Olfaction and peripheral olfactory connections in methimazole-treated rats

Methimazole has been reported to produce extensive degenerative changes in olfactory epithelium and a severe deficit in odor detection [Genter BM, Owens DM, Carlone HB, Crofton KM. Fundam. Appl. Toxicol. 1996;29:71-77; Genter BM, Owens DM, Deamer NJ, Blake BL, Wesley DS, Levi PE. Toxicol. Pathol. 1995;23:477-486.]. To examine this further, rats were tested on olfactory detection and discrimination problems before and after intraperitoneal injection of 300 mg/kg methimazole. In the first 2 days after treatment, experimental rats had nasal congestion and a modest decrement on odor detection and odor mixture discrimination tasks. They performed almost as well as control rats on the third post injection day. In a separate group of rats, anterograde transport of horseradish peroxidase from olfactory epithelium to the bulb was examined 1, 2, 3, and 5 days after administration of methimazole. The treatment produced a modest but progressive disruption of bulbar input: 2 days after administration only approximately 10% of bulbar glomeruli had reduced levels of reaction product while 30-40% of glomeruli had little or no reaction product in 3-5 day survival rats. These results indicate that methimazole is not a particularly effective olfactotoxin and does not produce anosmia or even a severe hyposmia.