Stress-induced sensitization of the hypothalamic-pituitary adrenal axis is associated with alterations of hypothalamic and pituitary gene expression

Two models of inescapable stress increase tph2 mRNA expression in the anxiety-related dorsomedial part of the dorsal raphe nucleus

Expression of TPH2, the rate-limiting enzyme for brain serotonin synthesis, is elevated in the dorsal raphe nucleus (DR) of depressed suicide victims. One hypothesis is that this increase in TPH2 expression is stress-induced. Here, we used an established animal model to address whether exposure to an acute stressor, inescapable tail shock (IS), increases tph2 mRNA and Tph2 protein expression, and if IS sensitizes the DR to a subsequent, heterotypic stressor. In Experiment 1, we measured tph2 mRNA expression 4 h after IS or home cage (HC) control conditions in male rats, using in situ hybridization histochemistry. In Experiment 2, we measured Tph2 protein expression 12 h or 24 h after IS using western blot. In Experiment 3, we measured tph2 mRNA expression following IS on Day 1, and cold swim stress (10 min, 15 °C) on Day 2. Inescapable tail shock was sufficient to increase tph2 mRNA expression 4 h and 28 h later, selectively in the dorsomedial DR (caudal aspect of the dorsal DR, cDRD; an area just rostral to the caudal DR, DRC) and increased Tph2 protein expression in the DRD (rostral and caudal aspects of the dorsal DR combined) 24 h later. Cold swim increased tph2 mRNA expression in the dorsomedial DR (cDRD) 4 h later. These effects were associated with increased immobility during cold swim, elevated plasma corticosterone, and a proinflammatory plasma cytokine milieu (increased interleukin (IL)-6, decreased IL-10). Our data demonstrate that two models of inescapable stress, IS and cold swim, increase tph2 mRNA expression selectively in the anxiety-related dorsomedial DR (cDRD).

Evidence for Ancestral Programming of Resilience in a Two-Hit Stress Model

In a continuously stressful environment, the effects of recurrent prenatal stress (PS) may accumulate across generations and alter stress vulnerability and resilience. Here, we report in female rats that a family history of recurrent ancestral PS facilitates certain aspects of movement performance, and that these benefits are abolished by the experience of a second hit, induced by a silent ischemia during adulthood. Female F4-generation rats with and without a family history of cumulative multigenerational PS (MPS) were tested for skilled motor function before and after the induction of a minor ischemic insult by endothelin-1 infusion into the primary motor cortex. MPS resulted in improved skilled motor abilities and blunted hypothalamic-pituitary-adrenal (HPA) axis function compared to non-stressed rats. Deep sequencing revealed downregulation of miR-708 in MPS rats along with upregulation of its predicted target genes Mapk10 and Rasd2. Through miR-708 stress may regulate mitogen-activated protein kinase (MAPK) pathway activity. Hair trace elemental analysis revealed an increased Na/K ratio, which suggests a chronic shift in adrenal gland function. The ischemic lesion activated the HPA axis in MPS rats only; the lesion, however, abolished the advantage of MPS in skilled reaching. The findings indicate that MPS generates adaptive flexibility in movement, which is challenged by a second stressor, such as a neuropathological condition. Thus, a second “hit” by a stressor may limit behavioral flexibility and neural plasticity associated with ancestral stress.

Learning about stress: neural, endocrine and behavioral adaptations

In this review, nonassociative learning is advanced as an organizing principle to draw together findings from both sympathetic-adrenal medullary and hypothalamic-pituitary-adrenocortical (HPA) axis responses to chronic intermittent exposure to a variety of stressors. Studies of habituation, facilitation and sensitization of stress effector systems are reviewed and linked to an animal's prior experience with a given stressor, the intensity of the stressor and the appraisal by the animal of its ability to mobilize physiological systems to adapt to the stressor. Brain pathways that regulate physiological and behavioral responses to stress are discussed, especially in light of their regulation of nonassociative processes in chronic intermittent stress. These findings may have special relevance to various psychiatric diseases, including depression and post-traumatic stress disorder (PTSD).

Critical features of acute stress-induced cross-sensitization identified through the hypothalamic-pituitary-adrenal axis output

Stress-induced sensitization represents a process whereby prior exposure to severe stressors leaves animals or humans in a hyper-responsive state to further stressors. Indeed, this phenomenon is assumed to be the basis of certain stress-associated pathologies, including post-traumatic stress disorder and psychosis. One biological system particularly prone to sensitization is the hypothalamic-pituitary-adrenal (HPA) axis, the prototypic stress system. It is well established that under certain conditions, prior exposure of animals to acute and chronic (triggering) stressors enhances HPA responses to novel (heterotypic) stressors on subsequent days (e.g. raised plasma ACTH and corticosterone levels). However, such changes remain somewhat controversial and thus, the present study aimed to identify the critical characteristics of the triggering and challenging stressors that affect acute stress-induced HPA cross-sensitization in adult rats. We found that HPA cross-sensitization is markedly influenced by the intensity of the triggering stressor, whereas the length of exposure mainly affects its persistence. Importantly, HPA sensitization is more evident with mild than strong challenging stressors, and it may remain unnoticed if exposure to the challenging stressor is prolonged beyond 15 min. We speculate that heterotypic HPA sensitization might have developed to optimize biologically adaptive responses to further brief stressors.

Schisandra chinensis and Rhodiola rosea exert an anti-stress effect on the HPA axis and reduce hypothalamic c-Fos expression in rats subjected to repeated stress

The aim of the present study was to investigate the effects of Schisandra chinensis (S. chinensis) and Rhodiola rosea (R. rosea) on rats subjected to 5 h of stress, induced by water-floating followed by treadmill exercise. Hypothalamus-pituitary-adrenal (HPA) activity and c-Fos and Fos-related antigen 2 (Fra-2) mRNA expression levels in the hypothalamus of the rats were evaluated. Rats were distributed into four groups: S. chinensis (n=12), R. rosea (n=10), stress control (n=10) and quiet control (n=8). Following a training period of 6 consecutive days, the S. chinensis, R. rosea and stress control groups underwent a 3-h water-floating session in the presence of feline predators immediately followed by 2 h treadmill running to induce psychological and physical stress. Following compound stress induction, the serum levels of corticosterone (CORT), adrenocorticotropic hormone and interleukin-1β and the mRNA expression levels of hypothalamic corticotropin-releasing hormone (CRH), neuropeptide-Y, c-Fos and Fra-2 were evaluated using enzyme-linked immunosorbent assay, radioimmunoassay and quantitative polymerase chain reaction, respectively. The results indicated that S. chinensis and R. rosea markedly decreased the stress-induced elevation of CRH and peripheral CORT levels. The mRNA expression levels of c-Fos and Fra-2 in the hypothalamus were significantly increased after 5 h compound stress, and reduced levels of c-Fos expression were detected in rats treated with R. rosea. Thus, S. chinensis and R. rosea exert an anti-stress effect in rats subjected to stress by balancing the HPA axis, and possibly by reducing the expression of c-Fos in the hypothalamus.

Stress-induced sensitization: the hypothalamic-pituitary-adrenal axis and beyond

Exposure to certain acute and chronic stressors results in an immediate behavioral and physiological response to the situation followed by a period of days when cross-sensitization to further novel stressors is observed. Cross-sensitization affects to different behavioral and physiological systems, more particularly to the hypothalamus-pituitary-adrenal (HPA) axis. It appears that the nature of the initial (triggering) stressor plays a major role, HPA cross-sensitization being more widely observed with systemic or high-intensity emotional stressors. Less important appears to be the nature of the novel (challenging) stressor, although HPA cross-sensitization is better observed with short duration (5-15 min) challenging stressors. In some studies with acute immune stressors, HPA sensitization appears to develop over time (incubation), but most results indicate a strong initial sensitization that progressively declines over the days. Sensitization can affect other physiological system (i.e. plasma catecholamines, brain monoamines), but it is not a general phenomenon. When studied concurrently, behavioral sensitization appears to persist longer than that of the HPA axis, a finding of interest regarding long-term consequences of traumatic stress. In many cases, behavioral and physiological consequences of prior stress can only be observed following imposition of a new stressor, suggesting long-term latent effects of the initial exposure.

Sex differences in the long-lasting effects of a single exposure to immobilization stress in rats

In male rats, a single exposure to a severe stressor such as immobilization (IMO) results in marked activation of the HPA axis and reduction of body weight gain. In addition, the HPA response to the same (homotypic) stressor is reduced, whereas the response to a different (heterotypic) stressor is enhanced for days. Although sex differences in the responsiveness of the HPA axis have been described, there are few studies about the influence of sex on long-lasting effects of stress. Thus, we have compared the consequences of a single exposure to IMO in male and female rats. Females showed a similar ACTH response to the first IMO associated with higher corticosterone, but they were more resistant than males to stress-induced loss of body weight. Unstressed females showed higher resting levels of ACTH and corticosterone, but they did not show the increase in the resting levels of HPA hormones observed in males on the day after IMO. During exposure to a different stressor (open-field) two days after IMO, enhanced corticosterone response and hypoactivity was observed in males, but not in females. Finally, a second exposure to IMO 8 days after the first one resulted in a reduction of the HPA response and of the negative impact on body weight as compared to the first exposure, and this protective effect was greater in females. In sum, IMO-exposed females showed a greater reduction of the response to a second IMO and appear to be more resistant than males to some of the negative impacts of IMO.

Preferential loss of dorsal-hippocampus synapses underlies memory impairments provoked by short, multimodal stress

The cognitive effects of stress are profound, yet it is unknown if the consequences of concurrent multiple stresses on learning and memory differ from those of a single stress of equal intensity and duration. We compared the effects on hippocampus-dependent memory of concurrent, hours-long light, loud noise, jostling and restraint (multimodal stress) with those of restraint or of loud noise alone. We then examined if differences in memory impairment following these two stress types might derive from their differential impact on hippocampal synapses, distinguishing dorsal and ventral hippocampus. Mice exposed to hours-long restraint or loud noise were modestly or minimally impaired in novel object recognition, whereas similar-duration multimodal stress provoked severe deficits. Differences in memory were not explained by differences in plasma corticosterone levels or numbers of Fos-labeled neurons in stress-sensitive hypothalamic neurons. However, although synapses in hippocampal CA3 were impacted by both restraint and multimodal stress, multimodal stress alone reduced synapse numbers severely in dorsal CA1, a region crucial for hippocampus-dependent memory. Ventral CA1 synapses were not significantly affected by either stress modality. Probing the basis of the preferential loss of dorsal synapses after multimodal stress, we found differential patterns of neuronal activation by the two stress types. Cross-correlation matrices, reflecting functional connectivity among activated regions, demonstrated that multimodal stress reduced hippocampal correlations with septum and thalamus and increased correlations with amygdala and BST. Thus, despite similar effects on plasma corticosterone and on hypothalamic stress-sensitive cells, multimodal and restraint stress differ in their activation of brain networks and in their impact on hippocampal synapses. Both of these processes might contribute to amplified memory impairments following short, multimodal stress.

Repeated fear-induced diurnal rhythm disruptions predict PTSD-like sensitized physiological acute stress responses in F344 rats

AIM

To identify objective factors that can predict future sensitized stress responses, thus allowing for effective intervention prior to developing sensitization and subsequent stress-related disorders, including post-traumatic stress disorder (PTSD).

METHODS

Adult male F344 rats implanted with biotelemetry devices were exposed to repeated conditioned fear or control conditions for 22 days followed by exposure to either no, mild or severe acute stress on day 23. Diurnal rhythms of locomotor activity (LA), heart rate (HR) and core body temperature (CBT) were biotelemetrically monitored throughout the study. In a subset of rat not implanted, corticosterone and indices of chronic stress were measured immediately following stress.

RESULTS

Rats exposed to repeated fear had fear-evoked increases in behavioural freezing and HR/CBT during exposure to the fear environment and displayed indices of chronic stress. Repeated fear produced flattening of diurnal rhythms in LA, HR and CBT. Repeated fear did not sensitize the corticosterone response to acute stress, but produced sensitized HR/CBT responses following acute stress, relative to the effect of acute stress in the absence of a history of repeated fear. Greater diurnal rhythm disruptions during repeated fear predicted sensitized acute stress-induced physiological responses. Rats exposed to repeated fear also displayed flattened diurnal LA and basal increases in HR.

CONCLUSIONS

Exposure to repeated fear produces outcomes consistent with those observed in PTSD. The results suggest that diurnal rhythm disruptions during chronic stressors may help predict sensitized physiological stress responses following traumatic events. Monitoring diurnal disruptions during repeated stress may thus help predict susceptibility to PTSD.

Comparison of the effects of single and daily repeated immobilization stress on resting activity and heterotypic sensitization of the hypothalamic-pituitary-adrenal axis

Acute exposure to severe stressors causes marked activation of the hypothalamic-pituitary-adrenal (HPA) axis that is reflected on the day after higher resting levels of HPA hormones and sensitization of the HPA response to novel (heterotypic) stressors. However, whether a single exposure to a severe stressor or daily repeated exposure to the same (homotypic) stressor modifies these responses to the same extent has not been studied. In this experiment, we studied this issue in adult male Sprague-Dawley rats daily exposed for seven days to a severe stressor such as immobilization on boards (IMO). A first exposure to 1 h IMO resulted in a marked activation of the HPA axis as reflected in plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone, and such activation was significantly reduced after the seventh IMO. On the day after the first IMO, higher resting levels of ACTH and corticosterone and sensitization of their responses to a short exposure to an open-field (OF) were observed, together with a marked hypoactivity in this environment. Repeated exposure to IMO partially reduced hypoactivity, the increase in resting levels of HPA hormones and the ACTH responsiveness to the OF on the day after the last exposure to IMO. In contrast, corticosterone response was gradually increased, suggesting partial dissociation from ACTH. These results indicate that daily repeated exposure to the same stressor partially reduced the HPA response to the homotypic stressor as well as the sensitization of HPA axis activity observed the day after chronic stress cessation.

Acute stress-induced sensitization of the pituitary-adrenal response to heterotypic stressors: independence of glucocorticoid release and activation of CRH1 receptors

A single exposure to some severe stressors causes sensitization of the hypothalamic-pituitary-adrenal (HPA) response to novel stressors. However, the putative factors involved in stress-induced sensitization are not known. In the present work we studied in adult male rats the possible role of glucocorticoids and CRH type 1 receptor (CRH-R1), using an inhibitor of glucocorticoid synthesis (metyrapone, MET), the glucocorticoid receptor (GR) antagonist RU38486 (mifepristone) and the non-peptide CRH-R1 antagonist R121919. In a first experiment we demonstrated with different doses of MET (40-150 mg/kg) that the highest dose acted as a pharmacological stressor greatly increasing ACTH release and altering the normal circadian pattern of HPA hormones, but no dose affected ACTH responsiveness to a novel environment as assessed 3 days after drug administration. In a second experiment, we found that MET, at a dose (75 mg/kg) that blocked the corticosterone response to immobilization (IMO), did not alter IMO-induced ACTH sensitization. Finally, neither the GR nor the CRH-R1 antagonists blocked IMO-induced ACTH sensitization on the day after IMO. Thus, a high dose of MET, in contrast to IMO, was unable to sensitize the HPA response to a novel environment despite the huge activation of the HPA axis caused by the drug. Neither a moderate dose of MET that markedly reduced corticosterone response to IMO, nor the blockade of GR or CRH-R1 receptors was able to alter stress-induced HPA sensitization. Therefore, stress-induced sensitization is not the mere consequence of a marked HPA activation and does not involve activation of glucocorticoid or CRH-R1 receptors.

Prior exposure to glucocorticoids potentiates lipopolysaccharide induced mechanical allodynia and spinal neuroinflammation

While stress and stress-induced glucocorticoids are classically considered immunosuppressive, they can also enhance proinflammatory responses to subsequent challenges. Corticosterone (CORT) primes rat immune cells, exacerbating pro-inflammatory responses to subsequent immune challenges. Stress can also sensitize pain. One possibility is that stress primes spinal immune cells, predominantly glia, which are key mediators in pain enhancement through their release of proinflammatory cytokines. Therefore, we aimed to identify whether prior CORT sensitizes spinal cord glia such that a potentiated pro-inflammatory response occurs to later intrathecal (IT) lipopolysaccharide (LPS), thereby enhancing pain. Rats received subcutaneous CORT/vehicle 24 h before IT LPS/vehicle. Hind paw pain thresholds were measured before CORT/vehicle, before and up to 48 h after IT LPS/vehicle. In separate rats treated as above, lumbar spinal cord tissue was collected and processed for proinflammatory mediators. CORT alone had no effect on pain responses, nor on any pro-inflammatory cytokines measured. LPS induced allodynia (decreased pain threshold) lasting <4 h and elevated spinal IL-1β and IL-6 protein. Prior CORT potentiated allodynia, lasting >24 h following LPS and potentiated spinal IL-1 and IL-6 protein. Coadministration of IL-1 receptor antagonist with LPS IT completely blocked the allodynia irrespective of whether the system was primed by CORT or not. At 24 h, TLR2, TLR4, MD2, and CD14 mRNAs were significantly elevated within the spinal cord in the CORT+LPS group compared to all other groups. Prior CORT before a direct spinal immune challenge is able to potentiate pain responses and pro-inflammatory cytokine production.

MDMA (ecstasy) enhances loud noise-induced morphofunctional alterations in heart and adrenal gland

Noise is an environmental stressor increasingly more present in modern life and, in particular, in a variety of recreational contexts. The aim of this work is to show the effects of noise on the myocardium and adrenal gland, through a careful review of the literature dealing with the peripheral effects of noise exposure in experimental and clinical studies. Noise induces adverse effects in human health, principally involving the cardiovascular and autonomic nervous systems, and the endocrine apparatus. Several factors in recreational environments potentially worsen the effects induced by loud noise. Among these, the intake of 3,4-methylenedioxymethamphetamine (MDMA) is frequently associated with noise exposure in recreational situations, because of its high compliance within social and relaxation settings. For this reason, MDMA is defined as a club drug--as its intake by young people often occurs in association with other factors, such as aggregation, high temperatures, and noise. It is known that self-administration of MDMA by humans causes severe toxicity. In particular, the myocardium is affected early after MDMA intake--resulting in tachycardia, hypertension, and arrhythmia. Furthermore, MDMA alters the activity of the adrenal glands by elevating catecholamines and corticosterone levels. This review shows that combining MDMA and loud noise exposure potentiates the effects that are produced by each single stimulant alone as seen in experimental animal models. The convergence of the effects of prolonged loud noise exposure and the consumption of MDMA on the same system might explain the sudden fatal events that happen in recreational situations.

Corticosterone pretreatment suppresses stress-induced hypothalamic-pituitary-adrenal axis activity via multiple actions that vary with time, site of action, and de novo protein synthesis

Glucocorticoid regulation of the hypothalamic-pituitary-adrenal (HPA) axis is believed to depend on multiple actions operative within discrete time domains. However, the underlying cellular and molecular mechanisms for those glucocorticoid actions remain undetermined. Moreover, there is absence of in vivo studies examining whether there are multiple glucocorticoid effects on HPA axis-related function within an intermediate feedback time frame (1-3  h after glucocorticoid elevation), and whether those effects depend on de novo protein synthesis. We examined in rats the effects of protein synthesis inhibition on HPA axis response to restraint (15 min) after 1 and 3  h phasic corticosterone (CORT) pretreatment. We measured HPA axis hormones (ACTH and CORT) and gene expression in the paraventricular nucleus (c-fos and crh genes), as well as gene expression in the anterior and intermediate pituitaries (c-fos and pomc genes). Both CORT pretreatment intervals produced inhibition of stress-induced ACTH secretion, but no inhibition was observed in the presence of protein synthesis inhibition. CORT pretreatment produced inhibitory effects on stress-induced gene expression that varied for each gene depending on the anatomical site, pretreatment time, and protein synthesis dependency. Taken together, the ACTH and gene expression patterns support the presence of multiple independent glucocorticoid actions initiated during the intermediate glucocorticoid negative feedback phase. Moreover, we conclude that those effects are exerted predominantly on the intrinsic anatomical elements of the HPA axis, and some of those effects depend on CORT induction of the expression of one or more regulatory gene products.

Stress-induced priming of glutamate synapses unmasks associative short-term plasticity

Exposure to a stressor sensitizes or 'primes' the hypothalamic-pituitary-adrenal axis to a subsequent novel stressor. The synaptic mechanisms underlying this priming, however, are not known. We found that exposing a rat to a single stressor primed glutamate synapses in the paraventricular nucleus of the hypothalamus and allowed them to undergo a short-term potentiation (STP) following a burst of high-frequency afferent activity. This transient potentiation requires a corticotrophin-releasing hormone-dependent depression of postsynaptic NMDA receptors (NMDARs). The long-term depression of NMDAR function after stress prevented the vesicular release of an inhibitory retrograde messenger that, in control conditions, arrests STP. Following stress, STP manifested as an increase in the release probability of glutamate that was sufficient to induce multivesicular release. Our findings indicate that the priming of synapses to express STP is a synaptic correlate to stress-induced behavioral and neuroendocrine sensitization.

Inescapable but not escapable stress leads to increased struggling behavior and basolateral amygdala c-fos gene expression in response to subsequent novel stress challenge

Control over an aversive experience can greatly impact the organism's response to subsequent stressors. We compared the effects of escapable (ES) and yoked inescapable (IS) electric tail shocks on the hypothalamic-pituitary-adrenal (HPA) axis hormonal (corticosterone and adrenocorticotropic hormone (ACTH)), neural (c-fos mRNA) and behavioral (struggling) response to subsequent restraint. We found that although the HPA axis response during restraint of both previously stressed groups were higher than stress-naïve rats and not different from each other, lack of control over the tailshock experience led to an increase in restraint-induced struggling behavior of the IS rats compared to both stress-naïve and ES rats. Additionally, c-fos expression in the basolateral amygdala was increased selectively in the IS group, and relative c-fos mRNA expression in the basolateral amygdala positively correlated with struggling behavior. Restraint-induced c-fos expression in the medial prefrontal cortex, a brain area critical for mediating some of the differential neurochemical and behavioral effects of ES and IS, was surprisingly similar in both ES and IS groups, lower than that of stress-naïve rats, and did not correlate with struggling behavior. Our findings indicate that basolateral amygdala activity may be connected with the differential effects of ES and IS on subsequent behavioral responses to restraint, without contributing to the concurrent HPA axis hormone response.

The role of oestradiol in sexually dimorphic hypothalamic-pituitary-adrena axis responses to intracerebroventricular ethanol administration in the rat

Systemic ethanol (EtOH) administration activates the hypothalamic-pituitary-adrenal (HPA) axis of rats in a sexually dimorphic manner. The present studies tested the role played by the central nervous system (CNS) in this phenomenon. To localise the effects of the drug to the brain, we utilised an experimental paradigm whereby a small, nontoxic amount of the drug was delivered via intracerebroventricular (i.c.v.) injection. EtoH administered i.c.v. rapidly diffuses throughout the cerebrospinal fluid and brain, and does not cause neuronal damage or have any long-term physiological or behavioural effects. Experimental groups included intact males, intact cycling females, and ovariectomised (OVX) animals with or without replacement oestradiol (E(2)). Intracerebroventricular EtOH-induced HPA hormonal activation was determined by measuring plasma adrenocorticotrophin (ACTH) levels. Activation of brain areas that both regulate HPA function and are responsive to gonadal hormones was determined using expression of the transcription factor c-fos (Fos) as a marker of neuronal activity. We observed sex- and oestrous cycle- dependent differences in HPA activation by EtOH as measured by both these parameters. ACTH secretion was highest in females in pro-oestrus or oestrus, just prior to or after the endogenous peak of E(2), as was Fos expression in the paraventricular nucleus of the hypothalamus (PVN) and the locus coreuleus (LC) of the brainstem. In OVX animals, E(2) replacement caused an increase in PVN and LC Fos expression in response to i.c.v. EtOH compared to OVX controls, but a decrease in ACTH secretion. Taken together, these results indicate that at the level of the CNS, EtOH stimulates HPA activity more robustly at times when the effects of E(2) are high, but that E(2) alone is not responsible for this effect. The data further suggest that the LC plays an important role in the circuitry, which appears to be different from that activated following the systemic administration of EtOH.

Cat odor causes long-lasting contextual fear conditioning and increased pituitary-adrenal activation, without modifying anxiety

A single exposure to a cat or cat odors has been reported by some groups to induce contextual and auditory fear conditioning and long-lasting changes in anxiety-like behaviour, but there is no evidence for parallel changes in biological stress markers. In the present study we demonstrated in male rats that exposure to a novel environment containing a cloth impregnated with cat fur odor resulted in avoidance of the odor, lower levels of activity and higher pituitary-adrenal (PA) response as compared to those exposed to the novel environment containing a clean cloth, suggesting increased levels of stress in the former animals. When re-exposed 9 days later to the same environment with a clean cloth, previously cat fur exposed rats again showed avoidance of the cloth area and lower levels of activity, suggesting development of contextual fear conditioning, which again was associated with a higher PA activation. In contrast, unaltered both anxiety-like behaviour and PA responsiveness to an elevated plus-maze were found 7 days after cat odor exposure. It is concluded that: (i) PA activation is able to reflect both the stressful properties of cat fur odor and odor-induced contextual fear conditioning; (ii) development of cat odor-induced contextual fear conditioning is independent of the induction of long-lasting changes in anxiety-like behaviour; and (iii) greater PA activation during exposure to the odor context is not explained by non-specific sensitization of the PA axis caused by previous exposure to cat fur odor.

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.

Exposure to severe stressors causes long-lasting dysregulation of resting and stress-induced activation of the hypothalamic-pituitary-adrenal axis

Exposure to some predominantly emotional (electric shock) and systemic (interleukin-1beta) stressors has been found to induce long-term sensitization of the hypothalamic-pituitary-adrenal (HPA) responsiveness to further superimposed stressors. Since exposure to immobilization on wooden boards (IMO) is a severe stressor and may have interest regarding putative animal models of post-traumatic stress disorders (PTSD), we have characterized long-lasting effects of a single exposure to IMO and other stressors on the HPA response to the same (homotypic) and to novel (heterotypic) stressors and the putative mechanisms involved. A single exposure to IMO caused a long-lasting reduction of peripheral and central responses of the HPA axis, likely to be mediated by some brain areas, such as the lateral septum and the medial amygdala. This desensitization is not explained by changes in negative glucocorticoid feedback, and, surprisingly, it is positively related to the intensity of the stressors. In contrast, the HPA response to heterotypic stressors (novel environments) was enhanced, with maximal sensitization on the day after IMO. Sensitization progressively vanished over the course of 1-2 weeks and was not modulated by IMO-induced corticosterone release. Moreover, it could not be explained by changes in the sensitivity of the HPA axis to fast or intermediate/delayed negative feedback, as evaluated 1 week after exposure to IMO, using shock as the heterotypic stressor. Long-lasting stress-induced behavioral changes reminiscent of enhanced anxiety and HPA sensitization are likely to be parallel but partially independent phenomena, the former being apparently not related to the intensity of stressors.

Marked dissociation between hypothalamic-pituitary-adrenal activation and long-term behavioral effects in rats exposed to immobilization or cat odor

Exposure of rodents to cats or certain cat odors results in long-term behavioral effects reminiscent of enhanced anxiety that have been considered to model post-traumatic stress disorder. However, other severe stressors such as tail-shock or immobilization in wooden boards (IMO) appear to induce shorter lasting changes in anxiety. In addition, there are controversial results regarding the effects of urine/feces odors. In the present work, we studied in two experiments the relationship between the degree of stress experienced by the animals during exposure to IMO, urine odors or fur odors (as assessed by hypothalamic-pituitary-adrenal activation and plasma glucose) and the short- and long-term behavioral consequences. In the first experiment, rats were individually exposed for 15 min to a novel environment (white large cages) containing either clean cat litter (controls) or litter soiled by cats (urine odors). Half of the rats in each condition were left to freely explore the environment whereas the others were subjected to immobilization (IMO) within the cages. Although ACTH, corticosterone and glucose responses to IMO were much stronger than those to the white cages with clean litter or urine odors (which did not differ from each other), no effect of treatments on anxiety-like behavior in the elevated plus-maze (EPM) were found one week later. However, previous IMO exposure did cause sensitization of the ACTH response to the EPM. In the second experiment, the response to white large cages containing either no odor (controls), litter soiled by cats (urine odor) or a cloth impregnated with cat odor (fur odor) was compared. Urine and fur odors elicited similar ACTH and corticosterone responses that were higher than those of controls, but plasma glucose levels were slightly higher in rats exposed to fur odor. When compared to controls, activity was only diminished in the novel cages containing fur odor. Similarly, fur odor-exposed rats, but not those exposed to urine odor, showed signs of enhanced anxiety in the EPM seven days later, although the ACTH response to the EPM was similar in the three groups. The present data demonstrate: (a) a marked dissociation between the degree of ACTH, corticosterone and glucose responses to stressors and their long-term anxiety-like effects; (b) that the type of cat odor is critical in determining the short-term and long-term physiological and behavioral consequences of exposure; and (c) that plasma ACTH released during brief exposure to the EPM does not appear to reflect anxiety-like behavior.

Persistent depressive state after chronic stress in rats is accompanied by HPA axis dysregulation and reduced prefrontal dopaminergic neurotransmission

Exposure to stress is thought to play an important role in the etiology of depression. Dysregulation of the hypothalamo-pituitary-adrenal (HPA) axis characterized by glucocorticoid negative feedback resistance is frequently observed in human depressives. Additionally, dysfunctions of the dopaminergic and serotonergic systems in the prefrontal cortex (PFC) are thought to be involved in the development of a depressive state. In rats, chronic stress induces a behaviorally depressive state, concomitant with dysregulation of the HPA axis and reductions in dopaminergic and serotonergic transmissions in the PFC. Considering that dysregulation of the HPA axis is associated with relapse and persistency of depression, it is possible that the chronic stress-induced depressive state persists during long-term rest after its exposure. In the present study, we examined this possibility in rats and found that the behaviorally depressive state in the rotarod test, negative feedback resistance in the dexamethasone suppression test, and a decrease in the extracellular concentration of dopamine but not serotonin in the PFC persisted for 3 months following a 4-week stress session. These results suggest that dysregulation of the HPA system and reduced dopaminergic transmission in the PFC underlies persistent behavioral depression following chronic stress.

Long-term neuroendocrine and behavioural effects of a single exposure to stress in adult animals

There is now considerable evidence for long-lasting sequels of stress. A single exposure to high intensity predominantly emotional stressors such as immobilisation in wooden-boards (IMO) induces long-term (days to weeks) desensitization of the hypothalamic-pituitary-adrenal (HPA) response to the same (homotypic) stressor, whereas the response to novel (heterotypic) stressors was enhanced. In addition, long-lasting changes in behaviour have been described after a single exposure to brief or more prolonged sessions of shocks, predator, predator odour, underwater stress or a combination of three stressors on 1 day. The most consistent changes are reduced entries into the open arms of the elevated plus-maze and enhanced acoustic startle response, both reflecting enhanced anxiety. However, it is unclear whether there is any relationship between the intensity of the stressors, as evaluated by the main physiological indexes of stress (e.g. HPA axis), the putative traumatic experience they represent and their long-term behavioural consequences. This is particularly critical when trying to model post-traumatic stress disorders (PTSD), which demands a great effort to validate such putative models.

Interannual variation in the adrenal responsiveness of black-legged kittiwake chicks (Rissa tridactyla)

Over the last decade, field endocrinologists have explored the efficacy of using plasma corticosterone concentrations of breeding seabirds as an indicator of food availability and predictor of breeding success. However, studies have been predominately confined to adults and the results have been inconsistent. We examined the relationship between the productivity of black-legged kittiwakes (Rissa tridactyla) and the baseline and stress-induced corticosterone levels of 12-15 day-old kittiwake chicks in a multi-year study in Chiniak Bay, Kodiak, Alaska. We predicted that corticosterone levels would negatively correlate with productivity. Productivity decreased across years from relatively high levels in 2002, to very low levels in 2004 and 2005. Baseline corticosterone levels of the chicks did not increase consistently across years. Stress-induced corticosterone levels were statistically indistinguishable in high productivity years but increased significantly in low productivity years. The decline in kittiwake productivity coincided with warming ocean conditions, which, historically, are linked to declines in forage abundance for kittiwakes. Inconsistent changes in baseline corticosterone levels suggest some adult kittiwakes were able to buffer their chicks from poor foraging conditions. However, large increases in stress-induced corticosterone levels during the low productivity years indicate chicks were physiologically stressed for some period prior to sampling. Our results suggest that stress-induced corticosterone levels of kittiwake chicks are effective indicators of productivity in poor years, whereas the changes in baseline corticosterone levels across years are not as pronounced and therefore may not be as reliable.

Decreased ACTH secretion during prolonged transportation stress is associated with reduced pituitary responsiveness to tropic hormone stimulation in cattle

The present study examined the effect of transportation stress on hypothalamic-pituitary-adrenal axis responsiveness to tropic hormone stimulation and on abundance of corticotropin releasing factor (CRF) receptor R1 (CRFR1) and arginine vasopressin (AVP) receptor V3 (V3) mRNAs in the anterior pituitary (AP) of cattle. Holstein steers were transported for 10 h or used as non-transported controls (NTC). Blood samples were collected at start of transportation and every 1-2h thereafter. To test AP responsiveness to tropic hormones, animals were challenged (i.v.) with CRF (0.5 microg/kg), AVP (1 microg/kg) or CRF plus AVP immediately after end of transportation and blood samples collected every 30 min for 3h. The AP of animals transported for 0, 4 or 10 h were harvested for mRNA analyses. Plasma ACTH in transported animals increased within 1h and remained elevated for 6 and 8h versus NTC and 0 h values, respectively. Plasma concentrations of cortisol increased in response to transportation and remained elevated throughout the transport period. Injection of CRF or AVP to NTC animals increased plasma ACTH, but ACTH secretion in response to CRF or AVP was dramatically reduced in transported animals. ACTH secretion following co-injection of CRF and AVP tended to be less in transported animals, but was almost 100% greater than when secretagogues were administered separately. Despite decreased AP responsiveness to CRF and AVP, AP CRFR1 and V3 mRNAs were increased after 10 h transportation. Results indicate decreased AP responsiveness to CRF and AVP may regulate duration of ACTH secretion in response to transportation stress in cattle.

Differential responses of hypothalamus-pituitary-adrenal axis immediate early genes to corticosterone and circadian drive

The hypothalamus-pituitary-adrenal (HPA) axis diurnal cycle of activity is manifest in circadian rhythms of ACTH and corticosterone secretion, which in the rat peak around the onset of the dark period. This cycle is thought to be driven by daily fluctuations in activity of CRH neurons within the paraventricular nucleus of the hypothalamus (PVN), controlled by suprachiasmatic nucleus inputs. In this study we examined whether the circadian drive that regulates ACTH and corticosterone basal secretion in the rat is reflected in PVN immediate early gene expression and, if so, whether different genes respond uniformly or uniquely to circadian stimulatory input. In addition, we examined how circadian drive and acute stress, two categories of stimuli that induce HPA axis activation, comparatively affect gene expression within different components of the HPA axis (c-fos mRNA, CRH heteronuclear RNA, and zif268 mRNA in PVN; c-fos mRNA, proopiomelanocortin heteronuclear RNA, and zinc finger 268 mRNA in anterior pituitary; c-fos mRNA and nerve growth factor I-B mRNA in adrenal cortex). Finally, we examined whether circadian differences in gene expression depend on endogenous glucocorticoids and, if so, whether the dependence is on an acute or permissive influence of the hormone. We found that a circadian drive that regulates HPA axis basal hormone secretion is also manifest on basal c-fos gene expression in the PVN. Moreover, we show that different immediate early genes within the HPA axis anatomical components display different diurnal patterns of gene expression. These differential patterns result, in part, from gene-specific responses to circadian signals and acute and/or permissive glucocorticoid actions.

Increase in clusterin-containing follicles in the adenohypophysis of drug abusers

The hypothalamic-pituitary-adrenocortical (HPA) system in drug abusers may be affected due to disorders of the hypothalamic dopaminergic system. The present study investigated alterations in the adenohypophysis of middle-aged drug abusers (40-60 years of age), using clusterin-containing mixed cell-follicles as the indicator, in which clusterin (apolipoprotein J) is a multifunctional glycoprotein related to neurodegeneration. The paraffin-embedded adenohypophyses of methamphetamine and psychotropic drug abusers (n = 76) were compared with those of non-abusers (n = 82). The number of follicles was larger in drug abusers independent of the immediate cause of death, although the size was not significantly different. When cell types forming the follicles were immunohistochemically examined, drug abusers showed an increase of prolactin (PRL) cells and gonadotroph cells and a reciprocal decrease of growth hormone cells, suggesting hypofunction of dopaminergic neurons in the hypothalamus, while there was no change in the adrenocorticotropic hormone and thyroid-stimulating hormone cells. These increases of the clusterin-containing follicles and PRL cells in the follicles may be related to the dysfunction of dopaminergic neurons in the hypothalamus of chronic drug abusers and may be useful for investigating drug abuse in forensic casework.

Systemic Administration of Alcohol to Adult Rats Inhibits Leydig Cell Activity: Time Course of Effect and Role of Nitric Oxide

BACKGROUND

Alcohol has been shown to interfere with testosterone (T) release from Leydig cells. However, the mechanisms responsible for this phenomenon, which may include decreased activity of the luteinizing hormone-releasing hormone (LHRH)-LH axis, as well as a direct influence of the drug on the testes, are not fully understood. In this work, we investigated the influence of alcohol, administered intragastrically (i.g.) or delivered via vapors, on Leydig cell activity and T release. Leydig cell function was studied by measuring changes in the levels of the steroidogenic proteins steroidogenic acute regulatory (StAR), the peripheral-type benzodiazepine receptor (PBR), and the cytochrome P450 side-chain cleavage enzyme (P450scc). Testosterone release was studied under basal conditions or in response to human chorionic gonadotropin (hCG). Finally, to identify potential factors mediating the influence of alcohol, we measured the testicular variant of the neuronal nitric oxide (NO) synthase (NOS), TnNOS, in semipurified Leydig cells.

METHODS

Adult male Sprague-Dawley rats were either injected with alcohol i.g. once or exposed to alcohol vapors (4 h/d) for 1 or 5 days. Controls received the vehicle (i.g. model) or were kept in boxes through which no vapors were circulated. Following these treatments, one series of experiments was devoted to investigate Leydig cell responsiveness by measuring plasma T levels before or after the intravenous injection of hCG (1 U/kg). In another series of experiments, we used semipurified Leydig cell preparations to measure StAR, PBR, P450scc, and TnNOS by Western blot analysis.

RESULTS

In the i.g. model, the T response to hCG was blunted for 12 hours following alcohol injection, but showed a rebound at 48 hours. Levels of StAR protein and of PBR, but not of P450scc, were significantly decreased within 10 minutes of drug administration. While StAR then remained depressed for 24 hours, PBR values were variable over this time course. By 48 hours, StAR, PBR, and P450scc levels had increased above control values. Both StAR and PBR levels showed correlations with plasma T levels. In the alcohol vapor models, both regimens of the drug also significantly depressed StAR and PBR protein concentrations, blunted the T response to hCG, and did not alter P450scc. Finally, we observed that alcohol delivered i.g. or via vapors up-regulated TnNOS levels in Leydig cells, but that blockade of NO formation failed to restore a normal T response to hCG.

CONCLUSIONS

Collectively, these results suggest that (a) the ability of Leydig cells to release T does not show a simple correlation with changes in StAR, PBR, and P450scc levels; (b) the time course of the alcohol-induced changes were protein-specific; and (c) despite the ability of alcohol to stimulate TnNOS expression, NO does not appear to mediate the inhibitory influence of this drug on testicular steroidogenesis in the models that we studied.

Regulation of proopiomelanocortin gene transcription during single and repeated immobilization stress

We have previously reported that repeated immobilization produces persistent activation of the hypothalamic-pituitary-adrenocortical axis in rats. In an attempt to assess whether any adaptational responses occur at the pituitary level, we examined the detailed time courses of proopiomelanocortin (POMC) gene transcription in the anterior pituitary (AP) in comparison with those of corticotropin-releasing hormone (CRH) gene transcription in the hypothalamic paraventricular nucleus (PVN) during single and repeated immobilization using both intronic and exonic probes. During single immobilization, there was a robust and rapid increase in both CRH heteronuclear RNA (hnRNA) in the PVN and POMC hnRNA in the AP, together with a slower increase in CRH mRNA, but no significant increase in POMC mRNA. Single immobilization also caused significant increases in the plasma concentrations of both ACTH and corticosterone. Daily immobilization for 6 days increased the basal levels of CRH hnRNA and CRH mRNA in the PVN and POMC mRNA in the AP. Both CRH hnRNA and POMC hnRNA responded rapidly to a final episode of acute immobilization on day 7, whereas the peak values of CRH hnRNA and POMC hnRNA after 15 min of the final stress were smaller than those during single immobilization. In contrast to single stress, CRH mRNA did not change significantly, whereas POMC mRNA robustly increased after the final immobilization on day 7. Plasma ACTH increased to a similar degree to single stress, but its initial increase at 5 min was significantly higher than that during single immobilization. The increase in the plasma corticosterone concentration was higher during final immobilization than during single stress. These results suggest that, in response to the hypothalamic drive during repeated immobilization stress, pituitary corticotrophs are capable of upregulating the basal and stress-induced POMC mRNA levels via increased efficiency of the posttranscriptional processing of the hnRNA and/or increased mRNA stability.

The bed nucleus of the stria terminalis is critically involved in enhancing associative learning after stressful experience

Exposure to an acute stressful event enhances trace eyeblink conditioning in male rats, even when rats begin training days after the stressor (Shors, 2001). The authors examined whether the bed nucleus of the stria terminalis (BNST), an area involved in stress and anxiety, is critically involved in this effect and, if so, when. The authors found that excitotoxic lesions to the BNST prevented the enhanced conditioning after stressor exposure. In addition, temporary inactivation of the BNST during the stressor did not alter enhanced responding, whereas inactivation during training prevented the enhancement. These data indicate that stressful experience induces persistent changes in the BNST that are necessary for enhancing learning well after the stressful event has ceased.