From aversive associations to defensive programs: experience-dependent synaptic modifications in the central amygdala

Plasticity elicited by fear conditioning (FC) is thought to support the storage of aversive associative memories. Although work over the past decade has revealed FC-induced plasticity beyond canonical sites in the basolateral complex of the amygdala (BLA), it is not known whether modifications across distributed circuits make equivalent or distinct contributions to aversive memory. Here, we review evidence demonstrating that experience-dependent synaptic plasticity in the central nucleus of the amygdala (CeA) has a circumscribed role in memory expression per se, guiding the selection of defensive programs in response to acquired threats. We argue that the CeA may be a key example of a broader phenomenon by which synaptic plasticity at specific nodes of a distributed network makes a complementary contribution to distinct memory processes.

Adolescent alcohol exposure alters threat avoidance in adulthood

Adolescent binge-like alcohol exposure impairs cognitive function and decision making in adulthood and may be associated with dysfunction of threat avoidance, a critical mechanism of survival which relies upon executive function. The present study investigated the impact of binge-like alcohol exposure during adolescence on active avoidance in adulthood. Male and female rats were subjected to adolescent intermittent ethanol (AIE) exposure by vapor inhalation and then tested in adulthood using a platform-mediated avoidance task. After training to press a lever to receive a sucrose reward, the rats were conditioned to a tone that co-terminated with a foot-shock. A motivational conflict was introduced by the presence of an escape platform that isolated the rat from the shock, but also prevented access to the sucrose reward while the rat was on the platform. During the task training phase, both male and female rats exhibited progressive increases in active avoidance (platform escape) in response to the conditioned tone, whereas innate fear behavior (freezing) remained relatively constant over training days. A history of AIE exposure did not impact either active avoidance or freezing behavior during task acquisition. On the test day following platform acquisition training, female rats exhibited higher levels of both active avoidance and freezing compared to male rats, while AIE-exposed male but not female rats exhibited significantly greater levels of active avoidance compared to controls. In contrast, neither male nor female AIE-exposed rats exhibited alterations in freezing compared to controls. Following 5 days of extinction training, female rats continued to display higher levels of active avoidance and freezing during tone presentation compared to males. Male AIE-exposed rats also had higher levels of both active avoidance and freezing compared to the male control rats. Together, the results demonstrate that female rats exhibit elevated levels of active avoidance and freezing compared to males and further reveal a sex-specific impact of AIE on threat responding in adulthood.

Dynamic θ Frequency Coordination within and between the Prefrontal Cortex-Hippocampus Circuit during Learning of a Spatial Avoidance Task

θ-Scale coordination of prelimbic medial prefrontal cortex (mPFC) local field potentials (LFPs) and its influence via direct or indirect projections to the ventral hippocampus (vHC) and dorsal hippocampus (dHC) during spatial learning remains poorly understood. We hypothesized that θ frequency coordination dynamics within and between the mPFC, dHC, and vHC would be predetermined by the level of connectivity rather than reflecting differing circuit throughput relationships depending on cognitive demands. Moreover, we hypothesized that coherence levels would not change during learning of a complex spatial avoidance task. Adult male rats were bilaterally implanted with EEG electrodes and LFPs recorded in each structure. Contrary to predictions, θ coherence averaged across "Early" or "Late" training sessions in the mPFC-HC, mPFC-mPFC, and HC-HC increased as a function of task learning. Coherence levels were also highest between the indirectly connected mPFC-dHC circuit, particularly during early training. Although mPFC postacquisition coherence remained higher with dHC than vHC, dynamic mPFC coherence patterns with both hippocampal poles across avoidance epochs were similar. In the 3 s before avoidance, a regional temporal sequence of transitory coherence peaks emerged between the mPFC-mPFC, the mPFC-HC, and then dHC-dHC. During this sequence, coherence within θ bandwidth fluctuated between epochs at distinct subfrequencies, suggesting frequency-specific roles for the propagation of task-relevant processing. On a second timescale, coherence frequency within and between the mPFC and hippocampal septotemporal axis change as a function of avoidance learning and cognitive demand. The results support a role for θ coherence subbandwidths, and specifically an 8- to 9-Hz mPFC θ signal, for generating and processing qualitatively different types of information in the organization of spatial avoidance behavior in the mPFC-HC circuit.

Action-based organization of a cerebellar module specialized for predictive control of multiple body parts

The role of the cerebellum in predictive motor control and coordination has been thoroughly studied during movements of a single body part. In the real world, however, actions are often more complex. Here, we show that a small area in the rostral anterior interpositus nucleus (rAIN) of the mouse cerebellum is responsible for generating a predictive motor synergy that serves to protect the eye by precisely coordinating muscles of the eyelid, neck, and forelimb. Within the rAIN region, we discovered a new functional category of neurons with unique properties specialized for control of motor synergies. These neurons integrated inhibitory cutaneous inputs from multiple parts of the body, and their activity was correlated with the vigor of the defensive motor synergy on a trial-by-trial basis. We propose that some regions of the cerebellum are organized in poly-somatotopic "action maps" to reduce dimensionality and simplify motor control during ethologically relevant behaviors.

ERP Indicators of Self-Pain and Other Pain Reductions due to Placebo Analgesia Responding: The Moderating Role of the Fight-Flight-Freeze System

This study evaluates the modulation of phasic pain and empathy for pain induced by placebo analgesia during pain and empathy for pain tasks. Because pain can be conceptualized as a dangerous stimulus that generates avoidance, we evaluated how approach and avoidance personality traits modulate pain and empathy for pain responses. We induced placebo analgesia to test whether this also reduces self-pain and other pain. Amplitude measures of the N1, P2, and P3 ERPs components, elicited by electric stimulations, were obtained during a painful control, as well as during a placebo treatment expected to induce placebo analgesia. The placebo treatment produced a reduction in pain and unpleasantness perceived, whereas we observed a decrease in the empathy unpleasantness alone during the empathy pain condition. The moderator effects of the fight-flight-freeze system (FFFS) in the relationships linking P2 and P3 amplitude changes with pain reduction were both significant among low to moderate FFFS values. These observations are consistent with the idea that lower FFFS (active avoidance) scores can predict placebo-induced pain reduction. Finally, in line with the revised Reinforcement Sensitivity Theory (r-RST), we can assume that phasic pain is an aversive stimulus activating the active-avoidance behavior to bring the system back to homeostasis.

Involvement of Cerebellar Neural Circuits in Active Avoidance Conditioning in Zebrafish

When animals repeatedly receive a combination of neutral conditional stimulus (CS) and aversive unconditional stimulus (US), they learn the relationship between CS and US, and show conditioned fear responses after CS. They show passive responses such as freezing or panic movements (classical or Pavlovian fear conditioning), or active behavioral responses to avoid aversive stimuli (active avoidance). Previous studies suggested the roles of the cerebellum in classical fear conditioning but it remains elusive whether the cerebellum is involved in active avoidance conditioning. In this study, we analyzed the roles of cerebellar neural circuits during active avoidance in adult zebrafish. When pairs of CS (light) and US (electric shock) were administered to wild-type zebrafish, about half of them displayed active avoidance. The expression of botulinum toxin, which inhibits the release of neurotransmitters, in cerebellar granule cells (GCs) or Purkinje cells (PCs) did not affect conditioning-independent swimming behaviors, but did inhibit active avoidance conditioning. Nitroreductase (NTR)-mediated ablation of PCs in adult zebrafish also impaired active avoidance. Furthermore, the inhibited transmission of GCs or PCs resulted in reduced fear-conditioned Pavlovian fear responses. Our findings suggest that the zebrafish cerebellum plays an active role in active avoidance conditioning.

Revisiting the role of anxiety in the initial acquisition of two-way active avoidance: pharmacological, behavioural and neuroanatomical convergence

Two-way active avoidance (TWAA) acquisition constitutes a particular case of approach -avoidance conflict for laboratory rodents. The present article reviews behavioural, psychopharmacological and neuroanatomical evidence accumulated along more than fifty years that provides strong support to the contention that anxiety is critical in the transition from CS (conditioned stimulus)-induced freezing to escape/avoidance responses during the initial stages of TWAA acquisition. Thus, anxiolytic drugs of different types accelerate avoidance acquisition, anxiogenic drugs impair it, and avoidance during these initial acquisition stages is negatively associated with other typical measures of anxiety. In addition behavioural and developmental treatments that reduce or increase anxiety/stress respectively facilitate or impair TWAA acquisition. Finally, evidence for the regulation of TWAA acquisition by septo-hippocampal and amygdala-related mechanisms is discussed. Collectively, the reviewed evidence gives support to the initial acquisition of TWAA as a paradigm with considerable predictive and (in particular) construct validity as an approach-avoidance conflict-based rodent anxiety model.

Functional and Dysfunctional Neuroplasticity in Learning to Cope with Stress

In this brief review, we present evidence of the primary role of learning-associated plasticity in the development of either adaptive or maladaptive coping strategies. Successful interactions with novel stressors foster plasticity within the neural circuits supporting acquisition, consolidation, retrieval, and extinction of instrumental learning leading to development of a rich repertoire of flexible and context-specific adaptive coping responses, whereas prolonged or repeated exposure to inescapable/uncontrollable stressors fosters dysfunctional plasticity within the learning circuits leading to perseverant and inflexible maladaptive coping strategies. Finally, the results collected using an animal model of genotype-specific coping styles indicate the engagement of different molecular networks and the opposite direction of stress effects (reduced vs. enhanced gene expression) in stressed animals, as well as different behavioral alterations, in line with differences in the symptoms profile associated with post-traumatic stress disorder.

Éloge de la Fuite: Neural Circuits for Avoiding Dangerous Situations

When facing threats, animals not only innately freeze or flee, but can also learn to avoid harmful situations, a behaviour known as 'active avoidance'. A recent study by Hormigo et al. (J. Neurosci., 2019) provides new insights into the neural circuit responsible for this behaviour, placing the pedunculopontine tegmental nucleus (PPT) at its centre.

Pattern of dopamine signaling during aversive events predicts active avoidance learning

Learning to avoid aversive outcomes is an adaptive strategy to limit one's future exposure to stressful events. However, there is considerable variance in active avoidance learning across a population. The mesolimbic dopamine system contributes to behaviors elicited by aversive stimuli, although it is unclear if the heterogeneity in active avoidance learning is explained by differences in dopamine transmission. Furthermore, it is not known how dopamine signals evolve throughout active avoidance learning. To address these questions, we performed voltammetry recordings of dopamine release in the ventral medial striatum throughout training on inescapable footshock and signaled active avoidance tasks. This approach revealed differences in the pattern of dopamine signaling during the aversive cue and the safety period that corresponded to subsequent task performance. Dopamine transmission throughout the footshock bout did not predict performance but rather was modulated by the prior stress exposure. Additionally, we demonstrate that dopamine encodes a safety prediction error signal, which illustrates that ventral medial striatal dopamine release conveys a learning signal during both appetitive and aversive conditions.

Rats bred for high anxiety exhibit distinct fear-related coping behavior, hippocampal physiology, and synaptic plasticity-related gene expression

The hippocampus is essential for learning and memory but also regulates emotional behavior. We previously identified the hippocampus as a major brain region that differs in rats bred for emotionality differences. Rats bred for low novelty response (LRs) exhibit high levels of anxiety- and depression-like behavior compared to high novelty responder (HR) rats. Manipulating the hippocampus of high-anxiety LR rats improves their behavior, although no work to date has examined possible HR/LR differences in hippocampal synaptic physiology. Thus, the current study examined hippocampal slice electrophysiology, dendritic spine density, and transcriptome profiling in HR/LR hippocampus, and compared performance on three hippocampus-dependent tasks: The Morris water maze, contextual fear conditioning, and active avoidance. Our physiology experiments revealed increased long-term potentiation (LTP) at CA3-CA1 synapses in HR versus LR hippocampus, and Golgi analysis found an increased number of dendritic spines in basal layer of CA1 pyramidal cells in HR versus LR rats. Transcriptome data revealed glutamate neurotransmission as the top functional pathway differing in the HR/LR hippocampus. Our behavioral experiments showed that HR/LR rats exhibit similar learning and memory capability in the Morris water maze, although the groups differed in fear-related tasks. LR rats displayed greater freezing behavior in the fear-conditioning task, and HR/LR rats adopted distinct behavioral strategies in the active avoidance task. In the active avoidance task, HRs avoided footshock stress by pressing a lever when presented with a warning cue; LR rats, on the other hand, waited until footshocks began before pressing the lever to stop them. Taken together, these findings concur with prior observations of HR rats generally exhibiting active stress coping behavior while LRs exhibit reactive coping. Overall, our current findings coupled with previous work suggest that HR/LR differences in stress reactivity and stress coping may derive, at least in part, from differences in the developing and adult hippocampus.

Repeated Cocaine Exposure Attenuates the Desire to Actively Avoid: A Novel Active Avoidance Runway Task

Drug addiction is a disorder in which drug seeking persists despite aversive consequences. While it is well documented in animal models of drug sensitization that repeated drug exposure enhances positive incentive motivation for drug and natural reinforcers, its effect on negative incentive motivation, defined here as the motivation to avoid a cued aversive outcome, remains an open question. In the present study, we designed a novel active avoidance (AA) runway paradigm to assess the effects of repeated cocaine exposure on the motivation to avoid an aversive outcome. Cocaine and saline pre-exposed rats were first trained to perform a conditioned AA lever press response to prevent the occurrence of foot shock administrations. The rats were subsequently tested in a runway apparatus, wherein they were required to traverse the length of a straight alley maze to reach the lever and emit a conditioned AA response. Run times were measured as an indication of negative incentive motivation. Cocaine pre-exposed rats demonstrated longer latencies to emit the conditioned AA response but showed no differences in latency to initiate runway behavior, nor in their acquisition of the AA response compared to the saline pre-exposed controls. Subsequent testing in an elevated plus maze revealed no differences in the expression of anxiety in cocaine pre-exposed rats compared to saline pre-exposed controls. Our results indicate that prior repeated cocaine exposure attenuated cued negative incentive motivation, which suggests that drug addiction may be attributable to a decrease in motivation to avoid aversive consequences associated with drug use.

Age-Related Decrease in Stress Responsiveness and Proactive Coping in Male Mice

Coping is a strategic approach to dealing with stressful situations. Those who use proactive coping strategies tend to accept changes and act before changes are expected. In contrast, those who use reactive coping are less flexible and more likely to act in response to changes. However, little research has assessed how coping style changes with age. This study investigated age-related changes in coping strategies and stress responsiveness and the influence of age on the processing of conditioned fear memory in 2-, 12- and 23-month-old male mice. Coping strategy was measured by comparing the escape latency in an active avoidance test and by comparing responses to a shock prod. The results showed that proactivity in coping response gradually decreased with age. Stress responsiveness, measured by stress-induced concentration of corticosterone, was also highest in 2-month-old mice and decreased with age. Consolidation of fear memory was highest in 12-month-old mice and was negatively correlated with the degree of stress responsiveness and proactivity in coping. Fear extinction did not differ among age groups and was not correlated with stress responsiveness or the proactivity of coping. However, the maintenance of extinct fear memory, which was best in 2-month-old mice and worst in 12-month-old mice, was negatively correlated with stress responsiveness but not with coping style. Age-dependent changes in the expression of glucocorticoid receptor (GR) and its regulatory co-chaperones, which are accepted mechanisms for stress hormone stimulation, were measured in the hippocampus. The expression of GR was increased at 12 months compared to other age groups. There were no differences in Hsp70 and BAG1 expression by age. These results can be summarized as follows: (1) stress responsiveness and proactivity in coping decreased with age class; (2) consolidation of fear memory was negatively correlated with both stress responsiveness and proactivity; however, maintenance of extinct fear memory was negatively correlated with stress responsiveness only; and (3) consolidation and maintenance of extinct fear memory appeared to be more influenced by factors other than stress reactivity and proactivity in coping, such as the amount of hippocampal glucocorticoid expression.

Schizophrenia-Related Microdeletion Impairs Emotional Memory through MicroRNA-Dependent Disruption of Thalamic Inputs to the Amygdala

Individuals with 22q11.2 deletion syndrome (22q11DS) are at high risk of developing psychiatric diseases such as schizophrenia. Individuals with 22q11DS and schizophrenia are impaired in emotional memory, anticipating, recalling, and assigning a correct context to emotions. The neuronal circuits responsible for these emotional memory deficits are unknown. Here, we show that 22q11DS mouse models have disrupted synaptic transmission at thalamic inputs to the lateral amygdala (thalamo-LA projections). This synaptic deficit is caused by haploinsufficiency of the 22q11DS gene Dgcr8, which is involved in microRNA processing, and is mediated by the increased dopamine receptor Drd2 levels in the thalamus and by reduced probability of glutamate release from thalamic inputs. This deficit in thalamo-LA synaptic transmission is sufficient to cause fear memory deficits. Our results suggest that dysregulation of the Dgcr8–Drd2 mechanism at thalamic inputs to the amygdala underlies emotional memory deficits in 22q11DS.

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Thalamic inputs to the lateral amygdala (LA) are impaired in 22q11DS mice

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Thalamo-LA disruption is sufficient to cause associative fear memory deficits

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Deficiency in microRNA-processing Dgcr8 causes thalamo-LA and fear memory deficits

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Fear memory deficits in 22q11DS mice are rescued by thalamic Drd2 inhibition

Central Amygdala Somatostatin Neurons Gate Passive and Active Defensive Behaviors

The central amygdala (CeA) has a key role in learning and expression of defensive responses. Recent studies indicate that somatostatin-expressing (SOM(+)) neurons in the lateral division of the CeA (CeL) are essential for the acquisition and recall of conditioned freezing behavior, which has been used as an index of defensive response in laboratory animals during Pavlovian fear conditioning. However, how exactly these neurons participate in fear conditioning and whether they contribute to the generation of defensive responses other than freezing remain unknown. Here, using fiber-optic photometry combined with optogenetic and molecular techniques in behaving mice, we show that SOM(+) CeL neurons are activated by threat-predicting sensory cues after fear conditioning and that activation of these neurons suppresses ongoing actions and converts an active defensive behavior to a passive response. Furthermore, inhibition of these neurons using optogenetic or molecular methods promotes active defensive behaviors. Our results provide the first in vivo evidence that SOM(+) neurons represent a CeL population that acquires learning-dependent sensory responsiveness during fear conditioning and furthermore reveal an important role of these neurons in gating passive versus active defensive behaviors in animals confronted with threat.

SIGNIFICANCE STATEMENT

The ability to develop adaptive behavioral responses to threat is fundamental for survival. Recent studies indicate that the central lateral amygdala (CeL), in particular its somatostatin-expressing neurons, is crucial for both learning and the expression of defensive response. However, how exactly these neurons participate in such processes remains unclear. Here we show for the first time in behaving mice that the somatostatin-expressing neurons in the CeL acquire learning-dependent responsiveness to sensory cues predicting a threat. Furthermore, our results indicate that these neurons gate the behavioral output of an animal: whereas high activity in these neurons biases toward passive defensive responses, low activity in these neurons allows the expression of active defensive responses.

Ventral Pallidum Encodes Contextual Information and Controls Aversive Behaviors

Successful avoidance of aversive outcomes is crucial for the survival of animals. Although accumulating evidence indicates that an indirect pathway in the basal ganglia is involved in aversive behavior, the ventral pallidum (VP), which is an important component of this pathway, has so far been implicated primarily in appetitive behavior. In this study, we used single-cell recordings and bicuculline (GABAA antagonist) injections to elucidate the role of VP both in the encoding of aversive context and in active avoidance. We found 2 populations of neurons that were preferentially activated by appetitive and aversive conditioned stimuli (CSs). In addition, VP showed appetitive and aversive outcome anticipatory activities. These activity patterns indicate that VP is involved in encoding and maintaining CS-induced aversive contextual information. Furthermore, the disturbance of VP activity by bicuculline injection increased the number of error trials in aversive trials. In particular, the subjects released the response bar prematurely, showed no response at all, or failed to avoid the aversive outcome. Overall, these results suggest that VP plays a central role in controlling CS-induced negative motivation to produce avoidance behavior.

Active Avoidance: Neural Mechanisms and Attenuation of Pavlovian Conditioned Responding

Patients with anxiety disorders often experience a relapse in symptoms after exposure therapy. Similarly, threat responses acquired during Pavlovian threat conditioning often return after extinction learning. Accordingly, there is a need for alternative methods to persistently reduce threat responding. Studies in rodents have suggested that exercising behavioral control over an aversive stimulus can persistently diminish threat responses, and that these effects are mediated by the amygdala, ventromedial prefrontal cortex, and striatum. In this fMRI study, we attempted to translate these findings to humans. Subjects first underwent threat conditioning. We then contrasted two forms of safety learning: active avoidance, in which participants could prevent the shock through an action, and yoked extinction, with shock presentation matched to the active condition, but without instrumental control. The following day, we assessed subjects' threat responses (measured by skin conductance) to the conditioned stimuli without shock. Subjects next underwent threat conditioning with novel stimuli. Yoked extinction subjects showed an increase in conditioned response to stimuli from the previous day, but the active avoidance group did not. Additionally, active avoidance subjects showed reduced conditioned responding during novel threat conditioning, but the extinction group did not. We observed between-group differences in striatal BOLD responses to shock omission in Avoidance/Extinction. These findings suggest a differential role for the striatum in human active avoidance versus extinction learning, and indicate that active avoidance may be more effective than extinction in persistently diminishing threat responses.SIGNIFICANCE STATEMENT Extinguished threat responses often reemerge with time, highlighting the importance of identifying more enduring means of attenuation. We compared the effects of active avoidance learning and yoked extinction on threat responses in humans and contrasted the neural circuitry engaged by these two processes. Subjects who learned to prevent a shock through an action maintained low threat responses after safety learning and showed attenuated threat conditioning with novel stimuli, in contrast to those who underwent yoked extinction. The results suggest that experiences of active control over threat engage the striatum and promote a shift from expression of innate defensive responses toward more adaptive behavioral responses to threatening stimuli.

Why Are Children With Epileptic Encephalopathies Encephalopathic?

The epileptic encephalopathies are devastating conditions characterized by frequent seizures, severely abnormal electroencephalograms (EEGs), and cognitive slowing or regression. The cognitive impairment in the epileptic encephalopathies may be more concerning to the patient and parents than the epilepsy itself. There is increasing recognition that the cognitive comorbidity can be both chronic, primarily due to the underlying etiology of the epilepsy, and dynamic or evolving because of recurrent seizures, interictal spikes, and antiepileptic drugs. Much of scholars' understanding of the neurophysiological underpinnings of cognitive dysfunction in the epileptic encephalopathies comes from rodent studies. Frequent seizures and interictal EEG discharges in rats lead to considerable spatial and social-cognitive deficits. Paralleling these cognitive deficits are dyscoordination of dynamic neural activity within and between the neural networks that subserve normal cognitive processes.

The effect of ingested sulfite on active avoidance in normal and sulfite oxidase-deficient aged rats

The aim of this study was to investigate the possible toxic effects of sulfite on neurons by measuring active avoidance learning in normal and sulfite oxidase (SOX)-deficient aged rats. Twenty-four months of age Wistar rats were divided into four groups: control (C), sulfite-treated group (S), SOX-deficient group (D) and SOX-deficient + sulfite-treated group (DS). SOX deficiency was established by feeding rats with a low molybdenum (Mo) diet and adding 200 ppm tungsten (W) to their drinking water. Sulfite in the form of sodium metabisulfite (25 mg/kg) was given by gavage for six weeks. Active avoidance responses were determined by using an automated shuttle box. Hepatic SOX activity was measured to confirm SOX deficiency. The hippocampus was used for determining the activity of cyclooxygenase (COX) and caspase-3 enzymes and the level of prostaglandin E2 (PGE2) and nitrate/nitrite. SOX-deficient rats had an approximately 10-fold decrease in hepatic SOX activity compared with normal rats. Sulfite did not induce impairment of active avoidance learning in SOX-deficient rats and in normal rats compared with their control groups. Sulfite had no effect on the activity of COX and caspase-3 in the hippocampus. Treatment with sulfite did not significantly increase the level of PGE2 and nitrate/nitrite in the hippocampus.

Strain dependent effects of conditioned fear in adult C57Bl/6 and Balb/C mice following postnatal exposure to chlorpyrifos: relation to expression of brain acetylcholinesterase mRNA

Following reports of emotional psychopathology in children and adults exposed to organophosphates, the effects of postnatal chlorpyrifos (CPF) on fear-conditioning and depression-like behaviors were tested in adult mice. Concomitant changes in expression of mRNA for synaptic and soluble splice variants of acetylcholinesterase (AChE) were examined in mouse pups and adults of the Balb/C and C57Bl/6 (B6) strains, which differ in their behavioral and hormonal stress response. Mice were injected subcutaneously with 1 mg/kg CPF on postnatal days 4–10 and tested as adults for conditioned fear, sucrose preference, and forced swim. Acetylcholinesterase activity was assessed in the brains of pups on the first and last day of treatment. Expression of soluble and synaptic AChE mRNA was assessed in brains of treated pups and fear-conditioned adults using real-time PCR. Adult Balb/C mice exposed postnatally to CPF showed exacerbated fear-conditioning and impaired active avoidance. Adult B6 mice exposed postnatally to CPF showed a more specific fear response to tones and less freezing in the inter-tone intervals, in contrast to the vehicle-pretreated mice. Chlorpyrifos also attenuated sweet preference and enhanced climbing in the forced swim test. Chlorpyrifos-treated mice had increased expression of both synaptic and readthrough AChE transcripts in the hippocampus of Balb/C mice and decreased expression in the amygdala following fear-conditioning. In conclusion, postnatal CPF had long-term effects on fear and depression, as well as on expression of AChE mRNA. These changes may be related to alteration in the interaction between hippocampus and amygdala in regulating negative emotions.

Impact of an additional chronic BDNF reduction on learning performance in an Alzheimer mouse model

There is increasing evidence that brain-derived neurotrophic factor (BDNF) plays a crucial role in Alzheimer’s disease (AD) pathology. A number of studies demonstrated that AD patients exhibit reduced BDNF levels in the brain and the blood serum, and in addition, several animal-based studies indicated a potential protective effect of BDNF against Aβ-induced neurotoxicity. In order to further investigate the role of BDNF in the etiology of AD, we created a novel mouse model by crossing a well-established AD mouse model (APP/PS1) with a mouse exhibiting a chronic BDNF deficiency (BDNF^+/−). This new triple transgenic mouse model enabled us to further analyze the role of BDNF in AD in vivo. We reasoned that in case BDNF has a protective effect against AD pathology, an AD-like phenotype in our new mouse model should occur earlier and/or in more severity than in the APP/PS1-mice. Indeed, the behavioral analysis revealed that the APP/PS1-BDNF^+/−-mice show an earlier onset of learning impairments in a two-way active avoidance task in comparison to APP/PS1- and BDNF^+/−-mice. However in the Morris water maze (MWM) test, we could not observe an overall aggrevated impairment in spatial learning and also short-term memory in an object recognition task remained intact in all tested mouse lines. In addition to the behavioral experiments, we analyzed the amyloid plaque pathology in the APP/PS1 and APP/PS1-BDNF^+/−-mice and observed a comparable plaque density in the two genotypes. Moreover, our results revealed a higher plaque density in prefrontal cortical compared to hippocampal brain regions. Our data reveal that higher cognitive tasks requiring the recruitment of cortical networks appear to be more severely affected in our new mouse model than learning tasks requiring mainly sub-cortical networks. Furthermore, our observations of an accelerated impairment in active avoidance learning in APP/PS1-BDNF^+/−-mice further supports the hypothesis that BDNF deficiency amplifies AD-related cognitive dysfunctions.

Taking action in the face of threat: neural synchronization predicts adaptive coping

The ability to take action in the face of threat is highly diverse across individuals. What are the neural processes that determine individual differences in the ability to cope with danger? We hypothesized that the extent of synchronization between amygdala, striatum, and medial prefrontal cortex (mPFC) would predict successful active coping performance. To test this, we developed a novel computer task based on the principals of Sidman avoidance. Healthy human participants learned through trial and error to move a marker between virtual game board compartments once every 3 s to avoid mild shocks. Behaviorally, participants exhibited large individual differences. Strikingly, both amygdala-mPFC and caudate-mPFC coupling during active coping trials covaried with final active coping performance across participants. These findings indicate that synchronization between mPFC subregions, and both amygdala and caudate predicts whether individuals will achieve successful active coping performance by the end of training. Thus, successful performance of adaptive actions in the face of threat requires functional synchronization of a neural circuit consisting of mPFC, striatum, and amygdala. Malfunction in the crosstalk between these components might underlie anxiety symptoms and impair individuals' ability to actively cope under stress. This opens an array of possibilities for therapeutic targets for fear and anxiety disorders.

Protective effects of glucose-6-phosphate dehydrogenase on neurotoxicity of aluminium applied into the CA1 sector of rat hippocampus

BACKGROUND & OBJECTIVES

Aluminum (Al) toxicity is closely linked to the pathogenesis of Alzheimer's disease (AD). This experimental study was aimed to investigate the active avoidance behaviour of rats after intrahippocampal injection of Al, and biochemical and immunohistochemical changes in three bilateral brain structures namely, forebrain cortex (FBCx), hippocampus and basal forebrain (BF).

METHODS

Seven days after intra-hippocampal (CA1 sector) injection of AlCl3 into adult male Wistar rats they were subjected to two-way active avoidance (AA) tests over five consecutive days. Control rats were treated with 0.9% w/v saline. The animals were decapitated on the day 12 post-injection. The activities of acetylcholinesterase (AChE) and glucose-6-phosphate dehydrogenase (G6PDH) were measured in the FBCx, hippocampus and BF. Immunohistochemical staining was performed for transferrin receptors, amyloid β and tau protein.

RESULTS

The activities of both AChE and G6PDH were found to be decreased bilaterally in the FBCx, hippocampus and basal forebrain compared to those of control rats. The number of correct AA responses was reduced by AlCl3 treatment. G6PDH administered prior to AlCl 3 resulted in a reversal of the effects of AlCl3 on both biochemical and behavioural parameters. Strong immunohistochemical staining of transferrin receptors was found bilaterally in the FBCx and the hippocampus in all three study groups. In addition, very strong amyloid β staining was detected bilaterally in all structures in AlCl3-treated rats but was moderate in G6PDH/AlCl3-treated rats. Strong tau staining was noted bilaterally in AlCl3-treated rats. In contrast, tau staining was only moderate in G6PDH/AlCl3-treated rats.

INTERPRETATION & CONCLUSIONS

Our findings indicated that the G6PDH alleviated the signs of behavioural and biochemical effects of AlCl3-treatment suggesting its involvement in the pathogenesis of Al neurotoxicity and its potential therapeutic benefit. The present model could serve as a useful tool in AD investigations.

Influence of the green tea leaf extract on neurotoxicity of aluminium chloride in rats

Aluminium may have an important role in the aetiology/pathogenesis/precipitation of Alzheimer's disease. Because green tea (Camellia sinensis L.) reportedly has health-promoting effects in the central nervous system, we evaluated the effects of green tea leaf extract (GTLE) on aluminium chloride (AlCl3 ) neurotoxicity in rats. All solutions were injected into the cornu ammonis region 1 hippocampal region. We measured the performance of active avoidance (AA) tasks, various enzyme activities and total glutathione content (TGC) in the forebrain cortex (FbC), striatum, basal forebrain (BFb), hippocampus, brain stem and cerebellum. AlCl3 markedly reduced AA performance and activities of cytochrome c oxidase (COX) and acetylcholinesterase (AChE) in all regions. It decreased TGC in the FbC, striatum, BFb, hippocampus, brain stem and cerebellum, and increased superoxide dismutase activity in the FbC, cerebellum and BFb. GTLE pretreatment completely reversed the damaging effects of AlCl3 on AA and superoxide dismutase activity, markedly corrected COX and AChE activities, and moderately improved TGC. GTLE alone increased COX and AChE activities in almost all regions. GTLE reduces AlCl3 neurotoxicity probably via antioxidative effects and improves mitochondrial and cholinergic synaptic functions through the actions of (-)-epigallocatechin gallate and (-)-epicatechin, compounds most abundantly found in GTLE. Our results suggest that green tea might be beneficial in Alzheimer's disease.

Involvement of cyclin-dependent kinase-like 2 in cognitive function required for contextual and spatial learning in mice

Cyclin-dependent kinase-like 2 (Cdkl2) is a cdc2-related serine/threonine protein kinase that is postnatally expressed in various brain regions, including the cerebral cortex, entorhinal cortex, hippocampus, amygdala, and dorsal thalamus. The extremely high Cdkl2 expression in these regions suggests that it has a role in cognition and emotion. Recent genetic studies indicate that mutations of Cdkl family kinases are associated with neurodevelopmental and neuropsychiatric disorders in humans. To elucidate the physiologic role of Cdkl2, we behaviorally analyzed Cdkl2(LacZ/LacZ) mice lacking Cdkl2. Cdkl2(LacZ/LacZ) mice had reduced latencies to enter the dark compartment after electric footshock in an inhibitory avoidance task and attenuated contextual fear responses when exposed to mild training conditions. Hippocampal spatial learning in the Morris water maze was slightly anomalous with mice exhibiting an abnormal swimming pattern. The aversive response in a two-way avoidance task was slightly, but not significantly, enhanced. On the other hand, Cdkl2(LacZ/LacZ) mice did not exhibit altered sensitivity to aversive stimuli, such as electric footshock and heat, or deficits in the elevated plus maze or rotating rod test. These findings suggest that Cdkl2 is involved in cognitive function and provide in vivo evidence for the function of Cdkl family kinases expressed in terminally differentiated neurons in mice.

Nicotinic modulation of tone-evoked responses in auditory cortex reflects the strength of prior auditory learning

Nicotinic acetylcholine receptors (nAChRs) contribute to sensory-cognitive function, as demonstrated by evidence that nAChR activation enhances, and nAChR blockade impairs, neural processing of sensory stimuli and sensory-cognitive behavior. To better understand the relationship between nAChR function and behavior, here we compare the strength of nAChR-mediated physiology in individual animals to their prior auditory behavioral performance. Adult rats were trained on an auditory-cued, active avoidance task over 4 days and classified as "good," "intermediate" or "poor" performers based on their initial rate of learning and eventual level of performance. Animals were then anesthetized, and tone-evoked local field potentials (LFPs) recorded in layer 4 of auditory cortex (ACx) before and after a test dose of nicotine (0.7mg/kg, s.c.) or saline. In "good" performers, nicotine enhanced LFP amplitude and decreased response threshold to characteristic frequency (CF) stimuli, yet had opposite effects (decreased amplitude, increased threshold) on responses to spectrally distant stimuli; i.e., cortical receptive fields became more selective for CF stimuli. In contrast, nicotine had little effect on LFP amplitude in "intermediate" or "poor" performing animals. Nicotine did, however, reduce LFP onset latency in all three groups, indicating that all received an effective dose of the drug. Our findings suggest that nicotinic regulation of cortical receptive fields may be a distinguishing feature of the best-performing animals, and may facilitate sensory-related learning by enhancing receptive field selectivity.