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132 EFFECTS OF EARLY MALTREATMENT AND STRESSof stress elevations in glucocorticoids is less well studied but is likely related toevidence that frequent or chronic exposure to high levels of glucocorticoids lowersthe threshold for activation of the amygdala-LC reverberating threat circuit,making it easier for milder threats to activate and maintain anxious, vigilant defensiveresponses (see review, Rosen & Schulkin, 1998).In summary, the emotion-stress system likely evolved to foster survival in theface of threatening or potentially threatening events. The capacity to activate thissystem, thus, is essential for adaptation. Basal levels of glucocorticoids that areextremely low and stress responses that are extremely high pose risks to healthand adaptive functioning because they result in failure of the HPA system to serveits permissive, stimulatory, suppressive, and preparatory roles in stress and adaptation.However, the HPA system is a powerful system that needs to be regulatedsuch that it is activated only when needed and returned to normal basal levels ofactivity when threat has passed. Frequent stress activation, particularly when itproduces high and prolonged glucocorticoid levels, can impair brain growth andorganization (e.g., produce damage to the hippocampus, thus impairing its role inlearning and memory) and chronically suppress immune functioning, thus increasingvulnerability to infectious diseases. It may also lower the threshold and increasethe vigor of the neural systems underlying fear and anxiety, increasing risksof affective disorders. In sum, the relation between activity of the HPA systemand adaptive functioning is typically seen as that of an inverted-U shaped functionwhereby both low and high levels of reactivity are associated with poor mentaland emotional health, and moderate and well-timed response are associatedwith health and adaptive functioning (Sapolsky, 1997).Early Experiences in the Laboratory Ratand Development of the Stress-Emotion SystemMuch of our understanding of the impact of early experiences on the stressemotionsystem comes from rat research. The rat exhibits a curiously hyporesponsiveHPA system for several weeks postbirth (Sapolsky & Meaney, 1986).This stress hyporesponsive period (SHRP) lasts from approximately day 4 today 14 of life, during which time stimuli that would normally activate the axisproduce modest, if any, responses. Other facets of the stress-emotion system(e.g., amygdala CRH system) are not hyporesponsive during this time, and therat pup is quite capable of emitting distress vocalizations. Thus the relative stresshyporesponsive period refers only to relative hyporesponsiveness of the HPAaxis (Smith, Kim, Van Oers, & Levine, 1997). Maternal behaviors, in particularlicking and grooming of the pup and provision of milk into the gut, maintainthe HPA axis in this relatively quiescent state (Rosenfield, Suchecki, & Levine,1992). The SHRP roughly defines a period when disturbing the nest by separat-