Prelimbic cortical BDNF is required for memory of learned fear but not extinction or innate fear

Fear conditioning, synaptic plasticity and the amygdala: implications for posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is an anxiety disorder that can develop after a traumatic experience such as domestic violence, natural disasters or combat-related trauma. The cost of such disorders on society and the individual can be tremendous. In this article, we review how the neural circuitry implicated in PTSD in humans is related to the neural circuitry of fear. We then discuss how fear conditioning is a suitable model for studying the molecular mechanisms of the fear components that underlie PTSD, and the biology of fear conditioning with a particular focus on the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (TrkB), GABAergic and glutamatergic ligand-receptor systems. We then summarize how such approaches might help to inform our understanding of PTSD and other stress-related disorders and provide insight to new pharmacological avenues of treatment of PTSD.

Epigenetic modification of hippocampal Bdnf DNA in adult rats in an animal model of post-traumatic stress disorder

Epigenetic alterations of the brain-derived neurotrophic factor (Bdnf) gene have been linked with memory, stress, and neuropsychiatric disorders. Here we examined whether there was a link between an established rat model of post-traumatic stress disorder (PTSD) and Bdnf DNA methylation. Adult male Sprague-Dawley rats were given psychosocial stress composed of two acute cat exposures in conjunction with 31 days of daily social instability. These manipulations have been shown previously to produce physiological and behavioral sequelae in rats that are comparable to symptoms observed in traumatized people with PTSD. We then assessed Bdnf DNA methylation patterns (at exon IV) and gene expression. We have found here that the psychosocial stress regimen significantly increased Bdnf DNA methylation in the dorsal hippocampus, with the most robust hypermethylation detected in the dorsal CA1 subregion. Conversely, the psychosocial stress regimen significantly decreased methylation in the ventral hippocampus (CA3). No changes in Bdnf DNA methylation were detected in the medial prefrontal cortex or basolateral amygdala. In addition, there were decreased levels of Bdnf mRNA in both the dorsal and ventral CA1. These results provide evidence that traumatic stress occurring in adulthood can induce CNS gene methylation, and specifically, support the hypothesis that epigenetic marking of the Bdnf gene may underlie hippocampal dysfunction in response to traumatic stress. Furthermore, this work provides support for the speculative notion that altered hippocampal Bdnf DNA methylation is a cellular mechanism underlying the persistent cognitive deficits which are prominent features of the pathophysiology of PTSD.

Phencyclidine (PCP)-induced disruption in cognitive performance is gender-specific and associated with a reduction in brain-derived neurotrophic factor (BDNF) in specific regions of the female rat brain

Phencyclidine (PCP), used to mimic certain aspects of schizophrenia, induces sexually dimorphic, cognitive deficits in rats. In this study, the effects of sub-chronic PCP on expression of brain-derived neurotrophic factor (BDNF), a neurotrophic factor implicated in the pathogenesis of schizophrenia, have been evaluated in male and female rats. Male and female hooded-Lister rats received vehicle or PCP (n=8 per group; 2 mg/kg i.p. twice daily for 7 days) and were tested in the attentional set shifting task prior to being sacrificed (6 weeks post-treatment). Levels of BDNF mRNA were measured in specific brain regions using in situ hybridisation. Male rats were less sensitive to PCP-induced deficits in the extra-dimensional shift stage of the attentional set shifting task compared to female rats. Quantitative analysis of brain regions demonstrated reduced BDNF levels in the medial prefrontal cortex (p<0.05), motor cortex (p<0.01), orbital cortex (p<0.01), olfactory bulb (p<0.05), retrosplenial cortex (p<0.001), frontal cortex (p<0.01), parietal cortex (p<0.01), CA1 (p<0.05) and polymorphic layer of dentate gyrus (p<0.05) of the hippocampus and the central (p<0.01), lateral (p<0.05) and basolateral (p<0.05) regions of the amygdaloid nucleus in female PCP-treated rats compared with controls. In contrast, BDNF was significantly reduced only in the orbital cortex and central amygdaloid region of male rats (p<0.05). Results suggest that blockade of NMDA receptors by sub-chronic PCP administration has a long-lasting down-regulatory effect on BDNF mRNA expression in the female rat brain which may underlie some of the behavioural deficits observed post PCP administration.

PTSD and gene variants: new pathways and new thinking

Posttraumatic Stress Disorder (PTSD) is an anxiety disorder which can develop as a result of exposure to a traumatic event and is associated with significant functional impairment. Family and twin studies have found that risk for PTSD is associated with an underlying genetic vulnerability and that more than 30% of the variance associated with PTSD is related to a heritable component. Using a fear conditioning model to conceptualize the neurobiology of PTSD, three primary neuronal systems have been investigated - the hypothalamic-pituitary-adrenal axis, the locus coeruleus-noradrenergic system, and neurocircuitry interconnecting the limbic system and frontal cortex. The majority of the initial investigations into main effects of candidate genes hypothesized to be associated with PTSD risk have been negative, but studies examining the interaction of genetic polymorphisms with specific environments in predicting PTSD have produced several positive results which have increased our understanding of the determinants of risk and resilience in the aftermath of trauma. Promising avenues of inquiry into the role of epigenetic modification have also been proposed to explain the enduring impact of environmental exposures which occur during key, often early, developmental periods on gene expression. Studies of PTSD endophenotypes, which are heritable biomarkers associated with a circumscribed trait within the more complex psychiatric disorder, may be more directly amenable to analysis of the underlying genetics and neural pathways and have provided promising targets for elucidating the neurobiology of PTSD. Knowledge of the genetic underpinnings and neuronal pathways involved in the etiology and maintenance of PTSD will allow for improved targeting of primary prevention amongst vulnerable individuals or populations, as well as timely, targeted treatment interventions. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Memory facilitating effects of agomelatine in the novel object recognition memory paradigm in the rat

The aim of the present study was to evaluate the effects of agomelatine, an antidepressant with melatonergic agonist and 5-HT(2C) antagonist properties, in the rat novel object recognition (NOR) task, a model of short-term episodic memory. To assess the potential involvement of its chronobiotic activity, single intraperitoneal administration of agomelatine and NOR testing were performed either in the evening or in the morning. In both conditions, using a 24h retention interval, vehicle-treated rats did not discriminate between the novel and the familiar object (recognition index was not different from chance performance) while object memory performance of rats treated with agomelatine either in the evening (10 and 40mg/kg) or in the morning (2.5, 10, and 40mg/kg) was significantly improved. Moreover, the selective 5-HT(2C) antagonist SB 242,084 (0.63, 2.5, and 10mg/kg) and melatonin (2.5, 10, and 40mg/kg) displayed also memory facilitating effects in both administration conditions. Finally, thioperamide used as positive reference compound to validate the experimental conditions, demonstrated a memory facilitating effect. In conclusion, agomelatine was shown to possess memory facilitating effects in the rat NOR task and both melatonergic agonist and 5-HT(2C) antagonist properties could be involved in these effects.

Effect of 7,8-dihydroxyflavone, a small-molecule TrkB agonist, on emotional learning

OBJECTIVE

Despite increasing awareness of the many important roles played by brain-derived neurotrophic factor (BDNF) activation of TrkB, a fuller understanding of this system and the use of potential TrkB-acting therapeutic agents has been limited by the lack of any identified small-molecule TrkB agonists that fully mimic the actions of BDNF at brain TrkB receptors in vivo. However, 7,8-dihydroxyflavone (7,8-DHF) has recently been identified as a specific TrkB agonist that crosses the blood-brain barrier after oral or intraperitoneal administration. The authors combined pharmacological, biochemical, and behavioral approaches in a preclinical study examining the role of 7,8-DHF in modulating emotional memory in mice.

METHOD

The authors first examined the ability of systemic 7,8-DHF to activate TrkB receptors in the amygdala. They then examined the effects of systemic 7,8-DHF on acquisition and extinction of conditioned fear, using specific and well-characterized BDNF-dependent learning paradigms in several models using naive mice and mice with prior traumatic stress exposure.

RESULTS

Amygdala TrkB receptors, which have previously been shown to be required for emotional learning, were activated by systemic 7,8-DHF (at 5 mg/kg i.p.). 7,8-DHF enhanced both the acquisition of fear and its extinction. It also appeared to rescue an extinction deficit in mice with a history of immobilization stress.

CONCLUSIONS

These data suggest that 7,8-DHF may be an excellent agent for use in understanding the effects of TrkB activation in learning and memory paradigms and may be attractive for use in reversing learning and extinction deficits associated with psychopathology.

The use of cognitive enhancers in animal models of fear extinction

In anxiety disorders, such as posttraumatic stress disorders and phobias, classical conditioning pairs natural (unconditioned) fear-eliciting stimuli with contextual or discrete cues resulting in enduring fear responses to multiple stimuli. Extinction is an active learning process that results in a reduction of conditioned fear responses after conditioned stimuli are no longer paired with unconditioned stimuli. Fear extinction often produces incomplete effects and this highlights the relative permanence of bonds between conditioned stimuli and conditioned fear responses. The animal research literature is rich in its demonstration of cognitive enhancing agents that alter fear extinction. This review specifically examines the fear extinguishing effects of cognitive enhancers that act on gamma-aminobutyric acid (GABA), glutamatergic, cholinergic, adrenergic, dopaminergic, and cannabinoid signaling pathways. It also examines the effects of compounds that alter epigenetic and neurotrophic mechanisms in fear extinction. Of these cognitive enhancers, glutamatergic N-methyl d-aspartate (NMDA) receptor agonists, such as D-cycloserine, have enhanced fear extinction in a context-, dose- and time-dependent manner. Agents that function as glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonists, alpha2-adrenergic receptor antagonists (such as yohimbine), neurotrophic factors (brain derived neurotrophic factor or BDNF) and histone deacetylase inhibitors (valproate and sodium butyrate) also improve fear extinction in animals. However, some have anxiogenic effects and their contextual and temporal effects need to be more reliably demonstrated. Various cognitive enhancers produce changes in cortico-amygdala synaptic plasticity through multiple mechanisms and these neural changes enhance fear extinction. We need to better define the changes in neural plasticity produced by these agents in order to develop more effective compounds. In the clinical setting, such use of effective cognitive enhancers with cue exposure therapy, using compounds derived from animal model studies, provides great hope for the future treatment of anxiety disorders.

Rapid brain-derived neurotrophic factor-dependent sequestration of amygdala and hippocampal GABA(A) receptors via different tyrosine receptor kinase B-mediated phosphorylation pathways

During the consolidation of fear memory, it has been shown that GABA(A) receptors (GABA(A)R) are rapidly downregulated in amygdala. This rapid decrease in GABA(A)R functioning may permit transient hyperexcitablity, contributing to cellular mechanisms of memory consolidation. Memory consolidation also requires brain-derived neurotrophic factor (BDNF) activation of tyrosine receptor kinase B (TrkB) receptors in the amygdala and hippocampus. We hypothesized that rapid internalization of GABA(A)Rα1 is mediated via TrkB activation of PKA and PKC-dependent processes. Primary neuronal cell cultures, from postnatal day 14-21 mouse amygdala and hippocampus, were analyzed with immunofluorescence using cell-surface, whole-cell permeabilization, and antibody internalization techniques, as well as with (3)H-muscimol binding assays. In both hippocampal and amygdala cultures, we found a >60% reduction in surface GABA(A)Rα1 within 5 min of BDNF treatment. Notably, the rapid decrease in surface GABA(A)Rα1 was confirmed biochemically using surface biotinylation assays followed by western blotting. This rapid effect was accompanied by TrkB phosphorylation and increased internal GABA(A)Rα1 immunofluorescence, and was blocked by k252a, a broad-spectrum tyrosine kinase antagonist. To further demonstrate TrkB specificity, we used previously characterized TrkB(F616A) mice, in which the highly selective TrkB-mutant specific antagonist, 1NMPP1, prevented the BDNF-dependent GABA(A)Rα1 internalization. In hippocampus, we found both PKA and PKC inhibition, using Rp-8-Br-cAMP and Calphostin C, respectively, blocked GABA(A)Rα1 internalization, whereas inhibition of MAPK (U0126) and PI3K (LY294002) did not prevent rapid internalization. By contrast in amygdala cultures, Rp-8-Br-cAMP had no effect. Together, these data suggest that rapid GABA(A)R internalization during memory consolidation is BDNF-TrkB dependent. Further, it appears that hippocampal GABA(A)R internalization is PKA and PKC dependent, while it may be primarily PKC dependent in amygdala, implying differential roles for TrkB-dependent kinase activation in BDNF-dependent memory formation.

Prefrontal control of fear: more than just extinction

Although fear research has largely focused on the amygdala, recent findings highlight cortical control of the amygdala in the service of fear regulation. In rodent models, it is becoming well established that the infralimbic (IL) prefrontal cortex plays a key role in extinction learning, and recent findings are uncovering molecular mechanisms involved in extinction-related plasticity. Furthermore, mounting evidence implicates the prelimbic (PL) prefrontal cortex in the production of fear responses. Both IL and PL integrate inputs from the amygdala, as well as other structures to gate the expression of fear via projections to inhibitory or excitatory circuits within the amygdala. We suggest that dual control of the amygdala by separate prefrontal modules increases the flexibility of an organism's response to danger cues.