68 Fig. 3 Cross sections of the cords at the injury site staining with GFAP and OX-42 from guanosine-treated and vehicle-treated rats 14 days after treatment following chronic spinal cord injury. a, b Demonstrate examples of GFAP-positive astrocytes (in red) in cross sections from vehicle- (a) and guanosine- (b) treated animals at the injury site 14 days after treatment. c, d Show OX-42-immunofluorescent staining of cross sections of spinal cords from vehicle- (c) and guanosine- (d) treated animals at the lesion site 14 days after treatment. Quantifica- tion showed more OX-42-immunolabelled cells in cords of guanosine- treated rats (d, e) compared to cords of vehicle-treated rats (c, e; p < 0.05). Scale bar=50 μm for all Purinergic Signalling (2008) 4:61–71 numbers of OX-42+ microglia/macrophages until 14 days after treatment. At day 14, guanosine significantly in- creased OX-42+ cells compared to vehicle treatment (Fig. 3c–e; p < 0.05). Guanosine inhibits apoptosis due to a number of stimuli [50–52]. After stroke and spinal cord injury, intrinsic stem cells and progenitors proliferate and differentiate but many die through apoptosis [15, 53–55]. In these experiments, guanosine did not prevent cell death (measured by TUNEL) (Fig. 4), indicating that the protective effects of guanosine do not result from a reduction in precursor cell apoptosis. Before the intraperitoneal injection of guanosine, there were detectable amounts of guanosine and its metabolic product guanine in the spinal cords of injured rats (time 0, Fig. 5a,b). There were no significant differences in guanosine or guanine in the samples of the spinal cords above, at or below the lesion. After guanosine injection, the amount of both guanosine and guanine increased, reaching a maximum by 30 min; the amounts of guanosine and guanine were about twofold and fivefold higher than those detected before the injection (Fig. 5a,b). As with the basal levels (time 0), there was no difference in the amounts of guanosine above, at or below the lesion. Interestingly, guanine levels were always higher than those of guanosine at each of the times tested after injection (Fig. 5a,b). New oligodendrocytes in injured spinal cords come from a population of endogenous progenitor cells present in the CNS that can differentiate into mature cells capable of myelinating axons [3, 12, 13, 56]. The data from the present study showed that the increase in the number of NG2+ cells is associated with an increase in Rip+ cells. Our data indicated that guanosine treatment induced an increase of mature oligodendroglia (Rip+ cells), likely due to prolifer- ation and differentiation of their progenitor (NG2+) cells. Therefore, one would expect that Rip+ cells increase after NG2+ cells. However, this was not observed. But guanosine may well stimulate the proliferation of oligodendroglial Fig. 4 Guanosine treatment did not affect apoptosis in the cords with chronic, stable spinal cord injury. For detection of apoptotic cells, a terminal deoxynucleotidyltransferase (TdT)-mediated dUTP nick end labelling (TUNEL) stain was performed using the ‘In situ Cell Death Detection Kit, Fluorescein’. No obvious difference in number of TUNEL-positive cells was observed between vehicle- (a) or guano- sine- (b) treated spinal cord sections. Scale bar=50 μm for all