Control of prothoracic gland activity by juvenile hormone in fourth instar Manduca sexta larvae

In Manduca sexta, larval-larval molting of the IVth instar ensues in response to a large pulse of ecdysteroid. Ecdysone release by the prothoracic glands (PG), in turn, is stimulated by prothoracicotropic hormone (PTTH) from the brain. We show that juvenile hormone (JH) is necessary and its titer need not fall for PTTH release to occur. Application of the JH analog methoprene to larvae did not shift the hemolymph ecdysteroid titer profile nor did it affect PG activity in vitro. On the other hand, allatectomy abolished the ecdysteroid pulse which could be restored by application of methoprene. Neck-ligation and debraining experiments show that this effect of methoprene is mediated by the brain. In neck-ligated larvae, i.e. when PTTH is eliminated, the ecdysteroid hemolymph titer rises gradually and leads to a larval-pupal molt. In this situation, methoprene suppressed the ecdysteroid titer completely, presumably by direct inhibition of the PG. Possible mechanisms of ecdysteroid regulation in intact and neck-ligated IVth instar larvae are discussed.