STUDIES ON THE UPTAKE OF MORPHINE BY THE CHOROID PLEXUS IN VITRO

¹ Department of Pharmacology, University of Minnesota, College of Medical Sciences, Minneapolis, Minnesota

When choroid plexuses from rabbits are incubated aerobically at 37°C with morphine, the tissue takes up morphine by a process which satisfies the usual criteria for active transport. Morphine is taken up against an apparent concentration gradient and high concentrations of morphine can saturate the transport system. The uptake of morphine can be inhibited by low temperature, metabolic inhibitors and compounds with similar structures, such as codeine and levallorphan. Morphine is transported much more readily at alkaline pH and the transport process is dependent on the presence of Ca⁺⁺ and Mg⁺⁺ ions. Morphine is released exponentially from the choroid plexus at an initial fast rate and a secondary slow rate. Only the slower rate is influenced by metabolic inhibitors or levallorphan. Since hexamethonium does not inhibit the uptake of morphine, the active transport system in the choroid plexus for quaternary amines appears to differ from that for morphine, a tertiary amine. Other differences between the two systems are discussed. The implication of this choroidal uptake system in the transport of morphine into or out of the cerebrospinal fluid is also discussed.