Inhibition of dopa decarboxylase synthesis by 20-hydroxyecdysone during the last larval moult of Manduca sexta

Abstract

During the last third of fourth/fifth larval instar moult of the tobacco hornworm Manduca sexta, the level of epidermal dopa decarboxylase (DDC) activity increased four- or seven-fold, depending on whether the epidermis was taken from intact or allatectomized larvae, respectively. This increase in activity was prevented by infusion of 1.2 μg/hr 20-hydroxyecdysone for 24 hr from the time of head capsule slippage, which maintained an ecdysteroid titre of about 6 μg/ml 20-OH-ecdysone equivalents, but not by infusion of 0.15 μg 20-OH-ecdysone/hr. In the absence of ecdysteroid, the epidermal DDC activity steadily increased in vitro for 15 hr to a maximal level approx. three-fold higher than the maximum attained in vivo. Incubation with α-fluoromethyldopa, an irreversible inhibitor of DDC, for 7 hr eliminated DDC activity which could be restored by incubation in inhibitor-free medium for 17 hr. The presence of 1 μg/ml 20-OH-ecdysone or of actinomycin D, α-amanitin or cycloheximide (ED 50 of 0.8, 1.2 and 3.8 μg/ml respectively) during this 17 hr period prevented the reappearance of DDC activity. Thus, the decline in ecdysteroid titre at the end of a larval moult is necessary to allow the onset of synthesis of DDC.