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.
Copyright 2010 Elsevier Ireland Ltd. All rights reserved.