Modulation of ecdysis in the moth Manduca sexta: the roles of the suboesophageal and thoracic ganglia

Abstract

The sequential behaviours shown by insects at ecdysis are due to the sequential release of various hormones, but the transition from one phase to the next can be fine-tuned by inhibitory influences. The ecdysis sequence in the moth Manduca sexta was initiated by injecting sensitive animals with the neuropeptide ecdysis-triggering hormone (ETH). Exposure to ETH stimulates the release of eclosion hormone (EH) which, in turn, activates a set of neurons containing crustacean cardioactive peptide (CCAP) by elevating their levels of intracellular cyclic GMP. We characterized a set of non-CCAP containing neurons that also appear to be EH targets because of their response to cyclic GMP at ecdysis. The neurons did not display leucokinin-, diuretic-hormone- or FMRFamide-like immunoreactivity. They are probably the bursicon-containing cells described previously. After release of EH, there is a transient inhibition of the abdominal centers responsible for ecdysis. Transection experiments suggested that this suppression is via descending inhibitory units from the suboesophageal and thoracic ganglia. The duration of this inhibition appears to depend on the levels of cyclic GMP and can be extended by pharmacologically suppressing cyclic GMP breakdown. We further found that brief exposure to CO(2) caused premature ecdysis. Since the CO(2) treatment was effective only after EH release, it probably acts by suppressing descending inhibition. Studies on adult eclosion suggest that CO(2), given at the appropriate time, can uncouple the basic larval motor program from modulatory influences provided by the adult pterothoracic ganglion. CO(2) therefore appears to be a novel and non-invasive tool for studies of ecdysis behavior in insects.