Habituation and cross-sensitization of stress-induced hypothalamic-pituitary-adrenal activity: effect of lesions in the paraventricular nucleus of the thalamus or bed nuclei of the stria terminalis

Influences of Stress and Sex on the Paraventricular Thalamus: Implications for Motivated Behavior

The paraventricular nucleus of the thalamus (PVT) is a critical neural hub for the regulation of a variety of motivated behaviors, integrating stress and reward information from environmental stimuli to guide discrete behaviors via several limbic projections. Neurons in the PVT are activated by acute and chronic stressors, however several roles of the PVT in behavior modulation emerge only following repeated stress exposure, pointing to a role for hypothalamic pituitary adrenal (HPA) axis modulation of PVT function. Further, there may be a reciprocal relationship between the PVT and HPA axis in which chronic stress-induced recruitment of the PVT elicits an additional role for the PVT to regulate motivated behavior by modulating HPA physiology and thus the neuroendocrine response to stress itself. This complex interaction may make the PVT and its role in influencing motivated behavior particularly susceptible to chronic stress-induced plasticity in the PVT, especially in females who display increased susceptibility to stress-induced maladaptive behaviors associated with neuropsychiatric diseases. Though literature is describing the sex-specific effects of acute and chronic stress exposure on HPA axis activation and motivated behaviors, the impact of sex on the role of the PVT in modulating the behavioral and neuroendocrine response to stress is less well established. Here, we review what is currently known regarding the acute and chronic stress-induced activation and behavioral role of the PVT in male and female rodents. We further explore stress hormone and neuropeptide signaling mechanisms by which the HPA axis and PVT interact and discuss the implications for sex-dependent effects of chronic stress on the PVT's role in motivated behaviors.

Periaqueductal gray inputs to the paraventricular nucleus of the thalamus: Columnar topography and glucocorticoid (in)sensitivity

The ability to cope with a novel acute stressor in the context of ongoing chronic stress is of critical adaptive value. The hypothalamic-pituitary-adrenal (HPA) axis contributes to the integrated physiological and behavioural responses to stressors. Under conditions of chronic stress, the posterior portion of the paraventricular thalamic nucleus (pPVT) mediates the 'habituation' of HPA-axis responses, and also facilitates HPA-axis reactivation to novel acute stressors amidst this habituation. Since pPVT neurons are sensitive to the inhibitory effects of circulating glucocorticoids, a glucocorticoid-insensitive neural pathway to the pPVT is likely essential for this reactivation process. The pPVT receives substantial inputs from neurons of the periaqueductal gray (PAG) region, which is organised into longitudinal columns critical for processing acute and/or chronic stressors. We investigated the columnar organisation of PAG → pPVT projections and for the first time determined their glucocorticoid sensitivity. Retrograde tracer injections were made into different rostro-caudal regions of the pPVT, and their PAG columnar inputs compared. Glucocorticoid receptor immunoreactivity (GR-ir) was quantified in these projection neurons. We found that the dorsolateral PAG projected most strongly to rostral pPVT and the ventrolateral PAG most strongly to the caudal pPVT. Despite abundant GR-ir in the PAG, we report a striking absence of GR-ir in PAG → pPVT neurons. Our data suggests that these pathways, which are insensitive to the direct actions of circulating glucocorticoids, likely play an important role in both the habituation of HPA-axis to chronic stressors and its facilitation to acute stressors in chronically stressed rats.

Brain-Wide Mapping of Afferent Inputs to Accumbens Nucleus Core Subdomains and Accumbens Nucleus Subnuclei

The nucleus accumbens (NAc) is the ventral part of the striatum and the interface between cognition, emotion, and action. It is composed of three major subnuclei: i.e., NAc core (NAcC), lateral shell (NAcLS), and medial shell (NAcMS), which exhibit functional heterogeneity. Thus, determining the synaptic inputs of the subregions of the NAc is important for understanding the circuit mechanisms involved in regulating different functions. Here, we simultaneously labeled subregions of the NAc with cholera toxin subunit B conjugated with multicolor Alexa Fluor, then imaged serial sections of the whole brain with a fully automated slide scanning system. Using the interactive WholeBrain framework, we characterized brain-wide inputs to the NAcC subdomains, including the rostral, caudal, dorsal, and ventral subdomains (i.e., rNAcC, cNAcC, dNAcC, and vNAcC, respectively) and the NAc subnuclei. We found diverse brain regions, distributed from the cerebrum to brain stem, projecting to the NAc. Of the 57 brain regions projecting to the NAcC, the anterior olfactory nucleus (AON) exhibited the greatest inputs. The input neurons of rNAcC and cNAcC are two distinct populations but share similar distribution over the same upstream brain regions, whereas the input neurons of dNAcC and vNAcC exhibit slightly different distributions over the same upstream regions. Of the 55 brain regions projecting to the NAcLS, the piriform area contributed most of the inputs. Of the 72 brain regions projecting to the NAcMS, the lateral septal nucleus contributed most of the inputs. The input neurons of NAcC and NAcLS share similar distributions, whereas the NAcMS exhibited brain-wide distinct distribution. Thus, the NAcC subdomains appeared to share the same upstream brain regions, although with distinct input neuron populations and slight differences in the input proportions, whereas the NAcMS subnuclei received distinct inputs from multiple upstream brain regions. These results lay an anatomical foundation for understanding the different functions of NAcC subdomains and NAc subnuclei.

Sex differences in the regulation of brain IL-1β in response to chronic stress

Elevations in brain interleukin-1 beta (IL-1β) during chronic stress exposure have been implicated in behavioral and cognitive impairments associated with depression and anxiety. Two critical regulators of brain IL-1β production during times of stress are glucocorticoids and catecholamines. These hormones work in opposition to one another to inhibit (via glucocorticoid receptors) or stimulate (via beta-adrenergic receptors: β-AR) IL-1β production. While chronic stress often heightens both corticosterone and catecholamine levels, it remains unknown as to how chronic stress may affect the "yin-yang" balance between adrenergic stimulation and glucocorticoid suppression of brain IL-1β. To investigate this further, male and female rats underwent 4 days of stress exposure or served as non-stressed controls. On day 5, animals were administered propranolol (β-AR antagonist), metyrapone (a glucocorticoid synthesis inhibitor), vehicle, or both drugs and brain IL-1β mRNA was measured by rtPCR in limbic brain areas. In males, administration of propranolol had no effect on IL-1β expression in non-stressed controls but significantly reduced IL-1β in the hippocampus and amygdala of chronically stressed animals. In females, propranolol significantly reduced IL-1β in the amygdala and hypothalamus of both control and stressed rats. In male rats, metyrapone treatment significantly increased IL-1β mRNA regardless of stress treatment in all brain areas, while in female rats metyrapone only increased IL-1β in the hypothalamus. Interestingly, propranolol treatment blocked the metyrapone-induced increase in brain IL-1β indicating the increase in brain IL-1β following metyrapone treatment was due to increase β-AR activation. Additional studies revealed that metyrapone significantly increases norepinephrine turnover in the hypothalamus and medial prefrontal cortex in male rats and that microglia appear to be the cell type contributing to the production of IL-1β. Overall, data reveal that stress exposure in male rats affects the regulation of brain IL-1β by the norepinephrine-β-AR pathway, while stress had no effect in the regulation of brain IL-1β in female rats.

Relaxin-3/RXFP3 signalling in mouse hypothalamus: no effect of RXFP3 activation on corticosterone, despite reduced presynaptic excitatory input onto paraventricular CRH neurons in vitro

Relaxin-3/RXFP3 signalling is proposed to be involved in the neuromodulatory control of arousal- and stress-related neural circuits. Furthermore, previous studies in rats have led to the proposal that relaxin-3/RXFP3 signalling is associated with activation of the hypothalamic-pituitary-adrenal axis, but direct evidence for RXFP3-related actions on the activity of hypothalamic corticotropin-releasing hormone (CRH) neurons is lacking. In this study, we investigated characteristics of the relaxin-3/RXFP3 system in mouse hypothalamus. Administration of an RXFP3 agonist (RXFP3-A2) intra-cerebroventricularly or directly into the paraventricular nucleus of hypothalamus (PVN) of C57BL/6J mice did not alter corticosterone levels. Similarly, there were no differences between serum corticosterone levels in Rxfp3 knockout (C57BL/6J^RXFP3TM1) and wild-type mice at baseline and after stress, despite detection of the predicted stress-induced increases in serum corticosterone. We examined the nature of the relaxin-3 innervation of PVN in wild-type mice and in Crh-IRES-Cre;Ai14 mice that co-express the tdTomato fluorophore in CRH neurons, identifying abundant relaxin-3 fibres in the peri-PVN region, but only sparse fibres associated with densely packed CRH neurons. In whole-cell voltage-clamp recordings of tdTomato-positive CRH neurons in these mice, we observed a reduction in sEPSC frequency following local application of RXFP3-A2, consistent with an activation of RXFP3 on presynaptic glutamatergic afferents in the PVN region. These studies clarify the relationship between relaxin-3/RXFP3 inputs and CRH neurons in mouse PVN, with implications for the interpretation of current and previous in vivo studies and future investigations of this stress-related signalling network in normal and transgenic mice, under normal and pathological conditions.

Optimizing laboratory animal stress paradigms: The H-H* experimental design

Major advances in behavioral neuroscience have been facilitated by the development of consistent and highly reproducible experimental paradigms that have been widely adopted. In contrast, many different experimental approaches have been employed to expose laboratory mice and rats to acute versus chronic intermittent stress. An argument is advanced in this review that more consistent approaches to the design of chronic intermittent stress experiments would provide greater reproducibility of results across laboratories and greater reliability relating to various neural, endocrine, immune, genetic, and behavioral adaptations. As an example, the H-H* experimental design incorporates control, homotypic (H), and heterotypic (H*) groups and allows for comparisons across groups, where each animal is exposed to the same stressor, but that stressor has vastly different biological and behavioral effects depending upon each animal's prior stress history. Implementation of the H-H* experimental paradigm makes possible a delineation of transcriptional changes and neural, endocrine, and immune pathways that are activated in precisely defined stressor contexts.

Differential activation of c‑Fos in the paraventricular nuclei of the hypothalamus and thalamus following myocardial infarction in rats

Proto-oncogene c-Fos (c-Fos) is frequently used to detect a pathogenesis in central nervous system disorders. The present study examined changes in the immunoreactivity of c-Fos in the paraventricular nucleus of the hypothalamus (PVNH) and paraventricular nucleus of the thalamus (PVNT) following myocardial infarction (MI) in rats. Infarction in the left ventricle was examined by Masson's trichrome staining. Neuronal degeneration was monitored for 56 days after MI using crystal violet and Fluoro-Jade B histofluorescence staining. Changes in the immunoreactivity of c-Fos were determined using immunohistochemistry for c-Fos. The average infarct size of the left ventricle circumference was ~44% subsequent to MI. Neuronal degeneration was not detected in PVNH and PVNT following MI. c-Fos immunoreactive (⁺) cells were infrequently observed in the nuclei of the sham-group. However, the number of c-Fos⁺ cells was increased in the nuclei following MI and peaked in the PVNH and PVNT at 3 and 14 days, respectively. The number of c-Fos⁺ cells were comparable with the sham group at 56 days after MI. Therefore, MI may induce c-Fos immunoreactivity in PVNH and PVNT, this increase of c-Fos expression levels may be associated with the stress that occurs in the brain following MI.

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).

Central apelin mediates stress-induced gastrointestinal motor dysfunction in rats

Apelin, an endogenous ligand for APJ receptor, has been reported to be upregulated in paraventricular nucleus (PVN) following stress. Central apelin is known to stimulate release of corticotropin-releasing factor (CRF) via APJ receptor. We tested the hypothesis that stress-induced gastrointestinal (GI) dysfunction is mediated by central apelin. We also assessed the effect of exogenous apelin on GI motility under nonstressed (NS) conditions in conscious rats. Prior to solid gastric emptying (GE) and colon transit (CT) measurements, APJ receptor antagonist F13A was centrally administered under NS conditions and following acute stress (AS), chronic homotypic stress (CHS), and chronic heterotypic stress (CHeS). Plasma corticosterone was assayed. Strain gage transducers were implanted on serosal surfaces of antrum and distal colon to record postprandial motility. Stress exposure induced coexpression of c-Fos and apelin in hypothalamic PVN. Enhanced hypothalamic apelin and CRF levels in microdialysates were detected following AS and CHeS, which were negatively and positively correlated with GE and CT, respectively. Central F13A administration abolished delayed GE and accelerated CT induced by AS and CHeS. Central apelin-13 administration increased the plasma corticosterone and inhibited GE and CT by attenuating antral and colonic contractions. The inhibitory effect elicited by apelin-13 was abolished by central pretreatment of CRF antagonist CRF9-41 in antrum, but not in distal colon. Central endogenous apelin mediates stress-induced changes in gastric and colonic motor functions through APJ receptor. The inhibitory effects of central exogenous apelin-13 on GI motility appear to be partly CRF dependent. Apelin-13 inhibits colon motor functions through a CRF-independent pathway.

Targeting the modulation of neural circuitry for the treatment of anxiety disorders

Anxiety disorders are a major public health concern. Here, we examine the familiar area of anxiolysis in the context of a systems-level understanding that will hopefully lead to revealing an underlying pharmacological connectome. The introduction of benzodiazepines nearly half a century ago markedly improved the treatment of anxiety disorders. These agents reduce anxiety rapidly by allosterically enhancing the postsynaptic actions of GABA at inhibitory type A GABA receptors but side effects limit their use in chronic anxiety disorders. Selective serotonin reuptake inhibitors and serotonin/norepinephrine reuptake inhibitors have emerged as an effective first-line alternative treatment of such anxiety disorders. However, many individuals are not responsive and side effects can be limiting. Research into a relatively new class of agents known as neurosteroids has revealed novel modulatory sites and mechanisms of action that are providing insights into the pathophysiology of certain anxiety disorders, potentially bridging the gap between the GABAergic and serotonergic circuits underlying anxiety. However, translating the pharmacological activity of compounds targeted to specific receptor subtypes in rodent models of anxiety to effective therapeutics in human anxiety has not been entirely successful. Since modulating any one of several broad classes of receptor targets can produce anxiolysis, we posit that a systems-level discovery platform combined with an individualized medicine approach based on noninvasive brain imaging would substantially advance the development of more effective therapeutics.

Adaptation of the hypothalamus-pituitary-adrenal axis to daily repeated stress does not follow the rules of habituation: A new perspective

Repeated exposure to a wide range of stressors differing in nature and intensity results in a reduced response of prototypical stress markers (i.e. plasma levels of ACTH and adrenaline) after an acute challenge with the same (homotypic) stressor. This reduction has been considered to be a habituation-like phenomenon. However, direct experimental evidence for this assumption is scarce. In the present work we demonstrate in adult male rats that adaptation of the hypothalamus-pituitary-adrenal (HPA) axis to repeated stress does not follow some of the critical rules of habituation. Briefly, adaptation was stronger and faster with more severe stressors, maximally observed even with a single exposure to severe stressors, extremely long-lasting, negatively related to the interval between the exposures and positively related to the length of daily exposure. We offer a new theoretical view to explain adaptation to daily repeated stress.

Repeated exposure to conditioned fear stress increases anxiety and delays sleep recovery following exposure to an acute traumatic stressor

Repeated stressor exposure can sensitize physiological responses to novel stressors and facilitate the development of stress-related psychiatric disorders including anxiety. Disruptions in diurnal rhythms of sleep-wake behavior accompany stress-related psychiatric disorders and could contribute to their development. Complex stressors that include fear-eliciting stimuli can be a component of repeated stress experienced by human beings, but whether exposure to repeated fear can prime the development of anxiety and sleep disturbances is unknown. In the current study, adult male F344 rats were exposed to either control conditions or repeated contextual fear conditioning for 22 days followed by exposure to no, mild (10), or severe (100) acute uncontrollable tail shock stress. Exposure to acute stress produced anxiety-like behavior as measured by a reduction in juvenile social exploration and exaggerated shock-elicited freezing in a novel context. Prior exposure to repeated fear enhanced anxiety-like behavior as measured by shock-elicited freezing, but did not alter social exploratory behavior. The potentiation of anxiety produced by prior repeated fear was temporary; exaggerated fear was present 1 day but not 4 days following acute stress. Interestingly, exposure to acute stress reduced rapid eye movement (REM) and non-REM (NREM) sleep during the hours immediately following acute stress. This initial reduction in sleep was followed by robust REM rebound and diurnal rhythm flattening of sleep/wake behavior. Prior repeated fear extended the acute stress-induced REM and NREM sleep loss, impaired REM rebound, and prolonged the flattening of the diurnal rhythm of NREM sleep following acute stressor exposure. These data suggest that impaired recovery of sleep/wake behavior following acute stress could contribute to the mechanisms by which a history of prior repeated stress increases vulnerability to subsequent novel stressors and stress-related disorders.

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.

Sex differences in activation of the hypothalamic-pituitary-adrenal axis by methamphetamine

Dysregulation of hypothalamic-pituitary-adrenal (HPA) axis activation is associated with changes in addiction-related behaviors. In this study, we tested whether sex differences in the acute effects of methamphetamine (MA) exposure involve differential activation of the HPA axis. Male and female mice were injected with MA (1 mg/kg) or saline for comparison of plasma corticosterone and analysis of the immediate early gene c-Fos in brain. There was a prolonged elevation in corticosterone levels in female compared to male mice. C-Fos was elevated in both sexes following MA in HPA axis-associated regions, including the hypothalamic paraventricular nucleus (PVN), central amygdala, cingulate, and CA3 hippocampal region. MA increased the number of c-Fos and c-Fos/glucocorticoid receptor (GR) dual-labeled cells to a greater extent in males than females in the cingulate and CA3 regions. MA also increased the number of c-fos/vasopressin dual-labeled cells in the PVN as well as the number and percentage of c-Fos/GR dual-labeled cells in the PVN and central amygdala, although no sex differences in dual labeling were found in these regions. Thus, sex differences in MA-induced plasma corticosterone levels and activation of distinct brain regions and proteins involved in HPA axis regulation may contribute to sex differences in acute effects of MA on the brain. Methamphetamine induces a prolonged plasma corticosterone response in females compared to males. This may be mediated by increased neural activation, involving a greater activation of glucocorticoid receptor-positive cells, in males in the CA3 and cingulate brain regions, which are involved in negative feedback functions. These findings indicate a sex difference in the neural regulation of methamphetamine-induced plasma corticosterone release.

Exposure to high fat during early development impairs adaptations in dopamine and neuroendocrine responses to repeated stress

Perturbations in the perinatal environment have been shown to significantly alter mesolimbic dopamine (DA) and hypothalamic-pituitary-adrenal (HPA) responses to stressors in adulthood. We have previously demonstrated that adult offspring exposed to high fat during the last week of gestation and throughout lactation display permanent alterations in mesolimbic DA function and behavior. The goal of the present study was to investigate nucleus accumbens (NAc) DA and HPA responses to acute and repeated stress in high fat exposed (HFD, 30% fat) and control (CD, 5% fat) offspring. Using microdialysis to monitor extracellular DA, we report that adult HFD offspring show an enhanced NAc DA response to acute tail-pinch compared to CD offspring. With repeated tail-pinch, the response of the HFD animals remains unchanged while CD offspring exhibit a sensitized DA response. The pattern of the DA response to both acute and repeated stress is also significantly altered by early diet exposure with an earlier peak and faster return to baseline levels in CD compared with HFD offspring. Similarly, neuroendocrine adaptations to repeated tail-pinch are observed in CD animals, but not in HFD animals. While controls display a habituated adrenocorticotropic hormone (ACTH) response to repeated tail-pinch, and an exacerbated ACTH response to a novel stressor, this effect was not observed in the HFD offspring. Together, our data demonstrate that exposure to high fat during early development impairs adaptations in NAc DA and HPA responses usually observed with repeated stress.

Changes in hypothalamic-pituitary-adrenal stress responsiveness before and after puberty in rats

This article is part of a Special Issue "Puberty and Adolescence". Many endocrine changes are associated with pubertal and adolescent development. One such change is the responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to physical and/or psychological stressors. Recent human and non-human animal studies have shown that hormonal stress reactivity increases significantly throughout puberty and adolescence. Specifically, exposure to various stressors results in greater adrenocorticotropic hormone (ACTH) and glucocorticoid responses in peripubertal compared to adult animals. This review will focus on how stress reactivity changes throughout puberty and adolescence, as well as potential mechanisms that mediate these changes in stress responsiveness. Though the implications of these pubertal shifts in stress responsiveness are not fully understood, the significant increase in stress-related mental and physical dysfunctions during this stage of development highlights the importance of studying pubertal and adolescent maturation of HPA function and its reactivity to stress.

Divergent stress-induced neuroendocrine and behavioral responses prior to puberty

Following an acute stressor, pre-adolescent rats exhibit a protracted hormonal response compared to adults, while after repeated exposure to the same stressor (i.e., homotypic stress) prepubertal males fail to habituate like adults. Though the neurobehavioral implications of these changes are unknown, studying pubertal shifts in stress reactivity may help elucidate the mechanisms that underlie the increase in stress-related psychological and physiological disorders often observed during adolescence. Here, we investigated hormonal, behavioral, and neural responses of prepubertal (30d) and adult (77d) male rats before, during, or after acute stress (restraint), homotypic stress (repeated restraint) or heterotypic stress (repeated cold exposure followed by restraint). We found that prepubertal males exhibit prolonged corticosterone responses following acute and heterotypic stress, and higher adrenocorticotropic hormone and corticosterone responses after homotypic stress, compared to adults. Despite these significant age-dependent changes in hormonal responsiveness, we found that struggling behavior during restraint was similar at both ages, such that both prepubertal and adult animals exposed to homotypic stress struggled less than animals exposed to either acute or heterotypic stress. Across these different stress paradigms, we found greater neural activation, as indexed by FOS immunostaining, in the prepubertal compared to adult paraventricular nucleus of the hypothalamus, a nucleus integral for initiating the hormonal stress response. Interestingly, however, we did not find any influence of pubertal development on stress-induced activation of the posterior paraventricular thalamic nucleus, a brain region involved in experience-dependent changes in stress reactivity. Collectively, our data indicate that prepubertal and adult males display divergent hormonal, behavioral, and neural responses following a variety of stressful experiences, as well as a distinct dissociation between hormonal and behavioral reactivity in prepubertal males under homotypic conditions.

The effects of single and repeated exposure to 2.45 GHz radiofrequency fields on c-Fos protein expression in the paraventricular nucleus of rat hypothalamus

This study investigated the effects of microwave radiation on the PVN of the hypothalamus, extracted from rat brains. Expression of c-Fos was used to study the pattern of cellular activation in rats exposed once or repeatedly (ten times in 2 weeks) to 2.45 GHz radiation in a GTEM cell. The power intensities used were 3 and 12 W and the Finite Difference Time Domain calculation was used to determine the specific absorption rate (SAR). High SAR triggered an increase of the c-Fos marker 90 min or 24 h after radiation, and low SAR resulted in c-Fos counts higher than in control rats after 24 h. Repeated irradiation at 3 W increased cellular activation of PVN by more than 100% compared to animals subjected to acute irradiation and to repeated non-radiated repeated session control animals. The results suggest that PVN is sensitive to 2.45 GHz microwave radiation at non-thermal SAR levels.

Effects of reproductive status on behavioral and endocrine responses to acute stress in a biparental rodent, the California mouse (Peromyscus californicus)

In several mammalian species, lactating females show blunted neural, hormonal, and behavioral responses to stressors. It is not known whether new fathers also show stress hyporesponsiveness in species in which males provide infant care. To test this possibility, we determined the effects of male and female reproductive status on stress responsiveness in the biparental, monogamous California mouse (Peromyscus californicus). Breeding (N=8 females, 8 males), nonbreeding (N=10 females, 10 males) and virgin mice (N=12 females, 9 males) were exposed to a 5-min predator-urine stressor at two time points, corresponding to the early postpartum (5-7 days postpartum) and mid/late postpartum (19-21 days postpartum) phases, and blood samples were collected immediately afterwards. Baseline blood samples were obtained 2 days prior to each stress test. Baseline plasma corticosterone (CORT) concentrations did not differ among male or female groups. CORT responses to the stressor did not differ among female reproductive groups, and all three groups showed distinct behavioral responses to predator urine. Virgin males tended to increase their CORT response from the first to the second stress test, while breeding and nonbreeding males did not. Moreover, virgin and nonbreeding males showed significant behavioral changes in response to predator urine, whereas breeding males did not. These results suggest that adrenocortical responses to a repeated stressor in male California mice may be modulated by cohabitation with a female, whereas behavioral responses to stress may be blunted by parental status.

Immune challenge activates neural inputs to the ventrolateral bed nucleus of the stria terminalis

Hypothalamo-pituitary-adrenal (HPA) axis activation in response to infection is an important mechanism by which the nervous system can suppress inflammation. HPA output is controlled by the hypothalamic paraventricular nucleus (PVN). Previously, we determined that noradrenergic inputs to the PVN contribute to, but do not entirely account for, the ability of bacterial endotoxin (i.e., lipopolysacharide, LPS) to activate the HPA axis. The present study investigated LPS-induced recruitment of neural inputs to the ventrolateral bed nucleus of the stria terminalis (vlBNST). GABAergic projections from the vlBNST inhibit PVN neurons at the apex of the HPA axis; thus, we hypothesize that LPS treatment activates inhibitory inputs to the vlBNST to thereby "disinhibit" the PVN and increase HPA output. To test this hypothesis, retrograde neural tracer was iontophoretically delivered into the vlBNST of adult male rats to retrogradely label central sources of axonal input. After one week, rats were injected i.p. with either LPS (200 μg/kg BW) or saline vehicle, and then perfused with fixative 2.5h later. Brains were processed for immunohistochemical localization of retrograde tracer and the immediate-early gene product, Fos (a marker of neural activation). Brain regions that provide inhibitory input to the vlBNST (e.g., caudal nucleus of the solitary tract, central amygdala, dorsolateral BNST) were preferentially activated by LPS, whereas sources of excitatory input (e.g., paraventricular thalamus, medial prefrontal cortex) were not activated or were activated less robustly. These results suggest that LPS treatment recruits central neural systems that actively suppress vlBNST neural activity, thereby removing a potent source of inhibitory control over the HPA axis.

The role of the bed nucleus of the stria terminalis in stress-induced inhibition of pulsatile luteinising hormone secretion in the female rat

The bed nucleus of the stria terminalis (BNST) occupies a central position in the neural circuitry regulating the hypothalamic-pituitary-adrenocortical axis response to stress. The potential role of the BNST in stress-induced suppression of the gondotrophin-releasing hormone (GnRH) pulse generator, the central regulator of the reproductive system, was assessed by examining the effects of micro-infusion of corticotrophin-releasing factor (CRF) or its antagonist into the BNST on pulsatile luteinising hormone (LH) secretion or stress-induced inhibition of LH pulses, respectively. Ovariectomised oestrogen-treated rats were implanted chronically with bilateral cannulae in the dorsolateral BNST and i.v. catheters. CRF (25, 50 or 100 pmol in 200 nl of artificial cerebrospinal fluid) administered bilaterally into the BNST resulted in a dose-dependent decrease in LH pulse frequency, and induced Fos expression in glutamic acid decarboxylase immunostained neurones in the medial preoptic area. These results suggest that the activation of hypothalamic GABAergic neurones in response to intra-BNST administration of CRF may be involved in the suppression of LH pulses. Furthermore, administration of CRF antagonist (280 pmol astressin-B, three times at 20-min intervals) into the BNST effectively blocked the suppression of pulsatile LH secretion in response to restraint (1 h) but not hypoglycaemic (0.25 U insulin/kg, i.v.) stress. These data suggest that CRF innervation of the dorsolateral BNST plays a key, but differential, role in stress-induced suppression of the GnRH pulse generator.

Pubertal maturation and programming of hypothalamic-pituitary-adrenal reactivity

Modifications in neuroendocrine function are a hallmark of pubertal development. These changes have many short- and long-term implications for the physiological and neurobehavioral function of an individual. The purpose of the present review is to discuss our current understanding of how pubertal development and stress interact to affect the hypothalamic-pituitary-adrenal (HPA) axis, the major neuroendocrine axis that controls the hormonal stress response. A growing body of literature indicates that puberty is marked by dramatic transitions in stress reactivity. Moreover, recent studies indicate that exposure to stressors during pubertal maturation may result in enduring changes in HPA responsiveness in adulthood. As puberty is marked by a substantial increase in many stress-related psychological and physiological disorders (e.g., depression, anxiety, drug abuse), it is essential to understand the factors that regulate and modulate HPA function during this crucial period of development.

A comparison of two repeated restraint stress paradigms on hypothalamic-pituitary-adrenal axis habituation, gonadal status and central neuropeptide expression in adult male rats

The available evidence continues to illustrate an inhibitory influence of male gonadal activity on the hypothalamic-pituitary-adrenal (HPA) axis under acute stress. However, far less is known about how these systems interact during repeated stress. Because HPA output consistently declines across studies examining repeated restraint, the potential mechanisms mediating this habituation are often inferred as being equivalent, even though these studies use a spectrum of restraint durations and exposures. To test this generalisation, as well as to emphasise a potential influence of the male gonadal axis on the process of HPA habituation, we compared the effects of two commonly used paradigms of repeated restraint in the rodent: ten daily episodes of 0.5 h of restraint and five daily episodes of 3 h of restraint. Both paradigms produced comparable declines in adrenocorticotrophic hormone and corticosterone between the first and last day of testing. However, marked differences in testosterone levels, as well as corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) expression, occurred between the two stress groups. Plasma testosterone levels remained relatively higher in animals exposed to 0.5 h of restraint compared to 3 h of restraint, whereas forebrain gonadotrophin-releasing hormone (GnRH) cell counts increased in both groups. AVP mRNA was increased after 3 h, but not after 0.5 h of repeated restraint, in the medial parvicellular paraventricular nucleus and in the posterior bed nucleus of the stria terminalis (BST), and increased with 0.5 h of repeated restraint in the medial amygdala. CRH mRNA was increased after 3 h, but not after 0.5 h of repeated restraint, in the central amygdala and anterior BST. The data obtained illustrate that, despite comparable declines in HPA responses, the pathways recruited for stress adaptation appear to be distinct between restraint groups. Given the extreme sensitivity of limbic AVP to testosterone, and conversely CRH to circulating glucocorticoids, whether differences in endocrine profiles might explain these neuropeptide differences remains to be seen. Nonetheless, the present study provides several new entry points for testing gonadal influences on stress-specific HPA habituation.

Effects of exposure period and dose of diethylstilbestrol on pregnancy in rats

The aim of this study was to investigate the effect of exposure period and dose of diethylstilbestrol (DES), which has strong estrogenic activity, on pregnancy in rats. All rats with observed vaginal plugs or sperm in vaginal smear tests after mating were divided into 3 groups: those fed a normal diet, a diet mixed with DES throughout pregnancy and a diet mixed with DES from day 13 of pregnancy. DES was mixed into the diet at 0.1, 1, 10 and 100 ppm. All bred rats fed the normal diet and 0.1 ppm DES from day 13 of pregnancy delivered pups, while none of the rats treated with 1-100 ppm DES during pregnancy and 100 ppm DES from day 13 of pregnancy delivered any pups. The number of male and female pups born decreased in rats treated with 10 ppm DES from day 13 of pregnancy. These results suggest that DES could affect pregnancy and that the exposure period and dose may result in sterility, abortion, poor fetal growth and reduced number of pups born.

Neural regulation of endocrine and autonomic stress responses

The survival and well-being of all species requires appropriate physiological responses to environmental and homeostatic challenges. The re- establishment and maintenance of homeostasis entails the coordinated activation and control of neuroendocrine and autonomic stress systems. These collective stress responses are mediated by largely overlapping circuits in the limbic forebrain, the hypothalamus and the brainstem, so that the respective contributions of the neuroendocrine and autonomic systems are tuned in accordance with stressor modality and intensity. Limbic regions that are responsible for regulating stress responses intersect with circuits that are responsible for memory and reward, providing a means to tailor the stress response with respect to prior experience and anticipated outcomes.

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.

Buspirone induced acute and chronic changes of neural activation in the periaqueductal gray of rats

5-HT(1A) modulation within the midbrain periaqueductal gray (PAG) is closely associated with anxiety- or panic-like behavior. Several findings have demonstrated that the properties of buspirone (a 5-HT(1A) partial agonist) would function as either anxiolytic or panicolytic in both clinical and laboratory animal research. In this study, we have investigated the neuronal activity occurring within the different regions of the PAG induced by buspirone treatment. Twenty-eight albino Wistar rats (350-400 g) were injected with either acute or chronic saline/buspirone (each, n=7), respectively. Our results show that buspirone treatment reduced locomotor activity, body weight and fecal boli, particularly in the chronic buspirone group. Two-way ANOVA revealed a significant decrease of c-Fos-immunoreactive (ir) cells expression in all regions of the rostral PAG after both acute and chronic buspirone (acute buspirone (AB) and chronic buspirone (CB), respectively) treatment. However, no effects on c-Fos-ir were detected in the caudal lateral periaqueductal gray (lPAG) and ventrolateral periaqueductal gray (vlPAG) in both the AB and CB groups, and in the dorsolateral periaqueductal gray (dlPAG) of the CB group. Interestingly, c-Fos-ir cells in the dorsomedial periaqueductal gray (dmPAG) column were reduced consistently in both the rostral and caudal PAG in both AB and CB groups. Besides, in all regions the number of c-Fos-ir cells was higher in the AB than in the CB group with exception of the rostral lPAG. In conclusion, the main anxiolytic effect of buspirone was specifically localized in all regions of the rostral PAG and in the caudal dmPAG. However, the caudal dlPAG, lPAG and vlPAG were found to be ineffective to buspirone treatment, probably due to their distinctive function in mediating higher level of anxiety in defensive behavior. This indicates that the longitudinal anatomical structure of the PAG possesses a different level of receptor sensitivity of 5-HT(1A) in the pathophysiology of anxiety and panic disorder.

Acute stress differentially affects corticotropin-releasing hormone mRNA expression in the central amygdala of the "depressed" flinders sensitive line and the control flinders resistant line rats

Preclinical and clinical evidence suggests that neuropeptides play a role in the pathophysiology of mood disorders. In the present study, we investigated the involvement of the peptides corticotropin-releasing hormone (CRH), neuropeptide Y (NPY) and nociceptin/orphanin FQ (N/OFQ) and of their receptors in the regulation of emotional behaviours. In situ hybridization experiments were performed in order to evaluate the mRNA expression levels of these neuropeptidergic systems in limbic and limbic-related brain regions of the Flinders Sensitive Line (FSL) rats, a putative genetic animal model of depression. The FSL and their controls, the Flinders Resistant Line (FRL) rats, were subjected to one hour acute restraint and the effects of the stress exposure, including possible strain specific changes on these neuropeptidergic systems, were studied. In basal conditions, no significant differences between FSL and FRL rats in the CRH mRNA expression were found, however an upregulation of the CRH mRNA hybridization signal was detected in the central amygdala of the stressed FRL, compared to the non stressed FRL rats, but not in the FSL, suggesting a hypoactive mechanism of response to stressful stimuli in the "depressed" FSL rats. Baseline levels of NPY and N/OFQ mRNA were lower in the FSL rats compared to the FRL in the dentate gyrus of hippocampus and in the medial amygdala, respectively. However, the exposure to stress induced a significant upregulation of the N/OFQ mRNA levels in the paraventricular thalamic nucleus, while in the same nucleus the N/OFQ receptor mRNA expression was higher in the FSL rats. In conclusion, selective alterations of the NPY and N/OFQ mRNA in limbic and limbic-related regions of the FSL rats, a putative animal model of depression, provide further support for the involvement of these neuropeptides in depressive disorders. Moreover, the lack of CRH activation following stress in the "depressed" FSL rats suggests a form of allostatic load, that could alter their interpretation of environmental stimuli and influence their behavioural response to stressful situations.

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.

Recurrent intermittent restraint delays fed and fasting hyperglycemia and improves glucose return to baseline levels during glucose tolerance tests in the Zucker diabetic fatty rat--role of food intake and corticosterone

Short-term elevations of stress hormones cause an increase in glycemia. However, the effect of intermittent stress on development of type 2 diabetes mellitus is unclear. We hypothesized that recurrent intermittent restraint stress would deteriorate glycemia. Male, prediabetic Zucker diabetic fatty (ZDF) rats were restrained 1 hour per day, 5 days per week for 13 weeks and compared with unstressed, age-matched diabetic controls and lean nondiabetic rats. To differentiate the effects of recurrent restraint stress per se vs restraint-induced inhibition of food intake, a pair-fed group of rats was included. Surprisingly, recurrent restraint and pair feeding delayed fed and fasting hyperglycemia, such that they were lowered 50% by restraint and 30% by pair feeding after 13 weeks. Rats that were previously restrained or pair fed had lower glucose levels during a glucose tolerance test, but restraint further improved the return of glucose to baseline compared to pair feeding (P<.05). This was despite pair-fed rats having slightly lowered food intake and body weights compared with restrained rats. Restraint and pair feeding did not alter insulin responses to an intraperitoneal glucose tolerance test (IPGTT) or fasting insulin, and did not lower plasma lipids. Interestingly, restraint normalized basal corticosterone to one third that in control and pair-fed rats, prevented increases in pretreatment corticosterone seen with pair feeding, and led to habituation of restraint-induced corticosterone responses. After 13 weeks of treatment, multiple regression analysis showed that elevations in basal corticosterone could explain approximately 20% of the variance in fed glucose levels. In summary, intermittent restraint and its adaptations delayed hyperglycemia and improved glucose control in Zucker diabetic fatty rats. These benefits can be partially explained by restraint-induced lowering of food intake, but additional improvements compared to pair feeding may involve lower overall corticosterone exposure with repeated restraint. Paradoxically, these novel investigations suggest some types of occasional stress may limit development of diabetes.

Adaptation in the corticosterone and hyperthermic responses to stress following repeated stressor exposure

Previous studies have shown that repeated daily exposure to the same (homotypic) stressor results in habituation of the corticosterone (CORT) response. Others have found that the stress response to a more ethologically relevant stressor, social defeat, does not habituate and, in some cases, sensitisation has been observed. Similar observations have been noted when core temperature is examined. Although habituation and/or sensitisation have been reported during stressor exposure, little is known about the development of an anticipatory fever in response to daily stressor exposure. The aim of the present study was to compare systematically commonly used laboratory stressors (i.e. restraint, cage confinement and social defeat) using a common set of procedures and analyses. Specifically, we examined: (i) the development of an anticipatory fever to repeated (5 days) homotypic stressor exposure; (ii) the adaptation of the fever response during stressor exposure; and (iii) the resolution of the fever response to stressors presented at the same time each day. For comparison, adaptation of the CORT response was also examined to assess the degree to which habituation to repeated stressor exposure may represent a more general response observed across diverse physiological measures. Habituation was observed after restraint and cage confinement, but not observed in either the CORT or hyperthermic responses to repeated social defeat. Furthermore, no anticipatory fever response was observed with repeated exposure to restraint, cage confinement, or social defeat. These data suggest that habituation to repeated stressor exposure may not occur with all homotypic stressor paradigms. In addition, rats do not appear to entrain an anticipatory fever response to a stressor presented at the same time each day, at least not within 5-6 days of repeated exposure.

Social instability in adolescence alters the central and peripheral hypothalamic-pituitary-adrenal responses to a repeated homotypic stressor in male and female rats

There has been little research on effects of chronic stressors on neuroendocrine function in adolescence despite increasing evidence of enduring effects of stressors during this period on behaviour in adulthood. We previously reported that social stress (SS: daily 1 h isolation and new cage partner for 16 days) in adolescence altered locomotor responses to psychostimulants in adulthood. Here, we investigated neuroendocrine responses over the duration of the procedure that may underlie the enduring effects of SS. SS rats were compared to rats undergoing daily isolation only (ISO) and controls (CTL) to determine responses to acute and repeated isolation with and without social instability. At 30 days of age (first isolation), higher plasma corticosterone and corticotrophin-releasing hormone (CRH) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus and in the central nucleus of the amygdala (CeA) were found in males caged with a new partner (SS) after isolation than those returned to their original partner (ISO). On day 45, SS males and females showed less habituation (higher bioactive levels of corticosterone based on plasma corticosterone and corticosteroid binding globulin levels) to the 16th episode of isolation than did ISO. SS and ISO had higher baseline expression of CRH mRNA in the PVN on day 45 than did CTL, and only CTL had increased levels after isolation. CRH mRNA expression in the CeA increased to a first isolation in CTL and to a 16th isolation in SS but not in ISO males. Modest differences in social interactions were observed between SS and ISO when returned to their cages after isolation. The results suggest that mild social stressors in adolescence impede neuroendocrine adaptation to homotypic stressors. The resultant increase in exposure to glucocorticoids over adolescence may alter ongoing brain development and increase vulnerability to psychopathology.

Predator threat induces behavioral inhibition, pituitary-adrenal activation and changes in amygdala CRF-binding protein gene expression

Behavioral inhibition (BI) is an adaptive defensive response to threat; however, extreme BI is associated with anxiety-related psychopathology. When rats are exposed to a natural predator they display stress- and anxiety-related behavioral alterations and physiological activation. To develop a preclinical rodent model to study mechanisms underlying human BI and anxiety, we examined the extent to which ferret exposure elicits anxiety-related BI and HPA and amygdala activation of the CRF system. In the first experiment, BI and other behaviors were assessed in the presence or absence of a ferret. In the second experiment, ferret-induced corticosterone release and changes in brain c-fos expression were assessed. In the final experiment, gene chip and quantitative real time-PCR analyses were performed on amygdala tissue from control and ferret-exposed rats. Ferret exposure increased BI and submissive posturing, as well as plasma corticosterone and the number of Fos-positive cells in several brain regions including the amygdala. Gene expression analysis revealed increased amygdalar mRNA for CRF-binding protein, but not the CRF1 receptor, CRF2 receptor or CRF. In rodents, ferret exposure can be used to elicit anxiety-related BI, which is associated with HPA and amygdala activation. Since the amygdala and the CRF system have been implicated in adaptive and maladaptive anxiety responses in humans, these data support use of our rodent model to further investigate mechanisms underlying anxiety-related psychopathology in humans.

Glucocorticoids, chronic stress, and obesity

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

Innervation of the paraventricular nucleus of the thalamus from cocaine- and amphetamine-regulated transcript (CART) containing neurons of the hypothalamus

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus with heavy projections to the nucleus accumbens and other limbic regions. Previous studies have shown that the PVT contains fibers immunoreactive for cocaine- and amphetamine-related transcript (CART). The purpose of the present study was to determine the location of CART neurons innervating the PVT of the rat by using retrograde tracing with cholera toxin B (CTb) combined with immunofluorescence for CTb and CART (amino acid sequence 55-102). Immunohistochemical analysis of CART in the dorsal thalamus showed that the PVT is densely innervated by CART fibers whereas adjacent midline and intralaminar thalamic nuclei are unlabeled. Injections of CTb in the dorsal midline thalamus retrogradely labeled neurons in several areas of the hypothalamus and brainstem which also contained CART neurons. The largest number of double-labeled neurons (CTb/CART) was found in the arcuate nucleus of the hypothalamus. CTb/CART neurons were also found in the lateral hypothalamus, zona incerta, and periventricular hypothalamus. These results indicate that the arcuate nucleus is a major source of CART fibers in the PVT. CART neurons in the arcuate nucleus monitor circulating hormonal signals and may regulate food intake and hypothalamic-pituitary-adrenal (HPA) activity. Consequently, CART neurons in the arcuate nucleus may transmit signals to the PVT which in turn may influence limbic regions involved in regulating food intake and the HPA.

Heterogeneity in low voltage-activated Ca2+channel-evoked Ca2+responses within neurons of the thalamic paraventricular nucleus

Low voltage-activated Ca2+ channels (LVA or T-type Ca2+ channels) are crucial to burst firing and oscillations in thalamocortical relay cells and are exhibited by neurons in the paraventricular nucleus of thalamus (PVT), a dorsal midline nucleus deemed important in the neural representation of motivational behaviours. We used a functional approach (whole-cell patch-clamp electrophysiology combined with confocal laser scanning microscopy) to analyse the spatial distribution of LVA Ca2+ channel-evoked Ca2+ transients in PVT neurons. We observed that the magnitude of LVA Ca2+ channel-evoked Ca2+ transients was significantly greater in proximal dendrites (located up to 20 microm from the soma) than in the soma. In addition, the magnitudes of these Ca2+ transients varied significantly not only among different dendrites of the same cell but also within individual dendrites. These findings suggest that LVA Ca2+ channels are expressed (i) predominantly on the proximal dendrites and (ii) heterogeneously within individual dendrites of PVT neurons. The spatial characteristics of dendritic LVA Ca2+ channels in PVT neurons suggest that these channels may regulate burst firing by modulating dendritic afferent inputs.

Habituation to repeated restraint stress is associated with lack of stress-induced c-fos expression in primary sensory processing areas of the rat brain

Rats repeatedly exposed to restraint show a reduced hypothalamic-pituitary-adrenal axis response upon restraint re-exposure. This hypothalamic-pituitary-adrenal axis response habituation to restraint does not generalize to other novel stressors and is associated with a decrease in stress-induced c-fos expression in a number of stress-reactive brain regions. We examined whether habituation to repeated restraint is also associated with adaptation of immediate early gene expression in brain regions that process and relay primary sensory information. These brain regions may not be expected to show gene expression adaptation to repeated restraint because of their necessary role in experience discrimination. Rats were divided into a repeated restraint group (five 1-hour daily restraint sessions) and an unstressed group (restraint naïve). On the sixth day rats from each group were either killed with no additional stress experience or at 15, 30 or 60 min during restraint. Immediate early gene expression (corticotrophin-releasing hormone heteronuclear RNA, c-fos mRNA, zif268 mRNA) was determined by in situ hybridization. A reduction in stress-induced hypothalamic-pituitary-adrenal axis hormone secretion (plasma corticosterone and adrenocorticotropic hormone) and immediate early gene expression levels in the paraventricular nucleus of the hypothalamus, the lateral septum and the orbital cortex was observed in repeated restraint as compared with restraint naïve animals. This reduction was already evident at 15 min of restraint. Unexpectedly, we also found in repeated restraint rats a reduction in restraint-induced c-fos expression in primary sensory-processing brain areas (primary somatosensory cortex, and ventroposteriomedial and dorsolateral geniculate nuclei of thalamus). The overall levels of hippocampal mineralocorticoid receptor heteronuclear RNA or glucocorticoid receptor mRNA were not decreased by repeated restraint, as may occur in response to severe chronic stress. We propose that repeated restraint leads to a systems-level adaptation whereby re-exposure to restraint elicits a rapid inhibitory modulation of primary sensory processing (i.e. sensory gating), thereby producing a widespread attenuation of the neural response to restraint.

The paraventricular nucleus of the thalamus as an interface between the orexin and CART peptides and the shell of the nucleus accumbens

The paraventricular nucleus of the thalamus (PVT) receives afferents from the brainstem and has been thought to relay arousal related information to specific limbic forebrain areas, including the nucleus accumbens. More recent anatomical observations suggest that the PVT also receives afferents from various hypothalamic nuclei. The present anatomical experiments investigated the innervation of the PVT by fibers immunoreactive for orexin and cocaine and amphetamine related transcript (CART), two feeding-related peptides highly concentrated in the hypothalamus. Emphasis was placed on identifying the relationship between these neuropeptides and PVT neurons projecting to the shell of the nucleus accumbens (NacSh). Infusion of a retrograde tracer into the NacSh labeled numerous cells of the midline and intralaminar thalamus, most of which were restricted to the PVT. The retrograde tracer, orexin fibers, and CART fibers were immunopositive throughout the entire PVT whereas no overlap between signals was evident within adjacent thalamic regions. High-magnification light and confocal microscopy showed that both orexin and CART fibers made frequent contact with retrogradely labeled neurons throughout the anteroposterior PVT. Furthermore, single PVT cells retrogradely labeled from the NacSh were apposed by both orexin and CART fibers. The present experiments provide the first evidence suggesting a role for the PVT as a relay of hypothalamic activity to the nucleus accumbens. The PVT may function to link visceral arousal signals with limbic regions involved in behavioral responses.

Toluene inhalation-induced adrenocortical hypertrophy and endocrinological changes in rat

Rats were exposed to toluene (1,500 ppm for 4 hr per day) for 7 days. The body weight of the rats was significantly lower and the weight of the adrenal gland was significantly higher in the toluene inhalation group compared to the controls. Microscopically, there was no obvious change in the medulla, but hypertrophy of the cortex was observed in the toluene inhalation group. And, the size of adrenocortical cells in treated-rats was also significantly enlarged than the control. Immunohistochemical staining did not show a clear difference in localization of aldosterone-positive cells between the control and inhalation groups. Expansion of the corticosterone-positive area consistent with the cortical hypertrophy was recognized in the inhalation group. Enhancement of 72 kD-heat-shock protein (HSP70)-expression in the toluene inhalation group was not observed. Neither stress nor damage to cortical cells due directly to toluene exposure was observed in the cortex. Also, there was no obvious difference in the anti-proliferating cell nucleus antigen (PCNA)-immunostaining between control and inhalation groups. Thus, it is suspected that cortical hypertrophy was the result of cell enlargement due to the stimulation of the cortical cells. Corticotropin-releasing factor (CRF) immunoreactivity in the paraventricular nucleus (PVN) was increased in the inhalation group. Concentration of plasma ACTH was elevated significantly by toluene exposure. The amounts of mRNA of adrenocortical steroid metabolism gene, cytochrome side-chain cleavage (P450scc), was also increased by toluene inhalation. Toluene exposure might induce adrenocortical hypertrophy via the hypothalamus-pituitary-adrenal gland (HPA) axis.

Analysis of the stress response in rats trained in the water-maze: differential expression of corticotropin-releasing hormone, CRH-R1, glucocorticoid receptors and brain-derived neurotrophic factor in limbic regions

Glucocorticoids and corticotropin-releasing hormone (CRH) are key regulators of stress responses. Different types of stress activate the CRH system; in hypothalamus, CRH expression and release are increased by physical or psychological stressors while in amygdala, preferentially by psychological stress. Learning and memory processes are modulated by glucocorticoids and stress at different levels. To characterize the kind of stress provoked by a hippocampal-dependent task such as spatial learning, we compared the expression profile of glucocorticoid receptor (GR), pro-CRH and CRH-R1 mRNAs (analyzed by RT-PCR), in amygdala, hippocampus and hypothalamus and quantified serum corticosterone levels by radioimmunoassay at different stages of training. mRNA levels of brain-derived neurotrophic factor (BDNF) were also quantified due to its prominent role in learning and memory processes. Male Wistar rats trained for 1, 3 or 5 days in the Morris water-maze (10 trials/day) were sacrificed 5-60 min the after last trial. A strong stress response occurred at day one in both yoked and trained animals (increased corticosterone and hypothalamic pro-CRH and CRH-R1 mRNA levels); changes gradually diminished as the test progressed. In amygdala, pro-CRH mRNA levels decreased while those of BDNF augmented when stress was highest, in yoked and trained animals. Hippocampi, of both yoked and trained groups, had decreased levels of GR mRNA on days 1 and 3, normalizing by day 5, while those of pro-CRH and CRH-R1 increased after the 3rd day. Increased gene expression, specifically due to spatial learning, occurred only for hippocampal BDNF since day 3. These results show that the Morris water-maze paradigm induces a strong stress response that is gradually attenuated. Inhibition of CRH expression in amygdala suggests that the stress inflicted is of physical but not of psychological nature and could lead to reduced fear or anxiety.

Sex and repeated restraint stress interact to affect cat odor-induced defensive behavior in adult rats

The overall objective of the present experiment was to assess sex differences in the effects of repeated restraint stress on fear-induced defensive behavior and general emotional behavior. Groups of male and female Long-Evans rats received either daily restraint stress (stressed) or daily brief handling (nonstressed) for 21 consecutive days. On days 22-25, a number of behavioral tests were administered concluding with a test of defensive behavior in response to a predatory odor. Stressed and nonstressed males and females were exposed to a piece of cat collar previously worn by a female domestic cat (cat odor) or a piece of collar never worn by a cat (control odor) in a familiar open field containing a hide barrier. Rats displayed pronounced defensive behavior (increased hiding and risk assessment) and decreased nondefensive behavior (grooming, rearing) in response to the cat odor. Nonstressed females exposed to cat odor displayed less risk assessment behavior relative to nonstressed males exposed to cat odor. Restraint stress had little effect on defensive behavior in male rats but significantly increased risk assessment behaviors in females. Behavior on the Porsolt forced swim test (a measure of depression-like behavior) and the open field test (a measure of anxiety-like behavior) was not affected by stress or sex. These findings indicate the utility of the predator odor paradigm in detecting subtle shifts in naturally occurring anxiety-like behaviors that may occur differentially in males and females.

Effects of repeated restraint stress on hypothalamo-pituitary-adrenocortical function in vasopressin deficient Brattleboro rats

Arginine-vasopressin (AVP) has been proposed to be an important mediator during chronic stress in the regulation of the hypothalamo-pituitary-adrenal axis. In the present study we addressed the role of AVP in maintaining adrenocortical responsiveness during chronic stress using the AVP deficient mutant Brattleboro rat. Heterozygous Brattleboro rats (di/+) served as controls and were compared to homozygous rats (di/di) with diabetes insipidus. Sixty minutes daily restraint was repeated for 5, 8, 11 or 15 days and organ weights, plasma adrenocorticotropin (ACTH) and corticosterone levels and anterior pituitary proopiomelanocortin (POMC) mRNA and ACTH content were measured. The body, adrenal and thymus weight changes induced by chronic stress became significant between 5 and 8 repetition and AVP deficiency had no effect on these parameters. The first indication that AVP has a role to play appears after 11 repetitions. In the di/di group at the end of 11th restraint, the plasma ACTH was decreased when compared to the di/+ rats. In animals with indwelling cannulas some adaptation could be seen in ACTH response without any difference between di/+ and di/di rats after 15 restraints. The corticosterone- and prolactin-elevations induced by restraint did not habituate in the di/+ and the di/di rats. Chronic stress increased POMC mRNA in the anterior pituitary similarly in di/+ and di/di rats. Although AVP seems to be necessary for a full ACTH response, most of the other signs of chronic stress after repeated restraint occur unchanged in the absence of AVP in both genders. This suggests that either AVP is not indispensable for activating the hypothalamo-pituitary-adrenocortical system by chronic stress or the absence of AVP is compensated by other mediators in Brattleboro rats.

Thalamic paraventricular nucleus lesions facilitate central amygdala neuronal responses to acute psychological stress

The thalamic paraventricular nucleus (PVT) is activated by stress and projects to forebrain structures directly implicated in processing stress-related information. Accordingly, it seems likely the PVT plays an important role in modulating stress responses. We examined effects of excitotoxic PVT lesions on forebrain Fos expression patterns normally elicited by an acute psychological stressor. PVT lesions significantly increased stress-induced Fos in a key stress-processing region, the central amygdala.

Role of catecholaminergic inputs to the medial prefrontal cortex in local and subcortical expression of Fos after psychological stress

A wide variety of stressors elicit Fos expression in the medial prefrontal cortex (mPFC). No direct attempts, however, have been made to determine the role of the inputs that drive this response. We examined the effects of lesions of mPFC catecholamine terminals on local expression of Fos after exposure to air puff, a stimulus that in the rat acts as an acute psychological stressor. We also examined the effects of these lesions on Fos expression in a variety of subcortical neuronal populations implicated in the control of adrenocortical activation, one classic hallmark of the stress response. Lesions of the mPFC that were restricted to dopaminergic terminals significantly reduced numbers of Fos-immunoreactive (Fos-IR) cells seen in the mPFC after air puff, but had no significant effect on stress-induced Fos expression in the subcortical structures examined. Lesions of the mPFC that affected both dopaminergic and noradrenergic terminals also reduced numbers of Fos-IR cells observed in the mPFC after air puff. Additionally, these lesions resulted in a significant reduction in stress-induced Fos-IR in the ventral bed nucleus of the stria terminalis. These results demonstrate a role for catecholaminergic inputs to the mPFC, in the generation of both local and subcortical responses to psychological stress.