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.     

Hippocampal dendritic spines remodeling and fear memory are modulated by GABAergic signaling within the basolateral amygdala complex

GABAergic signaling in the basolateral amygdala complex (BLA) plays a crucial role on the modulation of the stress influence on fear memory. Moreover, accumulating evidence suggests that the dorsal hippocampus (DH) is a downstream target of BLA neurons in contextual fear. Given that hippocampal structural plasticity is proposed to provide a substrate for the storage of long‐term memories, the main aim of this study is to evaluate the modulation of GABA neurotransmission in the BLA on spine density in the DH following stress on contextual fear learning. The present findings show that prior stressful experience promoted contextual fear memory and enhanced spine density in the DH. Intra‐BLA infusion of midazolam, a positive modulator of GABAa sites, prevented the facilitating influence of stress on both fear retention and hippocampal dendritic spine remodeling. Similarly to the stress‐induced effects, the blockade of GABAa sites within the BLA ameliorated fear memory emergence and induced structural remodeling in the DH. These findings suggest that GABAergic transmission in BLA modulates the structural changes in DH associated to the influence of stress on fear memory. © 2015 Wiley Periodicals, Inc.     

Chronic cerebrolysin administration attenuates neuronal abnormalities in the basolateral amygdala induced by neonatal ventral hippocampus lesion in the rat

The neonatal ventral hippocampal lesion (nVHL) has emerged as a model of schizophrenia‐related behavior in the rat. Our previous report demonstrated that cerebrolysin (Cbl), a neuropeptide preparation which mimics the action of endogenous neurotrophic factors on brain protection and repair, promoted recovery of dendritic and neuronal damage of the prefrontal cortex and nucleus accumbens and behavioral improvements in postpubertal nVHL rats. We recently demonstrated that nVHL animals exhibit dendritic atrophy and spine loss in the basolateral amygdala (BLA). This study aimed to determine whether Cbl treatment was capable of reducing BLA neuronal alterations observed in nVHL rats. The morphological evaluation included examination of dendrites using the Golgi‐Cox procedure and stereology to quantify the total cell number in BLA. Golgi‐Cox staining revealed that nVHL induced dendritic retraction and spine loss in BLA pyramidal neurons. Stereological analysis demonstrated nVHL also produced a reduction in cells in BLA. Interestingly, repeated Cbl treatment ameliorated dendritic pathology and neuronal loss in the BLA of the nVHL rats. Our data show that Cbl may foster recovery of BLA damage in postpubertal nVHL rats and suggests that the use of neurotrophic agents for the management of some schizophrenia‐related symptoms may present an alternative therapeutic pathway in these disorders. Synapse, 68:31–38, 2014 . © 2013 Wiley Periodicals, Inc.     

Dendritic morphology changes in neurons from the ventral hippocampus,amygdala and nucleus accumbens in rats with neonatal lesions into the prefrontal cortex

Neonatal prefrontal cortex (nPFC) lesions in rats could be a potential animal model to study the early neurodevelopmental abnormalities associated with the behavioral and morphological brain changes observed in schizophrenia. Morphological alterations in pyramidal neurons from the ventral hippocampus (VH) have been observed in post‐mortem schizophrenic brains, mainly because of decreased dendritic arbor and spine density. We assessed the effects of nPFC‐lesions on the dendritic morphology of neurons from the VH, basolateral‐amygdala (BLA) and the nucleus accumbens (NAcc) in rats. nPFC lesions were made on postnatal day 7 (PD7), after dendritic morphology was studied by the Golgi‐Cox stain procedure followed by Sholl analysis at PD35 (prepubertal) and PD60 (adult) ages. We also evaluated the effects of PFC‐lesions on locomotor activity caused by a novel environment. Adult animals with nPFC lesions showed a decreased spine density in pyramidal neurons from the VH and in medium spiny cells from the NAcc. An increased locomotion was observed in a novel environment for adult animals with a PFC‐lesion. Our results indicate that PFC‐lesions alter the neuronal dendrite morphology of the NAcc and the VH, suggesting a disconnection between these limbic structures. The locomotion paradigms suggest that dopaminergic transmission is altered in the PFC lesion model. This could help to understand the consequences of an earlier PFC dysfunction in schizophrenia. To evaluate possible dendritic changes in neonatal prefrontal cortex lesions in schizophrenia‐related regions including nucleus accumbens, ventral hippocampus and basolateral amygdala, we used the Golgi‐Cox stain samples at PD35 and PD70. Our results suggest that neonatal prefrontal cortex damage alters dendritic parameters in limbic regions, and this has potential implications for schizophrenia. Synapse 69:314–325, 2015 . © 2015 Wiley Periodicals, Inc.     

Curcuma treatment prevents cognitive deficit and alteration of neuronal morphology in the limbic system of aging rats

Curcuma is a natural compound that has shown neuroprotective properties, and has been reported to prevent aging and improve memory. While the mechanism(s) underlying these effects are unclear, they may be related to increases in neural plasticity. Morphological changes have been reported in neuronal dendrites in the limbic system in animals and elderly humans with cognitive impairment. In this regard, there is a need to use alternative therapies that delay the onset of morphologies and behavioral characteristics of aging. Therefore, the objective of this study was to evaluate the effect of curcuma on cognitive processes and dendritic morphology of neurons in the prefrontal cortex (PFC), the CA1 and CA3 regions of the dorsal hippocampus, the dentate gyrus, and the basolateral amygdala (BLA) of aged rats. 18‐month‐old rats were administered curcuma (100 mg/kg) daily for 60 days. After treatment, recognition memory was assessed using the novel object recognition test. Curcuma‐treated rats showed a significant increase in the exploration quotient. Dendritic morphology was assessed by Golgi–Cox staining and followed by Sholl analysis. Curcuma‐treated rats showed a significant increase in dendritic spine density and dendritic length in pyramidal neurons of the PFC, the CA1 and CA3, and the BLA. The preservation of dendritic morphology was positively correlated with cognitive improvements. Our results suggest that curcuma induces modification of dendritic morphology in the aforementioned regions. These changes may explain how curcuma slows the aging process that has already begun in these animals, preventing deterioration in neuronal morphology of the limbic system and recognition memory.     

Adult microbiota‐deficient mice have distinct dendritic morphological changes: differential effects in the amygdala and hippocampus

Increasing evidence implicates the microbiota in the regulation of brain and behaviour. Germ‐free mice (GF; microbiota deficient from birth) exhibit altered stress hormone signalling and anxiety‐like behaviours as well as deficits in social cognition. Although the mechanisms underlying the ability of the gut microbiota to influence stress responsivity and behaviour remain unknown, many lines of evidence point to the amygdala and hippocampus as likely targets. Thus, the aim of this study was to determine if the volume and dendritic morphology of the amygdala and hippocampus differ in GF versus conventionally colonized (CC) mice. Volumetric estimates revealed significant amygdalar and hippocampal expansion in GF compared to CC mice. We also studied the effect of GF status on the level of single neurons in the basolateral amygdala (BLA) and ventral hippocampus. In the BLA, the aspiny interneurons and pyramidal neurons of GF mice exhibited dendritic hypertrophy. The BLA pyramidal neurons of GF mice had more thin, stubby and mushroom spines. In contrast, the ventral hippocampal pyramidal neurons of GF mice were shorter, less branched and had less stubby and mushroom spines. When compared to controls, dentate granule cells of GF mice were less branched but did not differ in spine density. These findings suggest that the microbiota is required for the normal gross morphology and ultrastructure of the amygdala and hippocampus and that this neural remodelling may contribute to the maladaptive stress responsivity and behavioural profile observed in GF mice.     

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.     

Loss of dendritic synapses in the medial amygdala associated with kindling

Kindling stimulation was given in the basolateral amygdala (BLA), the septal area or the corpus callosum in the right hemisphere of adult rats. The density of dendritic synapses was electron microscopically studied in the medial amygdaloid nucleus (MAN) ipsi- and contralateral to the stimulation side. The number of dendritic synapses was markedly decreased in both sides of the MAN of 3 groups of kindled rats. Such reduction occurred in both dendritic shaft and spine synapses. The most remarkable decrease was obtained in the BLA kindling. These results suggest that a decrease of synapses may provide a morphological basis for kindling.     

Enhancement of conditioned fear extinction by infusion of the GABA(A) agonist muscimol into the rat prefrontal cortex and amygdala

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 infra limbic prefrontal cortex (IL) and the basolateral amygdala (BLA) are involved in extinction. In this study, we examine the involvement of these two regions in extinction by manipulating the gamma-aminobutyric acid (GABA)ergic system, in the Sprague-Dawley rat. We microinfused a low dose of the GABA(A) agonist muscimol into the IL or BLA. Muscimol infused to IL before extinction training, but not after either a short (five-trials) or long (15-trials) extinction training, resulted in long-term facilitation of extinction. Infusion of muscimol to the BLA following a short (five-trial) extinction session facilitated extinction at least 48-h post-drug infusion. The differences in the temporal parameters of the effects of muscimol in the IL or BLA, suggest differential involvement of these structures in long-term extinction of fear memory. We propose a facilitating role for GABA(A) neurotransmission in the IL in triggering the onset of fear extinction and its maintenance, whereas in the BLA, GABA(A) neurotransmission facilitates extinction consolidation. The involvement of GABA(A) receptors in fear extinction in the prefrontal cortex and amygdala is of particular interest, because of the role of these areas in emotional processes, and the role of the GABA(A) receptors in anxiety states.     

Dendritic morphology of amygdala and hippocampal neurons in more and less predator stress responsive rats and more and less spontaneously anxious handled controls

We investigated the neurobiological bases of variation in response to predator stress (PS). Sixteen days after treatment (PS or handling), rats were grouped according to anxiety in the elevated plus maze (EPM). Acoustic startle was also measured. We examined the structure of dendritic trees of basolateral amygdala (BLA) output neurons (stellate and pyramidal cells) and of dorsal hippocampal (DHC) dentate granule cells of less anxious (LA) and more (extremely) anxious (MA) stressed animals (PSLA and PSMA). Handled controls (HC) which were less anxious (HCLA) and spontaneously more anxious (HCMA) equivalently to predator stressed subgroups were also studied. Golgi analysis revealed BLA output neurons of HCMA rats exhibited longer, more branched dendrites with higher spine density than the other groups of rats, which did not differ. Finally, spine density of DHC granule cells was equally depressed in HCMA and PSMA rats relative to HCLA and PSLA rats. Total dendritic length of BLA pyramidal and stellate cells (positive predictor) and DHC spine density (negative predictor) together accounted for 96% of the variance of anxiety of handled rats. DHC spine density was a negative predictor of PSMA and PSLA anxiety, accounting for 70% of the variance. Data are discussed in the context of morphological differences as phenotypic markers of a genetic predisposition to anxiety in handled controls, and a possible genetic vulnerability to predator stress expressed as reduced spine density in the DHC. Significance of findings for animal models of anxiety and hyperarousal comorbidities of PTSD are discussed.     

Photoperiod alters fear responses and basolateral amygdala neuronal spine density in white-footed mice (Peromyscus leucopus)

Photoperiodism is a biological phenomenon in which environmental day length is monitored to ascertain time of year to engage in seasonally appropriate adaptations. This trait is common among organisms living outside of the tropics. White-footed mice (Peromyscus leucopus) are small photoperiodic rodents which display a suite of adaptive responses to short day lengths, including reduced hippocampal volume, impairments in hippocampal-mediated memory, and enhanced hypothalamic-pituitary-adrenal axis reactivity. Because these photoperiodic changes in brain and behavior mirror some of the etiology of post-traumatic stress disorder (PTSD), we hypothesized that photoperiod may also alter fear memory and neuronal morphology within the hippocampus-basolateral amygdala-prefrontal cortex fear circuit. Ten weeks of exposure to short days increased fear memory in an auditory-cued fear conditioning test. Short days also increased dendritic spine density of the neurons of the basolateral amygdala, without affecting morphology of pyramidal neurons within the infralimbic region of the medial prefrontal cortex. Taken together, photoperiodic phenotypic changes in brain morphology and physiology induced by a single environmental factor, exposure to short day lengths, affect responses to fearful stimuli in white-footed mice. These results have potential implications for understanding seasonal changes in fear responsiveness, as well as for expanding translational animal models for studying gene-environment interactions underlying psychiatric diseases, such as PTSD.     

Chronic nicotine exposure produces lateralized,age‐dependent dendritic remodeling in the rodent basolateral amygdala

This study investigated the dendritic morphology of neurons located in the right and left basolateral amygdala (BLA) and infralimbic (IL) cortex following chronic nicotine exposure during adolescence or adulthood. Sprague–Dawley rats were administered subcutaneous injections of nicotine (0.5 mg/kg; free base) or saline three times per week for 2 weeks (six total injections). The dose period began on either postnatal day (P) 32 (adolescent) or P61 (adult). Twenty days following the end of dosing, brains were processed for Golgi‐Cox staining, and dendrites from principal neurons in the BLA and pyramidal neurons in the IL were digitally reconstructed in three dimensions. Morphometric analysis revealed a contrasting pattern of BLA dendritic morphology between the adolescent and adult pretreatment groups. In the adult control group, basilar dendritic length did not differ with respect to hemisphere. Nicotine induced robust hemispheric asymmetry by increasing dendritic length in the right hemisphere only. In contrast, adolescent nicotine exposure did not produce significant alteration of basilar dendritic morphology. There was, however, an indication that nicotine eliminated a naturally existing hemispheric asymmetry in the younger cohort. At both ages, nicotine produced a reduction in complexity of the apical tree of principal neurons. Chronic nicotine did not affect the dendritic morphology of pyramidal neurons from the IL in either age group, indicating another dimension of anatomical specificity. Collectively, these data implicate the BLA as a target for lasting neuroplasticity associated with chronic nicotine exposure. Further, hemispheric differences in dendritic morphology were uncovered that depended on the age of nicotine exposure, a finding that underscores the importance of considering laterality when investigating neurodevelopmental effects of drug exposure. Synapse 64:754–764, 2010. © 2010 Wiley‐Liss, Inc.     

Internuclear synapse in the amygdala is not facilitated in fear conditioning

The amygdala is essential for fear learning and memory. Synaptic transmission is enhanced in two pathways in the amygdala in fear conditioning. In this study we examined whether lateral (LA) to basolateral (BLA) amygdala synapses are potentiated and participate in intra-amygdala plasticity during the maintenance of fear memory. Our data showed that synaptic strength from the LA (ventrolateral) to the BLA (parvicellular) pathway was not increased after fear conditioning and suggests that this pathway does not integrate information relevant to the coding of memories in auditory fear learning.     

Decreased dendritic spine density of neurons of the prefrontal cortex and nucleus accumbens and enhanced amphetamine sensitivity in postpubertal rats after a neonatal amygdala lesion

A neonatal basolateral‐amygdala (nBLA) lesion in rats could be a potential animal model to study the early neurodevelopmental abnormalities associated with the behavioral and morphological brain changes observed in schizophrenia. Morphological alterations in pyramidal neurons from the prefrontal cortex (PFC) have been observed in postmortem schizophrenic brains, mainly because of decreased dendritic arbor and spine density. We assessed the effects of nBLA‐lesion on the dendritic morphology of neurons from the PFC and the nucleus accumbens (NAcc) in rats. nBLA lesions were made on postnatal day 7 (PD7), and later, the dendritic morphology was studied by the Golgi‐Cox stain procedure followed by Sholl analysis at PD35 (prepubertal) and PD60 (adult) ages. We also evaluated the effects of the nBLA‐lesion on locomotor activity caused by a novel environment, apomorphine, and amphetamine. Adult animals with nBLA lesions showed a decreased spine density in pyramidal neurons from the PFC and in medium spiny cells from the NAcc. An increased locomotion in a novel environment and in amphetamine‐treated adult animals with an nBLA‐lesion was observed. Our results indicate that nBLA‐lesion alters the neuronal dendrite morphology of the NAcc and PFC, suggesting a disconnection between these limbic structures. The locomotion paradigms support the idea that dopaminergic transmission is altered in the nBLA lesion model. This could help to understand the consequences of an earlier amygdala dysfunction in schizophrenia. Synapse 63:1143–1153, 2009. © 2009 Wiley‐Liss, Inc.     

Hitting Ras where it counts: Ras antagonism in the basolateral amygdala inhibits long-term fear memory

Several studies have implicated the Ras/mitogen-activated protein kinase (MAPK) pathway in Pavlovian fear conditioning. RasGRF1 knockout mice show significant deficits in acquisition of long-term fear memories and long-term potentaition (LTP) in the basolateral amygdala (BLA). MAPK kinase inhibition also impairs fear conditioning and amygdaloid LTP. However, there is no direct evidence to date for the involvement of Ras itself in fear conditioning. To address this issue, we examined the effects of intra-amygdala infusions of the selective Ras antagonist farnesylthiosalicylic acid (FTS) on the acquisition and expression of conditional freezing in rats. Micro-infusions of FTS into the BLA prior to contextual fear conditioning significantly impaired acquisition of long-term contextual fear memory in a dose-dependent manner. Post-training FTS infusions had no effect on acquisition of long-term fear memory. The effects of FTS on fear conditioning were specific for the BLA. Finally, intra-amygdala infusions of FTS inhibited MAPK activation in BLA. Collectively, these results provide further evidence for the involvement of amygdaloid Ras in the acquisition of long-term conditional fear memory.     

Lack of experience‐dependent intrinsic plasticity in the adolescent infralimbic medial prefrontal cortex

Fear extinction, an inhibitory learning that suppresses a previously learned fear memory, is diminished during adolescence. Earlier studies have shown that this suppressed fear extinction during adolescence involves an altered glutamatergic plasticity in infralimbic medial prefrontal cortical (IL‐mPFC) pyramidal neurons. However, it is unclear whether the excitability of IL‐mPFC pyramidal neurons plays a role in this development‐dependent suppression of fear extinction. Therefore, we examined whether fear conditioning and extinction affect the active and passive membrane properties of IL‐mPFC layer 5 pyramidal neurons in preadolescent, adolescent and adult mice. Both preadolescent and adult mice exhibited a bidirectional modulation of the excitability of IL‐mPFC layer 5 pyramidal neurons following fear conditioning and extinction, i.e., fear conditioning reduced membrane excitability, whereas fear extinction reversed this effect. However, the fear conditioning‐induced suppression of excitability was not reversed in adolescent mice following fear extinction training. Neither fear conditioning nor extinction affected GABAergic transmission in IL‐mPFC layer 5 pyramidal neurons, suggesting that GABAergic transmission did not play a role in experience‐dependent modulation of neuronal excitability. Our results suggest that the extinction‐specific modulation of excitability is impaired during adolescence.     

Dendritic morphology of neurons in prefrontal cortex and ventral hippocampus of rats with neonatal amygdala lesion

Neonatal basolateral amygdala (nBLA) lesions in rats have been widely used as a neurodevelopmental model that mimics schizophrenia-like behaviors. Recently, we reported that nBLA lesions result in significant decreases in the dendritic spine number of layer 3 prefrontal cortex (PFC) pyramidal cells and medium spiny neurons of the nucleus accumbens (NAcc), which all changes after puberty. At present, we aimed to evaluate the effect of this lesion in pyramidal neurons of CA1 of the ventral hippocampus (VH) and layer 5 of the PFC. In order to assess the effects of nBLA lesions on the dendritic morphology of the PFC and VH neurons, we carried out nBLA lesions in rats on postnatal day (PD) 7, and then we studied the dendritic morphology of these two limbic subregions at prepubertal (PD35) and postpubertal (PD60) ages. Dendritic characteristics were measured by Golgi-Cox procedure followed by Sholl analysis. We also evaluated the effects of nBLA lesions on the prepulse inhibition (PPI) and acoustic startle responses. The nBLA lesion induced a significant increase in dendritic length of layer 5 pyramidal neurons of the PFC at both ages, with a decrease in the dendritic spines density after puberty. The spine density of CA1 VH pyramidal neurons showed significant decreases at both ages. PPI was decreased in adulthood in the animals with an nBLA lesion. These results show that an nBLA lesion alters the dendritic morphology at the level of the PFC and VH in distinct ways before puberty, suggesting a disconnection between these limbic structures at an early age, and increasing our understanding of the implications of the VH in early amygdala dysfunction in schizophrenia.     

Associative Pavlovian conditioning leads to an increase in spinophilin-immunoreactive dendritic spines in the lateral amygdala

Changes in dendritic spine number and shape are believed to reflect structural plasticity consequent to learning. Previous studies have strongly suggested that the dorsal subnucleus of the lateral amygdala is an important site of physiological plasticity in Pavlovian fear conditioning. In the present study, we examined the effect of auditory fear conditioning on dendritic spine numbers in the dorsal subnucleus of the lateral amygdala using an immunolabelling procedure to visualize the spine-associated protein spinophilin. Associatively conditioned rats that received paired tone and shock presentations had 35% more total spinophilin-immunoreactive spines than animals that had unpaired stimulation, consistent with the idea that changes in the number of dendritic spines occur during learning and account in part for memory.     

Hemodynamic responses in amygdala and hippocampus distinguish between aversive and neutral cues during Pavlovian fear conditioning in behaving rats

Lesion and electrophysiological studies in rodents have identified the amygdala and hippocampus (HPC) as key structures for Pavlovian fear conditioning, but human functional neuroimaging studies have not consistently found activation of these structures. This could be because hemodynamic responses cannot detect the sparse neuronal activity proposed to underlie conditioned fear. Alternatively, differences in experimental design or fear levels could account for the discrepant findings between rodents and humans. To help distinguish between these alternatives, we used tissue oxygen amperometry to record hemodynamic responses from the basolateral amygdala (BLA), dorsal HPC (dHPC) and ventral HPC (vHPC) in freely‐moving rats during the acquisition and extinction of conditioned fear. To enable specific comparison with human studies we used a discriminative paradigm, with one auditory cue [conditioned stimulus (CS)+] that was always followed by footshock, and another auditory cue (CS?) that was never followed by footshock. BLA tissue oxygen signals were significantly higher during CS+ than CS? trials during training and early extinction. In contrast, they were lower during CS+ than CS? trials by the end of extinction. dHPC and vHPC tissue oxygen signals were significantly lower during CS+ than CS? trials throughout extinction. Thus, hemodynamic signals in the amygdala and HPC can detect the different patterns of neuronal activity evoked by threatening vs. neutral stimuli during fear conditioning. Discrepant neuroimaging findings may be due to differences in experimental design and/or fear levels evoked in participants. Our methodology offers a way to improve translation between rodent models and human neuroimaging.     

The Roles of Basolateral Amygdala Parvalbumin Neurons in Fear Learning

The basolateral amygdala (BLA) is obligatory for fear learning. This learning is linked to BLA excitatory projection neurons whose activity is regulated by complex networks of inhibitory interneurons, dominated by parvalbumin (PV)-expressing GABAergic neurons. The roles of these GABAergic interneurons in learning to fear and learning not to fear, activity profiles of these interneurons across the course of fear learning, and whether or how these change across the course of learning all remain poorly understood. Here, we used PV cell-type-specific recording and manipulation approaches in male transgenic PV-Cre rats during pavlovian fear conditioning to address these issues. We show that activity of BLA PV neurons during the moments of aversive reinforcement controls fear learning about aversive events, but activity during moments of nonreinforcement does not control fear extinction learning. Furthermore, we show expectation-modulation of BLA PV neurons during fear learning, with greater activity to an unexpected than expected aversive unconditioned stimulus (US). This expectation-modulation was specifically because of BLA PV neuron sensitivity to aversive prediction error. Finally, we show that BLA PV neuron function in fear learning is conserved across these variations in prediction error. We suggest that aversive prediction-error modulation of PV neurons could enable BLA fear-learning circuits to retain selectivity for specific sensory features of aversive USs despite variations in the strength of US inputs, thereby permitting the rapid updating of fear associations when these sensory features change.SIGNIFICANCE STATEMENT The capacity to learn about sources of danger in the environment is essential for survival. This learning depends on complex microcircuitries of inhibitory interneurons in the basolateral amygdala. Here, we show that parvalbumin-positive GABAergic interneurons in the rat basolateral amygdala are important for fear learning during moments of danger, but not for extinction learning during moments of safety, and that the activity of these neurons is modulated by expectation of danger. This may enable fear-learning circuits to retain selectivity for specific aversive events across variations in expectation, permitting the rapid updating of learning when aversive events change.