Diurnal expression patterns of neurohypophysial hormone genes in the brain of the threespot wrasse Halichoeres trimaculatus

The aim of this study was to determine the involvement of neurohypophysial hormones in the diurnal patterns of the threespot wrasse Halichoeres trimaculatus, which is common in coral reefs and exhibits daily behavioral periodicity. Prohormone cDNAs of the neurohypophysial peptides, arginine vasotocin (AVT) and isotocin (IT), were cloned by 3′- and 5′-rapid amplification of cDNA ends (RACE). The distribution and expression patterns of pro-AVT and -IT mRNAs in the brain were determined using reverse-transcription polymerase chain reaction (RT-PCR) and real-time quantitative PCR, respectively. The respective full-length cDNAs of pro-AVT and -IT were 945 and 755 bp in length, respectively. The deduced amino acid sequences for pro-AVT and pro-IT were 154 and 156 residues in length, respectively. Both pro-peptides contained a signal sequence followed by the respective hormones and neurophysin connected by a Gly-Lys-Arg bridge. Pro-AVT mRNA was detected only in the hypothalamus area, while pro-IT mRNA in the whole part of the brain. The relative abundance of pro-AVT and -IT mRNA varied according to time of day; it was significantly greater at 12:00 h than at 24:00 h. Following intraperitoneal administration of melatonin, pro-AVT mRNA abundance in the brain decreased, while pro-IT mRNA abundance remained unchanged. These results demonstrate that daily fluctuations of pro-AVT and pro-IT levels in the brain of threespot wrasse are differentially regulated.