Gonadotropin-Releasing Hormones and Their Receptors in the Marine Invertebrates: Aplysia californica and Branchiostoma floridae

Gonadotropin-releasing hormone (GnRH) is a neuroendocrine activator of the reproductive axis in vertebrates. GnRH was once thought to be a hormone that exists exclusively in vertebrates. However, there is now ample evidence that support the presence of GnRH-like molecules outside of vertebrate classes. Although the vertebrate GnRH systems have been well characterized, little is known about the anatomy and function of the invertebrate GnRH systems. The aim of this thesis was to examine the anatomical distribution and perform preliminary functional studies on GnRH and its receptors in two invertebrates, a gastropod mollusk, Aplysia californica, and a cephalochordate, Branchiostoma floridae. Chapter 1 examines the mRNA and peptide distribution of Aplysia GnRH (ap-GnRH) in the central and peripheral tissues of A. californica. Results revealed that ap-GnRH is expressed in three discrete central ganglia (abdominal, cerebral, and pedal) of A. californica, suggesting ap-GnRH may perform a wide array of functions unrelated to reproduction. Chapter 2 examines the tissue distribution and reproductive consequence of activating endogenous GnRH receptors (GnRH-R) in B. floridae. RT-PCR revealed that 3 amphioxus GnRH-Rs were expressed in several body segments, indicating that these receptors may be involved in regulation of diverse functions. In vivo functional study revealed that ap-GnRH had a stimulatory effect on gametogenesis, suggesting a conserved functional link to reproductive activation. Overall, work from this thesis illustrates the complex evolutionary trajectory of the GnRH system and suggests that the functional connection between GnRH and reproductive activation becomes more uncertain in organisms outside Phylum Chordata