Molecular basis for the development of individual differences in the hypothalamic-pituitary-adrenal stress response.

1. Several years ago, investigators described the effects of infantile handling on the development of hypothalamic-pituitary-adrenal (HPA) responses to stress in the rat. Rat pups exposed to brief periods of innocuous handling early in life showed reduced HPA responses to a wide variety of stressors, and the effect persists throughout the life of the animal. These effects are robust and provide an excellent model for understanding how early environmental stimuli, which are external to the organism, alter neural differentiation and, thus, neuroendocrine responsivity to stress. 2. This paper reviews the endocrine mechanisms affected by early handling and our current understanding of the neural transduction of environmental events and their effects at the level of the target neurons (in the hippocampus and frontal cortex). 3. In brief, handling serves to increase glucocorticoid receptor gene transcription, increasing sensitivity to glucocorticoid negative feedback regulation and, thus, altering the activity within hypothalamic corticotropin-releasing factor/vasopressin neurons. Together these changes serve to determine neuroendocrine responsivity to stress.