Urea effects on the olfactory epithelium of developing Danio rerio

Aim of this study was to evaluate the effects of urea on the molecules involved in olfactory signal reception and transduction, and on nitric oxide synthase (NOS) in the olfactory epithelium of vertebrates. The study is based on evolutionary and clinical clues. The physiologically uremic fishes (the Chondrichthyes class) lack olfactory cilia, olfactory receptors and the G protein alpha subunit olfactory-type (Gαolf) (Ferrando and Gallus, 2013). Chronic renal diseases cause olfactory impairment through an unknown mechanism (Landis et al., 2011). Danio rerio embryos were exposed to 75 mM urea concentration from 24 to 96 hours post fertilization. The endocytosis of Neutral Red dye in the olfactory mucosa was 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. Nitric oxide (NO) has important roles in the olfactory epithelium, meanwhile urea is known to affect its production. Thus we evaluated, in control and urea-treated larvae, the presence of NOS in the olfactory epithelium using immunohistochemistry and a histoenzymatic reaction. We also exposed D. reriolarvae to a NO-production inhibitor, (NG-L-Nitro-Arginine), and a NO donor (Nitroprusside) comparing them to the control and urea-treated fishes. The Neutral Red internalization, and possibly the olfactory receptors presence and functions, was unaffected by urea. Both Gαolf and NOS were increased in urea-treated fishes than in control. The treatment with NO-production inhibitor and NO donor gave interesting clues allowing some considerations about the role of NO in the olfactory transduction molecules. Further investigations are needed in order to understand the actual effects of urea on the olfactory system of pathologically and physiologically uremic vertebrates.