The final step in the moulting of all insects is ecdysis, the shedding of the cuticle of the previous instar, which is triggered in Lepidop-tera by the neurosecretory peptide eclosion hormone1. This hormone acts directly on the nervous system to release the appropriate motor patterns for larval, pupal and adult ecdyses2-5, but there are only brief periods near the end of each moult when the nervous system is competent to respond to the hormone. Previous experiments have shown that the action of eclosion hormone on the nervous system at pupal ecdysis in the tobacco hornworm, Manduca sexta, is mediated by the second messenger cyclic GMP1. Here we report that the hormone-stimulated increase in cGMP results in the phosphorylation of two proteins, each with an apparent relative molecular mass (Mr) of 54,000. Moreover, the brief periods during which the central nervous system (CNS) is responsive to eclosion hormone seem to result from the transient presence of these substrate proteins within the nervous system. This provides a novel mechanism by which hormonal responsiveness can be regulated.
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