Blood to brain and brain to blood passage of native horseradish peroxidase, wheat germ agglutinin, and albumin: pharmacokinetic and morphological assessments

Water permeability of capillaries in the subfornical organ of rats determined by Gd-DTPA(2-) enhanced 1H magnetic resonance imaging

The water permeability of capillaries in the subfornical organ (SFO) of rat was measured by a (1)H nuclear magnetic resonance method in combination with a venous injection of a relaxation reagent, gadolinium-diethylene triamine-N,N,N',N",N"-pentaacetic acid (Gd-DTPA(2-)), which could not pass through the blood-brain barrier (BBB). Judging from results of Gd-DTPA(2-) dose dependency in the intact brain and the BBB-permeabilized brain, Gd-DTPA(2-) could not have leaked out from the capillaries in the cortex, thalamus or SFO, but it could have been extravasated in the posterior lobe of the pituitary gland. The longitudinal (T(1)) relaxation time of water in the SFO region was measured by inversion-recovery magnetic resonance imaging at 4.7 T. The T(1) relaxation rates (1/T(1)) before and after Gd-DTPA(2-) infusion were 0.70 +/- 0.02 s(-1) (mean +/- S.E.M., n = 9) and 1.53 +/- 0.11 s(-1) (n = 9), respectively. The rate constant for water influx to the capillaries was estimated to be 0.84 +/- 0.11 s(-1) (n = 9) which corresponds with a diffusive membrane permeability (P(d)) of 3.7 x 10(-3) cm s(-1). Compared with values found in the literature available on this subject, this P(d) value for the capillaries in the SFO was the same order of magnitude as that for transmembrane permeability of water for the vasa recta, and it may be 10-100 times larger than that of the blood-brain barrier in the cortex. Areas of the cortex and thalamus showed minimal changes in the T(1) relaxation rate (ca 0.09 s(-1)), but these values were not statistically significant and they corresponded to P(d) values much smaller than those found in the SFO. From these results, we conclude that the capillaries in the SFO have one of the highest water permeability values among all of the capillaries in the brain. It is also suggested that this magnetic resonance imaging, based on T(1) relaxation rate, is a useful method to detect local water permeability in situ.

Extent and direction of ghrelin transport across the blood-brain barrier is determined by its unique primary structure

The novel hormone ghrelin is a potent orexigen that may counterbalance leptin. Ghrelin is the only secreted molecule requiring post-translational acylation with octanoic acid to ensure bioactivity. Ghrelin, predominantly derived from the stomach, may target neuroendocrine networks within the central nervous system (CNS) to regulate energy homeostasis. This would require ghrelin to cross the blood-brain barrier (BBB). In mice, we examined whether ghrelin crosses the BBB and whether its lipophilic side chain is involved in this process. We found that saturable systems transported human ghrelin from brain-to-blood and from blood-to-brain. Mouse ghrelin, differing from human ghrelin by two amino acids, was a substrate for the brain-to-blood but not for the blood-to-brain transporter and so entered the brain to a far lesser degree. des-Octanoyl ghrelin entered the brain by nonsaturable transmembrane diffusion and was sequestered once within the CNS. In summary, we show that ghrelin transport across the BBB is a complex, highly regulated bidirectional process. The direction and extent of passage are determined by the primary structure of ghrelin, defining a new role for the unique post-translational octanoylation.

Regional variation in brain capillary density and vascular response to ischemia

Differences in brain neuroarchitecture have been extensively studied and recent results demonstrated that regional differences in the physiological properties of glial cells are equally common. Relatively little is known on the topographic differences in vascular supply, distribution and density of brain capillaries in different CNS regions. We developed a simple method consisting of intravascular injection of fluorescent dyes coupled to immunocytochemical techniques that allows for simultaneous observation of glia-neuronal-vascular interactions in immersion-fixed brain specimens from small rodents. This technique permits quantitative evaluation of regional differences in glial/neuronal distribution and the study of their relationship to vascular densities. Variations of this technique also allow the detection of abnormal microvasculature (i.e. 'leaky' vessels), a useful feature for studies of blood-brain barrier function in health and disease. By use of quantitative confocal microscopy, the three-dimensional geometry of cortical and hippocampal structures revealed remarkable differences in vascularization between cortical gray/white matter junction, and hippocampal formation (CA1 and CA3 regions). Significant differences were also observed within the same investigative region: CA1 was characterized by low capillary density compared to neighboring CA3. Following an ischemic insult, CA1 vessels had more extensive blood-brain barrier leakage than CA3 vessels. We conclude that in addition to neuronal and glial heterogeneity, cortical structures are also endowed with region-specific vascular patterns characterized by distinct pathophysiological responses.

Ultrastructural and permeability features of microvessels in the periventricular area of senescence-accelerated mice (SAM)

Brain transfer of intravenously injected horseradish peroxidase (HRP) and the ultrastructural features of the vessels were examined in periventricular areas in senescence-accelerated mice (SAMP8), which show age-related deficits in learning and memory, and senescence-accelerated resistant mice (SAMR1), which do not show age-related deficits. In all mice examined with light microscopy, staining reaction for HRP was seen in the periventricular area adjacent to the medial side of the lateral ventricle. Electron microscopic examination in the periventricular area of young and old mice of both strains showed that the staining reaction for HRP appeared in the vesicular profiles of the endothelial cytoplasm, the cytoplasm of the perivascular cells, the basal lamina, and the adjoining extracellular spaces of the white matter, suggesting an incomplete blood-brain barrier (BBB) in the periventricular white matter. In addition, irregularly thickened endothelial cell cytoplasm, membranous inclusions within the basal lamina, and electron-dense endothelial cell cytoplasm were occasionally seen in aged SAMP8 mice. These findings were not observed in 3-month-old SAMP8 mice and 3- and 13-month-old SAMR1 mice. Perivascular collagen deposits were also frequently seen in aged SAMP8 mice. These findings indicate that the endothelial cells and pericytes in the periventricular white matter in aged SAMP8 mice have an ultrastructure with damaged BBB function. Intravascular substances can easily penetrate the periventricular white matter and the BBB of the vessels in the area can be deteriorated with aging in SAMP8 mice.

Transport of human immunodeficiency virus type 1 pseudoviruses across the blood-brain barrier: role of envelope proteins and adsorptive endocytosis

Blood-borne human immunodeficiency virus type 1 (HIV-1) crosses the blood-brain barrier (BBB) to induce brain dysfunction. How HIV-1 crosses the BBB is unclear. Most work has focused on the ability of infected immune cells to cross the BBB, with less attention devoted to the study of free virus. Since the HIV-1 coat glycoprotein gp120 can cross the BBB, we postulated that gp120 might be key in determining whether free virus can cross the BBB. We used radioactive virions which do (Env+) or do not (Env-) bear the envelope proteins to characterize the ability of HIV-1 to be taken up by the murine BBB. In vivo and in vitro studies showed that the envelope proteins are key to the uptake of free virus and that uptake was enhanced by wheat germ agglutinin, strongly suggesting that the envelope proteins induce viral adsorptive endocytosis and transcytosis in brain endothelia. Capillary depletion showed that Env+ virus completely crossed the vascular BBB to enter the parenchyma of the brain. Virus also entered the cerebrospinal fluid, suggesting passage across the choroid plexus as well. About 0.22% of the intravenously injected dose was taken up per g of brain. In vitro studies showed that postinternalization membrane cohesion (membrane binding not reversed with acid wash or cell lysis) was a regulated event. Intact virus was recovered from the brain endothelial cytosol and was effluxed from the endothelial cells. These results show that free HIV-1 can cross the BBB by an event related to adsorptive endocytosis and mediated by the envelope proteins.

Spontaneous hemorrhagic stroke in a mouse model of cerebral amyloid angiopathy

A high risk factor for spontaneous and often fatal lobar hemorrhage is cerebral amyloid angiopathy (CAA). We now report that CAA in an amyloid precursor protein transgenic mouse model (APP23 mice) leads to a loss of vascular smooth muscle cells, aneurysmal vasodilatation, and in rare cases, vessel obliteration and severe vasculitis. This weakening of the vessel wall is followed by rupture and bleedings that range from multiple, recurrent microhemorrhages to large hematomas. Our results demonstrate that, in APP transgenic mice, the extracellular deposition of neuron-derived beta-amyloid in the vessel wall is the cause of vessel wall disruption, which eventually leads to parenchymal hemorrhage. This first mouse model of CAA-associated hemorrhagic stroke will now allow development of diagnostic and therapeutic strategies.

Diffusion and action of intracerebroventricularly injected interleukin-1 in the CNS

Interleukin-1beta acts on the CNS to induce fever, neuroendocrine activation and behavioural depression. We have previously demonstrated that interleukin-1beta is synthesized in glial cells and macrophages of circumventricular organs and choroid plexus after intraperitoneal administration of bacterial lipopolysaccharide. Whether, and how, interleukin-1beta produced in glial cells affects neuronal functioning is unknown. Diffusion throughout the extracellular space is an important pathway by which factors produced by glial cells act on distant cells, a phenomenon coined "volume transmission". The present study assessed diffusion of recombinant rat interleukin-1beta, recombinant human interleukin-1 receptor antagonist and 10mol. wt dexran in the rat CNS after intracerebroventricular administration to model interleukin-1beta release from choroid plexus. Immunocytochemistry with specific antibodies directed against interleukin-1beta and interleukin-1 receptor antagonist revealed that these molecules rapidly penetrated into periventricular tissue and spread along white matter fibre bundles and blood vessels in the caudoputamen, hypothalamus and amygdala. The transcription factor nuclear factor kappa B and the immediate-early gene product Fos were detected immunocytochemically to reveal interleukin-1beta action. Intracerebroventricular infusion of interleukin-1beta induced nuclear factor kappa B translocation in choroid plexus, ependymal cells, basolateral amygdala, cerebral vasculature and meninges. Fos immunoreactivity was found in the supraoptic and paraventricular hypothalamus and central amygdala. We propose that intracerebroventricular injected interleukin-1beta can enter the brain parenchyma and act as a "volume transmission" signal in, for example, the basolateral amygdala where it might activate a neuronal projection to the central amygdala.

Improved bioavailability to the brain of glycosylated Met-enkephalin analogs

The blood-brain barrier prevents the entry of many potentially therapeutic peptide drugs to the brain. Glycosylation has shown potential as a methodology for improving delivery to the CNS. Previous studies have shown improved bioavailability and improved centrally mediated analgesia of glycosylated opioids. In this study we investigate the effect of glycosylation on the cyclic opioid peptide [D-Cys(2,5),Ser(6),Gly(7)] enkephalin. The peptide was glycosylated on the Ser(6) via an O-linkage with various sugar moieties and alignments. The peptides were then investigated for receptor binding, physiochemical attributes, in situ brain uptake in female Sprague-Dawley rats and antinociception in male ICR mice. Glycosylation resulted in a slight decrease in affinity to the delta-opioid receptor, and mixed effect on binding to the mu-opioid receptor. There was a significant decrease in lipophilicity resulting from glycosylation and a slight reduction in binding to bovine serum albumin. In situ perfusion showed that brain uptake was improved by up to 98% for several of the glycosylated peptides, and the nociceptive profiles of the peptides, in general, followed the rank order of peptide entry to the brain with up to a 39-fold increase in A.U.C.

Endothelial vesicles in the blood-brain barrier: are they related to permeability?

1. Macromolecules cross capillary walls via large vascular pores that are thought to be formed by plasmalemmal vesicles. Early hypotheses suggested that vesicles transferred plasma constituents across the endothelial wall either by a "shuttle" mechanism or by fusing to form transient patent channels for diffusion. Recent evidence shows that the transcytotic pathway involves both movement of vesicles within the cell and a series of fusions and fissions of the vesicular and cellular membranes. 2. The transfer of macromolecules across the capillary wall is highly specific and is mediated by receptors incorporated into specific membrane domains. Therefore, despite their morphological similarity, endothelial vesicles from heterogeneous populations in which the predominant receptor proteins incorporated in their membranes define the functions of individual vesicles. 3. Blood-brain barrier capillaries have very low permeabilities to most hydrophilic molecules. Their low permeability to macromolecules has been presumed to be due to an inhibition of the transcytotic mechanism, resulting in a low density of endothelial vesicles. 4. A comparison of vesicular densities and protein permeabilities in a number of vascular beds shows only a very weak correlation, therefore vesicle numbers alone cannot be used to predict permeability to macromolecules. 5. Blood-brain barrier capillaries are fully capable of transcytosing specific proteins, for example, insulin and transferrin, although the details are still somewhat controversial. 6. It has recently been shown that the albumin binding protein gp60 (also known as albondin), which facilitates the transcytosis of native albumin in other vascular beds, is virtually absent in brain capillaries. 7. It seems likely that the low blood-brain barrier permeability to macromolecules may be due to a low level of expression of specific receptors, rather than to an inhibition of the transcytosis mechanism.

4-(2-Aminoethyl)benzenesulfonyl fluoride attenuates tumor-necrosis-factor-alpha-induced blood-brain barrier opening

The effect of serine protease inhibitor 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF) was investigated on the prevention of tumor-necrosis-factor-alpha (TNF-alpha)-induced blood-brain barrier opening. TNF-alpha (10,000 IU) was injected intracarotidly to newborn pigs pretreated with 0, 2.4, 4.8, 9.6 and 19.2 mg/kg AEBSF (n = 6 in each group). AEBSF dose-dependently inhibited the TNF-alpha-induced increase in the blood-brain barrier permeability for sodium fluorescein (MW = 376) in all of the five brain regions examined, while only 19.2 mg/kg AEBSF could significantly (P < 0.05) decrease the change in Evan's blue-albumin (MW = 67,000) transport in two regions. In conclusion, AEBSF attenuates vasogenic brain edema formation.

Transmission routes of HIV-1 gp120 from brain to lymphoid tissues

The blood-brain barrier (BBB) restricts the entry of antiviral agents into the CNS thereby facilitating the creation of a reservoir of HIV that could potentially reinfect peripheral tissues. We characterized the efflux from brain of radioactively labeled viral coat HIV-1 gp120 (I-gp120) after intracerebroventricular (i.c.v.) injection. The half-time disappearance rate of I-gp120 from brain was 12.6 min, which was faster than could be explained by the reabsorption of cerebrospinal fluid into blood but could not be explained by a saturable transporter. After i.c.v. injection, I-gp120 appeared in the serum and was sequestered by spleen and the cervical nodes, demonstrating a potential for virus within the CNS to reinfect peripheral tissues. However, the amount of I-gp120 appearing in serum was less than that expected based on the efflux rate, whereas uptake by the cervical nodes was much greater after i. c.v. than after i.v. injection of I-gp120. These findings were explained by drainage from the brain directly to the cervical lymph nodes through the brain's primitive lymphatic system. These lymphatics potentially provide a pathway through which CNS reservoirs of HIV-1 could directly reinfect lymphoid tissue without being exposed to circulating antiviral agents.

Adsorptive endocytosis of HIV-1gp120 by blood-brain barrier is enhanced by lipopolysaccharide

Previous work suggests that gp120 mediates the passage of HIV-1 and infected immune cells across the blood-brain barrier (BBB) by induction of adsorptive endocytosis (AE) in brain endothelial cells. Other work has suggested that cytokines may increase the permeability of the BBB to free virus or infected immune cells. Here, we investigated the ability of lipopolysaccharide (LPS), a bacterial wall toxin that stimulates the release of cytokines, to increase gp120 passage across the BBB by enhancement of AE and/or induction of BBB disruption. We found that LPS enhanced the passage of gp120 radioactively labeled with 125I (I-gp120) in a reversible, time-dependent, prostaglandin-independent manner that was not completely explained by disruption of the BBB. LPS also enhanced wheatgerm agglutinin mediated uptake of I-gp120 almost exclusively through the potentiation of AE. These results show that LPS or cytokines released by LPS can have a major effect on the permeability of the BBB to HIV-1gp120 both by stimulating AE and by inducing a disruption of the BBB. This suggests that bacterial infection or other inflammatory states could facilitate invasion of the CNS by HIV-1.

Transport of brain-derived neurotrophic factor across the blood-brain barrier

Brain-derived neurotrophic factor (BDNF) is a potential therapeutic agent for degenerative disorders of the central nervous system. In this report, we investigated the ability of BDNF to cross the blood-brain barrier (BBB). BDNF was stable in blood up to 60 min after i.v. injection, with evidence for aggregation, and had an early, rapid influx into brain. By 10 min, most of the BDNF sequestered by the cerebral cortex was associated with the parenchyma rather than with the endothelial cells, demonstrating complete passage across the BBB. A small dose of unlabeled BDNF enhanced the entry of 125I-BDNF from blood to brain after an i.v. bolus injection, whereas larger doses had no effect. In contrast, a large dose of unlabeled BDNF inhibited the influx of 125I-BDNF during in situ brain perfusion. After intracerebroventricular injection, the efflux of BDNF from brain to blood occurred at a rate similar to that for reabsorption of cerebrospinal fluid, and no evidence for self-inhibition was found. Therefore, we conclude that intact BDNF in the peripheral circulation crosses the BBB by a high-capacity, saturable transport system.

Blood-brain barrier damage in reperfusion following ischemia in the hippocampus of the Mongolian gerbil brain

Vascular permeability to intravenously injected horseradish peroxidase (HRP) was qualitatively examined in the hippocampus of ischemic Mongolian gerbil brains by light and electron microscopy. After 30 min of right common carotid artery occlusion followed by 90 min of reperfusion, the animal was perfused with a fixative and killed. Before the perfusion of the fixative, HRP was injected into the femoral vein. HRP was visualized with tetramethyl benzidine (TMB) and diamino-benzidine (DAB) for light and electron microscopy, respectively. Staining reaction with TMB for HRP appeared in medial or dorsal portions of the operated side of the hippocampus, especially around some vessels along the hippocampal fissure. Ultrastructural examination in the vessels along hippocampal fissure revealed that the endothelial cytoplasm contained HRP-filled vesicles or vacuoles in close proximity to the basal lamina, and seemed to be slightly electron-dense. Swollen pericytes, swollen astrocytic foot processes and perivascular cells with HRP-filled cytoplasm were also observed in that area. In this study, it was clearly demonstrated that intravascular macromolecules leaked transendothelially, through vessel walls in the hippocampal fissure, from the blood stream in the medial portions of the hippocampus during reperfusion following ischemia. These findings suggest that the blood-brain barrier in some vessels along the hippocampal fissure in the medial parts of the hippocampus is more vulnerable to ischemic insults than those in other brain areas.

Characterization of lectin-mediated brain uptake of HIV-1 GP120

The viral coat of the HIV-1 virus, gp120, has been shown to cross the blood-brain barrier (BBB) in lectin-like fashion by inducing adsorptive endocytosis (AE), a vesicular mechanism that could provide pathways into and across brain endothelial cells for virus and infected immune cells. Here, we extended those findings to show that gp120 slowly crossed the BBB with about 0.15% of an intravenously injected dose entering the brain after about 2 hr. The plant lectin glycoprotein wheat germ agglutinin (WGA) greatly enhanced gp120 crossing without disrupting the BBB. WGA enhanced the uptake of gp120 into all peripheral tissues studied, but the greatest percent increase occurred for brain, whereas another barrier tissue, the testis, had the least increase. Five other plant lectins tested had little or no effect on gp120 uptake by brain, suggesting a key role for sialic acid and N-acetyl-beta-D-glucosaminyl acid, the sugars to which WGA binds, in the uptake of gp120 by brain endothelial cells. WGA did not enhance the uptake of nonglycosylated gp120 and the uptake of gp120 was not self-inhibitable or altered by pretreatment of mice with aluminum. In conclusion, these studies show that gp120 crosses the BBB by a lectin-like mechanism resembling AE that is likely mediated by binding to specific sugar moieties and is rather selective for brain.

Adsorptive endocytosis mediates the passage of HIV-1 across the blood-brain barrier: evidence for a post-internalization coreceptor

HIV-1 induces the AIDS dementia complex and infects brain endothelial and glial cells. Because the endothelial cells comprising the blood-brain barrier (BBB) do not possess CD4 receptors or galactosylceramide binding sites, it is unclear how HIV-1 negotiates the BBB. Previous work has suggested that gp120, the glycoprotein viral coat of HIV-1, is capable of inducing adsorptive endocytosis. Glycoprotein lectins like wheatgerm agglutinin induce adsorptive endocytosis and greatly potentiate the uptake by and passage across mouse endothelial cells in vivo and in vitro. We show here that the wheatgerm agglutinin-induced binding of gp120 is dose-dependent and involves components of the cytoskeleton. The uptake is partially dependent on temperature and energy and is modestly enhanced by potassium depletion. Glycosylation of gp120 is critical for its uptake by adsorptive endocytosis since the non-glycosylated form of gp120 is unaffected by wheatgerm agglutinin. Evidence is presented for the existence of a coreceptor sensitive to protamine sulfate that is primarily involved in membrane fusion after 125I-gp120 has bound to the cell membrane and is probably activated after internalization. This coreceptor probably contains a negatively charged heparin sulfate group and could be a member of the chemokine receptor family.

Interactions of beta-amyloids with the blood-brain barrier

Blood-borne beta-amyloids (A beta s) could affect brain function by (1) crossing the BBB to directly interact with brain tissues or (2) altering BBB function by interacting with the brain capillaries that make up the BBB. Several radioactively labeled A beta s have been examined for such interactions. Blood-borne A beta 1-28 is hindered from accumulating in brain by a slow rate of passage across the BBB and by robust enzymatic degradation. A beta 1-40, but not A beta 40-1 or A beta 1-42, is sequestered by brain capillaries, raising the possibility that it could affect BBB function. Small amounts of circulating A beta 1-40 are recovered intact from CSF and brain. A beta 1-40 is degraded by aluminum-sensitive, calcium-dependent intracellular enzymes. Apo-J, which can bind A beta, has been shown with an in situ method to be transported by a saturable system across the BBB. However, our recent work has shown that this system is not operable in vivo, probably because the transporter is saturated at physiological blood levels. In conclusion, A beta s have been shown to interact with and to cross the BBB.

Acute modulation of active carrier-mediated brain-to-blood transport of corticotropin-releasing hormone

The unidirectional brain-to-blood transport system for corticotropin-releasing hormone (CRH) across the blood-brain barrier could be instrumental in the homeostasis of central CRH. To characterize this system, the intracerebroventricular injection of 125I-CRH was used in mice. CRH was rapidly transported out of the brain with a half-time disappearance (t1/2) of 15 min, much faster than albumin (t1/2 = 50 min). Kinetic analysis revealed a saturable component with a low maximum velocity (apaproximately 0.020 nmol x min(-1) x brain(-1)) and low capacity (Michaelis constant approximately 1.4 nmol/brain). Transport was inhibited by verapamil, ouabain, and colchicine but not by cyclosporin. Transport was increased by corticosterone and inhibited by tumor necrosis factor-alpha and beta-endorphin. These results suggest that the specific unidirectional brain-to-blood transport system for CRH is dependent on energy and calcium channels, involves microtubules, is independent of the P-glycoprotein transporter, and is acutely modulated by adrenal steroids, cytokines, and endogenous opiates. This suggests its participation in the control of the stress response.

HIV-1 protein gp120 crosses the blood-brain barrier: role of adsorptive endocytosis

HIV-1 infects the brain and leads to AIDS dementia complex. The viral coat glycoprotein, gp120, may facilitate the passage of HIV-1 and HIV-infected immune cells across the blood-brain barrier (BBB). Since the endothelial cells of the BBB do not possess the CD4 or galactosylceramide binding sites used by gp120 to induce HIV-1 uptake into other cell types, how gp120 mediates entry into brain is unknown. We postulate that gp120 crosses the BBB and does so by acting as a weak lectin to induce adsorptive endocytosis (AE) in a fashion similar to other glycoproteins like wheatgerm agglutinin (WGA). We found in vivo that gp120 crosses the BBB and its passage is enhanced 18.7-fold by WGA. In vitro studies confirm that WGA enhances uptake of gp120 by brain endothelia; most of the uptake is membrane-associated, as expected in AE. Uptake is not dependent on clatharin, caveolae, calcium channels, or endosomal acidification. Our results suggest that gp120 crosses the BBB and does so by acting as a lectin to induce AE.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

Blood-endothelial cell and blood-brain transport of L-proline, alpha-aminoisobutyric acid, and L-alanine

We report the application of multiple time regression analysis with the in situ brain perfusion technique to measure the rates of passage between blood and brain for [14C] L-proline, [14C] L-alanine, and [14C] alpha-aminoisobutyric acid (AIB) and their rapidly reversible volumes following perfusion of these amino acids from 10 to 60 seconds. We also report on their mechanism of transport. Proline diffused through the blood-brain barrier with a transfer coefficient (Kin) of 0.55 +/- 0.15 x 10(-4) ml/s/g and had no reversible compartment. AIB had a low Kin of 0.68 +/- 0.14 x 10(-4) ml/s/g and a significant reversible volume of 4.34 +/- 0.51 x 10(-3) ml/g in parietal cortex. L-alanine had the highest transfer coefficient, 3.11 +/- 0.26 x 10(-4) ml/s/g, and a reversible volume of 10.03 +/- 0.93 x 10(-3) ml/g in the same cerebral region. Postwash procedures which remove any radiotracer in the vasculature and capillary depletion were performed for alanine and AIB, as they had significant reversible compartments, to test the possibility of rapid efflux from the endothelial cells. Results obtained from wash and capillary depletion procedures suggest that a rapid efflux could occur from endothelial cells after entry of alanine and AIB. Mechanisms of transport for L-alanine and AIB were investigated using amino acids (5 mM) as substrates and inhibitors of different amino acid transport systems. AIB transport was reduced by plasma and L-leucine and unchanged by sodium-free buffer, confirming its passage by the L1 system. L-alanine uptake was sodium-independent and not reduced by plasma. L-serine, L-cysteine, L-leucine and L-phenylalanine produced similar inhibition (66%) while L-alanine produced a lower inhibition (41%). L-arginine increased alanine uptake in cortex and thalamus. Adding L-serine to L-phenylalanine reduced the uptake only in cortex and hippocampus. These data suggest that L-alanine is transported by another L transport system different from the L1 system at the luminal membrane.

Reversible association of the cytokines MIP-1 alpha and MIP-1 beta with the endothelia of the blood-brain barrier

Macrophage inflammatory proteins (MIP)-1 alpha and -1 beta have been postulated to exert their pyrogenic effects by acting directly at sites within the brain. Such activity would require circulating MIP-1s to cross the blood-brain barrier (BBB). We examined the ability of the monomer and polymer of MIP-1 alpha and the polymer of MIP-1 beta radioactively labeled with 125iodine (I-MIP-1) to cross the BBB. These I-MIP-1s behaved very similarly to each other but in a manner not previously seen for other cytokines. The I-MIP-1s immediately associated to a high degree and in a reversible manner with the vascular space of the brain. This association did not increase over time nor was it self-inhibitable. These results make it unlikely that the MIP-1s are transported into the brain by saturable transport systems in the manner found for some of the other cytokines. Other mechanisms, such as interactions with brain endothelia, leakage into brain through extracellular pathways, and binding at circumventricular organs, are more likely to provide the mechanisms through which blood-borne MIP-1s affect the central nervous system.

Immunocytochemical evaluation of the blood-brain barrier to endogenous albumin in the olfactory bulb and pons of senescence-accelerated mice (SAM)

The blood-brain barrier (BBB) to endogenous albumin was studied in the olfactory bulb and pons of the senescence-accelerated prone (SAMP8) mouse and senescence-accelerated resistant (SAMR1) mouse strains by using a quantitative immunocytochemical procedure. Ultrathin sections of Lowicryl K4M-embedded samples were exposed to anti-mouse albumin antiserum followed by protein A-gold. Morphometric analysis of the electron micrographs revealed that in the olfactory bulb of both groups of animals, especially in the internal granular layer, some percentage of capillaries and slightly larger microvessels showed leakage of albumin. However, this percentage was larger in SAMP8 than in SAMR1 mice. In the pons, no significant differences in the permeability of blood microvessels were observed in both groups of mice, although a small fraction of capillaries in SAMP8 mice showed limited extravasation of blood plasma albumin. These observations indicate that the BBB in the olfactory bulb of control and SAMP8 mice is not as tight as it is in the pons or in the previously examined cerebral cortex. The labelling density of the neuropil was slightly higher than in the cerebral cortex, suggesting that albumin may have extravasated locally, in addition to having acces to the parenchyma of the olfactory bulb and pons from neighbouring areas supplied with the non-BBB-type of microvasculature. Furthermore, the data obtained suggest that there is limited (segmental), premature age-related impairment of the BBB function in SAMP8 mice.

Penetration of interleukin-6 across the murine blood-brain barrier

Interleukin-6 (IL-6) can alter brain function after peripheral administration, suggesting that it, like IL-1 alpha, IL-1 beta and TNF-alpha, might be able to cross the blood-brain barrier (BBB). We used multiple-time regression analysis to measure the unidirectional influx constant (Ki) into brain of radioactively labeled murine and human IL-6 given i.v. Ki values ranged from 3.05 to 4.54 (10(-4)) ml/g/min and were inhibited by unlabeled IL-6 but not IL-1 alpha or TNF-alpha, showing that the transport system for IL-6 is distinct from those for IL-1 alpha and TNF-alpha. Approximately 0.2% of the dose injected i.v. entered each gram of brain. The capillary depletion method showed that most of the IL-6 taken up by brain entered the parenchyma. However, only approximately 16% of the radioactivity recovered eluted as intact I-IL-6 in brain and approximately 50% in CSF after chromatographic separation by HPLC/Sephadex. The efflux rate for IL-6 injected into the lateral ventricle of the brain suggests that it enters the blood with the reabsorption of CSF. These results suggest that blood-borne IL-6 can reach sites behind the BBB, but that susceptibility to enzymatic degradation may limit contact time within the CNS.

Blood-borne interleukin-1 alpha is transported across the endothelial blood-spinal cord barrier of mice

1. Previous work has shown that one mechanism by which blood-borne interleukin-1 alpha (IL-1) may be able to affect the central nervous system (CNS) is by direct transport into the brain across the blood-brain barrier (BBB). The BBB of the brain consists of endothelial (between blood and interstitial fluid) and ependymal (between blood and cerebrospinal fluid) barriers. Which of these barriers IL-1 can cross has not previously been investigated. At the spinal cord, which could be the site of action for some of the effects of IL-1 such as analgesia, the BBB consists only of the endothelial barrier. 2. We show here that IL-1 labelled with 125I (I-IL) is transported across the BBB of the spinal cord by a saturable system similar to the one previously described for the brain. High performance liquid chromatography (HPLC) showed that most of the material entering the spinal cord represented intact I-IL. The BBB of the spinal cord was no more leaky to radioactively labelled albumin than the BBB of the brain and was not disrupted by 50 micrograms kg-1 of IL-1. 3. Capillary depletion showed that most of the I-IL entered the parenchymal-interstitial fluid space of the spinal cord with only a modest amount being sequestered by the endothelial cells of its BBB. 4. I-IL entered the cervical, thoracic and lumbar regions of the spinal cord equally well. I-IL entering at the brain and diffusing caudally was estimated only to account for about 1% of the total radioactivity found in the spinal cord after i.v. injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Passage of human amyloid beta-protein 1-40 across the murine blood-brain barrier

Previous studies have suggested that the amyloid beta-protein present in the brains of patients with Alzheimer's disease may be derived in part from peripheral blood. We determined that after IV injection of synthetic amyloid beta-protein 1-40 (A beta), labeled with radioactive 125I (I-A beta), radioactivity accumulated in the brains of mice by a nonsaturable mechanism. Radioactivity also accumulated in the brain after the i.v. injection of radioiodinated reverse amyloid beta-protein 40-1 (I-rA beta). Capillary depletion techniques, however, showed I-A beta to have a much greater degree of association with brain capillaries than I-rA beta. Acid precipitation of radioactivity in CSF samples and recovery from cortical homogenates suggested the presence of intact I-A beta within the CNS after peripheral administration. HPLC analysis of cortical homogenates confirmed the presence of intact I-A beta. Gel electrophoresis of the CSF acid precipitates and of the HPLC fractions further verified the presence of intact blood-derived I-A beta peptide in CNS. These results suggest that endogenous bloodborne A beta can enter the CNS after associating with the capillary endothelium to accumulate intact within the parenchymal and CSF spaces of the brain.