Duress without stress: Cryptobia infection results in HPI axis dysfunction in rainbow trout Barry N Madison, Patrick T K Woo and Nicholas J Bernier Department of Integrative Biology, University of Guelph, 50 Stone Road East, Guelph, Ontario, Canada N1G 2W1 Correspondence should be addressed to N J Bernier Email nbernier@uoguelph.ca Abstract Despite clear physiological duress, rainbow trout (Oncorhynchus mykiss) infected with the pathogenic haemoflagellate Cryptobia salmositica do not appear to mount a cortisol stress response. Therefore, we hypothesized that the infection suppresses the stress response by inhibiting the key effectors of the hypothalamic–pituitary–interrenal (HPI) axis. To test this, we characterized the basal activity of the HPI axis and the cortisol response to air exposure in saline- and parasite-injected fish. All fish were sampled at 4 and 6 weeks post-injection (wpi). While both the treatment groups had resting plasma cortisol levels, the parasite-infected fish had lower levels of plasma ACTH than the control fish. Relative to the control fish, the infected fish had higher mRNA levels of brain pre-optic area corticotrophin-releasing factor (CRF) and pituitary CRF receptor type 1, no change in pituitary POMC-A1, -A2 and -B gene expression, higher and lower head kidney melanocortin 2 receptor mRNA levels at 4 and 6 wpi respectively and reduced gene expression of key proteins regulating interrenal steroidogenesis: StAR, cytochrome P450scc and 11b-hydroxylase. The parasite-infected fish also had a reduced plasma cortisol response to a 60-s air exposure stressor. Superfusion of the head kidney tissues of the parasite-infected fish led to significantly lower ACTH-stimulated cortisol release rates than that observed in the control fish. These novel findings show that infection of rainbow trout with C. salmositica results in complex changes in the transcriptional activity of both central and peripheral regulators of the HPI axis and in a reduction in the interrenal capacity to synthesize cortisol. Key Words " parasite infection " cortisol synthesis " hypothalamic–pituitary– interrenal axis " stress Journal of Endocrinology (2013) 218, 287–297 Introduction Challenges that disturb the homoeostasis of an animal can be met by an activation of the stress response. A key component of this response in fish involves the stimu- lation of the hypothalamic–pituitary–interrenal (HPI) axis (Wendelaar Bonga 1997). Corticotrophin-releasing factor (CRF) from the pre-optic area (POA) is the principle hypothalamic regulator of the HPI axis (Bernier et al. 2009). CRF stimulates the secretion of the pro-opiomelanocortin (POMC)-derived peptide ACTH from the anterior pituitary via the CRF type 1 receptor (CRF-R1; Flik et al. 2006). In turn, ACTH binds to the melanocortin type 2 receptor (MC2R; Aluru & Vijayan 2008) of the interrenal cells in the head kidney and regulates the synthesis and secretion of cortisol, the primary stress hormone in teleosts (Barton 2002). The key rate-limiting steps for the production of cortisol Journal of Endocrinology Research B N MADISON and others Stress response of parasite-infected trout 218 :3 287–297 http://joe.endocrinology-journals.org Ñ 2013 Society for Endocrinology DOI: 10.1530/JOE-13-0155 Printed in Great Britain Published by Bioscientifica Ltd.