Neurons in a variety of molluscs react to antibodies raised against the VD₁/RPD₂ α-neuropeptide of the pond snail Lymnaea stagnalis

The VD₁ and RPD₂ neurons of Lymnaea stagnalis innervate other central neurons, certain skin areas, the pneumostome area, and the auricle of the heart. Recently, a set of four (λ, ω, α, β) neuropeptides produced by these giant neurons and by certain other central neurons has been characterized. Although alternative splicing of the preprohormone of these neurons yields at least 10 different α neuropeptides, an affinity-purified antiserum directed against a domain common to all α neuropeptides has previously been shown to be highly selective in staining VD₁, RPD₂ and other neurons that produce the preprohormone. Since the gene encoding the neuropeptides is structurally similar to that expressed in R15 of the marine opisthobranch Aplysia californica, we have used the affinity purified antiserum as a marker for VD₁/RPD₂-related systems in other molluscs. Immunopositive neurons and fibers are observed in the central nervous systems of all species studied (Achatina fulica, Anodonta sp., Aplysia brasiliana, A. californica, Bulinus truncatus, Cepea sp., Eobania vermiculata, Helix aspersa, H. pomatia, Limax maximus, Mytilus edulis, Nassarius reticulatus, Viviparus viviparus). Several medium-sized and small neurons and 1-4 giant neurons are found in the pulmonates and opisthobranchs. The giant neurons in pulmonates have locations in the subesophageal ganglion, axonal branching patterns, and terminal arborizations in the auricle of the heart; all these characteristics are similar to those of VD₁ and RPD₂. Double-labelling (Lucifer yellow injection, immunocytochemistry) confirms that the two giant neurons in Helix pomatia are Br and Br. The immunoreactive cells in A. fulica appear to include the VIN and PON neurons. The antiserum also stains cells that appear to be the R15 neurons in two Aplysia species. The small and medium-sized neurons are distributed widely over the central ganglia of opisthobranchs and pulmonates. In prosobranchs and bivalves, small neurons are found in the cerebral and abdominal ganglia. No evidence has been found for innervation of the heart in these latter groups but in M. edulis, immunoreactive terminals can be observed in the gill. The results suggest the evolutionary conservation of immunoreactive peptides and the neurons that produce them, and thus support and extend previous hypotheses regarding the homology of certain giant neurons across molluscan species.