Ultrastructural aspects of peptidergic modulation in the peripheral nervous system of Helix pomatia

The ultrastructural characteristics of peptidergic peripheral contacts in t he snail, Helix pomatia, were investigated, with special attention to the i nnervation of the heart, buccal mass, and salivary gland by Mytilus inhibit ory peptide-immunoreactive neurons. Following the application of correlativ e light- and electron-microscopic pre-embedding immunocytochemistry, the pe ripheral tissues reveal a rich innervation by Mytilus inhibitory peptide-im munoreactive elements. These neurons establish three types of neuromuscular contacts in the heart and buccal mass: (1) close (16-20 nm) unspecialized membrane contacts; (2) contacts with a relative wide (40-100 nm) intersynap tic cleft; and (3) labeled varicosties located freely in the extracellular space, far (0.5-several mu m) from the muscle cells. In the salivary gland, the immunoractive profiles contact both the muscular and glandular element s with close (type 1) and wider (type 2) membrane attachments. The great ma jority of Mytilus inhibitory peptide-immunoreactive profiles contain an ult rastructurally uniform population of large (120-150 nm) electron dense gran ules. The ultrastructural features of the innervation by Mytilus inhibitory peptide-immunoreactive elements are compared with those established by imm unogold labelled FMRFamide-containing profiles in the heart and salivary gl and. These latter display similarities in forming the different kinds of in tercellular contacts, and differences in the morphological variability of t he content of granules in the immunolabeled profiles. The results suggest d iverse, non-synaptic modulatory roles of neuropeptides in the peripheral ne rvous system of Helix pomatia, including localized membrane effects and neu rohormonal-like remote global controls, that may also be of significance in orchestrating the effects of neuropeptides released at the same time on di fferent targets. (C) 2000 Wiley-Liss, Inc.