THE NATURE OF THE DECREASE IN BLOOD CEREBROSPINAL-FLUID BARRIER EXCHANGE DURING POSTNATAL BRAIN-DEVELOPMENT IN THE RAT

1. The blood-cerebrospinal fluid (CSF) exchange of a wide range of pas sively transported lipid insoluble compounds (0.43-5.4 nm molecular ra dius) has been investigated in rats at different stages of postnatal d evelopment (2 days old to adult). A novel 'litter-based' model for inv estigating blood-CSF barrier exchange in immature animals is described . 2. At each age investigated there was a clear inverse correlation be tween molecular radius and blood-CSF barrier exchange, in addition to an overall decrease in blood-CSF barrier exchange with increasing age. 3. The decrease in blood-CSF barrier exchange with age was not consis tent with a reduction in pore diameters, nor does it appear to be due to an increase in the CSF sink effect with age. It seems likely to be due to a relative decrease in the number of a population of large diam eter pores.