Re-emergence of extinguished auditory-cued conditioned fear following a sub-conditioning procedure: Effects of hippocampal and prefrontal tetanic stimulations

Deep Brain Stimulation of the dorsal raphe induces anxiolytic and panicolytic-like effects and alters serotonin immunoreactivity

Previously we showed that Deep Brain Stimulation (DBS) of the dorsal region (DRD) and of the lateral wings of the dorsal raphe (lwDR) respectively decreases anxiety and panic-like responses in the elevated T-maze (ETM). This study investigates neurobiological alterations which might respond for these behavioral effects. Male Wistar rats were submitted to high-frequency stimulation (100µA, 100Hz) of the DRD or of the lwDR for 1h, and subsequently tested in the avoidance or escape tasks of the ETM. Since serotonin (5-HT) reuptake inhibitors are first line pharmacological treatment for anxiety disorders, we also tested the effects of chronic fluoxetine administration (10mg/kg, IP, 21 days) on a separate group of rats. An open field was used for locomotor activity assessment. Additionally, we evaluated c-Fos immunoreactivity (Fos-ir) in serotonergic cells of the dorsal raphe (DR). Results showed that DBS of the DRD decreases avoidance reactions, an anxiolytic-like effect, without altering escape or locomotor activity. Both fluoxetine and DBS of the lwDR decreased escape responses in the ETM, a panicolytic-like effect, without altering avoidance measurements or locomotor activity. While DBS of the DRD decreased double immunostaining in the DRD, DBS of the lwDR increased Fos-ir and double immunostaining in the DRD and lwDR. Fluoxetine also increased double immunostaining in the lwDR and in the DRV but decreased it in the DRD. These results suggest that both the anxiolytic and panicolytic-like effects of DBS and fluoxetine are related to 5-HT modulation in different subnuclei of the DR.

Application of Deep Brain Stimulation in Refractory Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a mental disorder that produces crippling anxiety and occurs in response to an extreme, traumatic stressor. Compared to the prevalence of PTSD in the general population, the prevalence of PTSD in at-risk populations (e.g., army veterans, those affected by environmental calamities, and others) can reach up to threefold. The conventional treatment of PTSD involves using SSRIs (serotonin reuptake inhibitors) and other anti-depressants along with psychotherapy such as debriefing and CBT (cognitive behavioral therapy). Due to increasing resistance to conventional treatment, more novel treatment options, such as stellate ganglion block shots and neuromodulation, are being explored. These neuromodulation techniques include transcranial magnetic stimulation (TMS), transcranial direct current stimulation (TDS), and deep brain stimulation (DBS). The rationale behind employing these techniques in refractory PTSD is the altered neurocircuitry seen in PTSD patients, which can be visualized on imaging. Studies involving the use of DBS for PTSD primarily target specific areas in the brain: the amygdala, the prefrontal cortex, the hippocampus, and the hypothalamus. This article aims to provide a brief overview of the various neuromodulation techniques currently employed in the management of treatment-resistant PTSD and an in-depth review of the available literature on animal models in which DBS for PTSD has been researched. We also shed light on the human clinical trials conducted for the same.

Unexpected Consequences of Noise-Induced Hearing Loss: Impaired Hippocampal Neurogenesis, Memory, and Stress

Noise-induced hearing loss (NIHL), caused by direct damage to the cochlea, reduces the flow of auditory information to the central nervous system, depriving higher order structures, such as the hippocampus with vital sensory information needed to carry out complex, higher order functions. Although the hippocampus lies outside the classical auditory pathway, it nevertheless receives acoustic information that influence its activity. Here we review recent results that illustrate how NIHL and other types of cochlear hearing loss disrupt hippocampal function. The hippocampus, which continues to generate new neurons (neurogenesis) in adulthood, plays an important role in spatial navigation, memory, and emotion. The hippocampus, which contains place cells that respond when a subject enters a specific location in the environment, integrates information from multiple sensory systems, including the auditory system, to develop cognitive spatial maps to aid in navigation. Acute exposure to intense noise disrupts the place-specific firing patterns of hippocampal neurons, “spatially disorienting” the cells for days. More traumatic sound exposures that result in permanent NIHL chronically suppresses cell proliferation and neurogenesis in the hippocampus; these structural changes are associated with long-term spatial memory deficits. Hippocampal neurons, which contain numerous glucocorticoid hormone receptors, are part of a complex feedback network connected to the hypothalamic-pituitary (HPA) axis. Chronic exposure to intense intermittent noise results in prolonged stress which can cause a persistent increase in corticosterone, a rodent stress hormone known to suppress neurogenesis. In contrast, a single intense noise exposure sufficient to cause permanent hearing loss produces only a transient increase in corticosterone hormone. Although basal corticosterone levels return to normal after the noise exposure, glucocorticoid receptors (GRs) in the hippocampus remain chronically elevated. Thus, NIHL disrupts negative feedback from the hippocampus to the HPA axis which regulates the release of corticosterone. Preclinical studies suggest that the noise-induced changes in hippocampal place cells, neurogenesis, spatial memory, and glucocorticoid receptors may be ameliorated by therapeutic interventions that reduce oxidative stress and inflammation. These experimental results may provide new insights on why hearing loss is a risk factor for cognitive decline and suggest methods for preventing this decline.

Deep Brain Stimulation for Post-Traumatic Stress Disorder: A Review of the Experimental and Clinical Literature

INTRODUCTION

Up to 30% of patients with post-traumatic stress disorder (PTSD), especially combat veterans, remain refractory to conventional treatment. For them, deep brain stimulation (DBS) has been suggested. Here, we review the literature on animal models of PTSD in which DBS has been used to treat PTSD-type behavior, and we review and discuss patient reports of DBS for PTSD.

METHODS

A broad search was performed to find experimental animal articles and clinical reports on PubMed, Ovid MEDLINE, Cochrane Library, and PsycINFO, using combinations and variations of search words pertinent to DBS and PTSD.

RESULTS

The search yielded 30 articles, 24 on DBS in rat models of PTSD, and 6 publications between 2016 and 2020 reporting on a total of 3 patients. DBS in rat models targeted 4 brain areas: medial prefrontal cortex (mPFC), ventral striatum, amygdala, and hippocampus. Clinical publications reported on 2 male combat veterans who received DBS in basolateral amygdala, and 1 female with PTSD due to domestic abuse, who received DBS of mPFC. All 3 patients benefitted to various extents from DBS, at follow-ups of 4 years, 6 months, and 7 months, respectively.

CONCLUSIONS

PTSD is the only potential clinical indication for DBS that shows extensive animal research prior to human applications. Nevertheless, DBS for PTSD remains highly investigational. Despite several years of government funding of DBS research in view of treating severe PTSD in combat veterans, ethical dilemmas, recruitment difficulties, and issues related to use of DBS in such a complex and heterogenous disorder remain prevalent.

Repeated cocaine exposure prior to fear conditioning induces persistency of PTSD-like symptoms and enhancement of hippocampal and amygdala cell density in male rats

Pre- and post-trauma drug use can interfere with recovery from post-traumatic stress disorder (PTSD). However, the biological underpinnings of this interference are poorly understood. Here we examined the effect of pre-fear conditioning cocaine self-administration on PTSD-like symptoms in male rats, and defined impairment of fear extinction as difficulty to recover from PTSD. We also examined cell density changes in brain regions suspected of being involved in resistance to PTSD recovery. Before footshock stress testing, rats were trained to self-administer cocaine during 20 consecutive days, after which they were exposed to footshocks, while other rats continued to self-administer cocaine until the end of the experiment. Upon assessment of three PTSD-like symptoms (fear during situational reminders, anxiety-like behavior, and impairment of recognition memory) and fear extinction learning and memory, changes in cell density were measured in the medial prefrontal cortex, hippocampus, and amygdala. Results show that pre-footshock cocaine exposure did not affect fear during situational reminders. Fear conditioning did not lead to an increase in cocaine consumption. However, in footshock stressed rats, cocaine induced a reduction of anxiety-like behavior, an aggravation of recognition memory decline, and an impairment of extinction memory. These behavioral alterations were associated with increased cell density in the hippocampal CA1, CA2, and CA3 regions and basolateral amygdala, but not in the medial prefrontal cortex. Our findings suggest that enhancement of cell density in the hippocampus and amygdala may be changes associated with drug use, interfering with PTSD recovery.

Contributions of prelimbic cortex, dorsal and ventral hippocampus, and basolateral amygdala to fear return induced by elevated platform stress in rats

Fear relapse is a major challenge in the treatment of stress-related mental disorders. Most investigations have focused on fear return induced by stimuli associated with the initial fear learning, while little attention has been paid to fear return evoked after exposure to an unconditioned stressor. This study explored the neural mechanisms of fear return induced by elevated platform (EP) stressor in Sprague-Dawley rats initially subjected to auditory fear conditioning. The contributions of the prelimbic cortex (PL), dorsal hippocampus (DH), ventral hippocampus (VH), and basolateral amygdala (BLA) were examined by targeted bilateral intracerebral injection of the GABAA agonist muscimol after elevated platform (EP) stressor. Muscimol-induced inactivation of PL or BLA significantly impaired the return of conditioning fear, while inactivation of the DH or VH had no effect. These results suggest that fear return induced by non-associative stressor may depend on the PL and BLA but not on the hippocampus.

Deep Brain Stimulation of the dorsal raphe abolishes serotonin 1A facilitation of AMPA receptor-mediated synaptic currents in the ventral hippocampus

In a previous study we showed that Deep Brain Stimulation (DBS) of the rat dorsal subregion of the dorsal raphe (DRD), which sends serotonergic projections to forebrain areas, such as the ventral hippocampus, induces anxiolytic-like effects. The purpose of the present study was to investigate neurobiological alterations which might underline these behavioral effects. For that, we tested the influence of DBS upon the neuromodulatory action of serotonin on excitatory post-synaptic currents (EPSCs) in the ventral hippocampus. Male Wistar rats were submitted to high-frequency stimulation (100 μA, 100 Hz) of the DRD for 1 h during three consecutive days. On the third day, immediately after the DBS procedure, animals were euthanized. Slices of the ventral hippocampus were processed for whole cell patch clamp recordings of AMPA-receptor (AMPAR) mediated EPSCs in the CA1 area. As reported by others, we confirmed that in pre-weaning rats a high affinity 5-HT1A receptor agonist (8-OH-PIPAT, 0.5-5nM) inhibits EPSCs. However, in adult rats (non-operated or sham-operated), 8-OH-PIPAT (0.5-5 nM) increased EPSC amplitude, an effect blocked by the 5-HT1A antagonist WAY-100,635 (200 nM). Importantly, in adult rats exposed to DBS, the 5-HT1A agonist was devoid of effect. Taken together these results show that: 1) changes in 5-HT1A receptor-mediated hippocampal synaptic transmission occur with age; 2) these changes lead to a facilitatory effect of 5-HT1A receptors; 3) DBS blocks this serotonergic facilitatory action. These observations suggest that an alteration in serotonin modulation of limbic areas may underlie the psychotherapeutic effects of DBS.

Acute Chemogenetic Activation of CamKIIα-Positive Forebrain Excitatory Neurons Regulates Anxiety-Like Behaviour in Mice

Anxiety disorders are amongst the most prevalent mental health disorders. Several lines of evidence have implicated cortical regions such as the medial prefrontal cortex, orbitofrontal cortex, and insular cortex along with the hippocampus in the top–down modulation of anxiety-like behaviour in animal models. Both rodent models of anxiety, as well as treatment with anxiolytic drugs, result in the concomitant activation of multiple forebrain regions. Here, we sought to examine the effects of chemogenetic activation or inhibition of forebrain principal neurons on anxiety and despair-like behaviour. We acutely activated or inhibited Ca²⁺/calmodulin-dependent protein kinase II α (CamKIIα)-positive forebrain excitatory neurons using the hM3Dq or the hM4Di Designer Receptor Exclusively Activated by Designer Drug (DREADD) respectively. Circuit activation was confirmed via an increase in expression of the immediate early gene, c-Fos, within both the hippocampus and the neocortex. We then examined the influence of DREADD-mediated activation of forebrain excitatory neurons on behavioural tests for anxiety and despair-like behaviour. Our results indicate that acute hM3Dq DREADD activation of forebrain excitatory neurons resulted in a significant decline in anxiety-like behaviour on the open field, light–dark avoidance, and the elevated plus maze test. In contrast, hM3Dq DREADD activation of forebrain excitatory neurons did not alter despair-like behaviour on either the tail suspension or forced swim tests. Acute hM4Di DREADD inhibition of CamKIIα-positive forebrain excitatory neurons did not modify either anxiety or despair-like behaviour. Taken together, our results demonstrate that chemogenetic activation of excitatory neurons in the forebrain decreases anxiety-like behaviour in mice.

Behavioral and immunohistochemical characterization of rapid reconditioning following extinction of contextual fear

A fundamental property of extinction is that the behavior that is suppressed during extinction can be unmasked through a number of postextinction procedures. Of the commonly studied unmasking procedures (spontaneous recovery, reinstatement, contextual renewal, and rapid reacquisition), rapid reacquisition is the only approach that allows a direct comparison between the impact of a conditioning trial before or after extinction. Thus, it provides an opportunity to evaluate the ways in which extinction changes a subsequent learning experience. In five experiments, we investigate the behavioral and neurobiological mechanisms of postextinction reconditioning. We show that rapid reconditioning of unsignaled contextual fear after extinction in male Long-Evans rats is associative and not affected by the number or duration of extinction sessions that we examined. We then evaluate c-Fos expression and histone acetylation (H4K8) in the hippocampus, amygdala, prefrontal cortex, and bed nucleus of the stria terminalis. We find that in general, initial conditioning has a stronger impact on c-Fos expression and acetylation than does reconditioning after extinction. We discuss implications of these results for theories of extinction and the neurobiology of conditioning and extinction.

Neuromodulation Strategies in Post-Traumatic Stress Disorder: From Preclinical Models to Clinical Applications

Post-traumatic stress disorder (PTSD) is an often debilitating disease with a lifetime prevalence rate between 5⁻8%. In war veterans, these numbers are even higher, reaching approximately 10% to 25%. Although most patients benefit from the use of medications and psychotherapy, approximately 20% to 30% do not have an adequate response to conventional treatments. Neuromodulation strategies have been investigated for various psychiatric disorders with promising results, and may represent an important treatment option for individuals with difficult-to-treat forms of PTSD. We review the relevant neurocircuitry and preclinical stimulation studies in models of fear and anxiety, as well as clinical data on the use of transcranial direct current stimulation (tDCS), repetitive transcranial magnetic stimulation (rTMS), and deep brain stimulation (DBS) for the treatment of PTSD.

High-frequency stimulation of the infralimbic cortex, following behavioral suppression of PTSD-like symptoms, prevents symptom relapse in mice

BACKGROUND

We have previously demonstrated, in mice, that antidepressant treatment can prevent relapse of PTSD-like behaviors (avoidance, hyperarousal, and anxiety) through increased activation in the infralimbic cortex (IL) of the medial prefrontal cortex.

OBJECTIVE

Here, we examined whether direct high-frequency stimulation (HFS) of the IL, provoking its heightened activation (i.e., long-term potentiation, LTP), would also prevent the return of PTSD-like symptoms.

METHODS

A 1.5-mA foot-shock was used to generate PTSD-like symptoms in Swiss mice. In Experiment 1, local field potentials were recorded in the IL to test whether normal IL LTP can be induced after the suppression of PTSD-like symptoms. In Experiment 2, IL HFS was applied after symptom suppression, but prior to the provocation of relapse, to test HFS effect on symptom return.

RESULTS

We observed that PTSD-like state was associated with impairment in IL HFS-induced IL LTP. However, IL LTP induction was near normal when PTSD-like symptoms were suppressed. We then found that IL HFS, applied after symptom suppression, prevented symptom return.

CONCLUSIONS

Increased activation of the IL may be a key mechanism preventing PTSD relapse. Prefrontal cortex deep brain stimulation may, therefore, be relevant for preventing PTSD symptom return in remitted high-risk patients.

β-Adrenoceptor Blockade in the Basolateral Amygdala, But Not the Medial Prefrontal Cortex, Rescues the Immediate Extinction Deficit

Early psychological interventions, such as exposure therapy, rely on extinction learning to reduce the development of stress- and trauma-related disorders. However, recent research suggests that extinction often fails to reduce fear when administered soon after trauma. This immediate extinction deficit (IED) may be due to stress-induced dysregulation of neural circuits involved in extinction learning. We have shown that systemic β-adrenoceptor blockade with propranolol rescues the IED, but impairs delayed extinction. Here we sought to determine the neural locus of these effects. Rats underwent auditory fear conditioning and then received either immediate (30 min) or delayed (24 h) extinction training. We used bilateral intracranial infusions of propranolol into either the infralimbic division of the medial prefrontal cortex (mPFC) or the basolateral amygdala (BLA) to examine the effects of β-adrenoceptor blockade on immediate and delayed extinction learning. Interestingly, intra-BLA, but not intra-mPFC, propranolol rescued the IED; animals receiving intra-BLA propranolol prior to immediate extinction showed less spontaneous recovery of fear during extinction retrieval. Importantly, this was not due to impaired consolidation of the conditioning memory. In contrast, neither intra-BLA nor intra-mPFC propranolol affected delayed extinction learning. Overall, these data contribute to a growing literature suggesting dissociable roles for key nodes in the fear extinction circuit depending on the timing of extinction relative to conditioning. These data also suggest that heightened noradrenergic activity in the BLA underlies stress-induced extinction deficits. Propranolol may be a useful adjunct to behavioral therapeutic interventions in recently traumatized individuals who are at risk for developing trauma-related disorders.

Posttraumatic Stress Disorder: Perspectives for the Use of Deep Brain Stimulation

OBJECTIVES

Deep Brain Stimulation (DBS) has been either approved or is currently under investigation for a number of psychiatric disorders.

MATERIALS AND METHODS

We review clinical and preclinical concepts as well as the neurocircuitry that may be of relevance for the implementation of DBS in posttraumatic stress disorder (PTSD).

RESULTS

PTSD is a chronic and debilitating illness associated with dysfunction in well-established neural circuits, including the amygdala and prefrontal cortex. Although most patients often improve with medications and/or psychotherapy, approximately 20-30% are considered to be refractory to conventional treatments. In other psychiatric disorders, DBS has been investigated in treatment-refractory patients. To date, preclinical work suggests that stimulation at high frequency delivered at particular timeframes to different targets, including the amygdala, ventral striatum, hippocampus, and prefrontal cortex may improve fear extinction and anxiety-like behavior in rodents. In the only clinical report published so far, a patient implanted with electrodes in the amygdala has shown striking improvements in PTSD symptoms.

CONCLUSIONS

Neuroimaging, preclinical, and preliminary clinical data suggest that the use of DBS for the treatment of PTSD may be practical but the field requires further investigation.

Deep Brain Stimulation in Animal Models of Fear, Anxiety, and Posttraumatic Stress Disorder

Although medications and psychotherapy are often effective for the treatment of posttraumatic stress disorder (PTSD), 20-30% of patients do not respond to these conventional therapies. In psychiatry, DBS has been either approved or is currently under investigation for different disorders. At present, whether DBS may be used to treat PTSD remains unknown. Preclinical research may provide the scientific rationale for helping conceive and further improve such trials. Some of the animal models commonly used to date are more suitable for investigating mechanisms of anxiety and fear than the long-lasting behavior that characterized PTSD. That said, mechanisms and neurocircuits involved in paradigms such as fear conditioning and extinction share several common features with those of PTSD. In this article, we review preclinical studies in which electrical stimulation has been delivered to animal models of PTSD-like behavior. In those studies, commonly targeted regions were the basolateral amygdala, ventral striatum, hippocampus, and prefrontal cortex. Overall, stimulation delivered at high frequencies to most of these targets improved fear extinction and anxiety-like behavior. Though further research is certainly needed, promising findings from DBS studies in animals are encouraging and suggest a positive future perspective for the field.

Paroxetine treatment, following behavioral suppression of PTSD-like symptoms in mice, prevents relapse by activating the infralimbic cortex

Clinical studies have shown that post-traumatic stress disorder (PTSD) remission, induced by selective serotonin reuptake inhibitor (SSRI) treatment, is associated with increased prefrontal activation during post-treatment symptom provocation. Other studies have shown that continuation SSRI treatment after remitting from PTSD reduces the rate of relapse. The aim of the present preclinical study was to investigate the relationship between post-treatment prefrontal changes and PTSD relapse prevention. Avoidance conditioning (with a 1.5-mA foot-shock), avoidance extinction and a trauma priming exposure (with a 0.3-mA foot-shock) were used in mice to induce, suppress and reactivate PTSD-like symptoms (including avoidance, fear sensitization, enhanced contextual fear, and anxiety-like behavior), respectively. Paroxetine, injected at 8 mg/kg/day (7 days), was used as SSRI treatment. PTSD-like symptoms were present for at least 30 days and resistant to paroxetine treatment. However, after extinction training (suppressing all PTSD-like symptoms), paroxetine treatment prevented symptom reactivation. Paroxetine treatment also induced infralimbic neuronal activation. However, infralimbic functional tetrodotoxin inactivation abolished the preventive effect of paroxetine treatment on symptom reactivation. The data reveal a potential ability of treatments inducing infralimbic activation to provide prophylactic protection against PTSD relapse.

Region-specific roles of the prelimbic cortex, the dorsal CA1, the ventral DG and ventral CA1 of the hippocampus in the fear return evoked by a sub-conditioning procedure in rats

The return of learned fear is an important issue in anxiety disorder research since an analogous process may contribute to long-term fear maintenance or clinical relapse. A number of studies demonstrate that mPFC and hippocampus are important in the modulation of post-extinction re-expression of fear memory. However, the region-specific role of these structures in the fear return evoked by a sub-threshold conditioning (SC) is not known. In the present experiments, we first examined specific roles of the prelimbic cortex (PL), the dorsal hippocampus (DH, the dorsal CA1 area in particular), the ventral hippocampus (the ventral dentate gyrus (vDG) and the ventral CA1 area in particular) in this fear return process. Then we examined the role of connections between PL and vCA1 with this behavioral approach. Rats were subjected to five tone-shock pairings (1.0-mA shock) to induce conditioned fear (freezing), followed by three fear extinction sessions (25 tone-alone trials each session). After a post-test for extinction memory, some rats were retrained with the SC procedure to reinstate tone-evoked freezing. Rat groups were injected with low doses of the GABAA agonist muscimol to selectively inactivate PL, DH, vDG, or vCA1 120 min before the fear return test. A disconnection paradigm with ipsilateral or contralateral muscimol injection of the PL and the vCA1 was used to examine the role of this pathway in the fear return. We found that transient inactivation of these areas significantly impaired fear return (freezing): inactivation of the prelimbic cortex blocked SC-evoked fear return in particular but did not influence fear expression in general; inactivation of the DH area impaired fear return, but had no effect on the extinction retrieval process; both ventral DG and ventral CA1 are required for the return of extinguished fear whereas only ventral DG is required for the extinction retrieval. These findings suggest that PL, DH, vDG, and vCA1 all contribute to the fear return and connections between PL and vCA1 may be involved in the modulation of this process.

Stress and Fear Extinction

Stress has a critical role in the development and expression of many psychiatric disorders, and is a defining feature of posttraumatic stress disorder (PTSD). Stress also limits the efficacy of behavioral therapies aimed at limiting pathological fear, such as exposure therapy. Here we examine emerging evidence that stress impairs recovery from trauma by impairing fear extinction, a form of learning thought to underlie the suppression of trauma-related fear memories. We describe the major structural and functional abnormalities in brain regions that are particularly vulnerable to stress, including the amygdala, prefrontal cortex, and hippocampus, which may underlie stress-induced impairments in extinction. We also discuss some of the stress-induced neurochemical and molecular alterations in these brain regions that are associated with extinction deficits, and the potential for targeting these changes to prevent or reverse impaired extinction. A better understanding of the neurobiological basis of stress effects on extinction promises to yield novel approaches to improving therapeutic outcomes for PTSD and other anxiety and trauma-related disorders.

Post-training corticosterone inhibits the return of fear evoked by platform stress and a subthreshold conditioning procedure in Sprague-Dawley rats

The return of fear is an important issue in anxiety disorder research. Each time a fear memory is reactivated, it may further strengthen overactivation of the fear circuit, which may contribute to long-term maintenance of the fear memory. Recent evidence indicates that glucocorticoids may help attenuate pathological fear, but its role in the return of fear is unclear. In the present study, systemic corticosterone (CORT; 25mg/kg) administration 1h after fear conditioning did not impair the consolidation process but significantly suppressed the return of fear evoked by a subthreshold conditioning (SC) procedure and elevated platform (EP) stress. Compared with the SC-induced return of fear, acute stress-induced return was state-dependent. In addition, post-training CORT treatment increased the adrenocorticotropic response after EP stress, which indicates that the drug-induced suppression of the return of fear may possibly derive from its regulation effect of the hypothalamic-pituitary-adrenal axis reactivity to stress. These results suggest that post-training CORT administration may help inhibit the return of fear evoked by EP or SC stress. The possible mechanisms involved in the high-dose CORT-induced suppression of the SC- and EP-induced return of fear are discussed.

High-frequency stimulation of the hippocampus blocks fear learning sensitization and return of extinguished fear

Patients with post-traumatic stress disorder (PTSD) present hippocampal (HPC) dysfunction, which may facilitate fear-related phenomena such as fear learning sensitization (i.e. potentiation of fear acquisition by initial fear conditioning (FC1)) and fear return (i.e. reactivation of extinguished fear). Fear return is sensitive to HPC high-frequency stimulation (HFS) in rats. The goal of the present study was to examine whether fear learning sensitization is also sensitive to HPC HFS in rats. We found in control conditions that, after FC1 (with 15 shock administrations) and extinction, conditioning in a different context with one shock administration was potentiated (proactive effect) and associated with fear return in the initial context (retroactive effect). Both phenomena were prevented by HPC HFS applied before the second conditioning. We also found that the effect of HPC HFS on fear learning sensitization required initial extinction. These findings suggest a pivotal role of the HPC in preventing proactive and retroactive effects of successive fear conditionings. These data also support the concept that HPC deactivation may be involved in fear learning sensitization and fear return in PTSD patients.

Hippocampal neurogenesis protects against cocaine-primed relapse

Accumulating evidence demonstrates a functional role for the hippocampus in mediating relapse to cocaine-seeking behavior and extinction-induced inhibition of cocaine seeking, and dentate gyrus neurogenesis in the hippocampus may have a role. Here, we tested the hypothesis that disruption of normal hippocampal activity during extinction alters relapse to cocaine-seeking behavior as a function of dentate gyrus neurogenesis. Adult rats were trained to self-administer cocaine on a fixed-ratio schedule, followed by extinction and cocaine-primed reinstatement testing. Some rats received low-frequency stimulation (LFS; 2 Hz for 25 minutes) after each extinction session in the dorsal or ventral hippocampal formation. All rats received an injection of the mitotic marker 5-bromo-2'-deoxyuridine (BrdU) to label developing dentate gyrus neurons during self-administration, as well as before or after extinction and LFS. We found that LFS during extinction did not alter extinction behavior but enhanced cocaine-primed reinstatement. Cocaine self-administration reduced levels of 24-day-old BrdU cells and dentate gyrus neurogenesis, which was normalized by extinction. LFS during extinction prevented extinction-induced normalization of dentate gyrus neurogenesis and potentiated cocaine-induced reinstatement of drug seeking. LFS inhibition of extinction-induced neurogenesis was not due to enhanced cell death, revealed by quantification of activated caspase3-labeled cells. These data suggest that LFS during extinction disrupts hippocampal networking by disrupting neurogenesis and also strengthens relapse-like behaviors. Thus, newly born dentate gyrus neurons during withdrawal and extinction learning facilitate hippocampal networking that mediates extinction-induced inhibition of cocaine seeking and may play a key role in preventing relapse.

Prefrontal high-frequency stimulation prevents sub-conditioning procedure-provoked, but not acute stress-provoked, reemergence of extinguished fear

We have recently shown that post-extinction exposure of rats to a sub-conditioning procedure (SCP, i.e., retraining with a shock intensity that is too weak to induce by itself significant fear conditioning) or to acute stress provokes reemergence of extinguished fear. Furthermore, this SCP effect can be abolished by high-frequency stimulation (HFS) of the medial prefrontal cortex (mPFC), when applied following the SCP. The aim of the present study was to test whether HFS of the mPFC is effective in preventing both SCP-induced and acute stress-provoked fear reemergence. Rats implanted with stimulating electrodes in the mPFC were trained to acquire high levels of freezing to conditioned auditory cue. This fear response was then extinguished. Three weeks later, no spontaneous recovery was observed, but rats exposed to either the SCP or acute stress again exhibited high levels of freezing. HFS of the mPFC, applied before provoking fear reemergence, prevented the effects of SCP, but not acute stress. These data suggest that acute stress may have more impact on functions of the mPFC and/or associated structures than a situational reminder of fear conditioning.

Low-frequency stimulation of the ventral hippocampus facilitates extinction of contextual fear

Difficulties to treat fear-associated disorders, including posttraumatic stress disorder, are thought to result from dysfunction in fear extinction learning and/or memory. Animal studies on extinction modulation are therefore promising for the development of new treatments. Recent rat studies, including ones using low-frequency stimulation (LFS), have demonstrated that the ventral hippocampus (VH) modulates extinction memory. The present study explores whether the VH also modulates extinction learning. For this, rats were implanted with stimulating electrodes in the VH and experienced contextual fear conditioning, followed 6 or 24 h later by VH LFS and three sessions of extinction training. We found that, whatever the delay used (6 or 24 h), animals that received VH LFS displayed persistent low levels of freezing from the second extinction session, whereas control rats showed low levels of freezing only during the third session. In animals submitted to a stress condition (provoked by a single inescapable foot-shock followed by three sessions of situational reminders) prior to fear conditioning, VH LFS also reduced freezing levels, which, in contrast, remained high in control rats during the course of extinction training. These data suggest that LFS, targeting the VH, may be useful in reducing fear responses during extinction learning.

Post-extinction fluoxetine treatment prevents stress-induced reemergence of extinguished fear

RATIONALE

The post-extinction exposure of rats to a sub-conditioning procedure (SCP; i.e., retraining with a shock intensity that is too weak to induce by itself significant fear conditioning) has been reported to provoke the reemergence of extinguished fear. This phenomenon can be prevented by chronic fluoxetine treatment.

OBJECTIVES

We sought to examine another potential inducer of fear reemergence, acute stress, in rats and determine whether fluoxetine prevents this phenomenon.

METHODS

Because in previous studies fluoxetine was administered before extinction, we first analyzed its effect on the SCP-associated reemergence of auditory-cued conditioned fear in rats injected after extinction to avoid any interaction between fluoxetine and extinction learning. Next, we used the same protocol but replaced the SCP with acute stress.

RESULTS

We found that the SCP and acute stress, which were carried out 3 weeks after fear extinction, similarly provoked the reemergence of extinguished fear in rats injected with vehicle during the 3-week period. In contrast, the animals treated with fluoxetine during this period behaved similarly to those not exposed to an inducer of fear reemergence.

CONCLUSIONS

Our data establish acute stress as an inducer of fear reemergence. The results provide further support for the hypothesis that fluoxetine interfered with mechanisms that reactivated extinguished fear, even when administered after fear extinction.

Serotonergic modulation of conditioned fear

Conditioned fear plays a key role in anxiety disorders as well as depression and other neuropsychiatric conditions. Understanding how neuromodulators drive the associated learning and memory processes, including memory consolidation, retrieval/expression, and extinction (recall), is essential in the understanding of (individual differences in vulnerability to) these disorders and their treatment. The human and rodent studies I review here together reveal, amongst others, that acute selective serotonin reuptake inhibitor (SSRI) treatment facilitates fear conditioning, reduces contextual fear, and increases cued fear, chronic SSRI treatment reduces both contextual and cued fear, 5-HT1A receptors inhibit the acquisition and expression of contextual fear, 5-HT2A receptors facilitates the consolidation of cued and contextual fear, inactivation of 5-HT2C receptors facilitate the retrieval of cued fear memory, the 5-HT3 receptor mediates contextual fear, genetically induced increases in serotonin levels are associated with increased fear conditioning, impaired cued fear extinction, or impaired extinction recall, and that genetically induced 5-HT depletion increases fear conditioning and contextual fear. Several explanations are presented to reconcile seemingly paradoxical relationships between serotonin levels and conditioned fear.

Sex differences, gonadal hormones and the fear extinction network: implications for anxiety disorders

Convergent data from rodents and human studies have led to the development of models describing the neural mechanisms of fear extinction. Key components of the now well-characterized fear extinction network include the amygdala, hippocampus, and medial prefrontal cortical regions. These models are fueling novel hypotheses that are currently being tested with much refined experimental tools to examine the interactions within this network. Lagging far behind, however, is the examination of sex differences in this network and how sex hormones influence the functional activity and reactivity of these brain regions in the context of fear inhibition. Indeed, there is a large body of literature suggesting that sex hormones, such as estrogen, do modulate neural plasticity within the fear extinction network, especially in the hippocampus.After a brief overview of the fear extinction network, we summarize what is currently known about sex differences in fear extinction and the influence of gonadal hormones on the fear extinction network. We then go on to propose possible mechanisms by which sex hormones, such as estrogen, may influence neural plasticity within the fear extinction network. We end with a discussion of how knowledge to be gained from developing this line of research may have significant ramifications towards the etiology, epidemiology and treatment of anxiety disorders.