TRANSMITTER-SPECIFIC SUBSETS OF SENSORY ELEMENTS IN THE PROSOBRANCH OSPHRADIUM

The osphradium is a putative chemosensory organ of aquatic molluscs. P reviously, we identified two distinct types of primary sensory neurons in the osphradial ganglion of freshwater pulmonates, one immunoreacti ve to leucine-enkephalin (LEnk-ir) and another to FMRFamide (FMRFa-ir) . In addition, NADPH diaphorase (NADPHd)-positive elements apparently producing nitric oxide (NO) were demonstrated in the organ. In the pre sent study, prosobranch molluscs, which have retained the osphradial s ensory neurons within the epithelium, were studied. Both types of pept idergic neurons, as well as NADPHd-positive cells, were found within t he epithelium or in a basiepithelial position in the relatively simple osphradium of the mesogastropod Littorina littorea and in the complex , bipectinate osphradium of the neogastropod Buccinum undatum. Similar evidence was also obtained for another mesogastropod, Ampullarius sp. Transmitter-specific sensory cell types like those discovered in the osphradium are also present as single neuroepithelial cells in other o rgans of the mantle complex in prosobranchs and in the pelecypod Anodo nta cygnea. We suggest that evolutionarily conservative, transmitter-s pecific types of epithelial and neuroepithelial sensory cells predated the osphradium, which developed as the site of their concentration, w hile retaining characteristic subsets of sensory neurons.