A modified method for measurement of radiotracer permeation across the rat blood--brain barrier: the problem of correcting brain uptake for intravascular tracer

Methods used for the measurement of blood-brain barrier integrity

The blood-brain barrier (BBB) comprises the interface between blood, brain and cerebrospinal fluid. Its primary function, which is mainly carried out by tight junctions, is to stabilize the tightly controlled microenvironment of the brain. To study the development and maintenance of the BBB, as well as various roles their intrinsic mechanisms that play in neurological disorders, suitable measurements are required to demonstrate integrity and functional changes at the interfaces between the blood and brain tissue. Markers and plasma proteins with different molecular weight (MW) are used to measure the permeability of BBB. In addition, the expression changes of tight-junction proteins form the basic structure of BBB, and imaging modalities are available to study the disruption of BBB. In the present review, above mentioned methods are depicted in details, together with the pros and cons as well as the differences between these methods, which maybe benefit research studies focused on the detection of BBB breakdown.

Post-ischemic hypothermia attenuates loss of the vascular basement membrane proteins, agrin and SPARC, and the blood-brain barrier disruption after global cerebral ischemia

Vascular basement membrane (BM) stabilizes brain vessels and inhibits endothelial cell cycle. Cerebral ischemia causes BM breakdown with the loss of structural BM components including collagens and laminins. In this study, the expression changes of the BM proteoglycan agrin, and the non-structural BM constituent SPARC (BM-40, osteonectin), were studied in brain vessels after global cerebral ischemia. A transient 20-min forebrain ischemia followed by 1, 6 or 24 h of reperfusion was induced in adult Sprague-Dawley rats by combined bilateral common carotid artery occlusion and hypotension (42-45 mm Hg). In a separate group of animals, a mild (32 degrees C) post-ischemic hypothermia was induced for 6 h, starting immediately after ischemia. RNA from approximately 500 brain vessels (20-100 microm) extracted by laser-capture microdissection (LCM) microscopy was used to determine the expression of proteoglycans agrin and SPARC mRNAs by quantitative PCR (Q-PCR). Protein expression was determined by immunohistochemistry in adjacent tissue sections. The BBB permeability was assessed using (3)H-sucrose as an in vivo tracer and by examining fibrinogen immunoreactivity in tissue sections. A transient global brain ischemia resulted in a significant (ANOVA, p<0.05; 6 animals/group) reduction in agrin and SPARC mRNAs in LCM-captured brain vessels 24 h after reperfusion. A time-dependent loss of agrin and SPARC from the BM during reperfusion was also observed by immunochemistry. A 6-h post-ischemic hypothermia reduced SPARC and agrin mRNA and protein losses, BBB transfer constant for (3)H-sucrose as well as fibrinogen extravasation 24 h after reperfusion. It is conluded that a transient post-ischemic hypothermia stabilizes brain vessels and reduces BBB disruption in part by preventing proteolytic degradation of regulatory BM constituents, SPARC and agrin.

Graded reversible opening of the rat blood-brain barrier by intracarotid infusion of sodium caprate

The fatty acid salt, sodium caprate (C10) is a well recognized drug absorption enhancer in intestine because of its ability to widen tight junctions in the epithelial cell lining. Caprate's potential usefulness to similarly alter the blood-brain barrier (BBB) tight junctions of brain vasculature and enhance CNS drug delivery has undergone little investigation. Adult SD rats were anesthetized and C10 was infused into the left internal carotid artery (dosing parameters: 10-30 mM, 1 or 2 ml min(-1), for 0.5-1.5 min). Beginning 5 or 60 min after infusion an i.v. bolus of [3H]mannitol was allowed to circulate for 30 min and degree of BBB leakiness measured as magnitude of the transfer constant (Ki, nl g(-1)s(-1)) for blood to brain mannitol permeation determined from brain and plasma samples. In initial experiments identical C10 infusions caused dramatic BBB opening in some rats, e.g., 10-fold increase in Ki, but not in others. Higher dosing produced consistent opening measured 5-35 or 60-90 min post-infusion but was also toxic as shown by severe brain edema and cardio-respiratory failure. The variable effect of moderate doses was attributed to the fact that arterial blood pressure markedly increased during C10 infusion and may have altered the flow dynamics of cerebrovascular caprate distribution from rat to rat. We modified the procedure by temporarily withdrawing blood to produce hypovolemia and systemic arterial hypotension during C10 infusion. Caprate infusions of 15-25 mM, 2 ml min(-1) for 1 min, produced reliable dose-related openings that lasted as much as an hour, were reversible, and accompanied by little or moderate edema, depending on dose. These findings confirm an earlier report showing that intracarotid caprate infusion opens the BBB but also show that control of the temporary hypertensive response produced by intracarotid caprate infusion is key to tailoring the dosage to consistently achieve graded, reversible BBB opening.

Characterization of vascular protein expression patterns in cerebral ischemia/reperfusion using laser capture microdissection and ICAT‐nanoLC‐MS/MS

Cerebral ischemia rapidly initiates structural and functional changes in brain vessels, including blood-brain barrier disruption, inflammation, and angiogenesis. Molecular events that accompany these changes were investigated in brain microvessels extracted using laser-capture microdissection (LCM) from Sprague-Dawley rats subjected to a 20 min transient global cerebral ischemia followed by 1, 6, or 24 h reperfusion. Proteins extracted from approximately 300 LCM captured microvessels (20-100 microm) were ICAT-labeled and analyzed by nanoLC-MS. In-house software was used to identify paired ICAT peaks, which were then sequenced by nanoLC-MS/MS. Pattern analyses using k-means clustering method classified 57 differentially expressed proteins in 7 distinct dynamic patterns. Protein function was assigned using Panther Classification system. Early reperfusion (1 h) was characterized by down-regulation of ion pumps, nutrient transporters, and cell structure/motility proteins, and up-regulation of transcription factors, signal transduction molecules and proteins involved in carbohydrate metabolism. The up-regulation of inflammatory cytokines and proteins involved in the extracellular matrix remodeling and anti-oxidative defense was observed in late reperfusion (6-24 h). The up-regulation of IL-1beta and TGF-1beta in ischemic brain vessels was confirmed by ELISA, quantitative PCR, and/or immunohistochemistry. A biphasic postischemic (1 and 24 h) BBB opening for (3)H-sucrose was evident in the same model. Differentially expressed proteins identified in brain vessels during reperfusion are likely involved in orchestrating functional vascular responses to ischemia, including the observed BBB disruption.

Inefficient central nervous system delivery limits the use of ibuprofen in neurodegenerative diseases

Chronic use of non-steroidal anti-inflammatory drugs may reduce the risk or delay the onset of Alzheimer's disease. To date, only limited information exists on the brain distribution of these drugs. The objective of this study was to determine the absolute brain delivery of ibuprofen by using constant in vivo infusion in rats. Ibuprofen was infused to steady-state concentrations both in plasma and brain tissue. Ibuprofen levels in plasma and brain tissue were measured by RP-HPLC after the plasma and the brain samples were purified by protein precipitation and solid phase extraction, respectively. Results indicate that both plasma and brain concentrations reached steady-state within 6h, and that the brain to plasma ratio of ibuprofen was only 0.02. Thus, limited brain penetration prevents the possible use of ibuprofen in treating or preventing neurodegenerative disorders such as Alzheimer's disease.

A two-hour window for hypothermic modulation of early events that impact delayed opening of the rat blood-brain barrier after ischemia

Opening of the blood-brain barrier (BBB) and consequent edema are known to intensify 24-72 h after ischemic stroke, and research on potential ameliorative therapies in animal models may lead to improved clinical treatments to prevent brain swelling and the secondary damage it causes. In this study, post-ischemic hypothermia treatment, which is an established neuroprotective strategy, was examined for its ability to prevent delayed BBB opening in a rat model of global ischemia. Anesthetized, normothermic SD rats (340-380 g) underwent 20 min of two-vessel (carotid) occlusion plus hypotension (2VO ischemia, between 0900-1100 h). Marked cortical BBB leakiness, which developed overnight, was indicated at sacrifice 24 h post-2VO by an average six- to eightfold increase above baseline in transfer constant values (K(i) ) for rate of blood to brain diffusion of intravenously delivered [(3)H]sucrose. A post-2VO treatment involving whole body cooling to 31.5 degrees-32.5 degrees C, maintenance for 6 h and rewarming to normothermia, significantly reduced BBB leakiness at 24 h, whether cooling was initiated immediately after reperfusion, or after a 1-h delay, but not after 2-h delay. Immediate hypothermia treatment reduced overall tissue injury at 24 h as evidenced by an assay of mitochondrial succinate dehydrogenase activity, and also reduced brain edema. By contrast, treatment of rats with the anti-inflammatory drugs cyclosporine A or minocycline offered no protection of BBB or mitochondria. It is concluded that hypothermic alteration of critical events during the first 2 h after prolonged ischemia powerfully mitigates the BBB damage and associated events that normally develop 24 h later.

Characteristics of sustained blood-brain barrier opening and tissue injury in a model for focal trauma in the rat

Minor stab wounding of rodent brain by needle or razor blade is a standard model for immunohistochemical investigations of secondary neuronal degeneration and scarring. Opening of the blood-brain barrier (BBB) to plasma molecules and inflammatory cells is integral to the secondary injury process. To facilitate quantitative study of these BBB phenomena, we tested the utility of a stereotaxic wire knife as a minimally invasive way for modeling of focal trauma and bleeding in brain parenchyma and substantial, reproducible BBB damage. Adult rats were anesthetized, and through a skull burr hole, the 0.3-mm dia guide cannula housing a laterally extendable tungsten wire (0.13 mm dia) was inserted into the right striatum. A layering of horizontal disk-like cuts (3 mm dia) was made, producing a cylindrical lesion of approximately 18 mm3 volume, approximately 2.7% of the cerebral hemisphere. Transfer constants (Ki) for blood to brain permeation of [3H]sucrose measured from 30 min to 2 weeks postlesion showed sustained BBB leakiness; for example, mean Ki +/- SEM (nL.g(-1) x s(-1)) for a standard, matrix-dissected forebrain sample enclosing the lesion were 7.2 +/- 1.2 (day 1 postlesion), 8.1 +/-1.4 (day 3), 5.4 +/- 0.8 (day 14) compared with values for contralateral nonlesioned forebrain ranging from 1.3 +/- 0.05 to 1.6 +/- 0.3 (n = 3-4 samples per time point). Analysis of the simultaneous transport of [14C]sucrose (MW = 342 Da) and [3H]inulin (MW approximately 5,000) showed significantly larger upward increments in Ki for sucrose than inulin, indicating a pore-like opening mechanism. Significant edema was measured 3 days postlesion. A reactive glial response was indicated by an increase in S100beta by 6 h and a glial scar forming around the lesion by 7 days. Secondary brain injury was indicated by a 10% loss of hemisphere mass, measured at 2 months. The wire knife enabled tailoring of interstitial trauma with a minimum of extraneous injury and supported reproducible measurements of sustained BBB injury using relatively few animals.

Biphasic opening of the blood-brain barrier following transient focal ischemia: effects of hypothermia

OBJECTIVE

Tracer constants (Ki) for blood-to-brain diffusion of sucrose were measured in the rat to profile the time course of blood-brain barrier injury after temporary focal ischemia, and to determine the influence of post-ischemic hypothermia.

METHODS

Spontaneously hypertensive rats were subjected to transient (2 hours) clip occlusion of the right middle cerebral artery. Reperfusion times ranged from 1.5 min to 46 hours, and i.v. 3H-sucrose was circulated for 30 min prior to each time point (1 h, 4 h, 22 h, and 46 h; n = 5-7 per time point). Ki was calculated from the ratio of parenchymal tracer uptake and the time-integrated plasma concentration. Additional groups of rats (n = 7-8) were maintained either normothermic (37.5 degrees C) or hypothermic (32.5 degrees C or 28.5 degrees C) for the first 6 hours of reperfusion, and Ki was measured at 46 hours.

RESULTS

Rats injected after 1.5-2 min exhibited a 10-fold increase in Ki for cortical regions supplied by the right middle cerebral artery (p < 0.01). This barrier opening had closed within 1 to 4 hours post-reperfusion. By 22 hours, the blood-brain barrier had re-opened, with further opening 22 and 46 hours (p < 0.01), resulting in edema. Whole body hypothermia (28 degrees C-29 degrees C) during the first six hours of reperfusion prevented opening, reducing Ki by over 50% (p < 0.05).

CONCLUSIONS

Transient middle cerebral artery occlusion evokes a marked biphasic opening of the cortical blood-brain barrier, the second phase of which causes vasogenic edema. Hypothermic treatment reduced infarct volume and the late opening of the blood-brain barrier. This opening of the blood-brain barrier may enhance delivery of low permeability neuroprotective agents.

Preparation and biodistribution of copper-67 complexes with tetradentate Schiff-base ligands

Uncharged, lipophilic, low molecular weight copper complexes labeled with generator-produced copper-62 are of interest as potential radiopharmaceutials for imaging the brain with positron emission tomography (PET). We report here the synthesis and biodistribution of a series of [67Cu]copper(II) complexes with tetradentate N2O2(2-)Schiff-base ligands. The compounds studied varied in lipophilicity from log P = 1.7 to log P = 3.6, where P is the octanol/water partition coefficient. In rat biodistribution studies the tracers were generally found to penetrate the blood-brain barrier following intravenous injection, but some far better than others. For closely related compounds brain uptake at 1 min postinjection increased with increasing lipophilicity, although log P was clearly not the sole determinant of high brain uptake. Substantial variations were also observed in the rate at which these various compounds are cleared from brain, with a few exhibiting the prolonged cerebral retention of tracer that would be desired for imaging with 62Cu and PET.

Diffusion into rat brain of contrast and shift reagents for magnetic resonance imaging and spectroscopy

A sensitive radiotracer technique was used to measure transfer constants (Kis) for blood to brain diffusion of the MR contrast reagent gadolinium diethylenetriaminepentaacetate (GdDTPA2-) and the MR shift reagent dysprosium triethylenetetraminehexaacetate (DyTTHA3-) across the normal and the ischemically injured blood-brain barrier (BBB) of rats. In rats with a normal BBB mean Kis (nL/g/s) for these reagents ranged from 0.3 to 1.4 across eight brain regions and were significantly lower in each region than Kis for sucrose (1.5-3.2), a substance known to be a poor permeant of the intact BBB. Kis measured 6 h after a 10 min period of normothermic forebrain ischemia were increased to 4.0-6.2 (reagents) and 6.6-7.5 (sucrose) in two brain regions, striatum and hippocampus, known to be especially vulnerable to ischemic injury. Measurements of BBB permeability to DyTTHA3- after osmotic opening of the barrier with hypertonic arabinose gave Kis of 25-30 in forebrain regions. Estimates of reagent concentrations in brain interstitial fluid 30 min after dosing the animals indicated that both an extremely high dose of DyTTHA3- and severe disruption of the BBB would be required to shift the resonance frequency of extracellular Na+ appreciably. With the moderate degrees of BBB injury produced by short-term ischemia, a dose of GdDTPA2- about 25 times the usual clinical dose of 0.1 mmol/kg would be required to quantify the injury by dynamic MRI.

Three openings of the blood-brain barrier produced by forebrain ischemia in the rat

A sensitive radiotracer method was used to explore the time course and regional pattern of blood-brain barrier (BBB) opening that occurs in a rat forebrain ischemia model that mimics temporary cardiac arrest. Immediately following 10 min of ischemia, transfer constants (Ki) for blood to brain permeation of [3H]sucrose were augmented severalfold, indicating widespread BBB opening. After 6 h, a delayed intensification of opening was evident in striatum and hippocampus, regions known to undergo selective, delayed neuronal death. There was generalized BBB recovery by 24 h except in experiments that involved prolonged ischemia (25 min) or concomitant brain hyperthermia (41 degrees C, 10 min). These protocols evoked a third opening; a marked upward increment in Ki and % H2O developed in neocortex between 6 and 24 h post-ischemia. Pharmacological or other manipulations of these temporal and regional patterns of altered transfer constants may aid understanding of the interplay between microvascular damage, edema, and neuronal death following brain ischemia.

Central nervous system targeting of 2',3'-dideoxyinosine via adenosine deaminase-activated 6-halo-dideoxypurine prodrugs

AIDS dementia complex is a neurologic disorder, characterized by increasingly severe cognitive, behavioral, and motor impairment, which is associated with human immunodeficiency virus (HIV) infection in the central nervous system (CNS). Many of the dideoxynucleosides effective systemically in the treatment of HIV infections, such as 2',3'-dideoxyinosine (ddI), exhibit limited penetration into the CNS and limited or variable effectiveness in reversing the symptoms of AIDS dementia. Thus, approaches for increasing the CNS uptake of ddI and other dideoxynucleosides are needed. The CNS uptake of a series of 6-halo-2',3'-dideoxypurine ribofuranosides (6-halo-ddPs) previously shown to be active against HIV because of their conversion to ddI through the action of adenosine deaminase was examined in rats. In vitro studies in rat blood and brain tissue homogenate suggested a favorable selectivity for bioconversion in brain tissue, but with bioconversion half-lives varying widely within the series. In vivo infusions of 6-chloro-ddP (6-Cl-ddP), 6-bromo-ddP (6-Br-ddP), and 6-iodo-ddP (6-I-ddP) resulted in significant increases (20- to 34-fold) in the ddI concentration ratios in brain parenchyma/plasma when compared with those after an infusion of ddI alone. Absolute concentrations of ddI in brain parenchyma were increased 10- and 4-fold, respectively, following 30-min infusions of 6-Cl-ddP or 6-Br-ddP, but were 2.4-fold lower after an infusion of 6-I-ddP relative to that after a control infusion of ddI. Detailed studies of the plasma pharmacokinetics, CNS uptake kinetics, and bioconversion of 6-Cl-ddP were conducted to compare in vivo transport and bioconversion parameters with those predicted from in vitro measurements and to rationalize the efficiency of CNS delivery of ddI from 6-Cl-ddP. The results show that increased lipophilicity alone does not ensure that a given prodrug will deliver higher levels of a parent compound to the CNS. Both the selectivity and absolute rate of bioconversion in the brain are important factors.

Transfer constants for blood-brain barrier permeation of the neuroexcitatory shellfish toxin, domoic acid

The cause of the toxic mussel poisoning episode in 1987 was traced to a plankton-produced excitotoxin, domoic acid. Experiments were undertaken to quantitate the degree to which blood-borne domoic acid can permeate the microvasculature to enter the brain. Pentobarbital-anesthetized, adult rats received an i.v. injection of 3H-domoic acid which was permitted to circulate for 3-60 min. Transfer constants (Ki) describing blood-to-brain diffusion of tracer were calculated from analysis of the relationship between brain vs plasma radioactivity with time. Mean values (mL.g-1.s-1 X 10(6] for permeation into 7 brain regions (n = 10 rats) ranged from 1.60 +/- 0.13 (SE) to 1.86 +/- 0.33 (cortex, pons-medulla respectively), and carrier transport or regional selectivity in uptake were not evident. Nephrectomy prior to domoic acid injection resulted in the elevation of circulating plasma tracer level and brain uptake. The Ki values are comparable to those for other polar compounds such as sucrose, and indicate that the blood-brain barrier greatly limits the amount of toxin that enters the brain. Together with absorbed dosage, integrity of the cerebrovascular barrier and normal kidney function are important to the outcome of accidentally ingesting domoic acid.

Brain and cerebrospinal fluid uptake of zidovudine (AZT) in rats after intravenous injection

Uptake kinetics of zidovudine into cerebrospinal fluid (CSF) and brain tissue were determined in adult Sprague Dawley male rats after single intravenous injection of 6.7 mg/kg (25 mumol/kg). The drug kinetics in plasma followed biexponential disposition with an initial distribution half-life of approximately 11 minutes and an elimination half-life of 40 minutes. Over the plasma concentration range of 0.2 to 10 micrograms/ml, the cerebrospinal fluid to plasma ratio averaged 14.8 +/- 1.9% whereas the mean brain tissue to plasma ratio was 8.2 +/- 1.2% (uncorrected) or 2.3 +/- 1.8% (corrected) for the brain vascular space contribution. Simultaneous nonlinear regression analysis of brain, CSF and plasma concentration data indicate that the overall rate constant for efflux of drug from brain is approximately 75-fold higher and from CSF is 8-fold higher than the respective rate constants for influx. Thus, the ratio of the efflux to influx appears to be the predominant factor in determining the net accumulation of drug into CSF and brain parenchymal tissue.

Selective, delayed increase in transfer constants for cerebrovascular permeation of blood-borne 3H-sucrose following forebrain ischaemia in the rat

Experiments were conducted to explore the time course of changes in blood-brain barrier (BBB) permeability that may occur in the 2-vessel occlusion model of stroke in the rat. Anaesthetized Sprague-Dawley rats underwent 10 min of cerebral ischaemia produced by bilateral carotid occlusion plus haemorrhagic hypotension. After 6 min, or 3, 6, 18, 24, 48 h recovery and re-anaesthetization, an i.v. injection of 3H-sucrose was permitted to circulate for 30 min. Regional transfer constants (Ki) for BBB permeation of sucrose were calculated from the ratio of sucrose concentration in parenchyma relative to the time-integrated plasma concentration. In the 6-min group, all cerebral regions showed evidence of early BBB leakiness (increase in Ki above non-stroke baseline) which was maximal in forebrain cortex. This effect was diminished at subsequent time points, except in striatum and hippocampus which exhibited delayed intensification of opening, maximal in the 6 h group. Ki values had largely normalized by 24 h. Ki values were also determined 6 min, 6 h and 24 h after a 20-min stroke procedure. Early and regionally selective, delayed BBB openings were also seen, but recovery was not evident in cerebral regions at 24 h. Cortex exhibited a large increase in Ki indicating that a delayed, marked deterioration of BBB integrity had developed between the 6 h and 24 h time points. It is concluded that the combination of transfer constant measurements and the 2-vessel occlusion model could provide a sensitive means for investigating the cerebrovascular consequences and therapy of stroke.

Does magnetic resonance imaging compromise integrity of the blood-brain barrier?

It has been reported that a standard clinical procedure of magnetic resonance imaging (MRI) carried out experimentally in the rat caused temporary opening of the blood-brain barrier. Electron micrography indicated blood-to-brain movement of horseradish peroxidase, a protein tracer that does not normally permeate the barrier. In the present study, permeability-area products (PA), describing the limited permeation of blood-borne [14C]sucrose into brain parenchyma, were measured in anesthetized rats subjected either to 23 min of MRI, to osmotic barrier opening by intracarotid infusion of hypertonic arabinose, or to control conditions. Osmotic opening caused manyfold increases in PA whereas MRI produced no change. The proposal that MRI can compromise integrity of the blood-brain barrier is not supported by these findings.

Technetium-99m spiperone dithiocarbamate: a potential radiopharmaceutical for dopamine receptor imaging with SPECT

Spiperone dithiocarbamate (SPDC) was prepared by reacting spiperone with carbon disulfide followed by sodium hydroxide. SPDC was labelled with 99mTc by reduction of pertechnetate with formamidine sulfinic acid or sodium dithionite at alkaline pH, resulting in approximately 40% incorporation of 99mTc. The lipophilic complex was conveniently isolated at high specific activity and high radiochemical purity by extraction into dichloromethane, which was then evaporated and the residue was redissolved in a 1:3 mixture of ethanol and saline containing 0.1 mg/mL gentisic acid. Biodistribution studies following i.p. injection in rats showed low uptake of radioactivity in the brain, but striatum/cortex and striatum/cerebellum ratios were reduced by pretreatment with haloperidol. This agent may allow imaging of dopamine D-2 receptors using single-photon emission computed tomography (SPECT).

Plasma, cerebrospinal fluid, and brain distribution of 14C-melatonin in rat: a biochemical and autoradiographic study

The distribution of 14C-Melatonin (14C-MT) after systemic injection was studied in the plasma, cerebrospinal fluid (CSF), and brain of rats. Chromatographic analysis (thin-layer chromatography and high-performance liquid chromatography) indicated that the radioactivity from biological samples taken at various times following the injection of label was mainly associated with 14C-MT. Computer analysis of plasma 14C-MT kinetics showed a three-compartment system with half-lives of 0.21 +/- 0.05, 5.97 +/- 1.11, and 47.52 +/- 8.86 min. The volume of distribution and the clearance were 1,736 +/- 349 ml.kg-1 and 25.1 +/- 1.7 ml.min-1.kg-1 respectively. The entry of 14C-MT into the CSF was rapid and reached a maximum at 5 min. The decay followed a two-compartment system with half-lives of 16.5 +/- 2.9 and 47.3 +/- 8.6 min. The CSF/plasma concentration ratio was 0.38 at the steady state (30 min). At 2 min the level of 14C-MT in the brain was 3.8 higher than in the CSF. Representative autoradiograms revealed an heterogeneous localization of 14C-MT in the grey matter. The highest regional values, as evaluated by the permeability area product technique, were found in cortex, thalamic nuclei, medial geniculate nucleus, anterior pretectal area, paraventricular nucleus of the hypothalamus, choroid plexuses, and bulb-pons. Thirty minutes later 14C-MT was still detected in most of the brain regions analyzed. These results point to a low but rapid penetration of circulating MT into the brain and the CSF. The heterogeneous distribution and the partial retention of 14C-MT in the brain are compatible with the hypothesis of a central action of this hormone mediated via binding sites.

The effects of galactosamine-induced hepatic failure upon blood-brain barrier permeability

The role of changes in blood-brain barrier permeability in the pathogenesis of hepatic encephalopathy remains uncertain. To test the hypothesis that brain microvessel permeability is nonselectively increased in hepatic encephalopathy we measured the blood-brain barrier permeability-surface area product in rats with acute liver failure induced by intraperitoneal injection of galactosamine. The permeability-surface area products to the diffusion-limited tracers, sucrose and methylaminoisobutyric acid, were determined as a measure of blood-brain barrier permeability. Animals were examined 24, 36 and 42 hr after injection, at times when they were stuporous, but not comatose. No significant elevations of the permeability-surface area products for either compound were detected in clinically affected experimental animals when compared to controls. Our results indicate there is no generalized increase in brain vascular permeability during hepatic insufficiency in precomatose animals.

Transport of sodium from blood to brain in ischemic brain edema

Brain water and sodium increase during ischemia, suggesting that the blood-brain barrier permeability to sodium is increased. To test this hypothesis we measured the permeability-surface area products of 22Na and [3H]sucrose in gerbils following 3 hours of unilateral ischemia. In animals with neurologic symptoms, unilateral carotid occlusion reduced the cerebral blood flow in the ipsilateral cerebral hemisphere to 13 +/- 4 ml/100 g/min (n = 6). The water content of the ischemic hemisphere increased from 79.0 +/- 0.6 to 80.8 +/- 0.2% (n = 7, p less than 0.001) and tissue sodium content increased from 231 +/- 17 to 359 +/- 23 mEq/kg (p less than 0.0001). However, there was a 40% reduction in the sodium permeability-surface area product of the ischemic hemisphere compared with the control side (1.65 +/- 0.44 vs 2.79 +/- 0.29 microliter/g/min, n = 6, p less than 0.001). The sucrose permeability-surface area product, a measure of blood-brain barrier integrity, was unchanged. Although ischemia was less severe in the diencephalon, the tissue water and sodium contents increased significantly on the ischemic side. In contrast to the cerebral hemisphere, however, the permeability-surface area products for both sodium and sucrose were unchanged in the ischemic diencephalon. These results suggest that the increase in tissue sodium seen in ischemic edema is not due to enhanced sodium uptake; we speculate that it results, in part, from a reduction in sodium and water clearance from the tissue.

Defining the lower limits of blood-brain barrier permeability: factors affecting the magnitude and interpretation of permeability-area products

Experimental alteration in the restricted permeability of the blood-brain barrier to polar, blood-borne molecules is often quantitated in the rat with use of 14C-sucrose or 3H-mannitol delivered as a test substance into the circulation. The underlying principle is to relate the quantity of saccharide that has permeated into brain parenchyma, after an arbitrary time period, to some index of the circulating tracer level. This study indicates that to correct the radioactivity level in the brain tissue for intravascular tracer, it is an erroneous practice to estimate the latter as the product of tissue blood volume and the tracer concentration measured in a systematic blood sample. Dissected brain tissue was found to have a lower hematocrit and thereby larger plasma/tracer compartment per unit blood volume than femoral arterial blood. It is further shown that, although commercially supplied stocks of 14C-sucrose or 3H-mannitol may contain only small quantities of radioactive impurities, their inclusion in injectates and preferential uptake into brain may cause significant overestimation of permeability to the parent tracer. It is also confirmed that magnitude of permeability-area (PA) products for permeation of purified sucrose or mannitol into brain varies inversely with the length of time allotted for tracer circulation in the bloodstream. This finding is at variance with the assumptions of a two-compartment (plasma/brain) diffusion model underlying such measurements and supports a recently published model for blood-to-brain transfer based on multiple uptake compartments in brain parenchyma. The factors compromising PA measurement identified in this study may partly underlie variations in PA values published from several laboratories that had been attributed to genetic differences in laboratory rats.

Cerebral blood flow and blood-brain influx of some neutral amino acids in control and hypothyroid 16-day-old rats

Rats were made hypothyroid by a daily subcutaneous injection of propylthiouracil beginning the first day after birth. CBF, brain plasma volume, blood-brain extraction, and influx of some neutral amino acids were studied in 16-day-old animals. In hypothyroid rats, the brain plasma volume was decreased by approximately 30%. CBF was decreased by greater than 50%. This decrease was the highest in cerebellum. Blood-brain extraction of small neutral amino acids (alanine, serine, cysteine) was greatly enhanced. This greater extraction compensated for the decreased supply of alanine brought about by its decreased plasma concentration and the lower CBF. In contrast, the extraction of the large amino acids tested (leucine, phenylalanine) was hardly increased, and the influx of phenylalanine was slightly decreased. These results suggest an alteration in the maturation of the brain capillary bed and capillary transport for neutral amino acids in hypothyroidism. The differential effect of hypothyroidism on some small and large amino acids is an additional argument for the existence of two systems of transport for neutral amino acids at the luminal membrane of brain capillary endothelial cells of immature rats.

The involvement of GABA-transaminase in the blood-brain barrier to radiolabelled GABA

The involvement of GABA-transaminase (GABA-T) in the blood-brain barrier to GABA was investigated using a sensitive radiotracer technique which allows measurement of tracer permeation as a cerebrovascular permeability-area product, PA. In Sprague-Dawley rats subjected to peripheral GABA-T inhibition and occlusion of hepatic circulation, chromatographic (TLC) analysis together with lyophylized plasma samples showed the radioactivity measured in brain parenchyma was derived from the permeation of unchanged [3H]GABA and not its blood-borne metabolites. Permeation (PA) of i.v. injected [3H]GABA was poor in all brain regions examined. Pretreatment of these rats with various compounds known to inhibit endothelial GABA-T activity did not cause any increased permeation of [3H]GABA into the brain. These results suggest that the poor permeation of GABA into the rat brain is unrelated to the activity of GABA-T at endothelial sites.

Reduced permeation of 14C-sucrose, 3H-mannitol and 3H-inulin across blood-brain barrier in nephrectomized rats

Experiments were carried out to determine if changes in the concentration-time profile of a blood-borne radiotracer such as 14C-sucrose would spuriously alter measurements of its permeation across the blood-brain barrier (permeability-area product, PA) based on a 2-compartment (plasma/brain) simple diffusion model. Anesthetized rats which were bilaterally nephrectomized and given a standard intravenous bolus injection of 14C-sucrose, 3H-mannitol or 3H-inulin exhibited an elevated plasma tracer concentration compared to control animals. However, tracer concentration measured in brain parenchyma after 30 min was not proportionally elevated, and PA calculated from the ratio, parenchymal tracer concentration: plasma concentration-time integral, was significantly reduced below control values. In control rats, distortion and elevation of the plasma 14C-sucrose profile by continuous intravenous infusion did not result in lowered PA values. This suggested that the lowering of PA by nephrectomy reflected reduced cerebrovascular permeability or area or other cerebral influence rather than a deficiency in the 2-compartment model for PA measurement.