The Effect of Continuous Corticosterone Administration On Lymphocyte Subpopulations in the Peripheral Blood of the Fischer 344 Rat as Determined by Two Color Flow Cytometric Analyses

This study was undertaken to determine the effects of continuous corticosterone administration on lymphocyte subsets in the peripheral blood of Fischer 344 rats. Pellets which released corticosterone over a 21 day period (0.07 mg/day, 0.48 mg/day and 4.8 mg/day) were implanted subcutaneously in male rats. Control rats received pellets containing only the excipient carrier. Rats in the test and control groups were sacrificed at 7, 14 and 21 days. Lymphocyte subsets were enumerated by dual color flow cytometry and the data expressed in absolute numbers/mm3. Effects were observed only in the animals treated with the highest dose which was 70,000 times the normal plasma level. The spleen, thymus and lymph nodes were examined for histopathological changes. At the seven day sacrifice there was a statistically significant decrease in total white blood cells and selective decrements in lymphocytes with reductions in the absolute numbers of the T helper/amplifier, T cytotoxic/suppressor and B cells. Only numbers of natural killer cells were within normal limits. Histopathological data from animals treated with the high dose corticosterone for seven days demonstrated decreased thymic weights and a loss of thymic lymphocytes. At 14 and 21 days, the numbers of lymphocytes returned to the normal range, but the numbers of total T cells remained decreased. Also, thymic weights were reduced but not histological abnormalities were observed in the thymus. The data suggest that corticosterone induced a persistent decrease in total T cells, but only a transient effect on total lymphocytes.