Hypothalamo-pituitary-adrenal Response Research Articles

Opposing effects of pituitary leukemia inhibitory factor and SOCS-3 on the ACTH axis response to inflammation.

We have shown that leukemia inhibitory factor (LIF) and suppressor of cytokine signaling (SOCS)-3 are expressed in the hypothalamus and pituitary and that LIF induces proopiomelanocortin (POMC) and ACTH, whereas SOCS-3 abrogates corticotroph POMC gene transcription and ACTH secretion. Here, we determined the role of pituitary LIF and SOCS-3 in regulating hypothalamo-pituitary-adrenal (HPA) axis inflammatory responses. Murine pituitary LIF expression was induced up to eightfold after intraperitoneal injection of lipopolysaccharide or tumor necrosis factor-alpha, concordant with elevated plasma levels of ACTH and corticosterone. In LIF knockout (LIFKO) mice, induction of both ACTH and corticosterone were attenuated. LIF deletion was associated with elevated (P < 0.05) levels of pituitary TNF-alpha, interleukin (IL)-1beta, and IL-6 mRNA and cytokine-inducible pituitary SOCS-3 expression. Abrogation of the HPA axis stress response and higher pituitary levels of proinflammatory cytokines observed in LIFKO mice resulted in a stronger inflammatory process, as evidenced by elevated erythrocyte sedimentation rate and increased serum amyloid A levels (P < 0.05). The results indicate that, although LIF induces ACTH, SOCS-3 acts to counterregulate the HPA axis response to inflammation.

Alprazolam, a benzodiazepine, does not modify the ACTH and cortisol response to hCRH and AVP, but blunts the cortisol response to ACTH in humans

Alprazolam (AL), a benzodiazepine which activates gamma-amino butyrric acid (GABA)-ergic receptors, exerts a clear inhibitory effect on the activity of the hypothalamo-pituitary-adrenal (HPA) axis and is able to markedly reduce the ACTH response to metyrapone-induced inhibition of glucocorticoid feedback. It has been suggested that its inhibitory action could be regulated by CRH or AVP mediated mechanisms. However, the effect of benzodiazepines on the HPA response to CRH or AVP is contradictory. It has been shown that benzodiazepines have specific receptors on the adrenal gland but it is unclear if they mediate biological effects in humans. In order to further clarify the mechanisms underlying the inhibitory effect of benzodiazepine on HPA axis in humans, we studied the effect of AL (0.02 mg/kg po at -90 min) or placebo in 7 healthy young volunteers (7 female, age: 26-34 yr; wt: 50-58 kg, BMI 20-22 kg/m2) on: 1) the ACTH and cortisol responses to hCRH (2.0 microg/kg iv at 0 min) or AVP (0.17 U/kg im at 0 min); 2) the cortisol, aldosterone and DHEA responses to ACTH 1-24 (0.06 and 250 microg iv at 0 and 60 min, respectively). After placebo, the ACTH and cortisol responses to hCRH (peaks, mean+/-SE: 29.8+/-4.4 pg/ml and 199.3+/-19.6 microg/l) were similar to those recorded after AVP (31.7+/-6.5 pg/ml and 164.8+/-18.0 microg/l); the cortisol response to 0.06 microg ACTH (190.4+/-11.8 microg/l) was similar to that recorded after hCRH and AVP but lower (p<0.01) than that after 250 microg ACTH (260.6+/-17.4 microg/l). AL did not modify the ACTH response to both hCRH (42.5+/-7.1 pg/ml) and AVP (33.3+/-2.7 pg/ml), which even showed a trend toward increase. AL also failed to significantly modify the cortisol response to both hCRH (156.3+/-12.7 microg/l) and AVP (119.4+/-14.5 microg/l), which, on the other hand, showed a trend toward decrease. The cortisol peaks after 0.06 microg ACTH were significantly reduced (p<0.02) by AL pre-treatment (115.0+/-7.7 microg/l) which, in turn, did not modify the cortisol response to the subsequent ACTH bolus (214.7+/-16.6 microg/l). The DHEA and aldosterone responses to all the ACTH doses were not significantly modified by AL. In conclusion, these data show that the HPA response to AVP as well as to hCRH is refractory to the inhibitory effect of AL which, in turn, blunts the cortisol response to low ACTH dose. These findings suggest that both CRH- and AVP-mediated mechanisms could underlie the CNS-mediated inhibitory effect of AL on HPA axis; in the meantime, these results suggest that benzodiazepines could also act on adrenal gland by blunting the sensitivity of the fasciculata zone to ACTH.

Adrenomedullin augments the neurohumoral response to haemorrhage in non-pregnant but not in pregnant sheep.

Adrenomedullin (ADM) is a novel peptide with actions which include reduction of arterial pressure and interaction with a number of hormone systems. In order to assess possible interactions with the renin-angiotensin system (RAS) and the hypothalamo-pituitary-adrenal (HPA) axis, we have examined neurohumoral responses to hypotensive haemorrhage (15 ml/kg over 15 min) with or without co-infusions of ADM (5.5 pmol/kg per min) in six non-pregnant and eight pregnant conscious sheep. Haemorrhage induced a greater decrease in arterial pressure, but a blunted increase in heart rate in pregnant sheep. There was no significant effect of ADM on haemodynamic responses to haemorrhage in either group. In non-pregnant sheep, haemorrhage-induced activation of both RAS and HPA was significantly augmented by ADM, as indicated by greater increases in plasma renin activity (P<0.01), angiotensin II (P<0.05) and arginine vasopressin (P<0.01). In contrast, ADM did not augment these responses to haemorrhage in pregnant sheep. Rather, plasma concentrations of aldosterone (P=0.039) and adrenocorticotrophic hormone (P=0.012) were decreased by ADM. In conclusion, ADM-induced augmentation of the RAS and HPA responses to hypotensive haemorrhage is abolished in the pregnant state.

Combined Treatment with Corticosteroids and Moclobemide Favors Normalization of Hypothalamo-Pituitary-Adrenal Axis Dysregulation in Relapsing-Remitting Multiple Sclerosis: A Randomized, Double Blind Trial

Hyperresponsiveness of the hypothalamo-pituitary-adrenal (HPA) axis in multiple sclerosis (MS), an autoimmune inflammatory disease of the central nervous system, is presumably due to diminished corticosteroid receptor function. It probably influences the immune response, but its clinical significance is not clear. Similar HPA dysregulation occurs in depression and is reversible with successful antidepressant treatment. We conducted a double blind, placebo-controlled trial to evaluate the neuroendocrine effect of cotreatment with the antidepressant moclobemide as an adjunct to oral corticosteroids in MS. Twenty-one patients with definite relapsing-remitting MS (11 females, aged 33.9 ± 2.0 yr; Expanded Disability Status Scale score of neurological impairment, 2.0–6.5) in acute relapse were treated with placebo (n = 13) or 300 mg moclobemide (reversible monoamine oxidase A inhibitor; n = 8) for 75 days. All received oral fluocortolone from day 7 on, and the dose was tapered until day 29. Effects were evaluated using the combined dexamethasone-CRH test and clinically on days 1, 30, and 75. At baseline, the HPA axis was mildly activated, comparably for treatment groups [area under the curve for cortisol (AUC-Cort), 213.8 ± 76.8 arbitrary units in the moclobemide group vs. 225.8 ± 65.1 in the steroid alone group; mean ± sem]. In a group of healthy controls with comparable demographic characteristics, the AUC-Cort was 107.4 ± 14.1. Moclobemide cotreatment resulted in normalization of the HPA axis response, whereas the HPA system hyperresponse was maintained with steroids alone (AUC-Cort on day 30, 85.9 ± 22.8 vs.177.1 ± 68.5; on day 75, 111.0 ± 46.0 vs. 199.2 ± 64.6). The change in Expanded Disability Status Scale was comparable for both groups. Although corticosteroids alone had no effect on the HPA response using the dexamethasone-CRH test, treatment with moclobemide combined with corticosteroids favors normalization of the HPA response in relapsing-remitting MS.

Stress and Inflammatory Disease: Widening Roles for Serotonin and Substance P

Serotonin has been implicated in mediating the hypothalamo-pituitary-adrenal (HPA) axis response to stress and is an important therapeutic target for a number of psychiatric disorders including depression. The neurokinin substance P has been shown to inhibit stress-induced HPA axis activity and we have demonstrated that endogenous substance P is able to reduce the duration of the HPA axis response to stress suggesting an important role in the termination of the stress response. This may be important in controlling the transition from acute to chronic stress and substance P has recently attracted attention as a potential antidepressant.In addition to these central effects, serotonin and substance P are considered to be pro-inflammatory agents. Despite being implicated in mediating inflammation there have been few studies investigating the effects of manipulations of serotonergic or substance P systems on chronic inflammatory disease. Treatment of rats with adjuvant-induced arthritis (AA), a model of chronic inflammatory stress, with a substance P antagonist specific for the NK1 receptor subtype resulted in a reduction in hind paw inflammation suggesting substance P may influence inflammation. We have noted that depletion of whole body serotonin and selective central depletion of serotonin results in a decrease in the severity of inflammation in rats with adjuvant arthritis. Furthermore, treatment with a selective serotonin reuptake inhibitor results in an earlier onset and increased severity of inflammation in adjuvant arthritis, confirming a pro-inflammatory role for serotonin. Serotonin is also present in the immune tissues and concentrations in the spleen fall following the development of inflammation in adjuvant arthritis. Concentrations of serotonin are significantly higher in normal female spleen than in males, and this may underlie the greater predisposition of females to certain autoimmune diseases.There is increasing evidence of a role for transmitters such as serotonin and substance P, both centrally and peripherally, in mediating a wide variety of inflammatory and psychiatric disorders. A better understanding of the mechanisms of action of these transmitters and the development of suitable drugs targeting specific receptor subtypes has great potential to impact on clinical practice in the near future. The purpose of this review is to consider the separate roles of serotonin and substance P in relation to HPA axis stress responses, in the context of a model of chronic inflammatory disease, highlighting novel directions of current research for each of these transmitters.

Chapter 10 Alterations in behavioral and neuroendocrine stress coping strategies in pregnant, parturient and lactating rats

In the present chapter the behavioral and neuroendocrine alterations accompanying pregnancy and lactation will be discussed. It will be shown that many are dependent on the innate level of emotionality of the rats. In late pregnancy the level of anxiety, as measured on the elevated plus-maze is increased in rats with both high and low level of innate anxiety-related behavior, whereas lactating rats display less anxiety in such tests and higher degrees of aggressive behavior in tests for agonistic behavior. There is a dramatic reduction in the responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis to various physical or emotional stimuli in both pregnant and lactating rats. This appears to be due to changes throughout the HPA axis. Oxytocin has been implicated in the control of the axis at this time, but the inhibitory action of central oxytocin on ACTH or corticosterone secretion seen in virgin female rats is not evident during pregnancy and lactation. However, central oxytocin is involved in the regulation of emotionality at this time. In addition to its anxiolytic effect, prolactin, acting at brain prolactin receptors, seems to exert an inhibitory effect on HPA axis responsiveness. At the time of parturition, the HPA axis is not stimulated by parturition-related stimuli and is under strong inhibition by endogenous opioids as revealed by the application of the opioid receptor antagonist naloxone.

Evidence that a single exposure to aversive stimuli triggers long‐lasting effects in the hypothalamus‐pituitary‐adrenal axis that consolidate with time

AbstractBecause of its use as a negative reinforcer in animal studies and its potential pathological impact (e.g. post‐traumatic stress disorder and depression), exposure to aversive stimuli is a relevant model for studying CNS plasticity. We present evidence that a single exposure to two predominantly emotional stressors [restraint in tubes and immobilization on wooden boards (IMO)] can modify the response of the hypothalamo–pituitary–adrenal (HPA) axis to a subsequent exposure to the same stressor days later in that a more rapid return to the baseline was observed in the poststress period. In addition, the effect was greater with IMO, the more severe stressor. Using IMO, we have further demonstrated that the effect of a previous single exposure to the stressor (i) increased with days elapsed between the two exposures; (ii) was specific for the previously experienced stressor; and (iii) was mediated via central‐mediated effects [corticotropin‐releasing factor (CRF) mRNA in the paraventricular nucleus of the hypothalamus]. These data suggest that animals retain memory about a single experience with stressors, resulting in an acceleration of the poststress recovery of the HPA axis that enhances progressively over a period of weeks. The extent to which the present data are relevant regarding post‐traumatic stress disorders is unclear, but the study of the HPA response to severe stressors may be suitable for the study of the neurobiological basis of the progressive consolidation of learning over a long period of time (days to weeks).

Chapter 8 Peripartum plasticity within the hypothalamo-pituitary-adrenal axis

The hypothalamo-pituitary-adrenal (HPA) axis plays important roles in the adaptive changes in physiology that occur during pregnancy and lactation. Although the axis still exhibits a pulsatile pattern of secretion, the normal diurnal rhythm of pulse amplitude is lost during lactation, such that mean basal levels remain constant throughout the day. In addition, the peripartum period is associated with a remarkable plasticity in stress-induced HPA activity, in that the increase of HPA activity normally seen in response to either physical or psychological stresses in the non-reproductive state become severely attenuated or absent in the lactating animal. This stabilization of both basal and stress-induced HPA activity may be important for maintaining a constant endocrine environment, thereby preventing any programming effects on the developing offspring. Attenuation of the stress response is initiated in late pregnancy and is temporally associated with luteolysis, indicating possible steroid hormone involvement. Indeed, mimicking the luteolytic changes in oestrogen and progesterone levels in non-pregnant animals induces a similar attenuation of the stress response. Furthermore down-regulation of the stress response is, at least in part, centrally mediated since in the period following luteolysis rats will show a decreased level of stress-induced neuronal activation of the PVN, as measured by the expression of either c-fos or CRH mRNAs. Persistence of this adapted state is dependent upon the continued suckling stimulus, as removal of the offspring litter rapidly leads to resumption of HPA responses to and the appearance of an exaggerated diurnal rhythm. The underlying mechanisms responsible for this stress hyporesponsiveness may include plasticity of noradrenergic and oxytocin pathways. In view of its role in other reproductive behaviors, a stress-inhibiting effect of oxytocin may reflect a more widespread co-ordinating role in the peripartum animal.

Evidence that a single exposure to aversive stimuli triggers long-lasting effects in the hypothalamus-pituitary-adrenal axis that consolidate with time

Because of its use as a negative reinforcer in animal studies and its potential pathological impact (e.g. post-traumatic stress disorder and depression), exposure to aversive stimuli is a relevant model for studying CNS plasticity. We present evidence that a single exposure to two predominantly emotional stressors [restraint in tubes and immobilization on wooden boards (IMO)] can modify the response of the hypothalamo-pituitary-adrenal (HPA) axis to a subsequent exposure to the same stressor days later in that a more rapid return to the baseline was observed in the poststress period. In addition, the effect was greater with IMO, the more severe stressor. Using IMO, we have further demonstrated that the effect of a previous single exposure to the stressor (i) increased with days elapsed between the two exposures; (ii) was specific for the previously experienced stressor; and (iii) was mediated via central-mediated effects [corticotropin-releasing factor (CRF) mRNA in the paraventricular nucleus of the hypothalamus]. These data suggest that animals retain memory about a single experience with stressors, resulting in an acceleration of the poststress recovery of the HPA axis that enhances progressively over a period of weeks. The extent to which the present data are relevant regarding post-traumatic stress disorders is unclear, but the study of the HPA response to severe stressors may be suitable for the study of the neurobiological basis of the progressive consolidation of learning over a long period of time (days to weeks).

Post-surgical recovery and time-of-day mask potentiated responses of ACTH to repeated moderate hemorrhage in conscious rats

We determined how changes in the responsiveness of the hypothalamo-pituitary-adrenal (HPA) system that accompany experimentation affect facilitation of HPA responses to hemorrhage. Hemorrhage (10 ml/kg over 3 min) was performed in conscious, chronically prepared rats. Blood was sampled over 1 h followed by reinfusion of shed blood. Hemorrhage was performed either once or twice separated by 24 h in different groups of animals. To test the effect of the circadian variation in responsiveness, rats were hemorrhaged on days 4 and 5 after surgery either in the morning (AM) or in the afternoon (PM). The response of ACTH to hemorrhage on day 4 was greater in the PM than in the AM (P<0.01). The ACTH response to the second hemorrhage on day 5 was greater than that to hemorrhage on day 4 only in the AM group (P<0.01). Thus, facilitation of ACTH responses by prior hemorrhage was evident only in the AM. To determine the effects of surgical recovery, additional experiments were done in the AM either early (days 3 and 4) or later (days 6 and 7) after surgery. In these experiments, hemorrhage was performed in all rats on days 4 and 7 and either hemorrhage or blood sampling alone was performed on day 3 and 6. ACTH did not increase in rats with sampling and no hemorrhage. ACTH increased more after an initial hemorrhage on day 3 than on day 6 (P<0.01). ACTH response to hemorrhage on day 4 was greater when preceded by hemorrhage vs sampling on day 3 (P<0.01). ACTH response to hemorrhage in rats bled twice did not differ on day 3 and day 4. On day 7, the response of ACTH in rats that had hemorrhage on day 6 was greater than both their own response on day 6 and the response of a control group with sampling on day 6 (P<0.01). These results demonstrate potentiation of ACTH responses to hemorrhage by an earlier similar hemorrhage, but clearly indicate that enhanced sensitivity of the HPA to hemorrhage either by circadian factors or by surgery can mask this effect.

Significance of pulsatility in the HPA axis.

A stress-free automated blood sampling system has been employed to demonstrate pulsatile hypothalamo-pituitary-adrenal (HPA) activity in the rat. In females, pulses of corticosterone secretion occur approximately once/hour throughout the 24 h cycle, with variation in pulse amplitude underlying a diurnal rhythm. Males show smaller pulses of secretion which become widely spaced during the early light phase nadir. Ageing does not affect the occurrence of pulses but the diurnal variation is lost. Analysis of the relationship between the HPA response to an acute noise stress and its coincidence with the various phases of the pulse, suggests that pulsatile activity arises from alternating periods of activation and suppression. Responses to i.v. corticotropin-releasing factor are not affected by pulse phase, indicating that this relationship is not generated at the pituitary-adrenal level. This phase relationship holds for all strains of rat except the hyperresponsive Fischer-344 in which an exaggerated stress response arises from a lack of phase-dependent suppression. Patterns of pulsatile activity are also modulated by neonatal programming or chronic HPA activation arising from adjuvant-induced arthritis, with consequent impact upon the response to acute stimuli. Thus, variations in the patterns of pulsatile activity are important determinants of both basal secretion and acute responses of the HPA axis.

The Pulsatile Characteristics of Hypothalamo-Pituitary-Adrenal Activity in Female Lewis and Fischer 344 Rats and Its Relationship to Differential Stress Responses

The dynamic patterns of basal and stimulated hypothalamo-pituitary-adrenal (HPA) activity of freely moving female Lewis and Fischer 344 rats were compared using an automated blood-sampling system. Both strains showed pulsatile corticosterone release throughout the 24 h cycle. Lewis rats showed clear circadian variation in both pulse frequency (8.4 ± 0.4 pulses between 1700–2300 h vs. 5.3 ± 0.8 pulses between 0500–1100 h; P < 0.05) and height (198 ± 27 ng/ml between 1700–2300 h vs. 107 ± 14 ng/ml between 0500–1100 h; P < 0.05). Fischer rats exhibited pulses of similar frequency and height to those in Lewis rats during the evening, but showed no circadian variation, resulting in higher mean daily corticosterone concentrations. Although both strains showed behavioral and HPA responses to white noise stress (10 min; 114 dB), Fischer rats showed much greater increases in total activity, grooming, and rearings, and two important differences in the corticosterone responses were observed. First, in Lewis rats a clear relationship existed between basal and stimulated HPA activities, in that a significant response was seen only when the stress coincided with the rising (secretory active) phase of a basal pulse. Noise stress coinciding with a falling (nonsecretory) phase elicited no significant response. In contrast, Fischer rats showed similar responses regardless of the underlying pulse phase. Second, after the peak response at 20 min (Lewis, 237 ± 67 ng/ml; Fischer, 390 ± 57 ng/ml), corticosterone levels fell rapidly in Lewis rats, but remained maximally elevated for 20 min in Fischer rats, resulting in a significantly greater integrated response. The corticosterone response to iv CRF was unaffected by pulse phase in both strains, suggesting that a suprapituitary mechanism mediates the phase-dependent response to stress in the Lewis strain. CRF-induced corticosterone levels rose more rapidly in Fischer rats, peaking at 10 min (473 ± 95 ng/ml) compared with 30 min (390 ± 75 ng/ml) in Lewis rats, suggesting greater pituitary sensitivity in this strain. Thus, differences in both central and pituitary control of the HPA axis contribute to the strain difference in stress responsiveness between female Lewis and Fischer rats.

The pulsatile characteristics of hypothalamo-pituitary-adrenal activity in female Lewis and Fischer 344 rats and its relationship to differential stress responses.

The dynamic patterns of basal and stimulated hypothalamo-pituitary-adrenal (HPA) activity of freely moving female Lewis and Fischer 344 rats were compared using an automated blood-sampling system. Both strains showed pulsatile corticosterone release throughout the 24 h cycle. Lewis rats showed clear circadian variation in both pulse frequency (8.4 +/- 0.4 pulses between 1700-2300 h vs. 5.3 +/- 0.8 pulses between 0500-1100 h; P < 0.05) and height (198 +/- 27 ng/ml between 1700-2300 h vs. 107 +/- 14 ng/ml between 0500-1100 h; P < 0.05). Fischer rats exhibited pulses of similar frequency and height to those in Lewis rats during the evening, but showed no circadian variation, resulting in higher mean daily corticosterone concentrations. Although both strains showed behavioral and HPA responses to white noise stress (10 min; 114 dB), Fischer rats showed much greater increases in total activity, grooming, and rearings, and two important differences in the corticosterone responses were observed. First, in Lewis rats a clear relationship existed between basal and stimulated HPA activities, in that a significant response was seen only when the stress coincided with the rising (secretory active) phase of a basal pulse. Noise stress coinciding with a falling (nonsecretory) phase elicited no significant response. In contrast, Fischer rats showed similar responses regardless of the underlying pulse phase. Second, after the peak response at 20 min (Lewis, 237 +/- 67 ng/ml; Fischer, 390 +/- 57 ng/ml), corticosterone levels fell rapidly in Lewis rats, but remained maximally elevated for 20 min in Fischer rats, resulting in a significantly greater integrated response. The corticosterone response to i.v. CRF was unaffected by pulse phase in both strains, suggesting that a suprapituitary mechanism mediates the phase-dependent response to stress in the Lewis strain. CRF-induced corticosterone levels rose more rapidly in Fischer rats, peaking at 10 min (473 +/- 95 ng/ml) compared with 30 min (390 +/- 75 ng/ml) in Lewis rats, suggesting greater pituitary sensitivity in this strain. Thus, differences in both central and pituitary control of the HPA axis contribute to the strain difference in stress responsiveness between female Lewis and Fischer rats.

The role of endogenous opioids in neurohypophysial and hypothalamo-pituitary-adrenal axis hormone secretory responses to stress in pregnant rats.

Endogenous opioid regulation of neurohypophysial and hypothalamo-pituitary-adrenal (HPA) axis hormone secretion in response to forced swimming (90 s in deep water at 19 degrees C) was investigated in virgin and 21-day-pregnant rats. There was no difference in basal plasma oxytocin concentrations between pregnant and virgin rats, but the opioid antagonist, naloxone, increased basal oxytocin secretion in the pregnant rats. Forced swimming increased oxytocin secretion similarly in pregnant and virgin rats, and this response was enhanced by naloxone. In pregnant rats naloxone had a greater effect (by 3.1-fold) than in virgins, showing stronger endogenous opioid restraint of an enhanced oxytocin secretory response to stress in pregnancy. Vasopressin secretion was not increased with forced swimming in virgin or pregnant rats, and naloxone had no effect. ACTH and corticosterone secretion in response to forced swimming was attenuated in pregnant rats compared to virgin rats, measured at 5 min. Naloxone had no effect on basal plasma ACTH or corticosterone concentration, but it reduced ACTH secretion in virgin rats 5 min after forced swimming; in pregnant rats naloxone had no such effect. Naloxone removed the pregnancy-related attenuation in corticosterone secretion measured at 5 min after forced swimming. Fifteen minutes after forced swimming, plasma corticosterone concentrations were not different between groups. In the late-pregnant rats, the increases in plasma ACTH and corticosterone induced by forced swimming were significantly prolonged compared to virgins. The results show that endogenous opioid inhibition emerges in pregnancy to restrict the responses of oxytocin neurones to a stressor. In contrast, the endogenous opioid enhancement of mechanisms regulating HPA axis secretory responses in virgin rats is not evident during pregnancy.

Attenuated neuroendocrine responses to emotional and physical stressors in pregnant rats involve adenohypophysial changes.

1. The responsiveness of the rat hypothalamo-pituitary-adrenal (HPA) axis and hypothalamo-neurohypophysial system (HNS) to emotional (elevated plus-maze) and physical (forced swimming) stressors and to administration of synthetic corticotrophin-releasing hormone (CRH) was investigated during pregnancy and lactation. In addition to pregnancy-related adaptations at the adenohypophysial level, behavioural responses accompanying the neuroendocrine changes were studied. 2. Whereas basal (a.m.) plasma corticosterone, but not corticotrophin (adrenocorticotrophic hormone; ACTH), levels were increased on the last day (i.e. on day 22) of pregnancy, the stress-induced rise in both plasma hormone concentrations was increasingly attenuated with the progression of pregnancy beginning on day 15 and reaching a minimum on day 21 compared with virgin control rats. A similar attenuation of responses to both emotional and physical stressors was found in lactating rats. 3. Although the basal plasma oxytocin concentration was elevated in late pregnancy, the stress-induced rise in oxytocin secretion was slightly lower in day 21 pregnant rats. In contrast to vasopressin, oxytocin secretion was increased by forced swimming in virgin and early pregnant rats indicating a differential stress response of these neurohypophysial hormones. 4. The blunted HPA response to stressful stimuli is partly due to alterations at the level of corticotrophs in the adenohypophysis, as ACTH secretion in response to CRH in vivo (40 ng kg-1, i.v.) was reduced with the progression of pregnancy and during lactation. In vitro measurement of cAMP levels in pituitary segments demonstrated reduced basal levels of cAMP and a lower increase after CRH stimulation (10 nM, 10 min) in day 21 pregnant compared with virgin rats, further indicating reduced corticotroph responsiveness to CRH in pregnancy. 5. The reduced pituitary response to CRH in late pregnancy is likely to be a consequence of a reduction in CRH receptor binding as revealed by receptor autoradiography. [125I] CRH binding in the anterior pituitary was significantly reduced in day 11, 17 and 22 pregnant rats compared with virgin controls. 6. Anxiety-related behaviour of the animals as revealed by the time on and entries into the open arms of the elevated plus-maze was different between virgin and pregnant rats with decreased number of entries indicating increased anxiety with the progression of pregnancy (except on pregnancy day 18). The emotional behaviour, however, was not correlated with the neuroendocrine responses. 7. The results indicate that the reduced response of the HPA axis to stressors described previously during lactation is already manifested around day 15 of pregnancy in the rat and involves physiological adaptations at the adenohypophysial level. However, alterations in stressor perception at higher brain levels with the progression of pregnancy may also be involved.

Reduced response of the hypothalamo-pituitary-adrenal axis to alpha1-agonist stimulation during lactation.

To determine whether altered noradrenergic activation of the hypothalamo-pituitary-adrenal (HPA) axis contributes to the attenuated neuroendocrine response to stress observed during lactation, the effect of intracerebroventricular injection of the alpha1-agonist methoxamine (100 microg) was compared between virgin and lactating rats. Virgin rats showed significant increases in plasma corticosterone after methoxamine, reaching 317 +/- 44 ng/ml at 10 min and remaining significantly elevated for more than 120 min, but lactating rats showed no significant increase in corticosterone levels. Furthermore, methoxamine induced an increase in paraventricular nucleus (PVN) CRF messenger RNA expression in virgin, but not lactating, animals. Both groups of rats exhibited comparable elevations in plasma PRL after methoxamine treatment. Arginine vasopressin messenger RNA expression within the parvocellular PVN was greater in the lactating animals than in the virgin controls, but methoxamine injection was without further effect. Studies performed on ovariectomized virgin rats and ovariectomized rats receiving estradiol or progesterone replacement failed to reproduce the attenuated HPA responses seen after methoxamine treatment, although methoxamine-induced PRL levels were greatly increased by estradiol, probably arising from an effect on hormone synthesis. In vitro electrophysiological recordings of PVN neurons in hypothalamic slices from proestrous virgin and lactating rats showed that 45-52% of neurons in both groups exhibited excitatory responses to 10(-4) M methoxamine, but there was a differential response to 10(-5) M methoxamine, with PVN neurons from lactating animals failing to show a response. These data show a selective down-regulation of alpha1-mediated activation of the HPA axis in lactating animals. This may contribute to the attenuated stress-induced activation of the HPA axis during lactation.

Dopamine-beta-hydroxylase activity is necessary for hypothalamo-pituitary-adrenal (HPA) responses to ether, and stress-induced facilitation of subsequent HPA responses to acute ether emerges as HPA responses are inhibited by increasing corticosterone (B).

To determine a role of norepinephrine (NE) in stress-induced HPA function, young male rats were treated with diethyldithiocarbamide (DDC) which inhibits dopamine-beta-hydroxylase, the enzyme that synthesizes NE from dopamine (DA). DDC injected 5 h prior to ether stress stimulated ACTH and corticosterone (B) during this time, and there was no further HPA response to ether. To control for elevated B feedback in DDC effects on HPA responses to ether, rats were adrenalectomized (Adx) and replaced with no (0% B), moderate (40% B) and high (80% B) levels of steroid 5 d prior to DDC or saline with ether stress 5 h later; Sham-Adx rats were included. In Adx rats increasing B inhibited thymus weight, median eminence CRF content, pituitary and plasma ACTH. In saline-treated rats, ether 5 h later caused increased CRF content and plasma ACTH in Sham-Adx and Adx, 0% B, increased ACTH in Adx, 40% B, and no response in Adx, 80% B. B treatment did not alter catecholamine content, and DDC treatment reduced NE content in the paraventricular nuclei by 50-60% in all groups. 5 h after DDC, pituitary ACTH was decreased in all rats with B and plasma ACTH was increased in sham-Adx and Adx, 40% B; thus DDC caused significant, prolonged stress which should facilitate subsequent HPA responses to acute stress. There was no HPA response to ether in Sham-Adx, Adx, 0% or 40% B groups, but there was a marked ACTH response to ether in the Adx, 80% B group treated with DDC. We conclude that: 1) the HPA response to ether stress is probably mediated by catecholamines; 2) DDC does not stimulate responses in the HPA axis in the absence of B; and, 3) facilitation of HPA responses to acute stress depends on increased steady-state B signals. Facilitated responses are probably not mediated by catecholamines. The consequence of facilitation is that under conditions of chronic stress and elevated B concentrations, as in depression or anorexia nervosa in man, or adjuvent-induced arthritis in rats, the HPA axis is continually responsive to new stimuli.

Gender difference in cocaine-induced HPA axis activation

The study investigates the influence of gender on the activation of the hypothalamo-pituitary-adrenal (HPA) axis by cocaine and other monamine uptake inhibitors. Drug-induced secretion of adrenocorticotrophic hormone (ACTH) was used to pursue this question. Cocaine produced a dose-related rise in serum ACTH levels in both male and female rats, but the rise was significantly greater in females than in males. Females also showed enhanced HPA axis activation following administration of the dopamine uptake inhibitor GBR 12909, the serotonin uptake inhibitor fluoxetine, and the norepinephrine uptake inhibitor desipramine. No gender difference in HPA axis activation was observed in a dose-response study of cocaine-induced HPA activation in 10 day old rats. Ovariectomy of females decreased the ACTH response to cocaine, but castration did not affect the response of males. To evaluate the role of the pituitary in this gender difference, ACTH levels were determined after corticotropin-releasing factor (CRF) administration. Females showed greater ACTH responses to CRF than males. These studies demonstrate that female rats show exaggerated HPA responses to monoamine uptake inhibitors and that enhanced pituitary responsivity to CRF contributes to this gender difference.

A hypercaloric load induces thermogenesis but inhibits stress responses in the SNS and HPA system.

Caloric overingestion generates a sympathetic nervous system (SNS)-mediated increase in brown adipose tissue (BAT) thermogenesis; its effect on the hypothalamo-pituitary-adrenal (HPA) axis is unknown. To determine whether metabolic activation affects the HPA axis, male rats were provided palatable sucrose ad libitum. After 5 or 10 days of sucrose ingestion, BAT and basal and restraint-induced HPA variables were measured. Some rats were instrumented with temperature probes. BAT temperature and HPA axis responses to restraint were measured. Although caloric intake increased > or = 18%, body weight gain did not change after sucrose ingestion; DNA, protein, and uncoupling protein increased in BAT depots, and white adipose tissues were heavier after both 5 and 10 days. During days 5-10, the BAT-core temperature difference was +0.30 degrees C in sucrose rats and -0.46 degrees C in controls (P < 0.05); this, together with the biochemical changes, shows persistent activation of BAT by excess calories. Basal HPA measures were not altered. The sucrose group exhibited smaller BAT temperature and HPA responses to restraint on day 10; there was no HPA difference on day 5. We conclude that calorically mediated increases in BAT thermogenesis are independent of basal HPA activity; however, both systems respond concordantly to restraint stress. The diminished response to restraint in both systems in sucrose-fed rats may result from signals indicating increased energy stores.

Hypothalamo-pituitary-adrenal axis response to coronary artery embolization: an ovine model of acute myocardial infarction.

Although previous studies have described the hypothalamo-pituitary-adrenal (HPA) response to the stress of acute myocardial infarction, it is not possible to study the hormone changes immediately after infarction in humans. Accordingly, we have examined the HPA response to microembolization of coronary arteries in 13 sheep compared with 5 sham control sheep. Plasma vasopressin (AVP; P < 0.001), ACTH (P = 0.005) and cortisol (P = 0.005) were all increased 2 h (first sample time) after embolization. Plasma ACTH and cortisol levels returned to baseline levels by 6 h but plasma AVP levels did not return to baseline levels until more than 12 h after embolization. Plasma corticotrophin-releasing hormone (CRH) showed no significant change in response to embolization. In a subset of six animals which were sampled more frequently, the peak responses for plasma AVP, ACTH and cortisol occurred at 40 min after embolization. The maximum responses in any individual sheep observed at this time point were 744 pmol/l for AVP, 144 pmol/l for ACTH and 492 nmol/l for cortisol. CRH levels tended to increase across the first hour but these changes were not statistically significant. In conclusion, the stress hormone responses to microembolization of the coronary arteries have been defined in an ovine model of myocardial infarction. This model is suitable for studying the effects of novel treatments to reduce the stress of myocardial infarction.