The importance of CXC-receptors CXCR1-2 and CXCR4 for adaptive regulation of the stress axis in teleost fish

In an ever-changing environment, an adaptive stress response is the pivotal regulatory mechanism to maintain allostasis. Physiologic responses to stressors enable to overcome potential threat. Glucocorticoid effects can be considered compensatory and adaptive, however prolonged or excessive glucocorticoid secretion can be also maladaptive and detrimental. Therefore, it must be tightly regulated. Apart from the essential hormonal feedback regulation, evidence accrues that cytokines, e.g., proinflammatory interleukin 1β (IL-1β), also play an important regulatory role in the stress axis. Here we focused on the potential role of CXC chemokines (CXCL8 and CXCL12) and their receptors (CXCR1, 2 and 4) in the regulation of the stress response in common carp. We studied changes in gene expression of CXC chemokines and CXCRs in the stress axis organs (hypothalamus-pituitary gland-head kidney) upon 11 h of restraint stress and we established how CXCR blocking affects the activation of the stress axis and the synthesis/conversion of cortisol. During restraint stress, gene expression of the majority of the proinflammatory CXCL8 and homeostatic CXCL12 chemokines and their receptors was upregulated in the stress axis organs. Inhibition of CXCR1-2 and CXCR4 differentially affected the expression of genes encoding stress-related molecules: hormones, binding proteins, receptors as well as expression of genes encoding IL-1β and its receptor. Moreover, we observed that CXC chemokines, via interaction with their respective CXCRs, regulate gene expression of molecules involved in cortisol synthesis and conversion and consistently affect the level of cortisol released into the circulation during the stress response. We revealed that in fish, CXC chemokines and their receptors are important regulators of the stress response at multiple levels of the stress axis, with particularly pronounced effects on steroidogenesis and cortisol conversion in the head kidney.