Temperament and activity of the hypothalamo-pituitary-adrenal axis in sheep

Stacey Rietema

    Research output: ThesisDoctoral Thesis

    317 Downloads (Pure)

    Abstract

    Temperament is highly variable, even within a species and gender, and a more ‘reactive’ temperament has been associated with greater activity of the hypothalamic-pituitary-adrenal (HPA) axis, a key system in the response to stress, both in the presence and absence of stressors. Although the role of the HPA axis in stress is generally thought of as being protective, an excess of cortisol can be harmful to the individual as it suppresses important processes for longer than necessary to overcome the stressor. Therefore, individuals with a highly responsive temperament may be at a greater risk of compromised health and welfare due to stress, as they may more frequently experience greater concentrations of cortisol both in the absence and presence of stressors.

    Work in this thesis tests whether, in sheep with genetically-based difference in temperament, hyper-reactive, or ‘nervous’, animals have a more active and responsive HPA axis than hypo-reactive, or ‘calm’, animals in the absence and presence of stimuli, and if there is a consequent effect on metabolic hormone balance. These sheep have been bred for 2 decades for their divergent behavioural responses to the stressors of isolation and human presence. These animals also have associated differences in their HPA axis responses to stressors, and are known to differ in some parameters of metabolism.

    The series of experiments compares the two temperament groups, looking at, in brief, 1) the diurnal rhythm of HPA activity, and its relationship with metabolic hormone patterns; 2) the effect of repeated exposure to acute stressors on HPA axis activity in the presence and absence of stressors; 3) the responsiveness of the adrenal gland to pituitary stimulation; and 4) the responsiveness of the pituitary-adrenal axis to hypothalamic stimulation.

    Firstly, selection for temperament did not affect the diurnal rhythm of secretion of cortisol, or prolactin, a stress responsive hormone. However, ‘calm’ animals had greater concentrations of insulin during the afternoon than ‘nervous’ animals. A similar tendency was seen in the rhythm of leptin secretion. Therefore, selection for temperament did not affect resting activity of the HPA axis, but it did appear to affect insulin patterns, independently of the HPA axis.

    Secondly, nervous animals do not seem to be more prone to chronic stress at the level of stressor intensity used in this study. There was no effect of the repeated stressor treatment on cortisol and insulin concentrations in the absence of stressors, or on the behavioural and cortisol response to isolation, suggesting that our stressor model was insufficient to induce chronic stress. There were no differences between the temperament groups in cortisol or insulin concentrations in the absence of stressors, but ‘nervous’ animals showed a longer cortisol response and greater behavioural response to the acute stressor than ‘calm’ animals, indicating that temperament-based differences in the HPA axis may only be apparent during a stressful stimulus.

    Finally, we showed that temperament did not affect the responsiveness of the pituitary-adrenal axis to stimulation with hypothalamic peptides, or of the adrenal glands to pituitary stimulation. Therefore, it is clear that ‘nervous’ animals have a greater HPA axis response to acute stressors than calm animals, but this is not due to changes in the responsiveness of the pituitary-adrenal axis. Furthermore, selection for temperament has not affected the resting activity of the HPA axis. Instead, the difference in the HPA axis response to stressors between the temperaments might lay in the neural circuits that process perception of the stressor and connect it to the activation of hypothalamic peptide secretion. Additionally, there is some evidence that temperament is associated with differences in concentrations of the metabolic hormone, insulin, but such effects are independent of the HPA axis, and require further study. This thesis furthers the understanding of the link between temperament and stress physiology, which can help promote health and welfare.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2015

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