Effects of interleukin-1 on the stress-responsive and -nonresponsive subtypes of corticotropin-releasing hormone neurosecretory axons

Review: Endotoxin and the hypothalamo-pituitary-adrenal (HPA) axis

Endotoxin is considered to be a systemic (immunological) stressor eliciting a prolonged activation of the hypothalamo-pituitary-adrenal (HPA) axis. The HPA-axis response after an endotoxin challenge is mainly due to released cytokines (IL-1, IL-6 and TNF-α) from stimulated peripheral immune cells, which in turn stimulate different levels of the HPA axis. Controversy exists regarding the main locus of action of endotoxin on glucocorticoid secretion, since the effect of endotoxin on this neuro-endocrine axis has been observed in intact animals and after ablation of the hypothalamus; however, a lack of LPS effect has been described at both pituitary and adrenocortical levels. The resulting increase in adrenal glucocorticoids has well-documented inhibitory effects on the inflammatory process and on inflammatory cytokine release. Therefore, immune activation of the adrenal gland by endotoxin is thought to occur by cytokine stimulation of corticosteroid-releasing hormone (CRH) production in the median eminence of the hypothalamus, which, in turn stimulates the secretion of ACTH from the pituitary. Acute administration of endotoxin stimulates ACTH and cortisol secretion and the release of CRH and vasopressin (AVP) in the hypophysial portal blood. During repeated endotoxemia, tolerance of both immune and HPA function develops, with a crucial role for glucocorticoids in the modulation of the HPA axis. A single exposure to a high dose of LPS can induce a long-lasting state of tolerance to a second exposure of LPS, affecting the response of plasma TNF-α and HPA hormones. Although there are gender differences in the HPA response to endotoxin and IL-1, these responses are enhanced by castration and attenuated by androgen and estrogen replacement. Estrogens attenuate the endotoxin-induced stimulation of IL-6, TNF-α and IL-1ra release and subsequent activation in postmenopausal women. There appears to be a temporal and functional relation between the HPA-axis response to endotoxin and nitric oxide formation in the neuro-endocrine hypothalamus, suggesting a stimulatory role for nitric oxide in modulating the HPA response to immune challenges.

Hormetic Dose-Response Relationships in Immunology: Occurrence, Quantitative Features of the Dose Response, Mechanistic Foundations, and Clinical Implications

This article provides an assessment of the occurrence of immune-system-related hormetic-like biphasic dose-response relationships. Such dose-response relationships are extensive, with over 90 different immune response-related endpoints reported, induced by over 70 endogenous agonists, over 100 drugs, and over 40 environmental contaminants. Such hormetic responses were reported in over 30 animal models, over a dozen mammalian and human cell lines. These findings demonstrate that immune-system-related hormetic-like biphasic dose-response relationships are common and highly generalizable according to model, endpoint, and chemical class. The quantitative features of the dose response are generally consistent with previously published examples of hormetic dose responses for other biological endpoints. These findings were generally recognized and explicitly discussed by the original authors, often with consideration given to possible mechanistic foundations as well as numerous clinical implications. Despite the recognition by individual authors of the hormetic nature of these observed responses, the overall widespread nature of immune-related hormetic responses has been only little appreciated, with a general lack of insight into the highly generalizable nature of this phenomenon as well as the complex regulatory networks affecting biological switching mechanisms that result in the hormetic responses.

The functional neuroanatomy of corticotropin-releasing factor

Descriptions of the central distribution of corticotropin-releasing factor (CRF) have been taken as generally supporting the proposition that this neuropeptide is involved in the mediation of complementary neuroendocrine, autonomic and behavioural responses to stress. The hypothalamic paraventricular nucleus (PVN) is recognized as the principal source of CRF in hypophysial portal plasma; CRF mRNA and peptide expression in parvocellular neurosecretory neurons are regulated negatively by adrenal steroids and positively by many stressors. Consistent with the latter, the hypophysiotropic zone of the PVN receives a rich, and biochemically differentiated, afferent supply that provides visceral, somatic and special sensory systems with access to the 'CRF neuron'. Within the PVN, CRF is also expressed, and differentially regulated, in oxytocinergic magnocellular neurosecretory neurons and in autonomic-related projection neurons. CRF expression in at least some extrahypothalamic cell groups (olfactory bulb, Barrington's nucleus) is responsive to certain stressful stimuli, but not to perturbations of the steroid environment. Refinement of our understanding of the central distribution of CRF has been provided by the recognition that most CRF antisera cross-react with an amidated dipeptide encoded by the melanin-concentrating hormone precursor, and by the likelihood that some central sites of CRF peptide expression may be muted or masked by the presence of a CRF-binding protein (CRF-BP). The CRF-BP is expressed prominently in the telencephalon, where it is co-localized with CRF in some neurons, and in anterior pituitary corticotrophs.

Sensitization to the neuroendocrine, central monoamine and behavioural effects of murine tumor necrosis factor-alpha: peripheral and central mechanisms

Systemic administration of murine tumour necrosis factor-alpha (mTNF-alpha; 0.1-2.0 microg, i.p.) dose-dependently increased plasma corticosterone and augmented monoamine utilization within the paraventricular nucleus of the hypothalamus (PVN), locus coeruleus, medial prefrontal cortex (PFC), central and medial amygdala. A time-dependent sensitization was induced in mice, wherein reexposure to mTNF-alpha 28 days (but not 1 day) following the initial cytokine treatment provoked marked signs of illness (diminished activity, ptosis, piloerection) and increased plasma corticosterone levels. Serotonin (5-HT) activity was augmented upon mTNF-alpha reexposure at the 1- or 28-day intervals in the PFC and medial amygdala, respectively. Intracerebroventricular (i.c.v.; 1-500 ng) mTNF-alpha did not promote illness, but modestly increased plasma corticosterone levels. Neither the illness nor the corticosterone changes were subject to a sensitization upon i.c.v. cytokine reexposure. Acute i.c.v. mTNF-alpha increased norepinephrine (NE), 5-HT and dopamine (DA) activity within the PVN and median eminence/arcuate nucleus complex (ME/ARC), and NE utilization within the central amygdala. Subsequent i.c.v. mTNF-alpha further enhanced the hypothalamic monoamine variations. Finally, systemic (i.p.) mTNF-alpha pretreatment did not proactively influence sickness or corticosterone responses upon later i.c.v. cytokine challenge, but augmented locus coeruleus NE activity and 5-HT and DA utilization within the ME/ARC. It is suggested that the sensitization with respect to sickness and corticosterone activity in response to mTNF-alpha reflect the involvement of peripheral mechanisms. Moreover, it appears that mTNF-alpha promotes central neurochemical plasticity through independent central and peripheral mechanisms.

Anhedonic and anxiogenic effects of cytokine exposure

Systemic interleukin IL-1 beta, TNF alpha, and IL-2 profoundly influenced central monoamine activity, as well as behavioral outputs. The effects of the various cytokines were clearly distinguishable from one another, although synergistic effects were detected between several of these cytokines and between the actions of cytokines and stressors. Acutely applied IL-2 appeared to affect reward processes, but did not affect anxiety. When chronically administered, this cytokine markedly influenced working memory in a spatial learning test. In contrast to IL-2, both IL-1 beta and TNF alpha appeared to provoke an anxiogenic action, and provoked clear signs of illness. While these cytokines induced anorexia, they did not appear to affect reward processes. IL-1 beta and TNF alpha were found to act synergistically, and the TNF alpha provoked a sensitization with respect to the action of subsequent TNF alpha treatment. The findings indicated that cytokine treatments profoundly influence extrahypothalamic neurochemical functioning and may thus impact on behavioral outputs. Analyses of the behavioral and neurochemical changes elicited by cytokines, and particularly TNF alpha, need to consider not only the immediate impact of such treatments, but also the proactive effects that may be engendered.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

High intensity exercise promotes escape of adrenocorticotropin and cortisol from suppression by dexamethasone: sexually dimorphic responses

Exercise promotes escape of ACTH and cortisol from suppression by dexamethasone (DEX) in some healthy men and women. To determine whether stimulus strength, diurnal rhythmicity, or gender influences neuroendocrine escape during DEX suppression, we studied men (n = 5) and women (n = 5) during high intensity exercise tests after taking 4 mg DEX: two tests (one at 90% and one at 100% of maximal aerobic capacity) were conducted in the morning and two were performed in the afternoon on nonconsecutive days. Plasma ACTH and cortisol showed significantly greater increases with the 100% compared to the 90% intensity exercise (ACTH: 90%, 2 +/- 0.4; 100%, 3 +/- 0.5 pmol/L; cortisol: 90%, 53 +/- 5.3; 100% 93 +/- 23.6 nmol/L). Plasma cortisol responses were significantly higher in women than in men (P < 0.01). Plasma arginine vasopressin (AVP) exhibited significant intensity-dependent increases, with higher responses in women than men (P < 0.01). In conclusion, despite high dose glucocorticoid pretreatment, intense exercise can override the glucocorticoid negative feedback of hypothalamic-pituitary-adrenal activation in most normal men and women. This ability to override cortisol negative feedback inhibition may relate to the magnitude of the AVP response, the potency/specificity of the stressor to elicit a CRH/AVP response, and/or the sensitivity of the glucocorticoid negative feedback system at the time of the stress.

Subdiaphragmatic vagotomy inhibits intra-abdominal interleukin-1 beta stimulation of adrenocorticotropin secretion

Although interleukin (IL)-1 beta activates the hypothalamic-pituitary-adrenal (HPA) axis, the mechanisms by which peripheral IL-1 beta acutely stimulates adrenocorticotropin (ACTH) secretion are not clear. Recently, the vagus has been implicated in mediating peripheral cytokine signalling of the brain. To investigate a possible central mechanism for peripheral cytokine stimulation of the HPA axis, we tested the hypothesis that the vagus mediates IL-1 beta activation of the HPA axis by an intra-abdominal stimulus. We studied the effect of subdiaphragmatic vagotomy on plasma ACTH stimulation in rats by intraperitoneal (i.p.) IL-1 beta. Adult male Sprague-Dawley rats underwent subdiaphragmatic vagotomy or sham surgery 1 week prior to study. Rats were killed 1 and 2 h after i.p. saline (control) and low- (4 micrograms/kg) and high-dose (20 micrograms/kg) IL-1 beta. Vagotomy markedly attenuated plasma ACTH secretion at 2 h after high-dose IL-1 beta stimulation and abolished plasma ACTH secretion at 2 h after low-dose IL-1 beta stimulation. At 1 h after low-dose IL-1 beta, stimulation of plasma ACTH in vagotomized animals was also markedly diminished compared to sham animals. However, vagotomy did not alter stimulation of plasma corticosterone at 1 or 2 h after low-dose IL-1 beta or at 2 h after high-dose IL-1 beta. In addition, vagotomy did not alter stimulation of plasma ACTH or corticosterone secretion by insulin-induced hypoglycemia. We conclude that: (1) the vagus plays an important role in stimulation of ACTH secretion by intra-abdominal (i.p.) IL-1 beta; (2) stimulation of corticosterone secretion by i.p. IL-1 beta is not altered by vagotomy; and (3) the inhibitory effect of vagotomy on activation of the HPA axis appears to be specific for immune stimulation by cytokines.

Acute stimulation of the hypothalamic-pituitary-adrenal axis by IL-1 beta, TNF alpha and IL-6: a dose response study

We investigated the effects of i.v. and intracerebroventricular (i.c.v) administration of increasing doses of recombinant human IL-1 beta, TNF alpha and IL-6 on plasma corticosterone (B) levels in rats. Rats were equipped with a jugular cannula for repeated blood sampling anda subgroup of rats also received an i.c.v implanted cannula. I.v. administration of IL-1 beta, TNF alpha or IL-6 and i.c.v administration of IL-1 beta and IL-6 induced a significant dose-dependent increase in plasma B levels, whereas i.c.v injection of TNF alpha in doses up to 1000 ng/rat was not effective. I.v. pretreatment of rats with anti-CRH antiserum had no significant overall effect on the plasma B response to i.v. administered IL-1 beta (500 and 3000 ng/rat), whereas the plasma B response to i.v. TNF alpha or IL-6 administration (3000 ng/rat) were significantly reduced. I.v. pretreatment of the animals with recombinant human IL-1 receptor antagonist (IL-1ra) significantly blocked the plasma B response to i.v. treatment with IL-1 beta, whereas the TNF alpha- and IL-6-induced increases in plasma B levels were not affected. Our data show that 1) i.v. administration of IL-beta, TNF alpha or IL-6 and i.c.v administration of IL-1 beta or IL-6 dose-dependently stimulate the HPA axis; 2) when given i.v. or i.c.v, IL-1 beta is more powerful than TNF alpha and IL-6 in activating the HPA axis; 3) endogenous CRH is involved in the activation of the HPA axis by acute i.v. administration of TNF alpha and IL-6. It is most likely that in case of i.v. treatment with IL-1 beta a CRH-independent mechanism is involved. This study provides no arguments for the involvement of endogenous IL-1 in TNF alpha- or IL-6-induced activation of the HPA axis.

Intracerebroventricularly Transplanted Embryonic Neuronal Tissue from Inflammatory-Resistant F344/N Rats Decreases Acoustic Startle Responses in Inflammatory-Susceptible Lew/N Rats

Recently, we have shown that intracerebral transplantation of fetal F344/N hypothalamic tissue into LEW/N rats converts the LEW/N inflammatory-susceptible phenotype into an inflammatory-resistant phenotype in LEW/N hosts. Because LEW/N rats also exhibit relatively high acoustic startle responses (ASRs) compared to F344/N rats, in the present study we examined the effects on ASR of transplantation of F344/N hypothalamic tissue into the third ventricle of LEW/N rats. Dissected neuronal tissue from F344/N rats (Day E15-16) was implanted into the third ventricle of LEW/N rats. After 4 wk of postoperative survival, the animals' responses to acoustic startle stimuli were tested. Compared to naive and sham-operated animals, LEW/N rats transplanted with hypothalamic tissue exhibited significant decreases in ASR amplitudes. A similar decrease in ASR amplitude was observed in the group of LEW/N rats transplanted with embryonic striatal tissue. Our results indicate that the third ventricular neuronal grafts may modulate behavioral responses in the LEW/N rats. Although the mechanism of this effect is unknown, these studies suggest that intracerebral neuronal transplantation is a viable method with which to explore mechanisms of behavioral, neuroendocrine, and inflammatory response associations.

Interleukin-1 beta stimulates both central and peripheral release of vasopressin and oxytocin in the rat

Simultaneous microdialysis in the brain and blood was used to monitor the release of vasopressin and oxytocin within the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei and into the systemic circulation of urethane-anaesthetized male rats before and after central administration of interleukin-1 beta (IL-1 beta). Following intracerebroventricular infusion of the cytokine (200 ng/5 microliters), the content of vasopressin (up to 278% compared to vehicle-treated control, P < 0.01 compared to vehicle-treated control and preinfusion baseline) but not oxytocin (up to 148%, not significant) in 30-min blood microdialysates was found to be increased. This peripheral release was accompanied by a transient rise in vasopressin (up to 163%, P < 0.05) and oxytocin (up to 182%, P < 0.05) release within the SON, the peak typically occurring during the first and second 30-min collection intervals after IL-1 beta respectively. In contrast, in the simultaneously microdialysed PVN, both vasopressin and oxytocin failed to respond to intracerebroventricular IL-1 beta. In another series of experiments, IL-1 beta was directly infused (20 ng/0.5 microliters) into either the SON or PVN during microdialysis of the corresponding nucleus. The cytokine caused a significant and immediate rise in intra-SON release of both vasopressin (up to 225%, P < 0.01) and oxytocin (up to 178%, P < 0.05). Again, in the PVN, nonapeptide release, although tending to be stimulated in response to intranuclear IL-1 beta, failed to reach statistical significance. The cytokine-induced central and peripheral release pattern appeared to be independent of the rise in body temperature observed after IL-1 beta administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of cytokine expression by hypericum extract

The effect of hypericum extract LI 160 on the stimulated cytokine expression was investigated in vitro in a whole blood culture system. Blood samples were taken from five healthy volunteers and four depressive patients. The release of interleukin-6 (IL-6), interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) was measured quantitatively after an incubation time of 24 hours on microtiter plates. A massive suppression of the interleukin-6 release was found for PHA-stimulated hypericum extract. Possible relations to the antidepressive effects of hypericum extract are discussed.

Release of hypothalamic corticotropin-releasing hormone and arginine-vasopressin by interleukin 1 beta and alpha MSH: studies in rats with different susceptibility to inflammatory disease

The susceptibility of Lewis rats is related to blunted hypothalamic-pituitary-adrenal (HPA) axis responsiveness to a variety of inflammatory and neuroendocrine stimuli. In contrast resistance to inflammatory disease of histocompatible Fischer rats is associated with their intact HPA axis responses to the same stimuli. We have examined the contribution of IL-1 beta to in vitro corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) release from hypothalamic explants derived from LEW/N and F344/N rats. The same animal model has been used to investigate the regulatory effect of alpha MSH, an immunosuppressive neurohormone, on IL-1 beta stimulated CRH and AVP secretion. CRH basal release in both strains was similar. However, LEW/N hypothalamic AVP basal secretion was significantly elevated. CRH relative response of LEW/N hypothalamic explants to IL-1 beta stimulation was lower compared to Fischer, which is consistent with their hyporesponsiveness to inflammatory mediators. AVP secretion however, was significantly decreased in hypothalamic explants from both strains after 40 min exposure to IL-1 beta. alpha MSH suppressed basal CRH and AVP release in both LEW/N and F344/N rats and prevented IL-1 beta stimulated CRH secretion in these strains. AVP was further diminished in F344/N explants following incubation with alpha MSH + IL-1 beta, while LEW/N level was significantly elevated. However, AVP levels remained significantly below baseline in explants from both strains after final incubation with IL-1 beta. Although our findings indicate a modulatory action of alpha MSH in HPA axis regulation in vitro, the physiological importance of this phenomenon in Lewis and Fischer rats requires further investigation.

Functional implications of brain corticosteroid receptor diversity

1. Corticosteroids readily enter the brain and control gene expression in nerve cells via binding to intracellular receptors, which act as gene transcription factors. In the rat brain corticosterone binds to mineralocorticoid receptors (MRs) with a 10-fold higher affinity than to glucocorticoid receptors (GRs). As a consequence, these MRs are extensively occupied under basal resting conditions, while substantial GR occupation occurs at the circadian peak and following stress. Both receptors are colocalized in most, but not all, hippocampal neurons. In addition, some neurons contain aldosterone-selective MRs, if corticosterone is enzymatically inactivated. These aldosterone target neurons are presumably localized in the anterior hypothalamus, where they underlie central control of salt appetite and cardiovascular regulation. 2. The data show that MR- and GR-mediated effects proceed in a coordinate and often antagonistic mode of action: (i) in hippocampus MR activation maintains excitability, while GR occupancy suppresses excitability, which is transiently raised by excitatory stimuli; (ii) central MRs participate in control of the sensitivity of the neuroendocrine stress response system, while GRs are involved in termination of the stress response; (iii) MRs in the hippocampus have a role in regulation of behavioral reactivity and response selection. GR-mediated effects facilitate storage of information. 3. On the basis of these data, we propose that a relative deficiency or excess of MR- over GR-mediated neuronal effects may lead to a condition of enhanced or reduced responsiveness to environmental influences, alter behavioral adaptation, and promote susceptibility to stress. The findings may serve development of novel therapeutic strategies for treatment of stress-related brain diseases.