Onset of glucocorticoid responsiveness of anterior pituitary corticotrophs during development is scheduled by corticotropin-releasing factor

Fetal and Neonatal HPA Axis

Stress is an integral part of life. Activation of the hypothalamus-pituitary-adrenal (HPA) axis in the adult can be viewed as mostly adaptive to restore homeostasis in the short term. When stress occurs during development, and specifically during periods of vulnerability in maturing systems, it can significantly reprogram function, leading to pathologies in the adult. Thus, it is critical to understand how the HPA axis is regulated during developmental periods and what are the factors contributing to shape its activity and reactivity to environmental stressors. The HPA axis is not a passive system. It can actively participate in critical physiological regulation, inducing parturition in the sheep for instance or being a center stage actor in the preparation of the fetus to aerobic life (lung maturation). It is also a major player in orchestrating mental function, metabolic, and cardiovascular function often reprogrammed by stressors even prior to conception through epigenetic modifications of gametes. In this review, we review the ontogeny of the HPA axis with an emphasis on two species that have been widely studied-sheep and rodents-because they each share many similar regulatory mechanism applicable to our understanding of the human HPA axis. The studies discussed in this review should ultimately inform us about windows of susceptibility in the developing brain and the crucial importance of early preconception, prenatal, and postnatal interventions designed to improve parental competence and offspring outcome. Only through informed studies will our public health system be able to curb the expansion of many stress-related or stress-induced pathologies and forge a better future for upcoming generations.

Vasopressin deficiency diminishes acute and long-term consequences of maternal deprivation in male rat pups

Early life events have special importance in the development as postnatal environmental alterations may permanently affect the lifetime vulnerability to diseases. For the interpretation of the long-term consequences it is important to understand the immediate effects. As the role of vasopressin in hypothalamic-pituitary-adrenal axis regulation as well as in affective disorders seem to be important we addressed the question whether the congenital lack of vasopressin will modify the stress reactivity of the pups and will influence the later consequences of single 24h maternal deprivation (MD) on both stress-reactivity and stress-related behavioral changes. Vasopressin-producing (di/+) and deficient (di/di) Brattleboro rat were used. In 10-day-old pups MD induced a remarkable corticosterone rise in both genotypes without adrenocorticotropin (ACTH) increase in di/di rats. Studying the later consequences at around weaning (25-35-day-old rats) we found somatic and hormonal alterations (body weight reduction, dysregulation of the stress axis) which were not that obvious in di/di rats. The more anxious state of MD rats was not detectable in di/di rats both at weaning and in adulthood (7-12-week-old). The lack of vasopressin abolished all chronic stress and anxiety-like tendencies both at weaning and in adulthood probably as a consequence of reduced ACTH rise immediately after MD in pups. This finding suggests that postnatal stress-induced ACTH rise may have long-term developmental consequences.

Differential contribution of CBP:CREB binding to corticotropin-releasing hormone expression in the infant and adult hypothalamus

Corticotropin-releasing hormone (CRH) contributes crucially to the regulation of central and peripheral responses to stress. Because of the importance of a finely tuned stress system, CRH expression is tightly regulated in an organ- and brain region-specific manner. Thus, in the hypothalamus, CRH is constitutively expressed and this expression is further enhanced by stress; however, the underlying regulatory mechanisms are not fully understood. The regulatory region of the crh gene contains several elements, including the cyclic-AMP response element (CRE), and the role of the CRE interaction with the cyclic-AMP response element binding protein (CREB) in CRH expression has been a focus of intensive research. Notably, whereas thousands of genes contain a CRE, the functional regulation of gene expression by the CRE:CREB system is limited to ∼100 genes, and likely requires additional proteins. Here, we investigated the role of a member of the CREB complex, CREB binding protein (CBP), in basal and stress-induced CRH expression during development and in the adult. Using mice with a deficient CREB-binding site on CBP, we found that CBP:CREB interaction is necessary for normal basal CRH expression at the mRNA and protein level in the nine-day-old mouse, prior to onset of functional regulation of hypothalamic CRH expression by glucocorticoids. This interaction, which functions directly on crh or indirectly via regulation of other genes, was no longer required for maintenance of basal CRH expression levels in the adult. However, CBP:CREB binding contributed to stress-induced CRH expression in the adult, enabling rapid CRH synthesis in hypothalamus. CBP:CREB binding deficiency did not disrupt basal corticosterone plasma levels or acute stress-evoked corticosterone release. Because dysregulation of CRH expression occurs in stress-related disorders including depression, a full understanding of the complex regulation of this gene is important in both health and disease.

The role of the glucocorticoids in developing resilience to stress and addiction

There is emerging evidence that individuals have the capacity to learn to be resilient by developing protective mechanisms that prevent them from the maladaptive effects of stress that can contribute to addiction. The emerging field of the neuroscience of resilience is beginning to uncover the circuits and molecules that protect against stress-related neuropsychiatric diseases, such as addiction. Glucocorticoids (GCs) are important regulators of basal and stress-related homeostasis in all higher organisms and influence a wide array of genes in almost every organ and tissue. GCs, therefore, are ideally situated to either promote or prevent adaptation to stress. In this review, we will focus on the role of GCs in the hypothalamic-pituitary adrenocortical axis and extra-hypothalamic regions in regulating basal and chronic stress responses. GCs interact with a large number of neurotransmitter and neuropeptide systems that are associated with the development of addiction. Additionally, the review will focus on the orexinergic and cholinergic pathways and highlight their role in stress and addiction. GCs play a key role in promoting the development of resilience or susceptibility and represent important pharmacotherapeutic targets that can reduce the impact of a maladapted stress system for the treatment of stress-induced addiction.

Participation of endocannabinoids in rapid suppression of stress responses by glucocorticoids in neonates

In adult rodents, endocannabinoids (eCBs) regulate fast glucocorticoid (GC) feedback in the hypothalamus-pituitary adrenal (HPA) axis, acting as retrograde messengers that bind to cannabinoid receptors (CB1R) and inhibit glutamate release from presynaptic CRH neurons in the paraventricular nucleus of the hypothalamus (PVN). During the first two weeks of life, rat pups exhibit significant CRH and ACTH responses to stress although the adrenal GC output remains reduced. At the same time, pups also display increased sensitivity to GC feedback, but it is unclear whether eCBs play a role in mediating fast GC feedback in neonatal life. In our studies, we examined the role of eCBs in the rapid suppression of anoxia-induced ACTH release and determined whether eCB action could be modulated by the levels of circulating GCs present at the time of stress. PND8 pups were subjected to 3-min anoxia with AM251, a CB1R blocker, injected 30 min prior to stress onset. The effects of either metyrapone (MET) (a steroidogenic 11 beta-hydroxylase blocker) or methylprednisolone (PRED) (a synthetic GC) pretreatment on AM251 effect and the stress response were evaluated. Treatment with AM251 before stress onset tended to increase overall ACTH and CORT secretion, and also delayed the return to baseline ACTH. The AM251 effect on ACTH in PND8 pups was lost in MET-treated pups, who exhibited high basal and stimulated ACTH release and no CORT response to stress. Methylprednisolone suppressed ACTH stress responses although AM251 still delayed restoration of ACTH levels to the baseline. This suggests that the eCB effect on ACTH secretion in neonates is most evident when there is a dynamic fluctuation of corticosterone levels. Interestingly, AM251 increased basal and stimulated corticosterone secretion in all treatments including MET, suggestive of a direct action of CB1R blockade on adrenal steroidogenesis.

Effects of early life stress on neuroendocrine and neurobehavior: mechanisms and implications

Evidence continues to mount that adverse experiences early in life have an impact on brain functions. Early life stress can program the development of the hypothalamic-pituitary-adrenal axis and cause alterations of neurochemistry and signaling pathways involved in regulating neuroplasticity, with resultant neurobehavioral changes. Early life experiences and genetic factors appear to interact in determining the individual vulnerability to mental health disorders. We reviewed the effects of early life stress on neuroendocrine regulation and the relevance to neurobehavioral development.

Early and delayed effects of hypoxia during the infantile period on behavioral and hormonal reactions of rats

We studied the early and delayed effects of hypoxia during the infantile period on the behavioral reactions and corticosterone concentration in male rats. The elevation of corticosterone concentration, decrease in the immobility time (forced swimming test), and increase in the nociceptive response (formalin test) were observed in 7-day-old rats immediately after hypoxia. Adult animals exposed to hypoxia at the age of 7 days exhibited elevated basal corticosterone level and lengthened immobility time. Hypoxia had the same effect on plasma corticosterone concentration in 7-day-old and adult rats. Changes in corticosterone concentration after forced swimming were shown to differ in hypoxic animals and non-hypoxic specimens. Studying the dynamics of age-related variations in the test parameters will contribute to the understanding of pathogenetic mechanisms and development of new methods for pharmacological correction of postnatal changes in CNS after hypoxia during early ontogeny.

Heterogeneity of the infant stage of rat development: inflammatory pain response, depression-related behavior, and effects of prenatal stress

The infant stage of rat development is a very important period for potential correction of adverse consequences produced by negative prenatal events. However the age limit for this correction needs to be investigated. The last prenatal and first two weeks after birth are "critical" for maturation of the nociceptive and emotional systems. Clinical observations suggest a correlation between persistent pain response and emotional behavior. In infant male rats of different ages, we studied indices of the inflammatory pain response (the number of flexes+shakes in the formalin test), depression-related behavior (immobility in the forced swim test) and the relations between them, as well as the effects of prenatal stress on these indices. Furthermore, we assessed the trend of body weight and the relations between body weight and the depression- and pain-related behaviors. We demonstrate heterogeneity of the infant stage: control prenatally non-stressed rat pups showed significantly lower immobility at 7 days of age than at 10 days; prenatal stress caused an increase of immobility and the number of flexes+shakes in 7-8-day-old pups but not in 10-11-day-olds. These findings should be taken into account in the treatment of abnormalities of emotional and inflammatory pain-related behaviors produced by prenatal stressful events. The present data and our previous findings indicate that the deficiency of body weight in prenatally stressed newborns may predict the development of abnormalities in inflammatory pain-related responses during postnatal ontogeny.

Control of the hypothalamo-pituitary-adrenal axis in the neonatal period: adrenocorticotropin and corticosterone stress responses dissociate in vasopressin-deficient brattleboro rats

In adulthood the hypothalamo-pituitary-adrenal axis is controlled by both CRH and arginine vasopressin (AVP). However, in neonates CRH secretion is very low, whereas AVP secretion is fully functional. This suggests that the role of AVP is more pronounced in young than in adult rats. We investigated the role of AVP by studying stress responses in 5, 10, and 20-d-old AVP-deficient Brattleboro rats. Two different stressors were applied: 24-h maternal separation and Hypnorm Grove Oxford UK injections. In heterozygous controls (that do express AVP), both stressors increased plasma ACTH and corticosterone. The ACTH stress response disappeared in AVP-deficient rats, demonstrating that during the perinatal period, the secretion of this hormone is controlled by AVP. Surprisingly, corticosterone responses remained intact in AVP-deficient rats. Similar findings were obtained after 1-, 4-, 12-, and 24-h long maternal separations. Thus, preserved corticosterone stress responses were not explained by changes in the timing of ACTH secretion. In vitro experiments suggested that the dissociation of ACTH and corticosterone stress responses can only be partly explained by higher ACTH responsiveness of the adrenal cortex in AVP-deficient rats. Together, our results show that in neonatal periods, AVP is crucial for the expression of ACTH stress responses, but neither AVP nor ACTH is necessary for the induction of corticosterone stress responses. Discrepant ACTH and corticosterone stress responses may reflect compensatory mechanisms activated by AVP deficiency, but disparate findings suggest that they rather depict a neonate-specific mechanism of hypothalamo-pituitary-adrenal-axis control.

Stress, genes and the mechanism of programming the brain for later life

Adverse conditions during early life are a risk factor for stress-related diseases such as depression and post-traumatic stress disorder (PTSD). How this long-term effect of early adversity occurs is not known, although evidence accumulates that the action of stress hormones is an important determinant. In rodents after a variety of experiences, even minor ones, during postnatal life permanent changes in emotional and neuroendocrine reactivity have been observed. Also stressful events occurring prenatally and even the pre-implantation hormonal conditions can have permanent consequences. Here we will focus on evidence obtained from (i) the blastocyst implantation during conditions of ovarian hyperstimulation, which is commonly used in the generation of transgenic mice; (ii) the stress system activity in the newborn under various conditions of maternal care; (iii) the long-term consequences of maternal separation procedures. The results clearly demonstrate that early experiences trigger immediate changes in the stress system that may permanently alter brain and behaviour.

Postnatal diet-induced obesity in rats upregulates systemic and adipose tissue glucocorticoid metabolism during development and in adulthood: its relationship with the metabolic syndrome

In humans, a hyperactivity of glucocorticoid metabolism was postulated to be involved in the intrauterine programming of the metabolic syndrome in adulthood. We studied in rats the effects of overfeeding, obtained by reducing the size of the litter in the immediate postnatal period, a time crucial for neuroendocrine maturation such as late gestation in humans. Overfeeding induced early-onset obesity and accelerated the maturation of the hypothalamo-pituitary-adrenal (HPA) axis together with an upregulation of adipose tissue glucocorticoid receptor (GR) mRNA. In adulthood, neonatally overfed rats presented with moderate increases in basal and stress-induced corticosterone secretion and striking changes in visceral adipose tissue glucocorticoid signaling, that is, enhanced GR and 11beta-hydroxysteroid dehydrogenase type 1 mRNA levels. The above-mentioned alterations in the endocrine status of overfed rats were accompanied by a moderate overweight status and significant metabolic disturbances comparable to those described in the metabolic syndrome. Our data demonstrate for the first time that postnatal overfeeding accelerates the maturation of the HPA axis and leads to permanent upregulation of the HPA axis and increased adipose tissue glucocorticoid sensitivity. Thus, the experimental paradigm of postnatal overfeeding is a powerful tool to understand the pathophysiology of glucocorticoid-induced programming of metabolic axes.

Region-specific onset of handling-induced changes in corticotropin-releasing factor and glucocorticoid receptor expression

Early-life experience including maternal care profoundly influences hormonal stress responses during adulthood. Daily handling on postnatal day (P) 2-9, eliciting augmented maternal care upon returning pups to their cage, permanently modifies the expression of the stress neuromodulators corticotropin-releasing factor (CRF) and glucocorticoid receptor (GR). We have previously demonstrated reduced hypothalamic CRF expression already at the end of the handling period, followed by enhanced hippocampal GR mRNA levels (by P45). However, the initial site(s) and time of onset of these enduring changes have remained unclear. Therefore, we used semiquantitative in situ hybridization to delineate the spatiotemporal evolution of CRF and GR expression throughout stress-regulatory brain regions in handled (compared with undisturbed) pups. Enhanced CRF mRNA expression was apparent in the amygdaloid central nucleus (ACe) of handled pups already by P6. By P9, the augmented CRF mRNA levels persisted in ACe, accompanied by increased peptide expression in the bed nucleus of the stria terminalis and reduced expression in the paraventricular nucleus. The earliest change in GR consisted of reduced expression in the ACe of handled pups on P9, a time point when hippocampal GR expression was not yet affected. Thus, altered gene expression in ACe, bed nucleus of the stria terminalis as well as paraventricular nucleus may contribute to the molecular cascade by which handling (and increased maternal care) influences the stress response long term.

Regulation of the hypothalamic-pituitary-adrenal axis in the neonatal rat: the role of maternal behavior

There is a growing literature that indicates that exposure to elevated levels of glucocorticoids can result in long term consequences for the developing brain. In the developing rodent there is a period from about day 4-14 when the adrenal response to stress is either minimal or non-existent thus resulting in stable low levels of circulating glucocorticoids. This has been designated as the stress hypo-responsive period (SHRP). Numerous experiments have demonstrated that maternal factors are critical for the regulation of the pup's hypothalamic-pituitarty adrenal (HPA) axis and the maintenance of the SHRP. Following 24 h of maternal deprivation the neonatal rat shows elevated basal levels of corticosterone and exhibits a robust corticosterone and ACTH response to mild stress. Further c-fos mRNA in the paraventricular nucleus is enhanced following stress in deprived pups. At least three aspects of maternal behavior play a role in the regulation of the HPA axis during development. Tactile stimulation appears capable of inhibiting most of the brain-related changes that occur following maternal deprivation. Feeding is essential for maintaining the adrenal unresponsive and reduces the sensitivity of the adrenal to ACTH. Passive contact suppresses the response to stress. In the adult corticotropin-releasing hormone (CRH) is the major neuropeptide that controls pituitary ACTH secretion. In the maternally deprived pup CRH gene transcription is down regulated and arginine vasopressin (AVP) appears to assume the major regulatory hormone that modulates ACTH. These data all indicate that maternal factors are responsible for actively inhibiting the endocrine responses to stress postnatally. Further, maternal deprivation also results in increased cell death in several brain regions. Thus during development most of the peripheral and central stress responsive systems are capable of being activated. However, under conditions of normal dam-pup interactions these responses are mostly suppressed by the dam's behavioral interaction with the pups thus preventing the potential toxic effects of increased secretion of glucocorticoids during critical periods of brain development.

Primary social relationships influence the development of the hypothalamic--pituitary--adrenal axis in the rat

In the developing rodent, there is a period from about days 4 to 14 when the adrenal response to stress is either minimal or non-existent. This has been designated as the stress hyporesponsive period (SHRP). Numerous experiments have demonstrated that maternal factors are critical for the regulation of the pup's hypothalamic--pituitary--adrenal (HPA) axis and the maintenance of the SHRP. Following 24 h of maternal deprivation, the neonatal rat shows elevated basal levels of corticosterone (CORT) and exhibits a robust CORT and ACTH response to mild stress. Further, c-fos mRNA in the paraventricular nucleus (PVN) is enhanced following stress in deprived pups. At least three aspects of maternal behavior play a role in the regulation of the HPA axis during development. Tactile stimulation appears capable of inhibiting most of the brain-related changes that occur following maternal deprivation. Feeding is essential for maintaining the adrenal-unresponsive and reduces the sensitivity of the adrenal to ACTH. Passive contact suppresses the response to stress. In the adult, corticotropin-releasing hormone (CRH) is the major neuropeptide that controls pituitary ACTH secretion. In the maternally deprived pup, CRH gene transcription is downregulated and arginine vasopressin (AVP) appears to assume the major regulatory hormone that modulates ACTH. These data all indicate that maternal factors are responsible for actively inhibiting the endocrine responses to stress postnatally. Thus, during development, most of the peripheral and central stress-responsive systems are capable of being activated. However, under conditions of normal dam-pup interactions, these responses are mostly suppressed by the dam's behavioral interaction with the pups.

Thyroxine modulates corticotropin-releasing factor but not arginine vasopressin gene expression in the hypothalamic paraventricular nucleus of the developing Rat

Neonatal rats were daily injected with 100 microg/kg T4 and killed at 4, 8 or 15 days. Circulating corticosterone and corticosteroid binding globulin concentrations increased in 8- and 15-day-old rats after T4 treatment. Plasma adrenocorticotropic hormone (ACTH) concentrations, pituitary ACTH content and pro-opiomelanocortin mRNA expression were unaffected in T4-treated rats. T4 treatment induced an increase in corticotropin-releasing factor (CRF) mRNA expression in the whole population of CRF synthesizing cells of the paraventricular nucleus (PVN) that became significant at day 8 and disappeared at day 15. Double labelling in situ hybridization revealed that CRF gene expression in the CRF+/arginine vasopressin (AVP)+ subpopulation was increased at days 4 and 8 and decreased at day 15. CRF immunoreactivity in the zona externa of the median eminence increased with age but was not affected by the experimental hyperthyroidism. The degree of CRF and AVP colocalization, the concentration of AVP mRNA in the parvo and magnocellular cell bodies of the PVN and the density of immunoreactive AVP in the zona interna or zona externa of the median eminence did not change after T4 treatment. Our data demonstrate that experimental hyperthyroidism accelerates the maturation of hypothalamic CRF gene expression, including in particular in the CRF+/AVP+ subpopulation, during the stress hyporesponsive period. These observations suggest that the physiological peak of plasma thyroxine that occurs between days 8-12 may participate in the maturation of hypothalamic CRF cells.

Effect of hibernation, thyroid hormones and dexamethasone on cytosolic and mitochondrial glycerol-3-phosphate dehydrogenase from jerboa (Jaculus orientalis)

Tissue distribution of the cytosolic and mitochondrial glycerol-3-phosphate dehydrogenase (cGPDH and mGPDH) activities in jerboa (Jaculus orientalis), a hibernator, shows the highest level of enzyme activity in skeletal muscle and brown adipose tissue, respectively. The effect of hibernation on cGPDH indicates an increase of activity in all tissues examined. In contrast, hibernation decreases mGPDH activity in all tissues, except skeletal muscle. The effect of thyroid hormones on GPDH activity was tissue specific: in kidneys, cGPDH activity doubled in euthermic jerboas treated with T4. In contrast, 6-n-propyl-2-thiouracil treatment provokes an increase of enzyme activity in brown adipose tissue, liver and brain. T4 treatment leads to a 2.7-fold increase in liver mGPDH activity. 6-n-propyl-2-thiouracil treatment decreases mGPDH activity in the skeletal muscle whereas the opposite effect was observed in brain. Dexamethasone stimulates cGPDH in all tissues examined, except skeletal muscle and kidneys. In the case of mGPDH activity, this increase was observed only for brown adipose tissue and brain. Our results suggest that hibernation, thyroid hormones and dexamethasone probably play a role in the regulation of cGPDH and mGPDH activities in jerboa. Our findings confirm that these enzymes are involved in metabolic adaptation to thermal stress in Jaculus orientalis.

Corticotrophin-releasing hormone and fetal responses in human pregnancy

During human pregnancy, maternal and fetal compartments of the human placenta produce and release corticotrophic-releasing hormone (CRH). Elevations of placental CRH are associated with decreased gestational length (including preterm delivery). The effects of elevated placental CRH on human fetal neurological development are not known. Pregnant women in the 31st and 32nd week of gestation consented to procedures for collection of blood and measurement of fetal heart rate (FHR) in response to a series of 40 vibro-acoustic stimuli (VAS). Measures of habituation and dishabituation were calculated from the FHR. All subjects were followed to delivery. Fetuses (N = 33) of women with highly elevated CRH were least responsive (p < .03) to stimulation after presentation of a novel (dishabituating) stimulus with control for parity, fetal gender, medical (antepartum) risk, and gestational length at term. In a larger sample (N = 156) a polynomial model predicted the pattern of FHR reactivity for the first 15 trials. Placental CRH concentration significantly predicted FHR reactivity after controlling for the effects of trial number, baseline FHR, inter-trial interval, and presence of uterine contractions. Increased maternal CRH levels were significantly related to the length of gestation after controlling for the effects of fetal gender, parity, and medical risk (p = .05). The relationship between length of gestation and FHR was not significant suggesting separate actions of CRH on these events. Elevated placental CRH appears to accelerate certain developmental events (gestational length) and may influence the fetal nervous system. The impaired fetal responses to novelty and increased arousal observed in this study suggest that neurological systems may be targets for placental CRH during sensitive developmental periods.

Maternal corticotropin-releasing hormone and habituation in the human fetus

Elevated concentrations of maternal corticotrophin-releasing hormone (CRH) during the 2nd and early 3rd trimester of human pregnancy are associated with spontaneous preterm birth, but the effects of maternal CRH on the fetus are unknown. Maternal plasma was collected for analysis of CRH concentration, m = 156.24 +/- 130.91 pg/ml, from 33 pregnant women during Weeks 31-33 of gestation. Immediately after collection of plasma, fetal heart rate (FHR) measures were obtained in response to a challenge with a series of vibroacoustic stimuli. Fetuses of mothers with highly elevated CRH did not respond significantly to the presence of a novel stimulus in a repeated series, p = 0.016. These effects on the FHR response were not related to parity, fetal gender, medical (antepartum) risk, or eventual birth outcomes. Impaired dishabituation in these fetuses of mothers with high concentrations of CRH suggests that neurological systems rich with CRH receptors that support learning and memory, such as parahippocampal regions, may be targets for maternal/placental CRH, with implications for fetal neurological development.

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.

Ontogeny of prohormone convertases in rat prenatal development

It has been well established that peptide precursors usually undergo limited proteolysis at pairs or single basic amino acids during their biosynthetic process. This posttranslational modification paradigm is common for numerous membrane-spanning and secreted proteins, neuropeptides, and peptide hormones of physiological significance, in which endoproteolytic cleavage is invariably essential for the accurate biosynthesis and full activity of the mature products. Establishment of an effective peptide profile is dependent on not only the presence of peptide precursor, but also the presence and the enzymatic specificities of cleavage enzymes. We have, therefore, characterized the spatial and temporal patterns of six subtilisin-like serine endoproteases known to be involved in proprotein processing, including furin, PC1, PC2, PC4, PC5, and PACE4, in rat prenatal development and related the results to the expression patterns of several peptide precursors. We have observed largely distinct and sometimes complementary expression patterns of individual PCs in various embryonic structures, suggesting PCs may be functionally distinct in processing different sets of proprotein substrates in development. From these studies, numerous tentative enzyme-substrate relationships in various embryonic structures have been proposed and should encourage more studies to test the in vitro cleavage potentialities of individual PCs toward these precursors. In the future, knowledge gained from these studies, when combined with insights gained from in vivo perturbation and genetic ablation studies, should lead to final comprehensive understanding of specific precursors cleaved by specific enzymes at specific cleavage sites in known spatial and temporal expression patterns during development.

Developmental profile of messenger RNA for the corticotropin-releasing hormone receptor in the rat limbic system

The ontogeny of corticotropin-releasing hormone (CRH) receptor messenger ribonucleic acid (mRNA) in rat brain, using in situ hybridization, is the focus of this study. The developmental profile of CRH receptor using binding assays and receptor autoradiography has been reported, but may be confounded by the presence of a binding protein. The recent cloning of the rat CRH receptor gene has permitted the use of in situ hybridization histochemistry to map the distribution of cells expressing CRH receptor mRNA in the developing brain. We used antisense 35S-labeled oligodeoxynucleotide probes for the two reported splice-variants of the CRH receptor mRNA, which yielded essentially identical localization patterns. CRH receptor mRNA was clearly detectable in infant brain starting on the second postnatal day. Signal in hippocampal CA1, CA2 and CA3a increased to 300-600% of adult levels by postnatal day 6 with a subsequent gradual decline. In the amygdala, in contrast, CRH receptor mRNA abundance increased steadily between the second and the ninth postnatal days, to levels twice higher than those in the adult. In the cortex, CRH receptor mRNA levels were high on postnatal day 2 and decreased to adult levels by day 12. Transient signal over the hypothalamic paraventricular nucleus, observed on the second postnatal day, was not evident at older ages. These results demonstrate robust synthesis of CRH receptor as early as on the second postnatal day and unique region-specific developmental profiles for CRH receptor gene expression.

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.

Anomalous adrenalectomy-induced Fos-like immunoreactivity in the hypothalamic paraventricular nucleus of stress-hyporesponsive rats

Cellular activity in the paraventricular nucleus of the hypothalamus (PVN) in response to adrenalectomy (ADX) and sham-ADX was measured in adult male rats, lactating females, and nursing pups using c-fos immunocytochemistry. Increased Fos-like immunoreactivity (FLI) was seen in the PVN of male rats at 4 h following ADX or sham-ADX but this increase was transient, and basal values were restored within 24 h. In suckling pups, ADX induced a marked FLI response in the PVN at 3 days and at 18 days postpartum. At 11 days postpartum, however, the FLI response was attenuated relative to adult animals, and 3- and 18-day-old pups. Similarly, in nursing mothers, ADX induced FLI at 3 days, but this response disappeared by 7 days and did not reappear by the end of the suckling period (day 21). These data indicate that c-fos expression is a sensitive indicator of hyporesponsiveness in the hypothalamic-pituitary-adrenal (HPA) system.

Corticotroph, somatotroph and mammotroph cell kinetics in the postnatal infant female rat

The aim of this work was to detect if hypothalamic-pituitary maturation was accompanied by significant proliferation changes in differentiated pituitary cell pools. For this purpose, pituitary corticotroph (Ct), mammotroph (Mt) and somatotroph (St) proliferation activities were scanned in intact female rats during the postnatal (P) period (1-35 postnatal days). The techniques of tritiated thymidine labelling, immunostaining and autoradiography were combined to visualize DNA synthesis of hormone containing cells. Immunoreactive cell densities were measured using image analysis, and double labelled cells were counted. Corticotroph proliferation activity increased significantly on day P12, followed by an increase in the Ct proportion on days P13-14. This is the first observation of a spontaneous change of corticotroph proliferation at the end of the stress nonresponsive period. The mammotroph density and proliferation rate increased gradually during postnatal maturation, until the Mt pool overran other cell types of the female hypophysis on day 35. The somatotroph pool was the most numerous until day P20; the proliferation rate remained constant while St proportions increased reaching a plateau between days P13 and 20, then decreased to the adult level. Each cell type examined showed a characteristic, individual density and proliferation pattern.

Effects of a specific glucocorticoid receptor antagonist on corticotropin releasing hormone gene expression in the paraventricular nucleus of the neonatal rat

Mechanisms controlling the synthesis of corticotropin releasing hormone (CRH) in neonatal rats, and the ontogeny of glucocorticoid (GC) feedback control of hypothalamic CRH remain unknown. Specific issues are whether stress induces up-regulation of CRH gene expression during the first postnatal week, and the role of GC feedback, at the hypothalamic level, in the stress-hyporesponsive period. We studied the ontogeny of the negative feedback regulation of CRH gene expression by GC in the paraventricular nucleus (PVN). We implanted chronic cannulae containing a GC-receptor antagonist, RU 38486, in rats on postnatal days 3 to 13. Three days later, animals were sacrificed, and brains were analyzed for CRH-messenger RNA (CRH-mRNA), using semi-quantitative in situ hybridization. Animals implanted with cholesterol-containing cannulae served to evaluate the stressful effect of implantation on CRH-mRNA abundance. The presence of GC receptor messenger RNA (GR-mRNA) in the PVN of neonatal rats was also determined. RU 38486 did not increase CRH-mRNA abundance during the first postnatal week, despite the presence of GR-mRNA in the PVN. Chronic-implantation stress also failed to increase CRH synthesis. CRH gene expression in the PVN was enhanced in infant rats implanted with RU-38486 on postnatal day 9 or later. Cholesterol implantation on days 9, 10 (but not later), resulted in increased PVN-CRH-mRNA. Thus, CRH-mRNA is up-regulated by chronic blockade of GC receptors only subsequent to the eighth postnatal day. Furthermore, such blockade does not affect the response of CRH-mRNA to chronic stress in the neonatal rat.

Effects of repeated maternal separations on the adrenocortical response to stress of preweanling rats

Previous data indicate that the infant rat shows a marked increase in adrenocortical responsiveness to stress immediately following prolonged maternal separation. In Experiment 1 we studied the immediate effects of repeated maternal deprivation. Our results indicate that the increase in basal as well as stress-induced corticosterone levels is a direct function of the length of the deprivation period immediately preceding testing, and is not cumulative. In Experiment 2 we examined the long-term consequences of maternal deprivation on adrenal responsivity. Four days following a single 24-h period of maternal deprivation, pups remained hyperresponsive to stress, although their basal levels of corticosterone had returned to control values. Shorter periods of deprivation (which did result in increased responsivity immediately following deprivation) did not have persistent effects. Our data suggest: 1) short periods of deprivation do not have a cumulative effect, and 2) there is a critical length of deprivation beyond which persistent changes in adrenocortical responsivity ensue.

Systemic NMDA treatment increases plasma ACTH in the neonatal female and male rat

The present study was designed to examine the adrenocorticotropic hormone (ACTH) response to N-methyl-D-aspartate (NMDA) in neonatal rats. Subcutaneous injection of NMDA (30 mg/kg) was found to increase plasma ACTH concentrations two-fold after 15 min in 9-10 and 20-21 day-old female and male rats. Pretreatment with the competitive NMDA receptor antagonist CPP (10 mg/kg) failed to attenuate the ACTH response to NMDA in the younger rats, yet reduced the response in older ones. These findings indicate that NMDA can elevate plasma ACTH in both female and male neonatal rats, however this response is not sensitive to CPP antagonism until the end of the neonatal period.

Ontogeny of expression and glucocorticoid regulation of the arginine vasopressin gene in the rat hypothalamic paraventricular nucleus

Corticotropin-releasing factor and arginine vasopressin (AVP) are the two major hypothalamic factors that regulate anterior pituitary adrenocorticotropin secretion during stress. We have previously reported that the expression of the corticotröpin-releasing factor gene in the hypothalamus and its regulation by glucocorticoids were not mature during the first week of life in the rat, i.e. during the stress non-responsive period. In this report, we studied the ontogeny of expression of the AVP gene in the hypothalamic paraventricular nucleus in rats using in situ hybridization. AVP mRNA was detected as early as day 20 of gestation (E20) both in the parvocellular and the magnocellular portion of the paraventricular nucleus. The levels of expression of the AVP gene increased steadily from E20 to the third day after birth (P3) and remained stable from P3 to P14. Bilateral surgical adrenalectomy induced an increase in AVP mRNA levels in the parvocellular portion of the paraventricular nucleus, but not in the magnocellular portion, in both 7-day-old and 14-day-old rats, suggesting that the glucocorticoid regulation of the AVP synthesizing neurons of the paraventricular nucleus is mature in the developing postnatal rat.

Arthritis-susceptible Lewis rats fail to emerge from the stress hyporesponsive period

Susceptibility to streptococcal cell wall (SCW)-induced arthritis in 4- to 6-week-old Lewis (LEW/N) rats is associated with blunted glucocorticoid production secondary to a profound defect in inflammatory mediator-induced hypothalamic corticotropin-releasing hormone (CRH) biosynthesis and secretion. The relative SCW arthritis resistance in histocompatible Fischer (F344/N) rats, on the other hand, is associated with robust hypothalamic-pituitary-adrenal (HPA) axis responses to inflammatory mediators. In this study, we investigated HPA axis responses to SCW during the postnatal developmental period in LEW/N and F344/N rats. We found that SCW-induced plasma corticosterone (CORT) responses do not significantly increase during development in LEW/N, while such responses clearly appear at postnatal day 14 in F344/N and outbred Harlan-Sprague-Dawley (HSD) rats. Additionally, LEW/N rats fail to exhibit the normal ontogenic increase in CRH mRNA levels in the paraventricular nucleus (PVN), whereas their SCW-induced PVN CRH mRNA responses are blunted compared to F344/N at postnatal day 14. Taken together, these results suggest that LEW/N rats fail to emerge completely from their stress hyporesponsive period. This may account for the lack of stress responsiveness in young adult LEW/N rats, and consequently, for their susceptibility to SCW-induced arthritis and other inflammatory diseases.

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

The hypothalamic-pituitary-adrenal system shows an overall diminished responsiveness throughout ontogeny. Thus, during this period, the sensitivity of the adrenal gland to ACTH is markedly reduced. Furthermore, basal and stress-induced concentrations of corticosterone (CORT), ACTH and hypothalamic secretagogues remain at very low levels. Both structural immaturity and active inhibitory processes appear to underlie this overall hyporesponsiveness. The available data indicate that the characteristic developmental pattern of the HPA system results from multiple regulatory factors acting in conjunction at various levels of the axis. The primary rate-limiting steps, however, are probably at the brain and adrenal levels. The ultimate "goal" appears to be to keep CORT levels within the narrow range of concentrations required for normal development.

Responses of brain and pituitary immunoreactive corticotropin-releasing factor in surgically interrenalectomized eels: immunocytochemical study

Interrenal (IR) cell islets, mainly located in the anterior and posterior cardinal veins, can be surgically removed in the eel. Immunocytochemical studies showed that sham operation slightly affected the distribution of immunoreactive corticotropin-releasing factor (ir-CRF) in the preoptic nucleus (PON) and neurohypophysis (NH). Surgically interrenalectomized (IRX) eels showed a significant increase in the cross-sectional area of the magno- and parvocellular perikarya of the PON and a noticeable decrease in their immunostaining after 8-9 days. In the pituitary, the immunostaining of the rostral nerve fibers facing the corticotropic (ACTH) cells was greatly reduced whereas ir-CRF in the caudal NH was not clearly affected. Immunostaining of the ACTH cells was strongly reduced and that of melanocorticotrops slightly decreased. These data suggest that the suppression of cortisol synthesis is responsible for the stimulation of the CRF ir-perikarya and the CRF release along the rostral pars distalis, inducing ACTH cell degranulation. However, the stressed condition of these eels may accentuate the response to IRX. The neurointermediate lobe, although slightly affected, does not seem to be directly controlled by the plasma cortisol level. These results are compared to those obtained after a pharmacological IRX produced by metopirone.

Corticotropin releasing factor stimulates growth hormone secretion in neonatal rats

Corticotropin releasing factor (CRF) both stimulates ACTH secretion from the pituitary and inhibits secretion of growth hormone (GH) in adult rats through actions in the CNS. The purpose of the present study was to evaluate these pituitary and central actions of CRF in neonatal rats, in which the hypothalamo- pituitary adrenal (HPA) axis is relatively hypo-functional. The results of this study show that central or peripheral administration of CRF evokes a marked dose-related rise in serum corticosterone in 6-day old rats. The same doses of CRF stimulate, rather than inhibit GH secretion. These results suggest that CRF has unique central actions early in ontogeny.

Glucose homeostasis and hypothalamic-pituitary-adrenocortical axis during development in rats

Glucoprivation represents a model stress in which activation of different stress responses at different ages can be monitored both in vivo and in vitro. Physiological data indicate rat brain contains a liver/pancreas-type glucose sensor, yet no biochemical or immunocytochemical evidence exists for such a sensor. Young rats appear to lack normal hypothalamic glucose-sensing ability and do not show typical secretory patterns of corticotropin-releasing factor, adrenocorticotropic hormone, or corticosterone after experimentally induced glucoprivation. However, they hypersecrete catecholamines and glucagon (compared with adults) and thrive on fuel sources other than glucose that are abundant after birth. High steroid levels during the first 24 h after birth may be critical for inducing gluconeogenic enzymes and promoting differentiation of tissues like pancreas. Neonatal rats also have unique control systems to combat the damaging effects of other stresses like hypoxia; these systems may disappear in adults. Thus the definition of stress may change during development, and the compensatory mechanisms employed to combat stress change from neonatal to adult life and are intricately related to the metabolic needs of the animal.