Localization and Functional Characterization of a Novel Adipokinetic Hormone in the Mollusk, <i>Aplysia californica</i>

<div><p>Increasing evidence suggests that gonadotropin-releasing hormone (GnRH), corazonin, adipokinetic hormone (AKH), and red pigment-concentrating hormone all share common ancestry to form a GnRH superfamily. Despite the wide presence of these peptides in protostomes, their biological effects remain poorly characterized in many taxa. This study had three goals. First, we cloned the full-length sequence of a novel AKH, termed <i>Aplysia</i>-AKH, and examined its distribution in an opisthobranch mollusk, <i>Aplysia californica</i>. Second, we investigated <i>in vivo</i> biological effects of <i>Aplysia</i>-AKH. Lastly, we compared the effects of <i>Aplysia</i>-AKH to a related <i>A. californica</i> peptide, <i>Aplysia</i>-GnRH. Results suggest that <i>Aplysia</i>-AKH mRNA and peptide are localized exclusively in central tissues, with abdominal, cerebral, and pleural ganglia being the primary sites of <i>Aplysia</i>-AKH production. However, <i>Aplysia</i>-AKH-positive fibers were found in all central ganglia, suggesting diverse neuromodulatory roles. Injections of <i>A. californica</i> with <i>Aplysia</i>-AKH significantly inhibited feeding, reduced body mass, increased excretion of feces, and reduced gonadal mass and oocyte diameter. The <i>in vivo</i> effects of <i>Aplysia</i>-AKH differed substantially from <i>Aplysia</i>-GnRH. Overall, the distribution and biological effects of <i>Aplysia</i>-AKH suggest it has diverged functionally from <i>Aplysia</i>-GnRH over the course of evolution. Further, that both <i>Aplysia</i>-AKH and <i>Aplysia</i>-GnRH failed to activate reproduction suggest the critical role of GnRH as a reproductive activator may be a phenomenon unique to vertebrates.</p>