In vivo visualization of olfactory pathophysiology induced by intranasal cadmium instillation in mice

Abstract

Intranasal exposure to cadmium has been related to olfactory dysfunction in humans and to nasal epithelial damage and altered odorant-guided behavior in rodent models. The pathophysiology underlying these deficits has not been fully elucidated. Here we use optical imaging techniques to visualize odorant-evoked neurotransmitter release from the olfactory nerve into the brain’s olfactory bulbs in vivo in mice. Intranasal cadmium chloride instillations reduced this sensory activity by up to 91% in a dose-dependent manner. In the olfactory bulbs, afferents from the olfactory epithelium could be quantified by their expression of a genetically-encoded fluorescent marker for olfactory marker protein. At the highest dose tested, cadmium exposure reduced the density of these projections by 20%. In a behavioral psychophysical task, mice were trained to sample from an odor port and make a response when they detected an odorant against a background of room air. After intranasal cadmium exposure, mice were unable to detect the target odor. These experiments serve as proof of concept for a new approach to the study of the neural effects of inhaled toxicants. The use of in vivo functional imaging of the neuronal populations exposed to the toxicant permits the direct observation of primary pathophysiology. In this study optical imaging revealed significant reductions in odorant-evoked release from the olfactory nerve at a cadmium chloride dose two orders of magnitude less than that required to induce morphological changes in the nerve in the same animals, demonstrating that it is a more sensitive technique for assessing the consequences of intranasal neurotoxicant exposure. This approach is potentially useful in exploring the effects of any putative neurotoxicant that can be delivered intranasally.

Odor-Specific, Olfactory Marker Protein-Mediated Sparsening of Primary Olfactory Input to the Brain after Odor Exposure

“…Adult (4 -9 months old) male and female OMP-spH mice were used as subjects in the imaging experiments Kass et al, 2013b). In this mouse line, spH is expressed under the OMP promoter in all OSN terminals, and spH signals indicate neurotransmitter release from the olfactory nerve into olfactory bulb glomeruli (Bozza et al, 2004). Mice heterozygous for both OMP and spH were generated by crossing OMP Ϫ/Ϫ -spH mice with wild-type 129 mice as described previously Kass et al, 2013b).…”

“…Chronic cranial windows were implanted bilaterally as reported previously (Bozza et al, 2004). Briefly, mice were anesthetized with pentobarbital and dosed with atropine to reduce intranasal mucous secretion.…”

“…Each OSN in the nasal epithelium expresses one of a repertoire of ϳ1000 olfactory receptors in the mouse, and as the axons reach the bulb, they segregate by receptor type such that all of the OSNs expressing a given receptor project to one or two specific glomeruli (spherical structures in which OSN axons converge and synapse onto postsynaptic neurons) on the surface of the olfactory bulb (Mombaerts, 2006). An odorant binding to a subset of odorant receptor types in the epithelium stimulates those OSNs to fire action potentials and thus evokes neurotransmitter release from OSN synaptic terminals into a corresponding subset of olfactory bulb glomeruli.…”

“…An odorant binding to a subset of odorant receptor types in the epithelium stimulates those OSNs to fire action potentials and thus evokes neurotransmitter release from OSN synaptic terminals into a corresponding subset of olfactory bulb glomeruli. This synaptic output from OSNs can be visualized optically in vivo in gene-targeted mice that express the fluorescent exocytosis indicator synaptopHluorin (spH) from the locus for olfactory marker protein (OMP), resulting in expression in OSN presynaptic terminals (OMP-spH mice; Bozza et al, 2004). The patterns of synaptic input evoked by odorant presentation are odorant specific, stable across time (Bozza et al, 2004), and similar across mice (Soucy et al, 2009), thus providing a model system in which experience-dependent plasticity in the primary sensory input to the CNS can be observed in the same individual over time.…”

Effect of Cadmium on Human Middle Ear Epithelial Cells

“…We believe that cadmium can access the middle ear space through the Eustachian tube, like other air pollutants or smoke, or can enter the middle ear space via systemic circulation. Cadmium has been found in ossicles after systemic exposure [13] , and its uptake has also been demonstrated in nasal mucosa or olfactory pathways [26] . There is no treatment for cadmium toxicity.…”