Infralimbic Prefrontal Cortex Research Articles

Infralimbic prefrontal cortex interacts with nucleus accumbens shell to unmask expression of outcome-selective Pavlovian-to-instrumental transfer.

Although several studies have examined the subcortical circuitry underlying Pavlovian-to-instrumental transfer (PIT), the role of medial prefrontal cortex in this behavior is largely unknown. Elucidating the cortical contributions to PIT will be key for understanding how reward-paired cues control behavior in both adaptive and maladaptive context (i.e., addiction). Here we use bilateral lesions in a rat model to show that infralimbic prefrontal cortex (ilPFC) is necessary for appropriate expression of PIT. Further, we show that ilPFC mediates this effect via functional connectivity with nucleus accumbens shell (NAcS). Together, these data provide the first demonstration that a specific cortico-striatal circuit is necessary for cue-invigorated reward seeking during specific PIT.

Persistent active avoidance correlates with activity in prelimbic cortex and ventral striatum.

Persistent avoidance is a prominent symptom of anxiety disorders and is often resistant to extinction-based therapies. Little is known about the circuitry mediating persistent avoidance. Using a recently described platform-mediated active avoidance task, we assessed activity in several structures with c-Fos immuno-labeling. In Task 1, rats were conditioned to avoid a tone-signaled shock by moving to a safe platform, and then were extinguished over two days. One day later, failure to retrieve extinction correlated with increased activity in the prelimbic prefrontal cortex (PL), ventral striatum (VS), and basal amygdala (BA), and decreased activity in infralimbic prefrontal cortex (IL), consistent with pharmacological inactivation studies. In Task 2, the platform was removed during extinction training and fear (suppression of bar pressing) was extinguished to criterion over 3–5 days. The platform was then returned in a post-extinction test. Under these conditions, avoidance levels were equivalent to Experiment 1 and correlated with increased activity in PL and VS, but there was no correlation with activity in IL or BA. Thus, persistent avoidance can occur independently of deficits in fear extinction and its associated structures.

Astroglial Control of the Antidepressant-Like Effects of Prefrontal Cortex Deep Brain Stimulation

Although deep brain stimulation (DBS) shows promising efficacy as a therapy for intractable depression, the neurobiological bases underlying its therapeutic action remain largely unknown. The present study was aimed at characterizing the effects of infralimbic prefrontal cortex (IL-PFC) DBS on several pre-clinical markers of the antidepressant-like response and at investigating putative non-neuronal mechanism underlying DBS action. We found that DBS induced an antidepressant-like response that was prevented by IL-PFC neuronal lesion and by adenosine A1 receptor antagonists including caffeine. Moreover, high frequency DBS induced a rapid increase of hippocampal mitosis and reversed the effects of stress on hippocampal synaptic metaplasticity. In addition, DBS increased spontaneous IL-PFC low-frequency oscillations and both raphe 5-HT firing activity and synaptogenesis. Unambiguously, a local glial lesion counteracted all these neurobiological effects of DBS. Further in vivo electrophysiological results revealed that this astrocytic modulation of DBS involved adenosine A1 receptors and K+ buffering system. Finally, a glial lesion within the site of stimulation failed to counteract the beneficial effects of low frequency (30Hz) DBS. It is proposed that an unaltered neuronal–glial system constitutes a major prerequisite to optimize antidepressant DBS efficacy. It is also suggested that decreasing frequency could heighten antidepressant response of partial responders.

Activation of AMPA Receptors Mediates the Antidepressant Action of Deep Brain Stimulation of the Infralimbic Prefrontal Cortex.

Although deep brain stimulation (DBS) has been used with success in treatment-resistant depression, little is known about its mechanism of action. We examined the antidepressant-like activity of short (1 h) DBS applied to the infralimbic prefrontal cortex in the forced swim test (FST) and the novelty-suppressed feeding test (NSFT). We also used in vivo microdialysis to evaluate the release of glutamate, γ-aminobutyric acid, serotonin, dopamine, and noradrenaline in the prefrontal cortex and c-Fos immunohistochemistry to determine the brain regions activated by DBS. One hour of DBS of the infralimbic prefrontal cortex has antidepressant-like effects in FST and NSFT, and increases prefrontal efflux of glutamate, which would activate AMPA receptors (AMPARs). This effect is specific of the infralimbic area since it is not observed after DBS of the prelimbic subregion. The activation of prefrontal AMPARs would result in a stimulation of prefrontal output to the brainstem, thus increasing serotonin, dopamine, and noradrenaline in the prefrontal cortex. Further, the activation of prefrontal AMPARs is necessary and sufficient condition for the antidepressant response of 1 h DBS.

Optogenetic stimulation of infralimbic PFC reproduces ketamine's rapid and sustained antidepressant actions.

Ketamine produces rapid and sustained antidepressant actions in depressed patients, but the precise cellular mechanisms underlying these effects have not been identified. Here we determined if modulation of neuronal activity in the infralimbic prefrontal cortex (IL-PFC) underlies the antidepressant and anxiolytic actions of ketamine. We found that neuronal inactivation of the IL-PFC completely blocked the antidepressant and anxiolytic effects of systemic ketamine in rodent models and that ketamine microinfusion into IL-PFC reproduced these behavioral actions of systemic ketamine. We also found that optogenetic stimulation of the IL-PFC produced rapid and long-lasting antidepressant and anxiolytic effects and that these effects are associated with increased number and function of spine synapses of layer V pyramidal neurons. The results demonstrate that ketamine infusions or optogenetic stimulation of IL-PFC are sufficient to produce long-lasting antidepressant behavioral and synaptic responses similar to the effects of systemic ketamine administration.

Prenatal ethanol exposure alters ethanol-induced Fos immunoreactivity and dopaminergic activity in the mesocorticolimbic pathway of the adolescent brain

Prenatal ethanol exposure (PEE) promotes alcohol intake during adolescence, as shown in clinical and pre-clinical animal models. The mechanisms underlying this effect of prenatal ethanol exposure on postnatal ethanol intake remain, however, mostly unknown. Few studies assessed the effects of moderate doses of prenatal ethanol on spontaneous and ethanol-induced brain activity on adolescence. This study measured, in adolescent (female) Wistar rats prenatally exposed to ethanol (0.0 or 2.0g/kg/day, gestational days 17–20) or non-manipulated (NM group) throughout pregnancy, baseline and ethanol-induced cathecolaminergic activity (i.e., colocalization of c-Fos and tyrosine hydroxylase) in ventral tegmental area (VTA), and baseline and ethanol-induced Fos immunoreactivity (ir) in nucleus accumbens shell and core (AcbSh and AcbC, respectively) and prelimbic (PrL) and infralimbic (IL) prefrontal cortex. The rats were challenged with ethanol (dose: 0.0, 1.25, 2.5 or 3.25g/kg, i.p.) at postnatal day 37. Rats exposed to vehicle prenatally (VE group) exhibited reduced baseline dopaminergic tone in VTA; an effect that was inhibited by prenatal ethanol exposure (PEE group). Dopaminergic activity in VTA after the postnatal ethanol challenge was greater in PEE than in VE or NM animals. Ethanol-induced Fos-ir at AcbSh was found after 1.25g/kg and 2.5g/kg ethanol, in VE and PEE rats, respectively. PEE did not alter ethanol-induced Fos-ir at IL but reduced ethanol-induced Fos-ir at PrL. These results suggest that prenatal ethanol exposure heightens dopaminergic activity in the VTA and alters the response of the mesocorticolimbic pathway to postnatal ethanol exposure. These effects may underlie the enhanced vulnerability to develop alcohol-use disorders of adolescents with a history of in utero ethanol exposure.

Emotional modulation of synapses, circuits and memory.

Emotion is a powerful tool for change in the central nervous sys-tem. Whereas, mostlong-term memoriesare stored after practiceor rehearsal, an emotionally arousing memory can be consoli-dated after a single experience. Research examining the influenceof emotion on synaptic function provides a window of oppor-tunity for exploring the mechanisms of memory consolidationduring the minutes to hours after a single, well-rememberedexperience. It can also shed light on numerous psychiatric con-ditions, bringing the field closer to preventing or treating thoseconditions that stem from traumatic memories. This researchtopic brings together leading experts who share their recentfindings and perspectives on how emotion may influence brainfunction.In this issue, four articles review or describe evidence of aspecialized influence of emotion on synaptic function. SheenaJosselyn’slaboratorypreviouslyfoundthatneuronswithhighlev-els of cAMP Responsive Element Binding Protein (CREB) at thetime of training are preferentially allocated to the memory trace(Han et al., 2007). In this issue, Sargin et al. (2013) provide acellular mechanism as to why this may be. They report that over-expression of CREB within neurons of the lateral nucleus of theamygdala (LA), leads to an increase in dendritic spine density,whereas LA neurons with low CREB activity exhibit a decreaseindendriticspinedensity.ThesedatasupportthehypothesisthatCREB may increase a neuron’s propensity for being included in amemory trace by increasing dendritic spine density.Young and Williams addressed the issue of lateralization ofamygdala recruitment during memory consolidation. Imagingstudies in humans indicate that activity in the right amygdala isassociated with aversive memories (Morris et al., 1999). Othersindicatethattheleftamygdalaispreferentiallyinvolvedinencod-ing memories that have a positive valence (Zalla et al., 2000;Hamann et al., 2002). In this issue, Young and Williams (2013)examine expression of a marker of synaptic plasticity in the ratamygdala. They report that the synaptic plasticity-related pro-tein Arc is specifically elevated in the right amygdala followingtraining on an aversive task, and in the left amygdala followingtraining on an appetitive task. Considering that Arc expressionin the amygdala is necessary for consolidation of conditionedfear (Ploski et al., 2008), these findings indicate that, memory-related synaptic plasticity in the amygdala is lateralized andvalence-dependent.Headley and Pare (2013) add a temporal dimension to thisresearch topic. They review literature on temporal synchronyof firing across brain regions involved in emotional memory.They describe gamma oscillations and emotional memory, citingrecent reports of enhanced gamma oscillations in the neocor-tex and amygdala during emotional situations, and evidence thatgamma oscillations have predictive value for synaptic plasticityand emotional memory.Gronli et al. (2013) take a different view on the effects ofemotion on memory and plasticity. These authors review thecellular and molecular evidence that stress may impair cogni-tion by interfering with sleep. They describe the importance ofsleep for the cellular and molecular processes that contribute tomemoryconsolidation,andsuggestthatstressthatinterfereswithsleep is associated with deficient synaptic plasticity and impairedcognitive performance.Several of the contributions to this issue examined synapticchanges related to extinction of conditioned fear in rats. Bothpost-traumatic stress disorder (PTSD) and obsessive compulsivedisorder (OCD) are characterized by avoidance of stimuli thatare perceived as threatening, even in the absence of real dan-ger. Impaired extinction of conditioned fear could contributeto the persistence of maladaptive behaviors seen in these dis-orders (Milad et al., 2008, 2013). It was recently demonstratedthat vagus nerve stimulation (VNS) enhances extinction of con-ditioned fear in rats (Pena et al., 2013). In this issue, (Pena etal.,2014)reportthatextinction-enhancingVNSreversessynapticdepression in the infralimbic prefrontal cortex basolateral amyg-dala pathway, a circuit that is implicated in extinction memory(Sierra-Mercado et al., 2011). Collectively, these findings sug-gest that VNS could be paired with exposure therapy to facilitateextinction of conditioned fear and reverse pathological synap-tic function seen in anxiety disorders. Another contributioninvestigated the effects of deep brain stimulation (DBS) on theextinction circuitry. In 2010, several groups reported that stimu-lation of the ventral capsule/ventral striatum reduced symptomsofOCDinrefractorypatients(Denysetal.,2010;Goodmanetal.,2010; Greenberg et al., 2010). In rats, stimulation of the ventralstriatum enhances extinction of conditioned fear (Rodriguez-Romaguera et al., 2012). In this issue, research from Greg Quirk’slab demonstrates that extinction-enhancing stimulation of theventral striatum increases expression of the brain-derived neu-rotrophic factor protein (BDNF) in the medial prefrontal cortex,indicating that the clinical benefits of DBS may be mediatedby BDNF-associated synaptic changes in the extinction pathway(Do-Monte et al., 2013).One of the limitations of exposure therapy is that condi-tionedfearcanreturnevenaftersuccessfulextinctionlearning.In

Differential effects of cocaine on extracellular signal-regulated kinase phosphorylation in nuclei of the extended amygdala and prefrontal cortex of psychogenetically selected Roman high- and low-avoidance rats.

Roman high (RHA)- and low (RLA)-avoidance rats are selectively bred for rapid vs. poor acquisition of active avoidance, respectively, and differ markedly in emotional reactivity, coping style, and behavioral and neurochemical responses to morphine and psychostimulants. Accordingly, acute cocaine induces more robust increments in locomotion and dopamine output in the nucleus accumbens shell (AcbSh) of RHA than of RLA rats. Cocaine induces short- and long-term neuronal plasticity via activation of the extracellular signal-regulated kinase (ERK) pathway. This study compares the effects of acute cocaine on ERK phosphorylation (pERK) in limbic brain areas of Roman rats. In RHA but not RLA rats, cocaine (5 mg/kg) increased pERK in the infralimbic prefrontal cortex and AcbSh, two areas involved in its acute effects, but did not modify pERK in the prelimbic prefrontal cortex and Acb core, which mediate the chronic effects of cocaine. Moreover, cocaine failed to affect pERK immunolabeling in the bed nucleus of stria terminalis pars lateralis and central amygdala of either line but increased it in the basolateral amygdala of RLA rats. These results extend to pERK expression previous findings on the greater sensitivity to acute cocaine of RHA vs. RLA rats and confirm the notion that genetic factors influence the differential responses of the Roman lines to addictive drugs. Moreover, they support the view that the Roman lines are a useful tool to investigate the molecular underpinnings of individual vulnerability to drug addiction.

Brain-derived neurotrophic factor and addiction: Pathological versus therapeutic effects on drug seeking

Many abused drugs lead to changes in endogenous brain-derived neurotrophic factor (BDNF) expression in neural circuits responsible for addictive behaviors. BDNF is a known molecular mediator of memory consolidation processes, evident at both behavioral and neurophysiological levels. Specific neural circuits are responsible for storing and executing drug-procuring motor programs, whereas other neural circuits are responsible for the active suppression of these “seeking” systems. These seeking-circuits are established as associations are formed between drug-associated cues and the conditioned responses they elicit. Such conditioned responses (e.g. drug seeking) can be diminished either through a passive weakening of seeking- circuits or an active suppression of those circuits through extinction. Extinction learning occurs when the association between cues and drug are violated, for example, by cue exposure without the drug present. Cue exposure therapy has been proposed as a therapeutic avenue for the treatment of addictions. Here we explore the role of BDNF in extinction circuits, compared to seeking-circuits that “incubate” over prolonged withdrawal periods. We begin by discussing the role of BDNF in extinction memory for fear and cocaine-seeking behaviors, where extinction circuits overlap in infralimbic prefrontal cortex (PFC). We highlight the ability of estrogen to promote BDNF-like effects in hippocampal–prefrontal circuits and consider the role of sex differences in extinction and incubation of drug-seeking behaviors. Finally, we examine how opiates and alcohol “break the mold” in terms of BDNF function in extinction circuits.This article is part of a Special Issue entitled SI:Addiction circuits.

Medial amygdala lesions selectively block aversive pavlovian-instrumental transfer in rats.

Pavlovian conditioned stimuli (CSs) play an important role in the reinforcement and motivation of instrumental active avoidance (AA). Conditioned threats can also invigorate ongoing AA responding [aversive Pavlovian–instrumental transfer (PIT)]. The neural circuits mediating AA are poorly understood, although lesion studies suggest that lateral, basal, and central amygdala nuclei, as well as infralimbic prefrontal cortex, make key, and sometimes opposing, contributions. We recently completed an extensive analysis of brain c-Fos expression in good vs. poor avoiders following an AA test (Martinez et al., 2013, Learning and Memory). This analysis identified medial amygdala (MeA) as a potentially important region for Pavlovian motivation of instrumental actions. MeA is known to mediate defensive responding to innate threats as well as social behaviors, but its role in mediating aversive Pavlovian–instrumental interactions is unknown. We evaluated the effect of MeA lesions on Pavlovian conditioning, Sidman two-way AA conditioning (shuttling) and aversive PIT in rats. Mild footshocks served as the unconditioned stimulus in all conditioning phases. MeA lesions had no effect on AA but blocked the expression of aversive PIT and 22 kHz ultrasonic vocalizations in the AA context. Interestingly, MeA lesions failed to affect Pavlovian freezing to discrete threats but reduced freezing to contextual threats when assessed outside of the AA chamber. These findings differentiate MeA from lateral and central amygdala, as lesions of these nuclei disrupt Pavlovian freezing and aversive PIT, but have opposite effects on AA performance. Taken together, these results suggest that MeA plays a selective role in the motivation of instrumental avoidance by general or uncertain Pavlovian threats.

BDNF signaling potentiates transmission of information from the basolateral amygdala to infralimbic prefrontal cortex during conditioned taste aversion extinction.

Conditioned taste aversion (CTA) is a form of associative learning in which a palatable taste [e.g., saccharin; conditioned stimulus (CS)] is paired with a toxic unconditioned stimulus (e.g., injection of lithium chloride), resulting in a reduced preference for the CS. Extinction is a form learning

A unifying model of the role of the infralimbic cortex in extinction and habits.

The infralimbic prefrontal cortex (IL) has been shown to be critical for the regulation of flexible behavior, but its precise function remains unclear. This region has been shown to be critical for the acquisition, consolidation, and expression of extinction learning, leading many to hypothesize that IL suppresses behavior as part of a “stop” network. However, this framework is at odds with IL function in habitual behavior in which the IL has been shown to be required for the expression and acquisition of ongoing habitual behavior. Here, we will review the current state of knowledge of IL anatomy and function in behavioral flexibility and provide a testable framework for a single IL mechanism underlying its function in both extinction and habit learning.

Involvement of BDNF signaling transmission from basolateral amygdala to infralimbic prefrontal cortex in conditioned taste aversion extinction.

Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase receptor B (TrkB), play a critical role in memory extinction. However, the detailed role of BDNF in memory extinction on the basis of neural circuit has not been fully understood. Here, we aim to investigate the role of BDNF signaling circuit in mediating conditioned taste aversion (CTA) memory extinction of the rats. We found region-specific changes in BDNF gene expression during CTA extinction. CTA extinction led to increased BDNF gene expression in the basolateral amygdala (BLA) and infralimbic prefrontal cortex (IL) but not in the central amygdaloid nucleus (CeA) and hippocampus (HIP). Moreover, blocking BDNF signaling or exogenous microinjection of BDNF into the BLA or IL could disrupt or enhance CTA extinction, which suggested that BDNF signaling in the BLA and IL is necessary and sufficient for CTA extinction. Interestingly, we found that microinjection of BDNF-neutralizing antibody into the BLA could abolish the extinction training-induced BDNF mRNA level increase in the IL, but not vice versa, demonstrating that BDNF signaling is transmitted from the BLA to IL during extinction. Finally, the accelerated extinction learning by infusion of exogenous BDNF in the BLA could also be blocked by IL infusion of BDNF-neutralizing antibody rather than vice versa, indicating that the IL, but not BLA, is the primary action site of BDNF in CTA extinction. Together, these data suggest that BLA-IL circuit regulates CTA memory extinction by identifying BDNF as a key regulator.

Differential sensitivity of ethanol-elicited ERK phosphorylation in nucleus accumbens of Sardinian alcohol-preferring and -non preferring rats

Sardinian alcohol-preferring (sP) and -non preferring (sNP) rats have been selectively bred for opposite ethanol preference and consumption; sP rats represent a validated experimental tool to model several aspects of excessive ethanol drinking in humans. Phosphorylated Extracellular signal-Regulated Kinase (pERK) in dopamine-rich terminal areas plays a critical role in several psychopharmacological effects of addictive drugs, including ethanol. This study was aimed at investigating whether ethanol-elicited ERK activation may differ in key brain areas of ethanol-naïve sP and sNP rats. To this end, the effects of ethanol (0, 0.5, 1, and 2 g/kg, administered intra-gastrically [i.g.]) on ERK phosphorylation were assessed by pERK immunohistochemistry in the shell (AcbSh) and core (AcbC) of the nucleus accumbens (Acb) as well as in the prelimbic (PrL) and infralimbic (IL) prefrontal cortex (PFCx), in the bed nucleus of stria terminalis (BSTL) and in the central nucleus of the amygdala (CeA). Ethanol (1 g/kg) significantly increased pERK immunoreactivity in AcbSh and AcbC of sP but not sNP rats. Conversely, ethanol failed to affect pERK expression in PrL and IL PFCx as well as in BSTL and CeA of both sP and sNP rats. These results suggest that selective breeding of these rat lines results in differential effects of acute ethanol on ERK phosphorylation in brain regions critical for the psychopharmacological effects of ethanol.

Maternal separation exaggerates spontaneous recovery of extinguished contextual fear in adult female rats

Early life stress increases the risk of posttraumatic stress disorders (PTSD). Patients with PTSD show impaired extinction of traumatic memory, and in women, this occurs more often when PTSD is preceded by child trauma. However, it is still unclear how early life stress accounts for extinction impairment. Here, we studied the effects of maternal separation (MS, postnatal day 2 to 14) on contextual fear extinction in adult female rats. Additionally, to examine changes in synaptic function affected by MS, we measured long-term potentiation (LTP) in prefrontal cortex and hippocampus in vitro, both of which have been implicated in fear extinction. We found that adult female rats had been subjected to MS exhibited significant spontaneous recovery of fear to the extinguished context. Furthermore, MS exposure resulted in LTP impairment in both infralimbic prefrontal cortex layer 2/3-layer 5 and hippocampal SC-CA1 pathways. Interestingly, no obvious effects of MS on contextual fear conditioning, fear recall as well as extinction training and recall were observed. Innate fear in the elevated plus maze or open field test remained nearly unaffected. These findings provided the first evidence that MS may exaggerate spontaneous recovery after contextual fear extinction, for which LTP impairment in the medial prefrontal cortex and hippocampus may be responsible, thereby possibly leading to impaired extinction associated with PTSD.

Hippocampal–Prefrontal BDNF and Memory for Fear Extinction

Infusing brain-derived neurotrophic factor (BDNF) into the infralimbic (IL) prefrontal cortex is capable of inducing extinction. Little is known, however, about the circuits mediating BDNF effects on extinction or the extent to which extinction requires BDNF in IL. Using local pharmacological infusion of BDNF protein, or an antibody against BDNF, we found that BDNF in the IL, but not prelimbic (PL) prefrontal cortex, is both necessary and sufficient for fear extinction. Furthermore, we report that BDNF in IL can induce extinction of older fear memories (14 days) as well as recent fear memories (1 day). Using immunocytochemistry, we show that BDNF is increased in the ventral hippocampus (vHPC), but not IL or PL, following extinction training. Finally, we observed that infusing BDNF into the vHPC increased the firing rate of IL, but not PL neurons in fear conditioned rats. These findings indicate that an extinction-induced increase in BDNF within the vHPC enhances excitability in IL targets, thereby supporting extinction memories.

Differential Contributions of Infralimbic Prefrontal Cortex and Nucleus Accumbens during Reward-Based Learning and Extinction

Using environmental cues for the prediction of future events is essential for survival. Such cue-outcome associations are thought to depend on mesolimbic circuitry involving the nucleus accumbens (NAc) and prefrontal cortex (PFC). Several studies have identified roles for both NAc and PFC in the expression of stable goal-directed behaviors, but much remains unknown about their roles during learning of such behaviors. To further address this question, we used in vivo oxygen amperometry, a proxy for blood oxygen level-dependent (BOLD) signal measurement in human functional magnetic resonance imaging, in rats performing a cued lever-pressing task requiring discrimination between a rewarded and nonrewarded cue. Simultaneous oxygen recordings were obtained from infralimbic PFC (IFC) and NAc throughout both acquisition and extinction of this task. Activation of NAc was specifically observed following rewarded cue onset during the entire acquisition phase and also during the first days of extinction. In contrast, IFC activated only during the earliest periods of acquisition and extinction, more specifically to the nonrewarded cue. Thus, in vivo oxygen amperometry permits a novel, stable form of longitudinal analysis of brain activity in behaving animals, allowing dissociation of the roles of different brain regions over time during learning of reward-driven instrumental action. The present results offer a unique temporal perspective on how NAc may promote actions directed toward anticipated positive outcome throughout learning, while IFC might suppress actions that no longer result in reward, but only during critical periods of learning.

Directional Theta Coherence in Prefrontal Cortical to Amygdalo-Hippocampal Pathways Signals Fear Extinction

Theta oscillations are considered crucial mechanisms in neuronal communication across brain areas, required for consolidation and retrieval of fear memories. One form of inhibitory learning allowing adaptive control of fear memory is extinction, a deficit of which leads to maladaptive fear expression potentially leading to anxiety disorders. Behavioral responses after extinction training are thought to reflect a balance of recall from extinction memory and initial fear memory traces. Therefore, we hypothesized that the initial fear memory circuits impact behavioral fear after extinction, and more specifically, that the dynamics of theta synchrony in these pathways signal the individual fear response. Simultaneous multi-channel local field and unit recordings were obtained from the infralimbic prefrontal cortex, the hippocampal CA1 and the lateral amygdala in mice. Data revealed that the pattern of theta coherence and directionality within and across regions correlated with individual behavioral responses. Upon conditioned freezing, units were phase-locked to synchronized theta oscillations in these pathways, characterizing states of fear memory retrieval. When the conditioned stimulus evoked no fear during extinction recall, theta interactions were directional with prefrontal cortical spike firing leading hippocampal and amygdalar theta oscillations. These results indicate that the directional dynamics of theta-entrained activity across these areas guide changes in appraisal of threatening stimuli during fear memory and extinction retrieval. Given that exposure therapy involves procedures and pathways similar to those during extinction of conditioned fear, one therapeutical extension might be useful that imposes artificial theta activity to prefrontal cortical-amygdalo-hippocampal pathways that mimics the directionality signaling successful extinction recall.

Role of Prefrontal Cortex Glucocorticoid Receptors in Stress and Emotion

Stress-related disorders (e.g., depression) are associated with hypothalamic-pituitary-adrenocortical axis dysregulation and prefrontal cortex (PFC) dysfunction, suggesting a functional link between aberrant prefrontal corticosteroid signaling and mood regulation. We used a virally mediated knockdown strategy (short hairpin RNA targeting the glucocorticoid receptor [GR]) to attenuate PFC GR signaling in the rat PFC. Adult male rats received bilateral microinjections of vector control or short hairpin RNA targeting the GR into the prelimbic (n = 44) or infralimbic (n = 52) cortices. Half of the animals from each injection group underwent chronic variable stress, and all were subjected to novel restraint. The first 2 days of chronic variable stress were used to assess depression- and anxiety-like behavior in the forced swim test and open field. The GR knockdown confined to the infralimbic PFC caused acute stress hyper-responsiveness, sensitization of stress responses after chronic variable stress, and induced depression-like behavior (increased immobility in the forced swim test). Knockdown of GR in the neighboring prelimbic PFC increased hypothalamic-pituitary-adrenocortical axis responses to acute stress and caused hyperlocomotion in the open field, but did not affect stress sensitization or helplessness behavior. The data indicate a marked functional heterogeneity of glucocorticoid action in the PFC and highlight a prominent role for the infralimbic GR in appropriate stress adaptation, emotional control, and mood regulation.

Fear extinction induces mGluR5-mediated synaptic and intrinsic plasticity in infralimbic neurons.

Studies suggest that plasticity in the infralimbic prefrontal cortex (IL) in rodents and its homolog in humans is necessary for inhibition of fear during the recall of fear extinction. The recall of extinction is impaired by locally blocking metabotropic glutamate receptor type 5 (mGluR5) activation in IL during extinction training. This finding suggests that mGluR5 stimulation may lead to IL plasticity needed for fear extinction. To test this hypothesis, we recorded AMPA and NMDA currents, AMPA receptor (AMPAR) rectification, and intrinsic excitability in IL pyramidal neurons in slices from trained rats using whole-cell patch-clamp recording. We observed that fear extinction increases the AMPA/NMDA ratio, consistent with insertion of AMPARs into IL synapses. In addition, extinction training increased inward rectification, suggesting that extinction induces the insertion of calcium-permeable (GluA2-lacking) AMPARs into IL synapses. Consistent with this, selectively blocking calcium-permeable AMPARs with Naspm reduced the AMPA EPSCs in IL neurons to a larger degree after extinction. Extinction-induced changes in AMPA/NMDA ratio, rectification, and intrinsic excitability were blocked with an mGluR5 antagonist. These findings suggest that mGluR5 activation leads to consolidation of fear extinction by regulating the intrinsic excitability of IL neurons and modifying the composition of AMPARs in IL synapses. Therefore, impaired mGluR5 activity in IL synapses could be one factor that causes inappropriate modulation of fear expression leading to anxiety disorders.