Neurosteroid Role in the Integration of Olfactory Immune Responses in Rainbow Trout (Oncorhynchus Mykiss)
Authors: Billah Mohammad Maruf , Mancha Fabiola , Ellis Laura , Huertas Mar 
Published: May 2022

The objective of this study is to unravel the integration of olfactory signals involved in the nasal detection of pathogens and the activation of associated physiological responses in Rainbow trout.

The infectious pathogen Yersinia ruckeri causes enteric redmouth disease in aquaculture farms worldwide. Recent research shows that olfactory organ of rainbow trout is highly susceptible external tissue like gills or skin due to direct contact with the microbiome‐rich environment. Recent findings have demonstrated that olfactory sensory cells play a key role in early pathogen detection and immune cell activation along with their primary role in detecting environmental stimuli.

We hypothesize that olfactory immune responses elicited by Y ruckeri are modulated by neurosteroids. To test our hypothesis, we nasally exposed rainbow trout to Y. ruckeri vaccine or saline and took nose and brain samples at four different time periods (4 hours, 24 hours, 7 days, and 14 days) after exposure. We analyzed fish neurosteroids (cortisol, estradiol, and progesterone) and gene expression of steroidogenic enzymes (p450, 3β HSD, 11β HSD, and 17 β HSD) in nose, olfactory bulb, and rest of the brain. Exposure to Y. rukeri vaccine caused significant changes in tissue neurosteroid production and steroidogenic enzyme expression at different times after exposure.

Within the olfactory epithelium (OE), we detected a significant decrease (p = 0.003) in estradiol after 15 min exposure with 26.1 pg/mg tissue and 391.0 pg/mg tissue in treated and control individual respectively. Progesterone (174.83 pg/mg tissue) was significantly (p = 0.004) higher after 4 hours compared to the control (26.5 pg/mg of tissue) and cortisol (10.95 pg/mg of tissue) was significantly lower (p = 0.003) after 14 days in Y. ruckeri‐treated fish compared to the control (78.76 pg/mg of tissue). Within the olfactory bulb (OB), lower concentrations of cortisol and progesterone (5.65 pg/mg tissue and 1.6 pg/mg tissue respectively) were observed after 15 minutes in the Y. ruckeri‐treated fish compared to the control (22.8 pg/mg of tissue and 1.9 pg/mg of tissue, respectively). There was also a higher concentration of estradiol after 15 minutes and 4 hours compared to the control. Lastly, within the rest of brain cortisol (9.1 pg/mg tissue) was significantly higher after 4 hours in vaccine treated fish compared to the control (4.9 pg/mg tissue) and estradiol was significantly lower after 15 minutes in the vaccine group (5.5 pg/mg of tissue), compared to the control (55.9 pg/mg of tissue). However, neurosteroid concentration in the rest of brain was relatively low compared to the OE and OB. The neurosteroids: cortisol, estradiol, and progesterone play many important roles in fish physiology including stress response, cognitive behavioral modulation, and immunological response. Our preliminary results also show a significant differential expression of steroidogenic enzymes after exposure to the vaccine. Altogether, our results demonstrate that neurosteroids play a critical role in the integration of immune olfactory responses in vertebrates.