Long-Term Effects of a Single Exposure to Immobilization on the Hypothalamic-Pituitary-Adrenal Axis: Neurobiologic Mechanisms

Pre-pregnancy stress induces maternal vascular dysfunction during pregnancy and postpartum

An estimated 20% of women suffer from a stress-related mood disorder including depression and anxiety during and after pregnancy, making these disorders among the most common complications of pregnancy. These stress-related disorders are associated with adverse pregnancy outcomes including gestational hypertension and preeclampsia, which are associated with poor cardiometabolic health postpartum. Despite these associations, the direct impact of stress and related disorders on maternal vascular health, and contributing mechanisms, remain understudied. The aim of this study was to investigate the effect of pre-pregnancy stress on maternal vascular outcomes in a BALB/c mouse model of chronic unpredictable stress. Maternal blood pressure and ex-vivo vascular function were investigated during pregnancy and postpartum. Offspring characteristics were assessed at the end of pregnancy and postpartum. Main findings show that pre-pregnancy stress exposure increased blood pressure during mid and late pregnancy and impaired ex vivo vascular function at the end of pregnancy. These effects persisted into the postpartum period, suggesting a long-term effect of stress on maternal vascular health, which appear to be partially attributable to disruptions in nitric oxide (NO) pathway signaling. These data suggest exposure to stress and related disorders, even prior to pregnancy, can contribute to vascular complications during pregnancy and postpartum.

Animal models of PTSD: Comparison of the neuroendocrine and behavioral sequelae of immobilization and a modified single prolonged stress procedure that includes immobilization

A single exposure to some stressors results in long-lasting consequences reminiscent of those found in post-traumatic stress disorder (PTSD), but results are very often controversial. Although there is no consensus regarding the best animal models of PTSD, the single prolonged stress (SPS) model, consisting of sequential exposure within the same day to various stressors (typically restraint, forced swim, and ether), has gained acceptance. However, results, particularly those related to the hypothalamic-pituitary-adrenal (HPA) axis, are inconsistent and there is no evidence that SPS is clearly distinct from models using a single severe stressor. In the present study, we compared in male rats the behavioral and neuroendocrine (HPA) consequences of exposure to immobilization on boards (IMO) with a SPS-like model (SPSi) in which IMO and isoflurane were substituted for restraint and ether, respectively. Both procedures caused a similar impact on food intake and body weight as well as on sensitization of the HPA response to a novel environment (hole-board) on the following day. Reduction of activity/exploration in the hole-board was also similar with both stressors, although the impact of sudden noise was higher in SPSi than IMO. Neither IMO nor SPSi significantly affected contextual fear conditioning acquisition, although a similar trend for impaired fear extinction was observed compared to controls. Exposure to additional stressors in the SPSi did not interfere with homotypic adaptation of the HPA axis to IMO. Thus, only modest neuroendocrine and behavioral differences were observed between IMO and SPSi and more studies comparing putative PTSD models are needed.

Anatomy, development, and plasticity of the neurosecretory hypothalamus in zebrafish

The paraventricular nucleus (PVN) of the hypothalamus harbors diverse neurosecretory cells with critical physiological roles for the homeostasis. Decades of research in rodents have provided a large amount of information on the anatomy, development, and function of this important hypothalamic nucleus. However, since the hypothalamus lies deep within the brain in mammals and is difficult to access, many questions regarding development and plasticity of this nucleus still remain. In particular, how different environmental conditions, including stress exposure, shape the development of this important nucleus has been difficult to address in animals that develop in utero. To address these open questions, the transparent larval zebrafish with its rapid external development and excellent genetic toolbox offers exciting opportunities. In this review, we summarize recent information on the anatomy and development of the neurosecretory preoptic area (NPO), which represents a similar structure to the mammalian PVN in zebrafish. We will then review recent studies on the development of different cell types in the neurosecretory hypothalamus both in mouse and in fish. Lastly, we discuss stress-induced plasticity of the PVN mainly discussing the data obtained in rodents, but pointing out tools and approaches available in zebrafish for future studies. This review serves as a primer for the currently available information relevant for studying the development and plasticity of this important brain region using zebrafish.

Noninvasive assessment of altered activity following restraint in mice using an automated physiological monitoring system

In the laboratory setting, typical endocrine and targeted behavioral tests are limited in their ability to provide a direct assessment of stress in animals housed in undisturbed conditions. We hypothesized that an automated phenotyping system would allow the detection of subtle stress-related behavioral changes well beyond the time-frames examined using conventional methods. In this study, we have utilized the TSE PhenoMaster system to continuously record basal behaviors and physiological parameters including activity, body weight, food intake and oxygen consumption in undisturbed and stressed C57Bl/6J male mice (n = 12/group), with a pharmacological intervention using the conventional anxiolytic, diazepam (5 mg kg^-1 i.p.; n = 8/group). We observed significant 20-30% reductions in locomotor activity in the dark phase, with subtle reductions in light phase activity for up to 96 h following a single 2 h episode of restraint stress. A single administration of diazepam reduced plasma corticosterone concentrations by 30-35% during stress exposure when compared to mice treated with vehicle. This treatment did not result in significantly different locomotor activity compared to vehicle within the first 48 h following restraint stress. However, diazepam treatment facilitated restoration of locomotor activity at 72 and 96 h after restraint stress exposure in comparison to vehicle-treated mice. Hence, the use of an automated phenotyping system allows a real time assessment of basal behaviors and empirical metabolism following exposure to restraint stress and demonstrates major and subtle changes in activity persist for several days after stress exposure.

Stress-related synaptic plasticity in the hypothalamus

Stress necessitates an immediate engagement of multiple neural and endocrine systems. However, exposure to a single stressor causes adaptive changes that modify responses to subsequent stressors. Recent studies examining synapses onto neuroendocrine cells in the paraventricular nucleus of the hypothalamus demonstrate that stressful experiences leave indelible marks that alter the ability of these synapses to undergo plasticity. These adaptations include a unique form of metaplasticity at glutamatergic synapses, bidirectional changes in endocannabinoid signalling and bidirectional changes in strength at GABAergic synapses that rely on distinct temporal windows following stress. This rich repertoire of plasticity is likely to represent an important building block for dynamic, experience-dependent modulation of neuroendocrine stress adaptation.

Factors associated with cortisol levels and health in 5-6-year-old children

OBJECTIVES

This study assesses the relationships between social context (family and inter-peer context), stress, and illness in 5-6-year-old children.

METHODS

To this end, data were collected on spontaneous social interpeer behavior; families provided data on stress, anxiety, and parental acceptance-rejection; and the children's pediatricians provided data relative to their health. Data on stress-related hormones (cortisol) were collected using saliva samples.

RESULTS

The results revealed that none of the variables examined were significantly related to illness development in the subjects in the sample group. Cortisol levels, however, were positively associated with a record of chronic or congenital illnesses, the manifestation of behaviors related to the search for leadership status in the group and the presence of stressful events in the family environment.

CONCLUSIONS

Despite finding no relationship between children's level of adrenocortical activity and the contracting or contingent development of diseases, we did find that chronic/congenital diseases may constitute a source of early stress in childhood. Certain conditions of uncertainty in the social context (family and interpeer) also seem to constitute different sources of stress.

A minimal stress model for the assessment of electroacupuncture analgesia in rats under halothane

The use of anesthetics in acupuncture analgesia is controversial. We evaluate a steady-state light anesthesia model to test whether minimal stress manipulation and reliable measurement of analgesia could be simultaneously achieved during electroacupuncture (EA) in animals. A series of experiments were performed. Firstly, EA compliance and tail-flick latencies (TFL) were compared in rats under 0.1%, 0.3%, 0.5%, 0.7%, or 1.1% halothane for 120min. Under 0.5% halothane, TFL were then measured in groups receiving EA at intensity of 3, 10 or 20 volt (V), 1 or 2mg/kg morphine, 20V EA plus naloxone, or control. Subsequently, the effect of EA on formalin-induced hyperalgesia was tested and c-fos expression in the spinal dorsal horn was analyzed. Rats exhibited profound irritable behaviors and highly variable TFL under 0.1% or 0.3% halothane, as well as a time-dependent increase of TFL under 0.7% or 1.1% halothane. TFL remained constant at 0.5% halothane, and needle insertion and electrical stimulation were well tolerated. Under 0.5% halothane, EA increased TFL and suppressed formalin-induced hyperalgesia in an intensity-dependent and naloxone-reversible manner. EA of 20V prolonged TFL by 74%, suppressed formalin-induced hyperalgesia by 32.6% and decreased c-fos expression by 29.7% at the superficial and deep dorsal horn with statistically significant difference. In conclusion, 0.5% halothane provides a steady-state anesthetic level which enables the humane application of EA stimulus with the least interference on analgesic assessment. This condition serves as a minimal stress EA model in animals devoid of stress-induced analgesia while maintaining physiological and biochemical response in the experiment.

Social defeat promotes specific cytokine variations within the prefrontal cortex upon subsequent aggressive or endotoxin challenges

Stressful experiences typically have short-lived neuroendocrine and neurochemical effects, but the processes leading to these biological alterations may be sensitized so that later challenges promote exaggerated responses. As stressors and immunogenic insults have both been associated with inflammatory immune variations within the brain, we assessed whether a social defeat stressor would result in augmented corticosterone release and mRNA expression of pro-inflammatory cytokines within the prefrontal cortex (PFC) upon later social defeat (sensitization) or endotoxin (lipopolysaccharide: LPS) challenges (cross-sensitization). In the absence of a prior stressor experience, the social defeat challenge did not affect prefrontal interleukin (IL)-1β or tumor necrosis factor (TNF)-α mRNA expression, but increased that of IL-6, whereas LPS increased the expression of each cytokine. Among mice that had initially been repeatedly defeated, IL-1β and TNF-α expression was enhanced after the social defeat challenge, whereas this was not evident in response to the LPS challenge. In contrast, the initial social defeat stressor had protracted effects in that increase of IL-6 expression was limited upon subsequent challenge with either social defeat or LPS. Previous social stressor experiences also limited the corticosterone rise ordinarily elicited by either social defeat or LPS treatment. It seems that a powerful stressor, such as social defeat, may have persistent effects on later corticosterone and cytokine responses to different types of stressful insults (social versus systemic challenges), but the nature of the effects varies with the specific process assessed.

Hypothalamic-pituitary-adrenal axis activity and upper respiratory tract infection in young children transitioning to primary school

RATIONALE

We have previously reported an increase in salivary cortisol in a cohort of 4-year-old children transitioning to primary school. We hypothesised that increased cortisol in response to this acute naturalistic stress in early development may be immunostimulatory and associated with positive health outcomes.

OBJECTIVES

We tested this hypothesis by measuring upper respiratory tract infection (URI) across the first 6 months of school, in relation to salivary cortisol at the end of the second week following school transition

METHODS

Seventy children supplied morning and evening saliva samples for cortisol assay. Children were psychologically assessed for temperament and behavioural adaptation. Symptoms of URI were recorded in diary form, and variables relating to URI occurrence, duration and severity were assessed.

RESULTS

Children with higher evening cortisol at school transition experienced significantly fewer episodes of URI over the following 6 months. Diurnal cortisol change was negatively correlated with number of illnesses across the 6 months, indicating an association between a greater decline in cortisol across the day and a greater number of colds. URI severity was associated with the greatest resistance to URI infection in children who were less socially isolated and who had a smaller diurnal change in cortisol across the day.

CONCLUSIONS

Our results showing that higher cortisol is associated with lower URI may be explained by proposing that increased cortisol in response to the naturalistic stress of school transition may prime the immune system to develop resistance to URI at this critical stage of a child's development.

Early life stress enhances behavioral vulnerability to stress through the activation of REST4-mediated gene transcription in the medial prefrontal cortex of rodents

There is growing evidence suggesting that early life events have long-term effects on the neuroendocrine and behavioral developments of rodents. However, little is known about the involvement of early life events in the susceptibility to subsequent stress exposure during adulthood. The present study characterized the effect of maternal separation, an animal model of early life adversity, on the behavioral response to repeated restraint stress in adult rats and investigated the molecular mechanism underlying behavioral vulnerability to chronic stress induced by the maternal separation. Rat pups were separated from the dams for 180 min per day from postnatal day 2 through 14 (HMS180 rats). We found that, as young adults, HMS180 rats showed a greater hypothalamic-pituitary-adrenal axis response to acute restraint stress than nonseparated control rats. In addition, repeatedly restrained HMS180 rats showed increased depression-like behavior and an anhedonic response compared with nonrestrained HMS180 rats. Furthermore, HMS180 rats showed increased expression of REST4, a neuron-specific splicing variant of the transcriptional repressor REST (repressor element-1 silencing transcription factor), and a variety of REST target gene mRNAs and microRNAs in the medial prefrontal cortex (mPFC). Finally, REST4 overexpression in the mPFC of neonatal mice via polyethyleneimine-mediated gene transfer enhanced the expression of its target genes as well as behavioral vulnerability to repeated restraint stress. In contrast, REST4 overexpression in the mPFC of adult mice did not affect depression-like behaviors after repeated stress exposure. These results suggest that the activation of REST4-mediated gene regulation in the mPFC during postnatal development is involved in stress vulnerability.

Influence of age on behavioral, immune and endocrine responses to the T-cell superantigen staphylococcal enterotoxin A

Aged subjects are more vulnerable to administration of the endotoxin lipopolysaccharide, but research on age-associated sensitivity to other immune stimulants has been limited. The current study examined the effects of administering the superantigen, staphylococcal enterotoxin A (SEA), to young (4-month-old) and aged (20-month-old) male C57BL/6J mice on consumption of a novel liquid, cytokine production, corticosterone levels, and expression of central mRNA levels of cytokines and corticotropin-releasing hormone. SEA produced exaggerated hypophagia in aged mice, as they showed decreased consumption that persisted for 24 h. SEA increased hypothalamic mRNA levels of interleukin-1beta in the aged, but not the young, mice 2 h after administration. No differences in cytokine expression were observed 24 h after SEA. Both age groups showed increased plasma corticosterone levels 2 h after SEA administration. However, 24 h after SEA exposure the aged, but not the young, mice showed an augmented corticosterone response to the consumption test. Collectively, these data show that aging may exacerbate the behavioral and neuroinflammatory response to superantigen exposure. Further, the present study suggests that immune activation may result in delayed alterations in stress-induced corticosterone production in aged subjects.

Repeated ferret odor exposure induces different temporal patterns of same-stressor habituation and novel-stressor sensitization in both hypothalamic-pituitary-adrenal axis activity and forebrain c-fos expression in the rat

Repeated exposure to a moderately intense stressor typically produces attenuation of the hypothalamic-pituitary-adrenal (HPA) axis response (habituation) on re-presentation of the same stressor; however, if a novel stressor is presented to the same animals, the HPA axis response may be augmented (sensitization). The extent to which this adaptation is also evident within neural activity patterns is unknown. This study tested whether repeated ferret odor (FO) exposure, a moderately intense psychological stressor for rats, leads to both same-stressor habituation and novel-stressor sensitization of the HPA axis response and neuronal activity as determined by immediate early gene induction (c-fos mRNA). Rats were presented with FO in their home cages for 30 min a day for up to 2 wk and subsequently challenged with FO or restraint. Rats displayed HPA axis activity habituation and widespread habituation of c-fos mRNA expression (in situ hybridization) throughout the brain in as few as three repeated presentations of FO. However, repeated FO exposure led to a more gradual development of sensitized HPA-axis and c-fos mRNA responses to restraint that were not fully evident until after 14 d of prior FO exposure. The sensitized response was evident in many of the same brain regions that displayed habituation, including primary sensory cortices and the prefrontal cortex. The shared spatial expression but distinct temporal development of habituation and sensitization neural response patterns suggests two independent processes with opposing influences across overlapping brain systems.

Corticosterone and corticosteroid binding globulin in birds: relation to urbanization in a desert city

As cities expand worldwide, understanding how species adapt to novel urban habitats will become increasingly important to conservation. The adrenocortical stress response enables vertebrates to cope with novel environmental challenges to homeostasis. We examined total and estimates of free baseline and stress-induced corticosterone (CORT) concentrations and CORT binding globulin (CBG) levels in five passerine species within and around Phoenix, Arizona. We tested whether baseline and stress-induced CORT patterns differed among species living at varying densities in Phoenix and tested the hypothesis that, for species capable of successfully colonizing cities, individuals living in urban areas have a decreased acute stress response compared to individuals living in native desert. Baseline total CORT levels were generally similar in urban and rural birds. Capture and handling stress typically produced greater total CORT responses in urban birds than in rural birds, although these responses differed as a function of the life history stage (non-breeding, breeding or molt). CBG binding capacity did not change with life history stage or locality. Estimated free CORT concentrations differed less between groups than total CORT concentrations. Urban birds showed less variability in stress responses across life history stages than rural birds. We propose that more predictable resources in the city than in rural areas may decrease the need to vary stress responsiveness across life history stages. The results highlight the species-specific effects of urbanization on stress physiology and the difficulty to predict how urbanization impacts organisms.

Effects of perinatal oxycodone exposure on the response to CRH in late adolescent rats

We hypothesized that prenatal oxycodone exposure suppresses the Hypothalamic-Pituitary-Adrenal (HPA) response to stress in late adolescence. Dark Agouti rats were given either intravenous oxycodone or vehicle (controls, CON) daily from gestation day 8 until postnatal day (PD) 5. At PD 45, the male and female offspring received intravenously either ovine corticotropin releasing hormone (CRH) or saline. Plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels were determined before, and 15, 30, and 60 min after injection. Prenatal oxycodone had no effect on baseline ACTH values; CRH elicited a greater ACTH response than saline. In males, prenatal oxycodone delayed and enhanced the peak ACTH response to CRH, but had no effect in females. The CORT response to CRH was not different between oxycodone and CON; however mean CORT levels in females were significantly higher than those in males at baseline and after stimulation. These results demonstrate that prenatal oxycodone increases pituitary response to CRH in late adolescent male rats, but not in females. The absence of an enhanced adrenal response in oxycodone-exposed males suggests either desensitization or maximal adrenal response to a high CRH dose. The mechanisms of postnatal sex-specific HPA dysregulation following prenatal oxycodone remain to be elucidated.

Attenuation of amydgala and frontal cortical responses to low blood glucose concentration in asymptomatic hypoglycemia in type 1 diabetes: a new player in hypoglycemia unawareness?

OBJECTIVE

Loss of ability to recognize hypoglycemia (hypoglycemia unawareness) increases risk of severe hypoglycemia threefold in insulin-treated diabetes. We set out to investigate the cerebral correlates of unawareness in type 1 patients.

RESEARCH DESIGN AND METHODS

Regional changes in brain glucose kinetics were measured using [(18)F]-fluorodeoxyglucose (FDG) positron emission tomography (PET), in 13 men with type 1 diabetes--6 with hypoglycemia awareness and 7 with hypoglycemia unawareness--at euglycemia (5 mmol/l) and hypoglycemia (2.6 mmol/l), in random order.

RESULTS

Epinephrine responses to hypoglycemia were reduced in hypoglycemia unawareness (P < 0.0003), as were symptoms. Statistical parametric mapping (SPM) of FDG uptake using SPM2 at a statistical threshold of P < 0.005 showed increased FDG uptake in left amygdala in hypoglycemia awareness, but not in hypoglycemia unawareness (region of interest analysis -0.40 +/- 1.03 vs. 3.66 +/- 0.42, respectively; P = 0.007), and robust increase in bilateral ventral striatum during hypoglycemia (region of interest analysis hypoglycemia unawareness 3.52 +/- 1.02 vs. awareness 6.1 +/- 0.53; P = 0.054). Further analysis at the statistical threshold of P < 0.01 showed bilateral attenuated activation of brain stem regions and less deactivation in lateral orbitofrontal cortex in hypoglycemia unawareness.

CONCLUSIONS

Ventral striatal, amygdala, brain stem, and orbitofrontal responses to hypoglycemia indicate engagement of appetitive motivational networks, associated with integrated behavioral responses to hypoglycemia. Reduced responses in these networks in hypoglycemia unawareness, particularly failure of amygdala and orbifrontal cortex responses, suggest habituation of higher behavioral responses to hypoglycemia as a basis for unawareness. New approaches may be needed to restore awareness effectively in practice.

Long-term effects of a single exposure to immobilization: a c-fos mRNA study of the response to the homotypic stressor in the rat brain

A single exposure to a severe emotional stressor such as immobilization in wooden boards (IMO) causes long-term (days to weeks) peripheral and central desensitization of the hypothalamic-pituitary-adrenal (HPA) response to the same (homotypic) stressor. However, the brain areas putatively involved in long-term desensitization are unknown. In the present experiment, adult male rats were subjected to 2 h of IMO and, 1 or 4 weeks later, exposed again to 1 h IMO together with stress-naive rats. C-fos mRNA activation just after IMO and 1 h after the termination of IMO (post-IMO) were evaluated by in situ hybridization. Whereas in most brain areas c-fos mRNA induction caused by the last IMO session was similar in stress-naive (controls) and previously immobilized rats, a few brain areas showed a reduced c-fos mRNA response: ventral lateral septum (LSv), medial amygdala (MeA), parvocellular region of the paraventricular hypothalamic nucleus (pPVN), and locus coeruleus (LC). In contrast, an enhanced expression was observed in the medial division of the bed nucleus stria terminalis (BSTMv). The present work demonstrates that a previous experience with a stressor can induce changes in c-fos mRNA expression in different brain areas in response to the homotypic stressor and suggests that LSv, MeA, and BSTMv may be important for providing signals to lower diencephalic (pPVN) and brainstem (LC) nuclei, which results in a lower physiological response to the homotypic stressor.

Impaired adaptation to repeated restraint and decreased response to cold in urocortin 1 knockout mice

Urocortin 1 (UCN1) is a corticotropin-releasing factor (CRF)-like peptide whose role in stress is not well characterized. To study the physiological role of UCN1 in the response of the hypothalamic-pituitary-adrenal (HPA) axis to stress, we generated UCN1-knockout (KO) mice and examined their adaptation to repeated restraint and to cold environment. Wild-type (WT) and UCN1-KO animals were restrained hourly for 15 min from 9 AM to 2 PM, and blood samples were obtained for corticosterone measurement. WT animals adapted to repeated restraint with a decreased corticosterone response; the restraint-stimulated corticosterone levels fell from 215 +/- 31 ng/ml in naïve animals to 142 +/- 50 ng/ml in mice subjected to repeated restraint (P < 0.01) and from 552 +/- 98 to 314 +/- 58 ng/ml (P < 0.001) in males and females, respectively. Male UCN1-KO mice did not show any adaptation to repeated restraint; instead, restraint-stimulated corticosterone levels were increased from 274 +/- 80 ng/ml in naïve animals to 480 +/- 75 ng/ml in mice subjected to repeated restraint (P < 0.001). Female UCN1-KO mice showed only a partial adaptation to repeated restraint, with a decrease in the restraint-stimulated corticosterone response from 631 +/- 102 ng/ml in naïve animals to 467 +/- 78 ng/ml in mice subjected to repeated restraint (P < 0.01). In addition, UCN1-KO mice showed no corticosterone response to 2-h cold environment. These data demonstrate an important role for UCN1 in the HPA axis adaptation to repeated restraint and in the corticosterone response to a cold environment.

Influence of prior experience with homotypic or heterotypic stressor on stress reactivity in catecholaminergic systems

Here we review how prior experience with stress alters the response to a subsequent homotypic or heterotypic stressor, focusing on the catecholaminergic systems in the adrenal medulla and the locus coeruleus (LC). The changes in response to homotypic stress differ depending on the stressor applied. With immobilization stress (IMO), transcriptional responses in the adrenal medulla to a single exposure are pronounced and several of the transcription factors and signaling kinases induced or activated are reviewed and compared to the longer term alterations with repeated stress, consistent with persistent activation of gene expression of catecholamine (CA) biosynthetic enzymes. In the LC, transcriptional and post-transcriptional activation of gene expression are shown to be important. Repeated IMO stress triggers further activation of a number of signalling pathways. Neither adrenal medulla nor LC display habituation to long term repeated stress. In contrast, gene expression for CA biosynthetic enzymes habituates to prolonged cold stress in the adrenal medulla and LC, but displays an exaggerated response with exposure to a novel or heterotypic stressor such as IMO. Some of the transcriptional pathways displaying sensitization are described.

Protective effects of endotoxin in a rat model of chronic inflammation are accompanied by suppressed secretion of pro-inflammatory cytokines and biphasic alteration in hypothalamo-pituitary-adrenal axis activity

We have previously demonstrated that Gram-negative bacterial endotoxin can exert long-term protective effects against the chronic inflammatory disease adjuvant arthritis in rats. The present study was designed to investigate the mechanisms and time-course of hypothalamo-pituitary-adrenocortical (HPA) axis activity and cytokine secretion underlying this phenomenon. Rats were injected with endotoxin (lipopolysaccharide) and blood was collected either 7 or 21 days later. Priming with endotoxin induced a biphasic alteration in secretion of adrenocorticotrophic hormone and corticosterone in response to a second injection of endotoxin, with decreased secretion observed after 7 days whereas robust secretion was observed at 21 days. Seven days following priming with endotoxin, plasma concentrations of pro-inflammatory cytokines interleukin (IL)-6 and interferon (IFN)-gamma were reduced by 90%, and tumour necrosis factor (TNF)-alpha by 70%, compared to saline-treated rats, whereas robust secretion of the anti-inflammatory cytokine IL-10 was maintained in both groups. A similar net change favouring an anti-inflammatory cytokine secretory milieu was also observed 21 days following priming with endotoxin. This study provides evidence that the long-term protective effects of endotoxin on inflammation are associated with a sustained reduction in secretion of pro-inflammatory cytokines. HPA axis hypoactivity at 7 days suggests that corticosterone is not involved in suppressing IL-6, IFN-gamma and TNF-alpha at this time point. Conversely, hypersecretion of corticosterone at 21 days may underlie synchronous suppression of IL-6 and IFN-gamma. These data provide novel insight into interactions between HPA axis activity and cytokine secretion following endotoxin priming prior to induction of inflammatory disease.

Individual variation in cortisol responses to acute "on-back" restraint in an outbred hamster

An outbred species of dwarf hamster (Phodopus campbelli) was used to assess between-individual variability in the response to, and recovery from, a one-time stressor of 6 min of physical restraint in a subordinate, on-back, position. Four repeated plasma samples were drawn under home-cage isoflurane anesthesia from 33 males and 38 females 50 min before, and then 10, 60, and 120 min after the stress onset. Plasma cortisol concentrations were higher in females than males, but there was no evidence for a sex difference in response to the stressor. The expected cross-sectional increase ( approximately 50 ng/ml) in response to the stressor, followed by recovery, was seen. However, there was extensive individual variation, ranging from no reaction to continuous decline from the initial to the final sample. Results were expressed in four ways (absolute concentration, relative concentration, and area under the curve relative to ground and relative to the stress-induced increase) and also standardized and subjected to hierarchical cluster analysis. Clusters failed to effectively partition the between-individual variation and did not cluster by sex, age, or housing conditions. The current study cautions against ignoring individual differences and suggests that outbred animal models might be particularly relevant to understanding stress-related pathological conditions.

Immobilization stress causes increases in tetrahydrobiopterin, dopamine, and neuromelanin and oxidative damage in the nigrostriatal system

Oxidative stress is believed to contribute to the pathophysiology of Parkinson's disease, in which nigrostriatal dopaminergic (DA) neurons undergo degeneration. Identification of endogenous molecules that contribute to generation of oxidative stress and vulnerability of these cells is critical in understanding the etiology of this disease. Exposure to tetrahydrobiopterin (BH4), the obligatory cofactor for DA synthesis, was observed previously to cause oxidative damage in DA cells. To demonstrate the physiological relevance of this observation, we investigated whether an overproduction of BH4 and DA might actually occur in vivo, and, if it did, whether this might lead to oxidative damage to the nigrostriatal system. Immobilization stress (IMO) elevated BH4 and DA and their synthesizing enzymes, tyrosine hydroxylase and GTP cyclohydrolase I. This was accompanied by elevation of lipid peroxidation and protein-bound quinone, and activities of antioxidant enzymes. These increases in the indices of oxidative stress appeared to be due to increased BH4 synthesis because they were abolished following administration of the BH4 synthesis inhibitor, 2,4-diamino-6-hydroxy-pyrimidine. IMO also caused accumulation of neuromelanin and degeneration of the nigrostriatal system. These results demonstrate that a severe stress can increase BH4 and DA and cause oxidative damages to the DA neurons in vivo, suggesting relevance to Parkinson's disease.

Role of mitogen-activated protein kinases during recovery from head-shake response habituation in rats

Habituation is defined as a decrease in responsiveness to a repeatedly presented stimulus. The head-shake response (HSR) consists of a rapid twisting of the head about the front-to-rear axis elicited by a stream of air to the ear. This response demonstrates several fundamental properties of habituation including sensitivity to the frequency and intensity of stimulation, and spontaneous recovery. Despite an abundance of behavioral data on the HSR, relatively little is known about its physiological mechanism(s). To address this issue, changes in mitogen-activated protein kinases (MAPK) were assessed 5 min and 2, 6, and 24 h following the habituation of the HSR. Three cascades of MAPK activity were measured in the cerebellum and hippocampal, prefrontal, and piriform cortices, including extracellular-response kinase (ERK), p-38 kinase (p-38), and stress-activated protein kinase (SAPK). Significant activation of p-38 and SAPK was observed in all four brain structures, accompanied by modest changes in ERK activity. Recovery of the HSR was characterized by decreasing MAPK activation with control levels re-established 24 h after habituation. The present results suggest that MAPK activation mediates recovery from habituation; however, these findings may also support alternative interpretations such that MAPK activation reflects the encoding of spatial cues associated with the testing environment and/or are due to stress induced by the habituation protocol.