A population of pedal-buccal projection neurons associated with appetitive components of Aplysia feeding behavior

Molecular characterization and expression of a two-pore domain potassium channel in the CNS of Aplysia californica

A cDNA encoding a two-pore domain potassium (K2p) channel subunit, AcK2p2, was cloned from the CNS of the marine opisthobranch Aplysia californica. This is the second K2p subunit to be identified in molluscs. Like the K2p subunit cloned previously from Aplysia, AcK2p2 appears to be more closely related to human K2p channels than to any from Drosphila melanogaster or Caenorhabditis elegans. However, the overall identity is much lower (24% with human TALK-1) and phylogenetic analysis indicates that AcK2p2 cannot be grouped into any established mammalian subclass. We analyzed the distribution of this channel by in situ hybridization in whole mount preparations of the CNS. Less than a dozen of the approximately 20,000 neurons in the CNS expressed AcK2p2 at high levels, with the consistently intense labeling seen in a single bilaterally symmetrical pair of pedal neurons. The neuron-specific expression pattern seen for this channel is consistent with data from a variety of organisms that implicate K2p channels as determinants of neuronal phenotype and function.

Identification of a new neuropeptide precursor reveals a novel source of extrinsic modulation in the feeding system of Aplysia

The Aplysia feeding system is advantageous for investigating the role of neuropeptides in behavioral plasticity. One family of Aplysia neuropeptides is the myomodulins (MMs), originally purified from one of the feeding muscles, the accessory radula closer (ARC). However, two MMs, MMc and MMe, are not encoded on the only known MM gene. Here, we identify MM gene 2 (MMG2), which encodes MMc and MMe and four new neuropeptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to verify that these novel MMG2-derived peptides (MMG2-DPs), as well as MMc and MMe, are synthesized from the precursor. Using antibodies against the MMG2-DPs, we demonstrate that neuronal processes that stain for MMG2-DPs are found in the buccal ganglion, which contains the feeding network, and in the buccal musculature including the ARC muscle. Surprisingly, however, no immunostaining is observed in buccal neurons including the ARC motoneurons. In situ hybridization reveals only few MMG2-expressing neurons that are mostly located in the pedal ganglion. Using immunohistochemical and electrophysiological techniques, we demonstrate that some of these pedal neurons project to the buccal ganglion and are the likely source of the MMG2-DP innervation of the feeding network and musculature. We show that the MMG2-DPs are bioactive both centrally and peripherally: they bias egestive feeding programs toward ingestive ones, and they modulate ARC muscle contractions. The multiple actions of the MMG2-DPs suggest that these peptides play a broad role in behavioral plasticity and that the pedal-buccal projection neurons that express them are a novel source of extrinsic modulation of the feeding system of Aplysia.