Pituitary-adrenocortical response to stress in the neonatal rat

Isoflurane stress induces region-specific glucocorticoid levels in neonatal mouse brain

The profound programming effects of early life stress (ELS) on brain and behavior are thought to be primarily mediated by adrenal glucocorticoids (GCs). However, in mice, stressors are often administered between postnatal days 2 and 12 (PND2-12), during the stress hyporesponsive period (SHRP), when adrenal GC production is greatly reduced at baseline and in response to stressors. During the SHRP, specific brain regions produce GCs at baseline, but it is unknown if brain GC production increases in response to stressors. We treated mice at PND1 (pre-SHRP), PND5 (SHRP), PND9 (SHRP), and PND13 (post-SHRP) with an acute stressor (isoflurane anesthesia), vehicle control (oxygen), or neither (baseline). We measured a panel of progesterone and six GCs in the blood, hippocampus, cerebral cortex, and hypothalamus via liquid chromatography tandem mass spectrometry. At PND1, baseline corticosterone levels were high and did not increase in response to stress. At PND5, baseline corticosterone levels were very low, increases in brain corticosterone levels were greater than the increase in blood corticosterone levels, and stress had region-specific effects. At PND9, baseline corticosterone levels were low and increased similarly and moderately in response to stress. At PND13, blood corticosterone levels were higher than those at PND9, and corticosterone levels were higher in blood than in brain regions. These data illustrate the rapid and profound changes in stress physiology during neonatal development and suggest that neurosteroid production is a possible mechanism by which ELS has enduring effects on brain and behavior.

Effects of maternal separation during early postnatal development on male sexual behavior and female reproductive function

The endocrine response to stress is an important homoeostatic mechanism, and the secretion of glucocorticoids from the adrenal cortex is a central feature of this response. During early postnatal development, the neonatal rat displays a reduced hypothalamic-pituitary-adrenal (HPA) response to stress. This early period has been termed the 'stress hyporesponsive period' (SHRP). Maternal separation (Sep) of neonates from their mothers during early postnatal development alters the HPA response to stress. In this study, we report the effects of Sep during the SHRP. Female rats were time mated and randomly divided into control or Sep groups before birth. The Sep litters were removed from the mothers during the dark cycle for 6 h per day from postnatal day (PND) 2 to 10. On PND 28, the pups from both groups were weighed, the anogenital distance (AGD) was measured and the animals weaned. At 40 days of age, male and female animals from both groups were tested for open-field activity. As the animals matured, vaginal opening and estrous cycles were measured in females, and males were tested for male sexual behavior at adulthood. Basal, stress, and stress recovery serum corticosterone levels were measured from control and Sep male and female animals. Open-field activity was not significantly different between control or Sep male or female animals. Sep did not affect either vaginal opening or estrous cycles in female animals. Corticosterone secretion in response to stress was similar in control and Sep males and females; however, the recovery levels were significantly higher in Sep females than in Sep males or female control values. In male sexual behavior tests, Sep males had significantly longer mount latencies (time to the first mount), longer intromission latencies (time to the first intromission) and a significant reduction in the percent of animals ejaculating versus control values (controls 84 and Sep 50%). Therefore, Sep males as adults displayed altered reproductive behavior, whereas their stress recovery levels of corticosterone returned to near basal levels in a similar fashion to that observed for control non-handled males. In contrast, females displayed normal reproductive physiology, while their recovery levels of corticosterone remained high, unlike that observed with control females. Thus, significant gender differences in response to Sep (during the dark phase of the circadian cycle) were observed in the paradigm used in the present study.

Differential effects of prenatal and postnatal ACTH or nicotine exposure on 5-HT high affinity uptake in the neonatal rat brain

These studies were designed to examine the differential effects of prenatal or postnatal administration of ACTH 1-39 and nicotine, on 5-HT high affinity uptake in brainstem and hippocampal synaptosomes. ACTH was administered prenatally (to pregnant dams) and postnatally to the neonates. Postnatal administration of ACTH significantly increased high-affinity 5-HT uptake in the hippocampus and especially the brainstem at both 7 and 21 days after birth. Prenatal ACTH, on the other hand, transiently increased 5-HT uptake in only the brainstem at 7 days, a change that was reversed at 21 days. While the effects of postnatal nicotine administration were essentially the same as those of postnatal ACTH treatment, prenatal nicotine, unlike ACTH, did not alter 5-HT uptake in 7-day-old rats but did reduce uptake in both tissues at 21 days. The observation that postnatal nicotine mimics the effects of postnatal ACTH and that nicotine stimulates ACTH release, suggests that the postnatal effects of nicotine may be exerted through ACTH.

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.

Inhibition of aromatase activity during pregnancy affects sex-related adrenocorticotropic hormone release in response to ether-inhalation in juvenile rats

The pituitary release of adrenocorticotropic hormone (ACTH) induced by ether inhalation was investigated on day 30 postpartum in male and female young rats from mothers treated daily with the aromatase inhibitor ATD (1,4,6-androstatriene 3,17-dione) from day 17 to day 21 of gestation, in males from mothers injected daily with solvent during the same period of gestation. In spite of a surge in plasma testosterone levels at birth, the males from ATD-treated mothers showed on day 30 postpartum a pituitary response characteristic of the females and not of the males. Thirty minutes after the end of the ether inhalation, the males from ATD-treated mothers showed a surge in plasma ACTH similar to that of littermate females but significantly higher than that of males from solvent-treated mothers. These data suggest that the aromatization of endogenous testosterone to estrogens during the critical period of the fetal life, when a transient surge of plasma testosterone was observed, has a significant organizational effect on the sexually dimorphic ACTH response to the ether stress.

Endocrine responses to mother-infant separation in developing rats

Separation of neonatal rat pups from the dam have been reported to elicit two endocrine responses in the pup: a fall in growth hormone secretion and a rise in corticosterone secretion. However, the temporal, ontogenetic, and behavioral determinants of these responses have not been compared. In the present study, we report that these two responses can be differentiated on each of these criteria. Growth hormone secretion falls rapidly immediately upon separation of pups from the dam, while robust rises in corticosterone secretion are delayed for many hours. In addition, growth hormone responses are observed earlier in ontogeny. Finally, active maternal behavior is required for normal growth hormone secretion in 10-day-old rat pups, while passive sensory stimuli associated with the dam can significantly reduce the corticosterone response to separation.

Prenatal exposure to ethanol alters the ontogeny of the beta-endorphin response to stress

To determine whether prenatal exposure to ethanol alters the response of the beta-endorphin (beta-EP) system to stress, the effect of two types of stressful stimuli, ether and cold, was examined in the offspring of rats which during pregnancy were: (a) fed with an ethanol-containing diet; (b) pair-fed with an isocaloric sucrose diet; and (c) fed ad libitum with standard lab chow (basic control group). The effect of stress on the content of beta-EP in the serum, pituitary gland and hypothalamus, as well as on the serum corticosterone and hypothalamic corticotropin-releasing factor (CRF) content was examined. Pups prenatally exposed to ethanol had significantly higher serum beta-EP levels on Day 1 and higher serum corticosterone levels on Days 1-3 when compared to their pair-fed or basic controls. On all days tested pituitary beta-EP content was lower in the offspring of the ethanol-treated rats than in the control groups. There was no difference in the total hypothalamic beta-EP content between the three treatment groups; however, during the first 10 days of life a higher concentration (ng/mg protein) of beta-EP was observed in the hypothalami of the ethanol and the pair-fed group when compared to the basic control pups. Hypothalamic CRF levels, though significantly lower in the pups exposed to ethanol in utero than in the control groups on Day 3, increased significantly in the ethanol group between Days 14 and 22, while no significant change was observed during this period in either of the control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary-adrenal response to bacterial endotoxin in developing rats

The neonatal rat is very sensitive to the lethal effects of bacterial endotoxin. Because of the adaptive importance of pituitary-adrenal secretions to stress, this study examined the ontogeny of the plasma corticosterone and adrenocorticotropic hormone (ACTH) responses to endotoxin. The lethal sensitivity of young rats to endotoxin ranged from 0.5 to 30 mg/kg (ip) in the 1- to 21-day-old rat. After endotoxin treatment, the 1- and 2-day-old rat showed marked elevations of corticosterone similar in magnitude to that seen in 21-day-old and adult rats; however, significantly depressed corticosterone increments were observed in the 5-, 10-, and 14-day-old rats. This age-related pattern of adrenocortical secretion was correlated with the developing rat's corticosterone response to exogenous ACTH. In contrast, endotoxin administered to 5-, 10-, and 14-day-old rats resulted in increments of plasma ACTH similar to those observed in the 21-day-old and adult rats. Although plasma ACTH levels increased by 84-127% in the 1- and 2-day-old rats, these increases were significantly less than those of rats at all other ages tested. Thus the newborn rat mounts an effective corticosterone response to endotoxin, loses this ability between ages 5-14 days, and regains this response at 21 days of age. Because the hyporesponsive ages exhibit a marked increase in ACTH secretion, the loss of the adrenocortical response to endotoxin appears to be a result of a depressed responsiveness of the adrenal cortex to ACTH.

Effect of neonatal handling on age-related impairments associated with the hippocampus

In rats, an environmental manipulation occurring early in life resulted in changes in the adrenocortical axis that persisted throughout the entire life of the animals and attenuated certain deficits associated with aging. Rats handled during infancy had a permanent increase in concentrations of receptors for glucocorticoids in the hippocampus, a critical region in the negative-feedback inhibition of adrenocortical activity. Increased receptor concentrations led to greater hippocampal sensitivity to glucocorticoids and enhanced negative-feedback efficacy in the handled rats. Thus, at all ages tested, rats that were not handled secreted more glucocorticoids in response to stress than did handled rats. At later ages, nonhandled rats also showed elevated basal glucocorticoid levels, with the result that there was a greater cumulative exposure to glucocorticoids in nonhandled rats. Increased exposure to adrenal glucocorticoids can accelerate hippocampal neuron loss and cognitive impairments in aging. Hippocampal cell loss and pronounced spatial memory deficits emerged with age in the nonhandled rats, but were almost absent in the handled rats. Previous work showed that glucocorticoid hypersecretion, hippocampal neuron death, and cognitive impairments form a complex degenerative cascade of aging in the rat. The present study shows that a subtle manipulation early in life can retard the emergence of this cascade.

The development of the glucocorticoid receptor system in the rat limbic brain. I. Ontogeny and autoregulation

In order to characterize the development of the glucocorticoid receptor system in the brain, we examined [3H]dexamethasone binding in rat pups at various ages. Using an in vitro, cytosol, receptor assay we found evidence for low levels of glucocorticoid receptors perinatally with a subsequent increase in receptor concentrations that began by about the end of the first week of life. We have also shown that receptors during this period have a ligand specificity similar to that of receptors in adult animals. The postnatal increase in receptor levels parallels an increase in circulating corticosterone titers. Thus, receptor and hormone levels increase coincidentally. In adult animals, however, increasing levels of corticosterone are associated with a decrease in receptor levels and vice versa, such that corticosterone is thought to regulate its own receptor (i.e. autoregulation). This suggested an absence of autoregulation during development. We then determined hippocampal receptor concentrations of rats treated for 5 days with corticosterone, or adrenalectomized (ADX) 5 days prior to assay, examining whether up- or down-regulation occurs throughout development. In adults corticosterone treatment decreased (-45%) and long-term adrenalectomy increased (211%) glucocorticoid receptor concentrations. In contrast, at the youngest age tested (Day 10), the effects of manipulations of corticosterone titers on receptor concentrations were negligible. The potential for autoregulation emerged gradually throughout development. Thus, it appears that corticosterone regulation of its own receptors emerges only by about the time of puberty, and that this permits an increase in receptor levels to occur despite the concurrently increasing levels of circulating corticosterone.

The development of the glucocorticoid receptor system in the rat limbic brain. II. An autoradiographic study

We report here the results of an autoradiographic analysis of the postnatal development of the hippocampal glucocorticoid receptor system in the rat brain. Quantitative analysis of the autoradiograms revealed a varied pattern of gradual development towards adult receptor concentrations during the second week of life. Receptor concentrations in the dentate gyrus increased dramatically between Days 9 and 15, while the changes during this period in the pyramidal layers of Ammon's horn seemed to reflect both structural changes in these regions as well as increases in receptor concentrations.

Opiate receptor ontogeny and morphine-induced effects: influence of chronic footshock stress in preweanling rats

The ontogeny of opiate receptors was examined in various CNS regions of preweanling rats which received either daily inescapable footshock stress, exposure to a footshock apparatus without shock, or no handling from birth to 21 days of age. At 21 days of age, each of these treatment groups was also assessed for morphine-induced changes in activity, hot-plate paw-lick latency, and core body temperature. Marked regional differences in [3H]naloxone binding capacity were observed from 7 to 21 days of age in spinal cord, medulla-pons, midbrain, hypothalamus, striatum, and cortex. Caudal regions approached adult-like [3H]naloxone binding before rostral regions. The normal ontogeny of opiate receptors was not significantly influenced by the chronic footshock treatment. However, footshock treatment significantly reduced the efficacy of morphine (2 mg/kg) in producing hypoactive and antinociceptive effects, but not in producing a hyperthermic effect. These results demonstrate that stress-related changes in the behavioral efficacy of morphine do not necessarily depend upon changes in those opiate receptor populations that bind naloxone.

Effects of early experience and dexamethasone on adrenocortical reactivity

This investigation tested the hypothesis that changes in circulating adrenocortical hormones mediate the long term effects of shocking infant rats. Dexamethasone administered through the mothers' milk was used to block adrenocortical responses to peripheral shock stimulation by the young. Control experiments indicated that the dexamethasone was present in the pups as early as 2 days after the mothers began drinking water containing dexamethasone. Shock reduced adrenocortical reactivity under conditions where the mother and the young were capable of an adrenocortical response to the shock, where either the mother or the young could have a response, and where neither the mother nor the young could have an adrenocortical response to this early stimulation. These data do not support the hypothesis that a pituitary-adrenocortical response of either the mother or the young mediates the effects of stimulation in infancy on subsequent adrenocortical reactivity.