Gonadotropin-releasing hormone （GnRH） is a crucial peptide for the regulation of reproduction. Using immunological techniques, we investigated the presence of GnRH in horseshoe crab Tachypleus tridentatus. Octopus GnRH-like immunoreactivity, tunicate GnRH-like immunoreactivity, and lamprey GnRH-I-like immunoreactivity were detected in the neurons and fibers of the protocerebrum. However, no mammal GnRH-like immunoreactivity or lamprey GnRH-LII-like immunoreactivity was observed. Our results suggest that a GnRH-like factor, an ancient peptide, existed in the brain of T. tridentatus and may be involved in the reproductive endocrine system.
