SPECIFIC BINDING OF [H-3] IMIPRAMINE INDICATES THE PRESENCE OF A SPECIFIC SEROTONIN TRANSPORT-SYSTEM ON ENDOTHELIAL-CELLS OF PORCINE BRAIN

The blood-brain barrier (BBB) represents a complex epithelial interface in vertebrates that separates the blood compartment from the extracellular fluid compartment of the brain, Isolated microvessels are a tool to study the function of this interface in vitro. Here we report on attempts to demonstrate the presence of the serotonin transporter on microvessels from the porcine brain. For comparison, membrane preparations of brain tissue were used. The enrichment of the microvessel fraction determined by measurement of alkaline phosphatase activity was about 30-fold. In saturation experiments high- and low-affinity binding of [H-3]imipramine could be demonstrated on brain microvessels. Different concentrations of unlabelled imipramine were used to inhibit the binding of [H-3]imipramine in brain tissue and microvessels. Comparison of both preparations revealed a two-fold higher density of the high-affinity binding site, while the density of the low-affinity binding site was 28-fold higher in brain microvessels. Imipramine binding could be inhibited by potent non-tricyclic inhibitors of the serotonin transporter such as paroxetine and fluoxetine but also by the tricyclic antidepressant drugs clomipramine and desipramine. Therefore, it is concluded that [H-3]imipramine labels serotonin uptake sites localized on porcine brain microvessels.