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Abstract
Cuticular sclerotization in insects requires dopamine derivatives and thus the presence
of dopa decarboxylase (DDC), the enzyme which converts dopa to dopamine. During the
last half of the larval molt of the tobacco hornworm, Manduca sexta, beginning at
16 hr after head capsule slippage, the epidermal DDC activity increased fourfold.
By contrast, allatectomized larvae which were destined to produce a melanized cuticle
showed a sevenfold increase. This increase in DDC activity was prevented by infusion
of 20-hydroxyecdysone (20HE) into the larva, indicating that the fall of the ecdysteroid
titer is necessary for the increase. In vitro 20HE also prevented the increase in
a dose-dependent manner when the epidermis was explanted at 16 hr after head capsule
slippage but had less effect on epidermis explanted 3 hr later. Both 5 micrograms/ml
alpha-amanitin and 100 micrograms/ml cycloheximide also prevented the increase. Application
of juvenile hormone I showed that the critical period for determination of the level
of the later increase in DDC activity was about 4 hr after head capsule slippage at
the peak of the ecdysteroid titer. Apparently then the rise and fall of ecdysteroid
regulate different aspects of DDC synthesis, the rise determining its later appearance
and the fall timing this appearance.