Pituitary-adrenal axis and behavioural responses of maternally deprived juvenile rats to the open field

Adult rats submitted to maternal deprivation (DEP) on post-natal day (pnd) 11 show smaller corticosterone (CORT) response to a saline injection than non-deprived (NDEP) rats, mainly at 30 days of age. In the present study we sought to investigate the pituitary-adrenal axis response of 30-day-old DEP rats to a 5-min open-field session, with lights and sound on. Hormone levels were assessed immediately or 20 min after the end of the stress, and were compared to basal levels. The immediate ACTH response of DEP females was the highest; this difference was no longer observed at 20 min, but hormone levels were still higher than basal. Both males and females showed an augmented CORT secretion immediately after the open field session, although only the response of NDEP females was higher than that of male counterparts. Moreover, DEP females showed a lower CORT response than their NDEP counterparts immediately after the stress. The CORT stress response remained equally elevated in males and females at the 20-min time point. Finally, DEP animals, regardless of the gender, ambulated more in the centre of the open field and displayed less grooming behaviour than NDEP pups, suggesting that DEP rats are less emotional than NDEP animals.     

Corticosterone release in response to repeated, short episodes of neonatal isolation : evidence of sensitization

Repeated isolation of neonatal rats produces persistent changes in physiology and behavior. In Experiment 1, we examined changes in plasma corticosterone (CORT) levels as a possible mechanism for the effects of isolation. Pups that were isolated from their mother and the nest for 1 h per day on postnatal days (PND) 2–9 were compared to control litters of pups that were either nonhandled or handled but not isolated. On PND 2, compared to nonhandled pups, handled pups had elevated CORT levels that returned to baseline levels within 30 to 60 min of return to the home cage. No significant elevation of CORT levels were found in handled pups on PND 9. The CORT levels of isolated pups were over twice those of nonhandled pups on PND 2 and four times those of nonhandled pups on PND 9. In Experiment 2, we investigated whether the increased CORT release in response to isolation on PND 9 was the result of the pups treatment on the previous six days as against an effect of maturation. Plasma CORT levels were measured in rat pups that were either isolated, handled or nonhandled on PNDs 2–8 during the conditions of isolation, handling and nonhandling on PND 9. There were no differences among the groups in basal plasma levels of CORT. Handling on PND 9 did not result in elevated CORT levels in any of the groups. All three groups showed a significant increase in plasma CORT levels after isolation on PND 9. However, the CORT response to isolation of pups previously isolated on PND 2–8 were significantly higher than pups that were either handled or nonhandled on PNDs 2–8. Thus, daily episodes of isolation potentiate the hypothalamic-pituitary-adrenal response to stress.     

Long-Term Selective Down-Regulation of Brain Glucocorticoid Receptors after Neonatal Dexamethasone Treatment in Rats

The development of corticosteroid receptors may be permanently modified by perinatal hormonal influences. Changes in binding characteristics of corticosteroid receptors were examined in rats treated sc. with 1  μg/g dexamethasone (DEX) on postnatal day (PND) 1 and subsequently two more times on PND 3 and 5 in several brain areas. [³H]Corticosterone (CORT) binding capacity (B_max) and affinity (K_d) were determined at 3 weeks old and adult ages by using saturation analysis. The mineralocorticoid type receptor (MR) and the glucocorticoid receptor (GR) sites were measured separately with single point analysis applying a selective glucocorticoid ligand RU 28362 saturating GR. The decrease in CORT binding was due to a selective GR decrement in all structures and the MR concentration was not changed considerably. The basal levels of plasma CORT were not permanently influenced by neonatal DEX. In conclusion, DEX given during the first week of life resulted in long-term and selective down-regulation of GR and this decrement was independent of the actual circulating CORT level.     

Blunted pituitary-adrenocortical stress response in adult rats following neonatal dexamethasone treatment

Glucocorticoids have a prominent impact on the maturation of the stress-related neuroendocrine system and on the postnatal establishment of adaptive behaviour. The present study aimed at investigating the stress responsiveness of the hypothalamo-pituitary-adrenocortical (HPA) axis in young and adult rats after neonatal treatment with the synthetic glucocorticoid agonist, dexamethasone. Newborn male Wistar rats were injected s.c. with 1 microg/g dexamethasone on postnatal days 1, 3 and 5. Circulating adrenocorticotropic hormone (ACTH) and corticosterone concentrations were measured in the resting state and following a 30-min cold stress at the age of 10 days, as well as after a 30-min restraint stress at the age of 14 weeks. Also in adults, pituitary and adrenocortical hormone responsiveness was evaluated after i.v. administration of 2 microg/kg corticotropin releasing hormone (CRH). In addition, glucocorticoid (GR) and mineralocorticoid receptor (MR) binding capacities were assessed in the pituitaries of adult rats. The results showed that at day 10 basal ACTH concentration was elevated while the cold stress-evoked ACTH response was attenuated in the dexamethasone-treated rats. As adults, treated rats showed a suppressed elevation of both ACTH and corticosterone plasma concentrations in response to restraint, while basal hormonal concentrations were not altered. There was no difference in the magnitude of the CRH-induced elevation of ACTH and corticosterone concentrations initially; however, the dexamethasone-treated animals showed a prolonged secretion of both hormones. These animals also showed a selective decrease in pituitary GR binding capacity. Neonatal dexamethasone treatment strongly suppressed body weight gain, and adrenal and thymus weights in the early phase of postnatal development. By adulthood, the body and adrenal weights were normalized while thymus weight was greater than in controls. These findings indicate that neonatal dexamethasone treatment permanently alters HPA axis activity by reducing stress responses to cold and restraint probably through supra-pituitary actions, and by decreasing the effectiveness of feedback through a diminished GR binding in the pituitary.     

Combined pre- and postnatal environmental enrichment programs the HPA axis differentially in male and female rats

Experimental environmental enrichment (EE) is usually applied in adulthood or immediately after weaning, with robust effects on physiology and behaviour. To investigate the effects of EE earlier in life, female rats were maintained under moderate enrichment during pregnancy and, together with their pups, during lactation until weaning. A separate group of dams housed under standard conditions during pregnancy and lactation served as controls. Dams housed under EE exhibited fewer nursing episodes and were off the nest more often, but the frequency of pup licking was not affected on postnatal days 3-5. EE effects on hypothalamus-pituitary-adrenal (HPA) axis responses to an acute stressor were determined in adult male and female offspring with and without previous exposure to the chronic stressor of constant light. In female offspring, chronic stress significantly increased basal corticosterone (CORT) levels, but not if rats had been exposed to early EE. Furthermore, while control females exposed to chronic stress showed a greatly reduced adrenocorticotropin (ACTH) response to an acute stressor, EE females did not display this desensitization. There was no significant effect of EE on basal ACTH and CORT levels in adult male offspring, nor did it alter their response to acute stress. Maternal licking frequency was moderately but significantly correlated with net corticosterone increases in response to acute stress, the direction of the correlation crucially depending on the offspring's sex and stress conditions. This study shows that EE during pregnancy and lactation has long-lasting effects on reactivity to acute and chronic stress in offspring and that these effects are dependent on the offspring's sex but not greatly on early postpartum maternal behaviour.     

Consequences of repeated early isolation in domestic piglets (Sus scrofa) on their behavioural, neuroendocrine, and immunological responses

Stress in the form of intermittent maternal deprivation and social isolation during early postnatal life in rats and monkeys produces persistent changes in physiology and behaviour. In farm animals physiological consequences of disrupting mother-infant interactions with respect to health and animal welfare are relatively unknown. Therefore, the aim of the present study was to investigate the behavioural, neuroendocrine and immunological consequences of a 2 h daily social isolation from day 3 to day 11 of age in domestic piglets as well as potential long-term effects on the brain-endocrine-immune regulation. Repeated social isolation resulted in significantly decreased open-field activity (locomotion, vocalization) during the isolation period, increased basal cortisol concentrations and decreased lymphocyte proliferation in response to concanavalin A and pokeweed mitogen one day after the isolation. There was also a significant increase of interleukin-1beta (IL-1beta) concentration in hippocampus in isolated piglets compared to controls at this time. Six weeks after isolation significant enhanced basal ACTH concentrations as well as higher IL-1beta content and glucocorticoid receptor (GR) binding in hippocampus were found. These endocrine and immune responses were associated with decreased CRH levels in the hypothalamus and increased CRH content in the amygdala. The present data indicate that early social isolation in pigs may cause changes in behavioural, neuroendocrine, and immune regulation and produce long-term effects not only on the activity of the hypothalamic-pituitary-adrenal (HPA) system, but also on the immune-brain circuitry with possible negative consequences in health and welfare of commercial pigs. Using the pig as a suitable animal model, the finding of this study may also have some implications for the etiology of anxiety and depression in humans.     

Periodic maternal separation decreases hippocampal neurogenesis without affecting basal corticosterone during the stress hyporesponsive period, but alters HPA axis and coping behavior in adulthood

Although not directly evaluated, the early rise of glucocorticoid (GC) levels, as occur after exposure to adverse early life experience, are assumed to affect hippocampal ontogeny by altering the hippocampus negative feedback on adult HPA axis. To test whether hippocampal ontogeny is affected by early exposure to stress we estimated the survival of recently formed hippocampal granule cells in rat pups subjected to periodic maternal separation (180 min/day; MS180) from postnatal days (PND) 1 to 14. Accordingly, MS180 pups injected with bromodeoxyuridine (BrdU, 50 mg/kg, ip) at PND 5 showed decreased density of doublecortin (DCX) positive BrdU-labeled cells at PND 15. MS180 and AFR pups showed similar corticosterone (CORT) basal levels between PND 3 and 12, whereas adult MS180 rats presented with higher CORT levels than AFR adults. Nonetheless, both AFR and MS180 pups and adults showed similar transient increments of CORT levels in response to stress. In addition, MS180 had no effect on the adult anxiety-like behavior evaluated in the elevated plus maze, but evoked a passive coping strategy in the forced swimming test. The data show that the decrease in hippocampal neurogenesis is an early onset phenomenon, and suggests that adverse experiences alter hippocampal ontogeny without chronic elevation of GC levels.     

Corticosterone controls the developmental emergence of fear and amygdala function to predator odors in infant rat pups

In many altricial species, fear responses such as freezing do not emerge until sometime later in development. In infant rats, fear to natural predator odors emerges around postnatal day (PN) 10 when infant rats begin walking. The behavioral emergence of fear is correlated with two physiological events: functional emergence of the amygdala and increasing corticosterone (CORT) levels. Here, we hypothesize that increasing corticosterone levels influence amygdala activity to permit the emergence of fear expression. We assessed the relationship between fear expression (immobility similar to freezing), amygdala function (c-fos) and the level of corticosterone in pups in response to presentation of novel male odor (predator), littermate odor and no odor. CORT levels were increased in PN8 pups (no fear, normally low CORT) by exogenous CORT (3 mg/kg) and decreased in PN12 pups (express fear, CORT levels higher) through adrenalectomy and CORT replacement. Results showed that PN8 expression of fear to a predator odor and basolateral/lateral amygdala activity could be prematurely evoked with exogenous CORT, while adrenalectomy in PN12 pups prevented both fear expression and amygdala activation. These results suggest that low neonatal CORT level serves to protect pups from responding to fear inducing stimuli and attenuate amygdala activation. This suggests that alteration of the neonatal CORT system by environmental insults such as alcohol, stress and illegal drugs, may also alter the neonatal fear system and its underlying neural control.     

Amplified behavioral and endocrine responses to forced swim stress in the Wistar-Kyoto rat

The Wistar Kyoto (WKY) rat may be a useful model for the study of depressive behavior because they exhibit exaggerated responses to a number of stressors. These studies compared the behavioral and endocrine responses to swimming stress in WKY rats with Sprague-Dawley (SD) rats. In the first experiment, the onset of behavioral immobility and the endocrine stress responses (adrenocorticotropin hormone (ACTH) and corticosterone (CORT)) were examined as the duration of a swimming session was increased. In the second experiment, WKY and SD rats were swum for 15 min, then sacrificed at different intervals after completion of the swim, to examine the time course of endocrine stress responses. The final experiment compared the suppression of ACTH and CORT secretion by dexamethasone of peak diurnal ACTH and CORT levels in WKY and SD rats. Behaviorally, the WKY rats displayed early and prolonged immobility compared to SD rats regardless of the length of the swim stress. Plasma CORT and ACTH increased in WKY and SD rats as the duration of the stressor lengthened. The swim stress (15 min) produced higher levels of ACTH and CORT secretion at the end of the stress interval that persisted after termination of the stressor in WKY compared to SD rats. Peak diurnal CORT levels, but not ACTH levels, were higher in WKY than in SD rats. Dexamethasone suppressed ACTH levels less in WKY than in SD rats. These results indicate that the WKY rat that displays increased behavioral immobility also demonstrates exaggerated secretion of stress hormones during swimming stress, and the results may be due, in part, to reduced sensitivity of glucocorticoid receptors that supply negative feedback to the hypothalamic-pituitary-adrenal axis. The exaggerated behavioral and endocrine stress responses in the WKY rat support its potential usefulness as a model for studying stress-evoked depressive behavior.     

Function of neuropeptide Y and agouti-related protein at weaning: relation to corticosterone, dietary carbohydrate and body weight

Neuropeptide Y (NPY) and agouti-related protein (AgRP), potent stimulants of feeding, have been linked in adult rats to both corticosterone (CORT) and dietary carbohydrate. To understand the significance of this relationship early in life, measurements were taken of these parameters at different ages around weaning, in rats given a choice of macronutrient diets or maintained on a carbohydrate-rich diet. The results demonstrate that, in both male and female rat pups, the expression and production of NPY and AgRP in the arcuate nucleus (ARC) peak on postnatal day 21 (P21), compared to P15 before weaning and P27 after weaning. These elevated levels of peptide were associated with peak levels of CORT and glucose and also a strong, natural preference for carbohydrate at weaning, which accounted for 55-65% of the pups' total diet. In subgroups defined by their body weight at these stages, rats with as little as 4% lower body weight (compared to higher weight pups) had 30-60% greater expression of NPY and AgRP in the ARC and elevated levels of CORT, with no difference in leptin or insulin. This response was significantly more pronounced at P21 than at P15 or P27. The importance of carbohydrate during this stage was suggested by additional results showing elevated NPY expression, CORT levels, body weight and inguinal fat pad weights in P27 pups raised on a 65% carbohydrate diet vs. 45% carbohydrate. These results suggest that hypothalamic NPY and AgRP, together with CORT, have glucoregulatory as well as feeding stimulatory functions that help mediate the transition from suckling of a fat-rich diet to independent feeding of a carbohydrate-rich diet. During this critical period, the carbohydrate together with the peptides and CORT provide the important signals, including elevated glucose, that promote de novo lipogenesis and enable weanling animals to survive periods of food deprivation.     

Development of glucocorticoid receptor regulation in the rat forebrain: implications for adverse effects of glucocorticoids in preterm infants

Glucocorticoids are the consensus treatment to avoid respiratory distress in preterm infants but there is accumulating evidence that these agents evoke long-term neurobehavioral deficits. Earlier, we showed that the developing rat forebrain is far more sensitive to glucocorticoid-induced disruption in the fetus than in the neonate. Feedback regulation of glucocorticoid receptors (GRs) is an essential homeostatic mechanism and we therefore examined the development of GR downregulation in the perinatal period. Pregnant rats or newborn pups were given dexamethasone daily (gestational days 17–19, postnatal days 1–3, or postnatal days 7–9), ranging from doses below that recommended for use in preterm infants (0.05 mg/kg) to therapeutic doses (0.2 or 0.8 mg/kg). Twenty-four hours after the last injection, we determined forebrain GR protein by Western blotting. Although postnatal dexamethasone treatment downregulated GRs at all doses, the fetal forebrain failed to show any decrement and instead exhibited slight GR upregulation. In controls, forebrain GR levels also showed a large increment over the course from late gestation through the second postnatal week, despite the fact that circulating glucocorticoid levels increase substantially during this period. Our results suggest that GR homeostasis develops primarily postnatally and that fetal inability to downregulate GRs in the face of exogenous glucocorticoid administration plays a role in the vulnerability of key neural circuits to developmental disruption. Since this developmental phase in the rat corresponds to the critical period in which glucocorticoids are used in preterm infants, adverse effects on brain development may be inescapable.     

Neuroendocrine responses to experimentally-induced psychological stress in healthy humans

Previous studies of hormonal and neurophysiological changes in response to psychological stress in humans have produced contrasting findings due to differing experimental procedures and consistent individual variability. Habituation effects, which influence physiological coping in response to exposure to repeated stress, need to be investigated more extensively. In the present study, twenty healthy male subjects were each exposed twice to the same psychosocial stressor (Stroop Color Word Interference task, public speaking and mental arithmetic in front of an audience) during a first session (day 1) and a second session (day 8). Plasma concentrations of norepinephrine (NE), epinephrine (EPI), adrenocorticotropic hormone (ACTH), cortisol (CORT) and prolactin (PRL) were measured immediately before the beginning of the tests and at their end, 30 min later, on both experimental days. For the total group, NE, EPI, ACTH, and CORT levels were significantly elevated, and PRL levels were significantly decreased, after stress exposure on day 1. ACTH and CORT levels showed less significant increases after stress on day 8. In contrast, NE and EPI responses to stress were not significantly blunted, and PRL response was unchanged on day 8. Cluster analysis revealed two groups of subjects who showed different habituation patterns for ACTH and CORT. The first group (n=12) of subjects showed a reduction of ACTH and CORT responses to stress on day 8. The subjects of the second group (n=8) displayed a significant increase of ACTH and cortisol in response to stress on day 8, without any habituation effect. These results increase the evidence concerning the involvement of the HPA axis and catecholamines in response to psychological stress, and suggest that possible individual differences in the neuroendocrine coping mechanisms may affect mood regulation and the state of health.     

Circadian rhythm of corticosterone secretion in the neonatal rabbit

Maternal care in the rabbit is restricted to a single 3- to 5-min period of nursing each day. One to three hours prior to the arrival of the mother, pups exhibit anticipatory behavioral arousal and elevation of core body temperature. In rodents, the hypothalamic-pituitary-adrenal axis is quiescent and levels of the adrenal hormone, corticosterone (CORT), are stable and extremely low during the first days of life, which are known collectively as the stress-hyporesponsive period (SHRP). It has been proposed that the rabbit also has an SHRP. However, we decided to examine CORT changes associated with the daily nursing bout, since behavioral arousal, core body temperature, and CORT are elevated prior to the onset of feeding in many other species. We scheduled the nursing bout at 1000 h and analyzed CORT concentration using RIA in pups sampled at various intervals after suckling. CORT levels showed a circadian variation that peaked at the time of the daily nursing bout. After nursing, values steadily decreased to a nadir at 2200 h and then rose again in advance of the next scheduled nursing. The three-fold difference between the highest (1000 h) and the lowest (2200 h) CORT values suggests that either the SHRP does not exist in rabbit pups or else it has properties that differ from those described for rodents. Unlike rodents, rabbit pups show a circadian secretion of CORT synchronized to the scheduled nursing bout.     

Differential and Age-Dependent Effects of Maternal Deprivation on the Hypothalamic-Pituitary-Adrenal Axis of Brown Norway Rats from Youth to Senescence

In this study, the hypothesis was tested that infants deprived from maternal care show persistent changes in hypothalamic-pituitary-adrenal activity. For this purpose, we studied the effect of maternal deprivation in one cohort of the healthy ageing Brown Norway rat strain showing still more than 80% survival rate at 32 months of age. Three-day-old male Brown Norway rats were either maternally deprived for 24 h or remained with the dam. In 3, 12 and 30-32 months (young, adult, senescent) deprived rats and their nondeprived littermates (controls), we determined basal resting and stress-induced plasma adrenocorticotropic hormone (ACTH) and corticosterone as well as corticotropin releasing hormone (CRH) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus. Mineralocorticoid (MR) and glucocorticoid receptors (GR) in hippocampus and PVN were also assessed using in vitro cytosol binding and in situ hybridization. The effect of ageing per se showed that in the control nondeprived Brown Norway rats, basal corticosterone and ACTH concentrations did not change during life. However, with age, the corticosterone response to novelty stress became progressively attenuated, but prolonged, while there was an age-related increase in the ACTH response. CRH mRNA expression in PVN decreased with age. Hippocampal MR binding and MR mRNA expression in the dentate gyrus were reduced at senescence, as were the GR binding capacities in hippocampus and hypothalamus. Maternal deprivation did not affect survival rate, body weight, nor adrenal weight of the ageing Brown Norway rats. Basal corticosterone and ACTH levels were not affected by deprivation, except for a rise in basal corticosterone concentrations at 3 months. At this age, the corticosterone output in response to novelty was attenuated in the deprived rats. In contrast, a striking surge in novelty stress-induced corticosterone output occurred at midlife while, at senescence, the corticosterone and ACTH responses were attenuated again in the deprived animals, particularly after the more severe restraint stressor. CRH mRNA expression was reduced only during adulthood in the deprived animals. After maternal deprivation, the MR mRNA in dentate gyrus showed a transient midlife rise. GR binding in hypothalamus and hippocampus GR binding was reduced in young rats while, in the senescent deprived animals, a reduced GRmRNA expression was observed in PVN and hippocampal CA1. In conclusion, in the Brown Norway rat, ageing causes a progressive decline in corticosterone output after stress, which is paralleled at senescence by decreased MR and GR mRNA expression in hippocampus and hypothalamus. The long-term effects of maternal deprivation become manifest differently at different ages and depend on test conditions. The deprivation effect culminates in a midlife corticosterone surge and results at senescence in a strongly reduced corticosterone output.     

Effects of sub-chronic nandrolone administration on hormonal adaptive response to acute stress in rats

Androgenic-anabolic steroid (AAS) misuse has been associated with depression. It has been proposed that stress has a role in depression and that serotonin is involved in both endocrine responses to stress and depressive physiopathology. Although reports demonstrate that AAS chronic administration modifies components of stress-responsive hypothalamic-pituitary-adrenal axis (HPAA), no study has evaluated AAS effect on the response to stressful stimuli. We studied the effects of the subchronic administration (once a day for 14 days in rats) of a supratherapeutical dose of nandrolone decanoate (ND) on HPAA and cortical serotoninergic system response to acute restraint stress (RS). Acute RS produced the following effects: increase in CORT (in blood) and ACTH (both in blood and in pituitary corticotropes), GR depletion in hippocampus and hypothalamus cytosol and GR translocation in hippocampus nuclear fraction, cortical serotonin re-uptake stimulation and hippocampus cytosolic ERK2 activation. ND by itself, i.e. in non-stressed rats, did not modify these parameters, except for a decrease of plasma CORT and ACTH levels and an increase in hippocampus cytosolic phospho-ERK1/2. On the contrary, in stressed rats ND affected stress-induced plasma ACTH increase and prevented all other above reported stress effects, except the increase in pituitary ACTH positive cell density. Our results show that the prolonged administration of a supratherapeutical dose of ND in rats, albeit did not affect in a notable way HPAA and serotonin transporter activity in the absence of stress, may deregulate the stress-induced hormonal cascade which plays a crucial role in depressive psychopathology.     

Long-Lasting Deficient Dexamethasone Suppression of Hypothalamic-Pituitary-Adrenocortical Activation Following Peripheral CRF Challenge in Socially Defeated Rats

The present study focuses on the long-term changes in the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis following two short-lasting episodes of intensive stress in the rat stress model of social defeat and the possible similarities with HPA functioning in human affective disorders. Male Wistar rats experienced social defeats on 2 consecutive days by an aggressive male conspecific. The long-term effect of these defeats on resting and ovine corticotropin-releasing factor (oCRF; intravenous (i.v.) 0. 5 microg/kg) induced levels of plasma ACTH and corticosterone (CORT) were measured 1 and 3 weeks later. In a second experiment the glucocorticoid feedback regulation of HPA function was tested in a combined dexamethasone (DEX)/CRF test (DEX; 25 microg/kg s.c., 90 min before oCRF injection, 0.5 microg/kg). The oCRF challenges were performed between 11.00 and 13.00 h (about three hours after start of the light phase). One week after defeat the ACTH response to CRF was significantly enhanced in defeated rats as compared to controls. Three weeks after defeat the ACTH response was back to control levels. The increased ACTH response 1 week after the stressor was not reflected in higher CORT levels. Neither were baseline ACTH and CORT levels affected by the prior stress exposure. DEX pretreatment inhibited pituitary adrenocortical activity, reflected both in reduced baseline and response values of ACTH and CORT. The ACTH response to CRF following DEX administration was significantly higher in defeated rats as compared to controls both at one and three weeks after defeat. A reduced DEX suppression of baseline secretion of ACTH appeared 3 weeks after defeat. The same tendency was apparent in response and baseline values of CORT. The differences in CORT between socially stressed and control treated rats, however, did not reach significance. The possible role of changes in glucocorticoid-(GR) and mineralocorticoid receptor (MR) binding in the altered regulation of HPA activity following defeat were studied in brain and pituitary of male Wistar rats 1 and 3 weeks after defeat. One week after defeat GR-binding decreased in hippocampus and hypothalamus. No changes were observed in GR-binding in the pituitary nor in MR-binding in any of the regions analysed. Three weeks after defeat GR-binding recovered in hippocampus and hypothalamus but at this time MR-binding in hippocampal tissue was seriously decreased. In a fourth experiment vasopressin (AVP) and CRF stores in the external zone of the median eminence (ZEME) were measured by quantitative immunocytochemistry one and three weeks after defeat and compared with controls. Social defeat failed to induce a change in the immunocytochemical stores of AVP or CRF. The present findings show that in rats short-lasting stressors like defeat induce long-lasting, temporal dynamic changes in the regulation of the HPA axis. Since these changes in time are reflected in GRs and MRs in different brain areas an altered corticosteroid receptor binding might play an important role in the affected HPA activity following defeat.     

Inhibitory effects of corticosterone in the hypothalamic paraventricular nucleus (PVN) on stress-induced adrenocorticotrophic hormone secretion and gene expression in the PVN and anterior pituitary

Endogenous glucocorticoid negative-feedback influence on the hypothalamic-pituitary-adrenal (HPA) axis depends on glucocorticoid actions exerted on multiple glucocorticoid-sensitive tissues and differential glucocorticoid effects that are expressed within several distinct temporal domains. The relative contribution and underlying molecular mechanisms of action for the effects of location and timing of glucocorticoid exposure on HPA axis activity remain to be determined. In the present study, we examined the effects of acute exposure to corticosterone (CORT) at the level of the paraventricular nucleus (PVN) on the HPA axis response to a subsequent stressor in a short-term (1 h) timeframe. Intra-PVN CORT microinjection 1 h before restraint suppressed the adrenocorticotrophic hormone (ACTH) response and blunted restraint-induced corticotrophin-releasing hormone (CRH) heterogeneous nuclear (hn)RNA expression in the PVN and pro-opiomelanocortin hnRNA expression in the anterior pituitary (AP); however, it had no effect on restraint-induced plasma prolactin levels and c-fos mRNA expression (PVN and AP). This pattern of results suggests that CORT acts locally at the level of the PVN within a short-term timeframe to suppress stress-induced excitation-exocytosis coupling within CRH neurones and CRH gene induction without altering the stress-associated trans-synaptic input and intracellular signal transduction that converges on PVN c-fos gene induction. The present study is the first to demonstrate that an acute infusion of CORT into the PVN is sufficient to suppress the ACTH response to stress initiated 1 h after CORT infusion.     

Effects of foraging demand on maternal behaviour and adult offspring anxiety and stress response in C57BL/6 mice

It has been proposed that developmental plasticity of anxiety and stress responses in rodents is mediated by environment-dependent variations in maternal behaviour, but recent evidence indicates that other factors must be involved. To examine this further, we exposed lactating C57BL/6 mice to environmental conditions that imposed high- (HFD), low- (LFD) or variable-foraging demand (VFD) from postnatal day 1-13, depending on the amount and predictability of food supply. While nest attendance was unaffected by treatment, both HFD and VFD-dams showed increased active maternal care compared to LFD-dams. Anxiety in adult male and female offspring was examined on an elevated-O-maze (EZM) and in the open-field test, while hypothalamo-pituitary-adrenal (HPA) reactivity to 20min novelty/isolation stress was determined based on changes in plasma corticosterone (CORT) levels. There were no persistent treatment effects on the offspring's CORT response to novelty/isolation stress. However VFD-males, but not HFD-males, behaved more anxious than LFD males. Their reduced activities throughout the behavioural tests are indicative of a more passive coping style. Conversely, VFD-females, but not HFD-females, behaved less anxious than LFD-females. Our results demonstrate (1) that maternal behaviour in C57BL/6 mice is sensitive to specific characteristics of the environment, (2) that even subtle environment-dependent variations in maternal behaviour can have persistent effects on the offspring's behavioural phenotype, (3) that other factors besides active maternal care must have contributed to these effects, and (4) that male and female offspring may be differentially sensitive to early maternal and/or environmental cues.     

Stress-induced alterations in locus coeruleus gene expression during ontogeny

Brainstem noradrenergic neurons, particularly the locus-coeruleus (LC), play a pivotal role in modulating the central stress response and have been implicated in regulating the hypothalamic-pituitary-adrenal (HPA) axis. In adult rats, acute stress causes an increase in LC firing and tyrosine hydroxylase (TH) gene expression. While the role of the LC-norepinephrine (LC-NE) system in the adult stress response has been well characterized, there is limited evidence for its participation during development. Previous studies described the neonatal HPA axis as hyporeactive because of stimulus-selective pituitary activation. However, maternal deprivation does reinstate stress-induced endocrine activity and can amplify the neural stress response. Considering that LC neurons can modulate neuroendocrine activity, we hypothesized that the LC-NE system would be stress-responsive during development. Because maternal deprivation (DEP) can alter the central stress response, we examined the LC-NE stress response in both DEP and non-deprived (NDEP) pups. Following an isotonic saline injection (stressor) the time course of TH, c-fos and glucocorticoid receptor (GR) mRNA was examined. Stress-induced TH mRNA was increased in DEP pups at postnatal day (pnd) 12 and in both NDEP and DEP pups at pnd 18. At 15, 30 and 240 min c-fos mRNA was markedly increased in all groups examined. GR mRNA was not altered at pnd 12; however, at pnd 18 NDEP pups showed reduced GR mRNA expression. These data indicate that during ontogeny the LC-NE system is stress-responsive to an acute mild challenge. Activation of LC-NE neurons suggests that this system may participate in modulating the neuroendocrine stress response during development.