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Hypothalamo-pituitary-adrenal responses to social defeat are reduced in pregnant rats

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Original languageEnglish
JournalJournal of Physiology
Issue numberO32
Publication statusPublished - 2003


The responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis to a range of 'emotional/psychological' stressors is reduced in late pregnancy in the rat. Here we employed the social defeat model of 'maternal defence' (previously validated as a relevant emotional stressor for female rats (Neumann et al. 2001) to test whether pregnant rats demonstrate attenuated HPA axis responses to an acute social stressor relevant to females. The test relies upon aggression displayed by a lactating rat directed against an intruder, male or female, that approaches the nest.

On the morning of the experiment pregnant (day 21) or virgin Sprague-Dawley rats were transferred from their home cages into the cage of a lactating resident and exposed to 'maternal defence' for 30 min, and then killed immediately by decapitation (in accordance with the UK Home Office Animals (Scientific Procedures) Act 1986). Control rats were undisturbed before killing. Trunk blood was collected for determination of plasma ACTH and corticosterone concentrations by radioimmunoassay. Brains were removed and processed by in situ hybridisation (ISH) for the immediate early gene: nerve growth factor induced gene-B (NGFI-B) mRNA expression in the parvocellular region of the paraventricular nucleus (pPVN).

Aggressive behaviour displayed by the lactating resident (number of attacks and latency to attack) was not significantly affected by the reproductive status of the intruder rat (Student's unpaired t test). Exposure to maternal defence induced a significant increase in plasma ACTH concentration in the virgin group (2.3-fold increase, 2-way ANOVA; P < 0.01, n = 7), but not in the pregnant group (1.4-fold increase, 2-way ANOVA; n.s., n = 6). Similarly, the maternal defence test induced a significant increase in corticosterone secretion only in the virgin group (36.0 % increase, 2-way ANOVA; P < 0.01) and had no effect in the pregnant rats (1.7 % increase, n.s.). ISH revealed a significant increase in expression of NGFI-B mRNA in the pPVN of virgin rats in response to social defeat but not in the pregnant rats (2.3-fold vs. 1.5-fold increase, respectively; P < 0.01; 2-way ANOVA).

Thus the responsiveness of the HPA axis to social defeat is reduced in late pregnant rats consistent with our previous studies using other emotional stressors. This is a consequence of reduced activation of the pPVN corticotropin-releasing hormone and/or vasopressin neurones in the pregnant rats.

This work was support by PJB, Faculty of Medicine Scholarship and BBSRC.

ID: 20159050