Abstract
Pacific salmon employ a semelparous reproductive strategy where sexual maturation is followed by rapid senescence and death. Cortisol overproduction has been implicated as the central physiologic event responsible for the post-spawning demise of these fish. Cortisol homeostasis is regulated through the action of hormones of the hypothalamus-pituitary-interrenal (HPI) axis. These include corticotropin-releasing factor (CRF) and urotensin-I (UI). In the present study, masu salmon (Oncorhynchus masou) were assayed for changes in the levels CRF-I and UI mRNA transcripts by quantitative real-time PCR (qRT-PCR). These results were compared to plasma cortisol levels in juvenile, adult, and spawning masu salmon to identify specific regulatory factors that appear to be functionally associated with changes in cortisol levels. Intramuscular implantation of GnRH analog (GnRHa) capsules was also used to determine whether GnRH influences stress hormone levels. In both male and female masu salmon, spawning fish experienced a 5- to 7-fold increase in plasma cortisol levels relative to juvenile non-spawning salmon. Changes in CRF-I mRNA levels were characterized by 1-2 distinctive short-term surges in adult masu salmon. Conversely, seasonal changes in UI mRNA levels displayed broad and sustained increases during the pre-spawning and spawning periods. The increases in UI mRNA levels were positively correlated (R(2)=0.21 male and 0.26 female, p<0.0001) with levels of plasma cortisol in the pre-spawning and spawning periods. Despite the importance of GnRH in sexual maturation and reproduction, the administration of GnRHa to test animals failed to produce broad changes in CRF-I, UI or plasma cortisol levels. These findings suggest a more direct role for UI than for CRF-I in the regulation of cortisol levels in spawning Pacific salmon.