Inhibition of infant glucocorticoid stress response: specific role of maternal cues

Previous work from this laboratory has shown that passive contact with an anesthetized lactating dam eliminates the corticosteroid stress response to novelty in 12-, 16-, and 20-day-old rat pups. In the present study, we further examined some of the sensory and developmental aspects of this phenomenon. In two experiments, we asked whether inhibition of the corticosteroid stress response is related exclusively to contact with a maternal stimulus as opposed to other social stimuli. Experiment I compared the effectiveness of contact with an anesthetized dam vs, an anesthetized adult male rat. At 12 and 16 days of age, contact with an adult male was somewhat effective in reducing stress responsiveness, whereas at 20 days, contact with an adult male was entirely ineffective. At all ages, contact with an anesthetized dam was an effective inhibitor of the stress response. Experiment II compared the relative effectiveness of contact with an anesthetized dam vs. an anesthetized sibling pup. At all ages, contact with a dam was very effective, whereas contact with an anesthetized pup was entirely ineffective in inhibiting the glucocorticoid stress response. In Experiment III, the effect of maternal contact was assessed during a period of development when the biological, and presumably social, significance of the lactating-dam stimulus is changing. In this study, 20-, 24-, and 28-day-old rat pups revealed a progressive decrement in the ability of maternal contact to inhibit stress responsiveness. These findings replicate and extend our previous ones by showing that maternal stimuli play a special role in the modulation of pituitary-adrenal activity during development.     

Maternal regulation of the adrenocortical response in preweanling rats.

In the following studies, we investigated the effects of 24-h maternal deprivation on the infant's hypothalamic-pituitary-adrenal system. Experiment 1 examined the effect of deprivation on the infant's corticosterone (CORT) response to adrenocorticotropin hormone (ACTH) injection. At all ages studied, deprivation resulted in a potentiation of the response. At some ages, deprived nontreated pups had higher CORT levels than nondeprived pups. Experiment 2 examined the ontogeny of the deprivation-induced stress response, and the capacity of the mother to inhibit it. From 8 days of age onwards, deprived animals showed a CORT response to saline injection that was either absent or far smaller in nondeprived pups. Saline-induced CORT secretion was diminished, or prevented, by returning the infant to its dam. Maternal reunion had no effect on ACTH-induced CORT elevations. Finally, Experiment 3 investigated the effects of deprivation over a more extended period of time. In maternally deprived pups, ACTH-induced CORT elevations persisted for at least 2 h following reunion, but by 6 h had returned to baseline. These data suggest that maternal factors are involved in the regulation of the responsiveness of the pup's hypothalamic-pituitary-adrenal system.     

Time course of the effect of maternal deprivation on the hypothalamic-pituitary-adrenal axis in the infant rat.

Prolonged (i.e., 24-hr) maternal deprivation leads to a marked disinhibition of the infant rat's adrenocortical response to stress and/or ACTH. In the following study we examined the time course over which these effects develop. Pups were maternally deprived for varying lengths of time (i.e., 0, 2, 4, 8, & 24 hr); at the end of this period, corticosterone (CORT) secretion in response to stress (novelty or novelty plus saline injection) and ACTH injection was measured. Basal levels of CORT increased progressively over time in 7- and 11- (but not 3-) day-old pups. CORT release in response to stress followed a similar pattern. In contrast, ACTH injection resulted in marked increases in CORT levels regardless of the length of maternal deprivation in 3-day-old animals; at older ages, however, 24 hr of deprivation led to a much larger increase. These findings support the hypothesis that the hypothalamic-pituitary-adrenal axis of the neonatal rat is subject to maternal regulation.     

Long-lasting behavioral and neuroendocrine changes provoked in rabbits by cancelling a single nursing bout in early lactation

To restore estrus in lactating doe rabbits, nursing was prevented on lactation day 10 and allowed again from day 12 onward. This “biostimulation” procedure, used to accelerate reproduction, allows kit survival despite a 48 hr fast. Yet, the consequences of “biostimulation” on their psychobiological and neuroendocrine development are largely unknown. Therefore, we determined, in adult males and females, tested in the morning or afternoon: corticosteroid secretion (baseline and reactive), sexual behavior, and responses in six tests measuring stress/anxiety. The latter were not affected by maternal deprivation or time of testing in either sex. The lordosis quotient was reduced in deprived does (relative to controls), an effect eliminated when only non-kins (of both groups) were compared. Deprived males showed altered sexual behavior, evidenced by a large number of mounts not culminating in ejaculation. Corticosterone and cortisol secretion increased (relative to baseline) following i.m. saline in all groups, but the magnitude of the response was affected by maternal deprivation, time of day, and sex. Results indicate that particular behaviors and reactivity to stress have different thresholds regarding the effects of mother–litter separation in early lactation.     

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.     

An investigation of the effects of maternal separation and novelty on central mechanisms mediating pituitary-adrenal activity in infant guinea pigs (Cavia porcellus)

In mammalian species in which the young exhibit a strong filial attachment (e.g., monkeys, guinea pigs), numerous studies have shown that even brief separation from the attachment figure potently elevates circulating concentrations of glucocorticoids and adrenocorticotropic hormone (ACTH). However, effects of separation on central regulation of this stress response are not known. Therefore, we investigated central mechanisms mediating pituitary-adrenal activation during maternal separation and novelty exposure in guinea pig (Cavia porcellus) pups. Corticotropin-releasing factor (CRF) mRNA expression in the hypothalamic paraventricular nucleus (PVN), and plasma cortisol and ACTH levels, were elevated only during separation in a novel environment. C-Fos activity was elevated in the medial amygdala (MeA) and reduced in the bed nucleus of the stria terminalis (BNST) during novelty exposure, regardless of separation. On the other hand, c-Fos activity was elevated in the PVN during separation, regardless of novelty exposure. These results demonstrate independent and combined effects of separation and novelty in regions of the guinea pig CNS that regulate pituitary-adrenal activity. Moreover, they suggest that a pathway from MeA to BNST to PVN mediates responses to novelty in the guinea pig pup, as in the adult rat, though inputs from other cell populations appear required to fully account for the HPA activity observed here.     

Age-dependent effects of maternal deprivation on oxidative stress in infant rat brain

Developing brain is much more sensitive to all kind of stressors than the developed brain. Early maternal deprivation causes some behavioural and physiological effects on rats. After the birth, there is no endocrinological response to stressors between post-natal 4 and 14th days, which is called stress-hyporesponsive period (SHRP) in rats. This hypo-responsiveness is time- and stressor-specific, as some more severe stressors have been shown to induce a stress response. The present study examined the effects of maternal deprivation on oxidative stress in the hippocampus, prefrontal cortex (PFC) and striatum regions of the brain both during and after SHRP of the infant rats. The results showed that maternal deprivation in SHRP increased antioxidant enzyme activities and reduced lipid peroxidation in infant rat brain. However, by the termination of SHRP, maternal deprivation reduced enzyme activities and increased lipid peroxidation. The results indicated that infant brain might be protected in SHRP from maternal deprivation-induced oxidative stress.     

Pituitary-adrenal effects on avoidance-of-attack in mice: separation of the effects of ACTH and corticosterone.

Two experiments were conducted to separate the effects of ACTH and corticosterone on avoidance-of-attack. In Experiment 1, treatment with corticosterone restored the avoidance performance of hypophysectomized mice to normal levels. In Experiment 2, ACTH treatment increased the avoidance retention of intact mice but did not affect the avoidance responding of mice which were adrenalectomized and treated with a replacement dosage of corticosterone (controlled-corticosterone condition). These two studies suggest that corticosterone is the critical hormone of the pituitary-adrenal axis in the control of avoidance responding in agonistic situations. In addition, these studies demonstrate that (1) ACTH and corticosterone affect avoidance-of-attack differently from the way they have been shown to affect avoidance responding in shock-mediated situations, and (2) different hormones of the pituitary-adrenal axis are involved in the control of aggressiveness and avoidance-of-attack.     

Thermotaxis in neonatal rat pups

Deprived and nondeprived 3- and 6-day-old rat pups were tested on a thermal gradient for thermotaxis. Deprived 3-day olds demonstrated a marked thermotaxic responsiveness that was not observed in nondeprived 3-day olds or in deprived and nondeprived 6-day olds. Deprived 3-day olds did not show a strong thermal orientation immediately, but the distance moved by pups increased with repeated trials on the gradient. These findings demonstrate that neonatal rats are capable of thermotaxis, but that factors of age, deprivation and repeated handling determine pups' responsiveness to thermal gradients.     

Effect of age, food deprivation and stress on gastric erosions in the rat

Two experiments were conducted in which rats were subjected to food deprivation and/or cold-restraint stress. In Experiment 1, 40- and 570-day old rats exhibited a significant degree of glandular gastric erosion due to the stress and to the deprivation. Both of these were attributable to the single condition wherein deprivation and stress were combined. No effect involving age was significant. In Experiment 2, 22-day old weanlings exposed to the same conditions evinced a significant glandular erosion effect of the deprivation, but neither the stress nor the interaction effects were significant. It was concluded that 3 hr of cold restraint in the sated mature rat has a relatively minor effect on the stomach. Forty-eight hours of food deprivation, also ineffective by itself in such animals, renders the stomach vulnerable to the effects of stress. In the weanling, in contrast, deprivation alone can cause glandular erosion but its potentiating effect is lacking. With respect to ruminal lesions, it was hypothesized that time without food rather than initial body weight or nutritional deficit is the critical variable.     

Bidirectional responsiveness of the pituitary-adrenal system in old and young male and female rats

Bidirectional responsiveness of the pituitary-adrenal system was examined in young adult (3-5 months) and old (24-26 months) male and female rats. In Experiment 1, an approach-avoidance conflict was created by exposing food- and water-deprived subjects to a flavored solution which had previously been paired with lithium chloride-induced illness in a conditioned taste aversion paradigm. Young and old males and young females elevated plasma corticosterone in response to deprivation; males showed a further elevation when reexposed to the solution, whereas young females did not. Old females did not exhibit a corticosterone elevation to deprivation or upon reexposure. In Experiment 2, water was restricted to a fixed period in the morning when corticosterone levels are typically low. Plasma corticosterone was measured before and after watering. Young and old males and young females reentrained their circadian corticosterone rhythm so that levels were elevated just prior to watering; consumption was followed by a drop in corticosterone. Old females failed to reentrain or to suppress corticosterone secretion upon drinking. These findings indicate that the lability of the pituitary-adrenal system is more markedly affected by senescence in females than in males.     

Increased hypothalamic neuropeptide Y expression in deprived preweanling rats is reversed by intragastric infusion of milk

Lean, preweanling Zucker rat pups increase neuropeptide Y (NPY) expression in the hypothalamic arcuate nucleus in response to a 24-h deprivation of food, water, and maternal interaction as early as postnatal day 2 (P2). In this study, we examined if replacing nutritive or tactile aspects of maternal behavior to deprived rat pups could block the increased expression of hypothalamic NPY measured by in situ hybridization. On P2, P12, or P15, littermates were assigned to one of four treatment groups: (1) left with the dam for 24 h, (2) deprived of the dam for 24 h and given tactile stimulation in the form of periodic anogenital stroking to elicit urination and defecation, (3) deprived of the dam and given periodic anogenital stroking plus continuous gastric infusion of milk for 24 h, or (4) deprived of the dam and given periodic anogenital stroking plus continuous infusion of water for 24 h. We found that gastric infusions of milk normalized NPY expression at all three ages, gastric infusions of water did not on P2 and P15, and anogenital stroking alone had no effect. We suggest that the lack of milk is the major cause of increased hypothalamic NPY expression during maternal deprivation in lean Zucker pups.     

Behavioral mechanisms for active maternal potentiation of isolation calling in rat pups

The ultrasonic vocalization (USV) response of the isolated infant rat is a promising model for studying the neurobiology of an early anxiety state, and potentiation of the USV response after brief maternal encounters is a newly discovered behavioral regulator of this state. Using experimental variations in the contexts and patterns of maternal behavior during pup encounters and manual simulations of maternal behavior, we have identified several specific mother-pup interactions that cause potentiation. When one of these, pup carrying, was manually simulated, potentiation followed only if a characteristic postural transport response had been fully expressed by the pup. These behavioral mechanisms and other data suggest hypotheses for the development and adaptive role of maternal potentiation.     

人参皂甙通过垂体—肾上腺轴调整应激期NK细胞的受抑作用

本文主要探讨了人参皂甙抗手术应激时,NK细胞活性的调节与垂体—肾上腺激素分泌有关的机理。实验证实,人参皂甙能部分拮抗应激所致的小鼠脾NK细胞活性下降。其可能机制是,手术应激引起小鼠外周血ACTH、皮质酮含量增加,同时,脾细胞上高亲和力ACTHR在应激期表达也增加,因此,应激期升高的ACTH、皮质酮等作用在脾脏免疫活性细胞,导致免疫功能抑制;而人参皂甙能部分抑制应激期ACTH、皮质酮的增加,提高免疫细胞功能。由此而知,人参皂甙拮抗应激期NK细胞活性下降,与降低垂体—肾上腺轴活动有关。     

Integral as well as proportional adrenal responses to ACTH

To characterize adrenal responses to ACTH, conscious dogs were infused for 40 min with normal saline or 20-1,500 ng/min alpha-ACTH. Plasma ACTH and corticosteroid levels were measured during the infusion and for 80 min after the end of the infusion. Half-maximal plateau corticosteroid responses were achieved with ACTH levels of 105 pg/ml (23 pM); a maximal plateau corticosteroid response (10.2 micrograms/100 ml) resulted from ACTH levels greater than 335 pg/ml (74 pM). When levels of ACTH between 335 and 3,000 pg/ml were achieved during the 40-min infusion, the duration of maximal corticosteroid levels was prolonged beyond the duration of the ACTH infusion. Plasma ACTH concentration decreased after the end of the infusion with a half-disappearance time of 3.6 +/- 0.2 min, but plasma corticosteroid concentrations and cortisol and corticosteroid secretory rates remained at or above the plateau secretory rates during the infusion and for about 80 min after the end of an infusion of 1,000 ng ACTH/min. When the total corticosteroid responses to the ACTH infusions were estimated, the relationship between the total corticosteroid response and the logarithm of the ACTH concentration was linear for 80-3,000 pg ACTH/ml.     

The long-term effect of high-dose beclomethasone dipropionate on the pituitary-adrenal function]

The pituitary-adrenal function was studied in seven asthmatic subjects who had been received daily inhalations of 800 to 1,000 micrograms of beclomethasone dipropionate (BDP) over a year. None of the subjects had taken oral corticosteroids for at least six months prior to the study. As indices of pituitary-adrenal function, 1) circadian rhythm of plasma ACTH and cortisol, 2) urine 17-OHCS and 17-KS, and 3) the response of cortisol in rapid ACTH test were examined. All subjects showed normal circadian rhythms of plasma ACTH and cortisol levels. Urinary 17-OHCS and 17-KS excretions over a 24-hour period were also within the normal range. Plasma cortisol levels in the rapid ACTH test were significantly increased and judged as normal responses in all subjects. These results indicate that long-term treatment with BDP ranging from 800 to 1,000 micrograms/day induces no suppressive effect on the pituitary-adrenal function.     

Regulation of the developing hypothalamic-pituitary-adrenal axis in corticotropin releasing hormone receptor 1-deficient mice

During postnatal development, mice undergo a so-called stress hyporesponsive period, which is characterized by low basal corticosterone levels and the inability of mild stressors to induce a corticosterone response. The stress hyporesponsiveness is in part regulated by maternal factors. Twenty-four hours of deprivation results in an activation of basal and stress-induced corticosterone and a down-regulation of corticotropin releasing hormone (CRH), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) expression in the brain. It has been hypothesized that the CRH receptor 1 (CRHr1) may play an important regulatory role during development by mediating the effects of maternal deprivation. Using CRHr1-deficient mice we examined the role of this receptor on the maternal deprivation effects and in regulating the expression of hypothalamic-pituitary-adrenal axis-related genes. We could demonstrate that the CRHr1 is essential for the activation of the corticosterone response following maternal deprivation, most likely due to the lack of the receptor in the pituitary. Furthermore, we could show that the CRHr1 is regulating the expression of CRH and MRs. In contrast, effects of maternal deprivation during postnatal development on GRs are not mediated by this receptor.     

Adrenocortical response following acute neurogenic stimuli is mediated by CRF-41

The present study examined whether neurogenic stimuli activate the pituitary-adrenal axis via CRF-41. Adult male rats were exposed to photic, acoustic or sciatic nerve stimulation. At 4, 15, and 30 min following the onset of stress, animals were sacrificed, trunk blood collected and the median eminence removed. At 4 min following the stress onset, there was a significant decrease in CRF-41 content of the median eminence, which persisted for 30 min. Concomitant with the decrease in CRF-41 content, serum adrenocorticotropic hormone (ACTH) and corticosterone levels increased. Thus, this study demonstrates that CRF-41 released from the median eminence plays a dynamic role in mediating the ACTH and corticosterone response to neurogenic stimuli.     

Adrenocortical response to stress in fasted and unfasted artificially reared 12-day-old rat pups

Recent studies have compared artificially reared (AR) rats with dam-reared rats on behavioral outcomes but, despite the fact that they are deprived of their mothers during the stress hyporesponsive period (SHRP), little is known about the effects of AR on the development of the stress response. In this study, the corticosterone (CORT) response to a stressor (saline injection ip) on postnatal Day 12 was assessed in rat pups that had been either dam-reared (DR) or artificially reared since Day 5. In the preceding 24 hr, half the pups in the DR group were maternally deprived (DEP). To control for the food deprivation consequent to maternal deprivation in these groups, half the pups in the AR groups also underwent 24-hr food deprivation (DEP). In the nondeprived condition AR pups did not differ from DR pups on untreated CORT levels or on levels at 1-hr poststress (i.e., all rats demonstrated low levels of CORT characteristic of the SHRP). In contrast, both maternally deprived DR pups and food-deprived AR pups exhibited increased untreated CORT levels as well as a significant increase at 30-min poststress, but CORT elevations were lower in the AR groups than in the DR groups. Thus, long-term maternal deprivation through artificial rearing in rats does not affect the reduced CORT levels and reduced CORT responsiveness associated with the SHRP; however, if animals are food deprived, then all show increased basal CORT levels and a greater CORT response to stress, although this response is lower in AR groups than in DR groups. These results suggest that rat pups artificially reared with adequate nutrition will still exhibit the SHRP.