Norepinephrine release in the amygdala in response to footshock stimulation

Noradrenergic modulation of stress induced catecholamine release: Opposing influence of FG7142 and yohimbine

Life stress modulates decision making, particularly in the face of risk, in some cases prompting vulnerable populations to make suboptimal, life-altering choices. In the brain, stress is known to alter the extracellular release of catecholamines in structures such as basolateral amygdala (BLA) and nucleus accumbens (NAc). To study the role of catecholamines in risky decision-making in rats, we combined a touch screen task, systemic neuropharmacological manipulation, and direct measurement of norepinephrine (NE) and dopamine (DA) release using fiber photometry. Long-Evans rats were trained on an operant touchscreen decision-making task in which they chose between a safe stimulus that delivered a certain 50µl sucrose, or a risky stimulus that delivered either a 'loss' (10µl sucrose 75% of the time) or 'win' (170µl sucrose 25% of the time). Following the pharmacological induction of stress by administration of the inverse GABA A agonist, FG7142, rats were biased in their decisions towards safe choices and the avoidance of loss. This exaggerated loss aversion was blocked by co-treatment with the α2 A receptor antagonist, yohimbine. Direct optical measurement of NE release in the BLA and DA release in the NAc revealed temporal dynamics time-locked to the task events and directly related to the outcome of each trial. In both structures, pharmacological stress altered catecholamine release, with systemic yohimbine showing opposing modulation. These findings highlight the catecholamine basis of loss aversion and neuromodulation of critical brain structures during stress.

Activation of the basolateral or the central amygdala dampened the incentive motivation for food reward on high fixed-ratio schedules

The amygdala plays crucial roles in emotional processing, motivated behaviors, and stress responses. It receives sensory information and modulates fear- and anxiety-related behaviors. Neuronal activations are induced in the basolateral complex of the amygdala (BLA) and the central nucleus of the amygdala (CeA) when exposing to acute stress, leading to increased alertness and proper behavioral adaptation. Previous studies have shown that animals displayed a decrease in appetitive motivated behaviors under stress conditions. However, whether the hyperactive amygdala is responsible for the decrease in appetitive motivated behaviors remains unknown. In this study, we aimed to examine the role of BLA or CeA activation in effort-based motivated behavior. We pharmacologically activated the BLA or the CeA with N-methyl-D-aspartate (NMDA) before the lever-pressing for food reward test on different fixed-ratio (FR) schedules (FR1, FR16, or FR32) in male Long-Evans rats. Our data showed that activation of either the BLA or the CeA with NMDA (0.05 μg in 0.5 μl per site) decreased the lever-pressing behavior on higher FR schedules of FR16 and FR32, but not on the FR1 test. Importantly, locomotor activity and free-feeding food intake were intact under amygdala activation, suggesting that the decrease in lever-pressing behavior was not due to motor disablement or decreased appetite. These results suggested that activation of the BLA or the CeA negatively impaired the effort-based motivated behavior that the animals were less willing to work for food reward.

Local extracellular K+ in cortex regulates norepinephrine levels, network state, and behavioral output

Extracellular potassium concentration ([K+]e) is known to increase as a function of arousal. [K+]e is also a potent modulator of transmitter release. Yet, it is not known whether [K+]e is involved in the neuromodulator release associated with behavioral transitions. We here show that manipulating [K+]e controls the local release of monoaminergic neuromodulators, including norepinephrine (NE), serotonin, and dopamine. Imposing a [K+]e increase is adequate to boost local NE levels, and conversely, lowering [K+]e can attenuate local NE. Electroencephalography analysis and behavioral assays revealed that manipulation of cortical [K+]e was sufficient to alter the sleep-wake cycle and behavior of mice. These observations point to the concept that NE levels in the cortex are not solely determined by subcortical release, but that local [K+]e dynamics have a strong impact on cortical NE. Thus, cortical [K+]e is an underappreciated regulator of behavioral transitions.

Functional neuroanatomy of monoaminergic systems in the basolateral nuclear complex of the amygdala: Neuronal targets, receptors, and circuits

This review discusses neuroanatomical aspects of the three main monoaminergic systems innervating the basolateral nuclear complex (BNC) of the amygdala (serotonergic, noradrenergic, and dopaminergic systems). It mainly focuses on immunohistochemical (IHC) and in situ hybridization (ISH) studies that have analyzed the relationship of specific monoaminergic inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of the monoaminergic modulation of BNC circuitry. First, light and electron microscopic IHC investigations identifying the main BNC neuronal subpopulations and characterizing their local circuitry, including connections with discrete PN compartments and other INs, are reviewed. Then, the relationships of each of the three monoaminergic systems to distinct PN and IN cell types, are examined in detail. For each system, the neuronal targets and their receptor expression are discussed. In addition, pertinent electrophysiological investigations are discussed. The last section of the review compares and contrasts various aspects of each of the three monoaminergic systems. It is concluded that the large number of different receptors, each with a distinct mode of action, expressed by distinct cell types with different connections and functions, should offer innumerable ways to subtlety regulate the activity of the BNC by therapeutic drugs in psychiatric diseases in which there are alterations of BNC monoaminergic modulatory systems, such as in anxiety disorders, depression, and drug addiction. It is suggested that an important area for future studies is to investigate how the three systems interact in concert at the neuronal and neuronal network levels.

Norepinephrine release in the cerebellum contributes to aversive learning

The modulation of dopamine release from midbrain projections to the striatum has long been demonstrated in reward-based learning, but the synaptic basis of aversive learning is far less characterized. The cerebellum receives axonal projections from the locus coeruleus, and norepinephrine release is implicated in states of arousal and stress, but whether aversive learning relies on plastic changes in norepinephrine release in the cerebellum is unknown. Here we report that in mice, norepinephrine is released in the cerebellum following an unpredicted noxious event (a foot-shock) and that this norepinephrine release is potentiated powerfully with fear acquisition as animals learn that a previously neutral stimulus (tone) predicts the aversive event. Importantly, both chemogenetic and optogenetic inhibition of the locus coeruleus-cerebellum pathway block fear memory without impairing motor function. Thus, norepinephrine release in the cerebellum is modulated by experience and underlies aversive learning.

Impact of acute psychosocial stress on attentional control in humans. A study of evoked potentials and pupillary response

Psychosocial stress has increased considerably in our modern lifestyle, affecting global mental health. Deficits in attentional control are cardinal features of stress disorders and pathological anxiety. Studies suggest that changes in the locus coeruleus-norepinephrine system could underlie the effects of stress on top-down attentional control. However, the impact of psychosocial stress on attentional processes and its underlying neural mechanisms are poorly understood. This study aims to investigate the effect of psychosocial stress on attentional processing and brain signatures. Evoked potentials and pupillary activity related to the oddball auditory paradigm were recorded before and after applying the Montreal Imaging Stress Task (MIST). Electrocardiogram (ECG), salivary cortisol, and subjective anxiety/stress levels were measured at different experimental periods. The control group experienced the same physical and cognitive effort but without the psychosocial stress component. The results showed that stressed subjects exhibited decreased P3a and P3b amplitude, pupil phasic response, and correct responses. On the other hand, they displayed an increase in Mismatch Negativity (MMN). N1 amplitude after MIST only decreased in the control group. We found that differences in P3b amplitude between the first and second oddball were significantly correlated with pupillary dilation and salivary cortisol levels. Our results suggest that under social-evaluative threat, basal activity of the coeruleus-norepinephrine system increases, enhancing alertness and decreasing voluntary attentional resources for the cognitive task. These findings contribute to understanding the neurobiological basis of attentional changes in pathologies associated with chronic psychosocial stress.

Enhancement of memory consolidation by an avoidance conditioned stimulus: Modulation by the D3 receptor

Conditioned stimuli (CS) paired with foot-shock can enhance memory consolidation. Because the dopamine D3R has been implicated in mediating various responses to CSs, the current study explored its potential role in modulation of memory consolidation by an avoidance CS. Male Sprague-Dawley rats trained to avoid foot-shocks in a two-way signalled active avoidance task (8 sessions, 30 trials per session, 0.8 mA foot-shock) were pre-treated with the D3R antagonist NGB-2904 (Vehicle, 0.1 or 5 mg/kg) and exposed to the CS immediately after the sample phase of an object recognition memory task. Discrimination ratios were assessed 72 h later. Immediate, but not delayed (6 h), post-sample exposure to the CS enhanced object recognition memory and this effect was dose-dependently blocked by NGB-2904. Control experiments with the beta-noradrenergic receptor antagonist propranolol (10 or 20 mg/kg) and D2R antagonist pimozide (0.2 or 0.6 mg/kg) indicated that NGB-2904 targeted post-training memory consolidation. Exploring the pharmacological selectivity of the D3R effect, it was found that: 1) 5 mg/kg NGB-2904 blocked conditioned memory modulation produced by post-sample exposure to a "weak" CS (one day of avoidance training) and concurrent stimulation of catecholamine activity by 10 mg/kg bupropion; 2) post-sample exposure to a "weak" CS and concurrent administration of the D3R agonist 7-OH-DPAT (1 mg/kg) enhanced consolidation of object memory. Finally, because 5 mg/kg NGB-2904 had no effect on modulation by avoidance training in the presence of foot-shocks, the findings herein support the hypothesis that the D3R plays an important role in modulation of memory consolidation by CSs.

Time-dependent effects of acute stress on working memory performance: A systematic review and hypothesis

Laboratory procedures such as the Trier Social Stress Test or the (Socially Evaluated) Cold Pressor Test have been used to investigate working memory performance under stress. Researchers so far have reported a diverse spectrum of stress effects (including the lack thereof) on working memory tasks. We conducted a systematic review of the effect acute stress on working memory performance in standardized laboratory procedures. An overview of the existing literature suggests that acute stress affects working memory in a time-dependent manner, presumably due to the differing time scales of the main stress-reactive hormones involved. Based on the empirical evidence, we hypothesize that the immediate stress-induced release of noradrenaline decreases working memory performance within the first 10 min post stress. In addition, rapid cortisol effects impair working memory at a later time-interval beginning about 25 min post stress. We outline future research directions which could further explore the implications of our insights, as for example combined pharmacological and naturalistic stressor interventions.

Transforming experiences: Neurobiology of memory updating/editing

Long-term memory is achieved through a consolidation process where structural and molecular changes integrate information into a stable memory. However, environmental conditions constantly change, and organisms must adapt their behavior by updating their memories, providing dynamic flexibility for adaptive responses. Consequently, novel stimulation/experiences can be integrated during memory retrieval; where consolidated memories are updated by a dynamic process after the appearance of a prediction error or by the exposure to new information, generating edited memories. This review will discuss the neurobiological systems involved in memory updating including recognition memory and emotional memories. In this regard, we will review the salient and emotional experiences that promote the gradual shifting from displeasure to pleasure (or vice versa), leading to hedonic or aversive responses, throughout memory updating. Finally, we will discuss evidence regarding memory updating and its potential clinical implication in drug addiction, phobias, and post-traumatic stress disorder.

The Contribution of the Locus Coeruleus-Noradrenaline System Degeneration during the Progression of Alzheimer's Disease

Alzheimer's disease (AD), which is characterized by extracellular accumulation of amyloid-beta peptide and intracellular aggregation of hyperphosphorylated tau, is the most common form of dementia. Memory loss, cognitive decline and disorientation are the ultimate consequences of neuronal death, synapse loss and neuroinflammation in AD. In general, there are many brain regions affected but neuronal loss in the locus coeruleus (LC) is one of the earliest indicators of neurodegeneration in AD. Since the LC is the main source of noradrenaline (NA) in the brain, degeneration of the LC in AD leads to decreased NA levels, causing increased neuroinflammation, enhanced amyloid and tau burden, decreased phagocytosis and impairment in cognition and long-term synaptic plasticity. In this review, we summarized current findings on the locus coeruleus-noradrenaline system and consequences of its dysfunction which is now recognized as an important contributor to AD progression.

Dynamics of Dopamine and Other Monoamines Content in Rat Brain after Single Low-Dose Carbon Nuclei Irradiation

Space radiation, presented primarily by high-charge and -energy particles (HZEs), has a substantial impact on the central nervous system (CNS) of astronauts. This impact, surprisingly, has not only negative but also positive effects on CNS functions. Despite the fact that the mechanisms of this effect have not yet been elucidated, several studies indicate a key role for monoaminergic networks underlying these effects. Here, we investigated the effects of acute irradiation with 450 MeV/n carbon (12C) nuclei at a dose of 0.14 Gy on Wistar rats; a state of anxiety was accessed using a light–dark box, spatial memory in a Morris water maze, and the dynamics of monoamine metabolism in several brain morphological structures using HPLC. No behavioral changes were observed. Irradiation led to the immediate suppression of dopamine turnover in the prefrontal cortex, hypothalamus, and striatum, while a decrease in the level of norepinephrine was detected in the amygdala. However, these effects were transient. The deferred effect of dopamine turnover increase was found in the hippocampus. These data underscore the ability of even low-dose 12C irradiation to affect monoaminergic networks. However, this impact is transient and is not accompanied by behavioral alterations.

RTL1/PEG11 imprinted in human and mouse brain mediates anxiety-like and social behaviors and regulates neuronal excitability in the locus coeruleus

RTL1/PEG11, which has been associated with anxiety disorders, is a retrotransposon-derived imprinted gene in the placenta. However, imprinting patterns and functions of RTL1 in the brain have not been well-investigated. We found Rtl1 was paternally, but not maternally, expressed in brain stem, thalamus, and hypothalamus of mice, and imprinting status of RTL1 was maintained in human brain. Paternal Rtl1 knockout (Rtl1^m+/p-) mice had higher neonatal death rates due to impaired suckling, and low body weights beginning on embryonic day 16.5. High paternal expression of Rtl1 was detected in the locus coeruleus (LC) and Rtl1^m+/p- mice showed an increased delay in time of onset for action potentials and inward currents with decreased neuronal excitability of LC neurons. Importantly, Rtl1^m+/p- mice exhibited behaviors associated with anxiety, depression, fear-related learning and memory, social dominance, and low locomotor activity. Taken together, our findings demonstrate RTL1 is imprinted in brain, mediates emotional and social behaviors, and regulates excitability in LC neurons.

Unrelenting Fear Under Stress: Neural Circuits and Mechanisms for the Immediate Extinction Deficit

Therapeutic interventions for disorders of fear and anxiety rely on behavioral approaches that reduce pathological fear memories. For example, learning that threat-predictive stimuli are no longer associated with aversive outcomes is central to the extinction of conditioned fear responses. Unfortunately, fear memories are durable, long-lasting, and resistant to extinction, particularly under high levels of stress. This is illustrated by the “immediate extinction deficit,” which is characterized by a poor long-term reduction of conditioned fear when extinction procedures are attempted within hours of fear conditioning. Here, I will review recent work that has provided new insight into the neural mechanisms underlying resistance to fear extinction. Emerging studies reveal that locus coeruleus norepinephrine modulates amygdala-prefrontal cortical circuits that are critical for extinction learning. These data suggest that stress-induced activation of brain neuromodulatory systems biases fear memory at the expense of extinction learning. Behavioral and pharmacological strategies to reduce stress in patients undergoing exposure therapy might improve therapeutic outcomes.

Co-released norepinephrine and galanin act on different timescales to promote stress-induced anxiety-like behavior

Both the noradrenergic and galaninergic systems have been implicated in stress-related neuropsychiatric disorders, and these two neuromodulators are co-released from the stress-responsive locus coeruleus (LC); however, the individual contributions of LC-derived norepinephrine (NE) and galanin to behavioral stress responses are unclear. Here we aimed to disentangle the functional roles of co-released NE and galanin in stress-induced behavior. We used foot shock, optogenetics, and behavioral pharmacology in wild-type (WT) mice and mice lacking either NE (Dbh^-/-) or galanin (Gal^cKO-Dbh) specifically in noradrenergic neurons to isolate the roles of these co-transmitters in regulating anxiety-like behavior in the elevated zero maze (EZM) either immediately or 24 h following stress. Foot shock and optogenetic LC stimulation produced immediate anxiety-like behavior in WT mice, and the effects of foot shock persisted for 24 h. NE-deficient mice were resistant to the anxiogenic effects of acute stress and optogenetic LC stimulation, while mice lacking noradrenergic-derived galanin displayed typical increases in anxiety-like behavior. However, when tested 24 h after foot shock, both Dbh^-/- and Gal^cKO-Dbh mice lacked normal expression of anxiety-like behavior. Pharmacological rescue of NE, but not galanin, in knockout mice during EZM testing was anxiogenic. In contrast, restoring galanin, but not NE, signaling during foot shock normalized stress-induced anxiety-like behavior 24 h later. These results indicate that NE and noradrenergic-derived galanin play complementary, but distinguishable roles in behavioral responses to stress. NE is required for the expression of acute stress-induced anxiety, while noradrenergic-derived galanin mediates the development of more persistent responses following a stressor.

Cortical neurochemical signaling of gustatory stimuli and their visceral consequences during the acquisition and consolidation of taste aversion memory

The insular cortex (IC) has a crucial role in taste recognition memory, including conditioned taste aversion (CTA). CTA is a learning paradigm in which a novel taste stimulus (CS) is associated with gastric malaise (US), inducing aversion to the CS in future encounters. The role of the IC in CTA memory formation has been extensively studied. However, the functional significance of neurotransmitter release during the presentation of taste stimuli and gastric malaise-inducing agents remains unclear. Using microdialysis in free-moving animals, we evaluated simultaneous changes in glutamate, norepinephrine and dopamine release in response to the presentation of an innate appetitive or aversive gustatory novel stimulus, as well as after i.p. administration of isotonic or hypertonic gastric malaise-inducing solutions. Our results demonstrate that the presentation of novel stimuli, regardless of their innate valence, induces an elevation of norepinephrine and dopamine. Administration of a gastric malaise inducing agent (LiCl) promotes an elevation of glutamate regardless of its concentration. In comparison, norepinephrine release is related to the LiCl concentration and its equimolar NaCl control. Additionally, we evaluated their functional role on short and long-term taste aversion memory. Results indicate that the blockade of noradrenergic β1,2 receptors in the IC spares CTA acquisition and memory consolidation. In contrast, blockade of dopamine D1/D5 receptors impaired CTA consolidation, whereas the NMDA receptor blockade impedes both acquisition and consolidation of CTA. These results suggest that dopaminergic and noradrenergic release are related to the salience of conditioned taste stimuli. However, only cortical D1/D5 dopaminergic activity, but not the noradrenergic β1,2 activity, is involved in the acquisition and consolidation of taste memory formation. Additionally, glutamatergic activity signals visceral distress caused by LiCl administration and activates NMDA receptors necessary for the acquisition and consolidation of long-lasting taste aversion memory.

Analysis of memory modulation by conditioned stimuli

Conditioned stimuli (CS) have multiple psychological functions that can potentially contribute to their effect on memory formation. It is generally believed that CS-induced memory modulation is primarily due to conditioned emotional responses, however, well-learned CSs not only generate the appropriate behavioral and physiological reactions required to best respond to an upcoming unconditioned stimulus (US), but they also serve as signals that the US is about to occur. Therefore, it is possible that CSs can impact memory consolidation even when their ability to elicit conditioned emotional arousal is significantly reduced. To test this, male Sprague–Dawley rats trained on a signaled active avoidance task were divided into “Avoider” and “Non-Avoider” subgroups on the basis of percentage avoidance after 6 d of training. Subgroup differences in responding to the CS complex were maintained during a test carried out in the absence of the US. Moreover, the subgroups displayed significant differences in stress-induced analgesia (hot-plate test) immediately after this test, suggesting significant subgroup differences in conditioned emotionality. Importantly, using the spontaneous object recognition task, it was found that immediate post-sample exposure to the avoidance CS complex had a similar enhancing effect on object memory in the two subgroups. Therefore, to our knowledge, this is the first study to demonstrate that a significant conditioned emotional response is not necessary for the action of a predictive CS on modulation of memory consolidation.

Corticotropin releasing factor, but not alcohol, modulates norepinephrine release in the rat central nucleus of the amygdala

Alcohol misuse and dependence is a widespread health problem. The central nucleus of the amygdala (CeA) plays important roles in both the anxiety associated with alcohol (ethanol) dependence and the increased alcohol intake that is observed during withdrawal in dependent animals. We and others have shown the essential involvement of the corticotropin releasing factor (CRF) system in alcohol's synaptic effects on the CeA and in the development of ethanol dependence. Another system that has been shown to be critically involved in the molecular underpinnings of alcohol dependence is the norepinephrine (NE) system originating in the locus coeruleus. Both the CRF and NE systems act in concert to facilitate a stress response: central amygdalar afferents release CRF in the locus coeruleus promoting widespread release of NE. In this study, we are the first to use fast-scan cyclic voltammetry to classify local electrically-evoked NE release in the CeA and to determine if acute alcohol and CRF modulate it. Evoked NE release is action potential dependent, is abolished after depletion of monoaminergic vesicles, differs pharmacologically from dopamine release, is insensitive to acute alcohol, and decreases in response to locally applied CRF. Taken together, these results indicate that NE release in the CeA is released canonically in a vesicular-dependent manner, and that while acute alcohol does not directly alter NE release, CRF decreases it. Our results suggest that CRF acts locally on NE terminals as negative feedback and potentially prevents hyperactivation of the CRF-norepinephrine stress pathway.

Opposite effects of noradrenergic and glucocorticoid activation on accuracy of an episodic-like memory

Stressful and emotionally arousing experiences activate hormonal systems that create strong memories. It remains unclear, however, how this strengthening affects the quality of such memories. In the present study, we examined whether the noradrenergic and glucocorticoid hormonal systems affect accuracy of episodic-like memory. We trained male Sprague-Dawley rats on an episodic-like association task, termed inhibitory avoidance discrimination task, in which they explored two different contexts, but shock was given only in the latter context. Forty-eight hours later, retention latencies were tested in the two training contexts as well as in a novel context. The noradrenergic stimulant yohimbine, administered systemically immediately after the training session, enhanced both accuracy and strength of the memory, as shown by long latencies specific to the shock context. By contrast, the glucocorticoid corticosterone induced a generalized strengthening of memory and enhanced latencies in both the shock and non-shock training contexts. Retention latencies in the novel context were not significantly affected. These findings indicate that the noradrenergic and glucocorticoid systems, while both strengthening memory of the shock experience per se, produce opposite effects on accuracy of the shock-context association.

Stress-induced microglial activation occurs through β-adrenergic receptor: noradrenaline as a key neurotransmitter in microglial activation

Background

The involvement of microglia in neuroinflammatory responses has been extensively demonstrated. Recent animal studies have shown that exposure to either acute or chronic stress induces robust microglial activation in the brain. In the present study, we investigated the underlying mechanism of brain microglial activation by acute stress.

Methods

We first looked at the spatial distribution of the noradrenaline (NA)-synthesizing enzyme, DBH (dopamine β-hydroxylase), in comparison with NA receptors—β1, β2, and β3 adrenergic receptors (β1-AR, β2-AR, and β3-AR)—after which we examined the effects of the β-blocker propranolol and α-blockers prazosin and yohimbine on stress-induced microglial activation. Finally, we compared stress-induced microglial activation between wild-type (WT) mice and double-knockout (DKO) mice lacking β1-AR and β2-AR.

Results

The results demonstrated that (1) microglial activation occurred in most studied brain regions, including the hippocampus (HC), thalamus (TM), and hypothalamus (HT); (2) within these three brain regions, the NA-synthesizing enzyme DBH was densely stained in the neuronal fibers; (3) β1-AR and β2-AR, but not β3-AR, are detected in the whole brain, and β1-AR and β2-AR are co-localized with microglial cells, as observed by laser scanning microscopy; (4) β-blocker treatment inhibited microglial activation in terms of morphology and count through the whole brain; α-blockers did not show such effect; (5) unlike WT mice, DKO mice exhibited substantial inhibition of stress-induced microglial activation in the brain.

Conclusions

We demonstrate that neurons/microglia may interact with NA via β1-AR and β2-AR.

Acute prazosin administration does not reduce stressor reactivity in healthy adults

RATIONALE

Norepinephrine plays a critical role in the stress response. Clarifying the psychopharmacological effects of norepinephrine manipulation on stress reactivity in humans has important implications for basic neuroscience and treatment of stress-related psychiatric disorders, such as posttraumatic stress disorder and alcohol use disorders. Preclinical research implicates the norepinephrine alpha-1 receptor in responses to stressors. The No Shock, Predictable Shock, Unpredictable Shock (NPU) task is a human laboratory paradigm that is well positioned to test cross-species neurobiological stress mechanisms and advance experimental therapeutic approaches to clinical trials testing novel treatments for psychiatric disorders.

OBJECTIVES

We hypothesized that acute administration of prazosin, a noradrenergic alpha-1 antagonist, would have a larger effect on reducing stress reactivity during unpredictable, compared to predictable, stressors in the NPU task.

METHODS

We conducted a double-blind, placebo-controlled, crossover randomized controlled trial in which 64 healthy adults (32 female) completed the NPU task at two visits (2 mg prazosin vs. placebo).

RESULTS

A single acute dose of 2 mg prazosin did not reduce stress reactivity in a healthy adult sample. Neither NPU startle potentiation nor self-reported anxiety was reduced by prazosin (vs. placebo) during unpredictable (vs. predictable) stressors.

CONCLUSIONS

Further research is needed to determine whether this failure to translate preclinical neuroscience to human laboratory models is due to methodological factors (e.g., acute vs. chronic drug administration, brain penetration, study population) and/or suggests limited clinical utility of noradrenergic alpha-1 antagonists for treating stress-related psychiatric disorders.

Impact of Chronic BDNF Depletion on GABAergic Synaptic Transmission in the Lateral Amygdala

Brain-derived neurotrophic factor (BDNF) has previously been shown to play an important role in glutamatergic synaptic plasticity in the amygdala, correlating with cued fear learning. While glutamatergic neurotransmission is facilitated by BDNF signaling in the amygdala, its mechanism of action at inhibitory synapses in this nucleus is far less understood. We therefore analyzed the impact of chronic BDNF depletion on GABA_A-mediated synaptic transmission in BDNF heterozygous knockout mice (BDNF^+/−). Analysis of miniature and evoked inhibitory postsynaptic currents (IPSCs) in the lateral amygdala (LA) revealed neither pre- nor postsynaptic differences in BDNF^+/− mice compared to wild-type littermates. In addition, long-term potentiation (LTP) of IPSCs was similar in both genotypes. In contrast, facilitation of spontaneous IPSCs (sIPSCs) by norepinephrine (NE) was significantly reduced in BDNF^+/− mice. These results argue against a generally impaired efficacy and plasticity at GABAergic synapses due to a chronic BDNF deficit. Importantly, the increase in GABAergic tone mediated by NE is reduced in BDNF^+/− mice. As release of NE is elevated during aversive behavioral states in the amygdala, effects of a chronic BDNF deficit on GABAergic inhibition may become evident in response to states of high arousal, leading to amygdala hyper-excitability and impaired amygdala function.

A-Kinase Anchoring Protein 150 and Protein Kinase A Complex in the Basolateral Amygdala Contributes to Depressive-like Behaviors Induced by Chronic Restraint Stress

BACKGROUND

The basolateral amygdala (BLA) has been widely implicated in the pathophysiology of major depressive disorder. A-kinase anchoring protein 150 (AKAP150) directs kinases and phosphatases to synaptic glutamate receptors, controlling synaptic transmission and plasticity. However, the role of the AKAP150 in the BLA in major depressive disorder remains poorly understood.

METHODS

Depressive-like behaviors in C57BL/6J mice were developed by chronic restraint stress (CRS). Mice received either intra-BLA injection of lentivirus-expressing Akap5 short hairpin RNA or Ht-31, a peptide to disrupt the interaction of AKAP150 and protein kinase A (PKA), followed by depressive-like behavioral tests. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid glutamate receptor (AMPAR)-mediated miniature excitatory postsynaptic currents were recorded by whole-cell patch-clamp techniques.

RESULTS

Chronic stress exposure induced depressive-like behaviors, which were accompanied by an increase in total and synaptic AKAP150 expression in the BLA. Accordingly, CRS facilitated the association of AKAP150 with PKA, but not of calcineurin in the BLA. Intra-BLA infusion of lentivirus-expressing Akap5 short hairpin RNA or Ht-31 prevented depressive-like behaviors and normalized phosphorylation of serine 845 and surface expression of AMPAR subunit 1 (GluA1) in the BLA of CRS mice. Finally, blockage of AKAP150-PKA complex signaling rescued the changes in AMPAR-mediated miniature excitatory postsynaptic currents in depressive-like mice.

CONCLUSIONS

These results suggest that AKAP150-PKA directly modulates BLA neuronal synaptic strength, and that AKAP150-PKA-GluA1 streamline signaling complex is responsible for CRS-induced disruption of synaptic AMPAR-mediated transmission and depressive-like behaviors in mice.

Chemogenetic Silencing of the Locus Coeruleus-Basolateral Amygdala Pathway Abolishes Pain-Induced Anxiety and Enhanced Aversive Learning in Rats

BACKGROUND

Pain affects both sensory and emotional aversive responses, often provoking anxiety-related diseases when chronic. However, the neural mechanisms underlying the interactions between anxiety and chronic pain remain unclear.

METHODS

We characterized the sensory, emotional, and cognitive consequences of neuropathic pain (chronic constriction injury) in a rat model. Moreover, we determined the role of the locus coeruleus (LC) neurons that project to the basolateral amygdala (BLA) using a DREADD (designer receptor exclusively activated by designer drugs).

RESULTS

Chronic constriction injury led to sensorial hypersensitivity in both the short term and long term. Otherwise, long-term pain led to an anxiety-like profile (in the elevated zero maze and open field tests), as well as increased responses to learn aversive situations (in the passive avoidance and fear conditioning tests) and an impairment of nonemotional cognitive tasks (in the novel object recognition and object pattern of separation tests). Chemogenetic blockade of the LC-BLA pathway and intra-BLA or systemic antagonism of beta-adrenergic receptors abolished both long-term pain-induced anxiety and enhanced fear learning. By contrast, chemogenetic activation of this pathway induced anxiety-like behaviors and enhanced the aversive learning and memory index in sham animals, although it had little effect on short- and long-term chronic constriction injury animals. Interestingly, modulation of LC-BLA activity did not modify sensorial perception or episodic memory.

CONCLUSIONS

Our results indicate that dimensions associated with pain are processed by independent pathways and that there is an overactivation of the LC-BLA pathway when anxiety and chronic pain are comorbid, which involves the activity of beta-adrenergic receptors.

Vagus nerve stimulation as a tool for enhancing extinction in exposure-based therapies

RATIONALE

Emotionally traumatic experiences can lead to maladaptive memories that are enduring and intrusive. The goal of exposure-based therapies is to extinguish conditioned fears through repeated, unreinforced exposures to reminders of traumatic events. The extinction of conditioned fear depends upon the consolidation of new memories made during exposure to reminders. An impairment in extinction recall, observed in certain patient populations, can interfere with progress in exposure-based therapies, and the drive to avoid thoughts and reminders of the trauma can undermine compliance and increase dropout rate. Effective adjuncts to exposure-based therapies should improve the consolidation and maintenance of the extinction memory or improve the tolerability of the therapy. Under stressful conditions, the vagus nerve responds to elevations in epinephrine and signals the brain to facilitate the storage of new memories while, as part of the parasympathetic nervous system, it slows the sympathetic response.

OBJECTIVE

Here, we review studies relevant to fear extinction, describing the anatomical and functional characteristics of the vagus nerve and mechanisms of vagus nerve stimulation (VNS)-induced memory enhancement and plasticity.

RESULTS

We propose that stimulation of the left cervical vagus nerve during exposure to conditioned cues signals the brain to store new memories just as epinephrine or emotional arousal would do, but bypasses the peripheral sympathetic "fight-or-flight" response.

CONCLUSIONS

In support of this hypothesis, we have found that VNS accelerates extinction and prevents reinstatement of conditioned fear in rats. Finally, we propose future studies targeting the optimization of stimulation parameters and the search for biomarkers of VNS effectiveness that may improve exposure therapy outcomes.

The Impact of Stressor Exposure and Glucocorticoids on Anxiety and Fear

Anxiety disorders and trauma- and stressor-related disorders, such as posttraumatic stress disorder (PTSD), are common and are associated with significant economic and social burdens. Although trauma and stressor exposure are recognized as a risk factors for development of anxiety disorders and trauma or stressor exposure is recognized as essential for diagnosis of PTSD, the mechanisms through which trauma and stressor exposure lead to these disorders are not well characterized. An improved understanding of the mechanisms through which trauma or stressor exposure leads to development and persistence of anxiety disorders or PTSD may result in novel therapeutic approaches for the treatment of these disorders. Here, we review the current state-of-the-art theories, with respect to mechanisms through which stressor exposure leads to acute or chronic exaggeration of avoidance or anxiety-like defensive behavioral responses and fear, endophenotypes in both anxiety disorders and trauma- and stressor-related psychiatric disorders. In this chapter, we will explore physiological responses and neural circuits involved in the development of acute and chronic exaggeration of anxiety-like defensive behavioral responses and fear states, focusing on the role of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoid hormones.

Pheromone-Induced Odor Associative Fear Learning in Rats

Alarm pheromones alert conspecifics to the presence of danger. Can pheromone communication aid in learning specific cues? Such facilitation has an evident evolutionary advantage. We use two associative learning paradigms to test this hypothesis. The first is stressed cage mate-induced conditioning. One pair-housed adult rat received 4 pairings of terpinene + shock over 30 min. Ten minutes after return to the home cage, its companion rat was removed and exposed to terpinene. Single-housed controls were exposed to either terpinene or shock only. Companion rats showed terpinene-specific freezing, which was prevented by β-adrenoceptor blockade. Using Arc to index neuronal activation in response to terpinene re-exposure, stressed cage-mate induced associative learning was measured. Companion rats showed increased neuronal activity in the accessory olfactory bulb, while terpinene + shock-conditioned rats showed increased activity in the main olfactory bulb. Both groups had enhanced activity in the anterior basolateral amygdala and central amygdala. To test involvement of pheromone mediation, in the 2nd paradigm, we paired terpinene with soiled bedding from odor + shock rats or a rat alarm pheromone. Both conditioning increased rats’ freezing to terpinene. Blocking NMDA receptors in the basolateral amygdala prevented odor-specific learning suggesting shock and pheromone-paired pathways converge in the amygdala. An alarm pheromone thus enables cue-specific learning as well as signalling danger.

Differential effects of emotion induced after encoding on item memory and reality-monitoring source memory

Although studies have examined the effect of emotional stimuli on reality-monitoring source memory, it is poorly understood whether the effect observed would remain if emotion is induced after encoding. In addition, although there has been evidence that post-encoding emotion enhances item memory but not external monitoring source memory, it is unclear whether such a null effect extends to other types of source memory. To address these gaps, in the current study, participants encoded a list of words. For half of the words they were asked to think about the corresponding opposite words, and for the remaining half of words they viewed the corresponding opposite words. Following encoding they watched a neutral, positive or negative video. Replicating prior studies, both positive and negative emotions enhanced consolidation of item memory. Furthermore, participants at a high level of state anxiety, trait anxiety and depression were more likely to benefit from the enhancement effect of post-encoding emotion. However, no significant effect was observed on reality-monitoring source memory. Taken together the current study suggests that the enhancement effect of post-encoding emotion on item memory does not necessarily extend to reality-monitoring source memory.

Bidirectional influence of amygdala β1-adrenoceptors blockade on cannabinoid signaling in contextual and auditory fear memory

The basolateral amygdala (BLA) is a major target and modulator of stress and has a critical role in the neural circuitry presenting learned fear behaviors. On the other hand, both the endocannabinoid and noradrenergic systems may be involved in regulating the stress responses, fear, and anxiety. Considering the aforementioned, we have investigated the involvement of the BLA β₁-adrenoceptors in conditioned fear responses induced by ACPA, a CB1 receptor (CB1R) agonist. In adult male NMRI mice, freezing responses to context and cue were measured using a Pavlovian fear conditioning apparatus. Pre-training intra-BLA microinjection of xamoterol (0.01 and 0.02 µg/mouse), a partial β₁-adrenoceptor agonist, or atenolol (0.5 µg/mouse), a β₁-adrenoceptor antagonist, decreased freezing behavior, which suggests an impairment of contextual and auditory fear retrieval. Similar results were found with pre-training intraperitoneal administration of ACPA (0.5 mg/kg). A sub-threshold dose of xamoterol, infused into the BLA, decreased ACPA (0.005 and 0.05 mg/kg) effect on both memories, while atenolol increased ACPA response to the context at the middle dose and decreased ACPA response to the tone at the lower dose. It can be concluded that the blockade of BLA β₁-adrenoceptors differentially affects ACPA response on the contextual and auditory conditioned fear memories.

Propranolol produces short-term facilitation of extinction in a rabbit model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a learning-based anxiety disorder with significant public health challenges due to difficulties in treating the complex, multiple symptomology. We have developed an animal model of PTSD, based on Pavlovian eyeblink conditioning in rabbits, that addresses two key features: conditioned responses (CRs) to cues associated with an aversive event and a form of conditioned hyperarousal referred to as conditioning-specific reflex modification (CRM). We have found previously that unpaired extinction is ideal for reducing both CRs and CRM simultaneously and shows sensitivity to systemic serotonergic and glutamatergic manipulations. The following study aimed to extend our work to examine the role of the noradrenergic system, dysregulation of which is strongly implicated as part of the neurobiology of PTSD and which may also play a role in the balance shift from fear reconsolidation to extinction during treatment. The goal of the following two studies was to examine whether the β-adrenergic receptor antagonist propranolol combined with either a full or brief course of unpaired extinction treatment could enhance extinction of CRs and/or CRM. Results showed a within-session facilitation of propranolol on extinction of CRs, particularly during the first extinction session, and a short-term enhancement of extinction of CRM when extinction treatment was brief. However, neither benefit translated to long-term extinction retention for the majority of subjects. Findings suggest that propranolol may provide the most therapeutic benefit in situations of high arousal early in treatment, which may be more important for future patient compliance rather than long-term treatment outcomes.

Presynaptic Regulation of Tonic Inhibition by Neuromodulatory Transmitters in the Basal Amygdala

Tonic inhibition mediated by ambient levels of GABA that activate extrasynaptic GABA_A receptors emerges as an essential factor that tunes neuronal network excitability in vitro and shapes behavioral responses in vivo. To address the role of neuromodulatory transmitter systems on this type of inhibition, we employed patch clamp recordings in mouse amygdala slice preparations. Our results show that the current amplitude of tonic inhibition (I_tonic) in projection neurons of the basal amygdala (BA) is increased by preincubation with the neurosteroid THDOC, while the benzodiazepine diazepam is ineffective. This suggests involvement of THDOC sensitive δ subunit containing GABA_A receptors in mediating tonic inhibition. Moreover, we provide evidence that the neuromodulatory transmitters NE, 5HT, and ACh strongly enhance spontaneous IPSCs as well as I_tonic in the BA. As the increase in frequency, amplitude, and charge of sIPSCs by these neuromodulatory transmitters strongly correlated with the amplitude of I_tonic, we conclude that spill-over of synaptic GABA leads to activation of I_tonic and thereby to dampening of amygdala excitability. Since local injection of THDOC, as a positive modulator of tonic inhibition, into the BA interfered with the expression of contextual fear memory, our results point to a prominent role of I_tonic in fear learning.

Noradrenergic Modulation of Fear Conditioning and Extinction

The locus coeruleus norepinephrine (LC-NE) system plays a broad role in learning and memory. Here we begin with an overview of the LC-NE system. We then consider how both direct and indirect manipulations of the LC-NE system affect cued and contextual aversive learning and memory. We propose that NE dynamically modulates Pavlovian conditioning and extinction, either promoting or impairing learning aversive processes under different levels of behavioral arousal. We suggest that under high levels of stress (e.g., during/soon after fear conditioning) the locus coeruleus (LC) promotes cued fear learning by enhancing amygdala function while simultaneously blunting prefrontal function. Under low levels of arousal, the LC promotes PFC function to promote downstream inhibition of the amygdala and foster the extinction of cued fear. Thus, LC-NE action on the medial prefrontal cortex (mPFC) might be described by an inverted-U function such that it can either enhance or hinder learning depending on arousal states. In addition, LC-NE seems to be particularly important for the acquisition, consolidation and extinction of contextual fear memories. This may be due to dense adrenoceptor expression in the hippocampus (HPC) which encodes contextual information, and the ability of NE to regulate long-term potentiation (LTP). Moreover, recent work reveals that the diversity of LC-NE functions in aversive learning and memory are mediated by functionally heterogeneous populations of LC neurons that are defined by their projection targets. Hence, LC-NE function in learning and memory is determined by projection-specific neuromodulation that accompanies various states of behavioral arousal.

β-Adrenoceptor Blockade in the Basolateral Amygdala, But Not the Medial Prefrontal Cortex, Rescues the Immediate Extinction Deficit

Early psychological interventions, such as exposure therapy, rely on extinction learning to reduce the development of stress- and trauma-related disorders. However, recent research suggests that extinction often fails to reduce fear when administered soon after trauma. This immediate extinction deficit (IED) may be due to stress-induced dysregulation of neural circuits involved in extinction learning. We have shown that systemic β-adrenoceptor blockade with propranolol rescues the IED, but impairs delayed extinction. Here we sought to determine the neural locus of these effects. Rats underwent auditory fear conditioning and then received either immediate (30 min) or delayed (24 h) extinction training. We used bilateral intracranial infusions of propranolol into either the infralimbic division of the medial prefrontal cortex (mPFC) or the basolateral amygdala (BLA) to examine the effects of β-adrenoceptor blockade on immediate and delayed extinction learning. Interestingly, intra-BLA, but not intra-mPFC, propranolol rescued the IED; animals receiving intra-BLA propranolol prior to immediate extinction showed less spontaneous recovery of fear during extinction retrieval. Importantly, this was not due to impaired consolidation of the conditioning memory. In contrast, neither intra-BLA nor intra-mPFC propranolol affected delayed extinction learning. Overall, these data contribute to a growing literature suggesting dissociable roles for key nodes in the fear extinction circuit depending on the timing of extinction relative to conditioning. These data also suggest that heightened noradrenergic activity in the BLA underlies stress-induced extinction deficits. Propranolol may be a useful adjunct to behavioral therapeutic interventions in recently traumatized individuals who are at risk for developing trauma-related disorders.

Characterization of the amplificatory effect of norepinephrine in the acquisition of Pavlovian threat associations

The creation of auditory threat Pavlovian memory requires an initial learning stage in which a neutral conditioned stimulus (CS), such as a tone, is paired with an aversive one (US), such as a shock. In this phase, the CS acquires the capacity of predicting the occurrence of the US and therefore elicits conditioned defense responses. Norepinephrine (NE), through β-adrenergic receptors in the amygdala, enhances threat memory by facilitating the acquisition of the CS–US association, but the nature of this effect has not been described. Here we show that NE release, induced by the footshock of the first conditioning trial, promotes the subsequent enhancement of learning. Consequently, blocking NE transmission disrupts multitrial but not one-trial conditioning. We further found that increasing the time between the conditioning trials eliminates the amplificatory effect of NE. Similarly, an unsignaled footshock delivered in a separate context immediately before conditioning can enhance learning. These results help define the conditions under which NE should and should not be expected to alter threat processing and fill an important gap in the understanding of the neural processes relevant to the pathophysiology of stress and anxiety disorders.

Basolateral amygdala and stress-induced hyperexcitability affect motivated behaviors and addiction

The amygdala integrates and processes incoming information pertinent to reward and to emotions such as fear and anxiety that promote survival by warning of potential danger. Basolateral amygdala (BLA) communicates bi-directionally with brain regions affecting cognition, motivation and stress responses including prefrontal cortex, hippocampus, nucleus accumbens and hindbrain regions that trigger norepinephrine-mediated stress responses. Disruption of intrinsic amygdala and BLA regulatory neurocircuits is often caused by dysfunctional neuroplasticity frequently due to molecular alterations in local GABAergic circuits and principal glutamatergic output neurons. Changes in local regulation of BLA excitability underlie behavioral disturbances characteristic of disorders including post-traumatic stress syndrome (PTSD), autism, attention-deficit hyperactivity disorder (ADHD) and stress-induced relapse to drug use. In this Review, we discuss molecular mechanisms and neural circuits that regulate physiological and stress-induced dysfunction of BLA/amygdala and its principal output neurons. We consider effects of stress on motivated behaviors that depend on BLA; these include drug taking and drug seeking, with emphasis on nicotine-dependent behaviors. Throughout, we take a translational approach by integrating decades of addiction research on animal models and human trials. We show that changes in BLA function identified in animal addiction models illuminate human brain imaging and behavioral studies by more precisely delineating BLA mechanisms. In summary, BLA is required to promote responding for natural reward and respond to second-order drug-conditioned cues; reinstate cue-dependent drug seeking; express stress-enhanced reacquisition of nicotine intake; and drive anxiety and fear. Converging evidence indicates that chronic stress causes BLA principal output neurons to become hyperexcitable.

Locus coeruleus to basolateral amygdala noradrenergic projections promote anxiety-like behavior

Increased tonic activity of locus coeruleus noradrenergic (LC-NE) neurons induces anxiety-like and aversive behavior. While some information is known about the afferent circuitry that endogenously drives this neural activity and behavior, the downstream receptors and anatomical projections that mediate these acute risk aversive behavioral states via the LC-NE system remain unresolved. Here we use a combination of retrograde tracing, fast-scan cyclic voltammetry, electrophysiology, and in vivo optogenetics with localized pharmacology to identify neural substrates downstream of increased tonic LC-NE activity in mice. We demonstrate that photostimulation of LC-NE fibers in the BLA evokes norepinephrine release in the basolateral amygdala (BLA), alters BLA neuronal activity, conditions aversion, and increases anxiety-like behavior. Additionally, we report that β-adrenergic receptors mediate the anxiety-like phenotype of increased NE release in the BLA. These studies begin to illustrate how the complex efferent system of the LC-NE system selectively mediates behavior through distinct receptor and projection-selective mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.18247.001

Localization of the delta opioid receptor and corticotropin-releasing factor in the amygdalar complex: role in anxiety

It is well established that central nervous system norepinephrine (NE) and corticotropin-releasing factor (CRF) systems are important mediators of behavioral responses to stressors. More recent studies have defined a role for delta opioid receptors (DOPR) in maintaining emotional valence including anxiety. The amygdala plays an important role in processing emotional stimuli, and has been implicated in the development of anxiety disorders. Activation of DOPR or inhibition of CRF in the amygdala reduces baseline and stress-induced anxiety-like responses. It is not known whether CRF- and DOPR-containing amygdalar neurons interact or whether they are regulated by NE afferents. Therefore, this study sought to better define interactions between the CRF, DOPR and NE systems in the basolateral (BLA) and central nucleus of the amygdala (CeA) of the male rat using anatomical and functional approaches. Irrespective of the amygdalar subregion, dual immunofluorescence microscopy showed that DOPR was present in CRF-containing neurons. Immunoelectron microscopy confirmed that DOPR was localized to both dendritic processes and axon terminals in the BLA and CeA. Semi-quantitative dual immunoelectron microscopy analysis of gold-silver labeling for DOPR and immunoperoxidase labeling for CRF revealed that 55 % of the CRF neurons analyzed contained DOPR in the BLA while 67 % of the CRF neurons analyzed contained DOPR in the CeA. Furthermore, approximately 41 % of DOPR-labeled axon terminals targeted BLA neurons that expressed CRF while 29 % of DOPR-labeled axon terminals targeted CeA neurons that expressed CRF. Triple label immunofluorescence microscopy revealed that DOPR and CRF were co-localized in common cellular profiles that were in close proximity to NE-containing fibers in both subregions. These anatomical results indicate significant interactions between DOPR and CRF in this critical limbic region and reveal that NE is poised to regulate these peptidergic systems in the amygdala. Functional studies were performed to determine if activation of DOPR could inhibit the anxiety produced by elevation of NE in the amygdala using the pharmacological stressor yohimbine. Administration of the DOPR agonist, SNC80, significantly attenuated elevated anxiogenic behaviors produced by yohimbine as measured in the rat on the elevated zero maze. Taken together, results from this study demonstrate the convergence of three important systems, NE, CRF, and DOPR, in the amygdala and provide insight into their functional role in modulating stress and anxiety responses.

Chemogenetic and Optogenetic Activation of Gαs Signaling in the Basolateral Amygdala Induces Acute and Social Anxiety-Like States

Anxiety disorders are debilitating psychiatric illnesses with detrimental effects on human health. These heightened states of arousal are often in the absence of obvious threatening cues and are difficult to treat owing to a lack of understanding of the neural circuitry and cellular machinery mediating these conditions. Activation of noradrenergic circuitry in the basolateral amygdala is thought to have a role in stress, fear, and anxiety, and the specific cell and receptor types responsible is an active area of investigation. Here we take advantage of two novel cellular approaches to dissect the contributions of G-protein signaling in acute and social anxiety-like states. We used a chemogenetic approach utilizing the Gαs DREADD (rM3Ds) receptor and show that selective activation of generic Gαs signaling is sufficient to induce acute and social anxiety-like behavioral states in mice. Second, we use a recently characterized chimeric receptor composed of rhodopsin and the β2-adrenergic receptor (Opto-β2AR) with in vivo optogenetic techniques to selectively activate Gαs β-adrenergic signaling exclusively within excitatory neurons of the basolateral amygdala. We found that optogenetic induction of β-adrenergic signaling in the basolateral amygdala is sufficient to induce acute and social anxiety-like behavior. These findings support the conclusion that activation of Gαs signaling in the basolateral amygdala has a role in anxiety. These data also suggest that acute and social anxiety-like states may be mediated through signaling pathways identical to β-adrenergic receptors, thus providing support that inhibition of this system may be an effective anxiolytic therapy.

Adolescent-onset of cocaine use is associated with heightened stress-induced reinstatement of cocaine seeking

Adolescent rats take cocaine more readily than adults, are more sensitive to lower doses of the drug and work harder for it. It remains unknown if adolescent-onset of cocaine use has long-term consequences on adult relapse liability. Therefore, we tested if self-administering cocaine during adolescence impacts subsequent stress-induced reinstatement to cocaine seeking and taking, after a prolonged drug-free period. Adolescent (~P42) or adult (P88) rats self-administered cocaine (0.6 or 1.2 mg/kg/infusion) for 7 or 10 days. Then, they underwent a prolonged drug-free period (21-40 days), after which they were tested for reinstatement of cocaine-seeking (i.e. responding in the absence of cocaine) induced by the stress hormone corticosterone, the pharmacological stressor yohimbine or electric footshock. Studies employed either single extinction session (within-session extinction/reinstatement) or repeated extinction prior to reinstatement (between-session extinction/reinstatement). Finally, in a separate set of experiments, rats underwent a prolonged drug-free period (~40 days) and were then allowed to self-administer cocaine again, using progressive-ratio procedures that appraise the reinforcing efficacy of cocaine. Rats with adolescent-onset of cocaine use showed greater stress-induced reinstatement of cocaine seeking than rats with adult-onset of cocaine use. This was observed across conditions, providing external validity to these results. Groups did not differ on drug taking in progressive-ratio tests. Our studies indicate that experiencing cocaine during adolescence renders subjects particularly responsive to the subsequent effects of stress on drug seeking. This heightened propensity for reinstatement puts adolescent-onset drug users at heightened risk for relapse.

Revisiting propranolol and PTSD: Memory erasure or extinction enhancement?

Posttraumatic stress disorder (PTSD) has been described as the only neuropsychiatric disorder with a known cause, yet effective behavioral and pharmacotherapies remain elusive for many afflicted individuals. PTSD is characterized by heightened noradrenergic signaling, as well as a resistance to extinction learning. Research aimed at promoting more effective treatment of PTSD has focused on memory erasure (disrupting reconsolidation) and/or enhancing extinction retention through pharmacological manipulations. Propranolol, a β-adrenoceptor antagonist, has received considerable attention for its therapeutic potential in PTSD, although its impact on patients is not always effective. In this review, we briefly examine the consequences of β-noradrenergic manipulations on both reconsolidation and extinction learning in rodents and in humans. We suggest that propranolol is effective as a fear-reducing agent when paired with behavioral therapy soon after trauma when psychological stress is high, possibly preventing or dampening the later development of PTSD. In individuals who have already suffered from PTSD for a significant period of time, propranolol may be less effective at disrupting reconsolidation of strong fear memories. Also, when PTSD has already developed, chronic treatment with propranolol may be more effective than acute intervention, given that individuals with PTSD tend to experience long-term, elevated noradrenergic hyperarousal.

Light and electron microscopic analysis of enkephalin-like immunoreactivity in the basolateral amygdala, including evidence for convergence of enkephalin-containing axon terminals and norepinephrine transporter-containing axon terminals onto common targets

Modulatory interactions of opioids and norepinephrine (NE) in the anterior subdivision of the basolateral nucleus of the amygdala (BLa) are critical for the consolidation of memories of emotionally arousing experiences. Although there have been several studies of the noradrenergic system in the amygdalar basolateral nuclear complex (BLC), little is known about the chemical neuroanatomy of opioid systems in this region. To address this knowledge gap the present study first examined the distribution of met-enkephalin-like immunoreactivity (ENK-ir) in the BLC at the light microscopic level, and then utilized dual-labeling immunocytochemistry combined with electron microscopy to investigate the extent of convergence of NE and ENK terminals onto common structures in the BLa. Antibodies to ENK and the norepinephrine transporter (NET) were used in these studies. Light microscopic examination revealed that a subpopulation of small nonpyramidal neurons expressed ENK-ir in all nuclei of the BLC. In addition, the somata of some pyramidal cells exhibited light to moderate ENK-ir. ENK+ axon terminals were also observed. Ultrastructural analysis confined to the BLa revealed that most ENK+ axon terminals formed asymmetrical synapses that mainly contacted spines and shafts of thin dendrites. ENK+ terminals forming symmetrical synapses mainly contacted dendritic shafts. Approximately 20% of NET+ terminals contacted a structure that was also contacted by an ENK+ terminal and 6% of NET+ terminals contacted an ENK+ terminal. These findings suggest that ENK and NE terminals in the BLa may interact by targeting common dendrites and by direct interactions between the two types of terminals.

Regulation of dopamine system responsivity and its adaptive and pathological response to stress

Although, historically, the norepinephrine system has attracted the majority of attention in the study of the stress response, the dopamine system has also been consistently implicated. It has long been established that stress plays a crucial role in the pathogenesis of psychiatric disorders. However, the neurobiological mechanisms that mediate the stress response and its effect in psychiatric diseases are not well understood. The dopamine system can play distinct roles in stress and psychiatric disorders. It is hypothesized that, even though the dopamine (DA) system forms the basis for a number of psychiatric disorders, the pathology is likely to originate in the afferent structures that are inducing dysregulation of the DA system. This review explores the current knowledge of afferent modulation of the stress/DA circuitry, and presents recent data focusing on the effect of stress on the DA system and its relevance to psychiatric disorders.

Propranolol decreases retention of fear memory by modulating the stability of surface glutamate receptor GluA1 subunits in the lateral amygdala

BACKGROUND AND PURPOSE

Posttraumatic stress disorder (PTSD) is a mental disorder with enhanced retention of fear memory and has profound impact on quality of life for millions of people worldwide. The β-adrenoceptor antagonist propranolol has been used in preclinical and clinical studies for the treatment of PTSD, but the mechanisms underlying its potential efficacy on fear memory retention remain to be elucidated.

EXPERIMENTAL APPROACH

We investigated the action of propranolol on the retention of conditioned fear memory, the surface expression of glutamate receptor GluA1 subunits of AMPA receptors and synaptic adaptation in the lateral amygdala (LA) of rats.

KEY RESULTS

Propranolol attenuated reactivation-induced strengthening of fear retention while reducing enhanced surface expression of GluA1 subunits and restoring the impaired long-term depression in LA. These effects of propranolol were mediated by antagonizing reactivation-induced enhancement of adrenergic signalling, which activates PKA and calcium/calmodulin-dependent protein kinase II and then regulates the trafficking of AMPA receptors via phosphorylation of GluA1 subunits at the C-terminus. Both i.p. injection and intra-amygdala infusion of propranolol attenuated reactivation-induced enhancement of fear retention.

CONCLUSIONS AND IMPLICATIONS

Reactivation strengthens fear retention by increasing the level of noradrenaline and promotes the surface expression of GluA1 subunits and the excitatory synaptic transmission in LA. These findings uncover one mechanism underlying the efficiency of propranolol on retention of fear memories and suggest that β-adrenoceptor antagonists, which act centrally, may be more suitable for the treatment of PTSD.

Projection specificity in heterogeneous locus coeruleus cell populations: implications for learning and memory

Noradrenergic neurons in the locus coeruleus (LC) play a critical role in many functions including learning and memory. This relatively small population of cells sends widespread projections throughout the brain including to a number of regions such as the amygdala which is involved in emotional associative learning and the medial prefrontal cortex which is important for facilitating flexibility when learning rules change. LC noradrenergic cells participate in both of these functions, but it is not clear how this small population of neurons modulates these partially distinct processes. Here we review anatomical, behavioral, and electrophysiological studies to assess how LC noradrenergic neurons regulate these different aspects of learning and memory. Previous work has demonstrated that subpopulations of LC noradrenergic cells innervate specific brain regions suggesting heterogeneity of function in LC neurons. Furthermore, noradrenaline in mPFC and amygdala has distinct effects on emotional learning and cognitive flexibility. Finally, neural recording data show that LC neurons respond during associative learning and when previously learned task contingencies change. Together, these studies suggest a working model in which distinct and potentially opposing subsets of LC neurons modulate particular learning functions through restricted efferent connectivity with amygdala or mPFC. This type of model may provide a general framework for understanding other neuromodulatory systems, which also exhibit cell type heterogeneity and projection specificity.

An Overview of Translationally Informed Treatments for Posttraumatic Stress Disorder: Animal Models of Pavlovian Fear Conditioning to Human Clinical Trials

Posttraumatic stress disorder manifests after exposure to a traumatic event and is characterized by avoidance/numbing, intrusive symptoms and flashbacks, mood and cognitive disruptions, and hyperarousal/reactivity symptoms. These symptoms reflect dysregulation of the fear system likely caused by poor fear inhibition/extinction, increased generalization, and/or enhanced consolidation or acquisition of fear. These phenotypes can be modeled in animal subjects using Pavlovian fear conditioning, allowing investigation of the underlying neurobiology of normative and pathological fear. Preclinical studies reveal a number of neurotransmitter systems and circuits critical for aversive learning and memory that have informed the development of therapies used in human clinical trials. In this review, we discuss the evidence for a number of established and emerging pharmacotherapies and device-based treatments for posttraumatic stress disorder that have been developed via a bench to bedside translational model.

Biochemical Markers of Physical Exercise on Mild Cognitive Impairment and Dementia: Systematic Review and Perspectives

BACKGROUND

The cognitive effects of physical exercise in patients with dementia disorders or mild cognitive impairment have been examined in various studies; however the biochemical effects of exercise from intervention studies are largely unknown. The objective of this systematic review is to investigate the published results on biomarkers in physical exercise intervention studies in patients with MCI or dementia.

METHODS

The PubMed database was searched for studies from 1976 to February 2015. We included intervention studies investigating the effect of physical exercise activity on biomarkers in patients with MCI or dementia.

RESULTS

A total of eight studies were identified (n = 447 patients) evaluating exercise regimes with variable duration (single session-three sessions/week for 26 weeks) and intensity (light-resistance training-high-intensity aerobic exercise). Various biomarkers were measured before and after intervention. Seven of the eight studies found a significant effect on their selected biomarkers with a positive effect of exercise on brain-derived neurotrophic factor, cholesterol, testosterone, estradiol, dehydroepiadrosterone, and insulin in the intervention groups compared with controls.

CONCLUSION

Although few studies suggest a beneficial effect on selected biomarkers, we need more knowledge of the biochemical effect of physical exercise in dementia or MCI.

Hypofunction of prefrontal cortex NMDA receptors does not change stress-induced release of dopamine and noradrenaline in amygdala but disrupts aversive memory

RATIONALE

A dysfunction of prefrontal cortex has been associated with the exacerbated response to stress observed in schizophrenic patients and high-risk individuals to develop psychosis. The hypofunction of NMDA glutamatergic receptors induced by NMDA antagonists produces cortico-limbic hyperactivity, and this is used as an experimental model to resemble behavioural abnormalities observed in schizophrenia.

OBJECTIVES

The aim of the present study was to investigate whether injections of NMDA antagonists into the medial prefrontal cortex of the rat change (1) the increases of dopamine, noradrenaline and corticosterone concentrations produced by acute stress in amygdala, and (2) the acquisition of aversive memory related to a stressful event.

METHODS

Male Wistar rats were implanted with guide cannulae to perform microdialysis and bilateral microinjections (0.5 μl/side) of the NMDA antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phophonic acid (CPP) (25 and 100 ng). Prefrontal injections were performed 60 min before restraint stress in microdialysis experiments, or training (footshock; 0.6 mA, 2 s) in inhibitory avoidance test. Retention latency was evaluated 24 h after training as an index of aversive memory.

RESULTS

Acute stress increased amygdala dialysate concentrations of dopamine (160% of baseline), noradrenaline (145% of baseline) and corticosterone (170% of baseline). Prefrontal injections of CPP did not change the increases of dopamine, noradrenaline or corticosterone produced by stress. In contrast, CPP significantly reduced the retention latency in the inhibitory avoidance test.

CONCLUSIONS

These results suggest that the hypofunction of prefrontal NMDA receptors does not change the sensitivity to acute stress of dopamine and noradrenaline projections to amygdala but impairs the acquisition of aversive memory.