Stress and the adaptive self-organization of neuronal connectivity during early childhood.

A conceptual framework is proposed for a better understanding of the biological role of the stress-response and the relationship between stress and brain development. According to this concept environmental stimuli (in children mainly psychosocial challenges and demands) exert profound effects on neuronal connectivity through repeated or long-lasting changes in the release of especially such transmitters and hormones which contribute, as trophic, organizing signals, to the stabilization or destabilization of neuronal networks in the developing brain. The increased release of noradrenaline associated with the repeated short-lasting activation of the central stress-responsive systems in the course of the stress-reaction-process to psychosocial challenges which are felt to be controllable acts as a trigger for the stabilization and facilitation of those synaptic and neuronal pathways which are activated in the course of the cognitive, behavioral and emotional response to such stressors. The long-lasting activation of the central stress-responsive systems elicited by uncontrollable psychosocial conflicts in conjunction with the activation of glucocorticoid receptors by the sustained elevation of circulating glucocorticoid levels favors the destabilization of already established synaptic connections and neuronal pathways in associative cortical and limbic brain structures. The facilitation and stabilization of neuronal pathways triggered by the experience of controllable stress is thus opposed, attenuated or even reversed in the course of lon-lasting uncontrollable stress. This destabilization of previously established synaptic connections and neuronal pathways in cortical and limbic brain structures is a prerequisite for the acquisition of novel patterns of appraisal and coping and for the reorganization of the neuronal connectivity in the developing brain. Alternating experiences of repeated controllable stress and of long-lasting uncontrollable stress are therefore needed for the "self-adjustment" of neuronal connectivity and information processing the developing brain to changing environmental (psychosocial) demands during childhood. The brain structures and neuronal circuits involved in the regulation of behavioral responding become thus repeatedly reoptimized and refitted, not the changing conditions of life per se but rather to those conditions which are still able to activate the central stress responsive systems of an individual at a certain developmental stage.