17β‐Oestradiol Modulates Glucocorticoid, Neural and Behavioural Adaptations to Repeated Restraint Stress in Female Rats

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Abstract
Sex steroids have a role in modulating responses that extends beyond reproduction. The current study investigated the influence of the sex steroid 17β‐oestradiol on hypothalamic‐pituitary‐adrenal (HPA) and behavioural responses to acute or repeated restraint stress. Ovariectomized rats treated with 17β‐oestradiol or peanut oil via a subcutaneous silastic capsule were subjected to daily handling (non stressed), acute (single, 1 h) or daily (10 days, 1 h/day) restraint stress. Blood collected at the end of stress revealed that 17β‐oestradiol treatment augmented the corticosterone response to acute restraint. After daily exposure to restraint, the corticosterone response was noticeably diminished in untreated females but 17β‐oestradiol‐treated rats still showed an exaggerated response compared to castrated, untreated females. Brain tissue collected 3 h after the end of restraint was probed using isotopic in situ hybridization for corticotropin‐releasing factor (CRF) and vasopressin gene expression in the paraventricular nucleus (PVN) of the hypothalamus. 17β‐oestradiol treatment at the higher dose (120 µg/ml) decreased basal CRF mRNA. Stress caused an increase in CRF mRNA expression in 17β‐oestradiol‐treated rats but not in the vehicle group. Repeated restraint stress caused an increase in PVN parvocellular vasopressin gene expression, which was more pronounced in 17β‐oestradiol‐replaced rats. Animals were exposed to the elevated plus maze for 5 min as a test for anxiety. Non‐stressed control rats with or without 17β‐oestradiol replacement spent the same percentage amount of time exploring the open arms of the maze. Previous exposure to acute restraint stress caused a marked reduction in the time spent exploring the open arms, indicating an increase in anxiety levels in these rats; this effect was observed in both vehicle and 17β‐oestradiol‐treated rats. After repeated restraint stress, 17β‐oestradiol‐replaced rats spent as much time exploring the open arms of the maze as controls, indicating adaptation. By contrast, nonreplaced rats were still showing a significant reduction in open arm exploration after repeated restraint. The present study presents novel data showing that the HPA axis remains reactive to repeated stress in 17β‐oestradiol‐treated ovariectomized rats, but stress‐induced anxiety behaviour is reduced.

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