Neurocircuitry of stress: central control of the hypothalamo-pituitary-adrenocortical axis

Chemical stimulation of the laryngopharynx increases Fos-like immunoreactivity in the rat hypothalamus and amygdala

Using immunohistochemical detection of the Fos protein as a cellular marker of neuronal activation, we examined forebrain areas that may be activated upon chemical stimulation of the laryngeal opening. Anesthetized rats were subject to multiple infusions of a chemical solution into the laryngopharynx. These animals were compared to two control groups: a surgical control group in which the animals were subject to the surgical procedure but received no stimulus infusions and a flow control group in which physiological saline replaced the chemical stimulus. Comparing the numbers of Fos-like-immunoreactive neurons in regions of the forebrain across groups revealed that infusing the chemical stimulus solution into the laryngopharyngeal opening selectively increased the number of Fos-like-immunoreactive nuclei in the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala, two autonomic-visceral related forebrain regions. Within the paraventricular nucleus of the hypothalamus, Fos-like-immunoreactive nuclei were significantly increased in the parvocellular subdivision while in the central nucleus of the amygdala, significant increases in Fos-like-immunoreactive nuclei were limited to the lateral capsular subdivision. These data suggest that in the rat laryngopharyngeal chemosensory stimulation activates forebrain regions that receive oral sensory information and are involved in visceral and autonomic functions.

Distribution of fibroblast growth factor-5 in rat hypothalamus, and its possible role as a regulator of feeding behaviour

We previously reported that a transcript of fibroblast growth factor-5 (FGF-5) was more abundant in the brain of postnatal and adult mice than in the embryonic brain. This suggested that FGF-5 plays some role in the mature brain. Here, we have investigated the spatiotemporal expression and function of FGF-5 in the adult rat hypothalamus with the emphasis on feeding behaviour. In situ hybridization experiments demonstrated that, in both adequately fed and fasted (20 h) rats, FGF-5 transcripts were present within several nuclei in the hypothalamus (viz. the magnocellular part of the paraventricular nucleus, supraoptic nucleus, arcuate nucleus, median eminence, and ventromedial hypothalamic nucleus), but not in the lateral hypothalamic area. Quantitative detection of FGF-5 mRNA in the hypothalamus (especially in the paraventricular nucleus) indicated that food deprivation (20 h) reduced the expression of this gene to almost one-half of that seen in the control (fed) rats. The expression recovered to the control level after 1 h re-feeding, and this recovery persisted for several hours. Furthermore, FGF-5, when infused into the third ventricle, consistently reduced food intake, water intake and body weight gain, all in a dose-dependent manner. These results suggest that FGF-5 in the hypothalamus acts as a physiological regulator of feeding behaviour, and that its decreased expression during food deprivation may be important in stimulating appetite.

Corticosteroids, the Aging Brain and Cognition

The hippocampal region of the brain is a useful model system for understanding the plasticity and resilience of brain cells to stress hormone action and aging. Hippocampal neurons show both structural and functional plasticity, and individual differences in hippocampal function are shaped by early life experiences. For human brain aging, there are new non-invasive imaging tools to relate to the animal models, and these can help to assess the vulnerability of the aging hippocampus in relation to stress and Alzheimer's disease.

Susceptibility of Lewis and Fischer rats to stress-induced worsening of TNB-colitis: protective role of brain CRF

We assessed the role of central corticotropin-releasing factor (CRF) in stress-induced worsening of colitis in inbred rat strains with hypo (Lewis/N) and hyper (Fischer344/N) CRF responses to stress. Intracolonic administration of 2,4,6-trinitrobenzenesulfonic acid (TNB) induced colitis of similar severity in both strains as assessed on day 7 by macroscopic scoring, histological evaluation, tissue myeloperoxidase (MPO) activity, and decrease in food intake and body weight. Colitis was inhibited by daily intracerebroventricular injections of CRF in both strains. Chronic stress (3 h/day, water avoidance or wrap restraint on alternate days for 6 days) aggravated colitis more in Lewis than Fischer rats (71 and 22% further increase in MPO activity, respectively). The CRF antagonist astressin injected intracerebroventricularly enhanced the colitis response to stress and caused mortality in both strains. Fischer rats had higher plasma corticosterone levels 20 min after stress alone on day 1 and after TNB plus stress on days 1 and 3 compared with Lewis. These data show that central CRF restrains the proinflammatory action of stress in experimental colitis.

Changes in brain corticotropin-releasing factor messenger RNA expression in aged Fischer 344 rats

Adaptation in aging may become impaired from abnormal expression of corticotropin-releasing factor (CRF) and altered CRF receptor function. In this study, we measured CRF mRNA levels in Fischer 344 rats at various ages. The brains of these rats were processed for in situ hybridization. Relative to 3-month-old rats, levels of CRF mRNA were significantly decreased in the following brain areas at the following ages: at 24 months in the paraventricular hypothalamus, at 11, 17, and 24 months in the amygdala and at 17 and 24 months in the bed nucleus of the stria terminalis. These changes may contribute to impaired adaptations to stress, cognitive decline and other pathophysiological processes during aging.

Functional neuroanatomy of the parvocellular vasopressinergic system: transcriptional responses to stress and glucocorticoid feedback

This chapter summarizes the regulation of vasopressin (VP) transcription within the parvocellular neurosecretory cells of the hypothalamic paraventricular nucleus in vivo, with special reference to stress-response and glucocorticoid feedback. VP is commonly held as the first and the most potent among the co-secretagogues that act synergistically with corticotropin-releasing factor (CRF-41) to induce adrenocorticotropin (ACTH) from the anterior pituitary in response to various internal and external stimuli. Cellular levels of the primary transcripts of VP and CRF genes, revealed by in situ hybridization histochemistry using probes complementary to intronic sequences, are increased after acute challenges with different time courses. In contrast to the rapid stress-induced upregulation of CRF gene expression, VP transcription shows a delayed increase suggesting different regulatory mechanisms governing the two main ACTH releasing neuropeptides in the parvocellular neurosecretory neurons. With respect of transcription factors that may mediate these effects, besides rapid phosphorylation of the cAMP-response element-binding protein (CREB), VP activation in the parvocellular neurons requires additional newly synthesized factors such as those encoded by immediate-early genes, like c-fos. In addition, it has recently been revealed that glucocorticoid negative feedback during stress, selectively targets vasopressin transcription in the parvocellular neurons that is likely mediated by interaction of glucocorticoid receptors and immediate-early gene products. These data speak for the emerging consensus that VP is the principal factor that imparts situation-specific drive and represents the regulated variable governing hypothalamo-pituitary-adrenocortical axis during stress.

Role of 5-HT2A receptors in the stress-induced down-regulation of brain-derived neurotrophic factor expression in rat hippocampus

Immobilization stress decreases the expression of BDNF mRNA in the rat hippocampus, and this effect could contribute to the atrophy of hippocampal neurons. This study examines the influence of selective 5-HT, as well as norepinephrine, receptor antagonists on the stress-induced down-regulation of BDNF mRNA. Pretreatment with a selective 5-HT2A receptor antagonist, MDL100,907, significantly blocked the influence of stress on expression of BDNF mRNA. In contrast, pretreatment with either a selective 5-HT2C or 5-HT1A receptor antagonist did not influence the stress-induced decrease in levels of BDNF mRNA. The stress-induced decrease was also not influenced by pretreatment with antagonists of beta(1/2)- or alpha1-adrenergic, or CRF-R1 receptors. The results demonstrate that 5-HT2A receptors mediate, at least in part, the stress-induced down-regulation of BDNF expression in the rat hippocampus.

Behavioral and neurochemical consequences of lipopolysaccharide in mice: anxiogenic-like effects

Systemic administration of lipopolysaccharide (LPS) induces sickness behaviors, as well as alterations of hypothalamic-pituitary-adrenal functioning commonly associated with stressors. In the present investigation, it was demonstrated that systemic LPS treatment induced a sickness-like behavioral profile (reduced active behaviors, soporific effects, piloerection, ptosis), which appeared to be dependent upon the novelty of the environmental context in which animals were tested. As well, LPS induced anxiogenic-like responses, including decreased time spent in the illuminated portion of a light-dark box, reduced open-arm entries in a plus-maze test, and decreased contact with a novel stimulus object in an open-field situation. The behavioral changes were accompanied by increased plasma ACTH and corticosterone levels. As well, LPS induced increased turnover of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in the paraventricular nucleus (PVN), median eminence plus arcuate nucleus, hippocampus, as well as NE turnover within the locus coeruleus and DA turnover within the nucleus accumbens. Although these neurochemical variations were reminiscent of those elicited by stressors, LPS was not particularly effective in modifying DA activity within the prefrontal cortex or NE within the amygdala, variations readily induced by stressors. Whether the LPS-induced anxiogenic-like responses were secondary to the illness engendered by the endotoxin remains to be determined. Nevertheless, it ought to be considered that bacterial endotoxin challenge, and the ensuing cytokine changes, may contribute to emotionality and perhaps even anxiety-related behavioral disturbances.

Regional expressions of Fos-like immunoreactivity in rat cerebral cortex after stress; restraint and intraperitoneal lipopolysaccharide

To demonstrate regional activation in the rat cerebral cortex related to stress-evoked neuroendocrine response, Fos expression in both the cerebral cortex and hypothalamic paraventricular nucleus (PVN) was immunohistochemically examined in two experimental groups; a lipopolysaccharide (LPS) intraperitoneally injected group for inflammatory stress and a restraint group for emotional stress. The LPS injection (100 microg/100 g b.w.) and restraint (for 30 min) had similar effect on Fos-like immunoreactivity (Fos-LI) in PVN with regard to the number of immunoreactive nuclei and their distribution pattern, while the times to maximize Fos-LI were different. Numerical analysis of cortical Fos-LI in untreated rats showed a distinct region-specific pattern. Statistical analysis revealed no significant increase in Fos-LI density in any cortical regions in the LPS group, but restraint resulted in a dramatic and region-specific increase. A significant increase was detected in the prefrontal cortex (the cingulate, orbital and agranular insular cortex), the frontal area 2, the agranular retrosplenial cortex, the parietal cortex, and the medial and lateral occipital area 2. These results indicate that cortical activation relevant to specific functions may be involved in stress-specific neural circuitry.

Neural plasticity, neuropeptides and anxiety in animals--implications for understanding and treating affective disorder following traumatic stress in humans

Exposure of rats to cats (predator stress) lastingly increases rodent anxiety-like behavior (ALB) in the elevated plus-maze. Previous work shows that lasting changes in ALB following predator stress depend on NMDA and CCKB receptors. In this paper we describe the effects of differing degrees of predator exposure on behavior. Effects depend on the behavioral measure. In general, exposure to predator odor is less provocative of lasting change in ALB than is unprotected exposure to a cat. In addition, we examine the development of effects of unprotected predator exposure over time. Lasting effects on ALB begin at 30 min to 1 h after predator stress and persist for at least 3 weeks. We also report a complex pattern of effects of predator stress on neuroendocrine and stress peptide (bombesin, CRF and AVP) levels in a variety of brain areas. Not surprisingly, predator exposure increases plasma levels of corticosterone and ACTH. Central changes in peptide content in the hypothalamo-pituitary axis, related hypothalamic nuclei, limbic and brain stem areas are also noted. Finally, path analysis demonstrates a replicable relationship between cat behavior, rat defensive behavior and degree of increase in ALB one week later. It is proposed that behavioral changes following predator stress may model anxiety associated with PTSD.

Stress and the aging hippocampus

The "glucocorticoid cascade hypothesis" of hippocampal aging has stimulated a great deal of research into the neuroendocrine aspects of aging and the role of glucocorticoids, in particular. Besides strengthening the methods for investigating the aging brain, this research has revealed that the interactions between glucocorticoids and hippocampal neurons are far more complicated than originally envisioned and involve the participation of neurotransmitter systems, particularly the excitatory amino acids, as well as calcium ions and neurotrophins. New information has provided insights into the role of early experience in determining individual differences in brain and body aging by setting the reactivity of the hypothalamopituitary-adrenal axis and the autonomic nervous system. As a result of this research and advances in neuroscience and the study of aging, we now have a far more sophisticated view of the interactions among genes, early development, and environmental influences, as well as a greater appreciation of events at the cellular and molecular levels which protect neurons, and a greater appreciation of pathways of neuronal damage and destruction. While documenting the ultimate vulnerability of the brain to stressful challenges and to the aging process, the net result of this research has highlighted the resilience of the brain and offered new hope for treatment strategies for promoting the health of the aging brain.

Differential immune responses in mice with left- and right-turning preference

Humoral and cell-mediated immune responses of inbred BALB/c male mice were assayed for differential reactivities associated with behavioral sidedness, which was evaluated by spontaneous rotational behavior in a circular cage model system. Mice with left-turning preference had lower in vivo primary IgM and IgG anti-Keyhole Limpet Hemocyanin (KLH) antibody responses, delayed-type hypersensitivity (DTH) responses, and host-resistance against the intracellular bacteria, Listeria monocytogenes, than mice with right-turning preference. The only immune parameter not shown to be associated with turning preference was the secondary humoral immune response to KLH. The weak innate immune response of left-turners for clearance of Listeria showed close intercorrelation with elevated serum IL-6 levels. Serum corticosterone and splenic norepinephrine levels were differentially increased and decreased by infection, respectively. We suggest that the observed differential immune reactivities of individual animals with same age, gender, and genetic background are associated with functional asymmetries within the brain, that the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic innervation are involved in the regulatory brain: immune interconnection after infection, and that the HPA axis and sympathetic nervous system are involved in the brain laterality effects on immune responses.

Norepinephrine Injection into the Paraventricular Nucleus Induces a Reduced Modification of Eating Behavior and Thermogenesis in Brattleboro Rats

Intake of carbohydrates, lipids, proteins and total calories, temperature of interscapular brown adipose tissue, and oxygen consumption were monitored in vasopressin-containing and vasopressin-deficient rats. These variables were measured after a 20 nmol norepinephrine (NE) or saline injection into the paraventricular nucleus (PVN) of the hypothalamus. NE increased the intake of carbohydrates, lipids and total calories, decreased brown adipose tissue temperature and oxygen consumption in vasopressin-containing rats. NE reduced the intake of carbohydrates, while it increased the consumption of lipids in vasopressin-deficient rats. These findings indicate that vasopressin is involved in the modifications of eating behavioral and thermogenesis induced by NE injection into the hypothalamic PVN.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Co-localization of corticotropin-releasing hormone with glutamate decarboxylase and calcium-binding proteins in infant rat neocortical interneurons

Corticotropin releasing hormone (CRH) has been localized to interneurons of the mammalian cerebral cortex, but these neurons have not been fully characterized. The present study determined the extent of co-localization of CRH with glutamate decarboxylase (GAD) and calcium-binding proteins in the infant rat neocortex using immunocytochemistry. CRH-immunoreactive (ir) neurons were classified into two major groups. The first group was larger and consisted of densely CRH-immunostained small bipolar cells, predominantly localized to layers II and III. The second group of CRH-ir cells was lightly labeled and included multipolar neurons mainly found in deep cortical layers. Co-localization studies indicated that the vast majority of CRH-ir neurons, including both bipolar and multipolar types, was co-immunolabeled for GAD-65 and GAD-67. Most multipolar, but only some bipolar, CRH-ir neurons also contained parvalbumin, while CRH-ir neurons rarely contained calbindin or calretinin. These results indicate that virtually all CRH-ir neurons in the rat cerebral cortex are GABAergic. Furthermore, since parvalbumin is expressed by cortical basket and chandelier cells, the co-localization of CRH and parvalbumin suggests that some cortical CRH-ir neurons may belong to these two cell types.

The source of origin of PACAP- and VIP-immunoreactive fibers in the laterodorsal division of the bed nucleus of the stria terminalis in the rat

The bed nucleus of the stria terminalis (BSTL), which is known to be involved in the modulation of stress responses, exhibits a dense network of pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) immunoreactive (ir) fibers. The origin of the PACAP-ir fibers is unknown, and the origin of the VIP-ir fibers remains uncertain. The most important brain regions connected to the BSTL are the amygdaloid nuclei, the paraventricular and ventromedial hypothalamic nuclei, mesencephalic periaqueductal grey, the dorsal and linear raphe nuclei, the parabrachial nucleus, and the dorsal vagal complex. After microinjecting cholera toxin B subunit (CTB) in the BSTL as a retrograde tracer, neurons were double labeled for CTB and PACAP or VIP immunohistochemistry and the cells from which the PACAP- and VIP-ir fiber networks in the BSTL originated were identified. Cholera toxin B subunit labeled and VIP-ir cells were found in the mesencephalic periaqueductal grey and the dorsal and linear raphe nuclei, but no double labeled cells were seen in the amygdaloid nuclei or the hypothalamic region. CTB- and PACAP-ir neurons were observed in the paraventricular nucleus and the dorsal vagal complex. No double labeled perikarya were seen in the parabrachial nucleus or in the amygdaloid nuclei.

Postmortem anterograde tracing of intrahypothalamic projections of the human dorsomedial nucleus of the hypothalamus

Together with the paraventricular nucleus (PVN), the dorsomedial nucleus of the hypothalamus (DMH) acts as one of the hypothalamic centers that integrate autonomic and central information. The DMH in the rat brain has extensive intrahypothalamic connections and is implicated in a wide variety of functions. Up until now, no knowledge has been available to indicate that the human DMH might have functions similar to those of the rat DMH. In the present study, intrahypothalamic efferent projections of the human DMH were revealed by a recently developed in vitro postmortem tracing method. It was found that the most densely innervated areas are the PVN, the ventromedial nucleus of the hypothalamus, and the area below the PVN. Other significant terminal fields include the periventricular nucleus, the lateral hypothalamic area, and the medial part of the anteroventral hypothalamic area. Scarce fibers project to the suprachiasmatic nucleus, infundibular nucleus, posterior hypothalamic nucleus, and posterior part of the bed nucleus of the stria terminals. The projections of the ventral and dorsal part of the DMH show some differences. The dorsal part of the DMH has denser projections to the dorsal part of the PVN than to the ventral part of the PVN. In contrast, the ventral part of the DMH has denser projections to the ventral part of the PVN. Labeled fibers in the PVN from ventral and dorsal DMH appear to run near many vasopressin and oxytocin neurons of different sizes, and also near some corticotropin- releasing hormone neurons, suggesting that the DMH neurons may directly affect the functioning of these PVN neurons. In many aspects, the observed projections of the human DMH resemble those of the rat, indicating that the organization of DMH intrahypothalamic projections of human is similar to that of rat. The functional significance of DMH intrahypothalamic connections is discussed.

Involvement of capsaicin sensitive primary afferents in thymulin-induced hyperalgesia

Intraplantar (5 ng) or intraperitoneal (50 ng) injections of thymulin, produced both thermal and mechanical hyperalgesia in rats. In this report, we show that ablation of capsaicin sensitive primary afferents (CSPA) can alter or abolish thymulin-induced hyperalgesia. Different groups of rats were subjected to either treatment with capsaicin or to surgical subdiaphragmatic vagotomy (SDV). Both capsaicin and SDV reduced significantly thymulin-induced hyperalgesia. On the other hand, these treatments elicited differential effects on the modulation by thymulin of the levels of nerve growth factor and interleukin 1beta. We conclude that the hyperalgesic effects of i.p. thymulin are mainly mediated through the CSPA fibers.

Neuropeptide-mediated excitability: a key triggering mechanism for seizure generation in the developing brain

Most human seizures occur early in life,consistent with established excitability-promoting features of the developing brain. Surprisingly, the majority of developmental seizures are not spontaneous but are provoked by injurious or stressful stimuli. What mechanisms mediate'triggering' of seizures and limit such reactive seizures to early postnatal life? Recent evidence implicates the excitatory neuropeptide, corticotropin-releasing hormone (CRH). Stress activates expression of the CRH gene in several limbic regions, and CRH-expressing neurons are strategically localized in the immature rat hippocampus, in which this neuropeptide increases the excitability of pyramidal cells in vitro. Indeed, in vivo, activation of CRH receptors--maximally expressed in hippocampus and amygdala during the developmental period which is characterized by peak susceptibility to 'provoked' convulsions--induces severe, age-dependent seizures. Thus, converging data indicate that activation of expression of CRH constitutes an important mechanism for generating developmentally regulated, triggered seizures, with considerable clinical relevance.

Evaluation of RU28318 and RU40555 as selective mineralocorticoid receptor and glucocorticoid receptor antagonists, respectively: receptor measures and functional studies

Corticosterone regulates a wide range of physiological parameters. Two receptors for corticosterone have been identified, the mineralocorticoid (type I) receptor (MR) and the glucocorticoid (type II) receptor (GR). To determine the relative role of these two receptors in mediating the effects of endogenous corticosterone, many studies have relied on the use of putative selective corticosteroid receptor antagonists. This study further examined the in vivo receptor selectivity of two compounds, RU28318 and RU40555 that are believed to be selective antagonists for MR and GR, respectively. Acute treatment of adrenalectomized rats with RU28318 (10-50 mg/kg) selectively decreased ex-vivo available MR binding in the hippocampus, whereas acute treatment with RU40555 (10-30 mg/kg) selectively decreased available GR binding in the hippocampus and pituitary. These receptor binding measures suggest that RU28318 in vivo selectively occupied MR, and that RU40555 in vivo selectively occupied GR. In functional studies, RU28318 (50 mg/kg) blocked the normalizing effect of aldosterone (120 microg/kg) on saline intake of adrenalectomized rats. RU40555 (30 mg/kg) blocked the suppressive effect of dexamethasone (50 microg/kg) on acute stress-induced corticosterone secretion. These studies further support the in vivo corticosteroid receptor selectivity of these two compounds and confirms their effective corticosteroid antagonistic properties.

Corticotropin releasing hormone, receptor regulation and the stress response

Corticotropin releasing hormone (CRH) coordinates behavioral, autonomic and hormonal responses to stress, including activation of the hypothalamic-pituitary-adrenal (HPA) axis with stimulation of adrenocorticotropin (ACTH) and glucocorticoids. Differential changes of expression of CRH and vasopressin(VP) in the parvicellular hypothalamic paraventricular nucleus (PVN), as well as regulation of CRH and VP receptors, are critical for the responsiveness of the HPA axis during stress. Pituitary CRH receptor (CRH-R)expression and content is controlled by the coordinated action of CRH, VP and glucocorticoids. Marked changes in hypothalamic and pituitary CRH-R expression support a key regulatory role for CRH in the HPA axis and the integrated stress response.

Ventral subiculum regulates hypothalamo-pituitary-adrenocortical and behavioural responses to cognitive stressors

The hippocampus plays an important role in central stress integration. The present study tests the hypothesis that the ventral subiculum, as a principal source of hippocampal efferents, is involved in co-ordination of hypothalamo-pituitary-adrenocortical and behavioural responses to cognitively-processed information. Basal hypothalamo-pituitary-adrenocortical activation appears to be normal in ventral subiculum lesion rats, as basal corticosterone and adrenocorticotropic hormone secretion, anterior pituitary pro-opiomelanocortin and type 1 corticotropin-releasing hormone receptor messenger RNA expression, adrenal and thymus weight, and splenic mitogen activity are not affected by lesion. Lesions of the ventral subiculum induce glucocorticoid hypersecretion following restraint stress or open field exposure, whereas responses to ether inhalation are unaffected. Interestingly, ventral subiculum lesion does not affect fast glucocorticoid negative feedback inhibition of restraint-induced adrenocorticotropic hormone release. Corticotropin-releasing hormone immunoreactivity is increased in the hypothalamic paraventricular nucleus of ventral subiculum lesion rats, and is differentially depleted by acute stress exposure (relative to sham-lesion rats). However, ventral subiculum lesion does not affect basal and stress-induced corticotropin-releasing hormone, arginine vasopressin and cFOS messenger RNA expression in paraventricular nucleus neurons. Behavioural analysis reveals that ventral subiculum lesion rats are hyper-responsive to open field exposure, showing decreased total ambulation and reduced incidence of central square entry. The results suggest that the ventral subiculum plays a specific role in integrating cognitively-processed stimuli (e.g., restraint and open field exposure) into appropriate neuroendocrine and behavioural responses to stress. Enhanced stress-induced glucocorticoid secretion and increased corticotropin-releasing hormone biosynthesis are likely due to removal of oligosynaptic inhibitory input to the paraventricular nucleus subsequent to ventral subiculum lesion.

Region-specific regulation of glutamic acid decarboxylase (GAD) mRNA expression in central stress circuits

Neurocircuit inhibition of hypothalamic paraventricular nucleus (PVN) neurons controlling hypothalamo-pituitary-adrenocortical (HPA) activity prominently involves GABAergic cell groups of the hypothalamus and basal forebrain. In the present study, stress responsiveness of GABAergic regions implicated in HPA inhibition was assessed by in situ hybridization, using probes recognizing the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD65 and GAD67 isoforms). Acute restraint preferentially increased GAD67 mRNA expression in several stress-relevant brain regions, including the arcuate nucleus, dorsomedial hypothalamic nucleus, medial preoptic area, bed nucleus of the stria terminalis (BST) and hippocampus (CA1 and dentate gyrus). In all cases GAD67 mRNA peaked at 1 hr after stress and returned to unstimulated levels by 2 hr. GAD65 mRNA upregulation was only observed in the BST and dentate gyrus. In contrast, chronic intermittent stress increased GAD65 mRNA in the anterior hypothalamic area, dorsomedial nucleus, medial preoptic area, suprachiasmatic nucleus, anterior BST, perifornical nucleus, and periparaventricular nucleus region. GAD67 mRNA increases were only observed in the medial preoptic area, anterior BST, and hippocampus. Acute and chronic stress did not affect GAD65 or GAD67 mRNA expression in the caudate nucleus, reticular thalamus, or parietal cortex. Overall, the results indicate preferential upregulation of GAD in central circuitry responsible for direct (hypothalamus, BST) or multisynaptic (hippocampus) control of HPA activity. The distinct patterns of GAD65 and GAD67 by acute versus chronic stress suggest stimulus duration-dependent control of GAD biosynthesis. Chronic stress-induced increases in GAD65 mRNA expression predict enhanced availability of GAD65 apoenzyme after prolonged stimulation, whereas acute stress-specific GAD67 upregulation is consistent with de novo synthesis of active enzyme by discrete stressful stimuli.

Aversive and appetitive events evoke the release of corticotropin-releasing hormone and bombesin-like peptides at the central nucleus of the amygdala

There is wide agreement that corticotropin-releasing hormone (CRH) systems within the brain are activated by stressful stimuli. There is also mounting evidence for the role of bombesin (BN)-like peptides in the mediation of the stress response. To date, however, the extent to which other stimuli increase the activity of these peptidergic systems has received little attention. In the present investigation we validated and used in vivo microdialysis sampling followed by ex vivo radioimmunoassays to monitor the release of CRH and BN-like peptides during appetitive (food intake) and stressful (restraint) events. It is demonstrated for the first time that the in vivo release of CRH and BN-like peptides at the central nucleus of the amygdala was markedly increased by both stressor exposure and food ingestion. In fact, the meal-elicited rise of CRH release was as great as that associated with 20 min of restraint stress. Paralleling these findings, circulating ACTH and corticosterone levels were also increased in response to both food intake and restraint. Contrary to the current views, these results indicate that either food ingestion is interpreted as a "stressful" event by certain neural circuits involving the central amygdala or that the CRH- and BN-related peptidergic systems may serve a much broader role than previously envisioned. Rather than evoking feelings of fear and anxiety, these systems may serve to draw attention to events or cues of biological significance, such as those associated with food availability as well as those posing a threat to survival.

Stressor-induced corticotropin-releasing hormone, bombesin, ACTH and corticosterone variations in strains of mice differentially responsive to stressors

The effects of brief stressor exposure on hypothalamic-pituitary-adrenal (HPA) functioning was assessed in two strains of mice shown to be differentially responsive to stressors. Mild stress (1 min of cold swim, 20 C) led to marked elevations of plasma ACTH and corticosterone concentrations in the stress-reactive BALB/cByJ and the stress-resistant C57Bl/6ByJ mice. Moreover, it was observed that the strains differed in basal CRH content within the amygdala and the paraventricullar nucleus (PVN). Within 1 min of cold swim, the CRH changes were detected in these brain regions in BALB/cByJ mice, but were less apparent in C57Bl/6ByJ mice. Following a chronic stressor regimen, the marked elevations of plasma ACTH associated with acute stressors in BALB/cByJ mice were diminished. In contrast, in C57Bl/6ByJ mice in which acute stressors hardly affected ACTH concentrations, the chronic stressor regimen lead to a marked increase of plasma ACTH. Taken together, data indicate that the stress reactivity differences seen in the two strains of mice are not limited to ACTH and corticosterone, but are also detected with respect to CRH within the amygdala and PVN. Furthermore, the suggestion is offered that the reactivity differences in the two strains of mice may have lead to different profiles of ACTH secretagogues and hence the response profile to later acute and chronic stressors differed in these strains of mice.

Psychoneuroendocrinology of depression. Hypothalamic-pituitary-adrenal axis

Among the more consistent observations in patients with major depression is dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis presenting as elevation of basal cortisol, dexamethasone-mediated negative feedback resistance, increased cerebrospinal fluid levels of corticotropin-releasing factor (CRF), and a blunted adrenocorticotropic hormone (ACTH) response to challenge with exogenous CRF. These features appear to be state, rather than trait markers, and are normalized upon successful treatment. These pathophysiologic adaptations may arise from defects in central drive to the neuroendocrine hypothalamus, disruption of normal adrenocortical hormone receptor function or a modification of HPA axis function at any level. Functional assessment of the HPA axis is thought to provide a window into central nervous system operation that may be of diagnostic value in this and other affective disorders regardless of whether CRF and glucocorticoids are directly involved in the origin of major depression or merely exacerbate the consequences of other primary defects.

Do early-life events permanently alter behavioral and hormonal responses to stressors?

Early-life stimulation (e.g., brief handling) attenuates the behavioral and neuroendocrine responses to stressors encountered in adulthood, particularly with respect to activation of hypothalamic-pituitary-adrenal (HPA) activity. In contrast, if neonates were subjected to a more severe stressor, such as protracted separation from the dam or exposure to an endotoxin, then the adult response to a stressor was exaggerated. These early-life experiences program HPA functioning, including negative feedback derived from stimulation of hippocampal glucocorticoid receptors, and corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) coexpression in PVN neurons, to modify the response to subsequent stressor experiences. The persistent variations of HPA activity observed in handled/stimulated animals may stem from alterations in dam-pup interactions (e.g. increased arched-back feeding, licking, grooming). In addition genetic makeup is critical in determining stress reactivity. For instance, BALB/cByJ mice are more reactive to stressors than C57BL/6ByJ mice, exhibiting greater HPA hormonal alterations and behavioral disturbances. BALB/cByJ also fail to acquire a spatial learning response in a Morris water-maze paradigm, which has been shown to be correlated with hippocampal cell loss associated with aging. Early-life handling of BALB/cByJ mice prevented these performance deficits and attenuated the hypersecretion of ACTH and corticosterone elicited by stressors. The stressor reactivity may have been related to maternal and genetic factors. When BALB/cByJ mice were raised by a C57BL/6ByJ dam, the excessive stress-elicited HPA activity was reduced, as were the behavioral impairments. However, cross-fostering the more resilient C57BL/6ByJ mice to a BALB/cByJ dam failed to elicit the behavioral disturbances. It is suggested that genetic factors may influence dam-pup interactive styles and may thus proactively influence the response to subsequent stressors among vulnerable animals. In contrast, in relatively hardy animals the early-life manipulations may have less obvious effects.

Reduced activity of hypothalamic corticotropin-releasing hormone neurons in transgenic mice with impaired glucocorticoid receptor function

Loss of central glucocorticoid receptor (GR) function is thought to be involved in the development of neuroendocrine and psychiatric disorders associated with corticotropin-releasing hormone (CRH) hyperactivity. The possible causal relationship between defective GR function and altered activity of CRH neurons was studied in transgenic mice (TG) expressing antisense RNA against GR. Immunocytochemical studies showed significant reductions in CRH immunoreactive neurons in the paraventricular nucleus (PVN) and in CRH and vasopressin (AVP) stores in the external zone of the median eminence. Concomitantly, stimulus-evoked CRH secretion from mediobasal hypothalami of TG mice in vitro was reduced significantly. However, CRH mRNA levels in the PVN of TG mice were marginally lower than those in wild-type (WT) mice. 125I-CRH binding autoradiography revealed no differences between WT and TG animals in any of the brain regions that were studied. Basal plasma corticosterone (cort) levels and 125I-CRH binding, CRH-R1 mRNA, POMC mRNA, and POMC hnRNA levels in the anterior pituitary gland were similar in WT and TG mice. Intraperitoneal injection of interleukin-1beta (IL-1beta) increased plasma cort levels, CRH mRNA in the PVN, and anterior pituitary POMC hnRNA similarly in WT and TG mice. The injection of saline significantly reduced anterior pituitary CRH-R1 mRNA levels in WT mice, but not in TG mice, whereas IL-1beta produced a decrease in these mRNA levels in both strains. The data show that long-term GR dysfunction can be associated with reduced activity of CRH neurons in the PVN and decreased sensitivity of pituitary CRH-R1 mRNA to stimulus-induced downregulation. Moreover, the hypothalamic changes observed in this model suggest that impaired GR function, at least if present since early embryonic life, does not necessarily result in CRH hyperexpression characteristics of disorders such as major depression.

Brain corticosteroid receptor balance in health and disease

In this review, we have described the function of MR and GR in hippocampal neurons. The balance in actions mediated by the two corticosteroid receptor types in these neurons appears critical for neuronal excitability, stress responsiveness, and behavioral adaptation. Dysregulation of this MR/GR balance brings neurons in a vulnerable state with consequences for regulation of the stress response and enhanced vulnerability to disease in genetically predisposed individuals. The following specific inferences can be made on the basis of the currently available facts. 1. Corticosterone binds with high affinity to MRs predominantly localized in limbic brain (hippocampus) and with a 10-fold lower affinity to GRs that are widely distributed in brain. MRs are close to saturated with low basal concentrations of corticosterone, while high corticosterone concentrations during stress occupy both MRs and GRs. 2. The neuronal effects of corticosterone, mediated by MRs and GRs, are long-lasting, site-specific, and conditional. The action depends on cellular context, which is in part determined by other signals that can activate their own transcription factors interacting with MR and GR. These interactions provide an impressive diversity and complexity to corticosteroid modulation of gene expression. 3. Conditions of predominant MR activation, i.e., at the circadian trough at rest, are associated with the maintenance of excitability so that steady excitatory inputs to the hippocampal CA1 area result in considerable excitatory hippocampal output. By contrast, additional GR activation, e.g., after acute stress, generally depresses the CA1 hippocampal output. A similar effect is seen after adrenalectomy, indicating a U-shaped dose-response dependency of these cellular responses after the exposure to corticosterone. 4. Corticosterone through GR blocks the stress-induced HPA activation in hypothalamic CRH neurons and modulates the activity of the excitatory and inhibitory neural inputs to these neurons. Limbic (e.g., hippocampal) MRs mediate the effect of corticosterone on the maintenance of basal HPA activity and are of relevance for the sensitivity or threshold of the central stress response system. How this control occurs is not known, but it probably involves a steady excitatory hippocampal output, which regulates a GABA-ergic inhibitory tone on PVN neurons. Colocalized hippocampal GRs mediate a counteracting (i.e., disinhibitory) influence. Through GRs in ascending aminergic pathways, corticosterone potentiates the effect of stressors and arousal on HPA activation. The functional interaction between these corticosteroid-responsive inputs at the level of the PVN is probably the key to understanding HPA dysregulation associated with stress-related brain disorders. 5. Fine-tuning of HPA regulation occurs through MR- and GR-mediated effects on the processing of information in higher brain structures. Under healthy conditions, hippocampal MRs are involved in processes underlying integration of sensory information, interpretation of environmental information, and execution of appropriate behavioral reactions. Activation of hippocampal GRs facilitates storage of information and promotes elimination of inadequate behavioral responses. These behavioral effects mediated by MR and GR are linked, but how they influence endocrine regulation is not well understood. 6. Dexamethasone preferentially targets the pituitary in the blockade of stress-induced HPA activation. The brain penetration of this synthetic glucocorticoid is hampered by the mdr1a P-glycoprotein in the blood-brain barrier. Administration of moderate amounts of dexamethasone partially depletes the brain of corticosterone, and this has destabilizing consequences for excitability and information processing. 7. The set points of HPA regulation and MR/GR balance are genetically programmed, but can be reset by early life experiences involving mother-infant interaction. 8. (ABSTRACT TRUNCATED)

Postnatal maternal separation during the stress hyporesponsive period enhances the adrenocortical response to novelty in adult rats by affecting feedback regulation in the CA1 hippocampal field

The aim of the present experiment was to study the effects of early postnatal maternal separation on behavioural and adrenocortical responses to novelty in rats tested as adults. Sprague-Dawley rat pups were exposed to daily maternal separation (5 h/day) from postnatal day 2 to 6, during the stress hyporesponsive period. Since this procedure requires physical contact with the animals, a first control group of daily handled pups was introduced. A second control group, consisting of pups never handled or separated from the mother, was also considered. At postnatal day 45, the rats were tested in a two-compartment exploratory apparatus: the maternally separated and the non-handled rats, whose behavioural performance did not differ, showed higher emotional behaviour when compared with the handled rats (P < 0.05), suggesting that the handling procedure but not maternal separation improved the capacity to cope with novelty. Corticosterone plasma levels were found to be higher in the maternally separated rats than in the other two groups (P < 0.05), either at resting conditions or at 30 min after novelty exposure (P < 0.05). Levels of nuclear glucocorticoid receptor immunoreactivity in the CA1 hippocampal field were shown to be regulated by novelty exposure, as expected, in both the handled and the non-handled rats but not in the maternally separated rats. In conclusion, repeated maternal separation periods of 5 h/day during the first week of life produced long-lasting effects on the hippocampal regulation of the hypothalamic-pituitary-adrenocortical axis, which appear to be associated with increased responsiveness to stress stimuli in adulthood.

Stress, adaptation, and disease. Allostasis and allostatic load

Adaptation in the face of potentially stressful challenges involves activation of neural, neuroendocrine and neuroendocrine-immune mechanisms. This has been called "allostasis" or "stability through change" by Sterling and Eyer (Fisher S., Reason J. (eds): Handbook of Life Stress, Cognition and Health. J. Wiley Ltd. 1988, p. 631), and allostasis is an essential component of maintaining homeostasis. When these adaptive systems are turned on and turned off again efficiently and not too frequently, the body is able to cope effectively with challenges that it might not otherwise survive. However, there are a number of circumstances in which allostatic systems may either be overstimulated or not perform normally, and this condition has been termed "allostatic load" or the price of adaptation (McEwen and Stellar, Arch. Int. Med. 1993; 153: 2093.). Allostatic load can lead to disease over long periods. Types of allostatic load include (1) frequent activation of allostatic systems; (2) failure to shut off allostatic activity after stress; (3) inadequate response of allostatic systems leading to elevated activity of other, normally counter-regulated allostatic systems after stress. Examples will be given for each type of allostatic load from research pertaining to autonomic, CNS, neuroendocrine, and immune system activity. The relationship of allostatic load to genetic and developmental predispositions to disease is also considered.

Dynamic changes in glucocorticoid and mineralocorticoid receptor mRNA in the developing guinea pig brain

The guinea pig has a high degree of neurological maturity at birth. Since glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) are central to several aspects of brain and neuroendocrine development, we examined the hypothesis that development of central GR and MR systems takes place during fetal life, in species which give birth to mature young. Fetal guinea pigs were retrieved on gestational days (gd) 40-45, 50-55, 60-65. A group of 7-day old neonates was also euthanized. Levels of GR and MR mRNA were determined by in situ hybridization followed by computerized image analysis. MR mRNA was confined to limbic structures, and was present at high levels in the hippocampus and dentate gyrus by gd40. Hippocampal MR mRNA levels decreased with the progression of gestation. GR mRNA was more widely distributed, with highest levels being expressed in the cingulate cortex, hippocampus, amygdala and hypothalamic paraventricular nucleus (PVN). In the hippocampus, GR mRNA levels increased with progression of gestation, attaining highest levels near term. In contrast to the hippocampus, GR mRNA levels were highest in the PVN at gd40-45, but decreased dramatically in the last 25 days of gestation. In conclusion, there are dynamic site-specific changes in the expression of corticosteroid receptors in the brain of the fetal guinea pig, at the time of most rapid brain growth. The decreases in GR mRNA levels in the PVN in late gestation likely facilitate the simultaneous increases in ACTH and cortisol that occur near term, and which are critical for the delivery of viable young.

Induction of corticotropin-releasing hormone gene expression by glucocorticoids: implication for understanding the states of fear and anxiety and allostatic load

Evidence supports the idea of two distinct corticotropin-releasing hormone (CRH) systems in the brain: one which is constrained by glucocorticoids and the other which is not. It is this latter system that includes two primary sites (central nucleus of the amygdala and the lateral bed nucleus of the stria terminalis) in which the regulation of CRH gene expression can be disassociated from that of the paraventricular nucleus of the hypothalamus. It is this other system that we think is linked to fear and anxiety and to clinical syndromes (excessively shy fearful children, melancholic depression, post-traumatic stress disorder and self-administration of psychotropic drugs). The excess glucocorticoids and CRH, and the state of anticipatory anxiety, contribute to allostatic load, a new term that refers to the wear and tear on the body and brain arising from attempts to adapt to adversity.

Neural networks for generation and suppression of alpha rhythm: a PET study

To study neuronal activities that influence the generation of the alpha rhythm, we used positron emission tomography and simultaneous recording of the electroencephalogram (EEG) in normal volunteers and under passive conditions. A negative correlation between regional cerebral blood flow and alpha power was found in the occipital cortex, consistent with the visual modality-specific reactivity of the alpha rhythm. A positive correlation was found in the pons, midbrain, hypothalamus, amygdala, the basal prefrontal cortex, insula and the right dorsal premotor cortex. Neuronal activities of the brain stem and limbic system that are positively correlated with alpha power may provide an anatomical basis for studies of the relationship between emotional state and brain rhythm in health and disease.

Are bombesin-like peptides involved in the mediation of stress response?

The neurochemical mechanisms underlying the coincident activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system in response to stress remain unclear. Central injection of the neuropeptide bombesin (BN) potently stimulates the release of epinephrine from the adrenal medulla, adrenocorticotropic hormone (ACTH) from the pituitary gland, and elicits behaviors typically associated with increased emotionality and arousal. The current studies assessed whether stress is associated with 1) fluctuations in the endogenous regional levels of BN-like peptides and/or 2) changes in BN receptor density. Male Sprague-Dawley rats received either no treatment or were subjected to acute immobilization stress for 10, 30 or 120 min. Plasma ACTH levels increased in response to stress, peaking at 30 min. BN-like immunoreactivity increased significantly at the hypothalamus and medulla, within 30 min; however with more sustained immobilization (120 min) BN-like immunoreactivity declined to control levels. Levels of BN-like peptides remained unchanged in several other regions, including the hippocampus, striatum, midbrain, pituitary, and pons. Autoradiographic analysis revealed that the density of BN receptor varied in a regionally specific manner. Significant stress related increases in binding were found at the nucleus of the solitary tract (at 30 and 120 min), and at the paraventricular (at 120 min) and arcuate nuclei (at 120 min) of the hypothalamus. These data indicate the BN-like peptides may play a role in the mediation and/or modulation of response to stress.

Intracerebroventricular injection-induced increase in plasma corticosterone levels in the mouse: a stress model

The method of intracerebroventricular (i.c.v.) injection of drugs to conscious mice is a simple and useful technique for studying the central actions of drugs in mice. However, the use of this technique to dissect the central regulatory mechanisms of stress-activated hypothalamo-pituitary-adrenocortical (HPA) axis may produce confusing results difficult to interpret, because i.c.v. injection itself induces an increase in plasma corticosterone in mice due to the traumatic nature of the technique. Here we propose to use the i.c.v. injection itself as a stress stimulus in mice. An i.c.v. saline injection induced an increase in plasma corticosterone levels in mice, which reached a maximum of 38.0+/-1.9 microg/100 ml at 30 min after the i.c.v. injection. Alpha-helical corticotropin-releasing factor (CRF) 9-41, a CRF antagonist, injected i.c.v. (1, 3 microg), effectively inhibited the injection stress-induced rise in plasma corticosterone levels, suggesting the involvement of CRF in this response. This i.c.v. injection stress model permits the evaluation of the effects of drugs administered i.c.v. simultaneously.

Influence of a psychogenic and a neurogenic stressor on several indices of immune functioning in different strains of mice

It is demonstrated that cell proliferation in response to mitogens, natural killer cell (NK) activity, and macrophage functioning of mice may be influenced by either a neurogenic stressor (footshock) or a psychogenic stressor (exposing the mouse to a predator, namely a rat). The nature and magnitude of the immune changes, however, varied across three strains of mice (BALB/cByJ, C57BL/6ByJ, and CD-1), differing in reactivity to stressors and also as a function of the type of stressor employed. While footshock reduced mitogen-stimulated B-cell proliferation in BALB/cByJ mice, it had the opposite effect in the CD-1 strain. Exposure to the predator, however, had little effect in any of the strains. Macrophage activity and NK cytotoxicity were reduced in response to both stressors in a strain-dependent fashion. Plasma corticosterone in response to footshock was greater in BALB/cByJ than in C57BL/6ByJ mice; however, the strain difference was not evident in response to the psychogenic stressor. It is suggested that analyses of stressor effects on immune functioning need to consider the specific strain/species employed, the particular immune parameters being examined, and the nature of the stressor employed.

Elevation of brain 5-HT activity, POMC expression, and plasma cortisol in socially subordinate rainbow trout

Agonistic behavior, brain concentrations of serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA, the main 5-HT metabolite), plasma cortisol levels, and the pituitary expression of pro-opiomelanocortin (POMC) A and B mRNA were determined in socially dominant and subordinate rainbow trout after 1 or 7 days of social interaction. Telencephalic and brain stem 5-HIAA/5-HT ratios, plasma cortisol levels, and pituitary POMC mRNA concentrations were elevated in fish being subordinate for 1 day. Furthermore, neither telencephalic 5-HIAA/5-HT ratios nor pituitary POMC A or POMC B mRNA expression showed any decline after 7 days of social interaction. By contrast, plasma cortisol concentrations of subordinate fish declined after 7 days but were still significantly higher than in dominant fish. Furthermore, in subordinate fish, hypothalamic 5-HIAA/5-HT ratios and plasma cortisol levels were highly correlated, suggesting an important role of hypothalamic 5-HT in the regulation of the teleost hypothalamic-pituitary-interrenal (HPI) axis. The number of aggressive acts received and plasma cortisol levels were highly correlated in 1-day subordinates, a relationship not seen in fish subjected to 1 wk of subordination. Thus the chronic stress experienced by subordinates in established dominance hierarchies appears to be more closely related to the threat imposed by the presence of the dominant fish than to actual aggressive encounters. The sustained elevation of pituitary POMC mRNA expression, an effect mainly related to an increase of melanotropic POMC expression, in subordinates could be a mechanism serving to maintain HPI axis excitability and promote acclimation in these individuals.

Effects of electric footshock and water immersion restraint stresses on fibrinolytic parameters in the plasma of rats

Wistar rats were exposed to electric footshock (ES) or water immersion restraint stress (WS). Blood was taken immediately after, 24, or 48 hours after the stress. The stomachs of rats taken 1 hour after stress application indicate that there were many bleeding spots in the stomachs of WS rats, but practically no visible bleeding spots in the stomachs of ES rats. Plasma levels of t-PA antigens increased in ES rats up to 24 hours after the stress, but the t-PA antigen levels decreased up to 48 hours in ES rats. There were no changes in t-PA activities in plasma of WS rats, but the levels in ES rats decreased immediately and 48 hours after the stress. PAI activity did not change immediately after WS but increased 24 hours after the stress. There was no change in PAI activity in ES rats up to 48 hours. ELT did not change in ES rats, but prolonged in WS rats at 24 hours after the stress. There were significant negative correlations between t-PA antigen levels or activities and ELT in control rats. No correlation was observed in ES or WS rats between t-PA antigen levels and ELT, and no correlation was shown in WS rats between t-PA activities and ELT. Plasma levels of catecholamines increased at the 20-minute period during ES, which may not explain the delayed effects of ES on hemostatic balance. Plasma levels of arginine vasopressin increased significantly immediately after the shock up to 2 hours, indicating that the stress was conveyed to the hypothalamus during the stress application. These results may indicate that some stressors induce an increase or decrease in the local balance of fibrinolytic activities, resulting in bleeding or thrombosis in the local vessels. Such changes may not be detected in the general circulation due to the neutralization of locally induced fibrinolytic changes or the involvement of other hepatically originated hemostatic factors induced by stressors.

Corticotropin-releasing hormone (CRH)-containing neurons in the immature rat hippocampal formation: light and electron microscopic features and colocalization with glutamate decarboxylase and parvalbumin

Corticotropin-releasing hormone (CRH) excites hippocampal neurons and induces death of selected CA3 pyramidal cells in immature rats. These actions of CRH require activation of specific receptors that are abundant in CA3 during early postnatal development. Given the dramatic effects of CRH on hippocampal neurons and the absence of CRH-containing afferents to this region, we hypothesized that a significant population of CRHergic neurons exists in developing rat hippocampus. This study defined and characterized hippocampal CRH-containing cells by using immunocytochemistry, ultrastructural examination, and colocalization with gamma-aminobutyric acid (GABA)-synthesizing enzyme and calcium-binding proteins. Numerous, large CRH-immunoreactive (ir) neurons were demonstrated in CA3 strata pyramidale and oriens, fewer were observed in the corresponding layers of CA1, and smaller CRH-ir cells were found in stratum lacunosum-moleculare of Ammon's horn. In the dentate gyrus, CRH-ir somata resided in the granule cell layer and hilus. Ultrastructurally, CRH-ir neurons had aspiny dendrites and were postsynaptic to both asymmetric and symmetric synapses. CRH-ir axon terminals formed axosomatic and axodendritic symmetric synapses with pyramidal and granule cells. Other CRH-ir terminals synapsed on axon initial segments of principal neurons. Most CRH-ir neurons were coimmunolabeled for glutamate decarboxylase (GAD)-65 and GAD-67 and the majority also contained parvalbumin, but none were labeled for calbindin. These results confirm the identity of hippocampal CRH-ir cells as GABAergic interneurons. Further, a subpopulation of neurons immunoreactive for both CRH and parvalbumin and located within and adjacent to the principal cell layers consists of basket and chandelier cells. Thus, axon terminals of CRH-ir interneurons are strategically positioned to influence the excitability of the principal hippocampal neurons via release of both CRH and GABA.

Corticosteroids in the brain. Cellular and molecular actions

The rat adrenal hormone corticosterone reaches the brain and binds to intracellular receptors. These receptors comprise high-affinity mineralocorticoid and lower-affinity glucocorticoid receptors that, upon activation, affect the transcription rate of specific genes. The two receptor types are discretely localized in the brain, with particularly high expression levels in the hippocampus. Here we review recent studies showing that electrical properties and structural aspects of hippocampal principal neurons are specifically regulated by mineralocorticoid- or glucocorticoid-receptor activation. The molecular mechanisms by which these cellular effects could be accomplished are discussed.

Orphanin FQ acts as an anxiolytic to attenuate behavioral responses to stress

Orphanin FQ (OFQ, Nociceptin) is a recently discovered 17-amino acid neuropeptide that is structurally related to the opioid peptides but does not bind opioid receptors. OFQ has been proposed to act as an anti-opioid peptide, but its widespread sites of action in the brain suggest that it may have more general functions. Here we show that OFQ plays an important role in higher brain functions because it can act as an anxiolytic to attenuate the behavioral inhibition of animals acutely exposed to stressful/anxiogenic environmental conditions. OFQ anxiolytic-like effects were consistent across several behavioral paradigms generating different types of anxiety states in animals (light-dark preference, elevated plus-maze, exploratory behavior of an unfamiliar environment, pharmacological anxiogenesis, operant conflict) and were observed at low nonsedating doses (0.1-3 nmol, intracerebroventricular). Like conventional anxiolytics, OFQ interfered with regular sensorimotor function at high doses (>3 nmol). Our results show that an important role of OFQ is to act as an endogenous regulator of acute anxiety responses. OFQ, probably in concert with other major neuropeptides, exerts a modulatory role on the central integration of stressful stimuli and, thereby, may modulate anxiety states generated by acute stress.

Facilitation of feedback inhibition through blockade of glucocorticoid receptors in the hippocampus

In the present study the effects of intracerebroventricular (i.c.v.) and intrahippocampal administration of corticosteroid antagonists on basal hypothalamic-pituitary-adrenal (HPA) activity around the diurnal peak were compared in male Wistar rats. In two separate experiments the glucocorticoid receptor (GR) antagonist RU 38486 and the mineralocorticoid receptor (MR) antagonist RU 28318 were tested. One hour after GR antagonist injection, significant increases in plasma ACTH and corticosterone levels were observed in the i.c.v. treated rats, when compared to vehicle. In contrast, a significant decrease in ACTH levels, and a slight, but non-significant decrease in corticosterone concentrations were attained one hour after intrahippocampal injection of the GR antagonist. Injection of the MR antagonist, on the other hand, resulted in enhanced ACTH and corticosterone levels irrespective of the site of injection. These findings suggest that negative feedback inhibition at the circadian peak involves hippocampal MRs and extrahippocampal (hypothalamic) GRs. The latter feedback inhibition overrides a positive feedback influence exerted by endogenous corticosteroids through hippocampal GRs.

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

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