PERIPHERAL CONNECTIONS OF FMRFAMIDE-LIKE IMMUNOREACTIVE NEURONS IN THE SNAIL, HELIX-POMATIA - AN IMMUNOGOLD ELECTRON-MICROSCOPIC STUDY

Using a postembedding immunogold electron microscopic method, the ultr astructure and synaptic connections of FMRFamide-like immunoreactive v aricosities were investigated in different peripheral organs of the sn ail Helix pomatia, including the heart (auricle), intestine, hepatopan creas, upper tentacle and salivary gland. The FMRFamide-like immunorea ctive varicosities contained granules and vesicles as described in a p revious study of the CNS of this species, and additionally, based on t heir granule content, two novel types of varicosities were found in th e auricle. A selective accumulation of gold particles over the granule s could be demonstrated. The FMRFamide-like immunoreactive varicositie s formed unspecialized contacts with postsynaptic target cells in all peripheral organs investigated, with the exception of the tentacle ret ractor muscle. Both the neuro-muscular and the neuro-glandular contact s were characterized by either unspecialized close apposition of the ' pre- and postsynaptic' membranes or the immunoreactive elements faced the target cell(s) across a relatively wide extracellular space. In th e tentacle retractor muscle some of the neuromuscular contacts showed appositions of electron dense material along the presynaptic membrane, clustering of agranular synaptic vesicles and intersynaptic cleft mat erial. The present observations support previous electrophysiological findings and suggest a versatile modulatory role of FMRFamide and rela ted substances in the Helix PNS.