GHRP-6 mimics ghrelin-induced stimulation of food intake and suppression of locomotor activity in goldfish

Ghrelin was first identified and characterized from rat stomach as an endogenous ligand for the growth hormone secretagogue (GHS) receptor (GHS-R). Ghrelin also acts as an orexigenic factor and regulates energy balance in rodents. In goldfish, native ghrelin consists of 11 molecular variants, the major form being a 17-residue peptide with n-octanoic acid modification (n-octanoyl ghrelin17), and intraperitoneal (IP) administration of n-octanoyl ghrelin17 induces central actions such as stimulation of food intake and suppression of locomotor activity through capsaicin-sensitive afferents. Four types of GHS-Rs (1a-1, 1a-2, 2a-1 and 2a-2) have been identified in goldfish, and one GHS, GHRP-6, can activate only GHS-R2a-1 in vitro. However, there is no information about the effect of GHRP-6 on food intake and locomotor activity in goldfish in vivo. Therefore, in the present study, we examined whether IP-administered GHRP-6 would mimic the orexigenic action of n-octanoyl ghrelin17 and its suppression of locomotor activity. IP administration of GHRP-6 at 1pmol/g body weight (BW) stimulated food intake, and was equipotent to the orexigenic action of n-octanoyl ghrelin17 at 10 pmol/g BW. IP-injected GHRP-6 at 1 pmol/g BW also induced a significant decrease of locomotor activity, as was the case for IP-injected n-octanoyl ghrelin17 at 10 pmol/g BW. The action of GHRP-6 was blocked by IP-preinjected capsaicin at 160 nmol/g BW. These results suggest that the central action of GHRP-6 might be mediated via the GHS-R2a-1-signaling pathway, and subsequently through capsaicin-sensitive afferents in goldfish.