| Summary: | 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 decline 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 molting. Plasma cortisol levels showed an increase in concentration during the spring in all groups. The rise in cortisol concentration, however, was significantly greater in the wilds molts than the hatchery smolts. Timing of maximal Gil levels in the plasma during the spring were similar to the surge in cortisol. The absolute differences between the groups and sampling times during the spring, however, were much smaller than those observed for cortisol. CR concentration and affinity decreased during the spring. The wild fish consistently possessed the greatest number of CR. The change in affinity was similar for the hatchery and wild fish during the spring of 1991. In 1992, the hatchery fish also showed a gradual increase in the dissociation constant (kr,). In contrast, the wild fish did not show an increase in dissociation constant until May. The changes in cortisol concentration in the plasma and the CR dissociation constant occurred synchronously with the increase in Na+K+ATPase activity of the wild fish. The wild fish showed the greatest increase in km plasma cortisol concentration and Na+1C+ATPase activity. Although the hatchery fish were much larger than their wild or colonized counterparts, they consistently showed a reduced saltwater tolerance as assessed by a much greater perturbation in plasma sodium concentration following transfer to saltwater. Within each group there was no relationship between the size of the fish and saltwater tolerance. Following transfer to salt water, the hatchery fish showed a significant increase in circulating plasma cortisol concentration and changes in plasma GH level that were associated with the osmotic stress experienced by the fish. These changes were not seen in the wild smolts. The hatchery fish possess fewer chloride cells and lower specific activities of the enzymes Na+IVATPase and citratesynthase. The weaker osmoregulatory ability of the hatchery fish was associated with a greater mortality following abrupt transfer to 35 %o sea water. The research presented indicates that in coho salmon, gill CR change seasonally. CR concentration reflects smolting and development of saltwater tolerance. The affinity and concentration of CR are regulated by levels of cortisol and growth hormone in the plasma. Rearing conditions that cause a change in these hormone levels also affects the sensitivity of the gills to cortisol. Rearing environment, therefore, has a large effect on corticosteroid receptor dynamics and smolting. The changes in CR that are associated with rearing environment contribute to the differences in saltwater tolerances seen between the hatchery and wild coho smolts.
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