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

In our previous experiments, we demonstrated that xylazine, an α 2 -adrenergic agonist,stimulated proliferation of thymocytes triggered by concanayalin A. In contrast, higher concentrations of xylazine were inhibitory. In this work, we studied the mechanisms; involved in immunosuppresssion of xlazine and found that the compound at concentrations between 100 μM and 500 μM induced apoptosis of rat thymocytes in vitro. In addition, xylazine at concentrations higher than 50 μM also induced apoptosisof a thymocyte hybridoma (BWRT8) and increased apoptosis of the line triggered by T cell receptor (TCR) cross-linking. Apoptosis wss confirmed by morphologicalanalysis 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 α-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.