals with lesions of the DH were similar to controls with respect to the tone-mediated discrimination, yet were delayed in the initial expression of a location-specific fear response. Conversely, amygdala lesions did not affect place learning; however, these animals were impaired in their suppression of the fear response following repeated unpaired trials.

Time-dependent involvement of the dorsal hippocampus in trace fear conditioning in mice

Hippocampal and amygdaloid neuroplasticity are important substrates for Pavlovian fear conditioning. The hippocampus has been implicated in trace fear conditioning. However, a systematic investigation of the significance of the trace interval has not yet been performed. Therefore, this study analyzed the time-dependent involvement of N-methyl-D-aspartate (NMDA) receptors in the dorsal hippocampus in one-trial auditory trace fear conditioning in C57BL/6J mice. The NMDA receptor antagonist APV was injected bilaterally into the dorsal hippocampus 15 min before training. Mice were exposed to tone (conditioned stimulus [CS]) and footshock (unconditioned stimulus [US]) in the conditioning context without delay (0 s) or with CS-US (trace) intervals of 1-45 s. Conditioned auditory fear was determined 24 h after training by the assessment of freezing and computerized evaluation of inactivity in a new context; 2 h later, context-dependent memory was tested in the conditioning context. NMDA receptor blockade by APV markedly impaired conditioned auditory fear at trace intervals of 15 s and 30 s, but not at shorter trace intervals. A 45-s trace interval prevented the formation of conditioned tone-dependent fear. Context-dependent memory was always impaired by APV treatment independent of the trace interval. The results indicate that the dorsal hippocampus and its NMDA receptors play an important role in auditory trace fear conditioning at trace intervals of 15-30-s length. In contrast, NMDA receptors in the dorsal hippocampus are unequivocally involved in contextual fear conditioning independent of the trace interval. The results point at a time-dependent role of the dorsal hippocampus in encoding of noncontingent explicit stimuli. Preprocessing of long CS-US contingencies in the hippocampus appears to be important for the final information processing and execution of fear memories through amygdala circuits.

SNAP-25 in hippocampal CA1 region is involved in memory consolidation

As a synaptosomal protein, SNAP-25 plays a role in a number of neuronal functions including axonal growth, dendrite formation, fusion of synaptic vesicles with membrane and the expression of long-term potentiation (LTP) in the hippocampus. Using a learning/memory behavior screening, we identified SNAP-25 as one of the differentially expressed genes in the hippocampus upon behavioral training. The inhibition of SNAP-25 with intracerebroventricular antisense oligonucleotide caused a deficit in long- but not short-term memory for step-down inhibitory avoidance. Intra-CA1 infusion of the SNAP-25 antisense oligonucleotide impaired long-term contextual fear memory and spatial memory and interfered with the LTP of synaptic transmission in the CA1 region. The inhibitory effect on LTP was not mediated by a pre-synaptic mechanism because paired pulse facilitation of synaptic transmission was not affected after administration of the antisense oligonucleotide. Together, the results suggest that SNAP-25 in the CA1 region is involved in memory consolidation.

Conditioned suppression and freezing as measures of aversive Pavlovian conditioning: effects of discrete amygdala lesions and overtraining

Freezing and suppression are measures of conditioned fear that correlate in unlesioned animals. Both the basolateral (BLA) and central (CeN) nuclei of the amygdala are required for conditioned freezing, though there can be recovery with overtraining. The neuroanatomical substrates of conditioned suppression are less clear, with evidence both for a specific requirement of the CeN and for disruption by BLA lesions. The present study investigated the impact of selective excitotoxic lesions of the BLA and CeN upon the acquisition and expression of conditioned fear, measured by freezing and both on-baseline and off-baseline conditioned suppression in the same rats. BLA and CeN lesions both abolished all measures of conditioned fear after 9 trials of fear conditioning. However, when conditioning was extended to 33 trials, whereas rats with combined lesions of both the BLA and CeN continued to show no conditioned fear responses, there was a pattern of recovery observed after selective lesions. There was a partial recovery of freezing with both lesions, and full recovery of conditioned suppression, except for off-baseline suppression in CeN lesioned rats. These results indicate that with few conditioning trials, both the BLA and CeN are required in a serial manner for conditioned fear responses, but that overtraining can mitigate such impairments, likely involving parallel pathways in and through the amygdala.

Contributions of the anterior cingulate cortex and amygdala to pain- and fear-conditioned place avoidance in rats

The pain experience includes a sensory-discriminative and an affective-emotional component. The sensory component of pain has been extensively studied, while data about the negative affective component of pain are quite limited. The anterior cingulate cortex (ACC), and amygdala are thought to be key neural substrates underlying emotional responses. Using formalin-induced conditioned place avoidance (F-CPA) and electric foot-shock conditioned place avoidance (S-CPA) models, the present study observed the effects of bilateral excitotoxic (quinolinic acid 200 nmol/microl) lesions of the ACC and amygdala on pain and fear induced negative emotion, as well as on sensory component of pain. In the place-conditioning paradigm, both intraplantar (i.pl.) injection of formalin and electric foot-shock produced conditioned place avoidance. Excitotoxin-induced lesion of either the ACC or amygdala significantly reduced the magnitude of F-CPA. However, the decrease in the magnitude of S-CPA occurred only in the amygdala, but not ACC lesioned animals. Neither ACC nor amygdala lesion significantly changed formalin-induced acute nociceptive behaviors. These results suggest that the amygdala is involved in both pain- and fear-related negative emotion, and the ACC might play a critical role in the expression of pain-related negative emotion.

Acute amygdalar activation induces an upregulation of multiple monoamine G protein coupled pathways in rat hippocampus

A "partial" rodent model for schizophrenia has been used to characterize the regulation of hippocampal genes in response to amygdalar activation. At 96 h after the administration of picrotoxin into the basolateral nucleus, we have observed an increase in the expression of genes associated with 18 different monoamine (ie adrenergic alpha 1, alpha 2 and beta 2, serotonergic 5HT5b and 5HT6, dopamine D4 and muscarinic m1, m2 and m3) and peptide (CCK A and B, angiotensin 1A, mu and kappa opiate, FSH, TSH, LH, GNRH, and neuropeptide Y) G-protein coupled receptors (GPCRs). These latter receptors are associated with three different G protein signaling pathways (Gq, Gs, and Gi) in which significant changes in gene expression were also noted for adenylate cyclase (AC4), phosphodiesterase (PDE4D), protein kinase A (PKA), and protein kinase C (PKC). Quantitative RT-PCR was used to validate the results and demonstrated that there were predictable increases of three GPCRs selected for this analysis, including the dopamine D4, alpha 1b, and CCK-B receptors. Eight out of the nine monoamine receptors showing these changes have moderate to high affinity for the atypical antipsychotic, clozapine. Taken together, these results suggest that amygdalar activation may play a role in the pathophysiology and treatment of psychosis by regulating the activity of multiple GPCR and metabolic pathways in hippocampal cells.

The hippocampus and inhibitory learning: a 'Gray' area?

Gray's approach to understanding hippocampal functioning [The Neuropsychology of Anxiety: An Enquiry into the Function of the Septo-hippocampal System, 1982; The Neuropsychology of Anxiety, 2000] departs from the prevailing view of that structure as a substrate for memory. Instead, Gray and McNaughton have proposed that hippocampus is involved with a function that is more fundamental than memory, namely the resolution of conflict between competing approach and avoidance tendencies. The present paper attempts to advance this perspective by describing how the effects of selective lesions of the hippocampus on performance in both relatively simple Pavlovian conditioning tasks and in more complex radial maze problems could be a consequence of an impairment in a simple form of inhibitory learning. Specifically, we consider the idea that the hippocampus is needed to form simple inhibitory associations between events that are concurrently embedded in simple excitatory associations [Behav Brain Res 119 (2001) 111]. This idea is compared with the conflict resolution hypothesis offered by Gray and McNaughton and avenues of integration are noted. In addition, the potential role for inhibitory learning in hippocampal-dependent spatial and contextual information processing is also discussed.

Effect of social isolation on stress-related behavioural and neuroendocrine state in the rat

The present study investigated the effects of post-weaning social isolation (SI) on behavioural and neuroendocrine reactivity to stress of male and female rats. Innate aspects of fear and anxiety were assessed in the open field and elevated plus maze tests. Spontaneous startle reflex and conditioned fear response were further investigated. The neuroendocrine response of isolates was examined by measuring basal and stress release of ACTH and corticosterone and by evaluating the mRNA expression of mineralocorticoid (MR) and glucocorticoid (GR) receptors using in situ hybridization. Locomotor activity in the open field was not modified by chronic SI. In males, but not females, SI produced an anxiogenic profile in the elevated plus maze. Male isolates showed a trend towards increased startle reflex amplitude relative to socially-reared controls. Moreover, SI in males produced alterations of the HPA axis functioning as reflected by higher basal levels of ACTH, and enhanced release of ACTH and corticosterone following stress. In contrast, startle response or HPA axis functioning were not altered in female isolates. Social isolates from both genders showed reduced contextual fear-conditioning. Finally, the mRNA expression of MR and GR was not modified by SI. The results of the present study suggest that chronic SI increases emotional reactivity to stress and produces a hyperfunction of the HPA axis in adult rats, particularly in males.

Emotional perseveration: an update on prefrontal-amygdala interactions in fear extinction

Fear extinction refers to the ability to adapt as situations change by learning to suppress a previously learned fear. This process involves a gradual reduction in the capacity of a fear-conditioned stimulus to elicit fear by presenting the conditioned stimulus repeatedly on its own. Fear extinction is context-dependent and is generally considered to involve the establishment of inhibitory control of the prefrontal cortex over amygdala-based fear processes. In this paper, we review research progress on the neural basis of fear extinction with a focus on the role of the amygdala and the prefrontal cortex. We evaluate two competing hypotheses for how the medial prefrontal cortex inhibits amygdala output. In addition, we present new findings showing that lesions of the basal amygdala do not affect fear extinction. Based on this result, we propose an updated model for integrating hippocampal-based contextual information with prefrontal-amygdala circuitry.

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.

Influence of chronic corticosterone and glucocorticoid receptor antagonism in the amygdala on fear conditioning

Glucocorticoid receptor activation within the basolateral amygdala (BLA) during fear conditioning may mediate enhancement in rats chronically exposed to stress levels of corticosterone. Male Sprague-Dawley rats received corticosterone (400 microg/ml) in their drinking water (days 1-21), a manipulation that was previously shown to cause hippocampal CA3 dendritic retraction. Subsequently, rats were adapted to the fear conditioning chamber (day 22), then trained (day 23), and tested for conditioned fear to context and tone (day 25). Training consisted of two tone (20s) and footshock (500 ms, 0.25 mA) pairings. In Experiment 1, muscimol (4.4 nmol/0.5 microl/side), a GABAergic agonist, was microinfused to temporarily inactivate the BLA during training. Rats given chronic corticosterone showed enhanced freezing to context, but not tone, compared to vehicle-supplemented rats. Moreover, BLA inactivation impaired contextual and tone conditioning, regardless of corticosterone treatment. In Experiment 2, RU486 (0, 0.3, and 3.0 ng/0.2 microl/side) was infused on training day to antagonize glucocorticoid receptors in the BLA. Corticosterone treatment enhanced fear conditioning to context and tone when analyzed together, but not separately. Moreover, RU486 (3.0 ng/side) selectively exacerbated freezing to context in chronic corticosterone-exposed rats only, but failed to alter tone conditioning. Serum corticosterone levels were negatively correlated with contextual, not tone, conditioning. Altogether, these suggest that chronic corticosterone influences fear conditioning differently than chronic stress as shown previously. Moreover, chronic exposure to corticosteroids alters BLA functioning in a non-linear fashion and that contextual conditioning is influenced more than tone conditioning by chronic corticosterone and BLA glucocorticoid receptor stimulation.

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.

Hippocampus and Pavlovian fear conditioning in rats: muscimol infusions into the ventral, but not dorsal, hippocampus impair the acquisition of conditional freezing to an auditory conditional stimulus

The authors compared the effects of pharmacological inactivation of the dorsal hippocampus (DH) or ventral hippocampus (VH) on Pavlovian fear conditioning in rats. Freezing behavior served as the measure of fear. Pretraining infusions of muscimol, a GABAA receptor agonist, into the VH disrupted auditory, but not contextual, fear conditioning; DH infusions did not affect fear conditioning. Pretesting inactivation of the VH or DH did not affect the expression of conditional freezing. Pretraining electrolytic lesions of the VH reproduced the effects of muscimol infusions, whereas posttraining VH lesions disrupted both auditory and contextual freezing. Hence, neurons in the VH are importantly involved in the acquisition of auditory fear conditioning and the expression of auditory and contextual fear under some conditions.

Perirhinal cortex supports delay fear conditioning to rat ultrasonic social signals

Auditory information can reach the lateral nucleus of the amygdala (LA) through a monosynaptic thalamic projection or a polysynaptic cortical route. The polymodal input from the perirhinal cortex (PR) is a major informational gateway to the LA and nearby structures. Pretraining PR lesions impair fear conditioning to a context, but there have been no reports that they cause deficits in delay conditioning to an auditory cue. The direct subcortical projection to the LA seems sufficient to support delay conditioning to a tone conditional stimulus (CS). We examined the effect of PR lesions on delay conditioning to two different tone conditional stimuli (4 and 22 kHz tones; both 10 sec duration) and two different rat ultrasonic vocalization (USV) conditional stimuli (10 sec of "22 kHz USVs"). The two USV conditional stimuli were multi-call segments that were recorded (digitized at 100 kHz) from two different rats. One USV CS was a continuous sequence of eight calls, and the other was a portion of a continuous sequence of six calls. PR lesions significantly impaired conditioning to both USV conditional stimuli and to the training context but had no significant effect on conditioning to either tone CS. The role of PR in fear conditioning appears not to be determined by whether the conditional stimuli serve as contexts or cues, but instead by the nature or complexity of the stimuli or stimulus configurations. These cue-specific effects of PR lesions are suggested to reflect differences in the stimulus features that are encoded in the two CS pathways to the LA.

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

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

Unique neural circuitry for neonatal olfactory learning

Imprinting ensures that the infant forms the caregiver attachment necessary for altricial species survival. In our mammalian model of imprinting, neonatal rats rapidly learn the odor-based maternal attachment. This rapid learning requires reward-evoked locus ceruleus (LC) release of copious amounts of norepinephrine (NE) into the olfactory bulb. This imprinting ends at postnatal day 10 (P10) and is associated with a dramatic reduction in reward-evoked LC NE release. Here we assess whether the functional emergence of LC alpha2 inhibitory autoreceptors and the downregulation of LC alpha1 excitatory autoreceptors underlie the dramatic reduction in NE release associated with termination of the sensitive period. Postsensitive period pups (P12) were implanted with either LC or olfactory bulb cannulas, classically conditioned with intracranial drug infusions (P14), and tested for an odor preference (P15). During conditioning, a novel odor was paired with either olfactory bulb infusion of abeta-receptor agonist (isoproterenol) to assess the target effects of NE or direct LC cholinergic stimulation combined with alpha2 antagonists and alpha1 agonists in a mixture to reinstate neonatal levels of LC autoreceptor activity to assess the source of NE. Pups learned an odor preference when the odor was paired with either olfactory bulb isoproterenol infusion or reinstatement of neonatal LC receptor activity. These results suggest that LC autoreceptor functional changes rather than olfactory bulb changes underlie sensitive period termination.

Basolateral amygdala lesions do not prevent memory of context-footshock training

The present studies examined the effects of basolateral amygdala (BLA) lesions induced prior to or after context-footshock training on 48-h memory, using several retention measures. In experiment 1, male Sprague-Dawley rats with bilateral BLA lesions (NMDA, 12.5 mg/mL, 0.2 microL) were given footshock training in one compartment of a two-compartment alley. Rats were habituated to the alley and 24 h later were given two footshocks in the shock compartment. Retention was tested 48 h later, using latency to enter the shock compartment and time spent freezing as measures of memory. Two days later, they were tested again and received a footshock on each re-entry of the shock compartment prior to remaining in the safe compartment for 200 consecutive seconds. The BLA lesions did not block retention as assessed by freezing or number of re-entries of the shock compartment. In experiment 2, no prior habituation was given, and only one footshock was used for the training. BLA lesions did not block retention, as indicated by latencies to enter the shock compartment on a 48-h test or by number of entries of the shock compartment. Experiment 3 examined the effects of the GABAA agonist muscimol infused into the BLA prior to the 48-h retention test. The muscimol infusions decreased retention test entrance latencies but did not block retention as assessed by the number of subsequent entries of the shock compartment. These findings provide additional evidence that an intact BLA is not required for the acquisition or retention of context-footshock training.

C-fos activation patterns in rat prefrontal cortex during acquisition of a cued classical conditioning task

The prefrontal cortex (PFC) is known to be involved in associative learning; however, its specific role in acquisition of cued classical conditioning has not yet been determined. Furthermore, the role of regional differences within the PFC in the acquisition of cued conditioning is not well described. These issues were addressed by exposing rats to either one or four sessions of a cued classical conditioning task, and subsequently examining c-fos immunoreactivity in various areas of the PFC. Differences in patterns of c-fos immunopositive nuclei were found when comparing the PFC areas examined. No significant differences were found between rats presented with a temporally contingent conditioned stimulus (CS) light and food (paired groups) and those presented with the same stimuli temporally non-contingently (unpaired groups). In lateral and orbital PFC, both the paired and unpaired groups showed more c-fos immunopositive nuclei than control groups exposed only to the behavioral setup (context exposed groups), and all groups showed a drop in c-fos immunopositive nuclei from session 1 to session 4. In dorsal medial PFC, no differences were seen between the paired, unpaired and context exposed groups. These groups did, however, differ from naive animals, an effect that was not seen in the ventral medial PFC. The results of this study do not support a role for the PFC in the acquisition of a cued classical conditioning task. The differences seen between paired, unpaired and context exposed groups in orbital and lateral PFC could be due to contextual conditioning or reward-related effects.

Lesions to the basolateral amygdala and the orbitofrontal cortex but not to the medial prefrontal cortex produce an abnormally persistent latent inhibition in rats

Repeated nonreinforced preexposure to a stimulus interferes with the establishment of conditioned responding to this stimulus when it is subsequently paired with reinforcement. This stimulus-preexposure effect is known as latent inhibition (LI). Rather remarkably, LI appears to be resistant to the effects of numerous lesions, including the prefrontal cortex (PFC) and the basolateral amygdala (BLA). However, intact behavioral expression of LI following damage to given brain regions does not preclude the possibility that such regions participate in the regulation of LI expression in the intact brain. The present study showed that lesions of the BLA and the orbitofrontal cortex (OFC) but not of the medial PFC (mPFC) led to an abnormally persistent LI which emerged under conditions that disrupted LI in control rats. LI was measured in a thirst motivated conditioned emotional response procedure by comparing suppression of drinking in response to a tone in rats which received 0 (nonpreexposed) or 40 tone presentations (preexposed) followed by either two or five tone-shock pairings. Control rats showed LI with 40 preexposures and two conditioning trials, but raising the number of conditioning trials to five disrupted LI. OFC- and BLA-lesioned rats showed LI under the former condition but in addition persisted in exhibiting LI under the latter condition. Rats with lesion of the mPFC did not show persistent LI. Thus, although LI does not depend on the integrity of BLA and OFC (because it is present in BLA- and OFC- lesioned rats even under conditions disrupting the phenomenon in normal rats), these regions play an important role in the modulation of its expression, more specifically, in the control of the non-expression of LI when the impact of conditioning increases beyond a certain level.

AUTOMATED MEASUREMENT OF FREEZING TIME TO CONTEXTUAL AND AUDITORY CUES IN FEAR CONDITIONING AS A SIMPLE SCREENING METHOD TO ASSESS LEARNING AND MEMORY ABILITIES IN RATS

In fear-conditioning paradigms, rats display a freezing response not only upon presentation of a conditioned stimulus (usually tone), but also when returned to a conditioning chamber (context) in which they received an unconditioned stimulus (usually a foot shock). These paradigms have intrinsic advantages for screening effects of chemicals on learning and memory, although the time-consuming monitoring of freezing time by human observers may be problematic. In this study, an automated apparatus was developed to optimize a fear-conditioning paradigm for screening. We developed an apparatus that records freezing time measured from body movements detected by passive infrared (PIR) sensors. The apparatus detected freezing time as accurately as the human scoring method, and these data were used to determine learning parameters (freezing time to context or to tone) and a non-learning parameter (freezing time to novel context) in the rats. Rats orally administered the neurotoxic compound trimethyltin (TMT) exhibited decreased freezing levels to context, but not to tone or to novel context. These results suggest that this automated method can be an effective part of a screening program.

Effects of β-Adrenoreceptor Blockade During Chronic Exercise on Contextual Fear Conditioning and mRNA for Galanin and Brain-Derived Neurotrophic Factor

The authors examined the effects of activity wheel running (AWR) and propranolol on contextual fear conditioning (CFC) and messenger RNA (mRNA) for galanin (GAL) in the locus coeruleus (LC) and brain-derived neurotrophic factor (BDNF) in the hippocampal formation (HF) in rats. Freezing behavior during the testing session of the CFC protocol was elevated in the AWR-placebo group compared to sedentary-placebo and AWR-propranolol groups. AWR increased GAL mRNA in the LC. CFC increased BDNF mRNA in the HF. These results suggest that exercise enhances CFC and that antagonism of the beta-adrenoreceptors attenuates this effect. The exercise-related induction of GAL gene expression in the LC may influence noradrenergic transmission to facilitate CFC.

The challenges of understanding mammalian cognition and memory-based behaviours: an interactive learning and memory systems approach

Various research problems are presented to illustrate the utility of using the interactive multiple learning and memory systems view to better understand normal and abnormal manifestations of mammalian behaviour. Evidence for incidental learning and memory processes is presented and various implications of this work are discussed. Empirical and theoretical work directed at understanding the cognitive and non-cognitive processes associated with place learning in the water task and context conditioning during aversive events is also presented.

Medial Septal Lesions Mimic Effects of Both Selective Dorsal and Ventral Hippocampal Lesions

Electrolytic medial septal (MS) lesions, which depleted acetylcholinesterase staining in both dorsal and ventral hippocampus, produced a constellation of behaviors, combining aspects of both selective dorsal and ventral hippocampal lesion effects. MS lesions impaired spatial working memory on the T maze, thus resembling the effects of dorsal hippocampal lesions. In addition, MS lesions reduced anxiety during successive alleys (a modified form of the elevated plus-maze), social interaction, and hyponeophagia tests. MS lesions also reduced postshock freezing. These effects more closely resemble those of ventral hippocampal lesions. Therefore, the effects of electrolytic MS lesions derive from the resulting combined deafferentation of dorsal and ventral hippocampal regions, suggesting that previously reported effects of cytotoxic dorsal hippocampal lesions are unlikely to be due to a demyelination of fibers of passage coursing through the septal pole.

Dorsal hippocampus and classical fear conditioning to tone and context in rats: effects of local NMDA-receptor blockade and stimulation

Consistent with the importance of the hippocampus in learning more complex stimulus relations, but not in simple associative learning, the dorsal hippocampus has commonly been implicated in classical fear conditioning to context, but not to discrete stimuli, such as a tone. In particular, a specific and central role in contextual fear conditioning has been attributed to mechanisms mediated by dorsal hippocampal N-methyl-D-aspartate (NMDA)-type glutamate receptors. The present study characterized the effects of blockade or tonic stimulation of dorsal hippocampal NMDA receptors by bilateral local infusion of the noncompetitive NMDA receptor antagonist MK-801 (dizocilpine maleate; 6.25 microg/side) or of NMDA (0.7 microg/side), respectively, on classical fear conditioning to tone and context in Wistar rats. Freezing was used to measure conditioned fear. Regardless of whether conditioning was conducted with tone-shock pairings or unsignaled footshocks (background or foreground contextual conditioning), both NMDA and MK-801 infusion before conditioning resulted in reduced freezing during subsequent exposure to the conditioning context. Freezing during subsequent tone presentation in a new context, normally resulting from conditioning with tone-shock pairings, was not impaired by MK-801 but was strongly reduced by NMDA infusion before conditioning; this freezing was also reduced by NMDA infusion before tone presentation (in an experiment involving NMDA infusions before conditioning and subsequent tone presentation to assess the role of state-dependent learning). It was assessed whether unspecific infusion effects (altered sensorimotor functions, state dependency) or infusion-induced dorsal hippocampal damage contributed to the observed reductions in conditioned freezing. Our data suggest that formation of fear conditioning to context, but not tone, requires NMDA receptor-mediated mechanisms in the dorsal hippocampus. As indicated by the effects of NMDA, some dorsal hippocampal processes may also contribute to fear conditioning to tone. The role of the dorsal hippocampus and local NMDA receptor-mediated processes in fear conditioning to tone and context is discussed in comparison with ventral hippocampal processes.

Neurochemical correlates of relapse to d-amphetamine self-administration by rats induced by stimulation of the ventral subiculum

RATIONALE AND OBJECTIVES

Previous studies show that electrical stimulation of the ventral subiculum (vSub) can reinstate drug-seeking behavior in rats following extinction. This study examined whether vSub stimulation could also evoke reinitiation of d-amphetamine (d-AMPH) self-administration during voluntary abstinence following a prolonged bout of drug intake. Dynamic changes in extracellular levels of dopamine (DA) and metabolites in the nucleus accumbens (NAc) during 48-h unlimited access to d-AMPH by rats were monitored. Neurochemical correlates of relapse to d-AMPH administration induced by vSub stimulation or by experimenter administered d-AMPH infusions were also examined during voluntary abstinence in separate experiments.

METHODS AND RESULTS

In vivo microdialysis using high-pressure liquid chromatography with electrochemical detector (HPLC-EC) was used to monitor changes in DA and metabolite efflux in the NAc during a continuous access-abstinence-relapse cycle of d-AMPH self-administration in a 48-h test. The initial pattern of drug intake was associated with significant increases in DA efflux in the NAc. During the abstinence phase, DA efflux was near pre-session baseline values. Electrical stimulation at the vSub after 2 h of abstinence immediately induced a significant increase in DA efflux and reinstatement of drug self-administration behavior. Evoked DA release and responses on drug-paired lever induced by vSub stimulation were significantly greater than those induced by experimenter-administered d-AMPH.

CONCLUSIONS

Relapse to drug-taking behavior can be triggered by activation of the subicular glutamatergic pathway to the NAc. This study also confirmed that during abstinence the neurochemical response of the mesolimbic DA system to d-AMPH is attenuated and this can be reversed by vSub stimulation.

Evaluation of an improved automated analysis of freezing behaviour in rats and its use in trace fear conditioning

Pavlovian fear conditioning in rodents often involves the assessment of freezing behaviour. Since human-based scoring is labour-intensive and subject to biases, several automated methods have been proposed, but few of them have been assessed for temporal definition. An accurate method for the automated analysis of freezing in rats is presented. It is based on a comparison of contrast from successive video images (Learn. Mem. 7 (2000) 58). A major improvement involves subtracting a previously recorded image of the empty background from each frame before processing. This simple operation greatly improves reliability, since it cancels the influence of video noise outside the region of interest in the image (the rat), and can also take advantage of the movements of a patterned animal over a textured background. The technique is compared with manual scoring and with the Anagnostaras method in a trace fear conditioning experiment. The method is appropriate to monitor the time course of freezing behaviour. The same system with a different threshold can also score locomotor activity. A quantitative evaluation of the residual errors inherent to automated methods is attempted.

Mnemonic functions of dorsal striatum and hippocampus in aversive conditioning

These experiments examined the mnemonic functions of hippocampus and dorsal striatum in Pavlovian aversive conditioning. Rats were trained in a single session by presenting three paired tones and footshocks. Immediately after training, they were given microinjections of D-amphetamine or vehicle into either dorsal hippocampus or dorsal striatum. Twenty-four hours later conditioned freezing (measured as cessation of movement by infrared motion detectors) to the experimental context and to the tone were measured separately. Compared to vehicle injections, amphetamine injections into dorsal hippocampus significantly increased conditioned freezing to the context but not to the tone. Injections into dorsal striatum increased conditioned freezing to both the context and the tone. These results converge with findings from lesion experiments suggesting that hippocampus is involved in aversive conditioning with contextual CSs only, and that dorsal striatum is involved in aversive conditioning with both contextual and discrete cue CSs. The functions of the these two structures in relation to that of the amygdala in the mediation of conditioned freezing are discussed.

Differential involvement of hippocampal and amygdalar NMDA receptors in contextual and aversive aspects of inhibitory avoidance memory in rats

Adult male rats bilaterally implanted with guide canullae aimed either at the dorsal hippocampus (dHIP) or the basolateral nucleus of the amygdala (BLA) were trained in a step-down inhibitory avoidance task (IA) and tested for retention 24 h after training. Immediately after training, animals were given a bilateral infusion of the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP5) (5.0 microg) into the dHIP or the BLA. Both intrahippocampal and intraamygdala infusions of AP5 blocked IA retention. Preexposure to the training box, but not to a different environment 24 h prior to training prevented the impairing effect of intrahippocampal infusion of AP5 on retention. Preexposure did not affect the retention impairment induced by intraamygdala infusion of AP5. These data suggest that hippocampal NMDA receptors might be involved in the contextual and spatial aspects, while amygdalar NMDA receptors might be involved in the aversive aspects of memory for IA.

The effects of ibotenic hippocampal lesions on discriminative fear conditioning to context in mice: impairment or facilitation depending on the associative value of a phasic explicit cue

To what extent the hippocampus is required for contextual conditioning remains a matter of debate. The present experiments examined the effects of ibotenate hippocampal lesions on discriminative fear conditioning to context in mice using measures of freezing in two conditioning paradigms. In both paradigms animals received foot shock as the unconditional stimulus (US) when placed in the (conditioning) context and no foot-shock when placed in the other (neutral) context. In both contexts, animals were presented with a tone as the conditioned stimulus (CS). In the conditioning context there was either no interval (delay condition) or a 30-s interval (trace condition) between tone CS end and shock US onset. These two paradigms were used because theory predicts that in the trace condition animals would learn more about contextual cues as predictors, or not, of shock US occurrence than in the delay condition. In agreement with this, we observed that sham-operated mice learned the context discrimination faster in the trace than in the delay condition. Lesions of the hippocampus significantly retarded, but did not prevent, the acquisition of the context discrimination in the trace condition. In contrast, lesions produced an opposite (facilitatory) effect in the delay condition, which was mainly observed during tone CS presentation. The data suggest that mice used two distinct competing strategies in solving this discrimination task: (i) a strategy relying on the processing of background contextual stimuli allowing direct establishment of context-US associations of different strengths, and (ii) a conditional cue (tone)-based strategy. Hence, hippocampal lesions may impair the use of the former strategy while exacerbating (unmasking) the use of the latter.

NMDA receptor blockade and hippocampal neuronal loss impair fear conditioning and position habit reversal in C57Bl/6 mice

The interpretation of learning and memory deficits in transgenic mice has largely involved theories of NMDA receptor and/or hippocampal function. However, there is little empirical data that describes what NMDA receptors or the hippocampus do in mice. This research assessed the effects of different doses of the NMDA receptor antagonist, MK-801, or different-sized hippocampal lesions on several behavioral parameters in adult male C57Bl/6 mice. In the first set of experiments, different doses of MK-801 (0.05-0.3mg/kg, s.c.) were assayed in fear conditioning, shock sensitivity, locomotion, anxiety, and position habit reversal tests. Contextual and cued fear conditioning, and position habit reversal were impaired in a dose-dependent manner. Locomotor activity was increased immediately after injection of the highest dose of MK-801. A second set of experiments determined the behavioral effects of a moderate and large excitotoxic hippocampal lesion. Both lesions impaired contextual conditioning, while the larger lesion interfered with cued conditioning. Reversal learning was significantly diminished by the large lesion, while the moderate lesion had a detrimental effect at a trend level (P<0.10). These results provide important reference data for studies involving genetic manipulations of NMDA receptor or hippocampal function in mice. Furthermore, they serve as a basis for a non-transgenic mouse model of the NMDA receptor or hippocampal dysfunction hypothesized to occur in human cognitive disorders.

Post-training excitotoxic lesions of the dorsal hippocampus attenuate forward trace, backward trace, and delay fear conditioning in a temporally specific manner

The present study sought to determine whether post-training excitotoxic lesions of the dorsal hippocampus would disrupt retention of fear conditioned using a trace procedure. Rats were trained using one of six procedures. Forward trace conditioning consisted of 10 trials in which a 16-s tone conditional stimulus (CS) was followed by a 28-s stimulus-free trace interval and then a mild footshock unconditional stimulus (US). We used two forms of delay conditioning where the tone and footshock co-terminated. Short delay used a 16-s tone and long delay used a 46-s tone. Backward trace conditioning was the same as forward trace, except that the order of the CS and US was reversed. CS-only and US-only were similar to forward trace except that the footshock or tone, respectively, was eliminated. One day later, animals received either an N-methyl-D-aspartate (NMDA)-induced lesion of the dorsal hippocampus or sham surgery. One week later, the rats were tested for freezing to the tone in a novel context. The next day, they were tested for freezing to the original training context. Hippocampal lesioned trace conditioned rats showed significantly less freezing during the tone compared with their sham lesioned controls. The lesion did not affect freezing during the tone in delay conditioning, nor in the other training conditions. During the 1-min period after tone offset, there was a trend in all hippocampal lesioned animals toward a deficit in freezing, compared with their corresponding sham lesioned controls, although only short delay, forward and backward trace groups showed a significant deficit. Hippocampal lesions also attenuated contextual conditioning. Thus, the hippocampus is critical for the consolidation and/or expression of a trace fear conditioned stimulus.

Memory retrieval after contextual fear conditioning induces c-Fos and JunB expression in CA1 hippocampus

Using specific polyclonal antisera against c-Fos, JunB, c-Jun and JunD, we tried to identify the candidate transcription factors of the immediate early gene family which may contribute to the molecular processes during contextual memory reconsolidation. For that purpose we analyzed the expression of these proteins in the hippocampus after contextual memory retrieval in a mouse model of fear conditioning. A single exposure to a foot shock of 0.8 mA was sufficient to induce robust contextual fear conditioning in C57BI/6N mice. In these mice context dependent memory retrieval evoked a marked induction of c-Fos and JunB, but not of c-Jun and JunD, in pyramidal CA1 neurons of the dorsal hippocampus. In contrast, mice exposed and re-exposed only to the context, without foot shock, did not show behavioral signs of contextual fear conditioning and exhibited significantly less expression of c-Fos and JunB in CA1 neurons. Mice which received a foot shock but were not re-exposed to the context revealed no immediate early gene induction. These results demonstrate that contextual memory retrieval is associated with de novo synthesis of specific members of the Fos/Jun transcription factor family. Therefore we suggest that these genes may contribute to plasticity and reconsolidation accompanying the retrieval process. The specific activation of CA1 neurons during the retrieval of contextual fear associations supports the postulated concept of a mnemonic role of this hippocampal subsector during the retrieval of contextual informations.

The human amygdala and the emotional evaluation of sensory stimuli

A wealth of animal data implicates the amygdala in aspects of emotional processing. In recent years, functional neuroimaging and neuropsychological studies have begun to refine our understanding of the functions of the amygdala in humans. This literature offers insights into the types of stimuli that engage the amygdala and the functional consequences that result from this engagement. Specific conclusions and hypotheses include: (1) the amygdala activates during exposure to aversive stimuli from multiple sensory modalities; (2) the amygdala responds to positively valenced stimuli, but these responses are less consistent than those induced by aversive stimuli; (3) amygdala responses are modulated by the arousal level, hedonic strength or current motivational value of stimuli; (4) amygdala responses are subject to rapid habituation; (5) the temporal characteristics of amygdala responses vary across stimulus categories and subject populations; (6) emotionally valenced stimuli need not reach conscious awareness to engage amygdala processing; (7) conscious hedonic appraisals do not require amygdala activation; (8) activation of the amygdala is associated with modulation of motor readiness, autonomic functions, and cognitive processes including attention and memory; (9) amygdala activations do not conform to traditional models of the lateralization of emotion; and (10) the extent and laterality of amygdala activations are related to factors including psychiatric status, gender and personality. The strengths and weakness of these hypotheses and conclusions are discussed with reference to the animal literature.

High and low responders to novelty and mesolimbic noradrenaline: effects of noradrenergic agents on radial-maze performance

The authors used high and low responders to novelty (HRs and LRs, respectively) to examine the effects of noradrenergic injections into the nucleus accumbens using a special radial-maze task. During the 5 successive test days, solvent-treated HRs acquired this task faster than LRs. Isoproterenol (beta-agonist) combined with phenylephrine (alpha-agonist) improved acquisition in LRs but not in HRs; this effect was counteracted by propranolol (beta-antagonist) and phentolamine (alpha-antagonist). Propranolol combined with phentolamine, as well as phentolamine alone, disrupted acquisition in HRs but not in LRs. Data show that the effects of noradrenergic agents in HRs and LRs are due to differences in acquisition directed by type-specific differences in functional mesolimbic noradrenaline.

Smaller hippocampal volume predicts pathologic vulnerability to psychological trauma

In animals, exposure to severe stress can damage the hippocampus. Recent human studies show smaller hippocampal volume in individuals with the stress-related psychiatric condition posttraumatic stress disorder (PTSD). Does this represent the neurotoxic effect of trauma, or is smaller hippocampal volume a pre-existing condition that renders the brain more vulnerable to the development of pathological stress responses? In monozygotic twins discordant for trauma exposure, we found evidence that smaller hippocampi indeed constitute a risk factor for the development of stress-related psychopathology. Disorder severity in PTSD patients who were exposed to trauma was negatively correlated with the hippocampal volume of both the patients and the patients' trauma-unexposed identical co-twin. Furthermore, severe PTSD twin pairs-both the trauma-exposed and unexposed members-had significantly smaller hippocampi than non-PTSD pairs.

Cellular imaging with zif268 expression in the rat nucleus accumbens and frontal cortex further dissociates the neural pathways activated following the retrieval of contextual and cued fear memory

Quantitative in situ hybridization revealed that the expression of the plasticity-associated gene zif268 was increased in specific regions of the rat frontal cortex and nucleus accumbens following fear memory retrieval. Increased expression of zif268 was observed in neurons in the core of the nucleus accumbens during the retrieval of contextual and discrete cued fear associations. In contrast, zif268 expression was additionally induced in neurons of the nucleus accumbens shell and the anterior cingulate cortex during the retrieval of contextual but not cued fear memories. No changes in the expression of this gene were seen in the ventral medial prefrontal cortex or ventral and lateral regions of the orbitofrontal cortex that were correlated specifically with the retrieval of fear memory. These experiments demonstrate the specific and dissociable activation of limbic cortical-ventral striatal regions that accompanies cued and contextual fear. These data, together with those previously published by our laboratory (Hall, J., Thomas, K.L. & Everitt, B.J. (2001) J. Neurosci., 21, 2186-2193), suggest that retrieval of contextual fear memories activates a wider limbic cortical-ventral striatal neural circuitry than does retrieval of cued fear memories. Moreover, the expression of zif268 may contribute to plasticity and reconsolidation of fear memory in these dissociable pathways.

Chronic exposure to trichloroethylene affects neuronal plasticity in rat hippocampal slices

Inhalational exposure to organic solvents is known to exert neurotoxic effects. Using the new multielectrode dish system (Panasonic) the effects of chronic exposure to trichloroethylene (TCE) on neuronal plasticity were assessed in different regions of the adult rat brain. Two groups of Long-Evans rats were exposed to 0 ppm or 500 ppm TCE, respectively, 6 h/day, 5 days/week for 6 months. Long-term potentiation (LTP) as well as paired-pulse potentiation/inhibition were assessed in slices from the visual cortex and the hippocampus. In addition, several behavioral tests were performed. Trichloroethanol concentrations were measured in blood and trichloroacetic acid concentrations were determined in urine. While TCE exposure impaired LTP as well as paired-pulse potentiation in hippocampal slices, no effects were seen in cortical slices. Our data demonstrate brain region specific functional changes following TCE exposure with the hippocampus being more vulnerable than the visual cortex. The behavioral measurements revealed no TCE related effects.

Right-sided human prefrontal brain activation during acquisition of conditioned fear

This H2(15)O positron emission tomography (PET) study reports on relative regional cerebral blood flow (rCBF) alterations during fear conditioning in humans. In the PET scanner, subjects viewed a TV screen with either visual white noise or snake videotapes displayed alone, then with electric shocks, followed by final presentations of white noise and snakes. Autonomic nervous system responses confirmed fear conditioning only to snakes. To reveal neural activation during acquisition, while equating sensory stimulation, scans during snakes with shocks and white noise alone were contrasted against white noise with shocks and snakes alone. During acquisition, rCBF increased in the right medial frontal gyrus, supporting a role for the prefrontal cortex in fear conditioning to unmasked evolutionary fear-relevant stimuli.

Reduced fear expression after lesions of the ventral hippocampus

The hippocampus has a critical role in several fundamental memory operations, including the conditioning of fear to contextual information. We show that the hippocampus is necessary also for unconditioned fear, and that the involved circuitry is at the ventral pole of the hippocampus. Rats with selective hippocampal lesions failed to avoid open arms in an elevated plus-maze and had decreased neuroendocrine stress responses during confinement to a brightly lit chamber. These effects were reproduced by lesions of the ventral half of the hippocampus, but not by damage to the dorsal three-quarters of the hippocampus or the amygdala. Ventral lesions failed to impair contextual fear conditioning or spatial navigation, suggesting that the ventral hippocampus may specifically influence some types of defensive fear-related behavior.

Disruption of Pavlovian contextual conditioning by excitotoxic lesions of the nucleus accumbens core

Nucleus accumbens (NAcc) core lesions were performed either before or after Pavlovian aversive conditioning. NAcc core lesions had no effect on discrete-cue or contextual conditioned freezing during acquisition. During retention testing, neither pre- nor posttraining lesions had any effect on conditioned freezing to the discrete cue. However, pretraining lesions resulted in a profound impairment of contextual conditioned freezing in a retention test, and posttraining lesions resulted in a smaller impairment. NAcc core lesions had no effect on sensory or motor processes, as measured by shock reactivity and spontaneous locomotor activity. These results suggest that during acquisition, processes independent of the NAcc core mediate contextual conditioned freezing, but that the NAcc is implicated in the retention of this aversive memory.