Thymidine transport in the central nervous system.

The mechanisms by which thymidine enters and leaves brain, choroid plexus, and CSF were investigated by injecting [3H]thymidine intravenously and intraventricularly. [3H]thymidine, with and without unlabeled thymidine, was infused at a constant rate into conscious adult rabbits. At 150 min, [3H]thymidine readily entered CSF, choroid plexus, and brain. In brain, approximately 45% of the nonvolatile radioactivity was [3H]thymidine phosphates. The addition of 0.21 mmol/kg unlabeled thymidine to the infusion syringe decreased the phosphorylation of [3H]thymidine in brain by approximately 85%; the addition of 2.1 mmol/kg of unlabeled thymidine to the infusion syringe decreased the relative entry of [3H]thymidine into CSF and brain by 40 and 78%, respectively. Two h after intraventricular injection of [3H]thymidine, [3H]thymidine was rapidly cleared from CSF, in part, to brain, where approximately 40% of the [3H]thymidine was converted to [3H]thymidine phosphates. The intraventricular injection of unlabeled thymidine (21 mumol) with the [3H]thymidine abolished the phosphorylation of [3H]thymidine in brain and significantly decreased the clearance of [3H]thymidine from the CSF. Rabbit brain slices accumulated [3H]thymidine by an energy-dependent, saturable high-affinity system that depended, in part, on intracellular phosphorylation of the [3H]thymidine. These results were interpreted as showing that the entry of thymidine from blood into CSF and presumably the extracellular space of brain and then into brain cells involves one or more saturable transport and/or metabolic steps.