Effects of anesthetic agents on brain blood oxygenation level revealed with ultra-high field MRI

Anesthesia-related brain microstructure modulations detected by diffusion magnetic resonance imaging

Recent studies have shown significant changes to brain microstructure during sleep and anesthesia. In vivo optical microscopy and magnetic resonance imaging (MRI) studies have attributed these changes to anesthesia and sleep-related modulation of the brain's extracellular space (ECS). Isoflurane anesthesia is widely used in preclinical diffusion MRI (dMRI) and it is therefore important to investigate if the brain's microstructure is affected by anesthesia to an extent detectable with dMRI. Here, we employ diffusion kurtosis imaging (DKI) to assess brain microstructure in the awake and anesthetized mouse brain (n = 22). We find both mean diffusivity (MD) and mean kurtosis (MK) to be significantly decreased in the anesthetized mouse brain compared with the awake state (p < 0.001 for both). This effect is observed in both gray matter and white matter. To further investigate the time course of these changes we introduce a method for time-resolved fast DKI. With this, we show the time course of the microstructural alterations in mice (n = 5) as they transition between states in an awake-anesthesia-awake paradigm. We find that the decrease in MD and MK occurs rapidly after delivery of gas isoflurane anesthesia and that values normalize only slowly when the animals return to the awake state. Finally, time-resolved fast DKI is employed in an experimental mouse model of brain edema (n = 4), where cell swelling causes the ECS volume to decrease. Our results show that isoflurane affects DKI parameters and metrics of brain microstructure and point to isoflurane causing a reduction in the ECS volume. The demonstrated DKI methods are suitable for in-bore perturbation studies, for example, for investigating microstructural modulations related to sleep/wake-dependent functions of the glymphatic system. Importantly, our study shows an effect of isoflurane anesthesia on rodent brain microstructure that has broad relevance to preclinical dMRI.

Promising Cerebral Blood Flow Enhancers in Acute Ischemic Stroke

Ischemic stroke presents a major global economic and public health burden. Although recent advances in available endovascular therapies show improved functional outcome, a good number of stroke patients are either ineligible or do not have access to these treatments. Also, robust collateral flow during acute ischemic stroke independently predicts the success of endovascular therapies and the outcome of stroke. Hence, adjunctive therapies for cerebral blood flow (CBF) enhancement are urgently needed. A very clear overview of the pial collaterals and the role of genetics are presented in this review. We review available evidence and advancement for potential therapies aimed at improving CBF during acute ischemic stroke. We identified heme-free soluble guanylate cyclase activators; Sanguinate, remote ischemic perconditioning; Fasudil, S1P agonists; and stimulation of the sphenopalatine ganglion as promising potential CBF-enhancing therapeutics requiring further investigation. Additionally, we outline and discuss the critical steps required to advance research strategies for clinically translatable CBF-enhancing agents in the context of acute ischemic stroke models.

Spatial and temporal frequency band changes during infarct induction, infarct progression, and spreading depolarizations in the gyrencephalic brain

Aim

To describe the spatial and temporal electrocorticographic (ECoG) changes after middle cerebral artery occlusion (MCAo), including those caused by spreading depolarization (SD) in the pig brain.

Methods

The left middle cerebral arteries (MCAs) were clipped in six pigs. The clipping procedure lasted between 8 and 12 min, achieving a permanent occlusion (MCAo). Five-contact ECoG stripes were placed bilaterally over the frontoparietal cortices corresponding to the irrigation territory of the MCA and anterior cerebral artery (ACA). ECoG recordings were performed around 24 h: 1 h before and 23 h after the MCAo, and SDs were quantified. Five-minute ECoG signal segments were sampled before, 5 min, and 4, 8, and 12 h after cerebral artery occlusion and before, during, and after the negative direct current shift of the SDs. The power spectrum of the signals was decomposed into delta, theta, alpha, beta, and gamma bands. Descriptive statistics, Wilcoxon matched-pairs signed-rank tests, and Friedman tests were performed.

Results

Electrodes close to the MCAo showed instant decay in all frequency bands and SD onset during the first 5 h. Electrodes far from the MCAo exhibited immediate loss of fast frequencies and progressive decline of slow frequencies with an increased SD incidence between 6 and 14 h. After 8 h, the ACA electrode reported a secondary reduction of all frequency bands except gamma and high SD incidence within 12-17 h. During the SD, all electrodes showed a decline in all frequency bands. After SD passage, frequency band recovery was impaired only in MCA electrodes.

Conclusion

ECoG can identify infarct progression and secondary brain injury. Severe disturbances in all the frequency bands are generated in the cortices where the SDs are passing by.

Optical measurement of microvascular oxygenation and blood flow responses in awake mouse cortex during functional activation

The cerebral cortex has a number of conserved morphological and functional characteristics across brain regions and species. Among them, the laminar differences in microvascular density and mitochondrial cytochrome c oxidase staining suggest potential laminar variability in the baseline O2 metabolism and/or laminar variability in both O2 demand and hemodynamic response. Here, we investigate the laminar profile of stimulus-induced intravascular partial pressure of O2 (pO2) transients to stimulus-induced neuronal activation in fully awake mice using two-photon phosphorescence lifetime microscopy. Our results demonstrate that stimulus-induced changes in intravascular pO2 are conserved across cortical layers I-IV, suggesting a tightly controlled neurovascular response to provide adequate O2 supply across cortical depth. In addition, we observed a larger change in venular O2 saturation (ΔsO2) compared to arterioles, a gradual increase in venular ΔsO2 response towards the cortical surface, and absence of the intravascular "initial dip" previously reported under anesthesia. This study paves the way for quantification of layer-specific cerebral O2 metabolic responses, facilitating investigation of brain energetics in health and disease and informed interpretation of laminar blood oxygen level dependent functional magnetic resonance imaging signals.

Ultra-Early Cerebral Thrombosis Formation After Experimental Subarachnoid Hemorrhage Detected on T2* Magnetic Resonance Imaging

BACKGROUND AND PURPOSE

The mechanisms of brain damage during ultra-early subarachnoid hemorrhage (SAH) have not been well studied. The current study examined the SAH-induced hyperacute brain damage at 4 hours using magnetic resonance imaging and brain histology in a mouse model.

METHODS

SAH was induced by endovascular perforation in adult mice. First, adult male wild-type mice underwent magnetic resonance imaging T2 and T2* 4 hours after an endovascular perforation or a sham operation and were euthanized to assess brain histology. Second, male and female adult lipocalin-2 knockout mice had SAH. All animals underwent magnetic resonance imaging at 4 hours, and the brains were harvested for brain histology.

RESULTS

T2* hypointensity vessels were observed in the brain 4 hours after SAH in male wild-type mice. The numbers of T2*-positive vessels were significantly higher in SAH brains than in sham-operated mice. Brain histology showed thrombosis and erythrocyte plugs in the T2*-positive cerebral vessels which may be venules. The number of T2*-positive vessels correlated with SAH grade and the presence of T2 lesions. Brain thrombosis was also accompanied by albumin leakage and neuronal injury. LCN2 deficient male mice had lower numbers of T2*-positive vessels after SAH compared with wild-type male mice.

CONCLUSIONS

SAH causes ultra-early brain vessel thrombosis that can be detected by T2* gradient-echo sequence at 4 hours after SAH. LCN2 deficiency decreased the number of T2*-positive vessels.

Brain networks of rats under anesthesia using resting-state fMRI: comparison with dead rats, random noise and generative models of networks

OBJECTIVE

Connectivity networks are crucial to understand the brain resting-state activity using functional magnetic resonance imaging (rs-fMRI). Alterations of these brain networks may highlight important findings concerning the resilience of the brain to different disorders. The focus of this paper is to evaluate the robustness of brain network estimations, discriminate them under anesthesia and compare them to generative models.

APPROACH

The extraction of brain functional connectivity (FC) networks is difficult and biased due to the properties of the data: low signal to noise ratio, high dimension low sample size. We propose to use wavelet correlations to assess FC between brain areas under anesthesia using four anesthetics (isoflurane, etomidate, medetomidine, urethane). The networks are then deduced from the functional connectivity matrices by applying statistical thresholds computed using the number of samples at a given scale of wavelet decomposition. Graph measures are extracted and extensive comparisons with generative models of structured networks are conducted.

MAIN RESULTS

The sample size and filtering are critical to obtain significant correlations values and thereby detect connections between regions. This is necessary to construct networks different from random ones as shown using rs-fMRI brain networks of dead rats. Brain networks under anesthesia on rats have topological features that are mixing small-world, scale-free and random networks. Betweenness centrality indicates that hubs are present in brain networks obtained from anesthetized rats but locations of these hubs are altered by anesthesia.

SIGNIFICANCE

Understanding the effects of anesthesia on brain areas is of particular importance in the context of animal research since animal models are commonly used to explore functions, evaluate lesions or illnesses, and test new drugs. More generally, results indicate that the use of correlations in the context of fMRI signals is robust but must be treated with caution. Solutions are proposed in order to control spurious correlations by setting them to zero.

Propofol/Remifentanil Anesthesia Might Not Alter the Middle Cerebral Artery Diameter by Digital Subtraction Angiography

INTRODUCTION

Transcranial Doppler (TCD) of the middle cerebral artery (MCA) enables the measurement of the mean blood velocity (MCA_Vm) and the estimation of the cerebral blood flow (CBF), provided that no significant changes occur in the MCA diameter (MCA_Diam). Previous studies described a decrease in the MCA_Vm associated with the induction of total intravenous anesthesia (TIVA) by propofol and remifentanil. This decrease in blood velocity might be interpreted as a decrease in the CBF only where the MCA_Diam is not modified across TCD examinations.

METHODS

In this observational study, we measured the MCA_Diam of 24 subjects (almost exclusively females) on digital subtraction angiography under awake and TIVA conditions.

RESULTS

Across the two phases, we observed a decrease in the mean arterial blood pressure (from 84 ± 9 to 71 ± 6 mmHg; p < 0.001) and heart rate (76 ± 10 vs. 65 ± 8 beats/min; p < 0.001), and a concomitant decrease in the MCA_Vm (61 vs. 42 cm/s; p < 0.001). In contrast, the MCA_Diam did not vary in association with TIVA (2.3 ± 0.2 vs. 2.3 ± 0.2 mm; p = 0.52).

CONCLUSIONS

Those results suggested that in this population, no significant changes in the MCA_Diam are associated with TIVA.

Cerebrovascular MRI in the mouse without an exogenous contrast agent

PURPOSE

To assess the effect of a variety of anesthetic regimes on T 2 ∗ -weighted MRI of the mouse brain and to determine the optimal regimes to perform T 2 ∗ -weighted MRI of the mouse cerebrovasculature without a contrast agent.

METHODS

Twenty mice were imaged with a 3D T 2 ∗ -weighted sequence under isoflurane, dexmedetomidine, or ketamine-xylazine anesthesia with a fraction of inspired oxygen varied between 10% and 95% + 5% CO2 . Some mice were also imaged after an injection of an iron oxide contrast agent as a positive control. For every regime, whole brain vessel conspicuity was visually assessed and the apparent vessel density in the cortex was quantified and compared.

RESULTS

The commonly used isoflurane anesthetic leads to poor vessel conspicuity for fraction of inspired oxygen higher or equal to 21%. Dexmedetomidine and ketamine-xylazine enable the visualization of a significantly larger portion of the vasculature for the same breathing gas. Under isoflurane anesthesia, the fraction of inspired oxygen must be lowered to between 10% and 14% to obtain similar vessel conspicuity. Initial results on automatic segmentation of veins and arteries using the iron oxide positive control are also reported.

CONCLUSION

T 2 ∗ -weighted MRI in combination with an appropriate anesthetic regime can be used to visualize the mouse cerebrovasculature without a contrast agent. The differences observed between regimes are most likely caused by blood-oxygen level dependent effects, highlighting the important impact of the anesthetic regimes on cerebral blood oxygenation of the mouse brain at rest.

Animal Functional Magnetic Resonance Imaging: Trends and Path Toward Standardization

Animal whole-brain functional magnetic resonance imaging (fMRI) provides a non-invasive window into brain activity. A collection of associated methods aims to replicate observations made in humans and to identify the mechanisms underlying the distributed neuronal activity in the healthy and disordered brain. Animal fMRI studies have developed rapidly over the past years, fueled by the development of resting-state fMRI connectivity and genetically encoded neuromodulatory tools. Yet, comparisons between sites remain hampered by lack of standardization. Recently, we highlighted that mouse resting-state functional connectivity converges across centers, although large discrepancies in sensitivity and specificity remained. Here, we explore past and present trends within the animal fMRI community and highlight critical aspects in study design, data acquisition, and post-processing operations, that may affect the results and influence the comparability between studies. We also suggest practices aimed to promote the adoption of standards within the community and improve between-lab reproducibility. The implementation of standardized animal neuroimaging protocols will facilitate animal population imaging efforts as well as meta-analysis and replication studies, the gold standards in evidence-based science.

Multi-slice passband bSSFP for human and rodent fMRI at ultra-high field

Balanced steady-state free precession (bSSFP) can be used as an alternative to gradient-echo (GE) EPI for BOLD functional MRI when image distortions and signal drop-outs are severe such as at ultra-high field. However, 3D-bSSFP acquisitions have distinct drawbacks on either human or animal MR systems. On clinical scanners, 3D imaging is suboptimal for localized fMRI applications. It can also display distortions when acceleration methods such as spiral read-outs are used, and, compared to multi-slice acquisitions, suffers from increased sensitivity to motion or physiological noise which further results in blurring. On pre-clinical systems, 3D acquisitions have low temporal resolution due to limited acceleration options, while single slice often results in insufficient coverage. The aim of the present study was to implement a multi-slice bSSFP acquisition with Cartesian read-out to obtain non-distorted BOLD fMRI activation maps in the human and rat brain at ultra-high field. We show that, when using a new pseudo-steady-state, the bSSFP signal characteristics are preserved. In the human brain at 7 T, we demonstrate that both task- and resting-state fMRI can be performed with multi-slice bSSFP, with a temporal SNR that matches that of 3D-bSSFP, resulting in - at least - equal performance. In the rat brain at 14 T, we show that the multi-slice bSSFP protocol has similar sensitivity to gradient-echo EPI for task fMRI, while benefitting from much reduced distortions and drop-outs. The advantages of passband bSSFP at 14 T in comparison with GE-EPI are expected to be even more marked for mouse brain.

Fentanyl-Induced Brain Hypoxia Triggers Brain Hyperglycemia and Biphasic Changes in Brain Temperature

Fentanyl is a potent synthetic opioid used extensively in humans for general anesthesia and analgesia. Fentanyl has emerged as a recreational drug, often in combination with heroin, and can result in lethality during overdose. Fentanyl is well characterized as an anesthetic, but the basic physiological effects of fentanyl in the brain when taken as a drug of abuse are largely unknown. We used high-speed amperometry in freely moving rats to examine the effects of intravenous fentanyl at doses within the range of possible human intake (3-40 μg/kg) on oxygen and glucose levels in nucleus accumbens (NAc). Fentanyl induced a rapid, dose-dependent decrease in NAc oxygen followed by a more delayed and prolonged increase in NAc glucose. Fentanyl induced similar oxygen decreases in the basolateral amygdala, indicating that brain hypoxia could be a generalized phenomenon. We used oxygen recordings in the subcutaneous space to confirm that fentanyl-induced brain hypoxia results from decreases in blood oxygen levels caused by drug-induced respiratory depression. Temperature recordings in the NAc, muscle, and skin showed that fentanyl induces biphasic changes in brain temperature, with an initial decrease that results primarily from peripheral vasodilation, and a subsequent increase driven by metabolic brain activation. The initial vasodilation appears caused by respiratory depression-induced hypoxia and a subsequent rise in CO₂ that drives fentanyl-induced increases in NAc glucose. Together, these data suggest that fentanyl-induced respiratory depression triggers brain hypoxia and subsequent hyperglycemia, both of which precede slower changes in brain temperature and metabolic brain activity.

Comparison of Atipamezole with Yohimbine for Antagonism of Xylazine in Mice Anesthetized with Ketamine and Xylazine

The α2 adrenergic agonist xylazine produces a sedative effect and is typically combined with ketamine and used for anesthesia or chemical restraint of laboratory mice. Xylazine's sedative effect-and its undesirable side effects of bradycardia, hypotension, and poor tissue perfusion-can be reversed by administration of α2 antagonists, such as atipamezole or yohimbine. Although atipamezole and yohimbine dosing guidelines are available for mice, no controlled comparison has been performed to guide the lab animal community in the selection of one over the other. This study is a single-dose crossover comparison of these 2 antagonist drugs, given intraperitoneally at clinically recommended doses, to determine which results in more rapid recovery of mice from xylazine-ketamine anesthesia. Time to return of righting reflex was used as the primary outcome measure. Mice were anesthetized with xylazine (10 mg/kg IP) and ketamine (80 mg/kg IP), followed 15 min later by injection of an α2 antagonist or saline (control). Time to return of righting reflex differed significantly among groups, with mice recovering in an average of 10.3 min after administration of atipamezole (1 mg/kg IP) as compared with 21.3 min after yohimbine (1.5 mg/kg IP) and 38.2 min after saline. When rapid recovery of mice after xylazine-ketamine anesthesia is desirable, administration of an antagonist to reverse the effects of the xylazine is indicated. When injection of the antagonist by the technically simple intraperitoneal route is desirable, our data indicate that (at the doses evaluated) atipamezole is more effective than yohimbine.

Light controls cerebral blood flow in naive animals

Optogenetics is increasingly used to map brain activation using techniques that rely on functional hyperaemia, such as opto-fMRI. Here we test whether light stimulation protocols similar to those commonly used in opto-fMRI or to study neurovascular coupling modulate blood flow in mice that do not express light sensitive proteins. Combining two-photon laser scanning microscopy and ultrafast functional ultrasound imaging, we report that in the naive mouse brain, light per se causes a calcium decrease in arteriolar smooth muscle cells, leading to pronounced vasodilation, without excitation of neurons and astrocytes. This photodilation is reversible, reproducible and energy-dependent, appearing at about 0.5 mJ. These results impose careful consideration on the use of photo-activation in studies involving blood flow regulation, as well as in studies requiring prolonged and repetitive stimulations to correct cellular defects in pathological models. They also suggest that light could be used to locally increase blood flow in a controlled fashion.

Combination of optogenetics and BOLD fMRI is routinely used to map neuronal activity upon photostimulation. Here the authors show that light, shone at intensities used in optogenetic studies, dilates vessels and increases blood flow independently of exogenous light-sensitive proteins in the mouse brain.

Comparing the Effects of Isoflurane and Alpha Chloralose upon Mouse Physiology

Functional magnetic resonance imaging of mice requires that the physiology of the mouse (body temperature, respiration and heart rates, blood pH level) be maintained in order to prevent changes affecting the outcomes of functional scanning, namely blood oxygenation level dependent (BOLD) measures and cerebral blood flow (CBF). The anesthetic used to sedate mice for scanning can have major effects on physiology. While alpha chloralose has been commonly used for functional imaging of rats, its effects on physiology are not well characterized in the literature for any species. In this study, we anesthetized or sedated mice with isoflurane or alpha chloralose for up to two hours, and monitored physiological parameters and arterial blood gasses. We found that, when normal body temperature is maintained, breathing rates for both drugs decrease over the course of two hours. In addition, alpha chloralose causes a substantial drop in heart rate and blood pH with severe hypercapnia (elevated blood CO2) that is not seen in isoflurane-treated animals. We suggest that alpha chloralose does not maintain normal mouse physiology adequately for functional brain imaging outcome measures.

Factoring the brain signatures of anesthesia concentration and level of arousal across individuals

Combining resting-state functional magnetic resonance imaging (fMRI) connectivity and behavioral analysis during sedation, we factored out general effects of the anesthetic drug propofol and a specific index of conscious report, participants' level of responsiveness. The factorial analysis shows that increasing concentration of propofol in blood specifically decreases the connectivity strength of fronto-parietal cortical loops. In contrast, loss of responsiveness is indexed by a functional disconnection between the thalamus and the frontal cortex, balanced by an increase in connectivity strength of the thalamus to the occipital and temporal regions of the cortex.

Temporal assessment of vascular reactivity and functionality using MRI during postischemic proangiogenenic vascular remodeling

Postischemic angiogenesis is an important recovery mechanism. Both arteries and veins are upregulated during angiogenesis, but eventually there are more angiogenic veins than arteries in terms of number and length. It is critical to understand how the veins are modulated after ischemia and then transitioned into angiogenic vessels during the proangiogenic stage to finally serve as a restorative strength to the injured area. Using a rat model of transient focal cerebral ischemia, the hypercapnic blood oxygen level-dependent (BOLD) response was used to evaluate vascular reactivity, while the hyperoxic BOLD and tissue oxygen level-dependent (TOLD) responses were used to evaluate the vascular functionality at 1, 3, and 7days after ischemia. Vessel-like venous signals appeared on R2* maps on days 3 and 7, but not on day 1. The large hypercapnic BOLD responses on days 3 and 7 indicated that these areas have high vascular reactivity. The temporal correlation between vascular reactivity and the immunoreactivity to desmin and VEGF further indicates that the integrity of vascular reactivity is associated with the pericyte coverage as regulated by the VEGF level. Vascular functionality remained low on days 1, 3, and 7, as reflected by the small hyperoxic BOLD and large hyperoxic TOLD responses, indicating the low oxygen consumption of the ischemic tissues. These functional changes in proangiogenic veins may be critical for angiogenesis.

Gadolinium-Loaded Viral Capsids as Magnetic Resonance Imaging Contrast Agents

Polymeric nanohybrid P22 virus capsids were used as templates for high density Gd³⁺ loading to explore magnetic field-dependent (0.5-7.0 T) proton relaxivity. The field-dependence of relaxivity by the spatially constrained Gd³⁺ in the capsids was similar when either the loading of the capsids or the concentration of capsids was varied. The ionic longitudinal relaxivity, r₁, decreased from 25-32 mM^-1 s^-1 at 0.5 T to 6-10 mM^-1 s^-1 at 7 T. The ionic transverse relaxivity, r₂, increased from 28-37 mM^-1 s^-1 at 0.5 T to 39-50 mM^-1 s^-1 at 7 T. The r₂/r₁ ratio increased linearly with increasing magnetic field from about 1 at 0.5 T, which is typical of T₁ contrast agents, to 5-8 at 7 T, which is approaching the ratios for T₂ contrast agents. Increases in electron paramagnetic resonance line widths at 80 and 150 K and higher microwave powers required for signal saturation indicate enhanced Gd³⁺ electron spin relaxation rates for the Gd³⁺-loaded capsids than for low concentration Gd³⁺. The largest r₂/r₁ at 7 T was for the highest cage loading, which suggests that Gd³⁺-Gd³⁺ interactions within the capsid enhance r₂ more than r₁.

Differentiating BOLD and non-BOLD signals in fMRI time series from anesthetized rats using multi-echo EPI at 11.7 T

The study of spontaneous brain activity using fMRI is central to mapping brain networks. However, current fMRI methodology has limitations in the study of small animal brain organization using ultra-high field fMRI experiments, as imaging artifacts are difficult to control and the relationship between classical neuroanatomy and spontaneous functional BOLD activity is not fully established. Challenges are especially prevalent during the fMRI study of individual rodent brains, which could be instrumental to studies of disease progression and pharmacology. A recent advance in fMRI methodology enables unbiased, accurate, and comprehensive identification of functional BOLD signals by interfacing multi-echo (ME) fMRI acquisition, NMR signal decay analysis, and independent components analysis (ICA), in a procedure called ME-ICA. Here we present a pilot study on the suitability of ME-ICA for ultra high field animal fMRI studies of spontaneous brain activity under anesthesia. ME-ICA applied to 11.7 T fMRI data of rats first showed robust performance in automatic high dimensionality estimation and ICA decomposition, similar to that previously reported for 3.0 T human data. ME sequence optimization for 11.7 T indicated that 3 echoes, 0.5mm isotropic voxel size and TR=3s was adequate for sensitive and specific BOLD signal acquisition. Next, in seeking optimal inhaled isoflurane anesthesia dosage, we report that progressive increase in anesthesia goes with concomitant decrease in statistical complexity of "global" functional activity, as measured by the number of BOLD components, or degrees of freedom (DOF). Finally, BOLD functional connectivity maps for individual rodents at the component level show that spontaneous BOLD activity follows classical neuroanatomy, and seed-based analysis shows plausible cortical-cortical and cortical-subcortical functional interactions.

Sedation agents differentially modulate cortical and subcortical blood oxygenation: evidence from ultra-high field MRI at 17.2 T

BACKGROUND

Sedation agents affect brain hemodynamic and metabolism leading to specific modifications of the cerebral blood oxygenation level. We previously demonstrated that ultra-high field (UHF) MRI detects changes in cortical blood oxygenation following the administration of sedation drugs commonly used in animal research. Here we applied the UHF-MRI method to study clinically relevant sedation drugs for their effects on cortical and subcortical (thalamus, striatum) oxygenation levels.

METHODS

We acquired T2*-weighted images of Sprague-Dawley rat brains at 17.2T in vivo. During each MRI session, rats were first anesthetized with isoflurane, then with a second sedative agent (sevoflurane, propofol, midazolam, medetomidine or ketamine-xylazine) after stopping isoflurane. We computed a T2*-oxygenation-ratio that aimed at estimating cerebral blood oxygenation level for each sedative agent in each region of interest: cortex, hippocampus, thalamus and striatum.

RESULTS

The T2*-oxygenation-ratio was consistent across scan sessions. This ratio was higher with inhalational agents than with intravenous agents. Under sevoflurane and medetomidine, T2*-oxygenation-ratio was homogenous across the brain regions. Intravenous agents (except medetomidine) induced a T2*-oxygenation-ratio imbalance between cortex and subcortical regions: T2*-oxygenation-ratio was higher in the cortex than the subcortical areas under ketamine-xylazine; T2*-oxygenation-ratio was higher in subcortical regions than in the cortex under propofol or midazolam.

CONCLUSION

Preclinical UHF MRI is a powerful method to monitor the changes in cerebral blood oxygenation level induced by sedative agents across brain structures. This approach also allows for a classification of sedative agents based on their differential effects on cerebral blood oxygenation level.

Cerebral mechanisms of general anesthesia

How does general anesthesia (GA) work? Anesthetics are pharmacological agents that target specific central nervous system receptors. Once they bind to their brain receptors, anesthetics modulate remote brain areas and end up interfering with global neuronal networks, leading to a controlled and reversible loss of consciousness. This remarkable manipulation of consciousness allows millions of people every year to undergo surgery safely most of the time. However, despite all the progress that has been made, we still lack a clear and comprehensive insight into the specific neurophysiological mechanisms of GA, from the molecular level to the global brain propagation. During the last decade, the exponential progress in neuroscience and neuro-imaging led to a significant step in the understanding of the neural correlates of consciousness, with direct consequences for clinical anesthesia. Far from shutting down all brain activity, anesthetics lead to a shift in the brain state to a distinct, highly specific and complex state, which is being increasingly characterized by modern neuro-imaging techniques. There are several clinical consequences and challenges that are arising from the current efforts to dissect GA mechanisms: the improvement of anesthetic depth monitoring, the characterization and avoidance of intra-operative awareness and post-anesthesia cognitive disorders, and the development of future generations of anesthetics.

Functional connectivity fMRI in mouse brain at 7T using isoflurane

Although many resting state fMRI human studies have been published, the number of such rodent studies is considerably less. The reason for this is the severe technical challenge of high magnetic field small rodent imaging. Local magnetic field susceptibility changes at air tissue boundaries cause image distortion and signal losses. The current study reports measures of functional connectivity in mice using only isoflurane for the anesthetic. Because all anesthetic agents will alter cerebral blood flow and cerebral metabolism, the impact these changes have on neuronal connectivity has yet to be fully understood, however this work reports for the first time that reliable functional connectivity measures in mouse brain can be obtained with isoflurane.