The role of angiotensin II in regulation of cerebrospinal fluid formation in rabbits

Mini-Review: Role of Drugs Affecting Renin-Angiotensin System (RAS) in Traumatic Brain Injury (TBI): What We Know and What We Should Know

Traumatic brain injuries (TBIs) are among the most important clinical and research areas in neurosurgery, owing to their devastating effects and high prevalence. Over the last few decades, there has been increasing research on the complex pathophysiology of TBI and secondary injuries following TBI. A growing body of evidence has shown that the renin-angiotensin system (RAS), a well-known cardiovascular regulatory pathway, plays a role in TBI pathophysiology. Acknowledging these complex and poorly understood pathways and their role in TBI could help design new clinical trials involving drugs that alter the RAS network, most notably angiotensin receptor blockers and angiotensin-converting enzyme inhibitors. This study aimed to briefly review the molecular, animal, and human studies on these drugs in TBI and provide a clear vision for researchers to fill knowledge gaps in the future.

The legacy of Malcolm Beverley Segal (1937-2019) on the science and fields concerned with choroid plexus and cerebrospinal fluid physiology

This article highlights the scientific achievements, professional career, and personal interactions of Malcolm B. Segal who passed away in July this year. Born in 1937 in Goodmayes, Essex, UK, Segal rose to the Chairman position in the Division of Physiology at United Medical and Dental School of Guy’s and St. Thomas’ Hospitals, retiring in 2006 after his long professional career in biomedical science. Being trained in Hugh Davson’s laboratory, Segal became one of the pioneers in research on cerebrospinal fluid physiology and the choroid plexus. During the course of his career, Segal himself trained a number of young scientists and collaborated with many colleagues around the world, making long-lasting friendships along the way. In addition to his professional accomplishments as a researcher and educator, Segal was an avid sailor and wine connoisseur, and enjoyed teaching classes on navigation and wine tasting.

Effects of head-down tilt on the intracranial pressure in conscious rabbits

Head-down tilt (HDT) causes a fluid shift towards the upper body, which increases intracranial pressure (ICP). In the present study, the time course of ICP changes during prolonged exposure to HDT was investigated in conscious rabbits through a catheter chronically implanted into the subarachnoid space. The production of cerebrospinal fluid (CSF) after exposure to 7-days HDT was also examined by a ventriculo-cisternal perfusion method. The ICP increased from 4.3+/-0.4 (mean+/-S.E.M.) mmHg to 8.0+/-0.8 mmHg immediately after the onset of 45 degrees HDT, reached a peak value of 15.8+/-1.9 mmHg at 11 h, and then decreased to 10.4+/-1.1 mmHg at 24 h. During 7-days HDT, it also increased from 4.8+/-0.9 mmHg to 9.2+/-1.6 mmHg immediately after the onset of 45 degrees HDT, reached a peak value of 12.8+/-2.5 mmHg at 12 h of HDT, and then decreased gradually towards the pre-HDT baseline value for 7 days. The rate of CSF production was 10.1+/-0.6 microl/min in rabbits exposed to 7-days HDT, and 9.7+/-0.5 microl/min in control rabbits. These results suggest that the rabbits begin to adapt to HDT within a few days and that the production of CSF is preserved after exposure to 7-days HDT. The time course of ICP changes during HDT in conscious rabbits seems to be considerably different from that in anesthetized rabbits.

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.

Angiotensin II modulates respiratory and acid-base responses to prolonged hypoxia in conscious dogs

We tested the hypothesis that angiotensin II (ANG II) contributes to ventilatory and acid-base adaptations during 3-4 h of hypoxia (partial pressure of O2 in arterial blood approximately 43 Torr) in the conscious dog. Three protocols were carried out over 3-4 h in five dogs: 1) air control, 2) 12% O2 breathing, and 3) 12% O2 breathing with ANG II receptors blocked by infusion of saralasin (0. 5 microg . kg-1 . min-1). After 2 h of hypoxia, expired ventilation and alveolar ventilation progressively increased, and the partial pressure of CO2 in arterial blood and the difference between the arterial concentrations of strong cations and strong anions ([SID]) decreased. When the hypoxic chemoreceptor drive to breathe was abolished transiently for 30 s with 100% O2, the resultant central apneic time decreased between 0.5 and 2.5 h of hypoxia. All these adaptive responses to hypoxia were abolished by ANG II receptor block. Because plasma ANG II levels were lower during hypoxia and hypoxic release of arginine vasopressin from the pituitary into the plasma was prevented by ANG II receptor block, the brain renin-angiotensin system was likely involved. It is possible that ANG II mediates ventilatory and acid-base adaptive responses to prolonged hypoxia via alterations in ion transport to decrease [SID] in brain extracellular fluid rather than acting by a direct neural mechanism.

Vasopressin mediates the inhibitory effect of central angiotensin II on cerebrospinal fluid formation

Angiotensin II infused at low doses into the cerebral ventricles decreases cerebrospinal fluid (CSF) production. Since central angiotensin II also activates the sympathetic nervous system and promotes vasopressin release, the roles of these two factors in mediating the inhibitory effect of angiotensin II on CSF formation were studied. CSF production was measured in rats by the ventriculocisternal perfusion method. During central angiotensin II infusion (5 pg min(-1)), the following adrenoceptor antagonists were administered intravenously (i.v.): phentolamine (alpha1/alpha2, 2 mg/kg per h), prazosin (alpha1, 1 mg/kg per h), and propranolol (beta, 1 mg/kg per h). None of these agents affected the inhibitory effect of angiotensin II on CSF formation. In comparison, in animals administered i.v., the vasopressin V1 receptor antagonist, d(CH2)5Tyr(Me)Arg-vasopressin (10 microg/kg per h), the angiotensin II-induced decrease in CSF production was abolished. Our observations indicate, therefore, that vasopressin mediates the inhibitory effect of central angiotensin II on CSF formation.

Distribution of N-type Ca2+ channel binding sites in rabbit brain following central administration of omega-conotoxin GVIA

Central administration of the N-type Ca2+ channel blocker omega-conotoxin GVIA in conscious rabbits has previously been shown to result in a slowly developing hypotensive and sympatholytic effect, with peak changes observed after 48 h. The aim of the current study was to examine the distribution of [125I] omega-conotoxin GVIA binding in rabbit brain alone or following a prior i.c.v. injection of omega-conotoxin GVIA to determine the site(s) of action of centrally administered omega-conotoxin GVIA. Brains were removed from rabbits 2 or 48 h after central administration of vehicle or non-labelled omega-conotoxin GVIA (30 pmol/kg, i.c.v.). Brain sections were then incubated with [125I] omega-conotoxin GVIA (50 pM) and the density of specific [125I] omega-conotoxin GVIA binding measured in dpm/mm2 was determined by quantitative receptor autoradiography. In the vehicle group, highest densities of [125I] omega-conotoxin GVIA binding sites (> 20 dpm/mm2) were detected in cortex, caudate, putamen, and the stratum oriens and stratum radiatum of the hippocampus. Prior (48 h) i.c.v. injection of omega-conotoxin GVIA resulted in a decrease in specific binding of [125I] omega-conotoxin GVIA, particularly in cortex and some portions of the caudate and hippocampus. Lesser effects were observed with a prior (2 h) i.c.v. injection of omega-conotoxin GVIA. Central administration of omega-conotoxin GVIA may be acting to disrupt neurotransmission in higher brain regions which may, in turn, affect cardiovascular control mechanisms in the rabbit.

The role of angiotensin II in the regulation of blood flow to choroid plexuses and cerebrospinal fluid formation in the rat

The effect of peripherally administered angiotensin II (AII) on blood flow to choroid plexuses was examined in pentobarbital-anesthetized rats. The indicator fractionation method with 123I- or 125I-N-isopropyl-p-iodoamphetamine as the marker was employed to measure blood flow. Basal blood flow to choroid plexus of the lateral cerebral ventricle (LVCP) (3.19 +/- 0.23 ml g-1 min-1) was lower than that to choroid plexuses of the third (3VCP) and fourth (4VCP) ventricles (3.90 +/- 0.38 and 3.95 +/- 0.36 ml g-1 min-1, respectively). The effect of AII on choroidal blood flow varied depending on peptide dose and anatomical location of the choroidal tissue. AII infused intravenously at rates of 30 and 50 ng kg-1 min-1 decreased blood flow to both LVCP and 4VCP by 12-20%. Both lower (10 ng kg-1 min-1) and higher (100 and 300 ng kg-1 min-1) AII doses did not alter blood flow to LVCP and 4VCP. Blood flow to the 3VCP was not affected by any dose of the peptide used. In comparison, blood flow to cerebral cortex increased by 33% during intravenous AII infusion at a rate of 300 ng kg-1 min-1. The choroidal blood flow-lowering effect of moderate AII doses was abolished by both AT1 (losartan) and AT2 (PD 123319) receptor subtype antagonists (3 mg kg-1 i.v.). To determine whether the hemodynamic changes observed in choroid plexuses with moderate AII doses influence CSF formation, the ventriculocisternal perfusion was performed in rats (under the experimental conditions described) with Blue Dextran 2000 as the indicator.(ABSTRACT TRUNCATED AT 250 WORDS)

AT1 receptor subtype mediates the inhibitory effect of central angiotensin II on cerebrospinal fluid formation in the rat

The effect of central administration of angiotensin II (AII) on cerebrospinal fluid (CSF) formation was studied in pentobarbital-anesthetized, artificially-ventilated rats. CSF production was measured by the ventriculocisternal perfusion method with Blue Dextran 2000 as the indicator. Baseline value of CSF production was 3.35 +/- 0.08 microliters/min. Intracerebroventricular (i.c.v.) infusion of AII at rates of 0.5 and 5 pg/min significantly lowered (P < 0.01) CSF formation by 23% and 16%, respectively. In comparison, high peptide doses (50 and 500 pg/min) did not alter this parameter. The inhibitory effect of low AII doses on CSF formation was blocked by the i.c.v. AT1 receptor subtype antagonists, losartan and SK&F 108566 (2.4 and 2.7 ng/min, respectively), but not by the AT2 receptor subtype-specific agent, PD 123319 (3.8 ng/min). Peptide AII antagonists, [Sar1,Ile8]AII (5 ng/min), which binds to both AT1 and AT2 receptors, had a similar effect to those of AT1-specific blockers. It is concluded that AII, by controlling CSF formation, may influence the water and electrolyte balance in the brain.

Endogenous angiotensin II inhibits production of cerebrospinal fluid during posthypoxemic reoxygenation in the rabbit

BACKGROUND AND PURPOSE

The choroid plexus, the major source of cerebrospinal fluid (CSF), contains receptors for angiotensin II and a very high concentration of angiotensin converting enzyme. Circulating angiotensin II decreases blood flow to the choroid plexus and the production of CSF. During recovery from hypoxia, marked increases in circulating angiotensin II have been described in some studies. We tested the hypothesis that CSF production decreases during posthypoxemic reoxygenation and examined related changes in plasma concentrations of angiotensin II. We also determined whether effect of posthypoxic reoxygenation on production of CSF is due to endogenous release of angiotensin II.

METHODS

We measured production of CSF in chloralose-anesthetized rabbits using ventriculocisternal perfusion of artificial CSF containing blue dextran. After control measurements, rabbits were subjected to one of the following interventions: (1) 30 minutes of hypoxia (PaO2 = 36 +/- 1 mmHg [mean +/- SE]) followed by 90 minutes of reoxygenation; (2) 30 minutes of hypoxia (PaO2 = 37 +/- 2 mmHg) followed by 90 minutes of reoxygenation in the presence of the angiotensin II antagonist saralasin; (3) hypoxia for 120 minutes (PaO2 = 35 +/- 1 mmHg); and (4) infusion of vehicle under normoxic conditions for 120 minutes (time control). Plasma concentrations of angiotensin II were also measured (radioimmunoassay) under control conditions, during hypoxia, and during posthypoxic reoxygenation (first intervention) and at corresponding time intervals in time control animals (fourth intervention).

RESULTS

Under control conditions, the rate of production of CSF averaged 6.7 +/- 0.1 microL.min-1. During posthypoxemic reoxygenation, production of CSF decreased by 31 +/- 4% (P < .05). In the presence of sarlasin, CSF production did not change significantly during posthypoxemic reoxygenation (-12 +/- 6%, P > .05). In time control animals and during prolonged hypoxia, CSF production did not change significantly (-12 +/- 5% [P > .05] and 9 +/- 7% [P > .05], respectively). Plasma concentrations of angiotensin were below the threshold of sensitivity of the assay under control conditions and during interventions in animals that were made hypoxic and then reoxygenated and in time control animals.

CONCLUSIONS

CSF production decreases during posthpoxemic reoxygenation. Since plasma concentrations of angiotensin II did not change during posthypoxic reoxygenation, this effect does not appear to be mediated by increases in circulating angiotensin II. We speculate that endogenous release of angiotensin II, perhaps in the choroid plexus epithelium, decreases production of CSF after hypoxic brain injury.