THE PASSAGE OF BROMIDE, IODIDE AND THIOCYANATE INTO AND OUT OF THE CEREBROSPINAL FLUID

¹ From the Department of Pharmacology, New York University College of Medicine

1. The purpose of the experiments reported here on dogs was to study the rates and amounts of bromide, iodide and thiocyanate passing from the serum into the cerebrospinal fluid and from the cerebrospinal fluid into the serum.

2. After intravenous injections these ions begin to pass promptly into the cerebrospinal fluid, drawn from the cisterna magna. The maximum passage from the serum to the cerebrospinal fluid requires 7 hours or more at which time there is a constant ratio between the serum and the cerebrospinal fluid. Subsequently, as the serum level falls, the serum/CSF quotient rises. The passage of these ions into the cerebrospinal fluid differs from their passage into transudates and most other tissue fluids in two respects; (a) the cerebrospinal fluid concentration never reaches that of the serum; (b) the rate of passage is slower.

3. The difference in the passage of bromide, iodide and thiocyanate into the cerebrospinal fluid and other body fluids is most strikingly shown by the threshold figures. The cerebrospinal fluid may contain no iodide or thiocyanate at a time when the serum contains measurable quantities. In other words the serum must attain a certain minimum concentration of iodide or thiocyanate before these ions pass into the cerebrospinal fluid. The bromide may also have a threshold value, but if so it is much lower than the threshold value for the others. It follows that with low concentrations in serum the difference between the individual passage of bromide, iodide and thiocyanate into the cerebrospinal fluid is much more pronounced than with large doses. The above facts bring out sharply the selective capacity of the barrier between serum and central nervous system.

4. In the passage from the cerebrospinal fluid into the circulation no appreciable difference is seen between the ions. When injected into the cistern they all pass out quickly at the same rate. There is thus no evidence of any barrier regulating their passage from cerebrospinal fluid to blood.