2018 Study Highlights
- Olfaction is critical in tadpoles for predator avoidance and food location.
- Thyroid hormones (THs) change olfactory tissues dramatically during metamorphosis.
- T3 & municipal wastewater effluent, but not T4 or E2, disrupt avoidance behavior.
- Thra and thibz mRNAs are significantly higher in T3 versus T4 olfactory epithelium.
- Further bioindicator candidates are needed to link to adverse behavior outcomes.
Olfaction is critical for survival, facilitating predator avoidance and food location. The nature of the olfactory system changes during amphibian metamorphosis as the aquatic herbivorous tadpole transitions to a terrestrial, carnivorous frog. Metamorphosis is principally dependent on the action of thyroid hormones (THs), l-thyroxine (T4) and 3,5,3′-triiodothyronine (T3), yet little is known about their influence on olfaction during this phase of postembryonic development.
We exposed Taylor Kollros stage I-XIII Rana (Lithobates) catesbeiana tadpoles to physiological concentrations of T4, T3, or 17-beta-estradiol (E2) for 48 h and evaluated a predator cue avoidance response.
The avoidance response in T3-exposed tadpoles was abolished while T4- or E2-exposed tadpoles were unaffected compared to control tadpoles. qPCR analyses on classic TH-response gene transcripts (thra, thrb, and thibz) in the olfactory epithelium demonstrated that, while both THs produced molecular responses, T3 elicited greater responses than T4. Municipal wastewater feed stock was spiked with a defined pharmaceutical and personal care product (PPCP) cocktail and treated with an anaerobic membrane bioreactor (AnMBR). Despite substantially reduced PPCP levels, exposure to this effluent abolished avoidance behavior relative to AnMBR effluent whose feed stock was spiked with vehicle. Thibz transcript levels increased upon exposure to either effluent indicating TH mimic activity.
The present work is the first to demonstrate differential TH responsiveness of the frog tadpole olfactory system with both behavioral and molecular alterations. A systems-based analysis is warranted to further elucidate the mechanism of action on the olfactory epithelium and identify further molecular bioindicators linked to behavioral response disruption.