Transcriptional responses of the vasopressin and corticotropin-releasing hormone genes to acute and repeated intraperitoneal hypertonic saline injection in rats

Role of glucocorticoid negative feedback in the regulation of HPA axis pulsatility

The hypothalamic-pituitary-adrenal (HPA) axis is the major neuroendocrine axis regulating homeostasis in mammals. Glucocorticoid hormones are rapidly synthesized and secreted from the adrenal gland in response to stress. In addition, under basal conditions glucocorticoids are released rhythmically with both a circadian and an ultradian (pulsatile) pattern. These rhythms are important not only for normal function of glucocorticoid target organs, but also for the HPA axis responses to stress. Several studies have shown that disruption of glucocorticoid rhythms is associated with disease both in humans and in rodents. In this review, we will discuss our knowledge of the negative feedback mechanisms that regulate basal ultradian synthesis and secretion of glucocorticoids, including the role of glucocorticoid and mineralocorticoid receptors and their chaperone protein FKBP51. Moreover, in light of recent findings, we will also discuss the importance of intra-adrenal glucocorticoid receptor signaling in regulating glucocorticoid synthesis.

Novel stress increases hypothalamic-pituitary-adrenal activity in mice with a raised bite

BACKGROUND AND OBJECTIVE

In humans, occlusal disharmony may cause various physical complaints, including head and neck ache, stiffness in the shoulder and neck, and arthrosis of the temporomandibular joints. Occlusal disharmony induced by raising the bite in rodents, increases plasma corticosterone levels, which leads to morphologic changes in the hippocampus and altered hippocampus-related behavior. The paraventricular nucleus (PVN) of the hypothalamus regulates the hypothalamic-pituitary-adrenal system. Chronically stressed animals exposed to a novel stress exhibit higher adrenocorticotropic hormone levels than naive control animals. We hypothesized that there would be different response of the corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) to a novel acute stress with occlusal disharmony.

DESIGN

In order to investigate how exposure of mice with occlusal disharmony to a novel acute stress (restraint stress) affects the PVN, we induced occlusal disharmony by raising the vertical dimension of the bite (bite-raised condition) and examined the expression of corticotrophin releasing hormone (CRH) mRNA and arginine vasopressin (AVP) mRNA in mouse PVN.

RESULTS

CRH mRNA expression was increased in the PVN of the bite-raised group 90min after the bite-raising procedure, but the expression was recovered to the control level at 14days. AVP mRNA expression in the PVN was normal at 90min, and increased significantly 14days after the bite-raising procedure. Exposure to restraint stress in the bite-raised mice induced a significant increase in CRH mRNA expression in the PVN.

CONCLUSIONS

The bite-raising procedure induced a rapid CRH mRNA response and a slower AVP mRNA response in the parvocellular PVN of the hypothalamus. Exposure to a novel stress following the bite-raising procedure further reinforced the CRH stress response. Thus, occlusal disharmony, such as that induced by raising the bite, may be a risk factor for hypersensitivity to a novel stress.

Regulation of the hypothalamic-pituitary-adrenal axis by neuropeptides

The major endocrine response to stress occurs via activation of the hypothalamic-pituitary-adrenal (HPA) axis, leading ultimately to increases in circulating glucocorticoids, which are essential for the metabolic adaptation to stress. The major players in the HPA axis are the hypothalamic neuropeptide, corticotropin releasing hormone (CRH), the pituitary hormone adrenocorticotropic hormone, and the negative feedback effects of adrenal glucocorticoids. In addition, a number of other neuropeptides, including vasopressin (VP), angiotensin II, oxytocin, pituitary adenylate cyclase activating peptide, orexin and cholecystokinin, and nesfatin can affect HPA axis activity by influencing the expression and secretion of CRH, and also by modulating pituitary corticotroph function or adrenal steroidogenesis. Of these peptides, VP co-secreted with CRH from axonal terminals in the external zone of the median eminence plays a prominent role by potentiating the stimulatory effect of CRH and by increasing the number of pituitary corticotrophs during chronic challenge. Although the precise role and significance of many of these neuropeptides in regulating HPA axis activity requires further investigation, it is likely that they are part of a multifactorial system mediating the fine tuning of HPA axis activity during adaptation to a variety of physiological and stressful conditions.

The impact of ventral noradrenergic bundle lesions on increased IL-1 in the PVN and hormonal responses to stress in male sprague dawley rats

The impact of acute stress on inflammatory signaling within the central nervous system is of interest because these factors influence neuroendocrine function both directly and indirectly. Exposure to certain stressors increases expression of the proinflammatory cytokine, Il-1β in the hypothalamus. Increased IL-1 is reciprocally regulated by norepinephrine (stimulatory) and corticosterone (inhibitory), yet neural pathways underlying increased IL-1 have not been clarified. These experiments explored the impact of bilateral lesions of the ventral noradrenergic bundle (VNAB) on IL-1 expression in the paraventricular nucleus of the hypothalamus (PVN) after foot shock. Adult male Sprague Dawley rats received bilateral 6-hydroxydopamine lesions of the VNAB (VNABx) and were exposed to intermittent foot shock. VNABx depleted approximately 64% of norepinephrine in the PVN and attenuated the IL-1 response produced by foot shock. However, characterization of the hypothalamic-pituitary-adrenal response, a crucial prerequisite for interpreting the effect of VNABx on IL-1 expression, revealed a profound dissociation between ACTH and corticosterone. Specifically, VNABx blocked the intronic CRH response in the PVN and the increase in plasma ACTH, whereas corticosterone was unaffected at all time points examined. Additionally, foot shock led to a rapid and profound increase in cyclooxygenase-2 and IL-1 expression within the adrenal glands, whereas more subtle effects were observed in the pituitary gland. Together the findings were the 1) demonstration that exposure to acute stress increased expression of inflammatory factors more broadly throughout the hypothalamic-pituitary-adrenal axis; 2) implication of a modest role for norepinephrine-containing fibers of the VNAB as an upstream regulator of PVN IL-1; and 3) suggestion of an ACTH-independent mechanism controlling the release of corticosterone in VNABx rats.

Stress responsiveness of the hypothalamic-pituitary-adrenal axis: age-related features of the vasopressinergic regulation

The hypothalamic-pituitary-adrenal (HPA) axis plays a key role in adaptation to environmental stresses. Parvicellular neurons of the hypothalamic paraventricular nucleus secrete corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) into pituitary portal system; CRH and AVP stimulate adrenocorticotropic hormone (ACTH) release through specific G-protein-coupled membrane receptors on pituitary corticotrophs, CRHR1 for CRH and V1b for AVP; the adrenal gland cortex secretes glucocorticoids in response to ACTH. The glucocorticoids activate specific receptors in brain and peripheral tissues thereby triggering the necessary metabolic, immune, neuromodulatory, and behavioral changes to resist stress. While importance of CRH, as a key hypothalamic factor of HPA axis regulation in basal and stress conditions in most species, is generally recognized, role of AVP remains to be clarified. This review focuses on the role of AVP in the regulation of stress responsiveness of the HPA axis with emphasis on the effects of aging on vasopressinergic regulation of HPA axis stress responsiveness. Under most of the known stressors, AVP is necessary for acute ACTH secretion but in a context-specific manner. The current data on the AVP role in regulation of HPA responsiveness to chronic stress in adulthood are rather contradictory. The importance of the vasopressinergic regulation of the HPA stress responsiveness is greatest during fetal development, in neonatal period, and in the lactating adult. Aging associated with increased variability in several parameters of HPA function including basal state, responsiveness to stressors, and special testing. Reports on the possible role of the AVP/V1b receptor system in the increase of HPA axis hyperactivity with aging are contradictory and requires further research. Many contradictory results may be due to age and species differences in the HPA function of rodents and primates.

Vasopressin modulates hypothalamo-pituitary activity by paracrine action during acute and chronic immobilization stress in rats

The aim of the study was to examine changes in corticotropin releasing hormone (CRH), vasopressin (VP), adrenocorticotropin (ACTH) as well as VP3 receptor protein concentration in the hypothalamus and the pituitary of rats exposed to acute (3 h) and chronically repeated (3 h daily for 7 days) immobilization stress. Our results show that, unlike the increase in ACTH, there were no changes in serum VP despite the significant changes in this hormone and its receptor concentrations in the hypothalamus and pituitary (HP). This suggests that VP regulates HP activity by predominantly acting in a paracrine manner under the examined stress condition.

The parvocellular vasopressinergic system and responsiveness of the hypothalamic pituitary adrenal axis during chronic stress

Vasopressin (VP) secreted from parvocellular neurons of the hypothalamic paraventricular nucleus (PVN) stimulates pituitary adrenocorticotropic hormone (ACTH) secretion, through interaction with receptors of the V1b subtype (V1bR) in the pituitary corticotroph, mainly by potentiating the stimulatory effects of corticotrophin-releasing hormone (CRH). Chronic stress paradigms associated with corticotroph hyperresponsiveness lead to preferential expression of hypothalamic VP over CRH and upregulation of pituitary V1bR, suggesting that VP has a primary role during adaptation of the hypothalamic pituitary adrenal (HPA) axis to long-term stimulation. However, studies using pharmacological or genetic ablation of V1bR have shown that VP is required for full ACTH responses to some stressors, but not for the sensitization of ACTH responses to a novel stress observed during chronic stress. Studies using minipump infusion of a peptide V1 antagonist in long-term adrenalectomized rats have revealed that VP mediates proliferative responses in the pituitary. Nevertheless, only a minor proportion of cells undergoing mitogenesis co-express markers for differentiated corticotrophs or precursors, suggesting that new corticotrophs are recruited from yet undifferentiated cells. The overall evidence supports a limited role of VP regulating acute ACTH responses to some acute stressors and points to cell proliferation and pituitary remodelling as alternative roles for the marked increases in parvocellular vasopressinergic activity during prolonged activation of the HPA axis.

Vasopressin does not mediate hypersensitivity of the hypothalamic pituitary adrenal axis during chronic stress

The hypothesis that vasopressin (VP) becomes the main mediator of pituitary corticotroph responsiveness during chronic hypothalamic pituitary adrenal (HPA) axis activation was tested by examining the effect of pharmacologic VP receptor blockade on the adrenocorticotropic hormone (ACTH) and corticosterone responses of 14-day repeatedly restrained rats. In spite of the increased vasopressinergic activity, repeatedly restrained rats showed lower ACTH and corticosterone responses to 10 min white noise compared with handled controls. These responses were unchanged by injection of the nonpeptide-selective V1b receptor antagonist SSR149415 i.v., 1 h before noise application. In contrast to noise stress, plasma ACTH responses to i.p. hypertonic saline injection were enhanced in the repeatedly restrained rats compared with handled controls, but responses were also unaffected by SSR149415 administered orally, daily 1 h before restraint. Since SSR149415 effectiveness was low, we used minipump infusion of the peptide V1 receptor antagonist, dGly[Phaa1,D-tyr(et), Lys, Arg]VP (V1-Ant) for 14 days, which effectively blocked ACTH responses to exogenous VP. Chronic V1-Ant infusion reduced plasma ACTH responses to i.p. hypertonic saline in handled controls but not in repeatedly restrained rats. These data suggest that the increased vasopressinergic activity characteristic of chronic stress plays roles other than mediating the hypersensitivity of the HPA axis to a novel stress.

Evolutionarily conserved glucocorticoid regulation of corticotropin-releasing factor expression

Glucocorticoids (GCs) exert feedback regulation on corticotropin-releasing factor (CRF) neurons in mammals. The nature of GC actions is cell-type specific, being either inhibitory (e.g. paraventricular nucleus) or stimulatory (e.g. amygdala and bed nucleus of the stria terminalis). Nothing is known about differential regulation of CRF gene expression by GCs in nonmammalian vertebrates. We studied the actions of GCs on CRF expression in discrete brain regions of the frog Xenopus laevis. Treatment with corticosterone (CORT) decreased, whereas the corticosteroid synthesis inhibitor metyrapone increased CRF expression in the anterior preoptic area (homolog of the mammalian paraventricular nucleus), as measured by CRF primary transcript, mRNA, and CRF immunoreactivity (ir) (by immunocytochemistry). By contrast to the preoptic area, CORT increased CRF-ir in the medial amygdala and bed nucleus of the stria terminalis, whereas metyrapone decreased CRF-ir in the medial amygdala. CRF-ir and glucocorticoid receptor-ir were colocalized in cells in the frog brain. In transient transfection assays in PC-12 cells, GCs decreased forskolin-induced activation of the frog CRF promoters. Treatment with CORT also reduced CRF promoter activity in transfected tadpole brain in vivo. Frog glucocorticoid receptor bound with high-affinity in vitro to regions in the proximal promoters of frog CRF genes that are homologous with the human CRF gene. Our findings suggest that the neural cell-type specificity and molecular mechanisms of GC-dependent regulation of CRF are phylogenetically ancient, and that the limbic pathways mediating behavioral and physiological responses to stressors were likely present in the earliest land-dwelling vertebrates.

Pituitary-adrenal response to acute and repeated mild restraint, forced swim and change in environment stress in arginine vasopressin receptor 1b knockout mice

Arginine vasopressin and corticotrophin-releasing hormone synthesised and released from the hypothalamic paraventricular nucleus are the prime mediators of the hypothalamic-pituitary-adrenal (HPA) axis response to stress. These neurohormones act synergistically to stimulate adrenocorticotophin (ACTH) secretion from the anterior pituitary, culminating in an increase in circulating glucocorticoids. Arginine vasopressin mediates this action at the arginine vasopressin 1b receptor (Avpr1b) located on pituitary corticotrophs. Arginine vasopressin is regarded as a minor ACTH secretagogue in rodents but evidence suggests that it has a role in mediating the neuroendocrine response to some acute and chronic stressors. To investigate the role of the Avpr1b in the HPA axis response to an acute and chronic (repeated) stress, we measured the plasma ACTH and corticosterone concentrations in three stress paradigms in both Avpr1b knockout and wild-type mice. Single acute exposure to restraint, forced swim and change in environment stressors elevated both plasma ACTH and corticosterone concentrations in wild-type animals. Conversely, the ACTH response to the acute stressors was significantly attenuated in Avpr1b knockout mice compared to their wild-type counterparts. Plasma corticosterone concentrations were reduced in Avpr1b knockout mice in response to change in environment but not to mild restraint or forced swim stress. Irrespective of genotype, there was no difference in the plasma ACTH or corticosterone concentrations in response to acute and repeated stressors. The data show that a functional Avpr1b is required for an intact pituitary ACTH response to the acute and chronic stressors used in this study. Furthermore, the normal corticosterone response to repeated exposure to change in environment stress also requires the Avpr1b to drive HPA axis responsiveness.

Negative regulation of corticotropin releasing factor expression and limitation of stress response

Corticotropin releasing factor (CRF) coordinates behavioral, autonomic and hormonal responses to stress. Activation of the hypothalamic pituitary adrenal (HPA) axis with stimulation of CRF and vasopressin (VP) release from hypothalamic parvocellular neurons, and consequent secretion of ACTH from the anterior pituitary and glucocorticoid from the adrenal cortex, is the major endocrine response to stress. Current evidence indicates that the main regulator of ACTH secretion in acute and chronic conditions is CRF, in spite of the fact that the selective increases in expression of parvocellular VP and pituitary VP V1b receptors observed during prolonged activation of the HPA axis have suggested that VP becomes the predominant regulator. Following CRF release, activation of CRF transcription is required to restore mRNA and peptide levels, but termination of the response is essential to prevent pathology associated with chronic elevation of CRF and glucocorticoid production. While glucocorticoid feedback plays an important role in regulating CRF expression, the relative importance of direct transcriptional repression of the CRF gene by glucocorticoids in the overall feedback mechanism is not clear. In addition to glucocorticoids, intracellular feedback mechanisms in the CRF neuron, involving induction of repressor forms of cAMP response element modulator (CREM) limit CRF transcriptional responses by competing with the positive regulator, phospho-CREB. Rapid repression of CRF transcription following stress-induced activation is likely to contribute to limiting the stress response and to preventing disorders associated with excessive CRF production.

Induction of the galanin-like peptide gene expression in the posterior pituitary gland after acute osmotic stimulus in rats

Galanin-like peptide (GALP) is a 60 amino-acid peptide, and the GALP mRNA is restricted to pituicytes in the posterior pituitary gland (PP) and neurons in the hypothalamic arcuate nucleus (Arc). We examined whether the GALP gene expression in the PP and Arc would be induced after intraperitoneal (i.p.) administration of hypertonic saline, that is, acute osmotic stimulus, in rats. The dose-response (2.8, 4.5, 6.0 and 9.0% NaCl) and time-course (6.0% NaCl, 1, 3, 6, 12 and 24h) effects of acute osmotic stimulus on GALP mRNA levels in the PP and Arc were examined in rats by using in situ hybridization histochemistry. Plasma osmolality and plasma sodium concentration increased significantly at 1h, and returned to control level at 6h after i.p. administration of hypertonic saline (6.0% NaCl). The GALP mRNA level in the PP increased significantly 3 and 6h after i.p. administration of hypertonic saline (6.0% NaCl), but the level in the Arc did not change. These results showed that acute osmotic stimulus-induced GALP gene expression in the pituicyte of the PP, but not in the neurons in the Arc, and the gene expression in the pituicyte might be regulated by plasma osmolality and/or plasma sodium concentration.

Tóth Z, House SB, Gainer H. Depolarization and neurotransmitter regulation of vasopressin gene expression in the rat suprachiasmatic nucleus in vitro

Vasopressin (VP) transcription in the rat suprachiasmatic nucleus (SCN) in organotypic culture was studied by in situ hybridization histochemistry using an intron-specific VP heteronuclear RNA probe. The circadian peak of VP gene transcription in the SCN in vitro is completely blocked by a 2 h exposure to tetrodotoxin (TTX) in the culture medium, and this TTX inhibition of VP gene transcription is reversed by exposure of the SCN to either forskolin or potassium depolarization. This suggests that an intrinsic, spontaneously active neuronal mechanism in the SCN is responsible for the cAMP- and depolarization-dependent pathways involved in maintaining peak VP gene transcription. In this paper, we evaluate a variety of neurotransmitter candidates, membrane receptors, and signal-transduction cascades that might constitute the mechanisms responsible for the peak of VP gene transcription. We find that vasoactive intestinal peptide (VIP) and a VPAC2 (VIP receptor subtype 2) receptor-specific agonist, Ro-25-1553, are the most effective ligands tested in evoking a cAMP-mitogen-activated protein kinase signal transduction cascade leading to an increase in VP gene transcription in the SCN. In addition, a second independent pathway involving depolarization activating L-type voltage-gated calcium channels and a Ca-dependent kinase pathway [inhibited by KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine)] rescues VP gene transcription in the presence of TTX. In the absence of TTX, these independent pathways appear to act in a cooperative manner to generate the circadian peak of VP gene transcription in the SCN.

Angiotensin II AT1 receptor blockade prevents the hypothalamic corticotropin-releasing factor response to isolation stress

Sustained pretreatment with angiotensin II AT(1) receptor antagonists prevents the sympathoadrenal and hormonal responses to 24 h isolation stress. To elucidate the mechanism of the anti-stress effects of AT(1) receptor antagonism, we examined the effect of subcutaneous infusion of candesartan, a non-competitive AT(1) receptor antagonist, 0.5 mg/kg/day for 14 days, to Wistar rats on the hypothalamic pituitary adrenal (HPA) axis after 24 h isolation stress. In the morning of day 15, we measured AT(1) receptors corticotropin-releasing factor (CRF) mRNA and immunoreactive CRF in the paraventricular nucleus (PVN), the pituitary adrenocorticotropin hormone (ACTH) and adrenal corticosterone content, and the urinary corticosterone excretion. In rats not treated with candesartan, 24 h isolation stress increased pituitary ACTH, adrenal corticosterone content and AT(1) receptor binding in the PVN but decreased CRF mRNA and CRF content in the PVN. This indicates enhanced CRF utilization not compensated by CRF gene transcription and effective glucocorticoid feedback inhibition in spite of the increase in AT(1) receptor expression. The effects of stress on HPA axis activation and CRF mRNA and content in the PVN were prevented by candesartan pretreatment, suggesting that activation of AT(1) receptors is required for the HPA axis response to isolation. Our results support the hypothesis that the activity of PVN AT(1) receptors is part of the mechanism necessary for development of a full stress-induced HPA axis activation. Inhibition of central AT(1) receptors limits the CRF response to stress and should be considered as a therapeutic tool to preserve homeostasis under chronic stress conditions.

Role of glucocorticoids and cAMP-mediated repression in limiting corticotropin-releasing hormone transcription during stress

The role of glucocorticoids and the repressor isoform of cAMP response element (CRE) modulator (CREM), inducible cAMP early repressor (ICER), in limiting corticotropin-releasing hormone (CRH) transcription during restraint stress were examined in both intact and adrenalectomized rats receiving glucocorticoid replacement. CRH primary transcript, measured by intronic in situ hybridization, increased after 30 min of restraint and returned to basal levels by 90 min, despite the persistent stressor. The decline was independent of circulating glucocorticoids, because adrenalectomized rats displayed an identical pattern. ICER mRNA in the hypothalamic paraventricular nucleus (PVN) increased after 30 min and remained elevated for up to 4 h in a glucocorticoid-independent manner. Western blot and electrophoretic mobility shift assay analyses showed increases in endogenous ICER in the PVN of rats subjected to restraint stress for 3 h. Chromatin immunoprecipitation assays showed the recruitment of CREM by the CRH CRE in conjunction with decreases in RNA polymerase II (Pol II) binding in the PVN region of rats restrained for 3 h. These data show that stress-induced glucocorticoids do not mediate the limitation of CRH transcription. Furthermore, the ability of CREM to bind the CRH CRE and the time relationship between elevated CREM and reduced Pol II recruitment by the CRH promoter suggest that inhibitory isoforms of CREM induced during stress contribute to the decline in CRH gene transcription during persistent stimulation.

Vasopressin gene transcription increases in response to decreases in plasma volume, but not to increases in plasma osmolality, in chronically dehydrated rats

The synthesis of arginine vasopressin (AVP) in the magnocellular neurons of the supraoptic (SON) and paraventricular nuclei (PVN) is physiologically regulated by plasma osmolality and volume. To clarify how the regulation of AVP gene transcription is affected by chronic dehydration, we examined changes in the transcriptional activities of AVP gene by plasma osmolality and volume in both euhydrated and dehydrated conditions. Euhydrated rats had free access to water, whereas dehydrated rats had been deprived of water for 3 days before experiments. Rats in both conditions were subjected to acute hypertonic stimuli or hypovolemia, and changes in AVP heteronuclear (hn)RNA levels, an indicator of gene transcription, in the SON and PVN were examined with in situ hybridization. The intraperitoneal (i.p.) injection (2% body wt) of hypertonic (1.5 M) saline increased plasma Na levels by approximately 40 meq/l in both euhydrated and dehydrated conditions. However, expression levels of AVP hnRNA in the SON and PVN were increased only in euhydrated, not dehydrated, rats. On the other hand, i.p. injection of polyethylene glycol decreased the plasma volume by approximately 16-20%, and AVP hnRNA levels in the SON and PVN were significantly increased in both conditions. Thus it is demonstrated that signaling pathways regulating AVP gene transcription in the magnocellular neurons were completely refractory to acute osmotic stimuli under the chronic dehydration and that AVP gene transcription could probably respond to acute hypovolemia through different intracellular signal transduction pathways from those for osmoregulation.

Inhibition of corticotrophin-releasing hormone transcription by inducible cAMP-early repressor in the hypothalamic cell line, 4B

We have shown recently that the rapid decline in corticotrophin-releasing hormone (CRH) transcription following activation by stress is associated with induction and binding to the CRH promoter of the repressor isoforms of cAMP responsive element modulator (CREM), inducible cAMP early repressor (ICER). The ability of ICER to inhibit CRH transcription was examined in the hypothalamic cell line, 4B, which expresses CRH. Co-transfection of the inhibitory isoforms of CREM, ICER I and II and CREMbeta, and CRH promoter-luciferase constructs in 4B cells blunted basal and forskolin-stimulated CRH promoter activity, an effect which was abolished by mutation of the CRE of the CRH promoter. Western blot analyses and electromobility gel-shift and super-shift showed increases in endogenous ICER after 3 h of incubation with forskolin. Consistent with an inhibitory effect of CREM on CRH transcription, chromatin immunoprecipitation assays in cells transfected with ICER I revealed recruitment of CREM by the CRH promoter in conjunction with decreases in Pol II association. The study shows that generation of ICER following prolonged stimulation with forskolin, or transfection of an ICER expression vector in hypothalamic cell lines expressing CRH, is associated with CREM binding to the CRH promoter and transcriptional repression. The data support the hypothesis that induction of repressor isoforms of CREM is part of an intracellular feedback mechanism contributing to the termination of CRH transcription during stimulation.

Regulation of proopiomelanocortin gene transcription during single and repeated immobilization stress

We have previously reported that repeated immobilization produces persistent activation of the hypothalamic-pituitary-adrenocortical axis in rats. In an attempt to assess whether any adaptational responses occur at the pituitary level, we examined the detailed time courses of proopiomelanocortin (POMC) gene transcription in the anterior pituitary (AP) in comparison with those of corticotropin-releasing hormone (CRH) gene transcription in the hypothalamic paraventricular nucleus (PVN) during single and repeated immobilization using both intronic and exonic probes. During single immobilization, there was a robust and rapid increase in both CRH heteronuclear RNA (hnRNA) in the PVN and POMC hnRNA in the AP, together with a slower increase in CRH mRNA, but no significant increase in POMC mRNA. Single immobilization also caused significant increases in the plasma concentrations of both ACTH and corticosterone. Daily immobilization for 6 days increased the basal levels of CRH hnRNA and CRH mRNA in the PVN and POMC mRNA in the AP. Both CRH hnRNA and POMC hnRNA responded rapidly to a final episode of acute immobilization on day 7, whereas the peak values of CRH hnRNA and POMC hnRNA after 15 min of the final stress were smaller than those during single immobilization. In contrast to single stress, CRH mRNA did not change significantly, whereas POMC mRNA robustly increased after the final immobilization on day 7. Plasma ACTH increased to a similar degree to single stress, but its initial increase at 5 min was significantly higher than that during single immobilization. The increase in the plasma corticosterone concentration was higher during final immobilization than during single stress. These results suggest that, in response to the hypothalamic drive during repeated immobilization stress, pituitary corticotrophs are capable of upregulating the basal and stress-induced POMC mRNA levels via increased efficiency of the posttranscriptional processing of the hnRNA and/or increased mRNA stability.

Time-dependent alterations in mRNA expression of brain neuropeptides regulating energy balance and hypothalamo-pituitary-adrenal activity after withdrawal from intermittent morphine treatment

Chronic stressors alter brain function and may leave traces after their relief. We used intermittent morphine treatment to examine the relationships between stress-induced changes in energy balance and hypothalamo-pituitary-adrenal (HPA) activity and the recovery thereafter. We studied the effects of morphine injections on energy balance, hormones and fat stores, brain neuropeptide expression, and the ACTH and corticosterone responses to restraint 12 hr after the final injection and 8 d later during recovery. Weight gain, food intake, and caloric efficiency decreased at morphine onset, and these were maintained throughout the morphine injections. At 12 hr, fat stores, leptin, insulin, and testosterone concentrations were reduced. Subsequently, body weight gain and food intake increased and caloric efficiency was above control during the final days. By the eighth recovery day, fat stores and peripheral hormones were no longer depressed. At 12 hr, an over-response of CRF mRNA to restraint occurred in the hypothalamus, similar to the facilitated ACTH and corticosterone responses. On day 8, the hypothalamic CRF mRNA response to restraint was still facilitated, opposite to inhibited ACTH responses. Hypothalamic CRF mRNA correlated highly with mesenteric fat weight in morphine-treated rats. We conclude that there is a prolonged recovery from chronic stressors involving interrelated changes in energy balance and HPA activity. Nonetheless, 8 d after withdrawal from morphine, rats still display facilitated central stress responses, similar to the HPA symptoms described in posttraumatic stress disorder patients. Repeated partial withdrawal associated with intermittent morphine treatment, compounded by complete withdrawal associated with termination of the treatment, is likely required for these metabolic and HPA derangements.

Molecular cloning and developmental expression of corticotropin-releasing factor in the chicken

We have characterized the structure of the chicken corticotropin-releasing factor (CRF) gene through cDNA cloning and genomic sequence analysis, and we analyzed the expression of CRF mRNA and peptide in the diencephalon of the chick throughout embryonic development. The structure of the chicken CRF gene is similar to other vertebrate CRF genes and contains two exons and a single intron. The primary structure of the mature chicken CRF peptide is identical to human and rat CRF. This is the first archosaurian CRF gene to be characterized. We used RIAs to analyze CRF peptide content in the diencephalon and the median eminence and plasma corticosterone during the last week of embryonic development. We also developed a semiquantitative RT-PCR method to analyze the expression of CRF mRNA during the same period. CRF peptide content in the diencephalon increased, whereas peptide content in the ME decreased just before hatching, suggesting that release and biosynthesis are coupled. Plasma corticosterone concentration significantly increased between embryonic d 20 and the first day post hatch. By contrast, CRF mRNA levels in the diencephalon decreased just before hatching. Changes in CRF production just before hatching may be causally related to the regulation of the thyroid and interrenal axes at this stage of chicken development.

Chronic stress attenuates GABAergic inhibition and alters gene expression of parvocellular neurons in rat hypothalamus

Chronic stress causes disinhibition of the hypothalamus-pituitary-adrenal axis. Consequently, the brain is overexposed to glucocorticoids which in humans may precipitate stress-related disorders, e.g. depression. The hypothalamus-pituitary-adrenal activity is strongly regulated by GABAergic input to parvocellular neurons in the hypothalamic paraventricular nucleus. We here report a reduced frequency of miniature inhibitory postsynaptic currents (mIPSCs) in parvocellular neurons of rats exposed to 3 weeks of unpredictable stress. The mIPSC amplitude and kinetic properties were unchanged, pointing to a presynaptic change caused by chronic stress. Because paired-pulse inhibition was unaffected by chronic stress, the number of functional GABAergic synaptic contacts rather than the release probability seems to be reduced after chronic stress. Linearly amplified RNA from postsynaptic cells was hybridized with multiple cDNA clones of interest, including most GABA(A) receptor subunits. In agreement with the electrophysiological observations, relative expression of the prevalent GABA(A)alpha1, alpha3, gamma1 and gamma2 receptor subunits, which largely contribute to the recorded responses, was not altered after chronic stress. However, expression of the extra-synaptic GABA(A)alpha5 subunit, earlier linked to depression in humans, and of the delta receptor subunit were found to be significantly changed. In conclusion, chronic stress leads to presynaptic functional alterations in GABAergic input to the paraventricular nucleus which could contribute to the observed disinhibition of the hypothalamus-pituitary-adrenal axis; additionally other aspects of GABAergic transmission may also be changed due to transcriptional regulation of specific receptor subunits in the parvocellular neurons.

A single exposure to severe stressors causes long-term desensitisation of the physiological response to the homotypic stressor

Although some laboratories have reported that a single session of stress is able to induce a long-lasting sensitisation of the hypothalamic-pituitary-adrenal (HPA) response to further exposures to stress, we have found that a single exposure to severe emotional (immobilisation, restraint or shock) or systemic (endotoxin) stressors reduces the responsiveness of the HPA to the same, but not to a novel (heterotypic), stressor, in which case a slight sensitisation was observed. Long-term desensitisation has been found to reduce not only secretion of peripheral HPA hormones (ACTH and corticosterone), but also to reduce responses of central components of the HPA axis (c-fos and CRF gene expression at the level of the paraventricular nucleus of the hypothalamus, PVN). In addition, desensitisation also applies to the impact of the stressor on food intake and, probably, to stress-induced hyperglycaemia. The development of long-term desensitisation of the HPA axis does not appear to be a universal consequence of exposure to severe stressors as it was not observed in response to insulin-induced hypoglycaemia. Whether or not the development of long-term effects of stress depend on the specific pathways activated by particular stressors remains to be tested. The observed desensitisation of the HPA axis in response to the homotypic stressor shows two special features which makes it difficult to be interpreted in terms of an habituation-like process: (a) the effect increased with time (days to weeks) elapsed between the first and second exposure to the stressor, suggesting a progressive maturational process; and (b) the stronger the stressor the greater the long-term desensitisation. Therefore, it is possible that desensitisation of the HPA axis is the sum of two different phenomena: long-term effects and habituation-like processes. The contribution of the former may be more relevant with severe stressors and longer inter-stress intervals, and that of the latter with mild stressors and repeated exposures. Long-term stress-induced changes may not take place at the level of the PVN itself, but in brain nuclei showing synaptic plasticity and putatively involved in the control of the HPA axis and other physiological responses. As for the precise areas involved, these remain to be characterized.

Effects of repeated restraint stress on hypothalamo-pituitary-adrenocortical function in vasopressin deficient Brattleboro rats

Arginine-vasopressin (AVP) has been proposed to be an important mediator during chronic stress in the regulation of the hypothalamo-pituitary-adrenal axis. In the present study we addressed the role of AVP in maintaining adrenocortical responsiveness during chronic stress using the AVP deficient mutant Brattleboro rat. Heterozygous Brattleboro rats (di/+) served as controls and were compared to homozygous rats (di/di) with diabetes insipidus. Sixty minutes daily restraint was repeated for 5, 8, 11 or 15 days and organ weights, plasma adrenocorticotropin (ACTH) and corticosterone levels and anterior pituitary proopiomelanocortin (POMC) mRNA and ACTH content were measured. The body, adrenal and thymus weight changes induced by chronic stress became significant between 5 and 8 repetition and AVP deficiency had no effect on these parameters. The first indication that AVP has a role to play appears after 11 repetitions. In the di/di group at the end of 11th restraint, the plasma ACTH was decreased when compared to the di/+ rats. In animals with indwelling cannulas some adaptation could be seen in ACTH response without any difference between di/+ and di/di rats after 15 restraints. The corticosterone- and prolactin-elevations induced by restraint did not habituate in the di/+ and the di/di rats. Chronic stress increased POMC mRNA in the anterior pituitary similarly in di/+ and di/di rats. Although AVP seems to be necessary for a full ACTH response, most of the other signs of chronic stress after repeated restraint occur unchanged in the absence of AVP in both genders. This suggests that either AVP is not indispensable for activating the hypothalamo-pituitary-adrenocortical system by chronic stress or the absence of AVP is compensated by other mediators in Brattleboro rats.

The Role of Vasopressin in Hypothalamo-Pituitary-Adrenal Axis Activation during Stress: An Assessment of the Evidence

The hypothalamo-pituitary-adrenal (HPA) axis is a key component of the stress reaction. Most contemporary reviews mention the corticotropin-releasing hormone and arginine vasopressin (AVP)-containing parvocellular neurons of the hypothalamic paraventricular nucleus as the endocrinomotor component of the system. Although there are many studies about the role of AVP in the stress activation, there is evidence consistent and inconsistent with the general view on the importance of AVP. We propose a list of experiments that may provide critical evidence for or against the widely held opinion. The naturally AVP-deficient Brattleboro rat seems to be a good tool for studying the role of AVP. Our experiments on Brattleboro rats with restraint and ip hypertonic saline injection did not support the prominent role of AVP in acute stress, although in forced swim the lack of AVP influenced the HPA axis activation. Among different chronic stress situations (14 days' restraint, chronic morphine or ip hypertonic saline treatment, streptozotocin-induced diabetes mellitus), the role of AVP was not confirmed by changes in somatic parameter (i.e., body, thymus, and adrenal weight changes).

Differential effects of forced swim-stress on the corticotropin-releasing hormone and vasopressin gene transcription in the parvocellular division of the paraventricular nucleus of rat hypothalamus

Corticotropin-releasing hormone (CRH) and vasopressin (AVP) colocalize in the parvocellular division of the paraventricular nucleus of the hypothalamus (PVN). We examined the effect of forced swim-stress on the CRH and AVP primary transcript (hnRNA) levels in the rat PVN by semi-quantitative in situ hybridization. CRH hnRNA increased markedly following 10-min swim-stress and returned to the basal level by 2 h. AVP hnRNA in the parvocellular division of the PVN, where AVP colocalizes with CRH, did not change significantly immediately after the swim-stress, but it did increase significantly 2 h after the stress. Pretreatment with dexamethasone abolished the increases in CRH and AVP hnRNA levels after the swim-stress. The present results demonstrate the differential effects of forced swim-stress on the CRH and AVP gene transcription in the parvocellular PVN, confirming the diverse response of the dual peptide-containing system in the face of acute stressful events.

Osmoregulation of vasopressin release and gene transcription under acute and chronic hypovolemia in rats

Although acute decreases in plasma volume are known to enhance the osmotically induced arginine vasopressin (AVP) release, it is unclear whether there is also such interaction at the level of gene transcription. It also remains to be established how sustained changes in plasma volume affect the osmoregulation. In this study, we examined how acute and chronic decreases in blood volume affected the osmoregulation of AVP release and gene transcription in rats. Acute hypovolemia was induced by intraperitoneal injection of polyethylene glycol (PEG), and chronic hypovolemia was induced by 3 days of water deprivation (WD) or 12 days of salt loading (SL). Rats were injected with isotonic or hypertonic saline, and plasma AVP levels and AVP heteronuclear (hn)RNA expression in the supraoptic and paraventricular nuclei, an indicator of gene transcription, were examined in relation to plasma osmolality in each group. Plasma AVP levels were correlated with plasma Na levels in all groups. Whereas the regression lines relating plasma AVP to Na were almost identical among control, WD, and SL groups, the thresholds of plasma Na for AVP release were significantly decreased only in the PEG group. AVP hnRNA levels were also correlated with plasma Na levels in control and PEG groups, and the thresholds were significantly decreased in the PEG group. In contrast, there was no significant correlation of AVP hnRNA and plasma Na levels in WD and SL groups. Thus it was demonstrated that acute and chronic reduction in plasma volume affected the osmoregulation of AVP release and gene transcription in different ways.

Intermittent morphine administration induces dependence and is a chronic stressor in rats

Although constant treatment with morphine (implanted pellets) does not activate the hypothalamic-pituitary-adrenal (HPA) axis, intermittent injections of morphine may constitute a chronic stressor in rats. To test this hypothesis, we compared the effects of morphine in escalating doses (10-40 mg/kg, s.c.) or saline injected twice daily for 4 days on energy balance, hormones, HPA responses to novel restraint and central corticotropin-releasing factor (CRF) mRNA 12 h and 8 days after the last morphine injection in adult male Sprague-Dawley rats. Weight gain stopped at the onset of morphine, weight loss was marked 36 h postmorphine; thereafter, body weight gain paralleled saline controls. At 12 h, insulin, leptin, and testosterone concentrations were reduced but normalized by 8 days. Restraint and tail nicks caused facilitated ACTH responses at 12 h, under-responsiveness at 8 days. CRF mRNA, measured only at 12 h, was increased in the paraventricular (PVN) and Barrington's nuclei (BAR), decreased in the bed nuclei of the stria terminalis (BNST) and unchanged in the amygdala (CeA) in morphine-treated rats. After stress, CRF mRNA increased in PVN in both groups, increased in BAR and decreased in BNST in saline but not morphine groups, and was unchanged in CeA in both groups. Results from all variables characterize intermittent morphine injections as a chronic stressor. In contrast to constant treatment, injected morphine probably allows some withdrawal during each 12 h interval, causing repeated stress. Drug addicts treat themselves intermittently, and stress causes relapse after withdrawal. Thus, intermittent morphine, itself, may promote relapse.

Long-term effects of a single exposure to immobilization stress on the hypothalamic-pituitary-adrenal axis: transcriptional evidence for a progressive desensitization process

In the present work we have characterized the long-term influence of a single exposure to the stress of immobilization (IMO) on the hypothalamic-pituitary-adrenal (HPA) axis of adult rats. Rats without prior stress (control) and rats exposed to IMO for 2 h on day 1 (IMO+4wk) or on day 21 (IMO+1wk) were killed on day 28, either without stress (basal), immediately after IMO for 1 h (IMO), or 1 h after termination of IMO (post-IMO). IMO caused a strong activation of c-fos mRNA and corticotropin-releasing factor (CRF) and vasopressin (AVP) heteronuclear RNA (hnRNA) in the paraventricular nucleus of the hypothalamus in control rats; this activation was essentially maintained in the post-IMO period. The overall AVP hnRNA response to day 28 stress was not affected by prior stress. Post-IMO c-fos mRNA and CRF hnRNA levels were lower in previously stressed rats, as compared with controls. Whereas the effect of prior IMO on both peripheral HPA hormones and c-fos mRNA was maximal in IMO+1wk rats, the effect of prior stress on CRF hnRNA was only observed in IMO+4wk rats. The present data indicate that prior single IMO triggers a process of desensitization of the HPA responsiveness to IMO over the course of the following weeks. Although the various components of the HPA axis were modified in the same direction, a clear temporal dissociation was found among them, revealing the fine tuning of stress-induced activation of the HPA axis.

Effects of glucagon-like peptide-1 (7-36) amide on neurohypophysial hormone secretion induced by acute hyperosmotic challenge

This study was designed to investigate possible effects of glucagon-like peptide-1 (7-36) amide on the vasopressin and oxytocin release induced by acute peripheral or central osmotic stimulation. In the first series of experiments, rats were injected intraperitoneally with the isotonic (0.15 M) or hypertonic (1.5 M) NaCl solution and then, intracerebroventricularly, with either 1 microg glucagon-like peptide-1 (7-36) amide dissolved in 5 microl of isotonic saline or with the vehicle only. In the second study, 1 microg glucagon-like peptide-1 (7-36) amide, dissolved in isotonic or hypertonic (0.6 M) saline, was injected into the cerebroventricular system. Control rats were treated with isotonic or hypertonic saline only. All the animals were decapitated 10 min after the intracerebroventricular injection. Glucagon-like peptide-1 (7-36) amide enhanced significantly the basal secretion of vasopressin and oxytocin. Moreover, this peptide increased additionally the release of both neurohypophysial hormones stimulated previously by peripheral osmotic challenge. On the other hand, the peptide increased the oxytocin but not vasopressin secretion brought about by an intracerebroventricular injection of hypertonic saline thus suggesting that the central osmotic stimulation decreases the sensitivity of vasopressin neurons to glucagon-like peptide-1 (7-36) amide. It is concluded that glucagon-like peptide-1 (7-36) amide may affect the secretory activity of the hypothalamo-neurohypophysial system under acute osmotic challenge.

Lipopolysaccharide endotoxin potentiates the effect of osmotic stimulation on vasopressin synthesis and secretion in the rat hypothalamus

Vasopressin secreted by magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei is essential for water balance. In this study, we examined magnocellular neurone responses to osmotic stimulation in vehicle-injected controls or rats receiving an intraperitoneal (i.p.) injection of 250 microg/100 g of lipopolysaccharide (LPS), 3 h or 6 h earlier. LPS injection had no effect on plasma vasopressin concentrations in control rats but it caused marked and transient potentiation of the responses to a single i.p. injection of hypertonic saline (five- and two-fold, 3 and 6 h after LPS, respectively). The enhancement of plasma vasopressin responses was independent of plasma sodium concentrations or changes in blood pressure. Basal vasopressin mRNA expression in the paraventricular and supraoptic nuclei decreased slightly 6 h after LPS injection, without changes in vasopressin transcription as indicated by vasopressin heteronuclear (hn) RNA levels. Parvocellular neurones showed expected increases in vasopressin hnRNA expression following LPS injection and a further increase after i.p. hypertonic saline injection (due to the painful component). In contrast to magnocellular vasopressin mRNA expression, the effects of LPS and hypertonic saline injections in parvocellular neurones were additive and not synergistic. Light microscopic immunohistochemical examination revealed an increase in size of vasopressin but not oxytocin axonal terminals in the neural lobe 3 h after LPS injection. Osmotic stimulation caused marked depletion of vasopressin immunoreactivity in axonal terminals of the neural lobe in both control and LPS-pretreated rats. The changes in vasopressin axon terminals were accompanied by induction of interleukin (IL)-1 beta and IL-6 in the posterior pituitary. The data show that endotoxemia causes morphological and functional alterations of the hypothalamic neurohypophyseal system, resulting in facilitation rather than inhibition of vasopressin synthesis, and secretion in response to osmotic stimulation.

Ceramide mediates age-associated increase in macrophage cyclooxygenase-2 expression

Previously, we showed that macrophages (MØ) from old mice have significantly higher levels of lipopolysaccharide (LPS)-induced prostaglandin E(2) (PGE(2)) production than young mice, due to increased cyclooxygenase-2 (COX-2) mRNA levels. The aim of the current study was to determine the underlying mechanisms of age-associated increase in COX-2 gene expression. The results demonstrate that increased COX-2 mRNA expression in the old mice is due to a higher rate of transcription rather than increased stability of COX-2 mRNA. Furthermore, the results show that LPS-induced ceramide levels from the old mice are significantly higher than those of young mice, whereas there is no age-related difference in concentration of its down stream metabolite, sphingosine. The addition of ceramide in the presence or absence of LPS resulted in a significant increase in PGE(2) production in a dose- and time-dependent manner. Inhibition of ceramide conversion to sphingosine had no effect on this ceramide-induced effect. The ceramide-induced up-regulation in PGE(2) production was mediated through increase in COX activity and transcriptional up-regulation of COX-2 mRNA. Collectively, these data suggest that the age-associated increase in MØ COX-2 mRNA is due to transcriptional up-regulation. Furthermore, this increase in transcription is mediated by higher cellular ceramide concentration in old MØ compared with that of young MØ.

Increases in plasma ACTH and cortisol after hypertonic saline infusion in patients with central diabetes insipidus

To clarify the mechanism for the potentiation of CRH-induced ACTH response by the infusion of hypertonic saline, we investigated changes in plasma ACTH concentration after infusion of 5% hypertonic saline in five patients with untreated central diabetes insipidus (DI). Basal levels of plasma ACTH and cortisol in the DI group were not significantly different from those in normal control subjects. The infusion of hypertonic saline produced an increase in plasma arginine vasopressin (AVP) in controls, but did not elevate ACTH. However, in patients with DI, the plasma AVP concentration did not change, but circulating ACTH increased 3.6-fold (7.7 +/- 1.5 to 23.0 +/- 2.7 pmol/liter; P < 0.01), and plasma cortisol also increased significantly (298 +/- 99 to 538 +/- 124 nmol/liter; P < 0.05). Moreover, a positive correlation was observed between plasma ACTH and osmolality (r = 0.72; P < 0.005). These results indicate that ACTH secretion in DI patients is regulated by a mechanism distinct from that in healthy subjects. It seems possible that the increase in plasma osmolality promotes ACTH secretion in DI patients through AVP and/or urocortin via the hypophyseal portal system, independent of the AVP secretion from magnocellular neurons.

Colitis induces CRF expression in hypothalamic magnocellular neurons and blunts CRF gene response to stress in rats

We investigated hypothalamic neuronal corticotropin-releasing factor (CRF) gene expression changes in response to visceral inflammation induced by 2,4,6-trinitrobenzenesulfonic acid (TNB) and acute stress. Seven days after TNB, rats were subjected to water-avoidance stress (WAS) or restraint for 30 min and euthanized. Hypothalamic CRF primary transcripts (heteronuclear RNA, hnRNA) and CRF and arginine vasopressin (AVP) mRNAs were assessed by in situ hybridization. Antisense (35)S-labeled cRNA probes against CRF mRNA intronic and exonic sequences and an oligonucleotide probe against the AVP mRNA were used. TNB induced macroscopic lesions and a fivefold elevation in myeloperoxidase activity in the colon. Colitis increased CRF hnRNA and mRNA signals in the magnocellular part of the paraventricular nucleus of the hypothalamus (PVN) and supraoptic neurons, whereas AVP mRNA was not altered. Colitis did not modify CRF hnRNA signal in the parvocellular part of the PVN (pPVN), plasma corticosterone, and serum osmolarity levels. However, CRF hnRNA expression in the pPVN and the rise in corticosterone and defecation induced by WAS or restraint were blunted in colitic rats. These data show that colitis upregulates CRF gene synthesis in magnocellular hypothalamic neurons but dampens CRF gene transcription in the pPVN and plasma corticosterone responses to environmental acute stressors.

Vasopressin gene expression: experimental models and strategies

The intergenic region (IGR) separating the genes for vasopressin (VP) and oxytocin (OT) has been shown to be critical for the cell-specific expression of these peptide genes in hypothalamic neurons. To date, the most relevant information about the putative cis-elements in the IGR that might determine cell-specific gene expression has come from studies in transgenic models. As a first step toward increasing the efficiency of the IGR sequence deletion studies in transgenic animals, a comparative genomics approach comparing the IGR sequence in humans versus mice was used to identify conserved sequences that might be candidate regulatory elements. The nucleotide sequence of the IGR between the human VP and OT genes was determined and compared to the mouse IGR, and 26 conserved sequences in three distinct clusters were found. These conserved sequences and motifs may be important for the cell-specific expression of the VP and OT genes. However, before further significant progress can be made, a "high-throughput" method for the analysis of deletion constructs in relevant cell types in vitro is needed. It is proposed here that organotypic culture models combined with the use of particle-mediated gene transfer methods may provide an effective, general strategy for the study of cell-specific expression in the central nervous system.

Regulation of corticotropin-releasing hormone (CRH) transcription and CRH mRNA stability by glucocorticoids

1. The increases in corticotropin-releasing hormone (CRH) mRNA following long-term adrenalectomy are associated with low levels of CRH gene transcription, suggesting that glucocorticoids regulate CRH mRNA at the posttranscriptional level. In this study we determined the time course of transcriptional activation after early adrenalectomy by intronic in situ hybridization, and evaluated the effects of glucocorticoids on CRH mRNA stability. 2. Plasma corticosterone was undetectable 3 h after adrenalectomy, but CRH hnRNA increased only by 12 h, and remained elevated for the next 72 h. CRH mRNA increased 18 h after adrenalectomy and reached a plateau lasting from 2 to 6 days, despite very low CRH hnRNA levels. 3. Assessment of CRH mRNA stability, by incubation of slide-mounted hypothalamic sections in an intracellular-like medium at 37 degrees C, prior to measuring CRH mRNA levels by in situ hybridization, revealed a half-life (t1/2) of 11.5 min in sham-operated rats, and a slower decrease adrenalectomized rats (t1/2--26.3 min). Corticosterone administration for 3 days markedly decreased CRH mRNA t1/2 in both sham-operated and adrenalectomized rats (6.5 and 5.0 min, respectively). 4. The data show that adrenalectomy causes transient increases in CRH mRNA transcription, followed by decreases in the rate of CRH mRNA degradation. This suggests that glucocorticoids regulate CRH mRNA at two sites, by inhibiting transcription and by decreasing mRNA stability.

Effect of repeated lipopolysaccharide administration on tissue cytokine expression and hypothalamic-pituitary-adrenal axis activity in rats

The effects of chronic immune challenge on cytokine expression and hypothalamic-pituitary-adrenal axis (HPA) axis responses to stress were studied in Wistar rats after administration of increasing doses of lipopolysaccharide (LPS). Repeated LPS (R-LPS) decreased body weight and increased adrenal weight and pituitary pro-opiomelanocortin mRNA levels. LPS injection increased plasma adrenocorticotropic hormone (ACTH) and corticosterone but the effect was attenuated in R-LPS. Plasma corticosterone but not ACTH responses to restraint were also reduced in R-LPS. Basal and restraint-stimulated corticotropin releasing hormone (CRH) mRNA levels were lower in R-LPS, but responses to a new LPS injection were similar to controls. In contrast, type 1 CRH receptor (CRH-R1) mRNA responses to both LPS and restraint were blunted in R-LPS. Vasopressin mRNA levels in parvocellular neurones were higher in R-LPS, and increased further after restraint but not after a new LPS injection. Glucocorticoid receptor (GR) levels in the paraventricular nucleus (PVN) increased after a single LPS or R-LPS (24 h after the last injection) but declined after a new injection in R-LPS. Interleukin (IL)-1beta and IL-6 mRNAs increased in the pituitary, spleen and circumventricular organs after single or R-LPS, suggesting that cytokines may contribute to the activation of the HPA axis though pathways from the circumventricular organs as well as paracrine effects in the pituitary. The data show that (i) adaptation of the HPA axis during repeated LPS injection involves increases in vasopressin : CRH expression ratios in parvocellular neurones; (ii) that hypothalamic CRH and vasopressin responses to acute stimulation are independent of CRH-R1 expression in the PVN; and (iii) there is a dissociation between pituitary and adrenal responses to acute stress suggesting a decrease of adrenal sensitivity to ACTH.

Hypothalamic and amygdaloid corticotropin-releasing hormone (CRH) and CRH receptor-1 mRNA expression in the stress-hyporesponsive late pregnant and early lactating rat

This study investigated the expression of corticotropin releasing hormone (CRH) and its receptor CRHR-1, and arginine vasopressin (AVP) mRNAs during the stress hyporesponsive periods of late pregnancy and lactation (day-3) and in virgin stress-responsive females. In situ hybridization histochemistry showed that basal CRH mRNA in the paraventricular nucleus (PVN) decreased in pregnant and increased in lactating rats (compared with virgin controls), whereas it increased after restraint stress only in virgin rats. Basal PVN CRHR-1 mRNA increased markedly in all groups but reached lower levels in pregnant rats. Basal AVP mRNA in the parvocellular PVN was higher in lactating rats, and in contrast to CRH mRNA, it increased after stress in all groups. In medial preoptic area (MPOA) CRH mRNA levels were higher in lactating females compared with virgin and pregnant rats, and unexpectedly they decreased markedly after stress only in virgin rats. CRH mRNA levels in the central and medial nuclei of the amygdala were higher in lactating rats than in virgin or pregnant ones, and stress had no effect in either group. These data suggest that these stress hyporesponsive periods: (1) do not depend on basal CRH mRNA expression in the PVN; (2) appear to have intact stress-activated afferent pathways to the PVN, as shown by preservation of CRHR-1 and AVP responses to stress, but the information may be differently processed; (3) are associated with an alteration in a CRH mediated pathway from the MPOA.

The central effects of orexin-A in the hypothalamic-pituitary-adrenal axis in vivo and in vitro in male rats

Orexin-A is synthesized in the posterolateral hypothalamus and immunoreactive fibres project to many central nervous system structures, including the paraventricular nucleus, which is rich in corticotropin releasing factor (CRF) neurones and neuropeptide Y (NPY) innervation. We investigated the central effects of orexin-A on the hypothalamic-pituitary-adrenal (HPA) axis by measuring plasma concentrations of corticosterone and adrenocorticotropic hormone (ACTH) in vivo. We explored the potential neuropeptide pathways involved by investigating the effects of orexin-A on CRF, NPY, arginine vasopressin (AVP) and noradrenaline release from hypothalamic explants in vitro. Intracerebroventricular (i.c.v.) injection of orexin-A (3 nmol) in male rats stimulated increases in plasma concentrations of corticosterone between 10 and 40 min after injection, and of plasma ACTH at 20 and 90 min after injection. Orexin-A significantly stimulated CRF and NPY release from hypothalamic explants in vitro. Orexin-A did not stimulate CRF release in the presence of the selective NPY Y1 receptor antagonist, BIBP3226. BIBP3226 alone did not alter CRF release from hypothalamic explants. Orexin-A had no effect in vitro on the release of other neuropeptides, AVP and noradrenaline, involved in the central regulation of the HPA axis. These results suggest that orexin-A is involved in activation of the HPA axis, and that these effects could be mediated via the release of NPY.

Forced swimming stimulates the expression of vasopressin and oxytocin in magnocellular neurons of the rat hypothalamic paraventricular nucleus

Previous studies have shown that a 10-min forced swimming session triggers the release of both vasopressin and oxytocin into the extracellular fluid of the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON) in rats. At the same time oxytocin, but not vasopressin, was released from the axon terminals into the blood. Here we combined forced swimming with in situ hybridization to investigate whether (i) the stressor-induced release of vasopressin and oxytocin within the PVN originates from parvo- or magnocellular neurons of the nucleus, and (ii) central release with or without concomitant peripheral secretion is followed by changes in the synthesis of vasopressin and/or oxytocin. Adult male Wistar rats were killed 2, 4 or 8 h after a 10-min forced swimming session and their brains processed for in situ hybridization using 35S-labelled oligonucleotide probes. As measured on photo-emulsion-coated slides, cellular vasopressin mRNA concentration increased in magnocellular PVN neurons 2 and 4 h after swimming (P < 0.05). Similarly, oxytocin mRNA concentration was significantly increased in magnocellular neurons of the PVN at 2 and 8 h (P < 0.05). We failed to observe significant effects on vasopressin and oxytocin mRNA levels in the parvocellular PVN and in the SON. Taken together with results from previous studies, our data suggest that magnocellular neurons are the predominant source of vasopressin and oxytocin released within PVN in response to forced swimming. Furthermore, in the case of vasopressin, central release in the absence of peripheral secretion is followed by increased mRNA levels, implying a refill of depleted somato-dendritic vasopressin stores. Within the SON, however, mRNA levels are poor indicators of the secretory activity of magnocellular neurons during stress.

Time course of Fos and Fras expression in the hypothalamic supraoptic neurons during chronic osmotic stimulation

The Fos family comprises Fos and several subtypes of Fos-related proteins (Fras) such as FosB, Fra-1, Fra-2, DeltaFosB, and chronic Fras. Changes in the expression of Fos family proteins with time are not well elucidated, particularly during chronic stimulation. In the present experiments, we investigated quantitatively the time course changes in Fos, FosB and Fras immunoreactivity in the magnocellular neurons of the supraoptic nucleus (SON) during acute and chronic osmotic stimulation. A small number of Fos- and FosB-positive neurons were observed in the SON of control rats, while many Fras-positive neurons were seen in control animals. Significant increases in the numbers of Fos-, FosB-, and Fras-positive neurons were observed 2 h after acute osmotic stimulation by intraperitoneal (i.p.) injection of 3% NaCl solution. Although the number of Fos-positive neurons returned to the control level 4 h after i.p. injection, a significant number of FosB- and Fras-positive neurons were still observed 8 h after i.p. injection. During chronic osmotic stimulation by giving 2% NaCl solution for 2 and 5 days, a large number of Fos-positive neurons were observed, but the cessation of chronic osmotic stimulation by normal water drinking immediately decreased the number of Fos-positive neurons to the control level within 2 h. The number of FosB-positive neurons was increased with period of chronic osmotic stimulation, and a significant number were observed 2-8 h after the cessation of the stimulation. The number of Fras-positive neurons was also significantly higher during chronic osmotic stimulation, and this number was significantly high 2-8 h after the cessation of the stimulation. RT-PCR analysis demonstrated the persistent expression of c-fos mRNA in the SON during chronic osmotic stimulation. These results suggest that c-fos mRNA and Fos protein are constitutively elevated during chronic osmotic stimulation and the time course changes in Fos are different from those seen in FosB and Fras.

Corticosterone differentially modulates expression of corticotropin releasing factor and arginine vasopressin mRNA in the hypothalamic paraventricular nucleus following either acute or repeated restraint stress

Exposing rats to repeated restraint stress induces well-characterized adaptations in the expression of either corticotropin-releasing factor (CRF) or arginine-vasopressin (AVP) mRNA in the parvocellular neurons of the hypothalamic paraventricular nucleus (PVN). The effects of regulating corticosterone levels on this adaptation was studied in male rats. In intact rats, acute restraint stress increased the expression of CRF mRNA whilst AVP mRNA expression was no different to control. Repeated exposure resulted in habituation of CRF expression, whereas AVP mRNA increased above that seen in either non stressed or acutely stressed animals. In adrenalectomised rats with replacement pellets of corticosterone that replicated blood levels approximating to the daily trough (mean levels 37--65 ng/mL), basal CRF expression levels were raised, but the response to acute stress was still observed. However, the habituation seen in normal animals that had been repeatedly stressed was prevented, so that CRF mRNA levels continued to be raised after repeated stress. By contrast, the AVP response to both acute and repeated stress was unaltered in these low-dose corticosterone-treated rats compared with controls. Higher dose pellets, which resulted in blood levels around those of the daily maximum (mean 118--141 ng/mL) had the opposite effects. There was no change compared to intact rats in the expression of CRF mRNA following either acute or repeated stress, but the expected increase in AVP following repeated restraint was prevented. These experiments show that corticosterone has important modulating effects on the adaptive pattern of both CRF and AVP mRNA expression in the parvocellular PVN. The 'set-point' of corticosterone differs; for CRF, experiencing higher levels is necessary for subsequent adaptation to repeated restraint to occur, whereas for AVP a return to lower levels is necessary to allow this peptide to respond to repeated stress.

Expression of kalirin, a neuronal GDP/GTP exchange factor of the trio family, in the central nervous system of the adult rat

Kalirin is a multifunctional protein identified by its interaction with peptidylglycine alpha-amidating monooxygenase, an enzyme essential for neuropeptide biosynthesis. Several forms of Kalirin exist, all containing spectrin-like repeats, a Dbl homology (DH) domain, and an adjacent pleckstrin homology (PH) domain; several different COOH-termini provide additional DH/PH domains and a putative protein kinase. Kalirin binds Rac1 and affects cytoskeletal organization, neuropeptide secretion, and iNOS activity. By in situ hybridization, the highest levels of Kalirin mRNA were found in the cerebral cortex, hippocampal formation, and Purkinje cells, with high levels also in thalamus, caudate putamen, septal nucleus, nucleus accumbens, amygdala, and anterior olfactory nucleus. Low levels of Kalirin mRNA were detected in the paraventricular, supraoptic, and reticular thalamic nuclei and in the ventromedial hypothalamic nucleus. Brain areas with high levels of Kalirin mRNA showed strong Kalirin-like immunoreactivity. Pyramidal neurons with strongly staining soma and long dendrites were observed primarily in layer 5 of the cerebral cortex. In the hippocampus, a uniform distribution of neurons with fine dendritic staining was observed in the pyramidal cell layer, in the granule cell layer, and in the hilar cells of the dentate gyrus as well as in isolated interneurons. Cerebellar Purkinje neurons exhibited intense staining in the soma and in extensive dendritic arbors extending to the surface of the molecular layer. During embryonic development, Trio, the Drosophila orthologue of Kalirin, plays an essential role in axon guidance; localization of Kalirin to the somatodendritic region of adult neurons provides the basis for future studies of regulation and function.

Vasopressinergic regulation of the hypothalamic-pituitary-adrenal axis: implications for stress adaptation

In addition to its role on water conservation, vasopressin (VP) regulates pituitary ACTH secretion by potentiating the stimulatory effects of corticotropin releasing hormone (CRH). The pituitary actions of VP are mediated by plasma membrane receptors of the V1b subtype, coupled to calcium-phospholipid signaling systems. VP is critical for adaptation of the hypothalamic-pituitary-adrenal (HPA) axis to stress as indicated by preferential expression of VP over CRH in parvocellular neurons of the hypothalamic paraventricular nucleus, and the upregulation of pituitary VP receptors during stress paradigms associated with corticotroph hyperresponsiveness. V1b receptor mRNA levels and coupling of the receptor to phospolipase C are stimulated by glucocorticoids, effects which may contribute to the refractoriness of VP-stimulated ACTH secretion to glucocorticoid feedback. The data suggest that vasopressinergic regulation of the HPA axis is critical for sustaining corticotroph responsiveness in the presence of high circulating glucocorticoid levels during chronic stress.

Characterisation of vasopressin V(1A), angiotensin AT(1) and AT(2) receptor distribution and density in normotensive and hypertensive rat brain stem and kidney: effects of restraint stress

In the present study, we have examined neurochemical correlates that may be involved in the differential cardiovascular responses observed in normotensive and hypertensive rats during stress. Using a restraint stress paradigm, both normotensive Wistar Kyoto (WKY) and Spontaneously Hypertensive rats (SHR) underwent acute (1 h restraint in a perspex tube), chronic (1 h restraint for ten consecutive days) or no restraint (control) stress. Following cessation of restraint, rats were processed by incubating sections of brain stem and kidney with [125I]-HO-LVA (0.03 nM) or [125I]Sar(1)Ile(8)-AngiotensinII (0.5 nM), in the presence of PD123319 (10 microM) or losartan (10 microM), to determine the distribution and density of vasopressin V(1A), angiotensin AT(1) and AT(2) receptors, respectively. Analysis of autoradiograms indicated changes in the density of radioligand binding in acutely and chronically-stressed rats, as compared to controls. For example, V(1A) binding in the medial nucleus tractus solitarius (SolM) decreased in the WKY but increased in the SHR. AT(1) binding in SolM did not significantly change in the WKY but decreased in the SHR with repeated restraint. In kidney slices, AT(1) binding decreased with stress in the WKY (-17%) but increased in SHR (+10-15%). AT(2) binding in the kidney showed a pattern similar to that of AT(1) binding in SHR, but not WKY. Graded increases in V(1A) binding were measured in kidney medulla and cortex of both strains (+50-60% with chronic restraint). These results suggest that physiological adaptation to restraint is associated with specific changes in V(1A), AT(1) and AT(2) receptor density within brain nuclei and kidney.

Effect of chronic psychogenic stress exposure on enkephalin neuronal activity and expression in the rat hypothalamic paraventricular nucleus

This study tested the hypothesis that the activation pattern of enkephalinergic (ENKergic) neurons within the paraventricular nucleus of the hypothalamus (PVH) in response to psychogenic stress is identical whether in response to repeated exposure to the same stress (homotypic; immobilization) or to a novel stress (heterotypic; air jet puff). Rats were assigned to either acute or chronic immobilization stress paradigms (90 min/day for 1 or 10 days, respectively). The chronic group was then subjected to an additional 90-min session of either heterotypic or homotypic stress. A single 90-min stress session (immobilization or air jet) increased PVH-ENK heteronuclear (hn) RNA expression. In chronically stressed rats, exposure to an additional stress session (whether homotypic or heterotypic) continued to stimulate ENK hnRNA expression. Acute immobilization caused a marked increase in the numbers of Fos-immunoreactive and Fos-ENK double-labeled cells in the dorsal and ventral medial parvicellular, and lateral parvicellular subdivisions of the PVH. Chronic immobilization caused an attenuated Fos response ( approximately 66%) to subsequent immobilization. In contrast, chronic immobilization did not impair ENKergic neuron activation within the PVH following homotypic or heterotypic stress. These results indicate that within the PVH, chronic psychogenic stress markedly attenuates the Fos response, whereas ENKergic neurons resist habituation, principally within the ventral neuroendocrine portion of the nucleus. This suggests an increase in ENK effect during chronic stress exposure. Homotypic (immobilization) and heterotypic (air jet) psychogenic stressors produce similar responses, including Fos, ENK-Fos, and ENK hnRNA, within each subdivision of the PVH, suggesting similar processing for painless neurogenic stimuli.