Serotoninlike immunoreactivity in the central and peripheral nervous system of the scale worm Harmothoe imbricata (polychaeta)

The distribution of serotoninergic neurons in the nervous system of the scale worm Harmothoe imbricata was visualized in the anterior half of the body by the peroxidase‐antiperoxidase (PAP) immunohistochemical method with a specific antiserotonin antibody. Immunoreactive neuronal somata were localized in discrete ganglion cell masses of the dorsally situated cerebral ganglion and in segmental ganglia of the ventral nerve cord. They also make up the majority of neurons present in the parapodial ganglia. Large and small varicose fibers stained in the neuropile of all the above‐mentioned ganglia but also in interganglionic connectives and segmental nerves. On the basis of soma size and location and of fiber distribution, the reactive neurons were identified as primarily interneuronal with a few motoneurons and presumptive afferent neurons. The presence of a motor component was substantiated by observations of several reactive varicose fibers spread over longitudinal muscle layers of the trunk. In addition, neurites of the subepidermal nerve plexus and enterochromaffinlike cells of the gut epithelium reacted with the serotonin antibody. It is concluded that serotoninergic pathways are ubiquitous elements in the organization of the central and peripheral nervous system of this polychaete. The significance of these findings in relation to other annelid groups and to the physiological role of serotonin is discussed.