Activation of the hypothalamic-pituitary axis in adrenalectomised rats: potentiation by chronic stress

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

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

Effects of chronic restraint stress on body weight, food intake, and hypothalamic gene expressions in mice

BACKGROUND

Stress affects body weight and food intake, but the underlying mechanisms are not well understood.

METHODS

We evaluated the changes in body weight and food intake of ICR male mice subjected to daily 2 hours restraint stress for 15 days. Hypothalamic gene expression profiling was analyzed by cDNA microarray.

RESULTS

Daily body weight and food intake measurements revealed that both parameters decreased rapidly after initiating daily restraint stress. Body weights of stressed mice then remained significantly lower than the control body weights, even though food intake slowly recovered to 90% of the control intake at the end of the experiment. cDNA microarray analysis revealed that chronic restraint stress affects the expression of hypothalamic genes possibly related to body weight control. Since decreases of daily food intake and body weight were remarkable in days 1 to 4 of restraint, we examined the expression of food intake-related genes in the hypothalamus. During these periods, the expressions of ghrelin and pro-opiomelanocortin mRNA were significantly changed in mice undergoing restraint stress. Moreover, daily serum corticosterone levels gradually increased, while leptin levels significantly decreased.

CONCLUSION

The present study demonstrates that restraint stress affects body weight and food intake by initially modifying canonical food intake-related genes and then later modifying other genes involved in energy metabolism. These genetic changes appear to be mediated, at least in part, by corticosterone.

Coping with chronic social stress in mice: hypothalamic-pituitary-adrenal/ sympathetic-adrenal-medullary axis activity, behavioral changes and effects of antalarmin treatment: implications for the study of stress-related psychopathologies

The aim of this study was to analyze the individual differences that lead to the development of psychopathological changes in response to chronic social stress. We also assessed the ability of an antagonist of the corticotrophin-releasing hormone (CRH) receptors to reverse the effects of stress. Male adult mice were exposed to repeated defeat experiences for 21 days using a sensorial contact model. After 18 days of defeat, two groups of subjects were established (active and passive), according to their behaviors during social confrontation. Antalarmin treatment was given for 4 and 6 days. The results corroborated previous data indicating that subjects who adopted a passive coping strategy had higher corticosterone levels after 21 days of defeat and decreased resting levels 3 days later. Moreover, they showed higher resting expression levels of hypothalamic CRH than their active counterparts. On day 24, the experimental animals were subjected to another social defeat to determine whether the stress response remained. The increase in corticosterone and hypothalamic CRH levels was similar for all of the stressed subjects, but the passive subjects also had a greater CRH response in the amygdala. Passive subjects had decreased levels of adrenal dopamine β-hydroxylase, tyrosine hydroxylase and plasma adrenaline compared to the active subjects, and lower plasma noradrenaline levels than manipulated controls. The passive profile of physiological changes in both the hypothalamic-pituitary-adrenal and sympathetic-adrenal-medullary (SAM) axes has been associated with changes related to mood disorders, such as posttraumatic stress disorder and depression. The active coping profile is characterized by similar corticosterone resting levels to controls and increased SAM activity. Both profiles showed alterations in the novel palatable and forced swimming tests, with the passive profile being the most vulnerable to the effects of stress in this last test. Pharmacological treatment with antalarmin failed to reverse the effects of stress.

Effects of chronic manipulation of adrenocorticotropic hormone levels in Chinook salmon on expression of interrenal steroidogenic acute regulatory protein and steroidogenic enzymes

The effects of chronic exposure to adrenocorticotropic hormone (ACTH) or the synthetic glucocorticoid dexamethasone (DEX) on the expression of genes involved in cortisol synthesis were examined using quantitative RT-PCR and immunohistochemistry. Juvenile Chinook salmon were treated with either ACTH via micro-osmotic pumps or with DEX via a lipid-based sustained release vehicle. Plasma cortisol levels were significantly elevated in ACTH-treated fish after 1 day, with a significant reduction in this effect with increasing treatment duration. ACTH also appeared to cause progressive hyperplasia of interrenal cells. Steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc) transcripts but not 3β-hydroxysteroid dehydrogenase-isomerase (3β-HSD) or cytochrome P450 11β-hydroxylase (P45011β) transcripts in head kidneys significantly increased after 5 days of ACTH treatment. Significant linear relationships between plasma cortisol levels and transcript levels were identified at day 1 and day 5 for StAR, and day 5 for P450scc. Increased immunoreactivity for P450scc was observed in interrenal cells of ACTH-treated fish after 5 and 10 days. No effect of ACTH on 3β-HSD immunoreactivity was apparent at any time point. P45011β immunoreactivity was more intense after 5 days treatment with ACTH. DEX significantly reduced resting plasma cortisol levels and induced interrenal cell atrophy. Although no significant effect of treatment with DEX was found for any transcript, immunoreactivity for P450scc and P45011β appeared to be reduced. These results indicate that StAR and P450scc are subject to transcriptional regulation by chronic changes in ACTH levels.

The importance of corticosterone in mediating restraint-induced weight loss in rats

I. J. Scherer, P. V. Holmes, R. B.S. Harris. The importance of corticosterone in mediating restraint-induced weight loss in rats. PHYSIOL BEHAV 00 (0) 000-000, 2010. Rats restrained for 3 h/day for 3d ays (RR) lose weight and do not return to the weight of non-restrained controls once restraint has ended. This study tested the importance of restraint-induced corticosterone release in mediating the change in body weight by injecting ADX rats with 2.0mg corticosterone/kg before each restraint to replicate the restraint-induced surge in circulating corticosterone. Restrained adrenalectomized (ADX) rats injected with corticosterone had the same initial weight loss as intact restrained rats, whereas corticosterone injection in non-restrained ADX rats and restraint of ADX rats injected with saline each produced only half as much initial weight loss. Sustained weight loss, measured for 14 days after the end of RR, was the same for restrained intact rats and restrained ADX rats injected with corticosterone whereas restrained ADX rats injected with saline achieved the same weight gain as their controls. Corticosterone injections had no effect on weight gain of non-restrained intact rats. In situ hybridization showed that corticotropin releasing factor (CRF) mRNA expression in the paraventricular nucleus of the hypothalamus (PVN) was increased by the same degree in ADX rats and restrained intact rats and was not modified by corticosterone injections. There was no significant effect of restraint, ADX or corticosterone injection on PVN arginine vasopressin (AVP) mRNA expression. These data indicate that a surge in corticosterone causes sustained weight loss in ADX rats through a mechanism that can be compensated for in intact rats and is independent of changes in PVN CRF or AVP mRNA expression.

Thymocyte-synthesized glucocorticoids play a role in thymocyte homeostasis and are down-regulated by adrenocorticotropic hormone

Thymocytes from adult mice synthesize glucocorticoids (GCs), and some data indicate a role for this hormone production in thymic homeostasis. Here we present further support for this view by showing that the dramatic increase in thymocyte number seen after adrenalectomy (ADX) does not correlate with the decrease in systemic GCs but rather with an ACTH-mediated down-regulation of GC synthesis in thymocytes. High ACTH concentrations caused by ADX in wild-type mice down-regulated CYP11B1 mRNA expression, encoding the last enzyme required for corticosterone synthesis and as a consequence reduced GC synthesis in thymocytes. This was not seen in IL-1beta/IL-18 double-knockout mice unable to respond to ADX with high ACTH levels. However, if ADX IL-1beta/IL-18 double-knockout mice were treated with ACTH, this led to a down-regulation of CYP11B1 and GC synthesis in thymocytes. In addition, in vivo treatment of mice with the CYP11B1 antagonist metyrapone, without affecting the systemic corticosterone level, increased thymocyte numbers and in vitro treatment of isolated thymocytes prevented thymocyte loss. Furthermore, in vitro experiments showed that both ACTH and its receptor-induced second-messenger molecule cAMP down-regulated mRNA expression of critical enzymes in GC steroidogenesis and GC synthesis in thymocytes. We conclude that thymocyte-produced GCs are important for the homeostasis of adult mouse thymocytes and that high ACTH level, in contrast to stimulating GC synthesis in the adrenal glands, has the opposite effect in thymocytes.

Quetiapine regulates the stress-induced increase in corticotropin-releasing factor mRNA expression in the rat hypothalamus

Corticotropin-releasing factor (CRF) is a key regulator of the stress response. We investigated the effects of the atypical antipsychotic drug quetiapine on CRF mRNA expression in the rat hypothalamus following immobilization stress. Pretreatment with 10 mg/kg quetiapine significantly reduced the immobilization stress-induced increase in CRF mRNA expression in the paraventricular nucleus of the hypothalamus. These results suggest that quetiapine may modulate the stress response via regulation of CRF mRNA expression.

Desipramine prevents the sustained increase in corticotropin-releasing hormone-like immunoreactivity induced by repeated immobilization stress in the rat central extended amygdala

Clinical and experimental studies have shown that the activation of corticotropin-releasing hormone (CRH) and noradrenergic systems mediate stress-induced anxiety. Repeated immobilization stress (RIS) has been shown to induce long-lasting anxiety behavior and changes in noradrenaline turnover. The present work was aimed at studying the effect of RIS on the in situ expression of CRH-LI in the central extended amygdala and paraventricular nucleus of the hypothalamus (PVN). Our results showed that RIS for 15 days induces a significant increase of CRH-LI expression in the central extended amygdala. The increase in CRH-LI expression in the central extended amygdala was sustained even after a 25-day stress-free period. The concomitant administration of desipramine (DMI), a specific noradrenaline uptake inhibitor, fully prevented the RIS-induced increase in CRH expression. RIS also induced an increase of CRH-LI expression in the PVN that was prevented by the concomitant DMI administration. In contrast to the sustained effect observed in the central extended amygdala, the RIS-induced increase of CRH-LI expression in the PVN was nonlasting. DMI administration also prevented the RIS-induced increase of adrenal gland weight. The present findings showing that RIS induces a sustained increase of CRH expression in the central extended amygdala suggest that the repeated activation of CRH neurons and CRH receptors in the central extended amygdala may underlie the long-lasting anxiety behavior induced by RIS. Further studies should address the mechanisms involved in the effect of DMI and its eventual relevance in the therapeutic actions of DMI.

Insidious adrenocortical insufficiency underlies neuroendocrine dysregulation in TIF-2 deficient mice

The transcription-intermediary-factor-2 (TIF-2) is a coactivator of the glucocorticoid receptor (GR), and its disruption would be expected to influence glucocorticoid-mediated control of the hypothalamo-pituitary-adrenal (HPA) axis. Here, we show that its targeted deletion in mice is associated with altered expression of several glucocorticoid-dependent components of HPA regulation (e.g., corticotropin-releasing hormone, vasopressin, ACTH, glucocorticoid receptors), suggestive of hyperactivity under basal conditions. At the same time, TIF-2(-/-) mice display significantly lower basal corticosterone levels and a sluggish and blunted initial secretory response to brief emotional and prolonged physical stress. Subsequent analysis revealed this discrepancy to result from pronounced aberrations in the structure and function of the adrenal gland, including the cytoarchitectural organization of the zona fasciculata and basal and stress-induced expression of key elements of steroid hormone synthesis, such as the steroidogenic acute regulatory (StAR) protein and 3beta-hydroxysteroid dehydrogenase (3beta-HSD). In addition, altered expression levels of two nuclear receptors, DAX-1 and steroidogenic factor 1 (SF-1), in the adrenal cortex strengthen the view that TIF-2 deletion disrupts adrenocortical development and steroid biosynthesis. Thus, hyperactivity of the hypothalamo-pituitary unit is ascribed to insidious adrenal insufficiency and impaired glucocorticoid feedback.

Glucocorticoids, chronic stress, and obesity

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

Housing in pyramid counteracts neuroendocrine and oxidative stress caused by chronic restraint in rats

The space within the great pyramid and its smaller replicas is believed to have an antistress effect. Research has shown that the energy field within the pyramid can protect the hippocampal neurons of mice from stress-induced atrophy and also reduce neuroendocrine stress, oxidative stress and increase antioxidant defence in rats. In this study, we have, for the first time, attempted to study the antistress effects of pyramid exposure on the status of cortisol level, oxidative damage and antioxidant status in rats during chronic restraint stress. Adult female Wistar rats were divided into four groups as follows: normal controls (NC) housed in home cage and left in the laboratory; restrained rats (with three subgroups) subject to chronic restraint stress by placing in a wire mesh restrainer for 6 h per day for 14 days, the restrained controls (RC) having their restrainers kept in the laboratory; restrained pyramid rats (RP) being kept in the pyramid; and restrained square box rats (RS) in the square box during the period of restraint stress everyday. Erythrocyte malondialdehyde (MDA) and plasma cortisol levels were significantly increased and erythrocyte-reduced glutathione (GSH) levels, erythrocyte glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) activities were significantly decreased in RC and RS rats as compared to NC. However, these parameters were maintained to near normal levels in RP rats which showed significantly decreased erythrocyte MDA and plasma cortisol and significantly increased erythrocyte GSH levels, erythrocyte GSH-Px and SOD activities when compared with RS rats. The results showed that housing in pyramid counteracts neuroendocrine and oxidative stress caused by chronic restraint in rats.

Effects of repeated tianeptine treatment on CRF mRNA expression in non-stressed and chronic mild stress-exposed rats

Accumulating evidence suggests that dysregulation of corticotropin-releasing factor (CRF) may play a role in depression and that this dysregulation may be corrected by antidepressant drug treatment. Here, we examined whether chronic mild stress (CMS) alters CRF mRNA levels in stress-related brain areas including the bed nucleus of the stria terminalis (BNST) and the central nucleus of amygdala (CeA), and whether repeated tianeptine treatment can attenuate CMS-induced changes in CRF mRNA levels. Male rats were exposed to CMS for 19 days, and control animals were subjected to brief handling. Both groups were injected daily with tianeptine or saline. CMS significantly increased CRF mRNA levels in the dorsal BNST (dBNST), but not in other areas. Repeated tianeptine treatment prevented the CMS-induced increase in CRF mRNA levels in the dBNST, and reduced CRF mRNA levels in dBNST in non-stressed controls. Moreover, repeated tianeptine treatment significantly decreased CRF mRNA levels in the ventral BNST and CeA of non-stressed controls as well as CMS-exposed rats. These results show that CMS induces a rather selective increase of CRF mRNA in the dBNST. In addition, these results suggest that repeated tianeptine treatment diminishes the basal activity of CRF neurons and reduces their sensitivity to stress.

The hypothalamic–pituitary–adrenal and glucose responses to daily repeated immobilisation stress in rats: individual differences

It is accepted that there are important individual differences in the vulnerability to stress-induced pathologies, most of them associated to the hypothalamic-pituitary and sympatho-medullo-adrenal axes, the two prototypical stress-responsive systems. However, there are few studies specifically aimed at characterising individual differences in the physiological response to daily repeated stress in rats. In the present work, male rats were submitted to repeated immobilisation (IMO) stress (1 h daily for 13 days) and several samples were taken at specific days and time points. Animals only subjected to blood sampling procedure served as controls. Daily adrenocorticotropic-hormone (ACTH), corticosterone and glucose responses to immobilisation (that included the post-immobilisation period) progressively declined over the days. In addition, repeated immobilisation resulted in decreased relative thymus weight, increased relative adrenal weight, elevated corticotropin-releasing factor (CRF) mRNA levels in the hypothalamic paraventricular nucleus (PVN), and down-regulation of glucocorticoid receptor gene transcription in hippocampus CA1. However, only CRF mRNA levels in the paraventricular nucleus correlated with the ACTH (on day 1) and corticosterone responses (from day 4-13) to immobilisation. When the animals were classified in three groups on the basis of their plasma ACTH levels immediately after the first immobilisation, individual differences in the ACTH response progressively disappeared on successive exposures to the stressor, whereas those in corticosterone and glucose were more sustained. The present results suggest that there are individual differences in the physiological response to stress that tend to be reduced rather than accentuated by repeated exposure to the stressor. Nevertheless, this buffering effect of repeated stress was dependent on the particular variable studied.

Long-lasting delayed hyperalgesia after chronic restraint stress in rats-effect of morphine administration

Different effects upon the nociceptive response have been observed with exposure to acute and chronic stress in rats. In the present study we repeatedly submitted rats to restraint for 40 days, inducing hyperalgesia using the tail-flick test. A new session of acute stress was applied at the end of 40 days period, and the chronically-stressed animals demonstrated analgesia after forced swimming, but not after restraint. The effect of stress interruption for 14 or 28 days on the nociceptive threshold was then investigated. The basal tail-flick latency remained decreased for at least 28 days (hyperalgesic effect). Following the periods of suspension, the animals were submitted to new session of acute restraint, and stress-induced analgesia was observed only after 28 days of stress interruption. Thus, the mechanisms involved in the long-lasting hyperalgesia presented in this study are not exactly the same as those responsible for the analgesia induced by acute stressors. After 40 days of chronic stress treatment, morphine was injected i.p. (1.0, 5.0 mg/kg or saline). The repeatedly stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. The tolerance of the response to morphine agrees with previous studies suggesting that chronic restraint stress could modify the activity of opioid systems.

Body weight gain and diurnal differences of corticosterone changes in response to acute and chronic stress in rats

Plasmatic levels of corticosterone display a circadian rhythm, with the higher values occurring during the dark phase in nocturnally feeding animals. Stressful situations induce a rise of corticosterone levels and this endocrine response to stress also presents circadian variations. The higher increase of corticosterone in response to stress occurs when the hormone is in its lower circadian level, and the minimum responses occurring at the peak. Since it has been shown that plasma hormones respond differently to different stressors, in the present study, we compared the acute and chronic effects of four different stressors: electric foot shocks (3 mA, 1/s, 5 min), immobilization during two hours or six hours, and immersion in cold water (15 degrees C) for 15 min. Stressors were applied, both acutely and chronically (during 4, 12 and 20 days) at the onset of the light phase as well as at the onset of the dark phase of the light/dark cycle. Body weight was assessed every day, and at the end of the manipulations plasmatic corticosterone levels were determined from the trunk blood. Adrenal and testicular weights were also assessed. Acute exposure to stressors increased plasmatic corticosterone levels significantly when the stressors were applied at the beginning of the light phase of the cycle. In the dark phase, only two hours of immobilization and immersion in cold water caused an increase in plasmatic corticosterone. With repeated exposure, electric foot shocks failed to induce significant changes in corticosterone levels in any phase of the light-dark cycle. Immobilization stress induced a significant rise in corticosterone levels only when the stressor was applied during the light phase. Immersion in cold water elicited a clear increase in plasmatic corticosterone levels in all the periods tested, regardless of the time of the cycle in which the stressor was applied. We did not observe a loss in body weight, but rather a smaller weight gain in stressed rats. Body weight gain was minimum in rats exposed to immersion and 6 hours of immobilization. Adrenal hypertrophy was observed in rats exposed to these same stressors. We conclude that: 1) the activation of the hypothalamus-pituitary-adrenal axis by stress depends mainly on the characteristics of the stressor; 2) the response of this axis to stress also depends on the time of day in which the stressor is applied.

Habituation and cross-sensitization of stress-induced hypothalamic-pituitary-adrenal activity: effect of lesions in the paraventricular nucleus of the thalamus or bed nuclei of the stria terminalis

Habituation of the hypothalamic-pituitary-adrenal (HPA) response to chronic intermittent restraint stress (30 min/day for 15 days) and the cross-sensitization to a heterotypic stress [i.p. lipopolysaccharide (LPS)] were investigated in intact male Sprague Dawley rats, and in rats bearing quinolinic acid lesions to the medial anterior bed nuclei of the stria terminalis (BST) or anterior region of the paraventricular nucleus of the thalamus (PVT). In intact animals, a single period of restraint increased plasma corticosterone levels at 30 min and led to an increase in corticotropin-releasing hormone (CRH) mRNA levels in the PVN at 3 h. LPS had a smaller effect on corticosterone and more variable effect on CRH mRNA. Chronic intermittent restraint stress caused a decrease in body weight and increase in adrenal weights, with concomitant increase in basal corticosterone levels. These animals also displayed marked habituation of the corticosterone and CRH mRNA responses to the homotypic stress of restraint, but no loss of the corticosterone response to the heterotypic stress of LPS and a cross-sensitization of the CRH mRNA response. This pattern of stress responses in control and chronically stressed animals was not significantly affected by lesions to the PVT or BST, two areas which have been implicated in the coping response to stress. Thus, these data provide evidence for independent adaptive mechanisms regulating HPA responses to psychological and immune stressors, but suggest that neither the medial anterior BST nor the anterior PVT participate in the mechanisms of habituation or cross-sensitization.

Acute effects of acephate and methamidophos and interleukin-1 on corticotropin-releasing factor (CRF) synthesis in and release from the hypothalamus in vitro

Acute effects of Ace, Meth and IL-1 on AChE activity, ACh and CRF mRNA levels in, and CRF-release from the hypothalamus were studied in vitro. The hypothalamus samples were dissected from the rat brain and were incubated in vitro with IL-1, Ace or Meth in the presence or absence of Dex, Atrop, PTL, PROP and GABA. Ace and Meth, but not IL-1, inhibited AChE activity, while all three compounds; (1) increased ACh and CRF mRNA levels in and CRF release from; (2) activated the CRE promoter region of CRF-gene in: and (3) increased cFos binding to the AP-1 region of the CRF-gene in the hypothalamus. Dex suppressed the effects of IL-1, possibly by inducing the nGRE regulatory sites of the CRF-gene. Dex, however, did not modulate the effects of Ace and Meth on the hypothalamus, which may be attributed to the failure of Dex to modulate the CRF-gene's nGRE regulatory sites. Atrop caused 80-90% inhibition of the effects of IL-1, but caused only 50-65% inhibition of the effects of Ace or Meth on CRF mRNA levels in and CRF release from the hypothalamus. PTL did not affect, while PROP slightly attenuated the effects of IL-1 and the insecticides on the hypothalamus. GABA attenuated the effects of the insecticides but not the effects of IL-1 on the hypothalamus. This suggests that the IL-1-induced augmentation of CRF synthesis in and release from the hypothalamus is mediated through a cholinergic pathway, while the insecticide-induced augmentation of CRF synthesis in and release from the hypothalamus is mediated through the cholinergic and GABAergic pathways. The insecticides, but not IL-1, disrupt feedback regulation of CRF synthesis in and release from the hypothalamus.

Decreased corticotropin-releasing factor receptor expression and adrenocorticotropic hormone responsiveness in anterior pituitary cells of Wistar-Kyoto rats

The Wistar-Kyoto (WKY) rat shows signs of persistent activation of the hypothalamic-pituitary-adrenal axis, but the cause and site of this activation is not yet known. Chronically activated corticotrophs generally show blunted adrenocorticotropic hormone (ACTH) response to corticotropin releasing factor (CRF); therefore, the anterior pituitary responsiveness to ACTH secretagogues, CRF and vasopressin, was compared in male WKY and Wistar rats. Anterior pituitary CRF binding and CRF receptor mRNA expression was significantly decreased in WKY rats. ACTH response to CRF or vasopressin was markedly impaired, and vasopressin failed to potentiate the CRF-stimulated ACTH release in cultured WKY anterior pituitary cells. In contrast, CRF and vasopressin alone and in combination stimulated large, concentration-dependent increases in ACTH release in Wistar anterior pituitary cells. By contrast to the decreased ACTH secretory responses, steady-state anterior pituitary pro-opiomelanocortin mRNA levels were approximately 12-fold greater in WKY rats compared to Wistar rats, and they further increased in response to CRF stimulation. These findings suggest that, although the WKY rat corticotroph is under a chronic state of activation or disinhibition, the in vitro secretory responses to classic ACTH secretagogues are impaired.

Corticotropin-releasing hormone and animal models of anxiety: gene-environment interactions

The study of the neural substrates underlying stress and anxiety has in recent years been enriched by a burgeoning pool of genetic information gathered from rodent studies. Two general approaches have been used to characterize the interaction of genetic and environmental factors in stress regulation: the evaluation of stress-related behavioral and endocrine responses in animals with targeted deletion or overexpression of specific genes and the evaluation of changes in central nervous system gene expression in response to environmental perturbations. We review recent studies that have used molecular biology and genetic engineering techniques such as in situ hybridization, transgenic animal, and antisense oligonucleotide gene-targeting methodologies to characterize the function of corticotropin-releasing hormone (CRH) system genes in stress. The effects of genetic manipulations of each element of the CRH system (CRH, its two receptors, and its binding protein) on stress-related responses are summarized. In addition, the effects of stress (acute, repeated, or developmental) on CRH system gene expression are described. The results from these studies indicate that experimentally engineered or stress-induced dysregulation of gene expression within the CRH system is associated with aberrant responses to environmental contingencies. These results are discussed in the context of how CRH system dysfunction might contribute to stress-related psychopathology and are presented in conjunction with clinical findings of CRH system dysregulation in psychiatric illness. Finally, future research strategies (i.e., high-throughput gene screening and novel gene-targeting methodologies) that may be used to gain a fuller understanding of how CRH system gene expression affects stress-related functioning are discussed.