Pathogen odors elicit distinct neurosteroid and behavioral responses in Rainbow trout (Oncorhynchus mykiss)
Authors: Huertas Mar , Billah Maruf Mohammad 
Published: May 2023

Rainbow trout can smell virus and bacteria cues that in turn activate an immune response in the brain in absence of a systemic response (i.e. the immune response is activated by the nervous system instead of the pathogen inside tissues or bloodstream). However, little is known if pathogen odors can elicit different physiological responses (i.e. behavioral and immune) and how are pathogen odors integrated into different responses in the brain. We hypothesize that pathogen odors not only activate immune responses but also induce avoidance behavior. Moreover, we hypothesized that pathogen olfactory responses are modulated by neurosteroids at different times in different parts of the brain. We used a two-choice maze test to determine whether inactivated Yersinia ruckeri elicits aversive behavior. Furthermore, to check if behavioral and immune responses are modulated by neurosteroids, we performed a second experiment where we nasally administered inactivated Y. ruckeri to rainbow trout. After that, the nose, olfactory bulb, cerebellum, and optic tectum were collected at four different time points (15 min, 4 hours, 24 hours, and 7 days) and samples were extracted to measure gene expression of several steroidogenic enzymes ( cyp19a1, 3βHSD, 11HSD, and 17HSD) and fish production of neurosteroids (e.g. cortisol, estradiol, and progesterone). We found that rainbow trout actively avoids pathogen odors. Moreover, we found a different expression of neuro steroidogenic enzymes at different brain areas and times post-exposure. Fish exposed to pathogen odor express a significant increase (ANOVA 2 factor, P<0.05) of cyp19a1 (involved in estradiol synthesis) as soon as 15 min in the nose and olfactory bulb, and 4 hours post-exposure in the cerebellum and optic tectum. This gene expression was parallel to a significant increase of estradiol in the nose, bulb, and rest of the brain between 15 min and 4 hours. Moreover, there was a significant increase of 11HSD and 17HSD (related to the production of testosterone, cortisol, and corticosterone) 4 hours after exposure in the nose and cerebellum but a significant decrease in the olfactory bulb and optic tectum. These gene expression patterns were parallel o an increase of cortisol in the nose and olfactory bulb. Finally, we found a significant increase of 3βHSD (involved in the first steps of steroidogenesis) after 24 hours in the nose and olfactory bulb, although we didn’t find an increase in steroid production in our preliminary assays. Our results showed that exposure to pathogen odors activates different neuro steroidogenic pathways at different times after exposure, and at different areas of the brain. These findings pinpoint the use of the sense of smell by fish to control behavioral and immune responses to pathogens, with neurosteroids playing a key role in modulating and integrating these olfactory responses. This material is based upon work supported by the National Science Foundation under Grant No. 1755348 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.