Gonadotropin-releasing hormone-like molecule is not an acute reproductive activator in the gastropod, Aplysia californica

Gonadotropin-releasing hormone (GnRH) is indispensable for reproductive activation in all vertebrates. Although several GnRH-like molecules have been isolated from non-chordates, the function of GnRH in these taxa remains unclear. We previously isolated the full-length cDNA sequence of a prohormone containing a GnRH-like molecule, termed ap-GnRH, from the gastropod mollusk, Aplysia californica. In this study, we characterized the distribution and quantity of ap-GnRH peptide in several central and peripheral tissues of A. californica. Further, we performed in vivo and in vitro studies to explore the function of ap-GnRH in these animals. Immunohistochemistry and radioimmunoassay using specific antisera against ap-GnRH showed that pedal ganglia contained the highest level of ap-GnRH peptide, followed by cerebral ganglia, abdominal ganglia, and then buccal ganglia. Ovotestis did not contain detectable levels of ap-GnRH peptide. Injection of sexually mature and immature animals with synthetic ap-GnRH over a course of 10–14 days had no effects on ovotestis mass, reproductive tract mass, egg-laying, and penile eversion. ap-GnRH also failed to alter oocyte growth and egg-laying hormone accumulation and secretion. Interestingly, ap-GnRH injection triggered acute behavioral responses including the stimulation of parapodial opening, inhibition of feeding, and promotion of substrate attachment. Our results showed that in A. californica, ap-GnRH could modulate a wide range of behavioral attributes. Most strikingly, ap-GnRH is not involved in the acute activation of reproduction in a fashion similar to vertebrate GnRH.