Seymour Levine

Developmental determinants of sensitivity and resistance to stress

The purpose of this paper is two fold. First, to revisit the issue of the definition of stress and to highlight the difficulties with the contemporary definitions and, second, to review the literature on the influence of early experiences on the endocrine stress responses and behavior in rodents, sub-human primates and humans. Early experiences, usually involving some manipulation that results in disruption of the mother-infant relationship, have been shown to have long-term influences on the behavioral and endocrine responses to stress. In the rodent, brief periods of separation result in an attenuated adrenal response to stress (reduced secretion of corticosterone). In contrast, longer periods of separation result in an exaggerated response and several behavioral anomalies i.e. increased alcohol consumption, increased startle response etc. However, the effects of disruptions of the mother-infant relationships in primates reveal a pattern of behavioral disturbance but little influence on the endocrine response. Brief maternal separations result in a blunted cortisol response in juvenile squirrel monkeys. The long-term effects of early experiences in humans are very difficult to interpret. It is not possible to determine the length and severity of the experiences, and when in development the experiences were imposed on the child. Despite these limitations, there is a general consensus that adverse early experiences contribute to adult psychopathology.

Influence of psychological variables on the activity of the hypothalamic-pituitary-adrenal axis.

Psychobiology is the discipline that attempts to integrate the impact of environmental and psychological variables on biological systems. This paper focuses on the psychobiology of the hypothalamic-pituitary-adrenal (HPA) axis and illustrates several processes that influence the response of the HPA axis. The interaction of the developing rodent or primate with their primary care giver has permanent long-term effects on the HPA axis. Manipulations that alter maternal behavior during critical periods of development permanently modify the HPA axis. The HPA axis can be programmed to be hypo-responsive or hyper-responsive as a function of time and length of maternal separation. In the adult organism, the HPA response to stress is highly dependent on specific psychological factors such as control, predictability, and feedback. In primates, social variables have been shown to diminish or exacerbate the HPA stress response. During the post-natal period of development, the mother appears to actively inhibit the pups HPA axis. Different aspects of maternal behavior regulate different components of the HPA system.

Stress and salivary cortisol during pregnancy.

The purpose of this study was to determine whether exposure to stressful life events was associated with changes in levels of circulating cortisol during pregnancy in a population of 603 pregnant women. The participating pregnant women filled out a questionnaire and collected a morning and evening sample of saliva in early pregnancy (median 14th gestational week) and in late pregnancy (median and 30th gestational week). They were asked to report the number of life events experienced during first and second trimester, respectively, and were asked to rate the intensity of the experienced events. Complications related to the pregnancy such as vaginal bleeding and suspected growth retardation were registered and the women were asked about concerns about their pregnancy. The salivary samples were analyzed for cortisol and the levels were higher in late than in early pregnancy. In late pregnancy women exposed to more than one life event or were concerned about pregnancy complications during second trimester had a higher evening cortisol level, whereas morning values were unaffected. After adjustment for smoking women who experienced more than one very stressful life event had 27% higher evening cortisol concentrations (95% confidence intervals: 1-59%). Women with worries about pregnancy complications had 27% (95% confidence intervals: 2-57%) higher levels. In early pregnancy women reporting stressful life events did not have higher evening cortisol levels, but tended to have a blunted morning HPA response. In conclusion, we found differences in the associations between chronic stress in early and late pregnancy and cortisol levels indicating that the response to chronic stress is dependent on the stage of the pregnancy.

Morning cortisol Levels in preschool-aged foster children: Differential effects of maltreatment type†

Maltreated foster children are subjected to a range of early adverse experiences, including neglect, abuse, and multiple caregiver disruptions. Research suggests that such disturbances alter the development and subsequent functioning of the hypothalamic-pituitary-adrenocortical system. The current study was designed to investigate morning cortisol levels in 117 foster children and 60 low-income, nonmaltreated children. Maltreatment and foster care placement experiences were coded from official records. Analyses revealed that the foster children were significantly more likely than the nonmaltreated children to have low morning cortisol levels. Additionally, specific maltreatment experiences were significantly associated with the foster childrens morning cortisol levels. Foster children with low morning cortisol levels experienced more severe physical neglect than the other foster children. In contrast, foster children with high morning cortisol levels experienced more severe emotional maltreatment. These results suggest that specific early adverse experiences have differential effects on the functioning of the hypothalamic-pituitary-adrenocortical system.

The postnatal development of the hypothalamic-pituitary-adrenal axis in the mouse.

The main characteristic of the postnatal development of the stress system in the rat is the so‐called stress hypo‐responsive period (SHRP). Lasting from postnatal day (pnd) 4 to pnd 14, this period is characterized by very low basal corticosterone levels and an inability of mild stressors to induce an enhanced ACTH and corticosterone release. During the last years, the mouse has become a generally used animal in stress research, also due to the wide availability of genetically modified mouse strains. However, very few data are available on the ontogeny of the stress system in the mouse. This study therefore describes the postnatal ontogeny of peripheral and central aspects of the hypothalamic–pituitary–adrenal (HPA) axis in the mouse. We measured ACTH and corticosterone in blood and CRH, urocortin 3 (UCN3), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) transcripts in the brain at postnatal days 1, 2, 4, 6, 9, 12, 14 and 16. Our results show that we can subdivide the postnatal development of the HPA axis in the mouse in two phases. The first phase corresponds to the SHRP in the rat and lasts from right after birth (pnd 1) until pnd 12. Basal corticosterone levels were low and novelty exposure did not enhance corticosterone or ACTH levels. This period is further characterized by a high expression of CRH in the paraventricular nucleus (PVN) of the hypothalamus. Expression levels of GR in the hippocampus and UCN3 in the perifornical area are low at birth but increase significantly during the SHRP, both reaching the highest expression level at pnd 12. In the second phase, the mice have developed past the SHRP and were now exhibiting enhanced corticosterone basal levels and a response of ACTH and corticosterone to mild novelty stress. CRH expression was decreased significantly, while expression of UCN3 and GR remained high, with a small decrease at pnd 16. The expression of MR in the hippocampus was very dynamic throughout the postnatal development of the HPA axis and changed in a time and subregion specific manner. These results demonstrate for the first time the correlation between the postnatal endocrine development of the mouse and gene expression changes of central regulators of HPA axis function.

Dual Circuitry for Odor–Shock Conditioning during Infancy: Corticosterone Switches between Fear and Attraction via Amygdala

Rat pups must learn maternal odor to support attachment behaviors, including nursing and orientation toward the mother. Neonates have a sensitive period for rapid, robust odor learning characterized by increased ability to learn odor preferences and decreased ability to learn odor aversions. Specifically, odor–0.5 mA shock association paradoxically causes an odor preference and coincident failure of amygdala activation in pups until postnatal day 10 (P10). Because sensitive-period termination coincides with a declining “stress hyporesponsive period” when corticosterone release is attenuated, we explored the role of corticosterone in sensitive-period termination. Odor was paired with 0.5 mA shock in either sensitive-period (P8) or postsensitive-period (P12) pups while manipulating corticosterone. We then assessed preference/aversion learning and the olfactory neural circuitry underlying its acquisition. Although sensitive-period control paired odor–shock pups learned an odor preference without amygdala participation, systemic (3 mg/kg, i.p.; 24 h and 30 min before training) or intra-amygdala corticosterone (50 or 100 ng; during training) permitted precocious odor-aversion learning and evoked amygdala neural activity similar to that expressed by older pups. In postsensitive-period (P12) pups, control paired odor–shock pups showed an odor aversion and amygdala activation, whereas corticosterone-depleted (adrenalectomized) paired odor–shock pups showed odor-preference learning and activation of an odor learning circuit characteristic of the sensitive period. Intra-amygdala corticosterone receptor antagonist (0.3 ng; during training) infused into postsensitive-period (P12) paired odor–shock pups also showed odor-preference learning. These results suggest corticosterone is important in sensitive-period termination and developmental emergence of olfactory fear conditioning, acting via the amygdala as a switch between fear and attraction. Because maternal stimulation of pups modulates the pups endogenous corticosterone, this suggests maternal care quality may alter sensitive-period duration.

Activation and inhibition of the hypothalamic-pituitary-adrenal axis of the neonatal rat: Effects of maternal deprivation.

These studies investigated the activation and inhibition (negative feedback) of the neonatal rat. The ACTH response of maternally deprived pups is persistently elevated for 30 min, suggesting a deficiency in the negative feedback system. In Experiment 1, we examined the time-course of corticosterone (CORT) and ACTH responses to a saline injection over a 120-min period during development. In deprived pups, CORT and ACTH were persistently elevated throughout the testing period, whereas only 15-day-old nondeprived pups showed ACTH and CORT elevations. Further nondeprived and deprived pups were exposed twice to ether (Experiment 2) or saline injections (Experiment 3) separated by a 1-h interval. Nondeprived pups showed an augmented ACTH response to double exposure to ether, but not to saline. No CORT response to either stimulus was observed. In response to one exposure of each stimulus, deprived pups showed increased ACTH and CORT values and no further elevation to repeated exposure. These results suggest the negative feedback system of neonates is immature, but partially functional in deprived pups. Moreover, nondeprived pups show a stressor-specific response to stress, whereas deprived animals show a similar response to different stimuli.

Comparing Cortisol, Stress and Sensory Sensitivity in Children with Autism

Previously we reported that children with autism show significant variability in cortisol. The current investigation was designed to extend these findings by exploring plausible relationships between cortisol and psychological measures of stress and sensory functioning. Salivary cortisol values for diurnal rhythms and response to stress in children with and without autism were compared to parent‐report measures of child stress, the Stress Survey Schedule (SSS), sensory functioning, Short Sensory Profile (SSP), and Parenting Stress Index. In autism, a negative relationship between morning cortisol and the SSS revealed that higher observed symptoms of stress were related to lower cortisol. Lower cortisol is seen in conditions of chronic stress and in social situations characterized by unstable social relationships. Sensory sensitivity painted a more complicated picture, in that some aspects of SSP were associated with higher while others were associated with lower cortisol. We propose that increased sensory sensitivity may enhance the autistic childs susceptibility to the influence of zeitgeibers reflected in variable cortisol secretion. Evening cortisol was positively associated with SSS such that the higher the level of evening cortisol, the higher the childs parent‐reported daily stress, especially to changes, such as in daily routine. Regarding the response to stress, the psychological and parent variables did not differentiate the groups; rather, discrete subgroups of cortisol responders and nonresponders were revealed in both the autism and neurotypical children. The results support a complex interplay between physiological and behavioral stress and sensory sensitivity in autism and plausible developmental factors influencing stress reactivity across the groups.

The HPA system during the postnatal development of CD1 mice and the effects of maternal deprivation.

In this study we describe in 9- and 18-day-old CD1 mice (i) the basal and stress-induced activity of markers of the HPA system in blood, brain and pituitary, (ii) the effects of a 24-h maternal deprivation and (iii) the influence of anogenital stimulation (stroking) in deprived pups on these markers. We found low basal concentrations of ACTH and corticosterone at postnatal day (pnd) 9 and no or little response to a mild stressor at this age, confirming the existence of a SHRP in mice. At pnd 18 the mice displayed an adult-like ACTH and corticosterone response following a mild stressor. Maternal deprivation resulted in enhanced basal and stress levels of corticosterone at both ages. Interestingly, basal ACTH levels were elevated following maternal deprivation at pnd 9. At pnd 18 maternal deprivation resulted in a blunted ACTH response. Maternal deprivation resulted in a down-regulation of GR, MR, CRH and POMC transcript in the brain. However, maternally deprived 18-day-old pups displayed increased levels of CRH and POMC transcript, while GR and MR mRNA was also down-regulated. Anogenital stroking could reverse maternal deprivation effects on ACTH and MR mRNA, but not CRH mRNA. We conclude that (i) at the two measured time points the HPA axis develops similar in mice as in rats, (ii) maternal deprivation has pronounced effects in mice, which are similar to that found in the rat and (iii) there are a number of significant differences, which mainly concern the central CRH-ACTH components of the axis.

The dynamics of the hypothalamic-pituitary-adrenal axis during maternal deprivation.

A close contact between the dam and the litter is essential for the normal development of the hypothalamic‐pituitary‐adrenal (HPA) axis in rats and mice. Maternal signals, as licking and feeding, have been shown to sustain the HPA axis of the pups in a hypo‐responsive state. Disruption of this mother–pup interaction by 24u2003h of maternal deprivation activates the otherwise quiescent stress system of the neonates, resulting in an enhanced adrenal sensitivity to adrenocorticotropic hormone (ACTH) and a decreased expression of central HPA markers, such as corticotropin‐releasing hormone (CRH). However, the dynamics of these central and peripheral changes over the 24h period are largely unknown. In this study, we examined the time course of some of the central and peripheral indices of HPA activity during 24u2003h of maternal deprivation. We measured corticosterone and ACTH in the blood as well as CRH, mineralocorticoid and glucocorticoid receptor expression in the brain. Our results demonstrate that each of the components of the HPA axis responds to maternal deprivation at different time points following removal of the mother and with a very specific time course. The main activation of the HPA axis occurred between 4u2003h and 8u2003h of maternal absence. By contrast, during the second half of the deprivation period, negativefeedback mechanisms restrained the further increase in ACTH and corticosterone release. We conclude that maternal deprivation triggers a cascade of sequential changes at the various levels of the stress system, and that measuring only one aspect of the system at one time point does not accurately reflect the dynamic alterations of the HPA axis.