Effects of urea on the olfactory reception in zebrafish (Danio rerio)

The effects of uremia on human olfactory functions have been clinically evaluated in various studies, even if to date it is not completely clarified which uremic toxins mediate these processes. Surprisingly, the role of the main molecule involved in uremia, urea indeed, has not been adequately investigated as other possible molecules may also be involved in uremic anosmia. The effects of urea on the olfaction have been evaluated in some clinical studies, but this is the first attempt to determine a direct action of urea on the olfactory epithelium of a vertebrate. Danio rerio adults were exposed to urea in different experiments to assess the effects on olfactory sensitivity and signal transduction. The analysis of the swimming speed has been used to evaluate the response to hypoxanthine 3-N-oxide (H3NO), a molecule that is known to elicit an olfactory-mediated alarm reaction in D. rerio. The presence and distribution of the G protein alpha subunit coupled to the olfactory receptors (Gα_olf) has been immunohistochemically investigated in the olfactory epithelium of control and urea-exposed D. rerio. Our findings showed that urea alters the response to H3NO of D. rerio with a quite rapid and reversible effect that appears to be independent from a mere interference of urea on the receptor-ligand binding. The Gα_olf protein resulted increases after urea treatment, suggesting an effect of urea on its expression or degradation.