GnRH-immunoreactive neuronal system in the presumptive ancestral chordate, Ciona intestinalis (Ascidian)

Gonadotropin-releasing hormone (GnRH) of the vertebrate brain, which has originally been identified as a peptidergic hypophysiotropic hormone, is now believed to act also as a neuromodulator. It has recently been shown that a vertebrate-like GnRH-gonadotropin system occurs in the urochordates, which are considered to be the presumptive ancestral chordate. In this paper, we examined the morphology of the GnRH neuronal system of ascidian, Ciona intestinalis, by immunocytochemistry and found many GnRH-immunoreactive neuronal cells and fibers in a specific surface area of the cerebral ganglion, along the inner wall of the dorsal blood sinus, as well as on the anterior surface of the ovary.