Isoflurane-induced cerebral hyperemia in neuronal nitric oxide synthase gene deficient mice

Amyloid beta accumulation in TgF344-AD rats is associated with reduced cerebral capillary endothelial Kir2.1 expression and neurovascular uncoupling

Alzheimer's disease (AD) exerts a tremendous socio-economic burden worldwide. Although reduced cerebral blood flow is an early and persistent symptom that precedes the loss of cognitive function in AD, the underlying molecular and cellular mechanisms remain unclear. The present study investigated whether capillary endothelial inward rectifier potassium 2 (Kir2.1) expression is reduced in TgF344-AD (AD) rats and contributes to neurovascular uncoupling and cognitive deficits in AD. Three- to fourteen-month-old AD rats expressing mutant human APP and PS1 and age-matched wild-type (WT) F344 rats were studied. AD rats exhibited higher amyloid beta (Aβ) expression in the brain as early as 3 months of age and amyloid plaques by 4 months of age. Functional hyperemic responses induced by whisker stimulation were impaired at 4 months of age, which were exacerbated in 6-month- and 14-month-old AD rats. The expression of Kir2.1 protein was significantly lower in the brains of 6-month-old AD versus WT rats, and Kir2.1 coverage was lower in the cerebral microvasculature of AD than in WT rats. Aβ1-42 reduced the Kir2.1 expression in cultured capillary endothelial cells. Cerebral parenchymal arterioles with attached capillaries exhibited a reduced vasodilator in response to 10 mM K+ applied to capillaries, and constricted less following administration of a Kir2.1 channel blocker, compared to WT vessels. These results indicate that capillary endothelial Kir2.1 expression is reduced and contributes to impaired functional hyperemia in AD rats at early ages, perhaps secondary to elevated Aβ expression.

Effect of sphenopalatine ganglion block on intracranial pressure and cerebral venous outflow oxygenation during craniotomy for supratentorial brain tumours

BACKGROUND

Intraoperative intracranial pressure (ICP) control continues to be a challenge for anaesthetists during craniotomies. Although many standard brain-dehydrating protocols are available, they may be ineffective in certain surgical situations and may result in harm either to the systemic or cerebral circulation. Sphenopalatine ganglion block (SPGB) can reverse the vasodilatory effects of anaesthesia during craniotomy.

METHODS

This prospective randomised study was carried from June 2020 to February 2021. Fifty-two patients were randomly allocated into two groups, the block group (B) and the non-block control group (Non). Twenty-six patients were enrolled in the (B) group and received a bilateral transnasal SPG block with 2% lidocaine using a hallow culture swab prior to anaesthesia induction. Intraoperative monitoring was performed using standard American Society of Anesthesiologists (ASA) monitors in addition to invasive monitoring using intra-arterial cannulas and jugular venous bulb catheters. Subdural ICP monitors were also employed. The arterio-jugular oxygen difference in mmol/l (AjvDO2) was then calculated. Mean flow velocity cm/s (MFV) and pulsatility index (PI) were monitored in both groups using Transcranial Doppler. Haemodynamic data were recorded every 30 min from induction of anaesthesia until the closure of the dura.

RESULTS

There was a significant difference in ICP prior to the dural opening between the block group (B), mean ± sd 7.58 ± 1.47, and the non-block group (Non), mean ± sd (11.69 ± 1.72), p-value < 0.001. There was no significant difference in MFV between (B) group, mean ± sd 72.65 ± 2.28 and (Non) group, mean ± sd 71.19 ± 3.09 before intubation (baseline values). While there was a significant difference after intubation between block group, mean ± sd 72.12 ± 1.77 and non - block group, mean ± sd 74.62 ± 5.07, p-value = 0.02. There was an insignificant difference between (B) and (Non) groups before intubation regarding PI values, while PI was significantly higher in (B) group than the (Non) group after intubation where mean ± sd was 1.17 ± 0.05 versus 0.96 ± 0.09, respectively, p-value = 0.001. There was no significant difference regarding cerebral oxygenation between the groups.

CONCLUSIONS

SPGB can control factors that increase CBF during anaesthesia by the block of parasympathetic vasodilatory fibres to the arterial system in the anterior cerebral circulation, while neither hindering cerebral venous drainage nor impairing cerebral oxygenation, as it gives no supply to cerebral veins and does not affect basal CBF. Additionally, it does not affect systemic circulation.

Impaired capillary-to-arteriolar electrical signaling after traumatic brain injury

Traumatic brain injury (TBI) acutely impairs dynamic regulation of local cerebral blood flow, but long-term (>72 h) effects on functional hyperemia are unknown. Functional hyperemia depends on capillary endothelial cell inward rectifier potassium channels (Kir2.1) responding to potassium (K+) released during neuronal activity to produce a regenerative, hyperpolarizing electrical signal that propagates from capillaries to dilate upstream penetrating arterioles. We hypothesized that TBI causes widespread disruption of electrical signaling from capillaries-to-arterioles through impairment of Kir2.1 channel function. We randomized mice to TBI or control groups and allowed them to recover for 4 to 7 days post-injury. We measured in vivo cerebral hemodynamics and arteriolar responses to local stimulation of capillaries with 10 mM K+ using multiphoton laser scanning microscopy through a cranial window under urethane and α-chloralose anesthesia. Capillary angio-architecture was not significantly affected following injury. However, K+-induced hyperemia was significantly impaired. Electrophysiology recordings in freshly isolated capillary endothelial cells revealed diminished Ba2+-sensitive Kir2.1 currents, consistent with a reduction in channel function. In pressurized cerebral arteries isolated from TBI mice, K+ failed to elicit the vasodilation seen in controls. We conclude that disruption of endothelial Kir2.1 channel function impairs capillary-to-arteriole electrical signaling, contributing to altered cerebral hemodynamics after TBI.

Role of Endothelial Nitric Oxide Synthase in Isoflurane Conditioning-Induced Neurovascular Protection in Subarachnoid Hemorrhage

Background

Delayed cerebral ischemia remains a common and profound risk factor for poor outcome after subarachnoid hemorrhage (SAH). The aim of our current study is to define the role of endothelial nitric oxide synthase (eNOS) in isoflurane conditioning‐induced neurovascular protection after SAH.

Methods and Results

Ten‐ to 14‐week‐old male wild‐type mice (C57BL/6) as controls and eNOS knockout male mice (strain # 002684) were obtained for the study. Animals underwent either sham surgery, SAH surgery, or SAH with isoflurane conditioning. Anesthetic post conditioning was performed with isoflurane 2% for 1 hour, 1 hour after SAH. Normothermia was maintained with the homeothermic blanket. In a separate cohort, nitric oxide synthase was inhibited by a pan nitric oxide synthase inhibitor, L‐nitroarginine methyl ester. Vasospasm measurement was assessed 72 hours after SAH and neurological function was assessed daily. Isoflurane‐induced changes in the eNOS protein expression were measured. eNOS protein expression was significantly increased by isoflurane conditioning in naïve mice as well as mice subjected to SAH. Vasospasm of the middle cerebral artery and neurological deficits were evident following SAH versus sham surgery, both in wild‐type mice and eNOS knockout mice. Isoflurane conditioning attenuated vasospasm and neurological deficits in wild‐type mice. This delayed cerebral ischemia protection was lost in L‐nitroarginine methyl ester ‐administered mice and eNOS knockout mice.

Conclusions

Our data indicate isoflurane conditioning provides robust protection against SAH‐induced vasospasm and neurological deficits, and that this delayed cerebral ischemia protection is critically mediated via isoflurane‐induced augmentation of eNOS.

Conditional Human Immunodeficiency Virus Transactivator of Transcription Protein Expression Induces Depression-like Effects and Oxidative Stress

BACKGROUND

The prevalence of major depression in those with HIV/AIDS is substantially higher than in the general population. Mechanisms underlying this comorbidity are poorly understood. HIV-transactivator of transcription (Tat) protein, produced and excreted by HIV, could be involved. We determined whether conditional Tat protein expression in mice is sufficient to induce depression-like behaviors and oxidative stress. Further, as oxidative stress is associated with depression, we determined whether decreasing or increasing oxidative stress by administering methylsulfonylmethane (MSM) or diethylmaleate (DEM), respectively, altered depression-like behavior.

METHODS

GT-tg bigenic mice received intraperitoneal saline or doxycycline (Dox, 25-100 mg/kg/day) to induce Tat expression. G-tg mice, which do not express Tat protein, also received Dox. Depression-like behavior was assessed with the tail suspension test (TST) and the two-bottle saccharin/water consumption task. Reactive oxygen/nitrogen species (ROS/RNS) were assessed ex vivo. Medial frontal cortex (MFC) oxidative stress and temperature were measured in vivo with 9.4-Tesla proton magnetic resonance spectroscopy (MRS).

RESULTS

Tat expression increased TST immobility time in an exposure-dependent manner and reduced saccharin consumption. MSM decreased immobility time while DEM increased it in saline-treated GT-tg mice. Tat and MSM behavioral effects persisted for 28 days. Tat and DEM increased while MSM decreased ROS/RNS levels. Tat expression increased MFC glutathione levels and temperature.

CONCLUSIONS

Tat expression induced rapid and enduring depression-like behaviors and oxidative stress. Increasing/decreasing oxidative stress increased/decreased, respectively, depression-like behavior. Thus, Tat produced by HIV may contribute to the high depression prevalence among those with HIV. Further, mitigation of oxidative stress could reduce depression severity.

Isoflurane induces endothelial apoptosis of the post-hypoxic blood-brain barrier in a transdifferentiated human umbilical vein endothelial cell model

Isoflurane is a popular volatile anesthetic agent used in humans as well as in experimental animal research. In previous animal studies of the blood-brain barrier (BBB), observations towards an increased permeability after exposure to isoflurane are reported. In this study we investigated the effect of a 2-hour isoflurane exposure on apoptosis of the cerebral endothelium following 24 hours of hypoxia in an in vitro BBB model using astrocyte-conditioned human umbilical vein endothelial cells (AC-HUVECs). Apoptosis of AC-HUVECs was investigated using light microscopy of the native culture for morphological changes, Western blot (WB) analysis of Bax and Bcl-2, and a TUNEL assay. Treatment of AC-HUVECs with isoflurane resulted in severe cellular morphological changes and a significant dose-dependent increase in DNA fragmentation, which was observed during the TUNEL assay analysis. WB analysis confirmed increases in pro-apoptotic Bax levels at 4 hours and 24 hours and decreases in anti-apoptotic Bcl-2 in a dose-dependent manner compared with the control group. These negative effects of isoflurane on the BBB after a hypoxic challenge need to be taken into account not only in experimental stroke research, but possibly also in clinical practice.

Cerebral perfusion MRI in mice

Perfusion MRI is a tool to assess the spatial distribution of microvascular blood flow. Arterial spin labeling (ASL) is shown here to be advantageous for quantification of cerebral microvascular blood flow (CBF) in rodents. This technique is today ready for assessment of a variety of murine models of human pathology including those associated with diffuse microvascular dysfunction. This chapter provides an introduction to the principles of CBF measurements by MRI along with a short overview over applications in which these measurements were found useful. The basics of commonly employed specific arterial spin-labeling techniques are described and theory is outlined in order to give the reader the ability to set up adequate post-processing tools. Three typical MR protocols for pulsed ASL on two different MRI systems are described in detail along with all necessary sequence parameters and technical requirements. The importance of the different parameters entering theory is discussed. Particular steps for animal preparation and maintenance during the experiment are given, since CBF regulation is sensitive to a number of experimental physiological parameters and influenced mainly by anesthesia and body temperature.

Experimental comparison of four FAIR arterial spin labeling techniques for quantification of mouse cerebral blood flow at 4.7 T

Pulsed arterial spin labeling (ASL) is an attractive and robust method for quantification of rodent cerebral blood flow (CBF) in particular, although there is a need for sensitivity optimization. Look-Locker flow-sensitive alternating inversion recovery (FAIR) echo planar imaging (EPI) (LLFAIREPI) was expected to be a likely candidate for assessing sensitivity, although it has not yet been applied to rodents. In this study, the performance of two FAIR techniques and two Look-Locker FAIR techniques were compared in mouse brain at 4.7 T. FAIR-EPI (single inversion time, FAIREPI-1TI), FAIR-EPI (eight inversion times, FAIREPI-8TI), LLFAIREPI and Look-Locker FAIR gradient echo (LLFAIRGE) sequences were implemented with equal spatial resolution and equal FAIR preparation modules. Measurements were carried out sequentially on the brain in 10 healthy mice, and quantitative CBF maps were obtained after different acquisition times up to 23 min. All methods gave similar group variability in CBF. Especially at shorter acquisition times, LLFAIREPI gave lower relative variations in CBF within selected brain regions than the other techniques at the same acquisition time. The Look-Locker techniques, however, overestimated CBF compared with classical FAIR-EPI, which was attributed to bulk flow in arterioles and T(2) effects. The image quality with LLFAIREPI was less reproducible within the group. Both FAIREPI-1TI and LLFAIREPI appear to be good candidates for serial rapid measurement of CBF, but LLFAIREPI has the additional advantage that apparent T(1) can be measured simultaneously with CBF.

Sevoflurane Impairs Cerebral Blood Flow Autoregulation in Rats: Reversal by Nonselective Nitric Oxide Synthase Inhibition

In this study, we investigated the effects of 1.0 and 2.0 minimum alveolar anesthetic concentration (MAC) sevoflurane on cerebral blood flow (CBF) autoregulation before and after nonselective inhibition of nitric oxide (NO) synthase in rats. Rats were randomly assigned as follows: Group 1 (n = 8): 1.0 MAC sevoflurane; Groups 2 and 3 (n = 8 per group): 2.0 MAC sevoflurane. Assessment of autoregulation within a mean arterial blood pressure range of 140-60 mm Hg was performed by graded hemorrhage before and after administration of l-arginine methyl ester (l-NAME, 30 mg/kg IV, Groups 1 and 2) or during hypocapnia (Group 3). In 10 additional animals, brain tissue NO(2)(-) concentrations were measured at 1.0 and 2.0 MAC sevoflurane. CBF autoregulation was maintained with 1.0 MAC sevoflurane (Group 1) regardless of NO synthase status indicating that CBF autoregulation might not be related to NO availability. Sevoflurane dose-dependently increased brain tissue NO(2)(-) and impaired CBF autoregulation. Administration of l-NAME (Group 2) but not hypocapnia (Group 3) restored CBF autoregulation. This suggests that sevoflurane impairs the autoregulatory capacity secondary to an increase of the perivascular NO availability and questions the importance of basal cerebrovascular tone in terms of vasodilatory capacity during hypotensive challenges.

IMPLICATIONS

The present study suggests that the volatile anesthetic sevoflurane dose-dependently impairs cerebrovascular autoregulation by mechanisms secondary to increase of perivascular nitric oxide availability.

Role of nitric oxide in cerebral blood flow changes during kainate seizures in mice: genetic and pharmacological approaches

The role of neuronal nitric oxide (NO) in the cerebrovascular response to partial seizures induced by intrahippocampal injection of kainate (KA) was investigated in mice deleted for the neuronal NO synthase gene (nNOS-/-) and in wild-type controls (WT). A second group of WT mice received the nNOS inhibitor, 7-nitroindazole (WT-7NI). Local cerebral blood flow (LCBF) was measured using the quantitative (14)C-iodoantipyrine method. Within the epileptic focus, all three groups of seizing mice (WT, WT-7NI, and nNOS-/-) showed significant 26-88% LCBF increases in ipsilateral hippocampus, compared to saline-injected mice. Contralaterally to the epileptic focus, KA seizures induced a 21-47% LCBF decreases in hippocampus and limbic cortex of WT mice and in most contralateral brain structures of nNOS-/- mice, while WT-7NI mice showed no contralateral CBF change. Neuronal NO appears to be not involved in the cerebrovascular response within the epileptic focus, but may rather have a role in the maintenance of distant LCBF regulation during seizures.

Laser speckle flowmetry for the study of cerebrovascular physiology in normal and ischemic mouse cortex

Laser speckle flowmetry (LSF) is useful to assess noninvasively two-dimensional cerebral blood flow (CBF) with high temporal and spatial resolution. The authors show that LSF can image the spatiotemporal dynamics of CBF changes in mice through an intact skull. When measured by LSF, peak CBF increases during whisker stimulation closely correlated with simultaneous laser-Doppler flowmetry (LDF) measurements, and were greater within the branches of the middle cerebral artery supplying barrel cortex than within barrel cortex capillary bed itself. When LSF was used to study the response to inhaled CO2 (5%), the flow increase was similar to the response reported using LDF. For the upper and lower limits of autoregulation, mean arterial pressure values were 110 and 40 mm Hg, respectively. They also show a linear relationship between absolute resting CBF, as determined by [C]iodoamphetamine technique, and 1/tau(c) values obtained using LSF, and used 1/tau(c) values to compare resting CBF between different animals. Finally, the authors studied CBF changes after distal middle cerebral artery ligation, and developed a model to investigate the spatial distribution and hemodynamics of moderate to severely ischemic cortex. In summary, LSF has distinct advantages over LDF for CBF monitoring because of high spatial resolution.

Miniaturization: an overview of biotechnologies for monitoring the physiology and pathophysiology of rodent animal models

Recent advances in bioengineering technologies have made it possible to collect high-quality reproducible data quantitatively in a wide range of laboratory animal species, including rodents. Several of these technologies are incorporated into a plan called Miniaturization, which aims to design, develop, and maintain rodent animal models to study the pathophysiology and therapy of human diseases. Laser Doppler flowmetry, digital sonomicrometry, bioelectrical impedance, and microdialysis are some of the most widely used methods under the plan because they cause minimal pain and distress, reduce the number of animals used in biomedical research, and allow chronic, nonterminal assessment of physiological parameters in rodents. An overview of each of these technologies and their major applications in rodents used for biomedical research is provided.

Cerebral perfusion during anesthesia with fentanyl, isoflurane, or pentobarbital in normal rats studied by arterial spin-labeled MRI

The influence of anesthetic agents on cerebral blood flow (CBF) was tested in normal rats. CBF is quantified with arterial spin-labeled MRI in rats anesthetized with either an opiate (fentanyl), a potent inhalation anesthetic agent (isoflurane), or a barbiturate (pentobarbital) using doses commonly employed in experimental paradigms. CBF values were found to be about 2.5-3 times lower in most regions analyzed during anesthesia with either fentanyl (with N(2)O/O(2)) or pentobarbital vs. isoflurane (with N(2)O/O(2)), in agreement with findings utilizing invasive measurement techniques. CBF was heterogeneous in rats anesthetized with isoflurane (with N(2)O/O(2)), but relatively homogeneous in rats anesthetized with either fentanyl (with N(2)O/O(2)) or pentobarbital, also in agreement with studies using other techniques. Magn Reson Med 46:202-206, 2001.

Endotoxin augments cerebral hyperemic response to halothane by inducing nitric oxide synthase and cyclooxygenase

We examined the cerebral hyperemic response to halothane after treatment with bacterial lipopolysaccharide (LPS). To determine the involvement of inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX-2), we tested whether the effect of LPS on halothane-induced hyperemia was altered by pretreatment with the selective iNOS inhibitor, aminoguanidine (100 mg/kg), COX-2 inhibitor, NS-398 (5 mg/kg), or enzyme expression inhibitor, dexamethasone (4 mg/kg). Further, we examined whether the administration of a nitric oxide donor, diethylamine NONOate, would change the cerebral hyperemic response of halothane. Sprague-Dawley rats were anesthetized with 0.5 minimum alveolar anesthetic concentration of halothane and artificially ventilated. Regional cerebrocortical blood flow (rCBF) was assessed by laser-Doppler flowmetry. LPS (1 mg/kg) was administered intracerebroventricularly; artificial cerebrospinal fluid was used in controls. Four hours after LPS infusion, iNOS and COX-2 messenger ribonucleic acid (mRNA) levels (reverse transcription-polymerase chain reaction) and enzyme activities (arginine-citrulline conversion and prostaglandin E(2) enzyme immunoassay) were significantly increased. LPS enhanced halothane-induced 3.9 and 1.6-fold increases in rCBF at 1.0 and 1.5 minimum alveolar concentration, respectively. Co-treatment with NS-398 attenuated, but aminoguanidine or dexamethasone abolished the effect of LPS on halothane-induced rCBF increase. Diethylamine NONOate mimicked the enhanced rCBF response to halothane. These results suggest that LPS augmented halothane-induced cerebrocortical hyperemia by induction of iNOS and COX-2.

The effect of low concentrations of halothane on the cerebrovascular circulation in young children

To determine the effect of halothane on cerebral blood flow velocity measured by transcranial Doppler, 23 healthy young children were studied during surgery. Anaesthesia was induced with thiopental, fentanyl and vecuronium, and maintained with halothane in 70% nitrous oxide in oxygen. A continuous epidural anaesthesia with 0. 25% bupivacaine was performed. End-tidal carbon dioxide pressure, temperature, heart rate and systolic blood pressure were kept constant. Three minimal alveolar concentrations (MAC; 0.5, 1.0 and 1. 5) of halothane were administered in stepwise increases. The cerebral blood flow velocity increased significantly at 1.0 (p < 0. 01) and 1.5 MAC (p < 0.001) compared with the value at 0.5 MAC. No further change in cerebral blood flow velocity was seen between 1.0 and 1.5 MAC. These data show that maximal changes in cerebral blood flow velocity are obtained at 1.0 MAC and that further increases in halothane concentration do not modify the cerebral circulation. It is suggested that young children differ from adults in that the maximal effect of halothane occurs at lower concentrations.

7-Nitroindazole impedes erythrocyte flow response to isovolemic hemodilution in the cerebral capillary circulation

The role of nitric oxide (NO) in the mechanism of hemodilution-induced cerebral hyperemia is unclear. Based on findings in hypoxemia, the authors hypothesize that NO of neuronal origin contributes to an increase in velocity of erythrocytes in the cerebral microcirculation during anemia produced by isovolemic hemodilution. The change in erythrocyte velocity in cerebrocortical capillaries was assessed by intravital fluorescence video microscopy. A closed cranial window was implanted over the frontoparietal cortex of barbiturate-anesthetized, ventilated adult rats. Erythrocytes were labeled in vitro with fluorescein isothiocyanate and infused intravenously, and their velocity in subsurface capillaries was measured by frame-to-frame image tracking. Arterial blood was withdrawn in increments of 2 mL and replaced by serum albumin; arterial blood pressure was maintained at control level with an infusion of methoxamine. Erythrocyte velocity increased progressively, reaching 215% of baseline, as arterial hematocrit was reduced from 45% to 17%. Pretreatment of a separate group of rats with 7-nitroindazole (20 mg/kg intraperitoneally), a relatively selective inhibitor of neuronal NO synthase, abolished the increase in velocity at hematocrits greater than 20%, but the maximum velocity attained at the lowest hematocrit was similar to that in the control group. The results suggest that NO from neuronal source may contribute to the increase in capillary erythrocyte flow during moderate isovolemic hemodilution.

Neural mechanisms in nitric-oxide-deficient hypertension

Nitric oxide is hypothesized to be an inhibitory modulator of central sympathetic nervous outflow, and deficient neuronal nitric oxide production to cause sympathetic overactivity, which then contributes to nitric-oxide-deficient hypertension. The biochemical and neuroanatomical basis for this concept revolves around nitric oxide modulation of glutamatergic neurotransmission within brainstem vasomotor centers. The functional consequence of neuronal nitric oxide in blood pressure regulation is, however, marked by an apparent conflict in the literature. On one hand, conscious animal studies using sympathetic blockade suggest a significant role for neuronal nitric oxide deficiency in the development of nitric-oxide-deficient hypertension, and on the other hand, there is evidence against such a role derived from 'knock-out' mice lacking nitric-oxide synthase 1, the major source of neuronal nitric oxide.