Stimulatory effect of ghrelin on food intake in bullfrog larvae

Ghrelin is a potent orexigenic peptide implicated in appetite regulation in rodents. However, except for teleost fish, the involvement of ghrelin in the regulation of feeding in non-mammalian vertebrates has not been well studied. Anuran amphibian larvae feed and grow during the pre- and prometamorphic stages, but, thereafter they stop feeding as the metamorphic climax approaches. Therefore, orexigenic factors seem to play important roles in growing larvae. In the present study, we examined the effect of intraperitoneal (IP) or intracerebroventricular (ICV) administration of synthetic bullfrog ghrelin (n-octanoylated 28-amino acid form) on food intake in larvae at the prometamorphic stages. Cumulative food intake was significantly increased by IP (8 and 16 pmol/g body weight (BW)) or ICV (0.5 and 1 pmol/g BW) administration of ghrelin during a 15-min observation period. The orexigenic action of ghrelin at 8 pmol/g BW (IP) or at 0.5 pmol/g BW (ICV) was blocked by treatment with a growth hormone secretagogue-receptor antagonist, [D-Lys3]GHRP-6 at 80 pmol/g BW (IP) or at 5 pmol/g BW (ICV). We then investigated the effect of feeding status on expression levels of the ghrelin transcript in the hypothalamus and gastrointestinal tract. Ghrelin mRNA levels in both were decreased 15 and 60 min after feeding. These results indicate that ghrelin acts as an orexigenic factor in bullfrog larvae.