Basal regulation of HPA and dopamine systems is altered differentially in males and females by prenatal alcohol exposure and chronic variable stress
Introduction
Alcohol consumption during pregnancy can result in fetal alcohol spectrum disorder (FASD) in the children, with a prevalence of 9/1000 births in North America (Thanh and Jonsson, 2010). The adverse effects of FASD depend on dose, timing and duration of alcohol exposure, and include impairments in cognition, self-regulation, and adaptive functioning (Riley et al., 2003, Streissguth et al., 2004). Numerous ‘secondary’ disabilities, including substance use disorders (SUD), have been reported following prenatal alcohol exposure (PAE) (O’Connor and Paley, 2009). Consistent with these findings, rodent models of PAE show enhanced preference for alcohol (Barbier et al., 2009) and increased voluntary alcohol consumption in PAE offspring (Chotro et al., 2007), supporting the hypothesis that PAE results in neurobiological vulnerability to SUD. The present study focuses on elucidating the effects of PAE on neurobiological mechanisms underlying vulnerability to SUD.
Dysregulation of hypothalamic–pituitary–adrenal (HPA) function is associated with SUD (Lovallo, 2006, Koob and Kreek, 2007). Moreover, data indicate that PAE reprograms the fetal HPA axis such that basal HPA tone is increased, and greater HPA activation and/or delayed or deficient recovery following stress are observed (Haley et al., 2006, Weinberg et al., 2008). Importantly, it appears that maturation of HPA responsivity may have a different developmental trajectory in alcohol-exposed compared to non-exposed infants during the critical first year of life (Ramsay et al., 1996, Jacobson et al., 1999, Haley et al., 2006).
While stress-induced HPA activation is adaptive in the short term, long-term increases in basal HPA tone and/or frequent or sustained HPA activation can lead to HPA dysregulation and alterations in physiological responsiveness and behavior (McEwen, 2008). Thus, the ability to maintain basal hormone levels within the normal physiological range and to turn off a stress response once initiated are as important as the ability to respond to stress initially. The finding that all facets of HPA regulation are altered by PAE has important implications for healthy offspring development.
The mesocorticolimbic DA pathway is implicated in functions such as motivation, attention, reinforcement, executive function and emotional regulation. Importantly, PAE produces significant alterations in DA systems by delaying development, reducing the size, structure and electrical activity of DA neurons, and reducing DA synthesis, binding, and metabolism (Rathbun and Druse, 1985, Cooper and Rudeen, 1988, Druse et al., 1990, Shetty et al., 1993, Shen et al., 1995, Shen et al., 1999). These findings support the suggestion that PAE results in reduced overall baseline activity of DA systems. Of relevance to the present study, there is significant overlap between central stress and DA pathways, resulting in a bi-directional interaction between stress and DA systems (Cabib and Puglisi-Allegra, 2012). For example, stress sensitizes healthy individuals to the rewarding effects of substance use, and can induce relapse after abstinence (Sarnyai et al., 2001), suggesting that HPA alterations provide a pathway for increased vulnerability to SUD.
Both HPA and DA systems are important for resilience against, or vulnerability to, addiction (Sinha, 2009); however, effects of PAE on the interaction between HPA and DA systems have not been investigated. In the present study we examined PAE effects on basal regulation of central stress and DA systems in key brain regions where these systems intersect (medial prefrontal cortex [mPFC], bed nucleus stria terminalis [BNST], striatum, nucleus accumbens [NAc], central nucleus of the amygdala (CeA), hippocampus [HPC]). Basal levels of mRNA for corticotropin releasing hormone (CRH) and glucocorticoid receptors (mineralocorticoid, MR; glucocorticoid, GR) were measured as indicators of dysregulation of the stress system, as alterations in these measures of stress system function have been implicated in vulnerability to SUD (Koob and Kreek, 2007, Le Moal, 2009, Sinha, 2011). Alterations in basal expression of DA receptors (DA-R) are also implicated in SUD (Everitt et al., 2008, Volkow et al., 2011), and were investigated. All neural measures were examined under basal conditions, either with or without prior exposure to chronic variable stress (CVS). Additionally, we investigated sex differences in outcome as: (1) there are sex differences in HPA function across species, with females typically showing greater HPA responses and greater resistance to negative feedback compared to males (Young, 1998); (2) PAE differentially alters HPA activity in males and females (Haley et al., 2006, Weinberg et al., 2008); and (3) sex differences are observed in SUD (Becker and Hu, 2008). Given the links between HPA and DA systems, and the effects of PAE on these systems, we hypothesized that PAE will enhance the adverse effects of CVS on basal/tonic regulation of HPA and DA systems in the brain, and that sex differences will be present in the effects of both PAE and CVS, with greater effects of CVS on basal regulation in females compared to males.
Section snippets
Breeding, experimental diets and feeding
Animal procedures were in accordance with the National Institutes of Health guidelines, and were approved by the institutional Animal Care Committee. Adult virgin female (225–260 g; n = 31) and male (275–300 g; n = 12) Sprague-Dawley rats were obtained from Charles River (St Constant, PQ, Canada) and put onto a 08:00–20:00 light schedule. Briefly, a male and female were paired, and the presence of vaginal plugs indicated day 1 of gestation (GD 1), which followed previous procedures (Hellemans et al.,
Developmental data
Reduced body weight in PAE and PF compared to control dams during gestation but not lactation: Analysis of maternal weight throughout pregnancy revealed a significant prenatal group × gestation day interaction (F6,72 = 8.62, p < 0.001; Table 1). PAE weighed less than C dams on GD 7 and 21 (p's < 0.02), and PF weighed less than C dams throughout pregnancy (GD 7–21) (p's < 0.03), and less than PAE dams on GD 21 (p < 0.03). There were no effects of prenatal group on maternal body weights during lactation (p's >
Discussion
Overall, prior CVS exposure unmasked alterations in basal HPA and DA circuitry in PAE subjects, revealing more widespread changes in basal regulation of central stress circuitry, but a loss of the typical changes in basal DA-R expression in PAE compared to control animals (Table 3). Specifically, PAE increased sensitivity to stress, and altered interactions between HPA and DA systems in a sexually dimorphic manner, as shown by differential effects of CVS on: (1) body weights and patterns of
Conclusions
The present data demonstrate long-lasting alterations in both HPA and DA systems, as well as in HPA–DA interactions, in PAE males and females. Specifically, PAE animals show widespread changes in basal regulation of central stress circuitry, but a loss of the typical changes in basal DA-R expression compared to controls. It is possible that these changes in central regulation of basal HPA activity may underlie the increased stress responsiveness that is consistently observed in PAE animals and
Role of the funding sources
This research was funded by grants from the Canadian Foundation for Fetal Alcohol Research (CFFAR) to JW and LAMG, and NIH/NIAAA R37 AA007789 to JW. KAU was funded by IMPART (CIHR STIHR program).
Conflict of interest
No financial support from any individual or corporate body has been received for compensation of work over the past three years; therefore the authors declare no potential conflict of interest.
Acknowledgements
We would like to thank Dr. Douglas Allan and Luba Veverytsa at the Facility for Synaptic Imaging at the University of British Columbia for assistance with and use of the confocal microscope for imaging of dopamine receptors. We would like to thank Stephanie Lieblich for her expert assistance with tissue processing, Wayne Yu for his expert assistance with RIAs, as well as Caitlin Bauermeister, Welan Dionela, and Jimmy Yan for their valuable contributions to this study.
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Equal senior authors.