Prenatal stress does not alter innate novelty-seeking behavioral traits, but differentially affects individual differences in neuroendocrine stress responsivity

Summary

Exposure to stress during prenatal or early postnatal life can dramatically impact adult behavior and neuroendocrine function. We recently began to selectively breed Sprague-Dawley rats for high (high responder, HR) and low (low responder, LR) novelty-seeking behavior, a trait that predicts a variety of differences in emotional reactivity, including differences in neuroendocrine stress response, fear- and anxiety-like behavior, aggression, and propensity to self-administer drugs of abuse. We evaluated genetic–early environment interactions by exposing HR- and LR-bred animals to prenatal stress (PS) from pregnancy day 3–20, hypothesizing that PS exposure would differentially impact HR versus LR behavior and neuroendocrine reactivity. We evaluated novelty-induced locomotion, anxiety-like behavior, and corticosterone stress response in weanling (25-day-old) and adult HR–LR stressed and control males. Exposure to PS did not alter HR–LR differences in locomotion, but did impact anxiety-like behavior, specifically in LR animals. Surprisingly, LR animals exposed to PS exhibited less anxiety than LR controls. HR rats were not affected by PS, with both stress and control groups showing low levels of anxiety. PS differentially impacted neuroendocrine stress reactivity in young versus adult HR–LR animals, leading to an exaggerated corticosterone response in LR pups compared to LR controls, while HRs pups were unaffected. In contrast, exposure to PS produced an exaggerated stress response in HR adults, compared to HR controls, while LR animals were not significantly affected. These findings highlight how genetic predisposition may shape individual's response to early life stressors, and furthermore, show that a history of early life stress may differentially impact an organism at different points in life. Future work will explore neural mechanisms which underlie the different behavioral and neuroendocrine consequences of PS in HR versus LR animals.

Introduction

Inborn differences in personality and emotional reactivity strongly shape how individuals respond to stress, and may predispose certain people to develop psychiatric disorders such as depression, anxiety, and drug addiction. Exposure to early life stress is known to increase the risk for developing emotional disorders (Weinstock, 1997; Johnson et al., 2001; Mueser et al., 2002), but this risk is greatly magnified if an individual also carries a genetic predisposition for psychiatric illness (Caspi et al., 2002, Caspi et al., 2003). While such observations in humans are compelling, the neural mechanisms that underlie this interactive pathway to disease remain poorly understood. Therefore, rodent models offer a powerful tool to examine how such gene–environment interactions shape brain development, stress-sensitivity, and emotional behavior.

Numerous rodent studies demonstrate that stress during pregnancy produces offspring with a variety of long-term neurobiological (Maccari et al., 1995; Barbazanges et al., 1996; Lemaire et al., 2000; Morley-Fletcher et al., 2004; Van den Hove et al., 2006), behavioral (Ward and Stehm, 1991; Szuran et al., 1994; Vallee et al., 1996, Vallee et al., 1997; Morley-Fletcher et al., 2003; Patin et al., 2005), and endocrine (Fride et al., 1986; Takahashi et al., 1990; Takahashi and Kalin, 1991; Henry et al., 1994; Maccari et al., 2003; Morley-Fletcher et al., 2003) abnormalities. While prenatal stress (PS) has clear deleterious effects on offspring, the specific types and severity of stress-induced alterations vary from study to study. Such inconsistent results are likely due to a host of technical factors, including use of different stressors (e.g. restraint, footshock, forced swimming, predator odor, noise), and application of stressors at different points in pregnancy, such as throughout the entire pregnancy (Rojo et al., 1985; Fride et al., 1986), during the first (Suchecki and Palermo Neto, 1991; Tazumi et al., 2005; Bosch et al., 2006) or second half of pregnancy (Maccari et al., 1995; Vallee et al., 1996; Lehmann et al., 2000; Lemaire et al., 2000; Morley-Fletcher et al., 2003; Poltyrev et al., 2005; Van den Hove et al., 2006). Different rat strains are frequently used across experiments, which is important since each strain may be differentially vulnerable to early life stress (Stohr et al., 1998). Furthermore, besides inter-strain differences, it is also important to consider that rats within a given strain may exhibit individual differences in stress-reactivity, making them more or less sensitive to early life stress (Neumann et al., 2005; Bosch et al., 2006).

We recently began to selectively breed Sprague-Dawley rats based on their innate differences in “novelty-seeking”—a trait in rodents which predicts several key facets of emotional reactivity, including fear- and anxiety-like behavior (Kabbaj et al., 2000; Stead et al., 2006), and propensity to self-administer drugs of abuse (Piazza et al., 1989; Kabbaj et al., 2001). Outbred rats, like humans and all other organisms, show a wide range of emotional response to environmental challenges. When exposed to a novelty, some rats (high responders, HR) vigorously explore the new environment, while others (low responders, LR), are inhibited and exhibit very little activity. HR–LR rats exhibit a variety of other interesting behavioral differences, including differences in anxiety-like behavior (Kabbaj et al., 2000; Stead et al., 2006), aggression (Abraham et al., 2006), stress responsiveness (Piazza et al., 1989, Piazza et al., 1991a, Piazza et al., 1993; Kabbaj et al., 2000), and willingness to self-administer psychostimulants (Piazza et al., 1989; Kabbaj et al., 2001). For example, HR animals show reduced anxiety-like behavior across multiple tests, including the elevated plus maze, light dark box, and open field test, compared to LRs (Kabbaj et al., 2000; Stead et al., 2006; Mallo et al., 2007; White et al., 2007), although prior benzodiazepine treatment eliminates these differences (Stead et al., 2006). HR males also show enhanced aggressive behavior (Abraham et al., 2006) and greater behavioral response to cocaine and amphetamine (Deminiere et al., 1989; Piazza et al., 1989; Kabbaj et al., 2001; Alttoa et al., 2007) compared to LRs. Taken together, these data suggest that HR and LR animals exhibit fundamental differences in emotional reactivity, interacting differently with their environment across numerous conditions.

Neurochemical and neural gene expression differences contribute, at least in part, to the HR–LR behavioral phenotypes (Piazza et al., 1991b; Hooks et al., 1994a, Hooks et al., 1994b; Kabbaj et al., 2000; Kabbaj, 2004). For example, HR rats exhibit decreased hippocampal glucocorticoid receptor (GR) mRNA expression compared to LR rats, and local infusion of the GR antagonist RU38486 into the hippocampus equalizes HR–LR behavioral differences, leading LRs to explore more and show decreased anxiety-like behavior (Kabbaj et al., 2000). Another study using Affymetrix microarrays identified numerous putative gene expression differences in the hippocampus of HR–LR rats both basally and following psychosocial stress, highlighting differences in a range of molecules involved in intracellular signal transduction pathways and neurogenesis (Kabbaj et al., 2004).

The first goal of the present study was to confirm whether our selectively bred HR–LR rats show hypothalamic–pituitary–adrenal (HPA) axis differences similar to those reported in commercially purchased HR–LR animals. Specifically, we used in situ hybridization to measure mRNA expression of GR and the mineralocorticoid receptor (MR) in the hippocampus of selectively bred HR–LR males, and evaluated their patterns of corticosterone secretion following novelty stress. Our results show that selectively bred HR rats, like purchased HRs, express lower levels of hippocampal GR mRNA, and also exhibit exaggerated stress-induced corticosterone secretion compared to LRs.

Since HR–LR animals show clear baseline differences in HPA axis reactivity, we hypothesized that they would differentially react to PS. Moreover, because the HR–LR traits appear to involve a strong genetic component (Stead et al., 2006), we also wanted to explore possible gene–environment interactions which may differentially impact behavioral and/or neuroendocrine aspects of the HR–LR phenotypes. Therefore, the second major goal of the present study was to expose selectively bred HR–LR animals to PS and assess its impact on behavior and neuroendocrine reactivity in both weanling and adult offspring. We chose to evaluate the impact of PS in both young and adult animals to determine (a) whether the HR–LR phenotype is present in early life and (b) whether PS differentially impacted behavior and stress-reactivity in young versus adult animals. Our results show that while PS only subtly influences the HR–LR behavioral phenotypes, it exerts greater influence on neuroendocrine stress reactivity, with differential effects in young versus adult HR–LR offspring.