Pathophysiology of the opening of the blood-brain and blood-cerebrospinal fluid barriers in acute hypertension

PROTECTION OF BLOOD-BRAIN BARRIER BREAKDOWN BY NIFEDIPINE IN ADRENALINE-INDUCED ACUTE HYPERTENSION

The question of whether influxes of ionic Ca+2 into cerebral endothelium plays an important role in increased vascular permeability consequent to an acute hypertension is not accurately resolved. We tested the effect of nifedipine, a calcium entry blocker, on the cerebrovascular permeability for proteins in adrenalin-induced acute hypertension. The experiments were carried out on male Wistar rats. The experimental groups consisted of normotensive saline controls, adrenaline-induced hypertensive rats, and adrenalin-induced hypertensive rats as pre-treated or post-treated with a bolus of nifedipine. Brains of hypertensive rats showed increased permeability to Evans Blue-Albumin complex, when blood pressure elevated rapidly to more than 170 mmHg. The number and size of areas of Evans-Blue extravasation were smaller if an increase in blood pressure was prevented. The short lasting elevation of blood pressure did not result in protein extravasation in brains of hypertensive rats. The results suggest that nifedipine can modify the permeability disruptions observed in acutely hypertensive rats. The data also support the hypothesis that Ca+2 may be responsible for the changes in permeability of BBB in hypertension by mediating the contraction of vascular muscles.

Scalp-recorded slow EEG responses generated in response to hemodynamic changes in the human brain

OBJECTIVE

To study whether hemodynamic changes in human brain generate scalp-EEG responses.

METHODS

Direct current EEG (DC-EEG) was recorded from 12 subjects during 5 non-invasive manipulations that affect intracranial hemodynamics by different mechanisms: bilateral jugular vein compression (JVC), head-up tilt (HUT), head-down tilt (HDT), Valsalva maneuver (VM), and Mueller maneuver (MM). DC shifts were compared to changes in cerebral blood volume (CBV) measured by near-infrared spectroscopy (NIRS).

RESULTS

DC shifts were observed during all manipulations with highest amplitudes (up to 250 microV) at the midline electrodes, and the most pronounced changes (up to 15 microV/cm) in the DC voltage gradient around vertex. In spite of inter-individual variation in both amplitude and polarity, the DC shifts were consistent and reproducible for each subject and they showed a clear temporal correlation with changes in CBV.

CONCLUSIONS

Our results indicate that hemodynamic changes in human brain are associated with marked DC shifts that cannot be accounted for by intracortical neuronal or glial currents. Instead, the data are consistent with a non-neuronal generator mechanism that is associated with the blood-brain barrier.

SIGNIFICANCE

These findings have direct implications for mechanistic interpretation of slow EEG responses in various experimental paradigms.

Effects of acute and chronic hypertension on the labyrinthine barriers in rat

Hearing loss, vertigo, and tinnitus have been related to arterial hypertension. The aim of the present work was to study the permeability of the blood-perilymph and of the labyrinthine barrier, between endolymph and perilymph, to small molecules during chronic and acute hypertension. Experiments were performed in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Acute hypertension was induced by phenylephrine. Perilymph was sampled from the first turn of the scala vestibuli and the Na, K, urea, and radioactive concentrations ((14)C-urea and (3)H-mannitol) were measured. In another experimental set, the endocochlear potential was recorded from the basal turn of scala media, before and after phenylephrine injection. The composition of the perilymph and the kinetic constants for (14)C-urea and (3)H-mannitol were similar in WKY and SHR, and not modified after acute hypertension. In endolymph, the endocochlear potential in SHR (+80+/-2.7 mV, n=24) was lower (P<0.001) than in WKY (+98+/-1.5 mV, n=29). The endocochlear potential was decreased by 40 mV during acute hypertensive peak in seven out of 19 WKY but not in SHR rats (n=13). In conclusion, chronic or acute hypertension did not severely alter the permeability to small molecules of the blood-perilymph barrier. The relationship between the low endocochlear potential and hypertension in SHR remains to be evaluated. After acute hypertensive peak, the presence of vascular protective mechanisms in the cochlea could account for the stable endocochlear potential recorded in SHR and 60% of normotensive rats.

Chronic nicotine pretreatment protects the blood-brain barrier against nicotine-induced seizures in the rat

This study was designed to investigate the possible protective actions of nicotine on cerebrovascular permeability in convulsions during nicotine-induced seizures. We have measured the permeability changes in the blood-brain barrier (BBB) macroscopically and spectrophotometrically by using Evans blue dye. Specific gravity measurements were also performed to assess brain edema which develops after blood-brain barrier opening. The experiments were carried out on Wistar rats. Rats were divided into two groups. They received acutely a convulsive dose of nicotine 3, 5, 8 and 9 mg kg(-1)i.p. or pretreated with a low dose of nicotine (0.8 mg kg(-1)i. p.) for 21 days followed by the procedure mentioned in the first group. Acute nicotine injection induced a significant increase in blood pressure and Evans-blue passage, despite a decline in specific gravity values. Low doses of chronic nicotine administration markedly reduced both the leakage of dye, and brain water content. Chronic treatment with low doses of nicotine (0.8 mg kg(-1)day(-1)s. c.) lessened the intensity of tonic-clonic seizures observed with a single dose of 3, 5, 8 or 9 mg kg(-1)nicotine. The data presented here demonstrate that nicotine pretreatment results in decreased sensitivity to nicotine-induced seizures in rats.

Age-dependent alteration in regional cerebrovascular permeability during drug-induced epilepsy

Age-related changes in blood-brain barrier permeability were investigated during pentylenetetrazol-induced seizures in rats aged from 15 days to 120 days. Tracers such as [14C]sucrose and [3H]inulin which diffuse very slowly across the intact endothelium were simultaneously injected i.v. in rats treated with pentylenetetrazol (PTZ) or in control animals. Permeability-surface area products (PA) were determined in 9 brain regions. Pentylenetetrazol-induced seizures caused a significant increase in PA for both sucrose and inulin in all brain regions studied. Blood-brain barrier dysfunction was present only in animals in which the mean arterial blood pressure rose at seizure onset. Although increased blood-brain barrier permeability was found partly in similar areas in both young and adult rat brains, in adults the increase was the highest in the preoptic area, septum, colliculus inferior, hypothalamus and in the cerebellum while the increase was comparatively much smaller in the same areas of young brains. The increase in blood-brain barrier permeability was extremely high in the hippocampus, hypothalamus and cerebellum of 15-day-old rat brain and, was least affected in the corpus striatum and cerebral cortex in contrast to older rats. From the results obtained it may be concluded that the increased cerebrovascular permeability induced by pentylenetetrazol differs markedly in localization in young and adult rats. The age-dependent increased blood-brain barrier integrity is not over all dependent on variations in the blood pressure, but rather on progressive maturation of capillaries and changes in their internal structure, and local phenomena in neuronal activity during the seizures.

Elevation of taurine in hippocampal extracellular fluid and cerebrospinal fluid of acutely hypoosmotic rats: contribution by influx from blood?

Previous work has demonstrated that there is a selective increase in extracellular taurine in the brain during acute water intoxication. One aim of the present study was to investigate whether plasma taurine contributes to this increase. To this end, the concentrations of taurine, other amino acids, and ethanolamine (EA) were measured in plasma and CSF of urethane-anesthetized rats injected with 150 ml/kg body weight of distilled water. Blood pressure, blood gases, and pH, as well as plasma and CSF osmolality, were also measured. The CSF level of albumin was quantitated to study the function of the blood-CSF barrier. In separate experiments, hippocampal microdialysis was performed to determine the effects of acute plasma hypoosmolality on extracellular amino acids. Finally, the effect of water injection on hippocampal specific gravity and tissue amino acids was assessed. Blood gases and pH were essentially unchanged after water administration. Mean arterial blood pressure increased to peak levels approximately 50 mm Hg above control. Plasma osmolality decreased rapidly, whereas the depression of CSF osmolality was slower and less pronounced. The average volume of the hippocampus increased by 8%. Water injection was accompanied by a 25-fold elevation of taurine in plasma, whereas phosphoethanolamine (PEA) and EA increased moderately. A small fraction of the increase in plasma taurine might derive from blood cells because dilution of blood in vitro led to doubled plasma levels of the amino acid. Taurine, PEA, and EA increased consistently in CSF and hippocampal microdialysates. Plasma hypoosmolality transiently opened the blood-CSF barrier is reflected by augmented CSF concentrations of albumin.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in regional patterns of blood-brain barrier breakdown during epileptiform seizures induced by pentylenetetrazol

Age related changes in blood-brain barrier (BBB) permeability to macromolecules were investigated during seizures induced by pentylenetetrazol in rats aged from 6 to 120 days. Evans blue was used as a visual indicator of BBB integrity. BBB leakage due to seizures was present only in animals in which the mean arterial blood pressure (BP) rose with the seizure onset. Although considerable BBB damage was found partly in similar areas in young and adult rat brains, in adults the leakage of Evans blue was most intense in preoptic area, colliculus inferior, hypothalamus and cerebellum whereas the BBB opening was comparatively rare, in the same areas of young brains. In 6- and 15-day-old rats which did not differ in BP changes from the adults, the leakage was extremely intense in hypothalamus and hippocampus and in contrast to 30- or 120-day-old rats there was no leakage of Evans blue in preoptic area and cerebral cortex. From the results obtained, the conclusion may be drawn that the brain regions which are vulnerable to seizures induced by pentylenetetrazol differ markedly in developing and adult rats. On the other hand, in adult animals, either certain brain areas are more vulnerable to pentylenetetrazol or the BBB has an increased fragility particular to seizure activity. These results indicate that the sensitivity of BBB mechanisms may depend on proliferation of capillaries and changes in their internal structure and may emphasize it.

Water and electrolyte content of rabbit brain after opening the blood-brain barrier by acute hypertension

Changes in water and electrolyte content of the brain and edema formation after acute, drug-induced hypertension were studied in albino rabbits. Blood-brain and blood-cerebrospinal fluid (CSF) barriers opened to Evans blue-albumin when systemic blood pressure was elevated abruptly to more than 160 mm Hg by i.v. injection of Aramin. No statistically significant changes in sodium and potassium content of brain, muscle, and CSF were observed. Measurable brain edema did not develop. The results suggest that short-lasting hypertensive barrier opening does not cause brain edema, but may enhance a tendency for brain edema.