Effects of urea on zebrafish olfactory organ after environmental exposure.

Chronic renal disease is known to alter olfactory function. Of the uraemic toxins, high urea blood levels have been suspected to induce a hyposmic condition. However, with the exception of olfactometric analysis on human subjects, no other examinations have been conducted, particularly histomorphological observations of the olfactory organ. Even if hematic administration could better mimic pathologic conditions, as first approach we opted for direct nasal exposure, to minimize potential systemic interference. In this study, we describe the effects of environmental exposure to elevated concentrations of urea (7 g/L, 13.5 g/L and 20 g/L) on the sensory mucosa of zebrafish (considered a good predictive model of mammalian toxicity) in acute (48h and 96h) and chronic (30 days) toxicity tests. We observed that lamellae maintained structural integrity and epithelial thickness was slightly reduced only after 30 days at highest urea concentration. However the ratio between the volumes of sensory and non-sensory epithelial regions sensibly decreased during exposure. Pan-neuronal labelling with anti-Hu was negatively correlated with high doses of urea (13.5 g/L and 20 g/L), thus, we investigated whether distinct neuron subtypes were equally sensitive to the toxicant. Using densitometric analysis we evaluated and compared the immunolabelling of Golf -, TRPC2- and S100-expressing cells, as representatives of ciliated, microvillous and crypt neurons, respectively. The three subpopulations responded differently to urea. In particular, crypt cells were more severely affected than the other cell types, and Golf -immunoreactivity increased when fish were exposed to low doses of urea. The moderate sensory toxicity of urea seems to be in accordance with the olfactometric measurements reported in the literature. We counted also the dividing PCNA+-cells, whose density remained constant: it is possible that other non-neuronal cells substituted olfactory neurons preserving epithelial integrity.