TSAI, P.-S.*; MALDONADO, T.A.; PAK, T.R.; ZHANG, L.: Characterization of a Gonadotropin-Releasing Hormone System in an Opisthobranch Mollusk, Aplysia californica

Gonadotropin-releasing hormone (GnRH) is a neurohormone crucial for the regulation of reproductive and neural functions in vertebrates. Recent discoveries of GnRH immunoreactivity (IR) in a number of invertebrates raised the possibility that GnRH may be an ancient molecule that had arisen before the emergence of Phylum Chordata. We previously demonstrated the presence of a GnRH-IR similar to the mammalian (m) and tunicate I (tI) forms of GnRH in the hemolymph and ovotestis of an opisthobranch mollusk, Aplysia californica; however, the presence of GnRH in Aplysia central nervous system (CNS) could not be detected with the available antisera against various forms of chordate GnRH. In the present study, we generated an antiserum against a synthetic tI-GnRH analogue (97575). This antiserum cross-reacted strongly with native tI-GnRH and also detected GnRH-IR in extracts of ovotestis and hemolymph. Immunocytochemical analysis revealed that this antiserum detected strong GnRH-IR in defined neurons of Aplysia CNS, with the greatest number of GnRH-IR neurons present in pedal and pleural ganglia. In summary, we have identified a GnRH-IR very similar to tI-GnRH in the CNS, hemolymph and ovotestis of Aplysia californica. In view of our previous finding that vertebrate GnRH can alter the electrical activity of Aplysia neurons, the possibility exists that this GnRH-IR may be a factor released from the CNS and ovotestis into circulation to alter neural functions. (Supported by NSF Grant IBN-9996398 to PST).