An enzymic barrier mechanism for monoamine precursors in the newly-forming brain capillaries following electrolytic or mechanical lesions

Challenges for blood-brain barrier (BBB) screening

Whilst blood-brain barrier permeability is an important determinant in achieving efficacious central nervous system drug concentrations, it should not be viewed or measured in isolation. Recent studies have highlighted the need for an integrated approach where optimal central nervous system penetration is achieved through the correct balance of permeability, a low potential for active efflux, and the appropriate physicochemical properties that allow for drug partitioning and distribution into brain tissue. Integrating data from permeability studies performed incorporating an assessment of active efflux by P-glycoprotein in combination with drug-free fraction measurements in blood and brain has furthered the understanding of the impact of the blood-brain barrier on central nervous system uptake and the underlying physicochemical properties that contribute to central nervous system drug disposition. This approach moves away from screening and ranking compounds in assays designed to measure or predict central nervous system penetration in the somewhat arbitrary units of brain-blood (or plasma) ratios.

Age-related changes in the blood-brain barrier

Aging of the cerebral microcirculation results in significant alteration in the blood-brain barrier (BBB). The barrier function appears to remain intact in older animals, although it may be more susceptible to disruption by external factors (hypertension) and drugs (haloperidol). While overall transport processes do not change with age, aging animals and humans have altered BBB function of select carrier mediated transport systems including the transport of choline, glucose, butyrate and triiodothyronine. These age-related changes are the result of either alteration in the carrier molecules or the physiochemical properties of the cerebral microvessels. At the present time, it is not known whether changes in the BBB contribute to the age-related neurodegenerative diseases or are merely epiphenomena of aging.

Molecular anatomy of the blood-brain barrier as defined by immunocytochemistry

This review outlines the recent developments and improvements of our knowledge concerning the molecular composition of the BBB as revealed by immunocytochemistry. Data have been accumulated which show that the BBB exhibits a specific collection of structural and metabolic properties which are also found in tight transporting epithelia. This conclusion is substantiated by (i) the implementation of antibodies which recognize proteins of non-BBB origin, to show that these biochemical markers and the functions that they represent are localized in the BBB endothelium; and (ii) the characterization of target molecules to which polyclonal or monoclonal antibodies which have been generated to epitopes of the BBB endothelium or brain homogenates. According to these data the protein assemblies comprising the phenotypical appearance of the BBB can therefore be defined by the particular selection as well as topological expression of common epithelial antigens, rather than the expression of BBB-unique molecular species. In this respect the immunocytochemical data corroborate the physiological assumption that the BBB possesses the character of a specific polarized epithelium. Attention is also given to the description of developmental expression of BBB-related immunomarkers. By collecting the data from different sources we introduce a classification of the BBB marker proteins according to their developmental appearance. Three groups of proteins are classified with respect to their sequential expression around the time of BBB closure: Phase E (early) markers which appear before BBB closure, phase I (intermediate) markers which are expressed at the time of BBB tightening, and phase L (late) markers which are detectable after the closure of the BBB. Such a scheme may to be useful in better defining the maturation process of BBB, which apparently is not a momentary event in brain development, but rather consists of a temporally sequenced process of hierarchically structured gene expression which finally define the molecular properties of the BBB. This process continues even after parturition, especially with regard to the achievement of immunological properties of the mature BBB. By examining the developmental spatio-temporal expression of different BBB markers we conclude that the mechanisms governing the pattern of BBB maturation are not limited to the interactions occurring between glial and endothelial cells. We therefore suggest a heuristic model in a triangular interrelationship that includes differentiation effects of neurons on glia and of glia cells on the BBB endothelium.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Barrier mechanisms for neurotransmitter monoamines and their precursors at the blood-brain interface

The integrity of the endothelial cell lining of the cerebrovascular bed constitutes a morphological blood-brain barrier mechanism to neurotransmitter monoamines. Circulating monoamines are prevented from entering the brain primarily at the luminal membrane of the endothelial ling. The small percentage of amines that may pass this membrane is deaminated within the endothelial cells and pericytes of brain microvessels (capillaries, venules, and small veins) and, in the case of large parenchymal and pial vessels, in the smooth muscle layers, where O-methylation also takes place. In the choroid plexus a corresponding deamination and O-methylation takes place in the epithelial cells. The presence of these enzymes constitutes a further, enzymatic, blood-brain barrier in the brain vessels for these monoamines. The monoamine precursors L-3,4-dihydroxyphenylalanine (L-dopa) and L-5-hydroxytryptophan readily pass from the luminal endothelial cell membrane but are trapped by another enzymatic barrier mechanism. Within the endothelial cells and pericytes of the microvasculature, these compounds are decarboxylated to their corresponding amines and then immediately deaminated. One clinical implication of these enzymatic barrier mechanisms is the use of decarboxylase and monoamine oxidase inhibitors as adjuncts to L-dopa treatment of Parkinson disease; these substances facilitate the entry of L-dopa into brain and thus increase the amount of dopamine available at receptor sites. A brief hypertensive or hypertonic stimulus can transiently open the blood-brain barrier through an effect on endothelial cell linings. High circulating concentrations of monoamines can also open the morphological barrier, but probably only indirectly by inducing an acute rise in systemic blood pressure. Once the barrier is open, systemically administered monoamines enter the brain parenchyma, where they can induce pronounced changes in cerebral blood flow and metabolism.