Trace fear conditioning is reduced in the aging rat

Auditory trace fear conditioning is a hippocampus-dependent learning task that requires animals to associate an auditory conditioned stimulus (CS) and a fear-producing shock-unconditioned stimulus (US) that are separated by an empty 20-s trace interval. Previous studies have shown that aging impairs learning performance on hippocampus-dependent tasks. This study measured heart rate (HR) and freezing fear responses to determine if aging impairs hippocampus-dependent auditory trace fear conditioning in freely moving rats. Aging and Young rats received one long-trace fear conditioning session (10 trials). Each trial consisted of a tone-CS (5 s) and a shock-US separated by an empty 20-s trace interval. The next day rats received CS-alone retention trials. Young rats showed significantly larger HR and freezing responses on the initial CS-alone retention trials compared to the Aging rats. A control group of aging rats received fear conditioning trials with a short 1-s trace interval separating the CS and US. The Aging Short-Trace Group showed HR and freezing responses on the initial CS alone retention trials that were similar to the Young Long-Trace Group, but greater than the Aging Long-Trace Group. A second aging control group received unpaired CSs and USs, and showed no HR or freezing responses on CS-alone retention trials. These data show that HR and freezing are effective measures for detecting aging-related deficits in trace fear conditioning.     

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.     

The development of cued versus contextual conditioning in a predictable and an unpredictable human fear conditioning preparation

In this human fear conditioning study, the online development of conditioned US-expectancy to discrete cues and background contexts was measured in two groups. In the paired group (n=30), the CS was systematically followed by an aversive shock (US). In the unpaired group (n=30), CS and US were presented explicitly unpaired. Using US-expectancy ratings, we replicated the basic finding already illustrated in humans with startle modulation. In the paired group, the CS elicited more US-expectancy than the context, whereas in the unpaired group, the context elicited more US-expectancy than the CS. Interestingly, we also observed a trial-by-trial development of conditioning to the context in the unpaired group as indicated by a significant linear trend. This gradual development and the evidence for the role of US-expectancy in contextual fear add to the idea that cued and contextual fear rely on the same basic associative processes.     

Observational conditioning of fear to fear-relevant versus fear-irrelevant stimuli in rhesus monkeys

Two experiments examined whether superior observational conditioning of fear occurs in observer rhesus monkeys that watch model monkeys exhibit an intense fear of fear-relevant, as compared with fear-irrelevant, stimuli. In both experiments, videotapes of model monkeys behaving fearfully were spliced so that it appeared that the models were reacting fearfully either to fear-relevant stimuli (toy snakes or a toy crocodile), or to fear-irrelevant stimuli (flowers or a toy rabbit). Observer groups watched one of four kinds of videotapes for 12 sessions. Results indicated that observers acquired a fear of fear-relevant stimuli (toy snakes and toy crocodile), but not of fear-irrelevant stimuli (flowers and toy rabbit). Implications of the present results for the preparedness theory of phobias are discussed.     

The role of the nucleus basalis magnocellularis in fear conditioning consolidation in the rat

The nucleus basalis magnocellularis (NBM) is known to be involved in the memorization of several conditioned responses. To investigate the role of the NBM in fear conditioning memorization, this neural site was subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats that had undergone fear training to acoustic conditioned stimulus (CS) and context. TTX was stereotaxically administered to different groups of rats at increasing intervals after the acquisition session. Memory was assessed as the conditioned freezing duration measured during retention testing, always performed 72 and 96 h after TTX administration. In this way, there was no interference with normal NBM function during either acquisition or retrieval phases, allowing any amnesic effect to be due only to consolidation disruption. The results show that for contextual fear response memory consolidation, NBM functional integrity is necessary up to 24 h post-acquisition. On the other hand, NBM functional integrity was shown to be necessary for memory consolidation of the acoustic CS fear response only immediately after acquisition and not 24-h post-acquisition. The present findings help to elucidate the role of the NBM in memory consolidation and better define the neural circuits involved in fear memories.     

Olfactory fear conditioning induces field potential potentiation in rat olfactory cortex and amygdala

The widely used Pavlovian fear-conditioning paradigms used for studying the neurobiology of learning and memory have mainly used auditory cues as conditioned stimuli (CS). The present work assessed the neural network involved in olfactory fear conditioning, using olfactory bulb stimulation-induced field potential signal (EFP) as a marker of plasticity in the olfactory pathway. Training consisted of a single training session including six pairings of an odor CS with a mild foot-shock unconditioned stimulus (US). Twenty-four hours later, the animals were tested for retention of the CS as assessed by the amount of freezing exhibited in the presence of the learned odor. Behavioral data showed that trained animals exhibited a significantly higher level of freezing in response to the CS than control animals. In the same animals, EFPs were recorded in parallel in the anterior piriform cortex (aPC), posterior piriform cortex (pPC), cortical nucleus of the amygdala (CoA), and basolateral nucleus of the amygdala (BLA) following electrical stimulation of the olfactory bulb. Specifically, EFPs recorded before (baseline) and after (during the retention test) training revealed that trained animals exhibited a lasting increase (present before and during presentation of the CS) in EFP amplitude in CoA, which is the first amygdaloid target of olfactory information. In addition, a transient increase was observed in pPC and BLA during presentation of the CS. These data indicate that the olfactory and auditory fear-conditioning neural networks have both similarities and differences, and suggest that the fear-related behaviors in each sensory system may have at least some distinct characteristics.     

Scales for measuring fear of AIDS and homophobia

Two scales, one for measuring attitudes toward the fear of Acquired Immune Deficiency Syndrome (AIDS) and one for measuring attitude toward homosexuality, were constructed using Thurstone's method of equal-appearing intervals. After construction, the scales were given to 528 students at the University of Texas at Austin in the fall of 1985 to determine their respective reliabilities. Factor analyses were also done to determine what factors underlie the attitudes measured by the two scales and to determine if fear of AIDS and homophobia are in fact simply two facets of the same attitude: fear of homosexuals. The results indicated high reliabilities of both scales and a relatively low correlation between the two scales suggesting that the scales do measure different attitudes.     

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.     

Posttraining handling facilitates memory for auditory-cue fear conditioning in rats

A large number of studies have indicated that stress exposure or the administration of stress hormones and other neuroactive drugs immediately after a learning experience modulates the consolidation of long-term memory. However, there has been little investigation into how arousal induced by handling of the animals in order to administer these drugs affects memory. Therefore, the present study examined whether the posttraining injection or handling procedure per se affects memory of auditory-cue classical fear conditioning. Male Sprague-Dawley rats, which had been pre-handled on three days for 1 min each prior to conditioning, received three pairings of a single-frequency auditory stimulus and footshock, followed immediately by either a subcutaneous injection of a vehicle solution or brief handling without injection. A control group was placed back into their home cages without receiving any posttraining treatment. Retention was tested 24 h later in a novel chamber and suppression of ongoing motor behavior during a 10-s presentation of the auditory-cue served as the measure of conditioned fear. Animals that received posttraining injection or handling did not differ from each other but showed significantly less stimulus-induced movement compared to the non-handled control group. These findings thus indicate that the posttraining injection or handling procedure is sufficiently arousing or stressful to facilitate memory consolidation of auditory-cue classical fear conditioning.     

Dissociated roles for the lateral and medial septum in elemental and contextual fear conditioning

Extensive evidence indicates that the septum plays a predominant role in fear learning, yet the direction of this control is still a matter of debate. Increasing data suggest that the medial (MS) and lateral septum (LS) would be differentially required in fear conditioning depending on whether a discrete conditional stimulus (CS) predicts, or not, the occurrence of an aversive unconditional stimulus (US). Here, using a tone CS-US pairing (predictive discrete CS, context in background) or unpairing (context in foreground) conditioning procedure, we show, in mice, that pretraining inactivation of the LS totally disrupted tone fear conditioning, which, otherwise, was spared by inactivation of the MS. Inactivating the LS also reduced foreground contextual fear conditioning, while sparing the higher level of conditioned freezing to the foreground (CS-US unpairing) than to the background context (CS-US pairing). In contrast, inactivation of the MS totally abolished this training-dependent level of contextual freezing. Interestingly, inactivation of the MS enhanced background contextual conditioning under the pairing condition, whereas it reduced foreground contextual conditioning under the unpairing condition. Hence, the present findings reveal a functional dissociation between the LS and the MS in Pavlovian fear conditioning depending on the predictive value of the discrete CS. While the requirement of the LS is crucial for the appropriate processing of the tone CS-US association, the MS is crucial for an appropriate processing of contextual cues as foreground or background information.     

Histone acetylation rescues contextual fear conditioning in nNOS KO mice and accelerates extinction of cued fear conditioning in wild type mice

Epigenetic regulation of chromatin structure is an essential molecular mechanism that contributes to the formation of synaptic plasticity and long-term memory (LTM). An important regulatory process of chromatin structure is acetylation and deacetylation of histone proteins. Inhibition of histone deacetylase (HDAC) increases acetylation of histone proteins and facilitate learning and memory. Nitric oxide (NO) signaling pathway has a role in synaptic plasticity, LTM and regulation of histone acetylation. We have previously shown that NO signaling pathway is required for contextual fear conditioning. The present study investigated the effects of systemic administration of the HDAC inhibitor sodium butyrate (NaB) on fear conditioning in neuronal nitric oxide synthase (nNOS) knockout (KO) and wild type (WT) mice. The effect of single administration of NaB on total H3 and H4 histone acetylation in hippocampus and amygdala was also investigated. A single administration of NaB prior to fear conditioning (a) rescued contextual fear conditioning of nNOS KO mice and (b) had long-term (weeks) facilitatory effect on the extinction of cued fear memory of WT mice. The facilitatory effect of NaB on extinction of cued fear memory of WT mice was confirmed in a study whereupon NaB was administered during extinction. Results suggest that (a) the rescue of contextual fear conditioning in nNOS KO mice is associated with NaB-induced increase in H3 histone acetylation and (b) the accelerated extinction of cued fear memory in WT mice is associated with NaB-induced increase in H4 histone acetylation. Hence, a single administration of HDAC inhibitor may rescue NO-dependent cognitive deficits and afford a long-term accelerating effect on extinction of fear memory of WT mice.     

Anterograde amnesia for Pavlovian fear conditioning and the role of one-trial overshadowing: effects of preconditioning exposures to morphine in the rat

Four experiments studied anterograde deficits in Pavlovian fear conditioning following prolonged exposure to the mu-opioid receptor agonist morphine. Injections of morphine produced temporally graded anterograde amnesia characterized by deficits in contextual and conditioned-stimulus (CS) conditioning 1 or 7 days and selective impairment in CS conditioning 21 days after last injection. This anterograde deficit in conditioning did not recover across a retention interval, was absent when rats were tested immediately after conditioning, and required the presence of an auditory CS. These results suggest that anterograde deficits in Pavlovian fear conditioning emerged from differences in susceptibility to 1-trial overshadowing of context by CS.