Patterns of enkephalin immunolabeling in the pulmonate snail Cepaea nemoralis and related molluscs

In the land pulmonate snail Cepaea nemoralis, immunocytochemical localization of methionine- and leucine-enkephalin-like substances was demonstrated in specific populations of brain neurons. Methionine-enkephalin reactivity is also present in the peripheral nervous system as (i) abundant axonal projections of central neurons to the sole of the foot, and (ii) sparse local neurons of the head and mantle wall. Similar peripheral methionine-enkephalinergic elements were found immunocytochemically in all pulmonates (Helix aspersa, Lymnaea stagnalis) and prosobranchs (Littorina littorea, Acmaea testudinalis) examined and seem therefore conserved among gastropods. Their distribution was different from those of nervous elements labeled by antisera against serotonin and FMRFamide. The chiton Lepidopleurus asellus considered to be a more primitive mollusc, had both types of enkephalin-immunoreactive elements in the central nervous system whereas peripheral methionine-enkephalin-immunoreactive elements were lacking. It is suggested on the morphological evidence that enkephalinergic (especially methionine-enkephalinergic) neurons are part of a sensory system mediating analgetic effects in molluscs.