Purinergic Signalling (2008) 4:61–71 69 a 14 12 10 e u iss 8 t f o /g 6 g µ b 4 2 0 40 35 30 e 25 u s s i ft 20 o g / µg 15 10 5 0 Above the lesion At the lesion Below the lesion 0 7.5 30 Minutes 60 Above the lesion At the lesion Below the lesion 0 7.5 30 60 Minutes Fig. 5 Guanosine (a) and guanine (b) in spinal cord samples of rats with chronic spinal cord injury before and after intraperitoneal injection of guanosine. Guanosine (8 mg/kg) was administered intraperitoneally to rats 5 weeks after the spinal cord injury. Before or at the indicated time points after the injection, samples of spinal cord from 3–6 animals were taken from the site of the lesion and immediately above and below the lesion. The tissue content of guanosine and its metabolic product guanine was measured by HPLC and CE analysis. Data are the mean±SEM progenitor cells at several stages of development. This would account for the observed results. Additionally, we cannot exclude that other possible mechanisms are involved in the process; for example, mature oligodendroglia could migrate from the adjacent normal tissue to the injury site soon after guanosine treatment. Whether the effects on remyelination are due to guanosine, guanine or both is not clear. However, since a putative receptor for guanosine has been identified in the CNS, and since PNP (purine nucleoside phosphorylase) in CNS permits the interconversion of guanine and guano- sine enabling guanine to act as a reservoir for guanosine, it is possible that guanosine may be responsible for causing the proliferation of the endogenous progenitor cells and the subsequent remyelination process, although this is by no means certain. But, since guanine itself is difficult to administer because of its poor solubility, administration of guanosine is the most effective delivery mechanism. The mechanism by which remyelination is facilitated after chronic, traumatic SCI is unknown. Guanosine has been shown to stimulate the production and/or release of a variety of trophic factors [25]; for example bFGF, NGF and NT-3 have been demonstrated to increase the proliferation and survival of oligodendrocyte progenitors [3, 16, 57], induce oligodendroglial genesis [58–61], stimulate the proliferation of oligodendrocyte precursors and enhance remyelination after spinal cord traumatic injury [18, 19] and after toxin-induced demyelination [62]. There are no published reports of the effects of guanine in the vivo systems. However, in cultured cells, guanosine stimulates cell division apparently through a pertussis toxin-sensitive process involving MAP kinase [63]. Moreover, there is evidence of a G protein-coupled receptor in brain mem- branes that might be a candidate for a guanosine receptor [64]. Further studies will permit us to distinguish amongst these possibilities. The present findings are the first to show that systemic treatment with guanosine after chronic SCI induces an improvement in function that is accompanied by the formation of mature oligodendrocytes. This effect of guanosine/guanine may be attributable to direct or indirect stimulation of endogenous oligodendrocyte lineage progen- itors in the spinal cord and remyelination of axons at the injury site. These findings raise the possibility that the administra- tion of guanosine may be effective in treating spinal cord injuries and in the treatment of demyelinating diseases such as multiple sclerosis where quiescent progenitors exist in demyelinated plaques [65–70]. Acknowledgements We thank Dr. Raelene Kinlough-Rathbone for her helpful suggestions and critical appraisal of the manuscript, and Drs. Richard Borgens, Andrew Blight, and John Gruner for advice on the spinal crush model. We are grateful to Prof. Marie T. Filbin, Department of Biological Sciences, CUNY, NY, for kindly providing the anti-P0.