INVOLVEMENT OF CAMP AND CGMP IN THE MODE OF ACTION OF MOLT-INHIBITINGHORMONE (MIH) A NEUROPEPTIDE WHICH INHIBITS STEROIDOGENESIS IN A CRAB

In crustaceans, production of molting hormones (or ecdysteroids) by th e molting glands (Y-organs; YO), is under negative control exerted by a neuropeptide, the molt-inhibiting hormone (MIH). MIH of the crab Car cinus maenas inhibits in vitro steroidogenesis of basal (intermolt cra b) or activated (premolt crab) YO. MIH inhibits secretion of the two e cdysteroids synthesized by crab YO, ecdysone (E) secreted throughout t he molting cycle, and 25-deoxyecdysone (25dE), secreted during the pre molt period. At a MIH concentration of 10(-8) M, E is reduced about 50 % and 25dE 94%. Regardless of the molting stage, this inhibition of st eroidogenesis is reversible, dose dependent and measurable after 5 min . On intermolt YO, MIH induced cGMP increase and 8BrcGMP mimics the ef fect of MIH: at this stage cGMP seems to be involved with MIH inhibiti on of steroidogenesis. On premolt YO MIH induced a transient increase of cAMP (2-fold) and a long-lasting enhancement of cGMP (60-fold). On active YO, we demonstrated that a low concentration (10(-5) M) of dbcA MP, 8BrcAMP, 8BrcGMP, or agents increasing intracellular cAMP, mimic M IH effects and inhibit steroidogenesis. From these observations it is concluded that both cyclic nucleotides are involved in the mode of act ion of MIH on activated YO. At this premolt period, MIH/cAMP may act c ooperatively with MIH/cGMP in the inhibitory control of steroidogenesi s by crab YO.