Serotonin system in Tunicates: insight from morphological and molecular approaches

Serotonin (5 hydroxytryptamine, 5-HT) is a biogenic amine of ancient origin that
is widespread among animals. It plays multiple roles during development and in
adults as neurotransmitter at synaptic level and neuro hormone controlling
complex behaviors in both vertebrates and invertebrates. Tunicates occupy a
key phylogenetic position to understand the evolution of serotonin functions
since they are the sister group of vertebrates. The presence of serotonin in
tunicates was first reported in adults of the ascidian Ciona robusta (formerly
Ciona intestinalis) in the 1946. Since then, serotonin systems have been in many
tunicate species and its functions during embryogenesis and metamorphosis
explored. We reviewed the current knowledge about serotonin in these animals
first by comparing its presence and localization in larvae and adults of different
species. Then, we focused on the model organism Ciona for which data
regarding sequences and expression patterns of genes involved in serotonin
synthesis and function have been reported. Overall, we provided a
comprehensive overview of serotonergic machinery in tunicates and gave
hints for future studies in this field.