Immunocytochemical and ultrastructural differentiation between Met-enkephalin-, Leu-enkephalin-, and Met/Leu-enkephalin-immunoreactive neurons of feline gut.

Selective immunocytochemical procedures, applied at the light and electron microscopical level, have resulted in the identification of three types of enkephalin-immunoreactive nerves in the feline gastrointestinal tract, including neurons containing either Met- or Leu-enkephalin immunoreactivity or those simultaneously storing both Met- and Leu-enkephalin immunoreactants. The three different types of enkephalin nerves show a similar distribution in the gut with the exception of the small arteries which frequently appear to be innervated only by Met-enkephalin-immunoreactive terminals. Electron microscopical identification using optimally fixed, osmicated, and contrasted tissue revealed ultrastructural differences between the three types of nerve terminals. Thus, the terminals contained variable proportions of large granular and large opaque p-type vesicles. Large granular vesicles were associated predominantly with Met-enkephalin immunoreactions. It is presently impossible to decide whether the different p-type vesicles reflect a continuous processing of a precursor contained within them or whether they are truly different in peptide content. Thus, granular vesicles had cores of either high or medium electron density. Medium electron-dense granular vesicles were sometimes difficult to distinguish from large opaque vesicles suggesting the existence of possible transitional forms. All types of enkephalin terminals also contained small clear ("cholinergic-like") vesicles which were not immunoreactive to enkephalins. These data indicate that multiple enkephalin precursors/enkephalin-containing peptides occur in the gut or, alternatively or additionally, that a common Met/Leu-enkephalin precursor is processed differently in different neurons.