Modulation of foregut synaptic activity controls resorption of molting fluid during larval molts of the moth Manduca sexta.

We examined the role of the foregut in the resorption of molting fluid (MF) from the exuvial space during the last larval-larval molt of the moth Manduca sexta. In intermolt larvae, the activity of the foregut is characterized by robust peristaltic contractions. With the onset of the molt, MF is secreted into the exuvial space where it digests and weakens the old cuticle. The appearance of MF in the exuvial space is accompanied by a dramatic reduction in the amplitude of the foregut contractions. Foregut peristalsis returned about halfway through the molt, followed shortly by the appearance of MF in the gut. These observations suggested that larvae use their foreguts to remove MF from the exuvial space. Animals whose foreguts were surgically inactivated did not resorb their MF and most failed to successfully shed their old cuticles. The reduction in foregut motility at the onset of the molt was correlated with a sharp decline in the amplitude of the excitatory junctional potentials. With the onset of the molt there was also a decline in the number of presynaptic terminals on the foregut that loaded with the activity-dependent dye FM1-43. In the second half of the molt, the appearance of MF in the foregut and the return of foregut motility was correlated with an increase in FM1-43 loading. These data reveal that during a larval-larval molt, vesicle release and/or recycling of the presynaptic endings on the foregut muscles is modulated to assure the proper timing of MF resorption.