Urea affects the olfactory epithelium in developing Danio rerio: a possible role of nitric oxide

Chronic renal disease causes olfactory impairment through an unknown mechanism. Chondrichthyes, a class of vertebrates characterized by high plasma urea concentration, lack olfactory cilia and the G protein alpha subunit olfactory-type (Gαolf - marker of the olfactory receptor neurons in vertebrates) in their olfactory neuroepithelium (ONE). Despite of that, there is no data in the literature about a direct action of urea on the ONE. In this study, Danio rerio embryos were exposed to 75 mM urea concentrations from 24 to 96 hours post fertilization. The endocytosis of Neutral Red dye in the olfactory mucosa were detected in control and urea-exposed larvae as a marker of binding and internalization of the Olfactory Receptors. The presence and distribution of Gαolf were investigated in the olfactory epithelium of control and urea-exposed larvae, using a commercial antibody. As Nitric oxide (NO) has important roles in the ONE and urea is known to affect its production, we evaluated, in control and urea-treated larvae, the presence of NO synthase (NOS) in the ONE using immunohistochemistry and a histoenzymatic reaction. We also exposed D. rerio larvae to a NOS-inhibitory molecule, NG-L-Nitro-Arginine (L-NArg), comparing them to the control and urea-treated fishes. Both Gαolf and NOS were increased in urea- and L-NArg-treated fishes than in the control, while the Neutral Red internalization was unaffected. Different hypotheses can be made about the effect of urea on NO production in the olfactory epithelium and, more speculatively, also about the role of NO on the Gαolf synthesis/degradation. Further investigations are needed in order to understand the real effects of urea on the olfactory system of pathologically and physiologically uremic vertebrates.