Immunohistochemical detection of O6-ethyldeoxyguanosine in the rat brain after in vivo applications of N-ethyl-N-nitrosourea.

An immunohistochemical procedure was developed which allows the localization of the DNA lesion O6-ethyldeoxyguanosine (O6-EtdGuo) within tissues and organs. The method permits the detection of 24,000 residues of O6-EtdGuo per diploid nucleus. We have used this procedure to localize N-ethyl-N-nitrosourea (ENU)-induced O6-EtdGuo in the rat brain. Shortly after a single injection of ENU, we observed O6-EtdGuo in most or all of the rat brain nuclei. After repeated injections of small doses of ENU, with intervals of 1 or 2 weeks between the injections and between the last injection and sacrifice, we could demonstrate O6-EtdGuo only in part of the rat brain nuclei. Oligodendrocytes, granular neurons and endothelial cells, and part of the pyramidal neurons and astrocytes had accumulated O6-EtdGuo, while in all other cells this lesion was not detectable after repeated injections of small doses of ENU. We found no obvious correlation between the putative sensitivity of rat brain cells to tumor induction and the accumulation of O6-EtdGuo in their DNA.