Submicroscopic characterization of proctolin-like immunoreactivity in the nervous system of the cockroach Periplaneta americana L.

Summary Proctolin-like immunoreactivity (PLI) was found in different parts of the arthropod central nervous system and in nerve fibers of muscles. In order to examine whether this PLI is related to a uniform type of secretory vesicle, hindgut musculature and frontal and hypocerebral ganglia were examined with the immunogold technique. PLI occurs exclusively within membran-bounded secretory granules. Neither granular ER nor Golgi stacks show PLI. In some cases close relationships between PLI-bearing granules and lysosomes were observed. In presynaptic areas, PLI-reactive granules are associated with numerous clear synaptic vesicles and restricted to an area distinctly separate from the presynaptic membrane. Three types of granules were found, differing in diameter and electron density: (1) dense, 80 nm; (2) dense, 150 nm; (3) low density, 150 nm. The results demonstrate that: (1) the PLI of the produced peptide occurs shortly after its separation from the Golgi stack; (2) the occurrence of PLI in three different granule types could be the morphological expression of the common occurrence of proctolin with other neuroactive substances. However, a possible cross-reactivity with other, hitherto unknown substances must be considered as well.