Freezing response-independent facilitation of fear extinction memory in the prefrontal cortex

Pharmacological stimulation of infralimbic cortex after fear conditioning facilitates subsequent fear extinction

The infralimbic (IL) division of the medial prefrontal cortex (mPFC) is a crucial site for the extinction of conditioned fear memories in rodents. Recent work suggests that neuronal plasticity in the IL that occurs during (or soon after) fear conditioning enables subsequent IL-dependent extinction learning. We therefore hypothesized that pharmacological activation of the IL after fear conditioning would promote the extinction of conditioned fear. To test this hypothesis, we characterized the effects of post-conditioning infusions of the GABAA receptor antagonist, picrotoxin, into the IL on the extinction of auditory conditioned freezing in male and female rats. In four experiments, we found that picrotoxin injections performed immediately, 24 h, or 13 days after fear conditioning reduced conditioned freezing to the auditory conditioned stimulus (CS) during both extinction training and extinction retrieval; this effect was observed up to two weeks after picrotoxin infusions. Interestingly, inhibiting protein synthesis inhibition in the IL immediately after fear conditioning prevented the inhibition of freezing by picrotoxin injected 24 h later. Our data suggest that the IL encodes an inhibitory memory during the consolidation of fear conditioning that is necessary for future fear suppression.

Pharmacological stimulation of infralimbic cortex after fear conditioning facilitates subsequent fear extinction

The infralimbic (IL) division of the medial prefrontal cortex (mPFC) is a crucial site for extinction of conditioned fear memories in rodents. Recent work suggests that neuronal plasticity in the IL that occurs during (or soon after) fear conditioning enables subsequent IL-dependent extinction learning. We therefore hypothesized that pharmacological activation of the IL after fear conditioning would promote the extinction of conditioned fear. To test this hypothesis, we characterized the effects of post-conditioning infusions of the GABAA receptor antagonist, picrotoxin, into the IL on extinction of auditory conditioned freezing in male and female rats. In four experiments, we found that picrotoxin injections performed immediately, 24 hours, or 13 days after fear conditioning reduced conditioned freezing to the auditory conditioned stimulus (CS) during both extinction training and extinction retrieval; this effect was observed up to two weeks after picrotoxin infusions. Interestingly, inhibiting protein synthesis inhibition in the IL immediately after fear conditioning prevented the inhibition of freezing by picrotoxin injected 24 hours later. Our data suggest that the IL encodes an inhibitory memory during the consolidation of fear conditioning that is necessary for future fear suppression.

Intra-mPFC injection of sodium butyrate promotes BDNF expression and ameliorates extinction recall impairment in an experimental paradigm of post-traumatic stress disorder

Objectives

Therapeutic strategies that facilitate extinction are promising in the treatment of post-traumatic stress disorder (PTSD). Brain-derived neurotrophic factor (BDNF) has a crucial role in neural plasticity, a process needed for the retention of fear extinction. In this study, we investigated the effects of local administration of a histone deacetylase (HDAC) inhibitor, sodium butyrate (NaBu), on BDNF transcription and behavioral markers of extinction in the single prolonged stress (SPS) model of PTSD.

Materials and Methods

NaBu was infused into the infralimbic (IL) subregion of the medial prefrontal cortex (mPFC) of male rats. The freezing response was recorded as the criterion to assess fear strength on the day of extinction as well as 24 hr later in the retention test. Other behavioral tests were also measured to evaluate the anxiety level, locomotor activity, and working memory on the retention day. HDAC activity and BDNF mRNA expression were evaluated after the behavioral experiments.

Results

NaBu facilitated the recall of fear extinction in SPS rats (P<0.0001). SPS rats had higher HDAC activity (P<0.0001) and lower BDNF expression (P<0.05) than non-SPS animals. Also, anxiety was higher in the SPS group (P<0.0001), but locomotor activity (P=0.61) and working memory (P=0.36) were not different between SPS and Non-SPS groups.

Conclusion

Our findings provide evidence that the mechanism of action of NaBu in the improvement of extinction recall is mediated, in part, by enhancing histone acetylation and reviving BDNF expression in IL.

Sex differences in the immediate extinction deficit and renewal of extinguished fear in rats

Extinction learning is central to exposure-based behavioral therapies for reducing fear and anxiety in humans. However, patients with fear and anxiety disorders are often resistant to extinction. Moreover, trauma and stress-related disorders are highly prone to relapse and are twice as likely to occur in females compared to males, suggesting that females may be more susceptible to extinction deficits and fear relapse phenomena. In this report, we tested this hypothesis by examining sex differences in a stress-induced extinction learning impairment, the immediate extinction deficit (IED), and renewal, a common form of fear relapse. In contrast to our hypothesis, there were no sex differences in the magnitude of the immediate extinction deficit in two different rat strains (Long-Evans and Wistar). However, we did observe a sex difference in the renewal of fear when the extinguished conditioned stimulus was presented outside the extinction context. Male Wistar rats exhibited significantly greater renewal than female rats, a sex difference that has previously been reported after appetitive extinction. Collectively, these data reveal that stress-induced extinction impairments are similar in male and female rats, though the context-dependence of extinction is more pronounced in males.

Cerebellar 5HT-2A receptor mediates stress-induced onset of dystonia

Stress is a key risk factor for dystonia, a debilitating motor disorder characterized by cocontractions of muscles leading to abnormal body posture. While the serotonin (5HT) system is known to control emotional responses to stress, its role in dystonia remains unclear. Here, we reveal that 5HT neurons in the dorsal raphe nuclei (DRN) send projections to the fastigial deep cerebellar nuclei (fDCN) and that photostimulation of 5HT-fDCN induces dystonia in wild-type mice. Moreover, we report that photoinhibition of 5HT-fDCN reduces dystonia in a1A ^tot/tot mice, a genetic model of stress-induced dystonia, and administration of a 5HT-2A receptor inverse agonist (MDL100907; 0.1 to 1 mg/kg) or shRNA-mediated knockdown of the ht2ar gene in fDCN can notably reduce the onset of dystonia in a1A ^tot/tot mice. These results support the serotonin theory of dystonia and suggest strategies for alleviating symptoms in human patients by blocking 5HT-2A receptors.

Infralimbic cortex controls fear memory generalization and susceptibility to extinction during consolidation

Lesioning or inactivating the infralimbic (IL) subregion of the medial prefrontal cortex before acquisition produces more generalized and extinction-resistant fear memories. However, whether and how it modulates memory specificity and extinction susceptibility while consolidation takes place is still unknown. The present study aims to investigate these questions using muscimol-induced temporary inactivation and anisomycin-induced protein synthesis inhibition in the rat IL following contextual fear conditioning. Results indicate that the IL activity immediately after acquisition, but not six hours later, controls memory generalization over a week, regardless of its strength. Such IL function depends on the context-shock pairing since muscimol induced no changes in animals exposed to immediate shocks or the conditioning context only. Animals in which the IL was inactivated during consolidation extinguished similarly to controls within the session but were unable to recall the extinction memory the following day. Noteworthy, these post-acquisition IL inactivation-induced effects were not associated with changes in anxiety, as assessed in the elevated plus-maze test. Anisomycin results indicate that the IL protein synthesis during consolidation contributes more to producing extinction-sensitive fear memories than memory specificity. Collectively, present results provide evidence for the IL's role in controlling generalization and susceptibility to extinction during fear memory consolidation.

Fear not: recent advances in understanding the neural basis of fear memories and implications for treatment development

Fear is a highly adaptive emotion that has evolved to promote survival and reproductive fitness. However, maladaptive expression of fear can lead to debilitating stressor-related and anxiety disorders such as post-traumatic stress disorder. Although the neural basis of fear has been extensively researched for several decades, recent technological advances in pharmacogenetics and optogenetics have allowed greater resolution in understanding the neural circuits that underlie fear. Alongside conceptual advances in the understanding of fear memory, this increased knowledge has clarified mechanisms for some currently available therapies for post-traumatic stress disorder and has identified new potential treatment targets.

Neuroepigenetic mechanisms underlying fear extinction: emerging concepts

An understanding of how memory is acquired and how it can be modified in fear-related anxiety disorders, with the enhancement of failing memories on one side and a reduction or elimination of traumatic memories on the other, is a key unmet challenge in the fields of neuroscience and neuropsychiatry. The latter process depends on an important form of learning called fear extinction, where a previously acquired fear-related memory is decoupled from its ability to control behaviour through repeated non-reinforced exposure to the original fear-inducing cue. Although simple in description, fear extinction relies on a complex pattern of brain region and cell-type specific processes, some of which are unique to this form of learning and, for better or worse, contribute to the inherent instability of fear extinction memory. Here, we explore an emerging layer of biology that may compliment and enrich the synapse-centric perspective of fear extinction. As opposed to the more classically defined role of protein synthesis in the formation of fear extinction memory, a neuroepigenetic view of the experience-dependent gene expression involves an appreciation of dynamic changes in the state of the entire cell: from a transient change in plasticity at the level of the synapse, to potentially more persistent long-term effects within the nucleus. A deeper understanding of neuroepigenetic mechanisms and how they influence the formation and maintenance of fear extinction memory has the potential to enable the development of more effective treatment approaches for fear-related neuropsychiatric conditions.

Brain circuit dysfunction in post-traumatic stress disorder: from mouse to man

Post-traumatic stress disorder (PTSD) is a prevalent, debilitating and sometimes deadly consequence of exposure to severe psychological trauma. Although effective treatments exist for some individuals, they are limited. New approaches to intervention, treatment and prevention are therefore much needed. In the past few years, the field has rapidly developed a greater understanding of the dysfunctional brain circuits underlying PTSD, a shift in understanding that has been made possible by technological revolutions that have allowed the observation and perturbation of the macrocircuits and microcircuits thought to underlie PTSD-related symptoms. These advances have allowed us to gain a more translational knowledge of PTSD, have provided further insights into the mechanisms of risk and resilience and offer promising avenues for therapeutic discovery.