INVIVO APPROACHES FOR STUDYING PEPTIDE INTERACTIONS AT THE BLOOD-BRAIN-BARRIER

The hypothesis that the polarity and size of peptide and protein molecules would preclude their transport across the blood-brain barrier (BBB) received experimental support in brain uptake studies performed by means of the intracarotid injection technique in vivo in the rat. However, failure to demonstrate significant differences between brain uptake of peptides, proteins and inert polar molecules could also be attributed to the methodological inadequacies in the initial studies, such as the short exposure-time to the BBB (15 s). A subsequent in vivo approach involved intravenous administration of the test molecule and its presence in the general circulation for several minutes, but may not have been suitable for peptide research due to the possibility of systemic transformation of molecules before reaching the barrier. Recent studies have overcome the above technical problems by employing the vascular brain perfusion method in the guineapig, which permits the measurement of brain uptake over periods of up to 20 min under controlled conditions. The unidirectional transfer constant, Kiv of the radiolabelled peptide can be determined by multiple time-point/graphical analysis and Michaelis-Menten parameters for peptide uptake at the luminal side of the BBB can be estimated. Computer-assisted imaging of the perfused brain may provide further quantitative results regarding the preferential distribution of blood-borne molecules in the brain tissue analyzed by immunocytochemical methods. Participation of circumventricular organs (CVO) and choroid plexuses in overall brain uptake of circulating peptides can be assessed at the same time. Chromatographic analysis of the brain tissue following the perfusion can provide additional information regarding the metabolic turnover rate of a peptide once it has been taken up by the luminal side of the BBB. © 1990.