Abstract
Olfactory sensory neurons (OSNs) of fish belong to three main types: ciliated olfactory sensory neurons (cOSNs), microvillous olfactory sensory neurons (mOSNs), and crypt cells. Mercury is a toxic metal harmful for olfaction. We exposed the olfactory epithelium of zebrafish to three sublethal Hg2+ concentrations. Molecular markers specific for the different types of OSNs were immunohistochemically detected. Image analysis of treated sections enabled counting of marked cells and measurement of staining optical density indicative of the response of OSNs to Hg2+ exposure. The three types of OSNs reacted to mercury in a different way. Image analysis revealed that mOSNs are more susceptible to Hg2+ exposure than cOSNs and crypt cell density decreases. Moreover, while the ratio between sensory/nonsensory epithelium areas is unchanged, epithelium thickness drops, and dividing cells increase in the basal layer of the olfactory epithelium. Cell death but also reduction of apical processes and marker expression could account for changes in OSN immunostaining. Also, the differential results between dorsal and ventral halves of the olfactory rosette could derive from different water flows inside the olfactory chamber or different subpopulations in OSNs.
Highlights
Incubation with anti-calretinin antibody that stains cell bodies and dendrites of both mature ciliated olfactory sensory neurons (cOSNs) and microvillous olfactory sensory neurons (mOSNs), resulted in clear immunopositivity ranging from the basal to the apical layers of the olfactory epithelium (Figs. 4a–4d)
In the dorsal half of the olfactory rosette, Optical Density (OD) analysis revealed a condition similar to the whole rosette, with the addition of a significantly higher OD value for medium quantity (MQ) compared to high quantity (HQ) (Fig. 4f)
The value at HQ appeared significantly decreased compared to control, low quantity (LQ), and MQ in the entire rosette and in each dorsal and ventral half (Figs. 6a–6c)
Summary
The olfactory sensory neurons (OSNs) of fish are directly exposed to water and its pollutants, which can interact with the dendritic terminations, affecting their integrity and functions (Sorensen & Caprio, 1998; Laberge & Hara, 2001; Zielinski & Hara, 2006; Tierney et al, 2010; Azizishirazi et al, 2014; Lari et al, 2019). Heavy metals are common anthropogenic pollutants, persistent, and toxic at very low concentrations (Azizishirazi et al, 2014) They can cause histopathological (Bettini et al, 2006a, 2006b; Tierney et al, 2010; Lazzari et al, 2017, 2019) and functional (McIntyre et al, 2008; Tierney et al, 2010; Dew et al, 2012, 2014, 2016; Hentig & Byrd-Jacobs, 2016; Razmara et al, 2021) alterations to the olfactory system of fish, affecting various behaviors guided by olfaction, such as food searching, recognition of sexual partners, alarm cue, and predators avoidance (Scott et al, 2003; Sandahl et al, 2007; McIntyre et al, 2012; Azizishirazi et al, 2014; Abreu et al, 2016, 2017; Hentig & Byrd-Jacobs, 2016)
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