Effects of single or repeated immobilization on release of norepinephrine and its metabolites in the central nucleus of the amygdala in conscious rats

The Catecholaldehyde Hypothesis for the Pathogenesis of Catecholaminergic Neurodegeneration: What We Know and What We Do Not Know

3,4-Dihydroxyphenylacetaldehyde (DOPAL) is the focus of the catecholaldehyde hypothesis for the pathogenesis of Parkinson’s disease and other Lewy body diseases. The catecholaldehyde is produced via oxidative deamination catalyzed by monoamine oxidase (MAO) acting on cytoplasmic dopamine. DOPAL is autotoxic, in that it can harm the same cells in which it is produced. Normally, DOPAL is detoxified by aldehyde dehydrogenase (ALDH)-mediated conversion to 3,4-dihydroxyphenylacetic acid (DOPAC), which rapidly exits the neurons. Genetic, environmental, or drug-induced manipulations of ALDH that build up DOPAL promote catecholaminergic neurodegeneration. A concept derived from the catecholaldehyde hypothesis imputes deleterious interactions between DOPAL and the protein alpha-synuclein (αS), a major component of Lewy bodies. DOPAL potently oligomerizes αS, and αS oligomers impede vesicular and mitochondrial functions, shifting the fate of cytoplasmic dopamine toward the MAO-catalyzed formation of DOPAL—destabilizing vicious cycles. Direct and indirect effects of DOPAL and of DOPAL-induced misfolded proteins could “freeze” intraneuronal reactions, plasticity of which is required for neuronal homeostasis. The extent to which DOPAL toxicity is mediated by interactions with αS, and vice versa, is poorly understood. Because of numerous secondary effects such as augmented spontaneous oxidation of dopamine by MAO inhibition, there has been insufficient testing of the catecholaldehyde hypothesis in animal models. The clinical pathophysiological significance of genetics, emotional stress, environmental agents, and interactions with numerous proteins relevant to the catecholaldehyde hypothesis are matters for future research. The imposing complexity of intraneuronal catecholamine metabolism seems to require a computational modeling approach to elucidate clinical pathogenetic mechanisms and devise pathophysiology-based, individualized treatments.

A balancing act: the role of pro- and anti-stress peptides within the central amygdala in anxiety and alcohol use disorders

The central nucleus of the amygdala (CeA) is widely implicated as a structure that integrates both appetitive and aversive stimuli. While intrinsic CeA microcircuits primarily consist of GABAergic neurons that regulate amygdala output, a notable feature of the CeA is the heterogeneity of neuropeptides and neuropeptide/neuromodulator receptors that it expresses. There is growing interest in the role of the CeA in mediating psychopathologies, including stress and anxiety states and their interactions with alcohol use disorders. Within the CeA, neuropeptides and neuromodulators often exert pro- or anti- stress actions, which can influence anxiety and alcohol associated behaviours. In turn, alcohol use can cause adaptions within the CeA, which may render an individual more vulnerable to stress which is a major trigger of relapse to alcohol seeking. This review examines the neurocircuitry, neurochemical phenotypes and how pro- and anti-stress peptide systems act within the CeA to regulate anxiety and alcohol seeking, focusing on preclinical observations from animal models. Furthermore, literature exploring the targeting of genetically defined populations or neuronal ensembles and the role of the CeA in mediating sex differences in stress x alcohol interactions are explored.

How does homeostasis happen? Integrative physiological, systems biological, and evolutionary perspectives

Homeostasis is a founding principle of integrative physiology. In current systems biology, however, homeostasis seems almost invisible. Is homeostasis a key goal driving body processes, or is it an emergent mechanistic fact? In this perspective piece, I propose that the integrative physiological and systems biological viewpoints about homeostasis reflect different epistemologies, different philosophies of knowledge. Integrative physiology is concept driven. It attempts to explain biological phenomena by continuous formation of theories that experimentation or observation can test. In integrative physiology, "function" refers to goals or purposes. Systems biology is data driven. It explains biological phenomena in terms of "omics"-i.e., genomics, gene expression, epigenomics, proteomics, and metabolomics-it depicts the data in computer models of complex cascades or networks, and it makes predictions from the models. In systems biology, "function" refers more to mechanisms than to goals. The integrative physiologist emphasizes homeostasis of internal variables such as Pco₂ and blood pressure. The systems biologist views these emphases as teleological and unparsimonious in that the "regulated variable" (e.g., arterial Pco₂ and blood pressure) and the "regulator" (e.g., the "carbistat" and "barostat") are unobservable constructs. The integrative physiologist views systems biological explanations as not really explanations but descriptions that cannot account for phenomena we humans believe exist, although they cannot be observed directly, such as feelings and, ultimately, the conscious mind. This essay reviews the history of the two epistemologies, emphasizing autonomic neuroscience. I predict rapprochement of integrative physiology with systems biology. The resolution will avoid teleological purposiveness, transcend pure mechanism, and incorporate adaptiveness in evolution, i.e., "Darwinian medicine."

Biobehavioral Interactions Between Stress and Alcohol

In this review, the effects of stress on alcohol drinking are discussed. The interactions between biological stress systems and alcohol drinking are examined, with a focus on the hypothalamic pituitary adrenal axis, corticotropin releasing factor, dynorphin, neuropeptide Y, and norepinephrine systems. Findings from animal models suggest that these biological stress systems may be useful targets for medications development for alcohol use disorder and co-occurring stress-related disorders in humans.

Homeostatic systems, biocybernetics, and autonomic neuroscience

In this review we describe a series of major concepts introduced during the past 150years that have contributed to our current understanding about how physiological processes required for well-being and survival are regulated. One can theorize that hierarchical networks involving input-output relationships continuously orchestrate and learn adaptive patterns of observable behaviors, cognition, memory, mood, and autonomic systems. Taken together, these networks function as "good regulators" determining levels of internal variables and act as if there were homeostatic comparators ("homeostats"). The consequences of models with vs. without homeostats remain the same in terms of allostatic load and the eventual switch from stabilizing negative feedback loops to destabilizing, pathogenic positive feedback loops. Understanding this switch seems important for comprehending senescence-related, neurodegenerative disorders that involve the autonomic nervous system. Our general proposal is that disintegration of homeostatic systems causes disorders of regulation in degenerative diseases and that medical cybernetics can inspire and rationalize new approaches to treatment and prevention.

α2-adrenoceptor-mediated inhibition in the central amygdala blocks fear-conditioning

The central amygdala is critical for the acquisition and expression of fear memories. This region receives a dense innervation from brainstem noradrenergic cell groups and has a high level of α₂-adrenoceptor expression. Using whole-cell electrophysiological recordings from rat brain slices, we characterise the role of pre-synaptic α₂-adrenoceptor in modulating discrete inhibitory and excitatory connections within both the lateral and medial division of the central amygdala. The selective α₂-adrenoceptor agonist clonidine blocked the excitatory input from the pontine parabrachial neurons onto neurons of the lateral central amygdala. In addition, clonidine blocked inhibitory connections from the medial paracapsular intercalated cell mass onto both lateral and medial central amygdala neurons. To examine the behavioural consequence of α₂-adrenoceptor-mediated inhibition of these inputs, we infused clonidine into the central amygdala prior to contextual fear-conditioning. In contrast to vehicle-infused rats, clonidine-infused animals displayed reduced levels of freezing 24 hours after training, despite showing no difference in freezing during the training session. These results reveal a role for α₂-adrenoceptors within the central amygdala in the modulation of synaptic transmission and the formation of fear-memories. In addition, they provide further evidence for a role of the central amygdala in fear-memory formation.

Linking Stress, Catecholamine Autotoxicity, and Allostatic Load with Neurodegenerative Diseases: A Focused Review in Memory of Richard Kvetnansky

In this Focused Review, we provide an update about evolving concepts that may link chronic stress and catecholamine autotoxicity with neurodegenerative diseases such as Parkinson's disease. Richard Kvetnansky's contributions to the field of stress and catecholamine systems inspired some of the ideas presented here. We propose that coordination of catecholaminergic systems mediates adjustments maintaining health and that senescence-related disintegration of these systems leads to disorders of regulation and to neurodegenerative diseases such as Parkinson's disease. Chronically repeated episodes of stress-related catecholamine release and reuptake, with attendant increases in formation of the toxic dopamine metabolite 3,4-dihydroxyphenylacetaldehyde, might accelerate this process.

Enduring attenuation of norepinephrine synaptic availability and augmentation of the pharmacological and behavioral effects of desipramine by repeated immobilization stress

Here we provide evidence that repeated immobilization stress (RIS) in rats induces a persistent increase in noradrenergic activity in the anterior aspects of the anterolateral bed nucleus of the stria terminalis (alBNST). This increase in noradrenergic activity results from both enhanced synthesis and reuptake of norepinephrine (NE). It leads to a decrease in the synaptic availability of NE, which elicits an augmented noradrenergic response to the inhibitors of NE reuptake (NRIs), such as desipramine (DMI), an antidepressant. The enduring depression-like behavior and the augmentation of the climbing behavior seen in repeatedly stressed rats following subchronic administration of DMI in the forced swimming test (FST) might be explained by a dysregulation of noradrenergic transmission observed in alBNST. Taken together, we propose that dysregulation of noradrenergic transmission such as the one described in the present work may represent a mechanism underlying major depressive disorders (MDD) with melancholic features in humans.

Reboxetine Improves Auditory Attention and Increases Norepinephrine Levels in the Auditory Cortex of Chronically Stressed Rats

Chronic stress impairs auditory attention in rats and monoamines regulate neurotransmission in the primary auditory cortex (A1), a brain area that modulates auditory attention. In this context, we hypothesized that norepinephrine (NE) levels in A1 correlate with the auditory attention performance of chronically stressed rats. The first objective of this research was to evaluate whether chronic stress affects monoamines levels in A1. Male Sprague-Dawley rats were subjected to chronic stress (restraint stress) and monoamines levels were measured by high performance liquid chromatographer (HPLC)-electrochemical detection. Chronically stressed rats had lower levels of NE in A1 than did controls, while chronic stress did not affect serotonin (5-HT) and dopamine (DA) levels. The second aim was to determine the effects of reboxetine (a selective inhibitor of NE reuptake) on auditory attention and NE levels in A1. Rats were trained to discriminate between two tones of different frequencies in a two-alternative choice task (2-ACT), a behavioral paradigm to study auditory attention in rats. Trained animals that reached a performance of ≥80% correct trials in the 2-ACT were randomly assigned to control and stress experimental groups. To analyze the effects of chronic stress on the auditory task, trained rats of both groups were subjected to 50 2-ACT trials 1 day before and 1 day after of the chronic stress period. A difference score (DS) was determined by subtracting the number of correct trials after the chronic stress protocol from those before. An unexpected result was that vehicle-treated control rats and vehicle-treated chronically stressed rats had similar performances in the attentional task, suggesting that repeated injections with vehicle were stressful for control animals and deteriorated their auditory attention. In this regard, both auditory attention and NE levels in A1 were higher in chronically stressed rats treated with reboxetine than in vehicle-treated animals. These results indicate that NE has a key role in A1 and attention of stressed rats during tone discrimination.

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.

Direct targeting of peptidergic amygdalar neurons by noradrenergic afferents: linking stress-integrative circuitry

Amygdalar norepinephrine (NE) plays a key role in regulating neural responses to emotionally arousing stimuli and is involved in memory consolidation of emotionally charged events. Corticotropin-releasing factor (CRF) and dynorphin (DYN), two neuropeptides that mediate the physiological and behavioral responses to stress, are abundant in the central nucleus of the amygdala (CeA), and directly innervate brainstem noradrenergic locus coeruleus (LC) neurons. Whether the CRF- and DYN-containing amygdalar neurons receive direct noradrenergic innervation has not yet been elucidated. The present study sought to define cellular substrates underlying noradrenergic modulation of CRF- and DYN-containing neurons in the CeA using immunohistochemistry and electron microscopy. Ultrastructural analysis revealed that NE-labeled axon terminals form synapses with CRF- and DYN-containing neurons in the CeA. Semi-quantitative analysis showed that approximately 31 % of NET-labeled axon terminals targeted CeA neurons that co-expressed DYN and CRF. As a major source of CRF innervation to the LC, it is also not known whether CRF-containing CeA neurons are directly targeted by noradrenergic afferents. To test this, retrograde tract tracing using FluoroGold from the LC was combined with immunocytochemical detection of CRF and NET in the CeA. Our results revealed a population of LC-projecting CRF-containing CeA neurons that are directly innervated by NE afferents. Analysis showed that approximately 34 % of NET-labeled axon terminals targeted LC-projecting CeA neurons that contain CRF. Taken together, these results indicate significant interactions between NE, CRF and DYN in this critical limbic region and reveal direct synaptic interactions of NE with amygdalar CRF that influence the LC-NE arousal system.

Corticotropin releasing factor and catecholamines enhance glutamatergic neurotransmission in the lateral subdivision of the central amygdala

Glutamatergic neurotransmission in the central nucleus of the amygdala (CeA) plays an important role in many behaviors including anxiety, memory consolidation and cardiovascular responses. While these behaviors can be modulated by corticotropin releasing factor (CRF) and catecholamine signaling, the mechanism(s) by which these signals modify CeA glutamatergic neurotransmission remains unclear. Utilizing whole-cell patch-clamp electrophysiology recordings from neurons in the lateral subdivision of the CeA (CeAL), we show that CRF, dopamine (DA) and the β-adrenergic receptor agonist isoproterenol (ISO) all enhance the frequency of spontaneous excitatory postsynaptic currents (sEPSC) without altering sEPSC kinetics, suggesting they increase presynaptic glutamate release. The effect of CRF on sEPSCs was mediated by a combination of CRFR1 and CRFR2 receptors. While previous work from our lab suggests that CRFRs mediate the effect of catecholamines on excitatory transmission in other subregions of the extended amygdala, blockade of CRFRs in the CeAL failed to significantly alter effects of DA and ISO on glutamatergic transmission. These findings suggest that catecholamine and CRF enhancement of glutamatergic transmission onto CeAL neurons occurs via distinct mechanisms. While CRF increased spontaneous glutamate release in the CeAL, CRF caused no significant changes to optogenetically evoked glutamate release in this region. The dissociable effects of CRF on different types of glutamatergic neurotransmission suggest that CRF may specifically regulate spontaneous excitatory transmission.

Eugenol as an anti-stress agent: modulation of hypothalamic-pituitary-adrenal axis and brain monoaminergic systems in a rat model of stress

Stress is the leading psychopathological cause for several mental disorders. Physiological and psychological responses to stress are mediated by the hypothalamic?pituitary?adrenal (HPA), sympathoadrenal system (SAS), and brain monoaminergic systems (BMS). Eugenol is reported to substantially modulate brain functions by regulating voltage-gated cation channels and release of neurotransmitters. This study was designed to evaluate the anti-stress effect of eugenol in the 4-h restraint model using rats. Ulcer index was measured as a parameter of the stress response. HPA axis and the SAS were monitored by estimating plasma corticosterone and norepinephrine (NE), respectively. Analysis of NE, serotonin (5-HT), dopamine, and their metabolites in discrete brain regions was performed to understand the role of BMS in the anti-stress effect of eugenol. Stress exposure increased the ulcer index as well as plasma corticosterone and NE levels. Eugenol pretreatment for 7 days decreased the stress-induced increase in ulcer index and plasma corticosterone but not NE levels, indicating a preferential effect on the HPA axis. Furthermore, eugenol showed a ?U?-shaped dose?response curve in decreasing ulcer index and plasma corticosterone levels. Eugenol also reversed the stress-induced changes in 5-HT levels in all brain regions, whereas NE levels were reversed in all brain regions except hippocampus. These results suggest that eugenol possesses significant anti-stress activity in the 4-h restraint model and the effect is due to modulation of HPA and BMS.

Noradrenergic facilitation of shock-probe defensive burying in lateral septum of rats, and modulation by chronic treatment with desipramine

We have previously shown that acute stress-induced release of norepinephrine (NE) facilitates anxiety-like behavioral responses to stress, such as reduction in open-arm exploration on the elevated-plus maze and in social behavior on the social interaction test. Since these responses represent inhibition of ongoing behavior, it is important to also address whether NE facilitates a response that represents an activation of behavior. Correspondingly, it is unknown how a chronic elevation in tonic steady-state noradrenergic (NA) neurotransmission induced by NE reuptake blockade might alter this acute modulatory function, a regulatory process that may be pertinent to the anxiolytic effects of NE reuptake blockers such as desipramine (DMI). Therefore, in this study, we investigated noradrenergic modulation of the shock-probe defensive burying response in the lateral septum (LS). In experiment 1, shock-probe exposure induced an acute 3-fold increase in NE levels measured in LS of male Sprague-Dawley rats by microdialysis. Shock-probe exposure also induced a modest rise in plasma ACTH, taken as an indicator of perceived stress, that returned to baseline more rapidly in rats that were allowed to bury the probe compared to rats prevented from burying by providing them with minimal bedding, indicating that the active defensive burying behavior is an effective coping strategy that reduces the impact of acute shock probe-induced stress. In experiment 2, blockade of either alpha(1)- or beta-adrenergic receptors in LS by local antagonist microinjection immediately before testing reduced defensive burying and increased immobility. In the next experiment, chronic DMI treatment increased basal extracellular NE levels in LS, and attenuated the acute shock probe-induced increase in NE release in LS relative to baseline. Chronic DMI treatment decreased shock-probe defensive burying behavior in a time-dependent manner, apparent only after 2 weeks or more of drug treatment. Moreover, rats treated chronically with DMI showed no significant rise of plasma ACTH in response to shock-probe exposure. Thus, acute stress-induced release of NE in LS facilitated defensive burying, an active, adaptive behavioral coping response. Chronic treatment with the NE reuptake blocker and antidepressant drug DMI attenuated acute noradrenergic facilitation of the active burying response, and also attenuated the level of perceived stress driving that response. These results suggest that long-term regulation of the acute modulatory function of NE by chronic treatment with reuptake blockers may contribute to the mechanisms by which such drugs exert their anxiolytic effects in the treatment of stress-related psychiatric conditions, including depression and anxiety.

Desipramine prevents the sustained increase in corticotropin-releasing hormone-like immunoreactivity induced by repeated immobilization stress in the rat central extended amygdala

Clinical and experimental studies have shown that the activation of corticotropin-releasing hormone (CRH) and noradrenergic systems mediate stress-induced anxiety. Repeated immobilization stress (RIS) has been shown to induce long-lasting anxiety behavior and changes in noradrenaline turnover. The present work was aimed at studying the effect of RIS on the in situ expression of CRH-LI in the central extended amygdala and paraventricular nucleus of the hypothalamus (PVN). Our results showed that RIS for 15 days induces a significant increase of CRH-LI expression in the central extended amygdala. The increase in CRH-LI expression in the central extended amygdala was sustained even after a 25-day stress-free period. The concomitant administration of desipramine (DMI), a specific noradrenaline uptake inhibitor, fully prevented the RIS-induced increase in CRH expression. RIS also induced an increase of CRH-LI expression in the PVN that was prevented by the concomitant DMI administration. In contrast to the sustained effect observed in the central extended amygdala, the RIS-induced increase of CRH-LI expression in the PVN was nonlasting. DMI administration also prevented the RIS-induced increase of adrenal gland weight. The present findings showing that RIS induces a sustained increase of CRH expression in the central extended amygdala suggest that the repeated activation of CRH neurons and CRH receptors in the central extended amygdala may underlie the long-lasting anxiety behavior induced by RIS. Further studies should address the mechanisms involved in the effect of DMI and its eventual relevance in the therapeutic actions of DMI.

Locus Coeruleus Lesions Decrease Oxytocin and Vasopressin Release Induced by Hemorrhage

The role of the noradrenergic nucleus Locus Coeruleus (LC) on hemorrhage-induced vasopressin (AVP) and oxytocin (OT) secretion was examined. Rats with LC lesion were submitted to three 1-min hemorrhage sessions at 5-min intervals; 15% of the total blood volume was withdrawn in each session. OT and AVP were measured in plasma, paraventricular (PVN) and supraoptic (SON) nuclei and in posterior pituitary (PP). LC Lesion did not affect basal plasma AVP or OT levels, but partly blocked the increase in plasma AVP and OT induced by hemorrhage. Hemorrhage produced decreases in content of AVP and OT in the PVN and SON and increased levels in the PP. These responses were attenuated in the lesioned group, but only in the PVN and PP. Data suggest a stimulatory role of the inputs from LC to PVN neurons on hemorrhage-induced OT and AVP secretion and that, this pathway is critical in the hypo-volemic neuroendocrine reflex.

The effects of acute and chronic restraint stress on activation of ERK in the rostral ventromedial medulla and locus coeruleus

Extracellular signal-regulated kinase (ERK) is a key molecule in numerous cellular and physiological processes in the CNS. Exposure to stressors causes substantial effects on the perception and response to pain. The rostral ventromedial medulla (RVM) and the locus coeruleus (LC) play crucial roles in descending pain modulation system. In the present study, the activation of ERK in the RVM and the LC in rats following acute and chronic restraint stress was examined in order to characterize the mechanisms underlying stress induced analgesic and hyperalgesic responses. Rats were stressed by restraint 6h daily for 3 weeks. The acute and chronic restraint stresses produced analgesic and hyperalgesic reactions, respectively, to thermal stimuli applied to the tail. The phospho-ERK-immunoreactive (p-ERK-IR) neurons were observed in the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis pars alpha (GiA) and LC. In the RVM, the number of p-ERK-IR neurons per section in the 3-week restraint rats (14.3+/-1.2) was significantly higher than that in the control rats (8.9+/-0.7) [P<0.01]. About 75% of p-ERK-IR neurons in the RVM in the 3-week restraint rats were serotonergic neurons. Protein levels of tryptophan hydroxylase were significantly increased in the RVM region in the 3-week restraint rats. On the other hand, the chronic restraint stress significantly decreased p-ERK-IR in the LC [P<0.05]. These findings suggest that chronic restraint stress-induced activation of ERK in the RVM and the suppression in the LC may be involved in the modulation of the pain threshold by chronic stress.

Increases in extracellular levels of 5-HT and dopamine in the basolateral, but not in the central, nucleus of amygdala induced by aversive stimulation of the inferior colliculus

Consistent evidence has shown that dopamine release in the prefrontal cortex is increased by electrical stimulation of the inferior colliculus (IC) as unconditioned stimulus. Recent reports have also demonstrated that inactivation of the basolateral nucleus of the amygdala (BLA) with muscimol enhances the behavioural consequences of the aversive stimulation of the IC and reduces the dopamine release in the prefrontal cortex. Moreover, neurotoxic lesions of the BLA enhance whereas those of the central nucleus of the amygdala (CeA) reduce the aversiveness of the electrical stimulation of the IC. Based on these findings the present study examined the effects of the electrical stimulation of the IC on the extracellular levels of serotonin and dopamine in the BLA and CeA. To this end, rats implanted with a stimulation electrode in the IC also bore a microdialysis probe in the BLA or CeA for determination of the release of dopamine and serotonin. IC electrical stimulation at the freezing and escape thresholds increased the levels of serotonin ( approximately 70%) and dopamine ( approximately 60%) in the BLA related to the basal values. Similarly, the metabolites DOPAC and 5-HIAA increased in a parallel fashion in BLA. No significant changes could be detected in these biogenic amines and metabolites in CeA following IC aversive stimulation. These findings point to a differential role of serotonergic and dopaminergic mechanisms of the BLA and CeA in the setting up of adaptive responses to fear states generated at the inferior colliculus level.

Adrenalectomy decreases corticotropin-releasing hormone gene expression and increases noradrenaline and dopamine extracellular levels in the rat lateral bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) has a high density of corticotropin-releasing hormone (CRH)-containing neurons that are significantly innervated by noradrenergic and dopaminergic nerve terminals. This limbic structure is involved in the extrahypothalamic response to stress. The purpose of the present work is to study whether the absence of glucocorticoids, induced by a long-term adrenalectomy, regulates CRH gene expression and noradrenaline and dopamine extracellular levels in the rat BNST. The results showed that adrenalectomy decreases CRH mRNA in the dorsal lateral BNST but not in the ventral lateral BNST. Adrenalectomy also decreases CRH-like immunoreactivity both in BNST subnuclei and in the central nucleus of the amygdala. In addition, adrenalectomy significantly increases noradrenaline and dopamine extracellular levels in the lateral BNST. The present results suggest that adrenalectomy regulates CRH gene expression and noradrenaline and dopamine extracellular levels in the BNST in an opposite way. Thus, the present study adds novel evidence further supporting that the BNST and the central nucleus of the amygdala form part of an adrenal steroid-sensitive extrahypothalamic circuit that has been involved in fear and anxiety responses and in clinical syndromes such as melancholic depression, posttraumatic stress disorders, and addiction.

Regulation of hypoxia-induced release of corticotropin-releasing factor in the rat hypothalamus by norepinephrine

Corticotropin-releasing factor (CRF) peptide release was activated by hypoxia in the rat hypothalamus. The mechanisms, however, of the hypoxia-induced CRF release remains unclear. In this study, we demonstrated that the norepinephrine (NE) and its receptors in the paraventricular nucleus (PVN) mediated the CRF release in a simulated altitude hypoxia. When rats were exposed to 5 or 7 km altitude of hypoxia for a short or long term: (1) NE levels in the PVN and the CeA, using the HPLC analysis, were intensity and time course dependently increased, but the increase in the PVN were potential than in the CeA. Restraint-induced NE increase was much higher in both the PVN and the CeA, compared with hypoxia-induced response. (2) Hypoxia and restraint significantly enhanced CRF release in the ME and the PVN but not in the CeA, through RIA assay, which result in stimulating corticosterone secretion. (3) Hypoxia-induced CRF release was reversed by an injection of prazosin (i.c.v.), an alpha-1 adrenoceptor antagonist, while administration of yohimbine (i.c.v.), an alpha-2 receptor antagonist, facilitated further CRF release. These data suggested that hypoxia induced NE activation centrally, via alpha-1 and -2 receptors, leading to improving hypothalamic CRF release, which in turn stimulated pituitary and adrenal cortex. Restraint presented much potential action on NE activation than hypoxia.

Brain Circuits Involved in Corticotropin-Releasing Factor-Norepinephrine Interactions during Stress

Corticotropin-releasing factor (CRF)- and norepinephrine (NE)-containing neurons in the brain are activated during stress, and both have been implicated in the behavioral responses. NE neurons in the brain stem can stimulate CRF neurons in the hypothalamic paraventricular nucleus (PVN) to activate the hypothalamic-pituitary-adrenocortical axis and may affect other CRF neurons. CRF-containing neurons in the PVN, the amygdala, and other brain areas project to the area of the locus coeruleus (LC), and CRF injected into the LC alters the electrophysiologic activity of LC-NE neurons. Neurochemical studies have indicated that CRF applied intracerebroventricularly or locally activates the LC-NE system, and microdialysis and chronoamperometric measurements indicate increased NE release in LC-NE terminal fields. However, chronoamperometric studies indicated a significant delay in the increase in NE release, suggesting that the CRF input to LC-NE neurons is indirect. The reciprocal interactions between cerebral NE and CRF systems have been proposed to create a "feed-forward" loop. It has been postulated that a sensitization of such a feed-forward loop may underlie clinical depression. However, in the majority of studies, repeated or chronic stress has been shown to decrease the behavioral and the neurochemical responsivity to acute stressors. Repeated stress also seems to decrease the responsivity of LC neurons to CRF. These results do not provide support for a feed-forward hypothesis. However, a few studies using certain tasks have indicated sensitization, and some other studies have suggested that the effect of CRF may be dose dependent. Further investigations are necessary to establish the validity or otherwise of the feed-forward hypothesis.

Induction of FOS expression by acute immobilization stress is reduced in locus coeruleus and medial amygdala of Wistar–Kyoto rats compared to Sprague–Dawley rats

Activation of the brain noradrenergic system during acute stress is thought to play an important integrative function in coping and stress adaptation by facilitating transmission in many brain regions involved in regulating behavioral and physiologic components of the stress response. Compared with outbred control Sprague-Dawley (SD) rats, inbred Wistar-Kyoto (WKY) rats exhibit an exaggerated hypothalamic-pituitary-adrenal (HPA) response as well as increased susceptibility to certain forms of stress-related pathology. However, we have also shown previously that WKY rats exhibit reduced anxiety-like behavioral reactivity to acute stress, associated with reduced activation of the brain noradrenergic system. Thus, to understand better the possible neurobiological mechanisms underlying dysregulation of the stress response in WKY rats, we investigated potential strain differences in stress-induced neuronal activation in brain regions that are both involved in regulating behavioral and neuroendocrine stress responses, and are related to the noradrenergic system, either as targets of noradrenergic modulation or as sources of afferent innervation of noradrenergic neurons. This was accomplished by visualizing stress-induced expression of Fos immunoreactivity in the paraventricular nucleus of the hypothalamus, lateral bed nucleus of the stria terminalis, central nucleus of the amygdala, and medial nucleus of the amygdala (MeA), as well as the noradrenergic nucleus locus coeruleus (LC). Stress-induced Fos expression was found to be decreased in the LC and MeA of WKY rats compared with similarly stressed SD rats, whereas no strain differences were observed in any of the other brain regions. This suggests that strain-related differences in activation of the MeA may be involved in the abnormal neuroendocrine and behavioral stress responses exhibited by WKY rats. Moreover, as the MeA is both an afferent as well as an efferent target of the brainstem noradrenergic system, reduced MeA activation may either be a source of reduced noradrenergic reactivity seen in WKY rats, or possibly a consequence. Nonetheless, understanding the mechanisms underlying altered stress reactivity in models such as the WKY rat may contribute to a better understanding of stress-related psychopathologies such as depression, post-traumatic stress disorder or other anxiety disorders.

Effects of acute, repeated and chronic variable stress on in vivo tyrosine hydroxylase activity and on alpha(2)-adrenoceptor sensitivity in the rat brain

We assessed the effects of a single tail pinch and two chronic stress regimes, repeated and variable, on in vivo tyrosine hydroxylase activity and on alpha2-adrenoceptor sensitivity in two brain regions. After administering a 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, tyrosine hydroxylase activity, measured as the accumulation of DOPA, and noradrenaline (NA) content were determined by using high-performance liquid chromatography. A single tail pinch for 5 min induced an enhancement of DOPA content in hippocampus (28%) and hypothalamus (67%) which was still present 24 h later. This increase could account for the lack of changes in NA content in both regions after the application of this stressor. However, tyrosine hydroxylase activity was unmodified 24 h after exposure to both repeated (5 min of tail pinch, twice daily, for 14 days) and chronic variable stress (one of 5 different stressors, once daily, for 14 days) although there was an enhancement of NA levels in hippocampus (45 and 54%, respectively) and hypothalamus (24.5 and 36%, respectively). The sensitivity of the alpha2-adrenoceptors which regulate [3H]-NA release in hippocampal and hypothalamic synaptosomes was not modified by the acute or chronic stress protocols assayed. The results show that both paradigms of chronic stress had similar effects on the noradrenergic indices evaluated.

Interactions of norepinephrine and galanin in the central amygdala and lateral bed nucleus of the stria terminalis modulate the behavioral response to acute stress

Many aspects of drug abuse and addiction share neurobiological substrates with the modulatory processes underlying the response and adaptation to acute stress. In particular, the ascending noradrenergic system has been implicated in facilitating the response to stress, and in stress-induced reinstatement of drug seeking behavior. Thus, to better understand the link between stress and addictive behaviors, it would be informative to understand better the modulatory function of the ascending noradrenergic system, and its interaction with other neurotransmitters with which it is closely associated or co-localized, such as the neuropeptide galanin. In this paper, we review a series of studies investigating the functional interactions of norepinephrine and galanin in modulating the behavioral response to acute stress in two components of the extended amygdala, the central nucleus of the amygdala and the lateral bed nucleus of the stria terminalis. We showed that norepinephrine facilitates behavioral reactivity to stress on the elevated plus-maze and social interaction tests. However, when stress-induced activation of the noradrenergic system was enhanced by blocking inhibitory adrenergic autoreceptors, galanin release was recruited in the central amygdala, acting to attenuate the behavioral response to stress. By contrast, stress-induced galanin release in the lateral bed nucleus appeared to be independent of enhanced noradrenergic activation, and unlike the central amygdala, both galanin and norepinephrine facilitated behavioral stress reactivity in the bed nucleus. The different modes of interaction and differential region- and response-specificity of galanin and norepinephrine suggest that a complex neural circuit interconnecting these two regions is involved in the modulatory effects of norepinephrine and galanin on the behavioral response to stress. Such complexity may allow for flexibility and plasticity in stress adaptation, and may also contribute to behavioral changes induced by chronic drug administration. Thus, the interaction of galanin and norepinephrine may be a viable target for the future development of novel therapeutic strategies for treating behavioral disorders related to stress or drug abuse.

Role of the basolateral amygdala in memory consolidation

Memories of emotionally arousing events tend to be more vivid and to persist longer than do memories of neutral or trivial events. Moreover, memories of emotionally influenced information may endure after a single experience. Recent findings strongly suggest that the influence of emotional arousal on memory consolidation is mediated by the release of adrenal stress hormones (epinephrine and glucocorticoids) and neurotransmitters that converge in modulating the noradrenergic system within the amygdala. Considerable evidence also indicates that amygdala activation influences memory by regulating consolidation in other brain regions. The findings suggest further that this memory-modulatory system may be involved in the formation of traumatic memories and posttraumatic stress disorder in human subjects.

Rapid habituation of hippocampal serotonin and norepinephrine release and anxiety-related behaviors, but not plasma corticosterone levels, to repeated footshock stress in rats

Prior stress exposure is known to alter the activation response to a subsequent stressor. In the present study, we examined neurochemical, neuroendocrinological, and behavioral correlates of short-term adaptation to homotypic stressors administered 60 min apart. An initial electric footshock significantly induced extracellular levels of both serotonin (5-HT) and norepinephrine (NE) in the rat hippocampus (650% and 200% above baseline, respectively), as measured by in vivo microdialysis. A rapid habituation in this response was evident in the inability of a second footshock to evoke similar increases. In contrast, the hypothalamic-pituitary-adrenal (HPA) response was augmented further after the second shock session: plasma corticosterone (CORT) levels were 18.1, 316.5, and 441.6 mg/ml in nonstressed, one-footshock-, or two-footshock-treated rats, respectively. In a social interaction paradigm, rats subjected to a single footshock showed several fear- and anxiety-related behaviors such as increases in freezing and decreases in rearing and active approach for social interaction. Exposure to a second footshock completely blocked the freezing response and restored rearing behavior without affecting the disruption in social interactions. Taken together, these data raise the possibility that neurochemical and neuroendocrine adaptations to short-term homotypic stressors differentially contribute to expression of different fear and anxiety-like responses in the rat.

Modulatory effects of norepinephrine, acting on alpha 1 receptors in the central nucleus of the amygdala, on behavioral and neuroendocrine responses to acute immobilization stress

The central nucleus of the amygdala (CeA) is a component of the limbic fear-anxiety circuit, and has also been implicated in regulation of the hypothalamic-pituitary-adrenal (HPA) stress axis. The CeA receives dense noradrenergic innervation, and is rich in expression of alpha(1)-adrenergic receptors. We hypothesized that norepinephrine (NE), acting on alpha(1) receptors in CeA, may modulate stress-induced anxiety-like behavioral responses and HPA activation. To investigate the role of alpha(1) adrenergic receptors in CeA on stress-induced behavioral reactivity, the alpha(1) antagonist benoxathian was microinjected bilaterally into CeA of male Sprague-Dawley rats, and anxiety-like behavioral responses to acute immobilization stress were measured on the Social Interaction (SI) test and on the Elevated Plus-maze (EPMZ). Benoxathian dose dependently blocked the reduction in SI time induced by immobilization stress, whereas beta-receptor antagonists had no effect, consistent with an absence of beta-receptors in CeA. By contrast, in separate experiments, benoxathian had no effect on stress-induced reduction in open-arm exploratory behavior on the EPMZ, nor on stress-induced plasma ACTH secretion. These results confirm that the SI test and EPMZ measure different aspects of behavioral stress reactivity that can be modulated independently, and likewise, that noradrenergic modulation of behavioral stress reactivity can occur independently of modulation of the HPA axis.

Blockade of stress-induced but not cocaine-induced reinstatement by infusion of noradrenergic antagonists into the bed nucleus of the stria terminalis or the central nucleus of the amygdala

Experiments in our laboratory have shown that central noradrenergic (NA) activation plays a major role in stress-induced reinstatement of drug seeking in rats. In the present experiments, we investigated the effects of blockade of beta-NA adrenoceptors in the bed nucleus of the stria terminalis (BNST) and in the region of the central nucleus of the amygdala (CeA) on footshock- and cocaine-induced reinstatement. Rats were trained to self-administer cocaine (0.5 mg/kg, i.v.) for 9 d and, after a 5-7 d drug-free period, were given extinction sessions followed by a test for footshock stress-induced (15 min of intermittent footshock, 0.8 mA) or cocaine-induced (20 mg/kg, i.p.) reinstatement. Before the test, different groups of rats were given bilateral infusions of one of four doses of a mixture of the beta(1)- and beta(2)-receptor antagonists betaxolol and ICI-118,551 (vehicle, 0.25, 0.5, and 1 nmol of each compound in 0.5 microliter) into either the BNST or CeA. We observed a dose-dependent reduction of stress-induced reinstatement after infusions into the BNST and a complete blockade of stress-induced reinstatement after infusions into the CeA at all doses tested. The same treatments did not block cocaine-induced reinstatement when given at either site. These data suggest that stress-induced NA activation in the BNST and in the region of the CeA is critical to relapse to drug seeking induced by stress but not to relapse induced by priming injections of cocaine, and we hypothesize that NA activity leads to activation of corticotropin-releasing factor neurons in these regions.

Behavioral reactivity to stress: amplification of stress-induced noradrenergic activation elicits a galanin-mediated anxiolytic effect in central amygdala

Brain norepinephrine (NE) modulates many aspects of the stress response. The interaction between NE and neuropeptides such as galanin, with which it is closely associated and which may be released from noradrenergic terminals under conditions of high activity, has not been well studied. We therefore investigated the modulatory effects of galanin in the central nucleus of the amygdala (CeA) on behavioral responsivity to stress when activation of the noradrenergic system was amplified using the adrenergic autoreceptor antagonist yohimbine (2.5 mg/kg ip). Either immobilization stress or yohimbine alone had anxiogenic effects on rat behavior in the elevated plus maze. However, yohimbine pretreatment before stress produced a paradoxical anxiolytic response, which we hypothesized was attributable to galanin release in CeA. Microdialysis verified that yohimbine amplified NE release in CeA during immobilization stress, and also showed that whereas there was no detectable change in galanin release in CeA during stress alone, there was an increase during immobilization stress after yohimbine pretreatment. Bilateral administration of the galanin antagonist M40 into CeA before stress blocked the anxiolytic influence of yohimbine pretreatment. Exogenous galanin mimicked the anxiolytic effect of yohimbine pretreatment, and this too was blocked by M40. These results suggest that amplifying the noradrenergic response to stress can recruit galanin release in CeA, which buffers the anxiety-like behavioral response to acute stress. The balance between noradrenergic and peptidergic neurotransmission may be modified by prior stress, drug treatment or genetic variability, and may represent a novel target for treatment of stress-related neuropsychiatric disorders.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Effects of adrenergic and serotonergic agonists in the amygdala on the hypothalamo-pituitary-adrenocortical axis

The effect of direct administration of adrenergic and serotonergic (5-HT) agonists into the central nucleus of the amygdala (AMG) on the hypothalamo-pituitary-adrenal (HPA) axis have been studied in intact male rats and in animals with 6-hydroxydopamine (6-OHDA) or 5, 7-dihydroxytryptamine (5,7-DHT) neurotoxic lesions in the paraventricular nucleus of the hypothalamus (PVN). In intact animals, the administration of phenylephrine, an alpha1 adrenergic agonist or 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) a 5-HT(1A) agonist caused depletion of median eminence corticotropin releasing hormone and a rise in serum adrenocorticotrophic hormone (ACTH) and corticosterone (CS) levels. Isoproterenol a beta agonist was more effective than phenylephrine and a 5-HT(1B) agonist CP-93, 129 was less effective than 8-OH-DPAT on the adrenocortical activity. The 6-OHDA or 5,7-DHT hypothalamic lesions prevented the stimulatory effects of phenylephrine and 8-OH-DPAT, respectively, which where injected into the AMG, on serum ACTH and CS levels. In view of our previous studies on the effects of the adrenergic and 5-HT antagonists in the AMG and the present data, it is suggested that norepinephrine and 5-HT play an important role in the stimulatory effect of the AMG on the HPA axis. These effects depend on the presence of these excitatory neurotransmitters in the PVN.

Treadmill exercise training augments brain norepinephrine response to familiar and novel stress

In a test of hypothalamic-pituitary-adrenal (HPA) cortical and hypothalamic-pituitary-gonadal (HPG) interaction during familiar and novel stress, we previously reported that treadmill exercise training led to blunted plasma adrenocorticotrophin (ACTH) response to acute treadmill running but a hyper-responsiveness of ACTH after novel immobilization. In this follow-up analysis, we examined whether those results might be plausibly explained by a similar effect of treadmill exercise training on increased levels of norepinephrine (NE) in hypothalamic and limbic brain regions which synergize to modulate the release of ACTH during stress. Ovariectomized Sprague-Dawley rats that had been exercise trained by treadmill running or remained sedentary for 6 weeks received intramuscular injections of estradiol benzoate (Eb) or sesame oil on each of 3 days prior to 15 min of familiar treadmill running or novel immobilization. Treadmill exercise training, regardless of Eb treatment or type of stress, increased NE levels in the paraventricular (PVN), arcuate, medial preoptic, and ventromedial areas of the hypothalamus and protected against depletion of NE in the locus coeruleus, amygdala, and hippocampus. We conclude that treadmill exercise training has a hyperadrenergic effect in brain areas that modulate hypothalamic regulation of ACTH release during stress that is independent of HPA-HPG interaction and novelty of the stressor. To help elucidate these findings, the effects of treadmill exercise training on A1-A2 nuclei which innervate the PVN and their relationship with the limbic and hypothalamic responses we report require study.

Response of amygdalar norepinephrine to footshock and GABAergic drugs using in vivo microdialysis and HPLC

These studies used in vivo microdialysis and high-performance liquid chromatography (HPLC) to examine levels of norepinephrine (NE) within the amygdala in response to both footshock and GABAergic compounds. In Experiment 1, microdialysis probes were inserted into a previously implanted guide cannula aimed at the amygdala and the level of NE was examined in response to footshock. A 0.55 mA (2 s) footshock induced a significant increase in NE levels when the microdialysis probe was located within the amygdala; levels of NE did not differ from baseline in rats with microdialysis probes located outside the amygdala. Experiment 2 examined the effects of the GABAergic antagonist, picrotoxin, the GABAergic agonist, muscimol, and saline on NE levels in the amygdala. Rats treated systemically with picrotoxin showed a dramatic increase in levels of NE within the amygdala. In contrast, systemic injection of muscimol resulted in decreased levels of NE. These findings are consistent with the hypothesis that drugs that are capable of modulating memory do so by altering levels of NE within the amygdala.

Dopaminergic innervation of the amygdala is highly responsive to stress

The amygdala has been implicated in the neuronal sequelae of stress, although little is known about the neurochemical mechanisms underlying amygdala transmission. In vivo microdialysis was employed to measure extracellular levels of dopamine in the basolateral nucleus of the amygdala in awake rats. Once it was established that impulse-dependent release of dopamine could be measured reliably in the amygdala, the effect of stress, induced by mild handling, on amygdala dopamine release was compared with that in three other dopamine-innervated regions, the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. The magnitude of increase in dopamine in response to the handling stimulus was significantly greater in the amygdala than in the nucleus accumbens and prefrontal cortex. This increase was maximal during the application of stress and diminished after the cessation of stress. In contrast, the increases in extracellular dopamine levels in other regions, in particular the nucleus accumbens, were prolonged, reaching maximal values after the cessation of stress. These results suggest that dopaminergic innervation of the amygdala may be more responsive to stress than that of other dopamine-innervated regions of the limbic system, including the prefrontal cortex, and implicate amygdalar dopamine in normal and pathophysiological processes subserving an organism's response to stress.

Norepinephrine release in the amygdala in response to footshock and opioid peptidergic drugs

These experiments used in vivo microdialysis and high-performance liquid chromatography to examine, in rats, norepinephrine (NE) release in the amygdala induced by footshock and systemic administration of drugs affecting the opioid peptidergic system. A microdialysis probe was inserted into a previously implanted guide cannula aimed at the amygdala and the rat was placed in a box with a stainless-steel grid floor through which a single footshock was delivered. Samples were collected and analyzed at 15-min intervals. Footshock stimulation increased NE levels and the magnitude of the increase varied with footshock intensity. Relative to baseline levels, intensities of 0.3, 0.7 and 1.2 mA (3 s) induced increases of 41, 64 and 97%, respectively. NE levels returned to baseline within 30 min after footshock stimulation. The opioid peptidergic antagonist naloxone (1 mg/kg, i.p.) administered immediately after footshock (0.55 mA for 1 s) potentiated NE release. In contrast, the opioid peptidergic agonist beta-endorphin (10 microgram/kg, i.p.) administered after the footshock blocked the footshock-induced increase in NE levels. The magnitude of NE release was less when the drugs were administered without prior footshock and when the injections were given 30 min after footshock. The findings are consistent with previous evidence that acute, mildly stressful stimulation induces the release of NE in the amygdala as well as with extensive pharmacological evidence indicating that amygdala NE released by arousing stimulation is involved in regulating memory storage and that the opioid peptidergic system influences memory storage by modulating the release of NE in the amygdala.

Stress-induced expression of immediate early genes in the brain and peripheral organs of the rat

Stress causes rapid and transient expression of immediate early genes (IEGs) in the brain, and the monitoring of IEGs has enabled the visualization of the neurocircuitry of stress. Previous studies have postulated that stressors can be divided into two categories; processive and systemic. The neural circuits of brain activation differ between the two kinds of stressors. For example, processive stressors, such as immobilization (IMO), induce c-fos mRNA first in the cortical and limbic areas and then in the paraventricular hypothalamic nucleus (PVH), while c-fos expression in the PVH precedes that in other areas in animals subjected to systemic stressors. We further show that prior exposure to IMO stress for 6 days, or implantation of corticosterone pellets suppresses the induction of c-fos, fos B, jun B and NGFI-B, but not that of NGFI-A in the rat PVH. Plasma glucocorticoid may be an important factor regulating stress-induced IEG expression. It is well known that AP-1 and glucocorticoid receptors (GR) interact and suppress each other. Thus, decreased AP-1 levels in chronically stressed animals may help enhance the negative feedback effects of GR and prevent hypersecretion of glucocorticoid, which is implicated in the pathogenesis of stress-related diseases. IMO stress induces rapid expression of c-fos, c-jun and NGFI-A mRNAs in the heart and stomach. These were observed in the ventricular myocardium and coronary arteries, and in the epithelium, smooth muscles and arteries of the stomach after 30 min of IMO. IEG expression in the peripheral organs may provide a molecular basis for stress-induced psychosomatic disorders.

The effect of acute, chronic and chronic intermittent stress on the central noradrenergic system

The objective of this investigation was to examine the immediate and long term effects of acute, chronic and chronic intermittent stress on the central noradrenergic system of rats. Male Sprague-Dawley rats were subjected to one hour of physical immobilization stress either as a single exposure, or as 14 exposures applied either on consecutive days, or randomly over 60 days. Animals were sacrificed immediately, 6 h and 24 h following the last stressor. Levels of norepinephrine (NE) and 3-methoxy-4-hydroxyphenylethylene-glycol sulfate (MHPG-sulfate) were measured in the hypothalamus, hippocampus, cerebral cortex and locus coeruleus region and beta-adrenergic receptor (BAR) density was determined in the cortex. Immediately after acute stress, a significant reduction in hypothalamic NE levels and marked increases in MHPG-sulfate levels in all four brain regions were observed. In contrast immediately after the last stressor of a chronic or chronic intermittent stress regimen, no change in NE concentration was observed while levels of MHPG-sulfate in the four brain regions showed a smaller increase than that observed after an acute stressor. Acute stress induced changes normalized within 6 h while chronic and chronic intermittently stressed animals had altered NE or MHPG-sulfate levels in certain brain regions for up to 6-24 h. Cortical BAR binding parameters remained unchanged after all stress paradigms.

alpha1B adrenoceptors in rat paraventricular nucleus overlap with, but do not mediate, the induction of c-Fos expression by osmotic or restraint stress

A role has been suggested for hypothalamic alpha1 adrenoceptors in the acute stress-induced activation of the hypothalamic-pituitary-adrenal axis. Using a polyclonal antiserum against the rat alpha1B adrenergic receptor protein, we have demonstrated alpha1B receptor immunoreactivity in neurons and especially in punctate cell processes in the rat paraventricular nucleus. The distribution of alpha1B receptor immunoreactivity overlapped in part with the distributions of c-Fos immunoreactivity induced in the paraventricular nucleus by either restraint stress or hypertonic saline administration. However, intraperitoneal pretreatment with the alpha1 receptor antagonist prazosin (0.5 or 5.0 mg/kg) failed to attenuate stress-induced c-Fos expression in the paraventricular nucleus. Prazosin also failed to attenuate the secretion of corticosterone following restraint stress. Thus, we conclude that neither acute secretory activity nor activation of gene transcriptional responses mediated by c-Fos in the hypothalamic pituitary adrenal axis following these stressors are dependent upon hypothalamic alpha1 adrenergic receptors.

Reciprocal cross-desensitization of locus coeruleus electrophysiological responsivity to corticotropin-releasing factor and stress

While acutely administered corticotropin-releasing factor (CRF) and acute stress each activate neurons of the locus coeruleus (LC), desensitization to both develops with repeated treatment. The present experiments were designed to investigate whether cross-desensitization develops between CRF and stress. Because acute hemodynamic stress caused by intravenous infusion of sodium nitroprusside increases LC electrophysiological discharge rate via a CRF-dependent mechanism, it was hypothesized that repeated CRF administration would cause desensitization to the effect of this stressor on LC. For a complementary experiment, it was hypothesized that repeated stress, which presumably results in the repeated release of endogenous CRF, would result in desensitization to subsequent exogenous CRF. The results of the first experiment showed that repeated intracerebroventricular (i.c.v.) administration of CRF caused a significant attenuation of the sodium nitroprusside-induced increase in LC discharge rate seen in naive rats, although this pretreatment actually potentiated the decrease in blood pressure produced by sodium nitroprusside. In the second experiment, either one or eight sessions of white-noise stress attenuated the effect of CRF on LC activity 24 h after the last stress exposure, and this attenuation was more pronounced following eight sessions of stress than following one session. In a test of the specificity of this effect, stress-induced desensitization did not generalize to the LC electrophysiological response to clonidine (i.c.v.). One week following the last of eight sessions of stress, LC responsivity to CRF had recovered to control levels. These experiments demonstrate reciprocal cross-desensitization between CRF and stress using LC electrophysiological responsivity as an assay. This modifiability of the interaction between CRF and the LC may represent the operation of mechanisms mediating adaptive responding to stress.

Norepinephrine depletion in the amygdala inhibits CRF-41, ACTH, and corticosterone responses following photic simulation

The effects of amygdaloid norepinephrine depletion by 6-hydroxydopamine on changes in corticotropin releasing factor-41 (CRF-41) and serum adrenocorticotropic hormone (ACTH) and corticosterone levels, following neural stimuli were investigated. In intact animals, photic or acoustic stimulation caused CRF-41 depletion from the median eminence and a rise in serum ACTH and corticosterone levels. In rats with amygdalar norepinephrine depletion there were no changes in basal CRF-41, ACTH, or corticosterone levels. However, the above responses of the hypothalamo-pituitary adrenocortical axis were blocked following photic, but not acoustic, stimulation. These results indicate that the facilitatory role of the amygdala on the above responses following photic stimulation depends on the presence of norepinephrine in this region.

Amygdala efferents form inhibitory-type synapses with a subpopulation of catecholaminergic neurons in the rat Nucleus tractus solitarius

The central nucleus of the amygdala (CNA) integrates visceral responses to stress partially through efferent projections to portions of the medial nuclei of the solitary tracts (mNTS) containing catecholaminergic neurons. To determine anatomical sites for CNA modulation of these neurons, immunoperoxidase detection of anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) was combined with immunogold-silver labeling of the catecholamine-synthesizing enzyme, tyrosine hydroxylase, in adult rat mNTS. From 350 anterogradely labeled terminals identified within the intermediate mNTS, 30% formed symmetric, inhibitory-type synapses and the remainder lacked recognized junctions as seen within a single plane of section. Of the terminals forming symmetric synapses, 16% were presynaptic to tyrosine hydroxylase immunoreactive dendrites and the remainder to unlabeled dendrites. The level of tyrosine hydroxylase immunoreactivity as assessed by density of gold-silver particles was significantly lower in dendrites receiving synaptic input from CNA efferents as compared with dendrites of the same sizes (2.0 microns 2 in mean area) which received synapses from unlabeled terminals or lacked recognizable synaptic inputs. When separately examined without regard to afferent input, the medium- and larger-sized dendrites having mean cross-sectional areas of 1-3 microns 2 also contained significantly less tyrosine hydroxylase immunoreactivity than small (< 1 micron 2) dendrites. These results suggest that CNA efferents to the mNTS inhibit non-catecholamine-containing neurons and a subpopulation of catecholaminergic neurons distinguished by their low levels of tyrosine hydroxylase. The findings also indicate that small, presumably more distal, dendrites in the intermediate mNTS may synthesize and/or release catecholamines.

Brain microdialysis and its application for the study of animal behaviour

Microdialysis is a sampling method that is used to determine the extracellular concentration of neurotransmitters in the brain. The method can be applied to conscious and unrestrained animals and is very suitable for the study of the chemistry of endogenous behaviour. This article reviews the contribution that microdialysis made to our understanding of the chemistry of behaviour. Methodological and practical considerations such as the implantation time and the use of guide cannulas are reviewed. The question whether neurotransmitters and related metabolites in dialysates reflect true synaptic release is critically discussed. There is much evidence that dopamine, noradrenaline, acetylcholine and serotonin in dialysates are related to neurotransmission, but there is serious doubt whether this is the case with amino acid transmitters such as GABA, glutamate and aspartate. Until now far over 100 papers appeared that used microdialysis in behavioural studies. Behavioural activation, the sleep-awake cycle and diurnal rhythms were subject of several of these studies. Various workers have described neurochemical changes in the brain that are related to feeding. Other studies were concerned with sexual behaviour and the sexual cycle in females. Parturition, maternal behaviour and offspring recognition have been studied in a series of microdialysis studies carried out in sheep. An overview is given of the microdialysis studies that were carried out to understand the biochemistry of stress. In this respect dopamine and noradrenaline have received much attention. A great number of microdialysis studies dealt with the role of dopamine in self-stimulation, reward and aversive emotions. It is concluded that microdialysis is at presently the most versatile and practical method to study the chemistry of behaviour and it is to be expected that it will soon be a routine methodology in behavioural research. Finally, perspectives and possible future developments of the methods are discussed.

Effect of corticosterone on the enhancement of the acoustic startle reflex by corticotropin releasing factor (CRF)

The present study evaluated the effects of adrenalectomy and chronic administration of corticosterone on the ability of CRF given intraventricularly to increase the amplitude of the acoustic startle reflex in rats. Experiment 1 showed that CRF-enhanced startle was not affected by adrenalectomy, indicating a central effect independent of the integrity of the hypothalamic-pituitary-adrenal axis. In Experiment 2, chronic injection of corticosterone augmented CRF-enhanced startle using a dose of CRF (0.25 micrograms) that normally is ineffective in increasing startle amplitude. Chronic injection of corticosterone by itself did not increase startle amplitude (Experiment 3). We suggest that the potentiation of CRF-enhanced startle by corticosterone may result from an activation of CRF in the central nucleus of the amygdala.

Allostasis, amygdala, and anticipatory angst

Regions of the amygdala are involved in anticipation of negative events. Chronic anticipation of negative events leads to what we call allostatic load, or arousal pathology. Two hormones appear to be involved in arousal pathology; corticotropin-releasing hormone in the brain and glucocorticoids. We suggest that increases in corticotropin-releasing hormone, by stress or glucocorticoids, in the amygdala may have functional consequences for allostatic load. Whereas, corticotropin-releasing hormone in the parvocellular region of the paraventricular nucleus of the hypothalamus is decreased by glucocorticoids thereby under negative feedback and homeostatic control, the central nucleus of the amygdala is to some extent under positive feedback and is increased by glucocorticoids, and perhaps under allostatic control. The human and animal literature suggest that a variety of psychopathologies (e.g., melancholia) may be tied to neurohormonal signals activating regions of the amygdala.

Corticosterone effects on corticotropin-releasing hormone mRNA in the central nucleus of the amygdala and the parvocellular region of the paraventricular nucleus of the hypothalamus

Using in situ hybridization histochemistry, we report differential expression of corticotropin-releasing hormone (CRH) mRNA in the central nucleus of the amygdala (CEA) and the parvocellular region of the paraventricular nucleus of the hypothalamus (PVN) following systemic treatment with corticosterone (CORT) in adrenally-intact rats. Both injection of low (1 mg/kg/day) and high (5 mg/day) CORT reduced CRH mRNA expression in the PVN in a dose-dependent manner, although it returned to normal at the low dose by 14 days. By contrast, the high dose of CORT increased CRH mRNA transiently in the CEA at 4 days, although the low dose of CORT decreased it at 14 days. In a second experiment, we implanted a slowly-releasing CORT pellet for 2 weeks (200 mg, 60 day release) subcutaneously. This treatment produced an elevation of CRH mRNA in the CEA both at 1 and 2 weeks, whereas CRH mRNA in the PVN was decreased to a large extent as seen in the high CORT group of the first experiment. These results suggest that glucocorticoids can facilitate CRH mRNA expression in the CEA, a site implicated in anxiety and fear, while restraining the hypothalamic-pituitary-adrenal axis as indicated by the reduction in CRH mRNA in the PVN.