The steady-state amino acid fluxes across the perfused choroid plexus of the sheep

Causal association between 731 immunocyte phenotypes and liver cirrhosis: A bidirectional two-sample mendelian randomization analysis

BACKGROUND

Liver cirrhosis is a progressive hepatic disease whose immunological basis has attracted increasing attention. However, it remains unclear whether a concrete causal association exists between immunocyte phenotypes and liver cirrhosis.

AIM

To explore the concrete causal relationships between immunocyte phenotypes and liver cirrhosis through a mendelian randomization (MR) study.

METHODS

Data on 731 immunocyte phenotypes were obtained from genome-wide association studies. Liver cirrhosis data were derived from the Finn Gen dataset, which included 214403 individuals of European ancestry. We used inverse variable weighting as the primary analysis method to assess the causal relationship. Sensitivity analyses were conducted to evaluate heterogeneity and horizontal pleiotropy.

RESULTS

The MR analysis demonstrated that 11 immune cell phenotypes have a positive association with liver cirrhosis [P < 0.05, odds ratio (OR) > 1] and that 9 immunocyte phenotypes were negatively correlated with liver cirrhosis (P < 0.05, OR < 1). Liver cirrhosis was positively linked to 9 immune cell phenotypes (P < 0.05, OR > 1) and negatively linked to 10 immune cell phenotypes (P < 0.05; OR < 1). None of these associations showed heterogeneity or horizontally pleiotropy (P > 0.05).

CONCLUSION

This bidirectional two-sample MR study demonstrated a concrete causal association between immunocyte phenotypes and liver cirrhosis. These findings offer new directions for the treatment of liver cirrhosis.

Is the Brain Undernourished in Alzheimer's Disease?

Cerebrospinal fluid (CSF) amino acid (AA) levels and CSF/plasma AA ratios in Alzheimer Disease (AD) in relation to nutritional state are not known. Methods: In 30 fasting patients with AD (46% males, 74.4 ± 8.2 years; 3.4 ± 3.2 years from diagnosis) and nine control (CTRL) matched subjects, CSF and venous blood samples were drawn for AA measurements. Patients were stratified according to nutritional state (Mini Nutritional Assessment, MNA, scores). Results: Total CSF/plasma AA ratios were lower in the AD subpopulations than in NON-AD (p < 0.003 to 0.017. In combined malnourished (16.7%; MNA < 17) and at risk for malnutrition (36.6%, MNA 17−24) groups (CG), compared to CTRL, all essential amino acids (EAAs) and 30% of non-EAAs were lower (p < 0.018 to 0.0001), whereas in normo-nourished ADs (46.7%, MNA > 24) the CSF levels of 10% of EAAs and 25% of NON-EAAs were decreased (p < 0.05 to 0.00021). CG compared to normo-nourished ADs, had lower CSF aspartic acid, glutamic acid and Branched-Chain AA levels (all, p < 0.05 to 0.003). CSF/plasma AA ratios were <1 in NON-AD but even lower in the AD population. Conclusions: Compared to CTRL, ADs had decreased CSF AA Levels and CSF/plasma AA ratios, the degree of which depended on nutritional state.

Targeting the Choroid Plexuses for Protein Drug Delivery

Delivery of therapeutic agents to the central nervous system is challenged by the barriers in place to regulate brain homeostasis. This is especially true for protein therapeutics. Targeting the barrier formed by the choroid plexuses at the interfaces of the systemic circulation and ventricular system may be a surrogate brain delivery strategy to circumvent the blood-brain barrier. Heterogenous cell populations located at the choroid plexuses provide diverse functions in regulating the exchange of material within the ventricular space. Receptor-mediated transcytosis may be a promising mechanism to deliver protein therapeutics across the tight junctions formed by choroid plexus epithelial cells. However, cerebrospinal fluid flow and other barriers formed by ependymal cells and perivascular spaces should also be considered for evaluation of protein therapeutic disposition. Various preclinical methods have been applied to delineate protein transport across the choroid plexuses, including imaging strategies, ventriculocisternal perfusions, and primary choroid plexus epithelial cell models. When used in combination with simultaneous measures of cerebrospinal fluid dynamics, they can yield important insight into pharmacokinetic properties within the brain. This review aims to provide an overview of the choroid plexuses and ventricular system to address their function as a barrier to pharmaceutical interventions and relevance for central nervous system drug delivery of protein therapeutics. Protein therapeutics targeting the ventricular system may provide new approaches in treating central nervous system diseases.

The legacy of Malcolm Beverley Segal (1937-2019) on the science and fields concerned with choroid plexus and cerebrospinal fluid physiology

This article highlights the scientific achievements, professional career, and personal interactions of Malcolm B. Segal who passed away in July this year. Born in 1937 in Goodmayes, Essex, UK, Segal rose to the Chairman position in the Division of Physiology at United Medical and Dental School of Guy’s and St. Thomas’ Hospitals, retiring in 2006 after his long professional career in biomedical science. Being trained in Hugh Davson’s laboratory, Segal became one of the pioneers in research on cerebrospinal fluid physiology and the choroid plexus. During the course of his career, Segal himself trained a number of young scientists and collaborated with many colleagues around the world, making long-lasting friendships along the way. In addition to his professional accomplishments as a researcher and educator, Segal was an avid sailor and wine connoisseur, and enjoyed teaching classes on navigation and wine tasting.

Thyroxine (T4) Transfer from Blood to Cerebrospinal Fluid in Sheep Isolated Perfused Choroid Plexus: Role of Multidrug Resistance-Associated Proteins and Organic Anion Transporting Polypeptides

Thyroxine (T₄) enters the brain either directly across the blood–brain barrier (BBB) or indirectly via the choroid plexus (CP), which forms the blood–cerebrospinal fluid barrier (B-CSF-B). In this study, using isolated perfused CP of the sheep by single-circulation paired tracer and steady-state techniques, T4 transport mechanisms from blood into lateral ventricle CP has been characterized as the first step in the transfer across the B-CSF-B. After removal of sheep brain, the CPs were perfused with ¹²⁵I-T₄ and ¹⁴C-mannitol. Unlabeled T₄ was applied during single tracer technique to assess the mode of maximum uptake (U_max) and the net uptake (U_net) on the blood side of the CP. On the other hand, in order to characterize T₄ protein transporters, steady-state extraction of ¹²⁵I-T₄ was measured in presence of different inhibitors such as probenecid, verapamil, BCH, or indomethacin. Increasing the concentration of unlabeled-T₄ resulted in a significant reduction in U_max%, which was reflected by a complete inhibition of T₄ uptake into CP. In fact, the obtained U_net% decreased as the concentration of unlabeled-T₄ increased. The addition of probenecid caused a significant inhibition of T₄ transport, in comparison to control, reflecting the presence of a carrier mediated process at the basolateral side of the CP and the involvement of multidrug resistance-associated proteins (MRPs: MRP1 and MRP4) and organic anion transporting polypeptides (Oatp1, Oatp2, and Oatp14). Moreover, verapamil, the P-glycoprotein (P-gp) substrate, resulted in ~34% decrease in the net extraction of T₄, indicating that MDR1 contributes to T₄ entry into CSF. Finally, inhibition in the net extraction of T₄ caused by BCH or indomethacin suggests, respectively, a role for amino acid “L” system and MRP1/Oatp1 in mediating T₄ transfer. The presence of a carrier-mediated transport mechanism for cellular uptake on the basolateral membrane of the CP, mainly P-gp and Oatp2, would account for the efficient T₄ transport from blood to CSF. The current study highlights a carrier-mediated transport mechanism for T4 movement from blood to brain at the basolateral side of B-CSF-B/CP, as an alternative route to BBB.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

The influence of ageing in the cerebrospinal fluid concentrations of proteins that are derived from the choroid plexus, brain, and plasma

Studies have shown that ageing alone can cause increases in the concentrations of many cerebrospinal fluid (CSF) proteins. Therefore, CSF protein concentrations must be interpreted with caution before concluding that the increased concentrations of certain proteins can be used as disease-specific biomarkers. Age-related reduction in CSF turnover has been shown to have a significant concentrating effect on CSF proteins from young to old. As a result, CSF protein concentrations need to be corrected with age-specific turnovers first before performing any data comparisons between different ages. This study applied the concept of CSF/plasma concentration ratios of plasma-derived proteins that is frequently used in the investigation of brain barrier integrity to calculate the amount of protein that enters the CSF from the plasma side in different age groups. Based on our calculations, proteins with molecular weights greater than 91.92 kDa for the young, 109.51 kDa for the middle-aged and 120 kDa for the old should not be able to cross the brain barriers of the blood-brain and blood-CSF barriers to enter the CSF from the plasma side. For proteins that can be derived from the choroid plexus (CP), brain, and plasma, the amount that crosses the barriers to enter the CSF from the plasma side will contribute to their measured total protein concentrations in the CSF. CP and brain production of these proteins can be calculated when turnover corrected CSF protein concentrations are further corrected by the amount of protein that crosses the barriers. In this study, CP and brain produced concentrations of transthyretin, retinol binding protein, alpha-1-antitrypsin, gelsolin, and lactotransferrin were calculated. The production of these proteins decreased with age with alpha-1-antitrypsin protein revealing the most substantial decrease of 86% from young (0.14±0.01 mg·dL(-1)) to old (0.02 mg·dL(-1)). In conclusion, measured CSF protein concentrations for proteins that can be derived from the CP, brain, and plasma need to be corrected by age-specific CSF turnovers and by the amount of protein that crosses the brain barriers first before their concentrations can be compared logically between different ages.

Elevation of CSF albumin in old sheep: relations to CSF turnover and albumin extraction at blood-CSF barrier

Albumin is the most abundant protein in both CSF and plasma, and albumin quotient is often used to assess the functions of brain barriers especially that of the blood-CSF barrier [i.e. the choroid plexus (CP) which also secretes CSF]. In this study, we took albumin as a model molecule to investigate ageing-related alterations in the CSF-CP system in sheep. We found significant ageing-related increases in the weight of lateral CP [122.4 +/- 14.0 mg in the young, 198.6 +/- 35.4 mg in the middle aged, 286.1 +/- 25.1 mg in the old (p < 0.05)], in the CSF albumin as well as the albumin quotient. Albumin protein spots in old CSF displayed wider on 2D western immunoblotting images, and had higher densities on images of 2D large gels stained with Pro-Q Emerald 488 compared to the young samples, suggesting ageing-related post-translational modification in the albumin. CSF secretion was reduced with age: 0.148 +/- 0.013 mL/min/g in the young, 0.092 +/- 0.02 mL/min/g in the middle aged, 0.070 +/- 0.013 mL/min/g in the old (p < 0.05). The (125)I-BSA extraction was not different among the sheep groups, nor was altered by temperature reduction, monensin, nocodazole, anti-transforming growth factor beta receptor II antibody, as well as unlabelled albumins. In conclusion, elevation of albumin in old CSF is associated with reduced CSF secretion by the CP, which size increases with age. (125)I-BSA extract, reflecting the extracellular space rather than the active albumin uptake in the CP, is not different between ages. These early changes in health ageing may result in the accumulation and modifications of CSF proteins leading to neurotoxicity.

Changes in kinetics of amino acid uptake at the ageing ovine blood-cerebrospinal fluid barrier

Amino acids (AA) in brain are precisely controlled by blood-brain barriers, which undergo a host of changes in both morphology and function during ageing. The effect of these age-related changes on AA homeostasis in brain is not well described. This study investigated the kinetics of four AA (Leu, Phe, Ala and Lys) uptakes at young and old ovine choroid plexus (CP), the blood-cerebrospinal fluid (CSF) barrier (BCB), and measured AA concentrations in CSF and plasma samples. In old sheep, the weight of lateral CP increased, so did the ratio of CP/brain. The expansion of the CP is consistent with clinical observation of thicker leptomeninges in old age. AA concentrations in old CSF, plasma and their ratio were different from the young. Both V(max) and K(m) of Phe and Lys were significant higher compared to the young, indicating higher trans-stimulation in old BCB. Cross-competition and kinetic inhibition studies found the sensitivity and specificity of these transporters were impaired in old BCB. These changes may be the first signs of a compromised barrier system in ageing brain leading increased AA influx into the brain causing neurotoxicity.

Decrease of Transthyretin Synthesis at the Blood-Cerebrospinal Fluid Barrier of Old Sheep

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 Choroid Plexus‐Cerebrospinal Fluid System: From Development to Aging

The function of the cerebrospinal fluid (CSF) and the tissue that secretes it, the choroid plexus (CP), has traditionally been thought of as both providing physical protection to the brain through buoyancy and facilitating the removal of brain metabolites through the bulk drainage of CSF. More recent studies suggest, however, that the CP-CSF system plays a much more active role in the development, homeostasis, and repair of the central nervous system (CNS). The highly specialized choroidal tissue synthesizes trophic and angiogenic factors, chemorepellents, and carrier proteins, and is strategically positioned within the ventricular cavities to supply the CNS with these biologically active substances. Through polarized transport systems and receptor-mediated transcytosis across the choroidal epithelium, the CP, a part of the blood-CSF barrier (BCSFB), controls the entry of nutrients, such as amino acids and nucleosides, and peptide hormones, such as leptin and prolactin, from the periphery into the brain. The CP also plays an important role in the clearance of toxins and drugs. During CNS development, CP-derived growth factors, such as members of the transforming growth factor-beta superfamily and retinoic acid, play an important role in controlling the patterning of neuronal differentiation in various brain regions. In the adult CNS, the CP appears to be critically involved in neuronal repair processes and the restoration of the brain microenvironment after traumatic and ischemic brain injury. Furthermore, recent studies suggest that the CP acts as a nursery for neuronal and astrocytic progenitor cells. The advancement of our knowledge of the neuroprotective capabilities of the CP may therefore facilitate the development of novel therapies for ischemic stroke and traumatic brain injury. In the later stages of life, the CP-CSF axis shows a decline in all aspects of its function, including CSF secretion and protein synthesis, which may in themselves increase the risk for development of late-life diseases, such as normal pressure hydrocephalus and Alzheimer's disease. The understanding of the mechanisms that underlie the dysfunction of the CP-CSF system in the elderly may help discover the treatments needed to reverse the negative effects of aging that lead to global CNS failure.

Blood-mediated scavenging of cerebrospinal fluid glutamate

The maintenance of brain extracellular glutamate (Glu) at levels below its excitotoxic threshold is performed by Glu transporters present on glia and neurons as well as on brain capillary endothelial cells which remove brain Glu into blood. The feasibility of accelerating the naturally occurring brain-to-blood Glu efflux was studied using paradigms based on the fate of Glu present in the cerebrospinal fluid or infused into the brain ventricles and monitored before, during, and after decreasing blood Glu levels with pyruvate and oxaloacetate, the respective Glu co-substrates of the blood resident enzymes glutamate-pyruvate transaminase and glutamate-oxaloacetate transaminase. Results from cerebroventricular perfusions with [3H]Glu, intracerebroventricular injections of [3H]Glu, and measurements of the basal CSF Glu levels point out to the same conclusion that the intravenous administration of pyruvate and oxaloacetate which decreases blood Glu levels accelerates the brain-to-blood Glu efflux. We conclude that the brain extracellular Glu levels can be controlled in part by the blood Glu levels. The results may provide not only a rational explanation for the inhibition of Glu release and neuroprotective effects of parentally administered pyruvate in hemorrhagic shock and forebrain ischemia but could also outline a potential strategy for the removal of excess Glu in various neurodegenerative disorders.

Transport of L-[125I]thyroxine by in situ perfused ovine choroid plexus: inhibition by lead exposure

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.

Leptin transport at the blood--cerebrospinal fluid barrier using the perfused sheep choroid plexus model

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.

Transport of nutrients across the choroid plexus

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.

Theoretical pharmacokinetic and pharmacodynamic simulations of drug delivery mediated by blood--brain barrier transporters

Pharmacokinetic/pharmacodynamic simulations were performed to assess the feasibility of central nervous system (CNS) drug delivery via endogenous transporters resident at the blood-brain barrier (BBB). Pharmacokinetic models were derived for intravenous bolus dosing of a hypothetical drug in the absence or presence of an endogenous, competing transport inhibitor. These models were linked to CNS pharmacodynamic models where the effect sites were either cell surface receptors or intracellularly localized enzymes. The response of the dependent parameter, the duration of effect (t(dur)), was examined in relationship to changes in the independent parameters, i.e. dose, elimination rate constant (k(e1)), BBB transport parameters (K(m1) and V(max1)) and EC(50) (effective concentration that elicits a 50% response). As expected, t(dur) increased with (a) increases in drug doses, (b) decreases in k(e1) or (c) decreases in EC(50), irrespective of the effect site. Surprisingly, endogenous transport inhibition produced decreases in drug terminal half-life and corresponding decreases in t(dur). Interestingly, t(dur) was independent of assigned transporter K(m) and V(max) when the dose/EC(50) ratio (dose/EC(50)) was >1 (irrespective of endogenous transport inhibition), but highly dependent on K(m1) and V(max1) when dose/EC(50) was (a) <1 (no endogenous transport inhibition) or (b) equal to 1 (with endogenous transport inhibition). Oral input of the endogenous transport inhibitor produced a decrease in t(dur) when the dose/EC(50) range was 0.1-10. These simulations highlight that (a) systemic pharmacokinetic and BBB transport parameters influence t(dur), (b) drug terminal half-life is inversely related to circulating levels of endogenous inhibitors, and (c) oral ingestion of endogenous transport inhibitor(s) reduces t(dur). Overall, these simulations provide insight for the feasibility of rational CNS drug design/delivery via endogenous transporters.

The choroid plexuses and the barriers between the blood and the cerebrospinal fluid

1. The fluid homeostasis of the brain depends both on the endothelial blood-brain barrier and on the epithelial blood-cerebrospinal fluid (CSF) barrier located at the choroid plexuses and the outer arachnoid membrane. 2. The brain has two fluid environments: the brain interstitial fluid, which surrounds the neurons and glia, and the CSF, which fills the ventricles and external surfaces of the central nervous system. 3. CSF acts as a fluid cushion for the brain and as a drainage route for the waste products of cerebral metabolism. 4. Recent findings suggest that CSF may also act as a "third circulation" conveying substances secreted into the CSF rapidly to many brain regions.

Cerebrospinal fluid formation and absorption in dehydrated sheep

Cerebrospinal fluid (CSF) plays an important role in the brain's adaptive response to acute osmotic disturbances. In the present experiments, the effect of 48-h dehydration on CSF formation and absorption rates was studied in conscious adult sheep. Animals had cannulas chronically implanted into the lateral cerebral ventricles and cisterna magna to enable the ventriculocisternal perfusion. A 48-h water deprivation altered neither CSF production nor resistance to CSF absorption. However, in the water-depleted sheep, intraventricular pressure tended to be lower than that found under control conditions. This likely resulted from decreased extracellular fluid volume and a subsequent drop in central venous pressure occurring in dehydrated animals. In conclusion, our findings provide evidence for the maintenance of CSF production during mild dehydration, which may play a role in the regulation of fluid balance in the brain during chronic hyperosmotic stress.

The characteristics of basolateral nucleoside transport in the perfused sheep choroid plexus and the effect of nitric oxide inhibition on these processes

The single pass paired dilution technique was used to measure the uptake of nucleosides across the basolateral face of the isolated in situ perfused sheep choroid plexus (CP). The uptake of labelled adenosine and guanosine into the CP was large (approximately 35%) whereas that of thymidine was less (approximately 15%). The addition of 0.5 mM unlabelled adenosine to the perfusate inhibited the uptake of labelled adenosine by 66%, guanosine by 100% and that of thymidine by 50%, whereas the addition of 0.5 mM unlabelled thymidine caused complete self-inhibition. The backflux of adenosine was very small which may indicate a high rate of cellular metabolism or a flux into cerebrospinal fluid (CSF). The addition of 0.5 mM unlabelled adenosine did not alter the backflux of adenosine, but increased that of guanosine and thymidine. The entry of radioactivity derived from adenosine across the apical side of the CP cells into the newly formed CSF was determined as a 'CSF uptake index' relative to [14C]butanol and found to be about 25%; however, HPLC analysis revealed that the majority of this activity was hypoxanthine, and not adenosine. The complete inhibition of nitric oxide synthase caused a significant reduction in adenosine uptake into the CP and an increase in backflux for this molecule. It would appear that the uptake for adenosine by the CP is governed by the rate of cellular metabolism and not by the rate of transport into the cells of the choroid plexus whereas for guanosine and thymidine, transport is of greater importance.

Acidic amino acid accumulation by rat choroid plexus during development

Acidic amino acid accumulation by the choroid plexuses of the lateral ventricles was investigated using 1, 2, 3 week and adult (7-10 weeks old) rats. The accumulation from both blood and CSF sides of the choroid plexuses were investigated. The uptake from blood side was studied using the bilateral in situ brain perfusion, and time-dependent uptake profiles (2, 10, 20, and 30 min) of 14C-labelled aspartate, glutamate, and NMDA were measured. [3H]Mannitol was also included in perfusion fluid as a baseline for [14C]amino acid uptake into choroidal tissue. Uptake of [14C]aspartate and [14C]glutamate declined with age, while [14C]NMDA showed no significant uptake at any age. Twenty min [3H]mannitol uptake in the 1-week-old rat was significantly greater than the adult (P < 0.05). The K(m) for [14C]aspartate and [14C]glutamate obtained from multiple time uptake profiles also showed reduction with development but it was greater than that for mannitol. [14C]Aspartate declined from 69.8 +/- 21.1 microliters.min-1.g-1 in the neonate to 40.6 +/- 4.0 microliters.min-1.g-1 in the adult (P < 0.05), while glutamate showed a sharper decline from 78.9 +/- 24.2 microliters.min-1.g-1 to 17.7 +/- 5.4 microliters.min-1.g-1 (P < 0.01). Accumulation of 14C-labelled aspartate and glutamate by the choroid plexus from CSF side was also measured using ventriculo-cisternal perfusion. The accumulation in the adult was found to be 2-3 times greater than that in the neonatal rat (P < 0.05) for both amino acids. The uptake from either side was found to be saturable, stereospecific, not inhibited by neutral amino acid analogues, and shared by both aspartate and glutamate.

Changes in amino acid levels in rat plasma, cisternal cerebrospinal fluid, and brain tissue induced by intravenously infused arginine-vasopressin

Circulating arginine-vasopressin (AVP) is known to reduce the blood-to-brain transfer of large neutral amino acids (AA). As a first step to examine whether the reduced uptake by brain endothelial cells is reflected in changes in large neutral amino acid levels of the extracellular fluid environment of cells within the nervous tissue, we measured the concentrations of amino acids in plasma, cerebrospinal fluid (CSF), and hippocampal tissue of rats before and after infusion of AVP (34 and 68 ng/min/kg, respectively) over the time period of 60 min. AA levels changed in all compartments investigated during both saline and AVP infusions. Whereas in the saline-infused controls changes in CSF AA levels paralleled those in plasma, this correlation was abolished by raising AVP concentrations. The effect of AVP was found to be i) dependent on the AA, ii) different with respect to direction and iii) magnitude of changes in AA levels, and iv) in some cases dose dependent. In summary, AVP infusion increased plasma levels of 10 AA, but decreased all 15 AA measured by some 30% in CSF. In contrast to CSF, levels of AA were slightly enhanced in the hippocampal tissue. The results are not solely explicable by a reduced blood-to-brain transfer of AA. We conclude that further mechanisms by which AVP affects the availability of AA to the brain may exist. The physiological significance of the findings might be related to brain osmoregulation, especially in situations of stress.

Characterization of a sodium-dependent taurine transporter in rabbit choroid plexus

Taurine, a beta-amino acid, plays an important role as a neuromodulator and is necessary for the normal development of the brain. Since de novo synthesis of taurine in the brain is minimal and in vivo studies suggest that taurine does not cross the blood-brain barrier, we examined whether the choroid plexus, the blood-cerebrospinal fluid barrier, plays a role in taurine transport in the central nervous system. The uptake of [3H]taurine into ATP-depleted choroid plexus from rabbit was substantially greater in the presence of an inwardly directed Na+ gradient, whereas in the absence of a Na+ gradient taurine accumulation was negligible. A transient inside-negative potential gradient enhanced the Na(+)-driven uptake of taurine into the tissue slices, suggesting that the transport process is electrogenic. Na(+)-driven taurine uptake was saturable with an estimated Vmax of 111 +/- 20.2 nmol/g per 15 min and a Km of 99.8 +/- 29.9 microM. The estimated coupling ratio of Na+ and taurine was 1.80 +/- 0.122. Na(+)-dependent taurine uptake was significantly inhibited by beta-amino acids, but not by alpha-amino acids, indicating that the transporter is selective for beta-amino acids. Na(+)-dependent taurine uptake showed some selectivity for anions: the accumulation was comparable in the presence of Cl-, Br- and thiocyanate whereas I-, SO4(2-) and gluconate did not stimulate the uptake significantly. Collectively, our results demonstrate that taurine is transported in the choroid plexus via a Na(+)-dependent, saturable and apparently beta-amino acid selective mechanism. This process may be functionally relevant to taurine homeostasis in the brain.

The transport of L-6-fluorodopa and its metabolites from blood to cerebrospinal fluid and brain

The transport of L-6-fluorodopa and its major metabolites from the blood to the brain, cerebrospinal fluid (CSF), and muscle was studied in carbidopa-pretreated cynomolgus monkeys. A bolus intravenous injection of 18F-L-6-fluorodopa was followed by serial positron emission tomography scans and sampling of cisternal CSF and arterial blood. The relative concentrations of L-6-fluorodopa and its metabolites were determined in blood plasma and CSF by high-performance liquid chromatography. Raising the blood concentration of phenylalanine by intraperitoneal injection markedly reduced the accumulation of tracer in the brain. This indicates that L-6-fluorodopa and 3-O-methylfluorodopa, like native L-dopa and its O-methylated derivative, are transported at the brain capillary by the large neutral amino acid carrier-mediated system, which is subject to saturation and competition by other large neutral amino acids (such as phenylalanine) at physiological plasma concentrations. In contrast, administration of phenylalanine had no effect on the accumulation of tracer either in muscle, or as L-6-fluorodopa and 3-O-methylfluorodopa, in CSF. This suggests that the transport of L-dopa and its derivatives at the blood-CSF barrier differs from the transport at the blood-brain barrier and also that measurement of CSF L-dopa is not a good index of the transport and pharmacokinetics of L-dopa in the brain. However, the effect of phenylalanine administration in reducing the concentration of fluorohomovanillic acid in the CSF suggests that the concentration of homovanillic acid in the CSF is an accurate reflection of dopamine turnover in the brain.

Extracellular and cerebrospinal fluids

The mechanism of formation of extracellular fluid is first described, followed by an explanation of the relation between osmotic force, reflection coefficient and molecular size. The possible mechanism of brain extracellular fluid formation is then proposed in relation to the restriction offered by the blood-brain barrier. The functions and compositions of cerebrospinal fluid (CSF) are then described followed by sections on the process of formation of CSF, the non-electrolytes and proteins in CSF, the drainage mechanisms and protein synthesis by the choroid plexus.

Saturable uptake of [125I]L-triiodothyronine at the basolateral (blood) and apical (cerebrospinal fluid) sides of the isolated perfused sheep choroid plexus

The uptake of 125I-labelled L-triiodothyronine (T3) was measured on the blood side of the isolated perfused choroid plexus of the sheep using steady-state and single-circulation paired tracer techniques. The steady-state uptake of T3 was 33.5% (perfusion fluid protein content was 0.05 g.dl-1) which could be reduced to 9.4% in the presence of 500 microM unlabelled T3 showing partial saturation. The CSF to blood steady-state [125I]T3 measurements gave plasma/CSF ratio, R%, of 24.6 +/- 4.8% which was reduced to 9.8 +/- 2.1% in the presence of 500 microM unlabelled T3 in the mock CSF. The transport of T3 across the blood face of the choroid plexus and the CSF to blood transport, failed to show sodium dependence. Using the single circulation paired tracer technique, the initial uptake in less than 60 s, Umax of [125I]T3 was 50.4 +/- 3.9% relative to the extracellular marker [3H]D-mannitol. However, when 250 microM unlabelled T3 was present, Umax was reduced by 66%, although further significant inhibition at higher concentrations was not observed. Uptake of T3 at the blood side of the choroid plexus was partially saturated in the presence of unlabelled reverse T3 and DT3, suggesting little uptake stereospecificity. Unlabelled thyroxine (T4) and the amino acid analogues cycloleucine (aminocyclopentane-1-carboxylic acid) and BCH (beta-2-aminobicyclo-[2,2.1]-heptane-2-carboxylic acid) each reduced [125I]T3 uptake significantly, but not to the same degree as T3 stereoisomers. The neutral amino acids alanine and phenylalanine, had no effect on uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

The uptake of anionic and cationic amino acids by the isolated perfused sheep choroid plexus

The carrier-mediated uptake of anionic and cationic amino acids by the basolateral face (blood side) of the isolated perfused sheep choroid plexus was demonstrated using the paired-tracer dilution technique. The uptake of these two classes of amino acid was higher than at the blood-brain barrier with no cross-competition between them. In addition the uptake was markedly stereospecific with a high degree of selectivity and no interaction with the L-system amino acids.