Animal models of early life stress: Implications for understanding resilience

Abstract

In the mid-1950s, Levine and his colleagues reported that brief intermittent exposure to early life stress diminished indications of subsequent emotionality in rats. Here we review ongoing studies of a similar process in squirrel monkeys. Results from these animal models suggest that brief intermittent exposure to stress promotes the development of arousal regulation and resilience. Implications for programs designed to enhance resilience in human development are discussed.

FIGURE 1

Early intermittent separations diminish subsequent pituitary–adrenal responses to a novel test environment. Posttest plasma levels of cortisol (top panel) and adrenocortico-tropic hormone (bottom panel) are presented for previously separated and nonseparated monkeys (n = 9–10 monkeys per condition; mean ± SEM). Asterisks signify a significant (p <.05) separation effect (adapted from data presented in Parker et al., 2004).

FIGURE 2

Early intermittent separations increase subsequent novelty-seeking behavior. Measures of familiar versus novel object exploration are presented for previously separated and nonseparated monkeys (n = 9–10 monkeys per condition; mean ± SEM). Asterisks signify separation effects (p <.05) and the bracket shows that previously separated monkeys preferred to interact with the novel toy-like object (adapted from data presented in Parker et al., 2007).

FIGURE 3

Early intermittent separation-induced changes in neuroendocrine indications of resilience are not maternally mediated. The percentage of contact attempts initiated by mothers in the home cage (top panel) and plasma cortisol levels after exposure to a novel environment stress test (bottom panel) are presented for 9–11 monkeys in each treatment condition (mean ± SEM). Treatment conditions without shared superscript letters differ significantly from one another (p <.05) and conditions with shared letters do not (adapted from data presented in Parker et al., 2006).

FIGURE 4

Early intermittent separation-induced adaptations in prefrontal cortex. Ventromedial prefrontal cortical volumes are larger in monkeys exposed to early intermittent separations compared to nonseparated controls (n = 9–10 monkeys per condition; mean ± SEM). Separation effects were not discerned in dorsolateral prefrontal cortex (data not shown) and the separation-by-brain side interactions were not statistically significant (adapted from data presented in Katz et al., 2009).

FIGURE 5

Early intermittent separation-induced adaptations in prefrontal white matter. Ventromedial prefrontal white matter fractional anisotropy is greater in monkeys previously exposed to early intermittent separations compared to nonseparated controls (n = 9–10 monkeys per condition; mean ± SEM). Separation effects were not discerned in the white matter region underlying dorsolateral prefrontal cortex (data not shown) and the separation-by-brain side interactions were not statistically significant (adapted from data presented in Katz et al., 2009).