Acquisition Of Auditory Fear Conditioning Research Articles

Angiotensin type 1a receptors on corticotropin-releasing factor neurons contribute to the expression of conditioned fear.

Although generally associated with cardiovascular regulation, angiotensin II receptor type 1a (AT1a R) blockade in mouse models and humans has also been associated with enhanced fear extinction and decreased post-traumatic stress disorder (PTSD) symptom severity, respectively. The mechanisms mediating these effects remain unknown, but may involve alterations in the activities of corticotropin-releasing factor (CRF)-expressing cells, which are known to be involved in fear regulation. To test the hypothesis that AT1a R signaling in CRFergic neurons is involved in conditioned fear expression, we generated and characterized a conditional knockout mouse strain with a deletion of the AT1a R gene from its CRF-releasing cells (CRF-AT1a R((-/-)) ). These mice exhibit normal baseline heart rate, blood pressure, anxiety and locomotion, and freeze at normal levels during acquisition of auditory fear conditioning. However, CRF-AT1a R((-/-)) mice exhibit less freezing than wild-type mice during tests of conditioned fear expression-an effect that may be caused by a decrease in the consolidation of fear memory. These results suggest that central AT1a R activity in CRF-expressing cells plays a role in the expression of conditioned fear, and identify CRFergic cells as a population on which AT1 R antagonists may act to modulate fear extinction.

Abnormal emotional learning in a rat model of autism exposed to valproic acid in utero.

Autism Spectrum Disorders (ASD) are complex neurodevelopmental disorders characterized by repetitive behavior and impaired social communication and interactions. Apart from these core symptoms, a significant number of ASD individuals display higher levels of anxiety and some ASD individuals exhibit impaired emotional learning. We therefore sought to further examine anxiety and emotional learning in an environmentally induced animal model of ASD that utilizes the administration of the known teratogen, valproic acid (VPA) during gestation. Specifically we exposed dams to one of two different doses of VPA (500 and 600 mg/kg) or vehicle on day 12.5 of gestation and examined the resultant progeny. Our data indicate that animals exposed to VPA in utero exhibit enhanced anxiety in the open field test and normal object recognition memory compared to control animals. Animals exposed to 500 mg/kg of VPA displayed normal acquisition of auditory fear conditioning, and exhibited reduced extinction of fear memory and normal litter survival rates as compared to control animals. We observed that animals exposed to 600 mg/kg of VPA exhibited a significant reduction in the acquisition of fear conditioning, a significant reduction in social interaction and a significant reduction in litter survival rates as compared to control animals. VPA (600 mg/kg) exposed animals exhibited similar shock sensitivity and hearing as compared to control animals indicating the fear conditioning deficit observed in these animals was not likely due to sensory deficits, but rather due to deficits in learning or memory retrieval. In conclusion, considering that progeny from dams exposed to rather similar doses of VPA exhibit striking differences in emotional learning, the VPA model may serve as a useful tool to explore the molecular and cellular mechanisms that contribute to not only ASD, but also emotional learning.

Chronic Antidepressant Treatment Impairs the Acquisition of Fear Extinction

Like fear conditioning, the acquisition phase of extinction involves new learning that is mediated by the amygdala. During extinction training, the conditioned stimulus is repeatedly presented in the absence of the unconditioned stimulus, and the expression of previously learned fear gradually becomes suppressed. Our previous study revealed that chronic treatment with a selective serotonin reuptake inhibitor (SSRI) impairs the acquisition of auditory fear conditioning. To gain further insight into how SSRIs affect fear learning, we tested the effects of chronic SSRI treatment on the acquisition of extinction. Rats were treated chronically (22 days) or subchronically (9 days) with the SSRI citalopram (10 mg/kg/day) before extinction training. The results were compared with those after chronic and subchronic treatment with tianeptine (10 mg/kg/day), an antidepressant with a different method of action. The expression of the NR2B subunit of the N-methyl-D-aspartate receptor in the amygdala was examined after behavioral testing. Chronic but not subchronic administration of citalopram impaired the acquisition of extinction and downregulated the NR2B subunit of the N-methyl-D-aspartate receptor in the lateral and basal nuclei of the amygdala. Similar behavioral and molecular changes were found with tianeptine treatment. These results provide further evidence that chronic antidepressant treatment can impair amygdala-dependent learning. Our findings are consistent with a role for glutamatergic neurotransmission in the final common pathway of antidepressant treatment.

The amygdala is not necessary for unconditioned stimulus inflation after Pavlovian fear conditioning in rats

The basolateral complex (BLA) and central nucleus (CEA) of the amygdala play critical roles in associative learning, including Pavlovian conditioning. However, the precise role for these structures in Pavlovian conditioning is not clear. Recent work in appetitive conditioning paradigms suggests that the amygdala, particularly the BLA, has an important role in representing the value of the unconditioned stimulus (US). It is not known whether the amygdala performs such a function in aversive paradigms, such as Pavlovian fear conditioning in rats. To address this issue, Experiments 1 and 2 used temporary pharmacological inactivation of the amygdala prior to a US inflation procedure to assess its role in revaluing shock USs after either overtraining (Experiment 1) or limited training (Experiment 2), respectively. Inactivation of the BLA or CEA during the inflation session did not affect subsequent increases in conditioned freezing observed to either the tone conditioned stimulus (CS) or the conditioning context in either experiment. In Experiment 3, NBQX infusions into the BLA impaired the acquisition of auditory fear conditioning with an inflation-magnitude US, indicating that the amygdala is required for associative learning with intense USs. Together, these results suggest that the amygdala is not required for revaluing an aversive US despite being required for the acquisition of fear to that US.

CAMP/PKA signaling and RIM1α mediate presynaptic LTP in the lateral amygdala

NMDA receptor-dependent long-term potentiation (LTP) of glutamatergic synaptic transmission in sensory pathways from auditory thalamus or cortex to the lateral amygdala (LA) underlies the acquisition of auditory fear conditioning. Whereas the mechanisms of postsynaptic LTP at thalamo-LA synapses are well understood, much less is known about the sequence of events mediating presynaptic NMDA receptor-dependent LTP at cortico-LA synapses. Here, we show that presynaptic cortico-LA LTP can be entirely accounted for by a persistent increase in the vesicular release probability. At the molecular level, we found that signaling via the cAMP/PKA pathway is necessary and sufficient for LTP induction. Moreover, by using mice lacking the active-zone protein and PKA target RIM1alpha (RIM1alpha(-/-)), we demonstrate that RIM1alpha is required for both chemically and synaptically induced presynaptic LTP. Further analysis of cortico-LA synaptic transmission in RIM1alpha(-/-) mice revealed a deficit in Ca(2+)-release coupling leading to a lower baseline release probability. Our results reveal the molecular mechanisms underlying the induction of presynaptic LTP at cortico-LA synapses and indicate that RIM1alpha-dependent LTP may involve changes in Ca(2+)-release coupling.

The Rostral Anterior Cingulate Cortex Modulates the Efficiency of Amygdala-Dependent Fear Learning

The rostral anterior cingulate cortex (rACC) and the amygdala consistently emerge from neuroimaging studies as brain regions crucially involved in normal and abnormal fear processing. To date, however, the role of the rACC specifically during the acquisition of auditory fear conditioning still remains unknown. The aim of this study is to investigate a possible top-down control of a specific rACC sub-region over amygdala activation during pavlovian fear acquisition. We performed excitotoxic lesions, temporal inactivation, and activation of a specific sub-region of the rACC that we identified by tracing studies as supporting most of the connectivity with the basolateral amygdala (r(Amy)-ACC). The effects of these manipulations over amygdala function were investigated with a classical tone-shock associative fear conditioning paradigm in the rat. Excitotoxic lesions and transient inactivation of the r(Amy)-ACC pre-training selectively produced deficits in the acquisition of the tone-shock associative learning (but not context). This effect was specific for the acquisition phase. However, the deficit was found to be transient and could be overcome by overtraining. Conversely, pre-training transient activation of the r(Amy)-ACC facilitated associative learning and increased fear expression. Our results suggest that a subregion of the rACC is key to gating the efficiency of amygdala-dependent auditory fear conditioning learning. Because r(Amy)-ACC inputs were confirmed to be glutamatergic, we propose that recruitment of this brain area might modulate overall basolateral amygdala excitatory tone during conditioned stimulus-unconditioned stimulus concomitant processing. In the light of clinical research, our results provide new insight on the effect of inappropriate rACC recruitment during emotional events.

Acute Selective Serotonin Reuptake Inhibitors Increase Conditioned Fear Expression: Blockade With a 5-HT 2C Receptor Antagonist

Selective serotonin reuptake inhibitors (SSRIs) effectively treat various anxiety disorders, although symptoms of anxiety are often exacerbated during early stages of treatment. We previously reported that acute treatment with the SSRI citalopram enhances the acquisition of auditory fear conditioning, which is consistent with the initial anxiogenic effects reported clinically. Here, we extend our findings by assessing the effects of acute SSRI treatment on the expression of previously acquired conditioned fear. Rats underwent fear conditioning drug-free. Tone-evoked fear responses were tested after drug treatment the following day. This protocol more closely resembles the clinical setting than pre-conditioning treatment, because it evaluates effects of treatment on a pre-existing fear rather than on the formation of a new fear memory. A single pre-testing injection of the SSRIs citalopram or fluoxetine significantly increased fear expression. There was no effect of the antidepressant tianeptine or the norepinephrine reuptake inhibitor tomoxetine, indicating that this effect is specific to SSRIs. The SSRI-induced enhancement in fear expression was not blocked by tropisetron, a 5-HT(3) receptor antagonist, but was blocked by SB 242084, a specific 5-HT(2C) receptor antagonist. Enhanced activation of 5-HT(2C) receptors might be a mechanism for the anxiogenic effects of SSRIs observed initially during treatment.

Extinction of auditory fear conditioning requires MAPK/ERK activation in the basolateral amygdala

Whereas the neuronal substrates underlying the acquisition of auditory fear conditioning have been widely studied, the substrates and mechanisms mediating the acquisition of fear extinction remain largely elusive. Previous reports indicate that consolidation of fear extinction depends on the mitogen-activated protein kinase/extracellular-signal regulated kinase (MAPK/ERK) signalling pathway and on protein synthesis in the medial prefrontal cortex (mPFC). Based on experiments using the fear-potentiated startle paradigm suggesting a role for neuronal plasticity in the basolateral amygdala (BLA) during fear extinction, we directly addressed whether MAPK/ERK signalling in the basolateral amygdala is necessary for the acquisition of fear extinction using conditioned freezing as a read-out. First, we investigated the regional and temporal pattern of MAPK/ERK activation in the BLA following extinction learning in C57Bl/6J mice. Our results indicate that acquisition of extinction is associated with an increase of phosphorylated MAPK/ERK in the BLA. Moreover, we found that inhibition of the MAPK/ERK signalling pathway by intrabasolateral amygdala infusion of the MEK inhibitor, U0126, completely blocks acquisition of extinction. Thus, our results indicate that the MAPK/ERK signalling pathway is required for extinction of auditory fear conditioning in the BLA, and support a role for neuronal plasticity in the BLA during the acquisition of fear extinction.

The selective serotonin reuptake inhibitor citalopram increases fear after acute treatment but reduces fear with chronic treatment: a comparison with tianeptine

BackgroundSelective serotonin reuptake inhibitors (SSRIs) are efficacious in the treatment of a variety of fear or anxiety disorders. Although they inhibit the reuptake of serotonin within hours of administration, therapeutic improvement only occurs after several weeks. In this study, we used fear conditioning to begin to understand how acute and chronic SSRI treatment might differentially affect well-characterized fear circuits. MethodsWe evaluated the effects of acute and chronic treatment with the SSRI citalopram on the acquisition of auditory fear conditioning. To further understand the role of serotonin in modulating fear circuits, we compared these effects with those of acute and chronic administration of the antidepressant tianeptine, a purported serotonin reuptake enhancer. ResultsWe found that acute administration of the SSRI citalopram enhanced acquisition, whereas chronic treatment reduced the acquisition of auditory fear conditioning. In comparison, treatment with tianeptine had no effect acutely but also reduced the acquisition of tone conditioning when administered chronically. ConclusionsOur findings with citalopram are consistent with the clinical effects of SSRI treatment seen in patients with anxiety disorders, in which anxiety is often increased during early stages of treatment and decreased after several weeks of treatment. The findings also indicate that auditory fear conditioning can be a useful tool in understanding differences in the effects of short-term and long-term antidepressant treatment with serotonergic medications.

Damage to the lateral and central, but not other, amygdaloid nuclei prevents the acquisition of auditory fear conditioning.

It is well established that the amygdala plays an essential role in Pavlovian fear conditioning, with the lateral nucleus serving as the interface with sensory systems that transmit the conditioned stimulus and the central nucleus as the link with motor regions that control conditioned fear responses. The lateral nucleus connects with the central nucleus directly and by way of several other amygdala regions, including the basal, accessory basal, and medial nuclei. To determine which of these regions is necessary, and thus whether conditioning requires the direct or one of the indirect intra-amygdala pathways, we made lesions in rats of the lateral, central, basal, accessory basal, and medial nuclei, as well as combined lesions of the basal and accessory basal nuclei and of the entire amygdala. Animals subsequently underwent fear conditioning trials in which an auditory conditioned stimulus was paired with a footshock unconditioned stimulus. Animals that received lesions of the lateral or central nucleus, or of the entire amygdala, were dramatically impaired, whereas the other lesions had little effect. These findings show that only the lateral and central nuclei are necessary for the acquisition of conditioned fear response to an auditory conditioned stimulus.