Context modulates outcome of perinatal glucocorticoid action in the brain.

Prematurely born infants may be at risk, because of inadequate maturation of tissues. If there are signs of preterm birth, it has become common practice therefore to treat either antenatally the mother or postnatally the infant with glucocorticoids to accelerate tissue development, particularly of the lung. However, this life-saving early glucocorticoid treatment was found to increase the risk of adverse outcome in later life. In one animal study, the authors reported a 25% shorter lifespan of rats treated as newborns with the synthetic glucocorticoid dexamethasone, but so far this finding has not been replicated. After a brief clinical introduction, we discuss studies in rodents designed to examine how perinatal glucocorticoid action affects the developing brain. It appears that the perinatal action of the glucocorticoid depends on the context and the timing as well as the type of administered steroid. The type of steroid is important because the endogenous glucocorticoids cortisol and corticosterone bind to two distinct receptor populations, i.e., mineralocorticoid and glucocorticoid receptors (GR), while synthetic glucocorticoids predominantly bind to the GR. In addition, if given antenatally hydrocortisone is inactivated in the placenta by 11ß-HSD type 2, and dexamethasone is not. With respect to timing, the outcome of glucocorticoid effects is different in early vs. late phases of brain development. The context refers to the environmental input that can affect the susceptibility to glucocorticoid action in the newborn rodent brain; early handling of pups and maternal care obliterate effects of post-natal dexamethasone treatment. Context also refers to coping with environmental conditions in later life, for which the individual may have been programed epigenetically by early-life experience. This knowledge of determinants affecting the outcome of perinatal glucocorticoid exposure may have clinical implications for the treatment of prematurely born infants.

Early experience of a novel-environment in isolation primes a fearful phenotype characterized by persistent amygdala activation.

Prolonged maternal separation (MS) activates the neonate's hypothalamus-pituitary-adrenal axis causing elevated basal and stress-induced corticosterone levels that may initiate amygdala-dependent fear learning. Here we test the hypothesis that the adult fearful phenotype is programmed by the pup's stressful experience during prolonged MS rather than by prolonged maternal absence per se. For this purpose, Wistar rat pups were exposed, on postnatal-day (pnd) 3, to: (i) repeated-MS in home-environment (HOME-SEP), 8h-MS daily for three days with the pups remaining together in the home-cage; (ii) repeated-MS in a novel-environment (NOVEL-SEP), with the same separation procedure, but now the pups were individually housed in a novel-environment during the 8h dam's absence; (iii) repeated handling, which consisted of daily brief (15 min instead of 8h) MS in the home-altogether or in a novel-environment individually (HOME-HAN and NOVEL-HAN, respectively); (iv) no-separation/no-handling (NON-SEP/NON-HAN) control condition, in which pups were left undisturbed in their home-cage. Compared to HOME-SEP rats, the NOVEL-SEP rats showed one day after the last MS enhanced stress-induced amygdala c-Fos expression and ACTH-release, despite of reduced adrenal corticosterone secretion. The higher amygdala c-Fos expression, ACTH-release and reduced corticosterone output observed postnatally, persisted into adulthood of the NOVEL-SEP animals. Behaviorally, NOVEL-SEP juvenile rats displayed deficits in social play, had intact spatial memory in the peri-pubertal period and showed more contextual fear memory compared to HOME-SEP in adulthood. Finally, NOVEL-HAN, compared to HOME-HAN, displayed increased stress-induced corticosterone output, no deficits in social play and reduced contextual fear. In conclusion, programming of an adult fearful phenotype linked to amygdala priming develops if pups are repeatedly isolated from peers in a novel-environment, while away from the dam for a prolonged period of time.

Acute effects of neonatal dexamethasone treatment on proliferation and astrocyte immunoreactivity in hippocampus and corpus callosum: towards a rescue strategy.

Dexamethasone (DEX), a synthetic glucocorticoid, has been used to treat respiratory distress syndrome in prematurely born infants. Despite the important short-term benefit on lung function, there is growing concern about the long-term outcome of this treatment, since follow-up studies of prematurely born infants have shown lasting adverse neurodevelopmental effects. Since the mechanism underlying these neurodevelopmental impairments is largely unknown, the aim of the present study was (i) to investigate the acute effects of neonatal DEX treatment on the developing brain; and (ii) to block specifically the effects of DEX on the brain by central administration of the glucocorticoid receptor (GR) antagonist mifepristone. Long Evans rat pups were injected subcutaneously with tapering doses of DEX or saline (SAL) on postnatal days (pnd) 1, 2 and 3. Separate groups received intracerebroventricular injections with mifepristone prior to DEX treatment. On pnd 4 and 10, pups were sacrificed and brains collected for analysis of cell proliferation (Ki-67) and astrogliosis (GFAP). We report that neonatal DEX treatment reduced hippocampal cell proliferation on pnd 4, an effect that was normalized by pnd 10. Although on pnd 4, GFAP expression was not affected, DEX treatment caused a significant reduction in the number and density of astrocytes in hippocampus and corpus callosum on pnd 10, which was normalized by mifepristone pre-treatment. These acute alterations in the neonate brain might underlie later functional impairments reported in DEX-treated animals and humans and further illustrate the impact of early GR activation on brain development.

Early handling modulates outcome of neonatal dexamethasone exposure.

Synthetic glucocorticoids such as dexamethasone (DEX) are used to prevent or treat respiratory disorders in prematurely born infants. Besides the short-term benefit on lung development, numerous human and animal studies have reported adverse neurodevelopmental side effects. In contrast, maternal care is known to exert a positive influence on neurodevelopmental outcome in rodents. The aim of the current study was therefore to investigate whether neonatal handling (days 1-21), known to induce maternal care, might serve as an intervention strategy modulating the adverse effects of DEX treatment (days 1-3). For this purpose we have measured the outcome of these early-life manipulations on development as well as adult endocrine and behavioral phenotype of male rats. Maternal care was observed during the first week of life and indeed enhanced in response to handling. Eye opening was accelerated and body weight reduced in DEX-treated animals. In adulthood, we report that handling ameliorated impaired spatial learning observed in DEX treated non-handled animals in the T-maze. Additionally, handling reduced susceptibility to the impact of DEX treatment in the water maze. Although DEX treatment and handling both resulted in enhanced negative feedback of the stress-induced corticosterone response and both reduced startle reactivity, the acquisition of fear was only reduced by handling, without effect of DEX. Interestingly, handling had a beneficial effect on pre-pulse inhibition, which was diminished after DEX treatment. In conclusion, these findings indicate that handling of the neonate enhances maternal care and attenuates specific DEX-induced alterations in the adult behavioral phenotype.

The newborn rat's stress system readily habituates to repeated and prolonged maternal separation, while continuing to respond to stressors in context dependent fashion.

Adrenal corticosterone secretion of newborn mice rapidly desensitizes to repeated maternal absence. The present study investigated the effects of novelty exposure, maternal care and genotype on this phenomenon. Maternal separation (MS) took place on postnatal days (pnd) 3-5. In Wistar rats, the degree of novelty in the MS-environment was varied by exposing pups to: (i) "home separation": pups remained in the home cage; (ii) "novel separation": pups were placed individually in a novel cage. Maternal care was recorded on pnd 1 to 4. To investigate the effect of genotype, we also examined Long Evans in the "home separation" condition. Basal and stress-induced ACTH and corticosterone levels were measured. Adrenal tyrosine hydroxylase (TH) and melanocortin receptor-2 (MCR-2) proteins served as markers for adrenal function. We show, in both rat strains, that the rise in plasma corticosterone induced by a single 8h-MS on pnd 5 was abolished, when this separation procedure had also been performed on pnd 3 and 4. Habituation to maternal absence occurred irrespective of housing conditions. However, pups in the "home separation" condition received less maternal care upon reunion than those placed in the "novel separation". These "home separation" pups appeared more responsive to a subsequent acute novelty-stressor, and their adrenal TH and MCR-2 were higher. Long Evans rats appeared more stress responsive than the Wistars, in the home separation condition. In conclusion, separation environment, maternal care and genotype do not affect adrenal desensitization to repeated 8 h-MS itself, but may modulate the adrenal stress-responsiveness of separated pups.

Development of individual differences in stress responsiveness: an overview of factors mediating the outcome of early life experiences.

RATIONALE: Human epidemiology and animal studies have convincingly shown the long-lasting impact of early life experiences on the development of individual differences in stress responsiveness in later life. The interplay between genes and environment underlies this phenomenon. OBJECTIVES: We provide an overview of studies investigating the impact of early life experiences on the development of individual differences in neuroendocrine stress responsiveness in adulthood and address (1) impact of environment on later stress phenotypes, (2) role of genetic factors in modulating the outcome of environment, and (3) role of nonshared environmental experience in the outcome of gene × environment interplays. We present original findings where we investigated the influence of nonshared experiences in terms of individual differences in maternal care received, on the development of stress phenotype in later life in rats. RESULTS: Environmental influences in early life exert powerful effects on later stress phenotypes, but they do not always lead to expression of diseases. Heterogeneity in response is explained by the role of particular genetic factors in modulating the influence of environment. Nonshared experiences are important in the outcome of gene × environment interplays in humans. We show that nonshared experiences acquired through within-litter variation in maternal care in rats predict the stress phenotype of the offspring. CONCLUSION: The outcome of early experience is not deterministic and depends on several environmental and genetic factors interacting in an intricate manner to support stress adaptation. The degree of "match" and "mismatch" between early and later life environments predicts resilience and vulnerability to stress-related diseases, respectively.