Permeability of the developing blood-brain barrier to 14C-mannitol using the rat in situ brain perfusion technique

The brain penetration of 14C-mannitol was investigated using a bilateral in situ brain perfusion technique followed by capillary depletion analysis. This technique measures the uptake of slowly penetrating solutes in the absence of the systemic circulation, and separates accumulation in brain endothelial cells from uptake into brain parenchyma. Penetration of 14C-mannitol was linear up to 30 min in rats aged 1, 2, 3 weeks and in adults. The brain mannitol space was higher in 1-week-old neonatal rats compared with adults (P < 0.05) and was due to a greater initial volume of distribution (Vi) for mannitol in the neonates, and not due to an elevated transfer rate (K(in)). Thirty percent of mannitol in the neonatal brain was associated with the capillary containing fraction, whereas in the adult only 13% was found in this fraction. This suggests that the permeability of the blood-brain barrier to mannitol does not change significantly with development but that more mannitol is associated with endothelial cells in the neonate. An investigation of 14C-glycine uptake was also carried out, and unlike mannitol the K(in) was greater in the neonate compared to the adult suggesting an elevated rate of transfer for this amino acid into the neonatal rat brain.