Xylazine, an alpha(2)-adrenergic agonist, induces apoptosis of rat thymocytes and a thymocyte hybridoma line in vitro

Abstract

In our previous experiments, we demonstrated that xylazine, an alpha(2)-adrenergic agonist, stimulated proliferation of thymocytes triggered by concanavalin A. In contrast, higher concentrations of xylazine were inhibitory. In this work, we studied the mechanisms involved in immunosuppression of xylazine and found that the compound at concentrations between 100 muM and 500 muM induced apoptosis of rat thymocytes in vitro. In addition, xylazine at concentrations higher than 50 muM also induced apoptosis of a thymocyte hybridoma (BWRT8) and increased apoptosis of the line triggered by T cell receptor (TCR) cross-linking. Apoptosis was confirmed by morphological analysis staining with merocyanine 540 and propidium iodide and in cases of BWRT8 by fragmentation of DNA. The mechanisms of xylazine-induced apoptosis of the BWRT8 hybridoma were further examined. We demonstrated that the process in both nonactivated and activated (TCR cross-linking) BWRT8 cells was not prevented by yohimbine (a selective alpha-adrenergic antagonist) and by antibodies to Fas and Fas-L. In contrast, cell death was completely blocked by a caspase inhibitor Z-Val-Ala-Asp (OMe)-CH2F. Cyclosporine, a calcineurin blocker partly inhibited the xylazine-induced apoptosis of activated BWRT8 cells.