Figure 1.

Inverted U-shaped relationship between physiologic or behavioral “performance' and HPA axis output (corticosterone secretion). Secretion of glucocorticoids following stress is likely an adaptive function, supplying needed energy to meet real or potential threats. Underactive stress axis activation does not mobilize the resources needed to meet a challenge, resulting in suboptimal performance (left arm of the U-shaped curve). Excessive glucocorticoid secretion (right arm of the U-shaped curve) can cause excessive or prolonged catabolic responses, which can result in turn-off of vital stress counter-regulatory systems or energetic challenge in the CNS (see text).