Opposing influence of basolateral amygdala and footshock stimulation on neurons of the central amygdala

Multiple Memory Subsystems: Reconsidering Memory in the Mind and Brain

The multiple-memory-systems framework-that distinct types of memory are supported by distinct brain systems-has guided learning and memory research for decades. However, recent work challenges the one-to-one mapping between brain structures and memory types central to this taxonomy, with key memory-related structures supporting multiple functions across substructures. Here we integrate cross-species findings in the hippocampus, striatum, and amygdala to propose an updated framework of multiple memory subsystems (MMSS). We provide evidence for two organizational principles of the MMSS theory: First, opposing memory representations are colocated in the same brain structures; second, parallel memory representations are supported by distinct structures. We discuss why this burgeoning framework has the potential to provide a useful revision of classic theories of long-term memory, what evidence is needed to further validate the framework, and how this novel perspective on memory organization may guide future research.

Central Neural Circuits Orchestrating Thermogenesis, Sleep-Wakefulness States and General Anesthesia States

Great progress has been made in specifically identifying the central neural circuits (CNCs) of the core body temperature (Tcore), sleep-wakefulness states (SWs), and general anesthesia states (GAs), mainly utilizing optogenetic or chemogenetic manipulations. We summarize the neuronal populations and neural pathways of these three CNCs, which gives evidence for the orchestration within these three CNCs, and the integrative regulation of these three CNCs by different environmental light signals. We also outline some transient receptor potential (TRP) channels that function in the CNCs-Tcore and are modulated by some general anesthetics, which makes TRP channels possible targets for addressing the general-anesthetics-induced-hypothermia (GAIH). We suggest this review will provide new orientations for further consummating these CNCs and elucidating the central mechanisms of GAIH.

Bacopa monnieri alleviates aluminium chloride-induced anxiety by regulating plasma corticosterone level in Wistar rats

Objectives Aluminium is present in food preparations, antacids and many medications. It causes neurodegeneration thereby resulting in a spectrum of neurological disorders such as dementia, Alzheimer's disease and anxiety. Bacopa monnieri (BM) is widely used in ayurvedic medicine to improve memory functions. Its anxiolytic property was investigated in this study by using elevated plus maze (EPM) and plasma corticosterone level. Methods Thirty rats were assigned into five groups. Control group received distilled water, and 0.5% tween 80, AlCl3 group received Aluminium Chloride (AlCl3), Protective groups (BM100 + AlCl3 group and BM200 + AlCl3 group) received AlCl3 and BM at two different doses, and the BM200 group received BM. The EPM experiment was performed at the end of the 4th week of oral administration of BM and AlCl3 followed by the measurement of plasma corticosterone. Results Oral administration of AlCl3 to rats increases the levels of anxiety as seen in a decrease in the percentage of entries into the open arms of EPM, an increase in grooming frequency and defecation index. However, the rats in the protective groups shown an increase in the percentage of open arm entries and rearing frequency, and decreased grooming frequency and defecation index. AlCl3 alone treated group showed a significant increase in the plasma corticosterone levels compared to the control group. Whereas the protective groups have shown a significant decrease in the plasma corticosterone levels than the AlCl3 alone treated group. Conclusions Hence the BM has potential role in reverting the anxiogenic effect of AlCl3 in the amygdala as it is evident from the plasma corticosterone levels and the EPM parameters of different groups under study.

Effects of Optogenetic inhibition of BLA on Sleep Brief Optogenetic Inhibition of the Basolateral Amygdala in Mice Alters Effects of Stressful Experiences on Rapid Eye Movement Sleep

Study Objectives

Stressful events can directly produce significant alterations in subsequent sleep, in particular rapid eye movement sleep (REM); however, the neural mechanisms underlying the process are not fully known. Here, we investigated the role of the basolateral nuclei of the amygdala (BLA) in regulating the effects of stressful experience on sleep.

Methods

We used optogenetics to briefly inhibit glutamatergic cells in BLA during the presentation of inescapable footshock (IS) and assessed effects on sleep, the acute stress response, and fear memory. c-Fos expression was also assessed in the amygdala and the medial prefrontal cortex (mPFC), both regions involved in coping with stress, and in brain stem regions implicated in the regulation of REM.

Results

Compared to control mice, peri-shock inhibition of BLA attenuated an immediate reduction in REM after IS and produced a significant overall increase in REM. Moreover, upon exposure to the shock context alone, mice receiving peri-shock inhibition of BLA during training showed increased REM without altered freezing (an index of fear memory) or stress-induced hyperthermia (an index of acute stress response). Inhibition of BLA during REM under freely sleeping conditions enhanced REM only when body temperature was high, suggesting the effect was influenced by stress. Peri-shock inhibition of BLA also led to elevated c-Fos expression in the central nucleus of the amygdala and mPFC and differentially altered c-Fos activity in the selected brain stem regions.

Conclusions

Glutamatergic cells in BLA can modulate the effects of stress on REM and can mediate effects of fear memory on sleep that can be independent of behavioral fear.

Lifestyle Modulators of Neuroplasticity: How Physical Activity, Mental Engagement, and Diet Promote Cognitive Health during Aging

The number of the elderly across the globe will approximate 2.1 billion by 2050. Juxtaposed against this burgeoning segment of the population is evidence that nonpathological aging is associated with an increased risk for cognitive decline in a variety of domains, changes that can cause mild disability even before the onset of dementia. Given that pharmacological treatments that mitigate dementia are still outstanding, alternative therapeutic options are being investigated increasingly. The results from translational studies have shown that modifiable lifestyle factors-including physical activity, cognitive engagement, and diet-are a key strategy for maintaining brain health during aging. Indeed, a multiplicity of studies has demonstrated relationships between lifestyle factors, brain structure and function, and cognitive function in aging adults. For example, physical activity and diet modulate common neuroplasticity substrates (neurotrophic signaling, neurogenesis, inflammation, stress response, and antioxidant defense) in the brain whereas cognitive engagement enhances brain and cognitive reserve. The aims of this review are to evaluate the relationship between modifiable lifestyle factors, neuroplasticity, and optimal brain health during aging; to identify putative mechanisms that contribute positive brain aging; and to highlight future directions for scientists and clinicians. Undoubtedly, the translation of cutting-edge knowledge derived from the field of cognitive neuroscience will advance our understanding and enhance clinical treatment interventions as we endeavor to promote brain health during aging.

Dysregulation of the dopamine system in the pathophysiology of schizophrenia and depression

The dopamine system is unique among the brain's modulatory systems in that it has discrete projections to specific brain regions involved in motor behaviour, cognition and emotion. Dopamine neurons exhibit several activity patterns - including tonic and phasic firing - that are determined by a combination of endogenous pacemaker conductances and regulation by multiple afferent systems. Emerging evidence suggests that disruptions in these regulatory systems may underlie the pathophysiology of several psychiatric disorders, including schizophrenia and depression.

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.

Acute tianeptine treatment selectively modulates neuronal activation in the central nucleus of the amygdala and attenuates fear extinction

Antidepressant drugs are commonly prescribed treatments for anxiety disorders, and there is growing interest in understanding how these drugs impact fear extinction because extinction learning is pivotal to successful exposure-based therapy (EBT). A key objective within this domain is understanding how antidepressants alter the activation of specific elements of the limbic-based network that governs such fear processing. Chronic treatment with the antidepressant tianeptine has been shown to reduce the acquisition of extinction learning in rats, yet the drug's acute influence on activation in prefrontal and amygdalar regions, and on extinction learning are not well understood. To assess its influence on cellular activation, rats were injected with tianeptine and Fos immunoreactivity was measured in these regions. Acute tianeptine treatment selectively altered Fos expression within subdivisions of the central nucleus of the amygdala (CEA) in a bidirectional manner that varied in relation to ongoing activation within the capsular subdivision and its prefrontal and intra-amygdalar inputs. This pattern of results suggests that the drug can conditionally modulate the activation of CEA subdivisions, which contain microcircuits strongly implicated in fear processing. The effect of acute tianeptine was also examined with respect to the acquisition, consolidation and expression of fear extinction in rats. Acute tianeptine attenuated extinction learning as well as the recall of extinction memory, which underscores that acute dosing with the drug could alter learning during EBT. Together these findings provide a new perspective for understanding the mechanism supporting tianeptine's clinical efficacy, as well as its potential influence on CEA-based learning mechanisms.

Central amygdalar nucleus treated with orexin neuropeptides evoke differing feeding and grooming responses in the hamster

Interaction of the orexinergic (ORXergic) neuronal system with the excitatory (glutamate, l-Glu) or the inhibitory (GABA) neurosignaling complexes evokes major homeostatic physiological events. In this study, effects of the two ORXergic neuropeptides (ORX-A/B) on their receptor (ORX-2R) expression changes were correlated to feeding and grooming actions of the hibernating hamster (Mesocricetus auratus). Infusion of the central amygdala nucleus (CeA) with ORX-A caused hamsters to consume notable quantities of food, while ORX-B accounted for a moderate increase. Interestingly the latter neuropeptide was responsible for greater frequencies of grooming with respect to both controls and the hamsters treated with ORX-A. These distinct behavioral changes turned out to be even greater in the presence of l-Glu agonist (NMDA) while the α1 GABAA receptor agonist (zolpidem, Zol) greatly reduced ORX-A-dependent feeding bouts. Moreover, ORX-A+NMDA mainly promoted greater ORX-2R expression levels with respect to ORX-A-treated hamsters while ORX-B+Zol was instead largely responsible for a down-regulatory trend. Overall, these features point to CeA ORX-2R sites as key sensory limbic elements capable of regulating eating and grooming responses, which may provide useful insights regarding the type of molecular mechanism(s) operating during feeding bouts.

Stress, arousal, and sleep

Stress is considered to be an important cause of disrupted sleep and insomnia. However, controlled and experimental studies in rodents indicate that effects of stress on sleep-wake regulation are complex and may strongly depend on the nature of the stressor. While most stressors are associated with at least a brief period of arousal and wakefulness, the subsequent amount and architecture of recovery sleep can vary dramatically across conditions even though classical markers of acute stress such as corticosterone are virtually the same. Sleep after stress appears to be highly influenced by situational variables including whether the stressor was controllable and/or predictable, whether the individual had the possibility to learn and adapt, and by the relative resilience and vulnerability of the individual experiencing stress. There are multiple brain regions and neurochemical systems linking stress and sleep, and the specific balance and interactions between these systems may ultimately determine the alterations in sleep-wake architecture. Factors that appear to play an important role in stress-induced wakefulness and sleep changes include various monominergic neurotransmitters, hypocretins, corticotropin releasing factor, and prolactin. In addition to the brain regions directly involved in stress responses such as the hypothalamus, the locus coeruleus, and the amygdala, differential effects of stressor controllability on behavior and sleep may be mediated by the medial prefrontal cortex. These various brain regions interact and influence each other and in turn affect the activity of sleep-wake controlling centers in the brain. Also, these regions likely play significant roles in memory processes and participate in the way stressful memories may affect arousal and sleep. Finally, stress-induced changes in sleep-architecture may affect sleep-related neuronal plasticity processes and thereby contribute to cognitive dysfunction and psychiatric disorders.

The basolateral amygdala determines the effects of fear memory on sleep in an animal model of PTSD

Fear conditioning [inescapable shock training (ST)] and fearful context re-exposure (CR) alone can produce significant fear indicated by increased freezing and reductions in subsequent rapid eye movement (REM) sleep. Damage to or inactivation of the basolateral nucleus of the amygdala (BLA) prior to or after ST or prior to CR generally has been found to attenuate freezing in the shock training context. However, no one has examined the impact of BLA inactivation on fear-induced changes in sleep. Here, we used the GABAA agonist, muscimol (MUS), to inactivate BLA prior to ST, the period when fear is learned, and assessed sleep after ST and sleep and freezing after two CR sessions. Wistar rats (n = 14) were implanted with electrodes for recording sleep and with cannulae aimed bilaterally into BLA. After recovery, the animals were habituated to the injection procedure (handling) over 2 consecutive days and baseline sleep following handling was recorded. On experimental day 1, the rats were injected (0.5 μl) into BLA with either MUS (1.0 μM; n = 7) or vehicle (distilled water, n = 7) 30 min prior to ST (20 footshocks, 0.8 mA, 0.5-s duration, 60-s interstimulus interval). On experimental days 7 and 21, the animals experienced CR (CR1 and CR2, respectively) alone. Electroencephalogram and electromyogram were recorded for 8 h on each day, and the recording was scored for non-rapid eye movement sleep, REM sleep, and wakefulness. Freezing was examined during CR1 and CR2. MUS microinjections into BLA prior to ST blocked the post-training reduction in REM sleep seen in vehicle-treated rats. Furthermore, in MUS-treated rats, REM sleep after CR1 and CR2 was at baseline levels and freezing was significantly attenuated. Thus, BLA inactivation prior to ST blocks the effects of footshock stress on sleep and reduces fear memory, as indicated by the lack of freezing and changes in sleep after CR. These data indicate that BLA is an important regulator of stress-induced alterations in sleep and an important site for forming fear memories that can alter sleep.

Effects of repeated stress on excitatory drive of basal amygdala neurons in vivo

Chronic stress leads to heightened affective behaviors, and can precipitate the emergence of depression and anxiety. These disorders are associated with increased amygdala activity. In animal models, chronic stress leads to increased amygdala-dependent behaviors, as well as hyperactivity of amygdala neurons. However, it is not known whether increased excitatory synaptic drive after chronic stress contributes to hyperactivity of basolateral amygdala (BLA; comprised of basal, lateral, and accessory basal nuclei) neurons. This study tested whether repeated stress causes an increase in excitatory drive of basal amygdala (BA) neurons in vivo, and whether this is correlated with an increase in the number of dendritic spines and a shift in dendritic distribution. Using in vivo intracellular recordings, this study found that repeated restraint stress caused an increase in the frequency of spontaneous excitatory synaptic events in vivo, which correlated with the number of dendritic spines in reconstructed neurons. Furthermore, parallel changes in the kinetics of the synaptic events and the distribution of spines indicated a more prominent functional contribution of synaptic inputs from across the dendritic tree. The shift in spine distribution across the dendritic tree was further confirmed with the examination of Golgi-stained tissue. This abnormal physiological drive of BA neurons after repeated stress may contribute to heightened affective responses after chronic stress. A reduction in the impact of excitatory drive in the BA may therefore be a potential treatment for the harmful effects of chronic stress in psychiatric disorders.

GABA neurons of the VTA drive conditioned place aversion

Salient but aversive stimuli inhibit the majority of dopamine (DA) neurons in the ventral tegmental area (VTA) and cause conditioned place aversion (CPA). The cellular mechanism underlying DA neuron inhibition has not been investigated and the causal link to behavior remains elusive. Here, we show that GABA neurons of the VTA inhibit DA neurons through neurotransmission at GABA(A) receptors. We also observe that GABA neurons increase their firing in response to a footshock and provide evidence that driving GABA neurons with optogenetic effectors is sufficient to affect behavior. Taken together, our data demonstrate that synaptic inhibition of DA neurons drives place aversion.

Hippocampus, amygdala, and stress: interacting systems that affect susceptibility to addiction

Stress is one of the major factors in drug abuse, particularly in relapse and drug-seeking behavior. However, the mechanisms underlying the interactions between stress and drug abuse are unclear. For many years, studies have focused on the role of the dopaminergic reward system in drug abuse. Our results, for example, show that increased dopaminergic activity is induced by drug sensitization and different stressors via potentiation of the ventral subiculum-nucleus accumbens (NAc) pathway. Although the role of the norepinephrine (NE) system in stress is well known, its involvement in drug abuse has received less attention. This review explores the different mechanisms by which stressors can modulate the ventral subiculum-accumbens pathway, and how these modulations can induce alterations in the behavioral response to drug administration. In particular, we will focus on two main afferents to the NAc, the basolateral amygdala and the ventral subiculum of the hippocampus, and their interactions with the locus coeruleus-norepinephrine system.

Dopamine receptor mechanisms mediate corticotropin-releasing factor-induced long-term potentiation in the rat amygdala following cocaine withdrawal

Corticotropin-releasing factor (CRF) in the amygdala is involved in stress responses. Moreover, dopaminergic neurotransmission in the brain reward system including the amygdala plays a significant role in the pathology of cocaine addiction. The present study analysed CRF-induced synaptic plasticity, its pharmacological sensitivity and interactions with the dopamine (DA) system in the basolateral to lateral capsula central amygdala (lcCeA) pathway after a 2-week withdrawal from repeated cocaine administration. A physiologically relevant CRF concentration (25 nm) induced long-term potentiation (LTP) that was enhanced after cocaine withdrawal. In saline-treated rats, CRF-induced LTP was mediated through N-methyl-d-aspartate (NMDA) receptors, L-type voltage-gated calcium channels (L-VGCCs) and CRF(1) receptors. However, in cocaine-withdrawn animals, activation of CRF(1) and CRF(2) receptors was found to enhance LTP. This enhanced CRF-induced LTP after cocaine withdrawal was mediated through endogenous activation of both D1- and D2-like receptors. Furthermore, expression of the D1 receptor (D1R) but not the D2R, D3R, D4R or D5R was significantly increased after cocaine withdrawal. CRF(1) but not CRF(2) protein expression was increased, suggesting that elevated levels of these proteins contributed to the enhancement of CRF-induced LTP during cocaine withdrawal. CRF interactions with the DA system in the amygdala may represent a fundamental neurochemical and cellular mechanism linking stress to cocaine-induced neuronal plasticity.

Amygdala mechanisms of Pavlovian psychostimulant conditioning and relapse

Psychostimulant addiction often consists of periods of sustained drug abstinence disrupted by periods of relapse and renewed heavy drug use. Prevention of relapse remains the greatest challenge to the successful treatment of drug addiction. Drug-associated cues are a primary trigger for relapse, as they can elicit intense craving for the drug. These cues become associated with the drug reward through Pavlovian learning processes that develop over multiple drug-cue pairings. The amygdala (AMY) is critical for such drug-related learning. Intrinsic and extrinsic circuitry position the AMY to integrate cue and drug-related information and influence drug-seeking and drug-taking behaviors. Animal models of conditioned drug reward, drug use, and relapse have confirmed the necessary role of the AMY for drug conditioned cues to control motivated behavior. Neurons within the AMY are responsive to the primary effects of psychostimulants, and more critically, they also respond to the presentation of drug-associated cues. The mechanisms by which conditioned cues come to influence drug-seeking behavior likely involve long-term plasticity and neuroadaptations within the AMY. A greater understanding of the associative learning mechanisms that depend upon the AMY and related limbic and cortical structures, and the process by which drug cues come to gain control over behavior that maintains the addictive state, will facilitate the development of more effective addiction treatments.

Central, but not basolateral, amygdala is critical for control of feeding by aversive learned cues

Environmental factors contribute to the motivation to eat and can override homeostatic signals to stimulate eating in sated states, or inhibit eating in states of hunger. In particular, stress, fear, and anxiety have been linked to suppression of eating and anorexia nervosa. Here, we use a rodent model of an aversive cue-induced cessation of feeding. In this setting, food-deprived rats suppress eating when presented with a tone [conditioned stimulus (CS)] that was previously paired with footshocks [unconditioned stimulus (US)]. To begin to delineate the underlying neural circuitry we examined the two regions of the amygdala with well known roles in associative learning--the central nucleus (CEA) and the basolateral area (BLA; includes the basolateral, basomedial, and lateral nuclei). We produced selective, bilateral, neurotoxic lesions of the CEA or BLA, and then trained these rats together with sham-lesioned controls in a behavioral protocol that allowed a test for food consumption in the presence of an aversive CS. Both sham- and BLA-lesioned rats showed inhibition of eating when presented with the CS. In contrast, bilateral, neurotoxic lesions of the CEA abolished this effect. These results demonstrate that the CEA, but not BLA, is critical for control of feeding by an aversive CS. Previously we demonstrated that enhancement of eating by an appetitive CS is dependent on the integrity of BLA, but not CEA. Those findings together with the current results show a double dissociation between amygdalar subsystems that control food consumption by appetitive and aversive learned cues.

Anxiogenic modulation of spontaneous and evoked neuronal activity in the basolateral amygdala

The amygdala has a well-established role in stress, anxiety, and aversive learning, and anxiolytic and anxiogenic agents are thought to exert their behavioral actions via the amygdala. However, despite extensive behavioral data, the effects of noradrenergic anxiogenic drugs on neuronal activity within the amygdala have not been examined. The present experiments examined how administration of the anxiogenic drug yohimbine affects spontaneous and evoked neuronal activity in the basolateral amygdala (BLA) of rats. Yohimbine produced both excitatory and inhibitory effects on neurons of the BLA, with an increase in spontaneous activity being the predominant response in the lateral and basomedial nuclei of the BLA. Furthermore, yohimbine tended to facilitate neuronal responses evoked by electrical stimulation of the entorhinal cortex, with this facilitation seen more often in lateral and basomedial nuclei of the BLA. These data are the first to examine the effects of the anxiogenic agent yohimbine on BLA neuronal activity, and suggest that neurons in specific subnuclei of the amygdala exhibit unique responses to administration of such pharmacological agents.

Conditioned fear inhibits c-fos mRNA expression in the central extended amygdala

We have shown previously that unconditioned stressors inhibit neurons of the lateral/capsular division of the central nucleus of the amygdala (CEAl/c) and oval division of the bed nucleus of the stria terminalis (BSTov), which form part of the central extended amygdala. The current study investigated whether conditioned fear inhibits c-fos mRNA expression in these regions. Male rats were trained either to associate a visual stimulus (light) with footshock or were exposed to the light alone. After training, animals were replaced in the apparatus, and 2 h later injected remotely, via a catheter, with amphetamine (2 mg/kg i.p.), to induce c-fos mRNA and allow inhibition of expression to be measured. The rats were then presented with 15 visual stimuli over a 30 minute period. As expected, fear conditioned animals that were not injected with amphetamine, had extremely low levels of c-fos mRNA in the central extended amygdala. In contrast, animals that were trained with the light alone (no fear conditioning) and were injected with amphetamine had high levels of c-fos mRNA in the CEAl/c and BSTov. Animals that underwent fear conditioning, and were re-exposed to the conditioned stimulus after amphetamine injection had significantly reduced levels of c-fos mRNA in both the BSTov and CEAl/c, compared to the non-conditioned animals. These data suggest that conditioned fear can inhibit neurons of the central extended amygdala. Because these neurons are GABAergic, and project to the medial CEA (an amygdaloid output region), this may be a novel mechanism whereby conditioned fear potentiates amygdaloid output.

Reactivity and plasticity in the amygdala nuclei during opiate withdrawal conditioning: differential expression of c-fos and arc immediate early genes

Opiate withdrawal leads to the emergence of an aversive state that can be conditioned to a specific environment. Reactivation of these withdrawal memories has been suggested to be involved in relapse to drug-seeking of abstinent opiate addicts. Among the limbic areas that are likely to mediate these features of opiate dependence, amygdala nuclei represent critical neural substrates. Using a conditioned place aversion paradigm (CPA), we have previously shown specific opposite patterns of reactivity in the basolateral (BLA) and the central (CeA) amygdala, when comparing the experience of acute opiate withdrawal with the re-exposure to a withdrawal-paired environment. These data gave clues about the potential mechanisms by which amygdala nuclei may be involved in withdrawal memories. To extend these results, the present study aimed to assess the cellular reactivity and plasticity of amygdala nuclei during the opiate withdrawal conditioning process. For this, we have quantified c-fos and arc expression using in situ hybridization in rats, following each of the three conditioning sessions during CPA, and after re-exposure to the withdrawal-paired environment. BLA output neurons showed an increase in the expression of the plasticity-related arc gene during conditioning that was also increased by re-exposure to the withdrawal-paired environment. Interestingly, the CeA showed an opposite pattern of responding, and the intercalated cell masses (ITC), a possible inhibitory interface between the BLA and CeA, showed a persistent activation of c-fos and arc mRNA. We report here specific c-fos and arc patterns of reactivity in amygdala nuclei during withdrawal conditioning. These findings improve our understanding of the involvement of the amygdala network in the formation and retrieval of withdrawal memories. Plasticity processes within BLA output neurons during conditioning, may participate in increasing the BLA reactivity to conditioned stimuli, which could in turn (by the control of downstream nuclei) reinforce and drive the motivational properties of withdrawal over drug consumption.

Noradrenergic modulation of basolateral amygdala neuronal activity: opposing influences of alpha-2 and beta receptor activation

Substantial data exists demonstrating the importance of the amygdala and the locus ceruleus (LC) in responding to stress, aversive memory formation, and the development of stress-related disorders; however, little is known about the effects of norepinephrine (NE) on amygdala neuronal activity in vivo. The basolateral nucleus of the amygdala (BLA) receives dense NE projections from the LC, NE increases in the BLA in response to stress, and the BLA can also modulate the LC via reciprocal projections. These experiments examined the effects of noradrenergic agents on spontaneous and evoked responses of BLA neurons. NE iontophoresis inhibited spontaneous firing and decreased the responsiveness of BLA neurons to electrical stimulation of entorhinal cortex and sensory association cortex (Te3). Confirmed BLA projection neurons exhibited exclusively inhibitory responses to NE. Systemic administration of propranolol, a beta-receptor antagonist, decreased the spontaneous firing rate and potentiated the NE-evoked inhibition of BLA neurons. In addition, iontophoresis of the alpha-2 agonist clonidine, footshock administration, and LC stimulation mimicked the effects of NE iontophoresis on spontaneous activity. Furthermore, the effects of LC stimulation were partially blocked by systemic administration of alpha 2 and beta receptor antagonists. This is the first study to demonstrate the actions of directly applied and stimulus-evoked NE in the BLA in vivo, and provides a mechanism by which beta receptors can mediate the important behavioral consequences of NE within the BLA. The interaction between these two structures is particularly relevant with regard to their known involvement in stress responses and stress-related disorders.

Amygdala GABA-A receptor involvement in mediating sensory-discriminative and affective-motivational pain responses in a rat model of peripheral nerve injury

The contribution of the amygdala to neuropathic pain processing in animals has not been clearly acknowledged. To assess the relative contribution of amygdala GABA-A receptors in mediating sensory-discriminative and affective-motivational pain components, the GABA-A receptor agonist muscimol and the antagonist bicuculline (both 10-25 ng/microl) were administered by acute bilateral injection directly into the central amygdala in rats with a chronic constriction injury (CCI). Escape/avoidance behaviour reflecting the affective-motivational dimension of pain was measured using a light/dark chamber in combination with suprathreshold nociceptive stimulation, and was defined as a shift from the 'non-aversive' dark area of the chamber to the 'aversive' light area. Hindpaw mechanical allodynia and mechanical hyperalgesia thresholds reflecting the sensory-discriminative dimension of pain were determined prior to and following escape/avoidance testing. Muscimol administration into the amygdala attenuated escape/avoidance behaviour and reversed hindpaw mechanical hypersensitivity in CCI rats; the magnitude of reduction in escape/avoidance behaviour was 2- to 3-fold greater than mechanical allodynia. Surprisingly, administration of bicuculline also attenuated escape/avoidance behaviour but had no effect on nociceptive behaviours. The muscimol-induced reversal of hindpaw mechanical hypersensitivity was completely blocked by co-administration of bicuculline, in contrast to escape/avoidance behaviour. Motility behaviour was unaffected by injection of either drug as determined in the open field test. Thus, amygdala GABA-A receptors appear to play an important role in sensory and especially affective pain processing in neuropathic rats. Furthermore, after nerve injury reflex nociceptive behaviours appear to be under tonic control by descending inputs, which originate from or are modulated within the amygdala.

Increased limbic phosphorylated extracellular-regulated kinase 1 and 2 expression after chronic stress is reduced by cyclic 17β-estradiol administration

Chronic stress induced neuronal changes that may have consequences for subsequent stress responses. For example, chronic stress in rats rearranges dendritic branching patterns and disturbs the phosphorylation of extracellular-regulated kinase 1 and 2 (ERK) 1/2 throughout the limbic system. Stress-induced psychopathology occurs more often in women, however, most of studies have been done in male rats. Therefore, we studied the effect of stress in female rats. Other studies show that estradiol can modulate neuronal plasticity and might protect against stress-induced aberrations. To investigate the role of estradiol in stress responses we manipulated the hormone levels. Ovariectomized rats were cyclically treated with vehicle or with 17beta-estradiol-benzoate (1x in 4 days, 10 microg/250 g, s.c.) and subjected to either acute (3 days) or chronic (22 days) stress. In ovariectomized rats, the number of c-Fos positive cells in the infralimbic and prelimbic cortex of the prefrontal cortex and in the medial and basolateral amygdala was increased after acute stress. Moreover, acute stress reduced the number of phosphorylated ERK1/2 positive neurons in the prefrontal cortex of ovariectomized rats. Chronic stress, on the other hand, abolished normal patterns of c-Fos immunoreactivity in the prefrontal cortex and amygdala and increased the prefrontocortical phosphorylation of ERK1/2 in ovariectomized rats. Cyclic estradiol treatment preserved the neuronal reactivity in the infralimbic cortex after chronic stress and prevented sustained accumulation of phosphorylated ERK1/2. Therefore, cyclic estradiol administration apparently preserves the integrity of signal transduction cascades in limbic structures, which may protect against the harmful consequences of recurrent stress.

Disruption of cortical-limbic interaction as a substrate for comorbidity

The prefrontal cortex exerts a potent regulatory influence over subcortical systems that are involved in the regulation of affective states. In particular, the amygdala is a region that is known to play a prominent role in the expression of emotions, and this function is believed to be disrupted in affective disorders and drug abuse. In addition, dysfunction of the prefrontal cortex is believed to be a common element in many psychiatric disorders such as schizophrenia. Using electrophysiological recordings in rodents, we examined the interactions of the prefrontal cortex with the amygdala. Our studies showed that these areas are strongly interdependent, with the prefrontal cortex showing conditioned responses that depend on amygdala inputs, and in turn exerting a potent attenuation of activity within the amygdala. In particular, the ability of the prefrontal cortex to modulate amygdala activity is likely to play an important role in our ability to cope with stressors. We propose that a dysfunction within the prefrontal cortex disrupts the ability of this region to effectively modulate the amygdala, leaving the organism susceptible to detrimental effects of stressors. This would appear to be a common underlying process that may leave the individual susceptible to drug abuse and to the onset or exacerbation of psychiatric disorders such as schizophrenia and depression.