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

Glucocorticoids as prophylaxis against acute mountain sickness

OBJECTIVE

Acute mountain sickness (AMS) characterized by presence of symptoms including headache, nausea, excessive fatigue, loss of appetite, irritability and insomnia is a major impediment to work performance in human subjects who are rapidly inducted to high altitude (HA) during the initial phase of induction. The present study aims at to evaluate the efficacy of prophylactic administration of low dose glucocorticoids in prevention of AMS in normal healthy men who are inducted to HA by air.

DESIGN

Fifty healthy men were randomly divided into five groups of 10 each. Group I received prednisolone (Pred) 10 mg, Group II Pred 20 mg, Group III Pred 40 mg, Group IV dexamethasone 0.5 mg, Group V received placebo once a day in the morning for 2 days at sea level (SL) and for 3 days on arrival at an altitude of 3450 m by air.

MEASUREMENTS

The severity of AMS was assessed using Lake Louise AMS scoring system. Physiological parameters like blood pressure, respiratory rate, peripheral blood O2 saturation and heart rate were measured at sea level and on arrival at HA. Circulatory levels of cortisol and adrenocorticotropic hormone (ACTH) were measured by radioimmunoassay (RIA) and immunoradiometreic assay (IRMA), respectively.

RESULTS

In the placebo group, significant AMS could be detected at 12 h of arrival at HA, peaked by day 1 or 2 of stay and started declining thereafter. As compared to the placebo group, the steroid treated groups showed a significant (P < 0.01) reduction in daily AMS score. When compared with prednisolone 10 mg, 40 mg and dexamethasone groups, the prednisolone 20 mg group showed an optimal response in reduction of AMS symptoms. The O2 saturation showed a significant decline (P < 0.001) on arrival at HA, but the pattern of O2 saturation in placebo and glucocorticoid groups was identical. Similarly, the rise in heart rate and blood pressure and on day 3 of stay at HA was similar in placebo and glucocorticoid-treated groups. An increase in plasma cortisol in placebo group was observed on day 3 of stay at HA and continued to rise till day 8 of observations. The cortisol levels in Pred 10 mg and Pred 20 mg groups on day 1 and 3 of arrival at HA were not significantly different than the SL post-treatment values but were found to be significantly higher on day 8 of stay. Plasma cortisol in Pred 40 mg and dexamethasone groups was significantly lower (P < 0.01) on day 1 and 3 of stay but showed an increase by day 8 of stay. The ACTH levels were increased at HA in placebo group but did not show any significant change till day 3 of stay in steroid treated subjects and were found to be higher in all groups on day 8 of observations.

CONCLUSION

These observations suggest that administration of low-dose glucocorticoids can curtail acute mountain sickness significantly without influencing the normal adreno cortical response to hypoxia.

Hormonal influence on the permeability of the blood-brain barrier: effect of an analog of adrenocorticotropic hormone, beta 1-24 corticotrophin

Regional unidirectional transport of alpha-aminoisobutyric acid (AIB) (mol. wt.: 104) and sucrose (mol wt.: 342) which have a low permeability across the intact endothelium was investigated in brain of rats either treated with synacthène: an analog of ACTH, tetracosactide retard (beta-1-24 corticotrophin) or in brain of placebo-treated controls. Three days treatment with synacthène, reduced the rate of influx of AIB and sucrose in most of the brain regions studied especially in thalamus, hypothalamus, cortex, and caudate nucleus without affecting the vascular compartment. The brainstem, cerebellum and white matter were less affected. These experimental findings may suggest that ACTH exhibits significant influence on hormonal regulation of blood-brain barrier permeability. Thereby such a regulation may involve the entry of polar compounds into the CNS and may influence the central effects of diffusion-limited drugs.

Blood-brain barrier permeability: regional alterations after acute and chronic administration of ethinyl estradiol

In this study we measured the effect of acute and chronic estrogen treatment on cerebrovascular permeability to sucrose and inulin. Animals were subcutaneously injected once with 0.1 micrograms/rat of ethinyl estradiol or injected daily with the same drug dose for 3 weeks. Control rats received the same amount of arachis oil vehicle. Three weeks treatment but not the single injection of ethinyl estradiol produced significant increases in the cerebrovascular permeability-surface area product for sucrose and inulin in almost all brain regions.

Regional alterations in blood-to-brain transfer of alpha-aminoisobutyric acid and sucrose, after chronic administration and withdrawal of dexamethasone

The effect of dexamethasone administration and withdrawal was studied with respect to blood-brain barrier function. The tracers alpha-[3H]aminoisobutyric acid (AIB) (MW 104) and [14C]sucrose (MW 342), which have a low permeability across the intact endothelium, were simultaneously injected intravenously in rats treated with dexamethasone and placebo-treated control animals or in rats in which dexamethasone treatment was discontinued 3 days before the experiment. Unidirectional transfer constants (Ki) were determined in discrete brain regions. Steroid administration reduced the rate of influx of AIB and sucrose, whereas discontinuation of drug resulted in an increased permeability. These findings suggest that when exposure to glucocorticoids is prolonged, the efficiency of medical treatment of CNS diseases may decrease due to reduction of drug delivery to CNS. Thus, these experimental findings may have particular importance in the clinical setting of drug administration when considering the combination of steroids with other drugs, and may aid in understanding better the pathogenesis of some types of brain edema seen in patients from whom corticosteroid therapy has been withdrawn.

Blood-brain barrier breakdown in cold-injured brain is linked to a biphasic stimulation of ornithine decarboxylase activity and polyamine synthesis: both are coordinately inhibited by verapamil, dexamethasone, and aspirin

An early increase in ornithine decarboxylase (ODC) activity and polyamine levels in rat cerebral capillaries was previously implicated in the mediation of blood-brain barrier (BBB) breakdown in cold-injured brain. A time course study in rat cerebrum indicated that cold injury evokes a biphasic increase in ODC activity and polyamine levels in perilesional cortex. ODC activity rose sharply (fourfold) within 1 min, remained elevated for 5 min, and then returned to the basal level by 10 min. A transient rise in polyamine concentration followed in the rank order of putrescine greater than spermidine greater than spermine. A secondary rise in ODC activity commenced in perilesional tissue at 2-6 h and peaked (8.8-fold) at 48 h. Major increases in the content of putrescine (330%), spermidine (103%), and spermine (50%) developed at 48-72 h. alpha-Difluoromethylornithine (DFMO), a specific irreversible inhibitor of ODC, suppressed the evoked increase in ODC activity and abolished the associated increase in content of polyamines, findings indicating that the accumulation of polyamines in cryoinjured brain reflects enhanced synthesis resulting from an ODC-mediated increase in putrescine content. Cycloheximide and actinomycin D were without effect on the early increase in ODC activity but inhibited the delayed increase in ODC activity, an observation suggesting that the initial increase in activity reflects an activation of a cryptic ODC via a posttranslational process, whereas the delayed increase in activity results from ODC synthesis mainly under transcriptional control.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia-induced increases in pulmonary transvascular protein escape in rats. Modulation by glucocorticoids

Pulmonary edema after ascent to altitude is well recognized but its pathogenesis is poorly understood. To determine whether altitude exposure increases lung vascular permeability, we exposed rats to a simulated altitude of approximately 14,500 feet (barometric pressure [Pb] 450 Torr) and measured the pulmonary transvascular escape of radiolabeled 125I-albumin corrected for lung blood content with 51Cr-tagged red blood cells (protein leak index = PLI). Exposures of 24 and 48 h caused significant increases in PLI (2.30 +/- 0.08 and 2.40 +/- 0.06) compared with normoxic controls (1.76 +/- 0.06), but brief hypoxic exposures of 1-13 h produced no increase in PLI, despite comparable increases in pulmonary artery pressure. There were associated increases in gravimetric estimates of lung water in the altitude-exposed groups and perivascular edema cuffs on histologic examination. Normobaric hypoxia (48 h; fractional inspired oxygen concentration [FIO2] = 15%) also increased lung transvascular protein escape and lung water. Dexamethasone has been used to prevent and treat altitude-induced illnesses, but its mechanism of action is unclear. Dexamethasone (0.5 or 0.05 mg/kg per 12 h) started 12 h before and continued during 48 h of altitude exposure prevented the hypoxia-induced increases in transvascular protein escape and lung water. Hemodynamic measurements (mean pulmonary artery pressure and cardiac output) were unaffected by dexamethasone, suggesting that its effect was not due to a reduction in pulmonary artery pressure or flow. The role of endogenous glucocorticoid activity was assessed in adrenalectomized rats that showed augmented hypoxia-induced increases in transvascular protein escape, which were prevented by exogenous glucocorticoid replacement. In summary, subacute hypoxic exposures increased pulmonary transvascular protein escape and lung water in rats. Dexamethasone prevented these changes independent of reductions of mean pulmonary artery pressure or flow, whereas adrenalectomy increased pulmonary vascular permeability and edema at altitude. Increases in vascular permeability in hypoxia could contribute to the development of high-altitude pulmonary edema and endogenous glucocorticoids may have an important influence on pulmonary vascular permeability in hypoxia.

Effect of dexamethasone on transport of alpha-aminoisobutyric acid and sucrose across the blood-brain barrier

The effect of glucocorticoids on the blood-brain barrier (BBB) was studied in rats following a single injection or 3 days of dexamethasone administration. Tracers with a low permeability across the intact endothelium, [14C]sucrose and alpha-[3H]aminoisobutyric acid ([3H]AIB), were simultaneously injected intravenously in untreated rats or in rats treated with dexamethasone. Unidirectional blood-to-brain transfer constants (Ki) in 14 regions of the rat brain were determined. In regions of control brain, average Ki values for AIB and sucrose were approximately 0.0020 and 0.00060 ml g-1 min-1, respectively. The lowest transfer constants were found in caudate nucleus, hippocampus, white matter, and cerebellum. In dexamethasone-treated animals, Ki values for both sucrose and AIB markedly decreased by 30-50% in almost all brain regions. These results indicate that a single injection or 3 days of treatment with dexamethasone causes an apparent reduction in the normal BBB permeability, and dexamethasone may greatly interfere with drug delivery into brain. These observations may have an importance for the administration of drugs in brain disease in the presence of steroids.

Effect of dexamethasone on blood-brain barrier in the normal mouse

Dexamethasone ameliorates cerebral edema, but its effect on normal blood-brain barrier is unknown, as is the site of action. Sixteen normal mice were given dexamethasone, 3 mg/kg intramuscularly or 2 mg/kg intravenously. One to two hours later 10 mg of horseradish peroxidase (HRP) in 0.1 ml of saline was administered intravenously and allowed to circulate for 15 minutes. Brain slices were examined by light and electron microscopy. The number of HRP-permeable arteriolar segments per brain was less in dexamethasone-treated than in control mice (p less than 0.001). In addition, the number of small HRP-filled endothelial vesicles of capillaries and of HRP-permeable and -impermeable arterial segments was less in dexamethasone-treated mice than that for similar vessels in control mice (p less than 0.005, p less than 0.05, and p less than 0.01, respectively). The total number of small vesicles and the number of larger HRP-filled vesicles in arterioles was the same in treated and control mice. Dexamethasone therefore decreased the normal permeability of cerebral blood vessels to HRP. This decrease was accompanied by a decrease in the number of HRP-containing small endothelial vesicles. These data suggest that dexamethasone influences cerebral edema by decreasing the permeability of the cerebral vasculature for macromolecules.

Intravenous injection of horseradish peroxidase in the rat stimulates corticosterone and adrenocorticotropic hormone release

Wistar rats were given intravenous (i.v.) horseradish peroxidase (HRP) in saline in doses commonly used to study vascular permeability and the blood-brain barrier. Samples of blood were taken from conscious animals via indwelling catheters at intervals up to 6 h after the HRP injection. Plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone were determined and compared with levels in control rats injected with saline alone. Rats injected with saline only presented levels of hormones within the low limits of normal indicating an insignificant influence of stress induced by the experimental procedure. Within 30 min of the i.v. HRP injection, the plasma concentrations of both ACTH and corticosterone increased to very high levels and remained so throughout the period of observation, namely 6 h. The time course of the changes in the concentrations was the same for the two hormones and the actual numerical values were related to the dose of HRP injected. The i.v. injection of HRP in Wistar rats, therefore, induces a marked release of stress hormones which by themselves have profound physiological effects. This phenomenon must be taken into account, in studies on normal vascular permeability using HRP as a tracer and also in similar experiments exploring various pathological conditions of the blood-brain barrier.

Age-associated changes in the effects of amphetamine on the blood-brain barrier of rats

The effects of varying doses of amphetamine on the permeability of the blood-brain barrier (BBB) to albumin were studied in young and aged Fischer 344 rats. There was no change in the permeability of the BBB associated with aging as revealed by saline-treated control experiments. The changes in the permeability of the BBB did not correlate with the changes in blood pressure produced by corresponding doses of amphetamine in the young or aged animals. In the dose range reported the young and aged rats showed similar pressor responses to amphetamine. The aged rats showed a greater increase in the permeability of the BBB than the young, and this effect was seen with a lower dose of amphetamine in the aged rats. At the highest doses tested the permeability was restored to control levels in both the young and the aged rats. The enhanced vulnerability of the BBB of the aged rats may be related to a deterioration of the noradrenergic control of cerebrovascular permeability.

Therapeutic considerations in blood-brain barrier disturbances

Current methods of treatment of brain damage, as e.g. edema by steroids and barbiturates, have components which benefit the blood-brain barrier. Protection of the blood-brain barrier may result from: (a) prevention of endothelial lesions, perhaps pinocytosis (b) reduction of secondary necrosis formation, (c) interference with release, or activation of mediator compounds causing endothelial lesions such as: biogenic amines, free fatty acids, prostaglandins, free radicals, or kinins, (d) stabilization of lysosomal membranes, and (e) prevention of microcirculatory disturbances. Other methods, or compounds aiming at mechanisms of barrier damage have a therapeutic potential as shown with regard to indomethacin, free radical scavengers, or phenothiazines. However, further studies appear necessary to demonstrate the benefit of these compounds under clinical circumstances. Reversible opening of the blood-brain barrier may be considered as a therapeutic approach to provide access of drugs to brain tissue which are normally excluded by the barrier.

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.