Peptidergic modulation of a multi-functional central pattern generator in the pulmonate snail

Egg laying in pulmonate snails is a well orchestrated process that involves a period of reduced locomotion, followed by substrate cleaning with rhythmic rasping of the surface to make tiny grooves, into which eggs are deposited. While the neurohormonal control of initiating egg-laying has been well established, the signals that modulate the buccal central pattern generator to substrate cleaning during egg-laying are not known. Neuropeptides of the invertebrate Gonadotropin-Releasing Hormone/Corazonin family (invGnRH/CRZ) have been shown to be involved in reproduction and allied behaviors in many vertebrates and invertebrates. Here we show that the buccal motor pattern underlying substrate cleaning during egg-laying is altered by a vetebrate GnRH-agonist. Signals from the intestinal nerve innervating reproductive structures, previously shown to be both necessary and sufficient for egg-laying behaviors, are blocked by a vertebrate GnRH-antagonist. Further, the vertebrate GnRH triggered response elicits rhythmic, phase 2 and non-phase 2 activity in the buccal motor pattern, with a shut down of phase 3, indicative of repetitive rasping without accompanied swallowing behavior. Using immunohistochemistry, intracellular electrophysiology and extracellular nerve stimulation, we show that a member of the invGnRH/CRZ-family of neuropeptides could be the signal that contextually switches the multifunctional buccal CPG to a biphasic rasping rhythm that underlies substrate cleaning behavior during egg-laying in pulmonate snail Planorbella (Helisoma) trivolvis.