Cellular identity and Ca2+ signaling activity of the non-reproductive GnRH system in the Ciona larva

Tunicate larvae have a non-reproductive GnRH system with multiple ligands and receptor heterodimerization enabling complex regulation. In the Ciona larva, one of the gnrh genes, gnrh2, is conspicuously expressed in the motor ganglion and nerve cord, which are homologous structures to the hindbrain and spinal cord, respectively, of vertebrates. The gnrh2 gene is also expressed in the proto-placodal sensory neurons, which are the proposed homologue of vertebrate olfactory neurons. The tunicate larvae occupy a non-reproductive dispersal stage, yet the roles of their GnRH system remain elusive. In this study, we investigated neuronal types of gnrh2-expressing cells in the Ciona larva and visualized activity of these cells by fluorescence imaging using a calcium sensor protein. Some cholinergic neurons as well as dopaminergic cells express gnrh2, suggesting that a role of GnRH in the control of swimming behavior. By contrast, none of the gnrh2-expressing cells overlap with glycinergic or GABAergic neurons. A role in the motor control is also suggested by correlation between activity of some gnrh2-expressing cells and tail movements. Interestingly, gnrh2-positive ependymal cells in the nerve cord, known as a kind of glia cells, actively produced Ca2+ transients, suggesting that neuroendocrine signaling occurs in glia cells of the nerve cord.