Electrolytic lesions of the dorsal hippocampus disrupt renewal of conditional fear after extinction

There is a growing body of evidence that the hippocampus is critical for context-dependent memory retrieval. In the present study, we used Pavlovian fear conditioning in rats to examine the role of the dorsal hippocampus (DH) in the context-specific expression of fear memory after extinction (i.e., renewal). Pre-training electrolytic lesions of the DH blunted the expression of conditional freezing to an auditory conditional stimulus (CS), but did not affect the acquisition of extinction to that CS. In contrast, DH lesions impaired the context-specific expression of extinction, eliminating the renewal of fear normally observed to a CS presented outside of the extinction context. Post-extinction DH lesions also eliminated the context dependence of fear extinction. These results are consistent with those using pharmacological inactivation of the DH and suggest that the DH is required for using contextual stimuli to regulate the expression of fear to a Pavlovian CS after extinction.     

Role of the hippocampus in blocking and conditioned inhibition of the rabbit's nictitating membrane response.

Bilateral aspiration of the dorsal hippocampus produced a disrupttion of blocking of the rabbit's nictitating membrane response in Kamin's two-stage paradigm (Experiment 1) but had no effect on the formation of a Pavlovian conditioned inhibitor (Experiment 2). The results of Experiment 1 indicated that normal animals and those with cortical lesions given conditioning to a light-plus-tone conditioned stimulus (CS) gave conditioned responses (CRs) to both the light and the tone during nonreinforced presentations of each (test phase). If, however, compound conditioning was preceded by tone acquisition, only the tone elicited a CR during testing; that is, blocking was observed. In rabbits with hippocampal lesions, however, CRs were given to both the light and the tone during testing whether or not compound conditioning was preceded by tone acquisition. The data from Experiment 2 showed that rabbits with hippocampal lesions could discriminate as well as normal rabbits and those with cortical lesions between a light (CS+) and light plus tone (CS-). In addition, when the inhibitory tone was subsequently paired with the unconditioned stimulus in retardation testing, animals in all three lesion conditions acquired the CR at the same rate. Thus, it appears that hippocampal lesions do not disrupt conditioned inhibition. The results of these experiments were taken as support for the view that the hippocampus is responsible for "tuning out" stimuli that have no adaptive value to the organism.     

Infusion of lidocaine into the dorsal hippocampus before or after the shock training phase impaired conditioned freezing in a two-phase training task of contextual fear conditioning

Learning in a contextual fear conditioning task involves forming a context representation and associating it with a shock. The dorsal hippocampus (DH) is implicated in representing the context, but whether it also has a role in associating the context and shock is unclear. To address this issue, male Wistar rats were trained on the task by a two-phase training paradigm, in which rats learned the context representation on day 1 and then reactivated it to associate with the shock on day 2; conditioned freezing was tested on day 3. Lidocaine was infused into the DH at various times in each of the two training sessions. Results showed that intra-DH infusion of lidocaine shortly before or after the context training session on day 1 impaired conditioned freezing, attesting to the DH involvement in context representation. Intra-DH infusion of lidocaine shortly before or after the shock training session on day 2 also impaired conditioned freezing. This deficit was reproduced by infusing lidocaine or APV (alpha-amino-5-phosphonovaleric acid) into the DH after activation of the context memory but before shock administration. The deficit was not due to drug-induced state-dependency, decreased shock sensitivity or reconsolidation failure of the contextual memory. These results suggest that in contextual fear conditioning integrity of the DH is required for memory processing of not only context representation but also context-shock association.     

Multiunit changes in hippocampus and medial geniculate body in free-behaving rats during acquisition and retention of a conditioned response to a tone

Multiunit activity (MUA) was recorded chronically in the hippocampus (CA3) and the medial geniculate body (mMG) during habituation to a tone followed by conditioning (tone paired with footshock) or pseudoconditioning (tone/footshock unpaired) in rats previously trained in a lever-pressing for food task (VI 60). In the conditioned group pairing tones with footshocks rapidly induced an increase in the initial CS-evoked response in the mMG, followed by the emergence of a hippocampal response and a marked conditioned suppression of lever-pressing to the tone. In contrast, in the pseudoconditioned group, the stimulus induced only transient cellular changes in the hippocampus and in the mMG, while no behavioral suppression to the tone could be seen. Moreover, presentations of the CS 45 days later induced multiunit and behavioral responses in both structures, only in the conditioned group. These results are used for discussion of the role of learning-induced changes in the sensory structure (mMG) as compared with changes in an associative structure (hippocampus), during acquisition and retention of a conditioned response.     

Roles of hippocampal GABA(A) and muscarinic receptors in consolidation of context memory and context-shock association in contextual fear conditioning: a double dissociation study

Contextual fear conditioning involves forming a context representation and associating it to a shock, both of which involved the dorsal hippocampus (DH) according to our recent findings. This study tested further whether the two processes may rely on different neurotransmitter systems in the DH. Male Wistar rats with cannula implanted into the DH were subjected to a two-phase training paradigm of contextual fear conditioning to separate context learning from context-shock association in two consecutive days. Immediately after each training phase, different groups of rats received bilateral intra-DH infusion of the GABA(A) agonist muscimol, 5HT(1A) agonist 8-OH-DPAT, NMDA antagonist APV or muscarinic antagonist scopolamine at various doses. On the third day, freezing behavior was tested in the conditioning context. Results showed that intra-DH infusion of muscimol impaired conditioned freezing only if it was given after context learning. In contrast, scopolamine impaired conditioned freezing only if it was given after context-shock training. Posttraining infusion of 8-OH-DPAT or APV had no effect on conditioned freezing when the drug was given at either phase. These results showed double dissociation for the hippocampal GABAergic and cholinergic systems in memory consolidation of contextual fear conditioning: forming context memory required deactivation of the GABA(A) receptors, while forming context-shock memory involved activation of the muscarinic receptors.     

Temporary inactivation of dorsal hippocampus attenuates explicitly nonspatial, unimodal, contextual fear conditioning

The current study examined the effects of temporary inactivation of the DH on freezing, rearing, ambulating, grooming, and whisking behavior in an explicitly nonspatial contextual fear conditioning paradigm in which olfactory stimuli served as temporally and spatially diffuse contexts. Prior either to training, testing, or both, male Sprague–Dawley rats received bilateral microinfusions of saline or the GABA_A agonist muscimol into the DH. Results indicate that temporary inactivation of DH produced both anterograde and retrograde deficits in contextually conditioned freezing, while sparing the acquisition and expression of freezing to a discrete auditory or olfactory CS. These data suggest that there is a decidedly nonspatial component to the role of DH in contextual conditioning, and that olfactory contextual conditioning is a fruitful means of further exploring this function.     

Role of amygdala and hippocampus in the neural circuit subserving conditioned defeat in Syrian hamsters

We examined the roles of the amygdala and hippocampus in the formation of emotionally relevant memories using an ethological model of conditioned fear termed conditioned defeat (CD). Temporary inactivation of the ventral, but not dorsal hippocampus (VH, DH, respectively) using muscimol disrupted the acquisition of CD, whereas pretraining VH infusions of anisomycin, a protein synthesis inhibitor, failed to block CD. To test for a functional connection between the VH and basolateral amygdala (BLA), we used a classic functional connectivity design wherein injections are made unilaterally in brain areas either on the same or opposite sides of the brain. A functional connection between the BLA and VH necessary for the acquisition of CD could not be found because unilateral inactivation of either BLA alone (but not either VH alone) was sufficient to disrupt CD. This finding suggested instead that there may be a critical functional connection between the left and right BLA. In our final experiment, we infused muscimol unilaterally in the BLA and assessed Fos immunoreactivity on the contralateral side following exposure to social defeat. Inactivation of either BLA significantly reduced defeat-induced Fos immunoreactivity in the contralateral BLA. These experiments demonstrate for the first time that whereas the VH is necessary for the acquisition of CD, it does not appear to mediate the plastic changes underlying CD. There also appears to be a critical interaction between the two BLAs such that bilateral activation of this brain area must occur in order to support fear learning in this model, a finding that is unprecedented to date.Our laboratory has taken a novel approach to examine the behavioral and physiological changes that accompany social experience by studying a striking behavioral response that is exhibited following social defeat. When a Syrian hamster is paired with a larger, more aggressive opponent and is defeated, it subsequently becomes highly submissive and fails to defend its own home cage even against a smaller, nonaggressive intruder. We call this change in the behavior of the defeated hamster conditioned defeat (CD) (Portegal et al. 1993) and believe that it is a valuable model with which to study neural and behavioral plasticity following exposure to a biologically relevant stressor.One of the critical structures subserving CD is the amygdala; temporary inactivation of its major subnuclei, including the basolateral amygdala (BLA), blocks the acquisition of CD (Jasnow and Huhman 2001). Together with the findings that protein synthesis inhibition in the BLA effectively disrupts CD (Markham and Huhman 2008) and that overexpression of cAMP response element binding protein (CREB) in the BLA enhances CD (Jasnow et al. 2005), the data support the hypothesis that the BLA is a critical site for plasticity related to CD.One brain region that we have largely overlooked, but which receives considerable attention for its role in learning and memory, is the hippocampus. Several groups have now gathered anatomical and behavioral data demonstrating functionally specific dissociation between the dorsal (DH) and ventral (VH) regions of the hippocampus (Risold and Swanson 1996; Moser and Moser 1998; Bannerman et al. 2004; McEown and Treit 2009). While the DH is critical for spatial relationships (O''Keefe and Nadel 1978; Moser et al. 1993; Eichenbaum 1996) and has been shown to play an important role in social recognition in hamsters (Lai et al. 2005), the VH appears to be involved in the production of behaviors produced in response to aversive stimuli (Trivedi and Coover 2004; Pentkowski et al. 2006).Considering how critically important the hippocampus and amygdala are in relation to fear and memory, some studies are beginning to suggest that these areas may functionally interact to modulate memory function (Akirav and Richter-Levin 2002; McGaugh et al. 2002; McGaugh 2004; Vouimba et al. 2007). The BLA projects to the hippocampus (Amaral and Insausti 1992), and high-frequency stimulation of the BLA combined with tetanic stimulation of the perforant pathway facilitates hippocampal long-term potentiation (LTP) (Ikegaya et al. 1996). Additionally, electrolytic lesions of the VH produce a deficit in the acquisition of fear to a contextual conditioned stimulus, and NMDA lesions of the BLA cause a nonselective deficit in the acquisition of fear to both contextual and acoustic conditioned stimuli (Maren and Fanselow 1995). Although our laboratory has previously demonstrated that the BLA is critically involved in the acquisition of CD (Jasnow and Huhman 2001), the role of the hippocampus has yet to be investigated. The aim of the present study was to examine whether the VH and DH are involved in mediating CD and also to determine whether there is a functional interaction between the hippocampus and the amygdala in the acquisition of CD.     

Lesions of Periaqueductal Gray Dissociate-Conditioned Freezing From Conditioned Suppression Behavior in Rats

It is commonly assumed that suppression of an ongoing behavior is an indirect measure of freezing behavior. We tested whether conditioned suppression and freezing are the same or distinct conditioned responses. Rats were trained to press a bar for food and then given fear-conditioning sessions in which a tone was paired with a foot shock (two pairings a day for 2 days). They then received either sham or electrolytic lesions of the periaqueductal gray (PAG). Post-training PAG lesions blocked freezing to the conditioned stimulus (CS), but had no effect on the suppression of operant behavior to the same CS. Thus, conditioned suppression and freezing, which both cause a cessation in activity, appear to be mediated by separate processes.     

Hippocampal associative cellular responses: dissociation with behavioral responses revealed by a transfer-of-control technique

Multiunit activity was recorded in the CA3 field of the dorsal hippocampus in freely moving rats during classical conditioning and subsequent presentation of the CS on operant baselines for food reward as well as shock avoidance. Rats were first trained in a nonsignaled bar-pressing-dependent shock omission task and in a food-motivated lever-pressing task (60-s VI). Five sessions with presentations of a previously habituated tone as a CS paired with footshock as a US were then given. Testing was carried out by presenting the CS alone while behavioral responses were maintained by reinforcement in both instrumental tasks on alternate sessions. As expected, the CS induced a marked suppression of lever pressing for food reward and a marked enhancement of bar-pressing for shock avoidance. The analysis of the frequency of multiunit discharges to the CS revealed that the hippocampal cellular responses established during classical conditioning were maintained while two different behavioral responses were exhibited to the CS. The results showed that the associative response of hippocampal neurons may be dissociated from the Pavlovian conditioned responses the CS elicits. They support the hypothesis that hippocampal cellular responses represent a neural index of the acquired CS-US associative representation.     

Time Cues Block the CS, but the CS Does Not Block Time Cues

Analysis of conditioned defensive freezing in rats revealed that prior pairings of a tone conditioned stimulus (CS) and footshock in Context 1 at Time 1 failed to give that tone CS the power to block conditioning to Context 2 at Time 2. This failure of an excitatory CS to block conditioning of time cues was not reciprocal. When the stimulus roles were reversed, excitatory time cues blocked conditioning to the tone CS. This asymmetry in blocking is best explained by the notion that time cues have special access to the association-formation mechanism.     

Selective hippocampal lesions disrupt a novel cue effect but fail to eliminate blocking in rabbit eyeblink conditioning

The classical conditioning task of blocking involves the adding of a novel but redundant stimulus to a previously trained stimulus. Both blocking and novelty detection are thought to involve the hippocampus. Previously, Solomon (1977) found that nonselective aspiration lesions of the hippocampal region eliminated blocking in rabbit eyeblink conditioning. We tested the effects of selective ibotenic acid lesions of the hippocampus on blocking, as well as on novelty detection, when training is switched from a tone conditioned stimulus (CS) to a compound tone-light CS in eyeblink conditioning. Selective hippocampal lesions did not eliminate blocking but did lead to a facilitation of conditioned response (CR) acquisition to the tone and to the light, but not to the tone-light compound. Selective hippocampal lesions disrupted a CR decrement observed in sham surgical controls when transferred from tone training to tone-light training. It appears that although selective hippocampal lesions do not eliminate blocking in eyeblink conditioning, they do disrupt novelty detection and may facilitate learning to a previously blocked cue.     

Footshock intensity and generalization in contextual and auditory-cued fear conditioning in the rat

The relationship between US (footshock) intensity and the two conditioned freezing responses (to acoustic CS and to "context") was investigated in fear conditioning. Administered footshock intensity was 0.00, 0.15, 0.30, 0.60, 0.90, and 1.20 mA to six different groups of 70-day-old male Albino Wistar rats. To measure contextual freezing, the animals were again placed inside the conditioning apparatus without acoustic CS and US presentation. To measure acoustic CS freezing, the animals were placed in a totally different apparatus and only the acoustic CS was presented. The 0.15 mA footshock intensity was not sufficient to condition the animals, in fact no freezing was exhibited as in the non-shocked control group. The 0.30 mA footshock intensity was sufficient only to condition the animals to the acoustic CS, whereas the 0.60 mA was sufficient to condition the animals both to acoustic CS and to context. Footshock intensities (0.90 and 1.20 mA) did not elicit any significant increase in conditioned freezing for either acoustic CS or context but at the highest one the generalization phenomenon appeared (freezing in the different context before presentation of acoustic CS). Acoustic CS freezing to all over-threshold intensities was longer than that to context. In conclusion, freezing responses to acoustic CS and context after increasing footshock intensities follow distinct patterns, and intermediate footshock intensities (0.60 and 0.90 mA) appear to be the most useful for eliciting conditioned freezing responses to acoustic CS and to context without inducing a generalized fear status contamination.     

Temporal dynamics of Arc gene induction in hippocampus: relationship to context memory formation

Past studies have proposed a role for the hippocampus in the rapid encoding of context memories. Despite this, there is little data regarding the molecular processes underlying the stable formation of a context representation that occurs in the time window established through such behavioral studies. One task that is useful for investigating the rapid encoding of context is contextual fear conditioning (CFC). Behavioral studies demonstrate that animals require approximately 30 s of exploration prior to a footshock to form a contextual representation supporting CFC. Thus, any potential molecular process required for the stabilization of the cellular representation for context must be activated within this narrow and behaviorally defined time window. Detection of the immediate-early gene Arc presents an ideal method to assess the activation of specific neuronal ensembles, given past studies showing the context specific expression of Arc in CA3 and CA1 subfields and the role of Arc in hippocampal long-term synaptic plasticity. Therefore, we examined the temporal dynamics of Arc induction within the hippocampus after brief context exposure to determine whether experience-dependent Arc expression could be involved in the rapid encoding of incidental context memories. We found that the duration of context exposure differentially activated Arc expression in hippocampal subfields, with CA3 showing rapid engagement within as little as 3 s of exposure. By contrast, Arc induction in CA1 required 30 s of context exposure to reach maximal levels. A parallel behavioral experiment revealed that 30 s, but not 3 s, exposure to a context resulted in strong conditioned freezing 24 h later, consistent with past studies from other laboratories. The current study is the first to examine the rapid temporal dynamics of Arc induction in hippocampus in a well-defined context memory paradigm. These studies demonstrate within 30 s of context exposure Arc is fully activated in CA3 and CA1, suggesting that the engagement of plastic processes requiring Arc function (such as long-term potentiation) occurs within the same temporal domain as that required for behavioral conditioning.     

Social transmission of Pavlovian fear: fear-conditioning by-proxy in related female rats

Pairing a previously neutral conditioned stimulus (CS; e.g., a tone) to an aversive unconditioned stimulus (US; e.g., a foot-shock) leads to associative learning such that the tone alone will elicit a conditioned response (e.g., freezing). Individuals can also acquire fear from a social context, such as through observing the fear expression of a conspecific. In the current study, we examined the influence of kinship/familiarity on social transmission of fear in female rats. Rats were housed in triads with either sisters or non-related females. One rat from each cage was fear conditioned to a tone CS+ shock US. On day two, the conditioned rat was returned to the chamber accompanied by one of her cage mates. Both rats were allowed to behave freely, while the tone was played in the absence of the foot-shock. The previously untrained rat is referred to as the fear-conditioned by-proxy (FCbP) animal, as she would freeze based on observations of her cage-mate’s response rather than due to direct personal experience with the foot-shock. The third rat served as a cage-mate control. The third day, long-term memory tests to the CS were performed. Consistent with our previous application of this paradigm in male rats (Bruchey et al. in Behav Brain Res 214(1):80–84, 2010), our results revealed that social interactions between the fear conditioned and FCbP rats on day two contribute to freezing displayed by the FCbP rats on day three. In this experiment, prosocial behavior occurring at the termination of the cue on day two was significantly greater between sisters than their non-sister counterparts, and this behavior resulted in increased freezing on day three. Our results suggest that familiarity and/or kinship influences the social transmission of fear in female rats.     

Neonatal ventral hippocampus lesions disrupt extra-dimensional shift and alter dendritic spine density in the medial prefrontal cortex of juvenile rats

Recent data showed that neonatal ventral hippocampus (VH) lesions, an approach used to model schizophrenia symptoms in rodents, produce premature deficits of working memory believed to be associated with early medial prefrontal cortex (mPFC) maldevelopment. This experiment expands the investigation of mPFC integrity in juvenile rats with neonatal VH lesions by assessing behavioral flexibility and dendritic spine density. Sixteen Sprague-Dawley male pups received bilateral microinjections of ibotenic acid in the VH or SHAM surgery on postnatal day (PND) 6. On PND 29 and 30, rats were subjected to a spatial shift task in a cross-maze; an attentional set-shifting task was then administered on two consecutive days, between PND 33 and PND 35. Rats were sacrificed at PND 36 and dendritic spine density in the mPFC was assessed using Golgi-Cox staining procedure. Results revealed impaired extra-dimensional shift in VH-lesioned rats and inconsistent reversal discrimination outcomes. Although lesioned animals displayed intact performance in the spatial shift, rates of perseverative responses were higher than normal in this task. Neonatal VH damage resulted in lower dendritic spine density in the mPFC than measured in control brains; however, no significant correlation was found between this outcome and behavioral data. Juvenile morphological and cognitive perturbations are consistent with the early emergence of mPFC anomalies following neonatal VH lesions. Results are discussed in relation with potential common mechanisms linking pre- and post-pubertal onsets of behavioral dysfunction.     

Comparison of Single Unit Responses to Tone, Light, and Compound Conditioned Stimuli during Rabbit Classical Eyeblink Conditioning

Unit recordings and lesion studies have implicated the cerebellum as an essential site for the acquisition and maintenance of the conditioned eyeblink response. The current study looked at the neural characteristics of conditioned stimulus (CS) processing in the interpositus nucleus of the cerebellum after training New Zealand white rabbits (Oryctolagus cuniculus) in one of two conditioning paradigms: (a) compound conditioning (CMP), a compound CS consisting of light and tone paired with an air puff unconditioned stimulus (US); or (b) stimulus compounding (ALT), alternating blocks of tone CS and light CS trials paired with the air puff US. Single unit responses were recorded during five sessions after the animals had reached an asymptotic level of responding. Animals were tested for behavioral and neural responses to CS alone trials that included tone alone, light alone, and compound tone-light trials. For the CMP group, the compound CS elicited 80 to 90% conditioned eyeblink responses (CRs), whereas the individual tone and light CSs elicited only 40 to 50% CRs. For the ALT group, all three CSs (tone, light, and compound) elicited very high levels of responding of at least 80% CRs. For the CMP group, there were roughly equal numbers of cells responding to all of the CSs. This includes cells that responded exclusively to one, and only one, of the three stimuli and also those cells that responded to combinations of two or more. Cells from the ALT group were far more likely to respond exclusively to only one of the CSs. Both the behavioral and physiological results suggest that the compound tone-light stimulus was processed as a distinct stimulus, separate from the component tone and light. These results are discussed in the context of multisensory processing.     

Reevaluating the role of the medial prefrontal cortex in delay eyeblink conditioning

It has been proposed that the medial prefrontal cortex (mPFC) is not necessary for delay eyeblink conditioning (DEC). Here, we investigated the involvement of the mPFC in DEC with a soft or loud tone as the conditioned stimulus (CS) by using electrolytic lesions or muscimol inactivation of guinea pig mPFC. Interestingly, when a soft tone was used as a CS, electrolytic lesions of the mPFC significantly retarded acquisition of the conditioned response (CR), and muscimol infusions into mPFC distinctly inhibited the acquisition and expression of CR, but had no significant effect on consolidation of well-learned CR. In contrast, both electrolytic lesions and muscimol inactivation of mPFC produced no significant deficits in the CR when a loud tone was used as the CS, or in the unconditioned response (UR) when a soft or loud tone was used as the CS. These results demonstrate that the mPFC is essential for the DEC with the soft tone CS but not for the DEC with the loud tone CS.     

Overt behavior and ultrasonic vocalization in a fear conditioning paradigm: a dose-response study in the rat

The behavioral analysis of laboratory rats is usually confined to the level of overt behavior, like locomotion, behavioral inhibition, instrumental responses, and others. Apart from such visible outcome, however, behaviorally relevant information can also be obtained when analyzing the animals' ultrasonic vocalization, which is typically emitted in highly motivational situations, like 22-kHz calls in response to acute or conditioned threat. To further investigate such vocalizations and their relationship with overt behavior, we tested male Wistar rats in a paradigm of Pavlovian fear conditioning, where a tone stimulus (CS) was preceding an aversive foot-shock (US) in a distinct environment. Importantly, the shock dose was varied between groups (0-1.1 mA), and its acute and conditioned outcome were determined. The analysis of visible behavior confirms the usefulness of immobility as a measure of fear conditioning, especially when higher shock doses were used. Rearing and grooming, on the other hand, were more useful to detect conditioned effects with lower shock levels. Ultrasonic vocalization occurred less consistently than changes in overt behavior; however, dose-response relationships were also observed during the phase of conditioning, for example, in latency, call rate and lengths, intervals between calls, and sound amplitude. Furthermore, total calling time (and rate) were highly correlated with overt behavior, namely behavioral inhibition as measured through immobility. These correlations were observed during the phase of fear conditioning, and the subsequent tests. Importantly, conditioned effects in overt behavior were observed, both, to the context and to the CS presented in this context, whereas conditioned vocalization to the context was not observed (except for one rat). In support and extent of previous results, the present data show that a detailed analysis of ultrasonic vocalization can substantially broaden and refine the spectrum of analysis in behavioral work with rats, since it can provide information about situational-, state-, and subject-dependent factors which are partly distinct from what is visible to the experimenter.     

Fornix, medial prefrontal cortex, nucleus accumbens, and mediodorsal thalamic nucleus: roles in a fear-based context discrimination task

The goal of the present study was to evaluate the contributions of various brain structures anatomically and functionally linked to the hippocampus and amygdala in a fear-based context discrimination task. The brain areas of interest included the fornix, medial prefrontal cortex, mediodorsal (MD) thalamic nucleus, and nucleus accumbens. Damage to the MD thalamic nucleus and medial prefrontal cortex produced the largest impairment in context-specific fear responses. Damage to the fornix impaired some fear responses (freezing, ultrasonic vocalizations, defecation, and approach/avoidance) while leaving conditioned fear expression of heart rate and urination unaltered. Damage to the nucleus accumbens was also coupled with deficits in the discriminative expression of some (heart rate, urination, and ultrasonic vocalizations) but sparing of context-appropriate freezing, defecation, and approach/avoidance behaviors.     

Conditioned memory modulation, freezing, and avoidance as measures of amygdala-mediated conditioned fear

Three conditioned aversive responses were used to infer the existence of an unobservable central state of "conditioned fear," and the roles of certain amygdala subregions in producing these responses were investigated. Rats received tone-shock pairings in one compartment of a shuttle box and no tones or shocks in the other, distinctive, compartment. They were then trained to find food in one arm of a Y-maze. After the final training trial they were exposed to different sets of stimuli in the shuttle box with no shock. Twenty-four hours later rats that had received immediate posttraining exposure to the conditioned stimuli (in the shock-paired compartment) made significantly more correct responses on the Y-maze than rats that had been exposed to the neutral stimuli (in the no-shock compartment) or rats that had received delayed posttraining exposure to the conditioned stimuli. This constitutes a demonstration of posttraining memory modulation by conditioned aversive stimuli. Freezing increased during posttraining exposure to the conditioned stimuli compared to the neutral stimuli. When subsequently allowed to move freely between the two compartments, the rats in all groups also showed significant conditioned avoidance of the compartment containing the conditioned stimuli. In a second experiment the effects of lesions confined to specific parts of the amygdala on the three conditioned responses (memory modulation, freezing, avoidance) were tested. Lesions of the central nucleus impaired all three conditioned responses; lesions of the medial nucleus impaired conditioned modulation and avoidance. These lesions had no effect on freezing during the training trials. Lesions of the lateral and basolateral nuclei attenuated freezing during both training and testing. The findings suggest that the central and medial nuclei of the amygdala may be important parts of neural circuits mediating conditioned responses that constitute conditioned aversive states, but that conditioned freezing may be mediated independently.