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.     

Further evidence for the central effect of dexamethasone at the hypothalamic level in the negative feedback mechanism

The site of action of glucocorticoids (GC) in exerting negative feedback upon the hypothalamo-pituitary-adrenocortical (HPA) axis is not yet clear. In the present study we have examined whether dexamethasone (Dex) can inhibit the HPA axis stress responses by acting locally at the hypothalamic level in freely moving male rats. Local micro-injection of Dex in the paraventricular nuclei (PVN; 1 microg) prevented a decrease of CRH-41 content in the median eminence. The PVN Dex injections (0.25-1 microg) also inhibited the rise in plasma ACTH and corticosterone (CS) following short photic stimulation in a dose dependent manner. In PVN Dex-injected rats, i.v. injection of CRH-41 increased serum ACTH and CS levels similar to that observed in rats injected with saline into the PVN indicating normal sensitivity of the pituitary gland to CRH-41. Local injection of [3H]Dex in the PVN showed that only a negligible amount of radioactivity was found in the pituitary. These data indicate that minute amounts of Dex in the PVN, which did not affect the pituitary, blocked the HPA axis responses to photic stimulation. It is suggested that Dex may exert its inhibitory effect on the HPA axis at least in part at the hypothalamic level.     

Quantitative analysis of corticotropin-releasing factor and arginine vasopressin mRNA in the hypothalamus during chronic morphine treatment in rats: an in situ hybridization study

The content of corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) in the hypothalamic paraventricular nucleus (PVN) increases during chronic morphine treatment. Because these experiments cannot distinguish between increased synthesis or reduced release, the present study measured changes in CRF and AVP mRNAs in the PVN by in situ hybridization. Concomitantly, changes in noradrenaline turnover in the PVN and changes in plasma corticosterone release were determined. Male rats were implanted with placebo (naive) or morphine pellets for 7 days. On day 7, groups of rats received an acute injection of either saline i.p. or morphine (30 mg/kg, i.p.). Acute morphine injection did not change the total size of the labelled area for CRF mRNA in the PVN of naive or morphine-pelleted rats, indicating that the number of CRF-containing neurones was unchanged. On the other hand, in rats chronically treated with morphine, the intensity of labelling for CRF mRNA was significantly reduced, suggesting a decrease in the synthesis of CRF. In placebo rats, injection of saline or morphine did not affect the surface hybridized for AVP mRNA. By contrast, in the morphine-group injected with saline, there was a significant reduction in the number of labelled neurones, measured by the size of labelled area. Similarly, there was a decrease in intensity of AVP mRNA expression in the parvocellular and magnocellular neurones of the PVN in the morphine-group injected with saline, suggesting a decreased synthesis of AVP in these neurones. In parallel with the decrease in the expression of CRF and AVP mRNAs in the PVN, there was a pronounced decrease in noradrenaline turnover and in the release of corticosterone in the morphine-pelleted rats. In conclusion, present results show that, in addition to modifications in corticosterone secretion and in noradrenaline turnover, chronic morphine administration produces a reduction in the synthesis of CRF and AVP.     

Effect of direct injection of melanin-concentrating hormone into the paraventricular nucleus: further evidence for a stimulatory role in the adrenal axis via SLC-1

Melanin-concentrating hormone (MCH) is implicated in the control of a number of hormonal axes including the hypothalamic-pituitary adrenal (HPA) axis. Previous studies have shown that there is evidence for both a stimulatory and an inhibitory action on the HPA axis; therefore, we attempted to further characterize the effects of MCH on this axis. Intracerebroventricular injection of MCH increased circulating adrenocorticotropic hormone (ACTH) at 10 min post injection. Injection of MCH directly into the paraventricular nucleus (PVN) was found to increase both circulating ACTH and corticosterone 10 min after injection. Additionally, MCH was found to increase corticotropin-releasing factor (CRF) release from hypothalamic explants, and this effect was abolished by the specific SLC-1 antagonist SB-568849. Neuropeptide EI, a peptide from the same precursor as MCH was also found to increase CRF release from explants. These results suggest that MCH has a stimulatory role in the HPA axis via SLC-1, and that MCH exerts its effects predominantly through the PVN CRF neuronal populations     

The Role of Ascending Neuronal Pathways in Stress-Induced Release of Noradrenaline in the Hypothalamic Paraventricular Nucleus of Rats

Central catecholaminergic pathways carrying pain-related signals to the hypothalamic paraventricular nucleus (PVN) were investigated in laboratory rats. Four per cent formalin injected subcutaneously was employed as a stressful stimulus. Neuronal activity in brainstem catecholaminergic and paraventricular neurones was assessed by Fos immunohistochemistry. Stress-induced noradrenaline (NE) release from nerve terminals in the PVN was measured in extracellular fluid by in-vivo microdialysis. Within 30 min, formalin elicited a four- to sixfold increase in plasma ACTH and corticosterone concentrations and intense Fos-like activity was seen in the superficial zones of the lumbar spinal cord ipsilateral to the side of the formalin injection. In brainstem catecholaminergic neurones, the PVN, and midline thalamic nuclei, formalin-induced Fos-immunopositivity was equally present in the ipsi- and contralateral sides of the injection. An immediate elevation (4-5 times higher than baseline levels) of NE levels was measured in both the right and left PVN after a formalin injection into the right paw. Unilateral surgical transections at the medulla-spinal cord junction failed to affect formalin-induced elevations in NE levels in the PVN independently of the side of the formalin injection or the knife cut. Thus, this observation clearly shows that fibres carrying pain-evoked signals ascend bilaterally from the spinal cord to the brainstem and forebrain. Hemisections of the medulla oblongata between the level of A1-A2 NE cell groups and the locus coeruleus reduced but did not eliminate formalin-induced NE release from the PVN ipsilateral to the knife cut. This effect was independent of the side of the formalin injection. In the contralateral PVN, high and similar NE levels were measured in response to a formalin injection into the right or the left leg. The present study indicates that formalin-induced pain signals are carried by sensory fibres to the ipsilateral spinal cord. From there, axons of different dorsal horn neurones reach noradrenergic cells on both sides of the medulla oblongata. The majority of noradrenergic fibers ascend on the same side and innervate the ipsilateral PVN. Since formalin administration resulted in a moderate elevation of NE levels in the PVN on the operated side, the role of other ascending noradrenergic (from the locus coeruleus) or noncatecholaminergic fibres that could modulate NE release from the PVN should be considered.     

Norepinephrine in the paraventricular nucleus stimulates corticosterone release

Hypothalamic cells containing corticotropin-releasing factor are believed to be densely innervated by noradrenergic terminals. However, the role of norepinephrine (NE) in the control of the hypothalamo-pituitary-adrenal axis has remained undefined, with both excitatory and inhibitory effects suggested by the literature. The present experiments tested the effects of direct hypothalamic infusion of NE on the release of corticosterone (CORT) in awake and freely moving rats. Norepinephrine infusion into the paraventricular nucleus (PVN) produced a dose-dependent increase in circulating levels of CORT. In a mapping study, this stimulatory effect of NE was found to be anatomically localized. The strongest rise in CORT levels (up to 12 micrograms%) was observed after injection into the PVN, where NE acted in a dose-dependent fashion. A somewhat smaller effect was also detected with NE in the dorsomedial nucleus, while no response occurred after injection just dorsal to the PVN, into the ventromedial or supraoptic nuclei, or into the lateral or posterior hypothalamus. Serotonin infusion into the PVN produced a small but statistically reliable increase in circulating CORT levels. However, dopamine injection into this nucleus had no observable effect. These results agree with recent studies suggesting an excitatory function of PVN NE in the pituitary-adrenal axis.     

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.     

Immunotargeted Lesions of Paraventricular CRF and AVP Neurons in Developing Rats Reveal the Pattern of Maturation of these Systems and their Functional Importance

Pituitary ACTH secretion in the rat is controlled by a number of hypothalamic secretagogues, like CRF and AVP and by inhibitory feedback provided by glucocorticoids. During development, little is known about the precise regulation of ACTH release by hypothalamic neuropeptides and glucocorticoids. We used immunotargeted chemical PVN lesions to investigate the role of CRF and AVP neurons of the hypothalamic paraventricular nucleus (PVN) in the control of ACTH secretion in neonatal rats under basal conditions and 5 days after adrenalectomy (ADX). Neonates aged day (d) 4 or d14 were injected over the PVN with ricin A toxin associated with either non-specific antibodies (IgG/Tx), or monoclonal antibodies directed towards CRF (CRF/Tx) or AVP (AVP/Tx). Rats from each group received either sham surgery (SHAM) or were adrenalectomized (ADX). Pups were sacrificed 5 days after PVN treatment and adrenal surgery (d9 or 19). Plasma ACTH and corticosterone (B) levels were measured by RIAs. Changes in CRF and AVP expression in the PVN and other brain regions were determined by immunohistochemistry (ICC) and in situ hybridization. Injection of the toxin associated with IgGs did not have non specific effects on body weight gain, neuropeptide expression or plasma ACTH and B secretion compared to intact, uninjected rats. Lesions of CRF or AVP neurons greatly reduced peptide expression and mRNA levels in the PVN and median eminence at both ages. However, the specificity of the lesion was greater in older than in young pups. At both ages, we observed a dissociation between the morphological effects of the lesions and hormonal responses. In d14–19 pups, CRF and AVP lesions prevented ADX-induced changes in mRNA levels and peptide expression but did not reduce ACTH secretion under basal or stimulated (post ADX) conditions. However, CRF and AVP lesions increased the expression of CRF in the central amygdala and the bed nucleus of the stria terminalis. Lesions with AVP also stimulated CRF expression in the PVN. Thus, these compensatory changes could take over some of the hypophysiotropic actions of the damaged PVN neurons. In young pups (d4–9), we did not observe the typical increase in CRF and AVP mRNA levels and peptide expression found after ADX in older pups or adults. Lesions of the CRF neurons also affected the AVP system and reciprocally. We suggest that this could be explained by a high degree of colocalization of CRF and AVP observed in parvocellular and small, immature magnocellular neurons in young pups. The lesions did not affect basal or ADX-induced ACTH secretion, suggesting that during the early neonatal period, the pituitary is the major site of glucocorticoid inhibitory feedback on ACTH secretion and that the hypothalamus does not exert a tonic control over basal pituitary secretion. These results unravel ontogenetical differences in the regulation of ACTH secretion by hypothalamic CRF and AVP. During the first 10 days of life, within the adrenal stress hyporesponsive period, hypothalamic CRF and AVP neurons are not sensitive to glucocorticoid feedback and basal ACTH secretion appears to be relatively independent from hypothalamic input. After the second week of life, maturation of glucocorticoid receptors, neuronal phenotype and connections of the PVN to other brain structures (bed nucleus of the stria terminalis, central amygdala) allows for the full expression of corticosterone effect on hypothalamic neurons and for compensatory changes to occur following lesions. These results emphasize the extraordinary capacity of the developing central nervous system to adapt to changes in functionning of some neuronal areas critical for homeostatic balance and the important potential role of intra-hypothalamic and extrahypothalamic relationships in maintaining control over ACTH and glucocorticoid production during development.     

Effects of the short chain sugar acid 2-buten-4-olide on the hypothalamo-pituitary-adrenal axis in normal and adjuvant-induced arthritic rats

The effects of intraperitoneal (i.p.) administration of 2-buten-4-olide (2-B4O), an endogenous sugar acid, on the hypothalamo-adenohypophysial system were examined in Lewis rats that were normal and in adjuvant-induced arthritic (AA) rats. In comparison with vehicle-treated rats, the plasma corticosterone and c-fos mRNA levels in the paraventricular nucleus (PVN) of normal rats increased significantly after i.p. administration of 2-B4O. Dual immunostaining revealed that almost all corticotrophin-releasing factor (CRF)-immunopositive neurones in the parvocellular division of the PVN exhibited Fos-like immunoreactivity (LI) 120 min after i.p. administration of 2-B4O (100 mg/kg). In the AA rats, repeated i.p. administration of 2-B4O (100 mg/kg) after immunisation significantly suppressed the expression of clinical symptoms and significantly increased plasma concentrations of corticosterone. Further, repeated i.p. administration of 2-B4O significantly increased CRF mRNA levels in the PVN and pro-opiomelanocortin mRNA levels in the anterior pituitary; however, they did not change arginine vasopressin mRNA levels in the parvocellular division of the PVN. These results suggest that i.p. administration of 2-B4O activates the hypothalamo-pituitary-adrenal (HPA) axis via the activation of CRF neurones in the PVN, and the activation of the HPA axis by i.p. administration of 2-B4O may be associated with the inhibition of AA in rats.     

Intracerebroventricular administration of β-endorphin increases the expression of c-fos and of corticotropin-releasing factor messenger ribonucleic acid in the paraventricular nucleus of the rat

We evaluated the effects of intracerebroventricular (i.c.v.) administration of β-endorphin and naloxone, an opioid antagonist, on the induction of c-fos and corticotropin-releasing factor (CRF) mRNA to clarify the effects of β-endorphin on cellular activity and CRF gene expression in the paraventricular nucleus (PVN) of the rat using in situ hybridization. A significant induction of c-fos mRNA was noted in the PVN after i.c.v. injection of β-endorphin, compared to control. This induction was inhibited by the administration of naloxone. A significant increase in CRF mRNA levels in the PVN was observed 120 min after the i.c.v. injection of β-endorphin. This increase was partially, but significantly, inhibited by naloxone administration. In addition, i.c.v. administration of β-endorphin increased plasma ACTH concentration in freely moving rats, which was inhibited by intravenous injection of CRF antiserum. These results suggest that the i.c.v. injection of β-endorphin increases the neuronal activity and the biosynthesis of CRF in the PVN, and stimulates the secretion of ACTH by increasing CRF secretion. This effect on the PVN was mediated, at least in part, via the opioid receptor.     

Gender differences in corticotropin and corticosterone secretion and corticotropin-releasing factor mRNA expression in the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala in response to footshock stress or psychological stress in rats

Anorexia nervosa is mostly seen in adolescent females, although the gender-differentiation mechanism is unclear. Corticotropin-releasing factor (CRF), a key peptide for stress responses such as inhibition of food intake, increases in arousal and locomotor activity, and gonadal dysfunction, is thought to be involved in the pathophysiology of anorexia nervosa. CRF in the paraventricular nucleus of the hypothalamus (PVN) and CRF in the central nucleus of the amygdala (CeA) are involved in the regulation of stress responses, and gender differences in CRF mRNA expression in these regions in response to various stressors are controversial. We therefore examined CRF gene expression in the PVN and CeA as well as corticotropin (ACTH) and corticosterone secretion in response to a 60-min period of electric footshock (FS) or psychological stress (PS) induced by a communication box in both male and female rats in proestrus or diestrus in an effort to elucidate the mechanism underlying the gender difference in the activity of the hypothalamic-pituitary-adrenal (HPA) axis and the mechanism underlying the remarkable prevalence of anorexia nervosa in females. Female rats in proestrus showed higher basal plasma ACTH and CRF mRNA expression levels in the PVN and CeA than males. Females more rapidly showed higher plasma ACTH and corticosterone levels and a higher CRF mRNA expression level in the PVN in response to FS than males. Although females in both proestrus and diestrus showed significant increases in plasma ACTH and corticosterone and CRF mRNA expression in the PVN in response to PS, no significant responses of the HPA axis to PS were found in males. FS significantly increased CRF mRNA expression in the CeA in both females and males, with significantly higher peaks in females in proestrus than in males, while PS significantly increased CRF mRNA expression in the CeA only in males. These results suggest that gender affects differentially the function of the stress-related regions such as the PVN and CeA. The finding that CRF gene expression in the PVN responds to PS only in females may be a clue to elucidation of the neurobiological mechanism underlying the gender-differential prevalence of anorexia nervosa.     

Neuropeptide Y (NPY) May Integrate Responses of Hypothalamic Feeding Systems and the Hypothalamo-Pituitary-Adrenal Axis

Neuropeptide Y (NPY) is a powerful stimulus to food intake in the rat. Exogenous NPY given into the third ventricle or into the paraventricular nucleus (PVN) of the hypothalamus stimulates both food consumption as well as the hypothalamus-pituitary-adrenal (HPA) axis. Presumably NPY activates the adrenocortical system through direct stimulation of CRF containing cells in the PVN. Food intake is also a major regulator of adrenocortical activation. Rhythms in HPA axis activity follow rhythms in food consumption, and rats that have been food deprived overnight have inhibited HPA axis responses to restraint stress and corticosteroid feedback the following morning. To investigate the interaction of NPY with both feeding and HPA axis activation three sets of experiments were performed: Animals fed ad lib were injected icv with NPY (2.5 μg) and allowed access to food or not post injection; animals were fasted overnight prior to NPY injection; finally, dose response experiments were performed to examine the relative sensitivities of feeding and HPA axis activation to exogenous NPY. Ad lib fed animals allowed access to food after NPY injection had slightly greater ACTH responses to NPY while glucocorticoid and insulin responses were not significantly different from ad lib fed animals not allowed access to food post injection. Animals allowed to eat post injection had significantly decreased food consumption the night following injection, however, total 24 h food consumption was not different between these animals and those given food 8 h post NPY injection. In overnight fasted animals NPY injections produced ACTH responses of equal magnitude to those in ad lib fed animals. Insulin responses to NPY were significantly elevated compared to CSF controls in overnight fasted animals. Dose response studies revealed that the adrenocortical system responds to icv NPY with at least as great sensitivity as feeding systems. NPY is discussed as a potential integrator of feeding and responsiveness in the HPA axis.     

Central orexin-A activates hypothalamic-pituitary-adrenal axis and stimulates hypothalamic corticotropin releasing factor and arginine vasopressin neurones in conscious rats

The effects of centrally injected orexin-A on plasma adrenocorticotropin (ACTH) and corticosterone levels and corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA in the parvocellular cells of the paraventricular nucleus (PVN) of the rat were investigated. In animals implanted previously with a lateral brain ventricle and femoral artery cannula, a single i.c.v. injection of orexin-A (10 microg/rat) resulted in a rapid, significant increase in plasma ACTH and corticosterone concentrations. Plasma ACTH reached a peak (12.5-fold greater than basal levels) at 30 min, which was maintained over 120 min before declining towards control levels by 240 min. Plasma corticosterone concentrations reached a peak (6.7-fold greater than basal levels) at 30 min. Orexin-A at a higher dose (30 microg/rat) also produced a rapid increase in plasma ACTH and corticosterone concentrations. The area under the curve for plasma levels of ACTH was similar for both doses of orexin-A. In a second study, orexin-A (10 microg/rat) was injected i.c.v. and brains and pituitaries were rapidly removed after 240 min. In situ hybridization histochemistry revealed that CRF and AVP mRNA levels were significantly increased in the parvocellular cells of the PVN. Pro-opiomelanocortin mRNA levels in the pituitary gland were not significantly elevated in response to orexin-A. These results suggest that orexin-A is able to act centrally to activate the hypothalamic-pituitary-adrenal axis involving stimulation of both CRF and AVP expression.     

Stress-induced activation of CRF and c-fos mRNAs in the paraventricular nucleus are not affected by serotonin depletion

The role of serotonin in regulating the stress response is controversial. We have investigated the effects of serotonin depletion byp-chlorophenylalanine (PCPA) on corticotrophin-releasing factor (CRF) mRNA and c-fos mRNA responses in the paraventricular nucleus (PVN) together with circulating levels of ACTH and corticosterone to both physical and psychological stressors in the rat. PCPA pretreatment, which resulted in a 95% depletion in hypothalamic serotonin, had no effect on basal levels of ACTH or the increase in response to the physical stress of hypertonic saline. Plasma ACTH concentrations were also not affected by serotonin depletion in response to the predominantly psychological stress of restraint. Both basal and restraint stress-induced circulating corticosterone levels were however further stimulated in the PCPA-pretreated rats suggesting a possible inhibitory serotoninergic tone at the adrenal level. C-fos mRNA was undetectable in control animals. Activation of c-fos mRNA in response to stress was unaffected by serotonin depletion and the activation of magnocellular PVN and supraoptic nucleus cells was demonstrated to be stressor dependent. Basal and stress-induced levels of CRF mRNA were unaffected by PCPA pretreatment. It appears therefore that under these experimental conditions there is little if any involvement of serotonin in either basal levels or the stress-induced activation of the hypothalamo-pituitary-adrenal axis in vivo.     

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

The influence of chronic stress on the status of the hypothalamo-pituitary-adrenal (HPA) axis of sham-operated and adrenalectomised rats was assessed. Animals underwent bilateral adrenalectomy (ADX) and 3 days later they were either left undisturbed or subjected daily to immobilization for 2 h each morning for 14 days (chronic IMO). In situ hybridization histochemistry revealed that ADX increased corticotropin-releasing factor (CRF) mRNA levels in the paraventricular nucleus of the hypothalamus (PVN) and proopiomelanocortin (POMC) mRNA levels in the anterior pituitary, in both control and chronically stressed rats as measured on the day following the last exposure to stress. Chronic IMO increased CRF mRNA levels in the PVN and POMC mRNA levels in the anterior pituitary of sham-operated rats, as measured on the day following the last exposure to stress. Chronic IMO potentiated the increase in CRF mRNA in the PVN following ADX and resulted in further increases in CRF mRNA above levels seen in adrenal-intact animals. Finally, chronic stress, while not altering basal ACTH levels of ADX rats, reduced the ACTH response of these animals to a novel stressor (tail-shock for 30 min). These results suggest that chronic stress exerts a stimulatory influence at the hypothalamic level that is partially restrained by daily stress-induced glucocorticoid release. Despite the potentiation by chronic stress of CRF mRNA content in the PVN of ADX rats, a blunted circulating ACTH response to an acute short-term stressor was apparent in ADX-chronically stressed rats, suggesting that chronic stress might also alter POMC processing and/or ACTH secretory patterns in the anterior pituitary in ADX animals.     

Normal hypothalamo-pituitary-adrenal axis function in a rat model of peripheral neuropathic pain

Chronic pain conditions such as rheumatoid arthritis and fibromyalgia are associated with profound hypothalamo-pituitary-adrenal (HPA) axis dysfunction which may exacerbate symptoms of chronic pain. HPA axis dysfunction has also been well documented in animal models of chronic inflammatory pain. However, the role of the HPA axis in animal models of neuropathic pain is currently unknown. Rats with a chronic constriction injury (CCI) of the sciatic nerve that developed marked mechanical allodynia and hyperalgesia of the injured hindpaw were used to determine basal and stimulatory levels of HPA axis activity. Plasma ACTH and corticosterone levels were increased significantly (P < 0.05) in CCI rats after 20 min restraint stress compared with baseline; however, the magnitude of the increase was no different from sham rats. Furthermore, the temporal profile of ACTH release over the 60 min period after termination of restraint was similar between CCI and sham rats suggesting normal glucocorticoid-mediated feedback. Restraint stress also significantly increased (P < 0.05) expression of the immediate early genes c-Fos and FosB within the hypothalamic PVN to a similar extent in CCI and sham rats. Within the parvocellular PVN basal expression of both CRF and AVP mRNA was no different between CCI and sham rats; restraint stress induced a significant 2.5 fold increase (P < 0.05) in CRF mRNA expression in sham rats only. These results suggest that, in contrast to inflammatory immune-mediated pain models where HPA axis function is profoundly altered, in the CCI model of neuropathic pain, basal HPA axis function is unchanged. Furthermore, the HPA axis responds normally to a novel stressor in the face of ongoing nociceptive input, a stimulus known to activate the HPA axis.