Do thyroid hormones function in insects?

Earlier work demonstrated that phenoxy-phenyl compounds such as fenoxycarb and thyroxine mimicked the effects of JH III in causing a reduction in volu me of the follicle cells of Locusta migratoria. While these compounds were only moderately effective, a derivative of thyroxine, 3,3',5-triiodothyroni ne (T3) was as effective as JH III, and T3 has been shown to bind to the sa me membrane receptor and activate the same pathway as JH III. The current p aper shows that other thyroxine derivatives vary in activity. 3,3',5'-Triio dothyronine (reverse T3) is inactive. 3,5-Diiodothyronine (T2) is more acti ve than JH III, while its relatives (iodines at 3',5' or at 3,3') are inact ive. When follicles are exposed in vitro to rhodamine conjugated T3, the fl uorescent compound can be seen to enter the cells and accumulate there: thi s process is inhibited by cycloheximide or by a temperature of 0 degrees C. The accumulation is antagonised by JH III but not JH I (which does not bin d to the JH III membrane receptor) and by an antiserum raised against the p utative membrane receptor protein. The action of T3, but not T2, is inhibit ed by 6-n-propyl-2-thiouracil or by aurothioglucose, both known to inhibit deiodinases. The activity of T3, but not of T2, increases with time of expo sure to the follicle cells. These facts suggest that T3 enters the cells by receptor mediated endocytosis and is converted to a more active compound. Immunoreactivity to T3, but not thyroxine, can be detected in the haemolymp h of locusts, and the titre Varies slightly with the gonotrophic cycle. The food shows immunoreactivity for both thyroxine and T3. These findings sugg est that thyroid hormones are ingested by locusts and have the potential to be used as hormonal signals in the control of egg production. (C) 2000 Els evier Science Ltd. All rights reserved.