Neuroendocrine and behavioural effects of stress during pregnancy across two generations of rats

Abstract

Prenatal stress (PNS) has been shown to affect a range of different modalities, like stress responsiveness and affective traits in both animals and humans. Previously, Dr Paula Brunton’s lab has used a novel model of ethologically relevant prenatal social stress and had shown increased stress responsiveness in both first generation (F1) PNS males and females and increased anxiety-like trait in males, together with corresponding changes in mRNA expression for corticotrophin releasing hormone (Crh) and its receptors. The first aim on this project was to further explore the phenotype of PNS offspring created using this model in social context. F1 PNS females, but not males, displayed an impairment in social memory in comparison to control females, which was supported by lower vasopressin receptor type 1a (Avpr1a) mRNA expression in the anterior part of lateral septum and bed nucleus of stria terminalis. Acute stress exposure immediately prior to the social memory test, impaired social memory in control males and females, but had no effect in PNS males and markedly improved performance in PNS females. This facilitated learning in the PNS females was supported by the finding of higher Avpr1a mRNA expression in both target regions in the brain. Finally, olfactory memory for social but not non-social odours was also impaired in PNS females, compared to control females, indicating that deficits in social memory in PNS females are specific to social odours and not in the detection and/or processing of all odours. It has been shown previously that phenotypes observed in PNS animals can also be seen following disrupted maternal care in the early post-partum period and that stress can affect maternal behaviour. To investigate this possibility in the current model a maternal behaviour observation protocol was developed. Dams were observed during the first postnatal week, three times/day in 90 min blocks. Stressed dams showed an increase in pup-directed behaviours, together with an increase in arched back nursing specifically, compared to control dams. Furthermore, studies have shown that maternal behaviour patterns can be transferred from mother to daughters, therefore this possibility was also investigated here. There were no differences in maternal behaviour between F1 control and F1 PNS dams. The maternal behaviour experiment for the F1 PNS dams created a unique opportunity to study their offspring (F2). F2 PNS rats had lower body weights than their control counterparts throughout their lifetimes, while not differing significantly in their calorie intake. Increased anxiety-like behaviour was also observed in both the F2 males and females (but only during proestrous and estrous stages of their cycle). These changes were supported in males by increased Crh and Crh receptor type 1 and decreased Crh receptor type 2 mRNA expression in discrete regions of the amygdala. Furthermore, F2 PNS females exhibited exaggerated, and males attenuated ACTH and corticosterone secretion in response to acute stress, compared with controls. The reduced stress response in F2 PNS males was supported by higher glucocorticoid receptor (GR, Nr3c1) mRNA expression in field CA1 of hippocampus. In F2 PNS females, increased stress responses were associated with increased Crh and Avp mRNA expression in the medial parvocellular division of the paraventricular nucleus of the hypothalamus and lower basal Nr3c1 and mineralocorticoid receptor (MR, Nr3c2) mRNA expression in the hippocampus. As increased stress and anxiety-like responses have been linked to a depressive phenotype that possibility was also investigated. No changes were found in either sucrose preference or floating/swimming behaviour in the forced swim test between the F2 PNS and control rats, in either sex. Finally, the variation in individual stress responsiveness and anxiety-like trait and the relationship between these phenotypes was investigated in an outbred male Sprague Dawley population. It was found that three of the most commonly used anxiety tests: open field test, light dark box and elevated plus maze do not correlate as well with each other as could be expected. Secondly plasma corticosterone concentrations 30min after the onset of acute restraint stress were positively correlated with the amount of time the rats spent in the anxiogenic environments, showing perhaps counter-intuitively, that the higher the stress responses the less anxious the male rat is. The behavioural tests were also used as a selection procedure to compare gene expression by microarray in the amygdala of high and low anxious rats and hypothalamus of high and low stress-responsive rats. To summarise, stress during pregnancy has profound effects on the dams’ immediate maternal behaviour, as well as neuroendocrine and behavioural effects in both the F1 and F2 offspring. Furthermore, there is evidence of inter-individual variation in stress responsiveness and anxiety-like behaviour in an outbred rat population. This data could lead to further understanding of the origins of inter-individual variation and appreciation of the effect of stress throughout the life course.