NIACIN AND NIACINAMIDE TRANSPORT IN THE CENTRAL NERVOUS SYSTEM. IN VIVO STUDIES

Abstract– The concentration ol niacinamide in plasma and CSF was 0.5 and 0.7 μm respectively. The, mechanisms by which niacin and niacinamide, which are not synthesized in brain, enter brain, CSF and choroid plexus were investigated by injecting [¹⁴C]niacin or [¹⁴C]niacinamide intravenously and intraventricularly. [¹⁴C]Niacin or [¹⁴C]niacinamide, with or without unlabeled niacin or niacinamide, were infused intravenously at a constant rate into conscious rabbits. At 3 h, [¹⁴C]niacinamide, but not [¹⁴C]niacin, readily entered CSF, choroid plexus and brain. The addition of 4.1 mmol/kg niacinamide to the infusate markedly depressed the relative entry of [¹⁴C]niacinamide into choroid plexus and brain but not into CSF. After intraventricular injection, [¹⁴C]niacin was rapidly cleared from CSF and readily entered brain and choroid plexus. The addition of unlabeled niacin to the intraventricular injectate decreased the clearance of [¹⁴C]niacin from CSF and the entry of [¹⁴C]niacin into choroid plexus and brain. Unlike niacin, carrier niacinamide (82 μmol) in the injectate did not depress the extremely rapid clearance of intraventricularly injected [¹⁴C]niacinamide from CSF but did decrease the entry of [¹⁴C]niacinamide into brain. These results show that the control of entry and exit of niacinamide and niacin is the mechanism, at least in part, by which total niacin and NAD levels in brain cells are regulated. In the case of niacinamide which readily passes between CSF and plasma, the regulation of entry of niacinamide into brain cells by a high affinity accumulation system is an integral part of the homeostatic system. In the case of niacin, penetration into CSF and the extracellular space of brain from plasma as well as regulation of entry into brain cells by a saturable accumulation system are two distinct parts of the homeostatic system. In vivo, niacin that enters the central nervous system is converted to the principal plasma vitamer, niacinamide, in its free or bound forms such as NAD.