The hypothalamo-pituitary-adrenal (HPA) axis in sheep is attenuated during lactation in response to psychosocial and predator stress

C.R Ralph,A.J Tilbrook

doi:10.1016/j.domaniend.2015.11.003

Abstract

Activation of the hypothalamo-pituitary-adrenal (HPA) axis by psychosocial stress is attenuated during lactation. We tested the hypothesis that lactating ewes will have attenuated HPA axis responses to isolation and restraint but will have greater responses to predator stress in the form of barking dogs. We imposed two 4 h stressors: psychosocial stress (isolation and restraint of ewes) and predator stress (barking dogs). Blood was collected intravenous every 10 min from nonlactating ewes (n = 6), lactating ewes with lambs present but not able to be suckled (n = 6), and lactating ewes with lambs present and able to be suckled (n = 6). Plasma cortisol and oxytocin were measured. For nonlactating ewes, cortisol increased (P < 0.01) in response to both stressors, and these increases were greater (P < 0.01) than that in the lactating animals. For lactating ewes with lambs present but unable to be suckled, cortisol increased (P < 0.05) in response to both stressors with a greater response to barking dogs (P < 0.05). For lactating ewes with lambs present and able to be suckled, cortisol increased (P < 0.01) in response to barking dogs only. Plasma oxytocin was greater (P < 0.01) in lactating ewes than in nonlactating ewes and did not change in response to the stressors. In conclusion, lactating ewes are likely to have a greater HPA axis response to a stressor that may be perceived to threaten the welfare of themselves and/or their offspring. The role of oxytocin in attenuation of the HPA axis to stress in sheep is unclear from the current research and requires further investigation.

Highlights

Activation of the hypothalamo-pituitary-adrenal (HPA) axis by psychosocial stress is commonly attenuated during mid–late pregnancy and lactation in a range of species,y Present address: Division of Livestock and Farming Systems J.S
There is experimental evidence that oxytocin can act centrally to attenuate the activity of the HPA axis [for review see [14]], and we showed in the sheep that oxytocinergic neurons are located in close proximity to corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) neurons in the paraventricular nucleus (PVN) [4]
There is evidence that central oxytocin is involved in attenuation of the HPA axis [5], and we have shown that oxytocinergic neurons in the PVN are well placed to influence CRH and AVP neurons in sheep [19]

Summary

Introduction

Activation of the hypothalamo-pituitary-adrenal (HPA) axis by psychosocial stress is commonly attenuated during mid–late pregnancy and lactation in a range of species,y Present address: Division of Livestock and Farming Systems J.S. It has been suggested that this natural state of stress hyporesponsiveness in lactating females is important for the wellbeing and mental health of the dam, [5] which, in turn, contributes to her maternal behavior and ability to safely rear the offspring. This makes sense, there are clearly conditions where it would be beneficial for the lactating female to elicit stress responses to stimulate behavioral and physiological actions designed to protect the offspring in a threatening environment.

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Conclusion