The stress-response and threat-response systems and their afferent and efferent pathways that extend into the PFC are immature in young mammals. There is substantial evidence that these developing systems are particularly plastic and open to modification by early life experiences (Levine, 1994; Meaney et al., 1996; Suomi, 1997). Inadequate parental care shapes a more reactive stress-response system in infants of many species and might contribute to hypersensitivity of the developing threat appraisal system, thus increasing vulnerability to stressors throughout life. Conversely, impacts of early caregiving disruptions on the developing threat system, particularly on amygdalar CRH and CRH-receptor systems may result in a lower threshold for activation of the HPA axis. Hence, there are bi-directional and transactional arrows between the developing stress- and threat-response systems in Figure 1. The developmental anatomy and physiology of these systems is only beginning to be understood (Bachevalier, 1998; Vazquez, 1998). The maturity of these systems relative to parturition also differs markedly among species. In general, however, as shown in Figure 1, the developing stress- and threat-response systems develop earlier than systems involved in behavioral and
