Ultrastructural studies on cerebrovascular permeability in acute hypertension

The meningococcus and mechanisms of pathogenicity

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Phenothiazine-mediated protection of the blood-brain barrier during acute hypertension

The phenothiazine dixyrazine (5 mg . kg-1 i.v.) had minimal, transient hypotensive effects but significantly reduced the leakage of 125I labelled serum albumin in conscious rats subjected to acute hypertension provoked by i.v. adrenaline or bicuculline. By contrast, dixyrazine did not protect the blood-brain barrier during osmotic stress induced by intracarotid infusion of 2 M urea. The diameters of pial arteries and veins were continuously measured with a multichannel videoangiometer through a closed cranial window in anesthetized rats before and after i.v. injection of dixyrazine (5 mg . kg-1). No change in vessel diameter was observed except for a transient autoregulatory dilatation of arteries in response to a slight transitory decrease in the blood pressure. It is concluded that dixyrazine probably protects the blood-brain barrier during mechanical stress by modifying the endothelial cell membrane.

The permeability alteration of brain and spinal cord vasculature to horseradish peroxidase during experimental decompression sickness as compared to the alteration in permeability induced by hyperosmolar solution

The permeability of microvasculature in the cerebral cortex, neostriatum, and spinal cord to i.v. injected horseradish peroxidase (HRP) has been investigated in rats following experimental compression to 6.1 bars (abs.) air for 90 min, and subsequent decompression to the ambient pressure in 1 min. For comparison, 1 ml of 2.0 M urea was injected into the right common carotid artery of rats during 15 s. After exposure to compression-decompression, under the light microscope focal leaky areas were found in all the regions examined. The leakage was most prominent in the grey matter of the spinal cord, and the cerebral cortex. In decompressed rats, arterioles were most often the site of peroxidase extravasation, whereas extravasation of HRP was less frequently displayed by capillaries and venules. In urea-treated rats, capillaries and venules frequently displayed extravasation of HRP as well. Parenchymal cells accumulated the trace adjacent to the leaky areas. Under the electron microscope, the extravasation of HRP was associated with peroxidase-containing pleomorphic vesicular structures in the endothelium, both in decompressed and urea-injected rats. Moreover, in contrast to decompressed rats, the junctions between endothelial cells were penetrated by the trace in urea-treated rats. Accordingly, the results indicate that during decompression sickness the pathway for the extravasation of proteins is through vesicular transfer, whereas the injection of hyperosmolar urea induces extravasation, both through vesicular transfer and junctions between the endothelial cells.

Computed tomography and stroke edema: case report with an analysis of water in acute infarction

An index case and 13 other cases of acute ischemic cerebral infarction have been examined following the occurrence of death 1-6 days postictus. Histologic studies and water content assay involved both the infarct and peri-infarct tissue. The primary site of fluid accumulation was the infarcted white matter; however, after 3 days, edema in the adjacent white matter may be significant. The reduced attenuation of CT scans is caused only by edema since necrosis of the white matter is not present. Reactive vascular changes that occur in the cortex appear to be the basis of early contrast enhancement. Three different mechanisms of edema formation have been proposed to be involved in the mass effect.

Permeability of intracranial extracerebral vessels in stroke-prone SHR

Permeability of intracranial extracerebral arteries of stroke-prone spontaneously hypertensive rats (SHRSP) was studied using labeling techniques (ferritin and horseradish peroxidase), at the cellular level. In the arterial endothelial cells, the tracer molecules were slowly but constantly transported by the plasmalemmal vesicles to the subendothelial space. This endothelial transportation of the tracers into these cerebral arteries did not seem to be significantly influenced by aging, increased blood pressure, hyperlipidemia or the existence of cerebral bleeding and infarction. Around the adventitia, there were a great number of periadventitial capillaries, especially near bifurcations. In the periadventitial capillaries, the tracer molecules were readily trapped by endothelial cells and were quickly transported to pericapillary spaces. The tracer molecules were then detected in the phagocytes adjacent to the deeper layers of the media, and further in the medial smooth muscle cells. The possibility that large amounts of plasma components are supplied to the media from periadventitial capillaries in the intracranial extracerebral arteries has to be considered in the pathogenic mechanisms of cerebrovascular lesions.

Ultrastructural characteristics of endothelial permeability in chronic hypertension

This study examined characteristics of paracellular and pinocytotic permeability pathways across the middle cerebral artery endothelium of 12- to 16- month-old spontaneously hypertensive rats (SHR). Interendothelial junctions in SHR, like those of age-matched Wistar-Kyoto controls, were impermeable to lanthanum and horseradish peroxidase (HRP) tracers. Freeze-fracture preparations revealed that interendothelial junctions of chronically hypertensive rats are characterized by a twofold increase over controls in the number of tight junctional strands and the mean apical-basal depth. It is believed that this tight junctional hypertrophy may function to increase adhesive forces between neighboring endothelial cells, and may play a role inthe prevention of hypertension-induced paracellular permeability increases. Morphological and tracer studies of pinocytotic pathways indicated that, it the middle cerebral artery, endothelial vesicular transport activity is not increased during chronic hypertension. No evidence was found to indicate the presence of transendothelial permeability channels across control or hypertensive arterial endothelium. Thus, increased transendothelial permeability, commonly observed in acute hypertension, does not appear to occur during chronic hypertension, at least in the middle cerebral artery. Our findings suggest that the arterial endothelium may undergo structural (tight junctional) adaptation in response to prolonged hypertension.

Cerebrovascular permeability in mechanically induced hypertension

Our previous studies of cerebrovascular permeability in angiotensin-induced acute hypertension demonstrated that the principal mechanism resulting in increased permeability is enhanced pinocytosis. In order to exclude the possibility that the enhanced pinocytosis was a direct effect of exogenous angiotensin, cerebrovascular permeability alterations were studied in nonpharmacologically induced acute hypertension. Rats receiving horseradish peroxidase (HRP) intravenously, were sacrificed 2 1/2 minutes after the onset of hypertension induced by placing a clip on the abdominal aorta. These animals showed the same pattern of permeability alterations as had been observed previously in animals with angiotensin-induced acute hypertension. Focal segments of penetrating arterioles in the temporal and parietal cortex showed increased permeability to HRP. Permeable vessels showed increased numbers of pinocytotic vesicles and the interendothelial junctions revealed no alterations. Enhanced pinocytosis appears to be the principal mechanism resulting in increased cerebrovascular permeability in this model as well as suggesting that the alterations of cerebrovascular permeability observed previously in angio-tensin-induced acute hypertension occur due to the hypertensive state and are not a direct drug effect of exogenous angiotensin.

Ultracytochemical studies of the blood-meningeal barrier (BMB) in rat spinal cord

Alkaline phosphatase(AP),5'-nucleotidase(5'N) and nucleoside diphosphatase (NDPase) activities were studied by cytochemical methods applied to light and electron microscopy in the microvasculature of spinal cord leptomeningeal strips of normal and protamine sulfate (PS) treated rats. The increased permeability to intravenously injected horseradish peroxidase was observed in some segments of microvessels of PS treated rats. Enhanced formation of plasmalemmal pits and deep invaginations, formation of numerous pinocytic vesicles and the appearance of channel-like structures in the cytoplasm of endothelial cells were the most striking ultrastructural evidence of increased permeability of the affected microvessels. All of these structures also showed activity of AP, and to lesser extent, of NDPase; 5'N activity was mainly associated with the delimiting membranes of pinocytic vesicles. Our data present evidence that a shift of enzymatic activity from luminal to abluminal surface of affected endothelial cells results from membrane flow accompanying increased transport activity via formation of pinocytic vesicles and channel-like structures.

Opening of the blood-brain barrier by acute elevation of intracarotid pressure

A method for local opening of the blood-brain barrier in the territory of one internal carotid artery in the rat is described. A local hypertensive insult is induced by rapid infusion of blood into the internal carotid via the external carotid. The hemodynamic changes caused by the infusion, in particular relation to the threshold and extent of barrier opening, are analyzed. This mode of hypertensive barrier opening may be advantageous to those in which the insult is induced systematically, especially when studying the cerebrovascular effects of neurotransmitter catecholamines, since all the latter methods interfere with adrenergic mechanisms. Further, unilateral intracarotid infusion may allow the territory of the contralateral middle cerebral artery to be used as internal control.

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.

Vincristine reduces damage of the blood-brain barrier induced by high intravascular pressure

A reversible opening of the structural blood-brain barrier (BBB) was accomplished in rats by rapidly increasing the hydrostatic pressure either through forced intracarotid infusion of autologous blood or i.v. administration of angiotensin, whose effect was exaggerated by previous injection of the vasodilator, papaverine. The extent of barrier damage was evaluated in terms of the extravasation of Evans blue-albumin complex or the brain uptake of circulating [14C]inulin. Pretreatment of the animals with the antimitotic agent, vincristine, significantly reduced the barrier damage. This is interpreted as an effect of vincristine on intracellular microtubule systems, which may be involved in the vesicular transport across the endothelial cells of brain microvessels.

Characterization of the in vitro uptake of monoamines into brain microvessels

The ability of brain microvessels to take up and metabolize transmitter monoamines (DA, NA, A and 5-HT) was studied in tissues from several species (rat, guinea-pig, cat, dog, baboon, man) in vitro. Following incubation, slices were analyzed by fluorescence histochemistry and isolated microvessels by measurement of radioactivity from tritiated amine. Provided the MAO activity was inhibited a strong accumulation, similar for all animals tested, occurred in the wall (endothelial cells and pericytes) of capillaries and venules--but not in larger vessels--of all species. The degree of amine uptake and accumulation and the conditions under which it was inhibited suggested that it reflected a saturable, energy-dependent process, with characteristics of both the extraneuronal and neuronal type of uptake processes. The mechanism may serve as an inactivation of transmitter monoamines at the blood-brain interphase, thereby assisting in the control of transmitter levels in the cerebral extracellular compartment.

Cerebral venous pressure during actively induced hypertension and hypercapnia in cats

Superior sagittal sinus pressure, intracranial pressure and arterial pressure were recorded in an experimental series on 10 cats. During drug-induced, severe, acute arterial hypertension and parallel hypercapnia, venous pressure could exceed intracranial pressure in both the supra- and infratentorial compartment. From these data it is concluded that cerebral venous pressure during acute arterial hypertension may contribute to protein extravasation at the postcapillary-venular level.

Hemodynamic changes in brain caused by local infusion of hyperosomolar solutions, in particular relation to blood-brain barrier opening

The intracarotid infusion of hyperosmolar aqueous solutions such as urea is widely used to transiently open the blood-brain barrier in various animal species. In the present study in the rat an attempt was made to analyze the hemodynamic changes caused by the intracarotid infusions with special reference to possible mechanisms underlying the barrier opening. The cerebral blood flow (as reflected by the cerebral venous outflow rate) intracranial pressure, intracarotid pressure and systemic blood pressure were continuously measured during and following the infusions. It was found that intracarotid infusions of hyperosomolar solutions induce cerebral vasodilation and flow increase. Autoregulation is impaired. Probably, a direct vasodilator mechanism is involved but, depending on volume and osmolarity of the solution used, the changes may be heavily influenced by vascular distention due to intracarotid pressure effect, and by systemic pressure changes. These mechanisms are in common with other methods for barrier opening. The local vasomotor and systemic effects of a hyperosmolar solution of urea, capable of opening the barrier, are not normalized until about 6 min after the administration.

Induction of pinocytosis in cerebral vessels by acute hypertension and by hyperosmolar solutions

A new approach is described to study the mechanism of protein leakage through the endothelial cells in acute hypertension and after intracarotid infusion of hyperosmolar solutions. The luminarl surface of the cerebrovascular endothelial cells was labeled with choleragenoid-peroxidase before the blood-brain barrier (BBB) dysfunction was induced. Numerous pinocytotic vesicles and some transendothelial channels were formed and showed labeling of parts of their membranes. Our results indicate that the mechanism behind the protein leakage induced by hypertension and by hyperosmolar solutions is the same--ie, transendothelial pinocytosis.

Effect of an anion transport inhibitor on blood-brain barrier lesions during acute hypertension. Possible prevention of transendothelial vesicular transport

Opening of the blood-brain barrier (BBB) by a hypertensive insult comprises the formation of endothelial micropinocytosis. Constricted vessels are less vulnerable to the insult. In the present study SITS (4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonic acid disodium) was shown to prevent leakage across the BBB into the brain parenchyma following a hypertensive insult induced by a local increase of the intraluminal pressure in anesthetized rats and by i.v. administration of adrenaline or bicuculline in conscious unrestrained animals. Since SITS induced an increase in cerebral blood flow the protection cannot be explained by a constrictory action on the cerebral vessels. SITS is a drug with complex action on the cell membrane including an inhibitory effect on anion transport mechanisms and on some c-AMP mediated processes. It is possible that the protection of BBB observed in the present study is connected to a reduction of c-AMP but membrane stabilizing effect can at present not be excluded.

Blood-brain barrier opening to horseradish peroxidase in acute arterial hypertension

Acute arterial hypertension was induced in male Wistar rats using two experimental techniques: (1) i.v. injection of Aramine and (2) infusion of physiological saline as a bolus via internal carotid artery. Horseradish peroxidase (HRP) was injected i.v. prior to both experimental procedures and subsequently localized in the brain by light and electron microscopy. In the saline infusion (pressure pulse) model, colloidal lanthanum was also applied as a diffusion tracer following fixation of the cerebral endothelium. In the Aramine model, extravsation of HRP correlated with abrupt elevation of blood pressure. In the pressure pulse model HRP extravasation was consistently visualized in the affected hemisphere. Electron microscopy showed consistent labeling of plasmalemmal vesicles by HRP in segments of cerebral endothelium. However, HRP was also clearly visualized in junctional pools suggesting focal opening of endothelial tight junctions as a pathway for extravasation of this tracer in both hypertensive models. Colloidal lanthanum not transported by plasmalemmal vesicles across endothelium after fixation of the brain also bypassed consecutive membrane appositions of endothelial tight junctions indicating existance of interendothelial pathways to macromolecules in acute arterial hypertension.

New ultrastructural evidence for a protein transport system in endothelial cells of gerbil brains

Pathways for transfer of horseradish peroxidase (HRP) across the cerebral microvasculature were studied in Mongolian gerbils after inducing either unilateral carotid-artery ligation or intracarotid air embolism. Electron microscopy on samples from both ipsilateral and contralateral brain hemispheres showed the reaction product (HRP) in the endothelial cytoplasm of capillaries and arteriols in all animals; in the basal lamina, HRP was seen only after long (3--4 h) ischemic periods. HRP was seen within both spherical and tubular structures, with the latter appearing in obliquely or longitudinally sectioned blood vessels. These endothelial channel-like structures are part of a tubulovesicular network which seemingly exists in the normal endothelium of the gerbil brain. The tubules serve as pathways for proteins into the brain; protein transfer becomes enhanced following circulatory injuries such as ischemia.

Opening of tight junctions in cerebral endothelium. II. Effect of pressure-pulse induced acute arterial hypertension

Acute arterial hypertension was produced in male Wistar rats by pressure pulse through the right internal carotid artery. The pressure pulse was induced by infusion of physiological saline as a bolus, at a rate of 0.63 ml per second by syringe pump. Evans blue (Eb) was used to visualize the areas of blood-brain barrier opening. Intravenously injected horseradish peroxidase (HRP) was used to study the ultrastructural basis of permeability changes in cerebral endothelium. Eb outlined circumscribed areas of blood-brain barrier opening. HRP extravasation was found mainly around small arteries. The capillary network was affected to a much lesser extent. Electron microscopy showed that HRP crossed the endothelial cell layer by intercellular routes. Glutaraldehyde fixed brain samples permeated with colloidal lanthanum supported these observations.

Chlorpromazine treatment of blood-brain barrier dysfunction. A quantitative and fluorescence microscopical study on small cerebral stab wounds in the rat

The extravasation of albumin around small cerebral stab wounds in the rat frontal cortex was studied by fluorescence microscopy after i.v. injection of Evans blue-labeled albumin and by quantitative immunoelectrophoresis after i.v. injection of bovine albumin, in both nonintoxicated and ethanol-intoxicated rats. The extravasion of albumin was enhanced in ethanol-intoxicated compared to nonintoxicated rats, as studied both by immunoelectrophoresis and by fluorescence microscopy. Administration of chloropromazine (2 mg/kg body weight i.v.) 16 h after inflicting cerebral stab wounds decreased the area around the wound showing extravasation of Evans blue-labeled albumin as studied by fluorescence microscopy both in nonintoxicated and in ethanol-intoxicated rats. Quantitative studies showed a significanct decrease in the amount of extravasated albumin around cerebral stab wounds in ehtnaol-entoxicated rats, while the extravasation in nonintoxicated rats was not significantly changed by chlorpromizine treatment.

Quantitative estimate of pinocytosis in experimental acute hypertension

Cerebral cortical arterioles in focal neocortical areas develop increased permeability to plasma proteins and protein tracers in experimental hypertensive encephalopathy. The mechanism underlying this increased permeability has been the subject of several studies. In our previous studies of angiotensin-induced acute hypertension, pinocytosis appeared to be the principal mechanism for the increased blood-brain barrier (BBB) permeability observed. In the present study pinocytotic activity was assessed quantitatively to determine whether enhanced pinocytosis was confined to the permeable arteriolar segments of hypertensive animals. In addition, the effect of horseradish peroxidase (HRP) itself on the pinocytotic activity of normal cerebral cortical arteriolar endothelium was determined. In 12 rats following administration of HRP, hypertension was induced by an infusion of angiotensin. The animals were perfusion-fixed 90 s after the onset of the infusion. Control animals received saline only or HRP only. The area of arteriolar endothelium in cross section was determined by a planimeter from overlapping electron micrographs taken at a constant magnification around the circumference of the vessel wall. Results indicate a significant (P less than 0.001) increase in the number of pinocytotic vesicles in the permeable arteriolar segments of hypertensive animals as compared with nonpermeable arteriolar segments of the same animals and comparable segments of normotensive rats. In addition, eight times as many vesicles appear to be transporting tracer in the permeable arteriolar segments of hypertensive animals as compared to the nonpermeable segments of the same animals and normotensive animals. HRP alone did not affect the pinocytotic index, there being no difference (P greater than 0.05) in the number of vesicles in normotensive animals receiving saline only and those receiving HRP only. Our previous observation that disruption of endothelial cells or their tight junctions did not occur was confirmed.

Prevention of protein extravasation in the brain by an anion transport inhibitor in acute experimental hypertension in rats

Acute experimental hypertension induces protein leakage in the brain. The protein is thought to be, at least to a great extent, transported through the endothelial cells by pinocytosis. An anion transport inhibitor, 4 acetamido-4-isothiocyano-stilbene-2,2'-disulfonic acid, markedly reduced adrenaline or bicuculline-induced leakage of 125IHSA and Evans blue-albumin in all areas of the rat brain. The preventive effect of dexamethasone was less pronounced and no effect was seen of haloperidol.

Effects of increasing arterial pressure on cerebral blood flow in the baboon: influence of the sympathetic nervous system

The influence of stimulation of the cervical sympathetic chain on the response of cerebral blood flow to hypertension induced by the intravenous infusion of angiotensin was studied in anaesthetised baboons. Cerebral blood flow was measured by the intracarotid 133Xenon injection technique. Possible lesions of the blood-brain barrier were studied by injecting Evans blue towards the end of the experiment and ischaemic brain damage was assessed following perfusion fixation. In a control group of five baboons blood flow increased by 53 +/- 9% (mean +/- S.E.) from the base line values in the arterial pressure range 130-159 mm Hg. In four baboons subjected to unilateral sympathetic stimulation flow increased by 16 +/- 4% in the same pressure range. In three baboons subjected to bilateral sympathetic stimulation there were no significant increases in flow until the arterial pressure had increased above 159 mm Hg. Disruption of the blood-brain barrier in the parietooccipital regions was only seen in the control animals but not in the stimulated baboons. Ischaemic brain damage was not observed with the exception of one small lesion in a single stimulated baboon. These findings provide strong support for the observations of Bill and Linder (1976) that activation of the cervical sympathetic can modify the level at which breakthrough of cerebral blood flow occurs in association with systemic hypertension.

Ultrastructural studies on blood-brain barrier dysfunction after cerebral air embolism in the rat

Male albino rats were anaesthetized with diazepam, injected with horseradish peroxidase and Evans blue-labeled albumin and given an embolus of 0.01 ml air in the right common carotid artery after ligation of the external carotid branch. The pial arteries of the right cerebral hemisphere were stained blue, particularly the middle cerebral artery and its main arterial branchlets. Ultrastructurally, some endothelial cells in the right middle cerebral artery, small arteries and arterioles showed a diffuse distribution of horseradish peroxidase in their cytoplasm, although these vessels only occasionally showed peroxidase in their basement membranes. Other endothelial cells in these arterial branchlets showed few if any signs of a diffuse distribution of peroxidase but displayed several pinocytotic vesicles and occasionally trans-endothelial channels filled with peroxidase, sometimes with a slight leakage of peroxidase into adjacent basement membranes and neuropil. Scattered pinocytotic vesicles were observed in capillaries and venules, but there was usually no extravasation of peroxidase around these vessels.

Vascular permeability alterations to horseradish peroxidase in experimental brain injury

Protein uptake and transport within the brain stem vasculature of mechanically brain injured cats was studied by means of both light and electron microscopy utilizing intravenously injected horseradish peroxidase as the protein tracer. In animals sustaining low grade head injuries not of sufficient intensity to elicit either microscopic, intraparenchymal hemorrhages or subtle, neuropathological responses, peroxidase extravasation was noted both in the vascular walls and in the surrounding parenchyma of the ventromedial aspect of the brain stem. At the ultrastructural level as early as 3 min after brain injury, occasional arterioles, venules and capillaries displayed peroxidase leakage. In serial sections large endothelial segments of these vessels revealed the peroxidase reaction product within numerous vesicles which often shared continuity with tubular and vacuolar profiles. Such vesicular activity apparently moved the peroxidase from the luminal surface to extrude it into the basal lamina. From the perivascular basal lamina, the reaction product flooded the interstices of the surrounding brain stem parenchyma where occasional neural, glial and pericytic elements incorporated the peroxidase within coated invaginations, vesicles, tubules and vacuoles. In that protein leakage was consistently observed despite the apparent integrity of both the endothelial tight junctions and their cell membranes, it is concluded that the vesicular transport of horseradish peroxidase across the endothelia of the brain stem vasculature represents a possible mechanism of blood-brain barrier dysfunction in mechanical brain injury.

Hypertension-induced cerebral atherosclerosis in the cholesterol-fed rabbit

Foam cell lesions were found in cholesterol-fed rabbits with induced hypertension, particularly in intimal cushions at branching sites, where permeability to horseradish peroxidase was enhanced. Permeability to horseradish peroxidase was enhanced at the edge of intimal cushions without foam cell accumulation. This finding suggests that permeability is increased before foam cell infiltration. No foam cell lesions were observed in the intima of cerebral arteries distant from branching sites, but insudation of plasma constituents here caused endothelial cells to separate from the subendothelial matrices. Foam cell lesions were absent from the cerebral arteries in normotensive cholesterol-fed rabbits.

Ultrastructural studies on blood-brain barrier dysfunction around cerebral stab wounds, aggravated by acute ethanol intoxication

Ethanol-intoxicated and non-intoxicated albino rats subjected to cerebral stab wounds were injected with Evans blue-labelled albumin (EBA) and horseradish peroxidase (HRP) 24 hours after the injury. The tracers were injected 30 and 2 minutes before perfusion fixation with glutaraldehyde and then prepared for light microscopy and electron microscopy. There was prominent leakage of peroxidase through chains of pinocytotic vesicles and transendothelial channels from blood, through the endothelial cells, into the brain. There was no difference in the way of peroxidase leakage through the endothelial cells between ethanol-intoxicated and control rats, but the area containing leaking blood vessels was greater in intoxicated rats. Furthermore, the number of trans-endothelial channels, vesicles and caveolae per unit length of capillaries and venules was increased in ethanol-intoxicated rats. Injured neurons with a diffuse distribution of peroxidase in their cytoplasm were observed within a greater area around the stab wound in ethanol-intoxicated than in control rats.

Osmotic opening of the blood-brain barrier

The blood-brain barrier at cerebral blood vessels is due to a continuous lining of endothelial cells, which are connected by tight junctions that restrict intercellular diffusion. The endothelium excludes most water-soluble solutes and proteins but supports facilitated stereospecific transport of monosaccharides and large neutral and basic amino acids. The barrier in different species can be made permeable by infusing a hypertonic solution of urea or arabinose into the internal carotid artery. Endothelial cells presumably shrink and tight junctions between them widen to proteins and normally restricted solutes. Thus, intravascular protein tracers such as Evans' blue-albumin, 125I-labelled albumin, horseradish peroxidase (ED 1.11.1.7) and alpha-mannosidase (EC 3.2.1.24) are allowed into the brain, and uptake of [3H] norepinephrine (noradrenaline) is increased more than twofold above a normal rate of accumulation by brain. Osmotic barrier opening to amines has been used to demonstrate their effect on cerebral blood flow from within the brain parenchyma. Osmotic barrier opening is reversible, may be graded with respect to molecular size and is not followed by evidence of brain damage or of brain oedema (when measured two days after hypertonic infusion). Transient cerebral changes probably accompany osmotic opening, however, as glucose uptake and cerebral metabolism of glucose are increased after hypertonic infusion.

Effect of dexamethasone on protein extravasation in the brain in acute hypertension induced by amphetamine

Amphetamine productes protein leakage in the brain when given to rats under nitrous oxide anesthesia. The blood-brain barrier dysfunction is caused by the combined effect of blood pressure increase and vasodilatation. In the present experiments pretreatment with dexamethasone, 2 mg.kg-1, diminished the amphetamine-induced extravasation of Evans blue albumin and 125IHSA in the rats' brain. Possible explanations to the effect of dexamethasone on cerebrovascular permeability are discussed.

Regional brain uptake of noradrenaline following mechanical or osmotic opening of the blood-brain barrier

The passage of noradrenaline from the cerebrovascular circulation into the brain (vessel walls and parenchyma) was studied quantitatively and regionally by a modification of Oldendorf's technique for determination of brain uptake index. Using 14C-ethanol as the highly diffusible internal standard, the index for noradrenaline varied between 2.7 and 4.5% in different regions, confirming the poor penetration of this neurotransmitter. The barrier was impaired transiently as evidenced by Evans blue extravasation, through osmotic opening by internal carotid injection of a hyperosmolar urea solution or mechanical disruption by a short-lasting elevation of the intracarotid hydrostatic pressure. This resulted in a 3-4-fold increase in the passage of noradrenaline from the circulation into the brain.

The blood-brain barrier to horseradish peroxidase under normal and experimental conditions

This review paper deals with the transport of the protein tracer horseradish peroxidase across cerebral vessels under normal and various experimental conditions. Electronmicroscopical investigations have revealed that, under normal conditions, a minor vesicular transfer of intravenously injected peroxidase occurs across the endothelium in segments of arterioles, capillaries and venules, especially in arterioles with a diameter about 15-30 mu. This normally occurring vesicular transport is susceptible to various experimental conditions. Thus the transfer of tracer increases when a hypertonic solution is injected into the internal carotid artery presumably due to vesicular transport. Extensive acute hypertension of short duration also increases the vesicular transfer of peroxidase from blood to brain. Identical observations are obtained when the hypertension is evoked by intravenous injection of phentolamine and by electrically induced seizures. During the postischemic period, one hour after release of the occlusion of an internal carotid artery in the Mongolian gerbil the vesicular transport of peroxidase is increased across the endothelium of cerebral vessels. The explanation may be release of serotonin from blood platelets during the occlusion. The serotonin could then increase the blood pressure locally in the brain resulting in an enhanced permeability. Serotonin, after perfusion through the cerebral ventricular system, is also able to increase the normally occurring vesicular transfer. The most likely mechanism behind this phenomenon seems at the moment to be local hypertension evoked by serotonin-induced vasoconstriction of arterioles. Finally, the enhanced vesicular transport across cerebral endothelium caused by porto-caval anastomosis is mentioned and the possible role of disturbances in the metabolism of amines as responsible for the extravasation is discussed.

Ultrastructural characteristics of the brain and blood-brain barrier in experimental seizures

During experimental seizures, the blood-brain barrier (BBB) is broken; tracer substances such as I131-albumin, Evans blue and horseradish peroxidase (HRP) geographically locate the barrier breakdown primarily in the diencephalon. Using rats, we have induced seizures with electroshocks and demonstrated the breakdown of the BBB with Evans blue and HRP. We have shown that (1) the BBB breakdown is proportional to the number of electroconvulsant shocks (ES) given; (2) the mechanism of increased barrier permeability is primarily by micropinocytosis in the cerebral capillaries, arterioles, and, to a lesser extent, venules; and (3) the stimulus for micropinocytosis and hence BBB breakdown is associated with the abrupt rise in systemic blood pressure and cerebral vasodilatation that accompanies each ES. If the systolic hypertension is abolished via cervical cordotomy, there is little to no breakdown in the BBB.

Increased vesicular transfer of horseradish peroxidase across cerebral endothelium, evoked by acute hypertension

Acute hypertension in rats was produced by intravenous infusion of metaraminol bitartrate (Aramine). The permeability to intravenously injected horseradish peroxidase (HRP) was increased across the cerebral arterioles, capillaries and venules. From the basement membranes of the vessel walls the protein tracer moved into the extracellular spaces of the adjacent neuropil. No endothelial cell damage was observed. The tight junctions between endothelial cells were intact and prevented intercellular movement of peroxidase. Many HRP-labeled vesicles within the endothelial cells or connected with the luminal or abluminal surface, occurred in segments of the microvasculature. Otherwise the endothelium was unchanged. Diffuse uptake of HRP into the cytoplasm of neurons and glial cells was not observed. The alphablocker phentolamine (Regitin) was given to a group of rats simultaneously to Aramine. The increase in blood pressure was thus prevented; furthermore, the permeability remained as under normal conditions. The Aramine, Regitin and HRP did not significantly influence the pH, PO2 and pCO2 of the arterial blood. It is concluded that acute hypertension increases the vesicular transport of HRP across the endothelium of cerebral arterioles, venules and capillaries that normally occurs to a small extent only after intravenous injection of the tracer.

Effects of acutely induced hypertension in cats on pial arteriolar caliber, local cerebral blood flow, and the blood-brain barrier

Acute hypertension was induced in 19 anesthetized cats by the intravenous administration of angiotensin. The caliber of pial arteries was measured by a television image-splitting technique and local cerebral blood flow by the hydrogen clearance technique. As the blood pressure was increased, pail arterioles constricted and cerebral blood flow remained relatively constant, showing that autoregulation of cerebral blood flow was intact. At mean arterial pressures of more than 170 mm Hg arteriolar dilation appeared. In smaller arterioles (initial diameter less than 100 mum) a segmental dilation (the "sausage'string" phenomenon) frequently preceded uniform dilation. This arteriolar dilation was associated with a marked increase in local cerebral blood flow indicating that the upper level of autoregulation had been breached. In no cat was vasospasm or a decrease in blood flow observed during induced hypertension. Hypertension also caused dysfunction of the bloodbrain barrier since, in 17 out of 19 of the cats examined, there was extravasation of protein-bound Evans blue into brain tissue. In only one of the 19 cats subjected to neuropathological analysis was ischemic brain damage identified and this was restricted to minimal ischemic cell change. The results indicate that severe, induced hypertension in cats produces cerebral arteriolar dilation, an increase of cerebral blood flow, and dysfunction of the blood-brain barrier. These observations may be of importance in understanding the pathogenesis of hypertensive encephalopathy.