Dual ecdysteroid action on the epitracheal glands and central nervous system preceding ecdysis of Manduca sexta

Initiation of the ecdysis behavioural sequence in insects requires activati on of the central nervous system (CNS) by pre-ecdysis-triggering hormone (P ETH) and ecdysis-triggering hormone (ETH), which are released from the Inka cells of the epitracheal glands. Here, we show that the developmental even ts preceding larval and pupal ecdysis of Manduca sexta involve a dual actio n of ecdysteroids on the epitracheal glands and CNS. The low steroid levels in freshly ecdysed and feeding larvae are associated with small-sized epit racheal glands, reduced peptide production in Inka cells and insensitivity of the CNS to ETH. The elevated ecdysteroid levels before each ecdysis lead to a dramatic enlargement of Inka cells and increased production of peptid e hormones and their precursors. As blood ecdysteroids reach peak levels, t he CNS becomes responsive to Inka cell peptides. These effects of natural e cdysteroid pulses can ex induced by injection of 20-hydroxyecdysone or the ecdysteroid agonist tebufenozide (RH-5992) into ecdysed larvae, thus stimul ating peptide production in Inka cells and inducing CNS sensitivity to ETH. A direct steroid action on the CNS is demonstrated by subsequent treatment of isolated nerve cords from ecdysed larvae with 20-hydroxyecdysone and ET H, which results in preecdysis or ecdysis bursts. Our data show that ecdyst eroid-induced transcriptional activity in both the epitracheal glands and t he CNS are necessary events for the initiation of the ecdysis behavioural s equence.