Subsequent Fear Conditioning Research Articles

A mouse model of stress-enhanced fear learning demonstrates extinction-sensitive and extinction-resistant effects of footshock stress

Stressful experiences can cause long-lasting sensitization of fear and anxiety that extends beyond the circumstances of the initial trauma. The neural mechanisms of these stress effects have been studied extensively in rats using the stress-enhanced fear learning (SEFL) paradigm, in which exposure to footshock stress potentiates subsequent fear conditioning. Here we establish a mouse version of the SEFL paradigm. Male and female 129s6 mice received four 1-mA footshocks or equivalent context exposure without shock. Shock exposure induced Pavlovian fear conditioning to the shock context and produced three more general effects: (1) suppression of open field exploration, (2) potentiated unconditioned fear of a novel tone stimulus, and (3) enhanced fear conditioning in a novel context. To determine whether these effects of footshock stress reflect generalized Pavlovian fear conditioning versus nonassociative fear sensitization, some mice received extinction training in the footshock stress context, which reduced contextual fear to the levels of unstressed control mice. Extinction restored normal open field exploration, suggesting that this effect of stress reflects generalized Pavlovian fear. In contrast, extinction failed to attenuate stress-enhanced fear, indicating that stress-enhanced fear is nonassociative and mechanistically distinct from Pavlovian fear conditioning. The effects of footshock stress were similar in male and female mice, although female mice displayed larger acute responses to fear-inducing stimuli than did males. The results demonstrate that footshock stress influences emotional behavior through distinct associative and nonassociative mechanisms, which likely involve unique neural underpinnings.

Effects of Methamphetamine Exposure on Fear Learning and Memory in Adult and Adolescent Rats.

Methamphetamine (meth) use is often comorbid with anxiety disorders, with both conditions predominant during adolescence. Conditioned fear extinction is the most widely used model to study the fear learning and regulation that are relevant for anxiety disorders. The present study investigates how meth binge injections or meth self-administration affect subsequent fear conditioning, extinction and retrieval in adult and adolescent rats. In experiment 1, postnatal day 35 (P35-adolescent) and P70 (adult) rats were intraperitoneally injected with increasing doses of meth across 9days. At P50 or P85, they underwent fear conditioning followed by extinction and test. In experiments 2a-c, P35 or P70 rats self-administered meth for 11days then received fear conditioning at P50 or P85, followed by extinction and test. We observed that meth binge exposure caused a significant disruption of extinction retrieval in adult but not adolescent rats. Interestingly, meth self-administration in adolescence or adulthood disrupted acquisition of conditioned freezing in adulthood. Meth self-administration in adolescence did not affect conditioned freezing in adolescence. These results suggest that intraperitoneal injections of high doses of meth and meth self-administration have dissociated effects on fear conditioning and extinction during adulthood, while adolescent fear conditioning and extinction are unaffected.

Inflammatory neonatal pain disrupts maternal behavior and subsequent fear conditioning in a rodent model.

Infants spending extended time in the neonatal intensive care unit are at greater risk of developing a variety of mental health problems later in life, possibly due to exposure to painful/stressful events. We used a rodent model of inflammatory neonatal pain to explore effects on fear conditioning, somatosensory function and maternal behavior. Hindpaw injections of 2% λ-carrageenan on postnatal days 1 and 4 produced an attenuation in conditioned freezing during the postweaning period, similar to our previous work with acute pain, but did not cause lasting impacts on contextual freezing nor somatosensory function. Additionally, we assessed maternal behavior to observe dam-pup interactions during the neonatal period. Results showed dams of litters which experienced pain spent similar amounts of time with pups as undisturbed controls. However, the specific behaviors differed per condition. Dams of pain litters exhibited less time licking/grooming, but more time nursing than controls. These results suggest changes in maternal care following pain could be a contributing factor underlying the long-term effects of neonatal trauma. Furthermore, our laboratory has previously shown acute, but not inflammatory pain, disrupted conditioned freezing; the current experiment observed the long-term effects of neonatal inflammatory pain on conditioned fear using a weak conditioning protocol.

Facilitation of fear learning by prior and subsequent fear conditioning

Classical fear conditioning is perhaps the premier model system used to study the neurobiological basis of memory formation. Prior work has resulted in a good understanding of both the molecular mechanisms and neural circuits supporting this form of learning. However, much of what is known about these mechanisms comes from studies in which fear memory is acquired using a single, isolated training session. Given that we cannot divorce the acquisition of new information from the backdrop on which it occurs, studies are needed to determine how the acquisition of fear memory is affected by other learning events. Here, we used rats to describe the time course by which auditory fear conditioning can facilitate learning to a different fear learning event, which alone is insufficient to support long-term fear memory. First, we replicated previous findings showing that although a single trial of light and shock produces little evidence of memory, two identical trials spaced 60 min or 24 h apart support long-term memory. Next, we report that a typical auditory fear conditioning session facilitated memory formation when rats were subsequently exposed to a single trial of light and shock 60 min or 24 h, but not 4 min, later. Finally, we show that learning can be enhanced retroactively if auditory fear conditioning occurs 60 min, but not 24 h, after a single light-shock pairing. These data demonstrate that a weak fear conditioning trial can be enhanced by prior and subsequent fear conditioning depending on the timing between training events.

Sex differences in discriminating between cues predicting threat and safety

Post-traumatic stress disorder (PTSD) is more prevalent in women than men. PTSD is characterized by overgeneralization of fear to innocuous stimuli and involves impaired inhibition of learned fear by cues that predict safety. While evidence indicates that learned fear inhibition through extinction differs in males and females, less is known about sex differences in fear discrimination and safety learning. Here we examined auditory fear discrimination in male and female rats. In Experiment 1A, rats underwent 1–3days of discrimination training consisting of one tone predicting threat (CS+; presented with footshock) and another tone predicting safety (CS−; presented alone). Females, but not males, discriminated between the CS+ and CS− after one day of training. After 2–3days of training, however, males discriminated whereas females generalized between the CS+ and CS−. In Experiment 1B, females showed enhanced anxiety-like behaviour and locomotor activity in the open field, although these results were unlikely to explain the sex differences in fear discrimination. In Experiment 2, we found no differences in shock sensitivity between males and females. In Experiment 3, males and females again discriminated and generalized, respectively, after three days of training. Moreover, fear generalization in females resulted from impaired safety learning, as shown by a retardation test. Whereas subsequent fear conditioning to the previous CS− retarded learning in males, females showed no such retardation. These results suggest that, while females show fear discrimination with limited training, they show fear generalization with extended training due to impaired safety learning.

Induction and Expression of Fear Sensitization Caused by Acute Traumatic Stress.

Fear promotes adaptive responses to threats. However, when the level of fear is not proportional to the level of threat, maladaptive fear-related behaviors characteristic of anxiety disorders result. Post-traumatic stress disorder develops in response to a traumatic event, and patients often show sensitized reactions to mild stressors associated with the trauma. Stress-enhanced fear learning (SEFL) is a rodent model of this sensitized responding, in which exposure to a 15-shock stressor nonassociatively enhances subsequent fear conditioning training with only a single trial. We examined the role of corticosterone (CORT) in SEFL. Administration of the CORT synthesis blocker metyrapone prior to the stressor, but not at time points after, attenuated SEFL. Moreover, CORT co-administered with metyrapone rescued SEFL. However, CORT alone without the stressor was not sufficient to produce SEFL. In these same animals, we then looked for correlates of SEFL in terms of changes in excitatory receptor expression. Western blot analysis of the basolateral amygdala (BLA) revealed an increase in the GluA1 AMPA receptor subunit that correlated with SEFL. Thus, CORT is permissive to trauma-induced changes in BLA function.

Differential impact of the first and second wave of a stress response on subsequent fear conditioning in healthy men

Stress is a process of multiple neuroendocrine changes over time. We examined effects of the first-wave and second-wave stress response on acquisition and immediate extinction of differential fear conditioning, assessed by skin conductance responses. In Experiment 1, we placed acquisition either close to the (second-wave) salivary cortisol peak, induced by a psychosocial stressor (experimental group, EG), or after non-stressful pretreatment (control group, CG). Contrary to predictions, groups did not differ in differential responding. In the EG only, mean differential responding was negatively correlated with cortisol increases. In Experiment 2, we placed conditioning near the first-wave stress response, induced by a cold pressor test (CPT), or after a warm-water condition (CG). CPT-stress increased extinction resistance. Moreover, acquisition performance after CPT was positively correlated with first-wave blood pressure increases. Data suggest that mediators of the first-wave stress response enhance fear maintenance whereas second-wave cortisol responsivity to stress might attenuate fear learning.

Stress differentially affects fear conditioning in men and women

Stress and fear conditioning processes are both important vulnerability factors in the development of psychiatric disorders. In behavioral studies considerable sex differences in fear learning have been observed after increases of the stress hormone cortisol. But neuroimaging experiments, which give insights into the neurobiological correlates of stress × sex interactions in fear conditioning, are lacking so far. In the current functional magnetic resonance imaging (fMRI) study, we tested whether a psychosocial stressor (Trier Social Stress Test) compared to a control condition influenced subsequent fear conditioning in 48 men and 48 women taking oral contraceptives (OCs). One of two pictures of a geometrical figure was always paired (conditioned stimulus, CS+) or never paired (CS-) with an electrical stimulation (unconditioned stimulus). BOLD responses as well as skin conductance responses were assessed. Sex-independently, stress enhanced the CS+/CS- differentiation in the hippocampus in early acquisition but attenuated conditioned responses in the medial frontal cortex in late acquisition. In early acquisition, stress reduced the CS+/CS- differentiation in the nucleus accumbens in men, but enhanced it in OC women. In late acquisition, the same pattern (reduction in men, enhancement in OC women) was found in the amygdala as well as in the anterior cingulate. Thus, psychosocial stress impaired the neuronal correlates of fear learning and expression in men, but facilitated them in OC women. A sex-specific modulation of fear conditioning after stress might contribute to the divergent prevalence of men and women in developing psychiatric disorders.

Error Correction in Latent Inhibition and its Disruption by Opioid Receptor Blockade with Naloxone

Latent inhibition refers to the retardation in the development of conditioned responding when a pre-exposed stimulus is used to signal an unconditioned stimulus. This effect is described by error-correction models as an attentional deficit and is commonly used as an animal model of schizophrenia. A series of experiments studied the role of error-correction mechanism in latent inhibition and its interaction with the endogenous opioid system. Systemic administration of the competitive opioid receptor antagonist naloxone before rats were pre-exposed to a target stimulus prevented latent inhibition of its subsequent fear conditioning; it was without effect on a non-pre-exposed stimulus and did not produce state-dependent learning (Experiments 1a and 1b). Naloxone did not reverse the latent inhibitory effect already accrued to a pre-exposed target. However, it did prevent the enhancement of latent inhibition by a long retention interval interpolated between its initial exposure and re-exposure (Experiment 2) or by a novel stimulus compounded with the pre-exposed target during re-exposure (Experiment 3). These results provide evidence that attentional loss in latent inhibition is instructed by an opioid-mediated error signal which diminishes with repeated stimulus exposures but recovers with the passage of time or reintroduction of novelty.

Endogenous Noradrenaline and Fear Extinction Learning in Humans

Endogenous Noradrenaline and Fear Extinction Learning in Humans

Reinforced stimulus preexposure effects as a function of US intensity: Implications for understanding the Hall–Pearce effect

Three conditioned suppression experiments examined the Hall–Pearce (1979) negative transfer effect in rats. Experiment 1 replicated the effect: CS-US weak pairings retarded subsequent fear conditioning to the CS as a result of CS-US strong pairings. The size of this retardation was less than that produced by non-reinforced CS presentations (latent inhibition). When the magnitude of the US weak was reduced in Experiment 2, the Hall–Pearce effect was greater than latent inhibition. Experiment 3 confirmed the findings of the two previous experiments, and demonstrated that magnitude of the Hall–Pearce negative transfer effect is inversely related to the magnitude of the US weak. From these findings it is suggested that the Hall–Pearce effect consists of a balance between a positive transfer of associative strength, and negative transfer based on CS- and US-preexposure effects.

Controllable versus uncontrollable stressors bi-directionally modulate conditioned but not innate fear

Fear conditioning and fear extinction play key roles in the development and treatment of anxiety-related disorders, yet there is little information concerning experiential variables that modulate these processes. Here we examined the impact of exposure to a stressor in a different environment on subsequent fear conditioning and extinction, and whether the degree of behavioral control that the subject has over the stressor is of importance. Rats received a session of either escapable (controllable) tail shock (ES), yoked inescapable (uncontrollable) tail shock (IS), or control treatment (home cage, HC) 7 days before fear conditioning in which a tone and foot shock were paired. Conditioning was measured 24 h later. In a second experiment rats received ES, IS or HC 24 h after contextual fear conditioning. Extinction then occurred every day beginning 7 days later until a criterion was reached. Spontaneous recovery of fear was assessed 14 days after extinction. IS potentiated fear conditioning when given before fear conditioning, and potentiated fear responding during extinction when given after conditioning. Importantly, ES potently interfered with later fear conditioning, decreased fear responding during fear extinction, and prevented spontaneous recovery of fear. Additionally, we examined if the activation of the ventral medial prefrontal cortex (mPFCv) by ES is critical for the protective effects of ES on later fear conditioning. Inactivation of the mPFCv with muscimol at the time of the initial experience with control prevented ES-induced reductions in later contextual and auditory fear conditioning. Finally, we explored if the protective effects of ES extended to an unconditioned fear stimulus, ferret odor. Unlike conditioned fear, prior ES increased the fear response to ferret odor to the same degree as did IS.

Cross-modal transfer of conditioned suppression in rats: Effects of US intensity and extinction of the initial conditioning task

In conditioned suppression of water licking behavior by rats, we obtained data indicating general transfer of fear conditioning. A series of experiments resulted in two major findings. First, pairing of a neutral stimulus with a shock in the initial conditioning task facilitated acquisition of subsequent fear conditioning to another neutral stimulus, if the conditioned fear of the initial task was extinguished prior to the second task and if equally strong shocks were employed in both conditioning tasks. Second, omission of the extinction treatment or employment of weaker shocks in the initial task resulted in retardation, rather than facilitation, of the second conditioning task. An application to human clinical settings is discussed.

Mechanisms of action of midazolam on expression of contextual fear in rats

Midazolam may suppress conditioned fear after an aversive event by disrupting the memory trace formed during conditioning, by altering the emotional part of the aversive event, or by the combination of both effects. The purpose of the present study was to determine whether affective-related processes contribute to the amnesic-like effects of midazolam on aversive events. The effects of acute administration of low doses of midazolam (0.37-3 mg kg(-1)) on fear conditioning (association between a neutral context and an aversive stimulus) and on innate anxiety in fearful surroundings were examined in rats. The effect of midazolam on the deleterious consequences of pre-exposure to the context (a non-aversive event) for subsequent fear conditioning was then compared with its effect on fear conditioning. The role of midazolam as an affective context was assessed by performing the testing phase under midazolam. Possible locomotor impairment or long-term effects of midazolam were controlled in additional experiments. Midazolam reduced both contextual fear conditioning and spontaneous fear. The deleterious effect of midazolam on pre-exposure to the context was of the same magnitude as its effect on the acquisition phase of fear conditioning. The effects of midazolam on both pre-exposure to the context and fear conditioning were unchanged when rats received a second injection of midazolam before the retention phase. Low doses of midazolam that do not impair locomotion suppress conditioned fear to the context by acting on memory processes rather than on affective or anxiolytic processes.

Fear conditioned potentiation of the acoustic blink reflex in patients with cerebellar lesions

OBJECTIVETo investigate whether the human cerebellum takes part in fear conditioned potentiation of the acoustic blink reflex.METHODSA group of 10 cerebellar patients (eight patients with lesions involving the medial cerebellum,...