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Zinc sulfate is a known olfactory toxicant, although its effects on the olfactory epithelium of zebrafish have not been examined. Previous observations utilizing Triton X-100 found selective retention of some olfactory sensory neurons and loss of others. Based on those findings, we hypothesized that the retention of the ability to sense amino acids, detected by microvillous olfactory sensory neurons, and loss of the ability to perceive bile salts, detected by ciliated olfactory sensory neurons, is a universal response to chemical ablation. Fish were treated with zinc sulfate and survived for 2, 5, 10 and 14 days post treatment and processed for histological, immunohistochemical, ultrastructural, and behavioral analysis.
Severe morphological changes to the olfactory organ were observed 2 days following exposure, accompanied by a significant decrease in anti-calretinin staining in the olfactory epithelium. Lamellae of the olfactory organ appeared fused, and there was obvious inflammation of the epithelium, with large vacuoles within the cells. The structure of the olfactory organ returned to near-control morphology with a lamellar arrangement by 5 days after chemical exposure; however, the amount of anti-calretinin labeling in the olfactory epithelium did not return to control levels until 10 days after exposure.
Scanning electron microscopy revealed the sensory region was absent of ciliated structures, but microvilli could be seen on the apical surface at 2 days after zinc sulfate exposure. Sensory ciliated structures began to return 5 days after exposure, though the population was scarce and the structures appeared thinner. Treated organs returned to near control structure by 10 days following exposure.
An assay quantifying the fish response to an amino acid mixture or a bile salt mixture was utilized to examine olfactory-mediated behavior. Behavior before and after odor exposure was recorded and number of turns was compared. Unlesioned controls responded by significantly increasing their turning behavior in response to both amino acids and bile salt mixtures. At 2 days following exposure, the fish appeared to detect amino acids but their turning behavior was not significantly different from pre-odor behavior. The ability to perceive amino acids returned 10 days after exposure, while the ability to perceive bile salts remained lost. When given 14 days of recovery, the ability to perceive bile salts returned. Thus, chemical ablation of the olfactory epithelium with zinc sulfate results in degeneration of the olfactory organ and removal of most olfactory sensory neurons within 2 days. The ability to perceive amino acids was regained prior to the ability to detect bile salts. Thus, zinc sulfate appears to have a greater affect on ciliated olfactory sensory neurons than on microvillous olfactory sensory neurons.
Hentig, James, "Injury-Induced Neuronal Turnover with Zinc Sulfate Affects Cilitated Olfactory Sensory Neurons more than Microvillous Olfactory Sensory Neurons in the Adult Zebrafish" (2016). Honors Theses. 2685.
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