Corticotropin-releasing factor-containing axon terminals synapse onto catecholamine dendrites and may presynaptically modulate other afferents in the rostral pole of the nucleus locus coeruleus in the rat brain

Corticotropin-releasing factor and noradrenergic signalling exert reciprocal control over startle reactivity

Corticotropin-releasing factor (CRF) and norepinephrine (NE) levels are altered in post-traumatic stress disorder and may be related to symptoms of hyperarousal, including exaggerated startle, in these patients. In animals, activation of both systems modulates anxiety behaviours including startle plasticity; however, it is unknown if they exert their actions orthogonally or dependently. We tested the hypothesis that NE receptor activation is required for CRF effects on startle and that CRF1 receptor activation is required for NE effects on startle. The study examined the effects of: (1) α2 agonist clonidine (0.18 mg/kg i.p.), α1 antagonist prazosin (0.8 mg/kg), and β1/2 antagonist propranolol (0.8, 8.0 mg/kg) pretreatment on ovine-CRF (oCRF)- (0.6 nmol) induced increases in startle reactivity and disruption of prepulse inhibition (PPI); (2) α2 antagonist atipamezole (1-30 mg/kg) and α1 agonist cirazoline (0.025-1.0 mg/kg) treatment on startle; (3) CRF1 antagonist (antalarmin, 14 mg/kg) pretreatment on atipamezole- (10.0 mg/kg) induced increases in startle. oCRF robustly increased startle and reduced PPI. Pretreatment with clonidine or prazosin, but not propranolol, blocked oCRF-induced increases in startle but had no effect on oCRF-induced disruptions in PPI. Atipamezole treatment increased startle, which was partially attenuated by CRF1 antagonist pretreatment. Cirazoline treatment did not increase startle. These findings suggest that CRF modulation of startle, but not PPI, requires activation of α1 adrenergic receptors, while CRF1 activation also contributes to NE modulation of startle. These data support a bi-directional model of CRF-NE modulation of stress responses and suggest that both systems must be activated to induce stress effects on startle reactivity.

Amygdalar peptidergic circuits regulating noradrenergic locus coeruleus neurons: linking limbic and arousal centers

The endogenous opioid peptides, met- or leu-enkephalin, and corticotropin-releasing factor (CRF) regulate noradrenergic neurons in the locus coeruleus (LC) in a convergent manner via projections from distinct brain areas. In contrast, the opioid peptide dynorphin (DYN) has been shown to serve as a co-transmitter with CRF in afferents to the LC. To further define anatomical substrates targeting noradrenergic neurons by DYN afferents originating from limbic sources, anterograde tract-tracing of biotinylated dextran amine (BDA) from the central amygdaloid complex was combined with immunocytochemical detection of DYN and tyrosine hydroxylase (TH) in the same section of tissue. Triple labeling immunocytochemistry was combined with electron microscopy in the LC where BDA was identified using an immunoperoxidase marker, and DYN and TH were distinguished by the use of sequential immunogold labeling and silver enhancement to produce different sized gold particles. Results show direct evidence of a monosynaptic pathway linking amygdalar DYN afferents with LC neurons. To determine whether DYN-containing amygdalar LC-projecting neurons colocalize CRF, retrograde tract-tracing using fluorescent latex microspheres injected into the LC was combined with immunocytochemical detection of DYN and CRF in single sections in the central amygdala. Retrogradely labeled neurons from the LC were distributed throughout the rostro-caudal extent of the central nucleus of the amygdala (CeA) as previously described. Cell counts showed that approximately 42% of LC-projecting neurons in the CeA contained both DYN and CRF. Taken with our previous studies showing monosynaptic projections from amygdalar CRF neurons to noradrenergic LC cells, the present study extends this by showing that DYN and CRF are co-transmitters in monosynaptic projections to the LC and are poised to coordinately impact LC neuronal activity.

Molecular and cellular sex differences at the intersection of stress and arousal

Elucidating the mechanisms underlying sex biases in the prevalence and severity of diseases can advance our understanding of their pathophysiological basis and serve as a guide for developing treatments. A well-established sex difference in psychiatry is the higher incidence of mood and anxiety disorders in females. These disorders share stress as a potential etiological contributor and hyperarousal as a core symptom, suggesting that the distinction between sexes lies at the intersection of stress and arousal systems. This review focuses on the link between the stress axis and the brain norepinephrine arousal system as a key point at which sex differences occur and are translated to differences in the expression of mood disorders. Evidence for a circuit designed to relay emotion-related information via the limbic corticotropin-releasing factor (CRF) system to the locus coeruleus (LC)-norepinephrine arousal system is reviewed. This is followed by recent novel findings of sex differences in CRF receptor signaling and trafficking that would result in an enhanced arousal response and a compromised ability to adapt to chronic stress in females. Finally, we discuss the evidence for sex differences in LC dendritic structure that allow for an increased receipt and processing of limbic information in females compared to males. Together these complementary sets of data suggest that in females, the LC arousal system is poised to process more limbic information and to respond to some of this information in an enhanced manner compared to males. The clinical and therapeutic considerations arising from this perspective are discussed. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Localization of GABA-A receptor alpha subunits on neurochemically distinct cell types in the rat locus coeruleus

The locus coeruleus (LC) provides the major source of noradrenaline to the central nervous system and is modulated by neurochemically diverse afferents. LC function is central to arousal, memory, cognition and the stress response, with dysfunction of the LC-noradrenergic axis implicated in debilitating psychiatric disorders. The precise targeting of neurotransmitter receptors within the LC is essential for processing the information contained in diverse afferents and thus LC output. The inhibitory modulation of LC neurons is thought to be effected mainly through GABA-A receptors (GABA(A)Rs). Diverse GABA(A)Rs are pentameric complexes assembled from a repertoire of subunits resulting in substantial diversity in their molecular, functional and pharmacological properties throughout the brain. The precise location of distinct GABA(A) R subunits in subregions of the LC, and the neurochemical identity of the cells that express them, remains to be determined. Here, we show that the GABA(A)R alpha1 subunit is expressed exclusively in neurochemically and morphologically diverse non-noradrenergic cell types within the LC, which may innervate the principal noradrenergic cells. Thus, the GABA(A)R alpha1 subunit could provide a neurochemical signature for a pool of local circuit interneurons in the LC. In contrast, non-overlapping GABA(A)R alpha2 and alpha3 subunit-immunoreactive puncta were enriched on noradrenergic dendrites and, to a lesser extent, on somata. The study reveals a cell-type- and domain-specific expression pattern of distinct GABA(A)R subunits in the LC. These data will serve as a template for understanding inhibitory modulation of this region and facilitate more directed pharmacological strategies for disorders arising from the impairment of LC function.

Sexual dimorphism in locus coeruleus dendritic morphology: a structural basis for sex differences in emotional arousal

Stress-related psychiatric disorders, such as depression and anxiety, affect a disproportionate number of women. We previously demonstrated that the major brain norepinephrine (NE)-containing nucleus, locus coeruleus (LC) is more sensitive to stressors and to the stress-related neuropeptide, corticotropin-releasing factor (CRF) in female compared to male rats. Because the LC-NE system is a stress-responsive system that is thought to be dysregulated in affective disorders, sex differences in LC structure or function could play a role in female vulnerability to these diseases. The present study used different approaches to compare LC dendritic characteristics between male and female rats. Immunofluorescence labeling of tyrosine hydroxylase, the norepinephrine synthetic enzyme, revealed that LC dendrites of female rats extend further into the peri-LC region, covering a significantly greater area than those of males. Optical density measurements of dendrites in the peri-LC revealed increased dendritic density in females compared to their male counterparts. Additionally, immunoreactivity for synaptophysin, a synaptic vesicle protein, was significantly greater in the LC in female rats, suggesting an increased number of synaptic contacts onto LC processes. Individual LC neurons were juxtacellularly labeled with neurobiotin in vivo for morphological analysis. LC dendritic trees of females were longer and had more branch points and ends. Consistent with this, Sholl analysis determined that, compared to males, LC dendrites of females had a more complex pattern of branching. The greater dendritic extension and complexity seen in females predicts a higher probability of communication with diverse afferents that terminate in the peri-LC. This may be a structural basis for heightened arousal in females, an effect which may, in part, account for the sex bias in incidence of stress-related psychiatric disorders.

The bladder-brain connection: putative role of corticotropin-releasing factor

The coordination of pelvic visceral activity with appropriate elimination behaviors is a complex task that requires reciprocal communication between the brain and pelvic organs. Barrington's nucleus, located in the pons, is central to a circuit involved in this function. Barrington's nucleus neurons project to both pelvic visceral motorneurons and cerebral norepinephrine neurons that modulate behavior. This circuit coordinates the descending limb of the micturition reflex with a central limb that initiates arousal and shifts the focus of attention to facilitate elimination behavior. The same circuitry that links the bladder and brain enables pathological processes in one target of the circuit to be expressed in the other. Urological disorders can, therefore, have cognitive and behavioral consequences by affecting components of this circuit; and in the opposing direction, psychosocial stressors can produce voiding dysfunctions and bladder pathology. The stress-related neuropeptide, corticotropin-releasing factor, which is prominent in Barrington's nucleus neurons, is a potential mediator of these effects.

Sex differences in corticotropin-releasing factor receptor signaling and trafficking: potential role in female vulnerability to stress-related psychopathology

Although the higher incidence of stress-related psychiatric disorders in females is well documented, its basis is unknown. Here, we show that the receptor for corticotropin-releasing factor (CRF), the neuropeptide that orchestrates the stress response, signals and is trafficked differently in female rats in a manner that could result in a greater response and decreased adaptation to stressors. Most cellular responses to CRF in the brain are mediated by CRF receptor (CRFr) association with the GTP-binding protein, G(s). Receptor immunoprecipitation studies revealed enhanced CRFr-G(s) coupling in cortical tissue of unstressed female rats. Previous stressor exposure abolished this sex difference by increasing CRFr-G(s) coupling selectively in males. These molecular results mirrored the effects of sex and stress on sensitivity of locus ceruleus (LC)-norepinephrine neurons to CRF. Differences in CRFr trafficking were also identified that could compromise stress adaptation in females. Specifically, stress-induced CRFr association with beta-arrestin2, an integral step in receptor internalization, occurred only in male rats. Immunoelectron microscopy confirmed that stress elicited CRFr internalization in LC neurons of male rats exclusively, consistent with reported electrophysiological evidence for stress-induced desensitization to CRF in males. Together, these studies identified two aspects of CRFr function, increased cellular signaling and compromised internalization, which render CRF-receptive neurons of females more sensitive to low levels of CRF and less adaptable to high levels of CRF. CRFr dysfunction in females may underlie their increased vulnerability to develop stress-related pathology, particularly that related to increased activity of the LC-norepinephrine system, such as depression or post-traumatic stress disorder.

Roles for gamma-aminobutyric acid in the development of the paraventricular nucleus of the hypothalamus

The development of the hypothalamic paraventricular nucleus (PVN) involves several factors that work together to establish a cell group that regulates neuroendocrine functions and behaviors. Several molecular markers were noted within the developing PVN, including estrogen receptors (ER), neuronal nitric oxide synthase (nNOS), and brain-derived neurotrophic factor (BDNF). By contrast, immunoreactive gamma-aminobutyric acid (GABA) was found in cells and fibers surrounding the PVN. Two animal models were used to test the hypothesis that GABA works through GABA(A) and GABA(B) receptors to influence the development of the PVN. Treatment with bicuculline to decrease GABA(A) receptor signaling from embryonic day (E) 10 to E17 resulted in fewer cells containing immunoreactive (ir) ERalpha in the region of the PVN vs. control. GABA(B)R1 receptor subunit knockout mice were used to examine the PVN at P0 without GABA(B) signaling. In female but not male GABA(B)R1 subunit knockout mice, the positions of cells containing ir ERalpha shifted from medial to lateral compared with wild-type controls, whereas the total number of ir ERalpha-containing cells was unchanged. In E17 knockout mice, ir nNOS cells and fibers were spread over a greater area. There was also a significant decrease in ir BDNF in the knockout mice in a region-dependent manner. Changes in cell position and protein expression subsequent to disruption of GABA signaling may be due, in part, to changes in nNOS and BDNF signaling. Based on the current study, the PVN can be added as another site where GABA exerts morphogenetic actions in development.

The locus coeruleus: A key nucleus where stress and opioids intersect to mediate vulnerability to opiate abuse

The interaction between the stress axis and endogenous opioid systems has gained substantial clinical attention as it is increasingly recognized that stress predisposes to opiate abuse. For example, stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Numerous reports indicating that stress alters individual sensitivity to opiates suggest that prior stress can influence the pharmacodynamics of opiates that are used in clinical settings. Conversely, the effects of opiates on different components of the stress axis can impact on individual responsivity to stressors and potentially predispose individuals to stress-related psychiatric disorders. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC), the major brain norepinephrine (NE)-containing nucleus. This review summarizes our current knowledge regarding the anatomical and neurochemical afferent regulation of the LC. It then presents physiological studies demonstrating opposing interactions between opioids and stress-related neuropeptides in the LC and summarizes results showing that chronic morphine exposure sensitizes the LC-NE system to corticotropin releasing factor and stress. Finally, new evidence for novel presynaptic actions of kappa-opioids on LC afferents is provided that adds another dimension to our model of how this central NE system is co-regulated by opioids and stress-related peptides.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Social stress-induced bladder dysfunction: potential role of corticotropin-releasing factor

Psychological stress can impact on visceral function with pathological consequences, although the mechanisms underlying this are poorly understood. Here we demonstrate that social stress produces marked changes in bladder structure and function. Male rats were subjected to repeated (7 days) social defeat stress using the resident-intruder model. Measurement of the voiding pattern indicated that social stress produced urinary retention. Consistent with this, bladder size was increased in rats exposed to social stress. Moreover, this was negatively correlated to the latency to assume a subordinate posture, implying an association between passive behavior and bladder dysfunction. In vivo cystometry revealed distinct changes in urodynamic function in rats exposed to social stress, including increased bladder capacity, micturition volume, intermicturition interval, and the presence of non-micturition-related contractions, resembling overactive bladder. In contrast to social stress, repeated restraint (7 days) did not affect voiding, bladder weight, or urodynamics. The stress-related neuropeptide corticotropin-releasing factor (CRF) is present in spinal projections of Barrington's nucleus that regulate the micturition reflex and has an inhibitory influence in this pathway. Social stress, but not restraint, increased the number of CRF-immunoreactive neurons in Barrington's nucleus. Additionally, social stress increased CRF mRNA in Barrington's nucleus. Together, the results imply that social stress-induced CRF upregulation in Barrington's nucleus neurons results in urinary retention and, eventually, bladder dysfunction, perhaps as a visceral component of a behavioral coping response. This mechanism may underlie dysfunctional voiding in children and/or contribute to the development of stress-induced bladder disorders in adulthood.

Type 1 corticotropin-releasing factor receptor expression reported in BAC transgenic mice: implications for reconciling ligand-receptor mismatch in the central corticotropin-releasing factor system

In addition to its established role in initiating the endocrine arm of the stress response, corticotropin-releasing factor (CRF) can act in the brain to modulate neural pathways that effect coordinated physiological and behavioral adjustments to stress. Although CRF is expressed in a set of interconnected limbic and autonomic cell groups implicated as primary sites of stress-related peptide action, most of these are lacking or impoverished in CRF receptor (CRFR) expression. Understanding the distribution of functional receptor expression has been hindered by the low resolution of ligand binding approaches and the lack of specific antisera, which have supported immunolocalizations at odds with analyses at the mRNA level. We have generated a transgenic mouse that shows expression of the principal, or type 1, CRFR (CRFR1). This mouse expresses GFP in a cellular distribution that largely mimics that of CRFR1 mRNA and is extensively colocalized with it in individual neurons. GFP-labeled cells display indices of activation (Fos induction) in response to central CRF injection. At the cellular level, GFP labeling marks somatic and proximal dendritic morphology with high resolution and is also localized to axonal projections of at least some labeled cell groups. This includes a presence in synaptic inputs to central autonomic structures such as the central amygdalar nucleus, which is implicated as a stress-related site of CRF action, but lacks cellular CRFR1 expression. These findings validate a new tool for pursuing the role of central CRFR signaling in stress adaptation and suggest means by which the pervasive ligand-receptor mismatch in this system may be reconciled.

Differential blockade of CRF-evoked behaviors by depletion of norepinephrine and serotonin in rats

RATIONALE

Central administration of corticotropin-releasing factor (CRF) elicits a specific pattern of behavioral responses resembling a stress-like state and is anxiogenic in rodent models of anxiety.

OBJECTIVES

Specific behaviors evoked by the administration of CRF were measured. The roles of CRF receptor subtypes and that of serotonergic and noradrenergic systems in mediating these responses were studied.

MATERIALS AND METHODS

Burying, grooming, and head shakes were quantified in rats following intracerebroventricular administration of CRF and urocortin II and after pretreatment with antagonists. The role of forebrain norepinephrine in the behavioral responses to CRF (0.3 microg) was examined following pretreatment with the neurotoxin DSP-4 and that of serotonin after depletion using systemic administration of para-chlorophenylalanine (p-CPA).

RESULTS

CRF at 0.3 and 3.0 microg caused robust increases in burying, grooming, and head shakes, but urocortin II was ineffective. Pretreatment with either antalarmin or propranolol significantly attenuated the CRF-evoked behaviors. Destruction of forebrain norepinephrine pathways blocked spontaneous burying behavior elicited by CRF and conditioned burying directed towards an electrified shock probe. In contrast, depletion of 5-HT selectively attenuated CRF-evoked grooming.

CONCLUSIONS

Overt behavioral responses produced by CRF, burying, grooming, and head shakes appeared to be mediated through the CRF(1) receptor. Spontaneous burying behavior evoked by CRF or conditioned burying directed towards a shock probe was disrupted by lesion of the dorsal noradrenergic bundle and may represent anxiety-like behavior caused by CRF activation of the locus ceruleus. In contrast, CRF-evoked increases in grooming were dependent on serotonin.

Dynorphin and stress-related peptides in rat locus coeruleus: contribution of amygdalar efferents

The interaction between the stress axis and endogenous opioid systems has gained substantial attention, because it is increasingly recognized that stress alters individual sensitivity to opiates. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC). We have previously described interactions of several opioid peptides [e.g., proopiomelanocortin, enkephalin (ENK)] with the stress-related peptide corticotropin-releasing factor (CRF) in the LC. To examine further the interactions among dynorphin (DYN), ENK, and CRF in the LC, sections were processed for detection of DYN and CRF or DYN and ENK in rat brain. DYN- and CRF-containing axon terminals overlapped noradrenergic dendrites in this region. Dual immunoelectron microscopy showed coexistence of DYN and CRF; 35% of axon terminals containing DYN were also immunoreactive for CRF. In contrast, few axon terminals contained both DYN and ENK. A potential DYN/CRF afferent is the central nucleus of the amygdala (CeA). Dual in situ hybridization showed that, in CeA neurons, 31% of DYN mRNA-positive cells colocalized with CRF mRNA, whereas 53% of CRF mRNA-containing cells colocalized with DYN mRNA. Finally, to determine whether limbic DYN afferents target the LC, the CeA was electrolytically lesioned. Light-level densitometry of DYN labeling in the LC showed a significant decrease in immunoreactivity on the side of the lesion. Taken together, these data indicate that DYN- and CRF-labeled axon terminals, most likely arising from amygdalar sources, are positioned dually to affect LC function, whereas DYN and ENK function in parallel.

The role of corticotropin-releasing factor and noradrenaline in stress-related responses, and the inter-relationships between the two systems

Substantial evidence indicates that brain neurons containing and secreting noradrenaline and corticotropin-releasing factor (CRF) are activated during stress, and that physiological and behavioural responses observed during stress can be induced by exogenous administration of CRF and adrenoceptor agonists. This review focusses on the evidence for the involvement of these two factors in stress-related responses, and the inter-relationships between them. The possible abnormalities of these two systems in depressive illness are also discussed.

Convergent regulation of locus coeruleus activity as an adaptive response to stress

Although hypothalamic-pituitary-adrenal axis activation is generally considered to be the hallmark of the stress response, many of the same stimuli that initiate this response also activate the locus coeruleus-norepinephrine system. Given its functional attributes, the parallel engagement of the locus coeruleus-norepinephrine system with the hypothalamic-pituitary-adrenal axis serves to coordinate endocrine and cognitive limbs of the stress response. The elucidation of stress-related afferents to the locus coeruleus and the electrophysiological characterization of these inputs are revealing how the activity of this system is fine-tuned by stressors to facilitate adaptive cognitive responses. Emerging from these studies, is a picture of complex interactions between the stress-related neuropeptide, corticotropin-releasing factor (CRF), endogenous opioids and the excitatory amino acid neurotransmitter, glutamate. The net effect of these interactions is to adjust the activity and reactivity of the locus coeruleus-norepinephrine system such that state of arousal and processing of sensory stimuli are modified to facilitate adaptive behavioral responses to stressors. This review begins with an introduction to the basic anatomical and physiological characteristics of locus coeruleus neurons. The concept that locus coeruleus neurons operate through two activity modes, i.e., tonic vs. phasic, that determine distinct behavioral strategies is emphasized in light of its relevance to stress. Anatomical and physiological evidence are then presented suggesting that interactions between stress-related neurotransmitters that converge on locus coeruleus neurons regulate shifts between these modes of discharge in response to the challenge of a stressor. This review focuses specifically on the locus coeruleus because it is the major source of norepinephrine to the forebrain and has been implicated in behavioral and cognitive aspects of stress responses.

The central corticotropin releasing factor system during development and adulthood

Corticotropin releasing factor (CRH) has been shown to contribute critically to molecular and neuroendocrine responses to stress during both adulthood and development. This peptide and its receptors are expressed in the hypothalamus, as well as in limbic brain areas including amygdala and hippocampus. This is consistent with roles for CRH in mediating the influence of stress on emotional behavior and cognitive function. The expression of CRH and of its receptors in hypothalamus, amygdala and hippocampus is age-dependent, and is modulated by stress throughout life (including the first postnatal weeks). Uniquely during development, the cardinal influence of maternal care on the central stress response governs the levels of central CRH expression, and may alter the 'set-point' of CRH-gene sensitivity to stress in a lasting manner.

Stress-induced intracellular trafficking of corticotropin-releasing factor receptors in rat locus coeruleus neurons

Corticotropin-releasing factor (CRF) activates locus coeruleus (LC)-norepinephrine neurons during stress. Previous stress or CRF administration attenuates the magnitude of this response by decreasing postsynaptic sensitivity to CRF. Here we describe the fate of CRF receptors (CRFr) in LC neurons after stress. Rats were exposed to swim stress or handling and perfused 1 or 24 h later. Sections through the LC were processed for immunogold-silver labeling of CRFr. CRFr in LC dendrites was present on the plasma membrane and within the cytoplasm. In control rats, the ratio of cytoplasmic to total dendritic labeling was 0.55 +/- 0.01. Swim stress increased this ratio to 0.77 +/- 0.01 and 0.80 +/- 0.02 at 1 and 24 h after stress, respectively. Internalized CRFr was associated with different organelles at different times after stress. At 1 h after stress, CRFr was often associated with early endosomes in dendrites and perikarya. By 24 h, more CRFr was associated with multivesicular bodies, suggesting that some of the internalized receptor is targeted for degradation. In perikarya, more internalized CRFr was associated with Golgi apparatus 24 vs. 1 h after stress. This is suggestive of changes in CRFr synthesis. Alternatively, this may indicate communication between multivesicular bodies and Golgi apparatus in the process of recycling. Administration of the selective CRF(1) antagonist, antalarmin, before swim stress attenuated CRFr internalization. The present demonstration of stress-induced internalization of CRFr in LC neurons provides evidence that CRF is released in the LC during swim stress to activate this system and initiate cellular trafficking of the receptor that determines subsequent sensitivity of LC neurons to CRF.

Role of brain adrenoceptors in the corticortopin-releasing factor-induced central activation of sympatho-adrenomedullary outflow in rats

We investigated the role played by catecholamine-dependent pathways in modulating the ability of centrally administered corticotropin releasing factor (CRF) to activate sympatho-adrenomedullay outflow, using urethane-anesthetized rats. The CRF (1.5 nmol/animal, i.c.v.)-induced elevations of both plasma noradrenaline and adrenaline were attenuated by phentolamine (a non-selective alpha adrenoceptor antagonist) [125 and 250 microg (0.33 and 0.66 micromol)/animal], Heat (a selective alpha(1) adrenoceptor antagonist) [10 and 30 microg (30 and 90 nmol)/animal, i.c.v.] and clonidine (a selective alpha(2) adrenoceptor agonist) [100 microg (0.375 micromol)/animal, i.c.v.]. On the other hand, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of both catecholamines was not influenced by RS 79948 (a selective alpha(2) adrenoceptor antagonist) [10 and 30 microg (7.2 and 72 nmol)/animal, i.c.v.]. Furthermore, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of noradrenaline was attenuated by sotalol (a non-selective beta adrenoceptor antagonist) [125 and 250 microg (0.4 and 0.8 micromol)/animal, i.c.v.], while that of adrenaline was not influenced by sotalol. These results suggest that centrally administered CRF-induced elevation of plasma noradrenaline is mediated by an activation of alpha(1) and beta adrenoceptors in the brain, and that of plasma adrenaline is mediated by an activation of alpha(1) adrenoceptors in the brain. Furthermore, central alpha(2) adrenoceptors are involved in modulating the CRF-induced elevation of both plasma catecholamines.

Co-localization of corticotropin-releasing factor and vesicular glutamate transporters within axon terminals of the rat dorsal raphe nucleus

Electrophysiological, microdialysis and behavioral studies support a modulatory role for corticotropin-releasing factor (CRF) in regulating the dorsal raphe nucleus (DRN)-serotonin (5-HT) system. CRF and 5-HT are implicated in the pathophysiology of depression, thus neuroanatomical substrates of CRF-DRN-5-HT interactions are of interest. Identification of co-transmitters within DRN CRF axon terminals is important for elucidating the complex effects underlying CRF afferent regulation of DRN neurons. This study investigated whether CRF-labeled axon terminals within the DRN contain immunoreactivity for vesicular glutamate transporters (isoforms vGlut1 and vGlut2) indicative of the excitatory neurotransmitter glutamate. Dual immunohistochemistry for CRF and either vGlut1 or vGlut2 was conducted within the same tissue section and immunofluorescence results indicated patterns of immunoreactivity consistent with previous reports. Abundant vGlut1- and vGlut2-immunoreactivity was found in puncta exhibiting a largely uniform distribution, whereas CRF-immunoreactivity was localized to topographically distributed varicose processes within the DRN. Profiles containing both CRF- and either vGlut1- or vGlut2-immunoreactivity were apparent in the DRN. Electron microscopy confirmed that immunoreactivity for CRF and vGlut1 was localized primarily to separate axon terminals in the DRN, with a subset co-localizing CRF and vGlut1. Examination of CRF and vGlut2 immunoreactivities in the DRN indicated that CRF and vGlut2 were found within the same axon terminal more frequently than CRF and vGlut1. Overall, these anatomical findings suggest that CRF may function, in part, with the excitatory neurotransmitter glutamate in the modulation of neuronal activity in the DRN.

Dynorphin-containing axons directly innervate noradrenergic neurons in the rat nucleus locus coeruleus

Stress causes increased dynorphin (DYN) expression in limbic brain regions and antagonism of kappa-opioid receptors may offer therapeutic potential for the treatment of depression. A potential site of DYN action relevant to stress and related neuropsychiatric disorders is the locus coeruleus (LC), the primary source of forebrain norepinephrine. Therefore, using immunofluorescence and immunoelectron microscopic analyses, we characterized the cellular substrates for interactions between DYN and tyrosine hydroxylase (TH), a catecholamine synthesizing enzyme in single sections through the rat LC. Light microscopic analysis of DYN immunoreactivity indicated that DYN fibers are distributed within the core and pericoerulear subregions of the LC. Using electron microscopy, immunoperoxidase labeling for DYN was primarily found in axon terminals, although in some cases was diffusely localized to somatodendritic processes. When DYN-containing axons formed synaptic contacts, they typically (89%) exhibited an asymmetric morphology. Almost a third (28%) of the postsynaptic targets of DYN-containing axons contained immunogold labeling for TH. These findings reveal some diversity as to the localization of DYN in the LC within axons that contact both TH and non-TH containing dendrites. However, the present data provide the first ultrastructural evidence that DYN-containing axon terminals directly innervate catecholaminergic LC dendrites. Moreover, DYN axon terminals targeting catecholaminergic LC dendrites via asymmetric synapses are consistent with localization within excitatory type afferents to the LC. Therefore, direct modulation of catacholaminergic LC neurons maybe an important site of action for DYN relevant to stress and stress-related disorders.

Ultrastructural evidence for co-localization of corticotropin-releasing factor receptor and mu-opioid receptor in the rat nucleus locus coeruleus

Previous studies have shown that corticotropin-releasing factor (CRF), an integral mediator of the stress response, and opioids regulate the activity of the locus-coeruleus-norepinephrine (LC-NE) system during stress in a reciprocal manner. Furthermore, repeated opiate exposure sensitizes noradrenergic neurons to CRF. Previous studies have shown that mu-opioid receptors (muORs) are prominently distributed within somatodendritic processes of catecholaminergic neurons in the LC and axon terminals containing opioid peptides and CRF converge within the LC. To further examine cellular sites for interactions between CRF receptor type 1 (CRFr) and muOR, immunofluorescence and electron microscopic analysis of the rat LC was conducted. Triple immunofluorescence showed prominent co-localization of the CRFr and muOR in noradrenergic somata in the LC. Ultrastructural analysis confirmed dual localization of CRFr and muOR in common dendritic processes in the LC. Semi-quantitative analysis showed that of the dendrites exhibiting CRFr immunolabeling, 57% expressed muOR immunoreactivity. These data provide ultrastructural evidence that CRFr and muOR are co-localized in LC neurons, a cellular substrate that may underlie opiate-induced sensitization of brain noradrenergic neurons to CRF.

Agonist-induced internalization of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Corticotropin-releasing factor (CRF) acts within the locus coeruleus (LC), to modulate activity of the LC-norepinephrine (NE) system. Combining molecular and cellular approaches, we demonstrate CRF receptor (CRFr) mRNA expression in Sprague-Dawley rat LC and provide the first in vivo evidence for agonist-induced internalization of CRFr. CRFr mRNA was detected in LC micropunches by RT-PCR. In dual labelling immunofluorescence studies, tyrosine hydroxylase (TH) containing neurons exhibited CRFr labelling. At the ultrastructural level, immunogold-silver labelling for CRFr was localized to the plasma membrane of TH-immunoperoxidase labelled dendrites. CRF (100 ng) injection into the LC produced a robust neuronal activation that peaked 10-15 min after injection and was maintained for the duration of the recording. This was associated with CRFr internalization in LC neurons that was apparent at 5 and 30 min after injection. By 5 min after injection the ratio of cytoplasmic to total dendritic CRFr-labelling was 0.81 +/- 0.01 in rats injected with CRF and 0.59 +/- 0.02 in rats injected with artificial cerebrospinal fluid (ACSF; P < 0.0001). Enhanced internalization of CRFr was maintained at 30 min after CRF injection, with the ratio being 0.86 +/- 0.02 for CRF-injected cases and 0.57 +/- 0.03 for ACSF-injected cases (P < 0.0001). Internalized CRFr was associated with early endosomes, indicative of degradation or recycling. Agonist-induced CRFr internalization in LC neurons may underlie acute desensitization to CRF or stress. This process may be a pivotal target by which stressors or pharmacological agents regulate the sensitivity of the LC-NE system to CRF and subsequent stressors.

Increased anxiety-like behavior during the post-stress period in mice exposed to repeated restraint stress

Mice exposed to repeated restraint (RR: 2 h of restraint on each of 3 consecutive days) lose weight and do not return to the weight of non-stressed controls after restraint ends. These mice also exhibit an exaggerated endocrine response to mild stressors in the post-stress period. To determine if other aspects of the stress response are altered, NIH Swiss mice were repeatedly restrained then evaluated for anxiety-like behavior in various behavioral tests. Twelve days after the end of RR half of the control and RR mice were subjected to the mild stress of an intraperitoneal injection of saline before placement in an elevated plus maze. RR mice not subjected to mild stress showed the same level of anxiety as the control and RR mice exposed to mild stress. Placement in a light-dark box 20 days after restraint also indicated an increase in anxiety-like behavior in RR mice that had not been exposed to mild stress. In contrast, RR mice displayed no increase in anxiety-like behavior in the defensive withdrawal apparatus and the marble burying test 6 and 17 days, respectively, after restraint. RR mice released more corticosterone than non-restrained controls exposed to defensive withdrawal or EPM apparatus although baseline corticosterone remained at control levels. These results suggest that RR induces an exaggeration of both endocrine and behavioral responses to subsequent mild stressors. This post-stress hypersensitivity to mild stress may contribute to the sustained reduction in the body weight of RR animals.

Paraventricular-coerulear interactions: role in hypertension induced by prenatal undernutrition in the rat

Rats submitted to fetal growth retardation by in utero malnutrition develop hypertension when adult, showing increased hypothalamic mRNA expression for corticotropin-releasing hormone (CRH) and increased central noradrenergic activity. As hypothalamic CRH serves as an excitatory neurotransmitter within the locus coeruleus (LC) and coerulear norepinephrine plays a similar role within the paraventricular nucleus (PVN) of the hypothalamus, we studied, in both normal and prenatally undernourished 40-day-old anesthetized rats, the effects of intra-LC microinjection of CRH and intra-PVN microinjection of the alpha(1)-adrenoceptor antagonist prazosin on multiunit neuronal activity recorded simultaneously from the two nuclei, as well as the effects on systolic pressure. Undernutrition was induced during fetal life by restricting the diet of pregnant mothers to 10 g daily, whereas mothers of control rats received the same diet ad libitum. At day 40 of postnatal life: (i) undernourished rats showed increased neuronal activity in the PVN and LC, as well as increased systolic pressure; (ii) intra-LC CRH stimulated LC and PVN neurons and increased systolic pressure only in normal rats; (iii) intra-PVN prazosin decreased LC and PVN neuronal activity and systolic pressure only in undernourished rats; and (iv) in normal rats, prazosin prevented the stimulatory effect of CRH only in PVN activity; in undernourished rats, prazosin allowed CRH to regain its stimulatory effects. The results point to the existence of an excitatory PVN-LC closed loop, which seems to be hyperactive in prenatally undernourished rats as a consequence of fetal programming; this loop could be responsible, in part, for the hypertension developed by these animals.

Pro-opiomelanocortin colocalizes with corticotropin- releasing factor in axon terminals of the noradrenergic nucleus locus coeruleus

We previously demonstrated that the opioid peptide enkephalin and corticotropin-releasing factor (CRF) are occasionally colocalized in individual axon terminals but more frequently converge on common dendrites in the locus coeruleus (LC). To further examine potential opioid cotransmitters in CRF afferents we investigated the distribution of pro-opiomelanocortin (POMC), the precursor that yields the potent bioactive peptide beta-endorphin, with respect to CRF immunoreactivity using immunofluorescence and immunoelectron microscopic analyses of the LC. Coronal sections were collected through the dorsal pontine tegmentum of rat brain and processed for immunocytochemical detection of POMC and CRF or tyrosine hydroxylase (TH). POMC-immunoreactive processes exhibited a distinct distribution within the LC as compared to the enkephalin family of opioid peptides. Specifically, POMC fibers were enriched in the ventromedial aspect of the LC with fewer fibers present dorsolaterally. Immunofluorescence microscopy showed frequent coexistence of POMC and CRF in varicose processes that overlapped TH-containing somatodendritic processes in the LC. Ultrastructural analysis showed POMC immunoreactivity in unmyelinated axons and axon terminals. Axon terminals containing POMC were filled with numerous large dense-core vesicles. In sections processed for POMC and TH, approximately 29% of POMC-containing axon terminals (n = 405) targeted dendrites that exhibited immunogold-silver labeling for TH. In contrast, sections processed for POMC and CRF showed that 27% of POMC-labeled axon terminals (n = 657) also exhibited CRF immunoreactivity. Taken together, these data indicate that a subset of CRF afferents targeting the LC contain POMC and may be positioned to dually impact LC activity.

Corticotropin-releasing factor neurons in the hypothalamic paraventricular nucleus are involved in arousal/yawning response of rats

Our previous studies have suggested that activation of the hypothalamic paraventricular (PVN) descending oxytocinergic projections is involved in the induction of yawning accompanied by an arousal response, but the possibility that neural systems other than the oxytocinergic system in the PVN also mediate the arousal/yawning response cannot be ruled out. We assessed the activity of corticotropin-releasing factor (CRF) neurons during yawning induced by the PVN stimulation in anesthetized, spontaneously breathing rats using double-staining for c-Fos and CRF. Yawning response was evaluated by monitoring an intercostals electromyogram as an index of inspiratory activity and a digastric electromyogram as an indicator of mouth opening. We also recorded the electrocorticogram (ECoG) to determine the arousal response during yawning. Microinjection of l-glutamate (2-5 nmol) into the PVN produced a frequent yawning accompanied by an arousal shift in the ECoG, and these behavioral effects were associated with a significant increase of c-Fos positive CRF neurons in the medial parvocellular subdivision of the PVN. In addition, a marked enhancement in the c-Fos expression was found in the both locus coeruleus (LC) and global area in the cortex when the frequency of yawning response was increased by the PVN stimulation, suggesting that the arousal response during yawning might be mediated by the activation of LC neurons. The present study suggests that an activation of CRF neurons in the PVN is responsible for the arousal response accompanied by yawning behavior.

The phenotypic differentiation of locus ceruleus noradrenergic neurons mediated by brain-derived neurotrophic factor is enhanced by corticotropin releasing factor through the activation of a cAMP-dependent signaling pathway

We have developed a model system of locus ceruleus (LC) neurons in culture, in which brain-derived neurotrophic factor (BDNF) induces the emergence of noradrenergic neurons attested by the presence of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase and the absence of phenylethanolamine N-methyl-transferase. Although inactive in itself, the neuropeptide corticotropin releasing factor (CRF) strongly amplified the effect of BDNF, increasing the number of cells expressing TH and the active accumulation of noradrenaline by a factor of 2 to 3 via a mechanism that was nonmitogenic. CRF also acted cooperatively with neurotrophin-4, which like BDNF is a selective ligand of the TrkB tyrosine kinase receptor. The effect of CRF but not that of BDNF was prevented by astressin, a nonselective CRF-1/CRF-2 receptor antagonist. However, only CRF-1 receptor transcripts were detectable in LC cultures, suggesting that this receptor subtype mediated the effect of CRF. Consistent with the positive coupling of CRF-1 receptors to adenylate cyclase, the trophic action of CRF was mimicked by cAMP elevating agents. Epac, a guanine nucleotide exchange factor directly activated by cAMP, contributed to the effect of CRF through the stimulation of extracellular signal-regulated kinases (ERKs) 1/2. However, downstream of ERK1/2 activation by CRF, the phenotypic induction of noradrenergic neurons relied upon the stimulation of the phosphatidylinositol-3-kinase/Akt transduction pathway by BDNF. Together, our results suggest that CRF participates to the phenotypic differentiation of LC noradrenergic neurons during development. Whether similar mechanisms account for the high degree of plasticity of these neurons in the adult brain remains to be established.

Maternal programming of defensive responses through sustained effects on gene expression

There are profound maternal effects on individual differences in defensive responses and reproductive strategies in species ranging literally from plants to insects to birds. Maternal effects commonly reflect the quality of the environment and are most likely mediated by the quality of the maternal provision (egg, propagule, etc.), which in turn determines growth rates and adult phenotype. In this paper we review data from the rat that suggest comparable forms of maternal effects on defensive responses stress, which are mediated by the effects of variations in maternal behavior on gene expression. Under conditions of environmental adversity maternal effects enhance the capacity for defensive responses in the offspring. In mammals, these effects appear to 'program' emotional, cognitive and endocrine systems towards increased sensitivity to adversity. In environments with an increased level of adversity, such effects can be considered adaptive, enhancing the probability of offspring survival to sexual maturity; the cost is that of an increased risk for multiple forms of pathology in later life.

Environmental programming of stress responses through DNA methylation: life at the interface between a dynamic environment and a fixed genome

Early experience permanently alters behavior and physiology. These effects are, in part, mediated by sustained alterations in gene expression in selected brain regions. The critical question concerns the mechanism of these environmental “programming” effects. We examine this issue with an animal model that studies the consequences of variations in mother-infant interactions on the development of individual differences in behavioral and endocrine responses to stress in adulthood. Increased levels of pup licking/grooming by rat mothers in the first week of life alter DNA structure at a glucocorticoid receptor gene promoter in the hippocampus of the offspring. Differences in the DNA methylation pattern between the offspring of high- and low-lickinglgrooming mothers emerge over the first week of life; they are reversed with cross-fostering; they persist into adulthood; and they are associated with altered histone acetylation and transcription factor (nerve growth factor-induced clone A [NGFIA]) binding to the glucocorticoid receptor promoter. DNA methylation alters glucocorticoid receptor expression through modifications of chromatin structure. Pharmacological reversal of the effects on chromatin structure completely eliminates the effects of maternal care on glucocorticoid receptor expression and hypothalamic-pituitary-adrenal (HPA) responses to stress, thus suggesting a causal relation between the maternally induced, epigenetic modification of the glucocorticoid receptor gene and the effects on stress responses in the offspring. These findings demonstrate that the structural modifications of the DNA can be established through environmental programming and that, in spite of the inherent stability of this epigenomic marker, it is dynamic and potentially reversible.

Maternal programming of steroid receptor expression and phenotype through DNA methylation in the rat

Increased levels of pup licking/grooming and arched-back nursing by rat mothers over the first week of life alter the epigenome at a glucocorticoid receptor gene promoter in the hippocampus of the offspring. Differences in the DNA methylation pattern between the offspring of High and Low licking/grooming--arched-back mothers emerge over the first week of life, are reversed with cross-fostering, persist into adulthood and are associated with altered histone acetylation and transcription factor (NGFI-A) binding to the glucocorticoid receptor promoter. Central infusion of the adult offspring with the histone deacetylase inhibitor trichostatin A removes the previously defined epigenomic group differences in histone acetylation, DNA methylation, NGFI-A binding, glucocorticoid receptor expression, and hypothalamic-pituitary-adrenal responses to stress, thus suggesting a causal relation between the epigenomic state, glucocorticoid receptor expression and the effects of maternal care on stress responses in the offspring. These findings demonstrate that an epigenomic state of a gene can be established through a behavioral mode of programming and that in spite of the inherent stability of this epigenomic mark, it is dynamic and potentially reversible.

Hypothalamic projections to locus coeruleus neurons in rat brain

Locus coeruleus (LC) neurons respond to autonomic and visceral stimuli and discharge in parallel with peripheral sympathetic nerves. The present study characterized the synaptic organization of hypothalamic afferents with catecholaminergic neurons in the LC using electron microscopy. Peroxidase labeling of axon terminals that were anterogradely labeled from the paraventricular nucleus (PVN) was combined with gold-silver labeling of tyrosine hydroxylase in the LC. Approximately 19% of the anterogradely labeled axon terminals formed synaptic specializations with tyrosine hydroxylase-immunoreactive dendrites in the LC. Retrograde transport from the LC combined with immunocytochemical detection of enkephalin and corticotropin-releasing factor (CRF) suggested that most of the LC-projecting PVN neurons (30%) were CRF immunoreactive and few (2%) were enkephalin immunoreactive. Finally, dual retrograde tracing from the LC and median eminence revealed that PVN neurons that project to the LC are a population distinct from that projecting to the median eminence. The present data suggest that a population of hypothalamic neurons is poised to directly modulate the activity of LC neurons and may integrate autonomic responses in brain by influencing LC neurons. Moreover, PVN neurons that use CRF as a neurohormone are distinct from those that use CRF as a neuromodulator to impact on the LC.

Effects of psychologic stress on fetal development and pregnancy outcome

Data from animal studies show that maternal stress is associated with disturbances in pregnancy outcomes and offspring development and behavior, possibly as a result of permanent structural and functional changes termed "early-life programming." There is growing interest in whether similar relationships are present in humans. Here we review recent significant findings from the literature on the impact of prenatal psychologic stressors on pregnancy outcome and offspring development, with a particular focus on the developing brain. Relevant papers were searched using PubMed, and reference lists from obtained articles were checked. In humans, prenatal stress is associated with pregnancy complications, developmental, cognitive, and behavioral disorders, and possible onset of psychopathology in later life. In contrast to the available research done in animals, virtually nothing is known about the effects of prenatal stress on morphologic fetal brain development, and the mechanisms underlying subsequent associated behavioral changes.

Ultrastructural evidence for a role of gamma-aminobutyric acid in mediating the effects of corticotropin-releasing factor on the rat dorsal raphe serotonin system

The dorsal raphe nucleus (DRN) serotonin (5-HT) system has been implicated in acute responses to stress and in stress-related psychiatric disorders such as anxiety and depression. Substantial findings suggest that the neuropeptide corticotropin-releasing factor (CRF) is instrumental in modulating the activity of this system during stress. Because the DRN is neurochemically heterogeneous, dual immunoelectron microscopy was used to examine cellular substrates for interactions between CRF and either 5-HT or gamma-aminobutyric acid (GABA) in the dorsolateral and ventromedial DRN. CRF immunoreactivity was identified primarily within axon terminals, where immunolabeling was particularly enriched in dense-core vesicles. Although CRF terminals targeted 5-HT-containing dendrites in the dorsolateral DRN (16%; n = 251 terminals), synaptic contacts with dendrites that lacked detectable 5-HT immunolabeling were more numerous (48%). In contrast, dual labeling for CRF and GABA (n = 240 terminals) in the dorsolateral DRN revealed that substantially more CRF terminals contacted GABA dendrites (42%) as opposed to unlabeled dendrites (29%). In the ventromedial DRN, contacts between CRF axon terminals and either 5-HT-labeled dendrites or GABA-containing dendrites were fewer than in the dorsolateral DRN. As in the dorsolateral DRN, CRF terminals more frequently contacted GABA dendrites than 5-HT dendrites (30% vs. 8%, respectively). The findings support physiological studies suggesting that CRF has both direct and indirect effects on DRN-5-HT neurons and further implicate GABA as a primary mediator by which CRF and stressors alter the activity of the DRN-5-HT system.

Organization of hypocretin/orexin efferents to locus coeruleus and basal forebrain arousal-related structures

Hypocretin/orexin neurons give rise to an extensive projection system, portions of which innervate multiple regions associated with the regulation of behavioral state. These regions include the locus coeruleus, medial septal area, medial preoptic area, and substantia innominata. Evidence indicates that hypocretin modulates behavioral state via actions within each of these terminal fields. To understand better the circuitry underlying hypocretin-dependent modulation of behavioral state, the present study characterized the degree to which there exists: 1) lateralization of hypocretin efferents to basal forebrain and brainstem arousal-related regions, 2) topographic organization of basal forebrain- and brainstem-projecting hypocretin neurons, and 3) collateralization of individual hypocretin neurons to these arousal-related terminal fields. These studies utilized combined immunohistochemical identification of hypocretin neurons with single or double retrograde tracing from the locus coeruleus, medial preoptic area, medial septal area, and substantia innominata. Results indicate that approximately 80% of hypocretin efferents to basal forebrain regions project ipsilaterally, whereas projections to the locus coeruleus are more bilateral (65%). There was a slight preference for basal forebrain-projecting hypocretin neurons to be distributed within the medial half of the hypocretin cell group. In contrast, hypocretin neurons projecting to the locus coeruleus were located primarily within the dorsal half of the hypocretin cell group. Finally, a large proportion of hypocretin neurons appear to project simultaneously to at least two of the examined terminal fields. These latter observations suggest coordinated actions of hypocretin across multiple arousal-related regions.

The role of the locus coeruleus in corticotropin-releasing hormone and stress-induced suppression of pulsatile luteinizing hormone secretion in the female rat

Despite a wealth of evidence for CRH mediating stress-induced suppression of the hypothalamic GnRH pulse generator, and hence reproductive dysfunction, the site and mechanism of action remains elusive. The locus coeruleus (LC), a prominent noradrenergic brain stem nucleus, is innervated by CRH neurons, mediates several behavioral stress responses, and is implicated in the control of pulsatile LH secretion. The aim of this study was to test the hypothesis that LC CRH has a critical role in mediating stress-induced suppression of pulsatile LH secretion in the rat. Ovariectomized rats with 17beta-estradiol or oil-filled s.c. capsules were implanted with bilateral LC and i.v. cannulae. Central administration of CRH (10 ng to 1 microg) resulted in a dose-dependent suppression of LH pulses, which was reversed by a CRH receptor antagonist (alpha-helical CRF(9-41), 1 microg). The induction of c-fos expression in glutamic acid decarboxylase67 immunostained neurons in the preoptic area suggests activation of the secretion of gamma-aminobutyric acid in response to intracoerulear administration of CRH; 17beta-estradiol further increased the percentage of glutamic acid decarboxylase67-positive neurons that expressed fos and augmented suppression of LH pulses. Furthermore, intracoerulear administration of alpha-helical CRF(9-41) completely blocked restraint stress-induced suppression of LH pulses, without affecting the inhibitory response to hypoglycemia. These results suggest that CRH innervation of the LC may play a pivotal, but differential, role in the normal physiological response of stress-induced suppression of the GnRH pulse generator and hence the reproductive system.

Neuropeptides in anxiety modulation

This review is focused on the involvement of neuropeptides in the modulation of physiological and pathological anxiety. Neuropeptides play a major role as endogenous modulators of complex behaviours, including anxiety-related behaviour and psychopathology, particularly due to their high number and diversity, the dynamics of release patterns in distinct brain areas and the multiple and variable modes of interneuronal communication they are involved in. Manipulations of central neuropeptidergic systems to reveal their role in anxiety (and often comorbid depression-like behaviour) include a broad spectrum of loss-of-function and gain-of-function approaches. This article concentrates on those neuropeptides for which an involvement as endogenous anxiolytic or anxiogenic modulators is well established by such complementary approaches. Particular attention is paid to corticotropin-releasing hormone (CRH) and vasopressin (AVP) which, closely linked to stress, neuroendocrine regulation, social behaviour and learning/memory, play critical roles in the regulation of anxiety-related behaviour of rodents. Provided that their neurobiology, neuroendocrinology and molecular-genetic background are well characterized, these and other neuropeptidergic systems may be promising targets for future anxiolytic strategies.

The programming of individual differences in defensive responses and reproductive strategies in the rat through variations in maternal care

There are profound maternal effects on individual differences in defensive responses and reproductive strategies in species ranging literally from plants to insects to birds. Maternal effects commonly reflect the quality of the environment and are most likely mediated by the quality of the maternal provision (egg, propagule, etc.), which in turn determines growth rates and adult phenotype. In this paper, we review data from the rat that suggest comparable forms of maternal effects on both defensive responses to threat and reproductive behavior and which are mediated by variations in maternal behavior. Ultimately, we will need to contend with the reality that neural development, function and health are defined by social and economic influences.

Maternal Programming of Individual Differences in Defensive Responses in the Rat

This paper describes the results of a series of studies showing that variations in mother-pup interactions program the development of individual differences in behavioral and endocrine stress responses in the rat. These effects are associated with altered expression of genes in brain regions, such as the amygdala, hippocampus, and hypothalamus, that regulate the expression of stress responses. Studies from evolutionary biology suggest that such "maternal effects" are common and often associated with variations in the quality of the maternal environment. Together these findings suggest an epigenetic process whereby the experience of the mother alters the nature of the parent-offspring interactions and thus the phenotype of the offspring.

Gene expression in autonomic areas of the medulla and the central nucleus of the amygdala in rats during and after space flight

During space flight astronauts show vestibular-related changes in balance, eye movements, and spontaneous and reflex control of cardiovascular, respiratory and gastrointestinal function, sometimes associated with space motion sickness. These symptoms undergo compensation over time. Here we used changes in the expression of two immediate-early gene (IEG) products to identify cellular and molecular changes occurring in autonomic brainstem regions of adult male albino rats killed at different times during the Neurolab Space Mission (STS-90). Both direct effects of gravitational changes, as well as indirect effects of gravitational changes on responses to light exposure were examined. Regions under the direct control of vestibular afferents such as the area postrema and the caudal part of the nucleus of the tractus solitarius (NTSC) were both directly and indirectly affected by gravity changes. These areas showed no changes in the expression of IEG products during exposure to microgravity with respect to ground controls, but did show a significant increase 24 h after return to 1 G (gravity). Exposure to microgravity significantly inhibited gene responses to light exposure seen after return to 1 G. A similar direct and indirect response pattern was also shown by the central nucleus of the amygdala, a basal forebrain structure anatomically and functionally related to the NTS. The rostral part of the NTS (NTSR) receives different afferent projections than the NTSC. This region did not show any direct gravity-related changes in IEG expression, but showed an indirect effect of gravity on IEG responses to light. A similar pattern was also obtained in the intermediate reticular nucleus and the parvocellular reticular nucleus. Two other medullary reticular structures, the dorsal and the ventral medullary reticular nuclei showed a less well defined pattern of responses that differed from those seen in the NTSC and NTSR. The short- and long-lasting molecular changes in medullary and basal forebrain gene expression described here are thought to play an important role in the integration of autonomic and vestibular signals that ultimately regulate neural adaptations to space flight.

Inducible cAMP early repressor regulates corticosterone suppression after tricyclic antidepressant treatment

The cAMP-response element binding protein (CREB) is involved in antidepressant action, but the role of related CRE-binding transcription factors in the behavioral and endocrine responses to antidepressants is unclear. Alternative transcription of the cAMP response element-modulator (CREM) gene yields activator and repressor isoforms, including the strong repressor inducible cAMP early repressor (ICER). ICER is highly expressed in hypothalamic tissues and upregulated after electroconvulsive seizure. Thus, ICER may be a novel mediator of antidepressant action at endocrine and/or behavioral levels. Here we establish that both subchronic and chronic desipramine (DMI) treatments upregulate hypothalamic ICER expression in wild-type mice. Behavioral responses to DMI in the forced swim and tail suspension tests are unchanged in mice lacking ICER. However, the ability of DMI to suppress an acute corticosterone response after swim stress is compromised in ICER-deficient mice, suggesting that increased hypothalamic ICER mRNA after DMI treatment may be required for suppression of corticosterone release. To investigate the mechanism underlying this response, we measured corticotropin releasing factor (CRF), an upstream modulator of corticosterone release. Using real-time quantitative PCR, we establish that hypothalamic CRF expression is significantly reduced after swim exposure in DMI-treated wild-type mice, however DMI is unable to blunt hypothalamic CRF expression in ICER-deficient mice. Furthermore, we demonstrate that ICER is enriched in CRF-expressing neurons in the paraventricular nucleus of the hypothalamus. These data indicate that ICER is required for DMI to reduce stress-induced corticosterone release through regulation of hypothalamic CRF expression, revealing a novel role for ICER in antidepressant regulation of the hypothalamic-pituitary adrenal axis.

Cellular interactions between axon terminals containing endogenous opioid peptides or corticotropin-releasing factor in the rat locus coeruleus and surrounding dorsal pontine tegmentum

Recent evidence suggests that certain stressors release both endogenous opioids and corticotropin-releasing factor (CRF) to modulate activity of the locus coeruleus (LC)-norepinephrine (NE) system. In ultrastructural studies, axon terminals containing methionine(5)-enkephalin (ENK) or CRF have been shown to target LC dendrites. These findings suggested the hypothesis that both neuropeptides may coexist in common axon terminals that are positioned to have an impact on the LC. This possibility was examined by using immunofluorescence and immunoelectron microscopic analysis of the rat LC and neighboring dorsal pontine tegmentum. Ultrastructural analysis indicated that CRF- and ENK-containing axon terminals were abundant in similar portions of the neuropil and that approximately 16% of the axon terminals containing ENK were also immunoreactive for CRF. Dually labeled terminals were more frequently encountered in the "core" of the LC vs. its extranuclear dendritic zone, which included the medial parabrachial nucleus (mPB). Triple labeling for ENK, CRF, and tyrosine hydroxylase (TH) showed convergence of opioid and CRF axon terminals with noradrenergic dendrites as well as evidence for inputs to TH-labeled dendrites from dually labeled opioid/CRF axon terminals. One potential source of ENK and CRF in the dorsal pons is the central nucleus of the amygdala (CNA). To determine the relative contribution of ENK and CRF terminals from the CNA, the CNA was electrolytically lesioned. Light-level densitometry revealed robust decreases in CRF immunoreactivity in the LC and mPB on the side ipsilateral to the lesion but little or no change in ENK immunoreactivity, confirming previous studies of the mPB. Degenerating terminals from the CNA in lesioned rats were found to be in direct contact with TH-labeled dendrites. Together, these data indicate that ENK and CRF may be colocalized to a subset of individual axon terminals in the LC "core." The finding that the CNA provides, to dendrites in the area examined, a robust CRF innervation, but little or no opioid innervation, suggests that ENK and CRF axon terminals impacting LC neurons originate from distinct sources and that terminals that colocalize ENK and CRF are not from the CNA.

Elevated concentrations of CRF in the locus coeruleus of depressed subjects

Research evidence that corticotropin-releasing factor (CRF) plays a role in the pathophysiology of major depressive disorder (MDD) has accumulated over the past 20 years. The elevation of lumbar cerebrospinal fluid (CSF) concentrations of CRF decreased responsiveness of pituitary CRF receptors to challenge with synthetic CRF, and increased levels of serum cortisol in MDD subjects support the hypothesis that CRF is chronically hypersecreted in at least the endocrine circuits of the hypothalamic-pituitary-adrenal (HPA) axis and may also involve other CRF brain circuits mediating emotional responses and/or arousal. One such circuit includes the excitatory CRF input to the locus coeruleus (LC), the major source of norepinephrine in the brain. Furthermore, there are now reports of decreased levels of CRF in lumbar CSF from MDD patients after symptom relief from chronic treatment with antidepressant drugs or electroconvulsive therapy. Whether this normalization reflects therapeutic effects on both endocrine- and limbic-associated CRF circuits has not yet been effectively addressed. In this brief report, we describe increased concentrations of CRF-like immunoreactivity in micropunches of post-mortem LC from subjects with MDD symptoms as established by retrospective psychiatric diagnosis compared to nondepressed subjects matched for age and sex.

Differential impact of audiogenic stressors on Lewis and Fischer rats: behavioral, neurochemical, and endocrine variations

Exposure to intense noise can trigger a cascade of neuroendocrine events reminiscent of a stress response, including activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Using male Fischer and Lewis rats, which exhibit differences in their corticosterone response to stressors, this investigation assessed effects of acute noise exposure on neurochemical and neuroendocrine responses. In response to the noise exposure, Fischer rats displayed greater plasma adrenocorticotropin-releasing hormone (ACTH) and corticosterone responses than their Lewis counterparts. However, both strains responded with similar increases of plasma prolactin, suggesting that strain differences in the HPA response were not likely because of differences in noise perception. Post-mortem analyses revealed that noise exposure induced strain-dependent variations of corticotropin-releasing hormone (CRH) across several brain regions. These effects were evident irrespective of whether the rats were noise exposed in a familiar (home cage) or unfamiliar environment. In vivo, dynamic assessment of immunoreactive (ir)-CRH at the pituitary gland revealed that noise exposure elicited an immediate rise in ir-CRH among Fischer rats, relative to the delayed response in Lewis rats. Similarly, the rise in local interstitial corticosterone was more rapid and pronounced in Fischer rats. In contrast to these differences, ir-CRH released at the central nucleus of the amygdala (CeA) was gradual and protracted following noise exposure in both strains. Behaviorally, the Fischer rats displayed an active stress response, whereas the Lewis strain adopted freezing as a defensive style. The role of CRH in the genesis of the overall strain-dependent response to stressors is discussed.

Phasic activation of locus ceruleus neurons by the central nucleus of the amygdala

The role of the central nucleus of the amygdala (CeN) in modulating output of noradrenaline in the forebrain was evaluated by recording extracellular, single-unit activity from the noradrenergic nucleus locus ceruleus (LC) during stimulation of the CeN. Short high-frequency trains (200 Hz) delivered at 800 microA in the CeN evoked phasic responses in 90% of the neurons recorded in LC. Single pulses were also effective but less reliably. The responses were complex, multiphasic with an initial latency of 10-20 msec. This early peak was diminished or, in some cases, completely blocked by local or intracerebroventricular application of the corticotrophin releasing factor antagonist alpha helical CRF (9-41). The later excitatory peak and subsequent inhibition were not effected by the drug treatment. The results underline the reciprocal functional relationship between the amygdaloid complex and the LC and suggest that the LC might be an important "effector" of CeN activation during learning.

Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats

Although it is well documented that stress can increase the activity of central dopamine and norepinephrine neurons, little is known about the role of other neurotransmitters in modulating this response. Previous studies have implicated corticotropin-releasing hormone in modulating stress-evoked changes in the activity of locus coeruleus neurons. The present study examines whether corticotropin-releasing hormone contributes to stress-evoked increases in extracellular norepinephrine and dopamine in rat medial prefrontal cortex, as monitored by in vivo microdialysis. As noted previously, 30 min of tail-shock increased extracellular levels of norepinephrine and dopamine in the medial prefrontal cortex of naïve rats, and this was enhanced in rats previously exposed to chronic cold ( approximately 5 degrees C for 2-3 weeks). Previous intraventricular administration of a corticotropin-releasing hormone antagonist (D-Phe-corticotropin-releasing hormone; 3 and 9 microg) did not alter the tail-shock evoked in increase in extracellular levels of norepinephrine and dopamine in either naïve or chronically cold-exposed rats. Intraventricular administration of 3 microg of D-Phe-corticotropin-releasing hormone attenuated the increase in extracellular norepinephrine induced by co-administration of 3 microg of corticotropin-releasing hormone, confirming the efficacy of this compound. Results of the present study suggest that endogenous corticotropin-releasing hormone does not play a role in modulating the release of norepinephrine and dopamine occurring in response to acute tail-shock or the expression of a potentiated response to tail-shock in rats exposed chronically to cold.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Alteration of locus coeruleus neuronal activity by interleukin-1 and the involvement of endogenous corticotropin-releasing hormone

Activity of the locus coeruleus (LC), which is the source of most of the norepinephrine in the brain, may participate in effects of the cytokine interleukin (IL)-1. This report describes the influence of IL-1 beta on the electrophysiological single-unit activity of LC neurons. When microinjected into the LC, human recombinant IL-1 beta (50 pg to 5 ng) increased the activity of LC neurons, predominantly by increasing 'burst' firing, which occurs in response to a sensory stimulus. At the higher doses and/or with longer time delays after injection, the spontaneous depolarization rate was also increased. This excitation (1). did not occur if IL-1 beta was microinjected nearby but outside of the LC and (2). could be reversed by administration of IL-1 receptor antagonist (IL-1 RA). In contrast to excitatory effects, microinjection of a very low dose of IL-1 beta (5 pg) into the LC inhibited LC activity, and this change could also be blocked by IL-1 RA. In view of earlier findings that (1). LC electrophysiological activity could be inhibited by microinjection of corticotropin-releasing hormone (CRH) into the LC region and (2). IL-1 beta in the brain stimulates the release of CRH, the hypothesis was tested that the inhibition of LC activity produced by the low dose of IL-1 was mediated by CRH. Microinfusion of the CRH receptor antagonist alpha-helical CRH(9-41) blocked the inhibition of LC activity otherwise produced by 5 pg of IL-1 beta, thus indicating that IL-1 beta also influences the activity of LC neurons via CRH. Finally, microinjection of IL-1 RA alone was found to decrease LC activity, raising the possibility that LC neurons are under the influence of tonic excitation by IL-1 in the brain. In summary, the findings described here show that the activity of LC neurons can be influenced by IL-1 beta through stimulation of IL-1 beta receptors. The potential involvement of IL-1 beta in stress responses by means of this cytokine influencing the activity of LC neurons is discussed.