Ecdysteroid biosynthesis in crayfish Y-organs: Feedback regulation by circulating ecdysteroids

In crustaceans, ecdysteroid synthesis in the Y-organs is negatively regulat ed by the molt-inhibiting hormone (MIH). Reduction or cessation of MIH rele ase from the sinus gland in the eyestalk, probably due to environmental cue s, is one of possibly several signals for an increase of edysteroid product ion and subsequently enhancement of 20-hydroxyecdysone (20E) levels in the hemolymph. The present study asks the question whether the 20E peak in prem oult stages D-2/D-3 is explained solely by the cessation of MIH release or whether positive feedback mechanisms are also involved. Ecdysteroid product ion by the Y-organ of the crayfish Orconectes limosus was found to be under negative feedback control by circulating ecdysteroids. Exogenous 20-hydrox yecdysone (20E) as well as RH-5849, a nonsteroidal ecdysteroid agonist, red uced ecdysteroid synthesis significantly when injected into intermoult anim als. A direct, short loop inhibitory feedback effect was demonstrated by in vitro incubations of Y-organs with RH-5849. Thus, the results presented he re do not point to a stimulatory effect of 20E on Y-organ activity but sugg est that during intermolt a negative feedback by ecdysteroids plays a role in addition to MIH. Arch. (C) 1999 Wiley-Liss. Inc.