Multifactorial regulation of the hypothalamic-pituitary-adrenal axis during development

Anterior pituitary response to stress: time-related changes and adaptation

A wide array of physical and psychological stressors alter the secretion of anterior pituitary hormones. However, both the qualitative and the quantitative features of the stressors as well as its duration markedly influence the final endocrine response. In addition, among all anterior pituitary hormones, only ACTH and prolactin levels appear to reflect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, the response of the pituitary-adrenal (PA) and sympathomedulloadrenal axes are common to almost all stressors. After an initial stimulatory effect of stress, an inhibition of all anterior pituitary hormones, except ACTH, can be found provided the stressor is intense enough. The mechanisms responsible for this biphasic response to stress are likely to be located at sites above the pituitary. When the animals are repeatedly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the animals become adapted to the stimulus. This process has been also termed habituation. Among all the pituitary hormones, only ACTH and prolactin levels are reduced as a consequence of repeated exposure to the same (homotypic) stressor, although some negative results have been reported. However, it has been recently reported that subtle changes in the characteristics of the stressors or in their regularity can greatly influence adaptation, and these factors might explain failure to find adaptation of ACTH and prolactin in some works. Habituation of ACTH and prolactin, when observed, appears to be specific for the chronically applied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the case of the PA axis, an intact or potentiated response to a novel stressor is observed in spite of presumably negative feedback exerted by daily stress-induced glucocorticoid release and the high resting levels of glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of the PA axis, developmental aspects of anterior pituitary response to stress have been poorly studied, available data suggest that dramatic changes occur in some hormones during weaning, with some, but less profound, change thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones.

Postnatal maternal separation during the stress hyporesponsive period enhances the adrenocortical response to novelty in adult rats by affecting feedback regulation in the CA1 hippocampal field

The aim of the present experiment was to study the effects of early postnatal maternal separation on behavioural and adrenocortical responses to novelty in rats tested as adults. Sprague-Dawley rat pups were exposed to daily maternal separation (5 h/day) from postnatal day 2 to 6, during the stress hyporesponsive period. Since this procedure requires physical contact with the animals, a first control group of daily handled pups was introduced. A second control group, consisting of pups never handled or separated from the mother, was also considered. At postnatal day 45, the rats were tested in a two-compartment exploratory apparatus: the maternally separated and the non-handled rats, whose behavioural performance did not differ, showed higher emotional behaviour when compared with the handled rats (P < 0.05), suggesting that the handling procedure but not maternal separation improved the capacity to cope with novelty. Corticosterone plasma levels were found to be higher in the maternally separated rats than in the other two groups (P < 0.05), either at resting conditions or at 30 min after novelty exposure (P < 0.05). Levels of nuclear glucocorticoid receptor immunoreactivity in the CA1 hippocampal field were shown to be regulated by novelty exposure, as expected, in both the handled and the non-handled rats but not in the maternally separated rats. In conclusion, repeated maternal separation periods of 5 h/day during the first week of life produced long-lasting effects on the hippocampal regulation of the hypothalamic-pituitary-adrenocortical axis, which appear to be associated with increased responsiveness to stress stimuli in adulthood.

Stress-induced transcriptional regulation in the developing rat brain involves increased cyclic adenosine 3',5'-monophosphate-regulatory element binding activity

The cAMP-regulatory element (CRE) binding protein (CREB) functions as a trans-acting regulator of genes containing the CRE sequence in their promoter. These include a number of critical genes, such as CRF, involved in the hypothalamic response to stressful stimuli in the adult. The ability of the developing rat (during the first 2 postnatal weeks) to mount the full complement of this stress response has been questioned. We have previously demonstrated the stress-induced up-regulation of the transcription of hypothalamic CRF during the second postnatal week in the rat. The focus of the current study was to explore the mechanism of transcriptional regulation in response to stress through the physiological induction of transcriptional trans-activators that bind to the CRE in the developing rat brain. CRE-binding activity was detected via gel shift analysis in extracts from both the hypothalamus and the cerebral cortex of the developing rat. CREB was identified in these extracts by Western blot analysis and was shown to be the major contributor to the CRE-binding activity by gel shift analysis with two specific antibodies directed against CREB. After acute hypothermic stress, the abundance of CRE-binding activity (but not of total immunoreactive CREB), increased in hypothalamic extracts. This enhanced CRE-binding activity was blocked by an antiserum directed against CREB and was accompanied by an apparent increase in CREB phosphorylation. These results indicate that posttranslational enhancement of CRE-binding activity is likely to constitute an important mechanism for up-regulation of genes possessing the CRE sequence in the developing rat hypothalamus by adverse external signals.

The Brown Norway rat displays enhanced stress-induced ACTH reactivity at day 18 after 24-h maternal deprivation at day 3

The effect of a 24-h maternal deprivation at day 3 was studied on the hypothalamus-pituitary-adrenal axis of 18-day-old Brown Norway pups, whose stress-hyporesponsive period is similar to other rat strains. Deprivation resulted at day 18 in reduced basal ACTH levels. The rate of onset and the duration of stress-induced ACTH release were enhanced. CRH mRNA, brain corticosteroid mRNA levels and corticosterone receptor levels were not affected by deprivation, but adrenal weight was increased. It is concluded that maternal deprivation has persistently diminished adrenocortical function in containment of the ACTH response to stress.

Maternal deprivation and stress induce immediate early genes in the infant rat brain

The hypothalamic-pituitary-adrenal (HPA) axis is normally quiescent during the stress-hyporesponsive period (SHRP) from day 4-14 in infant rats. However, maternal deprivation (DEP) can disinhibit the HPA axis, thus enabling neonatal rats to respond to mild stressors. In an effort to understand how DEP may alter HPA axis sensitivity, we used in situ hybridization to measure changes in the expression of stress-responsive genes in the brains of neonatal rats. Despite the minimal HPA axis response in nondeprived rats during the SHRP (postnatal day 12), the mild stress of a saline injection significantly increased messenger RNA levels of two immediate-early genes (IEGs), c-fos and NGFI-B, in the hypothalamic paraventricular nucleus (PVN) and in the cerebral cortex. Following 24 h of DEP, the induction of IEGs in response to stress was greatly potentiated in the PVN of P12 neonates. In contrast, DEP attenuated the effects of stress on IEG induction in rats that had matured beyond the SHRP (P20). Surprisingly, DEP decreased basal levels of CRH messenger RNA in the PVN at P12 and P20. Thus the SHRP most accurately refers to HPA axis insensitivity to stress because the brain itself readily responds to stress as evidenced by the induction of IEGs.

Long-term effects of maternal deprivation on the corticosterone response to stress in rats

Twenty-four hours of maternal deprivation result in activation of the infant rat's adrenocortical axis. In the present study we examined the long-term effects of maternal deprivation on the corticosterone (Cort) response to stress. Pups were maternally deprived (Dep) on postnatal day (PND) 11 and tested immediately (PND 12) or returned to their mothers and tested at later ages. Testing consisted of a time course of the Cort response to a saline injection (5, 15, 30, and 60 min). At PND 12, the response of Dep pups was higher than that of nondeprived (non-Dep) pups. No group differences were observed at PND 16 and 22. On PND 30, Dep rats showed lower Cort levels than non-Dep pups at 0, 5, and 30 min after saline. At PND 60, non-Dep females showed higher Cort levels than males at 5, 15, and 30 min. This gender difference for Dep pups was observed only at 5 min. Male and female Dep animals presented lower Cort levels than non-Dep counterparts at 60 and 30 min after saline, respectively. These findings indicate that maternal deprivation effects on Cort secretion are long lasting. Dep rats showed a smaller adrenal response to stress at PND 30, whereas as adults the stress response was similar but the turnoff was different.

Effect of Interleukin-1beta on Pituitary-Adrenal Responses and Body Weight in Neonatal Rats: Interaction with Maternal Deprivation

The hypothalamic-pituitary-adrenal (HPA) axis of the infant rat is normally hyporesponsive during postnatal days (pnd) four to fourteen. This interval is termed the stress hyporesponsive period (SHRP). The HPA axis, however, does respond to selective stimuli, such as interleukin-1beta (IL-1beta) during this period. Furthermore, maternal deprivation has been shown to alter the system so that it is responsive to mild stimuli. The present studies examined the interaction between 24 h of maternal deprivation and intraperitoneal administration of recombinant human (rh) IL-1beta (4 µg/kg) at 3 ages (i.e., pnd 6, 12, 18) during or after the SHRP. The results demonstrate that maternal deprivation modifies the response of the HPA axis induced by IL-1beta in an age-dependent fashion: 1) a greater response at pnd 6; 2) a quicker response at pnd 12; and 3) a suppressed response pattern at pnd 18. Moreover, these responses across ages differ as a function of maternal contact postinjection: 1) deprivation augments the ACTH and CORT response and maternal contact postinjection further augments this response at pnd 6; 2) deprivation increases the ACTH and CORT response to vehicle and the CORT response to IL-1beta in 12 day-old pups and the mother has a modest inhibitory effect; and 3) at pnd 18 deprivation leads to lower ACTH concentrations, but higher overall CORT levels and maternal contact postinjection effectively suppresses both the ACTH and CORT response to IL-1. These differences in the HPA response do not appear to be due to differences in the immune response. Plasma concentrations of endogenous rat IL-1beta determined 1 and 2 h after injection of rhIL-1beta were not modified by deprivation and were reduced at pnd 18 compared to pnd 6 and 12 in NDEP pups. Finally, IL-1beta reduced food intake, as reflected by a decrease in body weight, in deprived pups at all 3 ages. The findings in the present experiments suggest that there are additional pathways through which IL-1beta can act on the CNS to activate the HPA axis besides direct action at the hypothalamus.

Persistent, but Paradoxical, Effects on HPA Regulation of Infants Maternally Deprived at Different Ages

Twenty-four hours of maternal deprivation have been shown to have immediate and long-term effects on the hypothalamic-pituitary-adrenal (HPA) axis. In the first experiment the influence of such a maternal deprivation period (pnd 11-12) on basal and stress-induced ACTH and CORT levels 4 and 8 days following reunion was investigated. The results revealed a suppression of the ACTH response in the previously deprived animals which was not reflected in the CORT response. In the second experiment these persistent effects were studied in animals deprived during different stages of development. Deprivation early in development (pnd 3-4) produced an animal with a hyperreactive ACTH response whereas deprivation later (pnd 7-8, pnd 11-12) resulted in a hyporeactive ACTH response to stress at pnd 20. To study further the possible mechanisms leading to these different ACTH responses, we used in situ hybridization to investigate hippocampal mineralocorticoid (MR) and glucocorticoid receptor (GR) gene expression and corticotropin-releasing hormone (CRH) and GR mRNA levels in the paraventricular nucleus of the hypothalamus (PVN) of these 20-day old animals. Permanent changes in hippocampal GR mRNA were seen only in the later deprived pups, whereas GR mRNA was reduced in the PVN in all deprived pups. In conclusion, maternal deprivation during the neonatal period produces alterations in the ACTH response to a mild stress and sustained changes in GR transcript levels. The direction and magnitude of these effects are dependent upon the age at which maternal deprivation is experienced.

Neonatal treatment of rats with the neuroactive steroid tetrahydrodeoxycorticosterone (THDOC) abolishes the behavioral and neuroendocrine consequences of adverse early life events

Stressful experience during early brain development has been shown to produce profound alterations in several mechanisms of adaptation, while several signs of behavioral and neuroendocrine impairment resulting from neonatal exposure to stress resemble symptoms of dysregulation associated with major depression. This study demonstrates that when applied concomitantly with the stressful challenge, the steroid GABA(A) receptor agonist 3,21-dihydropregnan-20-one (tetrahydrodeoxycorticosterone, THDOC) can attenuate the behavioral and neuroendocrine consequences of repeated maternal separation during early life, e.g., increased anxiety, an exaggerated adrenocortical secretory response to stress, impaired responsiveness to glucocorticoid feedback, and altered transcription of the genes encoding corticotropin-releasing hormone (CRH) in the hypothalamus and glucocorticoid receptors in the hippocampus. These data indicate that neuroactive steroid derivatives with GABA-agonistic properties may exert persisting stress-protective effects in the developing brain, and may form the basis for therapeutic agents which have the potential to prevent mental disorders resulting from adverse experience during neonatal life.

Hypothalamic-pituitary-adrenal responses to brief social separation

Involuntary separation from close social companions is widely held to lead to pathophysiological outcomes. Presumably, the relationship with, or category of, the separated individual determines the nature of the physiological response. Here, experiments examining the consequences of brief involuntary separation on the activity of the stress-responsive hypothalamic-pituitary-adrenal (HPA) system are reviewed. Only those studies designed specifically to assess the effect of the absence of the social partner are considered. Evidence for HPA activation in response to social separation has been obtained in a number of species; yet, many studies find no effect of separation of affiliative partners on HPA activity. The occurrence of an HPA response to separation does not appear to be related to the phylogenetic position or cognitive capacity of the species studied, nor is it a universal response to mother-infant separation. Rather, it is suggested that the pattern of results can be largely understood in the context of attachment. Separation of partners exhibiting signs of emotional attachment leads to an immediate and persistent HPA response, whereas separation of partners that are affiliative, but do not exhibit attachment, has little or no effect on HPA activity.

Regulation of glucocorticoid and mineralocorticoid receptor mRNAs in the hippocampus of the maternally deprived infant rat

The hypothalamic-pituitary-adrenal (HPA) axis in the developing rat has a limited response to acute challenges between days 3 and 14 of life. Several hypotheses have been proposed to explain this quiescent state. Immaturity of brain, pituitary and adrenal elements or excessive feedback inhibition are common explanations. Recently, a series of studies by Levine and co-workers has shown that prolonged maternal deprivation (24 h) results in increased basal and stress induced corticosterone (CS) levels. An increased adrenal response to ACTH along with an enhanced and sustained ACTH response have been implicated in this phenomenon. A brain structure that appears to be important for normal HPA function is the hippocampus, a structure rich in corticosteroid receptors, which has been hypothesized to play a role in the basal tone of the HPA and in the magnitude and duration of stress responses. Thus, to study further the possible mechanisms leading to an enhanced and sustained ACTH response that is seen in maternally deprived pups, we used in situ hybridization to investigate hippocampal mineralocorticoid (MR) and glucocorticoid receptor (GR) gene expression in 12 groups of animals: six groups involved 24 h maternally deprived (DEP) and non-deprived (NDEP) rat pups at three ages (6-, 9-, and 12-days-old); the other six groups included pups similarly treated, but challenged with an exposure to a mild stressor (saline injection) and sacrificed 1 h thereafter. We found: (1) an age effect for almost every hippocampal subfield for both MR and GR mRNAs: MR increases with age, while GR decreases: (2) down-regulation of MR mRNA in CA1 region in the DEP animals; and (3) down-regulation of GR mRNA, also in CA1, in the saline-injected DEP and NDEP animals. Our results indicate that corticoid receptors in the developing CA1 hippocampal region appear to be sensitive to circulating CS. They also suggest that the relative ratio of GR and MR in the CA1 region may contribute to the enhanced and sustained CS response seen after a mild stressor in deprived animals.

Effects of excitatory amino acids on the hypothalamic-pituitary-adrenal axis of the neonatal rat

Most stimuli that elicit a response by the hypothalamic-pituitary-adrenal (HPA) axis of adult rats fail to do so in infant rats aged 4-14 postnatal days (pnd). This interval is termed the stress hyporesponsive period (SHRP). The present study examined the development of the HPA response to the excitatory amino acids (EAAs), N-methyl-D-aspartic acid (NMDA) and kainic acid (KA), at 3 ages (i.e., pnd 6, 12, 18) during or immediately after the SHRP. Results indicate that intraperitoneal (i.p.) administration of 2.5 mg/kg KA or 5 mg/kg NMDA is capable of inducing age- and time-dependent elevations of ACTH and CORT, with KA being the more potent of the two EAAs. In contrast to other stimuli which are capable of eliciting an HPA response during the SHRP, NMDA and KA appear to possess more potent effects at earlier ages. Administration of lower doses of these EAAs did not elicit an HPA response. Pretreatment with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.69 mg/kg i.p.), a KA receptor-specific antagonist, attenuated the effects of KA. These results suggest that KA exerts its effects via the KA receptor and that this receptor appears to be mature at both pnd 12 and 18. In contrast, pretreatment with D,L-2-amino-5 phosphonovaleric acid (APV; 7.5 mg/kg i.p.), an NMDA receptor-specific antagonist, was only effective at pnd 18 suggesting that the NMDA receptor is not yet mature at pnd 12. Finally, EAAs induce age- and time-dependent behavioral modifications (i.e., hindpaw scratching and hyperlocomotion). These effects, however, appear to only contribute to, but not cause, the endocrine responses.

Brain-corticosteroid hormone dialogue: slow and persistent

1. The stress response system is shaped by genetic factors and life experiences, of which the effect of a neonatal life event is among the most persistent. Here we report studies focused on the "nature-nurture" question using rat lines genetically selected for extreme differences in dopamine phenotype as well as rats exposed as infants to the traumatic experience of maternal deprivation. 2. As key to the endocrine and behavioural adaptations occurring in these two animal models the hormone corticosterone (CORT) is considered. The stress hormone exerts slow and persistent genomic control over neuronal activity underlying the stress response system via high affinity mineralocorticoid (MR) and glucocorticoid receptors (GR). This action is exerted in a coordinate manner and involves after stress due to the rising CORT levels progressive activation of both receptor types. 3. Short periods of maternal separation (neonatal handling) trigger subsequently enhanced maternal care and sensory stimulation. However, a prolonged period (24 h) of depriving the infant of maternal care disrupts the stress hyporesponsive period (SHRP) and causes an inappropriate rise in CORT. During development exposure to CORT and to sensory stimulation has longlasting consequences for organization of the stress response system. 4. We find that these factors embodied by mother-pup interaction are critical for dopamine phenotype, CORT receptor dynamics and neuroendocrine regulation in adult life. The findings provide a conceptual framework to study dopamine-related psychopathology against a background of genetic predisposition, early life events, stress hormones and brain development.

Activation of the HPA axis by immune insults: roles and interactions of cytokines, eicosanoids, glucocorticoids

It is now well established that challenges to the immune system (e.g., infection, inflammation) initiate diverse changes in neuroendocrine function, the most overt of which is activation of the hypothalamo-pituitary-adrenocortical (HPA) axis. The glucocorticoids that are released as a consequence fulfill a vital role in the maintenance of homeostasis that is effected in part through their ability to quench the immune/inflammatory response and thereby prevent them accelerating to a point where they become hazardous to the host. This article discusses the putative mechanisms by which immune insults stimulate the HPA axis, with particular reference to the roles and interactions of the interleukins, eicosanoids and glucocorticoids.

Long-term effects of neonatal maternal deprivation and ACTH on hippocampal mineralocorticoid and glucocorticoid receptors

In the brain, corticosteroids bind to two types of receptors, the classical glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). The effects of different manipulations taking place during early ontogeny on GR and MR binding properties were examined in the adult hippocampus. Infant rats at postnatal day (pnd) 3 were deprived of maternal contact for 24 h and injected with saline or ACTH1-24 at the end of the deprivation period. They were then returned to their dams and weaned on pnd 21. At pnd 48, they were sacrificed (24 h post adrenalectomy) and the hippocampal MR and GR measured using an in vitro cytosol binding assay. In the male rats, deprivation and deprivation + ACTH resulted in a reduction of GRs. MRs were also significantly down regulated in the deprived males. In the female rats, saline injections in deprived female rats resulted in increased GR capacity and ACTH injections led to a further up-regulation of the GRs. None of the early manipulations influenced the regulation of the MRs in females. The binding affinity for corticosteroid receptors was also altered by some of the early manipulations. These results in adult (7-week-old) rats indicate that the receptor systems for corticosteroids in the brain are sensitive to brief manipulations occurring early in development. These changes in receptor capacity and/or affinity may affect corticosteroid-mediated processes in the adult rat.

Development of divergence in dopamine responsiveness in genetically selected rat lines is preceded by changes in pituitary-adrenal activity

Two pharmacogenetically selected Wistar rat lines have been used as a model for individual variability in behavioral and neuroendocrine responses. As a selection criterion the behavioral responsiveness for the dopamine agonist apomorphine was used, giving rise to the apomorphine-susceptible (apo-sus) and apomorphine-unsusceptible (apo-unsus) rat lines. This selection has been maintained over 16 generations. Recent studies have shown that adult rats of these selection lines also show pronounced differences in responsiveness of the hypothalamic-pituitary-adrenal (HPA) system. In this study we analyzed to what extent the divergence in dopamine phenotype and HPA responsiveness, as observed in adult rats, are linked to possible differences, within both systems, during early postnatal development. Therefore, we measured in neonatal female rats of 10 and 18 days of age several parameters of the dopamine and HPA system which show significant differences in adult rats. These include tyrosine hydroxylase (TH) and dopamine D1 and D2 receptor mRNA levels, which were determined within the nigrostriatal system since this system shows the most pronounced differences between adult rats of both selection lines. As indices of HPA activity we measured CRH mRNA, ACTH and total and free corticosterone plasma concentrations under basal conditions in the morning. Transcripts of the two types of corticosteroid receptors, mineralocorticoid (MR) and glucocorticoid (GR) receptor were measured in hippocampus and paraventricular nucleus. In 10-day-old rats all dopamine and HPA parameters were similar in rats of the two selection lines, except for GR mRNA in the parvocellular neurons of the paraventricular nucleus of the hypothalamus (PVN) of apo-sus rats, which was significantly higher than in apo-unsus rats. Eighteen-day-old apo-sus rats, however, showed significantly higher ACTH, comparable total corticosterone and a trend towards lower free corticosterone plasma levels. This HPA profile resembles the situation in adult apo-sus rats as compared with adult apo-unsus rats. Hippocampal GR mRNA expression and thymus weight were also higher in apo-sus rats. In addition, these rats showed an age-related increase in hippocampal MR mRNA expression, while in apo-unsus rats MR mRNA levels did not change between pnd 10 and 18. The measures of the nigrostriatal dopamine system at day 18 were still similar in rats of both lines. In conclusion, divergence in the dopamine systems of the two pharmacogenetically selected rat lines emerges subsequent to divergence in pituitary-adrenal activity.

Stress reactivity and attachment security

Seventy-three 18-month-olds were tested in the Ainsworth Strange Situation. These children were a subset of 83 infants tested at 2, 4, 6, and 15 months during their well-baby examinations with inoculations. Salivary cortisol, behavioral distress, and maternal responsiveness measures obtained during these clinic visits were examined in relation to attachment classifications. In addition, parental report measures of the children's social fearfulness in the 2nd year of life were used to classify the children into high-fearful versus average- to low-fearful groups. In the 2nd year, the combination of high fearfulness and insecure versus secure attachment was associated with higher cortisol responses to both the clinic exam-inoculation situation and the Strange Situation. Thus, attachment security moderates the physiological consequences of fearful, inhibited temperament. Regarding the 2-, 4-, and 6-month data, later attachment security was related to greater maternal responsiveness and lower cortisol baselines. Neither cortisol nor behavioral reactivity to the inoculations predicted later attachment classifications. There was some suggestion, however, that at their 2-month checkup, infants who would later be classified as insecurely attached exhibited larger dissociations between the magnitude of their behavioral and hormonal response to the inoculations. Greater differences between internal (hormonal) and external (crying) responses were also negatively correlated with maternal responsiveness and positively correlated with pretest cortisol levels during these early months of life.

Changes in plasma adrenocorticotropin, corticosterone, corticosteroid-binding globulin, and hippocampal glucocorticoid receptor occupancy/translocation in rat pups in response to stress

Pituitary-adrenal responses to stress in the neonatal rat have been reported to be substantially reduced compared to older animals (i.e. a stress hyporesponsive period). This supposed period of endocrine quiescence is characterized by reduced stress-induced increases in both plasma ACTH and corticosterone. At the same time a number of authors have noted the decreased plasma corticosteroid-binding globulin (CBG) levels of the neonate, and there is evidence for an increased percentage of free corticosterone as well as age-related changes in the volume of distribution for corticosterone. These findings suggest that the reduced CBG levels might enhance the biological significance of existing glucocorticoid levels, beyond that assumed on the basis of plasma total corticosterone levels. We examined this question by estimating hippocampal glucocorticoid receptor occupancy and 'translocation' in Day 6, Day 15, and adult animals under basal and stressful conditions. The results showed that: 1) plasma ACTH levels were elevated in Day 6 animals in response to acute exposure to ether, maternal separation, and maternal separation + ether, however, ACTH responses were substantially lower than in Day 15 or adult animals; 2) Plasma total corticosterone levels followed a similar pattern; most noteworthy was the potent glucocorticoid response in Day 15 animals to the combination of maternal separation + ether; 3) Plasma CBG levels in Day 6 animals were extremely low (< 3% adult values); by Day 15 CBG levels were about 25% of adult levels. Interestingly, maternal separation was associated with a substantial decrease in plasma CBG levels; 4) Hippocampal glucocorticoid receptor occupancy/translocation was similar at all ages under both basal and stress conditions. The only notable exception occurred during maternal separation in Day 15 animals, where the percentage of hippocampal glucocorticoid receptor occupancy/translocation was higher than that observed at any time in either Day 6 or adult animals. This finding is likely related to the decrease in plasma CBG that occurs following separation of Day 15 pups from the dam. Thus, despite the higher corticosterone level in the adult, the increase in glucocorticoid receptor occupancy/translocation was generally comparable across all ages either under basal conditions, or following stress. These receptor data underscore the importance of developmental changes in plasma CBG levels.

Stability and change in cortisol and behavioral response to stress during the first 18 months of life

Observation of cortisol and behavioral responses to routine inoculation was conducted at 18 months for infants in a longitudinal sample whose stress responses had been observed at 2, 4, and 6 months of age. At 18 months, infants showed an increase in cortisol level over base to the perturbation. The magnitude of this response did not differ from the 6-month response. Moreover, level of cortisol response at 18 months was related to level of cortisol response at 6 months, but not at 2 or 4 months of age. In light of previous findings for a decline in cortisol response between 2 and 6 months had for the emergence of consistent individual differences in cortisol response by 4 to 6 months, the present findings indicate that a developmental shift in adrenocortical functioning has occurred by 6 months of age.

Developmental change in infants' responses to stress

Infant stress responses to a well-baby physical examination and inoculation were observed longitudinally at 2, 4, and 6 months of age. In general, there were cortisol increases over base to the procedures. Cortisol level and cortisol response decreased with age. These data indicate a developmental shift in adrenocortical functioning between 2 and 6 months of age. Further evidence for this shift was seen in the stability of individual responses between 4 and 6 months of age. Individual differences in both cortisol and behavioral responses showed the most stability between these 2 ages. Moreover, diurnal variation in baseline cortisol level was present only at 6 months of age. While a sizable minority of infants showed stress-related cortisol decreases to the procedures at a given age, there was no evidence for cross-age consistency in individual infants showing these cortisol decreases.

Dimensional classification and behavioral pharmacology of personality disorders; a review and hypothesis

Nosological orientation in psychiatry has severely hampered the progress of research in biological psychiatry, especially in the case of personality disorders. Dimensional approaches have redefined the characteristics of these disorders and their possible pathogenetic factors. The significance of arousal and stress, so far relatively neglected in clinical research, and its important function in adaptive and coping strategies has to be included in the study of the behavioral pharmacology of personality disorders. Some preliminary clinical data are available suggesting the potential therapeutic use of serotonin modulating agents in the key symptomatology of certain personality disorders such as disturbed impulsive regulation and increased stress reactivity.

Growth and metabolic parameters in pups of undernourished lactating rats

Effects of reduced nutrition during postnatal development were studied by measuring body weight, plasma corticosterone, corticosterone-binding globulin (CBP), and metabolic parameters (plasma glucose and body composition) in preweanling Long Evans rat pups. Control litters consisted of dams that were fed chow ad lib while restricted dams were provided 1/3 the amount of chow given controls (25-30 g/day). At 11 days, body weight of the restricted pups dropped below that of controls. Percent body lipid, plasma corticosterone and glucose of restricted pups were lower than controls. Binding capacity of CBP in controls increased without a change in binding affinity (Kd). By contrast, binding capacity of CBP of restricted pups never matched that of controls but the Kd increased. Nutritional deprivation during lactation affected production of CBP that effectively reduced the biological activity of corticosterone in circulation. Deprived of an effective cellular response to an impoverished supply of food, pups were left with little capacity to manage endogenous fuels during the growth spurt that normally occurs during the suckling period.

Activation and inhibition of the hypothalamic-pituitary-adrenal axis of the neonatal rat: effects of maternal deprivation

These studies investigated the activation and inhibition (negative feedback) of the neonatal rat. The ACTH response of maternally deprived pups is persistently elevated for 30 min, suggesting a deficiency in the negative feedback system. In Experiment 1, we examined the time-course of corticosterone (CORT) and ACTH responses to a saline injection over a 120-min period during development. In deprived pups, CORT and ACTH were persistently elevated throughout the testing period, whereas only 15-day-old nondeprived pups showed ACTH and CORT elevations. Further nondeprived and deprived pups were exposed twice to ether (Experiment 2) or saline injections (Experiment 3) separated by a 1-h interval. Nondeprived pups showed an augmented ACTH response to double exposure to ether, but not to saline. No CORT response to either stimulus was observed. In response to one exposure of each stimulus, deprived pups showed increased ACTH and CORT values and no further elevation to repeated exposure. These results suggest the negative feedback system of neonates is immature, but partially functional in deprived pups. Moreover, nondeprived pups show a stressor-specific response to stress, whereas deprived animals show a similar response to different stimuli.

Developmental profile of the hypothalamo-pituitary-adrenal axis response to nerve growth factor

The developmental profile of the hypothalamo-pituitary-adrenal axis (HPAA) response to nerve growth factor (NGF) in the rat was determined. NGF induced HPAA activity as assayed by increased serum corticosterone levels in pups that were > or = 15 days of age. Since the development of an HPAA response to NGF is parallel to synaptogenesis in the hippocampus and to the regulation of HPAA function by the hippocampus, these findings support the hypothesis that serum NGF activates the HPAA by acting within the CNS.

Corticotropin-releasing hormone mediates the response to cold stress in the neonatal rat without compensatory enhancement of the peptide's gene expression

A variety of stressors activate the hypothalamic-pituitary-adrenal axis, with secretion and compensatory enhanced synthesis of hypothalamic corticotropin-releasing hormone (CRH). Whether CRH is a major effector in the stress response of the neonatal rat and whether the peptide's gene expression is subsequently up-regulated are not fully understood. We studied the effect of cold-separation stress on plasma corticosterone (CORT) levels and CRH messenger RNA (CRH-mRNA) abundance in the paraventricular nucleus. Rats (4-16 days old) were subjected to maximal tolerated cold-separation. CORT and CRH-mRNA abundance were measured before and at several time points after stress. Cold-separation stress resulted in a significant plasma CORT increase in all age groups studied. This was abolished by the administration of an antiserum to CRH on both postnatal days 6 and 9. CRH-mRNA increased in rats aged 9 days or older, but not in 6-day-old rats, by 4 h after stress. These results suggest the presence of robust CRH-mediated adrenal responses to cold-separation stress in neonatal rats. Before postnatal day 9, however, the compensatory increase in CRH-mRNA abundance is minimal.

Behavioral and hormonal responses to stress in the newborn mouse: effects of maternal deprivation and chlordiazepoxide

These studies investigated behavioral and hormonal responses to stress in developing mice. Experiment 1 examined the effects of 24-hr maternal deprivation on corticosterone (CORT) secretion and ultrasonic vocalization (UVZ) rate in 4-, 8-, and 12-day-old mice. At these ages, exposure to a novel environment resulted in minimal changes in CORT secretion. Maternal deprivation increased pups' CORT secretion in an age-dependent fashion but did not affect their UVZ rate. The aim of experiment 2 was to test the effects of chlordiazepoxide (CDP), an anxiolytic compound, on CORT secretion and UVZ in both normally reared and in maternally deprived 8-day-old mice. CDP administration elevated CORT secretion in a dose-dependent fashion, producing larger CORT increases in deprived (DEP) animals. CDP affected UVZ only in nondeprived (NDEP) animals: UVZ rate was decreased by high CDP doses. Overall, these findings demonstrate that the infant mouse shows a period of stress hyporesponsiveness similar to the rat and that maternal presence contributes to inhibit adrenocortical activity. CDP administration, but not novelty exposure, increased CORT secretion in 8-day-old normally reared mice suggesting that during the stress hyporesponsive period, the HPA axis is capable of responding only to specific stimuli. Changes in HPA axis activity and UVZ rate resulting from maternal deprivation and/or CDP challenge do not seem to be directly related.

Pituitary-adrenal and interleukin-6 responses to recombinant interleukin-1 in neonatal rats

The cytokine interleukin-1 (IL-1) is a potent activator of the hypothalamic-pituitary-adrenal (HPA) axis. During postnatal development, the rat appears to be hyporesponsive to many stimuli which activate the HPA system in adulthood. Since hyporesponsiveness depends to a large extent on the stimulus, these experiments investigated the ontogeny of the HPA axis and interleukin-6 (IL-6) responses to IL-1 beta. Six-, 9-, and 18-day-old pups were injected with human recombinant IL-1 beta and plasma ACTH, corticosterone (CORT) and IL-6 levels were measured. IL-1 beta administration resulted in age-dependent endocrine and immune responses. The younger neonates secreted less ACTH and CORT and more IL-6. This was not due to a lowered capacity of the pituitary to synthesize and secrete ACTH since peptide levels following adrenalectomy did not reveal age differences. These data suggest that the diminished response to IL-1 beta is due to the immaturity of neural circuits which may be required to fully activate the HPA axis to immune signals.

Maternal regulation of the hypothalamic-pituitary-adrenal axis in the infant rat: the roles of feeding and stroking

Twenty-four hours of maternal separation results in increased secretion of ACTH and corticosterone (CORT), suggesting the hypothalamic-pituitary-adrenal (HPA) axis is regulated by some aspect of maternal behavior. Previous results indicate that feeding plays a role in maintaining low levels of CORT in 12-day-old pups. In Experiment 1 basal and stress levels of CORT and ACTH were measured in maternally-deprived pups either provided or not with milk to determine whether: (1) feeding maintains ACTH secretion at low levels, and/or (2) feeding maintains the adrenal insensitive to ACTH. The results showed that, although ACTH levels were markedly low (compared to previous values reported by this laboratory) for both groups, only non-fed pups showed a robust increase in basal and stress CORT levels. During the deprivation period in Experiment 1, all pups were manually stroked to induce urination and defecation, suggesting an effect of stroking on ACTH secretion. Experiment 2 examined this hypothesis. Stroking suppressed stress-induced elevations of ACTH secretion due to maternal deprivation. CORT levels, however, were elevated in all deprived pups. The results indicate that maternal regulation of the infant's HPA axis occurs at multiple levels. Feeding appears to regulate adrenal sensitivity, whereas anogenital stroking inhibits the activation of centrally-controlled components of the axis.

Ontogeny of corticosteroid receptors in the brain

1. In the brain, glucocorticoids bind to both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). These receptors show clearly distinct developmental patterns in the infant rat. 2. Low levels of GRs are present around the time of birth throughout the brain. Concentrations rise slowly, and do not achieve adult levels until the third week of life, approximately. GR affinity for corticosterone is higher perinatally than at later ages. Receptor microdistribution changes dramatically during ontogeny. In particular, certain regions, such as the suprachiasmatic nucleus of the hypothalamus, express high levels of receptor only during the first week of life. GRs may show impaired capacity to undergo transformation and/or nuclear translocation during the second postnatal week. Environmental manipulations during early ontogeny (e.g., early handling) may have permanent effects on GR capacity. 3. MRs are present at very low concentrations in the first days of life. Binding capacity rises rapidly thereafter and resembles that found in the adult by the end of 1 week. Neither binding affinity in vitro nor overall distribution changes with age. As in the adult, low doses of corticosterone, in vivo, bind mainly to the MRs. Levels of corticosterone are low and relatively unperturbable in the intact infant rat. It is likely, therefore, that most of the physiological actions of this hormone during this period are mediated by the MR.