Intra-arterial injection of prolactin-releasing peptide elevates prolactin gene expression and plasma prolactin levels in rainbow trout

Prolactin-releasing peptide (PrRP), recently isolated from the brain of mammals and teleosts, is a strong candidate for being a stimulatory hormone of pituitary prolactin secretion. The present study examined whether or not PrRP is capable of inducing prolactin gene expression and elevating plasma prolactin levels in vivo in cannulated rainbow trout. Following a single intra-arterial injection of chum salmon PrRP (40 nmol kg−1) through a dorsal aorta catheter, plasma prolactin levels increased (P<0.05) rapidly (2 min and 30 min), and prolactin mRNA levels were elevated (P<0.05) in pituitaries sampled 8 h after the injection. In contrast, plasma levels of somatolactin were decreased (P<0.05) and growth hormone and somatolactin mRNA levels were not significantly affected by PrRP. Thus, PrRP appears to be a potent prolactin secretagogue as well as prolactin transcription inducer in vivo in the rainbow trout.