Serotonin immunoreactivity in the peripheral nervous system of oligochaeta.

The distribution of serotonin immunoreactive elements in the peripheral nervous system of three earthworm species (Lumbricus terrestris, Eisenia fetida and Tubifex tubifex) was studied using the peroxidase-antiperoxidase method. Most parts of the peripheral nervous system contain immunoreactive fibers. All segmental nerves as well as branches of the prostomial nerves show strong immunoreactivity. Muscles of the body wall contain serotonergic fibers, mostly between the circular and longitudinal layers. The best supplied area is the buccal musculature. All parts of the enteric nervous system contain serotonergic fibers, strongest staining was observed in the buccal and pharyngeal walls. Along the wall of the calciferous glands thin immunoreactive fibers were found. The excretory system, the metanephridial tubules also display immunoreactivity. Under the surface epithelium, a serotonergic subepidermal plexus was observed. No significant differences were observed between the species studied. On the basis of our morphological findings and functional studies performed by other authors it is suggested that serotonin plays an essential role in the function of the peripheral nervous system in Oligochaeta.