the conclusion that the infant is perhaps hyper-responsive rather than hyporesponsive to challenges (see Fig. 1).
Negative feedback has two modes of operation: (1) The "proactive" mode that involves the maintenance of basal levels of HPA activity and is mediated by the high-affinity mineralocorticoid receptors (MR) for CORT in higher brain regions and (2) the "reactive" mode that facilitates the termination of stress-induced HPA activity, which involves the lower-affinity glucocorticoid receptors (GR) localized in the PVN and the pituitary corticotrophs (de Kloet et al, 1998b). GR are widely distributed in the limbic-midbrain stress circuitry innervating the PVN, where it exerts facilitatory and inhibitory influences on the HPA axis. Both feedback modes are operative during the SHRP (van Oers et al, 1998). The proactive mode seems prominent, as ADX results in a high level of circulating ACTH. The reactive feedback seems to be a late developing process, as during neonatal life the stress-induced ACTH levels are not terminated as efficiently as in adulthood.
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