Confocal imaging of xenobiotic transport across the choroid plexus

Confocal microscopy is a tool by which the distribution of fluorescent compounds within living, complex tissues can be mapped at submicrometer resolution and quantitated. This laboratory has used confocal imaging and quantitative image analysis to visualize transport of xenobiotics across intact rat and mouse choroid plexus. For both organic anions and organic cations, transport from CSF to blood is a three-step process involving: uptake at the apical membrane of the epithelial cells, transcellular transport and efflux at the basolateral membrane. Both transmembrane steps are carrier-mediated and concentrative. In the cytoplasm of the epithelial cells, all fluorescent xenobiotics studied partition between diffuse and punctate compartments, some of which appear to be mobile. Use of confocal imaging in combination with transport inhibitors, treatments that alter metabolism and ion gradients and tissue from genetically altered mice, has allowed us to characterize transport at specific membrane sites and begin to identify the responsible transporters at the molecular level.     

Interplay of conjugating enzymes with OATP uptake transporters and ABCC/MRP efflux pumps in the elimination of drugs

BACKGROUND: Biliary excretion is a major elimination route of many drugs and their metabolites. Hepatobiliary elimination is a vectorial process involving uptake transporters in the basolateral hepatocyte membrane, possibly Phase I and Phase II metabolizing enzymes, and ATP-dependent efflux pumps in the apical hepatocyte membrane. OBJECTIVES: Because many drugs and their metabolites are anions, this review focuses on transporters involved in their hepatocellular uptake (members of the organic anion transporting polypeptide (OATP) family) and biliary elimination (apical conjugate efflux pump ABCC2/MRP2). METHODS: The molecular and functional characteristics of the human OATP and ABCC/MRP transporters are presented, including a detailed overview of endogenous and drug substrates. Examples illustrate the interplay of transporters with Phase II conjugating enzymes. Model systems to study the vectorial transport of organic anions are also discussed. RESULTS/CONCLUSIONS: OATP uptake transporters, conjugating enzymes, and ABCC2/MRP2 work in concert to enable the hepatobiliary elimination of anionic drugs and their metabolites. It is increasingly important to understand how genetic variants of these transporters and enzymes influence the interindividual variability of drug elimination.     

Analysis of xenobiotic detoxification system mediated by efflux transporters

The excretion of drugs mediated by transporters plays an important role in the detoxification of xenobiotics. In this article, I will summarize recent progress we have made in this field, particularly focusing on the roles of transporters responsible for exporting drugs. As far as the biliary excretion of xenobiotics is concerned, it has been suggested that canalicular multispecific organic anion transporter/multidrug resistance associated protein 2 (cMOAT/MRP2) is involved in the ATP-dependent export of organic anions across the bile canalicular membrane. By comparing the transport across this membrane between normal rats and Eisai hyperbilirubinemic rats whose cMOAT/MRP2 function is hereditarily defective, we were able to demonstrate the substrate specificity of cMOAT/MRP2. This includes non-conjugated anionic drugs, and glutathione- and glucuronide-conjugates of xenobiotics. The role of cMOAT/MRP2 in drug disposition has also been clarified. Moreover, the cDNA of cMOAT/MRP2 has been cloned and its functional analysis has been completed. Thus, it may be possible to predict in vivo transport across the bile canalicular membrane from in vitro data using the recombinant transporter. We also cloned MRP3 as an inducible transporter in the liver under the cholestatic conditions. Although MRP3 mediates the cellular export of non-conjugated organic anions and glucuronide-conjugates, the substrate specificity of MRP3 is different from that of cMOAT/MRP2 in that glutathione-conjugates are poor substrates for MRP3. It is possible that MRP3 plays an important role under certain pathological conditions in the liver. Since it has been shown that cMOAT/MRP2 and MRP 3 are expressed in the small intestine under physiological conditions, it seems reasonable that these transporters are responsible for the previously reported cellular extrusion of organic anions. We also found that there was MRP activity in the blood-brain and blood-cerebrospinal fluid barriers. RT-PCR resulted in the amplification of MRP1, 5 and 6 from freshly isolated rat cerebral endothelial cells. It has been suggested that there is basolateral localization of MRP1 in the choroid plexus. In conjunction with the P-glycoprotein located on the luminal membrane of cerebral endothelial cells, these transporters play significant roles in restricting the entry of xenobiotics from the circulating blood into the central nervous system. Regulation of the activity of these efflux transporters allows the disposition of drugs to be altered.     

Peptide and peptide analog transport systems at the blood-CSF barrier

In addition to being the main source of cerebrospinal fluid (CSF) secretion, the choroid plexuses are involved in the supply and distribution of peptides to brain, the removal of toxic metabolites, the excretion of xenobiotics, and the delivery of drugs as an alternative route to the blood-brain barrier (BBB). The discovery of proton-coupled oligopeptide transporters in choroid plexus has generated considerable interest regarding their physiologic role at the blood-cerebrospinal fluid interface and their potential for peptide/antagonist pharmacotherapy in the central nervous system. Many of the same factors that affect the disposition of naturally occurring peptides in brain will also affect the disposition of exogenously delivered peptide or peptidomimetic drugs. Therefore, this review addresses three main areas: (1) choroid plexus structure, physiology, and barrier function in relation to peptide transport; (2) polypeptide transport and secretion mechanisms into cerebrospinal fluid; and (3) molecular physiology, expression, and functional activity of proton-coupled oligopeptide transporters in choroid plexus.     

In-vivo and in-vitro evidence of a carrier-mediated efflux transport system for oestrone-3-sulphate across the blood-cerebrospinal fluid barrier

The efflux transport of oestrone-3-sulphate, a steroid hormone sulphate, across the blood-cerebrospinal fluid barrier has been examined following its intracerebroventricular administration. [3H]Oestrone-3-sulphate was eliminated from cerebrospinal fluid (CSF) with an apparent efflux clearance of 205 microL min(-1) per rat. There was 25% of unmetabolized [3H]oestrone-3-sulphate in the plasma 5 min after intracerebroventricular administration, indicating that at least a part of [3H]oestrone-3-sulphate is transported from CSF to the circulating blood across the blood-CSF barrier. This efflux transport was inhibited by co-administration of excess oestrone-3-sulphate (25 mM 10 microL = 0.25 micromol) into rat cerebral ventricle. To characterize the oestrone-3-sulphate transport process, an in-vitro uptake experiment was performed using isolated rat choroid plexus. Oestrone-3-sulphate uptake by isolated rat choroid plexus was found to be a saturable process with a Michaelis-Menten constant (Km) of 18.1 +/- 6.3 microM, and a maximum uptake rate (Vmax) of 48.0 +/- 15.1 pmol min(-1) microL(-1) of tissue. The oestrone-3-sulphate transport process was temperature dependent and was inhibited by metabolic inhibitors such as 2,4-dinitrophenol and rotenone, suggesting an energy dependence. This uptake process was also inhibited by steroid hormone sulphates (1 mM dehydroepiandrosterone sulphate and 1 mM oestrone sulphate), bile acids (1 mM taurocholic acid and 1 mM cholic acid) and organic anions (1 mM sulphobromophthalein and 1 mM phenolsulphonphthalein), whereas 1 mM p-aminohippuric acid, 1 mM p-nitrophenol sulphate, 0.1 mM methotrexate and the cardiac glycoside, 2.5 microM digoxin, had little effect. In conclusion, these results provide evidence that oestrone-3-sulphate is transported from CSF to the circulating blood across the blood-CSF barrier via a carrier-mediated efflux transport system.     

Effects of 2,4,5-trichlorophenoxyacetic acid and quinolinic acid on 5-hydroxy-3-indoleacetic acid transport by the rabbit choroid plexus: Pharmacology and electron microscopic cytochemistry

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) reduced the uptake of 5-hydroxy-3-indoleacetic acid (5-HIAA) by the choroid plexus in a dose-related manner, while treatment with quinolinic acid at comparable concentrations did not inhibit 5-HIAA uptake. The role of carrier-mediated transport in the clearance of 5-HIAA from cerebrospinal fluid (CSF) was also evaluated in vivo by ventriculocisternal perfusion. Steady-state clearance of 5-HIAA from CSF exceeded that of inulin and was reduced competitively in the presence of 2,4,5-T. However, the clearance was not affected by quinolinic acid. The effect of 2,4,5-T on transport enzyme systems was also studied by electron microscopic cytochemistry. Na+-K+-ATPase and cytochrome oxidase activities in the choroid plexus were reduced by 2,4,5-T. Since this transport system in the choroid plexus is normally responsible for the excretion of the serotonin metabolite from the brain to the plasma, accumulation of endogenously produced organic acids in the CSF and the brain, secondary to reduced clearance by the choroid plexus, could be a contributing factor in the development of neurotoxicity.     

Expression of tight junction proteins and transporters for xenobiotic metabolism at the blood–CSF barrier during development in the nonhuman primate (P. hamadryas)

The choroid plexus (CP) is rich in barrier mechanisms including transporters and enzymes which can influence drug disposition between blood and brain. We have limited knowledge of their state in fetus. We have studied barrier mechanisms along with metabolism and transporters influencing xenobiotics, using RNAseq and protein analysis, in the CP during the second-half of gestation in a nonhuman primate (Papio hamadryas). There were no differences in the expression of the tight-junctions at the CP suggesting a well-formed fetal blood–CSF barrier during this period of gestation. Further, the fetal CP express many enzymes for phase I–III metabolisms as well as transporters suggesting that it can greatly influence drug disposition and has a significant machinery to deactivate reactive molecules with only minor gestational changes. In summary, the study suggests that from, at least, midgestation, the CP in the nonhuman primate is restrictive and express most known genes associated with barrier function and transport.     

Characterization of renal excretion mechanism for a novel diuretic, M17055, in rats

M17055 was developed as a novel diuretic that inhibits both Na(+), K(+), and 2Cl(-) cotransport at the thick ascending Henle's loop and Na(+) reuptake at the distal tubule. It is secreted at the renal proximal tubules. The purpose of the present study was to characterize the renal excretion mechanism of M17055. We used the renal cortical slices and brush border membrane vesicles (BBMVs) to investigate the transport mechanisms across the basolateral and brush border membranes, respectively. M17055 uptake by rat renal slices increased with time and was saturable. Several organic anions including probenecid, para-aminohippurate (PAH), and estrone-3-sulfate, decreased M17055 uptake. The uptake of M17055 was also observed into HEK293 cells expressing rat OAT1, and was inhibited by PAH. M17055 uptake by BBMVs was time-dependent, saturable, osmolarity-sensitive, and inhibited by several organic anions, but not by PAH. These results suggest that plural organic anion transport systems are involved in M17055 transport via both basolateral and brush border membranes of proximal tubule epithelial cells, a part of the renal uptake being mediated by OAT1.     

Expression and functional characterization of rat organic anion transporter 3 (rOat3) in the choroid plexus

We reported previously that an efficient efflux system for benzylpenicillin (PCG) is located on the choroid plexus (CP). In this study, we investigated the involvement of rat organic anion transporter 1 (rOat1; Slc22a6) and rOat3 (Slc22a8) in the uptake of PCG and p-aminohippurate (PAH) by the CP. Western blot analysis indicates the expression of rOat3, but not rOat1, on the CP, and immunohistochemical staining shows that rOat3 is localized on the brush border membrane of the choroid epithelial cells. PCG and PAH were found to be taken up by isolated rat CP, with K(m) values of 111 and 354 microM, respectively. A mutual inhibition study suggests that the same transporter is responsible for the uptake of PCG and PAH by isolated rat CP. This was confirmed by examining the effect of organic anions and cimetidine on their uptake. Estradiol-17beta-glucuronide and cimetidine were found to be selective inhibitors of rOat3. The inhibition constants of the inhibitors including estradiol-17beta-glucuronide and cimetidine were comparable for the uptake of PAH and PCG by isolated rat CP. In addition, these values were also comparable with those for rOat3, but not with those for rOat1. These results suggest that rOat3 is mainly responsible for the uptake of PCG and PAH by isolated rat CP, and it functions as one of the detoxification systems on the CP by removing its substrates from the cerebrospinal fluid.     

Modulatory effects of hormones,drugs, and toxic events on renal organic anion transport

The human body is exposed continuously to a wide variety of exogenous compounds, many of which are anionic compounds. In addition, products of phase II biotransformation reactions are negatively charged, viz. glucuronides, sulfate esters, or glutathiones. Renal transport of organic anions is an important defense mechanism of the organism against foreign substances. The combination of the rate of uptake and efflux and the intracellular disposition of organic anions in the proximal tubule determines the intracellular concentration and the nephrotoxic potential of a compound. Modulation of organic anion secretion is observed after exposure of proximal tubules to various hormones, and the subsequent receptor-mediated response is signaled by protein kinases. Transport of anionic compounds across the basolateral as well as the luminal membrane is modified by activation or inhibition of protein kinases. Protein kinase C activation reduces the uptake of organic anions mediated by the organic anion transporter 1 (OAT1/Oat1) and Oat3 and reduces Mrp2-mediated efflux. In addition, activation of protein kinase C has been shown to inhibit transport by the organic anion transporting polypeptide 1 (Oatp1) across the luminal membrane. Additional protein kinases have been implicated in the regulation of organic anion transport, and the role of nuclear factors in xenobiotic excretion is an emerging field. The physiological regulation of organic anion transporters may also be influenced by exogenous factors, such as exposure to xenobiotics and cellular stress. This commentary discusses the current knowledge of endogenous and exogenous influences on renal anionic xenobiotic excretion.     

Usefulness and limitation of primary cultured porcine choroid plexus epithelial cells as an in vitro model to study drug transport at the blood-CSF barrier

The epithelial cells of the choroid plexus form the anatomical structure responsible for the blood-cerebrospinal fluid (CSF) barrier. Here we present our recent progress in the application of porcine choroid plexus epithelial cells in the investigation of permeability and transport properties of this tissue in vitro. Isolated cells are seeded on permeable supports where they grow to confluent monolayers. The cell differentiation is significantly increased under serum-free culture conditions, verifiable by an improvement of barrier properties and enhanced characteristics of the epithelial phenotype. We underline the importance of a tight model system for the investigation of transport processes by showing that permeability and transport properties critically depend on the electrical tightness of the monolayer. The mechanisms of vectorial transfer of micronutrients across the epithelial layer have been investigated in detail for ascorbic acid and myo-inositol transport. Additionally, we describe the transfer of organic anions and the expression of the corresponding transport proteins in vitro. The model system was applied to determine permeation rates of various drugs into the CSF. In conclusion our porcine in vitro model of the blood-CSF barrier provides a reliable system to study the transport characteristics of the choroid plexus epithelium and to probe the passage of drugs.     

Drug uptake systems in liver and kidney

The hepatobiliary system and the kidneys are the main routes by which drugs and their metabolites leave the body. Compounds that are mainly excreted into bile in general have relatively high molecular weights, are amphipathic and highly bound to plasma proteins. In contrast, compounds that are predominantly excreted into urine have relatively low molecular weights, are more hydrophilic and generally less protein bound. The first step in drug elimination in liver and kidney is uptake into hepatocytes or into proximal tubular cells. The substrate specificity and affinity of the uptake carriers expressed at the basolateral membranes of hepatocytes and proximal tubular cells could therefore play an important role for the determination of the main elimination route of a compound. This review discusses the tissue distribution, substrate specificity, transport mechanism, and regulation of the members of the organic anion transporting polypeptide (Oatp/OATP) superfamily (solute carrier family SLC21A) and the SLC22A family containing transporters for organic cations (OCTs) and organic anions (OATs). The Oatps/OATPs are mainly important for the hepatic uptake of large amphipathic organic anions, organic cations and uncharged substrates, whereas OCTs and OATs mediate uptake of predominantly small organic cations and anions in liver and kidney.     

Constitutive expression of various xenobiotic and endobiotic transporter mRNAs in the choroid plexus of rats.

The aim of this study was to quantitatively determine the constitutive expression levels of various transporter mRNAs in rat choroid plexus. To provide a reference for the relative expression levels, the expression of various transporter mRNAs in choroid plexus were compared with that in liver, kidney, and ileum. The mRNA levels of multidrug resistance protein (Mrp)1, 2, 3, 4, 5, and 6; multidrug resistance (Mdr)1a, 1b, and 2; organic anion transporting polypeptide (Oatp)1, 2, 3, 4, 5, 9, 12, and Oat-K (1/2); organic anion transporter (Oat)1, 2, and 3; organic cation transporter (Oct)1, 2, 3, N1, and N2; bile acid transporters sodium taurocholate cotransporting polypeptide (Ntcp), bile salt excretory protein (Bsep), and ileal bile acid transporter (Ibat); divalent metal transporter 1 (DMT1), Menke's and Wilson's metal transporters; equilibrative nucleotide transporters (Ent) 1 and 2, and constitutive nucleotide transporters (Cnt)1 and 2; peptide transporters (Pept)1 and 2; as well as ATP-binding cassette (Abc)G5 and 8 were measured in choroid plexus by the branched DNA signal amplification method. Mrp1, 4, and 5, Oatp3, Menke's transporter, DMT1, Ent1, and Pept2 mRNAs were expressed in choroid plexus at higher levels than in liver, kidney, or ileum. OctN1 and N2, Oatp2, Oat2 and 3, and Cnt1 and 2 mRNAs expressions were detectable in choroid plexus, but the levels were lower compared with that in liver, kidney, or ileum. The remaining transporters [Mrp2, Mrp3, Oct1, Oct2, Oatp1, Oatp4, Oatp5, Oatp12, Oat-K (1/2), Ntcp, Bsep, Ibat, Mdr1a, Mdr1b, Mdr2, Oat1, Ent2, Pept1, AbcG5, AbcG8] were expressed at very low levels in choroid plexus. The constitutive expression levels of different transporters in choroid plexus may provide an insight into the range of xenobiotics that can potentially be transported by the choroid plexus, thereby providing a means of xenobiotic detoxification in the brain.     

Carrier-mediated uptake of H2-receptor antagonists by the rat choroid plexus: involvement of rat organic anion transporter 3.

The choroid plexus (CP) acts as a site for the elimination of xenobiotic organic compounds from the cerebrospinal fluid (CSF). The purpose of the present study is to investigate the role of rat organic anion transporter 3 (rOat3; Slc22a8) in the uptake of H(2)-receptor antagonists (cimetidine, ranitidine, and famotidine) by the isolated rat CP. Saturable uptake of cimetidine and ranitidine was observed in rOat3-LLC with K(m) values of 80 and 120 microM, respectively, whereas famotidine was found to be a poor substrate. The steady-state concentration of the H(2)-receptor antagonists in the CSF was significantly increased by simultaneously administered probenecid, although it did not affect their brain and plasma concentrations. Saturable uptake of cimetidine and ranitidine was observed in the isolated rat CP with K(m) values of 93 and 170 microM, respectively, whereas 50% of the uptake of famotidine remained at the highest concentration examined (1 mM). The K(i) value of ranitidine for the uptake of cimetidine by the isolated CP (50 microM) was similar to its own K(m) value, suggesting that they share the same transporter for their uptake. The inhibition potency of organic anions such as benzylpenicillin, estradiol 17beta-glucuronide, p-aminohippurate, and estrone sulfate for the uptake of cimetidine by the isolated rat CP was similar to that for benzylpenicillin, the uptake of which has been hypothesized to be mediated by rOat3, whereas a minimal effect by tetraethylammonium excludes involvement of organic cation transporter(s). These results suggest that rOat3 is the most likely candidate transporter involved in regulating the CSF concentration of H(2)-receptor antagonists at the CP.     

Hepatobiliary disposition in primary cultures of dog and monkey hepatocytes

Hepatobiliary transporters are a major route for elimination of xenobiotics and endogenous products. In vitro hepatobiliary models have been reported for human and rat, but not for the other preclinical species used in safety evaluation. We have established methodologies for culturing dog and monkey hepatocytes with optimal bile canalicular formation and function, using a sandwich culture comprising rigid collagen substratum and gelled collagen overlay. Hepatic uptake utilizing sinusoidal transporters and biliary excretion through canalicular transporters were assessed using the bile salt taurocholate, salicylate (negative control), and the Bsep inhibitors cyclosporin A (CsA) and glyburide. There was significant taurocholate and salicylate canalicular efflux in dog and monkey hepatocytes, although the amount of salicylate transported was one thousandth that of taurocholate. Species differences were observed, as glyburide significantly inhibited taurocholate uptake in monkey (64% at 10 microM) but not dog hepatocytes, and inhibited taurocholate efflux in dog (100% at 10 microM) but not monkey hepatocytes. CsA did not inhibit bile salt uptake and significantly inhibited canalicular efflux in dog (at 0.1 microM) and monkey (at 1 and 10 microM) hepatocyte cultures. These results suggest that glyburide is a bile salt uptake inhibitor in monkey but not in dog hepatocytes and that CsA inhibits bile salt canalicular efflux but not basolateral uptake in these species. We have established dog and monkey hepatocytes in sandwich culture with intact bile canalicular formation and function. The differences observed in taurocholate transport between dog and monkey hepatocytes may be indicative of in vivo species differences.     

A new in vitro model for blood-cerebrospinal fluid barrier transport studies: an immortalized choroid plexus epithelial cell line derived from the tsA58 SV40 large T-antigen gene transgenic rat

The blood-cerebrospinal fluid barrier (BCSFB) plays a key role in the influx and efflux transport of drugs and endogenous substrates in the cerebrospinal fluid (CSF). To clarify the molecular mechanism of the BCSFB transport system, a new in vitro BCSFB model, i.e. an immortalized rat choroid plexus epithelial cell line (TR-CSFB), has been established from transgenic rats harboring a temperature-sensitive simian virus 40 large T-antigen gene. TR-CSFB cells grow well at 33 degrees C because of activation of the temperature-sensitive large T-antigen. These cells have a polygonal epithelial cell morphology and express typical choroid plexus epithelial cell markers, such as transthyretin (TTR) and Na+, K+ -ATPase, as well as the transporters, system A and ABCC1/mrp1. The localization of Na+, K+ -ATPase, and the transport direction of system A are polarized in TR-CSFB cells as is the case in vivo. TR-CSFB cells exhibit L-proline and L-glutamic acid uptake activities and may reflect the CSF-to-blood efflux transport functions involving these amino acids in vivo. Using TR-CSFB cells, we found for the first time that oatp3 is expressed at the BCSFB. TR-CSFB cells appear to be a useful in vitro model of the BCSFB for the study of drug transport, BCSFB transporters, and the regulation of BCSFB functions.     

Vectorial transport of fexofenadine across Caco-2 cells: involvement of apical uptake and basolateral efflux transporters

Fexofenadine is a nonsedative antihistamine that exhibits good oral bioavailability despite its zwitterionic chemical structure and efflux by P-gp. Evidence exists that multiple uptake and efflux transporters play a role in hepatic disposition of fexofenadine. However, the roles of specific transporters and their interrelationship in intestinal absorption of this drug are unclear. This study was designed to elucidate vectorial absorptive transport of fexofenadine across Caco-2 cells involving specific apical uptake and efflux transporters as well as basolateral efflux transporters. Studies with cellular models expressing single transporters showed that OATP2B1 expression stimulated uptake of fexofenadine at pH 6.0. Apical uptake of fexofenadine into Caco-2 cells was decreased by 45% by pretreatment with estrone 3-sulfate, an OATP inhibitor, at pH 6.0 but not at pH 7.4, indicating that OATP2B1 mediates apical uptake of fexofenadine into these cells. Examination of fexofenadine efflux from preloaded Caco-2 cells in the presence or absence of (i) the MRP inhibitor MK-571 and (ii) the P-gp inhibitor GW918 showed that apical efflux is predominantly mediated by P-gp, with a small contribution by MRP2, whereas basolateral efflux is predominantly mediated by MRP3. These results also showed that while OSTαβ is functionally active in the basolateral membrane of Caco-2 cells, it does not play a role in the export of fexofenadine. MK-571 decreased the absorptive transport of fexofenadine by 17%. However, the decrease in absorptive transport by MK-571 was 42% when P-gp was inhibited by GW918. The results provide a novel insight into a vectorial transport system mainly consisting of apical OATP2B1 and basolateral MRP3 that may play an important role in delivering hydrophilic anionic and zwitterionic drugs such as pravastatin and fexofenadine into systemic circulation upon oral administration.     

Mechanism of the reduced elimination clearance of benzylpenicillin from cerebrospinal fluid in rats with intracisternal administration of lipopolysaccharide.

The mechanism responsible for the reduced clearance of benzylpenicillin (BPC) from the cerebrospinal fluid (CSF) was investigated in rats that received an intracisternal administration of lipopolysaccharide (LPS). BPC was intraventricularly injected and its elimination from the CSF studied. During the inflammation created by the LPS administration to the cisterna magna, the clearance of BPC and taurine from the CSF was significantly reduced but reverted to the control level when N-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor, was intracisternally administered. The in vitro uptake of BPC and taurine was significantly reduced in the choroid plexus (CP, the blood-CSF barrier) of rats with experimental inflammation and in control CP that had been pretreated with sodium nitroprusside (SNP, an NO donor). Interestingly, the clearance and CP uptake of formycin B, a substrate for a nucleoside transporter, were not affected by the experimental inflammation or by pretreatement with SNP. These observations suggest that the BPC transporter, and probably other transport systems as well, is functionally sensitive to NO in the blood-CSF barrier. Therefore, functional impairment of BPC transport in the CP by NO may be partly responsible for the increase in BPC concentration in the CSF during inflammation such as that caused by meningitis.