Histamine and Gamma-aminobutyric acid in the nervous system of Pygospio elegans (Annelida: Spionidae). Structure and recovery during reparative regeneration

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.