Functional feature of a novel model of blood brain barrier: studies on permeation of test compounds.
Drug delivery to the central nervous system (CNS) is subject to the permeability limitations imposed by the blood-brain barrier (BBB). Several systems in vitro have been described to reproduce the physical and biochemical behavior of intact BBB, most of which lack the feature of the in vivo barrier. We developed a fully formed monolayer of RBE4.B immortalized rat brain microvessel endothelial cells (ECs), grown on top of polycarbonate filter inserts with cortical neuronal cells grown on the outside. Neurons induce ECs to synthesize and sort occludin to the cell periphery. Occludin localization is regulated by both compositions of the substratum and soluble signals released by cortical co-cultured neurons. The observed effects do not require strict physical contact among cells and neurons. To assess the physiological function of the barrier we examined the transendothelial transfer of three test compounds: dopamine, L-tryptophan and L-DOPA. Polycarbonate filter inserts, where ECs were co-cultured with neurons, were assumed as open two compartments vertical dynamic models. Permeation studies demonstrated that the ECs/neurons co-cultures possess permeability characteristics approaching those of a functional BBB: the system behaved as a selective interface that excludes dopamine permeation, yet permits L-tryptophan and L-DOPA to cross. The movement of test compounds from the donor to the acceptor compartment was observed at a distinct time from the start of co-culture. Transfer was determined using standard kinetic equations. Different performance was observed after 5 and 7 days of co-culture. After 5 days dopamine, L-tryptophan and L-DOPA passively permeate through the membrane as indicated by fittings with a first-order kinetic process equation. After 7 days of co-culture, occludin localizes at ECs periphery, dopamine does not cross the barrier to any further extent, while the transfer of L-tryptophan and L-DOPA fits well with a saturable Michaelis-Menten kinetic process, thus indicating the involvement of a specific carrier-mediated transport mechanism. Permeation studies confirmed that culture of ECs in the presence of neurons induces the characteristic permeability limitations of a functional BBB.