REGION-SPECIFIC DNA-SYNTHESIS IN BRAINS OF F344 RATS FOLLOWING A 6-DAY BROMODEOXYURIDINE INFUSION

Prolonged exposure to certain alkylating chemicals induces glial and m eningeal tumours in rats, probably resulting from DNA damage to dividi ng neural cells. The present work evaluated DNA synthesis in the brain s of untreated, young adult male F344 rats in order to define a BrdUrd infusion protocol to more adequately assess proliferation in slowly d ividing neural cell populations. BrdUrd (2.5 to 160 mg/ml) was adminis tered for 6 days via subcutaneous osmotic pumps. Clinical toxicity was not observed at any dose. The labelling index (LI; % of cells per bra in area that incorporated BrdUrd) and unit length labelling index (ULL I; % of cells per meningeal length that incorporated BrdUrd) were calc ulated for selected regions by counting labelled neural cells in defin ed areas of the right hemisphere in coronal brain sections. Intensely stained cells were numerous in the cerebral subependymal layer (LI=35. 8%); scattered in cerebral white matter tracts (e.g. corpus callosum a nd internal capsule; LI=6.2%) as well as cerebral (ULLI=4.2%) and cere bellar (ULLI=3.6%) meninges; and rare in the hippocampus (LI>0.1%). Mi ldy stained cells were dispersed in the pens (LI=2.1%), deep cerebral (LI=1.8%) and cerebellar (LI=1.0%) grey matter, and thalamus (LI=0.3%) . Phenotypically, BrdUrd-positive cells in neuropil were glial cell pr ecursors and their progeny, while those associated with meninges were usually located in the superficial subarachnoid space and appeared to be fibrocytes. Using BrdUrd infusion, LI for glial precursors at these sites ranged from two- to 10-fold higher than those reported previous ly after a brief parenteral pulse dose, These data indicate that conti nuous BrdUrd infusion for 6 days by subcutaneous osmotic pump is an ef ficient means of labelling neural cells throughout the brain.