62 Early-Life Experiences: Enduring Behavioral,Neurological, and Endocrinological ConsequencesR D Romeo, Barnard College, New York, NY, USAA C Tang, University of New Mexico, Albuquerque, NM, USAR M Sullivan, Nathan Kline University and New York University Langone Medical Center, Orangeburg, NY, USA
2009 Elsevier Inc. All rights reserved.
Chapter Outline
62.1 Introduction 1976
62.2 HypothalamicPituitaryAdrenal Axis 1976
62.2.1 Development of the HPA Axis 1977
62.2.1.1 Neonatal development of the HPA axis 1977
62.2.1.2 Pubertal development of the HPA axis 1978
62.3 Neonatal Experiences and Enduring Behavioral, Neurological,
and Endocrinological Consequences 1980
62.3.1 Individual Differences in the Development of the HPA Axis and
Neonatal Experience 1980
62.3.2 Neonatal Handling 1981
62.3.2.1 Neonatal handling and behavior 1981
62.3.2.2 Neonatal handling and brain 1983
62.3.2.3 Neonatal handling and endocrine function 1984
62.3.3 Neonatal Novelty Exposure 1984
62.3.3.1 Neonatal novelty exposure and behavior 1985
62.3.3.2 Neonatal novelty exposure and brain 1987
62.3.3.3 Neonatal novelty exposure and endocrine function 1987
62.3.3.4 Neonatal novelty exposure and maternal influence 1989
62.3.4 Maternal Deprivation 1989
62.3.4.1 Maternal deprivation and behavior 1990
62.3.4.2 Maternal deprivation and brain 1990
62.3.4.3 Maternal deprivation and endocrine function 1991
62.3.5 Pain, Fear Conditioning, and Context of Early-Life Adversity 1992
62.3.5.1 Odorshock conditioning and behavior 1992
62.3.5.2 Odorshock conditioning and brain 1993
62.3.5.3 Odorshock conditioning and endocrine function 1993
62.3.6 Functional Consequences of Early-Life Experiences 1993
62.4 Pubertal Experiences and Enduring Behavioral and Endocrine Consequences 1994
62.4.1 Pubertal Experience and Behavior 1994
62.4.2 Pubertal Experience and Brain 1995
62.4.3 Pubertal Experience and Endocrine Function 1995
62.5 Adolescence as a Period of Intervention to Mitigate Early Developmental Insults 1996
62.5.1 Reversals of Perinatal Insults through Pubertal Environmental Enrichment 1996
62.5.2 Mitigation of Perinatal Brain Damage through Pubertal Environmental Enrichment 1996
62.6 Conclusions 1997
References 1997
1975
1976 Early-Life Experiences
Glossaryadrenocorticotropic hormone (ACTH) A peptide
hormone released from the anterior
pituitary gland that mediates the production
and secretion of hormones of the adrenal
cortex.
arginine vasopressin (AVP) A peptide hormone
that participates in the release of
stress-related hormones, such as ACTH.
corticosterone (CORT) A steroid hormone
released by the adrenal cortex in response to
stress.
corticotropin-releasing hormone (CRH)
A peptide hormone that participates in the
release of stress-related hormones, such as
ACTH. This hormone is also referred to as
corticotropin-releasing factor (CRF).
glucocorticoid receptor (GR) A low-affinity
steroid hormone receptor for the corticoid
steroids, such as CORT.
hypothalamicpituitaryadrenal axis (HPA axis)
The major neuroendocrine axis that
mediates the hormonal stress response.
mineralocorticoid receptor (MR) A high-affinity
steroid hormone receptor for the corticoid
steroids, such as CORT.
paraventricular nucleus of the hypothalamus
(PVN) A nucleus in the hypothalamus that contains
CRH and AVP neurosecretory cells that
regulate the release of stress-related
hormones.
62.1 Introduction
Though individuals function in the present, we carrywith us previous experiences that can fundamen-tally change how we respond physiologically andbehaviorally to internal and external challenges.Since the seminal work of Weininger (1954) on gen-tling and Levine (1957) on infantile experiences inrats, an ever-growing body of literature has indictedthat experiences early in development can have long-lasting effects on an individuals physiological andbehavioral potentials. In fact, some of these effects ofearly experience are so enduring that they can betransgenerational (Denenberg and Rosenberg, 1967).The purpose of this chapter is to highlight some recentstudies regarding how experiences neonatally and/orpubertally can modulate later adult functioning.
Specifically, we emphasize the role of early experi-ence such as neonatal handling, novelty exposure,maternal deprivation, and odorshock conditioningon immediate and long-term emotionality and cogni-tive abilities. As the neonatal period is not the onlydevelopmental stage when individuals are susceptibleto both positive and negative influences, we also dis-cuss how exposure to stressors during adolescencemodifies later stress responsiveness and emotionalbehavior. We conclude by briefly describing someprovocative experiments which indicate that experi-ences during puberty can offset or mitigate develop-mental insults that occur perinatally. These dataindicate that at least some enduring consequences ofearly-life experience remain malleable, evenwell intoadolescence and adulthood.
The hormones released during stressful eventsappear to be a common thread in how early experi-ences during the neonatal or pubertal stage of develop-ment affect an individuals immediate and long-termphysiological and behavioral function. Thus, we beginthis chapter by briefly examining the hypothalamicpituitaryadrenal (HPA) axis, the major neuroendo-crine axis mediating the hormonal stress response(Herman and Cullinan, 1997; Herman et al., 2003).Below, we discuss the components that comprise thisaxis and its neonatal and pubertal maturation.
62.2 HypothalamicPituitaryAdrenal Axis
The release of stress-related hormones by the HPAaxis is driven by a cascade of signals beginning withthe release of the neuropeptides corticotropin-releasinghormone (CRH) and arginine vasopressin (AVP) fromthe paraventricular nucleus of the hypothalamus(PVN). CRH and AVP are released into the hypophy-seal-portal system, which bring about the release ofadrenocorticotropic hormone (ACTH) from theanterior pituitary. ACTH then stimulates the produc-tion and secretion of glucocorticoids (i.e., cortisol inprimates and corticosterone (CORT) in many rodentspecies) from the cortex of the adrenal gland. Thehormones secreted by the HPA axis control theirown secretion through a neuroendocrine negativefeedback loop. That is, glucocorticoids feed back onthe PVN and extrahypothalamic sites (e.g., pituitary,hippocampus, and prefrontal cortex (PFC)) to inhibitthe further release of hypothalamic CRH and AVP(Herman et al., 2003; Figure 1).
Paraventricular nucleus of the hypothalamus(PVN)
CRH/AVP
ACTH
CORT
()
()
(+)
(+)
AP
CM
Hypothalamus
Pituitary
Adrenal
Figure 1 A diagram of the HPA axis. ACTH,adrenocorticotropin hormone; AP, anterior pituitary; AVP,
arginine vasopressin; C, cortex; CORT, corticosterone;CRH, corticotropin-releasing hormone; M, medulla; PVN,
paraventricular nucleus of the hypothalamus; (), positivedrive; (), negative feedback.
Early-Life Experiences 1977
Two receptors (the mineralocorticoid receptor(MR) and the glucocorticoid receptor (GR)) mediatethe actions of glucocorticoids in the central nervoussystem. These steroid receptors are found in relativelyhigh concentrations throughout the neuralpituitarynetwork that controls both negative feedback andactivation of the HPA axis (Sapolsky et al., 2000). Thehigh-affinity MR is typically saturated at basal gluco-corticoid levels, while the low-affinity GR is primarilyoccupied only when elevated concentrations of gluco-corticoids are present (deKloet et al., 1998).Thus, whenglucocorticoid levels rise in response to stressors, thenegative feedback on the HPA axis is primarilymediated by the GR (de Kloet et al., 1998). However,the MR also appears to play a role in glucocorticoid-mediated negative feedback under mildly stressful con-ditions (Pace and Spencer, 2005).
The hormonal stress response is essential to sur-vival as it allows an organism to cope with the inter-nal and external demands imposed by a challengingevent. This response attempts to restore the organismto homeostasis, a process termed allostasis (McEwenand Stellar, 1993). However, prolonged or morechronic exposures to stress and stress-induced hor-mones can lead to allostatic overload, resulting in anumber of negative effects, particularly in regard toneurobiological and emotional function (Herbert
et al., 2006; McEwen, 2003, 2004; McEwen andStellar, 1993; Sapolsky, 1999; van Praag, 2004).
Both the magnitude and duration of the hormonalstress response change dramatically throughout anorganisms life span. For instance, neonatal animalsshow reduced stress reactivity in response to stressorsthat typically elicit robust stress responses in adults(Sapolsky and Meaney, 1986). The reduced stressreactivity experienced by neonates has been positedto protect the developing organism from the negativeinfluences of stress hormones (Sapolsky and Meaney,1986). Conversely, aged animals show heightened andmore prolonged stress responses compared to youngeradults (Sapolsky, 1999). This has been proposed tocontribute to the decline in neurophysiological andcognitive function observed during aging (Sapolsky,1999; Sapolsky et al., 1985). Thus, parameters thatchange the responsiveness of the HPA axis, such asdevelopment, may have profound consequences onwhether stressors lead to adaptive or maladaptiveresponses. The next section briefly describes some ofthe major changes that occur in HPA function duringneonat
