Localization and Functional Characterization of a Novel Adipokinetic Hormone in the Mollusk, Aplysia californica

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 Aplysia -AKH, and examined its distribution in an opisthobranch mollusk, Aplysia californica . Second, we investigated in vivo biological effects of Aplysia -AKH. Lastly, we compared the effects of Aplysia -AKH to a related A. californica peptide, Aplysia -GnRH. Results suggest that Aplysia -AKH mRNA and peptide are localized exclusively in central tissues, with abdominal, cerebral, and pleural ganglia being the primary sites of Aplysia -AKH production. However, Aplysia -AKH-positive fibers were found in all central ganglia, suggesting diverse neuromodulatory roles. Injections of A. californica with Aplysia -AKH significantly inhibited feeding, reduced body mass, increased excretion of feces, and reduced gonadal mass and oocyte diameter. The in vivo effects of Aplysia -AKH differed substantially from Aplysia -GnRH. Overall, the distribution and biological effects of Aplysia -AKH suggest it has diverged functionally from Aplysia -GnRH over the course of evolution. Further, that both Aplysia -AKH and Aplysia -GnRH failed to activate reproduction suggest the critical role of GnRH as a reproductive activator may be a phenomenon unique to vertebrates.