No abstract

Background In recent two decades, studies of the annelid nervous systems were revolutionized by modern cell labeling techniques and state-of-the-art microscopy techniques. However, there are still huge gaps in our knowledge on the organization and functioning of their nervous system. Most of the recent studies have focused on the distribution of serotonin and FMRFamide, while the data about many other basic neurotransmitters such as histamine (HA) and gamma-aminobutyric acid (GABA) are scarce. Results Using immunohistochemistry and confocal microscopy we studied the distribution of histamine and gamma-aminobutyric acid in the nervous system of a spionid annelid Pygospio elegans and traced their redevelopment during reparative regeneration. Both neurotransmitters show specific patterns in central and peripheral nervous systems. HA-positive cells are concentrated mostly in the brain, while GABA-positive cell somata contribute equally to brain and segmental ganglia. Some immunoreactive elements were found in peripheral nerves. Both substances were revealed in high numbers in bipolar sensory cells in the palps. The first signs of regenerating HAergic and GABAergic systems were detected only by 3 days after the amputation. Further redevelopment of GABAergic system proceeds faster than that of HAergic one. Conclusions Comparisons with other annelids and mollusks examined in this respect revealed a number of general similarities in distribution patterns of HAergic and GABAergic cells in different species. Overall, the differences in the full redevelopment of various neurotransmitters correlate with neuronal development during embryogenesis. Our results highlight the importance of investigating the distribution of different neurotransmitters in comparative morphological and developmental studies.

Background The nervous system of siboglinids has been studied mainly in Osedax and some Vestimentifera, while data in Frenulata – one of the four pogonophoran main branches – is still fragmentary. In most of the studies, the focus is almost always on the central nervous system, while the peripheral nervous system has traditionally received little attention. In contrast to other annelids, the structure and diversity of sensory structures in siboglinids are still quite undescribed. Meanwhile, the peripheral nervous system, as well as sensory elements, are extremely evolutionarily labile, and information about their organization is of high importance to understand lifestyles and behavior as well as main trends that lead siboglinids to their peculiar organization. Results The structure of the peripheric nervous system, sensory elements, and neuromuscular relationships of Oligobrachia haakonmosbiensis were studied using both scanning electron and confocal laser microscopy. A significant number of monociliary sensory cells, as well as sensory complexes located diffusely in the epithelium of the whole body were revealed. The latter include the cephalic tentacles, sensory cells accumulations along the dorsal furrow and ciliary band, areas of the openings of the tubiparous glands, and papillae. The oval ciliary spot located on the cephalic lobe at the base of the tentacles can also be regarded as a sensory organ. Most of the detected sensory cells show immunoreactivity to substance P and/or acetylated α-tubulin. FMRFamide- and serotonin-like immunoreactivity are manifested by neurons that mainly innervate tentacles, muscles, body wall epithelium, skin glands, tubiparous glands, and papillae. In the larva of O. haakonmosbiensis, monociliary sensory elements were revealed in the region of the apical organ, along the body, and on the pygidium. Conclusions The diversity of sensory structures in O. haakonmosbiensis comprises epidermal solitary sensory cells, sensory spots around tubiparous glands openings, and putative sensory organs such as cephalic tentacles, an oval ciliary spot on the cephalic lobe, the dorsal furrow, and papillae. Sensory structures associated with papillae and tubiparous glands play presumable mechanosensory functions and are associated with regulation of tube building as well as anchorage of the worm inside the tube. Sensory structures of the dorsal furrow are presumably engaged in the regulation of reproductive behavior. An overall low level of morphological differentiation of O. haakonmosbiensis peripheral nervous system is not typical even for annelids with the intraepithelial nervous system. This can be considered as a plesiomorphic feature of its peripheral plexus’s organization, or as evidence for the neotenic origin of Siboglinidae.

Catecholamines (CA) are known to play an important role in the physiology of most invertebrates. Here we provide the results on catecholaminergic systems study of two annelid species: Pygospio elegans (Spionidae) and Platynereis dumerilii (Nereididae). The nervous system was studied with the histochemical method of monoamine condensation with glyoxylic acid in combination with confocal laser scanning microscopy. Both animals possess developed catecholaminergic systems with similar general organization. However, in P. dumerilii considerably fewer cells were detected. In both species, CApositive cells were found in the brain, ventral nerve cord as well as in the stomatogastric system, body wall, palps, prostomium, and regions around the chaetae. According to the cell morphology and their location in the most agile parts of the animal, we suggest CA-positive cells to be mostly proprio-or mechanoreceptors.

Background In recent two decades, studies of the annelid nervous systems were revolutionized by modern cell labeling techniques and state-of-the-art microscopy techniques. However, there are still huge white fields in our knowledge on nervous system organization and functioning. Though most recent studies were focused on the distribution of now routine serotonin and FMRFamide, data about many basic neurotransmitters such as histamine (HA) and gamma-aminobutyric acid (GABA) are scarce. Results Using immunohistochemistry and confocal microscopy we studied distribution of histamine and gamma-aminobutyric acid in nervous system of a spionid annelid Pygospio elegans and followed their redevelopment during reparative regeneration. Both neurotransmitters show specific patterns in central and peripheral nervous systems. HA-positive cells are concentrated mostly in the brain, while GABA-positive cell somata contribute equally to brain and segmental ganglia. Some immunoreactive elements were found in peripheral nerves. Both substances revealed high number of bipolar sensory cells in the palps. The first signs of regenerating HA- and GABA-ergic systems were detected only by 3 days after the amputation. Further redevelopment of GABA-ergic system proceeds faster than HA-ergic. Conclusions Comparisons with other studied annelids and mollusks reveal both similar and unique features in HA- and GABA-ergic systems organizations. Overall, difference between the full redevelopment of various neurotransmitters correlates with neuronal development during embryogenesis. All these results state the high importance of investigating different neurotransmitter distribution in comparative morphological and developmental studies.