Corticosterone responses to stressors and the regulation of hypothalmic-pituitary-adrenal axis in Japanese quail : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physiology at Massey University, Palmerston North, New Zealand
Corticosterone is the major adrenal glucocorticoid in birds. It is secreted in response to stressors, with plasma concentrations typically rising to a peak within 10 to 15 minutes and then declining over 30 to 60 minutes once the stressor is removed. The increase in corticosterone is thought to help the animal to adjust to the stressor, although corticosterone may also inhibit the reproductive axis. The corticosterone response to a stressor varies between individuals, although the level of the hypothalamic-pituitary-adrenal axis at which this variation arises in birds has not been identified. The goals of this research were to determine in Japanese quail (Coturnix coturnix japonica): (1) the effects of corticosterone and of various stressors on sex steroid secretion; (2) corticosterone responses to 5, 10 or 15 min exposure to manual restraint and 15 min exposure to different stimuli; (3) quantifying individual variation in the corticosterone response; and (4) whether individual variation in the corticosterone response to stimuli used in goal 2 is regulated by differences in pituitary or adrenal sensitivity to corticotropin releasing factor (CRF) and adrenocorticotropic hormone (ACTH) respectively. A prolonged elevation in plasma corticosterone concentrations can inhibit reproduction. The effects of a short elevation in corticosterone are less understood, so the relationship between corticosterone and sex steroid secretion in quail was characterised by administering 1.2 mg corticosterone injections. Corticosterone treatment stimulated an increase in plasma corticosterone, while testosterone declined in both corticosterone-treated and control birds. Plasma LH was unaffected by challenge with corticosterone. Collectively, these data suggest that repeated handling associated with frequent blood sampling inhibited testosterone secretion directly at the testis. Corticosterone responses to a variety of novel stimuli were characterised. Manual restraint for 10 or 15 min caused a significant response whereas restraint for 5 min or less did not lead to an increase in corticosterone 15 minutes from the start of the stressor. Corticosterone responses to 15 min of mechanical restraint varied between birds whereas 15 min of manual restraint elicited a maximal plasma corticosterone response in all birds. Manual and mechanical restraint caused declines in plasma testosterone of a similar magnitude. There was more variation within than between birds in their corticosterone response to mechanical restraint. Despite this, the general pattern of the corticosterone response was repeatable for individuals. Quail with low or high plasma corticosterone responses to 15 minutes of mechanical restraint were injected intrajugularly with a dose of ACTH that stimulated a sub-maximal corticosterone response. The plasma corticosterone response to ACTH did not differ between birds with low or high corticosterone responses to mechanical restraint, indicating that variation in corticosterone responses to restraint did not arise at the level of the adrenal gland. A preliminary study showed that injections of ovine CRF stimulated corticosterone secretion. However, CRF did not consistently stimulate an increase in plasma corticosterone in these birds and hence it was not possible to determine if pituitary responsiveness to CRF differed between birds with low or high corticosterone responses to restraint. These results indicate that variation in the corticosterone response between individual Japanese quail arises above the level of the adrenal gland in the HPA-axis, and may occur at the pituitary gland or due to differences in activation of neural pathways in the brain.