Morphological Description of Olfactory Organ Changes after Acute Nitrite Exposure in Xiphophorus couchianus

Abstract

This study will explore the effects of acute nitrite exposure in the olfactory organ of Xiphophorus couchianusthrough histological studies. Exposure to aquatic nitrite is known to affect cardiovascular functionality, respiration, and the endocrine and nervous systems in vertebrates. Fish have proven to be beneficial models for studying the effects of aquatic nitrites on vertebrate models. Specifically, the direct interaction of the olfactory system with the aquatic environment offers the opportunity to explore the direct effect of nitrites on the vertebrate nervous system. The olfactory system in fish is incredibly sensitive and allows them the ability to detect odorants across long distances. Thus, the sense of smell in fish is used in a variety of physiological functions (foraging, migration, reproduction, etc.) critical for the survival of the species. For this study, we propose Xiphophorus couchianus as the model species, a live‐bearing fish that uses chemical communication to coordinate reproductive events. We predict that there will be morphological and functional changes in the olfactory tract of X. couchianus due to acute nitrite exposure.To assess the effects of nitrite on olfaction, male X. couchianus were exposed to sublethal levels (0.005 mg/L) of nitrite for forty‐eight hours. The control group consisted of several male X. couchianus that were not previously exposed to nitrite. After forty‐eight hours, their heads were sampled and fixed in bouin. The heads were oriented in paraffin blocks and a microtome was used to cut their noses into 7µm cross sections. The olfactory tract cross sections of the exposed fish were then stained with alcian blue and Gallegos Trichrome staining, and then compared to the olfactory tract of the control group. We found that exposure to nitrite in olfactory tissues changes the morphology of the olfactory organ. More specifically, olfactory epithelium showed signs of inflammation and changes in the morphology of olfactory cells (i.e., hypertrophy and hyperplasia of mucus, several types of receptor cells and lamina propia).To assess the effects of nitrite on olfactory organ function, behavior tests were conducted for both the control and treatment groups. Fish were placed in a two‐choice maze where a food odorant was presented. Fish who were exposed to nitrite showed no preference, whereas the control group clearly preferred the side with the food odorant.Based on the results of this study it can be concluded that nitrite exposure has adverse effects on the morphology of the olfactory tract in X. couchianus and, consequently, hinders typical functioning of relevant olfactory components.