Time-course changes in neural cell apoptosis in the rat fetal brain from dams treated with 6-mercaptopurine (6-MP)

6-Mercaptopurine (6-MP), one of the major drugs for the therapy of acute lymphoblastic leukemia and autoimmune diseases, is incorporated as thioguanine in nucleic acid and it induces cytotoxicity and fetotoxicity. In the present study, pregnant rats were treated with 50 mg/kg of 6-MP on 13 embryonic days (E), and fetuses were collected from 12 to 96 h after the treatment to examine the mechanism and time-course changes in neural cell death in the developing brain. The weights of fetal telencephalon and the thickness of the dorsal telencephalic wall of the fetuses were significantly reduced at 96 h. The number of pyknotic neural cells in the fetal telencephalon began to increase at 24 h, peaked at 36 h, and then gradually decreased toward 72 h. The nuclei of most of these pyknotic cells were stained positively by TUNEL method, which detects DNA fragmentation. Moreover, pyknotic cells were immunohistochemically positive for cleaved caspase-3, one of the key executioners of apoptosis, and the increased expression of the protein from 30 to 48 h was confirmed by using Western blot analysis. Also, electron microscopical features of the pyknotic cells showed ultrastructural characteristics of apoptosis. On the other hand, the number of mitotic and BrdU-positive neural cells in the telencephalon decreased from 30 to 72 h. These results suggest that 6-MP induced apoptotic cell death in neural cells in the rat fetal brain is probably due to cytotoxic action of 6-MP.