Regulation of Hypothalamic-Pituitary-Interrenal Axis Function During Chronic Social Stress in Rainbow Trout (Oncorhynchus mykiss)

Abstract

The hypothalamic-pituitary-interrenal (HPI) axis in fishes controls the production of cortisol in response to a stressor. To study HPI axis regulation during chronic stress, pairs of juvenile rainbow trout (Oncorhynchus mykiss) were allowed to form dominance hierarchies. Subordinate trout experience chronic social stress associated with a sustained elevation in circulating cortisol levels. Circulating cortisol levels reflect the balance between negative feedback regulation of the HPI axis and cortisol clearance, both of which lower cortisol levels, and cortisol production via HPI axis activation. The capacity for negative feedback and cortisol clearance appeared to be intact in subordinate fish, based on the rapid decline in cortisol after cortisol injection or during recovery from social stress. In addition, corticosteroid receptor transcript and protein abundances throughout the HPI axis did not differ between dominant and subordinate trout. Elevated transcript abundances of 11β-hydroxysteroid dehydrogenase type 2 (11βhsd2) in the preoptic area (POA) and pituitary of subordinate fish suggested an enhanced role for this cortisol-inactivating enzyme during chronic social stress. Steroidogenesis occurs in the interrenal cells of the head kidney, and appeared to be a primary driver of increased plasma cortisol levels in subordinate trout. Transcript abundances of the HPI axis effectors corticotropin releasing factor (crf) and pro-opiomelanocortin (pomc) were not elevated in the POA or pituitary, respectively, of subordinate trout. However, rate-limiting components of head kidney steroidogenesis were elevated, including steroidogenic acute regulatory protein (star) and P450 side chain cleavage enzyme (p450scc). A novel finding in the head kidney was increased transcript abundance of steroidogenic factor 1, which regulates steroidogenic enzyme transcription in mammals. Accordingly, head kidney preparations from subordinate trout exhibited increased basal cortisol production in vitro. Despite this elevation in basal steroidogenesis, subordinate rainbow trout demonstrated attenuated cortisol production in response to stimulation by exogenous adrenocorticotropic hormone (ACTH) both in vivo and in vitro. A similar attenuation was observed with cAMP, suggesting that impairment of acute cortisol production arises downstream of cAMP in the ACTH signalling pathway. The regulation of crf in the POA during chronic social stress was probed in more detail by measuring the abundance of selected microRNAs (miRNA) predicted to target and reduce crf or 11βhsd2 mRNA, but these were not found to play a role. Methylation of the crf promoter revealed significantly decreased methylation at two loci in dominant trout, but the functional significance of this methylation pattern requires further study. Collectively, the present thesis used an array of experimental approaches to provide a comprehensive picture of HPI axis regulation during chronic social stress in rainbow trout.