Full Length ArticleImpact of preweaning stress on long-term neurobehavioral outcomes in Sprague-Dawley rats: Differential effects of barren cage rearing, pup isolation, and the combination
Introduction
Neurodevelopmental cognitive disorders are the result of gene x environment interactions of which stress is a significant environmental factor. Stress during early childhood, such as poverty, neglect, and low socioeconomic status (SES), may alter neurodevelopment (Rebello et al., 2018). Children raised in very low SES households may experience psychosocial stress from neglect, family disruption, substandard housing, hunger, and violence (Evans, 2004; Taylor et al., 1997). Developmental exposure to such factors is associated with potentiated physiological stress. Elevated cortisol levels are found in children living in low SES households and in those in family environments characterized by conflict and neglect (Cohen et al., 2006; Evans and English, 2002; Evans and Kim, 2007; Lupien et al., 2001; Repetti et al., 2002). Chronic developmental stress is neurotoxic (van Bodegom et al., 2017) and induces dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis (Lupien et al., 2009; McEwen, 1998). Several days after birth in rats, the HPA axis goes through the stress hyporesponsive period (SHRP) that is characterized by a downregulated adrenal response and low basal glucocorticoid levels. The SHRP is hypothesized to be a neuroprotective mechanism that defends the developing brain from stress-induced excitotoxicity (De Kloet et al., 1988; Schmidt et al., 2002; Schmidt, 2019). However, some stressors overcome the buffering capacity of the SHRP and result in a maladaptive programming of HPA axis responsiveness and adverse functional outcomes (Levine, 2001; Mishra et al., 2019; Pihoker et al., 1993; van Bodegom et al., 2017).
Models of early life stress in rodents such as barren cage (also referred to as limited nesting) stress and maternal separation/pup isolation cause HPA axis dysregulation and long-term behavioral alterations (Bolton et al., 2017; Gilles et al., 1996; Lajud et al., 2012; Tractenberg et al., 2016). Barren cage rearing, as a model of environmental impoverishment, was used to induce chronic stress during development in numerous studies (Brunson et al., 2005; Gilles et al., 1996; Ivy et al., 2008; Rice et al., 2008; Sprowles et al., 2018; Vorhees et al., 2014; Walker et al., 2017). Barren cage rearing caused impaired learning and memory (Brunson et al., 2005; Molet et al., 2016b; Sprowles et al., 2018), emotion-related behavioral changes (Dalle Molle et al., 2012; Molet et al., 2016a; Sprowles et al., 2018), alterations in basal corticosterone levels (Arp et al., 2016; Avishai-Eliner et al., 2001; Rice et al., 2008), and abnormal hippocampal volume and structure (Molet et al., 2016b). The maternal separation stress paradigm, as a model of neglect wherein pups are separated from the dam, is one of the most frequently used neonatal stress protocols (van Bodegom et al., 2017). Maternal separation stress causes learning and memory deficits (Aisa et al., 2007; Grassi-Oliveira et al., 2016), increased anxiety-like behavior (Aisa et al., 2007; Gracia-Rubio et al., 2016), increased depressive-like behavior (Aisa et al., 2007; Kundakovic et al., 2013), HPA axis hyper-reactivity (van Bodegom et al., 2017), epigenetic changes, and altered expression of hippocampal and striatal molecular markers (Tractenberg et al., 2016). We chose maternal separation without pup warming.
Exposure to multiple early stressors may pose greater neurodevelopmental effects compared with one stressor alone. The purpose of this study was to assess whether simultaneous exposure to two stressors interact to adversely affect cognition and behavior more than either one separately. We used barren cage rearing and maternal separation/pup isolation. Both models of developmental stress cause long-term effects (Bolton et al., 2017; Gilles et al., 1996; Ivy et al., 2008; Rice et al., 2008; Tractenberg et al., 2016; van Bodegom et al., 2017). Rats were reared in barren or standard cages and/or were subjected to daily periods of isolation from their dam or remained with their dam prior to weaning, a period of brain development analogous to late gestation to early childhood in humans (Clancy et al., 2001; Clancy et al., 2007a; Clancy et al., 2007b). The hypothesis was that developmental exposure to a combination of stressors would produce greater cognitive and anxiety-related changes compared with either stressor alone.
Section snippets
Animals
Male and nulliparous female (175–200 g when received) Sprague-Dawley CD/IGS rats (strain 001) were obtained from Charles River Laboratories, Raleigh, NC and acclimated for not less than one week before breeding. Rats were maintained in an AAALAC International accredited vivarium on a 14:10 h light-dark cycle (lights on at 600 h) with controlled temperature (19 ± 1 °C) and humidity (50 ± 10%). Rats were provided with ad lib NIH-07 diet (LabDiet, Richmond, IN) and reverse osmosis, UV filtered
Growth
Rats were weighed prior to weaning and as adults. Repeated measures ANOVA on body weight showed effects of cage (F(1,38.7) = 29.29, p < 0.0001), isolation (F(1,272) = 423.30, p < 0.0001), cage x day (F(5,1183) = 82.15, p < 0.0001), isolation x day (F(5,1183) = 145.57, p < 0.0001), and cage x isolation x day (F(5,1183) = 3.76, p < 0.005). The data are shown in Fig. 1A,B. All experimental groups differed significantly from Std-Norm starting on P11; differences increased thereafter through weaning
Discussion
Table 1 summarizes the primary effects of the developmental stress conditions and the combination of both together. While there were several main effects of barren cage and maternal separation, most of the effects were interactions of the two. The interactions, however, did not support the hypothesis that these two stressors would interact synergistically to cause greater effects than either one alone. Instead, the interactions were specific to particular combinations and were seen where
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Supported by NIH grant R01 ES015689 and training grant T32 ES007051.
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