Distribution and characterization of pedal peptide immunoreactivity in Aplysia

Pedal peptide (Pep) is a very abundant neuropeptide in Aplysia. A radioimmunoassay (RIA) was developed to quantify Pep‐like immunoreactivity (IR‐Pep) in tissue extracts. IR‐Pep was present in very high concentrations in the central nervous system (CNS) and two peripheral tissues: the large hermaphroditic duct (LHD) and the foot. RIA of fractions from high‐pressure liquid chromatography (HPLC) indicated that Pep itself was the predominant immunoreactive species in each of these tissues. Lower concentrations of Pep were found in a number of other peripheral tissues. Incorporation of labelled amino acid indicated that Pep was synthesized in the LHD, whereas Pep in the foot was synthesized primarily in central neurons and transported to the foot. IR‐Pep was further localized by immunocytology. All peripheral IR‐Pep appeared to be associated with neuronal fibers, most commonly varicose axons. Immunoreactive innervation of the LHD and foot was particularly dense but positive staining was also observed in other tissues including tegument, gill, gut, and heart. IR‐Pep innervation in all tissues including the LHD appeared to be localized predominantly in muscular portions of the tissue. Spontaneous contractions of isolated LHD were accelerated by the application of Pep. Pep appears to act as a transmitter or neuromodulator at a number of different sites in Aplysia.