NEVITT, Gabrielle ; LEMA, Sean; Univ. of California, Davis; Univ. of California, Davis: Testing a new model for olfactory imprinting in salmon: Evidence for proliferation of olfactory receptor neurons in response to thyroid hormone.

Salmon are famous for their ability to imprint and home to natal stream odors, yet the mechanisms driving olfactory imprinting remain obscure. Our laboratory has suggested that olfactory imprinting involves a physiological tuning of the peripheral olfactory system to odors associated with the home stream. According to our model, this tuning should be initiated by a proliferation of olfactory receptor neurons in response to surges in thyroid hormone – a hormone linked to imprinting ability. Here we begin to test this idea by determining whether thyroid hormone influences cell proliferation in the olfactory epithelium of coho salmon parr (Oncorhynchus kisutch). We experimentally elevated circulating levels of the thyroid hormone 3,5,3�-triiodothyronine (T₃) and examined cellular proliferation in the olfactory epithelium using the 5-bromo-2�-deoxyuridine (BrdU) cell birth-dating technique and computer imaging. We found that the total number of BrdU-labeled cells in the olfactory epithelium was significantly greater in fish implanted with T₃ than with placebo pellets (Mann-Whitney U test, P = 0.028). The difference in proliferation rates was restricted to the basal region of the olfactory epithelium where olfactory receptor neurons are born and begin to differentiate. These results support the first prediction of our model in establishing a link between the thyroid hormone axis and measurable anatomical changes in the peripheral olfactory system of salmon.