IMMUNOHISTOCHEMICAL LOCALIZATION OF ANNETOCIN, AN EARTHWORM OXYTOCIN-RELATED PEPTIDE, AND IDENTIFICATION AND ULTRASTRUCTURAL CHARACTERISTICS OF THE ANNETOCIN-SECRETORY CELLS IN THE OLIGOCHAETE EARTHWORM EISENIA-FOETIDA

Annetocin is an egg-laying-inducing oxytocin-related peptide which we have previously isolated from the earthworm, Eisenia foetida. Here we report the results of immunohistochemical and ultrastructural studies on annetocin-secretory cells in the earthworm. Annetocin-immunoreactiv e (IR) cell-somata were located mainly at the ventro-lateral side of t he subesophageal ganglion, Only four annetocin-IR cells were seen in t he cerebral ganglion. Some annetocin-IR cells displayed unipolar-like structure with a process directing to the core region (the neuropile) of the ganglion. Annetocin-IR fibers were also observed in the neuropi le of the ventral ganglia and the ventral nerve cord between the 4th a nd the 30th segments including the clitellum, but not in the posterior segments (31-55th). The number of annetocin-IR fibers decreased from the 4th to the 30th segment. The annetocin-secretory cells were identi fied by the immunogold staining, and filled with gold-labeled vesicles , 200-250 nm in diameter, which included moderately electron dense mat erial. The annetocin-secretory cells possessed a euchromatic nucleus, well-developed rough endoplasmic reticulum and Golgi apparatus. Some o f the annetocin-secretory cells were found to form a neurohemal-like s tructure, where somata or fibers with loose glial investment came in c ontact with the coelomic space at the ventral side of the subesophagea l ganglion. The results suggest that annetocin is a neuropeptide produ ced and secreted by the neuron in the cerebral and subesophageal gangl ia of the earthworm.