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Zinc is both an essential and potentially toxic metal. It is widely believed that oral zinc supplementation can reduce the effects of the common cold; however, there is strong clinical evidence that intranasal (IN) zinc gluconate (ZG) treatment for this purpose causes anosmia, or the loss of the sense of smell, in humans. Using the rat olfactory neuron cell line, Odora, we investigated the mechanism by which zinc is toxic to olfactory neurons. Following treatment of Odora cells with 100 and 200 µM ZG for 0-24 hr, we performed RNAseq and in silico analyses and found up-regulation of pathways associated with zinc metal response, oxidative stress, and ATP production. We also observed that Odora cells could recover from zinc-induced oxidative stress, but ATP depletion persisted with longer exposure to ZG. We also found that ZG exposure caused an increase in NLRP3 and Il-1ß protein levels in a time-dependent manner, suggesting that zinc causes an inflammasome-mediated cell death, pyroptosis, in olfactory neurons. The role of metallothionein (Mt), a metal response protein whose polymorphisms are associated with metal toxicity, particularly in developing nations, was also investigated. In vitro studies using shRNA knockdown of Mt1/2 demonstrated basal elevated level of markers of reactive oxygen species and depressed levels of reduced glutathione; however, the cells were still able to recover from excess zinc and showed a similar LC50 with untransfected Odora cells. In vivo studies utilizing wild-type, MT1/2 knockout mice (KO), and heterozygotes administered ZG by IN instillation showed profound loss of the olfactory mucosa in the nasal cavity. Recovery was monitored, and a lower percentage of the mice were able to smell 28 d after treatment; however, no significant difference was observed in the average thickness of the olfactory epithelium between KO and wild type mice. Both in vitro and in vivo studies showed down-regulation of actinin levels, which may be the trigger for ZG-induced olfactory mucosal damage.

Zinc has been touted as a panacea for the common cold. However, there has been some controversy over whether Zicam, an intranasal zinc gluconate gel purported to fight colds, causes anosmia, or the loss of the sense of smell. Historical evidence has shown that zinc sulfate solutions can cause anosmia in humans along with significant damage to the olfactory epithelium in rodents. However, more recent work has claimed to show that zinc gluconate is less toxic than zinc sulfate. Using an in vitro system to compare the toxicity of zinc sulfate and zinc gluconate on immature and mature rat olfactory sensory neurons, it was found that the toxicity of both zinc salts was similar with zinc sulfate being slightly more toxic than zinc gluconate and occurred at significantly lower concentrations than that found in Zicam nasal gel, which strengthens the epidemiological link between intranasal zinc exposure and anosmia. Mechanistic studies disproved the hypothesis that zinc toxicity was caused by inhibition of the HVCN1 proton channel which would have led to acidosis and apoptotic cell death. It was found that these immature rat olfactory sensory neurons are able to maintain their intracellular pH through a Na+/H+ exchanger, specifically NHE1, and a Cl-/HCO3- exchanger. Zinc sulfate, at non-toxic levels, had no impact on intracellular pH via proton transport either after acute exposure or after 24 hours incubation with the cells. In conclusion, zinc toxicity is not mediated through an acidification of intracellular pH.