Facilitation of fear extinction by novelty depends on dopamine acting on D1-subtype dopamine receptors in hippocampus

Acute physical exercise prevents memory amnesia caused by protein synthesis inhibition in rats' hippocampus

The benefits of physical exercise (PE) on memory consolidation have been well-documented in both healthy and memory-impaired animals. However, the underlying mechanisms through which PE exerts these effects are still unclear. In this study, we aimed to investigate the role of hippocampal protein synthesis in memory modulation by acute PE in rats. After novel object recognition (NOR) training, rats were subjected to a 30-min moderate-intensity acute PE on the treadmill, while control animals did not undergo any procedures. Using anisomycin (ANI) and rapamycin (RAPA), compounds that inhibit protein synthesis through different mechanisms, we manipulated protein synthesis in the CA1 region of the hippocampus to examine its contribution to memory consolidation. Memory was assessed on days 1, 7, and 14 post-training. Our results showed that inhibiting protein synthesis by ANI or RAPA impaired NOR memory consolidation in control animals. However, acute PE prevented this impairment without affecting memory persistence. We also evaluated brain-derived neurotrophic factor (BDNF) levels after acute PE at 0.5h, 2h, and 12h afterward and found no differences in levels compared to animals that did not engage in acute PE or were only habituated to the treadmill. Therefore, our findings suggest that acute PE could serve as a non-pharmacological intervention to enhance memory consolidation and prevent memory loss in conditions associated with hippocampal protein synthesis inhibition. This mechanism appears not to depend on BDNF synthesis in the early hours after exercise.

A partial habituation method to test for anterograde and retrograde amnestic treatment effects: Evidence that antagonism of the NMDA receptor can induce anterograde but not retrograde amnestic effects

BACKGROUND

A progressive decrease in spontaneous locomotion with repeated exposure to a novel environment has been assessed using both within and between-session measures. While both are well-established and reliable measurements, neither are useful alone as methods to concurrently assess treatment effects on acquisition and retention of habituation.

NEW METHOD

We report a behavioral method that measures habituation by combining the within and between measurements of locomotion. We used a 30 min session divided into 6 five min blocks. In the first novel environment session activity was maximal in the first 5 min block but was reduced to a low level by the sixth block, indicative of within-session habituation. Using 8 daily sessions, we showed that this terminal block low level of activity progressed incrementally to the first block to achieve complete habituation.

RESULTS/COMPARISON WITH EXISTING METHODS

Within-session activity across sessions was used to identify different stages of between session habituation. It was then possible to assess drug treatment effects from partial to complete habituation, so that treatment effects on retention of the previously acquired partial habituation, expressed as a reversion to an earlier within session habituation pattern (retrograde amnesia assessment), as well as the effects on new learning by the failure in subsequent sessions to acquire complete between-session habituation (anterograde amnesia assessment).

CONCLUSIONS

The use of spontaneous motor activity to assess learning and memory effects provides the opportunity to assess direct treatment effects on behavior and motor activity in contrast to many learning and memory models.

Novelty facilitates the persistence of aversive memory extinction by dopamine regulation in the hippocampus and ventral tegmental area

Aversive memory extinction comprises a novel learning that blocks retrieving a previously formed traumatic memory. In this sense, aversive memory extinction is an excellent tool for decreasing fear responses. However, this tool it's not effective in the long term because of original memory spontaneous recovery. Thus, searching for alternative strategies that strengthen extinction learning is essential. In the current study, we evaluated the effects of a novel context (i.e., novelty) exposure on aversive memory extinction enhancement over days and the dopaminergic system requirement. Given the purpose, experiments were conducted using 3-month-old male Wistar rats. Animals were trained in inhibitory avoidance (IA). Twenty-four hours later, rats were submitted to a weak extinction protocol. Still, 30 min before the first extinction session, animals were submitted to an exploration of a novel context for 5 min. After, memory retention and persistence were evaluated 24 h, 3, 7, 14, and 21 days later. The exposition of a novel context caused a decrease in aversive responses in all days analyzed and an increase in dopamine levels in the hippocampus. The intrahippocampal infusion of dopamine in the CA1 area or the stimulation of the ventral tegmental area (VTA) by a glutamatergic agonist (NMDA) showed similar effects of novelty. In contrast, VTA inhibition by a gabaergic agonist (muscimol) impaired the persistence of extinction learning induced by novelty exposition and caused a decrease in hippocampal dopamine levels. In summary, we show that novel context exposure promotes persistent aversive memory extinction, revealing the significant role of the dopaminergic system.

A behavioral tagging account of kinase contribution to memory formation after spaced aversive training

Long-term memory (LTM) can be induced by repeated spaced training trials. Using the weak inhibitory avoidance (wIA) task, we showed that one wIA session does not lead to a 24-h LTM, whereas two identical wIA sessions spaced by 15 min to 6 h induce a 24-h LTM. This LTM promotion depends both on hippocampal protein synthesis and the activity of several kinases. In agreement with the behavioral tagging (BT) hypothesis, our results suggest that the two training sessions induce transient learning tags and lead, via a cooperative effect, to the synthesis of plasticity-related proteins (PRPs) that become available and captured by the tag from the second session. Although ERKs1/2 are needed for PRPs synthesis and CaMKs are required for tag setting, PKA participates in both processes. We conclude that the BT mechanism accounts for the molecular constraints underlying the classic effect of spaced learning on LTM formation.

Hippocampal cholinergic receptors and the mTOR participation in fear-motivated inhibitory avoidance extinction memory

Evidence has demonstrated the hippocampal cholinergic system and the mammalian target of rapamycin (mTOR) participation during the memory formation of aversive events. This study assessed the role of these systems in the hippocampus for the extinction memory process by submitting male Wistar rats to fear-motivated step-down inhibitory avoidance (IA). The post-extinction session administration of the nicotinic and muscarinic cholinergic receptor antagonists, mecamylamine and scopolamine, respectively, both at doses of 2 µg/µl/side, and rapamycin, an mTOR inhibitor (0.02 µg/µl/side), into the CA1 region of the dorsal hippocampus, impaired the IA extinction memory. Furthermore, the nicotinic and muscarinic cholinergic receptor agonists, nicotine and muscarine, respectively, had a dose-dependent effect on the IA extinction memory when administered intra-CA1, immediately after the extinction session. Nicotine (0.6 µg/µl/side) and muscarine (0.02 µg/µl/side), respectively, had no effect, while the higher doses (6 and 2 µg/µl/side, respectively) impaired the IA extinction memory. Interestingly, the co-administration of muscarine at the lower dose blocked the impairment that was induced by rapamycin. This effect was not observed when nicotine at the lower dose was co-administered. These results have demonstrated the participation of the cholinergic receptors and mTOR in the hippocampus for IA extinction, and that the cholinergic agonists had a dose-dependent effect on the IA extinction memory. This study provides insights related to the behavioural aspects and the neurobiological properties underlying the early stage of fear-motivated IA extinction memory consolidation and suggests that there is hippocampal muscarinic receptor participation independent of mTOR in this memory process.

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Locus coeruleus input-modulated reactivation of dentate gyrus opioid-withdrawal engrams promotes extinction

Extinction training during the reconsolidation window following memory recall is an effective behavioral pattern for promoting the extinction of pathological memory. However, promoted extinction by recall-extinction procedure has not been universally replicated in different studies. One potential reason for this may relate to whether initially acquired memory is successfully activated. Thus, the methods for inducing the memory into an active or plastic condition may contribute to promoting its extinction. The aim of this study is to find and demonstrate a manipulatable neural circuit that engages in the memory recall process and where its activation improves the extinction process through recall-extinction procedure. Here, naloxone-precipitated conditioned place aversion (CPA) in morphine-dependent mice was mainly used as a pathological memory model. We found that the locus coeruleus (LC)-dentate gyrus (DG) circuit was necessary for CPA memory recall and that artificial activation of LC inputs to the DG just prior to initiating a recall-extinction procedure significantly promoted extinction learning. We also found that activating this circuit caused an increase in the ensemble size of DG engram cells activated during the extinction, which was confirmed by a cFos targeted strategy to label cells combined with immunohistochemical and in vivo calcium imaging techniques. Collectively, our data uncover that the recall experience is important for updating the memory during the reconsolidation window; they also suggest a promising neural circuit or target based on the recall-extinction procedure for weakening pathological aversion memory, such as opioid withdrawal memory and fear memory.

Tag and capture: how salient experiences target and rescue nearby events in memory

The long-term fate of a memory is not exclusively determined by the events occurring at the moment of encoding. Research at the cellular, circuit, and behavioral levels is beginning to reveal how neurochemical activations in the moments surrounding an event can retroactively and proactively rescue weak memory for seemingly mundane experiences. We review emerging evidence showing enhancement of weakly formed memories encoded minutes to hours before or after a related motivationally relevant experience. We discuss proposed neurobiological mechanisms for strengthening weak memories formed in temporal proximity to a strong event, and how this knowledge could be leveraged to improve memory for information that is prone to forgetting.

The role of hippocampus in memory reactivation: an implication for a therapeutic target against opioid use disorder

Purpose of the review

The abuse of opioids induces many terrible problems in human health and social stability. For opioid-dependent individuals, withdrawal memory can be reactivated by context, which is then associated with extremely unpleasant physical and emotional feelings during opioid withdrawal. The reactivation of withdrawal memory is considered one of the most important reasons for opioid relapse, and it also allows for memory modulation based on the reconsolidation phenomenon. However, studies exploring withdrawal memory modulation during the reconsolidation window are lacking. By summarizing the previous findings about the reactivation of negative emotional memories, we are going to suggest potential neural regions and systems for modulating opioid withdrawal memory.

Recent findings

Here, we first present the role of memory reactivation in its modification, discuss how the hippocampus participates in memory reactivation, and discuss the importance of noradrenergic signaling in the hippocampus for memory reactivation. Then, we review the engagement of other limbic regions receiving noradrenergic signaling in memory reactivation. We suggest that noradrenergic signaling targeting hippocampus neurons might play a potential role in strengthening the disruptive effect of withdrawal memory extinction by facilitating the degree of memory reactivation.

Summary

This review will contribute to a better understanding of the mechanisms underlying reactivation-dependent memory malleability and will provide new therapeutic avenues for treating opioid use disorders.

A new method to study learning and memory using spontaneous locomotor activity in an open-field arena

BACKGROUND

Reduced locomotion with repeated exposure to a novel environment is often used as a measure of the basic adaptive learning process of habituation. While this is a well-established and reliable measure of habituation, it is not useful for the investigation of neurobiological changes before and after habituation because of the uncontrolled differential activity levels in a novel versus habituated environment. In this study we report a behavioral method that uses spontaneous locomotion to measure habituation, in which the total spontaneous locomotion in an initially novel environment does not change with repeated testing but, the ratio of central to peripheral activity does change and is indicative of habituation. The test sessions are brief (5 min) and the locomotion is measured in 2 separate zones. The peripheral zone comprises 8/9 of the test arena and the central zone 1/9 of the arena.

RESULTS/COMPARISON WITH EXISTING METHODS

In contrast to methods that use between-session reductions in locomotion to assess habituation, this method employs brief test sessions in which overall activity between sessions does not change, but the distribution of locomotion in the periphery versus the central zone of the arena does change. The brevity of the test session also enables us to utilize post-trial drug treatment protocols to impact memory consolidation.

CONCLUSIONS

The progressive change in the central/peripheral activity ratio with repeated testing can be determined independently of total activity and provides a habituation acquisition function that permits the measurement of neurobiological changes without the complication of effects related to changes in locomotor activity per se. The present report also presents evidence that this method can be used with post-trial drug treatment protocols to study the learning and memory effects of the post-trial treatments without the use of explicit rewards and punishments.

Novelty promotes recognition memory persistence by D1 dopamine receptor and protein kinase A signalling in rat hippocampus

Strategies for improving memory are increasingly studied, and exposure to a novel experience can be an efficient neuromodulator. Novelty effects on memory depend on D1-family dopamine receptors (D1Rs) activation. Here, we evaluated the novelty effect on memory persistence of Wistar rats and investigated the contribution of D1Rs and their signalling pathways by protein kinase A (PKA) and C (PKC). Animals with infusion cannulae inserted into the CA1 hippocampus area were trained on the novel object recognition (NOR) task, which involved exploring two different objects. After training, some rats received intrahippocampal infusions of vehicle or D1Rs agonist; others explored a novel environment for 5 min and were infused with a variety of drugs targeting D1Rs and their signalling pathways. We demonstrated that pharmacological stimulation of D1Rs or novelty exposure promoted NOR memory persistence for 14 days and that the novelty effect depended on D1Rs activation. To determine if the D1 and D5 receptor subtypes were necessary for the impact of novelty exposure on memory, we blocked or stimulated PKA or PKC-protein kinases activated mainly by D1 and D5, respectively. Only PKA inhibition impaired the effect of novelty on memory persistence. After novelty and D1Rs blocking, PKA but not PKC stimulation maintained the memory persistence effect. Thus, we concluded that novelty promoted memory persistence by a mechanism-dependent on activating hippocampal D1Rs and PKA pathway.

Comparing three extinction methods to reduce fear expression and generalization

Fear extinction is easy to achieve but difficult to maintain, as evidenced by the relapse of fear after extinction. Counterconditioning and novelty-facilitated extinction have been shown to interfere with fear expression without erasing it. Because of the similarity between the two extinction paradigms, we extended the standard extinction, which merely omitted the expected threat outcomes after exposure to original threat cues. The modified paradigm provided a stimulus (neutral picture or positive picture) to replace the omitted threat outcomes during extinction. Sixty-four healthy volunteers were randomized into three groups for a three-day procedure: fear acquisition (day 1), fear extinction (day 2), and fear recall and generalization test (day 3). Our results showed the modified extinction paradigm failed to prevent fear expression in spontaneous recovery and reinstatement tests. However, novelty-facilitated extinction showed powerful effects in preventing fear generalization. Besides, there was a negative correlation between spontaneous recovery index and emotion regulation scores. We speculated that emotion and prediction error may be important factors influencing fear extinction and affect fear recall and generalization. Overall, this study suggests that novelty-facilitated extinction had a superior effect in preventing fear generalization, providing new perspectives for enhancing the effect of exposure therapy.

Infralimbic and prelimbic prefrontal cortex activation is necessary to the enhancement of aversive memory extinction promoted by reactivation

Memory extinction has been used in behavioral therapy to treat post-traumatic stress disorders. It was demonstrated that memory reactivation before extinction could facilitate this process. However, the mechanisms involved are still unclear. Here, we investigated the participation of two regions of the ventromedial prefrontal cortex (vmPFC), the infralimbic (IL) and prelimbic (PL), in the memory reactivation modulatory effect of fear extinction. We confirmed that the reactivation facilitates the fear extinction in an inhibitory aversive task; however, when the muscimol (a GABAergic agonist) is infused in IL or PL vmPFC after reactivation, extinction's facilitation was not observed. These findings support the idea that the reactivation can modulate the fear extinction process, facilitating it, and that this effect requires the activation of both IL and PL regions of vmPFC.

DNA methylation changes following narrative exposure therapy in a randomized controlled trial with female former child soldiers

The aftermath of traumatization lives on in the neural and epigenetic traces creating a momentum of affliction in the psychological and social realm. Can psychotherapy reorganise these memories through changes in DNA methylation signatures? Using a randomised controlled parallel group design, we examined methylome-wide changes in saliva samples of 84 female former child soldiers from Eastern DR Congo before and six months after Narrative Exposure Therapy. Treatment predicted differentially methylated positions (DMPs) related to ALCAM, RIPOR2, AFAP1 and MOCOS. In addition, treatment associations overlapped at gene level with baseline clinical and social outcomes. Treatment related DMPs are involved in memory formation-the key agent in trauma focused treatments-and enriched for molecular pathways commonly affected by trauma related disorders. Results were partially replicated in an independent sample of 53 female former child soldiers from Northern Uganda. Our results suggest a molecular impact of psychological treatment in women with war-related childhood trauma.Trial registration: Addressing Heightened Levels of Aggression in Traumatized Offenders With Psychotherapeutic Means (ClinicalTrials.gov Identifier: NCT02992561, 14/12/2016).

Gender dimorphic effect of dopamine D2 and muscarinic cholinergic receptors on memory retrieval

Episodic memory retrieval is fundamental for daily activities of humans and animals. Muscarinic cholinergic signaling is important for memory functioning and shows gender-dependent response in episodic memory retrieval. Dopamine D2 receptors influence memory formation and retrieval by influencing cholinergic signaling in the brain. This study aimed to determine the gender-dependent effects of D2 and muscarinic activity on memory retrieval. Male and female mice were trained for Morris water maze test and contextual fear conditioning. Memory retrieval was assessed following sub-chronic treatment (for 5 days) with D2 antagonist (risperidone 2.5 mg/kg) alone or in combination with scopolamine (1 mg/kg) or donepezil (1 mg/kg). Open field test was performed prior to the retrieval test to evaluate effects of risperidone treatment on locomotor activity and exploratory behavior. Risperidone co-treatment with donepezil impaired spatial memory retrieval in males only. Muscarinic and D2 simultaneous antagonism tend to impair fear retrieval in males but significantly enhanced retrieval of fear memories in female mice. These results suggest that D2 signaling influence muscarinic receptor activity during memory retrieval in gender-dependent manner.

Elevated activity in the dorsal dentate gyrus reduces expression of fear memory after fear extinction training

BACKGROUND

Effectively reducing the expression of certain aversive memories (fear or trauma memories) with extinction training is generally viewed to be therapeutically important. A deeper understanding of the biological basis for a more effective extinction process is also of high scientific importance.

METHODS

Our study involved intraventricular injection or local injection into the dorsal dentate gyrus of anti-neuregulin 1 antibodies (anti-NRG1) before fear extinction training, followed by testing the expression of fear memory 24 hours afterward or 9 days later. We used local injection of chemogenetic or optogenetic viruses into the dorsal dentate gyrus to manipulate the activity of the dorsal dentate gyrus and test the expression of fear memory. We also examined the effect of deep brain stimulation in the dorsal dentate gyrus on the expression of fear memory.

RESULTS

Mice that received intraventricular injection with anti-NRG1 antibodies exhibited lower expression of fear memory and increased density of activated excitatory neurons in the dorsal dentate gyrus. Injection of anti-NRG1 antibodies directly into the dorsal dentate gyrus also led to lower expression of fear memory and more activated neurons in the dorsal dentate gyrus. Inhibiting the activity of dorsal dentate gyrus excitatory neurons using an inhibitory designer receptor exclusively activated by designer drugs (DREADD) eliminated the effects of the anti-NRG1 antibodies. Enhancing the activity of the dorsal dentate gyrus with an excitatory DREADD or optogenetic stimulation resulted in lower expression of fear memory in mice that did not receive infusion of anti-NRG1 antibodies. Deep brain stimulation in the dorsal dentate gyrus effectively suppressed expression of fear memory, both during and after fear extinction training.

LIMITATIONS

The mechanism for the contribution of the dorsal dentate gyrus to the expression of fear memory needs further exploration.

CONCLUSION

Activation of the dorsal dentate gyrus may play an important role in modulating the expression of fear memory; its potential use in fear memory extinction is worthy of further exploration.

Dopamine: The Neuromodulator of Long-Term Synaptic Plasticity, Reward and Movement Control

Dopamine (DA) is a key neurotransmitter involved in multiple physiological functions including motor control, modulation of affective and emotional states, reward mechanisms, reinforcement of behavior, and selected higher cognitive functions. Dysfunction in dopaminergic transmission is recognized as a core alteration in several devastating neurological and psychiatric disorders, including Parkinson's disease (PD), schizophrenia, bipolar disorder, attention deficit hyperactivity disorder (ADHD) and addiction. Here we will discuss the current insights on the role of DA in motor control and reward learning mechanisms and its involvement in the modulation of synaptic dynamics through different pathways. In particular, we will consider the role of DA as neuromodulator of two forms of synaptic plasticity, known as long-term potentiation (LTP) and long-term depression (LTD) in several cortical and subcortical areas. Finally, we will delineate how the effect of DA on dendritic spines places this molecule at the interface between the motor and the cognitive systems. Specifically, we will be focusing on PD, vascular dementia, and schizophrenia.

One single physical exercise session improves memory persistence by hippocampal activation of D1 dopamine receptors and PKA signaling in rats

Previously, we demonstrated that one single physical exercise session could positively modulate recognition memory persistence by D1/D5 activation. Here, we aim to investigate whether the effect of physical exercise on memory occurs due to the activation of both receptors, D1 and D5, or only one of them. Adult male Wistar rats were habituated on a treadmill one week before experiments. After learning session in the object recognition task, some animals received intrahippocampal infusions of the vehicle or a D1/D5 agonist (SKF 38393, 12.5 μg/μL/side), whereas others performed a single session of physical exercise on a treadmill (30 min at an intensity of 60-70% of indirect VO₂ max.). Immediately after physical exercise, some animals received intrahippocampal infusions of vehicle or D1/D5 antagonist (SCH 23390, 1 μg/μL/side). Signaling pathways of D1 and D5 receptors in the hippocampus were evaluated by pharmacological activation or inactivation of protein kinases A (PKA) and C (PKC), respectively. According to previous findings, D1/D5 agonist and a single physical exercise session after learning promoted memory persistence, and D1/D5 block impaired physical exercise effect. Importantly, here we demonstrated for the first time that PKA inhibition, but not PKC, impairs the effect of acute physical exercise on memory persistence. Besides, PKA stimulation can promote its effects on memory. Therefore, we provide evidence that corroborates the idea that D1-like dopaminergic receptors, by activation of the PKA pathway, are involved in the effects of acute physical exercise on memory.

Contribution of D1R-expressing neurons of the dorsal dentate gyrus and Cav1.2 channels in extinction of cocaine conditioned place preference

Cocaine-associated contextual cues can trigger relapse behavior by recruiting the hippocampus. Extinction of cocaine-associated contextual memories can reduce cocaine-seeking behavior, however the molecular mechanisms within the hippocampus that underlie contextual extinction behavior and subsequent reinstatement remain poorly understood. Here, we extend our previous findings for a role of Ca_v1.2 L-type Ca²⁺ channels in dopamine 1 receptor (D1R)-expressing cells in extinction of cocaine conditioned place preference (CPP) in adult male mice. We report that attenuated cocaine CPP extinction in mice lacking Ca_v1.2 channels in D1R-expressing cells (D1^cre, Ca_v1.2^fl/fl) can be rescued through chemogenetic activation of D1R-expressing cells within the dorsal dentate gyrus (dDG), but not the dorsal CA1 (dCA1). This is supported by the finding that Ca_v1.2 channels are required in excitatory cells of the dDG, but not in the dCA1, for cocaine CPP extinction. Examination of the role of S1928 phosphorylation of Ca_v1.2, a protein kinase A (PKA) site using S1928A Ca_v1.2 phosphomutant mice revealed no extinction deficit, likely due to homeostatic scaling up of extinction-dependent S845 GluA1 phosphorylation in the dDG. However, phosphomutant mice failed to show cocaine-primed reinstatement which can be reversed by chemogenetic manipulation of excitatory cells in the dDG during extinction training. These findings outline an essential role for the interaction between D1R, Ca_v1.2, and GluA1 signaling in the dDG for extinction of cocaine-associated contextual memories.

Behavioral tagging underlies memory reconsolidation

Memory reconsolidation occurs when a retrieving event destabilizes transiently a consolidated memory, triggering thereby a new process of restabilization that ensures memory persistence. Although this phenomenon has received wide attention, the effect of new information cooccurring with the reconsolidation process has been less explored. Here we demonstrate that a memory-retrieving event sets a neural tag, which enables the reconsolidation of memory after binding proteins provided by the original or a different contiguous experience. We characterized the specific temporal window during which this association is effective and identified the protein kinase A (PKA) and the extracellular signal-regulated kinase 1 and 2 (ERK 1/2) pathways as the mechanisms related to the setting of the reconsolidation tag and the synthesis of proteins. Our results show, therefore, that memory reconsolidation is mediated by a "behavioral tagging" process, which is common to different memory forms. They represent a significant advance in understanding the fate of memories reconsolidated while being adjacent to other events, and provide a tool for designing noninvasive strategies to attenuate (pathological/traumatic) or improve (education-related) memories.

Morphine administered post-trial induces potent morphine conditioned effects if the context is novel but not if the context is familiar

Morphine administered shortly after exposure to a novel environment induces potent locomotor stimulant conditioning. Environmental novelty is important as pre-exposure (PE) to a stimulus can attenuate the capacity to acquire conditioned stimulus (CS). Here, the importance of environmental novelty for the efficacy of an open-field to become a CS for elicitation of a morphine conditioned response was assessed by comparing the effects of morphine administered post-trial following a 5 min exposure to a novel environment versus a PE environment. Four groups of rats (2 vehicle and 2 morphine groups) were used. Two groups received ten daily 5 min non-drug PEs to an open-field arena and the other two groups were not pre-exposed to the environment. Subsequently, all groups received post-trial injections of either vehicle or morphine immediately after each of five daily 5 min sessions in the open-field. Importantly, on the first day of testing prior to the first post-test morphine administration, the locomotor activity of the novel and PE groups was not different. Over the 5 post-trial morphine treatments, the activity of the PE morphine group, the PE vehicle and the novel environment vehicle groups did not change and were equivalent. In contrast, in the novel environment morphine group, a conditioned hyper-activity response increased with repeated post-trial morphine treatments. For the morphine group it is suggested that the novel environment initiated a post-trial stimulus trace that occurred in temporal contiguity with the post-trial drug response and enabled the trace to become a CS for the morphine unconditioned response. In contrast, PE induced a latent inhibition effect in the PE morphine group, thus the post-trial CS trace was insufficient to become associated to the morphine response and no conditioning occurred. In addition to conventional drug induced Pavlovian delay conditioning, the findings are suggestive of drug induced Pavlovian trace conditioning.

One physical exercise session promotes recognition learning in rats with cognitive deficits related to amyloid beta neurotoxicity

Alzheimer's disease is a progressive neurodegenerative pathological process that causes memory loss and cognitive impairment. One of the pathological characteristics of Alzheimer's disease is the amyloid-β protein aggregation on the brain. The regular practice of physical exercise is a consolidated strategy on the prevention of cognitive deficits; however, little is known about the effects of acute exercise on memory. We hypothesize that one physical exercise session could act as a modulator of learning. Here we investigated the effects of one single session of running (aerobic) or strength (anaerobic) exercise on memory deficits related to neurotoxicity induced by amyloid-β. Male Wistar rats were submitted to stereotaxic surgery to intrahippocampal infusion of amyloid-β protein or saline (control). Ten days after the surgery the rats were submitted to the object recognition (OR) memory task. Immediately after the OR learning session, some rats were submitted to one treadmill running or strength exercise session. Then, the animals were submitted to memory tests 24 h, 7, and 14 days after the OR learning. We demonstrated that one physical exercise session, both aerobic as anaerobic, performed after learning improves learning and memory, promoting memory persistence in control rats and memory consolidation in rats submitted to amyloid-β neurotoxicity model. Notably, the effects of the aerobic exercise session seem to be more prominent, since they also reflect in an improvement of object discrimination index for 7 days in control animals. We verified that the mechanisms involved in the effects of aerobic exercise include the dopaminergic system activation. The mechanisms involved in the anaerobic exercise effects seem to be others since no alterations on hippocampal dopamine or noradrenaline levels were detected.

Dopamine: from prediction error to psychotherapy

Dopamine, one of the main neurotransmitters in the mammalian brain, has been implicated in the coding of prediction errors that govern reward learning as well as fear extinction learning. Psychotherapy too can be viewed as a form of error-based learning, because it challenges erroneous beliefs and behavioral patterns in order to induce long-term changes in emotions, cognitions, and behaviors. Exposure therapy, for example, relies in part on fear extinction principles to violate erroneous expectancies of danger and induce novel safety learning that inhibits and therefore reduces fear in the long term. As most forms of psychotherapy, however, exposure therapy suffers from non-response, dropout, and relapse. This narrative review focuses on the role of midbrain and prefrontal dopamine in novel safety learning and investigates possible pathways through which dopamine-based interventions could be used as an adjunct to improve both the response and the long-term effects of the therapy. Convincing evidence exists for an involvement of the midbrain dopamine system in the acquisition of new, safe memories. Additionally, prefrontal dopamine is emerging as a key ingredient for the consolidation of fear extinction. We propose that applying a dopamine prediction error perspective to psychotherapy can inspire both pharmacological and non-pharmacological studies aimed at discovering innovative ways to enhance the acquisition of safety memories. Additionally, we call for further empirical investigations on dopamine-oriented drugs that might be able to maximize consolidation of successful fear extinction and its long-term retention after therapy, and we propose to also include investigations on non-pharmacological interventions with putative prefrontal dopaminergic effects, like working memory training.

Dopaminergic Plasticity in the Bilateral Hippocampus Following Threat Reversal in Humans

When a cue no longer predicts a threat, a diminished ability to extinguish or reverse this association is thought to increase risk for stress-related disorders. Despite the clear clinical relevance, the mediating neurochemical mechanisms of threat reversal have received relatively little study. One neurotransmitter implicated in rodent research of changing associations with threat is dopamine. To study whether dopamine is involved in threat reversal in humans, we used high-resolution positron emission tomography (PET) coupled with 18F-fallypride. Twelve healthy volunteers (6 F/6 M) underwent three PET scans: (i) at baseline, (ii) following threat conditioning (the response to a cue associated with electric wrist shock), and (iii) following threat reversal (the response to the same cue now associated with safety). We observed moderate evidence of reduced dopamine D2/3 receptor availability, consistent with greater dopamine release, in the bilateral anterior hippocampus following threat reversal, in response to a safety cue that was previously associated with threat, as compared to both baseline and during exposure to the same cue prior to threat reversal. These findings offer the first preliminary evidence that the response to a previously threatening cue that has since become associated with safety involves dopaminergic neurotransmission within the hippocampus in healthy humans.

Maternal Deprivation Induces Memory Deficits That Are Reduced by One Aerobic Exercise Shot Performed after the Learning Session

During the neonatal period, the brain is susceptible to external influences. Exposure to stressful events during this phase of life influences brain development and impacts adult life. In animals, the maternal deprivation (MD) model is effective in mimicking stress in the early stages of development. In contrast, physical exercise seems to be able to prevent deficits in memory consolidation. Although the effects of chronic exercise in cognition are already well established, little is known about the effects of acute aerobic exercise. Here, male Wistar rats divided into deprived (MD) and nondeprived (NMD) rats were submitted to the object recognition (OR) memory test. Immediately after OR training, some of the rats were submitted to a single aerobic exercise session for 30 minutes. Memory consolidation and persistence were evaluated by retention tests performed 24 h and 7, 14, and 21 days after OR training. We show that a single physical exercise session is able to modulate learning by promoting memory consolidation and persistence in rats with cognitive deficits induced by MD. Hippocampal dopamine levels, measured by HPLC, were not altered after OR training in rats that performed and in rats that did not perform an exercise session; on the other hand, while OR training promoted increase of hippocampal norepinephrine in NMD rats, the MD rats did not present this increase, regardless of the practice or not of exercise.

Catecholaminergic hippocampal activation is necessary for object recognition memory persistence induced by one-single physical exercise session

Previously we demonstrated that a single physical exercise session promotes the persistence of object recognition (OR) memory and this effect involves the activation of the noradrenergic system. Here, using adult male Wistar rats (3 months old) we confirm that an aerobic single physical exercise session (30 min of treadmill running at an intensity of 60-70 % of indirect VO₂ max.) after OR learning promotes memory persistence. We also demonstrate that this effect involves the dopaminergic system, since it is blocked when a D1-family receptor antagonist (SCH-23390, 1μg/μl) is infused into the hippocampus after the physical exercise session. Additionally, through HPLC experiments we demonstrate that a physical exercise session increases the hippocampal dopamine levels. Taken together, our results demonstrate that acute post-learning physical exercise is able to promote the persistence of OR memory, inducing the release of dopamine in hippocampus, which is necessary for the modulation of memory persistence. This work brings new evidences on the benefit of a single physical exercise session to memory, as well as suggests that catecholaminergic mechanisms are behind this effect.

Noradrenergic and dopaminergic involvement in novelty modulation of aversive memory generalization of adult rats

The generalization of aversive memory can be defined as the phenomenon in which a situation similar to (but distinctive from) a previous aversive event triggers an avoidance response. This phenomenon has been suggested to play a role in several psychological disorders. In this study, we investigate the effects of novelty on the generalization of fear memories, and the involvement of noradrenergic and dopaminergic systems in this process. For this study we used male Wistar rats (3 months old, 300-400 g). The participation of each neurotransmitter system was evaluated separately (two set of experiments). In each experimental set, the animals were divided in groups (8 animals each): (i) control, (ii) novelty, and, (iii) antagonist + novelty group (timolol, a β-adrenergic antagonist, or SCH23390, a D1/D5 dopaminergic antagonist, in the first and in the second set of experiments, respectively). Additionaly, to investigate whether novelty exposure increases the levels of noradrenaline and/or dopamine in the hippocampus fifteen animals were divided in three groups (5 animals each).: (i) naïve, (ii) control; and, (iii) novelty. To examine aversive memory, and generalization of aversive memory, we trained adult male Wistar rats in an inhibitory avoidance (IA) memory task and after in a modified inhibitory avoidance (MIA). Before the MIA training some of the animals were exposed to environmental novelty (open field). Immediately before this novelty exposure, some animals received intrahippocampal infusion of timolol (β-adrenergic antagonist), SCH23390 (D1/D5 antagonist) or vehicle to evaluate the involvement of noradrenergic and dopaminergic systems. Finally, to evaluate aversive memory and generalization of aversive memory respectively, half of the animals in each group were tested on IA and half on MIA. We confirmed that the exposure to novelty blocks the generalization of aversive memory, but moreover, demonstrated that this process involves activation of β-adrenergic and dopaminergic D1/D5 pathways. We additionally observed that exposure to novelty raises hippocampal levels of noradrenaline and dopamine. This suggests that these neurotransmitters not only influence long-term memory (LTM) as such, but also aversive memory generalization.

Sex differences in fear extinction

Despite the exponential increase in fear research during the last years, few studies have included female subjects in their design. The need to include females arises from the knowledge gap of mechanistic processes underlying the behavioral and neural differences observed in fear extinction. Moreover, the exact contribution of sex and hormones in relation to learning and behavior is still largely unknown. Insights from this field could be beneficial as fear-related disorders are twice as prevalent in women compared to men. Here, we review an up-to-date summary of animal and human studies in adulthood that report sex differences in fear extinction from a structural and functional approach. Furthermore, we describe how these factors could contribute to the observed sex differences in fear extinction during normal and pathological conditions.

Novelty exposure hinders aversive memory generalization and depends on hippocampal protein synthesis

Fear generalization is defined as the transferring of fear experienced during a traumatic event to safe conditions resembling or not the traumatic event. It has been related to several psychological disorders. Here we set out to determine whether novelty exposure can be effective to avoid fear generalization. We evaluated the effect of a novelty exposure on fear memory generalization using an aversive memory task, the inhibitory avoidance (IA). Male Wistar rats were trained in IA (day 1) and 24 h after (day 2) they were exposed to a new context similar to the original (modified IA - MIA), with some rats being exposed to a novelty just before the exposure to the MIA, while others were not (controls). On day 3, retention tests for IA and MIA contexts were performed. The control rats generalized the memory, expressing aversive behavioral in both contexts whereas rats exposed to novelty only expressed aversion on IA. Furthermore, both anisomycin, an inhibitor of ribosomal protein synthesis, and rapamycin, an inhibitor of mTOR-mediated protein synthesis, injected in the CA1 region of dorsal hippocampus blocked the novelty effect, promoting memory generalization. We conclude that novelty exposure hinders aversive memory generalization depending on hippocampal protein synthesis.

Subchronic administration of aripiprazole improves fear extinction retrieval of Pavlovian conditioning paradigm in rats experiencing psychological trauma

People may suffer from an intruded fear memory when the attributable traumatic events no longer exist. This is of highly clinical relevance to trauma-induced mental disorders, such as posttraumatic stress disorder (PTSD). Mechanism underlying PTSD largely lies in the abnormal process of fear extinction and a functional imbalance within amygdala associated fear circuit areas. Previous evidence suggested central dopamine plays a key role in the regulation of the fear memory process, yet it remains unclear whether the intervention of dopamine modulators would be beneficial for the fear extinction abnormalities. The present study examined the performance of Pavlovian conditioned fear and the changes of dopamine profiles following a subchronic 14-day regimen of aripiprazole (a partial agonist of dopamine D2 receptors to normalize the condition caused by dopamine imbalance) in rats previously experienced a psychologically traumatic procedure of single prolonged stress (SPS). The results demonstrated that aripiprazole at 5.0 mg/kg reversed the SPS-impaired fear memory dysfunction and the SPS-reduced dopamine efflux in the amygdala. The present study suggests a therapeutic potential of subchronic treatment with aripiprazole in managing patients suffered from fear extinction problem.

Acute Low Alcohol Disrupts Hippocampus-Striatum Neural Correlate of Learning Strategy by Inhibition of PKA/CREB Pathway in Rats

The hippocampus and striatum guide place-strategy and response-strategy learning, respectively, and they have dissociable roles in memory systems, which could compensate in case of temporary or permanent damage. Although acute alcohol (AA) treatment had been shown to have adverse effects on hippocampal function, whether it causes the functional compensation and the underlying mechanisms is unknown. In this study, rats treated with a low dose of AA avoided a hippocampus-dependent spatial strategy, instead preferring a striatum-dependent response strategy. Consistently, the learning-induced increase in hippocampal, but not striatal, pCREB was rendered less pronounced due to diminished activity of pPKA, but not pERK or pCaMKII. As rats approached the turn-decision area, Sp-cAMP, a PKA activator, was found to mitigate the inhibitory effect of AA on intra- and cross-structure synchronized neuronal oscillations, and rescue response-strategy bias and spatial learning deficits. Our study provides strong evidence of the critical link between neural couplings and strategy selection. Moreover, the PKA/CREB-signaling pathway is involved in the suppressive effect of AA on neural correlates of place-learning strategy. The novel important evidence provided here shows the functional couplings between the hippocampus and striatum in spatial learning processing and suggests possible avenues for therapeutic intervention.

A translational perspective on neural circuits of fear extinction: Current promises and challenges

Fear extinction is the well-known process of fear reduction through repeated re-exposure to a feared stimulus without the aversive outcome. The last two decades have witnessed a surge of interest in extinction learning. First, extinction learning is observed across species, and especially research on rodents has made great strides in characterising the physical substrate underlying extinction learning. Second, extinction learning is considered of great clinical significance since it constitutes a crucial component of exposure treatment. While effective in reducing fear responding in the short term, extinction learning can lose its grip, resulting in a return of fear (i.e., laboratory model for relapse of anxiety symptoms in patients). Optimization of extinction learning is, therefore, the subject of intense investigation. It is thought that the success of extinction learning is, at least partly, determined by the mismatch between what is expected and what actually happens (prediction error). However, while much of our knowledge about the neural circuitry of extinction learning and factors that contribute to successful extinction learning comes from animal models, translating these findings to humans has been challenging for a number of reasons. Here, we present an overview of what is known about the animal circuitry underlying extinction of fear, and the role of prediction error. In addition, we conducted a systematic literature search to evaluate the degree to which state-of-the-art neuroimaging methods have contributed to translating these findings to humans. Results show substantial overlap between networks in animals and humans at a macroscale, but current imaging techniques preclude comparisons at a smaller scale, especially in sub-cortical areas that are functionally heterogeneous. Moreover, human neuroimaging shows the involvement of numerous areas that are not typically studied in animals. Results obtained in research aimed to map the extinction circuit are largely dependent on the methods employed, not only across species, but also across human neuroimaging studies. Directions for future research are discussed.

Modulating reconsolidation and extinction to regulate drug reward memory

Drug addiction is an aberrant memory that shares the same memory processes as other memories. Brief exposure to drug-associated cues could result in reconsolidation, a hypothetical process during which original memory could be updated. In contrast, longer exposure times to drug-associated cues could trigger extinction, a process that decreases the conditioned responding. In this review, we discuss the pharmacological and non-pharmacological manipulations on the reconsolidation and extinction that could be used to interfere with drug reward memories. Pharmacological agents such as β-adrenergic receptor antagonist propranolol can interfere with reconsolidation to disrupt drug reward memory. Pharmacological agents such as the NMDA receptor glycine site agonists d-cycloserine and d-serine can facilitate extinction and then attenuate the expression of drug reward memory. Besides pharmacological interventions, drug-free behavioral approaches by utilizing the reconsolidation and extinction, such as 'post-retrieval extinction' and 'UCS-retrieval extinction', are also effective to erase or inhibit the recall of drug reward memory. Taken together, pharmacological modulation and non-pharmacological modulation of reconsolidation and extinction are promising approaches to regulate drug reward memory and prevent relapse.

Atypical fetal development: Fetal alcohol syndrome, nutritional deprivation, teratogens, and risk for neurodevelopmental disorders and psychopathology

Accumulating evidence indicates that the fetal environment plays an important role in brain development and sets the brain on a trajectory across the life span. An abnormal fetal environment results when factors that should be present during a critical period of development are absent or when factors that should not be in the developing brain are present. While these factors may acutely disrupt brain function, the real cost to society resides in the long-term effects, which include important mental health issues. We review the effects of three factors, fetal alcohol exposure, teratogen exposure, and nutrient deficiencies, on the developing brain and the consequent risk for developmental psychopathology. Each is reviewed with respect to the evidence found in epidemiological and clinical studies in humans as well as preclinical molecular and cellular studies that explicate mechanisms of action.

Environmental novelty modulates the induction and expression of single injection-induced behavioral sensitization to morphine

Opioid addiction is a growing public health problem, being currently considered an epidemic in the United States. Investigating the behavioral effects of opioids and the factors influencing their development becomes of major importance. In animals, the effects of drugs of abuse can be assessed using the behavioral sensitization model, which shares similar neuronal substrates with drug craving in humans. Importantly, novelty plays a critical role on the development of behavioral sensitization. The aim of the present study was to investigate the influence of a new environment on both the induction and expression phases of morphine (Mor)-induced behavioral sensitization in the two-injection protocol. Mice were initially treated with saline, 15 or 30 mg/kg Mor (induction phase), and subsequently challenged 7 days later with 15 mg/Kg Mor (expression phase). Locomotor frequency was evaluated during behavioral sessions, performed as follow: induction session on a novel environment and expression on a familiar open-filed apparatus; induction session on animals' home-cage (familiar environment) and expression session on an unknown open-filed apparatus; both sessions on novel environments; and both sessions on familiar contexts. Mor-induced behavioral sensitization was only observed when animals were exclusively exposed to novelty during the induction phase, not being observed when both the induction and expression sessions were performed on similar (novel or familiar) environments. Our results suggest that the development of behavioral sensitization to Mor depends on the exposure to novelty during the induction phase and absence of novelty during the expression phase, indicating a complex relationship between novelty and Mor-induced behavioral effects.

Dopaminergic innervation and modulation of hippocampal networks

The catecholamine dopamine plays an important role in hippocampus-dependent plasticity and related learning and memory processes. Dopamine secretion in the hippocampus is activated by, e.g., salient or novel stimuli, thereby helping to establish and to stabilize hippocampus-dependent memories. Disturbed dopaminergic function in the hippocampus leads to severe pathophysiological conditions. While the role and importance of dopaminergic modulation of hippocampal networks have been unequivocally proven, there is still a lack of detailed molecular and cellular mechanistic understanding of how dopamine orchestrates these hippocampal processes. In this chapter of the special issue "Hippocampal structure and function," we will discuss the current understanding of dopaminergic modulation of basal synaptic transmission and long-lasting, activity-dependent potentiation or depression.

Core Mechanisms of Cognitive Behavioral Therapy for Anxiety and Depression: A Review

This article reviews the extant literature on mediators of change in cognitive behavioral therapy (CBT) for anxiety and depression. The authors briefly discuss the efficacy of CBT for anxiety and depression and methods of mediation analysis and detection. Then the authors discuss fear extinction in anxiety treatment and cognitive change in depression treatment.

Extinction of Contextual Cocaine Memories Requires Cav1.2 within D1R-Expressing Cells and Recruits Hippocampal Cav1.2-Dependent Signaling Mechanisms

Exposure to cocaine-associated contextual cues contributes significantly to relapse. Extinction of these contextual associations, which involves a new form of learning, reduces cocaine-seeking behavior; however, the molecular mechanisms underlying this process remain largely unknown. We report that extinction, but not acquisition, of cocaine conditioned place preference (CPP) in male mice increased Ca_v1.2 L-type Ca²⁺ channel mRNA and protein in postsynaptic density (PSD) fractions of the hippocampus, a brain region involved in drug-context associations. Moreover, viral-mediated deletion of Ca_v1.2 in the dorsal hippocampus attenuated extinction of cocaine CPP. Molecular studies examining downstream Ca_v1.2 targets revealed that extinction recruited calcium/calmodulin (Ca²⁺/CaMK)-dependent protein kinase II (CaMKII) to the hippocampal PSD. This occurred in parallel with an increase in phosphorylation of the AMPA GluA1 receptor subunit at serine 831 (S831), a CaMKII site, along with an increase in total PSD GluA1. The necessity of S831 GluA1 was further demonstrated by the lack of extinction in S831A GluA1 phosphomutant mice. Of note hippocampal GluA1 levels remained unaltered at the PSD, but were reduced near the PSD and at perisynaptic sites of dendritic spines in extinction-resistant S831A mutant mice. Finally, conditional knock-out of Ca_v1.2 in dopamine D1 receptor (D1R)-expressing cells resulted in attenuation of cocaine CPP extinction and lack of extinction-dependent changes in hippocampal PSD CaMKII expression and S831 GluA1 phosphorylation. In summary, we demonstrate an essential role for the hippocampal Ca_v1.2/CaMKII/S831 GluA1 pathway in cocaine CPP extinction, with data supporting contribution of hippocampal D1R-expressing cells in this process. These findings demonstrate a novel role for Ca_v1.2 channels in extinction of contextual cocaine-associated memories.SIGNIFICANCE STATEMENT Continued drug-seeking behavior, a defining characteristic of cocaine addiction, can be precipitated by contextual cues, yet the molecular mechanisms required for extinction of these context-specific memories remain poorly understood. Here, we have uncovered a novel and selective role of the Ca_v1.2 L-type Ca²⁺ channel and its downstream signaling pathway in the hippocampus that mediate extinction of cocaine conditioned place preference (CPP). We additionally provide evidence that supports a role of Ca_v1.2 within dopamine D1 receptor-expressing cells of the hippocampus for extinction of cocaine CPP. Therefore, these findings reveal a previously unknown role of Ca_v1.2 channels within the hippocampus and in D1 receptor-expressing cells in extinction of cocaine-associated memories, providing a framework for further exploration of mechanisms underlying extinction of cocaine-seeking behavior.

Extinction of avoidance behavior by safety learning depends on endocannabinoid signaling in the hippocampus

The development of exaggerated avoidance behavior is largely responsible for the decreased quality of life in patients suffering from anxiety disorders. Studies using animal models have contributed to the understanding of the neural mechanisms underlying the acquisition of avoidance responses. However, much less is known about its extinction. Here we provide evidence in mice that learning about the safety of an environment (i.e., safety learning) rather than repeated execution of the avoided response in absence of negative consequences (i.e., response extinction) allowed the animals to overcome their avoidance behavior in a step-down avoidance task. This process was context-dependent and could be blocked by pharmacological (3 mg/kg, s.c.; SR141716) or genetic (lack of cannabinoid CB1 receptors in neurons expressing dopamine D1 receptors) inactivation of CB1 receptors. In turn, the endocannabinoid reuptake inhibitor AM404 (3 mg/kg, i.p.) facilitated safety learning in a CB1-dependent manner and attenuated the relapse of avoidance behavior 28 days after conditioning. Safety learning crucially depended on endocannabinoid signaling at level of the hippocampus, since intrahippocampal SR141716 treatment impaired, whereas AM404 facilitated safety learning. Other than AM404, treatment with diazepam (1 mg/kg, i.p.) impaired safety learning. Drug effects on behavior were directly mirrored by drug effects on evoked activity propagation through the hippocampal trisynaptic circuit in brain slices: As revealed by voltage-sensitive dye imaging, diazepam impaired whereas AM404 facilitated activity propagation to CA1 in a CB1-dependent manner. In line with this, systemic AM404 enhanced safety learning-induced expression of Egr1 at level of CA1. Together, our data render it likely that AM404 promotes safety learning by enhancing information flow through the trisynaptic circuit to CA1.

The acquisition, extinction and spontaneous recovery of Pavlovian drug conditioning induced by post-trial dopaminergic stimulation/inhibition

In contextual drug conditioning, the onset of the drug treatment is contiguous with the contextual cues. Evidence suggests that drug conditioning also can occur if there is a discontinuity between the onset of the drug effect and offset of the contextual cues. Here we examine whether post-trial contextual drug conditioning conforms to several Pavlovian conditioning tenets namely: acquisition, extinction and spontaneous recovery. Six groups of rats received apomorphine (0.05 or 2.0mg/kg) and vehicle immediately or after a 15min delay following a 5min non-drug exposure to an open-field during three successive days (conditioning phase). The extinction phase occurred on days 4-8, in which all post-trial treatments were vehicle injections. After 2days of non-testing, the final test was performed. The results showed that on the first test day, the activity levels of the 6 groups were statistically equivalent. On test day 2, there were marked differences in activity levels selectively between the two immediate post-trial apomorphine treatment groups. The immediate low dose apomorphine group displayed a reduction in activity and the immediate high dose group an increase in activity relative to their day 1 levels. The activity levels of both vehicle groups and both apomorphine delay groups remained equivalent to their day 1 activity levels. On test day 3, the differences in activity levels between the two immediate post-trial apomorphine groups increased but the activity levels of the vehicle groups and the 15min delay post-trial apomorphine groups remained unchanged. In the extinction phase, the conditioned activity differences between the two immediate post-trial apomorphine groups were gradually eliminated. During the final test, the activity differences between the immediate post-trial apomorphine groups were partially restored, indicative of spontaneous recovery. These findings are consistent with several basic elements of Pavlovian conditioning and are supportive of drug induced trace conditioning.

Reduced Ventral Tegmental Area-Hippocampal Connectivity in Children and Adolescents Exposed to Early Threat

BACKGROUND

Preclinical data suggest that early life stress has detrimental effects on the brain's dopaminergic system, particularly the mesocorticolimbic pathway. Altered dopamine function is thought to contribute to the development of stress-related pathologies; yet, little is known about the impact of early stress on dopamine systems during childhood and adolescence, when stress-related disorders frequently emerge. Here, we evaluate the impact of early threat exposure (violence, abuse) on functional connectivity of putative dopaminergic midbrain regions, the ventral tegmental area (VTA) and substantia nigra (SN), giving rise to mesocorticolimbic and nigrostriatal pathways, respectively.

METHODS

Resting-state functional magnetic resonance imaging scans were completed in 43 trauma-exposed and 43 matched comparison youth (ages 7-17). Functional connectivity of the VTA and SN were compared between groups.

RESULTS

The trauma group demonstrated lower functional connectivity between the VTA and hippocampus. No group differences in SN connectivity were observed. Across all participants, there were age-related decreases in connectivity of both VTA and SN with the hippocampus, suggesting that age-related attenuations in VTA-hippocampal circuitry may be exacerbated in trauma-exposed youth. Higher levels of anxiety symptomology were associated with reduced SN-nucleus accumbens connectivity.

CONCLUSIONS

Prior research suggests that VTA-hippocampal circuitry is critical for the gating of new information into long-term memory. Lower connectivity in this circuitry suggests a novel mechanism that may serve to adaptively prevent the overwriting of a previously stored trauma memory, but at the same time contribute to the broad range of cognitive and emotional difficulties linked to early stress exposure.

COMT Val158Met polymorphism moderates the association between PTSD symptom severity and hippocampal volume

BACKGROUND

Memory-based alterations are among the hallmark symptoms of posttraumatic stress disorder (PTSD) and may be associated with the integrity of the hippocampus. However, neuroimaging studies of hippocampal volume in individuals with PTSD have yielded inconsistent results, raising the possibility that various moderators, such as genetic factors, may influence this association. We examined whether the catechol-O-methyltransferase (COMT) Val158Met polymorphism, which has previously been shown to be associated with hippocampal volume in healthy individuals, moderates the association between PTSD and hippocampal volume.

METHODS

Recent war veterans underwent structural MRI on a 3 T scanner. We extracted volumes of the right and left hippocampus using FreeSurfer and adjusted them for individual differences in intracranial volume. We assessed PTSD severity using the Clinician-Administered PTSD Scale. Hierarchical linear regression was used to model the genotype (Val158Met polymorphism) × PTSD severity interaction and its association with hippocampal volume.

RESULTS

We included 146 white, non-Hispanic recent war veterans (90% male, 53% with diagnosed PTSD) in our analyses. A significant genotype × PTSD symptom severity interaction emerged such that individuals with greater current PTSD symptom severity who were homozygous for the Val allele showed significant reductions in left hippocampal volume.

LIMITATIONS

The direction of proposed effects is unknown, thus precluding definitive assessment of whether differences in hippocampal volume reflect a consequence of PTSD, a pre-existing characteristic, or both.

CONCLUSION

Our findings suggest that the COMT polymorphism moderates the association between PTSD and hippocampal volume. These results highlight the role that the dopaminergic system has in brain structure and suggest a possible mechanism for memory disturbance in individuals with PTSD.