Differential behavioral and endocrinological effects of corticotropin-releasing hormone (CRH) in the syracuse high- and low-avoidance rats

The Syracuse high- and low-avoidance rats, which have been selectively bred for good (SHA/Bru) or poor (SLA/Bru) avoidance learning in a two-way shuttle box, differ in emotionality: This experiment investigated the effect of corticotropin-releasing hormone (CRH), administered centrally (0, 0.1, 0.5, and 1.0 mu g), on conditioned suppression and on the hypothalamic-pituitary-adrenocortical system. Three groups of animals were used: SHA/Bru rats conditioned at 0.21 or 0.43 mA and SLA/Bru rats conditioned at 0.21 mA. The results confirm those of previous studies which found that SLA/Bru rats show greater conditioned suppression than the SHA/Bru rats at the low shock intensity and that at 0.43 mA, the SHA/Bru animals acquire a level of conditioning comparable to that of the SLA/Bru animals at 0.21 mA. The results show that the nonlinear behavioral effect of CRH is independent of strain and produces comparable effects in animals of both strains, but only when level of conditioning is equated. Adrenal and plasma concentrations of corticosterone increased in all three groups of animals as a direct linear function of dose of CRH. Both greater levels of conditioning and larger amounts of CRH increase the synthesis of corticosterone more in SHA/Bru animals than in the SLA/Bru animals. Thus, genetic variation, which differentiates the behavioral and endocrinological characteristics of these animals, shows that these effects of CRH can be independent of each other and suggests that some minimal level of conditioned fear is necessary for CRH to exert its anxiogenic effect. (C) 1998 Academic Press.