ICV-CRH alters stress-induced freezing behavior without affecting pain sensitivity

Central regulation of stress-evoked peripheral immune responses

Stress-linked psychiatric disorders, including anxiety and major depressive disorder, are associated with systemic inflammation. Recent studies have reported stress-induced alterations in haematopoiesis that result in monocytosis, neutrophilia, lymphocytopenia and, consequently, in the upregulation of pro-inflammatory processes in immunologically relevant peripheral tissues. There is now evidence that this peripheral inflammation contributes to the development of psychiatric symptoms as well as to common co-morbidities of psychiatric disorders such as metabolic syndrome and immunosuppression. Here, we review the specific brain and spinal regions, and the neuronal populations within them, that respond to stress and transmit signals to peripheral tissues via the autonomic nervous system or neuroendocrine pathways to influence immunological function. We comprehensively summarize studies that have employed retrograde tracing to define neurocircuits linking the brain to the bone marrow, spleen, gut, adipose tissue and liver. Moreover, we highlight studies that have used chemogenetic or optogenetic manipulation or intracerebroventricular administration of peptide hormones to control somatic immune responses. Collectively, this growing body of literature illustrates potential mechanisms through which stress signals are conveyed from the CNS to immune cells to regulate stress-relevant behaviours and comorbid pathophysiology.

Human defensive freezing: Associations with hair cortisol and trait anxiety

The anticipation of threat facilitates innate defensive behaviours including freezing reactions. Freezing in humans is characterised by reductions in body sway and heart rate. Limited evidence suggests that individual differences in freezing reactions are associated with predictors of anxiety-related psychopathology including trait anxiety and hypothalamic-pituitary-adrenal (HPA) axis activity. However, previous human studies focused on acutely circulating cortisol levels, leaving the link between freezing and more stable, individual trait markers of HPA axis activity unclear. We investigated whether individual differences in anticipatory freezing reactions are predicted by accumulated hair cortisol concentrations (HCC) and trait anxiety, in a well-powered mixed sample of police recruits at the start of the police training, and age, sex and education matched controls (total N = 419, mean age = 24, N_women = 106, N_{police recruits} = 337). Freezing-related reactions were assessed with posturographic and heart rate measurements during an active shooting task under threat of shock. The anticipation of threat of shock elicited the expected reductions in body sway and heart rate, indicative of human freezing. Individual differences in threat-related reductions in body sway, but not heart rate, were related to lower HCC and higher trait anxiety. The observed links between postural freezing and predictors of anxiety-related psychopathology suggest the potential value of defensive freezing as a somatic marker for individual differences in stress-vulnerability and resilience. DATA AVAILABILITY: The datasets analysed during the current study are available from the corresponding authors upon reasonable request.

Role of corticotropin-releasing factor in alcohol and nicotine addiction

The two most prevalent substance use disorders involve alcohol and nicotine, which are often co-abused. Robust preclinical and translational evidence indicates that individuals initiate drug use for the acute rewarding effects of the substance. The development of negative emotional states is key for the transition from recreational use to substance use disorders as subjects seek the substance to obtain relief from the negative emotional states of acute withdrawal and protracted abstinence. The neuropeptide corticotropin-releasing factor (CRF) is a major regulator of the brain stress system and key in the development of negative affective states. The present review examines the role of CRF in preclinical models of alcohol and nicotine abuse and explores links between CRF and anxiety-like, dysphoria-like, and other negative affective states. Finally, the present review discusses preclinical models of nicotine and alcohol use with regard to the CRF system, advances in molecular and genetic manipulations of CRF, and the importance of examining both males and females in this field of research.

Freeze for action: neurobiological mechanisms in animal and human freezing

Upon increasing levels of threat, animals activate qualitatively different defensive modes, including freezing and active fight-or-flight reactions. Whereas freezing is a form of behavioural inhibition accompanied by parasympathetically dominated heart rate deceleration, fight-or-flight reactions are associated with sympathetically driven heart rate acceleration. Despite the potential relevance of freezing for human stress-coping, its phenomenology and neurobiological underpinnings remain largely unexplored in humans. Studies in rodents have shown that freezing depends on amygdala projections to the brainstem (periaqueductal grey). Recent neuroimaging studies in humans have indicated that similar brain regions may be involved in human freezing. In addition, flexibly shifting between freezing and active defensive modes is critical for adequate stress-coping and relies on fronto-amygdala connections. This review paper presents a model detailing these neural mechanisms involved in freezing and the shift to fight-or-flight action. Freezing is not a passive state but rather a parasympathetic brake on the motor system, relevant to perception and action preparation. Study of these defensive responses in humans may advance insights into human stress-related psychopathologies characterized by rigidity in behavioural stress reactions. The paper therefore concludes with a research agenda to stimulate translational animal–human research in this emerging field of human defensive stress responses.

This article is part of the themed issue ‘Movement suppression: brain mechanisms for stopping and stillness’.

Defensive freezing links Hypothalamic-Pituitary-Adrenal-axis activity and internalizing symptoms in humans

The Hypothalamic-Pituitary-Adrenal (HPA)-axis plays an important role in the expression of defensive freezing. Adaptive freezing reactivity, characterized by an immediate increase in acute stress and timely termination upon threat offset or need to act, is essential for adequate stress coping. Blunted HPA-axis activity in animals is associated with blunted freezing reactivity and internalizing symptoms. Despite their potential relevance, it remains unknown whether these mechanisms apply to humans and human psychopathology. Using a well-established method combining electrocardiography and posturography, we assessed freezing before, immediately after, and one hour after a stress induction in 92 human adolescents. In line with animal models, human adolescents showed stress-induced freezing, as quantified by relative reductions in heart rate and body sway after, as compared to before, stress. Moreover, relatively lower basal cortisol was associated with reduced stress-induced freezing reactivity (i.e., less immediate freezing and less recovery). Path analyses showed that decreased freezing recovery in individuals with reduced cortisol levels was associated with increased levels of internalizing symptoms. These findings suggest that reduced freezing recovery may be a promising marker for the etiology of internalizing symptoms.

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.

GABA and NMDA receptors in CRF neurons have opposing effects in fear acquisition and anxiety in central amygdala vs. bed nucleus of the stria terminalis

This article is part of a Special Issue "SBN 2014". Beginning with Vale and Colleagues in 1981, corticotropin releasing factor (CRF) also called corticotropin releasing hormone (CRH) has repeatedly been identified as an important contributor to fear and anxiety behavior. These findings have proven useful to further our understanding of disorders that have significant fear-dysregulation, such as post-traumatic stress, as well as other stress- and anxiety-related disorders. Unfortunately, the data are not all in agreement. In particular the role of CRF in fear learning is controversial, with studies pointing to contradictory effects from CRF manipulation even within the same brain structure. Further, very few studies address the potentially promising role of CRF manipulation in fear extinction behavior. Here, we briefly review the role of CRF in anxiety, fear learning and extinction, focusing on recent cell-type and neurotransmitter-specific studies in the amygdala and bed nucleus of the stria terminalis (BNST) that may help to synthesize the available data on the role of CRF in fear and anxiety-related behaviors.

Drug withdrawal conceptualized as a stressor

Drug withdrawal is often conceptualized as an aversive state that motivates drug-seeking and drug-taking behaviors in humans. Stress is more difficult to define, but is also frequently associated with aversive states. Here we describe evidence for the simple theory that drug withdrawal is a stress-like state, on the basis of common effects on behavioral, neurochemical, and molecular endpoints. We also describe data suggesting a more complex relationship between drug withdrawal and stress. As one example, we will highlight evidence that, depending on drug class, components of withdrawal can produce effects that have characteristics consistent with mood elevation. In addition, some stressors can act as positive reinforcers, defined as having the ability to increase the probability of a behavior that produces it. As such, accumulating evidence supports the general principles of opponent process theory, whereby processes that have an affective valence are followed in time by an opponent process that has the opposite valence. Throughout, we identify gaps in knowledge and propose future directions for research. A better understanding of the similarities, differences, and overlaps between drug withdrawal and stress will lead to the development of improved treatments for addiction, as well as for a vast array of neuropsychiatric conditions that are triggered or exacerbated by stress.

Effects of intracerebroventricular corticotropin releasing factor on sensory-evoked responses in the rat visual thalamus

Corticotropin releasing factor (CRF) coordinates the brain׳s responses to stress. Recent evidence suggests that CRF-mediated activation of the locus coeruleus-norepinephrine (LC-NE) system contributes to alterations in sensory signal processing during stress. However, it remains unclear whether these actions are dependent upon the degree of CRF release. Using intracerebroventricular (ICV) infusions, we examine the dose-dependent actions of CRF on sensory-evoked discharges of neurons in the dorsal lateral geniculate nucleus of the thalamus (dLGN). The LGN is the primary relay for visual signals from retina to cortex, receiving noradrenergic modulation from the LC. In vivo extracellular recording in anesthetized rats was used to monitor single dLGN neuron responses to light flashes at three different stimulus intensities before and after administration of CRF (0.1, 0.3, 1.0, 3.0 or 10.0 μg). CRF produced three main effects on dLGN stimulus evoked activity: (1) increased magnitude of sensory evoked discharges at moderate doses, (2) decreased response latency, and (3) dose-dependent increases in the number of cells responding to a previously sub-threshold (low intensity) stimulus. These modulatory actions were blocked or attenuated by intra-LC infusion of a CRF antagonist prior to ICV CRF administration. Moreover, intra-LC administration of CRF (10 ng) mimicked the facilitating effects of moderate doses of ICV CRF on dLGN neuron responsiveness to light stimuli. These findings suggest that stressor-induced changes in sensory signal processing cannot be defined in terms of a singular modulatory effect, but rather are multi-dimensional and dictated by variable degrees of activation of the CRF-LC-NE system.

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.

Behavioral Studies and Genetic Alterations in Corticotropin-Releasing Hormone (CRH) Neurocircuitry: Insights into Human Psychiatric Disorders

To maintain well-being, all organisms require the ability to re-establish homeostasis in the presence of adverse physiological or psychological experiences. The regulation of the hypothalamic-pituitary adrenal (HPA) axis during stress is important in preventing maladaptive responses that may increase susceptibility to affective disorders. Corticotropin-releasing hormone (CRH) is a central stress hormone in the HPA axis pathway and has been implicated in stress-induced psychiatric disorders, reproductive and cardiac function, as well as energy metabolism. In the context of psychiatric disorders, CRH dysfunction is associated with the occurrence of post-traumatic stress disorder, major depression, anorexia nervosa, and anxiety disorders. Here, we review the synthesis, molecular signaling and regulation, as well as synaptic activity of CRH. We go on to summarize studies of altered CRH signaling in mutant animal models. This assembled data demonstrate an important role for CRH in neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation. Next, we present findings regarding human genetic polymorphisms in CRH pathway genes that are associated with stress and psychiatric disorders. Finally, we discuss a role for regulators of CRH activity as potential sites for therapeutic intervention aimed at treating maladaptive behaviors associated with stress.

Therapeutic utility of non-peptidic CRF1 receptor antagonists in anxiety, depression, and stress-related disorders: evidence from animal models

Adaptive responding to threatening stressors is of fundamental importance for survival. Dysfunctional hyperactivation of corticotropin releasing factor type-1 (CRF(1)) receptors in stress response system pathways is linked to stress-related psychopathology and CRF(1) receptor antagonists (CRAs) have been proposed as novel therapeutic agents. CRA effects in diverse animal models of stress that detect anxiolytics and/or antidepressants are reviewed, with the goal of evaluating their potential therapeutic utility in depression, anxiety, and other stress-related disorders. CRAs have a distinct phenotype in animals that has similarities to, and differences from, those of classic antidepressants and anxiolytics. CRAs are generally behaviorally silent, indicating that CRF(1) receptors are normally in a state of low basal activation. CRAs reduce stressor-induced HPA axis activation by blocking pituitary and possibly brain CRF(1) receptors which may ameliorate chronic stress-induced pathology. In animal models sensitive to anxiolytics and/or antidepressants, CRAs are generally more active in those with high stress levels, conditions which may maximize CRF(1) receptor hyperactivation. Clinically, CRAs have demonstrated good tolerability and safety, but have thus far lacked compelling efficacy in major depressive disorder, generalized anxiety disorder, or irritable bowel syndrome. CRAs may be best suited for disorders in which stressors clearly contribute to the underlying pathology (e.g. posttraumatic stress disorder, early life trauma, withdrawal/abstinence from addictive substances), though much work is needed to explore these possibilities. An evolving literature exploring the genetic, developmental and environmental factors linking CRF(1) receptor dysfunction to stress-related psychopathology is discussed in the context of improving the translational value of current animal models.

Transient forebrain over-expression of CRF induces plasma corticosterone and mild behavioural changes in adult conditional CRF transgenic mice

BACKGROUND

Converging findings support a role for extra-hypothalamic CRF in the mediation of the stress response. The influence of CRF in the amygdala is well established, while less is known of its role in other areas of the forebrain where CRF and CRF(1) receptors are also expressed. In the present study CRF was genetically induced to allow forebrain-restricted expression in a temporally-defined manner at any time during the mouse lifespan. This mouse model may offer the possibility to establish a model of the pathogenesis of recurrent episodes of depression.

METHODS

Mice were engineered to carry both the rtTA transcription factor driven by the CamKII alpha promoter and the doxycycline-regulated operator (tetO) upstream of the CRF coding sequence. Molecular, biochemical and behavioural characterisation of this mouse is described.

RESULTS

Following a three-week period of transcriptional induction, double transgenic mice showed approximately 2-fold increased expression of CRF mRNA in the hippocampus and cortex, but not hypothalamus. These changes were associated with 2-fold increase in morning corticosterone levels, although responses to the dexamethasone suppression test or acute stress were unaffected. In contrast, induced mice displayed modestly altered behaviour in the Light and Dark test and Forced Swim test.

CONCLUSIONS

Transient induction of CRF expression in mouse forebrain was associated with endocrine and mild anxiety-like behavioural changes consistent with enhanced central CRF neurotransmission. This mouse allows the implementation of regimens with longer or repeated periods of induction which may model the initial stages of the pathology underlying recurrent depressive disorders.

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.

Effects of chlordiazepoxide on footshock- and corticotropin-releasing factor-induced increases in cortical and hypothalamic norepinephrine secretion in rats

Noradrenergic and corticotropin-releasing factor (CRF) neuronal systems within the brain have been implicated in stress and anxiety. Synaptic release of cerebral norepinephrine (NE) is increased during stress, and following intracerebral CRF administration. Benzodiazepines are commonly used anxiolytic drugs but information on their effects on the stress- and CRF-related release of NE is limited. We have used in vivo microdialysis to test the effects of the benzodiazepine, chlordiazepoxide (CDP) on the noradrenergic responses to footshock and intracerebroventricular CRF in the medial hypothalamus and the medial prefrontal cortex (PFM) of freely moving rats. Footshock (60 x 0.1-0.2 mA shocks in 20 min) significantly increased microdialysate concentrations of NE in the first sample collected after initiating the footshock. In the hypothalamus, microdialysate NE was augmented 64% above baseline. A second footshock session (100 min after the first footshock) increased microdialysate NE to 313% of the baseline. Thus the noradrenergic responses to footshock were enhanced by preceding footshocks. CRF (100 ng) administered into the locus coeruleus (LC) almost tripled microdialysate concentrations of NE in the PFM. CDP (5mg/kg, i.p.) had no statistically significant effects on the basal dialysate concentrations of NE, but it significantly attenuated both footshock- and CRF-induced increases in dialysate NE. CDP may exert a direct inhibitory effect on the noradrenergic neurons, alter the input to LC noradrenergic neurons, or alter the ability of CRF to activate the LC noradrenergic system.

Inhibition of inflammatory pain by CRF at peripheral, spinal and supraspinal sites: involvement of areas coexpressing CRF receptors and opioid peptides

There is conflicting evidence on the antinociceptive effects of corticotropin-releasing factor (CRF) along the neuraxis of pain transmission and the responsible anatomical sites of CRF's action at the level of the brain, spinal cord and periphery. In an animal model of tonic pain, that is, Freunds complete adjuvant (FCA) hindpaw inflammation, we systematically investigated CRF's ability to modulate inflammatory pain at those three levels of pain transmission by algesiometry following the intracerebroventricular, intrathecal, and intraplantar application of low, systemically inactive doses of CRF. At each level, CRF elicits potent antinociceptive effects, which are dose dependent and antagonized by local, but not systemic CRF receptor antagonist alpha-helical CRF indicating CRF receptor specificity. Consistently, we have identified by immunohistochemistry multiple brain areas, inhibitory interneurons within the dorsal horn of the spinal cord as well as immune cells within subcutaneous tissue--but not peripheral sensory neurons--that coexpress both CRF receptors and opioid peptides. In line with these anatomical findings, local administration of CRF together with the opioid receptor antagonist naloxone dose-dependently reversed CRF's antinociceptive effects at each of these three levels of pain transmission. Therefore, local application of low, systemically inactive doses of CRF at the level of the brain, spinal cord and periphery inhibits tonic inflammatory pain most likely through an activation of CRF receptors on cells that coexpress opioid peptides which results in opioid-mediated pain inhibition. Future studies have to delineate whether endogenous CRF at these three levels contributes to the body's response to cope with the stressful stimulus pain in an opioid-mediated manner.

The role of corticotropin-releasing factor in behavioural responses to stress

Corticotropin-releasing factor (CRF), when administered directly into the CNS, can have activating properties on behaviour and can enhance behavioural responses to stress. CRF injected intraventricularly produces a dose-dependent increase in locomotor activity and increased responsiveness to an acoustic startle stimulus. However, this profile of activation changes to enhanced suppression of behaviour in stressful situations and includes increased freezing, increased conditioned suppression, increased conflict, decreased feeding and decreased behaviour in a novel open field. These effects of CRF are independent of the pituitary-adrenal axis and can be reversed by the CRF antagonist alpha-helical CRF(9-41). More importantly, the CRF antagonist can also reverse many behavioural responses to stressors. alpha-Helical CRF(9-41) reverses stress-induced fighting behaviour, stress-induced freezing, stress-induced suppression of feeding, stress-induced decreases in exploration of an elevated plus maze, fear-potentiated startle and the development of conditioned suppression. Intracerebral microinjections suggest that the amygdala may be an important site for the anti-stress effects of alpha-helical CRF(9-41). These results suggest that endogenous CRF systems in the CNS may have a role in mediating behavioural responses to stress and further suggest that CRF in the brain may function as a fundamental behavioural activating system. This CRF system may be particularly important in situations where an organism must mobilize not only the pituitary-adrenal system but also the CNS in response to environmental challenge.

Long-term consequences of neonatal rearing on central corticotropin-releasing factor systems in adult male rat offspring

In a series of studies on the long-term consequences of neonatal rearing, we compared hypothalamic and extrahypothalamic central corticotropin-releasing factor (CRF) systems in male rats reared under conditions of animal facility rearing, nonhandling (HMS0), handling with brief maternal separation for 15 min (HMS15), or handling with moderate maternal separation for 180 min (HMS180) daily from postnatal days 2-14. CRF-like immunoreactivity (CRFir) was elevated in lumbar cerebrospinal fluid of adult HMS180 and HMS0 rats relative to the other groups. In the paraventricular nucleus, central nucleus of the amygdala, bed nucleus of the stria terminalis, and locus coeruleus, CRFir and CRF mRNA levels were significantly elevated in HMS0 and HMS180 rats. Neonatal maternal separation was associated with regionally specific alterations in CRF receptor type 1 (CRF1) mRNA density in HMS180 rats. No rearing-associated differences in CRF2alpha binding were apparent in either the lateral septum or the ventromedial hypothalamus. These findings indicate that early rearing conditions can permanently alter the developmental set-point of central CRF systems, and potentially influence the expression of behavioral and endocrine responses to stress throughout life, thereby providing a possible neurobiological substrate for the relationship between early life events and increased vulnerability for hypothalamic-pituitary-adrenal axis and coping skill alterations and the frequency of mood disorders in patients with a history of such experiences.

The therapeutic potential of CRF1 antagonists for anxiety

Preclinical studies suggest that the brain corticotropin-releasing factor (CRF) systems mediate anxiety-like behavioural and somatic responses through actions at the CRF1 receptor. CRF1 antagonists block the anxiogenic-like effects of CRF and stress in animal models. Cerebrospinal fluid levels of CRF are elevated in some anxiety disorders and normalise with effective treatment, further implicating CRF systems as a therapeutic target. Prototypical CRF1 antagonists are highly lipophilic, non-competitive antagonists of peptide ligands. Modification of the chemotype and the identification of novel pharmacophores are yielding more drug-like structures with increased hydrophilicity at physiological pHs. Newer compounds exhibit improved solubility, pharmacokinetic properties, potency and efficacy. Several clinical candidates have entered Phase I/II trials. However, unmet challenges await resolution during further discovery, clinical development and therapeutic application of CRF1 antagonists.

Effects of the traditional Japanese medicine Unkei-to on the corticotropin-releasing factor-induced increase in locomotor activity

The effect of Unkei-to, a traditional Japanese herbal medicine and strong in vitro releaser of cytokine-induced neutrophil chemoattractant (CINC), on the increase in locomotor activity induced by intracerebroventricular (icv) injection of corticotropin-releasing factor (CRF) in male rats in a familiar environment was investigated. Oral administration of Unkei-to (100 mg/kg) for 1 week significantly attenuated the CRF-induced increase in locomotor activity. Unkei-to also reduced the CRF-induced accumulation of hypothalamic CINC, which has a functional antagonistic action on the response to CRF; the reduction may reflect an increased release of CINC. These results suggest that Unkei-to has an alleviative effect on the action induced by brain CRF and the mechanism of this effect may partly involve CINC.

Corticotropin-Releasing Factor in Brain: A Role in Activation, Arousal, and Affect Regulation

Organisms exposed to challenging stimuli that alter the status quo inside or outside of the body are required for survival purposes to generate appropriate coping responses that counteract departures from homeostasis. Identification of an executive control mechanism within the brain capable of coordinating the multitude of endocrine, physiological, and functional coping responses has high utility for understanding the response of the organism to stressor exposure under normal or pathological conditions. The corticotropin-releasing factor (CRF)/urocortin family of neuropeptides and receptors constitutes an affective regulatory system due to the integral role it plays in controlling neural substrates of arousal, emotionality, and aversive processes. In particular, available evidence from pharmacological intervention in multiple species and phenotyping of mutant mice shows that CRF/urocortin systems mediate motor and psychic activation, stimulus avoidance, and threat recognition responses to aversive stimulus exposure. It is suggested that affective regulation is exerted by CRF/urocortin systems within the brain based upon the sensitivity of local brain sites to CRF/urocortin ligand administration and the appearance of hypothalamo-pituitary-adrenocortical activation following stressor exposure. Moreover, these same stress neuropeptides may constitute a mechanism for learning to avoid noxious stimuli by facilitating the formation of so-called emotional memories. A conceptual framework is provided for extrapolation of animal model findings to humans and for viewing CRF/urocortin activation as a continuum measure linking normal and pathological states.

Light-enhanced and fear-potentiated startle: temporal characteristics and effects of alpha-helical corticotropin-releasing hormone

BACKGROUND

It has been suggested that the light-enhanced startle paradigm (LES) is an animal model for anxiety, because of the unconditioned and nonspecific cue and the long-term effect. In contrast, the fear-potentiated startle (FPS) is suggested to model fear. In the present study, we assessed in detail the time course of LES and FPS and investigated whether corticotropin-releasing hormone (CRH) is differentially involved in these two models.

METHODS

In experiment 1, the amplitude of the startle response was tracked in the presence of the light and after light offset, in both models. In experiment 2, the effects of intracerebroventricular administration of the CRH-receptor antagonist alpha-helical CRH (0, 1, 5, and 25 microg) on LES and FPS were studied.

RESULTS

In LES, light onset resulted in a long-lasting potentiation of the startle response and a slow return to baseline after light offset. In FPS, the potentiation of the startle response returned to baseline almost immediately after light offset. Alpha-helical CRH reduced the potentiation in LES at the 5-microg dose but not at 25 microg. In FPS, alpha-helical CRH had no effect.

CONCLUSIONS

The results show that the time course of LES is markedly different from that of FPS, which together with the differences in eliciting stimuli suggest that they model anxiety and fear, respectively. Moreover, the results suggest that CRH is involved in LES and not in FPS.

Hippocampal serotonergic system is involved in anxiety-like behavior induced by corticotropin-releasing factor

To clarify the interaction between anxiety-like behavior produced by corticotropin-releasing factor (CRF) and the 5-HT system, we investigated the effects of intracerebroventricular (i.c.v.) administration of CRF on an elevated plus-maze performance as indices of anxiety, measuring extracellular levels of 5-HT in the ventral hippocampus using an in vivo brain dialysis method in rats. The time spent in the open arms of the maze and the number of open arm entries were decreased in a dose-dependent manner by the administration of CRF (0.3-1.0 microg/rat). These effects of CRF were prevented by pretreatment with a 5-HT(1A) receptor agonist, 8-OH-DPAT (0.5 mg/kg, s.c.). In biochemical studies, CRF increased 5-HT release about 150-250% above baseline in the ventral hippocampus and this elevation was significantly inhibited by a CRF receptor antagonist, alpha-Helical CRF(9-41) (50 mug/rat), and 5-HT(1A) receptor agonist, 8-OH-DPAT (0.5 mg/kg, s.c.). These results suggested that the anxiety-like effect produced by CRF may have involved increased 5-HT transmission in the ventral hippocampus. Taken with the evidence for hypersecretion of CRF in patients with depression and anxiety-related disorders, our findings lead to the intriguing hypothesis that interaction between CRF and 5-HT, especially in the ventral hippocampus, plays a role in the etiology of affective and anxiety disorders.

The pharmacology of CP-154,526, a non-peptide antagonist of the CRH1 receptor: a review

Since CRH has been shown to mediate stress-induced physiological and behavioral changes, it has been hypothesized that CRH receptor antagonists may have therapeutic potential in disorders that involve excessive CRH activity. CP-154,526 and its close analog antalarmin are potent, brain-penetrable, selective nonpeptide CRH1 receptor antagonists that were discovered in an effort to develop compounds with efficacy in CNS disorders precipitated by stress. Since its discovery many investigators have used CP-154,526 as a tool to study the pharmacology of CRH and its receptors and to evaluate its therapeutic potential in a variety of CNS and peripheral disorders. Systemically-administered CP-154,526 has been demonstrated to antagonize CRH- and stress-induced neuroendocrine, neurochemical, electrophysiological, and behavioral effects. These findings support the hypothesis that CRH1 receptor antagonists may have therapeutic utility in a number of neuropsychiatric disorders. CP-154,526, as well as other CRH1 receptor antagonists that have since been discovered, have also shown activity in several preclinical models of anxiety, depression, and substance abuse, while having little effect on locomotor activity and motor function. Although these effects are on occasion inconsistent among different laboratories, clinical evaluation of CRH1 antagonists appears justified on the basis of these and clinical data implicating the involvement of CRH in several CNS disorders. The effects of CRH1 antagonists on cognition, neurodegeneration, inflammation, and the gastrointestinal system have not been as extensively characterized and additional studies will be necessary to evaluate their therapeutic potential in these areas.

An inhibitory effect of cytokine-induced neutrophil chemoattractant on corticotropin-releasing factor-induced increase in locomotor activity

To investigate whether cytokine-induced neutrophil chemoattractant (CINC) has an influence on corticotropin-releasing factor (CRF) in the central nervous system, the effects of intracerebroventricular (i.c.v.) injection of CINC on CRF-induced behavior were examined. Intracerebroventricular CRF injection produced an increase in locomotor activity, which was significantly reduced by i.c.v. injection of CINC. The intravenous injection of CINC did not alter CRF-induced locomotor hyperactivity. These results suggested that CINC has a functional antagonistic action on the response to CRF and may attenuate stress responses.

Dissociation of arousal-like from anxiogenic-like actions of brain corticotropin-releasing factor receptor ligands in rats

Behavioral actions of centrally administered corticotropin-releasing factor (CRF) are likely subserved by multiple brain targets and functional effector systems. The present studies compared effects of two CRF ligands, a full, post-synaptic CRF receptor agonist (rat/human CRF(1-41)) and a CRF binding protein ligand inhibitor (rat/human CRF(6-33)) in a behavioral testing battery sensitive to arousal, fear-like and aversive processes in Wistar rats. The profile of global efficacy for the centrally administered CRF receptor agonist was characterized by low dose (0.5-1.0 microg) arousal-like effects in locomotor and conditioned ambulation contexts and by high dose (5-25 microg) conditioned immobility, taste aversion and place aversion. In contrast, a profile of limited efficacy for the centrally administered CRF binding protein ligand inhibitor included only dose dependent motor activating and facilitation of fear conditioning effects without any of the anxiogenic-like or aversive properties of CRF agonist administration. These results suggest that arousal-like activation is a fundamental, physiologically relevant consequence of brain CRF system stimulation whereas aversive and anxiety-like effects reflect pharmacological actions of a CRF receptor agonist.

Attenuation of fear conditioning by antisense inhibition of brain corticotropin releasing factor-2 receptor

Corticotropin releasing factor (CRF) is an important regulator of the endocrine, behavioral, autonomic and immune responses to stress. Two high affinity CRF receptors have been identified, which are distributed in distinct anatomical regions. CRF(1) receptors have been relatively well characterized and antagonists to this receptor effectively block stress-induced behaviors in rodents. The function of CRF(2) receptors, which are highly expressed in limbic brain regions, is less well understood. Therefore, an antisense oligonucleotide approach was used to study the role of CRF(2) receptors in the lateral septum in rats. An antisense oligonucleotide directed against the CRF(2) receptor mRNA reduced expression of CRF(2) receptors by 60--80%. In shock-induced freezing tests, animals administered the antisense oligonucleotide exhibited a significant reduction in freezing duration. However, pain sensitivity and locomotor activity were unaltered. A four-base mismatch of the antisense sequence had no significant effects on CRF(2) receptor density and on freezing behavior. These data support the involvement of CRF(2) receptors in fear conditioning. CRF(1) receptor antagonists also reduce freezing in this test. Additional studies to determine the effects of simultaneous inhibition of both receptor subtypes show that rats receiving both CRF(2) receptor antisense oligonucleotide and CRF(1) receptor antagonist froze significantly less than animals treated with either agent alone. These results provide additional evidence for the role of CRF(2) receptors in mediating the stress-induced actions of endogenous CRF.

The effect of prenatal cocaine exposure on the stress response of adult mice

The neurotoxic consequences of intrauterine exposure to drugs of abuse, including cocaine, may include compromised fetal brain development with associated lasting behavioral alterations. Some infants exposed to cocaine in utero demonstrate impairments in reactivity and altered behavioral responses to stressful conditions. Alterations in arousal regulation can impact on socialization, adaptation, and educability. Moreover, such alterations may render cocaine-exposed children more vulnerable to the adverse developmental impact of stressful situations, with implications for subsequent behavior and psychopathology. Animal models facilitate the independent analysis and identification of genetic, intrauterine, and postnatal environmental factors in contributing to cocaine-induced alterations in behavioral and neurochemical responses to stressors. Utilizing a prenatal mouse model of gestational cocaine exposure we have identified a behavioral alteration evident as decreased duration of footshock-induced immobility termed "freezing" in cocaine-exposed adults as compared with controls. However, this attenuated behavioral response was not accompanied by demonstrable alterations in corticosterone response, nor was the corticosterone response altered in cocaine-exposed adults following a more protracted restraint-induced stress. The dissociation of these behavioral and neurochemical indices of altered response to stressors may provide insights regarding brain mechanisms underlying alterations in behavioral reactivity to stressful conditions following in utero cocaine exposure. In addition, this preclinical study may have implications for improved diagnostics and therapeutics for infants and children exposed to cocaine in the womb.

Stress, corticotropin-releasing factor, and drug addiction

The neuropeptide corticotropin-releasing factor (CRE) and related neuropeptides not only mediate hormonal responses to stressors but also have a neurotropic role in the central nervous system to mediate behavioral responses to stressors. CRF antagonists effectively block CRF responses and have effects opposite those of CRF in many stress-related situations. Recent advances suggest that in addition to CRF itself there is another CRF-related neuropeptide, urocortin, that may be involved in stress-related responses, particularly those involving appetite. At least two CRF receptors have been discovered to date, CRF-1 and CRF-2. CRF may be involved in various aspects of the addiction cycle associated with drugs of abuse. CRF appears to be activated during stress-induced reinstatement of drug taking as well as acute withdrawal from all major drugs of abuse. CRF is hypothesized to be part of an allostatic change leading to vulnerability to relapse during prolonged abstinence from drugs of abuse.

The role of corticotropin-releasing factor in pain and analgesia

Corticotropin-releasing factor (CRF) is a peptide that is released from the hypothalamus and in widespread areas of the brain following exposure to stressors. It is considered to be a mediator of many of the effects of stress, and its analgesic properties have been demonstrated in many studies. However, for primarily methodological reasons, the effects of CRF in the central nervous system have been neglected whereas the peripheral effects of CRF have been overemphasized. We present evidence that: (1) CRF can act at all levels of the neuraxis to produce analgesia; (2) the release of beta-endorphin does not explain the analgesia following intravenous or intracranial CRF administration; (3) inflammation must be present for local CRF to evoke analgesia and (4) the analgesic effects of CRF show specificity for prolonged pain. These findings suggest that CRF may have a significant role in chronic pain syndromes associated with hypothalamic-pituitary-adrenal axis abnormalities. Furthermore, CRF may represent a new class of analgesics that merits further study. Implications for the relationship between stress and pain are discussed.

A role for corticotropin releasing factor and urocortin in behavioral responses to stressors

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides have an important role in the central nervous system to mediate behavioral responses to stressors. CRF receptor antagonists are very effective in reversing stress-induced suppression and activation in behavior. An additional CRF-like neuropeptide, urocortin, has been identified in the brain and has a high affinity for the CRF-2 receptor in addition to the CRF-1 receptor. Urocortin has many of the effects of CRF but also is significantly more potent than CRF in decreasing feeding in both meal-deprived and free-feeding rats. In mouse genetic models, mice over-expressing CRF show anxiogenic-like responses compared to wild-type mice, and mice lacking the CRF-1 receptor showed an anxiolytic-like behavioral profile compared to wild-type mice. Results to date have led to the hypothesis that CRF-1 receptors may mediate CRF-like neuropeptide effects on behavioral responses to stressors, but CRF-2 receptors may mediate the suppression of feeding produced by CRF-like neuropeptides. Brain sites for the behavioral effects of CRF include the locus coeruleus (LC), paraventricular nucleus (PVN) of the hypothalamus, the bed nucleus of the stria terminalis (BNST), and the central nucleus of the amygdala. CRF may also be activated during acute withdrawal from all major drugs of abuse, and recent data suggest that CRF may contribute to the dependence and vulnerability to relapse associated with chronic administration of drugs of abuse. These data suggest that CRF systems in the brain have a unique role in mediating behavioral responses to diverse stressors. These systems may be particularly important in situations were an organism must mobilize not only the pituitary adrenal system, but also the central nervous system in response to environmental challenge. Clearly, dysfunction in such a fundamental brain-activating system may be the key to a variety of pathophysiological conditions involving abnormal responses to stressors such as anxiety disorders, affective disorders, and anorexia nervosa.

Corticotropin-releasing factor, norepinephrine, and stress

Corticotropin-releasing factor (CRF) and related peptides in the central nervous system appears to have activating properties on behavior and to enhance behavioral responses to stressors. CRF and urocortin injected into the brain produces increases in arousal as measured by locomotor activation and increased responsiveness to stressful stimuli. These effects of CRF appear to be independent of the pituitary adrenal axis and can be reversed by specific and selective CRF antagonists alpha-helical CRF9-41 and D-Phe CRF12-41. Perhaps more importantly, CRF antagonists can reverse behavioral responses to many stressors. These results suggest that endogenous CRF systems in the brain may have a role in mediating behavioral responses to stressors. Norepinephrine systems emanating from the nucleus locus coeruleus also long have been hypothesized to be involved in mediating behavioral constructs associated with alertness, arousal, and stress. Pharmacologic, physiologic, and neuroanatomic evidence supports an important role for a CRF-norepinephrine interaction in the region of the locus coeruleus in response to stressors that may be modality-specific where CRF neurons activate the locus coeruleus. One may hypothesize that another norepinephrine-CRF interaction may occur in the terminal projections of the forebrain norepinephrine systems in the paraventricular nucleus of the hypothalamus, the bed nucleus of the stria terminalis, and the central nucleus of the amygdala where norepinephrine stimulates CRF release. Such a feed-forward system may be particularly important in situations where an organism must mobilize not only the pituitary adrenal system but also the central nervous system, in response to environmental challenge. However, such a feed-forward mechanism in a fundamental brain-activating system may be particularly vulnerable to dysfunction and thus, may be the key to a variety of pathophysiologic conditions involving abnormal responses to stressors such as anorexia nervosa, anxiety, and affective disorders.

The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety

Neuroendocrine studies strongly suggest that dysregulation of the hypothalamic pituitary-adrenocortical (HPA) system plays a causal role in the development and course of depression. Whereas the initial mechanism resulting in HPA hyperdrive remains to be elucidated, evidence has emerged that corticosteroid receptor function is impaired in many patients with depression and in many healthy individuals at increased genetic risk for an depressive disorder. Assuming such impaired receptor function, then central secretion of CRH would be enhanced in many brain areas, which would account for a variety of depressive symptoms. As shown in rats and also in transgenic mice with impaired glucocorticoid receptor function, antidepressants enhance the signaling through corticosteroid receptors. This mechanism of action can be amplified through blocking central mechanisms that drive the HPA system. Animal experiments using antisense oligodeoxynucleotides directed against the mRNA of both CRH receptor subtypes identified the CRH1 receptor as the mediator of the anxiogenic effects of CRH. Studies in mouse mutants in which this receptor subtype had been deleted extended these findings as the animals were less anxious than wild-type mice when experimentally stressed. Thus, patients with clinical conditions that are causally related to HPA hyperactivity may profit from treatment with a CRH1 receptor antagonist.

Is there a future for neuropeptide receptor ligands in the treatment of anxiety disorders?

This review provides an overview of preclinical and clinical evidence of a role for the neuroactive peptides cholecystokinin (CCK), corticotropin-releasing factor (CRF), neuropeptide Y (NPY), tachykinins (i.e., substance P, neurokinin [NK] A and B), and natriuretic peptides in anxiety and/or stress-related disorders. Results obtained with CCK receptor antagonists in animal studies have been highly variable, and clinical trials with several of these compounds in anxiety disorders have been unsuccessful so far. However, future investigations using CCK receptor antagonists with better pharmacokinetic characteristics and animal models other than those validated with the classical anxiolytics benzodiazepines may permit a more precise evaluation of the potential of these compounds as anti-anxiety agents. Results obtained with peptide CRF receptor antagonists in animal models of anxiety convincingly demonstrated that the blockade of central CRF receptors may yield anxiolytic-like activity. However, the discovery of nonpeptide and more lipophilic CRF receptor antagonists is essential for the development of these agents as anxiolytics. Similarly, there is clear preclinical evidence that the central infusion of NPY and NPY fragments selective for the Y1 receptor display anxiolytic-like effects in a variety of tests. However, synthetic nonpeptide NPY receptor agonists are still lacking, thereby hampering the development of NPY anxiolytics. Unlike selective NK1 receptor antagonists, which have variable effects in anxiety models, peripheral administration of selective NK2 receptor antagonists and central infusion of natriuretic peptides produce clear anxiolytic-like activity. Taken as a whole, these findings suggest that compounds targeting specific neuropeptide receptors may become an alternative to benzodiazepines for the treatment of anxiety disorders.

Differential effects of CRH infusion into the central nucleus of the amygdala in the Roman high-avoidance and low-avoidance rats

Roman-high (RHA/Verh) and low (RLA/Verh) avoidance rats are selected and bred for rapid learning versus non-acquisition of two-way, active avoidance behavior in a shuttle box. RHA/Verh rats generally show a more active coping style than do their RLA/Verh counterparts when exposed to various environmental challenges. The central nucleus of the amygdala (CeA) is known to be involved in the regulation of autonomic, neuroendocrine and behavioural responses to stress and stress-free conditions, and it is considered in relation to coping strategies. Corticotropin-releasing hormone (CRH) seems to be a key factor in the control of the CeA output. Neuroanatomical studies have revealed that the majority of CRH fibers from the CeA have direct connections with autonomic regulatory nuclei in the brainstem, e.g. lateral parabrachial nucleus (lPB), ventrolateral periaquaductal gray (vlPAG). The modulating effects of CRH (30 ng) on CeA activity were studied by infusion of CRH into the CeA in freely moving male RHA/Verh and RLA/Verh rats under stress-free conditions. Heart-rate and behavioural activities were repeatedly measured before, during and after local administration of CRH or vehicle, after which early gene product FOS immunocytochemistry and CRH-mRNA in situ hybridisation were carried out in selected brain areas. CRH infusion into the CeA caused a long lasting increase in heart-rate and behavioural activation in the RHA/Verh rats, leaving the RLA/Verh rats unaffected. As a result of CRH infusion, the number of FOS positive cells in the CeA and lPB of RLA/Verh rats was increased whereas an opposite response was found in the RHA/Verh rats. However, CRH into the CeA of the Roman rat lines induced no pronounced effects on FOS staining in the vlPAG and CRH mRNA levels in the CeA. These results indicate that the CRH system of the CeA, connected with the output brainstem areas, is differentially involved in cardiovascular and behavioural responses.

Locus coeruleus electrophysiological activity and responsivity to corticotropin-releasing factor in inbred hypertensive and normotensive rats

The spontaneously hypertensive rat (SHR) and its normotensive progenitor, the Wistar-Kyoto rat (WKY), have been shown to be differentially responsive to the behavioral and endocrine effects of both stress and corticotropin-releasing factor (CRF), both of which increase locus coeruleus (LC) electrophysiological activity. However, the effect of central administration of CRF in these rat strains has yet to be examined. In the present studies, LC electrophysiological responsivity to intracerebroventricular infusions of CRF was assessed in SHR, an inbred strain of WKY rats (the WKY[LJ] rat), and an outbred normotensive rat strain, Sprague-Dawley (SD) rats. Spontaneous LC discharge rate, mean arterial blood pressure and heart rate were also examined. LC activity was increased to the same extent in the three rat strains in response to a 3 microg dose of CRF. However, WKY(LJ) rats showed an exaggerated LC in response to a 1 microg dose of CRF in comparison to the other rat strains tested at this dose. Spontaneous discharge rates of individual LC neurons were lower in both SHR and WKY[LJ] rats than in SD rats. Further, the variability of the discharge rates of LC neurons was greater in WKY[LJ] rats than in the other two strains. These results indicate that the WKY[LJ] rat may provide a useful model for assessing the role of sensitivity to CRF in stress responsiveness.

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.

Serotonin reuptake inhibitors reduce conditioned fear stress-induced freezing behavior in rats

Conditioned fear stress (CFS)-induced freezing behavior has been proposed as an animal model of anxiety. In the present study, freezing was used to determine the anxiolytic activity of selective serotonin reuptake inhibitors (SSRIs), which are reported to be clinically effective in anxiety disorders. The duration of freezing behavior was reduced by acute treatment with the SSRIs citalopram (1-10 mg/kg) and fluvoxamine (3-30 mg/kg). Acute treatment with the serotonin (5-HT)/noradrenaline (NA) mixed reuptake inhibitor milnacipran (3-30 mg/kg) also attenuated CFS-induced freezing, while acute treatment with the NA reuptake inhibitors maprotiline and ORG4428, and the dopamine (DA) reuptake inhibitor GBR12909 failed to alter CFS-induced freezing. These results indicate that facilitation of 5-HT availability in the brain produced by 5-HT reuptake inhibition reduces CFS-induced freezing behavior. CFS may be a useful model for detecting the anxiolytic potential of 5-HT reuptake inhibitors.

Effect of cocaine metabolites on behavior: possible neuroendocrine mechanisms

The predominant cocaine metabolites were tested for central nervous system effects by intracerebroventricular (ICV) administration in rats. We found two types of responses: cocaine, norcocaine (NC), benzoylecgonine (BE), and benzoylnorecgonine (Nor BE) produced stimulatory effects, whereas ecgonine methyl ester (EME) and ecgonine (EC) resulted in no specific effect or sedation. A novel metabolite interaction was revealed when rats were pretreated with EME, which inhibited both analgesia and seizures by subsequently administered cocaine. Pretreatment with EC inhibited both cocaine and BE seizures and seizure-associated death. Direct injection of EME into the nucleus accumbens significantly suppressed systemic cocaine potentiation of intracranial electrical self-stimulation of the ventral tegmental area, whereas corticotropin releasing hormone injected ICV selectively potentiated BE-induced seizures and death. These results confirm multiple, metabolite-mediated activities in the central nervous system. Pharmacological interactions of the metabolites with each other and/or with neurohormones may help explain some of the pathophysiological effects seen in human chronic cocaine abuse.

Chronic infusion of a CRH1 receptor antisense oligodeoxynucleotide into the central nucleus of the amygdala reduced anxiety-related behavior in socially defeated rats

We studied the role of central amygdala CRH receptors in behavioral responses to an anxiogenic stimulus. An antisense oligodeoxynucleotide corresponding to the rat CRH1 receptor mRNA was infused chronically into the central amygdaloid nucleus of male rats via osmotic minipumps (0.25 micrograms/0.5 microliters/h). Control groups received infusions of either a scrambled sequence oligodeoxynucleotide or vehicle. On the 4th day of treatment, rats were subjected to 10 min of social defeat and immediately afterwards tested on the elevated plus-maze. Antisense oligodeoxynucleotide-treated rats spent significantly more time exploring the open arms of the plus-maze than scrambled sequence- and vehicle-treated animals, both of which did not differ from each other. The social discrimination test, on the other hand, revealed no difference in juvenile recognition abilities among the treatment groups. Using in situ hybridization and receptor autoradiography, we were not able to detect clear signals of CRH1 receptor mRNA and CRH binding sites in the central amygdaloid nucleus of either group, confirming the reportedly low expression and density of CRH receptors in this brain area. The present data support the view that CRH receptors in the central nucleus of the amygdala are involved in the mediation and expression of anxiety-related behavior, but simultaneously raise questions as to the mechanisms of antisense oligodeoxynucleotide action.

Behavioural activation produced by CRH but not alpha-helical CRH (CRH-receptor antagonist) when microinfused into the central nucleus of the amygdala under stress-free conditions

The central nucleus of the amygdala (CeA) is known to be involved in the regulation of autonomic, neuroendocrine, and behavioural responses in stress situations. The CeA contains large numbers of corticotropin-releasing hormone (CRH)-containing cell bodies and terminals. In the present study we examined (by continuous behaviour observations) the effects of a high dose of CRH (150 ng) and two doses of the CRH-receptor antagonist (alpha-hCRH: 1.0 and 0.1 micrograms) after microinfusion into the CeA in freely moving male Wistar rats under stress-free conditions. In comparison with control, alpha-hCRH infusion did not cause any behavioural activation. In contrast CRH-infusion revealed a long-lasting increase in grooming and exploration with a concomitant decrease in behaviours specified as resting. These results indicate that the CRH system in the CeA does not seem to be activated in stress-free conditions, but its activation is of importance for active behavioural responses.

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.

Corticotropin-releasing factor and neuropeptide Y: role in emotional integration

The amygdala complex integrates stressful stimuli and is critical in transducing their aversive value into autonomic, endocrine and behavioural responses. Stimulation within the amygdala complex produces signs of fear without a relevant external object, while lesions in this region abolish normal fear responses. In a manner characteristic of phylogenetically old limbic brain areas, the complex neurochemical anatomy of the amygdala involves a large number of phylogenetically old peptide mediators. The distribution and connectivity of these peptide systems have been extensively studied, but less is known about their functional role. Recent evidence suggests that two neuropeptides, corticotropin-releasing factor (CRF) and neuropeptide Y (NPY) exert a reciprocal regulation of responsiveness to stressful stimuli, possibly via an interaction of these two systems in the amygdala.

Depression and anxiety: role of the locus coeruleus and corticotropin-releasing factor

Based on studies of depression and anxiety using animal (rat) models, it is suggested that, contrary to a widely accepted theory, increased activity of locus coeruleus (LC) neurons does not appear to potentiate anxiety; instead, the influence of LC activity may be opposite to this. First, studies are described that indicate that behavioral changes resembling what is seen in human clinical depression occur in rats exposed to highly stressful conditions, and the research is then traced, which links this stress-induced depression to disturbance of normal noradrenergic regulation of LC activity. Second, the potential role of corticotrophin releasing factor (CRF) in stress-induced behavioral depression is explored. CRF infused into the LC did not produce behavioral depression in the swim test but did increase anxiety; by comparison, CRF infused into the parabrachial nucleus lateral to LC increased both depression and anxiety. Finally, to further explore the relationship between LC activity and anxiety, drugs were infused into LC region to attempt to specifically activate or depress firing of LC neurons. In contrast to expectations, infusion to decrease firing of LC cells increased anxious behavior, while infusion to increase firing decreased anxious behavior. Several other studies are discussed that point to a similar conclusion. It is suggested that, at least in rats, the capacity of stress-inducing or aversive stimuli to activate LC neurons does not potentiate anxiety under environmental conditions that elicit this response, but, rather, the increased activity of the LC/dorsal noradrenergic system under such conditions may exert a counterbalancing, antianxiety influence.

Corticotropin-releasing hormone microinfusion in the central amygdala diminishes a cardiac parasympathetic outflow under stress-free conditions

The central nucleus of the amygdala (CeA) is known to be involved in the regulation of autonomic, neuroendocrine and behavioural responses in stress situations. The CeA contains large numbers of corticotropin-releasing hormone (CRH) cell bodies. Neuroanatomical studies revealed that the majority of the CRH fibres from the CeA have direct connections with autonomic regulatory nuclei in the brainstem. In the present study, the effects of locally infused CRH (30 ng) into the CeA, in freely moving male Wistar rats under stress-free conditions, were examined. Heart rate, endocrine parameters and behavioural activity were repeatedly measured before, during and after local administration of CRH, pretreated with either artificial CSF or the CRH-receptor antagonist, alpha-helical CRH (alpha-hCRH). CRH infusion alone caused a long-lasting increase in heart rate without affecting plasma adrenaline and noradrenaline as indicators of sympathetic activity. This CRH-induced tachycardia was effectively blocked by pretreatment with a high dose (1 microgram) alpha-hCRH locally into the CeA, while the pretreatment with low dose (0.1 microgram) of the alpha-hCRH caused a minor blockade of the CRH-induced tachycardia. The results suggest that CRH mechanisms in the CeA regulate the autonomic changes probably only by affecting parasympathetic but not sympathetic output systems. Because CRH is given at the level of the cell body of the CRH neurons in the CeA, we suggest that the reduction of the parasympathetic output may be explained as an autoreceptor-mediated inhibition of CRH neurons from the CeA with parasympathetic-regulating brainstem nuclei.

Attenuation of stress-induced behavior by antagonism of corticotropin-releasing factor receptors in the central amygdala in the rat

Research suggests that endogenous corticotropin-releasing factor (CRF) in the amygdala plays a role in the expression of stress-induced behavior. This study examined in rats whether antagonism of CRF receptors in the central amygdala (CA) region using alpha-helical CRF9-41, a CRF antagonist, was effective in attenuating the occurrence of stress-induced freezing. Bilateral infusions of 50, 100, or 200 ng of the CRF antagonist were made in the CA region using 33-gauge cannula immediately prior to testing. Freezing was measured in two test conditions. In one condition, the effects of the CRF antagonist on freezing was assessed immediately after exposure to electric foot shock. In the other condition, freezing was examined in shock-experienced rats that were re-exposed to the shock environment. Results suggested that 50 and 100 ng of the CRF antagonist were effective in reducing the duration of freezing in the immediate post-shock period. In addition, the 100 ng dose produced a significant reduction in freezing duration after rats were re-exposed to the shock environment. Collectively, data suggest that antagonizing the action of endogenous CRF in the CA region contributes to a general alleviation of stress-induced freezing.