Pituitary gene expression of somatolactin, prolactin, and growth hormone during Atlantic salmon parr-smolt transformation

Somatolactin (SL), prolactin (PRL) and growth hormone (GH) belong to the same family of adenohypophyseal hormones. In fish, these hormones have all been linked to several important physiological functions. During salmon smoltification, GH has an important role in stimulating growth and improving hypoosmoregulatory ability. The role of PRL in fish osmoregulation has mostly been linked to survival in fresh water (FW), whereas the physiological functions of SL is less clear, having been linked to reproduction, stress, Ca-regulation and acid - base balance. To obtain more information on the roles and interactions of these hormones during the parr - smolt transformation, juvenile Atlantic salmon in FW were sampled from early-February until late-June. Approximately every 2 weeks, fish were sacrificed and pituitaries sampled for assessment of SL, PRL, and GH mRNA. In addition, fish were transferred to seawater (SW) in late-May, and sampled after 1 and 4 weeks in SW. Somatolactin mRNA expression did not change during the parr - smolt transformation of the Atlantic salmon, but increased transiently during the first week in SW, a response which may be correlated to stress and/or Ca-balance. PRL and GH mRNA expression increased transiently during the latter part of the parr - smolt transformation of Atlantic salmon. Following SW entry, the PRL mRNA expression decreased, while GH mRNA expression increased further. This indicates that PRL is of importance during the parr - smolt transformation of the Atlantic salmon, probably by counteracting the hypoosmoregulatory effects of GH and cortisol, allowing the fish to maintain osmotic balance during the smoltification process.