Neurotransmitters in the gastropod CNS: comparative immunocytochemistry.

The number, distribution, morphology, and projection areas of immunoreactive neurons labelled with different antisera were analyzed in the gastropod species, Aplysia californica, Lymnaea stagnalis and Helix pomatia, representing different levels of evolutionary development of the central nervous system. Our results show that the number of small-size peptidergic interneurons increases considerably in the cerebral ganglia of Helix, when compared to serotonin-immunoreactive neurons. This phenomenon might be connected to the change from aquatic to terrestrial life, involving also a change in the composition and quality of sensory input, reaching the animal from the surrounding.