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-immunoreactive (IR) cell-somata were located mainly at the ventro-lateral side of the subesophageal ganglion. Only four annetocin-IR cells were seen in the 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 neuropile of the ventral ganglia and the ventral nerve cord between the 4th and 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 identified by the immunogold staining, and filled with gold-labeled vesicles, 200-250 nm in diameter, which included moderately electron dense material. The annetocin-secretory cells possessed a euchromatic nucleus, well-developed rough endoplasmic reticulum and Golgi apparatus. Some of the annetocin-secretory cells were found to form a neurohemal-like structure, where somata or fibers with loose glial investment came in contact with the coelomic space at the ventral side of the subesophageal ganglion. The results suggest that annetocin is a neuropeptide produced and secreted by the neuron in the cerebral and subesophageal ganglia of the earthworm.