ALBUMIN TRANSFER ACROSS THE CHOROID-PLEXUS OF SOUTH-AMERICAN OPOSSUM (MONODELPHIS-DOMESTICA)

1. Blood-cerebrospinal fluid (CSF) transfer of various exogenous album ins has been investigated in developing Monodelphis domestica (South A merican grey short-tailed opossum) and compared with the steady-state CSF:plasma ratios for endogenous (Monodelphis) albumin. Ratios for Mon odelphis albumin and human albumin were similar and were the highest a t postnatal day 5 (P5) (48.2 +/- 4.4 and 40.6 +/- 4.5%, respectively). The ratio for bovine albumin uas similar to the steady-state ratio fo r Monodelphis albumin at P7-8 but became consistently lower than the M onodelphis albumin ratio at all other ages until P32-36 when all album ins tested attained a similar low ratio. The CSF:plasma ratio of chemi cally modified (succinylated) bovine albumin was always significantly lower than that of other albumins, except at the oldest age examined ( P32-36). 2. Immunocytochemistry showed that within the brain, albumin was confined to the lumen and endothelial cells of blood vessels. In t he choroid plexus only a small proportion (0.2-1.7% of the total cell number) of epithelial cells was positive for albumin, both endogenous and exogenous, at all ages studied (except the 3rd ventricle where cel ls were only positive from P8). The CSF was strongly positive for all albumins. The peak proportion of positive cells and of albumin concent rations in CSF occurred at P8. These findings suggest that the primary route for penetration of albumin into CSF is directly across the chor oid plexus rather than via the brain. 3. Double-labelling immunocytoch emistry revealed that the same epithelial cells contained both endogen ous (Monodelphis) and exogenous (human) albumin. In contrast, for succ inylated albumin, at P7 only about 35% (lateral ventricle) and 50% (4t h ventricle) of Monodelphis albumin positive cells were also positive for succinylated albumin, but by P30 this proportion increased to 90% at both sites. 4. Thus the developing choroid plexus distinguishes bet ween different albumins. Chemical modification of albumin (succinylati on) disrupts this mechanism. It is proposed that in older animals (P32 -36) all of the albumin in the CSF is derived from plasma by diffusion (as in adult animals). At earlier stages of development, a proportion of the albumin in CSF also appears to be transferred from the plasma by diffusion with an additional component transferred by a mechanism t hat can distinguish between different species of albumin. The main rou te of entry of albumin to CSF seems likely to be via the choroid plexu s epithelial cells.