roles of central and peripheral eclosion hormone release in the control of ecdysis behavior in Manduca sexta

Ecdysis, a behavior by which insects shed the old cuticle at the culmination of each molt, is triggered by a unique peptide hormone, eclosion hormone (EH). In pupal Manduca sexta, EH is released into the hemolymph just prior to ecdysis, and circulating hormone is sufficient to elicit this behavior. Removal of the proctodeal nerves in prepupal animals eliminated the appearance of blood-borne EH, but ecdysis behavior occurred on schedule. Therefore, circulating EH is not necessary for the triggering of ecdysis. In contrast, a set of dermal glands failed to show their expected bout of secretion after proctodeal nerve removal. Injection of exogenous EH rescued this secretion. Thus, circulating EH appears necessary for action on peripheral but not central targets. A major reduction in EH immunostaining is seen in the proctodeal nerves just preceding ecdysis; this coincides with a > 90% reduction in extractable EH from this structure and the appearance of circulating EH. A similar, concomitant reduction was seen in central EH cell processes, suggesting release of peptide within the CNS. Antidromic stimulation of the proctodeal nerve stumps following proctodeal nerve removal triggered precocious ecdysis. This result further supports the conclusion that centrally released EH is sufficient to trigger the motor program.