M B Segal

King's College London, Londinium, England, United Kingdom

Are you M B Segal?

Claim your profile

Publications (54)149.06 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dysfunction of the choroid plexuses (CPs) and the blood-cerebrospinal fluid barrier (BCSFB) might contribute to age-related cognitive decline and neurodegenerative disease. We used the CPs from young (1-2 years), middle-aged (3-6 years) and old (7-10 years) sheep to explore effects of aging on various aspects of CP and BCSFB functions. Total protein in the cerebrospinal fluid (CSF) was significantly higher in old compared to young sheep and CSF secretion by the CP perfused in situ was significantly lower in both old and middle-aged when compared to young sheep, which correlated with reduced (22)Na(+) uptake and efflux by the CP. Steady-state extractions of a low and medium size molecular weight extracellular space marker, (14)C-mannitol and (3)H-polyethylene glycol, respectively, were significantly higher in CPs from old compared to young animals; however, there was no significant difference in steady-state extraction of a high molecular weight marker, (125)I-bovine serum albumin. This indicates increased passive BCSFB permeability for small and medium sized molecules in old sheep. CP redox activity was significantly lower in the old animals as assessed by the MTT assay, however, there was no significant difference in ATP content and energy charge of the CP with age suggesting adequate baseline energy reserve capacity. These data indicate that normal aging processes alter protein content in the CSF, CSF secretion, integrity of the BCSFB and Na(+) flux in the epithelial layer, which could impact on CSF homeostasis and turnover.
    Experimental gerontology 01/2009; 44(4):289-96. · 3.34 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Sheep choroid plexus epithelium expresses equilibrative nucleoside transporters (ENT) 1 and 2 and concentrative nucleoside transporter 2 at the transcript level. This study aimed to explore the kinetics and functional role of these transporters at the basolateral side of the sheep choroid plexus epithelium perfused in situ. The cellular uptake of [(3)H]adenosine and [(3)H]uridine was insensitive to 1 microm nitrobenzylthioinosine (NBTI), and the uptake of [(3)H]adenosine was reduced significantly when 10 microm NBTI was present in low-Na(+) Ringer solution. This might suggest that ENT2, a transporter sensitive to micromolar NBTI, is functionally active at the basolateral side of the choroid plexus epithelium while ENT1, a transporter sensitive to nanomolar NBTI, is not active. When low-Na(+) Ringer solution was used for the in situ perfusion, the Na(+) concentration in the venous effluent decreased to 14 mm; under these conditions the maximal uptake (U(max)) of [(3)H]adenosine and [(3)H]uridine did not change significantly when compared with the U(max) obtained when Ringer solution that contained 145 mm Na(+) was used. Kinetic analysis revealed apparent Michaelis-Menten constants (K(m,app)) for cellular uptake of [(3)H]adenosine, [(3)H]inosine and [(3)H]thymidine of 1.2 +/- 0.2, 15.7 +/- 2.6 and 3.8 +/- 0.9 microm, respectively. The HPLC and HPLC-fluorometric analysis of the sheep plasma and cerebrospinal fluid revealed nanomolar concentrations of adenosine and thymidine and micromolar levels of inosine and nucleobases. Considering the estimated K(m,app) values, it appears that under normal conditions inosine is the more important nucleoside substrate for uptake by the basolateral membrane of the choroid plexus epithelium than other nucleosides.
    Experimental Physiology 04/2008; 93(3):325-33. · 2.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Hypoxanthine is the main product of purine metabolic degradation and previous studies have revealed that it is present in the sheep CSF and plasma in micromolar concentrations. The aim of this study was to elucidate the transport of this molecule across the sheep choroid plexus epithelium (CPE) as a monolayer in primary culture, to explore the mechanism of uptake by the apical side of the CPE and investigate the metabolic changes inside the cell. The estimated permeability of the CPE monolayer for [14C]hypoxanthine, [14C]adenine and [14C]guanine was low and comparable to the permeability towards the extracellular space markers. The study of [14C]hypoxanthine uptake by the CPE revealed two components: Na+-dependent and Na+-independent, the latter being partially mediated by the equilibrative nucleoside transporter 2. HPLC with simultaneous detection of radioactivity revealed that the majority of [14C]hypoxanthine inside the CPE is metabolised into [14C]nucleotides and [14C]inosine. The remaining intact [14C]hypoxanthine was transported across the opposite, basolateral side of CPE and appeared in the lower chamber buffer together with [14C]inosine. These findings indicate two possible roles of hypoxanthine uptake from the CSF by the CP epithelium in vivo: to provide material for nucleotide synthesis through the salvage pathways in the CPE, as well as to transfer excess hypoxanthine from CSF to blood.
    Neuroscience Letters 02/2008; 431(2):135-40. · 2.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This study investigated the transfer of T4 from cerebrospinal fluid (CSF) into the choroid plexuses (CP) and ventricular brain regions, and the role of P-glycoprotein (P-gp), multidrug resistance protein 1 (mrp1) and organic anion transporting polypeptides (oatps). During in vivo ventriculo-cisternal (V-C) perfusion in the anesthetized rabbit (meditomidine hydrochloride 0.5 mg kg(-1), pentobarbitone 10 mg kg(-1) i.v.), 125I-T4 was perfused continuously into ventricular CSF with reference molecules 14C-mannitol and blue dextran. Over 2 h, 36.9+/-4.6% 125I-T4 was recovered in cisternal CSF. Addition of P-gp substrate verapamil increased CSF 125I-T4 recovery to 51.4+/-2.8%, although mrp1 and oatp substrates had no significant effect. In brain, 125I-T4 showed greatest accumulation in the CP (1.52+/-0.31 ml g(-1)), followed by ventricular regions (caudate putamen, ependyma, hippocampus, 0.05-0.14 ml g(-1)). At the CP, verapamil and probenecid (but not indomethacin) significantly increased 125I-T4 accumulation, implicating a role for P-gp and oatps. Of other brain regions, all three drugs increased accumulation in caudate putamen 3-5 times, and indomethacin and probenecid increased accumulation in ependyma 4-5 times. The role of P-gp was investigated further in isolated incubated CPs using 5 microg/ml C219 anti-P-gp antibody. Both 125I-T4 and 3H-cyclosporin accumulation increased by 80%, suggesting that P-gp is functional in the CP and has a role in removal of T4. Combined with the in vivo results, these studies suggest that P-gp provides a local homeostatic mechanism, maintaining CSF T4 levels. We conclude that P-gp and oatps contribute to the transfer of 125I-T4 between the CSF, CP and brain, hence regulating 125I-T4 availability in CSF.
    Brain Research 12/2007; 1181:44-50. · 2.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The transport of 125I-labeled thyroxine (T4) from the cerebrospinal fluid (CSF) into brain and choroid plexus (CP) was measured in anesthetized rabbit [0.5 mg/kg medetomidine (Domitor) and 10 mg/kg pentobarbitonal sodium (Sagatal) iv] using the ventriculocisternal (V-C) perfusion technique. 125I-labeled T4 contained in artificial CSF was continually perfused into the lateral ventricles for up to 4 h and recovered from the cisterna magna. The %recovery of 125I-labeled T4 from the aCSF was 47.2+/-5.6% (n=10), indicating removal of 125I-labeled T4 from the CSF. The recovery increased to 53.2+/-6.3% (n=4) and 57.8+/-14.8% (n=3), in the presence of 100 and 200 microM unlabeled-T4, respectively (P<0.05), indicating a saturable component to T4 removal from CSF. There was a large accumulation of 125I-labeled T4 in the CP, and this was reduced by 80% in the presence of 200 microM unlabeled T4, showing saturation. In the presence of the thyroid-binding protein transthyretin (TTR), more 125I-labeled T4 was recovered from CSF, indicating that the binding protein acted to retain T4 in CSF. However, 125I-labeled T4 uptake into the ependymal region (ER) of the frontal cortex also increased by 13 times compared with control conditions. Elevation was also seen in the hippocampus (HC) and brain stem. Uptake was significantly inhibited by the presence of endocytosis inhibitors nocodazole and monensin by >50%. These data suggest that the distribution of T4 from CSF into brain and CP is carrier mediated, TTR dependent, and via RME. These results support a role for TTR in the distribution of T4 from CSF into brain sites around the ventricular system, indicating those areas involved in neurogenesis (ER and HC).
    AJP Regulatory Integrative and Comparative Physiology 12/2006; 291(5):R1310-5. · 3.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Efflux transport of adenosine across the choroid plexus (CP) epithelium might contribute to the homeostasis of this neuromodulator in the extracellular fluids of the brain. The aim of this study was to explore adenosine transport across sheep CP epithelial cell monolayers in primary culture. To explore transport of adenosine across the CP epithelium, we have developed a method for primary culture of the sheep choroid plexus epithelial cells (CPEC) on plastic permeable supports and analysed [14C] adenosine transport across this cellular layer, [14C] adenosine metabolism inside the cells, and cellular uptake of [14C] adenosine from either of the chambers. The primary cell culture consisted of an enriched epithelial cell fraction from the sheep fourth ventricle CP and was grown on laminin-precoated filter inserts. CPEC grew as monolayers forming typical polygonal islands, reaching optical confluence on the third day after the seeding. Transepithelial electrical resistance increased over the time after seeding up to 85 +/- 9 Omega cm2 at day 8, while permeability towards [14C] sucrose, a marker of paracellular diffusion, simultaneously decreased. These cells expressed some features typical of the CPEC in situ, including three nucleoside transporters at the transcript level that normally mediate adenosine transport across cellular membranes. The estimated permeability of these monolayers towards [14C] adenosine was low and the same order of magnitude as for the markers of paracellular diffusion.However, inhibition of the intracellular enzymes, adenosine kinase and adenosine deaminase, led to a significant increase in transcellular permeability, indicating that intracellular phosphorylation into nucleotides might be a reason for the low transcellular permeability. HPLC analysis with simultaneous detection of radioactivity revealed that [14C] radioactivity which appeared in the acceptor chamber after the incubation of CPEC monolayers with [14C] adenosine in the donor chamber was mostly present as [14C] hypoxanthine, a product of adenosine metabolic degradation. Therefore, it appears that CPEC in primary cultures act as an enzymatic barrier towards adenosine. Cellular uptake studies revealed that concentrative uptake of [14C] adenosine was confined only to the side of these cells facing the upper or apical chamber, indicating uneven distribution of nucleoside transporters.
    Cerebrospinal Fluid Research 02/2006; 3:4. · 1.81 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Transthyretin (TTR), synthesized by the choroid plexus (CP) and secreted into cerebrospinal fluid (CSF), is involved in thyroxine (T4) transport and chelation of beta-amyloid peptide, attenuating neurotoxicity. To characterize age-related changes in TTR synthesis, CSF and CPs were collected from young adult (1-2 years) and old (>8 years) sheep anesthetized with thiopentone sodium. TTR in old sheep CSF was low compared to young (n = 4 each); however, CP messenger RNA (mRNA) for TTR did not change. CPs were perfused with Ringer containing 14C-leucine to assess de novo protein synthesis, or with 125I-T4 to assess T4 transport. Protein synthesis, including TTR, was reduced in old sheep CP and in newly secreted CSF. 125I-T4 Vmax and Kd (but not Km) were reduced in old sheep CP. These age-related changes suggest reduced capacity of CP to maintain CSF T4 homeostasis and could also reduce chelation of beta-amyloid and be an added risk for Alzheimer's disease.
    The Journals of Gerontology Series A Biological Sciences and Medical Sciences 08/2005; 60(7):852-8. · 4.31 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ammonia may be the major cerebral intoxicant responsible for the increased general or passive permeability of the blood-brain barrier (BBB) leading to the cerebral edema associated with acute liver failure. The present study investigated the effects of ammonia, as NH4+, on Na+ (22Na), K+ (86Rb), and 14C-mannitol uptake in the BBB. An in situ isolated perfused rat brain preparation was used to study the action of 1 mM ammonium acetate in Krebs'-Ringer perfusate. Passive water transport in the brain was studied by 14C-labeled mannitol uptake, a usually nondiffusible marker and active water transport by 22Na and 86Rb uptake. NH4+ significantly reduced 14C-mannitol uptake into the choroid plexus (P < 0.001) and increased it in the CSF (P < 0.05). Decreased 86Rb was measured in whole brain (P < 0.01) and CSF. However, no effect was observed in brain parenchyma, endothelium or choroid plexus thereby suggesting an increased efflux of 86Rb to the interstitial fluid. NH+ increased Na+ uptake into all areas of the brain studied. NH4+ does not increase the passive permeability into the BBB and was decreased in the choroid plexus. The increased 22Na+ uptake was substantiated by the observed decreases in 86Rb uptake in whole brain and CSF. This suggested NH4+ stimulates the Na+/K+ pump and increases extracellular Na+ concentrations and possibly intracellular concentrations with a concomitant decrease in K+ concentrations. These observations may provide a basis for the explanation of NH+ toxicity during hepatic encephalopathy and liver failure-induced cerebral edema.
    Metabolic Brain Disease 04/2005; 20(1):19-33. · 2.33 Impact Factor
  • Zoran B Redzic, Malcolm B Segal
    [Show abstract] [Hide abstract]
    ABSTRACT: The choroid plexuses (CPs) are leaf-like highly vascular structures laying in the ventricles. The main function of choroid plexuses is the production of the cerebrospinal fluid (CSF). Although CPs have a unique distribution of ion transporters/channels, the mechanism of CSF production is similar to the production of fluids in other epithelia and is based on energy released from ATP hydrolysis, which drives unidirectional flux of ions accompanied by movement of water by osmosis. The CPs have an important role in the homeostasis of nutrients in the CSF since the kinetic parameters of glucose and amino acid (AA) transport across the CPs are the main reason for the low concentration of these molecules in the CSF. The CPs appear to be source of CSF-borne hormones and growth factors, including insulin-like growth factor II (IGF II), vasopressin (VP) and transforming growth factor beta1 (TGF-beta1). The CPs also synthesise the thyroid transporting protein transthyretin and transferrin and can chelate heavy metals.
    Advanced Drug Delivery Reviews 11/2004; 56(12):1695-716. · 12.89 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Lead (Pb) exposure hinders brain development in children by mechanisms that remain unknown. Previous evidence shows that sequestration of Pb in the choroid plexus lowers the production and secretion of transthyretin (TTR), a thyroxine (T4) transport protein, from the choroid plexus into the cerebrospinal fluid (CSF). This study was undertaken to characterize the uptake kinetics of T4 by the choroid plexus and to determine if in vivo Pb exposure altered the T4 uptake in an in situ perfused ovine choroid plexus model. Sheep received i.p. injections of Pb acetate (20 mg Pb/kg) or Na acetate (as the controls) every 48 h for a period of 16 d. The [125I]T4 uptake was determined by a paired-tracer perfusion method using 0.5 microCi [125I]T4 and 2 microCi [14C]mannitol at various concentrations of unlabeled T4 (trace to 20 microM). The flux of [125I]T4 into the choroid plexus followed Michaelis-Menten kinetics with the maximum flux (Vmax) of 56.6 nmol/min/g and half-saturation constant (Km) of 10.7 mumol/L, suggesting an evident saturable influx of T4 into the choroid epithelium. In vivo Pb exposure in these sheep resulted in a significant accumulation of Pb in the choroid plexus and hippocampus. Pb treatment diminished the Vmax by 63.7% of control, but did not alter Km. The maximal cellular uptake (Umax) and net uptake (Unet) in Pb-treated animals were 2.1-fold and 1.9-fold, respectively, lower than those of control. Exposure to Pb, however, did not significantly change the flow rate through the choroid plexus. Data suggest that the choroid plexus may serve as a significant site for T4 transport into the CSF, and Pb exposure may hinder the influx of T4 from the blood into the choroid plexus.
    Journal of Toxicology and Environmental Health Part A 04/2003; 66(5):435-51. · 1.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The aim of this study was to analyse the uptake of the synthetic nucleoside tiazofurin and glucoso-linker-tiazofurin conjugate (GLTC) into rat C6 glioma cells in vitro. Results indicated that C6 cells accumulated [3H] tiazofurin slowly with time and that accumulation was reduced by the presence of unlabelled GLTC in the medium which implies that GLTC competes with tiazofurin for transport sites. Uptake of [14C] 2 deoxy-glucose into these cells was very rapid and was not affected by the presence of unlabelled GLTC. To prove the true rate of uptake, the HPLC analysis of cellular extract was performed. After the 360 min of incubation in medium that contained 0.15 mM of tiazofurin, the sum of the concentration of tiazofurin and it's metabolite thiazole-adenine dinucleotide (TAD) in the cells was a total of approximately 4.8% of the amount added to each flask. After the same period of incubation in medium which contained 0.15 mM of GLTC, the sum of concentrations of conjugate, free tiazofurin and TAD represented less than 1/3 of the total concentration measured after the incubation with free tiazofurin and was further reduced in the presence of dipyridamole. Therefore, it can be concluded that GLTC shows some affinity for the nucleoside transporter, but the actual rate of uptake is low.
    Journal of Drug Targeting 01/2003; 10(8):633-6. · 2.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The efflux of purine nucleobases and their nucleosides from the rat brain was investigated using the brain efflux index (BEI) method. Calculated BEI values showed that purine nucleobases had very rapid initial efflux after the intracerebral injection, which was followed by the slower efflux due to the intracellular trapping of labelled molecules and confirmed by the capillary depletion technique. The efflux of ribonucleosides was much slower than the efflux of nucleobases and the structure of the sugar moiety seemed to be important, since a significant difference in the efflux velocity between ribo- and deoxyribonucleosides was observed. The results of self- and cross-inhibition studies suggested that the efflux of test molecules was saturable and that purines shared the same transport system on the abluminal side of the blood-brain barrier.
    Neuroscience Letters 02/2002; 318(2):65-8. · 2.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The uptake of principal salvageable nucleobase hypoxanthine was investigated across the basolateral membrane of the sheep choroid plexus (CP) perfused in situ. The results suggest that hypoxanthine uptake was Na+-independent, which means that transport system on the basolateral membrane can mediate the transport in both directions. Although the unlabelled nucleosides adenosine and inosine markedly reduce the transport it seems that this inhibition was due to nucleoside degradation into nucleobases in the cells, since non-metabolised nucleoside analogue NBTI did not inhibit the transport. The presence of adenine also inhibits hypoxanthine uptake while the addition of the pyrimidines does not show any effect, so it seems that the transport of purine nucleobases through basolateral membrane is mediated via a common transporter which is different from the nucleoside transporters. The inclusion of allopurinol in the perfusion fluid did not change the value and general shape of the curve for the uptake which suggest that degradation of hypoxanthine into xanthine and uric acid does not occur in the CP. The capacity of the CP basolateral membrane to transport hypoxanthine is high (90.63+/-3.79 nM/min/g) and close to the values obtained for some essential amino acids by the CP and blood-brain barrier, while the free diffusion is negligible. The derived value of Km (20.72+/-2.42 microM) is higher than the concentration of hypoxanthine in the sheep plasma (15.61+/-2.28 microM) but less than a half of the concentration in the CSF, which indicates that the transport system at basolateral membrane mostly mediates the efflux of hypoxanthine from the cerebrospinal fluid in vivo.
    Brain Research 02/2002; 925(2):169-75. · 2.88 Impact Factor
  • S A Thomas, A Bye, M B Segal
    [Show abstract] [Hide abstract]
    ABSTRACT: The role of the blood-brain and blood-cerebrospinal fluid (CSF) barriers in the distribution of anti-human immunodeficiency virus (HIV) drugs is integral to the design of effective treatment regimens for HIV infection within the brain. Abacavir (formerly 1592U89) is a nucleoside analog reverse transcriptase inhibitor, which has activity against HIV. The ability of this drug to reach the brain at therapeutic concentrations has been explored by means of an established bilateral in situ brain perfusion model in combination with high-performance liquid chromatography analysis in the anesthetized guinea pig. The influence of other drugs on the entry of abacavir into the brain was also investigated and is of special significance with the use of three of more anti-HIV drugs as the recommended treatment for HIV infection. The results of this study indicate that intact [(14)C]abacavir can cross the blood-brain and blood-CSF barriers and enter the brain and cisternal CSF. Further studies, at a perfusion time of 10 min, revealed that the uptake (R(cerebrum)) of this (14)C-labeled drug (10.1 +/- 0.6%) was not affected by the presence of 0.86 to 200 microM unlabeled abacavir (6.8 microM; 11.0 +/- 1.4%), the nucleoside transport inhibitor [10 microM 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine; 9.7 +/- 3.3%], or a substrate for the nucleobase transporter (100 microM adenine; 12.7 +/- 3.0%). This would suggest that the entry of abacavir into the brain would not be affected by the presence of other anti-HIV drugs. The results of this animal study indicate that abacavir would be a useful addition to a treatment regimen against HIV-infection within the brain.
    Journal of Pharmacology and Experimental Therapeutics 10/2001; 298(3):947-53. · 3.89 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The brain efflux of radiolabelled hypoxanthine in the rat was rapid in the first minute after injection [K(eff)(i)=0.21+/-0.06 min(-1)], which was saturable with a V(max)=13.08+/-0.81 nM min(-1) g(-1), and a high K(m,app) (67.2+/-13.4 microM); the K(i,app) for inosine was 31.5+/-7.6 microM. Capillary depletion analysis indicated that hypoxanthine accumulates in neurons and glia with the time. From cross-inhibition studies with different purines and pyrimidines, it suggests that these molecules could also be important substrates for this carrier.
    Brain Research 05/2001; 899(1-2):248-50. · 2.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Leptin is secreted by adipose tissue and thought to regulate appetite at the central level. Several studies have explored the central nervous system (CNS) entry of this peptide across the blood-brain and blood-cerebrospinal fluid (CSF) barriers in parallel, but this is the first to explore the transport kinetics of leptin across the choroid plexus (blood-CSF barrier) in isolation from the blood-brain barrier (BBB). This is important as the presence of both barriers can lead to ambiguous results from transport studies. The model used was the isolated Ringer perfused sheep choroid plexus. The steady-state extraction of [(125)I]leptin (7.5 pmol l(-1)) at the blood face of the choroid plexus was 21.1+/-5.7%, which was greater than extraction of the extracellular marker, giving a net cellular uptake for [(125)I]leptin (14.0+/-3.7%). In addition, trichloroacetic acid precipitable [(125)I] was detected in newly formed CSF, indicating intact protein transfer across the blood-CSF barrier. Human plasma concentrations of leptin are reported to be 0.5 nM. Experiments using 0.5 nM leptin in the Ringer produced a concentration of leptin in the CSF of 12 pM (similar to that measured in humans). [(125)I]Leptin uptake at the blood-plexus interface using the single-circulation paired tracer dilution technique (uptake in <60 s) indicated the presence of a saturable transport system, which followed Michaelis-Menten-type kinetics (K(m)=16.3+/-1.8 nM, V(max)=41.2+/-1.4 pmol min(-1) g(-1)), and a non-saturable component (K(d)=0.065+/-0.002 ml min(-1) g(-1)). In addition, secretion of new CSF by the choroid plexuses was significantly decreased with leptin present. This study indicates that leptin transport at the blood-CSF barrier is via saturable and non-saturable mechanisms and that the choroid plexus is involved in the regulation of leptin availability to the brain.
    Brain Research 03/2001; 895(1-2):283-90. · 2.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The uptake of nucleobases was investigated across the basolateral membrane of the sheep choroid plexus perfused in situ. The maximal uptake (U(max)) for hypoxanthine and adenine, was 35.51+/-1.50% and 30.71+/-0.49% and for guanine, thymine and uracil was 12.00+/-0.53%, 13.07+/-0.48% and 12.30+/-0.55%, respectively with a negligible backflux, except for that of thymine (35.11+/-5.37% of the U(max)). HPLC analysis revealed that the purine nucleobase hypoxanthine and the pyrimidine nucleobase thymine can pass intact through the choroid plexus and enter the cerebrospinal fluid CSF so the lack of backflux for hypoxanthine was not a result of metabolic trapping in the cell. Competition studies revealed that hypoxanthine, adenine and thymine shared the same transport system, while guanine and uracil were transported by a separate mechanism and that nucleosides can partially share the same transporter. HPLC analysis of sheep CSF collected in vivo revealed only two nucleobases were present adenine and hypoxanthine; with an R(CSF/Plasma) 0.19+/-0.02 and 3.43+/-0.20, respectively. Xanthine and urate, the final products of purine catabolism, could not be detected in the CSF even in trace amounts. These results suggest that the activity of xanthine oxidase in the brain of the sheep is very low so the metabolic degradation of purines is carried out only as far as hypoxanthine which then accumulates in the CSF. In conclusion, the presence of saturable transport systems for nucleobases at the basolateral membrane of the choroidal epithelium was demonstrated, which could be important for the distribution of the salvageable nucleobases, adenine and hypoxanthine in the central nervous system.
    Brain Research 02/2001; 888(1):66-74. · 2.88 Impact Factor
  • M B Segal
    [Show abstract] [Hide abstract]
    ABSTRACT: A brief outline is given first of the early history of the ventricles and the strange ideas of their functions from Galen to the enlightenment of the Renaissance with the work of Versalius. This is followed by a description of the histology of the choroid plexuses (CP) and discussion on the functions of the choroid plexus and on the composition of cerebrospinal fluid (CSF). The methods of measuring the rate of secretion of CSF will be outlined and the possible nutritive functions of the choroid plexuses will be considered. The role of the choroid plexuses in the control of the concentration of glucose and amino acids in CSF will be compared with data from in vitro experiments to that from the isolated vascularly perfused choroid plexuses. The handling of peptides and proteins by the CP and the synthesis of these molecules by this tissue is then discussed and the effects of lead on the synthesis of transthyretin by this tissue. Finally, reference will be made to the extensive neuro-endocrine role of the CP and efflux systems across the tissue for lipid soluble molecules.
    Microscopy Research and Technique 02/2001; 52(1):38-48. · 1.59 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: (S)-2-ethyl-7-fluoro-3-oxo-3, 4-dihydro-2H-quinoxaline-carboxylic acid isopropylester (GW420867X) inhibits HIV-1 reverse transcriptase and could be used for the treatment of HIV infection. This study quantified the movement of [14C]GW420867X into the CNS by means of a guinea-pig brain perfusion technique. Results indicated that [14C]GW420867X can enter the brain (Kin: 38.4 ± 7.7 μl min-1 g-1) and cerebrospinal fluid (CSF; Kin: 1.2 ± 0.1 μl min-1 g-1). Self-inhibition studies also suggested the presence of a saturable transport system for [14C]GW420867X at the blood-brain barrier (BBB). Thus [14C]GW420867X can enter the brain via the BBB and, compared with the blood-CSF barrier, this route is the predominant pathway for the brain entry of this drug. This would suggest that GW420867X is a promising drug for the treatment of HIV infection within the brain.
    Neuroreport 11/2000; 11(17):3811-3815. · 1.40 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The integrity of the blood-brain barrier (BBB) was measured in male Sprague Dawley rats subjected to 16 weeks of portacaval shunting (PCS), the optimal time required for the cerebral changes to develop, by using an in situ brain perfusion technique. The penetration of a vascular space marker 14C mannitol, and labelled amino acids 3H-phenylalanine or 3H-glutamate were measured in brain and cerebrospinal fluid (CSF) using an in situ brain perfusion technique, over 2 or 20 minutes. The patency of the surgical shunt was confirmed by measurement of significantly increased plasma ammonia (131.5 +/- 14.8 micromol x l(-1)) and AST (159.5 +/- 19.9 IU x l(-1)) concentrations compared to controls 39.9 +/- 3.7*, and 82.5 +/- 6.6* respectively. Brain and CSF 14C-mannitol space (ml x 100g(-1)), was not increased by PCS where brain space was 1.31 +/- 0.27 mL x 100g(-1) compared to control 1.19 +/- 0.49 mL x 100g(-1), and CSF was 0.14 +/- 0.06 mL x 100g(-1) compared to control 0.15 +/- 0.05 (PCS n=10, control n=8). The uptake for 3H-glutamate, which is required for cerebral ammonia detoxification, was also unchanged in both brain and CSF. However, brain uptake of 3H-phenylalanine was significantly reduced from 871 +/- 80 microL x min(-1) x g(-1) to 356 +/- 154* microl x min(-1) x g(-1) (n=4), although there was no change in CSF uptake. These data suggest that there is no generalized breakdown of the blood-brain or blood-CSF barriers during PCS as assessed by mannitol penetration. The reduction in phenylalanine uptake into the brain may help stabilize high cerebral aromatic amino acid levels. *P<0.05, Two-tailed, Student's unpaired t-test.
    Metabolic Brain Disease 07/2000; 15(2):93-103. · 2.33 Impact Factor

Publication Stats

718 Citations
149.06 Total Impact Points

Institutions

  • 2000–2009
    • King's College London
      • • School of Biomedical Sciences
      • • Wolfson Centre for Age-Related Diseases
      • • Institute of Gerontology
      • • Department of Physiology
      • • Academic Department of Surgery
      Londinium, England, United Kingdom
  • 2003
    • Columbia University
      • Department of Environmental Health Sciences
      New York City, NY, United States
  • 2001
    • The Kings College
      Thomas, Oklahoma, United States
  • 1997
    • University of London
      Londinium, England, United Kingdom
  • 1991
    • Children's Hospital Los Angeles
      • Division of Neurosurgery
      Los Angeles, California, United States
  • 1989
    • University of Belgrade
      • Institute of Pathological Physiology
      Belgrade, SE, Serbia