Ventriculo-cisternal perfusion of twelve amino acids in the rabbit

Age dependent contribution of entry via the CSF to the overall brain entry of small and large hydrophilic markers

Background

Apparent permeability of the blood brain barrier to hydrophilic markers has been shown to be higher in the developing brain. Apart from synthesis in situ, any substance detected in the brain parenchyma can originate from two sources: directly through blood vessels of brain vasculature and/or indirectly by entry from the cerebrospinal fluid (CSF) after transfer across the choroid plexuses. The relative quantitative contribution of these two routes to the overall brain entry remains unclear.

Methods

In rats at embryonic day 16, 19 and postnatal day 4 and young adults, a small (sucrose, mw. 342 Da) or a large (dextran, mw. 70 kDa) radiolabelled hydrophilic marker was injected intravenously for very short periods of time (30 s to 5 min) before collection of plasma, cerebrospinal fluid (CSF) and brain samples. Results are presented as concentration ratios between radioactivity measured in CSF or brain and that in plasma (%).

Results

The dextran brain/plasma ratio five minutes post injection was similar (2–4%) from E16 to adulthood whereas the sucrose brain/plasma ratio was significantly higher in fetal brains, but was comparable to dextran values in the adult. Sucrose CSF/plasma ratios were also significantly higher in fetal animals and decreased with age. In very short experiments involving fetal animals, entry of sucrose into the CSF after only 30 s was similar to that of dextran and both markers showed similar brain/plasma ratios.

Conclusions

In the developing brain the apparent higher brain entry of a small hydrophilic marker such as sucrose can be attributed to its higher entry into the CSF and subsequent diffusion into the brain. By contrast, movement of a larger marker like 70 kDa dextran is restricted firstly by choroid plexus epithelial tight junctions and secondly by specialised junctions in the neuroependymal interface between the CSF and brain. Brain/plasma ratios of 70 kDa dextran were similar in fetal and adult rats. Therefore 70 kDa dextran should be considered an appropriate marker if brain residual vascular space is to be measured, especially in younger animals.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Blood glutamate scavenging: insight into neuroprotection

Brain insults are characterized by a multitude of complex processes, of which glutamate release plays a major role. Deleterious excess of glutamate in the brain's extracellular fluids stimulates glutamate receptors, which in turn lead to cell swelling, apoptosis, and neuronal death. These exacerbate neurological outcome. Approaches aimed at antagonizing the astrocytic and glial glutamate receptors have failed to demonstrate clinical benefit. Alternatively, eliminating excess glutamate from brain interstitial fluids by making use of the naturally occurring brain-to-blood glutamate efflux has been shown to be effective in various animal studies. This is facilitated by gradient driven transport across brain capillary endothelial glutamate transporters. Blood glutamate scavengers enhance this naturally occurring mechanism by reducing the blood glutamate concentration, thus increasing the rate at which excess glutamate is cleared. Blood glutamate scavenging is achieved by several mechanisms including: catalyzation of the enzymatic process involved in glutamate metabolism, redistribution of glutamate into tissue, and acute stress response. Regardless of the mechanism involved, decreased blood glutamate concentration is associated with improved neurological outcome. This review focuses on the physiological, mechanistic and clinical roles of blood glutamate scavenging, particularly in the context of acute and chronic CNS injury. We discuss the details of brain-to-blood glutamate efflux, auto-regulation mechanisms of blood glutamate, natural and exogenous blood glutamate scavenging systems, and redistribution of glutamate. We then propose different applied methodologies to reduce blood and brain glutamate concentrations and discuss the neuroprotective role of blood glutamate scavenging.

Dynamics of cerebrospinal fluid flow in slit ventricle syndrome

INTRODUCTION

Although slit ventricle syndrome (SVS) is identified as a serious complication in shunt-treated hydrocephalus, cerebral spinal fluid (CSF) flow via external ventricular drainage (EVD) or shunts in SVS have not been studied up to now.

MATERIAL AND METHODS

A new apparatus (LiquoGuard(®); Möller-Medical, Fulda, Germany) was used for EVD in a child with SVS. The LiquoGuard actively controls CSF drainage, based on intracranial pressure (ICP).

RESULTS

To achieve well-tolerated clinical conditions, an ICP level of 4 mmHg was necessary; realizable by drainage rates between 0 and 35 mL/h. Drainage rate variations typically occurred with repetitive time intervals of 2 h causing a "saw tooth" shaped CSF flow pattern throughout 24 h.

DISCUSSION

SVS seems to be characterized largely by quickly varying CSF drainage demands. Whether this is a general phenomenon or just true for this case has still to be studied and needs further clarification.

Mechanisms of glutamate efflux at the blood-brain barrier: involvement of glial cells

At high concentrations, glutamate (Glu) exerts potent neurotoxic properties, leading to irreversible brain damages found in numerous neurological disorders. The accepted notion that Glu homeostasis in brain interstitial fluid is maintained primarily through the activity of Glu transporters present on glial cells does not take into account the possible contribution of endothelial cells constituting the blood-brain barrier (BBB) to this process. Here, we present evidence for the presence of the Glu transporters, excitatory amino-acid transporters (EAATs) 1 to 3, in porcine brain endothelial cells (PBECs) and show their participation in Glu uptake into PBECs. Moreover, transport of Glu across three in vitro models of the BBB is investigated for the first time, and evidence for Glu transport across the BBB in both directions is presented. Our results provide evidence that the BBB can function in the efflux mode to selectively remove Glu, via specific transporters, from the abluminal side (brain) into the luminal compartment (blood). Furthermore, we found that glial cells lining the BBB have an active role in the efflux process by taking up Glu and releasing it, through hemichannels, anion channels, and possibly the reversal of its EAATs, in close proximity to ECs, which in turn take up Glu and release it to the blood.

The blood-brain barrier in Alzheimer's disease: novel therapeutic targets and nanodrug delivery

Treatment strategies for Alzheimer's disease (AD) are still elusive. Thus, new strategies are needed to understand the pathogenesis of AD in order to provide suitable therapeutic measures. Available evidences suggest that in AD, passage across the blood-brain barrier (BBB) and transport exchanges for amyloid-β-peptide (ABP) between blood and the central nervous system (CNS) compartments play an important regulatory role for the deposition of brain ABP. New evidences suggest that BBB is altered in AD. Studies favoring transport theory clearly show that ABP putative receptors at the BBB control the level of soluble isoform of ABP in brain. This is achieved by regulating influx of circulating ABP into brain via specific receptor for advanced glycation end products (RAGE) and gp330/megalin-mediated transcytosis. On the other hand, the efflux of brain-derived ABP into the circulation across the vascular system via BBB is accomplished by low-density receptor-related protein-1 (LRP1). Furthermore, an increased BBB permeability in AD is also likely since structural damage of endothelial cells is quite frequent in AD brain. Thus, enhanced drug delivery in AD is needed to induce neuroprotection and therapeutic success. For this purpose, nanodrug delivery could be one of the available options that require active consideration for novel therapeutic strategies to treat AD cases. This review is focused on these aspects and provides new data showing that BBB plays an important role in AD-induced neurodegeneration and neurorepair.

Thyroid transcription factor-1 facilitates cerebrospinal fluid formation by regulating aquaporin-1 synthesis in the brain

In the brain, aquaporin-1 (AQP-1), a water channel for high osmotic water permeability, is mainly expressed in the apical membrane of the ventricular choroid plexus and regulates formation of cerebrospinal fluid (CSF). Although the physiology of AQP-1 has been the subject of several publications, much less is known about the trans-acting factors involved in the control of AQP-1 gene expression. Here we report that TTF-1, a homeodomain-containing transcriptional regulator, is coexpressed with AQP-1 in the rat brain choroid plexus and enhances AQP-1 gene transcription by binding to conserved core TTF-1-binding motifs in the 5'-flanking region of the AQP-1 gene. Intracerebroventricular administration of an antisense TTF-1 oligodeoxynucleotide significantly decreased AQP-1 synthesis and reduced CSF formation. In addition, blockade of TTF-1 synthesis increased survival of the animals following acute water intoxication-induced brain edema. These results suggest that TTF-1 is physiologically involved in the transcriptional control of AQP-1, which is required for CSF formation.

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.

Increased brain uptake and CSF clearance of 14C-glutamate in spontaneously hypertensive rats

Blood-to-brain and blood-to-CSF transport kinetics of 14C-glutamate in the spontaneously hypertensive rats (SHR) were studied using the in situ brain perfusion technique. Also, clearance of 14C-glutamate from CSF of SHR was studied using the ventriculo-cisternal (VC) perfusion technique. Blood-to-brain and blood-to-CSF transport kinetics showed greater rate of maximal transport into both brain and CSF of SHR compared to normotensive Wistar Kyoto (WKY) rats (p>0.05). Uptake into CSF of WKY and uptakes into brains of WKY and SHR did not show any significant diffusion (K(d)) of 14C-glutamate (p<0.05). However, some diffusion of 14C-glutamate only into CSF of SHR was observed, 0.031+0.006 microl min(-1) g(-1). Clearance of 14C-glutamate from CSF was greater in the SHR (28.33+/-6.9 microl min(-1)) compared to that in WKY rats (19.42+/-4.7 microl min(-1)). However, 14C-glutamate uptake by brain from CSF side was not significantly different between SHR and WKY rats (p>0.05). These results suggest that the greater blood-to-brain and blood-to-CSF entry of 14C-glutamate during hypertension may be balanced by greater removal of 14C-glutamate from CSF back to blood.

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.

Effect of hypertension on the integrity of blood brain and blood CSF barriers, cerebral blood flow and CSF secretion in the rat

Hypertension has been related to the development of brain damage, dementia and other CNS dysfunctions. Disruption of the blood-brain barrier (BBB) is thought to contribute to these disorders. In this study, the integrity of both blood-brain and blood-CSF barriers during chronic hypertension was investigated. For this, the entry of [14C]sucrose and of lanthanum into brain tissue, choroid plexus, and CSF was studied. Also brain regional blood flow and brain [14C]sucrose volume of distribution were measured using indicator fractionation and ventriculo-cisternal perfusion methods, respectively. The above measurements were performed in normotensive (WKY) rats and in the spontaneously hypertensive rats (SHR). Choroid plexus and CSF uptakes of [14C]sucrose were found to be significantly greater in SHR compared to WKY rats (P<0.05). Intercellular entry of lanthanum was observed in choroidal tissue of SHR but not in that of WKY rats and at the BBB. Choroid plexus blood flow was significantly greater in SHR, 2.82+/-0.21 ml g(-1) min(-1), compared to 2.4+/-0.08 ml g(-1) min(-1) in WKY (P<0.05). There were no significant differences (P>0.05) in brain % water content and extracellular fluid [14C]sucrose volume of distribution between SHR and WKY rats. However, choroid plexus showed greater % water content in SHR (85.7+/-1.9%) compared to the WKY rats (81.5+/-1.7%). These results suggest that chronic hypertension in SHR may cause more pronounced defects in the integrity of the blood-CSF barrier than in the BBB.

Stereoselective transport of histidine in rat lung microvascular endothelial cells

The transport characteristics of L- and D-histidine through the blood-lung barrier were studied in cultured rat lung microvascular endothelial cells (LMECs). L-Histidine uptake was a saturable process. The addition of metabolic inhibitors [2,4-dinitrophenol (DNP) and rotenone] reduced the uptake rate of L-histidine. Ouabain, an inhibitor of Na(+)-K(+)-ATPase, also reduced uptake of L-histidine. Moreover, the initial L-histidine uptake rate was reduced by the substitution of Na(+) with choline chloride and choline bicarbonate in the incubation buffer. The system N substrate, L-glutamic acid gamma-monohydroxamate, also inhibited uptake of L-histidine. However, system N-mediated transport was not pH sensitive. These results demonstrated that L-histidine is actively taken up by a system N transport mechanism into rat LMECs, with energy supplied by Na(+). Moreover, the Na(+)-independent system L substrate, 2-amino-2-norbornanecarboxylic acid (BCH), had an inhibitory effect on L-histidine uptake in Na(+) removal, indicating facilitated diffusion by a Na(+)-independent system L transport into the rat LMECs. These results provide evidence for there being at least two pathways for L-histidine uptake into rat LMECs, a Na(+)-dependent system N and Na(+)-independent system L process. On the other hand, the uptake of D-histidine into rat LMECs was not reduced by the addition of DNP, rotenone, or ouabain, or by Na(+) replacement. Although the uptake of D-histidine was reduced in the presence of BCH, the addition of L-glutamic acid gamma-monohydroxamate did not significantly decrease uptake of D-histidine. These results suggest that the uptake of D-histidine by rat LMECs has different characteristics compared with its isomer, L-histidine, indicating that system N transport did not involve D-histidine uptake.

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.

Cerebrospinal fluid concentrations of tryptophan and 5-hydroxyindoleacetic acid in Macaca mulatta: diurnal variations and response to chronic changes in dietary protein intake

In rats, dietary protein is known to influence brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis. However, few studies have examined this relationship in primates (including humans). We therefore studied the effect in monkeys of changes in chronic protein intake on plasma and cerebrospinal fluid (CSF) concentrations of TRP and 5-hydroxyindoleacetic acid (5HIAA), the principal 5HT metabolite. Juvenile male monkeys (Macacca mulatta) consumed for sequential 4-week periods diets differing in protein content (approximately 23%-->approximately 16%--> approximately 10%-->approximately 6% protein [%-energy/day]). Each day, food was presented as a morning meal of fruit, and an afternoon meal consisting of a pelleted, commercial diet and fruit. During week 4 on each diet, blood and CSF were sampled diurnally via indwelling catheters. Plasma and CSF TRP varied diurnally and with dietary protein content. On all diets, CSF TRP declined modestly in the morning, and increased in the afternoon; the magnitude of the increments varied directly with dietary protein content. Diurnal variations were absent for CSF 5HIAA; however, CSF 5HIAA varied directly with chronic dietary protein content. We conclude that dietary protein content can chronically influence CSF TRP concentrations in monkeys. The variation in CSF 5HIAA suggests chronic protein intake may influence serotonin synthesis and turnover, perhaps via changes in TRP concentrations.

Acidic amino acid clearance from CSF in the neonatal versus adult rat using ventriculo-cisternal perfusion

The acidic amino acids aspartate and glutamate are excitatory neurotransmitters in the CNS. The clearance of this group of amino acids from CSF of adult and neonatal (7-day-old) rats was investigated. Ventriculo-cisternal perfusions with 14C-amino acids and 3H-dextran were carried out for up to 90 min. Uptake of the amino acids by the whole brain was measured, and the loss to blood was calculated. 3H-Dextran was included in the perfusate for measurement of CSF secretion rate. After 90-min perfusion, both aspartate and glutamate showed a similar uptake into the whole brain, and this did not change with age (p>0.05). However, clearance from CSF was greater in the adult, as was entry into blood from CSF. Addition of 5 mM excess unlabelled amino acid resulted in reduction in the brain uptake of both 14C-amino acids in the adult rat. In the neonate, addition of aspartate also reduced brain aspartate uptake, whereas addition of glutamate increased brain neonatal [14C]glutamate uptake. The rate of CSF secretion was significantly greater in the adult, 1.26+/-0.18 microl x min(-1) x g(-1), than in the neonate, 0.62+/-0.08 microl x min(-1) x g(-1), and the turnover of CSF was greater in adults (p<0.01). In summary, both aspartate and glutamate showed greater clearances from CSF in the adult than the neonate. This clearance was found to be by carrier-mediated mechanisms.

Choroid plexus histidine transport

System-N transport plays an important role in l-glutamine uptake into isolated rat choroid plexus but its role in the transport of another System-N substrate, l-histidine, has yet to be determined. Similarly, the possible effects on System-N mediated l-histidine transport of changes in pH and extracellular l-glutamine, such as occur in cerebral ischemia and hepatic encephalopathy, have yet to be examined. In the absence of competing amino acids, l-[3H]histidine uptake in isolated rat choroid plexus was mediated by both Na+-independent and Na+-dependent transport. The former was inhibited by 2-amino-2-norbornane carboxlic acid, indicating System-L transport, while the latter appears System-N mediated as it was inhibited by three System-N substrates but not substrates for System-A and -ASC. The Na+-dependent uptake had a Km of 0.2 mM and a Vmax of 1.4 nmol/mg/min. It accounted for 30% of l-histidine uptake in the presence of physiological concentrations of amino acids. Reductions in pH markedly inhibited Na+-dependent but not Na+-independent transport indicating that, as in liver but not neurons, System-N mediated transport at the choroid plexus is pH sensitive. Increases in l-glutamine concentration in the pathophysiological range reduced l-histidine uptake via both System-L and -N.

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.

Whole cell Cl- conductances in mouse choroid plexus epithelial cells do not require CFTR expression

Whole cell patch-clamp studies were performed with tissue isolated from the cystic fibrosis (CF) transgenic Cftrm1cam mouse, to determine whether anion currents in choroid plexus epithelial cells require the expression of cystic fibrosis transmembrane conductance regulator (CFTR). Inclusion of 0.25 mM adenosine 3',5'-cyclic monophosphate (cAMP) and 375 nM protein kinase A (PKA) in the pipette solution caused a significant activation of a Cl(-)-selective, inward-rectifying conductance in cells from wild-type and CF mice. The small, outward currents observed in wild-type and CF animals, however, were not activated by cAMP-PKA. There were no significant differences in the size of currents between wild-type, heterozygote, and CF cells in the presence or absence of cAMP-PKA. A second whole cell conductance was activated when cells from wild-type mice were swollen. These volume-activated currents were Cl- selective and exhibited outward rectification. They were Ca2+ independent and ATP dependent and blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 5-nitro-2-(3-phenylpropylamino)benzoic acid. The volume-activated channels were also activated in CF mutant cells, and there was no significant difference in the size of the volume-activated currents between wild-type, heterozygote, and CF cells. It is concluded that CFTR neither contributes to the whole cell conductance nor regulates the other anion conductances in choroid plexus epithelial cells.

The blood-brain barrier: principles for targeting peptides and drugs to the central nervous system

The presence of the blood-brain barrier (BBB), reduces the brain uptake of many drugs, peptides and other solutes from blood. Strategies for increasing the uptake of drugs and peptide-based drugs include; structural modifications to increase plasma half-life; improving passive penetration of the BBB by increasing the lipophilicity of the molecule; designing drugs which react with transporters present in the BBB; and reducing turnover and efflux from the central nervous system (CNS).

Zidovudine transport in the rabbit brain during intravenous and intracerebroventricular infusion

The distribution of zidovudine (AZT) between plasma, brain extracellular fluid (ECF), and cerebrospinal fluid (CSF) was investigated in a crossover design study (n = 5) in unanesthetized rabbits. Drug was administered by intravenous (iv) and intracerebroventricular (icv) infusions at the same infusion rate (1.5 mg/h.kg). The concentrations of AZT in ECF and CSF were measured by HPLC with microdialysis sampling. Plasma concentrations of AZT were quantitated by HPLC. Following iv infusion, the ECF- and CSF-to-plasma concentration ratios at steady state (SS), were 0.19 +/- 0.05 and 0.29 +/- 0.06, respectively. These values were less than unity, indicating the existence of active transport processes for the transport of AZT from brain to plasma across the blood-brain barrier (BBB) or blood-CSF barrier (BCB). The transport processes were modeled by compartmental model analysis, and the results suggest that the transport efficiency of AZT across the BBB is asymmetric; that is, the efflux clearance was five times greater than the influx clearance. Similarly, the efflux clearance from CSF is three times larger than the influx clearance into CSF. The SS concentrations of AZT in brain ECF in the same animals that received an icv infusion of AZT in the crossover design study were approximately two orders of magnitude greater than those in animals following iv infusion at the same dosing rate. Nevertheless, the SS plasma concentrations of AZT were similar for both routes of administration (1.2 +/- 0.19 and 1.2 +/- 0.13 micrograms/mL for iv and icv routes, respectively), confirming that the brain is not an organ that exhibits first-pass metabolism under the present experimental conditions.

Brain-to-blood transport of peptides and the alcohol withdrawal syndrome

Brain-to-blood transport, or efflux, systems play important roles in brain functions and can affect the CNS uptake and activity of endogenous and exogenous blood-borne substances. Several efflux systems have been described for peptides. These efflux systems may play important roles in communication between the CNS and peripheral tissues and may be important in conditions such as alcoholism.

High activities of glutamine transaminase K (dichlorovinylcysteine beta-lyase) and omega-amidase in the choroid plexus of rat brain

Certain halogenated hydrocarbons, e.g., dichloroacetylene, are nephrotoxic to experimental animals and neurotoxic to humans; cysteine-S-conjugate beta-lyases may play a role in the nephrotoxicity. We now show that with dichlorovinylcysteine as substrate the only detectable cysteine-S-conjugate beta-lyase in rat brain homogenates is identical to glutamine transaminase K. The predominant (mitochondrial) form of glutamine transaminase K in rat brain was shown to be immunologically distinct from the predominant (cytosolic) form of the enzyme in rat kidney. Glutamine transaminase K and omega-amidase (constituents of the glutaminase II pathway) activities were shown to be widespread throughout the rat brain. However, the highest specific activities of these enzymes were found in the choroid plexus. The high activity of glutamine transaminase K in choroid plexus was also demonstrated by means of an immunohistochemical staining procedure. Glutamine transaminase K has a broad specificity toward amino acid and alpha-keto acid substrates. The omega-amidase also has a broad specificity; presumably, however, the natural substrates are alpha-ketoglutaramate and alpha-ketosuccinamate, the alpha-keto acid analogues of glutamine and asparagine, respectively. The high activities of both glutamine transaminase K and omega-amidase in the choroid plexus suggest that the two enzymes are linked metabolically and perhaps are coordinately expressed in that organ. The data suggest that the natural substrate of glutamine transaminase K in rat brain is indeed glutamine and that the metabolism of glutamine through the glutaminase II pathway (i.e., L-glutamine and alpha-keto acid-->alpha-ketoglutarate and L-amino acid + ammonia) is an important function of the choroid plexus. Moreover, the present findings also suggest that any explanation of the neurotoxicity of halogenated xenobiotics must take into account the role of glutamine transminase K and its presence in the choroid plexus.

Formycin B elimination from the cerebrospinal fluid of the rat

The goal of this study was to determine whether specific transport systems are involved in nucleoside elimination from the cerebrospinal fluid (CSF). First, in vitro studies were carried out in isolated choroid plexus tissue slices from rat to ascertain the mechanisms of transport of formycin B, a model nucleoside analogue. 3H-Formycin B accumulated against a concentration gradient in the presence of an Na+ gradient in the isolated ATP-depleted choroid plexus tissue slices. This accumulation was reduced by high concentrations of unlabeled formycin B. Nitrobenzylthioinosine (NBMPR), an equilibrative nucleoside transport inhibitor, inhibited the uptake of formycin B in the absence of an Na+ gradient. These data suggest that both equilibrative and secondary active Na(+)-nucleoside transport systems are present in rat choroid plexus. In vivo, formycin B, together with inulin as a bulk flow marker, was injected into the lateral ventricle of the anesthetized rat with the aid of a stereotaxic device, and CSF was sampled from the cisterna magna at various times after injection. Twelve rats were randomized and divided into a low- and a high-dose group. The CSF clearance (CLCSF) of formycin B was significantly higher than the CLCSF of inulin in both animal groups (P < 0.01), indicating that formycin B is cleared from CSF by a pathway(s) in addition to bulk flow. Formycin B CLCSF was significantly lower in the high-dose group than in the low-dose group (P < 0.05), suggesting a saturable CSF elimination. The CLCSF of formycin B was also significantly reduced in animals treated with NBMPR (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Bidirectional saturable transport of LHRH across the blood-brain barrier

Systemic administration of luteinizing hormone-releasing hormone (LHRH) in rats has been found to influence behavior independently of pituitary or ovarian function. A previous study has shown that LHRH can cross the blood-brain barrier in one direction, but it was not known whether this was due to a saturable transport system. The rate of entry of 125I-labeled LHRH from blood to brain was determined by two different single-pass methods of carotid perfusion. The first, a multiple time point method, measures Ki from the slope of the linear regression when brain-to-blood ratios of radioiodinated LHRH are plotted against time. Saturable transport was determined by the difference between the Ki of rats perfused with 125I-LHRH (12.51 X 10(-3) mg.g-1.min-1) vs. rats perfused with 125I-LHRH and unlabeled LHRH (10 nmol/ml; 2.20 X 10(-3) ml.g-1.min-1). The inhibition by the unlabeled peptide was statistically significant (P less than 0.001). The second method, a single time point technique, measures the cerebrovascular permeability-surface area coefficient (PA). Saturable transport was determined in rats by the competition of unlabeled LHRH with 125I-LHRH. The PA value for 125I-LHRH (20.00 X 10(-3) ml.g-1.min-1) was significantly greater (P less than 0.05) than for 125I-LHRH with the addition of 10 nmol/ml unlabeled LHRH (4.14 X 10(-3) ml.g-1.min-1). Saturable transport of LHRH from brain to blood in mice was also determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Leucine modulates peptide transport system-1 across the blood-brain barrier at the stereospecific site within the central nervous system

Previous results have shown that leucine injected into a cerebral ventricle (i.c.v.) can act as an allosteric regulator of peptide transport system-1 (PTS-1), the system that transports Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1) and the enkephalins out of the central nervous system (CNS). D-Leucine appeared more potent than L-leucine. In the current study, dose-response curves were constructed for each compound after both intravenous (i.v.) and i.c.v. injection. Based on ED50 values after i.c.v. injection, D-leucine was about 200 times more potent than L-leucine in its inhibition of PTS-1, thereby confirming stereospecificity of the allosteric site. D- and L-Leucine were also more potent when given i.c.v. than when given i.v., suggesting that the site is located on the CNS side of the blood-brain barrier (BBB). The finding that D-leucine was less potent than L-leucine when given i.v. is also consistent with a CNS site of action because the L-isomer of leucine has been shown to be preferentially transported into the brain. These findings agree with the previous suggestion that some of the neurotoxic effects of leucine may be mediated through PTS-1 and could help explain how D-amino acids can exert opiate-related effects on the CNS.

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

The steady-state flux of labelled amino acids was investigated across the isolated perfused choroid plexus of the sheep. The extraction of anionic, cationic, small and large neutral amino acids by the blood side of the choroid plexus was demonstrated. However, there was no uptake of the analogue MeAIB, confirming the absence of the 'A' carrier system on this side of the blood--CSF barrier. The direction of the net flux of amino acids across the tissue varied depending on the amino acid and its concentration. At a concentration of 0.01 mM the net movement for phenylalanine, serine, aspartate and glycine was from blood to CSF. When the concentration of amino acid was increased to 0.1 mM, the net flux of phenylalanine and serine remained from blood to CSF whereas the net flux of the transmitters, aspartate and glycine, was in the opposite direction, from CSF to blood. When the level was raised further, to 1 mM, all four amino acids showed a net CSF to blood flux. The concentration of amino acids is newly formed CSF was calculated from the blood to CSF fluxes and was found to be between 2 and 200 microM, similar to that found in mammalian bulk phase CSF.

Saturable mechanism for delta sleep-inducing peptide (DSIP) at the blood-brain barrier of the vascularly perfused guinea pig brain

Cellular uptake of [125I] labelled DSIP at the luminal interface of the blood-brain barrier (BBB) was studied in the ipsilateral perfused in situ guinea pig forebrain. Regional unidirectional transfer constants (Kin) calculated from the multiple-time brain uptake analysis were 0.93, 1.33 and 1.66 microliter.min-1 g-1 for the parietal cortex, caudate nucleus and hippocampus, respectively. In the presence of 7 microM unlabelled DSIP the brain uptake of [125I]-DSIP (0.3 nM) was inhibited, the values of Kin being reduced to 0.23-0.38 microliter.min-1 g-1, values that were comparable with the Kin for mannitol. The rapidly equilibrating space of brain, measured from the intercept of the line describing brain uptake versus time on the brain uptake ordinate, Vi, was greater for [125I]-DSIP than for mannitol; in the presence of unlabelled DSIP this was reduced to that of mannitol, and it was suggested that the larger volume for [125I]-DSIP represented binding at specific sites on the brain capillary membrane. L-tryptophan, the N-terminal residue of DSIP, in concentrations of 7 microM and 1 mM, inhibited Kin without affecting Vi. A moderate inhibition of Kin was obtained by vasopressin ([Arg8]-VP), but only at a concentration as high as 0.2 mM. The results suggest the presence of a high affinity saturable mechanism for transport of DSIP across the blood-brain barrier, with subsequent uptake at brain sites that are highly sensitive to L-tryptophan, and may be modulated by [Arg8]-VP.

Neutral amino acid uptake by the isolated perfused sheep choroid plexus

1. The uptake of neutral amino acids from the blood into the cells of the choroid plexus was studied by means of the rapid (less than 40 s) single-circulation paired-tracer dilution technique in the isolated perfused choroid plexus of the sheep. 2. The study provides the first direct evidence for the carrier-mediated entry of neutral amino acids from blood into the cells of the choroid plexus. 3. In the terms of Christensen's classification the presence of L-amino acid carrier systems for large neutral amino acids with bulky side chains has been demonstrated. 4. No measurable uptake of [14C]methyl amino isobutyric acid ([14C]MeAIB) during a single passage through the choroid plexus circulation was demonstrated which indicates the probable absence of a significant 'A' transport system. 5. The uptake of small neutral amino acids such as glycine and L-alanine was shown to be carrier-mediated. Results suggest that these amino acids are mainly transported by the glycine and ASC carrier systems, respectively. 6. The results suggest that there is a similarity between the transport systems for neutral amino acids on the blood side of both the blood-brain barrier and blood-cerebrospinal fluid barrier, the exception being for the presence of a glycine carrier on the blood side of the choroid plexus.

Passage of delta sleep-inducing peptide (DSIP) across the blood-cerebrospinal fluid barrier

Unidirectional flux of 125I-labeled DSIP at the blood-tissue interface of the blood-cerebrospinal fluid (CSF) barrier was studied in the perfused in situ choroid plexuses of the lateral ventricles of the sheep. Arterio-venous loss of 125I-radioactivity suggested a low-to-moderate permeability of the choroid epithelium to the intact peptide from the blood side. A saturable mechanism with Michaelis-Menten type kinetics with high affinity and very low capacity (approximate values: Kt = 5.0 +/- 0.4 nM; Vmax = 272 +/- 10 fmol.min-1) was demonstrated at the blood-tissue interface of the choroid plexus. The clearance of DSIP from the ventricles during ventriculo-cisternal perfusion in the rabbit indicated no significant flux of the intact peptide out of the CSF. The results suggest that DSIP crosses the blood-CSF barrier, while the system lacks the specific mechanisms for removal from the CSF found with most, if not all, amino acids and several peptides.

Unidirectional uptake of enkephalins at the blood-tissue interface of the blood-cerebrospinal fluid barrier: a saturable mechanism

The cellular uptake at the blood-tissue interface of the blood-cerebrospinal fluid (CSF) barrier to tyrosyl-3,5-[3H]enkephalin-[5-L-leucine] (abbreviated to Leu-enkephalin) and of its synthetic analogue D-alanine2-tyrosyl-3,5-[3H]enkephalin-[5-D-leucine] (abbreviated to D-Ala2-D-Leu5-enkephalin) was studied in the isolated perfused choroid plexuses from the lateral ventricles of the sheep, using the rapid (less than 30 s), single circulation, paired-tracer dilution technique, in which D-[14C]-mannitol serves as an extracellular marker. Cellular uptake of peptides was estimated by directly comparing venous dilution profiles of [3H] and [14C] radioactivities in the absence and presence of unlabelled peptide, the N-terminal amino acid (L-tyrosine), the typical L-transport system substrate, 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH) and the inhibitor of aminopeptidase activity, bacitracin. The cellular uptake of both enkephalins was strongly (65-76%) but not completely inhibited by the addition of 5 mM unlabelled peptide to the bolus; the self-inhibition was significantly higher for D-Ala2-D-Leu5-enkephalin than for Leu-enkephalin. The addition to the bolus of L-tyrosine (5 mM), BCH (10 mM) or bacitracin (2 mM) reduced the 3H-radioactivity uptake by the choroid plexus of both enkephalins by 20-40%, the degree of inhibition being greater for [3H]-Leu-enkephalin than for its analogue. It is concluded that during single passage of enkephalins through the choroid plexus circulation, unidirectional uptake at the blood-tissue interface of the blood-CSF barrier consists of two components; a saturable component, which represents uptake of the intact peptide by the choroid epithelium, and a non-saturable component, which reflects enzymatic degradation of peptide in the blood and/or at the barrier, with a liberation of the N-terminal tyrosyl residue. Higher penetration of the blood-CSF barrier by D-Ala2-D-Leu5-enkephalin can be attributed to its greater resistance to hydrolysis.

The effects of aluminum chloride on the rate of secretion of the cerebrospinal fluid

The claim that AlCl3 could produce 100% inhibition of cerebrospinal fluid secretion was investigated using ventriculocisternal perfusion in the rabbit, and it was shown that a large part of this inhibition was an artefact due to a pH sensitivity of Blue Dextran, used as an indiffusible marker, caused by AlCl3. Thus the true inhibition found by us, using [3H]labeled markers, was about 33% and usually only partly reversible. Penetration of 22Na from blood into the perfused ventricles was partially inhibited by AlCl3. The effects of some other acid buffer systems, namely acetate and phosphate, on rate of secretion were measured; the results were highly variable. When mean arterial pressure was measured, it was found to be unaffected by AlCl3 but elevated with acetate and phosphate buffer mixtures.

Transport of leucine-enkephalin across the blood-brain barrier in the perfused guinea pig brain

Transport of [tyrosyl-3,5-3H]enkephalin-(5-L-leucine) [( 3H]Leu-enkephalin) across the blood-brain barrier was studied in the adult guinea pig, by means of vascular perfusion of the head in vivo. The unidirectional transfer constant (Kin) estimated from the multiple-time uptake data for [3H]Leu-enkephalin ranged from 3.62 X 10(-3) to 3.63 X 10(-3) ml min-1 g-1 in the parietal cortex, caudate nucleus, and hippocampus. Transport of [3H]Leu-enkephalin was not inhibited by unlabelled L-tyrosine (the N-terminal amino acid) at a concentration as high as 5 mM, or by the inhibitor of aminopeptidase activity bacitracin (2 mM), suggesting that there was no enzymatic degradation of peptide at the blood-brain barrier. By contrast, 2 mM unlabelled Leu-enkephalin strongly inhibited the unidirectional blood-to-brain transport of [3H]Leu-enkephalin by 74-78% in the parietal cortex, caudate nucleus, and hippocampus. The tetrapeptide tyrosyl-glycyl-glycyl-phenylalanine (without the C-terminal leucine of Leu-enkephalin), at a concentration of 5 mM, caused a moderate inhibition ranging from 15 to 29% in the brain regions studied, whereas the tetrapeptide glycyl-glycyl-phenylalanyl-leucine (without the N-terminal tyrosine) at 5 mM was without effect on Leu-enkephalin transport. Unidirectional brain uptake of Leu-enkephalin was not altered in the presence of naloxone at a concentration as high as 3 mM (1 mg/ml), suggesting that there is no binding of Leu-enkephalin to opioid receptors at the blood-brain barrier. It is concluded that there is a specific transport mechanism for Leu-enkephalin at the blood-brain barrier in the guinea pig.(ABSTRACT TRUNCATED AT 250 WORDS)

Steady-state distribution of cycloleucine and alpha-aminoisobutyric acid between plasma and cerebrospinal fluid

Estimates of the steady-state distribution ratios of two nonmetabolizable amino acids, alpha-aminoisobutyric acid and aminocyclopentane carboxylic acid (cycloleucine), between plasma and cerebrospinal fluid were made with a view to establishing whether or not the low values found with metabolizable amino acids, such as glycine or leucine, could be accounted for by uptake and metabolism by the brain. The estimates, based on the ratios found after i.p. injections either in bolus form or by implantation of "osmotic pumps" containing the labeled amino acids, were comparable with those found for metabolizable amino acids.

Permeability of the blood-cerebrospinal fluid and blood-brain barriers to thyrotropin-releasing hormone

The permeability of the blood-cerebrospinal fluid (CSF) barrier to 3H-labelled thyrotropin-releasing hormone (TRH), was studied at the blood-tissue interface of the isolated perfused choroid plexus of the sheep, using a rapid (less than 30 s), single circulation paired-tracer dilution technique, in which D-[14C]mannitol serves as an extracellular marker. Arterio-venous loss of 14C radioactivity reflects the percentage of the D-mannitol dose that crosses the blood-CSF barrier using a non-specific pathway. This loss suggests that the choroidal epithelium is moderately leaky. Cellular uptake of TRH, estimated by directly comparing venous dilution profiles of [3H]TRH and D-[14C]mannitol was independent of this leakiness. The unidirectional transport of TRH could not be saturated with unlabelled TRH at a concentration as high as 10 mM, but was markedly reduced by 10 mM proline and by the inhibitor of amidase and aminopeptidase activity, bacitracin (2 mM). Permeability of the blood-brain barrier to [3H]TRH was studied in the adult rat, employing the intracarotid injection technique of Oldendorf in which [14C]butanol served as an 'internal standard'. Brain-uptake of 3H radioactivity corrected for residual vascular space indicated a low extraction from the blood of TRH during a 15 s period of exposure to the peptide. Self-inhibition of [3H]TRH uptake by unlabelled TRH (10 mM) could not be demonstrated, but L-proline (10 mM) and bacitracin (2 mM) strongly inhibited this uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood-brain barrier permeability to leucine-enkephalin, D-alanine2-D-leucine5-enkephalin and their N-terminal amino acid (tyrosine)

The permeability of the blood-brain barrier to [tyrosyl-3,5-3H]enkephalin-(5-L-leucine) (abbreviated to Leu-Enk) and of its synthetic analogue D-alanine2-[tyrosyl-3,5-3H]enkephalin-(5-D-leucine) (abbreviated to D-Ala2-D-Leu5-Enk) was studied, in the adult rat, by means of Oldendorf's27 intracarotid injection technique. The brain uptake index (BUI) corrected for residual vascular radioactivity was about the same for both peptides, indicating a low extraction from the blood during a 5- or 15-s period of exposure to the peptides. Transport of Leu-Enk was not saturated by unlabelled Enk at a concentration as high as 5 mM but was completely abolished by 5mM tyrosine and by the inhibitor of aminopeptidase activity, bacitracin (2 mM). Also the typical L-transport system substrate, 2-aminobicyclo(2,2,1)heptane-2 carboxylic acid (BCH)9 at 10 mM concentration markedly reduced (by 80%) Leu-Enk uptake by the brain. In contrast, brain uptake of D-Ala2-D-Leu5-Enk was reduced only to about one-half of its control value by bacitracin or by 25% by BCH. Brain uptake for L-tyrosine was typically large and markedly inhibited by BCH but not inhibited by 5 mM unlabelled Leu-Enk. These results show that the measurable but low first-pass extractions for enkephalins are not representative of the uptake of these peptides into the brain, but rather reflect their extreme sensitivity to enzymatic degradation with a release of the N-terminal tyrosine residue. The results also suggest that small amounts of D-Ala2-D-Leu5-Enk might cross the blood-brain barrier in an intact form.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood-brain barrier permeability to dipeptides and their constituent amino acids

The penetration of two [14C]-labelled dipeptides, glycyl-L-phenylalanine and glycyl-L-leucine, and of their constituent amino acids into the brain of the rat was measured employing an intracarotid injection technique. The brain-uptakes of the dipeptides were about equal to that of sucrose suggesting a negligible extraction from the blood during the 15-s period of exposure to the peptides. Brain uptakes for L-phenylalanine and L-leucine were large and in agreement with earlier work on these amino acids; self-inhibition by unlabelled amino acids was marked as also inhibition by the typical L-transport system substrate, 2-aminobicyclo (2, 2, 1) heptane-2 carboxylic acid (BCH), whilst the substrate for the A-system, N-methyl-L-aminoisobutyric acid (MeAIB) was without effect. Uptake of L-phenylalanine and L-leucine was not inhibited by dipeptides in 10 mM concentration. The uptakes of [14C]-labelled MeAIB and glycine were not significantly different from that of sucrose. It is concluded that peptide formation effectively excludes the rapidly penetrating L-system amino acids, L-leucine and L-phenylalanine, from access to the L-system channel.