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

Central amygdala CRF+ neurons promote heightened threat reactivity following early life adversity in mice

Failure to appropriately predict and titrate reactivity to threat is a core feature of fear and anxiety-related disorders and is common following early life adversity (ELA). A population of neurons in the lateral central amygdala (CeAL) expressing corticotropin releasing factor (CRF) have been proposed to be key in processing threat of different intensities to mediate active fear expression. Here, we use in vivo fiber photometry to show that ELA results in sex-specific changes in the activity of CeAL CRF+ neurons, yielding divergent mechanisms underlying the augmented startle in ELA mice, a translationally relevant behavior indicative of heightened threat reactivity and hypervigilance. Further, chemogenic inhibition of CeAL CRF+ neurons selectively diminishes startle and produces a long-lasting suppression of threat reactivity. These findings identify a mechanism for sex-differences in susceptibility for anxiety following ELA and have broad implications for understanding the neural circuitry that encodes and gates the behavioral expression of fear.

Early-Life Stress Impairs Perception and Neural Encoding of Rapid Signals in the Auditory Pathway

During developmental critical periods (CPs), early-life stress (ELS) induces cognitive deficits and alters neural circuitry in regions underlying learning, memory, and attention. Mechanisms underlying critical period plasticity are shared by sensory cortices and these higher neural regions, suggesting that sensory processing may also be vulnerable to ELS. In particular, the perception and auditory cortical (ACx) encoding of temporally-varying sounds both mature gradually, even into adolescence, providing an extended postnatal window of susceptibility. To examine the effects of ELS on temporal processing, we developed a model of ELS in the Mongolian gerbil, a well-established model for auditory processing. In both male and female animals, ELS induction impaired the behavioral detection of short gaps in sound, which are critical for speech perception. This was accompanied by reduced neural responses to gaps in auditory cortex, the auditory periphery, and auditory brainstem. ELS thus degrades the fidelity of sensory representations available to higher regions, and could contribute to well-known ELS-induced problems with cognition.SIGNIFICANCE STATEMENT In children and animal models, early-life stress (ELS) leads to deficits in cognition, including problems with learning, memory, and attention. Such problems could arise in part from a low-fidelity representation of sensory information available to higher-level neural regions. Here, we demonstrate that ELS degrades sensory responses to rapid variations in sound at multiple levels of the auditory pathway, and concurrently impairs perception of these rapidly-varying sounds. As these sound variations are intrinsic to speech, ELS may thus pose a challenge to communication and cognition through impaired sensory encoding.

Chronic stress induced loudness hyperacusis, sound avoidance and auditory cortex hyperactivity

Hyperacusis, a debilitating loudness intolerance disorder, has been linked to chronic stress and adrenal insufficiency. To investigate the role of chronic stress, rats were chronically treated with corticosterone (CORT) stress hormone. Chronic CORT produced behavioral evidence of loudness hyperacusis, sound avoidance hyperacusis, and abnormal temporal integration of loudness. CORT treatment did not disrupt cochlear or brainstem function as reflected by normal distortion product otoacoustic emissions, compound action potentials, acoustic startle reflexex, and auditory brainstem responses. In contrast, the evoked response from the auditory cortex was enhanced up to three fold after CORT treatment. This hyperactivity was associated with a significant increase in glucocorticoid receptors in auditory cortex layers II/III and VI. Basal serum CORT levels remained normal after chronic CORT stress whereas reactive serum CORT levels evoked by acute restraint stress were blunted (reduced) after chronic CORT stress; similar changes were observed after chronic, intense noise stress. Taken together, our results show for the first time that chronic stress can induce hyperacusis and sound avoidance. A model is proposed in which chronic stress creates a subclinical state of adrenal insufficiency that establishes the necessary conditions for inducing hyperacusis.

Age- and sex-specific effects of maternal separation on the acoustic startle reflex in rats: early baseline enhancement in females and blunted response to ambiguous threat

Early life adversity (ELA) increases the incidence of later-life anxiety disorders. Dysregulated threat processing, including responsivity to ambiguous threats, is an indicator of anxiety disorders and can be influenced by childhood experiences. The acoustic startle response is a defensive reflex displayed by mammals when exposed to sudden intense stimuli reflecting individual variations in vigilance. These measures can be altered by previous experience and experimental modifications, including the introduction of unconditioned aversive stimuli. Rats emit ultrasonic vocalizations (USVs) in the 22 KHz range in negative contexts. As such, 22 KHz USVs are an ethologically relevant social cue of environmental threat shown to induce anxiety-like behavior in recipient rats. Because the timing of symptom manifestation after early life adversity can differ between sexes, the current study sought to identify the age- and sex-specific effects of daily maternal separation (MS) on responsivity to ambiguous threat in rats. In Experiment 1, rat pups underwent MS or control rearing from postnatal day (P) 2–20, then underwent behavioral testing beginning on P24, 34, or 54 to determine whether MS modified the baseline startle response or the modulation of startle by 22 KHz USVs. In Experiment 2, rats were tested in a light-enhanced startle paradigm at P54 after MS or control rearing to determine whether MS influenced light-enhanced startle. Results show an enhancement of the baseline startle magnitude by MS in females at P34. At P54, MS reduced the modulation of the startle response by 22 KHz USVs and prevented light-enhanced startle, indicating an MS-induced deficit in defensive responsivity when exposed to potential threat.

Effects of handling on the behavioural phenotype of the neuregulin 1 type III transgenic mouse model for schizophrenia

Handling of laboratory mice affects animal wellbeing and behavioural test outcomes. However, present research has focused on handling effects in common strains of laboratory mice despite the knowledge that environmental factors can modify established phenotypes of genetic mouse models. Thus, we examined the impact of handling on the face validity of a transgenic mouse model for the schizophrenia risk gene neuregulin 1 (i.e. Nrg1 type III overexpression). Nrg1 III tg and wild type-like (WT) control mice of both sexes underwent tail or tunnel handling before being assessed in the open field (OF), elevated plus maze (EPM), social preference/novelty, prepulse inhibition, and fear conditioning tests. Tunnel-handling reduced the startle response in all mice, increased OF locomotion and exploration in males and reduced anxiety in males (OF) and females (EPM) compared to tail-handling. Importantly, tunnel handling induced a more pronounced startle response to increasing startle stimuli in Nrg1 III tg females compared to respective controls, a phenomenon absent in tail-handled females. Finally, Nrg1 III tg males displayed reduced OF exploration and centre locomotion and Nrg1 III tg females displayed increased cue freezing over time compared to controls. In conclusion, handling methods have a significant impact on a variety of behavioural domains thus the impact of routine handling procedures need be considered when testing behavioural phenotypes. Handling did not change the main schizophrenia-relevant characteristics of Nrg1 III tg mice but affected the acoustic startle-response in a genotype- and sex-specific manner. Future research should evaluate the effect of handling on other genetic models.

Effects of psychological or physical prenatal stress on attention and locomotion in juvenile rats

Background: Prenatal stress has been shown to affect the cognition of offspring, including memory and learning abilities.Methods: In the current study, the long-term effects of chronic prenatal exposure to the physical or psychological stress on locomotion and attention were evaluated by using open field test (OFT) and prepulse inhibition (PPI) of the acoustic startle reflex (ASR). In addition, the level of corticosterone was measured after the ASR trial.Results: Male and female rodents that underwent prenatal physical and psychological stress had an augmented velocity in OFT, and only male animals showed an increased ASR. Neither male nor female offsprings had an alteration in the level of corticosterone and PPI values regardless of the stress type.Conclusion: Our results revealed that exposure to stress during the development of fetus increases ASR in a sex-dependent manner. This finding might implicate the effect of prenatal stress on attention in male offspring regardless of the stress type.

Children with chronic stress-induced recurrent muscle pain have enhanced startle reaction

BACKGROUND

Children with recurrent pain of negative chronic stress origin from different locations have a characteristic pattern of tender points in the temporal, trapezoid, great pectoral and abdominal muscles. We tested the hypothesis that the startle reaction is activated in these children and that some of the startle-activated muscles are related to the tender point pattern and the recurrent pain.

METHODS

In children/adolescents, aged 10-17 years, 19 with recurrent psychosomatic pain (PAIN) and 23 controls (CON) we measured and analysed resting activity and acoustic startle response with electromyography (EMG) for the muscles involved in the pattern of tender points and also the lumbar erector spinae.

RESULTS

The PAIN group showed higher resting activity and higher acoustic startle response values than the CON group for all six muscles together regarding the mean amplitude in the initial 200 ms, and during the burst of activity, and longer burst duration and shorter burst latency. For PAIN versus CON, all separate muscles showed generally higher values of EMG amplitudes and burst durations, and shorter latencies for the burst onset in all measures; with significance or strong trends for several parameters and muscles.

CONCLUSION

For the first time in children with recurrent psychosomatic pain, increased resting activity and potentiated startle response were demonstrated in the muscles involved in the stress tender point pattern.

SIGNIFICANCE

This study demonstrates in adolescents how recurrent pain of negative stress origin from the head, stomach, back and chest is related to increased startle reaction and increased muscular tension in these regions. This study contributes to the understanding of the mechanisms underlying the global burden of recurrent pain.

Sensitive Periods of Emotion Regulation: Influences of Parental Care on Frontoamygdala Circuitry and Plasticity

Early caregiving experiences play a central role in shaping emotional development, stress physiology, and refinement of limbic circuitry. Converging evidence across species delineates a sensitive period of heightened neuroplasticity when frontoamygdala circuitry is especially amenable to caregiver inputs early in life. During this period, parental buffering regulates emotional behaviors and stress physiology as emotion regulation circuitry continues to mature. By contrast, disorganized or poor quality caregiving has profound and lasting consequences on the maturation of frontoamygdala circuitry essential for emotion regulation, even following termination of this early life stressor (e.g., adoption from orphanage). This article highlights how interactions between caregiving experiences and the biological state of the developing brain have broad implications for long-term health.

HPA Axis Interactions with Behavioral Systems

Perhaps the most salient behaviors that individuals engage in involve the avoidance of aversive experiences and the pursuit of pleasurable experiences. Engagement in these behaviors is regulated to a significant extent by an individual's hormonal milieu. For example, glucocorticoid hormones are produced by the hypothalamic-pituitary-adrenocortical (HPA) axis, and influence most aspects of behavior. In turn, many behaviors can influence HPA axis activity. These bidirectional interactions not only coordinate an individual's physiological and behavioral states to each other, but can also tune them to environmental conditions thereby optimizing survival. The present review details the influence of the HPA axis on many types of behavior, including appetitively-motivated behaviors (e.g., food intake and drug use), aversively-motivated behaviors (e.g., anxiety-related and depressive-like) and cognitive behaviors (e.g., learning and memory). Conversely, the manuscript also describes how engaging in various behaviors influences HPA axis activity. Our current understanding of the neuronal and/or hormonal mechanisms that underlie these interactions is also summarized. © 2016 American Physiological Society. Compr Physiol 6:1897-1934, 2016.

Separation at weaning from the family is stressful for naturally group-living, but not solitary-living, male African striped mice Rhabdomys

Early separation from a family is stressful for young mammals, but might be more stressful for group-living than solitary species. Using juvenile males of three African striped mice Rhabdomys taxa that are either group (R. pumilio) or solitary (R. dilectus dilectus and R. d. chakae) living, we predicted greater separation anxiety in R. pumilio than R. dilectus because group-living could reduce anxiety in R. pumilio. Three brothers from each of 10 litters per taxon were randomly assigned soon after natural weaning (25 days) to one of three treatments for 10 days: (1) remained with the family (philopatric); (2) separated from the family by a wire mesh barrier (separated); and (3) isolated from the family (isolated). Males were individually tested in a four-arm maze to assess their anxiety responses and sampled for corticosterone concentrations 20 mins and 10 days later. Compared to R. dilectus males, R. pumilio males showed a greater treatment response to separation: philopatric males used the light arms of the maze less and had higher corticosterone concentrations compared to isolated males, which spent the most time in the light arms and had the lowest corticosterone concentrations overall; separated males showed an intermediate behavioural response, but had similar corticosterone concentrations to philopatric males. Thus, separation from a family group is more stressful in group-living Rhabdomys and this stress response dissipates with time. Philopatry and group-living may be more important for young R. pumilio, whereas dispersal at weaning is an important life history event for solitary R. dilectus.

Postnatal ablation of POMC neurons induces an obese phenotype characterized by decreased food intake and enhanced anxiety-like behavior

Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus are central components of systems regulating appetite and energy homeostasis. Here we report on the establishment of a mouse model in which the ribonuclease III ribonuclease Dicer-1 has been specifically deleted from POMC-expressing neurons (POMC(ΔDCR)), leading to postnatal cell death. Mice are born phenotypically normal, at the expected genetic ratio and with normal hypothalamic POMC-mRNA levels. At 6 weeks of age, no POMC neurons/cells could be detected either in the arcuate nucleus or in the pituitary of POMC(ΔDCR) mice. POMC(ΔDCR) develop progressive obesity secondary to decreased energy expenditure but unrelated to food intake, which was surprisingly lower than in control mice. Reduced expression of AgRP and ghrelin receptor in the hypothalamus and reduced uncoupling protein 1 expression in brown adipose tissue can potentially explain the decreased food intake and decreased heat production, respectively, in these mice. Fasting glucose levels were dramatically elevated in POMC(ΔDCR) mice and the glucose tolerance test revealed marked glucose intolerance in these mice. Secondary to corticotrope ablation, basal and stress-induced corticosterone levels were undetectable in POMC(ΔDCR) mice. Despite this lack of activation of the neuroendocrine stress response, POMC(ΔDCR) mice exhibited an anxiogenic phenotype, which was accompanied with elevated levels of hypothalamic corticotropin-releasing factor and arginine-vasopressin transcripts. In conclusion, postnatal ablation of POMC neurons leads to enhanced anxiety and the development of obesity despite decreased food intake and glucocorticoid deficiency.

Effects of continuously enhanced corticotropin releasing factor expression within the bed nucleus of the stria terminalis on conditioned and unconditioned anxiety

The lateral division of the bed nucleus of the stria terminalis (BNST), which forms part of the circuitry regulating fear and anxiety, contains a large number of neurons expressing corticotropin releasing factor (CRF), a neuropeptide that has a prominent role in the etiology of fear- and anxiety-related psychopathologies. Stress increases CRF expression within BNST neurons, implicating these cells in stress- and anxiety-related behaviors. These experiments examined the effect of chronically enhanced CRF expression within BNST neurons on conditioned and unconditioned anxiety-related behavior by using a lentiviral vector containing a promoter that targets CRF gene overexpression (OE) to CRFergic cells. We found that BNST CRF-OE did not affect unconditioned anxiety-like responses in the elevated plus maze or basal acoustic startle amplitude. CRF-OE induced before training weakened sustained fear (conditioned anxiety); when induced after conditioning, CRF-OE increased expression of the conditioned emotional memory. Increased BNST CRF expression did not affect plasma corticosterone concentration but did decrease CRFR1 receptor density within the BNST and CRFR2 receptor density within the dorsal portion of the caudal dorsal raphe nucleus. These data raise the possibility that the observed behavioral effects may be mediated by enhanced CRF receptor signaling or compensatory changes in CRF receptor density within these structures. Together, these studies demonstrate that CRF neurons within the lateral BNST modulate conditioned anxiety-like behaviors and also suggest that enhanced CRF expression within these neurons may contribute to inappropriate regulation of emotional memories.

Epigenetic Risk Factors in PTSD and Depression

Epidemiological and clinical studies have shown that children exposed to adverse experiences are at increased risk for the development of depression, anxiety disorders, and posttraumatic stress disorder (PTSD). A history of child abuse and maltreatment increases the likelihood of being subsequently exposed to traumatic events or of developing PTSD as an adult. The brain is highly plastic during early life and encodes acquired information into lasting memories that normally subserve adaptation. Translational studies in rodents showed that enduring sensitization of neuronal and neuroendocrine circuits in response to early life adversity are likely risk factors of life time vulnerability to stress. Hereby, the hypothalamic-pituitary-adrenal (HPA) axis integrates cognitive, behavioral, and emotional responses to early-life stress and can be epigenetically programed during sensitive windows of development. Epigenetic mechanisms, comprising reciprocal regulation of chromatin structure and DNA methylation, are important to establish and maintain sustained, yet potentially reversible, changes in gene transcription. The relevance of these findings for the development of PTSD requires further studies in humans where experience-dependent epigenetic programing can additionally depend on genetic variation in the underlying substrates which may protect from or advance disease development. Overall, identification of early-life stress-associated epigenetic risk markers informing on previous stress history can help to advance early diagnosis, personalized prevention, and timely therapeutic interventions, thus reducing long-term social and health costs.

The psychobiology of resilience and vulnerability to anxiety disorders: implications for prevention and treatment

Much of the research on the neurobiology of human anxiety disorders has focused on psychopaihological abnormalities in patients with anxiety disorders. While this line of research is obviously important, more investigation is needed to elucidate the psychobiology of resilience to extreme stress. Study of the psychobiology of resilience has the potential to identify neurochemical, neuropeptide, and hormonal mediators of vulnerability and resilience to severe stress. In addition, the relevance of neural mechanisms of reward and motivation, fear responsiveness, and social behavior to character traits associated with risk and resistance to anxiety disorders may be clarified. These areas of investigation should lead to improved methods of diagnosis, novel approaches to prevention, and new targets for antianxiety drug discovery.

Human amygdala development in the absence of species-expected caregiving

In altricial species, like the human, caregiver presence is necessary for typical emotional development. Children who have been raised in institutional care early in life experience caregiver deprivation and are at significantly elevated risk for emotional difficulties. The current manuscript examines the non-human and human literatures on amygdala development following caregiver deprivation and presents an argument that in the absence of the species-expected caregiver presence, human amygdala development exhibits rapid development and perhaps premature engagement that results in some of the emotional phenotypes observed following early institutional care.

Hydrocortisone suppression of the fear-potentiated startle response and posttraumatic stress disorder

This study examined the effects of oral administration of 20mg hydrocortisone on baseline and fear-potentiated startle in 63 male veterans with or without PTSD. The procedure was based on a two-session, within-subject design in which acoustic startle eyeblink responses were recorded during intervals of threat or no threat of electric shock. Results showed that the magnitude of the difference between startle responses recorded during anticipation of imminent shock compared to "safe" periods was reduced after hydrocortisone administration relative to placebo. This effect did not vary as a function of PTSD group nor were there were any significant group differences in other indices startle amplitude. Findings suggest that the acute elevations in systemic cortisol produced by hydrocortisone administration may have fear-inhibiting effects. This finding may have implications for understanding the role of hypothalamic-pituitary-adrenal (HPA)-axis function in vulnerability and resilience to traumatic stress.

Adrenal activity during repeated long-access cocaine self-administration is required for later CRF-Induced and CRF-dependent stressor-induced reinstatement in rats

Understanding the neurobiological processes that contribute to the establishment and expression of stress-induced regulation of cocaine use in addicted individuals is important for the development of new and better treatment approaches. It has been previously shown that rats self-administering cocaine under long-access conditions (6 h daily) display heightened susceptibility to the reinstatement of extinguished cocaine seeking by a stressor, electric footshock, or i.c.v. administration of the stressor-responsive neuropeptide, corticotropin-releasing factor (CRF). This study tested the hypothesis that adrenal responsiveness during earlier long-access cocaine self-administration (SA) is necessary for the establishment of later CRF-dependent stress-induced reinstatement. Reinstatement by footshock, but not a cocaine challenge (10 mg/kg, i.p.) following long-access SA, was blocked by i.c.v. administration of the CRF receptor antagonist, α-helical CRF(9-41) (10 μg). Elimination of SA-induced adrenal responses through surgical adrenalectomy and diurnal corticosterone replacement (ADX/C) before 14 days of SA under long-access conditions had minimal impact on cocaine SA, but blocked later footshock-induced reinstatement. By contrast, ADX/C after SA, but before extinction and reinstatement testing, failed to reduce footshock-induced reinstatement. Likewise, ADX/C before 14 days long-access SA prevented later reinstatement by i.c.v. CRF (0.5 or 1.0 μg). However, significant CRF-induced reinstatement was observed when rats underwent ADX/C following SA, but before extinction and reinstatement testing, although a modest but statistically nonsignificant reduction in sensitivity to CRF's reinstating effects was observed. Taken together, these findings suggest that adrenal-dependent neuroadaptations in CRF responsiveness underlie the increased susceptibility to stress-induced relapse that emerges with repeated cocaine use.

Influence of diurnal phase on startle response in adult rats exposed to dexamethasone in utero

Depression and pathological anxiety disorders are among the most prevalent neurological diseases in the world and can be precipitated and exacerbated by stress. Prenatal stress alters both behavioral and endocrine responses to stressful stimuli in later life. We have previously observed increased basal acoustic startle response (ASR) in Wistar rats exposed to stress or dexamethasone (DEX) in utero when tested during the light phase of the circadian rhythm, and decreased prepulse inhibition (PPI) in similar animals tested during the dark phase of the cycle. We speculated that this observation of increased basal startle might be influenced by diurnal phase. In the present study, adult female Sprague Dawley rats, stressed prenatally with DEX (200 μg/kg, gestational days 14-21) and postnatally by blood sampling under restraint, were tested for the ASR during both circadian phases (light and dark). Basal startle was increased in animals tested both during the light and the dark phases of the cycle. We hereby replicated our earlier findings in a new strain and laboratory, thus strengthening the validity of our model regarding prenatal stress effects on ASR in female offspring. Our results indicate that observation of increased basal ASR is not solely dependent on diurnal phase. We found no difference in hippocampal glucocorticoid and mineral corticoid receptor expression between groups.

A review of adversity, the amygdala and the hippocampus: a consideration of developmental timing

A review of the human developmental neuroimaging literature that investigates outcomes following exposure to psychosocial adversity is presented with a focus on two subcortical structures – the hippocampus and the amygdala. Throughout this review, we discuss how a consideration of developmental timing of adverse experiences and age at measurement might provide insight into the seemingly discrepant findings across studies. We use findings from animal studies to suggest some mechanisms through which timing of experiences may result in differences across time and studies. The literature suggests that early life may be a time of heightened susceptibility to environmental stressors, but that expression of these effects will vary by age at measurement.

Phasic vs sustained fear in rats and humans: role of the extended amygdala in fear vs anxiety

Data will be reviewed using the acoustic startle reflex in rats and humans based on our attempts to operationally define fear vs anxiety. Although the symptoms of fear and anxiety are very similar, they also differ. Fear is a generally adaptive state of apprehension that begins rapidly and dissipates quickly once the threat is removed (phasic fear). Anxiety is elicited by less specific and less predictable threats, or by those that are physically or psychologically more distant. Thus, anxiety is a more long-lasting state of apprehension (sustained fear). Rodent studies suggest that phasic fear is mediated by the amygdala, which sends outputs to the hypothalamus and brainstem to produce symptoms of fear. Sustained fear is also mediated by the amygdala, which releases corticotropin-releasing factor, a stress hormone that acts on receptors in the bed nucleus of the stria terminalis (BNST), a part of the so-called 'extended amygdala.' The amygdala and BNST send outputs to the same hypothalamic and brainstem targets to produce phasic and sustained fear, respectively. In rats, sustained fear is more sensitive to anxiolytic drugs. In humans, symptoms of clinical anxiety are better detected in sustained rather than phasic fear paradigms.

Glucocorticoid inhibition in the treatment of depression: can we think outside the endocrine hypothalamus?

BACKGROUND

Major depressive disorder affects a substantial percentage of the U.S. population, and can be highly debilitating. Selective serotonin reuptake inhibitors are commonly prescribed to treat depression, but may not be as effective for more severe or persistent depression.

METHODS

The authors review data concerning the effects of corticosteroid synthesis inhibitors (CSIs) in the management of depressive disorders, present a hypothesis as to their possible mechanisms of action based on recent data suggesting synergistic effects of glucocorticoids on extrahypothalamic corticotropin-releasing hormone (CRH), and consider alternative hypotheses. Published reports evaluating the efficacy of CSIs in treating depression are reviewed and presented in light of recent findings regarding actions of glucocorticoids on the central CRH system.

RESULTS

Results from open label and double-blind studies by several groups have indicated that CSIs may be efficacious or of adjunctive value in some patients with depression, including those refractory to other agents; however, there is a need for more controlled studies. Several lines of data suggest that the mechanism of action of these agents may not be solely a function of inhibition of adrenal cortisol production.

CONCLUSIONS

The authors propose that CSIs may be efficacious in part by reducing glucocorticoid enhancement of CRH action in neurons of the central nucleus of the amygdala and other structures outside the endocrine hypothalamus. Possible effects of systemically administered CSIs on glucocorticoid receptor regulation, neuroactive steroids, and classical monoamine systems are also discussed. We conclude that available clinical data suggest a potential role for CSIs in the management of depressive disorders, especially major depression with psychotic features.

Stress and the neuroendocrinology of anxiety disorders

Stress is a risk factor for depressive and anxiety disorders. Changes in lifestyle patterns that are associated with increased stress therefore place a greater burden on mental health. Stress challenges the organism's homeostatic mechanisms, triggering a cascade of events that should, normally, maintain or allow a return to equilibrium. Stressful events are perceived by sensory systems in the brain, facilitating evaluation and comparison of the existing and previous stimuli as well as the activation of hormones responsible for energy mobilization. The limbic system coordinates the release of corticosteroids, the primary stress hormones, by modulating activation of the hypothalamic paraventricular nucleus (PVN). The amygdala, a limbic structure related to emotional behavior, has a putative role in the evaluation of emotional events and formation of fearful memories; it is also a target of the neurochemical and hormonal mediators of stress. Clinical and experimental data have correlated changes in the structure/function of the amygdala with emotional disorders such as anxiety. In this chapter we review the neuroendocrinology of the stress response, focusing on the role of the limbic system in its establishment and supplementing that information with new experimental data that demonstrates the relationship between stress and anxiety disorders; we also discuss the structural changes that occur in the amygdala after stress.

Models and mechanisms of anxiety: evidence from startle studies

RATIONALE

Preclinical data indicates that threat stimuli elicit two classes of defensive behaviors, those that are associated with imminent danger and are characterized by flight or fight (fear), and those that are associated with temporally uncertain danger and are characterized by sustained apprehension and hypervigilance (anxiety).

OBJECTIVE

The objectives of the study are to (1) review evidence for a distinction between fear and anxiety in animal and human experimental models using the startle reflex as an operational measure of aversive states, (2) describe experimental models of anxiety, as opposed to fear, in humans, (3) examine the relevance of these models to clinical anxiety.

RESULTS

The distinction between phasic fear to imminent threat and sustained anxiety to temporally uncertain danger is suggested by psychopharmacological and behavioral evidence from ethological studies and can be traced back to distinct neuroanatomical systems, the amygdala and the bed nucleus of the stria terminalis. Experimental models of anxiety, not fear, are relevant to non-phobic anxiety disorders.

CONCLUSIONS

Progress in our understanding of normal and abnormal anxiety is critically dependent on our ability to model sustained aversive states to temporally uncertain threat.

Neuropeptide Y in the amygdala induces long-term resilience to stress-induced reductions in social responses but not hypothalamic-adrenal-pituitary axis activity or hyperthermia

Resilience to mental and physical stress is a key determinant for the survival and functioning of mammals. Although the importance of stress resilience has been recognized, the underlying neural mediators have not yet been identified. Neuropeptide Y (NPY) is a peptide known for its anti-anxiety-like effects mediated via the amygdala. The results of our current study demonstrate, for the first time that repeated administration of NPY directly into the basolateral nucleus of the amygdala (BLA) produces selective stress-resilient behavioral responses to an acute restraint challenge as measured in the social interaction test, but has no effect on hypothalamic-adrenal-pituitary axis activity or stress-induced hyperthermia. More importantly, the resilient behaviors observed in the NPY-treated animals were present for up to 8 weeks. Antagonizing the activity of calcineurin, a protein phosphatase involved in neuronal remodeling and present in NPY receptor containing neurons within the BLA, blocked the development of long-term, but not the acute increases in social interaction responses induced by NPY administration. This suggests that the NPY-induced long-term behavioral resilience to restraint stress may occur via mechanisms involving neuronal plasticity. These studies suggest one putative physiologic mechanism underlying stress resilience and could identify novel targets for development of therapies that can augment the ability to cope with stress.

CRF(1) receptor antagonists attenuate escalated cocaine self-administration in rats

RATIONALE

Previous work suggests a role for stress-related corticotropin-releasing factor (CRF) systems in cocaine dependence. However, the involvement of activation of CRF(1) receptors in rats self-administering cocaine with extended access is unknown.

OBJECTIVE

The current study examined whether CRF(1) receptor antagonist administration alters cocaine self-administration in animals given extended access.

MATERIALS AND METHODS

Wistar rats (n = 32) acquired cocaine self-administration (0.66 mg/kg per infusion) in 1 h sessions for up to 11 days. Rats then were assigned to receive either daily short (1 h, ShA) or long (6 h, LgA) access to cocaine self-administration (n = 7-9 per group). Following escalation of intake, animals received one of two selective CRF(1) antagonists: antalarmin (6.3-25 mg/kg, i.p.) or N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo[1,5a]pyrimidin-7-amine (MPZP; 3.6-27.5 mg/kg, s.c.).

RESULTS

By day 11 of the escalation period, LgA rats increased their cocaine intake, reaching an intake level of 15.1 mg/kg, compared to 11.1 mg/kg in ShA rats, during the first hour of sessions. Antalarmin reduced cocaine self-administration at the highest dose selectively in the LgA group but not the ShA group. MPZP reduced cocaine intake both in LgA and ShA rats. However, MPZP did so at a lower dose in LgA rats than in ShA rats. Within the LgA group, MPZP decreased cocaine intake in the first 10 min (loading phase) as well as in the latter session intake (maintenance phase).

CONCLUSION

The data suggest that hypersensitivity of the CRF system occurs with extended access to cocaine self-administration and that this altered CRF system may contribute to the increased motivation to self-administer cocaine that develops during psychostimulant dependence.

Estradiol regulates corticotropin-releasing hormone gene (crh) expression in a rapid and phasic manner that parallels estrogen receptor-alpha and -beta recruitment to a 3',5'-cyclic adenosine 5'-monophosphate regulatory region of the proximal crh promoter

In the central nervous system, CRH regulates several affective states. Dysregulation of neuronal crh expression in the paraventricular nucleus of the hypothalamus correlates with some forms of depression, and amygdalar crh expression may modulate levels of anxiety. Because estrogens modulate these states, we sought to determine 17beta-estradiol (E2) effects on crh expression. CRH mRNA levels were measured in the AR-5 amygdaloid cell line by RT-PCR analysis. They increased by 1 min of E2 treatment, suggesting that crh behaves as an immediate-early gene. After peaking at 3 min, CRH mRNA returned to basal levels and then increased by 60 min. To dissect some of the molecular mechanisms underlying these events, we measured occupancy of the crh promoter by estrogen receptors (ERs) and coactivators, using chromatin immunoprecipitation. Because this promoter does not contain palindromic estrogen response elements, we targeted the region of a cAMP regulatory element (CRE), implicated in crh regulation. The temporal pattern of the mRNA response was mimicked by recruitment of ERalpha and -beta, phospho-CRE-binding protein, coactivators steroid receptor coactivator-1 and CRE-binding protein-binding protein (CBP), and an increase in histone 3 and 4 acetylation. Lastly, ERalpha and -beta loading were temporally dissociated, peaking at 1 and 3 min, respectively. The ER peaks were associated with coactivators and acetylation patterns. ERalpha associated with phospho-CRE-binding protein, CBP, steroid receptor coactivator-1, and increased acetylated histone 3. ERbeta associated with CBP and increased acetylated histone 4. The tight temporal correlation between E2-induced CRH mRNA levels and promoter occupancy by ERs strongly suggest that E2 regulates crh expression through an ERalpha- and/or ERbeta-CRE alternate pathway.

Angst and the amygdala

Fear is an adaptation to danger, but excessive fear underlies diverse forms of mental anguish and pathology. One neural site linked to a sense of adversity is the amygdala, and one neuropeptide, corticotropin-releasing hormone (CRH), is localized within the central nucleus of the amygdala. Glucocorticoids enhance the production of CRH in this region of the brain, resulting in increased attention to external events and, when sustained for longer periods of times, perhaps contributing to anxious depression.

Role of dopamine D1 and D2 receptors in CRF-induced disruption of sensorimotor gating

Corticotropin-releasing factor (CRF), a neuropeptide released during stress, has been reported to modulate startle behavior, including reducing the threshold for acoustic startle responding and reducing prepulse inhibition (PPI). The central mechanisms mediating CRF system regulation of startle and PPI are still unclear. Some antipsychotic drugs attenuate CRF-induced deficits in PPI in rats and mice. Here we tested the hypothesis that indirect activation of DA(1)-receptors (D(1)) and DA(2)-receptors (D(2)) contributes to the effects of CRF on PPI. We compared the effect of central administration of h/r-CRF (0.2-0.6 nmol) on PPI in mice with either a D(1) or D(2) receptor null mutation (knockout, KO) or in mice pretreated with D(1) or D(2) receptor antagonists SCH23390 (1 mg/kg) or haloperidol (1 mg/kg). D(1) and D(2) KO mice exhibited no significant differences in their sensitivity to CRF-induced disruptions of PPI. Similarly, neither SCH23390 nor haloperidol pretreatment altered the CRF-induced disruption in PPI, although both increased PPI at baseline. CRF-induced increases in startle also remained unchanged by any of the DA receptor manipulations. These results indicate that neither D(1)- nor D(2)-receptor activation is necessary for CRF to exert its effects on acoustic startle and PPI in mice.

Role of corticotropin releasing factor in anxiety disorders: a translational research perspective

Anxiety disorders are a group of mental disorders that include generalized anxiety disorder (GAD), panic disorder, phobic disorders (e.g., specific phobias, agoraphobia, social phobia) and posttraumatic stress disorder (PTSD). Anxiety disorders are among the most common of all mental disorders and, when coupled with an awareness of the disability and reduced quality of life they convey, they must be recognized as a serious public health problem. Over 20 years of preclinical studies point to a role for the CRF system in anxiety and stress responses. Clinical studies have supported a model of CRF dysfunction in depression and more recently a potential contribution to specific anxiety disorders (i.e., panic disorder and PTSD). Much work remains in both the clinical and preclinical fields to inform models of CRF function and its contribution to anxiety. First, we will review the current findings of CRF and HPA axis abnormalities in anxiety disorders. Second, we will discuss startle reflex measures as a tool for translational research to determine the role of the CRF system in development and maintenance of clinical anxiety.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Behavioral and anatomical interactions between dopamine and corticotropin-releasing factor in the rat

The neuropeptide corticotropin-releasing factor (CRF) is believed to play a role in a number of psychiatric conditions, including anxiety disorders and depression. In the present study, male Sprague Dawley rats were used to examine the behavioral effects of altering dopamine transmission on CRF-enhanced startle, a behavioral assay believed to reflect stress- or anxiety-like states. Systemic administration of the selective dopamine D1 receptor antagonist SCH 23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride] (0, 0.01, 0.05, 0.1, 0.5 mg/kg) dose dependently blocked the effect of CRF (1 microg, i.c.v.) on startle at doses that had no effect on baseline startle response. Immunohistochemical studies showed that most CRF-containing cells in the dorsolateral division of the bed nucleus of the stria terminalis (BSTld), part of the critical brain area mediating CRF-enhanced startle, are surrounded by a dense plexus of tyrosine hydroxylase (TH)-positive fibers. Intra-BSTld injections of the retrograde tracer Fluorogold (FG) into the TH field identified neurons in the major dopaminergic areas (A8-A10), but not the major noradrenergic areas [A5, A6 (locus ceruleus), A7], as a significant source of TH-positive innervation. The majority of FG-filled cells double-labeled for TH were found in the dorsocaudal A10 cell group (A10dc) located in the periaqueductal gray area. Together, these data suggest that neuronal regulation of the BSTld by specific dopaminergic pathways and receptors may be an important mechanism for controlling CRF-dependent moods and affective states. These data also suggest that compounds with D1 receptor antagonist properties might have anxiolytic-like effects that could be useful for treating conditions associated with hyperactive CRF systems.

Acute cortisol elevations cause heightened arousal ratings of objectively nonarousing stimuli

To test the effects of cortisol on affective experience, the authors orally administered a placebo, 20 mg cortisol, or 40 mg cortisol to 85 men. Participants' affective responses to negative and neutral stimuli were measured. Self-reported affective state was also assessed. Participants in the 40-mg group (showing extreme cortisol elevations within the physiological range) rated neutral stimuli as more highly arousing than did participants in the placebo and 20-mg groups. Furthermore, within the 20-mg group, individuals with higher cortisol elevations made higher arousal ratings of neutral stimuli. However, cortisol was unrelated to self-reported affective state. Thus, findings indicate that acute cortisol elevations cause heightened arousal in response to objectively nonarousing stimuli, in the absence of effects on mood.

Role of stress, corticotrophin releasing factor (CRF) and amygdala plasticity in chronic anxiety

Stress initiates a series of neuronal responses that prepare an organism to adapt to new environmental challenges. However, chronic stress may lead to maladaptive responses that can result in psychiatric syndromes such as anxiety and depressive disorders. Corticotropin-releasing factor (CRF) has been identified as a key neuropeptide responsible for initiating many of the endocrine, autonomic and behavioral responses to stress. The amygdala expresses high concentrations of CRF receptors and is itself a major extrahypothalamic source of CRF containing neurons. Within the amygdala, the basolateral nucleus (BLA) has an important role in regulating anxiety and affective responses. During periods of stress, CRF is released into the amygdala and local CRF receptor activation has been postulated as a substrate for stress-induced alterations in affective behavior. Previous studies have suggested that synaptic plasticity in the BLA contributes to mechanisms underlying long-term changes in the regulation of affective behaviors. Several studies have shown that acute glutamate receptor-mediated activation, by either GABA-mediated disinhibition or CRF-mediated excitation, induces long-term synaptic plasticity and increases the excitability of BLA neurons. This review summarizes some of the data supporting the hypotheses that stress induced plasticity within the amygdala may be a critical step in the pathophysiology of the development of chronic anxiety states. It is further proposed that such a change in the limbic neural circuitry is involved in the transition from normal vigilance responses to pathological anxiety, leading to syndromes such as panic and post-traumatic stress disorders.

Modulation of anxiety circuits by serotonergic systems

Anxiety is a complex emotional state associated with sustained heightened autonomic and behavioral arousal and an increase in avoidance behavior. Anxiety-related behavior is a form of risk assessment behavior that is associated with a level of uncertainty or unpredictability regarding the outcome of emotionally salient events, often when both rewarding and aversive outcomes are possible. In this review, we highlight recent advances in our understanding of the neural circuits regulating anxiety states and anxiety-related behavior with an emphasis on the role of brainstem serotonergic systems in modulating anxiety-related circuits. In particular, we explore the possibility that the regulation of anxiety states and anxiety-related behavior by serotonergic systems is dependent on a specific, topographically organized mesolimbocortical serotonergic system that originates in the mid-rostrocaudal and caudal parts of the dorsal raphe nucleus.

A neuroendocrine mechanism for sustaining fear

Fear is an adaptive response to recognition of a potentially dangerous event. Glucocorticoids are essential for maintaining a wide variety of behavioral events by their regulation of numerous genes; one such gene encodes corticotrophin-releasing hormone (CRH). CRH is involved in diverse behavioral responses to changing environmental demands. In this review, we focus on one aspect of glucocorticoid regulation of CRH--namely, fear-related responses to diverse classes of adverse events, such as those represented by contextual and cue-specific stimuli. Three extra-hypothalamic forebrain sites appear crucial for fear-related behavioral responses: the amygdala and the bed nucleus of the stria terminalis for sustaining adaptive fear-related behaviors, and the medial prefrontal cortex for modulating fear-related behaviors. Central regulation of CRH by glucocorticoids is important for adaptive and sustained fear-related behaviors, and its aberration is associated with anxiety and depressive disorders.

Tissue plasminogen activator in the bed nucleus of stria terminalis regulates acoustic startle

The bed nucleus of stria terminalis is a basal forebrain region involved in regulation of hormonal and behavioral responses to stress. In this report we demonstrate that bed nucleus of stria terminalis has a high and localized expression of tissue plasminogen activator, a serine protease with neuromodulatory properties and implicated in neuronal plasticity. Tissue plasminogen activator activity in the bed nucleus of stria terminalis is transiently increased in response to acute restraint stress or i.c.v. administration of a major stress mediator, corticotropin-releasing factor. We show that tissue plasminogen activator is important in bed nucleus of stria terminalis function using two criteria: 1, Neuronal activation in this region as measured by c-fos induction is reduced in tissue plasminogen activator-deficient mice; and 2, a bed nucleus of stria terminalis-dependent behavior, potentiation of acoustic startle by corticotropin-releasing factor, is attenuated in tissue plasminogen activator-deficient mice. These studies identify a novel site of tissue plasminogen activator expression in the mouse brain and demonstrate a functional role for this protease in the bed nucleus of stria terminalis.

Progesterone attenuates corticotropin-releasing factor-enhanced but not fear-potentiated startle via the activity of its neuroactive metabolite, allopregnanolone

Intact female rats and ovariectomized (OVX) rats with different ovarian steroid replacement regimens were tested for changes in corticotropin-releasing factor (CRF)-enhanced startle (increased acoustic startle amplitude after intracerebroventricular infusion of 1 mug of CRF). OVX rats injected with estradiol (E) followed by progesterone (P) showed a blunted CRF-enhanced startle effect compared with OVX and E-injected rats. CRF-enhanced startle also was reduced significantly in lactating females (high endogenous P levels) compared with cycling rats (low to moderate P levels), as well as in non-E-primed rats when P was administered acutely (4 hr before testing) or chronically (7 d P replacement). The ability of P to attenuate CRF-enhanced startle was probably mediated by its metabolite allopregnanolone [tetrahydroprogesterone (THP)], because THP itself had a similar effect, and chronic administration of medroxyprogesterone, which is not metabolized to THP, did not blunt CRF-enhanced startle but instead slightly increased it. These data suggest that P blunts CRF-enhanced startle through a mechanism involving its neuroactive metabolite THP, although a role for the P receptor cannot be completely ruled out. Finally, neither chronic P replacement nor acute THP affected fear-potentiated startle, suggesting that P metabolites have an effect on the bed nucleus of the stria terminalis and anxiety rather than on the amygdala and stimulus-specific fear.

Stress-induced reductions of sensory reactivity in female rats depend on ovarian hormones and the application of a painful stressor

The current experiments occurred in the context of disputes in the literature concerning whether inescapable stress causes differential changes in sensory reactivity, which could lead to differences in many learning procedures. We tested rats for differences in sensitivity and responsivity to acoustic stimuli through the use of the acoustic startle response (ASR) 2 h after stressor exposure and ambulatory activity 24 h later in the open field. Stressed females showed reduced responsivity to acoustic stimuli with no apparent shift in stimulus sensitivity. Males did not show differences in either reactivity index following stressor exposure. Reduced responsivity did not occur if females had been OVX (OVX alone did not effect stimulus responsivity or sensitivity). All groups that experienced tailshock stress also had reduced open field activity 24 h later. Restraint for 2 h did not reduce stimulus responsivity in the ASR or open field activity in female rats. Acute reductions in ASRs after a painful stressor appear to be a feature specific to females, with an apparent role of ovarian hormones as a modulator of the effect. Possible hormone and/or immunological mechanisms of these sex-specific effects are discussed. Understanding the mechanisms of this stressor-induced reduction in sensory reactivity could advance our knowledge of how individual differences in ovarian hormone levels influence the physical and psychological processes by which females acutely respond and later recover from traumatic events.

A stress-induced anxious state in male rats: corticotropin-releasing hormone induces persistent changes in associative learning and startle reactivity

BACKGROUND

Exposure to intense inescapable stressors induces a persistent anxious state in rats. The anxious state is evident as increased sensory reactivity and enhanced associative learning.

METHODS

We examine whether similar neurobehavioral changes are observed after intracerebroventricular (ICV) administration of corticotropin releasing hormone (CRH). Two behaviors were observed: acoustic startle responses (ASRs) and acquisition of the classically conditioned eyeblink response. Male Sprague-Dawley rats were administered ICV CRH either in a single dose (1.0 microg/rat) or in three doses each separated by 30 min.

RESULTS

Exaggerated ASRs were evident 2 hours after either CRH treatment; however, only the rats given three injections exhibited a persistently exaggerated ASR apparent 24 hours after CRH treatment. Rats administered three injections of CRH also exhibited faster acquisition of the eyeblink conditioned response beginning 24 hours after treatment. Yet, we did not find evidence for a persistent activation of the HPA-axis response; three CRH injections did not lead to elevated basal plasma corticosterone levels the following morning.

CONCLUSIONS

Repeated treatment with CRH over a 1.5-hour period models some of the behavioral changes observed after exposure to intense inescapable stressors.

Buprenorphine and a CRF1 antagonist block the acquisition of opiate withdrawal-induced conditioned place aversion in rats

Conditioned place aversion in rats has face validity as a measure of the aversive stimulus effects of opiate withdrawal that reflects an important motivational component of opiate dependence. The purpose of the present study was to validate conditioned place aversion as sensitive to medications that will alleviate the aversive stimulus effects of opiate withdrawal in humans, and to extend this model to the exploration of the neuropharmacological basis of the motivational effects of opiate withdrawal. Male Sprague-Dawley rats were implanted with two subcutaneous morphine pellets and 5 days later began place conditioning training following subcutaneous administration of a low dose of naloxone. Animals were subjected to three pairings of a low dose of naloxone (15 microg/kg, s.c.) to one arm of a three-chambered place conditioning apparatus. Buprenorphine administered prior to each pairing dose-dependently blocked the place aversion produced by precipitated opiate withdrawal. A corticotropin-releasing factor-1 (CRF1) receptor antagonist (antalarmin) also reversed the place aversion produced by precipitated opiate withdrawal. Antalarmin did not produce a place preference or place aversion by itself in morphine-dependent rats. No effect was observed with pretreatment of the dopamine partial agonist terguride or the selective serotonin reuptake inhibitor fluoxetine. Also, chronic pretreatment with acamprosate (a glutamate receptor modulator used to prevent relapse in alcohol dependence) did not alter naloxone-induced place aversion. Buprenorphine by itself in dependent rats produced a mild place preference at low doses and a mild place aversion at higher doses. These results suggest that buprenorphine blocks the aversive stimulus effects of precipitated opiate withdrawal in rats and provides some validity for the use of place conditioning as a measure that is sensitive to potential opiate-dependence medications. In addition, these results suggest that CRF1 antagonists can block the aversive stimulus effects of opiate withdrawal and may be potential therapeutic targets for opiate dependence.

Stress induces CRF release in the paraventricular nucleus, and both CRF and GABA release in the amygdala

In the hypothalamus, corticotropin-releasing factor (CRF) initiates the hypothalamic-pituitary-adrenal (HPA) axis response to stress, resulting in the release of glucocorticoids, including cortisol. Extrahypothalamic CRF, particularly in the limbic system, also appears to play a role in the stress response. To further define brain CRF response to stress, immunosensor-based microdialysis probes were used to measure the extracellular levels of CRF in the paraventricular nucleus of the hypothalamus (PVN) and in the amygdala of sheep during a predator (dog) exposure stress. In addition, gamma amino butyric acid (GABA) was measured in the amygdala and cortisol was measured in venous blood. Exposure to the predator stress increased CRF in the PVN and both CRF and GABA in the amygdala. These were followed in time by a rise in venous cortisol. Application of a CRF antagonist to the amygdala, immediately prior to stress, had a small effect on the subsequent observed stress responses. This treatment, however, significantly reduced the responses to a repeat stress administered 2 days later, compared to nontreated animals. Application of a GABA antagonist to the amygdala prior to stress had no effect on the subsequent observed stress response but increased the response to the stress repeated 2 days later. Perfusion with 4-aminopyridine, a neuronal depolarising agent, into the PVN induced a release of CRF accompanied shortly thereafter by a small increase in CRF in the amygdala, and 5-10 min later by an increase in venous cortisol. Perfusion into the amygdala increased the levels of both CRF and GABA but had no effect on either PVN CRF or venous cortisol. These data support roles for both the PVN and amygdala in stress responsiveness. It suggests further that actions at the amygdala can strongly influence subsequent responsiveness to a further stress, mediated in part by both CRF and GABA actions.

Corticotrophin releasing factor-induced synaptic plasticity in the amygdala translates stress into emotional disorders

The amygdala is involved in the associative processes for both appetitive and aversive emotions, and its function is modulated by stress hormones. The neuropeptide corticotrophin releasing factor (CRF) is released during stress and has been linked to many stress-related behavioral, autonomic, and endocrine responses. In the present study, nonanxiety-inducing doses of a potent CRF type 1 and 2 receptor agonist, urocortin (Ucn), was infused locally into the basolateral amygdala (BLA) of rats. After 5 daily injections of Ucn, the animals developed anxiety-like responses in behavioral tests. Intravenous administration of the anxiogenic agent sodium lactate elicited robust increases in blood pressure, respiratory rate, and heart rate. Furthermore, in the absence of any additional Ucn treatment, these behavioral and autonomic responses persisted for >30 d. Whole-cell patch-clamp recordings from BLA neurons of these hyper-reactive animals revealed a pronounced reduction in both spontaneous and stimulation-evoked IPSPs, leading to a hyperexcitability of the BLA network. This Ucn-induced plasticity appears to be dependent on NMDA receptor and subsequent calcium-calmodulin-dependent protein kinase II (CaMKII) activation, because it is blocked by pretreatment with NMDA receptor antagonists and by coadministration of CaMKII inhibitors. Our results show for the first time a stress peptide-induced behavioral syndrome that can be correlated with cellular mechanisms of neural plasticity, a novel mechanism that may explain the etiological role of stress in several chronic psychiatric and medical disorders.

Corticosterone controls the developmental emergence of fear and amygdala function to predator odors in infant rat pups

In many altricial species, fear responses such as freezing do not emerge until sometime later in development. In infant rats, fear to natural predator odors emerges around postnatal day (PN) 10 when infant rats begin walking. The behavioral emergence of fear is correlated with two physiological events: functional emergence of the amygdala and increasing corticosterone (CORT) levels. Here, we hypothesize that increasing corticosterone levels influence amygdala activity to permit the emergence of fear expression. We assessed the relationship between fear expression (immobility similar to freezing), amygdala function (c-fos) and the level of corticosterone in pups in response to presentation of novel male odor (predator), littermate odor and no odor. CORT levels were increased in PN8 pups (no fear, normally low CORT) by exogenous CORT (3 mg/kg) and decreased in PN12 pups (express fear, CORT levels higher) through adrenalectomy and CORT replacement. Results showed that PN8 expression of fear to a predator odor and basolateral/lateral amygdala activity could be prematurely evoked with exogenous CORT, while adrenalectomy in PN12 pups prevented both fear expression and amygdala activation. These results suggest that low neonatal CORT level serves to protect pups from responding to fear inducing stimuli and attenuate amygdala activation. This suggests that alteration of the neonatal CORT system by environmental insults such as alcohol, stress and illegal drugs, may also alter the neonatal fear system and its underlying neural control.