Persistent effects of maternal deprivation on HPA regulation can be reversed by feeding and stroking, but not by dexamethasone.

Maternal deprivation of neonatal rats for 24 h has immediate and persistent effects on hypothalamic-pituitary-adrenal (HPA) regulation. Immediately after deprivation corticosterone (CORT) is elevated. The primary purpose of our experiments was to determine if, by preventing this CORT elevation, the persistent effects could be reversed. In experiment 1, pups were injected with dexamethasone at the onset of the 24-h deprivation period on postnatal day 11 to suppress the rise in CORT. In experiment 2 some aspects of maternal behaviour known to suppress CORT levels were mimicked during deprivation from postnatal days 11-12. The pups were either: (1) left undisturbed; (2) stroked periodically; or (3) stroked and episodically fed. At postnatal day 20 basal and stress-induced adrenocorticotrophic hormone (ACTH) and CORT levels were measured as well as brain mineralocorticoid (MR) and glucocorticoid receptors (GR). Neonatal rats receiving dexamethasone prior to the onset of the deprivation on day 11 did not differ on day 20 from deprived pups that were exposed to elevated CORT levels. There were no detectable changes in the non-deprived pups that were treated with dexamethasone. In contrast, feeding and stroking during the period of deprivation obliterated the persistent effects both with regard to the reduced ACTH response and the decreased GR mRNA in hippocampus and hypothalamus. Stroking alone appears to have no influence. In conclusion, the persistent reduction of the ACTH response to mild stress and the decrease of GR mRNA is not mediated by deprivation-induced elevations in CORT, but appears to be reversible by reinstating specific aspects of the dam's nurturing behaviour.

Early vs. late maternal deprivation differentially alters the endocrine and hypothalamic responses to stress.

Twenty-four hours of maternal deprivation results in persistent changes in the ACTH response to mild stress. These effects are dependent on the age of the neonate at the time of deprivation. Pups that were separated from the dam at postnatal days (pnd) 3-4 showed an enhanced stress-induced ACTH response at age 20, while pups deprived at pnd 11-12 displayed an attenuated ACTH response to stress at that time. The present study was designed to test the hypothesis that the immediate effects of deprivation at pnd 3 vs. pnd 11 would provide an explanation for these paradoxical effects observed at day 20. For this purpose, we measured the basal and the stress-induced ACTH and corticosterone (CORT) response at days 4 and 12, following 24 h of maternal deprivation. Furthermore, we examined whether similar differences in c-fos and CRH mRNA expression in the paraventricular nucleus (PVN) accompanied the differences in response characteristics of ACTH at pnd 20. The results indicate that changes in the ACTH and CORT responses were minimal after 24 h of maternal deprivation at day 4, whereas these hormones were markedly elevated following deprivation at day 12. The persistent effects also showed age-dependency: pups deprived early showed at pnd 20 an exaggerated ACTH response. Late deprived juveniles exhibited an attenuated ACTH response to stress at pnd 20 while in both conditions the CORT response was not different from the non-deprived litter mates. These persistent endocrine changes were accompanied by the changes in the neural stress markers. The expressions of c-fos and CRH mRNA in the PVN were at pnd 20 significantly greater in their controls in early-deprived animals. The late deprived juveniles showed a reduced response in these neural markers. In conclusion, the persistent alterations in ACTH are reflected by changes in c-fos and CRH mRNA, but these changes in endocrine and in neural stress markers do not appear to be related to any of the hormonal changes that occur at the time of maternal deprivation.

Maternal deprivation effect on the infant's neural stress markers is reversed by tactile stimulation and feeding but not by suppressing corticosterone.

After 24 hr of maternal deprivation, significant elevations in ACTH and the naturally occurring glucocorticoid corticosterone (CORT) are observed during the stress-hyporesponsive period. The deprived pups also showed in the paraventricular nucleus (PVN) a marked increase of stress-induced c-fos mRNA and a reduction of corticotropin-releasing hormone (CRH) and glucocorticoid receptor (GR) mRNA; in hippocampal CA1, a reduction of the mineralocorticoid receptor (MR) and GR was observed. Here, we examined whether these changes are reversed by (1) preventing the elevations of CORT characteristic for the 11-d-old deprived pups by administering the synthetic glucocorticoid dexamethasone (DEX); or (2) reinstating some aspects of maternal behavior. The pups were either (1) left undisturbed, (2) stroked, or (3) stroked and episodically fed by cheek cannulation. At postnatal day 12, peripheral and neural stress markers were measured. Nondeprived animals served as controls. Experiment 1 demonstrates that although CORT was kept low by DEX, the central effects on CORT receptors, CRH, and c-fos mRNA were still present, except for MR in hippocampal CA1. Experiment 2 shows that stroking alone prevented the stress-induced rise in ACTH and c-fos mRNA and in the reduction in CRH and MR mRNA. In pups that were fed and stroked, CORT and GR mRNA resembled nondeprived controls. In conclusion, the changes in peripheral endocrine responses and in the brain cannot be attributed to the effect of elevated CORT concentrations, which are characteristic of the maternally deprived neonate. However, reinstating some components of the dams' nurturing behavior can reverse the effects evoked by maternal deprivation.

The ontogeny of glucocorticoid negative feedback: influence of maternal deprivation.

Glucocorticoid feedback can be viewed as having two modes of operation: proactive and reactive. "Proactive" feedback maintains basal activity of the hypothalamic-pituitary-adrenal axis, whereas the termination of stress-induced hypothalamic-pituitary-adrenal activity is facilitated by "reactive" feedback. In the present study we studied the ontogeny of both feedback modes and tested the hypothesis that the development of feedback depends on mother-pup interaction. On postnatal day 9 or 12, pups were deprived (DEP) of the dam for 24 h; nondeprived pups of the same age served as controls. The pups were adrenalectomized (ADX) at the end of deprivation and given corticosterone (CORT) replacement by either injection or pellet implants using the following two designs: first at the time of adrenalectomy (ADX) to test the role of CORT in the maintenance of basal ACTH levels, and then 3 h after ADX, to investigate CORT suppression of elevated ACTH levels induced by prior ADX. Regarding proactive feedback, the results showed that injection of CORT at the time of ADX was only partially effective in preventing ACTH elevations, whereas CORT pellets maintained basal levels of ACTH in all ADX pups. The reactive mode of negative feedback in nondeprived pups was resistant to CORT injection, whereas the CORT pellet resulted in a return to basal levels within 60 min. Maternal deprivation did not affect proactive feedback, but caused a more sustained increase in ACTH levels and a failure to return to basal levels 3 h after ADX despite significantly higher levels of circulating CORT in these DEP pups. It is concluded that 1) proactive and reactive modes of negative feedback are operative, provided the pups are maintained on chronic replacement with CORT; 2) DEP impairs the reactive, rather than the proactive, mode of feedback inhibition in the neonate.

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.

Differential effect of ACTH1-24 and alpha-MSH induced grooming in the paraventricular nucleus of the hypothalamus.

Injection of ACTH1-24 as well as alpha-MSH in the paraventricular nucleus of the hypothalamus (PVH) induces intense grooming in the rat. While comparing the details of MSH, ACTH and control grooming, we found that the induction of grooming was highly site specific. Even injection of saline in that specific area produced some grooming, possibly due to the release of endogenous substances. To distinguish between effects caused by the peptides and the effects caused by the injection procedure, we compared the behavioural effects of saline and peptide injections in sites with exactly the same location in the PVH, in a post-hoc matched pairs design. Using this design we found that the grooming response induced by saline is of a limited duration. ACTH1-24 and alpha-MSH prolong grooming beyond that period. Interestingly, rats receiving alpha-MSH continued to groom, while rats receiving ACTH1-24 changed to scratching. This confirms earlier findings suggesting that grooming and scratching have a differential organization at the level of the PVH. Whether the peptides also have a role in the initiation of the grooming response, or just prolong a response caused by other local factors requires another experimental approach.