The immediate and long term effects of exposure to early life stress (ELS) have been documented in humans and animal models. Even relatively brief periods of stress during the first 10 days of life in rodents can impact later behavioral regulation and the vulnerability to develop adult pathologies, in particular an impairment of cognitive functions and neurogenesis, but also modified social, emotional and conditioned fear responses. The development of preclinical models of ELS exposure allows the examination of mechanisms and testing of therapeutic approaches that are not possible in humans. Here we describe limited bedding and nesting (LBN) procedures, with models that produce altered maternal behavior ranging from fragmentation of care to maltreatment of infants. The purpose of this paper is to discuss important issues related to the implementation of this chronic ELS procedure and to describe some of the most prominent endpoints and consequences, focusing on areas of convergence between laboratories. Effects on the hypothalamic-pituitary adrenal (HPA) axis, gut axis and metabolism are presented in addition to changes in cognitive and emotional functions. Interestingly, recent data have suggested a strong sex difference in some of the reported consequences of the LBN paradigm, with females being more resilient in general than males. As both the chronic and intermittent variants of the LBN procedure have profound consequences on the offspring with minimal external intervention from the investigator, this model is advantageous ecologically and has a large translational potential. In addition to the direct effect of ELS on neurodevelopmental outcomes, exposure to adverse early environments can also have intergenerational impacts on mental health and function in subsequent generation offspring. Thus, advancing our understanding of the effect of ELS on brain and behavioral development is of critical concern for the health and wellbeing of both the current population, and for generations to come.

We showed previously that brief footshock stress and priming injections of heroin reinstate heroin-seeking after prolonged drug-free periods. Here, we examined whether the adrenal hormone, corticosterone, and brain corticotropin-releasing factor (CRF) were involved in such reinstatement. We tested the effects of adrenalectomy, chronic exposure to the corticosterone synthesis inhibitor metyrapone (100 mg/kg, s.c., twice daily), acute exposure to metyrapone, acute intracerebroventricular injections of CRF (0.3 and 1.0 microgram), and intracerebroventricular injections of the CRF antagonist alpha-helical CRF (3 and 10 micrograms). Rats were trained to self-administer heroin (100 micrograms/kg/infusion, i.v.) for 12-14 d. Extinction sessions were given for 4-8 d (saline substituted for heroin). Tests for reinstatement were given after priming injections of saline and of heroin (0.25 mg/kg, s.c.), and after intermittent footshock (15 or 30 min, 0.5 mA). Adrenalectomy (performed after training) did not affect reinstatement by heroin but appeared to potentiate the reinstatement by footshock. Chronic exposure to metyrapone (from the beginning of extinction) or an acute injection of metyrapone (3 hr before testing) did not alter the reinstatement of heroin-seeking induced by footshock or heroin. Acute exposure to metyrapone alone potently reinstated heroin-seeking. In addition, acute exposure to CRF reinstated heroin-seeking, and the CRF antagonist alpha-helical CRF attenuated stress-induced relapse. The effect of the CRF antagonist on reinstatement by heroin was less consistent. These results suggest that CRF, a major brain peptide involved in stress, contributes to relapse to heroin-seeking induced by stressors.

The neonatal rat shows a period of decreased responsiveness to noxious stimuli during the first 3 weeks of life, but the nature of this impairment is still controversial. To test the functionality of the hypothalamus-pituitary-adrenal axis during this period, we studied pituitary and adrenal responsiveness to exogenous ovine CRF and the ability of various stressors (ether vapors, electroshocks, and hypoxia) to elicit ACTH and corticosterone secretion. We also measured hypothalamic CRF content and pituitary ACTH content as well as CRF-binding sites in the anterior pituitary. From days 3-10, small elevations in plasma ACTH and corticosterone levels were observed after a 3-min exposure to ether vapors or electroshocks. In contrast, during this period, a 20-min exposure to hypoxia (5% O2 in N2) was unable to trigger measurable ACTH secretion, while corticosterone was significantly elevated. From days 14-21, plasma ACTH and corticosterone levels increased significantly after exposure to ether stress, hypoxia, and, to a lesser extent, electroshocks. By contrast, administration of urethane (1.2 g/kg BW) caused a significant increase in ACTH secretion on days 3, 5, and 10, an effect that was partially suppressed by pretreatment with an anti-CRF serum. This suggests that endogenous CRF can be released by at least some stimuli as early as day 3. Direct stimulation of the pituitary with synthetic oCRF (10 micrograms/kg BW) caused significant elevations in plasma ACTH levels at all ages tested (days 3 through 21), though these increases were significantly (P less than or equal to 0.01) smaller on day 3 (2.7-fold) than on day 21 (4.3-fold). Hypothalamic CRF content as well as ACTH content increased gradually with age, but the values reached by the third week of life were still low compared to the values on day 45. Finally, anterior pituitary CRF-binding sites averaged 317 +/- 48 fmol/mg protein on day 5 and 158 +/- 22 fmol/mg protein on day 17. The affinity (Kd) of the receptor for CRF was not significantly different on day 5, 17, or 45. These results show that although pituitary corticotrophs appear to be functional at birth, exposure to stress does not elicit marked increases in plasma ACTH until day 14 of age.