Bi-directional communication between the hypothalamus–pituitary–adrenal (HPA)-axis and the sympathetic nervous system with the immune system is crucial to ensure homeostasis. Shared use of ligands and especially receptors forms a key component of this bi-directional interaction. Glucocorticoids (GC), the major end products of the HPA-axis differentially modulate immune function. Cytokines, especially interleukin-1 (IL-1), tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6), ensure immune signalling to the neuroendocrine system. In addition, hormones from leukocyte origin such as corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and β-endorphin, as well as centrally synthesised and secreted cytokines, contribute to the communication network. In teleost fish cortisol is the major product of the hypothalamus–pituitary–interrenal (HPI)-axis which is the teleost equivalent of the HPA-axis. Moderate and substantial increases in cortisol during stressful circumstances negatively affect B-lymphocytes, whereas rescue of neutrophilic granulocytes may support innate immunity. Recent elucidation of lower vertebrate cytokine sequences has facilitated research into neuroendocrine–immune interactions in teleosts and the first evidence for a significant function of interleukin-1 in the bi-directional communication is discussed.
Engelsma, M. Y., Huising, M. O., van Muiswinkel, W. B., Flik, G., Kwang, J., & Verburg-van Kemenade, B. M. L. (2002). Neuroendocrine-immune interactions in fish : a role for interleukin-1. Veterinary Immunology and Immunopathology, 87, 467-479. https://doi.org/10.1016/S0165-2427(02)00077-6