Deoxycytidine Transport and Metabolism in the Central Nervous System

The mechanisms by which deoxycytidine enters and leaves brain, choroid plexus, and CSF were investigated by injecting [ 3 H]deoxycytidine intraarterially, intravenously, and intraventricularly. After intracarotid injection of deoxycytidine (1.0 μ M ) into rats, deoxycytidine did not pass through the blood‐brain barrier at a faster rate than sucrose. [ 3 H]Deoxycytidine, either alone or together with unlabeled deoxycytidine, was infused at a constant rate into conscious adult rabbits. At 130 min, [ 3 H]deoxycytidine readily entered CSF, choroid plexus, and brain. In brain, approx. 60% of the nonvolatile radioactivity was attributable to [ 3 H]deoxycytidine phosphates. The addition of 0.22 mmol/kg unlabeled deoxycytidine to the infusion syringe decreased the phosphorylation of [ 3 H]deoxycytidine in brain by approx. 50%; the addition of 2.2 mmol/kg of unlabeled deoxycytidine to the infusion syringe decreased the relative entry of [ 3 H]deoxycytidine into CSF and brain by approx. 50 and 75%, respectively. Two hours after the intraventricular injection of [ 3 H]deoxycytidine, [ 3 H]deoxycytidine was rapidly cleared from CSF, in part, to brain, where approx. 65% of the [ 3 H]deoxycytidine was converted to [ 3 H]deoxycytidine phosphates. The intraventricular injection of unlabeled deoxycytidine with the [ 3 H]deoxycytidine decreased the phosphorylation of [ 3 H]deoxycytidine in the brain significantly and also decreased the clearance of [ 3 H]deoxycytidine from the CSF. These results were interpreted as showing that the entry of deoxycytidine from blood into CSF occurs by a saturable transport system within the choroid plexus. Once within the CSF, the deoxycytidine can enter brain, undergo phosphorylation to deoxycytidine phosphates, and subsequently be incorporated into DNA.