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270 EFFECTS OF STRESSevidence of a pubertal increase in activity of the HPA axis. Cross-sectional studiesof normal children reveal a gradual rise in salivary and urinary cortisol duringmiddle childhood, then a marked increase that begins around 13 years of age andcontinues through adolescence (Kenny, Gancayo, Heald, & Hung, 1966; Kenny,Preeyasombat, & Migeon, 1966; Kiess et al., 1995; Lupien et al., 2002; Wingo,2002). Recent longitudinal studies have also revealed increases in cortisol releaseduring adolescence, with the most significant augmentation occurring at 13 years(Wajs-Kuto, De Beeck, Rooman, & Caju, 1999; Walker et al., 2002). Studies thathave examined pubertal stage indicate that the changes are strongly linked withsexual maturation (Kenny, Gancayo, et al., 1966; Kiess et al., 1995; Tornhage,2002).It has been proposed that the HPA axis, in particular pituitary release of ACTHand adrenal release of cortisol, may be involved in triggering sexual maturation(Weber, Clark, Perry, Honour, & Savage, 1997). Consistent with this, Weber et al.(1997) found that individuals with familial glucocorticoid deficiency were morelikely to manifest a lack of adrenarche. Conversely, children who have an earlyonset of adrenarche, as measured by the early (before age 8) appearance of Tannerstage II–III, show signs of heightened HPA activity (Dorn, Hitt, & Rotenstein,1999). When compared to age-matched controls, girls with premature adrenarcheshow significantly higher concentrations of cortisol, as well as estradiol, thyroidstimulatinghormone, and adrenal androgens.Changes in circulating steroid hormones are important for brain structure andfunction because they are transported through the bloodstream, then trigger cellularactivity and regulate a range of physiological functions (Beach, 1975; Kawata,1995; Keenan & Soleymani, 2001). Hormones affect the way neurons functionbecause they modulate the response of neurons to neurotransmitters (Mesce, 2002).They do this by diffusing in the space surrounding neurons, and thereby influenceindividual neurons, as well as the structure and activity of neuronal circuits.Two general classes of hormonal effects on brain have been described: activationaland organizational (Arnold & Breedlove, 1985; Charmandari et al., 2003).Activational effects are conceptualized as transient inductions of time-limited,functional changes in neural circuitry. Hormones can have activational influenceson sensory processes, autonomic nervous system activity, and enzyme systems(and thus, cellular permeability to electrolytes, water, and nutrients). In contrast,organizational effects are those that result in changes in the way the brain is organized—itsstructural characteristics (Buchanan, Eccles, & Becker, 1992).Until recently, it was generally assumed that activational effects occur duringadulthood, whereas organizational effects are restricted to the fetal neurodevelopment.But it now appears that some organizational effects of hormonesoccur later in life, including adolescence (Arnold & Breedlove, 1985; Charmandariet al., 2003). The magnitude of these effects is suggested by accumulating longitudinalstudies of human adolescents that demonstrate that the brain undergoes