Enkephalin-like immunoreactive neurons in the central nervous system of gastropods (Helix pomatia, Lymnaea stagnalis, Aplysia californica): a comparative immunocytochemical study

Abstract

The distribution and morphology of met-enkephalin-like immunoreactive (MetLI) and leu-enkephalin-like immunoreactive (LeuLI) neurons were investigated in the central nervous system of three gastropod species, Helix pomatia, Lymnaea stagnalis and Aplysia californica. Differences between the three species were observed in (1) the characteristics of immunostaining with antibodies to met-enkephalin and leu-enkephalin antibodies; (2) the number of immunostained neurons; (3) the projection areas of imunostained elements; (4) the specificity of immunostaining. Differences in the appearance of MetLI and LeuLI neurons were apparent: in Aplysia, both MetLI and LeuLI neurons could be observed, whereas in Lymnaea LeuLI was only found in fibers; only MetLI neurons occurred in Helix. According to an absorbtion control specificity test, a part of the LeuLI seen in the neuropil of Aplysia ganglia did not represent authentic leu-enkephalin. In Helix pomatia, significantly more MetLI neurons were present than in the CNS of Lymnaea and Aplysia; the majority of these neurons were concentrated in the cerebral ganglia and were small-size (12–25 μm) interneurons. In addition to central and peripheral projections observed in the three species, the connective tissue sheath around the ganglia and peripheral nerves contained MetLI varicose axons only in Helix, where a possible neurohormonal role could be attributed to these substance. The mapping and detailed chemical-neuroanatomical description of enkephalin-immunoreactive elements may furnish a chemical-neuroanatomical background to facilitate further neurophysiologic and pharmacologic analysis of enkephalinergic mechanisms in the gastropod CNS.