High blood cortisol levels and low cortisol receptor affinity: is the chub, Leuciscus cephalus, a cortisol-resistant teleost?

In contrast to the relatively minor intra- and inter-species differences in blood cortisol levels reported for salmonid species, there is a more pronounced distinction between cortisol levels among the Salmonidae and Cyprinidae, with both basal and stress-induced cortisol levels markedly higher in the latter. This study shows that in the chub, Leuciscus cephalus, a widely distributed European cyprinid, mean blood cortisol levels during stress (1500 ng ml-1) exceeded those reported for most other species of fish and even in unstressed chub, cortisol levels (50 – 100 ng ml-1) were within the range known to cause immunosuppression, growth retardation and reproductive dysfunction in salmonid fish. The chub appears to be atypical only with respect to plasma cortisol levels; the levels of plasma glucose and plasma lactate in unstressed and stressed chub are similar to those reported for other species. Plasma levels of 11-ketotestosterone in males and 17beta-estradiol in females are lower than reported for salmonids but similar to other cyprinid species and display clear stress-induced reduction. Comparative analysis of the binding characteristics of the trout and chub gill cortisol receptor revealed that the total number of binding sites in gill tissue for each species was similar (Bmax; approx 50 – 100 fmol mg-1 protein). However, the affinity of the binding site for cortisol displayed an 8-fold difference between the species (rainbow trout: Kd approx. 6 nM; chub: Kd approx. 50 nM). Therefore, the potentially adverse effects of high circulating levels of cortisol found both at rest and under conditions of stress in chub may be offset by the lower affinity of the cortisol receptor, rather than the abundance of target-tissue receptor sites. This strategy is similar to that reported for some glucocorticoid-resistant rodent species and New World primates.