Extinction of cued fear memory involves a distinct form of depotentiation at cortical input synapses onto the lateral amygdala

The amygdala is known to be a critical storage site of conditioned fear memory. Among the two major pathways to the lateral amygdala (LA), the cortical pathway is known to display a presynaptic long‐term potentiation which is occluded with fear conditioning. Here we show that fear extinction results in a net depression of conditioning‐induced potentiation at cortical input synapses onto the LA (C‐LA synapses). Fear conditioning induced a significant potentiation of excitatory postsynaptic currents at C‐LA synapses compared with naïve and unpaired controls, whereas extinction apparently reversed this potentiation. Paired‐pulse low‐frequency stimulation (pp‐LFS) induced synaptic depression in the C‐LA pathway of fear‐conditioned rats, but not in naïve or unpaired controls, indicating that the pp‐LFS‐induced depression is specific to associative learning‐induced changes (pp‐LFS‐induced depotentiation_{ex vivo}). Importantly, extinction occluded pp‐LFS‐induced depotentiation_{ex vivo}, suggesting that extinction shares some mechanisms with the depotentiation. pp‐LFS‐induced depotentiation_{ex vivo} required NMDA receptor (NMDAR) activity, consistent with a previous finding that blockade of amygdala NMDARs impaired fear extinction. In addition, pp‐LFS‐induced depotentiation_{ex vivo} required activity of group II metabotropic glutamate receptors (mGluRs), known to be present at presynaptic terminals, but not AMPAR internalization, consistent with a presynaptic mechanism for pp‐LFS‐induced depotentiation_{ex vivo}. This result is in contrast with another form of ex vivo depotentiation in the thalamic pathway that requires both group I mGluR activity and AMPAR internalization. We thus suggest that extinction of conditioned fear involves a distinct form of depotentiation at C‐LA synapses, which depends upon both NMDARs and group II mGluRs.     

NMDA receptor antagonism in the basolateral but not central amygdala blocks the extinction of Pavlovian fear conditioning in rats

Glutamate receptors in the basolateral complex of the amygdala (BLA) are essential for the acquisition, expression and extinction of Pavlovian fear conditioning in rats. Recent work has revealed that glutamate receptors in the central nucleus of the amygdala (CEA) are also involved in the acquisition of conditional fear, but it is not known whether they play a role in fear extinction. Here we examine this issue by infusing glutamate receptor antagonists into the BLA or CEA prior to the extinction of fear to an auditory conditioned stimulus (CS) in rats. Infusion of the α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate (AMPA) receptor antagonist, 2,3‐dihydroxy‐6‐nitro‐7‐sulfamoyl‐benzo[f]quinoxaline‐2,3‐dione (NBQX), into either the CEA or BLA impaired the expression of conditioned freezing to the auditory CS, but did not impair the formation of a long‐term extinction memory to that CS. In contrast, infusion of the N‐methyl‐d ‐aspartate (NMDA) receptor antagonist, d,l ‐2‐amino‐5‐phosphonopentanoic acid (APV), into the amygdala, spared the expression of fear to the CS during extinction training, but impaired the acquisition of a long‐term extinction memory. Importantly, only APV infusions into the BLA impaired extinction memory. These results reveal that AMPA and NMDA receptors within the amygdala make dissociable contributions to the expression and extinction of conditioned fear, respectively. Moreover, they indicate that NMDA receptor‐dependent processes involved in extinction learning are localized to the BLA. Together with previous work, these results reveal that NMDA receptors in the CEA have a selective role acquisition of fear memory.     

Impaired fear extinction in mice lacking protease nexin‐1

The serine protease inhibitor protease‐nexin‐1 (PN‐1) has been shown to modulate N‐methyl‐d ‐aspartate receptor (NMDAR)‐mediated synaptic currents and NMDAR‐dependent long‐term potentiation of synaptic transmission. Here, we analysed the role of PN‐1 in the acquisition and extinction of classical auditory fear conditioning, two distinct forms of learning that both depend on NMDAR activity in the amygdala. Immunostaining revealed that PN‐1 is expressed throughout the amygdala, primarily in γ‐aminobutyric acid containing neurons of the central amygdala and intercalated cell masses (ITCs) and in glia. Fear extinction was severely impaired in mice lacking PN‐1 (PN‐1 KO). Consistent with a role for the basal nucleus of the amygdala in fear extinction, we found that, compared with wild‐type (WT) littermate controls, PN‐1 KO mice exhibited decreased numbers of Fos‐positive neurons in the basal nucleus after extinction. Moreover, immunoblot analysis of laser‐microdissected amygdala sub‐nuclei revealed specific extinction‐induced increases in the level of phosphorylated alpha‐calcium/calmodulin protein kinase II in the medial ITCs and in the lateral subdivision of the central amygdala in WT mice. These responses were altered in PN‐1 KO mice. Together, these data indicate that lack of extinction in PN‐1 KO mice is associated with distinct changes in neuronal activity across the circuitry of the basal and central nuclei and the ITCs, supporting a differential impact on fear extinction of these amygdala substructures. They also suggest a new role for serine protease inhibitors such as PN‐1 in modulating fear conditioning and extinction.     

Inhibition of amygdaloid dopamine D2 receptors impairs emotional learning measured with fear-potentiated startle

Considerable advances have been made in understanding the neurocircuitry underlying the acquisition and expression of Pavlovian conditioned fear responses. Within the complex cellular and molecular processes mediating fearfulness, amygdaloid dopamine (DA), originating from cells in the ventral tegmental area (VTA) of the midbrain, is thought to contribute to fear-motivated responding. Considering that blockade of DA D₂ receptors is a common mechanism of action for antipsychotic agents, we hypothesized that inhibition of D₂ receptors in the amygdala may be involved in the antiparanoid effects of these drugs. To assess the role of amygdaloid DA D₂ receptors in aversive emotionality, the D₂ receptor antagonist raclopride was infused into the amygdala prior to Pavlovian fear conditioning. Potentiated startle was used as a behavioral indicator of fear and anxiety. Classical fear conditioning and acoustic startle testing were conducted in a single session allowing for the concomitant assessment of shock reactivity with startle enhancement. Depending on dose, the results found conditioned fear acquisition and retention to be impaired following administration of raclopride into the amygdala. Additionally, the learning deficit was dissociated from shock detection and from fear expression assessed with the shock sensitization of acoustic startle. These findings further refine the known neural mechanisms of amygdala-based emotional learning and memory and were interpreted to suggest that, along with D₁ receptors, D₂ receptors in the amygdala may mediate the formation and the retention of newly-acquired fear associations.     

Clonidine injections into the lateral nucleus of the amygdala block acquisition and expression of fear-potentiated startle

Numerous studies of aversive learning with different animal models have shown that the noradrenergic system has an important role in the acquisition, consolidation and expression of aversive learning. We used intracerebral clonidine injections to investigate the role of the noradrenergic amygdaloid system in the fear-potentiated startle paradigm. Clonidine is a noradrenergic alpha2-receptor agonist which can decrease noradrenergic transmission by stimulating presynaptic alpha2-receptors. Rats received injections of 0, 2.5, 5 and 10 nmol clonidine into the lateral amygdala (i) before fear-conditioning, (ii) immediately after fear-conditioning, (iii) before testing and (iv) before both fear-conditioning and the testing of conditioned fear. Clonidine injections blocked the acquisition and expression of conditioned fear. The effect on acquisition was not caused by state-dependency or possible side-effects of clonidine on consolidation. Given that clonidine decreases amygdaloid noradrenaline release, these results show a crucial role of noradrenergic transmission within the amygdala in classical fear-conditioning. Surprisingly, both the acquisition and the expression of conditioned fear were blocked after amygdaloid injections of clonidine, suggesting that amygdaloid noradrenaline is necessary to induce both unconditioned and conditioned fear.     

Prolonged fear responses in mice lacking dopamine D1 receptor

Dopamine is an important neurotransmitter involved in learning and memory including emotional memory. The involvement of dopamine in conditioned fear has been widely documented. However, little is known about the molecular mechanisms that underlie contextual fear conditioning and memory consolidation. To address this issue, we used dopamine D₁-deficient mice (D₁−/−) and their wild-type (D₁+/+) and heterozygote (D₁+/−) siblings to assess aversive learning and memory. We quantified two different aspects of fear responses to an environment where the mice have previously received unsignaled footshocks. Using one-trial step-through passive avoidance and conditioned freezing paradigms, mice were conditioned to receive mild inescapable footshocks then tested for acquisition, retention and extinction of conditioned fear responses 5 min after and up to 45–90 days post-training. No differences were observed among any of the genotypes in the acquisition of passive avoidance response or fear-induced freezing behavior. However, with extended testing, D₁−/− mice exhibited prolonged retention and delayed extinction of conditioned fear responses in both tasks, suggesting that D₁−/− mice are capable of acquiring aversive learning normally. These findings demonstrate that the dopamine D₁ receptor is not important for acquisition or consolidation of aversive learning and memory but has an important role in modulating the extinction of fear memory.     

In vivo knockdown of GAD67 in the amygdala disrupts fear extinction and the anxiolytic-like effect of diazepam in mice

In mammals, γ-aminobutyric acid (GABA) transmission in the amygdala is particularly important for controlling levels of fear and anxiety. Most GABA synthesis in the brain is catalyzed in inhibitory neurons from ℒ-glutamic acid by the enzyme glutamic acid decarboxylase 67 (GAD67). In the current study, we sought to examine the acquisition and extinction of conditioned fear in mice with knocked down expression of the GABA synthesizing enzyme GAD67 in the amygdala using a lentiviral-based (LV) RNA interference strategy to locally induce loss-of-function. In vitro experiments revealed that our LV-siRNA-GAD67 construct diminished the expression of GAD67 as determined with western blot and fluorescent immunocytochemical analyses. In vivo experiments, in which male C57BL/6J mice received bilateral amygdala microinjections, revealed that LV-siRNA-GAD67 injections produce significant inhibition of endogenous GAD67 when compared with control injections. In contrast, no significant changes in GAD65 expression were detected in the amygdala, validating the specificity of LV knockdown. Behavioral experiments showed that LV knockdown of GAD67 results in a deficit in the extinction, but not the acquisition or retention, of fear as measured by conditioned freezing. GAD67 knockdown did not affect baseline locomotion or basal measures of anxiety as measured in open field apparatus. However, diminished GAD67 in the amygdala blunted the anxiolytic-like effect of diazepam (1.5 mg kg^–1) as measured in the elevated plus maze. Together, these studies suggest that of GABAergic transmission in amygdala mediates the inhibition of conditioned fear and the anxiolytic-like effect of diazepam in adult mice.     

Amygdaloid D1 dopamine receptor involvement in Pavlovian fear conditioning

The amygdala has long been implicated in conditioned fear. The mesencephalic dopaminergic system provides a rich innervation to the amygdala [J.H. Fallon, P. Ciofi, Distribution of monoamines within the amygdala, in: J.P. Aggleton (Ed.), The Amygdala: Neurobiological Aspects of Emotion, Memory and Mental Dysfunction, Wiley, New York, 1992, pp. 97–114; L.J. Freedman, M.D. Cassell, Distribution of dopaminergic fibers in the central division of the extended amygdala of the rat. Brain Research 633 (1994) 243–252; E. Asan, The catecholaminergic innervation of the rat amygdala. Advances in Anatomy Embryology and Cell Biology 142 (1996) 1–107]. Specific activation of the mesoamygdaloid dopaminergic system has been reported to occur in response to conditioned fear-arousing stimuli [M.L. Coco, C.M. Kuhn, T.D. Ely, C.D. Kilts, Selective activation of mesoamygdaloid dopamine neurons by conditioned stress: attenuation by diazepam. Brain Research 590 (1992) 39–47] suggesting that dopamine release in the amygdala may contribute to the acquisition and/or expression of conditioned fear. Using a 2×2 factorial design, Experiment 1A investigated the effects of bilateral intra-amygdaloid infusions of the selective D₁ receptor antagonist, SCH 23390 (2.0 μg 0.5 μl ⁻¹ side⁻¹), on the acquisition and expression of Pavlovian conditioned fear measured by freezing to acoustic and background contextual stimuli. Infusions of SCH 23390 prior to acquisition training, prior to retention testing or prior to both significantly attenuated conditioned freezing during retention testing. Experiment 1B investigated the dose-dependent effects of pre-training infusions of SCH 23390 (0.5, 1.0 and 2.0 μg) on conditioned fear. Pre-training infusions of SCH 23390 dose-dependently attenuated conditioned freezing during retention testing. Experiment 2A investigated the effects of bilateral infusions of the selective D₁ receptor agonist, SKF 82958 (2.0 μg 0.5 μl⁻¹ side⁻¹) on the acquisition and expression of conditioned fear. Infusions of SKF 82958 prior to training facilitated conditioned freezing during retention testing. Experiment 2B investigated the dose-dependent effects of pre-training infusions of SKF 82958 (1.0, 2.0 and 4.0 μg) on conditioned fear. Pre-training infusions of SKF 82958 dose-dependently facilitated conditioned freezing during retention testing. In conclusion, these results suggest that dopamine transmission within the amygdala contributes to the acquisition and expression of Pavlovian fear conditioning.     

Extinction of auditory fear conditioning requires MAPK/ERK activation in the basolateral amygdala

Whereas the neuronal substrates underlying the acquisition of auditory fear conditioning have been widely studied, the substrates and mechanisms mediating the acquisition of fear extinction remain largely elusive. Previous reports indicate that consolidation of fear extinction depends on the mitogen-activated protein kinase/extracellular-signal regulated kinase (MAPK/ERK) signalling pathway and on protein synthesis in the medial prefrontal cortex (mPFC). Based on experiments using the fear-potentiated startle paradigm suggesting a role for neuronal plasticity in the basolateral amygdala (BLA) during fear extinction, we directly addressed whether MAPK/ERK signalling in the basolateral amygdala is necessary for the acquisition of fear extinction using conditioned freezing as a read-out. First, we investigated the regional and temporal pattern of MAPK/ERK activation in the BLA following extinction learning in C57Bl/6J mice. Our results indicate that acquisition of extinction is associated with an increase of phosphorylated MAPK/ERK in the BLA. Moreover, we found that inhibition of the MAPK/ERK signalling pathway by intrabasolateral amygdala infusion of the MEK inhibitor, U0126, completely blocks acquisition of extinction. Thus, our results indicate that the MAPK/ERK signalling pathway is required for extinction of auditory fear conditioning in the BLA, and support a role for neuronal plasticity in the BLA during the acquisition of fear extinction.     

Amygdaloid metabotropic glutamate receptor subtype 7 is involved in the acquisition of conditioned fear

The metabotropic glutamate receptor subtype 7 (mGluR7) is presynaptically located and modulates transmitter release. An earlier study from our group demonstrated that systemic administration of N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082), a selective allosteric mGluR7 agonist, attenuates the acquisition of conditioned fear measured by fear-potentiated startle. Aim of this study was to explore whether this effect is mediated by the basolateral amygdala, a crucial brain structure for acquisition of conditioned fear. Therefore, AMN082 was locally injected into the basolateral amygdala of rats and the effects of these injections on the acquisition of conditioned fear was measured. Our data clearly show that intra-amygdala injection of AMN082 impairs fear acquisition. This finding demonstrates that amygdaloid mGluR7 controls the learning of conditioned fear.     

Robust BOLD Responses to Faces But Not to Conditioned Threat: Challenging the Amygdala's Reputation in Human Fear and Extinction Learning

Most of our knowledge about human emotional memory comes from animal research. Based on this work, the amygdala is often labeled the brain''s “fear center”, but it is unclear to what degree neural circuitries underlying fear and extinction learning are conserved across species. Neuroimaging studies in humans yield conflicting findings, with many studies failing to show amygdala activation in response to learned threat. Such null findings are often treated as resulting from MRI-specific problems related to measuring deep brain structures. Here we test this assumption in a mega-analysis of three studies on fear acquisition (n = 98; 68 female) and extinction learning (n = 79; 53 female). The conditioning procedure involved the presentation of two pictures of faces and two pictures of houses: one of each pair was followed by an electric shock [a conditioned stimulus (CS⁺)], the other one was never followed by a shock (CS^–), and participants were instructed to learn these contingencies. Results revealed widespread responses to the CS⁺ compared with the CS^– in the fear network, including anterior insula, midcingulate cortex, thalamus, and bed nucleus of the stria terminalis, but not the amygdala, which actually responded stronger to the CS^–. Results were independent of spatial smoothing, and of individual differences in trait anxiety and conditioned pupil responses. In contrast, robust amygdala activation distinguished faces from houses, refuting the idea that a poor signal could account for the absence of effects. Moving forward, we suggest that, apart from imaging larger samples at higher resolution, alternative statistical approaches may be used to identify cross-species similarities in fear and extinction learning.SIGNIFICANCE STATEMENT The science of emotional memory provides the foundation of numerous theories on psychopathology, including stress and anxiety disorders. This field relies heavily on animal research, which suggests a central role of the amygdala in fear learning and memory. However, this finding is not strongly corroborated by neuroimaging evidence in humans, and null findings are too easily explained away by methodological limitations inherent to imaging deep brain structures. In a large nonclinical sample, we find widespread BOLD activation in response to learned fear, but not in the amygdala. A poor signal could not account for the absence of effects. While these findings do not disprove the involvement of the amygdala in human fear learning, they challenge its typical portrayals and illustrate the complexities of translational science.     

The role of the amygdala in the extinction of conditioned fear.

The amygdala has long been known to play a central role in the acquisition and expression of fear. More recently, convergent evidence has implicated the amygdala in the extinction of fear as well. In rodents, some of this evidence comes from the infusion of drugs directly into the amygdala and, in particular, into the basolateral complex of the amygdala, during or after extinction learning. In vivo electrophysiology has identified cellular correlates of extinction learning and memory in the lateral nucleus of that structure. Human imaging experiments also indicate that amygdaloid activity correlates with extinction training. In addition, some studies have directly identified changes in molecular constituents of the basolateral amygdala. Together these experiments strongly indicate that the basolateral amygdala plays a crucial role in extinction learning. Interpreted in the light of these findings, several recent in vitro electrophysiology studies in amygdala-containing brain slices are suggestive of potential synaptic and circuit bases of extinction learning.     

Fear extinction deficits following acute stress associate with increased spine density and dendritic retraction in basolateral amygdala neurons

Stress‐sensitive psychopathologies such as post‐traumatic stress disorder are characterized by deficits in fear extinction and dysfunction of corticolimbic circuits mediating extinction. Chronic stress facilitates fear conditioning, impairs extinction, and produces dendritic proliferation in the basolateral amygdala (BLA), a critical site of plasticity for extinction. Acute stress impairs extinction, alters plasticity in the medial prefrontal cortex‐to‐BLA circuit, and causes dendritic retraction in the medial prefrontal cortex. Here, we examined extinction learning and basolateral amygdala pyramidal neuron morphology in adult male rats following a single elevated platform stress. Acute stress impaired extinction acquisition and memory, and produced dendritic retraction and increased mushroom spine density in basolateral amygdala neurons in the right hemisphere. Unexpectedly, irrespective of stress, rats that underwent fear and extinction testing showed basolateral amygdala dendritic retraction and altered spine density relative to non‐conditioned rats, particularly in the left hemisphere. Thus, extinction deficits produced by acute stress are associated with increased spine density and dendritic retraction in basolateral amygdala pyramidal neurons. Furthermore, the finding that conditioning and extinction as such was sufficient to alter basolateral amygdala morphology and spine density illustrates the sensitivity of basolateral amygdala morphology to behavioral manipulation. These findings may have implications for elucidating the role of the amygdala in the pathophysiology of stress‐related disorders.     

Noradrenergic enhancement of reconsolidation in the amygdala impairs extinction of conditioned fear in rats--a possible mechanism for the persistence of traumatic memories in PTSD

Background: Posttraumatic stress disorder (PTSD) is associated with enhanced noradrenergic activity. Animal and human studies demonstrate that noradrenergic stimulation augments consolidation of fear learning. Retrieval of well‐established memories by presenting a learned fear cue triggers reconsolidation processes during which memories may be updated, weakened, or strengthened. We previously reported that noradrenergic blockade in the rat amygdala impairs reconsolidation of fear memories. Here we investigated the effects of noradrenergic enhancement on reconsolidation of learned fear. Methods: Using auditory fear conditioning in rats, we tested the effects of postretrieval intraamygdala infusion of the β‐adrenergic receptor agonist isoproterenol or the antagonist propranolol on conditioned fear in the amygdala. Results: A single intraamygdala infusion of isoproterenol following a retrieval of a well‐consolidated memory enhanced fear memory elicited by the learned fear stimulus and impaired extinction of this memory 48 hr later. Intraamygdala infusion of the β‐adrenergic receptor antagonist propranolol following a consecutive retrieval trial blocked the enhancing effects of isoproterenol on fear memory. Conclusions: Postretrieval β‐adrenergic stimulation in the amygdala enhances reconsolidation of fear memories, making them resistant to extinction. Noradrenergic augmentation during retrieval of fear memories may thus contribute to persistence and severity of traumatic memories. Reconsolidation may be a useful tool in understanding the pathology of PTSD and may thus help in developing new and in modifying existing treatments of traumatic memories. Depression and Anxiety 28:186–193, 2011. © 2011 Wiley‐Liss, Inc.     

Extinction of fear-potentiated startle: blockade by infusion of an NMDA antagonist into the amygdala.

Data derived from in vitro preparations indicate that NMDA receptors play a critical role in synaptic plasticity in the CNS. More recently, in vivo pharmacological manipulations have suggested that an NMDA-dependent process may be involved in specific forms of behavioral plasticity. All of the work thus far has focused on the possible role of NMDA receptors in the acquisition of responses. However, there are many examples in the behavioral literature of learning-induced changes that involve the reduction or elimination of a previously acquired response. Experimental extinction is a primary example of the elimination of a learned response. Experimental extinction is well described in the behavioral literature, but has not received the same attention in the neurobiological literature. As a result, the neural mechanisms that underlie this important form of learning are not at all understood. In the present experiments, the fear-potentiated startle paradigm was employed to begin to investigate neural mechanisms of extinction. The results show that infusion of the NMDA antagonist D,L-2-amino-5-phosphonovaleric acid (AP5) into the amygdala, a limbic structure known to be important for fear conditioning, dose-dependently blocked extinction of conditioned fear. Control experiments showed that the blockade of extinction was neither the result of the permanent disruption of amygdaloid function nor the result of decreased sensitivity of the animals to the conditioned stimulus. Infusion of AP5 into the interpositus nucleus of the cerebellum, a control site, did not block extinction. Finally, intra-amygdala infusion of a selected dose of the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione did not block extinction of conditioned fear. These results, together with a previous report from our laboratory (Miserendino et al., 1990), demonstrate the importance of the amygdala in the elaboration of conditioned fear and suggest that an NMDA-dependent process might underlie the extinction of conditioned fear.     

Inhibition of amygdaloid dopamine D2 receptors impairs emotional learning measured with fear-potentiated startle

Considerable advances have been made in understanding the neurocircuitry underlying the acquisition and expression of Pavlovian conditioned fear responses. Within the complex cellular and molecular processes mediating fearfulness, amygdaloid dopamine (DA), originating from cells in the ventral tegmental area (VTA) of the midbrain, is thought to contribute to fear-motivated responding. Considering that blockade of DA D(2) receptors is a common mechanism of action for antipsychotic agents, we hypothesized that inhibition of D(2) receptors in the amygdala may be involved in the antiparanoid effects of these drugs. To assess the role of amygdaloid DA D(2) receptors in aversive emotionality, the D(2) receptor antagonist raclopride was infused into the amygdala prior to Pavlovian fear conditioning. Potentiated startle was used as a behavioral indicator of fear and anxiety. Classical fear conditioning and acoustic startle testing were conducted in a single session allowing for the concomitant assessment of shock reactivity with startle enhancement. Depending on dose, the results found conditioned fear acquisition and retention to be impaired following administration of raclopride into the amygdala. Additionally, the learning deficit was dissociated from shock detection and from fear expression assessed with the shock sensitization of acoustic startle. These findings further refine the known neural mechanisms of amygdala-based emotional learning and memory and were interpreted to suggest that, along with D(1) receptors, D(2) receptors in the amygdala may mediate the formation and the retention of newly-acquired fear associations.     

Prolonged fear responses in mice lacking dopamine D1 receptor

Dopamine is an important neurotransmitter involved in learning and memory including emotional memory. The involvement of dopamine in conditioned fear has been widely documented. However, little is known about the molecular mechanisms that underlie contextual fear conditioning and memory consolidation. To address this issue, we used dopamine D1-deficient mice (D1-/-) and their wild-type (D1+/+) and heterozygote (D1+/-) siblings to assess aversive learning and memory. We quantified two different aspects of fear responses to an environment where the mice have previously received unsignaled footshocks. Using one-trial step-through passive avoidance and conditioned freezing paradigms, mice were conditioned to receive mild inescapable footshocks then tested for acquisition, retention and extinction of conditioned fear responses 5 min after and up to 45-90 days post-training. No differences were observed among any of the genotypes in the acquisition of passive avoidance response or fear-induced freezing behavior. However, with extended testing, D1-/- mice exhibited prolonged retention and delayed extinction of conditioned fear responses in both tasks, suggesting that D1-/- mice are capable of acquiring aversive learning normally. These findings demonstrate that the dopamine D1 receptor is not important for acquisition or consolidation of aversive learning and memory but has an important role in modulating the extinction of fear memory.     

The amygdala and appraisal processes: stimulus and response complexity as an organizing factor

The amygdala has been implicated in a variety of functions, ranging from attention to memory to emotion. In theories about the amygdala's role in conditioned fear, the lateral amygdala (LA) is the primary, perhaps unique, interface for incoming conditioned sensory stimuli and the central nucleus is the major output station. Recent studies indicate, however, that amygdala output pathways may be dissociated as a function of the type of conditioned fear behavior. Based on behavioral, electrophysiological and anatomical evidence, the present discussion proposes a modification of the traditional model of input pathways to the amygdala such that the LA activation as a sensory interface is limited to relatively simple, unimodal conditioned stimulus features whereas the basal amygdaloid nucleus (B) may serve as an amygdaloid sensory interface for complex, configural conditioned stimulus information. We further argue that the partition of amygdalar nuclei according to a complexity dimension appears to correspond both for input and output pathways and thus constitutes a common organizing factor in the functional anatomy of the amygdala. The extensive intra-amygdala wiring is assumed to underlie the computations necessary to perform behavioral decisions of various levels of complexity. Collectively, these results endow the amygdala with a more sophisticated role in guiding motivation and behavior.     

Impaired extinction of fear and maintained amygdala-hippocampal theta synchrony in a mouse model of temporal lobe epilepsy

Purpose: The relationship between epilepsy and fear has received much attention. However, seizure‐modulated fear and physiologic or structural correlates have not been examined systematically, and the underlying basics of network levels remain unclear to date. Therefore, this project was set up to characterize the neurophysiologic basis of seizure‐related fear and the contribution of the amygdala‐hippocampus system. Methods: The experimental strategy was composed of the following steps: (1) use of the mouse pilocarpine model of temporal lobe epilepsy (TLE); (2) behavioral analyses of anxiety states in the elevated plus maze test, light–dark avoidance test, and Pavlovian fear conditioning; and (3) probing neurophysiologic activity patterns in amygdala‐hippocampal circuits in freely behaving mice. Results: Our results displayed no significant differences in basic anxiety levels comparing mice that developed spontaneous recurrent seizures (SRS) and controls. Furthermore, conditioned fear memory retrieval was not influenced in SRS mice. However, during fear memory extinction, SRS mice showed an extended freezing behavior and a maintained amygdala‐hippocampal theta frequency synchronization compared to controls. Discussion: These results indicate specific alterations in conditioned fear behavior and related neurophysiologic activities in the amygdala‐hippocampal network contributing to impaired fear memory extinction in mice with TLE. Clinically, the nonextinguished fear memories may well contribute to the experience of fear in patients with TLE.     

The T‐type calcium channel blocker Z944 reduces conditioned fear in Genetic Absence Epilepsy Rats from Strasbourg and the non‐epileptic control strain

Genetic Absence Epilepsy Rats from Strasbourg (GAERS) are a rodent model of childhood absence epilepsy (CAE) that display a gain‐of‐function mutation in the gene encoding the Cav3.2 T‐type calcium channel. GAERS demonstrate heightened learning and delayed extinction of fear conditioning. Our objective in the present study was to examine the effects of the pan‐T‐type calcium channel blocker Z944 on the acquisition, consolidation and extinction of conditioned fear in GAERS and the non‐epileptic control (NEC) strain. Z944 (10 mg/kg; ip) was administered 15 min prior to either acquisition, extinction day 1 (24 hr later), acquisition and extinction day 1, or during the consolidation (post‐acquisition) of tone‐cued fear conditioning. Extinction was examined 24 and 48 hr after conditioning. In drug naïve GAERS, increased freezing during the acquisition and extinction phases of fear conditioning was found. Short‐term effects of Z944 on performance were observed as Z944 increased freezing during testing on the day it was administered. Z944 administered prior to the acquisition phase had a long‐term effect on extinction. Specifically, both GAERS and NECs showed a decrease in freezing during extinction relative to drug naïve GAERS and NEC rats respectively. Regardless of strain or treatment, female rats showed reduced extinction of fear relative to male rats. These results demonstrate that T‐type calcium channels contribute to the neural systems that mediate the learning and memory of conditioned fear. Overall, these findings suggest that T‐type calcium channel blockers show promise in the treatment of learning impairments observed in disorders such as CAE.