The control of anterior foregut motility during a larval molt of the moth Manduca sexta involves the modulation of presynaptic activity
2006
SUMMARY In the moth, Manduca sexta , anterior foregut motility is modulated
during the larval–larval molts in order to control the timing of molting
fluid (MF) ingestion. MF is the enzymatic mixture that destroys the outer
cuticle so that it can be shed at the end of the molt. The onset of the
larval–larval molt is characterized by a dramatic decline in the
amplitude of the anterior foregut contractions so that MF is not prematurely
ingested. As the end of the molt approaches, the robust contractions of the
anterior foregut return and the MF is ingested, enabling the larva to free
itself from its old cuticle. In the present study we examine possible
mechanisms involved in modulating anterior foregut motility during a
larval–larval molt. Our results reveal that the release of a blood-borne
factor plays a role in the decline in anterior foregut peristaltic activity
during the molt. This blood-borne factor reduces the efficacy of the
presynaptic endings of the motorneurons, resulting in a reduction in the
amplitude of the excitatory junctional potential (EJP) recorded from the
anterior foregut musculature. We also present evidence that crustacean
cardioactive peptide (CCAP) targets the motorneuron terminals and its actions
are sufficient to trigger the dramatic increase in EJP amplitude and anterior
foregut contractions. Finally, the surgical ablation of the subesophageal
ganglion, which has been previously described to be a source of CCAP neurons
and the CCAP projections to the anterior foregut region, blocks both the
increase in anterior foregut motility and the ingestion of MF that normally
occur at the end of a larval–larval molt.
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