ACTH release induced in rats by noradrenaline is mediated by prostaglandin E2

Effect of selective cyclooxygenase inhibitors on animal behaviour and monoaminergic systems of the rat brain

The long-term effects of cyclooxygenase 1 and 2 (COX-1/2) inhibitors are usually tested in terms of the periphery of the organism. Therefore, we studied the effects of SC560 (selective COX-1 inhibitor) and celecoxib (selective COX-2 inhibitor) on the activity of brain monoaminergic systems and animal behaviour. Additionally, we tested the effect of these inhibitors during inflammation. We have observed that long-term administration of celecoxib reduces the activity of the noradrenergic system, increases the activity of dopaminergic and serotonergic systems, increases locomotor activity, and enhances the exploratory behaviour of rats. Administration of SC560 also increases the activity of dopaminergic and serotonergic systems but reduces locomotor activity and impairs the exploratory behaviour of rats. The mechanism responsible for decreased activity of the noradrenergic system may be related to the weakening of activity of the positive feedback loop between the paraventricular nucleus and coeruleus locus. We suggest that the effect of used inhibitors on the dopaminergic system is associated with a possible increase in anandamide concentration and its effect on dopamine reuptake in synaptic clefts. It also appears that cyclooxygenase peroxidase activity may play a role in this process. In turn, changes in the activity of the serotonergic system may be related to the activity of indoleamine-2,3-dioxygenase, which decreases because of the decreased concentration of pro-inflammatory compounds. We believe that behavioural changes induced by COX inhibitors are the result of the modified activity of monoaminergic CNS systems in the brainstem, hypothalamus, and medial prefrontal cortex.

The role of α2-adrenergic receptors in hypertensive preeclampsia: A hypothesis

Preeclampsia, a major disorder of human pregnancy, manifests as persistent hypertension and proteinuria presenting after 20 weeks of pregnancy. Multiple systemic symptoms might be associated with preeclampsia including thrombocytopenia, liver impairment, pulmonary edema, and cerebral disturbances. However, vascular dysfunction remains the core pathological driver of preeclampsia. Defective placental implantation followed by dysfunctional placental spiral artery development promotes a hypoxic environment. Massive endothelial dysfunction characterized by reduced vasodilation, augmented vasoconstriction, and increased vascular permeability and inflammation ensues. Interestingly, the same signaling and inflammatory pathways implicated in preeclampsia appear to be shared with other vascular disorders involving alteration of α₂ -AR function. The role of α₂ -ARs in the regulation of microcirculatory function has long been recognized, thus raising the question of whether they are involved in the pathogenesis of vascular dysfunction in preeclampsia. Here, we review possible interplay between signaling and inflammatory pathways common to preeclampsia and α₂ -AR function/regulation. We speculate on the potential contribution of these receptors to the observed phenotype and the potential role for their pharmacological modulators as therapeutic interventions with preeclampsia.

Time-dependent mediators of HPA axis activation following live Escherichia coli

The hypothalamus-pituitary-adrenal (HPA) axis is activated during an immune challenge to liberate energy and modulate immune responses via feedback and regulatory mechanisms. Inflammatory cytokines and prostaglandins are known contributors to HPA activation; however, most previous studies only looked at specific time points following LPS administration. Since whole bacteria have different immune stimulatory properties compared with LPS, the aim of the present studies was to determine whether different immune products contribute to HPA activation at different times following live Escherichia coli challenge. Sprague-Dawley rats were injected intraperitoneally with E. coli (2.5 × 10(7) CFU) and a time course of circulating corticosterone, ACTH, inflammatory cytokines, and PGE(2) was developed. Plasma corticosterone peaked 0.5 h after E. coli and steadily returned to baseline by 4 h. Plasma PGE(2) correlated with the early rise in plasma corticosterone, whereas inflammatory cytokines were not detected until 2 h. Pretreatment with indomethacin, a nonselective cyclooxygenase inhibitor, completely blocked the early rise in plasma corticosterone, but not at 2 h, whereas pretreatment with IL-6 antibodies had no effect on the early rise in corticosterone but attenuated corticosterone at 2 h. Interestingly, indomethacin pretreatment did not completely block the early rise in corticosterone following a higher concentration of E. coli (2.5 × 10(8) CFU). Further studies revealed that only animals receiving indomethacin prior to E. coli displayed elevated plasma and liver cytokines at early time points (0.5 and 1 h), suggesting prostaglandins suppress early inflammatory cytokine production. Overall, these data indicate prostaglandins largely mediate the early rise in plasma corticosterone, while inflammatory cytokines contribute to maintaining levels of corticosterone at later time points.

Hormonal influences of early postnatal indomethacin and ibuprofen in neonatal rats

OBJECTIVE

Indomethacin and ibuprofen are administered to preterm neonates for symptomatic patent ductus arteriosus. The drugs suppress prostaglandins (PGs) which modulate growth and secretion of various hormones. We examined the hypothesis that early postnatal indomethacin and ibuprofen influence growth and GH-IGF-I-insulin and HPA axes in neonatal rats.

DESIGN

Rat pups received IP injections of saline (Sal) on P1, P2, and P3; 10mg/kg ibuprofen on P1 followed by 5mg/kg on P2 and P3; or 0.2mg/kg indomethacin on P1 followed by 0.1mg/kg on P2 and P3. Serum and hepatic GH, GHBP and IGF-I; and serum corticosterone and insulin levels were determined.

RESULTS

Ibuprofen suppressed somatic growth in the sucking rats, but the effect was transient, resolving by P14. Indomethacin had an opposite, latent effect on body weight and liver to body weight ratios in weanling rats. Both indomethacin and ibuprofen had profound hormonal effects that differed in magnitude and timing. Indomethacin resulted in a sustained elevation in corticosterone levels at P21, while ibuprofen increased serum and hepatic GH levels. Both drugs suppressed GHBP in serum at P7 and P14; and liver at P4 and P7, but a rebound increase in serum GHBP was noted at P21 with Ibuprofen only. Both drugs increased serum IGF-I at P7. The effect remained sustained with indomethacin.

CONCLUSIONS

These results provide evidence for an involvement of PGs in the regulation of growth as well as the GH-IGF and HPA axes. Therefore, early postnatal exposure to PG inhibitors may further exacerbate postnatal growth restriction and ability to cope with stress.

Hyperthermia alters kidney function and renal scintigraphy

BACKGROUND/AIM

Fever can be caused by different reasons such as environmental conditions, acute rejection after kidney transplantation and bacterial diseases including kidney and urinary tract infections. The present study represents a novel idea of investigating the direct effect of body temperature elevation on kidney function to determine whether hyperthermia alters the kidney function transiently leading to inaccurate findings and possible misinterpretation of the radionuclide (99mTc-MAG-3) renography studies.

METHODS

Renography studies were performed on New Zealand White rabbits weighing approximately 3-3.5 kg. Each rabbit was inject with 48.1 MBq (1.3 mCi) technetium-99m-mercaptoacetyltriglycine (99mTc-MAG-3). Studies were acquired using a gamma camera equipped with a low-energy, high-resolution collimator interfaced with a computer. Dynamic images were acquired as 2-s frames for the first 1 min and every 30 s for the next 30 min on a matrix of 64 x 64. Regions of interest were drawn over the whole kidneys. Radioactivity time curves were generated from the regions of interest. Time to peak activity (Tmax), time from peak to 50% activity (T1/2), and the uptake slope of each kidney were calculated from the renograms. Three days later the same protocol was repeated for the same rabbit but with a higher body temperature by 2 degrees C. Then it was repeated with a higher body temperature by 3 degrees C, then 4 degrees C with the same interval period. Blood pressure was measured using a catheter inserted into the femoral artery connected to a Lectromid recorder at normal temperature and during increasing the temperature by 2, 3 and 4 degrees C. Renal blood flow was also measured via the renal artery using an electromagnetic blood flow sensor connected to a flowmeter. Creatinine and blood urea nitrogen (BUN) in blood were measured in control and hyperthermic rabbits.

RESULTS

During hyperthermia the experimental curves shifted to the right of the control curves indicating that there was a delayed renal uptake of 99mTc-MAG-3 and clearance of radioactivity. This delay was proportional to body temperature. Calculated averages were: Tmax 1.6 +/- 0.1, 2.8 +/- 0.3, 8.8 +/- 1, 15 +/- 4 min; T1/2 2.77 +/- 0.2, 3 +/- 0.4, 8.9 +/- 1.1, 20 +/- 3.4 min, and perfusion index 190 +/- 5, 201 +/- 4, 218 +/- 7, 224 +/- 9 of control and hyperthermic (elevation of temperature 2, 3, and 4 degrees C) rabbits, respectively (n = 6; p < 0.05). Mean arterial pressure and renal blood flow did not significantly change during hyperthermia. Creatinine and BUN were proportionally elevated to high temperature.

CONCLUSIONS

Our results indicate that hyperthermia causes a transient alteration in the function of the kidney and scintigraphic pattern on radionuclide renography. Radionuclide renography studies may be performed at normothermic temperature since interpretation at higher body temperature could lead to misleading results, and temperature should be checked and recorded for single and follow-up radionuclide renography studies.

Medial prefrontal cortex control of the paraventricular hypothalamic nucleus response to psychological stress: possible role of the bed nucleus of the stria terminalis

The medial prefrontal cortex (mPFC) has been strongly implicated in control of the paraventricular nucleus of the hypothalamus (PVN) response to stress. Because of the paucity of direct projections from the mPFC to the PVN, we sought to investigate possible brain regions that might act as a relay between the two during psychological stress. Bilateral ibotenic acid lesions of the rat mPFC enhanced the number of Fos-immunoreactive cells seen in the PVN after exposure to the psychological stressor, air puff. Altered neuronal recruitment was seen in only one of the candidate relay populations examined, the ventral bed nucleus of the stria terminalis (vBNST). Furthermore, bilateral ibotenic acid lesions of the BNST caused a significant attenuation of the PVN response to air puff. To better characterize the structural relationships between the mPFC and PVN, retrograde tracing studies were conducted examining Fos expression in cells retrogradely labeled with cholera toxin b subunit (CTb) from the PVN and the BNST. Results obtained were consistent with an important role for both the mPFC and BNST in the mpPVN CRF cell response to air puff. We suggest a set of connections whereby a direct PVN projection from the ipsilateral vBNST is involved in the mpPVN response to air puff and this may, in turn, be modulated by an indirect projection from the mPFC to the BNST.

Differential involvement of rat medial prefrontal cortex dopamine receptors in modulation of hypothalamic- pituitary-adrenal axis responses to different stressors

Recent investigations have implicated the medial prefrontal cortex (mPFC) in modulation of subcortical pathways that contribute to the generation of behavioural, autonomic and endocrine responses to stress. However, little is known of the mechanisms involved. One of the key neurotransmitters involved in mPFC function is dopamine, and we therefore aimed, in this investigation, to examine the role of mPFC dopamine in response to stress in Wistar rats. In this regard, we infused dopamine antagonists SCH23390 or sulpiride into the mPFC via retrodialysis. We then examined changes in numbers of cells expressing the c-fos immediate-early gene protein product, Fos, in subcortical neuronal populations associated with regulation of hypothalamic-pituitary-adrenal (HPA) axis stress responses in response to either of two stressors; systemic injection of interleukin-1 beta, or air puff. The D1 antagonist, SCH23390, and the D2 antagonist, sulpiride, both attenuated expression of Fos in the medial parvocellular hypothalamic paraventricular nucleus (mpPVN) corticotropin-releasing factor cells at the apex of the HPA axis, as well as in most extra-hypothalamic brain regions examined in response to interleukin-1 beta. By contrast, SCH23390 failed to affect Fos expression in response to air puff in any brain region examined, while sulpiride resulted in an attenuation of the air puff-induced response in only the mpPVN and the bed nucleus of the stria terminalis. These results indicate that the mPFC differentially processes the response to different stressors and that the two types of dopamine receptor may have different roles.

Regulation of ACTH levels in anterior pituitary cells during stimulated secretion: evidence for aspartyl and cysteine proteases in the cellular metabolism of ACTH

The regulation of cellular levels of adrenocorticotropin hormone (ACTH) in response to stimulated secretion was investigated to define the extent of cellular depletion of ACTH and subsequent increases to replenish ACTH levels in anterior pituitary cells (in primary culture). Treatment of cells with secretagogues for short-term incubation times (hours) resulted in extensive depletion of cellular ACTH. Corticotropin releasing factor (CRF) induced depletion of cellular levels of ACTH by 60-70% of control levels. The CRF-induced reduction of cellular ACTH was inhibited by the glucocorticoid dexamethasone. Phorbol myristate acetate (PMA), which stimulates protein kinase C (PKC), reduced ACTH levels by 50-60%. Forskolin, a stimulator of cAMP production, produced a moderate reduction in cellular ACTH. During prolonged incubation of cells (2 days) with these secretagogues, further reduction of ACTH levels by 70-80% was observed. However, increased cellular levels of ACTH occurred with continued treatment of cells with secretagogues, which provided nearly complete replenishment of cellular ACTH after 5 days treatment with secretagogues. Notably, the rising levels of cellular ACTH were inhibited by the aspartyl protease inhibitor acetyl-pepstatin A, and by the cysteine protease inhibitor E64d. These results demonstrate that depletion and recovery of ACTH levels are coordinately regulated, and that the increases in cellular levels of ACTH during the recovery phase involves participation of aspartyl and cysteine proteases. Thus, aspartyl and cysteine proteases may be involved in the cellular metabolism of ACTH.

Centrally applied nitric oxide donor elevates plasma corticosterone by activation of the hypothalamic noradrenergic neurons in rats

The present study was undertaken to investigate the mechanisms involved in a nitric oxide donor [3-morpholino-sydnonimine (SIN-1)]-induced activation of hypothalamic-pituitary-adrenal axis in urethane- and alpha-chloralose-anesthetized rats. Intracerebroventricularly (i.c.v.) administered SIN-1 (250 and 500 microg/animal) effectively and dose-dependently elevated plasma levels of corticosterone. Pretreatment with phentolamine (250 microg/animal, i.c.v.), an alpha-adrenoceptor antagonist, attenuated the elevation of plasma corticosterone evoked by SIN-1, but sotalol (300 microg/animal, i.c.v.), a beta-adrenoceptor antagonist, was without effects. The same doses of SIN-1 also increased the release of noradrenaline in the hypothalamic paraventricular nucleus (PVN) measuring microdialysis technique, and this increase was abolished by tetrodotoxin (1 microM) administered into the perfusion solution of the PVN. Furthermore, pretreatment with indomethacin (500 microg/animal, i.c.v.), a cyclooxygenase inhibitor, abolished the SIN-1-induced elevations of both noradrenaline in the PVN and plasma corticosterone. These results suggest that i.c.v. administered SIN-1 activates central noradrenergic neurons innervating the PVN by prostaglandin-mediated mechanisms. Released noradrenaline in the PVN elevates plasma levels of corticosterone via an activation of the central alpha-adrenoreceptors in rats.

Site and mechanism of action of dynorphin A-(1-13) and N-methyl-D-aspartate on ACTH release in fetal sheep

Dynorphin A (Dyn A) stimulates the release of ACTH in fetal sheep, a response that involves N-methyl-D-aspartate (NMDA) receptors but not the secretogogues corticotropin-releasing hormone or arginine vasopressin. We now find that neither Dyn A-(1-13) (0.5 mg/kg, i.v.) nor NMDA (4 mg/kg, i.v.) elicits ACTH release in postnatal lambs. This led us to hypothesize that Dyn A-(1-13) and NMDA might act to release placental ACTH. However, the ability of Dyn A-(1-13), NMDA, and the kappa-opioid receptor agonist U-50488H (1 mg/kg, i.v.) to release ACTH was lost after either fetal hypophysectomy (n = 4) or hypothalamo-pituitary disconnection (n = 4). These results indicate that neither the placenta nor the fetal pituitary is the site of action for these agonists and suggest a hypothalamic or suprahypothalamic site of action. Furthermore, the release of ACTH by Dyn A-(1-13) and NMDA was abolished after pretreatment with indomethacin, suggesting that they might cause the release of a prostanoid, possibly from the placenta, that subsequently acts at the hypothalamus or serves as a permissive factor in the action of Dyn A-(1-13) and NMDA at the hypothalamus.

Multihormonal responses to clonidine in patients with affective and psychotic symptoms

The neuroendocrine responses to the alpha(2)-adrenoreceptor agonist clonidine (CLO) (0.35 mg if body weight <65 kg or 0.375 mg if body weight> or =65 kg, PO) were studied in a large group of subjects: 134 drug-free inpatients--with either DSM-IV schizophrenia (SCZ, n=31), schizoaffective disorder (SAD, n=16), or major depressive episode (MDE, n=87) - and 22 hospitalized controls (HCs). Comparison with a previous placebo test performed in a subgroup of 92 subjects (46 MDEs, 20 SCZs, 8 SADs, and 18 HCs) showed that CLO induced a significant increase of growth hormone, prolactin (PRL) and thyrotropin (TSH) levels but no significant change in adrenocorticotropin and cortisol release. According to diagnostic categories, we found significantly lower GH stimulation in MDEs and in SADs compared to HCs or to SCZs. In addition, we found significantly lower CLO induced PRL and TSH stimulations in paranoid SCZ patients compared to controls and disorganized SCZ patients. Taken together, these results suggest a hyposensitivity of noradrenergic alpha(2)-receptors in patients with affective symptoms.

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.

Food for thought: a critique on the hypothesis that endogenous cholecystokinin acts as a physiological satiety factor

This review evaluates the various lines of evidence supporting the hypothesis that cholecystokinin (CCK) released from the small intestine during feeding plays a physiological satiety. Issues considered include, the effects of systemic injection of CCK on consummatory and operant feeding, the role of the vagus nerve, the effects of CCKB receptor antagonists, and the neuroendocrine responses to exogenous CCK. A critical appraisal of this research indicates that while it is clearly demonstratable that exogenous peripheral CCK can alter food intake by acting on CCKA receptors, the mechanism involved may be more closely related to the induction if aversion and nausea, rather than satiety. With regard to peripheral endogenous CCK, the available evidence also does not seem to support a role for the hormone in satiety. In particular, it is doubtful whether plasma concentrations of CCK following a meal are sufficiently high to inhibit feeding. Moreover, CCKA receptor antagonist which do not cross the blood brain barrier fail to increase meal size, as would be expected if peripheral CCK was an effective satiety factor. In addition, the recent literature concerned with the possibility that CCK may have a direct action within the brain in the control of food intake has been reviewed. These studies show that CCK administered intracerebroventicularly, or by micoinjection into discrete brain regions, also inhibits feeding via a CCKA receptor mechanism. However, the physiological relevance of these findings have yet to be determined.

Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. In the present review we argue that noradrenaline affects aggression on three different levels: the hormonal level, the sympathetic autonomous nervous system, and the central nervous system (CNS), in different, but functionally synergistic ways. Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. Indirect CNS effects include: the shift of attention towards socially relevant stimuli; the enhancement of olfaction (a major source of information in rodents); the decrease in pain sensitivity; and the enhancement of memory (an aggressive encounter is very relevant for the future of the animal). Concerning more aggression-specific effects one may notice that a slight activation of the central noradrenergic system stimulates aggression, while a strong activation decreases fight readiness. This biphasic effect may allow the animal to engage or to avoid the conflict, depending on the strength of social challenge. A hypothesis is presented regarding the relevance of different adrenoceptors in controlling aggression. It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors. The latter phenomenon may be dependent on a noradrenaline-induced corticosterone secretion. It appears that by activating very different mechanisms the systems working with adrenaline and/or noradrenaline prepare the animal in a very complex way to answer the demands imposed by, and to endure the effects caused by, fights. It is a challenge for future research to elucidate how precisely these mechanisms interact to contribute to functionally relevant and adaptive aggressive behaviour.

A comparative study of adrenocorticotropin-releasing activity of prostaglandins E1, E2, F2 alpha and D2 in the rat

There is substantial evidence to indicate that prostaglandin (PG) E2 exerts a stimulatory effect on the hypothalamo-pituitary-adrenal axis in rodents. However, little is known regarding the possibility that other PGs play a similar role in regulating the endocrine axis. Therefore, in this study we compared the effects of intravenous administration of PGs E1, E2, F2 alpha and D2 on adrenocorticotropin (ACTH) secretion in conscious male rats. Each PG was administered at two doses of 0.1 and 1.0 mg/kg body weight, and blood samples were collected sequentially up to 120 min postinjection. Although PGD2 was without effect on ACTH secretion at either dose, PGs E1, E2 and F2 alpha all significantly stimulated the hormonal response at both doses. Interestingly, PGs E1, E2 and F2 alpha were largely equipotent in stimulating ACTH release. To the best of our knowledge, this is the first study to demonstrate significant ACTH-releasing activity of intravenously administered PGs E1 and F2 alpha in the rat. These results suggest that PGE2 might not be the only prostanoid playing a role in regulating the hypothalamo-pituitary-adrenal axis and, thus, multiple PGs may be involved in the endocrine axis.

Effect of indomethacin on the pituitary-adrenocortical response to adrenergic stimulation

The role of prostaglandins (PGs) in stimulation of the hypothalamic-pituitary-adrenal (HPA) axis by adrenergic agonists and catecholamines was investigated in nonanesthetized rats. The cyclooxygenase and PGs synthesis inhibitor indomethacin was given systemically or intracerebroventricularly (icv) 15 min prior to phenylephrine (30 micrograms), clonidine (10 micrograms), and isoproterenol (20 micrograms), an alpha 1-, alpha 2-, and beta-adrenergic receptor agonists, respectively, or noradrenaline (10 micrograms) and adrenaline (10 micrograms). Indomethacin given ip (2 mg/kg) or icv (10 micrograms) almost abolished the increase in corticosterone secretion elicited by phenylephrine, considerably reduced the response to clonidine but did not markedly affect the response to isoproterenol. Pretreatment with indomethacin by either route strongly suppressed the corticosterone response to noradrenaline, but did not substantially affect the hormonal response to adrenaline. The above data indicate that prostaglandins considerably mediate the HPA axis response to central stimulation of alpha 1- and alpha 2-, but not beta-adrenergic receptors. They also point to significant involvement of prostaglandins in the noradrenaline-, but not adrenaline-induced HPA axis predominantly via alpha 1-and alpha 2-adrenergic receptors, whereas adrenaline exerts stimulation manly via beta-adrenergic receptors.

Activation of the HPA axis by immune insults: roles and interactions of cytokines, eicosanoids, glucocorticoids

It is now well established that challenges to the immune system (e.g., infection, inflammation) initiate diverse changes in neuroendocrine function, the most overt of which is activation of the hypothalamo-pituitary-adrenocortical (HPA) axis. The glucocorticoids that are released as a consequence fulfill a vital role in the maintenance of homeostasis that is effected in part through their ability to quench the immune/inflammatory response and thereby prevent them accelerating to a point where they become hazardous to the host. This article discusses the putative mechanisms by which immune insults stimulate the HPA axis, with particular reference to the roles and interactions of the interleukins, eicosanoids and glucocorticoids.

Elevation of plasma ACTH concentration in rabbits made febrile by systemic injection of bacterial endotoxin

This study was carried out to investigate the adrenocorticotropic hormone (ACTH) response in rabbits made febrile by systemic injection of lipopolysaccharide (LPS, Salmonella typhosa endotoxin). Intravenous (i.v.) injection of LPS (0.1 microgram/kg and 1.0 microgram/kg) increased rectal temperature (biphasic fever) and the plasma concentration of ACTH (ACTH response) in a dose-related manner. These responses were suppressed by pretreatment with indomethacin (20 mg/kg, subcutaneously). Intracerebroventricular (i.c.v.) administration of indomethacin (400 micrograms) had no effect on the ACTH response to LPS, although it significantly suppressed febrile response. Small increases in plasma concentration of ACTH and significant fevers followed i.c.v. administration of prostaglandin E2 (2 micrograms) or F2 alpha (2 micrograms). I.v. administration of corticotropin releasing factor (CRF) antagonist [alpha-helical CRF (9-41) (200 micrograms/kg)] partly suppressed the ACTH increase induced in plasma by i.v. LPS. These results suggest that prostaglandins synthesized outside the blood-brain barrier play an important role in the ACTH response and that the mechanism for induction of the ACTH response is not exactly the same as that for the febrile response, although prostaglandins are involved in both responses.

Alpha 2-adrenoceptor blockade, pituitary-adrenal hormones, and agonistic interactions in rats

The effects of adrenergic activation on aggressiveness and the aggression induced endocrine changes were tested in rats. Alpha 2 adrenoceptor blockers were used for enhancing activation of the adrenergic system, and changes in aggressiveness were tested in resident-intruder contests. Three experiments were conducted. In experiment 1, saline injected rats responded to the presence of an opponent by aggression and the increase in plasma ACTH and corticosterone. Intraperitoneal administration of 1 mg/kg CH-38083 (an alpha 2 adrenoceptor antagonist) produced a several fold increase in clinch fighting and mutual upright scores, and also further enhanced the plasma ACTH and corticosterone response. In experiment 2, the effect of three doses (0.5, 1 and 2 mg/kg) of three different alpha 2 adrenoceptor blockers CH-38083, idazoxan and yohimbine were tested. All the substances increased aggression at 0.5 and 1 mg/kg; at 2 mg/kg the effect of idazoxan and yohimbine disappeared, while with CH-38083 an additional increase was obtained. In yohimbine treated animals the enhancement of aggression was reduced already at 1 mg/kg. In experiment 3, indomethacin, a potent inhibitor of the catecholamine-induced ACTH release completely abolished the effects of the alpha 2 adrenoceptor antagonist CH-38083: the intensity of agonistic interactions, as well as ACTH and corticosterone plasma concentrations, returned to control levels. The possible role of catecholamines and the stress hormones in the activation of aggression is discussed.

ACTH response induced in capsaicin-desensitized rats by intravenous injection of interleukin-1 or prostaglandin E

1. We investigated whether afferent nerves are involved in the development of adrenocorticotrophic hormone (ACTH) responses induced either by systemic administration of interleukin-1 beta (IL-1 beta) and prostaglandin E2, or by psychological stress. The capsaicin desensitization method was used to impair afferent C fibres and we compared the ACTH responses between capsaicin desensitized and vehicle pretreated control rats. 2. The present results showed that the capsaicin desensitized rats had significantly smaller increases in plasma ACTH than the control rats in response to intravenous injection of IL-1 beta or prostaglandin E2. 3. There were no significant differences between the capsaicin desensitized and control rats in the ACTH responses induced by cage switch stress. 4. The capsaicin desensitized rats responded to intravenous injection of corticotrophin releasing factor (CRF) with a greater increase in the plasma level of ACTH than the control rats, indicating that capsaicin pretreatment resulted in augmentation of pituitary gland sensitivity to CRF. 5. These results suggest that afferent neurons play an important role in the ACTH responses induced by systemic injection of IL-1 beta or prostaglandin E2.

Evaluation for roles of brain prostaglandins in the catecholamine-induced vasopressin secretion in conscious rats

To evaluate roles of prostaglandins (PGs) in vasopressin (AVP) secretion elicited by stimulating alpha-adrenergic and dopaminergic receptors in the periventricular region, we examined in conscious rats the effects of intracerebroventricular (i.c.v.) injections of a cyclooxygenase inhibitor meclofenamate on the plasma AVP responses to i.c.v. applications of angiotensin II (ANG II), phenylephrine and dopamine. I.c.v. injections of 58 pmol ANG II produced, 5 and 15 min later, augmentations of plasma AVP accompanied by elevations of arterial pressure and tendencies of reduction in heart rate. Similarly, the administrations of 0.53 mumol phenylephrine or dopamine enhanced plasma AVP 5 min later, without altering arterial pressure and heart rate significantly. Meclofenamate (0.31 mumol) applied i.c.v. 30 min prior to the administrations of ANG II remarkably inhibited the AVP and pressor responses to this peptide. However, the responses of plasma AVP, arterial pressure and heart rate to phenylephrine or dopamine were not affected by the i.c.v. administrations of 0.31 mumol meclofenamate. The injections of meclofenamate followed by the administrations of a vehicle for ANG II and the catecholamines were without effect on plasma AVP and the cardiovascular parameters. Plasma osmolality, sodium, potassium and chloride in all the groups mentioned above were not significantly changed during experiments. These results suggest that PGs generated in the periventricular region, despite their probable stimulatory roles in the ANG II-evoked AVP secretion, may not participate in the AVP-releasing mechanisms activated by dopaminergic and alpha-adrenergic receptors, supporting the view that PGs and the catecholamines may facilitate AVP release via separate pathways.

Possible involvement of prostaglandins in psychological stress-induced responses in rats

1. We investigated the effect of pre-treatment with intraperitoneal (I.P.) injection of indomethacin, an inhibitor of prostaglandin synthesis, on psychological stress-induced responses including cardiovascular, thermoregulatory and hormonal responses in free-moving rats. 2. Psychological stress was induced by cage-switch stress. After the rats were placed in the novel environment, blood pressure, heart rate and body temperature significantly increased. Plasma levels of adrenocorticotrophic hormone (ACTH) and prostaglandin E2 were significantly higher 30 min after exposure to stress, in comparison to normal levels. 3. Pre-treatment with I.P. indomethacin significantly suppressed the increases in body temperature induced by cage-switch stress, but had no effect on increases in blood pressure and heart rate induced by this stress. Indomethacin also significantly suppressed the increases in the plasma levels of ACTH and prostaglandin E2 induced by cage-switch stress. 4. The present results suggest that prostaglandins are involved in the development of hyperthermia and the ACTH response induced by psychological stress.