Differential responses to stress stimuli of Lewis and Fischer rats at the pituitary and adrenocortical level.

OBJECTIVE

Histocompatible rat strains Lewis (LEW) and Fischer 344 (F344) are often used to study hypothalamic-pituitary-adrenocortical axis function in relation to immune system activity. It has been suggested that LEW rats have a defect in the hypothalamic production of corticotropin-releasing hormone. The aim of this study was to clarify differential responsiveness of LEW and F344 rat strains to acute stress by measuring adrenocorticotropic hormone (ACTH) and corticosterone concentrations in plasma, corticosterone in adrenal cortex and proopiomelanocortin (POMC) gene expression in the pituitary and spleen.

METHODS

Two separate experiments were performed. In the first, indwelling catethers were used for blood sampling from conscious animals during immobilization stress. In the second experiment, rats were immobilized for two hours and decapitated after another 3 hrs for organ collection.

RESULTS

Our results show that LEW strain hyporeactivity was evident from significantly lower ACTH and corticosterone levels compared to those in F344 at all time intervals during stress studied. Measurement of POMC gene expression in the pituitary revealed that the difference in hormone secretion was consistent with POMC mRNA concentrations in these strains of rats. On the other hand, corticosterone concentrations in the adrenal cortex after stress were significantly higher in LEW rats compared to F344. No differences in spleen POMC mRNA concentrations between LEW and F344 rats were found.

CONCLUSION

The results show that differential reactivity of LEW and F344 rats is associated with different POMC gene expression in the pituitary and probably other factors on the adrenocortical level.