Effects of reproductive condition on HPG-HPA axis interaction

In vertebrates, activation of the stress response, via the hypothalamic-pituitary-adrenal (HPA) axis often results in the down regulation of processes not immediately necessary for that individual's survival including reproduction (regulated via the hypothalamic-pituitary-gonadal (HPG) axis). The nature of the relationship between reproductive (testosterone (T)) and stress (corticosterone (B)) hormones depends upon the reproductive phase of the animal. As part of a larger project, we investigated whether artificially stimulating the HPA axis would alter plasma T and B concentrations during breeding or quiescence in male blue-tongued lizards, Tiliqua nigrolutea. We measured several parameters relating to HPA and HPG axis function: the increase in circulating B during both breeding and quiescence confirms the role of ACTH in this species as a stimulator of B response. The increase in blood glucose levels following ACTH injection supports the role of the HPA axis in facilitating the male T. nigrolutea response to stress acting to mobilise energy reserves, a role B plays in many reptilian species. A decrease in plasma T in response to activation of the HPA axis suggests ACTH suppresses, directly or indirectly (via elevation of B concentrations), concentrations of T, a critical reproductive hormone.