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Extracellular adenosine (Ado) and ATP stimulate astrocyte proliferation through activation of P(1) and P(2) purinoceptors. Extracellular GTP and guanosine (Guo), however, that do not bind strongly to these receptors, are more effective mitogens than ATP and Ado. Exogenous Guo, like GTP and 5'-guanosine-betagamma-imidotriphosphate (GMP-PNP), dose-dependently stimulated proliferation of rat cultured astrocytes; potency order GMP-PNP > GTP > or = Guo. The mitogenic effect of Guo was independent of the extracellular breakdown of GTP to Guo, because GMP-PNP, a GTP analogue resistant to hydrolysis, was the most mitogenic. In addition to a direct effect on astrocytes, Guo exerts its proliferative activity involving Ado. Exogenous Guo, indeed, enhanced the extracellular levels of endogenous Ado assayed by HPLC in the medium of cultured astrocytes. Culture pretreatment with Ado deaminase (ADA), that converts Ado into inosine, reduced but did not abolish Guo-induced astrocyte proliferation whereas erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), that inhibits ADA activity, amplified Guo effect. Moreover, the mitogenic activity of Guo was partly inhibited by 8-cyclopentyl-1,3-dipropylxanthine and alloxazine, antagonists of Ado A(1) and A(2B) receptors, respectively. Also microglia seem to be a target for the action of Guo. Indeed, the mitogenic effect of Guo on astrocytes was: i) increased proportionally to the number of microglial cells present in the astrocyte cultures; ii) amplified when purified cultures of astrocytes were supplemented with conditioned medium deriving from Guo-pretreated microglial cultures. These data indicate that the mitogenic effects exerted by exogenous Guo on rat astrocytes are mediated via complex mechanisms involving extracellular Ado and microglia-derived soluble factors., (Copyright 2000 Wiley-Liss, Inc.)

Brain ischemia stimulates release from astrocytes of adenine-based purines, particularly adenosine, which is neuroprotective. Guanosine, which has trophic properties that may aid recovery following neurological damage, is present in high local concentrations for several days after focal cerebral ischemia. We investigated whether guanine-based purines, like their adenine-based counterparts, were released from astrocytes and whether their release increased following hypoxia/hypoglycemia. HPLC analysis of culture medium of rat astrocytes showed spontaneous release of endogenous guanine-based purines at a higher rate than their adenine-based counterparts. The concentration of guanosine (approximately 120 nM) and adenosine (approximately 43 nM) in the culture medium remained constant, whereas concentrations of adenine and guanine nucleotides, particularly GMP, and their metabolites increased with time. Exposure of the cultures to hypoxia/hypoglycemia for 30 min increased the extracellular concentration of adenine-based purines by 2.5-fold and of guanine-based purines by 3.5-fold. Following hypoxia/hypoglycemia extracellular adenine nucleotide levels increased further. Adenosine concentration increased, but not proportionally to nucleotide levels. Accumulation of adenosine metabolites indicated it was rapidly metabolized. Conversely, the concentrations of extracellular guanine-based nucleotides remained elevated and the concentration of guanosine continued to increase. These data indicate that astrocytes are a major source of guanine-based purines, the release of which is markedly increased following hypoxia/hypoglycemia, permitting them to exert neurotrophic effects.

Proliferation of brain astrocytes as a result of cell death has been well documented in vivo. Dying cells release purine and pyrimidine nucleosides and nucleotides and their deoxy derivatives both from soluble intracellular pools and from DNA and RNA. Previously, we have observed that purine nucleosides and nucleotides stimulate chick astrocyte proliferation in vitro. To further our analysis, we questioned whether pyrimidines or the deoxy derivatives of purine nucleosides and nucleotides might also be astrocyte mitogens. Pyrimidine nucleosides, nucleotides, and their deoxynucleotide derivatives were uniformly inactive. In contrast, deoxyguanosine, deoxyadenosine, and their mono-, di-, and triphosphates stimulated thymidine incorporation into astrocytes at concentrations similar to those at which their ribonucleoside and ribonucleotide analogues were active. Inosine, IMP, ITP, and hypoxanthine were active, whereas xanthine and xanthosine were not. However, XMP, XDP, and XTP stimulated thymidine incorporation. The effects of the nucleosides and deoxynucleosides were inhibited by antagonists of adenosine A2 receptors. These data indicate that most purine nucleosides, deoxynucleosides, and their 5' mono, di-, and triphosphate derivatives released from damaged cells are capable of stimulating astrocyte proliferation in vitro and may contribute to astrocyte proliferation in vivo following injury to the CNS.

Contrast enhancement resulting from surgical trauma may mimic residual enhancing tumor, thereby complicating the interpretation of postoperative computed tomographic scans. We assessed the natural history of postoperative enhancement in 10 patients with brain tumor. Contrast enhancement distinguishable from residual enhancing tumor appeared along the operative margin following tumor resections but not lobectomies. Enhancement appeared as early as the fifth postoperative day, was most intense at two weeks, and persisted for several months. Prior to the fifth postoperative day, enhancement reflected residual tumor. Edema and artifacts were more prominent on the first and second postoperative days than on the third and fourth. We recommend that postoperative computed tomographic scans to assess residual enhancing tumor be performed on the third or fourth postoperative day. This timing avoids postoperative enhancement and minimizes interpretative difficulties caused by artifacts.

The regenerating amphibian limb serves as a useful model for studying factors influencing cell proliferation and differentiation. In particular, peripheral nerves are thought to provide a stimulus for growth of the blastema, presumably via the elaboration of an as yet unidentified neurotrophic factor. In the present study, pressure ultrafiltration coupled with chromatofocusing have proven to be effective methods of partially purifying a neurotrophic factor from adult chicken brains. This chick brain growth factor (CBGF) appears to be a heat-stable, basic peptide of low molecular weight (less than 6,000). It is a potent mitogen in vitro, at nanomolar concentrations, for both blastema cells and Swiss mouse 3T3 fibroblasts. CBGF is apparently distinct from other peptide mitogens and/or neuromodulators that have been reported to stimulate blastema growth in vivo and in vitro. These include substance P, FGF from bovine brain and pituitary, EGF, transferrin (sciatin), and spinal cord growth factor (SCGF). The possible relationship of CBGF to other neural regulatory molecules is discussed.