MIP-immunoreactive innervation of the snail, Helix pomatia, heart. An ultrastructural study

The ultrastructural characteristics of the innervation established by MIP-( Mytilus inhibitory peptide) immunoreactive neurons was investigated in the heart of the snail, Helix pomatia, applying correlative light- and electron microscopic pre-embedding immunocytochemistry on Vibratome-slices. In both the auricle and ventricle, the muscle fibers receive a rich innervation by MIP-immunoreactive (IR) varicose fibers. However, the innervation is seaso nally changing in the two parts of the heart. The varicosities, containing a morphologically uniform population of large (120-150 nm) electron-dense g ranules, can be found in three different positions in relation to the muscl e fibers: (i) close (15-20 nm) but unspecialized membrane connections betwe en MIP-(IR) varicosities and muscle fibers; (ii) MIP-IR varicosities locate d relatively far (0.5-several mu m) from the muscles fibers; (iii) MIP-IR p rofiles localized freely in the extracellular space among the loosely arran ged muscle fibers. A general modulatory role of MIP in regulating the heart activity of Helix is suggested.