Hugh Davson

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)

An introduction to the blood-brain barrier

History of basic concepts transport of glucose and amino-acids in the central nervous system peptides and proteins transport of some precursors of nucleotides and some vitamins experimental models in the study of pathology of the blood-brain barrier.

A saturable mechanism for transport of immunoglobulin G across the blood-brain barrier of the guinea pig

The existence of an immunological blood-brain barrier to homologous blood-borne immunoglobulin G (IgG) was investigated in the guinea pig using a vascular brain perfusion technique in situ. Cerebrovascular unidirectional transfer constants (Kin) for 125I-labeled IgG (2.5 micrograms/ml) estimated from the multiple-time brain uptake data, ranged from 0.53 to 0.58 ml min-1 g-1 X 10(3) in the parietal cortex, hippocampus, and caudate nucleus, the transfer rate being some 10 times higher than that for [3H]dextran (MW 70,000). In the presence of 4 mg/ml unlabeled IgG, unidirectional blood to brain transfer of 125I-IgG was markedly inhibited. Immunohistochemical analysis of the brain tissue after vascular perfusion with unlabeled IgG revealed a distribution of the blood-borne immunoglobulin in the endothelial cells of microvessels and in the surrounding perivascular tissue. It is concluded that there is a specific transfer mechanism for IgG at the blood-brain barrier in the guinea pig, which is saturated at physiological plasma levels of IgG.

Blood-brain barrier permeability changes during acute allergic encephalomyelitis induced in the guinea pig.

Blood-brain barrier permeability to homologous serum125I-IgG and to D-[3H]mannitol was studied by means of the brain vascular perfusion method in guinea pigs with experimental allergic encephalomyelitis (EAE). EAE was induced with homologous myelin basic protein (MBP) after pretreatment with foreign protein and muramyl dipeptide (MDP). The results suggest a significant comparable increase in IgG blood-to-brain clearance in the parietal cortex, hippocampus, and caudate nucleus, during vascular perfusion of the brains of animals, after 7 and 20 days of EAE. On the other hand, unidirectional transfer of mannitol in the same period of EAE was markedly augmented only in the hippocampus, but no significant changes in the parietal cortex or caudate nucleus were observed. Cerebrospinal fluid (CSF)/serum ratios for IgG and albumin were both significantly increased, suggesting an increase in blood-CSF barrier permeability, but more for albumin than for IgG. The results were confirmed by immunohistochemical determination of the IgG deposits in the brains of EAE animals, during vascular perfusion with unlabeled homologous IgG. An important role of the blood-brain barrier for the central nervous system immunoglobulin homeostasis during EAE is suggested.

Chronic amphetamine intoxication and the blood-brain barrier permeability to inert polar molecules studied in the vascularly perfused guinea pig brain

The brain vascular perfusion method, with a multiple-time brain uptake analysis, has been employed to study the effects of chronic amphetamine intoxication on the kinetics of entry of 2 inert polar molecules, D-[14C]mannitol (mol.wt. 180) and [3H]polyethylene glycol (PEG, mol.wt. 4000) into the forebrain of the guinea pig. The unidirectional transfer constants, Kin, determined from graphic analysis 14 and 20 days after chronic amphetamine treatment (5 mg/kg daily, i.p.) showed a marked time-dependent progressive enhancement of transfer for both molecules. The kinetic features of this entry suggest the opening up of pathways through the blood-brain barrier (BBB) which allows mannitol and PEG to pass into the brain at rates which are irrespective of their molecular size and/or lipophilia and these changes cannot be attributed to simple mechanical factors such as hypertension. This opening of the BBB was associated with changes in behaviour (increased locomotor activity, stereotypy, hypervigilance, social withdrawal, and loss of weight) seen in 14- and 20-day amphetamine-treated animals. At 7 and 28 days after the withdrawal of the amphetamine treatment, the behavioural manifestations were absent, and the Kin values for both molecules were not significantly different from those measured in normal control animals which had been treated with placebo injections. The present results suggest a reversible dysfunction of the BBB as a consequence of the chronic amphetamine intoxication which correlates with the behavioural syndrome induced in the guinea pig.

Steady-state distribution of cycloleucine and α-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.

History and Basic Concepts

The concept of the blood-brain barrier derives from the classical studies of the pioneers in chemotherapy, such as Ehrlich, who administered dyestuffs parenterally in the hope that they would attack infective organisms. Thus Ehrlich observed that many dyes, after intravenous injection, stained the tissues of practically the whole body, while the brain was spared. Later, Lewandowsky (1900) showed that the Prussian blue reagents (iron salt and potassium ferrocyanide) did not pass from blood to brain, and he formulated clearly the concept of the blood-brain barrier (Bluthirnschranke). The more definitive demonstration of the barrier we owe to Goldmann, who showed (1909) that, after intravenous injection with trypan blue, the brain was unstained; the dye did not enter the cerebrospinal fluid (CSF), although the choroid plexuses and meninges were stained. In a second paper (Goldmann, 1913), he described experiments in which trypan blue was injected into the CSF; in this event, the brain tissue was strongly stained, so that Goldmann rightly concluded that there was, indeed, a barrier between blood, on the one hand, and brain tissue on the other. Any argument that the failure to stain the brain with trypan blue after intravenous injection was due to a peculiar staining feature of the nervous tissue was negated by this fundamental ‘second experiment’, the first experiment being the demonstration that nervous tissue was unstained after intravenous injection.

Effects of sensory-motor cortical lesions on blood-brain permeability in guinea pigs

Effects of sensory-motor cortical lesions on the function of the blood-brain barrier in distant brain areas are poorly understood. Therefore a brain vascular perfusion method has been used to measure simultaneously the kinetics of entry of two inert polar molecules, D[14C]mannitol (MW 180) and [3H]polyethylene glycol (PEG; MW 4000), into the parietal cortex, hippocampus, and caudate nucleus in guinea pigs with ipsilateral and contralateral sensorymotor cortical lesions. The graphically determined cerebral capillary unidirectional constant,Kin, indicated a marked increase in blood-to-brain transport of both molecules in all regions studied, the changes being significantly higher after contralateral lesion. The mannitol/PEG cerebrovascular permeability constant ratio,Pman/PPEG, suggested the opening up of channels that permit a flow of fluid carrying substances either with respect to (2 days after ipsilateral lesion) or irrespective of their molecular size, depending on the time after lesion. Amphetamine treatment in the guinea pigs with sensory-motor lesions induced more pronounced blood-brain barrier permeability changes for both molecules in distant brain areas.

Transport of Glucose and Amino Acids in the Central Nervous System

Glucose is the main source of energy for the central nervous system, so that the dynamics of its supply from blood are of great interest, especially since the central nervous tissue does not store glycogen to any great extent. Thus, the continuous metabolism of glucose must be adequately maintained by a continuous supply of the metabolite from the blood. Sugars, like amino acids, are highly water-soluble, and, being relatively large molecules, they are unlikely to penetrate the capillary membrane of nervous tissue to any extent in the absence of special mechanisms. That special mechanisms exist for both classes of metabolite has already been made clear, the transport from blood to nervous tissue being of the carrier-mediated, or facilitated, type, exhibiting Michaelis-Menten kinetics from which the two parameters, Km and Vmax may be derived, Km being regarded as the reciprocal of the affinity of the molecule for the hypothetical carrier in the capillary membrane, or, more specifically, the concentration of the solute at which the carrier is half-saturated. Vmax is the maximum rate of transport across the capillary, obtained by extrapolating the rate to zero solute concentration, and is an index to the number of carrier sites available to the solute.

Blood—Brain Barrier Permeability to Peptides and Proteins

The neurons, glial cells, brain extracellular fluid and cerebrospinal fluid are separated from the blood by the blood-brain and blood-cerebrospinal fluid barriers (Davson, 1976). The blood-brain barrier is well characterized morphologically as a complete and continuous cellular layer of the endothelial cells which are sealed by tight junctions (Brightman, 1977). Normal cell-to-cell communications between astrocytes, pericytes, endothelial cells and surrounding neuropil are essential for the expression of blood-brain barrier phenomena and its homoeostatic mechanisms (Davson and Oldendorf, 1967; Brightman, 1989). Transport, enzymatic and receptor-mediated functions of the blood-brain barrier and blood-cerebrospinal fluid barrier are highly developed, playing a central role in the regulation of the composition of brain extracellular fluid and cerebrospinal fluid. The free movement of circulating hydrophilic substrates from blood to brain extracellular and cerebrospinal fluids is markedly retarded, and it has been accepted that any molecule, above a limiting size, circulating in the blood may gain access to the brain interstitial space only if there is a specific transport system for that molecule localized in the brain capillary endothelium (Oldendorf, 1987; Betz and Goldstein, 1986; Pardridge, 1988).