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First published online August 23, 2004
Journal of Experimental Biology 207, 3317-3327 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01143

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Evidence that thyroid hormone induces olfactory cellular proliferation in salmon during a sensitive period for imprinting

Sean C. Lema^* and Gabrielle A. Nevitt^*

Center for Animal Behavior and Section of Neurobiology, Physiology and Behavior, One Shields Avenue, University of California at Davis, Davis, CA 95616, USA

^* Author for correspondence (e-mail: sclema{at}ucdavis.edu or ganevitt{at}ucdavis.edu)

Accepted 15 June 2004

Salmon have long been known to imprint and home to natal stream odors, yet the mechanisms driving olfactory imprinting remain obscure. The timing of imprinting is associated with elevations in plasma thyroid hormone levels, with possible effects on growth and proliferation of the peripheral olfactory system. Here, we begin to test this idea by determining whether experimentally elevated plasma levels of 3,5,3'-triiodothyronine (T₃) influence cell proliferation as detected by the 5-bromo-2'-deoxyuridine (BrdU) cell birth-dating technique in the olfactory epithelium of juvenile coho salmon (Oncorhynchus kisutch). We also explore how natural fluctuations in thyroxine (T₄) relate to proliferation in the epithelium during the parr-smolt transformation. In both studies, we found that BrdU labeled both single and clusters of mitotic cells. The total number of BrdU-labeled cells in the olfactory epithelium was significantly greater in fish with artificially elevated T₃ compared with placebo controls. This difference in proliferation was restricted to the basal region of the olfactory epithelium, where multipotent progenitor cells differentiate into olfactory receptor neurons. The distributions of mitotic cluster sizes differed significantly from a Poisson distribution for both T₃ and placebo treatments, suggesting that proliferation tends to be non-random. Over the course of the parr-smolt transformation, changes in the density of BrdU cells showed a positive relationship with natural fluctuations in plasma T₄. This relationship suggests that even small changes in thyroid activity can stimulate the proliferation of neural progenitor cells in the salmon epithelium. Taken together, our results establish a link between the thyroid hormone axis and measurable anatomical changes in the peripheral olfactory system.

Key words: thyroid hormone, olfactory imprinting, cell proliferation, neural progenitor cell, peripheral olfactory system, parr-smolt transformation, salmon, Oncorhynchus kisutch

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