Consolidation of fear extinction requires NMDA receptor-dependent bursting in the ventromedial prefrontal cortex

Glutamate receptors in the medial geniculate nucleus are necessary for expression and extinction of conditioned fear in rats

Auditory fear conditioning requires anatomical projections from the medial geniculate nucleus (MGN) of the thalamus to the amygdala. Several lines of work indicate that the MGN is a critical sensory relay for auditory information during conditioning, but is not itself involved in the encoding of long-term fear memories. In the present experiments, we examined whether the MGN plays a similar role in the extinction of conditioned fear. Twenty-four hours after Pavlovian fear conditioning, rats received bilateral intra-thalamic infusions of either with NBQX (an AMPA receptor antagonist; Experiment 1) or MK-801 (an NMDA receptor antagonist; Experiment 1), anisomycin (a protein synthesis inhibitor; Experiment 2) or U0126 (a MEK inhibitor; Experiment 3) immediately prior to an extinction session in a novel context. The next day rats received a tone test in a drug-free state to assess their extinction memory; freezing served as an index of fear. Glutamate receptor antagonism prevented both the expression and extinction of conditioned fear. In contrast, neither anisomycin nor U0126 affected extinction. These results suggest that the MGN is a critical sensory relay for auditory information during extinction training, but is not itself a site of plasticity underlying the formation of the extinction memory.

Sustained conditioned responses in prelimbic prefrontal neurons are correlated with fear expression and extinction failure

During auditory fear conditioning, it is well established that lateral amygdala (LA) neurons potentiate their response to the tone conditioned stimulus, and that this potentiation is required for conditioned fear behavior. Conditioned tone responses in LA, however, last only a few hundred milliseconds and cannot be responsible for sustained fear responses to a tone lasting tens of seconds. Recent evidence from inactivation and stimulation studies suggests that the prelimbic (PL) prefrontal cortex is necessary for expression of learned fears, but the timing of PL tone responses and correlations with fear behavior have not been studied. Using multichannel unit recording techniques in behaving rats, we observed sustained conditioned tone responses in PL that were correlated with freezing behavior on a second-to-second basis during the presentation of a 30 s tone. PL tone responses were also correlated with conditioned freezing across different experimental phases (habituation, conditioning, extinction). Moreover, the persistence of PL responses after extinction training was associated with failure to express extinction memory. Together with previous inactivation findings, the present results suggest that PL transforms transient amygdala inputs to a sustained output that drives conditioned fear responses and gates the expression of extinction. Given the relatively long latency of conditioned responses we observed in PL (approximately 100 ms after tone onset), we propose that PL integrates inputs from the amygdala, hippocampus, and other cortical sources to regulate the expression of fear memories.

Amygdala inhibitory circuits and the control of fear memory

Classical fear conditioning is a powerful behavioral paradigm that is widely used to study the neuronal substrates of learning and memory. Previous studies have clearly identified the amygdala as a key brain structure for acquisition and storage of fear memory traces. Whereas the majority of this work has focused on principal cells and glutamatergic transmission and its plasticity, recent studies have started to shed light on the intricate roles of local inhibitory circuits. Here, we review current understanding and emerging concepts of how local inhibitory circuits in the amygdala control the acquisition, expression, and extinction of conditioned fear at different levels.

The role of context in the re-extinction of learned fear

Extinction of learned fear is both amygdala- and NMDA receptor (NMDAr)-dependent. Recent studies, however, have shown that extinction the second time (re-extinction) does not involve the amygdala and is NMDAr-independent. The present study compared the effects of context change on extinction and re-extinction in adult Sprague-Dawley rats. Experiment 1 showed that both extinction and re-extinction are context-specific with a renewal effect occurring in both cases. Experiment 2 then examined whether the transition from an NMDAr-dependent to an NMDAr-independent process was context-specific. As expected, the results showed that MK-801 (0.1 mg/kg) impaired initial extinction but did not impair re-extinction (i.e., re-extinction was found to be NMDAr-independent). A novel finding was that if re-extinction occurred in a context different from initial extinction, then MK-801 impaired re-extinction. In other words, re-extinction is NMDAr-dependent (i.e., like initial extinction) when it occurs in a different context to initial extinction. Therefore, the switch from NMDAr-dependent to NMDAr-independent extinction is both stimulus [Langton, J.M., Richardson, R. (2008). D-cycloserine facilitates extinction the first time but not the second time: An examination of the role of NMDA across the course of repeated extinction sessions. Neuropsychopharmacology, 33, 3096-3102.] and context-specific (the present study). The precise conditions that govern whether extinction requires NMDAr activation are of considerable theoretical interest and remain to be fully characterized.

Systemic propranolol acts centrally to reduce conditioned fear in rats without impairing extinction

BACKGROUND

Previous work has implicated noradrenergic beta-receptors in the consolidation and reconsolidation of conditioned fear. Less is known, however, about their role in fear expression and extinction. The beta-receptor blocker propranolol has been used clinically to reduce anxiety. With an auditory fear conditioning task in rats, we assessed the effects of systemic propranolol on the expression and extinction of two measures of conditioned fear: freezing and suppression of bar-pressing.

METHODS

One day after receiving auditory fear conditioning, rats were injected with saline, propranolol, or peripheral beta-receptor blocker sotalol (both 10 mg/kg, IP). Twenty minutes after injection, rats were given either 6 or 12 extinction trials and were tested for extinction retention the following day. The effect of propranolol on the firing rate of neurons in prelimbic (PL) prefrontal cortex was also assessed.

RESULTS

Propranolol reduced freezing by more than 50%, an effect that was evident from the first extinction trial. Suppression was also significantly reduced. Despite this, propranolol had no effect on the acquisition or retention of extinction. Unlike propranolol, sotalol did not affect fear expression, although both drugs significantly reduced heart rate. This suggests that propranolol acts centrally to reduce fear. Consistent with this, propranolol reduced the firing rate of PL neurons.

CONCLUSION

Propranolol reduced the expression of conditioned fear, without interfering with extinction learning. Reduced fear with intact extinction suggests a possible use for propranolol in reducing anxiety during extinction-based exposure therapies, without interfering with long-term clinical response.

mGluR5 has a critical role in inhibitory learning

The mechanisms that contribute to the extinction of previously acquired memories are not well understood. These processes, often referred to as inhibitory learning, are thought to be parallel learning mechanisms that require a reacquisition of new information and suppression of previously acquired experiences in order to adapt to novel situations. Using newly generated metabotropic glutamate receptor 5 (mGluR5) knock-out mice, we investigated the role of mGluR5 in the acquisition and reversal of an associative conditioned task and a spatial reference task. We found that acquisition of fear conditioning is partially impaired in mice lacking mGluR5. More markedly, we found that extinction of both contextual and auditory fear was completely abolished in mGluR5 knock-out mice. In the Morris Water Maze test (MWM), mGluR5 knock-out mice exhibited mild deficits in the rate of acquisition of the regular water maze task, but again had significant deficits in the reversal task, despite overall spatial memory being intact. Together, these results demonstrate that mGluR5 is critical to the function of neural circuits that are required for inhibitory learning mechanisms, and suggest that targeting metabotropic receptors may be useful in treating psychiatric disorders in which aversive memories are inappropriately retained.

Extinction circuits for fear and addiction overlap in prefrontal cortex

Extinction is a form of inhibitory learning that suppresses a previously conditioned response. Both fear and drug seeking are conditioned responses that can lead to maladaptive behavior when expressed inappropriately, manifesting as anxiety disorders and addiction, respectively. Recent evidence indicates that the medial prefrontal cortex (mPFC) is critical for the extinction of both fear and drug-seeking behaviors. Moreover, a dorsal-ventral distinction is apparent within the mPFC, such that the prelimbic (PL-mPFC) cortex drives the expression of fear and drug seeking, whereas the infralimbic (IL-mPFC) cortex suppresses these behaviors after extinction. For conditioned fear, the dorsal-ventral dichotomy is accomplished via divergent projections to different subregions of the amygdala, whereas for drug seeking, it is accomplished via divergent projections to the subregions of the nucleus accumbens. Given that the mPFC represents a common node in the extinction circuit for these behaviors, treatments that target this region may help alleviate symptoms of both anxiety and addictive disorders by enhancing extinction memory.

Brief neonatal maternal separation alters extinction of conditioned fear and corticolimbic glucocorticoid and NMDA receptor expression in adult rats

Neonatal maternal separation alters adult HPA axis responsiveness to stress, adult emotionality, and glucocorticoid receptor (GR) concentrations in forebrain regions such as hippocampus. To investigate effects of neonatal maternal separation on emotion regulation and its neural substrates, we assessed acquisition and extinction of conditioned fear in adult rats that underwent neonatal maternal separation. Corticolimbic structures including basolateral amygdala and medial prefrontal cortex are critical for acquisition and extinction of conditioned fear, and such learning is N-methyl-D-aspartic acid (NMDA) receptor-dependent. Thus, we used immunohistochemistry to assess expression of the GR and the NR1 subunit of the NMDA receptor in basolateral amygdala and medial prefrontal cortex. On postnatal days 2-14, pups underwent control rearing or maternal separation for 15 min per day. Fear conditioning and extinction in adulthood were then assessed in male rats. Rats received five tone-alone habituation trials, then seven tone/footshock pairings. After 1 h, rats received tone-alone extinction trials to criterion, and 15 recall of extinction trials the next day. Brains were processed for immunohistochemical labeling of GR and NR1, and staining was quantified. Brief maternal separation did not alter acquisition or initial extinction, but impaired extinction recall. Brief maternal separation did not alter GR or NR1 expression in basolateral amygdala. However, brief maternal separation increased GR and decreased NR1 expression specifically in the infralimbic region of medial prefrontal cortex, consistent with work implicating this area in extinction recall. Thus, brief maternal separation impaired extinction recall and altered GR and NR1 expression in its neural substrate in adults.

Brain region-specific gene expression activation required for reconsolidation and extinction of contextual fear memory

During fear conditioning, animals learn an association between a previously neutral or conditioned stimulus (CS) and an aversive or unconditioned stimulus (US). Subsequent reexposure to the CS alone triggers two competing processes. Brief reexposure to the CS initiates reconsolidation processes that serve to stabilize or maintain the original CS-US memory. In contrast, more prolonged reexposure to the CS leads to the formation of an inhibitory extinction (CS-no US) memory. Previous studies have established that both reconsolidation and extinction require gene expression. Consistent with this, here we first show that genetic disruption of cAMP-responsive element-binding protein (CREB)-mediated transcription blocks both reconsolidation and long-term extinction of contextual fear memory. We next asked whether reconsolidation and extinction engage CREB-mediated transcription in distinct brain regions. Accordingly, we used immunohistochemical approaches to characterize the activation of the transcription factor CREB [as well as the expression of the CREB-dependent gene Arc (activity-regulated cytoskeleton-associated protein)] after brief versus prolonged reexposure to a previously conditioned context. After brief reexposure, we observed significant activation of CREB-mediated gene expression in the hippocampus and amygdala. In contrast, after the prolonged reexposure, we observed significant activation of CREB-mediated gene expression in the amygdala and prefrontal cortex. Finally, we showed that blocking protein synthesis in either the hippocampus or the amygdala blocked reconsolidation of contextual fear memory, whereas similar blockade in the amygdala and prefrontal cortex prevented the formation of extinction memory. These experiments establish that reactivated contextual fear memories undergo CREB-dependent reconsolidation or extinction in distinct brain regions.

Cholinergic direct inhibition of N-methyl-D aspartate receptor-mediated currents in the rat neocortex

Acetylcholine (ACh) and N-methyl-D aspartate receptors (NMDARs) interact in the regulation of multiple important brain functions. NMDAR activation is indirectly modulated by ACh through the activation of muscarinic or nicotinic receptors. Scant information is available on whether ACh directly interacts with the NMDAR. By using a cortical brain slice preparation we found that the application of ACh and of other drugs acting on muscarinic or nicotinic receptors induces an acute and reversible reduction of NMDAR-mediated currents (I(NMDA)), ranging from 20 to 90% of the control amplitude. The reduction displayed similar features in synaptic I(NMDA) in brain slices, as well as in currents evoked by NMDA application in brain slices or from acutely dissociated cortical cells, demonstrating its postsynaptic nature. The cholinergic inhibition of I(NMDA) displayed an onset-offset rate in the order of a second, and was resistant to the presence of the muscarinic antagonist atropine (10 microM) in the extracellular solution, and of G-protein blocker GDP(beta)S (500 microM) and activator GTP(gamma)S (400 microM) in the intracellular solution, indicating that it was not G-protein dependent. Recording at depolarized or hyperpolarized holding voltages reduced NMDAR-mediated currents to similar extents, suggesting that the inhibition was voltage-independent, whereas the reduction was markedly more pronounced in the presence of glycine (20 microM). A detailed analysis of the effects of tubocurarine suggested that at least this drug interfered with glycine-dependent NMDAR-activity. We conclude that NMDAR-mediated current scan be inhibited directly by cholinergic drugs, possibly by direct interaction within one or more subunits of the NMDAR. Our results could supply a new interpretation to previous studies on the role of ACh at the glutamatergic synapse.

Extinction memory is impaired in schizophrenia

BACKGROUND

Schizophrenia is associated with abnormalities in emotional processing and social cognition, which might result from disruption of the underlying neural mechanism(s) governing emotional learning and memory. To investigate this possibility, we measured the acquisition and extinction of conditioned fear responses and delayed recall of extinction in schizophrenia and control subjects.

METHODS

Twenty-eight schizophrenia and 18 demographically matched control subjects underwent a 2-day fear conditioning, extinction learning, and extinction recall procedure, in which skin conductance response (SCR) magnitude was used as the index of conditioned responses.

RESULTS

During fear acquisition, 83% of the control subjects and 57% of the patients showed autonomic responsivity ("responders"), and the patients showed larger SCRs to the stimulus that was not paired with the unconditioned stimulus (CS-) than the control subjects. Within the responder group, there was no difference between the patients and control subjects in levels of extinction learning; however, the schizophrenia patients showed significant impairment, relative to the control subjects, in context-dependent recall of the extinction memory. In addition, delusion severity in the patients correlated with baseline skin conductance levels.

CONCLUSIONS

These data are consistent with prior evidence for a heightened neural response to innocuous stimuli in schizophrenia and elevated arousal levels in psychosis. The finding of deficient extinction recall in schizophrenia patients who showed intact extinction learning suggests that schizophrenia is associated with a disturbance in the neural processes supporting emotional memory.

A history of corticosterone exposure regulates fear extinction and cortical NR2B, GluR2/3, and BDNF

A history of exposure to stressors may be a predisposing factor for developing posttraumatic stress disorder (PTSD) after trauma. Extinction of conditioned fear appears to be impaired in PTSD, but the consequences of prior stress or excess glucocorticoid exposure for extinction learning are not known. We report that prior chronic exposure to the stress hormone, corticosterone (CORT), decreases endogenous CORT secretion upon context reexposure and impairs extinction after contextual fear conditioning in rats, while leaving fear memory acquisition and expression intact. Posttraining administration of the glucocorticoid receptor (GR) antagonist, RU38486, partially mimicked prior CORT exposure effects on freezing during fear extinction training. Extinction of conditioned fear is an active learning process thought to involve glutamatergic targets--including specific NMDA and AMPA receptor subunits--in the ventromedial prefrontal cortex (vmPFC), which includes the prelimbic, infralimbic, and medial orbitofrontal cortices. After CORT exposure, decreases in the NMDA receptor NR2B subunit and AMPA receptor subunits, GluR2/3, as well as brain-derived neurotrophic factor, were detected in cortical regions, but not dorsal hippocampus (CA1). Receptor subunit expression levels in the vmPFC correlated with freezing during training. In addition, prior CORT selectively decreased sucrose preference, consistent with established models of anhedonia and with blunted affect in PTSD. Together, these data suggest a cellular mechanism by which chronically elevated glucocorticoid exposure--as may be experienced during repeated exposure to stressors--interferes with the neural systems that modulate behavioral flexibility and may thereby contribute to psychopathological fear states.

The facilitative effects of D-cycloserine on extinction of a cocaine-induced conditioned place preference can be long lasting and resistant to reinstatement

RATIONALE AND OBJECTIVE

The N-methyl-D-aspartate receptor agonist, D-cycloserine (DCS), accelerates extinction of a cocaine-induced conditioned place preference (CPP) when given after daily extinction tests. Here, we studied the effects of DCS in rats given spaced-extinction sessions at 3- or 7-day intervals using two different extinction procedures.

MATERIALS AND METHODS

Rats were trained on a CPP (four cocaine, 10 mg/kg, i.p., and four saline pairings with one of two compartments). Immediately following the CPP test and all extinction tests (days 4, 7, 10, and 24, experiment 1), DCS (15 mg/kg, i.p.) or saline was administered. In experiment 2, extinction was conducted by exposing rats to the drug-paired cues for 2 or 20 min, three times, at 7-day intervals followed immediately by DCS or saline. After extinction, tests for retention and cocaine-induced reinstatement were given.

RESULTS

In experiment 1, rats given DCS lost the cocaine CPP after one extinction trial, an effect that persisted for 2 weeks after the last DCS injection and that was resistant to cocaine-induced reinstatement. In experiment 2, extinction was facilitated by DCS compared to saline when rats received 2-min exposures to the conditioned stimulus. Longer 20-min exposures minus/plus repeated testing led to retention of extinction in both groups regardless of DCS treatment.

CONCLUSIONS

Extinction of appetitive conditioning is facilitated by DCS after 1-3 post-spaced trial injections, and retention is lasting and resistant to reinstatement. The facilitative effects appear early in extinction, but when extinction procedures are intensive, DCS appears to have no additional benefit.

Chronic cannabinoid administration in vivo compromises extinction of fear memory

Endocannabinoids are critically involved in the extinction of fear memory. Here we examined the effects of repeated cannabinoid administration on the extinction of fear memory in rats and on inhibitory synaptic transmission in medial prefrontal cortex (mPFC) slices. Rats were treated with the CB1 receptor agonist WIN55212-2 (WIN 10 mg/kg, i.p.) once per day for 7 d. On day 8, the rats were submitted to a standard fear conditioning procedure, and retention of memory was measured with potentiated startle paradigm. We found that (1) WIN-pretreated rats exhibited much less extinction to cue alone presentations; (2) the reduction of fear-potentiated startle normally seen when the CB1 receptor agonists were infused into the mPFC was absent in the WIN-pretreated rats; (3) WIN-induced inhibition of GABAergic transmission was significantly less in slices from the WIN-pretreated rats than that from the vehicle-pretreated control; (4) WIN failed to induce extracellular signal-regulated kinases (ERKs) phosphorylation in the WIN-pretreated rats; and (5) the level of CB1 receptor in the WIN-pretreated rats was lower than that of vehicle-pretreated rats. These results suggest that endocannabinoids within the mPFC play an important role in the extinction of conditioned fear. However, long-term marijuana use may limit its clinical efficacy for the treatment of anxiety disorders.

D-cycloserine facilitates extinction the first time but not the second time: an examination of the role of NMDA across the course of repeated extinction sessions

Extinction of learned fear is facilitated by the partial NMDA agonist D-cycloserine (DCS). However, some studies suggest that the involvement of NMDA in learning differs depending on whether learning is for the first or second time. The current study aimed to extend these findings by examining the role of NMDA in extinction for the first and the second time. Specifically, the present series of experiments used Pavlovian fear conditioning and extinction paradigms to compare the effect of DCS on extinction of fear to a light CS the first and second time around. As found previously, DCS facilitated extinction of learned fear (Experiment 1). A novel finding, however, was that DCS did not facilitate the re-extinction of fear to this same CS following retraining (Experiments 2A and 2B). Finally, it was demonstrated that the transition from NMDA-dependent to NMDA-independent extinction was stimulus specific (Experiment 3). That is, rats were first trained to fear a CS (light); this fear was then extinguished. Following this, rats were then retrained to fear the same CS (light) or a new CS (white noise). When given a second extinction session, DCS was found to facilitate extinction of the new CS but not the original CS. The results of this series of experiments suggest that the role of NMDA in extinction depends on whether extinction is new learning (first extinction) or retrieval of a previous extinction memory (re-extinction).

Consolidation patterns of human motor memory

Can memories be unlearned, or is unlearning a form of acquiring a new memory that competes with the old, effectively masking it? We considered motor memories that were acquired when people learned to use a novel tool. We trained people to reach with tool A and quantified recall in error-clamp trials, i.e., trials in which the memory was reactivated but error-dependent learning was minimized. We measured both the magnitude of the memory and its resistance to change. With passage of time between acquisition and reactivation (up to 24 h), memory of A slowly declined, but with reactivation remained resistant to change. After learning of tool A, brief exposure to tool B brought performance back to baseline, i.e., apparent extinction. Yet, for up to a few minutes after A+B training, output in error-clamp trials increased from baseline to match those who had trained only in A. This spontaneous recovery and convergence demonstrated that B did not produce any unlearning of A. Rather, it masked A with a new memory that was very fragile. We tracked the memory of B as a function of time and found that within minutes it was transformed from a fragile to a more stable state. Therefore, a sudden performance error in a well-learned motor task does not produce unlearning, but rather installs a competing but fragile memory that with passage of time acquires stability. Learning not only engages processes that adapt at multiple timescales, but once practice ends, the fast states are partially transformed into slower states.

Impaired extinction of learned fear in rats selectively bred for high anxiety--evidence of altered neuronal processing in prefrontal-amygdala pathways

The impaired extinction of acquired fear is a core symptom of anxiety disorders, such as post-traumatic stress disorder, phobias or panic disorder, and is known to be particularly resistant to existing pharmacotherapy. We provide here evidence that a similar relationship between trait anxiety and resistance to extinction of fear memory can be mimicked in a psychopathologic animal model. Wistar rat lines selectively bred for high (HAB) or low (LAB) anxiety-related behaviour were tested in a classical cued fear conditioning task utilizing freezing responses as a measure of fear. Fear acquisition was similar in both lines. In the extinction trial, however, HAB rats showed a marked deficit in the attenuation of freezing responses to repeated auditory conditioned stimulus presentations as compared with LAB rats, which exhibited rapid extinction. To gain information concerning the putatively altered neuronal processing associated with the differential behavioural response between HAB and LAB rats, c-Fos expression was investigated in the main prefrontal-amygdala pathways important for cued fear extinction. HAB compared to LAB rats showed an attenuated c-Fos response to repeated conditioned stimulus presentations in infralimbic and cingulate cortices, as well as in the lateral amygdala, but facilitated the c-Fos response in the medial part of the central amygdala. In conclusion, the present results support the notion that impaired extinction in high anxiety rats is accompanied by an aberrant activation profile in extinction-relevant prefrontal-amygdala circuits. Thus, HAB rats may represent a clinically relevant model to study the mechanisms and potential targets to accelerate delayed extinction processes in subjects with enhanced trait anxiety.

Plasticity and memory in the prefrontal cortex

Neuropsychological and neuroimaging studies in humans have shown that the prefrontal cortex (PFC) is involved in long-term memory functioning. In general, the participation of the PFC in long-term memory has been attributed to its role in executive control rather than information storage. Accumulating data from recent animal studies, however, suggest the possible role of the PFC in the storage of long-term memory. In support of this view, there is evidence that various projection systems in the PFC support long-term synaptic plasticity. Recording studies have further demonstrated neural correlates of learning in various animal species. Lastly, behavioral and physiological studies indicate that the PFC is critically involved in memory consolidation, retrieval and extinction processes. These studies then suggest that the PFC is an integral part of the neural network where long-term memory trace is stored and retrieved. Though decisive evidence is still lacking at present, we propose here to assign a term 'control memory' (i.e., memory for top-down control processes) as a new type of memory function for the PFC. This new principle of PFC-long-term memory can help organize existing data and provide novel insights into future empirical studies.

Disruption of AMPA receptor endocytosis impairs the extinction, but not acquisition of learned fear

Synaptic plasticity in the form of long-term potentiation (LTP) plays a critical role in the formation of a Pavlovian fear association. However, the role that synaptic plasticity plays in the suppression of a learned fear response remains to be clarified. Here, we assessed the role that long-term depression (LTD) plays in the acquisition, expression, and extinction of a conditioned fear response. We report that blockade of LTD with a GluR2-derived peptide (Tat-GluR2(3Y); 1.5 micromol/kg, i.v.) that blocks regulated alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptor endocytosis during an initial extinction training session disrupted both the expression and recall of extinction learning. A similar impairment of extinction during training, but not recall, was observed when NMDA receptor-dependent LTD was inhibited through the selective blockade of NMDA NR2B receptors with Ro 25-6981. In contrast, blockade of LTD with Tat-GluR2(3Y) during fear conditioning or during a fear recall test did not effect the expression or recall of either contextual or cue-induced conditioned fear. Similarly, administration of Tat-GluR2(3Y) prior to an extinction recall test did not affect spontaneous recovery or rate of re-extinction in previously extinguished rats. These data demonstrate that AMPA receptor endocytosis does not mediate acquisition or expression of conditioned fear, but may play a role in the extinction of fear memories. Furthermore, these findings suggest that LTD may be a molecular mechanism that facilitates the selective modification of a learned association while leaving intact the ability to form a new memory.

Distinct contributions of the basolateral amygdala and the medial prefrontal cortex to learning and relearning extinction of context conditioned fear

We studied the roles of the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) in learning and relearning to inhibit context conditioned fear (freezing) in extinction. In Experiment 1, pre-extinction BLA infusion of the NMDA receptor (NMDAr) antagonist, ifenprodil, impaired the development and retention of inhibition but post-extinction infusion spared retention. Pre-extinction infusion of the GABA(A) agonist, muscimol, depressed freezing and impaired retention as did post-extinction infusion. In Experiment 2, pre-extinction mPFC infusion of ifenprodil spared the development of inhibition whereas muscimol depressed freezing. Both impaired retention when infused pre- or post-extinction. Thus, the development of inhibition involves NMDAr activation in the BLA, whereas its consolidation involves both NMDAr activation in the mPFC and NMDAr-independent mechanisms in the BLA. In Experiment 3, BLA infusion of ifenprodil impaired relearning and retention of inhibition when infused before but did not impair retention when infused after re-extinction. BLA infusion of muscimol depressed freezing but did not impair retention when infused before or after re-extinction. In Experiment 4, mPFC infusion of ifenprodil impaired relearning when infused before re-extinction, whereas muscimol depressed responses. Both drugs impaired retention when infused into the mPFC before or after re-extinction. Thus, relearning to inhibit fear responses involves NMDAr activation in both the BLA and mPFC and consolidation of the inhibitory memory involves NMDAr activation in the mPFC. However, relearning and consolidation occur in the absence of neuronal activity within the BLA. We propose that NMDAr in the mPFC supports relearning inhibition when the BLA is inactivated.

Neurophysiological responses to traumatic reminders in the acute aftermath of serious motor vehicle collisions using [15O]-H2O positron emission tomography

BACKGROUND

Neuroimaging studies report that individuals with posttraumatic stress disorder show abnormal responses in the amygdala and medial prefrontal cortex (mPFC)/anterior cingulate cortex (ACC) during exposure to traumatic reminders. However, neural responses arising in the early aftermath of a traumatic event have not been studied.

METHODS

Twenty-two motor vehicle collision survivors and 12 nontraumatized control subjects participated. Regional cerebral blood flow (rCBF) was measured using [(15)O]-H(2)O positron emission tomography (PET) at rest and as subjects listened to scripts of traumatic and neutral events. Self-report measures rated emotional responses to the scripts; standardized assessments (Impact of Events--Revised) evaluated acute stress symptoms at scanning and at 3-month follow-up. Most subjects improved symptomatically.

RESULTS

At rest, trauma subjects showed hyperperfusion in right mPFC/ACC and hypoperfusion in right amygdala compared with control subjects. In trauma subjects, listening to trauma scripts versus neutral scripts resulted in decreased flow in the right amygdala and left amygdala/perirhinal cortex, and symptom scores correlated negatively with right hippocampal flow changes. Symptom improvement at 3 months correlated negatively with rCBF changes in right perirhinal cortex and hippocampus during the trauma versus neutral script contrast. Subjective disturbance during the trauma versus neutral contrast correlated positively with rCBF changes in right amygdala and left mPFC. Functional connectivity analyses of rCBF changes during trauma versus neutral scripts demonstrated left amygdala coupling with right ACC and bilateral anterior insula, as well as coupling between the amygdala and contralateral hippocampus.

CONCLUSIONS

In recently traumatized subjects functional interactions between the amygdala, perirhinal cortex and ACC/mPFC that occur during exposure to traumatic reminders may underlie adaptive/recuperative processes.

Targeting extinction and reconsolidation mechanisms to combat the impact of drug cues on addiction

Drug addiction is a progressive and compulsive disorder, where recurrent craving and relapse to drug-seeking occur even after long periods of abstinence. A major contributing factor to relapse is drug-associated cues. Here we review behavioral and pharmacological studies outlining novel methods of effective and persistent reductions in cue-induced relapse behavior in animal models. We focus on extinction and reconsolidation of cue-drug associations as the memory processes that are the most likely targets for interventions. Extinction involves the formation of new inhibitory memories rather than memory erasure; thus, it should be possible to facilitate the extinction of cue-drug memories to reduce relapse. We propose that context-dependency of extinction might be altered by mnemonic agents, thereby enhancing the efficacy of cue-exposure therapy as treatment strategy. In contrast, interfering with memory reconsolidation processes can disrupt the integrity or strength of specific cue-drug memories. Reconsolidation is argued to be a distinct process that occurs over a brief time period after memory is reactivated/retrieved - when the memory becomes labile and vulnerable to disruption. Reconsolidation is thought to be an independent, perhaps opposing, process to extinction and disruption of reconsolidation has recently been shown to directly affect subsequent cue-drug memory retrieval in an animal model of relapse. We hypothesize that a combined approach aimed at both enhancing the consolidation of cue-drug extinction and interfering with the reconsolidation of cue-drug memories will have a greater potential for persistently inhibiting cue-induced relapse than either treatment alone.

Amygdala intercalated neurons are required for expression of fear extinction

Congruent findings from studies of fear learning in animals and humans indicate that research on the circuits mediating fear constitutes our best hope of understanding human anxiety disorders. In mammals, repeated presentations of a conditioned stimulus that was previously paired to a noxious stimulus leads to the gradual disappearance of conditioned fear responses. Although much evidence suggests that this extinction process depends on plastic events in the amygdala, the underlying mechanisms remain unclear. Intercalated (ITC) amygdala neurons constitute probable mediators of extinction because they receive information about the conditioned stimulus from the basolateral amygdala (BLA), and contribute inhibitory projections to the central nucleus (CEA), the main output station of the amygdala for conditioned fear responses. Thus, after extinction training, ITC cells could reduce the impact of conditioned-stimulus-related BLA inputs to the CEA by means of feed-forward inhibition. Here we test the hypothesis that ITC neurons mediate extinction by lesioning them with a toxin that selectively targets cells expressing micro-opioid receptors (microORs). Electron microscopic observations revealed that the incidence of microOR-immunoreactive synapses is much higher in ITC cell clusters than in the BLA or CEA and that microORs typically have a post-synaptic location in ITC cells. In keeping with this, bilateral infusions of the microOR agonist dermorphin conjugated to the toxin saporin in the vicinity of ITC neurons caused a 34% reduction in the number of ITC cells but no significant cell loss in surrounding nuclei. Moreover, ITC lesions caused a marked deficit in the expression of extinction that correlated negatively with the number of surviving ITC neurons but not CEA cells. Because ITC cells exhibit an unusual pattern of receptor expression, these findings open new avenues for the treatment of anxiety disorders.

Dissociable roles for the ventromedial prefrontal cortex and amygdala in fear extinction: NR2B contribution

Fear extinction, which involves learning to suppress the expression of previously learned fear, requires N-methyl-D-aspartate receptors (NMDARs) and is mediated by the amygdala and ventromedial prefrontal cortex (vmPFC). Like other types of learning, extinction involves acquisition and consolidation phases. We recently demonstrated that NR2B-containing NMDARs (NR2Bs) in the lateral amygdala (LA) are required for extinction acquisition, but whether they are involved in consolidation is not known. Further, although it has been shown that NMDARs in the vmPFC are required for extinction consolidation, whether NR2Bs in vmPFC are involved in consolidation is not known. In this report, we investigated the possible role of LA and vmPFC NR2Bs in the consolidation of fear extinction using the NR2B-selective antagonist ifenprodil. We show that systemic treatment with ifenprodil immediately after extinction training disrupts extinction consolidation. Ifenprodil infusion into vmPFC, but not the LA, immediately after extinction training also disrupts extinction consolidation. In contrast, we also show pre-extinction training infusions into vmPFC has no effect. These results, together with our previous findings showing that LA NR2Bs are required during the acquisition phase in extinction, indicate a double dissociation for the phase-dependent role of NR2Bs in the LA (acquisition, not consolidation) and vmPFC (consolidation, not acquisition).

The role of prefrontal cortex CB1 receptors in the modulation of fear memory

Understanding the mechanism of how fear memory can be extinguished could provide potential therapeutic strategies for the treatment of posttraumatic stress disorders. Here we show that infusion of CB1 receptor antagonist into the infralimbic (IL) subregion of the medial prefrontal cortex (mPFC) retarded cue-alone-induced reduction of fear-potentiated startle. Conversely, cannabinoid agonist WIN55212-2 (WIN) facilitated the extinction. Unexpectedly, administration of WIN without cue-alone trials reduced startle potentiation in a dose-dependent manner. The effect of cannabinoid agonists was mimicked by endocannabinoid uptake or fatty acid amide hydrolase inhibitors. Rats were trained with 10 conditioned stimulus (CS(+)) (yellow light)-shock pairings. Extinction training with CS(+) (yellow light)-alone but not CS(-) (blue light)-alone trials decreased fear-potentiated startle. Intra-IL infusion of WIN before CS(-)-alone trials decreased startle potentiation, suggesting that the cannabinoid agonist decreased conditioned fear irrespective of whether the rats underwent CS(+)- or CS(-)-alone trials. Cannabinoid agonists activated extracellular signal-regulated kinases (ERKs) in mPFC slices, and ERK inhibitor blocked the effect of cannabinoid agonists on fear-potentiated startle. These results suggest that CB1 receptors acting through the phosphorylation of ERK are involved not only in the extinction of conditioned fear but also in the adaptation to aversive situations in general.

Fear conditioning and extinction differentially modify the intrinsic excitability of infralimbic neurons

Extinction of conditioned fear is an active learning process involving inhibition of fear expression. It has been proposed that fear extinction potentiates neurons in the infralimbic (IL) prefrontal cortex, but the cellular mechanisms underlying this potentiation remain unknown. It is also not known whether this potentiation occurs locally in IL neurons as opposed to IL afferents. To determine whether extinction enhances the intrinsic excitability of IL pyramidal neurons in layers II/III and V, we performed whole-cell patch-clamp recordings in slices from naive, conditioned, or conditioned-extinguished rats. We observed that conditioning depressed IL excitability compared with slices from naive animals, as evidenced by a decreased number of spikes evoked by injected current and an increase in the slow afterhyperpolarizing potential (sAHP). Extinction reversed these conditioning-induced effects. Furthermore, IL neurons from extinguished rats showed increased burst spiking compared with naive rats, which was correlated with extinction recall. These changes were specific to IL prefrontal cortex and were not observed in prelimbic prefrontal cortex. Together, these findings suggest that IL intrinsic excitability is reduced to allow for expression of conditioning memory and enhanced for expression of extinction memory through the modulation of Ca(2+)-gated K(+) channels underlying the sAHP. Inappropriate modulation of these intrinsic mechanisms may underlie anxiety disorders, characterized by exaggerated fear and deficient extinction.

The basolateral amygdala is necessary for learning but not relearning extinction of context conditioned fear

Extinction of conditioned fear involves new learning that inhibits but does not eliminate the original fear memory. This inhibitory learning is thought to require activation of NMDA receptors (NMDAr) within the basolateral amygdala (BLA). However, once extinction has been learned, the role played by the BLA during subsequent extinction procedures remains unknown. The present study examined the role of neuronal activity and NMDAr activation in rats receiving their first or second extinction of context fear. We found that BLA infusion of DL-APV, a competitive antagonist of NMDAr, depressed fear responses at both the first and second extinction. It impaired learning extinction but spared and even facilitated relearning extinction. BLA infusion of muscimol, a GABA(A) agonist, produced a similar outcome, suggesting that DL-APV not only blocked NMDAr-dependent plasticity but also disrupted neuronal activity. In contrast, infusion of ifenprodil, a more selective antagonist of NMDAr containing the NR2B subunit, did not depress fear responses but impaired short- and long-term inhibition of fear at both the first and second extinction. Therefore, we suggest that relearning extinction normally requires NMDAr containing the NR2B subunit in the BLA. However, simultaneous blockade of these receptors and neuronal activity in the BLA results in compensatory learning that is able to promote long-term re-extinction. These data are consistent with a current model that attributes fear extinction to interactions between several neural substrates, including the amygdala and the medial prefrontal cortex.

Microinfusion of the D1 receptor antagonist, SCH23390 into the IL but not the BLA impairs consolidation of extinction of auditory fear conditioning

In auditory fear conditioning, repeated presentation of the tone in the absence of the shock leads to extinction of the acquired fear response. Both the medial prefrontal cortex (mPFC) and the basolateral amygdala (BLA) are involved in extinction. Here we examined this involvement by antagonizing D1 receptors in both regions, in the rat. We microinfused the D1 receptor antagonist, SCH23390, into the infra-limbic part of the mPFC (IL) or BLA at different time points. SCH23390 mircoinfused into the IL either before extinction acquisition or following short extinction training resulted in impairment of extinction consolidation. Microinfusion of SCH23390 into the BLA, prior to acquisition of extinction caused impairment in acquisition of extinction without affecting extinction consolidation. This is supported by the results showing that microinfusion of SCH23390 into the BLA following a short-training session did not affect consolidation. These results further strengthen the role of mPFC in consolidation of extinction while highlighting the role of the D1 receptors in this process.

Extinction learning: neurological features, therapeutic applications and the effect of aging

Extinction learning consists of the usually gradual inhibition of the retrieval of a previously learned response or behavior. It is widely used for the treatment of syndromes of learned fear, such as phobias and post-traumatic stress disorder. It relies on well-identified molecular processes in the hippocampus, basolateral amygdala, ventromedial prefrontal cortex (vmPFC) and entorhinal cortex. In humans, thickness of the orbital cortex, vmPFC and the anterior cingulate cortex correlates with the capacity to extinguish. The three regions are functionally inter-related (see below). The development of learned fear syndromes in humans is viewed by many as being due to a deficit of extinction, and so of the capacity to deal with fear. Blockade of NMDA receptors, inhibition of protein synthesis in the vmPFC or blockade of protein synthesis or of various molecular signaling cascades in the hippocampus, amygdala or entorhinal cortex impairs extinction. d-cycloserine, a partial agonist at NMDA receptors, enhances extinction in animals and humans and may help extinction to exert its therapeutic effect. Cannabinoids also enhance extinction, acting through CB1 receptors, but their therapeutic use is not warranted.

GABA(A) receptors determine the temporal dynamics of memory retention

Four experiments studied the role of GABA(A) receptors in the temporal dynamics of memory retention. Memory for an active avoidance response was a nonmonotonic function of the retention interval. When rats were tested shortly (2 min) or some time (24 h) after training, retention was excellent, but when they were tested at intermediate intervals (1-4 h), retention was poor. Activity at GABA(A) receptors was critical for impairing memory retention at the intermediate intervals because injection of the GABA(A) receptor partial inverse agonist FG7142 prior to test significantly improved performance. These retention enhancing effects of FG7142 were dose-dependent and not due to any nonspecific effects of FG7142 on activity. Our results suggest that the temporal dynamics of memory retention may be caused by variations in neurotransmission through the GABA(A) receptor in the post-training period.

Noradrenergic signaling in infralimbic cortex increases cell excitability and strengthens memory for fear extinction

Emotional arousal strengthens memory. This is most apparent in aversive conditioning, in which the stress-related neurotransmitter norepinephrine (NE) enhances associations between sensory stimuli and fear-inducing events. In contrast to conditioning, extinction decreases fear responses, and is thought to form a new memory. It is not known, however, whether NE is necessary for extinction learning. Previous work has shown that the infralimbic prefrontal cortex (IL) is a site of extinction consolidation. Here, we show that blocking noradrenergic beta-receptors in IL before extinction training impaired retrieval of extinction the following day, consistent with a weakened extinction memory. We further found that the sequelae of beta-receptor activation, including protein kinase A (PKA), gene transcription and translation in IL, are necessary for extinction. To determine whether activation of this cascade modulates IL excitability, we measured the response of IL pyramidal neurons to injected current. NE increased the excitability of IL neurons in a beta-receptor- and PKA-dependent manner. We suggest that NE released in IL during fear extinction activates a PKA-mediated molecular cascade that strengthens extinction memory. Thus, emotional arousal evoked by conditioned fear paradoxically promotes the subsequent extinction of that fear, thereby ensuring behavioral flexibility.

The role of the entorhinal cortex in extinction: influences of aging

The entorhinal cortex is perhaps the area of the brain in which neurofibrillary tangles and amyloid plaques are first detectable in old age with or without mild cognitive impairment, and very particularly in Alzheimer's disease. It plays a key role in memory formation, retrieval, and extinction, as part of circuits that include the hippocampus, the amygdaloid nucleus, and several regions of the neocortex, in particular of the prefrontal cortex. Lesions or biochemical impairments of the entorhinal cortex hinder extinction. Microinfusion experiments have shown that glutamate NMDA receptors, calcium and calmodulin-dependent protein kinase II, and protein synthesis in the entorhinal cortex are involved in and required for extinction. Aging also hinders extinction; it is possible that its effect may be in part mediated by the entorhinal cortex.

Neural mechanisms of extinction learning and retrieval

Emotional learning is necessary for individuals to survive and prosper. Once acquired, however, emotional associations are not always expressed. Indeed, the regulation of emotional expression under varying environmental conditions is essential for mental health. The simplest form of emotional regulation is extinction, in which conditioned responding to a stimulus decreases when the reinforcer is omitted. Two decades of research on the neural mechanisms of fear conditioning have laid the groundwork for understanding extinction. In this review, we summarize recent work on the neural mechanisms of extinction learning. Like other forms of learning, extinction occurs in three phases: acquisition, consolidation, and retrieval, each of which depends on specific structures (amygdala, prefrontal cortex, hippocampus) and molecular mechanisms (receptors and signaling pathways). Pharmacological methods to facilitate consolidation and retrieval of extinction, for both aversive and appetitive conditioning, are setting the stage for novel treatments for anxiety disorders and addictions.

The expression of contextual fear conditioning involves activation of an NMDA receptor-nitric oxide pathway in the medial prefrontal cortex

The ventral portion of medial prefrontal cortex (vMPFC) is involved in contextual fear-conditioning expression in rats. In the present study, we investigated the role of local N-methyl-D-aspartic acid (NMDA) glutamate receptors and nitric oxide (NO) in vMPFC on the behavioral (freezing) and cardiovascular (increase of arterial pressure and heart rate) responses of rats exposed to a context fear conditioning. The results showed that both freezing and cardiovascular responses to contextual fear conditioning were reduced by bilateral administration of NMDA receptor antagonist LY235959 (4 nmol/200 nL) into the vMPFC before reexposition to conditioned chamber. Bilateral inhibition of neuronal NO synthase (nNOS) by local vMPFC administration of the Nω-propyl-L-arginine (N-propyl, 0.04 nmol/200 nL) or the NO scavenger carboxy-PTIO (1 nmol/200 nL) caused similar results, inhibiting the fear responses. We also investigated the effects of inhibiting glutamate- and NO-mediated neurotransmission in the vMPFC at the time of aversive context exposure on reexposure to the same context. It was observed that the 1st exposure results in a significant attenuation of the fear responses on reexposure in vehicle-treated animals, which was not modified by the drugs. The present results suggest that a vMPFC NMDA–NO pathway may play an important role on expression of contextual fear conditioning.

Selective activation of medial prefrontal-to-accumbens projection neurons by amygdala stimulation and Pavlovian conditioned stimuli

Medial prefrontal cortex (mPFC) neurons respond to Pavlovian conditioned stimuli, and these responses depend on input from the basolateral amygdala (BLA). In this study, we examined the mPFC efferent circuits mediating conditioned responding by testing whether specific subsets of mPFC projection neurons receive BLA input and respond to conditioned stimuli. In urethane-anesthetized rats, we identified mPFC neurons that projected to the nucleus accumbens (NAcc) or to the contralateral mPFC (cmPFC) using antidromic activation. Stimulation of the BLA and Pavlovian conditioned odors selectively activated a subpopulation of ventral mPFC neurons that projected to NAcc, but elicited virtually no activation in mPFC neurons that projected to cmPFC. BLA stimulation typically evoked inhibitory responses among nonactivated neurons projecting to either site. These results suggest that the ventral mPFC-to-NAcc pathway may support behavioral responses to conditioned cues. Furthermore, because projections from the BLA (which also encode affective information) and the mPFC converge within the NAcc, the BLA may recruit the mPFC to drive specific sets of NAcc neurons, and thereby exert control over prefrontal cortical-striato-thalamocortical information flow.

Hippocampal low-frequency stimulation and chronic mild stress similarly disrupt fear extinction memory in rats

Disruptions of fear extinction-related potentiation of synaptic efficacy in the connection between the hippocampus (HPC) and the medial prefrontal cortex (mPFC) have been shown to impair the recall of extinction memory. This study was undertaken to examine if chronic mild stress (CMS), which is known to alter induction of HPC-mPFC long-term potentiation, would also interfere with both extinction-related HPC-mPFC potentiation and extinction memory. Following fear conditioning (5 tone-shock pairings), rats were submitted to fear extinction (20 tone-alone presentations), which produced an increase in the amplitude of HPC-mPFC field potentials. HPC low-frequency stimulation (LFS), applied immediately after training, suppressed these changes and induced fear return during the retention test (5 tone-alone presentations). CMS, delivered before fear conditioning, did not interfere with fear extinction but blocked the development of extinction-related potentiation in the HPC-mPFC pathway and impaired the recall of extinction. These findings suggest that HPC LFS may provoke metaplastic changes in HPC outputs that may mimic alterations associated with a history of chronic stress.

Isolation rearing impairs wound healing and is associated with increased locomotion and decreased immediate early gene expression in the medial prefrontal cortex of juvenile rats

In addition to its maladaptive effects on psychiatric function, psychosocial deprivation impairs recovery from physical illness. Previously, we found that psychosocial deprivation, modeled by isolation rearing, depressed immediate early gene (IEG) expression in the medial prefrontal cortex (mPFC) and increased locomotion in the open field test [Levine JB, Youngs RM, et al. (2007) Isolation rearing and hyperlocomotion are associated with reduced immediate early gene expression levels in the medial prefrontal cortex. Neuroscience 145(1):42-55]. In the present study, we examined whether similar changes in behavior and gene expression are associated with the maladaptive effects of psychosocial deprivation on physical injury healing. After weaning, anesthetized rats were subjected to a 20% total body surface area third degree burn injury and were subsequently either group or isolation reared. After 4 weeks of either isolation or group rearing (a period that encompasses post-wearing and early adolescence), rats were killed, and their healing and gene expression in the mPFC were assessed. Locomotion in the open field test was examined at 3 weeks post-burn injury. We found that: 1) gross wound healing was significantly impaired in isolation-reared rats compared with group-reared rats, 2) locomotion was increased and IEG expression was suppressed for isolation-reared rats during burn injury healing, 3) the decreased activity in the open field and increased IEG expression was greater for burn injury healing group-reared rats than for uninjured group-reared rats, 4) the degree of hyperactivity and IEG suppression was relatively similar between isolation-reared rats during burn injury compared with uninjured isolation-reared rats. Thus, in the mPFC, behavioral hyperactivity to novelty (the open field test) along with IEG suppression may constitute a detectable biomarker of isolation rearing during traumatic physical injury. Implications of the findings for understanding, assessing, and treating the maladaptive effects of psychosocial deprivation on physical healing during childhood are discussed.

Medial prefrontal cortex infusions of bupivacaine or AP-5 block extinction of amphetamine conditioned place preference

The present experiments used reversible lesion techniques and intra-mPFC infusions of the n-methyl D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid (AP-5) to examine the role of the mPFC in extinction of an amphetamine conditioned place preference (CPP). Following initial training and testing for an amphetamine (2 mg/kg) CPP, adult male Long-Evans rats were given extinction trials that were identical to training, except in the absence of peripheral amphetamine injections. Immediately prior to each extinction trial, rats received intra-mPFC infusions of the anesthetic drug bupivacaine (0.75% solution/0.5 microl), AP-5 (1.25, 2.5, 5.0 microg/0.5 microl), or saline. Following extinction training, rats were given a second CPP test session. Rats receiving intra-mPFC infusions of saline displayed extinction of CPP behavior. In contrast, intra-mPFC infusions of bupivacaine or AP-5 (2.5, 5.0 microg) blocked CPP extinction. The findings indicate (1) the mPFC mediates extinction of approach behavior to drug-associated environmental contexts, and (2) NMDA receptor blockade within the mPFC is sufficient to block extinction of amphetamine CPP behavior.

The effect of the NMDA receptor antagonist MK-801 on the acquisition and extinction of learned fear in the developing rat

Recent findings reveal qualitative developmental differences in extinction of learned fear. The present study explored potential developmental differences in the role of NMDA in acquisition and extinction. Rats were injected with MK-801 prior to fear conditioning or extinction training. Acquisition was found to be NMDA dependent in both age groups, whereas extinction was found to be NMDA dependent in 23-day-old rats, but NMDA independent in 16-day-old rats. These results illustrate another fundamental developmental difference in extinction as well as a dissociation in the role of NMDA in the acquisition and extinction of fear early in development.

A hippocampal Cdk5 pathway regulates extinction of contextual fear

Treatment of emotional disorders involves the promotion of extinction processes, which are defined as the learned reduction of fear. The molecular mechanisms underlying extinction have only begun to be elucidated. By employing genetic and pharmacological approaches in mice, we show here that extinction requires downregulation of Rac-1 and cyclin-dependent kinase 5 (Cdk5), and upregulation of p21 activated kinase-1 (PAK-1) activity. This is physiologically achieved by a Rac-1-dependent relocation of the Cdk5 activator p35 from the membrane to the cytosol and dissociation of p35 from PAK-1. Moreover, our data suggest that Cdk5/p35 activity prevents extinction in part by inhibition of PAK-1 activity in a Rac-1-dependent manner. We propose that extinction of contextual fear is regulated by counteracting components of a molecular pathway involving Rac-1, Cdk5 and PAK-1. Our data suggest that this pathway could provide a suitable target for therapeutic treatment of emotional disorders.

Inhibition of mRNA synthesis in the hippocampus impairs consolidation and reconsolidation of spatial memory

Using two different mRNA synthesis inhibitors, we show that blockade of hippocampal gene expression during restricted posttraining or postretrieval time windows hinders retention of long-term spatial memory for the Morris water maze task, without affecting short-term memory, nonspatial learning, or the functionality of the hippocampus. Our results indicate that spatial memory consolidation induces the activation of the hippocampal transcriptional machinery and suggest the existence of a gene expression-dependent reconsolidation process that operates in the dorsal hippocampus at the moment of retrieval to stabilize the reactivated mnemonic trace.