Periodic Activity of Gnrh-Expressing Neurons in the Ciona Brain: Insights into the Origin of Neural Crest Derived GnRH Neurons in Vertebrates

Gonadotropin-releasing hormone (GnRH) is an important neuropeptide that regulates reproductive function in vertebrates. GnRH-secreting neurons (GnRH neurons) are derived from neurogenic placodes and the neural crest during embryonic development. In the larva of the ascidian <i>Ciona intestinalis</i> type A (<i>Ciona robusta</i>), one of the <i>gnrh</i> genes, <i>gnrh2</i>, is expressed in the protoplacodederived sensory neurons as well as the central nervous system including the brain vesicle, the motor ganglion, and the nerve cord. In this study, we identified <i>gnrh2</i>-expressing cells in the motor ganglion and visualized their activity in tailbud embryos using fluorescent Ca² ⁺ indicator protein G-CaMP8. Two distinct types of cholinergic neurons, <i>vsx</i>+ /<i>pitx</i>+ interneurons (MGIN2s) and <i>islet</i>+ motor neurons (MN2s) were shown to express <i>gnrh2</i> in the motor ganglion. MN2s showed periodic Ca² ⁺ transients. By contrast, intracellular Ca² ⁺ was static in MGIN2s during development. Periodic Ca² ⁺ transients in MN2s were observed at 14 hours post-fertilization and continued during development. Previous studies demonstrated that MN2s originate from the neural plate border and migrate to the motor ganglion during neural tube formation. The unique properties of the MN2 motor neurons in <i>Ciona</i> are reminiscent of GnRH neurons with pacemaker activity in vertebrates in three aspects: first, they show spontaneous oscillatory activity, second, they migrate apart from the original position in the neural tube during development, and third, they originate from the neural plate border. These similarities suggest that the origin of the neural plate border-derived oscillatory GnRH neurons predates the divergence between vertebrates and tunicates.