The effects of volatile anesthetics on synaptic and extrasynaptic GABA-induced neurotransmission. Brain Res Bull. 2013;93:69-79. [PMID: 22925739 DOI: 10.1016/j.brainresbull.2012.08.001]

Impairment of synaptic plasticity in the primary somatosensory cortex in a model of diabetic mice

Type 1 and type 2 diabetic patients experience alterations in the Central Nervous System, leading to cognitive deficits. Cognitive deficits have been also observed in animal models of diabetes such as impaired sensory perception, as well as deficits in working and spatial memory functions. It has been suggested that a reduction of insulin-like growth factor-I (IGF-I) and/or insulin levels may induce these neurological disorders. We have studied synaptic plasticity in the primary somatosensory cortex of young streptozotocin (STZ)-diabetic mice. We focused on the influence of reduced IGF-I brain levels on cortical synaptic plasticity. Unit recordings were conducted in layer 2/3 neurons of the primary somatosensory (S1) cortex in both control and STZ-diabetic mice under isoflurane anesthesia. Synaptic plasticity was induced by repetitive whisker stimulation. Results showed that repetitive stimulation of whiskers (8 Hz induction train) elicited a long-term potentiation (LTP) in layer 2/3 neurons of the S1 cortex of control mice. In contrast, the same induction train elicited a long-term depression (LTD) in STZ-diabetic mice that was dependent on NMDA and metabotropic glutamatergic receptors. The reduction of IGF-I brain levels in diabetes could be responsible of synaptic plasticity impairment, as evidenced by improved response facilitation in STZ-diabetic mice following the application of IGF-I. This hypothesis was further supported by immunochemical techniques, which revealed a reduction in IGF-I receptors in the layer 2/3 of the S1 cortex in STZ-diabetic animals. The observed synaptic plasticity impairments in STZ-diabetic animals were accompanied by decreased performance in a whisker discrimination task, along with reductions in IGF-I, GluR1, and NMDA receptors observed in immunochemical studies. In conclusion, impaired synaptic plasticity in the S1 cortex may stem from reduced IGF-I signaling, leading to decreased intracellular signal pathways and thus, glutamatergic receptor numbers in the cellular membrane.

Extrasynaptic GABAA receptors in central medial thalamus mediate anesthesia in rats

Neuronal depression in the thalamus underlies anesthetic-induced loss of consciousness, while the precise sub-thalamus nuclei and molecular targets involved remain to be elucidated. The present study investigated the role of extrasynaptic GABAA receptors in the central medial thalamic nucleus (CM) in anesthesia induced by gaboxadol (THIP) and diazepam (DZP) in rats. Local lesion of the CM led to a decrease in the duration of loss of righting reflex induced by THIP and DZP. CM microinjection of THIP but not DZP induced anesthesia. The absence of righting reflex in THIP-treated rats was consistent with the increase of low frequency oscillations in the delta band in the medial prefrontal cortex. CM microinjection of GABAA receptor antagonist SR95531 significantly attenuated the anesthesia induced by systemically-administered THIP, but not DZP. Moreover, the rats with declined expression of GABAA receptor δ-subunit in the CM were less responsive to THIP or DZP. These findings explained a novel mechanism of THIP-induced loss of consciousness and highlighted the role of CM extrasynaptic GABAA receptors in mediating anesthesia.

The Effects of Volatile Anesthetics on Renal Sympathetic and Phrenic Nerve Activity during Acute Intermittent Hypoxia in Rats

Coordinated activation of sympathetic and respiratory nervous systems is crucial in responses to noxious stimuli such as intermittent hypoxia. Acute intermittent hypoxia (AIH) is a valuable model for studying obstructive sleep apnea (OSA) pathophysiology, and stimulation of breathing during AIH is known to elicit long-term changes in respiratory and sympathetic functions. The aim of this study was to record the renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA) during the AIH protocol in rats exposed to monoanesthesia with sevoflurane or isoflurane. Adult male Sprague-Dawley rats (n = 24; weight: 280-360 g) were selected and randomly divided into three groups: two experimental groups (sevoflurane group, n = 6; isoflurane group, n = 6) and a control group (urethane group, n = 12). The AIH protocol was identical in all studied groups and consisted in delivering five 3 min-long hypoxic episodes (fraction of inspired oxygen, FiO2 = 0.09), separated by 3 min recovery intervals at FiO2 = 0.5. Volatile anesthetics, isoflurane and sevoflurane, blunted the RSNA response to AIH in comparison to urethane anesthesia. Additionally, the PNA response to acute intermittent hypoxia was preserved, indicating that the respiratory system might be more robust than the sympathetic system response during exposure to acute intermittent hypoxia.

α5GABAA receptor: A potential therapeutic target for perioperative neurocognitive disorders, a review of preclinical studies

Perioperative neurocognitive disorders (PND) are a common complication in elderly patients following surgery, which not only prolongs the recovery period but also affects their future quality of life and imposes a significant burden on their family and society. Multiple factors, including aging, vulnerability, anesthetic drugs, cerebral oxygen desaturation, and severe pain, have been associated with PND. Unfortunately, no effective drug is currently available to prevent PND. α5 γ-aminobutyric acid subtype A (α5GABAA) receptors have been implicated in cognitive function modulation. Positive or negative allosteric modulators of α5GABAA receptors have been found to improve cognitive impairment under different conditions. Therefore, targeting α5GABAA receptors may represent a promising treatment strategy for PND. This review focuses on preclinical studies of α5GABAA receptors and the risk factors associated with PND, primarily including aging, anesthetics, and neuroinflammation. Specifically, positive allosteric modulators of α5GABAA receptors have improved cognitive function in aged experimental animals. In contrast, negative allosteric modulators of α5GABAA receptors have been found to facilitate cognitive recovery in aged or adult experimental animals undergoing anesthesia and surgery but not in aged experimental animals under anesthesia alone. The reasons for the discordant findings have yet to be elucidated. In preclinical studies, different strategies of drug administration, as well as various behavioral tests, may influence the stability of the results. These issues need to be carefully considered in future studies.

Sevoflurane induces neurotoxic effects on developing neurons through the WNK1/NKCC1/Ca2+ /Drp-1 signalling pathway

Children repeatedly exposed to anaesthesia have a high risk of cognitive impairment, but the mechanism of its regulation in this context is unknown. The objective of this study was to investigate the possible toxic mechanism of sevoflurane through the WNK1/NKCC1/Ca²⁺ /Drp-1 signalling pathway. The hippocampal neuronal HT22 cell line was used in this study. The intervention group was treated with the WNK1 inhibitor WNK-463, CaN inhibitor FK506 and Drp-1 inhibitor Mdivi-1 respectively in the medium for 30 min before sevoflurane anaesthesia. The sevofluane group and all intervention group treated with 4.1% sevoflurane for 6 h. Compared with the control group, sevoflurane treatment decreased cell viability and increased cellular apoptosis. Our study found that WNK-463, FK506 and Mdivi-1 can all alleviate the sevoflurane-induced reduction in cell viability, decrease the cell apoptosis. In addition, WNK-463 pretreatment could inhibit the increase of WNK1 kinase and NKCC1 protein concentration caused by sevoflurane. Further, sevoflurane anaesthesia causes intracellular calcium overload, increases the expression of CaN and induces the dephosphorylation of Drp-1 protein at ser637, while CaN inhibitor FK506 pretreatment could reduce the dephosphorylation of Drp-1. Therefore, the WNK1/NKCC1/Ca²⁺ /Drp-1 signalling pathway plays an important role in sevoflurane-related neurotoxicity. Reducing intracellular calcium influx may be one of the important mechanism to ameliorate sevoflurane toxicity.

Effects of isoflurane anesthesia on addictive behaviors in rats

RATIONALE

Recently, it has been suggested that isoflurane might reduce dopamine release from rat midbrain dopaminergic neurons, the neurobiological substrate implicated in the reinforcing effects of abused drugs and nondrug rewards. However, little is known about effects of isoflurane on neurobehavioral activity associated with chronic exposure to psychoactive substances.

OBJECTIVE

The present study was designed to investigate the effects of isoflurane on cocaine-reinforced behavior. Using behavioral paradigm in rats, we evaluated the effects of isoflurane on cocaine self-administration under fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement. We also tested the effects of isoflurane on lever responding by nondrug reinforcers (sucrose and food) in drug-naive rats to control for the nonselective effects of isoflurane on cocaine- and nicotine-taking behavior. To further assess the ability of isoflurane to modulate the motivation for taking a drug, we evaluated the effects of isoflurane on nicotine self-administration. Using different groups of rats, the effects of isoflurane on the locomotor activity induced by a single intraperitoneal injection of cocaine (15 mg/kg) were also examined.

RESULTS

Isoflurane significantly suppressed the self-administration of cocaine and nicotine without affecting food consumption. Unlike food-reinforced responding, responding for sucrose reinforcement was decreased by isoflurane. Isoflurane reduced breaking points under a PR schedule of reinforcement in a dose-dependent manner, indicating its efficacy in decreasing the incentive value of cocaine. Isoflurane also attenuated acute cocaine-induced hyperlocomotion.

CONCLUSIONS

The results provided evidence that isoflurane decreases cocaine- and nicotine-reinforced responses, while isoflurane effect is not selective for cocaine- and nicotine-maintained responding. These results suggest that isoflurane inhibitions of cocaine- and nicotine-maintenance responses may be related to decreased effects of dopamine, and further investigation will need to elucidate this relationship.

Role of GABAA receptor depolarization-mediated VGCC activation in sevoflurane-induced cognitive impairment in neonatal mice

Background

In neonatal mice, anesthesia with sevoflurane depolarizes the GABA Type A receptor (GABAAR), which leads to cognitive impairment. Calcium accumulation in neurons can lead to neurotoxicity. Voltage-gated calcium channels (VGCCs) can increase intracellular calcium concentration under isoflurane and hypoxic conditions. The underlying mechanisms remain largely unknown.

Methods

Six-day-old mice were anesthetized with 3% sevoflurane for 2 h/day for 3 days. The Y-Maze, new object recognition (NOR) test, the Barnes maze test, immunoassay, immunoblotting, the TUNEL test, and Golgi-Cox staining were used to assess cognition, calcium concentration, inflammatory response, GABAAR activation, VGCC expression, apoptosis, and proliferation of hippocampal nerve cells in mice and HT22 cells.

Results

Compared with the control group, mice in the sevoflurane group had impaired cognitive function. In the sevoflurane group, the expression of Gabrb3 and Cav1.2 in the hippocampal neurons increased (p < 0.01), the concentration of calcium ions increased (p < 0.01), inflammatory reaction and apoptosis of neurons increased (p < 0.01), the proliferation of neurons in the DG area decreased (p < 0.01), and dendritic spine density decreased (p < 0.05). However, the inhibition of Gabrb3 and Cav1.2 alleviated cognitive impairment and reduced neurotoxicity.

Conclusions

Sevoflurane activates VGCCs by inducing GABAAR depolarization, resulting in cognitive impairment. Activated VGCCs cause an increase in intracellular calcium concentration and an inflammatory response, resulting in neurotoxicity and cognitive impairment.

Signatures of Thalamocortical Alpha Oscillations and Synchronization With Increased Anesthetic Depths Under Isoflurane

Background: Electroencephalography (EEG) recordings under propofol exhibit an increase in slow and alpha oscillation power and dose-dependent phase–amplitude coupling (PAC), which underlie GABAA potentiation and the central role of thalamocortical entrainment. However, the exact EEG signatures elicited by volatile anesthetics and the possible neurophysiological mechanisms remain unclear.

Methods: Cortical EEG signals and thalamic local field potential (LFP) were recorded in a mouse model to detect EEG signatures induced by 0.9%, 1.5%, and 2.0% isoflurane. Then, the power of the EEG spectrum, thalamocortical coherence, and slow–alpha phase–amplitude coupling were analyzed. A computational model based on the thalamic network was used to determine the primary neurophysiological mechanisms of alpha spiking of thalamocortical neurons under isoflurane anesthesia.

Results: Isoflurane at 0.9% (light anesthesia) increased the power of slow and delta oscillations both in cortical EEG and in thalamic LFP. Isoflurane at 1.5% (surgery anesthesia) increased the power of alpha oscillations both in cortical EEG and in thalamic LFP. Isoflurane at 2% (deep anesthesia) further increased the power of cortical alpha oscillations, while thalamic alpha oscillations were unchanged. Thalamocortical coherence of alpha oscillation only exhibited a significant increase under 1.5% isoflurane. Isoflurane-induced PAC modulation remained unchanged throughout under various concentrations of isoflurane. By adjusting the parameters in the computational model, isoflurane-induced alpha spiking in thalamocortical neurons was simulated, which revealed the potential molecular targets and the thalamic network involved in isoflurane-induced alpha spiking in thalamocortical neurons.

Conclusion: The EEG changes in the cortical alpha oscillation, thalamocortical coherence, and slow–alpha PAC may provide neurophysiological signatures for monitoring isoflurane anesthesia at various depths.

Translational Stroke Research Review: Using the Mouse to Model Human Futile Recanalization and Reperfusion Injury in Ischemic Brain Tissue

The approach to reperfusion therapies in stroke patients is rapidly evolving, but there is still no explanation why a substantial proportion of patients have a poor clinical prognosis despite successful flow restoration. This issue of futile recanalization is explained here by three clinical cases, which, despite complete recanalization, have very different outcomes. Preclinical research is particularly suited to characterize the highly dynamic changes in acute ischemic stroke and identify potential treatment targets useful for clinical translation. This review surveys the efforts taken so far to achieve mouse models capable of investigating the neurovascular underpinnings of futile recanalization. We highlight the translational potential of targeting tissue reperfusion in fully recanalized mouse models and of investigating the underlying pathophysiological mechanisms from subcellular to tissue scale. We suggest that stroke preclinical research should increasingly drive forward a continuous and circular dialogue with clinical research. When the preclinical and the clinical stroke research are consistent, translational success will follow.

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Gabra6100Q allele Sprague-Dawley rats have a higher sensitivity to hypnosis induced by isoflurane and ethanol than the wild type rats

BACKGROUND

The neurobiological mechanisms underlying how general anesthetics render a patient's unconsciousness (hypnosis) remains elusive. The role of the cerebellum in hypnosis induced by general anesthetics is unknown. Gabra6^100Q allele Sprague-Dawley (SD) rats have a naturally occurring single nucleotide polymorphism in the GABA_A receptor α6 subunit gene that is expressed exclusively in cerebellum granule cells.

METHODS

We examined the loss of righting reflex (LORR) induced by isoflurane, and ethanol in Gabra6^100Q rats compared with those in wild type (WT) SD rats. We also examined the change of c-Fos expression induced by isoflurane exposure in cerebellum granule cells of both mutant and WT rats.

RESULTS

Gabra6^100Q rats are more sensitive than WT rats to the LORR induced by isoflurane and ethanol. Moreover, isoflurane exposure induced a greater reduction in c-Fos expression in cerebellum granule cells of Gabra6^100Q rats than WT rats.

CONCLUSIONS

Based on these data, we speculate that cerebellum may be involved in the hypnosis induced by some general anesthetics and thus may represent a novel target of general anesthetics.

The potential role of stress and sex steroids in heritable effects of sevoflurane

Most surgical procedures require general anesthesia, which is a reversible deep sedation state lacking all perception. The induction of this state is possible because of complex molecular and neuronal network actions of general anesthetics (GAs) and other pharmacological agents. Laboratory and clinical studies indicate that the effects of GAs may not be completely reversible upon anesthesia withdrawal. The long-term neurocognitive effects of GAs, especially when administered at the extremes of ages, are an increasingly recognized health concern and the subject of extensive laboratory and clinical research. Initial studies in rodents suggest that the adverse effects of GAs, whose actions involve enhancement of GABA type A receptor activity (GABAergic GAs), can also extend to future unexposed offspring. Importantly, experimental findings show that GABAergic GAs may induce heritable effects when administered from the early postnatal period to at least young adulthood, covering nearly all age groups that may have children after exposure to anesthesia. More studies are needed to understand when and how the clinical use of GAs in a large and growing population of patients can result in lower resilience to diseases in the even larger population of their unexposed offspring. This minireview is focused on the authors' published results and data in the literature supporting the notion that GABAergic GAs, in particular sevoflurane, may upregulate systemic levels of stress and sex steroids and alter expressions of genes that are essential for the functioning of these steroid systems. The authors hypothesize that stress and sex steroids are involved in the mediation of sex-specific heritable effects of sevoflurane.

The effects of the general anesthetic sevoflurane on neurotransmission: an experimental and computational study

The brain functions can be reversibly modulated by the action of general anesthetics. Despite a wide number of pharmacological studies, an extensive analysis of the cellular determinants of anesthesia at the microcircuits level is still missing. Here, by combining patch-clamp recordings and mathematical modeling, we examined the impact of sevoflurane, a general anesthetic widely employed in the clinical practice, on neuronal communication. The cerebellar microcircuit was used as a benchmark to analyze the action mechanisms of sevoflurane while a biologically realistic mathematical model was employed to explore at fine grain the molecular targets of anesthetic analyzing its impact on neuronal activity. The sevoflurane altered neurotransmission by strongly increasing GABAergic inhibition while decreasing glutamatergic NMDA activity. These changes caused a notable reduction of spike discharge in cerebellar granule cells (GrCs) following repetitive activation by excitatory mossy fibers (mfs). Unexpectedly, sevoflurane altered GrCs intrinsic excitability promoting action potential generation. Computational modelling revealed that this effect was triggered by an acceleration of persistent sodium current kinetics and by an increase in voltage dependent potassium current conductance. The overall effect was a reduced variability of GrCs responses elicited by mfs supporting the idea that sevoflurane shapes neuronal communication without silencing neural circuits.

Inhibition of cAMP-phosphodiesterase 4 (PDE4) potentiates the anesthetic effects of Isoflurane in mice

Despite major advances, there remains a need for novel anesthetic drugs or drug combinations with improved efficacy and safety profiles. Here, we show that inhibition of cAMP-phosphodiesterase 4 (PDE4), while not inducing anesthesia by itself, potently enhances the anesthetic effects of Isoflurane in mice. Treatment with several distinct PAN-PDE4 inhibitors, including Rolipram, Piclamilast, Roflumilast, and RS25344, significantly delayed the time-to-righting after Isoflurane anesthesia. Conversely, treatment with a PDE3 inhibitor, Cilostamide, or treatment with the potent, but non-brain-penetrant PDE4 inhibitor YM976, had no effect. These findings suggest that potentiation of Isoflurane hypnosis is a class effect of brain-penetrant PDE4 inhibitors, and that they act by synergizing with Isoflurane in inhibiting neuronal activity. The PDE4 family comprises four PDE4 subtypes, PDE4A to PDE4D. Genetic deletion of any of the four PDE4 subtypes in mice did not affect Isoflurane anesthesia per se. However, PDE4D knockout mice are largely protected from the effect of pharmacologic PDE4 inhibition, suggesting that PDE4D is the predominant, but not the sole PDE4 subtype involved in potentiating Isoflurane anesthesia. Pretreatment with Naloxone or Propranolol alleviated the potentiating effect of PDE4 inhibition, implicating opioid- and β-adrenoceptor signaling in mediating PDE4 inhibitor-induced augmentation of Isoflurane anesthesia. Conversely, stimulation or blockade of α₁-adrenergic, α₂-adrenergic or serotonergic signaling did not affect the potentiation of Isoflurane hypnosis by PDE4 inhibition. We further show that pretreatment with a PDE4 inhibitor boosts the delivery of bacteria into the lungs of mice after intranasal infection under Isoflurane, thus providing a first example that PDE4 inhibitor-induced potentiation of Isoflurane anesthesia can critically impact animal models and must be considered as a factor in experimental design. Our findings suggest that PDE4/PDE4D inhibition may serve as a tool to delineate the exact molecular mechanisms of Isoflurane anesthesia, which remain poorly understood, and may potentially be exploited to reduce the clinical doses of Isoflurane required to maintain hypnosis.

Temporal Progression of Excitotoxic Calcium Following Distal Middle Cerebral Artery Occlusion in Freely Moving Mice

Ischemic stroke is recognized as one of the leading causes of adult disability, morbidity, and death worldwide. Following stroke, acute neuronal excitotoxicity can lead to many deleterious consequences, one of which is the dysregulation of intracellular calcium ultimately culminating in cell death. However, to develop neuroprotective treatments that target neuronal excitotoxicity, it is essential to know the therapeutic time window for intervention following an ischemic event. To address this question, the current study aimed to characterize the magnitude and temporal progression of neuronal intracellular calcium observed following distal middle cerebral artery occlusion (dMCAO) in mice. Using the calcium fluorescence indicator, GCaMP, we tracked neuronal population response in freely moving animals immediately following dMCAO in both the core infarct and peri-infarct regions. Our results demonstrate that calcium excitotoxicity following artery occlusion can be generally characterized by two phases: a transient increase in activity that lasts tens of minutes, followed by a long, slow sustained increase in fluorescence signal. The first phase is primarily thought to represent neuronal hyperexcitability, defining our therapeutic window, while the second may represent gradual cell death. Importantly, we show that the level of intracellular calcium following artery occlusion correlated with the infarct size at 24 h demonstrating a direct connection between excitotoxicity and cell death in our stroke model. In addition, we show that administration of the NMDA antagonist MK-801 resulted in both a decrease in calcium signal and a subsequent reduction in the infarct size. Altogether, this study represents the first demonstration in freely moving animals characterizing the temporal progression of toxic calcium signaling following artery occlusion. In addition, these results define a critical time window for neuroprotective therapeutic intervention in mice.

Isoflurane decreases substantia gelatinosa neuron excitability and synaptic transmission from periphery in the rat spinal dorsal horn

Isoflurane is an inhaled anesthetic, though its actions at the cellular level remain controversial. By using acute spinal cord slices from adult rats and the whole-cell recording technique, we found that aqueous isoflurane at the minimum alveolar concentration decreased postsynaptic neural excitability and enhanced membrane conductance, while suppressing glutamate release from presynaptic afferent onto substantia gelatinosa (lamina II) neurons in the dorsal horn. The data demonstrate that isoflurane modulates synaptic transmission from peripheral to the spinal cord via both pre- and postsynaptic effects and these actions may underlie its spinal anesthesia.

Sevoflurane depresses neurons in the medial parabrachial nucleus by potentiating postsynaptic GABAA receptors and background potassium channels

Despite persistent clinical use for over 170 years, the neuronal mechanisms by which general anesthetics produce hypnosis remain unclear. Previous studies suggest that anesthetics exert hypnotic effects by acting on endogenous arousal circuits. Recently, it has been shown that the medial parabrachial nucleus (MPB) is a novel wake-promoting component in the dorsolateral pons. However, it is not known whether and how the MPB contributes to anesthetic-induced hypnosis. Here, we investigated the action of sevoflurane, a widely used volatile anesthetic agent that best represents the drug class of halogenated ethers, on MPB neurons in mice. Using in vivo fiber photometry, we found that the population activities of MPB neurons were inhibited during sevoflurane-induced loss of consciousness. Using in vitro whole-cell patch-clamp recordings, we revealed that sevoflurane suppressed the firing rate of MPB neurons in concentration-dependent and reversible manners. At a concentration equal to MAC of hypnosis, sevoflurane potentiated synaptic GABA_A receptors (GABA_A-Rs), and the inhibitory effect of sevoflurane on the firing rate of MPB neurons was completely abolished by picrotoxin, which is a selective GABA_A-R antagonist. At a concentration equivalent to MAC of immobility, sevoflurane directly hyperpolarized MPB neurons and induced a significant decrease in membrane input resistance by increasing a basal potassium conductance. Moreover, pharmacological blockade of GABA_A-Rs in the MPB prolongs induction and shortens emergence under sevoflurane inhalation at MAC of hypnosis. These results indicate that sevoflurane inhibits MPB neurons through postsynaptic GABA_A-Rs and background potassium channels, which contributes to sevoflurane-induced hypnosis.

The Estradiol Synthesis Inhibitor Formestane Diminishes the Ability of Sevoflurane to Induce Neurodevelopmental Abnormalities in Male Rats

Background

In rodents, the period of increased vulnerability to the developmental effects of general anesthetics coincides with the period of age-specific organizing (masculinizing) effects of the major female sex hormone 17β-estradiol (E2) in the male brain and excitatory GABA type A receptor (GABA_AR) signaling. We studied whether E2 synthesis and excitatory GABA_AR signaling are involved in the mediation of the developmental effects of sevoflurane in male rats.

Methods

Male Sprague-Dawley rats were pretreated with the inhibitors of E2 synthesis, formestane, or the Na⁺-K⁺-2Cl^– (NKCC1) Cl^– importer, bumetanide, prior to sevoflurane exposure for 6 h on postnatal (P) day 4, P5, or P6. We tested whether a subsequent exposure of these rats to sevoflurane on P∼10 would cause electroencephalography (EEG)-detectable seizures. We also evaluated their behavior during the elevated plus maze (EPM) test on P∼60, prepulse inhibition (PPI) of acoustic startle responses on P∼70, and corticosterone secretion to physical restraint on P∼80.

Results

The rats neonatally exposed to sevoflurane responded to repeated exposure to sevoflurane with increased EEG-detectable seizures (F₍₃,₂₄₎ = 7.445, P = 0.001) and exhibited deficiencies during the EPM (F₍₃,₅₅₎ = 4.397, P = 0.008) and PPI (F₍₃,₁₁₀₎ = 5.222, P = 0.003) tests. They also responded to physical restraint with heightened secretion of corticosterone (F₍₃,₁₆₎ = 11.906, P < 0.001). These parameters in the sevoflurane-exposed rats that were pretreated with formestane or bumetanide were not different from those in the control rats.

Conclusion

These results, along with previously published data, suggest that sevoflurane-enhanced E2 synthesis and excitatory GABA_AR signaling at the time of sevoflurane anesthesia are involved in the mediation of the neurodevelopmental effects of the anesthetic in male rats.

Neuroendocrine, epigenetic, and intergenerational effects of general anesthetics

The progress of modern medicine would be impossible without the use of general anesthetics (GAs). Despite advancements in refining anesthesia approaches, the effects of GAs are not fully reversible upon GA withdrawal. Neurocognitive deficiencies attributed to GA exposure may persist in neonates or endure for weeks to years in the elderly. Human studies on the mechanisms of the long-term adverse effects of GAs are needed to improve the safety of general anesthesia but they are hampered not only by ethical limitations specific to human research, but also by a lack of specific biological markers that can be used in human studies to safely and objectively study such effects. The latter can primarily be attributed to an insufficient understanding of the full range of the biological effects induced by GAs and the molecular mechanisms mediating such effects even in rodents, which are far more extensively studied than any other species. Our most recent experimental findings in rodents suggest that GAs may adversely affect many more people than is currently anticipated. Specifically, we have shown that anesthesia with the commonly used GA sevoflurane induces in exposed animals not only neuroendocrine abnormalities (somatic effects), but also epigenetic reprogramming of germ cells (germ cell effects). The latter may pass the neurobehavioral effects of parental sevoflurane exposure to the offspring, who may be affected even at levels of anesthesia that are not harmful to the exposed parents. The large number of patients who require general anesthesia, the even larger number of their future unexposed offspring whose health may be affected, and a growing number of neurodevelopmental disorders of unknown etiology underscore the translational importance of investigating the intergenerational effects of GAs. In this mini review, we discuss emerging experimental findings on neuroendocrine, epigenetic, and intergenerational effects of GAs.

Intergenerational Effects of Sevoflurane in Young Adult Rats

BACKGROUND

Sevoflurane administered to neonatal rats induces neurobehavioral abnormalities and epigenetic reprogramming of their germ cells; the latter can pass adverse effects of sevoflurane to future offspring. As germ cells are susceptible to reprogramming by environmental factors across the lifespan, the authors hypothesized that sevoflurane administered to adult rats could induce neurobehavioral abnormalities in future offspring, but not in the exposed rats themselves.

METHODS

Sprague-Dawley rats were anesthetized with 2.1% sevoflurane for 3 h every other day between postnatal days 56 and 60. Twenty-five days later, exposed rats and nonexposed controls were mated to produce offspring.

RESULTS

Adult male but not female offspring of exposed parents of either sex exhibited deficiencies in elevated plus maze (mean ± SD, offspring of both exposed parents vs. offspring of control parents, 35 ± 12 vs. 15 ± 15 s, P < 0.001) and prepulse inhibition of acoustic startle (offspring of both exposed parents vs. offspring of control parents, 46.504 ± 13.448 vs. 25.838 ± 22.866%, P = 0.009), and increased methylation and reduced expression of the potassium ion-chloride ion cotransporter KCC2 gene (Kcc2) in the hypothalamus. Kcc2 was also hypermethylated in sperm and ovary of the exposed rats. Surprisingly, exposed male rats also exhibited long-term abnormalities in functioning of the hypothalamic-pituitary-gonadal and -adrenal axes, reduced expression of hypothalamic and hippocampal Kcc2, and deficiencies in elevated plus maze (sevoflurane vs. control, 40 ± 24 vs. 25 ± 12 s, P = 0.038) and prepulse inhibition of startle (sevoflurane vs. control, 39.905 ± 21.507 vs. 29.193 ± 24.263%, P < 0.050).

CONCLUSIONS

Adult sevoflurane exposure affects brain development in male offspring by epigenetically reprogramming both parental germ cells, while it induces neuroendocrine and behavioral abnormalities only in exposed males. Sex steroids may be required for mediation of the adverse effects of adult sevoflurane in exposed males.

Involvement of homodomain interacting protein kinase 2-c-Jun N-terminal kinase/c-Jun cascade in the long-term synaptic toxicity and cognition impairment induced by neonatal Sevoflurane exposure

Sevoflurane is one of the most widely used anesthetics with recent concerns rising about its pediatric application. The synaptic toxicity and mechanisms underlying its long‐term cognition impairment remain unclear. In this study, we investigated the expression and roles of homeodomain interacting protein kinase 2 (HIPK2), a stress activating kinase involved in neuronal survival and synaptic plasticity, and its downstream c‐Jun N‐terminal kinase (JNK)/c‐Jun signaling in the long‐term toxicity of neonatal Sevoflurane exposure. Our data showed that neonatal Sevoflurane exposure results in impairment of memory, enhancement of anxiety, less number of excitatory synapses and lower levels of synaptic proteins in the hippocampus of adult rats without significant changes of hippocampal neuron numbers. Up‐regulation of HIPK2 and JNK/c‐Jun was observed in hippocampal granular neurons shortly after Sevoflurane exposure and persisted to adult. 5‐((6‐Oxo‐5‐(6‐(piperazin‐1‐yl)pyridin‐3‐yl)‐1,6‐dihydropyridin‐3‐yl)methylene)thiazolidine‐2,4‐dione trifluoroacetate, antagonist of HIPK2, could significantly rescue the cognition impairment, decrease in long‐term potentiation, reduction in spine density and activation of JNK/c‐Jun induced by Sevoflurane. JNK antagonist SP600125 partially restored synapse development and cognitive function without affecting the expression of HIPK2. These data, in together, revealed a novel role of HIPK2‐JNK/c‐Jun signaling in the long‐term synaptic toxicity and cognition impairment of neonatal Sevoflurane exposure, indicating HIPK2‐JNK/c‐Jun cascade as a potential target for reducing the synaptic toxicity of Sevoflurane.

Cover Image for this issue: doi: 10.1111/jnc.14757.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Sevoflurane activates hippocampal CA3 kainate receptors (Gluk2) to induce hyperactivity during induction and recovery in a mouse model

BACKGROUND

In addition to general anaesthetic effects, sevoflurane can also induce hyperactive behaviours during induction and recovery, which may contribute to neurotoxicity; however, the mechanism of such effects is unclear. Volatile anaesthetics including isoflurane have been found to activate the kainate (GluK2) receptor. We developed a novel mouse model and further explored the involvement of kainate (GluK2) receptors in sevoflurane-induced hyperactivity.

METHODS

Maximal speed, mean speed, total movement distance and resting percentage of C57BL/6 mice were quantitatively measured using behavioural tracking software before and after sevoflurane anaesthesia. Age dependence of this model was also analysed and sevoflurane-induced hyperactivity was evaluated after intracerebral injection of the GluK2 receptor blocker NS-102. Neurones from the hippocampal CA3 region were used to undertake in vitro electrophysiological measurement of kainate currents and miniature excitatory postsynaptic potential (mEPSP).

RESULTS

Sevoflurane induced significant hyperactivities in mice under sevoflurane 1% anaesthesia and during the recovery period, characterized as increased movement speed and total distance. The hyperactivity was significantly increased in young mice compared with adults (P<0.01) and pre-injection of NS-102 significantly prevented this sevoflurane-induced hyperactivity. In electrophysiological experiments, sevoflurane significantly increased the frequency of mEPSP at low concentrations and evoked kainate currents at high concentrations.

CONCLUSIONS

We developed a behavioural model in mice that enabled characterization of sevoflurane-induced hyperactivity. The kainate (GluK2) receptor antagonist attenuated these sevoflurane-induced hyperactivities in vivo, suggesting that kainate receptors might be the underlying therapeutic targets for sevoflurane-induced hyperactivities in general anaesthesia.

Targeted ischemic stroke induction and mesoscopic imaging assessment of blood flow and ischemic depolarization in awake mice

Despite advances in experimental stroke models, confounding factors such as anesthetics used during stroke induction remain. Furthermore, imaging of blood flow during stroke is not routinely done. We take advantage of in vivo bihemispheric transcranial windows for longitudinal mesoscopic imaging of cortical function to establish a protocol for focal ischemic stroke induction in target brain regions using photothrombosis in awake head-fixed mice. Our protocol does not require any surgical steps at the time of stroke induction or anesthetics during either head fixation or photoactivation. In addition, we performed laser speckle contrast imaging and wide-field calcium imaging to reveal the effect of cortical spreading ischemic depolarization after stroke in both anesthetized and awake animals over a spatial scale encompassing both hemispheres. With our combined approach, we observed ischemic depolarizing waves (3 to [Formula: see text]) propagating across the cortex 1 to 5 min after stroke induction in genetically encoded calcium indicator mice. Measures of blood flow by laser speckle were correlated with neurological impairment and lesion volume, suggesting a metric for reducing experimental variability. The ability to follow brain dynamics immediately after stroke as well as during recovery may provide a valuable guide to develop activity-dependent therapeutic interventions to be performed shortly after stroke induction.

In vivo field recordings effectively monitor the mouse cortex and hippocampus under isoflurane anesthesia

Isoflurane is a widely used inhaled anesthetic in the clinical setting. However, the mechanism underlying its effect on consciousness is under discussion. Therefore, we investigated the effect of isoflurane on the hippocampus and cortex using an in vivo field recording approach. Our results showed that 1.3%, 0.8%, and 0.4% isoflurane exerted an inhibitory influence on the mouse hippocampus and cortex. Further, high frequency bands in the cortex and hippocampus showed greater suppression with increasing isoflurane concentration. Our findings suggest that in vivo field recordings can monitor the effect of isoflurane anesthesia on the mouse cortex and hippocampus.

The potential for animal models to provide insight into mild traumatic brain injury: Translational challenges and strategies

Mild traumatic brain injury (mTBI) is a common health problem. There is tremendous variability and heterogeneity in human mTBI, including mechanisms of injury, biomechanical forces, injury severity, spatial and temporal pathophysiology, genetic factors, pre-injury vulnerability and resilience factors, and clinical outcomes. Animal models greatly reduce this variability and heterogeneity, and provide a means to study mTBI in a rigorous, controlled, and efficient manner. Rodent models, in particular, are time- and cost-efficient, and they allow researchers to measure morphological, cellular, molecular, and behavioral variables in a single study. However, inter-species differences in anatomy, morphology, metabolism, neurobiology, and lifespan create translational challenges. Although the term "mild" TBI is used often in the pre-clinical literature, clearly defined criteria for mild, moderate, and severe TBI in animal models have not been agreed upon. In this review, we introduce current issues facing the mTBI field, summarize the available research methodologies and previous studies in mTBI animal models, and discuss how a translational research approach may be useful in advancing our understanding and management of mTBI.

Exposure to Sevoflurane Affects the Development of Parvalbumin Interneurons in the Main Olfactory Bulb in Mice

Sevoflurane is widely used in adult and pediatric patients during clinical surgeries. Although studies have shown that exposure to sevoflurane impairs solfactory memory after an operation, the neuropathological changes underlying this effect are not clear. This study detected the effect of sevoflurane exposure on the development of calcium-binding proteins-expressing interneurons in the main olfactory bulb (MOB). We exposed neonatal mice to 2% sevoflurane at two different developmental time points and found that exposing mice to sevoflurane at postnatal day (PD) 7 significantly decreased the expression of GAD67 and parvalbumin (PV) in the olfactory bulb (OB) but did not alter the expression of calretinin (CR) or calbindin D28k (CB). The number and dendritic morphology of PV-expressing interneurons in the MOB were impaired by exposure to sevoflurane at PD7. However, exposure to sevoflurane at PD10 had no effect on calcium-binding protein expression or the number and dendritic morphology of PV-expressing interneurons in the MOB. These results suggest that exposing neonatal mice to sevoflurane during a critical period of olfactory development affects the development of PV-expressing interneurons in the MOB.

Effects of isoflurane and ethanol administration on c-Fos immunoreactivity in mice

Noninvasive functional imaging holds great promise for the future of translational research, due to the ability to directly compare between preclinical and clinical models of psychiatric disorders. Despite this potential, concerns have been raised regarding the necessity to anesthetize rodent and monkey subjects during these procedures, because anesthetics may alter neuronal activity. For example, in studies on drugs of abuse and alcohol, it is not clear to what extent anesthesia can interfere with drug-induced neural activity. Therefore, the current study investigated whole-brain c-Fos activation following isoflurane anesthesia as well as ethanol-induced activation of c-Fos in anesthetized mice. In the first experiment, we examined effects of one or three sessions of gaseous isoflurane on c-Fos activation across the brain in male C57BL/6J mice. Isoflurane administration led to c-Fos activation in several areas, including the piriform cortex and lateral septum. Lower or similar levels of activation in these areas were detected after three sessions of isoflurane, suggesting that multiple exposures may eliminate some of the enhanced neuronal activation caused by acute isoflurane. In the second experiment, we investigated the ability of ethanol injection (1.5 or 2.5g/kgi.p.) to induce c-Fos activation under anesthesia. Following three sessions of isoflurane, 1.5g/kg of ethanol induced c-Fos in the central nucleus of amygdala and the centrally-projecting Edinger-Westphal nucleus (EWcp). This induction was lower after 2.5g/kg of ethanol. These results demonstrate that ethanol-induced neural activation can be detected in the presence of isoflurane anesthesia. They also suggest, that while habituation to isoflurane helps reduce neuronal activation, interaction between effects of anesthesia and alcohol can occur. Studies using fMRI imaging could benefit from using habituated animals and dose-response analyses.

Nitrous oxide directly inhibits action potential-dependent neurotransmission from single presynaptic boutons adhering to rat hippocampal CA3 neurons

We evaluated the effects of N2O on synaptic transmission using a preparation of mechanically dissociated rat hippocampal CA3 neurons that allowed assays of single bouton responses evoked from native functional nerve endings. We studied the effects of N2O on GABAA, glutamate, AMPA and NMDA receptor-mediated currents (IGABA, IGlu, IAMPA and INMDA) elicited by exogenous application of GABA, glutamate, (S)-AMPA, and NMDA and spontaneous, miniature, and evoked GABAergic inhibitory and glutamatergic excitatory postsynaptic current (sIPSC, mIPSC, eIPSC, sEPSC, mEPSC and eEPSC) in mechanically dissociated CA3 neurons. eIPSC and eEPSC were evoked by focal electrical stimulation of a single bouton. Administration of 70% N2O altered neither IGABA nor the frequency and amplitude of both sIPSCs and mIPSCs. In contrast, N2O decreased the amplitude of eIPSCs, while increasing failure rates (Rf) and paired-pulse ratios (PPR) in a concentration-dependent manner. On the other hand, N2O decreased IGlu, IAMPA and INMDA. Again N2O did not change the frequency and amplitude of either sEPSCs of mEPSCs. N2O also decreased amplitudes of eEPSCs with increased Rf and PPR. The decay phases of all synaptic responses were unchanged. The present results indicated that N2O inhibits the activation of AMPA/KA and NMDA receptors and also that N2O preferentially depress the action potential-dependent GABA and glutamate releases but had little effects on spontaneous and miniature releases.

Early alterations in functional connectivity and white matter structure in a transgenic mouse model of cerebral amyloidosis

Impairment of brain functional connectivity (FC) is thought to be an early event occurring in diseases with cerebral amyloidosis, such as Alzheimer's disease. Regions sustaining altered functional networks have been shown to colocalize with regions marked with amyloid plaques burden suggesting a strong link between FC and amyloidosis. Whether the decline in FC precedes amyloid plaque deposition or is a consequence thereof is currently unknown. The sequence of events during early stages of the disease is difficult to capture in humans due to the difficulties in providing an early diagnosis and also in view of the heterogeneity among patients. Transgenic mouse lines overexpressing amyloid precursor proteins develop cerebral amyloidosis and constitute an attractive model system for studying the relationship between plaque and functional changes. In this study, ArcAβ transgenic and wild-type mice were imaged using resting-state fMRI methods across their life-span in a cross-sectional design to analyze changes in FC in relation to the pathology. Transgenic mice show compromised development of FC during the first months of postnatal life compared with wild-type animals, resulting in functional impairments that affect in particular the sensory-motor cortex already in preplaque stage. These functional alterations were accompanied by structural changes as reflected by reduced fractional anisotropy values, as derived from diffusion tensor imaging. Our results suggest cerebral amyloidosis in mice is preceded by impairment of neuronal networks and white matter structures. FC analysis in mice is an attractive tool for studying the implications of impaired neuronal networks in models of cerebral amyloid pathology.

Redox Changes Induced by General Anesthesia in Critically Ill Patients with Multiple Traumas

The critically ill polytrauma patient is a constant challenge for the trauma team due to the complexity of the complications presented. Intense inflammatory response and infections, as well as multiple organ dysfunctions, significantly increase the rate of morbidity and mortality in these patients. Moreover, due to the physiological and biochemical imbalances present in this type of patients, the bioproduction of free radicals is significantly accelerated, thus installing the oxidative stress. In the therapeutic management of such patients, multiple surgical interventions are required and therefore they are being subjected to repeated general anesthesia. In this paper, we want to present the pathophysiological implications of oxidative stress in critically ill patients with multiple traumas and the implications of general anesthesia on the redox mechanisms of the cell. We also want to summarize the antioxidant treatments able to reduce the intensity of oxidative stress by modulating the biochemical activity of some cellular mechanisms.

Distinctive recruitment of endogenous sleep-promoting neurons by volatile anesthetics and a nonimmobilizer

BACKGROUND

Numerous studies demonstrate that anesthetic-induced unconsciousness is accompanied by activation of hypothalamic sleep-promoting neurons, which occurs through both pre- and postsynaptic mechanisms. However, the correlation between drug exposure, neuronal activation, and onset of hypnosis remains incompletely understood. Moreover, the degree to which anesthetics activate both endogenous populations of γ-aminobutyric acid (GABA)ergic sleep-promoting neurons within the ventrolateral preoptic (VLPO) and median preoptic nuclei remains unknown.

METHODS

Mice were exposed to oxygen, hypnotic doses of isoflurane or halothane, or 1,2-dichlorohexafluorocyclobutane (F6), a nonimmobilizer. Hypothalamic brain slices prepared from anesthetic-naive mice were also exposed to oxygen, volatile anesthetics, or F6 ex vivo, both in the presence and absence of tetrodotoxin. Double-label immunohistochemistry was performed to quantify the number of c-Fos-immunoreactive nuclei in the GABAergic subpopulation of neurons in the VLPO and the median preoptic areas to test the hypothesis that volatile anesthetics, but not nonimmobilizers, activate sleep-promoting neurons in both nuclei.

RESULTS

In vivo exposure to isoflurane and halothane doubled the fraction of active, c-Fos-expressing GABAergic neurons in the VLPO, whereas F6 failed to affect VLPO c-Fos expression. Both in the presence and absence of tetrodotoxin, isoflurane dose-dependently increased c-Fos expression in GABAergic neurons ex vivo, whereas F6 failed to alter expression. In GABAergic neurons of the median preoptic area, c-Fos expression increased with isoflurane and F6, but not with halothane exposure.

CONCLUSIONS

Anesthetic unconsciousness is not accompanied by global activation of all putative sleep-promoting neurons. However, within the VLPO hypnotic doses of volatile anesthetics, but not nonimmobilizers, activate putative sleep-promoting neurons, correlating with the appearance of the hypnotic state.

Effects of the volatile anesthetic sevoflurane on tonic GABA currents in the mouse striatum during postnatal development

The volatile anesthetic sevoflurane, which is widely used in pediatric surgery, has proposed effects on GABAA receptor-mediated extrasynaptic tonic inhibition. In the developing striatum, medium-sized spiny projection neurons have tonic GABA currents, which function in the excitatory/inhibitory balance and maturation of striatal neural circuits. In this study, we examined the effects of sevoflurane on the tonic GABA currents of medium spiny neurons in developing striatal slices. Sevoflurane strongly increased GABAA receptor-mediated tonic conductance at postnatal days 3-35. The antagonist of the GABA transporter-1, 1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride further increased tonic GABA conductance during the application of sevoflurane, thereby increasing the total magnitude of tonic currents. Both GABA (5 μM) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol hydrochloride, the δ-subunit-containing GABAA receptor agonist, induced tonic GABA currents in medium spiny neurons but not in cholinergic neurons. However, sevoflurane additively potentiated the tonic GABA currents in both cells. Interestingly, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol hydrochloride-sensitive neurons made a large current response to sevoflurane, indicating the contribution of the δ-subunit on sevoflurane-enhanced tonic GABA currents. Our findings suggest that sevoflurane can affect the tone of tonic GABA inhibition in a developing striatal neural network.

Induction of ischemic stroke in awake freely moving mice reveals that isoflurane anesthesia can mask the benefits of a neuroprotection therapy

Anesthetics such as isoflurane are commonly used to sedate experimental animals during the induction of stroke. Since these agents are known to modulate synaptic excitability, inflammation and blood flow, they could hinder the development and discovery of new neuroprotection therapies. To address this issue, we developed a protocol for inducing photothrombotic occlusion of cerebral vessels in fully conscious mice and tested two potential neuroprotectant drugs (a GluN2B or α4β2 nicotinic receptor antagonist). Our data show in vehicle treated mice that just 20 min of exposure to isoflurane during stroke induction can significantly reduce ischemic cortical damage relative to mice that were awake during stroke. When comparing potential stroke therapies, none provided any level of neuroprotection if the stroke was induced with anesthesia. However, if mice were fully conscious during stroke, the α4β2 nicotinic receptor antagonist reduced ischemic damage by 23% relative to vehicle treated controls, whereas the GluN2B antagonist had no significant effect. These results suggest that isoflurane anesthesia can occlude the benefits of certain stroke treatments and warrant caution when using anesthetics for pre-clinical testing of neuroprotective agents.

The effects of volatile anesthetics on the extracellular accumulation of [(3)H]GABA in rat brain cortical slices

GABA is an inhibitory neurotransmitter that appears to be associated with the action of volatile anesthetics. These anesthetics potentiate GABA-induced postsynaptic currents by synaptic GABAA receptors, although recent evidence suggests that these agents also significantly affect extrasynaptic GABA receptors. However, the effect of volatile anesthetics on the extracellular concentration of GABA in the central nervous system has not been fully established. In the present study, rat brain cortical slices loaded with [(3)H]GABA were used to investigate the effect of halothane and sevoflurane on the extracellular accumulation of this neurotransmitter. The accumulation of [(3)H]GABA was significantly increased by sevoflurane (0.058, 0.11, 0.23, 0.46, and 0.93 mM) and halothane (0.006, 0.012, 0.024, 0.048, 0072, and 0.096 mM) with an EC50 of 0.26 mM and 35 μM, respectively. TTX (blocker of voltage-dependent Na(+) channels), EGTA (an extracellular Ca(2+) chelator) and BAPTA-AM (an intracellular Ca(2+) chelator) did not interfere with the accumulation of [(3)H]GABA induced by 0.23 mM sevoflurane and 0.048 mM halothane. SKF 89976A, a GABA transporter type 1 (GAT-1) inhibitor, reduced the sevoflurane- and halothane-induced increase in the accumulation of GABA by 57 and 63 %, respectively. Incubation of brain cortical slices at low temperature (17 °C), a condition that inhibits GAT function and reduces GABA release through reverse transport, reduced the sevoflurane- and halothane-induced increase in the accumulation of [(3)H]GABA by 82 and 75 %, respectively, relative to that at normal temperature (37 °C). Ouabain, a Na(+)/K(+) ATPase pump inhibitor, which is known to induce GABA release through reverse transport, abolished the sevoflurane and halothane effects on the accumulation of [(3)H]GABA. The effect of sevoflurane and halothane did not involve glial transporters because β-alanine, a blocker of GAT-2 and GAT-3, did not inhibit the effect of the anesthetics. In conclusion, the present study suggests that sevoflurane and halothane increase the accumulation of GABA by inducing the reverse transport of this neurotransmitter. Therefore, volatile anesthetics could interfere with neuronal excitability by increasing the action of GABA on synaptic and extrasynaptic GABA receptors.

Awake nonhuman primate brain PET imaging with minimal head restraint: evaluation of GABAA-benzodiazepine binding with 11C-flumazenil in awake and anesthetized animals

Neuroreceptor imaging in the nonhuman primate (NHP) is valuable for translational research approaches in humans. However, most NHP studies are conducted under anesthesia, which affects the interpretability of receptor binding measures. The aims of this study were to develop awake NHP imaging with minimal head restraint and to compare in vivo binding of the γ-aminobutyric acid type A (GABAA)-benzodiazepine radiotracer (11)C-flumazenil under anesthetized and awake conditions. We hypothesized that (11)C-flumazenil binding potential (BPND) would be higher in isoflurane-anesthetized monkeys.

METHODS

The small animal PET scanner was fitted to a mechanical device that raised and tilted the scanner 45° while the awake NHP was tilted back 35° in a custom chair for optimal brain positioning, which required acclimation of the animals to the chair, touch-screen tasks, intravenous catheter insertion, and tilting. For PET studies, the bolus-plus-constant infusion method was used for (11)C-flumazenil administration. Two rhesus monkeys were scanned under the awake (n = 6 scans) and isoflurane-anesthetized (n = 4 scans) conditions. An infrared camera was used to track head motion during PET scans. Under the awake condition, emission and head motion-tracking data were acquired for 40-75 min after injection. Anesthetized monkeys were scanned for 90 min. Cortisol measurements were acquired during awake and anesthetized scans. Equilibrium analysis was used for both the anesthetized (n = 4) and the awake (n = 5) datasets to compute mean BPND images in NHP template space, using the pons as a reference region. The percentage change per minute in radioactivity concentration was calculated in high- and low-binding regions to assess the quality of equilibrium.

RESULTS

The monkeys acclimated to procedures in the NHP chair necessary to perform awake PET imaging. Image quality was comparable between awake and anesthetized conditions. The relationship between awake and anesthetized values was BPND (awake) = 0.94 BPND (anesthetized) + 0.36 (r(2) = 0.95). Cortisol levels were significantly higher under the awake condition (P < 0.05).

CONCLUSION

We successfully performed awake NHP imaging with minimal head restraint. There was close agreement in (11)C-flumazenil BPND values between awake and anesthetized conditions.

Early life exposure to sevoflurane impairs adulthood spatial memory in the rat

Sevoflurane is a general anesthetic commonly used in the pediatric setting because it is sweet-smelling, nonflammable, fast acting and has a very short recovery time. Although recent clinical data suggest that early anesthesia exposure is associated with subsequent learning and memory problems, it is difficult to determine the exact scope of developmental neurotoxicity associated with exposure to specific anesthetics such as sevoflurane. This is largely due to inconsistencies in the literature. Thus, in the present studies we evaluated the effect of early life exposure to sevoflurane (1%, 2%, 3% or 4%) on adulthood memory impairment in Sprague-Dawley rats. Animals were exposed to different regimens of sevoflurane anesthesia on postnatal days (PNDs) 3, 7, or 14 or at 7 weeks (P7W) of age and spatial memory performance was assessed in adulthood using the Morris Water Maze (MWM). Rats exposed to sevoflurane exhibited significant memory impairment which was concentration and exposure duration dependent. Disruption of MWM performance was more severe in animals exposed on both PNDs 3 and 7 than in animals exposed on both PNDs 3 and 14. The younger the animal's age at the time of exposure, the more significant the effect on later MWM performance. Compared to the neonates, animals exposed at P7W were relatively insensitive to sevoflurane: memory was impaired in this group only after repeated exposures to low doses or single exposures to high doses. Early life exposure to sevoflurane can result in spatial memory impairments in adulthood and the shorter the interval between exposures, the greater the deficit.

Glutamate transporter type 3 mediates isoflurane preconditioning-induced acute phase of neuroprotection in mice

A pre-exposure to isoflurane reduces ischemic brain injury in rodents (isoflurane preconditioning). This neuroprotection has acute and delayed phases. Our previous in vitro studies suggest that the acute phase may involve excitatory amino acid transporters (EAATs). We determine whether this protection involves EAAT3, the major neuronal EAAT. Adult male EAAT3 knockout mice and their wild-type littermates were exposed or were not exposed to 1.5% isoflurane for 30 min. Sixty minutes later, they were subjected to a 90- or 60-min middle cerebral arterial occlusion (MCAO). Their neurological outcomes were evaluated 24h after the MCAO. In another experiment, cerebral cortex was harvested for Western blotting at 30 min after animals were exposed to 1.5% isoflurane for 30 min. Here, we showed that isoflurane reduced brain infarct volumes and improved neurological functions of wild-type mice after a 90-min MCAO. However, isoflurane pre-exposure did not change the neurological outcome of EAAT3 knockout mice no matter whether the MCAO was for 90 min or 60 min. Isoflurane increased phospho-Akt, a survival-promoting protein, in the wild-type mice but not in the EAAT3 knockout mice. The isoflurane-induced neuroprotection in the wild-type mice was abolished by LY294004, an Akt activation inhibitor. LY294004 alone did not affect the neurological outcome of the wild-type or EAAT3 knockout mice after focal brain ischemia. These results suggest that the isoflurane preconditioning-induced acute phase of neuroprotection involves EAAT3. The downstream event includes Akt activation.