Scientists found four different ghrelin receptors in goldfish, split into two main types. One of these receptors can be blocked by a modified version of the peptide GHRP-6, but this was only shown in fish, not humans. Fasting changed the levels of some of these receptors, hinting they help control energy use.

Here we report the identification and characterization of ghrelin (GRLN) receptors in goldfish Carassius auratus. We identified four distinct mRNAs generated from four different genes. Those were roughly divided into two types, based on the number of amino acids and amino acid sequence similarity; one composed of 360-amino acids, which is similar to zebrafish GHS-R1a (showing 94-96% identity) and the other encodes a 366- or 367-amino acid protein, which demonstrated 95% identity to zebrafish GHS-R2a. We therefore designated these proteins as goldfish GHS-R1a type 1 (1a-1) and type 2 (1a-2) and GHS-R2a type 1 (2a-1) and type 2 (2a-2). GHS-R1a and 2a proteins share 74% sequence identity with each other. In functional analyses, three of these four receptors (except 2a-2 receptor), were activated by goldfish GRLN or GHS. The GRLN activity was inhibited by [D-Lys(3)] GHRP-6 but not by des-acyl goldfish GRLN. Expression levels of GHS-R1a mRNA were 2- to 50-folds higher than those of GHS-R2a, and GHS-R2a-2 mRNA expression was 1/25 of GHS-R2a-1. GHS-R1a-1 and 1a-2 mRNAs were mainly detected in the central nervous system (CNS), pituitary, liver, intestine and testis, whereas GHS-R2a-1 and 2a-2 mRNAs were predominantly expressed in the CNS, body kidney, ovary and testis. A 7-day fasting led to a decrease in GHS-R1a-1 mRNA expression in the vagal lobe, but stimulated GHS-R1a-2 mRNA in the liver, although no change was observed in GHS-R2a mRNAs. These results indicate that goldfish has four GHS-Ra that is divided into two types, 1a and 2a; and each receptor expression is separately regulated with GHS-R1a acts on energy metabolism.