As noted above, the biology of stress involves the interaction of multiple systems of equilibrium maintenance (i.e. allostasis); therefore, the box within Figure 1 labeled “stress neurobiology” refers conceptually to the multiple systems. However, for most of the work relevant to this discussion, this box refers to activity of the hypothalamic-pituitary-adrenocortical (HPA) axis and its neuroactive petides and hormones. Note that the other major effector arm of the mammalian stress system is the sympatho-adrenal system (Sapolsky et al., 2000). Although HPA activity is sometimes measured solely as an index of stress, this neuroendocrine system has multiple effects on brain development that make it an attractive target for ELS research. Additionally, animal models that form the basis of the preclinical information on ELS began with a focus on the HPA system (see review, Levine, 2005). Glucorticoids (GCs; cortisol in primates; corticosterone in rodents) are gene transcription factors (i.e., influence gene expression; Meijer, 2006), that can be measured non-invasively in young children. GCs are permissive, their presence allowing or enhancing other neural, molecular, or biochemical events. Acutely elevated GCs help to terminate
