Understanding behavioral effects of early life stress using the reactive scope and allostatic load models

Abstract

The mechanisms through which early life stress leads to psychopathology are thought to involve allostatic load, the "wear and tear" an organism is subjected to as a consequence of sustained elevated levels of glucocorticoids caused by repeated/prolonged stress activations. The allostatic load model described this phenomenon, but has been criticized as inadequate to explain alterations associated with early adverse experience in some systems, including behavior, which cannot be entirely explained from an energy balance perspective. The reactive scope model has been more recently proposed and focuses less on energy balance and more on dynamic ranges of physiological and behavioral mediators. In this review we examine the mechanisms underlying the behavioral consequences of early life stress in the context of both these models. We focus on adverse experiences that involve mother-infant relationship disruption, and dissect those mechanisms involving maternal care as a regulator of development of neural circuits that control emotional and social behaviors in the offspring. We also discuss the evolutionary purpose of the plasticity in behavioral development, which has a clear adaptive value in a changing environment.

Figure 1

Reactive scope model could be interpreted as an inverted U-shaped curve. The dashed line represents the levels of mediators (including physiological and experiential such as maternal care) that can affect developmental outcomes of early life experiences, including stress reactivity and emotionality. Exposure to levels of mediators that fall within the “normal reactive scope” results in positive outcomes, whereas exposure to levels of mediators in either the “too high” range (“homeostatic overload”) or “too low” range (“homeostatic failure”) results in negative developmental outcomes. Adapted from “The Reactive Scope Model—A New Model Integrating Homeostasis, Allostatis, and Stress,” by L. M. Romero, M. J. Dickens, and N. E. Cyr, 2009, Hormones and Behavior, 55. Copyright 2009 by Elsevier. Adapted with permission. [A color version of this figure can be viewed online at journals. cambridge.org/dpp]