Seasonal changes in corticosteroid receptors (CR) in the gills of coho salmon (Oncorhynchus kisutch) were examined to determine what effect smolting and rearing environment had on gill tissue sensitivity to cortisol. CR concentration and affinity were found to change seasonally with the increases in gill Na+K+ATPase activity and the development of saltwater tolerance. At times of the year when fish showed increased saltwater tolerance, the gill CR concentration and affinity decreased. The

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... e in receptor numbers and affinity was concurrent with increases in plasma cortisol concentration. Endocrine control of CR by cortisol and growth hormone (GH) was examined. Cortisol downregulated CR in the gills. Acute administration of cortisol resulted in a reduction in CR numbers for 72 hr with no change in affinity. Chronic cortisol treatment resulted in a decrease in CR concentration and affinity The change in affinity occurred only while plasma cortisol levels remained elevated, but CR population remained significantly reduced for at least 10 days following cessation of hormone treatment. Repeated handling stresses resulted in a similar reduction in CR numbers, but without an apparent change in affinity. The chronic or repeated elevation in plasmacortisol down regulates the sensitivity of the gills to cortisol by a persistent reduction in CR concentration, despite the return to non-stress levels of circulating cortisol. In contrast to cortisol, Gil upregulated CR in the gills. Two bovine hormones were used in the study, growth hormone (bGH) and placental lactogen (bPL). These hormones caused a dose dependent increase in concentration of CR and Na+K+ATPase activity in the gills. The upregulation of CR by bPL and bGH enhanced the gill sensitivity to cortisol and may partially account for the greater saltwater tolerance exhibited by the Gil treated fish. The effect of rearing environment on cortisol dynamics and Gil changes were examined in juvenile hatchery and wild coho salmon over the spring when the fish were moltin [...]