CORTICOTROPIN RELEASING HORMONE RELATED BEHAVIORAL AND NEUROENDOCRINE RESPONSES TO STRESS IN LEWIS AND FISCHER RATS

We have recently shown that susceptibility to streptococcal cell wall (SCW)-induced arthritis in Lewis (LEW/N) rats is related to a lack of glucocorticoid restraint of inflammation while the relative SCW arthritis resistance in histocompatible Fischer (F344/N) rats is related to their greater hypothalamic-pituitary-adrenal (HPA) axis response. The difference in pituitary-adrenal responsiveness results from decreased inflammatory mediator-induced hypothalamic corticotropin-releasing hormone (CRH) biosynthesis and secretion in LEW/N rats. Because CRH not only activates the pituitary-adrenal axis, but also is associated with behavioral responses that are adaptive during stressful situations, we wished to determine if the differential LEW/N and F344/N CRH responsiveness to inflammatory mediators could also be associated with differences in neuroendocrine and behavioral responses to physical and emotional stressors. In this study, LEW/N rats exhibited significant differences compared to F344/N rats, in plasma adrenocorticotropin hormone (ACTH) and corticosterone responses during exposure to an open field, swim stress, restraint or ether. Furthermore, hypothalamic paraventricular CRH mRNA expression was also significantly lower in LEW/N compared to F344/N rats after restraint. These differences in neuroendocrine responses were associated with differences in behavioral responses in LEW/N compared to F344/N rats in the open field. Outbred HSD rats, which have intermediate and overlapping arthritis susceptibility compared to LEW/N and F344/N rats, exhibited intermediate and overlapping plasma corticosterone and behavioral responses to stressful stimuli compared to the two inbred strains. These data suggest that the differences in CRH responses in these strains may contribute to the behavioral and neuroendocrine differences we have observed. Therefore these strains may provide a useful animal model for studying the relationship between behavior, neuroendocrine and inflammatory responses.