1. Endocrinology and neuroendocrinology of protochordates: Evolutionary views and potentials as new model organisms

The critical phylogenetic position of ascidians suggests that evolutionary origins of neuropeptides and hormones of vertebrates are highly likely to be conserved in ascidians, and the cosmopolitan species, Ciona intestinalis, is expected to be an excellent deuterostome invertebrate model for studies on neuropeptides and hormones of vertebrates. Nevertheless, molecular and functional characterization of Ciona neuropeptides and hormone peptides was restricted to a few peptides. In the past few years, mass spectrometry-based peptidomic analyses and database-searching detected Ciona orthologs or prototypes of vertebrate peptides and their receptors, including tachykinin, insulin/relaxin, calcitonin, galanin, and neurotensin. Furthermore, several Ciona peptides, including vasopressin and a novel gonadotropin-releasing hormone (GnRH)-related peptide, have been shown to acquire ascidian-specific molecular forms and/or biological functions. These findings provided indisputable Correspondence/Reprint request: Dr. Honoo Satake, Suntory Foundation for Life Sciences, Bioorganic Research Institute, 1-1-1 Wakayamadai, Shimamoto, Mishima, Osaka 618-8503, Japan E-mail: [email protected] Honoo Satake et al. 2 evidence that ascidians, unlike other invertebrates including a nematode and fruifly, possess neuropeptides and peptide hormones structurally and functionally related to those of vertebrates, and that several ascidian peptides were diverged in ascidianunique evolutionary lineages. Moreover, recent functional analyses revealed a novel tachykininand neurotensin-regulated oocyte growth pathway in the ovary. Heterodimerization among GnRH receptor paralogs was also suggested to be involved in the direct regulation of oocyte growth by GnRHs in C. intestinalis. This article provides the basic and recent findings concerning molecular species and biological functions of ascidian neuropeptides and peptide hormones.